Carbonyl sulfide (COS) was added to oxygen as the additive etch gas for etching of amorphous carbon layers (ACL), and its effect on the etching characteristics of ACLs as the etch mask for high aspect ratio contact SiO2 etching was investigated. When a 50 nm amorphous carbon hole was etched in a gas mixture of O2 + 5% COS, not only did the etch profile of the ACL change more anisotropically but also the top/bottom opening ratio of the etch profile was improved by about 37% compared to those etched without COS. The improved ACL etch characteristics were related to the sidewall passivation of the amorphous carbon hole by the carbon sulfide related layer during the etching of the ACL. The distortion of the amorphous carbon hole was also reduced by about 6% due to the uniform deposition of the carbon sulfide related layer on the sidewall of the amorphous carbon hole. This uniform deposition improved the etch profile and opening ratio of the amorphous carbon hole, ultimately resulting in the enhanced contact oxide etching characteristics as evidenced by 5% improvement in the contact oxide opening and 20% improvement in the mask etch selectivity during the etching of an oxide having 20:1 high aspect ratio.