Microlithography is used for the production of microstructured components such as for example integrated circuits or LCDs. The microlithography process is carried out in what is referred to as a projection exposure apparatus having an illumination system and a projection objective. The image of a mask (=reticle) is illuminated via the illumination system, and is projected via the projection objective onto a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective in order to transfer the mask structure on to the light-sensitive coating on the substrate.
Mirrors are used as optical components for the imaging process in projection objectives designed for the EUV range (wavelengths of for example about 13 nm or about 7 nm) due to the general lack of availability of suitable translucent refractive materials. Particularly in high-aperture projection objectives (for example with values of the numerical aperture of greater than 0.4), a small spacing (for example in the range of 20-50 mm) is present between the mirror arranged closest to the wafer and the wafer so that this mirror is comparatively thin (that is to say with low values for the ratio of thickness to diameter of for example 10 or more).
With thin mirrors of that kind, unwanted surface deformation phenomena can occur, for example due to the force of gravity or caused by mounting or coating effects. Mirror deformation phenomena caused by parasitic moments or forces or moments or forces which disturb static definiteness can be, for example, 50% of the surface deformation which is allowed for the mirrors in total.
Increased mirror dimensions, involved with increasing numerical apertures, to comply with the desired specifications, involve increasing natural frequencies to rigidly maintain the positions of the optical elements relative to each other even upon the occurrence of external vibration and to afford sufficient stiffness in relation to parasitic and mass-dependent moments or interference forces which are generally produced by screwing the mirrors after polishing.
For mirrors in positions other than the position that is last at the image plane side in the projection objective, when relatively large mirror diameters are involved, the attainable natural frequency is often limited independently of the mirror thickness. In particular for example natural frequencies which have been attained in an EUV projection objective equipped with six mirrors may not be adequate to sufficiently suppress interference influences due to external vibration to achieve the desired overlay specifications.
A large number of approaches using decoupling elements is known for resolving issues arising from mechanical stresses to optical elements such as mirrors or lenses.
It is known to provide decoupling elements in a kinematic three-point arrangement to achieve good reproducibility of the position by virtue of clearly defining the plane by three points.
WO 2005/054953 A2 discloses among other things a holding arrangement for an optical element in a projection objective. Between a holder and the optical element is a reinforcing element whose coefficient of thermal expansion substantially corresponds to that of the optical element.
US 2009/0122428 A1 discloses among other things optical elements of a projection exposure apparatus which are designed so that thermally induced deformation phenomena are reduced, for which purpose among other things a correction plate provided with a ribbed structure is also applied to the rear side of a mirror.