An integrated circuit structure includes a semiconductor substrate, a semiconductor strip over a portion of the semiconductor substrate, and a Shallow Trench Isolation (STI) region on a side of the semiconductor strip. The STI region includes a dielectric layer, which includes a sidewall portion on a sidewall of the semiconductor strip and a bottom portion. The dielectric layer has a first etching rate when etched using a diluted HF solution. The STI region further includes a dielectric region over the bottom portion of the dielectric layer. The dielectric region has an edge contacting an edge of the sidewall portion of the dielectric layer. The dielectric region has a second etching rate when etched using the diluted HF solution, wherein the second etching rate is smaller than the first etching rate.