Optics: measuring and testing – By alignment in lateral direction – With light detector
Reexamination Certificate
1999-04-16
2002-06-11
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By alignment in lateral direction
With light detector
C250S492220
Reexamination Certificate
active
06404499
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of lithography. In particular, the present invention provides a device for optimising the uniformity of an image in a lithographic projection system.
2. Discussion of 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 and catadioptric systems, 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”. Any refractive, reflective or catadioptric elements in the radiation or illumination systems may be based on a substrate of glass or another suitable material, and may be provided with either single- or multi-layer coatings as desired. 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 stage” 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 apparatuses are described in International Patent Applications WO98/28665 and WO98/40791.
In lithography apparatuses, a mask (reticle) is irradiated. The mask has a pattern made from regions which either transmit radiation or block radiation; alternatively, so-called phase modulation (PSM) may be employed. The pattern on the mask is thus projected onto a substrate, typically of semiconductor material. The substrate (e.g. a wafer) is covered with a radiation-sensitive layer. Hence, the pattern on the mask is transferred onto the substrate.
A photolithographic apparatus of the construction described above and being usable, inter alia, in the manufacture of integrated circuits (ICs), is known, for example, from U.S. Pat. No. 5,194,893.
Due to the demand for an increasingly large number of electronic components in one IC, increasingly smaller details, also referred to as line widths, must be imaged by means of the projection apparatus in each area of the substrate on which an IC must be formed, which area is also referred to as IC area or “die”. One also wants to enlarge these IC areas so as to increase the number of components per IC. For the projection lens system this means, on the other hand, that the resolving power, hence its numerical aperture, must be raised and, on the other hand, that the image field must be enlarged. It is difficult to unite these two disparate requirements in a conventional lens.
This dilemma can be circumvented by changing from a so-called step-projection apparatus to a step-and-scan apparatus as described in U.S. Pat. No. 5,194,893 for example. In a step-projection apparatus, the complete mask pattern is illuminated and imaged as a whole on a target area on the substrate, for example, a wafer die or IC area. Subsequently, a step is made, i.e. the substrate is moved with respect to the projection lens system and the mask pattern, until a second target area is present opposite the mask pattern and within the image field of the projection lens system, and a second image of the mask pattern is formed in that area. Subsequently, the apparatus steps to a third target area and the mask pattern is imaged again, and so forth, until images of the mask pattern have been formed in all target areas.
In a step-and-scan apparatus, the same stepping movements are performed, but each time only a small part of the mask pattern is imaged on a corresponding sub-area of the target area. By imaging successive parts of the mask pattern on successive sub-areas of the target area in a scanning motion, an image of the complete mask pattern is obtained on a target area.
To this end, the pattern on the mask is illuminated with a projection beam which forms a small, for example, rectangular or arcuate illumination spot at the location of the mask pattern. In order to move the mask and the substrate, the mask is held on a mask table and the substrate is held on a substrate table. The mask table and the substrate table are moved (scanned) in the same direction or in mutually opposite directions along the scan direction, with respect to the projection lens system and the projection beam, the speed of the substrate being M times that of the mask table, where M is the magnification with which the mask pattern is imaged onto the substrate.
A commonly used value for M is {fraction (
1
/
4
)} or ⅕. Other values of M, for example 1, are alternatively possible. Said movement of the mask table and the substrate table with respect to the illumination spot is referred to as the scan movement. The illumination spot has its largest dimension in the direction transverse to the scan direction. This dimension may be equal to the width of the mask pattern, so that this pattern is imaged in one scan movement. However, it is alternatively possible that said dimension is half the mask pattern width or even smaller. In that case, the complete mask pattern can be imaged by performing two, or a larger number of, opposed scan movements.
It should then be ensured that the mask and the substrate have the correct mutual position and speed at any moment, which can be realised by means of a very accurate synchronisation of the movements of the mask and substrate tables, i.e. the speed V
sub
of the substrate should always be equal to M times the speed v
ma
of the mask.
To this end the apparatus may, for example, comprise a first and a second interferometer system for continuously measuring, during each imaging operation, the mutual position of the mask and the substrate.
Lithographic projection apparatuses may employ a projection beam of electromagnetic radiation, such as UV radiation, e.g. with a wavelength of 365 nm, 248 nm, 193 nm or 157 nm, or extreme UV, with a wavelength of the order of 15 nm. Alternatively such apparatus may employ a projection beam of charged-particle radiation, such as electron radiation or ion radiation, in which case an associated field-lens projection system is used.
More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO97/33205, for example.
There is a need to produce smaller and smaller semiconductor devices, and thus a corresponding need to improve the so-called critical dimension (CD) uniformity. Thus, these lithography apparatuses are being pushed to their resolution limits. Therefore, there is a need to minimise factors which can affect the resolution of the apparatus.
It is critical to produce a uniform light distribution at substrate level to achieve a high CD uniformity at high resolutions. It is an object of the present invention to produce a uniform light distribution at substrate level (in the absence of a mask).
Many different factors can affect the uniformity of the light produced at the substrate level. For example, the uniformity is dependent on factors such as the presence of a diaphragm e.g. so called REMA (reticle masking) blades, films formed on optical elements e.g. anti-reflective coatings, water, organic films formed on the illuminator, back reflections and contamination. Therefore, it is desirable if the effect of these factors on the light distribution at substrate level can be minimised.
It is known to place devices in the path of the employed radiation beam to vary the light intensity incident on the reticle. For example, U.S. Pat. No. 5,486,896 uses a device which decreases the light intensity at the sides of the beam. This device has two diaphragms which intercept the edges of the beam. In one embodiment, small filter strips are placed on the inner edges of the diaphragm to
Fey Franciscus H. A. G.
Stoeldraijer Judocus M. D.
Sytsma Joost
Ten Cate Jan W. R.
ASML Netherlands B.V.
Font Frank G.
Natividad Philip Sana
Pillsbury & Winthrop LLP
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