his case, gold (Au) and tungsten (W) are used because the difference of the atomic number is adequate to get a good material contrast by the following TEM investigations.
Another reason to choose these target materials was the fact that the characteristic x-ray peaks, which have the highest intensities, do not overlap and an EDX line scan can be performed through the multilayer system. Out of the EDX line scan the thickness values can be measured and compared by the result of the built-in quartz. The results can be seen in Table 1.
|Material||dLeica [nm]||dmeasured [nm]||dmeasured – dLeica [%]|
Tab. 1: Comparison of the measured thickness and the expectation value. dLeica = setted thickness, whereat the sputter process will stop in nm; dmeasured = arithmetic averaged thickness value of all measurement methods in nm; dmeasured – dLeica = difference between expectation value and measured value in %.
Improvement of the sputtered films
To get a smooth and even surface, the sputter parameters which have the most influence on the film growth are systematically varied.
Thickness, working distance, current and argon pressure are changed in a certain range. The effect on the morphology by changing these parameters is more precisely described in . The most important parameter which changes the morphology of the thin film is the argon pressure in the chamber of the coater. A decreasing of the argon pressure to values below 1e–2 mbar leads to a even film abruptly. This behaviour was observed for different target materials and it was shown that this effect is independent of the used target and also the critical argon pressure is nearly the same for every material.
To understand the physics behind the formation of the even film by sputtering at lower argon pressure, the film growth of a thin film which is sputtered with ditches and without ditches was compared and observed by TEM investigations (see Figure 6). Based on these findings the sputtered layer must be at least 2 nm thick to get an even film and the growth of thin films which are sputtered at higher argon pressure gets stopped at this stage where secondary nucleation should appear.
Low Argon pressure (p < 1e–2 mbar) leads to a smaller number of argon atoms in the chamber and therefore the amount of collisions between target atoms and argon atoms is reduced, the scattering angle is smaller and the kinetic energy of the target atoms is lower. Because of this smaller scattering angle the target atoms fall into the ditches which lead to secondary nucleation till the ditches are filled (see Figure 7, ). Additionally, the kinetic energy of the target atoms increases as well and this influences the mobility of the adatoms.
Characterisation of grown films
After the ability to sputter even metallic thin films, the quality of those films has to be quantised. Every film surface has characteristic dimensions of the formed islands. Therefore, atomic force microscopy was used to observe the topography of the films and measure the dimensions to express a quantity and evaluate the best material for HR-SEM applications.
To extract these dimensions from the AFM measurements several numerical methods were evaluated, like particle analysis tools by using the threshold technique, the power spectral density function, the radial autocorrelation function and an image processing tool based on the watershed algorithm. The sizes of such islands are in the nanometer range, hence a high lateral resolution has to be chosen for the quantisation. The resolution of the AFM image is set to the following conditions: 500 nm by 1024 px. Higher resolution would be possible but not applicable in this case of such small dimensions, because it would lead to drift effects which falsify the result of the lateral dimension (three examples of these images can be seen in Figure 8).
With this image processing tool and also with manual measurements of 100 different islands, a ranking of the mean island radius for different target materials was calculated (see Table 2). Besides the determination of the mean island radius, t