Si Nitride Etch
Quali-Surf On-line Si Analysis of Hot Phosphoric Solutions
Phosphoric Acid at 160C is used to selectively etch Silicon Nitride mask while preserving Silicon Dioxide. Selectivity and etch rate are controlled by the concentration of dissolved Si. The old practice is to “season” new etching solution by processing several dummy wafers, which introduces Si. The process is “blind”. Concentration of Si is increased over the life time of the solution causing variability of process characteristics. There is a need for fast and accurate on-line measurement of Silicon (ranging from 0-100 ppm) in the etching solution.