1. A method for etching material on a substrate, comprising: maintaining a reduced-pressure environment around a substrate holder for holding a substrate having a first material, a second material, and a surface exposing said first material and/or said second material; holding said substrate securely within said reduced-pressure environment; selecting one or more target etch process metrics, said target etch process metrics including an etch rate of said first material, an etch rate of said second material, an etch selectivity between said first material and said second material, a surface roughness of said first material, a surface roughness of said second material, an etch profile of said first material, and an etch profile of said second material; forming a gas cluster ion beam (GCIB) from a pressurized gas containing at least one etching gas; setting one or more GCIB properties of a GCIB process condition for said GCIB to achieve said one or more target etch process metrics; accelerating said GCIB through said reduced-pressure environment; and irradiating said GCIB onto at least a portion of said surface of said substrate to etch at least one of said first material and said second material.