GB2152691A

GB2152691A - Infra-red detector

Info

Publication number: GB2152691A
Application number: GB08420564A
Authority: GB
Inventors: David William Satchell
Original assignee: Standard Telephone and Cables PLC
Current assignee: STC PLC
Priority date: 1983-08-12
Filing date: 1984-08-13
Publication date: 1985-08-07
Also published as: GB2152691B

Abstract

An infra-red image sensor comprises a support grid structure 11 on one surface of which a liquid crystal 13 is disposed. Local temperature changes of the liquid crystal in response to an infra-red image projected thereon produce corresponding changes in the transmission of polarised light through the array. Typically the support grid is formed of single crystal silicon. <IMAGE>

Description

SPECIFICATION Infra-red detector This invention relates to infra-red detectors and to infra-red image converters e.g. for use in night vision equipment.

Infra-red sensors are used in a variety of applications, particularly in the military field where they are employed in heat sensors and night vision devices. The efficiency of these sensors is characterised by a figure of merit known as the detectivity (D) which is calculated from the expression D = V5 z/A (Af) V5 W where V5 is the r.m.s. output derived from the infra-red signal, V5 is the r.m.s. output generated by noise, W is the r.m.s. incident power, A is the effective detector area and Af is the bandwidth.

At present the most efficient detectors are photoconductive devices which have a detectivity of the order of 1010. Whilst these devices are widely used they suffer from the disadvantage that they require cooling to low temperatures. This restricts the portability and ease of operation of these devices.

Attempts have been made to overcome this problem by the use of bolometers or of pyroelectric devices which operate at ambient temperature. However these have a much lower detectivity than photoconductive sensors. The maximum value reported is 1 Os. The advantages of ambient temperature operation cannot therefore be fully realised with devices available at present.

The object of the present invention is to minimise or to overcome this disadvantage.

Our co-pending application No. 8321807 (D.W. Satchell 1 0) relates to an infra-red sensor including an array of horn aerial elements formed in a body of single crystal silicon, and bolometer elements one for each horn whereby in use radiation received by that horn is detected.

According to one aspect of the present invention there is provided an infra-red sensor array, including a laminar support grid structure, a continuous membrane on one surface of the grid structure, and a plurality of sensor elements disposed on the membrane and each in register with a grid opening, wherein each said element comprises a liquid crystal device.

According to a further aspect of the invention there is provided a method of making a liquid crystal device, the method including abrading a major surface of a planar silicon body, oxidising the surface to provide a layer conforming to the silicon surface configuration, selectively removing the silicon from the oxide, and applying a liquid crystal to the surface.

Advantageously the support grid structure is formed from single crystal silicon.

Typically the aerial horn array is formed by etching pyramidal pits in the surface of a single crystal sillicon body. The horns have a substantial gain relative to single dipoles and so provide a sensor of high detectivity.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a cross-section of a portion of the infra-red sensor array; Figure 2 is a sectional view of the support grid of the detector array of Fig. 1; Figure 3 shows an alternative array construction, and Figure 4 is a schematic diagram of an infrared image converter using the array of Figs. 1 and 2 or Figs. 2 and 3.

Referring to Figs. 1 and 2 the sensor array is formed on a support structure II comprising a grid formed from a laminar body, e.g. of selectively etched single crystal silicon. The grid is etched to form an array of pyramidal pits Ill which pits function at infra-red wavelengths as horn aerial elements. The surface of the wafer coinciding with the throats of the pits Ill is covered with a transparent membrane 1 2 typically of silicon dioxide or silicon nitride. The manufacture of the support grid structure may be effected by a selective doping and etching process such as that described in our co-pending application No.

8413225 (T.M. Jackson-D.W. Satchell 79-13).

The thermally sensitive elements of the array are provided by a layer of a liquid crystal 1 3 disposed on the membrane 1 2 and retained thereon by a further transparent membrane 14, e.g. of a plastics material.

The membrane 1 4 may be formed by freezing the liquid crystal 1 3 and applying an even coating of a liquid monomer to its surface.

The monomer is then polymerised to a solid layer, e.g. by the application of ionising radiation or by chemical reaction with a previously mixed polymerisation initiator. Alternatively the membrane 1 4 can be formed from a solution or an emulsion of the polymer, the liquid vehicle being removed by evaporation to leave the solid film. When the liquid crystal is then allowed to melt and resume its normal condition it is retained by the membrane 14.

An alternative construction is shown in Fig.

3 in which the liquid crystal 11 3 is retained by a membrane 114 on the underside of the membrane 12, i.e. the liquid crystal is disposed in the throat of each pyramidal pit 111.

Such an arrangement may be preferred in some applications as it has a higher thermal sensitivity than the arrangement of Fig. 1.

The sensor arrays of Figs. 1 and 3 may be employed in infra-red imaging applications.

Absorption of an infra-red image results in local temperature changes in the liquid crystal and thus alters its ability to rotate the plane of polarisation of optical radiation. If the sensor array is disposed between optical polarising elements and uniformly illuminated the amount of visible light transmitted by the array elements will depend on the variations of temperature produced by an infra-red image projected on to the array. This generates an optical image corresponding to the infrared image. The optical image may be viewed directly or may be processed and amplified by suitable electronic equipment.

An image converter operating on this principle is shown in Fig. 4. The converter includes a sensor array 41 of the type described with reference to Figs. 1 and 2 or Figs. 2 and 3.

Infra-red light from a source to be scanned is focussed by a lens 42 on to the sensor array 41 via an input polariser 43. The array is also flood lit from an optical source 44. Visual light passing through the array is transmitted via an output polariser 45 and a further lens 46 to an optical imaging device. The amount of light transmitted by each cell or pixel of the array is determined by the temperature of the corresponding liquid crystal element which temperature is in turn defined by the configuration of the infra-red image. Typically the arrangement of Fig. 4 may be employed in night vision equipment.

In order for the liquid crystal to function efficiently in rotating the plane of incident polarised light it is necessary to align the molecules parallel to some common axis in the plane of the substrate. This may be achieved by suitable preparation of the support structure. Where a silicon structure is employed we have found that this may be provided with an array of fine parallel microgrooves by grinding or abrasion. Coating such a surface with a membrane of silica or silicon nitride provides a layer which replicates the grooved surface beneath. Relative removal of the silicon by etching them leaves a corrugated membrane having a microgroove structure on both its lower and upper surfaces.

Liquid crystal molecules in contact with such a layer are aligned parallel to a common axis.

This technique avoids the need for additives to the liquid crystal and/or for treatment of the membrane subsequent to deposition.

Claims

1. An infra-red sensor array, including a laminar support grid structure, a continuous membrane on one surface of the grid structure, and a plurality of sensor elements disposed on the membrane and each in register with a grid opening, wherein each said element comprises a liquid crystal device.

2. A sensor array as claimed in claim 1, wherein said support structure comprises an integral body formed from single crystal silicon.

3. A sensor array as claimed in claims 1 and 2, whereby the membrane is of silica or silicon nitride.

4. A sensor array as claimed in claims 1, 2, 3 whereby the membrane has a grooved surface whereby alignment of the liquid crystal is effected.

5. A sensor array as claimed in claim 1 or 2, wherein said liquid crystal is retained on the array by a plastics membrane.

6. An infra-red sensor array substantially as described herein with reference to Figs. 1 and 2 or to Figs. 2 and 3 of the accompanying drawings.

7. An infra-red image converter incorporating a sensor array as claimed in any one of claims 1 to 6.

8. Night vision apparatus incorporating an image converter as claimed in claim 7.

9. A method of making a liquid crystal device, the method including abrading a major surface of a planar silicon body, oxidising the surface to provide a layer conforming to the silicon surface configuration, selectively removing the silicon from the oxide, and applying a liquid crystal to the surface.

10. A method as claimed in claim 9, wherein the liquid crystal is foreseen prior to the application of a transparent film whereby the liquid crystal is retained on the surface.

11. A method of making a liquid crystal device substantially as described herein with reference to the accompanying drawings.