Parylene-C Deposition

During the manufacturing processes of devices as sensors and integrated circuits, the materials such as semiconductors, dielectrics and metals are exposed to different processes, where the physical integrity of these may get disturbed and hence, it is not convenient. At the CIDESI Queretaro clean room, we have an apparatus called Chemical Vapor Deposition for Parylene-C deposition, this material is a polymer that helps us to protect the materials of interest during the development of the device.

One of the apparatus that CIDESI Queretaro has, is a “CVD-Parylene Coater” (Chemical Deposition in vapour phase) system, of the Specialty Coating Systems brand. This system is used by diffetent persons on the research projects and sensors and integrated circuits manufacturing, see Fig. 1.

Fig. 1. The deposition system of Parylene-C, is located in the CIDESI Queretaro clean room. This system is used in several research projects and technological development as sensors and integrated circuits.

How do we coat Parylene-C films?

The Parylene-C is deposited through a process similar to the material coatings in vacuum, nevertheless, these apparatus as Sputtering or e-beam, the coating is carried on at pressure conditions of 0-3 a 10-6 Torr, commonly, and in the Parylene-C coating, pressures around 0.1Torr are required. The deposition is in vapour phase, thus, during the coating the whole area of the object get impregnated uniformly, which results in a Parylene-C coating really uniform, conformal and without porous.

The coating process of Parylene-C consists of three steps, as described below.

  1. Vapourization of the solid dimer, approximately at 150°C.
  2. pyrolysis, the second step is the quantitative split (pyrolysis) of the dimer vapour on the two methylene-methylene bonds at approximately 680°C, that produces the stable monomeric bi-radical, for xilene.
  3. Polymerization, the monomeric vapour gets inside of the deposition chamber at room temperature where it gets polymered spontaneously over the substrate. The temperature of the substrate never gets higher than a few degrees upon the room temperature.

Fig. 2. a) Parylene coater. b) Coating process of Parylene-C in its different stages: vapourization, pyrolysis and polymerization.

In any moment during the coating process a liquid phase is generated, therefore, Parylene-C does not suffer from the effects characteristical of a liquid that may cause accumulation, flow, drag or defects caused by the edge effect. Also, Parylene-C does not either contain solvents, catalysts or plastifiers that could leach out or degas the coating.

What are the Parylene-C qualities?

Parylene-C has a high dielectrical resistance (5000V/mm)*, a really low dissipation factor, excellent mechanical resistance, very high surface and volume resistivities and some other electric properties higher that remain virtually constant.

In the current commercial applications, Partylene-C is deposited in thicknesses from a few nanometers to approximaely 75 microns, depending on the function the Parylene-C film has to perform. This provides physical barrier properties similar or more than 2-6 thousand epoxic, silicons or conventional coatings that generally require several applications to delete the porous.

Parylene-C does not require catalyst or solvent, which eliminates the environmental worries. Due very low static and dinamic friction coefficients, that are practically the same, parylene-C might work as a dry film lubricant that helps a lot in devices as tiny servomotors, where the start could be a problem. The Parylene-C helps in several purposes, including thermal isolation, humity and chemical isolation, mechanic protection, improved lubricity and the consolidation of the surface to avoid descaling or dust.

In the table 1 we show the main electric, barrier, mechanic and thermal properties of Parylene-C. These properties are compared to those reported for other conformal coating materials such as epoxics, acrylics and silicons.

One of the characteristics of Parylene-C coating is that extremely thin layers can be deposited, they have excellent dielectric voltages. It is also demostrated that the rupture of voltage per unity of thickness increases as the film thickness decreases. Moreover, the water vapour transmission rate (WVTR) is compared with other conformal coating material. In this case, the WVTR for Parylene-C is higher than that of the common polymeric materials, which decreases the surface corrosion and salinization.

Table 1. Main physic characteristics of Parylene-C coatings.

Parylene-C applications

The conformal coatings are used in a wide range of industries, including electronci, medical devices, automotive, military and aerospace. They are used for protection, biostability and surface alteration to improve the general reliability of the components assembly or final product. In the Microtechnology Management Clean Room, most of the times Parylene-C coatings are used as a protector film for semiconductors, dielectric and metals, that could get affected during the acid or plasma attack processes that are widely used during the devices manufacturing.

Fig. 3. In the image David Velarde is observed student of the Universidad Autonoma de Queretaro of the Nanotechnology degree during his professional internship. Currently, David has been incorporated as project engineer in the Microtechnologies Management.

Some of the applications used in the industry are listed below:

Fig. 4. Applications of Parylene films.

Medical Industry: Parylene-C is recognized by the Food and Drug Administration (FDA) and provides an ideal surface alteration for implantable and non implantable devices, as a catheter, seal, cochlear implants, surgical tools, pacemaker and components. The coating protects devices and components from biofluids, humity and biogas, and it works as a biocompatible surface to touch with tissues. See Fig. 4 a).

Automotive Industry: The Parylene-C coating protects critical sensors, circuit plates and other electronic components used in consumer cars, such as motors and heavy duty systems. The coatings provide a barrier against aggressive chemical products, fluids and gases at high temperatures and during long term use. See Fig. 4 b).

Aerospace Industry: This polymere offers an unmatched protection for several military and aerospacial applications, including vehicles and deffense equipment for international space programms. The coatings provide reliable barriers against elements such as dust, sand, moist and chemical and biologic agents. See Fig. 4 c).

Industria electrónica: The Parylene-C coating is conformal and uniform, which guarantees a full cover of circuits plates, ferrite nuclei and other electronic packages, as MEMS, laboratory technology on a chip (lab-on-a-chip) and sensors, at the Microtechnology Management there are projects running with the last three mentioned devices. It has been shown that Parylene-C coatings mitigate the metallic whiskers formation, eruptions of strange material forms and dendrites. See Fig. 4 d).

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