Flattening Technique of (551) Silicon Surface Using Xe/H2 Plasma
Authors: Suwa Tomoyuki , Teramoto Akinobu , Sugawa Shigetoshi , Ohmi Tadahiro 
Published: February 2020

The interface microroughness between the insulator film and the silicon substrate is one of the critical issues to the gate insulator film reliability and the performance of MOSFETs [1,2]. The atomically flat surface of Si(100) has been obtained by annealing wafers at the temperature of 800 ºC or above [3-7]. The MOSFETs having atomically flat interface between the silicon substrate and the radical gate SiO2 film show higher performances than the MOSFETs having conventional interfaces [3,4,7]. Furthermore, the speed performance of LSI is enhanced by introducing the MOS transistor fabricated on (551) surface silicon substrate [8]. In order to utilize (551) surface silicon substrate, flattening process of (551) silicon surface is required. In this paper, flattening technique of (551) silicon surface are developed by using Xe/H2Plasma.
Boron–doped Cz Si(551) wafers (8-12 Ω·cm) were used for the flattening experiment. Before flattening these wafers, the native oxides were removed by 0.5% dilute HF (DHF) and the surface microroughness (Ra) were about 0.12 nm.
(I) We applied the flattening technique by the thermal annealing in the ultraclean Ar ambience at the atmospheric pressure. Figure 1 shows 3×3 μm2AFM images of silicon surface after 850 °C annealing for 3 hours. Ra on each terrace of Si(100) surface is 0.03~0.04 nm, which is AFM noise level, and step height has only mono atomic step of 0.135 nm in Fig.1(a). On the other hand, Ra of Si(551) surface is 0.640 nm and step height has a few atomic step in Fig.1(b). Therefore Si(551) surface is drastically roughened by the ultraclean Ar annealing which can flatten Si(100) surface. Figure 2 shows the annealing temperature dependence of Ra of Si(551) surface. With decreasing the annealing temperature from 850 °C to 600 °C, Ra of Si(551) surface is decreased. It is considered that Si atoms on Si(551) surface easily migrate at lower temperature than Si(100). However, atomically flat silicon surface cannot be observed on Si(551) yet.
(II) We performed Xe/H2 plasma process at low temperature less than 600 °C in order to flatten Si(551) surface. Figure 3 shows 3×3 μm2 AFM images before and after flattening Si(551) surface by using Xe/H2 plasma. Plasma process was carried out in a microwave-excited high-density Xe (90%)/H2 (10%) mixture plasma at a pressure of 266 Pa at 400 °C for 3 min. Ra of Si(551) surface was decreased by Xe/H2 plasma flattening. Here, Ra of Si(551) surface did not change by annealing in 100% H2 ambient at a pressure of 266 Pa at 400 °C for 10 min without plasma excitation. Therefore it is considered that the surface migration of Si atoms on Si(551) surface increasingly progresses because Si(551) surface became activated by Xe/H2plasma.
This work was supported by JSPS KAKENHI Grant Number 22000010.

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