Optical: systems and elements – Having significant infrared or ultraviolet property – Having ultraviolet absorbing or shielding property
Reexamination Certificate
2000-06-28
2002-09-10
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Having significant infrared or ultraviolet property
Having ultraviolet absorbing or shielding property
C359S359000, C359S360000, C359S585000, C378S084000, C428S141000
Reexamination Certificate
active
06449086
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to multilayer mirrors for extreme ultraviolet radiation. More particularly, the invention relates to the use of such mirrors in lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table provided with a mask holder for holding a mask;
a second object table provided with a substrate holder for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
DESCRIPTION OF THE RELATED ART
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The illumination system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam, and such elements may also be referred to below, collectively or singularly, as a “lens”. In addition, the first and second object tables may be referred to as the “mask table” and the “substrate table”, respectively.
In the present document, the invention is described using a reference system of orthogonal X, Y and Z directions and rotation about an axis parallel to the I direction is denoted Ri. Further, unless the context otherwise requires, the term “vertical” (Z) used herein is intended to refer to the direction normal to the substrate or mask surface or parallel to the optical axis of an optical system, rather than implying any particular orientation of the apparatus. Similarly, the term “horizontal” refers to a direction parallel to the substrate or mask surface or perpendicular to the optical axis, and thus normal to the “vertical” direction.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto an exposure area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies which are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die at once such an apparatus is commonly referred to as a wafer stepper. In an alternative apparatus—which is commonly referred to as a step-and-scan apparatus—each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO97/33205, for example.
Until very recently, lithographic apparatus contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently moveable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO98/28665 and WO98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is at the exposure position underneath the projection system for exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge a previously exposed substrate, pick up a new substrate, perform some initial measurements on the new substrate and then stand ready to transfer the new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed; the cycle then repeats. In this manner it is possible to increase substantially the machine throughput, which in turn improves the cost of ownership of the machine. It should be understood that the same principle could be used with just one substrate table which is moved between exposure and measurement positions.
In a lithographic apparatus the size of features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation of around 13nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray and possible sources include laser plasma sources or synchrotron radiation from electron storage rings. An outline design of a lithographic projection apparatus using synchrotron radiation is described in “Synchrotron radiation sources and condensers for projection x-ray lithography”, J B Murphy et al, Applied Optics Vol. 32 No. 24 pp 6920-6929 (1993).
In the EUV spectral region high reflectivity mirrors, apart from grazing incidence mirrors, must necessarily be multilayered thin film designs. The predominant designs are composed of distrbuted Bragg reflectors resembling quarter wavelength stacks with constant film thicknesses throughout. For the 11-16 nm wavelength region two designs predominate: Mo/Be for the 11.3 nm window consisting typically of 80 periods and the Mo/Si system for the 13.4 nm window of 40-50 periods, both with a partition ratio &Ggr;=0.4, where &Ggr;=d
Mo
/(d
Mo
+d
Si(Be)
). In general, the partition ratio is defined as the ratio of the thickness of the material having the higher extinction coefficient, k, to the total thickness of the two layers. These designs yield maximum theoretical reflectivities of R~0.78 for the Mo/Be stack, and R~0.74 for the Mo/Si stack while taking into account a highly absorbing ~2 nm native oxide on the surface Si layer. These reflectivity values (which are amongst the best for multilayer reflectors in the EUV region), while insert adequate for optical systems with a few reflectors, will dramatically diminish the output optical intensity to 6-10% of that directly before the first mirror in, for example, a nine-mirror system. The significance of nine mirrors is that this is the number envisaged for an EUV lithographic system; two in the illumination optics, six in the imaging optics plus the reflecting reticle. It is therefore evident that even a “small” increase of 1-2% in the peak reflectivity of a single mirror will yield a significant light throughput enhancement of the optical system.
It is an object of the present invention to provide multilayer mirrors for extreme ultraviolet radiation (EUV) that have higher reflectivities at desired wavelengths.
SUMMARY OF THE INVENTION
According to the present invention, this and other objects are achieved in a reflector for reflecting radiation in a desired wavelength range, the reflector including a stack of alternating layers of a first and a second material, said first material having a lower real refractive index in said desired wavelength range than said second material, characterised by:
at least one layer of a third material interposed in said stack, said third material being selected from the group comprising Rb, RbCl, RbBr, Sr, Y, Zr, Ru, Rh, Tc, Pd, Nb and Be as well as alloys and compounds of such materials.
In preferred embodiments of the invention, a layer of said third material is interposed between each pair of layers of said first and second materials, and optionally at least one layer of a fourth material may be interposed in said stack, said
ASML Netherlands B.V.
Assaf Fayez
Spyrou Cassandra
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