Method of parallel shift operation of multiple reactors

A method of parallel shift operation of multiple reactors in which substrates are placed for processing, wherein n main gas lines numbered 1 to n and n gas ports numbered 1 to n for each main gas line are provided wherein n is an integer greater than one, each main gas line branches into n branch gas lines numbered 1 to n connected to the n gas ports, respectively, each branch gas line is provided with a valve for closing and opening the branch gas line, and each reactor is connected to a different gas ports, wherein one of branch gas lines 1 to n of each main gas line is in an open state at a time while alt the other branch gas lines of the main gas lines are in a closed state,

• said method comprising:
• (i) continuously supplying n gases numbered 1 to n simultaneously to the n gas ports via the n main gas lines, respectively, at a constant flow rate, wherein one of the branch gas lines of each main gas line is in an open state, said one of the branch gas lines being such that the inflow rates of the respective reactors are equal, and the outflow rates of the respective reactors are equal;
• and
• (ii) while maintaining the inflow rates and the outflow rates of the respective reactors, simultaneously closing the one of the branch gas lines of each main gas line in the open state and opening another one of the branch gas lines of each main gas line so that each numbered gas is continuously supplied to different reactors in sequence wherein all of the numbered gases are switched by the valves from one reactor to another simultaneously through the processing of the substrates without changing the inflow rates and the outflow rates of the reactors,
• further comprising switching gases constituting at least one of the numbered gases flowing through the main gas line upstream of the branch gas lines without interruption at the same total flow rate during step (i), where said switching of gases is synchronized to the switching of the numbered gases.