A method includes performing a first planarization step to remove portions of a semiconductor region over isolation regions. The first planarization step has a first selectivity, with the first selectivity being a ratio of a first removal rate of the semiconductor region to a second removal rate of the isolation regions. After the isolation regions are exposed, a second planarization step is performed on the isolation regions and a portion of the semiconductor region between the isolation regions. The second planarization step has a second selectivity lower than the first selectivity, with the second selectivity being a ratio of a third removal rate of the portion of semiconductor region to a fourth removal rate of the isolation regions.