4. 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
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 4
[6, 19]
5. Feature Size
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: 𝑑 sin 𝜃 = 𝑚λ
Resolution:
◦ Smallest resolvable feature
◦ Resolution limit: 𝑅 = 𝑘λ/𝑁𝐴
Today we will be discussing efforts to reducing feature size by
reducing the wavelength (λ), specifically to Extreme Ultraviolet
(EUV) regime
Mask Photoresist
Fraunhofer Diffraction
Resolving Diffracted Signals
Resolvable Unresolvable
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 5
[4]
6. 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
7. 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 let’s go even shorter!
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 7
8. [9,11,12,15,22]
Enter Extreme Ultraviolet
Engineers and scientists knew directionally where to
go, but there was no light source.
One would need to be invented.
“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
spending $4.1 billion to continue with Moore's Law.” – Business Insider, 2012
“In 2012, ASML also obtained a combined total of $1.9 billion in R&D funds from Intel, Samsung
and TSMC.” – Semiconductor Engineering, 2014
…30+ years later
… with the whole world working on it
… And billions and billions of dollars
We’re still using 193nm lasers…
What gives?!
“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
rising to $3 billion in 2019.” – VLSI Research, 2017
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 8
10. EUV Challenges
or “Why are we still waiting?!”
Resists Resists need more photons than they are getting
Power
Requirements
Massive power consumption for few photons
Debris
Plasma generation requires blasting solid Sn with laser
Splattered Sn everywhere
Plasmas
Using plasmas to generate high energy light
Efficiency in this production is fairly low
Masks/Pellicles
Most things are highly absorbent
A lot of heat is generated during absorption
Optics Most things are highly absorbent (so is air)
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 10
11. Optics - Refraction
Lenses:
◦ Light refracts through transparent materials of differing indexes (n)
◦ Bragg’s Law: 𝑛1𝑠𝑖𝑛 𝜃1 = 𝑛2𝑠𝑖𝑛 𝜃2
Problems:
◦ All useful optical materials are strongly absorbing
◦ Refractive index is a function of wavelength
◦ For X-rays, n ~ 1 for everything (no refraction)
Need new optics system!
Need to run in vacuum!
Heat management problems!
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 11
[4]
12. 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: 𝑑 = 𝑚𝜆/2cos(𝜃), m = 1, 2, 3,...
Design Considerations:
◦ Materials cannot absorb the EUV rays
◦ Mirrors need to be manufacturable
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 12
Note: any defect in the
layers causes a dark spot!
[8]
13. 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
This is how λ was chosen!
Mo/Si experimental vs theoretical reflection.
Process has since been optimized to ~70-72% or
within a few percent of optimal value.
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 13
[3,6]
14. Optics - System
Mirrors are important!
Problems:
◦ Relatively low NA (.33)
◦ Reflectance goes as a function of ~.72N where N is the number of mirrors
0
10
20
30
40
50
60
70
80
0 2 4 6 8 10 12
Reflectance
(%)
# of Mirrors
Design with 11 mirrors
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 14
[19]
15. 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
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 15
[6]
16. Plasmas
Plasma Light Production:
1) Heat material until electrons have more thermal energy than bonding energy
2) Atoms shed their higher orbital electrons
3) Ions are created where certain electron transitions dominate (for example Xe+10)
4) These electron transitions emit characteristic wavelength (E = c/λ)
Need a method to heat the material
Need the right emitting ions
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 16
[2]
18. 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
19. 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
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 19
LPP with Sn droplets =
Best known method!
[18]
20. 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
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 20
[6]
21. Debris - Ions
Ion Containment:
1) Sn is ionized when it becomes a plasma
2) H-field contains ionized Sn
3) Sn is guided down into ion collector
Problems:
◦ This does nothing to contain non-ionized Sn
◦ Only really effective if Sn is 100% plasma
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 21
Magnetic field
guides ions into
collector
Ion collector
H-field
[18]
22. Debris - Neutrals
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)
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 22
H2 flow away
from multilayer
collector
H etches remaining Sn
SnH4 is pumped out
Sn SnH4
H
[18]
23. 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
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 23
[21]
24. 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
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 24
EUV Source
[10]
26. Power Requirements
Efficiency Estimates:
PIF = 210W (best ASML reported)
ηmirror = .72 (N = 10)
ηpellicle = .85
Pout ~ 6W
ηfinal = 6W/250kW = .000024
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 26
Pout = (ηmirror)N x (ηpellicle)2 x PIF
193nm ArF provide 40W
Watts ≠ Photons
EUV photons << 193nm
[18]
27. 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
RESOLUTION
SENSITIVITY
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 27
29. 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
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 29
[3]
30. Current Status -
Power
FIGURE RIGHT SHOWS ALL
ROADMAPS 2010+ THAT HAVE
BEEN DELAYED DUE TO EUV
POWER
ASML REPORTED POWER SHOWS
CONSISTENT FAILURE EARLY ON,
BUT LARGE RECENT GAINS
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 30
2016
[17]
31. Current Status - Power
Recent Gains
◦ ASML reports that the introduction and optimization of the dual pulse technology is leading to massive
increases in efficiency
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 31
[6]
32. Current Status – Tool Sales
ASML’s 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)
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 32
[1]
33. 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
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
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
34. 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
36. 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/
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 36
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-this-tech-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
high-volume 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 Moore’s 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)
39. 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: 𝐸 = ℎ𝑐/λ
E* Ei
Et
λ ~ 1/(ΔE )
E*
Et
Ei
Excitation
energy
Photoemission Basics
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 39
40. Extreme Ultraviolet Naming
Early EUV System from
Lawrence Livermore National Lab
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 40
“SOFT X-RAY PROJECTION LITHOGRAPHY” WAS WHAT WE
ORIGINALLY NAMED IT UNTIL DARPA ASKED US TO GET THE “X -RAY”
OUT OF THE NAME IN 1993. SO IT WAS RENAMED “EXTREME
ULTRAVIOLET LITHOGRAPHY.”
I SUGGESTED THE NAME BECAUSE I KNEW BERKELEY HAD AN
“EXTREME ULTRAVIOLET ASTRONOMY” GROUP. AT THE TIME,
NOBODY IN OUR GROUP EVEN KNEW WHAT THE WAVELENGTHS OF
EUV WERE – BUT WE NEEDED A NEW NAME… QUICK.
-Natale Ceglio, Lawrence Livermore National Laboratory
41. 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
42. Plasmas - DPP
DPP Technique:
1) Changes in current induce magnetic field
2) Magnetic field “pinches” plasma
3) Current flowing through plasma faces increased resistance
4) Higher resistance induces more heat
Efficiency:
◦ Power scaling is limited by thermal management
◦ Does not scale up to necessary powers
Availability:
◦ Electrodes erode
◦ Erosion produces contamination
Two schematics of pinching
a) Z-pinch
b) Θ-pinch
DPP with Sn-plated disc
UNIVERSITY OF TEXAS - EUV SOURCES - M. HRDY 42
Editor's Notes
Arc lamps website
kobayashi lab
Takeaway – cannot use refractive optics
Takeaway – multilayer mirror considerations
Takeaway – final mirror determined wavelength
Takeaway – mirror reflectivity is a major source of power loss
Takeaway – Heat management issues
Takeaways – better efficiency than Xe
Takeaway – LPP
Takeaway – another source of power degradation
Takeaway – SOO MUCH POWER!
Takeaway – This is all about the photons. Resist innovations could help save the day.
