1. Evanescent Wave Imaging Using Optical Lithography
Reinaldo Vega
EE235
2/13/08
© 2008, Reinaldo Vega UC Berkeley

2. Motivation for Immersion Lithography
Conventional lithography:
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
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 = nm
Immersion Lithography
Putting fluid between lens and wafer. Fluid acts as a “final lens.”
Can achieve NA > 1 (hyper-NA)
Increases minimum refractive index => half-pitch drops.
Consider water (n = 1.44).
For classical NA = 0.9, immersion NA =
Half-pitch = nm.
Other fluids:
Aluminum chloride (n = 1.6)
Hydrogen phosphate (n = 1.54)
Sodium sulfate (n = 1.49)
© 2008, Reinaldo Vega UC Berkeley

3. 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 = 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

4. 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

5. Evanescent Wave Assist Features (EWAF)
TIR not needed to form evanescent waves.
Surface bound evanescent waves useful for enhancing image quality.
Example: contact holes.
62nm wide
Surface bound evanescent waves, constructive interference.
Exposing radiation
© 2008, Reinaldo Vega UC Berkeley

6. © 2008, Reinaldo Vega UC Berkeley
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

7. © 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