lithography

A change is underway in the manufacturing sector as the use of curvilinear shapes on photomasks grows, leading to the real possibility of curvilinear shapes in designs. It may just be the start of a revolution away from Manhattan or rectangular shapes to curvilinear shapes. Changing the physical design infrastructure to be curvilinear seems too daunting a task. Are curvilinear shapes in designs a real possibility? I turned to Aki Fujimura, CEO of D2S, a member of the ESD Alliance, a SEMI Technology Community, to further explain the shape of the future. Smith: What is the difference between Manhattan and curvilinear shapes? Fujimura: Manufactured masks and wafers are all curvilinear, even if the input CAD geometries are rectilinear (shown in Figure 1). It’s always been true that nature can’t make 90-degree turns, so sharp corners were always a matter of how closely you looked. These days, at the leading-edge nodes and their required resolutions, wafers and even masks are all visibly curvilinear as you can see in the graphic on the left in Figure 2. Since the 1980s, both chip design and chip manufacturing systems have used axis-aligned rectangles, or “Manhattan” geometries, because 1) that was sufficient to design transistors and interconnect for the most part, and 2) CPU-based computer algorithms can be made much more efficient for Manhattan geometries. Curvilinear shapes can be piecewise linear polygons of some resolution, or spline-like formats that are curvilinear at any resolution, or specific curved patterns like circles and ovals. Figure 1: All shapes on masks and wafers are curvilinear, even if the input geometries are Manhattan. Source: D2S Smith: What are the benefits of curvilinear masks? Fujimura: The manufacturing side of the semiconductor community knows that the best possible process window for wafer lithography is obtained by using curvilinear correction of mask shapes instead of Manhattan shapes. There have been numerous studies on the topic over several decades. The technique to generate purely curvilinear mask shapes is known as inverse lithography technology or ILT and is an advanced form of optical proximity correction (OPC). At a February 2020 eBeam Initiative event, Micron Technology presented a study showing process window improvement up to 85% for advanced memory designs as a result of using curvilinear ILT (shown in Figure 2). Additionally, Ryan Pearman from D2S presented a study at Photomask Japan 2019 showing that it is preferable to move toward a completely curvilinear paradigm, not only because ILT is better, but because the mask manufactured will have reduced variability. Figure 2: Micron Technology explained the benefits of curvilinear mask shapes for advanced memory at the eBeam Initiative event during 2020 SPIE Advanced Lithography Conference. Source: Micron Technology Smith: If the benefits have been known for decades, why is it happening only now? Fujimura: Several things happened at the same time. Multi-beam mask writing is now available. GPU acceleration for general computing has become mainstream. And wafer process window (resilience to manufacturing variation) is increasingly a problem for the leading-edge nodes as we are in the 5nm node, going to 3nm. Curvilinear ILT is needed much more now than before, will soon be needed for EUV lithography too, and is now possible because of multi-beam mask writing and GPU acceleration. Smith: Curvilinear mask shapes enable curvilinear design shapes too? Fujimura: Adoption of curvilinear mask shapes is the first step in targeting curvilinear shapes on wafers. Without curvilinear masks, it is difficult to target and reliably manufacture curvilinear designs. Curvilinear ILT works in the pixel-space to output the desired mask shape to maximize the process window for wafer lithography. A side effect of curvilinear ILT is that it can also take curvilinear targets as input. ILT, most likely GPU-accelerated ILT, works with rasterized input data, so the ILT algorithm itself is not affected even in runtime by having any amount of curvilinear design data. The resulting mask shapes are written in multi-beam mask writers, which write pixels with doses. They too will write curvilinear masks at the same speed as Manhattan masks. Suddenly now, curvilinear designs can be handled by chip manufacturing equally well for the first time in about 30 years. Smith: But curvilinear designs would be hard, right? There are a lot of tools that depend on the Manhattan assumption. Fujimura: Yes, you’re right. We’re not going to suddenly see chips that have curvilinear routing all over the place, or curvilinear intra-connect in standard cells or memory cells. The entire physical design infrastructure that includes place and route, timing, custom layout, parasitic extraction and design rule checking moving to curvilinear design all at once is extremely unlikely. Could portions of these problems be tackled for specific cases over time as “hot spot” solutions? With GPU-accelerated SPICE being available now, as an example, if GPU acceleration is adopted for design, the same transformation that happened in manufacturing can (gradually) happen in design too. The key question is whether it’s worth the trouble. Smith: Is it worth the trouble? Fujimura: I don’t know if it’s worth the trouble for the entire infrastructure. For hot spots, “hot” for various reasons, there are certainly benefits. Jogging a 32-bit bus by one grid is certainly much more economical space-wise with curvilinear shapes. Inside standard cells or memory cells, there are certain types of features that pack better with curvilinear designs. In general, interconnect is the limiter to chip size of course, but there are always critical areas that could use help to shrink. There are manufacturability benefits as well. In general, when something changes so drastically as this for the first time in 30 years, there’s bound to be some innovation that takes advantage of the discontinuity. Let’s see what the combined capitalistic power of the entire community might be able to come up with. The first thing is to let everyone know that curvilinear designs will be manufacturable today. Hear insights from other leading electronic system design industry CEOs at the SEMI ESD Alliance CEO Outlook on May 18, 2021, 2:00pm-3:00pm PDT. Panelists will discuss the state of the industry along with their views of the outlook for the coming years. Registration is free for SEMI members. About Bob Smith Robert (Bob) Smith is executive director of the ESD Alliance, a SEMI Technology Community. He is responsible for the management and operations of the ESD Alliance, an international association of companies providing goods and services throughout the semiconductor design ecosystem.

This article is the fifth and final in a series highlighting the vital importance of SEMI Standards to commemorate the publication of the 1000th SEMI Standard in July 2019. Find the entire series here.As we define industry standards for managing data in the fab and beyond, we are creating a virtuous circle. More data create better processes. Better processes generate more good data, and more good data lead to better processes. It becomes a cycle of continuous improvement, and we are only just beginning to realize its potential. To dive deeper we interviewed Alan Weber, vice president, New Product Innovations at Cimetrix, and an active member of SEMI Standards Information and Control Committee (IC C).“Industry standards are critical in allowing us to collect information across the fab and use it in increasingly sophisticated control algorithms for the equipment,” said Weber. “The last few years have been about analysis applications that leverage big data in the fab. What started at the lot level is now applied at the wafer level, and for a process like lithography, it’s down to the shot or die level. We’re now collecting enough data variables at individual process and recipe steps to model for predictive maintenance and virtual metrology.”The migration from using data as rearview mirror for identifying and addressing fab issues to using data to head off issues preemptively represents a paradigm shift with immense advantages. This is the starting point for realizing a virtuous data circle.The benefits of a virtuous data circle are simple and compelling: higher yields, faster time to market, more revenue and greater profitability. Our optimism, however, is tempered by major obstacles to this promising future.Multilingual ManufacturingWeber points out that the electronics industry is becoming a multilingual standards world with more than 1,000 fab equipment vendors and several layers of protocols that present the challenge of seamlessly handling multiple protocols. His IC C Committee is out to tackle this challenge.“While SEMI Standards efforts first began in the front end, our standards program now encompasses the back end with test and packaging as well as other device areas including MEMS, sensors and displays,” said James Amano, senior director, International Standards and EHS, SEMI. “We’re going to see data connectivity from the front end to the back end to the final assembly of multi-chip products and that needs standards,” Weber explained. “We’ll need more connected equipment throughout the global, multi-site manufacturing process if we are to support the full traceability requirements of the most demanding markets such as automotive.”The industry will benefit from greater collaboration. Weber predicts that companies will team to create integrated supply chains within broader industry supply chains.Getting the Right People at the Right Time“As we lead the development cycle of a standard from concept to realization, one of the most important jobs of our standards task forces and committees is to coordinate competing companies and build an industry consensus,” Amano said. “This is the case for data in particular, where we rely on industry professionals like Weber and his colleagues, who are working to bring people together to collaborate on developing standards for connectivity and data sharing. It is that critical human element that allows SEMI to sustain our commitment to introducing standards that move the industry forward.”Will Companies Share Data If It Is Secure? Weber contends that when it comes to securing and sharing the data, the biggest challenge is to change the industry’s information-sharing culture.“Finance and defense are already finding ways to deal with data security,” said Weber. “While we will always have problems that require technology fixes, like dealing with new types of computer viruses, I am confident that we will be able to create standards that enable the free, secure flow of information. The key to making progress and better leveraging data is to get companies to see the potential of sharing data while investing in the standards.”SEMI recently launched a project to optimize data sharing across two critical process steps – lithography and plasma etch – to accelerate the adoption of data-driven AI methodologies. The results will help to establish data transfer and management standards crucial to the trusted exchange of trade secrets, IP and other sensitive information. Tools and materials from several SEMI members will be used for the project at Cornell University’s NanoScale Science Technology Facility (CNF). SEMI members are invited to join the project review team. Contact Pushkar Apte at SEMI ([email protected]) for more information on the initiative.Advantages Are Too Great to IgnoreTraditional cultural obstacles aside, the advantages of creating virtuous data circles are simply too great to ignore. Now that it’s accepted wisdom for fabs, factories and supply chains to continuously leverage interconnected data to get smarter, the time has come to extend those advantages throughout the full manufacturing process. Without these data circles, we’ll slow the development of new technologies and applications.We can only speculate where the lines of sharing data are drawn and will be redrawn in the future. But, without doubt, technology innovations such as AI will spawn new information business models that vertically and horizontally integrate companies in ways previously unimaginable. Data standards will underpin this structural transformation.Use your voice to affect standardization in and around the microelectronics industry. Learn about SEMI International Standards – and become part of the solution. Heidi Hoffman is senior director of technology communities marketing at SEMI. Hoffman and her team shine a spotlight on the work of the more than 20 technology communities under the SEMI electronics manufacturing supply chain collaboration platform. Actively engaging community members in marketing programs that showcase their unique value, Hoffman’s team helps companies to grow and prosper through the power of connection, collaboration and innovation.

Materials innovation has always been vital to the semiconductor industry. In the past, it was high-κ gate dielectrics. Today, Cobalt is seen as a replacement for Tungsten in middle-of-line (MOL) contacts.What materials innovation will the future bring?A likely answer is Graphene, the wonder material discovered in 2004.Graphene is one atomic layer of carbon, the thinnest and strongest material that has ever existed. It is 200 times stronger than steel and the lightest material known to man (1 square meter weighing around 0.77 mg). It is an excellent electrical and thermal conductor at room temperature with an electron mobility of ~ 200,000cm2.V-1.s-1. At one atomic layer, graphene is flexible and transparent. Other notable properties of Graphene are its uniform absorption of light across the visible and near infrared spectrum and its applicability towards spintronics-based devices.Graphene and Moore’s LawMoore’s Law scaling can be broken down into 4 key areas: Lithography FET Advanced Packaging (2.5D and 3D IC) Interconnect Material Solutions for upcoming nodes are starting to emerge in the first two areas (EUV and Nanowire- or Nanosheet-based FET respectively). Graphene play an important role in the latter two areas. For advanced packaging, Graphene can be used as a heat spreader (to lower overall thermal resistance), or as an EM shield (to lower crosstalk) as part of a 3D IC package.Active Graphene device layers can potentially be stacked on top of each other using a low-temperature transfer process ( 400°C) to allow for a dense heterogeneous “memory near compute” configuration. This is an area DARPA is actively researching as part of its new $1.5 billion Electronics Resurgence Initiative.Regarding interconnects, Copper interconnects are running out of steam and becoming a major IC bottleneck (projected 40% total delay for 7 nm node). Graphene’s high electron mobility and thermal conductivity make it an attractive interconnect material for MOL and back-end-of-line (BEOL), especially at line widths 30 nm.Graphene Device ApplicationsGraphene-based semiconductor applications are already starting to hit the market. A fully integrated optical transceiver (with a Graphene modulator and photodetector) operating at 25 Gb/s/channel was on display at the recent Mobile World Congress in Barcelona. San Diego-based Nanomedical Diagnostics is selling a medical device that uses a Graphene biosensor. Europe-based Emberion is building Graphene optoelectronic sensors that might find a home in LIDAR applications, where there is currently a focus on improving sensing in low-light conditions.What will the overall Graphene roadmap in the semiconductor industry look like? The history of ion implantation serves as a good example of how a fundamental scientific discovery moves from the lab to the foundry floor.The dominant view in the semiconductor industry at the time was that ion implantation would not work in practice (vs. thermal diffusion) and that, if it did, it would only marginally improve the manufacturing yields of existing products. There was nothing obvious about the transfer of ion bombardment techniques from nuclear physics research to semiconductor production.Varian (led by British physicist Peter Rose) built a new, advanced ion implant tool that Mostek (DRAM manufacturer based in Texas) was able to use to create MOS ICs with clear competitive advantages. The successful collaboration between Varian and Mostek was the turning point in the development of ion implantation as a major semiconductor manufacturing process. Over the next few years, semiconductor firms used ion implantation in a growing number of process steps and, by the late 1970s, it became one of the main processes used in semiconductor manufacturing.Likewise, the Graphene world needs to work closely with the semiconductor industry to develop the tools and techniques required to solve fundamental issues around Graphene growth (good uniformity over large area, low defect density) and Graphene transfer (high throughput, CMOS compatible). It is only then will we fully realize a future that includes 2D materials.The first step in this process is cross-industry education and initiating the dialogue between semiconductor industry and graphene companies. The National Graphene Association will be hosting the largest gathering of graphene companies and commercial stakeholders at the Global Graphene Expo Conference, October 15-17, 2018, in Austin, Texas.Learn more about graphene at the upcoming Global Graphene Expo Conference with dedicated panels of experts and investors, and roundtable discussions on how Graphene will impact the semiconductor industry. The event promo code is SEMINGA. About the AuthorAnand Chamarthy is the CEO and Co-Founder of Lab 91, an Austin-based startup that is working towards Graphene/CMOS integration at the foundry level. Anand can be reached at [email protected]. About the National Graphene AssociationThe National Graphene Association is the main organization and body in the U.S. promoting and advocating for commercialization of graphene and addressing critical issues such as standards and policy development.

The Advanced Lithography TechXPOT at this year’s SEMICON West will explore progress in extreme ultraviolet lithography (EUVL), its economic viability for high-volume manufacturing (HVM) and other lithography solutions that will address the march to 5nm and onward to 3nm.As a prelude to the event, SEMI asked Neeraj Khanna, Global Head of the Patterning Customer Engagement Team at KLA-Tencor, a speaker at the TechXPOT, for insights about the readiness of inspection and metrology tools for EUVL applications at 5nm and 3nm. For a full list of speakers and program agenda, visit http://www.semiconwest.org/programs-catalog/lithography-5nm-and-below.Neeraj Khanna, Global Head of the Patterning Customer Engagement Team at KLA-TencorSEMI: In general, how would you characterize the readiness of inspection and metrology tools intended for EUVL applications at 5nm and 3nm? In particular, what are some of the remaining research and development challenges that need to be addressed for each of these nodes?Neeraj Khanna: KLA-Tencor is working closely with its customers to qualify and ramp EUV. Our suite of inspection, metrology and data analytic solutions are being implemented to enable EUV infrastructure readiness including, for example, new reticle and resist qualification, scanner qualification, and EUV ramp preparation. These EUV integration activities require process control systems that support a wide range of applications, including hotspot discovery, lithography modeling, focus/dose process window qualification, reticle print check, mask blank inspection, and process and tool monitoring.As with any major technology inflection, it is critical to understand sources of process variation to enable ramp at optimal yield. For example, stochastics result in random pattern variations, which have a major impact on EUV yield. To manage stochastics, IC manufacturers are deploying process control solutions that support fast modeling of stochastic variations coupled with high-sensitivity, high-coverage wafer defect inspection. Another example is a methodology called hybrid scanner utilization whereby, when EUV scanners are implemented in production, they will only be used for a few layers, while all other layers will be patterned with 193i scanners. This technique requires tighter control and monitoring of overlay budgets.SEMI: How are you able to achieve this tighter control and monitoring?NK: To understand why hybrid scanner utilization requires tighter control and monitoring of overlay budgets, it’s important to outline how this differs from current scanner implementation. For critical layers in current process flows using 193i lithography, pattern layers for a given wafer are printed using the same stage/chuck on the same scanner. The overlay performance achieved using this lithography strategy is called dedicated chuck overlay (DCO). Use of a dedicated scanner and chuck for lithography reduces inter-scanner and inter-chuck distortion effects, resulting in DCO overlay error of less than 1nm. When EUVL is first implemented in production, it will be used for a few layers – likely, cut masks and contacts with eventual migration to metal 1 layers. All other layers will be patterned with 193i scanners. This hybrid scanner operation eliminates any possibility of using a dedicated scanner and dedicated chuck to support tight overlay performance specifications. Instead, fabs will be forced to optimize mix-and-match overlay (MMO), with the overlay performance obtained using different scanners for printing different layers on a given wafer.With overlay specifications for advanced DRAM and logic at ~2.5nm, fabs will need to implement strict 193i-to-EUV scanner matching strategies or risk consuming 60 to100 percent of the overlay budget on just MMO. To achieve tighter overlay control and monitoring required for MMO, fabs need to implement dense, in-field overlay error measurements that feed into scanner fleet management systems. KLA-Tencor’s ATL™ overlay metrology system supports a high measurement speed and the use of small in-die targets, enabling dense in-field overlay measurements with high accuracy. Our 5D Analyzer® data analytic and management system includes scanner fleet management capability that enables automatic product-based corrections to minimize MMO error, helping fabs reduce the risk to yield loss associated with a 193i-EUV mixed scanner implementation. SEMI: What other challenges do you see coming to the fore at 5nm and 3nm?NK: Overall, the 5nm and beyond design nodes will face challenges associated with new lithography technology, potential new device structures and smaller pattern pitches. IC manufacturers will require process control solutions that not only identify process windows, but also monitor patterning parameters and defectivity at multiple points to identify process shifts. To monitor dynamic processes at these advanced nodes, inspection and metrology tools will need to have both sensitivity to critical parameters/defects and robustness to process variation in order to provide IC engineers with smart feedback for efficient control of their processes.SEMI: Could you elaborate on what will be required to monitor patterning parameters and defectivity at multiple points? How different will the techniques be at 5nm/3nm vs. at say, 7nm or 10nm?NK: As an example of monitoring at multiple points, consider the transition to EUV lithography. With EUV, the cost per scan goes up dramatically. Thus, IC manufacturers will monitor parameters at multiple points to maximize yield and minimize risk: EUV reticle qualification requires inspection and metrology throughout the entire flow from mask blank manufacturing, to the mask shop, to the IC fab. For the advanced design nodes associated with EUV, wafer qualification requires monitoring and control of wafer defectivity, shape and geometry throughout the wafer manufacturing process. It also requires control of fab incoming wafer qualification while ensuring that fab-wide processes are meeting defect and shape standards necessary for printing smaller feature sizes. EUV resist characterization and qualification requires comprehensive lithography simulation, wafer defect inspection, and film thickness and uniformity measurements to help reduce development time and prepare the litho stacks for production ramp. As EUV scanners are ramping, fabs are faced with finding any unknown particle sources within the new scanner chambers. This situation is driving the need for tighter and more frequent PWP (particles per wafer pass) chamber monitors to ensure scanner cleanliness. In addition, EUV scanner qualification requires reticle front and backside particle checks, and hotspot discovery and process window qualification using optical wafer defect inspection and e-beam review. EUV process monitoring encompasses overlay error monitoring, focus/dose monitoring, critical dimension (CD) and 3D device shape monitoring, and continuous process window monitoring. EUV inline defect monitoring and tool monitoring is important for reducing baseline defectivity in the litho cell for faster ramp, and for early identification of litho excursions in production. A critical part of this monitoring strategy is inline after develop inspection (ADI) monitoring, which is defect inspection on patterned wafers after printing and development of the resist ADI. Inline ADI innovations that find yield-critical defects allow fabs to reduce process issues, prevent at-risk wafers early in the process, and enable rework when excursions are found in production. As with past technology transitions, the implementation of multiple monitoring steps as part of a comprehensive process control strategy will be critical for a fab’s successful ramp of EUV.Debra Vogler, SEMI

This year’s Advanced Lithography TechXPOT at SEMICON West will explore the progress on extreme ultra-violet lithography (EUVL) and its economic viability for high-volume manufacturing (HVM), as well as other lithography solutions that can address the march to 5nm and onward to 3nm. Several session speakers offered their insights into the readiness of EUVL for 5nm and how other lithography solutions will enable 3nm. See the full list of speakers and program agenda at http://www.semiconwest.org/programs-catalog/lithography-5nm-and-below.Diverging viewpoints on EUVL readiness for 5nmMike Lercel, Director of Strategic Marketing at ASMLASML expects its first customer to start volume manufacturing with EUV at the 7nm logic node and the mid-10nm DRAM node in the 2018/2019 timeframe. “EUV will replace the most difficult layers that require multiple patterning, and many layers will continue to be allocated to immersion tools for the foreseeable future,” said Lercel. “For the 5nm logic node, more layers are expected to migrate to EUV.”Three ASML customers have early-access versions of the next-generation TWINSCAN NXT:2000i for the development of advanced logic and DRAM nodes. “This system delivers 2.0nm cross-matched on-product overlay, achieved through several hardware advancements,” noted Lercel. “It is also significant because this mix-and-match use with EUV features a significantly different hardware platform.” TWINSCAN NXT:2000i features a new alignment sensor and improved wafer table flatness, endurance, and clamping mechanism to enhance matching to EUV.ASML has achieved good industrialization progress of its pellicle, with tests confirming that pellicles can withstand 245W source power and an offline power lifetime test indicating 400W capability. Compared to the 7nm logic node, the requirements for EUV masks will become tighter at 5nm, but Lercel noted that ASML sees good progress with the industry infrastructure to support 5nm in areas such as reducing mask blank defects. “We will continue to improve pellicle transmission for enhanced throughput, but there are no fundamental changes in pellicle requirements for 5-3nm logic nodes. We see no infrastructure showstoppers for the introduction of EUVL at the 5nm node.”Stephen Renwick, Director of Imaging Physics at Nikon Research Corporation of AmericaRenwick said that the 7nm logic node is expected to be fabbed mostly using 193i lithography. “EUV will struggle to be ready for 5nm, limited by yield issues caused by stochastic effects in the resist,” said Renwick. “Ready or not, though, it will be used.” Renwick suggests that introducing multiple-patterning with EUV may be needed but would increase costs. “193i lithography will continue to be used with quadruple-patterning and in combination with other techniques – there is no single solution.”Figure 1. Normalized cost/layer vs. lithography method. SOURCE: Nikon Research Corporation of America When choosing between immersion lithography and EUV for different customer segments at 5nm, Renwick noted that the cost depends on the layer. “Some time ago, we calculated that the costs of either 193i triple-patterning or 193i SADP with two cuts were roughly equal to single-patterning with EUV,” explained Renwick (Figure 1). “That agreed with chipmakers' public estimates and meant that the choice of lithography method depended more on the performance tradeoffs involved, such as 193i's better line-edge roughness. At the 5nm node, we are probably faced with quad-patterning from 193i, double-patterning from existing EUV tools, or single-patterning from as-yet undelivered high-numerical aperture (NA) EUV tools.” Renwick believes that the competition between low-NA EUV double-patterning and 193i quad-patterning will be similar to the current situation (i.e., comparison of 193i triple-patterning or 193i SADP with two cuts vs. single-patterning with EUV), but for high-NA EUV tools he believes it's too early to say.Other challenges Renwick sees on the horizon for EUVL at 5nm are stochastic effects in EUV resists. “They cause yield problems on contact arrays and unacceptable line-edge roughness on line/space patterns,” said Renwick. “It's unlikely that these effects will go away without increasing the litho dose, which will further challenge throughput performance.” He also questions whether EUV pellicles, though under development, will be “ready for prime time.”Harry Levinson, Sr. Director of Strategic Lithography Technology and Sr. Fellow at GLOBALFOUNDRIESLevinson said additional fundamental engineering work is needed to ready EUV lithography for 5nm. “Among the top problems are stochastics-induced resist defects, which increase significantly as dimensions shrink below those for 7nm,” explained Levinson (Figure 2). “Higher exposure doses will be required to address these issues related to stochastics at 5nm, which will require higher source output” (than 7nm).Levinson said there will be greater motivation to use EUVL at the 5nm node vs. at 7nm to offset the large number of exposures associated with 193nm immersion multiple-patterning solutions. “The primary application of EUV lithography at 7nm will be for contact, via and cut layers,” Levinson noted. “It will be important to enable EUVL for metal masks at the 5nm node, which increases the need for an ample supply of very low defect EUV mask blanks.” Levinson added that the 7nm node is already stressing defect inspection capabilities, and no actinic defect inspection system is yet available for patterned masks. “This situation becomes more problematic with widespread application of EUVL to metal layers.” Mask development for 5nmChristopher C. Progler, CTO Strategic Planning at PhotronicsProgler said that the basic infrastructure for delivering EUV masks is available, especially for dark field layers and near in nodes. “The interconnected or more open frame patterns will need refinements to the processes and two to three nodes out will need certain new infrastructure,” said Progler. Overall, the main challenges for initial insertion are about creating a cost-effective and rapid-turn EUV mask process, he said. “The industry can certainly deliver EUV masks in some form. It is more a question of doing it efficiently and productively to match the stated value proposition of EUV over other lithographic methods. We don’t want a pick two of ‘cost, cycle time, capability’ sort of mask solution.” More specifically, Progler explained that after the initial EUV mask development for 5nm focused on contacts and block layers, the major push for N5 switched to delivering single-exposure EUV metal patterning as early as possible. “This has opened some new challenges for masks given the resolution, critical pattern density and tight pitch defect requirements of the re-aggregated single-layer metal mask designs,” said Progler. “For example, on the resolution side, we are accelerating the insertion of higher dose photoresists and also driving patterning module improvements in CD control, mask LER and sidewall angle.” Progler added that at N5, the mask 3D structure itself – including the sidewall – will have a greater impact on lithography because it is tied to stochastic error rates on the wafer.“Reliable, wide-area metrology for some of these 2D and 3D mask parameters is currently hard to come by. We may see an evolution of the blank structure at some point in N5, including hard mask options for pattern stability and expect earlier insertion of EUV mask process correction with model-based hot spot detection and rule checking as well. We also hope mask-scanner dedication is not needed, but there are some indications process sensitivity may push us earlier in this direction.” He added that to reduce metal layer defects, more attention needs to be devoted to advanced repair and model-based validation. “We are, unfortunately, still in a situation of blurry vision and high native defect counts alongside possible in situ contamination during mask changes.” Figure 2. Resist stochastics-induced defects. Graph courtesy of Peter DeBisschop, imec; SOURCE: GLOBALFOUNDRIES Progler pointed out that, with the advent at 5nm, metal masks will require some level of actinic blank inspection for yield, increasing the cost of an already expensive mask technology. “So, unless we want to contend with double and triple photomasks’ starts to deliver a single metal layer, it will be very important to tighten the multi-sensor inspection, defect abatement, and repair loops,” said Progler. He does see some clouds forming around high-volume manufacturing pellicles for metal layers. “This remains an open question, mainly for thermal and materials reasons, not to mention cost and cycle time,” Progler said. “We may be pessimistic, but we do not see an HVM pellicle solution converging in the required timeframe, which means leaning even more on a wafer-level inspection in the validation loop.” He believes that streamlining validation will be a differentiator. “I can imagine one losing most of the EUV cycle time benefits by endlessly circling masks around if this is not done well.” How does the industry get to 3nm? ASML plans to ship its first high-NA EUV prototope/pilot systems between 2020 and 2023 to support 3-2nm process development. “System designs are now being finalized and the platform is starting to come to life,” said Lercel. ASML supplier ZEISS is building a high-NA cleanroom for optics production. ASML believes that EUV, high-NA and DUV systems will be used together at the most advanced nodes and is designing to account for this mixed environment. “As chipmakers drive toward smaller geometries in the most advanced nodes like 3nm, they face unprecedented challenges in devices and materials. This will make the process control requirements even more challenging.” ASML is tackling these challenges with its YieldStar metrology platform, e-beam metrology (HMI) and computational lithography solutions that are designed to expand the process window, enhance process control, and improve patterning defect detection. “This ‘Holistic Lithography’ approach will become increasingly important to ensure throughput and yield at the most advanced nodes.”Levinson said that the issues he projects for 5nm will need to be addressed further at 3nm. “The challenges associated with resists at 3nm dimensions are such that it isn’t clear that chemically amplified resists will be capable of meeting requirements,” said Levinson. “If true, we would be seeing the most significant change in resist platforms in a quarter of a century. Potentially cost-reducing technologies such as directed self-assembly (DSA) are always welcome, but EUVL will be the lithographic workhorse through the 3nm node, and likely beyond.”At 3nm, mask makers will confront the realities of higher EUV NA tools. “We will need to implement thinner mask absorbers, new films, and perhaps hard masks,” Progler said. “This puts us in a new materials regime for masks, and history has shown us the mask industry takes a long time to refine processes and tools for new mask materials.” He explained that the small scale of the mask ecosystem and the small number of large suppliers available to address the challenges accounts for this lengthy time frame. Still, looking ahead, Progler noted that Photronics has already done a few studies on the impact of proposed half-field, high NA anamorphic optics on masks. "We uncovered some challenges that need to be addressed, particularly at boundaries and within the overall mask flow,” said Progler. As mask resolution continues to scale down, the industry will need fundamentally higher resolution mask making and inspection processes, requiring next-generation multi-beam mask writing and electron beam inspection, he explained.At 3nm and below, Progler noted that the metrology needs for masks, while not as severe as that for wafers at these nodes, will test the mask equipment infrastructure in ways that could challenge the relatively small mask industry. “Of course, EUV multi-patterning comes into play as well, and with that, the SRAF sizes will drop below 20nm, requiring an asymmetric compensation over a much wider influence area than the OPC people are used to considering.” With EUV multi-patterning, Progler explained that it will be increasingly important to match or pair EUV masks and to consider how 3D effects and stochastics will drive new technology to enable new requirements for high-speed metrology and simulation components. “All the justifiable hand-wringing over EPE with ArF multi-patterning today gets introduced to the EUV scene when masks are ganged together to make a single device layer,” said Progler.Debra Vogler, SEMI