2 May May 2006 Determining Optical Constants for ThO 2 Thin Films Sputtered Under Different Bias Voltages from 1.2 to 6.5 eV by Spectroscopic Ellipsometry William R. Evans, David D. Allred Brigham Young University International Conference on Metallurgical Coatings and Thin Films

2 May Our Goal – EUV Applications Extreme Ultraviolet Optics has many applications. These Include: –EUV Lithography –EUV Astronomy –Soft X-ray Microscopes A Better Understanding of materials for EUV applications is needed. EUV Lithography EUV Astronomy The Earth’s magnetosphere in the EUV Soft X-ray Microscopes

2 May 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.

2 May 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 torr. Bias voltages were between 0 and -70 V DC.

2 May 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.

2 May 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.

2 May 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.

2 May Results: n

2 May n not related to Bias Voltage or Thickness

2 May Results: Absorption There is a narrow absorption feature at about 6.2 eV, with full width half max of about 0.4 eV.

2 May 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.

2 May What we think might be going on... If the middle band were centered at about -9.8 eV in stead of 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.

2 May Conclusions First of all, we have shown that reactive 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.

2 May Acknowledgements R.S. Turley and The BYU EUV Thin Film Optics Group, past and present Student Travel Scholarship from ICMCTF BYU Department of Physics and Astronomy BYU Office of Research and Creative Activities Rocky Mountain NASA Space Grant Consortium Kristin Evans