Plasma ultracleaning of metal surfaces

Following industrial treatment, virtually any surface will be contaminated and will have to be cleaned. Increasing technical demands for substrate surface quality, for example in preparation for high-quality coatings, require microscopically clean surfaces from which all monomolecular layers have been removed. This is, for example, an essential prerequisite for good adhesion.

Attractive solutions to this problem can be found through plasma ultrapurification, developed a number of years ago and since refined by Fraunhofer IGB.

In a low-pressure plasma, the parts to be purified are exposed in a vacuum chamber at pressures of about 0.1 to 2 mbar to a plasma, that is to say a partially ionized gas. Through the effect of the proportion of gas particles in the plasma gas, which are much more reactive than gases in the normal state, impurities are reactively removed from the surface.

IGB has carried out comparative investigation of a variety of types of plasma reactors (an example of a laboratory setup is shown in the following figure), which differ for example by the nature of the plasma generation. An important aspect for samples which are sensitive to radiation or electromagnetic fields, for example electronic components, is the possibility of a down-stream process, in which the samples can be placed outside the compartment where the plasma is excited.

As an all-in-one treatment, plasma ultrapurification is, however, unsuitable for:

• particulate impurities such as swarf or metal flakes
• a number of inorganic impurities, for example salts or fingerprints
• thick layers of contamination, for example in the case of heavily oiled parts or oil-filled bores.

In cases such as these, the parts must be precleaned, for example with aqueous or hydrocarbon-based agents.

By way of example, Figure 2 shows a comparison between different methods for cleaning steel in terms of the removal of carbon. The values were determined using ESCA (Electron Spectroscopy for Chemical Analysis). The best purification is achieved by combinations of precleaning and plasma ultrapurification. If extremely clean surfaces are desired, plasma-assisted processes which permit minimization to below 10 percent residual carbon are appropriate.

Systematic studies show that, depending on storage conditions, freshly purified samples become recontaminated by adsorption of molecules from the gas phase. This is shown in the figure below. The intervening storage time should therefore be as short as possible.