(Dr. Yogendra K. Joshi, advisor)
"Characterization of Temperature Variation During Wire Bonding Process"
Decreasing pitch and increasing number of input output pads on semiconductor chips have made optimization of the wire bonding process a time consuming step in the packaging of wire-bonded chips. This has necessitated the development of in-situ sensors, which can streamline the optimization process. The main focus of this study was to monitor the variation of transient temperature during the wire bonding process under the bond pad using embedded thermopile sensors. Two designs of thermopile sensors were laid out, characterized and tested. The temperature dependent Seebeck coefficient of Aluminum-Polysilicon junction was characterized using an on chip calibration device. Temperature measurements using two devices were performed at different input powers, ball size, and substrate temperatures. Thermal modeling was utilized to interpret these measurements. The comparisons of the temperature response under the bonded ball and away from the bonded ball revealed that the temperature rise under the bond was higher than that around the bond. The thermal response under the bond pad could be used to detect the formation of microwelds at the interface. It was observed that higher temperature rise due to the ultrasonic were accompanied with a higher amount of intermetallic formation at the bonding interface. Also, the temperature rise during the ultrasonic application showed less scatter than the shear strength measurements of the bonded balls.
The unique aspect of these sensors is that they measure temperature at a known radial location, unlike the resistive sensors reported in the recent past, which measured an average temperature over a radial distance around the bond pad. Higher sensitivity of the thermopiles sensors allows the measurement of the device response without any amplification. The temperature measurements were performed at substrate temperatures of 150 °C and above for the first time, using an on-chip sensor. The temperature rise due to the ultrasonic can be correlated to the growth of microwelds at the interface, and the sensor could be used in a better estimation of optimum bonding parameters than that possible with shear test measurements. Post processing of the thermal response can be automated and human intervention can be minimized to speed up the optimization process.