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Directed Self-Assembly Requirements for Sub 20nm Lithography C. Michael Garner, Daniel Herr, and Christof Krautschik Abstract: This paper summarizes a set of critical requirements that directed self-assembly materials and methods must satisfy before this emerging technology warrants consideration as a potential patterning solution for sub-20 nm technology. These requirements transcend specific material systems and are intended to guide research, which identifies promising families of self-assembling materials that enable the fabrication and extensibility of charge based technology to its ultimate limits. If one family of smart resist materials is found that simultaneously meets these requirements, then directed self-assembly may move from the realm of academic exercise to that of an enabling option. Also, provided in Appendix I is an overview of the state-of-the-art in directed self assembly for one class of self-assembling polymeric materials, diblock copolymers. Introduction: Figure 1 reflects current industry consensus on lithography exposure tool potential solutions for enabling top down patterning through the 16 nm DRAM half-pitch node, in 2019. It suggests that EUV and innovative 193 nm optical immersion technologies will dominate between the 32 nm and the 22 nm technology nodes. At the 22 nm technology node, the industry recognizes a need to consider innovative patterning options. This potential solutions roadmap suggests that research on such innovative options must be launched by 2008 to impact this node‘s development insertion window, in 2011. 2007 2010 2013 2016 2019 Notes: EPL is a 2005 2006 2008 2009 2011 2012 2014 2015 2017 2018 2020 2021 potential solution DRAM 1/2 Pitch 65nm 45nm 32nm 22nm 16nm for 65-nm ½ pitch in one geographical DRAM Half-pitch region. RET and lithography friendly 193 nm 65 193 nm immersion with water Flash Half-pitch design rules will be 193 nm immersion with water used with all optical lithography Narrow 45 193 nm immersion with other fluids options EUV, ML2 solutions, including EUV with immersion; i 32 193 nm immersion with other fluids and lens material Innovative 193 nm immersion with water Narrow options therefore, they are Imprint, ML2 not explicitly noted. EUV RET—resolution 22 Innovative 193 nm immersion Imprint, ML2, innovative technology Narrow options enhancement technology EUV— extreme ultraviolet EPL—electron 16 Innovative technology Narrow Innovative EUV, imprint, ML2 options projection lithography ML2— Research Required Development Underway Qualification/Pre-Production Continuous Improvement maskless This legend indicates the time during which research, development, and qualification/pre-production should be taking place for the solution. lithography PEL— proximity electron lithography. Figure 1. 2005 ITRS Lithography Exposure Tool Potential Solutions1a While identified exposure tool technologies, augmented with RETs, may satisfy average resolution specifications through the 32 nm node, alternate approaches are needed to achieve the projected patterning dimensional and positioning tolerance requirements. Consider, for example, the recent projected ITRS MPU gate CD, line edge roughness, and overlay control requirements, shown in Figure 2. A key message from this table is that the lithography community foresees no known potential solutions File: DSAR10 031606 DH.doc 1 for achieving the projected 2007 MPU gate CD control requirements. By 2010, it expects to face a ‗red brick wall‘, with respect to MPU gate CD, line edge roughness, and overlay control requirements. Year of Production 2005 2008 2011 2014 2017 2020 MPU physical gate length (nm) [after etch] 32 23 16 11 8 6 MPU gate in resist length (nm) 53 38 27 19 13 9 Resist meets requirements for gate 3.3 2.3 1.7 1.1 0.8 0.6 resolution and gate CD control (nm, 3 sigma) Line width roughness: 2.6 1.8 1.3 0.9 0.6 0.5 (nm, 3 sigma) <8% of CD Overlay (3 sigma) (nm) 15 10 7.1 5.1 3.6 2.5 Figure 2. Selected ITRS lithography variability control requirements1b Significant targeted research progress must be made over the next four to five years for directed self- assembly to become a viable sub 20nm patterning technology option. A fundamental question is ―how can these intermolecular attractive and repulsive forces be harnessed to fabricate features, which are aligned with top down generated patterns and make critical patterns at the required sizes, with low defect densities, and with reasonable annealing times?‖ High Level Requirements: For the semiconductor industry to seriously consider this technology as a viable potential solution for sub 20 nm lithography (ITRS 2016), several fundamental capabilities need to be demonstrated by 2010. This decision point is based on the assumption that, prior to leading edge production, it would take at least six years to develop the necessary infrastructure within the supply base (chemical and CAD Tool) and semiconductor companies. Key directed self-assembly research milestones include: 1. Improved long range dimensional control [~1.2 nm 3 by 2010] and low frequency line edge roughness (LER) [~1.5 nm 3 by 2010] that satisfy projected ITRS requirements; 2. Improved resolution and linear density by at least a factor of two over that achieved by top down lithography [~11 nm resolution by 2010]; 3. Demonstrated essential set of sub-lithographic isolated and periodic lines and circular openings; 4. Fabricated features with multiple sizes and pitches* in the same layer in different regions of a chip; 5. Characterized and quantified defects levels; developed foundational understanding of material and process related defect mechanisms, and established guiding principles for ameliorating and managing defects; 6. Satisfied projected ITRS alignment and registration tolerances [Overlay of ~5.1 nm by 2010]; 7. Demonstrated a proof of concept for achieving projected ITRS throughput requirements, either via single wafer or batch processing [Average net throughput of ~1 wafer per 1-2 minutes]. Can batch processing achieve the required patterns at a rate of minutes per wafer? 8. Demonstrated potential extensibility beyond the 10 nm ITRS technology generation; 9. Satisfied projected ITRS etch and pattern transfer requirements for fabricating electronically useful features; 10. Demonstrated ease of integration and an enhanced process window. *Note: 1) Pitch is effectively the period of repeated features, which corresponds to the sum of a line‘s width and an adjacent space. 2) This paper considers self-assembling materials as smart resists, which may offer enhanced dimensional control and sub-lithographic pattern formation. Additionally, but not considered here, self-assembling materials may be designed to express electronically useful functionality that can be integrated within the structure of a device. File: DSAR10 031606 DH.doc 2 Key challenges: There are several fundamental challenges that must be addressed for this technique to be useful in extreme lithography and these were discussed earlier. As was shown above, self assembly appears to be making progress in controlling line edge roughness and mechanisms may be emerging to force alignment, but significant progress is needed in these and the other areas. By 2010, directed self-assembly must: 1. Demonstrate the feasibility of achieving projected 2017 low frequency Line Width Roughness requirements of 1.1 nm 3 by 2013. 2. Determine limits of sub-lithographic resolution extensibility. 3. Be able to fabricate a basis set of essential features, shown in Figure 6. The biggest challenges will be the controlled self assembly these features with 2X higher linear density than lithography and having multiple sizes and pitches within a given mask layer. This corresponds to at least 11 nm dense lines and spaces demonstrated by 2010. While achieving this 2X linear density for contacts should be reasonable, this may be much more difficult for the isolated lines, contacts, and the periodic lines, because new techniques will need to be developed to enable these capabilities.2 (a) (b) (c) (d) Figure 6. The most fundamental features needed are a) isolated lines, and c) contact openings and b) periodic lines and d) periodic hexagonal arrays of contact openings. In addition to this essential basis set of shapes, the following additional features are considered desirable.2 (a) (b) (c) (d) (e) Figure 7. Additional features considered desirable: a) T-shapes, b) L-shapes, c) boxes, d) periodic square arrays of contact openings, and e) dense lines with embedded jog and T structures. 4. Demonstrate an ability to form features with multiple sizes and pitches in different locations on the same layer. The size of features and spaces between them is controlled by the molecular weight of the different components of the block co-polymers and the relative volume occupied by the different components used. Thus, new mechanisms may need to be developed to modulate the co-polymer size controllably in different regions of the same layer. 5. Develop a foundational understanding of material and process related defectivity mechanisms and establish strategies for ameliorating and managing defects. Characterization of defect levels and defectivity mechanisms are needed. File: DSAR10 031606 DH.doc 3 6. Demonstrate a path to satisfy projected alignment and registration requirements: The next major research capability is to demonstrate mechanisms to align features from different layers, such as contacts to or between lines as shown in Figure 8a. Directed self assembly would be most useful in fabrication of transistors, memory cells, and the highest density interconnects that are present in the lower levels. Thus, there would be a need to align to a second layer of interconnects to the ―contacts‖ in 8a, as shown in 8b. a) b) Figure 8. The alignment of self assembled contacts to pre-existing lines is illustrated in a) and it should be noted that the self assembly of ―contacts will be in hexagonal patterns unless new processes constrain it to other order. In b) a second interconnect layer is then being aligned to one set of the contacts in the previous layer. These examples are only to illustrate the type of alignment that may be needed to fabricate devices and interconnects with directed self assembly. Furthermore, the alignment between contacts and lines in different levels needs to meet the ITRS goal of ~30% to achieve the required improvements in density and this may require development of new structures and mechanisms. While the work of Ross and co-workers demonstrates that introducing sharp structures can cause alignment of the ―contacts or dots‖, significant work must be done to translate this to level-to-level registration. 7. Demonstrate anneal times that are competitive with projected ITRS throughput requirements. Since these processes are driven by molecular repulsion, thermodynamics, diffusion, and interfacial energy, defects may be generated and the time to reach equilibrium may take considerable time. Also, the time required for the directed self assembly to reach completion needs to be less than an hour and preferably be on the order of minutes, so mechanisms to accelerate this need to be studied. 8. Demonstrate sub-10 nm pattern formation. Can directed self assembly be extended to the patterning of sub 10nm features, with a 20 nm pitch? For this technology to be of interest to industry, it must be demonstrated that it can be extended multiple generations. Since the size of features is controlled by the size of polymers in one of the constituents, decreasing the number of blocks will be needed, but it is not clear how this affects defect density and reproducibility. 9. Demonstrate competitive etch resistance, sufficient to satisfy projected pattern transfer requirements. 10. Compatibility with Si processing and enable affordable extensibility of patterning charge based technology to its ultimate limits. File: DSAR10 031606 DH.doc 4 Please refer to Appendix II for the most recent quantitative set of projected directed self-assembly research requirements. Current research has been using di-block copolymers, but research may need to investigate use of more complex tri-block co-polymers and other families of materials to achieve the control of structures and properties required for this technology to be useful. Furthermore, as this research is progressing, models of the mechanisms will need to be developed to accelerate progress. Future families of materials may utilize more advanced means of self assembly than the hydrophilic/ hydrophobic block approach suggested here. For example, it is possible to synthesize objects with asymmetric chemical reactivity that causes randomly dispersed arrays of such objects to spontaneously assemble into predetermined, programmed sequences or arrangements. The process spontaneously creates order out of chaos and can, in principle produce arbitrary shapes. In one demonstration, for example, DNA was demonstrated to be a useful ―glue‖ for bringing nanoscale 3 and microscale4 building blocks together with high specificity and functionality. DNA assemblies have been made that range in complexity from simple aggregates4 to periodic crystals,5 lattices,6 nets,7 and even the letters of the alphabet (i.e. D-N-A).8 Other molecules with high information content, such as RNA, 9 peptides,10 and peptide-mimetics11 have been used for similar purposes. Macroscopic components have been shown to undergo analogous types of assembly on an air-water interface if their shape and surface energies are designed appropriately.12,13 The term ―programmed‖, or ―algorithmic‖ self assembly has been coined to describe these new types of self assembling systems. Work in this field is barely ten years old, but it is developing a rich theoretical basis.14-16 The state of this field and the length scale of objects that can be assembled in this way suggest that it could play a very important role in many nanoscale electronics applications. The key challenge is to devise designs for the chemical interactions that allow these systems to spontaneously create order from chaos without the use of a physical template. We need to learn how to incorporate the appropriate chemical reactivity into functional objects so that at equilibrium, they spontaneously assemble into highly aperiodic structures. The principle is clear but the design of the process is lacking. New equilibrium interactions based on cooperative hydrogen bonding must be developed in order to demonstrate the full potential of this very powerful technique. Guiding principles and positional/orientation/conformational control. Finally, if this research is successful, the design community will need CAD tools to develop design rules and alignment strategies to achieve the required dimensional and placement control. Furthermore, resist suppliers and chemical companies will need to develop sustainable, benign, high performance material formulations with required control of features, defects, cure time, and alignment. File: DSAR10 031606 DH.doc 5 Appendix I Background: Industry concern for these potential show stoppers is driving research on alternate approaches that exhibit the potential of extending top down methods to their ultimate limits. This paper focuses on recent advances in directed self-assembly materials that impact these critical patterning requirements, specifically dimensional and positional control. Within this context, phase segregating block copolymers represent a promising evolutionary step forward in resist technology. These high information content resist materials are designed to make use of additional nanoscale chemical processes that can be tuned to enhance dimensional control. The design of a directed self-assembly technology requires an understanding of three interdependent factors: A material assembly instruction set, an internal assembly tool box, and a set of processes that guide how a given material interacts with its local environment. For example, consider the directed epitaxial self-assembly of block copolymers on lithographically defined nanopatterned substrates. 17 A copolymer is any polymer that is built from more than one monomer type. The family of copolymers made from two monomer types, A and B, can exist as one of three forms: Random, alternating or block. A random copolymer is characterized by a random ordering of the A and B monomer units in the polymeric chain, such as BBABAAABAABBBAB. Alternating copolymers exhibit a regular alternating pattern of monomer units in the polymeric chain, such as ABABABABABAB. Block copolymers represent an unusual class of polymers that are synthesized from two or more distinct homogeneous polymeric blocks. For the case of a diblock copolymer, the structure might correspond to AAAAAAAAAAA- BBBBBBBBB and be represented by the blue and red sections of the polymer chain shown in the last frame of Figure 3. Figure 3. Generic structure of a linear AAAAAAAAAAA-BBBBBBBBB (Blue-Red) diblock copolymer. Each block is homogeneous and may exhibit similar or dissimilar, oil-like (blue) vs. water-like (red), behaviors. A diblock copolymer contains information that determines its preferred mode of assembly into a film. It contains information that codes for whether the blocks will remain miscible or tend to phase segregate into sheets, lamellae, embedded rods or cylinders, or embedded spheres. The fundamental mechanism that drives self assembly in this family of polymers is molecular attraction and repulsion, such as that between oil and water. Each block also codes for the dimensions of the preferred phase segregated structures, preferred orientation with respect to the local external environment, and other useful macroscopic properties. For example, the volume fraction of one block determines its morphology within the polymer matrix. A polymeric block that makes up a volume fraction of 15% may exhibit a spherical morphology, while a corresponding block design to make up a volume fraction of 30% may yield cylindrical structures. Additionally, the interaction energy between the blocks controls the sharpness of the phase segregated interface. Hence, a diblock copolymer contains a rudimentary internal instruction set that influences its internal assembly options. Could the mechanisms that drive the phase segregation of these blocks extend lithographic technology by enabling the controlled formation of sub 20nm patterns to extend lithography? If recent research progress continues, the answer is a definite maybe. While the size of phase segregated structures tend to scale with the size of the polymeric blocks, the driving force for phase segregation also diminishes. The self assembling behavior of these thin film systems in the sub-10 nm regime has yet to be demonstrated. Progress to date: File: DSAR10 031606 DH.doc 6 1. Dimensional control: Profs. Nealey‘s and DePablo‘s teams, at the University of Wisconsin at Madison, recently compared the post etch LER of features formed via the phase segregation of a polystyrene-PMMA block copolymer with that of lithographically patterned and etched features in PMMA. The directed self-assembly process exhibited reduced post etch LER, compared with a lithographically patterned and etched feature in PMMA. Quantitatively, the measured post etch line edge roughness of ~2.4 nm (3) in the diblock copolymeric resist is about half of that measured in PMMA, ~4.65 nm (3). These early results are shown in Figure 4. Figure 4. Post etched dense line/space structures in a) PMMA and in b) phase segregated PS-PMMA diblock copolymers.18 A number of directed self-assembly methods get their directions from a pre-established pattern, which is generated lithographically. This naturally leads to the question: How can self-assembly be better than the lithography from which it originates? It is like wafer patterns being limited by the masks. P. Nealey‘s team applied self assembling materials of rather uniform size over a lithographically patterned surface of constant pitch, with features that varied in width. They report that the final feature size appeared to be locked in by and reflect the size of the polymer more than that of the underlying lithography. In this case, they argue that the size of the polymer in these phase segregating systems drives the long range dimensions of features and tends to smooth out and reduce some of the variability in the underlying lithographically defined template. They infer that dimensional tuning of the polymeric blocks and the phase segregation process may enable enhanced long range dimensional control. Their results suggest that this family of materials exhibit a self-healing attribute during the bake, i.e. anneal, step. While this work is suggestive, it remains to be seen whether an optimally designed family of self- assembling materials offers improved LER and long range dimensional control over that observed with conventional resist systems, under all essential and competitive patterning situations. 2. Directed sub-lithographic feature formation: At MIT, Prof. Ross‘s team has shown that sub- lithographic features can be formed and ordered, via templated self-assembly, when the block copolymer is constrained within a lithographically formed trench. An early example of directing the self-assembly of features with sub-lithographic resolution is shown in Figure 5. Can we embed and control the position of electronically useful materials within these features? Her group also was able to force alignment of these sub-lithographic features to small patterned alignment structures, such as the small wall defect in circled in Figure 5, with ~6 nm positional control over a distance of ~1 micron. Thus, this may be a first step to establishing mechanisms to force the self assembled structures to align to top down structures or structures generated by earlier process steps. File: DSAR10 031606 DH.doc 7 Figure 5. Demonstration of sub-lithographic feature registration induced by a programmed ~20 nm side wall defect.19 3. Essential shapes: Demonstrated a limited set of shapes required by the design community, such as: a) Dense linear arrays of lines and spaces, b) dense L-shaped arrays of lines and spaces; c) dense circular arrays of lines and spaces, and d) dense linear arrays of hexagonally packed spheres. 20 Directed self-assembly has yet to demonstrate the formation of isolated structures. This topic warrants additional focus. 4. Multiple sizes and pitches on the same level: This has yet to be demonstrated. 5. Defectivity: This topic has received limited research attention within the context of patterning and warrants significant emphasis. However, the polymer physics community has a built a considerable knowledgebase on defects within bulk polymeric materials.21 6. Alignment and Registration: Figure 5 suggests a proof of concept. This topic requires considerable focus. 7. Throughput: Current anneal times range from minutes to days. There are reports that some polymers anneal at ambient temperatures and their anneal times exhibit some tunability.22 This topic requires significant emphasis. 8. Extensibility to 10 nm and beyond: Preliminary models suggest that directed self-assembly may be extensible to the sub-10 nm regime. However, this topic warrants considerable research focus to gain a more fundamental understanding of the thermodynamic limits of this technology. Shorter polymer chains imply shorter phase segregation periods and a lower ordering temperature, which may require that the process be quenched to lock features in place. However, decreasing the size of the polymer blocks also reduces the driving force for pattern formation, i.e. phase segregation. 9. Etch resistance: Early results suggest resist-like etch resistance. However, this topic warrants considerable focus. 10. Integration and process window: Results are suggestive thus far. This topic warrants further emphasis. File: DSAR10 031606 DH.doc 8 Appendix II. Directed Self-Assembly Research Requirements for 2011-2012 Metric Requirement Defects and defect management strategies <0.02 20 nm defects/cm2 Line Edge Roughness ~2.1 nm 3 Gate CD Control ~1.7 nm 3 Resolution 11 nm Essential shapes Dense and Isolated L/S, circles, hexagonal arrays Overlay and registration 5.1-7.1 nm 3 Throughput 1 W/Min Etch and pattern transfer ~CARs Placement and orientation Under development Multiple Sizes-Pitches/Layer Overall Performance 2-3/layer Other ESH Impact? Selected References 1) International Technology Roadmap for Semiconductors, Santa Clara, CA: Semiconductor Industry Association - a) p. 470; b) pp. 700-701  2) SRC Patterning TTAB, Consensus position on a basis set of essential and desired shapes, March 2006. 3) Mirkin, C., Programming the assembly of two- and three- dimensional architectures with DNA and nanoscale inorganic building blocks. Inorg Chem., 2000. 39(11): p. 2258-72. 4) Valignat, M., et al., Reversible self-assembly and directed assembly of DNA-linked micrometer-sized colloids. PNAS, 2005. 102(12): p. 4225-9. 5) Winfree, E., et al., Design and self assembly of two-dimensional DNA crystals. Nature, 1998. 394(6693): p. 539-544. 6) Paukstelis, P., et al., Crystal Structure of a continuous three-dimensional DNA lattice. Chem. Biol., 2004: p. 1119-11126. 7) Li, H., et al., DNA templated self-assembly of protein and nanoparticle linear arrays. JACS, 2004. 126(2): p. 418-9. 8) Park, S., et al., Finite-size, fully addressable DNA tile lattices formed by hierarchical assembly procedures. Angew Chem Int Ed Engl., 2006. 45(5): p. 735-9. 9) Chworos, A., et al., Building programmable jigsaw puzzles with RNA. Science, 2004. 306(5704): p. 2068-2072. 10) Kotch, F. and R. RT, Self-assembly of synthetic collagen triple helices. PNAS, 2006. Epub ahead of print. 11) Percec, V., et al., Self-assembly of amphiphilic dendritic dipeptides into helical pores. Nature, 2004. 430(7001): p. 764-8. 12) Bowden, N., et al., Self-Assembly of Mesoscale Objects into Ordered Two-Dimensional Arrays. Science, 1997. 276(5310): p. 233-5. 13) Rothemund, P., Using lateral capillary forces to compute by self-assembly. Proceedings of the National Academy of Science, 2000. 97(3): p. 984-989. 14) Adleman, L., Toward a Mathematical Theory of Self Assembly. 1999, University of Southern California: Los Angeles. 15) Adleman, L., et al. 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Minoret, CEA Acknowledgements The authors wish to thank the following people for their review, insightful feedback, and contributions to this requirements document: Charles Black, Luigi Capodieci, Ralph Cavin, Juan DePablo, John Hartley, Harry Levinson, Paul Nealey, Chris Ober, Mihri Ozkan, Caroline Ross, Thomas Russell, Frank Schellenberg, Marc Ulrich, James Watkins, C. Grant Willson, H.-S. Philip Wong, Victor Zhirnov, and the SRC‘s Patterning Thrust Technical Advisory Board. File: DSAR10 031606 DH.doc 10