© 2010 Ravindra Mukhiya.
IIT Kharagpur
C
HAPTER1
INTRODUCTION
This chapter starts with introduction to Micro-Electro-Mechanical Systems (MEMS) and a brief on the applications of MEMS accelerometer sensors. In subsequent sections the research motivation and the objectives are discussed followed by the contributions made in this thesis. Finally, the organization of the dissertation is described.
© 2010 Ravindra Mukhiya.
IIT Kharagpur
© 2010 Ravindra Mukhiya.
IIT Kharagpur
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1.1 Introduction
Evolutionary development of integrated circuit (IC) and micromachining technologies in the past few decades is the key factor in the development of today’s high-technology systems. Integrated circuits have provided compact, reliable, low power and low cost solutions for complex electronic systems by miniaturization techniques. These techniques are then adapted to the fabrication of miniaturized actuators and sensors allowing miniaturization of the complete electromechanical systems. Such advanced systems have physical dimensions in the range of micrometers and classified as Micro-Electro- Mechanical Systems (MEMS). MEMS technology has opened up a wide variety of potential applications not only in the inertial measurement sector, but also spanning areas such as communications, biomedical and computer peripherals [1]-[4].
MEMS inertial sensors, accelerometers and gyroscopes, started to appear in the market due to their low cost, small size, low power consumption and promising performances [1]. Inertial sensors have wide varieties of applications in military such as navigation, guided missile and tracking [1], [2]; industrial applications like testing and conditioning of equipment [5], [6]; medical applications include heart monitoring and physical body motion [3], [4]; consumer applications involve automobile, entertainment and consumer navigation applications [4], [7].
In this thesis work, research has been focused on the design and development of a single-axis MEMS piezoresistive accelerometer using CMOS (Complementary Metal Oxide Semiconductor) compatible bulk micromachining technique. The current work includes the design of the MEMS accelerometer and its CMOS compatible fabrication for low cost consumer or industrial applications. Post-process CMOS compatible wet chemical bulk micromachining makes it suitable for on-chip integration with electronics and facilitates low cost batch fabrication. MEMS accelerometer based a novel fuel control unit has been proposed and designed. The proposed control unit uses MEMS accelerometer to measure acceleration and velocity, which controls the fuel injection electronically to ensure the complete combustion. It reduces the automobile pollution by
© 2010 Ravindra Mukhiya.
IIT Kharagpur
controlling the amount of unburnt hydrocarbons, nitrogen oxides and carbon mono- oxides [8], [9].
1.2 Research Motivation
MEMS inertial sensors have varieties of applications ranging from low-g to high-g with low-grade to high-grade performance devices. Most of these devices have been realized mainly by bulk micromachining and surface micromachining. Bulk micromachined devices are commercially available in the market but, most of them are in multi-chip module which makes it costly, because of added packaging cost of the electronic chip. On the other hand, the advancement of the technologies in the last decade made it possible to realize monolithically integrated surface micromachined devices. On-chip integration and smaller size make the surface micromachined devices cheaper than their counter part bulk micromachined devices. However, surface micromachined devices suffer from the technological limitations of beam and mass thickness, which puts constraints on the device dimensions and design. In bulk micromachining as such there is no technological limitations on the device dimensions and large mass can be realized which increases the sensitivity of the devices. This gives freedom to the design and also meets the desired specifications accurately for high-grade performance devices.
The major constrain of the bulk micromachined devices is its monolithic integration with the electronics. This is the prime motivation for the thesis work. Hence, research has been carried out to design and develop a low cost CMOS compatible bulk micromachined accelerometer. Accelerometer inertial sensor is opted in this work which has new application in automobile fuel control system.
The most promising concern for the automobile industry, these days, is efficient fuel utilization and compliance with the stringent emission norms. Air pollution is caused by the excess fuel, which is not burnt completely, is left in the combustion chamber of the engine [10]-[14]. The problem of air pollution due to automobiles has become very prominent and steps towards curbing it are being taken now by both governments as well as automobile companies, in many countries. The Society of Automotive Engineers
© 2010 Ravindra Mukhiya.
IIT Kharagpur
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(SAE) which consists of major automobile companies of the world like Ford, Toyota, BMW etc. puts “increasing fuel efficiency” and “reducing pollution from automobiles”
among its top most priorities [15].
Keeping in mind the number of cars running on roads that keep on increasing day by day, the pollution can be decreased substantially by decreasing harmful emissions even by a little amount. Similarly, improving on fuel efficiency in the cars can save a lot of fuel. These two facts are key factors behind the current research work leading towards the development of a novel fuel control system using accelerometer.
1.3 Research Objectives
The aim of the research work is towards the development of a smart sensor, which consists of a sensor integrated with the electronics on a single chip. In this context, this work is a kind of prototype in order to realize a full custom integrated and compact design. The specific objectives of this thesis to achieve the final goal are listed as follows:
¾ Electromechanical design and simulation of a single-axis MEMS piezoresistive accelerometer.
¾ Development of a low cost, CMOS compatible wet chemical bulk micromachining process for realization of the micro accelerometer structure.
¾ Fabrication of the designed accelerometer using post-process CMOS compatible wet bulk micromachining technique.
¾ Characterization of the fabricated accelerometer for static and dynamic behavior.
¾ Design and simulation of an accelerometer based fuel control unit for automobile applications.
1.4 Contributions of the Thesis
Major contributions of the thesis are summarized here:
© 2010 Ravindra Mukhiya.
IIT Kharagpur
1. Mathematical modeling, design and FEM simulation of a single-axis bulk micromachined piezoresistive accelerometer are presented. Based on simple electrical equivalent model new expressions for frequency and time domain analysis have been derived. Analytically estimated and FEM simulated results are compared and analyzed critically.
2. Extensive experimental study has been carried out to realize quad beam symmetric accelerometer structure using CMOS compatible 25% wt. TMAH.
Corner compensation structures and their etch morphology study have been performed thoroughly. Based on experiments, new empirical design equations for corner compensation structures have been derived.
3. Accelerometer is fabricated using processes derived from standard CMOS technology. Accelerometer is realized by a new indigenously developed single- step double-sided bulk micromachining process using post-process CMOS compatible dual-dope 5% TMAH.
4. Characterization of the fabricated accelerometer is performed using new, non- conventional and non-destructive techniques. Nano-indentation technique is used for static and maximum load evaluation. Laser Doppler Vibrometer technique is used with base excitation using simple speaker for dynamic behavior.
5. A novel accelerometer based fuel control system is proposed. The proposed fuel control unit is designed and simulated using MATLAB Simulink module.
1.5 Organization of the Thesis
After this introductory Chapter, Chapter 2 describes the literature review. It summarizes the important milestones achieved in the development of MEMS piezoresistive accelerometers.
© 2010 Ravindra Mukhiya.
IIT Kharagpur
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Chapter 3 focuses on the design, mathematical modeling and simulation of a MEMS accelerometer prototype. The operating principle and working of an accelerometer have been explained in the beginning. Subsequently the design topology and considerations for fabrication imperfections have been discussed. Finally, the details of the electromechanical design and simulation of the MEMS accelerometer is presented.
Chapter 4 deals with the detailed experimental study of the CMOS compatible bulk micromachining process using TMAH for the realization of the accelerometer structure. The problem of corner undercutting associated with wet bulk micromachining to fabricate convex corner structure of accelerometer is investigated and reported.
Proposed corner compensation structures are studied and analyzed to protect convex corners and mesa structures.
Chapter 5 presents the fabrication of the accelerometer. The accelerometer configuration with sensing elements positioning and interconnection is explained first.
Afterward, the fabrication process and process parameters are defined followed by post- process CMOS compatible bulk micromachining for aluminum passivation. Finally, the glass micromachining and bonding of the device to realize a three layer glass/silicon/glass accelerometer is described.
Chapter 6 reveals the characterization of the fabricated accelerometer. Simple, non traditional and non destructive techniques for static and dynamic testing using a Nano-indentor and Laser Doppler Vibrometer are explained. Experimental results obtained are furnished and analyzed critically.
Chapter 7 explains the design and simulation of an accelerometer based fuel control unit for automobile applications as a case study.
Finally, Chapter 8 gives the conclusions of the thesis and suggests future scope of the work to improve the device and system performance.
© 2010 Ravindra Mukhiya.
IIT Kharagpur
