1. Field of the Invention
The present invention relates to lithographic apparatus and methods
2. Description of the Related Art
The term “patterning structure” as here employed should be broadly interpreted as referring to a structure that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. Generally, the pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning structure include:                A mask. 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. In the case of a mask, the support structure will generally be a mask table, which 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;        A programmable mirror array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such a device is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation devices. Once again, the mirrors are matrix-addressable, such that addressed mirrors and unaddressed mirrors will reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The matrix-addressing can, for example, be performed using suitable electronic devices. In both of the situations described hereabove, the patterning structure can include one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and        A programmable liquid-crystal display (LCD) panel. An example of such a device is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.        
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning structure as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning structure 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. including 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 patterning 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 wafer stepper or step-and-repeat apparatus. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by scanning the mask pattern under the beam of radiation 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 apparatus as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a device 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, whereby any of these types of projection system may either be suitable for conventional imaging or be suitable for imaging in the presence of an immersion fluid. 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” machines 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. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, both incorporated herein by reference.
At several locations in conventional lithographic apparatus, moveable slide members are used for carrying objects. Such objects can be machine parts mounted on the slide member, but also objects that are exchanged with other objects of the same type regularly, such as a patterning structure or a substrate such as a silicon wafer. Some of the slide members are adapted to move over a running surface of a base. The running surface of the base can be a flat plane, but it can also have a different shape. A bearing system is provided to allow movements of the slide member over the running surface.
It is desirable that the slide member carries out its movements without friction between the slide member and the running surface. In order to achieve that, bearing system can be used to create a distance between the slide member and the running surface (the so-called “gap”) during the movements of the slide member, so that contact between the slide member and the running surface is avoided. The distance is created by the bearing system by applying a repelling force between the slide member and the running surface of the base. When the slide member is located above the running surface, creating the distance between the slide member and the running surface requires lifting the slide member relative to the running surface with a lifting force.
Generally, the bearing system may include magnets or air bearings in order to create a distance between the slide member and the running surface. In order for both the magnet and air systems to function properly and reliably, it is desirable to closely control the distance between the side of the bearing system facing the running surface and the running surface itself (that is: the width of the gap). Therefore, it is desirable that the face of the bearing system that faces the running surface, and also the free surface of the slide member directly adjacent to it, be even.
When an air bearing is used, a gas film is created and maintained between the slide member and the running surface during operation of the bearing system. The stiffness of this gas film during operation is a relevant parameter in the dynamic behavior of the slide member, especially with regard to the vibrations of the slide member, which vibrations may give rise to unallowable inaccuracies in positioning of objects carried by the slide member.
Depending on the design of the face of the air bearing that creates and maintains the gas film, an optimal or sufficient stiffness of the gas film may be achieved at a certain width of the gap, in combination with a corresponding nominal load on the slide member.
In order to produce this particular gap width, attraction devices may be provided in the bearing system. These attraction devices are adapted to provide an attracting force between the slide member and the running surface to balance the repelling force generated by the air bearings or magnets, which repelling force creates a distance between the slide member and the running surface. Therefore, known bearing systems do not only include repelling devices, that are adapted to provide a repelling force between the slide member and the running surface, but also attraction devices.
Conventional bearing systems configured to allow a slide member to move over a running surface without friction are generally heavy and large. As conventional bearing systems are mounted at least partly in the slide member, they generally cause the slide members to be heavy and large too. This may be disadvantageous, since new developments ask for faster moving slide members with higher acceleration levels, while the maximum allowable motor current in the motor that drives the slide member is limited.