GB2043908A

GB2043908A - Humidity Sensor Element

Info

Publication number: GB2043908A
Application number: GB7908447A
Authority: GB
Original assignee: Moisture Control & Mesurement
Current assignee: Moisture Control & Mesurement
Priority date: 1979-03-09
Filing date: 1979-03-09
Publication date: 1980-10-08

Abstract

A sensor element for an electrical hygrometer may be made by forming a layer 7 of hygroscopic silicon oxide on a substrate of silicon by anodic oxidation of the surface of the substrate. The substrate is rendered conductive by doping so that it forms a first electrode and a porous metallic layer 13 is deposited on the hygroscopic layer to form a second electrode. Advantageously a portion 9 of the surface of the substrate is also oxidised to form a layer of non- hygroscopic silicon oxide alongside the hygroscopic layer. <IMAGE>

Description

SPECIFICATION Humidity Sensor This invention relates to a humidity sensor with improved stability over a long period of time.

For many years humidity sensors have been produced comprising a hygroscopic layer of Aluminium Oxide on the surface of aluminium foil or wire. The sensors have been constructed to give an electrical signal in the form of a conductivity change, a capacity change, or a combination of both, and the electrical change has been made to indicate a change in humidity, or to control a process, or a combination of both.

More recently it has been proposed to produce a humidity sensor on a substrate of silicon. In such sensors, in which the silicon is insulated by a nonhygroscopic layer of silicon oxide, the hygroscopic layer has still been of aluminium oxide or possibly of calcium fluoride, and the present invention stems from the realization that silicon could not only provide a substrate or base for the sensor but also contribute to the formation of a hygroscopic layer thereupon.

According to one aspect of the invention there is provided a sensor or detector element for an electrical hygrometer comprising a layer of hygroscopic silicon oxide supported on a substrate of silicon. Preferably the silicon oxide is formed by oxidation of the surface of the substrate. Alongside the layer of hygroscopic silicon oxide and also supported on the silicon substrate there may be an insulating layer of nonhygroscopic silicon oxide. The substrate may be rendered conductive, and the detector element may further comprise a conductive layer overlying both the hygroscopic and non-hygroscopic layers of silicon oxide whereby the conductive layer and the substrate may act as opposed electrodes and the hygroscopic layer as an intermediate dielectric.According to a further aspect, the invention provides a method of use in the manufacture of sensors or detector elements for electrical hygrometers comprising forming layer of hygroscopic silicon oxide supported on a silicon substrate. Preferably the hygroscopic silicon oxide is formed by oxidation of the substrate, and the method preferably further comprises the step of forming a layer of non-hygroscopic silicon oxide alongside the hygroscopic layer and likewise supported by the substrate. Preferably nonhygroscopic silicon oxide is formed over the whole of a face of a silicon base and subsequently removed from a portion or portions of the face and replaced by hygroscopic silicon oxide.

Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings of which: Fig. 1 shows an embodiment of a humidity detector element in vertical section in which the scale is the vertical direction is grossly exaggerated; Fig. 2 is a plan view of the embodiment of Fig.

Fig. 3 is a plan view of a simplified embodiment.

As shown in the figures, the detector elements comprise a substrate or base 1 of silicon doped for example with boron or antimony to render the silicon electrically conductive. The upper face of the substrate as seen in Fig. 1 is polished to a brilliant finish whilst the lower face is as sawn from a crystal ingot. The thickness of the substrate is of the order of 0.01 5 to 0.20 inches.

The lower face is coated with a layer of metal 3 to which a connecting wire 5 is soldered. On the polished upper face of the substrate there is formed a rectangular layer 7 of hygroscopic silicon oxide and alongside that layer a further layer 9 of non-hygroscopic silicon oxide. In Figure 2 the non-hygroscopic oxide is shown to extend to form a border around the hygroscopic oxide but this refinement is omitted in the embodiment shown in Figure 3.

The layers 7 and 9 are shown as having substantially equal thickness, though this may not be the optimum arrangement.

A reinforcing disc 11 of metal is formed on the upper surface of layer 9, the shape of the reinforcement being convenient rather than critical.

A final metal layer 13 is formed to overlie the disc 11 and a substantial portion of each of the layers 7 and 9 but stopping short of the edge of those layers to avoid accidentally making direct electrical connection with the substrate 1.

A connector wire 1 5 is soldered to the layer 1 3 where it is reinforced by the disc 11.

The layer 1 3 and the substrate 1 form opposed electrodes of a capacitor whilst the layers 7 and 9 constitute the dielectric. The layer 1 3 is formed in such a way as to allow atmospheric moisture to pass through it to the hygroscopic layer 7, and the dielectric property of the capacitor varies according to the water content of layer 7 which in turn depends on the humidity of the atmosphere.

The detector does not, however, operate wholly capacitively because there is an electrical conductive path between layer 13 and substrate 1 via moisture held in layer 7 and the conductivity of this path has to be taken into consideration in interpreting the electrical behaviour of the detector.

The detector element described above typically measure 4 mmx2 mm in practice some 300 may be formed from a single silicon slice of 3 inches diameter.

A convenient method of producing the detector elements comprises the initial step of oxidising a doped silicon slice, of which one surface is polished, by a thermal technique such as is known in the manufacture of semi-conductor devices. By this technique a film of non-hygroscopic silicon oxide of thickness 5000-6000A is formed over the whole surface of the slice.

After cooling, the oxidised slice is cleaned with alcohol and coated with a photo-resist resin. A mask which is opaque except in areas corresponding to areas of the portions 9 of the detector elements to be produced from the slice is placed over the surface of the slice which was originally polished, and both sides of the slice are then exposed to ultra-violet light which hardens the resin in the un-masked areas. The slice is then washed with a suitable developer which removes unhardened resin, after which the remaining photo-resist is still further hardened by heat treatment.

After cooling, the slice is treated with hydrofluoric acid and ammonium bifluoride which removes the silicon oxide except from the areas protected by the photo-resist. The photo-resist is then dissolved away with chloroform.

The un-polished side of the slice is then cleaned and vacuum coated firstly with chromium to a thickness of about 100500 and then with gold to about 1000--5000a. The function of the chromium is to make the gold adhere to the silicon and the gold provides a surface to which connecting wire 5 can be soldered.

The polished side of the slice is then oxidised anodicaliy whereby a hygroscopic layer of silicon oxide is formed over the whole surface apart from those areas where the slice has been rendered non-conductive by the areas of non-hygroscopic silicon oxide.

After cleaning and drying, the oxidised surface of the slice is vacuum coated firstly with chromium and secondly with gold through a mask which limits the deposition of metal to the circular areas 11 shown in the figures. The thicknesses of the layers of chromium and gold are similar to those deposited on the other side of the slice.

By using a further mask a further coating of gold is deposited to give the layer 1 3 of each detector element.

The silicon slice is cut with a diamond stylus into the individual detector elements, care being taken not to spread gold from the layer 13 over the edge of the oxide layers, because direct metal ccntact between layer 1 3 and substrate 1 would effectively short circuit the capacitor defined by substrate 1 and layer 1 3 as electrodes and oxide layers 7 and 9 as dielectric.

Finally, a wire connector 5 is soft soldered to the under side of the element and a fine gold wire is ultrasonically welded to the layer 13 over the centre of the reinforcing disc 11.

In a less preferred embodiment not shown, the silicon substrate takes the form of a wire or rod of which the circumferential surface along adjacent length portions is coated respectively with hygroscopic and non-hygroscopic silicon oxide, the whole being overlaid with a metallic layer or a helical winding of metallic wire. The result is a substantially cylindrical capacitor with coaxial electrodes, which, however, it may not be easy to manufacture in large numbers.

Claims

1. A sensor for an electrical hygrometer comprising a layer of hygroscopic silicon oxide supported on a substrate of silicon.

2. A sensor as claimed in Claim 1 wherein the hygroscopic silicon oxide is formed by oxidation of the substrate at the surface thereof.

3. A sensor as claimed in Claim 1 or Claim 2 wherein the substrate also supports a layer of non-hygroscopic silicon oxide adjacent said layer of hygroscopic silicon oxide.

4. A sensor as claimed in Claim 3 further comprising a conductive layer overlying both said hygroscopic and non-hygroscopic layer.

5. A method of making a sensor for an electrical hygrometer having a layer of hygroscopic silicon oxide supported on a substrate of silicon comprises forming the said layer by oxidation of the substrate at the surface thereof.

6. A method according to Claim 5 wherein the surface of the substrate is oxidised to form a layer of non-hygroscopic silicon oxide adjacent said hygroscopic layer.

7. A method according to Claim 5 wherein the non-hygroscopic layer is first formed at the surface of the substrate, the non-hygroscopic silicon oxide is removed from a portion of the surface and said hygroscopic layer is formed on said portion.

8. A sensor for an electrical hygrometer substantially as described with reference to the drawings.

9. A method of making a sensor for an electrical hygrometer having a layer of hygroscopic silicon oxide supported on a substrate of silicon substantially as described.