Max acceptable resistance between IC ground and PCB ground plane
For 2OZ copper, 1cm of 20mil track is about 5mOhm. Assuming your mcu sinks a max current of 100ma, you'll get a vdrop of about 500uV. This is pretty tiny. The inductance will likely not be a big deal either. With microcontrollers, the most important thing is how quick the current draw changes. You can mitigate this by putting a small decoupling cap as close to the IC supply pins as possible. This is what inductance affects, so you want to make the loop as small as possible (i.e. the length is more important than the width).