Advancing Rotatable Sputter Target Coatings

December 1, 2010
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Target coating quality issues can be alleviated through the elimination of blended powders.



Figure 1. SEM photograph showing morphology of silicon powder "as received" (100x magnification).

Rotatable magnetron sputter deposition has become the most widely used coating technology for large area deposition. Silicon is typically used for coating glass in applications such as anti-reflection coatings, all-dielectric mirrors, beam dividers, band pass filters, and polarizers (used for architectural, automotive, and monitor display glass). Thin films of SiO2 and Si2N4 are sputtered from SiAl targets that comprise a blend of aluminum within a silicon base.

Figure 2. SEM photograph showing morphology of silicon powder coated with 10% aluminum (200x magnification).

The Basics

The production of SiAl targets by thermal spray processing is a state-of-the-art process, the success of which is due to several main factors:
  • Flexibility for target geometry and sizes allows for a wide range of shapes, diameters, and lengths. Today, cylindrical rotatable magnetrons can be up to 18 in. dia. x 152 in. in length, covering from 9-13 mm (350-515 mils) in thickness.
  • High deposition rates with good deposition thickness control and coating uniformity.
  • Possible remanufacturing of the target surface of at least two times or more.
  • Raw feed powder materials can be tailored (i.e., Si with 6% Al) to produce stoichiometric and non-stoichiometric compositions, thus allowing the glass coater to develop their own specific coatings for specific applications.
Despite these benefits to the thermal spray process (along with fixed process control), limiting factors can still affect the coated target. The desired functionality of the sputter target can only be achieved by a well-designed coating that consists of a fully dispersed, dense, and uniform coating structure.

Out of the several millions of square meters of glass being coated each year with a SiAl target, the most popular raw powder form is a blended composition of silicon with aluminum ranging from 0-19% by weight. Mechanically blending two dissimilar materials is quite useful, but it can provide many quality issues within the coating, including separation of particles in the plasma flame and overheating (evaporation) of lower melting point (MP) metals such as aluminum (MP = 660°C) within a silicon base (MP = 1410°C). This can result in increasing porosity levels and an inhomogeneous coating composition.

Figure 3. EDAX analysis of Si-10%Al coating aluminum map.

An Experiment

One solution to combat target coating quality issues is to eliminate the use of blended powders. This can be accomplished through the manufacture of unique powder formulations that combine both Si and Al into a single particle consisting of the proper chemical composition required for the target.

In a recent experiment, thermal spray coatings of powder formulation(s) were produced to provide a powder analysis with comparisons between the current blended powders and the single particles. Thermal sprayed coating cross- sections, density, and hardness results were reported, and coating integrity and coating composition were compared using EDAX and image analysis techniques.

Figure 4. EDAX analysis of Si-10%Al coating silicon map.

The technique of chemical vapor deposition in a fluidized bed reactor (CVD-FBR) is a well-known, useful process that provides a coating (clad) onto a substrate powder; it is typically performed under 600øC. The coated or clad powder is shown as a sphere with coating on the outside. This coating can be metal or ceramic, or even multiple layers. The process prevents segregation and enables control of powder reactivity (which can be either increased or reduced) during processing.

At low temperatures, clad coatings of aluminum can be deposited onto silicon without changing their microstructural properties. Clad coatings improve the chemical properties of substrate powders and their high-temperature durability in corrosive environments. In the case of silicon, the aluminum clad provides a built-in bonding material that allows the silicon to "stick" to the rotatable target surface while ensuring a uniform distribution of aluminum throughout the silicon matrix.

Figure 5. EDAX analysis of Si-10%Al coating SEM.

Coated powders are conceptually easy to visualize, but it is difficult to fully comprehend all of the endless possibilities. What makes this process intriguing is its ability to marry two dissimilar materials to form a homogenous coating structure.

Ten pounds of silicon powder was coated with 10% aluminum. A chemical analysis of the Al-coated powder was performed to determine the amount of aluminum coating, and particle size was analyzed prior to and after Al coating was conducted.

The coated powder was plasma sprayed using a plasma gun with nitrogen/hydrogen arc gas at 450A and 76V. The powder feed rate was at 35 g/min (approximately 5 lbs/hr). Coating samples were cut, mounted and polished. The microstructure was determined using SEM as well as an optical microscope at 200 x magnification. Chemical analyses on the coating-including a component distribution map-were performed using SEM and EDAX.

Results and Conclusions

Figures 1 and 2 show the morphology of the raw silicon powders as received and coated with 10% aluminum. The chemical analysis is shown in Table 1. The results indicate that the powder was coated with 10% aluminum (consisting of 90% silicon with 10% aluminum) and that the coating consisted of an even distribution of 91% silicon with 9% aluminum, as can be seen in Figures 3, 4, and 5.

Aluminum clad can be formed on pure silicon powder below 600øC. The aluminum clad is over 99% uniform around each silicon powder particle, and the addition of 10% aluminum clad onto silicon powder increases particle size only slightly.

A 10% aluminum clad silicon powder can be plasma sprayed to thicknesses greater than 2500 mm. In addition, aluminum 10% silicon forms a dense coating structure with a fine and homogenous dispersion of Al. Although not reported above, the spray results for deposition efficiency proved to be ~ 60% DE.

For additional details, contact HAI Advanced Material Specialists, Inc. at 1688 Sierra Madre Circle, Placentia, CA 92870; (877) 411-8971; fax (877) 411-8778; or visit www.haiams.com.

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