Patent ID: 8693098

Claim:
A projection objective, comprising: a first mirror comprising: a first mirror substrate; and a first reflective coating carried by the first mirror substrate; and a second mirror comprising: a second mirror substrate; and a second reflective coating carried by the second mirror substrate, wherein: when irradiated by EUV light having otherwise equal conditions, the first mirror substrate compacts less than the second mirror substrate under an effect of the EUV light, a total of M mirrors S 1 , S 2 , . . . , S M are arranged successively in the projection objective; m mirrors are first mirrors; n=M−m are second mirrors; for the m first mirrors, SA i <k·0.6 (i-1)/2 ; for the n second mirrors, SA i ≧k·0.6 (i-1)/2 ; SA i is an average subaperture diameter for the i th mirror M i ; and k is a threshold value factor of between 50 mm and 200 mm, which is set as a function of a strength of an EUV light source of the projection exposure apparatus, and the projection objective is configured to be used in a microlithographic EUV projection exposure apparatus.