We demonstrated the proof of principle for the fabrication of trenches with widths down to 0.1 mu m on a silicon substrate using photolithographically prefabricated starting mask structures having a spacing of 0.6 mu m. The corresponding trench width of 0.15 mu m was achieved using starting mask structures having a spacing of 1.0 mu m. 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.