Patent ID: 6265831
Filing Date: 2001-07-24
Classification: H01J,H03H

Abstract:
A method of controlling an a.c. plasma treating a workpiece in a vacuum plasma processing chamber, the workpiece being on a workpiece holder having an electrode, the method being performed in accordance with a recipe having a set of parameters including at least one gas supplied to the chamber, flow rate of the gas supplied to the chamber, pressure range in the chamber, power range supplied to an excitation reactance for the plasma and power range supplied to the electrode, the method comprisingsupplying a.c. from a first a.c. source to the plasma via a first circuit including an excitation reactance and a first matching network including first and second variable reactances,sensing a first function related to a load including the plasma as seen by the first circuit,controlling the first and second reactances in response to the sensed first function to maintain a predetermined relation between functions of (a) the output impedance of the first source and (b) the impedance the first source drives,supplying a.c. from a second a.c. source to the plasma via the electrode and a second matching network including third and fourth variable reactances,sensing a second function related to the impedance of the plasma,controlling the third and fourth variable reactances in response to the sensed second function to maintain a substantially matched condition between the output impedance of the second source and the impedance the second source drives, power reflected back to the second source having a tendency to have a step change as a result of a small variation in the value of at least one of the third and fourth reactances from the values which provide a match between the output impedance of the second source and the load the second source drives, the at least one small change resulting from an incremental change in at least one of the parameters while the one parameter remains in a range consistent with the recipe, andovercoming the tendency for the step change by varying the output power of the second source in response to a function of plasma impedance, the second source output power being varied at a speed much faster than the speed that the third and fourth reactances change to produce the tendency to have the step change.