Appl. Phys. A (2014) 117:1607–1614 DOI 10.1007/s00339-014-8601-2
INVITED PAPER
Industrial perspective on focused electron beam-induced processes Tristan Bret • Thorsten Hofmann • Klaus Edinger
Received: 6 May 2014 / Accepted: 1 July 2014 / Published online: 27 July 2014 ! Springer-Verlag Berlin Heidelberg 2014
Abstract After a short overview of the historical developments of the technique of gas-assisted focused electron beam-induced processing (mostly deposition and etching), this paper deals with the applications of this technology to photolithographic mask repair. A commented list of results is shown on different mask types, for different types of defects, and at different node generations. The scope of this article is double: summarize the state of the art in a fastpaced highly specific industrial environment driven by ‘‘Moore’s law’’ and feedback to academic researchers some technologically relevant directions for further investigations.
1 Introduction Solid samples can be modified at high resolution when hit by a focused particle beam in the presence of reactive gases. Depending on the nature of the gas, the process can result for instance in a deposit on the sample (‘‘additive nanolithography’’). In contrary, if the reactive gas combines with the sample to yield volatile compounds, the outcome of the process can be the local removal of material from the sample (‘‘etching’’). A schematic representation is proposed in Fig. 1. Historical developments, applications, models, and summaries of published studies were reviewed extensively in previous texts [1–3]. Let us stress out just a few points here. Firstly, many investigations on etching and deposition were conducted in parallel (compare for instance [4–7] T. Bret (&) ! T. Hofmann ! K. Edinger Carl Zeiss SMS GmbH, Betriebssta¨tte Rossdorf, Industriestrasse 1, 64380 Rossdorf, Germany e-mail:
[email protected]
or [8, 9]). Secondly, many improvements and ideas were triggered by technological needs (from developments in space science to scanning probe microscopy or magnetic data storage). Thirdly, many aspects of gas-assisted electron beam-induced processes resemble ion beam-induced processes, but they differ in the particle cross sections, implantation properties and momentum transfer, so that electron beam-induced processes are slower and milder than their ion beam counterparts. The absence of Gainduced staining to transparent samples [10] allowed focused electron beams (FEBs) to take over a technologically important function of focused ion beams (FIBs), namely photolithographic mask repair, when the critical dimensions (CD) of the mask features shrunk below the Ga-ion beam resolution limit. The feasibility of the repair of clear defects on photolithographic masks by electron beam-induced contamination deposition was mentioned in 1986 [11]. The first commercial system based on this technology and competitive with FIB-based systems to repair 65-nm node masks in an actual industrial environment was delivered in 2003 [12]. To our knowledge, among the many possible applications of FEB-induced processing, mask repair is among the most demanding. Yet, the application is economically viable: it is now state of the art or even yield-critical in front-end mask-shops worldwide [13]. This will be the main topic of this article.
2 Photolithographic mask defects and focused electron beam-induced repair 2.1 Masks and origins of defectivity A lithographic ‘‘photomask,’’ or ‘‘reticle,’’ is a critical component in the manufacturing process of semiconductor
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Fig. 1 Schematic representation of FEB-induced processing
devices, such as chips or memories. A master pattern is defined lithographically on the mask, which corresponds to a given layer. Chips are built from more than 30 layers nowadays, among which more than 10 can be ‘‘critical’’ [14]. Once the mask is successfully qualified, the pattern can be reproduced onto large number of wafers (like projecting photographies from a negative), with a size reduction factor of 4. But it is (at least) a chip yield issue if a printing defect remains on the mask: every layer is locally different from the design (for instance, a connector line becomes an open circuit). Despite the extreme care given to the mask production and handling process, isolated defects can arise from several sources: for instance, a bit error in the electron lithography step, an inhomogeneity in the e-beam resist, or a particle adder before pattern transfer into the mask absorber layer. Mask-shops usually track down the defect root causes with powerful inspection tools and correlation algorithms, and the low-defect generation probabilities are continuously improved to ever-lower levels. However, the parallel increase in pattern densities, reductions in the critical features sizes (from 65 nm in 2006 down to 22 nm in 2012), and ever-tighter lithographic specifications (for instance, on line-edge roughness) turn once negligible defects into ever more severe concerns. A series of repair technologies was thus developed to serially erase the defects: laser-based ablation, mechanical nanomachining, and gas-assisted particle beam-induced processing. Depending on the defect numbers and sizes, as well as target resolution, process throughput, and side effects inherent to every technique, mask-shops learnt how to optimally integrate repair technologies in their production workflows. Rewriting masks or further increasing designed redundancies are no long-term alternatives. Gas-assisted FEB-induced deposition and etching allow for removal of excess mask material and deposition of missing material. It is slower than FIB-induced processing, but allows for higher resolution and no beam-induced mask staining, which becomes critical when having to deposit
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193-nm transparent materials. With typical beam sizes of a few nanometers, FEBs offer higher resolution than diffraction-limited laser-based processes. When using low energies to reduce the scattering ranges, electron scattering side effects (such as ‘‘riverbedding’’ at edges) can be avoided [13]. As compared to contact-probe-based nanomachining [15], FEB-based processing allows for fast processing. The layer-by-layer removal of sample atoms as volatile compounds instead of solid debris, as well as a flexible beam deflection and low beam divergence angles, makes it a versatile tool able to etch through 60 to 90 nm thickness of a 500 9 500 nm2 absorber layer area of complex shape within minutes and with sidewall angles above 85". FEB-induced mask repair is industrially represented by the MeRiT# Mask Repair Tool product line by Carl Zeiss Semiconductor Metrology Systems (SMS) GmbH (formerly NaWoTec GmbH), based in Jena and Rossdorf, Germany. For mask qualification, the repairs have to be inspected by at-wavelength microscopy across several focal planes, by emulating the specific scanner illumination conditions. The availability of such a tool (named AIMSTM for Aerial Imaging Metrology System) within the industrial portfolio of Carl Zeiss SMS allows for fast feedback in the early stages of process development and for closed-loop repair and qualification workflows. 2.2 Binary masks Binary masks represent a historically and industrially important class of photomasks, down to the 32-nm node (half-pitch, on wafer level). A thin metallic layer, usually Cr covered with Cr oxide as anti-reflection layer, is patterned. In order to etch the hard absorber defects, proprietary processes were developed and optimized in order to meet customer needs. Tight specifications are set, with respect (among other parameters) to minimal defect size, etch selectivity versus the fused silica (popularly named ‘‘quartz’’ or ‘‘glass’’) substrate, throughput, maximal light
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intensity deviation from reference across several focal planes, and repeatability on programmed defects. Examples of MeRiT# etch results, shown in Fig. 2, illustrate the versatility, high-resolution, and final scope of the process. If absorber is missing, a complementary FEB-induced process can locally add material. Beyond 193-nm light opacity, deposits are subject to further specifications, such as extreme stability under the actinic illumination conditions and robustness against repeated mask cleaning. Since the purpose of mask cleaning is to efficiently remove all the ‘‘soft adders’’ to the mask surface, such as organic particles or salts condensed from traces of volatile compounds present in the cleanrooms, special care must be given to the adhesion properties of the deposit to the surface and to its chemical composition. 2.3 Attenuated phase-shift masks A second important class of photomasks is composed by attenuated phase-shift masks (‘‘APSMs’’). In this later mask generation, the incoming light is only partially absorbed. A phase shift of 180" leads the attenuated wave to interfere destructively with the transmitted light. Larger lithographic process windows can be obtained with such
Fig. 2 Left top-view SEM image in MeRiT# tool of a binary mask (Cr/CrOx on SiO2) absorber layer etch result. Compare the apparent Cr film graininess with the smoothness of the quartz substrate. Right repair example on a programmed defect on a binary mask (top row,
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masks. From a material point of view, the absorbing layers, or ‘‘blanks,’’ are built from Molybdenum-containing Silicon oxynitrides (also popularly known as ‘‘MoSi’’). The Mo fraction (usually 5–20 %) leads to strong absorption of the UV light (usually 80–94 %) and confers some electrical conductivity to the films. The real part of the refractive index of Si nitrides at 193 nm (around 2.5) is higher than that of Si oxides (around 1.6). 180" phase shift can be obtained from thinner layers (with typically 60 nm thickness) by incorporating nitrogen than by using purely oxygen based films. Thinner layers reduce optical 3D effects under the wide-angle illumination conditions used. They allow for reduced etching times upon processing and offer a material contrast with the substrate. Some oxygen (a few atomic percent) is usually present in the films to relax the Si-N network and avoid the formation of too large or anisotropic crystalline grains. Similarly to the processes described on binary masks, both FEB-induced etching and deposition are necessary to remove opaque and clear defects, respectively. The deposition process must yield an optically controlled material in order to match the absorption and phase-shifting properties of the reference film. In the case of complex or large defects, it can be simpler to combine etch and deposition processes in order
SEM views before/after repair; bottom row, corresponding light distribution maps—‘‘aerial images’’—at 193 nm. Color scale is blue for low intensity, red for high intensity)
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to get a practicable workflow and get rid of edge effects. A striking example of a two-step repair (adapted from [16]), shown in Fig. 3, illustrates the applicability to arbitrary structures while maintaining high resolution.
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mask repair and subsequent wafer printing was demonstrated and reported down to 32-nm logic patterns in 2009 [17]. An adapted example is shown in Fig. 4. 2.3.2 Masks for extreme ultraviolet lithography
2.3.1 Nanoimprint Lithography masks NanoImprint Lithography (NIL) is a further lithography generation. Whereas optical masks are usually projected on wafer with a reduction factor of 49 (i.e., the projection of a 128-nm-wide mask pattern is 32 nm wide on the wafer), patterned-quartz-based NIL masks are transferred at full scale in all dimensions. The resolution specifications are tighter, as well as the height (resp. depth) and flatness control requirements. The feasibility of FEB-induced NIL
With the planned extension of optical lithography to extreme ultraviolet (EUV) high-volume manufacturing, new types of reticles must be addressed. The envisioned 13.5-nm wavelength is strongly absorbed by most materials: Reflective masks were selected. A reflectivity of *70 % is obtained from an optimized Bragg mirror (from alternating Mo and Si layers, which can regretfully lead to confusion with the ‘‘MoSi’’ phase-shifting films mentioned above). The mirror is ‘‘capped’’ by a thin Ru layer, on top
Fig. 3 Left top-view SEM image in MeRiT# tool of a complex defect on a 193 nm, 6 % transmission APSM mask. Middle result after removing the 5 defective absorber lines (‘‘MoSi etch’’). Right result after depositing 5 straight lines of optically matched absorber (‘‘PSM depo’’)
Fig. 4 Repair and 1:1 printing qualification in 3D on resist of a programmed defect on a NIL mask
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Fig. 5 EUV mask repair examples, qualified by wafer printing in a EUV stepper (NXE:3100 by ASML). Left SEM tilted view of a EUV absorber etch result. Notice the smooth bottom and vertical side walls. Middle SEM top-views on mask (top) and resist-patterned wafer (bottom, scale 1/4) before and after FEB-induced removal of a partial-
height Ta-based opaque defect in 27-nm half-pitch (on wafer) lines and spaces (‘‘EUV etch’’). Right SEM top-views on mask (top) and resist-patterned wafer (bottom, scale 1/4) before and after FEBinduced addition of complementary absorber over 3 lines (‘‘EUV depo’’)
of which a thicker (typically 50–90 nm) Ta-based absorber with a TaBxNy anti-reflective coating can be accurately patterned [18]. The increased complexity of the stacking, and the chemically different nature of the involved materials, required the development of new etch and deposition processes. The smaller pitches and more stringent cleaning durability requirements increase the need for robust, selective, and artifact-free processes. In the absence of actinic inspection microscopes (an AIMSTM EUV tool is under construction [19]), the repairs must be qualified by wafer printing. Successful repair of real defects on EUV reticles was demonstrated over several nodes, from 40 nm down to 25 nm [20], as examples show in Fig. 5. New types of substrate defects can be encountered on EUV masks, which are not even visible in SEM. Deviations of the multilayer from planarity by as little as 3 nm (as a quarter of the actinic wavelength) induce lower reflectivities, i.e., printing defects, which must be addressed. Such defects can be localized and analyzed by Atomic Force Microscopy (AFM), which is a powerful add-on to the mask repair tool. A process named ‘‘compensation repair’’ experimentally demonstrated a restoration of the lithographic process window [21, 22]. An example of the workflow is shown in Fig. 6.
flows cannot dislodge them as easily as from free surfaces or long grooves. Some defects, especially made of amorphous carbon or residuals from polymer resists, can be easily removed by FEB-induced etching [9, 23]. Two selected examples of removed adders on EUV masks are shown in Fig. 7: A chemically well-defined polystyrene particle placed on purpose, and an unknown contaminant, previously identified as carbon-rich by backscattered electron imaging. Either a complete removal can be reached, or the quantitative volume reduction leads to defect-free printing.
2.3.3 Soft defects/particle adders Finally, let us mention the possibility of removing soft defects from mask surfaces. Particles chemically different from the constitutive mask materials can remain at printing positions. They can be difficult to remove by wet cleaning from small, deep patterns such as contact holes, where hydrodynamic
3 Challenges Several mask-specific challenges were resolved in the course of technology development. First of all, local charge buildup was one of the first show-stoppers to have been addressed on insulating quartz-based photomasks. By optimizing column design, precursor gas flow, and beam properties, most of the charging effects were satisfactorily tackled [12]. Secondly, stable beam-induced etching can be performed only if the competing contamination level is low enough (zero contamination is usually beyond reach). Good practices were developed to control contamination, among which quality and cleanliness standards by the assembly of components, residual gas control by strictly avoiding leakages and unnecessary venting, plasma-based processing steps, and constant monitoring. Regarding resolution, the use of good electron optical columns (designs of the Gemini and Ultra models, by Carl Zeiss NTS) allowed for sharp beams even at relatively low landing energies, where the interaction volumes are small. Thermal
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Fig. 6 Example of ‘‘EUV compensational repair’’ workflow. From top left, a defect is found by SEM inspection of printed wafers; no defect is seen by SEM inspection; AFM reveals a localized ‘‘bump’’ in the Bragg mirror; after absorber edition around the defective area
as shown in SEM top-view, the AFM tip reaches the defect sides more easily and confirms good repair placement; in the SEM top-view of the EUV exposed and developed wafer, the compensated defect does not print any more
drifts are reduced by performing all operations as close to possible to the temperature of the cleanroom in which the tool is located, and by separating all heat sources from the main chamber. As mentioned above, issues like deposit adhesion to the substrates and cleaning durability had to be addressed, which could be done only with access to the actual cleaning equipment used by the mask-shops, since every cleaning process is tailored to the mask type in use, and usually proprietary. For good etching performance, side effects like spontaneous ‘‘under-etch’’ seen on some materials could be avoided by carefully tuning process parameters (as in parallel reported by other groups [24, 25]). Process termination also had to be addressed. Either an etch selectivity can be chemically obtained from different layers, as demonstrated in the above examples, or the process must be stopped based on a robust end-point detection criterion. Along the course of development, the process stability was improved by processing not only the secondary electron signal, but also the backscattered electron signal collected from the sample. However, several technical challenges remain, which might become more and more relevant as the critical dimensions continue to shrink. For instance, new SixNybased phase-shifting materials are under development by the blank vendors, with increased durability. These materials have reduced Mo, reduced O, and increased N
contents as compared to previous generations and tend to become more difficult to etch by FEB while maintaining good performance with regard to smoothness, throughput, and selectivity. Although many good studies were reported on the mechanisms of etching and deposition of silicon oxides (see for instance [7, 26]), no such detail is available on Si nitrides (or oxynitrides, which exist in a variety of stable phases). Process development could benefit from academic input regarding the homogeneity, crystallinity, and etch mechanism of these materials. The same seems to be true regarding deposition studies on Si nitrides: Local deposition of optically dense materials with controlled fractions of other elements (such as C, H, O, Cl, or even Mo) remains, to our knowledge, too scarcely studied as compared to the many possible applications of these materials. Regarding the pure elements, the deposition of dense, flat, and pure layers of the elements found on EUV masks (i.e., Ta, Ru, Mo, or Si [18]) could also be technologically useful. Although precursors are known for CVD or ALD of many of these elements, temperature constraints on the mask, chamber, and precursor source limit the available vapor pressures. The fundamental limits to the process resolution (i.e. the minimal width for 60–90 nm deep vertical holes or grooves, respectively, pillars or walls) remain an open question: 11-nm-wide grooves and 25-nm-wide free-standing bridges were reported in 2004
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Fig. 7 Examples of FEB-induced particle etching from EUV mask surfaces. Left monodisperse polystyrene particles placed on purpose on the surface of an EUV mask can be individually removed without
affecting the surroundings. Right unknown organic contaminant can be quantitatively removed from 32-nm hp contact hole patterns by a similar recipe, as successfully qualified by NXE:3100 wafer printing
[12], but improved hardware and further optimization of the process parameters might in principle allow for smaller features. From the theoretical point of view, the relative roles of all phenomena involved in the activation of the precursor and the subsequent reaction steps could benefit from deeper investigations. Depending on the system under study, several mechanisms might be possible, between which the available experimental results do not allow discriminating: ionization of adsorbed precursor by primary, scattered, or secondary electrons, but also interaction of precursor molecular orbitals with tunneling ‘‘hot electrons,’’ resonant plasmons or radicals evolving below the surface. Many substrate modifications can occur under the electron beam, such as cross-linking, crystallization, element sputtering, or local establishment of electric field gradients. All these effects might be responsible for an initial ‘‘incubation,’’ which is difficult to include into simulation models without detailed evidence, but might have deep implications on the process performance, and could influence the design of future systems.
(‘‘MoSi’’) masks, nanoimprint lithography masks, and EUV masks. The etch and deposition processes were illustrated, including compensation repair on SEM-invisible but AFM-located EUV multilayer defects and soft particle defects. The importance of repair qualification by aerial image or printed wafer metrology was stressed upon. A list of already addressed challenges was given (charging, contamination, drift control, process selectivity, and cleaning durability), as well as possible directions for future investigations (etching and deposition of SiN-based materials, deposition of the EUV-relevant elements Ru, Ta, Mo, Si, study of the ultimate resolution on mask absorberrelevant thicknesses and more detailed investigations of the involved surface mechanisms). Acknowledgments The authors wish to thank all their colleagues at Carl Zeiss SMS GmbH for contributing to the presented results. Many thanks as well to external partners (EPFL/Lausanne, Switzerland; IMEC/Leuven, Belgium; IMS/Stuttgart, Germany; Intel Corporation/ Santa Clara, USA) for providing test masks on which the presented experiments were realized. Partial funding of the presented results by European research projects (FANTASTIC, CATRENE/EXEPT) and by the German Federal Ministry for Education and Research is also gratefully acknowledged.
4 Conclusions In this article, we summarized the historical developments of FEB-induced processing and highlighted the need for its application to mask repair. We reported up-to-date highresolution repair results on several mask types, namely binary (‘‘Cr on quartz’’) masks, attenuated phase-shift
References 1. I. Utke, J. Melngailis, P. Hoffmann, J. Vac. Sci. Technol., B 26(4), 1197–1276 (2008) 2. A. Botman, J.J.L. Mulders, C.W. Hagen, Nanotechnology 20(37), 372001 (2009)
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1614 3. T. Bret, Ph. D. manuscript, EPFL thesis no 3321 (2005). Available online from http://infoscience.epfl.ch/record/53644/files/ EPFL_TH3321.pdf. Accessed 28 April 2014 4. R.W. Christy, J. Appl. Phys. 31, 1680–1683 (1960) 5. K.-H. Lo¨ffler, Ph. D. manuscript, Technical University Berlin (1964) 6. S. Matsui, K. Mori, J. Vac. Sci. Technol., B 4(1), 299–304 (1986) 7. D.J. Oostra, A. Haring, A.E. de Vries, J. Vac. Sci. Technol., B 4, 1278 (1986) 8. H.W.P. Koops et al., J. Vac. Sci. Technol., B 13, 2400–2403 (1995) 9. K.T. Kohlmann-von Platen et al., J. Vac. Sci. Technol. B 14, 4262–4265 (1996) 10. Z. Cui, P.D. Prewett, J.G. Watson, J. Vac. Sci. Technol., B 14(6), 3942 (1996) 11. U.F.W. Behringer, P. Vettiger, J. Vac. Sci. Technol., B 4(1), 94–99 (1986) 12. K. Edinger et al., J. Vac. Sci. Technol., B 22(6), 2902–2906 (2004)
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T. Bret et al. 13. T. Hofmann, N. Auth, K. Edinger, in Nanofabrication using focused ion and electron beams, ed. by I. Utke, S. Moshkalev, P. Russell (Oxford University Press, Oxford, 2012), pp. 349–379 14. K. Jeong, A.B. Kahng, C.J. Progler, Proc. SPIE 8081, 80810P (2011) 15. T. Robinson et al., Proc. SPIE 6730, 67301Y (2007) 16. A. Garetto et al., Proc. SPIE 7488, 74880H (2009) 17. M. Pritschow et al., Proc. SPIE 7488, 74880V (2009) 18. V. Bakshi (ed.), EUV Lithography (SPIE Press, USA, 2009) 19. D. Hellweg et al., Proc. SPIE 8322, 83220L (2012) 20. M. Waiblinger et al., Proc. SPIE 8522, 85221M (2012) 21. R. Jonckheere et al., Proc. SPIE 8166, 81661G (2011) 22. T. Bret et al., Proc. SPIE 8322, 83220C–83221C (2012) 23. H. Miyazoe et al., Appl. Phys. Lett. 92, 043124 (2008) 24. M.G. Lassiter, T. Liang, P.D. Rack, J. Vac. Sci. Technol., B 26, 963 (2008) 25. T. Amano et al., Proc. SPIE 7122, 71222H (2008) 26. A. Perentes, P. Hoffmann, Chem. Vap. Deposition 13, 176–184 (2007)
