An apparatus for performing a deposition process on a semiconductor wafer includes a chamber, a wafer holder, and a shielding structure. The chamber contains a reaction area, the wafer holder is disposed in the chamber to hold the semiconductor wafer, and the reaction area is above the semiconductor wafer. The shielding structure is disposed in the chamber and isolates an inner sidewall of the chamber from the reaction area. The shielding structure includes a base member, a first member, and a second member. The base member is disposed between the inner sidewall of the chamber and the wafer holder. The first member is disposed on the base member and is windowless. The second member is disposed on the base member and within the first member, and the second member includes a sidewall provided with a first window to transfer the semiconductor wafer.