Submicron displacement measurements of MEMS using optical microphotographs in aqueous media: Enhancement using color ima
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Submicron displacement measurements of MEMS using optical microphotographs in aqueous media: Enhancement using color image processing Stephan Warnat, Hunter King, Rachael Schwartz, Marek Kujath and Ted Hubbard Dalhousie University, Mechanical Engineering Department, 1360 Barrington St. Halifax, NS B3J 1Z1, Canada. ABSTRACT The measurement of precise submicron displacements is essential in several MEMS applications. For instance, the measurement of the mechanical parameters of biological cells requires repeatable measurement of displacements in the nanometer regime. This paper presents a method to make displacement measurements in an aqueous MEMS environment with a ± 10 nm accuracy by using the blue channel of RGB pictures in combination with a FFT phase shift analysis. INTRODUCTION The accurate measurement of size and motion is a significant challenge of MEMS due to the small feature size: Several techniques including SEM, STM have been used to measure MEMS structures precisely; Capacitance measurements are commonly used in inertial measurement units (IMU) to achieve high in-plane resolution of any motion. However, an optical displacement measurement technique with a submicron resolution is very beneficial in biological cell applications since electrophysiological changes can be observed simultaneously. Periodic structures such as combs are suitable MEMS structures to measure displacements optically by measuring the gap between a moving and static comb. The measurement resolution relies on the optical microscope used. A typical optical microscope is diffraction limited to approximately 250 nm [1]. Pattern matching techniques allow for resolutions of about 100 nm [2] which is still too coarse to measure cell properties. Yamahata presented a FFT based optical measurement technique for in-plane displacement measurements with sub-nanometer resolution [3]. Yamahata’s work was conducted in air. We previously demonstrated that this FFT technique allows PolyMUMPs [4] displacement measurements with an accuracy of ± 10 nm in air and ± 30 nm in aqueous media [5]. Gray-scale photographs were used in this previous investigation. This current paper presents experimental investigations of the FFT displacements measurement technique in an aqueous PolyMUMPs environment by using RGB color pictures. THEORY A nanometer accurate displacement measurement technique was initially introduced by Yamahata et al and is based on the FFT phase shift theorem [3]. The technique relies on capturing static microphotographs of a moving structure with incorporated periodic pattern such as combs. These pictures were analyzed as followed: (1) an area of interest (ROI) (M columns x N rows) was selected from the first captured photograph and was used for all taken pictures; (2) the column gray scale intensity was calculated for all photographs based on the selected ROI and the column average was subtracted. This results in a periodic intensity profile with defined zero crossings for each picture; (3) the FFT was calculated for all intensity profiles and t
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