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

近年来，高功率脉冲磁控溅射(HIPIMS)无论是在研究上还是在工业上都受到越来越多的关注。在HIPIMS中，为了保持靶的完整性，靶被提供极强的短脉冲，而平均功率与传统的磁控溅射(MS)相当。这样的条件使得有可能使溅射的金属蒸气电离，并获得高于70%的电离分数，即比标准MS中高出约两个数量级。离子在等离子体电位或外部偏置电压中的加速，尤其允许沉积具有更高硬度、密度、折射率、导电性以及更好附着力的薄膜。此外，每个粒子的高能量和离子和电子在衬底上复合时的局部热释放允许在远低于热平衡或传统MS获得的温度下沉积晶相，包括亚稳相。同时，到达衬底的平均能量低于正常MS，这显著减少了衬底加热。因此，HIPIMS对于在聚合物和其他热敏衬底上沉积高质量薄膜非常有意义，因为在沉积过程中或沉积后不允许进行热处理。尽管人们的兴趣迅速增长，但对HIPIMS过程中的低温形核和生长过程知之甚少，也没有现场研究可用。在目前的联合提案中，我们旨在了解低温下HiPIMS在热敏衬底上形成薄膜的过程，包括成核、生长、结晶和颗粒结构演变。我们的方法是基于基于同步加速器的掠入射小角度X射线散射(GISAXS)的高时间和空间分辨率的实时监测。我们已经演示了亚毫秒尺度的时间分辨率，这是HiPIMS中单个脉冲的尺度。同时广角散射(GIWAXS)将使原位测量相形成和转变过程成为可能。除了实时测量，还将通过高分辨率电子显微镜、原子力显微镜、X射线光电子能谱、光学光谱和功能薄膜特性的表征来进行结构研究。由于基础和技术方面的兴趣，我们将研究作为金属系统的Au和Ag的沉积以及作为氧化物的TiO2和MoO_3的沉积。在研究选择的聚合物上更复杂的成膜过程之前，我们将使用硅作为参考衬底。为了进行比较，我们计划进行传统的MS，材料组合是针对有机光伏和光催化的目标应用而选择的，相关器件将根据溅射层的性能进行研究。

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.