A method includes forming a first gate, a second gate, a third gate, and a fourth gate over a substrate, in which a first distance between the first gate and the second gate is less than a second distance between the third gate and the fourth gate. A first spacer over a sidewall of the first gate, a second spacer over a sidewall of the second gate, a third spacer over a sidewall of the third gate, and a fourth spacer over a sidewall of the fourth gate are formed. A mask layer over the first and second spacers is formed, in which the third and fourth spacers are exposed from the mask layer. The exposed third and fourth spacers are trimmed.