High Accuracy EUV Reflectometry and Scattering at the Advanced Light Source Eric Gullikson Lawrence Berkeley National Laboratory 1
Reflectometry and Scattering Beamline (ALS 6.3.2) Commissioned Fall 1994 High accuracy reflectometry High spectral purity Novel VLS-PGM design 2
Reflectometry of EUVL Optics MET Secondary Mirror ETS C1 condensor mirror.
High Precision and Accuracy is Required The thickness control of the multilayer coatings must be better than the figure tolerance of the optics (~0.1 nm). This requires precise reflectometry. So as not to add figure errors, the multilayer coating thickness must be carefully controlled. The wavelength precision required to measure the multilayer thickness error is ±0.03% (3σ) for a figure tolerance of 0.1 nm.
High wavelength precision and accuracy are required for EUV mask reflectometry Mask tolerance driven by the illumination uniformity requirement. 1.01 1.00 Throughput 0.99 0.98 +- 0.7% +- 0.05 nm 0.97 0.96 13.25 13.30 13.35 13.40 13.45 13.50 Mask Wavelength (nm) Mask Uniformity < 1% (3σ) Wavelength Precision: 0.09% Wavelength Accuracy: 0.09% Reflectance Precision: 0.36%
High precision and accuracy is required for the reflectometry of multilayer coatings Minimum uncertainties ETS Beta-tool Ultimate Optics (rms Figure) 0.25 nm 0.1 nm 0.05 nm Wavelength Precision 0.06% 0.03% 0.015% Wavelength Accuracy 0.09% 0.09% 0.06% Mask (3σ Uniformity) 1% 1% 0.5% Wavelength Precision 0.09% 0.09% 0.06% Wavelength Accuracy 0.09% 0.09% 0.06% Reflectance Precision 0.36% 0.36% 0.18% 6
High precision is achieved 13.344 M1-010503A1D FS4038 Reflectance 0.8 0.6 0.4 0.2 M1-010503A1D_FS4038_PRE Wavelength (nm) 13.342 13.340 13.338 13.336 +/- 2 pm 13.334 2006 2007 2008 2009 2010 Date 0.0 13.0 13.2 13.4 13.6 13.8 Wavelength (nm) Reflectivity measurements of a standard multilayer mirror demonstrate the high precision in reflectivity and wavelength. Peak reflectance (%) 67.9 67.8 67.7 67.6 67.5 M1-010503A1D FS4038 +/- 0.10% (rel) 67.4 2006 2007 2008 2009 2010 Date 7
Accuracy tested in International Reflectivity Round Robins ALS BL 6.3.2 provided the standard measurements in the 2001 and 2006 international round-robins of EUV reflectometers. Most recently organized by SEMATECH the participants included NIST, PTB, New Subaru, NSLS (NRL), AIXUV and EUV Technology. The results were presented at the Fall 2006 International EUVL Symposium. Peak Reflectance Sample CXRO Sigma Other Sigma Difference FS9328 68.64 0.01 70.37 0.02 1.73 FS9112 68.55 0.038 68.6 0.05 0.05 FS9310 68.56 0.009 68.43 0.018-0.13 FS9329 68.62 0.007 69.12 0.104 0.5 FS9110 68.62 0.012 70.08 0.216 1.45 FS9326 68.57 0.061 68.54 0.261-0.03 Wavelength Sample CXRO Sigma Other Sigma Difference FS9328 13.443 0.001 13.437 0.000-0.006 FS9112 13.491 0.001 13.491 0.001 0.000 FS9310 13.500 0.000 13.501 0.001 0.001 FS9329 13.491 0.001 13.495 0.001 0.004 FS9110 13.445 0.001 13.433 0.002-0.013 FS9326 13.442 0.001 13.467 0.003 0.025 8
EUV optics lifetime Contamination is a serious concern for the optics in EUVL steppers due to the high power and outgassing from resists or vacuum components. Debris from the source limits the lifetime of collector optics. Accelerated contamination testing results in very small changes in reflectivity. Reflectivity measurements with high accuracy and spatial resolution are required. 9
Scattering from normal incidence optics As the wavelength is reduced scattering increases roughly as 1/λ 2. Roughness of optical surfaces must be controlled or scattering will result in loss of contrast. Scattering can significantly impact the ability to utilize the light from high emmitance sources. 10
EUV scattering from can be predicted from measured surface roughness
The effects of scattering have been verified for a multi-element EUV imaging system image = PSF * object The PSF is proportional to the angular scattering distribution. PSF sc ( r) = 1 2 ρ 1 P R 0 dp dω For a multi-element imaging system: 2 16π S 2 α nr PSF ( r) = A( r) S + α n PSD ( ) 4 2 n λ ρ λρ The flare was measured for the assembled systems at ALS 12.0.1. Excellent agreement with predictions was demonstrated. Flare (%) 25 20 15 10 5 0 Predicted Measured 1 10 100 Linewidth (μm) 12
EUV scattering provides an independent test of standard roughness measurements Improvements in the roughness optics and mask substrates will push the limits of the standard roughness metrology tools (AFM and optical micro-interferometry). EUV scattering can provide the independent measurements of surface roughness needed to validate the standard metrology tools. Power Spectral Density (nm 4 ) 1000 100 10 1 0.1 θ 1 =5 deg θ 1 =10 deg AFM HSQ313UB 0.01 1E-4 1E-3 0.01 0.1 Spatial Frequency (1/nm) 13
Scattered Light Limits Spectral Purity Scattering from a monochromator grating leads to spectrally impure light which limits reflectance measurement accuracy. Photodiode Current (na) 1 0.1 0.01 Monochromatic Peak Scattered Light Background 11.3 nm 13.4 nm 14.5 nm 1E-3 0 5 10 15 20 Wavelength (nm) Measurements of spectral contamination in BL6.3.2 (2001)
In a collaborative effort with Hitachi both diffraction efficiency and scattered light were improved 50 Hitachi 200 l/mm gratings 1 hν=92 ev 40 0.1 Efficiency (%) 30 20 Intensity 0.01 1E-3 10 Old grating New grating 0 0 50 100 150 200 Photon energy (ev) 1E-4 Old grating New grating 1E-5 0 2 4 6 8 Angle (deg)
The reduced scattered light led to improved reflectance accuracy 0.68 0.67 AP1 in AP1 out Old Grating CXRO4023 0.68 0.67 AP1 in AP1 out New Grating CXRO4023 Reflectance 0.66 0.65 Reflectance 0.66 0.65 0.64 0.64 0.63 13.2 13.3 13.4 13.5 13.6 Wavelength (nm) Using the new grating there is no observable change in reflectivity using an aperture to block the scattered light. Peak reflectance (%) 0.63 13.2 13.3 13.4 13.5 13.6 67.60 67.55 67.50 67.45 Wavelength (nm) Repeatability 28 July 2005 AP1 in 67.51% σ=0.011% (abs) AP1 out 67.51% σ=0.008% (abs) 67.40 0 5 10 15 20 Measurement CXRO4023
X-Ray Interactions with Matter website December 1993 Mosaic browser released. October 1994 First Netscape browser. October 1995 X-ray interactions with matter website introduced at the SRI95 meeting at the APS. Today ~ 1 million hits per year. ~ 4000 users each month. 17
The atomic scattering factors are revised as new measurements are made Zirconium (Z=40) At BL 6.3.2, new measurements have been made for 14 elements. Many studies performed as part of student thesis research. Absorption coefficient (cm 2 /gm) 10 5 10 4 Accessible with new 80 l/mm grating CXRO Tables ALS 6.3.2 H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Measured on BL632 Absorption coefficient (cm 2 /gm) 10 3 10 100 1000 Photon energy (ev) 10 5 10 4 Tantalum (Z=73) CXRO Tables Measured (ALS 6.3.2) 10 3 10 100 1000 Photon energy (ev) 18
EUV optical properties of photoresists Particular interest in the the EUV optical properties of materials for: photoresists mask materials filters Reflectance PMMA (λ=13.5 nm) 1 δ=0.0242, β=0.0054 0.1 0.01 1E-3 1E-4 d = 286.5 nm 1E-5 0 10 20 30 40 50 60 70 80 90 Angle (deg) Kwark Young-Je, JP Bravo-Vasquez, M Chandhok, H Cao, Deng Hai, E Gullikson, CK Ober, "Absorbance measurement of polymers at extreme ultraviolet wavelength: correlation between experimental and theoretical calculations." JVST B 24, 1822 (July 2006). 19
Accuracy of the scattering factors can be improved for many elements in the soft x-ray and EUV range Lots of room for improvement. H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Good Accuracy Moderate Poor Accuracy 20
Summary Accurate reflectometry has been pushed forward by the developments in EUV lithography. EUV scattering have provided confidence in our ability to predict the flare of EUV optics. Spectral purity is an important issue for reflectometry. Improvements in diffraction gratings have allowed for improved spectral purity of EUV monochromators. Accurate measurements of EUV optical constants are important for EUV masks, filters, photoresists,...
Thank you! 22