An Interdisciplinary Approach to M-icroinstrumentation ~
Gilbert Mugen
Lawrence Llvermore National Laboratory Livermore. Calif. 94550
Gary H W e
Depanmem of Chemistry Indiana University Blmmingon, 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 ( 1 ) . We are witnessing the same phenomenon in the field of chemical analysis. The recent emphasis on sensors, on
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 possi0003-2700188/0360423Al$01 SO10
@ 1987 American Chemical Society
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ble to employ disposable systems that are self-indicating and that constitute the modern equivalent ofiitmus 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).
Mkrdnstrumentati, benefits, and objectives Many of the benefits of microinstrumentation 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
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rlyure 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 lrom 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 10m; trained dogs can uncover clandestine material with a selectivity of one part in 10‘7 or 10’8. 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 mi24A
xosampler should produce tiny (P moiter 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 ndividual hasis or in rapid sequence so ;hat 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 he drawn. As the needle penetrates the liquid (frame 2) and subsequently withdraws (frame 31, 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 collaose into a tinv microdroolet (frame 6). The seouence of ooerations deoicted 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 a t 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 (121, 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.
Microsemors 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 piezobalanee. 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
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 piezohalance elements with different absorbing layers. Although no single layer will he 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 l o 5 individual compounds. The next stage of miniaturization of this piezobalance device would be a single crystal driven a t 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 hv variations in its temuerature or aging. An experiment with a specific elasto-
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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 mm3 in volume) and easily embedded into gasket material. The necessary measurement of Q requires only a solid-state frequencymeter (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 i t is important to develop small devices for indicating temperature or, better, for recording it on a continuous basis. T o 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 pro. files of such a detector can be employed t u deduce ih 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-
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tegral of the time multiplied by an exponential function of the temperature. As shown in Figure 3, this degree of diffusion can be measured by the change in surface resistance or capacitance that it produces. A 1%change in temperature at a mean temperature of 373 K 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 dooant concentration Drofile within two hours. Because the diffusion rate depends on an activation energy, one c a i alter the sensitivity of the integrating temperature microsensor through choice of the semiconductor material or the dopant. If several combinations of semiconductor and dopant are used that possess a range of activation energies, the combination of readouts corresponds to a set of integral equations that can be inverted mathematically (inverse Laplace transform) to produce the full curve of time vs. temperature. Of course, the number of time-resolution elements that can be obtained in this way is limited to the number of different activation energies of dopanhemiconductor combinations that are employed. A dopant layer of variable initial thickness can be used over limited temperature ranges instead of changes in dopant or semiconductor composition. This kind of device is portrayed in Figure 4. Chemiresistor. Very thin layers of semiconductor materials exhibit suhstantial 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, individual thin-film materials show different levels of response, not only from changes in the amount of vapor absorbed or adsorbed on them, hut 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 he made in extremely thin layers, they have a high intrinsic response to adsorbed material, and they have no tendency to form surface protective layers during exposure 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 order of picograms. As with piezosensors, the selectivity of a chemiresistor depends 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 required to produce easily measured currents. Such high voltages can cause electrical breakdown and, even a t 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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such as those used In measuring highenergy radlatlon can be used as an lntegrating temperature recorder. A wrlftedlayer win varying nianm IS e m ployed. AI S h lhatlon along this layer. ne t e m peraturahlsm of the devlce a n be expr& a8 an Integralequrnlon (onrlghl). lnterrogatlngthe layer at a numba (n) of lhatlons then permns the temperatureat ndloaem time intervals to be cab OUl&d.
driving voltage. Unfortunately, calculations show that the optimal length of the resulting chemiresistor is on the order of only a few hundred angstromswell 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 A) 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 a t which the films recrystallize into arrays of gold microcrystallites separated by some tens of ang28A
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posited on top of the gold “layer,” a sensitive and stable detector is produced. In a manner similar to that employed in the development of a selective piezosensor array, the thin-film semiconductor sensor is made selective through the use of arrays of devices. Such an array can be constructed by microfabrications simply on a single sheet of quartz substrate material (25).The array of electrodes on the quartz substrate is then separated by thin strips of recrystallized gold and coated with different semiconductor (phtbalocyanines) layers. Again, a pattern of responses is produced that is independent of sample concentration but that indicates the identity of a particular sample constituent (see Figure 6). Working curves showing the simultaneous detection of ammonia and water on an electron-tunneling gas sensor are shown in Figure 6 (19).
meiuture
Flpurr 5. A “cherniresistor” can be fabricated by heating a thin film of gold to produce closely spaced rnicrocrystallites on a thin layer of phthalocyanine. Quantum tunneling betweenthe microcrystals then prododuces a measurable current that is affected strongly by even submanolayer &posits of a target compound.
stroms. This arrangement is shown schematically in Figure 5. In such a discontinuous thin layer,current can travel between the electrodes by quantum tunneling across the narrow gaps between gold crystallites. Currents up co
1 pA have been demonstrated by using
1-2 V across the electrodes-a voltage low enough to avoid instability and shnrt device lifetimes. When the thin film (a few monolayers at most) of semiconductor (phthalocyanine) is de-
demonstrates selectivity. me test vapors are water, ammonia. and etMl aIcohol. aOA
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The foregoing sections have provided a glimpse of new developments in microinstrumentation. However, the applications of microengineering are far more abundant than those we have discussed here (3, 4-6, 8). Our findings have been encouraging, however, and several projects have made the transition from research activity to direct application. In addition, the development of microsensors and microdevices of various kinds is proceeding in an increasing number of laboratories throughout the world. The use of microengineering as a tool for materials and systems design is just beginning: The potential of this exciting tech-
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 that 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 tomicroinstrumentation 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 studiesand 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 t o 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 (2627). 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
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exciting new world of microinstrumentation. This paper is dedicated w the late Tamas Hirsehfeld, who headed the micmstiucture project at Lawenee Livermore National Labratory and who initiated many of the concepts and devices described here. Work was performed under the auspices ofthe US. Department of Energy by the Lawrence Livermore National Labratory under Contract no. W-7405-ENG-48. Research was sup’ ported in part by the Office ofNaval Research and by the Upjohn Co.
Washington. D.C.. 1984.
(271 Angel. S. M. Sperlrorrop~ 1987. 2(4L
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(28) Venkatesh, S.; Culshaw, B. Proe. SPIE Annu. Tech. Symp. 1985,566,110,
Reteremes (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., Ed%.; Solid State Chemical Sensors; Academic: New York, 1985. (4) Schuetzle. D.; Hsmmerle, R.; Butler, J. W., Eds.; Fundamentals ond A plicotions of Chemical Sensors; ACS lymposium Series 309 American Chemical Society: Washington, D.C., 1986. ( 5 ) Lauks, 1. Mierascience; SRI International Menlo Park, Calif.; Vol. 5,1983. (6b,yhltjen, H. And. Chem. 1984, 56,
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(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.: Hieftie. G . M. Anal. Chem. 1984,56,2 (12) Hieftje, G. M.; Chem. 1968, (13) Lemke. ’ Chim. Aeth 1982. (14) Alder, J. F.; 8 (London) 1983, lua, I I W . (15) Ballantine, D. S., Jr.; Rose, S. L.; Grate. J. W.: Wohltien. ~. H. Anal. Chem. 1986,is,3 0 5 ~ ~ 6 6 . (16) Fraser, S. M.; Edmonds, T.E.; West, T. S. Analyst (London) 1986,111,118388. (17) Olness, D.; Hirschfeld,. T. “Sorption
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Gilbert Haugen is a senior scientist with the Chemistry and Materials Science Department nt 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, a n d remote sensing with fib1 optics spectroscopy.
Detector System for Chemical Agents Detection aid Recognition”; Report No, CRDC-TR-84086; Chemical Research and Develooment Center. US.Armv Armament, M’unitions, and Chemical cammand, Aberdeen Proving Ground, Md., 1984.
Carey, W. P.; Kowalski, B. R. Anal. Chem. 1986,58,3077-84. (19) Olness, D.; Hirschfeld. T. “ R e c a i tion and Detection of Chemical Vaoor Mixtures by Sorption on Multiple Piezoelectric Crystals”: Report No. UCRL91513: Lawrence Livermore National Laboratory: Liv&m&, Calif., 1984. (20)Parzen. B. Design of Crystal and Other Harmonrc Oscillators; John Wiley: New York. 1983. (21) Hirschfeld, T.CHEMTECH February (18)
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(22) Carey, W. P.; Beeke, K. R.; Kowalski.
B.; Telman, D.; Hirschfeld, T. Anal.
Chem. 1986.58,149. (23) Olness, D. Presented at the Pittsbureh
Exposition on Analyti&l Chemiwsand Applied Spectroscopy,At. lanlir City, N J , March 9-13, 1987 I241 Hauren. G , Lawrrnce Livermore Na. tionrd Iyahoratory. unpublished results. (251Rrc,die. I Murav, .I .I The Ph,otcs o/ Conference and
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M i r r o f a b n e o l t o n . Plenum NQWYork, go,?” ail”...
(26) Haugen, G. Presented at the Memorial
Symposium for Dr. Tomas Hirschfeld New Frontiersin Analytical Science,Anaheim, Calif.; American Chemical Society:
Gary Hieftje i s distinguished professor of chemistryat Indiana University in Bloomington. He received a n 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 a t the Illinois S t a t e Geological Survey (Urbana). I n 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 inuestigation of basic mechanisms in atomic emission, absorption, and fluorescence spectrometric analysis and the deuelopment of atomic methods of analysis.
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