Introduction to Microsystem Design

Frontmatter

Introduction

Abstract

This book was developed from lectures held at RWTH Aachen University, Germany and Tsinghua University at Beijing, P.R. China. It may be used as a basis for similar lectures on designing of microsystems. For this purpose, it is recommended to follow the sequence of this book, because it is arranged such that following chapters are building up on previous ones. Students are also strongly recommended to solve the problems included in this book, because this is important for getting familiar with the units and orders of magnitude to be expected in microtechnique. Besides this, often the relevance of the lessons becomes clear much more, when an example calculation shows the importance of the subjects taught.

Werner Karl Schomburg

Scaling Laws

Abstract

If a cube is to be separated into smaller ones, in such a way, that each edge is cut into two equal pieces (cf. Fig. 1), three cuts through the cube need to be made. Each cut generates a surface area equal to the surfaces of the original cube parallel to the cut. All together, the overall surface is duplicated while the volume remains to be the same. If each edge is cut into n pieces, the surface is increased by a factor n. Thus, the ratio of surface to volume is changing by the inverse of the factor s_f by which the dimensions of a cube (and in general any object) is scaled up or down.

Werner Karl Schomburg

Elastic Deformations

Abstract

Before the mechanics of basic elements of microtechnique is described, it is necessary to introduce the main parameters which govern the elastic deformation of rigid bodies under the action of forces. There are mainly three parameters: Stress due to straining, residual stress, and bending.

Werner Karl Schomburg

Thin Films

Abstract

Thin films are an important basic element of microsystem design. They are used for masking in etching processes, as a diffusion stop layer, and as functional elements such as electrical conductors, membranes, and beams. Thin films may consist of nearly every rigid material. Typical examples are metals, polymers, oxides, and nitrides. The thickness of thin films typically is in the range of 50 nm – 10 μm. The lower limit is due to the problem that layers with an average thickness of less than 50 nm hardly are made homogeneously, because they tend to form separated clusters. The upper limit is a kind of convention. Films that are thicker than 10 μm are no longer considered to be thin in microtechnology, and they cannot be generated easily by processes such as sputtering and evaporation but need to be produced by, e.g., electroplating.

Werner Karl Schomburg

Conductor Paths

Abstract

Conductor paths are either thin films on a substrate patterned to get the shape of the desired paths or the substrate itself is made conductive along the desired paths. The latter is obtained by doping of a semiconductor. Doping is obtained by techniques such as diffusion of chemical species such as boron or phosphorus out of a gas phase into a semiconductor along the paths where a conductor is to be formed or ions are implanted there. For more details on doping of semiconductors, refer books on micromachining such as refs. [3–6].

Werner Karl Schomburg

Membranes

Abstract

Besides conductor paths, membranes are another special type of thin films. Membranes are an important mechanical basic element in microtechnique. They are the microscopic correspondence to macroscopic gaskets, bearings, and springs. They are made of silicon, oxides, nitrides, glasses, polymers, and metals. Their thickness typically is in the range of 0.5–500 μm. Membranes which are thinner than 0.5 μm are very hard to manufacture without holes and are generally not strong enough to withstand usual loads. The upper limit is given by the fact that thicker membranes are no longer a microscopic element. The lateral dimensions of membranes are typically in the range between 100 μm and 10 mm. Again the lower limit is defined by the possibilities of fabrication, while the upper limit is approximately the limit to the macroscopic world. However, all equations discussed here are valid in the macroscopic world also.

Werner Karl Schomburg

Strain Gauges on Membranes

Abstract

The deflection of membranes is most often measured by strain gauges on or in the membrane. The application used most frequently is the deflection of membranes in silicon pressure sensors. In pressure sensors, deflections of the membrane of less than a micrometer need to be detected and the strain generated by these deflections is on the order of 10⁻⁴. As a consequence, the resistance of conductor paths that are employed as strain gauges may change much more due to temperature changes than due to strain. Therefore, the deflection of such a membrane cannot be measured reliably without temperature compensation. The usual way for temperature compensation is to build up a Wheatstone bridge from two or four strain gauges as shown in Fig.  39.

Werner Karl Schomburg

Beams

Abstract

Beams are an important mechanical basic element in microtechnique. They are the microscopic correspondence to macroscopic bearings and springs. They can be made of nearly every rigid material such as silicon, oxides, nitrides, glasses, polymers, and metals. Their thickness typically is in the range of 1–500 μm. Beams which are thinner than 1 μm are very hard to manufacture and are not strong enough, in general. The upper limit is given by the fact that thicker beams are no longer a microscopic element. The width of beams is not much smaller than their thickness but may become a factor of 100 larger than it. When it is bent transversally, the dimension of the beam in the direction of bending is called its thickness and the dimensions perpendicular to this are called width and length. The length of beams typically is in the range between 10 μm and 20 mm. Again the lower limit is defined by the possibilities of fabrication, while the upper limit is approximately the limit to the macroscopic world. However, all equations discussed here are valid in the macroscopic world also.

Werner Karl Schomburg

Vibrations

Abstract

Vibrations of membranes and beams are important in microtechnique. The frequency range of pressure sensors and microphones is limited by the resonance frequencies of their membrane. In a similar way, the resonance frequency of beams limits the possible applications of acceleration sensors. On the other hand, the resonance frequency of a vibrating element may be proportional to the measurand and allow measurements less affected by noise.

Werner Karl Schomburg

Capillaries

Abstract

Microfluidic is one of the most promising fields of microtechnique. A variety of microfluidic devices has been developed such as micropumps (page 229), microvalves (page 203), pressure sensors (page 275), flow sensors (page 289), and analysis systems. The most important basic element of microfluidics is the capillary. It is needed to link components and to direct the flow of a fluid. The notion “fluid” includes both gasses and liquids. Besides this, capillaries can be employed for the separation of different ingredients.

Werner Karl Schomburg

Capacitive Forces

Abstract

Capacitive forces transform a voltage directly into a movement and they are insensitive to temperature changes. These are the main reasons why capacitive forces are employed so often in microtechnique.

Werner Karl Schomburg

Piezoelectric Effect

Abstract

The piezoelectric effect is widely employed in microtechnique. Especially, piezos used as an actuator are appreciated because of the large force which can be generated in a small volume. But piezos can be employed also as sensors which provide a large output voltage.

Werner Karl Schomburg

Thermal Actuators

Abstract

If the linear dimensions of a device are reduced, its mass decreases with the third power, while its surface decreases only proportional to the square (cf. page 3). As a consequence, the ratio of surface to volume or mass is very large for microdevices. This means, that a microdevice is heated up much more quickly and with less energy consumption, because of its small mass, and is cooling down more quickly due to its large surface to mass ratio. Therefore, heating up is a suitable actuation principle for a lot of microdevices, while it is not a good solution for macroscopic devices. The ink-jet printer is a good example (cf. page 251).

Werner Karl Schomburg

Microoptics

Abstract

Miniaturized optical components are very popular nowadays. For example, lenses in cellular phones need to be small and light weight and shall be equipped with an optical zoom. Micro-optical components are employed also in sensors, for data transmission, and chemical analysis.

Werner Karl Schomburg

Diffusion

Abstract

If two liquids are to be mixed in macroscopic applications, it is usual to generate some turbulence which facilitates mixing very much. Turbulence is achieved when the Reynolds’ number Re becomes larger than approximately 1,500. The Reynolds’ number is the ratio of inertial and friction forces in a flow. The inertial forces are described by the product of the density ρ _F of the liquid, a characteristic length L of the vessel or a rigid structure in interaction with the flow, and the velocity v of the flow. The frictional forces are described by the viscosity η of the fluid:

$$\rm Re: = \frac{{{\rho_{\rm{F}}}\ L}}{\eta }v. $$

(335)

Werner Karl Schomburg

Microvalves

Abstract

Microvalves have a lot of potential applications. They can be employed for dosing of small liquid volumes, controlling pneumatically or hydraulically driven robots or machines, and as a pilot valve which switches a larger valve.

Werner Karl Schomburg

Micropumps

Abstract

The main applications for which micropumps have been developed are drug delivery and taking and transporting of samples for analysis. It is much easier to build a micropump than a microvalve, and much more publications exist on micropumps than on microvalves. The problem with micropumps is that up to now there is no large market for them. Many tasks which could be done by micropumps can be done by even easier devices. For example, drug delivery can be done by an infusion bottle hanged up.

Werner Karl Schomburg

Microdosing

Abstract

Dosing of small amounts of liquids is a widespread application of microfluidics. The most common application is ink-jet printers, but dosing of adhesives and lubricants, and chemical and biological substances in pharmaceutical industry also occur often. Obviously, smaller amounts of a liquid can be dosed precisely when miniature devices are used.

Werner Karl Schomburg

Analogies of Physical Domains

Abstract

In microtechnique, often several basic components are combined to devices and several devices to systems. The basic elements employed in a system can belong to different physical domains such as mechanical, fluidic, electrical, and thermal. This chapter describes how to calculate the behavior of coupled elements and systems with the help of analogies.

Werner Karl Schomburg

Mechanical Devices for Electronics

Abstract

Electromechanical switches, also called relays, are devices which are designed to switch an electrical current by a voltage employing some mechanical means. For most applications, currents are switched by a transistor and this is the cheapest known way to control the flow of a current.

Werner Karl Schomburg

Pressure Sensors

Abstract

Pressure sensors play an important role in microsystem industries. They are employed in a lot of different application such as triggering for side air bags and monitoring tire pressure in cars, detecting the elevation for hiking or paragliding, and flow measuring (cf. page 299f).

Werner Karl Schomburg

Flow Sensors

Abstract

Flow sensor general The measurement of volume flow or flow velocity has a lot of applications in modern life. For example, for the control of the engine of a car with low exhaust emission, it is essential to know the fuel and air flow to the engine. In industry, it is necessary to apply well-defined amounts of lubricant into the bearing of gear wheels of watches or to dose glue onto small areas next to positions where no bond is desired. It is evident that, in chemical and pharmaceutical industry, mixing of substances in the correct ratio is important.

Werner Karl Schomburg

Inertial Sensors

Abstract

Inertial sensors are both sensors of acceleration and yaw rate (angular velocity). The first large application of Acceleration sensor general acceleration sensors was triggering air bags in cars. Nowadays, there are a lot of other applications such as the detection of the direction in which a cellular phone is held, controllers of computer games, and detection of the position of a GPS device as long as the contact to the satellites is lost.

Werner Karl Schomburg

Backmatter