In this paper, the anisotropic etching process of Si(100) wafers in tetramethyl ammonium hydroxide (TMAH) solution with isopropyl alcohol (IPA) is investigated in detail. An inverted trapezoidal pattern is developed. A series of experiments are performed by changing TMAH concentration, IPA concentration, etching temperature and etching time. The structure of inverted trapezoidal patterns and roughness of the bottom surface are characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The results show that with TMAH concentration increases, the roughness of bottom surface will decrease. The addition of IPA into TMAH solution improves the morphology of the bottom surface significantly. Low temperature is beneficial to get a smooth bottom surface. Furthermore, etching time can change the bottom surface roughness. A model is proposed to explain the etching processes. The hillock area ratio of the bottom surface has the same tendency as the etching area ratio. Finally, smooth silicon inverted trapezoidal patterns are obtained for epitaxial growth of GaN-based light emitting diode (LED) devices.