Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2000-04-19
2004-04-13
Fuller, Rodney (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S117000, C355S030000, C355S072000, C355S073000
Reexamination Certificate
active
06721035
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a preparatory station, in particular for a lithographic projection apparatus.
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 that 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 at once; 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 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, for example.
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 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 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.
Lithographic apparatus may employ various types of projection radiation, such as ultra-violet light (UV), extreme UV, X-rays, ion beams or electron beams, for example. Depending on the type of radiation used and the particular design requirements of the apparatus, the projection system may be refractive, reflective or catadioptric, for example, and may comprise vitreous components, grazing-incidence mirrors, selective multi-layer coatings, magnetic and/or electrostatic field lenses, etc; for simplicity, such components may be loosely referred to in this text, either singly or collectively, as a “lens”. The apparatus may comprise components that are operated in vacuum, and are correspondingly vacuum-compatible. As mentioned in the previous paragraph, the apparatus may have more than one substrate table and/or mask table.
In a manufacturing process using a lithographic projection apparatus, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy sensitive material (resist). For this process it is necessary to provide the substrate to the substrate table and to hold the substrate firmly in a fixed position on said table during the process. The substrate can be held in this fixed position with a substrate holder comprising means for applying a vacuum to a major surface of the substrate table. The vacuum will suck the substrate firmly to the substrate table. Before the substrate is provided to the substrate table it will often be processed (e.g. spincoated with resist) in a process track and therefore the temperature of the substrate can be different to the temperature of the substrate table. This temperature difference can give a problem, because the temperature of the substrate can change after the substrate is provided to the substrate table. The substrate table can cool the substrate, so that the substrate would tend to shrink. However, the substrate is held firmly by the substrate holder, which does not allow the substrate to shrink. The substrate can only shrink when the tension inside the substrate is higher than the friction between the substrate and the surface of the substrate table. If this occurs, a part of the substrate will slip over the surface of the substrate table to release the tension inside the substrate. This slip movement can give an error in the super-positioning of two concurrent images exposed on successive layers on the substrate, leading to so-called overlay errors. In general, the super-positioning of two concurrent images is very accurately achieved by aligning a mark on the substrate to a reference mark (e.g. on the mask, or on a fiducial on the substrate table). If the substrate slips after aligning the substrate to said reference mark, the super-positioning of two concurrent images on the slipped part of the substrate can fail. Similar considerations apply to the case whereby the substrate is colder than the substrate table and is warmed by the substrate table. The substrate in that case would like to expand and can also slip over the surface of the substrate table.
SUMMARY OF THE INVENTION
It is an object of the invention to alleviate, at least partially, the above problems. Accordingly the present invention provides an apparatus having an intermediate table comprising a major surface provided with a plurality of apertures, and gas bearing means for generating a gas bearing between said major surface and a substrate located thereon.
The gas bearing substantially removes the friction between the substrate and the major surface of the intermediate table. The substrate can easily expand and shrink on the gas bearing when the temperature of the substrate changes. Another advantage of using a gas bearing between the surface of the intermediate table and the substrate is that contamination of the backside of the substrate by foreign particles present on the major surface of the intermediate table is avoided. Particles already collected on the backside can even be blown away from the backside by the gas bearing.
The gas used in the gas bearing can be air and the gas source can be provided with a filter for the gas (e.g. air from outside) such that it is substantially free of foreign particles. Alternatively, other gases can be used, for example, nitrogen or helium. As will be appreciated by the skilled artisan, one can control the gas bearing by having apertures for the inflow of gas to the gas bearing and also apertures for the evacuation of gas from the gas bearing. A particular pressure for the inflow of gas may be between about 1.1 and 1.5 bar whereas a reduced gas pressure for the evacuation of the gas may be between about 0.5 and the 0.9 bar, for example. The gas bearing may have a thickness less than about 150 &mgr;m, for example.
The preparatory station can comprise a gas ionizer to ionize the gas used to create the gas bearing By using the gas ionizer the substrate can be gradually discharged from any initially charged state (since a statically charged part of the substrate will attract ions with an opposite charge, so that the charged part will be neutralized by the ions). This gradual discharging is advantageous, because it prevents a sudden discharge of the substrate, e.g. if it comes into the neighborhood of a conductor. A sudden discharge, for example with a spark, can cause damage to the substrate or to the sensitive structures already created thereon. As will be appreciated by one of ordinary skill in the art, the gas ionizer can, for exam
Bijnagte Anton A.
Boon Rudolf M.
Jacobs Fransiscus M.
Segers Hubert M.
ASML Netherlands B.V.
Fuller Rodney
Pillsbury & Winthrop LLP
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