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4). Sensitivity to edgedamage was expected to vary across the three resist types. A dedicated exposure jobspatially separated the areas where flakes are expected and where they are not expected.One section, consisting of two rows of 11 fields, was exposed close to the wafer edge at theopposite side of the notch. A similar area of two rows of 11 fields was exposed in the regionof the notch. During the exposure of these 2 × 11 sections at both locations near the waferwww.intechopen.comInfluence of Immersion Lithography on Wafer Edge Defectivit37edge (Region II), the IH makes continuous up- and down-scans over the wafer edge area,increasing the probability of defect generation. The exposure job was also designed so thaton another part of the wafer (Region I, on the right hand side), the immersion hood did notpass over the wafer edge.
In Region I, no flake-like defects were expected.Fig. 4. IH exposure sequence for edge flake characterization.7. QualificationThe specular images of regions with resist residues clearly showed differences in reflectedintensity: dark areas in the resist residues refer to thick layers, while light areas indicatemuch thinner layers.
The results obtained from Resist Type A are detailed below.We compared the SideScan images of areas where the IH did and did not pass. Figure 5a is atypical SideScan specular image for region I (where the IH did not pass). Differences in thickand thin resist residues are visible, but no fragments of the resist residues are evident. Incontrast, in Fig. 5b, taken from Region II, indicated that parts from the thick residue at thebottom of the apex were released. The close-up in the image indicates that some of theseedge flakes were re-deposited on the apex closer to the top.To determine whether any of these edge flakes end up on the top region (where edge diecan be damaged), we analyzed the TopScan image of the corresponding areas of Fig.
5a and5b using the scatter channel, as shown in Figs. 5c and 5d. In Region II, a lot of particles weredetected, while in Region I, no particles were observed in the images. This observation wasencouraging for further ADC work.Classification of the edge region flakes was found to vary by the defect location. Redeposited edge flakes on the apex side were best classified using their signal in the specularwww.intechopen.com38LithographyFig.
5. Specular and scatter images from Region I (a and c) and Region II (b and d) of a testwafer. IH damage is more likely in Region II.Fig. 6. Immersion characterization (quantification of edge flakes at wafer edge); the flakes onthe apex and top near edge can be quantified by composite analysis of SideScan andTopScan signals.www.intechopen.comInfluence of Immersion Lithography on Wafer Edge Defectivit39channel of the SideScan image. For re-deposited defects in the top near-edge region, acombination of signals in the specular and scatter channels gave more accurateclassification. Once all the measurement parameters for both areas are fixed, they can becombined in a single measurement sequence that provides defect classification and mappingfor all the wafer edge areas of interest.
(Fig. 6).8. Immersion process characterization and optimizationHaving qualified the inspection to classify and map edge flake defects, we used our resultsin a design of experiment to improve our understanding of this kind of defect source and itskey impact parameters.As indicated above, Resist A tends to generate flakes when the IH is passing over its edge.The non-optimized coating process left residues for two other resists, resist B and C;however the residue morphology was different.When the same immersion exposure was used, significantly fewer edge flakes were detectedin the near top region for Resist B and C than for Resist A (Fig. 7). Moreover, the residualdefects were less confined to the exposure zone, so some of these defects might be caused bycoating and wafer handling.
In the TopScan images, no clear sign of damage was seen.Clearly the choice of resist chemistry can be important to prevent these kinds of defects.As indicated earlier, resist residues can be optimized by changing the EBR recipe on the coattrack. Resist A showed several hundred defect flakes with the regular (short) EBR sequence.After optimization, this resist achieved defect values similar to the background valuesobtained with the non-flaking resists B and C.Fig. 7. Edge flake defects as a function of resist chemistry and EBR recipe.9.
Further wafer edge challengesMore kinds of defects besides the edge region flakes can be important in immersion litho.This section discusses other possible defect sources.www.intechopen.com40Lithography10. Wafer handling marks and resist rework processA variety of artifacts were seen even in fresh Si wafers, primarily on the bevel and apexregion. These wafers had very limited processing and handling, but damage was visible inthe form of particles in the apex/bevel region. This introduces an additional concern withtransport-related artifacts, and illustrates the need for an assessment of wafer edge qualityand handling before introduction to the immersion process.11.
Resist rework processesAt IMEC, resist work is typically done by a combination of a dry ashing step, followed by awet clean. In some cases, rework may be indicated to address an out-of-spec condition.Wafers used for monitoring of focus/dose/CD or overlay processes may be reworked daily.Limited rework typically results in an increased presence of scratches (typically at the lowerbottom bevel), and an overall increase in reflectivity variation, indicating degraded surfacequality.
When wafers are reworked ~10 times or more, the bevel/apex area is much moreaffected. These defects could pose a risk when the immersion hood is passing over thewafer.12. ConclusionIn this paper, we investigated the impact of immersion lithography on wafer edgedefectivity. In the past, such work has been limited to inspection of the flat top part of thewafer edge due to the inspection challenges at the curved wafer edge and lack of acomprehensive defect inspection solution. Our study used a new automated edge inspectionsystem that provides full wafer edge imaging and automatic defect classification.The work revealed several key challenges to controlling wafer edge-related defectivity,including choice of resist, optimization of EBR recipes, and wafer handling.13. AcknowledgmentsThe authors thank Diziana Vangoidsenhoven, Christie Delvaux, Bart Baudemprez, and TomVandeweyer for help in processing and wafer selection; Thomas Hoffmann for help inimmersion soak time simulations; and Philippe Foubert, Dieter Van Den Heuvel, ShinichiHatakeyama (TEL), Kathleen Nafus (TEL), Sean O’Brien (TI), Mireille Maenhoudt, andRichard Bruls (ASML) for helpful discussions on immersion tools and related defectivity.VisEdge is a trademark of KLA-Tencor Corp.14.
References[1] B. Smith, H. Kang, A. Bourov, F. Cropanese, Y. Fan, “Water Immersion OpticalLithography for the 45nm Node,” Proc. SPIE, Vol. 5040, p. 679-689, 2003.[2] M. Kocsis et al., “Immersion-specific Defect Mechanisms: Findings andRecommendations for Their Control,” Proc. SPIE, 6154, 6154-180, 2006.[3] M. Maenhoudt et al., Jour. of Photopolymer Science and Technology, 19, 585, 2006.[4] M.
Ercken et al., Jour. of Photopolymer Science and Technology, 19, 539, 2006.[5] I. Pollentier et al., Proc. SPIE, 5754, 129, 2005.www.intechopen.comLithographyEdited by Michael WangISBN 978-953-307-064-3Hard cover, 656 pagesPublisher InTechPublished online 01, February, 2010Published in print edition February, 2010Lithography, the fundamental fabrication process of semiconductor devices, plays a critical role in micro- andnano-fabrications and the revolution in high density integrated circuits. This book is the result of inspirationsand contributions from many researchers worldwide. Although the inclusion of the book chapters may not be acomplete representation of all lithographic arts, it does represent a good collection of contributions in this field.We hope readers will enjoy reading the book as much as we have enjoyed bringing it together.
We would liketo thank all contributors and authors of this book.How to referenceIn order to correctly reference this scholarly work, feel free to copy and paste the following:K. Jami, I. Pollentier, S. Vedula and G. Blumenstock (2010). Influence of Immersion Lithography on WaferEdge Defectivity, Lithography, Michael Wang (Ed.), ISBN: 978-953-307-064-3, InTech, Available from:http://www.intechopen.com/books/lithography/influence-of-immersion-lithography-on-wafer-edge-defectivityInTech EuropeUniversity Campus STeP RiSlavka Krautzeka 83/A51000 Rijeka, CroatiaPhone: +385 (51) 770 447Fax: +385 (51) 686 166www.intechopen.comInTech ChinaUnit 405, Office Block, Hotel Equatorial ShanghaiNo.65, Yan An Road (West), Shanghai, 200040, ChinaPhone: +86-21-62489820Fax: +86-21-62489821.
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