Patent Publication Number: US-2006012779-A1

Title: Lithographic apparatus and device manufacturing method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation-in-part of U.S. Ser. No. 10/889,211, filed Jul. 13, 2004, which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND  
      1. Field of the Invention  
      The present invention relates to a lithographic apparatus and a device manufacturing method.  
      2. Related Art  
      A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern. For example, the patterning means can be, but is not limited to, a reflective or transmissive contrast device, such as a spatial light modulator, a digital mirror device, a grating light valve, a liquid crystal display, or the like.  
      In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing a pattern onto the target portion in each exposure period. Other known lithographic apparatus include scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.  
      A metrology target generally refers to a type of target that may form part of the pattern written to the substrate, but which does not actually contribute directly to the functional or structural form of the device being manufactured. Usually, the function of a metrology target is to facilitate aspects of the manufacturing process itself, such as alignment of a substrate to the projection system, verification of overlay and/or imaging properties, etc. Metrology targets may therefore include alignment marks and targets used in “offline” metrology equipment associated with or within the lithography apparatus. Offline generally refers to metrology equipment designed to process a substrate separately from, and at a different time to, the main lithography processes used to pattern the substrate, while inline metrology refers to processes carried out at the same time and/or position. For the purposes of this description, protective structures for the above alignment marks and targets are themselves to be understood as types of metrology target.  
      In one example using mask-based systems, the metrology targets normally have to be defined before the mask is actually made. If it turns out that in manufacturing conditions the metrology target design is non-optimal, e.g., for overlay performance, a new mask or set of masks has to be produced before an improved metrology target design can be implemented. This hampers the speed at which the potential of new metrology target designs can be evaluated and leads to increased costs for the customer.  
      In another example, using either mask-based or maskless systems, variation between substrates within a batch to be exposed can mean that metrology information derived inline from one substrate can not accurately represent the characteristics of a following substrate. In such a situation, the exposure settings set for the second substrate, based on inspection of the first substrate, can not be optimal. This can be solved by re-working each substrate after metrology target inspection so that it can be printed a second time with the optimal exposure settings. However, this re-working process reduces the efficiency of the apparatus and requires complex substrate handling apparatus.  
      Therefore, what is needed is a system and method that can optimize performance of metrology targets in lithographic devices. Additionally or alternatively, what is needed is a system and a method that can add flexibility in the choice of metrology target even after the product design has been finalized. Additionally or alternatively, what is needed is an efficient system for providing exposure settings when substrate properties vary within a batch.  
     SUMMARY  
      An embodiment of the present invention provides a device manufacturing method comprising the following steps. A first exposure comprising exposing a substrate to a first pattern for forming one or more metrology targets. Inspecting a latent image of the one or more metrology targets formed on the substrate and deriving therefrom an improved set of exposure settings. A second exposure comprising exposing the substrate to a second pattern for forming one or more product device features. The second exposure is carried out using the improved set of exposure settings.  
      Further embodiments, features, and advantages of the present inventions, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES  
      The accompanying drawings, which are incorporated herein and form apart of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention.  
       FIG. 1  depicts a lithographic apparatus, according to one embodiment of the invention.  
       FIG. 2   a  depicts a lithographic apparatus comprising a first exemplary arrangement of a product patterning device and a metrology target patterning according to one embodiment of the invention.  
       FIG. 2   b  depicts a lithographic apparatus comprising a second exemplary arrangement of a product patterning device and a metrology target patterning, according to one embodiment of the invention.  
       FIG. 3  depicts an alternative configuration of a lithographic, according to one embodiment of the invention, where the metrology target patterning device comprises an array of individually controllable elements.  
       FIG. 4  depicts a metrology target optimizing feedback loop, according to one embodiment of the invention.  
       FIG. 5  depicts an arrangement of metrology targets of different types on different target portions of a substrate, according to one embodiment of the invention.  
       FIG. 6  depicts an example metrology target design comprising a primary structure and a substructure, according to one embodiment of the invention.  
       FIG. 7  depicts protective structures for metrology targets positioned in the scribe lane, according to one embodiment of the invention.  
       FIG. 8  depicts protective structures for metrology targets positioned in the region between the dies and the edge of the substrate, according to one embodiment of the invention.  
       FIG. 9  depicts positioning of metrology targets to minimize cross-talk with product features, according to one embodiment of the invention.  
       FIG. 10  depicts a lithographic apparatus, according to one embodiment of the invention, comprising a control system for an array of individually controllable elements and a metrology target verification and adaptation device.  
       FIGS. 11   a  and  11   b  depict a die and collection of dies with metrology target patterns only, according to one embodiment of the present invention.  
       FIGS. 12   a  and  12   b  depict the die and collection of dies of  FIGS. 11   a  and  11   b  with product patterns and metrology target patterns after a second exposure with improved exposure settings, according to one embodiment of the present invention.  
       FIG. 13  depicts a lithography apparatus configured to print metrology targets only, derive improved exposure settings from inspection of a latent image of metrology targets, and then print product patterns with the new exposure settings, according to one embodiment of the present invention.  
      The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers can indicate identical or functionally similar elements. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
     Overview and Terminology  
      Although specific reference can be made in this text to the use of lithographic apparatus in the manufacture of integrated circuits (ICs), it should be understood that the lithographic apparatus described herein can have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, 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 can be considered as synonymous with the more general terms “substrate” or “target portion,” respectively. The substrate referred to herein can be processed, before or after exposure, in for example a track (e.g., a tool that typically applies a layer of resist to a substrate and develops the exposed resist) or a metrology or inspection tool. Where applicable, the disclosure herein can be applied to such and other substrate processing tools. Further, the substrate can be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein can also refer to a substrate that already contains multiple processed layers.  
      The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of 365, 355, 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.  
      The term “projection system” used herein should be broadly interpreted as encompassing various types of projection systems, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate, for example, for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” herein can be considered as synonymous with the more general term “projection system.” 
      The term “patterning means” used herein should be broadly interpreted as referring to means that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam can not exactly correspond to the desired pattern in the target portion of the substrate. 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.  
      Patterning means can be transmissive or reflective. Examples of patterning means 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; in this manner, the reflected beam is patterned. In each example of patterning means, the support structure can be a frame or table, for example, which can be fixed or movable as required and which can ensure that the patterning means is at a desired position, for example with respect to the projection system. Any use of the terms “reticle” or “mask” herein can be considered synonymous with the more general term “patterning means”.  
      The illumination system can also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components can also be referred to below, collectively or singularly, as a “lens.” 
      The lithographic apparatus can be of a type having two (e.g., dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables can be used in parallel, or preparatory steps can be carried out on one or more tables while one or more other tables are being used for exposure.  
      The lithographic apparatus can also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index (e.g., water), so as to fill a space between the final element of the projection system and the substrate. Immersion liquids can also be applied to other spaces in the lithographic apparatus, for example, between the substrate and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems.  
      Further, the apparatus can be provided with a fluid processing cell to allow interactions between a fluid and irradiated parts of the substrate (e.g., to selectively attach chemicals to the substrate or to selectively modify the surface structure of the substrate).  
     Exemplary Lithography System  
       FIG. 1  schematically depicts a lithographic apparatus  100 , according to one particular embodiment of the invention. Lithographic apparatus comprises a radiation source  102 , an illumination system  104 , a first support structure  106 , a substrate table  108 , and a projection system  110 .  
      Illumination system  104  (e.g., an illuminator) provides a radiation beam  112  comprising, for example, ultra violet (UV) or extreme ultra violet (EUV) radiation. Illuminator  104  receives a radiation beam from radiation source  102 .  
      First support structure  106  (e.g. a mask table) supports a patterning means  114  (e.g. a mask) and is connected to a first positioning means  116  for accurately positioning patterning means  114  with respect to projection system  110 .  
      Substrate table  108  (e.g. a wafer table) holds a substrate  118  (e.g. a resist-coated wafer) and is connected to a second positioning means  120  that accurately positions substrate  118  with respect projection system  110 .  
      Projection system  110  (e.g. a reflective projection lens) images a pattern imparted to radiation beam  112  via patterning means  114  onto a target portion  122  (C) (e.g. one or more dies) of substrate  118 .  
      In this embodiment, lithographic apparatus  100  is of a reflective type (e.g., employing a reflective mask or a programmable mirror array of a type as referred to above). Alternatively, lithographic apparatus  100  can be of a transmissive type (e.g., employing a transmissive mask).  
      In one embodiment, source  102  and lithographic apparatus  100  can be separate entities. For example, when source  102  is a plasma discharge source. In such cases, source  102  is not considered to form part of lithographic apparatus  100 , and radiation beam  112  is generally passed from source  102  to illuminator  104  with the aid of a radiation collector (not shown). The radiation collector can comprise, for example, but not limited to, suitable collecting mirrors and/or a spectral purity filter.  
      In other cases source  102  can be integral part of apparatus  100 . For example, when source  102  is a mercury lamp.  
      In one example, source  102  and illuminator  104  can be referred to as a radiation system.  
      Illuminator  104  can comprise adjusting means (not shown) that adjust an angular intensity distribution of beam  112 . 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 illuminator  102  can be adjusted. Illuminator  102  provides a conditioned beam of radiation, referred to as radiation beam  112 , having a desired uniformity and intensity distribution in its cross-section.  
      Radiation beam  112  is incident on mask  114 , which is held on mask table  106 . Being reflected by mask  114 , radiation beam  112  passes through projection system  110 , which focuses the beam onto target portion C of substrate  118 . With the aid of second positioning means  120  and a position sensor  124  (e.g. an interferometric device), substrate table  108  can be moved accurately, e.g. so as to position different target portions C in the path of beam  112 . Similarly, first positioning means  116  and a position sensor  126  can be used to accurately position mask  114  with respect to the path of beam  112 , e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of object tables  106  and  108  will be realized with the aid of a long-stroke module (coarse positioning) (not shown) and a short-stroke module (fine positioning) (not shown), which form part of positioning means  116  and  120 . However, in the case of a stepper (as opposed to a scanner) mask table  116  can be connected to a short stroke actuator only, or can be fixed. Mask  114  and substrate  118  can be aligned using mask alignment marks M 1 , M 2  and substrate alignment marks P 1 , P 2 , respectively.  
      In various example, apparatus  100  can be used step, scan, or other modes, examples of which are described below, but are not to be seen as an exhaustive list.  
      In step mode, mask table  106  and substrate table  108  are kept essentially stationary, while an entire pattern imparted to radiation beam  112  is projected onto a target portion C in one go (i.e., a single static exposure). Substrate table  108  is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, a maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.  
      In scan mode, mask table  106  and substrate table  108  are scanned synchronously, while a pattern imparted to radiation beam  112  is projected onto a target portion C (i.e., a single dynamic exposure). A velocity and direction of substrate table  108  relative to mask table  106  is determined by (de-)magnification and image reversal characteristics of projection system  110 . In scan mode, a maximum size of an exposure field limits a width (in the non-scanning direction) of target portion C in a single dynamic exposure, whereas a length of the scanning motion determines a height (in the scanning direction) of target portion C.  
      In another mode, mask table  106  is kept essentially stationary holding a programmable patterning means, and substrate table  108  is moved or scanned, while a pattern imparted to radiation beam  112  is projected onto target portion C. In this mode, generally a pulsed radiation source  102  is employed and patterning means  114  is updated as required after each movement of substrate table  108  or in between successive radiation pulses during a scan. This mode of operation can be readily applied to maskless lithography that utilizes a programmable patterning means for patterning means  114 , for example, but not limited to, a programmable mirror array of a type as referred to above.  
      Combinations and/or variations on the above described modes of use or entirely different modes of use can also be employed.  
      Exemplary Product and Target Patterning Means Arrangements  
       FIGS. 2   a  and  2   b  are close-up views of a lithographic apparatus  100  in a region of one or more mask tables  106 , according to one embodiment of the invention. Two alternative arrangements are shown in which lithographic apparatus  100  comprises a product patterning device  2 , for example a mask, and a metrology target patterning device  3 , for example a mask.  
      In one exemplary arrangement, shown in  FIG. 2   a , mask table  106  is configured to support a product patterning mask  114 - 1  and one or more metrology target patterning masks  114 - 3 .  
      In one exemplary arrangement, shown in  FIG. 2   b,  two mask tables  106 - 1  and  106 - 2  are used. Mask table  106 - 1  supports product patterning mask  114 - 1  and mask table  106 - 2  supports metrology target patterning mask  114 - 2 .  
      The patterning masks  114 - 1 ,  114 - 2 , and  114 - 3  are arranged to impart a pattern in the cross-section of radiation beam  112  generated by illumination system  104 .  
      Although a single radiation source  102  is illustrated in  FIG. 1 , illumination system  104  can comprise a plurality of radiation sources  102 . For example, this can be done to provide initially separate radiation beams  112  to be patterned by product patterning mask  114 - 1  and metrology target patterning mask  114 - 2 / 114 - 3 . In one example using product patterning mask  114 - 1 , this will correspond to functional or structural features in a layer of the product being manufactured, whereas for target patterning masks  114 - 2  and  114 - 3 , the pattern will correspond to metrology targets. For example, metrology targets can be, but are not limited to, alignment marks to align one patterned layer on substrate  118  with another, to align substrate  118  itself relative to projection system  110 , or for other functions.  
      In each of the arrangements shown in  FIGS. 2   a  and  2   b , the metrology target patterning mask(s)  114 - 2  and  114 - 3  can be operated (e.g., exchanged, etc.) independently from product patterning mask  114 - 1 . This arrangement allows for development of the metrology target design in product-like circumstances (i.e., during one of the normal stages of product manufacture) rather than in a separate procedure dedicated solely to metrology target improvement. In each case, they can interact with a mask storage device controller  5 , which executes mask exchange with a mask storage device  7 .  
      Second Exemplary Lithography Apparatus  
       FIG. 3  schematically depicts a lithographic projection apparatus  300  according to an embodiment of the invention. In this embodiment, patterning devices  2  and  3  comprises an array of individually controllable elements  6  (e.g., a programmable mirror array, a grating light valve, a liquid crystal display, a digital mirror device, or the light contrast device or pattern generator) for applying a pattern to radiation beam  110 .  
      Apparatus  300  includes at least a radiation system  302 , patterning devices  2  and  3 , an object table  306  (e.g., a substrate table), and a projection system (“lens”)  308 .  
      Radiation system  302  can be used for supplying a projection beam  310  of radiation (e.g., UV radiation), which in this particular case also comprises a radiation source  312 .  
      An array of patterning devices  2  and  3  (e.g., a programmable mirror array) can be used for applying a pattern to projection beam  310 . In general, the position of the array of patterning devices  2  and  3  can be fixed relative to projection system  308 . However, in an alternative arrangement, an array of patterning devices  2  and  3  can be connected to a positioning device (not shown) for accurately positioning it with respect to projection system  308 . As here depicted, patterning devices  2  and  3  are of a reflective type (e.g., have a reflective array of individually controllable elements).  
      Object table  306  can be provided with a substrate holder (not specifically shown) for holding a substrate  314  (e.g., a resist coated silicon wafer or glass substrate) and object table  306  can be connected to a positioning device  316  for accurately positioning substrate  314  with respect to projection system  308 .  
      Projection system  308  (e.g., a quartz and/or (CaF2 lens system or a catadioptric system comprising lens elements made from such materials, or a mirror system) can be used for projecting the patterned beam received from a beam splitter  318  onto a target portion  320  (e.g., one or more dies) of substrate  314 . Projection system  308  can project an image of the array of patterning devices  2  and  3  onto substrate  314 . Alternatively, projection system  308  can project images of secondary sources for which the elements of the array of patterning devices  2  and  3  act as shutters. Projection system  308  can also comprise a micro lens array (MLA) to form the secondary sources and to project microspots onto substrate  314 .  
      Source  312  (e.g., an excimer laser) can produce a beam of radiation  322 . Beam  322  is fed into an illumination system (illuminator)  324 , either directly or after having traversed conditioning device  326 , such as a beam expander  326 , for example. Illuminator  324  can comprise an adjusting device  328  for setting the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in beam  322 . In addition, illuminator  324  will generally include various other components, such as an integrator  330  and a condenser  332 . In this way, projection beam  310  impinging on the array of patterning devices  2  and  3  has a desired uniformity and intensity distribution in its cross section.  
      It should be noted, with regard to  FIG. 3 , that source  312  can be within the housing of lithographic projection apparatus  300  (as is often the case when source  312  is a mercury lamp, for example). In alternative embodiments, source  312  can also be remote from lithographic projection apparatus  300 . In this case, radiation beam  322  would be directed into apparatus  300  (e.g., with the aid of suitable directing mirrors). This latter scenario is often the case when source  312  is an excimer laser. It is to be appreciated that both of these scenarios are contemplated within the scope of the present invention.  
      Beam  310  subsequently intercepts the array of patterning devices  2  and  3  after being directing using beam splitter  318 . Having been reflected by the array of patterning devices  2  and  3 , beam  310  passes through projection system  308 , which focuses beam  310  onto a target portion  320  of the substrate  314 .  
      With the aid of positioning device  316  (and optionally interferometric measuring device  334  on a base plate  336  that receives interferometric beams  338  via beam splitter  340 ), substrate table  306  can be moved accurately, so as to position different target portions  320  in the path of beam  310 . Where used, the positioning device for the array of patterning devices  2  and  3  can be used to accurately correct the position of the array of patterning devices  2  and  3  with respect to the path of beam  310 , e.g., during a scan. In general, movement of object table  306  is realized with the aid of a long-stroke module (course positioning) and a short-stroke module (fine positioning), which are not explicitly depicted in  FIG. 3 . A similar system can also be used to position the array of patterning devices  2  and  3 . It will be appreciated that projection beam  310  can alternatively/additionally be moveable, while object table  306  and/or the array of patterning devices  2  and  3  can have a fixed position to provide the required relative movement.  
      In an alternative configuration of the embodiment, substrate table  306  can be fixed, with substrate  314  being moveable over substrate table  306 . Where this is done, substrate table  306  is provided with a multitude of openings on a flat uppermost surface, gas being fed through the openings to provide a gas cushion which is capable of supporting substrate  314 . This is conventionally referred to as an air bearing arrangement. Substrate  314  is moved over substrate table  306  using one or more actuators (not shown), which are capable of accurately positioning substrate  314  with respect to the path of beam  310 . Alternatively, substrate  314  can be moved over substrate table  306  by selectively starting and stopping the passage of gas through the openings.  
      Although lithography apparatus  300  according to the invention is herein described as being for exposing a resist on a substrate, it will be appreciated that the invention is not limited to this use and apparatus  300  can be used to project a patterned projection beam  310  for use in resistless lithography.  
      In one example, at least one of patterning devices  2  and  3  comprises an array of individually controllable elements. In general, a position of patterning devices  2  and  3  will be fixed relative to projection system  308 . However, in other examples, at least one patterning device  2  or  3  can instead be connected to a positioning means for accurately positioning them with respect to projection system  308 .  
      In one example, as shown in  FIG. 3 , metrology target patterning device  6  comprises an array of individually controllable elements. Target patterning device  6  is connected to a metrology target patterning device controller  10 , which is configured to update a pattern represented by the array of individually controllable elements by determining and changing, if necessary, an activation state of each element in the array of individually controllable elements.  
      In one example, product patterning device  2  comprises a reflective mask  4 , which is supported and controlled by a mask table and controller  8 .  
      In one example, product patterning device  2  can also be arranged to comprise an array of individually controllable elements, in which case item  8  would function in a similar fashion to the metrology target patterning device controller  10 .  
      In one or more examples or embodiments, patterning the metrology targets using an array of individually controllable elements, independently from whichever process is used to pattern the product features, allows more efficient updates to be made to the metrology targets without affecting the throughput achieved in the product manufacturing cycle.  
      It is generally difficult to predict in advance how well a given metrology target will perform in practice. Performance can be improved by fine-tuning the properties of the metrology target, but this would normally require substantial expense and loss of time, particularly if a new reticle set has to be produced for each change of metrology target and if device/product manufacturing processes have to be interrupted and/or delayed in order to carry out these processes. One or more examples or embodiments of the present invention improves the situation by separating the metrology target pattern from the product feature pattern and, particularly where an array of individually controllable elements is used, facilitating the process of changing a metrology target pattern.  
       FIG. 4  depicts a metrology target optimizing feedback loop, according to one embodiment of the invention. This figure shows an arrangement of a metrology target patterning device controller  10 , which is arranged to interact with a feedback loop  18 . Lithography apparatus  1 , according to one embodiment of the invention, is arranged to print a pattern including at least one metrology target to a substrate W. Patterned substrate W is processed via processing station  20  to develop the metrology target(s) ready for testing. A substrate transportation device  19  is used to carry developed substrates W from processing station  20  to an inspection position to be inspected using a probe  14 , which is arranged to test the metrology target performance and send feedback to metrology target patterning device controller  10 . Based on information thus received, metrology target patterning device controller  10  calculates a correction to send to lithography apparatus  1  to prompt a change in the pattern imparted to the radiation beam by metrology target patterning device  3 .  
      In this embodiment, substrates developed with the updated metrology target are tested in the same way, and the cycle continues until the performance of the metrology targets falls within predetermined bounds of acceptability. The efficiency of this system allows not only optimization of metrology targets of a standard design type, but, because a larger number of trials are possible, also facilitates broader evaluation of alternative metrology target types.  
      Exemplary Arrangement of Metrology Targets  
       FIG. 5  depicts an arrangement of metrology targets of different types on different target portions of a substrate, according to one embodiment of the invention. In  FIG. 5 , metrology targets  22 ,  24 ,  25  and  27  of different types, which are illustrated schematically in the figure, but can in practice comprise a variety of designs, such as boxes, chevrons, horizontal or vertical gratings, etc., are arranged in different dies  23  on the substrate W.  
      In one example, metrology targets  22 ,  24 ,  25 , and  27  can be confined to a metrology target region (e.g., regions  35  and  39  in  FIGS. 7 and 8 , for example) around a periphery of the substrate W or along scribe lanes between dies.  
      However, in other examples, metrology targets  22 ,  24 ,  25 , and  27  can be distributed in a more complex fashion over the surface of the substrate W.  
      The number and size of metrology targets is limited by space considerations, since they sometimes take up room that might otherwise be used for product features. However, it is desirable that metrology targets be of a certain minimum size and that a plurality of different metrology target designs be printed. In a testing context, for example, to see which locations suffer least from cross-talk, this can be to allow more designs to be evaluated per substrate W. More generally, a number of metrology targets will be required to perform the variety of metrology steps required for accurate lithography. Another reason can be to include metrology target standards from a number of different manufacturers in order to allow different layers to be printed by different machines.  
      Various embodiments and/or examples of the present invention address the problem of limited space for the metrology targets. For example, a separately controllable metrology target patterning device  3 , which allows the metrology target to be easily varied, such as between one die and the next, without changing the pattern imparted by product patterning device  2 . High throughput can thus be maintained and, in the case where the metrology target is changed between one die and the next, unnecessary repetition of targets between dies is avoided, thus saving space without reducing the number of metrology targets used. For example, where it is necessary to have separate coarse and fine alignment marks, these can be located in corresponding regions of different dies. In this case, two types of exposure die would exist: a first for printing the product and the fine alignment mark, and a second for printing the product and the coarse alignment mark. The occupied area for the metrology marks is the same in each case and space is therefore saved.  
       FIG. 6  depicts an example metrology target design comprising a primary structure and a substructure, according to one embodiment of the invention. There are various types of metrology targets are likely to be useful. The performance of a given metrology target can be enhanced by including substructure in addition to the primary structure. An example of such an arrangement is shown in  FIG. 6 , which depicts a grating consisting of vertical lines  28  as a primary structure with a product-like pattern superimposed as a substructure  30 . In one example, substructure  30  can be at a relative length scale much smaller than that shown, which is intended for illustrative purposes.  
      In one example, when the metrology targets are used for alignment, they can be inspected at longer wavelengths than that used to image the product features, so that substructure  30  becomes invisible and does not interfere adversely with the operation of the metrology target as a whole. However, the presence of the product-like features ensures that the metrology targets image in a similar way to the actual product features of the device to be formed and do not suffer from different shifts or errors in the projection system.  
       FIG. 7  depicts protective structures for metrology targets positioned in the scribe lane, according to one embodiment of the invention. In one example, when metrology targets are positioned in isolated regions of the substrate or in areas with a significantly lower than average density of features, the metrology target can be vulnerable to excessive chemical or mechanical attack. This situation is illustrated in  FIG. 7 , where a metrology target  32  is isolated in a scribe lane  35  between dies  23 . The lower portion of  FIG. 7  illustrates how a similar metrology target  34  can be protected, according to one embodiment of the present invention by printing copies of a same metrology target  36  in a configuration surrounding target  34 .  
      Copies of the metrology target are shown in this example because this is an approach that can be favored economically to limit the overhead costs associated with applying protective structures, i.e., no new types of marks need to be made available.  
      It is to be appreciated that alternative structures can be used, particularly where it is possible to produce such structures without change to the product pattern.  
      In one example, dedicated protective structures are desired as they can be tailored more extensively to optimize their performance. The dedicated structures can be continuous, for example, rather than island-like, and be arranged to completely surround the metrology target to be protected.  
      The separation of the metrology target patterning device  3  and the product patterning device  2  allows a variety of configurations to be tested. Parameters that can be important include both the form of the surrounding structures and the separation between those structures and the structures to be protected. A balance can need to be struck between protecting the metrology target and leaving enough space around the metrology target to allow it to perform correctly.  
       FIG. 8  depicts protective structures for metrology targets positioned in the region between the dies and the edge of the substrate, according to one embodiment of the invention.  FIG. 8  shows the equivalent arrangement for metrology targets printed in a region  39  around the edge of the substrate outside of dies  23 . Again, metrology target  32  is likely to be exposed and vulnerable to attack, while metrology target  34  is protected by clone marks  36 .  
      In one example, although neighboring patterns (either deliberately added protective structures or nearby product features) can serve to protect a metrology target, they can also have a negative impact on performance if cross-talk occurs. It can be difficult to predict where cross-talk of this kind will be a problem.  
      In one example, a number of different positions for each type of metrology target are tried, and a deduction of which position is more desirable is determined.  
      In one example, an application is provided (e.g., implemented in software, firmware, or both,) that is arranged to analyze the product pattern and the desired metrology target pattern(s), and determine whether the intended metrology target location is optimal. For example, locations where the nearby product structure is most different to the metrology target are likely to be preferred.  
       FIG. 9  depicts positioning of metrology targets to minimize cross-talk with product features, according to one embodiment of the invention.  FIG. 9  illustrates a simple example of such decision making, which can be built into the metrology target patterning device  3 . Here, two product structures, a vertical grating  38  and a horizontal grating  40 , are shown near the edge of die  23 . Metrology target patterning device  3 , taking an input data that includes product structures  38  and  40  (e.g., this can be derived from the data set sent to the product patterning device  2 ) will position metrology target  42  at position (b) rather than (a), as the similarly oriented grating  38  is more likely to cause cross-talk effects than grating  40 .  
       FIG. 10  depicts a lithographic apparatus, according to one embodiment of the invention, comprising a control system for an array of individually controllable elements and a metrology target verification and adaptation device.  FIG. 10  shows an embodiment according to an alternative aspect of the invention, comprising a single array of individually controllable elements  17  for patterning both product device structures and metrology target structures onto the substrate W. The array of individually controllable elements  17  is controlled by a control system  29 , which is capable of actuating each element according to its address and one or more control signals. In this embodiment, control system  29  is configured to receive control signals comprising two separate data streams: a first data stream from a product pattern controller  5  via data path  13 , comprising a product pattern data representing features of a product device to be formed and a second data stream via data path  15   c  comprising metrology target pattern data representing an intended metrology target pattern and/or an intended metrology target location on the substrate.  
      In this embodiment, although it can be known what kinds of metrology target are likely to be needed for a given process layer, it can not be clear in advance how best to implement each metrology target for a given product pattern. The separation of the product pattern data from the metrology target pattern data, as described above, allows the implementation of a metrology target verification and adaptation device  7 , which is provided to facilitate the introduction of new kinds of metrology target by evaluating a proposed metrology target design and location on the substrate (input, for example, from a metrology target pattern controller  31  via data path  15   a ) while taking account of the product pattern to be printed (the relevant data being made available via data paths  13   a  and  15   a ). If judged necessary, the metrology target verification and adaptation device  7  calculates a suitable correction to either or both of the metrology target pattern or location and sends this correction as a feedback via data path  15   b.  Once the metrology target verification and adaptation device  7  judges that the likely performance of the metrology target is within acceptable limits, an updated metrology target pattern data is forwarded via data path  15   c  to control system  29 . In this way, the metrology target pattern can be updated in real-time without interrupting the product patterning process. The approach also facilitates the effective introduction of entirely new metrology targets in real time.  
      According to embodiments of the invention, metrology targets are printed onto the substrate W at the same time as product patterns. This is done to ensure a proper relationship between product structures and metrology targets. If the metrology targets on the mask can be used with inline metrology techniques (e.g., scatterometry), based on inspection of a “latent” image of metrology targets (i.e., metrology target patterns formed on the substrate by exposed radiation only, without any further processing of the substrate), a feedback loop of metrology information (e.g., overlay values) can be used to correct for errors in the imaging process for the next substrate to be processed. In practice, this correction involves modifying one or more so-called “exposure settings,” which can be any tunable parameter associated with elements of the lithography apparatus (including, for example, the illumination system, patterning means and projection system) that can affect image quality (as indicated by the inline readout of the metrology targets). The exposure settings can be parameterized in many different ways and can include, but are not limited to, magnification, translations in the substrate plane, focus, and radiation intensity.  
      In one example, a next substrate behaves in exactly the same way as the substrate used for correction of the exposure settings. In practice, this may not be the case and substrates within a given batch can vary significantly. This can be due to irregularities in previously formed device layers, or can arise due to other structural variations (for example, those caused by thermal offsets).  
      In one example, variation within a batch can be dealt with using the following process flow: (a) expose substrate; (b) readout metrology targets (inline); (c) re-work substrate (to prepare it for re-exposure of the product features, which would normally include removing a layer of exposed resist); and, (d) re-expose substrate with optimal exposure settings. The need for the substrate re-working step can severely hamper productivity and can make substrate flow in the factory very complex.  
      According to an embodiment of the invention, a more efficient optimization of exposure settings can be achieved using a system that can print metrology targets separately from product features. In particular, the present embodiment provides a system wherein a first pattern is printed to the substrate that consists mainly or entirely of metrology targets, without patterns corresponding to product features. Most of the substrate remains un-exposed after this step. An inspection out of the latent image of the metrology targets is then carried in order to measure metrology information (e.g., overlay values, etc.). In one example, “latent image” means an image detectable on the resist on the substrate after exposure with patterned radiation, but prior to any processing or development of the resist (e.g., a post-exposure bake).  
      In this example, the exposure settings of the lithography apparatus are improved by reference to the information derived in the inspection step. The product features are then exposed onto the substrate without having to carry out any re-working of the substrate. This is possible because the areas destined for product features were not affected by the metrology target writing step. Avoiding the re-working step greatly improves productivity and removes the need for additional substrate handling apparatus.  
       FIGS. 11   a  and  11   b  show schematically how such a first exposure pattern might be designed, according to one embodiment of the present invention.  FIG. 11   a  shows a single die after first exposure with four metrology targets  54  around the periphery of the die.  FIG. 11   b  shows how these dies can be distributed over the surface of the substrate W. Although  FIG. 11   b  shows all the dies represented, it one example it can be desired to pattern only a subset of the dies in the first exposure step, leaving the metrology targets associated with the remaining dies to be printed along with the product pattern in a later step (and, possibly, used in a final inspection step to evaluate the quality of the product pattern).  
       FIGS. 12   a  and  12   b  illustrate the pattern exposed on the substrate after the second optimized/compensation exposure has been made, including the product device features, according to one example of the present invention. The pattern corresponds to the same die and collection of dies as  FIGS. 11   a  and  11   b,  respectively.  FIGS. 11 and 12  show the substrate W having a circular form, but it can also be arranged to be rectangular (e.g., when the invention is applied to the manufacture of flat panel displays), or any other shape appropriate in the particular circumstances.  
      In one example, it may not be appropriate for the final inspection of the substrate (after the product image has been exposed) to be based on the same metrology targets as were used for the initial determination of the optimal exposure settings. Instead, other fields or other metrology targets in the same field can be selected for readout. As a variation (as mentioned above), in another example the second exposure (i.e., the exposure including product structure) can also comprise new metrology targets for use in the final inspection step.  
       FIG. 13  shows an apparatus suitable for carrying out the above method, according to one embodiment of the present invention. An illumination system  324  directs a projection beam  310  towards a beam splitter  318 . The projection beam  310  is then reflected from, and patterned by, a patterning device  2 ,  3  before being projected by projection system  308  onto a target portion of substrate W. The arrangement shown is intended for use with a maskless patterning device, but an analogous system using masks, such as that depicted in  FIGS. 2   a  and  2   b,  can also be used without departing from the scope of the invention.  
      After a first exposure with metrology targets, the substrate W can be moved from a position A, immediately below the axis of the projection system  308 , to a position B, which allows access to a metrology inspection device  60 . Arrow  64  is provided as a visual aid to show the transition between the positions A and B. In one example, metrology inspection device  60  can operate by scatterometry.  
      The metrology inspection device  60  is configured to inspect the latent image of metrology marks on the substrate W. The results of this inspection are analyzed in controller  62 , which calculates how to modify exposure settings for the illumination system  324 , patterning device  2 ,  3 , projection system  308 , and any other component that might affect metrology, in order to improve the imaging performance of the lithography apparatus for that particular substrate W.  
      In one example, when the lithographic apparatus comprises a number of optical columns, which can each comprise distinct patterning devices  2 ,  3  and projection systems  308 , etc., multiple sets of exposure settings (one set for each optical column) may need to be optimized/compensated, for example, by inspecting metrology targets generated by each column. Once this process is complete, the substrate W is replaced in the exposure position A ready for exposure of the actual product pattern with the optimized/compensated exposure settings.  
      In the example shown in  FIG. 13 , the substrate W moves between an exposure position and a metrology position. In another example, a metrology target inspection  60  device forms part of the projection system  308 , or is located adjacent thereto, in such a way that the latent metrology target images can be inspected while the substrate W is in an exposure position, beneath the axis of the projection system  308 .  
      The embodiment shown in  FIG. 13  allows true exposure settings optimization on an individual substrate basis (i.e., optimization for a given substrate is based on measurements of metrology targets on that substrate rather than on measurements of metrology targets on preceding substrates). This provides an efficient way of dealing with situations in which substrate properties vary substantially within a batch. More generally, the approach can also be used to provide an improved optimization even when this is not the case and/or for cost-saving purposes can allow tolerances related to substrate regularity to be relaxed. This arrangement can also enhance product yield per substrate.  
      In one example, for new product-starts, a “send-ahead” substrate (which is a calibration-only substrate sent in advance to determine suitable exposure parameters for the product substrates to follow) is no longer required. The spatial extent and location of the metrology targets necessary for determining optimal exposure settings are such that there is no great reduction in the space available for the product features.  
     CONCLUSION  
      While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.  
      It is to be appreciated that the Detailed Description section, and not the Summary and Abstract sections, is intended to be used to interpret the claims. The Summary and Abstract sections can set forth one or more, but not all exemplary embodiments of the present invention as contemplated by the inventor(s), and thus, are not intended to limit the present invention and the appended claims in any way.