Stochastic modeling of EUV resists: simulation of the effects of absorption, quantum yield and photoelectric blur on 18 nm contact hole performance John J. Biafore, Alessandro Vaglio Pret, Mark D. Smith, David A. Blankenship, Trey S. Graves IEUVI TWG Resist, Hiroshima, 10.2016
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Stochastic modeling of EUV resists: simulation of the effects of absorption,
quantum yield and photoelectric blur on 18 nm contact hole performance
John J. Biafore, Alessandro Vaglio Pret, Mark D. Smith, David A. Blankenship, Trey S. Graves
IEUVI TWG Resist, Hiroshima, 10.2016
2 KLA-Tencor Confidential. Copyright KLA-Tencor Corporation. All rights reserved.
Stochastic resist effects = local, random variability in the after-develop image
Examples: patterns printed in chemically-amplified photoresist with EUV [1]
[1] P. De Bisschop, J. Biafore, A.V. Pret, ‘Stochastic effects in EUV Lithography,’ ICPST, 2016
• Observed as variability in printed image or failure in electronic devices (LER, LWR, LCDU, etc.)
• Two components:
• Optical: Photon Shot Noise (PSN)
• Uncertainty in the amount of energy incident upon the resist
• Produces well-known dose dependence of variability
• Physical-Chemical: Effects specific to photoresists
• Uncertainty (noise) in the amount of energy absorbed by the resist
• Spatial blurring of absorbed energy by the photoelectric effect
• Noise in the number and positions of released acids
• Diffusional blurring during bake (but well-controlled in modern systems)
• Poor dissolution contrast (but well-controlled in modern systems)
6 KLA-Tencor Confidential. Copyright KLA-Tencor Corporation. All rights reserved.
Only way to repair PSN in resist (without raising dose) is to increase absorption
𝜶 = 6.5/um
𝜶 = 18.6/um
𝑭𝑻 = 30 nm
𝑭𝑻 = 50 nm
• Low EUV absorbance by organic resists increases PSN effects and wastes incident energy
• But very difficult to increase EUV absorption in organic resists
• Reducing organic resist thickness is also problematic
• Optical density of a CAR is about 0.20 at 30 nm FT (0.80 of photons pass through)
• Target optical density is closer to 0.45-0.50
𝑃𝑆𝑁 =1
𝑛 𝑎𝑏𝑠=
1
𝜶 𝑬𝒊𝒏𝒄 𝑽𝝀
ℎ𝑐
7 KLA-Tencor Confidential. Copyright KLA-Tencor Corporation. All rights reserved.
1. Direct photolysis: 1 absorbed photon releases at most 1 acid
Photon absorption by the generating molecule produces an electronically-excited state
favorable for conversion. [2] A. Reiser, ‘Photoreactive Polymers,’ Wiley, 1989
[3] N. Turro, ‘Modern Molecular Photochemistry,’ University Science Press, 1991
2. Electronic excitation: 1 scattering electron may release > 1 acid
Absorption of an EUV photon results in ionization and a cascade of secondary electrons.
Scattering electrons induce time-dependent electric fields whose individual Fourier
components may be treated using the method of virtual quanta. If a resonating system is
located within the field, it may interact with the passing charge. [4] E. Fermi, 1924
[5] Weizsacker, Williams, 1934
[6] J. D. Jackson, ‘Classical Electrodynamics’, Wiley, pp. 724-729, 1975
[7] A. Reiser, ‘Photoreactive Polymers,’ Wiley, 1989
[8] G. Han, F. Cerrina, ‘Energy transfer between electrons and photoresist,’ JVST B18(6), 3297, 2000
[9] (as applied to EUV resists) Mack, Thackeray, Biafore, Smith, ‘Stochastic Exposure Kinetics of EUV Photoresists: A Simulation
Study,’ SPIE 2011
3. Dissociative electron attachment: 1 scattering electron may release at most 1 acid
• Absorption of an EUV photon results in ionization and a cascade of secondary electrons.
The dissociative‐attachment peak cross section is a function of peak resonance energy [8]. [10] Christophorou, Stockdale, ‘Dissociative Electron Attachment to Molecules,’ JCP, 48-5, 1967
[11] (as applied to EUV resists) Kozawa et al, J. Vac. Sci. Technology. B22, 2004 3489
Supported EUV exposure mechanisms
• Simulation of these exposure mechanisms is supported by PROLITH
8 KLA-Tencor Confidential. Copyright KLA-Tencor Corporation. All rights reserved.
Electrons
PROLITH ™
Acids
PROLITH ™
Visualization of the photoelectric exposure blur in PROLITH X6
Discrete Acid PSF in XY plane
Discrete Electron PSF in XY plane
ℎ𝜈
ℎ𝜈
e
ℎ𝜈
𝑃𝑆𝑁 = 𝜎𝑛
𝑛=
0
1= 0
~60 𝑛𝑚
• Absorb 1 EUV photon at the center of a cube of resist
• Perform simulation of stochastic exposure within cube
• Record the number and positions of acids released
• Record the number and positions of electrons at 10 eV
• Re-initialize cubic domain
• Repeat for a specified number of randomized trials
• PSN component is zero – why?
• In each trial, we absorb 1 photon with probability = 100%
• Standard deviation of n absorbed photons is therefore 0
• PSN vanishes and the approach models a pure resist effect
9 KLA-Tencor Confidential. Copyright KLA-Tencor Corporation. All rights reserved.
Simulation of material-specific electron scattering properties in PROLITH X6
Resist electronic response characterization via dielectric function measurement [12-15]
• The dielectric (or loss) function ε(ω,0) is calculated by measuring the
extinction coefficient κ(ω) and the real refractive index n(ω).
• To cover a large enough range of energies, 3 techniques are used:
• CXRO website for n(E), κ(E) in the 1nm < λ < 41nm range[11]
• Spectroscopic ellipsometry for n(E), κ(E) in the 150nm < λ < 750nm range
• XAS, for n(E), κ(E) in the 4.5nm < λ < 450nm range (@ Elettra synchrotron)[12]
• Electron mean free path, CDSA energy loss and screening parameter can
be modeled from ε(ω,0)
• Scattering data can be loaded in PROLITH X6 to simulate the resist’s
electronic response to absorption of high energy photons [13-22].
• XAS can be also used to estimate other material properties such as Total
and Secondary Electron Yield (TEY, SEY), and used in PROLITH X6 [12] J. Biafore et al. “Pattern prediction in EUV resists”, Proc. SPIE 7520 (2009)
[13] A. Vaglio Pret et al. “Characterization and modeling electrical response to light for metal based EUV photoresists, Proc. SPIE 9779-5 (2015)
[14] P. De Schepper et al. XAS Photoresists Electron/Quantum yields study with synchrotron Light, Proc. SPIE 9425-6 (2015)
[15] CXRO website: http://www.cxro.lbl.gov/
[16] A. Giglia et al. “FEL multilayer optics damaged by multiple laser shots: experimental results and discussion”, Nucl. Instr. Meth. Phys. Res. A. 635,
530 (2011).
[17] J. C. Ashley et al. “Interaction of low-energy electrons with condensed matter stopping powers and inelastic mean free paths from optical data”, J.
Elec. Spectr. 46 (1988)
[18] M. Dapor et al. “Monte Carlo modeling in the low energy domain of the secondary electron emission of PMMA for CD-SEM”, JM3 9, (2010)
[19] D. R. Penn. “Electron mean-free-path calculations using a model dielectric function”, Phys. Rev. B, 35, (1987)
[20] V. P. Singh, et al. “Determination of Effective Atomic Numbers Using Different Methods for Some Low-Z Materials”, J. Nuc. Chem. (2014)
[21] H. Bethe, “Zur Theorie des Durchgangs schneller Korpuskularstrahlen durch Materie”, Annalen der Physik, 397(3), 325-400, (1930).
[22] M. Born, "Quantenmechanik der Stossvorgänge". Zeitschrift für Physik 38(11-12), 803-827, (1926).
[23] T. Schnattinger, “An Introduction to the Passage of Energetic Particles through Matter”, Taylor & Francis Group, LLC, CRC Press, Boca Raton,