A semiconductor device is formed by a multi-step etching process that produces trench openings in a silicon substrate immediately adjacent transistor gate structures formed over the substrate surface. The multi-step etching process is a Br-based etching operation with one step including nitrogen and a further step deficient of nitrogen. The etching process does not attack the transistor structure and forms the openings. The openings are bounded by upper surfaces that extend downwardly from the substrate surface and are substantially vertical, and lower surfaces that bulge outwardly from the upper vertical sections and undercut the transistor structure. The openings may be filled with a suitable source/drain material to produce SSD transistors with desirable Idsat characteristics.