Photocopying – Projection printing and copying cameras – Step and repeat
Reexamination Certificate
2000-11-27
2002-11-12
Dowling, William (Department: 2851)
Photocopying
Projection printing and copying cameras
Step and repeat
C355S072000, C355S073000, C355S074000, C355S075000, C355S076000, C355S077000, C250S492200, C250S492220, C430S311000
Reexamination Certificate
active
06480260
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 first object table for holding a mask;
a second object table for holding a substrate; and
a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate.
More particularly, the invention relates to a mask table for use in such apparatus.
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, 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 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. 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 table” 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. Twin stage lithographic apparatus are described in International Patent Applications WO 98/28665 and WO 98/40791, for example.
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 portion (comprising one or more dies) 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 target portions which are successively irradiated via the mask, one at a time. 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. In an alternative apparatus—which 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 as here described can be gleaned from International Patent Application WO 97/33205.
In the above apparatus, the mask must be securely held (“clamped”) so that it can be accurately positioned in the x, y and z direction and in rotational orientation about the x, y and z axes (referred to as the Rx, Ry and Rz directions). The z direction is defined as being the direction along an axis which is substantially parallel to the optical axis of the projection system, and the x and y directions are along axes which are substantially perpendicular to the z axis and to each other. The mask can be subjected to large accelerations in its plane (the xy plane), particularly in a step-and-scan apparatus where the acceleration can be around 5 g (where g is the gravitational acceleration). In the z direction, the mask can be positioned with a 100 Hz bandwidth actuator which requires a relatively high stiffness in the z direction. The mask clamping arrangement must be sufficiently secure to withstand such accelerations and also to provide the mask with the necessary stiffness in the xy plane.
However, previous mask clamping arrangements, such as a rigid vacuum clamp on the mask table, have the problem that deformation of the mask can be caused. This can be as a result of either or both of the mask and the vacuum clamp not being perfectly flat or because of contaminant particles being trapped between the mask and the clamp. For example, the mask may only be flat to within a few &mgr;m. The deformation of the mask leads to distortion of the projected image, and variation in distortion between different masks leads to overlay errors. Some deformation can be corrected for by adjustment of one or more lens elements in the apparatus; however, not all distortion can be corrected in this way and the deformation is generally not reproducible between different masks.
An object of the present invention is to provide an improved apparatus for holding a mask which avoids or alleviates the above problems.
According to the present invention there is provided a lithographic projection apparatus for imaging of a mask pattern in a mask onto a substrate provided with a radiation-sensitive layer, the apparatus comprising:
a radiation 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 irradiated portions of the mask onto target portions of the substrate; wherein:
said mask table comprises at least one compliant member for holding said mask such that said at least one compliant member yields to conform substantially to the profile of the mask.
The use of at least one compliant member enables the mask to be held, but without unwanted deformation by forcing it to adopt a particular shape. The member can yield to accommodate flatness variations in the mask. The member, preferably a membrane, has a stiffness in the xy plane such that thermal expansion of the mask can be accommodated by the flexibility of the member, but without slipping of the mask with respect to the member. Slipping is detrimental to overlay precision, more so than thermal expansion, because of its asymmetric occurrence.
By appropriate choice of material and thickness of the member, its stiffness can be determined such that any particles trapped between the mask and member will preferentially deform the member rather than the mask. This can reduce deflection of the mask caused by a contaminant particle by a factor of as much as 10,000 compared to conventional mask clamping arrangements.
Preferably said at least one member comprises a pair of parallel strips, each of which is supported along its length. This improves the stiffness of the member against sagging, and reduces material creep.
Preferably the apparatus comprises a recess in the member which can function as a vacuum space for holding the mask and the member against each other (see for example FIG.
4
). This arrangement is both secure and compact.
Preferably the apparatus comprises a plurality of support points for defining the position of the mask perpendicular to its plane, that is in the z, Rx and Ry directions. The, or each, member defines the x, y and Rz position of the mask, and support points define the remaining position of the mask without distorting it into a particular shape. Three support points define a plane, so are the minimum number needed without over-constraining the position of the mask. A fourth support point can also be provided to give more stability and stiffness, but preferably the fourth support point is, for example, a damped gas bearing to eliminate vibrations of the mask.
Preferably the apparatus further comprises a vacuum chamber in a table supporting said at least one member, for deforming the member. By bending the member down into the vacuum chamber a couple will be introduced on the mask supported on the opposite edge of the member which will compensate against sagging of the mask.
Preferably a gas cushion is provided for supporting said at least one member; such
Donders Sjoerd N. L.
van Empel Tjarko A. R.
ASML Netherlands B.V.
Brown Khaled
Dowling William
Pillsbury & Winthrop LLP
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