Exclusive Feature: WAFER TEST

Parametric test hardware for ultralow-current measurements

By: David Rose, Qi Wang, Keithley Instruments Inc., Cleveland, Ohio

Increasingly, parametric testing of MOSFETs in CMOS ICs requires very accurate measurement of various source, drain, and gate currents, and characterization of low current phenomena. The tester, cabling, and chucks used must be properly integrated from the perspective of the device under test and the specific instruments used. The overall approach must include attention to system grounding and shielding, minimization of system noise, consideration of measurement settling errors, and proper attention to measurement speed.

Parametric characterization of MOSFETs typically requires extremely sensitive measurements of the drain current during the off-state. Also, the transition from the on-state to the off-state (i.e., subthreshold current) is crucial to the performance of a CMOS circuit. Typically, off-state current is in femtoamps. Gate leakage current is also crucial in device performance. Furthermore, there are many low-current phenomena of particular importance to device reliability, such as gate-induced drain leakage, which must be characterized.

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