Detailed explanation of the deposition process and characteristics of parylene

Parylene conformal coatings have many attractive properties and can be widely used in a variety of applications
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Their low dielectric properties, high mechanical strength, transparency, biocompatibility, chemical inertness to all common acids, alkalis and organic solvents, low water/gas permeability and thermal properties make them popular in many industries
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Similarly, a pinhole-free parylene conformal coating with a thickness greater than 0.
1 μm is also possible, and it has been reported [1]
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Therefore, it is important to understand their deposition process and characteristics
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The parylene conformal coating is deposited under vacuum using the Gorham chemical vapor deposition (CVD) process
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As shown in the figure, the parylene (CVD) process is divided into three steps (sublimation, pyrolysis, and deposition)
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First, the dimer (precursor with two repeating units) is sublimated
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In the next step, the dimer is cleaved into two monomers by pyrolysis at a temperature higher than >500°C
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Finally, the monomers polymerize on the surface of the substrate (and the surface of the chamber) to form linear polymer chains
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The last step is carried out at room temperature to obtain a stress-free conformal coating
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The polymerization route of Parylene N-C 16 H 16 affects the deposition rate and thickness of parylene: The deposition rate and pressure of the parylene conformal coating control the final film during the coating process.
Uniformity and surface roughness
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According to reports, high pressure and high deposition rate will produce rough and uneven film coatings with poor dielectric properties [1]
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Jui-Mei Hsu et al.
reported that a higher precursor sublimation rate would result in a deposition rate ranging from 0.
015 to 0.
08 g/min.
The root mean square surface roughness was slightly higher from 5.
78 to 9.
53 nm [2 ]
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Therefore, extreme care must be taken when choosing the deposition rate, which in turn will affect the deposition rate of parylene
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According to the formula of Franz Selbmann et al.

.
(1) and (2) show that the parylene thickness s and deposition duration t increase with the increase of the dimer mass (the amount of dimer used) by about 0.
45 μm/g and 10.
80 min, respectively / G
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They studied the dimer mass between 1.
1 g and 10 g and found a correlation
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All depositions are at 130°C sublimation temperature and autogenous pressure <5.
5 Pa
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s = 0.
3386 µm + 0.
4528 µm / g·mD (Equation 1) t = 1.
55 minutes + 10.
80 minutes/g·mD (Equation 2) They also used different deposition parameters to understand the The effect on parylene coating
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Their sublimation temperature ranges from 100°C to 200°C, with a temperature step of 10°C
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They reported that the deposition rate varied between 20 nm/min at 100°C and 156 nm/min at 200°C
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They also showed that at a higher sublimation temperature, the layer thickness is slightly lower, which can be explained by the short residence time and short reaction time of the monomer molecules [2]
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The deposition duration, average deposition rate and layer thickness of parylene C at different sublimation temperatures between 100-200°C and constant dimer mass (6 g)
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Image reference: [2] Although parylene coatings are usually very thin, their deposition speed is relatively slow
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The fastest variant of parylene deposits-parylene C-usually deposits at a rate of 0.
2 mils or 5 microns per hour
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This means that a 75-micron coating takes about 15 hours
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Parylene N and D deposit more slowly
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Overall, the process may take several hours to more than 24 hours, depending on the coating thickness we want to achieve
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The time required to reach this thickness is controlled by the quality of the dimer, the deposition rate, temperature and other factors
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