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What is a plasma?
 Plasmas are said to be the fourth state of matter and are among the most widely studied subjects
in modern science. Stated simply, a plasma is an ionized gas that conducts electricity.
Plasmas come in many forms, ranging from a welding torch to a fluorescent light bulb. As
show in Fig. 01, they may be identified by their relationship between the neutral
gas temperature and the electron temperature in the plasma (1 eV ~11,000 ºC). In a torch,
an arc forms between the electrodes resulting in complete ionization of the gas and a
temperature rise >4,000 ºC. Atmospheric plasma spray guns take advantage of the high
temperatures found in arcs. These devices are able to vaporize many materials, and coat
them at high speed onto surfaces.
The fluorescent light bulb, by contrast, contains a low pressure glow discharge. The vacuum
inside the bulb is essential, because it allows the gas to be weakly ionized with low power
input. Such weakly ionized plasmas are not at thermal equilibrium, i.e., the neutral molecules
are close to 25 ºC, while the free electrons are above 10,000 ºC. The high-energy electrons
collide with the neutral molecules, dissociating them into atoms and radicals, thereby yielding
a reactive gas mixture. Most plasmas used for materials processing at low temperature are weakly
ionized and produced in a vacuum.
Using plasmas for materials processing requires expertise in chemistry. The Atomflo™ atmospheric
plasma is a source of atoms and radicals that are carefully selected for surface treatment,
cleaning, etching, or depositing thin coatings. For example, a hydrogen plasma may be used to
cure polymer films, whereas a nitrogen plasma may be combined with silane to deposit a silicon
nitride plasma coating. The key in all these applications is combining the right hardware with
the right chemistry to achieve the desired results.
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What hardware is used to generate plasma?
One must apply an electrical field to the gas to generate a plasma. In a vacuum chamber, where
the ions and electrons have long lifetimes, it is relatively easy to do this. Radio frequency
power can be applied to two metal plates immersed in the vessel creating a capacitive discharge.
Alternatively, RF power may be deposited into a coil mounted on the chamber walls, thereby
producing an inductively coupled plasma. The gas also may be ionized by application of microwave
power at 2.45 GHz to a specially designed cavity or horn.
In an atmospheric plasma, a variety of power supplies may be used from DC to RF. Here, the
challenge is to design the electrodes and gas flow to yield intimate contact between the reactive
gases and the substrate. At high pressure, the atoms and radicals are quickly consumed by
collisions in the gas, so the transit time from the plasma to the surface must be short. If one
wishes to achieve low temperature operation with atmospheric plasmas, then special precautions must
be taken to prevent arcing between the electrodes.
A wide variety of plasma tools are available today, reflecting the broad spectrum of materials
applications being tackled by these remarkable products. To the end user, the choices can be
bewildering, making the task of product selection challenging at best. We encourage you to
contact Surfx Technologies so that our engineers may assist you
with the selection process.
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How fast will the plasma treat the surface?
The speed depends on the application and the sample composition. Most plastic surfaces are activated for
adhesion in a few tenths of a second with no appreciable heating of the sample. Many customers are extremely
satisfied with the throughput of Surfx® plasma tools.
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Can I send you samples to treat and see if it works?
Yes! We offer one free demo to new customers and provide an Application Report when we send the samples back.
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Can I rent a system to test at my facility?
Yes! We offer monthly rentals of our systems. For a quote, please contact a Surfx® representative.
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How is surface activation measured?
The common method of measuring surface activation is water contact angle (WCA). The WCA is directly related to the surface energy
of the solid-water interface.
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Do any parts of the plasma source wear out?
No. Provided the Atomflo™ is operated properly, it will not wear out under continuous use in high volume manufacturing.
The RF plasma does not erode the electrodes such as occurs in thermal plasma torches.
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What types of gases can I feed to the plasma source?
Generally, you feed argon or helium and up to 2 vol.% of molecular gases, such as oxygen, nitrogen, hydrogen,
and carbon tetrafluoride. The molecules will be dissociated by the plasma into atoms, e.g., O, N, H or F.
The inert gas allows you to have complete control over the plasma chemistry.
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Where do I place the plasma source relative to my sample surface?
If possible, the plasma source should be placed within 5 mm of the sample surface. The reactive atoms generated
by the plasma are quickly consumed at atmospheric pressure. Moving the source farther away from the surface increases
the process time.
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What is the advantage of atmospheric pressure operation? Can I use the Atomflo™ in a vacuum system if I want to?
The advantage of atmospheric plasmas is their ease of use, cost effectiveness, and ability to continuously treat
materials of any size and shape. Furthermore, the reactive gas flows out of the plasma source and around and through
complex parts, such as MEMS. The Atomflo™ is unique in that it is a particle-free plasma source, and generates
significantly less particle contamination than vacuum plasmas.
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What is the difference between downstream and direct plasma processing?
That's a good question! Our pictures of the Atomflo™ show the plasma flowing out of
the device. Actually, at this point the gas is no longer strictly a plasma, because practically all of the electrons
and ions have been stripped away. Our products are used for downstream plasma processing. The plasma is sustained
between the electrodes inside the device. What flows out is primarily neutral species, i.e., atoms, radicals and
metastable molecules. These reactive neutral species are responsible for the reactions that occur on the substrate surface.
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Does the plasma generate UV light?
Ultraviolet light is generated by the plasma, but its intensity is very low, and we have not heard of it ever
causing any damage to treated parts.
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What safety concerns should I be aware of when operating the Atomflo™?
Oxygen plasmas generate a small amount of ozone, so you should operate the source under adequate ventilation,
such as inside a hood. The plasma source does not present an electrical shock hazard. However, it can get hot
to the touch. Please employ standard safety precautions as you would with any equipment that uses electricity
and compressed gases. Read the manual and follow the instructions. Always wear safety glasses!
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