The etched profile of the porous silica@footnote 1@ (produced in ULVAC Japan Ltd.) was obtained as an almost vertical structure. The etch rate was approximately 2 times higher than that of the SiO@sub 2@ film when linear saturated perfluoro carbon compounds were used,@footnote 2@ because the overall film density of the porous silica is low. However, in the case of C@sub 4@F@sub 8@ (Octafluorocyclobutane) plasma, the etch rate ratio to the SiO@sub 2@ was about 1.45. When C@sub 4@F@sub 6@ (CF@sub 2@=CFCF=CF@sub 2@: Hexafluorobutadiene) was used, the etch rate ratio was also very low (0.6). So the etch rate strongly depends on the gas structure, whereas the SiO@sub 2@ etch rate is almost constant. The Vpp of the RF bias was almost same for all kind of fluorocarbon gases. It was observed that CxFy@super +@ ions were main species in the C@sub 4@F@sub 8@ or C@sub 4@F@sub 6@ plasma (QMS) and a polymer film was formed on the etched surface (XPS). Therefore, it is considered that the fluorocarbon polymer formed in the pore suppresses the etch rate of the porous silica in the C@sub 4@F@sub 8@ or C@sub 4@F@sub 6@ plasmas. C@sub 3@F@sub 7@I (CF@sub 3@CFICF@sub 3@: 2-iodoheptafluoropropane) gas was examined. The etch rate selectivity of photo resist to porous silica went up about 50%, compared with C@sub 3@F@sub 8@. Negative F@super -@ ion in the C@sub 3@F@sub 7@I plasma was obtained as a very small peak, contrary in the C@sub 3@F@sub 8@ and other fluorocarbon plasmas. On the other hand, I@super -@ ion intensity was very strong. These results may imply that the iodine in the plasma plays some roles for etching. @FootnoteText@ @footnote 1@C. Tanaka and H. Murakami, Extended abstract (The 61st Autumn Meeting, 2000); The Japan Society of Applied Physics, 750 (4a-P4-27). @footnote 2@Y.Morikawa et al., J.Vac.Sci.Technol.A19(4), Jul/Aug (2001).