Plasma cleaning, ashing, and descum of silicon wafers
Cleaning, ashing and descum are terms that relate to the cleaning of silicon wafers:
- Wafer cleaning commonly refers to the removal of organic contamination.
- Wafer descum is the process for the removal of the remnants of photoresist (PR) that remain in unwanted areas of the wafer (see infographic below).
- Wafer ashing is where photoresist film is completely removed from the wafer, leaving a pristine surface for subsequent processing.
Traditional, multi-step wet cleaning methods use combinations of harsh chemicals with increasing complexity as the requirement moves from cleaning, to descum, to complete PR removal.
Cleaning of organic residues for example may involve multiple hot batch acetone washing steps followed by multiple isopropyl alcohol washing steps. Complete PR stripping on the other hand is often performed with Piranha solution, an aggressive hydrogen peroxide and sulphuric acid solution.
The Plasma Cleaning Method
Plasma cleaning is a simple, fast and inherently clean alternative wafer preparation treatment. Oxygen plasma generates UV light together with reactive oxygen species which are extremely effective at the removal of both organic contamination and also complete PR layers. Both cleaning and PR removal are performed in the same equipment under the same conditions, with only the time of exposure to the oxygen plasma being varied.
The PR removal process can be seen below, with complete removal of PR in under 30 minutes and without any waste products or chemical disposal requirements.
Oxygen plasma treatment also introduces polar functional groups, raising surface energy and producing a high surface wettability. This is essential in spin coating for example, where a contaminant-free wafer surface is a must to ensure complete and even spreading of the coating with correspondingly good adhesion