What types of materials can be parylene coated?

Table: Examples of devices that make use of Parylene conformal coatings.

Substrate surface / Parylene interaction / Substrate surface preparation, interface cleanliness, surface energy:

By Parylene CVD deposition it is possible to obtain stress-free and defect-free conformal coatings on almost all types of materials. Sensors used in the automotive industry can be prone to high temperatures, hydrocarbons, and moisture. Such environments cause the sensors Aluminum contact pads to corrode quickly. Use of parylene on these pads protects them and is commonly employed.

Parylene is also used as a masking material in MEMS devices due to its capability to fill the crevices, in the image it fills under the undercut and acts as a spacer to produce 100nm wide nanowires (inset). In that study, it forms interfaces with a polymeric resist, Silicon, Silicon Carbide and Platinum.

Parylene can be applied to replace the glass packaging leading to a smaller dimension and weight in the package. Also, the parylene can be coated as a stack of layers combined with other thin layers such as SiO2. The diversity in the applications and possibilities makes the Parylene a very attractive conformal coating.

Steps to follow before starting the deposition process

To prevent delamination, pinholes, cracks, wrinkles, blisters and other types of thin film defects different types of surfaces require different surface preparations.

Example Parylene/Material systems:

• PCB: The most common application area of Parylene is PCBs as a protective coating of the electronic components, bonds and the wires. It forms a protective layer against mechanical impact and diffusion of corrosive elements.
• Silicon wafer: Wafer bonding intermediate layers, as a mask for silicon chemical etching,
• Ceramic / glass: A-174 silane solution pretreatment can improve the adhesion of the Parylene-C thin film on flat glass substrate specimens.
• Metals / alloys: Parylene is widely used in coating sensor pads. (Figure (a) shows a plastic packaged silicon pressure sensor) Parylene suffers from poor adhesion to itself and noble metals, such as gold and platinum. To promote their adhesion a commercially available adhesion promoter can be used and it was reported that a 100-fold increase in adhesive strength at such interfaces was achieved and the corresponding improvements in resistance to moisture driven delamination were observed.
• Parylene – metal – parylene sandwich structures are produced as flexible electrodes, sensors, in general as biocompatible implants.
• Parylene – parylene deposition is possible but requires post-treatment to form a chemically stable interface by thermal treatment.
• Polymers: Parylene C on photoresist can be achieved by improving the adhesion of the resist and Parylene introducing a modified silanization pretreatment including heat treatment to remove hydroxyl groups from the surface of the resist.
• On oil: When the vapor pressure of the oil is low (<5 Pa) it is possible to deposit parylene on top. Parylene coated on oil exhibits a small stress and it can be ignored. Parylene-on-oil deposition (CVD) has been used for the preparation of long-term implantable pressure sensors, optical lenses out of oil and to make isolated parylene films by separating the parylene after deposition.

While there are many conformal coating options available, the diversity in the applications and possibilities makes the Parylene a very attractive conformal coating choice.