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Saturday, January 12, 2013

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
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.

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Your comments are welcome. I'd like to hear your thoughts and opinions. Many thanks for your readership. - Thomas D. Jay