Microwave bonding of silicon dies with thin metal films for MEMS applications

The selective, volumetric heating capabilities of microwave bonding are demonstrated using numerical analysis and verified using transient infrared imaging techniques. Two different silicon-to-silicon die designs were modeled. Both designs are 1 cm x 1 cm silicon die stacks bonded using a 024 mum thin gold film. Bonding in the first design occurred using a microwave cavity operating at 2.45 GHz. Less than 76W of power was applied for 15 seconds to achieve melting of the gold (with a melting point of 1064degreesC). Although only 12.6% of the input power is being absorbed by the die stack, numerical analysis shows that 98.3% of this absorbed energy in the die stack is being absorbed by the gold metal layer. Infrared images show that the die stack experiences uniform heating within the time window predicted by numerical analysis. Using the numerical results from the first design, the second design simulated the heating of a 5 mum x 5 mum wide by 0.5 mum thin gold pad located at the bottom of a 10 mum deep and 20 mum wide microchannel sealed within a 1 cm x 1 cm silicon-to-silicon die stack. The purpose of this design is to determine the ability of a living cell tethered to the gold pad to survive the microwave bonding process. Although only 10.6% of the input power is being absorbed by the die stack, numerical analysis shows that 98.5% of this absorbed energy in the die stack is being absorbed by the gold bonding layer and gold pad area. Less than a thousandth of a percent of the 98.5% of the energy absorbed was by the gold pad area.