Latchup-free fully-protected cmos on-chip esd protection circuit

Abstract

An ESD protection circuit fully protects the input stage of CMOS integrated circuits from four different ESD stress modes by providing four different ESD direct discharging paths. The ESD protection circuit has a primary ESD protection circuit, which has a first and a second thick-oxide MOS devices, and a secondary ESD protection circuit which has a resistor, a first and a second thin-oxide MOS devices. The resistor is connected between the primary and secondary ESD protection circuits. The primary and secondary ESD protection circuits each provide two ESD discharge paths from the input pad, and from the input of the internal circuits to be protected, to VDD and VSS voltage supply buses. The inventive ESD protection circuit also has merged latchup guard rings and protects against large ESDs, while occupying only a small layout area. Furthermore, the inventive ESD protection circuit clamps the voltage level of the input signal between 5.5 to -1 volts, the voltages applied to the thin-oxide CMOS input stage of the internal circuits.