Characterization of AlScN on CMOS

Abstract

In this paper, the bulk-wave excitation and sense properties of solidly mounted AlScN and AlN bulk-wave transducers itegrated onto CMOS substrates is presented. The AlN is fabricated at Silterra Malaysia, and the Al0.85Sc0.15N transducer is fabricated at the Institute of Microelectronics (IME), both on top of Global Foundries (GF) 180nm CMOS wafers. The transducers are deposited on a $725\mu m$ silicon substrate with the CMOS Back End of Line (BEOL) between the piezoelectric stack and the substrate silicon. In the continuous mode testing, the transducer is driven by a network analyzer to obtain the impedance response. We extracted the HBAR quality factor, electromechanical coupling, and the figure of merit from the impedance magnitude and phase. The composite quality factor of the AlN is found to be 17.4% higher than the AlScN, while the composite electromechanical coupling coefficient of the AlScN is 13% higher than the AlN. The larger electromechnaical coupling is useful for pulse response of the transducers used in ultrasonic imaging and sensing applications. In the pulsed mode testing, the transducer is driven by a 60 ns wide RF pulse packet with the carrier frequency swept from 700 MHz to 3 GHz, and the return is signal is used to calculate the transmit/receive transfer function. For the specific thin film stacks tested here, the voltage gain of the AlN and AlScN is comparable. The work also indicates the need for stack optimization to fully utilize the higher AlScN electromechanical coupling.