Diode laser interferometer for surface profilometry with nanometer resolution on lateral dimensions in the centimeter range

摘要

Conventional laser interferometers offers a nanometer resolution but their results are ambiguous if distance variations of more than half a wavelength occur between two measured points. This is a rather strong limitation for surface profilometry oil surfaces with steps larger than this value. By using multiple wavelengths the accessible range of unambiguousness can be extended to half the resulting synthetic wavelength. With three laser diodes emitting in the near infrared (780 nm and two diodes with ～825 nm) synthetic wavelengths of approx. 15 μm and 290 μm could be achieved. This allows calculating the phase of the optical wavelength unequivocal within 145 μm. A nanometer resolution was reached with a phase interpolation of 1/100 of the optical wavelength. The laser beams are coupled into an interferometer through a single monomode fibre and all interference signals are measured by one photo diode simultaneously. This leads to an easy alignment of the optical set-up and avoids the use of polarisation optics and retardation plates. The injection currents of the laser diodes are modulated with different frequencies around 1kHz. Using lock-in amplifiers the three interference signals are separated electronically. The high modulation frequencies allow a fast measuring rate of up to 10 kHz. The sample surface as one mirror of the interferometer is scanned by moving the sample with mechanical translation stages in x- and y-direction. These mechanical stages exhibit unwanted vertical movement of up to 250 nm on a travel of several millimeter. By combining the mechanical stages with a piezo driven stage this vertical movement can be corrected resulting in a nanometer resolution in z-direction over a lateral range of several centimeters.