Low deflection sputtering target assembly and methods of making same

Abstract

Described is a design and method for producing a sputtering target assembly with low deflection made from target material solder bonded to composite backing plate with coefficient of thermal expansion (CTE) matching the target material. The composite backing plate is composite configuration composed of at least two different materials with different CTE. The composite backing plate, after plastic deformation, if necessary, has a CTE matching the target material and low and desirable deflection in the bonding process, and therefore, resulting in a low deflection and low stress target material bonded to composite backing plate assembly. The method includes manufacturing composite backing plate with a flat bond surface, heat treating of target blank and composite backing plate to achieve desirable shape of bond surfaces, solder bonding target to a backing plate, and slowly cooling the assembly to room temperature. Matching CTE in both target material and backing plate eliminates the problem of CTE mismatch and prevents the assembly from deflection and internal stress.

Claims

1. A method of producing a sputter target assembly, said method comprising the steps of: a. providing a backing plate composed of a laminated assembly comprising a first layer having a first coefficient of thermal expansion (CTE) and a second layer having a second CTE; b. providing a sputtering target, said target composed of a third material having a CTE wherein the CTE of said second layer is about 2-5 times greater than the CTE of said first layer and the difference between the CTE of said third material and said first layer is about 7 or less; c. heating said first and second layers thereby forming a concavity along a top surface of said first layer; d. providing a liquid bonding medium in said concavity and bonding said target to said first layer along said concavity to provide said sputtering target assembly.

2. The method as recited in claim 1 wherein said first layer has a thickness TA and said second layer has a thickness TB; wherein TA:TB is from about 1:1 to about 100:1.

3. The method as recited in claim 2 wherein said target consists essentially of Si, said first layer consists essentially of Ti and said second layer consists essentially of Al or Al alloys.

4. The method as recited in claim 2 wherein said target consists essentially of W or WSi.sub.2, said first layer consists essentially of Ti and said second layer consists essentially of Al or Al alloys.

5. The method as recited in claim 2 wherein said target consists essentially of Si, said first layer consists essentially of Mo and said second layer consists essentially of Cu.

6. A method as recited in claim 1 wherein said liquid bonding medium is solder and said step d comprises solder bonding said target to said first layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic cross-sectional illustration of one embodiment of a target and laminated backing plate assembly in accordance with the invention;

(2) FIG. 2 is a schematic illustration of another embodiment in which a concavity or reservoir 10 is formed between the underside surface of the target and the top surface of the intermediate layer of the assembly during one stage of manufacture of the assembly;

(3) FIG. 3 is a schematic cross-sectional illustration of one embodiment of a target and laminated backing plate assembly in accordance with the invention;

(4) FIG. 4 is a process flow diagram contrasting a prior art target/backing plate manufacturing process and a target/backing plate manufacturing process in accordance with one aspect of the invention; and

(5) FIG. 5 is a fragmentary cross sectional illustration of one embodiment of the laminated or composite backing plate assembly and showing the thicknesses of the respective materials “A” and “B” that together define the composite backing plate assembly.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

(6) Turning first to FIG. 1 and FIG. 3 of the drawings, target/backing plate assemblies are shown in accordance with one embodiment of the invention. The assembly comprises a target material 2 having a surface composed of the desired sputtering material. A laminated or composite backing plate assembly is shown and comprises a first material 4, defining a first or top layer of the backing plate assembly that is superposed over a second backing plate material, material 6. In FIG. 1, a cooling chamber 8 is provided in heat exchange relation with the bottom surface of the second backing plate material 6 so as to aid in cooling of the assembly during the sputtering operation. As shown, the materials 4 and 6 can be bonded along their interface by conventional means such as solder bonding, diffusion bonding, brazing, coating, electroplating, etc. The target 2 may be bonded to the top side of first material 4 of the backing plate assembly by conventional means such as solder bonding, diffusion bonding, brazing, friction stir welding (FSW), etc.

(7) In accordance with one aspect of the invention, the CTEs of the target and the first material 4 are chosen so that they are compatible. Stated differently, the difference between the CTEs of the target 2 and first backing plate material 4 should be on the order of about 22 or less. In another embodiment, the difference between the CTEs may be 7 or less. Further, the CTE of the second material 6 of the laminated or composite backing plate assembly should be higher than the CTE of the material 4. Thus, upon heat treatment of the materials 4 and 6 such as in a bonding process or the like, a desired concavity can be formed along the top surface of the member 4. This concavity can serve as a reservoir for solder or the like, which is then used to bond the composite backing plate assembly to the target 2.

(8) With regard to FIG. 2, the concavity 10 is shown positioned along the top surface of first backing plate material 4. As shown in the drawing, the size of the concavity 10 is exaggerated for simplification purposes. Also, as shown in FIG. 2, liquid, such as heated solder or the like, is provided in the cavity as a bonding medium to bond the target 2 to the top surface of the first backing plate layer 4.

(9) FIG. 4 contrasts a conventional design 100 target/backing plate bonding method with a method in accordance with the invention shown as 200. In the conventional methods, a flat target blank and flat single backing plate material are provided as shown at step 110. Then, the flat target blank and flat backing plate are heated as shown at 120 and bonded together as shown at 130 via conventional means forming a flat bonded target/backing plate assembly. Upon cooling, due to the large CTE difference between the target and the backing plate, the entire assembly bows or becomes deformed as shown at 140. After the bonding process is concluded, the result is a bowed bonded assembly as shown at 150.

(10) In contrast, in accordance with one exemplary embodiment of the invention as shown at 210, a flat target blank is provided along with a flat laminated or composite backing plate of the type shown in FIG. 1 and FIG. 3. Then, the flat target blank and backing plate assemblies are heat treated as shown at 220. Due to the fact that the second material 6, as shown in FIG. 1, has a higher CTE than that provided in the first layer material, number 4, (FIG. 1), the backing plate assembly is warped into a controllable bowed position, and at the bonding temperature, this desirable concavity defines reservoir 10 shown in FIG. 2. This reservoir exists as a concavity along the top surface of first backing plate layer 4. Upon cooling as shown at 240 in FIG. 4, the entire assembly becomes flat, and after the bonding process, a flat bonded assembly 250 being provided.

(11) It is accordingly apparent then, that the choice of backing plate components 4 and 6 are made based on the target material properties. The target material CTE should be close to the CTE of the first backing plate layer 4, and as aforementioned, in certain exemplary embodiments, this difference should be on the order of about 22 or less. In another embodiment, the difference between the CTEs may be 7 or less. Further, the CTE of second backing plate layer 6 is greater than the CTE of the first backing plate layer 4. This helps to naturally form a concave surface which also may serve as a reservoir for solder bonding of the target to the backing plate assembly such as shown in FIG. 2.

(12) As illustrated in FIG. 5, different thickness ratios TA/TB may be chosen for the components 4, 6, respectively of the backing plate assembly so as to adjust the depth of the concave surface or reservoir 10.

(13) The layers 4, 6 of the backing plate assembly can be bonded together via diffusion bonding, brazing, soldering, friction stir welding, coating, electroplating, and other conventional methods. The target manufacturing method includes manufacturing the composite backing plate with a flat bond surface, heating of the target surface and composite plate to achieve a desirable shape of the bond surfaces, and in certain embodiments, a solder bonding of the target to the backing plate followed by slowly cooling the assembly to room temperature.

(14) In one exemplary embodiment, the target material 2 is Si, and the composite backing plate comprises top or first layer 4 of Ti and a second or bottom layer 6 of Al6061. The backing plate materials are diffusion bonded together with the Si target solder bonded to the composite backing plate.

(15) Another exemplary embodiment that can be mentioned is a Si target with a composite backing plate wherein the first layer 4 is Mo, and the second layer 6 is Cu. The Cu layer may have a thickness of greater than or equal to zero millimeters. When the thickness of the second layer is zero, it indicates that only the first layer is present. In other words, when the Cu layer thickness is zero, the backing plate has only a first layer of Mo.

(16) Another exemplary embodiment is a combination of a W target with a composite backing plate wherein the first backing plate layer 4 is Ti, and the second backing plate layer 6 is Al or Al alloys.

(17) Another exemplary embodiment is a combination of a WSi.sub.2 target with a composite backing plate wherein the first backing plate layer 4 is Ti and the second backing plate layer 6 is Al or Al alloys.

(18) The CTEs of exemplary target or backing plate layer materials are given in terms of Z×10.sup.−6 cm/cm/° C. at 20° C. wherein Z is the thermal expansion coefficient as listed below:

(19) TABLE-US-00002 Z (Thermal Expansion Coefficient) Mo 5.0 MoSi.sub.2 8.25 Si 2.49 Ti 8.4 Al 24.0 Ag 19.6 Co 12.5 Cu 16.4 TaSi.sub.2 8.8 TiW 4.0 Zr 5.8 Ta 6.5 W 4.4 WSi.sub.2 6.5 Nb 7.1 Ni 13.1 V 8.33

(20) In other exemplary embodiments, the target can be silicon, ceramic materials (including and not limited to oxides, nitrides, carbides, etc.), silicon-germanium, silicide (e.g., WSi.sub.2), and alloys thereof. The composite backing plate (members 4/6) may be composed of Ti/Al, Ti/Cu, Mo/Cu, Mo/Al, Ta/Al, Ta/Cu, etc. The materials 4/6 of the composite backing plate may be joined via diffusion bonding, soldering, brazing, friction stir welding, coating, electroplating, and other methods.

(21) It is apparent then that the present invention is directed toward a method of producing a sputter target assembly. In one embodiment, the method comprises the steps of providing a backing plate composed of a first layer having a first CTE and a second layer having a second CTE. A sputtering target is provided with a surface layer comprising material to be sputtered onto the desired substrate and an underlayer adapted to mate with the top surface of the first backing plate layer 4. The target is composed of a third material having a third CTE. The backing plate and target are solder bonded along the interfacial surfaces, i.e., bottom of the target to top surface of layer 4. In some embodiments, the first layer 4 of the backing plate as shown in the drawings, may be selected from the group consisting of Ti, Mo, Nb, Ta, Zr, and alloys thereof.

(22) In other exemplary embodiments, the second material, material 6 shown in the drawings may be selected from the group consisting of Al, Cu, Ti, Ni, V, and alloys thereof. In other aspects of the invention, the target material may be selected from the group consisting of Si, Si—Ge, silicide, W, Ti—W, Ti—Al, Ta, Co, Ni, Cu, rare earth metals, and alloys thereof, and ceramic materials including, but not limited to oxide, nitride, carbide, and carbon nitrides.

(23) In accordance with certain embodiments of the invention, the CTE of the first material 4 of the backing plate assembly is lower than the CTE of the second material 6 of the backing plate assembly. In certain embodiments, the CTE for layer 6 is about 2-5 times the CTE of layer 4. Stated differently, the ratio CTE layer 6/CTE layer 4 is about 2-5:1. The first and second materials 4 and 6 of the backing plate assembly can be joined together via diffusion bonding, brazing, soldering, friction stir welding, coating, electroplating, and other methods. Additionally, thermal plastic deformation can be applied to the backing plate assembly to achieve a flat first mating surface for the joint assembly of the materials 4, 6.

(24) In other aspects of the invention, a sputter target assembly is provided that comprises a backing plate with a first mating surface. The backing plate is composed of a first material having a first CTE and a second material having a second CTE. A sputter target with a second mating surface is provided, and the sputter target is composed of a third material having a third CTE. The target material is solder bonded to the backing plate along the first and second mating surfaces. The first material, 4, as shown in the drawings may be selected from the group consisting of Ti, Mo, Nb, Ta, Zr, and alloys thereof. Further, the second material, material 6, as shown in the drawings is selected from the group consisting of Al, Cu, Ti, Ni, V, and alloys thereof. The target may be composed of silicon, silicon-germanium, silicide, tungsten, titanium-tungsten, titanium-aluminum, tantalum, cobalt, nickel, copper, rare earth metals, and alloys thereof, and ceramic materials including and not limited to oxide, nitride, carbide, and carbonitride. In further embodiments, the target material may be intrinsic, p-type silicon or n-type silicon, and in other embodiments, the target material may be a single crystal silicon or polycrystalline silicon.

(25) It is apparent then that the present invention is capable of creating a composite backing plate with a low deflection to match the target materials, especially for low CTE brittle target materials. The provision of a component, such as a plural layer backing plate assembly reduces costs and provides for the provision of an inexpensive backing plate.

(26) In accordance with another aspect of the invention, the thickness ratio of the backing plate components TA/TB (see FIG. 5) is adjusted to adjust the depth of the reservoir 10 such as that shown in FIG. 2. In one example in which Ti is provided as the first material 4, with Al6061 utilized as the second material 6 for the backing plate assembly, a TA/TB ratio of 1:1 was provided. This Ti/Al6061 laminated backing plate was diffusion bonded together and a downward displacement of about 0.1 inch was observed at the center of the target. In another exemplary embodiment, Ti was again used as the first material 4 with Al6061 utilized as the second material 6. However, the TA/TB ratio was chosen to be on the order of about 5:1. In this case, the maximum downward displacement or bowing at the center of the assembly was found to be 0.04 inches. Exemplary TA/TB thickness ratios are on the order of about 1:1 to about 100:1.

(27) While we have shown and described herein certain embodiments of the present invention, it is intended that these 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.