Clean Etching without etching chemistry

Etching removes surfaces of substrates

  • in cleaning processes
  • for surface roughing
  • to form structures on surfaces

The etching substance must be able to attack the surface. Therefore it allways will be a chemically reactive substance with aggressive character at least on the substrate to be etched.

Most etching means however are aggressive on many substrate materials and therefore are harmfull to many materials different from those intended to be etched, Their handling, application, storage and waste disposal therefore requires particular attention and care.

Plasma etching

Almost any plasma with exception of an inert gas plasma is a very reactive and aggressive substance. A Low Pressure Plasma however only exists inside the hermetically isolated plasma chamber. The reactive medium are the Active Species:

  • Ions
  • Stimulated atoms
  • Radicals
  • UV-radiation

In the same instance when the plasma is flashed off all Active Species disappear immediately and the gas is no more aggressive at all. In general all important process gases such as Oxygene, Nitrogene, Argon, Hydrogene, Tetrafluormethane, Sulfur Hexafluoride are harmless substances if not ionisized.

Plasma etching in general does not require any particular safety procedures for health or inviromental protection. In some cases very small quantities of harmfull hydrocarbons can be generates by the etching process. In this case a charcoal filter will be integrated into the vacuum system which is able to absorb these substances.

 

 

Etching by Oxygene Plasma

The etching effect of an Oxygene plasma on Hydrocarbons is extensively used to remove oil, grease and release agents from surface.

In the same way an oxygene plasma is active on substrates which are based on hydrocarbons, most particullarly on allmost any polymeric (plastic) substrate.

Plasma cleaned plastic surface. Despite proper cleaning very often adhesion of coatings or paintings often is not satisfactory
As well as organic coatings and contaminations oxygen plasma also attacks organic substrates (plastics). This effect etches the surface
Surface layers are removed. The surface receives nanoscale structures. The increase of contact surface improves adhesion significantly

By Plasma etching in an Oxygene plasma a Nano-roughing of the surface is generated. The resulting increase of the effective contact surface optimizes the adhesion forces of any coating, glueing, printing or painting process.

By selecting appropriate intensity and time of an oxygene plasma treatment it can be determined if a surface will be cleaned, activates or even etched.

Oxygene plasma etching also enables glueing or printing of of POM surfaces (Delrin, Hostaform).

But still not of PTFE surfaces.

Etching PTFE

One of the most unstanding bernefits of completely fluorinated hydrocarbons like PTFE is its almost complete resistancy against any chemical attack. This characteristic however also prevents almost any chemical surface treatment. Moreover PTFE has the lowest surface energy of all solid materials, so that no wetting by adhesives, paintings or coatings takes place. Therefore it is considered to be practically impossible to glue, print or paint these substrates.

Even etching in an oxygene plasma does not improve adhesion to a PTFE surface

 

Even though also PTFE can be Plasma treated so that adhesion is provided.

This Plasma treatment process takes place in an hydrogen plasma. It can be considered to be an activation or an etching process.

The essential reason for the chemical resistancy of PTFE is the extreme stability of the C-F- bindings, which prevents the reaction with any other atom or molecule. Active species of the plasma however even crack C-F- bindings. Hydrogen -ions, -radicals and stimulated atoms join the Fluor-radicals which only exist for very small intervals of nanoseconds to form HF-molecules. Consequently unsaturated Carbon-bindings remain for restricted time, however at least for minutes, so that adhesion of molecules of adhesive mölecules is enabled.

Etching PTFE
Surface of a PTFE substrates. Fluorine atoms forming the surface layer prevent from any attack of chemicals.
Etching PTFE
Anyhow C-F-bindings are cracked in a hydrogen-plasma. Fluorine radicals join with hydrogen radicals to form HF-molecules. Carbon-bindings remain unsaturated.
Etching PTFE
Extrinsic molecles (adhesives, paintings can join to the unsaturated Carbon-links

Micro sandblasting

Oxygen plasma only can remove organic substances. However even anorganic matter can be removed by physical plasma etching. Physical plasma etching means etching by the impact of high energy projectiles. In case of plasma etching this means the bombardement by accelerated ions. Inert gas ions are used, preferably Argon, which do not form reactive Active species in a plasma.

The process is comparable with sandblasting except the fact, that the "sand particles" and the surface structures are of atomic dimensions.

Micro
A substrate with organic or anorganic surface contaminations
Argon ions of high energy hit the molecules and damage them just caused by their kinetic energy
This effect removes contaminations of any chemical character with low etching rates.
The surface of the substrate itself can also be etched by micro sandblasting
The bombardement of high energy ions releases molecules from the substrate
Independent from the substrate material the surface receives a nanoscale rough structure

Fluorine process gases

Saturated fluorine molecules generally are extremely stable. Tetrafluoromethane (CF4) or Sulfur hexafluoride (SF6) therefore usually are very inert gases, so that they are normally not involved in chemical reactions.

Plasma stimulation however even cracks these molecules so that fluorine radicals result. Remains are CF2- or CF3- radicals or SF3-, SF4- ... Radicals respectively. All of these Active Species are very reactive and attack even very inert solid surfaces such as Quartz Glas. 

Fluorine process gases are often used in microelectronic technologies to etch semiconductors and semiconductor oxides.