An investigation the defocus behavior of an optical microscope used for high precision length metrology

The microscope of the new vacuum comparator of the PTB had to be improved, be-cause the defocus influence of its microscope on the line position led to increased measurement uncertainties of a few 10 nm. A test setup was built up to investigate and characterise the optical microscope. It consists of a bridge, on which the custom made microscope is mounted and a high precision flexure stage, which provided the required accurate motion of the sample used to characterise the microscope. After replacing the critical with an appropriate Köhler illumination, which was optimised using an optical design program, and improving the alignment of the optical compo-nents with respect to each other and the sample, an illumination homogeneity better than 3% over the whole field of view and a repeatability of the detection of the posi-tion of a line below 1 nm were achieved. Furthermore, the defocus influence on the position of a line remained smaller than 6 nm while the sample was moved from - 3 um to 3 um with respect to the focus position. A successful comparison of the defo-cus influence on linewidth measurements between our microscope and a commercial linewidth microscope was also performed.