A method includes forming a plurality of trenches to define a fin, forming a first layer of insulating material in the trenches, forming a sidewall spacer on opposite sides of the fin above an upper surface of the first layer, removing the first layer and performing a fin-trimming etching process to define a plurality of increased-size trenches. The method also includes forming a first oxidation-blocking layer of insulating material in the increased-size trenches, forming a second layer of insulating material above the oxidation-blocking layer, and performing a thermal anneal process to convert at least a part of the portion of the fin that is in contact with the second layer of insulating material into an oxide fin isolation region.