Patent ID: 8248578

Claim:
A projection exposure method comprising: generating primary radiation having a center wavelength λ; guiding the primary radiation through an illumination system along an illumination path to generate illumination radiation incident in an illumination field on a mask bearing a pattern; projecting an image of the pattern onto an image field on a radiation-sensitive substrate through a projection objective with projection radiation along a projection path; varying the center wavelength within a wavelength variation range Δλ having a lower limit λ MIN ≦λ and an upper limit λ MAX ≧λ; wherein, within the wavelength variation range, a specific absorption coefficient k(λ) of at least one gaseous absorbent species, selected from the group consisting of oxygen (O 2 ), ozone (O 3 ) and water vapor (H 2 O) and present in at least one gas-filled space along at least one of the illumination path and the projection path, varies between a minimum absorption coefficient k MIN and a maximum absorption coefficient k MAX such that an absorption ratio (k MAX /k MIN ) exceeds 10; and controlling a concentration of the absorbent species within the gas-filled space such that an overall absorption variation effected by the absorbent species for all rays running along differing ray paths towards the image field is maintained below a predetermined absorption variation threshold value.