A free-space silicon one-dimensional photonic bandgap optical filter is designed and fabricated. A two-stage (110) wafer etching process is employed to form the extremely vertical, smooth, and high-aspect-ratio features that are essential for good optical properties. The (111) oriented planes of the wafer form <0.01 degrees off-vertical trenches that make up the Fabry-Perot filter. A simulation model is presented that analyzes the effect of verticality and predicts the measured spectrum well.