Ruifang Fang

Design and simulation of illuminator with micro scanning slit array for NA 0.75 lithography system

An illuminator with a micro scanning slit array for NA 0.75 lithography system has been proposed in this paper. In this illuminator, the beam is shaped by a diffractive optical element (DOE), a zoom and axicon lenses. The beam is homogenized by a micro intergrator rod array. The micro scanning slit array is used to substitute for the traditional internal rema. A micro lens array and a condenser image the micro scanning slit array onto the mask. This illuminator needs no illumination lens group and reduces the effect of the vibration introduced by the internal rema. The illuminator is designed and simulated. The result shows that the illuminating field on the mask has good uniformity and trapezoidal distribution along the scanning direction. The feasibility of the illuminator is verified.

Design of zoom system for pupil shaping of lithography tool

For the projection lithography, off-axis illumination has become one important resolution enhancement technique, which can also increase the depth of focus. It is realized by the pupil shaping unit to change the partial coherence factor. The pupil shaping unit is composed of a zoom system, diffractive optical elements (DOE) and axicons. By changing the focal length of the zoom system, a continuously adjustable diameter of the pupil is achieved. For the zoom system, its front focal plane and the back focal plane must be fixed and the telecentricity should be maintained in the process of zoom. In this paper, a zoom system with a zoom ratio of eight is designed. The system comprises only five lenses. The first lens works as the front fixed group and the last lens works as the back fixed group. The second lens is the zoom group to achieve the desired focal length, while the third and fourth lenses constitute the compensatory group to compensate the image shift. To eliminate the aberrations and obtain a compact structure, these five lenses are aspherical lenses. By using the multi-configuration design of ZEMAX software, the system is optimized to have a high imaging quality. Under the working wavelength of 193.368nm, the modulation transfer function is close to the diffraction limit. With this zoom system, the pupil shaping unit is simulated using ASAP software. The simulation results show that this zoom system can meet the requirement of pupil shaping of the lithography tool.

Real-time Measurement of Full Field Retardation near Quarter Wavelength

A real-time method to measure full field retardation near quarter wavelength is proposed. The circularly polarized beam passes through a sample with a large aperture. The measuring beam then goes through a quarter-wave plate and is then split by a Wollaston prism. An image with two sub-images is then detected by a high-speed image sensor. The full field retardation near quarter wavelength can be obtained in real time by processing the image. The measured retardation is independent of the fast axis angle of the sample and the fluctuation of the initial intensity. In experiments, a wedge waveplate is measured with different fast axis angle and initial intensity, and the full field retardations are acquired. The maximum and standard deviation of the full field retardation is 1.5° and 0.4°. The validity of the method is verified.