Photocopying – Projection printing and copying cameras – Distortion introducing or rectifying
Reexamination Certificate
2001-01-18
2003-02-18
Mathews, Alan A. (Department: 2851)
Photocopying
Projection printing and copying cameras
Distortion introducing or rectifying
C355S053000, C355S067000
Reexamination Certificate
active
06522387
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to telecentricity error compensation in a lithographic projection apparatus comprising:
a radiation system for supplying a projection beam of radiation;
patterning means, for patterning the projection beam according to a desired pattern;
a substrate table for holding a substrate; and
a projection system for imaging the patterned beam onto a target portion of the substrate.
2. Description of the Related Art
The term “patterning means” should be broadly interpreted as referring to means 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; the term “light valve” has also been used in this context. Generally, the said 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 means include:
A mask table for holding a mask. The concept of a mask is well known in lithography, and its 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 (“reticle”) 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.
A programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus 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 said 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-adressable surface. The required matrix addressing can be performed using suitable electronic means. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, which are incorporated herein by reference.
A programmable LCD array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference.
For the sake of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask table and mask; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.
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 generally comprises an illumination system (“illuminator”), which may also include elements operating according to any of these design types 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”. 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 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 in, for example, U.S. Pat. No. 5,969,441 and U.S. Ser. No. 09/180,011, filed Feb. 27, 1998, (WO 98/28665 and WO 98/40791), incorporated herein by reference.
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 (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 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. In an alternative apparatus (which is commonly referred to as a step-and-scan apparatus or scanner) 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, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
As demand for imaging ever-smaller features at higher densities increases, it is necessary to use shorter wavelength radiation, for example ultraviolet light with a wavelength of 157 nm or 126 nm. However, this can result in problems caused by chromatic aberration which can degrade the performance of the projection apparatus. Two reasons for this are that, firstly, radiation sources, such as lasers, for producing shorter wavelength radiation tend to have greater line widths, i.e. the source is less monochromatic and contains a broader spread of wavelengths; and, secondly, the dispersion relation of refractive index against wavelength for refractive media used for the lenses tends to have a steeper gradient at shorter wavelengths, and therefore the media are more dispersive, which results in increased chromatic aberration. One solution to this problem is to design a projection lens that is achromatic, for example by combining lens elements which have powers of opposite sign, and which are made of lens materials having different dispersion relations, such that the chromatic aberration is substantially canceled out. However, this increases the complexity and expense of the lens systems, since two different media are requited. Also, the number of possible refractive media decreases when light with a relatively short wavelength is used. This makes it very difficult to make a projection lens that is achromatic.
An alternative solution is to use a catadioptric lens system, which includes at least one reflective optical element. This enables a single material to be used for all the lenses. However, the use of reflective elements in some projection system designs means that an image must be projected off-axis to avoid part of it being obscured by certain elements in the system. This means that the projected image does not span the optical axis (i.e. the center) of the projection system. An example of a catadioptric lens can be found, for example, in U.S. Pat. No. 5,537,260, incorporated herein by reference.
However, such a projection system will generally have an intrinsic telecentricity error. There is a problem of simultaneously compensating for this error whilst minimizing the size of the illumination system lenses in an off-axis projection system.
SUMMARY OF THE INVENTION
An object of the present invention is to provide an improv
ASML Netherlands B.V.
Mathews Alan A.
Pillsbury & Winthrop LLP
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