Sputtering Target Bonding Service
Sputtering 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. The bonding process insures the thermal integrity of the interface between your system’s cooling assembly and the surface of the target, which experiences the most heat.
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.
DXAM provides a complete range of bonding services. Targets are sputter coated with two separate layers using an MRC Sputter Deposition System to match the expansion coefficient of the backing plate. Metallic bonding is available in various temperatures ranging from 150° to 225°C.
Types of Target Bonding
|Target Bonding Service||Introduction|
|Indium Metallic Bonding||Indium is the preferred method for bonding sputtering targets because it has the best thermal conductivity of all available bonds and is the most efficient at drawing heat away from the target. Indium is also more malleable than other bonding solders and therefore it is more forgiving. DXAM uses indium and indium based alloy materials for bonding. The softer solder allows some “give” when the target expands at a different rate than the backing plate. This reduces cracking that is caused by mismatch in the thermal expansion coefficients of the target and backing plate. The main limitation of the indium bond is the melting temperature of the indium solder. Indium has a melting point of 156.6°C so temperatures in excess of 150°C will cause the bond to melt and fail.|
|Epoxy Bonding||Epoxy bonding is other target bonding solutions, that is using epoxy to directly bond the target and the backing plate. Advantages of epoxy bonding are: it is not adapting to the traditional metallic solder, so it doesn’t need to consider the wettability of the solder in the target. And it can operate at room temperature, without worrying about the thermal expansion difference between of the target and the backing plate. It is especially suitable for large area target bonding, because there is no need to consider problems of welding deformation and welding stress. The disadvantage of epoxy bonding is that, due to the material limitations of the epoxy, the conductivity and thermal conductivity of epoxy bonding are worse than those of other.|
For applications that require a significantly higher melting point than Indium, we can use a Silver loaded epoxy. Our Electrically Conductive Silver Epoxy is specially designed for target bonding.
Sizes and Shapes of Target Bonding
DXAM can easily bond most targets, in any size or shape. This includes round targets from 0.5″ to 16″ in diameter, rectangular targets from 1″ x 1″ to 10″ x 60″, ‘Delta’ targets, and other custom shapes.
Backing Plate Material
|Oxygen-Free Copper||Oxygen Free Copper (OFHC) is the most common backing plate material. This metal has good electrical and thermal characteristics while also being easy to machine, easy to soften, and readily available at a low cost. Copper backing plates can be re-used, with care, 20 or more times.|
|Molybdenum||For applications where Copper is not appropriate, we often use Molybdenum instead. For some materials, such as ceramics, or even metals made via Powder Metallurgy, the coefficient of expansion for Copper is mismatched. For some applications required high temperature bond, Copper may oxidize badly or warp.|
|Copper and Stainless Steel Cups||DXAM specializes in the use of unique backing plate geometry for delicate ceramic targets ranging in size 0.5″ to 8″. The ‘cup’ design provides good mechanical support for the target on the back and around the edge. This improves both heat transfer to the cooling well of the cathode and increased mechanical integrity by providing a clamping point for the assembly which does not stress the target.|
Backing plates are available for the following systems
|Airco||Custom Designed Systems|
Types of Target 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.
Why Do We Use Target Bonding?
1. When the material is thinner, the material can transfer heat faster through its thickness. For most sputtering R&D guns, since the gun has a maximum thickness allowance, the thickness of the target is reduced to half when it is bonded to a backing plate. The copper backing plate comprises the other half of the thickness. The thinner target can cool more effectively than a thicker target because the distance that the heat generated on the surface of the target has to travel to reach the cooled side is reduced.
2. Target bonding makes that a ceramic material can cool more efficiently when bonded. The target is in intimate contact with the conductive solder layer which draws the heat from the target surface and into the copper backing plate. The copper backing plate is in contact with the water cooled gun so the heat is transferred through both pieces of copper and is removed through the cooling water.
3. Whether the target is bonded or the ramping procedure used to condition the target, some ceramic sputtering targets will crack during sputtering due to thermal shock. Bonded targets can usually continue to be used even after a target crack occurs, where typically an un-bonded target cannot.