US20120061244A1

US20120061244A1 - Method and device for the controlled electrolytic treatment of thin layers

Info

Publication number: US20120061244A1
Application number: US13/320,981
Authority: US
Inventors: Egon Engmark Hübel
Original assignee: Somont GmbH
Current assignee: Somont GmbH
Priority date: 2009-05-22
Filing date: 2010-05-18
Publication date: 2012-03-15
Also published as: DE102009023769A1; WO2010133220A3; WO2010133220A2; JP2012527524A; EP2432920A2; CN102459716A

Abstract

An electric contact in the outer edge thereof, is used for electroplating substrates, e.g. as a wafer in a cup plater. In order to obtain an even electroplating result, at least two electrolytic partial cells are formed which are supplied with current by respective electroplating current sources (9′) and (9″) that can be individually adjusted. The partial cathode (12′) and the associated partial anode (7′) are supplied in the region of the diametrically remote partial cathode (12″). Conversely, the partial cathode (12″) is supplied via the base layer of the partial cathode (12′). The required levelling of the layer thickness distribution is inter alia carried out by alternate different adjustments of the quantity of the electroplating current of the two partial cells and by the electrodes (7,12) that rotate relative to each other.

Description

The invention relates to electrochemical plating or deplating of thin and thus highly resistive layers on substrates. It is particularly suitable for electroplating of, for example, starting layers or seed layers on wafers. Horizontal or vertical electroplating cells are used inter alia for this purpose such as the so-called cup plater where the back of the wafer that is not to be treated is kept dry or covered. The starting layers consist, for example, of sputtered copper with a thickness of, for example, 0.1 μm. A high current density of, for example, 1 A/dm²or higher should be used for cost-effective electroplating. In this way, the layer thickness distribution obtained over the entire surface of the material has the smallest possible tolerance. The cathodic electroplating current can only be supplied to the material i.e. to the wafer, from the edge if the latter's center is not to be used for another electrical contact.
As a result of the initially high-impedance starting layer, the electroplating causes an electrical voltage drop on its way from the edge towards the center of the material. Accordingly, the thickness of the electro-deposited layer decreases as a radial gradient. With increasing diameter of the material, the problem of the radial gradient increases at a disproportionate rate.
There are proven solutions to remedy this problem.
The object of the invention is to describe a method and a device where the electrochemical treatment achieves a level layer thickness distribution on the material, even when the material has different starting conditions and parameters. In particular, it should be possible to electroplate a flat layer even in the case of large diameter material using a high global current density or to evenly etch a layer or surface electrochemically in a cup plater or similar electrolytic processing container.
By “large diameter material” is meant, for example, a wafer with a diameter of 200 mm or larger. Thin starting layers include sputtered or chemically deposited metal layers with a thickness of, for example, 0.1 μm. Depending on the electrochemical processes, high current densities can be in the region of 1 A/dm²but also 10 A/dm²or more.
The problem is solved by the method according to claim 1 and by the device according to claim 8. The dependent claims describe advantageous embodiments of the invention, which are generally described using the example of electroplating.
The invention provides for a division of the global electrode or anode of the cup plater into at least two partial cells that are electrically isolated from one another but that are identical or virtually identical in their size and shape. Each, preferably sector-like, partial cell is supplied from its own individually-assigned electroplating current source. The positive terminals of the electroplating current sources are connected to their assigned partial anodes. The negative terminals of the electroplating current sources are connected to their assigned partial cathodes according to the invention so that each is connected to, or contacts, the edge of the material furthest from the partial anodes assigned to the electroplating current sources. The flat material is located preferably in equal parts below and above the partial anodes. The above-mentioned polarities should be reversed in the case of electrochemical etching.

The invention is described below with reference to FIGS. 1 through 5 that are schematic and not to scale.

FIG. 1 shows a cross-sectional view of a processing container as the cup plater according to the invention for the electrolytic processing of the underside of a round material, such as a wafer for semiconductor manufacturing.

FIG. 2 shows a view looking upwards through two partial electrodes, which are located underneath in the processing container, in the direction of the underside of the material to be treated that is located on top of the cup.

FIG. 3 shows the view through four partial electrodes of the processing container as the cup plater looking in the direction of the underside of the material to be treated.

FIG. 4 shows measurement results that were determined on a resistance model of an electrolytic cell and a cell voltage/current density curve of a copper bath based on sulfuric acid.

FIG. 5 shows the qualitative characteristics of electrolytically deposited layers and layer thickness distributions on the material at various levels of electroplating current.

A cup plater with a global anode consisting of, for example, two partial anodes is initially considered for electroplating. These partial anodes each form a semicircle, less the space required for the mutual electrical isolation as shown in the schematic FIG. 2, and are described in more detail below. Details are given below on the partial anodes, the partial cathodes or electrodes, and the associated partial electrolytic cells, as well as the contacts at the edge of the material and the rectifier or electroplating current sources along with some comments.
The cup-plater in FIG. 1 consists essentially of a processing container 4 filled with electrolyte 5, the cup, a collection container 8 and a holder 2 for the material 1 that is to be electroplated in this example. The material 1, such as a wafer, is located on the holder 2 that holds and moves the side to be treated 3 of the wafer 1 above the processing container 4. Electrolyte 5 flows downwards into the process container 4 through an inlet 13 by means of a supply pump 6. The electrolyte 5 passes, for example, through apertures in the partial anodes 7′ and 7″ to the side to be treated 3 of the wafer 1 from where it then flows via overflows into the collection container 8 that also serves as the pump sump for the supply circuit. The two partial anodes 7′ and 7″ are insulated from one another, i.e. they do not have a low-resistance electrical connection. To increase the insulation, an electrically non-conducting partitioning wall 14 may be inserted between the partial anodes in order to prevent a continuous mutual plating or deplating of the electrodes.
The material 1 is arranged in parallel above the processing container 4 and extends over the entire cross-section of the processing container 4. The side to be treated (electroplated) 3 of the cathodic material 1 that represents a global cathode is likewise notionally divided into two partial areas, i.e. partial cathodes 12′ and 12″. At first the partial cathode 12′ is static with respect to the partial anode 7′ while the partial cathode 12″ is static at first with respect to the partial anode 7″. The partial anodes and partial cathodes respectively form electrolytic cells 11′ and 11″, which together form the global electrolytic cell 11 of the cup plater. The partial anode 7′ shown on the left is fed anodically by the electroplating current source 9′. The negative terminal of the electroplating current source 9′ is connected to the right edge of the wafer 1 by means of an electrical contact 10″ in the area of the partial cathode 12″. Conversely, the negative terminal of the electroplating current source 9″ is connected to the left edge of the wafer 1 in the area of the partial cathode 12′ that is vertically opposite the partial anode 7′. Sliding or rotary contacts and removable electrical contacts 10′ and 10″ are arranged diametrically to the material and are used to connect the negative terminal to the material 1. The electrical circuit of the electroplating current sources 9′ and 9″ and the position of the associated equipment are shown in FIG. 2. In this figure, the view is from underneath through the partial anodes 7′ and 7″ in the direction of the wafer.
The partial anode 7′ is supplied via a rectifier or an electroplating current source 9′ to form the electrolytic cell 11′ together with the underlying partial cathode 12′ that is not visible in this figure. The partial anode 7″ is supplied via a rectifier or an electroplating current source 9″ to form the electrolytic cell 11″ together with the underlying partial cathode 12″.
The electrical contact(s) 10′ are located in the marginal area of the cathodic material at the partial cathode 12′ and thus at the partial electrolytic cell 11′. The electrical contact(s) 10″ are located in the marginal area of the cathodic material at the partial cathode 12″ and thus at the electrolytic cell 11″.
According to the invention, the electroplating current source of the rectifier 9′ passes via the partial anode 7′, the left electrolytic partial cell 11′, and the partial cathode 12′ of the cathodic material 1, and from there through the base layer of the partial cathode 12″ of the material 1 via the electrical contact 10′. In the same way, the electroplating current source of the rectifier 9″ passes via the partial anode 7″, the right electrolytic partial cell 11″, and the partial cathode 12″ of the cathodic material 1, and from there through the base layer of the partial cathode 12′ of the material 1 via the electrical contact 10′.
The partial cathode 12′ of the material with the base layer on it thus serves as an electrical conductor for the cathodic current for the electrolytic partial cell 11″. The partial cathode 12″ of the material with the base layer on it serves correspondingly as an electrical conductor for the cathodic current for the electrolytic partial cell 11′.
Two limiting cases are considered in the example of FIG. 2 below to explain the invention:
The first limiting case is when only one of the two rectifiers is switched on for the time period t. The two respective rectifiers 9′, 9″ of the two sides or partial cathodes 12′, 12″ of the material which form a pair of rectifiers are switched on alternately. The duty cycle t1 of the rectifier of the one side 12′ and the duty cycle t2 of the rectifier of the other side 12″ are preferably of the same duration.
The second limiting case is when the rectifiers 9′, 9″ of a rectifier pair of the two partial anodes 7′, 7″ are switched on at the same time with the same current intensity I.
In the first limiting case, the first rectifier 9′ is switched off for a certain time t. The rectifier 9″ supplies the partial electrolytic cell 11″. The supply of the cathodic electroplating current I for the partial cathode 12″ of the partial electrolytic cell 11″ is effected solely via the contact 10′ or via the contact area 10′ and from there through the base layer of the partial cathode 12′. For this partial electrolytic cell 11″, this means that the supply of current into its base layer or into its partial cathode 12″ is preferably effected in the center of the material. In this partial electrolytic cell 11″, the voltage drop then increases in the base layer from the center of the material starting towards the edge, and is diametrical to the supply contact 10′. The locally effective cell voltage as well as the current density then decrease in the direction of this edge. This results in a radial gradient on the partial cathode 12″ of the material so that thickening occurs at the center of the material, although the material is electrically contacted at the edge, in fact at the edge that is not at the partial electrolytic cell 12″. The same situation then applies in this first limiting case to the partial electrolytic cell 12′ where the rectifier 9″ is preferably switched off for same time t and the rectifier 9′ is switched on to supply the partial cell 12′.
The electroplate deposition on the two partial cathodes 12′ and 12″ of the material 1 is effected successively in this example for the same length of exposure time, whereby the thinner electroplating sections occur surprisingly at the edges via which the current was supplied. This is in contrast to the state of the art by using very simple technical control measures for cost-effective leveling as is described below. According to the state of the art, however, the thicker inclined plane sections occur at the edges.
In the second limiting case, both rectifiers 9′ and 9″ were switched on with equal currents I at the same time, resulting in a complete symmetry of the currents I and the voltage drops in the partial cathode. The equally-sized currents I flow from both sides to the center of the material. In this case, the respective electrical conductor in the base layer and the symmetric voltage drop occurring there, is so effected that a thinner electroplating section occurs in the center of the material.
The controlled electroplating of the material 1 according to the invention takes place within one of these two limiting cases. By controlling or regulating the electroplating currents I of the rectifiers 9′ and 9″ alternately and simultaneously at different densities, the layer thickness distribution can be adjusted very accurately, i.e. leveling. The difference between the electroplating currents I at different densities that flow through the two electrolytic partial cells 11′ and 11″ alternately and preferably simultaneously, enables the radial section from the center of the material as far as the edges of the material to be level or even over-electroplated, although there are no contacts in the central section. Thus control technology takes into account the various parameters of the material to be electroplated, in particular base layers of various thicknesses, the thickness of the layer to be deposited and the average current density without having to adapt the cup plater. The same is true with reversed polarities of the equipment for an electrolytic etching process.
In the case of the preferred round material 1, there are different distances a and b from the contact 10″ or 10′ to the respectively assigned partial anode 7′ and 7″. Because of the longer distance b compared to the distance a, the edge section is electroplated less than the center of the material by the diametrical supply of the electroplating current. This also supports the method of the invention.
Layer thickness variations can occur on the material in certain areas such as near the insulating partitioning wall 14 in the case of static positioned electrodes. In order to level out locally occurring layer thickness differences, it is advantageous if a rotational relative movement or other movement takes place between the partial anodes 7′ and 7″ on the one hand and the material 1 on the other. Thus, for example, when using two partial anodes, a periodically reversing pivoting movement of at least ±90° of the material 1 is advantageous. The pivoting allows a technically simple power supply to the moving material or to the moving partial anode or partial electrodes via flexible electrical conductors. Technically elaborate sliding or rotary contacts are not required.
To level the layer deposited on the material, the material and the partial anodes can also be designed to rotate relative to one another. However, this requires e.g. in the case of two partial anodes, a two-pole rotating current transfer to the material and/or to the partial anode by means of sliding contacts 15 or rotary contacts. In the case of more than two partial anodes, these sliding contacts 15 have to be multi-pole. Therefore the cost-effective pivoting movement of the material and/or the partial anode around the vertical axis that passes through the center of the processing container 4 is preferable.
Another control means for leveling the deposited layer consists of non-linear pivotal or rotational movement of the material or the partial anodes. This means that the pivotal or rotational speed can be varied for certain relative positions of the material or the partial anodes, including a momentary rate of zero, i.e. temporary standstill. The contacts 10 of a pair of electrodes can supply the electroplating current to the material at the diametrical edge either at one point or through a limited arc. The edge portions of the wafer are preferably electroplated by increasing the arc length. Thus there is another way of leveling the deposited layer but that cannot be controlled electrically.
For additional needs-dependent control, the partial anodes can be connected to one another using at least one electrical switching contact 16 such as, for example, an electrical low-resistance relay contact. In this case, the electrolytic partial cells behave like a global electrolytic cell according to the state of the art, where, especially in the case of thin layers to be electroplated, the edge sections are preferably electroplated. This can be used, for example, for leveling towards the end of the exposure time if the starting layer is preferably electrochemically treated according to the inventive method for the center section. Conversely, for example, the treatment can be started by means of the closed circuit contact 16, and then leveling up to the thicker edge section is performed by raising the center section by means of the inventive measures described above.
Both methods can also be alternated several times during processing. An electrical resistance can be inserted in series with the switch contact 16. This serves as a further control means for a less abrupt transition from one switching state to another.
For the leveling of the layer to be treated, the methods and the devices according to the invention can be combined with the measures described above for uniform electrochemical treatment according to the state of the art
FIG. 3 shows four partial anodes 7′, 7″, 7′″ and 7″″ with four individual electroplating current sources 9′, 9″, 9′″ and 9″″. Here, again, the cathodic currents are supplied to the electrolytic partial cells 11′, 11″, 11′″ and 11″″ respectively via pairs of diametrically arranged contacts 10′, 10″, 10′″ and 10″″ in the material 1. These four partial anodes increase the control possibilities for the leveling of the deposited layer. A linear or nonlinear periodically reversing pivoting of the material and/or partial anodes by at least ±45° is required for the leveling of the deposited metal layer. Technically elaborate sliding contacts are not required for the pivoting. Simple flexible conductors such as stranded wires are sufficient. By using four sliding contacts 15 for the material and/or partial anodes, these can again be rotated evenly or unevenly with respect to one another; likewise with inserted breakpoints at points on the circular path that are determined, for example, by means of experiments.
In addition, an odd number of partial anodes from three pieces is possible. The pivoting angle can be reduced by increasing the number. As does an increased number of electroplating current sources. Likewise, the effort for the timely control of the level of the electroplating currents. The controlled switch contacts 16 also enable the edge electroplating to be preferred as needed in order to achieve an overall level layer thickness distribution on the material.
FIG. 4 shows the quantitative effect of the current differences AI in the electrolytic partial cells 11′, 11″. Because it is nearly impossible to measure the material in an electrolytic processing container filled with electrolyte, the data were determined using a resistance model. This model forms a relatively realistic typical electroplating material, such as a circuit board and an electrolytic cell.
A cell voltage/current density curve of a real electrolytic cell of a sulfuric acid copper bath is drawn in the diagram with the curves for the measurement points across the material on the X2 axis and the corresponding anode/cathode voltage on the Y2 axis. The cell voltage Uz is plotted on the Y1 axis, and the corresponding current densities i are plotted on the X1 axis. This Uz/i curve allows the determination of the real current density i for the anode/cathode voltages measured in the resistance model corresponding to the cell voltage Uz.
The current density is plotted in A/dm²and the cell voltage is plotted in volts. The typical course of the Uz/i curve shows that at Uz cell voltages below 1.5 V, there is almost no metal deposition. The cathodic current density i in this case is less than 0.2 A/dm². This is advantageously sufficient, however, to prevent re-dissolution of metal. In the range of Uz cell voltages from 1.5 V to 2.5 V, the current density increases from 0.2 A/dm²to 7.6 A/dm². The curves of the anode/cathode voltages show that in the case of currents in the left electrolytic cell 11′, which only carries up to 50% of the current in the right electrolytic cell 11″, anode/cathode voltages in the range of 1.5 V or less occur. At these small anode/cathode voltages or cell voltages, there is virtually no deposition. Thus electroplating only occurs in the right electrolytic cell 11″. The course of the anode/cathode voltages or cell voltages can be so adjusted so that a thickening or thinning of the deposited layer occurs in the center of the material. The thickest part occurs at 0% to the left and 100% to the right of the electroplating current.
A thinner part is formed in the center of the material at, for example, 70% to the left and 100% to the right of the electroplating current.
In the case of 100% electroplating current in both electrolytic partial cells 11′, 11″, the anode/cathode voltages are completely symmetrical. The greatest thinning occurs exactly in the center of the material. The diagram shows that the thinnest part is electroplated with a current density i of 4.6 A/dm²while the thickest part is electroplated with a current density i of 7.6 A/dm². The current density differences Δ1 thus amount to 3 A/dm²in this chosen example. This corresponds to the state of the art with a two-sided current supply.
The anode/cathode voltage curve with 30% to the left and 100% to the right, shows a nearly flat curve in the right partial cell 11″. The difference Δ2 of the anode/cathode voltages of about 0.1 V corresponds to the current density difference in the right electrolytic cell 11″ of about 0.6 A/dm². This signifies a virtually level electroplating on this half of the material.
The current is equally mirrored, i.e. 100% on the left side while the current is reduced on the right side, e.g. to 30%. The result is an almost completely flat deposition of the metal across the material.
The remaining small current density difference that occurs symmetrically on both sides and thus on a circular path on the material, can be leveled in accordance with the invention. In this case, electrodes 7′, 7″, for example, can be subjected to an oscillating movement in their plane by means of a drive. This motion is superimposed on the pivoting movement of the material. The result is a repetitive displacement of the dividing line(s) between the electrodes 7′, 7″ from the central axis of the processing container. Thus, the location of the slightly different treatment is displaced on the material. The result is completely level electroplating or completely uniform electrochemical etching.
FIG. 5 shows a simplified schematic of the gradients of the electroplating levels across the material and through its centerpoint. The situations at different current densities I are shown, in fact symbolically as a percentage after each of the partial steps at the times t1 and t2, as well as after complete plating of the material in the time t1+t2. The sum of the deposits on the two partial cathodes 12′ and 12″ that are static, i.e. without a relative movement of the electrodes in the times t1 and t2, is shown by the curve in FIG. 5 below. These are the results after electroplating of the material in the electrolytic cells 11′ and 11″.
FIG. 5 a shows the limiting case with the electroplating current sources 9′ and 9″, which were adjusted to equal current density I and switched on simultaneously. As a result, the thinnest parts of the radial gradient occur in the central section of the material or of the global cathode 12, consisting of the partial cathodes 12′, 12″. The layer thickness differences are designated by delta, which is maximum here.
FIG. 5 b shows the other limiting case where only the electroplating current source 9′ is switched on for the first time t1 with the partial cathode 12′ to supply the partial cell 11′ while in the subsequent time t2, only the electroplating current source 9″ is switched on with the partial cathode 12″ to supply the partial cell 11″. In sum, this results in a very advantageous thickening of the levels in the center of the material. Here also, delta is maximum. Because of the half exposure time compared to the case of FIG. 5 a, only about half of the metal is deposited during the same exposure time.
FIG. 5 c shows the situation between the two limiting cases with both electroplating current sources 9′ and 9″ switched on for the simultaneous supply of the partial cells 11′ and 11″, but alternately with different current densities I. These current densities I are so adjusted that the sum of the deposits is a level layer. Delta is zero here. The leveling of the layer takes place in this example after two periods of time, i.e. after t1 plus t2.
FIG. 5 d shows an imperfect correction or leveling of the layer thicknesses in both partial cells 11′ and 11″ of the material 1. The examples show, however, that only the inventive method enables individual profiles of the layer thickness distribution to be achieved by controlling the electroplating current I. The stated current densities I as a percentage only represent explanatory guidelines.
The times t1 and t2 can be in the range of a few milliseconds. They may also represent up to half of the total required exposure time. In practice, following initial experiments, empirical values are obtained for the adjusted times and current densities I of the rectifier and other parameters. The same is true for any necessary rotating or pivoting movements of the electrodes.

LIST OF REFERENCE NUMBERS

1 Material, wafer
2 Holder
3 Treatment side
4 Processing container
5 Electrolyte
6 Pump
7 Anodic electrode, global anode, partial anode, partial electrode
8 Collection container
9 Rectifiers, power source, electroplating current source,
10 Contact, contact area
11 Electrolytic cell, electrolytic partial cell, global cell
12 Cathodic electrode, counter electrode, global cathode, partial cathode
13 Inlet
14 Partitioning wall
15 Sliding contact, rotary contact
16 Switch contact with or without series resistance
I Current, direct current, electroplating current

Claims

1. Method for the electrochemical treatment of material (1) with at least an electrically conductive layer on the treatment side (3) such as, for example, a global cathode (12) and a global soluble or insoluble anode (7), which together form a global electrolytic cell (11), as well as with electrical contacts (10) of the material at the edge area, especially for electroplating or etching of substrates, such as a wafer in a cup plater or similar electrolytic processing container, where at least two diametrically arranged pairs of electrolytic partial cells (11′, 11″) are formed in the processing container, consisting of partial electrodes (7′, 7″) and partial counter cathodes (12′, 12″) each of which is supplied with electrolytic current (I) from adjustable current sources (9′, 9″), and where respective current (I) is supplied from the edge section to the material and which is located diametrically opposite on the material to the respective partial cell (11′, 11″).

2. Method according to claim 1, characterized in that in electroplating of the cathodic current (I) and electrolytic etching of the anodic current (I), the material is supplied via the contact area (10′, 10″) located diametrically opposite the respective partial cell (11′, 11″).

3. Method according to claim 1, characterized in that the electrolytic partial cells (11′ and 11″) are alternately supplied as pairs simultaneously or nearly simultaneously with currents of different densities (I).

4. Method according to claim 1, characterized in that the alternation of different currents (I) within the time range of one millisecond is effected for up to half of the total exposure time.

5. Method according to claim 1, characterized in that electrochemical treatment of the material in the center from the alternately switched power sources (9′, 9″) is the preferred treatment.

6. Method according to claim 1, characterized in that the leveling of the resultant preferred central treatment is effected by giving priority to edge electroplating whereby the electrolytic partial cells (11′ and 11″) are electrically connected with one another by means of switch contacts (16′, 16″) with or without a series resistance.

7. Method according to claim 1, characterized in that a rotating or reversing pivoting linear or nonlinear relative motion, including temporary standstill between the material (1) and the partial anodes (7′, 7″) or partial cathodes (12′, 12″), levels the deposited or etched metal layer.

8. Device for the electrochemical treatment of material (1) with at least an electrically conductive layer on the side to be treated (3) such as, for example, a global cathode (12) and a global soluble or insoluble anode (7), which together form a global electrolytic cell (11), as well as with electrical contacts (10) of the material at the edge area, especially for electroplating or etching of substrates, such as a wafer in a cup plater or similar electrolytic processing container, using the method according to claims 1 to 7, with at least two anodes (7′, 7″), which are arranged diametrically opposite one another, are electrically isolated from one another in the processing container (4) and are supplied with electroplating current from electroplating current sources (9′, 9″), and where the positive pole of the electroplating current source (9′) is on the partial anode (7′) and the negative pole of this electroplating current source (9′) is on the diametrically opposite edge of the material and is connected via the contact (10″) while the positive pole of the electroplating current source (9″) is on the partial anode (7″) and the negative pole of this electroplating current source (9″) is on the diametrically opposite edge of the material and is connected via the contact (10″).

9. Device according to claim 8, characterized in that there is at least one drive for the rotating or reversing pivoting relative movement between the partial electrode (7′, 7″) and the material.

10. Device according to claim 8, characterized in that the switch contacts (16) are electrically connected to the controlled partial electrodes (7′, 7″) with or without a series resistance.