Design of analogue CMOS VLSI MEMS sensor : a dissertation presented in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Engineering , Integrated Circuit Design at School of Engineering and Advanced Technology, Massey University, Albany campus

Abstract

There is an increasing demand of a highly sensitive and reliable pressure micro-sensor system, for implantable and non-implantable medical applications. The prerequisite of a miniaturized device for minimally invasive procedures, posed greater challenges in the complex integrated design of micro-system. Micro-sensor system designs in the recent advanced CMOS technologies are explored in this work for effective system miniaturization and improved performance. The material choices and geometry designs, which significantly influence the sensitivity and dynamic range of the micro-scale sensor devices, are well addressed. Cointegrations of MEMS devices with signal conditioning circuits that effectively reduce the parasitic effect are also performed for enhancing the overall system performance. In addition, system reliability is also improved with on-chip metal interconnections. The employed process technologies to a greater extent contributed to the high yield for these low cost micro-sensor systems. This research focuses on the design of integrated CMOS MEMS capacitive pressure sensors for diverse bio-medical applications. Two monolithically integrated capacitive pressure microsensor systems are designed, fabricated and experimentally verified. A novel micro-electromechanical capacitive pressure sensor in SiGeMEMS process, vertically integrated on top of a 0.18 μm TSMC CMOS processed die is proposed. The perforated elliptic diaphragm, which is edge clamped at the semi-major axis is developed using poly-SiGe material. High performance on-chip CMOS conditioning circuits are also designed to achieve better overall sensitivity. Experimental results indicate a high sensitivity of around 0.12 mV/hPa along with a nonlinearity of around 1% for the full scale range of applied pressure load. The L-clamp spring anchored diaphragm provided a wide dynamic range of around 900 hPa. Another integrated capacitive pressure micro-system, developed using the advanced standard IBM CMOS process in two geometrical designs is also proposed. A step-sided elliptic diaphragm that overcomes the CMOS process limitations is fabricated to achieve regulated membrane deflections and improved sensitivity. A foundry compatible post-process technique, for a lateral release length of 125 μm is also performed successfully on the 130 nm CMOS platform. A current cross mirroring technique is utilized to enhance the transconductance of an on-chip operational amplifier to achieve a high single stage gain. Sensitivities of the fluorosilicate sealed absolute pressure sensors were measured to be 0.07 mV/Pa and 0.05 mV/Pa for the elliptic and rectangular element, respectively. In addition, the linear capacitive transduction dynamic range was found to be 0.32 pF and 0.23 pF, respectively, for the elliptic and rectangular element.