Low-temperature bonding of optical chips using coined Au stud bumps and its application to micro laser Doppler velocimeter

Surface activated bonding of laser diode (LD) chips on coined Au stud bumps (thickness: 10~15 μm) with smooth surfaces (R_a <; 3.5 nm) was demonstrated for optical microsensor applications. Bonding of LD chips with Au thin films to the Au stud bumps on the Si substrates was performed in ambient air at low temperature (150 °C). Die-shear strength per unit area was three times as high as that obtained between Au thin films (thickness: 0.5 μm). Using this technique, compact and thin micro laser Doppler velocimeter (2.8 mm × 2.8 mm × 1 mm thick) was developed. The feasibility of measuring velocity was demonstrated for a moving Au wire (diameter: 22 μm).