WO2002018653A2

WO2002018653A2 - Method for depositing nitride layers

Info

Publication number: WO2002018653A2
Application number: PCT/US2001/026751
Authority: WO
Inventors: Suraj Rengarajan; Imran Hashim; Tony P. Chiang; Peijun Ding
Original assignee: Applied Materials, Inc.
Priority date: 2000-08-28
Filing date: 2001-08-28
Publication date: 2002-03-07
Also published as: WO2002018653A3; TW505988B

Abstract

A method of depositing a nitrogen rich nitride film, such as a tantalum nitride (TaN) film, which can be used with low-k dielectric films. An initial nitrogen rich layer is achieved by exposing a target to a nitrogen rich atmosphere prior to sputtering the target and deposition of the sputtered target material onto a substrate. Thereafter, the flow of N2 can be controlled during processing to create a desired nitrogen concentration in the film. The nitrogen flow may be held constant during processing, varied during processing, or terminated during processing to control the nitrogen concentration in the layer(s).

Description

NITROGEN RICH FILM FOR LO -K APPLICATIONS

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention generally relates to deposition of films onto a substrate. More particularly, the invention relates to deposition of nitride films, such as tantalum nitride (TaN) films, which can be used with low dielectric constant (low-k) films.

Background of the Related Art

In the fabrication of integrated circuits and other electronic devices on substrates, multiple deposition and etch steps are performed in sequence to fabricate desired _.electronic structures or devices on substrates. The trend in integrated circuit fabrication has been to reduce the overall size of the features which make up the devices and to increase the density of devices formed on a single die. The goal in designing and fabricating these electronic devices is to achieve an overall increase in the speed and capacity of the devices. As dimensions decrease, a need arises for materials with lower dielectric constants to act as insulators arid less resistive materials to serve as conductors. This trend has caused a significant amount of research to be undertaken in the area of materials to accomplish these goals.

One area of research has been conducted in the area of dielectric materials which have a lower dielectric constant. These materials are useful in preventing interference and cross talk between adjacent metal films, lines and other conducting features. The goal is to reduce the overall thickness of the dielectric material disposed between conducting features with improved insulating properties.

Barriers play a prominent role in the formation of multilevel metal structures which are present in many integrated circuits. Diffusion of materials between adjacent films in semiconductor devices is a particular concern to those in the semiconductor industry. Such diffusion or intermixing may be prevented by depositing a barrier film between the dielectric film and the conductive material. The barrier can be made of a single material or a stack of materials to prevent or retard the diffusion of the conductive material into the dielectric material.

With the recent progress in sub-quarter-micron copper interconnect technology, tantalum and tantalum nitride have become popular barrier materials in addition to titanium and titanium nitride. Depending on the application, a barrier may comprise a tantalum film, a tantalum nitride film, a tantalum tantalum nitride stack or other combinations of barrier materials. The barrier film is commonly deposited over a doped silicon oxide, such as a low-k fluorine-doped silicon oxide film (FSG), after openings for interconnect structures (e.g., contacts or vias) have been etched in the doped silicon oxide film. A metal, such as copper, is then deposited over the barrier to fill the interconnect feature.

During substrate processing, heat treatment steps in which a substrate is heated to a specified temperature for a specified time are employed for various reasons. For example, an anneal step may be used to repair damage to a substrate after a plasma processing step. However, when a low-k FSG film is subjected to a temperature greater than about 350°C, loosely-bonded fluorine atoms and residual fluorine atoms tend to be released from the FSG film. The released fluorine atoms from the FSG film react with the tantalum component of the tantalum nitride barrier film and form volatile tantalum fluoride (TaF₂). The TaF₂ formation can increase the resistance of an interconnect structure and can cause significant losses in the adhesion properties between the tantalum nitride film and the FSG film. The loss in adhesion properties can cause the tantalum nitride barrier film to peel off during subsequent processing of the substrate, resulting in the formation of defects. Similarly, for a titanium based barrier film, the released fluorine atoms react with the titanium to form titanium fluoride (TiF), which can lead to defect formations on the substrate as TiF₂.

Therefore, there is a need for a barrier film which promotes adhesion and a method of depositing the film for use with low-k dielectrics.

SUMMARY OF THE INVENTION

The present invention generally relates to a process of depositing a nitride film, such as tantalum nitride, on a substrate. In one aspect, the concentration of nitrogen is controlled to deposit a film with a desired nitrogen concentration. This control of nitrogen concentration allows a film varying in composition to be deposited on a substrate during a processing regime.

In another aspect, a method is provided in which a substrate is positioned in a processing chamber and a sputtering gas, such as argon, is introduced into the processing region. A reactive gas, such as nitrogen, is initially flowed into the processing region during pressure stabilization prior to plasma ignition. Next, a plasma is ignited. Then, a ratio of the reactive gas to the sputtering gas is selectively controlled, or "tuned," during sputtering of a target, such as a tantalum target, to deposit a film on the substrate having a desired concentration of nitrogen.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features, advantages and objects of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings.

It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

Figure 1 is a cross sectional view of an IMP chamber.

Figure 2 is a flowchart showing the processing steps of the present invention.

Figure 3 is a graph of the effect of N₂ pre-flow on TaN reactive sputtering with a constant nitrogen profile.

Figure 4 is a graph of the effect of N₂ pre-flow on TaN reactive sputtering with a non-constant nitrogen profile.

Figure 5 is a graph of the effect of N₂ pre-flow on TaN reactive sputtering with a less variant nitrogen profile.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The embodiments described below can be implemented using metal plasma process equipment, such as an ion metal plasma (IMP) processing chamber, known as an IMP VECTRA™ Chamber mounted on an Endura® platform, which is available from Applied Materials, Inc., located in Santa Clara, California. Generally, IMP is an extension of physical vapor deposition (PVD) technology that offers the benefit of highly directional deposition with good bottom coverage on high aspect ratio structures. Sputtered metal atoms are ionized by passing the atoms through a plasma generated in a deposition chamber between the target and the substrate. An electric field, or self-bias, develops in the boundary layer, or sheath, between the plasma and the substrate that accelerates the metal ions towards the substrate in a vector perpendicular to the substrate surface. The bias energy can be modulated on the substrate by an optional application of power, such as RF power. Because of the highly directional nature of the ions, the bottom coverage is substantially unaffected by feature width.

Figure 1 is a schematic cross-sectional view of an exemplary IMP chamber 100. The chamber 100 includes sidewalls 101, lid 102, and bottom 103. The lid 102 includes a target backing plate 104 which supports a target 105 of the material to be deposited. An opening 108 in the chamber 100 provides access for a robot (not shown) to deliver and retrieve substrates 110 to and from the chamber 100. The substrate support 112 is mounted on a lift motor 114 that raises and lowers the substrate support 112 and a substrate 110 disposed thereon. A lift plate 116 connected to a lift motor 118 is mounted in the chamber 100 and raises and lowers pins 120a, 120b mounted in the substrate support 112. The pins 120a, 120b raise and lower the substrate 110 from and to the surface of the substrate support 112. A coil 122 is mounted between the substrate support 112 and the target 105 and provides inductively-coupled magnetic fields in the chamber 100 to assist in generating and maintaining a plasma between the target 105 and substrate 110. Power supplied to the coil 122 densities the plasma which ionizes the sputtered material. The ionized material is then deposited on the substrate. A shield 124 is disposed in the chamber 100 to shield the chamber sidewalls 101 from the sputtered material. The shield 124 also supports the coil 122 by supports 126. The supports 126 electrically insulate the coil 122 from the shield 124 and the chamber 100. The clamp ring 128 is mounted between the coil 122 and the substrate support 112 and shields an outer edge and backside of the substrate from sputtered materials when the substrate is raised into a processing position to engage the lower portion of the clamp ring. In some chamber configurations, the shield 124 supports the clamp ring 128 when the substrate 110 is lowered below the shield 124 to enable substrate transfer.

Three power supplies are used in this type of inductively coupled sputtering chamber. A first power supply 130, preferably DC power for conductive materials, delivers power to the target 105 to cause the processing gas to form a plasma. Magnets 106a, 106b disposed behind the target backing plate 104 increase the density of electrons adjacent to the target 105, thus increasing ionization at the target. The magnets 106a, 106b increase the sputtering efficiency by generating magnetic field lines generally parallel to the face of the target. Electrons are trapped in spinning orbits around the magnetic field lines to increase the likelihood of a collision with, and ionization of, a gas atom for sputtering. A second power source 132, preferably a RF power source, supplies electrical power at about 13.56 mHz to the coil 122 to increase the density of the plasma. Another power source 134, typically a AC power source, biases the substrate support 112 with respect to the plasma and provides directional attraction of the ionized sputtered material toward the substrate 110.

Processing gases are supplied to the chamber 100 through a gas inlets 136, 137 from gas sources 138, 140 as metered by respective mass flow controllers 142, 144. A vacuum pump 146 is connected to the chamber 100 at an exhaust port 148 to exhaust the chamber 100 and maintain the desired pressure in the chamber 100.

A controller 149 controls the functions of the power supplies, lift motors, mass flow controllers for gas injection, vacuum pump, and other associated chamber components and functions. The controller 149 executes system control software stored in a memory, which in the preferred embodiment is a hard disk drive, and can include analog and digital input/output boards, interface boards, and stepper motor controller boards (not shown). Optical and/or magnetic sensors (not shown) are generally used to move and determine the position of movable mechanical assemblies.

In a typical IMP operation, a robot (not shown) delivers a substrate 110 to the chamber 100 through the opening 108. The pins 120a, 120b (only two of three are shown) are extended upward, lift the substrate 110 from the robot, and the robot retracts from the chamber 100. The pins 120a, 120b lower the substrate 110 to the surface of the substrate support 112. The substrate support 112 raises the substrate 110 to engage the clamp ring 128. A processing gas is injected into the chamber 100 and a plasma is generated between the target 105 and the substrate support 112 with power from the power source 130. The power source 132 delivers power to the coil, which densifies the plasma and ionizes the sputtered target material leaving the target 105 to form sputtered material ions. The sputtered material ions are accelerated toward the biased substrate 110. The process pressure may be operated from about 5 mTorr to about 100 mTorr to increase the ionization probability of the sputtered material atoms as the atoms travel through the plasma region. After deposition, the substrate support is lowered, the pins 120a, 120b are raised to lift the substrate 110, a robot (not shown) enters the chamber 100, retrieves the substrate 110, and if desired, delivers another substrate for processing.

Figure 2 is a flowchart representing a method for depositing a nitrogen rich tantalum/tantalum nitride film on a substrate. Generally in step 210, a substrate 110 is positioned in a chamber, such as the IMP chamber 100 shown in Figure 1, on a substrate support pedestal 112. Then in step 220, the chamber is pumped down to vacuum to remove any residual gas from the chamber prior to plasma strike. Next in step 230, an inert gas, such as argon, is introduced through gas inlet 136 into the processing chamber 100. The pressure within the processing chamber 100 is regulated between about 10 mTorr to about 60 mTorr, preferably between about 30 mTorr and about 50 mTorr, by vacuum source 146, such as a vacuum pump.

In step 240, a nitrogen source gas, such as N₂, is supplied to the processing chamber 100 through gas inlet 137 at a rate from about 8 seem to about 20 seem, but may be modified depending on chamber volume, pumping speed, and desired pressure, thereby forming a nitrogen rich gaseous mixture.

The N₂ flow may be held constant during processing, varied during processing, or terminated during processing depending on the nitrogen concentration desired in the film. Thus, controlling, tuning, or tailoring the N₂ flow during processing creates a nitrogen gradient through a film or stack of films from nitrogen rich to nitrogen deficient. During pressure stabilization and prior to plasma strike, a sufficient amount of N₂ is supplied to the chamber to ensure that there is adequate nitrogen to react with the tantalum target. An adequate amount of N₂ would be an amount sufficient to prevent poison mode. Poison mode results when the nitrogen available exceeds the stoichiometric concentration to produce a film having a nitrogen to tantalum ratio of more than 2:1. This results in a resistive dielectric film having a resistance in the MΩ range, as opposed to the desired nitrogen rich conductive film having a resistivity in the range of about 230 μΩ cm to a maximum of about 350 μΩ cm.

Next in step 250, the target 105 is exposed to the nitrogen rich gaseous mixture. In step 260, the nitrogen rich gaseous mixture is ignited into a plasma in the process chamber 100 by applying a bias from power sources 132 and 134, respectively, to a coil 122 and the substrate support pedestal 112. The coil is biased at about 1 kW to about 5 kW, preferably at about 2 kW to about 4 kW, and the substrate is biased at about 300 W to about 500 W, preferably at about 400 W to about 450 W. The target 105 is biased with a DC power supply 130.

In step 270, the flow of N₂ is then controlled, or "tuned," during processing to adjust a ratio of the N₂ to the sputtering gas to produce a film on the substrate having a desired concentration of nitrogen. For example, the film can be tailored so that the nitrogen concentration is rich at the dielectric interface, thus preventing the formation of TaF₂ between a film such as FSG with a subsequent film of tantalum. Then the flow of N₂ can be tapered off or cut off resulting in a nitrogen deficient, tantalum rich film at the conductive metal interface providing a good adhesive surface for subsequent deposition of the next film (i.e. seeding), such as copper.

Finally in step 280, a film having the desired concentration of nitrogen is deposited on the substrate 110. The resulting TaN film or stack of films has an appropriate nitrogen concentration to promote adhesion between a low-k dielectric interface, such as FSG, and a conductive metal interface, such as copper. The nitrogen deficient film at the metal interface, i.e. the bridge, also results in better orientation which can lessen the incidence of electromigration.

A nitrogen rich TaN film deposited according to aspects of the present invention is characterized by a smooth, amorphous like appearance which promotes the adhesive quality of the film and reduces the occurrence of peeling and other defects which may occur with a typical TaN film.

The steps shown in Figure 2 can be executed in response to instructions of a computer program executed by a microprocessor or computer controller for the system. For Example, a computer program product that runs on a conventional computer system comprising a central processor unit (CPU) interconnected to a memory system with peripheral control components, such as for example a 68400 microprocessor, commercially available from Synenergy Microsystems, California. The computer program code can be written in any conventional computer readable programming language such as for example 68000 assembly language, C, C++, or Pascal. Suitable program code is entered into a single file, or multiple files, using a conventional text editor, and stored or embodied in a computer usable medium, such as a memory system of the computer. If the entered code text is in a high level language, the code is compiled, and the resultant compiler code is then linked with an object code of precompiled windows library routines. To execute the linked compiled object code, the system user invokes the object code, causing the computer system to load the code in memory, from which the CPU reads and executes the code to perform the tasks identified in the program. EXAMPLE 1

In one example, a typical tantalum nitride film deposition process was performed on a substrate. The substrate was positioned in an IMP chamber where about 250 A of TaN was deposited on a silicon oxide wafer using conventional IMP techniques. The chamber pressure was maintained at about 35 mTorr and the chamber temperature was maintained from about ambient to about 100°C. Argon and nitrogen were flowed into the chamber at a rate of about 56 seem and about 36 seem, respectively. The gaseous mixture was ignited into a plasma in the process chamber by applying a bias to the coil of about 2.5 kW and a DC bias to a tantalum target of about 1 kW. Then the substrate was biased at about 400 W. The target was then sputtered and a TaN film was deposited on the wafer for about 20 seconds. In this example, the nitrogen flow was terminated during processing for about the last two seconds of deposition to allow the deposition of a tantalum rich interface, i.e., creating a tantalum post-flash.

EXAMPLE 2

In example two, a process according to one embodiment of the invention was performed on a substrate. The substrate was positioned in an IMP chamber where about 250 A of TaN was deposited on a silicon oxide wafer using IMP techniques. The chamber pressure was maintained at about 35 mTorr and the chamber temperature was maintained from about ambient to about 100°C. Argon was flowed into the chamber at a rate of about 56 seem. The nitrogen was pre-flowed into the chamber at a rate of about 20 seem prior to plasma strike and a target was exposed to the nitrogen rich gaseous mixture. Next, the nitrogen rich gaseous mixture was ignited into a plasma in the process chamber by applying a bias to the coil of about 2.5 kW and a DC bias to a tantalum target of about 1 kW. Then the substrate was biased at about 400 W. The target was then sputtered and a nitrogen rich TaN film was deposited on the wafer for about 20 seconds. This example was also run with about a two second tantalum post-flash.

EXAMPLE 3

In another example, a process according to an embodiment of the invention was performed on a substrate. The substrate was positioned in an IMP chamber where about 250 A of TaN was deposited on a silicon oxide wafer using IMP techniques. The chamber pressure was maintained at about 35 mTorr and the chamber temperature was maintained from about ambient to about 100°C. Argon was flowed into the chamber at a rate of about 56 seem. The nitrogen was pre-flowed into the chamber at a rate of about 8 seem prior to plasma strike and a target was exposed to the nitrogen rich gaseous mixture. Next, the nitrogen rich gaseous mixture was ignited into a plasma in the process chamber by applying a bias to the coil of about 2.5 kW and a DC bias to a tantalum target of about 1 kW. Then the substrate was biased at about 400 W. The target was then sputtered and a nitrogen rich TaN film was deposited on the wafer for about 20 seconds. This example was also run with about a two second tantalum post-flash.

Figures 3-5 illustrate analytical results from the deposited nitride films of the above examples, showing the effect of controlling the nitrogen pre-flow during tantalum nitride reactive sputtering. The concentration of tantalum to nitrogen over a silicon oxide wafer is represented by atomic percentages along the y-axis and film depth is measured in angstroms (A) along the x-axis.

Figure 3 is a graph illustrating the results of a traditional process, such as described in Example 1, in which there is no nitrogen pre-flow. In this example, the TaN film has a total depth of about 250 A having a tantalum concentration represented by line 310 of about 60%, plus or minus about 5%, and a nitrogen concentration represented by line 320 of about 40%, plus or minus about 5%, over a silicon oxide layer represented by line 300. Since the target has not been exposed to a nitrogen rich atmosphere prior to plasma strike, the resulting film has an approximately constant Ta:N ratio.

In contrast, Figure 4 is a graph illustrating the results of an embodiment of the process with a nitrogen pre-flow on TaN reactive sputtering deposition, such as described in Example 2. In this example, the film has a total depth of about 250 A deposited over a silicon oxide layer represented by line 400. The initial nitrogen concentration, represented by line 410, peaks at about 60% at a film depth of about 250 A and is then tuned to about 40%) near the silicon oxide interface. Whereas the tantalum concentration, represented by line 420, is nearly inversely proportional to the nitrogen concentration. At a film depth of about 250 A, the tantalum concentration is about 40% and about 60% near the silicon oxide interface. In comparison, Figure 5 is a graph illustrating another embodiment of the invention with a lower nitrogen pre-flow on TaN reactive sputtering deposition, such as described in Example 3. The tantalum nitride film is deposited over a silicon oxide layer represented by line 500 and has a film depth of about 250 A. The nitrogen concentration, represented by line 520, and the tantalum concentration, represented by line 510, at about 250 A are about 50%, plus or minus about 5%, and are about equal at the silicon oxide interface. The lower nitrogen concentration at the silicon oxide interface, as compared to Figure 4, is a result of the lower nitrogen pre-flow. As shown in comparison with Figure 4, the nitrogen concentration in the film can be modified to produce the desired film composition at desired depths.

While foregoing is directed to the preferred embodiment of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.

Claims

Claims:

1. A method for depositing a film on a substrate, comprising: flowing an inert gas into the chamber; flowing a reactive gas into the chamber and exposing a target thereto while igniting a plasma; controlling a ratio of the reactive gas to the inert gas; and sputtering the target to deposit a film onto the substrate.

2. The method of claim 1, wherein the reactive gas is nitrogen.

3. The method of claim 2, wherein the target is tantalum.

4. The method of claim 1, wherein the target is tantalum, titanium, or tungsten.

5. The method of claim 1, wherein the reactive gas flow is constant during processing.

6. The method of claim 1, wherein the reactive gas flow is varied during sputtering.

7. The method of claim 1, wherein the reactive gas flow is terminated during processing.

8. A method for depositing a nitrogen rich tantalum layer on a substrate, comprising: flowing an inert gas into the chamber; flowing nitrogen into the chamber and exposing a tantalum target thereto while igniting a plasma; controlling a ratio of said nitrogen to the inert gas; and sputtering the tantalum target to deposit a film onto the substrate.

9. The method of claim 8, wherein the nitrogen flow is constant during processing.

10. The method of claim 8, wherein the nitrogen flow is varied during processing.

11. The method of claim 8, wherein the nitrogen flow is terminated during processing.

12. The method of claim 8, wherein the inert gas is argon.

13. The method of claim 8, wherein the chamber is under a pressure of about 10 mTorr to about 60 mTorr.

14. The method of claim 14, wherein the nitrogen is flowed at a rate of about 8 seem to about 20 seem.

15. The method of claim 15, wherein a process regime is about 1 kilowatt to about 5 kilowatts.

16. The method of claim 16, wherein the substrate is a size of about 300 W to about 500 W.

17. An apparatus for depositing a nitrogen rich tantalum layer on a substrate, comprising: a processing chamber; a tantalum target; and a computer system connected to the processing chamber and comprising a processor and a memory containing a program code which, when executed by the processor, performs a method comprising: flowing an inert gas into the chamber; flowing nitrogen into the chamber and exposing the tantalum target thereto while igniting a plasma; controlling a ratio of said nitrogen to the inert gas; and sputtering the tantalum target to deposit a film onto the substrate.

18. The apparatus of claim 17, wherein the nitrogen flow is constant during processing.

19. The apparatus of claim 17, wherein the nitrogen flow is varied during processing.

20. The apparatus of claim 17, wherein the nitrogen flow is terminated during processing.