Analytical methods used for low temperature Cu?Cu wafer bonding process evaluation
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Metal wafer bonding is a well-established technology in the semiconductor industry. This technology gains more and more importance due to increasing market demands, particularly for three-dimensional integrated circuits and integration of complementary metal oxide semiconductor and microelectromechanical systems devices used in consumer electronics. Cu?Cu wafer bonding at low temperature is a very attractive process which can be used for both mechanical joints and electrical interconnects in wafer-level processes. In order to facilitate a permanent Cu?Cu bond at low temperature?in contrast to a typical ~400 °C Cu?Cu bonding temperature?the effect of material properties and bonding parameters has to be understood well. Therefore a comprehensive characterization including both, namely surface and interface analysis before and after wafer bonding of non-patterned Cu layers, respectively, is required in order to optimize the Cu?Cu metal wafer bonding process. Low temperature Cu?Cu bonding, namely ?200 °C, of ~500 nm sputter-deposited Cu films was demonstrated and characterized.Selected analytical methods, such as transmission electron microscopy, Auger electron spectroscopy and electron backscatter diffraction, and their corresponding specimen preparation techniques for microstructure and elemental composition evaluation are introduced. Experimental results of surfaces and interfaces investigation (materials properties with high impact on their bonding behavior), such as the (i) surface roughness, (ii) Cu native oxide, (iii) Cu (surface) purity and (iv) Cu grain size, as well as the influence of the (vi) bonding temperature are presented.