Click for full article:
http://electroiq.com/blog/2015/03/process-watch-know-your-enemy/#.VSMINmJzhCc.twitter

Variability is the Enemy of a Well Controlled Process.

In a wafer fab there are many different types of variability — all of them are bad.

• Variability in the lot arrival rate, the processing time and the downtime of processing tools, to name just a few sources, all contribute to increased cycle time
• Variability in the physical features (CD, film thickness, side-wall angle, etc.) contribute to increased leakage current, slower part speed, and yield loss
• Variability in the defect rate leads to variability in the final yield, in the infant mortality rate, and in long-term reliability
• Most importantly, variability degrades our ability to monitor small changes in the process – the signal must be greater than the noise in order to be detectable

...

It’s similar to looking at the stars on two different nights. Sometimes you see them all; sometimes you don’t. The stars are still there—it’s just that the conditions have changed. Something analogous happens with wafers. The exact same defects may be present but the conditions (film stack, CD, overlay, etc.) have changed. An inspection tool with a tunable wavelength allows you to filter out the background noise in the same way that a radio telescope allows you to see through the clouds. Inspection tools with flexible optical parameter settings (wavelength, aperture, polarization, etc.) produce robust inspections that effectively handle changes in background noise and take the variability out of the defect inspection process