We demonstrated the proof of principle for the fabrication of trenches with widths down to 0.1 μm on a silicon substrate using photolithographically prefabricated starting mask structures having a spacing of 0.6 μm. The corresponding trench width of 0.15 μm was achieved using starting mask structures having a spacing of 1.0 μm. Chemical‐beam deposition of metals was used to coat the starting mask structures and to narrow the aperture openings. Metal redeposition by ion‐beam sputtering directed perpendicular to the substrate further narrowed the spacing between the sidewalls of the mask structures. The subsequent chemically assisted ion‐beam etching of the underlying substrate through the constricted mask apertures produced the trenches with greatly reduced widths.