# Chemical Vapor Deposition of Longitudinal Homogeneous Parylene Thin-Films inside Narrow Tubes
## 3 Materials and Methods  
CVD experiments were performed in an in-house constructed deposition reactor (see Figure 1a), consisting of an evaporation chamber, a thermal cracker unit (heated by infrared radiation at 600 \({}^{\circ}\)C [41]), a monomer distributor (100 \({}^{\circ}\)C, above \(T_{\mathrm{c}}\) to avoid film growth) and a flow-through reaction chamber. The corresponding temperatures during the deposition process were set by a PID controller. The pressure in the reaction chamber was measured with a heated (90 \({}^{\circ}\)C, above \(T_{\mathrm{c}}\)) capacitance manometer. The mass flow of the diluent gas argon was set by a mass-flow controller with a mass flow of approximately 8.5 sccm resulting in 10 Pa base pressure. The argon gas inlet to the reactor was located between the cracker and the monomer distributor (see Figure 1a), ensuring a homogeneous mixture of both gas species. This allows the hot MPX to be cooled down in thermodynamic equilibrium to near room temperature before reaching the reaction chamber. For the whole reactor the gas dynamic regime is in the viscous flow within our pressure range. Therefore, the MPX partial pressure decrease in the reaction chamber due to surface polymerization on its walls is nearly negligible due to the low aspect ratio of close to one (see below). The PPX-N precursor was provided by Plasma Parylene Systems (Rosenheim, Germany). To ensure comparability, relevant process parameters were kept constant during all experiments. In order to suppress evaporation during the heating, the argon pressure was set to 100 Pa (130 sccm) and subsequently controlled with the mass flow controller to a base pressure of 10 Pa, where the deposition process was initiated. A precursor mass of 5.0 g was evaporated at a temperature of 115 \({}^{\circ}\)C. The capacitance manometer was placed as close as possible to the substrate, respectively inlet of the tubes, to best represent the concentration of the monomer in the gas phase above the substrate (see Figure 1a). The partial pressure of the monomer was determined from the measured total pressure, as the relation of argon mass flow and pressure is known for the reactor. However, since the precursor mass is limited and the open evaporator unit refers to a non-thermodynamic equilibrium system, and therefore cannot maintain a stable vapor pressure, the PPX-N mass flow and, respectively, partial pressure were not constant over time during the experiments. As the evaporation rate is proportional to the existing precursor surface, and this in turn to the partial pressure, an exponential progression of the PPX monomer partial pressure is expected (\((P_{\mathrm{M}}(t)\propto\exp(-\gamma t)\), with \(\gamma\) the evaporation rate constant). A typical pressure trend is depicted in Figure 1d. As can be seen, small deviations in the evaporator temperature control unit (\(\Delta T\approx 2\ ^{\circ}\)C) lead to minor oscillations in the pressure. A coating thickness of 2.6(3) \(\upmu\)m on a \(T_{\mathrm{ref}}=30\ ^{\circ}\)C heated soda lime glass, positioned in the center of the reaction chamber, was measured for all experiments. In order to determine the temperature dependence of the sticking coefficient, an aluminum slab attached to a resistive heating and water-cooled unit was placed with thermally connected soda lime glass microscope slides inside the reactor chamber. The setup is shown in Figure 1b. Due to the constant thermal flow within the aluminum slab, a linear temperature gradient is established on the microscope slides. The temperature was measured at two points with K-type thermocouple elements and may be interpolated for further analysis.