GB1570054A

GB1570054A - Sensor or detector element for an electrical hygrometer

Info

Publication number: GB1570054A
Application number: GB1155978A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1978-03-23
Filing date: 1978-03-23
Publication date: 1980-06-25

Description

(54) SENSOR OR DETECTOR ELEMENT FOR AN ELECTRICAL HYGROMETER (71) I, PETER ROCKLIFF, a British subject of 45 Heath Drive, Boston Spa, Yorkshire, do hereby declare the invention for which I pray that a Patent may be granted to me and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to a sensor or detector element of capacitance type for use in an electrical hygrometer and is intended particularly though not exclusively for meteorological purposes.

In recent years attempts have been made to measure atmospheric humidity accurately and at high speed in the field of Meteorology.

Hygrometers of the hair type in the early days have given way recently to electronic devices such as the aluminium oxide sensor. The aluminium oxide sensor has the required sensitivity, but it has the disadvantage of suffering deterioration over a period of storage and also the disadvantage of being difficult to heat to prevent saturation when exposed to sudden high humidities at low temperatures. The requirement for a very fast, stable, heated sensor has become manifest, as the possibility of using dropsondes (meteorological monitoring devices to be dropped from aircraft) is being examined.

The dropsonde starts its journey from a high altitude (approx. 30,000 ft.) where temperatures are low, humidity is low, and atmospheric pressure is low. Falling at great speed the response to changes of humidity has to be very fast. If the sensor were unheated, the first cloud layer encountered would probably produce saturation conditions, and the sensor upon becoming saturated would probably remain saturated due to the very low temperature encountered at high altitudes.

In my British Patent No. 1,513,371 I have described a sensor or detector element of the capacitance type for use in an electrical hygrometer, comprising a substrate or base on which is formed a first layer constituting one metal electrode, a second layer constituting the moisture sensitive dielectric, and a third layer constituting a second metal electrode capable of permitting free passage there through of water molecules, with means for connecting said first and third layers to a hygrometer circuit, said second layer comprising two films, the film adjacent the first layer being a barrier film which is non hygroscopic and which completely seals the first layer from the other film and the other film being a hygroscopic film having a rapid response to changes in humidity.

The chief object of the present invention is to provide a further improved sensor or detector element suitable for meteorological and for industrial use.

According to the invention there is provided a sensor or detector element of the capacitance type for use in an electrical hygrometer, comprising a non-metallic substrate or base of which at least a first layer at one surface is or is rendered at least electrically semi-conductive and constitutes a first and non-metallic or substantially non-metallic electrode, a second layer constituting a moisture sensitive dielectric formed on the first layer, and a third layer formed on the second layer, said third layer constituting a second electrode and being capable of permitting free passage therethrough of water molecules, with means for connecting said first and third layers in a hygrometer circuit, and said second layer comprising two superimposed films, the film adjacent the first layer being a barrier film which is non-hygroscopic and which completely seals the first layer from the other film and said other film being a water-insoluble hygroscopic film having a rapid response to changes in humidity.

Embodiments of the invention are now described by way of example with reference to the accompanying drawings, wherein Figs. 1 and 2 are respectively a plan and an edge view of one such sensor. The thickness of the sensor is exaggerated in Fig. 2 for clarity.

As shown in these drawings illustrating a first embodiment, a disc 1 of silicon is used as a support on a surface of which successive layers are formed or deposited to form a moisture sensitive device. The disc is of the order of 15-16m.m diameter and .003"- 008" thick, although thickness is not critical. The object of the successive layers is to produce a moisture sensitive capacitor rather than a resistance device, as tests over a period or years have established that the capacitive device is more stable than a device of which the resistance changes with moisture changes.

The first layer 2 is formed by doping the silicon substrate by a suitable dopant so as to render it at least at one surface, semi-conductive to electricity. The doping is effected by the deposition of materials such as arsenic, antimony and boron in minute quantities whereby to form a non-metallic or substantially non-metallic electrode.

The second layer 3 is the moisture sensitive dielectric of the capacitor. Practice has shown that in order to eliminate conductivity this layer should be in two parts, that is one film overlaid by another, namely; firstly a barrier (passive) film 3a which is not hygroscopic and which completely seals the first layer 2 and with molecules so tightly packed as to prevent water molecules from penetrating the layer and constituting a short circuit path at a later stage; and a hygroscopic section film 3b which will rapidly respond to changes of humidity.

The third layer 4 which is of metal and forms another electrode, has a good resistance to oxydisation, and should also be of a porous nature and capable of allowing free passage of water molecules through it.

A fourth layer (not shown) which may be deposited on the other side of the substrate is of metal or a ceramic/metal composition which will heat up with the passage of an electric current from a supply of potential of the order of 1.5 to 3 volts. This last layer may not always be required, but probably will be required for most meteorological purposes.

A sensor having the above features can be produced in the following manner: A disc 1 of doped silicon 15 m.m in diameter and .008" thick is coated under vacuum with the layers as follows; layer 3a, 3b consists of a film of silicon monoxide overlaid by a film of calcium fluoride, and the layer 4 is of gold. The thickness of these layers and films is commensurate with "thin film" deposition and maybe somewhat as follows: - passive film 3a-of the order of 0.5 to 1.0 micron; active film 3b-of the order of 3 to 4 microns; metal layer 4-of the order of 500 .

The materials used by the various layers are of very high purity, and are deposited under high vacuum (for example 10--4 torr) from a rotating turret of the type normally used by those skilled in the art of thin film deposition. The substrate is heated to a temperature of 200"C throughout the process. The vacuum deposition sequence is interrupted after layer 4 has been formed and the substrate disc is turned over in order to coat the other side with a nickel/chromium fourth layer.

In an alternative method a sheet of doped silicon material is similarly coated but using a series of masks to mask off areas not to be coated. When all the layers have been formed the sheet is divided into smaller elements by cutting between the coated areas.

The sensor according to the invention is not restricted to the specific materials men tioned above. For instance, the gold used for the third layer could be replaced by chromium, or any metal which is resistant to oxidisation; although the noble metals do have ideal properties.

The barrier (passive) film 3a could be any of a variety of ceramics which have the pro perties of forming a non-porous layer. This layer ensures that the sensor will operate solely by change in capacity and not by change in conductivity.

The hygroscopic (active) film 3b could con sist of almost any hygroscopic substance which can be vacuum deposited with the exception of hygroscopic water-soluble (i.e. deliquescent) substance.

The fourth layer could be of silicon mon oxide/chromium or chromium/nickel or of another electrically resistive alloy which would not dissociate upon vapourising.

In practice electrical connection with the other electrode, layer 4, is made by a small "tail" 5. The sensor is used in conjunction with a suitable capacity measuring circuit; the results obtained will be a change in capacity with a change in humidity.

The fourth layer which is used as a resistive heater warms up on passage therethrough of an electrical current. Connections are made to this layer by attaching two wires by suitable connector means. The sensor is only heated when external ambient temperature fluctuations are likely to affect capacity/humidity calibration and the fourth layer may be dispensed with if heating of the sensor will not be required, as may be the case in some in dustriat or other applications of the invention.

In a variation of the embodiment, the doped silicon is also oxidised over its entire surface by a suitable method. Doped silicon with an oxide coating has been found to provide an excellent substrate, the body of it being sufficiently electrically conductive and the surface of it being insulated by an oxide film.

The film provides the passive film 3a and readily accepts the deposition by vacuum coating thereon of the active film 3b.

In a second embodiment a substrate disc of graphite has applied to one surface, directly by vacuum coating or other suitable method, layer 3 consisting of films 3a and 3b, and layer 2 is dispensed with. The layer 4 is ap plied as described in connection with the first embodiment but if a further conductive layer is to be deposited at the opposite face of the substrate disc for the resistive heating of the sensor it will be electrically insulated from the substrate disc.

In a modification of the first described em bodiment and method of production there may be substituted for the silicon monoxide film 3a a borosilicate glass which forms a better bond with the support 1. Moreover, this borosilicate glass film may be applied by the technique of radio-frequency (RF) sputtering and this helps to ensure that the barrier (passive) film is free from pin-hole defects, which can occur when applying silicon monoxide by the vacuum deposition technique.

Moreover, the use of the RF technique enables more accurate control to be achieved over the application of the material, for example giving a more uniform thickness of deposition and a better control of the density of the layer material.

For improved commercial production the sensors may be produced in batches (say 25 or 100) cn one single substrate and subsequently divided into individual sensors by any suitable method e.g. by the scribe- and break method.

WHAT I CLAIM IS: 1. A sensor or detector element of the capacitance type for use in an electrical hygrometer, comprising a non-metallic sub strate or base of which at least a first layer at one surface is or is rendered at least electrically semi-conductive and constitutes a first and non-metallic or substantially non-metallic electrode, a second layer constituting a moist ure sensitive dielectric formed on the first layer, and a third layer formed on the second layer, said third layer constituting a second electrode and being capable of permitting free passage therethrough of water molecules, with means for connecting said first and third layers in a hygrometer circuit, and said second layer comprising two superimposed films, the film adjacent the first layer being a barrier film which is non-hygroscopic and which completely seals the first layer from the other film and said other film being a water-insoluble hygroscopic film having a rapid response to changes in humidity.

2. A sensor or detector element according to Claim 1, wherein second and third layers have a thickness in the "thin film" category.

3. A sensor or detector element according to Claim 1 or 2, wherein the third layer is of gold metal.

4. A sensor or detector element according to any one of the preceding claims, wherein the second layer is a film of silicon monoxide overlaid by a film of calcium fluoride.

5. A sensor or detector element according to Claim 4, but modified in that said film con stituting the barrier film is of borosilicate glass.

6. A sensor or detector element according to Claim 5, wherein the borosilicate film has been applied by radio-frequency sputtering.

7. A sensor or detector element according to any one of the preceding claims, wherein the thickness of the layers of films is of the order of 0.5 to 1.0 micron for the barrier film of the second layer, 3 to 4 microns for the hygroscopic film of the second layer and 500A for the third layer.

8. A sensor or detector element according to any one of the preceding Claims wherein each of the second and third layers of films has been produced by "thin film" deposition under high vacuum through a succession of masks, the substitute being heated throughout the process.

9. A sensor or detector element according to any one of the preceding Claims having a fourth layer applied to a further surface of the substrate and electrically insulated from the first layer, this fourth layer being connectible to a source of electric energy and adapted to constitute an electrically resistive heater for heating the sensor when the latter is in use.

10. A sensor or detector element according to any one of the preceding Claims wherein the substrate or base is rendered at least electrically semi-conductive at least in a layer defining said first layer at said one surface.

11. A sensor or detector element according to Claim 10, wherein the substrate or base comprises silicon rendered at least electrically semi-conductive by doping.

12. A sensor or detector element according to Claim 11, wherein the doped silicon substrate or base is oxidised to provide the barrier film portion of said second layer.

13. A sensor or detector element according to any one of Claims 1 to 9 wherein the substrate or base is at least substantially wholly composed of conductive non-metallic material.

14. A sensor or detector element according to Claim 13, wherein the substrate or base is of graphite.

15. A sensor or detector element of the capacitance type for use with an electrical hygrometer, constructed substantially as herein described with reference to, and as illustrated by, - the accompanying drawings.

16. Any one of the methods herein described of producing sensors or detector elements of the capacitance type for use with an electrical hygrometer, as claimed in any one of the preceding claims, and wherein a multiplicity of said sensors is produced in batches, each batch being formed on a single substrate which is subsequently divided into individual sensors.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. bodiment and method of production there may be substituted for the silicon monoxide film 3a a borosilicate glass which forms a better bond with the support 1. Moreover, this borosilicate glass film may be applied by the technique of radio-frequency (RF) sputtering and this helps to ensure that the barrier (passive) film is free from pin-hole defects, which can occur when applying silicon monoxide by the vacuum deposition technique. Moreover, the use of the RF technique enables more accurate control to be achieved over the application of the material, for example giving a more uniform thickness of deposition and a better control of the density of the layer material. For improved commercial production the sensors may be produced in batches (say 25 or 100) cn one single substrate and subsequently divided into individual sensors by any suitable method e.g. by the scribe- and break method. WHAT I CLAIM IS:

1. A sensor or detector element of the capacitance type for use in an electrical hygrometer, comprising a non-metallic sub strate or base of which at least a first layer at one surface is or is rendered at least electrically semi-conductive and constitutes a first and non-metallic or substantially non-metallic electrode, a second layer constituting a moist ure sensitive dielectric formed on the first layer, and a third layer formed on the second layer, said third layer constituting a second electrode and being capable of permitting free passage therethrough of water molecules, with means for connecting said first and third layers in a hygrometer circuit, and said second layer comprising two superimposed films, the film adjacent the first layer being a barrier film which is non-hygroscopic and which completely seals the first layer from the other film and said other film being a water-insoluble hygroscopic film having a rapid response to changes in humidity.