Silicon wet etching with surfactant-added TMAH solution has been investigated to achieve 45° micromirrors on a silicon concave structure used for a hot-embossing mold. The concentrations of TMAH solution and etching temperatures are important etching parameters that affect the surface morphology of both the (100) bottom plane and (110) inclined side walls. In this study, the concentrations of 10, 20, and 25% were used in an etching process at temperatures of 60, 65, 70, and 75 °C. In terms of the optical performance of the fabricated micromirror, the 25% TMAH solution was most effective at achieving a flat and smooth 45° micromirror. A silicon mold etched with Triton-X-added TMAH solution at 65 °C was used for fabricating a polymer microstructure with a 45°-inclined micromirror. The results of the reflection experiment revealed that the reflectivity of the Al-deposited polymer mirror improved as the concentration of TMAH solution used for silicon mold etching increased.