Metal working – Method of mechanical manufacture – Assembling or joining
Reexamination Certificate
1996-09-25
2001-03-13
Young, Lee (Department: 3726)
Metal working
Method of mechanical manufacture
Assembling or joining
C228S245000, C228S254000, C228S190000, C228S221000, C204S298120
Reexamination Certificate
active
06199259
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to techniques used to fabricate internally cooled sputtering target assemblies generally used in planar magnetron sputtering, and in particular to fabrication techniques used to enhance and assure parallelism between the surface of a target material and the substrate being sputter deposited.
BACKGROUND OF THE INVENTION
Sputtering describes a number of physical techniques commonly used in, for example, the semiconductor industry for the deposition of thin films of various metals such as aluminum, aluminum alloys, refractory metal suicides, gold, copper, titanium-tungsten, tungsten, molybdenum, tantalum, indium-tin-oxide (ITO) and less commonly silicon dioxide and silicon on an item (a substrate), for example a wafer or glass plate being processed. In general, the techniques involve producing a gas plasma of ionized inert gas “particles” (atoms or molecules) by using an electrical field in an evacuated chamber. The ionized particles are then directed toward a “target” and collide with it. As a result of the collisions, free atoms are released from the surface of the target as atom sized projectiles, essentially converting the target material to its gas phase. Most of the free atoms which escape the target surface condense (the atomic sized projectiles lodge on the surface of the substrate at impact) and form (deposit) a thin film on the surface of the object (e.g. wafer, substrate) being processed, which is located a relatively short distance from the target.
One common sputtering technique is magnetron sputtering. When processing wafers using magnetron sputtering, a magnetic field is used to concentrate sputtering action in the region of the magnetic field so that sputtering occurs at a higher rate and at a lower process pressure. The target itself is electrically biased with respect to the wafer and chamber, and functions as a cathode. The magnetic field's influence on the ions is proportional to its distance from the front of the target. Optimally a target assembly (the target and its backing plate) is thin to allow the magnetic field to have the greatest influence.
In generating the gas plasma and creating ion streams impacting on the cathode, considerable energy is used. This energy must be dissipated to avoid melting or nearly melting the structures and components involved. A technique used for cooling sputtering target assemblies is to pass water or other cooling liquid through fixed internal passages of the sputtering target assembly.
An example is shown in the simplified perspective sketch of
FIG. 1
, a sputtering system designed for large rectangular substrates, which includes a relatively thin sputtering target assembly with internal cooling passages. (Details of the chamber and its operation are described in earlier U.S. patent applications of the inventors: U.S. Ser. No. 08/157,763 filed Nov. 24, 1993 and U.S. Ser. No. 08/236,715 filed Apr. 29, 1994, now hereby incorporated by reference herein.) The processing/sputtering chamber
30
encloses a dark space ring
31
surrounding a substrate
32
to be sputter deposited. The upper flange of the sputtering chamber
30
supports a lower insulating ring
33
supporting a sputtering target assembly
40
. The target material on the sputtering target assembly is facing toward the substrate
32
to be sputtered. The target assembly is negatively biased relative to the substrate to effect the sputtering. Inlet cooling lines
36
and outlet cooling lines
37
connect to cooling passages in the sputtering target assembly
40
to cool the assembly during sputtering. The top of the sputtering target assembly
40
is enclosed by a top chamber
35
supported on the back of the sputtering target assembly by an upper insulating ring
34
. As fully discussed in the references previously cited, the top chamber
35
can house a moveable magnetron in an evacuated top chamber. The top chamber can be evacuated so that its pressure approaches the pressure of the process chamber. The force exerted on the area of the target assembly due to differential pressure between the process chamber and the top chamber is then minimal and easily restrained by the thin sputtering target assembly
40
.
A multi-layered sputtering target assembly
40
, as shown in
FIGS. 2 and 3
, is typically assembled according to the above mentioned patent applications using a two step process. In one step, a target material
48
is solder bonded to the backing plate
50
. In another step, a finned (or grooved) cover plate
52
is bonded to the back of the backing plate
50
using a structural epoxy based adhesive. The structural epoxy based adhesive is cured by putting it in position and raising the temperature of the pieces to be joined while at the same time applying a pressure to keep the parts in intimate contact throughout the heating cycle. The order in which the two steps are done is dependent on the melting temperature of the solder and the curing temperature of the structural epoxy. The higher temperature bonding process is done first so that the integrity of the first formed bond is not affected by the subsequent process.
The process and materials used in producing a structural epoxy bond generally create a good bond; however, the cooling fluid occasionally leaks due to imperfections in bonding thereby causing such sputtering target assemblies to be rejected. The factors affecting the structural epoxy bond integrity are 1) surface treatment of the pieces to be joined, 2) epoxy selection and curing procedure, and 3) mechanical fitting or mating of the surfaces being joined prior to adhesive cure.
Surface treatment removes mechanically weak or non-adherent surface film on the metal. For example, surface treatment may simply consist of mechanically abrading the surface to be bonded in order to obtain a “clean” metal surface. Or, for superior results, the procedure may involve a) degreasing, followed by b) an acid etch to remove any visible oxide film or scale, c) rinsing to remove all traces of the acid, d) a surface-conditioning step to deliberately form a corrosion film of controlled chemical composition and thickness which promotes primer adhesion, e) drying, and f) priming within an hour to seal the surface from atmospheric oxygen and moisture.
Epoxy selection is based on several factors including: type of carrier, strength of the adhesive, adhesion to the primed surface, curing temperature and pressure procedure, and ability of the adhesive to flow to create a leak-free joint.
Surface treatment, epoxy selection, and curing procedures are factors controlled by manufacturing rigor. However, good mechanical fitting or mating of the surfaces being joined is also required to achieve leak-free joints. Distortion and voids are introduced by the two-step soldering process presently used to join large areas (e.g. 643 mm×550 mm target material dimension) of a) dissimilar metals and/or b) non-uniformly heated or cooled similar metals. The present process includes the solder wetting of the two surfaces to be bonded. The target material is then heated and a pool of solder is created at the soldering location. The backing plate, also heated, is then slid into the pool of solder to avoid trapping the solder oxide that normally floats over the molten solder, and the weight of the piece and a light pressure cause the solder in the pool to spread out over the surfaces to be soldered and bring the two materials generally in close contact. The pieces are held aligned one to the other until the solder cools below its melting temperature and the two pieces are bonded.
For example, when solder bonding indium-tin-oxide (ITO) to a commercially pure titanium backing plate, during cooling from the soldering temperature (e.g., 156° C. for pure indium solder) to ambient temperature, the differential thermal contraction of the soldered connection tends to cause bending of the pieces. The edges of the target material, being the first to cool, initially form a stronger bond than the higher temperature center of the target. As a resu
Demaray Richard E.
Herrera Manuel
Applied Komatsu Technology Inc.
Butler Marc W.
Peters Verny Jones & Biksa, LLP
Young Lee
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