Interaction effects of cathode power, bias voltage, and mid-frequency on the structural and mechanical properties of sputtered amorphous carbon films

The Design of Experiments is a promising method to investigate the cause-effect relation between the mid-frequency magnetron sputtering parameters on the structural and mechanical properties of amorphous carbon (a-C) films. Based on the Central Composite Design, the cathode power of two graphite targets, bias voltage, and mid-frequency were simultaneously varied from 1500 to 4000 W, −100 to −200 V, and 20 to 50 kHz, respectively. The chemical bonding state was characterized using UV and visible Raman spectroscopy with excitation wavelengths of 266 and 532 nm. Corresponding measurements were performed by X-ray photoelectron spectroscopy (XPS) using synchrotron radiation. Additionally, hardness and elastic modulus of the sputtered a-C films were determined in nanoindentation tests. Multi-wavelength Raman spectroscopy identified an sp3 content below 20%, with most a-C films having an sp3 value in the range of 12 to 18%. The formation of sp3 bonded atoms is negatively influenced by a high cathode power and bias voltage, whereas the highest sp3 content is obtained with a-C films sputtered with a cathode power and bias voltage of 2750 W and −150 V. However, higher values of the cathode power and bias voltage result in a film delamination and decrease of the sp3 concentration. The bonding state affects the mechanical properties, as high hardness and elastic modulus result from a high sp3 content. Therefore, a targeted adjustment of cathode power and bias voltage is necessary to obtain a-C films with a high hardness. In contrast, the mid-frequency does not have a significant impact on the mechanical properties. In conclusion, the Central Composite Design has proven to be a suitable method to investigate the cause-effects of the sputtering parameters on the properties of the a-C film.