WO2001034339A1

WO2001034339A1 - Hermetic sealing of target/backing plate assemblies using electron beam melted indium or tin

Info

Publication number: WO2001034339A1
Application number: PCT/US2000/031037
Authority: WO
Inventors: David B. Smathers; Dorian Heimanson; Michael Gutkin
Original assignee: Tosoh Smd, Inc.
Priority date: 1999-11-09
Filing date: 2000-11-09
Publication date: 2001-05-17

Abstract

A method of sealing the edge of a solder bonded target assembly by heating the edge of the solder bond line to locally melt the solder. The target assembly is rotated to move the molten bead continuously around the assembly a full 360 degrees. The local melting of the solder may be acomplished through the use of an electron beam welder in a vacuum.

Description

Concise Technical Description of Invention

Descriptive Title: Hermetic Sealing of Target/Backing Plate Assemblies Using Electron Beam Melted Indium or Tin

A. General Purpose of the Invention

CVC approached TSMD with the customer driven need to reduce the impact of virtual leaks from solder bonded assemblies on vacuum quality in their sputtering system. The purpose is to cut the bond line off from the vacuum by a coherent, fully dense metal seal.

B. Prior Art (previous) Methods, Materials, or Devices Performing Function on this Invention

Solder bonding is generally done using either a tin eutectic alloy (such as tin 3.5 weight percent silver) or pure indium. The bonding process is performed in air or under a protective atmosphere and a solder flux may be used to break oxide barriers on the metal being bonded. Bonding in air or under a protective atmosphere can lead to trapped pockets containing gas. It is possible for porosity within the bond layer to allow the trapped gas to escape over at a low leak rate from the bond line between the target and the backing plate.

The most common approach to eliminate virtual leakage from a solder bond is to eliminate the solder bond entirely. This can be done several ways. The most simple is to make the target/backing plate a monolithic assembly, i.e. one piece. The second method is to diffusion bond the target to the backing plate. Diffusion bonding is performed under high pressure and temperature and a vacuum exists inside the diffusion bonding assembly prior to the application of heat and pressure. A third method involves electron beam welding the target to the backing plate. Electron beam welding works if the joint between the target and backing plate is ductile. It is not possible to directly weld aluminum to copper.

C. Disadvantages of Prior Art

It is not practical to solder bond in a vacuum. Thus, trapped pockets of gas are inevitable in a solder bond.

A monolithic assembly is possible when the value of the material is not too high and the target material is sufficiently strong and ductile to bear the structural demands of the backing plate. Deflection can be a problem if the target alloy is not very strong. Some materials are also brittle. Some backing plate designs involve internal cooling channels and this is the case with the current CVC design. A monolithic assembly would involve redesigning the target/backing plate to a two piece setup with channels cut in the back of the monolithic piece (similar to the Ulvac ZX-1000). Diffusion bonding is done at temperatures between 300 and 600 degrees C. This can be detrimental to the microstructure of some materials and also will leave residual stress in the assembly if the thermal expansion coefficients of the target and backing plate are different. Brittle target materials are difficult to diffusion bond because of the bending moment and stress caused by the differential thermal expansion and the application of pressure at high temperature. Not all materials can be diffusion bonded to copper, aluminum or molybdenum, the most common backing plate materials. The same issue regarding internal cooling channels exists for diffusion bonded targets.

Electron beam welding the target to the backing plate is possible only if the backing plate and target materials will form a ductile alloy.

D. Identification of Component Parts, or Steps, and Explanation of Mode of Operation of Invention

The invention is to use indium or pure tin to hermetically seal the target/backing plate interface.

Method:

1) Solder Bond target to backing plate using conventional bonding practice

2) Clean excess solder from the backing plate/target assembly

3) Lay a shim of the sealing material (In or Sn) at the target/backing plate interface

4) Use an Electron Beam welder (low power in vacuum) and run a molten bead around the target/backing plate interface until at least one complete revolution is made. The target will be on a rotating platter of some kind.

5) A laser may be substituted for the electron beam. ' -^{• •}' .-! . ; ^{• • ■}:-■

! , ■ • i- ; • i Biggest concern would be lack of vacuum to avoid trapped gas in the weld region.

The Indium metal (or Tin) will wet the surface of the backing plate and the target and the molten pool will solidify 100% dense because of the speed and vacuum. The welded bead will seal off the bond line and permanently close off the trapped gas areas between the target and backing plate.

Indium may be preferable to Tin because of its tendency to wet most materials. Indium wetting may be enhanced by rubbing pure indium against the area to be coated.

This method is different from electron beam welding the target to the backing plate because the target material and the backing plate material do not alloy together. There may be a very small region at the Indium (or Tin) metal interfaces form inter- metallics which are no different than if the indium or tin had been soldered to those surfaces in the traditional manner. In addition, the target may be removed by simply heating the assembly above the melting point of the solder and the backing plate can be re-used.

Electron beam welding is suggested here because a vacuum is required to operate the welder. Making the hermetic seal in a vacuum environment will reduce the chances of porosity impacting the quality of the seal. Trapped gas may cause an eruption which could compromise the seal.

TIG welding may also work to make this seal. There is a higher probability of trapped gas causing bubbles in the seal with TIG welding.

E. Alternate Embodiments of the Invention

1 ) Use of the same technology on other backing plate assemblies besides the CVC design.

2) TIG welding of the seal 3)

F. Advantages of the Invention Over the Prior Art

1) More materials can be bonded together with no virtual leak.

2) Grain structure of the target material can be preserved

3) The basic backing plate construction does not have to be modified to achieve the desired result. Backing plates with internal cooling channels can be used.

G. Features of the Invention Believed to be New

1) Modifying the backing plate/target dimensions to promote a hermetic seal with an electron beam welder.

2) Ignoring the content of the majority of the bond area and focusing only on the edge.

3) Use of an electron beam welder to run a solder bead in vacuum 4)

0.100"

Design option for trial. Make an internal resevoir by machining a trough near the outer edge of the target. After bonding, the e-beam will be used to heat the outer edge of the target zone until the solder starts to melt. The trough will allow solder to flow from both sides to fill the zone. While this may cause some undercutting at the very edge of the target, there will be no opening into trapped pockets under the target.

The trough is a groove cut with an outer diameter of 11.80" and a 5 degree taper to a depth of 0.020". The tolerance should not be too tight on the taper or the depth as it will be almost impossible to measure on repeat uses. Between uses, the trough can be re-cut prior to the next bonding attempt.

TSMD has an old CVC style backing plate which we can do a soldering experiment on. TSMD can not check the vacuum quality of the result.

The trial should be done with a target which will be 100% dense and not cause any distortion due to the bonding process. A 99.99% purity copper target fits this requirement.

It may be worth considering that we coat the copper backing plate with nickel or nickel- vanadium (non-magnetic) to prevent significant alloying of the solder with the backing plate. This would be a phase II modification after we investigate the closure of the gap by the e-beam for vacuum tightness. On work request 00W0152 dated 2/2/2000, Backing Plate 31950 was bonded to a 6N copper target, lot 8V3669-102 using indium solder. The backing plate was modified according to the plan descibed above.

Machined Profile of CVC Bond Experiment

.Measurement

5 8 6 6.2 64 66 7.2

Radial Position (inches)

The black triangle in the figure above shows the desired profile and the pink line shows the actual result. Machining this groove precisely is very difficult.

Machined Profile of CVC Bond Experiment i « Measurement ' F) ^ 5^"

O 01/34339

After bonding, the target was scanned for voids.

After scanning, the target was placed in a fixture in the EB welder to allow the target to be spun around its axis so that the Electron Beam could be scanned over the target/BP interface.

Close up images of the edge:

Before Welding After Welding

These images might suggest that there is more impact from the shape of the groove than from the welding process. Keep in mind that this was the first attempt at groove design and the welding process.

The outer surface of the interface was documented by macro-photography.

F⁾ cr /T>

Backing Plate

Bondline

Target Surface

Claims

The process of bonding followed by EB melting of the solder at the target/BP interface is feasible and no adverse impact is observed.

It is possible that the groove positioned at the outer diameter of the target acts as a sink for the solder and might by itself improve the solidification process to reduce the tendency for edge voids to extend into the bond line and subsequently act as virtual leaks.

Claims:

1) Sealing of the edge of a solder bonded target assembly by heating the edge of the solder bond line sufficient to melt the solder locally and rotating the assembly to move the molten bead continuously around the assembly a full 360 degrees.

2) Claim (1) where the continuous bead is accomplished while the entire assembly is held in vacuum.

3) Claim (1, 2) where a solder resevoir is built into the backing plate (or target) by developing a groove by machining or mechanical means. The resevoir must be thicker than the average bond line thickness by 2x to lOx. The groove is built into the part of the assembly which will be on the lower half during the bonding process. Some assemblies may be bonded in an inverted position.

4) Placement of a solder resevoir at the edge of a target assembly created by machining or other means a groove in the backing plate (or target) whereby edge voids will be trapped and not extend into the main solder layer.

5) Claim (1 ,2) where a solder shim is used to feed the excess solder for making the seal.

6) Claim (2,3) where a solder shim is used to feed the excess solder for making the seal.

7) Claim (1,2,3, 4 and 5) where the method of heating is an electron beam.

8) Claim (1,2,3, 4,and 5) where the method of heating is a laser beam.

9) Claim (1 , 2,3,4, and 5) where the method of heating is an inert gas weld with a high melting point electrode (ex. TIG).

10) All claims where the solder is a mixture of tin and a solute near the eutectic composition and the solute comes from the group (Copper, Silver, Aluminum, Magnesium, Gold and Nickel).

11) All claims except (10) where the solder is basically pure tin.

12) All claims except (10 and 11) where the solder is basically pure indium.

13) All claims except (10, 1 1 and 12) where the solder is tin alloyed with antimony, bismuth, indium or gallium or combinations thereof.

14) Claims (1 - 13) where the target and or backing plate are first coated with a protective layer of nickel or nickel alloy (such as Ni7V) before the solder bonding step.

15) Claim (3) where there is more than one concentric groove for the purpose of trapping voids.