| AVS 58th Annual International Symposium and Exhibition | |
| Plasma Science and Technology Division | Thursday Sessions |
| Session PS-ThM |
| Session: | Neutral Beam and Low Damage Processing |
| Presenter: | Asahiko Matsuda, Kyoto University, Japan |
| Authors: | A. Matsuda, Kyoto University, Japan Y. Nakakubo, Kyoto University, Japan Y. Takao, Kyoto University, Japan K. Eriguchi, Kyoto University, Japan K. Ono, Kyoto University, Japan |
| Correspondent: | Click to Email |
In etching processes during MOSFET fabrication, bombardment of high-energy ions from plasma creates damaged structures in layers near the substrate surface, characterized by defect sites in the crystalline structure such as vacancies, interstitials, displacements, and dangling bonds. The negative effects of the damage on device characteristics (such as drain current degradation) have become increasingly significant as devices scale down rapidly [1]. To realize low-damage process, an advanced damage evaluation technique is essential. Photoreflectance spectroscopy (PRS) is an optical technique capable of detecting damage with high sensitivity. The surface is perturbed with an electric field by a modulation laser beam, and the reflectivity of a probe beam is measured. PRS has been studied as an advanced technique for contactless damage evaluation.
In this study, we employed an improved technique, where the temperature of the sample is controlled with liquid N2 during PRS measurement. N-type Si (100) wafers were exposed to an inductively coupled plasma using argon gas, under various rf-bias powers to control the incident ion energies (Ei). At room temperature, the amplitude of the spectral peak decreased with the increase in Ei. This feature is explained in earlier literature that carriers trapped at defect sites lower the surface potential (Vs), resulting in a decrease of the amplitude [2]. Over Ei ≈ 400 eV, the peak became smaller than background fluctuation, rendering quantitative characterization impossible. When the sample was cooled to 90 K, the spectra were enhanced and the peak of the high-Ei sample was revealed. By fitting the spectra to a functional form [3] and modeling the parameters’ temperature dependences, we found that the amplitude enhancement is related to the temperature dependence of the spectral broadening parameter (Γ). We also found that, at a fixed temperature, Γ for damaged surfaces were larger than that of the control sample. These results show that the damage causes changes in Vs and Γ, but by controlling the sample temperature, we were able to lower Γ, which increased the amplitude and revealed the peak.
The PRS-based damage evaluation technique discussed here expands the range of the applicable plasma damage conditions. Furthermore, temperature dependences of the spectra and its parameters (e.g. Γ and/or optical band gap) give us an insight into the band structures of damaged Si. This PRS-based technique is expected to be potentially useful as a future in-situ monitoring technique.
[1] K. Eriguchi et al., IEDM Tech. Dig., 2008, p. 436
[2] H. Wada et al., J. Appl. Phys. 88, 2336 (2000)
[3] D. E. Aspnes, Surf. Sci. 37, 418 (1973)