Measurement of dynamic strain during ultrasonic au bump formation on Si chip

The strain generated at the surface of a Si chip during the ultrasonic formation of a Au bump was dynamically measured using strain gauges fabricated on the chip. The piezoresistive-type strain gauge was designed to be sensitive to unidirectional strain at the surface. Measured results show that the magnitude and direction of strain strongly depend on the relative position to the Au bump and bonding sequence. It has been found that the maximum strain is generated near the periphery of the bump during the application of ultrasonic vibration, while the maximum residual strain appears near the center of the bump. Analysis with the finite element method (FEM) has shown a good qualitative agreement with the measured results. Effects of direct bump formation on MOS devices were examined by time dependent dielectric breakdown (TDDB) tests and measurement of flat band voltage of metal oxide semiconductor (MOS) capacitors. The reliability of MOS gate oxide was found to decrease due to both ultrasonic and residual strain.