U.S. Philips Corporation

North American Philips Corporation

Andrew A Turnbull

Raymond F Hall

Edward H Stupp

Colin D Overall

Martin E Cooke

Andrew I Cox

Harry Sewell

An infrared detection device, e.g. for the 8 to 14 micrometer waveband, comprises a Langmuir-Blodgett or other very thin film (10) having pyroelectric or other temperature-dependent characteristics. At least one detector element having opposite electrical conductors (21 and 22) is formed in the film (10). The film (10) which may have a support layer (15) is mostly free of contact with a mount arrangement (25) which supports the film (10) in a very low thermally-conductive manner. In accordance with the present invention, the film (10) is very thin, but very efficient absorption of the radiation (31) is obtained in the following manner: the facing surface (26) of the mount arrangement (25) is reflective for the radiation (31); the sum of the optical thicknesses of the film (10), of any support layer (15) and (d) of a gap (28) forming the multiple layer detector-element structure along the radiation path to the reflector (26) is approximately one quarter of a selected wavelength (e.g. 9.6 micrometers) in the range to be detected; and the conductors (21 and 22) each comprise a resistive layer providing a sufficient resistance per square (Z1 and Z2, e.g. of twice the characteristic impedance of free space) as to absorb a substantial proportion of the incident radiation (31) and reflected radiation (32) in the wavelength range. Spaced supporting elements (252) below the film (10) can support the film (10) over the reflector (26) with a well-defined small gap (28) and carry an electrical connection to the second conductor(s) (22) below the film.

Pyroelectric and other infrared detection devices with thin films

In a thermal-radiation detection system, the thermal radiation (3) incident on a group of pyroelectric and/or ferroelectric infrared detector elements (20) is modulated at a frequency (f). The detector elements (20) are mounted on flexible platform areas of an insulating substrate (10) which may be of an elastic material such as a silicone rubber in, for example, an apertured or recessed mount (18,19). The grooves (15,16) forming the platform areas have a sufficient depth d and width w that the distance (2d+w) is at least as large as the thermal diffusion distance at the modulation frequency (f) so as to reduce thermal cross-talk between the detector elements (20). The elasticity and vibration absorption properties of an elastic substrate (10) and the flexibility and path length increased by the presence of the grooves (15,16) can provide a low microphony mounting arrangement with acoustic decoupling between detector elements (20). Preferably the groove depth d is at least as large as the thermal diffusion distance so as to reduce thermal losses from each detector element (20) and increase the acoustic path length and flexibility of the mounting. The substrate (10) may be molded with the grooves in its face (11), or the grooves may be cut with a saw.

Thermal-radiation detectors, detection systems and their manufacture

An infrared detector element (20) comprises a flexible continuous film (10) of polymer material which has pyroelectric or other temperature-dependent characteristics and which is also piezoelectric so also generating microphony when it vibrates. In accordance with the invention the mounting arrangement (30,31,32) for the film (10) comprises mutually-spaced supports (32), e.g. tapered ridges (32) of a moulded-plastics substrate (30); the spacing (x) between neighboring supports (32) adjacent to the detector element (20) is sufficiently small and the tension (P) of the film (10) is sufficiently large that the natural frequency of vibration of the parts of the polymer film (10) between these neighboring supports (32) is higher than the frequency bandwidth of the detector, e.g. as determined by the highest operating frequency of a read-out circuit to which the detector element (20) is connected. This arrangement significantly reduces the effects of microphony in contributing to the electrical output of the detector. The ridge portions (32) may form a continuous network around cavities (35) below the detector elements (20 ), and a conduit may extend into each cavity (35) with that at the opposite face (12) of the polymer film (10).

Pyroelectric and other thermal-radiation detectors with film element(s)

An infrared detector array or other electrical transducer device comprising an electrically-active film (10) of polymer material is manufactured with a pattern of electrodes (11) embedded at one face of the film (10). The electrodes (11) are formed on a support (4,5) as a photolithographically defined pattern of deposited material. These electrodes (11) are then transferred to the film (10) by depositing the film material on the support (4,5) and over the electrodes (11) and removing the support (4,5) at least at the area of the electrodes (11). The polymer material of the film (10) bonds well to the electrodes (11). In this manner, fine-geometry patterns of thin electrodes (11) can be formed embedded in the face of the film (10) and having reproduceable electrode characteristics for charge detection at the film faces in piezoelectric pyroelectric and ferroelectric devices. The bulk (5) of the support may be of glass dissolvable in HF, and it may have a conductive surface layer (4) which protects the film against the HF and connects the electrodes (11) together when applying an electric field across the film (10) to pole the polymer material, e.g. for a pyroelectric infrared detector.

Manufacture of electrical transducer devices, particularly infrared

Thermal radiation detection apparatus is provided comprising an array of pyroelectric detectors 1,2,3,4 in which compensation is provided for the effect of ambient temperature changes on the detector outputs and also for d.c. offsets which occur in source follower impedance converters 13 necessarily used with each detector. Single element detectors are used, the end pair of elements 3,4 being shielded from radiation and used as reference elements. Each element has a pair of diodes 11,12 connected in parallel in opposite sense to provide a d.c. path across the element. The reference element outputs are averaged 16,17,18 and fed via a high gain negative feedback loop 19 to the common connection 10 of all the elements. Offset voltages and element signals generated by ambient temperature changes are thereby compensated to the extent that the reference offsets and thermal voltages equal those of the active detector elements.

Temperature compensated thermal radiation detectors

A thermal radiation detection apparatus has an array of pyroelectric detector devices (10) for receiving radiation from a scene whose outputs, e.g. from associated source followers (12) are supplied via a multiplexer (14) in turn to a processing circuit (20) which is responsive to each detector device signal to produce an output which substantially faithfully reproduces the received radiation signal without requiring that a chopper be used. The processing circuit operates with a correction factor to produce an output signal with first and second components proportional respectively to the device signal and its rate of change, the relative proportions of the components being in a ratio according to the device's thermal time constant. Initial device voltage level and the device's electrical time constant effects can be corrected by additional processing in the circuit (20), producing a third component of the output proportional to the integral of the device's output and in a ratio to the first component according to the electrical time constant.

Thermal radiation detection apparatus

A thermal-image sensing device is manufactured with a 2-dimensional array of ferroelectric or pyroelectric infrared detector elements (38) mounted on a circuit substrate (1). The upper surface of the circuit substrate (1) is covered with a thick insulating layer (10) of plastics material in which a corresponding 2-dimensional array of bores (11) is formed. Each of the detector elements (38) is connected to a respective electrode (2) of the circuit substrate (1) by means of a metal coating over the wall of its respective bore (11) and over the respective electrode (2) of the circuit substrate (1). The invention permits a simplification in manufacture while forming a device structure with a high degree of ruggedness and good electrical and thermal insulation between the detector elements (38) and the circuit substrate (1). It also permits the detector element array to be formed in situ on the circuit substrate (1), for example by sawing grooves (36) through a metallised sheet (30,31,32) of the ferroelectric or pyroelectric material mounted on the metal-coated plastics material (10) and then providing top electrode connections (41) bridging the grooves (36).

Thermal-image sensing devices and their manufacture

Each element of an array of infrared detector elements comprises a capacitor formed by a body (11) of pyroelectric or ferroelectric material between a front electrode (14) and a back electrode (12). The elements have individual electrical connections (13) to their back electrode (12) from which there are derived electrical signals which differ as the temperature of the body (11) changes in response to incident infrared radiation (50). These individual electrical connections (13) are provided by a pattern of conductors (13) carried by a support (10) on which the elements are mounted. Each element has at least an infrared-collection area (21) of larger lateral dimensions (x) than at least the electrical connection (13) to the back electrode (12) whereby a peripheral part (23) of each element overhangs and is separated vertically from an underlying part of the support (10). In accordance with the invention, the elements are arranged in a 2-dimensional array with inner elements of the array surrounded by outer elements of the array. The conductors (13 ) to the inner elements extend on a part of the support (10) between the outer elements and below the overhanging peripheral parts (23) of these outer elements. These conductors (13) of the inner elements are separated from the back electrode (12) and overhanging peripheral parts (23) of the outer elements and from the individual electrical connections (13) of these outer elements by a gap (y,z) sufficient to avoid any troublesome parasitic capacitance across a detector element or any significant capacitive cross-talk between the detector elements. The width of the intermediate conductor tracks (13) and the dimensions (y,z) of the surrounding gap may be larger than the spacing between the overhanging peripheral parts (23) of adjacent elements and, for example, as large as or larger than the thickness of the element bodies (11). The array can be fabricated cheaply and reliably using thick-film techniques to form the conductors (13) on a good thermally insulative support (10), so also providing low thermal cross-talk between the detector elements.

Arrays of infrared detector elements

An infrared radiation detection device uses a pyroelectric polymer film (10), such as a poled vinylidene fluoride copolymer film, on one surface of which, facing incoming radiation, a front electrically resistive electrode (20) is disposed and on the opposite surface of which a back electrode (21) is disposed. Radiation absorption characteristics of the device for a given wavelength range of interest, for example 5 to 15 .mu.m, are optimized by giving the front electrode a high resistance per square value and forming the polymer film between the electrodes with an optical thickness substantially one quarter of a selected wavelength within the wavelength range at which wavelength radiation absorption at the front electrode is to be substantially a maximum. The back electrode preferably is reflective, having a low resistance per square. More detector elements may be provided defined by respective portions of a common polymer film.

Infrared radiation detection device

A thermal radiation detector suitable for detecting radiation in a given wavelength range, for example 5 to 15 micrometers, includes a detecting element (10), such as a pyroelectric element, and a structure coupled to the element for improving performance by optimizing the absorption characteristic of the detector over a wide wavelength range. The structure comprises two dielectric layers (12 and 18) preceding the detecting element with optical thicknesses substantially one quarter of a selected wavelength in the wavelength range and an intermediate resistive layer (17) for absorbing incident radiation in the wavelength range. A reflective layer (15) is preferably disposed between the element and the adjacent dielectric layer. The dielectric layers may both comprise ZnS or Ge. One layer may comprise a portion of a flexible film used to support the detector.

Thermal radiation detector

A thermal radiation detector comprises detecting means supported by a flexible film. To improve the absorption of incident radiation by the thermal radiation detector, the detector comprises an electrically resistive layer which is immediately adjacent the flexible film, and to which the detecting means are thermally coupled. The optical thickness of transparent dielectric material preceding the detecting means (suitably consisting only of the flexible film) is one fourth of a selected wavelength in a given wavelength range to be detected. The wavelength is selected so that the absorption of incident radiation by the resistive layer is substantially a maximum at the selected wavelength. The resistive layer may be between the flexible film and the detecting means, or it may precede the flexible film. In the latter case, an electrically conductive, reflecting layer is disposed between the flexible film and the detecting means.

A pyroelectric detector comprises an element (1) of pyroelectric material that responds to variations with time in the intensity of radiation incident on an operative surface region of the detector much larger than the area of a major face of the element (1). The Noise Equivalent Power of the detector is improved by providing a thermal diffusion means (10) to conduct thermal energy to the element from remote parts of the region. The element is resiliently supported on one or more flexible films (2,3) and the thermal diffusion means (10) is a thermally conductive layer on a film (3).

Pyroelectric detector

A bolometric detector comprising an optical immersion lens and a pyroelectric detector element mounted so as to be spaced from the lens. The detector may be mounted spaced from the lens by an air gap, by a foraminous spacer, or by a cement having a low thermal conductivity.

Bolometric detector

In order to reduce microphony due to the piezoelectric nature of a pyroelectric element (9) in a pyroelectric detector, the detector comprises one or more flexible films resiliently supporting the element (9) and two electrical connections thereto each comprising an electrically conductive layer (6,8) on the film(s). There are suitably two films with the element (9) therebetween. The films are secured to one another around the element (9) by an adhesive, thereby urging the films against the element (9) to hold it in position and maintain the electrical connections.

A photoconductive structure comprises an electrically insulating substrate bearing an evaporated metal electrode pattern, and a 60 to 150 nm thick polyimide film disposed between a sintered cadmium selenide layer and the substrate with the superposed electrode pattern. In the absence of the polyimide film, there had been unacceptable attack of the electrode structure and of silica substrates by the cadmium selenide.

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Reigate, United Kingdom

Andrew A Turnbull