The role of nitrogen addition in C4F8/Ar plasma to modulate the plasma process from polymerization to etching

Abstract

Low-pressure C4F8-based plasmas are widely employed during semiconductor fabrication processes, including deposition or etching with various gas mixtures. Oxygen gas, which suppresses the deposited layer via oxygen atom etching and increases the atomic fluorine density, is often added to the plasma to regulate polymer thickness. However, oxygen atoms are known to potentially cause severe damage to the devices and deteriorate the polymer-based surfaces. In this work we investigated the effects of substituting nitrogen for oxygen in C4F8/Ar plasmas. The addition of nitrogen dramatically reduced the thickness of the deposited layer on the Si substrate from 194.3 nm to 3.4 nm. Cross-sectional views of the Si substrate show that the interaction between the plasma and surface was switched from polymerization to etching by the introduction of nitrogen, as the CF2 radical density in the plasma decreased from 1.45 × 1020 m−3 to 1.18 × 1020 m−3 while the F atom density increased from 0.16 × 1020 m−3 to 0.43 × 1020 m−3. Plasma spectroscopy and surface analysis revealed that, the nitrogen atoms in the plasma were capable of scavenging the fluorocarbon molecules involved in polymer deposition through the formation of CN molecules and release of F atoms.