Anal. Chem. 1990, 62,542-544
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TECHNICAL NOTES Palladium Gate Metal-Oxide-Semiconductor Oxygen Sensors Jonas Karlsson, MArten Armgarth, Svante Odman,' and Ingemar Lundstrom* Laboratory of Applied Physics, Linkoping Institute of Technology, S-581 83 Linkoping, Sweden
INTRODUCTION Chemical sensors are electronic devices that are subjected to variations in the composition of their ambient. These variations may introduce not only the wanted electrical signal but also hysteresis and drift phenomena in the devices. We therefore found it of general interest to study the behavior of chemical sensors continuously operated close to steady state in terms of the response to the ambient. As an example we have studied the oxygen sensitivity of P d metal-oxide-semiconductor (PdMOS) field effect hydrogen sensors. This has been done for two reasons: first, the indirect oxygen sensitivity of the PdMOS hydrogen sensors is well established (I,2) and second, fast oxygen sensors with high-resolution capability should be very useful for measurements of small variations of oxygen concentration, e.g., in expired air. The hydrogen sensitivity of the PdMOS field effect transistor is explained by the fact that hydrogen gas adsorbs and dissociates on the catalytic P d surface ( I ) . Hydrogen atoms formed diffuse rapidly through the thin gate and adsorb at the metal-insulator interface, which gives rise to a change of the electrical field in the gate insulator of the FET, hence decreasing the threshold voltage of the device. It was observed that the response to hydrogen of this very sensitive and selective sensor decreased in the presence of oxygen in the ambient. A catalytic combustion of hydrogen by oxygen to water on the Pd surface explains the reduction of the response to hydrogen. The response of the PdMOS devices to hydrogen in oxygen is reported to follow a Langmuir-like expression in steady state ( I ) A V = AV,,C(PH,/P~,)'/~/(~ + c(PH,/PO,)~/~)(1) where AV is the change in threshold voltage of the PdMOS transistor. AVm, is about 0.5 V and c is a constant that depends on the rate constants governing the catalytic reactions between hydrogen and oxygen on the surface of the palladium gate. PH2and Po, denote the partial pressures of hydrogen and oxygen, respectively. With P H pin parts per million and Po, in percent, the constant c is typically about 0.1-0.5. Only a few attempts have been made earlier to use the oxygen sensitivity of the hydrogen sensor to determine oxygen. The reason for this is the lack of a practical, reliable method to counteract some drift phenomena induced by hydrogen in the PdMOS devices (3). On the other hand, there have been observations of large changes in the response to hydrogen due to changes in the oxygen concentration a t a given hydrogen to oxygen concentration ratio (PH,/Po, 0.4). This ratio is rationalized by the fact that the catalytic metal surface transforms from being oxygen to hydrogen covered a t that number ( 4 ) . The ratio may depend on the catalytic metal used (5). Also this effect turned out to be difficult to use due to, among other things, the hydrogen-induced drift mentioned above. The uncertainty of the response around the point of the steep change h a been estimated to be at least l o % , which does not give the necessary resolution in oxygen concentration. Center for Microtechnology, Linkoping Institute of Technology, S-581 83 Linkoping, Sweden.
We show here that small perturbations from a steady-state response can be used to detect oxygen with a high sensitivity. In such experiments, the sensor is exposed to a mixture of a given hydrogen gas concentration and an oxygen reference gas concentration within the actual measurement range. When the sensor signal has reached steady state, the gas to be tested and the oxygen reference gas are pulsed consecutively into the hydrogen gas stream, flowing continuously over the sensor (see the insert of Figure 1). The changes in threshold voltage are then in the order of f 1 0 mV, which is much lower than the 100 mV changes obtainable at the critical PH,/PO, ratio ( 4 ) . Still, these changes are well above the noise level of the threshold voltage. The threshold voltage change can be negative or positive depending on whether the oxygen concentration is highest in the test or reference gas as illustrated in Figure 1. The change in the threshold voltage is given by
where t , and t are the durations of the test gas and the reference gas pulses, respectively. Since AV in eq 2 is limited by the steady-state responses given by eq 1, then
It can readily be seen that the maximum response dV-, for a given difference in oxygen concentration of the test gas and the reference gas, is obtained by choosing the hydrogen concentration to
The maximum sensitivity is estimated to be 3 mV/% O2 change with an assumed POzrer= 20% and AV- = 0.5 V. The linear approximation in eq 5 introduces a relative error < 10% in the range of 16% to 24% 02. It should be pointed out that also other catalytic metals, like Pt and Ir, show hydrogen sensitivity in the presence of oxygen. A comparison between different metals (6) and especially between Pd and Pt (5) indicates that P d is the metal of choice for the present application.
EXPERIMENTAL DETAILS The hydrogen sensor used in the experiments was a PdMOS field effect transistor integrated with a temperature control circuit. The sensor was operated at 150 "C. The sensor was connected to a hydrogen leak detector (Sensistor Model 12N). It gives a voltage output which is a measure of the threshold voltage of the PdMOS hydrogen sensor. The gas manifold used was a computerized, general purpose system, designed for mixing different gases (from bottles) under well-controlled conditions. The different gas flows were regulated by mass flow controllers. The gas manifold was controlled by a PC (Ericsson), enabling automatic setting of the desired parameters building up a test sequence. The
C 1990 American Chemical Society 0003-2700/90/0362-0542$02.50/0
ANALYTICAL CHEMISTRY, VOL. 62, NO. 5, MARCH 1, 1990 543 H,/Ar
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Flgure 1. Example of the sensor response. The sensor, operating at 150 OC, is exposed to 850 ppm H, in argon (H2 on). After a few minutes, the 0,reference gas is mixed into the hydrogenlargon stream through the ejector. When a steady state is achieved, the sensor is exposed to alternating test and reference gas pulses. Response to 6 s long atternating exposures to test and reference gas, respectively. The 0, concentration in the test gas varied from 10 to 24%. The reference gas contained nominally 20% 0,.6Vdefined by eq 2 is indicated in the drawing. According to this definition 6 V > 0 when the test gas contains less oxygen than the reference gas. Insert: Ejector used to mix the hydrogen containing carrier gas with the oxygen containing gas (see the text for further description).
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O,(%I Figure 2. Sensor response 6 V (defined by eq 2 and in Figure 1) for 6 s long 0,pulses with randomized 0, concentration in the test gas pulses. The 0,concentration in the reference gas is determined to 20.7% from the drawing (at 6V = 0).
gas flows containing the oxygen/argon mixtures were stabilized, while fed to a gas drain, before they were switched over to the sensor. However, for pulse times shorter than about 1s, a complete stability of the gas concentrations was not obtained. The hydrogen- and oxygen-containing gases were mixed in an ejector before reaching the sensor. A schematic of the ejector is shown as an insert of Figure 1. It was made out of two steel tubes (actually surgical cannulas) where the inner one ended close to a hole in the outer tube. Hydrogen in argon with a flow rate of 60 mL/min passed through the inner tube. Measurements showed that flow rates above 40 mL/min were enough to cause a vacuum, which sucked gas through the hole in the wall. The gas stream outside the ejector consisting either of the test or the reference gas flowed through a wider tube with low resistance to flow. The flow rate through this tube was 260 mL/min. The amount of gas sucked through the hole in the ejector was not known. The gas composition a t the sensor was therefore unknown. It is, however, not necessary to know that composition in the present mode of operation, The measurement setup was tested for a number of different parameters, such as the duration of the test and reference gas pulses and hydrogen and oxygen flow rates. Each measurement series was initiated by exposing the sensor to a mixture of the oxygen reference gas and the hydrogen gas for a few minutes. With this procedure, the sensor response was allowed to reach almost steady state before the actual measurements started (see Figure 1).
The influence of pulse duration was, as expected, that the response increased with increased pulse duration. In the case of a 1-s pulse, the gas manifold system was too slow. However, since the responses for 6- and 11-s pulses were about the same, the response must saturate rather fast with time. In fact, the response curves have a nearly rectangular waveform (see Figure 1). This implies that greater pulse durations do not need to be well controlled, still having a high overall measurement accuracy. Secondly, the response of the hydrogen sensors is rather fast, upon a small change in the gas composition, compared with the response with rather long response times when the oxygen gas is introduced (see Figure 1). This difference can be explained by the fact that when a steady state has been obtained, both the hydrogen adsorption sites giving the Langmuir-like response and the sites responsible for the drift are in equilibrium with hydrogen in the palladium. The hydrogen-induced drift is attributed to hydrogen adsorption sites on the oxide side of the Pd-Si02 interface. They appear to be determined by (sodium) impurities in the oxide (3). The time constants for hydrogen adsorption and desorption in these sites are, however, much longer than the duration of the oxygen pulses. The occupancy of the slow sites does not change significantly during a single oxygen pulse. The pulse response is therefore rapid and reproducible even if there may be a slow base-line drift (as observed in Figure 1). A too low, or too high, hydrogen concentration will decrease the sensitivity. However, a large interval of useful hydrogen concentrations was found. As a consequence the hydrogen concentration does not have to be carefully controlled to maintain the maximum sensitivity. From eqs 3-5, it is found that the hydrogen concentration can be increased by a factor of 4, or decreased to one-fourth, of the optimum value, keeping the sensitivity within 90% of the maximum value. In the present mode of operation, the interference from other molecules is expected to be small. Hydrogen sulfide gives hydrogen-like responses. Molecules like ethanol and ethylene give only small response at an operation temperature of 150 "C. Since there is hydrogen in the ambient of the sensor all the time and since the hydrogen concentration is not critical, their influence will, however, be small. Oxidizing molecules like chlorine and hydrogen peroxide in the test gas may interfere with the oxygen-induced pulse response. It is not likely, however, that they will be present in large enough concentrations in the applications envisaged. Water vapor does not interfere with the Pd-gate device operated at 100-150 "C. At operation temperatures below 100 O C , water vapor may interfere with the oxygen signal.
RESULTS The results in the right-hand part of Figure 1were obtained with the oxygen concentration in the test gas changed in steps
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of 1% . No hysteresis was observed going up and down in the oxygen concentration. In Figure 2, the sensor was exposed to a number of test gas pulses with randomized oxygen concentrations keeping the oxygen concentration in the reference gas pulse constant. It was observed that the spread in the data occurred upon large successive changes in the oxygen concentration. The spread is assumed to be caused mainly by the gas manifold, due to hysteresis and delays in the valves. The sensitivity to oxygen can be estimated, from the slope of the curve in Figure 2, to 3.6 mV/ % O2 change, which should be compared to the estimated value of 3 mV/ % O2 from eq 5 assuming AV,, = 0.5 V. In conclusion we have shown that it is possible to use a PdMOS sensor for oxygen. This is achieved by the use of short alternating exposures of the sensor to a reference gas and the gas to be tested. Hence a high sensitivity and a fast detection of small changes in the oxygen concentration around 20% O2 were demonstrated. The concept may be described as keeping the sensor in a mode of operation that is always as close as possible to steady state. The hydrogen reference gas concentration and the length of the oxygen pulses do not have to be critically controlled. The range may be extended outside the 15-24% O2 studied here with a loss of linearity at large deviations from the reference gas concentration. The most
appropriate way to extend the dynamic range is, however, to change the oxygen concentration in the reference gas and the background hydrogen concentration. Further work will include improvements in the gas manifold, minimizing dead spaces in valves to optimize the time response. Furthermore, field tests are imperative to evaluate the possibilities of the described method for the monitoring of oxygen, e.g., in expired
LITERATURE CITED Lundstrom, I.; Svensson,C. I n Solid State Chemical Sensors; Janata, J., Huber, R. J., Eds.; Academic Press: New York, 1985; pp 1-63. Lundstrom, I.; Sijderberg, D. In Monitoring of Vital Parameters during Exhacorporeai Circulation, Kimmich, H. P., Ed.; Karger: Basel, 1981: pp 291-296. Nylander, C.; Armgarth, M.; Svensson, C. J. Appi. Phys. 1984, 5 6 , 1177- 1188. SWerberg. D.; Lundstrom. I. Solid State Commun. 1983, 45, 431-434. Armgarth, M.; Sijderberg, D.; Lundstrom, I. Appl. Phys. Left. 1982, 47, 654-855. Yamamoto, N.; Tonomura, S.; Matsuoka, T.; Tsubomura. H. Surf. Sci. 1880, 92, 400-406.
RECEIVEDfor review May 19, 1989. Revised manuscript received October 18,1989. Accepted November 27,1989. Our work on chemical sensors is supported by a grant from the National Swedish Board for Technical Development.
