Investigation of focus variation microscopy immunity to vibrations

Optical metrology plays a vital role in a wide range of research fields and for inspection in manufacturing industries. At present, the market offers a variety of optical metrology instruments, such as interferometers, confocal microscopes, and focus variation (FV) instruments. Although interferometers have the highest precision among optical metrology instruments, they are very sensitive to vibrations/environmental disturbances. On the other hand, focus variation technology is widely recognised for its robustness to vibrations (compared to interferometers), but so far, there is no study in place investigating the vibration frequency and amplitude limits within which focus variation instruments operate optimally in the presence of vibrations. To our knowledge, this article is the first study that aims to estimate quantitatively the immunity of focus variation instruments to vibration. This paper presents theoretical simulations to investigate how vibrations affect the FV principle of evaluating the best-focused images to calculate surface topography. The simulations were verified with practical results from an experimental FV setup built in the lab, and the two results match very well. Afterwards, vibration experiments were performed using the state-of-the-art focus variation instrument, Alicona InfiniteFocus G5, to measure the surface roughness of the Microusurf 334 comparator from Rubert & Co. LTD under vibrations induced by the P-840.2 piezoelectric actuator from Physik Instrumente (PI). The experiments were performed at different frequencies by incrementally changing the vibration amplitudes, pre-planned as a function of the depth of field (DoF) of each magnification lens (10x to 100x). It is observed that the FV system generates 100 % “bad-data” when the vibration amplitude exceeds three times the DoF of the used objective lens at low frequencies (i.e. as early as 5 Hz).