I see the following figure in Kenneth Martin textbook.
How does nwell underneath a digital interconnect shield the substrate? And, the nwell is connected to ground line instead of vdd line. I'm really confused about this. Shouldn't the nwell be connected to vdd instead of gnd? Please help me understand this.
Even if there's no reverse voltage between the n-well and p-substrate, there will be a small space charge region at the junction (mainly into the p-substrate). This reduces the coupling capacitance between the interconnect and the substrate, because it's in series to the interconnect-nwell capacitance.
Usually - if you have active devices in the n-well - yes, you're right. But in this case the n-well is just for shielding, so a GND connection is better if the signals to be shielded are GND referenced, because VDD might be more noisy (from switching circuitry).
For VDD referenced signals, however, the usual n-well connection to VDD would be better.