Shuji Nakao

Proposal for pattern layout rule in application of alternating phase-shift mask

An important pattern layout rule in application of alternating phase shift mask (PSM) is proposed. The images of semi- randomly aligned patterns show poor characteristics in defocus and mask fidelity. For an example, lines and spaces patterns with uniform bright and non-uniform dark widths showed significantly large CD variation with defocus. And lines and spaces patterns with uniform dark and non-uniform bright widths showed completely asymmetrical CD-focus characteristics. It has been revealed by the comparison between experimental data and the simulated results that the asymmetrical characteristics are caused by the spherical aberration in projection optics.

Extension of KrF lithography to sub-50-nm pattern formation

Sub-50 nm isolated line pattern is successfully formed by KrF lithography with DOF larger than 0.5 micrometers . This is performed by using a phase edge type phase shift mask, a special photo resist and a partial dry ashing process. Because all of these elemental techniques currently becomes mature, this method is one of promising candidates for sub- 50 nm isolated line pattern formation. As a conclusion, we consider KrF lithography can be extended to sub-50 nm high speed logic node.

0.10 µm Dense Hole Pattern Formation by Double Exposure Utilizing Alternating Phase Shift Mask Using KrF Excimer Laser as Exposure Light

Dense 0.10 µm hole pattern formation is achieved by optical lithography with a KrF excimer laser. A Double exposure utilizing two alternating phase shift masks (PSMs) of the line-and-space (L/S) pattern laid out in different directions produces a dense and small hole image in bright field with large focus and exposure latitude. Applying this method with a KrF excimer laser stepper and a chemically amplified negative-tone resist, a two-dimensional (2-D) 0.10 µm hole array with 0.40 µm pitch is resolved with a 0.6 µm depth of focus (DOF). The hole diameter and pitch in the resolution limit seem to be less than 0.10 µm and 0.28 µm, respectively. Also, 2-D hole arrays with different pitches in the x and y directions are easily formed using masks with different pitches in each direction. The hole pattern of an actual DRAM cell is successfully formed by this method. Because of the excellent patterning performance, this method will enable the fabrication of multi giga bit DRAMs by KrF excimer laser lithography.

Measurement of Spherical Aberration Utilizing an Alternating Phase Shift Mask

A spherical aberration function is measured utilizing an alternating phase shift mask (PSM). First, it is theoretically considered that with a highly coherent illumination, the focus of the alternating phase shift lines and spaces (L/S) pattern is equivalent to that of rays in a small annular area of the reference sphere when the aberration function is axially symmetric in the sphere. Then, the focus variation with the pattern pitch is obtained by a printing experiment that employs an alternating PSM. After a primitive mathematical operation, the aberration function is obtained from the experimentally observed focus variation. The aberration function is examined by investigating various anomalous phenomena observed in the printing with the various resolution enhancement techniques (RETs). The comparison between the experimentally observed phenomena and the predictions based on the calculated images with the measured aberration functions shows fair agreement. Consequently, the validity of this measurement method is confirmed.

Impact of Spherical Aberrations on Printing Characteristics of Irregularly Aligned Patterns of Alternating Phase Shift Mask

The printing characteristics of irregularly aligned patterns with alternating phase shift masks (PSMs) and the impact of spherical aberration on these characteristics are investigated by experiments and calculations of optical images. In experiments, very poor printing characteristics are observed for particular irregularly aligned patterns, even if there is no phase contradiction in the layout of the patterns. These patterns show small depth of focus (DOF) and/or poor mask fidelity. The phenomena are analyzed by beam configurations in interference and calculated optical images. For some patterns, the defocus characteristics are essentially poor even in non-aberrated optics. For some other patterns, the spherical aberration degrades the characteristics significantly, while non-aberrated characteristics are not so poor. In the application of alternating PSMs for device pattern formation, it is important to consider the pattern layout and lens aberration.

Innovative imaging of ultrafine line without using any strong RET

Sub-100 nm line pattern is easily formed with DOF larger than 0.9 micrometer by mature lithography technology in KrF wavelength. It is discovered by optical image calculations that a dark mask line between two bright mask lines with each dimensions of 0.20 to approximately 0.14 micrometer (measured on wafer scale) can be imaged with very fine width under a modified illumination. Also, at some conditions, iso-focal CD characteristics are observed for the very fine line image. The validity of this calculated characteristics is confirmed by experiments. The fine dark line pattern with the width finer than 100 nm is formed by the application of the image generated by this method. Moreover, the patterns formed by this method show high exposure latitude, low MEF and high immunity to lens aberration.

Simple and high sensitive focus monitoring utilizing an aperture on backside of photomask

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.

Random pattern formation by attenuated non-phase-shift assist pattern mask

A novel resolution enhancement technology (RET) for random pattern formation which utilizes attenuating non-phase-shift (Atten-NPS) assist pattern is proposed based on optical image calculation. By addition of Atten NPS assist pattern whose size is comparable to that of main pattern, much improvement of imaging characteristics is obtained for isolated feature under modified illumination. Modified illumination is optimized both for hole and line pattern. Also, transmission of Atten-NPS aperture is optimized to enhance imaging characteristics and not to be printed on resist. In the application of this RET, aperture size of assist pattern on mask can make similar to that of main pattern. Consequently, difficulty in mask fabrication for conventional assist pattern method , such as pattern delineation and defect inspection, will be overcome.

0.12 /spl mu/m hole pattern formation by KrF lithography for Giga bit DRAM

Dense 0.10 /spl mu/m hole pattern formation has been achieved by optical lithography with KrF wavelength. Double exposure utilizing two alternative phase shift masks of lines and spaces patterns produces dense small hole images with enough focus and exposure latitude. Applying this method with a KrF stepper and chemically-amplified negative-tone resist, a 2-dimensional 0.13 /spl mu/m hole array with a pitch of 0.40 /spl mu/m has been resolved with 1.0 /spl mu/m DOF, and resolution limit size and pitch are less than 0.10 /spl mu/m and 0.28 /spl mu/m, respectively.

Fast and accurate optical proximity correction based on aerial image simulation

Because optical lithography requires precise CD control, we developed a fast, accurate proximity correction method based on aerial image simulation. Simple formulas using a linear combination of simulated aerial image intensities both at and around mask edge were found effective for fast, precise CD prediction. Using the developed CD prediction and the fine biasing correction methods, we verified that various two-dimensional patterns printed by an i- line stepper using modified illumination and binary intensity mask are satisfactorily corrected; i.e., CD deviations from designed values, line shortening and feature deformations are effectively reduced.