Autofocusing of laser lithography through the crosshair projection method

Abstract

In laser direct writing lithography, there is not any image information from the sample surface, which makes it difficult to find the position of the focal plane. To overcome the problem, an autofocusing through the crosshair projection method is proposed in this work. The crosshair on the reticle is inserted into the lighting path and imaged onto the sample surface. The addition of the crosshair projection increases the image information from the sample surface, meeting the requirement for the image information in focusing and improving the focusing environment. Furthermore, this work presents what we believe to be a new division of the focusing curve based on the range of the perpendicular feature extracted from the crosshair projection during the focusing process. The perpendicular feature can be extracted from the crosshair projection in the focusing zone but not in the flat zone. Compared with the traditional division, this new division enables the use of the perpendicular feature to directly determine the zone of the current sample position and to find the focusing zone during the focusing process. This can completely filter out the interference of local fluctuations in the flat zone, greatly facilitating the sample focusing. The autofocusing process was designed based on this division, and experiments were carried out accordingly. The focusing accuracy is about 0.15 µm, which is in the range of the depth of focus of the optical system. The results show that the proposed method provides a good solution to achieve accurate focusing based on the crosshair projection image from the sample surface in laser lithography.