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 at least one of said first material and said second material; holding said substrate securely within said reduced-pressure environment; and performing a gas cluster ion beam (GCIB) etching process to remove at least a portion of said first material using a GCIB, said performing said GCIB etching process including: selecting target etch process metrics for said GCIB etching process, said target etch process metrics including an etch selectivity between said first material and said second material, and at least one of a surface roughness of said first material or a surface roughness of said second material; establishing a GCIB process condition including GCIB properties for said GCIB etching process to achieve said target etch process metrics, said establishing said GCIB process condition including setting a process composition, a flow rate of at least one constituent in said process composition, a beam acceleration potential, and at least one of a background gas pressure or a background gas flow rate for an increased pressure region through which said GCIB passes; forming said GCIB from a pressurized gas containing said process composition, said process composition including at least one etching gas; accelerating said GCIB through said reduced-pressure environment in accordance with said established GCIB process condition; and irradiating said GCIB onto at least a portion of said surface of said substrate to remove said at least a portion of said first material.