Effect of Composition Ratio on Erbium Silicide Work Function on Different Morphology of Si(100) Surface Changed by Alkaline Etching

I. Introduction

In order to reduce source/drain parasitic resistance (R_SD) in LSIs, it is essential to reduce contact resistance (R_c) [1, 2]. As the Schottky barrier height (SBH) between silicide and silicon must be low to achieve low contact resistivity [3]. On the other hand, some different orientations of Si must appear in the three dimensional MISFETs [4]. We have already reported that the low R_c with low SBH is realized using erbium silicide (ErSi_x) for n⁺ silicon on Si(100) and also reported that the properties of ErSi_x on Si(100), (111), (551) and alkaline etched Si(100) surfaces, and the SBHs is smaller on Si(111) than the others [5, 6, 7]. In this paper, we investigated the influence of Si(100) surface morphology for the composition ratio of ErSi_x.

II. Experimental

ErSi_x was used for a low work function material and the 2.38%-tetra-methyl ammonium hydroxide (TMAH) was used for an alkaline solution in this study. The Si(100) surface was etched by the TMAH at room temperature. In order to protect as well as Er or ErSi_x from being oxidized, two unique processes were employed for the silicidation. They are the total N₂ ambient surface cleaning and transfer process and the in-situ W metal capping on Er before silicidation [8]. Silicon wafers were loaded into the N₂ sealed cleaning chamber after the TMAH etching and the total room temperature five step cleaning [9] and a chemical oxide was formed by the dipping in the O₃ dissolved ultra-pure water. After the removing the chemical oxide with a diluted HF (0.5wt %) solution, the wafers were transferred to the clustered sputtering and lamp anneal equipment in a N₂ ambient. Er was deposited by the sputtering, followed by the lamp annealing to form ErSi_x. The silicidation annealing condition was 600^oC for 2 min. Processes from HF dipping to annealing step were performed in N₂ambient. The bare Si surface and W metal/Er surface were not exposure to clean room air.

III. Result and Discussion

Table 1 shows the SBHs of the fabricated Schottky barrier diodes as a function of the TMAH etching time. The SBHs extracted using the thermionic emission theory [3]. Although the value of SBH is not changed for the 10 min TMAH etched surface compared with the initial Si surface, the value of SBH decrease for the 60 min TMAH etched surface. We have already reported that the formed ErSi_x films have different physical and electrical properties by the Si surface orientations [5]. It suggests that another orientations such as Si(111) are appeared by alkaline solution etching and then, the orientation changing affect the SBH value. Fig. 1(a) and (b) show the hard X-ray photoelectron spectroscopy (HXPES) Si 1s spectra arising from ErSi_x/Si(100) (111) and 60 min TMAH etched Si(100). Spectra are measured at photo electron take-off angle of 80^o. The quantity of Er including in the ErSi_x film is fixed by the Er deposition thickness. The peak height of Si 1s is normalized by the peak height of Er 3d_5/2 spectrum. The Si 1s peak height of Si(100) is larger than that of Si(111). The etched Si(100) decreases compared with Si(100) and reaches to Si(111). The peak height is considered to indicate the Si/Er ratio. It means the quantities of Si atoms in the ErSi_x films are different. These HXPES peak height changes are related to the SBHs changes. Then, Si has higher work function than Er. It is considered that the ErSi_x contains more Si atoms, as a result; the work function is higher. Fig. 1(c) shows HXPES Er 3d_5/2 spectra arising from ErSi_x as a function of formed on various Si surfaces. The spectra of around 1408 eV are changed. It is considered that the formed ErSi_xfilm structure is changed by Si surface orientations. These electrical and physical properties change suggests that the Si(111) facet of Si surface generated by the alkaline solution etching and the SBH value reduce to that of Si(111) from that of Si(100).

IV. Conclusion

We have investigated that the SBH changes by alkaline etching effect of Si(100) surface. From HXPES results the composition ratio of ErSi_x on etched Si(100) surface is different from that of Si(100) and reaches to that of Si(111). The Si surface morphology affects the ErSi_x properties. A controlling the Si surface morphology is the key parameter for reducing R_cof high performance MISFETs.

V. Acknowledgment

This work was supported by JSPS KAKENHI Grant Number 22000010. The HXPES measurement was performed at beam line (BL46XU) of Super Photon Ring 8 GeV (SPring-8) (Project No. 2013A1628 and 2013A1836).

Reference

[1] S. Thompson et al., VLSI symp. p.132, 1998. [2] T. Ohmi et al., IEEE Trans. Electron Devices, p. 1471 2007. [3] S. M. Sze, “Physics of Semiconductor Devices” 2^nd Edition, Wiley-Interscience, 1981. [4] N. Collaert, IEEE Elec. Dev. Lett., p.568, 2004. [5] R. Kuroda et al., IEDM Tech. Dig., p.580, 2010. [6] H. Tanaka et al., ECS Transacions, p.371, 2012. [7] H. Tanaka et al., ECS Transacions, p.349, 2013. [8] T. Kuroda et al., SSDM proceedings., p.994, 2009. [9] T. Ohmi et al., J. Electrochem. Soc., p2957, 1996