Holography has a considerable advantage of retrieving three-dimensional information of an object from only one interference recording. However, twin images always appear in the reconstruction for the reason of symmetry. Especially, twin images significantly deteriorate the quality of the reconstructed information in on-axis configuration. A solution of the twin-image problem in a digital holographic microscope by using symmetry with quadrantal masks is suggested in this study. This method is effective to most of the measured area without any additional implements, and restrictions on sample or iterations, and is demonstrated by the simulation and experimental results. The ratio of the disturbed area by a twin-image to the total measured area is reduced to the value of 0.82% in a specific case.
A novel method is employed to eliminate the twin-images in the so called 'in-line' digital
holographic microscope. We could achieve digital holographic microscope which solve the
problems of overlapping of real and imaginary images and eliminating one of them by padding
and removing DC term by averaging method. The entire process needs only one digital
In recent, the analysis of the reason why CD uniformity error is produced on photomask has been performed and this is very important fact for the development of next generation mask. The analysis shows that current 6-inch 250 mil masks have systematic radial and side to side CD variation trend. In this paper, the reasons of these nonrandom errors of CD variation in mask making have been investigated by using some experimental methods. First, the verification of radial CD error has been performed with a viewpoint of chemical flow on mask in conjunction with spin process and dip process. The result shows that the radial error was largely dependent on blank mask rather than spin process effect. In that evidence, high temperature softbaking improved radial CD uniformity. Fundamentally, however, exposure contrast degradation is thought to be the major reason of the radial CD uniformity error. Secondly, side to side CD variation could be explained by chrome sputtering mechanism on blank mask.
Currently it is very popular to use off-axis illumination technique for higher resolution with wider depth of focus. However there are several problems in the technique, one of which is deterioration of image quality induced by the non- uniform effective source distribution. If the intensity distribution on the illumination aperture lacks of spatial symmetry, each diffraction order beam impinging on the wafer surface has angularly asymmetric distribution. This makes the optical system have pattern size dependent telecentricity error. For a simple line or grouped lines it gives rise to only the pattern displacement with defocus which can hardly be detected unless there are any reference. But the periodic island type patterns which have discrete features and multiple pitch components in one direction can be bent and deformed asymmetrically with defocus. Asymmetric imaging for island type patterns gives rise to also the pattern CD asymmetry with defocus. We present schematic explanation of the effects of non-uniform effective source and the simulation result. We also investigated the phenomena in a high density DRAM cell active layer of 460 nm minimum pitch and characterized it by various approach.
As pattern density is increased, it is difficult for current photolithography technology to meet the required resolution. Therefore, instead of the photolithography e-beam direct writing and x-ray lithography have been considered as next generation technology. However, due to the rapid development in photolithography, it will be employed in manufacturing the first generations of 1 or 4 Gb DRAM. As smaller design rules require tighter controllability of the mask CD uniformity, mask CD uniformity smaller than 20 nm (3 (sigma) ) will be required for next generation devices. This paper reports on evaluation of mask CD error using a raster scan type e-beam system. In our study, CD uniformity (global) is improved by the advanced blank mask and local CD uniformity requires consideration, in order to reach the 20 nm CD uniformity. In particular, the butting error is a major source of CD variation but it is difficult to monitor this in this real mask production. Thus, the MPP (multi phase printing), and voting method are an alternative plan for reducing the butting error. However, we found that the 2X MPP (4 pixel merged) produced some CD skews, between even address pattern and odd address pattern. The voting gave some improving in butting error, but at the expense of loss of writing time. To meet the needs of mask CD uniformity, not only the global CD variation but also the local CD variation requires consideration. These types of mask CD errors were studied herein.
Monte Carlo calculations, including secondary electron generation and development simulation, using a string algorithm have been carried out in order to estimate the process capabilities for a beam voltage of 50 keV with a rectangular shaped beam in electron beam lithography technology. The results for minimum resolution for the threshold energy density model and the development model were compared. A study of minimum resolution and process latitude with respect to the acceleration voltage, resist thickness, beam blur for commercial PBS resist was investigated. In addition, at 50 keV, the effects of (alpha) value, asymptotic slope at very high dose in the dissolution rate equation on minimum resolution and process latitude were examined. The results show that it is necessary to use resist with a higher (alpha) value, as well as a high acceleration voltage of 50 keV, in order to enhance process capabilities.