Questions about Diamond-Like Carbon
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.
Unlike real diamonds, Diamond-Like Carbon has no crystalline structures. The term Diamond Like Carbon (DLC) was created to differentiate this coating from other graphite coatings, which are lamellar, or layered. Dissociated cubic carbon (sp3 or diamond carbon) and hexagonal carbon (sp2 or graphite carbon) structures deposit arbitrarily on a surface creating a completely amorphous material solid coating.
The amorphous organization of DLC means the coating appears smooth to the naked eye while resembling a layer of pebbles microscopically. It also means DLC has no brittle fracture planes, which makes it a very tough material which inspires the name “Diamond-Like.”
Here’s a Scanning Electron Microscope (SEM) image of a gold-coated replica of a ta-C “diamond-like” coating.
(Image from https://www.wikiwand.com/en/Diamond-like_carbon)
In addition to being exceptionally hard, DLC has several other attractive properties including:
- Low coefficient of friction
- Abrasion resistance
- Chemically inert i.e. it is resistant to salts, acids, alkalis, and most organic solvents
- Biologically compatible
- Electrically insulating
- Optically transparent
These properties combined with smooth surfaces make DLC attractive for a broad range of applications from machining tools to motor and other frictional components to biomedical equipment to electronics and semi-conductor to solar to optics.
DLC is especially well-suited for high-performance infrared (IR) optics. It adheres well to a variety of infrared and ultraviolet substrates including Germanium (Ge), Silicon (Si), Zinc Selenide (ZnSe), Zinc Sulfide (ZnS), Fused Silica and Chalcogenides. It is an anti-reflective (AR) coating with high hardness and good stress resilience causing only minor losses (between 5-10%) in transmission. As a single-layer coating, DLC can be adjusted to tune it for specific spectral ranges.
Diamond-Like Carbon (DLC) coatings have been around since the late 1970s. These coatings are used in machining and automotive applications because of their exceptional durability, high hardness, low coefficient of friction and self-lubricating properties. Not only did the coating reduce wear on frictional components, it also increased the efficiency of motors. The same properties that make DLC an excellent coating for frictional applications also make it a superior coating for optical and electrical applications.
However, there are four common issues with the quality of a DLC coating on IR optics:
- Small defects called pinholes
- Non-uniform coating thickness
- Residual stress between the coating and the base material
- Poor coating adhesion
One source of these defects is the design of the DLC deposition process and the chamber configuration. Plasma Enhanced Chemical Vapor Deposition (PE-CVD) has emerged as a preferred process for addressing these flaws.