Highly selective etching of a silicon dioxide layer using a very thin physical-vapor-deposited amorphous carbon layer (PVD-ACL) was investigated in a dual-frequency superimposed capacitively coupled plasma etcher. The following process parameters of the C4F8/CH2F2/O-2/Ar plasmas were manipulated: CH2F2/(CH2F2+O-2) flow ratio, high frequency (HF) power (P-HF), and low frequency power (P-LF). A wide processing window existed to produce the ultrahigh etch selectivities of a SiO2 layer using the patterned PVD-ACL mask. The etch gas flow ratio played a critical role in determining the process window for ultrahigh silicon oxide/ ACL etch selectivity due to the disproportionate change in the degree of polymerization on the SiO2 and ACL surfaces. Etching of the ArF photoresist/bottom antireflective coating (BARC)/SiOx/ACL/silicon-oxide-stacked structure allows the use of a very thin PVD-ACL as an etch mask layer for the etching of high aspect ratio silicon dioxide patterns.