Diffractive optical elements and their potential role in high efficiency illuminators

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1 Diffractive optical elements and their potential role in high efficiency illuminators Patrick Naulleau Farhad Salmassi, Eric Gullikson, Erik Anderson Lawrence Berkeley National Laboratory Patrick Naulleau 2008 EUV Workshop 6/12/2008 1

2 Outline Motivation Theoretical efficiencies Fabrication and characterization of a relief HOE Fabrication and characterization of an etched multilayer null HOE Summary Patrick Naulleau 2008 EUV Workshop 6/12/2008 2

3 Motivation: modified illumination key element in lithographer s toolbox NA = 0.32 Modified illumination in lithography first proposed by: K. Kamon, et. al, Photolithography system using annular illumination, Japanese Journal of Applied Physics 30, (1991). Patrick Naulleau 2008 EUV Workshop 6/12/2008 3

4 Motivation 2 ASML roadmap shows off-axis illumination for 16-nm HP node (SPIE 2008) No details given on what off axis illumination and how it might be achieved Let s assumed condenser pupil plane aperture is used: Annular = 5x throughput loss Quadrupole = 4x throughput loss Dipole = 8x throughput loss Patrick Naulleau 2008 EUV Workshop 6/12/2008 4

5 Potential architectures for high-efficiency EUV diffractive/holographic elements Efficiency crucial for EUV applications, so we consider only phase devices Mo on Si Membrane Etched Mo/Si multilayer Mo/Si multilayer on relief substrate Mo on Mo/Si Multilayer Patrick Naulleau 2008 EUV Workshop 6/12/2008 5

6 Theoretical efficiencies Mo on Membrane π-phase-shift thickness = 86 nm (60% transmission) Assume 180-nm Si membrane (74% transmission) 1 st order efficiency = 24% Mo/Si on relief substrate and etched Mo/Si Assume 67% Mo/Si reflectivity in all regions 1 st order efficiency = 27% Mo on Mo/Si Round-trip thickness = 86 nm Assume 67% Mo/Si reflectivity 1 st order efficiency = 22% Patrick Naulleau 2008 EUV Workshop 6/12/2008 6

7 Symmetric pupil fills have potential for 2x efficiency Binary carrier holographic devices will produce symmetric diffraction orders When axially symmetric illumination patterns are desired, the total efficiency of the device can be double Near 50% or higher efficiency EUV! Mo on Membrane transmission example 24% 24% Total efficiency = 48% Patrick Naulleau 2008 EUV Workshop 6/12/2008 7

8 Higher efficiency through system design Previously stated efficiencies assume addition of discrete optical component to system Substrate losses can be avoided by integrating holographic element into existing component Transmission type could be integrated onto vacuum or spectral filtering membrane Reflection type could be integrated onto illuminator mirror Resulting efficiencies Pure Mo: 64% in +/-1 orders Mo/Si: 80% in +/-1 orders Patrick Naulleau 2008 EUV Workshop 6/12/2008 8

9 Is this just a pipe dream: is source etendue too large? 0.32-NA 4X optic, 6-mm slit height LPP: source size = 100 μm, collection half angle = 70 Minimum σ = 0.25 DPP: source size = 1 mm, collection half angle = 30 Minimum σ = 1 (NOT FEASIBLE) Patrick Naulleau 2008 EUV Workshop 6/12/2008 9

10 Design of example hologram 200 μm White = π phase shift Black = 0 phase shift 200 μm 20 μm 20 μm 1 mm carrier period = 392 nm Pattern NA = Pixel size = 98 nm Full hologram, 5x5 grid of 200x200μm subholgrams 1 mm Patrick Naulleau 2008 EUV Workshop 6/12/

$due to reduction in higher-order diffraction terms Patrick Naulleau 2008 EUV Workshop 6/12/2008$

11 Fabrication of a relief hologram AFM images of 5-μm patches of the hologram Before Multilayer Coating Coated hologram 5 μm 5 μm Multilayer smoothing no problem for the 392-nm period carrier and 98-nm pixel size chosen here Spectral analysis actually shows a benefit of the smoothing due to reduction in higher-order diffraction terms Patrick Naulleau 2008 EUV Workshop 6/12/

12 Characterization of relief hologram No zero order due to phaseonly architecture Predicted Measured +1 order only Image is stitched because CCD was too small to capture Patrick Naulleau 2008 EUV Workshop 6/12/

Relief hologram efficiency Null hologram used to determine efficiency because hologram NA too large for our reflectometer 0.25 Absolute efficiency 0.20 0.

13 Relief hologram efficiency Null hologram used to determine efficiency because hologram NA too large for our reflectometer 0.25 Absolute efficiency nd 1 st 0 th -1 st -2 nd Near ideal efficiency obtained (23%) Angle [deg] Patrick Naulleau 2008 EUV Workshop 6/12/

14 Etched multilayer implementation Multilayer smoothing will ultimately limit the resolution of relief implementation Etched methods avoid this problem Of etched methods, multilayer has higher potential efficiency Embedded chrome etch stop Photoresist After develop, etch, strip Patrick Naulleau 2008 EUV Workshop 6/12/

15 High reflectivity obtained from base multilayer even with embedded chrome G mul % 0.5 Reflectance Wavelength (nm) Patrick Naulleau 2008 EUV Workshop 6/12/

16 Etched multilayer implementation shows excellent potential for high resolution Etch depth = 115 nm (16 biliayers) π phase shift 500-nm period Chrome remaining in etched regions Patrick Naulleau 2008 EUV Workshop 6/12/

17 19% absolute efficiency obtained from null hologram Efficiency nm period Angle (deg) 19% By removing chrome etch stop, efficiency should increase to 23% Further improving base reflectivity, efficiencies > 25% can be expected Reflectance Patterning process caused 8% drop in base multilayer reflectivity, probably due to residual hardmask 55% Wavelength (nm) Multilayer Patrick Naulleau 2008 EUV Workshop 6/12/

18 Summary HOEs provide potential path to efficient EUV pupil-fill control Efficiency crucial in EUV regime (pure phase devices required) Ideal HOE architecture (transmission or reflection) depends on system design High-efficiency EUV HOE fabricated using relief substrate method 23% in +1 order (43% +/-1 orders) Etched ML or Mo methods enable higher resolution 19% single order efficiency etched ML null HOE fabricated 38% +/-1 orders Clear path to efficiencies as high as 25% (50% +/-1 orders) Patrick Naulleau 2008 EUV Workshop 6/12/

19 Future work Extension of HOE work Develop chrome etch stop strip process for ML Improve ML pattern/etch process Transfer high-resolution ML etch learning to pure Mo etch Implement full hologram devices with reflective etched ML and transmission Mo methods New directions Nanosmoothing techniques allowing low-cost diamond-turned optics to be used in the EUV Patrick Naulleau 2008 EUV Workshop 6/12/

20 Base multilayer closely matches expected performance 0.80 Ideal Interface roughness Interface roughness and diffusion 0.70 Reflectivity % 66% 63% Modeling data provided by Andy Aquila, LBNL, UC Berkeley Wavelength (nm) Patrick Naulleau 2008 EUV Workshop 6/12/

21 Iterative computation method used to generate phase-only hologram High efficiency requires both a phase only carrier and a phase-only modulating signal Phase-only modulating function for a F.T. hologram can be computed using iterative Gerchberg-Saxon method Initial guess FFT Discard phase Discard amplitude Multiply by target farfield image amplitude IFFT Resulting phase signal encoded onto binary carrier: h(x,y) = binary{sin[(2π f c + Φ(x,y)]} Patrick Naulleau 2008 EUV Workshop 6/12/

ELECTRONIC HOLOGRAPHY

ELECTRONIC HOLOGRAPHY CCD-camera replaces film as the recording medium. Electronic holography is better suited than film-based holography to quantitative applications including: - phase microscopy - metrology