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Über dieses Buch

This book on mechanical microsensors is based on a course organized by the Swiss Foundation for Research in Microtechnology (FSRM) in Neuchatel, Swit­ zerland, and developed and taught by the authors. Support by FSRM is herewith gratefully acknowledged. This book attempts to serve two purposes. First it gives an overview on me­ chanical microsensors (sensors for pressure, force, acceleration, angular rate and fluid flow, realized by silicon micromachining). Second, it serves as a textbook for engineers to give them a comprehensive introduction on the basic design issues of these sensors. Engineers active in sensor design are usually educated either in electrical engineering or mechanical engineering. These classical educa­ tional pro grams do not prepare the engineer for the challenging task of sensor design since sensors are instruments typically bridging the disciplines: one needs a rather deep understanding of both mechanics and electronics. Accordingly, the book contains discussion of the basic engineering sciences relevant to mechanical sensors, hopefully in a way that it is accessible for all colours of engineers. Engi­ rd th neering students in their 3 or 4 year should have enough knowledge to be able to follow the arguments presented in this book. In this sense, this book should be useful as textbook for students in courses on mechanical microsensors (as is CUf­ rently being done at the University ofTwente).

Inhaltsverzeichnis

Frontmatter

1. Introduction

Abstract
The use of silicon microsensors for pressure, acceleration, angular rate and fluid flow is increasing at high rates since micromachining has become a more or less mature technology. These sensors are used in great numbers especially in automobiles, process control, in the medical field and for scientific instrumentation. Market studies in the past years (mid nineties) have predicted an enormous increase in the need of these sensors. Recent predictions on market volumes of microcomponents (besides mechanical sensors ink jet printer heads and hard disk heads) are in the range of US$100 billion annually in Europe alone (Micromachine 1998).
Miko Elwenspoek, Remco Wiegerink

2. MEMS

Abstract
In many technical systems there is a strong trend for miniaturisation. This trend results on one hand from the fact that small components and systems perform differently: small systems can perform actions large systems cannot (example: minimal invasive surgery). In many cases a miniaturisation makes the systems more convenient (example: GSM telephone). On the other hand technology derived from IC-fabrication processes allows the production of miniature components in large volumes for low prices (examples: pressure sensors for automobile applications, ink jet printers).
Miko Elwenspoek, Remco Wiegerink

3. Silicon Micromachining

Abstract
The arsenal of technologies for silicon micromachining comprises photolithography, thin film deposition, doping, etching by wet chemicals and plasmas and waferbonding. Mainly for the sake of clarity and to describe the notions necessary to understand the limits and possibilities of micromachining we give a brief account of these fabrication methods. More information on silicon micromachining can be found in the literature mentioned in the text and in a few recent textbooks (Büttgenbach 1991, Elwenspoek 1999, Heuberger 1989, Kovacs 1998, Madou 1997, Menz 1993, Muller 1991, Muller 1999, Ristic 1994, Sze 1994, Tabib-Azar 1998).
Miko Elwenspoek, Remco Wiegerink

4. Mechanics of Beams and Diaphragms

Abstract
In this chapter we discuss and in part derive the equations that describe the mechanical deformation of construction elements found in mechanical sensors, as far as they are needed for the basic design of mechanical sensors. Derivations are given in cases that illuminate the basic ideas and methods of thinking about mechanics. In particular, we shall give derivations of the energy of a bent beam, including an axial load and stretching, and we shall show how partial differential equations can be found from energy expressions. These differential equations will give us the static and dynamic behaviour of membranes and beams. In many cases a closed analytical solution of the differential equation cannot be given, therefore we need ways to find reliable approximations. Therefore we also discuss how to obtain simple approximations for deflection amplitudes by energy minimisation. Finally we shall consider mechanical stability.
Miko Elwenspoek, Remco Wiegerink

5. Principles of Measuring Mechanical Quantities: Transduction of Deformation

Abstract
This chapter deals with the problem of the transfer of a deformation in a mechanical construction by an external load to an electrical signal. This is called “transduction”. The most important mechanisms for transduction of mechanical microsensors use the following effects: piezoresistivity, the dependency of the capacity on the geometric arrangement of conductors, piezoelectricity, optical resonance and optical interferometry. We concentrate here on the first two, most important, transducers. For flowsensors the thermal domain is of great importance, but we shall defer the discussion of transducers using heat to the chapter on flowsensors.
Miko Elwenspoek, Remco Wiegerink

6. Force and Pressure Sensors

Abstract
There are several techniques to measure forces and pressures. Very often, the force to be measured is converted into a change in length or height of a piece of material, the spring element. The change in dimensions is subsequently measured by a sensor element, e.g. a (piezo-) resistive or resonant strain gauge or a changing capacitance. This is illustrated in Fig. 6.1. Sometimes the sensor element and the spring element can not be distinguished, i.e. the sensor element itself is also the spring element. For example, in piezoelectric force transducers, the deformed crystal both supports the load and supplies the output signal. More sophisticated systems incorporate an electronic feedback to balance the external force or pressure by an equal but oppositely directed counterforce or pressure. The obvious advantage of such a system is that the spring element can be omitted, thus eliminating problems like linearity, creep and hysteresis related to the spring element. However, application of such systems is limited to relatively small forces and pressures because of the limited size of the counterforce or pressure that can be exerted.
Miko Elwenspoek, Remco Wiegerink

7. Acceleration and Angular Rate Sensors

Abstract
Micromachined inertial sensors, consisting of acceleration and angular rate sensors are produced in large quantities mainly for automotive applications, where they are used to activate safety systems, including air bags, and to implement vehicle stability systems and electronic suspension. Besides these automotive applications accelerometers are used in many other applications where low cost and small size are important, e.g. in biomedical applications for activity monitoring and in consumer applications such as the active stabilization of camcorder pictures.
Miko Elwenspoek, Remco Wiegerink

8. Flow Sensors

Abstract
Flow sensing is very complex. There are two distinct reasons for the complexity: First, flow is a science by itself and second, there are many phenomena that can be exploited for flow sensing. The designer of flow sensors must have a basic understanding of hydrodynamics. Micro flow sensors described so far are based on thermal principles or on the measurement of pressure distributions and shear stress. Optical and acoustical possibilities have not yet been explored.
Miko Elwenspoek, Remco Wiegerink

9. Resonant Sensors

Abstract
This chapter differs from the previous chapters in the sense that we will now discuss a class of sensors based on the operating principle of the sensor, i.e. resonant, and not on the measurand. In the previous chapters, resonant sensors have already been mentioned several times. This is not surprising as virtually every measurand can somehow be measured with a resonant sensor. In this chapter, we will present an overview of the operation and performance of resonating microsensors. A more complete discussion on the theory and applications of resonant sensors can be found in (Prak 1993) and (Wagner 1995). Other valuable papers discussing the operation of resonant sensors are (Tilmans 1992, Stemme 1991, Guckel 1990, Howe 1987). Resonant sensors are frequency output sensors and as such have several advantages:
  • The (digital) output signal can be directly connected to digital signal processing electronics. Analog to digital conversion is not necessary. Furthermore, a frequency signal can be transported over long distances with no loss of accuracy.
  • Large dynamic range. Time is by far the best measurand. Furthermore, the upper bound of the dynamic range is limited by the measurement time: increasing the measurement time automatically results in a larger dynamic range.
Miko Elwenspoek, Remco Wiegerink

10. Electronic Interfacing

Abstract
In this chapter we will discuss some basic topics with respect to the electronic interface circuits needed for microsensors. The chapter is divided in three parts, corresponding to the three most frequently used readout mechanisms, namely piezoresistive, capacitive and resonant.
Miko Elwenspoek, Remco Wiegerink

11. Packaging

Abstract
Packaging of microsensors presents special problems: part of the sensor requires environmental access while the rest may require protection from environmental influences. This is schematically illustrated in Fig. 11.1 (Senturia 1988). Due to this special role of the package, it is important that the package is designed simultaneously with the sensor. At the very least some time should be spent on examining the feasibility of a package. Otherwise, one may end up with a sensor that requires an extremely expensive package or cannot be packaged at all.
Miko Elwenspoek, Remco Wiegerink

Backmatter

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