Study On Digital Accelerometer Using MOS Ring Oscillators

Sensor technology is one of the most important parts of Information Technology (IT), served as the information acquirement unit. Accelerometers are pervasively used in various applications such as automobile airbag systems, industrial and military appliances. A novel Ring Oscillator (RO) accelerometer is studied and fabricated in this dissertation. The sensitive unit of this accelerometer is MOS ring oscillators located on silicon beams. The output of the accelerometer is frequency signal. Mixer is used as interior signal processor in order to improve characteristics of the output signal. The accelerometer is made up of two MOS ring oscillators with the same electrical parameters. The two ring oscillators locate on the stress sensitive region of the accelerometer beams, with different orientation of MOSFET channel. The principle of RO accelerometer is based on the piezoresistive effect of ring oscillator. MOS ring oscillator consists of an odd number of inverting gates connected in a ring. The mechanical stress generated by the applied acceleration changes the mobility of the charge carriers in the MOSFET of the ring oscillator due to the piezoresistive effect. Hence, the syntonic frequency of the ring oscillator becomes a function of acceleration. When acceleration applied, the syntonic frequencies of these two ring oscillators would change differently. The differential frequency is got from subtracting the syntonic frequency of these two ring oscillators. The subtracting circuit is realized by mixer in this dissertation.A novel 6-beam-mass structure of the accelerometer was studied and designed by FEA software ANSYS. The dimensions of mechanical structure were studied through ANSYS simulating. We analyzed and designed three kinds of RO: E/D NMOS RO, E/D PMOS RO and CMOS RO. The circuits of these three kinds of RO were studied by SPICE software OrCAD. The parameters of ring oscillator such as threshold voltage and W/L, were designed according to the simulation results. A novel Double Gate (DG) MOS mixer was studied and designed by OrCAD. For sine wave, the differential frequency of two inputs is got perfectly from the output signal of the DG MOS mixer.A novel fabrication process of RO accelerometer was studied and simulated by semiconductor process and device simulation software SILVACO. Using this process, the circuits of RO and mixer can be fabricated together with the bulk micromachining process that forms the mechanical structure of RO accelerometer. We designed the layout of RO accelerometer using L-EDIT. A convex corner compensation structure is presented in this dissertation. Using this structure, a convex corner was fabricated very well through KOH etch.The sample chip of RO accelerometer was fabricated in the MEMS lab of Institute of Microelectronics, Tsinghua University (IMETU). The characteristics of the sample chip were in fairly good agreement with the theoretical predictions. The test results of the sample chip indicate that the RO accelerometer has many perfect characteristics such as high sensitivity, low temperature coefficient and simple fabrication process. It will be used in many appliances in the future.