A new failure mechanism on analog I/O cell under ND-mode ESD stress in deep-submicron CMOS technology

A new ESD failure mechanism has been found in the analog pins with pure-diode protection scheme during ND-mode ESD stress. The failure is caused by the parasitic npn interaction between ESD protection diode and guard ring structure. The parasitic npn bipolar, which was constructed between the N+/PW diode and the N+/NW guard ring, provides the discharging path between the I/O pad to the grounded VDD under the ND-mode ESD stress to cause a low ESD robustness of the analog I/O cell. The solution to overcome this ESD failure is also proposed.