Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2000-04-19
2002-06-11
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S072000, C355S074000, C355S077000, C250S492200, C430S311000, C430S312000
Reexamination Certificate
active
06404483
ABSTRACT:
BACKGROUND IF 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 system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The radiation 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”.
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 substrate table (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 mask table (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 a 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 known lithography apparatus, substrates for exposure—such as wafers—can be first loaded from a wafer carrier or process track into a pre-align module, so as to prepare the substrates for exposure. One of the most important aspects of such preparation is a pre-alignment step. In this step, the wafer is placed on a pre-aligner turntable and its edge inspected by rotating it past an edge sensor, which may comprise an optical or capacitive sensor, for example. This enables automatic location of the notch or flat edge of the wafer, and also allows the eccentricity of the wafer on the turntable to be measured. In this way:
the notch or flat edge can be automatically oriented as desired, before the wafer is transferred to the substrate table;
It can be determined if the eccentricity of the wafer exceeds a threshold value which, when translated to the substrate table, would cause the wafer to fall outside the capture range of an alignment module employed at the substrate table. If such is the case, then the wafer can first be shifted on the turntable by a pre-calculated amount, so as to bring it within the threshold value.
Once these steps have been performed, a substrate handler can remove the wafer from the turntable and place it on the wafer table with a precision which will allow wafer capture by the alignment module.
However, the present inventors have determined that the various activities of the pre-aligner are a source of undesirable vibrations which can induce overlay errors if pre-alignment steps are carried out whilst another wafer is being exposed. It is undesirable to not perform pre-alignment concurrently with exposures as this reduces throughput. Also, if the pre-aligner is mechanically isolated from the wafer table to prevent transmission of vibration, their relative position will no longer be certain to a sufficient accuracy to allow the substrate handler to transfer the wafer to the wafer table with the required precision.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a substrate handler capable of transferring substrates from a pre-aligner to a substrate table in a lithographic projection apparatus whilst avoiding the transmission of vibrations to the substrate table when pre-alignment steps are carried out on one substrate concurrently with exposure of another substrate.
According to the present invention there is provided a lithographic projection apparatus comprising:
a radiation system constructed and arranged to supply a projection beam of radiation;
a mask table provided with a mask holder for holding a mask;
a substrate table provided with a substrate holder for holding a substrate;
a projection system constructed and arranged to image an irradiated portion of the mask onto a target portion of the substrate;
a pre-aligner constructed and arranged to transfer out an initial alignment of a substrate; and
a substrate handler constructed and arranged to transfer a substrate from said pre-aligner to said substrate table;
said pre-aligner is mechanically isolated from said substrate table; and
coupler constructed and arranged to coupler said substrate handler to said substrate table at a known relative position.
Because the pre-aligner of the present invention is mechanically isolated from the substrate table, which carries the substrate during exposure processes, vibrations caused by the pre-alignment steps are not transmitted to the substrate during exposure. To provide positional accuracy in the transfer of substrates, the substrate handler couples to the substrate table to define its position relative thereto and only then deposits the substrate in the substrate holder. The substrate handler can have a fixed position relative to the pre-aligner or may be mechanically isolated from it as well and couple to it to pick up the substrate. Coupling to the pre-aligner ensures that, even if the substrate handler is isolated from the pre-aligner, it is at a known, e.g. predetermined, position relative to the pre-aligner when the substrate is picked up so that it will be held in the substrate handler at a known position. Similarly, when the substrate handler is coupled to the substrate table, its relative position is known and the positional accuracy of the substrate when in the pre-aligner is preserved in the transfer. If the substrate handler couples to both the substrate table and the pre-aligner, its relative position is known relative to the pre-aligner when the substrate is picked up from that pre-aligner and its relative position is known relative to the substrate table when the substrate is deposits, such that the accuracy of the pre-aligning is maintained during the transfer.
In preferred embodiments, when the substrate handler approaches the pre-aligner or substrate table their relative positions are known only coarsely. Once the substrate handler ha
Bijnagte Anton A.
Boon Rudolf M.
Jacobs Fransiscus M.
Segers Hubert M.
Adams Russell
ASML Netherlands B.V.
Brown Khaled
Pillsbury & Winthrop LLP
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