In semiconductor dry etching processes, octafluorocyclobutane (c-C4F8) is widely used as an etching gas instead of conventional CF4, because the relatively high C/F ratio of C4F8 is believed to induce the formation of a C:F film on Si or SiN, which acts as an etching barrier, thus improving the selectivity. However, c-C4F8 is a PFC with a very high global-warming potential (GWP). So Shin team from Association of Super-advanced Electronics Technologies in Japan has investigated perfluoro-2-butene (l-C4F8) and Hexafluoropropylene (l-C3F6) as an alternative. Their results show that the oxide etching characteristics of linear chain gases will be more useful than of c-C4F8 as device structures become smaller. The results also show that the relatively high C/F ratio of C3F6 can induce the formation of a C:F film on Si or SiN, which acts as an etching barrier, thus improving the selectivity.

Applied materials inc. applied a patent that show a plasma etch process, particularly applicable to a self-aligned contact etch in a high-density plasma for selectively etching oxide over nitride, although selectivity to silicon is also achieved. In the process, a fluoropropane or a fluoropropylene is a principal etching gas in the presence of a substantial amount of an inactive gas such as argon. Good nitride selectivity has been achieved with hexafluoropropylene (C3F6), octafluoropropane (C3F8), heptafluoropropane (C3HF7), hexafluoropropane (C3H2F6). Difluoromethane (CH2F2) or other fluorocarbons may be combined with the above gases, particularly with C3F6 for optimum selectivity over other materials without the occurrence of etch stop in narrow contact holes and with a wide process window.

Taiyo Nippon Sanso Corporation applied a patent that showed a cleaning gas mixture for an apparatus and cleaning method. In the semiconductor processes, fluorocarbon gases which are short-lived in the atmosphere are easily polymerized, these gases cause a problem in that the etching rate generally decreases during cleaning of silicon oxide films, etc. Decreasing the etching rate causes problems in the removal times and in the removal efficiency of deposits inside the chamber in the cleaning process, and it is necessary to reduce the time so as to maintain the process efficiency. A cleaning gas of the present invention to achieve the object is a cleaning gas to remove deposits accumulated inside a semiconductor film forming device, and which includes a mixed gas containing a fluorocarbon gas, having at least one double bond per molecule, selected from the group comprising hexafluoropropylene(C3F6), octafluorocyclopentene (C5F8) and hexafluorobutadiene (C4F6); oxygen; and at least one selected from the group of nitrogen trifluoride, fluorine, nitrous oxide, nitrogen, and rare gases up to 10 % by volume based on the total gas volume.