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An Interdisciplinary Approach to Microinstrumentation Microinstrumentation

Gilbert Haugen Lawrence Livermore National Laboratory Livermore, Calif. 94550

Gary Hieftje Department of Chemistry Indiana University Bloomington, Ind. 47401

One of the clear trends in modern science is toward miniaturization. In the microelectronics industry, miniaturization began with the invention of the transistor and progressed through modern large-scale integrated-circuit technology. In a similar fashion, miniaturization and integration are occurring in optics. Image capture, complex signal processing, storage, and readout can all be carried out now in monolithic integrated-optical arrays; this new approach permits complex operations such as Fourier transformation to be carried out literally at the speed of light. Optical computers are envisioned as one of the important areas to be developed in the next few decades (i). We are witnessing the same phenomenon in the field of chemical analysis. The recent emphasis on sensors, on

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INSTRUMENTATION taking measurements at the sample location (in situ) rather than in the laboratory, and toward in vivo measurements urges that analytical instrumentation be made not only more sophisticated but also smaller, more reliable, and more self-sufficient. These goals can best be met through an approach that couples the concepts of microengineering, biology, chemistry, and modern chemical instrumentation. The design of chemical microinstrumentation will require a joint effort from scientists skilled in a range of disciplines, from engineering to chemistry and from biology to microcircuit fabrication. Although this task will not be straightforward, its benefits will be substantial. With properly designed microinstrumentation, it will be possi0003-2700/88/0360-023A/$01.50/0 © 1987 American Chemical Society

ble to employ disposable systems that are self-indicating and that constitute the modern equivalent oflitmus paper. Other sophisticated instruments might couple sensing and measurement with telemetry to permit long-term monitoring of in situ or in vivo processes. Already, pharmaceutical chemists and physicians are seeking ways of continuously monitoring the blood glucose level in a diabetic patient. Coupled with such a device might be an automatic dispenser of insulin, suggesting the ultimate application of microinstrumentation—to the real-time control of biological, environmental, or industrial processes (2). As far as chemistry is concerned, the field of microinstrumentation is still in its infancy. In this brief review we will

describe several undertakings into this new field, including devices designed for the sampling and direct sensing of chemical constituents and those that might form part of a larger system or instrument. It is important to recognize that these examples are meant to be illustrative only and represent mainly the experiences of the authors and activities in progress in the microstructure project of Lawrence Livermore National Laboratory. A great deal of additional activity is currently under way in the field of chemical microinstrumentation, particularly in the areas of field effect transistor sensors, metal oxide semiconductor sensors, pyroelectric sensors, and sensors utilizing immobilized microorganisms and electrochemical devices (3-7). Instrument manufacturers have not been idle, as is evident from some of the miniature devices that are available: gas chromatographs, hydrogen detectors, fiber-optics temperature probes, piezoelectric fans, and miniature refrigerators (8). Microinstrumentation—scope, benefits, and objectives Many of the benefits of microlnstrumentation are obvious—for example, reduced weight, power consumption, and volume. In addition, with reduced size comes increased ruggedness and response speed and reduced fabrication costs. Indeed, many of the strengths of microinstrumentation are derived from changes that occur as systems are scaled down in size. Phenomena that

ANALYTICAL CHEMISTRY, VOL. 60, NO. 1, JANUARY 1, 1988 · 23 A

Figure 1. Operational sequence during microsample dispensing: (1-3) formation of liquid filament, (4) filament detaches from glass needle, (5) filament collapse, (6) microsample droplet. (Reproduced with permission from Reference 9.)

are important in our conventional world—gravity, inertia, magnetism, flow, and thermal emission—become relatively unimportant in the microworld. Instead, small devices are affected more by electrostatic forces, surface tension, diffusion, van der Waals forces, and quantum effects. Thus the design, fabrication, and use of microinstrumentation must follow pathways different from those that are customary. For example, a propeller in the macroscopic world creates lift as it rotates. However, shrinking its dimensions proportionally does not yield a useful propeller in the microworld. As is exemplified by bacteria, a micrometer-sized helix provides a far more useful approach for propelling small systems. As this example suggests, the biological world has already solved some impressive microinstrumentation problems on its own. For instance, the gypsy moth can detect molecules with a sensitivity of one molecule per second and a selectivity approximately one part in 1020; trained dogs can uncover clandestine material with a selectivity of one part in 1017 or 1018. Other examples could be readily cited; scientists who are developing microinstrumentation should consider whether the biological world has already undertaken a similar task. Microdispensers In many situations it is necessary to obtain a representative microsample of a material before it can be chemically analyzed. Although such microsampling devices have long been available, only recently have they become capable of computer control. Ideally, a mi-

crosampler should produce tiny (r anoliter or below) aliquots of a sample solution on a demand basis. The system should be under electronic control so that computer manipulation is straightforward. Finally, microaliquots should be able to be dispensed on an individual basis or in rapid sequence so that signal averaging is possible. The unit displayed in Figure 1 (9) offers many of these attributes. The operation of the microdispenser is revealed in the sequence of frames. In frame one, a glass needle (lower left) is poised for insertion into the bulk liquid (upper right) from which the microsample is to be drawn. As the needle penetrates the liquid (frame 2) and subsequently withdraws (frame 3), it pulls with it a tiny filament of the sample solution. That filament then detaches from the glass needle (frame 4) and from the bulk liquid (frame 5) to collapse into a tiny microdroplet (frame 6). The sequence of operations depicted in Figure 1 can be carried out once or in rapid succession (up to several thousand times per second) so that droplets can be dispensed one at a time or in multiples for improved precision. Because each droplet is somewhat smaller than one nanoliter and can be generated with a relative standard deviation of about 2% (in diameter), it thus becomes possible to dispense microlitersized (thousand-droplet) aliquots at precision levels below 0.3% (9). This entire microdispenser is operated under computer control and already has been applied as a sample introduction device for atomic absorption spectrometry (10) and in titrimetry (11). Other droplet generators, based on a different principle (12), have been applied to similar fields. These generators are capable of even finer volume resolution and have also been used in a "pH-stat" instrument (13). We are likely to see further developments in the field of microdispensers in the near future. Microsensors Microsensors are often integrated systems that incorporate both a chemically selective element and a readout device (3-6). In other cases, microsensors require external components for amplifying or recording their output signals. In this paper we will briefly describe several kinds of sensors applicable to both chemical substances and physical phenomena. Multicomponent quartz piezobalance. An attractive approach to chemical sensing involves a quartz piezobalance coated with a chemically selective adsorbing or absorbing layer (14). Because the resonant frequency of the crystal depends on the total mass of the crystal and its coating, the absorption

24 A · ANALYTICAL CHEMISTRY, VOL. 60, NO. 1, JANUARY 1, 1988

of substances by the selective coating can be detected as a change in the crystal frequency. Of course, the selectivity of a typical quartz piezobalance arises only from that of the absorbing layer. Because few adsorbing or absorbing processes are completely specific, such detectors have suffered from a lack of selectivity. As a variation of this traditional approach, we and others (15-17) have coated an array of quartz piezobalance elements with different absorbing layers. Although no single layer will be entirely specific, the degree of selectivity attainable by monitoring the ensemble of resonant frequencies enables one to decode the response pattern for a single target species (18). Examples of such response patterns can be found in Figure 2, where the amplitude of response is plotted on the vertical axis and the identity of the absorbing layer on the horizontal axis. Obviously the selective layers should exhibit for the target substances affinities that are as different as possible. If eight degrees of freedom are used in the coating-selection process and 10 individual coatings are used, a measurement with 10% reproducibility in the response pattern would be sufficient to resolve some 800,000 different chemical vapors (19). Sensitivities to different vapor species are in the parts-permillion or subnanogram range, and the device is selective to more than 105 individual compounds. The next stage of miniaturization of this piezobalance device would be a single crystal driven at a high harmonic frequency. The pattern of vibration on the surface would be a standing wave (20); the locations of maximum amplitude would then each be covered with a different chemically selective layer. The resulting gas sensor would be approximately 1 cm2 in area and 2 mm thick. Qualitative measurement with such a device would be possible by matching an observed pattern of oscillation to premeasured patterns; quantitative measurements would be possible by examining the magnitude of the response (19,21, 22). Elasticity sensor. An illustration of a microsensor for a physical phenomenon is one intended to measure elasticity. A miniature piezoelectric vibrator in contact with an elastomer behaves as a forced harmonic oscillator with damping. At relatively high frequencies (e.g., 6 MHz), oscillations of such a combination are not completely damped, and the magnitude of oscillation depends on the load experienced by the piezoelectric element. In turn, this load is affected by changes in the modulus of elasticity of the elastomer caused by variations in its temperature or aging. An experiment with a specific elasto-

mer demonstrated a linear relationship between the Q of the oscillator and aging time. For every week of aging, the value of Q changed by 15%. The same elastomer, when exposed for 20 h to an atmosphere containing 8 ppm of ozone, exhibited an observable change in resonant frequency. The piezoelectric oscillator can be quite small (0.1 mm 3 in volume) and easily embedded into gasket material. The necessary measurement of Q requires only a solid-state frequency meter (23). A prototype elasticity sensor would require an encapsulated oscillator that compensates for ambient fluctuations in temperature and pressure.

Integrating temperature recorder. Microinstrumentation can be subject to small temperature variations that greatly exceed those encountered by larger systems. Consequently it is important to develop small devices for indicating temperature or, better, for recording it on a continuous basis. To be practical, such a temperature sensor should be inexpensive, should permit unattended operation, and should provide a complete temperature history when interrogated. A device that satisfies these criteria is based on the change in surface resistance and capacitance that occurs in doped semiconductors as their envi-

ronmental temperature is altered (21). Such devices, typified by the lithiumdrifted germanium or silicon detectors used in measuring high-energy photons, degrade in performance slowly as a dopant element (lithium here) diffuses farther and farther into the substrate. Because the diffusion rate is temperature-related, the dopant profiles of such a detector can be employed to deduce its temperature history. The dopant-semiconductor combination that is currently being tested (24) is germanium doped with lithium. The total extent of lithium diffusion that has occurred during a particular time interval will be defined by the in-

Figure 2. Response of a coated piezobalance to different vapor-phase substances. The identity of the adsorbing or absorbing layer (coating) is indicated on the horizontal axis; the magnitude of response (change in crystal resonant frequency) is shown on the vertical axis. The pattern produced by a particular vapor can be used to identify it; the magnitude of the pattern can be used to determine vapor concentration. (Adapted with permission from Reference 17.)


tegral of the time multiplied by an ex ponential function of the temperature. As shown in Figure 3, this degree of diffusion can be measured by the change in surface resistance or capaci tance that it produces. A 1% change in temperature at a mean temperature of 373 Κ causes a 7% change in surface concentration. Exposure of this wafer to a temperature step of 80 °C (293-373 K) causes nearly a factor of two change in dopant concentration profile within two hours. Because the diffusion rate depends on an activation energy, one can alter the sensitivity of the integrating tem perature microsensor through choice of the semiconductor material or the dop ant. If several combinations of semi conductor and dopant are used that possess a range of activation energies, the combination of readouts corre sponds to a set of integral equations that can be inverted mathematically (inverse LaPlace transform) to pro duce the full curve of time vs. tempera ture. Of course, the number of time-reso lution elements that can be obtained in this way is limited to the number of different activation energies of dop ant-semiconductor combinations that are employed. A dopant layer of vari able initial thickness can be used over limited temperature ranges instead of changes in dopant or semiconductor composition. This kind of device is por trayed in Figure 4. Chemiresistor. Very thin layers of semiconductor materials exhibit sub stantial changes in electrical resistance when even fractional monolayers of materials having a permanent or an in

duced dipole moment are deposited on them (4, 6). Not surprisingly, individ ual thin-film materials show different levels of response, not only from changes in the amount of vapor ab sorbed or adsorbed on them, but also because of variations in sensitivity to the effect. Such devices offer attractive alternatives as microsensors. Organic semiconductive layers are particularly suitable here; they can be made in ex tremely thin layers, they have a high intrinsic response to adsorbed materi al, and they have no tendency to form surface protective layers during expo sure to air. They can also be deposited easily in films of controllable thickness. Such a chemiresistor is extremely sensitive and can be used to detect less than a monolayer of adsorbed material. Current detection limits, even before complete optimization, are on the or der of picograms. As with piezosensors, the selectivity of a chemiresistor de pends on the characteristics of the thin-film material on which the target material deposits. Unfortunately, semiconductors that exhibit the strongest chemiresistance effect have extremely high resistivities. This fact, coupled with the very thin layers that are employed, yields a film of extremely high resistance. Therefore fairly high operating voltages are re quired to produce easily measured cur rents. Such high voltages can cause electrical breakdown and, even at lower levels, electrochemical decomposition. Obviously it would be desirable to reduce the length of the chemiresistive layer and employ electrodes that have a relatively large area so that currents can be as large as possible for a given

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Figure 4. A Li-drifted silicon detector such as those used in measuring highenergy radiation can be used as an inte grating temperature recorder. A Li-drifted layer with varying thickness is em ployed. At each location along this layer, the tem perature history of the device can be expressed as an integral equation (on right). Interrogating the layer at a number (n) of locations then permits the temperature at η discrete time intervals to be cal culated.

driving voltage. Unfortunately, calcu lations show that the optimal length of the resulting chemiresistor is on the or der of only a few hundred angstroms— well below that obtainable with current integrated-circuit technology. A new procedure has recently been developed to overcome this limitation. In this technique, a very thin (less than 300 Â) strip of gold is deposited on quartz between thicker contact strips of the same material. After these films have been vacuum-deposited, the layer is heated to approximately 120 °C, the temperature at which the films recrystallize into arrays of gold microcrystallites separated by some tens of ang-

posited on top of the gold "layer," a sensitive and stable detector is pro duced. In a manner similar to that employed in the development of a selective piezosensor array, the thin-film semicon ductor sensor is made selective through the use of arrays of devices. Such an array can be constructed by microfab rications simply on a single sheet of quartz substrate material (25). The ar ray of electrodes on the quartz sub strate is then separated by thin strips of recrystallized gold and coated with different semiconductor (phthalocyanines) layers. Again, a pattern of re sponses is produced that is indepen dent of sample concentration but that indicates the identity of a particular sample constituent (see Figure 6). Working curves showing the simulta neous detection of ammonia and water on an electron-tunneling gas sensor are shown in Figure 6 (19). The future

Figure 5. A "chemiresistor" can be fabricated by heating a thin film of gold to pro duce closely spaced microcrystallites on a thin layer of phthalocyanine. Quantum tunneling between the microcrystals then produces a measurable current that is affected strongly by even submonolayer deposits of a target compound.

stroms. This arrangement is shown schematically in Figure 5. In such a dis continuous thin layer, current can trav el between the electrodes by quantum tunneling across the narrow gaps be tween gold crystallites. Currents up to

1 μΑ have been demonstrated by using 1-2 V across the electrodes—a voltage low enough to avoid instability and short device lifetimes. When the thin film (a few monolayers at most) of semiconductor (phthalocyanine) is de

The foregoing sections have provided a glimpse of new developments in mi croinstrumentation. However, the ap plications of microengineering are far more abundant than those we have dis cussed here (3, 4-6, 8). Our findings have been encouraging, however, and several projects have made the transi tion from research activity to direct ap plication. In addition, the development of microsensors and microdevices of various kinds is proceeding in an in creasing number of laboratories throughout the world. The use of mi croengineering as a tool for materials and systems design is just beginning: The potential of this exciting tech-

Figure 6. (a) Working curves for ammonia and water vapor, obtained on the quantum-tunneling chemiresistor coated with phthalocyanine-Mg depicted in Figure 5. (b) An array of six quantum-tunneling chemiresistors, each having a different phthalocyanine, demonstrates selectivity. The test vapors are water, ammonia, and ethyl alcohol.


nique appears to be enormous and could bring about a revolution in microminiaturization of measurement systems. The recent announcement of high-temperature superconductors suggests that large arrays of microinstruments might be possible and that computer capability will increase dramatically in systems t h a t formerly were impossibly small in size. To realize success in the development of microinstrumentation, an interdisciplinary approach is extremely important. Basic research and scaling laws must be coupled with careful use of the literature in a broad range of areas (microelectronics, entomology, integrated optics, etc.) to generate new ideas. We hope that this "team" approach to microinstrumentation will be used in many other laboratories and that it might form the focus of an effective Science and Technology Research Center of the kind to be sponsored by the National Science Foundation. Although a substantial number of alternative devices could be discussed here, we close by listing a few systems whose concepts have been confirmed by feasibility studies and that offer new capabilities in microinstrumentation or microdevices. • A high-thermal-conductivity plastic has been designed around small-diameter glass capillaries that serve as an array of heat pipes. The resulting relatively inert unit possesses an equivalent thermal conductivity 3 orders of magnitude greater than that of an aluminum rod of similar dimensions (26). • New coatings have been developed that are resistant to both abrasion and impact. The coatings are designed from long filaments of materials such as carbon or aluminum oxide. The result is similar to the protective effect of mammalian hair (26). • A miniature dryer has been constructed that operates in a fashion similar to that of a human sweat gland. Operating with selective membranes made from Nafion and a fluorosilicone, the microdryer operates at 2 V and can dispose of nearly 2 g of water per year for each milliampere of current that it consumes (21,26) • A microcamera has been designed that is piezoelectrically scanned and can transmit entire images over a single optical fiber (26). • A microsensor is being explored that is capable of sensing organic vapors by changes in the numerical aperture of an optical fiber that the vapors induce (26,27). • A pocket-sized, low-voltage ionization detector that can detect organic vapors is being constructed (26). • An optically driven piezobalance is being explored (26,28). We hope that these concepts will encourage many readers to explore the

exciting new world of microinstrumentation. This paper is dedicated to the late Tomas Hirschfeld, who headed the microstructure project at Lawrence Livermore National Laboratory and who initiated many of the concepts and devices described here. Work was performed under the auspices of the U.S. Department of Energy by the Lawrence Livermore National Laboratory under contract no. W-7405-ENG-48. Research was supported in part by the Office of Naval Research and by the Upjohn Co.

Washington, D.C., 1984. (27) Angel, S. M. Spectroscopy 1987, 2(4), 38 (28) Venkatesh, S.; Culshaw, B. Proc. SPIE Annu. Tech. Symp. 1985,566,110.

References (1) Houston, J. B., Jr. Optics News 1986, 12(4), 5-28. (2) Hirschfeld, T.; Callis, J. B.; Kowalski, B. R. Science 1984,226, 312. (3) Janata, J.; Huber, R. J., Eds.; Solid State Chemical Sensors; Academic: New York, 1985. (4) Schuetzle, D.; Hammerle, R.; Butler, J. W., Eds.; Fundamentals and Applications of Chemical Sensors; ACS Symposium Series 309; American Chemical Society: Washington, D.C., 1986. (5) Lauks, I. Microscience; SRI International: Menlo Park, Calif.; Vol. 5,1983. (6) Wohltjen, H. Anal. Chem. 1984, 56, 87 A. (7) Hirschfeld, T. Science 1985,230, 286. (8) Chowdhury, J. Chem. Eng. News 1983, Feb. 21, p. 18. (9) Shabushnig, J. G.; Hieftje, G. M. Anal. Chim. Acta 1981,126,167-74. (10) Shabushnig, J. G.; Hieftje, G. M. Anal. Chim. Acta 1983,148,181-92. (11) Steele, A. W.; Hieftje, G. M. Anal. Chem. 1984,56,2884-88. (12) Hieftje, G. M.; Malmstadt, H. V. Anal. Chem. 1968,40,1860-67. (13) Lemke, R. E.; Hieftje, G. M. Anal. Chim. Acta 1982,141,173-86. (14) Alder, J. F.; McCallum, J. J. Analyst (London) 1983,108,1169. (15) Ballantine, D. S., Jr.; Rose, S. L.; Grate, J. W.; Wohltjen, H. Anal. Chem. 1986,50,3058-66. (16) Fraser, S. M.; Edmonds, T. E.; West, T. S. Analyst (London) 1986, 111, 118388. (17) Olness, D.; Hirschfeld, T. "Sorption Detector System for Chemical Agents Detection and Recognition"; Report No. CRDC-TR-84086; Chemical Research and Development Center, U.S. Army Armament, Munitions, and Chemical Command, Aberdeen Proving Ground, Md., 1984. (18) Carey, W. P.; Kowalski, B. R. Anal. Chem. 1986,58,3077-84. (19) Olness, D.; Hirschfeld, T. "Recognition and Detection of Chemical Vapor Mixtures by Sorption on Multiple Piezoelectric Crystals"; Report No. UCRL91573: Lawrence Livermore National Laboratory: Livermore, Calif., 1984. (20) Parzen, B. Design of Crystal and Other Harmonic Oscillators; John Wiley: New York, 1983. (21) Hirschfeld, T. CHEMTECH February 1986,118-23. (22) Carey, W. P.; Beeke, K. R.; Kowalski, B.; Telman, D.; Hirschfeld, T. Anal. Chem. 1986,58,149. (23) Olness, D. Presented at the Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy, Atlantic City, N.J.; March 9-13,1987. (24) Haugen, G., Lawrence Livermore National Laboratory, unpublished results. (25) Brodie, I.; Muray, J. J. The Physics of Microfabrication; Plenum: New York, 1984. (26) Haugen, G. Presented at the Memorial Symposium for Dr. Tomas Hirschfeld: New Frontiers in Analytical Science, Anaheim, Calif.; American Chemical Society:

Gilbert Haugen is a senior scientist with the Chemistry and Materials Science Department at Lawrence Livermore National Laboratory. He received his Ph.D. in chemistry from the University of Southern California in 1962. His research interests include chemical kinetics and mechanistic thermochemistry, photochemical and photophysical processes, optical instrumental and experimental design, and the application of information theory to chemical measurements. He is also interested in the application of lasers in chemical measurements and in time-resolved, correlation, Raman, luminescence, and remote sensing with fiber optics spectroscopy.

Gary Hieftje is distinguished professor of chemistry at Indiana University in Bloomington. He received an A.B. degree from Hope College (Holland, Mich.) in 1964 and a Ph.D. from the University of Illinois in 1969. From 1964 to 1965 he was a research associate in physical chemistry at the Illinois State Geological Survey (Urbana). In 1969 he was appointed assistant professor of chemistry at Indiana University and was subsequently promoted to associate professor and then to full professor in 1977. He received a special appointment to a distinguished professorship in 1985. His research interests include the investigation of basic mechanisms in atomic emission, absorption, and fluorescence spectrometric analysis and the development of atomic methods of analysis.


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