Influence of Macro- and Nanotopography, Thin Film Thermomechanical Behavior and Process Parameters on the Stability of T
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Influence of Macro- and Nanotopography, Thin Film Thermomechanical Behavior and Process Parameters on the Stability of Thermocompression Bonding. Konstantinos Stamoulis1 and S. Mark Spearing2,1 1 Massachusetts Institute of Technology, Cambridge, MA 02139, USA. 2 School of Engineering Sciences, University of Southampton, Southampton, United Kingdom. ABSTRACT The quality of wafer-level, gold thermocompression bonds is critically dependent on the interaction between the wafer topography, the thin film properties, the process parameters and tooling used to achieve the bonds. This study presents mechanics modeling of the effect of wafer topography. An analytical expression for the strain energy release rate associated with the elastic deformation required to overcome wafer bow is developed. Furthermore, a simple contact yielding criterion is used to examine the pressure and temperature conditions required to flatten nano-scale asperities in order to achieve bonding over the full apparent area. The analytical results combined with experimental data for the interface bond toughness obtained from fourpoint bend testing indicate that the overall wafer shape is a negligible contributor to bond quality. A micro-scale bond characterization based on microscopic observations and AFM measurements show that the bond yield is increased with increasing bond pressure. INTRODUCTION Device packaging is currently a complex and challenging process in microelectromechanical systems (MEMS) manufacture and is responsible for a large fraction of the total component cost. Wafer-level bonding offers an attractive option, as the cost of the process is distributed over the total number of devices on the wafer and at the same time, the device is protected prior to diesawing. Among the available wafer bonding methods, low temperature processes, such as anodic, thermocompression and eutectic bonding, are particularly attractive for this purpose since diffusion in previously fabricated components is minimized. Gold thermocompression bonding has the unique attributes that, it does not involve a liquid phase or the application of high voltages and results in a bond that is oxidation resistant and electrical conductive [1]. This process is a form of solid-state joining and requires the simultaneous application of temperature and pressure to wafers patterned with metallic thin films in order to bring the mating surfaces into atomic proximity. The quality of the resulting bond is dependent on the interaction between flatness deviations that range from wafer bow to surface roughness, the thin film properties and the process parameters. Hitherto there has been limited modeling applied to understand the relative contributions of these effects [2, 3]. The effect of wafer topography on the resulting bond quality is the focus of the current investigation. The modeling approach is complemented by experimental data for bond yield and toughness obtained from four-point bend delamination testing, microscopic observations and AFM measurements on the fractur
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