An improved scan mode in an electrostatic force microscope for surface profile measurement of micro-optics

A new scan mode referred to as the feed-forward controlled unidirectional scan mode is employed to improve the accuracy of surface profile measurement of micro-optics in a scanning electrostatic force microscope (SEFM) instead of the conventional feedback controlled bidirectional scan mode. Two kinds of error sources, the positioning deviation of the X-stage and the frequency shift detection delay due to the lock time of the PLL circuit, are analyzed theoretically. It is verified that large profile measurement error can be introduced by the error sources when steep structures are included in the surface profile. It is also demonstrated that the feed-forward controlled unidirectional scan mode can remove the influence of the positioning deviation error component. It can also reduce the influence of the frequency shift detection delay error component by shortening the lock time of the PLL circuit and increasing the bandwidth of the PLL circuit through a feed-forward control strategy of the scan traces with the PI controller of the Z-scanner turned off. Experiments of surface profile measurement are carried out on two diffraction grating samples with different grating structures. The feasibility of the new scan mode is confirmed by the experimental results.