Low temperature fabrication of immersion capacitive micromachined ultrasonic transducers on silicon and dielectric substrates

A maximum processing temperature of 250/spl deg/C is used to fabricate capacitive micromachined ultrasonic transducers (CMUTs) on silicon and quartz substrates for immersion applications. Fabrication on silicon provides a means for electronics integration via post-complementary metal oxide semiconductor (CMOS) processing without sacrificing device performance. Fabrication on quartz reduces parasitic capacitance and allows the use of optical displacement detection methods for CMUTs. The simple, low-temperature process uses metals both as the sacrificial layer for improved dimensional control, and as the bottom electrode for good electrical conductivity and optical reflectivity. This, combined with local sealing of the vacuum cavity by plasma-enhanced chemical-vapor deposition of silicon nitride, provides excellent control of lateral and vertical dimensions of the CMUTs for optimal device performance. In this paper, the fabrication process is described in detail, including process recipes and material characterization results. The CMUTs fabricated for intravascular ultrasound (IVUS) imaging in the 10-20 MHz range and interdigital CMUTs for microfluidic applications in the 5-20 MHz range are presented as device examples. Intra-array and wafer-to-wafer process uniformity is evaluated via electrical impedance measurements on 64-element ring annular IVUS imaging arrays fabricated on silicon and quartz wafers. The resonance frequency in air and collapse voltage variations are measured to be within 1% and 5%, respectively, for both cases. Acoustic pressure and pulse echo measurements also have been performed on 128 /spl mu/m/spl times/32 /spl mu/m IVUS array elements in water, which reveal a performance suitable for forward-looking IVUS imaging at about 16 MHz.

I. Introduction

Capacitive micromachined ultrasonic transducers (CMUTs) have been developed as an alternative to piezoelectric ultrasonic transducers, particularly for microscale and array applications [1]. Because CMUTs are surface micromachined, they can be fabricated into oneor two-dimensional arrays and customized for specific applications; and they can have performance comparable to piezoelectric transducers in terms of bandwidth and dynamic range [2]. A single element of a CMUT array consists of compliant membranes with electrodes suspended above an electrically conductive substrate. To transmit an acoustic wave, an alternating current (AC) signal and a large direct current (DC) bias are applied to the membrane. The DC voltage pulls down the membrane where the transduction is efficient and linearizes the device response. The AC voltage sets the membrane into motion at the desired frequency and generates an acoustic wave in the surrounding fluid. To receive an acoustic wave, the capacitance change is measured when an impinging acoustic wave sets the membrane into motion. If the elements of the CMUT array have a small, mechanically active area covered with an electrode, the change in capacitance also will be small and can be overwhelmed by parasitic capacitance.