Preparation And Theory Of Pyroelectric Lithium Tantalate Thin Film Infrared Detector

Infrared detectors are the key devices in infrared system.Uncooled infrared detectorsbecome the research focus of infrared detecting in recent years.And it has the importantactual sense of preparing sensing films and devices of uncooled infrared detector.Abated lithium tantalate crystal flat can be used to prepare uncoooled infrared detector.However,in bulk form it has limited applicability for high-performance system due tothe process,physical demension and rate of finished products.Therefore,thin filmforms of the lithium tantalate material with high quality increase its potential andavailability for pyroelectrie applications.So,the exploratory research of lithiumtantalate thin film and its pyroelectric infrared device started four years ago under thesupport of National Natural Science Fund of China.The good-performance pyroelectriclithium tantalate (LiTaO₃) thin film has been prepared by the sol-gel methods.Thereports about the preparation of the sol-gel derived LiTa₃O₈ film and its device can notbe found in literature before this work in China and Oversea.A novel sol-gel derivedLiTa₃O₈ thin film and its infrared detected device have been prepared by combined withthe MEMS process.The thermal conductivity and detectivity of the multilayer structureinfrared detector are simulated by MATLAB software.The simulated results provide thebasic structure design rules of device.The main results and research contents are as thefollowing:1.The theoretical model of multi-layer pyroelectric lithium tantalate thin filminfrared detector has been established based on one dimensional steady thermal conductequation and the actual boundary conditions of device application.And the relationsbetween the structure parameters of detector and the properties of detector have beendeeply discussed.The temperature distribution of multi-layer pyroelectric lithiumtantalate thin film infrared detector is simulated by MATLAB software under thesinusoidal modulated infrared radiation.The influences of lateral thermal diffusion onthe device performance are theoretically discussed.The voltage responsivity (Rv) andspecific detectivity (D^*) of detectors are calculated using the different structureparameters.And these provide the foundation of optimizing structure design of detector. 2.The key process of preparing LiTaO₃ thin film infrared detector device is thefabrication of the LiTaO₃ thin film.The big-area,smooth and crack-free LiTaO₃ thinfilm samples have been prepared on the different substrates by sol-gel methods.Thedifferent performances of the sol-gel LiTaO₃ thin films fabricated by different gel recipeof lithium tantalate are also given.The measurement results of crystalline orientation,surface morphology,dielectric and ferroelectric properties of the prepared sample areobtained.The influences of ITO,Pt,and TiN bottom electrodes on the crystal orientationand electrical properties of the sol-gel LiTaO₃ thin films are compared.3.The properties of preparing the lithium tantalate thin film device are decided bythe properties of leakage current and anti-breakdown ability of the lithium tantalate thinfilm.The good dielectric properties of lithium tantalate thin film are needed forpolarizing the sensing film of device.The theoretical analysis shows that,increasing theratio of Ta/Li in the lithium tantalate thin film is helpful to inhibition of the separationof Li,so as to decrease the leakage current of lithium tantalate thin film and improve theanti-breakdown ability when the film polarized.Novel lithium tantalate thin film on Ptsubstrate has been prepared by the sol-gel process and rapid thermal treatment using thenew gel recipe with the molar ratio of Li⁺:Ta⁵⁺=1:3.The compared LiTa₃O₈ ceramicpowder is prepared respectively by solid state reaction using the starting material ofTa₂O₅ and Li₂CO₃,and by sol-gel and sentering method using the staring material oflithium ethoxide and tantalum ethoxide.It is clear that,the main diffraction peak of thenovel prepared film on Pt substrate is match together with the prepared LiTa₃O₈ powder.Because the orientation of the prepared LiTa₃O₈ powder agrees with the orthogonalstrucure of LiTa₃O₈ powder sintered at 1330℃by Pouchard,the novel prepared film isverified the LiTa₃O₈ film and the novel prepared film also has orthogonal structure.Thedielectric,ferroelectric,leakage current and pyroelectric properites of the LiTaO₃ andLiTa₃O₈ films are measured.The pyroelectric coefficient is also measured by ourhome-made pyroelectric coefficient measurment system.The experimental results showthat,the remanent polarization of sol-gel LiTa₃O₈ films are larger than that of LiTaO₃films,and the leakage current of LiTa₃O₈ films are lower than that of the LiTaO₃ filmsprepared and measured under the same conditions.And anti-breakdown ability ofsol-gel LiTa₃O₈ films is higher than that of LiTaO₃ films.The pyroelectric coefficient ofLiTa₃O₈ films is about 14.07μC/m² K,smaller than that 122.80μC/m² K of the LiTaO₃ films.4.The new sol-gel derived LiTaO₃ and LiTa₃O₈ thin film infrared-detected unitdevice with silicon back etched suspended structure have been prepared respectively.And the thermal-electric response property of unit device has been measured either.Theprepared process of unit device based on MEMS and sol-gel technology is presented indetails.The thermal-electric response test system of device is constituted and the signalvoltage and noise voltage of the LiTaO₃ and LiTa₃O₈ thin film detectors are measured atthe frequency range of 10～1000Hz.The voltage response Rv,noise equivalent powerNEP and specific detecivity D^* of these detectors are calculated based on the measuredresults.To sol-gel LiTaO₃ thin film infrared detectors with silicon back etchedsuspended substrate,the maximal Rv of 6.30×10³ V/W and the maximal D^* of8.86×10⁷ cmHz^1/2/W have been measured and calculated at the measurement frequencyof 100Hz.The measured errors of Rv and D^* of the LiTaO₃ thin film detectors are 5%and 13% respectively.To sol-gel LiTa₃O₈ thin film infrared detectors with the samedevice structure and prepared process as the LiTaO₃ thin film detectors,the Rv and D^*of the LiTa₃O₈ thin film detectors are 4.99×10³ V/W and 1.95×10⁵cmHz^1/2/Wrespectively measured at 100Hz.The reasons of lower specific detectivity D^* of thesol-gel derived LiTa₃O₈ thin film infrared detectors are given in this paper.