Coating techniques in optical interferometric metrology.

In optical interferometry diffuse reflectivity of the surface under study should be high and homogeneous. Application of a white reflective coating can strongly improve measurement results. The optical properties of bronze powder, TiO(2) powder, white Chinese ink, and MgO coatings are discussed. Measurements of reflected intensity distribution show that white Chinese ink and MgO have superior optical characteristics, and electron microscopy shows that these coatings cause thickness artifacts of less than 7.5 and 17 mum, respectively. The effect on deformation measurements is demonstrated by moiré topography on a thin membrane that is deformed under small static pressures. The membrane center displacement varies from 15 to 100 mum, and within a measuring precision of 2.5 mum no artifacts on this deformation are found.