DEPOSITION OF THIN ELECTROCONDUCTIVE POLYMER FILM OF DESIRED RESISTANCE FOR GAS SENSING APPLICATIONS

Abstract: Electrodeposition of thin sensing polymers on microelectrodes, grown apparently to the same parameters, show wide resistance variations. Thicker films of more consistent resistance are too slow to respond, because of the time taken for the target gas to diffuse through them. These drawbacks may be overcome by tailoring thin electroconductive polymer films to the resistance required, by measuring the resistance at intervals during deposition and continuing to deposit film, until a high or predetermined resistance is obtained. This may be effected in a time multiplexed fashion, in which an electrodeposition current pulse is applied to the electrodes, followed by a period in which the inter-electrode is measured using a voltage below the threshold, at which electrodeposition occurs. Alternatively, an AC method, capable of measuring high impedance at low voltages may be employed and the measurement of resistance may be carried out simultaneously with the deposition. A typical electroconductive polymer is polypyrrole, which may ber deposited at a potential of 750 - 900 mV. Pyrrole forms an oligomer from solution prior to deposition and polymerisation is completed in situ. The substrate is preferably a microelectrode array, with elements about 1000 mu long, several mu wide and spaced about 10 mu apart, (i.e., within the distance, wherein the concentration decreases by a factor of 3). A counter electrode to supply the required current, in either a potentio or galvano-static mode is also required. In operation as a gas sensor, the power should be as low as possible to ensure that resistance changes represent a gas response alone, unaffected by measurement factors.

Description: DEPOSITION OF TfflN ELECTROCONDUCTIVE POLYMER FILM OF DESIRED RESISTANCE FOR GAS SENSING APPLICATIONS FIELD The invention comprises a method and apparatus for depositing conducting polymers iilms lor use in gas sensing applications and an apparatus for measuring resistance changes in thin conducting polymer films and gas sensing applications BACKGROUND The technique of gas or odour sensing using arrays of conducting polymers is a field in which much progress has been made in recent years, in it, the resistance of a thin film of an electronically conducting polymer such as polypyrrole, doped with an appropriate chemical, is observed to change in response to exposure to certain gases or odours. By constructing arrays of probes containing a polymer resistor, each with different dopant species, patterns of responses can be obtained which are characteristic of more complex gas mixtures or odours. Since the human nose is sensitive to odourants at the ppb (parts per billion) level, important considerations in the technology of odour sensing are methods which maximise detector sensitivity. A considerable difficulty in the technology is the production of films of repeatable electrical characteristics, in a form which maximises their chemical sensitivity. It is desirable that the film between the electrodes is as uniform as possible, and two basic methods are in use to achieve this; electrodeposition directly from solution, or chemical deposition of a base polymer film, followed by its patterning by lithographic techniques to provide a controlled geometry on which to electrodeposit the sensing polymer. Existing techniques for electrodepositing on microelectrodes suffer from repeatability problems. Due perhaps to differences in initiation times, or variations in oligomer diffusion before depositing, thin films grown to apparently the same parameters, such as constant charge, show wide resistance variations. This complicates the resistance measuring circuitry. For this reason, it is customary to grow thick films which show a smaller percentage variability m as-deposited resistance. Unfortunatelv, we have found such films to have slower responses than necessary, probably due to Lhe time needed for target gases to diffuse into them, and to exhibit a lower chemical sensitivity than thinner films. The resistance of very thin films has been shown to respond to pulses of target gas at speeds which apparently reflect the kinetics of the gas-polvmer interaction. The rate of response, in addition to the magnitude of the response, may thus become an additional parameter in identifying target compounds. In the operation of gas sensing arrays of conducting polymers, the measurement oi film resistance is complicated by the fact that frequently very small changes to a large resistance need to be measured; further, thin films heat very readily and change their resistance unless the measuring power is small. SUMMARY OF INVENTION The invention provides a method which enables a thin polymer film of controlled resistance to be deposited on a microelectrode array to form a resistance probe, and a method for operating such a gas sensing array in which the effect of self heating of the film can be minimised or avoided and inherent 1/f sensor noise minimised. In broad terms in one aspect the invention comprises a method for depositing a gas sensing polymer film, in which the resistance of the depositing polymer film is measured during the deposition process. This can be done in a time-multiplexed fashion in which an electrodepositing current pulse is applied to the electrodes followed by a period in which the inter-electrode resistance is measured using a voltage below the threshold at which electrodeposition occurs Alternatively an ac method capable of measuring high impedance at low voltages may be employed, and the measurement of resistance can then be carried out simultaneously with the deposition Thin films may be grown reliably to high resistances, or thicker films can be grown to predetermined resistance values, using the method of the invention. Preferably the direct electrodeposition is onto a microelectrode array in which the element spacing is of the order of ten microns, more or less. Microelectrodes are desirable because they enable more uniform films to be deposited; techniques which require a film to be grown many microns across a gap, between for example the ends of wires embedded in epoxy, tend to be very thick and non-uniform. Polymers such as polypyrrole form first by an electrode reaction which produces an oligomer in the solution surrounding the electrode; the oligomers then diffuse in the solution a distance before settling onto a surface and completing their polymerisation From likely diffusion coefficients in solution, and because diffusion proceeds as the square root of time, the characteristic diffusion distance de distance in which the concentration falls by perhaps a factor of three) is expected to be a lew tens of microns. Diffusion over larger distances will take much larger times and will likely be destroyed by convection. The implication is that in order to achieve maximum uniformity of deposition thickness in the gap between two electrodes, the gap should not exceed this diffusion length. In one preferred form the microelectrode pattern comprises four parallel gold tracks on the surface of an insulator such as layer of silicon dioxide on top of a silicon chip for example. Each electrode may be of the order of 1 OOOum long and several microns wide, separated from its neighbour by a similar distance. However, with appropriate scaling to preserve current densities, the methods described will apply to manv similar structures. In broad terms in another aspect the invention comprises a method whereby the resistance of a number of polymer film sensors of different kinds, exposed to a vapour, can be measured using powers low enough that self heating of the film (with consequent resistance changes) does not occur. Since it is generally the change of resistance which is required, and this is usually a small fraction of the total resistance, a stable and accurate method of offsetting using ac techniques is used. It is known that the resistance of polymer films is a strong function of temperature. We have found for example that the temperature coefficient is in the region of 2% per degree. We have also found that measuring powers of a milliwatt are sufficient to cause noticeable rises in film temperature. To achieve the maximum sensitivity to target gases de to be confident that resistance changes represent gas responses and not changes due to measurement artefacts) it is desirable to use the lowest power possible. In addition we have found that the measured resistance of the polymer films consists of two parts, a pair of contact resistances between the polymer and the metal ( usually gold) electrodes, in addition to the true film resistance between the electrodes. Measurements show that the contact resistance can be a significant component of the total resistance measured. Moreover, measurement with special electrode structures have shown that this contact resistance may or may not be modulated along with the bulk resistivity of the polymer upon exposure to target gases, but in the examples we have examined the modulation is always considerably less than the modulation of the bulk resistivity. A two-terminal measurement of the resistance change will thereiore exhibit less sensitivity than a method which probes bulk resistivity of the polymer. Measurements representative of bulk resistivity can be made by the four point probe, a technique used in other disciplines, in which two electrodes are used to inject a current, and the voltage drop between two other electrodes is measured at near-zero current to eliminate the contact impedance at those terminals. The preferred method of measuring film resistance in a way which maximises sensitivity therefore is to use a low voltage ac technique in conjunction with a four point probe, but the electrical measurement can be performed on a two electrode probe. Sensitivity increases of the order of 50% have been measured using a four point method compared with a two point method. A further advantage of the use of ac techniques is that the inherent 1/f sensor noise is circumvented. BRIEF DESCRIPTION OF DRAWINGS The invention will be further described with reference to the accompanying figures by way of example and without intending to be limiting, in which: Figure 1 schematically illustrates the deposition of a thin polymer film on a microelectrode sensor array using the method of the invention, Figure 2 shows examples including current and measured resistance wave forms using the system of Figure 1 , Figure 3 shows a dc bootstrap circuit that may be used in a system for depositing polymer film by the method of the invention, Figure 4 is a block diagram of a preferred form synchronous measuring system, Figure 5 shows examples of waveforms frequency applied voltage current and a mask for the measuring system of Figure 4, and Figure 6 shows a preferred form gas sensing system of the invention. DETAILED DESCRIPTION OF PREFERRED FORMS Conducting polymers are typically by electrodeposition from a solution. For example, polypyrrole may be deposited by applying a potential of between 750 and 900mV (relative to a Ag/AgCl reference) to the electrode upon which deposition is sought in a solution of 0.1 M pyrrole and an appropriate dopant ion salt (0.1 M), using a counter electrode to supply the required current in a either a potentiostatic or galvanostatic mode. We have found that it is possible to measure the resistance of the deposited film growing between electrodes while in solution because the inter-electrode impedance via the solution is higher than the resistance of the film except at the start of deposition. Figure 1 illustrates the method using a four-terminal probe. Figure 2 shows waveforms for current applied, excitation of relay RE. and measured resistance. Initially, relay RE ties together all electrodes. The deposition controller 1 then applies either a current pulse via the auxiliary mode. A typical pulse duration is 2 sec. Relay RE is then released and the resistance measuring circuit then operates in two-terminal mode applying a measuring voltage between and seeking a contact between the outer electrodes of the sensor electrode array 4 via a deposited film. Two terminal configuration is achieved using the solid state switches 5. If the applied voltage V is nominally lOmV, the actual voltage V means appearing across the electrodes, which is measured for the calculation of resistance may be less, depending upon whether a two or four terminal measurement is operating. The current is detected using a sensing resistor (E). The film resistance is calculated irom the voltage to current ratio, with maximum measurable value in excess of one megohm. The function of the series resistors R s is to ensure that deposition current is shared equally between the four electrode tracks. Should one track, for whatever reason, draw more current than the others, its series resistance R s causes a reduction of the deposition voltage at the metal-liquid interface, and in so doing, reduces the electrochemical current. The application of current pulses to deposit the film on sensor electrode array 4 and intermediate resistance measurements is continued, and after a small number of such deposit and measure cycles, the resistance is observed to fall to within the measuring range, indicating the presence of a continuous conducting film over and between the electrodes. During measure phases, with no deposition current) the resistance is seen to fall somewhat as ohgomers formed during the previous deposition pulse settle and polymerise on the film. Having detected a continuous film, the relay RE may be opened and (wo or four terminal resistance measurement performed. This is possible because the resistance measurement is done at ac, while the deposition is a dc process Precautions are taken however to ensure that the return path for the deposit current del cannot be through the ac voltage generator. This is achieved by the dc bootstrap shown in Figure 3. In the absence of amplifier 7, and deposition current entering the array in the vicinity of electrode a could flow to earth via the square wave source 6, disturbing its operation. Amplifier 7 and integrator 8 detect any dc current through the current sensing resistor 5, and applies a correction via the waveform generator 6. No dc can then flow through resistance 5, which appears like an infinite impedance to deposition currants. The current path through the solution has a large capacitive component at the electrode-liquid boundary. This component is frequency dependent, and therefore provides the least shunting effect if the measurement is confined to low frequencies. It is necessary to keep measuring voltages at a level well below that at which electrodeposition could be affected; currents arising from electroactive species in the solution would in any case appear as a resistive shunt across the desired film resistance and cause measurement errors. Here the measuring voltage level is of the order of l OmV. Special synchronous electronic techniques are necessary to measure accurately impedances of the order of a megohm with such low level signals. Figure 4 is a block diagram of a preferred form synchronous measuring system. Figure 5 shows waveforms for frequency, applied voltage, current, and a mask. Square wave voltages (line C) are generated from a crystal and applied to the electrodes. Because of the capacitive sheath surrounding the electrodes, currants 1+ and I- (line B ) are generated. In an experimental system the capacitive component had substantially decayed after 5msec. A measuring frequency of 50Hz therelore gave times of 5msec in both half cycles during which the current and voltages are approximately constant and both are ultimately averaged over these times using a mask In microprocessor both current and voltage are converted to a frequency, a process with extremely good linearity and resolution. DC offsets are removed by differencing the frequencies corresponding to V+ and V-, and 1+ and I-. The resistance value required is then the ratio R = Filter (V * - V ) / (I * - I ) where V, V-, I+, I- are now average values of the respective quantities, found by simply counting periods over their non-masked intervals. Sixteen bit resolution with excellent linearity is achieved. The measurement of film resistance during deposition enables various phases of film growth to be established, and deposition stopped at an appropriate position. In general, immediately following bridging of the gaps, the conductance rises to reach a maximum rate of change with time. Microscopic inspection shows this period correlates with the initial growths spreading between tracks and coalescing into a more-or-less uniform film. The film conductance now enters a period of more lιne<τr growth with time as the film thickens, which finally reduces to raies corresponding lo very thick films. Growth may be stopped at any appropriate stage, thin films generally exhibit greater chemical sensitivity. Figure 6 shows a system employing a thin conducting gas sensitive polymer film. The circumstances of measuring the resistance of the deposited films in air differs from their measurement during deposition because (a) The measurement must be made at low power to prevent film heating, rather than as previously at low voltage in order not to interfere with the electrodeposition. (b) The measurement can be made at frequencies up to at least 100kHz since the film is resistive and is not shunted by the reactive impedance of the solution as it was during film deposition. (c) The array of different polymers used in a complete sensor head ( usually) makes it desirable to measure several resistances simultaneously and make the resistance data available to a computer for display and analysis. (d) In most cases an accurate measurement of the change in film resistance i needed, rather than an absolute measurement. This requires a stable method of offsetting. In accordance with the invention the system of Figure 6, the measurement is performed as described in the following. Provision is made for multiple measurements with one ac current source, for example at a frequency of 1 kHz, and provision is also made for ranging and offset by subtracting synchronous ac currents and amplifying residual signals prior to detection. With reference to Figure 6 a measuring current of between 1 and l OOuA is injected to the outer terminals of a four terminal probe This current is chained through a number of other probes, for example eight. On each of the channels, the different voltage appearing across the inner electrodes is sensed This differential voltage may be offset by a programmable amplitude signal at the same frequency, and amplified, before synchronous detection and low pass filtering. In this way, very small sensor voltages can provide high resolution resistance measurements, because dc offsets arising from contact potentials, amplifier offset are eliminated, non synchronous noise is reduced, and the large 1/f noise of the sensors is effectively eliminated. The programmable offset and scale may be set so that the individual sensor response best suits the A-D converter range. In the example shown an octal converter can monitor eight channels simultaneously. A local microprocessor is used to perform further digital filtering before outputting the data to a computer. Gas responses are usually expressed in terms of the fractional change in film resistance upon exposure to the gas. It has been measured experimentally that the contact resistance is a somewhat variable parameter between sensors. In cases where the contact resistance is small compared with the film resistance, there is little increase in sensitivity of a four point probe method compared with a two point method. More generally, the contact resistance has been measured to be as much as 50% of the film resistance. It has been found to exhibit some gas sensitivity, but alwavs less as a percentage than the film resistance. Table 1 shows the measured film and contact resistances in nitrogen for five sensors, and the percentage improvement in sensitivity upon exposure to ethanol vapour when measured in four terminal mode compared with two terminal. Table Sensor 1 2 3 4 5 N2 Film Resistance 328 1015 550 346 352 N2 Contact Resistance 68 63 1 19 105 160 % Sensitivity Increase 44 33 32 55 47 The foregoing describes the invention including a preferred form thereof. Alterations and modifications as will be obvious to those skilled in the at are intended to be incorporated within the scope hereof as defined in the claims.

Claims: CLAIMS 1 . A method for depositing a gas sensing polymer film, comprising measuring the resistance during the deposition process and carrying out deposition until the desired film thickness or resistance has been achieved.

Inventors: ANDREWS MICHAEL KENNETH, HARRIS PAUL DAVID, PARTRIDGE ASHTON CYRIL, ANDREWS, MICHAEL, KENNETH, HARRIS, PAUL, DAVID, PARTRIDGE, ASHTON, CYRIL
Applicants: ANDREWS MICHAEL KENNETH, HARRIS PAUL DAVID, IND RES LTD, PARTRIDGE ASHTON CYRIL, ANDREWS, MICHAEL, KENNETH, HARRIS, PAUL, DAVID, INDUSTRIAL RESEARCH LIMITED, PARTRIDGE, ASHTON, CYRIL
Application year: 1997
Main classification: C25D13
IPC codes: C25D13, H01C17