A semiconductor structure includes a substrate, a semiconductor layer, a gate stack, two first gate spacers over two opposing sidewalls of the gate stack and extending above the gate stack; a second gate spacer over a sidewall of one of the first gate spacers and having an upper portion over a lower portion; an etch stop layer adjacent to the lower portion and spaced away from the upper portion; and a seal layer over the gate stack, the two first gate spacers and the second gate spacer, resulting in a first void and a second void below the first seal layer. The first void is above the lower portion of the second gate spacer and laterally between the etch stop layer and the upper portion of the second gate spacer. The second void is above the gate stack and laterally between the two first gate spacers.