AU733671B2 - Apparatus to deposit and measure the resistance charges of thin conducting polymer films in gas sensing applications - Google Patents

Description

RECEIVED 0 7 SEP 1998 APPARATUS TO DEPOSIT AND MEASURE THE RESISTANCE CHARGES OF THIN CONDUCTING POLYMER FILMS IN GAS SENSING APPLICATIONS

FIELD

The invention comprises a method and apparatus for depositing conducting polymer films for use in gas sensing applications and an apparatus for measuring resistance changes in thin conducting polymer films in 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.

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PCT/-.-Z 7 0 0 1 2 3 RECEIVED 0 7 SEP 1598 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 in as-deposited resistance. Unfortunately, we have found such films to have slower responses than necessary, probably due to the 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-polymer interaction. The rate of response, in addition to the magnitude of the response, may thus become an additional parameter in identifying target compounds.

Further, in the operation of gas sensing arrays of conducting polymers, the measurement of film resistance is complicated by the fact that frequently very small changes to a large resistance need to be measured. Also, 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 I/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

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RECEIVED 0 7 SEP 1998 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 (ie distance in which the concentration falls by perhaps a factor of three) is expected to be a few 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 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 -3- A EET RECEIVED 07 SEP 1998 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 (ie 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 therefore 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, 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 -4y AML',; ,.3HET P -9 7 0 0 2 3 ,rVeo C 0 7 r increases of the order of 50% have been measured using a four point method compared with a two point method.

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 of applied current and measured film resistance waveforms using the system of Figure 1, Figure 3 shows a dc bootstrap circuit that may be used in a system for depositing thin polymer films 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 As an example of the electrodeposition of a conducting polymer from a solution, 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 SAMENDi2 .SHEET Al 2 'AiAU rLTU/Zy U U I L 3 RECEIVED 0 7 SEP 1998 a solution of 0. 1M pyrrole and an appropriate dopant ion salt 1M), using a counter electrode to supply the required current in a either a potentiostatic or galvanostatic mode.

In the method of the invention the resistance of the deposited film growing between electrodes is measured while in solution. This is possible 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 RL, and measured resistance. A sensor array 4 comprising microelectrodes to which polymer film is to be applied are immersed in a solution. 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 1000um long and several microns wide, separated from its neighbour by a similar distance. Initially, relay RL ties together all electrodes of the array 4. The deposition controller 1 then applies either a current pulse via the auxiliary electrode 2. A typical pulse duration is 2 sec. Relay RL is then released and a resistance measuring circuit then operates in two-terminal mode applying an ac 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 10mV, 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 The film resistance is calculated from the voltage to current ratio, with maximum measurable value in excess of one megohm.

S T-6- 44 AMB"'.Di) SHEET zLsiP-/i P 97/ 00123 DCrfj7.: Pj 7 0 The application of current pulses to deposit the film on sensor electrode array 4 and intermediate resistance measurements are continued as a series of deposit and measure cycles, until 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 each measure cycle, with no deposition current the resistance is seen to fall somewhat as oligomers formed during the previous deposition pulse settle and polymerise on the film.

Having detected a continuous film, the relay RL may be opened and two or four terminal resistance measurement performed (possible because the resistance measurement is done at ac, while the deposition is a dc process). To ensure that the return path for the dc deposit current cannot be through the ac voltage generator, a dc bootstrap such as is shown in Figure 3 may be used. In the absence of amplifier 7, any deposition current entering the array 1 in the vicinity of electrode 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 apply a correction via the waveform generator 6. No dc can then flow through resistance 5, which appears as an infinite impedance to deposition currents.

Preferably the resistance measurement is carried out of low frequency. The current path through the solution has a large capacitive component at the electrode-liquid boundary, and therefore provides the least shunting effect if the measurement is confined to low frequencies. It is also necessary to keep the measuring voltage 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. The measuring voltage level may be of the order of 10mV for example.

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i-U^ P 9 7 0 0 12 3 RECEIVED 0 7 SEP3 Figure 4 is a block diagram of a preferred form synchronous measuring system which enables the accurate measurement of impedances of the order of a megohm at such low level signals. 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, currents I+ and I- (line B) are generated. In an experimental system the capacitive component had substantially decayed after 5msec. A measuring frequency of 50Hz therefore gave times of 5msec in both half cycles during which the current and voltages are approximately constant, and both were ultimately averaged over these times using a mask (line D).

In microprocessor uP both current and voltage are converted to a frequency. DC offsets are removed by differencing the frequencies corresponding to V+ and and and The resistance value required is then the ratio R Filter (V (I I-) where V, I- are average values of the respective quantities, found by simply counting periods over their non-masked intervals. Sixteen bit resolution with excellent linearity can be achieved.

Figure 6 shows a gas sensing 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. 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, and 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.

-8- 471OFA PCTrz9 7/ 001 2 3 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. Also in most cases an accurate measurement of the change in film resistance is 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 IkHz, 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 for example between 1 and 100uA, is injected to the outer terminals of a four terminal probe. The current is provided by the current driver circuit. 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 T -9- VI-. 5' 'r d PCT/NZ97/00123 Received 13 November 1998 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 always 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 1 Sensor 1 2 3 4 N2 Film Resistance 328 1015 550 346 352 N2 Contact Resistance 68 63 119 105 160 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 art are intended to be incorporated within the scope hereof as defined in the claims.

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Claims (9)

1. A method for depositing a gas sensing polymer film over an electrode array on a substrate comprising immersing the electrode array in a solution containing a monomer and a dissolved salt which will deposit on and between the electrodes upon passing current through the solution between the array and a counter electrode which supplies the current, during the electrodeposition measuring the resistance of the depositing film between the electrodes, and continuing the electrodeposition and resistance measurement until the desired film thickness or resistance has been achieved.

2. A method for depositing a gas sensing polymer film over an electrode array according to claim 1 including applying the electrodepositing current in cycles and periodically between cycles measuring the resistance between the electrodes of the array in solution, and carrying out the cycles of electrodepositing current and the resistance measurement until the desired film thickness or resistance has been achieved.

3. A method for depositing a gas sensing polymer film over an electrode array according to claim 2 wherein between said cycles of the electrodepositing current a voltage below the threshold at which deposition occurs is applied between the electrode and said another electrode to measure the resistance between the electrodes.

4. A method for depositing a gas sensing polymer film over an electrode array according to claim 1 including applying an ac voltage of lower amplitude than the electrodepositing voltage between the electrodes of the array simultaneously with said electrodepositing current, and measuring the resultant ac current to determine the resistance, and continuing to apply the electrodepositing current until the desired film thickness or resistance has been achieved.

A method according to any one of claims 1 to 4 wherein said electrode array comprises a multiple number of microelectrode elements spaced apart from each other by about 10 microns or less. <x

6. A method of measuring the resistance of a sensor comprising an electrode iay comprising gas sensing c efRO P1 in a chemical or gas sensing PCT/NZ97/00123 Received 07 January 1991 application, comprising applying an ac current to the sensor in the range 1 to 100 pA to minimise resistance charges due to self heating of the sensor.

7. A method according to claim 6 wherein the electrode array comprises four electrodes and the current is applied to the outer two electrodes and voltage is sensed across the inner two electrodes.

8. A method according to either one of claims 6 and 7 wherein the voltage charge is offset by a programmed amplitude signal at the same frequency, amplified, and detected.

9. A method according to claim 8 wherein a number of sensor arrays are monitored simultaneously over parallel channels by a microprocessor. AMENDED SHEET IPEA/AU