Photolithography based on optical mask is widely used in academic research laboratories due to its low-cost, simple mechanism, and ability to pattern in micron sized features on a wafer scale area. Since the resolution is bound by diffraction limits of the light source, nano-scale patterning using photolithography requires short wavelength light source combined with sophisticated optical elements, adding complexity and cost. In this paper, a novel method of sub-wavelength patterning process using conventional i-line mercury lamp is introduced, without the use of such advanced optical tools. The method utilizes the re-entrant geometry of image reversal photoresist produced from the developing process, where a secondary mask is generated by isotropically depositing a metal layer to cover the re-entrant profile of the photoresist. Removing the photoresist by applying ultrasonic vibrations in acetone bath uniformly cracks the metal layer at the sidewalls of the re-entrant profile, exposing the substrate with a reduced feature size. The width of the initial mask pattern can be reduced down by 400 nm in a controlled manner, regardless of the original width choice. As a result, the method is shown to achieve sub-100 nm scale linear patterns compatible for both subsequent deposition process and dry etching process. Our approach is applicable to various shapes of the patterns, and can be used in electronic device fabrication requiring nanoscale lithography patterning, such as the gate fabrication of AlGaN/GaN high electron mobility transistor (HEMT).