Fabrication and characteristics of Si piezoresistive micropressure sensors for tactile imaging devices

This paper describes the characteristics of a Si piezoresistive micropressure sensor fabricated by using a three-electrode electrochemical etch-stop to reduce the pressure sensitivity variation, as well as its application to both force and load distributions for instruments or tactile imaging devices. The Si microdiaphragm thickness is precisely controlled by using a natural etch-stop in aqueous tetramethyl ammonium hydroxide (TMAH) : isopropyl alcohol (IPA) : pyrazine solutions. Using the electrochemical etch-stop technique, we have made 801 microdiaphragms on a 5-inch Si wafer with 20-mu m-thick n-epi Si grown epitaxially on p-type substrates. The average thickness of the 801 microdiaphragms with sizes of 1.43 x 1.43 mm(2) has been measured to be 20.03 mu m with a standard deviation of +/- 0.26 mu m. The etch-stopped microdiaphragm surface is extremely flat without any noticeable taper or nonuniformity. The pressure sensitivity of the fabricated devices is 1.72 mV/(V.Kgf.cm(-2)), and its variation is less than +/- 2.3 %. This result indicates that the electrochemical etch-stop technique in TMAH : IPA: pyrazine solutions is useful for the manufacture of the uniformly thick microdiaphragms needed for producing micro-electro-mechanical-systems (MEMS) on a wafer scale.