Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-03-09
2002-12-24
Lee, John R. (Department: 2887)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492100, C250S50400H
Reexamination Certificate
active
06498351
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to illumination systems, e.g. for extreme ultraviolet radiation. More particularly, the invention relates to the application of such a device in lithographic projection apparatus comprising:
an illumination system constructed and arranged to supply a projection beam of radiation;
a first object table provided with a mask holder constructed to hold a mask;
a second object table provided with a substrate holder constructed to hold a substrate; and
a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate.
BACKGROUND OF THE INVENTION
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 systems, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. In addition, the first and second object tables may be referred to as the “mask table” and the “substrate table”, respectively. Further, the lithographic Apparatus may be of a type having two or more mask tables and/or two or more substrate tables. In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more stages while one or more other stages are being used for exposures. Twin stage lithographic apparatus are described in International Patent Applications WO 98/28665 and WO 98/40791, for example.
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 a target 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 WO 97/33205.
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. Whilst 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
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nm. 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). Although the synchrotron radiation emitted by a storage ring is well confined to the plane containing the circulating electron beam, it is emitted in all directions in that plane, and to generate a sufficiently intense projection beam it is necessary to collect synchrotron radiation from a wide range of angles. This results in an undesirably large device overall and in particular requires the provision of large collection mirrors.
So-called “undulators” and “wigglers” have been proposed as an alternative source of extreme ultraviolet radiation. In these devices, a beam of electrons traveling at high, usually relativistic, speeds, e.g. in a storage ring, is caused to traverse a series of regions in which magnetic fields perpendicular to the beam velocity are established. The directions of the magnetic field in adjacent regions are mutually opposite, so that the electrons follow an undulating path. The transverse accelerations of the electrons following the undulating path causes the emission of Maxwell radiation perpendicular to the direction of the accelerations, i.e. in the direction of the undeviated path. Such radiation sources generally have a moderate or small étendue, as compared to laser plasma sources, for example, which have a large étendue.
The term ‘étendue’ refers to the product of the size of the source and the solid emission angle.
It is an object of the present invention to provide an optical system that may be used to shape radiation emitted from a radiation source, especially extreme ultraviolet radiation, into an arch- or ring-shaped projection beam for a lithographic projection apparatus.
According to the present invention there is provided lithographic projection apparatus for imaging a mask pattern in a mask onto a substrate, the apparatus comprising:
an illumination system constructed and arranged to supply a projection beam of radiation;
a first object table provided with a mask holder constructed to hold a mask;
a second object table provided with a substrate holder constructed to hold a substrate; and
a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate; characterized in that said illumination system comprises:
scattering means constructed and arranged to control the divergence of said projection beam, said scattering means comprising a one-dimensional array of curved reflecting elements each conforming to a curved surface such as would reflect a narrow and collimated incident beam into a curved fan.
The present invention therefore provides an illumination system which can be used in a lithography apparatus to provide an arch or ring field illumination for the reticle and also good filling of the entrance pupil of the projection system. Furthermore, the invention enables provision to be made for the location of field masks (REMA) and pupil masks (for the control of the filling factor) in the system.
The arch shape of the illumination at the reticle is due to the scattering mirror which is a one-dimensional (at least) array of, for example, toroidal, cylindrical or conical mirrors. For light sources of large étendue, the scattering mirror is preferably a matrix of mirrors, each being a one-dimensional array of toroidal, cylindrical or conical mirrors. The individual mirrors of the matrix can be individually oriented to concentrate the radiation in the projection beam. For light sources with small
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tendue, a second scattering mirror can be introduced into the system to control and improve pupil filling. The second scattering mirror can be a two-dimensional array of aspherical mirrors.
In various embodiments of the invention, relay (or imaging) mirrors can be provided. For example, a relay or collector mirror in front of the first scattering mirror can be provided to collect light from the source and direct it to the first scattering mirror at an appropriate angle of incidence. Relay mirrors behind the first scattering mirror can be provided to produce conjugate planes for field and pupil masking, to direct the light to the reticle and the entrance pupil and to preserve the shape of the arched beam reflected by the first scattering mirror so that the illumination at the reticle has the shape of
Escudero Sanz Isabel
Kruizinga Borgert
ASML Netherlands B.V.
Lee John R.
Pillsbury & Winthrop LLP
Vanore David A.
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