2. PECVD reactors

The deposition of a-SiC layers in a Plasma Enhanced Chemical Vapor Deposition (PECVD) reactor is facilitated by the plasma generated between two electrodes (radio frequency-RFor DC discharge) in the presence of reacting gases, the substrate being connected at one of these electrodes. There is a very large diversity of types of PECVD reactors on the market, either for industrial applications or specially designed for dedicated R&D. Usually the R&D equipments are more complex but also may allow much more of flexibility and degrees of freedom in controlling the deposition, thus enabling to obtain different layers with distinct properties using the same reactor.

The key elements in the selection of a PECVD reactor can be summarized as follows:

Deposition chamber. The operating temperature for most of the PECVD reactors is between 200-400°C. In order to achieve a uniform deposition special attention has to be paid to the inlet of the reactive gases, which can be of three types: several inlets around the bottom chuck (electrode), or one inlet through the top electrode, or multiple inlets (shower) from the top electrode. The last solution seems to ensure a better distribution of reactive gases between the working electrodes with positive effect on the film's uniformity. Meanwhile, preheating the gases (using a heated gas distribution system) before their actual introduction into the deposition chamber may also improve the deposition uniformity. Heating the deposition chamber itself (usuallyat 50-100°C) generates gradients of temperature that avoid particle deposition on the substrate during processing. Of course, adding all these elements into a standard system would finally be reflected in a higher cost of the tool.

Loadlock system. Two types of reactors can be distinguished, depending on whether a loading system is present or not: open systems (without lock load, i.e. relatively cheap reactors used only in research labs) and closed systems. The presence of a vacuumed loading system is also a critical element for good PECVD deposition, for a stable and repetitive process. There are two main aspects related to the presence of the load lock: one is related to satety while the other one is related tothe quality of the deposited layer. For the safety aspect, the presence of the loading system avoids the contact of the operator with the by-products resulted during processing, some of which are carcinogenic. As for the process quality, the fact that the chamber is kept permanently under vacuum results in excellent film quality with outstanding reproducibility.

Reconfiguration of the chamber. Cleaning of the chamber is also an important element in achieving good-quality layers. Most of the PECVD reactors also allow a "plasma cleaning" process, which is applied once a certain thickness of the deposited layer is achieved (usually 5-10μm). This cleaning process is designed to remove the products deposited on the chamber's walls or on electrodes, and it is performed mainly using CF ./O2 or C .f ./O2 as reactive gasses. The process is followed by a short pre-deposition of the material desired to be deposited in the reactor. Mechanical cleaning must be also performed periodically.

Gas precursors. The PECVD systems trequently used in R&D are equipped with a large number of inlets for the reactive gases. In most of the cases, the equipment is used for multiple depositions such as SiO2 (doped and undoped), SigNy a-Si or even TEOS (using a special Liquid Delivery System -LDS). In our case, for the deposition of x-SiC layers, silane (SiH.) and methane (CH4) are the most often used gas precursors, although other precursors, e.g. methyltrichlorosilane (MTCS) [33] or SiH,/acetylene (C2H2) [34], were also studied.
