A channel-leakage monitor for silicon wafer scribe testing without mechanical access

A novel sensor topology has been developed for measuring the channel leakage current of a MOSFET in scribe test. Being integrated with a wireless beacon platform powered by indoor illumination, it eliminates all aspects of mechanical access including stepping, micro-metric alignment and contact. The sensor has been developed in the form of a three-stage ring oscillator, where each stage has been built with a MOSFET device-under-test whose m segments function as the load and one segment functions as the driver. Unlike the existing ring-oscillator type leakage sensors, its frequency is not corrupted by the highly uncertain channel leakage current of any MOSFET of complementary conductance. It therefore provides the most reliable measurement among the ring-oscillator class of leakage sensors. What enables the proposed leakage sensor to eliminate the conventional mechanical access in scribe test is its nanowatt-level power consumption, which can be supplied directly by photovoltaic conversion from environmental illumination. To this end, a test monitor that integrates the sensor with a platform of photovoltaic supply and two-way communication link has been designed. Downlink has been realized with visible light communication, which enables the user to select either PMOS or the NMOS version of the sensor. Uplink has been realized with a beacon of an ultra-wide-band impulse radio. It transmits bursts of a pulse train, whose repetition rate is equal to sensor frequency. The much slower repetition frequency of the bursts is determined by the acquisition time of adequate photovoltaic energy. A small loop antenna placed approximately 1 cm above the test monitor without any critical alignment is capable of detecting the transmitted pulses.