Trivedi, S., Yeo, Y. X., Sarafianou, M., Wai, E. L. C., Sharma, J., Chen, D. S.-H., Lal, A., & Chai, K. T. C. (2023, May 15). MEMS-Silicon Chip Bonding Verification using GHz Pulse-echo Simulation and Measurements. 2023 Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS). https://doi.org/10.1109/eftf/ifcs57587.2023.10272103
Abstract:
This research presents a novel method to differentiate the bonding quality of a MEMS chip to a silicon substrate by using bulk acoustic wave (BAW) transducer arrays to determine the GHz ultrasonic wave transmission through the bonding interface. Prior to bonding, one-port S-parameter measurements were conducted on the array device at the wafer level to determine the mean resonance frequency (1.487 GHz) and standard deviation (33.8 MHz). The MEMS chip was then bonded to a silicon substrate using Au-AuSn bonding at a temperature <300°C, and confocal scanning acoustic microscopy (CSAM) was used to verify the bond uniformity. The bonding completion was evaluated by exciting the MEMS transducers with an RF signal of 80 ns width centered at 1.47 GHz to generate a packet of bulk acoustic waves and assess the reflected echo signals. Finite element (FE) time-domain simulations were carried out for the 2D-array to analyze the pattern and peak-to-peak amplitudes of the first four echoes, as visual inspection alone was insufficient to confirm bonding. The simulation results revealed a unique signature that indicated bonding completion: the third-echo amplitude became larger than the second-echo when the bonding percentage was >70%, due to constructive interference of ultrasonic waves reflected from the bonding interface and the substrate bottom. For fully bonded cases, the fourth echo became larger or comparable to the second echo, and first echo levels diminish by at least 20%. Electro-acoustic measurements were performed before and after bonding to confirm these distinctive signatures and categorize the devices as fully bonded, partially bonded, or unbonded.
License type:
Publisher Copyright
Funding Info:
This research / project is supported by the A*STAR - Ultrasonic Wavefront Computing
Grant Reference no. : A19E8b0102