What is meant by shallow diffusion in silicon fabrication technology
In general, diffusion is one of the main ways in which doping changes are made in selected regions on the surface of a wafer. A fluid (gas or liquid) containing the desired dopant is flowed across the surface, which is masked appropriately. The time and the temperature of the wafer determine the depth of the diffusion.

Any particular fabrication "process" will typically involve several diffusion steps, with the deepest ones being done first, and the shallowest ones being done last. Other than that, there really isn't anything special about "shallow" diffusion.