Latchup suppression in fine-dimension shallow p-well CMOS circuits

The high packing density required for VLSI CMOS circuits leads to enhanced performance of the inherent parasitic bipolar devices, and thus latchup becomes a major problem. One of the most attractive techniques for overcoming this is to fabricate the devices on n-on-n+epitaxial substrate material. This paper deals with latchup suppression by such a technique in fine-dimension CMOS circuits based on very shallow p-wells. Experimental results demonstrate that latchup may be eliminated in structures with p-well depths as shallow as 0.8 µm at supply voltages up to 10 V and temperatures up to 140°C. Furthermore, this may be achieved with no significant degradation of other aspects of device or circuit performance. A simple lumped model equivalent circuit has been used to predict latchup characteristics where appropriate, and in general this gives good agreement with experiment.