On a conventional CMOS process (see figure 1), NMOS devices are formed in a P well or substrate connected to ground (or the most negative supply in the circuit). PMOS devices are formed in an N well connected to the most positive supply.
A typical CMOS inverter cross section. Substrate noise currents are shown as red lines.
Substrate noise caused by minority carrier injection into the substrate and well can be collected by the use of well taps and/or guard rings. An additional problem exists in that capacitive coupling of noise from the well to the substrate means more noise reaches the supply. In digital circuitry this is usually not a problem owing to the relatively high noise immunity of logic gates. However in analog design, for example a 12 bit ADC, noise can be a serious problem. A variety of techniques can be used to minimise this noise, for example by keeping analog devices surrounded by guard rings, or using a separate supply for the substrate/well taps. However guard rings alone cannot prevent noise coupling deep in the substrate, only surface currents.
Another problem is that it is not possible to isolate NMOS devices. So relatively noisy digital logic cannot be isolated completely from more sensitive analog areas.
A solution is to isolate the NMOS devices by using an extra well – a ‘deep N well’. So in figure 2 the NMOS device is fabricated in a P well or substrate completely surrounded by an N type diffusion.
A deep N well CMOS inverter cross section. Substrate noise currents are shown as red lines.
In this case, the deep N well is formed by a high energy ion implantation to give peak impurity concentration deep enough to un-affect the NMOS device performance1. Connection to the deep N well is formed by a N well ring that is connected to VDD. The deep N well has the effect of decreasing the noise coupling through it to the substrate and giving the advantage of fully isolated NMOS devices – which can in theory be at a different potential from ground.
The implications on layout are of course larger area for nmos devices due to the extra N well rings used to connect to the deep N well. However the noise performance improvements justify this for sensitive analog design.
In summary, the use of deep N well devices can significantly reduce noise coupling between sensitive analog areas and more noisy digital regions in mixed-signal designs.
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1 ESSDERC 2012