Future EUV/FEL Strategy – The Beam Line Approaches

As many of you know, I've been following the progress of EUV lithography over the years, observing and commenting on the program's many engineering successes as well as the delays in the convergence of EUV lithography and the anticipated HVM time line (High Volume Manufacturing). In spite of improvements in laser technology and the availability of low dose photoresists, the development of high power LPP (Laser Produced Plasma) EUV source technology >100 watts remains problematic. EUV HVM insertion remains elusive and unpredictable. The source power limitations and MTBF (Mean Time Between Failure) of LPP technology have gated the program. A few years ago many in the SPIE community sought to explore the potential of a FEL source for high power EUV applications. At the time, Free Electron Laser [1] Technology was not yet a topic for dinner table discussion so in September, 2014 I published Future FEL/EUV Strategy – The Light at the End of the Beam Line. [2] On-going FEL developments at SLAC and recent related commentary from companies like GLOBALFOUNDRIES have generated new interest in FEL technology. As many new engineers and investors have joined our ranks, I thought a more comprehensive review was in order, so I've excerpted portions of my 1994 primer on FEL/EUV strategy to point out the enabling feature/benefits of a high power, high reliability light source for EUV lithography.

What are the current obstacles to high power EUV?

In ASML's current LPP source designs, a solid state "pre-pulse" laser and a second, high energy CO2 laser are fired at micron sized tin (Sn) pellets, evaporating them and releasing EUV light as a byproduct. Knowledgeable sources have informed me that the currently employed CO2 lasers are at or near the maximum of their pulse rate capabilities, effectively limiting further power output. As more CO2 laser power becomes available, there may still be practical limitations on the scaling and feed rate of Sn (tin) target material. As determined by physics, the inherent energy conversion factor for tin approximates 4%, and further incremental improvements in efficiency are obtained with diminishing returns. Assuming additional laser power becomes available for ASML, further complications can result from higher LPP source power levels as the rate of residual particulate contamination from evaporated tin increases in approximate proportion with increased laser power. Critical beam line mirror surfaces and other source components rapidly lose their efficiencies as tin contamination accumulates, reducing the available up time of the stepper (MTBF). It was originally anticipated that an optimal LPP EUV source design would provide 13.5nm light at power levels >200 watts, providing current and future lithography requirements. However, more recent demands for even higher EUV power levels have been identified. ASML and Carl Zeiss acknowledged in an invited paper at SPIE Advanced Lithography 2015, that higher resolutions will require 60mJ/cm2 for half pitch nodes <8nm. [3] ASML's recent (2015) collaboration with Carl Zeiss has produced an optical system with a numeric aperture (NA) of 0.55 vs. ASML's current EUV NA of 0.33. The higher NA system will require 500 watts of EUV power to achieve the estimated 60 mJ/cm2 dosimetry required for throughput of 150 wafers/hour. While this concept extends the viability of 13.5nm lithography, the delivery of a reliable 500 watt EUV source remains a critical item on the agenda, meaning the availability of free electron laser technology will probably gate related programs. A recent article appearing in the SPIE News Room, Extending extreme-UV lithography technology, [4] suggests that power levels of 500 – 1000 watts may be required for a single stepper necessitating a large scale central source EUV FEL.

A Primer on Free Electron Lasers

What is a free electron laser and how is it different from conventional lasers and LPP systems? To answer this question we must entertain the convergence of the US DOE's high energy physics community with the semiconductor industry and discuss recent innovations in technology. In previous and current generation stepper and scanner systems, it's been common to utilize laser light sources producing the desired wavelengths required for semiconductor photolithography. In current 193nm lithography systems, an argon fluoride (ArF) laser produces the light. The laser light produced is monochromatic, of sufficient brilliance and provides many hours of trouble free uptime. It would seem this simplistic approach might be applied to EUV lithography. Why not build an EUV laser with a wavelength of 13.5 nanometers? This has not been possible due to limitations in physics. The highly reflective optics required for laser efficiencies have yet to be created for EUV spectra. Current Bragg cell mirrors reflect EUV with a closely approximated 90% efficiency. However, FEL is a game changer. Some history and an analogy:


The term LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. The first solid state, 694nm synthetic ruby laser produced at Hughes Research in 1960, [5] utilized xenon flash lamps to inject high energy photons throughout the core of a ruby rod, stimulating the emission of photons from its lattice structures. Lasers operate at specific wavelengths which are determined by the seed (or lazing) material's inherent spectral signature. The 694nm wavelength is derived from the band gap emissions of the ruby's crystalline composition. We might compare the ruby crystal in this laser with a quartz crystal in a radio which determines its operational frequency. Accordingly, we might otherwise assign “channel I” as an identifier of I-line photolithography operating at 365nm.

The Foundation Physics of FEL

 A Radio Logical Analogy

A radio transmitter's frequency has historically been controlled by quartz crystal elements. Y-cut quartz crystals oscillate (vibrate) at specific frequencies which are dependent upon their thickness. The thinner the crystal, the higher the frequency obtained. Inversely, the thicker the crystal, the lower the frequency obtained. Passing an electric current through a quartz crystal induces it to oscillate at its inherent resonant frequency, so determined by its thickness. The frequency produced is extremely stable and the resulting wave form is of high purity, providing an excellent medium for control of radio frequencies and instrumentation. Crystals also produce harmonic frequencies. A harmonic is a multiple of the crystal's fundamental resonant frequency. As such, a crystal oscillating at 3 MHz will also produce a weaker signal at 6MHz (its second harmonic frequency), and a still weaker third harmonic at 9MHz and so on. When impractical to manufacture crystals at their desired fundamental frequencies, "third overtone" crystals are often utilized to provide a harmonic frequency which can be sufficiently amplified and utilized as an effective fundamental frequency, thus extending the upper limits (and our usage) of the radio spectrum. Even with clever engineering, over the years radio frequency control became problematic as multi-channel communication systems evolved, requiring large banks of crystals to span a given range of frequencies; one crystal required for each channel frequency. Rather than utilize thousands of crystals to span the radio spectrum, communications equipment evolved to employ a frequency control device called a VFO: a Variable Frequency Oscillator. In this scenario, several fundamental frequency crystals and specially designed varactor diode/phased locked loop circuits comprise a heterodyne oscillator, sometimes known as an IF (Intermediate Frequency) mixer. Such an oscillator can generate a wide range of possible frequency combinations by mixing (heterodyning) the output obtained from the crystals to produce the desired sum/difference of their frequencies by way of constructive or destructive interference. As a VFO radio tuning dial is manipulated, it changes one of the mixing frequencies to produce the desired sum/difference operational frequency for both transmitter and receiver. The advantage to such a design is the elimination of separate transmitter/receiver controls, and thousands of individual crystals normally assigned for each desired radio channel. The conceptual use of both harmonic and sum/difference frequency synthesis has found its way into many applications in physics and electronics.

Free Electron Laser Fundamentals

Imagine that we might adjust and control a laser's wavelength using a concept similar to a radio's Variable Frequency Oscillator but with a different set of physics. By electronically tuning a laser's wavelength, we can eliminate the need for specialized crystalline, gaseous or other lazing materials and operate outside the spectral wavelength segments they are physically limited to. FEL technology can produce tunable wavelengths of light throughout the microwave, visible spectrum and x-ray regime. A free electron laser is comprised of a large beamline/electron source which accelerates electrons to near the speed of light. On opposite sides of the electron beam line are interposed field coils of opposing polarity called undulators or "wigglers", which when energized establish a transverse sinusoidal field across the beam path. Electrons accelerated into the transverse field produce incoherent photons in a mixed assortment of sinusoidal wavelengths sometimes referred to as “bunches”, emitting photons at wavelengths determined by their acceleration and the transverse field strength (synchrotron radiation). By adjusting the electron beam energy or the magnetic field strength of the undulators, the wavelength of the emitted photons can be tuned selectively to produce coherent light. Variations on this concept have evolved as follows:

A Tunable SASE FEL

A SASE FEL is able to produce laser light over a broad range of spectrum without the requirement for conventional lazing materials such as ruby crystal or argon fluoride etc. In a tunable SASE (Self Amplified Spontaneous Emission) FEL, high energy source electrons passing through an undulator can produce an assortment of incoherent photons (initially at randomly different wavelengths) which become bunched in the transverse sine wave and interact via constructive or destructive interference, producing incidental derivative wavelengths (spontaneous emission). That is to say the bunched photons add and subtract their wavelength values from one another producing new sum/difference valued photons at the mathematically resulting wavelengths. When tuned to a specific wavelength of interest by adjusting the electron beam energy or the magnetic field strength of the undulators, such subsequently produced photons arrive in phase (at the same wavelength) and cumulatively intensify to release high energy coherent laser light (self amplification). While a very useful concept for a variety of applications, the spontaneous emission in a SASE FEL can propagate statistical artifacts resulting from the inherent mathematical sum/difference phenomenon, and consequently can produce a beam exhibiting limited shot to shot reproducibility. As such, the utility of a SASE FEL might be limited in applications which require extremely accurate dosimetry. The limited shot to shot reproducibility might also contribute to the dosimetry phenomenon known as “shot noise”.

A Tunable HGHG FEL

FEL performance can be modified and improved by utilizing an external seed laser as a source wavelength. The seed laser is a conventional laser utilizing a material such as ruby crystal (one example) to produce a monochromatic feed source of photons. In an HGHG (High Gain Harmonic Generation) FEL, the seed laser interacts with the electron beam as it propagates through the first undulator (called a modulator), tuned to the seed laser's wavelength. The resulting interaction with the seed laser induces coherent modulation of the electron beam energy, creating photon bunching as well as consequential harmonic propagation (photons which are the mathematical multiples of the seed laser's wavelength). The micro-bunched beam of photons are then injected into a long undulator tuned to the desired harmonic wavelength. The desired wavelength comprised of harmonically produced photons arrive in phase and cumulatively intensify to release high energy coherent light at the newer, shorter wavelength of interest. A recent FERMI paper illustrates 500 shot reproducibility of 8th harmonic spectra at 32.5nm (obtained from a 260nm seed laser) exhibiting normalized photon/energy stability in the order of 7x10^-5 (root mean square), a marked improvement over previous SASE FEL data obtained over the same photon energy range. The high purity monochromatic spectra of an HGHG seed laser improves the system's shot to shot repeatability as its mode of operation does not incur the statistical deviation phenomena found in spontaneous emission spectra typically observed in a SASE FEL. As such, an HGHG FEL might be more advantageous for use in EUV applications requiring highly precise dosimetry, possibly reducing shot noise phenomenon. 

The EUV Source Challenge Ahead

Large scale projects are underway to build FEL systems to accommodate a wide range of wavelengths and scientific applications. FEL is next generation laser technology which is perhaps the best candidate to replace the LPP/EUV source designs currently offered by ASML.

Known for its work in actinic inspection at the 13.5 nm EUV wavelength, Lawrence Berkeley CXRO Lab [6] is also part of a DOE consortium currently working on LCLS-II (Linac Coherent Light Source-II) [7] at Lawrence Livermore and SLAC. Last June at the 2015 International Workshop on EUV Lithography in Maui, Aaron Tremaine of SLAC presented a comprehensive review of possible FEL designs that might be considered for EUV lithography and identified the consortium of DOE laboratories participating in the EUV FEL program. The Who's Who list of DOE participants includes SLAC, Lawrence Berkeley National Labs, Fermilab, Argonne National Lab, Cornell, UCLA, RadiaBeam, AES and Radiasoft. In order to appreciate the scale and capital intensity of this project it becomes necessary to review Aaron Tremaine's 2015 EUV Litho, Inc. Workshop presentation, LCLS-II and Free electron laser drivers for EUV Lithography [8]. The report describes FEL design considerations and recommends a “Straight Shooter” beamline configuration for semiconductor EUV lithography applications. The report addresses Erik R. Hosler's (GLOBALFOUNDRIES) 2015 SPIE publication, “Considerations for a free-electron laser based extreme-ultraviolet lithography program”, (Proc. of SPIE Vol. 9422, 94220D, 2015). The good news is that many new FEL programs are in progress [9] and the LCLS-II at SLAC might provide a viable, solution for HVM/EUV lithography. More recently, visible collaboration between GLOBALFOUNDRIES and SLAC has established the ground work for possible in-fab FEL source designs. The subsequent challenge for any future FEL/EUV initiative, is that once again the convergence of the semiconductor industry and our national laboratory community will be required to deliver future lithography source technology for EUV and beyond.

ASML has taken the lead in providing viable interim EUV technology permitting the characterization of materials, resists, masks and process precision required for future generation lithography. Double patterning techniques utilizing 193i lithography will continue to enable CDs =<10nm. We can also speculate how 13.5nm multiple patterning might enable future nodes and continued process development. In the interim, the current ASML LPP/EUV initiative has enabled the ground work our industry requires for future precision nanometer scale lithography.

Let's continue working together to secure next generation EUV and the preservation of Moore's Law.

During the course of researching this article I digested many components of the SLAC Conceptual Design Report for the LCLS-II (Linear Coherent Light Source).  Among many, two components of the LCLS-II are the SXR (Soft X-Ray) and HXR (Hard X-Ray) undulators and their respective beam lines.  It is with some amusement that my FCC designated amateur radio call sign is WA2HXR which I acquired in 1970. It should be noted that my amateur radio operations are restricted to applicable licensed amateur radio frequency spectra which excludes X-Ray wavelengths.  
CQ SLAC CQ SLAC CQ SLAC DE WA2HXR K.     73   

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Thomas D. Jay
Semiconductor Industry Consultant
Thomas.Dale.Jay@gmail.com
https://ThomasDaleJay.blogspot.com

Thomas D. Jay YouTube Channel

Visit my new Amateur Radio blog at:
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Corporate, private entities or publications referenced or linked in this article are the respective owners of their logos, trademarks, service marks, media content and intellectual property. Unless otherwise disclosed, Thomas D. Jay has no financial interest in companies referenced in blog articles or other published media communications. Thomas D. Jay is not a registered financial advisor. No representation is made to either buy or sell securities. Opinions expressed by Thomas D. Jay are his own. Thomas D. Jay does not employ or otherwise utilize/authorize third party agents to express his opinions, represent his interests or conduct business on his behalf except where formally contractually designated. Thomas D. Jay does not agree to indemnify or hold harmless vendors, clients or third parties to related contractual agreements and reserves the right to applicable legal remedies in lieu of arbitration. These terms are subject to change. Concerned parties should check this blog site for periodic updates.

Acknowledgments and Reference Links

[1] Free Electron Laser
Wikipedia

[2] Future FEL/EUV Strategy – The Light at the End of the Beam Line
Thomas D. Jay, Blog Publication, September 20, 2014

[3] ASML and Carl Zeiss acknowledged in an invited paper at SPIE Advanced Lithography 2015, that higher resolutions will require 60mJ/cm2 for half pitch nodes <8nm
SPIE Proceedings

[4] Extending extreme-UV lithography technology
SPIE News Room, Erik R. Hosler, Obert R. Wood II, Moshe E Preil

[5] The first solid state, 694nm synthetic ruby laser produced at Hughes Research in 1960
Wikipedia

[6] Lawrence Berkeley CXRO Lab
Lawrence Berkeley National Labs CXRO Web Site

[7] Linac Coherent Light Source-II
Stanford Linear Accelerator Web Site

[8] LCLS-II and Free electron laser drivers for EUV Lithography
Aaron Tremaine Presentation, SLAC
EUV Litho, Inc. Web Site, 2015 Program, Maui

[9] many new FEL programs are in progress
University of California, Santa Barbara FEL Web Site
Site Maintained by G. Ramian

A Defining Inflection in the EUV Continuum

An April 22, 2015 ASML press release [1] announced an agreement with one of its major US customers (believed to be Intel) to purchase a minimum quantity of fifteen of its NXE:3350B EUV lithography systems with two of the units slated for delivery year end 2015. Intel has previously invested over $4 Billion in ASML. Given the size of the order, it would appear that Intel will proceed with a large scale strategic commitment to EUV lithography for future production process nodes =<10nm, with a path to 7nm and smaller CDs.

Although financial details of the purchase were not released, a sizable capital equipment expenditure has been made after many years of delay and uncertainty in the EUV program. This purchase transaction represents a large scale, high profile commitment to what has been a capital intensive development program delayed by uncooperative laws of physics, semiconductor sector business cycles, and capital market dynamics. It seems that the semiconductor production road map has been sufficiently refined, concomitant with related process technologies, and that confidence has been restored in long term EUV/HVM convergence/insertion forecasting. Although Intel's eventual commitment to production EUV had been anticipated, the waiting game is over. The remaining field of industry players who have withheld their commitments to EUV might now be motivated to secure anticipated purchase positioning with ASML before delivery date extensions become a concern.

SPIE Advanced Lithography 2015 was likely a key catalyst triggering this defining inflection point as process experts from around the globe converged to announce new and encouraging breakthroughs in lithography and related technologies which have heretofore gated the EUV program. We might review some of the important observations and breakthroughs which comprise the critical mass of the inflection:

EUV Inflection Triggers

Probable Key Factors in Intel's EUV Decision

- While lithographers have entertained DSA and electron beam lithography as developmental candidates for 7nm scaling, given current evolution, only 13.5nm stepper/scanners can provide the image resolution and throughput required for both pilot line and future HVM.

- SEMATECH recently announced the development of a metal oxide based photoresist which reduces the EUV power output required for EUVL dosimetry (typically 15 – 20 mJ/cm2) to less than 2 - 3 mJ/cm2. [2] The new resist enables process development at reduced EUV power levels, but will not eliminate the future requirement for higher HVM source power. In the interim, it's possible that process solutions can be built around this low dose resist, enabling further, accelerated development of EUV HVM.

- During SPIE Advanced Lithography on February 24, ASML announced TSMC's confirmation that it had processed 1022 wafers in twenty four hours [3] on its NXE:3300B with a sustained source power of 90 watts. There is encouraging new data illustrating improved MTBF while sustaining EUV source operation at higher power levels.

- ASML has made significant over all progress in EUV development and has recently updated its time line for implementation of key milestones. ASML released many new updates on their EUV program at SPIE Advanced Lithography 2015 which were quite voluminous (see the post conference SPIE abstract summary).

- Obstacles to 7nm and future nodes have been addressed. ASML and Carl Zeiss acknowledged in an invited paper at SPIE Advanced Lithography 2015, that higher resolutions will require 60mJ/cm2 for half pitch nodes <8nm. [4] ASML's work with Carl Zeiss has produced an optical system with a numeric aperture (NA) of 0.55 vs. ASML's current EUV NA of 0.33. The higher NA system will require 500 watts of EUV power to achieve the estimated 60 mJ/cm2 dosimetry required for throughput of 150 wafers/hour. While this ASML/Carl Zeiss achievement paves the way for =<7nm process nodes, current R&D programs are also under way to provide a visible path to a >500 watt free electron laser EUV source, identifying one of the last major puzzle pieces in ASML's EUV endeavor. While ASML continues to refine Laser Produced Plasma source technology, the future availability of a >500 watt free electron laser EUV source remains a critical item on the agenda, and will probably gate the time lines of related programs.

- ASML has entered the pellicle business in the self interest of providing viable protection for EUV photomasks from particulate contamination. The polysilicon based pellicles are transparent to EUV with a one pass transmission loss approximating 14% and seem to exhibit sufficient durability for use in production. Previous uncertainty in EUV pellicle viability and availability have been resolved.

- As a key semiconductor industry supplier, Veeco Instruments has been successful in providing ion beam deposition tooling enabling EUV mask fabrication for advanced process nodes within acceptable defect limits. Defect free mask fabrication for advanced nodes has been a gating factor in the EUV program.

- Advancements in actinic inspection are progressing. Lawrence Berkeley National Laboratory's CXRO has been developing the SHARP EUV microscope in cooperation with SEMATECH . [5] A progress report on mask inspection was made at SPIE Advanced Lithography 2015. The SHARP EUV microscope is illuminated by an EUV synchrotron light source within the CXRO complex. The commercial availability of EUV obtained from free electron laser technology could enable the emulation of SHARP's capabilities given comparable optical and analytical performance.

- Future commercial availability of free electron laser EUV sources could also offset concerns gating the development of actinic inspection tools by KLA-Tencor and others. Given proper design, it's possible that a single free electron source beamline could provide EUV source illumination for both stepper/scanner clusters and in-line actinic inspection tools.

- Recent successes at TSMC with ASML's NXE:3300B EUV systems have prompted an additional order for two newer model NXE:3350B tools. As foundry commitments to EUV lithography continue, Intel has taken a major strategic step to ensure its competitive leadership positioning in the global wafer fabrication market.

No doubt there were many other considerations factored into Intel's EUV purchase decision. Ultimately the achievement and convergence of key process and equipment performance concerns have prompted a major commitment to both lithography and investment strategy over the longer term. In doing so, Intel has set a defining course for the semiconductor industry.

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Thomas D. Jay
Semiconductor Industry Consultant
Thomas.Dale.Jay@gmail.com
www.ThomasDaleJay.blogspot.com
Thomas D. Jay YouTube Channel

Corporate, private entities or publications referenced or linked in this article are the respective owners of their logos, trademarks, service marks, media content and intellectual property.  Unless otherwise disclosed, Thomas D. Jay has no financial interest in companies referenced in blog articles or other published media communications. No representation is made to either buy or sell securities. Opinions expressed by Thomas D. Jay are his own. Thomas D. Jay does not employ or otherwise utilize/authorize third party agents to express his opinions, represent his interests or conduct business on his behalf except where formally contractually designated.

Acknowledgements and Reference Links

[1] ASML press release 

Link to ASML web site press release.

[2] photoresist which reduces the EUV power output required for EUVL dosimetry
Link to SEMATECH web site.

[3] ASML announced TSMC's confirmation that is had processed 1022 wafers in twenty four hours
Link to ASML web site press release.

[4] ASML and Carl Zeiss acknowledged in an invited paper at SPIE Advanced Lithography 2015, that higher resolutions will require 60mJ/cm2 for half pitch nodes <8nm.
Link to SPIE Digital Library abstracts, SPIE Advanced Lithography 2015.

[5] Lawrence Berkeley National Laboratory's CXRO has been developing the SHARP EUV microscope in cooperation with SEMATECH
Link to SPIE Digital Library abstracts, SPIE Advanced Lithography 2015

The Shot Noise Heard Around the World

On January 9, I posted a discussion with the LinkedIn semiconductor Photolithography, Photolithography Specialist and EUV Lithography groups posing the question:

“What is the current shot noise/dose error performance for EUVL Sn/Laser source technology? Which secondary emissions comprise the largest dose error?”

For those outside the semiconductor industry, EUVL (Extreme Ultraviolet Lithography) is a next generation, extremely short wavelength light source (13.5 nanometers) providing improved photolithographic capability to print ever smaller, nanometer scale transistor circuit patterns on computer chips.  This technology will ensure your next smart phone will be even smarter. 

Although I anticipated a measured response to this topical question, three days have passed with no response to my posted inquiry. Perhaps this silence speaks the volumes to be presented and discussed at the SPIE (International Society for Optics and Photonics) Advanced Lithography Conference 2013, February 24-28 in San Jose, CA.  I suspect those with current answers to these questions are holding their thunder for the conference's Extreme Ultraviolet Lithography IV program. Among other topics, this conference will address the many current challenges in the development of production scale EUV light source and mask technologies critical to the successful on time delivery of 13.5 nanometer production lithography systems.

For world history buffs the “The shot heard around the world” is usually attributed to a pivotal event in history, the 1776 American revolution. In the global semiconductor community shot noise is a topic of considerable discussion at a pivotal point in the evolution of EUV lithography.   Shot noise is a term that has been used to describe the effects of energetic secondary particle emissions produced when surface materials are bombarded by high energy EUV light. The surface materials concerned are usually the EUV pattern mask or the EUV sensitive photoresist surface that nano-circuits are printed on during pattern mask exposure.   Secondary particle emissions can sometimes induce unwanted exposure dose errors and/or possible collateral exposure which can extend beyond the intended EUV image patterns. These errors can result in lower yields if pattern distortions are transferred when the circuits are printed. Advanced metrology systems are capable of detecting the effects of shot noise by measuring the change in patterns created by the exposure/dose error. There has been discussion and inquiry concerning the characterization of secondary emissions for many materials of interest inclusive of photoresists and masks. Typical questions concern the energy and range of secondary emissions and their particle make up. This phenomenon is a hot topic of discussion in EUV lithography and will be discussed at the SPIE Extreme Ultraviolet Lithography IV program in February.


The road map for 13.5 nanometer EUV technology was recently reinforced by Intel, Samsung and TSMC with an $8 billion dollar plus investment in ASML, a leading industry supplier of semiconductor lithography systems based in the Netherlands. Critical to ASML is the on time development of high power laser source technology provided by Cymer. ASML recently acquired Cymer for $2.6 billion to ensure the delivery of this critical system component.   Recently Cymer has addressed the issue of boosting EUV power output by the introduction of pre-pulse laser technology.  By directing an initial laser pulse on tin droplet laser source material, the droplet size is increased to a larger optimal "flat plate target" diameter for more efficient ionization by a second CO2 laser pulse.  Although no new power output levels have been quoted, Cymer reports this technique significantly improves EUV power output and performance as design improvements continue.   

The evolutionary path of EUV Lithography has attracted a significant level of industry investment spanning many years. These efforts are complemented by the supportive research at the SEMATECH Berkeley Micro Exposure Tool (MET) and the Lawrence Berkeley National Labs CXRO (Center for X-Ray Optics) who cooperatively work to provide metrology analysis and development of EUV mask and resist materials.  The CXRO has operated the AIT (Actinic Inspection Tool), a high resolution EUV Fresnel zoneplate microscope dedicated to photomask research.  The newer and improved version of the AIT has been named AIT5 or SHARP (Semiconductor High-NA Actinic Reticle review Project, and is currently scheduled to go on line in April, 2013 with enhanced performance specifications.  A new novel feature of interest is a Fourier Synthesis Illuminator, a concept originated by CXRO's Director, Patrick Naulleau, Ph.D.  AIT5/SHARP's updated hardware and performance specs are significant.  I encourage you to visit the AIT5 web site and download the presentation and technical paper describing the new system and upgrades (see the link below).  I suspect that lab time at AIT5/SHARP will be in demand when it goes online several weeks after the SPIE Advanced Lithography conference in February. 

Recently in cooperation with SEMATECH, researchers from UCF/CREOL (University of Central Florida/College of Optics and Photonics) conducted an evaluation of a high power Sn/EUV laser source.  The research evaluated plasma dynamics and the radiated emissions of Tin droplet targets while suggesting improvements in the instrumentation and methodologies required to enhance resolution of collected spectral, spatial and temporal data.  Additional UCF/CREOL research was conducted on Tin droplet target debris mitigation.  Collectively, the experiments made a significant contribution to the EUV R&D knowledge base.  A large number of well known semiconductor manufacturers, universities and national lab facilities all contribute to this EUV research effort yielding many technical papers. The list of credits is too numerous to mention here.

The current EUV initiative has centered around the wavelength of 13.5 nanometers which some have characterized as the domain of soft X-rays.  Some may debate this discussion of semantics but what are a few nanometers among friends?  In previous years research activity spanned the X-ray regime +/-  yielding similar observations of secondary emissions which have the potential to induce “noise” in lithography patterning. During the 1980's I had the opportunity to observe some of the first synchrotron X-ray lithography experiments at the Brookhaven National Laboratory's National Light Source. Interestingly at the time, two of my largest customers were plugged into the accelerator ring with research end stations, positioned side by side conducting lithography experiments. Analysis of the subsequent research prompted additional experimentation and it became apparent new directions were being plotted for future semiconductor lithography. Recognizing the significance of a seemingly historic moment in X-ray lithography I walked around the synchrotron ring seeking a suitable souvenir to commemorate the event. As most everything in sight was valuable instrumentation I asked my Brookhaven friends what items might be considered suitable as a commemorative take away without upsetting anyone in the accounting department. There was some suggestion and minimal discussion and with the permission of those concerned I claimed my souvenir. Somewhere in storage is a thirty pound lead brick I extracted from the surplus supply stacked near the synchrotron's radiation shield wall. My Brookhaven friends had assured me that there were plenty of lead bricks in the facility and no one would mind if I took one home.  A short time later the value of the synchrotron research became evident as viable customers were found for X-ray scale device technologies. Moore's Law marched on and much has transpired since then.

SEMI industry experts have maintained that for practical EUV source design purposes, 13.5 nanometers is a good fit and as the designated EUV wavelength, complementary tooling, mask and resist designs will follow.  It also seems agreed that the current successful implementation of 193 nanometer lithography will be around for quite a while, but that's another discussion.

As for the question posed in my original posting concerning shot noise, I anxiously await the publication of the EUV program discussions and technical papers to be presented at the SPIE Advanced Lithography Conference in February. A review of the scheduled program illustrates the many EUV issues being addressed. The list is quite extensive and is inclusive of EUV source power output, mask design, secondary emission characterization, and ultimate process quality at targeted nodes.

Note: I have updated this blog entry on January 15, to include mention of the SEMATECH Berkeley AIT5/SHARP (Actinic Inspection Tool) an important new EUV mask metrology capability.

Thomas D. Jay

Semiconductor Industry Consultant

Thomas.Dale.Jay@gmail.com

www.linkedin.com/pub/thomas-d-jay/26/aa3/499
www.ThomasDaleJay.blogspot.com

The Technology High Ground

For additional information on the SPIE Advanced Lithography 2013 - Extreme Ultraviolet Lithography IV program click on the link below:     

http://spie.org/app/program/index.cfm?fuseaction=conferencedetail&export_id=x12540&ID=x10947&redir=x10947.xml&conference_id=1039349&event_id=996835

For an update on current Cymer Pre-Pulse EUV source technology click on the link below:
http://www.cymer.com/pre_pulse/

For information on The SEMATECH Berkeley Microfield Exposure Tool (MET) click the link below:
http://cxro.lbl.gov/MET

For information on the SEMATECH Berkeley AIT5/SHARP (Actinic Inspection Tool) click on the link below:
http://ait5.lbl.gov/

For streaming updated technology news from Google, scroll to the bottom of this page.