Radiant energy – Irradiation of objects or material – Irradiation of semiconductor devices
Reexamination Certificate
2000-11-28
2003-07-15
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Irradiation of semiconductor devices
C250S492200, C250S492220, C250S442110, C430S395000, C430S296000
Reexamination Certificate
active
06593585
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to lithographic projection apparatus comprising:
an illumination system for supplying a projection beam of radiation;
a first object table for holding a mask;
a second object table for holding a substrate;
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
2. 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, and catadioptric systems, for example. The illumination system may also include elements operating according to any of these principles for directing, shaping or controlling the projection beam of radiation, 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. The mask table should be taken as any structure or device that may or does hold another structure or device, generally referred to as a mask, in which a pattern to be imaged is or can be formed. 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.
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 (comprising one or more dies) on a substrate (silicon wafer) which has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target areas which are successively irradiated via the mask, one at a time. In one type of lithographic projection apparatus, each target area is irradiated by exposing the entire mask pattern onto the target area 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 target area is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate 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 substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205.
In general, apparatus of this type contained a single first object (mask) table and a single second object (substrate) table. However, machines are 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 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 metrology steps 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 throughout, which in turn improves the cost of ownership of the machine.
To reduce the size of features that can be imaged, it is desirable to reduce the wavelength of the illumination beam. To such end, it has been proposed to use wavelengths of less than about 200 nm, for example 193 nm, 157 nm or 126 nm. Further reductions in the wavelength of the illumination radiation, e.g to about 10 to 20 nm, are also envisaged. Such wavelengths in particular are more conveniently focused and controlled by reflective optics, such as mirrors. However, mirrors in lithography apparatus must be positioned to especially high accuracy, as compared to refractive elements, because any rotational orientation errors are magnified by the total downstream optical path length. In an apparatus using very short wavelength radiation, the optical path length may be of the order of 2 m or more.
For example, to have a good overlay performance, it can be necessary to keep the position of an image of the irradiated portion of the mask stable at a given position at substrate level with an error (e) of less than about 1 nm (see
FIG. 3
of the accompanying drawings). If the distance between the mirror and the substrate (W) is 2 m the maximum permissible rotational error of the reflected beam, to keep the system within specification, is 28×10
−9
degrees (1×10
−9
m/2m=tan 28×10
−9
). Since, for a mirror, the angle of reflection equals the angle of incidence, a rotational error (da) in the position of the mirror will give rise to twice as large an error in the direction of the reflected beam. Thus the mirror must be positioned with an accuracy of 14×10
−9
degrees or better. If the mirror has a width of order 0.1 m and a rotating point at one side, that rotating point must be positioned to within 0.024 nm (tan 14×10
−9
×0.1=2.4×10
−11
). Clearly the accuracy with which such a mirror must be orientated is extremely high and will only increase as the specification for image accuracy increases. The accuracy requirements for position in X, Y and Z are less demanding as such errors are magnified less at substrate level.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a lithographic projection apparatus having an improved positioning system to accurately and dynamically position a mirror in the radiation or projection systems.
According to a first aspect of the present invention, there is provided a lithographic projection apparatus, including:
an illumination system constructed and arranged to supply a projection beam of radiation;
a first object table constructed and arranged to hold a mask;
a second object table constructed and arranged 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,
wherein at least one of said illumination system and said projection system comprises one or more reflective optical elements and positioning means for dynamically controlling a position and/or orientation of one or more of said reflective optical elements.
The one or more reflective optical elements may comprise a single element such as a mirror, a reflective grating, a reflective filter, etc. or a combination of such elements with or without other types of element. With the invention, the position of the reflective optics is controlled continuously or repeatedly during operation of the apparatus and the effects of vibrations and mechanical shocks, and thermal and mechanical drift thereby can be mitigated.
Preferably, the projection apparatus further comprises sensing means constructed and arranged to determine a change in position and/or orientation of one or more of said reflective optical elements, and to output one or more position signals indicative thereof; and said positioning means comprises:
drive means constructed and arranged to change a position and/or orientation of one or more of said reflecti
Loopstra Erik R.
Van Dijsseldonk Antonius J. J.
ASML Netherlands B.V.
Lee John R.
Pillsbury & Winthrop LLP
Wells Nikita
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