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Figure 2.2: Rutherford Back-Scattering, where projectile M1 with energy E1 hits target atom M2 scattering M1 at an angle θ with energy E2 . This equation is only valid if the angle between incident/scattered directions δ is very small and at the outermost surface of the sample. At greater depths stopping power leads to depth information. Stopping power causes the projectiles to loss energy in form of excitation or ionization of target electrons, therefore changing the kinematic ratio and target atom. Without any knowledge about the composition of the sample, it is hard to determine the target atom. Stopping power is a statistic process resulting in straggling. Counts/coulomb 200 160 120 80 40 0 E 0 1000 2000 3000 4000 4 MeV 4 He 165 E Silicon substrate Backscatter Energy (keV) C 14 at.% 17 at.% 0 500 1000 Channel - 10 - Thickness W 18 at.% Figure 2.3: An RBS spectrum of a 18% a-C:W film on a silicon substrate. The general thickness (atoms/cm 2 ) as well as the concentration can be determined by fitting a simulated spectrum.
Figure 2.3 shows a spectrum of a 18% a-C:W film on a silicon substrate. The atomic mass difference of the target atoms lead to different peak positions within the spectrum. The different target atoms and their concentration as well as the general layer thickness of the film can be obtained by fitting a simulated spectrum to the measured data. SIMNRA (SIMulated Nuclear Reaction Analysis) is such a program capable of simulating spectra to the measured data and is used in this thesis . In figure 2.3 two different simulated spectrums have been fitted to the measured data. A homogenous 18 at% W concentrated film is expressed by the red line, whereas the green line expresses a rough simulated fit to the measured data. As observed for the samples, the concentration in the lower layer is smaller than the surface concentration, due to start-up process of the sputter device. The specimens of the different production runs are named by their concentration levels at the surface, e.g. 18% a-C:W stands for 18 at% tungsten-doped amorphous carbon film. The lowering of the concentration as function of depth has to be kept in mind . 2.3 Scanning Electron Microscopy (SEM) A scanning electron microscope is a microscope that uses electrons rather than light to image surfaces. Since electrons are used, SEM has to operate in a vacuum environment and the specimen must have a certain electrical conductivity. The electrons produced by an electron gun are focused using electromagnetic lenses. Deflector plates are used to scan the very narrow electron beam in x and y direction to allow scans in raster pattern. The electrons interact with the atoms of the sample, producing signals containing information about the sample’s surface. The signals produced after interactions include: secondary-electrons (SE), Backscattered electrons (BSE) and Energy Dispersive X-ray (EDX). The signal are captured by detectors and processed into an image. - 11 -