Re: bird's beak..
Isolation between mos device in any microcircuit is obtained by LOCOS technique.
the problem arsing during this process, the oxidation of silicon slightly extents under the nitride. the second problem comes due to large mismatch in the thermal expansion coefficients of silicon nitride and silicon, which results in damage to the semiconductor during LOCOS.
This problems can be eliminated or reduced by growing thin sio2 layer prior to deposition of silicon nitride
this results in penetration of oxide under the nitride masked region results in structure in fig this is birds beak