Plasma impact onto surfaces

What is a plasma?

Plasmas are partially or completely ionized gases and vapors, which besides ions and electrons also contain chemical radicals and a large number of electronically excited particles. Far more than 99 percent of the universe known to us is in a plasma state. A plasma can be e.g. ignited and maintained by an electromagnetic field.

Characteristic of each plasma is its luminescence which, depending on the type of gas and its pressure, shines violet, blue, green, yellow, orange, or reddish. The plasma luminescence is used in fluorescent tubes, e.g. as advertising signs. Argon tubes shine blue, neon tubes produce an orange-red light. However, vaporous liquids may also be used as the illuminating gas, for example in fluorescent lamps, which are generally filled with a mercury-argon mixture.

Plasma-substrate interactions

The ions, molecules and atoms in electronically excited states, UV and light emissions, and high kinetic energy particles (especially ions) activate and/or etch surfaces, induce polymerization of many (especially organic) substances in the gas phase and/or on surfaces, and lead to film building on the substrate surface.

The plasmachemical reactions pathways and products differ appreciably from those with "conventional" chemistry. Plasmas can induce chemical reactions even with gases and surfaces which are totally inert under normal conditions – saturated hydrocarbons, nitrogen and inert gases are all activated or ionized in plasmas and participate in chemical reactions.

The following table illustrates some of the practically important forms of plasma-substrate interaction:

 

material change organic materials inorganic materials
removal

etching

cleaning (e. g. degreasing, sterilization)

change of the surface morphology (e. g. roughening)

 

etching

cleaning

 

modification

activation for subsequent processes

grafting

crosslinking

 

plasma oxidation, plasma nitriding
deposition plasma polymerization plasma-CVD

What are plasmas?

Plasma oxidation of polyethylene: animation.

The animation above demonstrates only one of the possible reaction pathways during plasmachemical oxidation of polyolefins. The reaction is initiated with atomic oxygen, which abstracts an hydrogen atom from the surface. This result in a free radical on the surface, which in turn can react with an oxygen molecule to produce a hydroperoxide group, which can decay into a keton group.

Besides keton groups, there also formed aldehyd, carboxyl, hydroperoxide, hydroxide, and ester groups (the picture above). The plasma oxidation results furthermore in breaking the polymer chains and material removal. A more or less specific surface functionalisation is still possible under precise control of the reaction conditions.