A structure and a formation method of a semiconductor device structure are provided. The method includes forming a gate stack over a semiconductor substrate. The method also includes forming a sealing layer over a sidewall of the gate stack using an atomic layer deposition process. The atomic layer deposition process includes alternately and sequentially introducing a first silicon-containing precursor gas and a second silicon-containing precursor gas over the sidewall of the gate stack to form the sealing layer. The second silicon-containing precursor gas has a different atomic concentration of carbon than that of the first silicon-containing precursor gas. The method further includes partially removing the sealing layer to form a sealing element over the sidewall of the gate stack.