A plasma confinement ring includes a lower ring, an upper ring on the lower ring, and a connection ring extended to connect the lower ring to the upper ring. The lower ring includes a lower center hole vertically penetrating the lower ring at a center of the lower ring and at least one slit penetrating the lower ring in a region outside the lower center hole. The slit is structured to pass a more amount of air or gas at a first portion closer to the center of the lower ring than at a second portion farther from the center of the lower ring.