Study of CDSEM measurement issue caused by wafer charging
Scanning Electron Microscope (SEM) image blurring issue is reported at via level after developing inspection (ADI) measurement in dual damascene process. The root cause is the existence of non-uniform electric field at measurement locations. To find the reason, wafer surface charge is measured at different steps. Electric field is almost uniform at hard mask (HM) deposition, metal ADI, HM open, and photoresist (PR) coating steps. Strangely, significant change has been found at via ADI step. This change is not simply caused by the developing process because SEM image is clear and the electric filed is uniform at ADI stage on bare wafer. It may be also relate to previous process especially the HM open step although the mechanism is not clear at this moment. Finally, auto-stigma function of CDSEM is applied to fix this issue. Critical dimension (CD) data is comparable between auto-stigma measurement method on non-uniform electric field wafer and normal measurement method on uniform electric field wafer.