Twin Tub Process:
A logical extension of the p-well and n-well approaches is the twin-tub fabrication process.
In this process, a substrate of high resistivity of n-type material is used and then in this n-type material both n-well and p-well regions are created.
By using this process, it is possible to preserve the performance of n-transistors without compromising the p-transistors.
The doping control is more rapidly achieved and some relaxation is manufacturing tolerances results.
This is particularly important as far as latch up is concerned.