The Optimum Design Of High Performance CMOS CO Sensor

The ultimate goal of wireless sensor networks (WSN) is to develop ultra-low-power and miniature devices that can be widely distributed in remotely area. The monolithic integration of sensors ensures mass manufacturing with low-cost. Our primary goal is to design a gas sensor integration with readout circuit on same chip which can be easily implemented with post-CMOS process steps and satisfy the demand of WSN. The selectivity, stability, power, noise and compatibility with CMOS process issues are presented in this thesis.A unique phenomenon that the surface temperature of platinum film oscillates when it is exposed to carbon monoxide can be facilitated to enhance the selectivity significantly, which can identify carbon monoxide exclusively among the gas mixture. However, it can't quantify the gases up to now. An approach is proposed to establish the relationship between the oscillation period and CO concentration based on non-linear system theory.The theoretic simulation results agree with the experiment results.The main challenge for gas sensor is the design of a micro-hotplate structure providing stable and uniform temperature to ensure a high sensitivity and selectivity to gases. Finite difference algorithm is developed to solve the problem of sensor temperature stability under ambient temperature interference. The simulation results are verified by PDEtool and ANSYS. It also reveals the uniformity of temperature profile on the sensing layer less than 0.4℃variation. A sub-threshold voltage circuit is proposed to adjust the sensor temperature to self-adapt to changes in environment temperature.It is vital for WSN life to lower the power of sensors. It is decreased by designing sub-threshold voltage circuit and MPH and adopting low operation temperature of sensor. The total power consumption of sensor and temperature-controlled circuit is about 332.9μW and 13.9μW, respectively.Furthermore, the circuits can operation stably immune to the variation of battery voltage. To further lower the power consumption, SON-MOSFET heater replaces the resistor heater, which monitors the temperature simultaneously. It isn't suitable for cmos amplifier to amplify the faint low-frequency small signal of the CO sensor, which is vulnerable to very serious 1/f noise problems. Noise reduction techniques are adopted by forward body bias or thinning oxide to reduce the trapped charge in gate oxide. The threshold voltage reduction means are also proposed.Aluminum was protected during Silicon anisotropic etching process Using TMAH with (NH4)2S2O8 and Si power, and lift-off was implemented successfully using reverse-pattern process and E-beam evaporation, which enable it compatible with CMOS process. Nano-size grain, 6~58nm, is obtained which helps to enhance gas sensitivity.