| With the rapid development of microelectronics industry,the demand for miniaturized,integrated,and high-performance electronic components is increasing.Perovskite-type(ABO3)ferroelectric thin films have become important material sources for these components due to their excellent non-volatile storage ferroelectricity,dielectric insulation,piezoelectricity and pyroelectricity.Piezoelectric cantilevers made from ferroelectric films are typically used as key components in multiple piezo-MEMS devices,including micro-sensors,micro-actuators and micro piezoelectric energy harvesters.Lead zirconate titanate(PZT)ferroelectric films are the most widely used perovskite ferroelectrics in scientific research and industrial application.PZT is an ideal material for micro-electro-mechanical system(MEMS)due to excellent piezoelectric properties.However,at present,the preparation temperature of PZT ferroelectric films on silicon substrate is relatively high(>500℃),which is incompatible with existing CMOS-Si processes.On the other hand,PZT ferroelectric films prepared at high temperatures currently contain many defects,leading to high leakage conductivity and low stability.In addition,their comprehensive electromechanical properties(such as high voltage combined with low dielectric)also need to be improved.Therefore,how to prepare PZT ferroelectric films with excellent piezoelectric and comprehensive electromechanical performances at lower temperatures,or at lower thermal budgets,has become an urgent research difficulty to be overcome.In the part of experimental research,lead zirconate titanate(PZT)ferroelectric films were prepared via multi-target RF magnetron sputtering deposition and rapid annealing technology at low temperature on crystalline Si substrates.We revealed the relationship between microstructure and piezoelectric properties by studying the orientation growth laws of the PZT films.The specific research contents of this thesis are as follows:(1)Pb(Zr0.53 Ti0.47)O3(PZT)films(～1μm-2μm)were prepared on LaNiO3 buffered(111)Pt/Ti/SiO2/(100)Si substrates,deposited by low-temperature(350℃)sputtering followed by a rapid thermal process(RTA).This(00l)-oriented perovskite PZT films have good overall electrical properties(ferroelectric,dielectric and piezoelectric),similar to the electrical properties of epitaxial PZT films,yet on a much reduced thermal budget.According to the microstructure and morphology analysis,this"two-step" method resulted in perovskite PZT films with a dense,fine-grain morphology and reduced strains,hence an optimal electrical performance intrinsic to the chemical composition was achieved.EDS and XPS analysis confirmed that the rapid annealing process can effectively remove the PbO impurity in the film to improve the electrical properties.Moreover,prototypical piezo-electric film cantilevers fabricated from the PZT/LaNiO3/Pt/Ti/SiO2/(100)Si heterostructure yielded a large transverse piezoelectric coefficient(e31,f)up to 15.7C/m2,together with a high coupling coefficient k2～0.23 for energy conversion efficiency.The PZT films showed high e31,f coefficients(～10C/m2)even when the RTA annealing time was reduced down to 2min.Such a piezoelectric performance is close to the highest ones reported in the literature from epitaxial or highly-oriented PZT films.These high quality PZT films open up many possibilities for the integration of piezoelectricity into Si-based micro-electromechanical systems(MEMS).(2)The "small-field" measurement was used to explore the nonlinear quantitative relationship between the intrinsic piezoelectric response of PZT ferroelectric films and the external electric field,which has been observed in the measured longitudinal piezoelectric responses.However,for transverse piezoelectric responses,the intrinsic piezoelectric response was hidden due to the limitation of "large-field" piezoelectric measurement methods.The response of transverse piezoelectric under different bias voltages corresponds to the evolution process of domain switching in this "small-field"measurement.In addition,Landau-Devonshire-type phenomenological thermodynamic theory was used to fit the obtained transverse piezoelectric coefficient e31,f(E),and it was verified that the piezoelectric response of poled PZT films with high(00l)orientation was basically derived from the intrinsic piezoelectric contributions,which also proved that the PZT ferroelectric films prepared by the "two-step" method had extremely high quality.(3)Highly(001)-oriented PZT thin films(～1.5μm)were prepared on Si(111)substrates via a multi-layer buffering technique,i.e.,through the use of a(111)Pt/Ti bilayer and a LaNiO3 buffer layer,at low temperature(350℃)combined with a 2-15 minute rapid annealing process.Integration of high-performance lead zirconate titanate(PZT)piezoelectric films onto Si(111)substrates is beneficial for the development of piezoelectric micro-electro-mechanical systems(piezo-MEMS)because of Si(111)’s isotropic mechanical properties and desirable etching characteristics.The influence of the competition mechanism between the disappearance of PbO impure phases and the increase of nanopores on the electrical properties of PZT ferroelectric films with the proliferation of annealing time was revealed in this work.A short RTA time of 2min proved to be sufficient to produce densely grained and high quality PZT films,and these films displayed excellent electrical and piezoelectric properties(e31,f～11.6C/m2).The high frequency loss of PZT films annealed for longer time was significantly reduced due to a significant reduction in internal defects.However,the disappearance of PbO impure phases created nanopores in the films,which reduced the piezoelectric and dielectric properties.These PZT films have great potential for the development of novel Si-based piezo-MEMS devices. |
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