Abstract:
A lithographic apparatus includes a uniformity correction system located at a plane and configured to receive a substantially constant pupil when illuminated with the beam of radiation. The uniformity correction system includes fingers that move into and out of intersection with a beam so as to correct an intensity of respective portions of the radiation beam. According to another embodiment, a method includes for: focusing a beam of radiation at a first plane to form pupil; adjusting the intensity of the beam near the first plane by moving fingers located near the first plane into and out of a path of the beam of radiation, wherein a width of a tip of each of the fingers is larger than that of corresponding actuating devices used to move each corresponding one of the fingers; patterning the beam; and projecting the patterned beam onto a substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit under 35 U.S.C. 119(e) to U.S. Provisional Application 61/475,156, filed Apr. 13, 2011, which is incorporated by reference herein in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a lithographic apparatus and illumination uniformity correction system. The present invention generally relates to lithography, and more particularly to a system and method for compensating for uniformity drift caused by, for example, illumination beam movement, optical column uniformity, uniformity compensator drift, etc. 
       BACKGROUND 
       [0003]    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). In that circumstance, a patterning device, 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, 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) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. 
         [0004]    A lithographic apparatus typically includes an illumination system, which is arranged to condition radiation generated by a radiation source before the radiation is incident upon a patterning device. The illumination system may, for example, modify one or more properties of the radiation, such as polarization and/or illumination mode. The illumination system may include a uniformity correction system, which is arranged to correct or reduce non-uniformities, e.g., intensity non-uniformities, present in the radiation. The uniformity correction devices may employ actuated fingers which are inserted into an edge of a radiation beam to correct intensity variations. However, a width of a spatial period of intensity variation in that can be corrected is dependent on a size of an actuating device used to move fingers of the uniformity correction system. Furthermore, in some instances, if a size or shape of the fingers used to correct irregularities of a radiation beam is modified, then the uniformity correction system may compromise or modify in an unwanted manner one or more properties of the radiation beam, such as a pupil formed by the radiation beam. 
         [0005]    Lithography is widely recognized as a key process in manufacturing integrated circuits (ICs) as well as other devices and/or structures. A lithographic apparatus is a machine, used during lithography, which applies a desired pattern onto a substrate, such as onto a target portion of the substrate. During manufacture of ICs with a lithographic apparatus, a patterning device (which is alternatively referred to as a mask or a reticle) generates a circuit pattern to be formed on an individual layer in an IC. This pattern may be transferred onto the target portion (e.g., comprising part of, one, or several dies) on the substrate (e.g., a silicon substrate). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (e.g., resist) provided on the substrate. In general, a single substrate contains a network of adjacent target portions that are successively patterned. To reduce manufacturing cost of ICs, it is customary to expose multiple substrates of each IC. Likewise, it is also customary that the lithographic apparatus is in almost constant use. That is, in order to keep manufacturing cost of all types of ICs at a potential minimum, the idle time between substrate exposures is also minimized. Thus, the lithographic apparatus absorbs heat which causes expansion of the apparatus&#39;s components leading to drift, movement, and uniformity changes. 
         [0006]    In order to ensure good imaging quality on the patterning device and the substrate, a controlled uniformity of the illumination beam is maintained. That is, the illumination beam before reflecting off of or transmitting through the patterning device potentially has a non-uniform intensity profile. It is desirable to the entire lithographic process that the illumination beam be controlled with at least some uniformity. Uniformity can refer to a constant intensity across the entire illumination beam, but can also refer to the ability to control the illumination to a target illumination. The target illumination uniformity has a flat or a non-flat profile. The patterning device imparts to a beam of radiation a pattern, which is then imaged onto a substrate. Image quality of this projected radiation beam is affected by the uniformity of the illumination beam. 
         [0007]    The market demands that the lithographic apparatus perform the lithography process as efficiently as possible to maximize manufacturing capacity and keep costs per device low. This means keeping manufacturing defects to a minimum, which is why the effect of the uniformity of the illumination beam needs to be minimized as much as practical. 
       SUMMARY 
       [0008]    It is desirable to provide a lithographic apparatus and method that overcome or mitigate one or more problems, whether identified herein or elsewhere. 
         [0009]    According to an embodiment of the present invention, there is provided a lithographic apparatus comprising an illumination system configured to condition a beam of radiation. The illumination system comprises a uniformity correction system located at a plane configured to receive a substantially constant pupil when illuminated with the beam of radiation. The uniformity correction system includes fingers configured to be movable into and out of intersection with a radiation beam so as to correct an intensity of respective portions of the radiation beam and actuating devices coupled to corresponding ones of the fingers and configured to move the corresponding fingers. 
         [0010]    According to an embodiment of the invention, a width of a tip of each of the fingers is larger than that of a width of an actuating device configured to move the tip. 
         [0011]    In one example, the lithography apparatus further includes a support structure, a substrate table, and a projection system. The support device is configured to hold a patterning device configured to impart the conditioned beam of radiation with a pattern in its cross-section. The substrate table is configured to hold a substrate. The projection system is configured to project the patterned radiation beam onto a target portion of the substrate. 
         [0012]    According to another embodiment of the invention, there is provided a method of lithography comprising the following steps: (1) focusing a beam of radiation at a first plane so as to form a substantially constant pupil at the first plane, (2) adjusting the intensity of the beam of radiation near the first plane by moving fingers located near the first plane into and out of a path of the beam of radiation, wherein a width of a tip of each of the fingers is larger than that of corresponding actuating devices used to move each corresponding one of the fingers, (3) directing the beam of radiation beam onto a patterning device to pattern the beam of radiation, and (4) projecting the patterned radiation beam onto a substrate. 
         [0013]    Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings. It is noted that the invention is not limited to the specific embodiments described herein. Such embodiments are presented herein for illustrative purposes only. Additional embodiments will be apparent to persons skilled in the relevant art(s) based on the teachings contained herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
         [0014]    The accompanying drawings, which are incorporated herein and form part 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 relevant art(s) to make and use the invention. 
           [0015]      FIGS. 1A and 1B  respectively depict reflective and transmissive lithographic apparatuses with uniformity compensators and associated sensors. 
           [0016]      FIG. 2  depicts an example extreme ultra violet (EUV) lithographic apparatus. 
           [0017]      FIG. 3  shows an example of a uniformity compensators with respect to the illumination beam slit. 
           [0018]      FIG. 4  shows an example of a illumination beam slit. 
           [0019]      FIGS. 5A and 5B  illustrate example reflective lithography systems containing uniformity compensators. 
           [0020]      FIGS. 6A and 6B  show example uniformity compensators. 
           [0021]      FIG. 7  is a three-dimensional model of an example uniformity compensator. 
           [0022]      FIG. 8  is a cut-away illustration of a mechanism that controls the movement of fingers in an example uniformity compensator. 
           [0023]      FIGS. 9A and 9B  illustrate example uniformity compensators with overlapping fingers having 4 mm and 2 mm pitch respectively. 
           [0024]      FIGS. 10A-10D  illustrates overlapping fingers of example uniformity compensation systems. 
           [0025]      FIG. 11  shows uniformity error performance of an example embodiment uniformity compensation system with 4 mm pitch. 
           [0026]      FIG. 12  shows uniformity error performance of an example embodiment uniformity compensation system with 2 mm pitch. 
           [0027]      FIG. 13  shows a flow for uniformity refresh. 
           [0028]      FIG. 14  depicts generalized main flow that is the combination of uniformity refresh and (optionally) offline calibration. 
           [0029]      FIG. 15  is an illustration of an example computer system  1500  in which embodiments of the present invention, or portions thereof, can be implemented as computer-readable code. 
       
    
    
       [0030]    The features and advantages of the present invention will become more apparent from the detailed description set forth below when taken in conjunction with the drawings, in which like reference characters identify corresponding elements throughout. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the corresponding reference number. 
       DETAILED DESCRIPTION 
       [0031]    The present invention is directed to methods using uniformity compensators to compensate for uniformity drift caused by, for example, illumination beam movement, optical column uniformity, uniformity compensator drift, etc. This specification discloses one or more embodiments that incorporate the features of this invention. The disclosed embodiment(s) merely exemplify the invention. The scope of the invention is not limited to the disclosed embodiment(s). The invention is defined by the claims appended hereto. 
         [0032]    The embodiment(s) described, and references in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment(s) described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
         [0033]    Before describing such embodiments in more detail, however, it is instructive to present an example environment in which embodiments of the present invention may be implemented. 
       I. An Example Lithographic Environment 
       [0034]    A. Example Reflective and Transmissive Lithographic Systems 
         [0035]      FIGS. 1A and 1B  schematically depict lithographic apparatus  100  and lithographic apparatus  100 ′, respectively. Lithographic apparatus  100  and lithographic apparatus  100 ′ each include: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g., DUV or EUV radiation); a support structure (e.g., a mask table) MT configured to support a patterning device (e.g., a mask, a reticle, or a dynamic patterning device) MA and connected to a first positioner PM configured to accurately position the patterning device MA; and a substrate table (e.g., a substrate table) WT configured to hold a substrate (e.g., a resist coated substrate) W and connected to a second positioner PW configured to accurately position the substrate W. Lithographic apparatuses  100  and  100 ′ also have a projection system PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion (e.g., comprising one or more dies) C of the substrate W. In lithographic apparatus  100  the patterning device MA and the projection system PS is reflective, and in lithographic apparatus  100 ′ the patterning device MA and the projection system PS is transmissive. 
         [0036]    The illumination system IL 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 the radiation B. The illumination system IL may also include an energy sensor ES that provides a measurement of the energy (per pulse), a measurement sensor MS for measuring the movement of the optical beam, and uniformity compensators UC that allow the illumination slit uniformity to be controlled. 
         [0037]    The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device MA, the design of the lithographic apparatuses  100  and  100 ′, and other conditions, such as for example whether or not the patterning device MA is held in a vacuum environment. The support structure MT may use mechanical, vacuum, electrostatic, or other clamping techniques to hold the patterning device MA. The support structure MT may be a frame or a table, for example, which may be fixed or movable, as required. The support structure MT may ensure that the patterning device is at a desired position, for example with respect to the projection system PS. 
         [0038]    The term “patterning device” MA should be broadly interpreted as referring to any device that may be used to impart a radiation beam B with a pattern in its cross-section, such as to create a pattern in the target portion C of the substrate W. The pattern imparted to the radiation beam B may correspond to a particular functional layer in a device being created in the target portion C, such as an integrated circuit. 
         [0039]    The patterning device MA may be transmissive (as in lithographic apparatus  100 ′ of  FIG. 1B ) or reflective (as in lithographic apparatus  100  of  FIG. 1A ). Examples of patterning devices MA include reticles, 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 may be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in the radiation beam B which is reflected by the mirror matrix. 
         [0040]    The term “projection system” PS may encompass 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. A vacuum environment may be used for EUV or electron beam radiation since other gases may absorb too much radiation or electrons. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps. 
         [0041]    Lithographic apparatus  100  and/or lithographic apparatus  100 ′ may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables) WT. In such “multiple stage” machines the additional substrate tables WT may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other substrate tables WT are being used for exposure. 
         [0042]    Referring to  FIGS. 1A and 1B , the illuminator IL receives a radiation beam from a radiation source SO. The source SO and the lithographic apparatuses  100 ,  100 ′ may be separate entities, for example when the source SO is an excimer laser. In such cases, the source SO is not considered to form part of the lithographic apparatuses  100  or  100 ′, and the radiation beam B passes from the source SO to the illuminator IL with the aid of a beam delivery system BD ( FIG. 1B ) comprising, for example, suitable directing mirrors and/or a beam expander. In other cases, the source SO may be an integral part of the lithographic apparatuses  100 ,  100 ′ for example when the source SO 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. 
         [0043]    The illuminator IL may comprise an adjuster AD ( FIG. 1B ) for adjusting 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 may be adjusted. In addition, the illuminator IL may comprise various other components ( FIG. 1B ), such as an integrator IN and a condenser CO. The illuminator IL may be used to condition the radiation beam B, to have a desired uniformity and intensity distribution in its cross section. This desired uniformity is may be maintained through the use of the energy sensors ES that divides-out the variation of the source output and the uniformity compensator UC that is comprised of a plurality of protrusions (e.g., fingers) that can be inserted into and removed from the illumination beam to modify its uniformity and intensity. 
         [0044]    Referring to  FIG. 1A , the radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device MA. In lithographic apparatus  100 , the radiation beam B is reflected from the patterning device (e.g., mask) MA. After being reflected from the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF 2  (e.g., an interferometric device, linear encoder, or capacitive sensor), the substrate table WT may be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor IF 1  may be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B. Patterning device (e.g., mask) MA and substrate W may be aligned using mask alignment marks M 1 , M 2  and substrate alignment marks P 1 , P 2 . 
         [0045]    Referring to  FIG. 1B , the radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the 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 positioner 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 positioner PM and another position sensor (which is not explicitly depicted in  FIG. 1B ) 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. Likewise, in  FIG. 2  there is a substrate stage slit sensor WS that on a per pulse basis in conjunction with the energy sensor ES produces normalized intensity data from the illumination system IL to the substrate W. 
         [0046]    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 positioner PM. Similarly, movement of the substrate table WT 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 (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. 
         [0047]    The lithographic apparatuses  100  and  100 ′ may be used in at least one of the following modes: 
         [0048]    1. In step mode, the support structure (e.g., mask table) MT and the substrate table WT are kept essentially stationary, while an entire pattern imparted to the radiation beam B is projected onto a target portion C at one time (i.e., a single static exposure). The substrate table WT is then shifted in the X and/or Y direction so that a different target portion C may be exposed. 
         [0049]    2. In scan mode, the support structure (e.g., mask table) MT and the substrate table WT are scanned synchronously while a pattern imparted to the radiation beam B is projected onto a target portion C (i.e., a single dynamic exposure). The velocity and direction of the substrate table WT relative to the support structure (e.g., mask table) MT may be determined by the (de-)magnification and image reversal characteristics of the projection system PS. 
         [0050]    3. In another mode, the support structure (e.g., mask table) MT is kept substantially stationary holding a programmable patterning device, and the substrate table WT is moved or scanned while a pattern imparted to the radiation beam B is projected onto a target portion C. A pulsed radiation source SO may be employed and the programmable patterning device is updated as required after each movement of the substrate table WT or in between successive radiation pulses during a scan. This mode of operation may be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to herein. 
         [0051]    Combinations and/or variations on the described modes of use or entirely different modes of use may also be employed. 
         [0052]    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 “substrate” 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. 
         [0053]    In a further embodiment, lithographic apparatus  100  includes an extreme ultraviolet (EUV) source, which is configured to generate a beam of EUV radiation for EUV lithography. In general, the EUV source is configured in a radiation system (see below), and a corresponding illumination system is configured to condition the EUV radiation beam of the EUV source. 
         [0054]    B. Example EUV Lithographic Apparatus 
         [0055]      FIG. 2  schematically depicts an exemplary EUV lithographic apparatus according to an embodiment of the present invention. In  FIG. 2 , EUV lithographic apparatus includes a radiation system  202 , an illumination optics unit  204 , and a projection system PS. The radiation system  202  includes a radiation source SO, in which a beam of radiation may be formed by a discharge plasma. In an embodiment, EUV radiation may be produced by a gas or vapor, for example, from Xe gas, Li vapor, or Sn vapor, in which a very hot plasma is created to emit radiation in the EUV range of the electromagnetic spectrum. The very hot plasma can be created by generating at least partially ionized plasma by, for example, an electrical discharge. Partial pressures of, for example, 10 Pa of Xe, Li, Sn vapor or any other suitable gas or vapor may be required for efficient generation of the radiation. The radiation emitted by radiation source SO is passed from a source chamber  206  into a collector chamber  208  via a gas barrier or contaminant trap  210  positioned in or behind an opening in source chamber  206 . In an embodiment, gas barrier  210  may include a channel structure. 
         [0056]    Collector chamber  208  includes a radiation collector  212  (which may also be called collector mirror or collector) that may be formed from a grazing incidence collector. Radiation collector  212  has an upstream radiation collector side  214  and a downstream radiation collector side  216 , and radiation passed by collector  212  can be reflected off a grating spectral filter  218  to be focused at a virtual source point  220  at an aperture in the collector chamber  208 . Radiation collectors  212  are known to skilled artisans. 
         [0057]    From collector chamber  208 , a beam of radiation  226  is reflected in illumination optics unit  204  via normal incidence reflectors  222  and  224  onto a reticle or mask (not shown) positioned on reticle or mask table MT. A patterned beam  228  is formed, which is imaged in projection system PS via reflective elements  230  and  232  onto a substrate (not shown) supported on substrate stage or substrate table WT. In various embodiments, illumination optics unit  204  and projection system PS may include more (or fewer) elements than depicted in  FIG. 2 . For example, illumination optics unit  204  may also include an energy sensor ES that provides a measurement of the energy (per pulse), a measurement sensor MS for measuring the movement of the optical beam, and uniformity compensators UC that allow the illumination slit uniformity to be controlled. Additionally, grating spectral filter  218  may optionally be present, depending upon the type of lithographic apparatus. Further, in an embodiment, illumination optics unit  204  and projection system PS may include more mirrors than those depicted in  FIG. 2 . For example, projection system PS may incorporate one to four reflective elements in addition to reflective elements  230  and  232 . In  FIG. 2 , reference number  240  indicates a space between two reflectors, e.g., a space between reflectors  234  and  236 . 
         [0058]    In an embodiment, collector mirror  212  may also include a normal incidence collector in place of or in addition to a grazing incidence mirror. Further, collector mirror  212 , although described in reference to a nested collector with reflectors  234 ,  236 , and  238 , is herein further used as example of a collector. 
         [0059]    Further, instead of a grating  218 , as schematically depicted in  FIG. 2 , a transmissive optical filter may also be applied. Optical filters transmissive for EUV, as well as optical filters less transmissive for or even substantially absorbing UV radiation, are known to skilled artisans. Hence, the use of “grating spectral purity filter” is herein further indicated interchangeably as a “spectral purity filter,” which includes gratings or transmissive filters. Although not depicted in  FIG. 2 , EUV transmissive optical filters may be included as additional optical elements, for example, configured upstream of collector mirror  212  or optical EUV transmissive filters in illumination unit  204  and/or projection system PS. 
         [0060]    The terms “upstream” and “downstream,” with respect to optical elements, indicate positions of one or more optical elements “optically upstream” and “optically downstream,” respectively, of one or more additional optical elements. Following the light path that a beam of radiation traverses through lithographic apparatus, a first optical elements closer to source SO than a second optical element is configured upstream of the second optical element; the second optical element is configured downstream of the first optical element. For example, collector mirror  212  is configured upstream of spectral filter  218 , whereas optical element  222  is configured downstream of spectral filter  218 . 
         [0061]    All optical elements depicted in  FIG. 2  (and additional optical elements not shown in the schematic drawing of this embodiment) may be vulnerable to deposition of contaminants produced by source SO, for example, Sn. Such may be the case for the radiation collector  212  and, if present, the spectral purity filter  218 . Hence, a cleaning device may be employed to clean one or more of these optical elements, as well as a cleaning method may be applied to those optical elements, but also to normal incidence reflectors  222  and  224  and reflective elements  230  and  232  or other optical elements, for example additional mirrors, gratings, etc. 
         [0062]    Radiation collector  212  can be a grazing incidence collector, and in such an embodiment, collector  212  is aligned along an optical axis O. The source SO, or an image thereof, may also be located along optical axis O. The radiation collector  212  may comprise reflectors  234 ,  236 , and  238  (also known as a “shell” or a Wolter-type reflector including several Wolter-type reflectors). Reflectors  234 ,  236 , and  238  may be nested and rotationally symmetric about optical axis O. In  FIG. 2 , an inner reflector is indicated by reference number  234 , an intermediate reflector is indicated by reference number  236 , and an outer reflector is indicated by reference number  238 . The radiation collector  212  encloses a certain volume, e.g., a volume within the outer reflector(s)  238 . Usually, the volume within outer reflector(s)  238  is circumferentially closed, although small openings may be present. 
         [0063]    Reflectors  234 ,  236 , and  238  respectively may include surfaces of which at least portion represents a reflective layer or a number of reflective layers. Hence, reflectors  234 ,  236 , and  238  (or additional reflectors in the embodiments of radiation collectors having more than three reflectors or shells) are at least partly designed for reflecting and collecting EUV radiation from source SO, and at least part of reflectors  234 ,  236 , and  238  may not be designed to reflect and collect EUV radiation. For example, at least part of the back side of the reflectors may not be designed to reflect and collect EUV radiation. On the surface of these reflective layers, there may in addition be a cap layer for protection or as optical filter provided on at least part of the surface of the reflective layers. 
         [0064]    The radiation collector  212  may be placed in the vicinity of the source SO or an image of the source SO. Each reflector  234 ,  236 , and  238  may comprise at least two adjacent reflecting surfaces, the reflecting surfaces further from the source SO being placed at smaller angles to the optical axis O than the reflecting surface that is closer to the source SO. In this way, a grazing incidence collector  212  is configured to generate a beam of EUV radiation propagating along the optical axis O. At least two reflectors may be placed substantially coaxially and extend substantially rotationally symmetric about the optical axis O. It should be appreciated that radiation collector  212  may have further features on the external surface of outer reflector  238  or further features around outer reflector  238 , for example a protective holder, a heater, etc. 
         [0065]    In the embodiments described herein, the terms “lens” and “lens element,” where the context allows, may refer to any one or combination of various types of optical components, comprising refractive, reflective, magnetic, electromagnetic and electrostatic optical components. 
         [0066]    Further, the terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, comprising ultraviolet (UV) radiation (e.g., having a wavelength λ of 365, 248, 193, 157 or 126 nm), extreme ultra-violet (EUV or soft X-ray) radiation (e.g., having a wavelength in the range of 5-20 nm, e.g., 13.5 nm), or hard X-ray working at less than 5 nm, as well as particle beams, such as ion beams or electron beams. Generally, radiation having wavelengths between about 780-3000 nm (or larger) is considered IR radiation. UV refers to radiation with wavelengths of approximately 100-400 nm. Within lithography, it is usually also applied to the wavelengths, which can be produced by a mercury discharge lamp: G-line 436 nm; H-line 405 nm; and/or I-line 365 nm. Vacuum UV, or VUV (i.e., UV absorbed by air), refers to radiation having a wavelength of approximately 100-200 nm. Deep UV (DUV) generally refers to radiation having wavelengths ranging from 126 nm to 428 nm, and in an embodiment, an excimer laser can generate DUV radiation used within lithographic apparatus. It should be appreciated that radiation having a wavelength in the range of, for example, 5-20 nm relates to radiation with a certain wavelength band, of which at least part is in the range of 5-20 nm. 
       II. System and Methods for Compensating for Drift in Illumination Beam Uniformity 
       [0067]      FIG. 3  illustrates a mechanical portion of a uniformity refresh (UR) correction system  300 , according to an embodiment of the present invention. In  FIG. 3 , the uniformity refresh (UR) correction system  300  includes energy sensors (ES)  310  and a plurality of uniformity compensators  320 . UR correction system  300  can modify the illumination beam during a lithographic operation. In at least one embodiment of the present invention, the illumination beam is shaped in an arc shape and is referred to as an illumination slit  330 . By controlling movement of the individual uniformity compensators  320  into and out of the illumination slit  330 , the uniformity of the illumination slit  330  can be controlled. The uniformity compensators  320  may also be referred to as fingers. An example operation of uniformity compensators may be found in commonly owned, co-pending U.S. Non-provisional patent application Ser. No. 12/789,795, filed May 28, 2010, which is incorporated by reference herein in its entirety. 
         [0068]    In one example, the fingers shown in  FIG. 3  may be individually controlled to modify the intensity of the illumination slit in order to achieve a target uniformity. 
         [0069]      FIG. 4  is an enlarged view of illumination slit  430 , according to one embodiment of the present invention. For example, in at least one embodiment,  FIG. 4  illustrates a size and shape of the illumination slit  430 .  FIG. 4  does not show fingers of the uniformity compensators that are inserted into and withdrawn from the path of the illumination slit in order to modify its intensity uniformity. In one embodiment, the uniformity compensators are only located on one side of the illumination slit. 
         [0070]      FIGS. 5A and 5B  illustrate example reflective lithography systems containing uniformity compensators  514  and  528  respectively. In the first example,  FIG. 5A  illustrates an illumination source  502  that provides an illumination beam that reflects off various mirrors  504 ,  506 ,  508 ,  510  and  512 . The beam interacts with fingers of a uniformity compensator  514  before hitting the reticle  516 . The reticle is reflective. Therefore, the patterned radiation beam  518  is reflected from the reticle  516  as it propagates toward the substrate (not shown). 
         [0071]    Another example reflective lithography system that uses a uniformity compensator system  528  is shown in  FIG. 5B . An illumination source  520  provides an illumination beam that is reflected from mirrors  522 ,  524 , and  526 . The beam interacts with fingers of a uniformity compensator  528  before hitting the reticle  530 . The reticle is reflective. Therefore the beam of radiation is patterned by the reticle  530  and is reflected as the patterned beam  532  as it propagates toward the substrate (not shown). 
         [0072]    An example uniformity compensator is shown in more detail in  FIGS. 6A and 6B .  FIG. 6A  shows an example, elevated view of the uniformity compensator system, looking downward from the reticle.  FIG. 6A  is similar to  FIG. 3  and shows an example uniformity compensator with fingers  602  and energy sensors  606 . By controlling the movement of the individual uniformity compensators  602  into and out of the beam  604 , uniformity of the illumination slit  608  can be controlled. 
         [0073]    A side view of the example uniformity compensator of  FIG. 6A  is shown in  FIG. 6B . The uniformity compensator fingers that are illustrated in  602  in  FIG. 6A  are shown  602  in  FIG. 6B . As shown in  FIG. 6B , the uniformity compensating fingers are below the reticle  616  and are separated by a distance  610 . The distance  610  between the fingertips and the reticle is on the order of several millimeters. In an example embodiment, the distance  610  may be between about 10-20 mm. 
         [0074]    Incident and reflected beams illustrated in  FIG. 6B  are seen interacting with the fingers  602  and the reticle  616  making an angle with the reticle as shown in  614  and  620 . The rest of the structure in  FIG. 6B  is associated with the actuators that control the movement of the fingers as will be discussed below. 
         [0075]    A three-dimensional model of the uniformity compensator is shown in  FIG. 7 . The uniformity compensator fingers  602  can be seen. The fingertips  602 , which extend into the beam can be moved into and out of the beam and are connected to finger necks  704 . The measurement and control of the movement of the fingers is controlled by an encoder box  706 . Exemplary mounting hardware is illustrated at  708 . 
         [0076]      FIG. 8  is a cut-away illustration of the encoder box  706 . As before, the fingertips  602  that move into and out of the beam are connected to finger necks  704 . The control circuitry is housed in the encoder box  706  as well as a measurement mechanism  808  that measures the displacement of the fingers. Each individual finger is moved (in a translational manner) by a linear motor  812  that utilizes magnets  810 . This cut-away-illustration also shows a finger body  814  as well as flexures  816  and  818 . 
         [0077]      FIG. 9A  illustrates a particular embodiment in which overlapping fingers gives rise to a predetermined finger pitch  906 . The fingers  602  of  FIG. 8  are shown in more detail as the features  902  and  904  in the left hand portion of  FIG. 9A , which is a top down view of the fingers. This collection of overlapping fingertips is shown in more detail in  FIGS. 10A and 10B . The right hand side of  FIG. 9A  is a schematic side view illustration corresponding to the cut-away structure of  FIG. 8 . In one embodiment, the predetermined finger pitch  906  is between 3-5 mm, and is preferably about 4 mm. 
         [0078]      FIG. 10A  shows a collection of overlapping fingertips corresponding to those on the left in  FIG. 9A . The shape of the individual fingers are shown in  FIG. 10B .  FIG. 10B  results from laterally separated the fingers of  FIG. 10A . The fingertips are chosen in “T” shape shown in  10 B so as to be overlapping, with the gray fingers on top of the white fingers. The width of the fingers  1008  and  1010  in this example embodiment is 7 mm. The pitch of this arrangement is 4 mm due to the overlapping placement as shown in  FIG. 10A . 
         [0079]    The detailed structure shown in the cut-away illustration of  FIG. 8  has been abstracted on the right hand side in  FIG. 9A . The finger necks  704  illustrated in  FIG. 8  is shown schematically in  9 A as feature  918  with the upper fingers  920  and lower finger  922  also schematically illustrated. The upper fingers  920  and lower fingers  922  on the right in  FIG. 9  are overlapping as shown by respective grey  902  and white  904  fingers in the top-down view on the left of  FIG. 9A . 
         [0080]    The magnets  810  of  FIG. 8  are abstracted as features  916  in  FIG. 9A , together with other details of the linear motor  914  ( 812 ). The encoder box  706  of  FIG. 8  is now abstracted as the gray rectangle  910  in  FIG. 9A . This encoder box uses a measurement sensor  912  to measure the movement of the finger actuators. The encoder box  910  also contains the circuitry that controls the movement of the fingers. 
         [0081]    The vertical finger necks  704  shown in  FIG. 8  now appear as the shaded region  908  in the top down view on the left of  FIG. 9A  and as  918  in the side view on the right of  FIG. 9A . The pitch  906  is determined as the distance between a right hand edge of one finger and a corresponding right hand edge of an adjacent finger. Although the width of a given finger in this example is 7 mm, because of the way that the fingers are overlapping, the pitch is smaller than that. In particular, in this case it is 4 mm. 
         [0082]      FIG. 9B  shows another example embodiment in which two sets of fingers are displaced from one another. The embodiment of  FIG. 9B  is generated by arranging (or otherwise configuring) two sets of fingers and displacing them one from the other. Thus, in the illustration on the left in  FIG. 9B , the gray fingers now correspond to all of the fingers that were illustrated in  FIG. 9A . The white fingers correspond to the arrangement of fingers shown in  9 A, after a lateral and vertical displacement as shown schematically on the right in  FIG. 9B . 
         [0083]    The upper set of fingers  928  in  FIG. 9B  is a copy of the complete set of fingers  920  and  922  of  FIG. 9A . Likewise, the lower set of fingers  930  in  FIG. 9B  is a similar copy of the complete set of fingers  920  and  922  of  FIG. 9A . As such, the example embodiment of  FIG. 9B  comprises twice as many fingers as that of  FIG. 9A . Such an arrangement can be accomplished by using a double-sided encoder box  940 . A motor mechanism for the top collection of fingers is shown in  948  with magnets  946 . Likewise, a lower motor  952  and magnet assembly  950  for the lower collection of fingers are shown. In this embodiment, the double-sided encoder box uses one encoder  942  to encode the positions of the upper set of fingers and another encoder  944  to encode the positions lower set of fingers. The necks of the two collections of fingers are shown by  938  on the right and shaded regions  934  and  936  in the top down view on the left hand side if  FIG. 9B . 
         [0084]    In an example embodiment as shown in  FIG. 9A , a set of 28 overlapping fingers each with finger width 7 mm resulting in a 4 mm pitch is provided. The corresponding embodiment of  FIG. 9B  comprises two sets of 28 fingers with each finger having width 3 mm. The second set can be displaced laterally by 2 mm such that the totality of 48 fingers has a pitch of 2 mm. In order for the configuration of  FIG. 9B  to be accommodated, the length of the fingers has to be different for the two sets of fingers  928  and  930  in  FIG. 9B . This is illustrated in more detail in  FIGS. 10A-D . 
         [0085]      FIG. 10C  illustrates the collection of overlapping fingers to achieve a 2 mm pitch. These comprise two overlapping sets of fingers. The fingers in  FIG. 10C  that are represented in gray, correspond to all of the fingers in  10 A, but the width of the finger tips in  FIGS. 10C and 10D  is 3 mm. Likewise, the fingers that are illustrated as  1014  in white in  FIG. 10C  also correspond to a duplicate set of all of the fingers shown in  10 A upon reduction of the width from 7 mm to 3 mm, for example. As mentioned previously, the two sets of fingers are displaced from one another by 2 mm. 
         [0086]    The performance of embodiments shown in  FIG. 9A and 9B  in terms of correcting the uniformity of illumination of a beam is shown in  FIGS. 11 and 12 . Both  FIGS. 11 and 12  plot the uniformity error for various types of illumination (1. quasi-cony; 2. large annular; 3. small annular; 4. dipole x 90; 5. dipole y 90; 6. quasar 45; 7. cquad 45; 8. dipole x 120; and 9. dipole y 120). 
         [0087]      FIG. 11  corresponds to the embodiment of  FIG. 9A  with a 4 mm pitch while  FIG. 12  illustrates the uniformity error for the embodiment of  FIG. 9B  that has 2 mm pitch. 
         [0088]    The uniformity error is defined as the ratio between the difference of the maximum and minimum intensity, and the sum of the maximum and minimum intensity. Ideally, this ratio should be zero for a completely uniform beam. For non-uniform illumination, this ratio quantifies the performance of the uniformity compensators. This explains why  FIGS. 11 and 12  show variations for different illuminations.  FIG. 12  is the measured uniformity error for the same sets of illumination patterns using the configuration of  FIG. 9B . Clearly the embodiment of  FIG. 9B , with 2 mm pitch shows improved uniformity performance as shown in  FIG. 12  relative to the embodiment of  FIG. 9A  with 4 mm pitch as shown in  FIG. 11 . This is because the smaller pitch of the embodiment of  FIG. 9B  provides for the ability to correct illumination variations with higher spatial frequency. 
         [0089]      FIG. 13  illustrates a method, according to an embodiment of the present invention for compensating for system uniformity drift. Such a method can be used to maximize manufacturing efficiencies by improving the quantity of successfully imaged devices on a substrate to substrate basis. 
         [0090]    In one example, method starts at a beginning of each lot  1310  of substrates. In step  1320 , the illumination slit uniformity is measured (e.g., by slit integrated intensity or by slit-scan average using discrete intensity samples along the slit). In step  1320 , the uniformity refresh (UR) correction system calculates uniformity compensators (e.g., fingers) positions based on a flat intensity profile across the slit. Optionally, in step  1340  the uniformity refresh (UR) correction system calculates uniformity compensators (e.g., fingers) positions based on a non-flat intensity profile (using a system such as DOSEMAPPER® or DoMa manufactured by ASML, Veldhoven, The Netherlands). Examples regarding DOSEMAPPER® embodiments may be found in U.S. Pat. No. 7,532,308, issued May 12, 2009, which is incorporated herein by reference in its entirety. In step  1350 , the uniformity refresh (UR) correction system sets positions of the plurality of uniformity compensators (e.g., fingers). In step  1360 , a substrate is exposed. In one example, during the exposure of each substrate, a number of different non-flat profiles (e.g., DOSEMAPPER® target illumination slit profiles) may be used (e.g., depending on the portion of the substrate being exposed). Thus, there may be uniformity compensator finger position changes even during the exposure of a single substrate. In step  1370 , it is determined whether or not another substrate is to be exposed in the lot. If step  1370  returns “yes,” the method returns to step  1320 . If step  1370  returns “no”, more substrates in the lot are to be exposed, at step  1390  method ends. 
         [0091]    In an embodiment of the present invention, during step  1310  the illumination slit uniformity is controlled (e.g., corrected) between subsequent substrates of a single lot, so that each substrate in the lot is exposed with an independently controlled uniform illumination slit. In step  1320 , a uniformity of the illumination slit is measured. For example, the uniformity of the illumination slit may change due to a number of factors, for example illumination beam movement, optical column uniformity, or uniformity compensator finger drift. 
         [0092]    In one example, the uniformity of the illumination slit is measured as a continuous intensity profile by integrating the illumination slit intensity across the entire slit. Additionally, or alternatively, the uniformity of the illumination slit may be measured as a slit-scan averaged intensity using discrete intensity samples along the slit. 
         [0093]    In step  1330 , using the measured illumination slit uniformity from step  1320 , the finger positions are calculated so as to produce a flat target illumination slit uniformity. Optionally, in step  1340 , non-flat (DoMa) uniformity profiles can be used, along with the measured illumination slit uniformity from step  1320 , to calculate the finger positions. In step  1350 , the calculated finger positions are set so that the illumination beam uniformity matches either the flat target profile or the non-flat target profile. In step  1360 , a substrate is exposed. 
         [0094]    In one embodiment, the fingers are moved during the exposure of the substrate so that different portions of the substrate are exposed using different illumination slit target profiles. 
         [0095]    In step  1370 , it is determined whether there are additional substrates in the lot to be exposed. If step  1370  returns yes, then method returns to step  1320 . In one example, measuring and correcting the uniformity of the illumination slit between substrates of a single lot is desirable because system movement, heat generation, and vibrations may have caused the uniformity of the illumination slit to change. If step  1370  returns no, method ends at step  1390 . 
         [0096]      FIG. 14  illustrates a method, according to an embodiment of the present invention for compensating for system uniformity drift. For example, this method can be used to maximize manufacturing efficiencies by improving the quantity of successfully imaged devices on a substrate to substrate basis. Method shown in  FIG. 14  can include an initial calibration step before a first substrate is processed. Subsequent substrates may not have a calibration step, but rather use the prior substrates ending measurement values as the initial measurement values. 
         [0097]    In step  1410 , an offline calibration of uniformity compensator positions is performed. In step  1415 , the uniformity compensators are mechanically adjusted. In step  1420 , a beam of radiation is produced. In step  1425 , the beam of radiation is passed through an optical system containing the uniformity compensators. In optional step  1430 , the beam movement is measured or beam movement is calculated. In step  1435 , the illumination slit uniformity is measured or calculated (e.g., if measured, this can be by slit integrated intensity or it can be by slit-scan average using discrete intensity samples along the slit). In step  1440 , positions of the uniformity compensator (e.g., finger) are determined based on current uniformity, offline data, and/or beam movement. In step  1445 , the uniformity compensator (e.g., finger) positions are adjusted. In step  1450 , a determination is made whether method shown in  FIG. 13  should be performed again. If step  1450  returns “yes,” method shown in  FIG. 14  returns to step  1420 . If step  1450  returns “no,” the method shown in  FIG. 14  moves to step  1455 , during which a substrate is exposed. 
         [0098]    In one example, during the exposure of each substrate, a number of different non-flat profiles (e.g., DOSEMAPPER® target illumination slit profiles) may be used depending on the portion of the substrate being exposed. For example, there may be uniformity compensator finger position changes even during the exposure of a single substrate. Examples regarding modeling of uniformity changes during heating and cooling cycles, calibrating associated parameters, and applying these results to actuation of individual attenuators may be found in U.S. Pat. No. 7,532,308, issued May 12, 2009, and U.S. Pat. No. 6,455,862, issued Sep. 24, 2002, both of which are incorporated herein by reference in their entireties. 
         [0099]    In an embodiment of the present invention, the illumination slit uniformity is controlled (e.g., corrected) between subsequent substrates as described with reference to  FIG. 13 . Also, in an embodiment of the present invention, the method for adjusting the uniformity compensators so that the illumination slit uniformity matches a target illumination slit uniformity comprises at least two measurements of the illumination slit uniformity per substrate. That is, when method shown in  FIG. 14  reaches step  1450  a “repeat” decision is made. The “repeat” decision is usually “yes” the first time for each substrate. When decision  1450  is “yes,” step  1420  is repeated and a new beam of radiation is produced. The new beam of radiation is passed, in step  1435 , through the optical system. Step  1435  measures the illumination slit uniformity, step  1440  determines the uniformity compensator positions, and step  1445  adjusts the uniformity compensators. 
         [0100]    During the repetition of method shown in  FIG. 14 , if the measured illumination slit uniformity is within a pre-determined tolerance of a target illumination slit intensity profile (flat or non-flat), than there will be no further need to repeat the compensating method and “no” will be chosen at step  1450 . Thereafter, at step  1455 , exposure of a substrate may occur as described with reference to  FIG. 13 . Method shown in  FIG. 14  may also be performed without a comparison to a pre-determined tolerance. In an embodiment, method shown in  FIG. 14  is performed only once and does not repeat to determine if the uniformity compensator adjustments, in step  1445 , cause the illumination slit uniformity to match the target illumination slit intensity profile (flat or non-flat). 
         [0101]    In another example, if the measured illumination slit uniformity is not within a pre-determined tolerance of a target illumination slit intensity profile (flat or non-flat), then the uniformity compensators may need further adjustment. In this case, “yes” will be chosen again at step  1450 . The illumination slit uniformity can be fine-tuned to be closer to the target illumination slit intensity profile. Alternatively, there may be no need to repeat the method, even after the first time for a substrate, if the measured illumination slit uniformity is within a pre-determined tolerance of a target illumination slit intensity profile. 
         [0102]    The control methods of the present invention illustrated in  FIGS. 13 and 14  may be implemented in software, firmware, hardware, or a combination thereof.  FIG. 15  is an illustration of an example computer system  1500  in which embodiments of the present invention, or portions thereof, can be implemented as computer-readable code. The methods illustrated by flowcharts of  FIGS. 13 and 14 , can be implemented in computer system  1500  that includes a display interface  1502  coupled to a display  1530 . Various embodiments of the invention are described in terms of this example computer system  1500 . After reading this description, it will become apparent to a person skilled in the relevant art how to implement embodiments of the invention using other computer systems and/or computer architectures. 
         [0103]    Computer system  1500  includes one or more processors, such as processor  1504 . Processor  1504  may be a special purpose or a general purpose processor. Processor  1504  is connected to a communication infrastructure  1506  (e.g., a bus or network). 
         [0104]    Computer system  1500  also includes a main memory  1505 , preferably random access memory (RAM), and may also include a secondary memory  1510 . Secondary memory  1510  can include, for example, a hard disk drive  1512 , a removable storage drive  1514 , and/or a memory stick. Removable storage drive  1514  can comprise a floppy disk drive, a magnetic tape drive, an optical disk drive, a flash memory, or the like. The removable storage drive  1514  reads from and/or writes to a removable storage unit  1518  in a well-known manner. Removable storage unit  1518  can include a floppy disk, magnetic tape, optical disk, etc. which is read by and written to by removable storage drive  1514 . As will be appreciated by persons skilled in the relevant art, removable storage unit  1518  includes a computer-usable storage medium having stored therein computer software and/or data. 
         [0105]    In alternative implementations, secondary memory  1510  can include other similar devices for allowing computer programs or other instructions to be loaded into computer system  1500 . Such devices can include, for example, a removable storage unit  1518  and an interface  1520 . Examples of such devices can include a program cartridge and cartridge interface (such as those found in video game devices), a removable memory chip (e.g., EPROM or PROM) and associated socket, and other removable storage units  1518  and interfaces  1520  which allow software and data to be transferred from the removable storage unit  1518  to computer system  1500 . 
         [0106]    Computer system  1500  can also include a communications interface  1524 . Communications interface  1524  allows software and data to be transferred between computer system  1500  and external devices. Communications interface  1524  can include a modem, a network interface (such as an Ethernet card), a communications port, a PCMCIA slot and card, or the like. Software and data transferred via communications interface  1524  are in the form of signals, which may be electronic, electromagnetic, optical, or other signals capable of being received by communications interface  1524 . These signals are provided to communications interface  1524  via a communications path  1526  and  1528 . Communications path  1526  and  1528  carries signals and may be implemented using wire or cable, fiber optics, a phone line, a cellular phone link, a RF link or other communications channels. 
         [0107]    In this document, the terms “computer program medium” and “computer-usable medium” are used to generally refer to media such as removable storage unit  1518 , removable storage unit  1518 , and a hard disk installed in hard disk drive  1512 . Computer program medium and computer-usable medium can also refer to memories, such as main memory  1505  and secondary memory  1510 , which can be memory semiconductors (e.g., DRAMs, etc.). These computer program products provide software to computer system  1500 . 
         [0108]    Computer programs (also called computer control logic) are stored in main memory  1505  and/or secondary memory  1510 . Computer programs may also be received via communications interface  1524 . Such computer programs, when executed, enable computer system  1500  to implement embodiments of the present invention as discussed herein. In particular, the computer programs, when executed, enable processor  1504  to implement processes of the present invention, such as the steps in the methods illustrated by flowchart of  FIG. 13 , discussed above. Accordingly, such computer programs represent controllers of the computer system  1500 . Where embodiments of the invention are implemented using software, the software can be stored in a computer program product and loaded into computer system  1500  using removable storage drive  1514 , interface  1520 , hard drive  1512  or communications interface  1524 . 
         [0109]    Embodiments of the invention are also directed to computer program products including software stored on any computer-usable medium. Such software, when executed in one or more data processing device, causes a data processing device(s) to operate as described herein. Embodiments of the invention employ any computer-usable or -readable medium, known now or in the future. Examples of computer-usable mediums include, but are not limited to, primary storage devices (e.g., any type of random access memory), secondary storage devices (e.g., hard drives, floppy disks, CD ROMS, ZIP disks, tapes, magnetic storage devices, optical storage devices, MEMS, nanotechnological storage devices, etc.), and communication mediums (e.g., wired and wireless communications networks, local area networks, wide area networks, intranets, etc.). 
       CONCLUSION 
       [0110]    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 may 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. 
         [0111]    The present invention has been described above with the aid of functional building blocks illustrating the implementation of specified functions and relationships thereof. The boundaries of these functional building blocks have been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. 
         [0112]    The foregoing description of the specific embodiments will so fully reveal the general nature of the invention that others can, by applying knowledge within the skill of the art, readily modify and/or adapt for various applications such specific embodiments, without undue experimentation, without departing from the general concept of the present invention. Therefore, such adaptations and modifications are intended to be within the meaning and range of equivalents of the disclosed embodiments, based on the teaching and guidance presented herein. It is to be understood that the phraseology or terminology herein is for the purpose of description and not of limitation, such that the terminology or phraseology of the present specification is to be interpreted by the skilled artisan in light of the teachings and guidance. 
         [0113]    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. 
         [0114]    The claims in the instant application are different than those of the parent application or other related applications. The Applicant therefore rescinds any disclaimer of claim scope made in the parent application or any predecessor application in relation to the instant application. The Examiner is therefore advised that any such previous disclaimer and the cited references that it was made to avoid, may need to be revisited. Further, the Examiner is also reminded that any disclaimer made in the instant application should not be read into or against the parent application.