ms 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, the roughness average value Ra and the ten-point height value RZ were used to estimate the height values of the islands. To get an even film a layer thickness of at least 2 nm is necessary for plane sample and the planetary drive stage and 5 nm for highly structured samples. The WD distance can be varied between 30 and 100 mm. At lower working distance the layer growth but also the heat impact in increased. The minimum sputter current for a stable plasma for different material is also shown in Table 2.
|Material||Island radius [nm]||Ra [nm]||RZ [nm]||pAr [mbar]||WD [mm]||Current [mA]|
|Au||13.5||0.74||3.7||< 1e–2||30–100||> 15 mA|
|W||8.3||0.14||0.7||< 1e–2||30–100||> 90 mA|
|Pt||5.9||0.37||1.6||< 1e–2||30–100||> 15 mA|
|Au/Pd||5.7||0.24||1.3||< 1e–2||30–100||> 15 mA|
|Pt/Pd||5.3||0.18||0.9||< 1e–2||30–100||> 15 mA|
|Ir||4.2||0.20||0.7||< 1e–2||30–100||> 80 mA|
Table 2: Summary of the optimized sputter parameters. Material = material of sputtered target; Island radius = mean island radius in nm; Ra = roughness average in nm; RZ = ten-point height in nm; PAr = Argon pressure in mbar; WD = working distance in mm; Current = sputter current in mA.
Highly demanding samples concerning sputtering processes are lipids. They are biological and non-conductive samples which are highly temperature sensitive. The surface structure of lipids is most important for the different applications, what makes the Leica Microsystems DC magnetron sputter coater to the ideal device to sputter a thin conductive layer. In Figure 9 a completely artefact free high resolution SEM image of a highly structured pharmaceutical lipid sample is shown (10 nm Au/Pd sputtered).
- Depla D, Mahieu S, and Greene JE: Sputter Deposition Processes.
- Venables JA: Introduction to surface and thin film processes. Cambridge, UK – New York: Cambridge University Press, 2000.
- Eisenmenger-Sittner C: Lecture notes of “Technologie Dünner Schichten”.
- Bertrand G: On topological watersheds. Journal of Mathematical Imaging and Vision 22 (2–3): 217–230 (2005).
- Melis