Abstract
Chemical Vapor Deposition (CVD) with its plasma-enhanced variation (PECVD) is a mighty instrument in the toolbox of surface refinement to cover it with a layer with very even thickness. Remarkable the lateral and vertical conformity which is second to none. Originating from the evaporation of elements, this was soon applied to deposit compound layers by simultaneous evaporation of two or three elemental sources and today, CVD is rather applied for vaporous reactants, whereas the evaporation of solid sources has almost completely shifted to epitaxial processes with even lower deposition rates but growth which is adapted to the crystalline substrate. CVD means first breaking of chemical bonds which is followed by an atomic reorientation. As result, a new compound has been generated. Breaking of bonds requires energy, i.e., heat. Therefore, it was a giant step forward to use plasmas for this rate-limiting step. In most cases, the maximum temperature could be significantly reduced, and eventually, also organic compounds moved into the preparative focus. Even molecules with saturated bonds (CH4) were subjected to plasmas—and the result was diamond! In this article, some of these strategies are portrayed. One issue is the variety of reaction paths which can happen in a low-pressure plasma. It can act as a source for deposition and etching which turn out to be two sides of the same medal. Therefore, the view is directed to the reasons for this behavior. The advantages and disadvantages of three of the widest-spread types, namely microwave-driven plasmas and the two types of radio frequency-driven plasmas denoted Capacitively-Coupled Plasmas (CCPs) and Inductively-Coupled Plasmas (ICPs) are described. The view is also directed towards the surface analytics of the deposited layers—a very delicate issue because carbon is the most prominent atom to form multiple bonds and branched polymers which causes multifold reaction paths in almost all cases. Purification of a mixture of volatile compounds is not at all an easy task, but it is impossible for solids. Therefore, the characterization of the film properties is often more orientated towards typical surface properties, e.g., hydrophobicity, or dielectric strength instead of chemical parameters, e.g., certain spectra which characterize the purity (infrared or Raman). Besides diamond and Carbon Nano Tubes, CNTs, one of the polymers which exhibit an almost threadlike character is poly-pxylylene, commercially denoted parylene, which has turned out a film with outstanding properties when compared to other synthetics. Therefore, CVD deposition of parylene is making inroads in several technical fields. Even applications demanding tight requirements on coating quality, like gate dielectrics for semiconductor industry and semi-permeable layers for drug eluting implants in medical science, are coming within its purview. Plasma-enhancement of chemical vapor deposition has opened the window for coatings with remarkable surface qualities. In the case of diamond and CNTs, their purity can be proven by spectroscopic methods. In all the other cases, quantitative measurements of other parameters of bulk or surface parameters, resp., are more appropriate to describe and to evaluate the quality of the coatings.
Highlights
In our reactor with a volume of 91 L (40 × 40 × 51 cm; Tevap = 130 ◦C), we find a flow into the reactor of 9.3 sccm after evaporation, and 18.6 sccm after complete dissociation (i.e., 2.5 × 1020 molecules/min or 1.51 × 1022 dimeric molecules/h and 3.02 × 1022 monomers/h), which is a total of 50 m moles or 5.22 g
The behavior for the non-substituted species (CVD) shows the typical conduct which is expected for aromatic polymers: lipophilic character which is combined with extremely hydrophobic properties
As can be drawn from the evaluation of the surface energy (Figure 34), this is nearly completely due to a large increase of the dispersive fraction in the Chemical Vapor Deposition (CVD) case which is opposed by a steep drop in the case of Plasma-Enhanced Chemical Vapor Deposition (PECVD), whereas the polar fraction remains almost constant in both cases
Summary
The first deposition techniques on molecular level were evaporation techniques. At extremely low vacuum, metals or oxides are evaporated and deposited at colder places of the reactor. Atomic sources are used but the result is a chemical compound, different from the reactants, and we have made the first step to Chemical Vapor Deposition (CVD) This technique is characterized by the fact that the evaporated solid is chemically different from the deposited layer. Applying two vapors as reactants completely avoids the presence of a solid The development of this branch of CVD began with the decomposition of SiH4 to form polycrystalline silicon, so-called polysilicon, at elevated temperatures and diluted with an inert gas (nitrogen or argon) typically by about a factor of 4 according to the stoichiometric sum reaction. The kinetic energy of the film-building species at substrate level had been reduced by collisions with the atoms of the inert gas [10] This concurrence between these two competing processes of reactions in the volume and at the surface is a main issue in epitaxy, i.e., crystal-orientated growth. In the case of depositing layers with a certain porosity, the exact control of the growth rate is mandatory and is possible by this epitaxial trick, leading to slowly grown, but high-quality, evenly layers with high a conformity (cf. Section 3) [12]
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