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Gerald Gerlach is currently a Professor for Solid-state Sensors at the Dresden University of Technology, Germany, a post he has held since 1996. His research interests include micromachined solid-state sensors (pressure, humidity, chemical) and sensor fabrication techniques, and he teaches courses in Microtechnology and Sensors. He has also held positions as a Researcher at different German companies making micromachined pressure sensors for biomedical applications, and is active in the sensor and measuring technology fields, having been Chairman of the German Association of University Professors in Measuring Technology (AHMT) and Vice-President of the German Society for Measurement and Control (GMA) since 2002. He has co-authored the German version of this book Einführung in die Mikrosystemtechnik (Hanser, 2006) and has contributed chapters to the book Functional Elements in Precision Engineering (Hanser) and Fabrication in Precision Engineering and Microtechnology (Hanser, 1995). He has also written over 250 journal and conference papers, and holds more than 35 patents.

Wolfram Dötzel is currently Professor for Microsystems and Precision Engineering at Chemnitz University of Technology, also holding the position of Vice-President for Research at the university. His main research fields are in the modelling, design and simulation of micromechanical components, characterization and testing of micromechanical components by experimental methods, and adaptation of methods and principles of precision engineering for microsystems. He teaches courses in Microsystems, reliability and the design of devices and has previously co-authored Einführung in die Mikrosystemtechnik (Hanser, 2006) with Gerald Gerlach. He has also authored a chapter in the book Manual of Data Acquisition (Verlag Technik, 1984), more than 130 publications in journals and conference proceedings on micromechanical and precision engineering components as well as modelling, simulation, and characterization, and holds 8 patents.

De la contraportada

Over half a century after the discovery of the piezoresistive effect, microsystem technology has experienced considerable developments. Expanding the opportunities of microelectronics to non-electronic systems, its number of application fields continues to increase. Microsensors are one of the most important fields, used in medical applications and micromechanics. Microfluidic systems are also a significant area, most commonly used in ink-jet printer heads.

This textbook focuses on the essentials of microsystems technology, providing a knowledgeable grounding and a clear path through this well-established scientific dicipline. With a methodical, student-orientated approach, Introduction to Microsystem Technology covers the following:

• microsystem materials (including silicon, polymers and thin films), and the scaling effects of going micro;
• fabrication techniques based on different material properties, descriptions of their limitations and functional and shape elements produced by these techniques;
• sensors and actuators based on elements such as mechanical, fluidic, and thermal (yaw rate sensor components are described);
• the influence of technology parameters on microsystem properties, examing, for example, the robust and safe operation of a microsystem.

The book presents problems at the end of each chapter so that you may test your understanding of the key concepts (full solutions for these are given on an accompanying website). Practical examples are included also, as well as case studies that enable a better understanding of the technology as a whole. With its extensive treatment on the fundamentals of microsystems, this book also serves as a compendium for engineers and technicians working with the technology.

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Introduction to Microsystem Technology

John Wiley & Sons

Chapter One

A radical change in the entire field of electronics began in 1947 when the transistor was invented; 11 years later in 1958 the first integrated semiconductor circuit was built. Ever since, electronics has turned almost completely into semiconductor electronics. Microelectronic manufacturing methods make it possible simultaneously to produce large numbers of similar components with dimensions that are much too small for precision mechanics. The discovery of the piezoresistive effect in 1953 (Figure 1.1, Table 1.1) created the precondition for also applying semiconductor materials and microelectronic production methods to non-electronic components. The first description of how to use a silicon membrane with integrated piezoresistors as mechanical deformation body dates back to 1962.

Uncountable, new miniaturized function and form elements, components and fabrication procedures have since been introduced, combining electrical and non-electrical functions and using semiconductor production technologies or even especially developed microtechnologies (Figure 1.2).

The term 'microsystem technology' has been used for a wide range of miniaturized technical solutions as well as for the corresponding manufacturing technologies and it has no universally acknowledged definition or differentiation.

Similar to microelectronics, the nonelectric domain uses the terms micromachining/ micromechanics, microfluidics or microoptics. Until the mid-1980s, the main focus of research and development was on miniaturized sensors, occasionally also on microactuators. Only after that were examples of complex miniaturized systems, such as micromechanical systems (MEMS) or microsystems, in general, introduced (e.g. gas chromatograph, ink-jet nozzles, force-balanced sensors, analysis systems).

1.1 WHAT IS A MICROSYSTEM?

This book will use the term 'microsystem technology' to mean the following:

Microsystem technology comprises the design, production and application of miniaturized technical systems with elements and components of a typical structural size in the range of micro and nanometers.

A microsystem can be characterized by the semantics of its word components 'micro' and 'system':

Components or elements of microsystems have a typical size in the submillimeter range and these sizes are determined by the components' or elements' functions. In general, the size lies in the range between micrometers and nanometers (Figure 1.3). Such small structural sizes can be achieved by directly using or adapting manufacturing methods of semiconductor technology as well as through specifically developed manufacturing processes that are close to microelectronics (Figure 1.4).

Recently, nanotechnology has started enjoying massive public attention. The prefix 'nano' is used there in two respects. On the one hand, nanotechnology can be applied to downscaling typical sizes, such as the thickness of function layers, from the micrometer down to the nanometer range. Today, typical gate thickness in microelectronic CMOS transistors is only a few dozen nanometers. Here, the term nanotechnology (nanoelectronics, nanoelectronic components) is used for an extremely diminished microtechnology where the known description and design procedures can be applied. On the other hand, the term nanotechnology is used for procedures and components which are only found at a certain miniaturized level. Examples are quantum effects (e.g. quantum dots) as well as tunnel effect devices or single-electron components. This textbook will not address such components.

Microsystems consist of several components which, in turn, consist of function and form elements (Figure 1.5). The components have specific functions, e.g. sensor, actuator, transformation, memory or signal processing functions and they can be constructively autonomous entities (e.g. an integrated circuit). Microsystems include both nonelectric and (micro-)electronic as well as electrical components. The system character is due to the fact that the system can only fulfil the total function if the components interact as a complex miniaturized unity.

Figure 1.6 shows the typical design of microsystems. Sensors and actuators as well as signal processing components that are suitable for system integration are - via appropriate interfaces - integrated with each other but also with the microsystem's environment, e.g. with a technical process that has to be controlled. The individual components consist each of a number of function and form elements that can be produced using corresponding materials and applying micro- and system technology. Microsystem technology is also used for the functional integration of the system components.

In summary, we can define 'microsystem' as follows:

A microsystem is an integrated, miniaturized system that

comprises electrical, mechanical and even other (e.g. optical, fluidic, chemical, biological) components;

is produced by means of semiconductor and microtechnological manufacturing processes;

contains sensor, actuator and signal functions;

comprises function elements and components in the range of micro- and nanometers and has itself dimensions in the range of micro- or millimeters.

This definition does not strictly distinguish between micro- and nanosystems. As microelectronics already uses ultrathin layers of only a few nanometers it has crossed the line to nanotechnology. Piezoresistive resistors are standard function elements in microsystem technology and they act as conduction areas for a two-dimensional electron gas if they are less than 10 nm thick. The resulting quantum effects lead to a substantial increase in the piezoresistive coefficients. Microsystems usually contain electrical and mechanical components as a minimum.

Thus, sensors have function elements for detecting non-electrical values (e.g. mechanical deformation values such as cantilevers or bending plates which are deformed by the effect of the measurand force or pressure), transformer elements for transforming the measurand into electrical values (e.g. piezoresistive resistors in the cantilever elements) as well as components for processing electrical signals. Vice versa, the same applies to electromechanical drives. Electrical functions und their corresponding microsystem components are used for signal extraction and processing as well as for power supply. At the same time, microsystems have - as a minimum - mechanical support functions, often even further reaching mechanical functionalities.

Coinciding with its purpose, a microsystem can also have other function elements in addition to the electrical and mechanical ones. The smallness of a microsystem's function components is often a prerequisite for applying a certain function principle. On the other hand, however, miniaturization makes a coupling to technical systems in our 'macroworld' more difficult. Therefore, complete microsystems often have dimensions in the range of millimeters which clearly facilitates their integration into other systems. Here the transition from the micro- to macroworld already takes place in the packaging of microsystems. However, even here the term microsystem is commonly used.

1.2 MICROELECTRONICS AND MICROSYSTEM TECHNOLOGY

The development of microsystem technology is the immediate result of microelectronics which shows two major drawbacks:

1. Microelectronics is limited to electronic devices and the integration of electronic functions. Usually, it is not possible to process non-electrical values. Building complex systems that are able to use sensors to read signals from the system environment and to affect the environment via actuators requires a combination of microelectronic components and classical components produced by precision mechanics. This reduces the miniaturizing potential and the level of integration that can be reached. Thus, reliability decreases.

2. Basically, the manufacturing process of semiconductor technology can only be used to produce two-dimensional but not three-dimensional structures. However, a number of functions - especially nonelectrical ones - require three-dimensional function components and their three-dimensional integration.

There are several reasons for the close connection of the development of microsystem technology with microelectronics:

Within microtechnologies such as micromechanics, microfluidics, microoptics etc., microelectronics has an outstanding position. Given the current state of the art, microsystems without microelectronic components for processing analogous or digital signals appear not to be meaningful.

Only semiconductor and thin film technology provides manufacturing processes that are able to produce structures in the range of micro- and nanometers. And there are additional advantages of microelectronic manufacturing processes that can be used: the parallel processing of identical elements or components within one and the same manufacturing process as well as the use of completely new physical-chemical procedures which differ substantially from classical manufacturing technologies.

Often, microsystem technology uses materials that are used in microelectronics. Both microsystem technology and microelectronics are dominated by silicon which has excellent characteristics in comparison with compound semiconductors, for instance. On the one hand, this is due to the fact that electronic components are very important in microsystems and therefore silicon particularly is qualified for integration technologies. On the other hand, silicon can be produced with the highest chemical purity and crystal perfection. And a large number of technological procedures and sensoric as well as actuating effects rely in particular on these crystal features.

Table 1.2 compares typical characteristics of microelectronics and microsystem technology. The given characteristics show that microsystem technology will even in the future mainly use microelectronic technologies. Large production numbers and a high standardization of components in microelectronics are due to the programming options of microprocessors and microcomputers as well as memory circuits. Therefore, silicon-based technology was able to attract development prospects that by far exceed those of microsystem technology. Due to the diversity and heterogeneity of microsystem technology, it will not be possible to find similarly standardized applications with similarly high production numbers. The only option here is to use highly developed fabrication methods of semiconductor technology. Original technological developments, such as the LIGA technology, are rather the exception. Currently, the following developments can be discerned regarding the use of microelectronic manufacturing methods:

transfer of two-dimensional structuring processes on to three-dimensional applications (e.g. surface and near-surface micromachining, see Chapter 4);

development of modified system integration technologies (e.g. packaging, see Chapter 5);

further development und adaptation of microelectronic design methods to complex heterogeneous systems, which are characteristic of microsystem technology (see Chapter 8).

1.3 AREAS OF APPLICATION AND TRENDS OF DEVELOPMENT

Initially, the development of microsystem technology was related to the study (1953-58) and commercialization (1958-72) of piezoresistive sensors [Gerlach05]. Since then, the range has become dramatically wider [Kovacs98]. In the beginning, the focus was on the advantages of miniaturization for new automotive applications (measuring manifold pressure of combustion engines in order to reduce emissions) and invasive biomedical sensors, for instance. Today, new areas of application are of major interest, allowing for large production numbers, low cost per unit and high reliability.

Currently, the following are important examples of the application of microsystems:

Automation technology: Modern cars contain a large number of new systems for improved driving safety and comfort. Microsystem technologies can be used to produce large volumes with low system cost and high reliability. Examples are acceleration sensors for ABS and airbag applications, yaw rate sensors for driving stability and airflow sensors for controlling air conditioning. More than half of all microsystem applications is used by the automotive industry. Medical technology: Microsystems with dimensions in the range of micro and millimeters can be widely used for invasive applications. Important examples are catheters for measuring heart pressure, probes for minimal invasive diagnostics and therapy as well as dosing systems.

Environmental technology, gene technology and biotechnology: Microanalysis and micro-dosing systems can be used for the chemical and biotechnological analysis of gases and fluids. Microreactors can be used for chemical processes involving very small volumes and other uncommon conditions. Microfluid systems: Ink jet nozzles can be produced at low cost using miniaturized integration of electrical, mechanical and fluidic functions.

Nanotechnology: The production, manipulation and characterization of nanostructures require tools for ultra-precise movement and positioning. Systems that are based on scanning tunneling and atomic force effects often use miniaturized cantilevers with tips in the range of nanometers. Microsystem technology can effectively produce such tools.

Considering the further development of microsystem technology, the following trends can be discerned:

Microsystem solutions require mainly applications with large production numbers.

The manufacturing of microsystems increasingly uses commercial semiconductor processes. The development of special technologies is only possible when large production numbers justify the costs or when there are no alternatives to microsystems and therefore a high per-unit price can be realized. This is the case of minimal-invasive medical applications, for instance.

Reliability and lifetime of microsystems as well as long-term stability and accuracy become ever more important, particularly regarding industrial applications of chemical and biological sensors and analysis systems.

The economic rather than the technological framework decides which integration technologies are used for producing microsystems. Whereas in the past the monolithic integration was a main goal, today almost exclusively hybrid integration is used for smaller production numbers. Nevertheless, there are substantial efforts to further develop monolithic integration methods, in particular those that try to integrate microtechnologies into commercial semiconductor manufacturing processes (e.g. CMOS processes). The three-dimensional design and integration of alternative methods of microsystem technology then takes place mainly as a back-end process following the conventional microelectronic manufacturing process.

Microsystems are based mainly on microelectronic materials and methods. Therefore, a main issue of the development of microsystems is the design process. Currently, there is a substantial need for developing appropriate design tools for modeling and simulating complex and heterogeneous systems (see Chapter 8).

1.4 EXAMPLE: YAW RATE SENSOR

In the following, we want to use an example to present a complex microsystem. We will show how a microsystem is constructed with various components and function elements. We also want to show that the characteristics of the material, manufacturing technology and design are closely interconnected.

1.4.1 Structure and Function

The rotating speed [OMEGA] (yaw rate) is an important parameter of bodies moving in space. Yaw rate sensors are therefore very important for the driving stability of cars. Figure 1.7 and Figure 1.9 show a yaw rate sensor that is based on the Coriolis principle.

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