Paper PS-ThP25
Evaluation of Surface Chemical Bonding State and Surface Roughness of Chemical Dry Etched Si using NO and F₂ Gas Mixture

Thursday, November 1, 2012, 6:00 pm, Room Central Hall

Economical Si etching is required to fabricate through silicon via ( TSV ) integration architecture for multi-layered packaging, micro-electro-mechanical system (MEMS), and surface patterning of solar panels. Chemical dry etching techniques by highly reactive ClF₃ gas [Ibbotson et al. J. Appl. Phys. 56 (1984) 2939] has been reported but the contamination by Cl is problematic. In this study, we have been investigating a new and economical Si chemical dry etching technique using nitric oxide (NO) and molecular fluorine (F₂) gas mixture. Atomic fluorine (F) is generated by mixing these gases at room temperature by the reaction of F₂ + NO à FNO + F. Kinetic energy of F at 0.8 eV and the change in post-reaction bonding energies of Si, SiF_x (x = 1~4), Si-NO, Si-NOF, and HF were calculated by B3LYP/6-311+G(d) in Gaussian 09. Si etch rate, etch directionality, etch selectivity, surface chemical bonding states, and surface roughness were evaluated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy, and atomic force microscopy to elucidate the Si etch dynamics in molecular level.

A prototype etching reactor was fabricated using the quartz tube with the inner diameter of 7.5 mm and the length of 150 mm. Flow rate of gases introduced into this chamber was Ar/5%F₂ ~ 38.5 sccm, F₂ ~ 1.9 sccm and NO ~ 1.5-5 sccm while maintaining the constant pressure at 620 Pa. The corresponding flow rate ratio, NO/NO + F₂, was 0.44 ~ 0.72. Etching was performed for 15 ~ 600 s. Single crystal and poly-Si samples were prepared to determine the etch rate, which is calculated from the cross-sectional SEM images. Preliminary results show that the etch rate was increased from 0.2 ~ 1.8 m m/min at NO/NO + F₂ = 0.44 ~ 0.72. This measured etch rate was more than 100 times faster than the estimated value by F atom reaction with H terminated Si. The etch rate sharply increased with NO up to NO/NO + F₂ ~0.57 and became almost constant when NO/NO + F₂ > 0.57. The etched surface after the exposure to the atmosphere mainly consisted of SiO₂, indicating that the reaction between Si-F_x, Si-NO, Si-NOF and H₂O in the air may occur rapidly. The detail analysis of the change in surface chemical bonding state during and after etching is in progress. The etched surface became rough and scalloped Si was formed when NO flow rate was increased. The surface roughening may be due to the presence of Si-NO and/or Si-NOF bond prior to the formation of Si-F_x that would eventually form SiF₄. From these results, Si etching is initiated not only the presence of F but also the existence of F₂, NO, and FNO.