In semiconductor manufacturing plasma ashing is the process of removing the photoresist from an etched wafer. Using a plasma source, a monatomic reactive specie is generated. Oxygen or fluorine are the most common reactive specie. The reactive specie combines with the photoresist to form ash which is removed with a vacuum pump.
Typically, monatomic (single atom) oxygen plasma is created by exposing oxygen gas (O2) to ionizing radiation. At the same time, many free radicals are formed which could damage the wafer. Newer, smaller circuitry is increasingly susceptible to these particles. Originally, plasma was generated in the process chamber, but as the need to get rid of free radicals has increased, many machines now use a downstream plasma configuration, where plasma is formed remotely and channeled to the wafer. This allows electrically charged particles time to recombine before they reach the wafer surface, and prevents damage to the wafer surface.
Monatomic oxygen is electrically neutral and although it does recombine during the channeling, it does so at a slower rate than the positively or negatively charged free radicals, which attract one another. Effectively, this means that when all of the free radicals have recombined, there is still a portion of the active specie available for process. Because a large portion of the active specie is lost to recombination, process times may take longer. To some extent, these longer process times can be mitigated by increasing the temperature of the reaction area.