PYROELECTRIC THIN-FILM SENSORS AND ARRAYS BASED ON P(VDF TRFE)/

After a short description of the structure and operation of a pyroelec tric sensor, the thermal conditions of the sensing element, the therma l-to-electrical conversion and the signal processing of pyroelectric t hin film sensors will be represented. By means of the complex normalis ed current responsivity T-R(j omega, s) and figures of merit M(V), M(I ), and M(D), an universal description of the sensor's internal operati on is obtained. The influence of electrothermal coupling effects on th e dielectric loss of the pyroelectric thin film is also discussed. Sub stantial requirements to the pyroelectric thin film and the sensor des ign are derived. A comparison of often used thin film ferroelectrics s hows that the application of P(VDF/TrFE) in low cost sensors can be ad vantageous although the figures of merit are lower. Copolymer film can be easily deposited onto a silicon wafer in post-processing after rea d out circuit fabrication, for instance by spin coating of a copolymer solution. Furthermore, the very low thermal conductivity provides goo d thermal insulation between the pyroelectric film and readout circuit ry. The chosen P(VDF/TrFE) with a molar content of 70 % VDF shows a sp ontaneous polarisation of 8 mu Ccm(-2) and a pyroelectric coefficient of 3.5 nCcm(-2)K(-1), a dielectric constant of 8 and a dielectric loss of about 0.018 at 25 degrees C. By a computer simulation, an optimum sensor design was achieved for single-element sensors and linear array s. The central feature is a self-supporting carrier membrane of Si3N4 (about 150 nm) and SiO2 (500 nn), processed by bottom side etching of silicone wafers covered with a spin coated P(VDF/TrFE) thin film of ab out (1...2) mu m in thickness. A radiation sensitive area (1x1) mm(2) and (2x2) mm(2) was choosen for the single-element sensor. In the line ar array 128 pixels are arranged with a pitch of 100 mu m, the pixel a rea is (90x100) mu m(2). A CCD read out circuit with gate modulation i nput structure is used as multiplexer. The specific detectivity D (50 0 K, 10 Hz) of the single-element sensor is 3.10(8) cmHz(1/2)W(-1). At a frequency of 40 Hz the linear array shows a NEP of 4.5 nW and a MTF of 0.32 at a spatial frequency of 3 lpmm(-1).