As a Product Manager in the Materials Division, I field a lot of questions from customers from many different industries with varying degrees of sputtering experience. One of the more common emails that I receive goes something along the lines of, "There's something wrong with my bonding because the indium has melted out of the sides of my target." Often times the user has waited for their target, spent money on it, only to use it one time and had to stop due to the melted bond. A frustrating situation for sure.
Bonding is recommended for materials that are not good at transferring heat that builds up on the face of the target during sputtering. Backing plates add stability and help to draw the heat away from the target. Some systems require a specific backing plate in order to install the target into the cathode.
Example of melted bond
Sometimes bonds will fail during sputtering for a variety of reasons. Most of the time, too much power has been applied. Bonded assemblies are limited in power to ~20watts/square inch or less depending on the target material. In some cases, the bond is the limiting factor. For instance, sometimes an expensive material like gold needs to be sputtered. In order to save costs, the user may prefer a very thin target. As such, bonding is required to meet a minimum thickness in the sputtering gun or to add stability to the thin target. Unbonded gold can be sputtered at ~100watts/square inch. Once gold is bonded, that drops the max power to ~20 watts/square inch. Many times, the bond is not factored in when determining the overall power and bonds can fail.
Example of melted bond
Indium melts at ~150°C. Because of the heat generated during sputtering, sometimes the bond will melt. It is important that a ramping procedure is followed. By slowly ramping the power instead of going straight to the desired power, the chances of melting the bond can be reduced. In general, we recommend not exceeding 20 watts/square inch on a bonded assembly. It is material specific so it's a good idea to review the max power density information found on our Sputtering Target product pages when determining appropriate power levels for your target.
If after using indium bonded targets, the bond frequently melts or fails altogether, one alternative would be to switch to an elastomer bond. Elastomer is rated to 250°C which allows more wiggle room in the chance that the bond sees temperatures in excess of the 150°C mark.
Example of melted bond
KJLC has over 20 years experience bonding targets from 1" diameter to larger, industrial configurations. There are varying levels of bond quality available in the market. It's important to select a high quality bond that offers both good adhesion and a good barrier between the target material and the bonding material.
Should you find yourself with a melted bond, please feel free to contact us. Often times we are able to clean up the target and re-bond the assembly.
Hopefully by following these guidelines, you can avoid the trouble of a melted bond!