Yutaka Ichihara

Spatial coherence of KrF excimer lasers

The spatial coherence and the beam divergence at 248 nm of a KrF excimer laser were obtained experimentally. These results are in good agreement with the theoretical calculations based on a simple pulse-laser model and the van Cittert-Zernike theorem. The relation between the spatial coherence and the beam divergence was obtained theoretically and supported by experimental results. This expression is given as a function of the wavelength of the laser but includes no parameters related to the laser structure. It is shown that these theoretical results are applicable to various kinds of pulse laser.

Absolute measurement of a spherical surface using a point diffraction interferometer

Absolute measurement of spherical surface by use of point diffraction interferometer (PDI) has been studied both theoretically and experimentally. By the estimation of optical error, 10-3 (lambda) rms can be expected as absolute accuracy. Experimental results with high accuracy have been obtained.

Illumination System Of An Excimer Laser Stepper

An illumination system suitable for an excimer laser stepper has been investigated. Unnecessary interference pattern(speckel) is reduced effectively by scanning the laser beam. We report spatial coherence of the lasers with different spectral line width, illumination system of the stepper, appearance of the interference pattern,its spacing and contrust and their relation to the illumination system and to the coherence of the laser. Then we report reduction of this pattern together with a simple method to measure its contrast.

Excimer Laser Stepper for Sub-half Micron Lithography

An excimer laser stepper having a flat image field was developed. Using a frequency stabilized, narrow band KrF 248nm laser (FWHM≈3μm), this stepper can print uniform patterns under 0.5 μm over a 15 mm square field. When the excimer laser output power is 2W, illumination intensity is more than 40 mW/cm2. Illumination uniformity is within ±2.5% over the field and exposure energy density is controlled within the precision of ±1.596, using an integrator sensor and a pulse number controller. A chromatic projection lens is adjusted so finely that aberrations are removed. In order to get imaging stability, a lens controller is used. An alignment system is off-axis, using He-Ne laser light spots and diffraction light detectors. By employing the EGA (Enhanced Global Alignment) method, overlay accuracy better than 0.18μm was obtained. Because the alignment detection light is insensitive to the resist, any types of resist may be used. The developed system has all the basic functions of a stepper. It can be used to improve DUV process and to fabricate sub-half micron devices in a laboratory.

Evaluation of the no-disparity realistic image from a sense of presence and low fatigue

We evaluated an observer’s fatigue and a sense of presence, in observing the no-disparity realistic image (NDR image) [1-3]. NDR image is consisted of two images (right and left image). Right image is created by shifting all pixels in left image same amount. Consequently, there are no disparities in all area of NDR image. NDR image which is reconfigured the contents that contain image with large disparity might have a possibility that it suppress an observer’s fatigue and let him feel high presence. Subjects observed three conditions videos (stereoscopic, NDR and 2D). Subjects observed two videos in each condition. Each video was 30 minutes. There were scenes with large disparity (more than 5 degrees) in videos. Subjects responded SSQ (Simulator Sickness Questionnaire), VAS (Visual Analogue Scale) for fatigue and questionnaire on realism and were measured CFF (Critical Frequency Fusion), accommodation tremor, stereoscopic vision test, ocular position measurement and eye sight test. Results showed that NDR image let observer feel high presence and an observers fatigue was low. NDR image is effective, even if contents which contain large disparity are converted into NDR image.