Optical method of measuring trench depth

A method for measuring a trench depth using optics, applicable to a substrate having a trench therein, wherein the substrate has a first surface, and the trench has a bottom surface to serve as a second surface as well as a depth, the method comprising steps of:

• measuring a total reflectance from the substrate using different wavelengths;
• measuring a first reflectance from the first surface using different wavelengths, wherein the total reflectance from the substrate is determined from a first actual reflectance from the first surface, a second actual reflectance from the second surface, and a scattering factor, wherein the first reflectance and a first area factor are directly proportional to the first actual reflectance, a second reflectance and a second area factor are directly proportional to the first actual reflectance, and the depth is determined from the second reflectance;
• and
• calculating the first area factor, the second area factor, the scattering factor, and the trench depth from results of the total reflectance and the corresponding first reflectance.