Self-curved diaphragms by stress engineering for highly responsive pMUT

Abstract

A process to make self-curved diaphragms by engineering residual stress in thin films has been developed to construct highly responsive piezoelectric micromachined ultrasonic transducers (pMUT). This process enables high device fill-factor for better than 95% area utilization with controlled formation of curved membranes. The placement of a 0.65 µm-thick, low stress silicon nitride (SiN) film with 650 MPa of tensile residual stress and a low temperature oxide (LTO) film with 180 MPa of compressive stress sitting on top of a 4 µm-thick silicon film has resulted in the desirable self-curved diaphragms. A curved pMUT with 200 µm in nominal radius, 2 µm-thick aluminum nitride (AlN) piezoelectric layer, and 50% SiN coverage has resulted in a 2.7 µm deflection at the center and resonance at 647 kHz. Low frequency and resonant deformation responses of 0.58 nm/V and 40nm/V at the center of the diaphragm have been measured, respectively. This process enables foundry-compatible CMOS process and potentially large fill-factor for pMUT applications.