Etching monitor device

An etching monitor device which monitors etching depth, the etching depth being a level difference between a surface to be etched and a top surface of a substrate in a processed region of the substrate, the top surface of the substrate being covered with a mask, and the surface to be etched not being covered by the mask, the device comprising:

• a light source which generates measurement light having a predetermined wavelength band;
• a measurement optical system which projects said measurement light onto said processed region of the substrate, and acquires reflected interference light containing three interference component signals which are due respectively to optical path differences of reflected light between three sets of surfaces, the three interference component signals including a first interference component based on an optical path difference between light reflected off of a top surface of the mask and light reflected off of the top surface of the substrate, a second interference component based on an optical path difference between light reflected off of the top surface of the substrate and light reflected off of a surface to be etched, and a third interference component based on an optical path between the light reflected off of the surface to be etched and the light reflected off of the mask surface;
• a spectrometric unit which disperses a wavelength of said reflected interference light and detects a reflected interference spectrum;
• an optical path difference analysis unit which performs extraction of the interference component signals, performs wavelength-wavenumber conversion on said reflected interference spectrum, performs optical path difference analysis based on reverse Fourier transform thereon to determine an intensity distribution for each optical path difference, and computes optical path difference analysis data containing one or two composite interference peaks due to superposition of the second interference component and the third interference component;
• an uncorrected second component interference distance computation unit which extracts a composite interference peak, containing mainly the second interference component signal, as an uncorrected peak from the optical path difference analysis data, and computes the optical path difference of a peak location of the uncorrected peak as an uncorrected second interference distance;
• a mask thickness measurement unit which computes a mask thickness independently from the uncorrected second interference distance computed from reflected interference light containing the three interference component signals;
• a correction table unit which stores a computed relationship between the mask thickness of said mask and the amount of offset of optical path difference which is imparted by said third interference component to said uncorrected second interference distance a correction table;
• a second interference distance true value computation unit which computes the amount of offset of said optical path difference based on said mask thickness and said correction table, and computes a corrected second interference distance wherein said uncorrected second interference distance is corrected by the amount of offset of the optical path difference;
• and
• an etching depth computation unit which computes the etching depth based on said corrected second interference distance and said mask thickness.