A gas supply unit includes a base plate, a plurality of gas supply regions protruding from the base plate and arranged on the base plate in a circumferential direction, and a plurality of sidewall trenches alternating with the gas supply regions on the base plate. A distance between opposing surfaces of the base plate increases in a radial direction from a center of the base plate in each of the plurality of sidewall trenches, so each of the plurality of sidewall trenches has a depth that decreases in the radial direction from the center of the base plate.