This paper presents the development of a new trench fabrication technique for silicon substrate utilizing undercutting and selective etching. The technique comprises (1) first aluminum film deposition and its etching with controlled undercut, (2) second aluminum film deposition, (3) second film liftoff, and (4) silicon substrate selective and anisotropic etching using the first and second films as etching masks. There are two predominant features of this technique. First, trench width is controlled by the first film undercutting. Second, highly selective and anisotropic etching of the silicon substrate is obtained employing the aluminum films as etching masks in configuration with planar plasma etching using CCl2F2gas. Under optimum conditions, trenches of approximately 0.12 μm in width and 0.6 μm in depth are successfully fabricated.