PLASMA

Good day to everyone who reads this blog.  I have started this informal communication with you to reach out to people who are interested in engineering surfaces, and want to find out what atmospheric pressure plasmas can do for them.  I am presenting this material with my hat on as Senior Vice President at Surfx Technologies, a subsidiary of Precision Mechatronics Pty Ltd.  Over the years, I have learned that people have a lot of questions concerning the use of plasmas to manufacture commercial products.

The main questions are: (1) what are the principal applications of plasmas in manufacturing; (2) how do I know which plasma to choose from the many types on the market, and (3) why would I select plasma over other types of surface treatment, such as abrasion, solvent wiping, or chemical etching.  Over the coming months I am going to take a shot at answering these questions.  I encourage readers of my blog to ask me questions, or select topics that they would like me to discuss in the areas of surfaces and plasma processing.  I sincerely hope you find this helpful as well as enjoyable to read.

Definition of Plasma
Before I start, let’s make it clear that we are talking about an ionized gas, and not a component of blood.  If you’re interested in blood, you’ve come to the wrong website.  An ionized gas, or plasma (or alternatively, a gas discharge), is comprised of free electrons, negatively and positively charged ions, and neutral molecules.  Plasmas can be created many ways, but the most common method is through the application of a sufficient voltage to strip electrons away from the neutral molecules, thereby ionizing the gas.  Plasmas conduct electricity, consume power (watts), and are sustained by the application of an electric potential (volts) and current (amps).

Applications of Plasma in Manufacturing
The principal applications of plasma in manufacturing are for making functional materials and surfaces.  Plasmas come in contact with materials at their surfaces.  Any change in material function must proceed through processes that occur at the surface.  The important applications are therefore:

(1)   Cleaning, i.e., contaminant removal.

(2)   Activation for wetting, i.e., adjustment of the surface energy.

(3)   Activation for adhesion.

(4)   Sterilization.

(5)   Etching of nanometer to micron scale features in materials.

(6)   Deposition of nanometer to micron thick coatings.

The six applications listed above are presented approximately in order of the time it takes to accomplish the task.  Cleaning can take a short or long time depending on the amount of contaminant on the surface.  “Gross contamination” corresponds to organic layers that are more than a micron thick.  This type of contamination is best handled by aqueous washing or solvent rinsing.  An exception to this rule is photoresist film removal, which is a standard plasma process carried out by the semiconductor industry.  “Fine contamination” is present on all surfaces, even after those recently cleaned.  This last layer on the surface is best removed by plasma, and it takes on the order of 0.1 to 10.0 seconds to complete.

Surface activation for wetting is the process of putting specific chemical groups on a material surface to precisely fix its energy.  Wetting refers to the spreading of water droplets onto a surface to make a continuous film.  If there is a low surface energy, droplets will not spread out, and the water contact angle between the droplet and the surface will be high, ~90^o.  By contrast, if the surface energy is high (say 70 dynes-cm), the droplets spread out and merge together easily, with the water contract angle below 20^o.  Wetting is necessary in some industries, such as printing, to get the desired coverage of the fluid on the solid surface.  Putting functional groups down on a surface is a fast process, because only one atomic layer is being changed.  This process is completed in the millisecond to second time range.

Surface activation for adhesion is also the process of putting specific chemical groups on a material surface, but this time the goal is to achieve strong bonds between the surface and an adhesive or glue.  As stated by Dr Mittal, “the strength of an adhesive bond increases with the quality and quantity of connections made at the interface.”  Since here as well we are only affecting roughly one atomic layer of the material, this process is fast, and can be completed in the millisecond to second time range.  If cleaning to remove fine contamination is required prior to activating the surface, then the process time can increases to tens of seconds.  Adhesively joining materials is ubiquitous in manufacturing, cutting across many industries from automotive and aerospace to packaging and electronics.  This is by far the largest industrial application of plasmas.

Sterilization is at present a relatively small application of ionized gas discharges.  Here we are killing microorganisms prior to packaging products which are destined for human consumption, either, food, drugs, medical devices, medical instrumentation, etc.  If the microorganisms are present on the product in thick film form than washing is probably the most effective cleaning route.  However, if you need to make sure that every last bug is killed down to the last layer on the surface, then plasma sterilization is a good way to go.  This process takes several seconds to several minutes to complete.  In this case, the job is finished when less than one biological organism remains out of more than a million that were present initially.

Etching nanometer to micron scale features on a surface is a crucial step in manufacturing integrated circuits, flat panel displays, microelectromechanical systems, and other microelectronic devices.  Vacuum plasmas are uniquely capable of etching these features, because one can direct the positively charge ions in the gas to bombard the surface with high energy, such that trenches with straight sidewalls are generated in the material.  Blanket etching is possible as well, as in the case of photoresist removal.  However, this process is relatively slow taking from a minute to as much as an hour to finish.

The last important application is the deposition onto materials of nanometer to micron thick coatings.  Thin film deposition is another crucial step in manufacturing integrated circuits, flat panel displays, microelectromechanical systems, medical devices, etc.  Plasmas are very valuable tools for this process, because they enable the coatings to be laid down at low temperatures where no thermal damage to the expensive electronic device can occur.  Since this process usually requires depositing many thousands of layers of atoms on the material one atomic layer at a time, it is a relatively slow process taking from a minute up to an hour to complete.