Fabrication of capacitive micromachined ultrasonic transducers using a boron etch-stop method

Abstract

Abstract-Capacitive Micromachined Ultrasonic Transducers (CMUTs) fabricated using Silicon-On-Insulator (SOI) wafers often have large thickness variation of the flexible plate, which causes variation in both pull-in voltage and resonant frequency across the CMUT array. This work presents a bond and boron etch-stop scheme for fabricating the flexible plate of a CMUT. The proposed fabrication method enables precise control of the plate thickness variation and is a low cost alternative to the SOI-based process. N-type silicon wafers are doped with boron to a surface concentration of > 10 20 cm −3 using solid planar diffusion predeposition at 1125• C for 30, 60, and 90 min. Process simulations are used to predict the boron doping profiles and validated with secondary ion mass spectrometry measurements. The doped wafers are fusion-bonded to a silicon dioxide surface and thinned down using an 80• C, 20 wt% potassium hydroxide solution with isopropyl alcohol added to increase the etch selectivity to the highly doped boron layer. The resulting plate thickness uniformity is estimated from scanning electron micrographs to a mean value of 2.00 µm ± 2.5%. The resonant frequency in air for a 1-D linear CMUT array is measured to 12 MHz ± 2.5%. Furthermore, hydrophone measurements show that the fabricated devices can be used to emit sound pressure in the ultrasonic frequency domain.

Assessing the Microfabrication-Related Variability of the Performance of CMUT Arrays

“…Up to now, the uniformity of the CMUT array performance has always been evaluated in terms of standard deviation of the biased transducer resonance frequency [26]- [29], and occasionally in terms of acoustic performance variability (e.g., sensitivity bandwidth and center frequency standard deviation) [12], [30]- [32]. Nevertheless, a quantitative evaluation of the effect of the fabrication-related variability of the geometrical parameters on the transducer performance variation has never been performed.…”