Real-time monitoring of low temperature nucleation and growth processes during high power impulse magnetron sputtering

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During the last years, high power impulse magnetron sputtering (HiPIMS) got increasing attention not only in research but also in industry. In HiPIMS, the target is supplied with short extremely intense pulses while the average power is comparable to conventional magnetron sputtering (MS) in order to preserve the integrity of the target. Such conditions make it possible to ionize the sputtered metal vapor and to achieve ionized fractions above 70 %, i.e. about two orders of magnitude higher than in standard MS. The acceleration of the ions in the plasma potential or an external bias voltage, inter alia, permits deposition of films with higher hardness, density, refractive index, and conductivity as well as better adhesion. Moreover, the high energy per particle and the local heat release during recombination of ions and electrons on the substrate allow for deposition of crystalline phases, including metastable ones, at temperatures much lower than those defined by thermal equilibrium or obtained by conventional MS. At the same time, the average energy arriving at the substrate is lower than in normal MS, which markedly reduces substrate heating. Thus, HiPIMS is very interesting for deposition of high-quality films on polymers and other heat-sensitive substrates, which do not allow heat treatment during or after deposition. Despite the rapidly growing interest, little is known about the low temperature nucleation and growth processes during HiPIMS, and no in-situ investigations are available. In the present joint proposal, we aim at understanding film formation during HiPIMS on heat-sensitive substrates at low temperatures including nucleation, growth, crystallization, and grain structure evolution. Our approach is based on real-time monitoring with synchrotron-based grazing incidence small angle X-ray scattering (GISAXS) with high temporal and spatial resolution. We already demonstrated a time-resolution on the sub-ms scale, which is the scale of single pulses in HiPIMS. Simultaneous wide angel scattering (GIWAXS) will permit in-situ measurement of phase formation and transformation processes. The real-time measurements will be complemented by structural investigations with high-resolution electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, optical spectroscopy, and characterization of the functional thin film properties. Because of fundamental as well as technological interest, we will study the deposition of Au and Ag as metallic systems and of TiO2 and MoO3 as oxides. Prior to the investigation of the more complex film formation processes on selected polymers, we will use Si as a reference substrate. For comparison, we plan to perform conventional MS. The material combinations are select concerning target applications in organic photovoltaics and photocatalysis and related devices will be investigated with respect to the performance of the sputtered layers.