Dynamic gas flow controller

Abstract

A method and apparatus for controlling the delivery of a gas from a reservoir to a semiconductor process chamber. In accordance with the present invention, a reservoir, having an inlet and outlet isolation valve and a known volume, is filled with a gas upon the initiation of process recipe step. The temperature and pressure of the gas in the reservoir are measured to determine an initial mass of the gas residing within the reservoir. The flow of gas from the reservoir to the process chamber is metered by a variable flow valve under the control of a self-calibrating, dynamic flow control circuit comprising a flow control servo loop (flow control circuit) and a calibration servo loop (calibration circuit). The variable flow valve is situated in a gas flow path between the reservoir and process chamber at a point upstream of a orifice. In operation, gas is delivered to the process chamber by releasing the gas from the reservoir and directing the gas through the variable flow valve and sonic orifice and into the process chamber. The flow control servo loop is used to dynamically control the variable flow valve in response to a measured gas flow rate. When the flow of gas to the process chamber is terminated, the temperature and pressure of the gas residing in the reservoir is again measured to determine the final mass of gas residing in the reservoir. The initial mass and final mass of gas values are compared to determine the actual mass of gas released from the reservoir during the recipe step. This value serves as an input to the calibration servo loop to update the system calibration constant. The execution of the calibration servo loop serves as a continuous self calibration of the dynamic servo loop.