Applied Materials publishes a case study on how its team helps to improve image sensor process at LFoundry Avezano, Italy fab. One of the stories talks about excessive leakage current due to improper oxide spacer etch step.
"The FabVantage team determined the root cause of the problem to be a recipe with low oxide-to-polysilicon sensitivity, which sometimes led to over-etching into the silicon substrate. Further complicating the matter, this overetching was not uniform across the tool’s chambers. Applied and the customer identified an optimized recipe as the best solution (see figure 1). It was also determined that the chamber lid temperature set point varied from chamber to chamber, which caused nonuniform etching across chambers. In addition, sensor trace analysis and Applied’s best known methods (BKMs) were used to optimize the etch process to eliminate voltage spikes that are known to lead to yield loss."
"The FabVantage team determined the root cause of the problem to be a recipe with low oxide-to-polysilicon sensitivity, which sometimes led to over-etching into the silicon substrate. Further complicating the matter, this overetching was not uniform across the tool’s chambers. Applied and the customer identified an optimized recipe as the best solution (see figure 1). It was also determined that the chamber lid temperature set point varied from chamber to chamber, which caused nonuniform etching across chambers. In addition, sensor trace analysis and Applied’s best known methods (BKMs) were used to optimize the etch process to eliminate voltage spikes that are known to lead to yield loss."
 |
| A recipe with low oxide-to-polysilicon sensitivity sometimes led to over-etching into the silicon substrate (figure 1a left). A new recipe eliminated the problem (figure 1a right). The graph compares the oxide-to-polysilicon sensitivity of several of Applied’s proposed recipe changes with the customer’s original recipe (figure 1b). |
