Transcript Evanescent Wave Lithography

Evanescent Wave Imaging
Using Optical Lithography
Reinaldo Vega
EE235
2/13/08
© 2008, Reinaldo Vega
UC Berkeley
Motivation for Immersion Lithography
F = , NA = 0
 Conventional lithography:
F = 0, NA = 1
Lens system
 Minimum half-pitch = k*/NA
 k = “process factor.” Empirically determined, but typically ~ 0.3.
 NA = numerical aperture.
 NA = n*sin
 n = lowest refractive index in the system
  = lens system half-angle

Focal
point (F)
 Physical NA limit = 1 in conventional systems with air (n=1) bet. lens and wafer.
 Min-half pitch = 57.9nm at 193nm litho, k=0.3, for “perfect” lens (NA = 1).
 Realistically NA ~ 0.9 => half pitch = 64.33 nm
 Immersion Lithography
 Putting fluid between lens and wafer. Fluid acts as a “final lens.”
 Consider water (n = 1.44).
 For classical NA = 0.9, immersion NA = 1.296.
 Half-pitch = 44.67 nm.
 Other fluids:
 Aluminum chloride (n = 1.6)
 Hydrogen phosphate (n = 1.54)
 Sodium sulfate (n = 1.49)
© 2008, Reinaldo Vega
UC Berkeley
Lens system
 Can achieve NA > 1 (hyper-NA)
 Increases minimum refractive index => half-pitch drops.
fluid
Fluid
(n>1)
air
Focal
point (F)
Limitations of Immersion Lithography
 High absorption in fluids other than water (~10-100x).
 Higher dose-to-clear.
 Lower wafer throughput.
 Resist swelling/contamination for thin films.
 n>1.6 difficult to achieve with fluids.
 Resolution limitation, half-pitch ~ 40 nm.
 Potential solution: Solid Immersion Lithography (SIL)
 Use solid final “lens” between lens system and wafer.
 Sapphire (Al2O3) is common (n = 1.92 @ 193nm).
 Half-pitch drops to 33.5 nm (for k = 0.3).
 Hyper-NA systems face problem of Total Internal Reflection (TIR).
 nsapphire > nphotoresist
 Critical angle related to interface between high (nH) and low (nL) index
media.
 c=sin-1(nL/nH)
 Example: NAclassical-lens = 0.9, resist index = 1.7, sapphire SIL with normal
light incidence from lens.
 c = sin-1(1.7/1.92) = 62.3 degrees.
 lens = sin-1(0.9) = 64.2 degrees!! TIR!!
 But photoresist still exposed! Why???
© 2008, Reinaldo Vega
UC Berkeley
Frustrated Total Internal Reflection
 Similar to “tunneling,” but for photons.
 Some transmission still occurs in low index medium under TIR.
 “evanescent wave” with exponentially decaying amplitude.
 Turns back into homogeneous wave upon confronting a higher index
medium (e.g., photoresist).
 Circumvents limitations of incidence angle on TIR.
 Highly sensitive to gap spacing between SIL layer and photoresist.
 Requires strong process control of resist, BARC, and TARC thicknesses.
 Significant gap spacing implications for wafer throughput.
© 2008, Reinaldo Vega
UC Berkeley
Evanescent Wave Assist Features
(EWAF)
 TIR not needed to form evanescent waves.
 Surface bound evanescent waves useful for
enhancing image quality.
 Example: contact holes.
Surface bound
evanescent waves,
constructive
interference.
Exposing
radiation
© 2008, Reinaldo Vega
UC Berkeley
62nm
wide
Conclusions
 Evanescent wave lithography (EWL) has wafer- and
mask-level applications.
 Wafer-level:
 Solid immersion lithography (SIL) with high index media.
 Total internal reflection (TIR) not a concern if small SIL-to-wafer
gap spacing can be achieved.
 Mask-level:
 Surface bound evanescent waves can be harnessed to improve
image contrast/sidewall angle.
 World-record imaging with EWL.
 Smallest half-pitch to-date (26 nm).
 Good for 32nm, 22nm nodes.
 Optical lithography isn’t dead yet!!!
© 2008, Reinaldo Vega
UC Berkeley
References
Bruce W. Smith et al., “Evanescent wave
imaging in optical lithography,” Proc.
SPIE, 6154 (2006)
Bruce W. Smith et al., “25nm Immersion
Lithography at a 193nm Wavelength,”
Proc. SPIE, 5754 (2005)
© 2008, Reinaldo Vega
UC Berkeley