A method includes, in a first etching step, etching a semiconductor substrate to form first recesses in a first device region and second recesses in a second device regions simultaneously. A first semiconductor strip is formed between the first recesses. A second semiconductor strip is formed between the second recesses. In a second etching step, the semiconductor substrate in the second device region is etched to extend the second recesses. The first recesses and the second recesses are filled with a dielectric material to form first and second isolation regions in the first and second recesses, respectively. The first isolation regions and the second isolation regions are recessed. Portions of the semiconductor substrate in the first and the second device regions protrude higher than top surfaces of the respective first and second isolation regions to form a first and a second semiconductor fin, respectively.