Cleaning of metal parts in oxygen radio frequency plasma: Process study

The removal of different lubricants (oils and greases) from metal surfaces by use of a capacitively coupled 13.56 MHz low-pressure oxygen discharge has been examined. Stainless steel, tool steel, brass, aluminum, and copper were used as a substrate materials. The ion bombardment and chemical reactions occurring at the metal surfaces in such a discharge allow for high cleaning speed. A hollow-cathode discharge has been used to increase the plasma density in contrast to a plane cathode discharge configuration. The cleaning process was characterized by use of emission spectroscopy and mass spectroscopy. Influence of rf power (up to 1 kW), pressure (from 0.01 to 1 mbar), and magnetic field (up to 20 mT) on the process duration and surface cleanliness has been investigated. The criteria for detection of the process end point are formulated and checked. The spectral information is useful for end point detection only in restricted regions of the process conditions. Results indicate a strong influence of the lubricant temperature on the process duration. The removal rate of N62 oil at 140 °C is twice as high as that at 50 °C. For temperatures higher than 140 °C the creation of a hardly removable solid residue occurs. The influence of the initial lubricant area and lubricant thickness on the process duration has been investigated. The increase of the lubricant area results in an increase of the process time (loading effect for cleaning process). The increase of the lubricant film thickness results in a proportional increase of the process time.