Material contrast identification and compositional contrast mapping using backscattered electron imaging

Scanning Electron Microscopy (SEM) is a vital high magnification and high resolution inspection tool in failure analysis of semiconductor devices. It provides an image of a sample by scanning the sample surface with a high-energy beam of electrons in a raster scan pattern. The electrons interact with the atoms that make up the sample producing different signals that contain information about the sample's surface topography and composition. Interaction of an accelerated electron beam with a sample target produces a variety of elastic and inelastic collisions between electrons and atoms within the sample. The signal produced by the elastic collisions is the backscattered electrons (BSE). BSE imaging is used to differentiate contrasts in a material comprising of different chemical compositions. It can locate regions of different atomic number and high atomic number impurities. It is also very helpful for obtaining high resolution compositional maps of a sample and for quickly distinguishing different material phases. This paper shows the importance of BSE imaging as one of the exemplary scanning electron microscopy technique in failure analysis of semiconductor devices. Fundamentals and concepts behind this technique are also discussed. Case studies show how BSE imaging played a vital role in giving more material information leading to the understanding of the real phenomenon behind the observed failures or material interactions.