|      Wafer Charging Monitors, Inc.
Introducing DamageMap Software!   In response to customer requests, WCM developed DamageMap : a new, simple method to compare CHARM®-2 results to wafer maps of product yield. DamageMap summarizes results that used to take several different wafer maps and J-V plots to display. This makes it much easier to assess the impact of wafer charging on product yield and to identify the fab equipment / process step causing the problem.
Better than Surface Potential Measurements
Before DamageMap, IC makers tried to use maps of surface potential to assess wafer charging damage. Sometimes these maps correlated with wafer damage, but often they did not. This discrepancy is no surprise, since the charge needed to produce significant surface potentials is several orders of magnitude lower than the breakdown charge of the gate oxide. So, high surface potentials alone do not always lead to damage. Moreover, the part of the wafer that sees the highest surface potentials is often not the part that experiences the maximum oxide current density responsible for device damage.
Better than J-V Plots
Current density, not surface potential, causes wafer charging damage. This is why CHARM®-2, with its unique ability to measure wafer charging current, has proved so superior to other monitors that only measure surface potential. However, comparing product yield wafer maps with J-V plots is cumbersome because, for example, hundreds of J-V plots may be needed to completely describe the charging characteristics on a 200 mm wafer (one J-V plot for each die location). Now, using DamageMap, this process is greatly simplified by generating a single wafer map that shows the values of damage-producing oxide current density.
Convenient and Practical
Previously, to assess wafer charging damage to gate oxides, you had to compare the gate oxide's Fowler-Nordheim characteristic with the corresponding J-V plot from CHARM®-2. If the two plots intersected, you could expect damage, otherwise not. However, in some wafer fabs, Fowler-Nordheim plots were not always available, but virtually every fab in the world measures gate oxide breakdown voltage. So, DamageMap uses gate oxide breakdown voltage as the input variable, an acceptable choice because charging damage in process tools takes place in seconds, thus at a high current density that occurs at nearly the breakdown voltage of the oxide.
Easy to InterpretDamageMap compares the wafer fab's gate oxide breadown data to J-V plots from CHARM®-2 and generates a wafer map showing damage-producing oxide current density at each die location. DamageMap wafer maps from a given process tool may be compared directly to product yield maps. If the DamageMap patterns correlate with the yield loss patterns and the DamageMap current densities are high, then the tool is a likely suspect. Spot wafer charging problems at a glance with DamageMap!
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