Batch transfer of microstructures using flip-chip solder bump 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 electroplated indium solder bumps to thick 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 ultimate tensile and shear strength of the solder bond 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