A miniature Fabry-Perot cavity consists of a 10 micron deep by 5 micron wide slit which is formed in a silica (SiO2) waveguide over silicon with the use of reactive ion etching. The multiple beam interference is due to successive reflections from both walls of the slit. The reflectance close to the critical angle exhibits a Finesse over 20. A model is developed to simulate the loss due to scattering from the etched wall roughness. The loss in amplitude is only 7% after each reflection. Thus, the etched walls are vertical and smooth and can be used as efficient mirrors in integrated-optical circuits and interferometers.