Pyroelectric thin film sensors and arrays based on P(VDF/TrFE)

Abstract

After a short description of the structure and operation of a pyroelectric sensor, the thermal conditions of the sensing element, the thermal-to-electrical conversion and the signal processing of pyroelectric thin film sensors will be represented. By means of the complex normalised current responsivity T_R(jω, s) and figures of merit M_v, M₁ and M_D, an universal description of the sensor's internal operation is obtained. The influence of electrothermal coupling effects on the dielectric loss of the pyroelectric thin film is also discussed. Substantial requirements to the pyroelectric thin film and the sensor design are derived. A comparison of often used thin film ferroelectrics shows that the application of P(VDF/TrFE) in low cost sensors can be advantageous although the figures of merit are lower. Copolymer film can be easily deposited onto a silicon wafer in post-processing after read out circuit fabrication, for instance by spin coating of a copolymer solution. Furthermore, the very low thermal conductivity provides good thermal insulation between the pyroelectric film and readout circuitry. The chosen P(VDF/TrFE) with a molar content of 70 % VDF shows a spontaneous polarisation of 8 μCcm⁻² and a pyroelectric coefficient of 3.5 nCcm⁻²K⁻¹, a dielectric constant of 8 and a dielectric loss of about 0.018 at 25 °C. By a computer simulation, an optimum sensor design was achieved for single-element sensors and linear arrays. The central feature is a self-supporting carrier membrane of Si₃N₄ (about 150 nm) and SiO₂ (500 nm), processed by bottom side etching of silicone wafers covered with a spin coated P(VDF/TrFE) thin film of about (1…2) μm in thickness. A radiation sensitive area (1×1) mm² and (2×2) mm² was choosen for the single-element sensor. In the linear array 128 pixels are arranged with a pitch of 100 μm, the pixel area is (90×100) μ². A CCD read out circuit with gate modulation input structure is used as multiplexer. The specific detectivity D∗ (500 K, 10 Hz) of the single-element sensor is 3·10⁸ cmHz^½W⁻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.