a method for in - line monitoring of via / contact etching process based on a test structure is described . the test structure is comprised of via / contact holes of different sizes and densities in a layout such that , for a certain process , the microloading or rie lag induced non - uniform etch rate produce under - etch in some regions and over - etch in others . a scanning electron microscope is used to distinguish these etching differences in voltage contrast images . image processing and simple calibration convert these voltage contrast images into a “ fingerprint ” image characterizing the etching process in terms of thickness over - etched or under - etched . tolerance of shifting or deformation of this image can be set for validating the process uniformity . this image can also be used as a measure to monitor long - term process parameter shifting , as well as wafer - to - wafer or lot - to - lot variations . advanced process control can be performed in - line with the guidance of this image so that potential electrical defects are avoided and process yield ramp accelerated .