Infrared sensor based on the monolithic structure Si-P(VDF/TrFE)

Abstract

This paper deals with thermal conditions of the sensing element and the signal processing of pyroelectric thin film sensors. A short description of the structure and operation of a pyroelectric sensor introduces the topic. By analyzing the complex normalized current responsivity T_R (jω) and specific detectivity D*, an optimum sensor design was achieved. In the case of single-element sensors and linear arrays, the sensor consists of a self supporting carrier membrane of about 500 nm Si₃N₄ and 150 nm SiO₂, made by back etching of silicone with a spin coated P(VDF/TrFE) film, with a thickness of 1–2 μm. In two dimensional arrays with high spatial resolution, a sandwich assembly would be preferred. Using a 10 μm thermal insulating layer with vias, the P(VDF/TrFE) layer's optimum thickness comes to about 10 μm for chopper frequencies above SO Hz. Spin coating is a simple and reliable method to produce thin copolymer films. A high rate of β form polar crystallites can be obtained in the films confirmed by X-ray diffraction patterns by melting and slowly cooling. Poling at bias fields above 100 Vμm results in a high spontaneous polarisation and a high pyroelectric coefficient, low dielectric constant and low dielectric loss. The spontaneous polarisation and the Curie point increase with rising content of VDF, while pyroelectric coefficient and dielectric constant decreases. Compositions with a VDF molar content of 70%–80% are best suited for applying to pyroelectric sensors. The chosen P(VDF/TrFE) 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. Single-element sensors and linear arrays fabricated in our laboratories distinguish themselves by a high voltage responsivity and a high signal-to-noise-ratio. Compared with other common materials P(VDF/TrFE) is an advantageous one for the application in low cost sensors.