This phenomenon is typically due to the spin coating process used to apply the photoresist. During spin coating, the wafer is placed on a vacuum chuck and the photoresist is dispensed onto the center of the wafer. As the chuck spins, centrifugal force spreads the photoresist towards the edges, and the excess is thrown off. The final thickness of the photoresist is influenced by factors such as the viscosity of the photoresist and the spin speed.

It may be due to a slight bending of your wafer. This bending comes from residual stress of the films you deposited on it. As a result, the gap between mask and wafer during exposure may change from center to edge, and this induces a change in your feature size.

What layers do you have on wafer and on what side of the wafer?

Cheers

It could be due to several factors; a) spin coating of photoresist resulting in uneven thickness, ensure the thickness is +-100nm across the wafer, b) does the uv light during the lithography process covering the whole wafer? Sometimes, the wafer edges are left which could results in different electrodes thickness.

c)ensure during baking the wafer is placed flat on the hotplate..sometimes when the wafer is not flat, the resist might flow towards the edges