1. Extreme Ultra-Violet Lithography
Matt Smith
Penn State University
EE 518, Spring 2006
Instructor: Dr. J. Ruzyllo

2. Outline Why do we need EUV lithography?
Brief overview of current technology
What exactly is EUV?
System diagram
Challenges associated with EUV
13.5nm source
Optics
Masks
Resists

3. Mask Maker’s Holiday: “large” k1 Mask Maker’s Burden: “small” k1
Why EUV?
k1*λ NA
Minimum lithographic feature size =
k1: “Process complexity factor” – includes “tricks” like phase-shift masks
λ: Exposure wavelength
NA: Numerical aperture of the lens – maximum of 1 in air, a little higher in immersion lithography (Higher NA means smaller depth of focus, though)
There are only so many “tricks” to increase this gap, and they are very expensive … we MUST go to a shorter wavelength!
Mask Maker’s Holiday: “large” k1
Mask Maker’s Burden: “small” k1
ftp://download.intel.com/research/silicon/EUV_Press_Foils_ pdf

4. Mask Maker’s Holiday: “large” k1 Mask Maker’s Burden: “small” k1
Why EUV? Why not the next excimer line?
Hg (G 436nm  Hg (H 405nm  Hg (I 365nm 
KrF 248nm  ArF 193nm  ???
157nm lithography based on the fluorine excimer laser has been largely shelved, which leaves 193nm with extensions for production
Below that, no laser line has the required output power
Excimer-based steppers expose 109 steps per 300mm wafer, and produce >100 wafers per hour – exposure times ~ 10-20ns
Additionally, fused silica and atmospheric oxygen become absorptive by 157nm – so even incremental decreases in wavelength start to require a major system overhaul
Mask Maker’s Holiday: “large” k1
Mask Maker’s Burden: “small” k1
ftp://download.intel.com/research/silicon/EUV_Press_Foils_ pdf

5. Why EUV? It’s all about the money.
By decreasing λ by a factor of 14, we take pressure off k1 – this makes the masks less complicated and expensive because we can skip the “tricks”
For example: a 90nm node mask set:
Pixels:
Number of pixels on 1 mask: 1012
Defects:
Size that must be found and repaired: 100nm (25nm as projected on wafer)
Number of such defects allowed: 0
Data:
Total file size needed for all layers: 200GB
Cost:
Cost for mask set (depreciation, labor, etc): ~$800k-1.3M
ftp://download.intel.com/technology/silicon/Chuck Gwyn Photomask Japan 0503.pdf

6. Current Lithographic Technology
Lenses are very effective and perfectly transparent for 193nm and above, so many are used
A single “lens” may be up to 60 fused silica surfaces
System maintained at atmospheric pressure
Lens NA ~
Up to 1.1 for immersion
Exposure field 26x32mm
Steppers capable of >100
300mm wafers per hour
at >100 exposures per
wafer
193 nm Excimer Laser Source
Computer Console
Exposure Column
(Lens)
Wafer
Reticle
(Mask)

7. Basic Technology for EUV
All solids, liquids, and gasses absorb 13.5nm – so system is under vacuum
Mask must be reflective and exceptionally defect-free
13.5nm photons generated by plasma source
All-reflective optics
(all lens materials are opaque)
ftp://download.intel.com/technology/silicon/EUV_Press_Foils_ pdf (both images)

8. 13.5nm Plasma Radiation Source
The only viable source for 13.5nm photons is a plasma
Powerful plasma required – temperature of up to 200,000oC, atoms ionized up to +20 state
Plasma must be pulsed – pulse length in pico- to nanosecond range
Argon
Pre-ionized plasma excited by powerful IR laser or electric arc of up to 60,000 A to cause emission
Tin

9. Plasma Compositions for 13.5nm
Argon
Tin
Argon
13.5nm photons only generated by one ion stage (Xe11+)
Even this stage emits 10 times more at 10.8nm than 13.5
Maximum population of this stage is 45%
On the plus side, Argon is very clean and easy to work with
 Argon is horribly inefficient: to produce 100W at 13.5nm, kilowatts of other wavelengths would have to be removed
Tin
Optimum emission when tin is a low-percentage impurity
All ion stages from Sn8+ to Sn13+ can contribute
Tin tends to condense on optics
 Tin is great as a 13.5nm source, if we can engineer a way to use it without destroying our optics

10. Where Plasma and Optics Meet
Ions in the source plasma have enough energy to sputter material off the lenses of the collector optics
If the source uses tin, that will deposit on the lenses as well
At the power levels required for real exposures,
collector optics have a lifetime of about a month
This is VERY bad for Cost of Ownership (CoO)
ftp://download.intel.com/technology/silicon/EUV_Press_Foils_ pdf

11. All-Reflective Optics
All solids, liquids, and gasses absorb 13.5nm photons
So fused silica lenses are OUT …
Indeed, all refracting lenses are OUT
Making EUV mirrors is no cakewalk, either …
50 or more alternating Mo/Si layers give the mirror its reflectivity
Each layer is 6.7nm thick and requires atomic precision
Since the angle of incidence changes across the mirror, so do the required Mo/Si layer thicknesses
Acceptable surface roughness: 0.2nm RMS
Aspheric
Net reflectance: ~70%

12. Optics System - Exposure Field
Full field: ~109 exposures per 300mm wafer
Development-size field: > 500,000 exposures per 300mm wafer
In July 2005, Carl Zeiss shipped the first 0.25NA full-field optics system to ASML for integration in an EUV system
Press release:
ftp://download.intel.com/research/library/IR-TR ChuckGwynPhotomaskJapan0503.pdf

13. EUV Masks
ftp://download.intel.com/research/library/IR-TR ChuckGwynPhotomaskJapan0503.pdf

14. EUV Masks NO defects are ever allowed in a completed mask
Extremely flat and defect-free substrate, perfected by smoothing layer
All defects in multilayer reflecting stack must be completely repaired
No defects allowed in absorber layer
All defects in final absorber pattern must be completely repaired
(No wonder mask sets are so expensive!)
ftp://download.intel.com/research/library/IR-TR ChuckGwynPhotomaskJapan0503.pdf

15. EUV Resists Best Positive Resist Best Negative Resist
2.3mJ/cm2 LER=7.2nm
Best Negative Resist
3.2mJ/cm2 LER=7.6nm
LER – Line Edge Roughness
39nm 3:1 (space:line)
ftp://download.intel.com/research/library/IR-TR ChuckGwynPhotomaskJapan0503.pdf

16. Conclusion Will 193nm ever die?
As recently as 2003, EUV was “the only viable solution” for the 45nm node
Now Intel wants EUV for the 32nm node, but it may be pushed back more:
“In a nutshell, many believe that EUV will NOT be ready for the 32-nm node in Some say the technology will get pushed out at the 22- nm node in Some even speculate that EUV will never work.”
- EE Times, Jan 19, 2006
My opinion: never say “never” about this industry…
A lot of work remains: increase output power of 13.5nm source, increase NA of reflective lenses, increase lifetime of collector optics (decrease cost of ownership)
But the potential payoff is sufficient that we will make it work