FAULT PATTERNS

Typical patterns of process instabilities and their major reasons.
Blue curve: normal wafers. Red curve: faulty wafers.

• Drifts of plasma parameters:

	Drifts of electron collision rate caused by changes in: Gas composition (e.g., leakage, MFC fault) Mask or film composition Drifts of electron density caused by changes in: RF-power in plasma   Best indicator: Mean of plasma parameter.
	Drifts of electron collision rate caused by changes in: Density of neutrals caused by temperature and pressure change The ions heat the gas and the gas the chamber parts. The temperature of the gas increases and the density of the gas decreases.  Reason: The pressure is kept constant. Thus the gas density and electron collision rate decreases. Result: First wafer effect
	Power variation leads to temperature change and so to changed gas density. Higher power  → higher temperature  → lower gas density  → smaller collision rate

• Endpoint shift caused by faulty in pre-process

Variations of etch rate and/or film thickness for endpoint controlled processes.   Please note: The mean for the individual  process steps does not change, whereas a significant change can be seen for the mean of all steps.

• Peaks in plasma parameters:

Peaks in electron collision rate caused by particles: A parasitic plasma or Arcing at shower head, at chamber wall, or wafer. Best  indicator: Coefficient of Variation (StdDev /Mean).  Please see also Process Fault - What Now?

• Peak at the beginning or the end of the process
  
  

  Ramp up and down of RF power causes peaks by running through different low-density discharge regimes. Normally not significant for the process. Solution: Cut of the first and last seconds of the process for calculation of mean.