Batch transfer of microstructures using flip-chip solder bonding

This paper describes a novel method for transfer and assembly of microstructures using sacrificial-layer micromachining and flip-chip bonding. The technique is performed at room temperature (cold weld) and at the back end of the process flow and may thus provide a commercially viable alternative to monolithic integration and costly hybrid packages. The transfer is achieved using break-away tethers and by cold welding (compression bonding) electroplated indium solder bumps to electroplated copper pads. Both high-aspect-ratio MEMS devices as well as surface-micromachined devices have been successfully transferred using this method with no observable misalignment between moving and stationary parts. The maximum tensile and shear stress the solder bond can withstand before failure is measured to be 11±3 MPa and 9±1 MPa, respectively. The contact resistance is measured to be of the order of 1.5 mΩ for a 65 μm×65 μm×4-μm indium bump