Additional evidence of EUV blank defects first seen by wafer printing

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1 Additional evidence of EUV blank defects first seen by wafer printing Rik Jonckheere, Dieter Van den Heuvel, Tristan Bret a, Thorsten Hofmann a, John Magana b, Israel Aharonson c, Doron Meshulach c, Eric Hendrickx, Kurt Ronse IMEC vzw, Kapeldreef 75, B-3001 Leuven, Belgium a Carl Zeiss SMS GmbH, Betriebstätte Rossdorf, Industriestrasse 1, Rossdorf, Germany b INTEL Corporation, SC1-03, 2200 Mission College Blvd., Santa Clara, CA c AMAT PDC, 4 Bergman Street, Rehovot, 76705, Israel ABSTRACT This paper is an add-on to the EMLC2011 contribution, which was awarded best paper and therefore invited at SPIE Photomask (BACUS) This manuscript focuses on additional results since the former conference. First experimental evidence is given that a second generation blank inspection tool has missed a number of printing reticle defects caused by an imperfection of its EUV mirror, i.e., so-called multi-layer defects (ML-defects). This work continued to use a combination of blank inspection (BI), patterned mask inspection (PMI) and wafer inspection (WI) to find as many as possible printing defects on EUV reticles. The application of more advanced wafer inspection, combined with a separate repeater analysis for each of the multiple focus conditions used for exposure on the ASML Alpha Demo Tool (ADT) at IMEC, has allowed to increase the detection capability for printing ML-defects. It exploits the previous finding that ML-defects may have a through-focus printing behavior. They cause a different grade of CD impact on the pattern in their neighborhood, depending on the focus condition. Subsequent reticle review is done on the corresponding locations with both SEM (Secondary Electron Microscope) and AFM (Atomic Force Microscope). This review methodology has allowed achieving clear evidence of printing ML defects missed by this BI tool, despite of a too high nuisance rate, reported before. This establishes a next step in the investigation how essential actinic blank inspection (ABI) is. Presently it is the only known technique whose detection capability is considered independent from the presence of a (residual) distortion of the multi-layer at the top surface. This is considered an important asset for blank inspection, because the printability of a ML-defect in EUV lithography is determined by the distortion throughout the multilayer, not that at the top surface. Keywords: EUV lithography, mask defectivity, natural defects, blank defects, multi-layer defects, defect inspection 1. INTRODUCTION Extreme ultra-violet lithography EUVL continues to be the lithography technique that is considered to have most promise to extend high-volume manufacturing to well below the 16nm half-pitch generation. Pre-production tools now in the field are expected to demonstrate the capability at 22nm level and to trigger the further development of the required infrastructure. Together with EUV sources, mask defectivity is considered one of the topics that still requires a major breakthrough to realize this. It includes two aspects. The first is the need for particle-free handling, as a pellicle is not obvious for EUVL. The second aspect of mask defectivity refers to the EUV specific type of defects, i.e., those of the multi-layer (ML) mirror, the so-called ML-defects. This paper deals with the latter. It is an extension of a previous publication 1 that triggered invitation to this conference. First a short summary of the previous publication is provided, but the focus is on the addition of new examples that make the conclusion even stronger. As shown before, EUV reticle defectivity has several aspects, beyond the conventional absorber type defects 2-4. The focus of this investigation continues the previously reported endeavors 4-9 to assess ML-defects. Prior work had demonstrated by simulation 3,4,6 that their printability is triggered from just nanometer height or depth onwards. In previous work 4,5,7,9 a number of such natural ML-defects were already visualized. Simulation work 3,4,6 has shown that Photomask Technology 2011, edited by Wilhelm Maurer, Frank E. Abboud, Proc. of SPIE Vol. 8166, 81660E 2011 SPIE CCC code: X/11/$18 doi: / Proc. of SPIE Vol E-1

2 the printability of ML-defects is not dominated by the phase shift caused by the shallow height distortion of the ML mirror. The printability of programmed ML-defects with a given height close to quarter wavelength, resp. half wavelength, considered to generate 180 degree phase shift, resp. 360 degree, did print largely similar 6. This indicates that the presence of an edge slope of a ML-defect has a dominant role in its printability. This is the reason why, unlike other publications by other researchers on this topic, this paper rather refers to these defects of the ML-mirror as MLdefects and not as phase defects. The printability of these ML-defects is found additionally due to the fact that they disturb the reflection of the incoming EUV light locally by scattering. This causes an intensity drop in the image, typical for an amplitude defect. The applied technique focuses on the use of wafer inspection (WI), followed by repeater analysis to identify the defects in the mask. The mask pattern consists of lines and spaces all across the reticle field. The list of defects detected by WI is then compared to the inspection results of patterned mask inspection (PMI) and blank inspection (BI) (= back-ward correlation). The former allows to thin-out the detections to those that correlate to the blank, based on previous evidence 5 that state-of-the-art PMI has (and is estimated to continue to have) detection capability for absorber related defects beyond what prints. Blank inspection done after ML-deposition is intended to detect the EUV-specific ML-defects. Especially for blank inspection the forward correlation to the printed wafer is also of interest. It can also reveal MLdefects on the mask that cause printing defects on the wafer that present WI is not yet capable to detect. One of the present limitations of WI is that the printed image on wafer typically still suffers from relatively high line-edge roughness (LER), such that it is difficult to distinguish it from the printing impact of a defect. The target of this forward - and backward correlation is to check for evidence of printing ML-defects that were missed by BI tools. Earlier work 5 had already demonstrated that a first generation blank inspection tool, as until relatively recently the only one in use by the blank vendors, had missed some printing ML defects. For a second generation tool no such evidence could be found before, although we had included the most advanced WI tools available. Yet, our work 5 has demonstrated that a high sensitivity of this BI tool is at the cost of a large set of detections that could not be confirmed to print by wafer review. This is for example a drawback in one of the intended mitigation approaches in which the absorber pattern is shifted, in order to cover the ML-defect to make it non-printable 11. As long as there are too many blank defects this route is not feasible. Once blank fabrication improvement provides a low blank defect density it is in reach to use such pattern shift technique. Ideally BI should be capable of detecting ALL printing ML-defects (and with low nuisance detections). The previous publication 1 already revealed four examples of printing ML-defects missed by the second generation BI tool. Here we add two and provide further analysis of previously found evidence. In a separate contribution to this conference 13 the feasibility of a second mitigation technique is discussed: If the most advanced blank inspection cannot avoid printing ML-defects that are first found on wafer, the paper shows what kind of reticle repair can be done to render the defects non-printable. 2. SUMMARY OF RESULTS OF THE PARENT PAPER 1 The main change of the working procedure has been to exploit the typical through-focus behavior of ML defects: ML bumps typically print most at negative focus, whereas ML pits print the strongest at positive focus, as predicted by Ref. 8. It was also found that the Sematech/Berkeley AIT showed the same behavior as found on wafer and confirmed the focus sign convention to be identical. Previous investigations 5 had only used wafer exposures done at best focus. This time the wafers were exposed with different focus settings, i.e., a number of dies for each focus. The exposure dose was fixed. Wafer inspection was done on AMAT s DUV laser based UVision4 WI tool, and the subsequent repeater analysis was carried out for each focus separately. In this way an additional number of printing defects was found. Subsequently focusing on defects that printed more severely out of focus was found an attractive way to narrow down to the candidates that could have been missed by the BI tool. This attempt was clearly successful. Already previously four examples have been shown 1, as obtained from two reticles. Here we restrict to give updated information. Erratum: Figures 6 to 9 in Ref. 1 refer to the incorrect Defect40FF reticle. A and B should be replaced by B and A respectively in each of these figure captions. Proc. of SPIE Vol E-2

3 -100nm Figure 1: Detailed analysis of a printing bump-type ML-defect that was missed during blank inspection on a M7360 (Reticle B, corresponding to figure 6 in Ref. 1). Top row, left: not visible in SEM, centre: top view as obtained by AFM, right: cross-sectional view in parallel and perpendicular direction to the lines, giving indications for height and width. This defect is re-estimated as a ~110nm wide, ~5 nm high bump. Bottom row: through-focus printing behavior on ADT. -100nm Figure 2: Detailed analysis of a second printing ML-defect that was missed during blank inspection on a M7360 (Reticle B, corresponding to figure 7 in Ref. 1). Previously it was not yet visualized by AFM, but Sematech/Berkeley AIT had confirmed its position information as accurate. Top row, left: not visible in SEM, centre: top view as obtained by AFM, right: cross-sectional view in parallel and perpendicular direction to the lines, giving indications for height and width. This defect is estimated as a ~140nm wide, ~3.5nm high bump. Bottom row: through-focus printing behavior on ADT. Proc. of SPIE Vol E-3

4 -100nm Figure 3: Detailed analysis of a first new (third total) printing ML-defect on Reticle A that was missed during blank inspection on a M7360. Top row, left: top view as obtained by AFM, right: cross-sectional view in parallel direction to the lines, giving indications for height and width. This defect is estimated as a ~100nm wide, ~3nm deep pit. Bottom row: through-focus printing behavior on ADT (Note: there is obviously no printing impact at negative focus). -100nm Figure 4: Detailed analysis of a second new (fourth total) printing ML-defect on Reticle A that was missed during blank inspection on a M7360. Top row, left: top view as obtained by AFM; right: cross-sectional view in parallel and perpendicular direction to the lines, view giving indications for height and width. This defect is estimated as a ~90nm wide, ~4.5nm deep. Bottom row: through-focus printing behaviour on ADT. Proc. of SPIE Vol E-4

5 3. ADDITIONAL RESULTS Figure 1 and 2 update the 2 defects shown in figures 6 and 7 in Ref. 1 for Defect40FF-B. They were our first evidence of blank defects that were found via the updated wafer inspection technique and not during the blank inspection on a Lasertec M7360. Both were not visible during mask review with SEM only. The defect of figure 7 could now also be visualized by AFM as a bump about 4nm high (see figure 2). Previously the position had been confirmed by Sematech/Berkeley AIT 1. As the defect of figure 1 it is also a bump, here with approximate height of 6nm. The throughfocus printing behavior, as included in Figs 1 and 2, illustrates that bumps typically print more severely at negative focus. For a the second reticle (Defect40FF-A) figures 8 and 9 in Ref. 1 already demonstrated two ML-defects as evidence of missers during blank inspection on the M7360. They were previously visualized as pits, respectively 4nm and 2nm deep. At the time of Ref. 1 there was still a set of further candidate ML defects missed during BI. Figure 3 and 4 detail the visualization of two more SEM non-visible ML-defects missed during BI. Also these have now been qualified as two pit defects, with a depth around 4nm. All 4 of these examples illustrate that pits typically print most severely at positive focus. The further candidate missers were found not to be of ML-type. Reticle A has dominantly pit-type of ML defects, while reticle B has mainly bump-type of defects. A more detailed analysis of all defects found on these two reticles, produced in the first half of 2009, gives the following results: On reticle A the total number of known defects is 117. Based on the correlation of all available results 42 were classified as ML-Type. This corresponds to a ML-defect density of 0.30/cm of those 42 were visualized by AFM as 1 bump and 18 pits. Of the latter 4 had been missed on M7360. The bump has no through-focus behavior, and is ~7-8nm high. The 18 pits all have through-focus behavior (typ nm high), except one (6nm deep), whereas one other, ~6nm deep, has through-focus behavior other ML-defect out of the known 42 that were not analyzed by AFM have been analyzed for through-focus printing behavior: 11 have a clear pit behavior (print more at positive focus), 3 have a rather flat through-focus behavior. On this reticle the number of pits in the total of ML-defects is estimated at least ~90%. On reticle B the number of known defects is 148. Based on the correlation of all available results 36 defects were classified as ML-type. This corresponds to a ML-defect density of 0.26/cm of those are SEM visible bumps with a height range 8nm 25nm. Just one of those one has a bump-typical through-focus behavior (and was found 15-20nm high). - 4 in 36 correlate to substrate inspection. Two are a pit and bump of 6-9nm depth, resp. height, but >~300nm wide. One is a 25nm high bump (~150nm laterally). The fourth was not reviewed in 36 are SEM non-visible. Two of those are the 6-9nm high/deep bump/pit in substrate inspection out of 26 others were visualized by AFM: 1 pit (2-3nm deep, through focus behavior), 15 bumps (*). Of the latter two were missed on M (*) Only 5 of those 15 print most at negative focus. Those typically were found 2-5nm high. Also some cases without pronounced printing at negative focus were found to have such shallow height. Others without are rather 5-8nm high out of 26 were not reviewed by AFM but checked for through-focus behavior: 2 are considered bumps (pronounced negative focus printing), 4 as pits, and 4 have a relatively flat (or somewhat irregular) throughfocus printing performance. On this reticle the number of pits in the total of ML-defects is estimated ~30% maximum, but probably rather ~15-20%. Additionally it was estimated why the M7360 might have missed these 6 ML-defects. For the analysis first the height, respectively depth, together with their lateral dimension is estimated from the AFM measurement. These data are then used to calculate the sphere equivalent diameter (SEVD) 14, as this is typically used as a measure for the sensitivity and capture rate of BI tools. For the ML defects that were detected on M7360 and that were visualized by AFM the minimum SEVD typically ranged between 35 and 40nm. The four M7360 missers on Reticle A (pits) include cases where the SEVD was estimated to as low as 25nm. For the two M7360 missers on reticle Reticle B (bumps)the SEVD was found in the range 50-55nm. Proc. of SPIE Vol E-5

6 4. CONCLUSIONS AND FINAL REMARKS In this extension of a previous publication 1, additional evidence was given for printing ML-defects, that were missed by the more advanced Lasertec M7360 blank inspection tool. The reticles used in this study were produced in early Our result has been achieved by a continued endeavor to further improve the total vision, by adding additional tools in the correlation between defect maps obtained by individual tools, and also by improved procedures. A clear example of the latter lies in the exploitation of the through-focus behavior of ML-defects, by the individual repeater analysis of the wafer inspection detection, made separately for each focus setting. This allowed collecting evidence for defects that could not be detected when only using exposures made at best focus. Most of our experimental data confirm the strongest printing behavior of bumps at negative focus, and for pits at the positive focus. Most of the ones without clearly pronounced printing behavior at a positive or negative focus are found typically deeper/higher than 5nm, although there are exceptions in both directions. At least no pit or bumps were found with most pronounced printing at opposite focus than expected. The M7360 missers are typically shallow and narrow pits or bumps, i.e., with height/depth and lateral size typically in the range 2-6nm and nm respectively. For a number of them the SEVD calculated based on the AFM data showed to be clearly lower than the smallest ones detected and visualized. It is expected that this also defines the class of printing ML-defects that is still missed by any technique. Unfortunately even best available WI still has limitations and is not yet capable of detecting all defects in the meaning of its definition: occurrences of CD deviations by more than 10%. Also the reader is reminded that we used 40nm hp printing, and one can logically expect that there is still a set of ML-defects that first prints at smaller half-pitch than the 40nm used here. Of the available blank inspection tools, clearly the 488nm - and 266nm wavelength based tools were not capable enough to detect all printing ML-defects. It is even more worry-some that for the M7360 this is at the cost of a relatively high nuisance rate of detections that cannot be confirmed to print 5. Two further candidate tool families are respectively using nm wavelength and actinic EUV wavelength. In view of the fact that the former is even more surface sensitive than those evaluated until now (because of the decreased penetration depth), and the fact that certain printing ML-defects may not even have surface topography, it is not unlikely that it can also miss printing ML-defects. Such evidence is not yet in place and is subject to further work. Neither could we include actinic blank inspection such as in Ref 12, but as it uses at least the same wavelength as the wafer scanner, it is less likely that it will fail to detect all printing ML-defects. These statements summarize our inspiration for future work. As a final conclusion this work shows an important limitation of state-of-the-art blank inspection: If it cannot detect ALL printing ML-defects and/or it is not capable of reaching a low enough false count rate, existing practices to deal with ML-defects by blank picking or design 11 will have too limited success. Yet, ML-defects not found in time (not on the blank, nor on the mask, but for the first time on printed wafer) likely can be overcome by compensation repair 6,10,. Such experimental work is presented separately at this conference 13. ACKNOWLEDGEMENTS The imec authors, as the steering party behind the reported activity, wish to explicitly express a warm thank you to their co-authors for bringing this work to such valuable level. That is truly due to the close collaboration. Imec and Carl Zeiss SMS GmbH are obliged to the Catrene Office and the local authorities (IWT and BM respectively) for the support to the EXEPT project, under which part of this work has been performed. At AMAT PDC the contributions by Moshe Rozentsvige, Robert Schreutelkamp, Gaetano Santoro, Shmoolik Mangan and Ilan Englard are very well appreciated. The authors are grateful to Kenneth Goldberg and Iacopo Mochi at Lawrence Berkeley National Laboratory (LBNL), Center for X-Ray Optics (CXRO), for their recommendations and for sharing their skills related to the application of the Sematech/Berkeley AIT in our work. At imec Rudi De Ruyter and Bart Baudemprez are acknowledged for their involvement. Proc. of SPIE Vol E-6

7 REFERENCES R. Jonckheere et al, Evidence of printing blank-related defects on EUV masks, missed by blank inspection, Proc. SPIE 7985, 31 (2011) R. Jonckheere et al, Mask defect printability in Full Field EUV Lithography Part 1, International Symposium on Extreme Ultraviolet Lithography, Sapporo (2007) R. Jonckheere et al, Mask defect printability in Full Field EUV Lithography Part 2, International Symposium on Extreme Ultraviolet Lithography, Lake Tahoe (2008) R. Jonckheere et al, Investigation of EUV Mask Defectivity via Full-Field Printing and Inspection on Wafer, Proc. SPIE 7379, (2009) D. Van den Heuvel et al, Natural EUV mask blank defects: evidence, timely detection, analysis and outlook, Proc. SPIE 7823, (2010) R. Jonckheere et al, Investigation of mask defect density in full field EUV lithography, International Symposium on Extreme Ultraviolet Lithography, Prague (2009) R. Jonckheere et al, Lessons Learned from Correlation between EUV Mask Inspection, Blank Inspection and Wafer Print Analysis, International Symposium on Extreme Ultraviolet Lithography, Prague (2009) Chris H. Clifford, Investigation of buried EUV mask defect printability using fast simulation at the 22nm and 16nm nodes, International Symposium on Extreme Ultraviolet Lithography, Prague (2009) D. Van den Heuvel et al, Comparison between existing inspection techniques for EUV mask defects, International Symposium on Extreme Ultraviolet Lithography, Kobe (2010) R. Jonckheere et al, EUV Mask Defectivity: Status and Mitigation Towards HVM, International Symposium on Extreme Ultraviolet Lithography, Kobe (2010) J. Burns et al, EUV mask defectivity mitigation through pattern placement, Proc SPIE 7823, (2010) T. Terasawa, Actinic phase defect detection and printability analysis for patterned EUVL mask, Proc. SPIE 7636, (2010) R. Jonckheere et al, Repair of natural EUV reticle defects, Proc. SPIE 8166, (to be published 2011) I.-Y. Kang et al, Printability and inspectability of programmed pit defects on the masks in EUV lithography, Proc. SPIE 7636, (2010) Proc. of SPIE Vol E-7