Kouichirou Tsujita

Ar ion implantation into resist for etching resistance improvement

Argon (Ar) ion implantation into resist pattern was investigated and the remarkable improvement of the etching resistance was confirmed on various films such as tungsten, aluminum copper, silicon oxide and silicon nitride. The possibility to make resist thickness thinner was proven. Ion dose more than 1E15/cm2 was necessary to obtain a sufficient effect, so that shrinking of resist thickness and pattern width occurred simultaneously. The dependence of pattern shrinking on the line width was observed. Line width uniformity within a wafer was improved because of high etching resistance by ion implantation process. Line edge roughest (LER) or resist pattern was also reduced by ion implantation and smooth etched pattern could be formed. Although same effects were obtained for ArF resist, the shrinkage of ArF resist after ion implantation was more than that of KrF resists. The improvements in etching resistance and critical dimension (CD) control will be discussed in this article.

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.

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.

Dry etching resistance of resist base polymer and its improvement

The etching rates of resist base polymers with several molecular weights were measured against fluorocarbon or chlorine plasma. The rate showed a minimum value at the weight-average molecular weight of approximately 10,000, and increased to a saturated value for higher molecular weights. For poly(p-hydroxystyrene) (PHS) protected with tert- butoxycarbonyl (BOC) or acetal (ACT) group, the rate became larger with increasing the protection ratio and the rate of BOC-protected PHS was lower than that of ACT-protected PHS. It was also found that the rate was significantly influenced by the prebake and increased as the temperature became higher. The etching rate of the resin with gradual cooling after the prebake was slower than that with quick cooling. These results may indicate that the dry etching resistance is independent of the density of resin film and influenced by the stability of arrangements of polymer molecules. The reduction of etching rate by the deep UV cure method has been tried. The rate decreased with the deep UV cure under N2 gas flow, while increased in the presence of O2 such as dry air. The films after deep UV cure under N2 and a dry air was scarcely dissolved in any organic solvents and this implies that the polymers are crosslinking. The absorbance band of carbonyl appeared in FT-IR spectra for the case under dry air may suggest that oxygen atoms in the polymer structure influence the dry etching resistance. We have tried deep UV cure under N2 gas flow for KrF chemically amplified PHS based resists and obtained the same level of etching rates as novolak resin. For ArF resists, the combination of the incorporation of acryl unit into the base polymer and deep UV cure under N2 gas flow may be an effective method for improving the dray etching resistance.

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.

Influence of aberration on performance during use of resolution enhancement technology

We have investigated the influence of a spherical aberration on the printing characteristics with modified illumination. At first, we have developed a simple method for measuring the aberration function with an alternating phase shift mask (PSM), and have measured that in the projection optics of a commercially available KrF stepper. Then the anomalous phenomena observed in the printing with modified illumination are examined with the simulated aerial images with the measured spherical aberration. As a result, we found good coincidence between the simulated images and the anomalies. In conclusion, the origin of the anomalies is ascribed to the spherical aberration in the projection optics.

Improvement of CD control and resolution by optimizing inorganic ARC process

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.

0.32-μm pitch random line pattern formation by dense dummy pattern and double exposure in KrF wavelength

0.32 micrometers pitch on-grid random line pattern formation by double exposure in KrF wavelength is proposed based on optical image calculations. For first exposure, mask patterns are generated by combination of designed patterns and dense dummy patterns, al of which are laid out at on- grid positions with a pitch of 0.32 micrometers . An attenuated phase shift mask and an annular illumination are applied. The imaging performance is significantly enhanced because all patterns in this mask are categorized dense with almost the same pitch of 0.32 micrometers . The mask pattern for second exposure are simply generated from the dummy patterns by tone inversion. The image size required for erasing dummy pattern in much larger than that for the first exposure. Consequently, large latitude, to erase the dummy pattern is much lager than that for the first exposure. Consequently, large latitude, to erase the dummy patterns and not to affect the designed patterns, can be obtained by conventional exposure method with low coherent illumination. Even in this method, OPC is required to obtain desirable CD. However, OPC in this method can be performed by simple rules based method, because on-grid restriction to layout much reduces the variation of pattern configuration. As a result, 0.32 micrometers pitch on-grid random line patterns are formed accurately with DOF larger than 0.6 micrometers in KrF wavelength.

Characterization of automatic overlay measurement technique for sub-half-micron devices

Measurement precision, especially the measurement offset of an automatic overlay measurement technique, was studied for application to sub-half micron device manufacturing. Experimental data showed that the measurement offset depended on the cross-section structure rather than the reflectivity or the roughness of the overlay marks. Dependence of the measurement offset upon equipment factors such as the incident angle of illumination was also studied. This paper also shows measurement offsets on critical levels of the sub-half micron device manufacturing.