Optical Constants of Sputtered Thoria Thin Films Useful in EUV Optics from IR to EUV

1. Optical Constants of Sputtered Thoria Thin Films Useful in EUV Optics from IR to EUV David D. Allred Brigham Young University

2. EUV Lithography EUV Astronomy Soft X-ray Microscopes The Earth’s magnetosphere in the EUV Our Goal – EUV Applications • Extreme Ultraviolet Optics has many applications. • These Include: • EUV Lithography • EUV Astronomy= image mission • Soft X-ray Microscopes • A Better Understanding ofEUV Optics & Materials for EUV applications is needed.

3. William R. Evans: senior (honors) thesis: Spectroscopic ellipsometry 1- 6.5 eV Niki F. Brimhall: senior (honors) thesis: EUV optical constants of Thoria (also Guillermo Acosta & Jed E. Johnson. ) Sarah C. Barton 10.2 eV reflectance (Monarch) R.S. Turley: most everything spectroscopic >10 eV. Michael Clemens: AFM XPS Amy B. Grigg and The BYU EUV Thin Film Optics Group, past and present who went to ALS : Jacque Jackson, Elise Martin, Lis Strein, Joseph Muhlestein Dr. Thomas Tiwald: JA Woollam Co: interpretation & extending range of Spec. Ellips. To IR and 9.5 eV Matt Linford’s Group (Chem.) Participants

4. Financial + Other Assistance • BYU Department of Physics and Astronomy shop & electronics • BYU Office of Research and Creative Activities • Rocky Mountain NASA Space Grant Consortium • V. Dean and Alice J. Allred, Marathon Oil Company • ALS time (DOE) and help @ beamline 6.3.2: Eric Gullikson, Andy Aquila

5. Outline • XUV Optics • Applications -Production • Review of Optics for EUV/ x-rays (E>15 eV) • Why Actinides in EUV? Why Oxides? • besides ML there are low-angle front surface mirrors • Optical constants from R and T • Measuring XUV OC with Reflectance & Transmission on “Absolute” X-ray diodes • Real Surfaces: Characterizing & Improving them. • Spectroscopic Ellipsometry 1- 6.5 eV: • Thoria has some leftover problems in solid state. • Index and band gap.

6. Extreme Ultraviolet Optics—What is our end goal? Multilayer Mirrors Astronomy Lithography Microscopy

7. Optics like n-IR, visible, & n-UV? First you need a light.

8. Optics like n-IR, visible, & n-UV? • How to manipulate light? • Lens? Prisms? Mirrors? Diff Gratings? ML interference coatings? • We need to have optical constants; • How to get in EUV? • Kramers-Kronig equations n () k () • Variable angle of reflection measurements, • Real samples aren’t good enough. Roughness

9. Absorption and Refraction • Optical properties characterized by index of refraction n • Visible • n real (often >>1) • n >0 (total internal reflection) • XUV and X-Rays • n complex; n=1-δ+iβ • Re(n) < 1 (but not by much)

10. Reflectance (normal)

11. Complex Index of Refraction • Real n • Complex n=1-δ+iβ • β = k

12. Multilayer Mirrors • Problems • Need constructive interference • Absorption in layers

13. Image Mirror

14. U/Si ML coating for EUV instrument • Picture (41 eV) is from EUV imager on the IMAGE Spacecraft. He (II) in magnetosphere • This was student powered project 1997-98 • Designed: needed 7 degree width off normal, 7.5 layer U/Si ML with U Oxide cap- peak R 25% • Coated & • Tested • Launched 2000 March 25

15. EUV Multilayer Optics 101 High reflectivity multilayer coatings require: • Refractive index (n = 1-δ+iβ) contrast at the interfaces: for most materials, these optical constants are not well known in this region. • Minimal absorption in the low-Z material • Interfaces which are chemically stable with time • Minimal interdiffusion at the interfaces • Thermal stability during illumination • Chemically stable vacuum interface Even with the very best designs, multilayer mirrors have only achieved a reflectivity of around 70% in the EUV.

16. The solution? Research of new materials with these properties Uranium: • Highly reflective in the region from 124-248 eV [1] • Not chemically stable with time Uranium Oxide: • Highly reflective in the region from 124-248 eV [1] • Not chemically stable with time Thorium: • Highly reflective in the region from 138-177 eV [2] • Not chemically stable with time, tho better than U. [1] RL Sandberg, DD Allred, JE Johnson, RS Turley, " A Comparison of Uranium Oxide and Nickel as Single-layer Reflectors", Proceedings of the SPIE, Volume 5193, pp. 191-203 (2004). [2] J. Johnson, D. Allred, R.S. Turley, W. Evans, R. Sandburg, “Thorium-based thin films as highly reflective mirrors in the EUV”, Materials Research Society Symposium Proceedings 893, 207-213, 2006.

17. Solutions • n=1-δ+iβ • Find materials with big δ and small β • Good candidates: High Density, High -Z materials like U. But Oxidation occurs. • Th as ThO2 has entrée.

18. How to Get OC from Data • Measure reflectance and/or transmission • Multiple wavelengths • Multiple angles • Fit data to a theoretical Model • film thicknesses • optical parameters • But reflectance is sensitive to surface- inhomogeneities roughness; oxidation

19. Transmissionk? • T = (Corrections) exp (-αd); • Corrections are due to R and can be small • At normal incidence R goes as [2 + β2]/4 • If film is close to detector scattering due to roughness etc. is less important. • But how to get an even, thin film? • A very thin membrane?

20. Measurements of reflectance and transmittance ~20 nm reactively sputtered ThO2 on a polyimide membrane (~100 nm, Moxtek) and a naturally oxidized silicon substrate.

21. Better procedures for fitting • Take several measurements—use each measurement to constrain those parameters to which it is most sensitive

22. A major problem with our first try • Measurements of thorium dioxide deposited on polyimide films gave unreliable data. Reflectances measured with different filter sets differed by as much as 32% of total reflectance. Absolute transmission measurements were uncertain by as much as 19%.

23. Optical Constants Even though our absolute transmission was uncertain to this degree, the energy of the incident light was known to 0.012%, and so even if the exact values of delta and beta are off, the edges won’t be.

24. A second method that worked • Thorium dioxide deposited on AXUV-100 silicon photodiodes (IRD).

25. Verification and a surprise • In delta: a peak shift to lower energies by 3 eV from 92.8 eV

26. Verification and a surprise • In beta: absorption edge shifts to lower energies from those of thorium by 4 eV from 105.6 eV and 2 eV from 91.5 eV

27. Summary and Conclusions • We report the optical constants of ThO2 from 50- 108 eV • We have used constraining techniques to fit optical constants including fitting film thickness using interference fringes in highly transmissive areas of the spectrum and fitting reflectance and transmittance data simultaneously • In delta we observed a peak shift to lower energies from that of thorium by 3 eV from 92.8 eV • In beta we observed absorption edge shifts to lower energies from those of thorium by 4 eV from 105.6 eV and 2 eV from 91.5 eV

28. Transmission thru a film on PI

29. But reflectance is a problem

30. The problem is waviness of substrate. Sample on Si does fine.

31. The Solution: Deposit the film on the detector • Uspenskii, Sealy and Korde showed that you could deposit a film sample directly onto an AXUV100 silicon photodiode (IRD) and determine the films transmission ( by ) from the ratio of the signals from various coated diodes with identical capping layers. • JOSA 21(2) 298-305 (2004).

32. Our group’s 1st approach • Measure the reflectance of the coated diode at the same time I am measuring the transmission. And • Measure both as a function of angle. And • Get the film thickness from the (R and T data) to check ellipsometry of witness.

33. Fitting T() to get dead layer thickness (6-7nm) on bare AXUV diode @=13.5nm

34. Focusing on the high reflectance & transmission had a problem

35. Comments • Either T or R have n and k data, but • Transmission has very little n data when δ is small (the EUV). • Reflection  n, k and when interference fringes are seen, and • It has thickness (z) data. What follows shows how we confirmed thickness for air-oxidized Sc sputter-coated AXUV diodes.

36. Our recent group’s approach • Measure the reflectance of the coated diode at the same time I am measuring the transmission. And • Measure both as a function of angle. And • Get the film thickness from the (R) interference fringes (@ high angles).

37. Interference in R (50<φ<700)  zfit=19.8 nm @ =4.7 nm

38. The complete set of R data (6<θ<200) zfit =28.1 nm @ =4.7 nm

39. We might gone with z= 24 nm, but

40. We looked at another = 7.7nm; needs z=29 nm

41. And the =4.7nm data is OK

42. Reflectance and transmittance of a ThO2-coated diode at 15 nm fitted simultaneously to obtain n&k • Green (blue) shows reflectance (transmission) as a function of grazing angle ()* • Noted the interference fringes at higher angles in R. * is always from grazing incidence

43. R &T of a ThO2-coated diode at 12.6 nm fitted simultaneously to obtain optical constants. • The fits were not very good at wavelengths where the transmission was lower than 4%. • All of these fits were trying to make the fit of transmission narrower than the data was.

44. Thin films of scandium oxide, 15-30 nm thick, were deposited on silicon photodiodes by Sputtering Sc from a target & letting it air oxidize OR reactively sputtering scandium in an oxygen environment. Similar thing was done with Thorium to make Thoria R and T Measured using synchrotron radiation at the als (Beamline 6.3.2), at LBNL over wavelengths from 2.5-40 nm at variable angles, were taken simultaneously. “Intermediate Conclusions”

45. ThO2 • A number of studies by our group have shown that thorium and thorium oxide (ThO2) have great potential as highly reflective coatings in the EUV. • In certain regions, ThO2 may be the best monolayer reflector that has yet been studied.

46. XUV Optics Production • Sputtering or Evaporation

47. Biased Sputtering • Our films were deposited by biased RF Magnetron Sputtering. • ThO2 was reactively sputtered off of a depleted thorium target with oxygen introduced in the chamber. • Chamber sputtering pressures were about 10-4 torr. • Bias voltages were between 0 and -70 V DC.

48. Film Characterization • Film composition was measured using x-ray photoelectron spectroscopy. Th % stayed between 60% and 70% with oxygen making up the balance of the composition. Only traces of other elements were detected. • X-ray diffraction was used 1) as a first measurement of film thickness and 2) to measure crystal structure. Orientations (111), (200), (220), and (311) were clearly visible, with other orientations being largely absent.

49. Spectroscopic Ellipsometry • Optical characteristics were measured using spectroscopic ellipsometry in the visible and near UV. • Ellipsometric data were taken from samples deposited on silicon between 1.2 and 6.5 eV at angles of every degree between 67° and 83°. • Normal incidence transmission data were taken over the same range of energies, from samples deposited on quartz slides.

50. Data Fitting • The data were modeled using the J. A. Woollam ellipsometry software. • n is modeled parametrically using a Sellmeier model which fits ε1 using poles in the complex plane. • The Sellmeier model by itself doesn’t account for absorption. (i.e. All poles are real.) • k can be added in separately, either by fitting point by point, or by modeling ε2 with parameterized oscillators.