US20090095637A1

US20090095637A1 - Electrochemical polishing method and polishing method

Info

Publication number: US20090095637A1
Application number: US12/285,549
Authority: US
Inventors: Yasushi Toma; Akira Kodera; Hirokuni Hiyama
Original assignee: Individual
Current assignee: Ebara Corp
Priority date: 2007-10-10
Filing date: 2008-10-08
Publication date: 2009-04-16
Also published as: TW200936309A

Abstract

The present invention provides an electrochemical polishing method capable of increasing a polishing speed while preventing excessive polishing, such as dishing or erosion. In the electrochemical polishing method, when a voltage applied to a conductive film formed on the surface of a substrate is increased at a contact surface pressure of 0 between the surface of the substrate and a polishing pad, a voltage at a first change point C that allows a current density to start to decrease after an increase is referred to as a minimum voltage. In addition, when the voltage is increased at a contact surface pressure having a finite value, a voltage at a second change point B that allows the current density to be maintained constant after the decrease is referred to as a maximum voltage. In this case, the surface of the conductive film is polished while maintaining the voltage to be not lower than the minimum voltage and not higher than the maximum voltage. Further, the present invention provides an electrochemical polishing method capable of rapidly removing a conductive film in regions other than a contact plug or wiring line forming region while preventing excessive polishing, such as dishing or erosion. In the electrochemical polishing method, in a step of increasing a voltage, when the voltage is increased at a contact surface pressure of 0, a voltage at a first change point C that allows a current density to start to decrease after an increase is referred to as a threshold voltage, and the voltage is increased such that a voltage in a region in which a barrier film is exposed is higher than the threshold voltage.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to an electrochemical polishing method and a polishing method.
Priority is claimed on Japanese Patent Application No. 2007-264468, filed Oct. 10, 2007, and Japanese Patent Application No. 2007-275684, filed Oct. 23, 2007, the content of which is incorporated herein by reference.
2. Description of Related Art
As a process of forming wiring lines of a semiconductor device, a so-called damascene process has been used which fills wiring concave portions, such as trenches or via holes, provided in an insulating film with a wiring metal material. As shown in FIG. 6A, in the damascene process, a contact plug or wiring line forming concave portion (hereinafter, referred to as a ‘concave portion’) 63 is formed in an insulating film (interlayer insulating film) 62 made of a so-called low-k material formed on a substrate, and a barrier metal film (hereinafter, referred to as a ‘barrier film’) 64, made of, for example, a titanium oxide, is formed on the entire surface of the interlayer insulating film 62 including the concave portion 63. Then, a conductive metal film (hereinafter, referred to as a ‘conductive film’) 66, made of, for example, tungsten, is formed on the barrier film 64 to fill the concave portion 63 with the conductive metal material. Then, a concave portion 67 is formed in the conductive film 66 so as to correspond to the concave portion 63 of the interlayer insulating film 62. Thereafter, the conductive film 66 and the barrier film 64 formed outside the concave portion 63 are removed.
The conductive film 66 made of tungsten is removed by chemical mechanical polishing (CMP). As shown in FIG. 11, the CMP is performed as follows. A slurry 52 is supplied to a conductive film formed on a substrate W, and the substrate W is moved relative to a polishing pad 101 while pressing the surface of the substrate W against the polishing pad 101, thereby polishing the surface of the conductive film (for example, see Non-Patent Document 1, that is, Toshiro Doi, “Details of Semiconductor CMP Technology”, Kogyo-chosakai, December, 2000, pp. 277-284). Abrasive grains and an oxidizing agent are added to the slurry 52, and a tungsten oxide film is formed on the surface of the conductive film 66 shown in FIG. 6A by the oxidizing agent. Since the tungsten oxide film formed on an upper part H (outside the concave portion 67) of the conductive film contacts the polishing pad and is then removed, the conductive film 66 on the upper part H is polished. In contrast, since the tungsten oxide film formed on a lower part L (inside the concave portion 67) of the conductive film does not contact the polishing pad, the tungsten oxide film on the lower part of the conductive film 66 is not polished. In this way, a step portion (concave portion 67) on the conductive film 66 is removed.
In order to increase the polishing speed, a high polishing pressure is needed in addition to slurry including high-density abrasive grains. However, in this case, damages, such as scratches, or dishing (over-polishing of the conductive film) is more likely to occur in the surface of the conductive film. In addition, the interlayer insulating film, which should not be polished, is also polished, and erosion (over-polishing of the insulating film) is more likely to occur.
An object of the present invention is to provide an electrochemical polishing method and a polishing method capable of rapidly removing a step portion on the surface of a conductive film while preventing over-polishing, such as dishing or erosion. The term ‘electrochemical polishing’ may be considered as a derived form of ‘electrolytic polishing’ and it may be included in the ‘electrolytic polishing’. However, in the specification, the electrolytic polishing and the electrochemical polishing are unified into the ‘electrochemical polishing’. The term ‘electrolytic polishing’ means a polishing method that electrically connects a conductive material and an opposite electrode by an electrolyte to polish the conductive material by an electrochemical reaction, which includes electrochemical mechanical polishing, which will be described below. In addition, chemical mechanical polishing (CMP) means a wet mechanical chemical polishing method that is developed to planarize an ultra LSI device (planarize an interlayer film of a multi-layer wiring line) and uses a solid-liquid reaction between a workpiece and a polishing agent. The electrochemical polishing means a polishing method that electrically connects a conductive material and an opposite electrode by an electrolyte to polish the conductive material by an electrochemical action and a mechanical action.
Another object of the present invention is to provide an electrochemical polishing method capable of rapidly removing a conductive film in regions other than a contact plug or wiring line forming region while preventing over-polishing, such as dishing or erosion.

SUMMARY OF THE INVENTION

The inventors have found the following. That is, when an electrolyte having a pH in the range of 4 to 10 contacts with a conductive film to apply a voltage to the conductive film, current density is sharply changed at a predetermined voltage. When the voltage applied to the conductive film is increased from 0 V, the current density is increased in proportion to the voltage. However, when the voltage is higher than a first change voltage at a first change point (for example, a point A of FIG. 7), the current density starts to decrease. When the voltage is higher than a second change voltage at a second change point (for example, a point B of FIG. 7), the decreasing rate of the current density is lowered, and the current density is maintained substantially at a constant level without being reduced any further. When the voltage that is lower than the first change voltage is applied, a protective film that is dissoluble in the electrolyte is formed on the surface of the conductive film. On the other hand, when the voltage that is higher than the second change voltage is applied, a protective film that is insoluble in the electrolyte is formed on the surface of the conductive film.
In the above-mentioned tungsten polishing method using CMP, an acid slurry having a pH of less than 4 is used to form a thick tungsten oxide film, and the tungsten oxide film is mechanically polished. Therefore, there is a concern that the polishing speed is lowered. In addition, a method of using a high polishing pressure in addition to slurry including high-density abrasive grains may be used in order to increase the polishing speed. However, it has been proved that this method also has problems.
The inventors have found that the first and second change voltages are changed by the contact surface pressure between the substrate and the polishing pad. As the contact surface pressure is increased, the first and second change voltages are increased, and the current density is also increased. When a voltage that is lower than a first change voltage (for example, a voltage at a point C of FIG. 7) when the contact surface pressure is 0 is applied, a protective film that is dissoluble in the electrolyte is formed on the entire surface of the conductive film. When a voltage that is higher than the second change voltage (for example, the voltage at the point B of FIG. 7) when the contact surface pressure has a finite value is applied, a protective film that is insoluble in the electrolyte is formed on the entire surface of the conductive film. The same results are obtained regardless of whether there is a step portion on the surface of the conductive film.
According to an aspect of the present invention, when the voltage is increased at a contact surface pressure of 0, a voltage (for example, a voltage at the point C of FIG. 7) that allows the current density to start to decrease after an increase is referred to as a minimum voltage. In addition, when the voltage is increased at a contact surface pressure having a finite value, a voltage (for example, a voltage at the point B of FIG. 7) that allows the current density not to decrease any further after the decrease is referred to as a maximum voltage. In this case, the surface of the conductive film is polished while maintaining the voltage to not lower than the minimum voltage and not higher than the maximum voltage.
According to this structure, when there is a step portion on the surface of the conductive film, a dissoluble protective film is formed on an upper part (for example, an upper part H of FIG. 8B) where the contact surface pressure between the conductive film and the polishing pad has a finite value, and the protective film is completely removed by polishing. In addition, an insoluble protective film is formed on a lower part (for example, a lower part L of FIG. 8B) where the contact surface pressure between the conductive film and the polishing pad is 0, and the protective film remains without being polished. In this way, in the upper part, the dissolution of the conductive film in the electrolyte is accelerated, and in the lower part, the dissolution of the conductive film is prevented. Therefore, it is possible to rapidly remove the step portion on the surface of the conductive film.
Since the polishing speed of the lower part covered by the protective film is very low, dishing is less likely to occur in the conductive film. In addition, it is possible to control the polishing speed of the upper part by adjusting the voltage applied. Therefore, it is possible to reduce the contact surface pressure between the polishing pad and the conductive film. As a result, it is possible to prevent erosion.
According to another aspect of the present invention, when the voltage is increased at a contact surface pressure of 0, a voltage (for example, a voltage at the point C of FIG. 7) that allows a current density to start to decrease after an increase is referred to as a minimum voltage. When the voltage is increased at a contact surface pressure having a finite value, a voltage (for example, a voltage at the point B of FIG. 7) that allows the current density to start to decrease after an increase is referred to as a maximum voltage. The surface of the conductive film is polished while maintaining the voltage at a level not lower than the minimum voltage and not higher than the maximum voltage.
According to this structure, when there is a step portion on the surface of the conductive film, it is possible to increase the difference between the current density of an upper part (for example, the upper part H of FIG. 8B) where the contact surface pressure between the conductive film and the polishing pad has a finite value and the current density of a lower part (for example, the lower part L of FIG. 8B) where the contact surface pressure between the conductive film and the polishing pad is 0. The difference between the current densities corresponds to a difference in polishing speed between the upper part and the lower part of the conductive film. For example, as the current density is increased, the polishing speed is increased. Therefore, as the difference between the current densities is increased, the difference between the polishing speeds is increased. In addition, as the current density is reduced, the difference between the polishing speeds is decreased. Therefore, it is possible to control the polishing speed by appropriately controlling the current density. As a result, it is possible to rapidly remove the step portion on the surface of the conductive film.
According to still another aspect of the present invention, the polishing speed of the conductive film may be controlled by adjusting the pH of the electrolyte.
As the pH of the electrolyte is increased, the dissolution of the conductive film is accelerated. Therefore, it is possible to control the polishing speed of the conductive film by adjusting the pH of the electrolyte.
According to yet another aspect of the present invention, the electrolyte may include an additive that reacts with the conductive film to generate an electrical insulating material, and the polishing speed of the conductive film may be controlled by adjusting the concentration of the additive.
As the concentration of the additive is increased, the rigidity of the protective film is increased, and it is not easy to polish and remove the protective film. Therefore, it is possible to control the polishing speed of the conductive film by adjusting the concentration of the additive.
According to still yet another aspect of the present invention, the polishing speed of the conductive film may be controlled by adjusting the number of revolutions of the polishing pad.
As the number of revolutions of the polishing pad is increased, the protective film is rapidly removed, and the dissolution of the conductive film is accelerated. Therefore, it is possible to control the polishing speed of the conductive film by adjusting the number of revolutions of the polishing pad.
According to yet still another aspect of the present invention, a polishing method includes: a protective film forming step of applying a voltage that is not lower than a threshold voltage to a conductive film contacting an electrolyte, without contacting a substrate with a polishing pad, thereby forming a protective film on the surface of the conductive film; and a polishing step of moving the substrate relative to the polishing pad while pressing the surface of the substrate against the polishing pad at a predetermined contact surface pressure, thereby polishing the conductive film formed on the surface of the substrate. In the polishing method, when the voltage that is applied to the conductive film contacting the electrolyte is increased at a contact surface pressure of 0 while moving the substrate relative to the polishing pad, a voltage that allows a current density not to decrease after an increase is referred to as the threshold voltage.
According to this structure, in the step of forming the protective film, a voltage that is higher than the threshold voltage is applied to form a protective film that is insoluble in the electrolyte. In the next polishing process, the protective film formed on the upper part of the conductive film is removed by polishing, but the protective film formed on the lower part of the conductive film remains without being polished. In this way, it is possible to rapidly remove a step portion on the surface of the conductive film.
According to still yet another aspect of the present invention, the polishing step may be a chemical mechanical polishing step or an electrochemical polishing step.
Since the protective film is formed before the polishing step, any of a chemical mechanical polishing method and an electrochemical polishing method can be used to rapidly remove a step portion on the surface of the conductive film.
The inventors have found that the polishing speed of each portion of the substrate is controlled by adjusting a voltage applied to each portion of the substrate. That is, the potential of each portion of the substrate is adjusted by using a difference in electric resistance between an underlying film, which is a low layer, and a conductive film, which is an upper layer, of a metal film formed on, the substrate. A voltage is applied to the surface of the substrate from a voltage application point provided on the surface of the metal film. When the conductive film remains on the entire surface of the substrate, a substantially constant voltage is applied to the entire surface of the substrate. However, when a portion of the conductive film in the vicinity of the voltage application point is removed to expose a portion of the underlying film, the voltage distribution of the surface of the substrate is changed. Specifically, since a voltage is applied to the conductive film through the underlying film having an electric resistance that is larger than that of the conductive film, the voltage applied to the conductive film is reduced as the distance from the voltage application point is increased.
According to yet still another aspect of the present invention, an electrochemical polishing method includes: a step of contacting an electrolyte with a metal film formed on the surface of a substrate to apply a voltage to the metal film, the electrolyte having a pH in the range of 4 to 10 and the metal film including an underlying film, which is a lower layer, and a conductive film, which is an upper layer, having an electric resistance that is lower than that of the underlying film; a step of moving the substrate relative to a polishing pad while pressing the surface of the substrate against the polishing pad at a predetermined contact surface pressure, thereby polishing the surface of the metal film; a step of removing a portion of the conductive film in the vicinity of a point where the voltage is applied first to expose the underlying film; and a step of increasing the voltage immediately before the exposure of the underlying film or after the exposure of the underlying film the underlying film is exposed. In the step of increasing the voltage, when the voltage is increased at a contact surface pressure of 0 while moving the substrate relative to the polishing pad, a voltage that allows a current density to start to decrease after an increase is referred to as a threshold voltage, and the voltage is increased such that the voltage in a region in which the underlying film is exposed is higher than the threshold voltage.
According to this structure, when the voltage in the exposed region of the underlying film is higher than the threshold voltage, the current density starts to decrease, and the polishing speed of the conductive film is reduced. Therefore, even when a conductive film forming, for example, a contact plug or a wiring line exists in the exposed region of the underlying layer, dishing is less likely to occur in the surface of the conductive film. In addition, it is not necessary to use a slurry having a high abrasive grain density in order to perform polishing using electrochemical dissolution, unlike in the related art, and it is also not necessary to perform polishing at a high contact surface pressure. Therefore, the ratio of a mechanical polishing operation is lowered, and it is possible to prevent the occurrence of erosion. The contact surface pressure has a finite value, for example, in the range of about 0.5 to 4 psi.
In the region in which the conductive film having an electric resistance that is lower than that of the underlying film remains, the voltage applied to the conductive film is reduced as a distance from the exposed portion of the underlying film is increased. Therefore, in the region, the current density is higher than that in the exposed region of the underlying film. In the region in which the conductive film remains, polishing is continuously performed, and it is possible to rapidly remove the conductive film while preventing damages, such as scratches, or dishing.
According to still yet another aspect of the present invention, in the step of increasing the voltage, when the voltage is increased at a contact surface pressure having a finite value, the voltage that allows the current density not to decrease any further after an increase is referred to as the maximum voltage, and the voltage is increased such that the voltage in regions other than the region in which the underlying film is exposed is higher than the threshold voltage and lower than the maximum voltage.
According to this structure, the voltage in regions other than the region in which the underlying film is exposed, that is, the voltage in the region in which the conductive film remains, is set to be higher than the threshold voltage and lower than the maximum voltage. In the region in which the conductive film remains, even when there is a conductive film forming a contact plug or a wiring line, the current density is maintained at a high level until the conductive film is polished and planarized so as to be flush with the underlying film.
Therefore, it is possible to continuously perform polishing to planarize the conductive film in the region in which the conductive film remains.
According to yet still another aspect of the present invention, in the step of exposing the underlying film, the contact surface pressure between the substrate and the polishing pad in the region in the vicinity of the voltage application point is higher than that in regions other than the region in the vicinity of the voltage application point.
According to this structure, the contact surface pressure in the region in the vicinity of the voltage application point is higher than that in regions other than the region in the vicinity of the voltage application point. In this way, the region in the vicinity of the voltage application point is polished more rapidly than regions other than the region in the vicinity of the voltage application point, and the conductive film in the region is removed first. Therefore, it is possible to obtain a voltage distribution in which the highest voltage is applied to a portion of the metal film in the vicinity of the voltage application point and the voltage is reduced as the distance from the voltage application point is increased. As a result, it is possible to accurately remove the remaining conductive film in the step of increasing the voltage after the underlying film is exposed.
According to still yet another aspect of the present invention, in the step of exposing the underlying film, an electrode opposite to the substrate is divided into a plurality of small electrodes that are concentrically arranged on the same plane, and the voltage applied to the divided electrodes is controlled such that a portion of the conductive film that is more frequently opposite to an outer portion of the substrate is more rapidly polished.
According to this structure, the voltage applied to the divided electrodes is controlled such that a portion of the conductive film that is more frequently opposite to the outer portion of the substrate is more rapidly polished. Therefore, polishing is performed so that the thickness of the remaining conductive film is increased from the outer portion to a central portion. As a result, it is possible to reliably expose the underlying film in the circumferential portion of the substrate.
According to yet still another aspect of the present invention, in the polishing step, polishing may be performed while measuring the thickness of the remaining conductive film using an overcurrent method.
According to this structure, the thickness of the remaining conductive film is measured by an overcurrent method. Therefore, it is possible to accurately measure the thickness of the conductive film.
According to yet still another aspect of the present invention, the conductive film may be a tungsten film.
According to this structure, it is possible to easily adjust the polishing speed by using the tungsten film as the conductive film.
According to still yet another aspect of the present invention, an electrochemical polishing method includes: a step of contacting an electrolyte with a metal film formed on the surface of a substrate to apply a voltage to the metal film, the electrolyte having a pH in the range of 4 to 10 and the metal film including an underlying film, which is a lower layer, and a conductive film, which is an upper layer, having an electric resistance that is lower than that of the underlying film; a step of moving the substrate relative to a polishing pad while pressing the surface of the substrate against the polishing pad at a predetermined contact surface pressure, thereby polishing the surface of the metal film; a step of arranging a point where the voltage is applied in a circumferential portion of the substrate and removing a portion of the conductive film in the circumferential portion first to expose the underlying film; and a step of moving the voltage application point from the circumferential portion to the center of the substrate to expose the underlying film.
According to this structure, after the underlying film is exposed, the voltage application point is moved from the circumferential portion to the central portion of the substrate. Therefore, it is possible to maintain the voltage applied to the exposed region of the underlying film at a high level. In this way, even when a conductive film forming, for example, a contact plug or a wiring line exists in the exposed region of the underlying layer, dishing is less likely to occur in the surface of the conductive film.
According to yet still another aspect of the present invention, an electrochemical polishing method includes: a step of contacting an electrolyte with a metal film formed on the surface of a substrate to apply a voltage to the metal film, the electrolyte having a pH in the range of 4 to 10 and the metal film including an underlying film, which is a lower layer, and a conductive film, which is an upper layer, having an electric resistance that is lower than that of the underlying film; a step of moving the substrate relative to a polishing pad while pressing the surface of the substrate against the polishing pad at a predetermined contact surface pressure, thereby polishing the surface of the metal film; a step of arranging a point where the voltage is applied in a circumferential portion of the substrate and removing a portion of the conductive film in the circumferential portion first to expose the underlying film; a step of moving the voltage application point from the circumferential portion to the center of the substrate to expose the underlying film; and a step of increasing the voltage immediately before or after the underlying film is exposed. In the step of increasing the voltage, when the voltage is increased at a contact surface pressure of 0 while moving the substrate relative to the polishing pad, a voltage that allows a current density to start to decrease after an increase is referred to as a threshold voltage, and the voltage is increased such that a voltage in a region in which the underlying film is exposed is higher than the threshold voltage.
According to this structure, it is possible to increase the voltage in the exposed region of the underlying film to be higher than the threshold voltage, without sharply increasing the voltage, by moving the voltage application point from the circumferential portion to the central portion of the substrate. In this way, even when a conductive film forming, for example, a contact plug or a wiring line exists in the exposed region of the underlying layer, dishing is less likely to occur in the surface of the conductive film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view illustrating the structure of a substrate processing apparatus;

FIG. 2 is a diagram schematically illustrating the structure of an electrochemical polishing device;

FIG. 3 is a cross-sectional view illustrating a substrate head;

FIG. 4 is a bottom view illustrating the substrate head;

FIG. 5A is a longitudinal sectional view schematically illustrating a main part of the electrochemical polishing device;

FIG. 5B is a longitudinal sectional view schematically illustrating the main part of the electrochemical polishing device;

FIG. 6A is a diagram illustrating electrochemical polishing;

FIG. 6B is a diagram illustrating the electrochemical polishing;

FIG. 6C is a diagram illustrating the electrochemical polishing;

FIG. 7 is a graph illustrating the relationship between the potential and the current density of a conductive film;

FIG. 8A is a diagram illustrating the state of a protective film when a voltage in a range α is applied to the conductive film;

FIG. 8B is a diagram illustrating the state of a protective film when a voltage in a range β is applied to the conductive film;

FIG. 8C is a diagram illustrating the state of a protective film when a voltage in a range γ is applied to the conductive film;

FIG. 9 is a graph illustrating the relationship between the current density and a voltage applied when the pH of an electrolyte is changed;

FIG. 10 is a graph illustrating the relationship between the current density and a voltage applied when the number of revolutions of a polishing pad is changed;

FIG. 11 is a longitudinal sectional view schematically illustrating a main part of a chemical mechanical polishing device;

FIG. 12A is a diagram illustrating electrochemical polishing;

FIG. 12B is a diagram illustrating the electrochemical polishing;

FIG. 12C is a diagram illustrating the electrochemical polishing; and

FIG. 13 is a plan view illustrating another structure of a polishing table.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

(Substrate Processing Apparatus)

FIG. 1 is a plan view illustrating the structure of a substrate processing apparatus including an electrochemical polishing apparatus according to the present invention. A substrate processing apparatus 300 includes a loading/unloading stage that accommodates substrate cassettes 204 stocking a plurality of substrates W. A transfer robot 202 having two hands is arranged on a traveling mechanism 200 such that it can reach all the substrate cassettes 204 in the loading/unloading stage. The traveling mechanism 200 includes a linear motor. The traveling mechanism including the linear motor makes it possible to rapidly and stably transport substrates having a large diameter and weight, for example, silicon wafers having a diameter of 450 mm. An ITM that measures the thickness of the substrate before or after polishing, that is, an in-line thickness monitor 224 is provided on an extension line of the traveling mechanism 200.
Two dry units 212 are provided opposite to the substrate cassettes 204 with the traveling mechanism 200 of the transfer robot 202 interposed therebetween. The dry units 212 are arranged such that the hands of the transfer robot 202 can reach the dry units. A substrate station 206 including four substrate loading tables is provided between the two dry units 212 at a position where the transfer robot 202 can reach the substrate station.
A transfer robot 208 is arranged such that it can reach the dry units 212 and the substrate station 206. Cleaning units 214 are provided adjacent to the dry units 212 at positions where the hands of the transfer robot 208 can reach the cleaning units. A rotary transporter 210 is provided at a position where the hands of the transfer robot 208 can reach the rotary transporter, and two electrochemical polishing devices 250 according to this embodiment are provided at positions where the electrochemical polishing devices can receive or transmit the substrates from or to the rotary transporter 210.
Each of the electrochemical polishing devices 250 include a substrate head 1, a polishing table 100, a polishing pad 101 (see FIG. 2), electrolyte supply nozzles (electrolyte supply portions) 102 that supply an electrolyte to the polishing pad 101, a dresser 218 for dressing the polishing pad 101, and a tank 222 for cleaning the dresser 218.

(Electrochemical Polishing Device)

FIG. 2 is a diagram schematically illustrating the structure of the electrochemical polishing device 250. As shown in FIG. 2, the substrate head 1 is connected to a head driving shaft 11 through a universal joint 10, and the head driving shaft 11 is connected to a head air cylinder 111 fixed to a tilting arm 110. The head air cylinder 111 moves the head driving shaft 11 in the vertical direction to lift up the substrate head 1 and press a retainer ring 3 fixed to the lower end of the head body 2 against the polishing table 100. The head air cylinder 111 is connected to a compressed air source 120 through a regulator RE1, and the regulator RE1 can adjust the pressure of fluid, such as the pressure of pressurized air supplied to the head air cylinder 111. In this way, it is possible to adjust the pressing force of the retainer ring 3 against the polishing pad 101.
The head driving shaft 11 is connected to a rotating cylinder 112 through a key (not shown). The rotating cylinder 112 has a timing pulley 113 in its outer circumferential portion. A head motor 114, which is a driving unit, is fixed to a tilting arm 110, and the timing pulley 113 is connected to a timing pulley 116 provided in the head motor 114 through a timing belt 115. Therefore, when the head motor 114 is driven, the rotating cylinder 112 and the head driving shaft 11 are integrally rotated by the timing pulley 116, the timing belt 115, and the timing pulley 113, and the substrate head 1 is rotated. The tilting arm 110 is supported by a shaft 117 fixed to a frame (not shown).

(Substrate Head)

FIG. 3 is a cross-sectional view illustrating the substrate head 1, and FIG. 4 is a bottom view illustrating the substrate head 1 shown in FIG. 3. As shown in FIG. 3, the substrate head 1 includes a cylindrical head body 2 having a space therein and the retainer ring 3 that is fixed to the lower end of the head body 2. The head body 2 is formed of a material having high strength and rigidity, such as metal or ceramics. The retainer ring 3 is formed of a resin having high rigidity, such as PPS (polyphenylene sulfide) or an insulating material, such as ceramics.
The head body 2 includes a cylindrical housing 2 a, a ring-shaped pressure sheet supporting portion 2 b that is fitted into a cylindrical portion of the housing 2 a, and a ring-shaped sealing portion 2 c that is fitted to an outer circumferential portion of the upper surface of the housing 2 a. A lower part of the retainer ring 3 fixed to the lower surface of the housing 2 a of the head body 2 protrudes inward. The retainer ring 3 may be formed integrally with the head body 2.
The head driving shaft 11 is provided above the center of the housing 2 a of the head body 2, and the head body 2 and the head driving shaft 11 are connected to each other by the universal joint 10. The universal joint 10 includes a spherical bearing mechanism that enables the head body 2 and the head driving shaft 11 to be tilted relative to each other and a rotation transmitting mechanism that transmits the rotation of the head driving shaft 11 to the head body 2. Therefore, the universal joint transmits the pressing force and the rotating force of the head driving shaft 11 to the head body 2 while allowing the tilting of the head body 2 relative to the head driving shaft 11.
The spherical bearing mechanism includes a spherical concave portion 11 a that is formed at the center of the lower surface of the head driving shaft 11, a spherical concave portion 2 d that is formed at the center of the upper surface of the housing 2 a, and a bearing ball 12 that is interposed between the concave portions 11 a and 2 d and is made of a material having high rigidity, such as ceramics. The rotation transmitting mechanism includes a driving pin (not shown) fixed to the head driving shaft 11 and a driven pin (not shown) fixed to the housing 2 a. Even when the head body 2 is inclined, the driven pin and the driving pin can be moved relative to each other in the vertical direction. Therefore, the driven pin and the driving pin are engaged with each other at different contact points, and the rotation transmitting mechanism reliably transmits the torque of the head driving shaft 11 to the head body 2.
An elastic pad 4 that comes into contact with the substrate W, such as a semiconductor wafer, held by the substrate head 1, a ring-shaped holder ring 5, and a chucking plate 6 that has a substantially disk shape and supports the elastic pad 4 are provided in a space defined by the head body 2 and the retainer ring 3 that is integrally fixed to the head body 2. The outer circumferential surface of the elastic pad 4 is interposed between the holder ring 5 and the chucking plate 6 fixed to the lower end of the holder ring 5, and covers the lower surface of the chucking plate 6. In this way, a space is formed between the elastic pad 4 and the chucking plate 6.
A pressure sheet 7, which is an elastic film, is provided between the holder ring 5 and the head body 2. The pressure sheet 7 is formed of a rubber material having high strength and durability, such as ethylene-propylene rubber (EPDM), polyurethane rubber, or silicon rubber. The pressure sheet 7 has one end that is interposed between the housing 2 a and the pressure sheet supporting portion 2 b of the head body 2 and the other end that is interposed between an upper part 5 a and a stopper 5 b of the holder ring 5. The head body 2, the chucking plate 6, the holder ring 5, and the pressure sheet 7 form a pressure chamber 21 inside the head body 2. As shown in FIG. 2, a fluid passage 31, such as a tube or a connector, extends from the pressure chamber 21, and the pressure chamber 21 is connected to the compressed air source 120 through a regulator RE2 provided in the fluid passage 31.
When the pressure sheet 7 shown in FIG. 3 is formed of an elastic material, such as rubber, and the pressure sheet 7 is fixed between the retainer ring 3 and the head body 2, it is difficult for the pressure sheet 7 formed of an elastic material to have a preferable flat surface below the lower surface of the retainer ring 3 due to elastic deformation. Therefore, in order to prevent the pressure sheet from being elastically deformed, in this embodiment, a pressure sheet supporting portion 2 b is additionally provided to fix the pressure sheet 7 between the pressure sheet supporting portion 2 b and the housing 2 a of the head body 2.
A center bag (center contact member) 8 and a ring tube (outside contact member) 9, which are contact members that contact the elastic pad 4, are provided in a space formed between the elastic pad 4 and the chucking plate 6. In this embodiment, as shown in FIGS. 3 and 4, the center bag 8 is provided at the center of the lower surface of the chucking plate 6, and the ring tube 9 is provided outside the center bag 8 so as to surround the center bag 8. The elastic pad 4, the center bag 8, and the ring tube 9 are each formed of a rubber material having high strength and durability, such as ethylene-propylene rubber (EPDM), polyurethane rubber, or silicon rubber, similar to the pressure sheet 7.
As shown in FIG. 3, the space formed between the chucking plate 6 and the elastic pad 4 is partitioned into a plurality of spaces by the center bag 8 and the ring tube 9. A pressure chamber 22 is formed between the center bag 8 and the ring tube 9, and a pressure chamber 23 is formed outside the ring tube 9.
The center bag 8 includes an elastic film 81 that contacts the upper surface of the elastic pad 4 and a center bag holder (holding portion) 82 that detachably holds the elastic film 81. A screw hole 82 a is formed in the center bag holder 82, and a screw 55 is threadably engaged with the screw hole 82 a such that the center bag 8 is detachably mounted to the center of the lower surface of the chucking plate 6. A center pressure chamber 24 is formed by the elastic film 81 and the center bag holder 82 in the center bag 8.
Similarly, the ring tube 9 includes an elastic film 91 that contacts the upper surface of the elastic pad 4 and a ring tube holder (holding portion) 92 that detachably holds the elastic film 91. A screw hole 92 a is formed in the ring tube holder 92, and a screw 56 is threadably engaged with the screw hole 92 a such that the ring tube 9 is detachably mounted to the lower surface of the chucking plate 6. An intermediate pressure chamber 25 is formed by the elastic film 91 and the ring tube holder 92 in the ring tube 9.
Fluid passages 33, 34, 35, and 36 each composed of a tube or a connector communicate with the pressure chambers 22 and 23, the central pressure chamber 24, and the intermediate pressure chamber 25, respectively. The pressure chambers 22 to 25 are connected to a compressed air source 120, which is a supply source, through regulators RE3, RE4, RE 5, and RE6 provided in the fluid passages 33 to 36, respectively. The fluid passages 31 and 33 to 36 are respectively connected to the regulators RE2 to RE6 through a rotary joint (not shown) that is provided at the upper end of the head driving shaft 11.
For example, pressurized fluid, such as pressurized air, is supplied to the pressure chamber 21 provided above the chucking plate 6, and the pressure chambers 22 to 25 through the fluid passages 31 and 33 to 36 communicating with the pressure chambers, respectively. As shown in FIG. 2, the regulators RE2 to RE6 provided above the fluid passages 31 and 33 to 36 of the pressure chambers 21 to 25 can adjust the pressure of pressurized liquid supplied to the pressure chambers. In this way, it is possible to individually control the internal pressure of the pressure chambers 21 to 25 to be the atmospheric pressure or vacuum.
As described above, the regulators RE2 to RE6 can individually control the internal pressure of the pressure chambers 21 to 25 to adjust the pressing force of a portion (a partitioned region) of each of the substrates W against the polishing pad 101 through the elastic pad.
As shown in FIG. 3, a plurality of convex portions 42 protrude from the chucking plate 6 to the pressure chambers 22 and 23. The leading end of each of the convex portions 42 is exposed from the surface of the elastic pad 4 through an opening 41. A fluid passage 43 extends from the leading end of each of the convex portions 42 and is connected to a vacuum source 121 shown in FIG. 2. In this way, it is possible to attract the substrate W to the leading end of each of the convex portions 42 shown in FIG. 3 by vacuum.
As shown in FIG. 2, a sensor coil 228 of an ITM 226, which is composed of, for example, an overcurrent sensor and measures the thickness of, for example, a conductive film of a substrate, is provided in the polishing table 100 of the electrochemical polishing device 250. Signals output from the ITM 226 are input to a control unit 310, and the control unit 310 outputs control signals to control the regulators RE3 to RE6.

(Polishing Table and Polishing Pad)

FIG. 5A is a longitudinal sectional view schematically illustrating a main part of the electrochemical polishing device. A disk-shaped supporting member 254 is fixed to the upper surface of the polishing table 100. The supporting member 254 is formed of a conductive material (for example, metal, alloy, or conductive plastic). A polishing pad 101 is mounted to the upper surface of the supporting member 254, and the upper surface of the polishing pad 101 is a polishing surface. The polishing table 100 is connected to a rotating mechanism (not shown). In this way, the polishing table 100 can be rotated integrally with the supporting member 254 and the polishing pad 101.
An electrolyte supply nozzle 102 extends in the radial direction of the polishing pad 101. An electrolyte supply port 102 a is provided at the leading end of the electrolyte supply nozzle 102. The electrolyte supply port 102 a is positioned above the center of the polishing pad 101, and an electrolyte is supplied from an electrolyte supply source (not shown) to the center of the polishing pad 101 through the electrolyte supply nozzle 102. When the polishing pad 101 is rotated, the electrolyte is spread to the outside and is then filled between the substrate head 1 and the polishing pad 101 and in a plurality of through holes 101 a of the polishing pad 101.
The supporting member 254 is connected to a negative electrode of a power supply 252, and serves as a first electrode (cathode). For example, a cam follower or a brush is provided at an electric contact between a wiring line extending from the power supply 252 and the supporting member (cathode) 254. For example, as shown in FIG. 5A, it is possible to bring an electric contact 262 into contact with the side surface of the supporting member 254. It is preferable that the electric contact 262 be formed of a soft metal material having low resistivity, such as gold, silver, copper, platinum, or palladium.
A second electrode (a feed electrode or a voltage application point) 264 is provided in the vicinity of the edge of the polishing pad 101 so as to be connected to the positive electrode of the power supply 252. The substrate head 1 is arranged such that the substrate W contacts the polishing surface with a portion of the substrate W protruding from the edge of the polishing pad 101, and the lower surface of the substrate W contacts the second electrode 264. In this way, power is supplied from the second electrode 264 to the conductive film of the substrate W. It is also possible to supply power from the second electrode 264 to the conductive film of the substrate W through the retainer ring. The supporting member 254, serving as a cathode, and the conductive film of the substrate W, serving as an anode, are electrically connected to each other by the electrolyte filled in through holes 101 a formed in the polishing pad 101.
FIG. 5B is a longitudinal sectional view schematically illustrating a main part of another electrochemical polishing device. A supporting member 254 of an electrochemical polishing device 250 includes as basic components a disk-shaped base 254 b and a cover 254 a that covers the upper surface of the base 254 b. As described above, since the supporting member 254 serves as a cathode (first electrode), at least one of the cover 254 a and the base 254 b is formed of a conductive material.
A plurality of communicating holes 255 are formed in the cover 254 a of the supporting member 254 at positions corresponding to the through holes 101 a of the polishing pad 101. In addition, a plurality of communicating holes 256 are formed in the lower surface of the cover 254 a so as to communicate with the communicating holes 255. The communicating holes may be formed in the upper surface of the base 254 b. A first electrolyte receiving hole 258A is formed at the center of the polishing pad 101 so as to pass through the polishing pad 101 in the vertical direction. In addition, a second electrolyte receiving hole 258B is formed in the cover 254 a at a position corresponding to the first electrolyte receiving hole 258A. The second electrolyte receiving hole 258B communicates with the plurality of communicating holes 256.
According to the above-mentioned structure, the electrolyte supplied from the supply port 102 a of the electrolyte supply nozzle 102 flow, in order, through the first electrolyte receiving hole 258A, the second electrolyte receiving hole 258B, the communicating hole 256, and the communicating hole 255 to reach the through hole 101 a. A vertical flow passage facing the polishing surface is formed in the through hole 101 a, and the electrolyte flows through the flow passage to be supplied to the polishing surface.

(Electrochemical Polishing Method According to First Embodiment)

Next, an electrochemical polishing method according to a first embodiment of the present invention will be described.
FIG. 6A is a diagram illustrating a substrate before polishing. First, the formation of the substrate W, which is a polishing target according to this embodiment, will be described. An interlayer insulating film 62 that is made of a so-called low-k material or an insulating material, such as SiO₂, SiOF, or SiOC, is formed on the surface of the substrate W made of, for example, silicon. A concave portion 63 for forming a wiring line or a contact plug is formed in the surface of the interlayer insulating film 62. A barrier film 64 that is made of, for example, titanium, tantalum, tungsten, ruthenium and/or alloys thereof, is formed with a thickness of about 10 nm on the surface of the interlayer insulating film 62 including the concave portion 63 formed therein. The barrier film 64 is provided in order to improve the adhesion between a conductive film 66, which will be described below, and the interlayer insulating film 62 and prevent a metal material forming the conductive film 66 from being spread to the substrate W. The conductive film 66 made of tungsten is formed with a thickness in the range of about 500 to 600 nm on the surface of the barrier film 64. When the conductive film 66 is formed by electrolytic plating, a seed film (not shown), serving as an electrode for electrolytic plating, is formed on the surface of the barrier film 64. In addition, a concave portion 67 having a height of about 300 nm and a width of about 100 μm is formed in the surface of the conductive film 66 so as to correspond to the concave portion 63 of the interlayer insulating film 62. The conductive film 66 may be formed of a conductive metal material, such as aluminum, copper, silver, gold, or alloys thereof, in addition to tungsten.
Since only a portion of the conductive film 66 filled in the concave portion 63 of the interlayer insulating film 62 serves as a contact plug or a metal wiring line, the other portions of the conductive film 66 formed outside the concave portion 63 are not needed. Therefore, the unnecessary portion of the conductive film 66 is removed by electrochemical polishing.
FIG. 6B is a diagram illustrating the electrochemical polishing. The electrochemical polishing is performed as follows: an electrolyte 50 comes into contact with the conductive film formed on the surface of the substrate W; a voltage is applied to the conductive film 66; and the substrate W is moved relative to the polishing pad 101 while the surface of the substrate W is pressed against the polishing pad 101, thereby polishing the surface of the conductive film 66.
It is necessary to planarize the surface of the substrate W after the unnecessary portion of the conductive film 66 is removed, in order to form a plurality of wiring lines on the substrate with the interlayer insulating film interposed therebetween. In the electrochemical polishing, a voltage is applied to the conductive film 66 to form a protective film 70 made of an electrical insulating material on the surface of the conductive film 66. The protective film formed on an upper part H (outside the concave portion 67) of the conductive film 66 contacts the polishing pad 101 and is then removed. In this way, the upper part H of the conductive film 66 is dissolved by the electrolyte 50 and then removed. In contrast, a lower part L (inside the concave portion 67) of the conductive film is shielded by the protective film 70 and is not dissolved by the electrolyte 50. Therefore, a step portion of the conductive film is removed. In this way, as shown in FIG. 6C, the surface of the conductive film 66 and the exposed surface of the barrier film 64 are flush with each other. As a result, the substrate W is planarized.
However, a tungsten polishing method using CMP adopts a mechanism that uses acid slurry having a pH of less than 4 to form a tungsten oxide film with a large thickness and mechanically polishes the tungsten oxide film. Therefore, the tungsten polishing method has a problem in that a polishing speed is low. In order to solve this problem, the electrochemical polishing method according to this embodiment uses an electrolyte having a pH in the range of 4 to 10 to form a protective film that is different from a tungsten oxide film according to the related art. In this embodiment, an electrolyte with a pH in the range of 4 to 10 is used before processing. However, during processing, the pH of the electrolyte is changed due to, for example, hydrogen generated during processing. Therefore, when the pH of the electrolyte is in the range of 4 to 10, the pH of the electrolyte falls within the range of 4 to 10 before, during, or after processing, and the pH of the electrolyte is in the above-mentioned range while the processing is actually performed. For example, a pH measuring device D51S (handy type) manufactured by HORIBA, Ltd., or a glass electrode method with a temperature correcting function (JIS No. 1) is used to measure the pH of the electrolyte. The measurement is performed substantially at a room temperature in the range of 20° C. to 30° C.
In this embodiment, any electrolyte can be used as long as it has an appropriate conductivity (about 1 mS/cm). A main electrolyte material contained in the electrolyte may have appropriate conductivity and may not erode the surface of the conductive film. In addition, it is preferable that the electrolyte form a complex or chelate with tungsten to accelerate the dissolution of tungsten and have, for example, an organic acid. For example, it is preferable to use as the organic acid at least one selected from the group consisting of glycolic acid, pyrophoric acid, phosphoric acid, citric acid, malic acid, maleic acid, malonic acid, lactic acid, tartaric acid, succinic acid, and the salts thereof. It is preferable that the organic acid have a pH in the range of 4 to 10 in which tungsten can easily generate a complex or chelate, which is a dissoluble compound.
A pH adjusting component may be included in the electrolyte. For example, alkali metal salt or alkaline-earth metal salt is used as the pH adjusting component. However, it is preferable that ammonium salt be used as the pH adjusting component in order to prevent metal pollution.
The electrolyte may include an additive that generates an electrical insulating material. A primary amine polymer is used as the additive that generates the electrical insulating material. Examples of the primary amine polymer include an allylamine polymer (a polymer having only a primary amino group in a side chain, which has a molecular weight in the range of 1000 to 60000), an allylamine hydrochloride polymer (a molecular weight in the range of 1000 to 60000), a copolymer of allylamine hydrochloride and diallylamine hydrochloride (a molecular weight in the range of 20000 to 100000), an allylamine amide sulfate polymer (a molecular weight of 12000), a copolymer of allylamine acetate and diallylamine acetate (a molecular weight of 100000), a copolymer of allylamine and demethyl allylamine (a molecular weight of 1000), a partially methoxy carbonylated polyallylamine (a molecular weight of 15000), and a partially methyl carbonylated allylamine acetate polymer (a molecular weight of 15000). In addition, polyethylenimine (a molecular weight in the range of 1000 to 70000) may be used as the primary amine polymer. The polyethylenimine is a polymer having a branched structure that simultaneously includes a primary amine, a secondary amine, and a tertiary amine in a molecule. It is effective for the primary amine to exist in the molecule.
The concentration of the additive that generates the electrical insulating material in the polyethylenimine is preferably in the range of 0.01 to 5% by weight, more preferably 0.1 to 1% by weight. If the concentration is lower than 0.01% by weight, a sufficient current suppression effect (electrical insulation) it not obtained. On the other hand, if the concentration is higher than 5% by weight, the current suppression effect tends to be lowered with an increase in the amount of additive. It is not necessary to increase the concentration of the additive to be higher than 5% by weight. The additive makes it possible to reduce the voltage required to remove a step portion, which will be described below.
In the case of CMP, the concentration of abrasive grains needs to be not lower than 10%. However, in this embodiment, it is possible to obtain a sufficient effect even when the concentration is not higher than 1%. A surface active agent may be added in order to improve the dispersibility of the abrasive grains. However, since the concentration of the abrasive grains is low, the surface active agent may not be added.
In this embodiment, ammonium citrate (pH8) is used as an example of the electrolytic solution.
The inventors conducted experiments and evaluation of the polishing speed of tungsten using the following method.
An electrolytic polishing device capable of processing only the substrate having a diameter of 40 mm was used to conduct the experiments. The device can control the electrode potential of a metal film formed on the substrate. The device applies a voltage and polishes the substrate with a polishing pad attached to the polishing table that rotates the exposed metal film. Specifically, the polishing speed is about 50 to 150 nm/min per current density of 10 mA/cm².
The electrode potential was measured by an electrochemical measuring system HZ-3000 (manufactured by HOKUTO DENKO CORPORATION), and a silver/silver chloride electrode (Ag/AgCl) was used as a reference electrode. A foamed polyurethane pad (IC 1000 single-layer pad with X-Y grooves manufactured by NITTA HAAS, INC.) having a lattice-shaped groove formed in its surface was used as the polishing pad.
The device was used to measure the polarization of an anode (gradually increasing the electrode potential of the substrate), thereby measuring the relationship between a voltage applied and a current flowing through the substrate.
FIG. 7 is a graph illustrating the relationship between the potential of a conductive film and current density. In FIG. 7, the vertical axis indicates the current density, that is, a current per unit area of the polishing surface of the substrate. If the current density is high, the amount of dissolution of the conductive film in the electrolyte is increased, and the polishing speed is increased. In addition, in FIG. 7, the horizontal axis indicates the electrode potential. However, in an actual polishing device, it is difficult to control the current with the electrode potential, and it is common to control the current with a voltage applied between the polishing substrate and a cathode. Therefore, in the following description, the term ‘voltage applied’ is used to control the electrode potential of the substrate. The term ‘voltage applied’ means ‘the electrode potential of the substrate, which is an anode, the electrode potential of a supporting member (polishing table), which is a cathode, and a potential difference between the anode and the cathode including the potential drop of the electrolytic solution’.
When an electrolyte having a pH in the range of 4 to 10 contacts the conductive film and the voltage applied to the conductive film is increased, the current density is rapidly changed at a predetermined voltage. When the voltage applied to the conductive film is increased from 0 V, the current density is increased in proportion to the voltage. However, when the voltage is increased to be higher than a first change voltage at a first change point (for example, a point A), the current density starts to decrease. When the voltage is increased to be higher than a second change voltage at a second change point (for example, a point B), the decreasing rate of the current density is reduced, and the current density is kept constant without being reduced. When a voltage that is lower than the first change voltage is applied, a protective film that is dissoluble in the electrolyte is formed on the surface of the conductive film. However, when a voltage that is higher than the second change voltage is applied, a protective film that is insoluble in the electrolyte is formed on the surface of the conductive film.
The inventors found that the first change voltage and the second change voltage were changed by a contact surface pressure between the substrate W and the polishing pad. In FIG. 7, a bold solid line indicates when the contact surface pressure is 0.5 psi (the contact surface pressure has a finite value, which corresponds to the polishing of an upper part of the step portion of the conductive film), and a bold dashed line indicates when the contact surface pressure is 0 psi (the contact surface pressure is 0, which corresponds to the polishing of a lower part of the step portion of the conductive film). The first change point when the contact surface pressure has a finite value is referred to as a point A, the second change point when the contact surface pressure has a finite value is referred to as a point B, the first change point when the contact surface pressure is 0 is referred to as a point C (threshold voltage), and the second change point when the contact surface pressure is 0 is referred to as a point D. As the contact surface pressure is increased, a change in the first and second change voltages is increased, and the current density is also increased. The first change voltage (voltage at the point C) and the second change voltage (voltage at the point D) when the contact surface pressure is 0 are about 0.5 V lower than the first change voltage (voltage at the point A) and the second change voltage (voltage at the point B) when the contact surface pressure has a finite value, respectively.
The voltage range of 0 V to the voltage at the point C is referred to as a range α, the voltage range of the voltage at the point C to the voltage at the point B is referred to as a range β, and the voltage range of more than the voltage at the point B is referred to as a range γ. In addition, the voltage range of the voltage at the point C to the voltage at the point A is referred to as a range δ.
FIGS. 8A to 8C are diagrams illustrating the state of the protective film when voltages in the above-mentioned ranges are applied to the conductive film. FIG. 8A is a diagram illustrating the state of the protective film when voltage in the range α is applied to the conductive film, FIG. 8B is a diagram illustrating the state of the protective film when voltage in the range β is applied to the conductive film, and FIG. 8C is a diagram illustrating the state of the protective film when voltage in the range γ is applied to the conductive film.
As shown in FIG. 8A, when voltage in the range α is applied to the conductive film, a protective film 71 that is dissoluble in the electrolyte 50 is formed. When there is a step portion on the surface of the conductive film, the protective film 71 on the upper part H where the contact surface pressure between the polishing pad 101 and the conductive film 66 has a finite value is completely removed by polishing.
As shown in FIG. 8C, when a voltage in the range γ is applied to the conductive film, a protective film 72 that is insoluble in the electrolyte 50 is formed. When there is a step portion on the surface of the conductive film, the protective film 72 on the lower part L where the contact surface pressure between the polishing pad 101 and the conductive film 66 is 0 remains without being polished.
In contrast, as shown in FIG. 8B, when a voltage in the range β is applied to the conductive film, an intermediate state between the states shown in FIGS. 8A and 8C is obtained. That is, a protective film that is dissoluble in the electrolyte is formed on the upper part H where the contact surface pressure between the polishing pad 101 and the conductive film 66 has a finite value, and the protective film is removed by polishing. In addition, the protective film 72 that is insoluble in the electrolyte is formed on the lower part L where the contact surface pressure between the polishing pad 101 and the conductive film 66 is 0, and the protective film 72 remains without being polished.
In this embodiment, a voltage in the range β is applied to perform electrochemical polishing. In this way, the dissolution of the conductive film 66 in the electrolyte is accelerated in the upper part H, but is prevented in the lower part L. Therefore, it is possible to rapidly remove a step portion formed on the surface of the conductive film 66.
When a voltage in the range β is applied, the polishing speed of the lower part L covered with the protective film 72 is excessively low, dishing is less likely to occur in the conductive film 66. In addition, it is possible to control the polishing speed of the upper part H by adjusting the voltage applied. Therefore, it is possible to reduce the contact surface pressure between the polishing pad 101 and the conductive film 66, and thus prevent erosion. For example, in the CMP method according to the related art, the contact surface pressure is about 4 psi. However, in this embodiment, it is possible to set the contact surface pressure to about 2 psi. The contact surface pressure is obtained by dividing the pressing force of the head against a rotating disk (which is also referred to as a platen) by the area of a wafer contacting the polishing pad, that is, a contact area. In this case, the area of the through holes formed in the polishing pad is not included in the contact area.
As described above, as the current density is increased, the polishing speed is increased.
As shown in FIG. 7, in the range α of 0 V to the voltage at the point C, both when the contact surface pressure has a finite value (0.5 psi) and when the contact surface pressure is 0 (0 psi), the current density is increased with increasing the voltage. In contrast, in the range δ of the voltage at the point C to the voltage at the point A, when the contact surface pressure has a finite value, the current density is continuously increased. Then, when the contact surface pressure becomes zero, the current density starts to decrease. Therefore, a difference ΔIδ between the current density when the contact surface pressure has a finite value and the current density when the contact surface pressure is 0 in the range δ is larger than a difference ΔIα between the current densities in the range α all the time. Actually, the difference between the current density when the contact surface pressure has a finite value and the current density when the contact surface pressure is 0 corresponds to a difference between the polishing speeds of the upper part and the lower part of a step portion on the surface of the conductive film. Therefore, when a voltage in the range δ is applied to perform polishing, the difference between the polishing speeds of the upper part and the lower part of the step portion on the surface of the conductive film is increased. As a result, it is possible to rapidly remove the step portion on the conductive film.
In FIG. 7, a bold line indicates when the additive for generating an electrical insulating material is not included in the electrolyte, and a thin line indicates when 1% by weight of polyethylenimine (PEI) is added as the additive for forming an electrical insulating material. In addition, a solid line indicates when the contact surface pressure between the polishing pad and the substrate is 0.5 psi, and a dashed line indicates when the contact surface pressure is 0 psi.
When the additive that generates an electrical insulating material is not added (or the concentration of the additive is low), the current density is increased. The reason is that, as the concentration of the additive that generates an electrical insulating material is increased, the rigidity of a protective film is increased, and it is difficult to remove the protective film by polishing. Therefore, an electrolyte including a low-concentration additive that generates an electrical insulating material may be used to increase the polishing speed. In contrast, an electrolyte including a high-concentration additive that generates an electrical insulating material may be used, in order to reduce the polishing speed to accurately polish a thin conductive film. As can be seen from FIG. 7, when the additive that generates an electrical insulating material is not added (or the concentration of the additive is low), the first and second change voltages are increased. Therefore, it is preferable to adjust the voltage in accordance with the concentration of the additive that generates an electrical insulating material.
FIG. 9 is a graph illustrating the relationship between the current density and a voltage applied when the pH of the electrolyte is changed. In the experiments shown in FIG. 9, electrolytes having a pH of 4, a pH of 6, a pH of 8, and a pH of 9 (without an additive that generates an electrical insulating material) are used.
As the pH of the electrolyte is increased, the current density is increased. The reason is that, as the pH of the electrolyte is increased, the dissolution of the conductive film is accelerated. Therefore, an electrolyte having a high pH may be used to increase the polishing speed. On the other hand, an electrolyte having a low pH may be used to decrease the polishing speed. As can be seen from FIG. 9, as the pH of the electrolyte is increased, the first and second change voltages are increased. Therefore, it is preferable to adjust a voltage applied in accordance with the pH of the electrolyte.
FIG. 10 is a graph illustrating the relationship between the current density and a voltage applied when the number of revolutions of the polishing pad is changed. In the experiments shown in FIG. 10, an electrolyte including no additive that generates an electrical insulating material is used. In FIG. 10, a bold line indicates when the number of revolutions is 250 rpm, and a thin line indicates when the number of revolutions is 50 rpm. In addition, a solid line indicates when the contact surface pressure between the polishing pad and the substrate is 0.5 psi, and a dashed line indicates when the contact surface pressure is 0 psi.
As the number of revolutions of the polishing pad is increased, the current density is increased. The reason is that, as the number of revolutions of the polishing pad is increased, the protective film is rapidly removed, and the dissolution of the conductive film is accelerated. Therefore, it is preferable to increase the number of revolutions of the polishing pad in order to increase the polishing speed. On the other hand, it is preferable to decrease the number of revolutions of the polishing pad in order to decrease the polishing speed. It is possible to change the number of revolutions of the polishing pad during polishing. As can be seen from FIG. 10, as the number of revolutions of the polishing pad is increased, the first and second change voltages are increased. Therefore, it is preferable to adjust the voltage applied in accordance with the number of revolutions of the polishing pad.

(Electrochemical Polishing Method According to Second Embodiment)

Next, an electrochemical polishing method according to a second embodiment of the present invention will be described.
In the first embodiment, the voltage applied to the conductive film is maintained in a predetermined range to perform a polishing process. However, the second embodiment differs from the first embodiment in that, before a polishing process, a protective film forming process of applying a predetermined voltage to a conductive film to form a protective film is performed. In the second embodiment, a detailed description of the components the same as those in the first embodiment will be omitted.
First, a protective film forming process of contacting an electrolyte with a conductive film formed on a substrate and applying a voltage to the conductive film to form a protective film on the surface of the conductive film is performed. In this process, a voltage that is higher than the second change voltage at the second change point (for example, the point D in FIG. 7) when the contact surface pressure is 0 is applied. In this way, as shown in FIG. 8C, the protective film 72 that is insoluble in the electrolyte 50 is formed. The protective film 72 is irreversibly formed on the entire surface of the conductive film 66. Therefore, the protective film is not changed even when a voltage that is lower than the second change voltage is applied.
After the protective film forming process, a general electrochemical polishing process is performed. Then, the protective film 72 formed on the upper part H of the conductive film 66 is removed by polishing, but the protective film 72 formed on the lower part L of the conductive film 66 remains without being polished. In this way, the dissolution of the conductive film 66 in the electrolyte is accelerated at the upper part H, but is prevented at the lower part L. As a result, it is possible to rapidly remove a step portion on the surface of the conductive film 66.

(Modifications)

After the protective film forming process, a chemical mechanical polishing (CMP) process may be performed.
FIG. 11 is a diagram schematically illustrating a CMP apparatus. In the CMP process, a slurry 52 is supplied to the conductive film formed on the surface of the substrate W, and the substrate W is moved relative to the polishing pad 101 while the surface of the substrate W is pressed against the polishing pad 101, thereby polishing the surface of the conductive film. When the CMP process is performed after the protective film forming process, as shown in FIG. 8C, the protective film 72 formed on the upper part H of the conductive film 66 is removed by polishing, but the protective film 72 formed on the lower part L of the conductive film 66 remains without being polished. In this way, the dissolution of the conductive film 66 in the electrolyte is accelerated at the upper part H, but is prevented at the lower part L. As a result, it is possible to rapidly remove a step portion on the surface of the conductive film 66.
The technical scope of the present invention is not limited to the above-described embodiments and the modifications thereof, but various changes and modifications of the present invention can be made without departing from the scope and spirit of the present invention. That is, the materials and the structures described in the embodiments are just illustrative, and they can be appropriately modified.

(Polishing Pad)

The following may be used as the polishing pad.
Examples of the polishing pad include an independently foamed polyurethane pad and a continuously foamed suede pad. When an electrolyte without abrasive grains is used, a fixed abrasive grain pad obtained by binding the abrasive grains using a binder may be used. Any of the following materials may be used as the abrasive grain: cerium oxide (CeO₂), alumina (Al₂O₃), silicon carbide (SiC), silicon oxide (SiO₂), zirconia (ZrO₂), iron oxide (FeO, Fe₂O₃, and Fe₃O₄), manganese oxide (MnO₂and Mn₂O₃), magnesium oxide (MgO), calcium oxide (CaO), barium oxide (BaO), zinc oxide (ZnO), barium carbide (BaCO₃), calcium carbide (CaCO₃), diamond (C), and composites thereof. In addition, any of the following may be used as the binder: phenolic resin, aminoplast resin, urethane resin, epoxy resin, acrylic resin, acrylic isocyanate resin, urea-formaldehyde resin, isocyanate resin, acrylic urethane resin, and acrylic epoxy resin. In order to ensure the supply of power to the surface of the conductive film to be polished, a conductive pad having a conductive film in at least a portion of the polishing surface may be used.
Any of the following grooves may be formed in the polishing pad: (1) concentric grooves; (2) eccentric grooves; (3) polygonal grooves (including lattice-shaped grooves); (4) spiral grooves; (5) radical grooves; (6) parallel grooves; (7) arc-shaped grooves; and combinations thereof. The shapes of the grooves have an effect on the storage and discharge of the electrolyte. For example, for the concentric groove or the eccentric groove, since a flow passage is closed, the electrolyte is held on the polishing pad. The polygonal groove or the radical groove has an effect of accelerating the inflow of the electrolyte to a polishing target and the outflow of the electrolyte to the outside of the polishing pad. In order to improve the inflow or outflow efficiency of the electrolyte to or from a polishing surface of the substrate and the holding efficiency of the electrolyte on the polishing surface, the width, pitch, and depth of the grooves in the surface of the polishing pad may be appropriately adjusted, thereby adjusting the groove density distribution in the polishing pad.
For example, it is preferable that the width and the depth of the groove be not smaller than 0.4 mm and the groove pitch be two times or larger than the width of the groove. It is more preferable that the width and the depth of the groove be not smaller than 0.6 mm in consideration of the flow of the electrolyte. In order to smooth the flow of the electrolyte between the grooves, auxiliary grooves may be provided between the grooves (for example, a plurality of fine grooves formed between the concentric grooves or fine grooves formed between large lattice grooves). The grooves may have a rectangular shape, a circular shape, or a V-shape in a cross-sectional view. In order to accelerate the discharge of the electrolyte from the grooves, forward grooves may be formed so as to be inclined to the downstream side in a direction in which the polishing table having a polishing pad mounted thereto is rotated. In contrast, in order to prevent the discharge of the electrolyte from the grooves, backward grooves may be formed so as to be inclined to the upstream side of the rotating direction. Further, in order to hold the electrolyte, one or more through holes may be formed in the polishing pad.
The mechanical removal of the protective film formed by an electrolytic reaction is affected by the shape of a contact surface between the polishing pad and a wafer. The contact surface may have a sharp shape, such as a conic shape, a pyramid shape, a polygonal pyramid shape, or a prism shape, in order to improve a mechanical action on the contact surface. When the shape of the contact surface is too sharp, scratches may occur in an object to be polished. Therefore, in order to prevent the scratches, the contact surface has a truncated conic shape or a truncated pyramid shape having a flat upper surface. In addition, in order to reduce the mechanical action on the contact surface, the contact surface has a cylindrical shape, an elliptic cylindrical shape, or a hemispheric shape. The protrusions formed on the contact surface may be arranged in a regular pattern, such as in a lattice, in a zigzag, or in a triangular pattern, or they may be randomly arranged. In addition, a plurality of protrusions having the above-mentioned shapes may be formed on the polishing surface of the polishing pad, and the density distribution thereof may be adjusted.

(Film Thickness Detecting Sensor)

In the embodiment shown in FIG. 2, the overcurrent sensor is used to detect the thickness of the conductive film. However, for example, an optical monitor, an X-ray fluorescence thickness meter, and a device for detecting a variation in voltage and current may be used to detect the thickness.
The optical monitor uses a variation in the intensity of reflected due to light interference. As a method of using the optical monitor, the following is used: a method of radiating light emitted from a light source provided in a table through the holes formed in the pad; and a method of measuring the thickness while the substrate overhangs outside the polishing table. Since a change start point varies depending on the wavelength of light used, the wavelength is appropriately selected according to the materials to be polished.
In the method of using the X-ray fluorescence thickness meter, the intensity of fluorescent X-rays generated when a primary X-ray is radiated to a target varies depending on the thickness of the target. The method uses variation in the intensity of the fluorescent X-rays. During polishing, an X-ray source provided in the table radiates X-rays to the conductive film to measure the thickness of the conductive film.
The method of detecting the variation in voltage and current uses a variation in the electric resistance of a conductive film of a target according to the thickness of the conductive film. The method measures a variation in current when a constant voltage is applied, or a variation in voltage when a constant current is applied, thereby measuring the thickness of the conductive film from the electric resistance. This method monitors the voltage and current applied during polishing to detect the pressure of the conductive film. Therefore, the method is easy to use.
For example, in addition to the above-mentioned film thickness detecting methods, as a method of detecting the completion (the end of polishing) of the polishing of the conductive film formed on the barrier film or the conductive film including the barrier film on the insulating film, the following methods may be used: a method of detecting a variation in the surface temperature of the polishing pad or the surface temperature of the substrate; a method of detecting a variation in the friction between the substrate and the polishing pad; a method of detecting a surface image variation; and a method of detecting a variation in components (the concentration of the oxide of a by-product and the concentration of ions caused by the conductive film) included in a slurry or an electrolyte.
In the method of detecting a variation in the surface temperature of the polishing pad or the surface temperature of the substrate, a radiation thermometer can be used to measure the surface temperature of the pad, or a radiation thermometer provided in the table can be used to measure the surface temperature of the substrate through the hole formed in the polishing pad.
In the method of detecting a variation in the friction between the substrate and the polishing pad, a variation in the driving current of a substrate holder or a table having the polishing pad mounted thereto can be measured, or a variation in the oscillation amplitude of a specific frequency with respect to the substrate holder over time can be measured.
In the method of detecting a surface image variation, a color sensor provided in the table can detect a variation in the color of the surface of the substrate through the hole formed in the polishing pad, or a CCD can measure a variation in the two-dimensional image of the surface of the substrate.
In the method of detecting a variation in components (the concentration of the oxide of a by-product and the concentration of ions caused by the conductive film) included in slurry or an electrolyte, a variation in the concentration of ions in a polishing agent discharged from the polishing table, which is caused by the conductive film, can be measured.
According to the present invention, when there is a step portion on the surface of the conductive film, a protective film that is dissoluble in the electrolyte is formed on the upper part where the contact surface pressure between the conductive film and the polishing pad has a finite value, and the protective film is completely removed by polishing. In addition, a protective film that is insoluble in the electrolyte is formed on the lower part where the contact surface pressure between the conductive film and the polishing pad is 0, and the protective film remains without being polished. In this way, the dissolution of the conductive film in the electrolyte is accelerated at the upper part, but is prevented at the lower part. As a result, it is possible to rapidly remove the step portion on the surface of the conductive film.
Next, a third embodiment of the present invention will be described with reference to the accompanying drawings.

THIRD EMBODIMENT

Substrate Processing Apparatus

In the third embodiment, the structure of a substrate processing apparatus is the same as that in the above-described embodiments, and thus a description of the same components as those in the above-described embodiments will be omitted.
As in the above-described embodiments, in FIG. 2, the regulators RE2 to RE6 can individually control the internal pressures of the pressure chambers 21 to 25 to adjust the pressing force (the contact surface pressure against the surface of the substrate) of the substrate W against the polishing pad 101 through the elastic pad.
That is, as shown in FIG. 4, during polishing, the pressure chambers 21 to 25 appropriately adjust the pressing force of the retainer ring 3 against the polishing pad 101 and the pressing force of the substrate W against the polishing pad 101 to obtain the desired distribution of the polishing pressures (the contact surface pressures against the surface of the substrate) of a central portion (C1 in FIG. 4) of the substrate W, an intermediate portion (C2) outside the central portion, an outer portion (C3), a circumferential portion (C4), and an outer circumferential portion of the retainer ring 3 arranged outside the substrate W.
As such, the substrate W is divided into four concentric circles and ring-shaped portions (C1 to C4), and it is possible to independently press the portions (pressure regions) with different pressing forces. The polishing speed depends on the pressing force of the substrate W against a polishing surface. However, as described above, since the pressing forces of the portions can be independently controlled, it is possible to independently control the polishing speeds of the four portions (C1 to C4) of the substrate W. Therefore, even when the thickness distribution of a polishing surface of the substrate W is uneven in the radial direction, it is possible to prevent the entire surface of the substrate W from being insufficiently or excessively polished.
That is, even when the polishing surface of the substrate W has different thicknesses in the radial direction of the substrate W, the pressure of a pressure chamber that is disposed above a thick portion of the surface of the substrate W among the pressure chambers 22 to 25 is set to be higher than that of the other pressure chambers, or the pressure of a pressure chamber arranged above a thin portion of the surface of the substrate W is set to be lower than that of the other pressure chambers, thereby making the pressing force of the thick portion against the polishing surface stronger than the pressing force of the thin portion against the polishing surface. Therefore, it is possible to selectively increase the polishing speed of the portions. In this way, it is possible to prevent the entire surface of the substrate W from being insufficiently or excessively polished, without depending on a film thickness distribution, during a film forming process.
In the polishing table according to this embodiment, a feed electrode (a voltage application point) 264 is provided in the vicinity of the edge of the polishing pad 101 so as to be connected to a positive electrode of a power supply 252. The substrate head 1 is arranged such that the substrate W contacts a polishing surface with a portion of the substrate W protruding from the edge of the polishing pad 101, and the circumferential portion C4 of the lower surface of the substrate W contacts the feed electrode 264. In this way, power is supplied from the feed electrode 264 to the conductive film 66 of the substrate W. It is also possible to supply power from the feed electrode 264 to the conductive film 66 of the substrate W through the retainer ring. A supporting member 254, serving as a cathode, and the conductive film of the substrate W, serving as an anode, are electrically connected to each other by an electrolyte filled in the through holes 101 a formed in the polishing pad 101.

(Electrochemical Polishing Method)

Next, an electrochemical polishing method according to this embodiment will be described. In the electrochemical polishing method according to this embodiment, a detailed description of the same processes as those in the electrochemical polishing method according to the first embodiment will be omitted.
FIGS. 12A to 12C are diagrams illustrating the electrochemical polishing method. In FIGS. 12A to 12C, the conductive film 66, which is a polishing surface, is arranged so as to face downward.
First, as shown in FIG. 12A, the electrolyte 50 comes into contact with the conductive film formed on the surface of the substrate W, and a voltage is applied to the conductive film 66. Then, the substrate W is moved (rotated) relative to the polishing pad 101 while the surface of the substrate W is pressed against the polishing pad 101, thereby polishing the surface of the conductive film 66.
It is necessary to planarize the surface of the substrate W after an unnecessary portion of the conductive film 66 is removed, in order to form a plurality of wiring lines on the substrate with an interlayer insulating film 62 interposed therebetween. In the electrochemical polishing, a voltage is applied to the conductive film 66 to form a protective film made of an electrical insulating material on the surface of the conductive film 66. The protective film formed on an upper part H (outside a concave portion 67) of the conductive film 66 contacts the polishing pad 101 and is then removed. In this way, the upper part H of the conductive film 66 is dissolved by the electrolyte 50 and then removed. In contrast, a lower part L (inside the concave portion 67) of the conductive film is shielded by the protective film and is not dissolved by the electrolyte 50. Therefore, a step portion of the conductive film is removed. In this way, the surface of the conductive film 66 and the exposed surface of the barrier film 64 are flush with each other. As a result, the substrate W is planarized.
The inventors found that it was possible to control the polishing speed of each portion of the surface of the substrate W by adjusting the potential of each portion of the surface of the substrate W. That is, there is a difference in electric resistance between metal films formed on the surface of the substrate W, that is, between the barrier film 64, which is a lower layer shown in FIG. 12A, and the conductive film 66, which is an upper layer, and the difference in electric resistance is used to adjust the potential of each portion of the substrate W. A voltage is applied from the feed electrode 264 provided on the surface of the metal film to the circumferential portion C4 of the substrate W. When the conductive film 66 remains on the entire surface of the substrate W, a substantially constant voltage is applied to the entire surface of the substrate. However, as shown in FIG. 12B, when the conductive film 66 is removed and a portion of the barrier film 64 (for example, a portion in the vicinity of the feed electrode 264) is exposed, the voltage distribution of the surface of the substrate W varies.
Specifically, after a portion of the barrier film 64 in a region corresponding to the circumferential portion C4 of the substrate W is exposed, the conductive film 66 having an electric resistance that is lower than that of the barrier film 64 remains in regions other than the circumferential portion C4 of the substrate W, that is, in regions corresponding to the portions C1 to C3 of the substrate W. Since a voltage is applied to the remaining conductive film 66 through the barrier film 64, the voltage applied to the conductive film 66 is reduced as a distance from the feed electrode 264 is increased.
In this embodiment, as shown in FIG. 12B, first, a portion of the conductive film 66 formed on the substrate W is removed to expose the barrier film 64. Specifically, polishing is performed while adjusting the flow rate of pressurized fluid supplied to the pressure chambers 22 to 25 (see FIG. 3) of the substrate head 1 such that the contact surface pressure between the substrate W and the polishing pad 101 has a maximum value in the circumferential portion C4 (see FIG. 4) of the substrate W.
It is preferable that the distribution of the contact surface pressures in portions other than the circumferential portion C4 of the substrate W be set such that the contact surface pressure is gradually reduced from the outer portion C3 to the central portion C1 (see FIG. 4) of the substrate W. Alternatively, the contact surface pressure may be constant from the outer portion C3 to the central portion C1 of the substrate W.
When polishing is performed in the distribution of the contact surface pressures, polishing is accelerated in a region corresponding to the circumferential portion C4 of the substrate W, that is, in the vicinity of the feed electrode 264, and the conductive film in the region is removed first. Then, a portion of the barrier film 64 in the region corresponding to the circumferential portion C4 of the substrate W is exposed. In this case, the conductive film 66 remains in regions other than the region corresponding to the circumferential portion C4 of the substrate W, that is, in regions corresponding to the outer portion C3 to the central portion C1 of the substrate W. In particular, when the distribution of the contact surface pressure of the substrate W is set such that the contact surface pressure is gradually reduced from the outer portion C3 to the central portion C1, the thickness of the remaining conductive film 66 is gradually increased from the outer portion C3 to the central portion C1.
As a method of removing a portion of the conductive film 66 formed on the substrate W to expose the barrier film 64, a method of dividing a cathode (an opposite electrode) facing the substrate W into a plurality of small cathodes (small electrodes) and using the divided cathodes (divided electrodes) may be used. As examples of the divided cathodes, as shown in FIG. 13, there are a plurality of divided cathodes K1 to K3 that are concentric with the center of the polishing table. When the voltage applied between the outermost cathode (the cathode K3 in FIG. 13) and the substrate W is lower than the voltage at the first change point A shown in FIG. 7 and is higher than the voltages of the cathodes K1 and K2, the circumferential portion C4 of the substrate W that faces the cathode K3 for a long time (frequency is high) is polished at the highest speed, and a portion of the conductive film 66 in a region corresponding to the circumferential portion is removed first. When a voltage applied to the divided cathodes is controlled so as to be gradually reduced from the outermost cathode K3 to the central cathode K1 (cathode K3>cathode K2>cathode K1 in FIG. 13), polishing is performed with the thickness of the remaining conductive film 66 being gradually increased from the outer portion C3 to the central portion C1 of the substrate W, and the barrier film 64 is gradually exposed from the circumferential portion C4 of the substrate W, which is preferable.
After the barrier film 64 is exposed, the distribution of the contact surface pressure applied to the substrate W is reversed. Specifically, the contact surface pressure applied to the circumferential portion C4 of the substrate W is reduced, or no pressure is applied to the circumferential portion, and the contact surface pressure applied to the outer portion C3 to the central portion C1 is increased. Alternatively, constant pressure may be applied to the entire surface of the substrate. In this way, it is possible to reliably prevent the occurrence of erosion due to mechanical polishing in the circumferential portion C4 of the exposed substrate W having the exposed barrier film 64.
In the electrochemical polishing method according to this embodiment, polishing is performed while the thickness of the remaining conductive film 66 is measured by a thickness sensor. An overcurrent sensor is preferably used as the thickness sensor. The overcurrent sensor detects the thickness of each portion using a change in synthetic impedance caused by the thickness of each portion. The overcurrent sensor provided in the polishing table 100 applies a high frequency to the conductive film to measure the thickness of the conductive film. Therefore, even when the conductive film 66 has a large thickness, it is possible to accurately measure the thickness of the conductive film.
When the barrier film 64 in a region corresponding to the circumferential portion C4 of the substrate W is exposed and the voltage distribution of the substrate W is changed, a voltage applied to the feed electrode 264 is increased. Specifically, a voltage applied to the region corresponding to the circumferential portion C4 of the substrate W is set to be higher than the voltage at the point C, preferably, the voltage at the point D (see FIG. 7). In this way, in the region in which the barrier film 64 has already been exposed (in the region corresponding to the circumferential portion C4 of the substrate W), polishing does not proceed any further. Therefore, even when the wiring conductive film 66 exists in the region corresponding to the circumferential portion C4 of the substrate W, dishing is less likely to occur in the surface of the conductive film.
In the regions corresponding to the central portion C1 to the outer portion C3 of the substrate W in which the conductive film 66 remains on the surface of the barrier film 64, a voltage is applied through the barrier film 64 having high resistance, and the voltage is reduced. However, it is possible to prevent a voltage drop in the regions corresponding to the central portion C1 to the outer portion C3 in which the conductive film 66 remains by increasing the voltage in the region corresponding to the circumferential portion C4 of the substrate W, and the current density is maintained at a high level. Specifically, the voltage applied to the regions corresponding to the central portion C1 to the outer portion C3 is set to be lower than the voltage at the point B of FIG. 7, preferably, lower than the voltage at the point A, more preferably, equal to the voltage in the region δ.
In this way, in the region corresponding to the circumferential portion C4 of the substrate W, for example, a current corresponding to the second change voltage (the voltage at the point D of FIG. 7) when the contact surface pressure is 0 flows. In the regions corresponding to the central portion C1 to the outer portion C3 of the substrate W, a current corresponding to the first change voltage (for example, the voltage at the point A of FIG. 7) flows. That is, in the region in which the barrier film 64 is exposed (in the region corresponding to the circumferential portion C4 of the substrate W), polishing does not proceed any further. However, in the regions corresponding to the central portion C1 to the outer portion C3 of the substrate W, polishing is accelerated.
As a result, as shown in FIG. 12C, the conductive film in the regions other than the concave portion 63 formed in the surface of the substrate W is removed.
As described above, in this embodiment, when the voltage of the exposed region of the barrier film 64 is higher than a threshold voltage, the current density is lowered, and the polishing speed of the conductive film 66 is reduced. Therefore, even when the conductive film 66 forming, for example, a contact plug or a wiring line exists in the exposed region of the barrier film 64, dishing is less likely to occur in the surface of the conductive film 66. In addition, it is not necessary to use slurry having a high contact surface pressure and a high abrasive grain density in order to perform polishing using an electrochemical dissolution, unlike the related art, and it is also not necessary to perform polishing at a high contact surface pressure. Therefore, the ratio of a mechanical polishing operation is lowered, and it is possible to prevent the occurrence of erosion. In the exposed region of the barrier film 64, the contact surface pressure between the polishing pad 101 and the conductive film 66 is low. Therefore, the ratio of the mechanical polishing operation is lowered, and it is possible to prevent the occurrence of dishing or erosion.
In the region in which the conductive film 66 having an electric resistance that is lower than that of the barrier film 64 remains, a voltage is lowered due to a voltage drop in the exposed portion of the barrier film 64, and the current density is higher than that in the exposed region of the barrier film 64. Therefore, in the region in which the conductive film 66 remains, polishing is continuously performed, and it is possible to rapidly remove the conductive film while preventing damages, such as scratches, or dishing.
The region in the vicinity of the feed electrode 264 is polished more rapidly than the other regions by increasing the contact surface pressure of the region in the vicinity of the feed electrode 264 to be higher than that of the other regions. Therefore, the conductive film 66 in the region is removed first. After the barrier film 64 is exposed, it is possible to obtain a voltage distribution in which the highest voltage is applied to a metal film in the vicinity of the feed electrode 264 and the voltage is reduced as the distance from the feed electrode increases. Therefore, it is possible to accurately remove the remaining conductive film 66 in a process of increasing the voltage after the barrier film 64 is exposed.
In FIG. 7, a bold line indicates when an additive that generates an electrical insulating material is not added to an electrolyte, and a thin line indicates when 1% by weight of polyethylenimine is added as the additive that generates an electrical insulating material. In addition, a solid line indicates when the contact surface pressure between the polishing pad and the substrate is 0.5 psi, and a dashed line indicates when the contact surface pressure is 0 psi.
It has been known that the current density is high when the additive that generates an electrical insulating material is not added (or when the concentration thereof is low). The reason is that, as the concentration of the additive that generates an electrical insulating material is increased, the rigidity of the protective film is increased, and it is difficult to remove the protective film by polishing. Therefore, it is preferable to use an electrolyte including a low-concentration additive that generates an electrical insulating material in order to increase the polishing speed. On the other hand, it is preferable to use an electrolyte including a high-concentration additive that generates an electrical insulating material, in order to decrease the polishing speed to accurately polish a thin conductive film or to accurately stop the polishing. As can be seen from FIG. 7, when the additive that generates an electrical insulating material is not added (or when the concentration thereof is low), the first and second change voltages are increased. Therefore, it is preferable to adjust a voltage applied in accordance with the concentration of the additive that generates an electrical insulating material.
The technical scope of the present invention is not limited to the above-described embodiments and the modifications thereof, but various changes and modifications of the present invention can be made without departing from the scope and spirit of the present invention. That is, the materials and the structures described in the embodiments are just illustrative, and they can be appropriately modified.
For example, in this embodiment, only one feed electrode is provided in the circumferential portion of the substrate, but a plurality of feed electrodes may be provided on the substrate. In this case, the area of a contact surface for voltage application is increased, and it is possible to reduce contact resistance. Therefore, it is possible to accurately control a voltage applied to the substrate. In this case, it is preferable that the feed electrodes be concentrically arranged on the substrate.
In addition, in this embodiment, after the barrier film is exposed, the voltage is increased. However, a thickness sensor may be used to detect immediately before the barrier film is exposed, and the voltage may be increased at the time.

(Polishing Pad)

The shape of the polishing pad and a material forming the polishing pad are the same as those in the second embodiment, and thus a detailed description thereof will be omitted.

FOURTH EMBODIMENT

Next, an electrochemical polishing method according to a fourth embodiment of the present invention will be described. In this embodiment, the components the same as those in the third embodiment shown in FIG. 7 are denoted by the same reference numerals, and a detailed description thereof will be omitted.
A voltage applied to the exposed region of the barrier film 64 and the remaining region of the conductive film 66 may be controlled by fixing the position of the feed electrode 264 and changing the position of the substrate head 1.
A voltage that is slightly higher than that at the point D of FIG. 7 (about 0.5 V) is applied, and the feed electrode (application point) 264 is disposed near the edge of the polishing table 100 (within a distance of 10 mm). Therefore, at the beginning, polishing is performed with the outermost portion of the polishing table overhanging the substrate W. Then, with an outermost portion of the barrier film 64 being exposed, the substrate head 1 is gradually moved in a direction in which it becomes distant from the center of the polishing table 100, that is, in a direction in which the overhang is increased. In this case, the exposed portion of the barrier film 64 contacts the feed electrode 264 all the time, and the conductive film 66 remains at a position where the polishing table does not overhang the conductive film.
According to this embodiment, with the barrier film 64 being exposed, the substrate head 1 is gradually moved in the direction in which it becomes distant from the center of the polishing table 100. Therefore, it is possible to maintain the voltage applied to the exposed region of the barrier film 64 at a high level. In this way, even when the conductive film 66 forming, for example, a contact plug or a wiring line exists in the exposed region of the barrier film 64, dishing is less likely to occur in the surface of the conductive film 66.
Further, after the substrate head 1 is moved, a voltage that is higher than a threshold voltage (a voltage at the point C of FIG. 7) may be applied to the exposed region of the barrier film 64, thereby increasing the voltage applied to the feed electrode 264.
According to this structure, it is possible to reliably make the voltage applied to the exposed region of the barrier film 64 higher than the threshold voltage without sharply increasing the voltage. In this way, even when the conductive film 66 forming, for example, a contact plug or a wiring line exists in the exposed region of the barrier film 64, dishing is less likely to occur in the surface of the conductive film 66. In addition, it is possible to reliably prevent further progression of polishing in the exposed region of the barrier film 64. Therefore, in the region in which the conductive film 66 remains, polishing is continuously performed, and in the exposed region of the barrier film 64, polishing is not performed. As a result, it is possible to rapidly remove the conductive film 66 while preventing damages, such as scratches, or dishing.
In this embodiment, in order to contact the feed electrode 264 with substantially the entire surface of the substrate W, the feed electrode 264 may be formed of a soft material, such as carbon resin, and a mechanism capable of generating no scratch in the substrate W, for example, a mechanism that is rotated with the rotation of the substrate W, such as a cam follower, may be used.
In addition, the feed electrode 264 is not necessarily provided near the edge of the polishing table 100. For example, the feed electrodes may be concentrically arranged in the polishing pad 4. In this case, even when the substrate W is gradually moved to the outside, it is possible to perform polishing without overhanging the substrate W.
In this embodiment, the feed electrode 264 is provided near the edge of the polishing table 100 (within a distance of 10 mm), but the present invention is not limited thereto. As another example of the feed electrode, a conductive pad having a concentric circle shape with a width of about 1 cm may be provided in the polishing pad 4. When the conductive pad is arranged at a distance that is larger than the radius of the substrate W from the edge of the polishing table 100, it is possible to stably perform polishing without overhanging the substrate even though the conductive pad reaches the center of the substrate.
According to the present invention, a voltage applied to the exposed region of an underlying film is set to be higher than a threshold voltage. Therefore, even when a conductive film forming, for example, a contact plug or a wiring line exists in the exposed region of the underlying film, dishing is less likely to occur in the surface of the conductive film. In addition, polishing is performed using electrochemical dissolution. Therefore, it is not necessary to use a slurry having a high abrasive grain density, unlike the related art, and it is also not necessary to perform polishing at a high surface pressure. As a result, the ratio of a mechanical polishing operation is reduced, and it is possible to prevent the occurrence of erosion.
In the region in which the conductive film having an electric resistance that is lower than that of the underlying film remains, a voltage that is higher than the threshold voltage and lower than the maximum voltage is applied. Therefore, in this region, the current density is maintained at a high level until the conductive film is polished and planarized such that the underlying film is exposed. In this way, even when the conductive film forming, for example, a contact plug or a wiring line exists in the exposed region of the underlying film, polishing is continuously performed. As a result, it is possible to rapidly remove the conductive film and planarize it while preventing damages, such as scratches, or dishing.
Although the exemplary embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments. Various modifications and changes of the present invention can be made without departing from the scope and spirit of the present invention. The present invention is not limited to the above-described embodiments, but is defined by only the appended claims.

Claims

1. An electrochemical polishing method comprising:

a step of contacting an electrolyte with a conductive film formed on the surface 5 of a substrate to apply a voltage to the conductive film; and

a step of moving the substrate relative to a polishing pad while pressing the surface of the substrate against the polishing pad at a predetermined contact surface pressure, thereby polishing the surface of the conductive film,

wherein the electrolyte has a pH in the range of 4 to 10,

when the voltage is increased at a contact surface pressure of 0, a voltage that allows a current density to start to decrease after an increase is referred to as a minimum voltage,

when the voltage is increased at a contact surface pressure having a finite value, a voltage that allows the current density not to decrease any further after the decrease is 15 referred to as a maximum voltage, and

the surface of the conductive film is polished while maintaining the voltage to be not lower than the minimum voltage and not higher than the maximum voltage.

2. An electrochemical polishing method comprising:

a step of contacting an electrolyte with a conductive film formed on the surface of a substrate to apply a voltage to the conductive film; and

wherein the electrolyte has a pH in the range of 4 to 10,

when the voltage is increased at a contact surface pressure having a finite value,

a voltage that allows the current density to start to decrease after an increase is referred to as a maximum voltage, and

3. The electrochemical polishing method according to claim 1, wherein the polishing speed of the conductive film is controlled by adjusting the pH of the electrolyte.

4. The electrochemical polishing method according to claim 1,

wherein the electrolyte includes an additive that reacts with the conductive film to generate an electrical insulating material, and

the polishing speed of the conductive film is controlled by adjusting the concentration of the additive.

5. The electrochemical polishing method according to claim 1,

wherein the polishing speed of the conductive film is controlled by adjusting the number of revolutions of the polishing pad.

6. The electrochemical polishing method according to claim 1,

wherein the conductive film is a tungsten film.

7. A polishing method comprising:

a protective firm forming step of applying a voltage that is not lower than a threshold voltage to a conductive film contacting an electrolyte, without contacting a 5 substrate with a polishing pad, thereby forming a protective film on the surface of the conductive film; and

a polishing step of moving the substrate relative to the polishing pad while pressing the surface of the substrate against the polishing pad at a predetermined contact surface pressure, thereby polishing the conductive film formed on the surface of the substrate,

wherein, when the voltage that is applied to the conductive film contacting the electrolyte is increased at a contact surface pressure of 0 while moving the substrate relative to the polishing pad, a voltage that allows a current density not to decrease after an increase is referred to as the threshold voltage.

8. The polishing method according to claim 7,

wherein the polishing step is a chemical mechanical polishing step or an electrochemical polishing step.

9. The polishing method according to claim 7, wherein the conductive film is a tungsten film.

10. An electrochemical polishing method comprising:

a step of contacting an electrolyte with a metal film formed on the surface of a 25 substrate to apply a voltage to the metal film, the electrolyte having a pH in the range of 4 to 10 and the metal film including an underlying film, which is a lower layer, and a conductive film, which is an upper layer, having an electric resistance that is lower than that of the underlying film;

a step of moving the substrate relative to a polishing pad while pressing the surface of the substrate against the polishing pad at a predetermined contact surface pressure, thereby polishing the surface of the metal film;

a step of removing a portion of the conductive film in the vicinity of a point where the voltage is applied first to expose the underlying film; and

a step of increasing the voltage immediately before the exposure of the 10 underlying film or after the exposure of the underlying film the underlying film is exposed,

wherein, in the step of increasing the voltage,

when the voltage is increased at a contact surface pressure of 0 while moving the substrate relative to the polishing pad, a voltage that allows a current density to start to decrease after an increase is referred to as a threshold voltage, and the voltage is increased such that a voltage in a region in which the underlying film is exposed is higher than the threshold voltage.

11. The electrochemical polishing method according to claim 10,

wherein, in the step of increasing the voltage,

when the voltage is increased at a contact surface pressure having a finite value, a voltage that allows the current density not to decrease after an increase is referred to as a maximum voltage, and

the voltage is increased such that a voltage in regions other than the region in which the underlying film is exposed is higher than the threshold voltage and lower than the maximum voltage.

12. The electrochemical polishing method according to claim 10,

wherein, in the step of exposing the underlying film, the contact surface pressure between the substrate and the polishing pad in the region in the vicinity of the voltage application point is higher than that in regions other than the region in the vicinity of the voltage application point.

13. The electrochemical polishing method according to claim 10,

wherein, in the step of exposing the underlying film,

an electrode opposite to the substrate is divided into a plurality of small electrodes that are concentrically arranged on the same plane, and

the voltage applied to the divided electrodes is controlled such that a portion of the conductive film that is more frequently opposite to an outer portion of the substrate is more rapidly polished.

14. The electrochemical polishing method according to claim 10,

wherein, in the polishing step, polishing is performed while measuring the thickness of the remaining conductive film using an overcurrent method.

15. The electrochemical polishing method according to claim 10,

wherein the conductive film is a tungsten film.

16. An electrochemical polishing method comprising:

a step of contacting an electrolyte with a metal film formed on the surface of a substrate to apply a voltage to the metal film, the electrolyte having a pH in the range of 4 to 10 and the metal film including an underlying film, which is a lower layer, and a conductive film, which is an upper layer, having an electric resistance that is lower than that of the underlying film;

a step of arranging a point where the voltage is applied in a circumferential portion of the substrate and removing the conductive film in the circumferential portion first to expose the underlying film; and

a step of moving the voltage application point from the circumferential portion to the center of the substrate to expose the underlying film.

17. An electrochemical polishing method comprising:

a step of arranging a point where the voltage is applied in a circumferential portion of the substrate and removing the conductive film in the circumferential portion first to expose the underlying film;

a step of moving the voltage application point from the circumferential portion 5 to the center of the substrate to expose the underlying film; and

a step of increasing the voltage immediately before the exposure of the underlying film or after the exposure of the underlying film the underlying film is exposed,

wherein, in the step of increasing the voltage,

when the voltage is increased at a contact surface pressure of 0 while moving the substrate relative to the polishing pad, a voltage that allows a current density to start to decrease after an increase is referred to as a threshold voltage, and

the voltage is increased such that a voltage in a region in which the underlying film is exposed is higher than the threshold voltage.

18. The electrochemical polishing method according to claim 1, wherein a positive voltage is applied to the conductive film.

19. The electrochemical polishing method according to claim 2,

wherein the polishing speed of the conductive film is controlled by adjusting the pH of the electrolyte.

20. The electrochemical polishing method according to claim 2,

21. The electrochemical polishing method according to claim 2,

22. The electrochemical polishing method according to claim 2,

wherein the conductive film is a tungsten film.

23. The polishing method according to claim 8, wherein the conductive film is a tungsten film.

24. The electrochemical polishing method according to claim 2, wherein a positive voltage is applied to the conductive film.

25. The electrochemical polishing method according to claim 7, wherein a positive voltage is applied to the conductive film.