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Evanescent Wave Imaging Using Optical Lithography

Evanescent Wave Imaging Using Optical Lithography. Reinaldo Vega EE235 2/13/08. 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

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Evanescent Wave Imaging Using Optical Lithography

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  1. Evanescent Wave Imaging Using Optical Lithography Reinaldo Vega EE235 2/13/08

  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 = 64.33 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 = 1.296. • Half-pitch = 44.67 nm. • Other fluids: • Aluminum chloride (n = 1.6) • Hydrogen phosphate (n = 1.54) • Sodium sulfate (n = 1.49)

  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 = 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???

  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.

  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

  6. 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!!!

  7. 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)

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