Vacuum deposition is a family of processes used to deposit layers atom-by-atom or molecule-by-molecule at sub-atmospheric pressure (vacuum) on a solid surface. The layers may be as thin as one atom to millimeters thick (freestanding structures). There may be multiple layers of different materials (e.g. optical coatings). A thickness of less than one micrometre is generally called a thin film while a thickness greater than one micrometre is called a coating. The vacuum environment may serve one or more purposes including:
- reducing the particle density so that the mean free path for collision is long
- reducing the particle density of undesirable atoms and molecules (contaminants)
- providing a low pressure plasma environment
- providing a means for controlling gas and vapor composition
- providing a means for mass flow control into the processing chamber.
Condensing particles may come from a variety of sources including:
- thermal evaporation, Evaporation (deposition)
- cathodic arc vaporization
- laser ablation
- decomposition of a chemical vapor precursor, chemical vapor deposition
When the vapor source is from a liquid or solid material the process is called physical vapor deposition (PVD). When the source is from a chemical vapor precursor the process is called low pressure chemical vapor deposition (LPCVD) or, if in a plasma, plasma enhanced CVD (PECVD) or "plasma assisted CVD" (PACVD). Often a combination of PVD and CVD processes are used in the same or connected processing chambers.
In reactive deposition the depositing material reacts either with a component of the gaseous environment (Ti + N → TiN) or with a co-depositing species (Ti + C → TiC). A plasma environment aids in activating gaseous species (N2 → 2N) and in decomposition of chemical vapor precursors (SiH4 → Si + 4H). The plasma may also be used to provide ions for vaporization by sputtering or for bombardment of the substrate for sputter cleaning and for bombardment of the depositing material to densify the structure and tailor properties (ion plating).