At its core, Chemical Vapor Deposition (CVD) deposits a thin film of precursor material onto the surface of a substrate through a thermally driven chemical process. A chemical reaction vaporizes a precursor material before exposing the substrate to the vapor. The precursor collects and solidifies into a thin coating on the substrate surface. Since it is a thermally driven process, CVD requires a process temperatures between 600°C and 800°C.
Plasma-Enhanced Chemical Vapor Deposition (PE-CVD) is a hybrid process that uses cold plasma rather than thermal energy to drive the chemical vaporization process. As a result, the PE-CVD process temperatures are between 250°C and 350°C. The lower process temperature of PE-CVD makes it attractive and ideal for depositing DLC on optical substrates.
In PE-CVD, DLCs are deposited on an optical substrate by exposing an ionizable gas such as methane (CH4) or Acetylene (C2H2) to a sustained glow discharge which is an inert plasma. The plasma exposure “cracks” or ionizes the ionizable gas into carbon and hydrogen ions. The carbon ions accelerate toward the optical substrate, which is attached to the grounding electrode, and a layer of amorphous carbon is deposit deposited on the substrate. Since methane is available in high purity forms, it is often the preferred ionizable gas for optical applications.
In most PE-CVD systems, the plasma gas and ionization gas are introduced at the top of the chamber while a vacuum pump evacuates the chamber from the bottom. In contrast, Dynasil’s PE-CVD system injects gases at the bottom of the chamber and evacuates them from the top. The result is a “Coat Up” process that minimizes many of the typical defects – pinholes, stress, uniformity and adhesion – found in DLC coatings.