Part of the  

Solid State Technology

  and   

The Confab

  Network

About  |  Contact

Posts Tagged ‘ASML’

Next Page »

EUVL Materials Readiness for HVM

Friday, June 2nd, 2017

thumbnail

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.

Lithographic Stochastic Limits on Resolution

Monday, April 3rd, 2017

thumbnail

By Ed Korczynski, Sr. Technical Editor

The physical and economic limits of Moore’s Law are being approached as the commercial IC fab industry continues reducing device features to the atomic-scale. Early signs of such limits are seen when attempting to pattern the smallest possible features using lithography. Stochastic variation in the composition of the photoresist as well as in the number of incident photons combine to destroy determinism for the smallest devices in R&D. The most advanced Extreme Ultra-Violet (EUV) exposure tools from ASML cannot avoid this problem without reducing throughputs, and thereby increasing the cost of manufacturing.

Since the beginning of IC manufacturing over 50 years ago, chip production has been based on deterministic control of fabrication (fab) processes. Variations within process parameters could be controlled with statistics to ensure that all transistors on a chip performed nearly identically. Design rules could be set based on assumed in-fab distributions of CD and misalignment between layers to determine the final performance of transistors.

As the IC fab industry has evolved from micron-scale to nanometer-scale device production, the control of lithographic patterning has evolved to be able to bend-light at 193nm wavelength using Off-Axis Illumination (OAI) of Optical-Proximity Correction (OPC) mask features as part of Reticle Enhancement Technology (RET) to be able to print <40nm half-pitch (HP) line arrays with good definition. The most advanced masks and 193nm-immersion (193i) steppers today are able to focus more photons into each cubic-nanometer of photoresist to improve the contrast between exposed and non-exposed regions in the areal image. To avoid escalating cost and complexity of multi-patterning with 193i, the industry needs Extreme Ultra-Violet Lithography (EUVL) technology.

Figure 1 shows Dr. Britt Turkot, who has been leading Intel’s integration of EUVL since 1996, reassuring a standing-room-only crowd during a 2017 SPIE Advanced Lithography (http://spie.org/conferences-and-exhibitions/advanced-lithography) keynote address that the availability for manufacturing of EUVL steppers has been steadily improving. The new tools are close to 80% available for manufacturing, but they may need to process fewer wafers per hour to ensure high yielding final chips.

Figure 1. Britt Turkot (Intel Corp.) gave a keynote presentation on "EUVL Readiness for High-Volume Manufacturing” during the 2017 SPIE Advanced Lithography conference. (Source: SPIE)

The KLA-Tencor Lithography Users Forum was held in San Jose on February 26 before the start of SPIE-AL; there, Turcot also provided a keynote address that mentioned the inherent stochastic issues associated with patterning 7nm-node device features. We must ensure zero defects within the 10 billion contacts needed in the most advanced ICs. Given 10 billion contacts it is statistically certain that some will be subject to 7-sigma fluctuations, and this leads to problems in controlling the limited number of EUV photons reaching the target area of a resist feature. The volume of resist material available to absorb EUV in a given area is reduced by the need to avoid pattern-collapse when aspect-ratios increase over 2:1; so 15nm half-pitch lines will generally be limited to just 30nm thick resist. “The current state of materials will not gate EUV,” said Turkot, “but we need better stochastics and control of shot-noise so that photoresist will not be a long-term limiter.”

TABLE:  EUVL stochastics due to scaled contact hole size. (Source: Intel Corp.)

CONTACT HOLE DIAMETER 24nm 16nm
INCIDENT EUV PHOTONS 4610 2050
# ABSORBED IN AREAL IMAGE 700 215

From the LithoGuru blog of gentleman scientist Chris Mack (http://www.lithoguru.com/scientist/essays/Tennants_Law.html):

One reason why smaller pixels are harder to control is the stochastic effects of exposure:  as you decrease the number of electrons (or photons) per pixel, the statistical uncertainty in the number of electrons or photons actually used goes up. The uncertainty produces line-width errors, most readily observed as line-width roughness (LWR). To combat the growing uncertainty in smaller pixels, a higher dose is required.

We define a “stochastic” or random process as a collection of random variables (https://en.wikipedia.org/wiki/Stochastic_process), and a Wiener process (https://en.wikipedia.org/wiki/Wiener_process) as a continuous-time stochastic process in honor of Norbert Wiener. Brownian motion and the thermally-driven diffusion of molecules exhibit such “random-walk” behavior. Stochastic phenomena in lithography include the following:

  • Photon count,
  • Photo-acid generator positions,
  • Photon absorption,
  • Photo-acid generation,
  • Polymer position and chain length,
  • Diffusion during post-exposure bake,
  • Dissolution/neutralization, and
  • Etching hard-mask.

Figure 2 shows the stochastics within EUVL start with direct photolysis and include ionization and scattering within a given discrete photoresist volume, as reported by Solid State Technology in 2010.

Figure 2. Discrete acid generation in an EUV resist is based on photolysis as well as ionization and electron scattering; stochastic variations of each must be considered in minimally scaled areal images. (Source: Solid State Technology)

Resist R&D

During SPIE-AL this year, ASML provided an overview of the state of the craft in EUV resist R&D. There has been steady resolution improvement over 10 years with Photo-sensitive Chemically-Amplified Resists (PCAR) from 45nm to 13nm HP; however, 13nm HP needed 58 mJ/cm2, and provided DoF of 99nm with 4.4nm LWR. The recent non-PCAR Metal-Oxide Resist (MOR) from Inpria has been shown to resolve 12nm HP with  4.7 LWR using 38 mJ/cm2, and increasing exposure to 70 mJ/cm2 has produced 10nm HP L/S patterns.

In the EUVL tool with variable pupil control, reducing the pupil fill increases the contrast such that 20nm diameter contact holes with 3nm Local Critical-Dimension Uniformity (LCDU) can be done. The challenge is to get LCDU to <2nm to meet the specification for future chips. ASML’s announced next-generation N.A. >0.5 EUVL stepper will use anamorphic mirrors and masks which will double the illumination intensity per cm2 compared to today’s 0.33 N.A. tools. This will inherently improve the stochastics, when eventually ready after 2020.

The newest generation EUVL steppers use a membrane between the wafer and the optics so that any resist out-gassing cannot contaminate the mirrors, and this allow a much wider range of materials to be used as resists. Regarding MOR, there are 3.5 times more absorbed photons and 8 times more electrons generated per photon compared to PCAR. Metal hard-masks (HM) and other under-layers create reflections that have a significant effect on the LWR, requiring tuning of the materials in resist stacks.

Default R&D hub of the world imec has been testing EUV resists from five different suppliers, targeting 20 mJ/cm2 sensitivity with 30nm thickness for PCAR and 18nm thickness for MOR. All suppliers were able to deliver the requested resolution of 16nm HP line/space (L/S) patterns, yet all resists showed LWR >5nm. In another experiment, the dose to size for imec’s “7nm-node” metal-2 (M2) vias with nominal pitch of 53nm was ~60mJ/cm2. All else equal, three times slower lithography costs three times as much per wafer pass.

—E.K.

Edge Placement Error Control in Multi-Patterning

Thursday, March 2nd, 2017

thumbnail

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.

High-NA EUV Lithography Investment

Monday, November 28th, 2016

thumbnail

By Ed Korczynski, Sr. Technical Editor

As covered in a recent press release, leading lithography OEM ASML invested EUR 1 billion in cash to buy 24.9% of ZEISS subsidiary Carl Zeiss SMT, and committed to spend EUR ~760 million over the next 6 years on capital expenditures and R&D of an entirely new high numerical aperture (NA) extreme ultra-violet (EUV) lithography tool. Targeting NA >0.5 to be able to print 8 nm half-pitch features, the planned tool will use anamorphic mirrors to reduce shadowing effects from nanometer-scale mask patterns. Clever design and engineering of the mirrors could allow this new NA >0.5 tool to be able to achieve wafer throughputs similar to ASML’s current generation of 0.33 NA tools for the same source power and resist speed.

The Numerical Aperture (NA) of an optical system is a dimensionless number that characterizes the range of angles over which the system can accept or emit light. Higher NA systems can resolve finer features by condensing light from a wider range of angles. Mirror surfaces to reflect EUV “light” are made from over 50 atomic-scale bi-layers of molybdenum (Mo) and silicon (Si), and increasing the width of mirrors to reach higher NA increases the angular spread of the light which results in shadows within patterns.

In the proceedings of last year’s European Mask and Lithography Conference, Zeiss researchers reported on  “Anamorphic high NA optics enabling EUV lithography with sub 8 nm resolution” (doi:10.1117/12.2196393). The abstract summarizes the inherent challenges of establishing high NA EUVL technology:

For such a high-NA optics a configuration of 4x magnification, full field size of 26 x 33 mm² and 6’’ mask is not feasible anymore. The increased chief ray angle and higher NA at reticle lead to non-acceptable mask shadowing effects. These shadowing effects can only be controlled by increasing the magnification, hence reducing the system productivity or demanding larger mask sizes. We demonstrate that the best compromise in imaging, productivity and field split is a so-called anamorphic magnification and a half field of 26 x 16.5 mm² but utilizing existing 6’’ mask infrastructure.

Figure 1 shows that ASML plans to introduce such a system after the year 2020, with a throughput of 185 wafers-per-hour (wph) and with overlay of <2 nm. Hans Meiling, ASML vice president of product management EUV, in an exclusive interview with Solid State Technology explained why >0.5 NA capability will not be upgradable on 0.33 NA tools, “the >0.5NA optical path is larger and will require a new platform. The anamorphic imaging will also require stage architectural changes.”

Fig.1: EUVL stepper product plans for wafers per hour (WPH) and overlay accuracy include change from 0.33 NA to a new >0.5 NA platform. (Source: ASML)

Overlay of <2 nm will be critical when patterning 8nm half-pitch features, particularly when stitching lines together between half-fields patterned by single-exposures of EUV. Minimal overlay is also needed for EUV to be used to cut grid lines that are initially formed by pitch-splitting ArFi. In addition to the high NA set of mirrors, engineers will have to improve many parts of the stepper to be able to improve on the 3 nm overlay capability promised for the NXE:3400B 0.33 NA tool ASML plans to ship next year.

“Achieving better overlay requires improvements in wafer and reticle stages regardless of NA,” explained Meiling. “The optics are one of the many components that contribute to overlay. Compare to ArF immersion lithography, where the optics NA has been at 1.35 for several generations but platform improvements have provided significant overlay improvements.”

Manufacturing Capability Plans

Figure 2 shows that anamorphic systems require anamorphic masks, so moving from 0.33 to >0.5 NA requires re-designed masks. For relatively large chips, two adjacent exposures with two different anamorphic masks will be needed to pattern the same field area which could be imaged with lower resolution by a single 0.33 NA exposure. Obviously, such adjacent exposures of one layer must be properly “stitched” together by design, which is another constraint on electronic design automation (EDA) software.

Fig.2: Anamorphic >0.5 NA EUVL system planned by ASML and Zeiss will magnify mask images by 4x in the x-direction and 8x in the y-direction. (Source: Carl Zeiss SMT)

Though large chips will require twice as many half-field masks, use of anamorphic imaging somewhat reduces the challenges of mask-making. Meiling reminds us that, “With the anamorphic imaging, the 8X direction conditions will actually relax, while the 4X direction will require incremental improvements such as have always been required node-on-node.”

ASML and Zeiss report that ideal holes which “obscure” the centers of mirrors can surprisingly allow for increased transmission of EUV by each mirror, up to twice that of the “unobscured” mirrors in the 0.33 NA tool. The holes allow the mirrors to reflect through each-other, so they all line up and reflect better. Theoretically then each >0.5 NA half-field can be exposed twice as fast as a 0.33 NA full-field, though it seems that some system throughput loss will be inevitable. Twice the number of steps across the wafer will have to slow down throughput by some percent.

White two stitched side-by-side >0.5 NA EUVL exposures will be challenging, the generally known alternatives seem likely to provide only lower throughputs and lower yields:

*   Double-exposure of full-field using 0.33 NA EUVL,

*   Octuple-exposure of full-field using ArFi, or

*   Quadruple-exposure of full-field using ArFi complemented by e-beam direct-writing (EbDW) or by directed self-assembly (DSA).

One ASML EUVL system for HVM is expected to cost ~US$100 million. As presented at the company’s October 31st Investor Day this year, ASML’s modeling indicates that a leading-edge logic fab running ~45k wafer starts per month (WSPM) would need to purchase 7-12 EUV systems to handle an anticipated 6-10 EUV layers within “7nm-node” designs. Assuming that each tool will cost >US$100 million, a leading logic fab would have to invest ~US$1 billion to be able to use EUV for critical lithography layers.

With near US$1 billion in capital investments needed to begin using EUVL, HVM fabs want to be able to get productive value out of the tools over more than a single IC product generation. If a logic fab invests US$1 billion to use 0.33 NA EUVL for the “7nm-node” there is risk that those tools will be unproductive for “5nm-node” designs expected a few years later. Some fabs may choose to push ArFi multi-patterning complemented by another lithography technology for a few years, and delay investment in EUVL until >0.5 NA tools become available.

—E.K.

Many Mixes to Match Litho Apps

Thursday, March 3rd, 2016

thumbnail

By Ed Korczynski, Sr. Technical Editor

“Mix and Match” has long been a mantra for lithographers in the deep-sub-wavelength era of IC device manufacturing. In general, forming patterns with resolution at minimum pitch as small as 1/4 the wavelength of light can be done using off-axis illumination (OAI) through reticle enhancement techniques (RET) on masks, using optical proximity correction (OPC) perhaps derived from inverse lithography technology (ILT). Lithographers can form 40-45nm wide lines and spaces at the same half-pitch using 193nm light (from ArF lasers) in a single exposure.

Figure 1 shows that application-specific tri-layer photoresists are used to reach the minimum resolution of 193nm-immersion (193i) steppers in a single exposure. Tighter half-pitch features can be created using all manner of multi-patterning processes, including Litho-Etch-Litho-Etch (LELE or LE2) using two masks for a single layer or Self-Aligned Double Patterning (SADP) using sidewall spacers to accomplish pitch-splitting. SADP has been used in high volume manufacturing (HVM) of logic and memory ICs for many years now, and Self-Aligned Quadruple Patterning (SAQP) has been used in HVM by at least one leading memory fab.

Fig.1: Basic tri-layer resist (TLR) technology uses thin Photoresist over silicon-containing Hard-Mask over Spin-On Carbon (SOC), for patterning critical layers of advanced ICs. (Source: Brewer Science)

Next-Generation Lithography (NGL) generally refers to any post-optical technology with at least some unique niche patterning capability of interest to IC fabs:  Extreme Ultra-Violet (EUV), Directed Self-Assembly (DSA), and Nano-Imprint Lithography (NIL). Though proponents of each NGL have dutifully shown capabilities for targeted mask layers for logic or memory, the capabilities of ArF dry and immersion (ArFi) scanners to process >250 wafers/hour with high uptime dominates the economics of HVM lithography.

The world’s leading lithographers gather each year in San Jose, California at SPIE’s Advanced Lithography conference to discuss how to extend optical lithography. So of all the NGL technologies, which will win out in the end?

It is looking most likely that the answer is “all of the above.” EUV and NIL could be used for single layers. For other unique patterning application, ArF/ArFi steppers will be used to create a basic grid/template which will be cut/trimmed using one of the available NGL. Each mask layer in an advanced fab will need application-specific patterning integration, and one of the rare commonalities between all integrated litho modules is the overwhelming need to improve pattern overlay performance.

Naga Chandrasekaran, Micron Corp. vice president of Process R&D, provided a fantastic overview of the patterning requirements for advanced memory chips in a presentation during Nikon’s LithoVision technical symposium held February 21st in San Jose, California prior to the start of SPIE-AL. While resolution improvements are always desired, in the mix-and-match era the greatest challenges involve pattern overlay issues. “In high volume manufacturing, every nanometer variation translates into yield loss, so what is the best overlay that we can deliver as a holistic solution not just considering stepper resolution?” asks Chandrasekaran. “We should talk about cost per nanometer overlay improvement.”

Extreme Ultra-Violet (EUV)

As touted by ASML at SPIE-AL, the brightness and stability and availability of tin-plasma EUV sources continues to improve to 200W in the lab “for one hour, with full dose control,” according to Michael Lercel, ASML’s director of strategic marketing. ASML’s new TWINSCAN NXE:3350B EUVL scanners are now being shipped with 125W power sources, and Intel and Samsung Electronics reported run their EUV power sources at 80W over extended periods.

During Nikon’s LithoVision event, Mark Phillips, Intel Fellow and Director of Lithography Technology Development for Logic, summarized recent progress of EUVL technology:  ~500 wafers-per-day is now standard, and ~1000 wafer-per-day can sometimes happen. However, since grids can be made with ArFi for 1/3 the cost of EUVL even assuming best productivity for the latter, ArFi multi-patterning will continue to be used for most layers. “Resolution is not the only challenge,” reminded Phillips. “Total edge-placement-error in patterning is the biggest challenge to device scaling, and this limit comes before the device physics limit.”

Directed Self-Assembly (DSA)

DSA seems most suited for patterning the periodic 2D arrays used in memory chips such as DRAMs. “Virtual fabrication using directed self-assembly for process optimization in a 14nm DRAM node” was the title of a presentation at SPIE-AL by researchers from Coventor, in which DSA compared favorably to SAQP.

Imec presented electrical results of DSA-formed vias, providing insight on DSA processing variations altering device results. In an exclusive interview with Solid State Technology and SemiMD, imec’s Advanced Patterning Department Director Greg McIntyre reminds us that DSA could save one mask in the patterning of vias which can all be combined into doublets/triplets, since two masks would otherwise be needed to use 193i to do LELE for such a via array. “There have been a lot of patterning tricks developed over the last few years to be able to reduce variability another few nanometers. So all sorts of self-alignments.”

While DSA can be used for shrinking vias that are not doubled/tripled, there are commercially proven spin-on shrink materials that cost much less to use as shown by Kaveri Jain and Scott Light from Micron in their SPIE-AL presentation, “Fundamental characterization of shrink techniques on negative-tone development based dense contact holes.” Chemical shrink processes primarily require control over times, temperatures, and ambients inside a litho track tool to be able repeatably shrink contact hole diameters by 15-25 nm.

Nano-Imprint Litho (NIL)

For advanced IC fab applications, the many different options for NIL technology have been narrowed to just one for IC HVM. The step-and-pattern technology that had been developed and trademarked as “Jet and Flash Imprint Lithography” or “J-FIL” by, has been commercialized for HVM by Canon NanoTechnologies, formerly known as Molecular Imprints. Canon shows improvements in the NIL mask-replication process, since each production mask will need to be replicated from a written master. To use NIL in HVM, mask image placement errors from replication will have to be reduced to ~1nm., while the currently available replication tool is reportedly capable of 2-3nm (3 sigma).

Figure 2 shows normalized costs modeled to produce 15nm half-pitch lines/spaces for different lithography technologies, assuming 125 wph for a single EUV stepper and 60 wph for a cluster of 4 NIL tools. Key to throughput is fast filling of the 26mmx33mm mold nano-cavities by the liquid resist, and proper jetting of resist drops over a thin adhesion layer enables filling times less than 1 second.

Fig.2: Relative estimated costs to pattern 15nm half-pitch lines/spaces for different lithography technologies, assuming 125 wph for a single EUV stepper and 60 wph for a cluster of 4 NIL tools. (Source: Canon)

Researchers from Toshiba and SK Hynix described evaluation results of a long-run defect test of NIL using the Canon FPA-1100 NZ2 pilot production tool, capable of 10 wafers per hour and 8nm overlay, in a presentation at SPIE-AL titled, “NIL defect performance toward high-volume mass production.” The team categorized defects that must be minimized into fundamentally different categories—template, non-filling, separation-related, and pattern collapse—and determined parallel paths to defect reduction to allow for using NIL in HVM of memory chips with <20nm half-pitch features.

—E.K.

ASML Has Record Revenue for 2015; Will Raise Dividend, Buy Back More Stock

Wednesday, January 20th, 2016

By Jeff Dorsch, Contributing Editor

ASML Holding today reported net income of about $1.5 billion on revenue of $6.855 billion for 2015. That compared with 2014’s net income of $1.3 billion on revenue of $6.385 billion.

In the fourth quarter, the company posted net income of $318.3 million on revenue of $1.563 billion. Those figures were lower than the third quarter.

ASML said it sold 144 new lithography systems last year, up from 116 in 2014. It also sold 25 used systems, against 20 units a year earlier.

The 2015 sales represented a record for the supplier of advanced lithography equipment. ASML forecast sales for 2016’s first quarter would exceed $1.4 billion.

The company said it would raise its annual dividend to shareholders by 50 percent, to 1.05 euros per share, and will spend an additional $1.09 billion in 2016-2017 repurchasing its shares.

“As we indicated three months ago, we expect our logic customers to take shipments of our leading-edge immersion tools in the second quarter in preparation of their 10-nanometer node ramp. As a result, we expect second-quarter sales to increase significantly from the first-quarter level,” ASML President and Chief Executive Officer Peter Wennink said in a statement.

ASML stated that its extreme-ultraviolet lithography line “met its 2015 productivity and availability targets. We had already achieved a productivity of more than 1,000 wafers per day early in 2015 on the NXE:3300B system and improved this to more than 1,250 wafers per day in the fourth quarter on the successor system, the NXE:3350B. In addition, the availability of systems in the field improved, with the majority of systems achieving a four-week availability of more than 70 percent in recent months; the best result was more than 80 percent over four weeks. We also shipped two of our latest NXE:3350B EUV systems and started shipping the third in 2015. They will be used in our customers’ fabs for preparing the introduction of EUV into volume production. Our goals for 2016 are to continue improving productivity and availability and shipping six to seven EUV systems.”

Regarding deep-ultraviolet lithography systems, “we began ramping shipments of the TWINSCAN NXT:1980, our most advanced immersion system, in the fourth quarter, shipping five systems. The installation of the first systems is complete,” ASML stated.

Holistic lithography “grew by over 20 percent in revenue in 2015; we saw increased adoption of our latest metrology systems and control software at both logic and memory customers. These applications play a more and more critical role in helping our customers achieve the best possible patterning performance on advanced nodes,” the company added.

“The first-quarter outlook disappoints,” SNS Securities analyst Edwin de Jong told Bloomberg News. “It is good that you return cash to shareholders, but you need to improve operationally.”

SPIE Advanced Lithography conference concludes

Friday, February 27th, 2015

By Jeff Dorsch, contributing editor

Exposures, and reducing their cost, were a theme running through the 2015 SPIE Advanced Lithography Symposium this week in San Jose, Calif., the center of Silicon Valley.

Doubts about the continued viability of Moore’s Law abound as the 50th anniversary of Gordon Moore’s historic article for Electronics magazine draws near. Lithographers are under immense industry pressure to lower the operating costs of lithography cells in the fab while increasing wafer throughput.

“Enabling,” “productivity,” and “stability” were watchwords frequently repeated throughout the conference. The various merits (and occasional demerits) of electron-beam, extreme-ultraviolet, 193i immersion and nanoimprint lithography technologies were debated and touted over four days.

One of the technical sessions closing out Wednesday at the San Jose Convention Center was devoted to papers on “Multibeam Lithography,” especially e-beam direct-write technology, which has been seen as “pie in the sky” for many years, yet seems closer to realization than before.

Hans Loeschner of IMS Nanofabrication described how his company’s e-beam tool has progressed from alpha to beta status this year, and predicted it would be ready for production applications in 2016. Altera, CEA-Leti, and MAPPER Lithography presented a total of three papers on MAPPER’s FLX-1200 e-beam direct-write system, saying it is better able to make chips with 20-nanometer features than an immersion lithography system.

The eBeam Initiative held its annual luncheon at SPIE Advanced Lithography on Tuesday, emphasizing how multibeam mask writing, model-based mask data preparation, and complex inverse lithography technology can enable continued density scaling at the 10-nanometer process node.

“We have reached a point with traditional rules-based designs where the rules are so conservative and the implementation costs are so high that the semiconductor industry has started to lose the economic benefits of scaling to smaller design nodes for system-on-chip designs,” D2S CEO Aki Fujimura said in a statement. “A simulation-based approach combining complex ILT, MB-MDP and existing variable shaped beam mask writers in parallel with the impending emergence of multibeam mask writing are providing platforms to enable the semiconductor industry to reverse this trend and reactivate the density benefits associated with Moore’s Law.”

EUV, another technology that has had a long gestation, was the subject of a conference track over all four days, with photomask and photoresist issues being discussed in several sessions.

The news that Taiwan Semiconductor Manufacturing was able to process 1,022 wafers in 24 hours with ASML Holding’s NXE:3300B scanner was the talk of the SPIE conference on Tuesday, the first day of the two-day exhibition, which had about 60 companies occupying booths. ASML didn’t declare an end to development of its EUV systems, saying there is more work to be done. This includes development of a pellicle for the scanner’s reticles and working with resist suppliers on formulas for EUV resists.

While improvements in all types of lithographies were discussed at the conference, there was increased interest in directed self-assembly, which employs polymers to get molecules to arrange themselves in lines and spaces with a patterning guide. Advances in reducing the defectivity of DSA were reported by imec, Merck, and Tokyo Electron.

Global interest in DSA over the past four years has accelerated due to “other things getting delayed,” said Tom Ferry of Synopsys. Among other initiatives, the electronic design automation software and services company was talking about how its S-Litho molecular simulator, S-Litho shape optimizer, and Proteus ILT guide patterning tool can help enable DSA research and development, design, and manufacturing.

The Belgium-based imec was a big contributor to conference presentations, with a first author on 18 papers and posters, and a co-author of 25 publications.

While EUV garnered headlines during SPIE Advanced Lithography, the Cymer subsidiary of ASML was at the conference to talk about its third-generation XLR 700ix light source for deep-ultraviolet lithography systems. Ted Cacouris of Cymer said, “10 nanometer is basically done with DUV. It could go to 7 nanometer; immersion could be extended. It could be complementary to EUV.”

Cymer also announced its DynaPulse program, an upgrade for its OnPulse subscription service for maintenance and repair of light sources. In 2012, prior to the company’s acquisition by ASML, Cymer derived nearly 70 percent of its light-source revenue from the OnPulse service program.

It’s been an interesting week, with about 2,400 attendees from around the world gathering for the premier lithography conference of the year. They will convene again a year from now to learn what’s new in lithography.

Proponents of EUV, immersion lithography face off at SPIE

Wednesday, February 25th, 2015

By Jeff Dorsch, contributing editor

The two main camps in optical lithography are arrayed for battle at the SPIE Advanced Lithography Symposium in San Jose, Calif.

Extreme-ultraviolet lithography, on one side, is represented by ASML Holding, its Cymer subsidiary, and ASML’s EUV customers, notably Intel, Samsung Electronics, and Taiwan Semiconductor Manufacturing.

On the other side is 193i immersion lithography, represented by Nikon and its customers, which also include Intel and other leading chipmakers.

There are other lithography technologies being discussed at the conference, of course. They are bit players in the drama, so to speak, although there is a lot of discussion and buzz about directed self-assembly technology this week.

ASML broke big news on Tuesday morning, reporting that Taiwan Semiconductor Manufacturing was able to expose more than 1,000 wafers in one day this year with ASML’s NXE:3300B EUV system. “During a recent test run on an NXE:3300B EUV system we exposed 1,022 wafers in 24 hours with sustained power of over 90 watts,” Anthony Yen, TSMC’s director of research and development, said at SPIE.

While ASML was obviously and justifiably proud of this milestone, after achieving its 2014 goal of producing 500 wafers per day, it cautioned that more development remains for EUV technology.

“The test run at TSMC demonstrates the capability of the NXE:3300B scanner, and moves us closer to our stated target of sustained output of 1,000 wafers per day in 2015,” ASML’s Hans Meiling, vice president service and product marketing EUV, said in a statement. “We must continue to increase source power, improve system availability, and show this result at multiple customers over multiple days.”

The day before, Cymer announced the first shipment of its XLR 700ix light source, which is said to improver scanner throughput and process stability for manufacturing chips with 14-nanometer features. The company also debuted DynaPulse as an upgrade option for its OnPulse customers. The XLR 700ix and DynaPulse together are said to offer better on-wafer critical dimension uniformity and provide stable on-wafer performance.

Another revelation at SPIE is that SK Hynix has been working with the NXE:3300, too, and is pleased with the system’s capabilities. According to Chang-Moon Lim, who spoke Monday morning, SK Hynix was recently able to expose 1,670 wafers over three days, with uptime of 86.3 percent over that period.

“Progress has been significant on various aspects, which should not be overshadowed by the delay of [light] sources,” he said of ASML’s EUV systems.

The Korean chipmaker is exploring how it could work without pellicles on the EUV reticle, Lim noted. ASML has been developing a pellicle, made with polycrystalline silicon, in cooperation with Intel and others.

Nikon Precision and other Nikon subsidiaries didn’t issue any press releases at SPIE. The companies presented much information at Sunday’s LithoVision 2015 event, held at the City National Civic auditorium, across the street from the San Jose Convention Center, where SPIE Advanced Lithography is staged.

On offer at the Nikon conference was the claimed superiority of 193i immersion lithography equipment to EUV systems for the 14nm, 7nm and future process nodes. Donis Flagello, Nikon Research Corp. of America’s president, CEO, and chief operating officer, emphasized that message on Tuesday morning with an invited paper on “Evolving optical lithography without EUV.”

Nikon’s champion machine is the NSR-S630D immersion scanner, which was touted throughout the LithoVision event. The system is capable of exposing 250 wafers per hour, according to Nikon’s Yuichi Shibazaki.

Ryoichi Kawaguchi of Nikon told attendees, “EUV lithography needs more stability and improvement.” He also brought up the topic of manufacturing on 450-millimeter wafers, which has mostly gone ignored in the lithography competition. Nikon will ship a 450mm system this spring to the Global 450 Consortium in Albany, N.Y., Kawaguchi said. The bigger substrates could provide “an alternative option to reduce cost,” he added.

Erik Byers of Micron Technology observed, “EUV is not a panacea.”

Which lithography technology will prevail in high-volume manufacturing? The question may not be definitively answered for some time.

Complexity is the Theme at Lithography Conference

Monday, February 23rd, 2015

By Jeff Dorsch, contributing editor

Nikon and KLA-Tencor put on separate conferences in San Jose, Calif., on Sunday, February 22, tackling issues in advanced optical lithography. The overarching theme in both sessions was the increased complexity of lithography as it approaches the 10-nanometer and 7nm process nodes.

“Complexity is much higher,” said Kevin Lucas of Synopsys at the Nikon event, LithoVision 2015. He noted that at the 28nm process node, lithographers could resort to five different options. For 14nm or 16nm, that expanded to eight options. There are 21 options available at 10nm, Lucas said, and at 7nm that explodes to more than 71 options.

“The increase in complexity is pretty dramatic,” he observed.

Electronic design automation vendors have “to provide more accurate modeling,” Lucas said. “We will have to go to better methods of [optical proximity correction].”

Ralph Dammel of EMD Performance Materials reviewed the situation in semiconductor materials as IC gate lengths continue to shrink. “We’re going to move from adding new elements to different forms of elements,” he said, such as graphene, silicine, black phosphorus, and molybdenum disulfide.

At the Lithography Users Forum, the event put on by KLA-Tencor, Mark Phillips of Intel said, “Scaling can continue, but it needs improved metrology.” He added, “We need side-by-side accuracy metrics.”

Phillips reported on Intel’s work with ASML Holding on developing pellicles for the reticles of ASML’s extreme-ultraviolet lithography systems. The companies have together come up with a prototype pellicle, which needs more development as a commercial product, he said.

Changes and Challenges Abound in Multi-patterning Lithography

Monday, January 26th, 2015

thumbnail

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.

Next Page »