Micromachined High-Aspect-Ratio Parylene Spring and Its Application to Low-Frequency Accelerometers

Abstract

A new microfabrication technology for high-aspect- ratio parylene structure has been developed for soft spring applications. Free-standing parylene beams with widths of 10–40 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$mu m$</tex> and aspect ratios of 10–20 have been successfully fabricated. Since parylene has a small Young's modulus, a high-aspect-ratio beam with a spring constant of the order of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1,times, 10 ^-3~ N/ m$</tex> has been realized. The large yield strain of parylene enables a test structure to have a large-amplitude oscillation of 600 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$mu m _ pmathchar"702D p$</tex> , without any failure of the high-aspect-ratio springs. An early prototype of in-plane capacitive accelerometer was also developed. It was found that its resonant frequency is as low as 37 Hz, and the noise spectral density is 64 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$mu g/( Hz)^0.5$</tex> .1606