STI CMP process

STI CMP (chemical-mechanical planarization) polishes the oxide-filled wafer surface flat after trench fill, removing excess oxide down to a defined stop point, commonly a nitride layer that was deposited specifically to serve as a CMP polish-stop before the trench fill step. This defined stop layer is what lets STI CMP achieve a precisely controlled final oxide height relative to the surrounding active silicon surface, rather than relying purely on polish time to estimate when the correct amount of oxide has been removed.