The present invention relates generally to illumination optical systems, and more particularly to an illumination apparatus, an exposure apparatus, a device fabricating method, and a device fabricated from an object to be exposed or a target. The illumination apparatuses and exposure apparatuses are used to fabricate various devices such as single crystal plates for semiconductor wafers, glass plates for liquid crystal displays (LCD), and the like. The present Invention is suitably applicable, for example, to an illumination apparatus used for an exposure apparatus that exposes single crystal plates for semiconductor wafers in a micro-lithography process for fabricating minute patterns.
Along with the recent demand on smaller and lower profile electronic devices, minute semiconductor devices to be mounted onto these electronic devices have been increasingly demanded. For example, a design rule for a mask pattern requires that an image with a size of a line and space (L&S) of 0.1 μm or less be extensively formed, and predictably, it will further move to a formation of circuit patterns of 80 nm or less in the future. L&S denotes an image projected to a wafer in exposure with equal line and space widths, and serves as an index of exposure resolution.
A projection exposure apparatus, which is a typical exposure apparatus for fabricating semiconductor devices, includes a projection optical system that projects and exposes a pattern drawn on a mask or a reticle (which are used interchangeably in this application) onto a wafer. Resolution R of a projection exposure apparatus (a minimum size which enables a precise transfer of an image) can be given by using a light-source wavelength λ and the number of apertures (NA) of the projection optical system as in the following equation:                     R        =                              k            1                    ×                      λ                          N              ⁢                                                           ⁢              A                                                          (        1        )            
Therefore, the shorter the wavelength becomes, and the higher the NA increases, the better the resolution becomes. In the meantime, a focusing range that maintains a desired image-forming performance is called a depth of focus (DOF), and the DOF is given in the following equation.                               D          ⁢                                           ⁢          O          ⁢                                           ⁢          F                =                              k            2                    ×                      λ                          N              ⁢                                                           ⁢                              A                2                                                                        (        2        )            
Therefore, the shorter the wavelength becomes, and the higher the NA increases, the smaller (shallower) the DOF becomes. A small DOF would make focus adjustment difficult, as well as requiring higher flatness for a plate and more precise focusing accuracy, and thus, a large DOF is basically desirable.
It can be understood from the equations (1) and (2) that in order to improve resolution while, on the other hand, preventing the DOF from becoming too small, a shortened wavelength will be more desirable than an increased NA. In recent years, a wavelength of an exposure light source is shifting from KrF excimer laser (with a wavelength of 248 nm) to ArF excimer laser (with a wavelength of 193 nm), and NA is from about 0.6 to 0.75. Further, a practical application of F2 laser (with a wavelength of 157 nm) is being promoted as an exposure light source.
Another factor that affects resolution could be uniform illuminance on an illuminated plane. High-resolution patterns are unavailable without satisfactorily uniform illuminance on the illuminated plane. Accordingly, a known technique for improving uniform illuminance is to arrange an optical integrator between a light source and an object to be illuminated, and the optical integrator may be, for example, a wave front splitting type optical integrator (for example, a fly-eye lens, and such terms are used interchangeably in this application) and a reflection type optical integrator (which includes a glass rod and hollow pipe, and is also called an optical pipe, and such terms are used interchangeably in this application).
Japanese Laid-Open Patent Application No. 7-201730, for example, proposes a method for improving uniform illuminance on the illuminated plane using an optical pipe (or a glass rod) as a reflection type optical integrator, thus making the optical pipe's edge plane of exit conjugate with a reticle plane. This reference keeps variable an angle of divergence (or angle of convergence) of a beam entering the optical pipe by driving an optical system provided at a front stage of the optical integrator.
Further, the instant assignee uses an optical pipe as a reflection type optical integrator in Japanese Laid-Open Patent Application No. 10-270312, and proposes a method for improving the uniform illuminance on the illuminated plane by arranging an angle-of-exit maintaining optical element at a front stage of the optical pipe for introducing a beam to the pipe with a specific angle of divergence. The reference arranges the optical pipe's edge plane of exit conjugate with a pupil plane of a projection optical system, thus improving the uniform light intensity distribution (i.e., an effective light source) formed by a fly-eye lens at a subsequent stage of the optical pipe as well as promoting the improved uniform illuminance on the illuminated plane (i.e., a reticle plane or a wafer plane).
However, it has not yet been verified that an illumination apparatus with a conventional reflection type optical integrator actually illuminates the illuminated plane uniformly and effectively (in other words, with desired illuminance). Uneven illumination would cause an insufficient transfer of a pattern onto a resist, thus deteriorating quality of semiconductor wafers, LCDs, thin-film magnetic heads, and the like. Further, a throughput would be lowered without illuminated at high illuminance.
A uniform illumination distribution (or light intensity distribution) at the optical pipe's edge plane of exit requires a polygonal sectional shape of the optical pipe so as to increase the number of internal-surface reflections. This would need a small section of the optical pipe or a long optical pipe in an axial direction. Nevertheless, the smaller section would condense light so much, facing a limitation in optical member's durability. On the other hand, the longer optical pipe would undesirably cause an increased loss of an in-pipe light amount (e.g., a loss resulted from a transmittance of a glass material in case of a glass rod, and a loss resulted from reflection efficiency on a reflection surface in case of a hollow rod), and a larger-sized apparatus.
Moreover, as mentioned above, an exposure apparatus for fabricating minute patterns, such as a semiconductor device, has recently sought a shorter exposure wavelength. The shorter exposure wavelength would restrict available glass materials having sufficiently high transmittance and coating materials with high reflection efficiency. Thus, redundancy of an optical pipe would lower light utilization efficiency, disadvantageously deteriorating the throughput in the long run.