Design and fabrication of Si diaphragms for capacitive pressure sensors in high-pressure microfluidics

Abstract: The pressure has a major impact when it comes to control of chemical processes. A method for integrating electrical pressure sensors in high-pressure microfluidic chips has, however, so far not been developed and the aim of this project was therefore to study how capacitive pressure sensors could be designed and fabricated to measure pressures between 1-100 bar. Based on analytical calculations and simulations of the thickness and the deflection of the diaphragms, touch mode capacitive pressure sensors were designed, and the fabrication involved exploring both isotropic plasma etching with SF6 and wet etching using KOH to fabricate a diaphragm on a Si wafer. The method which was found to be most suitable was to etch the diaphragm by plasma etching with SF6, even though a pressure resistance of only 23.4 bar could be accomplished, which is believed to be a consequence of the surface roughness, resulting in an Ra-value of 0.43 µm. The surface roughness lead to that chips had to be assembled by aligning them with an IR-camera and glued together with UV-epoxy, which enabled low precision. A method which do not leave any large etching effects on the diaphragm’s surface is therefore recommended for future work, to both potentially increase the pressure resistance and enable chips to be anodically bonded.