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Multi-Frequency Band Pyroelectric Sensors Sensors 2014, 14, 22180-22198; doi:10.3390/s141222180 OPEN ACCESS sensors ISSN 1424-8220 www.mdpi.com/journal/sensors Article Multi-Frequency Band Pyroelectric Sensors Chun-Ching Hsiao * and Sheng-Yi Liu Department of Mechanical Design Engineering, National Formosa University, No. 64, Wunhua Rd., Huwei Township, Yunlin County 632, Taiwan; E-Mail: [email protected] * Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.: +886-5-6315-557; Fax: +886-5-6363-010. External Editor: Vittorio M.N. Passaro Received: 23 September 2014; in revised form: 6 November 2014 / Accepted: 20 November 2014 / Published: 25 November 2014 Abstract: A methodology is proposed for designing a multi-frequency band pyroelectric sensor which can detect subjects with various frequencies or velocities. A structure with dual pyroelectric layers, consisting of a thinner sputtered ZnO layer and a thicker aerosol ZnO layer, proved helpful in the development of the proposed sensor. The thinner sputtered ZnO layer with a small thermal capacity and a rapid response accomplishes a high-frequency sensing task, while the thicker aerosol ZnO layer with a large thermal capacity and a tardy response is responsible for low-frequency sensing tasks. A multi-frequency band pyroelectric sensor is successfully designed, analyzed and fabricated in the present study. The range of the multi-frequency sensing can be estimated by means of the proposed design and analysis to match the thicknesses of the sputtered and the aerosol ZnO layers. The fabricated multi-frequency band pyroelectric sensor with a 1 μm thick sputtered ZnO layer and a 20 μm thick aerosol ZnO layer can sense a frequency band from 4000 to 40,000 Hz without tardy response and low voltage responsivity. Keywords: pyroelectricity; sensor; multi-frequency; zinc oxide; thin film Sensors 2014, 14 22181 1. Introduction Pyroelectricity, which is electric current generation from time-dependent temperature fluctuations, is useful in many applications, such as pollution monitoring, hot image detectors, intruder alarms, gas analysis and temperature sensors. A pyroelectric material exhibits a spontaneous polarization in the absence of an electric field. Thin-film pyroelectric sensors have many advantages, such as facile integration with on-chip circuitry, uncooled detection, room-temperature operation, speed, lower system costs, portability and a wide spectral response with high sensitivity [1–3]. Pyroelectric sensors have a pyroelectric layer sandwiched between top and bottom electrodes, which are built on thermally isolated structures or substrates to reduce heat loss. The principle of thin-film pyroelectric sensors is based on the pyroelectric effect, namely converting the heat transfer rates to the corresponding electrical signal. The pyroelectric effect is the property of selected dielectric materials with polar point symmetry, which show a spontaneous electrical polarization as a function of temperature. A change in the temperature of the material with respect to time (thermal fluctuations) causes a correspondent variation in the induced charge, thereby producing a pyroelectric current. The dynamic pyroelectric response current (ip) of pyroelectric sensors can be described by the expression [1]: ip = η × P × A × dT/dt (1) where η is the absorption coefficient of radiation, P is the pyroelectric coefficient of the pyroelectric film, A is the electrode area and dT/dt is the temperature variation rate of the pyroelectric film. When the materials and dimensions of the pyroelectric layers are defined, the absorption coefficient, the pyroelectric coefficient and the electrode area cannot be altered. However, the temperature variation rate has a huge maneuverability according to the design of the patterns, trenches, cavities and structures in the pyroelectric materials. ZnO is a wide-band gap semiconductor of the II-VI semiconductor group. This semiconductor has several favorable properties, including good transparency, high electron mobility, wide band gap and strong room-temperature luminescence. These properties are used in emerging applications for transparent electrodes in liquid crystal displays, in energy-saving or heat-protecting windows and in electronics as thin-film transistors and light-emitting diodes. ZnO has a relatively large direct band gap of ~3.3 eV at room temperature. The advantages associated with a large band gap include higher breakdown voltages, the ability to sustain large electric fields, lower electronic noise, and high-temperature and high-power operation. ZnO films have been synthesized by numerous methods, such as metal organic chemical vapor deposition, molecular beam epitaxy, magnetron sputtering, pulsed laser deposition, atomic layer deposition, spray pyrolysis, filtered cathodic vacuum arc technique, sol-gel process and aerosol deposition. The quality of ZnO films obtained by the above methods depends on the specific growth methods and conditions. Thus, the preferential orientation of ZnO films depends on the growth conditions. The pyroelectricity of ZnO is attributable to non-centrosymmetrical crystals, and so it has a specific polar axis along the direction of spontaneous polarization [1–4]. The most densely packed and thermodynamically favorable growth orientation in a ZnO wurtzite structure is one in which the c-axis is perpendicular to the substrate. ZnO films with the c-axis normal to the substrate are preferred in many applications, such as ZnO pyroelectric devices [3,4] and film bulk acoustic resonators [5]. When ZnO is subjected to temperature variations, its internal polarization will produce an electric field. Therefore, Sensors 2014, 14 22182 increasing the responsivity of a ZnO pyroelectric sensor depends on increasing the temperature variation rate of the ZnO layer, adopting a ZnO film with a strongly preferred orientation towards the c-axis, and using a high-performance thermal-isolation structure. The pyroelectric effect has been applied to environmental energy-harvesting systems. Pyroelectric energy conversion also offers a novel and direct way to convert time-dependent temperature fluctuations into electricity for micropower generators and low-energy-consumption systems [6,7]. The consideration of both the thermal and the electrical circuits is required in the analysis of pyroelectric devices. A radiation with power W(t) sinusoidally modulated at a frequency ω is incident on the top surface of the pyroelectric element (area A and thickness d), which has emissivity η. The pyroelectric element has a thermal capacity H and a thermal conductance to the surroundings GT; the thermal time constant can then be defined as τT = H/GT. The thermal capacity can be defined as H = c’dA, where c’ is the volume-specific heat. The electrical signal is further amplified by a high-input-impedance field effect transistor, which is used as an amplifier. RG is the gate resistor, and then the electrical time constant can be defined as τE = RG(CE + CA), where CE is the capacitance of the pyroelectric element; CA is the capacitance of the amplifier; and τT and τE are the fundamental factors which determine the frequency response of the pyroelectric sensors. The voltage responsivity (Rv) can be calculated by the following equation [1]: = (2) (1+ )(1+ ) −1 At a low frequency (ω << τT ), Rv is proportional to the frequency, and is shown as the following equation: = (3) Equation (3) can easily maximize Rv by minimizing GT (i.e., by adding a thermal insulation layer between the pyroelectric film and the substrates, adopting a suspended structure fabricated by a bulk micromachining technique using anisotropic silicon etching or by using a substrate with a low thermal −1 −1 conductivity). At a high frequency (ω >> τT ; ω >> τE ), Rv is inversely proportional to the frequency, and is shown as the following equation: = (4) ( +) Equation (4) can easily maximize Rv by minimizing the thermal capacity of the pyroelectric element H (i.e., decreasing the thickness of the pyroelectric element). Moreover, reducing the pyroelectric element’s thickness can retard the decline of the voltage responsivity at a high frequency. The frequency -1 at τT is a watershed to distinguish the ranges of low and high frequencies, and the pyroelectric element’s thickness determines the value of the thermal time constant (τT) under the decided pyroelectric materials and electrode areas. Therefore, a thicker pyroelectric element increases the thermal time constant, which is suitable as the sensor for a low-frequency range. Unlike the thicker element, a thinner pyroelectric element reduces the thermal time constant, which is suitable as the sensor for a high-frequency range. It is difficult to apply a pyroelectric sensor with a single pyroelectric layer to multi-frequency sensing tasks when the materials and dimensions of the pyroelectric layers are already fixed. In other words, the Sensors 2014, 14 22183 materials and dimensions of the pyroelectric layers directly determine the thermal time constant. Therefore, in this study, a structure with dual pyroelectric ZnO films was designed for a multi-frequency band pyroelectric sensor. The structure mainly comprised two ZnO pyroelectric layers: a thinner ZnO pyroelectric layer deposited by sputtering and a thicker ZnO pyroelectric layer deposited by aerosol deposition (AD). The thinner ZnO film was deposited by RF sputtering. Sputter deposition is a physical vapor deposition (PVD) method of depositing thin films by sputtering. Sputter deposited
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