Probeless voltage contrast using a focused ion beam for opens and shorts defect isolation of ultralarge scale integration technologies

Abstract

Defect isolation in large yield mazes for accurate and timely physical analysis continues to be a critical part of process development for 0.4 and 0.25 μm technologies. Shrinking dimensions have challenged common approaches such as light emission microscopy (LEM), optical beam induced current (OBIC), liquid crystal, and mechanical probing to the point where alternate techniques are needed. This article describes simple but powerful defect isolation techniques using probeless voltage contrast in a focused ion beam (FIB) tool. Fail modes to which the techniques are applicable include deep trench capacitor polysilicon to substrate leakage, gate conductor to self-aligned contact shorts, metal line opens, via chain opens, and metal line to line shorts. When possible, OBIC or LEM are initially used to determine the approximate fail location followed by precise isolation using the FIB. Typical examples of each fail mode will be shown. The basic principle employs the positive focused ion beam to charge floating structures which tend to attenuate the secondary electron image. This provides a suitable potential difference between grounded and ungrounded structures which can be easily observed in the electron imaging mode. Using this phenomenon along with leveraging the ability of the FIB to cut and deposit replaces the need for a probe. In addition, once a defect is isolated it can be precisely marked and then unlayered or cross sectioned in situ. This technique has provided the only practical solution to isolate defects in some situations while reducing the average turn around time for feedback to process engineering.