Wire-bonding process monitoring using thermopile temperature sensor

This work presents an approach to separate the thermal response due to ultrasonic excitation and ball deformation through novel application of aluminum-polysilicon thermopile sensors under the bond pad. These integrated thermopile sensors measure temperature at a radial distance under the bond pad, in contrast to the previously reported average measurements over the bond pad interface or around the bond pad over a radial distance. The high sensitivity and signal-to-noise ratio (SNR) of the sensor allow direct measurements of the signal, without any amplification or filtration. Transient temperature variations at two radial locations were obtained using two versions of thermopile sensor designs. The sensor response was interpreted using representative finite-element thermal modeling for the process. Results from modeling reveal that the thermal response is a strong function of radial location. These results also reveal that the thermal response due to interfacial heating is significantly higher under the bond pad, as compared to that around the bond pad. This is in agreement with the experimental observations. Critical points on the temperature variation curve were identified. These points can be used to correlate the sensor response to shear test data. Once the sensor response is calibrated, it can be used to monitor the bonding process. Measurements were performed at substrate temperatures of 150°C and 200°C, along with the microwelds characterization at the bonding interface. The comparison of the thermal response and the microwelds at the two substrate temperatures revealed that in order to correlate the sensor response to shear test data, the response must be obtained at the intended temperature of operation since the microwelds at two temperatures may be quite different, even though thermal responses may look similar.