A Reliability Failure Mode in Dynamic MOS Circuits - The Unopened Metal to Polysilicon Contact Window and the Floating Gate Transistor

Dynamic, p-channel 1024-bit RAM circuits have been observed to fail while in field operation (~5×103 hours) due to unopened metal-to-polysilicon contact windows and associated floating gate transistors. Capacitive coupling between metal and polysilicon permits enough transistor switching so that circuits can initially operate. After extended use, the floating gate collects a sufficient number of electrons so that the transistor remains in a permanent "on" state and the circuit operation ceases. Detailed experiments have determined the physical charging mechanism to be a hot-electron effect. Based upon time-to-failure vs. device operating voltage data, a successful testing technique has been developed for screening-out this type of latent defective circuit.