Vibration insensitive interferometer using sinusoidal phase-modulation and feedback control

Abstract

It is easy to extract a signal proportional to a phase fluctuation from a sinusoidally phase-modulated interference signal. This fluctuation is caused by mechanical vibration or air turbulence. In a sinusoidal phase-modulating interferometer using a laser diode (LD) the phase fluctuation is reduced by changing the injection current of the LD with a feedback control system. This control is very useful to a Fizeau type interferometer for surface profile measurement of a large size object. IC wafers of 100 mm diameter are measured with an interferometer insensitive to mechanical vibration and air turbulence. The phase fluctuation with nearly constant amplitude which corresponds to 30nm in surface height is reduced by about 10%. However the reduction in the phase fluctuation is not performed well for an instantaneous and large phase fluctuation. In order to be insensitive to all kinds of phase fluctuations the feedback signal is always observed, so that the interference signal is captured when the amplitude of the feedback signal is less than a specified level during the capturing time of 0.53 s. Thus surface profiles of the IC wafers can be measured with a high repeatability of a few nm even when any kind of vibration exists.