This research explores a novel in-situ technique for monitoring film thickness in the reactive etching process that incorporates a micromachined sensor. The sensor correlates film thickness with changes in resonant frequency that occurs in the micromachined platform during etching. The sensor consists of a platform that is suspended over drive and sense electrodes on the surface of the substrate. As material is etched from the platform, its resonant vibrational frequency shifts by an amount that is proportional to the amount of material etched, allowing etch rate to be inferred. This RIE monitoring methodology exploits the accuracy of resonant micromechanical structures, whereby shifts in the fundamental resonant frequency measure a physical parameter. A majority of these systems require free-standing mechanical movement and utilize a sacrificial layer process as the key technique to develop and release the structure on a substrate. A sacrificial layer technique that incorporates a low temperature sacrificial polymer was utilized to develop and release the suspended RIE sensor with excellent performance and is capable of fabricating other low cost, high performance and reliable suspended MEMS devices. The integration of sensors and electronic circuitry is a dominant trend in the semiconductor industry, and much work and research has been devoted to this effort. The RIE sensor relies on capacitive transduction to detect small capacitance changes and the resulting change in resonant frequency during the RIE process. The RIE sensor's overall performance is limited by the interface circuit, and integration with the proper circuit allows the RIE sensor to function as a highly sensitive measure of etch rate during the RIE process. A capacitive feedback charge amplifier interface circuit, when configured with the RIE senor at the input achieves very low noise sensing of capacitance changes and offers the potential for wide dynamic range and high sensitivity. As an application vehicle, process control was demonstrated in the PlasmaTherm SLR series RIE system located in the Georgia Tech Microelectronics Research Center.

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Hypersonic flow is both fascinating and intriguing mainly because of presence of strong entropy and viscous interactions in the flow field. Notwithstanding the tremendous advancements in numerical modeling in the last decade separated hypersonic flow still remains an area where considerable differences are observed between experiments and numerical results. Lack of reliable data base of surface static pressures with good spatial resolution in hypersonic separated flow field is one of the main motivations for the present study. The experiments in hypersonic shock tunnels has an advantage compared to wind tunnels for simulating the total energy content of the flow in addition to the Mach and Reynolds numbers. However the useful test time in shock tunnels is of the order of few milliseconds. Hence in shock tunnel experiments it is essential to have pressure measurement devices which has special features such as small in size, faster response time and the sensors in array form with improved spatial resolutions. Micro Electro Mechanical Systems (MEMS) is an emerging technology, which holds lot of promise in these types of applications. In view of the above requirement, MEMS based pressure sensor array was developed to measure the static pressure distribution. The study is comprised of two parts: one is on the development of MEMS based pressure sensor array, which can be used for hypersonic application and other is on experimental static pressure measurement using MEMS based sensors in separated hypersonic flow over a backward facing step model. Initially a static pressure sensor array with 25 sensors was developed. The static calibration of sensor array was carried out to characterize the sensor array for various characteristic parameters. The preliminary experimental study with cluster of 25 MEMS sensor array mounted on the flat plate did not provide reliable and repeatable results, but gave valuable inputs on the typical problems of using MEMS sensors in short duration hypersonic ground test facilities like shock tunnels. Incidentally, to the best of our knowledge this is first report on use of MEMS based pressure sensors in hypersonic shock tunnel. Later cluster of 5 sensor array was developed with improved electronic packaging and surface finish. The experiments were conducted with flat plate by mounting 5 sensor array shows good agreement in static pressure measurement compared with standard sensors. In the second part of the study a backward facing step model, which simulates the typical gasdynamic flow features associated with hypersonic flow separation is designed. Backward facing step model with step height of 3 mm was mounted with sensor array along the length of model. Just after the step, static pressure measurements were carried out with MEMS sensors. It is important to note that, in the space available in backward facing step model we could mount only one conventional Kulite pressure transducer. The experiments were conducted at Mach number of 6.3 and at stagnation enthalpy of 1.5 MJ/kg in hypersonic shock tunnel (HST-5) at IISc. Based on the static pressure measurement on backward facing step, the location of separation and reattachment points were clearly identified. The static pressure values show that reattachment of flow takes place at about 7 step heights. Numerical simulations were carried out using commercial CFD code, FLUENT for flat plate and backward facing step models to compliment the experiments. The experimental tests results match well with the illustrative numerical simulations results.

Now-a-days sensors are not limited only to industry or research laboratories but have come to common man’s usage. From kids toys to house hold equipment like washing machine, microwave oven as well as in automobiles, a wide variety of sensors and actuators can be easily seen. The aim of the present thesis work is to discuss the design, development, fabrication and testing of miniaturized piezoresistive, absolute type, low pressure sensor and flow sensor. Detailed performance study of these sensors in different ambient conditions (including harsh environment such as radiation, temperature etc.) has been reported. Extensive study on designing of thin silicon diaphragms and optimization of piezoresistor parameters is presented. Various experiments have been performed to optimize the fabrication and packaging processes. In the present work, two low range absolute type pressure sensors (0-0.5 bar and 0-1 bar) and a novel flow sensor (0-0.1 L min-1) for gas flow rate measurement are developed. The thesis is divided into following six chapters. Chapter 1: It gives a general introduction about miniaturization, MEMS technology and its applications in sensors area. A brief overview of different micromachining techniques is presented, giving their relative advantages and limitations. Literature survey of various types of MEMS based pressure sensors along with recent developments is presented. At the end, the motivation for the present work and organization of the thesis is discussed. Chapter 2: In this chapter, various design aspects of low, absolute type pressure sensors (0-0.5 bar and 0-1 bar) are discussed in detail. Static analysis of the silicon diaphragms has been carried out both analytically as well as through finite element simulations. Piezoresistive analysis is carried out to optimize the piezoresistor dimensions and locations for maximum sensitivity and minimum nonlinearity. All the Finite Element Analyses (FEA) were carried out using Coventorware software. A novel approach for the selection of resistor parameters (sheet resistance, length to width ratio) is reported . Finally, the expected performance of the designed sensors is summarized. Chapter 3: This chapter is divided into two parts. The first part presents the fabrication process flow adopted to develop these low range absolute pressure sensors. Two fabrication process approaches (wet etching and dry etching) which are used to fabricate the thin diaphragms are discussed in detail. Following an overall description, various aspects of the fabrication are elaborated on, like mask design, photolithography process, ion-implantation, bulk micromachining and wafer bonding. The required parameters for implantation doses, annealing cycles, low stress nitride deposition and anodic bonding are optimized through extensive experimental trials. The second part of this chapter discusses about the different levels of packaging involved in the realization of pressure sensors. Finite Element Analyses (FEA) of Level -0 and Level-1 packages has been carried out using ANSYS software to optimize the packaging materials. Exhaustive experimental studies on the selection of die attach materials and their characterization is carried out. Based upon these studies, the glass thickness and die-attach materials are selected. Chapter 4: The chapter discusses the measurement of the fabricated devices. The wafer level characterization which includes I-V characterization, measurement of offset and full scale output is discussed first. And then the temperature coefficient of resistance and offset is measured at wafer level itself. The performance characteristics like sensitivity, nonlinearity, hysteresis and offset of packaged pressure sensors is presented for all the variants (0.5 bar and 1 bar sensors fabricated by KOH and DRIE process) and their comparison with simulated values shows a close match. The measurement of dynamic characteristics using in-house developed test set-up are presented. The next section discussed detailed study about the stability of the developed sensors. The last part of this chapter reports the harsh environment characterization of the sensors viz. high temperature, humidity exposure, radiation testing etc. Chapter 5: The development of a novel micro-orifice based flow sensor for the flow rate measurement in the range of L min-1 is presented in this chapter. The sensing element is a thin silicon diaphragm having four piezoresistors at the edges. A detailed theoretical analysis showing the relationship between output voltage generated and flow rate has been discussed. The flow sensor is calibrated using an in-house developed testing set-up. Novelty of the design is that the differential pressure is measured at the orifice plate itself without the need of two pressure sensors or u-tube which is required otherwise. Chapter 6: This chapter summarizes the salient features of the work presented in this thesis with the conclusion. And then the scope for carrying out the further work is discussed.