Patent ID: 7525069

Claim:
A baking unit, comprising: a hot plate dedicated to support a substrate; a cover disposed over the hot plate such that a space in which a substrate is heated is defined between the cover and the hot plate; and a sensing device including a sensor unit mounted to one of the hot plate and the cover, and an optical element mounted to the other of the hot plate and the cover, the sensor unit comprising a photo emitter that emits light and a photo detector that detects light of the type emitted by the photo emitter, and wherein the optical element is positioned relative to the sensor unit and the photo detector is positioned relative to the photo emitter and the optical element such that the optical element receives light emitted from the photo emitter, the optical element transmits the received light to the photo detector, and the photo detector receives a predetermined quantity of light from the optical element only when at least the hot plate and the cover are level with respect to each other.