ELECTROCHEMICAL DEPOSITION DEVICE AND APPARATUS AND METHOD OF ELECTROCHEMICAL DEPOSITION USING THE SAME

Abstract: An electrochemical deposition device (10) comprises an electrode and a capillary tube (20) or rod (40) for retaining a hanging drop of electrolyte in contact with a region of a substrate surface of the electrode.

Description: ELECTROCHEMICAL DEPOSITION DEVICE AND APPARATUS AND METHOD OF ELECTROCHEMICAL DEPOSITION USING THE SAME The invention relates to an electrochemical deposition device and an electrochemical deposition apparatus and a method of using the device and apparatus. It is known to sense vapour compositions by the use of an array of sensors, each sensor comprising a substrate and an electrochemically deposited polymer region. The signals from an array of such sensors can be processed to discriminate, for example, between different types of beverage (e.g. coffees, beers etc) and foodstuffs and identify malodours and taints. Each sensor is made by producing a pattern of electrodes on an insulating substrate, for example by physically evaporating 99.99% pure gold onto an alumina tile substrate and etching an electrode pattern by conventional ultra violet lithography. The tile is then dipped into a bath containing electrolyte, monomer and solvent and a potential is applied between the gold layer and a reference electrode in a three electrode configuration or between the gold layer and a counter electrode in a two electrode configuration. In this way, polymer is deposited to form a sensor. A problem with the known method is that only one polymer can readily be applied to each tile and so a number of separate tiles may be used for an array, which is relatively bulky. According to one aspect of the invention there is provided an electrochemical deposition device comprising an electrode, and means for retaining electrochemical deposition liquid in contact with a region of a substrate surface of said electrode. As the liquid is retained in contact with a distinct region of a surface, polymer can be deposited in just that region and different polymers can be applied to different regions of a substrate surface to result in a compact sensor array. The device is of general application however and can be used to deposit an electroactive polymer (e.g. a conducting polymer) onto a variety of conducting surfaces. The conducting surface may be a high work function metal such as gold or platinum as conducting polymers are usually p-type. However, n-type polymers may also be made and these can be deposited onto low work function metals such as silver or copper. Conducting polymers can also be deposited onto heavily doped silicon, again usually p-type with a p-type polymer although you can also deposit a p-type polymer such as polypyrrole onto n-type silicon with an external light source. One important application of the technique is the deposition of conducting polymers onto small conducting areas as part of a microdevice (size of 10 microns up to 5mm). That is small devices that are manufactured using micromachining techniques commonly available in a silicon foundry. Semiconductor (e.g. silicon) processing is a mass-production technique used to make all microelectronic devices today. The device can be used to deposit polymers onto a planar structure - made using planar silicon processing - or on a curved surface made using silicon micromachining techniques. It would not be possible to use conventional techniques such as evaporation or sputtering to coat a re-entrant surface. Conducting polymers can be used as the active material in chemical sensors (gas, odour and liquid) based on a transduction principle such as resistance, potentiometric, amperometric, mass or heat production. They may also be used in types of physical sensors such as opto-electronic devices or even strain gauges. It is envisaged that conducting polymers may be used in microactuators. For example, in the preparation of a low friction or low wear-rate film for use in a slideway, bearing, non-stick valve or motor. In particular, small durable motors cannot have bearings made of silicon or an oxide or nitride thereof due to the high wear-rate. It is further envisaged that microelectronic components can be made with conducting polymers as the active material. That is you replace traditional group IV, III- V, II-VII semiconductors with polymers and then make active devices such as diodes, bipolar and FET transistors. The invention is not limited to polymers and the device could be used to deposit other suitable materials electrochemically, such as gold, for example. The retaining means may comprise a guide arranged to carry the liquid on the exterior surface thereof, for example, as a drop. The guide may be a rod. The guide may be arranged to contact a substrate or may be arranged to facilitate the formation of a hanging drop of liquid to contact the substrate when the guide is thereadjacent. The guide may incorporate or comprise the electrode. In another embodiment the retaining means def ines an orifice for the liquid. Preferably, the orifice has an appropriate size and shape such that a hanging drop of the electrochemical deposition liquid can be provided at the orifice. The orifice may be round. The means defining the orifice may incorporate or comprise the electrode. Alternatively, the electrode may be provided inside, or outside the orifice. A plurality of retaining means may be provided for different electrochemical deposition liquids. Means, such as a nozzle may be provided to apply a washing or cleaning liquid. Removal means may be provided for removing liquid from the working area, for example, by suction. Preferably, the device includes at least one reservoir for at least one electrochemical deposition liquid. Pump means is preferably provided to pump the liquid . from the reservoir to the working area. According to another aspect of the invention there is provided a method of electrochemical deposition on a substrate comprising the steps of moving a device according to the first aspect of the invention adjacent a conducting substrate, forming a drop of electrochemical deposition liquid to bridge the gap between the device and the substrate and applying a potential across an electrode contacting the substrate and the electrode of the device. The liquid can thus be brought into contact with a small droplet sized region of the substrate and material can be electrochemical ly deposited just on that region. By employing the method with a number of different electrochemical deposition liquids on a number of different regions of a substrate, an array of depositions can be produced. Where a plurality of retaining means is provided, liquids are preferably applied simultaneously to a plurality of regions of the substrate. Where removal means is provided, an applied drop may be removed by suction through a removal orifice. Where a nozzle for washing or cleaning liquid is provided, washing or cleaning liquid is suitably supplied to the region before and/or after the electrochemical deposition liquid. The device or substrate can be moved in use to create a deposition trail. The atmosphere in which the drop is formed may be controlled and in one embodiment the drop is formed in a liquid atmosphere. This prevents evaporation of the drop and enables greater overall control of conditions. According to a further aspect of the invention there is provided a sensor comprising at least one region of deposition on a surface and an electrical connection into and out of the deposition region, the sensor having been made by the method of the second aspect of the invention. According to another aspect of the invention, there is provided a microactuator comprising at least one low friction region deposited on a substrate by the method according to the second aspect of the invention. According to a further aspect of the invention there is provided a microelectronic device comprising at least one semiconducting region deposited on a substrate by the method according to the second aspect of the invention. According to another aspect of the invention there is provided an electrochemical deposition apparatus comprising at least one device according to the first aspect of the invention, and at least one electrode for connection to a conducting substrate. Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 shows an array of deposition devices in a first embodiment of the invention; Fig. 2 is a detail elevation of the tips of the deposition devices of Fig. 1, in contact with a tile; Fig. 3 is a circuit diagram of a first control circuit for the device of Fig. 1; Fig. 4 is a circuit diagram of a second control circuit for the device of Fig. 1; Fig. 5 is a circuit diagram of a third control circuit for the device of Fig. 1; and, Fig. 6 is an elevation in detail of a tip of a deposition device in a second embodiment of the invention. Fig. 1 shows a plurality of deposition devices 10 in the first embodiment of the invention, in an array. Each deposition device 10 comprises a motor 12 arranged to drive a plunger 14 into a cylindrical reservoir 16. A flexible tube 18 leads from the lower end of the cylindrical reservoir 16 to a metal tube 20. The tube 20 is a capillary tube which may have a diameter of about 0.5mm. Movement of the plunger 14 SUBSTITUTE SHEET {RULE 26) into the reservoir 16 causes a drop 9 to be formed at the end of the tube 20. The metal tube 20 is connected to an electrical supply and acts as a counter electrode. The tube 20 may be made of platinum. A reference electrode 22 is provided adjacent the outlet end of the reservoir 16 such that it does not interfere with the movement of the plunger 14. Each metal capillary tube 20 is rigidly held in an insulating block 24 so that the spacing of the capillary tubes 20 is predetermined. A pair of working electrodes 26 is also provided for connection to the substrate tile 8. Fig. 3 shows the electrode circuit of the device. A first operational amplifier 28 is connected at its positive terminal to earth and at its negative terminal to the reference electrode 22. The output of the operational amplifier 28 is connected to the counter electrode in the form of the metal tube 20. The pair of working electrodes 26 is connected to the negative input of a second operational amplifier 30 and a resistor 32 is connected across the negative input of the operational amplifier 30 and the output thereof. A wave form generator is connected to the positive input of the second operational amplifier 30. Deposition by means of the device is controlled by the potential waveform applied at the positive input to the second operational amplifier 30 and the deposition process can be monitored at the output of the second operational amplifier 30. In one method of use of the invention, a gold substrate layer is photo-1 ithographical ly patterned in known manner and the array of deposition devices 10 is brought adjacent the patterned tile. The motors 12 are operated to depress the plungers 14 and force liquid from each reservoir 16 through the flexible tube 18 and the capillary tube 20 to form a hanging drop at the end of the tube 20 such that the hanging drop, as it increases in size, comes into contact with the gold substrate. The motors 12 are then stopped and in this stable condition, in which the surface tension of the liquid holds the drop 9 in contact with the small area of the tile while remaining in contact with the capillary tube 20, a potential is applied at the positive input to the second operational amplifier 30 to initiate and control electrochemical deposition. As an example the deposition solution may comprise an aqueous solution of 0.1 mol/dm freshly purified pyrrole and 0.1 mol/dm sodium pentanesulphonate. The reference electrode 22 may be a saturated calomel electrode and the capillary tube 20 may be made from platinum. In use, the potential of the gold substrate may be stepped to 1.3 volts against the saturated calomel electrode 22 to initiate polymerisation. The polymer may be grown for 205 seconds before stepping back to 0 volts potential against the saturated calomel electrode 22. An area of less than 1mm 2 may be deposited upon in this way. Clearly a very small sensor array can be produced. Fig. 4 shows an alternative circuit arrangement. In this case, the reference electrode 22 is not included and the first operational amplifier 28 can therefore be left out of the circuit so that the capillary tube 20 is simply connected directly to earth. The remainder of the circuit remans the same as in Fig. 3. Fig. 5 shows a further alternative control circuit. In this circuit the controlled potential is applied through a resistor 34 to the negative terminal of a first operational amplifier 36. The positive input of the operational amplifier 36 is connected to earth. the negative input of the operational amplifier 36 is also connected to the capillary tube 20 while the output of the operational amplifier 36 is connected to the conducting substrate. The reference electrode is connected to the positive input of a second operational amplifier 38 of which the negative input and the output are shorted. While Figs. 3 and 4 illustrate potentiostatic control of growth, the circuit of Fig. 5 is an example of galvanostatic control of growth. The potential of the output of the second operation amplifier 38 can be used to monitor the working electrode potential. Fig. 6 shows a second embodiment of the tip of the deposition device of the invention. In this embodiment, the capillary tube 20 is not used as the counter electrode. The tube 20 is curved at its output end to touch or lie adjacent the end of an elongate counter electrode rod 40. A second tube 42 extends on the other side of the electrode 40 and is also curved at its end to touch or lie closely adjacent the end of the electrode 40. The tube 42 is connected to a suction pump (not shown). The tubes 20,42 may be made from glass. In use of this embodiment, the electrochemical deposition liquid comprising monomer, electrolyte and solvent as required is pumped down the capillary tube 20 and a drop is formed which contacts the tile 8 and also contacts the adjacent electrode 40. The size of the drop is increased so that it also contacts the end of the second tube 42 but remains in the pattern gap in the resist 46. A potential is applied across the counter electrode 40 and a working electrode pair 26 in contact with the tile 8 to result in deposition from the drop 9 and when deposition has finished, the drop 9 is removed by suction through the tube 42. A further tube may be provided to squirt a washing or cleaning liquid at the working area to remove any remaining electrochemical liquid. In a further, third embodiment similar to that of Fig. 6, the capillary tube 20 may be dispensed with and the flexible tube 18 may lead to the electrode 40 which may be a straight round section rod. The action of the motor may cause liquid to run onto the rod which will act as a guide so that the drop runs down the side of the rod 40 to form a hanging drop at the end so that electrochemical deposition can take place. The distance between the electrodes may be as little as 0.1 micrometres. In the third embodiment, the method may be carried out in a silicon oil bath. This enables more precise control of conditions and in particular prevents evaporation of the drop which may rapidly evaporate completely in air when it is very small. Also, the substrate may be held to be moved by a motor so that a trial of deposit is left. This is particularly useful when the deposited polymer is to serve as a low friction coating. Thus a raised pimple may be provided on a silicon wafer by known techniques and polymer deposited first on that pimple. Where a raised area of more complex shape is to be coated, the substrate may be moved to enable that.

Claims: CLA IMS 1. An electrochemical deposition device comprising an electrode, and means for retaining electrochemical deposition liquid in contact with a region of a substrate surface of said electrode. 2. A device as claimed in claim 1, wherein the retaining means comprises a guide arranged to carry the liquid on the exterior surface thereof. 3. A device as claimed in claim 2, wherein the guide is a rod. 4. A device as claimed in claim 2 or claim 3, wherein the guide is arranged to facilitate the formation of a hanging drop of liquid to contact the substrate when the guide is thereadjacent. 5. A device as claimed in claim 2, 3 or 4, wherein the guide incorporates or comprises the electrode. 6. A device as claimed in claim 1, wherein the retaining means defines an orifice for the liquid. 7. A device as claimed in claim 6, wherein the orifice has an appropriate size and shape such that a hanging drop of the electroche ical deposition liquid can be provided at the orifice. 8. A device as claimed in claim 6 or claim 7, wherein the means defining the orifice incorporates or comprises the electrode. 9. A device as claimed in any preceding claim, wherein a plurality of retaining means is provided for different electrochemical deposition liquids. 10. A device as claimed in any preceding claim, wherein means is provided to apply a washing or cleaning liquid. 11. A device as claimed in any preceding claim, wherein removal means is provided for removing liquid from the working area. 12. A device as claimed in claim 11, wherein the removal means acts by suction. 13. A device as claimed in any preceding claim, wherein the device includes at least one reservoir for at least one electrochemical deposition liquid. 14. A device as claimed in claim 13, wherein pump means is provided to pump the liquid from the reservoir to the working area. 15. A method of electrochemical deposition on a substrate comprising the steps of moving a device as claimed in any preceding claim adjacent a conducting substrate, forming a drop of electrochemical deposition liquid to bridge the gap between the device and the substrate and applying a potential across an electrode contacting the substrate and the electrode of the device. 16. A method as claimed in claim 15, wherein the steps are repeated with a number of different electrochemical deposition liquids on a number of different regions of a substrate to produce an array of depositions. 17. A method as claimed in claim 15, wherein where a plurality of retaining means is provided, liquids are applied simultaneously to a plurality of regions of the substrate. 18. A method as claimed in claim 15, 16 or 17, wherein where a nozzle for washing or cleaning liquid is provided, washing or cleaning liquid is supplied to the region before and/or after the electrochemical deposition liquid. 19. A method as claimed in any of claims 15 to 18, wherein one of the device and substrate is moved in use to create a deposition trail. 20. A method as claimed in any of claims 15 to 19, wherein a liquid atmosphere is provided in which the drop is formed. 21. An electrochemical deposition apparatus comprising at least one device as claimed in any of claims 1 to 14, and at least one electrode for connection to a conducting substrate. •» 22. A sensor comprising at least one region of deposition on a surface and an electrical connection into and out of the deposition region, the sensor having been made by a method as claimed in any of claims 15 to 20. 23. A microactuator comprising at least one low friction region deposited on a substrate by the method of any of claims 15 to 20. 24. A microelectronic device comprising at least SUBSTITUTE SHEET (RULE 26} one semi-conducting region deposited on a substrate by the method of any of claims 5 to 20. 25. An electrochemical deposition apparatus comprising at least one device as claimed in any of claims 1 to 14, and at least one electrode for connection to a conducting substrate.

Inventors: BARTLETT PHILIP NIGEL, GARDNER JULIAN WALTER, BARTLETT, PHILIP, NIGEL, GARDNER, JULIAN, WALTER
Applicants: BARTLETT PHILIP NIGEL, GARDNER JULIAN WALTER, UNIV WARWICK, BARTLETT, PHILIP, NIGEL, GARDNER, JULIAN, WALTER, UNIVERSITY OF WARWICK
Application year: 1995
Main classification: C25D5
IPC codes: C25D5, C25D13, C25D17