As the pattern feature size shrinks to sub-quarter micron, line edge roughness (LER) of hard mask becomes critical in line-width control for pattern transfer. The patterns with LER affect both device characteristics and electrical properties. Contributions to LER of resist have been carefully studied. However, understanding of dependence of LER on dry etching gas chemistry is still in the very early stage. LER of oxide hard mask has been observed after being patterned with deep UV (DUV) resist on a magnetically enhanced reactive ion etching tool, even though no LER was seen on DUV resist before hard mask patterning. Results show that the roughness depends very much on etching gas chemistry. Introduction of O@sub 2@ into process will cause very serious edge roughness, while CF@sub 4@ gas can reduce its formation on the line edge. Experiments also show that LER is first formed on the side wall of the resist and bottom anti-reflective coating layer, then it is transferred onto hard mask. Extending etching time will increase LER. Different etching behavior of gas chemistry has also been observed on resist. Top-view from critical dimension scanning electron microscopy shows a rough surface formed on resist after being etched using plasma containing either O@sub 2@ or Ar, but a much rougher surface has been seen on that from O@sub 2@ plasma. On the other hand, a combination of both Ar and O@sub 2@ plasma produces a very unform and smooth resist surface. Different etching behavior of gas chemistry, O@sub 2@, Ar, and CF@sub 4@ in hard mask opening is very helpful for us to understand the formation of the line edge roughness.