In order to probe the structural details of near surface and surface regimes, the grazing incidence small angle scattering (GI-SAXS) was introduced in 1987. The variation of incidence angle was used for non-destructive depth profiling, but this physical phenomenon is also used for X-ray reflectivity (XRR) measurements, a technique widely used for determining roughness, periodicity and thickness of thin amorphous and crystalline layers perpendicular to the surface normal.
In contrast to XRR, GI-SAXS measurements provide more direct information about in-plane correlations and roughnesses, both important parameters for the characterization of multilayer coatings.
The following experiment aims to study the thermal stability and decomposition process of nearly perfect X-Ray mirrors.
Experiment and Sample Material
These GI-SAXS investigations were done using a NANOSTAR-U equipped with a micro-focus X-ray source (IpS, Incoatec GmbH, Geesthacht, Germany) and a 2-dimensional VANTEC-2000 detector as shown in Figure 1. A high temperature heating chamber (DHS 900, Anton Paar, Graz, Austria) was implemented into the beam path. The measuring time was 10s per frame and the heating rate was set to 30°C/minute. The incidence angle was fixed at 0.6° just below the critical angle of total external reflection.
While the sample temperature was increased from 40°C up to 870°C, a series of 172 frames was recorded. The studied samples were W/C multilayers and the coating was deposited on single crystalline Silicon wafers by Ar-plasma sputtering technology (Incoatec, Geesthacht, Germany). The period of multilayer stacking (thickness of W and C layer) was 1.375nm and 80 individual layers were deposited.
SAXS NT software was used to perform the data reduction of the 2D frames in three different ways. Arc-like integration of the I(qyqz) patterns enabled calculation of the I(qz1) scattering curves. An opening angle of 10° was used and qz=0 was set at the center of the detector where the direct beam was located.
The multilayer period and the vertical correlation length were determined using the I(qz1) scattering curves. I (qy) scattering curves were calculated by pixel-wise integration between qz= 0.63 A-1 and qz= 0.42A-1. The I(qy) scattering curves were used to determine the lateral correlation length and the fractal dimension . The fractal dimension and the Hurst parameter are equal. The multilayer period and the vertical correlation length were determined by fitting the correlation peak with a pseudo-Voigt function, where the position represents the period and the width correlates to the size of the coherent scattering domains.
By applying the Scherrer formula on the FWHM of the obtained peak, the dimensions of coherent domains was determined. The FWHM was corrected for geometrical effects (large sample size), but the beam divergence was not considered. From the multilayer period (d) and Scherrer size (S) the effective number of multilayer periods in coherent domains (neff) was derived by neff=S/d.
The lateral correlation length was obtained by using Guinier's approximation, and the fractal dimension (Dmass) was obtained by fitting qy data at a large q with an exponential function. The integral intensities, referred as Iint, were calculated by integrating the Lorenz corrected scattering curves between qmin = 0.2 nm-1 and qmax = 18 nm- 1.
Figure 1. NANOSTAR-U with IpS (left), extended sample chamber with WAXS Image Plate detector (center), and a 20 x 20 cm2 active area vANTEC-2000 (right).
Figure 2. Scattering pattern of untreated mirror (right) and after heat treatment up to 900°C (left). A: reflected beam, B: main reflection of the multilayer, C: uncorrelated scatter (GI-SAXS signal).
The scattering pattern of an untreated mirror is dominated by the signal from the primary reflected beam and the first main reflection of the multilayer as shown in Figure 2. After the heat treatment, the main reflection peak essentially disappears, while the intensity of the primary reflected beam and the diffuse scattering strongly increases.
Figure 3 shows results of a series of in-situ measurements while increasing the sample temperature. The effective number of layers in coherent domains remains almost constant up to 490°C. Above this temperature, a linear decrease in n(eff) is observed. In parallel, the period of 1.375nm also slowly decreases with increasing temperature. Above 700°C the period rapidly drops down to 1.15nm. Above 840°C the signal caused by the period of the multilayer disappears from GI-SAXS signal.
Figure 3. Effective number of layers in coherent domains (black dots, left y-scale) and integrated scattering SAXS signal (Iint) (red dots, right y-scale) plotted against temperature. The vertical lines indicate temperatures where main structural changes occur.
The above described characteristics are clearly confirmed by the temperature dependence of the fractal dimensions and lateral correlation lengths as shown in Figure 4. Up to 700°C, the vertical correlation length remains approximately constant at 8nm. Above 700 °C it rapidly decreases with increasing temperatures down to 2nm. At the same temperature, the mass fractal dimension also decreases from 1.8 down to 0.8.
Figure 4. Fractal dimension and vertical correlation length plotted against temperature.
The thermal decomposition of a W/C multilayer coating with a period of 1.375nm was studied using the NANOSTAR U. The initial outcome of corrosion of the multilayer was the formation of surface roughness of the top layers.
Above 490°C the lateral correlation lengths became shorter and so did the effective number of layers in the coherent domains. Above 690°C vertical correlation lengths and fractal dimensions also changed dramatically, which finally led to a complete loss of order in the multilayer.
The publication clearly shows that in-situ GI-SAXS investigations with good time resolutions and up to high temperatures are possible with the lab-based NANOSTAR-U Small Angle X-ray scattering system using a micro-focus X-ray source.
About Bruker X-Ray Analysis
Bruker X-Ray Analysis designs and manufactures analytical X-ray systems for elemental analysis, materials research and structural investigations.
Our innovative solutions enable a wide range of customers in heavy industry, chemistry, pharmacy, semiconductor, life science and nanotechnology to make technological advancements and to accelerate their progress.
This information has been sourced, reviewed and adapted from materials provided by Bruker X-Ray Analysis.
For more information on this source, please visit Bruker X-Ray Analysis.