1.  Field of the Invention
The invention relates to a catadioptric projection objective for imaging of a pattern, which is arranged on the object plane of the projection objective, on the image plane of the projection objective. 
2.  Description of the Related Art
Projection objectives such as these are used in microlithography projection exposure systems for the production of semiconductor components and other finely structured components.  They are used to project patterns of photomasks or reticles which are referred to in the following text in a general form as masks or reticles, onto an object which is coated with a light-sensitive layer, with very high resolution and on a reduced scale. 
In this case, in order to produce ever finer structures, it is necessary on the one hand to enlarge the image-side numerical aperture (NA) of the projection objective, and on the other hand to use ever shorter wavelengths, preferably ultraviolet light at wavelengths of less than about 260 nm, for example 248 nm, 193 nm or 157 nm. 
In the past, purely refractive projection objectives have been predominantly used for optical lithography.  These are distinguished by a mechanically relatively simple, centered design, which has only a single optical axis, that is not folded.  Furthermore, it is possible to use object fields which are centered with respect to the optical axis, which minimize the light transmission level to be corrected, and simplify adjustment of the objective. 
However, the form of the refractive design is primarily characterized by two elementary imaging errors: the chromatic correction and the correction for the Petzval sum (image field curvature). 
Catadioptric designs, which have at least one catadioptric objective part and a hollow mirror or a concave mirror, are used to simplify the correction for the Petzval condition and to provide a capability for chromatic correction.  In this case, the Petzval correction is achieved by the curvature of the concave mirror and negative lenses in its vicinity, while the chromatic correction is achieved by the refractive power of the negative lenses upstream of the concave mirror (for CHL) as well as the diaphragm position with respect to the concave mirror (CHV). 
One disadvantage of catadioptric designs with beam splitting is, however, that it is necessary to work either with off-axis object fields, that is to say with an increased light conductance value (in systems using geometric beam splitting) or with physical beam splitter elements, which generally cause polarization problems.  The term “light conductance value” as used here refers to the Lagrange optical invariant or Etendue, which is defined here as the product of the image field diameter and the image-side numerical aperture. 
In the case of off-axis catadioptric systems, that is to say in the case of systems with geometric beam splitting, the requirements for the optical design can be formulated as follows: (1) reduce the light transmission level to the maximum extent, (2) design the geometry of the foldings (beam deflections) such that a mounting technology can be developed for this purpose, and (3) provide effective correction, in particular the capability to correct the Petzval sum and the chromatic aberrations jointly in the catadioptric mirror group. 
In order to keep the geometric light conductance value (Etendue) low, the folding of the design should in principle take place in the region of low NA, that is to say for example close to the object, or in the vicinity of a real intermediate image. 
However, as the numeric aperture increases, the object-side numerical aperture also increases, and thus the distance between the first folding mirror and the reticle, so that the light transmission level rises.  Furthermore, the diameter of the hollow mirror and the size of the folding mirror increase.  This can lead to physical installation space problems. 
These can be overcome by first of all imaging the reticle by means of a first relay system onto an intermediate image, and by carrying out the first folding in the area of the intermediate image.  A catadioptric system such as this is disclosed in EP 1 191 378 A1.  This has a refractive relay system, followed by a catadioptric objective part with a concave mirror.  The light falls from the object plane onto a folding mirror (deflection mirror) which is located in the vicinity of the first intermediate image, from there to the concave mirror and from there onto a refractive object part, with a second real intermediate image being generated in the vicinity of a second deflection mirror, and the refractive object part images the second intermediate image on the image plane (wafer).  Concatenated systems having, in that sequence, a refractive (R), a catadioptric (C), and a refractive (R) imaging subsystem will be denoted “R-C-R” type systems in the following. 
Systems of type R-C-R with a similar folding geometry are disclosed in WO 2004/019128 A, WO 03/036361 A1 and US 2002/019946 A1.  Patent application US 2004/0233405 A1 discloses R-C-R type projection objectives with different folding geometries including objectives where the first folding mirror is arranged optically downstream of the concave mirror to deflect radiation coming from the concave mirror towards the image plane. 
Other catadioptric systems with two real intermediate images are disclosed in JP 2002-372668 and in U.S. Pat.  No.  5,636,066.  WO 02/082159 A1 and WO 01/04682 disclose other catadioptric systems with more than one intermediate image.