Parylene C Conformal Coating: Optical Properties, Refractive Index and Application Areas

Parylene films are highly employed in different applications because they are chemically inert, transparent in the visible range of the solar spectrum, offer high dielectric strength and insulation resistance, low moisture vapor transmission and gas permeability rates [1]. They can also be deposited as void-free layers on complex geometries in deep, narrow crevices.

Parylene C Deposition Process

Parylene C conformal coatings are deposited from powders of chlorine substituted para-xylene dimers via chemical vapor deposition. The process takes place in a closed chamber in three main steps.

• Sublimation: First, the chlorine substituted para-xylene dimer is sublimed at a temperature of 150°C
• Pryrolysis: Second, the dimer is vaporized and the gas moves into the pyrolysis chamber where the dimer is converted into two monomers at ≈690°C.
• Deposition: In the final step, monomers move into the deposition chamber where it is deposited on the substrate surface. The deposition chamber is always kept at room temperature. At this point, a vacuum pressure gradient is formed to allow the gas flow to happen from the pyrolysis area to the deposition chamber. The rest of the Parylene is deposited on a cold trap that is cooled between -90°C and -120°C. It is responsible for removing all residual Parylene materials pulled through the coating chamber.
• Adhesion promoter: The deposition of Parylene C requires adhesion promoters such as A-174 (γ-methacryloxy-propyltrimethoxysilane) to chemically bond the monomers onto the substrate surfaces (eg. Steel, PCB, silicon wafer). A-174 can be directly applied to the chamber surfaces before the chamber is put under vacuum. Silane in the closed chamber evaporates and deposits as a monolayer onto the substrate surface leading to a very strong adhesion of Parylene C monomers. Advanced adhesion technologies like AdPro Plus® and AdPro Poly® from SCS can help with adhesion to difficult materials.
• Masking of the substrate: Parylene conformal coating adeptly fills crevices and gaps within the micron range. The Parylene monomer vapor infiltrates and polymerizes beneath the masking material. To avoid deposition where Parylene C is undesired, meticulous masking of substrate surfaces is imperative.

Parylene C Refractive Index

Refractive Index: Refractive index is measure of the bending (deflection) of a ray of light when passing from one medium into another. The refractive index (n) is calculated by dividing the velocity of light in vacuum (c) to that of the medium of interest (v).

The refractive index changes as a function of the wavelength therefore the wavelength (λ) is also reported among with the n. Parylene C conformal coating has a refractive index of 1.592- 1.639 reported at 632.8 nm. The refractive index of Parylene C is also dependent on temperature. The relationship of refractive index change to temperature change is the modification of crystallinity and morphology of the polymer.
The transmittance and absorbance data of Parylene C and N are illustrated in the Figure [2]. The high transmittance of the polymers in the visible region make them eligible for use in optical applications. Also, the thickness uniformity of the thin films do not affect Parylene C’s use as a conformal layer because of the broad emission spectra of the OLEDs.

Using Parylene above the glass transition temperature can lead to crystallization, especially when it’s meant for waveguide applications. To avoid undesired scattering in waveguides, ensure the Parylene does not exceed this temperature. On the other hand, OLEDs can benefit from increased crystallinity post-annealing, enhancing their transmission properties.

Parylene C’s Applications as an Optical Conformal Layer

Parylene type C or poly(chloro-p -xylene) is highly researched and used in the electronics industry in the manufacturing of capacitors, semiconductor passivation, as gate dielectric and as a conformal coating of PCB’s.

Applications of Parylene C in optical components such as antireflective coatings [3], optical waveguides[1], beam splitters [4], LED encapsulation due to its transparency (Transmission Visible = 90%) are also available[5]. The high visible or near infrared transmission (∼90-95%) of the films are suitable for LED/OLED encapsulation applications [1], [5]. The encapsulated LEDs haves been reported to show an improved lifetime of approximately 30% besides the visual appearance of Parylene layers are attractive to the customers.

Also, Parylene pellicles for use in beam splitters are available. Pellicles made of organic components available to split beams, polarize light and reflect images with negligible lateral shift effects [4]. And they offer advantages over traditional pellicles: minimize or eliminate ghost images, refractive errors, energy absorption and spherical and chromatic aberrations.

In summary, Parylene C serves as a excellent conformal coating, acting as an encapsulant to enhance the longevity of devices by safeguarding them against service conditions. Importantly, it achieves this without compromising the optical characteristics of the devices within the visible spectrum.

References
[1]     O. I. Szentesi and E. A. Noga, “Parylene C Films for Optical Waveguides,” Appl. Opt., vol. 13, no. 11, p. 2458, Nov. 1974.
[2]     Y. S. Jeong, B. Ratier, A. Moliton, and L. Guyard, “UV–visible and infrared characterization of poly(p-xylylene) films for waveguide applications and OLED encapsulation,” Synth. Met., vol. 127, no. 1, pp. 189–193, Mar. 2002.
[3]     A. J. Gatesman, J. Waldman, M. Ji, C. Musante, and S. Yagvesson, “An anti-reflection coating for silicon optics at terahertz frequencies,” IEEE Microw. Guid. Wave Lett., vol. 10, no. 7, pp. 264–266, Jul. 2000.
[4]     R. Machorro, L. E. Regalado, and J. M. Siqueiros, “Optical properties of Parylene and its use as substrate in beam splitters,” Appl. Opt., vol. 30, no. 19, p. 2778, Jul. 1991.
[5]     X. He, F. Zhang, and X. Zhang, “Effects of Parylene C layer on high power light emitting diodes,” Appl. Surf. Sci., vol. 256, no. 1, pp. 6–11, Oct. 2009.