GB2368188A - Infrared detector - Google Patents

Abstract

An infrared detector for operation at a cryogenic temperature has a photoconductive element (10) supported by a mounting substrate (26) which is formed as a sandwich comprising at least two superimposed thin layers (27, 28) of a material of high thermal diffusivity separated by a layer (29) of bonding cement which is sufficiently thin to leave the thermal properties of the substrate (27) substantially unaffected. The substrate (27) is very rapidly cooled by suitable cooling means (20), such as an open cycle Joule-Thompson mini-cooler (17), but its sandwich construction (27, 28, 29) reduces the incidence of electrical noise induced in the photoconductive element (10) thereby enabling increased performance. The element (10) may be epitaxially grown on a gallium arsenide layer (27).

Description

INFRARED DETECTOR This invention relates to an infrared detector for operation at a cryogenic temperature.

Such infrared detectors are well-known for use in missile seeker heads and missile guidance systems of the type generally termed optical beam riders, and for operational purposes typically need to be cooled from the ambient operational temperature to a cryogenic temperature of 77 K to 1000K in less than one second. To achieve such very rapid cooling, the detector and its supporting structure are designed to be of low thermal capacity and are very rapidly cooled by using, an open cycle Joule-Thompson mini-cooler. We have found that the performance of such detectors is adversely affected by electrical noise, that the frequency spectrum of this induced noise is especially rich at low frequencies (that is frequencies below a few KHz), and that the primary source of such low frequency noise is generated by the Joule-Thompson mini-cooler. We have also established that the severity of electrical noise increases systematically if the thickness of the supporting structure is systematically reduced to achieve low thermal capacity and shorter cool-down times.

An object of this invention is to provide an infrared detector, for operation at a cryogenic temperature, in which the incidence of electrical noise is reduced.

According to the invention an infrared detector, for operation at a cryogenic temperature, includes a photoconductive element supported by a mounting substrate, means for rapidly cooling the element by conduction through the substrate, the substrate being formed as a sandwich comprising at least two superimposed thin layers of a material of high thermal diffusivity separated by a layer of bonding cement which is sufficiently thin to leave the thermal properties of the substrate substantially unaffected. A layer of the substrate may be formed from sapphire, silicon, or any other material having the appropriate properties, and different layers may be formed from different materials.

The layer of bonding cement is preferably kept as thin as practical and typically would have a thickness of only a few microns.

The photoconductive element may be formed epitaxially on the substrate layer that is furthest away from the cooling means. In this event, the substrate layer may be formed from gallium arsenide. Alternatively, the photoconductive element may be secured by a thin layer of bonding cement to the substrate layer that is furthest away from the cooling means.

The cooling means is preferably the known open cycle Joule-Thompson cooler which is arranged to discharge coolant directly onto the face of the nearest substrate layer.

The invention is now described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is an axial section through a first form of known encapsulated infrared detector incorporating an open cycle Joule-Thompson mini cooler; Figure 2 is an axial section through another form of known encapsulated infrared detector, but with the mini-cooler omitted, and Figure 3 is an enlarged scrap section of a central portion of Figure 1 illustrating the present invention.

With reference to Figure 1, a photoconductive element 10 for detecting infrared radiation, is supported by a monolithic sapphire substrate 11 which is fixed in a recess 12 in a body 13 which defines a frusto-conical cooler interface 14. The photoconductive element 10 is encapsulated in a generally cylindrical housing 15 which locates and supports a window 16 to permit the reception of infrared radiation by the photoconductive element 10.

An open cycle Joule-Thompson mini-cooler is indicated generally by arrow 17 and is housed within the frusto-conical cooler interface 14. The mini-cooler 17 generally comprises a conical coil of tubing 18 of which the outer surface is a close fit against the cooler interface 14, and the inner surface is a close fit on a frusto-conical core 19 as shown. When the infrared detector is to be used, gas under high pressure is released through the tubing 18 and escapes through an orifice in the open end 20 to impinge on the adjacent surface of the substrate 11. The release of the gas through the orifice causes extremely rapid cooling of the gas which absorbs heat by conduction from the substrate 11 and the body 13, and escapes through the helical passage 21 defined between the tubing 18, the cooler interface 14 and the outer surface of the core 19, finally escaping as indicated by arrows 22. This arrangement operates as a counter-flow heat exchanger as the cooled escaping gas acts to precool the gas under pressure entering the coil 18. To enhance the heat exchange between the exhausting gas and the incoming gas, the tubing 18 is usually finned to increase the heat transfer surface. In operation, the counter flow heat exchanger is so effective that the gas in the coil 18 is very rapidly liquefied whereby the substrate 11 is cooled by the direct impingement of a jet of liquefied gas, thereby increasing the potential cooling rate by the latent heat of vaporisation of the liquefied gas. In the case where argon is used, the temperature of the liquid gas is about 900 Kelvin whereas, in the case of nitrogen being used, the temperature would be around 770 Kelvin. In either event, the operation of the mini-cooler 17 very rapidly reduces the temperature of the monolithic substrate 11 and its associated photoconductive element 10 to a cryogenic temperature, typically between 900 and 100 with argon. It will also be understood that the portions of body 13 immediately surrounding the substrate 11 provide a high degree of thermal isolation of the substrate by virtue of the very low thermal conductivity of the body 13.

We have found that the performance of infrared detectors of the type illustrated in Figure 1 is adversely effected by electrical noise, and that the frequency spectrum of the induced noise is especially rich at low frequencies. In use, such infrared detectors are commonly used to detect laser beams which are projected in a pattern that would typically pass the window 16 in about 20 microseconds. The detected infrared pulse contains frequencies in the range of OHz to 20KHz. It will therefore be appreciated that generation of any electrical noise in the photoconductive element between these frequencies is undesirable. The substrate 11 is typically only of four millimetres diameter and the exact mechanism by which electrical noise is generated and propagated is not fully understood. However, we have found that dominant low frequency noise is associated with the operation of the mini-cooler 17, and it appears that the high flow rate of the liquefied gas impinging on the monolithic substrate 11 generates vibration at acoustic frequencies which is propagated through the monolithic substrate 11 and appears as an electrical noise received by the photoconductive element 10. For operational use, it is essential that such an infrared detector becomes operative with the smallest possible delay, and it is for this reason that the substrate 11 is made thin and of a material of high thermal diffusivity, such as sapphire. In order to minimise the cooling period, one is faced with either increasing the flow rate of the jet impinging on the monolithic substrate 11 with an associated increase in the generation of electrical noise and a corresponding reduction in the operative efficiency of the detector, or of minimising the thermal mass. To achieve the latter, the thickness of the monolithic substrate 11 could be reduced to achieve shorter cool-down times, but we have found that the severity of the electrical noise imposed on the photoconductive element increases systematically with reduction in thickness of the detector substrate 11.

The alternative known construction, illustrated in Figure 2, has many features common with Figure 1 and the same reference numerals have been utilised to identify equivalent components. Indeed the difference between the two Figures resides solely in the replacement of the body 13 of Figure 1 with a thin-walled body 23 having an annular flange 24 sealingly secured to the bottom edge of the cylindrical housing 15. The thin-walled body 23 can be made as a pressing or by electroforming, and defines the outer frusto-conical cooler interface 14 for the open cycle Joule-Thompson mini-cooler which has been omitted from this Figure. It will be noted that the monolithic sapphire substrate 11 is secured to a wall portion 25 of the body 23.

Whilst this design minimises the mass of the body 23 locating the substrate 11 relative to the window 16, the jet of coolant will impinge first on the wall portion 25 which therefore increases the thermal mass to be cooled immediately underneath the photoconductive element 10. The generation of electrical noise in the photoconductor element 10 is generally the same as for the Figure 1 construction.

Apart from the features of the present invention, Figure 3 corresponds generally with the known arrangement already described with reference to Figure 1, and accordingly the same reference numerals have been employed to indicate equivalent components. The main point of difference is that the monolithic substrate 11 of Figure 1 has been replaced by a substrate 26 formed as a sandwich comprising two superimposed thin layers 27,28 of a material of high thermal diffusivity such as sapphire or silicon.

The layers 27,28 are separated by a layer 29 of bonding cement which is sufficiently thin to leave the thermal properties of the substrate 26 substantially the same as the substrate 11 in Figure 1. Because Figure 3 is drawn to a larger scale, the coil of tubing 18 has been sectioned to show the formation of the nozzle 30. This can be formed by drilling the tube, but may be made in any convenient manner.

For instance, it is practicable to set the coil 18 on a gas flow test rig and progressively crimp the open end 20 to calibrate the nozzle 30 to deliver a predetermined flow rate at a set pressure. From Figure 3, it will also be noted that the photoconductive element 10 is adhered to the upper layer 27 by a thin layer of bonding cement 31. Such bonding is preferably achieved by using a pure form of epoxy resin and a pure form of hardener in combination with solvents if necessary to enable the thickness of the bonding cement to be of the order of 1 micron. The layer 29 between the substrates 27 and 28 may be thicker and is typically a few microns thick, say, between 1 and 10 microns. Although the substrate 26 in Figure 3 is shown retained in a plain bore 32, it may of course be located in recess such as that indicated at 12 in Figure 1.

The substrate 26 is typically of 4 mm diameter with a thickness of about 500 microns split equally between the two layers 27 and 28. However, the architecture of the substrate 26 may be varied as required by using layers 27, 28 of differing thickness, and/or of differing materials such as sapphire, silicon, and gallium arsenide; also three or more layers maybe utilised if required. In particular, the upper layer 27, that is the layer that is furthest away from the cooling nozzle 30, may be formed of gallium arsenide with the photoconductive element 10 formed epitaxially on it.

With the arrangement illustrated in Figure 3, we have found that an open cycle Joule-Thompson mini-cooler generates substantially less electrical noise in the photoconductive element 10. Apart from increasing the performance of the photoconductive element 10 by reducing electrical noise, the invention also enables the cool-down time to be reduced for a given level of electrical noise generation, either by increasing the flow rate of the coolant, or by reducing the thickness of the substrate 26, or by a combination of both.

Claims (10)

1. An infrared detector for operation at a cryogenic temperature, including a photoconductive element supported by a mounting substrate, means for rapidly cooling the element by conduction through the substrate, the substrate being formed as a sandwich comprising at least two superimposed thin layers of a material of high thermal diffusivity separated by a layer of bonding cement which is sufficiently thin to leave the thermal properties of the substrate substantially unaffected.

2. An infrared detector, according to claim 1, in which at least one layer of the substrate is formed from sapphire.

3. An infrared detector, according to claim 1, in which at least one layer of the substrate is formed from silicon.

4. An infrared detector, according to any of claims 1 to 3, in which the layer of bonding cement has a thickness of less than ten microns.

5. An infrared detector, according to claim 4, in which the layer of bonding cement has a thickness of less than five microns.

6. An infrared detector, according to any preceding claim, in which the photoconductive element is formed epitaxially on the substrate layer that is furthest away from the cooling means.

7. An infrared detector, according to claim 6, in which the substrate layer furthest away from the cooling means is formed from gallium arsenide.

8. An infrared detector, according to any of claims 1 to 5, in which the photoconductive element is secured by a thin layer of bonding cement to the substrate layer that is furthest away from the cooling means.

9. An infrared detector, according to any preceding claim, in which the cooling means is an open cycle Joule-Thompson cooler arranged to discharge coolant directly onto the face of the nearest substrate layer.

10. An infrared controlled device incorporating an infrared detector in accordance with any preceding claim.

10. An infrared detector substantially as described with reference to Figure 3 of the accompanying drawings.

11. An infrared controlled device incorporating an infrared detector in accordance with any preceding claim. Amendments to the claims have been filed as follows 1. An infrared detector for operation at a cryogenic temperature, including a photoconductive element supported by a mounting substrate, an open cycle Joule Thomson cooler for rapidly cooling the element by conduction through the substrate, the substrate being formed as a sandwich comprising at least two superimposed thin layers of a material of high thermal diffusivity separated by a layer of bonding cement having a thckness of less than 10 microns so that the thermal properties of the substrate remain substantially unaffected.

2. An infrared detector, according to claim 1, in which at least one layer of the substrate is formed from sapphire.

3. An infrared detector, according to claim 1 or 2, in which at least one layer of the substrate is formed from silicon.

4. An infrared detector, according to any one of claims 1 to 3 in which the layer of bonding cement has a thickness of less than five microns.

5. An infrared detector, according to any preceding claim, in which the photoconductive element is formed epitaxially on the substrate layer that is furthest away from the cooling means.

6. An infrared detector, according to claim 5, in which the substrate layer furthest away from the cooling means is formed from gallium arsenie.

7. An infrared detector, according to any of claims 1 to 4, in which the photoconductive element is secured by a thin layer of bonding cement to the substrate layer that is furthest away from the cooling mena. s 8. An infrared detector, according to any preceding claim, in which the open cycle Joule Thomson cooler is arranged to discharge coolant directly onto the face of the nearest substrate layer.

9. An infrared detector substantially as described with reference to Figure 3 of the accompanying drawings.