Impacts of the Drain-side nWell Adding on ESD Robustness in 0.25-μm LV/HV nMOSTs

An n-channel MOS transistor (nMOST) no matter what low or high voltage processes are often used in I/O pads as ESD protection components. However, the contact spiking is a common caused leakage problems which deeply affect the ESD reliability capability of protection devices. Therefore, in this work, we proposed systematic experiments on the drain side: by adding an n-type Well (nWell) structure in the drain area for 0.25-μm low voltage (LV)/ high voltage (HV) processes. After measurement and analysis, it is found that for this LV process adding the nWell in drain side is bad for the I_t2 robustness of ESD capability, such as the lowest ESD capability condition (S= 9-μm) as compared with the reference group (none with the nWell) is decreased up to 42%, so in the ESD protection application should be avoided to add this structure; in the same token adding the nWell structure in the drain side of an HV nLDMOS, it was found that can expand an ESD current conduction cross-sectional area, which will not dissipate a lot of heat on the surface of the device led to burn, and then enhancing the ESD capability. Meanwhile, the ESD capability of a DUT with S= 9-μm as compared with the reference group (none with the nWell) is increased up to 8%, so adding an nWell structure in the drain side is good for ESD capability (I_t2 value) of HV MOS devices.