Grazing Incidence X-ray Scattering and Diffraction on Thin Films

Introduction
Grazing incidence X-ray scattering or diffraction (GIXS) (fig.1) refers to a method where the incident X-ray beam makes a small (typically about 1 degree) angle to the sample surface. This has an advantage, which is particularly important for thin films, of limiting the penetration depth of the X-rays into the sample with consequently low background scattering from the substrate. Often, the exit angle is also small and, thus, the scattering vector, Q, is in the plane of the sample; one measures diffraction from planes perpendicular to the sample surface. This geometry is shown in Figure 1. By varying the incidence angle, one can change the penetration depth of the X-rays from several nm up to typically several 100 nm (determined by the absorption length). In some cases, it is useful to have a small incidence angle, but a large exit angle. This preserves the low background from the grazing incidence geometry, but allows measurements with Q tilted with respect to the sample surface (i.e., the diffraction planes are tilted).

Instrumentation
Beamline 7-2 is well suited for GIXS measurements and is frequently used for these measurements. Either Soller slits or fixed slits are most commonly used to analyze the scattered beam (depending on resolution and sample size), but for high resolution a crystal analyzer can be used. It is advantageous to have a flat surface with an area of about one cm².

Analysis
The data analysis details are dependent on the goals of the experiments (e.g., depth profiling, strain determination, defect characterization). Some guidance can be found in the classic X-ray texts (Warren, Alexander). Also, see the references below for details or contact Mike Toney or Sean Brennan for more guidance. Here is a calculation (excel file) of the incident X-ray penetration depth as a function of incidence angle. Many thanks to Michael Chabinyc (PARC) for this calculation.

Applications of GIXS for materials science
GIXS is often used to examine the near surface structure of materials. Figure 2 shows an example for rubbed polymer films used in flat panel displays [1]. This set of experiments showed that buffing causes an alignment of the polymer molecules in the near surface region, which explains the alignment of liquid crystals placed in contact with the rubbed polymer. GIXS is also used to profile the strain distribution in a variety of thin films [2,3]. Since GIXS probes structure along the film surface, one learns about structural correlations and defects along this direction. This includes domain sizes [4], stacking faults [5], and chemical order [6], which can have important effects on materials properties.

Recent work on beam lines 11-3 and 7-2 have focused on semiconducting polymer thin films [7, 8, 9], pentacene thin films [10, 11, Bi and Cu nanoparticles [12], and Eu thin films [13].

Further General Reading:

• "Surface Sensitive X-ray Scattering", PH Fuoss and S Brennan, Annual Rev. Mater. Sci. 20, 365 (1990).

1. "The Near Surface Alignment of Polymers", M.F. Toney, T.P. Russell, J.A. Logan, H. Kikuchi, J. Sands, S.K. Kumar, Nature 374, 709 (1995).


2. "Depth Profile Study of Ferroelectric PbZr0.2Ti0.8O3 Films", Y. Li, V. Nagarajan, S. Aggarwal, R. Ramesh, L. G. Salamanca-Riba, and L. J. Martinez-Miranda, J. Appl. Phys. 92, 6762 (2002).


3. "X-ray Measurements of the Depth Dependence of Stress in Gold Films", Physica B 283, 125 (2000).


4. "O Thomas, Q Shen, P Schieffer, N Tournerie, B Lepine, Phys Rev. Lett. 90, 017205 (2003).


5. "Effects of Stacking Faults on Magnetic Viscosity in Thin Film Magnetic Recording Media", P. Dova, H. Laidler, K. O'Grady, M.F. Toney, M.F. Doerner, J. Appl. Phys. 85, 2775 (1999).


6. "Thickness Dependence of Exchange Bias and Structure in PtMn and NiMn Spin Valves", M.F. Toney, M.G. Samant, T. Lin, D. Mauri, Appl. Phys. Lett. 81, 4565 (2002).


7. “The Effect of Molecular Weight of Regioregular Poly(3-hexylthiophene) on Its Film Morphology and Field-Effect Mobility”, R.J. Kline, M. D. McGehee, E. N. Kadnikova, J. Liu, J.M.J. Fréchet, M.F. Toney, Macromolecules, 38, 3312 (2005).


8. “Highly Oriented Crystals at the Buried Interface in Polythiophene Thin Film Transistors”, R.J. Kline, M.D. McGehee, M.F. Toney, Nature Materials, in press (2005).


9. “Liquid crystalline semiconducting polymers with High charge carrier mobility”, I. McCulloch, M. Heeney, C. Bailey, K. Genevicius, I. MacDonald, M. Shkunov, D. Sparrowe, S. Tierney, R. Wagner, W. Zhang, M.L. Chabinyc, R.J. Kline, M. D. McGehee, M.F. Toney, in press (2005).


10. “Structural Characterization of a Pentacene Monolayer on an Amorphous SiO2 Substrate with Grazing Incidence X-ray Diffraction” S.E. Fritz, S.M. Martin, C.D. Frisbie, M.D. Ward, M.F. Toney, J. Amer. Chem. Soc. 126, 4084 (2004).


11. “Crystal Structure of Thermally Evaporated Pentacene Films by Grazing Incidence X-ray Diffraction and Semiempirical Calculations of Intermolecular Electronic Structure”, S.E. Fritz, M.F. Toney, D. da Silva Filho, J.-L. Brédas, C.D. Frisbie, M.D. Ward, submitted to JACS (2005).


12. “Structure of Oxidised Bismuth Nanoclusters and Electromigration Damage in Gold Interconnects,” K.J. Stevens, K.S. Cheong, D.M. Knowles, S.A. Brown, J. Partridge, A. Ayesh, M.F. Toney, M. Ryan, B. Ingham, R. Kemp, and R. Young, submitted to Nanotechnology, (2005).


13. “Transition Magneto-Structural dans les Films D'Eu Epitaxies: Diffraction a haute Resolution et en Incidence Rasante (GIXS) “K. Dumesnil, M. Toney, C. Dufour, Ph. Mangin”, see http://www.lmov.uvsq.fr/Neel2005/imagesneel2005/Programme_CLN2005.pdf.