TARGET BONDING ISSUES
One of the key parameters that determine the performance of sputtering targets is the integrity of the bond between the sputtering target and the backing plate. This bond can be critical to the performance and cost of the sputtering target. Studies have shown that bonded sputtering targets produce a more uniform deposition and demonstrate reduced target cracking at higher deposition power densities. As a leading supplier of sputtering targets to the semiconductor, optical coatings, LED, photonics, data storage, wireless, military, and medical industries, Materion’s customers depend on our bonding expertise. We are able to both supply the high quality traditional and custom alloy materials necessary for superior deposition, as well as maximize material performance in the manufacturing environment.
Producing Quality Bonds
The requirements for successful bonding are demanding. The bond must exhibit a low thermal resistance so that the highest possible power density can be applied to the target without the target overheating and cracking. There must also be minimal voiding in the bond to avoid hot spots which can be associated with cracking and non-uniform deposition. The bond must possess sufficient strength to prevent de-bonding of the target during operation, but be flexible enough to minimize the formation of residual stresses during bonding. The latter stresses can be associated with cracking of the target. To reduce the cost of ownership of the target assembly, it is important that the bond can be easily taken apart (reworked), so that the backing plate can be reused. Finally, the entire bonding process must be low cost.
Types of Bonding
Three types of bonding are commonly used: adhesive bonding, solder bonding and high temperature bonding (including brazing and diffusion bonding). By offering the full suite of these bonding techniques, Materion can provide the optimal bonding solution for our customer’s applications.
Silver Epoxy bonding is the lowest cost technique and involves bonding of the target to a backing plate using a specialized thermally and electrically conductive epoxy. This technique requires minimal surface preparation of the target and backing plate and the resulting bond has low residual stresses and is readily reworked. However, the disadvantage of epoxy bonding is that the bond has a relatively high thermal resistance which limits the power densities (and therefore deposition rates) at which the sputtering targets can be used.
Solder bonds inherently exhibit thermal resistance orders of a lower magnitude than adhesive bonds and consequently are able to operate at higher powers and therefore deposition rates. The most common solder filler material Materion uses is Indium which possesses certain advantages. Indium has high thermal conductivity, but also a relatively low melting point and elastic modulus which reduces the residual stresses in the resulting bond. Its low melting point also simplifies the rework of the bond and allows reuse of the backing plate. The entire bonding process can be carried out in air, thus reducing the cost of the bonding solution. The major disadvantages of solder bonding are that the bonds typically exhibit higher residual stresses than adhesive bonds, and that it is critical for the components to be wetted by the solder during the bonding process.
High temperature bonding techniques, such as brazing and diffusion bonding, are the third technique that can be employed. These processes are carried out at temperatures exceeding 500C and require the use of vacuum or controlled atmosphere ovens. For braze bonding, joining is achieved by melting a braze alloy in the bond to create the joint. For diffusion bonding, no filler is used and instead bonding is achieved by heating the components under high pressure to a temperature where interdiffusion occurs between the components creating the bond. The resulting bonds often exhibit very low thermal resistance, typically approaching or less than those of the parent alloys, and allow the highest possible power densities. Use of a high temperature bonding technique is limited by several factors. There is a greater expense associated with it, plus the fact that unless the materials expansion coefficients of the components are very well matched, the bond is extremely stressed. Also, the bonds cannot be reworked, which means the backing plate cannot be reused.
Materion’s Solder Bonding Approach
We have unique experience in optimizing the solder bonding process for a particular target/backing plate combination. Our bonding solutions include the use of thermal coefficient expansion-matched backing plate and target combinations (e.g. silicon sputtering targets on a molybdenum backing plate) to minimize the likelihood of target cracking from poorly matched material combinations. We also have extensive experience in the use of high temperature solders (e.g. silver tin) to allow higher deposition powers than is possible with indium solders. View our chart of Comparative Target Bonding Techniques.
Materion has strategically located target bonding service centers. Low temperature solder (Indium) bonding is carried out at our Buffalo, NY, Brewster, NY, Santa Clara, CA, Albuquerque, NM, Ireland, Taiwan, and Suzhou facilities. High temperature solder (silver tin) bonding is performed at our Buffalo, Brewster and Taiwan facilities. Epoxy bonding is done in Brewster, and in Taiwan, while our high temperature bonding is carried out in by our Brewster facility. Materion can partner with you to identify the best bonding solution for your application; for further information please contact Dr. Alan Duckham, Technical Manager (Buffalo Facility), Alan.Duckham@materion.com.