Semiconductor structures and fabrication methods are provided. An exemplary fabrication method includes providing a base substrate having an opening and forming a first gate layer in the opening. The first gate layer closes a top of the opening and includes a void. The method also includes forming a second gate layer on the first gate layer. An atomic radius of a material of the second gate layer is smaller than gaps among the atoms of the material of the first gate layer. Further, the method includes performing a thermal annealing process to cause atoms of the material of the second layer to pass through the first gate layer to fill the void.