Research Article Archive Versions 3 Vol 1 (2) : 18010204 2018
The 2017 IRDS Lithography Roadmap
: 2018 - 11 - 14
: 2018 - 12 - 27
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Abstract & Keywords
Abstract: Technology roadmaps have been a part of the semiconductor industry for many years. The first roadmap was Moore’s law, which started as an empirical observation that competitive forces then turned into a prediction that became an industry roadmap. Then the ITRS roadmap was developed and for many years was used by leading edge semiconductor producers to drive new technology they needed. Now there is the IRDS roadmap, which projects semiconductor end user requirements and develops a technology roadmap based on those requirements. The 2017 IRDS roadmap was just released. To prepare the roadmap, we received input from experts around the world. The roadmap predicts that the requirements of high performance logic will drive the development of different device structures in logic chips. Memory technology will also advance but is more focused on cost than high performance logic is. Because of this, there may be a split in the patterning roadmaps for different types of devices. Logic will adopt EUV and its extensions, while flash memory will consider nanoimprint. Directed self-assembly and direct write e-beam are also being developed. DSA has the potential to improve CD uniformity and lower costs. Direct write e-beam promises to make personalization of chips more feasible. DRAM memory will trail logic in critical dimensions and will adopt EUV when it becomes cost effective. The lithography community will both have to make EUV work and overcome the challenges of randomness in CDs and resist performance, while memory will try to make nanoimprint a reliable and low defect method of patterning. Long term, logic is expected to start focusing on 3D architectures in the late 2020’s. This will put a tremendous stress on the yield of patterning processes and on reducing the number of process steps that are required. It will also put more focus on hole type patterns, which will become one of the key patterning challenges in the future..
Keywords: lithography roadmap; IRDS; advanced patterning; EUV lithography; directed self-assembly (DSA); Ebeam direct write; Nanoimprint
1.   Introduction
The International Roadmap for Devices and Semiconductors assesses and forecasts semiconductor trends and performance. It does this by considering what the end uses of semiconductor chips are and estimating how these uses will drive improved semiconductor technology. The More Moore focus team does the roadmap for high performance semiconductor chips and the lithography team develops the lithographic roadmap. The mission of the lithography team is to forecast patterning technologies that will be used in manufacturing over the next fifteen year, to determine key patterning challenges and roadblock and to provide a usable parameter roadmap to the industry. Advances in lithography are driven by the needs new high-performance semiconductor chips, so the starting point for development of the lithography roadmap is the device requirements and dimensions as projected by the More Moore focus team. An international team of patterning experts then provides input and material that is developed into a final roadmap. The litho roadmap is codified into a spreadsheet and a white paper and is available on the IRDS website for anyone interested [1]. The latest complete version is the 2017 roadmap. This is the roadmap discussed below.
2.   Patterning Requirements and Drivers
Figure 1 shows the patterning requirements table of the 2017 roadmap. It is separated into sections for DRAMs, flash memory and high-performance logic chips. The cells are color coded. White means that the manufacturing technology exists that can meet the requirement, yellow means that technology that can meet the requirement is known, but not implemented in manufacturing, and red indicates that technology needs to be developed. DRAMs will need new technology for better resolution in 2024 and logic will need new technology for better resolution in 2021. The needs of high-performance logic chips is thus the driver for improved patterning resolution in the semiconductor industry. Memory technology is the driver for lower cost patterning methods. Flash memory used to drive lithography technology, but the industry turned to 3D flash which stacks memory cells on top of each other to enable higher bit densities instead of continuing to shrink cell sizes. 3D flash memory has many challenges of its own, especially in etch and deposition, but shrinking critical dimensions is not one of them. Thus all the cells in the requirements table for flash memory are white.
In the past, simple shrinks of critical dimensions for CMOS logic devices gave improved performance. Improvements in patterning technology directly gave improved devices. However, as device dimensions got smaller and smaller, electrical factors such as current leakage forced innovation in device design. In 2005, the rate of gate length change for logic device slowed down even though the rate of growth in devices per area of silicon did not slow down [2]. This was the result of device design innovations. In 2011, the FinFET was introduced to manufacturing, where the gate was redesigned into a vertical structure to improve leakage performance and give better performance. The requirement for device innovation at the same time as reduction in critical dimensions to give improved chip performance will continue. Figure 2 shows projected device structures from the More Moore Roadmap. FinFETs are expected to be replace by lateral gate all around structures (LGAA), then by vertical gate all around structures (VGAA), which will then scale in 3D instead of shrinking further. It is important to note that these design innovations are required in the future for the semiconductor needs of its target customers. Without introducing new types of devices, the industry will not able to provide the chip performance needed in the future. Projected key dimensions of logic chips over time are shown in Figure 3 [3]. Once 3D structures are introduced then horizontal scaling will stop. Thus, we can consider the short-term future to be the next seven to ten years where new logic device types are introduced simultaneously with shrinking critical dimensions and the long-term being 10 years or more from now where high-performance logic devices have switched to vertical scaling in order to increase device density in logic chips.

Figure 1.   The 2107 IRDS Roadmap Patterning requirements.

Figure 2.   Projected structures for high performance logic devices.

Figure 3.   Projected critical dimensions for high performance logic devices.
3. The State of Current Patterning and Possible Future Patterning Options
Multiple patterning using ArF immersion lithography is in use for all advanced chip designs today. Pattern multiplication has been qualified and in production since at least 2017. The resolution capabilities of multiple patterning are excellent, and resolution could be smaller than it is today, especially if more than four litho etch steps were used. However, shrinking features further or going beyond quadruple patterning creates difficult challenges in tolerance and overlay control and results in a process where the development time for the shrinking of for a new chip design is unacceptably long. Semiconductor makers want alternatives. The four new types of patterning under substantial development today and considered in this roadmap are directed self-assembly (DSA), nano-imprint (NIL), direct write ebeam lithography (maskless lithography or ML), and extreme ultraviolet lithography (EUV).
DSA is a technique that uses special polymers that separate into microphases when annealed. Guide structures are formed using lithography and the guide structures “direct” the polymer to separate into phases in usable arrangements during anneal, for example parallel vertical lamellae or vertical cylinders. The resulting pattern of phases is “developed” through etching to give the desired pattern. A cut level or other level is used to restrict the lines and spaces or the holes to the areas of the chip where they are desired. It promises to be inexpensive and to have high resolution. The first systems and processes used poly(styrene)-b-poly(methylmethacrylate) as the polymer and had a line and space minimum resolution of about 12nm to 13nm [4]. DRAM companies evaluated it, but in 2015 Micron reported that DSA did not give the same pattern quality for lines and spaces that their high-volume manufacturing technique, presumably multiple patterning did [5]. Process flows for other types of circuit patterns have been reported since then [6], but no successful implementation has occurred. Key issues are defects, pattern overlay and inspection. Defects are particularly hard to find, because they can be at the bottom of three-dimensional structure composed of different polymer phases where only the top of the structure is accessible to inspection before etch. Work on DSA materials that can achieve smaller dimensions has continued, and this work holds out hope for the future.
NIL involves creating a template with patterned grooves where lines will be and small pillars where contact holes will be. A master template is used to make secondary templates and the secondary templates are used to stamp a pattern into “resist” that has been applied to the wafer through an ink jet sort of printing process. The templates are transparent and UV light is used to cure the resist with the template still in place. Then the template is peeled away, leaving a relief pattern in the resist [7]. Because it is a method of contact printing the patterning can be affected by the accumulation of defects and this is why a system of disposable secondary templates is used. The templates are 1X, meaning that they are the same size as the patterns they print, unlike typical optical reticles. However, it promises low cost and has made great progress in achieving the performance needed for production of 3D NAND flash memory. Production tools are available and currently being qualified for potential use in memory production in 2019.
Direct Write Ebeam lithography uses focused beam of electrons to expose a resist in a pattern created by magnetically deflecting the beam to different locations. The resulting pattern is a function of software and doesn’t use a mask for pattern generation. If one electron beam is used the technique is suitable for slowly creating small numbers of circuits for research purposes but does not have the throughput to meet the needs of any volume chip maker. Massively parallel ebeam writing is needed to get a usable throughput for actual volume semiconductor production. Producing tools that can do this at the required dimensions has proved a big challenge. Work was reported on this technique for chip making in 2015 [8], but then apparent progress stopped. Developing a reliable small generator of many e-beams was difficult and delayed the program. But in 2018, the Mapper tool, FLX-1200 was described, which uses 65,000 parallel electron beams. It was operational and produced 42nm half pitch lines with a measured throughput of about 4 hours per wafer. A roadmap to a throughput of about one and a half hours per wafer was described [9]. While this dimension may seem large compared to the dimensions in Figure 1, and the throughput slow, it is still of interest to chip makers. If successful, it will enable cost effective production of very chip designs with very low production volumes and also enable the personalization of chips so that every chip on wafer has a unique ID.
EUV lithography is like traditional optical lithography in that light is projected on a wafer from a patterned mask and involves a mask that is four times larger than the printed pattern. The resolution is much better than that of ArF immersion and this could enable single exposures that replace multiple ArF immersion exposures. It reduces both development and manufacturing cycle time for semiconductor makers. It can also reduce cost if the productivity of the EUV tool is high enough. It is particularly cost effective for chip designs that have limited production, because the cost of the mask set per chip made is particularly high for such designs.
The EUV wavelength is so short that all the optics, including the mask, are reflective and the system operates in a high vacuum. Each mirror involved in the optical train has about 65% reflectance, that is, is absorbs 35% of the light. There are typically 6 mirrors in the lens, plus more in the illuminator and there is the mask also, the fraction of the light source that actually reaches the mask is something like 0.65 raised to the tenth power. This loss of power in the optical train puts a tremendous stress on the illumination source power. The lack of sufficiently powerful light sources limited throughput and held EUV back from production for a long time. After many years of development, ASML reported in 2018 that EUV exposure tools with 250 watts of power at intermediate focus were shipping in 2018 [10]. They reported that this power would enable a manufacturing level of throughput of 125 wafers per hour assuming a resist exposure dose of 20mJ/cm2. Although no production resist can provide sufficient quality imaging at this dose, resists are available that image at somewhat higher doses, and this is still enough power that both Samsung and TSMC have announced 7nm node logic process that will go into production in 2019. Challenges for EUV include defectivity, mask blank supply, pellicle development, line edge roughness, line width roughness and critical dimension uniformity.
A special mention about stochastics is needed. Stochastics refers to random effects due to the natural variations in the number of photons, the positions of key reactions in resist and the randomness of the actual placement of materials when looked at on a molecular scale. Stochastics are responsible for creating line edge and line width roughness and are responsible for some of the random critical dimension and positional variations of critical features in leading edge processes. As feature sizes shrink, stochastic effects do not shrink in magnitude. Instead they become a larger proportion of the error and tolerance budgets for critical dimensions. In ArF lithography, stochastics are responsible for line edge and line width roughness. In EUV, significant stochastic effects are driven not only be the random variations in resist chemistry, but also by random variations in the actual exposure dose due to the quantum nature of light. The number of photons in an EUV exposure is fourteen times smaller than the number of photons in a comparable ArF exposure. It is small enough that shot noise effects are a real issue for EUV, and an issue that will get worse as feature sizes continue to shrink. Stochastics can cause actual killer defects, not just critical dimension variations and feature roughness. Unfortunately, the number of opens and shorts and closed contact holes due to stochastic variations are larger (that is, worse in frequency) than one would predict from simple calculations assuming a normal distribution of critical dimensions with a given mean and sigma. This has been shown both by detailed simulation [11] and by experiment [12]. The extension of EUV to smaller feature sizes will be challenged more by the need to control stochastic effects than by anything else. Table 1 summarizes the status and challenges for each of the technologies discussed above.
Table 1.   Difficult challenges for each patterning technology.

4.   Possible Options
The potential insertion node of each of these technologies, the possible options for each upcoming leading-edge critical dimension, and the time frame needed for insertion into manufacturing is shown in Figures 4 and 5. Figure 4 shows possible options for line and space patterns and Figure 5 shows possible options for contact hole type patterns. Contact hole patterns have more challenges than line and space patterns. All projected line and space critical dimensions could possibly be achieved by EUV double patterning, however EUV double pattering will only support hole type patterns for the next two nodes. In 2024 something with better resolution than double patterning of EUV will be needed in manufacturing. The advent of vertical gate all around structures in 2027 will require resolving 6nm holes on a 14nm pitch. No known patterning technique can do this yet.

Figure 4.   Possible options for line and space patterns.

Figure 5.   Possible options for hole type patterns.
5.   Difficult Challenges
Near term challenges are the challenges faced in continuing to shrink dimensions and improve devices until VGAA structures are achieved. All potential patterning methods have worries about defectivity, inspection and metrology [13]. Overlay and edge placement error are also challenges that all patterning options face. Stochastics is a particular challenge for EUV. Research and development is already making major efforts to improve and to mitigate stochastics. But since it will be a bigger challenge as critical dimensions continue to shrink, work will have to continue to be done in this area. Patterning of small hole patterns required for VGAA structures is a challenge of its own. No current patterning technique can produce the size and type of patterns needed.
Long term challenges will arise from the shift to device stacking and growth in the vertical dimension instead of shrinking of key critical dimensions. The yield of each step will have to high to give an acceptable overall chip yield. Etch and deposition will become bigger challenges and patterning and overlay over topography could become significant issues.
6.   Conclusions
The IRDS lithography roadmap forecasts patterning challenges and achievements needed to support the IRDS More Moore roadmap. Logic devices are now driving high resolution patterning development. Memory devices are driving new approaches that lower cost. New device types will have to be implemented at the same time as pattern shrinks in order to keep delivering improved semiconductor devices in line with Moore’s law. Hole patterns will become more difficult challenges than line and space patterns. EUV with double patterning can support all projected line and space minimum pitches, but not all projected hole type critical dimensions. Innovation will be needed to find ways to print small hole patterns, particularly those needed for VGAA structures. In ten years, logic is projected to switch to vertical scaling instead of horizontal scaling. At that point challenges will shift from resolution, defectivity, and metrology to yield, process simplification, and possible involve patterning and overlay in the presence of substantial topography.
My thanks to the IRDS Litho team for their inputs and to Mustafa Badaroglu and the IRDS More Moore team for their inputs on chip designs and new device structures.
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Article and author information
Mark Neisser
Publication records
Published: Dec. 27, 2018 (Versions3
Journal of Microelectronic Manufacturing