1. 16 Aug 2006 16 Aug 2006 Optical Constants of Sputtered Thoria Thin Films Useful in EUV Optics from IR to EUV David D. Allred Brigham Young University

2. 21 Feb. 20072 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 of EUV Optics & Materials for EUV applications is needed. EUV Lithography EUV Astronomy The Earth’s magnetosphere in the EUV Soft X-ray Microscopes

3. 21 Feb. 20073 Participants 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.)

4. 21 Feb. 20074 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. 21 Feb. 20075 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. 21 Feb. 20076 Extreme Ultraviolet Optics—What is our end goal? Multilayer Mirrors Lithography Astronomy Microscopy

7. 21 Feb. 20077 Optics like n-IR, visible, & n- UV? First you need a light.

8. 21 Feb. 20078 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. 21 Feb. 20079 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. 21 Feb. 200710 Reflectance (normal)

11. 21 Feb. 200711 Complex Index of Refraction Real n Complex n=1-δ+iβ β = k

12. 21 Feb. 200712 Multilayer Mirrors Problems –Need constructive interference –Absorption in layers

13. 21 Feb. 200713 Image Mirror

14. 21 Feb. 200714 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. 21 Feb. 200715 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. 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. 21 Feb. 200717 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 ThO 2 has entrée.

18. 21 Feb. 200718 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. 21 Feb. 200719 Transmission  k? T = (Corrections) exp (-αd); Corrections are due to R and can be small At normal incidence R goes as [  2 + β 2 ]/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. 21 Feb. 200720 Measurements of reflectance and transmittance ~20 nm reactively sputtered ThO 2 on a polyimide membrane ( ~100 nm, Moxtek ) and a naturally oxidized silicon substrate.

21. 21 Feb. 200721 Better procedures for fitting  Take several measurements—use each measurement to constrain those parameters to which it is most sensitive

22. 21 Feb. 200722 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. 21 Feb. 200723 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. 21 Feb. 200724 A second method that worked  Thorium dioxide deposited on AXUV-100 silicon photodiodes (IRD).

25. 21 Feb. 200725 Verification and a surprise  In delta: a peak shift to lower energies by 3 eV from 92.8 eV

26. 21 Feb. 200726 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. 21 Feb. 200727 Summary and Conclusions  We report the optical constants of ThO 2 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. 21 Feb. 200728 Transmission thru a film on PI

29. 21 Feb. 200729 But reflectance is a problem

30. 21 Feb. 200730 The problem is waviness of substrate. Sample on Si does fine.

31. 21 Feb. 200731 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. 21 Feb. 200732 Our group’s 1 st approach 1.Measure the reflectance of the coated diode at the same time I am measuring the transmission. And 2.Measure both as a function of angle. And 3.Get the film thickness from the (R and T data) to check ellipsometry of witness.

33. 21 Feb. 200733 Fitting T(  ) to get dead layer thickness (6-7nm) on bare AXUV diode @ =13.5nm

34. 21 Feb. 200734 Focusing on the high reflectance & transmission had a problem

35. 21 Feb. 200735 Comments 1.Either T or R have n and k data, but 2.Transmission has very little n data when δ is small (the EUV). 3.Reflection  n, k and when interference fringes are seen, and 4.It has thickness (z) data. What follows shows how we confirmed thickness for air-oxidized Sc sputter- coated AXUV diodes.

36. 21 Feb. 200736 Our recent group’s approach 1.Measure the reflectance of the coated diode at the same time I am measuring the transmission. And 2.Measure both as a function of angle. And 3.Get the film thickness from the (R) interference fringes (@ high angles).

37. 21 Feb. 200737 Interference in R (50<φ<70 0 )  z fit =19.8 nm @ =4.7 nm

38. 21 Feb. 200738 The complete set of R data (6<θ<20 0 ) z fit =28.1 nm @ =4.7 nm

39. 21 Feb. 200739 We might gone with z= 24 nm, but

40. 21 Feb. 200740 We looked at another = 7.7nm; needs z=29 nm

41. 21 Feb. 200741 And the =4.7nm data is OK

42. 21 Feb. 200742 Reflectance and transmittance of a ThO 2 -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. 21 Feb. 200743 R &T of a ThO 2 -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. 21 Feb. 200744 “Intermediate Conclusions” 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.

45. 21 Feb. 200745 ThO 2 A number of studies by our group have shown that thorium and thorium oxide (ThO 2 ) have great potential as highly reflective coatings in the EUV. In certain regions, ThO 2 may be the best monolayer reflector that has yet been studied.

46. 21 Feb. 200746 XUV Optics Production Sputtering or Evaporation

47. 21 Feb. 200747 Biased Sputtering Our films were deposited by biased RF Magnetron Sputtering. ThO 2 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. 21 Feb. 200748 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. 21 Feb. 200749 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. 21 Feb. 200750 Data Fitting The data were modeled using the J. A. Woollam ellipsometry software. –n is modeled parametrically using a Sellmeier model which fits ε 1 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 ε 2 with parameterized oscillators.

51. 21 Feb. 200751 Results: n

52. 21 Feb. 200752 n not related to Bias Voltage or Thickness

53. 21 Feb. 200753 Results: Absorption There is a narrow absorption feature at about 6.2 eV, with full width half max of about 0.4 eV.

54. 21 Feb. 200754 Comparing to the Literature In reviewing the literature, there seems to be a couple of different band gaps that people detect: Graphic From: Rivas-Silva, et. al.

55. 21 Feb. 200755 Comparing to the Literature Sviridova & Suikovskaya measured sample thickness and absorption for several different wavelengths near where thorium goes transparent, for thorium chloride and thorium nitrate. From this we determine a band gap of ~5.92 eV. We obtained a value in the same range.

56. 21 Feb. 200756 Comparing to the Literature Mahmoud reports a very clear band gap of about 3.84 eV. However, his samples were deposited on glass of unspecified composition.

57. 21 Feb. 200757 What we think might be going on... If the middle band were centered at about -9.8 eV in stead of -11.8 eV, the ~6 eV band gap reported in the majority of the thin film sources would be explained as a jump from the valence band to the middle band. Also, if the conduction band started at about -6 eV in stead of about -7 eV, the ~4 eV band gap reported by Mahmoud and others could be explained by a transition from the middle band, which had some electrons in it due to mild doping, transitioning into the conduction band.

58. 21 Feb. 200758 Measurements at 10.2 eV We used a McPherson Vacuum monochromator at BYU to measure optical constants of our ThO 2 thin films at the 10.19 eV Kα line of Hydrogen.

59. 21 Feb. 200759 Measurements at 10.2 eV

60. 21 Feb. 200760 Measurements at 10.2 eV

61. 21 Feb. 200761 Conclusions: Vis & UV First of all, we have shown that DC Biased sputtering cannot be expected to significantly affect the optical constants of ThO 2 thin films. –This is not surprising considering the extremely high melting point of ThO 2. Secondly, exactly what is going on with the band gap of ThO 2 is still not really understood. –It appears that there are two fundamental band gaps in ThO 2, but more research is needed. –We are in the process of making additional measurements on ThO 2 between 6 and 9 eV.

62. 21 Feb. 200762 Questions?

63. 21 Feb. 200763 EUV measurements Our project was to see if we could get n as well as k from samples set up to measure transmission in the EUV. The films were deposited directly on Absolute EUV silicon photodiodes. IRD

64. 21 Feb. 200764 Total Reflection Snell’s Law Total Internal Reflection Total External Reflection n1n1 n2n2 n2n2 n1n1