Moiré Fringe analysis of accuracy of nano metrology in the SEM

Image based metrology for Scanning electron microscope (SEM) has become most important, versatile, and widely employed techniques for metrology as the 'Critical Dimension' scale has been reduced from micrometer to nanometers. However, the image that is captured and displayed by the SEM is not guaranteed to be true representation of the actual structure especially at nano meter level because, the factors that may be of little importance at micrometer level may induce substantial at nanometer level. One major problem in the measurement made using SEM is that the field of view of the image is calculated - based on the data provided by the manufacturer on beam energy, working distance current flowing through the scan coils -- rather than measured. As a result the two measurements of an object made by identical instruments under same instrument are likely to result in significantly different values of the sizes of same feature. Accurate metrology in the size range from nanometer to micrometer requires access to traceable artifacts of know size length. In this thesis a procedure for the calibration of the imaging field of view using a specially fabricated traceable standard artifact by moiré fringe technique is developed and demonstrated. The SEM is a mapping - rather than imaging - instrument so the accuracy of the geometric mapping of the image data from specimen to the display screen is crucial to the accuracy of the SEM image based metrology. Non-linearities in the raster depend on the scan speed an on the magnitude on the scan current so, consequently, the apparent size of the objects in the display may vary depending on the where in the field of the view they appear. In addition the X and Y axes of the raster scan may not be orthogonal, and the magnification of an object may vary with time if the specimen itself is charging. This thesis develops a novel technique using moiré fringes which permit such errors to detect and quantified.