# Chemical Vapor Deposition of Longitudinal Homogeneous Parylene Thin-Films inside Narrow Tubes
## 1 Introduction  
Chemical vapor deposition (CVD) is well known for forming high conformal coatings with uniform thickness on rugged surfaces with deep valleys or high mountains, on nearly all types of materials. Especially for modern applications as in medical [1; 2; 3; 4; 5] or in electronic devices [6; 7; 8], where constant film properties across the whole substrate are demanded, this type of processing has nowadays found widespread usage. In contrast to other deposition techniques, e.g., sputtering or evaporation techniques, which involve atoms as film-building species, reactive molecules or radicals in CVD form a polymeric film by a chemical reaction. According to [PERSON] and the law of mass action, chemical reactions always depend strongly on temperature [9; 10], and their manufacture is challenge with respect to process parameters and their stability [11; 12; 13]. For the coating of medical implants, one of the most promising polymers is poly-_para_-xylylene (PPX-N), or other related derivatives [14; 15], providing the ability to form high-precision thin-films on substrates. These thin-films have excellent properties such as transparency, high temperature resistance, mechanical flexibility, bio-compatibility, and chemical resistance and can even act as diffusion barriers [16; 8; 17]. Hereby, various methods are known to influence the film quality, properties and growth rate during PPX-N CVD. For example, diluting the monomer vapor with a non-reactive residual gas, such as argon, reduces the film growth rate but yields a higher optical transparency [18], and is therefore a state of the art technique in industry and research. Thin films of PPX-N are typically formed within the so-called Gorham process [19], where a precursor is evaporated under reduced pressure (temperatures between 120 \({}^{\circ}\)C and 150 \({}^{\circ}\)C, total pressure in the order of 10 Pa). The dimeric species is thermally crackedinto two di-radical monomers according to the simple sum reaction \(\,\mathrm{DPX}\to 2\,\mathrm{MPX}\), which flow through a monomer distributor into the reaction chamber. In principle, the process of polymerization is known to be a surface phenomenon [20], where the monomer particles are adsorbed and subsequently chemisorbed with a specific probability, so a polymerization on the substrate but also on the walls of the whole reaction chamber takes place [21]. Furthermore, the polymerization is determined to be kinetically controlled [10], which means that the film growth rate is proportional to the reactants surface concentration in the vapor phase (local partial pressure of the monomer) and the reaction kinetics during the polymerization, which are known to show a dependence on the surface temperature [22; 23; 24]. Experimentally, all authors have found a decrease of growth rate with an increase of temperature. Eventually, at the so-called ceiling temperature, no deposition is possible. The ceiling temperature for PPX-N has been communicated to be between 25 \({}^{\circ}\)C [23; 24; 25] and 68 \({}^{\circ}\)C [20]. In the past decades, different models for the deposition kinetics of PPX-N have been introduced, predicting the film growth rate of the polymer in dependence of the surface temperature of the substrate \(T_{\mathrm{s}}\) and the reactants concentration in the gas phase, as well as the partial pressure of the di-radical monomer. In short, these models show a dependence on the partial pressure \(p_{\mathrm{m}}\) to be proportional to \(p_{\mathrm{m}}^{\mathrm{n}}\) with values for \(n\) between one and two [22; 23; 25], where the linear model seems to represent the best compromise between complexity and predicting experiments [23]. In contrast to CVD processes which form inorganic films such as SiO\({}_{2}\), these models also predict a decreasing deposition rate with rising surface temperature which eventually vanishes. Despite its advantages of forming high conformal coatings, a homogeneous film thickness along an extended substrate can hardly be reached using CVD.