Purpose of field implant in semiconductor manufacturing

Question

I recently joined the semiconductor manufacturing industry.

In one of the process flows, I see a stage that has extra boron implantation step (called the field implant) in the P-WELL region.

I've never really understood the purpose of this extra implantation stage. To explain exactly what I am talking about, I'm including a picture below.

The light green region is what I am referring to. The field oxide will be grown on top of this in a later stage.

P.S. The orange region is nitride, where NMOS and PMOS transistors will be manufactured.

Answer 1

The purpose of field implant is to implant the area under field Oxide, thus the name "field" implant. This field implant will add gradual reduction of electrical field around the field oxide. This gradual reduction will avoid the abrupt change of electrical field at the interface area between active and field oxide area. As we know this abrupt change could cause heavy impact ionization and in result voltage breakdown.