Abstract: A wafer sensor includes a wafer holder, a liquid distribution system, and a pressure sensor. The wafer holder has a hole and is configured so that a wafer being held by the wafer holder restricts the flow of liquid from the liquid distribution system through the hole. The pressure sensor has a predetermined threshold and is configured to sense the pressure of the fluid at the hole. When no wafer is present in the wafer holder, the fluid flows essentially unrestricted through the hole in the wafer holder, resulting in a relatively low pressure of the liquid in the hole. When the wafer holder holds a wafer, the restricted flow through the hole causes an increase in pressure of the liquid in the hole. This increased pressure triggers the pressure sensor, thereby indicating the presence of the wafer in the wafer holder.