Design Of Long Depth Of Focus Projection Objective For Digital Grayscale Lithography

The photolithography technique is the core of manufacturing and processingMEMS and MOEMS. The digital gray lithography can produce continuous reliefsurface with high efficiency. At the same time, the costly conventional mask was saved.That is why the digital gray lithography is becoming an important direction ofdevelopment of the micro-nanofabrication.The three-dimensional continuous relief surface of micro-nano device fabricationmethod is pursuing the long depth of focus of the optical exposure system instead of theultimate resolution. Optical lithography of thick film resists is asking for longer depth offocus. However, it is well known that lens with lager NA may resolve small features butthe resulting images will have smaller depths of focus. Therefore it is necessary todesign a lithography objective lens that with a high resolution as well as large depth offocus.In this paper, we start with a short introduction to electromagnetic theory and basicdiffraction theory. By discussing the scalar diffraction theory, the light distribution ofdiffraction limited imaging system was researched. Then a binary phase mask isintroduced to modulate the phase of the incident light. By expanding with the normalizepupil function, contrast ratio and resolution has also been taken account as part of acomprehensive assessment of image quality. The optimal pupil mask was establishedwith the aid of Maple software. Using the optical design software ZEMAX, wedesigned an objective lens with the binary phase mask customized for digitalmicromirror.Simulation results shows that2.5times larger depth of focus was realized and onlywith slight change of the focus spot radius along the image space. The resolution hasachieved simultaneously throughout the entire exposure field while maintaining highcapability, and the defocus tolerance is improved. The experimental results is inaccording with theoretical analysis, which indicate the feasibility of the design.