Wafer charging quantification using image distortion

BackgroundElectron beam induced charging can introduce complications during defect inspection and metrology applications. Caution needs to be taken when choosing scanning electron microscope (SEM) condition during recipe set up.AimTo identify a reliable key performance indicator (KPI) based on SEM imaging that does not require repeated exposure. This KPI needs to be strongly correlated to charging and easily quantified. It is expected that the KPI can help simplify the recipe creation flow and increase the recipe robustness.ApproachHypothesized surface charging either impedes or facilitates the probe beam scanning, which would result in shrinking or expending field-of-view, respectively.ResultsTested the hypothesis on after-development-inspection wafers and observed image distortion being modulated by landing energy. Confirmed distortion, once presented, would increase with exposure repeats, which can be reasonably explained based on charging.ConclusionsImage distortion can be used as a reliable KPI to quantify wafer charging. This will greatly simplify recipe creation and improve recipe robustness. Also, the neutral condition in turn benefits the imaging and minimizes distortion.