Entwicklung von großflächigen PECVD-Prozessen zur kontrollierten, homogenen Abscheidung dünner Siliziumschichten für die Photovoltaik (2018)

Hydrogenated microcrystalline silicon (µc-Si:H) is used, among other things, as an absorber layer in solar cells. To produce µc-Si:H with homogeneous properties on a large-area, industrial scale, using plasma enhanced chemical vapor deposition, reproducible processes need to be established. In the first part of this thesis, the development of a reduced gas flow process to produce homogeneous, large-area microcrystalline solar cells is described. There are several ecological and economical advantages when the gas consumption is decreased. The costs for purchasing and storage of the feed gas are reduced. Due to the significantly lower feed gas flows, the rotational speed of the vacuum pumps can also be reduced. This results in lower electrical power consumption of the vacuum pumps, an extension to their maintenance intervals, and an overall increase in their service life. A vacuum system with a standard industrial electrode was used during these investigations. The geometry of the gas feed and the exhaust is of great importance when depositing µc-Si:H with a reduced gas flow. A mixture of silane and hydrogen gases is fed into the process volume through a so called showerhead electrode and is bilaterally exhausted. As the gas flow is reduced, the residence time of the gas molecules in the plasma increases. Simultaneously, a higher concentration of silane is used in the feed gases, resulting in a higher degree of silane molecule dissociation. A qualitative plasma-growth model for the large-area deposition of µc-Si:H describes the two main influences of reduced gas usage on the local silane concentration in the plasma:Silane back diffusion: At the beginning of the deposition, silane diffuses from the reactor dead volume into the plasma. Consequently, an incubation layer grows with a lower crystalline volume fraction. This effect can be overcome by using an optimized deposition process. Reduced silane concentration in the Plasma edge region: A net release of hydrogen in the plasma occurs due to the enhanced silane dissociation. Hydrogen that is released at the center of the plasma has to leave via the plasma edge region. This results in a reduced plasma silane concentration in these regions, leading to plasma conditions at the edge region which cause a silicon growth with a higher crystalline volume fraction compared to the center region. Different pumping speeds of silane and hydrogen intensify this effect where the gases are extracted. With the appropriate electrode, the gas usage during the deposition of µc-Si:H solar cells can be reduced by a factor of 10 compared to a standard deposition regime without affecting the solar cell conversion efficiency. A further reduction of the gas flow causes variations in the local plasma silane concentration leading to lateral inhomogeneous solar cell properties. Measurement of the optical plasma emission of the SiH* radicals can be used as a process control. A non-constant SiH* emission intensity in the initial deposition period indicates transient plasma effects. In this case the microstructure of the growing silicon layer changes with deposition time. In a solar cell, absorber layers with varying microstructures commonly result in lower efficiencies. As a result of reduced gas usage, powder production in the plasma increases. However, this has no influence on the solar cell properties produced with this deposition system. Nevertheless, for system maintenance, powder production is undesirable. An increased atmospheric contamination of the absorber layer with oxygen has been ruled out as a reason for the inhomogeneous solar cell properties. A simple method has been developed to identify in-situ powder-free deposition regimes. This is helpful since an uncontrolled powder production is undesirable in terms of maintenance and device quality. Two measures have been identified: the self-induced bias voltage at the electrodes or the ratio of the optical emission intensities from SiH* to H_beta in plasma. Both parameters show a characteristic behavior when the deposition conditions drift from powder-free to powder producing. In the deposition system used, a severe powder formation starts at a distinct residence time of the process gas in the plasma, independent of the deposition pressure. The second part of this thesis addresses the development of a novel experimental electrode. Utilizing this experimental electrode, new processes have been developed to deposit thin silicon layers over large areas. These processes include a process control to ensure a homogeneous deposition. There are five optical access points inside the experimental electrode. It is possible to detect and spatially resolve the optical plasma emission with these access points. Conclusions may then be drawn between the local plasma compositions and the layer properties. By measuring the plasma emissions at the optical access points the effect of silane back diffusion, which was seen with a standard electrode, was evaluated. The deposition rate of the growing layer can be determined in-situ with the help of the access points. Hence, the optical access points are a valuable tool for spatially resolved process control. In addition, a spatially resolved feed gas supply has been implemented into the experimental electrode. This allows the feed gas flow through the central region of the showerhead electrode to be adjusted independently to the feed gas flow through the edge regions. The silane concentration in the plasma edge region can then be readjusted in order to compensate the plasma effects, which were seen with the standard electrode. Silicon layers were deposited using the spatially resolved feed gas supply. As a result, the total feed gas flow was reduced by a factor of 10 compared to standard gas flows. It was demonstrated that the lateral homogeneity of the layer (in terms of crystalline volume fraction and thickness) can be improved significantly through an increased input silane concentration at the edge region.