at the mouth of each trench which further narrows the trench 39 s opening to between 60 nm and 70 nm in accordance with the present invention the wavelength of the incident light from source 5 is not selected to cause trenches 19 to act as waveguides for transmitting this light to the bases of the trenches the wavelength of the incident light is selected instead to be above the wavelength at which the trenches act as waveguides i e above the cutoff wavelength of the trenches for trenches 19 which are circular holes and rectangular trenches therefore the wavelength of the incident light is above respectively 1 71 times dimension a and 2 00 times dimension b fig4 in simulation studies the strength of the interferometric signal for semiconductor wafers having trenches with a critical dimension of 100 nm or less was found to be unexpectedly stronger for incident light at wavelengths substantially above the trenches 39 cutoff wavelength than at wavelengths below the trenches 39 cutoff wavelength this effect also was observed for trenches having a critical dimension substantially above 100 nm e g trenches having a critical dimension of approximately 200 nm a strong interferometric signal was observed from semiconductor wafers having trenches with a critical dimension of 200 nm or less using light having a wavelength of 470 nm or greater without limiting the invention to any particular scientific phenomenon for trenches having a critical dimension of 200 nm or less we believe this unexpected result is caused by the increased transparency of the semiconductor wafer to electromagnetic radiation at wavelengths above the trenches.