Sputtering targets can be simply clamped to the sputter source cathode inside the sputter depositing system. However, this procedure can prove costly due to cracking, warping or other damage to the target in use caused by inadequate cooling. Wherever possible, it is good practice to bond the sputter target to a strong, compatible backing plate.
Target Bonding is a critical process and the exact fabricating method employed can vary depending on the choice of the sputtering target material. A properly bonded sputtering target will normally give a longer working life than a non-bonded target, may enable the use of a higher power input to achieve faster sputtering rates and will enable thin film process parameters to be consistently repeated.
Shear stresses that develop between the target and backing plate, due to heating during sputtering, can be a cause of bonding failure when the backing plate and sputtering materials do not have compatible coefficients of thermal expansion. Depending on the dimensional tolerances allowed, the bond thickness can be adjusted to help compensate for these stresses.
As a complementary service to our Sputtering Target business, Testbourne proves target bonding by various metallic or silver epoxy techniques:
Indium-based Metallic Bonding
All Indium based metallic bonded targets include a sputtered metal intermediate bonding layer on the "bonded" side of the target, which acts as a barrier layer protecting the target material integrity from diffusion of the hot primary bonding metal alloy. The metal bonding technology offers excellent thermal and electrical conductivity and is compatible with either DC or RF power supplies. A properly bonded target will stay adhered to the backing plate providing there is a continuous supply of the sufficient cool water and the sputtering source cathode is not powered.
Metallic bonding can withstand temperatures up to 156 °C.
Elastomer bonding is a proprietary bonding method that was initially developed for bonding materials that were impossible to effectively bond with metals. Interest in this bonding technique has grown considerably in the last few years and now accounts for a high percentage of our business. As indium prices have increased, Elastomer bonding has become a cost effective alternative, especially where both the target and backing plate are consumable and indium reclaim is unrealistic.
The major advantages of Elastomer bonding are its low cure temperature (reducing internal bond stresses from thermal miss match) and high operation temperature (250 °C). Elastomer bonding has permitted new materials to be bonded effectively by increasing line yield from stress fractures. Targets have also been scaled to larger dimensions based on the Elastomer bonding technology. Multiple piece targets in excess of 120" x 110" and single piece targets of 110" x 100" have been bonded while maintaining flatness tolerances.
The Elastomer bonding has less than 1% total mass change at 250 °C for 24hrs at 1x10-7 Torr. The thermal conductivity of the bond is 54W/mK at 0 °C compared to Indium at 83.7W/mK, however the Elastomer bond will operate at 250 °C versus indium which melts at 156 °C.
Silver-filled Epoxy Bonding Kits
The silver filled epoxy bonding cements are high quality products which have been readily accepted as having acceptably low outgassing characteristics. They contain over 90% of high purity Silver for optimum thermal and electrical conductivity.
If you prefer to bond a target yourself, Testbourne supplies the silver epoxy bonding kits in 50g, 100g and 250g packs. We can advise the appropriate amounts of epoxy to use depending on the width of the bonding surface.
Oxygen free, high conductivity (OFHC) copper is a common backing plate material. It has good electrical and thermal characteristics, is easy to machine and is readily available at economic prices.
Aluminium, Molybdenum and Stainless Steel can also be used for backing plate fabrication when copper is not appropriate. As an example, the desired sputtering target material may have a non-compatible coefficient of expansion compared to that of copper which can cause the bonded target assembly to fail irrecoverably. In such case, the use of an alternative backing plate metal is essential.
In many instances, backing plates can be retrieved from spent target backing plate assemblies. The recovered backing plate is then inspected to ensure it is within specification, cleaned and re-bonded to a new sputtering target.