High Resolution 64-Element Pyroelectric Linear Array IR Detector

Abstract

High resolution 64- element pyroelectric linear array IR detectorAndrew A. Turnbull and Martin E. CookeMullard Southampton, Millbrook Industrial Estate, Southampton,S09 7BH, EnglandAbstractA 64- element pyroelectric linear array detector has been developed. Included within thedetector is a corresponding array of source followers together with a multiplexer andamplifier. High responsivity and high detectivity have been achieved together with a lowlevel of microphony.IntroductionSpecial problems arise in the development of pyroelectric linear array detectors when therequired resolution is high, there are a large number of elements and a very high performanceis required. This paper describes a design in which relatively low cost and a small overallsize were further significant aims.The detector comprises a pyroelectric array, a corresponding array of source followers,a multiplexer and an amplifier with a gain of 10. The novel feature of the array of sourcefollowers is the use of MOSFETs. The components of this detector are mounted within asealed 1.25 inch square flat pack, provided with a germanium window. The detector has beendesigned to yield high detectivity and high resolution using IR in the 8 to 14 micronwavelength band and chopped at frequencies in the region of 40 Hz.Basic constructionThe concept of the detector design is illustrated in the block diagram shown in Figure 1and the component layout illustrated in the photograph Figure 2.The pyroelectric array itself is mounted in one corner of the case. It is positionedaccurately with respect to three contact points on the outer edges of the case to facilitateeasy and accurate location in the camera by dowels. Apart from the pyroelectric array, allthe circuitry lies on an alumina substrate. This substrate incorporates printed circuitresistors and a gold print interconnect pattern. Other parts of the circuit comprise chipcapacitors and three IC chips, the largest of which, the source follower and multiplexerchip (7.3 x 3.7 mm) is mounted close to the edge of the alumina substrate to enable inter-connection to be made to the adjacent pyroelectric array.Pyroelectric array - Design principlesSome of the principles used in the construction of this array have been discussed in aprevious paper.(1) The active area of each element of the array is 100 microns (array width)by 80 microns, with 20 microns separation between the elements. To achie e the necessaryhigh degree of precision required, use was made of the fin concept in which the IRabsorbing active areas are thin film components. Of these components, the metal films aredefined by photolithography.A complete array is shown in Figure 3 and a schematic drawing showing one element of thearray is shown in Figure 4. Each element of the array is provided with a compensatingelement. The purpose of this compensating element is to minimise signals produced by rampsin ambient temperature. This is achieved by poling each pair of elements in oppositedirections and connecting the pair in parallel. The compensating elements are shielded fromincident IR by a highly reflective gold film. The complete array of pyroelectric elementsis fully reticulated and mounted between plastic membranes to afford low thermal cross -talkand low microphony.Pyroelectric array characteristicsOptical measurements carried out on the heat absorbing structure have shown that over thewavelength range 8 to 14 microns, IR absorption is high at about 95 %. Confirmation of thehigh IR absorption efficiency has been gained from measurements of responsivity at the sourcefollower outputs. On two measured samples, voltage responsivity has been 2.2 x 104V/W at10 Hz, which is within a few percent of the value predicted by calculations based on 100%absorption.92 /