Photocopying – Projection printing and copying cameras – Detailed holder for original
Reexamination Certificate
2001-03-21
2003-07-22
Adams, Russell (Department: 2851)
Photocopying
Projection printing and copying cameras
Detailed holder for original
C355S053000, C355S052000, C355S077000
Reexamination Certificate
active
06597434
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
a mask table for holding a mask on a mask bearing surface, the mask serving to pattern the projection beam according to a desired pattern;
a substrate table for holding a substrate; and
a projection system for projecting the patterned beam onto a target portion of the substrate.
2. Background of the Related Art
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning means may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing pattering by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion at once; such an apparatus is commonly referred to as a step-and-scan apparatus—each target portion 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 are here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
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 radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask 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 tables while one or more other tables are being used for exposures. Twin stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, both incorporated herein by reference.
The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in the radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. The mask table ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired.
Conventionally, the mask table has been positioned such that radiation is passed from the illumination system through the mask, the projection system and onto the substrate. Such masks are known as transmissive masks since they selectively allow the radiation from the illumination system to pass through, thereby forming a pattern on the substrate. Such masks must be supported so as to allow the transmission of light therethrough. This has conventionally been achieved by using a vacuum in the table underneath a perimeter zone of the mask so that the atmospheric air pressure clamps the mask to the table.
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 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray and possible sources include laser-produced plasma sources, discharge sources or synchrotron radiation sources.
When EUV radiation is used, the projection system will be a non-telecentric on the object side. Therefore, variations in the height of the mask will cause variations in the horizontal and vertical position of the image on the substrate. Also, it is necessary to use a vacuum in the light propagating path to avoid absorption of the light. Thus, the conventional vacuum clamping will not operate.
It is an object of the present invention to provide a lithographic apparatus comprising a mask table that may be used to accurately hold a mask to achieve correct positioning and improved flatness.
This and other objects are achieved according to the invention in a lithographic projection apparatus comprising:
a radiation system for providing a projection beam of radiation;
a mask table for holding a mask on a mask bearing surface, the mask serving to pattern the projection beam according to a desired pattern;
a substrate table for holding a substrate; and
a projection system for projecting the patterned beam onto a target portion of the substrate,
characterized in that said mask table comprises:
a compliant membrane comprising the mask bearing surface; and
at least one actuator operable to apply a force to the membrane so as to deform the membrane in a direction substantially perpendicular to the mask bearing surface.
Thus, variations in the surface of a reflective mask can be easily and accurately corrected.
Preferably, the actuators are operable on the backside surface, which opposes the mask bearing surface, of the membrane, and a number of such actuators may be used to increase the precision with which the membrane can be deformed. Further, springs may be
ASML Netherlands B.V.
Kim Peter B.
Pillsbury & Winthrop LLP
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