# Chemical Vapor Deposition of Longitudinal Homogeneous Parylene Thin-Films inside Narrow Tubes
## 4 Results and Discussion
### Measurement of the Temperature Dependence of the PPX-N Sticking Coefficient
As partial suppression of deposition at the inlet of the tube is necessary for a homogeneous coating, this temperature range remains of crucial interest for our work. Therefore, we use a smoothing spline to our experimental data instead of the chemisorption model for further calculations. Due to discrepant literature values, the ceiling temperature was further resolved using thin-film interferometry. To do so, we calculated the relative change in reflectance of the coated substrate compared to a bare soda lime microscope slide, which is given by \(\Delta R/R=(R_{\rm coated}-R_{\rm uncoated})/R_{\rm uncoated}\). Thus, in the absence of a PPX-N thin film on the substrate, \(\Delta R/R\) will remain zero over the entire spectral range (400 nm to 1000 nm). For \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \(T_{\rm s}\) & \(S_{\rm r}\) & \(T_{\rm s}\) & \(S_{\rm r}\) & \(T_{\rm s}\) & \(S_{\rm r}\) \\ \hline
14 & 2.02(13) & 32 & 0.93(07) & 50 & 0.37(03) \\
16 & 1.91(13) & 34 & 0.85(06) & 52 & 0.32(03) \\
18 & 1.76(12) & 36 & 0.78(06) & 54 & 0.29(02) \\
20 & 1.61(11) & 38 & 0.69(06) & 56 & 0.25(02) \\
22 & 1.46(10) & 40 & 0.62(04) & 58 & 0.22(02) \\
24 & 1.33(09) & 42 & 0.57(04) & 60 & 0.19(02) \\
26 & 1.22(09) & 44 & 0.51(04) & 62 & 0.16(01) \\
28 & 1.12(08) & 46 & 0.46(03) & 64 & 0.15(01) \\
30 & 1.00(07) & 48 & 0.41(03) & 66 & 0.14(01) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Measured relative sticking coefficient at 30 \({}^{\circ}\)C, over a wide range of varying surface temperature (\(\Delta T_{\rm s}\approx 50\)\({}^{\circ}\)C) up to the ceiling temperature. Temperature \(T_{\rm s}\) is given in units of \({}^{\circ}\)C and uncertainties are in brackets. selected temperatures around the ceiling temperature, the measured reflectance spectra are shown in Figure 2d. At the lowest temperature of 59.9 \({}^{\circ}\)C a maximum at about 550 nm is observed, but for the other temperatures is not. This is equivalent to the behavior known from thin-film interferometry, where with increasing film thickness the number of maxima as well as minima grows (\(R\propto\cos(nd/\lambda_{\rm p})\) with \(n\) the refractive index of the thin-film and \(\lambda_{\rm p}\) the photon wavelength [20; 43]. Consequently, this behavior is observed with decreasing surface temperatures (not shown here). The ceiling temperature is determined by taking the average of the lowest temperature with \(\Delta R/R=0\) and the highest temperature with \(\Delta R/R\
eq 0\). Thus, we can report the ceiling temperature of PPX-N on soda-time glass as 65.8(1.4) \({}^{\circ}\)C in the range of an average monomer partial pressure of approximately 2 Pa along 10 Pa argon base pressure. This value is in good agreement with our previous measurements with 70(2) \({}^{\circ}\)C [20], but strongly contrasts the values published by [PERSON] et al. and [PERSON] [10; 23; 24; 25], which are 30 \({}^{\circ}\)C and somewhat above 40 \({}^{\circ}\)C, respectively. We consider that the approach of determining \(T_{\rm c}\) by linear extrapolation of deposition rates from room temperature and below room temperature experiments causes the large discrepancies, since the actual, rather exponential, decay of \(S_{0}(T_{\rm s})\) is greatly overestimated within this method.