Anal. Chem. 1989, 61, 2355-2361
present system permits sensor operation at substantially milder potentials (+lo0 to +150 mV vs Ag/AgCl, see Table I), and hence the sensor output is contaminated to a lesser extent with redox reaction of indifferent substances. Dilution of samples is not preferred for practical use. In view of this, the linear range of the sensor should be extended up to levels of concentration in sera. A work is currently under way toward this end.
ACKNOWLEDGMENT The authors are grateful to Professors S. Mizusawa and T. Ohno and Dr. H. Kobayashi, Faculty of Engineering, Chiba University, for their help in the FIA experiments.
LITERATURE CITED Nagy, 0.; von Storp, L. H.; Guilbautt, 0. 0. Anal. Chim. Acta 1973, 66, 443-455. Aizawa. M.; Karube. I.: Suzuki, S. Anal. Chim. Acta 1974, 69, 43 1-437. Hahn, Y.; Olson, C. L. Anal. Chem. 1879, 51, 444-449. Kulys, J. J.; Pesliakiene, M. V.; Samalius, A. S. Bhlectrochem. Bioenerg. 1881, E , 81-88. ~ ~ lJ. yJ.;~Samelius, , A. s. Liet, TsR &ks/u Akad. Darb., ser, 1982, 3-9; Chem. Abstr. 1982, 97, 68449m.
2355
Kulys, J. J.; Samalius, A. S. Elektrokhimiya 1984, 20, 637-641. Frew, J. E.; Harmer, M. A.; Hill, H. A. 0.; Libor, S. I. J. Electroanel. Chem. 1988, 201, 1-10, Armstrong, F. A.; Lannon, A. M. J. Am. Chem. SOC. 1987. 109, 721 1-7212. Watanabe, T.; Okawa, Y.; Tsuzuki, H.; Yoshda, S.; Nfhei, Y. Chem. Lett. 1988. 1183-1186. Okawa, Y.; Tsuzuki, H.; Yoshida, S.; Watanabe, T. Anal. Sci., in press. Tsuzukl, H.; Watanabe, T.; Okawa, Y.; Yoshida, S.; Yano. S.; Koumoto, K.; Komiyama, M.; Nihei, Y. Chem. Lett. 1988, 1265-1268. Strojek, J. W.; Kuwana, T. J. Electroanal. Chem. 1968, 16,471-483. Armstrona, N. R.: Lin, A. W. C.; Fuiihira. M.; Kuwana. T. Anal. Chem. 1876, 48; 741-750. Moses, P. R.; Wier, L.; Murray, R. W. Anal. Chem. 1975, 47, 1882-1 086. Yamazaki, I.; Yokota, K. Mol. Cell. Biochem. 1973, 2 . 39-52. Maklno, K.; Maruo, S.; Morita, Y.; Takeuchi. T. Biotechnol. Bioeng. 1988, 31, 617-619. (17) Kirkeby, S . ; Jakobsen, P.; Moe. D. Anal. Lett. 1987, 20, 303-315. (18) Yamada, H.; Yamazaki, I. Arch. Biochem. Biophys. 1974, 165, 728-738.
RECEIVED for review June 8,1989. Accepted August 10,1989. This work was supported in part by the Asahi Glass Faundation and Grants-in-Aid from the Ministry of Education, Science and Culture of Japan (No. 63108001 and 63604517).
Interdigitated Gate Electrode Field Effect Transistor for the Selective Detection of Nitrogen Dioxide and Diisopropyl Methylphosphonate Edward S. Kolesar, Jr.,* and John M. Wiseman Air Force Institute of Technology, Department of Electrical and Computer Engineering, Wright-Patterson Air Force Base, Dayton, Ohio 45433-6583
An interdigitated gate electrode field effect translstor (IGEFET) coupled to an electron beam evaporated copper phthalocyanine thin flim was used to selectlveiy detect partper-biiiion concentration levels of nitrogen dloxlde (NO,) and dbopropyl methy@hoqhonate (DIMP). The sensor is exdted with a voltage pulse, and the time- and frequency-domain responses are measured. The envelopes of the magnitude of the normalized dlfference frequency spectrums reveal features that unambiguously distinguish NO, and DIMP exposures.
INTRODUCTION Two critical groups of environmentally sensitive contaminants are the oxides of nitrogen and the organophosphorus pesticides and their structurally affiliated compounds. It is now recognized that even trace amounts of these noxious pollutants may have an adverse effect on certain ecological systems. This recognition motivates the development of sensitive and selective personal monitoring instrumentation to detect subthreshold levels of these toxic compounds. The oxides of nitrogen, particularly nitrogen dioxide (NO2), are unintentionally emitted into the atmosphere from a number of industrial stacks and the exhaust of automobiles. In addition, NOz is known to evolve from the detonator chemical matrix in certain munitions as they age and cause corrosion of the electrical firing mechanism ( 1 ) .
In contrast to the NO2 pollutant, the organophosphorus pesticides and structurally related compounds are synthesized and valued for their deleterious effect and persistence, and their distribution is essential for their efficacy. A significant portion of the organophosphoruscontaminants contain either the phosphoryl or thiophosphoryl group. Since diisopropyl methylphosphonate (DIMP) is a phosphoryl-containingcompound, has low toxicity, and has been used in prior detector development investigations, it was selected as a model compound in this research (2-8). In recent years, coated bulk-wave piezoelectric quartz crystal microbalances (2-6,9, IO) and surface acoustic wave devices ( 11-15) have been investigated as candidate detector technologies for NO2 and the organophosphorus compounds. However, significant limitations associated with these technologies include the lack of selectivity, sensitivity to moisture, and overall response reproducibility. This paper reports the electronic properties that are modified when an interdigitated gate electrode field effect transistor (IGEFET) is coupled to an electron-beam evaporated copper phthalocyanine (CuPc) thin film which is used to selectively detect parts-per-billion (ppb) concentration levels of NO2 and DIMP. The IGEFET’s operation is based on the sensitivity of the field effect transistor’s output current to changes in the molecular structure or chemical composition of the thin film which covers the interdigitated gate electrode. These molecular and compositional changes are manifested as a correspondingchange in the gas-sensitive film’s dielectric relaxation function or, more succinctly, as a change in the
This article not subject to U.S. Copyright. Published 1989 by the American Chemical Society
2356
ANALYTICAL CHEMISTRY, VOL. 61, NO. 21. NOVEMBER 1, 1989
impedance of the thin film that couples the array elements comprising the interdigitated gate electrode structure (16,17). The concept of utilizing an interdigitated electrode chemiresistor structure to monitor impedance changes caused by a chemical reaction has been reported in the literature (18). Epoxy cure monitoring (ZS23) and the interaction of NOz and organophosphorus compounds with phthalocyanine films have been studied (24-29). The overwhelming majority of these investigations have focused on the direct current electrical conductivity changes manifested in the chemically active films, while a few have examined the alternating current impedance behavior a t specific frequencies. The influence of the chemically active film's dielectric relaxation behavior motivated the concept of the IGEFET sensor. Copper phthalocyanine is a member of the metal-substituted phthalocyanine family (which includes Ag, Be, Fe, Mg, Mn, Na, Ni, Pb, Pd, Pt, and Zn) whose electrical conductivity has been experimentally observed to increase upon exposure to electron acceptor challenge gases (for example, BCI,, BF,, CI,, F,, and NOz) (24,28, 30-32). The stability of CuPc is manifested by its low proton affinity, its resistance to dissolution by concentrated mineral acids, and the fact that it can he sublimed a t temperatures as high as 580 "C without decomposition (33, 34). The strong electron donor sites that comprise the planarly delocalized *-electron system in CuPc thin films have been postulated to be responsible for the experimentally observed electron-acceptor gas exposure interactions and the corresponding electrical conductivity changes (30-34). Since NO, is reversibly adsorbed on heated CuPc thin films (typically at temperatures spanning 100-170 "C), the interaction site has been identified to be at the film's intercrystallite interfaces (rather than being a true bulk diffusion mechanism) (32). The interaction betmeen the CuPc film and the electron-acceptor gas is likely to be a charge transfer interaction; that is, an interaction whose energy is stronger than a purely adsorptive process (less than 40 kJ/ mol), but which is weaker than a true covalent bond (approximately 300 kJ/mol) (14.32). Consequently, the selectivity of the interaction between a CuPc thin film and a specific challenge gas will he strongly influenced by the electronic and steric features of the film and gas. In this experimental investigation, both NO, and DIMP were reversibly adsorbed, and NOz induced a more pronounced electronic interaction for identical exposure concentrations.
SENSOR CONCEPT As illustrated in Figure 1, the IGEFET sensor consists of an interdigitated electrode structure which is coupled to the gate of a conventional metal-oxide-semiconductor field effect transistor (MOSFET). The interdigitated gate electrode structure is composed of a driven-electrode which envelopes the entire sensor and functions as a guard ring to minimize stray surface leakage currents. The corresponding floatingelectrode component is used to establish an electrical connection with the MOSFETs gate oxide. Electrical isolation between the driven- and floating-electrodes (greater than 100 Ma) is accomplished by supporting them on a thick (at least 1Fm), high-quality (resistivity greater than lOI4 a cm)silicon dioxide layer. The IGEFET's gas sensitivity is realized by depositing a chemically active membrane on the surface of, and between, the interdigitated electrode array elements. When the chemically active membrane is exposed to the challenge gas of interest, the membrane's electrical impedance is perturbed. These impedance perturbations can be observed and quantified when the IGEFET's driven-electrodeis excited with a voltage pulse. As a consequence of the floating-electrode being electrically connected to the MOSFET's gate oxide, charge is transferred through the chemically active membrane and manifests itself as a temporally dependent
I
I
*I 10.15
"Dlt.1
Interdsitated gate electrode Re!d effecttransistor (IGEFET) physical structure and electrical connections. Legend: (1) copper phthalocyanine (CuPc) thin film, (2) driven-electrode conductor, (3) floating-electrode conductor, (4) MOSFET drain contact. (5) MOSFET gate and floating-electrode contact, (6) MOSFET source contact, (7) drivenelectrode contact, (8) MOSFET source region, (9) MOSFET drain region, (IO)MOSFET drain-to-source channel, (11) host silicon substrate. Flgure 1.
potential applied to the high-input impedance gate contact of the MOSFET (35). The judicious specification of the MOSFET's materials, geometry, and operating bias conditions establishes its amplification characteristics (principally, ita gain, linearity, and cut-off frequency). As a result of the MOSFETs transfer function (36),the gas exposure induced potential perturbation appearing a t the MOSFET's gate terminal is manifested as a corresponding and amplified change in the device's drain-bsource current. By operating the MOSFET as a linear amplifier and connecting the drain-to-source terminals with an external series resistance, a potential corresponding to the temporally dependent drain-to-source current can he directly measured across the resistor. Thus, the electrical isolation afforded by the dielectric-supported interdigitated electrode and the gate oxide allows the MOSFET to manifest itself as an in situ "observation window" that monitors the electrical behavior of the chemically active membrane while minimizing its influence on the challenge gas induced changes. A significant limitation associated with the vast majority of electronic thin film gas detectors is that they only examine the sensor's response to a gas challenge a t a single operating point; that is, a direct current resistance measurement or the impedance a t a specific frequency. As a consequence, the detector's selectivity is often compromised because the electrical perturhation associated with the parameter being quantified could have been just as likely caused hy an interferant. Accordingly, the IGEFET sensor is operated to maximize its selectivity performance feature. The selectivity performance of the IGEFET sensor is realized by computing the normalized difference Fourier transform magnitude spectrum. The long-term excitation pulse characteristics, the purged CuPc-coated IGEFET response, and the equilibrated sensor response to a specific challenge gas concentration were experimentally observed to be reproducible and time-invariant. Consequently, the normalized differenceFourier transform magnitude spectrum was computed by Fourier transforming the time-domain voltage
ANALYTICAL CHEMISTRY, VOL. 61, NO. 21, NOVEMBER 1, 1989
2357
Table I. Critical Dimensions of the IGEFET Sensor Structure structure interdigitated gate electrode number of fingers in the floating electrode number of fingers in the driven electrode electrode finger width, pm electrode finger separation, pm interdigitated electrode array length, pm interdigitated electrode array width, pm MOSFET active gate length, pm active gate width, pm
feature 29 30 7.5 9 3792 921 12 15
response due to a challenge gas exposure and then normalizing it with an empirically determined factor that was also used to normalize the corresponding Fourier transform of the sensor’s purged time-domain voltage response. These two spectra were then subtracted from each other on a pointby-point basis at common frequencies, and the magnitude of the result was used to generate the sensor’s response envelope. In practice, the area beneath the normalized difference Fourier transform magnitude spectrum could be used to report the sensitivity of the sensor to a specific challenge gas, while the abundance of componentsthat define the shape of the spectral response envelope yields valuable information for unambiguously specifying the sensor’s selectivity. That is, as demonstrated experimentally, although different concentrations of NO2 and DIMP may manifest an identical change in the direct current resistance of a CuPc-coated interdigitated electrode structure, the shape of the normalized difference Fourier transform magnitude spectrum clearly distinguishes the dielectric relaxation behavior caused by the two challenge gases.
SENSOR FABRICATION The IGEFET was designed and fabricated by using the Metal Oxide SemiconductorImplementation Service (MOSIS) 3-pm, p-well, double-metal, complementary metal oxide semiconductor (CMOS) technology (37). An otherwise identical reference transistor, except that the gate electrode was solid metal, was located adjacent to each IGEFET sensor to facilitate postfabrication MOSFET performance characterization and the subsequent monitoring of operational stability in the thermostated gas challenge cell. A p-well was incorporated to enhance electrical isolation between the interdigitated electrode structure and the MOSFET, and n-channel enhancement-mode devices were utilized. Aluminum was used to implement the second-levelmetal interdigitated electrode array and the sensor’s bonding pads. Except for the interdigitated gate electrode structure and bonding pads, the surface of the IGEFET was passivated with a 3 pm thick layer of silicon dioxide. The overall dimensions of the IGEFET integrated circuit were 4466 X 6755 pm. The critical IGEFET structural dimensions are summarized in Table I. High-purity CuPc films (Fluke Chemical Corp., Ronkonkoma, NY) were deposited on the dielectric-supported interdigitated electrode structure using a helium cryogenically pumped, electron-beamthermal evaporation process operated at lo4 Torr vacuum (Denton Vacuum, Inc., Model DV-602, Cherry Hill, NJ). Compared to the bulk sample sublimation technique, the electron-beam evaporation process affords the opportunity to control both the deposition rate and the thickness of the CuPc film. In practice, the electron beam’s power was tuned for a nominal 10 Afs deposition rate, and the substrate was positioned 17 cm directly above the source. A mechanical shutter was used to isolate the IGEFET during the initial electron-beam tuning process, and it was subsequently used to terminate the deposition process at the desired
Interdigitnted Gate Electrode Structure
Floating Electrode
Electrode
I
I
Pulse Generator 1
Metal-OxldeSemiconductor Field Effecl Transistor
I
I I I
> I Gain/Phnse Analyzer
r
I
Fourier Transform Analyrcr
I
I
H
Oscilloscope
Microcomputer Data Acquisition System
Flgure 2. Instrumentation arrangement for measuring the electrical performance of the IGEFET sensor.
film thickness. An etched metal mask was used to confine the deposition within the boundaries of the interdigitated electrode structure. Nominal film thicknesses on the order of 1000 A were determined by using a precalibrated quartz crystal microbalance that was positioned coplanar with the interdigitated electrode structure. An ellipsometer (Gaertner Scientific Corp., Model L-117, Chicago, IL) was used to corroborate the deposited film’s thickness. Using the results reported by Renyuan et al. (38)concerning the refractive index and density of evaporated CuPc films, the results for the quartz crystal microbalance and ellipsometer compared favorably (to within 10%) for thin film samples. The extraordinary stability of vacuum sublimed CuPc thin films is reported in several investigations (14, 15, 28, 30-33). After the CuPc film was deposited, the discrete IGEFET sensor die were mounted in a standard 64-pin dual-in-line package (Kyocera Corp., part number KD-83578, Edina, MN). The package’s 16 bond pads that are distributed along one edge of the 300 X 300 mil cavity were sufficient to accommodate the required electrical connections to the IGEFET sensor and reference MOSFET.
INSTRUMENTATION The electrical instrumentation arrangement for characterizing the challenge gas exposure performance of the CuPc-coated IGEFET is illustrated in Figure 2. A 4-V peak-amplitude, 50 ps duration, 256 Hz repetition frequency pulse excitation signal was applied to the IGEFET’s drivenelectrode with a function generator (Wave-Tek Corp., Model 148, San Diego, CA). The frequency spectra of the IGEFET’s input excitation signal and output response were measured simultaneously with a low-frequency (25 kHz bandwidth) dual-channel Fourier transform analyzer (Bruel and Kjaer Instruments, Inc., Model 2032, Marlborough, MA). To complement the frequency-domain measurements, the corresponding time-domain spectra were captured on a dual-trace oscilloscope (Tektronix Corp., Model 475, Beaverton, OR). A gainfphase analyzer (Hewlett-Packard Corp., Model HP4194A, Palo Alto, CA) was incorporated to measure the voltage gain (20 log,, [V,,/Vi,l) and phase delay - &,) performance of the IGEFET using a sinusoidal test signal whose frequency spanned 10 Hz to 10 MHz. For completeness, the direct current resistance of the CuPc-coated interdigitated
2358
ANALYTICAL CHEMISTRY, VOL. 61, NO. 21, NOVEMBER 1, 1989
electrode structure was independently monitored with an electrometer (Keithley Instruments, Inc., Model 617, Cleveland, OH) to determine the sensor's equilibrium response during the cyclical challenge gas exposure and purge processes. Data collection was automated by use of a microcomputer (Zenith Data Systems Corp., Model 2-248, St. Joseph, MI) equipped with an IEEE-488 interface plug-in card (Capital Equipment Corp., Model 01000-60300, Burlington, MA). The design and operation of the dynamic gas generation and delivery system have been discussed by Kolesar in a previous paper (7). For this application, a manifold was included to facilitate switching between the NOz and DIMP permeation tubes (GC Industries, Models 23-7502 and 23-7392, Chatsworth, CAI. When the permeation tubes were thermostated at 50 "C, a broad challenge gas concentration range spanning 20-400 parts-per-billion (ppb) for NO2and 40-4000 ppb for DIMP was achieved. The temperature of the free-flow Pyrex sensor cell was thermostated at 125 f 2 "C by submersing it in a silicon oil bath. A network of silica gel beds were used to extract water vapor from the filtered laboratory air carrier (challenge gas diluent); the relative humidity was maintained at less than 2% throughout the IGEFET sensor characterization measurements. A tapered Teflon plug in the top of the sensor cell provided ports for the challenge gas delivery and exhaust, as well as the IGEFETs electrical wiring harness.
RESULTS AND DISCUSSION MOSFET Performance Characteristics. The electrical transfer function characteristics of the MOSFET portion of the IGEFET sensor and reference transistor were periodically monitored throughout the duration of the challenge gas exposure experiments. The average low frequency gain for an ensemble of six MOSFET's measured with the gain/phase analyzer was 12 f 1 dB. Independent confirmation of this value was derived from the direct current device characteristics measured with a semiconductor parameter analyzer (Hewlett-Packard Corp., Model HP4145, Palo Alto, CA). With a gate-to-source voltage ( VGS) of 10 V, the MOSFET's (), was calculated to be 0.133 mS from the transconductance g slope of the drain current (ID) versus gate-to-source voltage (vG$) plot. Correspondingly, with a V, of 2.5 V, the drainwas calculated to be 130 kQ from to-source on-resistance (9) the ID versus drain-to-source voltage (VDS) plot. By use of these g, and r D values, along with the externally connected 100-k!? drain bias resistor (Figure l),the direct current gain of the MOSFET was calculated to be approximately 15 dB (36). By comparison, the gain of the reference MOSFET was consistently 1dB greater than the gain of an adjacent IGEFET sensor, partially reflecting the differences between the two gate electrode geometries. Finally, the 3-dB cut-off frequency of the MOSFETs was on the order of 10 kHz, and the phase lag decreased from 180" at low frequencies to nearly 90" at 100 kHz. Direct Current Conductivity Measurements. Isothermal (125 "C) direct current conductivity measurements of the CuPc-coated interdigitated gate electrode structure were accomplished to determine the cyclical exposure and purge dynamics of the gas-sensitive film. In these measurements, the NOz and DIMP challenge gas concentrations were varied spanning 20-400 and 40-4000 ppb, respectively. The exposure portion of the cycle was 20 min in duration to ensure than an equilibrium response was attained. The corresponding purge cycle was similarly conducted for 30 min. The purge gas was filtered room air (less than 2% relative humidity). For each 1000 A thick film sample evaluated, it was observed that the initial and final values of the film's resistance were not reproducible for the first three exposure and purge cycles. However, a rapid convergence toward a set of repro-
a
a"
j
8
OO
1
0
20
60
40
TLp. Waut..)
Figure 3. Isothermal (125 "C) resistance of the interdigitated gate electrode structure coated with a 1000 A thick film of CuPc upon exposure to attemating purges and challenges of 40 ppb of NOpmixed with laboratory air (2% relative humidii). (The exposure and purge cycle depicted resulted after the preconditioning process; each data point represents the arithmetic average of four independent measurements). CUPS
.E9
ri-
'E9
mist-w C 3E3
IE9
whui ut.= uo2 .=u>E5
0
6I5
C lE9
C BE9
u 5E9
r C
1 8 . 0 5E5
. > 0 629 0 7E9 CUP= r r l r hsi.tmw
.
/
0 8E9 i o h u ) xft.r
,
,
8
0 5E5 QUIP
,
I
.
1
,
,
D
,
/
OE5
Flgure 4. Isothermal (125 "C) equilibrated electrical resistance of a 1000 A thick CuPc-coated interdigitated gate electrode structure as a function of the challenge gas concentration. (Each data point represents the arithmetic average of the film's resistance measured during the final 3 min of the 20-min exposure cycle, 2% relative humidity filtered laboratory air carrier.)
ducible initial-, final-, and delta-resistance values was noted after the films were "preconditioned" with a sequence of three, 30-min duration low-concentration level exposure and purge cycles. Additionally, the time increment required to attain equilibrium initial and final resistance values was observed to decrease after the preconditioning calibration procedure was accomplished. Figure 3 depicts the dynamics associated with one complete exposure and purge cycle for a 40 ppb NOz challenge which occurred after the "preconditioning" process was implemented. The DIMP response is similar except that the film's resistance change is much less than that observed for the same concentration of NO2. For both challengegases, the resistance of the film rapidly decreased and achieved 63% of its final value during the fiist 3 min of the exposure cycle; afterward, the resistance continued to decrease toward an equilibrium value, but at a much slower rate. When the challenge gas was purged, the film's resistance rapidly increased during the f i t 5 min of the cycle and achieved 63% of its final value; afterward, the resistance continued to increase toward an equilibrium value, but at a much slower rate. As depicted in Figure 4, the trend associated with the film's equilibrium resistance values resulting
ANALYTICAL CHEMISTRY. VOL. 61, NO. 21. NOVEMBER 1. 1989
2359
I I
1
! ier
if)
e
-
5
b
if) 6
2 4
G 4 $ 0
UPPERTRACE
0
LOWERTRACE
5 1.0
UPPERTRACE
z o 0
LOWER TRACE
0.4
0 1w 2w 3w 400 sw 0 100 200 300 4LM 5w
TIME IN MICROSECONDS
M I E IN MICROSECONDS
n"ain response of sensor when excned with me pulsed-vonage waveform as a lunclion of the DlMP challenge conc e n h a h (copper phthabcyanine 1000 A m(dc nim. 125 'C operamg temperature,and 2% relative humidity finered laboralory air carrier): (a1 preconditioned linered laboralw air response. (bl 100 ppb DIMP exposure, (c) purge with linered laboratory air, (d) 800 ppb DIMP exposure. (e) purge with likered laboratory air. (I)4000 ppb DIMP exposure. The uppar lrace in each frame is the pulsed-vonage excitation sigml(4-V amplitude. 50-fls pulse duration). The lower tram in each frame is the sensor's response.
response 01 me sensor when excned wlth me pulsed-voltage wavelwm as a lunctlon of the NO, challenge concanvatiw -( phthabcyanine IWO A Wck fiim. 125 OC operaw Iemperatue. and 2% relalive humidity finered labaalay air carrier): (a) precondkioned linered laboratory air response. (b) 100 ppb NO, exposure. (cl purge wnh linered laboratory air. (d) 200 ppb NO, exposure. (el puge IiHerec laboratory a t . (R4Wppb NO, exposure. The upper trace in each frame is the pulsed-vonage excnation signal ( 4 4 a m p W . 50-fls puke duration). The lower hace in each frame is the sensor's response. F I w a 5. l l " a l n
*
from five different NO?exposure concentrations (measured after the "preconditioning" process was implemented and 20 min had elapsed during the exposure cycle) is significantly more sensitive and highly nonlinear, while that for the less electronegative and physically larger DIMP species is much less sensitive (a corresponding logarithmically scaled plot of the film's resistance yields a linear relationship with respect to the DIMP challenge gas concentrations). From the slopes of the least-squares fitted NO? and DIMP exposure plots, an 800 ppb DIMP challenge concentration would be required to induce the equivalent resistance change that would otherwise be obtained from a 30 ppb NOZ challenge concentration. The preconditioning exposure behavior suggests that the challenge gases displace residually adsorbed, but less tightly bound species (for example, 0,) (32). Additionally, this characteristic and the observed shortening of the time increment required to attain a reversible response is consistent with the heterogeneous intercrystalline interface surface site adsorption model discussed earlier. Finally, in view of this kinetically limited adsorption behavior, two profound operating constraints will need to be incorporated into the sensor technology to achieve a reproducible and reversible response. First, depending upon the specific application. the films must be isothermally preconditioned with low concentrations of the
Figure 6.
challenge gas mixed with the filtered ambient atmosphere. Second, gas sampling will need to be accomplished by using a pulsed-mode technique to accommodate the variable response time in order to achieve a thermodynamic equilibrium surface coverage condition. Time- a n d Frequency-Domain Transient Response of the IGEFET. The isothermal (125 "C) time-domain voltage waveforms measured across the 100 kR drain-to-source external resistor (Figure 1) after cyclical exposures and purges with NO, and DIMP challenges are illustrated in Figure 5 and 6, respectively. The Same IGEFET was utilized to collect the time-domain challenge gas exposure responses. For each of the challenge gases, the preconditioning process was accomplished, followed by a 30-min purge and 20-min e x p u r e cycle to collect the equilibrated direct current performance data. For example, after the IGEFET was preconditioned with a low-level NOz exposure, the challenge gas performance data were collected (Figure 5). Following a 24-h purge with filtered laboratory air (less than 2% relative humidity), the DIMP challenge gas IGEFET performance data were likewise collected (Figure 6). Reproducible gas exposure results were obtained with sensors fabricated from the same batch and when the order of the challenge gas exposure trials was reversed. Additionally, by comparison of Figures 5a and 6a, the IGEFETs response following the 24-h purge between the challenge gas exposure trials yields an essentially identical
2360
ANALYTICAL CHEMISTRY, VOL. 61, NO. 21, NOVEMBER 1, 1989
COO:
E
s
rr-sy
15”CS
25xc
zr, 8.a. (8.1
Flgure 7. Magnitude of the normalized difference Fourier transform spectra for the NOp challenges depicted in Figure 5. (Each data point represents the arithmetic average of six independent measurements.)
Flgure 8. Magnitude of the normalized difference Fourier transform spectra for the DIMP CheHenges depicted in Figwe 6. (Each data point represents the arithmetic average of six independent measurements.)
base-line result. This characteristic attests to the reversibility kinetics of the CuPc film. For completeness, it is also noted that the IGEFET’s response returns to the initial “preexposure” status (Figures 5a and 6a, respectively) after each of the 30-min purges which follow a specific challenge gas concentration exposure. Although the time-domain IGEFET response for each of the exposure gas species and challenge concentrations are different (Figures 5 and 61, meaningful criteria for differentiating which gas species and the concentration being detected is not obvious due to the similarity of the response waveforms. To resolve this dilemma, the authors implemented the signal-processing technique referred to earlier in this paper as the calculation of the normalized difference Fourier transform magnitude spectrum. That is, because the relative curvatures of the leading and trailing edges of the IGEFET responses to NO2and DIMP are distinct and, further, because the peak of each response changes for different exposure concentrations, Fourier’s theorem was applied to determine the ensemble of frequency components that determine the unique timedomain dielectric relaxation response. The time-invariant and reversible behavior of the CuPc film to NOz and DIMP challenges was critical for implementing this signal-processing scheme because it provided a common base line for computing the normalized difference Fourier transform magnitude spectra. The corresponding normalized difference Fourier transform spectra associated with the NO2 and DIMP time-domain IGEFET responses (Figures 5 and 6, respectively) are shown in Figures 7 and 8, respectively. The critical advantage of this signal-proceasing scheme is that it produces a spectral envelope that unambiguously ascribes a selectivity feature to the IGEFET sensor. That is, if the seta of spectra in Figures 7 and 8 are superimposed, the envelopes only intersect at a few select points, and this characteristic clearly emphasizes the difference between the two challenge gas exposure responses. For emphasis, the first peak of the NO2 response occurs at approximately 1000 Hz, while that for DIMP occurs at approximately 3400 Hz. Additionally, the rise and decay rates associated with the first peak are more pronounced in the NOP spectra. Both spectra also contain a secondary peak; the NO2 secondary peak occurs a t approximately 8200 Hz, while that for DIMP occurs in the vicinity of 25 kHz. These low-frequency (acoustic) resonant peaks suggest that a long-range dielectric polarization interaction is being facilitated by the adsorbed challenge gases. Qualitatively, the electronic and steric features of the challenge gases, along with the interstitial crystallite adsorption model discussed earlier, are postulated to account for the observed interfacial dielectric relaxation behavior. A concise quantitative treatment of the interaction mechanism is not possible with this limited data set because
of the heterogeneous nature of the challenge gas adsorption process and the disordered grain-boundarynature of the CuPc films.
CONCLUSION Perturbation of the dielectric relaxation function of an electron-beamevaporated thin f i i of CuPc supported on the interdigitated gate electrode structure of a MOSFET in response to parts-per-billion levels of NO2and DIMP challenges was investigated. Calculation of the normalized difference Fourier transform magnitude spectra from the time-domain pulsed-voltage excitation responses of the IGEFET unambiguously revealed that the sensor concept could be utilized to selectively detect low concentrations of NOz and DIMP when either component is mixed with a common filtered laboratory air source (diluent). For the practical application of detecting corrosive NO2 emissions in the controlled environment of a hermetically sealed munitions detonator, this technique merits further exploitation. For more universal applications, this technology must also demonstrate a capacity to selectively detect two or more components in a multicomponent gas mixture. When compared to the conventional direct current or single-frequency alternating current modes of operating chemically active thin-film detectors, this new signal-processing technique yields an ensemble of frequency components which define a challenge gas response envelope. The ambiguity when attempting to differentiate specific challenge gas responses is minimized because the complete response envelopes are compared, rather than single data points, which are typically reported for the conventional direct current operating mode. While the spectral features for NO, and DIMP unambiguously differentiate between these two challenge gases, it is acknowledged that an interferant likely exists that will yield a spectral envelope that would be similar to either of the challenge gases. If the ambiguity level between sensor responses becomes a problem, it should be possible to minimize its influence by implementing a more sophisticated signalprocessing technique, such as first- and second-order differentiation of the spectral envelopes. For the 1000 A thick CuPc film evaluated, a reversible response to both challenge gases was achieved when the IGEFET was operated at 125 O C . Considering the chemical kinetics and the corresponding low-frequency (acoustic) electronic resonant behavior, a qualitative long-range, heterogeneous interfacial grain boundary adsorption site model was postulated as a likely mechanism. The results clearly motivate follow-on gas-sensitivity investigations with CuPc and the other metal-substituted phthalocyanine films that specifically examine the influence of temperature, film thickness, deposition technique (for example, electron-beam
ANALYTICAL CHEMISTRY, VOL. 61, NO. 21, NOVEMBER 1, 1989
evaporation and Langmuir-Blodgett), and the film’s morphology (especially the degree of polycrystallinity). Additionally, these investigations should contribute toward a more complete understanding of the relationship between the chemical kinetics and the observed electronic performance. Although the IGEFET sensor’s support electronics are more complicated when compared to the conventional direct current or single-frequency alternating current impedance bridge techniques, the required technology is currently available in discrete form and could readily be integrated into a unified design. For example, by fabricating the sensor using monolithic silicon integrated circuit technology, a diffused resistor (heat source) and diode (temperature sensor) combination could be utilized to provide a stable thermostated environment. Consequently, the IGEFET sensor concept offers significant promise as an alternative technology for detecting a host of gaseous contaminants when coupled with an appropriate chemically active film. By extension, an array of discrete IGEFETs, each supporting a different chemically active film, could be electronically multiplexed to measure different species. A dedicated microprocessor could also be employed to control the sensor’s pulsed-mode sampling technique and process the discrete responses. Utilization of pattern recognition algorithms would further enhance the IGEFET sensor’s selectivity.
LITERATURE CITED (1) Lavoie, J. M., EG&G Mound Appiled Technologies, Miamisburg, OH, February 10, 1988,penonal communlcatlon. (2) Schelde, E. P.; Gullbault, 0. G. Anal. Chem. 1972, 4 4 , 1764-1768. (3) Shackelford, W. M.; Gullbault, G. 0. Anal. Chim. Acta 1974, 73, 383-389. (4) Tomb, Y.; Guilbauit, 0. G. Anal. Chem. 1980, 52, 1484-1489. (5) Gullbauit, G. G.; Tomb, Y.; Kolesar, E. S., Jr. Sens. Actuators 1981,
2,43-57. (6) Gullbault, G. G.; Affolter, J.; Tomb, Y.; Koiesar, E. S., Jr. Anal. Chem. 1981,53,2067-2060. (7) Kolesar, E. S., Jr.; Waber, R. M. Anal. Chem. 1988, 60, 1731-1736. (8) Kolesar. E. S.,Jr.; Walser, R. M. Anal. Chem. 1988,60, 1737-1743. (9) Karmarkar, K. H.; Gullbault, G. G. Anel, Chlm. Acta 1975, 75, 1 1 1-117. (10) Webber, L. M.; Hiiavay, J.; Gullbault, G. 0. Mkrochim. Act8 1978, 7 , 351-358. (11) Barendsz, A. W.; VIS, J. C.; Nieuwenhulzen, M. S.; Nieuwkoop, E.; Vellekoop, M. J.; Ghljsen, W. J.; Venema, A. Roc. I € . € Ulbason. Symp. 1985. 586. (12) Nleuwenhulzen, M. S.;Barendsz, A. W. Electron Left. 1988. 22, 184- 165. (13) Venema, A.; Nleuwkoop, E.; Ghlhn, W. J.; Barendsz, A. W.; Nieuwenhulzen, M. S. I€€€ Tf8nS. UmZrson.,F m k h k s , Freq. Control 1987, WFC-34, 148-154. (14) Nleuwenhulzen, M. S.;Barendsz, A. W. Sens. Actuators 1987, 7 7 , 45-62.
2361
(15) Nieuwenhulzen, M. S.;Nederlof, A.; Barendsz, A. W. Anal. Chem. 1988, 60, 230-235. (16) Blythe, A. R. Ekfrlcal Ropwtbs of Polymers; Cambridge University Press: New York, 1979 Chapter 3; pp 38-67. (17) Impedance Spectroscopy-Emphaslzing W i d Materials and Systems: Macdonakl. J. R.. Ed.; John Wlley and Sons: New York, 1987; Chapter 2;pp 27-79. (18) Kutrler, F. W.; Barger, W.; Snow, A.; Wohltjen, H. Thin SOlM Films 1967. ..-., 155. .- - , 1-16. . .. . (19) Sheppard, N. F.; Garverlck, S. L.; Day, D. R.; Senturia, S. D. Roc. 26th Iuatl. S A M E Svmo.. 26 1961. 85-76. (20) Senturia, S.D.; Sheipa;d, N. F.; So&, Y. P.; Appeiman, H. R. Polym. Eng. Sci. 1981, 27, 113-118. (21) Senturia, S.D. S A M E J. 1983, 79, 22-26. (22) Day, D. R. Roc. Natl. SAMPESymp., 37sf 1986, 1095-1103. (23) Sentwla, S.D.; Sheppard, N. F. Dielectric Analysis of Thermoset Cure;
Epoxy Resins end Composkes I V ; Dusek, K., Ed.; Springer-Veriag: New York, 1986. (24) Baker, S.;Roberts, G. G.; Petty, M. C. IEE Roc.,Parf I : Solkl-State Electron Devices 1983, 130, 280-263. (25) Barger, W.; Wohltjen, H.; Snow, A. Roc. I€€€ Int. Conf. SoIMState Sens. Actuators 1985, 410-413. (26) Wohltjen, H.; Barger, W. R.; Snow, A. R.; Jarvis, N. L. IEEE Trans. E k t r . Dev. 1985. ED-32. 1170-1174. (27) Tregold, R. H.; Young, M. C. J.; Hodge, 8.; Hoorfar, A. I € € R o c . Perf I : solid-state Electron Devices 1985, 732,151-156. (28) Jones, T.; Bott, 8. Sens. Actuatws 1986,9 , 27-37. (29) Burr, P. M.; Jeffery, P.D.; Benjamin, J. D.; Uren, M. J. Thh Solid Films 1987, 757, Llll-L113. (30) van Ewyk. R. L.; Chadwick, A. V.; Wright, J. D. J . Chem . Soc ., Fare day Tf8nS. 7 IS80, 76, 2194-2205. (31) Wilson, A.; Collins, R. A. Sens. Actuators 1987, 12, 389-403. (32) Archer, P. B. M.; Chadwick, A. V.; Mlask, J. J.; Tamlzl, M.; Wright, J. 0.Sens. Actuetors 1989, 76, 379-392. (33) Martell, A. E.; Calvin, M. Chemlsby of the Metal Chelete Compounds: Prentice-Hall: New York, 1952;Chapter 4;pp 165-167. (34) org8nlc Semiconducting Polymers; Katon. J. E., Ed.; Marcel Dekker: New York, 1968;Chapter 4; pp 202-208;Chapter 5; pp 259-267. (35) Senturia, S. D.; Sechen, C. M.; Wishneusky, J. A. Appl. Phys. Left. 1977, 30, 106-108. (36) Sze, S. M. fhysics of Semlconductor Devlces, 2nd ed.;John Wiley and Sons: New York, 1981;Chapter 8;pp 431-455. (37) MOSIS-Metal Oxide Semiconductor Implementatlon Service User’s Manual. release 3.0 1988 InformationScience Institute. University of Southern Callfornia (USC/ISI), Marina del Rey, CA. (38) Renyuan, Q.; Zurong, S.; Zhongfu, C. Kexue Tanbeo 1981, 26,
-
522-525.
RECEIVED for review May 5,1989. Accepted August 14,1989. The authors wish to acknowledge the financial and material support for this research provided by the United States Air Force, Air Force Systems Command, Wright Research and Development Center, Flight Dynamics Laboratory, WrightPatterson Air Force Base, OH, under fund cite 616-88114364; the Armstrong Aerospace Medical Research Laboratory, Wright-Patterson Air Force Base, OH, under fund cite HY88-022; and EG&G Mound Applied Technologies, Miamisburg,
OH.
