A novel micromechanical digital integrated CMOS pressure sensor using delta-sigma noiseshaping

Abstract

This project presents a new CMOS-MEMS pressure sensor including an analog-to-digital converter on chip. The entire system is fabricated in a standard 0.6um CMOS process from Austria Micro Systems (AMS), with only one additional post-processing step required to form the MEMS diaphragm. This thesis integrates four different fields of science: mechanics, physics, chemistry and electronics.

The aim was to create a pressure sensor combining Frequency Delta Sigma Modulator (FDSM) in a MEMS solution. FDSM is a simpler solution than the traditional Delta Sigma Modulators, converting a frequency signal into a digital signal. By only operating in the digital domain, the FDSM solution has a great advantage. The basic concept of this pressure sensor is to take advantage of one of the behaviors of silicon (Si). When a silicon structure is exposed to stress there is a considerable change in the mobility. By placing several transistors on the edge of a thin silicon diaphragm, the current of these transistors will change proportional to the pressure applied to the diaphragm. By creating a Pressure Controlled Ring Oscillator including these transistors, the output of this circuit will be a frequency that changes proportional to the applied pressure. When pressure is being applied on the silicon diaphragm, this leads to stress in the diaphragm, which leads to a change in the silicon mobility, which again leads to a change in the output current of the transistors, and in the end it all leads to a change in the output frequency of the oscillator. This frequency signal is converted into a digital signal through the FDSM and the sinc decimation filter. Due to sensitivity to variations in the temperature and the power supply voltage, which are major drawbacks of inverter based ring oscillators, a differential solution was implemented.

The task has been to create a pressure sensor all the way from design and simulations, to the testing and measurements of the final chip. The measurement results showed that when pressure was increasing the oscillators, with transistor channels in the stress direction, decreased in frequency. And the opposite results where seen for the oscillators where the transistor channels where across the direction of the stress. The measurements indicated clearly a linear connection between the pressure and the output frequency.