Thermal imaging device

Abstract: A pyroelectric film 21 has a pattern of interconnected electrodes 22 formed on one major surface, and a pattern of discrete electrodes 23 and electrically conductive patches 24 formed on the other major surface. The film 21 is supported by pillars 26 connected to the patches 24, the pillars also producing a path for electrical signals between the patches 24 and a signal processing means 28. Each discrete electrode 23 is connected to a respective patch 24 by a respective strip of electrically conductive material.

Description: This invention relates to a thermal imaging device comprising an array of pyroelectric detector elements formed by electrodes carried on the opposing major surfaces of a thin film layer of pyroelectric material, the pyroelectric layer extending over a substrate layer carrying a plurality of electrical signal processing circuits and being supported from the substrate layer by a plurality of pillars which additionally provide electrical connection between the detector elements and the electrical circuits. In a known such device disclosed in EP-A-0173368 each thermal detector electrode is directly supported by a pillar containing conductive paths. This results in thermal conduction which impairs performance. An object of the present invention is to provide an alternative thermal imaging device in which the pyroelectric detector elements are supported by pillars but wherein thermal conduction between the pillars and detector elements is reduced. According to the invention a thermal imaging device as defined in the first paragraph above is characterised by the combination of: a) both major surfaces of the pyroelectric layer carry a plurality of discrete electrodes which cooperate to form the detector elements, the respective electrodes an each pyroelectric layer surface being spaced apart to leave non-electroded regions of the surfaces; b) the discrete electrodes on the pyroelectric layer surface which is remote from the substrate layer are electrically interconnected; c) the major surface of the pyroelectric layer which is adjacent to the substrate layer additionally carries a plurality of electrically conductive patches formed in a non-electroded region of the pyroelectric layer and laterally spaced from the detector elements , each patch being respectively electrically connected to an electrode on the same major surface by an elongate strip of electrically-conducting material; d) each elongate strip has a length to width ratio of at least 5; and e) the pillars extend from the patches to the substrate layer. The pyroelectric material may comprise material which is inherently pyroelectric, or material whose pyroelectric characteristics are induced (e.g. by appropriate poling of ferroelectric material). The layer is suitably a thin film of pyroelectric material. The elongate strip suitably has a length to width ratio of at least 5. Patent Abstracts of Japan vol.10 no. 232 p.486 discloses a planar self supporting array having elongate strips of electrically conducting material extending from the discrete electrodes to provide electrical connections close to the edge of the device, and achieves thermal separation between elements by drilling holes in the pyroelectric array. In one embodiment of the present invention the discrete electrodes are arranged in rows and columns and said electrically conductive regions are situated at the centres of respective rectangles formed by the centres of four adjacent discrete electrodes. In another embodiment the discrete electrodes are arranged in interleaved rows and interleaved columns and the electrically conductive regions are situated at the centres of respective triangles formed by the centres of three adjacent, discrete electrodes. In another embodiment, respective supplementary electrodes are provided in the non-electroded regions of the other major surface between each discrete electrode and the adjacent pillars, each supplementary electrode being connected to a respective pillar, the pillar having two independent electrically conductive paths to the signal processing means. By processing signals originating from the supplementary electrodes, an improved signal/noise ratio may be achieved. In another embodiment, received infrared radiation incident on the nonelectroded regions of the layer is diverted into the electroded regions by an optical arrangement. The use of a thin pyroelectric film, with pillars remote from the relevant discrete electrodes and long, narrow conductors to connect the discrete electrodes to the appropriate pillars via the patches ensures that the thermal conductance from each pixel to the relevant support pillar is small. Six thermal imaging devices in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figure 1(a) is a schematic sectional view of a part of the first device showing the discrete electrode structure; Figure 1(b) shows a section at AA in Figure 1(a). Figure 2 is a schematic sectional view of a part of the second device showing the discrete electrode structure; Figure 3 is a schematic sectional view of a part of the third device showing the discrete electrode structure; Figure 4(a) is a schematic sectional view of part of the fourth device showing the discrete electrode structure; Figure 4(b) shows a section along A-A in Figure 4(a); Figure 4(c) shows a sectional view of a modification of the device shown in Figure 4(b). Figure 5 is a schematic view of the part of the sixth device, illustrating the discrete electrode structure. In the embodiment shown in Figure 1, a pyroelectric film 21, of for example, polyvinylidene fluoride (PVDF) has a pattern of interconnected electrodes 22 formed on one side and a pattern of discrete electrodes 23 and electrically conductive patches 24 formed on the other side. The discrete electrodes 23 are linked to respective patches 24 by narrow electrically conductive strips 25. Electrically conductive pillars 26 are bonded at one end to respective patches 24 and at the other end to input pads 27 of an integrated circuit 28 formed on the substrate 29. As shown in Figure 1(a), the discrete electrodes 23 are arranged in rows and columns and the patches 24 are arranged in rows and columns which interleave those of the discrete electrodes 23. In the example of Figure 1, the electrodes 22 and 23 are each of equal size and the patches 24 and pillars 26 are also of equal size, but smaller than the electrodes. Also in this example the electrodes, patches and pillars are all square and arranged so that each of the inter-connected patches and electrodes has one face perpendicular to the conductive strip joining the patch and electrode. The inter-connected electrodes 22 are covered by an infrared absorbent layer 30, for example a layer of black gold. The inter-connections between these electrodes 22 should be narrow, thin and preferably meandered to increase their length. The interconnected electrodes are connected to ground. Referring now to Figure 2, in which as in all the drawings corresponding features to those shown in Figure 1 are correspondingly labelled, the discrete electrodes 23 are arranged in interleaved rows and columns and the patches 24 are arranged at the centres of the triangles formed by three adjacent electrodes 23, each patch being connected to a respective electrode by a narrow conductor 25. Apart from the changed positions of the electrodes, patches and pillars the construction of this embodiment is similar to that described hereinbefore with reference to Figure 1. The devices illustrated in Figures 1 and 2 suffer the disadvantage that each discrete electrode 23 has an area around it in which the received radiation is wasted. This is overcome in the third device to be described by the provision of a supplementary electrode around the electrode 23, as shown at 51 in Figure 3. In Figure 3, the arrangement of electrodes 23 is the same as in Figure 1, but the conducting patches 24 are each split into two patches 24a, 24b. Each electrode 23 is connected to a corresponding patch 24a by the narrow conductor 25 and each of the extra electrodes 51 is connected to the corresponding patch 24b by the narrow conductor 52. The two patches 24a, 24b are connected to a respective one of a respective pair of input pads of an integrated circuit either by a pair of electrically conductive pillars (not shown) or by an insulative pillar (also not shown) carrying two electrically conductive tracks. Both inputs to the processing means are pre-amplified, weighted and added. The supplementary electrode has higher thermal losses than the discrete electrode, so the supplementary electrode output is given a lower weighting than the discrete electrode output, but is still able to improve the signal/noise ratio. The fourth and fifth devices to be described both incorporate optical means for using some of the otherwise wasted received radiation. Referring to Figures 4(a) and 4(b) in the fourth device, a plate 61 transparent to infrared covers the array of inter-connected electrodes 30 and is profiled to provide convex surfaces 62 over each electrode area so that received radiation is concentrated towards the electrode area. In the fifth device, which is shown in Figure 4(c), the plate 61 is replaced by concave reflecting surfaces 63 positioned behind each discrete electrode 23, the plate reflecting the energy received around the electrodes back towards the electrodes. In some circumstances it may be advantageous to cover the discrete conductors 23 with an infrared absorbent layer, for example a layer of black gold, to absorb the energy reflected back by the concave surface 63. In the plan view of Figure 4a, the convex surfaces 62 and the concave reflecting surfaces 63 are shown schematically as circles around the discrete electrodes 23. It will be appreciated that whilst in the forth and fifth devices the electrode arrangement is the same as in Figure 1, this could equally well have been that of Figure 2. Referring now to Figure 5 the sixth device to be described includes two further means for reducing the thermal losses from the pyroelectric detector elements. Firstly the thermal conductance from the electrodes 23 to the corresponding patch 24 and pillar 26 is reduced by replacing the straight narrow conductors in the arrangements of Figures 1 and 2 by meandering conductors 71. Secondly, thermal conductance through the pyroelectric film 21 is reduced by forming holes 72 in the pyroelectric film. Either of these arrangements could also be used with the electrode arrangements of Figure 2. In Figure 5, the electrodes 23 and patches 24 are smaller than in Figure 1, facilitating the use of narrow strips of film 73 between the pairs of adjacent holes 72. In order to concentrate the received energy on the electrodes 23, a convex lens 62 transparent to infrared may be placed over each inter-connected electrode 22, as at 62 in Figure 4b, or a concave mirror 63 may be placed under each discrete electrode 23, as at 63 in Figure 4c. The lenses 62 or mirrors 63 are shown schematically as circles in Figure 5. Clearly, embodiments of the present invention are not restricted to the particular shapes of electrodes shown and described hereinbefore. Thus, the electrodes can be of a complex shape; in an advantageous form, the shape of the electrodes conforms to the isothermal-resistance contours of the device, such that the electrodes are located in regions wherein the thermal resistance to ground has a relatively high value (preferably being at its highest). In embodiments wherein one electrode at least partially encloses another electrode, preferably the outer boundary of the inner electrode and the inner boundary of the outer electrode have a similar shape in accordance with the isothermal-resistance contours of the locality, and not necessarily similar to the shape of the outer boundary of the outer electrode which is in accordance with the isothermal-resistance contours of the respective locality.

Claims: A thermal imaging device comprising an array of pyroelectric detector elements formed by electrodes (22,23) carried on the opposing major surfaces of a thin film layer (21) of pyroelectric material, the pyroelectric layer (21) extending over a substrate layer (29) carrying a plurality of electrical signal processing circuits (28) and being supported from the substrate layer (29) by a plurality of pillars (26) which additionally provide electrical connection between the detector elements and the electrical circuits (28), characterised by the combination of: a) both major surfaces of the pyroelectric layer (21) carry a plurality of discrete electrodes (22,23) which cooperate to form the detector elements, the respective electrodes on each pyroelectric layer surface being spaced apart to leave non-electroded regions of the surfaces; b) the discrete electrodes (22) on the pyroelectric layer surface which is remote from the substrate layer (29) are electrically interconnected; c) the major surface of the pyroelectric layer (21) which is adjacent to the substrate layer (29) additionally carries a plurality of electrically conductive patches (24) formed in a non-electroded region of the pyroelectric layer (21) and laterally spaced from the detector elements (21,22,23), each patch (24) being respectively electrically connected to an electrode (23) on the same major surface by an elongate strip (25) of electrically-conducting material; d) each elongate strip (25) has a length to width ratio of at least 5; and e) the pillars (26) extend from the patches (24) to the substrate layer (29). A device as claimed in claim 1, characterised by respective supplementary electrodes (51) provided around each electrode (23) on the major surface of the pyroelectric layer (21) which is adjacent to the substrate layer (29) and in a non-electroded surface region, each supplementary electrode (51) being connected to the same pillar (26) as the electrode (23) around which it is located, each pillar (26) having two independent electrically conductive paths to the pertaining electrical signal processing circuit (28). A device according to either preceding claim, including an optical arrangement (61,63) effective to divert infrared radiation incident on the non-electroded regions of the pyroelectric layer (21) into the electroded regions. A device according to Claim 3, in which the optical arrangement is a lens arrangement (61,62) interposed between the source of radiation and the polyelectric layer (21). A device according to Claim 3, in which the optical arrangement is a reflective arrangement (63) interposed between the pyroelectric layer (21) and the electrical signal processing circuits (28).

Inventors: VOLES ROGER, VOLES, ROGER
Applicants: PILKINGTON THORN OPTRONICS LTD, PILKINGTON THORN OPTRONICS LIMITED
Application year: 1989
Main classification: H01L27
IPC codes: H04N5, G01J5, G01J1, H01L27, H01L37