Extreme Ultraviolet Resist Outgassing and Its Effect on Nearby Optics

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1 Extreme Ultraviolet Resist Outgassing and Its Effect on Nearby Optics Rashi Garg College of Nanoscale Science and Engineering State University of New York, Albany June 11, International Workshop on EUV Lithography

2 Outline Degradation of EUV projection optics by loss in reflectivity Mechanisms involved Sources of this contamination EUV Resist Outgassing and exposure (ROX) system EUV resist outgassing results with a mass spectrometer Contamination results from injection of known resist outgassing species and effect on contamination rate of mirrors during exposure Witness plate experiments for resist outgassing measurements Chamber cleaning results with glow discharge plasma to reduce amount of contamination due to vacuum chamber 2

3 Reflective mask Capping layer EUV Reflective Optics 6º Mo-Si multilayer Projection optics Wafer Reflectivity (arb. units) Wavelength (nm) Lifetime of optics without capping layer is very short due to oxidation Maximum reflectivity of about 70% is achieved with Mo/Si multilayers at 13.5 nm 3

4 Contamination from resist outgassing Capping layer (Ru, Si, TiO 2 ) Resist hydrocarbons H 2 O Mo-Si multilayer optics EUV Water molecules and hydrocarbons from the vacuum chamber Resist Outgassing Wafer with resist Contamination of optics by resist outgassing leads to drop in reflectivity 4

5 Optics contamination: Mechanisms EUV EUV C x H y H 2 O e - e e- - e - e - e - Surface carbon growth Reversible Sub-surface oxidation Irreversible Carbon contamination results in surface carbon growth Water vapor environment results in sub-surface oxidation Mechanism of contamination may be dominated by either Photon dissociation Secondary electron dissociation 5

6 Role of Secondary Electron Yield (SEY) in optics contamination Theoretical results Secondary electron flux from a surface * I SE = 1 2 I 0 hν z M z μ e z z L dz 92.5 ev δ D hνl M C μc LRuM Ruμ 2 LC M C μc D exp L D + 1 exp L = C Ru Ru C hν : incident photon energy µ Ru, µ C : photoabsorption cross-section M Ru, M C : electron multiplication factors L Ru, L C : secondary electron escape depth D : thickness of growing hydrocarbon SEY for Ru dominates C around 92.5 ev If secondary electrons dominate the contamination process, then rate of growth on clean Ru may be faster than for carbon contaminated surface *J. Hollenshead and L. Klebanoff, "Modeling radiation-induced carbon contamination of extreme ultraviolet optics", J. Vac. Sci. Technol. B 24, (2006) 6

7 Resist outgassing is one of the a major concerns for optics contamination Detailed measurement is needed for Rate of outgassing of each species from resist Detailed understanding is needed for Rate of contamination of each outgassed species 7

8 EUV-ROX System EUV Resist Outgassing and exposure System Load Lock Zr/Si foil Loadlock Quadrupole Mass Spectrometer Sample EUV Quadrupole Mass spectrometer Injection of calibration species EUV Source 8

9 Chamber Resist outgassing background as measured by mass spectrometer CO+N 2 H 2 O CO+N 2 Benzene (C 6 H 6 ) CO 2 Diphenyl sulfide (C 12 H 10 S) Courtesy Prof. R. Brainard 9

10 What outgases from photoresist? K. Dean, G. Denbeaux, A. Wüest, R. Garg, EUV Resist Outgassing: How Much is Too Much?, Journal of Photopolymer Science and Technology, Vol. 20, pp (2007). 10

11 Chosen species for injection and exposure of mirrors to measure contamination Intended to represent known or similar structures that may outgas from resists 11

12 Contamination Studies of Injected Species We have directly injected a few species known to outgas from resist at high concentrations of approximately 1x10-6 Torr (about 100x higher pressure than during outgassing experiments) Benzene Tert-butanol Diphenyl Sulfide Then, we exposed a mirror to >30 J/cm 2 hydrocarbon environments (8 hours) in these high At these high pressures and modest doses, we can not measure reflectivity loss above the measurement accuracy We have yet to identify any of the outgassed species from resist that contribute significantly to optics contamination! 12

13 Reflectivity results due to contamination from these species Chamber Conditions Clean (background) Chamber Pressure (Torr) Exposure time (hours) Total Dose (J/cm 2 ) Number of pulses (millions) Reflectivity drop (ΔR/R%) 2.5 x Benzene 1 x Tert-Butanol 3 x Diphenyl Sulfide 1 x Diphenyl Sulfide 1 x Diphenyl Sulfide 1 x No significant reflectivity loss for these species at these pressures and doses 13

14 Optics contamination: Experimental configuration Two set of experiments done: Witness plate: The optics exposed to EUV in presence of resist sample Control witness plate: The optics exposed to EUV with no resist sample Optics: Mo/Si mirror at 6 degrees to the incident light Witness Control Plate Optics EUV Zr/Si Filter Resist sample Resist sample Energetiq Xenon Plasma EUV Source Vacuum Chamber 14

15 Glow Discharge Plasma Chamber Cleaning Partial Pressure (Torr) 1.00E E E E E-12 H2O CO,N2 43 CO Argon oxygen plasma cleaning at ~20 mtorr for 1 hour caused a drop in the mass spectrometer scan for high mass species E E E Mass number (amu) Before Cleaning 1.7e-8 Torr After Cleaning 1.9e-8 Torr 15

16 Witness Plate and Control Mirror Results Dose on mirror (J/cm2) Before chamber cleaning, there were large reflectivity losses and a wide spread in results After chamber cleaning, the results were improved The effect of the resist was subtle compared to chamber effects Reflectivity Loss dr/r Prior to chamber clean After chamber clean After Chamber Clean Pre Control After Chamber Clean Post Control After Chamber Clean Resist A Post Control High outgassing resist Pre Control - Bad filter 16

17 After Exposure and Reflectivity Loss, XPS Shows Primarily Carbon Unexposed Witness Plate Exposed Witness Plate Shows primarily an increase in carbon Si Mo C Si Mo Atomic % O C Atomic % O Thickness (nm) Thickness (nm) XPS with sputtering to look at materials through sample thickness 17

18 Conclusions Outgassing with a mass spectrometer works routinely However, without an understanding of which species are likely (if any) to contaminate optics, interpretation of the results for each resist is a challenge Witness plate work will provide a more direct understanding of the danger of each resist to the optics However, the current test has a low signal from the resist and a relatively high level of contamination due to the chamber so it is slow and challenging The hydrocarbon species injected into the system directly (so far) do not show large contamination There are bad components and there are bad chambers, but it is a challenge to see any effect of resist outgassing causing the contamination Either by injecting the outgassed species Or by witness plate exposures of mirrors near exposed resist 18

19 CNSE SEMATECH ASML Qimonda AMD IBM Acknowledgements Chimaobi Mbanaso, Justin Waterman, Leonid Yankulin, Alin Antohe, Yu-Jen Fan, Warren Montgomery, Robert Brainard, Greg Denbeaux Kim Dean, Kevin Orvek, Andrea Wüest Bill Pierson, Thomas Laursen, Sang-In Han, Robert Routh, Kevin Cummings Yayi Wei Obert Wood Chiew-Seng Koay CXRO, Berkeley Eric Gullikson, Andy Aquila NIST DESY Charles Tarrio, Steven Grantham Saša Bajt 19