Proximity effects of ´unused´ output buffers on ESD performance (CMOS)

A unique failure mechanism, identified on an unused output buffer located near a used input protection device, occurs when excessive substrate current is generated during an electrostatic discharge (ESD) event. This new mechanism, the proximity effect, plays an important role when the n moat region of an input ESD circuit is within 20 mu m of an unrelated n moat diffusion region contacted to the power supply, V/sub cc/. The operation of the most commonly used ESD input protection circuitry when stressed with respect to V/sub cc/ is reviewed. A laser cut experiment has verified that disconnecting the V/sub cc/ bus from the unused n moats eliminates this type of ESD failure. Device metal mask changes have confirmed these findings. This ESD failure mechanism, has been demonstrated on a variety of I/O buffer layouts, and a solution has been identified.<>