WO2000006793A1

WO2000006793A1 - Sputtering target assembly

Info

Publication number: WO2000006793A1
Application number: PCT/US1999/016734
Authority: WO
Inventors: Tony Chiang; Peijun Ding; Vikram Pavate; Richard Hong; Ruben Diaz; Barry Chin
Original assignee: Applied Materials Inc
Current assignee: Applied Materials Inc
Priority date: 1998-07-27
Filing date: 1999-07-22
Publication date: 2000-02-10
Anticipated expiration: 2001-01-27

Abstract

An improved tantalum sputtering target assembly is provided. The sputtering target assembly (11) has a tantalum sputtering target plate (13), and an aluminum backing plate (15) that are diffusion bonded via a titanium interlayer (17). Despite its high surface hardness and high melting point, titanium is a superior interlayer, interdiffusing readily with tantalum and reacting with aluminum to form an aluminum-titanium alloy. Accordingly, the sputtering target plate of the inventive sputtering target assembly is securely bonded to the aluminum backing plate.

Description

SPUTTERING TARGET ASSEMBLY

BACKGROUND OF THE INVENTION

This invention relates to sputtering targets, and more particularly to an improved tantalum sputtering target. Recent developments in the semiconductor device field such as the commercial development of copper interconnects, and the growing popularity of ionized metal plasma deposition, have increased not only the demand for cost effective tantalum sputtering targets, but also the demand for tantalum targets that are mechanically stable despite exposure to high temperature gradients and continuous thermal cycling. In a typical sputter apparatus a target formed of the to be deposited material is bombarded by energetic ions to dislodge atoms thereof for deposition on a substrate. The target often forms one wall of the vacuum deposition chamber and must therefore have a certain amount of strength. As the target is heated by the ion bombardment, its strength should not vary considerably at high temperatures. Two styles of planar target design are typically used: monolithic, wherein the target material is a solid piece, and a target assembly, where target material is affixed to a structural member, commonly called a backing plate. Sputter deposited tantalum and tantalum nitride are the choice materials for forming barrier layers in copper interconnects. However currently, sputter deposition of tantalum and tantalum nitride is far from cost effective. Specifically, tantalum itself is expensive, and its expense in addition to its weight (181 atomic mass units) essentially prohibits the use of a monolithic (i.e., a thick solid) sputtering target. Instead a "target assembly" must be used.

Most sputtering target assemblies include a backing plate, which typically provides the target with structural rigidity to deter bowing thereof, and a target material plate adhered to the frontside (i.e. the chamber facing side) of the backing plate. Often a cooling plate, having grooves forming a channel is affixed to the backside of the backing plate. During sputtering, to cool the target assembly, cooling fluid is flowed through the channels. Alternatively, a grooved backing plate may function as both a backing plate and a cooling cover plate, allowing cooling fluid to directly contact the back surface of the target material plate.

To produce a sputter deposited layer, a sputtering target assembly is preferably placed over an aperture formed in a top surface of a chamber such that the target backing plate forms a portion of the chamber enclosure, and such that the target material plate is exposed to the chamber's internal environment, as is the interface between the target material plate and the backing plate. A substrate such as a semiconductor wafer is placed in the chamber facing the sputtering target assembly and the chamber is evacuated to remove water and other contaminants from both the wafer and the chamber.

The chamber is then backfilled with an inert gas. For example, to sputter deposit a tantalum layer, a tantalum target is used and the chamber is back filled with an inert gas (e.g., argon), and to sputter deposit a tantalum nitride layer, a tantalum target is used and the chamber is backfilled with nitrogen and an inert gas (e.g., argon), the nitrogen forms a film on the surface of the tantalum sputtering target such that tantalum nitride is sputtered therefrom. Thereafter, a gas plasma is generated by exposing the gas to a high magnitude electric field which ionizes gas atoms or molecules forming "ionized particles". An electric field is applied to the target to attract the ionized particles to the target, where the ionized particles stride the target, dislodging atoms of target material. As a result of the collision-induced energy transfer between the ionized gas and target atoms, the sputtered target atoms leave the target and deposit on the semiconductor wafer forming a thin film. Because of the large energy exchange between the ionized gas atoms and the target material plate, the target plate must be continuously cooled during sputtering to prevent sagging or melting thereof.

While cooling a sputtering target prevents the target from melting, it also produces a large thermal gradient across the target as one side of the target is cooled (the side in contact with the cooling cover plate) and the other side of the target is heated by collisions with energetic gas particles. This thermal gradient can cause the target to bow as the heated side of the target expands more than the cooled side. The uniformity of the thickness of the film layer deposited is a function, in part, of the spacing between the target and the substrate. Thus, if only some regions of the target move relative to the deposition surface, the thickness uniformity of the deposited film will suffer. A substantial thermally induced strain results across the target which may eventually cause the target material plate to separate from the backing plate (i.e., to debond) during repeated sputtering operations. When target/backing plate debonding occurs the target material bows, resulting in unevenly deposited layers and uneven target erosion, marking the end of the target's useful life, often well before the available target material is deposited. Accordingly, debonding wastes a portion of the expensive tantalum target material, driving up processing costs. Moreover debonding and the resulting target bending may cause the target material to fracture—a catastrophic failure that can contaminate the sputtering chamber with large target particles and bonding material (e.g.*,, adhesive, brazing or soldering material). After such a catastrophic failure the sputtering chamber must be totally cleaned, resulting in significant downtime of the sputtering target as well as of upstream and downstream processing equipment which may have been contaminated.

The growing popularity of ionized metal plasma (IMP) deposition, a process employing an RF coil to increase deposition rates. Due to increased ion bombardment of the target, IMP deposition typically results in increased target temperatures, thermal gradients and thermal cycling strain, further abbreviating target lifetimes. Accordingly, because of the expense associated with wasted tantalum, a method of extending a tantalum target life is required.

One attempt to create a tantalum target having a longer useful life, is disclosed in commonly assigned US

Patent No. , Application Serial No. 08/511,824 (AMAT No.

642), filed August 7, 1995. The Patent (AMAT No. 642) discloses diffusion bonding a tantalum target plate to a titanium backing plate via an interlayer. Regarding the selection of the interlayer material, the Patent (AMAT

No. 642) indicates that the interlayer formed on the backing plate is preferably composed of the backing plate material or a high-diffusivity alloy thereof. Alternatively, the Patent (AMAT No. 642) indicates that a relatively soft-conformal material such as aluminum may form an interlayer where the titanium backing plate can readily diffuse. The interlayer is said to adhere metallurgically to the target plate, forming a continuous metallurgical extension of the target material which also levels the bonding surface of the target plate by filling any valleys formed therein.

While diffusion bonding of a tantalum target to a backing plate may, if successful, reduce debonding to some extent, the configuration of the Patent (AMAT No. 642) has a, few disadvantages. Namely the titanium backing plate itself is expensive, so although the bond may improve target life and reduce catastrophic failure rates, the thick titanium backing plate increases target assembly costs. Moreover, relatively high temperatures and long bonding times are required in order to achieve a sufficient interdiffusion depth between the tantalum and aluminum layers. Unfortunately, the high temperatures and long bond periods can cause the tantalum target plate to develop large grains. Such large grains in the target material can reduce target life by causing uneven target erosion and unevenly deposited layers. Further, the use of titanium adds significant weight to the overall target assembly.

Accordingly, a need exists for an improved tantalum sputtering target assembly that increases target lifetime without excessive cost and without affecting target material grain size.

SUMMARY OF THE INVENTION The inventors of the present sputtering target assembly have discovered that a diffusion bonded tantalum sputtering target assembly having a titanium interlayer and an aluminum backing plate provides a 500% increase in target life when compared to conventional tantalum targets diffusion bonded to an aluminum backing plate. The inventors have found that despite titanium' s high surface hardness values and despite the fact that both titanium and tantalum are refractory metals, titanium and tantalum readily inter-diffuse to a depth sufficient to withstand even the severe thermal gradients and thermal cycling imposed by IMP sputter deposition. Further, although titanium has a high melting point, the inventive tantalum sputtering target assembly bonds without requiring bonding times or temperatures that effect the tantalum's grain size. Because titanium is light-weight and inexpensive, alloys well with aluminum, and has been found to inter-diffuse readily with tantalum or tantalum nitride, the inventive target assembly is far superior to conventional tantalum target assemblies.

Specifically, an inventive tantalum sputtering target assembly is formed by etching the bonding surfaces of the titanium backing plate and the tantalum sputtering plate, providing an interlayer of titanium (e.g., by sputtering, plating, or providing a sheet of titanium, etc.) and placing the aluminum, titanium, and tantalum layers in a furnace, as further described below. The titanium and tantalum are preferably pure and the aluminum is preferably a commercial grade aluminum alloy such as 6061, 5051 or 5052. The titanium and aluminum inter- diffuse and react chemically forming an aluminum-titanium alloy, and the titanium and tantalum or tantalum nitride inter-diffuse to form a solid solution, resulting in a strong sputtering target assembly that gradually transitions from tantalum to aluminum thereby reducing the probability of debonding which otherwise may occur between the tantalum/interlayer interface. In fact, the interdiffused and alloyed region of the inventive sputtering target have a higher tensile strength than do the individual layers of aluminum, titanium or tantalum. Further, because backing plates are considerably thicker than interlayers, the inventive sputtering assembly is less expensive than conventional assemblies, as an inexpensive material (e.g., aluminum) is employed for the backing plate and only a small amount of titanium (a more expensive material) is required for the interlayer .

Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1A is a diagrammatic side view of an inventive sputtering target assembly prior to diffusion bonding thereof;

FIG. IB is a diagrammatic side view of the inventive sputtering target assembly of FIG. 1A after diffusion bonding thereof; and

FIG. 2 is a diagrammatic side sectional view of an IMP sputter deposition chamber that employs the inventive sputtering target assembly of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A is a diagrammatic side view of an inventive tantalum sputtering target assembly 11 prior to diffusion bonding thereof. The inventive sputtering target assembly 11 comprises a tantalum sputtering plate 13, an aluminum backing plate 15, and a titanium interlayer 17. The titanium and tantalum are preferably pure and the aluminum is preferably a commercial grade aluminum alloy such as 6061, 5051 or 5052. The sputtering plate 13 is as thick as possible based on weight considerations and on bond strength (described below) , as the thicker the sputtering plate 13, the greater the number of deposition cycles before sputtering plate replacement. Similarly, the thickness of the aluminum backing plate 15 is chosen to provide sufficient rigidity based on the weight of the target plate 13. The initial interlayer thickness (i.e., the thickness of the interlayer prior to diffusion bonding) is preferably chosen to be as thin as possible yet provide sufficient titanium to achieve the desired diffusion and alloying depths with the tantalum target plate 13 and aluminum backing plate 15, respectively. A one quarter inch target thickness, and an initial titanium layer having a thickness between 10-20 microns is presently preferred to provide optimal performance with minimal cost. The titanium interlayer 17 may be formed by conventional methods known in the art, such as by sputter deposition, providing a sheet of titanium between the titanium and tantalum members, plating, etc. The desired thickness of the titanium interlayer may be deposited (placed, plated, etc.) entirely on the bonding surface of either the tantalum sputtering plate 13 or the aluminum backing plate 15, or may be distributed therebetween in any proportion. Preferably the bonding surfaces of both the tantalum sputtering plate 13 and the aluminum backing plate 15 are cleaned to remove oxides (etched, sputter-etched, etc.) immediately before diffusion bonding, and are maintained in an oxygen free environment, preferably with a hydrogen or argon gas purge to remove contaminants. Any known methods and apparatuses for cleaning, transferring and diffusion bonding may be employed to form the inventive sputtering target 11, exemplary methods and apparatuses are disclosed in detail in US Patent No. , Application

Serial No. 08/511,824 (AMAT No. 642), the entirety of which is incorporated herein by this reference. To diffusion bond the tantalum sputtering plate 13 to the aluminum backing plate 15, the inventive target assembly of FIG. 1 is placed within a furnace and the furnace is heated to a temperature sufficient to cause the desired interdiffusion and alloying depths (e.g. 450-550°C) . A uniform compressive load may be applied to the nonbonding surfaces 13b, 15b of the tantalum sputtering target 13 and the aluminum backing plate 15, respectively, to speed diffusion and alloying and to reduce the required temperatures. A uniform compressive load of 145 pounds per square inch in a 350°C furnace provides an approximately 10- 20 microns interdiffusion depth, and an approximately 10-20 microns alloying depth.

FIG. IB is a diagrammatic side view of the inventive tantalum sputtering target assembly 11 after diffusion bonding thereof. As represented in FIG. IB an inter-diffused region 19 extends on either side of the initial boundary (the initial boundary being represented by dashed line 17a) of the titanium interlayer 17, and has an overall thickness sufficient to provide a strong bond between the tantalum sputtering target 13 and the titanium interlayer 17. The inter-diffused region 19 preferably has a thickness of less than 20 microns, with a thickness of less than 10 microns being preferred in order to provide an optimal balance of high bond strength, short bonding time, and minimal titanium expenditure.

Similarly, the alloyed region 21 extends on either side of the initial boundary (the initial boundary being represented by dashed line 17b) of the titanium interlayer 17, and has an overall thickness sufficient to provide a strong bond between the aluminum backing plate 15 and the titanium interlayer 17. The alloyed region 21 preferably has a thickness of less than 20 microns, with a thickness of less than 10 microns being preferred in order to provide an optimal balance of high bond strength, short bonding time, and minimal titanium expenditure.

Accordingly, the inventive tantalum sputtering target 11 gradually transitions from tantalum or tantalum nitride to aluminum. Regarding the interdiffusion region 19, the molecular structure and the coefficient of thermal expansion transitions gradually from substantially the molecular structure and coefficient of thermal expansion of tantalum or tantalum nitride to substantially the molecular structure and coefficient of thermal expansion of titanium. Similarly within the alloyed region 21 the aluminum-titanium alloy has a molecular structure and coefficient of thermal expansion intermediate the molecular structures and coefficients of thermal expansion of aluminum and titanium. Further the transition between aluminum or titanium and the aluminum-titanium alloy will occur gradually.

The similar molecular structures promote adhesion and the similar coefficients of thermal expansion reduce shear strain during thermal cycling. Further, because the inventive sputtering target assembly interdiffuses and alloys so readily, and because titanium and tantalum have such high melting temperatures, the times and temperatures required to achieve sufficient inter-diffused and alloyed depths do not affect the grain size of the sputtering surface 15a of the tantalum sputtering plate 15. Moreover, during exposure to high temperature gradients and repeated thermal cycling (such as during the IMP process described below with reference to FIG. 2) the inventive sputtering target assembly exhibits a surprisingly long useful life, 500% longer than that of conventional diffusion bonded tantalum target assemblies.

FIG. 2 is a diagrammatic illustration, in section, of the pertinent portions of an IMP sputtering chamber 23 that employs the inventive sputtering target 11 of FIG. 1. The inventive sputtering target 11 further comprises a cooling cover plate 25, having a plurality of cooling channels 27 formed therein, and an inlet hose 29 and an outlet hose 31 operatively coupled to the cooling channels 27 for supplying cooling fluid thereto. The IMP sputtering chamber 23 contains a wire coil

33 which is operatively coupled to an RF power supply 35. As shown in FIG. 2 the wire coil 13 is positioned along the inner surface of the IMP sputtering chamber 23, between the sputtering target assembly 11 and a substrate support pedestal 39. The substrate support pedestal 39 is positioned in the lower portion of the IMP sputtering chamber 23 and the sputtering target assembly 11 is mounted in the upper portion of the IMP sputtering chamber 23. The IMP sputtering chamber 23 generally includes a vacuum chamber enclosure wall 41 having at least one gas inlet 43 and having an exhaust outlet 45 operatively coupled to an exhaust pump 47. The sputtering target assembly 11 is electrically isolated from the enclosure wall 41. The enclosure wall 41 is preferably grounded so that a negative voltage (with respect to grounded enclosure wall 41) may be applied to the sputtering target assembly 11 via a DC power supply 49 operatively coupled between the sputtering target assembly 11 and the enclosure wall 41. A controller 51 is operatively coupled to the RF power supply 35, the DC power supply 49, the gas inlet 43 and the exhaust outlet 45.

In operation, a throttle valve (not shown) operatively coupled to the exhaust outlet 45 is placed in a mid-position in order to maintain the deposition chamber at a desired low vacuum level. A mixture of argon and nitrogen gas is flowed into the IMP sputtering chamber 23 via the gas inlet 43. After the gas stabilizes (approximately 10 seconds), both a DC power signal is applied to the sputtering target assembly 11 via the DC power supply 49 and an RF power signal is applied to the wire coil 33 via the RF power supply 35, while the gas mixture continues to flow into the IMP sputtering chamber 23 via the gas inlet 43. The DC power applied to the sputtering target assembly 11 causes the argon/nitrogen gas mixture to form a plasma and to generate energized plasma particles which strike the sputtering target assembly 11 causing target atoms to be ejected therefrom. The RF power applied to the coil 33 causes the IMP sputtering chamber 23 and its components to heat rapidly and additionally causes the ejected target atoms to ionize.

Because of the high concentration of ionized gas particles caused by the RF coil 33, a greater number of ions strike the tantalum plate 13 at any given time, resulting in a high deposition rate and high temperatures on the surface of the tantalum plate 15. A high temperature gradient therefore occurs across the sputtering target assembly 11. Moreover, the sputtering target assembly 11 thermally cycles (e.g., heats and cools) with each wafer that is processed in the chamber 23. Accordingly the temperature gradient and thermal cycling, together with the sheer weight of the tantalum plate 13, significantly strains the sputtering target assembly 11. However, the inventive sputtering target assembly 11 is able to withstand this strain, allowing the target material to be fully expended without premature target assembly failure. Accordingly, the present invention provides a cost effective tantalum sputtering target assembly having a long useful life and capable of consistent deposition of high quality layers.

The foregoing description discloses only the preferred embodiments of the invention, modifications of the above disclosed apparatus and method which fall within the scope of the invention will be readily apparent to those of ordinary skill in the art. For instance, other materials that alloy readily with titanium may be employed within the backing plate, and the sputtering target may comprise other tantalum compounds. Accordingly, while the present invention has been disclosed in connection with the preferred embodiments thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention, as defined by the following claims.

Claims

THE INVENTION CLAIMED IS:

1. A sputtering target assembly comprising: a sputtering plate comprising tantalum; a backing plate comprising a first material; and an interlayer comprising titanium, positioned between the sputtering plate and the backing plate for bonding with both the sputtering plate and the backing plate .

2. The sputtering target of claim 1 wherein the first material is aluminum.

3. The sputtering target assembly of claim 2 wherein the titanium interlayer further comprises: an inter-diffused region adjacent the sputtering plate comprising a solid solution of tantalum and titanium; and an alloyed region adjacent the backing plate comprising an alloy comprising titanium and aluminum; wherein the inter-diffused region and the alloyed region each have a thickness sufficient to resist debonding of the sputtering target assembly during thermal stress .

4. The sputtering target assembly of claim 3 wherein the inter-diffused region has a thickness of less than 20 microns, and the alloyed region has a thickness of less than 20 microns.

5. The sputtering target of claim 5 wherein the aluminum backing plate is sufficiently thick to provide structural rigidity for the sputtering plate, and wherein the thicknesses of the inter-diffused region and the alloyed regiqn are sufficient to support the weight of the sputtering plate.

6. The sputtering target assembly of claim 1 wherein the inter-diffused region and the alloyed region each have a thickness sufficient to resist debonding of the sputtering target assembly during thermal stress.

7. A sputter deposition chamber comprising: an enclosure wall; a wafer support, contained within the enclosure wall; the sputtering target assembly of claim 1, operatively coupled to the enclosure wall further comprising: a cooling cover plate coupled to the backing plate and having cooling fluid channels formed therein; a DC power supply operatively coupled to the sputtering target assembly; a gas supply inlet operatively coupled to the enclosure wall; and a gas outlet operatively coupled to the enclosure wall.

8. The sputtering deposition chamber of claim 7 further comprising a coil positioned between the sputtering target assembly and the wafer support, and located adjacent the inner surfaces of the enclosure wall; and an RF power supply operatively coupled to the coil.

9. A method of forming a sputtering target assembly comprising: providing a sputtering target plate of a first material; providing a backing plate of a second material; putting an interlayer of titanium between the sputtering target plate and the backing plate; heating the sputtering target plate, the backing plate and the interlayer to form a bond therebetween .

10. The method of claim 9 wherein the first material is tantalum.

11. The method of claim 10 wherein the second material is aluminum.

12. The method of claim 9 wherein heating the sputtering target plate, the backing plate and the interlayer comprises: causing the first material and the titanium interlayer to inter-diffuse to a first thickness; and causing the second material and the titanium interlayer to form an alloy having a second thickness; wherein the first thickness and the second thickness are sufficient to form bonds that resist debonding when the sputtering target assembly is exposed to thermal stress .

13. The method of claim 11 wherein heating the sputtering target plate, the backing plate and the interlayer comprises: causing the first material and the titanium interlayer to inter-diffuse to a first thickness; and causing the second material and the titanium interlayer to form an alloy having a second thickness; wherein the first thickness and the second thickness are sufficient to form bonds that resist debonding when the sputtering target assembly is exposed to thermal stress .

14. The method of claim 12 wherein the first thickness is in the range of 20 microns or less.

15. The method of claim 12 wherein the second thickness is in the range of 20 microns or less.

16. The method of claim 14 wherein the second thickness is in the range of 20 microns or less.

17. The method of claim 13 wherein the first thickness is in the range of 10 microns or less, and the second thickness is in the range of 10 microns or less.

18. The method of claim 12 wherein heating the sputtering target plate, the backing plate and the interlayer does not effect the grain size of the first material .

19. A sputtering target assembly comprising: a sputtering plate comprising tantalum; a backing plate comprising a first material; and an interlayer comprising titanium, positioned between the sputtering plate and the backing plate for bonding with both the sputtering plate and the backing plate, and further comprising: an inter-diffused region adjacent the sputtering plate comprising a solid solution comprising tantalum and titanium; and an alloyed region adjacent the backing plate comprising an alloy comprising titanium and the first material; wherein the inter-diffused region and the alloyed region each have a thickness sufficient to resist debonding of the sputtering target assembly during thermal stress.