Nanostructured thin films and surfaces have been attracting increased attention in recent years. Their range of applications includes many different fields, from porous materials, metals, and semiconductors to (bio-)polymers and soft matter materials. Classical nanoscale structural methods, such as atomic force microscopy (AFM) and transmission electron microscopy (TEM), provide highly precise local information about the nanostructured surface. However, with these methods, averaged (i.e. representative) results can hardly be obtained from a sample. Grazing-incidence small-angle X-ray scattering (GISAXS) ideally complements these microscopic methods since it readily provides representative structural information for a large sample area.
Grazing-incidence small-angle X-ray scattering (GISAXS)
Basics, applications, and complementary methods
The GISAXS method was originally introduced in 1989 by Joanna Levine and Jerry Cohen[1]. Since then, with the growing interest in studying the surface structure of nanosized thin films it has developed to be a frequently used scattering technique. GISAXS applications include the characterization of mesoporous thin films, surface-deposited nanoparticles, metal deposits on oxide surfaces, and – more recently – soft matter systems such as polymer/block copolymer thin films and biological materials which are attached to surfaces. More detailed information on GISAXS principles and applications can be found in [2] and [3].
GISAXS analyzes density correlations and the shape of nanostructured objects at surfaces or at buried (surface-near) interfaces. The GISAXS method combines features from small-angle X-ray scattering and diffuse X-ray reflectivity.
The GISAXS method analyzes scattering data at small scattering angles, typically up to 5° 2q Grazing-incidence wide-angle X-ray scattering (GIWAXS) uses the same principles of analysis and evaluation, but at wide scattering angles. Grazing-incidence X-ray diffraction (GIXD) is another related technique which is used to characterize crystalline thin-film structures. It analyzes Bragg reflections at higher scattering angles which correspond to distances at the atomic level.
Geometry of a GISAXS experiment
Fig. 1.: GISAXS geometry
The incident X-ray beam grazes the thin-film sample under a very small angle aI, typically below 1°. The advantage, particularly for thin films, is the limited penetration depth of the X-rays into the sample, with the benefit of low background scattering from the substrate. By varying the incident angle, the X-rays’ penetration depth can be changed from a few nanometers up to 100 nanometers. The scattered X-rays at small angles are recorded by a two-dimensional X-ray sensitive detector. Depending on the shape, size, and arrangement of objects at the surface, the GISAXS pattern comprises vertical (out-of-plane, qz) or lateral (in plane, qy) scattering intensities.
The GISAXS method has the following unique advantages:
- GISAXS provides averaged results which are representative of a large sample area.
- GISAXS in general requires no sample preparation.
- GISAXS studies can be performed in vacuum or under controlled atmosphere at ambient or non-ambient temperature.
GISAXS signatures
The resulting GISAXS pattern depends on the size, shape, and arrangement of the nanostructured surface.
Ordered normal and lateral density fluctuations
Fig. 2: GISAXS signatures of ordered density fluctuations
If the sample exhibits ordered lamellar structures, the scattering pattern shows distinct intensity stripes or peaks (Fig. 2). A classic example is PS-PMMA in a Si wafer covered with native oxide. If a sample has lamellae parallel to the substrate the GISAXS pattern exhibits intensity stripes at regular spacings along the out-of-plane axis. Samples with lamellae perpendicular to the substrate result in correlation peaks (typically rod-like shape) at regular spacings along the in-plane axis.
Disordered surface layers
In thick films the order induced at the interfaces may not prevail throughout the film and the interior of the film may exhibit a disordered three-dimensional powder bulk structure. The GISAXS pattern of such systems exhibits rings or partial rings (Fig. 3) which can indicate anything from complete disorder of the lamellar domains to partial order, such as lamellae with a finite distribution of tilt angles with respect to the interface.
Oriented thin films
Oriented (e.g. porous) thin films, such as mesostructured thin films of silica or other oxides, result in more complex diffraction patterns (Fig. 4).
Fig. 4: GISAXS signatures of oriented thin films
Fig. 3: GISAXS signatures of disordered surface layers
Conclusion
Compared to conventional scattering methods (small-angle X-ray, light, neutron scattering, and X-ray diffraction), GISAXS is a relatively new discipline which provides insights into the nanostructure of thin film samples. Today GISAXS is an important tool which provides representative information and therefore ideally complements high-resolution techniques such as AFM and TEM.
GISAXS covers many different applications, from porous materials and metals to polymers, soft matter materials, and many others. GISAXS is performed both on large synchrotron radiation beamlines and on laboratory GISAXS systems.
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References
- Levine, J. Cohen, J. Chung, Y. Georgopoulos, P. (1989). Grazing-incidence small-angle X-ray scattering: new tool for studying thin film growth. J. Appl. Cryst. (22), pp. 528–532.
- Smilgies, D. (2017). GISAXS – grazing-incidence small-angle scattering. In: The SAXS Guide, 4th ed. Graz: Anton Paar, pp. 109–123.
- Buschbaum, P. (2009). A basic introduction to Grazing-Incidence Small-Angle X-ray Scatterung, Lect. Notes Phys. (776), pp 61–89.
