ABSTRACTRIE is an important technique in obtaining anisotropie etch profile. This is a critical requirement for very deep etching which needs long etch duration. Among many factors which affect RIE characteristics in deep etching, the following are most concerned: (1) the etch mask: needs suitable plasma resistance without significant plasma attack for extended etch time; (2) Long time stability in etch rate and surface conditions for the sample; (3) Etch profile: should be anisotropie with tolerable undercut In this work, CCl2F2 and SiCl4 were used with CCl2F2 as the main etchant. Ar was used in the initial stage for sputtering away surface residues prior to actual etching was performed. Three types of etch masks were prepared and their performance such as plasma was investigated. Multilayer metal etch mask has very low etch rate in plasma and its etch rate selectivity is around 300. The etch selectivities of GaAs to Si3N4 and to photoresist are 35 and 23, respectively. Etch mask can be chosen depending on the thickness of etch mask and the required GaAs etch depth. The etch rate of GaAs was found significantly increased when metal mask was applied. While PR mask is easier for inducing surface coating.