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Posts Tagged ‘multi patterning’

EUVL Materials Readiness for HVM

Friday, June 2nd, 2017

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By Ed Korczynski, Sr. Technology Editor

Extreme-Ultra-Violet Lithography (EUVL)—based on ~13.5nm wavelength EM waves bouncing off mirrors in a vacuum—will finally be used in commercial IC fabrication by Intel, Samsung, and TSMC starting in 2018. In a recent quarterly earning calls ASML reported a backlog of orders for 21 EUVL tools. At the 2017 SPIE Advanced Lithography conference, presentations detailed how the source and mask and resist all are near targets for next year, while the mask pellicle still needs work. Actinic metrology for mask inspection still remains a known expensive issue to solve.

Figure 1 shows minimal pitch line/space grids and contact-hole arrays patterned with EUVL at global R&D hub IMEC in Belgium, as presented at the recent 2017 IMEC Technology Forum. While there is no way with photolithography to escape the trade-offs of the Resolution/Line-Width-Roughness/Sensitivity (RLS) triangle, patterning at the leading edge of possible pitches requires application-specific etch integration. The bottom row of SEMs in this figure all show dramatic improvements in LWR through atomic-scale etch and deposition treatments to patterned sidewalls.

Fig.1: SEM plan-view images of minimum pitch Resolution and Line-Width-Roughness and Sensitivity (RLS) for both Chemically-Amplified Resist (CAR) and Non-Chemically-Amplified Resist (NCAR, meaning metal-oxide solution from Inpria) formulations, showing that excessive LWR can be smoothed by various post-lithography deposition/etch treatments. (Source: IMEC)

ASML has recently claimed that as an indication of continued maturity, ASML’s NXE:33×0 steppers have now collectively surpassed one million processed wafers to date, and only correctly exposed wafers were included in the count. During the company’s 1Q17 earnings call, it was reported that three additional orders for NXE:3400B steppers were received in Q1 adding  to a total of 21 in backlog, worth nearly US$2.5B.

At $117M each NXE:3400B, assuming 10 years useful life it costs $32,000 each day and assuming 18 productive hours/day and 80 wafers/hour then it costs $22 per wafer-pass just for tool depreciation. In comparison, a $40M argon-fluoride immersion (ArFi) stepper over ten years with 21 available hours/day and 240 wafers/hour costs $2.2 per wafer-pass for depreciation. EUVL will always be an expensive high-value-add technology, even though a single EUVL exposure can replace 4-5 ArFi exposures.

Fabs that delay use of EUVL at the leading edge of device scaling will instead have to buy and facilitize many more ArFi tools, demanding more fab space and more optical lithography gases. SemiMD spoke with Paul Stockman, Linde Electronics’ Head of Market Development, about the global supply of specialty neon and xenon gas blends:  “Xenon is only a ppm level component of the neon-blend for Kr and Ar lasers, so there should be no concerns with Xenon supply for the industry. In our modeling we’ve realized the impact of multi-patterning on gas demand, and we’ve assumed that the industry would need multi-patterning in our forecasts.” said Stockman.

“From the Linde perspective, we manage supply carefully to meet anticipated customer demand,” reminded Stockman. “We recently added 40 million liters of neon capacity in the US, and continue to add significant supply with partners so that we can serve our customers regardless of the EUV scenario.” (Editor’s note: reported by SemiMD here.)

At SPIE Advanced Lithography 2017, SemiMD discussed multi-patterning process flows with Uday Mitra and Regina Freed of Applied Materials. “We need a lot of materials engineering now,” explained Freed. “We need new gap-fills and hard-masks, and we may need new materials for selective deposition. Regarding the etch, we need extreme selectivity with no damage, and ability to get into the smallest features to take out just one atomic layer at a time.”

Reminding us that IC fabs must be risk-averse when considering technology options, Mitra (formerly with Intel) commented, “You don’t do a technology change and a wafer size change at the same time. That’s how you risk manage, and you can imagine with something like EUVL that customers will first use it for limited patterning and check it out.”

Figure 2 lists the major issues in pattern-transfer using plasma etch tools, along with the process variables that must be controlled to ensure proper pattern fidelity. Applied Materials’ Sym3 etch chamber features hardware that provides pulsed energy at dual frequencies along with low residence time of reactant byproducts to allow for precise tuning of process parameters no matter what chemistry is needed.

Fig.2: Patterning issues and associated etch process variables which can be used for control thereof. (Source: Applied Materials)

Andrew Grenville, CEO of resist supplier Inpria, in an exclusive interview with SemiMD, commented on the infrastructure readiness for EUVL volume production. “We are building up our pilot line facility in Corvallis, Oregon. The timing for that is next year, and we are putting in place plans to continue to scale up the new materials at the same times as the quality control systems such as functional QC.” The end-users ask for quality control checks of more parameters, putting a burden on suppliers to invest in more metrology tools and even develop new measurement techniques. Inpria’s resist is based on SnOx nanoparticles, which provide for excellent etch resistance even with layers as thin as 20nm, but required the development of a new technique to measure ppb levels of trace metals in the presence of high tin signals.

“We believe that there is continued opportunity for improvement in the overall patterning performance based on the ancillaries, particularly in simplifying the under-layers. One of the core principles of our material is that we’re putting the ‘resist’ back in the resist,” enthused Grenville. “We can show the etch contrast of our material can really improve the Line-Width Roughness of the patterns because of what you can do in etch, and it’s not merely smoothing the resist. We can substantially improve the outcome by engineering the stack and the etch recipe using completely different chemistry than could be used with chemically-amplified resist.”

The 2017 EUVL Workshop (2017 International Workshop on EUV Lithography) will be held June 12-15 at The Center for X-ray Optics (CXRO) at Lawrence Berkeley National Laboratory in Berkeley, CA. This workshop, now in its tenth year, is focused on the fundamental science of EUV Lithography (EUVL). Travel and hotel information as well as on-line registration is available at https://euvlitho.com/.

[DISCLOSURE:  Ed Korczynski is also Sr. Analyst for TECHCET responsible for the Critical Materials Report (CMR) on Photoresists, Extensions & Ancillaries.]

—E.K.

Edge Placement Error Control in Multi-Patterning

Thursday, March 2nd, 2017

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By Ed Korczynski, Sr. Technical Editor

SPIE Advanced Lithography remains the technical conference where the leading edge of minimum resolution patterning is explored, even though photolithography is now only part of the story. Leading OEMs continue to impress the industry with more productive ArFi steppers, but the photoresist suppliers and the purveyors of vacuum deposition and etch tools now provide most of the new value-add. Tri-layer-resist (TLR) stacks, specialty hard-masks and anti-reflective coatings (ARC), and complex thin-film depositions and etches all combine to create application-specific lithography solutions tuned to each critical mask.

Multi-patterning using complementary lithography—using argon-fluoride immersion (ArFi) steppers to pattern 1D line arrays plus extreme ultra-violet (EUV) tools to do line cuts—is under development at all leading edge fabs today. Figure 1 shows that edge placement error (EPE) in lines, cut layers, and vias/contacts between two orthogonal patterned layers can result in shorts and opens. Consequently, EPE control is critical for yield within any multi-patterning process flow, including litho-etch-litho-etch (LELE), self-aligned double-patterning (SADP) and self-aligned quadruple-patterning (SAQP).

Fig.1: Plan view schematic of 10nm half-pitch vertical lines overlaid with lower horizontal lines, showing the potential for edge-placement error (EPE). (Source: Y. Borodovsky, SPIE)

Happening the day before the official start of SPIE-AL, Nikon’s LithoVision event featured a talk by Intel Fellow and director of lithography hardware solutions Mark Phillips on the big picture of how the industry may continue to pattern smaller IC device features. Regarding the timing of Intel’s planned use of EUV litho technology, Phillips re-iterated that, “It’s highly desirable for the 7nm node, but we’ll only use it when it’s ready. However, EUVL will remain expensive even at full productivity, so 193i and multi-patterning will continue to be used. In particular we’ll need continued improvement in the 193i tools to meet overlay.”

Yuichi Shibazaki— Nikon Fellow and the main architect of the current generation of Nikon steppers—explained that the current generation of 193i steppers, featuring throughputs of >200 wafers per hour, have already been optimized to the point of diminishing returns. “In order to improve a small amount of performance it requires a lot of expense. So just improving tool performance may not decrease chip costs.” Nikon’s latest productivity offering is a converted alignment station as a stand-alone tool, intended to measure every product wafer before lithography to allow for feed-forward tuning of any stepper; cost and cost-of-ownership may be disclosed after the first beta-site tool reaches a customer by the end of this year.

“The 193 immersion technology continues to make steady progress, but there are not as many new game-changing developments,” confided Michael Lercel, Director of Strategic Marketing for ASML in an exclusive interview with SemiMD. “A major theme of several SPIE papers is on EPE, which traditionally we looked at as dependent upon CD and overlay. Now we’re looking at EPE in patterning more holistically, with need to control the complexity with different error-variables. The more information we can get the more we can control.”

At LithoVision this year, John Sturtevant—SPIE Fellow, and director of RET product development in the Design to Silicon Division at Mentor Graphics—discussed the challenges of controlling variability in multi-layer patterning. “A key challenge is predicting and then mitigating total EPE control,” reminded Sturtevant. “We’ve always paid attention to it, but the budgets that are available today are smaller than ever. Edge-placement is very important ” At the leading edge, there are multiple steps within the basic litho flow that induce proximity/local-neighbor effects which must be accounted for in EDA:  mask making, photoresist exposure, post-exposure bake (PEB), pattern development, and CD-SEM inspection (wherein there is non-zero resist shrinkage).

Due to the inherent physics of EUV lithography, as well as the atomic-scale non-uniformities in the reflective mirrors focusing onto the wafer, EUV exposure tools show significant variation in exposure uniformities. “For any given slit position there can be significant differences between tools. In practice we have used a single model of OPC for all slit locations in all scanners in the fab, and that paradigm may have to change,” said Sturtevant. “It’s possible that because the variation across the scanner is as much as the variation across the slit, it could mean we’ll need scanner-specific cross-slit computational lithography.” More than 3nm variation has been seen across 4 EUVL steppers, and the possible need for tool-specific optical proximity correction (OPC) and source-mask optimization (SMO) would be horrible for managing masks in HVM.

Thin Films Extend Patterning Resolution

Applied Materials has led the industry in thin-film depositions and etches for decades, and the company’s production proven processing platforms are being used more and more to extend the resolution of lithography. For SADP and SAQP MP, there are tunable unit-processes established for sidewall-spacer depositions, and chemical downstream etching chambers for mandrel pull with extreme material selectivity. CVD of dielectric and metallic hard-masks when combined with highly anisotropic plasma etching allows for device-specific and mask-specific pattern transfers that can reduce the line width/edge roughness (LWR/LER) originally present in the photoresist. Figure 2 from the SPIE-AL presentation “Impact of Materials Engineering on Edge Placement Error” by Regina Freed, Ying Zhang, and Uday Mitra of Applied Materials, shows LER reduction from 3.4 to 1.3 nm is possible after etch. The company’s Sym3 chamber features very high gas conductance to prevent etch byproducts from dissociation and re-deposition on resist sidewalls.

Fig.2: 3D schematics (top) and plan view SEM images (bottom) showing that control of plasma parameters can tune the byproducts of etch processes to significantly reduce the line-width roughness (LWR) of minimally scaled lines. (Source: Applied Materials)

TEL’s new SAQP spacer-on-spacer process builds on the work shown last year, using oxide as first spacer and TiO2 as second spacer. Now TEL is exploring silicon as the mandrel, then silicon-nitride as the first spacer, and titanium-oxide as second spacer. This new flow can be tuned so that all-dry etch in a single plasma etch chamber can be used for the final mandrel pull and pattern transfer steps.

Coventor’s 3D modeling software allows companies to do process integration experiments in virtual space, allowing for estimation of yield-losses in pattern transfer due to variations in side-wall profiles and LER. A simulation of 9 SRAM cells with 54 transistors shows that photoresist sidewall taper angle determines both the size and the variability of the final fins. The final capacitance of low-k dielectric in dual-damascene copper metal interconnects can be simulated as a function of the initial photoresist profile in a SAQP flow.

—E.K.

Applied Materials Releases Selective Etch Tool

Wednesday, June 29th, 2016

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By Ed Korczynski, Sr. Technical Editor

Applied Materials has disclosed commercial availability of new Selectra(TM) selective etch twin-chamber hardware for the company’s high-volume manufacturing (HVM) Producer® platform. Using standard fluorine and chlorine gases already used in traditional Reactive Ion Etch (RIE) chambers, this new tool provides atomic-level precision in the selective removal of materials in 3D devices structures increasingly used for the most advanced silicon ICs. The tool is already in use at three customer fabs for finFET logic HVM, and at two memory fab customers, with a total of >350 chambers planned to have been shipped to many customers by the end of 2016.

Figure 1 shows a simplified cross-sectional schematic of the Selectra chamber, where the dashed white line indicates some manner of screening functionality so that “Ions are blocked, chemistry passes through” according to the company. In an exclusive interview with Solid State Technology, company representative refused to disclose any hardware details. “We are using typical chemistries that are used in the industry,” explained Ajay Bhatnagar, managing director of Selective Removal Products for Applied Materials. “If there are specific new applications needed than we can use new chemistry. We have a lot of IP on how we filter ions and how we allow radicals to combine on the wafer to create selectivity.”

FIG 1: Simplified cross-sectional schematic of a silicon wafer being etched by the neutral radicals downstream of the plasma in the Selectra chamber. (Source: Applied Materials)

From first principles we can assume that the ion filtering is accomplished with some manner of electrically-grounded metal screen. This etch technology accomplishes similar process results to Atomic Layer Etch (ALE) systems sold by Lam, while avoiding the need for specialized self-limiting chemistries and the accompanying chamber throughput reductions associated with pulse-purge process recipes.

“What we are doing is being able to control the amount of radicals coming to the wafer surface and controlling the removal rates very uniformly across the wafer surface,” asserted Bhatnagar. “If you have this level of atomic control then you don’t need the self-limiting capability. Most of our customers are controlling process with time, so we don’t need to use self-limiting chemistry.” Applied Materials claims that this allows the Selectra tool to have higher relative productivity compared to an ALE tool.

Due to the intrinsic 2D resolutions limits of optical lithography, leading IC fabs now use multi-patterning (MP) litho flows where sacrificial thin-films must be removed to create the final desired layout. Due to litho limits and CMOS device scaling limits, 2D logic transistors are being replaced by 3D finFETs and eventually Gate-All-Around (GAA) horizontal nanowires (NW). Due to dielectric leakage at the atomic scale, 2D NAND memory is being replaced by 3D-NAND stacks. All of these advanced IC fab processes require the removal of atomic-scale materials with extreme selectivity to remaining materials, so the Selectra chamber is expected to be a future work-horse for the industry.

When the industry moves to GAA-NW transistors, alternating layers of Si and SiGe will be grown on the wafer surface, 2D patterned into fins, and then the sacrificial SiGe must be selectively etched to form 3D arrays of NW. Figure 2 shows the SiGe etched from alternating Si/SiGe stacks using a Selectra tool, with sharp Si corners after etch indicating excellent selectivity.

FIG 2: SEM cross-section showing excellent etch of SiGe within alternating Si/SiGe layers, as will be needed for Gate-All-Around (GAA) horizontal NanoWire (NW) transistor formation. (Source: Applied Materials)

“One of the fundamental differences between this system and old downstream plasma ashers, is that it was designed to provide extreme selectivity to different materials,” said Matt Cogorno, global product manager of Selective Removal Products for Applied Materials. “With this system we can provide silicon to titanium-nitride selectivity at 5000:1, or silicon to silicon-nitride selectivity at 2000:1. This is accomplished with the unique hardware architecture in the chamber combined with how we mix the chemistries. Also, there is no polymer formation in the etch process, so after etching there are no additional processing issues with the need for ashing and/or a wet-etch step to remove polymers.”

Systems can also be used to provide dry cleaning and surface-preparation due to the extreme selectivity and damage-free material removal.  “You can control the removal rates,” explained Cogorno. “You don’t have ions on the wafer, but you can modulate the number of radicals coming down.” For HVM of ICs with atomic-scale device structures, this new tool can widen process windows and reduce costs compared to both dry RIE and wet etching.

—E.K.

Changes and Challenges Abound in Multi-patterning Lithography

Monday, January 26th, 2015

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By Jeff Dorsch

Multi-patterning lithography is a fact of life for many chipmakers. Experts in the fields of electronic design automation and lithography address the issues associated with the technology. Providing responses are David Abercrombie, Design for Manufacturing Program Manager, Mentor Graphics; Gary Zhang, Vice President Marketing, ASML Brion; and Dr. Donis Flagello of Nikon Research Corporation of America.

1. What are the significant considerations in semiconductor manufacturing and design with multi-patterning lithography?

David Abercrombie: Like most process/design trade-offs moving from one node to another it comes down to cost vs area and performance. Without multi-patterning or EUV you will struggle do design at 20nm or below limiting the opportunity to take advantage of design area and performance scaling. Essentially, Moore’s Law slows to a crawl without it. Multi-patterning affects almost all aspects of design and manufacturing. For physical design it adds additional design rule constraints and constrains cell placement and routing depending on cell architecture. For electrical design it adds additional parasitic variability to consider in timing analysis. For DFM it adds additional requirements for fill and lithographic checking. In manufacturing it adds additional masks, process steps and increases stepper utilization. All of these increase complexity and have an associated cost. It ultimately has to make business sense. Because of this you are seeing fewer companies moving to these advanced nodes as quickly as before, as they must have the volume and profit margins to justify the increased cost. Fortunately, there are products that do need the newest and most advanced process nodes, and because of those needs we continue to move forward into these new technology nodes on a regular schedule.

Gary Zhang: Multiple patterning (MPT) using immersion lithography is required for the semiconductor industry to continue device scaling until extreme ultraviolet (EUV) comes into full production (EUV is expected for a mid-node insertion in the 10nm logic node, and for 7nm node development and production in the 2015-2017 time frame). Multiple-patterning lithography brings the following new challenges from design to manufacturing. ASML has been collaborating with the chipmakers in a holistic lithography framework to tackle these challenges with innovative hardware and software solutions, including scanner systems, computational lithography, metrology and process control.

Integrated circuit designs have to be multiple patterning compatible. Industry has been developing methods to enable MPT-compatible designs via layout decomposition (coloring) and conflict resolution using multiple patterning rules as constraints. This applies to standard-cell libraries, cell boundaries, and placement and route to ensure full chip layouts meet all manufacturing requirements and can be decomposed into separate masks without any post-coloring MPT conflicts. Structured layouts with highly restricted design rules seem to be a key enabler for MPT-compliant designs.

The rule-based approach to MPT compatible designs tends to run the risk of pattern defects from design hot spots, especially when design rules are pushed aggressively for competitive die size. The lithography process window of these design hot spots can be enlarged using source-mask optimization (SMO). Brion’s Tachyon SMO has been routinely used to co-optimize scanner optics such as illumination source and projection lens wavefront and mask enhancements including sub-resolution assist features (SRAF) and optical proximity correction (OPC) for any given designs. Take triple patterning of a 10nm node metal layer as an example. Tachyon SMO enables a 23% larger process window for the selected SRAM and logic designs (Figure 1). By evaluating a range of design variations, SMO can help optimize design rules and MPT coloring rules to eliminate design hot spots in the technology development stage. For production mask data preparation, Brion’s multiple patterning OPC and LMC (Lithography Manufacturability Check) are widely used by the leading chipmakers to deliver the best full chip process window in wafer manufacturing. A combination of SMO, OPC and LMC makes up ASML’s process window enhancement solutions to the design hot spot problem.

Figure 1. Source-mask optimization (SMO) of a 10 nm node metal layer in triple patterning lithography. Overlapping process window of all three splits (masks) is improved by 23% for selected SRAM and logic patterns imaged with the same illumination setup.

Multiple patterning drives tighter CD, focus and overlay requirements to account for more process variations from the additional processing steps. Overlay is used here as an example to show the increasing complexity in multiple patterning process control from single exposure at 28nm node, to double patterning at 14nm node, to triple patterning at 10nm node (Figure 2). Tighter overlay specification has to be met for the exponentially increasing number of critical masks and metrology steps at 14nm and 10nm nodes. To deliver the required overlay control on product wafers, scanner matching and process control have to include high order corrections (Figure 3). ASML’s latest generation of immersion scanners have a large number of flexible actuators and are capable of sub-3 nm matched-machine overlay, dynamic lens heating and reticle heating corrections, and high-order interfield and intrafield corrections for imaging, focus and overlay.

Figure 2. A comparison of overlay metrology and control for single exposure at 28 nm node, double patterning at 14 nm node and triple patterning at 10 nm node, using the Metal 1 (M1) to Metal 2 (<2) process loop as an example.

Figure 3. On-product overlay roadmap showing the ever tighter specification from 28 nm node to 14 and 10 nm nodes and the requirement of advanced scanner correction capabilities (such as dynamic and high-order).Two different production scenarios are considered, namely scanner/chuck dedication and mix and match of different scanners.

With the introduction of multiple patterning below 28 nm node, the increasing number of masks and metrology steps translates to lower wafer throughput per scanner and longer wafer cycle time from start to finish. This then leads to cost per wafer significantly higher than the historical cost scaling trend from the previous technology nodes. ASML has been continuously driving the scanner innovation to increase the throughput and improve productivity in terms of wafer output per day. ASML’s YieldStar integrated metrology is another innovative solution to reduce wafer cycle time and improve on-product performance for effective productivity gain and overall cost benefit.

In summary, a full suite of design and manufacturing solutions are required to address the new challenges in multiple-patterning lithography. ASML has taken a holistic approach and worked in close collaboration with the chipmakers to optimize design, scanner, mask and process control altogether for the best manufacturability and yield. Figure 4 gives an example on how holistic lithography enables focus roadmap down to 1x nm node. In the design phase, process window enhancement solutions such as SMO, OPC and LMC are used to eliminate the design hot spots and maximize the full chip process window. In the wafer manufacturing phase, process window control solutions such as scanner matching and high order corrections are implemented to optimize CD, overlay and focus control dynamically from tool to tool, field to field, wafer to wafer and lot to lot. A combination of the largest process window and the tightest process control delivers the most robust manufacturability and yield in volume production.

Figure 4. An example of how holistic lithography enables focus roadmap down to 1x nm node (DPT: double patterning; MPT: multiple patterning). A combination of process window enhancement and process window control solutions delivers robust manufacturability and yield in volume production.

Donis Flagello: Multiple patterning brings a host of issues due to the added complexity associated with imaging and processing multiple patterns within the same design layer. From the exposure tool point of view, we need to ensure that the overall cost of ownership is maintained and the tool can enable further scaling. We are concentrating on many aspects of the technology. One of the most critical is overlay. This must be as low as possible such that the ensemble overlay of all the exposures within a layer is equal or better than a single exposure. Simultaneously, we need to increase the throughput of the tools to ensure that cost per wafer per hour is also continuously improved.  Both of these aspects drive a huge amount of innovation and technology development.

2. How do you deal with color assignment?

Abercrombie: The answer to that depends on the foundry and layer being discussed. Colorless, partial coloring and full coloring flows exist. In colorless flows the designer does not assign colors. There are specialized checks (like odd cycle checks in double patterning) that make sure the layout can be decomposed into multiple masks later once the design is taped-out to the foundry. In a partial coloring flow most of the layout follows the colorless flow, but the designer can manually assigns some parts of the layout to a particular color to manage subtle variation concerns. For instance, making sure matched circuitry also has matched coloring. In a fully colored flow the designer is responsible for producing the final mask assignments for all polygons in the layer. A GDS layer is dedicated to each mask. To assign a polygon to a given mask a copy of it is placed on the appropriate mask color layer. EDA companies provide various automation capabilities to assist with color assignment in custom, P&R and batch full chip applications.

It is best to use an EDA solution like Calibre that not only can address all different coloring flows but also provides the same checks/algorithms for all phases a design goes through from initial IP blocks to final full chip signoff.

Zhang: Layout decomposition or coloring has to deliver split patterns on separate masks which are free of any process rule violations and can then be patterned in single exposure with sufficient process window. A double patterning (DPT) using a litho-etch-litho-etch process is shown as an example (Figure 5). In the DPT coloring step, any non-native color conflicts are resolved in a layer aware implementation with stitches that are properly located away from the overlap region between layers (such as a metal line contacting a via) and have the least impact on the device performance and manufacturing yield. Process robust stitching must have sufficient overlap margin to tolerate misalignment between the exposures of the split masks. This is the concept of overlay aware stitching.

Figure 5. An example of design to manufacturing work flow for a litho-etch-litho-etch double patterning (DPT) process, from layer aware coloring to overlay aware stitching, to model based OPC, to the final contour after litho and etch processes.

Color balancing is another critical care-about in layout decomposition. MPT coloring not only needs to deliver split layouts free of MPT conflicts but also has to ensure the pattern density is balanced between the split masks. Color balancing is beneficial for litho and etch process control so that robust and uniform patterning qualities can be achieved.

Coloring can also be optimized for best process window using a model based approach, as described above in the “Design hot spots” section. Model-based coloring is not suitable for full chip application. It can be either used in source-mask optimization for MPT rule development or applied in local hot spot fix during the mask data preparation.

3. How does design rule check change? How is it the same?

Abercrombie: In a fully colored flow the design rules change slightly. First for every traditional spacing check there are essentially two checks for double patterning (DP): a minimum spacing for different colored polygons, and a larger minimum spacing for same colored polygons. In addition, there are usually additional density checks making sure the ratio between the colors is reasonably equal. In colorless flows specialized new checks have been developed to verify if a valid coloring exists for a given layout construct. In double patterning these specialized checks include odd cycle checks. For triple patterning (TP) and quadruple patterning (QP) new types of checks are required.

Zhang: Triple patterning (TPT) coloring is a lot more difficult and complex than DPT coloring. It is extremely hard to determine if a layout is TPT compatible, known as NP-complete problem in graph theory. There is no efficient way to find a solution on the full chip level. There are no existing methods for determining the number of conflicts and their locations.

Stitches are color-dependent in TPT and candidate stitch locations can be determined only after or during coloring.

Therefore it is important to ensure TPT compliance by design construct.

4. What are the complexities and issues in transitioning from double-patterning to triple-patterning?

Abercrombie: Although checking and decomposing a layout for two colors is complex, the algorithmic processing scales reasonably by design size. However, the generalized solution for triple and quadruple patterning has exponentially increasing run time as the number of polygons processed increases. This is, of course, is not a practical solution. So the problem must be constrained such that reasonable heuristic algorithmic approaches can be applied that provide reasonably scalable run times. So the complete set of design rules and design methodology need to be properly tuned to constrain the graph-complexity of the layouts produced so these checking and decomposition heuristic tools can be utilized. In addition, specialized checks may be needed so that layout constructs that do not meet the complexity constraints can be diverted from processing (to keep run time from exploding) and flagged to the user for modification until they can be properly processed.

The other challenge in moving from DP to TP and QP is colorless error visualization. If you are doing a colorless flow and need to check if the design can legally be colored, you need a way to highlight constructs for which no valid coloring solution exists in a way that the designer can understand so he/she can make changes in the layout to fix it. For DP this was odd cycle error visualization. An even-numbered cycle of interacting polygons can be colored and an odd numbered cycle of interacting polygons cannot. For TP and QP this is not the case. Any simple even or odd cycle can be colored. The constructs which cannot be colored are much more complex than in DP. In addition, narrowing down the implicated constructs to the “root” of the problem is more difficult. To address these issues Mentor Calibre is developing a new array of error visualization layers to help inform and guide the user to appropriate and productive fixes.

Flagello: Years ago many industry observers did not believe that double patterning was viable. Today double and triple patterning is being done. However, there are some key differences between the two. Depending on the technology used, double exposure from a tool perspective is more or less straightforward. Mask alignment is usually based on the previous layer mark. However, moving to triple exposure often results in much more of an optimization problem to determine the best alignment strategy. Sometimes, the previous layer alignment mark may have a poor signal depending on the number of films involved in the multiple-patterning schemes. While increasing the number of patterning steps increases some of the complexity, the solutions become more of an optimization and controls challenge.

5. What issues in IC design and verification emerge with multi-patterning?

Abercrombie: The designer should expect to see new design rules, more parasitic variation, more complexity in design and methodology constraints, increased wafer cost, and the need for new EDA tools and additional CPU hardware to process their designs. This is really not new as this increased complexity and cost has existed between every node transition. The difference is that the delta may be more than between previous nodes. It is important that design teams educate themselves early on the impacts of moving to multi-patterned process nodes. That includes getting information from the foundry and EDA partners as well as reading available material on the subject. I have a whole series of articles covering much of the questions in this round table in significant detail: http://www.mentor.com/solutions/foundry/solutions/multi-patterning

Zhang: In addition to the power, performance and area metrics, designers now have to ensure their IC designs are MPT compliant and free of design hot spots so that they can be manufactured cost effectively with the best yield using multiple-patterning lithography. From lithography point of view, design hot spots are the major yield detractor. Device performance such as RC timing delay, cross talk, leakage (such as IDDQ), breakdown voltage and final yield is heavily influenced by MPT process variations. Brion’s LMC has been used to evaluate the impact of realistic dose, focus, mask and overlay variations on MPT hot spots both intra-layer and interlayer. Identification of such MPT hot spots helps drive design and OPC improvements so that they can be eliminated in wafer manufacturing.