An opening such as a small-diameter via is formed in a semiconductor substrate such as a monocrystalline silicon chip or wafer by a high etch rate process which leaves the opening with a rough interior surface. A smoothing layer such as a polysilicon layer is applied over the interior surfaces of the openings. The smoothing layer presents a surface smoother than the original interior surface. An insulating layer is formed over the smoothing layer or formed from the smoothing layer, and a conductive element such as a metal is formed in the opening. In a variant, a glass-forming material such as BPSG is applied in the opening. The glass-forming material is reflowed to form a glassy insulating layer which presents a smooth surface. The interface between the metal conductive element and the insulating or glassy layer is smooth, which improves mechanical and electrical properties.