Combined optical and X-ray interferometry for high-precision dimensional metrology

The requirement for calibrating transducers having subnanometre displacemen t sensitivities stimulated the development of an instrument in which the di splacement is measured by a combination of optical and X-ray interferometry . The need to combine both types of interferometry arises from the fact tha t optical interferometry enables displacements corresponding to whole numbe rs of optical fringes to be measured very precisely, but subdivision of an optical fringe may give rise to errors that are significant at the subnanom etre level. The X-ray interferometer is used to subdivide the optical fring es. Traceability to the meter is achieved via traceable calibrations of the lattice parameter of silicon and of the laser frequency. Polarization encoding and phase modulation allow the optical interferometer to be precisely set on a specific position of the interference fringe-the null point setting. The null point settings in the interference fringe fiel d correspond to dark or bright hinges. Null measurement ensures maximum pos sible noise rejection. However, polarization encoding makes the interferome ter nonlinear, but all nonlinearity effects are effectively zero at the fri nge set point. The X-ray interferometer provides the means for linear subdi vision of optical fringes. Each X-ray fringe corresponds to a displacement that is equal to the lattice parameter of silicon, which is ca. 0.19 nm for the (220) lattice planes. For displacements up to 1 mu m the measurement u ncertainties at 95% confidence level are +/-30 pm, and for displacements up to 100 mu m and 1 mm the uncertainties are +/-35 and +/-170 pm, respective ly. Important features of the instrument, which is located at the National Phys ical Laboratory, are the silicon monolith interferometer that both diffract s X-rays and forms part of the optical interferometer, a totally reflecting parabolic collimator for enhancing the usable X-ray flux and the servo-con trol for the interferometers.