Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
1999-04-30
2002-10-01
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S030000
Reexamination Certificate
active
06459472
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a lithographic projection apparatus, and more particularly to a lithographic projection apparatus that is compatible with a vacuum or semi-vacuum environment.
2. Discussion of Related Art
An apparatus of this type 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 then 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 through 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 in one go; such an apparatus is commonly referred to as a waferstepper. 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 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.
Up to very recently, apparatus of this type contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such a multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial alignment measurements on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself; in this manner, it is possible to achieve a substantially increased machine throughput, which in turn improves the cost of ownership of the machine
In currently available lithographic devices, the employed radiation is generally ultra-violet (UV) light, which can be derived from an excimer laser or mercury lamp, for example; many such devices use UV light having a wavelength of 365 nm or 248 nm. However, the rapidly developing electronics industry continually demands lithographic devices which can achieve ever-higher resolutions, and this is forcing the industry toward even shorter-wavelength radiation, particularly UV light with a wavelength of 193 nm or 157 nm. Beyond this point there are several possible scenarios, including the use of extreme UV light (EUV: wavelength~50 nm and less, e.g. 13.4 nm or 11 nm), X-rays, ion beams or electron beams. All of these so-called next-generation radiations undergo absorption in air, so that it becomes necessary to at least partially evacuate the environment in which they are employed. This introduces considerable problems.
A general discussion of the use of EUV in lithographic projection apparatus can be found, for example, in the article by J. B. Murphy et al. in Applied Optics 32 (24), pp 6920-6929 (1993). Similar discussions with regard to electron-beam lithography can be found in U.S. Pat. No. 5,079,112 and U.S. Pat. No. 5,260,151, as well as in EP-A 98201997.8 (P-0113.000-EP).
SUMMARY OF THE INVENTION
It is an object of the invention to provide a lithographic projection apparatus as stated in the opening paragraph, which apparatus is compatible for use in a vacuum or semi-vacuum environment. In particular, it is an object of the invention that such an apparatus should be compatible with the use of radiation comprising EUV, charged particles or X-rays. More specifically, it is an object of the invention that such an apparatus should not suffer from significant “down-time” due to decrease of operational performance caused by degeneration of the projection system.
These and other objects are achieved in a lithographic projection apparatus that has a radiation system for supplying a projection beam of radiation; a mask table for holding a mask; a substrate table for holding a substrate; and a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate. Preferably, the lithographic projection apparatus according to the invention has the following characteristics;
a) the projection system is separated from the substrate table by an intervening space which can be at least partially evacuated and which is delimited at the location of the projection system by a solid surface from which the employed radiation is directed toward the substrate table;
b) the intervening space contains a hollow tube located between the solid surface and the substrate table and situated around the path of the radiation, the form and size of the tube being such that radiation focused by the projection system onto the substrate table does not intercept a wall of the hollow tube;
c) means are provided for continually flushing the inside of the hollow tube with a flow of gas.
The “solid surface” referred to under point (a) is, for example, the final mirror in the projection system from which the radiation is directed toward the substrate, or a (thin) optical flat (i.e. optical window) comprised of a vitreous material. The term vitreous should here be interpreted as encompassing such materials as silicates, quartz, various transparent oxides and fluorides (such as magnesium fluoride, for example) and other refractories.
In experiments leading to the invention, the inventors built a prototype device in which the radiation system delivered EUV (with a wavelength of approx. 13.4 nm). A projection system (comprising various mirrors) was used to focus the laser radiation onto a substrate table, onto which a test wafer could be mounted. A substantially evacuated enclosure, delimited (bounded) at one end by the exit aperture of the laser and at the other end by the substrate table, was provided around the projection system, so that the path of the radiation from source to substrate was substantially airless, including therefore the intervening space between the projection system and the substrate table. This intervening space was delimited on the side facing the substrate table by the last mirror in the projection system (the “solid surface” referred to hereabove). Such evacuation was necessary because of the fact that EUV undergoes significant absorption in air, and was aimed at avoiding substantial light-loss at substrate level.
In working with this prototype system, the inventors observed rapid degeneration of the resolution and definition of fine (submicron-sized) images projected onto a resist-coated wafer on the substrate table. Many different possible sources of this problem were sought and investigated before the inventors finally observed that the final optical surface (mirror) in the projection system had become unacceptably contaminated. Further analysis demonstrated that this contamination was caused by the presence of a spurious coating of organic material, which was subsequently identified as consisting of debris and bye-products from the resist layer on the wafer. Evidently, such material was being “sputtered” loose from the wafer by the EUV beam, and the evacuated intervening space between the wafer and the projection system allowed the released material to migrate toward the projection sys
De Jager Pieter Willem Herman
Van Zuylen Peter
Werij Henri Gerard Cato
Adams Russell
ASML Netherlands B.V.
Nguyen Hung Henry
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