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Introduction of AEM Target Bonding

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.

AEM 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.

Target Bonding

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. AEM 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

AEM 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

Material Introduction
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 AEM 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

Perkin Elmer Leybold
Airco Custom Designed Systems

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.