Gas compostion depends not only from the MFC's, they depends from the chamber wall status (deposition) too. The plasma parameters are enabled to mirror the gas composition. The tool parameters not!
First Wafer Effect
The first wafer effect mostly caused by increased gas temperatuer (decreased gas density).  The plasma parameters are enabled to mirror the gas density. The tool parameters (pressure) not! Optimization of warm up is possible.
  • Gas density is important, not the pressure!
  • Tool parameter does not reflect changes in gas density!
    Plasma parameters reflect changes in gas density
The plasma parameters represent the real tool (plasma) which treat the work pice (wafer). The recipe and hardware are only the presettings of the tool plasma.
The electron collsion rate is most sensitive to variation of chemistry (gas compostion), the plasma density indicates physical variations (hardeware: generator, match box, spare parts, reworked tool kids) more efficient.
Pre-process Issues
For example:
  • Mask errosion due to deposition fault (thickness, layer density - refractive index). The electron collision rate reacts on mask errosion due to changed gas composition.
  • Change of resist properties caused by too much delay between resist apply and exposure.
Chamber Matching
The plasma parameters represent the real tool (plasma). Differences in the plasma process are detectable by the plasma parameters.
Process Development
Only the plasma parameters can give information about the sensitivity of the plasma compared to the tool parameters.