Zirconium diffusion on tungsten (100) surface of Schottky electron sources

抄録

Introduction

Schottky electron sources are widely used as electron sources for high-resolution electron microscopes. Since electron emission is achieved by a combination of heat and electric field, tungsten, which has a high melting point, is used as the electron source material. On the other hand, because tungsten has a high work function of approximately 4.5 eV, its surface is coated with Zr-O to lower the work function. The Zr-O is supplied to the electron source by thermal diffusion from a ZrO₂ reservoir which is located several hundred micrometers away from the emission area. Although the coverage of Zr-O is closely related to the lowering of the work function, i.e., the performance of the electron source, the detailed diffusion mechanism is not understood. In this study, the diffusion state of Zr was investigated using AES.

Experimental

We investigated the diffusion of Zr and O at the electron emitting surface, W(100). A single crystal W(100) cut to 2×0.2×0.1 mm and ZrO₂ sintered in the middle was used as the measurement sample. The sample was heated in vacuum for more than 12 hours, and after quenching, the amount of Zr was measured using AES. The diffusion distance of Zr was investigated by measuring at different distances from the ZrO₂ sintered material. The heating temperature was varied around 1800 K, which is used for actual electron source, and in the range of 1600-2000 K to investigate the diffusion distance. The diffusion distance was also examined in terms of the dependence of the diffusion distance on the atmospheric vacuum by adjusting the sample by changing the vacuum level to 10^-8, 10^-7, and 10^-6 Pa.

Results and discussions

Figure 1(A) shows the measured AES spectrum. In the AES spectrum, signals from W of the base material and diffused Zr were observed. As the distance from the ZrO₂ reservoir increased, the Zr signal became weaker, and the W signal became stronger. Figure 1(B) shows a relationship between the distance from the ZrO₂ reservoir and the peak intensity ratio of Zr and W. The coverage is constant below 600 µm, but once the coverage becomes low, it is found to decrease rapidly. This behavior cannot be described by a simple equation consisting of a balance between diffusion and evaporation. One possible mechanism is that evaporation is suppressed in areas of high coverage and is stimulated when coverage decreases. Detailed mechanism will be reported in the presentation. The results of the heating temperature dependence showed that below 1800 K, there was no change in the coverage and diffusion was about 600 µm. However, as the temperature was increased to 1900K and 2000K, the coverage decreased immediately after the constant region disappeared. This behavior can be expressed by the diffusion-evaporation equation, and evaporation was dominant at these temperatures. The vacuum dependence of the Zr coverage was investigated, and it was found that the Zr diffusion was inhibited under poor vacuum conditions, as the coverage was no longer a constant region.

Summary

・We investigated Zr diffusion on W(100) surface by using AES.

・The diffusion of Zr cannot described simple equation based on diffusion-evaporation balance.

・The temperature and vacuum dependence of Zr coverage was also investigated. Diffusion of zirconia is inhibited at higher temperatures and poorer vacuum.

Reference

S. Matsunaga et al., J. Vac. Sci. Technol. B 39, 062806 (2021)