Dual-function Reflectance Standards for VUV and EUV Optics

1. Preparation, Storage, Characterization and Use of Two-surface Reflectance Standards for VUV and EUV Optics David D. Allred1, , E. Strein1, Nicole Brimhall1, Zach Strother2 and R. Steven Turley 1 Brigham Young University and 2Georgia Institute of Technology

3. IMAGE Extreme Ultraviolet Imager http://euv.lpl.arizona.edu/euv/index.html IMAGE Extreme Ultraviolet Imager Earth's plasmasphere at 30.4 nm. This image from the Extreme Ultraviolet Imager was taken at 07:34 UTC on 24 May 2000, at a range of 6.0 Earth radii from the center of Earth and a magnetic latitude of 73 N. The Sun is to the lower right, and Earth's shadow extends through the plasmasphere toward the upper left. The bright ring near the center is an aurora, and includes emissions at wavelengths other than 30.4 nm. (From Sandel, B. R., et al., Space Sci. Rev., 109, 25, 2003.) HeII is singly ionized He. Trapped in the Earth’s Magnetosphere it scatters light from sun’s corona due to 1s to 2p transition

4. Reflecting at 30.4 & Antireflecting at 58.4 nm   D.D. Allred, R. S. Turley, M. B. Squires, “Dual-function EUV multilayer mirrors for the IMAGE mission,” in EUV, X-Ray and Neutron Optics and Courses, Carolyn A. Macdonald, Kenneth A. Goldberg, Juan R. Maldonado, H. Heather Chen-Mayer, Stephen P. Vernon, Editors, Proceedings of SPIE Vol. 3767, 280-287 (1999). pdf

5. EUV Lithography EUV Astronomy Soft X-ray Microscopes The Earth’s magnetosphere in the EUV Our Goal – EUV Applications: there are scientists in China & US wanting Earth-observing Lunar. • Extreme Ultraviolet Optics has several applications. • These Include: • EUV Lithography • EUV Astronomy • Soft X-ray Microscopes • A Better Understanding ofmaterials for EUV applications is needed.

6. Introduction: Extreme Ultraviolet Optics and Optical Constants • Optical constants of compounds in the EUV are typically unknown, incomplete, or inaccurate. • This can be important for those designing EUV optics for applications such as astronomy, lithography, or microscopy. Two examples • IMAGE satellite 2000 (above) • ThO2 optical constants (right)

7. BYU researchers have addressed the following concerns for those who would do some VUV/EUV (Vacuum/Extreme Ultraviolet) optics in a conventional laboratory as opposed to a large synchrotron facility: • Standards- Zach Strother (NSF-REU 07) • Mirror cleaning and storage. Liz Strein • Modeling • Sources/ spectrometers

8. Standards: Desirable Qualities • Robust • Stable • Cleanable • Cheap • Easy to Fabricate • Easy to Characterize • First studied is SiO2 on Si- that is (thermally oxidized Si)

9. J. Tveekrem, “Contamination effects on EUV optics,” NASA Technical Report TP-1999-209264, 1999. Used with permission.

10. Calculated reflectance for 41.3nm (30 eV) light on silicon reflected light incident light organic SiO2 Si reflectance Workers in x ray and EUV frequently measure from glancing angle as in figure. E. Gullikson, X-Ray Interactions with Matter, http://henke.lbl.gov/optical_constants Accessed 27 Feb 2008. (calculated with the bilayer program)

11. Outline of cleaning section • Motivation: Dirt is clear in EUV • Techniques/Methods: VUV lamp • Results

12. Instrumentation Excimer UV lamp (cleans samples) X-ray Photoelectron Spectrometer (XPS) Evactron Plasma Cleaner (cleans XPS antechamber) Ellipsometer

13. Excimer Lamp • Cleaning technique • The excimer lamp creates ozone and atomic oxygen by exposing oxygen to 172nm photons. O • These products oxidize the adventitious carbon on the samples thus freeing the sample of its organic contamination organic SiO2 Adapted from http://ecl.web.psi.ch/NanoKat/Ni_Al2O3_ethanol_1.jpg

14. Ellipsometry • Looks at how polarized light changes when it reflects from a surface. • Used to determine the relative change in thickness for the “apparent oxide” on a sample organic “apparent oxide” layer SiO2 Si Substrate Adapted from http://users.aber.ac.uk/tej/ellipso5.gif

15. Before excimer lamp Si 2p After excimer lamp Si 2p

16. Correlation between characterization methods

17. how the “apparent oxide” thickness decreases with exposure time

18. Application to Source development

20. Comparison after cleanup

22. Take-home message of cleaning portion • 5 min under lamp cleans off most of the last couple of angstroms of AC • Correlation between characterization techniques (there are big problems when the characterization instruments change the nature of a sample) • Cleanliness is important but can be achieved. • Cleanup right before measurement is important.

24. Sources of EUV light • Synchrotron Source • High flux • Wide, continuous wavelength range • Not local, expensive to run, large footprint • Fixed polarization • High Harmonics: femtosecond laser • Fairly high flux • Wide wavelength range, good spacing of wavelengths throughout the range • Local • Easily rotatable linear polarization • Plasma Source • Low Flux • Wide wavelength range, only a few wavelengths in the range • Local • Unpolarized

25. Overview and Conclusions • We have constructed an extreme ultraviolet (EUV) polarimeter that employs laser-generated high-order harmonics as the light source. • This instrument represents a potential ‘in-house’ instrument at facilities developing EUV thin films. • The source has high flux, a wavelength range from 8-62 nm, and easily rotatable linear polarization. • The instrument has a versatile positioning system and can measure reflectance of multiple wavelengths of light simultaneously. • We have compared reflectance data with that taken at the Advanced Light Source (ALS) and with calculated data. These measurements agree well.

26. Introduction: Extreme Ultraviolet Optics and Optical Constants • Optical constants in the EUV are typically unknown, incomplete, or inaccurate. • This is important for those designing EUV optics for applications such as astronomy, lithography, or microscopy. Two examples • IMAGE satellite 2000 (above) • ThO2 optical constants (right)

27. Optical Constants EUV light sample incident angle (Θ) Optical constants are determined by measuring reflectance as a function of angle of a sample at a fixed wavelength and polarization, then fitting this data to the Fresnel equations.

28. Sources of EUV light • Synchrotron Source • High flux • Wide, continuous wavelength range • Not local, expensive to run, large footprint • Fixed polarization • High Harmonics • Fairly high flux • Wide wavelength range, good spacing of wavelengths throughout the range • Local • Easily rotatable linear polarization • Plasma Source • Low Flux • Wide wavelength range, only a few wavelengths in the range • Local • Unpolarized

29. High Harmonic Generation EUV Generation EUV Grating 800 nm, 30 fs, 10 mJ Laser Pulses EUV Light MCP Detector Gas (He, Ne, Ar) • Wavelength range from 8-62 nm • Flux of 6x108 photons/second • Easily rotatable linear polarization • Fairly high flux • Wide wavelength range with good spacing of wavelengths within the range • Easily rotatable linear polarization • Small footprint, low cost of operation • Potential ‘in-house’ instrument at facilities developing EUV thin films λ = 800 nm / q Orders 37 to 77 Wavelengths of 10-22 nm

30. Instrument Overview EUV generation f=100 cm focusing lens dual rotation stages turbo pumps secondary gas cell gas (He, Ne, Ar) sample 800 nm, 30 fs, 10 mJ laser pulses EUV grating aperture rotatable half-wave plate turbo pump turbo pump MCP CCD • Easily rotatable linear polarization • Ability to measure reflectance of multiple wavelengths simultaneously • Extensive scanning ability

31. Polarimeter Positioning System • The positioning system is made up of six motors, each controlled by a single computer. • The diffraction grating is placed after the sample, allowing simultaneous reflectance measurements at multiple wavelengths.

32. Controlled Harmonic Attenuator 90% 90% secondary gas cell 0.01% 0.01% We increase the dynamic range of our detection system with a secondary gas cell that acts as a controlled harmonic attenuator.

33. Laser Power Discriminator Stability of our high harmonic source is important to the accuracy of polarimetry measurements. • Shot-to-shot variations in the laser pulse energy lead to about 37% variation in harmonic signal. • Averaging 100 shots decreases variation to about 7%. • To further increase repeatability, we implemented a laser energy discriminator, decreasing variations to about 2%. A sample of the incident laser beam is imaged in real time simultaneously with harmonics to provide per-shot energy monitoring

34. Reflectance Measurements • Sample: • thermally oxidized silicon, 27.4 nm SiO2 layer. • High-harmonic generation parameters: • 100 torr helium gas • Measurement parameters: • all measurements averaged over 100 shots where the variation in the laser power was +/-5% • secondary gas cell pressures ranged in value from 0 to 2.8 torr (attenuation of about 3 orders of magnitude) • dark signal taken simultaneously with measurements • measurements taken on three separate days to examine possible systematics in repeatability.

35. Compare

36. Conclusions • We have constructed a new instrument that uses high-order harmonics to measure optical properties of materials in the EUV. • Our source has a wide wavelength range, high flux, and easily rotatable linear polarization. • Our instrument has a sophisticated positioning system and is efficient in that simultaneous reflectance measurements can be made at multiple wavelengths. • We have compared reflectance measurements with those taken at the ALS and computed data. These measurements agree.

37. Future Work • Investigate a new measurement technique • In some regions where reflectance is very low, it may be difficult to measure absolute reflectance accurately (at near-normal angles, absolute reflectance is often on the order of 10-4). • It may, however, be possible to measure a very accurate ratio of p- to s-polarized reflectance. Our instrument has the capability to quickly toggle between polarizations to measure a very accurate ratio. • Variation in the laser source or harmonic generation parameters over time scales longer than minutes will no longer be a concern. Also, dynamic range issues will no longer be a problem. • Measure optical properties of materials in this wavelength range • Optical constants • Bonding effects on optical properties • Oxidation rates • Roughness effects

38. Thank you We would like to recognize NSF grant PHY-0457316 and Brigham Young University for supporting this project.

39. High Harmonics: Schematic

40. High Harmonics: Horizontal Wavelength Sorting • Helium Gas

41. Synchrotron vs. Polarimeter Data

42. But there were some steps along the way:

43. J. Tveekrem, “Contamination effects on EUV optics,” NASA Technical Report TP-1999-209264, 1999. Used with permission.

44. Calculated reflectance for 41.3nm (30 eV) light on silicon reflected light incident light organic SiO2 Si reflectance E. Gullikson, X-Ray Interactions with Matter, http://henke.lbl.gov/optical_constants Accessed 27 Feb 2008. (calculated with the bilayer program)

45. Instrumentation X-ray Photoelectron Spectrometer (XPS) Excimer UV lamp (cleans samples) Evactron Plasma Cleaner (cleans XPS antechamber) Ellipsometer

46. Excimer Lamp • Cleaning technique • The excimer lamp creates ozone and oxygen radicals by exposing oxygen to 172nm photons. O • These products oxidize the adventitious carbon on the samples thus freeing the sample of its organic contamination organic SiO2 Adapted from http://ecl.web.psi.ch/NanoKat/Ni_Al2O3_ethanol_1.jpg

47. Ellipsometry • Looks at how polarized light changes when it reflects from a surface. • Used to determine the relative change in thickness for the “apparent oxide” on a sample organic “apparent oxide” layer SiO2 Si Substrate Adapted from http://users.aber.ac.uk/tej/ellipso5.gif

48. X-ray Photoelectron Spectroscopy(XPS) • Detects the speed of electrons ripped off from a sample’s surface by x rays. • Used to determine the chemical composition of a sample. http://www.almaden.ibm.com/st/scientific_services/materials_analysis/xps/XPS.gif

49. Need for Evactron: Deposition rate on the samples exposed to the XPS antechamber

50. Evactron C DeContaminator • Plasma clean the XPS chamber http://www.evactron.com/63193/image2.gif