Use of diodes to enable μLoop® test structures for buried defects and voltage to grayscale calibration

Two test structures that address key problems for voltage contrast inspection are introduced. These structures both utilize diodes to provide unique capability. The first structure illustrates how a two-pass area accelerated inspection can be used for detection of buried defects such as gate shorts or via opens. Like standard μLoop^® test structures, the first inspection pass is able to detect the existence of all the defects affecting the structure while only scanning a small fraction of the structure. The purpose of the second inspection pass is to look for physical signs of the defect, but none are generally visible for buried defects. The key to this structure is that diodes are used to indicate the location of the buried defect. The second structure is used to map voltage to grayscale. This structure takes advantage of the well-known fact that there is a 0.6V drop across a forward biased diode.