The silicon hardmask (Si-HM) is one of the key materials used in multilayer lithography for pattern transfer to a substrate using a fluorinated plasma etching process. Manufacturing of devices with smaller feature sizes introduces new challenges in defect control of all the critical layers, including the Si-HM layer used in photolithography. One of the major challenges of Si-HM materials includes intrinsic defect formations, which can be exacerbated by the presence of foreign contaminants such as soft and hard particles, organics, and metal-ionic contaminants. These contaminants are also known to induce defects by interfering with the plasma etch processes used in advanced patterning technologies. The contaminants can range from microns to angstroms in size. The identification and characterization of the defect adders is important to develop filtration methods capable of minimizing the number of on-wafer defects and consequently improving the quality. In this study, metal contaminants, liquid particle count and on-wafer defects of Si- HMs and filtration removal rates are monitored to determine the effect of filter type, pore size, media morphology, and cleanliness on filtration performance. 5-nm PTFE NTD2 filter having proprietary surface treatment used in this study shows lowest defect count.
Two versions of a specific 2nm rated filter containing filtration medium and all other components produced from high density polyethylene (HDPE), one subjected to standard cleaning, the other to specialized ultra-cleaning, were evaluated in terms of their cleanliness characteristics, and also defectivity of wafers processed with photoresist filtered through each. With respect to inherent cleanliness, the ultraclean version exhibited a 70% reduction in total metal extractables and 90% reduction in organics extractables compared to the standard clean version. In terms of particulate cleanliness, the ultraclean version achieved stability of effluent particles 30nm and larger in about half the time required by the standard clean version, also exhibiting effluent levels at stability almost 90% lower. In evaluating defectivity of blanket wafers processed with photoresist filtered through either version, initial defect density while using the ultraclean version was about half that observed when the standard clean version was in service, with defectivity also falling more rapidly during subsequent usage of the ultraclean version compared to the standard clean version. Similar behavior was observed for patterned wafers, where the enhanced defect reduction was primarily of bridging defects. The filter evaluation and actual process-oriented results demonstrate the extreme value in using filtration designed possessing the optimal intrinsic characteristics, but with further improvements possible through enhanced cleaning processes
Reduced tolerance for defectivity is a well-documented consequence of the semiconductor industry's constant
progress toward smaller IC device dimensions. Among all manufacturing functional areas, photolithography is arguably
the most sensitive to process defects, and thus, strongly influences manufacturing process yield. Microbridging is a
well-known type of "killer" defect that can become prevalent in KrF and ArF photoresist systems. When present in
BEOL lithography layers, bridge defects can manifest as catastrophic, single-line open circuit faults ("opens") in the
metal lines of the finished device. Previous work in BARC + resist systems has demonstrated the effectiveness of
improved filtration in reducing bridge defects. The present work evaluates the impact of improved filtration on both
litho defectivity and device yield. Application of asymmetric nylon 6,6 filters to a bi-layer resist yielded a significant
reduction in microbridge defects via removal of gel-like particle defect precursors. Ultimately, these changes are
responsible for two-thirds of baseline defectivity reduction in single-line opens. The same benefits are realized-though
via a different mechanism-when all-fluoropolymer filters are introduced for a post-develop rinse, which uses ultrapure
water (UPW) that is ozonated to 50ppb. Filtration is applied both to the water point-of-supply and at point-of-use. Over
time, a significant reduction in microbridge defects-caused by residual developed resist-was realized for several
BEOL KrF litho layers. As with the bi-layer resist process, enhanced filtration contributes significantly (36%) to
reduction in the single-line opens defect baseline.
The implementation of 193 nm lithography into production has been complicated by high defectivity issues. Many companies have been struggling with high defect densities, forcing process and lithography engineers to focus their efforts on chemical filtration instead of process development. After-etch defects have complicated the effort to reduce this problem. In particular it has been determined that chemical filtration at the 90 nm node and below is a crucial item which current industry standard pump recipes and material choices are not able to address. LSI Logic and Pall Corporation have been working together exploring alternative materials and resist pump process parameters to address these issues. These changes will free up process development time by reducing these high defect density issues. This paper provides a fundamental understanding of how 20nm filtration combined with optimized resist pump set-up and dispense can significantly reduce defects in 193nm lithography. The purpose of this study is to examine the effectiveness of 20 nanometer rated filters to reduce various defects observed in bottom anti reflective coating materials. Multiple filter types were installed on a Tokyo Electron Limited Clean Track ACT8 tool utilizing two-stage resist pumps. Lithographic performance of the filtered resist and defect analysis of patterned and non-patterned wafers were performed. Optimized pump start-up and dispense recipes also were evaluated to determine their effect on defect improvements. The track system used in this experiment was a standard production tool and was not modified from its original specifications.
A design of experiment (DOE) was implemented to show the effects of various point of use filters on the coat process. The DOE takes into account the filter media, pore size, and pumping means, such as dispense pressure, time, and spin speed. The coating was executed on a TEL Mark 8 coat track, with an IDI M450 pump, and PALL 16 stack Falcon filters. A KLA 2112 set at 0.69 μm pixel size was used to scan the wafers to detect and identify the defects. The process found for DUV42P to maintain a low defect coating irrespective of the filter or pore size is a high start pressure, low end pressure, low dispense time, and high dispense speed. The IDI M450 pump has the capability to compensate for bubble type defects by venting the defects out of the filter before the defects are in the dispense line and the variable dispense rate allows the material in the dispense line to slow down at the end of dispense and not create microbubbles in the dispense line or tip. Also the differential pressure sensor will alarm if the pressure differential across the filter increases over a user-determined setpoint. The pleat design allows more surface area in the same footprint to reduce the differential pressure across the filter and transport defects to the vent tube. The correct low defect coating process will maximize the advantage of reducing filter pore size or changing the filter media.
Photolithography is a key technology driver enabling next generation processes. As line widths decrease to 0.18 micrometer and below, the critical size of particulate contamination decreases proportionately. The implementation of filtration below 0.1 micrometer within existing dispense systems raises concern as the removal rating of the filter approaches the size of large molecular weight components of the photoresist. This study was undertaken in order to determine the effects, if any, of 0.05 micrometer and finer filtration on photoresist performance. Utilizing 248 nm DUV resist, filters were tested in two latest generation dispense pumps, one nitrogen pressurization, and the other having a stepper motor and diaphragm. The coated 200 mm wafers were exposed at increasing exposure dose and focus in a DUV scanner to produce 0.18 micrometer features. This study concludes that as the filter removal rating became finer, the resist performance in terms of photospeed, process window or thermal stability did not change. This indicates that, using existing dispense systems, photoresists can be filtered as fine as 0.03 micrometer without significant polymer shearing or the unintentional removal of important materials from the resist. Based on these data, appropriate protection in terms of particle removal is possible as line widths necessitate the use of finer filters in resist dispense pumps.
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