Optical charge-sensing method for testing and characterizing thin-film transistor arrays

Improved methods for testing thin-film transistor (TFT) arrays are becoming increasingly Important as TFT liquid-crystal flat-panel displays become the first choice for many applications. The optical charge-sensing method for testing TFT arrays described in this paper uses interface reflections to sense accumulative free carriers and to generate maps showing the type and location of line and point defects. The method is nondestructive and provides data which can be used to repair certain types of array defects. It can also be used to measure such parameters as threshold voltage and channel conductance of TFT's, and frequency response of driver circuits. Optical charge sensing depends not only on charge density but also on the structure of the TFT arrays. Charge-sensing sensitivity is reduced by reflections from the metal interconnects. An improvement in the net reflectance change is obtained by using polarized and angled incident light in spite of reduced reflectance changes compared with normal incidence