Precision 3D surface measurement of step-structures using mode-locked femtosecond pulses

Fast, precise 3-D measurement of step-structures fabricated on microelectronic products is essential for quality assurance of semiconductor, flat panel display and photovoltaic products. Optical interferometers have long been used, but not that wide-spread for step-structures due to their phase ambiguity or low spatial coherence. Femtosecond pulse lasers can provide novel possibilities to optical profilometry both in the time and the frequency domain. In the time domain, step-surfaces can be measured over wide area by exploiting low temporal but high spatial coherence of femtosecond pulses; in the frequency domain, multi-wavelength interferometry permits the absolute measurement over the discontinued surface profiles while maintaining the sub-wavelength measurement precision.