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Posts Tagged ‘advanced’

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.

EUV Resists and Stochastic Processes

Friday, March 4th, 2016

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By Ed Korczynski, Sr. Technical Editor

In an exclusive interview with Solid State Technology during SPIE-AL this year, imec Advanced Patterning Department Director Greg McIntyre said, “The big encouraging thing at the conference is the progress on EUV.” The event included a plenary presentation by TSMC Nanopatterning Technology Infrastructure Division Director and SPIE Fellow Anthony Yen on “EUV Lithography: From the Very Beginning to the Eve of Manufacturing.” TSMC is currently learning about EUVL using 10nm- and 7nm-node device test structures, with plans to deploy it for high volume manufacturing (HVM) of contact holes at the 5nm node. Intel researchers confirm that they plan to use EUVL in HVM for the 7nm node.

Recent improvements in EUV source technology— 80W source power had been shown by the end of 2014, 185W by the end of 2015, and 200W has now been shown by ASML—have been enabled by multiple laser pulses tuned to the best produce plasma from tin droplets. TSMC reports that 518 wafers per day were processed by their ASML EUV stepper, and the tool was available ~70% of the time. TSMC shows that a single EUVL process can create 46nm pitch lines/spaces using a complex 2D mask, as is needed for patterning the metal2 layer within multilevel on-chip interconnects.

To improve throughput in HVM, the resist sensitivity to the 13.54nm wavelength radiation of EUV needs to be improved, while the line-width roughness (LWR) specification must be held to low single-digit nm. With a 250W source and 25 mJ/cm2 resist sensitivity an EUV stepper should be able to process ~100 wafer-per-hour (wph), which should allow for affordable use when matched with other lithography technologies.

Researchers from Inpria—the company working on metal-oxide-based EUVL resists—looked at the absorption efficiencies of different resists, and found that the absorption of the metal oxide based resists was ≈ 4 to 5 times higher than that of the Chemically-Amplified Resist (CAR). The Figure shows that higher absorption allows for the use of proportionally thinner resist, which mitigates the issue of line collapse. Resist as thin as 18nm has been patterned over a 70nm thin Spin-On Carbon (SOC) layer without the need for another Bottom Anti-Reflective Coating (BARC). Inpria today can supply 26 mJ/cm2 resist that creates 4.6nm LWR over 140nm Depth of Focus (DoF).

To prevent pattern collapse, the thickness of resist is reduced proportionally to the minimum half-pitch (HP) of lines/spaces. (Source: JSR Micro)

JEIDEC researchers presented their summary of the trade-off between sensitivity and LWR for metal-oxide-based EUV resists:  ultra high sensitivity of 7 mJ/cm2 to pattern 17nm lines with 5.6nm LWR, or low sensitivity of 33 mJ/cm2 to pattern 23nm lines with 3.8nm LWR.

In a keynote presentation, Seong-Sue Kim of Samsung Electronics stated that, “Resist pattern defectivity remains the biggest issue. Metal-oxide resist development needs to be expedited.” The challenge is that defectivity at the nanometer-scale derives from “stochastics,” which means random processes that are not fully predictable.

Stochastics of Nanopatterning

Anna Lio, from Intel’s Portland Technology Development group, stated that the challenges of controlling resist stochastics, “could be the deal breaker.” Intel ran a 7-month test of vias made using EUVL, and found that via critical dimensions (CD), edge-placement-error (EPE), and chain resistances all showed good results compared to 193i. However, there are inherent control issues due to the random nature of phenomena involved in resist patterning:  incident “photons”, absorption, freed electrons, acid generation, acid quenching, protection groups, development processes, etc.

Stochastics for novel chemistries can only be controlled by understanding in detail the sources of variability. From first-principles, EUV resist reactions are not photon-chemistry, but are really radiation-chemistry with many different radiation paths and electrons which can be generated. If every via in an advanced logic IC must work then the failure rate must be on the order of 1 part-per-trillion (ppt), and stochastic variability from non-homogeneous chemistries must be eliminated.

Consider that for a CAR designed for 15mJ/cm2 sensitivity, there will be just:

145 photons/nm2 for 193, and

10 photons/nm2 for EUV.

To improve sensitivity and suppress failures from photon shot-noise, we need to increase resist absorption, and also re-consider chemical amplification mechanisms. “The requirements will be the same for any resist and any chemistry,” reminded Lio. “We need to evaluate all resists at the same exposure levels and at the same rules, and look at different features to show stochastics like in the tails of distributions. Resolution is important but stochastics will rule our world at the dimensions we’re dealing with.”

—E.K.

Many Mixes to Match Litho Apps

Thursday, March 3rd, 2016

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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.

Managing Dis-Aggregated Data for SiP Yield Ramp

Monday, August 24th, 2015

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By Ed Korczynski, Sr. Technical Editor

In general, there is an accelerating trend toward System-in-Package (SiP) chip designs including Package-On-Package (POP) and 3D/2.5D-stacks where complex mechanical forces—primarily driven by the many Coefficient of Thermal Expansion (CTE) mismatches within and between chips and packages—influence the electrical properties of ICs. In this era, the industry needs to be able to model and control the mechanical and thermal properties of the combined chip-package, and so we need ways to feed data back and forth between designers, chip fabs, and Out-Sourced Assembly and Test (OSAT) companies. With accelerated yield ramps needed for High Volume Manufacturing (HVM) of consumer mobile products, to minimize risk of expensive Work In Progress (WIP) moving through the supply chain a lot of data needs to feed-forward and feedback.

Calvin Cheung, ASE Group Vice President of Business Development & Engineering, discussed these trends in the “Scaling the Walls of Sub-14nm Manufacturing” keynote panel discussion during the recent SEMICON West 2015. “In the old days it used to take 12-18 months to ramp yield, but the product lifetime for mobile chips today can be only 9 months,” reminded Cheung. “In the old days we used to talk about ramping a few thousand chips, while today working with Qualcomm they want to ramp millions of chips quickly. From an OSAT point of view, we pride ourselves on being a virtual arm of the manufacturers and designers,” said Cheung, “but as technology gets more complex and ‘knowledge-base-centric” we see less release of information from foundries. We used to have larger teams in foundries.” Dick James of ChipWorks details the complexity of the SiP used in the Apple Watch in his recent blog post at SemiMD, and documents the details behind the assumption that ASE is the OSAT.

With single-chip System-on-Chip (SoC) designs the ‘final test’ can be at the wafer-level, but with SiP based on chips from multiple vendors the ‘final test’ now must happen at the package-level, and this changes the Design For Test (DFT) work flows. DRAM in a 3D stack (Figure 1) will have an interconnect test and memory Built-In Self-Test (BIST) applied from BIST resident on the logic die connected to the memory stack using Through-Silicon Vias (TSV).

Fig.1: Schematic cross-sections of different 3D System-in-Package (SiP) design types. (Source: Mentor Graphics)

“The test of dice in a package can mostly be just re-used die-level tests based on hierarchical pattern re-targeting which is used in many very large designs today,” said Ron Press, technical marketing director of Silicon Test Solutions, Mentor Graphics, in discussion with SemiMD. “Additional interconnect tests between die would be added using boundary scans at die inputs and outputs, or an equivalent method. We put together 2.5D and 3D methodologies that are in some of the foundry reference flows. It still isn’t certain if specialized tests will be required to monitor for TSV partial failures.”

“Many fabless semiconductor companies today use solutions like scan test diagnosis to identify product-specific yield problems, and these solutions require a combination of test fail data and design data,” explained Geir Edie, Mentor Graphics’ product marketing manager of Silicon Test Solutions. “Getting data from one part of the fabless organization to another can often be more challenging than what one should expect. So, what’s often needed is a set of ‘best practices’ that covers the entire yield learning flow across organizations.”

“We do need a standard for structuring and transmitting test and operations meta-data in a timely fashion between companies in this relatively new dis-aggregated semiconductor world across Fabless, Foundry, OSAT, and OEM,” asserted John Carulli, GLOBALFOUNDRIES’ deputy director of Test Development & Diagnosis, in an exclusive discussion with SemiMD. “Presently the databases are still proprietary – either internal to the company or as part of third-party vendors’ applications.” Most of the test-related vendors and users are supporting development of the new Rich Interactive Test Database (RITdb) data format to replace the Standard Test Data Format (STDF) originally developed by Teradyne.

“The collaboration across the semiconductor ecosystem placed features in RITdb that understand the end-to-end data needs including security/provenance,” explained Carulli. Figure 2 shows that since RITdb is a structured data construct, any data from anywhere in the supply chain could be easily communicated, supported, and scaled regardless of OSAT or Fabless customer test program infrastructure. “If RITdb is truly adopted and some certification system can be placed around it to keep it from diverging, then it provides a standard core to transmit data with known meaning across our dis-aggregated semiconductor world. Another key part is the Test Cell Communication Standard Working Group; when integrated with RITdb, the improved automation and control path would greatly reduce manually communicated understanding of operational practices/issues across companies that impact yield and quality.”

Fig.2: Structure of the Rich Interactive Test Database (RITdb) industry standard, showing how data can move through the supply chain. (Source: Texas Instruments)

Phil Nigh, GLOBALFOUNDRIES Senior Technical Staff, explained to SemiMD that for heterogeneous integration of different chip types the industry has on-chip temperature measurement circuits which can monitor temperature at a given time, but not necessarily identify issues cause by thermal/mechanical stresses. “During production testing, we should detect mechanical/thermal stress ‘failures’ using product testing methods such as IO leakage, chip leakage, and other chip performance measurements such as FMAX,” reminded Nigh.

Model but verify

Metrology tool supplier Nanometrics has unique perspective on the data needs of 3D packages since the company has delivered dozens of tools for TSV metrology to the world. The company’s UniFire 7900 Wafer-Scale Packaging (WSP) Metrology System uses white-light interferometry to measure critical dimensions (CD), overlay, and film thicknesses of TSV, micro-bumps, Re-Distribution Layer (RDL) structures, as well as the co-planarity of Cu bumps/pillars. Robert Fiordalice, Nanometrics’ Vice President of UniFire business group, mentioned to SemiMD in an exclusive interview that new TSV structures certainly bring about new yield loss mechanisms, even if electrical tests show standard results such as ‘partial open.’ Fiordalice said that, “we’ve had a lot of pull to take our TSV metrology tool, and develop a TSV inspection tool to check every via on every wafer.” TSV inspection tools are now in beta-tests at customers.

As reported at 3Dincites, Mentor Graphics showed results at DAC2015 of the use of Calibre 3DSTACK by an OSAT to create a rule file for their Fan-Out Wafer-Level Package (FOWLP) process. This rule file can be used by any designer targeting this package technology at this assembly house, and checks the manufacturing constraints of the package RDL and the connectivity through the package from die-to-die and die-to-BGA. Based on package information including die order, x/y position, rotation and orientation, Calibre 3DSTACK performs checks on the interface geometries between chips connected using bumps, pillars, and TSVs. An assembly design kit provides a standardized process both chip design companies and assembly houses can use to ensure the manufacturability and performance of 3D SiP.

—E.K.