Method for self-deciding cleaning process end point of reaction furnace

A method for self-deciding a cleaning process end point of a reaction furnace suitable for a low pressure chemical vapor deposition machine is provided. First, a cleaning gas is added into the reaction to occur a chemical reaction at a specific temperature. The temperature of a plurality of positions on the reaction furnace is measured continuously. The temperature variation of one of the positions is monitored until the temperature converges between the deflection amounts of the specific temperature. The temperature variation of all the positions is monitored until the temperature converges between the deflection amounts of the specific temperature. A period of over etching step is performed and then stopped the cleaning process of the reaction furnace.