Measurements of the ion species generated in the planar type neutral loop discharge of tetrafluoroethyl trifluoromethyl ether (CF3–O–CHF–CF3;HFE 227) demonstrate the presence of CFx+ species and CFx radicals, in a comparable manner to plasmas of more established gases such as C4F8, C2F6 or CHF3. However, as the etch selectivity of boron phosphorus silica glass (BPSG) to resist is quite poor in HFE 227 plasmas, organosilane Si(CH3)xH4-x additive gases are investigated as a source of Si to improve the BPSG/resist selectivity. The addition of 10% trimethylsilane to HFE 227 is capable of providing selectivity close to 5, which is desirable for the fabrication of deep holes. It is shown that the HFE 227/trimethylsilane plasma chemistry also preserves the resist hole pattern and does not cause hole expansion during the etching process. These latter observations are attributed to the deposition of polymer precursors induced by the trimethylsilane addition which reduces the top resist surface etch rate and balances the removal rate of the resist sidewalls. As a result, 0.15 μm diam holes with an aspect ratio of 15 are successfully fabricated. Mass spectrometry measurements of fluorocarbon radicals and ionic species imply that the high aspect ratio feature can be fabricated by having mostly CF3+ ions a- - nd lower density of fluorocarbon radicals. Furthermore, the microloading-free etching of contact holes is achieved at a bias voltage of -500 V for diameters down to 0.15 μm holes. The limit of the etching performance of the HFE 227/10% trimethylsilane plasma etch chemistry is investigated using very fine patterns defined by electron beam lithography. Trenches of 20 nm in width and mesh holes of 70 nm can be fabricated. © 2000 American Vacuum Society.