Fabrication of Capacitive Micromachined Ultrasonic Transducers Using a Boron Etch-Stop Method

Søren Elmin Diederichsen, Filip Sandborg-Olsen, Mathias Engholm, Anders Lei, Jørgen Arendt Jensen, Erik Vilain Thomsen

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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 > 1020 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 2.00μm±2.5%. The resonant frequency in air for a 1-D linear CMUT array is measured to 12MHz±2.5%. Furthermore, hydrophone measurements show that the fabricated devices can be used to emit sound pressure in the ultrasonic frequency domain.
Original languageEnglish
Title of host publication Proceedings of 2016 IEEE International Ultrasonics Symposium, 2016
Number of pages4
PublisherIEEE
Publication date2016
ISBN (Print)978-1-4673-9898-5
ISBN (Electronic)978-1-4673-9897-8
DOIs
Publication statusPublished - 2016
Event2016 IEEE International Ultrasonics Symposium - Convention Center Vinci Tours, Tours, France
Duration: 18 Sep 201621 Sep 2016

Conference

Conference2016 IEEE International Ultrasonics Symposium
LocationConvention Center Vinci Tours
CountryFrance
CityTours
Period18/09/201621/09/2016

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