Fabrication Of Na_0.5Bi_0.5TiO₃ Ferroelectric Thin Films And Its Etching

As is well known, ferroelectric materials containing much lead can cause environmental pollution and damage people's health during manufacture and use. In order to protect our environment from lead pollution, the scientists all over the world are engaged in lead-free or low lead content piezoelectric and ferroelectric materials. Among all these materials, Na_0.5Bi_0.5TiO₃ (NBT) is one of the main researching materials in recent years, but focuses on its ceramics and singer crystal. Thin films of Na_0.5Bi_0.5TiO₃ haven't been reported. In present work, we have prepared Na_0.5Bi_0.5TiO₃ thin films and studied its physics and chemical properties.Recent studies show that Na_0.5Bi_0.5TiO₃ has good ferroelectric property, its remanent polarization Pr=38 μ C/cm², and dielectric constant ( ε r) is 240-400 at room temperature, the ferroelectric Curie point is 320 ℃. These properties of Na_0.5Bi_0.5TiO₃ are advantages for preparing ferroelectric integration devices. In addition, Na_0.5Bi_0.5TiO₃ materials have prominent piezoelectric properties. In the rhombohedral-tetragonal morphotropic phase boundaries, 0.94 Na_0.5Bi_0.5TiO₃-0.06BaTiO₃ has piezoelectric constant d₃₃=125(10^-12C/N), the electromechanical coupling factor k₃₃=55%. In addition, Na_0.5Bi_0.5TiO₃ has favorable pyroelectric property, its pyroelectric coefficient p=2.5 × 10^-4C · m^-2 · K⁽-1(, and it changes little between 7℃ and 103℃. This permits many possible applications in thermoelectricity devices at room temperature as well as infrared detector arrays. Moreover, the pyroelectric property of Na_0.5Bi_0.5TiO₃ is similar to that of PT and PZT. The frequency coefficient of Na_0.5Bi_0.5TiO₃ is Np=3200 Hz · m, this properties are propitious to making sound wave device and ultrasonic sensor.We are witnessing the coming age of research of ferroelectric materials in the past century. Major advances have made in the last decade in research related to the synthesis, characterization, and determination ofprocessing-microstructure-property relationships of ferroelectric thin films and their integration into functional heterostructures. The high dielectric permittivities of perovskite-type materials can be used in dynamic random access memories (DRAM), while the large values of switchable remnant polarization of ferroelectric materials are suitable for non-volatile ferroelectric random access memories (NVFRAM). The research performed during the last decade has focused on developing both the scientific bases for the synthesis and characterization of ferroelectric capacitor heterostructures and the applications to ferroelectric film-based memories. In this work, we study the properties of Nao.sBio.sTiOs thin films and Au/NBT/Si(VSi/Au architecture.The technological uses of oxide thin films are not limited to ferroelectric memories, but extend to a wide range of applications in discrete devices, covering the full range of their properties, including dielectric, ferroelectric, piezoelectric, electrostrictive, pyroelectric, optical, electro-optic and magnetic properties, as well as electronic conduction, ionic conduction and superconductivity. Applications include multilayer capacitors for memories, piezoelectric film-based microelectro-mechanical systems(MEMS), boundary layer capacitors, varistors, gas sensors, radiation detectors, temperature sensors, transducers, switches, shutters, MHD electrodes, fuel cell electrolytes, concentration cell electrolytes and electrolytes for high energy density batteries. These ceramics represent an important world market, which has been experiencing steady growth.However, as for ferroelectric film and their integrated device. A manufacturing process for producing ferroelectric thin film-based devices should at least include the following characteristics: (1) applicability of the processes to deposition of ferroelectric films and integration with metallic or conductive oxide electrode layers with different physical and chemical properties; (2) compatibility with integrated device processing, including production of as-deposited films with specific microstructures (perovskite, for example) on substrates at the lowest possible temperature; (3)production of device-compatible, highly oriented or polycrystalline films and heterostructure with specific properties (e.g.fatigue-free for a large number of polarizationswitching cycles, long polarization retention times, and no polarization imprint effects); (4)reproducibility of the deposition process and (5) simple and low cost deposition processes with capacity for high deposition rates.It has been mentioned above that many demands about ferroelectric thin films science and devices, and have a large global market. So, studies on Nao.5Bio.5Ti03 thin films are also according to the requests and demand mentioned above. Such as, optimization of the deposition parameters and conditions, fatigue-free for a large number of polarization switching cycles, polarization retention times, compatibility with integrated device, reproducibility of the deposition process, simple and low cost deposition processes, all these problems will be addressed in this thesis.At present, SrBi2Ta2O9 thin films have prominent ferroelectric memory and fatigue-free properties, and it has been commerized. It's annealing temperature is high to 750—850 °C, which is a disadvantage for preparing related devices. It is necessary for exploring a novel kind ferroelectric film materials or new themal method. This is also one aim of my work in this thesis.Exploration of Nao.sBio.sTiC^ thin films has been carried out. Such as, the preparation of precursor solution, films fabrication and the x-ray diffraction analysis; P-E hysteresis loops research, e -T curve and e -f curves measurements; polarization switching cycles studies; wet etching and rective ion etching on the thin film; the surface morphologies of the films examined by atomic force microscopy (AFM) and the structure and disfigurement of TiC>2 studied by using high distinguish electron microscope.In recent years, there are lots of repors on low-lead Nao.sBio.sTiOs materials reported. Nao.sBio.sTiOa material containing 5%Sr and 5%Pb can be used in detector devices, Nao.sBio.sTiOa material containing 6.5 %Sr and 6.5% Pb which has low dielectric constant and high electromechanical coupling factor is suitable for ultrasonic sensor. In addition, Pb doping can increase the maximuim dielectric constant of NacsBiosTiOs and make phase transition temperature move to low temperature. Nao.sBio.sTiOs materials containing 13 — 17% Pb lies in the rhombohedral-tetragonal morphotropic phase boundaries.Similarity, NBT containing 5—6% Ba is also in the rhombohedral-tetragonal morphotropic phase boundaries. A recent study shows that its piezoelectric properties change dramatically from rhombohedra to tetragonal, piezoelectric constant d33 increases heavily, the electromechanical coupling factor increases dramatically. The physical properity near rhombohedra and tetragonal of Nao.5Bio.5Ti03 is also our main issue of this work.Our job is focused on lead-free 0.94Nao.5Bi0.5Ti03-0.06BaTi03, Nao.5Bio.5Ti03, and lead-small amount 0.87Nao.5Bio.5Ti03-0.13PbTi03 0.79Nao.5Bio.5Ti03-0.21 PbTiO3.The innovative points are as follows,At frist, NBT thin films have been successfully prepared for the first time. The thermal annealing temperature of NBT films is 600 °C, which is advantage for preparing devices. Their application parameters are satisfied to the use demands, and is prior to SrBi2Ta209 film.Second, the plasm etching of the wet etching and plasm etching make preparation for producing devices v/ith films. Reactive ion etching of Bi2Ti207 thin films using a SF6, CHF3 and SF6/CHF3 gas mixture, respectively, were studied for the first time. Studies showed that in the condition of HF: NH4F: H2O =2:1.2:3 at 50°C, the etching effect of Bi2Ti207 is good. We also used chemical solution of HF: NH4F: H2O =2:1.2:3 etching Nao.sBio.sTiOs and Bi2Ti2C>7 and at 50°C, the etching effect is good. The dry and wet etchings are compatible with the technics of semiconductor. This offers a key step to the device.Third, The polarization switching cycles of 0.79Nao.5Bio.5Ti03-0.21PbTiO3 film are 2.5X109, its spontaneous polarization is ll^C/cm2, which meet the commerced needs.Fourth, We prepared a buffer layer of 5nm-SiO2 on the Si substrate, and fabricate Au/NBT/SiO2/Si structure. On this structure, we obtained polarization-type switching and it is the desired mode for memory device. The thickness of SiO2 is measured by transmission electron microscope. The memory window of 0.87Nao.5Bio.5Ti03- 0.13PbTiO3 film is above 3V in theC-V curve. The memory window becomes larger with the increase of Pb content, and the symmetry of window is improved. These indicate the MFIS structure of NBT has good memory effect, and is promising for making FEFET memory.Fifth, From the ï¿¡ -T curve of 0.87Nao.5Bio.5Ti03-0.13PbTi03 film, we cansee the first phase transition is lowed from 220°C to 130°C because of the doped Pb. It agrees with the result of Nao.sBio.sTiOa ceramic worked by S. Kuharuangrong.Except the above innovative points, our work is at the flowing application and research fields.The resistivities of Nao.sBio.sTiOs is above 1.30 X 1013 Q cm in the voltage of ±4V. The leakage current density of 0.87Nao.5Bio.5Ti03-0.13PbTi03 is— ft *5below 10 A/cm in the voltage of +5V. The thickness of the films are about 3000—7000A. The film of 6000 A can endure 15V, and exhibit good insulting property and low leakage current.We analyzed the differences between MOSD + dipping using Si substrate and MOSD +Spin coating on Pt substrate. The results indicate the annealing temperature of NBT films using MOSD + dipping is 600 °C, which is very important for preparing devices.The surface morphologies of the films have been examined by atomic force microscopy (AFM). We observed the growth step and table-board-growth step-wrinkle which agrees with the growth theory of film. The film of Bi2Ti2O7 has a distinctly preferential (111) orientation by the analyzing of AFM and XRD.From the surface morphologies of the films examined by atomic force microscopy, we found that after thermal annealing the uncrystal grain would be crystalized, the surface are smooth and neat, Which indicate the effect of rapid thermal annealing.The C-V curves of Bi4Ti3Oi2 doped Na have symmetrical rectangle and larger window. This aim is to improve the C-V curve of MFIS structure, is suitable for making FEFET memory.