Abstract:
A lithographic apparatus exposes a radiation sensitive layer on a substrate to the pattern on a mask including pattern areas and opaque support. The apparatus uses a beam having a trapezoidal profile to provide a more uniform exposure at sub-field stitches in the event of any positional inaccuracies. The trapezoidal beam profile is generated by changing a characteristic, such as size or position, of the illumination beam on the mask during an exposure period.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a lithographic system, and more particularly to a manner of control of the illumination system in a lithographic apparatus. 
     2. Description 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 systems, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. The illumination 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 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 WO 98/28665 and WO 98/40791, for example. 
     Lithographic projection apparatuses 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 area (die) 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 dies which are successively irradiated via the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; 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 die is irradiated by progressively scanning the reticle pattern under the 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&lt;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. More information with regard to lithographic devices as here described can be gleaned from International Patent Application WO 97/33205. 
     In a lithographic apparatus, the size of features that can be imaged onto the wafer is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatuses employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use higher frequency (energy) radiation, e.g. EUV or X-rays, or particle beams, e.g. electrons or ions, as the illumination radiation in lithographic apparatuses. 
     However, the glass or quartz plates on which a conventional reticle pattern is defined are generally not transparent to some of these forms of illumination radiation. As an alternative in the case of charged-particle lithography, for example, the reticle is formed of a material, e.g. metal, that is opaque to the form of radiation used and in which apertures are cut to define the reticle pattern. To avoid the need to provide obscuring support arms to opaque islands in the pattern, the reticle pattern is divided into a plurality of sub-patterns separated by supporting struts. The complete pattern is correctly imaged on the wafer by introducing successive shifts in the illumination beam after it has passed through each sub-pattern. This type of arrangement is sometimes referred to as a “strutted mask” and an example is disclosed in U.S. Pat. No. 5,079,112. 
     General information with regard to the use of electron beams in lithography can be gleaned, for example, from U.S. Pat. No. 5,260,151. 
     As disclosed in EIPBN, May 1998 AE6, “Critical dimension control at stitched sub-field boundaries in a high-throughput SCALPEL system”, if the beam intensity profile of the illumination radiation is rectangular, then any positional inaccuracy in the stitching process will result in a substantial dose error. Such a stitching procedure using rectangular beam profiles is sometimes referred to as “simply-butted” and if the beam positions are not accurate there will be a region of no dose or double dose along the stitch seams. The EIPBN article therefore proposes the use of a trapezoidal beam profile and deliberate overlap in the stitching process. Positional inaccuracies then result in smaller dose errors. 
     The EIPBN article does not, however, disclose any method for generating an illumination beam having the desired trapezoidal intensity profile. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a convenient means of generating an illumination beam having a trapezoidal intensity profile in a lithographic projection apparatus. 
     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 mask having a plurality of transmissive regions bounded by opaque regions. The lithographic projection apparatus comprises: a radiation system comprising a radiation source and an illumination system for generating an illumination beam; a first movable object table provided with a mask holder for holding a mask; a second movable object table provided with a substrate holder for holding a substrate; and a projection system for imaging irradiated portions of the mask onto target portions of the substrate. The illumination system changes the size or position of the illumination beam on the mask during the course of an exposure period of at least part of a given transmissive region so as to generate an effective trapezoidal beam profile. 
     The term “transmissive region” is intended to refer to a region of the mask which is at least substantially transparent to the illumination radiation used. 
     With the present invention it is possible to easily generate the desired beam profile using electronic control of the beam size or position. 
     According to a yet further aspect of the invention there is provided a method of manufacturing a device using a lithographic projection apparatus comprising a radiation system comprising a radiation source and an illumination system for generating an illumination beam; a first movable object table provided with a mask holder for holding a mask; a second movable object table provided with a substrate holder for holding a projection system for imaging irradiated portions of the mask onto target portions of the substrate provided with a radiation-sensitive layer, so as to partially overlap. The size or position of the illumination beam on the mask is changed during the course of an exposure period of at least part of a given transmissive region so as to generate an effective trapezoidal beam profile. 
     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 specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “reticle”, “wafer” or “die” in this text should be considered as being replaced by the more general terms “masks, “substrate” and “target area”, respectively. 
     In the present document, the terms illumination radiation and illumination beam are used to encompass all types of electromagnetic radiation or particle flux, including, but not limited to, ultraviolet radiation, EUV, X-rays, electrons and ions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described below with reference to exemplary embodiments and the accompanying schematic drawings, in which: 
     FIG. 1 depicts a lithographic projection apparatus according to a first embodiment of the invention; 
     FIG. 2 is a plan view of a strutted reticle showing the scanning directions that may be used in embodiments of the invention; 
     FIG. 3 illustrates the overlap between beams in a stitching process; 
     FIGS. 4A,  4 B and  4 C illustrate the stitching process and resultant dose in the cases of correct and incorrect beam positioning; 
     FIG. 5 is a partial view of a reticle illustrating the manner of generation of the beam profile according to a first embodiment of the invention; 
     FIG. 6 is a partial view of a reticle illustrating the manner of generation of the beam profile according to a second embodiment of the invention; and 
     FIG. 7 is a partial view of a reticle illustrating the manner of generation of the beam profile according to a third embodiment of the invention. 
     In the drawings, like reference numerals indicate like parts. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
     FIG. 1 schematically depicts a lithographic projection apparatus according to the invention. The apparatus comprises a radiation system LA, Ex, IN, CO for supplying a projection beam PB of radiation (e.g. UV or EUV radiation); a first object table (mask table) MT provided with a mask holder for holding a mask MA (e.g. a reticle), and connected to first positioning device for accurately positioning the mask with respect to item PL; a second object table (substrate table) WT provided with a substrate holder for holding a substrate W (e.g. a resist-coated silicon wafer), and connected to a second positioning device for accurately positioning the substrate with respect to item PL: a projection system (“lens”) PL (e.g. a refractive or catadioptric system, a mirror group or an array of field deflectors) for imaging an irradiated portion of the mask MA onto a target portion C (die) of the substrate W. 
     As here depicted, the apparatus is of a transmissive type (i.e. has a transmissive mask). However, in general, it may also be of a reflective type, for example. 
     The radiation system comprises a source LA (e.g., a Hg lamp, excimer laser, an undulator provided around the path of an electron beam in a storage ring or synchrotron, or an electron or ion beam source) which produces a beam of radiation. 
     This beam is passed along various optical components comprised in the illumination system,—e.g. beam shaping optics Ex, an integrator IN and a condenser CO—so that the resultant beam PB is substantially collimated and uniformly intense throughout its cross-section. The beam PB subsequently intercepts the mask MA which is held in a mask holder on a mask table MT. Having passed through the mask MA, the beam PB passes through the lens PL, which focuses the beam PB onto a target area C of the substrate W. With the aid of the interferometric displacement and measuring device IF, the substrate table WT can be moved accurately, e.g. so as to position different target areas C in the path of the beam PB. Similarly, the first positioning device can be used to accurately position mask MA with respect to the path of the beam PB, e.g. after mechanical retrieval of the mask MA from a mask library. In general, movement of the object tables MT, WT will be realized with the aid of a long stroke module (course positioning) and a short stroke module (fine positioning), which are not explicitly depicted in FIG.  1 . The depicted apparatus can be used in two different modes: In step mode, the mask table MT is kept essentially stationary, and an entire mask image is projected in one go (i.e., a single “flash”) onto a target area C. The substrate table WT is then shifted in the x and/or y directions so that a different target area C can be irradiated by the beam PB; In scan mode, essentially the same scenario applies, except that a given target area C is not exposed in a single “flash”. Instead, the mask table MT is movable in a given direction (the so-called “scan direction”, e.g. the x direction) with a speed v, so that the projection beam PB is caused to scan over a mask image; concurrently, the substrate table WT is simultaneously moved in the same or opposite direction at a speed V=Mv, in which M is the magnification of the lens PL (typically, M=¼ or ⅕). In this manner, a relatively large target area C can be exposed, without having to compromise on resolution. 
     FIG. 2 shows a reticle in plan and the scheme by which it can be scanned with an illumination beam. As shown, the reticle  1  comprises an array of elongate sub-fields  2  (e.g. of length 12 mm) separated by struts  3 . The reticle is illuminated by, e.g., a square beam of 1×1 mm 2  size. The beam must illuminate each sub-field for a sufficient period to deliver a dose to expose the resist on the substrate wafer. By way of an example, if the resist sensitivity is 10 μC/cm 2 , the beam current 10 μA and the magnification ¼ then the time required to expose each (sub-) field is 625 μs. 
     The scanning scheme used, which is known in the prior art, is to scan across the widths of the sub-fields in the direction of arrow B, stepping across the struts to avoid excessive heating, and to mechanically scan the wafer and reticle along the lengths of the sub-fields, in the direction of arrow A (mutually parallel or anti-parallel). 
     As shown in FIG. 3, the beams  5   a  and  5   b  used to scan adjacent sub-fields  2  have trapezoidal (intensity) profiles. In the stitching process, the beams  5   a ,  5   b  are shifted sideways so that they overlap on the wafer  4  in region  6 . As shown in FIG. 4A, if the beams are correctly positioned then the net radiation dose  7   a  in the overlap region  6  is equal to that in the main beam portion. 
     The effects of small misalignments are shown in FIG. 4B and 4C. If the beams are slightly too far apart, then the net dose is slightly reduced in the overlap region, as shown by dashed line  7   b . If the beams overlap too much, then the net dose is slightly increased, as shown by dashed line  7   c . The slight under or over dose in either case is not as detrimental as the double dose or no dose that occurs in the event of misalignment when using a simply-butted stitching technique. 
     FIG. 5 is a partial view of one mask sub-field and illustrates how the desired beam profile is achieved according to one embodiment of the invention. 
     The sub-field  2  is surrounded by the strut  3  and has around its edge a blending area  21 , the pattern of which is repeated in the blending area of the adjacent sub-field. The inside edge of the strut  3  is provided with a skirt  31  which has no pattern on it. 
     The illumination system (not shown) projects a square illumination beam  5  of uniform-intensity illumination radiation on to the sub-field  2 . The illumination beam  5  is arranged to be equal in width (in the Y-direction) to the main sub-field portion  20  plus the width of one side of the blending area  21 . Thus the illumination beam is smaller than the distance between the skirts  31  of the struts  3 . 
     The desired trapezoidal beam profile is achieved according to the invention by scanning the beam rapidly within the sub-field in the direction of arrow B. i.e. perpendicular to the length of the sub-field  2 . The illumination profile shown in FIG. 3 is thus built up as the sum of a plurality of scans back and forth in direction B. 
     A trapezoidal profile along the longitudinal direction of the sub-field is also necessary if the beam is stepped in that direction, and this is achieved by effecting a movement in the direction of arrow A (i.e. along the length of the sub-field) that is much slower than that in the direction of arrow B. 
     Embodiment 2 
     FIG. 6 is a view similar to FIG. 5 but showing how the desired beam profile is achieved according to a second embodiment of the invention. In this embodiment, the shape of the illumination beam is defined by the overlap of two apertures  11 ,  12  in the illumination system, more precisely by the overlap of the image of one aperture on the other. The desired beam profile is obtained by shifting the image of the first aperture  11  on the second aperture  12  and by shifting the image of the second aperture on the reticle, both motions occurring along the diagonal direction C. In a preferred embodiment the illumination beam is initially set to cover only (part of) the main sub-field area  2  and is gradually enlarged to cover also (the corresponding part of) the blending area  21  during the course of the exposure. The illumination profile shown in FIG. 3 is thus built up as the sum (integration) of the illumination during the exposure period. As an alternative, the illumination beam may be initially set broadly and reduced in size during the course of the exposure. 
     Embodiment 3 
     A third embodiment of the invention is shown in FIG.  7  and is particularly adapted for use with square or nearly square rectangular sub-fields (e.g. with an aspect ratio of from 1:1 to 2:1) that may be flash-illuminated, e.g. in an electron beam direct-write apparatus. The square sub-field  2 ′ is illuminated by beam  5  which remains stationary but is changed in size during the exposure to create the effective trapezoidal beam profile. At the beginning of an exposure the beam is turned on with a size matching the non-overlapped area of the sub-field and is expanded steadily in both X and Y directions during the exposure until it additionally covers the blending area  21  around the periphery of the sub-field  2 ′. Alternatively, the apparatus can be arranged to project a large beam initially covering the whole sub-field  2 ′, including blending area  21 , and to reduce the size of the beam during the exposure. Throughput may be improved further by exposing successive sub-fields alternately with expanding and contracting beams. This avoids the need to reset the beam shaping elements between exposures. It may further be possible to expose a square or nearly square sub-field with an orbiting movement of a beam of constant size (so that there is oscillatory motion in both the X and Y directions). 
     In embodiments of the invention for use in electron or ion beam lithography, for example, the beam shape may be controlled electronically by shifting the images of two shaping apertures as described in, and using the apparatus of, co-pending European Patent application no. 98201997.8 (P-0113). 
     While we have described above a specific embodiment of the invention it will be appreciated that the invention may be practiced otherwise than described. The description is not intended to limit the invention.