Plasma Enhanced Chemical Vapor Deposition of Organic Polymers

At a Glance

Metadata	Details
Publication Date	2021-06-01
Journal	Processes
Authors	Gerhard Franz
Institutions	Munich University of Applied Sciences
Citations	25
Analysis	Full AI Review Included

Executive Summary

PECVD Value Proposition: Plasma Enhanced Chemical Vapor Deposition (PECVD) enables the deposition of highly conformal, uniform organic polymer layers at significantly reduced temperatures (often room temperature), overcoming the thermal limitations of conventional CVD.
Parylene (PPX) Deposition: The standard CVD process for Poly-p-xylylene (Parylene) involves a three-step process (sublimation, 700 °C thermal cracking, deposition). PECVD enhances the cracking efficiency, allowing layer formation to remain above the CVD rate even at low power inputs.
Mechanism Control: Polymerization is primarily radical-mediated. The growth rate exhibits a maximum as a function of plasma power (Yasuda’s critical composite parameter), separating the energy-deficient region from the passivation (fragmentation) region.
Purity and Analysis: Unlike inorganic films (SiO₂, Si₃N₄), organic polymer purity is difficult to verify chemically due to branching and non-stoichiometry. However, low-power PECVD Parylene films retain their aromatic ring structure, verifiable by FTIR spectroscopy (e.g., ring vibrations at 1495, 1609 cm^-1).
Surface Functionality Control: Surface properties can be precisely tuned. A mild oxygen plasma treatment (as short as 15 s) can switch the surface from hydrophobic to highly hydrophilic (contact angle < 30°).
Film Quality: Dilution of the monomer vapor with an inert gas (e.g., Argon) forces surface polymerization over volume polymerization, suppressing “snow” formation and resulting in high-quality, smoother films, albeit at a lower growth rate.
Advanced Carbon Materials: PECVD is essential for producing high-purity Diamond-Like Coatings (DLCs) and Carbon Nanotubes (CNTs), where purity is confirmed by characteristic Raman shifts (e.g., 1332 cm^-1 for diamond).

Technical Specifications

Parameter	Value	Unit	Context
Parylene N Sublimation T	100 to 150	°C	Standard CVD precursor evaporation (DPX).
Parylene Cracking T	Typically 700	°C	Thermal cleavage of DPX to monomer MPX radical.
SiH₄ Decomposition T (CVD)	600 to 650	°C	Low Pressure CVD (LPCVD) for polysilicon.
SiH₄ Decomposition T (PECVD)	120 or less	°C	Inductively Coupled Plasma (ICP) variant.
SiO₂ Deposition T (CVD)	500 to 750	°C	Conventional CVD using SiH₄ + O₂.
SiO₂ Deposition T (PECVD)	300	°C	Using SiH₄ + N₂O.
Standard RF Frequency (CCP)	13.56	MHz	Capacitively Coupled Plasma excitation.
Microwave Frequency (MW)	2.45	GHz	Standard industrial microwave plasma.
CCP Plasma Density (n_p)	~10¹⁰	cm^-3	Typical electron density in CCP reactors.
ICP/MW Plasma Density (n_p)	~10¹¹	cm^-3	High-density plasma regimes.
PECVD Base Vacuum (CVD)	~1	mTorr (0.13 Pa)	Achievable with rotary vane pumps.
RIE Base Vacuum (ICP/Turbopump)	< 10^-6	Torr (< 0.13 mPa)	Required for high-purity, mechanistic studies.
Parylene N Growth Rate (CVD)	54	nm/min	At T_evap 150 °C, 25 mTorr.
Parylene N Growth Rate (PECVD)	10 to 20	nm/min	At optimum MW power (125 W), 25 mTorr.
Optimum MW Power Density	~1.1	W/L	For maximum parylene growth yield (100 W in 91 L reactor).
Parylene N Contact Angle (Water)	80 to 90	°	CVD film (hydrophobic).
Parylene N Contact Angle (Water)	60 to 75	°	PECVD film (more hydrophilic).
Hydrophilization Time (O₂ Plasma)	15	seconds	Time required to switch parylene C to hydrophilic (contact angle < 30°).
Diamond Raman Shift	1332	cm^-1	Characteristic symmetric valence vibration proving spectroscopic purity.

Key Methodologies

The research focuses on comparing conventional Chemical Vapor Deposition (CVD) with Plasma Enhanced Chemical Vapor Deposition (PECVD), particularly using the p-xylylene dimer (DPX) as a precursor.

Conventional CVD (Gorham Method):
- Sublimation: Solid DPX precursor is sublimed at 100-150 °C under reduced pressure (a few Pascals).
- Pyrolysis (Cracking): The gaseous DPX is passed through a cracker unit heated to approximately 700 °C, cleaving the C-C bond to form the highly reactive monomer (MPX) double radical.
- Deposition: MPX is guided into a vacuum chamber at room temperature where it polymerizes heterogeneously on cold surfaces (surface polymerization) or homogeneously in the vapor phase (volume polymerization).
Plasma Enhanced CVD (PECVD):
- Activation: Microwave (MW) or Radio Frequency (RF) plasma is applied to the vaporous phase. Plasma electrons provide the energy for bond cleavage, significantly reducing the required thermal energy (eliminating the 700 °C cracker step in some variants).
- Process Control: The deposition rate and film quality are controlled by adjusting the MW power input and the total pressure. The optimum power input (e.g., 125 W for the tested reactor) maximizes radical generation while avoiding excessive fragmentation (passivation regime).
- Dilution Strategy: Inert gases (e.g., Argon) are added to the monomer flow. Dilution reduces the monomer density, suppressing second-order volume polymerization (which causes opaque, rough films) and favoring first-order surface polymerization, leading to smoother, higher-quality coatings.
Surface Functionalization:
- Hydrophilization: Deposited films (hydrophobic) are exposed to a mild oxygen (O₂) plasma treatment. This process replaces hydrophobic C-H bonds with hydrophilic C-OH bonds, rapidly reducing the water contact angle (e.g., 15 s treatment time).
- Hydrophobicity Enhancement: Doping the deposition atmosphere with fluorocarbon gases (e.g., CF₄) during PECVD increases the hydrophobic character of the resulting film.
Characterization:
- Chemical Structure: Fourier-Transformed Infrared Spectroscopy (FTIR) is used to confirm the preservation of the aromatic ring structure (key for parylene stability) and the presence of functional groups.
- Morphology: Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) are used to measure surface roughness, which is correlated with wettability (Wenzel and Cassie equations).
- Barrier Properties: Permeability against gases (e.g., N₂) and water vapor is measured to evaluate the film’s bulk quality.

Commercial Applications

The technology, particularly the deposition of Parylene and advanced carbon films via PECVD, is critical for several high-requirement engineering fields:

Industry/Application	Specific Use Case	Key Property Utilized
Medical Devices & Implants	Drug-eluting implants, neural probes, catheters.	FDA approval, biocompatibility, low gas/liquid permeability (barrier properties).
Semiconductor Manufacturing	Gate dielectrics, inter-layer insulation.	Excellent conformal coating, high dielectric strength, low mechanical stress.
Microelectromechanical Systems (MEMS)	Protective coatings for sensors and actuators.	Perfect conformity over highly rugged microscopic peaks and valleys.
Corrosion and Barrier Coatings	Protection of cold-rolled steel, electronic components.	Extremely low permeability against water vapor and corrosive gases.
Advanced Materials	Production of Diamond-Like Coatings (DLCs) and Carbon Nanotubes (CNTs).	Extreme hardness (DLCs), tailor-made electronic/optical properties (CNTs).
Surface Engineering	Creating superhydrophobic or hydrophilic surfaces.	Controllable surface roughness and chemical functionalization via plasma doping (O₂ or CF₄).
Acoustic Devices	Low-stress electret films for electroacoustic applications.	Controllable mechanical stress and dielectric properties.

View Original Abstract

Chemical Vapor Deposition (CVD) with its plasma-enhanced variation (PECVD) is a mighty instrument in the toolbox of surface refinement to cover it with a layer with very even thickness. Remarkable the lateral and vertical conformity which is second to none. Originating from the evaporation of elements, this was soon applied to deposit compound layers by simultaneous evaporation of two or three elemental sources and today, CVD is rather applied for vaporous reactants, whereas the evaporation of solid sources has almost completely shifted to epitaxial processes with even lower deposition rates but growth which is adapted to the crystalline substrate. CVD means first breaking of chemical bonds which is followed by an atomic reorientation. As result, a new compound has been generated. Breaking of bonds requires energy, i.e., heat. Therefore, it was a giant step forward to use plasmas for this rate-limiting step. In most cases, the maximum temperature could be significantly reduced, and eventually, also organic compounds moved into the preparative focus. Even molecules with saturated bonds (CH4) were subjected to plasmas—and the result was diamond! In this article, some of these strategies are portrayed. One issue is the variety of reaction paths which can happen in a low-pressure plasma. It can act as a source for deposition and etching which turn out to be two sides of the same medal. Therefore, the view is directed to the reasons for this behavior. The advantages and disadvantages of three of the widest-spread types, namely microwave-driven plasmas and the two types of radio frequency-driven plasmas denoted Capacitively-Coupled Plasmas (CCPs) and Inductively-Coupled Plasmas (ICPs) are described. The view is also directed towards the surface analytics of the deposited layers—a very delicate issue because carbon is the most prominent atom to form multiple bonds and branched polymers which causes multifold reaction paths in almost all cases. Purification of a mixture of volatile compounds is not at all an easy task, but it is impossible for solids. Therefore, the characterization of the film properties is often more orientated towards typical surface properties, e.g., hydrophobicity, or dielectric strength instead of chemical parameters, e.g., certain spectra which characterize the purity (infrared or Raman). Besides diamond and Carbon Nano Tubes, CNTs, one of the polymers which exhibit an almost threadlike character is poly-pxylylene, commercially denoted parylene, which has turned out a film with outstanding properties when compared to other synthetics. Therefore, CVD deposition of parylene is making inroads in several technical fields. Even applications demanding tight requirements on coating quality, like gate dielectrics for semiconductor industry and semi-permeable layers for drug eluting implants in medical science, are coming within its purview. Plasma-enhancement of chemical vapor deposition has opened the window for coatings with remarkable surface qualities. In the case of diamond and CNTs, their purity can be proven by spectroscopic methods. In all the other cases, quantitative measurements of other parameters of bulk or surface parameters, resp., are more appropriate to describe and to evaluate the quality of the coatings.

Tech Support

Original Source

DOI: https://doi.org/10.3390/pr9060980

References

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