GB2186741A

GB2186741A - Infrared detectors

Info

Publication number: GB2186741A
Application number: GB08603681A
Authority: GB
Inventors: William Andrew Ernest Dunn; David John Gowlett
Original assignee: Philips Electronic and Associated Industries Ltd
Current assignee: Philips Electronics UK Ltd
Priority date: 1986-02-14
Filing date: 1986-02-14
Publication date: 1987-08-19
Also published as: GB2186741B

Abstract

In an infrared detector having fast cool-down characteristics, the infrared detector element (2) is on the cooling- element side of a housing wall (12, 13) so as to be located in the space (20) into which the coolant (33) expands from the orifice (32) of a Joule-Thomson cooling element (3). The detector element (2) can be mounted on a low thermal conductivity circuit substrate (12) which forms part of the wall (12, 13) of the space (20) and through which the infrared radiation (50) may be transmitted to the detector element (2). The detector element mount (12) may be shaped to form an immersion lens for the detector element 2, or it may be, for example, an end face of the core (30) of the cooling element (3). <IMAGE>

Description

SPECIFICATION Infrared Detectors This invention relates to infrared detectors having fast cool-down characteristics, in which the infrared detector element or elements are cooled during operation by a Joule-Thomson cooling element.

Infrared detectors having fast cool-down characteristics are known, comprising a housing within which at least one infrared detector element and a Joule-Thomson cooling element are accommodated. One example of such a detector is shown in published United Kingdom patent application (GWA) 2 147 739A. The cooling element has an orifice from which coolant expands into a space on one side of a wall in the housing to effect cooling of the detector element during operation of the detector. The detector element is secured to a mount forming part of said wall. The design of the housing is such that the detector element can be cooled down very rapidly, for example within a few seconds, and the detector is used in situations where the cooled operating state need not be maintained for a very long time, for example less than 10 minutes.

In the detector shown in GB--A 2 147 739, the detector element is mounted on a carrier plate 11 which forms a part of an inner wall 11,13 of the housing and which is made of a material of good thermal conductivity but low thermal capacity, for example sapphire or molybdenum. These materials are expensive. This carrier plate 11 is adhered or soldered around its periphery, to a section 2 of the housing. As shown in the drawings of GWA 2 147 739, the side of the carrier plate 11 facing the cooling element 14,19 is formed with channels or fins to increase the surface area exposed to the expanded coolant from the orifice 14 so as to improve the cooling efficiency for the detector element 7.The detector element 7 is mounted on the opposite side of the carrier plate 11 and so its cooling relies on heat conduction through the carrier plate 11. This is in accordance with the conventional practice of locating the infrared detector element in either a vacuum or an hermetically sealed enclosure having a carefully controlled environment. Further illustrations of this conventional detector-element mounting practice arefound in, for example, published European patent applications (EP-A) 0 006 277 and 0 136 687, United States patents (US-A) 2 951 944 and 3 306 047 and published United Kingdom patent applications (GB--A) 2 119 071 and 2 148 051.

This conventional practice arose because it was found that the performance of infrared detector elements was degraded if operated in adverse environments for a long time (for example, in thermal imaging applications). Furthermore it was found that infrared detector elements of, for example, indium antimonide suffered from microphony and so would need protecting from the turbulence effects of rapidly expanding coolant from the orifice of a Joule-Thomson cooler. This long-standing conventional practice has now been adopted for fast cool-down detectors with short operating lives, for example a few minutes or less.

However, when a cheap, fast cool-down detector is required for use in situations where the cooled operating state is only maintained for a short time, the present inventors have found that this conventional practice can be rejected and that significant advantages can be obtained by providing the detector element on the other side (i.e. the cooling-element side) so as to be directly cooled in accordance with the Joule-Thomson effect by the coolant expanding from the cooling element.

Particularly with detector elements of a material such as cadmium mercurytelluride, no significant microphony problem arises, and the inventors have found that the passivation conventionally used for such detector elements will provide adequate environmental protection for such a detector element located in the coolant space. However, if desired, an additional protective film can be provided, sufficiently thin not to slow the fast cooldown.

Thus, according to the present invention, the infrared detector is characterised in that the at least one detector element is mounted on the coolingelement side of the wall of the housing so as to be located in the space into which the coolant expands from the orifice of the Joule-Thomson cooling element.

By mounting the detector element on the coolingelement side, an even more rapid cool-down of the detector element can be achieved and it is not necessary for the part of the wall on which the detector element is mounted to have good thermal conductance or to be provided with channels or fins for efficient heat transfer. On the contrary, the detector element can be cooled more efficiently if this mount part of the wall is of material having a low thermal conductance. Furthermore, such material can be comparatively cheap even when it is chosen to perform functions additional to the mounting of the detector element. Such additional functions are readily compatible with housing and mounting arrangements in accordance with the invention, and this also permits the design of more compact and simpler detector housings.

Thus, the mount which forms part of the wall may comprise a circuit substrate having an electrical conductor pattern providing electrical connections to the detector element. This substrate may be of an inexpensive electrically-insulating material, for example of glass or plastics material, on which the conductor pattern is formed by printed circuit definition techniques, for example using laminate etching, or plating or other deposition techniques.

The electrical connections may comprise throughconnections which extend through the thickness of the circuit substrate from the conductor pattern and which are connected to external leads of the housing. The through-connections may be formed by various known techniques, for example as conductive pins in the circuit substrate or more cheaply as, for example, plated metal vias at holes through the substrate.

The detector-element mount which forms part of the wall may be of infrared transmissive material to permit transmission or reflection of the infrared radiation to the detector element. This infrared transmissive mount may be coated so as to be highly reflective or it may be shaped to form an optical immersion lens for the detector element. An infrared transmissive part of sufficient thermal insulation and thickness may extend to a front outer surface of the housing, or it may be located behind an outer member through which the radiation is transmitted. This outer member may be for example, an infrared window, filter or lens and/or it may provide mechanical protection for the first part of the wall on which the detector element is mounted, and/or it may assist in thermally insulating this part of the wall from the ambient temperature outside the housing.The detector element mount may be formed by a core of the Joule-Thomson cooler, regardless of whether or not it is infrared transmission.

Although the mount to which the detector element is secured may be of glass, plastics, laminates, or other thermally insulating material, it can be advantageous to form this mount of silicon or germanium. These semiconductor materials have useful infrared transmissive properties, and they may be processed by known semiconductor device techniques to provide a circuit substrate providing connections and possibly even signal-processing circuitry for the detector element(s).

These and further features in accordance with the present invention will now be illustrated in particular embodiments of the invention, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figures 1,3 and 4 are cross-sectional views of parts of different infrared detectors in accordance with the invention, and Figure 2 is a cross-sectional view of the orifice of a cooling element adjacent a detector element on a circuit substrate in an infrared detector in accordance with the invention.

The drawings are diagrammatic and not drawn to scale. For the sake of convenience and clarity in the drawings, the dimensions and proportions of various features of these detectors have been shown exaggerated or diminished. The same reference signs used in one embodiment are generally used to refer to corresponding or similar parts of the other embodiments.

The infrared detector of Figure 1 comprises a housing 1 within which at least one infrared detector element 2 and a Joule-Thomson cooling element 3 are accommodated. The detector element 2 is secured to a mount 12 forming a part of one wall 12,13 of the housing. The cooling element 3 has an orifice 32 from which a coolant fluid expands (as indicated by arrows 33 in the drawing) into a space 20 on one side of the housing wall 12, 13. This expansion rapidly cools the fluid 33 in accordance with the Joule-Thomson effect and so effects cooling of the detector element 2 (for example to below 100"K or at least 120"K) during operation of the detector. It is only desired to operate the detectorfora maximum duration of, for example 1 to 3 minutes.

The Joule-Thomson cooling element 3 may be of any known form. Thus, for example, it may comprise a helical coil of metal tubing 31 wound on a frusto-conical core 30 the bulk of which may be of expanded polyurethane or other thermally insulating material. However, other shapes are possible, for example a cylindrical core 30 or even a planar geometry comprising a flat spiral coil 31. A source of pressurised fluid (for example argon or dry air) is connected to one end 34 of the tubing 31, and after expansion from the orifice 32 at the opposite end, the cooled fluid flows back over the outside of the tubing 31 so pre-cooling the pressurised fluid in the tubing 31 upstream of the orifice 32.This regenerative cooling effect rapidly reduces the temperature of the coolant fluid so that, for example, a coolant such as argon or air is liquified in the tubing 31 prior to its expansion from the orifice 32. To increase heat transfer between the tubing 31 and the venting coolant, the outer surface of the tubing 31 normally includes metal fins. A laminate foil 35 of superinsulation material (for example a laminate of oxidized aluminium and polyimide) may be present between the housing wall 13 and the finned metal tubing 31 to increase the efficiency of the regenerative cooling. However, the present invention may also be used with Joule Thomson coolers which have a structure with etched or cut interleaved channels instead of finned metal tubing.

In accordance with the present invention the detector element 2 is mounted on the cooling element side of the wall 12,13 so as to be located in the space 20 into which the coolant 33 expands from the orifice 32. This results in the detector element 2 being directly and rapidly cooled by the expanded coolant 33. For very fast cool-down of the detector element 2, the mount 12 forming the part of the wall 12,13 on which the detector element is mounted should have a low thermal conductance. The superinsulation foil 35 may also extend over the surface of the mount 12 except where the detector element 2 is exposed at a hole in the foil 35. The detector can be readily designed to have a cool down time of less than 1 second.The thermal insulation in the housing 1 is adequate to maintain operational cooling of the detector element 2 for its short maximum operating duration (at most 3 minutes), and during this time the performance of the detector element 2 is not adversely affected to any significant extent by being located in the coolant space 20.

Avariety of materials may be used for the mount 12. The particular choice depends on other functions which itis desired that the mount 12 should perform. In the detector of Figure 1, as well as forming part of the wall 12,13 and the mount for the detector element 2, the member 12 acts as a circuit substrate and as an infrared transmissive lens in the housing 1. Either a plastics material our a semiconductor material may be used for this purpose.

The detector of Figure 1 may be designed for detecting infrared wavelengths in, for example, the 8to 14um (micrometre) band. The infrared radiation 50 is incident via a window 11 in the housing 1. The window 11 which may be of germanium or a plastics material is attached at the front of the housing to a wall 15 which also bounds a cavity 40 through which the radiation 50 passes to the mount 12 and the detector element 2. The side of the infrared transmissive mount 12 opposite that on which the detector element 2 is mounted is shaped to form this mount 12 into a lens on which the detector element 2 is optically immersed. Most of the surface of the wall 15 in the cavity 40 may be coated with a highly reflective metal layer pattern 18 and 19 which acts as a light-pipe directing the radiation 50 towards the lens 12 and detector element 2.The surrounding volume 10 of the housing 1 between an outer side wall 14andthe walls 13 and 15 may be filled with expanded polyurethane or another thermally insulating material. The side wall 14 may have a flange at the back to which a spring-loaded retention plate 16 is clamped to secure the cooler 3 firmly in the housing 1. The members are shown separated in the drawings. The side wall 14 may be of a plastics material or of metal. The walls 13 and 15 may be of a plastics material.

On the outside of the housing 1 the external electrical connections for the detector element 2 may be formed by leads 38 and 39 soldered to conductors formed by parts 18 and 19 of the metal layer pattern on the wall 15. These metal layer parts 18 and 19 are insulated from each other by gaps 17 in the pattern. The conductors 18 and 19 may be connected to conductors of the circuit substrate 12 by solder or conductive epoxy.

The internal connections for the detector element 2 are carried by the circuit substrate 12 which also forms the lens. On the detector-element side, these connections may be a conductor pattern of gold or other suitable metal deposited on the substrate.

Two such conductors 21 and 22 are illustrated in Figure 2. In the case of a silicon or germanium substrate 12, its surface may be coated with an electrically insulating layer prior to the metal deposition. The connections may be taken to the opposite side of the substrate 12 in a variety of ways. Although Figure 2 illustrates two possibilities, it will normally be eonvenient and desirable to use only one method for all connections. Thus, for example, the metallization may be extended over the periphery of the substrate 12 and onto the opposite face as illustrate for conductor 22 in Figure 2. However, through-connections (such as connection 23 between conductors 21 and 24 in Figure 2) may be formed through the thickness of the substrate 12, for example by plating metal through via-holes in a plastics substrate 12.When the substrate 12 is of silicon or germanium, throughconnections 23 may be formed by localised doping across the substrate thickness, for example using known dopant thermomigration techniques.

The detector element(s) 2 may be mounted on the substrate 12 in known manner, for example by using techniques similarto those described in United Kingdom patent (GSA) 1 559 474. Thus, the detector element 2 which may be of cadmium mercury telluride having gold electrodes 5 and 6 may be secured to the substrate 12 by a layer of insulating epoxy adhesive 27. Gold layer interconnections 28 and 29 may then be deposited over edges of the detector element 2 to connect the electrodes 5 and 6 to the substrate conductors 21 and 22. As described in GB--A 1 559 474 and GB--A 1 568 958, the exposed faces of the detector element 2 are passivated with an anodic surface layer.Since in detectors in accordance with the invention, the detector element 2 is mounted in the space 20 into which the coolant 33 expands, it can be beneficial to provide a transparent, additional protective film 26 of, for example, thin plastics material over the detector element 2 (and possibly also over the interconnections 28 and 29 and conductors 21 and 22) to encapsulate the detector element 2 on the substrate 12. This provides an additional safeguard for the detector element 2, especially if a slightly longer operating duration is required. The film 26 may be formed by coating the desired area with a thin layer of insulating material or by placing an insulating adhesive film over the area.

As shown in the drawings, the detector element 2 is secured in a flat manner against the substrate 12 so as not to present a high profile to the rapidly expanding coolant flow 33. In order to secure a firm attachment, the adhesive 27 is provided over the whole back face of the element 2. Furthermore it is advisable to arrange the orifice 32 so that the expanding coolant 33 does not strain the attachment of the detector element 2 to the mount 12. In Figures 1 and 2 the orifice 32 is located directly opposite the detector element 2 so that the coolant impinges uniformally on the front surface of the element 2. Uniformity of expansion into the space 20 may be increased by providing a diffuser, for example in the form of a mesh 62 (Figure 2) over the orifice 32.In Figure 3, the orifice 32 is arranged parallel to the detector element 2, whereas in Figure 4 it is pointing away from the detector element 2.

In the arrangement illustrated in Figure 1 the infrared radiation 50 is incident via the opposite side of the infrared transmissive mount 12 to the detector element 2. However, an alternative arrangement is possible in which the radiation 50 is directly incident on the detector element 2 without passing through the mount 12. In this case, the core 30 of the cooler 3 may be hollow or at least transmissive of the infrared radiation 50, and the window 11 may be mounted on the bottom of the core 30 which now becomes the front of the housing 1. One example of such an arrangement is illustrated in Figure 3. In this detector there is a further infrared window 52 at the inside end face of the hollow core 30. The surface of the cavity in the hollow core 30 may be made highly reflective or highly absorbent as desired for any particular application. No cavity 40 is present in the bulk 10 of the housing 1. Insulated external leads 38 and 39 may now extend through the side wall 14 and through the expanded polyurethane 10 to be connected directly to the conductors 22,24 etc. of the circuit substrate 12 by solder or conductive epoxy. If desired, most of the back face of the substrate 12 may be coated to render it highly reflective at the wavelength(s) of the radiation 50, and the coated part of this back face may be convex so as to concentrate the reflected radiation onto the detector element 2.

Further modifications are illustrated in Figure 4. In this detector, the inside end face of the hollow core 30 is formed by an infrared transmissive circuit substrate 12,52 on which the detector element 2 is mounted. In this case the external leads 38 and 39 extend through the core 30 of the Joule-Thomson cooler 3 and are connected directly to the conductors on the circuit substrate 12. The Figure 4 housing structure can be made very compact and light weight.

Although Figure 2 illustrates a detector element 2 attached to a substrate 12 by epoxy adhesive 27, the detector element 2 may be formed in an epitaxial layer grown on a substrate 12 of, for example, cadmium telluride which may then be used to provide part of the wall defining the coolant expansion space 20 in which the detector element 2 is locally. Use may be made of the suitable infrared transmission properties and low electrical and thermal conductivity of such a cadmium telluride mount.

Claims

1. An infrared detector having fast cool-down characteristics, comprising a housing within which at least one infrared detector element and a Joule Thomson cooling element are accommodated, the cooling element having an orifice from which coolant expands into a space on one side of a wall in the housing to effect cooling of the detector element during operation of the detector, the detector element being secured to a mount forming part of said wall, characterised in that the at least one detector element is mounted on said one side of the wall so as to be located in the space into which the coolant expands firm the orifice of the Joule Thomson cooling element.

2. A detector as claimed in Claim 1, further characterised in that the mount is of material having a low thermal conductance.

3. A detector as claimed in Claim 1 or Claim 2, further characterised in that the mount comprises a circuit substrate having an electrical conductor pattern providing electrical connections to the detector element.

4. A detector as claimed in anyone of the preceding Claims, further characterised in that the mount is of infrared transmissive material to permit transmission of the infrared radiation to the detector element via said part of the wall.

5. A detector as claimed in Claim 4, further characterised in that the side of the infrared transmissive mount opposite the side on which the detector element is mounted is shaped to form said mount into a lens on which the detector element is optically immersed.

6. A detector as claimed in anyone of Claims 1 to 4, further characterised in that the Joule-Thomson cooling element has an infrared transmissive core through which the infrared radiation is incident on the detector element.

7. A detector as claimed in anyone of Claims 1 to 4, further characterised in that the Joule-Thomson cooling element has a core, an end face of which provides both the mount to which the detector element is secured and a part of the wall of the space.

8. A detector as claimed in anyone of the preceding Claims, further characterised in that the mount is of silicon or germanium.

9. A detector as claimed in anyone of the preceding Claims, further characterised in that a protective film is provided over the detector element to encapsulate the detector element on the wall.

10. A detector as claimed in anyone of the preceding Claims, further characterised in that the detector element is of passivated cadmium mercury telluride.

IlAninftateddetlita 30 20t222\h\! with reference to anyone of Figures 1 to 4 of the accompanying drawings.