Modeling of low-pressure CVD processes

The spread in layer thickness within a series of wafers simultaneously covered in an LPCVD process will, in general,have two distinct causes. First, a spread across a wafer may occur if the deposition process is carried out in a diffusioncontrolled growth regime. Second, a gradual depletion in the flow direction may cause a spread within the length of the boatcarrying the wafers. The latter phenomenon can be approximated with a mathematical model. This approach reveals thatthe thickness spread within a batch will be acceptably small if the gas flow velocity exceeds a certain value, determined bythe batch and wafer size, and also by the apparent order of the kinetics of the LPCVD process.