A method for opening a conformal layer at the bottom of a contact via on a substrate is described. The method includes providing a substrate having a first layer with a via pattern formed therein and a second layer conformally deposited on the first layer and within the via pattern to establish a contact via pattern characterized by an initial mid-critical dimension (CD). The method further includes etching through the second layer at the bottom of the contact via pattern to extend the contact via pattern through the second layer and form a contact via while retaining at least part of the second layer on the top surface of the first layer, the corner at the entrance to the via pattern, and the sidewalls of the via pattern, wherein the etching is performed by irradiating the substrate with a gas cluster ion beam (GCIB) according to a GCIB etching process.