Improvement of lateral resolution and reduction of batwings in vertical scanning white-light interferometry

The precise and fast acquisition of three-dimensional geometrical data of micro-components is mainly performed by two alternative measuring techniques, either vertical scanning white-light interferometry (SWLI) or confocal microscopy. Both are capable of measuring micro-structured surfaces with a very high precision while recording image sequences during a so-called depth scan. For most applications the axial resolution of these systems is sufficient. Though, in certain applications the lateral resolution of a measurement system is far more critical. In addition, there are several approaches to define the lateral resolution in 3D microscopy. In this paper, we discuss some physical approaches to improve lateral resolution and transfer characteristics in SWLI. As a contribution to the EC-funded project "NanoCMM" we developed a special kind of Linnik white-light interferometer, which provides a lateral resolution well below one micrometer even for working distances of more than 5 mm. The resolution enhancement was achieved by wavelength reduction, i.e. LEDs emitting in the blue and near UV range were used for illumination. We compare the results obtained from a silicon pitch standard based on different illumination sources with a conventional Mirau interferometer providing the same magnification. Finally, in another setup we show that the batwing effect can be successfully reduced by using a confocal aperture in the illumination path of the interferometer. The combination of these different modifications clearly improves SWLI measurement results in comparison with those performed by conventional white-light interferometers.