A novel epitaxial growth and micromachining technology were used to form a thin single-crystal silicon diaphragm for micromechanical sensors. Merged epitaxial lateral overgrowth (MELO) of silicon and SiO2 etch-stop technology were successfully used to fabricate a diaphragm with a precise thickness. Its implementation to the formation of a large area thin diaphragm is demonstrated. The silicon epitaxial growth rate is the only controlling parameter to define the diaphragm thickness. An average growth uniformity of the MELO film across the three-inch wafers was determined to be less than 5%. However, the average percentage variation of the growth at the same position on the wafer, from wafer to wafer in a single run, was measured to be within 2%. Diaphragms of 9 ± 0.05 μm thick and more than 200 μm wide and 1000 μm long were successfully fabricated using this technique.