IMPROVEMENTS IN PYROELECTRIC DETECTORS

Abstract: A pyroelectric detector in which an active pyroelectric infra-red detector element (10) is mounted adjacent to a block of more rigid material (20). The latter (20) is provided to reduce vibration included strain in the device element (10) by reducing lateral strain locally and by allowing strain relaxation through the thickness of the block (20). The block (20) may be of cylindrical or cuboid form and for optical effect has an aspect ratio of at least one. The block may be of high specific stiffness ceramic material, for example alu­ mina. Further improvement may be achieved by electrical compensation, there being provided a nonabsorbent com­ pensation element adjacent to the active element (10), this element being mounted upon the same block (20).

Description: IMPROVEMENTS IN OR RELATING TO PYROELECTRIC DETECTORS The present invention relates to pyroelectric detectors and more particularly to infra-red detector arrays. All pyroelectric materials are also piezoelectric and will therefore generate an output voltage when subject to longitudinal stresses or strains both along and perpendicular to a pyroelectrically active axis, defined as the Z-axis. Any accelerations causing stress or strain will generate a response that may interfere with the desired pyroelectric signal. For a number of applications of pyroelectric arrays, vibration within the operating environment may be substantial, thus any way of reducing stress or strain in the pyroelectric due to the acceleration associated with this vibration will be beneficial. Acceleration along the Z-axis of the pyroelectric results in stress along that axis as the pyroelectric material deforms udder its own mass and may also produce strains perpendiar to Z. In known practical device formats the pyroelectric mounting may be considered as a membrane or strut fixed at a number of points with pyroelectric material fixed to one face. Under acceleration this mounting bends under its own mass straining the pyroelectric, thus producing an output signal. It is an object of the present invention to minimise these effects. Thus a mounting arrangement that minimises straining of the pyroelectric will result in minimum piezoelectric output. This can be best achieved by mounting the pyroelectric itself directly or as rigidly as possible to a support that is inherently stiff reducing these strains. The present invention therefore provides an infra-red detector comprising a pyroelectric device mounted on a rigid structure, said rigid structure including a block of material which is substantially more stiffer than the pyroelectric device. In a preferred embodiment the pyroelectric device is mounted on a substrate, the substrate being mounted on a header or package and the header or package being mounted on the block of material comprising said rigid structure. The reduction of strain can be achieved in practice by the use of a stiff support block, this achieves strain reduction in the pyroelectric by two different effects: (1) The strain can relax through the thickness of the block so the strain on the face on which the pyroelecuric is mounted is less than the strain in the overall support it is bonded to; and, (2) The block stiffens the support locally reducing lateral strains in this overall support. The block ideally is a cylinder or cuboid, the type of device package. The amount of strain relaxation depends on the ratio between block thickness and half width or radius. An aspect ratio of approximately unity can give good relaxation and the optimum depends on the individual application. The material providing optimum performance depends on the following. The stiffening effect of the block is significant in cases where its thickness is not appreciably less than the thickness of the structure it is mounted on. If the structure is bending the block increases local rigidity thus decreasing strain generated. Similarly if it is being stretched laterally the increased stiffness reduces the strain in the area of the block by increasing it elsewhere. The block however adds to the overall mass of the structure increasing its tendency to bend. Thus a block material of high specific stiffness ie. a material of high elastic Young's Modulus (to give good stiffness) and low densisty (to reduce mass) is required. High values of specific stiffness are exhibited by ceramics such as alumina,silicon nitride, and silicon carbide. The bonds between the pyroelectric and support block and between the block and the substrate it is mounted on need to be uniform and stiff. If, for example, the block were mounted just at the centre, though it would be well isolated from the strains on the substrate, it would bend about this point, producing a strain in the pyroelectric that could be larger than the strain in the substrate. Any voids or non-uniformities would have a smaller effect through a similar mechanism. Such irregularities are not significant if they are on a scale that is small compared with the distance from bond to pyroelectric. Compared with an alternative technique of strain reduction by the inclusion of a compliant layer between device and substrate, the stiff block has a number of advantages. The movement of the device relative to its support is very small, thereby minimising the risk of defocussing an imaged device. Its high stiffness means that any mechanical resonances are of relatively high frequency and so are less likely to disturb the operation of a comparatively low frequency pyroelectric device, than the low-frequency resonances associated with a compliant layer. The pyroelectric, possibly with connecting leads attached can represent a non-uniform load to the structure it is attached to. If this structure includes a compliant layer this can cause bending and pyroelectric strain, as this layer cannot provide adequate support. This effect is minimised by the use of a stiff support. Further reduction in the piezoelectric response can be obtained using an electrical compensation scheme. Here a masked or non-absorbing compensation element, mounted adjacent to an active element and upon the same block, is connected in series with the active element such that the piezoelectric voltages generated cancel. For this to work well the strains in the two adjacent elements need to be identical or very similar. The common support block isolates each pyroelectric element from any irregularities in the mounting structure that might cause position dependent strains and attendant unbalanced voltages giving poor compensation. In the drawings acompanying this specification: Figure 1 and 2 show in perspective view and in crosssection, respectively, infra-red detector constructed in accordance with tthis invention. Detail of package structure is omitted in the perspective view, for clarity of presentation. Embodiments of this invention will now be described with reference to these drawings. In the infra-red detector shown, a piezo-electric ceramic device 10 is mounted on a metal substrate 12 and in turn mounted on an alumina package base 14. As shown in figure 2, the package includes a cover portion 16 and an infra-red transparent window 18, the latter aligned with the device 10. Solder bonded fly-leads 19 provide connection between the device and contact pads arranged on the base 14 about the periphery of the substrate 12. The whole is mounted on a stiffening ceramic block 20, the cross-section of which is matched in dimensions and shape to the base 14. It should be noted that the pyroelectric device 10 is bonded within this cross-section with no part over-hanging. In the embodiments shown the structure is of rectilinear geometry. The block 20 is provided in the form of a cube. The structure described is clamped or bonded onto a printed circuit board 22 and together with board connectors, is bolted onto a common mounting. Conductors 24 are provided adjacent to the side wall of the block 20 and connect the contact pads to points on the printed circuit board 22 Typical constructional details are summarised in the following table: Table I Piezo-electric device (10): Matrix array 16 x 16 elements, array 5 mm square, 30 mm thick material: electrically conducting ceramic based zirconate iron on the lead niobate titanate system, hot pressed, platinum black backed, gold electrode; or, linear array 40 elements, array 2 mm x 5 mm, 30 thick; substrate (12): 10 mm square, i to 12 mm thick, copper metal; package base (14): 25 mm square, 2 to 4 mm thick, alumina ceramic; block (20): 25 mm on each side, cube, alumina ceramic; printed circuit board; 50 mm square; bonding agent (provided at each mounting interface): Epoxy resin, low viscosity-eg. proprietary resin Araldite AV 100/HV100, bond thickness 100-150 To ensure uniformity of bonding, a low viscosity resin has been chosen. In the example given, the resin and hardner components are mixed and heated to reduce resin viscosity. The components are coated with resin, assembled and the resin allowed to set in a high pressure ambient, to reduce introduction of sizeable air voids. In addition, the mating surfaces of the ceramic components are accurately machined so that bonding layers of uniform thickness can be produced ( 20pM tolerance in thickness of 100-150to). Details of suitable high specific stiffness ceramics are given in the table following: Table II Alumina Silicon Silicon Silicon Carbide Nitride Nitride (machine - (Hot (Reaction grade) pressed) bonded) Young's 3.8 4.b 3.2 2.2-1.0 Modulus (x1010KG/mê) Density 3.8 3.1 3.2 2.7-2.1 (x10 Kg/m ) Specific 1.0 1.29 1.0 0.82-(0.48) Stiffness (xlO7m) For values of specific stiffness less than circa 0.75 x 10 m, the principle of block mounting becomes impracticable. The microphory level is as a result either inacceptably high, or the size of block required is too bulky an massive for practicle application. The specific stiffness value of 1.29 x 107m found for silicon carbide, is, it is beleived, the highest stiffness value to date for ceramics. It should be noted that the shape of block is not limited to that of a cube or cuboid. The precise shape will depend on the type of package used. It cold instead by cylindrical a typical geometry for butterly package constrction.

Claims: CLAIMS: 1. An infra-red detector comprising a pyroelectric device (10) characterised in that the device (10) is mounted on a rigid structure (12, 14 and 20), said rigid structure including a block of material (20) which is substantially stiffer than the pyroelectric device (10). 2. A detector, as claimed in claim 1, wherein the block of material (20) is in the form of a cylinder. 3. A detector, as claimed in claim 2, wherein said cylinder (20) has an aspect ratio - ie. the ratio of thickness to radius - of at least one. 4. A detector, as claimed in claim 1, wherein the block of material (20) is in the form of a cuboid. 5 A detector, as claimed in claim 4, wherein said cuboid (20) has an aspect ratio - ie. the ratio of thickness to half-width of at least one. 6. A detector, as claimed in one of the preceding claims wherein the material of the block (20) is a ceramic of high specific stiffness. 7. A detector, as claimed in claim 6, wherein said ceramic (20) is of alumina ceramic. 8. A detector, as claimed in claim 6, wherein said ceramic (20) is of silicon carbide ceramic 9. A detector, as claimed in claim 6, wherein said ceramic (20) is of silicon nitride ceramic. 10. A detector, as claimed in any of one of the preceding claims, wherein the device (10), together with a non-absorbing compensation element (10) adjacent thereto are mounted upon a common block (20). 9t. A detector, as claimed in any one of the preceding claims, wherein said rigid structure (12, 14 and 20) includes a substrate (12) and package structure (14) between the device (10) and the block (20), bonding being provided between each (10 and 12, 12 and 14, 14 and 20), said bonding being both uniform over the area of contact and stiff.

Inventors: LOVELUCK, JOHN EDWARD, ROBINSON, MARTIN KEITH, SHORROCKS, NICHOLAS MARTYN
Applicants: PLESSEY OVERSEAS LIMITED
Application year: 1984
Main classification: G01J5
IPC codes: G01J5