Extreme Ultraviolet Light Sources MARK HRDY 12/05/2017 Background/Motivation

Extreme Ultraviolet Light Sources MARK HRDY 12/05/2017 Background/Motivation

Extreme Ultraviolet Light Sources MARK HRDY 12/05/2017 Background/Motivation Resolution Limit UV Light Generation EUV Introduction Technical Challenges: Outline

Optics Masks/Pellicles Light Production Contamination Power Requirements Resists Current Outlook: Requirements Current Status Ongoing Concerns The Future of EUV Close:

References Questions UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 2 Background/ Motivation UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 3 Impact of Semiconductor Industry Semiconductor industry is massive and important A major factor in this growth has been the ability to define smaller and smaller feature sizes leading to more powerful and portable devices [6, 19]

UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 4 Feature Size Fraunhofer Diffraction Photolithography: Technique for transferring a given pattern to the substrate by projecting light through a patterned mask that alters the chemistry of a reactive substance (photoresist) on the substrate Diffraction: Light interferes with itself upon passing through some boundary Diffraction pattern: Resolution: Smallest resolvable feature

Resolution limit: Mask Photoresist Resolving Diffracted Signals Today we will be discussing efforts to reducing feature size by reducing the wavelength (), specifically to Extreme Ultraviolet (EUV) regime Resolvable UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY Unresolvable [4] 5 Early Sources

Mercury-vapor lamps are a good source of light at 365nm and 254nm Early lithography used 365nm light because of this existing source UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 6 Current Sources The invention of excimer lasers allowed for shorter wavelengths KrF excimer lasers provide a good source of 248nm Industry standard today is 193nm produced by ArF excimer lasers That was easy - So lets go even shorter! UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY

7 Enter Extreme Ultraviolet 30+ years later And billions and billions of dollars Those technologies are called extreme ultra-violet (EUV) lithography and 450-millimeter wafers and they will let Intel make smaller chips that drink less power. In other words, Intel is and scientists knew directionally where to2012 spending Engineers $4.1 billion to continue with Moore's Law. Business Insider, Were

using 193nm lasers go, butstill there was no light source. In 2012, ASML also obtained a combined total of $1.9 billion in R&D funds from Intel, Samsung and TSMC. Semiconductor Engineering, 2014 What gives?! One would need to be invented. ASML buys 24.9% of ZEISS subsidiary Carl Zeiss SMT for EUR 1 billion in cash. Start of development of entirely new High NA optical system for the future generation of EUV. ASML, 2016

forecasts $1.482 billion will be spent on EUV this year, up from $1.036 billion last year and [9,11,12,15,22] rising to $3 billion in 2019. VLSI Research, 2017 with the whole world working on it UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 8 Technical Challenges UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 9 EUV Challenges or Why are we still waiting?!

Optics Most things are highly absorbent (so is air) Masks/Pellicles Most things are highly absorbent A lot of heat is generated during absorption Plasmas Using plasmas to generate high energy light Efficiency in this production is fairly low Debris Power Requirements Resists

Plasma generation requires blasting solid Sn with laser Splattered Sn everywhere Massive power consumption for few photons Resists need more photons than they are getting UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 10 Optics - Refraction Lenses: Light refracts through transparent materials of differing indexes (n) Braggs Law: Need new optics system! Need to run in vacuum!

Heat management problems! Problems: All useful optical materials are strongly absorbing Refractive index is a function of wavelength For X-rays, n ~ 1 for everything (no refraction) [4] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 11 Optics - Reflection At EUV wavelengths, the small but measurable differences in refractive index can add up Multilayer Reflectors: By alternating layers of high-Z/high-n and low-Z/low-n materials, multiple reflections add up The periodicity of the bilayers needs to satisfy Bragg Condition so reflected waves will constructively interfere Bragg Condition: , m = 1, 2, 3,... Design Considerations:

Materials cannot absorb the EUV rays Mirrors need to be manufacturable Note: any defect in the layers causes a dark spot! [8] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 12 This is how was chosen! Optics - EUV Mirrors Final Mirror: Bilayers made of Mo/Si Periodicity of bilayers is 6.9nm

Up to 100 alternating layers (50 bilayers) Maximum reflectivity ~70-72% at ~13.5nm Mo/Si experimental vs theoretical reflection. Process has since been optimized to ~70-72% or within a few percent of optimal value. [3,6] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 13 Optics - System Mirrors are important! Problems: Relatively low NA (.33) Reflectance goes as a function of ~.72N where N is the number of mirrors 80 Design with 11 mirrors

Reflectance (%) 70 60 50 40 30 20 10 0 0 2 4 6 8

10 12 # of Mirrors [19] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 14 Masks/Pellicles Masks: Multilayer mirror with absorbing materials to generate contrast Problems: Defectivity of mirrors is an ongoing problem Needs pellicle or pristine tool Pellicles:

Fragile polysilicon film with relatively low absorption (~15%) Problems: Absorbed X-rays become heat! Low confidence in this film with higher power source (more x-rays, more heat) .852 reflectance, 28% more power loss [6] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 15 Plasmas Need a method to heat the material Plasma Light Production: 1) 2) 3)

4) Heat material until electrons have more thermal energy than bonding energy Atoms shed their higher orbital electrons Ions are created where certain electron transitions dominate (for example Xe +10) These electron transitions emit characteristic wavelength (E = c/) Need the right emitting ions [2] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 16 Plasmas Does not scale Materials

Heating Methods Xenon Tin Efficiency too low Discharge Produced Plasma (DPP) Laser Produced Plasma (LPP) UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 17 Plasmas - Sn Efficiency: Sn8+ to Sn12+ states contribute to emission Potential for much higher efficiencies than other sources

Availability: Solid metal contamination seriously problematic Reflectance drops off drastically with thin Sn layer on optics Regular cleaning and potentially part replacement necessary Despite Sn being horrible for contamination, the efficiency is better than Xe, so Sn is plasma of choice. UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 18 LPP with Sn droplets = Best known method! Plasmas LPP LPP Technique: 1) Powerful laser provides high energy photons 2) High energy photons transfer energy and heat target 3) Multilayer collector reflects produced light out

Efficiency: LPP still needs large amounts of power Needs dual-pulse system to be effective Availability: Sn contaminates multilayer collector which ruins the efficiency of the system LPP Sn was avoided for a long time due to this issue [18] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 19 Plasmas Dual-pulse System Dual-pulse 1) First pulse optimizes droplet shape/density 2) Second pulse converts newly formed droplet into emitting plasma Very important development for efficiency; does nothing to prevent Sn debris Various Ways to Optimize:

Laser frequency Pulse duration Laser power Droplet shape/density Droplet size Droplet stability Droplet opacity Droplet velocity [6] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 20 Debris - Ions Magnetic field guides ions into collector H-field Ion collector

Ion Containment: 1) 2) 3) Sn is ionized when it becomes a plasma H-field contains ionized Sn Sn is guided down into ion collector Problems: This does nothing to contain non-ionized Sn Only really effective if Sn is 100% plasma [18] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 21 Debris Neutrals

H2 flow away from multilayer collector H H etches remaining Sn SnH4 is pumped out Sn SnH4 Hydrogen Backfill: 1) Backfill with hydrogen 2) H2 pressure pushes Sn away from multilayer collector 3) H2 ionizes and etches Sn contamination Sn (s) + 4H (g) -> SnH4 (g) (stannane gas) 4) SnH4 gets pumped out of system Problems:

SnH4 breaks apart upon collisions and redeposits Sn O2 contamination leads to tin oxides which will not etch A number of other variables limit process window (carbon contamination, chamber temp, etc) [18] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 22 Debris - Degradation Debris mitigation effectiveness: Collector degradation has improved immensely Problems: Tool still requires a lot of maintenance 10% reflectance loss is still significant power loss [21] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY

23 Power Requirements Multistage kW CO2 laser Beam profile can be optimized for droplet (both pulses) Needs to deliver massive amounts of power (next slide) Maintenance on this is also a large source of downtime EUV Source [10] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 24 Power Requirements Pout = laser x Pin Efficiency Estimates: laser = .08

Pout = 20kW Pin = 250kW ArF run at 50kW 5x increase in power [10] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 25 Power Requirements Pout = (mirror) x (pellicle) x PIF N 2 Efficiency Estimates: PIF = 210W (best ASML reported) mirror = .72 (N = 10)

pellicle = .85 Pout ~ 6W final = 6W/250kW = .000024 193nm ArF provide 40W Watts Photons EUV photons << 193nm [18] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 26 Resists This is all about dose! Need to get more photons to resist for reaction. Current systems are still too slow, may be room for better resists. Variations in # of photons (shot noise) are also a problem LINE EDGE ROUGHNESS

SENSITIVITY RESOLUTIO N UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 27 Current Outlook UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 28 Requirements Source power has been major limiter and requirement has grown significantly Requirements below from November 2007 Requirement at intermediate focus is now 250W for 125 wafers per hour

[3] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 29 Current Status Power FIGURE RIGHT SHOWS ALL R O A D M A P S 2 0 1 0 + T H AT H AV E B E E N D E L AY E D D U E T O EUV POWER 2016 A S M L R E P O RT E D P O W E R S H O W S C O N S I S T E N T FA I LU R E E A R LY O N , B U T L A R G E RECENT GAINS [17]

UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 30 Current Status - Power Recent Gains ASML reports that the introduction and optimization of the dual pulse technology is leading to massive increases in efficiency [6] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 31 Current Status Tool Sales ASMLs TWINSCAN NXE:3400B is the current state of the art Reportedly at >125 wafers per hour with 13nm resolution Intended to support 7nm and 5nm nodes Order backlog of 27 systems valued at 2.8b euros ($3.3b)

[1] UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 32 Ongoing Concerns Optics: Relatively low NA (.33) Next generation will likely have more mirrors and more loss Production of mirrors is very slow Masks/Pellicles: Need greater availability of defect-free masks Needs to be able to withstand more power and more heat Power Requirements: Tools need to be working, not down for maintenance

Need to demonstrate source power in the field More power will be needed for next generations Resists: Need to be able to either get by with fewer photons or produce more Debris: Tools need to be working, not down for maintenance Uptime is still significantly lower than 193nm (70% compared to 95%) Concerns about lifetime of various components UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 33 The Future of EUV Concerns mentioned previously are still problematic Power and availability are ongoing issues Also hard to forget about all the missed goals of yesteryear

However -Generally, the attitude seems optimistic EUV orders are increasing Source power increases reported by ASML are encouraging UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 34 Close UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 35 References 1) ASML. (n.d.). TWINSCAN NXE:3350B. Retrieved December 04, 2017, from https://www.asml.com/products/systems/twinscan-nxe/twinscan-nxe3350b/en/s46772 ?dfp_product_id=9546 2) Attwood, D. (1999). Soft X-Rays and Extreme Ultraviolet Radiation: Principles and Applications. New York, NY: Cambridge University Press.

3) Bakshi, V. (Ed.). (2009). EUV Lithography. Hoboken, NJ: John Wiley & Sons. 4) Duree, G. (2011). Optics for dummies. Hoboken, NJ: Wiley 5) Elg, D. et al, "Magnetic mitigation of debris for EUV sources," Proc. SPIE 8679, Extreme Ultraviolet (EUV) Lithography IV, 86792M (1 April 2013) 6) Fomenkov, I., (2017, June 15). 2017 International Workshop on EUV Lithography. In EUV Lithography: Progress in LPP Source Power Scaling and Availability. Retrieved from https://www.euvlitho.com/2017/P5.pdf 7) Global semiconductor industry market size 2019 | Statistic. Retrieved December 04, 2017, from https://www.statista.com/statistics/266973/global-semiconductor-sales-since-1988/ 8) H. J. Levinson, Principles of Lithography, Second Edition, SPIE Press, Bellingham, WA (2005) 9) Lapedus, M. (2014, April 17). Billions And Billions Invested. Retrieved December 04, 2017, from https://semiengineering.com/billions-and-billions-invested/ 10) Lapedus, M. (2016, November 17). Why EUV Is So Difficult. Retrieved December 04, 2017, from https://semiengineering.com/why-euv-is-so-difficult/ 11) Lapedus, M. (2017, September 25). Looming Issues And Tradeoffs For EUV. Retrieved December 04, 2017, from https://semiengineering.com/issues-and-tradeoffs-for-euv/ 12) Merritt, R. (2017, October 10). Intel May Sit Out Race to EUV | EE Times. Retrieved December 04,

2017, from https://www.eetimes.com/document.asp?doc_id=1332420&page_number=1 13) Mizoguchiet, H., et al, "Performance of 250W high-power HVM LPP-EUV source," Proc. SPIE 10143, Extreme Ultraviolet (EUV) Lithography VIII, 101431J (27 March 2017) 14) Renk K.F. (2017) Gas Lasers. In: Basics of Laser Physics. Graduate Texts in Physics. Springer, Cham 15) Russell, K. (2013, October 30). Intel Is Investing Billions Of Dollars Into This Unproven Technology. Retrieved December 04, 2017, from http://www.businessinsider.com/intel-is-investing-billions-in-thistech-2013-10 16) Sporre, J. R., et al, "Collector optic in-situ Sn removal using hydrogen plasma," Proc. SPIE 8679, Extreme Ultraviolet (EUV) Lithography IV, 86792H (8 April 2013); doi: 10.1117/12.2012584 17) Tomie, T, "Tin laser-produced plasma as the light source for extreme ultraviolet lithography highvolume manufacturing: history, ideal plasma, present status, and prospects," J. Micro/Nanolith. 11(2) 021109 (21 May 2012) 18) Turkot, B., et al, "EUV progress toward HVM readiness," Proc. SPIE 9776, Extreme Ultraviolet (EUV) Lithography VII, 977602 (18 March 2016) 19) Wagner, C., & Harned, N. (2010). EUV lithography: Lithography gets extreme. Nature Photonics, 4(1), 24-26. doi:10.1038/nphoton.2009.251 20) Waldrop, M. M. (2016). The chips are down for Moores law. Nature News, 530(7589). Retrieved December 4, 2017, from http://www.nature.com/news/the-chips-are-down-for-moore-s-law-1.19338#/ref-link-5 21) Yabu, T., et al, "Key components development progress updates of the 250W high power LPP-EUV light source," Proc. SPIE 10450, International Conference on Extreme Ultraviolet Lithography 2017, 104501C (16 October 2017)

22) Yen, A, "EUV Lithography: From the Very Beginning to the Eve of Manufacturing," Proc. SPIE 9776, Extreme Ultraviolet (EUV) Lithography VII, 977632 (16 June 2016) UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 36 Questions UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 37 Supplemental UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 38 Wavelength Sources Photoelectric Effect: Excitation energy provides means for electrons to jump to higher energy orbitals

When the electrons drop down to a lower energy state, they release a photon inversely proportional the drop in energy Photon Energy: ~ 1/(E )E ) Photoemission Basics Et E* Excitation energy Et E* Ei

Ei UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 39 Extreme Ultraviolet Naming Early EUV System from Lawrence Livermore National Lab S O F T X- R AY P R O J E C T I O N L I T H O G R A P H Y WA S W H AT W E O R I G I N A L LY N A M E D I T U N T I L D A R PA A S K E D U S TO G E T T H E X- R AY O U T O F T H E N A M E I N 1 9 9 3 . S O I T WA S R E N A M E D E X T R E M E U LT R AV I O L E T L I T H O G R A P H Y. I SUGGESTED THE NAME BECAUSE I KNEW BERKELEY HAD AN E X T R E M E U LT R AV I O L E T A S T R O N O M Y G R O U P. AT T H E T I M E , N O B O D Y I N O U R G R O U P E V E N K N E W W H AT T H E WAV E L E N G T H S O F EUV WERE BUT WE NEEDED A NEW NAME QUICK. -Natale Ceglio, Lawrence Livermore National Laboratory

UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 40 Plasmas - Xe Efficiency: Relatively low Only one ionic state contributing to 13.5nm light (Xe10+) Availability: Little/no contamination from noble gas Some issues Xe ice fragments, largely resolved Ultimately, not used because efficiency is so low and it is very difficult to manage heat in vacuum UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 41

Plasmas - DPP DPP Technique: 1) 2) 3) 4) Two schematics of pinching a) Z-pinch b) -pinch Changes in current induce magnetic field Magnetic field pinches plasma Current flowing through plasma faces increased resistance Higher resistance induces more heat Efficiency: Power scaling is limited by thermal management Does not scale up to necessary powers

DPP with Sn-plated disc Availability: Electrodes erode Erosion produces contamination UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 42

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