Mismatch in diffusion resistors caused by photolithography

During the qualification of a 0.35-μm CMOS process, it was observed that diffusion resistors showed a systematic mismatch, depending on the position on the wafer. The mismatch increased from the center of the wafer to the outer regions. Various experiments showed that the mismatch was caused by spinning the wafer during the resist development process. Changing this process eliminated the systematic diffusion resistor mismatch.