A vertical wafer boat for a diffusion process is provided. The vertical wafer boat includes a plurality of wafer racks. Each of the plurality of wafer racks includes a vertical support member and a plurality of wafer support arms. The plurality of wafer support arms extends from a sidewall of the vertical support member. Each of the wafer support arms includes a support body and a ledge. The support body is located between the vertical support member and the ledge. Centers of the support body and the ledge are horizontally aligned. A vertical thickness of the ledge is smaller than a vertical thickness of the support body.