Method of fabricating a semiconductor device

A method of fabricating a semiconductor device, the method comprising:

• forming a parent pattern in a diffraction region and a first reflection region of a substrate;
• forming an upper thin film on the parent pattern and the substrate;
• forming a child pattern in the diffraction region and a second reflection region of the substrate;
• measuring a diffraction light from the parent and child patterns in the diffraction region to obtain an intensity difference curve of the diffraction light versus its wavelength;
• measuring a first reflection light from the parent pattern in the first reflection region to obtain a first reflectance versus its wavelength;
• measuring a second reflection light from the child pattern in the second reflection region to obtain a second reflectance versus its wavelength;
• and
• performing an overlay measurement process on the parent and child patterns in the diffraction region using a diffraction light that has a same wavelength as a peak of the intensity difference curve located near a first peak of the first reflectance or a second peak of the second reflectance, to obtain an overlay measurement value.