A method of manufacturing a semiconductor device includes providing a semiconductor substrate with gate structures. A sacrificial insulating layer is formed between the gate structures at a height lower than that of the gate structures such that a portion of each gate structure is exposed above the sacrificial insulating layer. Spacers are formed on sidewalls of the exposed portions of the gate structures. A portion of the sacrificial insulating layer between the spacers is exposed. The sacrificial insulating layer is removed, thereby forming spaces below the spacers. An insulating layer is formed to fill the spaces between the spacers such that air pockets are formed between the gate structures and below the spacers.