A method includes forming a first protruding fin and a second protruding fin over a base structure, with a trench located between the first protruding fin and the second protruding fin, depositing a trench-filling material extending into the trench, and performing a laser reflow process on the trench-filling material. In the reflow process, the trench-filling material has a temperature higher than a first melting point of the trench-filling material, and lower than a second melting point of the first protruding fin and the second protruding fin. After the laser reflow process, the trench-filling material is solidified. The method further includes patterning the trench-filling material, with a remaining portion of the trench-filling material forming a part of a gate stack, and forming a source/drain region on a side of the gate stack.