Etching Characteristics During Cleaning of Silicon Surfaces by NF 3 -added Hydrogen and Water-Vapor Plasma Downstream Tr
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ABSTRACT Hydrogen and water vapor plasma downstream treatment with the downstream injection of NF 3 removes native oxide from silicon surfaces. In contact hole cleaning, this dry processing is a promising alternative to HF wet treatment, which has a problem of expansion of hole diameter. We examined the etching characteristics of boron-phosphosilicate glass (B-PSG) films, as a typical oxide material for the side-walls of the contact holes and compared the etching depths of B-PSG and thermal oxide films. We found that the etching depths of B-PSG films fell to one third of those of thermal oxide films at lower treatment temperature. This result indicates that the expansion of contact hole diameters can be minimized. INTRODUCTION As ULSI devices shrink in size, the presence of native oxide on silicon surfaces increasingly affects qualities of thin films. For example, native oxide on contact areas increases contact resistance. Also, native oxide with thickness of 10 to 20 A degrades the electrical property of gate oxide films when those thickness reduced to less than 50 A. Therefore, native oxide must be removed from silicon surfaces prior to thin film growth. In addition, to prevent the silicon surfaces from being reoxidized after the native oxide removal, the silicon surface should be terminated by hydrogen. Conventionally, native oxide removal and hydrogen termination of the silicon surfaces are performed by hydrogen fluoride (HF) wet treatment. However, it is becoming more difficult to clean the silicon surfaces with finer patterns by the wet treatment because viscosity and surface tension of the solution prevent the solution from reaching the bottom of the finer patterns. Before the HF solution reaches the bottom of the patterns and removes the native oxide there, the oxide side-walls of the patterns tend to be etched. In contact hole cleaning, this results in the expansion of the hole diameter. To overcome the problem of wet treatment for contact hole cleaning, one alternative method is surface cleaning by dry processing. Another benefit of the dry processing is that, since the chamber for native oxide removal by the dry process is easily connected to multi-chamber equipment for subsequent thin film growth, it is possible to avoid exposure of the silicon surface after the native oxide removal to air which may cause reoxidation. Nishino et al. and Kusuki et al. reported the native oxide removal by dry processing using NF 3 + NH 3 and NF 3 + H 2 plasma downstream treatments, re167 Mat. Res. Soc. Symp. Proc. Vol. 477 01997 Materials Research Society
spectively.' ), 2) However, they also showed that fluorine remained on the silicon surfaces after the treatments. We have developed a non-damaging dry process for native oxide removal. We have reported that NF 3-added hydrogen and water vapor plasma downstream treat3 ment removed native oxide from silicon surfaces and formed hydrogen terminated surfaces. ) In this process, we obtained a large amount of hydrogen atoms in the downstream region of the plasma by adding wat
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