From thermal barriers and corrosion protection to optical applications and energy efficiency, coatings are used in many areas and can be applied in many ways. Application techniques can generally be characterized as wet or dry.

Wet methods, such as electroplating, usually involve toxic, flammable or otherwise dangerous solvents, acids and other chemicals that pose serious safety and disposal concerns.

Dry methods tend to be cleaner and less hazardous than wet methods. Dry methods include physical vapor deposition (PVD) techniques, such as:

Physical Vapor Deposition (PVD) Techniques:

• Evaporation
• Sputtering

Physical techniques offer little capability for controlling the chemistry of the resulting coating.

Chemical Techniques:

• Chemical vapor deposition (CVD)
• Plasma-enhanced CVD (PECVD)
• Plasma spray

Chemical techniques allow better quality and a much larger variety of coatings to be applied, particularly when plasma is involved.

Limitations of Conventional Plasma Techniques
Traditionally, PECVD must operate at reduced pressure within a vacuum chamber. This greatly restricts the size and type of substrates that can be coated and impacts the economics of the process. Plasma deposition at atmospheric pressure has relied on thermal plasma spray techniques, which are extremely hot. This limits their use to refractory materials. In addition, the process control and coating quality resulting from plasma spray can be inadequate, and adjacent components are at risk of thermal damage.

Non-thermal plasmas (NTP) offer better process control and little thermal exposure. Until now, however, deposition processes based on atmospheric pressure NTPs have been too slow to be of practical importance.

A Cost-Effective Alternative
The APPJ technology allows plasma power densities several orders of magnitude higher than conventional NTPs, such as corona and DBD, and offers process rates that make such applications economically feasible for the first time. Oxides of silicon, titanium and aluminum, as well as nitrides of silicon, have already been demonstrated using APPJ technology.

One of the benefits of APPJ deposition is that the deposition precursor does not have to be fed directly into the plasma. The high production rate of chemically reactive species, such as atomic oxygen, allows the precursor to be mixed downstream in the plasma afterglow, where it reacts with the reactive neutrals from the plasma to produce a coating on the substrate.

A major advantage of the APPJ technology is that it generates much higher reactive species concentrations and can be used in a remote or downstream mode. The figure below shows a flux of deposition species flowing from the output of the plasma jet and onto the substrate. In this case the substrate passes in front of the APPJ instead of through it, as occurs in the in-situ mode. This creates the possibility of mixing one or more precursor gases downstream into the plasma afterglow, where they are activated by the large flux of reactive species prior to depositing on the surface of the substrate - rather than onto the electrodes.

Downstream Operation Reduces Downtime and Improves Process Control
In conventional technologies, coatings build up on the electrodes and chamber wall. This build-up becomes a serious problem, because it produces detrimental particulate contamination that significantly affects coating quality. Downstream operation alleviates the need for the downtime associated with frequent chamber cleans.

Downstream operation also allows for better process control. The precursor does not experience excessive fragmentation, because it is not passed through the plasma. No electrons are present in the afterglow, so electron-induced dissociation of the precursor gas does not occur. Instead, a reaction takes place between the atomic and metastable species generated by the plasma and the undissociated precursor gas. As a result, polymerization and deposition of precursor gas are much more controlled. This approach avoids the scrambling effect of electron-induced dissociation of the precursor, which is difficult to predict and control. Thus, thin films deposited by the APPJ technology via radical initiation are generally more uniform in structure and composition than are thin films deposited by passing the precursor directly through a plasma source.

Flexibility for "Spot" Deposition
The downstream nature of the APPJ deposition technology also offers the opportunity for "spot" deposition of thin films. Unlike other "batch" treatment methods, the APPJ technology allows placement of a coating or a greater thickness of a coating in the region it is needed, without coating the entire substrate or object.

The technology supports the use of any reactive species besides O₂, which gives the promise of a wide variety of chemistries, such as:

• NF₃ to yield atomic fluorine
• H₂ to yield atomic H for reductive chemistry
• N₂ or ammonia to yield atomic nitrogen for the preparation of nitrides
• Hydrocarbons such as methane to yield carbides

Using the Deposition Flat Jet, films of SiO₂, TiO₂, Al2O₃ and SiNx have successfully been applied to various substrates at deposition rates exceeding one micron per minute.

These benefits demonstrate that APPJ technology has the potential to offer a new low-temperature, environmentally sound, highly controllable method of depositing thin-film coatings and expands the domain of thin-film deposition methods beyond the current state-of-the-art.