Generating high DOF light by using tapered hollow tube in a lithography system

Abstract

In this paper, we proposed a tapered hollow tube which can produce a near diffraction-limit spot and focus the incident light in far-field region. From previous researches, the sub-wavelength annular aperture (SAA) made on metallic film generates a Bessel beam in far-field region. Also, the traditional tapered fiber has been widely used in near-field scanning optical microscope (NSOM) to achieve super-resolution in near-field. Combining these two concepts, tapered hollow tube was shown to have great potential in creating a small sub-micron spot size and long depth of focus (DOF) emitted light beam. By using the commercially available capillary and fiber heat-pulling method, it was found that tube processed per design to be disclosed in this paper can achieve Bessel beam as well. It will be shown that the SAA-like structure was actually implemented by the geometry of the tube tip. From FDTD simulation and experiment, the emitted beam was identified to have more than 10 μm DOF and 250-300 nm focal spot excited by using the 408 nm laser source. These results not only can help us pursue lithography applied to create through silicon via (TSV) process in far-field region while maintaining near diffraction-limit spot size. The high throughput and side lobe became a serious problem when continuous incident light was used. To circumvent this problem, the incident light from was changed from continuous to pulse type and a suitable lithography experimental system designed by using three-axis displacement platform was developed. All results will be detailed in this paper.