Abstract The development of a fine grain microstructure in sputtering targets for thin film applications is a critical factor in the efficiency and robustness of the deposition process. This research was initiated to produce a practical method of producing superior fine grain sputtering targets. The SFGTM process has been developed to achieve a consistently superior fine grain in non-ferrous metals.
SFGTM targets produced from gold and silver were compared against industry standard targets. A comprehensive examination of the grain structure was performed to distinguish between the two microstructures. The film deposited by these targets was also investigated. The attributes measured include film thickness profile, deposition rate, and film uniformity.
The SFGTM process produced a consistently finer grain size when compared against the industry standard microstructure. The improved microstructure was observed in both gold and silver. The sputtered thin film thickness profile was also improved when using the SFGTM target. In addition, the sputter rate and the absolute difference in the thin film was superior when the SFGTM target was utilized. Similar results occur in other precious metals and non-ferrous alloys. Using a SFGTM target allows for a uniform erosion pattern to develop during the sputtering process. The randomly orientated grain structure developed eliminates any preferred sputtering of the target material. When preferential sputtering occurs, the target erodes unevenly. This leads to both a poorly sputtered thin film and less than optimal utilization of the target material. The SFGTM process produces a sputtering target that contributes to a superior thin film for optical and other deposition applications.
IntroductionThe development of a fine grain microstructure for deposition applications in the optical media and other thin film applications is a critical component in improving both final product quality and manufacturing efficiencies.
The SFG Super-Fine Grain ProcessWilliams Advanced Materials has developed a proprietary manufacturing process which develops superior fine grain size in precious and other non-ferrous materials. This process is known as SFGTM (Super-Fine Grain). The SFGTM microstructure provides superior performance in critical deposition applications.
The approach to producing fine grain size in the precious metals builds upon the similarities among the family of precious metals and other non-ferrous alloys. These metals share the Face-Centered Cubic (FCC) crystal structure. This close-packed structure allows a common manufacturing process to result in superior fine grain structure in the finished billet. This billet is then machined to exact specifications producing a superior sputtering target.
The goal of the development of the SFGTM process is to produce a target that gives superior performance when used in a thin film manufacturing process.
ResultsThe SFGTM targets were compared against industry standard targets. The target examinations compared grain size, sputtered film thickness profile, deposition rate, and thin film uniformity. The SFGTM manufacturing process shows superior performance using these criteria.
Gold and silver were chosen as typical nonferrous materials. The manufacturing process is applicable to a variety of nonferrous materials
Gold - Grain SizeTargets from the SFGTM and standard industry targets were examined both macroscopically and microscopically.
Figure 1 is a general macroscopic examination with the standard Au Target on the left and the SFGTM Target on the right. This photograph allows a general comparison of the superior grain structure of the SFGTM target.
When these gold targets are examined macroscopically but at magnification. The larger grain size of the industry standard target (Figure 2) can be compared to the SFGTM target (Figure 3).
Metallographic examination of the gold microstructure using a metallographic microscope allowed for measurement of the average grain size (Figure 4). This grain size was measured at 50 microns for gold.
Silver – Grain SizeA comparison was also made between an industry standard silver target (Figure 5) and the SFGTM process (Figure 6) for silver. A macroscopic examination confirms the superior grain size obtained by the SFGTM process. This leads to improved thin film characteristics.
The SFGTM silver target was also metallographically examined at magnification (Figure 7). The average grain size for the SFGTM target was determined to be 25 microns.Similar Results have been achieved for other precious and non-ferrous materials. Specific grain size is dependent on the alloy and the specific variant of the SFGTM process.
Film UniformityFilm Uniformity A comparison was made between an industry standard sputtering target and a SFGTM target to measure the in-chamber performance. Under identical sputtering conditions, the SFGTM target exhibited an improved thin film uniformity. Figure 8 gives the thickness profile obtained using a standard industry target on a polycarbonate substrate used for optical media.
Summarizing the performance of the two targets in the chamber, the SFGTM target had superior film uniformity, a higher sputter rate and the difference (D) between the thickest and thinnest part of the sputtered film was decreased. These results are summarized in Table 1.
Table 1. Comparison of properties of standard and SFGTM produced sputter targets.
Industry Standard SFGTM Film uniformity ± 8% to 10% ± 3% to 4% ∆Thickness 100-150Å 40-50 Å Sputter rate 29 Å/min 30 Å/min
ConclusionsThe development of a fine grain microstructure has been shown to be a critical factor in the performance of the target. A fine, randomly orientated microstructure produces a target that produces a more consistent thin film coating. Measurement of the film uniformity, sputter rate, and transfer coefficient all indicate that the superior grain structure produced by the SFGTM process leads to superior performance in the thin film deposition manufacturing process.
The fine grain microstructure developed during the SFGTM process allows for a uniform erosion pattern to develop on the sputtering target. When the energized argon atoms impinge on the target, the randomly orientated grain structure hinders any preferred sputtering of the target material. If preferential sputtering occurred, the target would erode unevenly. This leads to both an inferior sputtered thin film and less than optimal utilization of the target material.
The SFGTM manufacturing process has been proven using precious metal alloys such as gold, silver and platinum. The process is also successfully applied to other nonferrous alloys. These materials include aluminum, copper, and their alloys.
The SFGTM process has been shown to contribute to improved sputtering performance in a production environment. This improvement in thin film uniformity and process efficiency has a direct effect on the overall performance of the thin film manufacturing operation.