Abstract:
The invention relates to a lithographic apparatus arranged to transfer a pattern from a patterning device onto a substrate, wherein said apparatus is operable to measure higher-order distortions and/or image plane deviations of the patterning device, said apparatus comprising: a device for transmission image detection; and a processor configured and arranged to model higher-order distortions of the patterning device using signals received from the device for transmission image detection; wherein said patterning device has a main imaging field, and a perimeter and said apparatus is operable to model said higher-order distortions using signals resultant from alignment structures comprised in said perimeter and/or in the imaging field.

Description:
[0001]    This application claims priority of U.S. provisional application No. 61/235,562, the content of which is herewith incorporated by reference in its entirety. 
     
    
     BACKGROUND  
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a lithographic apparatus, a patterning device for use with such a lithographic apparatus, and a method for determining higher-order distortions of a patterning device of a lithographic apparatus. 
         [0004]    2. Description of the Related Art 
         [0005]    A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. 
         [0006]    The substrate (e.g. a wafer) is usually supported by a wafer stage. By moving the wafer stage, the substrate can be positioned relative to the reticle. In present lithographic apparatus, a device for transmission image detection is used in order to align a reticle to a wafer stage. Said device comprises a structure (e.g. a grating) on a reticle and a complementary structure on a transmission image detector plate of a transmission image detector. By transmitting a radiation beam through the structure on the reticle and detecting an image of said structure by the transmission image detector on the wafer stage the position and focus of the image can be determined. In practice, the transmission image detector comprises several structures. Beneath each structure a photodiode is located to detect the transmitted radiation beam. The transmission image detector is conventionally used to measure first-order positioning terms like translation, magnification and rotation of the reticle with respect to the wafer stage, so that the transmission image detector is able to measure first-order reticle distortions. 
         [0007]    However, higher-order reticle distortions remain unresolved and any reticle distortion, including these higher-order reticle distortions, may lead to image distortions of the pattern on the substrate resulting in overlay errors and focus errors. 
         [0008]    Overlay is an important factor in the yield, i.e. the percentage of correctly manufactured devices. Overlay is the accuracy within which layers are printed in relation to layers that have previously been formed. The overlay error budget will often be 10 nm or less, and to achieve such accuracy, the substrate must be aligned to the pattern on the reticle with great accuracy. 
         [0009]    Reticle distortions may have many different causes. To name a few, distortions may arise as a consequence of: 
         [0010]    reticle writing errors; 
         [0011]    reticle heating; 
         [0012]    holding the reticle in a reticle support (clamping effects and gravitational sag); 
         [0013]    reticle fabrication; 
         [0014]    assembling the reticle with other devices, such as a so-called pellicle. 
         [0015]    A pellicle is a device that prevents dust particles from landing on the reticle as it is easier and less risky to replace a “dirty” pellicle than to clean a “dirty” reticle, and keeps dust particles at an out-of-focus distance from the reticle, so that their influence on the pattern to be imaged on a wafer is minimal. A pellicle usually comprises a pellicle membrane which is transparent to the radiation beam, a pellicle frame, and an adhesive to attach the pellicle to the reticle. The pellicle is attached to the reticle such that it covers the pattern on the reticle and shields that part of the reticle from the rest of the lithographic apparatus. 
         [0016]    Because of the rigid nature of the pellicle frame, the reticle will experience mechanical distortions upon placement of the pellicle, resulting in higher-order image distortion. Furthermore, when the reticle is subsequently placed, i.e. chucked, on a reticle support to hold the reticle, this distortion becomes worse by gravitational sag and chucking stress. 
         [0017]    During the fabrication of a reticle several distortions may be introduced, which will be elucidated here. When a reticle blank or substrate is fabricated, it is not perfectly flat, but it will have a concave shape, a convex shape, or an even more complex shape. This non-flat shape will introduce distortions to the image. The reticle blank is further coated with one or more (absorber) layers and a resist. The stress in these layers may have an impact on the shape of the reticle. Subsequently, an image of the pattern which has to be transferred to the reticle is transferred to the resist on the reticle by a reticle writing tool. This can be done by means of laser radiation or charged particle (e.g. electron) radiation. Irradiation of the reticle can result in local heating effects and distort the reticle. The use of the charged particle radiation (e.g. an e-beam tool) can result in charging effects and distort the image during writing. Subsequently, the resist is processed and this processing can introduce further distortions of the image in the resist. The image in the resist is then transferred into the underlying layer (and if required into the substrate). The underlying (absorber) layer or layers (and if required the substrate) are locally removed or etched. This can introduce distortion of the image in the underlying layer or layer(and if required the substrate). Removal of material in the layer or layers (and if required the substrate) can result in relaxation of stress that was build up earlier by the deposition of the (absorber) layer(s) and the resist. As a consequence, the reticle can experience distortion of the image. 
         [0018]    When irradiating a reticle, some areas will absorb the radiation, and other parts will transmit the radiation, thereby forming a patterned radiation beam. However, absorbing the radiation will result in a local increase in temperature, thereby introducing distortions to the image field. 
         [0019]    The pellicle induced distortions, reticle fabrication induced distortions and the chucking induced distortions have a static, or quasi-static nature, i.e. there magnitude is relatively constant over time. As a pellicle may be replaced by another pellicle, the pellicle induced distortion may change abruptly due to this replacement. 
         [0020]    The distortions induced by heating have a dynamic nature and are dependent amongst others on the pattern on the reticle. The more radiation that is absorbed by the pattern, the more the reticle will deform due to heating. 
         [0021]    It has been shown that especially for high-throughput lithographic apparatus, the higher-order distortions have a large contribution in the overlay budget. 
       SUMMARY 
       [0022]    It is desirable to provide a lithographic apparatus with reduced overlay errors resulting from higher-order distortions of the patterning device. 
         [0023]    According to an aspect of the invention, there is provided a lithographic apparatus arranged to transfer a pattern from a patterning device onto a substrate, wherein said apparatus is operable to measure higher-order distortions and/or image plane deviations of the patterning device, said apparatus comprising:
   a device for transmission image detection; and   a processor configured and arranged to model higher-order distortions of the patterning device using signals received from the device for transmission image detection;
 
wherein said patterning device has a main imaging field, and a perimeter and said apparatus is operable to model said higher-order distortions using signals resultant from alignment structures comprised in said perimeter and/or in the imaging field.
   
 
         [0026]    According to an embodiment of the invention, there is provided a lithographic apparatus arranged to transfer a pattern from a patterning device onto a substrate, wherein said apparatus is operable to measure higher-order distortions and/or image plane deviations of the patterning device, said apparatus comprising:
   a device for transmission image detection; and   a processor configured and arranged to model higher-order distortions of the patterning device using signals received from the device for transmission image detection;
 
wherein said patterning device has a main imaging field, and a perimeter and said apparatus is operable to model said higher-order distortions using signals resultant from alignment structures comprised in at least three sides of said perimeter and/or in the imaging field.
   
 
         [0029]    According to another aspect of the invention, there is provided a method of determining higher-order distortions of a patterning device of a lithographic apparatus, said method comprising:
   imparting a radiation beam with a pattern in its cross-section to form a patterned radiation beam using a patterning device, said patterning device comprising a main imaging field, a perimeter and a plurality of alignment structures;   detecting transmission of radiation transmitted through said alignment structures of said patterning device and into a device for transmission image detection,   producing measurement signals from the detected radiation, and   determining higher-order distortions and/or image plane deviations of said patterning device using measurement signals resultant from radiation transmitted through alignment structures comprised in said perimeter and/or in said imaging field.   
 
         [0034]    According to an embodiment of the invention there is provided a method of determining higher-order distortions of a patterning device of a lithographic apparatus, said method comprising: creating a radiation beam; imparting the radiation beam with a pattern in its cross-section to form a patterned radiation beam using a patterning device, said patterning device comprising a main imaging field, a perimeter and a plurality of alignment structures; detecting transmission of radiation transmitted through said alignment structures of said patterning device and into a device for transmission image detection, producing measurement signals from the detected radiation, and determining higher-order distortions and/or image plane deviations of said patterning device using measurements signals resultant from radiation transmitted through alignment structures comprised in at least three sides of said perimeter and/or image field. 
         [0035]    According to a further aspect of the invention, there is provided a patterning device for use in a lithographic apparatus, said patterning device having a main imaging field, and a perimeter wherein said patterning device is provided with additional alignment structures for the improved measurement of distortion and/or field plane deviations of said patterning device. 
         [0036]    According to yet another aspect of the invention, there is provided an alignment apparatus comprising a patterning device for use in a lithographic apparatus, said patterning device having a main imaging field, and a perimeter wherein said patterning device is provided with additional alignment structures for the improved measurement of distortion and/or field plane deviations of said patterning device. 
         [0037]    Other features of the invention are as described in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]    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: 
           [0039]      FIG. 1  depicts a lithographic apparatus according to an embodiment of the invention; 
           [0040]      FIG. 2  depicts schematically a transmission image detector according to the state of the art; 
           [0041]      FIG. 3  shows an example of a transmission image detector according to the state of the art; 
           [0042]      FIG. 4  shows an example of a reticle that may be used with the transmission image detector of  FIGS. 2 and 3 ; 
           [0043]      FIG. 5  shows an example of a first enhanced reticle that may be used with the transmission image detector of  FIGS. 2 and 3 ; 
           [0044]      FIG. 6  shows an example of a second enhanced reticle that may be used with the transmission image detector of  FIGS. 2 and 3 ; 
           [0045]      FIG. 7  shows an example of a third enhanced reticle that may be used with the transmission image detector of  FIGS. 2 and 3 ; and 
           [0046]      FIG. 8  shows an example of a fourth enhanced reticle that may be used with the transmission image detector of  FIGS. 2 and 3 . 
       
    
    
     DETAILED DESCRIPTION 
       [0047]      FIG. 1  schematically depicts a lithographic apparatus according to one embodiment of the invention. The apparatus includes an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or any other suitable radiation), a mask support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioning device PM configured to accurately position the patterning device in accordance with certain parameters. The apparatus also includes a substrate table (e.g. a wafer table) WT or “substrate support” constructed to hold a substrate (e.g. a resist-coated wafer) W and connected to a second positioning device PW configured to accurately position the substrate in accordance with certain parameters. The apparatus further includes a projection system (e.g. a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g. including one or more dies) of the substrate W. 
         [0048]    The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation. 
         [0049]    The mask support structure supports, i.e. bears the weight of, the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The mask support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The mask support structure may be a frame or a table, for example, which may be fixed or movable as required. The mask support structure may ensure that the patterning device is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein may be considered synonymous with the more general term “patterning device.” 
         [0050]    The term “patterning device” used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section so as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example if the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit. 
         [0051]    The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix. 
         [0052]    The term “projection system” used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system”. 
         [0053]    As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above, or employing a reflective mask). 
         [0054]    The lithographic apparatus may be of a type having two (dual stage) or more substrate tables or “substrate supports” (and/or two or more mask tables or “mask supports”). In such “multiple stage” machines the additional tables or supports may be used in parallel, or preparatory steps may be carried out on one or more tables or supports while one or more other tables or supports are being used for exposure. 
         [0055]    The lithographic apparatus may also be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques can be used to increase the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that a liquid is located between the projection system and the substrate during exposure. 
         [0056]    Referring to  FIG. 1 , the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD including, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system. 
         [0057]    The illuminator IL may include an adjuster AD configured to adjust the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. In addition, the illuminator IL may include various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section. 
         [0058]    The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the mask support structure (e.g., mask table MT), and is patterned by the patterning device. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioning device PW and position sensor IF (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioning device PM and another position sensor (which is not explicitly depicted in  FIG. 1 ) can be used to accurately position the mask MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the mask table MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioning device PM. Similarly, movement of the substrate table WT or “substrate support” may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short-stroke actuator only, 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 . Although the substrate alignment marks as illustrated occupy dedicated target portions, they may be located in spaces between target portions (these are known as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks may be located between the dies. 
         [0059]    In an ideal situation, the mask MA would be perfectly flat without showing any distortions to ensure that the pattern comprised in the mask MA is transferred correctly to the target portion on the substrate. However, in practice, the mask MA will show distortions which may result from: 
         [0060]    mask writing errors; 
         [0061]    mask heating due to absorption of radiation; 
         [0062]    the mask support MT holding the mask; 
         [0063]    mask fabrication; and 
         [0064]    assembling the mask MA with other devices, such as pellicles. 
         [0065]    The distortions of the mask MA result in a distorted pattern transfer from mask to the substrate, i.e. overlay errors and focus errors. The distortions of the mask can be measured using the alignment marks M 1 , M 2  and additional alignment marks M 3 -M 10 . In  FIG. 1 , the alignment marks M 1 -M 10  are provided in two columns along the Y-direction, thereby allowing the determination of higher-order (≧2) distortions of the mask MA in the Y direction and first-order distortions in the X direction using a device for transmission image detection (not shown). The information obtained about the distortions can subsequently be used to compensate for the distortions and improve the overlay and focus of the apparatus. 
         [0066]    The depicted apparatus could be used in at least one of the following modes:
   1. In step mode, the mask table MT or “mask support” and the substrate table WT or “substrate support” are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT or “substrate support” is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.   2. In scan mode, the mask table MT or “mask support” and the substrate table WT or “substrate support” are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT or “substrate support” relative to the mask table MT or “mask support” may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.   3. In another mode, the mask table MT or “mask support” is kept essentially stationary holding a programmable patterning device, and the substrate table WT or “substrate support” is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or “substrate support” or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.   
 
         [0070]    Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed. 
         [0071]      FIG. 2  depicts schematically a device for transmission image detection, also referred to as a transmission image detector or transmission image sensor. A transmission image detector per se is known from the prior art. The radiation beam B is incident on a first object G 0  for example a grating in the mask MA. The first grating G 0  comprises a plurality of openings arranged for creating an image from the radiation beam B. The openings in the first grating G 0  each emit a radiation beam RB originating from the radiation beam B. The radiation beams RB emitted by the plurality of openings in G 0 , pass through a lens for example, the projection lens system PS. The optical properties of such projection lens system are such that an image of G 0 , G 0 ′, is formed at a given plane below the projection lens system PS. The transmission image detector TD is positioned below the projection lens system. The transmission image detector TD comprises a slot pattern G 1  and a photo sensor PH device. The slot pattern G 1  is an opening over the photo sensor PH device which has the shape of a slit or a square. Advantageously, applying a pattern on the opening over the photo sensor PH device increases the number of edges which may increase the signal level and thus the signal/noise ratio of the photo sensor PH. 
         [0072]    The transmission detector TD is arranged on the substrate table WT, see  FIG. 1 . The transmission image detector TD allows accurate positioning of the wafer relative to the position of the projection lens system PS and the mask MA in three orthogonal directions X, Y, Z. By scanning along these three directions the intensity of the image G 0 ′ can be mapped as a function of the XYZ position of the transmission image detector, for example in an image map (a 3D map), which comprises the coordinates of sampling locations and the intensity sampled at each location. From the 3D map, a computer or processor connected to the transmission image detector TD can derive the position of the image by using for example a parabolic fit of the top position using a least squares fitting method. 
         [0073]    One issue with the above apparatus, in use, which should be taken into account, is that higher-order distortions of the mask MA cannot be measured with only two measurement positions in the field. 
         [0074]      FIG. 3  shows an example of a transmission image detector  29  according to the state of the art with 4 gratings  30 ,  31 ,  32 ,  33  in a litho layer  34 . The litho layer  23  is manufactured on a quartz window  35 . Beneath each of the gratings  30 ,  31 ,  32 ,  33  an associated photodiode  36 ,  37 ,  38 ,  39  is provided. Using only  4  measurement positions in the corners of the image field as is standard, means that the transmission image detector  29  can only measure zero- and fist-order terms like translation, magnification, rotation, focus and focus tilt of the reticle with respect to the wafer stage and thus only first-order reticle distortions. Higher-order distortions, defined as some or all distortions of some or all polynomial orders above 1 (quadratic, cubic, etc.), generally remain undetected because these generally require more independent measurement positions in or around the image field. Such higher-order distortions may arise as a consequence of reticle heating and/or lens heating which is a dynamic distortion and/or (quasi-)static distortions such as mechanical induced distortions and may increase the overlay errors. 
         [0075]    Instead of measuring just a small number of measurement positions in the corners of the image field the transmission image detector may be used to measure the actual reticle deformation locally to reduce the overlay errors. Dependent on the required accuracy of the measured overlay errors, the number of measurement positions to measure the actual reticle deformation locally, with a transmission image detector, may be adjusted. The locally measured values can be put in a matrix and by using a feed-forward model, the deformation behavior of a reticle, including higher-order distortions, can be predicted so that smaller overlay errors can be obtained. This feed-forward model is an extension to the known ‘basis’ Reticle Align (RA) model with additional XY inputs. By using more measured independent inputs, this model is less sensitive to model assumptions than the known ‘basis’ Reticle Align (RA) model. 
         [0076]    The model may comprise a third order model with dX and dY terms as a function of X and Y, which means that 20 parameters are needed for a full decomposition, which in turn requires 20 independent measurements to avoid an undetermined system. In practice, some parameters (or terms) can be skipped or modeled from other parameters, reducing the required input measurements. However, it is still preferable to have dX, dY measurements both along the top/bottom (function of X) and left/right (function of Y) sides. 
         [0077]    In practice a third-order model is most practicable due to the correction potential of the machine (and the number of available independent input measurements, and noise propagation considerations), but this is not fundamental and in some cases (e.g. when the load is highly asymmetric) or in the future, it may indeed be worthwhile to include some fourth-order terms or higher. 
         [0078]    When the model is extended with additional XY inputs, the additional gratings to be measured at reticle level may be surrounded by very small chrome borders (e.g. as in scribe lanes) or even no chrome borders at all. In these cases spurious effects due to the limited or missing chrome borders for the intra-field gratings may occur and these effects should be suppressed. In these cases it can be advantageous to only use standard RA gratings for the actual first reticle align, while measuring and storing/calibrating the intra-field gratings for subsequent delta measurements/corrections. In such a scenario, a lower order correction model, using only the standard RA gratings, would be used for the first reticle align, while for subsequent wafers a relative higher-order correction model can be applied, using the standard RA gratings measurements in an absolute manner plus the additional XY grating measurements relative to the first reticle align. 
         [0079]      FIG. 4  shows a reticle that may be used in conjunction with a transmission image detector in an apparatus such as shown in  FIG. 1 . The standard reticle has an image field  400  and a perimeter  410 . The perimeter  410  is provided with four sets of x and y gratings  420 , one in each corner, with further x and y gratings  420  along the top and bottom and a number of single direction gratings  430  along the sides. Conventionally the corner gratings are used for basic alignment, while the other gratings are used in reticle shape correction for determining image plane deviation. 
         [0080]    According to a first embodiment of the invention, the reticle of  FIG. 4  can be used in this standard form with the apparatus depicted in  FIGS. 1 and 2 , and using the extended feed forward model described above, for the calculation of local reticle deformation. In particular, the use of gratings on all four sides (as opposed to just the top and bottom) of the reticle means that higher-order distortions, particularly in terms of y, can be calculated. 
         [0081]      FIG. 5  shows an enhanced reticle. One drawback with the reticle of  FIG. 4  is that the gratings found along the left and right hand sides are “y-gratings” which are designed to perform measurements along the y-axis. While this is useful, measurement along the x-axis would be better in case of a distortion that tends to distort more in the middle of the sides as opposed to the corners (barrel distortion), such as heating of the mask MA. Consequently, another embodiment uses a reticle, as shown in  FIG. 5 , with additional “x-gratings”  500  along each side, as well as along the top and bottom. This allows for a more extensive modeling of higher-order terms in x. 
         [0082]      FIG. 6  shows a further enhanced reticle having a matrix of gratings  600  which is provided in the image field itself. So as not to interfere with the image, it is proposed that the best place to locate these image field gratings is in the scribe lanes  610 , as shown. This enhanced reticle allows for the production of a denser matrix to allow for more accurate local corrections. When using this reticle, not only a more detailed third-order model can be achieved, but also higher-order modeling of dx and dy, as a function of both x and y, with a polynomial order depending on the overlay issue and/or correction potential, including x-y cross terms, is achievable. 
         [0083]      FIG. 7  shows an enhanced reticle with a matrix of gratings  600  in the image field scribe lanes  610 , and further gratings  700  in the image field  410  outside of the scribe lanes. This is possible particularly where the image is known, in which case the gratings can be placed anywhere in the image field that does not interfere with the image itself. 
         [0084]      FIG. 8  shows an enhanced reticle with a matrix of grating  800  outside of the scribe lanes allowing third-order corrections in X-direction and Y-direction. 
         [0085]    The scan should preferably be done during regular transmission image detector alignment to minimize the impact on throughput. In each case, the transmission image detector, as already described, comprises four gratings arranged on a litho layer (it is within the scope of the invention to use an improved transmission image detector, with an increased number of gratings, to increase throughput). The gratings are positioned so as to receive an image produced by the standard RA gratings on the reticle placed on the mask table MT of the lithographic apparatus, see  FIG. 1  (alignment marks M 1  to M 10 ). The transmission image detector further comprises radiation sensitive sensors arranged to receive radiation coming through one of the gratings and to produce a measurement signal. The litho layer may for example be a chrome layer patterned with a plurality of gratings arranged in a row. The measurement signals are input to a processing device which is arranged to determine higher-order distortions of the projection system (i.e. the lens) and/or of the patterning device. These distortions can be used to adjust the components of the lithographic apparatus, but they can also be used to improve alignment of the substrate table relative to the patterning device. 
         [0086]    While the above techniques have described the use of alignment gratings, the skilled person will appreciate that any alignment marks, patterns or structures may be used without departing from the scope of the invention. Furthermore the number and arrangement of the alignment marks and scribe lanes and the general arrangement of the reticles may differ significantly from the purely illustrative examples shown. Also by adding more chrome around the gratings to reduce stray light, accuracy can be improved. 
         [0087]    It should be noted that any type of image sensor or image detector may be used, and is not necessarily be limited to the transmission image detector as described. For example a wavefront detector or interferometer may be used. Also, improvements can be made to the transmission image detector such as improving the fitting algorithms to better deal with a varying signal environment. 
         [0088]    Note that the above description considers only the overlay terms. However, as the transmission image detector also yields focus results every scan, the disclosed concept can also be applied as an extension of reticle shape correction. This extension can be in the spatial domain by adding additional focus measurement points, or in the temporal domain by tracking changes in reticle shape correction over time (e.g. due to reticle heating). 
         [0089]    Furthermore the above techniques can also be used to measure image plane deviations caused by the inadequate flatness of the wafer table. 
         [0090]    It should be noted that the above techniques do not have to be performed for every wafer, as this would result in a throughput penalty that might be too great to be practical. Instead these measurements can be performed only once for a predetermined number of wafers, the number depending on the error range of the calculating/modeling software, which increases between recalibration. Alternatively, these measurements could be done off-line. In each case the results can be stored for further usage. 
         [0091]    To summarize the invention, information of the distortions of a reticle can be obtained by measuring the relative position of alignment structures on the reticle, preferably using a device for transmission image detection. A processor can then be used to model the distortions based on the measurement signals of the alignment structures. Depending on the amount of alignment structures and the desired order of modeling the distortions, a full order model can be obtained or a partial order model can be obtained. As an example, for a third order model in both x and y, 20 independent measurement results have to be available from the alignment structures. 
         [0092]    The model can be adjusted to match the compensation potential of the lithographic apparatus. In this way, the amount of alignment structures is optimized for the model that can be used to compensate for the distortions. In principle, the less amount of alignment structures, the less chance of throughput penalty. 
         [0093]    The model can also be obtained by using less alignment marks than necessary for directly deriving the model from the alignment marks and combining the measurement results from the alignment marks with a physical model of the distortion or in case of a (quasi-)static distortion results from an external measurement. 
         [0094]    The reticle can also be measured once externally to obtain the required model, and subsequently, the changes in the model can be derived from a limited set of alignment marks. This technique can be used for both dynamic and static distortions. 
         [0095]    Another way of reducing throughput penalty is to measure the alignment marks parallel. This can be achieved by properly placing the alignment marks so that full use can be made of the device for transmission image detection and/or by increasing the number of devices for transmission detection. 
         [0096]    Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “wafer” or “die” herein may be considered as synonymous with the more general terms “substrate” or “target portion”, respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers. 
         [0097]    Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device may be pressed into a layer of resist supplied to the substrate whereupon the resist is cured by applying electromagnetic radiation, heat, pressure or a combination thereof. The patterning device is moved out of the resist leaving a pattern in it after the resist is cured. 
         [0098]    The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. having a wavelength of or about 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams. 
         [0099]    The term “lens”, where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components. 
         [0100]    While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein. 
         [0101]    The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.