Abstract:
A method and apparatus, in particular for microlithographic exposure, comprising a radiation system for providing a projection beam of radiation; a support structure for supporting a patterning device, the patterning device serving to pattern the projection beam according to a desired pattern; a substrate table for holding a substrate; and a projection system for projecting the patterned beam onto a target portion of the substrate. Embodiments of the invention divide the projection beam into regions and select which features on the mask will be illuminated by which regions of the projection beam.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This application claims priority from EP application no. 03076421.1 filed May 12, 2003, the contents of which is incorporated herein in its entirety.  
           [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to a lithographic apparatus and method for its use.  
           [0004]    2. Description of the Related Art  
           [0005]    The term “patterning device” as here employed should be broadly interpreted as referring to devices 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” can also be used in this context. Generally, the 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 devices include:  
           [0006]    A mask. The concept of a mask is well known in lithography, and it 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 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 being incident on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which 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;  
           [0007]    A programmable mirror array. One 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 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-addressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuators. Again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic controllers. In both of the situations described above, the patterning device can comprise at least one programmable mirror array. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, the support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and  
           [0008]    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. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.  
           [0009]    For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as set forth above.  
           [0010]    Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising at least one dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that 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 one; such an apparatus is commonly referred to as a wafer stepper or step-and-repeat apparatus. In an alternative apparatus, 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 &lt;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.  
           [0011]    In a manufacturing process using a lithographic projection apparatus, a pattern (e.g., in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-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), metallization, 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, incorporated herein by reference.  
           [0012]    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 components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components 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 substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on at least one tables while at least one other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, both incorporated herein by reference.  
           [0013]    In the semiconductor manufacturing industry there is increasing demand for ever-smaller features and increased density of features. In other words the critical dimension (CD) is rapidly decreasing and becoming very close to the theoretical resolution limit of state-of-the-art exposure tools such as step-and-repeat and step-and-scan apparatus as described above. Consequently, the requirements for the variations in the relative position (overlay) of exposed features also become critical.  
           [0014]    Exposure tools typically comprise optical elements to manipulate the intensity and angular distribution of the projection beam being incident on the mask, creating regions of radiation with the required properties within the projection beam. Such regions may be substantially round (and called poles), but other shapes such as rings and bars are also possible. Different angular distributions are commonly called illumination modes, and they are selected to provide an optimal image on the substrate based upon the size and elongation direction (orientation) of features on the mask. However, a mask may typically comprise features of different sizes and orientations, which means that a single illumination mode may not provide the optimal exposure conditions for all features on the mask.  
           [0015]    This problem is typically solved using a multiple-exposure technique, in which mask features are grouped into similar sizes and/or orientations, and each group of features is placed onto a separate mask. Each mask is then exposed in turn with a suitable illumination mode onto the same target portion on the substrate. Typically, this technique is restricted to two steps only, and is called double-exposure.  
           [0016]    An example of double-exposure is shown in FIGS. 2A to  2 D. A projection beam PB 1 , PB 2  is shown in cross-section at a pupil plane in the illumination system, which is substantially perpendicular to optical axis A. Axes B and C define the pupil plane and are substantially perpendicular to each other and optical axis A. A mask MA 1 , MA 2  being illuminated is substantially perpendicular to optical axis A, and thus substantially parallel to the pupil plane BC. Prior to performing the double-exposure, all mask features have been separated onto two masks based upon orientation, namely a first group of features  110  with an orientation substantially parallel to axis C on mask MA 1 ; and a second group of features  210  with an orientation substantially parallel to axis B on mask MA 2 .  
           [0017]    For imaging of the first group of features  110 , it has been determined that illumination using a dipole of linearly-polarized light is most advantageous, in which the two poles which form the dipole are configured and arranged such that the axis joining its poles is substantially perpendicular to the elongation direction of the features  110 , and in which the illumination light being incident upon the features is linearly polarized in a direction substantially parallel to the elongation direction of the features  110 . Similarly, for imaging of the second group of features  210 , it has been determined that illumination using a dipole of linearly-polarized light is most advantageous, in which two poles which form the dipole are configured and arranged such that the axis joining its poles is substantially perpendicular to the elongation direction of the features  210 , and in which the illumination light being incident upon the features is linearly polarized in a direction substantially parallel to the elongation direction of the features  210 .  
           [0018]    Exposure then proceeds in two steps to ensure that each group of features is only exposed with the illumination mode that is most advantageous: in a first step, depicted in FIG. 2A, the first group of features  110  on mask MA 1  is exposed using the projection beam PB 1 . The projection beam PB 1  comprises a first region, namely a dipole of two poles  140  of linearly polarized light, disposed substantially symmetrically about optical axis A along axis B. The direction of polarization  145  of the light in the poles  140 , and the elongation direction of the first group of features  110 , are both substantially parallel to axis C. FIG. 2B shows a plan view of the first step viewed along optical axis A, illustrating the relative orientations of the poles  140 , the direction of polarization  145 , and the elongation direction of features  110  on the mask MA 1 .  
           [0019]    In a second step, depicted in FIG. 2C, the second group of features  210  on mask MA 2  is exposed using the projection beam PB 2 . The projection beam PB 2  comprises a second region, namely a dipole of two poles  240  of linearly polarized light, disposed substantially symmetrically about optical axis A along axis C. The direction of polarization  245  of the light in the poles  240  and the elongation direction of the features  210  are substantially parallel to axis B. FIG. 2D shows a plan view of the second step viewed along optical axis A, illustrating the relative orientations of the poles  240 , the direction of polarization  245 , and the elongation direction of features  210  on the mask MA 2 .  
           [0020]    If features  110  and  210  were disposed on the same mask (single-exposure), it would not be possible to image them with their own illumination modes—a compromise in illumination modes would have to be reached because light from both dipoles would be incident on each group of features. More details on double-exposure are given in European Patent Appl. No. EP 1,091,252, incorporated herein by reference.  
           [0021]    In general, multiple-exposure is recognized to have three main disadvantages: an increase in cost due to extra masks that need to be designed and manufactured; a considerable decrease in throughput of the lithographic projection apparatus due to extra mask exchanges and extra exposures; and extra overlay errors that may be introduced between the images produced by each mask.  
         SUMMARY OF THE INVENTION  
         [0022]    Embodiments of the present invention provide a lithographic projection apparatus and methods that maintain the advantages of multiple exposure, but may not incur the associated throughput and overlay penalties.  
           [0023]    This and other aspects are achieved according to embodiments of the invention by dividing the projection beam into parts, configuring the projection beam such that the parts of the projection beam correspond to regions in a pupil plane of the radiation system, and selecting which features on the mask will be illuminated by which regions of the projection beam. According to embodiments of the invention, there is provided a lithographic projection apparatus comprising: a radiation system for providing a projection beam of radiation; a support structure for supporting a patterning device, the patterning device serving to pattern the projection beam according to a desired pattern; a substrate table for holding a substrate; and a projection system for projecting the patterned beam onto a target portion of the substrate, wherein the radiation system is configured and arranged to provide the projection beam with a first part and a second part for cooperation with the patterning device providing a first patterning region and a second patterning region such that radiation in the second part of the projection beam is substantially prevented from being incident on the first patterning region of the patterning device.  
           [0024]    According to a further aspect of the invention there is provided a device manufacturing method comprising providing a substrate that is at least partially covered by a layer of radiation-sensitive material; providing a projection beam of radiation using a radiation system, the projection beam comprising a first part and a second part; using a patterning device that substantially prevents radiation from the second part of the projection beam being incident upon a first patterning region of the patterning device; endowing the projection beam with a pattern in its cross-section; and projecting the patterned beam of radiation onto a target portion of the layer of radiation-sensitive material.  
           [0025]    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 “mask,” “substrate” and “target portion,” respectively.  
           [0026]    In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., with a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range 5-20 nm). 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:  
         [0028]    [0028]FIG. 1 depicts a lithographic projection apparatus according to an embodiment of the invention;  
         [0029]    [0029]FIGS. 2A to  2 D depict a prior art double-exposure sequence;  
         [0030]    [0030]FIGS. 3A to  3 B depict an illumination mode and mask configuration according to the invention;  
         [0031]    [0031]FIGS. 4A to  4 B depict another embodiment of an illumination mode and mask configuration;  
         [0032]    [0032]FIGS. 5A to  5 B depict yet another embodiment of an illumination mode and mask configuration;  
         [0033]    [0033]FIGS. 6A to  6 B depict still another embodiment of an illumination mode and mask configuration;  
         [0034]    [0034]FIGS. 7A to  7 B depict a further embodiment of an illumination mode and mask configuration;  
         [0035]    [0035]FIGS. 8A to  8 D depict examples of mask construction according to the invention; and  
         [0036]    [0036]FIG. 9 depicts a device that may be used to create suitable illumination modes. 
     
    
     DETAILED DESCRIPTION  
     Embodiments  
       [0037]    [0037]FIG. 1 schematically depicts a lithographic projection apparatus  1  according to a particular embodiment of the invention. The apparatus comprises:  
         [0038]    a radiation system Ex, IL, for supplying a projection beam PB of radiation (e.g., DUV radiation). In this particular case, the radiation system also comprises a radiation source LA;  
         [0039]    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 positioner PM for accurately positioning the mask with respect to item PL;  
         [0040]    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 second positioner PW for accurately positioning the substrate with respect to item PL; and  
         [0041]    a projection system (“lens”) PL for imaging an irradiated portion of the mask MA onto a target portion C (e.g., comprising at least one die) of the substrate W.  
         [0042]    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 (with a reflective mask). Alternatively, the apparatus may employ another kind of patterning device, such as a programmable mirror array of a type as referred to above.  
         [0043]    The source LA produces a beam of radiation. This beam is fed into an illumination system (illuminator) IL, either directly or after having traversed conditioning means, such as a beam expander Ex, for example. The illuminator IL may comprise adjustable elements AM for setting the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in the beam. In addition, it will generally comprise various other components, such as an integrator IN and a condenser CO. In this way, the beam PB being incident on the mask MA has a desired uniformity and intensity distribution in its cross-section.  
         [0044]    It should be noted with regard to FIG. 1 that the source LA may be within the housing of the lithographic projection apparatus (as is often the case when the source LA is a mercury lamp, for example), but that it may also be remote from the lithographic projection apparatus, the radiation beam which it produces being led into the apparatus (e.g., with the aid of suitable directing mirrors); this latter scenario is often the case when the source LA is an excimer laser. The current invention and claims encompass both of these scenarios.  
         [0045]    The beam PB subsequently intercepts the mask MA, which is held on a mask table MT. Having traversed the mask MA, the beam PB passes through the lens PL, which focuses the beam PB onto a target portion C of the substrate W. With the aid of the second positioner PW (and interferometer or linear encoder), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the beam PB. Similarly, the first positioner PM can be used to accurately position the mask MA with respect to the path of the beam PB, e.g., after mechanical retrieval of the mask MA from a mask library, or during a scan. In general, movement of the object tables MT, WT will be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which are not explicitly depicted in FIG. 1. However, in the case of a wafer stepper (as opposed to a step-and-scan apparatus) the mask table MT may just be connected to a short stroke actuator, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M 1 , M 2  and substrate alignment marks P 1 , P 2 .  
         [0046]    The depicted apparatus can be used in two different modes:  
         [0047]    1. In step mode, the mask table MT is kept essentially stationary, and an entire mask image is projected at once (i.e., a single “flash”) onto a target portion C. The substrate table WT is then shifted in the x and/or y directions so that a different target portion C can be irradiated by the beam PB; and  
         [0048]    2. In scan mode, essentially the same scenario applies, except that a given target portion 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 y direction) with a speed ν, 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=Mν, in which M is the magnification of the lens PL (typically, M=¼ or ⅕). In this manner, a relatively large target portion C can be exposed, without having to compromise on resolution.  
         [0049]    According to the invention, FIG. 3A shows a projection beam PB 3  illuminating a mask MA 3 . The projection beam PB 3  is shown in cross-section at a pupil plane in the illumination system, which is substantially perpendicular to optical axis A. Axes B and C define the pupil plane and are substantially perpendicular to each other and to optical axis A. The mask MA 3  being illuminated is substantially perpendicular to optical axis A, and thus substantially parallel to the pupil plane BC.  
         [0050]    For imaging of a first group of features  10  on the mask MA 3 , it has been determined that illumination using a dipole of linearly-polarized light is most advantageous, in which two poles which form the dipole are configured and arranged such that the axis joining its poles is substantially perpendicular to the elongation direction of the features  10 , and in which the light being incident upon the features is linearly polarized in a direction substantially parallel to the elongation direction of the features  10 . Similarly, for imaging of a second group of features  310  on the mask MA 3 , it has been determined that illumination using a dipole of linearly-polarized light is most advantageous, in which two poles which form the dipole are configured and arranged such that the axis joining its poles is substantially perpendicular to the elongation direction of the features  310 , and in which the light being incident upon the group of features  310  is linearly polarized in a direction substantially parallel to the elongation direction of the features  310 .  
         [0051]    The mask MA 3  comprises the first group of features  10  which have an elongation in a direction substantially parallel to axis C; the second group of features  310  which have an elongation in a direction substantially parallel to axis B; a first polarization filter  60  configured and arranged such that it only allows light to be incident upon the first group of features  10  with a polarization direction  65 ; a second polarization filter  360  configured and arranged such that it only allows light to be incident upon the second group of features  310  with a polarization direction  365 . Direction  65  is substantially parallel to the elongation direction of the features  10 , and direction  365  is substantially parallel to the elongation direction of the features  310 .  
         [0052]    A cross-section through the mask MA 3  in the region of the second group of features  310  is depicted in FIG. 8A—the mask MA 3  comprises a substantially transparent substrate  380 ; a blocking layer  390 , typically made of chrome, comprising areas which prevent transmission of the projection beam PB 3 ; the second group of features  310  that transmit the projection beam PB 3 ; and the polarization filter  360  (polarization layer). The second group of features  310  is actually formed by gaps between the areas of the blocking layer  390 . In this configuration, the polarization filter  360  selects the radiation that can be incident upon the second group of features  310  based upon the polarization of the radiation. A suitable polarization filter  360  may be created, for instance, using lithographic processes such as deposition of a layer of polarizing or scattering material onto the substrate  380 , and selectively etching the layer to create a filter  360  that only covers the second group of features  310 . FIG. 8B shows an alternative construction in which a polarization filter  360  is created adjacent to the blocking layer  390 —in this configuration, the polarization layer  360  selects the radiation that can be transmitted by the second group of features  310  based upon the polarization of the radiation. All references to layers that selects the radiation being incident upon features should be broadly interpreted as also covering the configuration where the layer selects the radiation being transmitted by such features. FIG. 8C shows a further variation in which a polarizing filter  360  only covers individual features from the second group  310 , and which is shown in plan view along optical axis A in FIG. 8D. Although a mask of the binary type is depicted here, the same basic techniques can be used to create a polarizing layer on any type of mask, such as phase shift mask or reflective masks. Additionally, polarization filters can be created by forming suitable structures, such as gratings, on a surface of the mask.  
         [0053]    As shown in FIG. 3A, the projection beam PB 3  comprises a first region, namely a first dipole having poles  40  disposed substantially radially and symmetrically about the central optical axis A along axis C; and a second region, namely a second dipole having poles  340  disposed substantially radially and symmetrically about the central optical axis A along axis B. The linear polarization direction  45  of the radiation from the poles  40  is substantially parallel to the elongation direction of the features  310 , and the polarization direction  345  of the radiation from the poles  340  is substantially parallel to the elongation direction of the features  10 . Additionally, the poles  340  are configured to have substantially the same intensity of radiation as the poles  40 .  
         [0054]    [0054]FIG. 9 shows how the required projection beam configuration may be created. An aperture AP comprises a first filtering region  341 , configured and arranged to substantially block the illumination light; a second filtering region  342  comprised of two regions of linear-polarizing filter, configured and arranged to substantially transmit linearly-polarized light with a first polarization direction; and third filtering region  343  comprised of two regions of linear-polarizing filter, configured and arranged to substantially transmit linearly-polarized light with a second polarization direction. When the aperture AP is disposed substantially symmetrical about the optical axis A in a pupil plane of the illumination system, the second filtering region  342  creates the two poles  340  in the pupil plane BC and the third filtering region  343  creates two poles  40  in the pupil plane BC. Other combinations of illumination modes can be achieved by, for instance, adding additional filtering regions, changing the positions of the filtering regions or changing the shape of the filtering regions. Alternatively, the illumination modes can be created using diffractive optical elements (DOE&#39;s), polarization filters at the source or any combination of the methods shown.  
         [0055]    [0055]FIG. 3B shows a plan view along optical axis A, where the relative orientations of the poles  40  and  340 , the directions of polarization  45 ,  345 ,  65  and  365 , and the elongation direction of features  10  and  310  on the mask MA 3  are illustrated. During a single exposure, two groups of features are imaged using two illumination modes simultaneously without interference: the first group of features  10  are imaged using only radiation from the poles  340  of the first dipole, linearly-polarized in the elongation direction of the features  10 ; and the second group of features  310  are imaged using only radiation from the poles  40  of the second dipole, linearly-polarized in the direction elongation direction of the features  310 .  
         [0056]    A second embodiment of the invention, which may be the same as the first embodiment save as described below, is shown in FIG. 4A. According to the invention, a projection beam PB 4 , depicted in cross-section at a pupil plane of the illumination system, illuminates a mask MA 4 . For imaging of a first group of features  10  on the mask MA 4 , it has been determined that illumination using a dipole of linearly-polarized light is most advantageous, which is configured and arranged such that the axis joining its poles is substantially perpendicular to the elongation direction of the features  10 ; and the light being incident upon the group of features  10  is linearly polarized in a direction substantially parallel to the elongation direction of the features  10 . For imaging a second group of features  420  on the mask MA 4 , it has been determined that illumination using a single large pole  430  of randomly polarized light is most advantageous.  
         [0057]    The mask MA 4  comprises the first group of features  10  which have an elongation in a direction substantially parallel to axis C; the second group of features  420  which have a width that is substantially greater than the width of the features in the first group  10 ; a polarization filter  60  configured and arranged such that it only allows light with a polarization direction  65  to be incident upon the first group of features  10 ; and a neutral density filter  470  (gray filter) configured and arranged such that it reduces the amount of light being incident upon the second group of features  420 . The polarization direction  65  is arranged to be substantially parallel to the elongation direction of the features  10 . The neutral density filter  470  may be created and arranged in a similar way to that already indicated for the polarization filter.  
         [0058]    The projection beam PB 4  comprises a single pole  430 , disposed substantially symmetrically about the central optical axis A, supplying light with a polarization direction  435  which is substantially perpendicular to the direction of polarization  65  that the filter  60  transmits; and a dipole having poles  440  supplying randomly polarized radiation. The poles  440  are disposed substantially symmetrical about optical axis A along axis B, and configured to have substantially the same intensity of radiation as the single pole  430 .  
         [0059]    [0059]FIG. 4B shows a plan view along optical axis A, where the relative orientations of the poles  430  and  440 , the directions of polarization  435  and  65 , and the elongation direction of features  10  on the mask are illustrated. During a single exposure, the mask MA 4  is imaged using two illumination modes simultaneously without interference—the first group of features  10  is imaged using only part of the radiation from the dipole  440  that is transmitted through the filter  60 ; and the second group of features  420  is imaged using radiation from both the single pole  430  and the poles  440  of the dipole. Although both illumination modes  430 ,  440  are used to image the features  420 , the radiation intensities are substantially equal and the second group of features  420  is effectively illuminated with a single large pole. The neutral density filter  470  reduces the light intensity being incident on the features  420 , such that the exposure of the features  10  and  420  on the substrate can be performed simultaneously using the same dose.  
         [0060]    For this embodiment, the intensity of the radiation from poles  440  that is incident on the first group of features  10  may be increased by employing radiation in the poles  440  which is preferentially linearly polarized in a direction substantially parallel to the polarization direction  65 , and which is configured to produce the same intensity as the single pole  430 .  
         [0061]    A third embodiment of the invention, which may be the same as the previous embodiments save as described below, is shown in FIG. 5A. According to the invention, a projection beam PBS, depicted in cross-section at a pupil plane of the illumination system, illuminates a mask MA 5  which is disposed substantially perpendicular to optical axis A. For imaging of a first group of features  10  on the mask MA 5 , it has been determined that illumination using a dipole of linearly-polarized light is most advantageous, which is configured and arranged such that the axis joining its poles is substantially perpendicular to the elongation direction of the features  10 ; and the light being incident upon the features  10  is linearly polarized in a direction substantially parallel to the elongation direction of the features  10 . For imaging of a second group of features  520  on the mask MA 5 , it has been determined that illumination using a single annular ring  530  of randomly polarized light is most advantageous.  
         [0062]    The mask MA 5  comprises the first group of features  10  which have an elongation in a direction substantially parallel to axis C; the second group of features  520  which have a width substantially greater than the width of features in the first group  10 ; a polarization filter  60  configured and arranged such that it only allows light with a polarization direction  65  to be incident upon the first group of features  10 ; a neutral density filter  570  arranged such that it reduces the amount of light being incident upon the second group of features  520 . The polarization direction  65  is arranged to be substantially parallel to the elongation direction of the features  10 .  
         [0063]    The projection beam PB 5  comprises an annular ring  530 , disposed substantially symmetrically about the central optical axis A; and a dipole having poles  340  supplying randomly polarized radiation. The polarization direction  535  is substantially perpendicular to the direction of polarization  65  that the filter  60  transmits. The poles  340  are disposed substantially symmetrically about the optical axis A along axis B. Additionally, the poles  340  are configured to have substantially the same intensity of radiation as the annular ring  530 .  
         [0064]    [0064]FIG. 5B shows a plan view along optical axis A, where the relative orientations of the projection beam poles  340  and annular ring  530 , the directions of polarization  535  and  65 , and the elongation of features  10  on the mask are illustrated. During a single exposure, the mask MA 5  is imaged using two illumination modes simultaneously without interference—the first group of features  10  is imaged using only part of the radiation from the poles  340  that is transmitted by the polarization filter  60 ; and the second group of features  520  is imaged using radiation from both the annular ring  530  and the poles  340 . Although both illumination modes  340 ,  530  are used to image the features  520 , the radiation intensities are substantially equal and the second group of features  520  is effectively illuminated with a single annular ring. The neutral density filter  570  reduces the light intensity being incident on the features  520 , such that the exposure of the features  10  and  520  on the substrate can be performed simultaneously using the same dose.  
         [0065]    For this embodiment, the intensity of the radiation from poles  340  being incident on the first group of features  10  may be increased by employing radiation in the poles  340  which is preferentially linearly polarized in a direction substantially parallel to the polarization direction  65 , and which is configured to have the same intensity as the annular ring  530 .  
         [0066]    A fourth embodiment of the invention, which may be the same as the previous embodiments save as described below, is shown in FIG. 6A. According to the invention, a projection beam PB 6 , depicted in cross-section at a pupil plane of the illumination system, illuminates a mask MA 6  which is disposed substantially perpendicular to optical axis A. Axes D and E are mutually perpendicular, and are disposed in the plane BC at an angle of substantially 45-degrees to the axes B and C.  
         [0067]    For imaging of a first group of features  10  on the mask MA 6 , it has been determined that illumination using a dipole of linearly-polarized light is most advantageous, configured and arranged such that the axis joining its poles is substantially perpendicular to the elongation direction of the features  10 ; and when the light being incident upon the features  10  is linearly polarized in a direction substantially parallel to the elongation direction of the features  10 . For imaging of a second group of features  620  on the mask MA 6 , it has been determined that illumination using two dipoles of linearly-polarized light, arranged substantially perpendicular to each other (quadrupole), is most advantageous. The dipoles are configured and arranged such that the axes joining their respective poles are mutually orthogonal, and each axis is substantially at an angle of 45 degrees to the elongation direction of the features  10 . This latter mode is commonly referred to as quadrupole. When the poles of this quadrupole mode are rotated by 45 degrees, the resulting mode is commonly referred to as cross-quadrupole or c-quad.  
         [0068]    The mask MA 6  comprises the first group of features  10  which have an elongation in a direction substantially parallel to axis C; the second group of features  620  which have an elongation in a direction substantially parallel to axis B; a first polarization filter  60  configured and arranged such that it only allows light with a polarization direction  65  to be incident upon the first group of features  10 ; a second polarization filter  660  configured and arranged such that it only allows light with a polarization direction  665  to be incident upon the second group of features  620 ; and a neutral density filter  670  configured and arranged such that it reduces the amount of light being incident upon the second group of features  620 . The features in the second group  620  have a width that is substantially greater than the width of the features in the first group  10 . The polarization direction  65  is arranged to be substantially parallel to the elongation direction of the features  10 , and similarly the polarization direction  665  is arranged to be substantially parallel to the elongation direction of the second group of features  620 . In practice, it may be advantageous to combine the neutral density filter  670  and the second polarization filter  660  into a single filter layer.  
         [0069]    The projection beam PB 6  comprises a quadrupole having poles  640 , disposed substantially symmetrically about the central optical axis A along axes D and E; and a dipole having poles  340 , disposed substantially symmetrical about optical axis A along axis B. The radiation from the poles  640  has a polarization direction  645  that is substantially perpendicular to the direction of polarization  65  that the filter  60  transmits, and that is also substantially parallel to the direction of polarization  665  that the filter  660  transmits. The polarization direction  345  of the radiation from the poles  340  is substantially perpendicular to the polarization direction  345 .  
         [0070]    [0070]FIG. 6B shows a plan view along optical axis A, where the relative orientations of the poles  340  and  640 , the directions of polarization  645 ,  345 ,  65  and  665 , and the elongation direction of features  10  and  620  on the mask are illustrated. During a single exposure, the mask MA 6  is imaged using two illumination modes simultaneously without interference—the first group of features  10  are imaged using only radiation from the poles  340 , polarized in the elongation direction of the first group of features  10 ; and the second group of features  620  are imaged using only radiation from the poles  640 , polarized in the elongation direction of the second group of features  620 . The neutral density filter  670  reduces the light intensity being incident on the features  620 , such that the exposure of the features  10  and  620  on the substrate can be performed simultaneously using the same dose.  
         [0071]    A fifth embodiment of the invention, which may be the same as the previous embodiments save as described below, is shown in FIG. 7A. According to the invention, a projection beam PB 7 , depicted in cross-section at a pupil plane of the illumination system, illuminates a mask MA 7  which is disposed substantially perpendicular to optical axis A.  
         [0072]    For imaging of a first group of features  10  on the mask MA 7 , it has been determined that illumination is most advantageous using a quadrupole, configured and arranged such that the axes joining the respective poles of each dipole are mutually orthogonal, and each axis of each dipole is substantially disposed at 45 degrees to the elongation direction of the features  10 . For imaging of a second group of features  720  on the mask MA 7 , it has been determined that illumination using a single annular ring  730  is most advantageous. For imaging of a third group of features  710  on the mask MA 7 , it has been determined that illumination is most advantageous using a quadrupole, configured and arranged such that the axes joining the respective poles of each dipole are mutually orthogonal, and each axis of each dipole is substantially disposed at 45 degrees to the elongation direction of the features  710 .  
         [0073]    The mask MA 7  comprises the first group of features  10  which have an elongation in a direction substantially parallel to axis C; the second group of features  720  which contains both features with an elongation direction substantially parallel to axis B, and features with an elongation direction substantially parallel to axis C; a third group of features  710  which have an elongation direction substantially parallel to axis B; a first polarization filter  60 , configured and arranged such that it only allows light with a polarization direction  65  to be incident upon the first group of features  10 ; a second polarization filter  760  configured and arranged such that it only allows light with a polarization direction  765  to be incident upon the third group of features  710 ; and a neutral density filter  770  arranged such that it reduces the amount of light being incident upon the second group of features  720 . Both the polarization directions  65  and  765  are arranged such that they are substantially parallel to the axis B.  
         [0074]    The projection beam PB 7  comprises a cross-quadrupole having four poles  740 , disposed substantially symmetrically about the central optical axis A along axes D and E; and an annular ring  730 , disposed substantially symmetrically about the central optical axis A. The polarization direction  735  is substantially perpendicular to both the directions of polarization  65  and  765  that the filters  60  and  760  respectively transmit. The polarization direction  745  is substantially parallel to both directions of polarization  65  and  765  that the filters  60  and  760  respectively transmit. Additionally, the poles  740  are configured to have substantially the same intensity of radiation as the annular ring  730 .  
         [0075]    [0075]FIG. 7B shows a plan view along optical axis A, where the relative orientations of the poles  740  and ring  730 , the directions of polarization  745 ,  735 ,  65  and  765  are illustrated. During a single exposure, the mask MA 7  is imaged using two illumination modes simultaneously without interference—the first group of features  10  and the third group of features  710  are imaged using only radiation from the poles  740 ; and the second group of features  720  are imaged radiation from both the poles  740  and the annular ring  730 . Although both illumination modes  740 ,  730  are used to image the features  720 , the radiation intensities are substantially equal and the second group of features  720  is effectively illuminated with a single annular ring. The neutral density filter  770  reduces the light intensity being incident on the features  720 , such that the exposure of the features  10 ,  710  and  720  on the substrate can be performed simultaneously using the same dose.  
         [0076]    Although the embodiments above describe the use of linearly polarized radiation only, the skilled artisan will appreciate that other types of polarization, such as circular and elliptical polarization, may be utilized in isolation or in combination to create a similar effect. Additionally, the embodiments describe the situation where polarization and neutral density layers are applied to groups of features. It may, however, be advantageous to apply these layers to intersecting features, or even apply them to parts of features such as the ends.  
         [0077]    Applying a neutral density layer to a mask may also be employed to balance differences in doses due to the relative sizes of features, their relative proximity, or their density compared to other regions of the mask. For example, the light transmitted by relatively dense features may be reduced using a neutral density layer to balance the light transmitted by a relatively isolated feature.  
         [0078]    Although the embodiments describe the use of the invention in an apparatus utilizing transmissive optics, it will be obvious to the skilled artisan that the same basic principles can be also employed in an apparatus utilizing reflective optics.  
         [0079]    While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The description is not intended to limit the invention.