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Modern Electronic

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Measuring Systems

P.P.L. Regtien

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Published and distributed by Delft University Press Mijnbouwplein II

2628 RT Delft The Netherlands Telephone (0) 15 783254

Cover by Ben Aalbers, Voorburg, The Netherlands lIIustrations by G. van Berkel

English text reviewed by Mrs. C.A. Piggins

No part of this book may be reproduced in any form, by print, photoprint, microfilm or any óther means without written permissionfrom the publisher.

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Preface

.

A measuring system without electric or electronic parts is becoming an exceptional phenomenon. Data acquisition and signal handling are executed for the most part by electronic means, and are encountered in almost any situation where information about physical and physiological quantities is asked for. The application area of electrontc measuring systems is gradually increasing, far beyond the boundaries of the electric domain. Besides thls, there is rapid progress towards increasing accuracy, sensitivity, sp~ed, flexibility and so on in the development of new measuring systems. The non-electronically skilled users of electronic instruments need to be regularly informed about these developments, in order to reap the benefits in their own field of activity.

This is why the Department of Electrical Engineering at the Delft University of . Technology organized, at the initiative of the Afdeling voor Electrotechniek van het Koninklijk Instituut van Ingenieurs (Department of Electrical Engineering of the Royal Institute of Engineers), a symposium on Modern Electronic Measuring Sys-tems. Cooperative organizations we re the Nederlands Elektronica- en Radiogenoot-schap (Dutch Society for Electronics and Radionics), the Institute of Electrical and Electronic Engineering Benelux Section, the Afdeling Technische Physica van het Koninklijk Instituut van Ingenieurs (Department of Technical Physics of the Royal Institute of Engineers) and the Sektie Toegepaste Natuurkunde van de Nederlandse Natuurkundige Vereniging (Section Applied Physics of the Dutch Society of Physics).

The main subject was divided into four··sub-divisions: transducers, signal conditio-ning, micro-processors and displays. The state of the art and recent developments of each of these were treated.

The large number of participants in the symposium justified a publication of the lectures in book-form. The text of the lectures are adapted for thls form.

Modem Electronic Measurement Systems intends to inform the reader ab out recent and possible future developments in electronic measuring systems, and will give an impression of the possibilities as well as the restrictions of electronics in measure-ments and instrumentation.

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Introduction

Paul P. L. Regtien

*

From the ongm of mankind people have given measures to the objects of their communication. These measures were, for understandability, related to 'standard' quantities, which often consisted of familiar objects that could easily be reproduced or were always available.

In the course of time the basic quantities remained the same, but the exponen-tially growing demand for infonnation required more and more measurement values of a great variety of things and with ever increasing precision.

Man's senses are inadequate for this, but his inventiveness and creativeness are not. Once the usefulness of auxiliary measuring means was discerned, much progress was made in the development of measuring instrurnents and measuring techniques. This progress has been enonnously accelerated by the invention of the means for electronic amplification.

The great impact of electronics on measuring instrurnents is evident: electronic measuring systems offer great advantages, such as large power gain (109 ), high speed (picoseconds), no wear, no din and minute size, over non-electronic systems. The few disadvantages, such as restricted temperature range, low reliability and limited power, can be overcome in most cases. So it is not surprising that electronic measuring systems have come to stay in almast every field of research, developmentand pro-duction.

An electronic measuring system is nothing but an infonnation channel in which the information about a physical quantity is translated into a perceptible and inter-pretabie signal. Measuring a physical quantity starts with the conversion of the non-electrical quantity into an electric one by a transducer. The transducer aften fonns the weakest element of the whole measuring system, mainly due to the non-electric part of it. For this reason many attempts are made to design and construct trans-ducers which are composed for the most part of electronic components and mate-riaIs. Because silicon is the most widely used material for electronic components nowadays, it may be used as the sensor element for facilitating the bridging from non-electric to electric signals. An attendant asset is the possibility of integrating the transducer together with the signal conditioning circuitry. Amplification or modula-tion of the transducer signals irnmediately after the sensor, lowers the susceptibility to disturbing signals from the environment, when transmitting the measuring signals via long cables.

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If the trend in the cost, size, flexibility etc. of IC's also applies, to silicon trans-ducers,' then a new break-through in measuring techniques would be expected. Al-though recent publications on silicon-based transducers argue in favour to this de-velopment, it must be realized that the applicability of silicon for transducers is restricted. Some physical effects are too weak in silicon (e.g. photon-emission) or even do not exist. For example, silicon shows no piezoelectric effect, according to its pure symmetric lattice. In such cases compatible technology can be employed. If this is not possible or impracticable, one should ignore the idea of fully integrated trans-ducers, and try to improve an existent transducer by electronic means.

The sensitivity of a tr,ansducer to other quantities than the quantity that has to be measured, can be eliminated by, for instance, electronically correcting the transducer output with the outputs of auxiliary, low cost transducers for the most strongly disturbing quantities.

A transducer may also suffer from non-linearity, improper frequency response, etc. There is a trend towards solving these problems not by optimally designing the sensor itself (which is almost optimal already), but by ad ding e1ectronic circuits to reduce the main draw-backs (hybrid transducers). With this point of view in mind each step in transduction and signal modification can be optimized by choosing the optimal materials, construction and techniques.

Before an electric signal suits its purpose, it may have to be transported, amplified, converted, etc. Whatever the signal's purpose may be, electronic circuits are able to do the transporting, amplifying, converting etc., accurately, quickly and for a rela-tively low price. Thanks to IC technology, a complete general purpose operational amplifier has become as cheap as a transistor, since several functions are welded together on one single silicon crystal, and the whole amplifier is fabricated in one run.

However, also in this case, the material and technology restrict the attainable specifi-cations. Noise and non-linearity determine the dynamic range, capacitances limit the speed of voltage variations, temperature differences and tolerances in the masks cause offset voltages and offset currents.

With well-chosen signal handling techniques, many of these problems can be avoided (or at least cut down to minor proportions). It may seem unlikely but, with 'ordinary' strain gauges and electronic components, length variations of Ie ss than' 50

-pm can be detected by proper detection techniques.

The information ab out physical quantities obtained in electrical form must become available for either man or machine. In the latter case, the information is used for directly controlling a process. Here we will confme ourselves to the observation of measurement data by men who use it for getting insight into (more or less complex) processes.

The amount of measuring data available at one time, however, may exceed man's intellectual grasp. In fact, he may easily be snowed under by an abundance of information. Fortunately, not all information that comes to his disposal is relevant, so it is appropriate to throw away the redundant or irrelevant data, before presenting it. Furthermore, we are not likely to be interested in all original measurement data, and we are certainly not interested in all data at every moment. Moreover, we do not always require the data to have the highest degree of accuracy. Time discrete

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(sam-pIed) and amplitude discrete (quantized) data will do in most cases.

Besides quantization, further data reduction may be applied, depending on the particular purpose. For instance, only the trend of a measuring quantity, or the mean value over a certain time intervall needs to be represented, or only the result of a numerical calculation on a set of data is of interest.

Especially when a large number of data is present, the processing is preferably done by microprocessors. The development of microprocessors is in a fast ascending lift, which is rising even more rapidly than that of its analogue counterpart, the operational amplifier. One reason for this is its wide applicability in data processing systems, particularly in industrial processes.

In analogue signal handling, the operational amplifier cannot operate on its own:

it has to be supplemented by extern al components for performing specific tasks. The same holds to an even greater extent for the microprocessor. This is just one com·

ponent (true, a complex one), so it can only function in cooperation with supple-mentary elements. Unfortunately, costs of the peripheral instruments can be more than tenfold that of the microprocessor itself.

This unfavourable ratio, however, may undoubtly be improved by large scale integration. More and more functions are inc1uded on one silicon chip, thus reducing the amount of necessary peripheral circuitry. However, the flexibility may thim also decrease, and a compromise must be found.

Nevertheless, the microprocessor can execute a tremen do us number of operations within a short time, alleviating the operator's tasks. For instance, microprocessors are indispensable for overseeing highly complex processes. Reduction of data, in a man-ner appropriate to the varying circumstances is necessary for having a good view of the process under con trol, and for taking the right action at the right moment in case of fallures. Early detection of failures can avert a disaster.

Measurement data has to be presented in a perceptibleform, which is primarily visually, e.g. in a pictorial way, with characters (as mere numbers or as numbers on a scale) or a combination of these. The system's output must be adapted to the observer's visual input, the eye. Considered from the instrument point of view the information is translated into a visually perceptible quantity, e.g. illumination or displacement. An output transducer delivering such quantities, in a certain pattern so as to represent the proper information, is called a display.

Many physical principles can be employed for display purposes. The choice be-tween them depends upon criteria such as power consumption, response time, size and price. From the observer's point of view, the information must not only be perceived but also interpreted, preferably with the least effort possible and in a comfortable manner. Consequently, in designing displays one should not only con· sider the physical and technological aspects but also the ergonomical aspects.

A measuring system cannot be composed by merely connecting the inputs and out-puts of a set of subsystems for each of the operating functions to be performed. Implementing a measuring system starts with a precise and complete formulation of the measuring problem: what quantity has to be measured? of what measuring ob-ject? in what circumstances (temperature range, vibrations, shocks)? what accuracy is required? how must the data be presented? Then a functional scheme of the mea-suring system can be set up, conformabie to the derived formulation.

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-To, any system property the following statement applies: the/system is not better than the weakest link. Therefore, each subsystem should also be compared with all

relevant items mentioned above.

--To interprete weIl the measurement data from the realized system it is necessary to know the system properties, especially its shorteomings (e.g. bandwidth, non-linearity, noise, temperature coefficients). For this purpose the system is tested and calibrated under the operating conditions, to get information ab out the system's behaviour. From this information, and that from the system output, the information ab out the measuring object is fmally obtained.

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I. S

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1. Int

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When analyzing the needs of mankind it appears, that these needs can be arranged in three groups. The first group is related in general to the need of mankind for mate-riais, the second group is related to its need for energy and the third group is related to its need for information (Fig. 1). The usual way for man to obtain information is to use one of his senses or to employ measurement equipment in order to process information in such a way, that it becomes detectable to his senses.

Such an information processing system can be regarded as a triptych (Fig. 2). In the identification unit at the left the information is identified, e.g. the information contained in a picture is identified by a TV camera, or the information contained in a coin is identified by the money detector of a coffee machine. In the identification

Fig. 1. The three main groups ofmankind's needs.

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IDENTI-FICATION UNIT MODIFI-CATION UNIT

Fig. 2. The triptych ofinformation processing.

PRESEN-TATION

UNIT

unit the information is modulated onto a carrier and nowadays this very of ten is an electric carrier e.g. a current, a voltage, a frequency, or a pulsewidth. However, mechanic, hydraulic, or pneumatic information carriers are still used for industrial -applications. In the modification unit at the center of the triptych the information is modified e.g. a voltage is amplified or is modified into a pulse frequency if the carrier is of an electric nature. In the modification unit the carrier is of the same nature at in- and output, only the shape changes. In a mechanical system a modification unit could consist of a simple gear-box.

In the presentation unit at the-right the information is transferred into such a form, that man can detect it directly through one of his five senses. In the TV monitor the picture is transferred into an optical signal which we can ob serve with our eyes, It is more difficuIt to see the presentation function with respect to the

\ coffee machine but we can say that the valve mechanism serves this function and that the cup of coffee represents a material, an energy, and most importly, a flavour, the last being perceived by our sense of taste. -

--The field of application for an information processing system is broader in engi-neering than indicated above [1]. Such a system can also be used to convert informa-tion into an acinforma-tion, such as the closing of a valve. In that case the presentation unit is replaced by an implementation unit (Fig. 3). The information can also be stored, and, in that case, the last part of the triptych consists of a storage unit. Finally the system can also convert the information into a form such that it can be transmitted. In that case we need a transmission unit. In Fig. 3 the different possibilities are shown in a block-diagram.

In this review we will concentrate on modern electronic measuring systems and therefore we will only discuss systems in which the right part of the triptych is a presentation unit and the center part is a modifier of only electronic signals.

An identification unit like a TV camera is a complex instrument containing ele-ments in which the information carrying optical signal is transduced into another, in this case electronic, signal and elements in which only electronic signal modification

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PRESENTATION

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STORAGE

TRANSMISSION

Fig. 3. Block-diagram ofpossible information processing systems.

takes place. It is also true that a presentation unit can contain not only the LED display but also peripheral electronic circuits.

In order to enable a c1ear insight into the different signal conversions in a me a-surement system, we will represent such a system by a chain of three blocks (Fig. 4).

In the input transducer, also often called senSor [2], the physical or chemical signal is converted into an electron ic signal. In the modifier the electronic signal is modified, as the name indicates, and in the output transducer the electronic signal is converted into a signal that can be received by at least one of our five senses. The modifier can be as simpIe. as an amplifier, but can also consist of a microprocessor or even a large scale computer. The advances made in integrated circuit technology have made it possible to make extremely sophisticated modifiers at a very reasonable price. It is obvious that attempts have been made in the past ten years to apply integrated silicon technology to transducers as weIl. Besides the advantage of using a weIl developed technology suitable for mass production it also allows the integration of the transducer with a part of the modifier on one Si chip. Silicon microtransducers (SMT's), as they çan be called, started to appear on the market some years ago and it can be expected that in the next decade their development will be as phenomenal as the development of the integrated circuit during the last decade. The purpose of this review is to present a survey of effe cts in Si and to present the state of the art with respect to SMT's.

To begin with a diagram is presented in the next section (Sect. 2) in which all physical effects of interest for the construction of transducers can systematically be arranged. Further the difference between self-generating and modulating trarisducers

is explained. In later sections (Sect. 3 and 4) relevant features of silicon technology, physical effects which are usabIe in silicon and current SMT's are presented and discussed. The review is conc1uded (Se ct. 5) with a discussion of the advantages and disadvantages of SMT's and a consideration of the future outlook with respect to these devices.

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2.

P~ysica1

Effects for Transducers and Modifiers

In the input transducer a signal of one form is con~erted into an electrical signal. For instance in a photodiode an optical signal is converted into a current. When one tries to arrange the different input signals, it appears that six groups of input signals exist, depending upon the form of energy [3].

The signal applied to the input of a transducer can have the following energy

forms: ' 1. radiant 2. mechanical 3. thermal 4. electrical 5. magnetic 6. chemical.

All these energy forms can be converted into one of the others. For electronic measurement systems, only the conversion to an electronic signal is of interest. In the modifier the electronic signal might be amplified, fJltered, converted into a frequency etc., whereas in the output transducer the electronic signal is again converted into one of the six energy forms.

The effects and devices needed for transducers and modifiers can usually be arranged in a 6 x 6 matrix-like system (Fig. 5). In the column 'electrical' a few examples of effects or devices are given which can be used for input transducers. On the horizontal row 'electrical', devices and effects which can be used to convert the electrical signal into one of the six energy forms are shown.

On the main diagonal of the matrix, devices are indicated which can be used to modify one of the six signal forms. The signal at the input and output of a gearbox is of the same mechanical form, only the rotating speed of the shaft is changed. In the case represented by the junction of 'electrical in' and 'electrical out' on the matrix, each passive or active electrical network is a modifier. Even the central processing unit of a computer could fit very well in this case.

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Though a division into six energy groups is widely accepted, some authors prefer a more detailed grouping. They distinguish, for example, between molecular and chem-ical energy, between nuc1ear and electromagnetic radiation, or between mechanical and acoustic energy. An information signal causing some problems is time. However,

it appears that time or frequency is always measured with the help of one of the six groups of energy shown.

In order 10 describe transducers, the 36-element matrix of Fig. 5, in general, proves to be satisfactory.

Input transducers can be c1assified in two categories [4]:

1. selfgenerating transducers that generate an electrical output without an

aux-iliary source of energy e.g. asolar cell, a thennocouple, a piezoelectric crystal and

2. modulating transducers that can only convert one of the six input signal forms

into an electrical signal when an auxiliary source of energy is available, e.g. a photo-conductor, a strain gauge, a magnetoresistor.

Output transducers can also be divided into self-generating and modulating de-vices. So a heating-element is a self-generating transducer in which an electrical signal

is converted into a thennal signal without an auxiliary energy source, and a liquid crystal display is a modulating transducer, because an e1ectrical signal is converted into a radiant signal with the ambient light as auxiliary energy source. It appears that self-generating transducers are two port devices with respect to signals and to energy, whereas modulating transducers are two port devices with respect to signals and three port devices with respect to energy.

In order to improve one's insight into the different types and groups of trans-ducers, it is convenient to arrange transducers in a threedimensional diagram (Fig. 6) and to describe them with the help of the nomenc1ature used in crystallography. We use the x-axis as input, the y-axis as output and the z-axis as modulating signal axis. Self-generating transducers are located in the input-output plane. The energy as weIl as the signal flow goes from the input port to the output port. Aself-generating transducer can be best presented as shown in Fig .. 7a.

The input-output plane contains thirty-six different elements, six of which are modifiers (on the main diagonal) and thirty of which are self-generating transducers.

When we restrict ourselves to electronic measurement devices, we count one elec-tronic modifier, 5 self-generating input transducers and 5 self-generating output transducers.

A modulating transducer is presented by a vector whose component along the x-axis indicates the input energy form, the z-axis the modulating energy type -and the y-axis the output energy fonn. In Fig. 6 the vector of a temperature dependent solar cell is shown. In the solar cell radiant energy is converted into electrical energy. This conversion is modulated by the thermal energy. The signal flow is from the modula-tion axis to the output axis, whereas the main energy flow is from the input to the output axis. Such a transducer can be presented as shown in Fig. 7b. Because all 5 self-generating input transducers can be modulated by 5 types of signals, 25 modu-lating input transducers result. When an electrical signal is used for modulation, a modifier results because the signal input and output are electrical, whereas the aux-iliary energy source can be any of the 6 types. Along the same lines it can be seen,

that 25 modulating output transducers also exist.

In a liquid crystal display (LCD) the signal input is electrical, the energy input is radiant (ambient light) and the signal and energy output are both radiant.

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modo chem x chem magn el z y magn chem

Fig. 6. Three dimensional diagram, showing the selfgenerating and modulating transducers.

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Fig. 7. Presentation of a. selfgenerating transducers and b. a modulating transducer ('s' stands for signal and 'e' for energy).

On the analogy of the Miller indices describing crystal axes, transducers can be easily characterized by a sirllple notation. A modulating transducer is described by

[x, y, z] in which x is the energy input, y the energy and signal output and z the modulating signal input. The temperature dep ende nt solar cell is characterized by [rad, el, therm]. A self-generating transducer like a piezoelectric crystal as input device is described by [mech, el, 0] and as output device by [el, mech, 0].

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Table 1. Notation for transducers photoconductor thermocouple magnetoresistor pH meter transistor LED display LCD display coil lateral photodetector

[el, el, rad ] [therm, el, 0 ] [el, el, magn ] [chem, el, 0 ] [el, el, el ] [el, rad, 0 ] [rad, rad, el ] [el, magn, 0 ] [rad, el, mech]

To illustrate the feasibility of the above approach, the block diagram and notation of a digital thermometer with LCD display is given in Fig. 8. A temperature is converted into an electrical signal in the thermocouple. The signal is modified and the output signal of the modifier modulates a radiant energy flow to the eye.

3. Silicon Technology

3.1. Introduction

It was not clear to anyone 15 years ago that the invention of the integrated circuit by J.S. Kilby of Texas Instruments and J.A. Hoerni and R.N. Noyce of Fairchild would have such an impact on the electronics industry of today. The simple planar tech-nology, made possible by employing photolithographic methods and oxide window masking has the important feature that passive and active components as weil as the interconnections can be fabricated on one side of a silicon wafer. In the heginning only the military recognized the vast potential of plan ar technology. At that time the size aspect was less appealing to civil users of electronics. Today the market is flooded by integrated circuits and their penetration into originally non electronically oriented markets like those of the watch, calculator and automobile industry is rapidly increasing.

The reasons for this increasing popularity are also relevant to the transducer industry and will be explained in the following section.

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· 3.2. Features of Silicon Technology

3.2.1. Size

Photolithography makes it possible to pro duce integrated circuits with very fme

details. The resolution obtainable with photolithography is determined by the

wave-length of the light used. It appears that lines with a width of 0.2 I1m can ultimately

be made with this technique [5]. In the future smaller structures might be obtainable

with electron-beam techniques. Transistors can easily be fabricated in an area of 100

x 100 11m2, resistors are usually a little bit larger. A complete electronic circuit is

usually not much larger than a few mm2, much· smaller than discrete electronic or

thick-film circuits. Because of this small size not only electronic watches and pocket calculators, but also electronic thermometers, tire pressure gauges, doorlocks, chess-boards, controls in consumer products etc. are feasible. The progress with respect to the size of electronic elements is weU illustrated by Fig. 9. In this figure the number

of devices per mm2 for MOS technology is shown for different years [6]. Because the

chip size is also being increased some people predict that the 106 _{devices chip will}

become reality.

3.2.2. Complexity

Because of the large number of components, which can be produced on one chip, it became possible to integrate not only simple electronic functions, but also complete systems. So a memory chip contains not only memory cells but also ad dressing and sensing circuitry. This is a great advantage, because it relieves the interconnectiol).

problem. The frrst microprocessors were, in fact, only microcontrollers. Today

in-and output circuits, read-only memory in-and main memories are added so that true single-chip microcomputers appear on the market. For electronic measurement sys-tems it also seems likely, that transducing elements will be integrated with a large

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part of the modifier, so that, for example, a temperature is converted directly into a digital signal ready to connect to a digital display. The costs of the development of such a measurement chip are very high, and it is clear that the level of integration planned will depend on the size of the market for this special product. For small markets it is more probable that use will be made of general purpose microprocessor chips in comblnation with small-scale integrated transducers.

3.2.3. Power Consumption

Without a doubt electronic watches can also be constructed with vacuumtubes or discrete solid state circuits. However, apart from the size, the power consumption would be such, that the owner of an electronic watch would have to have a rather heavy battery on his back or on a separate handcart. In the last decade the invention of new circuit elements and the improvement of semiconductor technology has made it possible to pro duce signal processing integrated circuits, that need much less energy at equal processing speeds. Electronic watches with liquid-crystal display,

work continuously for more than 4 years on one small battery. It is self evident, that electronic thermometers, humidity testers, tire pressure gauges etc. can also be made with similarly Spartan power consumption. Most devices can work at low dissipation levels but this is usually obtained at the cost of large switching delays. Therefore in comparing log ic gates or memory cells, the practice of indicating the power-delay product has been adapted [7]. In Table 2 the power-delay product of some tech-nologies are compared [6]. The reduction of power consumption with time is ap-parent.

3.2.4. Price

Electronic measuring equipment was always rather expensive because of the many working-hours required to fabricate complex electronic components and circuits.

Moreover , due to human error the instrurnents very of ten did not work at the end of the line so th at very elaborate end-control was mandatory.

Table 2. LSI Technologies

TECHNOLOGY POWER- DELAY PRODUCTlpJ)

P - channel 450 metal qate p-channel 145 silicon gate n- channel 45

silicon gate C MOS 0.5 si licon gate SOS 0.1 C- MOS 12 L 0.01-1

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In contrast integrated circuits are made in half automated production lines and many çircuits are processed on one wafer simultaneously. This result in very low chip cost and it is often the case, that bonding and packaging constitute the largest share of the cost. Prices of integrated circuits have fallen to unbelievable depths and this also applies torather complex LSI circuits like memories and microprocessors.

Fig. 10 gives some indication of this development by showing the price in cents (U.S.) per gate P made with MOS technology for different years. One observes, that the price drops from 10 cents/gate for small-scale integration (SSI) to 1 cent/gate for medium-scale integration (MSI) to 0.1 cent/gate for large-scale integration (LSI) to perhaps 0.01 cent/gate for very-Iarge-scale integration (VLSI). The price now is 1000 times lower than one or two decades ago. There are few industrial products, that show such a dramatic development and it makes the fast penetration of elec-tronics into many industrial fields understandable.

3.2.5. Reliability

Vacuum-tube circuits were not very reliable and had a relatively short lifetirne. Were this acceptab1e for consumer products, such a poor performance could not be ac-cepted for computers and other professional equipment.

The introduction of the solid state component, which did not require a fJlament, presented a large improvement, but the mounting of these components on printed circuit boards remained troublesome. In integrated circuits the manually soldered joints are replaced by vacuum deposited and etched interconnections and when large integrated circuits are used, the number of outside bonded connections reduce with respect to the number of interconnections on the chip. Some problems remilin: first,

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the packaging and the final handling of the chip still reduces reliability; second, when a high current density flows through the deposited conductors, migration of metallic atoms which can lead to catastrophic interconnection bum-out, is observed and third, because of the increased number and complexity of integrated circuits, it becomes very difficult to test all the chips properly before assembly into packages. The reliability of an electronic component can be expressed in the Mean Time To Failure (MTTF). LSI circuits almost achieve an MTTF of 1011 hours per gate and this is 106 better than for vacuum-tube circuits imd 103 better than for discrete transistor circuits.

3.3. Conclusion

In conclusion one can say that integrated circuits show such an unbelievable combi-nation of virtues that it is very logical that silicon technology will be employed not only for the modification but also for the identification and presentation units of a measurement system, with the only restriction, that silicon show the required physi-cal effects to enable the construction of the desired transducers.

4. Physical Effects for Silicon Micro Transducers (SMT's)

In this sectiön the most important physical effects which occur in silicon and which can be used in silicon micro transducers for converting one of the six forms of energy into electrical energy will be discussed. In silicon different passive and active com-ponents like resistors, diodes, transistors etc. will show different sensitivities to the measurands and therefore will be included in this survey. When silicon does not show the required physical effect, e.g. silicon is not piezoelectric, compatible technology is employed.

Compatible technology means a technology which can be used in an integrated circuit plant without too many changes in the process. For instance the evaporation of magnetic Ni layers is not much different from that of conducting Al layers. Examples of effects and devices are shown in Table 3.

4.1. Radiant Energy SMT's

4.1.1. Photo-Electric Input Transducers

4.1.1.1. Self-Generating Input Transducers [rad, el, 0]

Photovoltaic effect

By means of this effect radiant energy can be converted into electrical energy with-out the use of an auxiliary energy source. The most important device is the sol ar cell, which consists of a pn junction (Fig. lla). Radiant energy (photons, -y-radiation, x-ray, neutrons etc.) creates electron-hole pairs, which lead to a potential difference across the junction. Because silicon has a bandgap of 1.1 eV such transducers are not sensitive to light with a wavelength longer than 1.1 J,Lm.

(24)

Table 3. Effects, Modifiers and Transducers in Si

Self- modo modo modo IC comp.

generating R,C,l diode transistor transducer techno

solar photo photo photo photo

CdS rad.

cell conductor diode transistor IC

piezo piezo piezo piezo CdS

mech.

-IC pi -FET resistor junction transistor

therm. Seebeck R=f('1') _{I rev}.=f('I') USE=f('1')

temperature metallayer

effect IC R=f('1')

el. diffused MOSFET uni junctien dual IC vac. dep.

R transistor gateMOSFET R

diftused magneto magneto Hall Hall vac. dep. magn.

coil resistor diode MOSFET IC magn. film

chem. galvanic C= f(cone.)

-

ISFET

-

Alz03

effect R=f(hum)

}

~

1

n-type p

"1

I

~

p

I.,

I

\

\.

pi,

..

(al (bi (cl

l

p

J

"--.

-I11I1

Idl (11 CdS

j".

( In

)

"

-Si (gl (hl

Fig. 11. Review of most important photoelectric silicon micro transducers: a. solar cel!, b. photo-conductor, c. photodiode, d. phototransistor, e. photodiode array,

f.

charge coupled device, g. CdS-Si-photoconductor, h. avalanche light emitting diode.

(25)

4.1.1.2. Modulating Input Transducers [el, el, rad] Photoconductor

A photoconductor in Si consists of a p-type strip diffused into an n-type substrate with ohmic contacts at the ends (Fig. 11 b). An auxiliary energy source is required to detect a change in conductivity due to radiation.

Ph 0 todiode

This device is a reverse-biased pn junction in which, because of the absorbed radia-tion, electron-hole pairs are generated in or near the depletion layer (Fig. llc). The reverse current is a measure of the light intensity. For different applications a large diversity of photodiodes is available.

Phototransistor

This is a junction transistor in which the base-collector junction is exposed to light (Fig. lId). When the base is not connected, the increase of the reverse current through this junction is amplified by the transistor resulting in a large change of the

collector current. MOSFET's are also sensitive to light. .

4.1.1.3. Integrated Photosensitive Devices Photo integrated circuit

It is obvious, that when light sensitive devices can be fabricated in silicon by the planar technology, by adding electronic circuits, light sensitive integrated circuits can also be produced. A number of companies indeed offer such devices.

Silicon photodiode array

Photodiodes can be arranged in two-dimensional arrays. Using x- and y-lines in com-bination with MOSFET switches (Fig. lle), such arrays can be used··for optical character recognition, facsimile, imaging etc. [11].

Charge-coupled device (CCD)

These devices consist of arrays of c10sely spaced metal electrodes on top of an oxidized silicon substrate (Fig. llt). Minority carriers can be stored in potential wells beneath the electrodes or can be moved from electrode to electrode by the suitable

application of voltages to the electrodes. Because the charge packets depend on the incident light intensity images can be stored and shifted [12

J.

4.1.1.4. Photo-Electric Transducers made in Compatible Technology

It is evident, that for certain applications the spectral response of silicon is not satisfactory. It is then conceivable that a higher or lower wavelength cutoff may be .

desirabie. It is possible to evaporate CdS (Xc = 0.7 Jlm) or InSb (r .. c = 7 Jlm) layers on top of the Si02 -covered silicon chip and to use the photo-conductive properties of these layers (Fig. lIg). The change in conductivity then can be detected by the underlying electronic circuits.

(26)

- - - _.

_---4.1.2. Photo-Electric Output :rransducers

4.1.2.1. Self-Generating Output Transducers [el, rad, 0]

When electron-hole pairs recombine, photons are emitted. In so-called direct band-gap materials (in which momentum is conserved) photons in accordance with the bandgap are emitted with high efficiency. By forward-biasing a pn junction made

of

such material, recombination and photon emission can be obtained. Unfortunately silicon is an indirect-band-gap semiconductor , and therefore· the electrical to radil,ult energy conversion is very inefficient. Moreover the light wavelength is in the infrared spectrum. The only method to produce light by means of a silicon structure is to use a pn junction at a large reverse voltage (Fig. llh). At a large voltage the charge carriers are so fast, that avalanche breakdown occurs. Using appropriate doping and suitable structures, devices can be made which produce white light at very low efficiency and rather short lifetime.

4.1.2.2. Modulating Output Transducers [rad, rad, eI]

At present no effects are known in silicon which make it possible to modulate its reflectivity or transparency at low electrical fields.

4.1.2.3. Output Transducers made in Compatible Technology

At present so called LED's and LCD's are employed in display devices. LED stands

j j j j

_;:.

_,

_l

_/,

++++++++++

0 LJ

---f

1 1 1

_Ibl lal

5 eJ

e b C n p n p . n+ / p (c) Idl (el in out 5 o (11

Fig. 12. Review of most important strain sensitive silicon micro transducers: a. piezo electric crystal, b. piezoresistor on diaphragm, c. piezojunction, d. piezotransis-tor, e. light spot position sensitive photodiode,

f.

ZnO pi-FET, g. CdS-Si SA W filter.

(27)

for light emitting diode. They are made of direct band-gap materials like GaAs (red light) or mixtures of direct and indirect band-gap materials like GaAs_{1 _}_{x Px (green} light) [4.5]. LCD stands for liquid-crystal display [4.6]. Between two·flat electrodes materials with slightly ordered long molecules calIed liquid crystals are placed.

One electrode is transparant and by means of rather small voltages on the plates the optical properties of the materials can be appreciably changed. It is conceivable, that in the future, by compatible techniques, LED and LCD materials can be applied directly on top of the silicon. Addressing of the segments in decimal displays then could be realized by circuits in the underlying silicon, thus avoiding the difficult interconnection problems of today's display devices.

4.2. Mechanical Energy SMT's

4.2.1. Self-Generating Input Transducers [meeh, el, 0]

Self-generating input and output transducers for mechanical or acoustical energy employ the piezoelectric effect (Fig. 12a). Piezoelectric effects only occur in crys-tals which lack a center of inversion symmetry. Unfortunately the lattice of silicon is symmetrie so that self-generating transducers cannot be realized in silicon.

4.2.2. Modulating Input Transducers [el, el, mech]

Piezoresistor

Because of the piezoresistive effect a silicon diffused resistor changes its value in response to strain. Therefore silicon is used for sensitive strain gauges. The disadvan-tage of silicon strain gauges is that the temperature dependence is relatively large compared to standard metal strain gauges.

For the measurement of pressure a very thin n-type silicon pressure diaphragm can be made in which four p-doped resistors are diffused in a wheatstone bridge arrangement (Fig. 12b). This device, which was first described by Gieles [15] is now offered by a number of companies in many configurations. The device and its appli-cations are amply described in the next paper.

Piezojunction

When a silicon pn junction is stressed, the I-V characteristic changes. This is due to the piezoresistive effect and band -gap changes caused by the strain [16]. Piezojunc-tions can be used as mechanical energy transducers, however, until now they have not been widely used (Fig. 12c).

Piezotransistor

When a junction is strain-sensitive it can be expected that transistors will also show such strain effects (Fig. 12d). This indeed is observed. Besides the change in charac-teristics the current amplification factor is also rather strain sensitive [17]. lt is found, that MOSFET's are also strain sensitive [18]. The effect is attributed to piezoresistive effects in the channel of the device.

(28)

Lateral photoeffect [rad, el, mech]

When ~ solar cell is non-uniformly illuminated, a voltage arises between two ohmic contacts at the periphery of the cell (Fig. 12e). When a light spot is used the voltage is a measure for the position of the light spot. With this effect large area two- diinen-sion al light spot position sensitive photodiodes can be fabricated [10].

4.2.3. Integrated Strain Sensitive Devices

Integrated pressure transducer (wheatstone bridge)

The possibility of making modulating strain sensitive devices in silicon opens the way to fuIl integration of transducers with signal processing circuits.

For reasons of yield and calibration, these devices are still not available at present. Yet some int~gration is realized by placing a transistor in the center of the wheat-stone bridge [19]. This transistor Can be used as a temperature sensor. lts output signal can be employed to compensate for the rather large temperature sensitivity of the silicon strain gauges.

Integrated pressure transducer (junction transistor)

Using a transistor pair in which one transistor is subjected to stress by pressing a diamond stylus on the emitter surface and ad ding current sources, operational

ampli-0 Idl T2 TI T) 0

0

0 flow~ Igl p Ibl lel ~i02

~

Ihl

11 I

ll-~

~-v lel lIJ

Fig. 13. Review of most important thermo silicon micro transducers: a. Seebeck

. effect, b. thermoresistor, c. thermodiode, d. thermotransistor, e. matched transistor pair,

f

flow sensor with wheatstone bridge, g. flow sensor with thermotransistors, h. metal thermoresistor, i. dotmatrix for thermal printer.

(29)

fiers etc. on the same chip, Veen [20] has shown that it is possible to make fully integrated pressure transducers with linear output characteristics.

4.2.4. Pressure Transducers made in Compatible Technology ZnO and CdS pi-FET

By depositing piezoelectric layers on top of a silicon chip, the disadvantage of silicon not being piezoelectric can be compensated for. When a piezoelectric ZnO or CdS layer is deposited between the channel of a MOSFET and the gate electrode, very sensitive pressure transducers have been constructed with very good high frequency (up to 20 MHz) properties (Fig. 12f). By stressing the piezo-FET (Pi-FET) the piezo -electric layer is polarized and the resulting -electric field modulates the channel [21]. Of course, in the future, peripheral electronic circuits can be added.

CdS-Si SA W filters

By means of interdigital transducers and piezoelectric materials surface-acoustic-wave filters can be made when a piezoelectric CdS or ZnO layer is deposited on a silicon substrate (Fig. 12g). SAW filters in the piezoelectric layer can be combined with signal processing circuits in the silicon substrate [22]. First experiments show that very versatile filters for microwave frequencies (2 GHz) can also be obtained.

4.3. Thermal Energy SMT's

4.3.1. Thermal Energy Input Transducers

4.3.1.1. Self-Generating Input Transducers [therm, el, 0]

When differently doped silicon structures are joined together in a circuit and the two junctions are at different temperatures, a current will flow in the circuit (Fig. 13a). This effect is called the Seebeck effect and the potential difference developed may be used to measure the temperature difference. No auxiliary power supply is required. 4.3.1.2 Modulating Input Transducers [el, el, therm]

Thermoresistor

In intrinsic semiconductors a temperature increase causes an increase of the current carrier density. The result is that the resistivity changes exponentially with tempera-ture and the effect can be used to monitor temperatempera-ture (Fig. l3b). In extrinsic semiconductors the donor or acceptor states at room temperature are already fully ionized and the temperature sensitivity is much smaller.

Thermodiode

Reverse as weIl as forward current in a pn junction are temperature dependent (Fig. l3c). At constant forward current this change is rather small, only -2mVtC.

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Thennotransistor

When the emitter current of a transistor is kept constant, the base-emitter voltage proves to be a linear function of the temperature (Fig. 13d). Sensitivities between lOmVtC and 200mVtC are obtainable [23].

4.3.1.3. Integrated Temperature Sensitive Devices Temperature integrated circuit

By bringing temperature sensitive transistors and signal processing together on one chip, temperature sensitive integrated circuits can be constructed. One such a device uses two matched transistors [24]. The difference in base-emitter voltages at different collector currents is proportional to the absolute temperature (Fig. l3e). The transis-tor pair is integrated with a zener voltage reference andan operational amplifier. The transducer can be used between -25°C and +8SoC and shows a linear output of

lOmVtC in this range. Integrated silicon flowmeter

A flowmeter can be inade by means of a wheatstone bridge consisting of four p-type diffused resistors in an n-type silicon substrate. When the chip is exposed to an air flow, the temperature of the resistors, which are heated by the bridge current, decreases (Fig. 13 f). The magnitude of this decrease depends on the orientation of the resistors with respect to the direction of the air flow. The signal due to the

.

~

tB

~

/ ' n

.J

0

p

0 B

B

_{cl {bi

0B

~

Cl 9 s~ d2 C2 {II {dl (el (gl (hl (il

Fig. 14. Review of the most important magnetic silicon micro transducers: a. di} jUsed coil, b. magnetoresistor, c. Corbino disk, d. Hall plate, e. magnistor,

f

magfet, g. Hall-MOSFET, h. integrated Hall plate with Schmitt trigger, i. Ni-Co magneto- resis-tor.

(31)

unbalance of the bridge is a measure of the flow [25]. A direction sensitive air

flowmeter can be constructed by using a heating transistor in the cent re of a chip and two thermotransistors at both edges (Fig. 13 g) [26].

4.3.1.4. Temperature Sensitive Transducers made in Compatible Technology When the temperature sensitivity of the thermosilicon components does not satisfy the required specifications, it is not difficult to deposit and etch metallayers on top of het Si02 (Fig. 13h). With metal layers very accurate and sta bie sensors can be

made, however, the temperature range remains rather small and is determined by the

underlying silicon circuits.

4.3.2. Thermal Energy Output Transducers

The conversion of e1ectrical energy into thermal energy is very simpie. When a

current passes through aresistor, the temperature of the resistor increases, due to the interaction of the current carriers with the lattice. The same applies to diodes and transistors. A matrix of such components in combination with heat sensitive paper can be used to make a thermal printer (Fig. 13i). The addressing electronics can eventually be integratedtogether with the dot elements on the same chip.

4.4. Magnetic Energy SMT's

4.4.1. Se1f-Generating Input Transducers [magn, el, 0]

When a silicon coil, made by diffusing p-type windings in an n-type substrate, is subjected to an alternating magnetic field, according to Faraday's inductiQn law, an alternating voltage is induced (Fig. 14a). Because of the existing technology only a small number of windings is feasible, but in combination with signal processing

circuits on the same chip, usabie output signals can be obtained.

4.4.2. Modulating Input Transducers [el, el, magn]

Magnetoresistor

According to the Gauss effect, the resistance of a current carrying conductor changes upon the application of a magnetic field (Fig. 14b). The. effect is rather small in silicon and depends strongly on the geometry of the diffused resistor [27]. The largest effect can be obtained with a so-called Corbino-disk (Fig. 14c).

Hall effect

When a current flows through a Hall plate, which is subjected to a magnetic field normal to the plate, a Hall voltage appears across the sample in a direction at right angles to both current and field direction (Fig. l4d). The Hall voltage polarity de -pends on the type of semiconductor and the direction of the current and magnetic field. Hall plates can easily be fabricated in silicon [28].

(32)

"

Magneto diode

A magneto diode consists of a pin sandwich. The carrier lifetime in the intrinsic

region (i) is long compared to the lifetime in other doped regions. The diode charac-teristics can be changed by a magnetic field, because current carriers are deflected from low'recombination to high recombination paths, due to the Lorentz force [28]. The device is more sensitive than the Hall plate but Ie ss linear and reproducable. . Magnistor

This device consists of a bipolar transistor structure having one emitter and two collectors [28]. Without a field the current divides equally between the collectors.

With a magnetic field a differential current occurs, producing a voltage between!he collectors, when load resistors are used (Fig. 14e).

Magfet

This device consists of a MOSFET with two drain contacts [28]. A magnetic field causes differential drain currents leading to an output voltage (Fig. 14f).

Hall MOSFET

The Hall voltage of a Hall plate is inversely proportional to the thickness of the plate.

In a MOSFET the inversion layer in the channel can be adjusted by means of the

gate voltage. A Hall MOSFET consists of a MOSFET with rather wide channel area and additional Hall contacts (Fig. 14g) [29].

4.4.3. Integrated Magnetic Transducers

Together with amplifiers, triggers, output stages, etc. a Hall plate can be integrated on one chip (Fig. 14h). Indeed a number of manufacturers offers such devices for use in magnetic recording heads, brushless motors, displacement transducers, contactless

push

buttons, etc. [30].

4.4.4. Magnetic Transducers made in Compatible Technology

When the temperature sensitivity of magnetic SM T's is unacceptable on top of the Si02 other layers can be deposited. The magnetoresistance effect in thin magnetic

•

p

(b) (c)

(a)

Fig. 15. Review of some chemical silicon micro transducers, a. humidity sensor, b.

(33)

Ni-Co or Ni-Fe films is rather large [31]. By means of the underlying electronic circuits the change of resistance can be processed (Fig. 14i).

4.5. Chemical Energy SMT's

4.5.1. Self-Generating Input Transducer [chem, el, 0]

From electrochemistry it is known that, by bringing suitable electrodes and solutions together, galvanic cells can be obtained. Putting a drop of some ionic solution on top . of a silicon chip with proper electrodes, a voltage will be generated, which is a

. measure of the concentration of ions. However, the solution might be destructive with respect to the other circuits on the chip.

4.5.2. Modulating Input Transducer [el, el, chem]

Ionic electrolytic conduction

A monolithic structure can be made with electrodes such that the resistance of an ionic solution on top of the silicon can be measured.

Humidity sensor

Using a silicon chip on top of a PeItier cooling element a humidity sensor can be constructed [32]. The silicon chip contains a polycrysta1line periodic structure (Fig. 15a) whose capacitance changes abruptly when water starts to con den se on top of the silicon chip.

Hydrogen sensitive MOSFET

MOSFET's with gate electrodes made of Pd can be used to sense hydrogen [33]. The threshold voltage of such a MOSFET is a function of the hydrogen pressure in the ambient atmosphere. Other gasses like ammonia and hydrogen sulfide can also be detected (Fig. 15b).

Ion sensitive field effect transistor (ISFET)

When the gate electrode of a MOSFET is removed, the device becomes sensitive for electronic charges on top of the oxide (Fig. 15c). When an ionic solution is brought in contact with the oxide, the electric field at the Si-Si0₂ interface will depend on the activity of the ions [34]. The device is very suitable for electrophysiological applications.

4.5.3. Integrated Chemical Transducer

Pollution abatement can only be succesful, when proper analysers are available. It is possible to form very powerful SM T's by combining the above transducers with signal processing circuits.

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5. Conc1usions andFuture Outlook

Without a doubt, by means of the planar silicon technology a large number of very suitablè transducers can be fabricated. Summarizing, the following advantages are apparent:

1. For the production of SMT's use can be made of a very weIl developed tech-nology; this leads to low costs per device. ,

2. The transducer can be integrated together with the signal processing circuits and this leads to

a. short connections between transducer and circuits and therefore a low dis-turb sensitivity.

b. on the chip correction of unwanted sensitivities (e.g. to temperature) ofthe transducer.

c. large sensitivities.

3. The transducers can be made very small, so that a. the transducers can be positioned almost anywhere.

b. improved characteristics (higher cut-off frequency) are obtainable. c. the transducer has only a small influence on the measurement object. 4. Because of the planar technology one and two dimensional structures can easily

be made, which enables the processing of images, e.g. pressure images, tempera-ture images.

Of course silicon micro transducers have also a number of disadvantages:

1. Silicon technology leads only to low-priced devices, when large numbers are processed. Vet when the price of an expensive instrument reduces considerably by using SMT's, the price of the SMT needs not to be an obstacle. .

2. The use of silicon restricts the range in which the transducer can be used. It is possible a. that the effect in silicon is very weak or non-existent, b. that the spectral sensitivity is impractical or c. that the usable temperature range

(-100°C

<

T

<

200°C) is too limited. '

3. The bonding technique has to be improved. Thin Al or Au wires very of ten will be destroyed in environments, in which transducers are used or contemplated (automobiles).

4. In order to use an SMT properly, it is often necessary to use an SMT in an open

-uncapsulated structure. Gasses, humidity and ionic solutions will certainly attack and eventually destroy the SMT's, when no special protection (e.g. glass layers)

is applied.

5. When transducer and signal processing circuits are integrated on one chip, it is possible (for example, in a temperature transducer), that unwanted feedback between circuits and transducer occurs.

In spite of the fact that SMT's show some important draw-backs, the future of SM T's looks rather exciting. Because the transducer industry is rather fragmented in com-parison to the integrated circuit market, progress will be slow, not to mention cau-tious. It will take some time before small companies with a non-electronic tradition will accept that SMT's are the way to go. However, the automotive, consumer prod-ucts, antipollution, biomedical technology industries in particular have a great demand for cheap, reliable and versatile transducers.

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It is not yet dear if transducers will be developed and manufactured by. the traditional transducer industries in the future or if the large integrated circuits indus-tries, due to their large technologicallead, will absorb these markets.

The authors expect, that large volume markets like automotive applications, elec-tronic watches, elecelec-tronic push buttons, elecelec-tronic barometers, electron ic tire-pres-sure gauges, electronic clinical thermometers etc. will be the domain of the large IC industries, whereas more specialized non-standard transducers for monitoring indus-trial processes will remain the field of the smaller companies which have built up, through years, a large market experience with respect to these application.

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(37)

II. Applications of

IC

Technology

In

Measuring Techniques

Anton Gieles*

1. Introduction

New physical effects, new technologies and new materials applied in metrology have causeda significant change in this field in the past years. Especially the application of procedures from planar silicon technology has opened the possibility of developing a new generation of small, reliable, accurate, and yet inexpensive, transducers. New application areas like car-electronics and domestic appliances are developing fast.

In this paper dealing with the impact of IC technology on measuring technique, the applications of IC technology will be divided into three groups:

- The first group comprises the application of general purpose integrated circuits to improve transducer characteristics.

- The second group contains applications of integrated circuits designed for one specific measurement, using standard IC technology.

- The third group adds a new dimension: the combination of standard plan ar tech-nology with some additional procedures specific for one type of transducer. Several examples of devices in each group will be given.

2. Applications of general purpose integrated circuits

A general trend in measuring systems is the shift of signal conditioning and signal preprocessing towards the transducer. This is desirabie for several reasons:

a. Modularity of a system. If all transducers in a large system deliver standardized signals, the central part of the system does not contain transducer-specific elec-tronics and can be standardized to a large extent.

b. The signal transport. The weak transducer outputs can not be transported over long distances without loss of information. Transmission in a standard digital format is much better in this respect.

c. The ease of installation and calibration.

Thanks to the availability of inexpensive, small and reliable general purpose inte-.

grated circuits the necessary functions like amplification, filtering, galvanic isolation and analog-to-digital conversion can be realized at the transducer.

. __ Also simple calculations can be executed at the transducer. For example, in an

Modern electronic measuring systems

Modern Electronic

Measuri

~g

Systems

~

1

'1'

11

~I

"

i

1

1/"

:

1"

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" {

.

"

-

i

iljIÎlüj~mllml'

Modern Electronic

.

Measuring Systems

P.P.L. Regtien

(ed.)

Contents

Preface

.

Introduction

*

- - - -

-

- - - -

-

-

-

- - - - -

- -

- - - -

-

---

-

-

I.

S

ilicon Micro-Transducers: a new

gen

e

ration

o

f m

e

asuring

e

l

e

m

e

nts

Simon Middelhoek* and Date

W

:

Noorlag

*

1.

Int

r

oduction

H

H

I---~~~:ENS

2.

Effects for Transducers and Modifiers

rx

3. Silicon Technology

3.1. Introduction

·

_.

Simon Middelhoek and Date*

_;:.

_,

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_/,