Selectivity
- The difference in etch rates of different
materials is characterized as selectivity.
- Selectivity is the ratio of the etch rates
between the different materials, especially the material that needs to be
etched compared with the material that we do not want to remove.
- During gate etch, the photoresist provides the
etch mask, and polysilicon is the material that needs to be etched.
- In the plasma etch process the photoresist
will also be etched to a certain degree. Therefore it is necessary to have
high polysilicon to photoresist selectivity to prevent excessive
photoresist loss before the etch process is finished.[2] [3]