# Immersion Lithography by 2i48203

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```									Immersion Lithography

ECE487 Spring 09

Raj Chakraborty
Outline
• Motivation for Immersion Lithography (need this
first to understand what immersion litho is)
• What are we immersing in?
• Implementation Issues
• Implementation Solutions
• Beyond 45nm node – The march towards 1.65
NA
• Key Players in Immersion Litho
• Q&A
Motivation for Immersion
Lithography
 Rayleigh equation:

K1: resolution/process factor (currently ~0.25-0.4). Function
of RET (Resolution Enhancement Techniques) and resist
quality
Lambda: wavelength
NA: Numerical Aperture

 Lambda:
– 1.2μm and larger linewidths: G-line output of mercury lamps (436nm)
was used
– 0.8μm linewidths: I-line output of mercury lamps (365nm) was
introduced for critical layers
– 0.35um linewidths: I-line use continued to the 350nm linewidth
– 130nm linewidths: Krypton Fluoride (KrF) Excimer Lasers (248nm)
– 90nm linewidths: KrF being replaced by Argon Fluoride (ArF) Excimer
lasers (193nm)
Motivation
 NA:
- Mid eighties ~ NA value of approximately 0.4 [2]
- Today ~ NA greater than 0.8. [2]
 KEY POINT: The physical theoretical limit to NA for
exposure systems using air as a medium between the
lens and the wafer is 1! Practical limit ~ 0.93
 The resolution limit for 193nm exposure systems may be
calculated using the Rayleigh equation
Lambda = 193nm, NA = 0.93 and k1 = 0.25

A highly optimized ArF exposure system has an absolute
maximum resolution of 52nm, sufficient for 65nm
linewidths, but not capable of meeting 45nm linewidths.
Motivation
 NA is actually determined by the acceptance angle of the
lens and the index of refraction of the medium
surrounding the lens and is given by:

where, n is the index of refraction of the medium
surrounding the lens and α is the acceptance angle of
the lens
 KEY POINT: What if a medium with a higher index of
refraction is substituted for air? Microscopy does this
currently..
What are we immersing in?
 The medium between the lens and the wafer being exposed needs
to have:
- index of refraction >1
- low optical absorption at 193nm
- be compatible with photoresist and the lens material
- be uniform and non-contaminating.
 Surprisingly, UltraPure Water (UPW) may meet all of these
requirements. Water has an index of refraction n ~1.47, absorption
of <5%, is compatible with photoresist and lens, and in it’s ultrapure
form is non-contaminating.

 Plugging in n = 1.47 and assuming sinα can reach 0.93 (NA = 1.36)
the resolution limits for 193nm immersion lithography are:
Implementation Issues
 The stage on a 193nm exposure tool steps from location to location
across the wafer scanning the reticle image for each field ~
Maintaining a consistent bubble free liquid between the lens and the
wafer is very difficult.
 Complete removal of liquid after exposure is completed
 Liquid Temperature control ~ variations in n. Maintaining
temperature uniformity with a rapidly moving stage and a pulsed
laser passing through the fluid is a challenge.
Implementation Issues
• Consistent Water purity (both particles and gasses)~
affects n
• Leaching (and swelling) from resist ~ need water
compatible resists [4]
• Outgassing from resist
• Conventional Lens material degradation [3]
Implementation Solutions
 Water puddle technique: dispense water between the
lens and the wafer with a specialized nozzle and rely on
surface tension to maintain a bubble-free puddle
 Specialized vacuum nozzle assembly to remove liquid
 Transparent top-coat layer on resist and low outgassing
resists
 Lutetium Aluminum Garnet (LuAG) lens material is more
compatible with immersion liquids and also has higher n.
[5]
 Point of Use (POU) temperature control unit for liquid
achieves temp control to within 0.01C
 Point of Use (POU) water microfiltration (down to 3nm)
and degassing of water.
Beyond 45nm node – The march
towards 1.65 NA
 Immersion applied to 157nm (F2 laser) exposure could carry optical
lithography even further (<30nm), although water is not a usable
medium at 157nm (because it absorbs too much) and suitable
mediums are still being researched. Exposure source stability is also
an issue at 157nm.
 Due to the issues with 157nm exposure, carrying immersion into the
32nm node will require higher NA scanners ~1.65. Current
immersion systems are at about 1.35. [5]
 Refractive index of medium, resist and lens need to be increased to
achieve 1.65 NA
 Doped water (mixing water with Sulfates, phosphates) is a
possibility (n~1.65) but slightly higher absorbance due to dopants.
Other exotic liquids.
 Resist polymer engineering to increase resist n.
 Challenges will include development of a suitable medium, high
index resist, and high index lens material with the required inter-
compatibility
Beyond 45nm node
 Strong contender with EUV for 32/22nm node. Matter of
fact, being implemented by Intel at 32nm (1268 process).
 Key concern will be defects. It has been estimated that
wet lithography (immersion) is statistically ~ 20% more
vulnerable to defect related issues compared to dry,
based on End of Line (EOL) yield data. [6]
 Defect sources will primarily be due to microbubbles and
particle carrying properties of liquids.
 Will require many vendors to work closely together to
resolve the impending issues (resist vendors, scanner
vendors, filtration and purification vendors, lens optics
vendors, laser vendors)
Key Players in Immersion Litho
•   Scanner OEM’s: ASML (Twinscan), Nikon, Canon
•   Track OEM’s: TEL (Lithius), DNS
•   Resist vendors: JSR Micro, TOK, Dupont, Fujifilm.
•   Laser OEM’s: Cymer
•   Filtration and Purification product vendors: Pall, Mykrolis
•   Lens and optics vendors: Carl Zeiss
•   Finally, IDM’s using immersion litho: Matsushita (at
45nm node), Intel (at 32nm), TSMC (at 90nm).
References
• www.solidstate.com/articles/article_display.html?id=20502
n_e/index.htm
n/
• spie.org/x8368.xml
• www.asml.com/immersion/
• www.eetimes.com/news/latest/showArticle.jhtml?articleID=
177103820
• www.future-fab.com/documents.asp?d_ID=1896
• www.nec.co.jp/techrep/en/journal/g09/n01/090114.html
Q&A

Thank You!

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