Selective etching of SiO2 over Si3N4 in a C5F8/O2/Ar plasma

The self-aligned contact (SAC) oxide etching is a key process in developing the next generation ultra large scale integrated (ULSI) device because transistor gate feature size is scaling down, and aspect ratio is becoming higher and higher. In this study, the SAC etching will be characterized by C5F8/O2/Ar plasmas as a function of C5F8/O2 gas mixing ratio, Ar flow rate, process pressure, and RF power. The oxide etch rate increases when C5F8 fractional flow rate increases from 38% to 62% and then abruptly drops at the C5F8 percentage above 62%. The selectivity of nitride to oxide and fluorocarbon polymer increased under the following conditions: high C5F8 flow rate, low Ar flow rate, high process pressure, and low RF power. However, when the ratio of O2 increases in the C5F8/O2 mixture, the amount of polymer decreases and the ability of contact etching increases. SAC patterned samples were characterized by top-down critical dimension scanning electron microscopy (CD-SEM) and transmission electron microscopy (TEM). To analyze the effect of C5F8/O2 gas mixing ratio, we investigated the chemical species in the gas phase with optical emission spectroscopy (OES). The components and thickness of fluorocarbon polymer on contact surface, bottom and sidewall were investigated with transmission electron microscopy-element mapping system (TEM-EELS) and X-ray photoelectron spectroscopy (XPS).