Photocopying – Projection printing and copying cameras – Illumination systems or details
Reexamination Certificate
2001-08-31
2004-03-23
Nguyen, Henry Hung (Department: 2851)
Photocopying
Projection printing and copying cameras
Illumination systems or details
C355S052000, C355S053000, C355S055000
Reexamination Certificate
active
06710856
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to lithographic projection systems and more particularly to lithographic projection systems incorporating an intensity distribution sensor.
2. Background 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, and catadioptric systems, for example. The radiation system also includes components operating according to any of these design types, and such components may also be referred to below, collectively or singularly, as a “lens”.
The radiation system as well as the projection system generally comprise components for directing, shaping or controlling the projection beam of radiation. Generally, the projection system comprises means to set the numerical aperture (commonly referred to as the “NA”) of the projection system. For example, an aperture adjustable NA-diaphragm is provided in a pupil of the projection system. The radiation system typically comprises adjusting means for setting the outer and/or inner radial extent (commonly referred to as &sgr;-outer and &sgr;-inner, respectively) of the intensity distribution upstream of the mask (in a pupil of the radiation system).
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 apparatus are described, for example, in U.S. Pat. No. 5,969,441 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 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 (which may comprise 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 reticle, one at a time. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire reticle pattern onto the target portion at once; such an apparatus is commonly referred to as a wafer stepper or just “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 reticle pattern under the (slitted) projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the wafer 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 wafer table is scanned will be a factor M times that at which the reticle table is scanned. 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.
In a manufacturing process using a lithographic projection apparatus according to the invention a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-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), metallisation, 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.
Although the spatial distribution of projection beam radiation at wafer level is measured accurately in a lithographic projection apparatus, the angular distribution is generally not monitored. Consequently, properties of the performance of the projection system, such as angular dependent lens transmission are unknown. Similarly, the projection system comprises a pupil. The spatial intensity distribution at this pupil is related to the angular intensity distribution at the reticle and the wafer. The spatial intensity distribution at said pupil is, in practice, very difficult to measure in situ and sufficiently fast (such as to not substantially impair the number of substrates that can be processed per unit of time), but potentially contains valuable information relating to the alignment, performance and optimisation of the lithographic projection apparatus. Conventional techniques which measure the intensity distribution at wafer level or reticle level do not enable an assessment of specific (residual) positional and angular alignment errors of corresponding specific optical components. These techniques can also be highly dependent on the &sgr;-outer and &sgr;-inner settings, which means that many measurements must be taken and an alignment procedure for said specific optical components is time-consuming.
Accordingly, the present invention provides a method of operating a lithographic projection apparatus comprising:
a radiation system, for providing a projection beam of radiation;
a first object table for holding a mask at a mask plane;
a second object table for holding a substrate at a substrate plane; and
a projection system for imaging at least a portion of the mask onto target portions of the substrate;
the method characterized by comprising the steps of:
forming at least one spot of radiation from at least a portion of said projection beam in said apparatus;
providing at least one radiation sensor, embodied for real time electronic analysis of an intensity distribution;
measuring, with said at least one sensor, the spatial variation in intensity of defocused radiation from said spot or from an image of said spot; and
determining properties of said apparatus from information obtained in said measuring step.
This method enables properties of the pupil in the projection system to be determined and the angular intensity distribution property of the apparatus to be determined.
The or each spot may be formed at at least one of the mask plane and the substrate plane.
The or each spot may be formed by using a substantially transmissive or substantially radiation blocking pinhole.
The method may further comprise generating radiation at particular angles using, at the or each spot, at least one radiation manipulation effect selected from the group of radiation manipulation effects comprising diffraction, scattering and diffusion of radiation.
By diffracting radiation at large angles the NA-diaphragm size and shape can be determined. Instead of diffraction effects, sc
Dierichs Marcel
Eurlings Markus Franciscus Antonius
Van Der Laan Hans
Van Greevenbroek Hendrikus Robertus Marie
ASML Netherlands B.V.
Nguyen Henry Hung
Pillsbury & Winthrop LLP
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