Fundamentals and Applications of Chemical Sensors - ACS Publications

10 Chemical Microsensors Based on Surface Impedance Changes Stephen D. Senturia

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: May 29, 1986 | doi: 10.1021/bk-1986-0309.ch010

Department of Electrical Engineering and Computer Science, Center for Material Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139

Sensing chemical species is a much more difficult task than the measurement of mechanical variables such as pressure, temperature, and flow, because in addition to requirements of accuracy, stability, and sensitivity, there is the requirement of specificity. In the search for chemically-specific interactions that an serve as the basis for a chemical sensor, investigators should be aware of a variety of possible sensor structures and transduction principles. This paper adresses one such structure, the charge-flow transistor, and its associated transductive principle, measurement of electrical surface impedance. The basic device and measurement are explained, and are then illustrated with data from moisture sensors based on thin films of hydrated aluminum oxide. Application of the technique to other sensing problems is discussed. Background. The term microsensor denotes a transducer that, in some fashion, exploits advanced miniaturization technology, whether an adaptation of integrated c i r c u i t technology, or some other microf a b r i c a t i o n technique. Within the past decade, a myriad of microsensors have been developed, with c a p a b i l i t i e s for measurement of temperature, pressure, flow, position, force, acceleration, chemical reactions, and the concentrations of chemical species. The l a t t e r measurements, of chemical species, are i n t r i n s i c a l l y more d i f f i c u l t than the measurement of mechanical variables because i n addition to requirements of accuracy, s t a b i l i t y , and s e n s i t i v i t y , there i s a requirement for s p e c i f i c i t y . Chemical sensors can be of the type that sense the species d i r e c t l y , such as ion-selective electrodes, or the type that sense the species i n d i r e c t l y through the change i n another physical property produced by the species. An example of this l a t t e r type i s the so-called "chemiresistor", i n which the presence of the species being sensed modifies the e l e c t r i c a l resistance of a transducer 0097-6156/ 86/ 0309-0166$06.00/ 0 © 1986 American Chemical Society

Schuetzle and Hammerle; Fundamentals and Applications of Chemical Sensors ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

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SENTURIA

Chemical Microsensors

167


material. This paper i s directed toward the chem ire si stor family of sensors, and s p e c i f i c a l l y toward applications of sensing gaseous chemical species i n gaseous ambient atmosphere. The domain of such applications includes routine monitoring of moisture or atmospheric pollutants, l i f e - s a f e t y applications such as smoke and/or gas sensing, and pesticide or other chemical-agent detection. In a l l such cases, one has a sensor material, usually i n the form of a thin f i l m , and a suitable set of electrodes with which the e l e c t r i c a l resistance of the f i l m can be monitored. This paper i l l u s t r a t e s a p a r t i c u l a r type of microsensor structure, called the charge-flow transistor, that i s i d e a l l y suited for chemiresistor application. H i s t o r i c a l Perspective. Development of the charge-flow transistor was stimulated by the report by Byrd 1 that polymer derivatives of poly(phenylacetylene) could be used i n t h i n - f i l m form to sense the presence of NH3 and S02* This led to several NASA-sponsored programs to explore i n a more systematic way the application of such polymers to l i f e - s a f e t y applications 2-4, including the possible development of advanced sensor structures that could be used with weakly conducting thin-film chemical-sensor materials. In 1977, the f i r s t charge-flow transistor device was reported 5.. This device consisted of a metal-oxide-semiconductor f i e l d - e f f e c t transistor (MOSFET) i n which a portion of the gate metal was replaced by the thin-film material. Because the thin f i l m was weakly conducting compared to the metal gate, turn-on of the device was l i m i t e d by how quickly charge could flow along the sheet of thin-film material, charging up the gate-to-semiconductor capacitance. Since the conduction i n the sheet of thin f i l m depended on the presence of s u i t able gases i n the ambient, the turn-on time of the device could be used to measure the presence of chemical species 6. The d i e l e c t r i c constant, l i k e index of r e f r a c t i o n , has been used to analyze binary mixtures for i n d u s t r i a l process control since the f i f t i e s 7 and used f o r humidity sensors shortly thereafter 8-14. In these humidity sensors, i n t e r d i g i t a l metal elements deposited on ceramic substrates were used to increase the s e n s i t i v i t y . These sensors used polymeric, inorganic or ceramic coatings on the interd i g i t a l structure to selectively adsorb water vapor. The idea that these structures could be miniturized by forming them on a s i l i c o n chip along with associated measuring c i r c u i t r y was reported i n 1977 as, f i r s t , discrete charge-flow transistors 5.6, then, discrete sense c i r c u i t s 15., and f i n a l l y , two f u l l y integrated monolithic sense c i r c u i t s 16.. In the work c i t e d above, in which the t h i n - f i l m material actually served as the gate of a portion of the FET, there were problems both i n s t a b i l i t y of the measurement, and i n the quant i t a t i v e interpretation of data. In the hope of finding improved measurement accuracy, an alternative structure, the floating-gate charge-flow transistor, was developed 17_, and has been applied to a variety of measurment problems, including (a) the study of moisture measurement both with thin polymer films 12. and thin films of hydrated aluminum oxide 18, and (b) i n - s i t u chemical reaction monitoring i n polymers 19,20. The goal of the present paper i s to review both the charge-flow transistor measurement concept and i t s applications to chemical sensing problems.


168

FUNDAMENTALS AND APPLICATIONS OF CHEMICAL SENSORS


The Floating-Gate

Charge-Flow Transistor (CFT)

The floating-gate charge-flow transistor (referred to hereafter as CFT) i s actually a small-scale s i l i c o n integrated c i r c u i t . I t combines a pair of i n t e r d i g i t a t e d electrodes with two f i e l d - e f f e c t t r a n s i s t o r s to y i e l d a transducer for the calibrated measurement of low-frequency sheet resistance and sheet suspectance (hence the term "surface impedance") of thin films. The heart of the device i s the electrode pair and t r a n s i s t o r i l l u s t r a t e d schematically i n Figure 1. One of the electrodes (the Driven Gate) has a signal applied to i t ; the other electrode (the Floating Gate) extends over the channel region of a f i e l d - e f f e c t t r a n s i s t o r but has no e x p l i c i t external connection. The only path for charge to reach the f l o a t i n g gate i s across the spaces between the two electrodes, and i t i s here that the thin f i l m i s placed, as i l l u s t r a t e d i n Figure 2. The e l e c t r i c a l equivalent c i r c u i t for the driven-gate-to-floating-gate transfer function can be understood with reference to Figures 2 and 3. The capacitance between the f l o a t i n g gate electrode and the conducting s i l i c o n substrate i s denoted by C . i t includes the input capacitance of the FET. The t o t a l charge reaching C , or, equivalently, the total voltage across C , serves as an input signal to the FET, modifying the conduction i n the channel of the FET. The coupling between the driven and f l o a t i n g gates cons i s t s of two parts. There i s a stray capacitive coupling through the a i r above the t h i n f i l m , denoted by C^, and there i s a d i s t r i b uted R-C transmission l i n e , denoted by R§, C , and Cj, which represents the combined effect of sheet conduction i n the thin f i l m (Rg), the film-to-substrate capacitance that must be charged by conduction i n the f i l m (Gj.), and a d i s t r i b u t e d capacitance that accounts for d i e l e c t r i c p o l a r i z a t i o n of the f i l m ( C ) . Mathematical analysis of the transfer function i s a straightforward problem i n l i n e a r c i r c u i t theory. In the sinusoidal steady state at frequency f, i f VfjG * the voltage phasor of the sinusoid applied to the driven gate, and VpQ i s the voltage phasor of the sinusoidal voltage appearing on the f l o a t i n g gate, then the transfer function has the following form: L

L

L

s

s

s

1 + (Cx/C )asinha _

Vpft — =

T

(1)

cosha + (CT/C t + C x / C x ) a s i n h a

V

DG

where

a

=

J(C

R

T

/C ) S

(2) J +

(2nfR C )S

1 J

S

The important point to note here i s that the capacitive terms determine the overall shape of the transfer function, whereas the f r e quency f and the sheet resistance Rg occur only within the parameter a, and only as a product. Therefore, an increase i n frequency at fixed sheet resistance can cause the same change i n transfer function as an increase i n sheet resistance at fixed frequency. This property can be exploited i n v i s u a l i z i n g the transfer function by p l o t t i n g the amplitude of Equation 1 against i t s phase s h i f t , using


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SENTURIA


Driven Gate

Electrodes Transistor Source[

Floating Gate

F i g u r e 1. Schematic top view of CFT d e v i c e , w i t h c i r c u i t symbol shown i n i n s e r t . Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 17. C o p y r i g h t 1982 I n s t i t u t e of E l e c t r i c a l and E l e c t r o n i c s E n g i n e e r s .

Thin film

Driven gate electrode

^Floating gate electrode

'.Si02V-::{:."-. p-Silicon

F i g u r e 2. Schematic c r o s s s e c t i o n of CFT d e v i c e through A-A', F i l m t h i c k n e s s i s exagerated r e l a t i v e to o t h e r dimensions. Reproduced w i t h p e r m i s s i o n from r e f e r e n c e 17. C o p y r i g h t 1982 I n s t i t u t e of E l e c t r i c a l and E l e c t r o n i c s E n g i n e e r s . the product f R as a parameter. Figure 4 i l l u s t r a t e s two different transfer functions for different assumed values of C , using actual device geometry to determine C and Cj. A microphotograph of an actual CFT i s shown in Figure 5. The overall chip size i s 2mm x 2mm, and the spacing between fingers of the interdigitated electrodes i s 12 |im. Note the additional FET on the chip, called the reference FET. Its function i s to compensate for process variations in the FET and for the temperature dependence of the transistor properties. A feedback c i r c u i t i n which the reference FET i s used i n conjunction with the sensing FET i s shown in Figure 6. Because the c i r c u i t requires the two transistor curs

§

x


170


DGo

/

Distributed RC line of length L

Substrate (ground plane) *


Figure 3. E l e c t r i c c i r c u i t model for CFT device. Reproduced with permission from reference 18. Copyright 1982 Institute of E l e c t r i c a l and Electronics Engineers.

GRIN ( d B )

I

.

.

1

1

1

1

1-180

Figure 4. Calculated transfer function for CFT device i l l u s t r a t i n g effect of sheet capacitance on shape of c a l i b r a t i o n curve. Reproduced with permission from reference 18. Copyright 1982 Institute of E l e c t r i c a l and Electronics Engineers.

Figure 5. Microphotograph of CFT chip. Reproduced with permission from reference 17. Copyright 1982 Institute of E l e c t r i c a l and Electronics Engineers.


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171


rents to be equal (via the feedback), and because the reference FET i s e l e c t r i c a l l y i d e n t i c a l to the sensing FET, the voltage applied to the reference FET gate must be equal to the voltage that apears on the floating gate through charge-flow processes. Thus, the entire measurement process consists of applying a sinusoid to the driven gate, and making an amplitude and phase measurement of the voltage that the feedback c i r c u i t applies to the gate of the reference FET. With the use of c a l i b r a t i o n curves of the type i l l u s t r a t e d i n Figure 4 (which, in practice, are stored as look-up tables i n our datalogging system), on-line real-time measurement of sheet resistance and sheet capacitance i s routinely possible. The range of frequencies presently i n use i s 0.1 to 10,000 Hz. The sheet resistance range accessible to this measurement technique i s between 10 and 10 Ohms/square. Thus, this sensor becomes a useful replacement for an electrometer when working with high impedance films, and has the added advantages of a wide measurement bandwidth and the routine use of AC measurement techniques to avoid electrode p o l a r i z a t i o n effects. The s e n s i t i v i t y of the sheet-resistance measurement i s a few percent. Absolute accuracy i s estimated at 10%. The s e n s i t i v i t y of the sheet capacitance measurement i s much lower, and depends d i r e c t l y on f i l m thickness. Our experience has been that sheet capacitance must be known to within 50% in order to provide an acceptably accurate c a l i b r a t i o n for sheet resistance, but that precisions much better than 50% are d i f f i c u l t to achieve. The temperature range over which the chip can operate i s very wide. We have made measurements between -200 and 300°C, although we do not yet know whether there i s a loss of c a l i b r a t i o n accuracy as temperature varies.


9

Application to Moisture Measurement with Hvdrated Aluminum Oxide Films The use of thin films of hydrated aluminum oxide to measure moisture i n the ambient i s well known 21, and serves as the basis for a number of commercial moisture-measurement products. The usual device configuration consists of a p a r a l l e l - p l a t e structure i n which an aluminum electrode i s i n contact with a hydrated aluminum oxide f i l m (see Figure 7a), over which a porous gold electrode i s placed. As moisture reaches the oxide layer, the conductivity of the hydrated oxide i s changed. This i s conventionally monitored by measuring the effective capacitance and/or conductance between the gold and aluminum electrodes. In the CFT approach, samples are prepared by evaporating a very thin aluminum f i l m (400-800 A) over a s i l i c o n wafer on which CFT device f a b r i c a t i o n has been carried out. The devices are then bonded to headers and wire bonded, after which immersion i n heated water serves to convert the thin aluminum f i l m to a moisture-sensitive hydrated oxide 22., yielding the crosssection i l l u s t r a t e d i n Figure 7b. That the aluminum-oxide CFT obeys the transfer function of Equations 1 and 2 i s i l l u s t r a t e d i n Figure 8, i n which the crosses represent actual room temperature measurements i n an ambient dew point of -16.9°C at a a variety of frequencies, and the c i r c l e s represent values calculated from the model f o r Rg = 1.7 x 10 Ohms/square, and C = 1.6 pF-square. Thus, by measuring the s


172

FUNDAMENTALS

A N D APPLICATIONS O F C H E M I C A L SENSORS

Gain(dB)

_ J III A

Phase(degrees)

HP3575A

1

*B"*

Gain-Phase Meter

A

B

+1 volt

SIM

i FET V


Signal /"S generator \~J

FG

4K

Figure 6. CFT feedback c i r c u i t and instrumentation for AC measurements. Reproduced with permission from reference 17. Copyright 1982 Institute of E l e c t r i c a l and Electronics Engineers.

POROUS GOLD

(a)

(b)

CONVENTIONAL

CHARGE-FLOW

TRANSISTOR

Figure 7. Schematic cross sections of (a) conventional aluminum oxide moisture sensor, and (b) CFT aluminum oxide moisture sensor.


10.

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173

transfer function for different ambient dew points, one can follow changes i n sheet resistance. Both aging (Figure 9) and hysteresis effects (Figure 10) have been observed with hydrated aluminum oxides 18,23.


In-Situ Measurement of Polymer Curing D i e l e c t r i c measurements have been established for nearly three decades as a technique for monitoring the cure of polymeric resins. Dramatic changes i n the d i e l e c t r i c properties of the material accompany the transformation of the resin from a viscous l i q u i d to a solid. The standard method for making measurements of d i e l e c t r i c properties i s to place a sample between closely spaced p a r a l l e l conducting plates, and to monitor the AC equivalent capacitance and d i s s i p a t i o n factor of the resulting capacitor. The capacitance i s proportional to the d i e l e c t r i c p e r m i t t i v i t y (e') at the measurement frequency, and the d i s s i p a t i o n factor i n combination with the value can be used to extract the d i e l e c t r i c loss factor (e") . The CFT device shown i n Figure 1 has been used to monitor d i e l e c t r i c properties of polymeric materials. The medium to be studied i s placed over the electrodes, either by application of a small sample or by embedding the entire integrated c i r c u i t i n the curing medium (see Figure 2 f o r a schematic cross-section through the electrode region). The resulting device and associated e l e c tronics i s called a microdielectrometer. The microdielectrometer has been used to monitor polymer curing i n - s i t u 22,23. The CFT device can make measurements at lower f r e quencies than could be achieved by conventional d i e l e c t r i c measurement techniques. Measurements at multiple frequencies can be made i n real-time. A Fourier transform equivalent of the microdielectrometer has been developed to extend the frequency range to as low as 0.005 Hz 24. Discussion The results i l l u s t r a t e d above show that the CFT method i s suitable for making chemical-sensor measurements using both bulk polymers and, i n particular, thin f i l m materials that are i n t r i n s i c a l l y weak conductors. Therefore, the CFT looks promising for such materials as poly (phenyl ace tylene) derivatives 24, for which carefully shielded electrometer measurements have been required i n the past because of current l e v e l s at the threshold of d e t e c t a b i l i t y . Furthermore, the fact that the CFT always makes AC measurements reduces the problem of DC p o l a r i z a t i o n of electrodes. In addition, the CFT approach should be suitable for other "chemiresistor" applications, such as the metal-substituted phthalocyanines proposed by Jarvis et. a l . 25. and f o r Langmuir-Blodgett films 26., which, because they are so thin, may prove impossible to use i n p a r a l l e l - p l a t e form, but which can be routinely used with the h i g h - s e n s i t i v i t y i n t e r d i g i tated-electrode approach provided by the CFT.


174


GRIN -60

-70

-50

-40


xDew P o i n t - -16.9°C

(dB) -30

-20

-10

O R s « 1.S7E14 Ohms/sq Cs= 1.55 pF-um

Figure 8. Typical comparison between experimental data (crosses) and calculated transfer function ( c i r c l e s ) for CFT coated with hydrated aluminum oxide, measured at room temperature in an ambient dew point of -16.9 °C. Reproduced with permission from reference 18. Copyright 1982 Institute of E l e c t r i c a l and Electronics Engineers.

-G5

-55 DEH

POINT

Figure 9. Sheet resistance data i l l u s t r a t i n g the effect of aging on aluminum oxide moisture sensor. Reproduced with permission from reference 18. Copyright 1982 Institute of E l e c t r i c a l and Electronics Engineers. Acknowledgment s This paper i s based on an invited presentation at the Symposium on Micro sensors f o r Chemical Detection held as part of the 17th MidA t l a n t i c Regional Meeting of the American Chemical Society (MARMACS), A p r i l 1983. The work was supported by the Thermal Processes D i v i s i o n of the National Bureau of Standards under Grant NB80-DADA1004 and by the National Science Foundation under Grant ECS-8114781. Devices used i n t h i s work were fabricated i n the MIT Microelectronics Laboratory, a Central F a c i l i t y of the Center for Materials Science and Engineering which i s sponsored i n part by the National Science Foundation under contract DMR-81-19295. Some of the measurement instrumentation was purchased under NSF Contract ENG-7717219 and under programs sponsored by the Office of Naval Research.



10. SENTURIA


175

-65 -55 -45 -35 -25 -15 -5 DEW POINT (°C) Figure 10. Sheet resistance data illustrating the hysteresis in aged aluminum oxide moisture sensor. Reproduced with permission from reference 18. Copyright 1982 Institute of Electrical and Electronics Engineers. Literature Cited 1.

Byrd, N. R. "Space Cabin Atmosphere Contaminat Detection Techniques"; Douglas Report SM-48446-F (Contract NAS 21-15); NTIS: Springfield, VA, 1968. 2. Byrd, N. R.; Sheratte, M. B. "Synthesis and Evaluation of Polymers"; NASA CR-134693 (Contract NAS 3-17515); NTIS: Springfield, VA, 1975. 3. Byrd, N. R.; Sheratte, M. B. "Semiconducting Polymers for Gas Detection"; NASA CR-134885 (Contract NAS 3-18919); NTIS: Springfield, VA, 1975. 4. Senturia, S. D. "Fabrication and Evaluation of Polymer Early-Warning Fire Detection Devices"; NASA CR-134764 (Contract NAS 3-17534); NTIS: Springfield, VA, 1975. 5. Senturia, S. D.; Sechen, C. M.; Wishneusky, J. A. Appl. Phys. Lett., (1977) 30, 106. 6. Senturia, S. D.; Huberman, M. G.; Van der Kloot, R. Proc. ARPA/NBS Workshop on Moisture Measurement in Integrated Circuit Packages, (1978) NBS Special Publication 400-69, p. 108. 7. Dee Snell, F.; Hilton, C. L. "Encyclopedia of Industrial Chemical Analysis", Section on Capacity and Dielectric Constant, Interscience Publishers, 1966, Vol. 1. 8. Musa, R. C.; Schnable, G. L. "Polyelectrolyte Electrical Resistance Humidity Elements", Paper 353 in Humidity and Moisture, Exler, A., Ed., Reinhold Publishing Corp., New York, 1965, Vol. 1. 9. Amdur, E. J.; Nelson, D. E. "A Ceramic Relative Humidity Sensor", Paper 38, ibid. 10. Regtien, P. P. L. Sensors and Actuators, 1981, 2, 85. 11. Wakabayashi, K.; Ohta, S.; Takemori, D.; shirae, K. "Non-Linear Behavior of Glass Substrate in High Humidity", in "Chemical Sensors", Seiyama, T.; Feuki, K.; Shiokawa, J.; Suzuki, S., Editors, Proceedings of the International Meeting on Chemical Sensors, Fukuoka, Japan, Analytical Chemistry Symposia Series-Volume 17, Elsevier, New York, pp. 439-444.


176 12. 13. 14. 15. 16. 17.


18. 19. 20. 21. 22. 23. 24.

25. 26. 27.

FUNDAMENTALS AND APPLICATIONS OF CHEMICAL SENSORS Uchikawa, F.; Miyao, K.; Horii, H.; Shimamoto, K. "Humidity Sensing with Silicone Composite Films", pp. 445-450, ibid. Miyoshi, S.; Sugihara, T.; Jinda, A.; Hijikigama, M. "Thin Film Humidity Sensor Composed of Cross-Linked Polyelectrolyte", pp. 451-456, ibid. Fleming, W. J. "A Physical Understanding of Solid State Humidity Sensors", Society of Automotive Engineers Paper No. 810432, Warrensdale, PA, 1981. Senturia, S. D.; Fertsch, M. T. IEEE J. Sol. St. Circuits, SC14 (1979) 753. Senturia, S. D.; Garverick, S. L.; Togashi, K. Sensors and Actuators, 1981/82, 2, 59. Garverick, S. L.; Senturia, S. D. IEEE Trans. Elec. Dev., ED29, 1982, 90. Davidson, T. M.; Senturia, S. D.; Proc. IEEE Int. Rel. Phys. Symp., 1982, pp. 249-252. Sheppard, Jr., N. F.; Coin, M. C. W.; Senturia, S. D. "Microdielectrometry: A New Method of In-Situ Cure Monitoring", Proc. 26th SAMPE Symposium, Los Angeles, CA, 1981, 26, 65-76. Senturia, S. D.; Sheppard, N. F.; Lee, Jr., H. L.; Day, D. R. J. Adhesion, 1982, 15, 69. Kovac, M. G., Chleck, D.; Goodman, P. Proc. IEEE Int. Rel. Phys. Symp., 1977, pp. 85-91. Vedder, W.; Vermilyea, D. A. Trans. Faraday Soc., 1969, 65, 561. Lin, C.-H.; Senturia, S. D. Proc. Solid State Transducers, 83, Delft, June, 1983; Sensors and Actuators, 1983, 4, 497. Coin, M. C.; Senturia, S. D. "The Application of Linear System Theory to Parametric Micro sensors", Proc. Third Int'l Conf on Solid-State Sensors and Actuators, Philadelphia, June 1985 (in press). Wentworth, S. E.; Libby, J. B.; Bergquist, P. R. Symp. on Micro sensors for Chemical Detection, 17th MARM-ACS, 1983. Wohltjen, H.; Jarvis, N. L.; Snow, A. Symp. on Microsensors for Chemical Detection, Chapter 20 in this book. Roberts, G. Sensors and Actuators, 1983, 4, 131.

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