Autofocusing of laser lithography through crosshair projection method

• Wei Wei, Jingsong Wei, Tianyu Gao, and Xiaozhong Xu
• received 03/05/2024; accepted 04/25/2024; posted 04/25/2024; Doc. ID 523160
• Abstract: In laser direct writing lithography, there is not any image information from samplesurface, which makes it difficult to find the position of the focal plane. To overcome theproblem, an autofocusing through crosshair projection method is proposed in this work. Thecrosshair 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 samplesurface, meeting the requirement for the image information in focusing and improving thefocusing environment. Furthermore, this work presents a new division of the focusing curvebased on the range of the perpendicular feature extracted from the crosshair projection duringthe focusing process. The perpendicular feature can be extracted from the crosshair projectionin the focusing zone but not in the flat zone. Compared with the traditional division, this newdivision enables the use of the perpendicular feature to directly determine the zone of thecurrent sample position and find the focusing zone during the focusing process. This cancompletely filter out the interference of local fluctuations in the flat zone, greatly facilitatingthe sample focusing. The autofocusing process was designed based on this division, andexperiments were carried out accordingly. The focusing accuracy is about 0.15 μm, which is inthe range of depth of focus of the optical system. The results show that the proposed methodprovides a good solution to achieve accurate focusing based on the crosshair projection imagefrom the sample surface in laser lithography