Tsutomu Atsuki

Preparation of Bi-Based Ferroelectric Thin Films by Sol-Gel Method

The effects of Bi/Ta and Sr/Ta mole ratios on ferroelectric properties of Srx Biy Ta2Oz [ SBIT(x/y/2.0); 0.7≤x≤1.0, 2.0≤y≤2.6] thin-film capacitors were investigated. The SBIT films were grown by the sol-gel method using a spin-on coating on Pt/Ta/SiO2/Si and Pt/Ti/SiO2/Si substrates. The films were annealed at 800° C for one hour in oxygen atmosphere. Remanent polarization (Pr) depended strongly on the Sr/Ta mole ratio, and increased with decrease of the Sr/Ta mole ratio. On the other hand, Pr was almost independent of the Bi/Ta mole ratio. Scanning electron microscopy (SEM) images show that crystal grains grew with decrease of the Sr/Ta mole ratio. Electron probe micro analyzer (EPMA) analysis revealed that Bi content in SBIT films increased with decrease in Sr composition from stoichiometry.

Surface morphology of lead-based thin films and their properties

Surface morphology of lead-zirconate-titanate [PZT(52/48)] thin films prepared by sol-gel processing on Pt/Ti/ SiO2/Si substrates was studied. When the atomic ratio [Pb/(Zr+Ti)] of PZT gel films was 1, rosette structure was observed in the films after annealing at 600° C for one hour. The crystal structure of the rosette was identified as perovskite and that of the other area was nonperovskite, by electron diffraction analysis. Lead deficiency in the non-perovskite phase caused by lead diffusion into the bottom electrode was detected by energy dispersive X-ray spectroscopy and Auger electron spectroscopy. In order to prepare PZT films with a smooth surface, the following four means were efficient: addition of lead excess, rapid thermal annealing (RTA), adoption of buffer layer, and preparation of crystal nucleus on the substrate.

Influence of buffer layers on microstructural and ferroelectric characteristics of sol-gel derived PbZrxTi1-xO3 thin films

Pb(Zr, Ti)O3 (PZT) films with very thin PbTiO3 (PT) and ( Ba0.5Sr0.5)TiO3 (BST) buffer layers inserted between the film and Pt/Ti/ SiO2/Si substrate were synthesized by the sol-gel technique, and effects of variation in thickness upon microstructural and ferroelectric characteristics were investigated. Insertion of buffer layers was found to exert marked effects. PT buffer layers with thicknesses as thin as 2–4 nm yield PZT films with dense, homogeneous microstructure and good ferroelectric properties such as large remanent polarization, small coercive field and low leakage current densities. On the other hand, use of (2–4)-nm-thick BST buffer layers yields PZT films with dense microstructure and good dielectric properties such as increased permittivity, small coercive field and good leakage current characteristics. However, fatigue behavior of PZT films was not improved through insertion of PT or BST buffer layer.

Preparation of Dielectric Thin Films from Photosensitive Sol-Gel Solution

Fine-patterned Bax Sr1-x TiO3 (BST) thin films were successfully fabricated from a photosensitive sol-gel solution by means of UV irradiation. This solution was prepared from barium acetate, strontium acetate, and titanium isopropoxide. BST gel films on substrates formed from the solution by spin-coating were irradiated with UV rays through a mask pattern and developed with a mixture of ethanol and water. The obtained patterns were a negative of the mask. The films were finally annealed for crystallization. The obtained films had perovskite structure and good properties.

Preparation of Pb(Zr,Ti)O3 films on Pt/Ti/Ta electrodes by sol-gel process

PbZr x Ti 1-x O 3 (PZT) films were prepared on Pt, Pt/Ti or Pt/Ti/Ta electrodes by the sol-gel process using rapid thermal annealing (RTA). In the case of Pt and Pt/Ti electrodes on poly-Si, thermal treatments of PZT films gave rise to interdiffusion between Pt and Si. This exerted an unfavorable influence on the preparation of PZT films with perovskite structure. The barrier effect of Ta films and the effect of Ti films on the crystallization of PZT films were investigated by X-ray diffraction (XRD) and Auger electron spectroscopy (AES). The electrical properties such as dielectric constants and P-E hysteresis curves were measured for PZT films with various Zr/Ti ratios on Pt/Ti/Ta/SiO 2 /Si(100) substrates

Evaluation of Self-Patterned PbZrxTi1-xO3 Thin Film from Photosensitive Solution

Micro-patterned PZT(20/80) films were fabricated on Pt/Ti/SiO2/Si substrate from a photosensitive solution by the sol-gel process. A coated film was exposed to UV rays (365 nm) at an intensity of 570 mJ/cm2 and developed with a 1:1 mixture of 2-methoxyethanol and isopropyl alcohol. The 170-nm-thick film with a smooth surface showed Pr of 31.9 µC/cm2 and Ec of 120.8 kV/cm. Films with smoother surface morphology and better electrical properties than conventional PZT films were obtained by this process.

Influence of Buffer Layers and Excess Pb/Zr+Ti Ratios on Fatigue Characteristics of Sol-Gel-Derived Pb(Zr, Ti)O3 Thin Films

Since fatigue of the Pb(Zr, Ti)O 3 (PZT) films is closely related to the movement of defects generated due to interdiffusion of atoms through the ferroelectric/electrode interface, studies on the effect of excess Pb content, defined as R=Pb/(Zr+Ti), in the PZT films or insertion of a PZT buffer layer with a large R or of a La-bearing PZT buffer layer between the films and electrode may be effective for diminution of fatigue. Another important aspect is lowering the annealing temperature as much as possible when making the films, so as to reduce interdiffusion. In the present work, annealing temperature for making sol-gel-derived ferroelectric films was fixed at 550°C, and fatigue tests of PZT films with and without buffer layers were carried out. Fatigue tests of the PZT(52/48) films with various values of R have shown that although polarization of the PZT(52/48) film increases with R, fatigue life does not significantly improve due to an increase in fatigue rate with increasing switching cycles. Insertion of the PbTiO 3 (PT) buffer layer, 4 nm thick, somewhat amplified the R dependence of ferroelectric properties of the films. Insertion of a 40-nm-thick PZT (120/52/48) buffer layer substantially increased (P r * -P r ) , the difference between switched and nonswitched polarizations, for the PZT(100-110/52/48) or PZT(110/70/30) film in early stages of the fatigue life profile ; however, an enhanced fatigue rate followed and eventually resulted in an insignificant improvement of fatigue life. AES depth profile analysis has revealed that diffusion of excess Pb in the buffer layer towards the surface removes or greatly reduces Pb deficiency which is typically generates in the vicinity of the surface of the films without buffer layers. The resulting reduction of the amount of the defects is thought to have caused substantial increase of (P r * -P r ) in early stages of the fatigue life profile. Insertion of double buffer layers of PZT(120/52/48) and PLZT(5/65/35), each 20 nm thick, produced a conspicuous bulge in the polarization fatigue life profile, although fatigue life was not much improved. The origin of the bulge was discussed.

Study on Pb-Based Ferroelectric Thin Films Prepared by Sol-Gel Method for Memory Application

Pb(Zr,Ti)O3 (PZT), PbTiO3 (PT), and (Pb,La)TiO3 (PLT) thin films were deposited by sol-gel method on Pt/IrO2 electrodes. Deposited films were annealed at 625–700° C by rapid thermal annealing (RTA). These prepared films were examined as to whether they were suitable for ferroelectric nonvolatile memory using a metal-ferroelectric-metal-insulator-semiconductor (MFMIS) structure. In the application to a MFMIS structure, widely used PZT films have some problems such as the cracking on SiO2 and too high a dielectric constant. In this study, we examined the ferroelectric thin films to solve these problems.

Crystallographic orientation control for ferroelectric SBT and SBTN thin films from photosensitive mod solutions

Abstract SBT and SBTN thin film capacitors on Pt/Ti/SiO2/Si have been fabricated from photosensitive MOD solutions with the composition Sr0.8Bi2.35Ta2−xNbx (x=0.0−0.8). Films prepared by a conventional spin-on and anneal process are essentially randomly oriented, while films that received UV radiation before annealing exhibit c-axis orientation. The degree of c-axis orientation is enhanced by short pyrolysis times. Ferroelectric properties were characterized by hysteresis and pulse measurements. A higher Qsw and lower V(90%) were measured for films with random orientation in comparison with films having the same composition but c-axis preferred oriented. The c-axis orientation can be controlled by substrate pre-annealing. UV irradiated SBT and SBTN films with random orientation can be prepared on the substrates that are pre-annealed at 650°C.