Realization and performance evaluation of high speed autofocusing for direct laser lithography

Abstract

The autofocusing is one of the important parts in the automated vision inspection or measurement using optical microscopes. Moreover, laser micromachining or laser lithography requires a high speed and precision autofocusing. In this paper, we propose and realize an autofocusing system using two cylindrical lenses, which is the enhanced version of the previous astigmatism method. It shows very good performances, especially very high speed and the largest range in comparison with the previous astigmatic methods. The performance of our autofocusing system was evaluated by tracing the linear stage whose position was monitored by a commercial laser interferometer. Then we applied the autofocusing to the direct laser lithographic system, and successfully fabricated circular symmetry patterns on a 300 mm diameter surface with the resolution of less than 1 microm within the defocusing range of +/-50 microm. The speed of the autofocusing was 150 Hz.

Precise autofocusing microscope with rapid response

“…By contrast, this kind of microscope uses a sensor to measure the distance between the lens and the sample, to correct for changes in the focal plane due to external factors such as vibrations, thermal drift, and so forth [15]. It has a more rapid response and achieves a higher focusing accuracy, therefore it is popularly used in many automated optical inspection and machining applications with a realtime requirement throughout industry [16][17][18][19][20][21][22].…”

Studying the impact of depth of focus on 3D profile of negative photoresist material: a simulation approach

“…Effective lithography system requires a precise laser focusing and a complete crosslinking of the photoresist [3]. Precise focusing enhances the performance and the resolution needed for the fabrication [4]. Working at the optimum depth of focus (DOF) is essential to realize better structural qualities [5].…”

A High Precision Laser-Based Autofocus Method Using Biased Image Plane for Microscopy

“…Distinguished from the image-based methods, opticbased autofocus methods adopt auxiliary optical paths for defocus detection, which have also been widely researched [6][7][8][9][10][11]. Hsu's team developed a laser-based autofocus system that had fast response characteristics [6].…”

“…The autofocus microscope system embedded an astigmatic lens to convert focus error signal to differential laser intensity on a fourquadrant photodiode. Rhee et al improved the astigmatism method using two cylindrical lenses to enhance the astigmatic imaging and a circuitry controller for larger detection range and higher speed performance [7]. Laser-based method is less sensitive to contents in the field of view, but requires high reflective surface, which means it is more suitable for industrial inspections, such as glass surface and TFT-LCD inspection.…”