Conference paper
Fabrication of Capacitive Micromachined Ultrasonic Transducers Using a Boron Etch-Stop Method
Department of Micro- and Nanotechnology, Technical University of Denmark1
MEMS-AppliedSensors, Department of Micro- and Nanotechnology, Technical University of Denmark2
Technical University of Denmark3
Department of Electrical Engineering, Technical University of Denmark4
Biomedical Engineering, Department of Electrical Engineering, Technical University of Denmark5
Center for Fast Ultrasound Imaging, Centers, Technical University of Denmark6
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.
Language: | English |
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Publisher: | IEEE |
Year: | 2016 |
Pages: | 1-4 |
Proceedings: | 2016 IEEE International Ultrasonics Symposium |
ISBN: | 1467398977 , 1467398985 , 9781467398978 and 9781467398985 |
ISSN: | 19485727 |
Types: | Conference paper |
DOI: | 10.1109/ULTSYM.2016.7728763 |
ORCIDs: | Diederichsen, Søren Elmin , Engholm, Mathias , Lei, Anders , Jensen, Jørgen Arendt and Thomsen, Erik Vilain |
1D linear CMUT array Bonding Boron Doping Resonant frequency Semiconductor process modeling SiO<sub>2</sub>:B Silicon Surface treatment boron boron doping profiles boron etch-stop method capacitive micromachined ultrasonic transducers capacitive transducers doping profiles etching isopropyl alcohol microfabrication micromachining potassium hydroxide solution resonant frequency scanning electron micrographs scanning electron microscopy silicon compounds silicon dioxide surface silicon-on-insulator wafers temperature 80 degC ultrasonic transducers