Isolated and dense patterns were formed at process layers from gate through to back-end on wafers using a 90 nm logic device process utilizing ArF lithography under various lithography conditions. Pattern placement errors (PPE) between AIM grating and BiB marks were characterized for line widths varying from 1000nm to 140nm. As pattern size was reduced, overlay discrepancies became larger, a tendency which was confirmed by optical simulation with simple coma aberration. Furthermore, incorporating such small patterns into conventional marks resulted in significant degradation in metrology performance while performance on small pattern segmented grating marks was excellent. Finally, the data also show good correlation between the grating mark and specialized design rule feature SEM
marks, with poorer correlation between conventional mark and SEM mark confirming that new grating mark significantly improves overlay metrology correlation with device patterns.
A simple and high sensitive focus monitoring has been developed utilizing an aperture in Cr film formed on backside of photomask. A special mask for focus monitoring is developed such that two mark patterns on the front side of the mask are irradiated by different illuminations. The different illuminations for the two marks are generated from usually used illumination with modulation by an aperture on the backside of the mask. In this work, two complementally halves of usually used illumination are effectively generated. Because illumination for each mark pattern on front side of the mask is strongly asymmetric in incident angle such that illumination beam impinged from only one side of the space, imaging of the large size mark pattern is carried out obliquely on the wafer. As a result, image is laterally shifted with focus. The direction of lateral image shift is opposite to that of another mark which is irradiated with illumination beams from opposite side of the space. Thus, the relative displacement between the two mark images may become a measure of focus. Because this focus monitor works under purely geometrical optics, focus monitoring of multiple steppers, which are working under different wavelength, can be performed with the same one photomask. In experiments, the two mark patterns, which are inner and outer box patterns, are printed with overlaying each other by double exposure with stepping of wafer stage. Then, mutual displacement of mark patterns is measured by a commercially available overlay measurement tool whose resolution is a few nm. Very high focus sensitivity (Δx/Δz) of ~0.9 is observed for NA=0.68 optics with strong annular illumination. Because of the high focus sensitivity and high resolution of overlay measurement, focus monitoring with very high resolution of a few nm can be achieved.
Simple focus monitoring method has been successfully developed by application of a special illumination aperture, which generates oblique illumination beam. By this method, very high sensitive focus monitoring has been achieved in a current stepper. In the stop of the illumination aperture, an opening is located at eccentric position near pupil edge. Then, illumination beam obliquely incidents to mark pattern on mask. Because of this configuration of illumination beam, imaging is carried out with oblique beams on wafer. As a result, imaging becomes non-telecentric. That is, image formed by this illumination laterally shifts almost proportional to focal deviation. To measure the lateral pattern shift, box-in-box mark is formed by double exposure. Inner box is formed by the oblique illumination in the first exposure and outer box is formed by conventional low coherent illumination in the second exposure overlaying inner box by stepping of wafer. Then, relative displacement of inner box to outer box is measured by commercially available overlay measurement system. Since sine of landing angle of imaging beams is approximately NA*sigma, which is over approximately 0.50 in a current stepper, the focus sensitivity, which is defined by a ratio of lateral pattern shift per unit defocus, may become approximately 0.50. Because resolution of lateral pattern shift is approximately 2 nm in current overlay measurement, the resolution of focus sensing becomes very high of approximately several nm.
For the convenience of practical use of phase shift focus monitor (PSFM), which has been developed by T. Brunner, imaging characteristics of PSFM are investigated under modified illumination by optical image calculations and printing experiments. Although the mechanism of pattern shift with focus offset under modified illumination is different from that for conventional high coherent illumination, sufficient sensitivity for precise focus monitoring is predicted by optical image calculations. Also, it is revealed that reduction of NA, i.e., localizing illumination at the peripheral part of pupil is effective to obtain higher sensitivity. By experiments, predicted characteristics are observed and similar sensitivity to that in conventional high coherent illumination is confirmed both for annular and quadrupole illuminations.
The comparison of organic and inorganic bottom antireflective coating (BARC) discussed, especially about resolution improvement considering dry etching characteristics of ARC. The target was 0.2micrometers gate layer with shallow trench isolation and KrF negative resist process was used. By evaluation of etching characteristics for hard mask, it was found that etching critical dimension (CD) shift from resist pattern was almost determined by neighboring space width. The CD shift of isolated line pattern was more than +50nm for organic ARC. When CD shift should be corrected accurately by mask correction, the problem occurred that depth of focus of resist pattern was extremely small to compensate such large CD shift. Inorganic ARC could reduce the CD shift of isolated line pattern by 15nm compared to organic ARC. Although CD-DOF for isolated line was deteriorated after etching in the case of organic ARC, that was maintained in the case of inorganic ARC. Organic ARC was superior for the resist line width control on substrate step and inorganic ARC was superior for line end shortening of resist pattern, while both ARCs showed nearly equal performance after etching. Considering the stage of post-etching, inorganic ARC improved resolution of 0.2micrometers isolated pattern by 20-30 nm and DOF with no worse line width control than organic ARC.
In this paper, the method to decrease lens aberration phenomena is presented. The performance of mask tone is compared experimentally and by simulation for actual conditions of 0.18um lithography. The stepper with some aberration was used with several modified illumination conditions. The effectiveness of negative tone lithography was confirmed experimentally. CD variations of sub-dense or isolated line patterns in intrafield are improved from about 30-40nm to 10-20nm for every illumination condition. For overlay, the placement error of isolate line patterns is improved from around 10nm to 5nm. By simulation, the phenomena are investigated for the same illumination conditions are experimental ones. It is proven that negative tone mask lithography is less sensitive to aberration rather than positive one, which corresponds to smaller intrafield CD and overlay variation errors. The reason why negative tone mask lithography is effective is investigated. From this study, it can be concluded that negative tone mask lithography is more robust for lens aberration phenomena than positive one for such pattern layer which consists mainly of sub-dense and isolated patterns as gate layer. In the viewpoint of production, with this method, even the stepper which has some aberration can get CD and overlay controllability to be suitable for 0.18um lithography.
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