Abstract
The projection lens is the core component of DMD-based maskless lithography and its imaging quality directly affects the transferal of exposure pattern. Based on the traditional projection lens system, we have designed diffractive optical element (DOE) and aspheric surfaces to optimize the refractive/diffractive hybrid projection lens system to improve its imaging quality. We found that the best effect is obtained when DOE is very close to the front lens group before the diaphragm of the hybrid system. Compared with the traditional projection lens system, this hybrid projection lens system has lower wave aberration with the help of DOE, and higher image quality owing to the modulation transfer function (MTF) value being improved. Finally, a hybrid projection lens system with working distance of 29.07 mm, image Space NA of 0.45, and total length of 196.97 mm is designed. We found that the maximum distortion and field curvature are 1.36 × 10−5% and 0.91 μm, respectively.
Highlights
The projection lens system is the key subsystem of digital micromirror device (DMD) based lithography, which directly determines the imaging quality
The actual optical transfer function (OTF) of the hybrid optical system can be obtained by calculating the product of the diffraction efficiency of diffractive optical element (DOE) and the modulation transfer function (MTF) curve given by the design software [33]
For a DMD-based maskless lithography projection optical system, we propose a refractive/diffractive hybrid projection system by using a singlealayer
Summary
The exposure light was reflected by DMD, which is controlled by a computer to generate a virtual mask in real time. Light modulated by DMD is imaged by the optical projection lens system into a reduced image, which is projected to the corresponding imaging position of the wafer surface for exposure imaging. The projection lens system is the key subsystem of DMD based lithography, which directly determines the imaging quality. It is necessary to improve the imaging quality of the projection system, so as to improve the lithography performance. The m’th order diffraction efficiency of the DOE in the oblique situation can be expressed as: φ(λ, θ ). 2π where sinc(x) = sin(πx)/πx, and m is the diffraction order
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