Thin Film Pyroelectric Detector Research Articles

Recording of Low-Power Nanosecond Radiation Pulses by a Detector Based on a Thin-Film Pyroelectric Structure

Results of an experimental study of a thin-film pyroelectric detector of radiation based on strontium barium niobate and used for detecting nanosecond radiation pulses are reported. The possibility of recording 30-ns radiation pulses with the sensitivity of 1 V/W (NEP = 8 · 10−7 W/Hz1/2) is demonstrated.

Characterization of a Large-Area Pyroelectric Detector from 300 GHz to 30 THz

The national metrology institute of Germany, the Physikalisch-Technische Bundesanstalt (PTB), together with the company Sensor and Lasertechnik (SLT), develops pyroelectric detectors for radiation in the terahertz (THz) spectral range. The intention of this development is to deliver a highly sensitive, accurately calibrated detector for power measurement in the power range of time-domain spectroscopy (TDS) systems. This work reports about a large-area thin-film pyroelectric (TFP) detector applicable within a wide spectral range from 300 GHz to 30 THz and its radiometric characterization by PTB’s THz radiation sources. Applying coherent synchrotron radiation from the Metrology Light Source (MLS), laser radiation from a molecular gas laser and blackbody radiation from a water-heated blackbody to this detector reveal its potential to be capable of spanning an even wider THz frequency range than covered by TDS systems. To demonstrate this, its spectral responsivity was measured at different frequencies between 300 GHz and 30 THz by means of those three THz radiation sources.

Pyroelectric Ta-modified LiNbO 3 thin films and devices for thermal infrared detection

High-performance pyroelectric infrared (IR) detectors have been fabricated using tantalate-doped lithium niobate LiNb 1 − x Ta x O 3 (abbreviated as LNT, with x = 0–1.0) thin films deposited on Pt(111)/Ti/SiO 2/Si(100) substrates by diol-based sol–gel processing, in which, tantalate (Ta) is adopted as dopant in lithium niobate. The randomly oriented LNT thin film exhibits a relatively small dielectric constant and a large pyroelectric coefficient. The pyroelectric characteristics of detectors with various tantalate contents, as a function of modulation frequency, were investigated. It was found that LiNb 0.8Ta 0.2O 3 had the largest voltage responsivity of 7020 (V/W) at 70 Hz, and a specific detectivity ( D ⁎) of 7.76 × 10 7 cm Hz 1/2/W at 200 Hz. These results indicate that the LNT thin film with x = 0.20 is most suitable for application as high-performance pyroelectric thin-film detectors.

The properties of tantalum modified lithium niobate thin films prepared by a diol-based sol-gel process

LiNbTa x O 3 thin films were successfully deposited on Pt(111)/Ti/SiO 2 /Si(100) substrates by spin coating with a diol-based sol-gel technology and rapid thermal annealing. The effects of various processing parameters, including Ta content (0) and heating temperature (500 ~ 800 °C), on the growth and properties of thin films were investigated. With the increase of Ta content, the grain size of film decreased slightly, and the maximum f factor (the degree of c -axis orientation) of the films were obtained in the composition of x = 0.2. As the composition of film varied from x = 0 to x = 1, the relative dielectric constant of film increased from 33 up to 62, and the dielectric loss factor (tan δ ) also increased from 0.00374 to 0.00686, respectively. The coercive field, Ec , and remanent polarization, Pr , decreased but pyroelectric coefficient, γ , increased from 2.76 × 10 −8 up to 4.51 × 10 −8 C/cm 2 K. The pyroelectric figures of merit, Fv and Fm , indicated that film with Ta content of 20 mol% and heating temperature of 700 °C exhibited the optimized pyroelectric characteristics and could be applied for high-performance pyroelectric thin-film detectors.

Pyroelectric Arrays Using Ceramics and Thin Films Integrated Radiation Collectors: Design Fabrication and Testing

Pyroelectric infra-red detectors have been of-interest for many years because of their wide wavelength response, good sensitivity and lack of need for cooling. Arrays of such detectors, comprising a pyroelectric material interfaced to an application specific integrated circuit for signal amplification and read out, provide an attractive solution to the problem of collecting spatial information on the IR distribution in a scene. Sol gel deposition provides an excellent technique for the growth of ferroelectric thin films and Mn-doped PZT30/70 films can be grown at 560°C with F D = 3.85 × 10−5 Pa−1/2 . A new concept is presented here: the use of arrays of thin film pyroelectric detector elements with integrated radiation collectors designed to enhance the intensity of the radiation falling on the element. Two collector designs are presented, one based on the use of wet-chemically-etched pyramidal cavities, the second based on the Compound Parabolic Concentrator (CPC). Approximations to truncated CPC structures were SF 6 -dry-etched into a silicon wafer, upon which had been defined pyroelectric IR sensors with low thermal conductance (spiral leg structure) fabricated in a high sensitivity PZT thin film. First experimental assessment of the performance of these structures is presented.

Pyroelectric Mg-modified PbTiO3 thin films and devices for thermal infrared detection

High-performance pyroelectric infrared (IR) detectors have been fabricated using magnesium-doped lead titanate (Pb1-xMgxTiO3 (PMT), with x=0∼0.08) thin films deposited on Pt/SiO2/Si substrates by diol-based sol–gel processing. The randomly oriented PMT thin film exhibits a relatively small dielectric constant and a large pyroelectric coefficient. The pyroelectric characteristics of detectors with various magnesium contents, as a function of modulation frequency, were conducted. It was found that PMT(6) has a large voltage responsivity of 5280 (V/W) at 20 Hz, and a specific detectivity (D*) of 5.12×107 cm Hz1/2/W at 100 Hz. These results indicate that the PMT thin film with x=0.06 was most suitable for application as high-performance pyroelectric thin-film detectors.

Rapid thermal processing of lithium tantalite thin films prepared by a diol-based sol–gel process

Lithium tantalite (LiTaO3) thin films have been deposited on Pt(111)/SiO2/Si(100) substrates by means of sol–gel spin-coating technology. Using a diol-based precursor solution and rapid thermal processing (RTP), highly c-axis oriented LiTaO3 thin films are obtained and the degree of orientation is increased with an increase of the heating rate. By changing the heating rate (600–3000 °C/min) and heating temperature (500–800 °C), the effects of various processing parameters on the growth of films are investigated. With the increase of heating rate, the grain size of LiTaO3 thin films decreases markedly, and the relative dielectric constant (er) increases from 28 up to 45.6. It was found that the dielectric loss factor (cosδ) decreased, and the ferroelectric properties were improved by the increase of heating rate. The figures of merit (Fv and Fm) indicate that the LiTaO3 thin film with a heating rate of 1800 °C/min is suitable for application as a high-performance pyroelectric thin-film detector.

Characterization of sol-gel-prepared magnesium-modified lead titanate thin films

Pb1-xMgxTiO3 (PMT) thin films (x=0–0.08) were deposited onto Pt/SiO2/Si substrates by diol-based sol-gel processing. The influence of various processing methods on the characteristics of the thin films was studied by varying the magnesium contents and heating temperatures (500–800 °C) of the samples. In the present study, the relative dielectric constant (er) of the PMT thin films increased from 25.8 to 91.8 as the magnesium content increased. The coercive field (Ec) and the remanent polarization (Pr) decreased as the magnesium content increased, but the dielectric loss factor (cosδ) and the pyroelectric coefficient (p) increased. The PMT thin films with x=0.06 exhibited the largest figures of merit for voltage responsivity and specific detectivity. These results indicate that the PMT thin film with x=0.06 was most suitable for application to high-performance pyroelectric thin-film detectors.

Influence of the silicon substrate thickness on the response of thin film pyroelectric detectors

The effect of silicon substrate thickness on the performance of thin film PZT pyroelectric detectors is theoretically and experimentally investigated. As the silicon substrate thickness increases, the pyroelectric current responsivity drastically decreases. The current responsivity of a pyroelectric thin film detector with no silicon substrate is about two orders higher than that of a similar detector with 450 μm thick silicon substrate. To verify the theoretical analysis, micromachined Pb(Zr 0.3Ti 0.7)O 3 (PZT30/70) thin film pyroelectric detectors with different silicon substrate thickness are fabricated and characterised. The experimental results agree well with the theoretical analysis.

Pyroelectric Thin Film Presence Detector Arrays with Micromachined Pixels

Lately, pyroelectric thin film detectors gain in significance in the field of presence detection. In the paper, different variants of Pb(ZrTi)O 3 based detector arrays with 8 to 256 pixels are introduced. The sensing elements are arranged on thin micromachined SiO 2 /Si 3 N 4 membranes [1-4] as mechanical supporting structure with both low thermal mass and low heat conductivity into the substrate to achieve a high detectivity. For shaping these membrane structures, the most promising technology is micromachining, either from back or from surface of the silicon substrate. Self-polarized PZT thin films with a pyroelectric coefficient p up to 2.1 * 10 m 4 C/(m 2 K), dielectric loss tan i of less than 0.01 and a dielectric coefficient l r of about 250 are deposited in a multi-target sputtering process at comparatively low substrate temperatures of about 520°C. In the paper, presence detectors using thin film pyroelectric IR detector arrays as well as detector design, detector simulation, detector manufacturing and detector characteristics are discussed. In addition, pyroelectric material investigations and preparation of micromachined structures in combination with the back-end process of the pyroelectric active capacitor structure are presented.

Sputtering of PZT thin films for surface micromachined IR-detector arrays

A planar, multi-target sputtering approach was used to deposit tetragonal Pb(Zr, Ti)O3(PZT) films with Zr/(Zr+Ti) = 0.25−0.3 on Pt bottom electrodes for the use in thin film pyroelectric IR detector arrays. Low stress ferroelectric capacitor stacks are needed in these devices, because the sensing elements are arranged on thin surface micromachined membranes. In-situ deposited films exhibited the best electrical properties. Typical values for the dielectric constant, the dielectric loss and the pyroelectric coefficient are 250, <0.01 and >2×10−4 C/(m2K), respectively. Very strong imprint effects have been found in these films. Even after a heat treatment above the Curie point a pyroelectric coefficient of > 2×10−4 C/(m2K) has been found. This is advantageous for possible high temperature processing steps after PZT deposition in pyroelectric detector device manufaction.

Gas spectrometry based on a pyroelectric thin film array

We present a far infrared gas spectrometer which uses a thin film pyroelectric detector integrated on silicon. The detector is in-house fabricated and consists of 1×50 elements of sol-gel deposited, zirconium rich lead titanate (PZT) which are suspended on a submicron membrane for thermal insulation. The detector shows a detectivity of 16 μA/W and is mounted into a Oriel monochromator fitted with gratings for the 3–5 μm and the 8–12 μm range, respectively. The infrared radiation is emitted by a hot filament and chopped mecanically with a frequency of 10 Hz. Absorption spectra of CO and CO2 gas as examples for the 3–5 μm wavelength range show detection limits of 15 ppm for CO2 and 200 ppm for CO. Line shape investigations show that the cross sensitivity of our instrument can be neglected. In the 8–12 μm wavelength range, we present absorption measurements for butane gas and N2O, respectively.

Dependence of Infrared Photoresponse of Pyroelectric Thin Film Detector with Respect to Chopper Frequency

In this paper, the dependence of infrared photoresponse of a pyroelectric thin film detector with respect to chopper frequency is studied theoretically and experimentally. The expressions of photocurrent, photovoltage and a special chopper frequency f m at which the maximum photovoltage is generated, are derived. A PMSZT/YBCO heterostructure is taken as the pyroelectric infrared detector and its photocurrent and photovoltage dependence on chopper frequency was measured. The experimental results are satisfactory and in agreement with theoretical analysis.

PZT thin films grown by multi-target sputtering: Analysis of thin film stress

A planar multi target sputtering approach was used to deposit Pb(Zr, Ti)O3 (PZT) films with different composition on Pt bottom electrodes for the use in thin film pyroelectric IR detector arrays. Low stress ferroelectric capacitor stacks are needed in these devices, because the sensing elements are arranged on thin micromachined membranes. PZT films with a Zr content of 10 at% (10/90) and 48 at% (48/52) were analyzed in terms of thin film stress. PZT (10/90) films were found in a slight compressed state and PZT (48/52) under tensile stress after deposition. Stress temperature curves were used to evaluate the Curie temperature of the films. From the stress temperature curves the spontaneous distortion of the ferroelectric given by the lattice parameters can be extracted. The lower Curie temperature in combination with the smaller spontaneous distortion of the PZT (48/52) film is the reason for the difference in the stress state after deposition.

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

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.

Potassium tantalate niobate thin films for spaceborne pyroelectric detectors

Potassium tantalate niobate (KTN) thin film pyroelectric detectors are being developed for broad band spaceborne sensors where passively cooled operating temperatures ∼ 90K are easily attained. The ability to tailor the phase transition temperature of KTN through composition control makes it an ideal candidate for such applications. Decreasing the operating temperature from 300K to 90K allows for over an order of magnitude increase in the background-limited sensitivity of a pyroelectric detector. Current research focuses on optimizing the composition and structure of laser deposited KTa0.91Nb0.09O3 (Tc = 93K) thin films. Preliminary studies of materials for bottom electrodes are being performed using Pt and conducting oxides with the perovskite structure: La0.5Sr0.5CoO3 (LSC), YBa2Cu3O7-∂ (YBCO) and SrRuO3. KTN films have been grown by pulsed laser deposition on LaAlO3, Pt/Si3N4/Si, LSC/LaAlO3, YBCO/SrTiO3, YBCO/LaAlO3, and YBCO/YSZ (yttria-stabilized ZrO2) substrates and heterostructures. The highest quality KTN films on (100)-pseudocubic LaAlO3 are grown at Po2 = 300–400 mTorr and 650–750°C exhibit in and out of plane orientation. The FWHM rocking curves of these films vary from 0.43–0.73°. The presence of undesirable pyrochlore phase, as evidenced by X-ray Diffraction, can be eliminated at partial oxygen pressures of 300 and 400 mTorr. Initially a potassium deficiency of 40–50% was measured in films using Rutherford Backscattering Spectrometry and X-ray Photoelectron Spectroscopy. This deficiency is eliminated by using a potassium enriched (117%) fully reacted ceramic target in addition to growing in higher oxygen partial pressures (400 mTorr). Devices have been fabricated from Pt, LSC and YBCO bottom electrode heterostructures. Results from these structures are discussed.

Deposition of self-polarized PZT films by planar multi-target sputtering

A planar multi target sputtering approach was used to deposit self polarized PZT films on TiO2/Pt bottom electrodes for the use in thin film pyroelectric IR detector arrays. By using elevated substrate temperatures of about 450°C “in situ” growth of tetragonal PZT could be achieved. The films exhibited pyroelectric currents without poling. The pyroelectric coefficient was 2×10−4 C/m2K, the dielectric constant was 300 and dielectric loss tan δ was 0.01. The self polarization disappears after heating the sample to 600°C. Stresses were studied in the thin film processing for the bottom electrode and the PZT film. The TiO2/Pt electrode is under high tensile stress of 900 MPa after preparation. PZT has a small compressive stress of -60 MPa, the whole TiO2/Pt/PZT stack has a tensile stress of +80 MPa. This low stress level together with the self polarization and the good electrical properties makes the films suitable for the use in pyroelectric detector arrays.

Heat Flow and Detectivity of Multilayer Thin-Film Pyroelectric Detector

The pyroelectric responses of a multilayer, thin-film pyroelectric detector under both isolated and heat-sink boundary conditions are completely analyzed. Exact mathematical solutions of the heat flow equation are derived. The multilayer detector is composed of pyroelectric layer/thermal isolation layer/silicon substrate. The analytical result shows that when the thickness of the isolation layer is increased, the detectivity, D*, at low frequencies will be improved, and as the thickness of the pyroelectric layer is decreased, the high-frequency response is improved. D* of a detector with 1 µm PLZT((Pb, La)(Zr, Ti)O3)/10 µm porous silica/0.5 mm silicon is higher than that of the detector with 10 µm PLZT/1 µm porous silica/0.5 mm silicon in a wide frequency range. In order to achieve better performance of a multilayer thin-film detector, the thicknesses of the two layers should be chosen very carefully. This analysis provides a guide for the preparation of multilayer thin-film pyroelectric detectors.

Properties of PLT thin film pyroelectric detectors I. Current responsivity

By solving the one-dimensional heat conduction equation, the dynamic pyroelectric response of a thin film pyroelectric detector to sinusoidal modulated radiation is represented by a matrix. The influence of the parameters of each layer in the detector on the current responsivity is analyzed. Theoretical results are compared with the experimental results for PLT thin film detectors.

Properties of PLT thin film pyroelectric detectors II. Noise and detectivity

Pyroelectric detectors are constructed with PLT 20 ferroelectric thin films. Noise, voltage response and specific detectivity of the detectors are measured. It is found that although the voltage response increases with the decreasing of modulating frequency, the detectivity is flat in the whole measured frequency band (10 Hz to 1000 Hz). This feature is determined by the noise-frequency property, which shows the same trend as voltage response. Sources of noise and possible ways to improve detectivity of the thin film pyroelectric detectors are discussed.