Patent ID: 9350133
Filing Date: 2016-05-24
CPC Classification: H01S

Claim Text:
1. A method of controlling a laser apparatus comprising: exchanging a laser gain medium in a laser chamber configured to output a laser beam by exciting the laser gain medium; first measuring, after the exchanging, pulse energy of a laser beam which is oscillated in the laser chamber under a specific gas pressure and a specific charge voltage; calculating an approximate expression indicating a relationship between the pulse energy of the laser beam and the gas pressure in the laser chamber and the charge voltage, or a table representing a correlationship between the pulse energy, the gas pressure and the charge voltage, based on the specific pressure, the specific charge voltage and the pulse energy in the first measuring; storing the approximate expression or the table; setting a lifetime of the laser chamber; calculating a reference ΔEpt of an energy reduction amount based on the lifetime of the laser chamber such that the reference ΔEpt is smaller at longer lifetime of the laser chamber; second measuring, after the first measuring, pulse energy Er of a laser beam oscillated in the laser chamber; calculating pulse energy Eec which is supposed to be obtained directly after the exchanging under the gas pressure and the charge voltage in the second measuring based on the approximate expression or the table; calculating a reduction amount ΔEd of pulse energy based on the pulse energy Eec and the pulse energy Er using ΔEd=Eec−Er; and conducting, when the reduction amount ΔEd of pulse energy is equal to or greater than the reference ΔEpt, partial gas exchange of the laser gas in the laser chamber, wherein the reference ΔEpt of an energy reduction amount is calculated by ΔEpt=Em−ΔEnt, where Em is calculated by Em=Emax−Eex, Emax is a value of maximum pulse energy in the data collected by the first measuring, and Eex is a value of maximum pulse energy instructed based on a request from an exposure apparatus, and where ΔEnt is calculated by ΔEnt=Δd×Bt, Ad is an energy reduction amount per unit of the number of discharges, and Bt is the number of discharges representing the lifetime of the laser chamber.