After etching, the resist has served its purpose and must be removed from the SiO2. "Plasma ashing" or "dry stripping" is usually the first step. The wafers are placed into a chamber under vacuum, and oxygen is introduced and subjected to radiofrequency power which creates oxygen radicals. The radicals react with the resist to oxidize it to water, carbon monoxide, and carbon dioxide. The ashing step is usually done to remove the top layer or "skin" of the resist, then additional wet or dry etching processes can be used to strip away the remaining resist (see Etching). Some wet and dry chemical constituents are shown in Table 4. After the stripping is complete, the wafers are rinsed with deionized water to remove any remaining chemicals or resist material.
The following are the potential hazards of photoresist stripping.
- Possible employee exposure to acids used for wet chemical etching/stripping (see Table 4).
- Possible employee exposure to solvents used for stripping and rinsing (see Table 4).
Radiofrequency (RF) Radiation
- Possible employee exposure to radiofrequency (RF) radiation used as a power source for dry stripping.
- See Possible Solutions: Radiofrequency (RF) and Infrared (IR) Radiation.
- Install interlocks and emergency shut-offs on etching equipment.