WO1992017622A1

WO1992017622A1 - Thermally compatible sputter target and backing plate assembly

Info

Publication number: WO1992017622A1
Application number: PCT/US1992/002679
Authority: WO
Inventors: William A. Manring; Sohail S. Qamar
Original assignee: Tosoh Smd, Inc.
Priority date: 1991-04-08
Filing date: 1992-04-03
Publication date: 1992-10-15

Abstract

A sputter target (4), backing plate (12), and backing plate-target assembly (12) are disclosed wherein the target (4) and backing plate (12) are closely thermally matched so as to minimize thermal stress at the backing plate-target interface. In a preferred embodiment, a laminated backing plate is provided, wherein an upper portion of the backing plate (10), adapted for close contact with the sputter target (4) is composed of a molybdenum layer (8).

Description

THER ALLY COMPATIBLE SPUTTER TARGET AND BACKING PLATE ASSEMBLY

Field of the Invention

The present invention pertains to a sputter target and backing plate wherein the target and backing plate are thermally compatible with each other so as to minimize thermal stress occurring along the target/backing plate interface.

Background of the Invention Sputtering as a means to deposit thin films of a desired material on a substrate has become important in the manufacture of semiconductor devices such as integrated circuits. In a sputtering system, material to be deposited on a substrate is removed from a sputter target by bombardment of the target with excited ions in the plasma state. Atoms or molecules of the target material are ejected from the target and are deposited on a workpiece or substrate. The ejection of these materials from the target is accompanied by significant heat build- up in the target area.

Normally, a sputtering system comprises a sputter source, a vacuum chamber, and means for positioning and holding the substrate in proximity to the sputter source. 'The sputter source normally comprises a target from which material is to be sputtered, means for holding the target in place, means for creating a plasma in the vicinity of the target, means for causing ions in the plasma to bombard the target, and means for cooling the target to prevent overheating.

Various means have been used in the past for holding sputter targets in place within the sputter sources. Such holding means must insure that the target maintains good thermal contact with the cooling means so that heat generated in the target may be dissipated away.

In some sputter sources, annular targets are removably mounted within a fixed backing plate and are retained in place by various hold down features, such as, clamps, springs, inserts, screws, etc. In these sputter source designs, the sputtering targets are independently removable from the fixed backing plates, for replacement of the targets due to target consumption, breakage etc. oftentimes the target is soldered to the backing plate, with the necessary requirement that the target be removed by melting the solder.

In order to cool the targets, cooling water is typically circulated through the backing plate positioned adjacent to the target. In some cases, the thermal contact between an outer rim of the target and the peripheral cooling wall of the backing plate is critical, and is maintained by thermal expansion of the target against the cooling wall. Examples of such arrangements are shown in U.S. patent No. 4,100,055; 4,385,979; 4,457,825 and 4,657,654.

In other systems, when the target is replaced, both the spent target and the corresponding backing plate are discarded. The backing plate in such designs typically includes an upper annularly shaped planar surface adapted for flush receipt against the lower surface of the target. The backing plate further includes an annular ring extending downwardly from the planar surface having an outer mounting flange at the lower end of the annular ring. The target plate and backing plate may be joined via soldering, welding, brazing or other metal bonding techniques. U.S. Patent 4,544,091 (Hidler et al ) discloses a target bonding process to secure target parts, such as a yttrium oxide target part, to a copper backing plate. A noble metal, such as platinum, is applied to the target to provide an oxide-free layer to which a solder layer, such as an indium/lead solder, is joined. A somewhat similar approach is discussed in U.S. Patent 4,341,816 (Lauterbach et al ) in which the target is attached to an associated cooling plate by plasma spraying the target surface with a thin compatible adhesive layer of from about 30 to 100 urn of, for example, a Ni , Ni/Cr,

Ni/Al, Ni/Al/Mo, Al/bronze, Mo, W, Al/Si, Zn, Cu, Cu/glass mixture, etc., and then coating the adhesive layer with a solderable layer and subsequently soldering the solderable layer onto the desired surface of the cooling plate. In Japanese Patent 0188680, a metal is melt- sprayed on one surface of a target base body formed of a difficult or impossible to solder metal, oxide, boride, carbide, nitride, silicide, or the like to form a solderable metallic film having a thickness of 20-300 urn. This metallic film surface and the backing plate are then joined by soldering.

Where the target is joined to the backing plate by a retaining member, such as a screw, or the like, U.S. Patent 4,448,652 teaches use of an intermediate layer of about 50 urn thickness, between the target and backing plate, comprising a heat conducting paste, metal powder, or metal foil to facilitate heat conduction from the target to the backing plate. Suitable metal powders comprise Cu, Al , bronze, or brass.

Another patent that is possibly pertinent is Japanese Patent 0093871 entitled, "Target for Magnetron Sputtering". Here , the target plate is soldered to cooling plate. Certain portions of the cooling plate are covered with an inhibitor layer, such as a polyimide tape. The cooling plate and target are immersed in Pb-Sn or Pb- Sn-Ag solder to solder-connect the assembly. The inhibitor layer is then removed. Due to the differing thermal expansion capacities of the target and backing plate members, the high heat levels attained by the target during high power (2-12 KW) sputtering result in excessive stress exerted along the target/backing plate interface. Warpage and differential shrinkage of both target and backing plate member may occur despite the cooling function, resulting in severely impeded cooling of the target with attendant target failure. Accordingly, 1t is an object of the invention to provide for close matching of the thermal expansion characteristics of both target and backing plate member so as to minimize thermal stress between these parts during high power sputtering operations. It is a further object to provide a structural arrangement for target and backing plate that achieves this purpose.

Summary of the Invention In accordance with the preferred aspect of the invention, thermal matching of target and backing plate members is accomplished by providing a target and a laminated backing plate assembly. The target and top layer of the laminated backing plate, adapted for contiguous mating with the target, have a difference in their thermal expansion coefficients of about 5 or less. By provision of such closely matching target and backing plate, thermal stress at the target/backing plate interface is minimized during sputtering operation.

A laminated backing plate assembly is provided with the top layer of the backing plate defining an intermediate layer between the target and backing plate. This intermediate layer is composed of a material that is thermally matched with the target material that is superposed thereover. In the preferred embodiment, the top layer of the backing plate is composed of molybdenum with the rest of the backing plate consisting of copper. This backing plate is ideally suited for use with Si targets although it can be used with all targets having thermal expansion coefficients less than 10 x 10^"β/cm/cm/'C.

In another aspect of the invention, a target and matching monolithic backing plate are provided wherein, similar to the preferred embodiment, the difference in the thermal expansion coefficients between the target and backing plate is 5 or less. In this embodiment, the target is chosen from Si, W-Ti , refractory metal silicides, such as Mo/Si, Ta/Si, V/Si , Nb/Si, Cr/Si, W/Si , and Ti/Si refractory metals. The backing plate in this embodiment is chosen from Mo, Zr, and Ta and alloys thereof.

The invention will be further described in conjunction with the appended drawing and detailed description.

Drawings In the drawings:

Fig. 1 is a schematic illustration showing the preferred embodiment of the invention in which an intermediate layer between the target and backing plate is provided; and

Fig. 2 is a schematic illustration of another embodiment of the invention in which a thermally matched target and a monolithic backing plate are provided. Detailed Description Turning now to Fig. 1, there is shown a target and backing plate assembly 2 in which target 4 comprising material to be sputter coated in accordance with conventional cathodic sputtering techniques is superposed over backing plate member 12. As is conventional in the art, a cooling medium, such as water 16 is circulated through heat exchange conduit 14 in heat exchange relation with backing plate member 12 to dissipate heat from target 4 that attains temperatures on the order of 70^*C to 360^*C during the sputtering process.

As shown, intermediate layer 8 is provided as a top laminate layer of backing plate 12 and is bonded to the lower layer 10 of the backing plate as hereinafter described. The intermediate layer 8 has a thickness of from about 1/8" to 3/16".

In accordance with the invention, the thermal expansion coefficients of the target 4 and intermediate layer 8 are matched so that target 4 and intermediate layer 8 are thermally compatible. That is, the target and intermediate layer materials are chosen so that the difference between the coefficients of thermal expansion for these members is 5 or less. In this regard, the thermal expansion coefficients of materials that may be used as target or intermediate layer are given in terms of Z«10"^β cm/cm/"C wherein Z is as defined.

Mo

Mo/Si Si

Ti Al Cu

Ta/Si W/Ti

Zr Ta

In accordance with Fig. 1, target 4 is preferably composed of silicon with the intermediate layer 8 comprising Mo or Mo alloy. The lower layer 10 of backing plate may be Cu.

Intermediate layer 8 is preferably bonded to lower layer 10 via explosion bonding techniques in which a sheet of explosive material progressively detonates above the layers to be joined. Upon detonation, a pressure wave moves along the layer interface with a small open angle being maintained between the two layers. As the pressure wave propagates, surface films liquify and spread out along the open interface. Clean metal surfaces are then forced together at high pressures, forming a solid-state bond with a wavy configuration along the interface. More on explosion bonding is reported in Mechanical Engineer's Reference Book, 11th Edition, A. Parrish, editor Butterworth's, 1973, pages 9-4 to 9-9. Other acceptable techniques for bonding the lower layer 10 to the intermediate layer 8 include silver brazing, solder bonding, friction and Inertial welding, diffusion bonding and roll bonding.

After the laminated backing plate has been made, it is soldered, via use of solder layer 6 to the target material 4. Although shown here as constituting a top layer over backing plate layer 10, intermediate layer 8 could be bonded directly to the underside of target 4, with appropriate bonding means then being used to bond the target-intermediate layer structure to the backing plate. In those instances in which the target or the backing plate are composed of a difficult to solder material, such as Si, Mo, etc., an adhesive material such as is conventionally known in the art may be coated onto the surface prior to soldering. Preferably, the adhesive material may comprise Ni , Ni/Cr, Ni/Al, Ni/Al/Mo,

Al/bronze, etc. Then, soldering by use of conventional lead/tin solders may be used to form the joint or bonding layer 6 along the target 4 - intermediate layer 8 interface. In accordance with the invention, the following materials are preferred for use in the backing plate/target construction shown in figure 1:

Intermediate layer 8 (Fig. 1) Target 4

Mo or Si

Mo alloy (available under TZM trademark - W/Ti Climax Specialty Metals - Cleveland, Ohio) Ta/Si W/Si

Mo/Si C B

B/N Ti/N

Zr or Zr alloy Si

W/Ti Ta/Si W/Si

Mo/Si C B B/N Ti/N

Ta or Ta alloy Si

W/Ti Ta/Si W/Si

Mo/Si C B B/N Ti/N

Most Preferred

Top layer 8 (Fig. 1) Target 4

Mo or Mo alloy Si

W/T1 Ta/S

Mo/Si

Accordingly, in the most preferred embodiment, and with respect to Fig. 1, a lower backing plate layer of Cu 10 is provided with a Mo or Mo alloy explosion bonded thereover as intermediate layer 8. The target 4 comprises silicon, refractory metal silicide, such as Ta/Si, or W/Si, or a tungsten-titanium target material. The underface of target 4 may be coated with nickel or a Cr/Ni alloy as is conventional in the art so as to enhance adhesion of the metal bonding layer 6, typically a solder layer.

The following example is illustrative of the invention and should not be construed as restricting the scope of the invention herein claimed. Example

In order to assess the efficacy of the invention in reducing stress levels in target-backing plate assemblies, a computer simulation using finite element analysis was undertaken with a test Si target. Results are shown in Table I.

TMax = Target Temperature during sputtering, 'C SIGE(Si) = equivalent stress dynes/cm² on Si target SX(Si) - tensile stress in the Si target dynes/cm²

In accordance with the above, it can been seen that the Cu/Mo backing plate combination with Mo provided as intermediate layer 8 provides significant improvement in reducing the stress components on the target in the test system when compared to the conventional all copper backing plate. In fact, in this simulation, stress is less than one-half of that shown in the all copper

(control) simulation. Enhanced results are also shown in conjunction with an all molybdenum backing plate as shown, wherein a 5/16" all molybdenum backing plate is used in combination with the Si target. Turning now to Fig. 2, there is shown a variant of the invention wherein the backing plate 12 is of monolithic structure. Backing plate 12 and target 4 are

Corresponds to 8 in Fig. 1 joined via conventional bonding layer 6 as aforementioned. Target 4 and backing plate 12 are thermally compatible with each other. That is, the difference in thermal expansion coefficients of the target and backing plate is 5 or less. One example of a structure in accordance with Fig. 2 is given above in the Table I example wherein an all molybdenum backing plate is used in combination with a Si target.

Other preferred combinations for target and backing plate in accordance with the Fig. 2 embodiment include: Target 4 Backing Plate 12 member chosen from member chosen from

Si, refractory metal Mo, Zr, Ta and alloys thereof silicides (i.e., Mo/Si, Ta/Si , V/Si , Nb/Si , Cr/Si , W/Si , or Ti/Si ) or a refractory metal

While we have shown and described herein certain embodiments of the present invention, it is intended that there be covered as well changes or modifications therein which may be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

Claims:

1. Assembly for use in a sputter coating system of the type for coating a substrate with a coating material comprising, in combination, a backing plate member and a target comprising said coating material, said target superposed on said backing plate, said target and said backing plates having coefficients of thermal expansion, such that the difference between said coefficients is 5 or less, wherein said backing plate comprises a member selected from molybdenum or alloy therewith, zirconium or alloy thereof, and tantalum and alloy thereof.

2. Assembly as recited in claim 1 wherein said target comprises a member selected from Si, tungsten- titanium alloys, refractory metal silicides and refractory metals.

3. Assembly as recited in claim 1 wherein said target is composed of Si.

4. Assembly as recited in claim 1 wherein said target is composed of tungsten-titanium alloy.

5. Assembly for use in a sputter coating system of the type for coating a substrate with coating material comprising, in combination, a sputter target including material to be coated onto said substrate, a cooling medium for dissipating heat from said target, a backing plate member for mounting of said target and disposed in heat exchange relationship with said cooling medium, an intermediate member positioned between said target and said backing plate, said target and said intermediate member having coefficients of thermal expansion, such that the difference between said coefficients is 5 or less.

6. Assembly as recited in claim 5 wherein said intermediate member comprises Mo or alloy thereof.

7. Assembly as recited in claim 6 wherein said target comprises a member selected from Si, tungsten- titanium, refractory metal silicides and refractory metals..

8. Assembly as recited in claim 7 wherein said backing plate comprises Cu.

9. Assembly as recited in claim 6 wherein said backing plate comprises Zr or alloy thereof.

10. Assembly as recited in claim 6 wherein said backing plate comprises Ta or alloy thereof.

11. Assembly as recited in claim 5, comprising a laminated backing plate, having a top layer and a second layer underlying said top layer, said intermediate member defining said top layer of said laminated backing plate, said top layer bonded to said second layer.

12. In a sputter system of the type for coating a substrate with a coating material ejected from a sputter target and a backing plate member chosen from zirconium, molybdenum, and tantalum and alloys thereof, disposed adjacent said sputter target, a sputter target having a coefficient of thermal expansion such that the difference between it and the coefficient of thermal expansion of said backing plate is 5 or less, said sputter target comprising a member selected from the group consisting of Si, W/Ti, refractory metal silicides and refractory metals.

13. In a sputter system of the type for coating a substrate with a coating material ejected from a sputter target and a laminated backing plate for mounting of said target and comprising a top layer adjacent said target and a bottom layer underlying said top layer, a sputter target having a coefficient of thermal expansion such that the difference between it and the coefficient of thermal expansion of the top layer is 5 or less.

14. A sputter target as recited in claim 13 comprising a member selected from the group consisting of Si, W/Ti, refractory metal silicides and refractory metals.

15. A sputter target as recited in claim 14 composed of Si .

16. A backing plate for use in sputter coating systems of the type adapted to coat material onto a substrate from a sputter target superposed over said backing plate, said backing plate comprising an upper and lower layer, said upper layer adapted for positioning adjacent said sputter target, said upper layer having a coefficient of thermal expansion such that the difference between it and the coefficient of thermal expansion of the target is 5 or less.

17. A backing plate as recited in claim 16 wherein said upper layer comprises Mo or Mo containing alloy.

18. A backing plate as recited in claim 17 wherein said lower layer comprises copper.