R. Köhler

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 TR(jω, s) and figures of merit Mv, M1 and MD, an universal description of the sensors 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...

Pyroelectric single-element and linear-array sensors based on P(VDF/TrFE) thin films

Abstract The application of pyroelectric poly(vinylidene fluoride/trifluoroethylene) (P(VDF/TrFE)) copolymer thin films simplifies the manufacture of pyroelectric single-element sensors and, above all, of sensor arrays with high sensitivity, low noise and slight crosstalk. The sensor consists of a self-supporting membrane that is a combination of silicon oxide and silicon nitride. This membrane is made by anisotropic etching on the back of a (100) silicon wafer. Onto the etched wafer a 1–2 μm thin P(VDF/TrFE) film is formed by spin coating. The pyroelectric copolymer is electrically connected by evaporated electrodes. In this way, both single-element sensors and sensor arrays are produced. Single-element sensors built up from P(VDF/TrFE) with a VDF content of 70–80% on an SiO2/Si3N4 membrane achieve a specific detectivity of up to 3.5×108 cm √Hz W−1 at a modulation frequency of 10 Hz and are temperature stable up to at least 55 °C. For linear arrays noise-equivalent power (NEP) values of 4.5 and 12 nW have been measured at chopper frequencies of 40 and 128 Hz, respectively. The average voltage responsivity Rv (500 K) of P(VDF/TrFE) linear arrays is about 110 000 V W−1 at 40 Hz and 40 000 V W−1 at 128 Hz.

Application of P(VDF/TrFE) thin films in pyroelectric detectors

Abstract Copolymer thin films can be cast by coating a thickness of 0.8 μm to 2.5 μm on solid silicon substrates as well as on unrestrained 800 nm thin SiO2/Si3N4 supporting membranes. By poling the P(VDF/TrFE) at about 140 V/μm, a high spontaneous polarization can be obtained without any stretching process. Measurements of dielectric constant, dielectric losses and pyroelectric coefficient confirm the ideal nature of these materials for pyroelectric detectors. Built-up single element detectors distinguish themselves by a high specific detectivity D* of 2 · 108 cm√Hz/W at a frequency of 10 Hz.

RF-sputtered PZT thin films for infrared sensor arrays

Abstract PZT thin films prepared by RF sputtering of a ceramic target of composition Pb(Zr0.25, Ti0.75)O3, show different textures with respect to sputtering conditions adopted. The films prepared were under high stress as shown by the stress measurements. PZT micro-structures with Pt electrodes sputtered on silicon wafers were investigated using the Raman peak of the single crystalline silicon. The Raman shift profiles were found to be dependent on the particular geometry of the investigated structures. Infrared sensor arrays described in this paper were fabricated with multitarget sputtered I μm PZT thin films. The array with 256 sensitive elements exhibits a noise equivalent power (NEP) of 0.42 nW at 20 Hz.

Investigation of the spatial polarization distribution of sputtered PZT thin films using limm

Abstract In this work, the laser-intensity-modulation method (LIMM), e.g. the determination of the spatial polarization profile from the pyroelectric current spectrum caused by the interaction of thermal waves generated by an intensity modulated laser and the unknown polarization distribution, is applied for investigation of sputtered PZT thin films on a thermally coupled bulk silicon substrate or a thermally isolating membrane, respectively. To investigate thin films with a thickness of about 1 μm the modulation frequency of the laser was extended up to 2 MHz. The reconstruction of the spatial polarization distribution requires to solve a Fredholms integral equation of the first kind which is a so called “ill-posed” problem. For deconvolution of the inverse LIMM problem several approaches like power series trial functions and Lagrange polynomials are used and compared with the original Fourier series approach for LIMM introduced by S.B. Lang. To obtain a non-oscillating solution of the inverse LIMM-prob...

Pyroelectric IR-detector arrays based on sputtered PZT and spin-coated P(VDF/TrFE) thin films

Abstract The choice of the material and an optimal thermal isolation of the sensitive elements are essential for a pyroelectric sensor with high detectivity. In this paper the application of two different ferroelectric materials, a copolymer [poly(vinylidene fluoride/trifluoroethylene), P(VDF/TrFE)] and a ceramic thin film (PZT) for pyroelectric IR sensing, is discussed. The polymer has a low Curie temperature (between 49°C and 135°C) and hence there is a risk of depolarization at temperatures higher than room temperature. In comparison PZT with a Zr content of 25 at% has a high Curie temperature (450°C) and the sputtered films exhibit self-polarization. Single element sensors fabricated from both materials, with a radiation sensitive area of 1 mm2 on SiO2/Si3N4 membranes exhibit at room temperature a specific detectivity of about 3×108 cm Hz½W−1. In addition, sensor arrays with up to 128 elements were also fabricated.

A 11×6 element pyroelectric detector array utilizing self-polarized pzt thin films grown by sputtering

Abstract A planar multi target sputtering approach was used to deposit PbTiO3 (PT) and Pb(Zr, Ti)O3 (PZT) films on TiO2/Pt bottom electrodes for the use in thin film pyroelectric IR detector arrays. PZT films with a Zr content of 28 at% (PZ28T) exhibited the best pyroelectric coefficient of typically 2×10−4 Cm−2K−1. The PZ28T films have been used for fabricating a two dimensional 11×6 pixel pyroelectric detector array on Si wafers. The array pixels with a sensitive area of 280 ×280 μm2 have a noise lequivalent power NEP of less than 0.7 nW at 1 Hz. It is planned to use the detector array in systems for motion detection.

Pyroelectric devices based on sputtered PZT thin films

Abstract After a short description of thermal conditions in a pyroelectric sensor, substantial requirements to pyroelectric thin film and sensor design are derived from the theoretical basics. A planar multi-target sputtering process is used to deposit PZT thin films for application in pyroelectric IR sensors. The self-polarized PZT are characterized by a pyroelectric coefficient p of 2-10−4 Cm−2K−1, a dielectric constant [Sgrave] of 300 and a dielectric loss tan δ of 0.01. These material properties, including a low tensile stress of the sensor layer stack of + 110 Mpa, as well as standard microelectronic technologies make the films suitable for the use in pyroelectric sensor arrays. Fabricated singleelement sensors have a specific detectivity D* (500K, 10 Hz) of 3.108 cmHz1/2W−1. An 11×6 array sensor has been developed for motion detection. The array pixels with a sensitive area of 0.0784 mm2 have a noise equivalent power NEP of less than 0.7 nW at 1 Hz.

Properties of sputter and Sol-Gel deposited PZT thin films for sensor and actuator applications: Preparation, stress and space charge distribution, self poling

Abstract In this paper, properties of sputter– and sol–gel deposited PZT thin films are reviewed and compared. It is shown that film properties are influenced by type and parameter of fabrication technology. By means of determining both stress and space charge distribution in the PZT films, it can be seen that an oxygen vacancy leads to a strong self–polarization which is stable under sensor operation conditions and makes these films very advantageous for pyroelectric IR sensors.

Pyroelectric sensors and arrays based on P(VDF/TRFE) copolymer films

Abstract The central problem of the sensor design is a good thermal insulation of the P(VDF/TrFE) thin film from the read-out circuit. Two variations were examined - thin carrier membranes of SiO2/Si3N4 produced through back etching and thick thermal insulating layers out of spin-coated polymers with via holes. Through simulation and measurements on single element sensors and linear arrays for both variants an optimal design could be established. Membrane sensors consisting of 1 μm P (VDF/TrFE) thin film deposited on a 0·65 μm thick membrane show within the frequency range of 10 Hz to 1 kHz comparatively higher values for responsivity and specific detectivity than insulating layer sensors. The insulating layer sensors have in the most favourable case a thermal cutoff frequency of about 100 Hz, below that the responsivity remains constant while the specific detectivity and NEP deteriorate again. The optimal layer thickness for the compound of BCB and P(VDF/TrFE) for a chopper frequency of about 100 Hz is a...