Recenty, it has been found that ferroelectric Bi layered structure oxides such as Sr Bi2 Ta2 O9 has excellent high fatigue resistance. Their crystal lattices are orthorhombic and consist of two connected layers of TaO6 octahedra, which are perovskite layers,separated by Bi2O2 layers as shown in Fig.3. Fig. 4 shows material shows fatigue character of Sr Bi2 Ta2 O9 thin film which keep remanent polarization (Pr) after 10^12 cycles. We are studying various Bi layer structured compounds using an improved sol-gel method.
1. Introduction
In recent years, aimed material development for ferroelectric nonvolatile memories has been actively developed in each research institution, and among them, SrBi 2 Ta 2 O 9 (SBT) oxide ferroelectric thin film is attracted much attention because fatigue phenomenon against polarization reversal hardly occurs. Many examples of this SBT thin film are produced by a coating thermal decomposition method using a carboxylic acid salt in many cases. For this heat treatment, relatively high temperature heat treatment has been required in the silicon process of 800 ° C. Therefore, we have developed total alkoxide type solution and investigated low temperature formation of SBT thin film.
Furthermore, the relationship between the electric characteristics and the firing temperature was examined. Bismuth layered ferroelectric crystal (BLSF) particles were formed with increasing temperature of 700 ° C and 750 ° C, and the amount of polarization increased. It has already been reported that ferroelectric BLSF particles are formed on the whole surface of a thin film calcined at 800 degrees, indicating sufficient polarization. Therefore, in order to prepare a ferroelectric thin film, a crystallization process changing from a fluorite structure to a layered structure is important. In particular, focusing on the change in the firing temperature of the microcrystalline region which has not changed to BLSF, in spite of the start of formation of BLSF at 700 ° C., in order to obtain a BLSF single phase film. It was aimed to clarify the reason why firing is necessary.
2. Experimental method
After applying the whole alkoxide type solution, preliminary firing was carried out, and the thin film which was actually sintered at 650, 700, 750 degrees was examined. As the alkoxide, one having 4 or less carbon atoms was used. Sr (OC 2 H 4 OC 2 H 5) 2 was used as the Sr source. Bi (O - nC 4 H 9) 3 and Ta (OC 2 H 5) 5 were used for Bi source and Ta source for each liquid. The charging ratio was Sr 0.7 Bi 2.3 Ta 2 O 9. For the substrate, Pt (60 nm) / SiO 2 (100 nm) / Si (6-inch wafer) formed by a sputtering method was used. Coating was performed with a spin coater. The coated film was dried at 150 ° C. for 30 minutes and then pre-baked at 450 ° C. The above process was repeated 8 times to obtain a thin film with a thickness of 0.2 μm. After repeating the above steps, the main firing was carried out in an oxygen atmosphere for 1 hour. The firing temperature was 800, 750, 700, 650 ° C. Electrical characteristics were evaluated using Radiant RT66A after forming the upper Pt electrode by sputtering. For the crystal structure analysis, a transmission electron microscope (TEM; H-8100, Hitachi, 200 kV) was used. For composition analysis, energy dispersive X-ray microanalyzer (EDX) attached to TEM was used.
3. Results and Discussion
First, we studied compositional change of microcrystal part present in thin film calcined at 650 ℃ to 750 ℃ due to rise of firing temperature. Composition analysis revealed that the bismuth concentration in the microcrystalline region decreases at 700 and 750 ° C compared to the layered structure crystal. And the decrease in the amount of bismuth in the microcrystalline region becomes significant due to the rise in the firing temperature.
Regarding the change in the microcrystalline part, crystal structure analysis was carried out by electron beam diffraction of the crystal region. The figure shows the limited field diffraction pattern from the microcrystalline region of the thin film fired at 650, 700, 750 ° C. It can be confirmed that the ring at 650 ° C. shows the 111, 200, 220, 311 faces of the fluorite structure. Next, at 700 ° C., the diffraction pattern likewise shows a fluorite structure, and no remarkable change is observed in the crystal structure. Here, in the thin film fired at 750 degrees, a diffraction pattern from the surface of about 6.2 A appears newly inside the fluorite structure diffraction ring. Therefore, a new phase formed at 750 ° C could be identified as a pyrochlore phase. In other words, it was confirmed that the bismuth composition decreased in the microcrystalline region and the crystal structure was changed from the fluorite to the pyrochlore structure.
4. Summary
In the crystallization of SBT, in the calcined film of 750 degrees, the microcrystalline part changes from a fluorite structure to a part to a BLSF crystal, and the rest changes to a pyrochlore structure. Because this pyrochlore is difficult to change to BLSF, it can be considered that it could not be a BLSF single phase film at 750 ° C firing. This structural change is thought to be due to the formation of a low bismuth region by the diffusion of bismuth and the structure change. Therefore, in order to suppress the phase transition which changes from the fluorite structure to the pyrochlore structure, it is necessary to suppress the migration of bismuth.
Study on SBT thin film with reduction resistance focusing Behavior of metal bismuth
[Introduction]
In order to achieve high integration of ferroelectric memories, it is necessary to enhance the process resistance of ferroelectric thin films, particularly chemical stability. Particularly, in the passivation process during the silicon process, the heat treatment atmosphere in the reducing atmosphere is exposed, so degradation such as leakage of the ferroelectric or lowering of the residual polarization has been regarded as a problem. Research on improving hydrogen tolerance by PZT thin film by improving electrode has been reported, but the resistance to Pt electrode is low. Furthermore, it has been reported that SBT with excellent switching characteristics has lower hydrogen resistance than PZT. We have reported that reduction of leakage current and RTA compatibility can be improved by using proprietary sol / gel type alkoxide solution as SBT thin film forming solution. We believe that these process resistance improvements are due to the fact that the Bi - rich component at the grain boundary of the SBT crystal could prevent precipitation. For the SBT thin film which improved the reduction resistance by improving the solution, chemical state analysis was carried out, and quantitative analysis of the oxidation state of bismuth was carried out in particular.
【Experiment】 Alkoxide mixed solution (composition Sr 0.7 Bi 2.3 Ta 2 O 9) solution and sol -gel solution (composition Sr 0.9 Bi 2. 1 Ta 2 O 9) was used. SBT thin films were formed on Pt / SiO 2 / Si substrates, respectively, and subjected to furnace processing (FA) at 800 ° C. and rapid heating (RTA). The surface chemical state was quantitatively analyzed by X-ray photoelectron spectroscopy (XPS).
【Result】
For the sample used in this analysis, comparison was made between a mixed alkoxide sample which can not be measured for ferroelectricity and a sol-gel sample which can be measured after hydrogen sintering (400 ° C., 15 minutes) as a reduction resistance test.
1, Influence of composition ratio
The figure shows the respective hysteresis loops of mixed alkoxide type and sol-gel method SBT. In the sol-gel film, it is a low bismuth composition film in which the excess amount of bismuth charged is reduced, but it is not different from the bismuth excess added film so far and shows a sufficient Pr value. The film quality when the composition ratio is changed from Sr 0.7 Bi 2 .3 Ta 2 O 9 to Sr 0.9 Bi 2. 1 Ta 2 O 9 is compared. The figure is a surface SEM image of each of mixed alkoxide type and sol-gel method SBT. In the mixed alkoxide system, stone wall-like particles are formed on the entire surface and a BLSF single phase film is formed, but the particles are formed like stone walls.
On the other hand, even with the sol-gel method, although the fine particle part remains slightly, the BLSF particles are formed on almost the entire surface, and therefore, it is considered that a sufficient Pr value is shown.
The stoichiometric value of the SBT crystal is SrBi 2 Ta 2 O 9, but at present other research institutes are currently formed with a composition of Sr 0.7 Bi 2 .3 Ta 2 O 9 by using a carboxylate. At this time, it is reported that there is an effect of replacing the bismuth excess with Sr and increasing the residual polarization Pr. Or it is supplemented as a burned-out portion. However, in our system, burn-out of bismuth composition is not observed even at 800 ℃ calcined film.
No significant increase in Pr due to substitution with strontium has been observed. Therefore, we considered that the role of excessive bismuth is important to help the phase transition from fluorite crystals to BLSF crystals.
Studies on the phase transition from the previous fluolite to BLSF report that the change to BLSF is inhibited as it is thermally stable once the pyrochlore phase is formed. Since this pyrochlore bismuth deficient composition tends to be formed when locally generated, it is easy to form a pyrochlore by reducing the excess amount of bismuth in the charged amount, making it difficult to obtain a single phase film of BLSF is expected. In this study, we also succeeded in promoting the formation of BLSF with this low bismuth composition by using the sol-gel method for the purpose of increasing the sinterability to the state close to the BLSF single phase even in the low bismuth composition film .
2. Metal bismuth
We have reported that the peak of metallic bismuth is observed from chemical state analysis of mixed alkoxide film. The figure further shows the XPS chart before and after slight dry etching of the surface by argon etching. It is found that the metal bismuth is abruptly increased by Ar etching. At this time, it is considered that there is a possibility that particles whose bismuth covers the outside of the metallic bismuth may be present as the reason why only bismuth is easily reduced.
The figure is an XPS chart of the mixed alkoxide film fired at 650 ° C. The bismuth metal exists in the fluorite fine particle film fired at 650 ° C, and the tendency for bismuth to be readily reduced by Ar etching is remarkable.
We point out the existence of particles that are excited by electron beams such as fine particles in these microcrystals or voids inside the BLSF and vibrate. This easily oscillating particle is considered to be metal bismuth particles reduced by Ar sputtering.
This bismuth metal behaves as a function of bismuth excess particles precipitated in grain boundaries and microcrystalline regions.
XPS analysis of the SBT thin film surface with the mixed alkoxide solution showed a peak of the metal state of Bi, in particular. Furthermore, it is confirmed that bismuth-rich fine particles are precipitated at the grain boundary. That is, as a feature of a sample having a low reduction resistance, the presence of a grain boundary instability component and the result of a quantitative analysis of Bi (metal) detected in a thin film formed by using two kinds of forming solutions are shown in the figure . In the mixed solution of alkoxide, the proportion (Bi-metal / (metal + oxide)) of metallic bismuth in the total bismuth amount of both FA and RTA shows a high value of 3% or more. On the other hand, the amount of metallic bismuth is kept extremely low in both the FA and RTA treatments in the SBT film made of the sol-gel solution having reduction resistance. This is consistent with the decrease in the amount of precipitated Bi particles at the grain boundaries observed by EDX. From the above, it can be judged that stabilization of the SBT grain boundary is made by sol-gel solution. At this time, metal bismuth is suppressed by the film having reduction resistance, and it can be judged that the amount of metal bismuth has a large influence on reduction resistance.
Based on the studies so far, the reduction site easily precipitates bismuth-rich particles segregated at the grain boundary or the metal state bismuth fine particles are present, and if a certain amount or more of leakage current exists, the dielectric strength is We believe that there is a critical amount that causes extreme drops.
In order to reduce bismuth in this metal, the merit of sol-gel method which can mix even oxygen atom and metal atom atomic level from solution is utilized.