Chemistry: electrical and wave energy – Apparatus – Coating – forming or etching by sputtering
Reexamination Certificate
2001-04-11
2004-09-28
McDonald, Rodney G. (Department: 1753)
Chemistry: electrical and wave energy
Apparatus
Coating, forming or etching by sputtering
C204S298120, C148S313000, C148S425000, C420S435000, C420S476000
Reexamination Certificate
active
06797137
ABSTRACT:
The invention relates to precious metal magnetic sputtering targets and the method of making the same. According to the invention, solid alloy powders manufactured via rapid solidification and elemental Pt are mechanically alloyed, densified, and machined into a sputtering target.
THE INVENTION
The objective of this invention is to achieve enhanced sputtering target characteristics from manufacturing and applications standpoints, through the utilization of innovative processing that enables novel microstructural design. The innovative process design has been developed with careful consideration of cost, lead time, and final product properties. The microstructural design has been developed with the intent to increase manufacturability and enhance product performance in application. In this invention, targets are manufactured using conventional processing steps such as gas atomization, powder mixing and milling, hot isostatic pressing, and machining, and, although the process steps themselves are not unique, the process steps have been strategically employed to achieve a superior sputtering target while maintaining competitive costs and lead-times in manufacturing. The novel microstructure created using the process described in this invention is characterized by a fine precipitate structure and a high degree of compositional homogeneity.
Gas atomization is a common method used to produce powdered metals for a broad range of industrial applications. It is generally recognized that this technique produces fine spherical powders with microstructures unique to rapidly solidified materials. Although atomization has been used in the sputtering target industry to make a range of alloy powders, atomization has not been used with the intent of reducing precipitate phase size in multiphase cobalt-based magnetic alloys. In this invention, gas atomization is used to produce alloy powders with fine microstructures, which lead to enhanced manufacturability during the mechanical working stage of the densified powders, and superior target microstructural and compositional homogeneity when compared to conventionally cast processing techniques. In general, the ductility of a multiphase metallic material is principally determined by the ductility of the continuous phase or phases in its microstructure. In a multiphase microstructure, the degree of continuity of a given phase is a function of its size and shape. For example, coarse microstructural features with high aspect ratios will become interconnected at much lower volume fractions than phases that are fine and spherical. This geometric fact can be summed up by stating that the percolation volume fraction limit of a given phase is inversely proportional to phase size and directly proportional to aspect ratio.
An example of this phenomenon occurs in CoCrPtB alloys containing greater than 6 atomic % B. When these materials are manufactured via conventional casting, they contain a brittle phase that is coarse and elongated. Because of its size and morphology, this phase is interconnected throughout the microstructure, and therefore dominates the mechanical behavior of the material and renders it brittle. In contrast, the same alloys, when manufactured in accordance with the invention tend to be much more ductile. This occurs because the brittle phases that are present in the microstructure are fine and equiaxed, and are therefore not continuous.
The microstructural differences that arise between the conventionally processed material, and the material processed using gas atomization are a result of the difference in the solidification rates of the two processes. Rapidly solidified materials tend to have much finer microstructures than conventionally cast materials. The intent of this invention is to strategically apply this phenomenon to the manufacture of cobalt-based sputtering targets, to promote superior mechanical working characteristics. The increases in ductility that are realized using the process steps outlined in the invention lead to high process yields during thermomechanical processing, which translate into manufacturing cost savings.
In addition, because the volume fraction of brittle phase in CoCrPtB alloys is a strong function of boron content, the relative increase in ductility, and therefore, the enhanced mechanical processing characteristics, for materials made in accordance with the invention becomes more pronounced as boron content increases. This amplifies the benefits of the invention as the requirements for increased boron content become more important in the media industry. Furthermore, in cases where the boron content of the finished target is above 10 at %, the invention becomes an enabling technology because conventional casting and mechanical working techniques become completely ineffective at these levels of boron.
Requirements for compositional homogeneity on thin film media have increased drastically over the last several years due to advances in head technology and disk storage capacity. This requirement has generated an industry need for multiphase sputtering targets with increased microstructural homogeneity, because increased target microstructure homogeneity reduces compositional gradients in sputtered films. In a paper by Harkness et al.,
J. Mater. Res
., Vol. 15, No. 12, December 2000, p. 2811), this result is clearly demonstrated in the case of CoCrPtTa alloys. Although the alloy system investigated in this reference did not contain boron, and although the target microstructural manipulation processes did not involve the rapid solidification techniques discussed herein, the general results are salient to supporting the art described herein. The current invention employs two primary methods for attaining excellent compositional homogeneity within sputtering targets with complex chemistries. The first method is rapid solidification of the base master alloy powders used to make the targets. Rapid solidification leads to chemically homogeneous fine powders containing fine precipitates. The small scale of the particles and precipitates promote excellent point-to-point chemical uniformity within powder mixtures and, in turn, within finished targets. The second method is the mechanical alloying of the base powders using ball milling, or some other mechanical alloying technique. Mechanical alloying leads to alloy powder mixtures with extremely low chemical variability, and is therefore considered to be the optimum method for mixing powders of varying composition to create chemically homogeneous powder mixtures. The combination of these methods enables the fabrication of sputtering targets with greater chemical and microstructural uniformity when compared to targets made using conventional casting technology.
To demonstrate the degree of increased point-to-point homogeneity in sputtering targets manufactured via the invention relative to those made using conventional casting techniques, two targets were compared. Target 1 was made according to the invention and Target 2 was made using conventional techniques. Eight material samples were extracted from the targets in random locations using a standard drilling technique. Each sample was chemically analyzed for Co, Cr, Pt, B and Ta. The results indicates that the point-to-point chemical variability of Target 1 was significantly less than that of Target 2. The averages and standard deviations for the constituents measured in each of the samples is shown in the Table set forth below.
TABLE
Comparison of Point-To-Point Compositional Uniformity
Co
Cr
Pt
B
Ta
Target 1 (Process X)
63.922
21.530
8.054
4.557
1.937
Average (at %)
Target 1 (Process X) Std.
0.067
0.040
0.035
0.034
0.004
Dev. (at %)
Target 2 (Cast) Average
64.423
21.780
8.078
3.708
2.011
(at %)
Target 2 (Cast) Std. Dev.
0.130
0.106
0.035
0.047
0.016
(at %)
The standard processing paradigm in the sputtering target industry is to melt and cast alloys with the finished product composition. Platinum-containing alloys produced for sputtering target applications can be manufactured in a cost-competitive manner using st
Corno Phillip
Kunkel Bernd
Sandlin Michael
Zhang Willy
Heraeus Inc.
McDermott Will & Emery LLP
McDonald Rodney G.
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