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Mechanistic Modeling of Silicon ALE for FinFETs

Tuesday, April 25th, 2017

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With billions of device features on the most advanced silicon CMOS ICs, the industry needs to be able to precisely etch atomic-scale features without over-etching. Atomic layer etching (ALE), can ideally remove uniform layers of material with consistent thickness in each cycle, and can improve uniformity, reduce damage, increase selectivity, and minimize aspect ratio dependent etching (ARDE) rates. Researchers Chad Huard et al. from the University of Michigan and Lam Research recently published “Atomic layer etching of 3D structures in silicon: Self-limiting and nonideal reactions” in the latest issue of the Journal of Vacuum Science & Technology A (http://dx.doi.org/10.1116/1.4979661). Proper control of sub-cycle pulse times is the key to preventing gas mixing that can degrade the fidelity of ALE.

The authors modeled non-idealities in the ALE of silicon using Ar/Cl2 plasmas:  passivation using Ar/Cl2 plasma resulting in a single layer of SiClx, followed by Ar-ion bombardment to remove the single passivated layer. Un-surprisingly, they found that ideal ALE requires self-limited processes during both steps. Decoupling passivation and etching allows for several advantages over continuous etching, including more ideal etch profiles, high selectivity, and low plasma-induced damage. Any continuous etching —when either or both process steps are not fully self-limited— can cause ARDE and surface roughness.

The gate etch in a finFET process requires that 3D corners be accurately resolved to maintain a uniform gate length along the height of the fin. In so doing, the roughness of the etch surface and the exact etch depth per cycle (EPC) are not as critical as the ability of ALE to be resistant to ARDE. The Figure shows that the geometry modeled was a periodic array of vertical crystalline silicon fins, each 10nm wide and 42nm high, set at a pitch of 42 nm. For continuous etching (a-c), simulations used a 70/30 mix of Ar/Cl gas and RF bias of 30V. Just before the etch-front touches the underlying SiO2 (a), the profile has tapered away from the trench sidewalls and the etch-front shows some micro-trenching produced by ions (or hot neutrals) specularly reflected from the tapered sidewalls. After a 25% over-etch (b), a significant amount of Si remains in the corners and on the sides of the fins. Even after an over-etch of 100% (c), Si still remains in the corners.

FIGURE CAPTION: Simulated profiles resulting from etching finFET gates with (a)–(c) a continuous etching process, or (d)–(f) an optimized ALE process. Time increases from left to right, and images represent equal over-etch (as a percentage of the time required to expose the bottom SiO2) not equal etch times. Times listed for the ALE process in (d)–(f) represent plasma-on, ignoring any purge or dwell times. (Source: J. Vac. Sci. Technol. A, Vol. 35, No. 3, May/Jun 2017)

In comparison, the ALE process (d-f) shows that after 25% over-etch (e) the bottom SiO2 surface would be almost completely cleared with minimal corner residues, and continuing to 100% over-etch results in little change to the profile. The ALE process times shown here do not include the gas purge and fill times between plasma pulses; to clear the feature using ALE required 200 pulses and assuming 5 seconds of purge time between each pulse results in a total process time of 15–20 min to clear the feature. This is a significant increase in total process time over the continuous etch (2 min).

One conclusion of this ALE modeling is that even small deviations from perfectly self-limited reactions significantly compromise the ideality of the ALE process. For example, having as little as 10 ppm Cl2 residual gas in the chamber during the ion bombardment phase produced non-idealities in the ALE. Introducing any source of continuous chemical etching into the ALE process leads to the onset of ARDE and roughening of the etch front. These trends have significant implications for both the design of specialized ALE chambers, and also for the use of ALE to control uniformity.

—E.K.

Lithographic Stochastic Limits on Resolution

Monday, April 3rd, 2017

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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

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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.

Picosun and Hitachi MECRALD Process

Friday, February 24th, 2017

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By Ed Korczynski, Sr. Technical Editor

A new microwave electron cyclotron resonance (MECR) atomic layer deposition (ALD) process technology has been co-developed by Hitachi High-Technologies Corporation and Picosun Oy to provide commercial semiconductor IC fabs with the ability to form dielectric films at lower temperatures. Silicon oxide and silicon nitride, aluminum oxide and aluminum nitride films have been deposited in the temperature range of 150-200 degrees C in the new 300-mm single-wafer plasma-enhanced ALD (PEALD) processing chamber.

With the device features within both logic and memory chips having been scaled to atomic dimensions, ALD technology has been increasingly enabling cost-effective high volume manufacturing (HVM) of the most advanced ICs. While the deposition rate will always be an important process parameter for HVM, the quality of the material deposited is far more important in ALD. The MECR plasma source provides a means of tunable energy to alter the reactivity of ALD precursors, thereby allowing for new degrees of freedom in controlling final film properties.

The Figure shows the MECRALD chamber— Hitachi High-Tech’s ECR plasma generator is integrated with Picosun’s digitally controlled ALD system—from an online video (https://youtu.be/SBmZxph-EE0) describing the process sequence:

1.  first precursor gas/vapor flows from a circumferential ring near the wafer chuck,

2.  first vacuum purge,

3.  second precursor gas/vapor is ionized as it flows down through the ECR zone above the circumferential ring, and

4.  second vacuum purge to complete one ALD cycle (which may be repeated).

Cross-sectional schematic of a new Microwave Electron Cyclotron Resonance (MECR) plasma source from Hitachi High-Technologies connected to a single-wafer Atomic Layer Deposition (ALD) processing chamber from Picosun. (Source: Picosun)

The development team claims that MECRALD films are superior to other PEALD films in terms of higher density, lower contamination of carbon and oxygen (in non-oxides), and also show excellent step-coverage as would be expected from a surface-driven ALD process. The relatively density of these films has been confirmed by lower wet etch rates. The single-wafer process non-uniformity on 300mm wafers is claimed at ~1% (1 sigma). The team is now exploring processes and precursors to be able to deposit additional films such as titanium nitride (TiN), tantalum nitride (TaN), and hafnium oxide (HfO). In an interview with Solid State Technology, a spokesperson from Hitachi High-Technologies explained that, “We are now at the development stage, and the final specifications mainly depend on future achievements.”

The MECR source has been used in Hitachi High-Tech’s plasma chamber for IC conductor etch for many years, and is able to generate a stable high-density plasma at very low pressure (< 0.1 Pa). MECR plasmas provide wide process windows through accurate plasma parameter management, such as plasma distribution or plasma position control. The same plasma technology is also used to control ions and radicals in the company’s dry cleaning chambers.

“I’m really impressed by the continuous development of ALD technology, after more than 40 years since the invention,” commented Dr. Tuomo Suntola, and the famous inventor and patentor of the Atomic Layer Deposition method in Finland in 1974, and member of the Picosun board of directors. “Now combining Hitachi and Picosun technologies means (there is) again a major breakthrough in advanced semiconductor manufacturing.”

MECRALD chambers can be clustered on a Picosun platform that features a Brooks robot handler. This technology is still under development, so it’s too soon to discuss manufacturing parameters such as tool cost and wafer throughput.

—E.K.

Vital Control in Fab Materials Supply-Chains – Part 2

Thursday, February 16th, 2017

By Ed Korczynski, Sr. Technical Editor

As detailed in Part 1 of this article published last month by SemiMD, the inaugural Critical Materials Council (CMC) Conference happened May 5-6 in Hillsboro, Oregon. Held just after the yearly private CMC meeting, the public CMC Conference provides a forum for the pre-competitive exchange of information to control the supply-chain of critical materials needed to run high-volume manufacturing (HVM) in IC fabs. The next CMC Conference will happen May 11-12 in Dallas, Texas.

At the end of the 2016 conference, a panel discussion moderated by Ed Korczynski was recorded and transcribed. The following is Part 2 of the conversation between the following industry experts:

  • Jean-Marc Girard, CTO and Director of R&D, Air Liquide Advanced Materials,
  • Jeff Hemphill, Staff Materials R&D Engineer, Intel Corporation,
  • Jonas Sundqvist, Sr. Scientist, Fraunhofer IKTS; and co-chair of ALD Conference, and
  • John Smythe, Distinguished Member of Technical Staff, Micron Technology.

FIGURE 1: 2016 CMC Conference expert panelists (from left to right) John Smyth, Jonas Sundqvist, Jeff Hemphill, and Jean-Marc Girard. (Source: TECHCET CA)

KORCZYNSKI:  We heard from David Thompson [EDITOR’S NOTE:  Director of Process Chemistry, Applied Materials presented on “Agony in New Material Introductions -  Minimizing and Correlating Variabilities”] today on what we must control, and he gave an example of a so-called trace-contaminant that was essential for the process performance of a precursor, where the trace compound helped prevent particles from flaking off chamber walls. Do we need to specify our contaminants?

GIRARD:  Yes. To David’s point this morning, every molecule is different. Some are very tolerant due to the molecular process associated with it, and some are not. I’ll give you an example of a cobalt material that’s been talked about, where it can be run in production at perhaps 95% in terms of assay, provided that one specific contaminant is less than a couple of parts-per-million. So it’s a combination of both, it’s not assay OR a specification of impurities. It’s a matter of specifying the trace components that really matter when you reach the point that the data you gather gives you that understanding, and obviously an assay within control limits.

HEMPHILL:  Talking about whether we’re over-specifying or not, the emphasis is not about putting the right number on known parameters like assay that are obvious to measure, the emphasis is on identifying and understanding what makes up the rest of it and in a sense trying over-specify that. You identify through mass-spectrometry and other techniques that some fraction of a percent is primarily say five different species, it’s finding out how to individually monitor and track and control those as separate parameters. So from a specification point of view what we want is not necessarily the lowest possible numbers, but it’s expanding how many things we’re looking at so that we’re capturing everything that’s there.

KORCZYNSKI:  Is that something that you’re starting to push out to your suppliers?

HEMPHILL:  Yes. It depends on the application we’re talking about, but we go into it with the assumption that just assay will not be enough. Whether a single molecule or a blend of things is supposed to be there, we know that just having those be controlled by specification will not be sufficient. We go under the assumption that we are going to identify what makes up the remaining part of the profile, and those components are going to need to be controlled as well.

KORCZYNSKI:  Is that something that has changed by node? Back when things were simpler say at 45nm and larger, were these aspects of processing that we could safely ignore as ‘noise’ but are now important ‘signals’?

HEMPHILL:  Yes, we certainly didn’t pay as close attention just a couple of generations ago.

KORCZYNSKI:  That seems to lead us to questions about single-sources versus dual-sourcing. There are many good reasons to do both, but not simultaneously. However, it seems that because of all of the challenges we’re heard about over the last day-and-a-half of this conference it creates greater burden on the suppliers, and for critical materials the fabs are moving toward more single-sourcing over time.

SMYTHE:  I think that it comes down to more of a concern over geographic risk. I’ll buy from one entity if that entity has more than one geographic location for the supply, so that I’m not exposed to a single ‘Act of God’ or a ‘random statistical occurrence of global warming.’ So for example I  need to ask if a supplier has a place in the US and a place in France that makes the same thing, so that if something bad happens in one location it can still be sourced? Or do you have an alternate-supply agreement that if you can’t supply it you have an agreement with Company-X to supply it so that you still have control? You can’t come to a Micron and say we want to make sure that we get at minimum 25% no matter what, because what typically happens with second-sourcing is Company-A gets 75% of the business while Company-B gets 25%. There are a lot of reasons that that doesn’t work so well, so people may have an impression that there’s a movement toward single-source but it’s ‘single flexible-source.’

HEMPHILL:  There are a lot of benefits of dual- or multiple-sourcing. The commercial benefits of competition can be positive and we’re for it when it works. The risk is that as things are progressing and we’re getting more sensitive to differences in materials it’s getting harder to maintain that. We have seen situations where historically we were successful with dual-sourcing a raw material coming from two different suppliers or even a single supplier using two different manufacturing lines and everything was fine and qualified and we could alternate sources invisibly. However, as our sensitivity has grown over time we can start to detect differences.

So the concept of being ‘copy-exactly’ that we use in our factories, we really need production lines to do that, and if we’re talking about two different companies producing the same material then we’re not going to get them to be copy-exactly. When that results in enough of a variation in the material that we can detect it in the factory then we cannot rely upon two sources. Our preference would be one company that maintains multiple production sites that are designed to be exactly the same, then we have a high degree of confidence that they will be able to produce the same material.

FIGURE 2: Jean-Marc Girard, Distinguished Member of Technical Staff of Micron Technology, provided the supplier perspective. (Source: SEMI)

GIRARD:  I can give you a supplier perspective on that. We are seeing very different policies from different customers, to the point that we’re seeing an increase in the number of customers doing single-sourcing with us, provided we can show the ability to maintain business continuity in case of a problem. I think that the industry became mature after the tragic earthquake and tsunami in Japan in 2011 with greater understanding of what business continuity means. We have the same discussions with our own suppliers, who may say that they have a dedicated reactor for a certain product with another backup reactor with a certain capacity on the same site, and we ask what happens if the plant goes on strike or there’s a fire there?

A situation where you might think the supply was stable involved silane in the United States. There are two large silane plants in the United States that are very far apart from each other and many Asian manufacturers dependent upon them. When the U.S. harbors went on strike for a long time there was no way that material could ship out of the U.S. customers. So, yes there were two plants but in such an event you wouldn’t have global supply. So there is no one way to manage our supply lines and we need to have conversations with our customers to discuss the risks. How much time would it take to rebuild a supply-chain source with someone else? If you can get that sort of constructive discussion going then customers are usually open to single-sourcing. One regional aspect is that Asian customers tend to favor dual-sourcing more, but that can lead to IP problems.

[DISCLOSURE:  Ed Korczynski is co-chair of the CMC Conference, and Marketing Director of TECHCET CA the advisory services firm that administers the Critical Materials Council (CMC).]

—E.K.

Vital Control in Fab Materials Supply-Chains

Wednesday, January 25th, 2017

By Ed Korczynski, Sr. Technical Editor

The inaugural Critical Materials Council (CMC) Conference, co-sponsored by Solid State Technology, happened May 5-6 in Hillsboro, Oregon. Held just after the yearly private CMC meeting, the public CMC Conference provides a forum for the pre-competitive exchange of information to control the supply-chain of critical materials needed to run high-volume manufacturing (HVM) in IC fabs. The next CMC Conference will happen May 11-12 in Dallas, Texas.

At the end of the 2016 conference, a panel discussion moderated by Ed Korczynski was recorded and transcribed. The following is an edited excerpt of the conversation between the following industry experts:

  • Jean-Marc Girard, CTO and Director of R&D, Air Liquide Advanced Materials,
  • Jonas Sundqvist, Sr. Scientist, Fraunhofer IKTS; and co-chair of ALD Conference, and
  • John Smythe, Distinguished Member of Technical Staff, Micron Technology.

KORCZYNSKI:  Let’s start with specifications: over-specifying, and under-specifying. Do we have the right methodologies to be able to estimate the approximate ‘ball-park’ range that the impurities need to be in?

GIRARD:  For determining the specifications, to some extent it doesn’t matter because we are out of the world of specs, where what matters is the control-limits. To Tim Hendry’s point in the Keynote yesterday [EDITOR’S NOTE:  Tim G. Hendrey, vice president of the Technology and Manufacturing Group and director of Fab Materials at Intel Corporation provided a conference keynote address on “Process Control Methods for Advanced Materials”], what was really interesting is instead of the common belief that we should start by supplying the product with the lowest possible variability, instead we should try to explore the window in which the product is working. So getting 10 containers from the same batch and introducing deliberate variability so that you know the process space in which you can play. That is the most important information to be able to reach the most reasonable and data-driven numbers to specify control limits. A lot of specs in the past were primarily determined by marketing decisions instead of data.

FIGURE 1: Jonas Sundqvist, Sr. Scientist of Fraunhofer IKTS, discusses collaboration with industry on application-specific ALD R&D. (Source: TECHCET CA)

SUNDQVIST:  Like the first introduction of what were called “super-clean” ALD precursors for the original MIS DRAM capacitors, Samsung used about 10nm of hafnium-aluminate and it would not matter if there was slight contamination in the precursors because you were not trying to control for a specific high-k phase. Whereas now you are doping very precisely and you have already scaled thinness so over time the specification for high-k precursors has become more important.

SMYTHE:  I think it comes down to the premise that when you are doing vapor transport through a bubbler that some would argue that that’s like a distillation column. So it’s a matter of thinking about what is transporting and what isn’t. In some cases the contaminant you’re concerned about is in the ampule but it never makes it to the process chamber, or the act of oxidizing destroys it as a volatile byproduct. So I think the bigger issue is change-management not necessarily the exact specification. You must know what you have, and agree that a single adjustment to improve the productivity of chemical synthesis requires that ‘fingerprinting’ must be done to show the same results. The argument is that you do not accept “less-than” as part of a specification, you only accept what it is.

AUDIENCE QUESTION:  The systems in which these precursors are used also have ‘memory’ based on the prior reactions in the chamber and byproducts that get absorbed on walls. When these byproducts come out in subsequent processing they can alter conditions so that you’re actually running in CVD-mode instead of ALD-mode. Chamber effects can wash-out a lot of value of having really pure chemicals moving through a delivery system into a chamber and picking up contaminants that you spent a whole lot of money taking out at the point of delivery. What do you think about that?

GIRARD:  Well, this is a ‘crisis!’ When something like this starts to happen in a fab or even during the development cycles, you can’t prioritize resources and approaches you just have to do everything. Sometimes it’s the tool, sometimes it’s the chemical, sometimes it’s the interaction of the two, sometimes it’s back-streaming from the vacuum sub-system…there are so many ways that things can go wrong. Certainly you have to clear up the chemistry part as early as possible.

SUNDQVIST:  We work with zirconium precursors for ALD, and you can develop a precursor that gives you a very pure ALD process that really works like an ALD process should. However, you can still use the TEMA-Zr precursor, that in processing has a CVD component which you can use that to gain throughput. So you can have a really good ALD precursor that gives low particle-counts and good process stability and ideal thermal processing range, but the growth rate goes down by 20% so you’re not very popular in the fab. Many things change when you make an ‘improved’ molecule to perfect the process, and sometime you want to use an imperfect part of the process.

FIGURE 2: John Smythe, Distinguished Member of Technical Staff of Micron Technology, explains approaches to controlling materials all the way to point-of-use. (Source: TECHCET CA)

SMYTHE:  What we’re doing a lot more these days is doing chamber finger-printing, where we’re putting a quad-filtered mass-spec on each chamber—not a cheap little RGA, but real analytical-grade—and it’s been enlightening. If you look at your chemistry moving through a delivery line using something like the Schrødenger software, it’s not a big deal to see that you can use the mass spec to see some synthesis happening in the line. We joke and call it ‘point of use synthesis’ but it’s not very funny. We are used to having spare delivery lines built-in so we can install tools to try to gain insights to prevent what we’ve been talking about.

KORCZYNSKI:  John, since Micron has fabs in Lehi and fabs in Singapore and other places, while they do run different product loads, do you have to worry about how long it takes things to travel on a slow boat to Singapore? Do you have to stockpile things more strategically these days, and does that effect your receiving department?

SMYTHE:  What we really need are a few good ocean-going hydrofoil ships! The most complete answer is we first identify which things need ‘batch-qual’ so if we do a batch-qual in Virginia and know that material is going to Taiwan that we have confidence it will pass batch-qual in Taiwan. There are certain materials that we require information on which synthesis batch, which production batch, and sometimes which bottling batch. Sometimes you take a yield hit because you didn’t have the right vision, and then you institute batch qual.

I think most of you are familiar with the concept of ‘ship-to-stock,’ when you have enough good statistical history and a good change management process with the supplier then you can do ship-to-stock and that reduces the batch-qual overhead. On a case by case basis you have to figure out how difficult that is. A small story I can tell is that with Block Co-Polymer (BCP) self-assembly we found one particular element that in concentration above 5 ppm prevented the poly-styrene from self-assembling in the same way, whereas other metal trace contaminants could be a hundred times higher and have no effect on the process. So this gets back to some of our earlier discussion that it’s not enough to know that your trace elements are below some level. Tell me the exact atoms and the exact counts and then we’ll talk about using them. The BCP R&D taught us that in some situations just changing from one batch to the next could increase defects a thousands times. So we will see a bigger push to counting atoms.

[DISCLOSURE:  Ed Korczynski is co-chair of the CMC Conference, and Marketing Director of TECHCET CA the advisory services firm that administers the Critical Materials Council (CMC).]

—E.K.

Multibeam Patents Direct Deposition & Direct Etch

Monday, November 14th, 2016

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By Ed Korczynski, Sr. Technical Editor

Multibeam Corporation of Santa Clara, California recently announced that its e-beam patent portfolio—36 filed and 25 issued—now includes two innovations that leverage the precision placement of electrons on the wafer to activate chemical processes such as deposition and etch. As per the company’s name, multi-column parallel processing chambers will be used to target throughputs usable for commercial high-volume manufacturing (HVM) though the company does not yet have a released product. These new patents add to the company’s work in developing Complementary E-Beam Lithography (CEBL) to reduce litho cost, Direct Electron Writing (DEW) to enhance device security, and E-Beam Inspection (EBI) to speed defect detection and yield ramp.

The IC fab industry’s quest to miniaturize circuit features has already reached atomic scales, and the temperature and pressure ranges found on the surface of our planet make atoms want to move around. We are rapidly leaving the known era of deterministic manufacturing, and entering an era of stochastic manufacturing where nothing is completely determined because atomic placements and transistor characteristics vary within distributions. In this new era, we will not be able to guarantee that two adjacent transistors will function the same, which can lead to circuit failures. Something new is needed. Either we will have to use new circuit design approaches that require more chip area such as “self-healing” or extreme redundancy, or the world will have to inspect and repair transistors within the billions on every HVM chip.

In an exclusive interview with Solid State Technology, David K. Lam, Multibeam Chairman, said, “We provide a high-throughput platform that uses electron beams as an activation mechanism. Each electron-beam column integrates gas injectors, as well as sensors, which enable highly localized control of material removal and deposition. We can etch material in a precise location to a precise depth. Same with deposition.” Lam (Sc.D. MIT) was the founder and first CEO of Lam Research where he led development and market penetration of the IC fab industry’s first fully automated plasma etch system, and was inducted into the Silicon Valley Engineering Hall of Fame in 2013.

“Precision deposition using miniature-column charged particle beam arrays” (Patent #9,453,281) describes patterning of IC layers by either creating a pattern specified by the design layout database in its entirety or in a complementary fashion with other patterning processes. Reducing the total number of process steps and eliminating lithography steps in localized material addition has the dual benefit of reducing manufacturing cycle time and increasing yield by lowering the probability of defect introduction. Furthermore, highly localized, precision material deposition allows for controlled variation of deposition rate and enables creation of 3D structures such as finFETs and NanoWire (NW) arrays.

Deposition can be performed using one or more multi-column charged particle beam systems using chemical vapor deposition (CVD) alone or in concert with other deposition techniques. Direct deposition can be performed either sequentially or simultaneously by multiple columns in an array, and different columns can be configured and/or optimized to perform the same or different material depositions, or other processes such as inspection and metrology.

“Precision substrate material removal using miniature-column charged particle beam arrays” (Patent #9,466,464) describes localized etch using activation electrons directed according to the design layout database so that etch masks are no longer needed. Figure 1 shows that costs are reduced and edge placement accuracy is improved by eliminating or reducing errors associated with photomasks, litho steps, and hard masks. With highly localized process control, etch depths can vary to accommodate advanced 3D device structures.

Fig.1: Comparison of (LEFT) the many steps needed to etch ICs using conventional wafer processing and (RIGHT) the two simple steps needed to do direct etching. (Source: Multibeam)

“We aren’t inventing new etch chemistries, precursors or reactants,” explained Lam. “In direct etch, we leverage developments in reactive ion etching and atomic layer etch. In direct deposition, we leverage work in atomic layer deposition. Several research groups are also developing processes specifically for e-beam assisted etch and deposition.”

The company continues to invent new hardware, and the latest critical components are “kinetic lens” which are arrangements of smooth and rigid surfaces configured to reflect gas particles. When fixed in position with respect to a gas injector outflow opening, gas particles directed at the kinetic lens are collimated or redirected (e.g., “focused”) towards a wafer surface or a gas detector. Generally, surfaces of a kinetic lens can be thought of as similar to optical mirrors, but for gas particles. A kinetic lens can be used to improve localization on a wafer surface so as to increase partial pressure of an injected gas in a target area. A kinetic lens can also be used to increase specificity and collection rate for a gas detector within a target frame.

Complementary Lithography

Complementary lithography is a cost-effective variant of multi-patterning where some other patterning technology is used with 193nm ArF immersion (ArFi) to extend the resolution limit of the latter. The company’s Pilot™ CEBL Systems work in coordination with ArFi lithography to pattern cuts (of lines in a “1D lines-and-cuts” layout) and holes (i.e., contacts and vias) with no masks. These CEBL systems can seamlessly incorporate multicolumn EBI to accelerate HVM yield ramps, using feedback and feedforward as well as die-to-database comparison.

Figure 2 shows that “1D” refers to 1D gridded design rule. In a 1D layout, optical pattern design is restricted to lines running in a single direction, with features perpendicular to the 1D optical design formed in a complementary lithography step known as “cutting”. The complementary step can be performed using a charged particle beam lithography tool such as Multibeam’s array of electrostatically-controlled miniature electron beam columns. Use of electron beam lithography for this complementary process is also called complementary e-beam lithography, or CEBL. The company claims that low pattern-density layers such as for cuts, one multi-column chamber can provide 5 wafers-per-hour (wph) throughput.

Fig.2: Complementary E-Beam Lithography (CEBL) can be used to “cut” the lines within a 1D grid array previously formed using ArF-immersion (ArFi) optical steppers. (Source: Multibeam)

Direct deposition can be used to locally interconnect 1D lines produced by optical lithography. This is similar in design principle to complementary lithography, but without using a resist layer during the charged particle beam phase, and without many of the steps required when using a resist layer. In some applications, such as restoring interconnect continuity, the activation electrons are directed to repair defects that are detected during EBI.

—E.K.

Air-Gaps for FinFETs Shown at IEDM

Friday, October 28th, 2016

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By Ed Korczynski, Sr. Technical Editor

Researchers from IBM and Globalfoundries will report on the first use of “air-gaps” as part of the dielectric insulation around active gates of “10nm-node” finFETs at the upcoming International Electron Devices Meeting (IEDM) of the IEEE (ieee-iedm.org). Happening in San Francisco in early December, IEDM 2016 will again provide a forum for the world’s leading R&D teams to show off their latest-greatest devices, including 7nm-node finFETs by IBM/Globalfoundries/Samsung and by TSMC. Air-gaps reduce the dielectric capacitance that slows down ICs, so their integration into transistor structures leads to faster logic chips.

History of Airgaps – ILD and IPD

As this editor recently covered at SemiMD, in 1998, Ben Shieh—then a researcher at Stanford University and now a foundry interface for Apple Corp.—first published (Shieh, Saraswat & McVittie. IEEE Electron Dev. Lett., January 1998) on the use of controlled pitch design combined with CVD dielectrics to form “pinched-off keyholes” in cross-sections of inter-layer dielectrics (ILD).

In 2007, IBM researchers showed a way to use sacrificial dielectric layers as part of a subtractive process that allows air-gaps to be integrated into any existing dielectric structure. In an interview with this editor at that time, IBM Fellow Dan Edelstein explained, “we use lithography to etch a narrow channel down so it will cap off, then deliberated damage the dielectric and etch so it looks like a balloon. We get a big gap with a drop in capacitance and then a small slot that gets pinched off.

Intel presented on their integration of air-gaps into on-chip interconnects at IITC in 2010 but delayed use until the company’s 14nm-node reached production in 2014. 2D-NAND fabs have been using air-gaps as part of the inter-poly dielectric (IPD) for many years, so there is precedent for integration near the gate-stack.

Airgaps for finFETs

Now researchers from IBM and Globalfoundries will report in (IEDM Paper #17.1, “Air Spacer for 10nm FinFET CMOS and Beyond,” K. Cheng et al) on the first air-gaps used at the transistor level in logic. Figure 1 shows that for these “10nm-node” finFETs the dielectric spacing—including the air-gap and both sides of the dielectric liner—is about 10 nm. The liner needs to be ~2nm thin so that ~1nm of ultra-low-k sacrificial dielectric remains on either side of the ~5nm air-gap.

Fig.1: Schematic of partial air-gaps only above fin tops using dielectric liners to protect gate stacks during air-gap formation for 10nm finFET CMOS and beyond. (source: IEDM 2016, Paper#17.1, Fig.12)

These air-gaps reduced capacitance at the transistor level by as much as 25%, and in a ring oscillator test circuit by as much as 15%. The researchers say a partial integration scheme—where the air-gaps are formed only above the tops of fin— minimizes damage to the FinFET, as does the high-selectivity etching process used to fabricate them.

Figure 2 shows a cross-section transmission electron micrograph (TEM) of what can go wrong with etch-back air-gaps when all of the processes are not properly controlled. Because there are inherent process:design interactions needed to form repeatable air-gaps of desired shapes, this integration scheme should be extendable “beyond” the “10-nm node” to finFETs formed at tighter pitches. However, it seems likely that “5nm-node” logic FETs will use arrays of horizontal silicon nano-wires (NW), for which more complex air-gap integration schemes would seem to be needed.

Fig.2: TEM image of FinFET transistor damage—specifically, erosion of the fin and source-drain epitaxy—by improper etch-back of the air-gaps at 10nm dimensions. (source: IEDM 2016, Paper#17.1, Fig.10)

—E.K.

D2S Releases 4th-Gen IC Computational Design Platform

Friday, September 30th, 2016

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By Ed Korczynski, Sr. Technical Editor

D2S (www.design2silicon.com) recently released the fourth generation of its computational design platform (CDP), which enables extremely fast (400 Teraflops) and precise simulations for semiconductor design and manufacturing. The new CDP is based on NVIDIA Tesla K80 GPUs and Intel Haswell CPUs, and is architected for 24×7 cleanroom production environments. To date, 14 CDPs across four platform generations are in use by customers around the globe, including six of the latest fourth generation. In an exclusive interview with SemiMD, D2S CEO Aki Fujimura stated, “Now that GPUs and CPUs are fast-enough, they can replace other hardware and thereby free up engineering resources to focus on adding value elsewhere.”

Mask data preparation (MDP) and other aspects of IC design and manufacturing require ever-increasing levels of speed and reliability as the data sets upon which they must operate grow larger and more complex with each device generation. The Figure shows a mask needed to print arrays of sub-wavelength features includes complex curvilinear shapes which must be precisely formed even though they do not print on the wafer. Such sub-resolution assist features (SRAF) increase in complexity and density as the half-pitch decreases, so the complexity of mask data increases far more than the density of printed features.

Sub-wavelength lithography using 193nm wavelength requires ever-more complex masks to repeatably print ever smaller half-pitch (HP) features, as shown by (LEFT) a typical mask composed of complex nested curves and dots which do not print (RIGHT) in the array of 32nm HP contacts/vias represented by the small red circles. (Source: D2S)

GPUs, which were first developed as processing engines for the complex graphical content of computer games, have since emerged as an attractive option for compute-intensive scientific applications due in part to their ability to run many more computing threads (up to 500x) compared to similar-generation CPUs. “Being able to process arbitrary shapes is something that mask shops will have to do,” explained Fujimura. “The world could go 193nm or EUV at any particular node, but either way there will be more features and higher complexity within the features, and all of that points to GPU acceleration.”

The D2S CDP is engineered for high reliability inside a cleanroom manufacturing environment. A few of the fab applications where CDPs are currently being used include:

  • model-based MDP for leading-edge designs that require increasingly complex mask shapes,
  • wafer plane analysis of SEM mask images to identify mask errors that print, and
  • inline thermal-effect correction of eBeam mask writers to lower write times.

“The amount of design data required to produce photomasks for leading-edge chip designs is increasing at an exponential rate, which puts more pressure on mask writing systems to maintain reasonable write times for these advanced masks. At the same time, writing these masks requires higher exposure doses and shot counts, which can cause resist proximity heating effects that lead to mask CD errors,” stated Noriaki Nakayamada, group manager at NuFlare Technology. “D2S GPU acceleration technology significantly reduces the calculation time required to correct these resist heating effects. By employing a resist heating correction that includes the use of the D2S CDP as an OEM option on our mask writers, NuFlare estimates that it can reduce CD errors by more than 60 percent, and reduce write times by more than 20 percent.”

In the E-beam Initiative 2015 survey, the most advanced reported mask-set contained >100 masks of which ~20% could be considered ‘critical’. The just released 2016 survey disclosed that the most complex single-layer mask design written last year required 16 TB of data, however platforms like D2S’ CDP have been used to accelerate writing such that the average reported write times have decreased to a weighted average of 4 hours. Meanwhile, the longest reported mask write time decreased from 72 to 48 hours.

3D-NAND Deposition and Etch Integration

Thursday, September 1st, 2016

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By Ed Korczynski, Sr. Technical Editor

3D-NAND chips are in production or pilot-line manufacturing at all major memory manufacturers, and they are expected to rapidly replace most 2D-NAND chips in most applications due to lower costs and greater reliability. Unlike 2D-NAND which was enabled by lithography, 3D-NAND is deposition and etch enabled. “With 3D-NAND you’re talking about 40nm devices, while the most advanced 2D-NAND is running out of steam due to the limited countable number of stored electrons-per-cell, and in terms of the repeatability due to parasitics between adjacent cells,” reminded Harmeet Singh, corporate vice president of Lam Research in an exclusive interview with SemiMD to discuss the company’s presentation at the Flash Memory Summit 2016.

“We’re in an era where deposition and etch uniquely define the customer roadmap,” said Singh,“and we are the leading supplier in 3D-NAND deposition and etch.” Though each NAND manufacturer has different terminology for their unique 3D variant, from a manufacturing process integration perspective they all share similar challenges in the following simplified process sequences:

1)    Deposition of 32-64 pairs of blanket “mold stack” thin-films,

2)    Word-line hole etch through all layers and selective fill of NAND cell materials, and

3)    Formation of “staircase” contacts to each cell layer.

Each of these unique process modules is needed to form the 3D arrays of NVM cells.

For the “mold stack” deposition of blanket alternating layers, it is vital for the blanket PECVD to be defect-free since any defects are mirrored and magnified in upper-layers. All layers must also be stress-free since the stress in each deposited layer accumulates as strain in the underlying silicon wafer, and with over 32 layers the additive strain can easily warp wafers so much that lithographic overlay mismatch induces significant yield loss. Controlled-stress backside thin-film depositions can also be used to balance the stress of front-side films.

Hole Etch

“The difficult etch of the hole, the materials are different so the challenges is different,” commented Singh about the different types of 3D-NAND now being manufactured by leading fabs. “During this conference, one of our customer presented that they do not see the hole diameters shrinking, so at this point it appears to us that shrinking hole diameters will not happen until after the stacking in z-dimension reaches some limit.”

Tri-Layer Resist (TLR) stacks for the hole patterning allow for the amorphous carbon hardmask material to be tuned for maximum etch resistance without having to compromise the resolution of the photo-active layer needed for patterning. Carbon mask is over 3 microns thick and carbon-etching is usually responsive to temperature, so Lam’s latest wafer-chuck for etching features >100 temperature control zones. “This is an example of where Lam is using it’s processes expertise to optimize both the hardmask etch as well as the actual hole etch,” explained Singh.

Staircase Etch

The Figure shows a simplified cross-sectional schematic of how the unique “staircase” wordline contacts are cost-effectively manufactured. The established process of record (POR) for forming the “stairs” uses a single mask exposure of thick KrF photoresist—at 248nm wavelength—to etch 8 sets of stairs controlled by a precise resist trim. The trimming step controls the location of the steps such that they align with the contact mask, and so must be tightly controlled to minimize any misalignment yield loss.

A) Simplified cross-sectional schematic of the staircase etch for 3D-NAND contacts using thick photoresist, B) which allows for controlled resist trimming to expose the next “stair” such that C) successive trimming creates 8-16 steps from a single initial photomask exposure. (Source: Ed Korczynski)

Lam is working on ways to tighten the trimming etch uniformity such that 16 sets of stairs can be repeatably etched from a single KrF mask exposure. Halving the relative rate of vertical etch to lateral etch of the KrF resist allows for the same resist thickness to be used for double the number of etches, saving lithography cost. “We see an amazing future ahead because we are just at the beginning of this technology,” commented Singh.

—E.K.

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