Research On CMOS Compatible Integrated Humidity Sensor

A CMOS compatible integrated humidity sensor has been designed, modeled, fabricated, packaged and tested in this thesis.Humidity sensors are widely used in military affairs, meteorology, agriculture, industry (especially in textile, electronics and foods), medical treatment, architecture and home appliances. With the high development in CMOS technology and MEMS (micro-electro-mechanical-systems), the low cost, batch fabrication IC technology has inspired the development of CMOS compatible sensor elements integrated with the measurement circuits on the same silicon chip. It stands for the trend of future intelligent sensors.Chapter 1 mainly describes the background, phylogeny and future trend of the humidity sensor, the main task is presented at the end of this chapter. Chapter 2 firstly introduces how to express relative humidity, looking back to several traditional humidity sensors, including hair hygrometer, dew point hygrometer and psychrometer, modern miniature humidity sensors based on CMOS and MEMS technique are emphasized at last, for example cantilever humidity sensor, silicon piezoresistive humidity sensors, integrated humidity sensor based on thermal conduction and capacitance variation. Their advantages and disadvantages are compared, considering the feasibility of the fabrication, CMOS processing plus simple post-processing is chosen to fabricate the integrated humidity sensor. In Chapter 3 a capacitive humidity sensor which is an interdigital lateral capacitance structure formed by the aluminum layers is thoroughly analyzed, the sensing model is developed, the sensors'performance is simulated using Coventorware software, the post-processing is of great importance which can determine the final performance of the sensor, so this chapter pays much attention on it. The CMOS compatible humidity sensor is tested in Chapter 4, including sensitivity, hysteresis, temperature coefficient, stability and response speed. In Chapter 5, several micro-capacitance detection circuits are introduced firstly, such as capacitance-frequency conversion circuit, capacitance-current conversion circuit and capacitance-voltage conversion circuit. Through comparing these circuits, switched-capacitance circuits is chose at last, the design flow of the circuit is analyzed particularly, the performance of the circuit is simulated using Orcad-Pspice software. The layout of the integrated humidity sensor is designed in Chapter 6, and fabrication processing flow is presented accordingly. The post-processing mainly includes deposition of polyimide, etching and heating periodically. The integrated humidity sensor is simply packaged using PVDF membrane as filter to against particles and other contamination in the air in Chapter 7. then the sensor is tested, and the results are as following: the resolutions of the circuits at 5℃,15℃,25℃å'Œ35℃are 5.32 mv/%RH,6.00 mv/%RH,6.67 mv/%RHå'Œ8.01mv/%RH respectively, and long stability is good. Chapter 8 gives the summary and expectation of this thesis.