Isolating light-sensitive defects using C-AFM

A soft failure, which is recoverable and sensitive to certain stresses, such as voltage, temperature and light, is defined as a failure, fault, defect, or error that results in a shift in the operating margin of a device. Several studies have been conducted into voltage or temperature dependent failures [1–3]. Research into light sensitive failures, however, has seldom been reported, as it is more difficult to isolate defects which are sensitive to light. Although certain global fault isolation techniques, such as photoelectric laser stimulation (PLS) have been developed to localize a wide range of potentially light sensitive defects, by means of the perturbation of the integrated circuit (IC) properties through carrier generation in the silicon, PLS cannot perform an exact failure localization on a single transistor or junction because of limited spatial resolution. This paper describes the use of a conductive atomic force microscope (C-AFM) within the failure analysis (FA) flow as a local fault isolation method in order to generate a more reliable failure hypothesis and successful physical root cause visualization for light-sensitive defects, and, using this technique, such failures, which pose potential reliability issues for devices as the affected circuit degrades over time or under stress, can be easily screened before any quality assurance test.