Wafer holder with stiffening rib

A wafer holder comprises a circular, disc-shaped main portion and a rib extending generally downward from a lower surface of the main portion. The rib encircles the vertical center axis of the wafer holder. The upper surface of the main portion has a wafer-receiving pocket defined by an inner pocket surface surrounded by an outer shoulder. The rib is closed to completely surround a vertical center axis of the main portion. The rib helps to prevent the main portion from inducing symmetric concavity during the manufacture of the wafer holder. In other words, the rib helps to maintain the flatness of the upper surface of the outer shoulder while the main portion is made symmetrically concave.