GAS SENSOR ASSEMBLY AND METHOD OF FABRICATION THEREOF

Abstract: There is described a method of fabricating a gas sensor assembly, particularly for sensing smells and aromas, the assembly comprising a board (26), a pair of conductive tracks (28, 30) on the board, a substrate (10) mounted on the board, a pair of spaced-apart contacts (12, 14) supported on the substrate, and a semi-conductive polymer (25) spanning the gap (16) between the contacts, which polymer is capable of interreacting with gases and/or volatile material to change its resistance, which method comprises: (1) mounting the substrate (10) on the board, the substrate comprising the said pair of spaced-apart contacts (12, 14) and a protective layer covering the contacts except for: (a) a region (22, 24) of each contact allowing electrical connection to be made to the contacts and (b) a region (20) spanning the gap (16) between the contacts corresponding to the location of the polymer (25), so that the electrical connection regions (22, 24) lie adjacent to the said conductive tracks (28, 30); (2) connecting the pair of conductive tracks (28, 30) to the connection regions (22, 24) of the respective pair of electrical contacts (12, 14) by electrical connections (36); (3) submerging substantially the whole of the substrate (10) in a monomer solution; and (4) applying a potential to the contacts (12, 14) to cause the monomer to polymerise in the said region (20) of the substrate spanning the gap (16). Such an arrangement allows the polymer to be grown consistently while at the same time reducing the resistance of the contacts (12, 14) by reducing their length.

Description: GAS SENSOR ASSEMBLY AND METHOD OF FABRICATION THEREOF TECHNICAL FIELD The present invention relates to a method of fabrication of a gas sensor assembly and also to the assembly itself. As used herein, the term "gas" will be taken to include vapours, particularly of volatile materials. BACKGROUND ART In International Patent Application No.WO93/03355, there is described a method of making a gas sensor of a type comprising a pair of spaced-apart electrical contacts and a semi-conductive polymer spanning the gap between the contacts. The semi-conductive polymer is capable of interreacting with gases and volatile materials and when it does so, the resistance of the polymer changes. The sensor can thus be used to detect the presence and/or nature (or other characteristic) of gases or volatile materials in an atmosphere being monitored by directly or indirectly measuring the resistance of the polymer between the two contacts. Sensors of this type are particularly useful for detecting aromas, smells and odours. The method of fabrication disclosed in WO93/03355 comprises vapour-depositing gold 500A thick on one side of a substrate to form a pair of spaced-apart contacts, depositing a photoresist over the whole of the substrate except for three windows; two of the windows (the "connection windows") are required to provide electrical connection to the two contacts and the third window (the "polymer window") is located in the region of the gap between the contacts. After electrical connections have been attached to the two contacts at the connection windows, the substrate is dipped into a solution of a monomer so that the polymer window is below the level of the solution while the connection windows are above the level of the solution. The contacts are then connected to a potentiostatic circuit to maintain the contacts at a potential that causes the monomer in the solution to polymerise in the polymer window and so span the gap between the contacts. By using microelectronic fabrication techniques, e.g. vapour deposition, it is possible to control the gap between the contacts accurately but such techniques result in the contacts being thin, which in turn causes their electrical resistance to be high. Even by increasing the thickness of the gold layer forming the contacts to lOOOA, the resistance of the contacts is still high compared to the resistance of the polymer and this can lead to inaccuracies in measurement. In order to minimise the resistance of the contacts, it is desirable to keep the distance between the polymer and the electrical connections to the contacts as small as possible. However, if the distance is reduced excessively, there is a problem submerging the polymer window well below the meniscus of the monomer solution (in order to obtain a reproducible thickness of polymer deposited in the polymer window) while at the same time maintaining the connection windows above the level of the solution (to avoid the polymer being deposited in the connection windows which has to be removed prior to connecting the sensor in a monitoring apparatus since otherwise the presence of polymer in the connection windows would lead to inaccuracies and inconsistencies from sensor to sensor). It is known to connect heated gas sensors of the tin oxide type to external circuitry by means of thin wires, but this is primarily to reduce the heat loss from the heated sensor and hence minimise the energy required to operate the sensor (see for example US-4584867 and EP-0333103). It is also known to connect such heated sensors using printed tracks (see EP-0116117 and US-4413502). DISCLOSURE OF THE INVENTION The present invention provides a method of fabricating gas sensors of the above type so that the length of the contacts between the connection windows and the gap window can be reduced to a sufficient extent that the resistance of the polymer constitutes a higher proportion of the overall resistance of the sensor than was achievable hereto without having to remove polymer deposited in the connection windows or compromise the quality of the polymer deposition by forming the polymer only just below the surface of the monomer solution. Preferably, the resistance of the polymer in the sensor made according to this method is at least 50% of the overall resistance of the sensor assembly and preferably at least 60% , e.g. at least 70%. According to the present invention, there is provided a mediod of fabricating a gas sensor assembly comprising a board (the board is preferably a printed circuit board), a pair of conductive tracks on the board, a substrate mounted on the board, a pair of spaced-apart contacts supported on the substrate, and a semi-conductive polymer spanning the gap between the contacts, which polymer is capable of interreacting with gases and/or volatile material to change its resistance, which method comprises: (1) mounting the substrate on the board, the substrate comprising the said pair of spaced-apart contacts and a protective layer covering the contacts except for (a) a region of each contact allowing electrical connection to be made to the contacts and (b) a region spanning the gap between the contacts corresponding to the location of the polymer, so that the electrical connection regions lie adjacent to the said conductive tracks, (2) connecting the pair of tracks to the connection regions of the respective pair of electrical contacts by electrical connections, preferably by wedge bonding thin wires (typically 5μm to 50μm in diameter) (3) submerging substantially the whole of the substrate in a monomer solution, and (4) applying a potential to the contacts to cause the monomer to polymerise in the said region of the substrate spanning the gap to form the polymer. Thus, according to the present invention, the electrical connections to the contacts that will be used during the operation of the sensor are formed before the polymer is deposited, rather than after the sensor is fully fabricated, as was the case with the prior art. This allows parts of the assembly to which potential is applied during polymer growth and that are, or are liable to be, submerged in the monomer solution to be electrically isolated with an insulating coating during polymer growth, thereby preventing polymer growth on those parts, which generally are the said electrical connections, the said connection regions of the contacts and the region of the conductive tracks adjacent to the substrate. It is not absolutely essential for the said electrical connections, the exposed connection region of the contacts or the adjacent region of the conductive tracks to be coated with an insulating coating since it is possible to tolerate deposition of polymer in some or all of these areas but it is desirable to do so since such polymer deposition can cause inconsistencies between supposedly identical sensors. The insulating coating is preferably an adhesive, e.g. an epoxy resin, which has the advantage that the connections are firmly bonded to the substrate which is advantageous when the connections are thin wires. The arrangement of the present invention dispenses with the need for maintaining a substantial distance between the polymer and die connection regions of the contacts and so allows microfabrication techniques to be used to form the contacts, even though the contacts formed in this way are thin. In the arrangement of the present invention, this distance is preferably less than 10mm e.g. less than 5mm and more preferably about 0.1 to 3mm (the "distance" is measured between the nearest points of the connections and the gap that is spanned by polymer). The electrical connections from the board to the external circuit during polymer growth should be located above the level of the monomer solution but this does not cause any problem because the conductive tracks can be made long enough to achieve this since the conductivity per unit length of the tracks on the board will be much higher than the corresponding conductivity of the contacts of the sensor and hence relatively long tracks can be provided without substantially increasing the resistance of the sensor assembly. The conductivity per unit length of the tracks is preferably at least ten times and preferably at least one hundred times that of the contacts. The length of the electrical connections between the conductive tracks and the contacts on the substrate should be as small as possible to minimise the resistance of the assembly. As used herein, the term "conductive track" is intended to mean an electrically conductive region or element and is not intended to imply necessarily that the region or element is elongate (although generally it will be) but it could be, for example, square. Preferably each track includes a first contact region adjacent to the substrate for bonding the electrical connections and a second contact region more remote from the substrate for making connections to an external circuit for applying potential to the contacts during polymerisation. In a preferred embodiment, the board may be severed between the first contact region and the second contact region of the conductive tracks after the polymer has been grown since the part of the board containing the second contact regions is superfluous after the polymer growth. The first contact region of each conductive track will then be used to supply electrical current to the sensor in use. In an alternative embodiment, the electrical connections may be connected to the contacts prior to the substrate being mounted on the board, in which case there is no reason why the electrical connection regions windows should not be covered by the protective layer. DESCRIPTION OF DRAWINGS The present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1A and B are, respectively, a plan view and a side view of a printed circuit board for use in the present invention; Figure 2 is a plan view of a gas sensor assembly in accordance with the present invention and incorporating the board of Figure 1 ; and Figure 3 is a plan view of a second embodiment of a gas sensor assembly in accordance with the present invention. BEST MODE FOR CARRYING OUT THE INVENTION Referring initially to Figures 1A and IB, a printed circuit board 26 is shown measuring 25mm x 8mm; it is of standard construction and includes three conductive tracks 28, 29 and 30, each connected to respective connection pins 40, 41 and 42 by solder joints 43, 44 and 45. Tracks 28 and 30 include connection pads 34 for electrical connection to a sensor chip 50 (shown in Figure 2). Track 29 is provided for connection to a compensating resistor but such a resistor is not used in the present embodiment. Referring now to Figure 2, a chip 50 is shown secured to the printed circuit board 26, for example, by adhesive (not shown). The chip 50 comprises a silicon substrate 10 measuring 3.5 x 3.5mm or 7 x 7mm (as shown) onto which has been deposited by vapour deposition a seed layer, for example of titanium, having the shape of electrical contacts 12, 14 (shown in dotted line); two lOOOA gold electrical contacts 12, 14 are then deposited by vapour deposition on top of the seed layer. The two contacts 12, 14 are spaced apart by a gap 16. A protective layer 21 is then deposited over the whole of the top surface of the substrate except for (a) window 20, which spans the gap 16, and (b) windows 22 and 24, which provide electrical connection points to the contacts 12, 14. The protective layer may be any insulating material, for example silicon nitride, polyamide or a simple photoresist. The protective layer may be applied through a mask or, if the protective material is a photoresist, by exposing photoresist material to light through a mask in which the areas corresponding to windows 20, 22 and 24 are opaque, and then removing the undeveloped photoresist material in areas 20, 22 and 24. Further details of making the substrate is described in International Patent Application No. WO93/03355, the contents of which are incorporated herein by reference. The substrate 10 is preferably made of silicon in order to utilise known fabrication techniques of the type used in the semi-conductor industry. Using such microfabrication techniques, it is possible to deposit the contacts 12. 14 in such a way that the gap 16 is accurately and reproducibly maintained from batch to batch. Preferably the width of the electrode gap is approximately 10μm but it can be of the order of 7 to 30μm. Alternatively, the substrate 10 may be made of a ceramic, for example alumina or an aluminium silicate, or quartz. The contacts 12, 14 may be deposited by other techniques, for example silkscreen printing. The chip 50 including (a) the substrate 10, (b) the contacts 12, 14 and (c) the protective layer 21 is mounted on the printed circuit board 26 shown in Figures I A and IB so that the windows 22, 24 are adjacent to connection pads 34 of the conductive tracks 28,30. The chip 50 may be mounted on the printed circuit board using any known chip mounting technique, e.g. by adhesive. The two pads 34 of the tracks 28, 30 are connected to the contacts 22, 24 by means of thin wires 36 (typically 5 to 50 μm in diameter). Each of the wires may be ultrasonically wedge bonded to its respective pad 34 and the widow areas 22,24 of the contacts 12,14. The wires 36 are then glued to the printed circuit board 26 using a suitable bonding glue, which is also used to cover the adjacent pads 34 and window areas 22 and 24, thereby insulating these parts. The bonding glue may also cover at least the lower pans of the tracks 28, 29 and 30. The distance between the wires 36 and the polymer window 20 is 4mm. A polymer is then grown in the window 20 using die technique described in International Patent Application No. WO93/03355; according to this technique, the pins 40,42 are connected to the same arm of a potentiostatic circuit (not shown). The printed circuit board 26 is then placed into a solution of a monomer that also contains a solvent and a counter-ion so that the end of the printed circuit board that contains the chip 50 is immersed up to approximately the level indicated by the arrows 52. If any parts of the conductive tracks 28, 30 are immersed, those pans should be covered by the bonding glue. A counter electrode and a reference electrode (not shown) are also immersed in die monomer solution and connected into die potentiostatic circuit which maintains a fixed potential between, on die one hand, contacts 12, 14 and, on die other hand, the reference electrode. Thus, the only parts of the assembly shown in Figure 2 that are both exposed to the monomer solution and maintained at a potential during polymer growth are the pans of the contacts 12, 14 exposed dirough the window 20 (which includes the gap 16) and so the monomer polymerises solely within this window. Since the window is well below die level of the monomer solution meniscus, an accurately reproducible diickness of polymer 25 can be grown across gap 16. Because the areas 22 and 24, the wires 36 and the pads 34 are covered by bonding glue, diey are not exposed to die monomer solution and accordingly no polymer is deposited there. In fact, it is not absolutely necessary to keep the areas 22 and 24, the wires 36 and die adjacent pads 34 free of polymer deposition since the polymer does not detrimentally affect the operation of the sensor assembly but it is preferred not to deposit polymer in these areas since such deposition makes the deposition of the polymer in the window 20 liable to inconsistencies. Once the desired thickness of polymer has been grown in area 20. the assembly is removed from the monomer solution, rinsed and the conductivity of die polymer 25 is tested. Although it may, in some cases, be possible to remove the protective layer 21 from the substrate 10 after the deposition of the polymer, it is preferred to retain it since is provides protection for the contacts 12, 14 and the polymer 25 in the window area 20. In use, the sensor is connected to an external electrical circuit by means of me pins 40,42. Referring now to Figure 3, there is shown a printed circuit board 26 measuring 19.5mm x 4.8mm on which a chip 50 measuring 3 x 3mm is mounted; apart from its size, the chip is identical to the chip 50 shown in Figure 2. Details of die chip are not shown in Figure 3. Like reference numbers are used in Figure 3 as in Figure 2 to denote corresponding parts and further description is dierefore superfluous. The distance between die wires 36 and die window 20 is 1.5mm. Instead of die pins 40, 42 of Figure 2, die assembly of Figure 3 includes contacts pads 54. 56: also, die pads 34 are include annular pin connectors around holes 60. The assembly of Figure 3 is made in substantially the same way as that described above in connection with Figure 2. The board 26 has a line of weakness 58 between the contact pads 34 and the pads 54, 56; after the polymer has been grown in window 20, the pads 54, 56 are superfluous and accordingly the top part 62 (as viewed in Figure 3) of the board 26 may be severed along line 58 so diat die assembly that is actually used is that part 64 of e sensor assembly below line 58, which includes the sensor chip 50. In use, the sensor is connected to an external electrical circuit by means of connection pins (not shown) that pass through the holes 60 in connection die pads 34; die electrical connection between die pins and the connection pads may be made by standard techniques, for example by soldering. The operation of the sensors is described in WO93/03355, but essentially an array of sensors is exposed to a vapour, e,g, an aroma, whereupon the resistance of die polymer 25 changes; the polymer resistance is measured by an external circuit by maintaining a potential difference between contact 12 and contact 14. The resistance of die polymer on exposure to a gas or vapour will depend on die nature of die gas or vapour and the nature of the polymer; thus by using several, e.g. 12, sensors each having different polymers and exposing them all to the gas or vapour, a pattern of responses from the sensors is obtained that is characteristic of die gas or vapour concerned. Figures 1 to 3 are drawn to scale.

Claims: Claims 1. A method of fabricating a gas sensor assembly comprising a board (26), a pair of conductive tracks (28,30) on the board, a substrate (10) mounted on the board, a pair of spaced-apart contacts (12,14) supported on the substrate, and a semi-conductive polymer (25) spanning the gap (16) between the contacts, which polymer is capable of interreacting with gases and/or volatile material to change its resistance, which method comprises: (1) mounting die substrate (10) on the board, the substrate bearing the said pair of spaced-apart contacts (12, 14) and a protective layer covering the contacts except for a region (20) spanning the gap between ie contacts corresponding to die location of die polymer (25) and optionally a region (22,24) of each contact allowing electrical connection to be made to d e contacts; (2) connecting the pair of conductive tracks (28,30) to the respective pair of electrical contacts (12, 14) by electrical connections (36), (3) submerging substantially the whole of die substrate (10) in a monomer solution, and (4) applying a potential to the contacts (12,14) to cause the monomer to polymerise in the said region (20) of the substrate spanning the gap (16) to form the polymer. 2. A method of fabricating a gas sensor assembly comprising a board (26), a pair of conductive tracks (28,30) on the board, a substrate (10) mounted on die board, a pair of spaced-apart contacts (12,14) supported on the substrate, and a semi-conductive polymer (25) spanning die gap (16) between the contacts, which polymer is capable of interreacting with gases and/or volatile material to change its resistance, which mediod comprises: (1) mounting the substrate (10) on the board, die substrate bearing the said pair of spaced-apart contacts (12.14) and a protective layer covering the contacts except for: (a) a region (22,24) of each contact allowing electrical connection to be made to the contacts and (b) a region (20) spanning die gap between the contacts corresponding to the location of the polymer (25), so that the electrical connection regions (22,24) lie adjacent to the said conductive tracks (28,30), (2) connecting the pair of conductive tracks (28,30) to the connection regions (22,24) of die respective pair of electrical contacts (12,14) by electrical connections (36), (3) submerging substantially the whole of the substrate (10) in a monomer solution, and (4) applying a potential to die contacts (12,14) to cause die monomer to polymerise in the said region (20) of die substrate spanning die gap (16) to form the polymer. 3. The method of claim 1 or claim 2, wherein the distance between die connections (36) and die polymer (25) spanning the gap (16) is less man 10mm, e.g. less man 5mm and more preferably about 0.1 to 3mm, the said distance being measured between die nearest points of the connections and the gap that is spanned by polymer. 4. The method of any one of claims 1 to 3, wherein the part of the conductive tracks remote from the substrate (10) is located above the level of the monomer solution during polymer growth. 5. A ediod of any one of claims 1 to 4, which includes coating at least die said connection regions (22,24) of the contacts with an insulating coating. 6. A method any one of claims 1 to 5, which includes coating the electrical connections (36) with an electrically insulating material. 7. A method of any one of claims 1 to 6, which includes coating die regions of die conductive tracks (28,30) diat lie adjacent to the connectors (36) with an insulating coating. 8. A method of any one of claims 1 to 7, wherein after step (4), die board (26) is severed part of die way along the conductive tracks (28,30) to provide a shortened board. 9. A mediod of any one of claims 1 to 8, wherein in step (4), die monomer is polymerised only in the said region (20) of die substrate spanning the gap (16) between the contacts. 10. A sensor assembly comprising a substrate (10) bearing a pair of contacts (12, 14) diat are spaced apart by a gap (16), a semi-conductive polymer (25) spanning the gap (16), a board having a pair of electrically conductive tracks (28, 30) located adjacent to die contacts on the substrate and means (36) for connecting each contact (22, 24) widi a respective conductive track (28,30), wherein the distance between each connection means (36) and die semi-conductive polymer (20) spanning e gap (16) is less dian 10mm, preferably less than 7mm and most preferably less man 5mm e.g. 0.1 to 3mm. 11- A sensor as claimed in claim 10, wherein the conductive tracks and die connection means (36) are covered by an electrically insulating material.

Inventors: MCINNES JAMES, MCINNES, JAMES
Applicants: MCINNES JAMES, NEOTRONICS LTD, MCINNES, JAMES, NEOTRONICS LIMITED
Application year: 1995
Main classification: G01N27
IPC codes: G01N27