Abstract:
A lithographic method and apparatus comprises an illumination system that supplies a beam of radiation, a patterning device that patterns the beam, and a projection system that projects the patterned beam onto a target portion of a substrate. A metrology system is provided adjacent the projection system for aligning the substrate with the projection system. Two or more movable chucks are each arranged to support a substrate and move between a loading device and the projection system. The chucks are independently movable so that one substrate can be passed through the metrology system and patterned beam while the other substrates are moved between the loading system and projection system.

Description:
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, can 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 can comprise an array of individually controllable elements that generate the circuit pattern. 
   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 an entire pattern onto the target portion in one go, and 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. 
   Many scanners employ a movable chuck for supporting the substrate that passes through the patterned beam. The chuck transports the substrate from a loader to an exposure location, and passes the transport beneath the scanning apparatus. In machines of this type, it is vital that the substrate is correctly aligned with the scanning apparatus before exposure begins. Alignment, preconditioning, and measurement of the substrate consumes a significant amount of time compared to the time taken to expose the substrate. A substrate will spend approximately one third (33%) of its total time in a typical machine in metrology (e.g., measuring) and internal handling and transport, and two thirds (67%) of its time being exposed. This represents a considerable inefficiency in the operation of the machine. 
   Lithography machines, in particular Integrated Circuit (IC) and Flat Panel Display (FPD) machines in which the projection systems are most expensive components of the machine, suffer particularly from this problem. If these components are idle for 33% of the time, this represents a significant waste of the capacity of the machine. FPD substrates are generally very large (e.g., typically 2-3 m along each side), and accordingly the machines required to expose such substrates have a very large footprint. 
   Therefore, what is needed is a method and apparatus that maximize an exposure time of substrates in a lithographic apparatus and/or to minimize an increase in footprint of the lithographic apparatus when large substrates are exposed. 
   SUMMARY 
   In accordance with one embodiment of the present invention, there is provided a lithographic apparatus comprising an illumination system, a patterning device, two or more moveable chucks, a projection system, a metrology system, and a loading device. The illumination system supplies a beam of radiation. The patterning device patterns the beam. The two or more movable chucks are each arranged to support one substrate each. The projection system projects the patterned beam onto a target portion of one of the substrates. The metrology system is adjacent the projection system and aligns the substrate with the projection system as it enters the patterned beam. The loading device places the substrates onto the chucks. The chucks are independently movable, so that one substrate can be passed through the patterned beam, while the or each other substrate is moved between the loading system and projection system. 
   Thus in this embodiment, while one substrate is exposed, another substrate (on its respective chuck) can be moved into position ready for exposure as soon as the first substrate is finished. The projection system is therefore in use almost all of the time, greatly increasing the efficiency of the apparatus. The “idle” chuck can be transported as close as possible to the chuck supporting the substrate being exposed. Immediately after exposure, the first substrate can be transported away from the projection apparatus, while the second substrate is exposed. Because the metrology system and projection system are adjacent to each other, metrology can begin on the second substrate even as the first substrate leaves the patterned beam. In one example, the projection system and metrology system are mounted on a single frame, improving the correlation between metrology and exposure. 
   In one embodiment, the patterning device can comprise an array of individually controllable elements and the projection system and patterning device can together comprise an array of projection elements. 
   In one embodiment, a preconditioning system is provided for thermally preconditioning one of the substrates, while another substrate is passed through the patterned beam. In one example, the preconditioning system is located physically proximate to the loading device so that preconditioning can take place while the substrate is located on the loading device. In another example, the preconditioning takes place after the substrate has been placed onto its chuck. 
   In one example, the projection system and metrology system can be located in a first carousel lane and the loading device located in a second carousel lane. This facilitates the passing of one of the substrates through the patterned beam while another substrate is loaded onto its corresponding chuck. It also allows for the “idle” chuck to be moved into the first carousel lane following loading, to align with the projection system ready for exposure of its substrate as soon as the exposure of the first substrate is complete. Furthermore, the first substrate can then be moved quickly back into the second carousel lane (and out of the path of the second substrate) as soon as its exposure is complete. The chucks can move substantially in a circular pattern, and can all follow the same path. 
   In this example, the provision of two or more carousel lanes also allows for the provision of an additional metrology system located in the second carousel lane. Preparatory measurements can be performed on each substrate while it is still in the second carousel lane before it is moved to the projection system. 
   In one example, a base frame is provided for supporting the projection system, metrology system and chucks. The chucks can be transported rapidly around the base frame when not being exposed. In one example, each chuck can be mounted on a long stroke frame movable about the base frame using a planar drive. In one example, each chuck is movable on its corresponding long stroke frame via a short stroke actuator. This allows for precise positioning of the substrates relative to the projection system. In one example, the planar drive can comprise a coil mounted in the base frame with at least one magnet mounted in each long stroke frame. This greatly reduces the problems of cabling in a carousel. Alternatively, the base frame can comprise a magnet plate. 
   It will be appreciated that other forms of actuators and bearings can also be used in place of a planar drive, and that a single stroke drive can be used in place of the long stroke frames and short stroke actuators. 
   In one example, the chucks can be moved along the carousel lanes by intra-lane drives, and between the carousel lanes by inter-lane drives. The inter- and intra-lane drives each move the chucks in one dimension, and this again simplifies the cabling. The base frame can be formed from a plurality of subsidiary base frames. 
   In one example, an unloading device can be provided for removing the substrates from their respective chucks. This can be located either immediately above or immediately below the loading device. The loading and unloading devices are arranged to move the substrates on and off their respective chucks from the same side. This is desirable because each chuck can include metrology features used by the metrology system to assist the initial alignment of the chuck (and thus the substrate) relative to the projection system. These features are often located at the front edge of the chuck with respect to its passage through the patterned beam, and typically project upwards from the chuck. It is therefore not simple to move a substrate on and off the chuck across the front end over the raised metrology features. It is therefore desirable to load and unload the substrates from the rear edge of the chuck. 
   In one example, an additional unloading device can also be provided for receiving one of the substrates from the unloading device and unloading it from the apparatus. This allows for substrates to be loaded into the apparatus at one end and unloaded at the opposite end. In one example, the projection system and metrology system are located in a first carousel lane, and the loading device, unloading device and additional unloading device are located in a second carousel lane. 
   In accordance with one embodiment of the present invention, there is provided a carousel for use in a lithographic apparatus comprising an illumination and projection system for projecting a patterned beam of radiation onto a target portion of a substrate and a metrology system adjacent the projection system for ensuring the substrate is aligned with the projection system. The carousel comprises two or more chucks, each arranged to support a substrate and a loading device for placing the substrates onto the chucks. The chucks are independently movable so that one substrate can be passed through the metrology system and patterned beam while the or each other substrate is moved between the loading system and projection system. 
   In accordance with one embodiment of the present invention, there is provided a device manufacturing method comprising the following steps. Patterning a beam of radiation and projecting the patterned beam of radiation onto a target portion of a substrate. Providing two or more movable chucks, each arranged to support one of the substrates. Aligning a first one of the substrates with the projection system using a metrology system adjacent the projection system. Projecting the patterned beam onto a target portion of the first substrate. Providing a loading device for placing the substrates onto the chucks. Moving a second one of the substrates between the loading device and the projection system, while the patterned beam is projected onto the first substrate. 
   In one example, the substrates can be for use in manufacturing flat panel displays. 
   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 a 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 pertinent art to make and use the invention. 
       FIG. 1  depicts a lithographic apparatus, according to one embodiment of the present invention. 
       FIG. 2  is a plan view of a lithographic apparatus including two chucks, according to one embodiment of the present invention. 
       FIG. 3  is a side view of the apparatus of  FIG. 2 . 
       FIGS. 4A to 4K  depict a sequence of stages during the operation of the apparatus of  FIG. 2 , according to one embodiment of the present invention. 
       FIG. 5  shows part of the apparatus of  FIG. 2 , according to one embodiment of the present invention. 
       FIG. 6  is a plan view of another lithographic apparatus, according to one embodiment of the present invention. 
       FIG. 7  is a side view of the apparatus of  FIG. 6 . 
     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 
   Throughout the remainder of this specification the terms “alignment mark” and “alignment marks” will be used to denote one or more individual, indiscrete alignment marks respectively, unless otherwise stated. By “individual” it is meant that each alignment mark is separate and distinct from others of its kind (i.e., from the other alignment marks). By “indiscrete” it is meant that each alignment mark is not divided into parts (e.g., each alignment mark is a single, undivided entity). A variety of such marks can be used in embodiments of the invention, and it will be appreciated that the dots, spots, and lines referred to in this specification are merely specific examples. Other forms can be used. 
   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, micro and macro fluidic devices, 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 term “array of individually controllable elements” as here employed should be broadly interpreted as referring to any device that can be used to endow an incoming radiation beam with a patterned cross-section, so that a desired pattern can be created in a target portion of the substrate. The terms “light valve” and “Spatial Light Modulator” (SLM) can also be used in this context. Examples of such patterning devices are discussed below. 
   A programmable mirror array can comprise a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that, for example, addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate spatial filter, the undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light to reach the substrate. In this manner, the beam becomes patterned according to the addressing pattern of the matrix-addressable surface. 
   It will be appreciated that, as an alternative, the filter can filter out the diffracted light, leaving the undiffracted light to reach the substrate. An array of diffractive optical micro electrical mechanical system (MEMS) devices can also be used in a corresponding manner. Each diffractive optical MEMS device can include a plurality of reflective ribbons that can be deformed relative to one another to form a grating that reflects incident light as diffracted light. 
   A further alternative embodiment can include a programmable mirror array employing a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation devices. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-addressable mirrors. The required matrix addressing can be performed using suitable electronic means. 
   In both of the situations described here above, the array of individually controllable elements can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference in their entireties. 
   A programmable LCD array can also be used. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference in its entirety. 
   It should be appreciated that where pre-biasing of features, optical proximity correction features, phase variation techniques and multiple exposure techniques are used. For example, the pattern “displayed” on the array of individually controllable elements can differ substantially from the pattern eventually transferred to a layer of or on the substrate. Similarly, the pattern eventually generated on the substrate can not correspond to the pattern formed at any one instant on the array of individually controllable elements. This can be the case in an arrangement in which the eventual pattern formed on each part of the substrate is built up over a given period of time or a given number of exposures during which the pattern on the array of individually controllable elements and/or the relative position of the substrate changes. 
   Although specific reference can 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 can have other applications, such as, for example, the manufacture of DNA chips, MEMS, MOEMS, 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 (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, 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 illumination system can also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the 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). 
   Lithographic Projection Apparatus 
     FIG. 1  schematically depicts a lithographic projection apparatus  100  according to an embodiment of the invention. Apparatus  100  includes at least a radiation system  102 , an array of individually controllable elements  104 , an object table  106  (e.g., a substrate table), and a projection system (“lens”)  108 . 
   Radiation system  102  can be used for supplying a beam  110  of radiation (e.g., UV radiation), which in this particular case also comprises a radiation source  112 . 
   An array of individually controllable elements  104  (e.g., a programmable mirror array) can be used for applying a pattern to beam  110 . In general, the position of the array of individually controllable elements  104  can be fixed relative to projection system  108 . However, in an alternative arrangement, an array of individually controllable elements  104  can be connected to a positioning device (not shown) for accurately positioning it with respect to projection system  108 . As here depicted, individually controllable elements  104  are of a reflective type (e.g., have a reflective array of individually controllable elements). 
   Object table  106  can be provided with a substrate holder (not specifically shown) for holding a substrate  114  (e.g., a resist coated silicon wafer or glass substrate) and object table  106  can be connected to a positioning device  116  for accurately positioning substrate  114  with respect to projection system  108 . 
   Projection system  108  (e.g., a quartz and/or CaF 2  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  118  onto a target portion  120  (e.g., one or more dies) of substrate  114 . Projection system  108  can project an image of the array of individually controllable elements  104  onto substrate  114 . Alternatively, projection system  108  can project images of secondary sources for which the elements of the array of individually controllable elements  104  act as shutters. Projection system  108  can also comprise a micro lens array (MLA) to form the secondary sources and to project microspots onto substrate  114 . 
   Source  112  (e.g., an excimer laser) can produce a beam of radiation  122 . Beam  122  is fed into an illumination system (illuminator)  124 , either directly or after having traversed conditioning device  126 , such as a beam expander, for example. Illuminator  124  can comprise an adjusting device  128  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  122 . In addition, illuminator  124  will generally include various other components, such as an integrator  130  and a condenser  132 . In this way, beam  110  impinging on the array of individually controllable elements  104  has a desired uniformity and intensity distribution in its cross section. 
   It should be noted, with regard to  FIG. 1 , that source  112  can be within the housing of lithographic projection apparatus  100  (as is often the case when source  112  is a mercury lamp, for example). In alternative embodiments, source  112  can also be remote from lithographic projection apparatus  100 . In this case, radiation beam  122  would be directed into apparatus  100  (e.g., with the aid of suitable directing mirrors). This latter scenario is often the case when source  112  is an excimer laser. It is to be appreciated that both of these scenarios are contemplated within the scope of the present invention. 
   Beam  110  subsequently intercepts the array of individually controllable elements  104  after being directed using beam splitter  118 . Having been reflected by the array of individually controllable elements  104 , beam  110  passes through projection system  108 , which focuses beam  110  onto a target portion  120  of the substrate  114 . 
   With the aid of positioning device  116  (and optionally interferometric measuring device  134  on a base plate  136  that receives interferometric beams  138  via beam splitter  140 ), substrate table  6  can be moved accurately, so as to position different target portions  120  in the path of beam  110 . Where used, the positioning device for the array of individually controllable elements  104  can be used to accurately correct the position of the array of individually controllable elements  104  with respect to the path of beam  110 , e.g., during a scan. In general, movement of object table  106  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. 1 . A similar system can also be used to position the array of individually controllable elements  104 . It will be appreciated that beam  110  can alternatively/additionally be moveable, while object table  106  and/or the array of individually controllable elements  104  can have a fixed position to provide the required relative movement. 
   In an alternative configuration of the embodiment, substrate table  106  can be fixed, with substrate  114  being moveable over substrate table  106 . Where this is done, substrate table  106  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  114 . This is conventionally referred to as an air bearing arrangement. Substrate  114  is moved over substrate table  106  using one or more actuators (not shown), which are capable of accurately positioning substrate  114  with respect to the path of beam  110 . Alternatively, substrate  114  can be moved over substrate table  106  by selectively starting and stopping the passage of gas through the openings. 
   Although lithography apparatus  100  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  100  can be used to project a patterned beam  110  for use in resistless lithography. 
   The depicted apparatus  100  can be used in four preferred modes: 
   1. Step mode: the entire pattern on the array of individually controllable elements  104  is projected in one go (i.e., a single “flash”) onto a target portion  120 . Substrate table  106  is then moved in the x and/or y directions to a different position for a different target portion  120  to be irradiated by patterned beam  110 . 
   2. Scan mode: essentially the same as step mode, except that a given target portion  120  is not exposed in a single “flash.” Instead, the array of individually controllable elements  104  is movable in a given direction (the so-called “scan direction”, e.g., the y direction) with a speed v, so that patterned beam  110  is caused to scan over the array of individually controllable elements  104 . Concurrently, substrate table  106  is simultaneously moved in the same or opposite direction at a speed V=Mv, in which M is the magnification of projection system  108 . In this manner, a relatively large target portion  120  can be exposed, without having to compromise on resolution. 
   3. Pulse mode: the array of individually controllable elements  104  is kept essentially stationary and the entire pattern is projected onto a target portion  120  of substrate  114  using pulsed radiation system  102 . Substrate table  106  is moved with an essentially constant speed such that patterned beam  110  is caused to scan a line across substrate  106 . The pattern on the array of individually controllable elements  104  is updated as required between pulses of radiation system  102  and the pulses are timed such that successive target portions  120  are exposed at the required locations on substrate  114 . Consequently, patterned beam  110  can scan across substrate  114  to expose the complete pattern for a strip of substrate  114 . The process is repeated until complete substrate  114  has been exposed line by line. 
   4. Continuous scan mode: essentially the same as pulse mode except that a substantially constant radiation system  102  is used and the pattern on the array of individually controllable elements  104  is updated as patterned beam  110  scans across substrate  114  and exposes it. 
   Combinations and/or variations on the above described modes of use or entirely different modes of use can also be employed. 
     FIGS. 2 and 3  are plan and side views, respectively, of a lithographic apparatus, according to one embodiment of the present invention. The apparatus comprises a substrate table WT comprising a base frame  2 , on which is mounted a long stroke (LS) magnet plate  3 . First and second chucks  4 ,  5  for supporting substrates are mounted on the LS magnet plate  3  via long stroke (LS) frames  6 ,  7 . The LS frames  6 ,  7  are coupled to the LS magnet plate using a planar drive (not shown), which provides contactless motion. This allows the LS frames  6 ,  7  to be moved around above the base frame with up to six degrees of freedom, although in one example two or three degrees of freedom are used. The chucks  4 ,  5  are mounted on the LS frames  6 ,  7  via short stroke (SS) actuators  8  which enable fine control of the positioning of the chuck. The SS actuators  8  provide, for example, three degrees of freedom for the each chuck  4 ,  5  with respect to the corresponding LS frame  6 ,  7 . 
   Each chuck  4 ,  5  is provided with sensors for detecting the alignment, positioning, and level of the chuck and substrate, and the energy falling on the substrate. These sensors are represented schematically as metrology features  9 ,  10  on each chuck. At the stage of operation shown in  FIGS. 1 and 2 , a first substrate  11  is supported by the first chuck  4 , but no substrate is supported by the second chuck  5 . 
   The apparatus of  FIGS. 2 and 3  additionally comprises a metroframe  13  mounted on the base frame  2  via three vibration isolation systems (VIS)  14 ,  15 ,  16 . The vibration isolation systems  14 ,  15 ,  16  are, for example, air mounts, which ensure that low frequency vibrations and force disturbances are not transmitted from the base frame  2  to the metroframe  13 . The metroframe  13  includes an array of projection elements  17  that perform the same function as the projection system PL and element array PPM shown in  FIG. 1 . The projection system elements  17  provide a patterned beam of radiation, which exposes the first substrate  11  as it passes beneath the metroframe  13 . The metroframe  13  also includes metrology systems  18 A,  18 B for interaction with the metrology features  9 ,  10  on each chuck and with the substrates  11 ,  12 . This allows for the alignment and positioning of each chuck  4 ,  5  and allows for substrate  11 ,  12  to be monitored as it passes beneath the metroframe  13 . 
   The apparatus of  FIGS. 2 and 3  also comprises a loader  19  and unloader  20  which are movable in the z-direction. At the stage shown in  FIGS. 1 and 2 , a second substrate  12  is located on the loader, having been placed there by an external robot (not shown). A preconditioning system (not shown) is located in the same region as the loader  19  and unloader  20  for thermal preconditioning of substrates. 
   The metroframe  13  is located in a first carousel lane  21  which runs in the −y-direction, whereas the loader  19  and the unloader  20  and preconditioning system are located in a second carousel lane  22  parallel to the first. 
   In use, the first substrate  11  mounted on the first chuck  4  moves slowly to the right along the first carousel lane  21  under the metroframe  13  and is exposed by the projection elements  17 . While this exposure takes place, a number of separate operations are performed on the second substrate  12 , as shown in  FIGS. 4A to 4K . It is to be appreciated that the apparatus shown in  FIGS. 4A to 4K  is the same as that shown in  FIGS. 2 and 3 , but illustrated in a simplified form for clarity. 
     FIGS. 4A to 4K  depict a sequence of stages during the operation of the apparatus of  FIG. 2 , according to one embodiment of the present invention. 
   As shown in  FIG. 4A , while exposure of the first substrate  11  takes place the second substrate  12  is placed on the loader  19  by the external robot. The loader  19  and unloader  20  are then lowered so that the top of the loader  19  is level with the top of the second chuck  5 . 
   As shown in  FIG. 4B , the second substrate  12  is transferred from the loader  19  to the second chuck  5 . The loader  19  and unloader  20  are then raised. 
   As shown in  FIG. 4C , the second chuck  5  (now supporting the second substrate  12 ) is moved to the left along the second carousel lane  22  until it is positioned underneath the loader  19  and in the preconditioning system, enabling the second substrate  12  to undergo thermal preconditioning. 
   As shown in  FIG. 4D , the second chuck  5  is moved in the x-direction out of the second carousel lane  22  and into the first carousel lane  21  behind the first chuck  4 . The second chuck  5  is then moved relatively rapidly in the y-direction along the first carousel lane  21  to close the gap to the first chuck  4 , as shown in  FIG. 4E . 
   As shown in  FIG. 4F , when the second chuck  5  reaches the metroframe  13  it starts moving slowly (at the same speed as the first chuck  4 ) so that alignment and metrology of the second chuck  5  can begin even as the exposure of the first substrate  11  is completed. 
   As shown in  FIG. 4G , as soon as the exposure of the first substrate  11  is complete, the first chuck  4  moves relatively rapidly in the y-direction along the first carousel lane  21 . Exposure of the second substrate  12  begins. 
   As shown in  FIG. 4H , the first chuck  4  moves in the x-direction from the first carousel lane  21  to the second carousel lane  22  so as to get out of the path of the second chuck  5  as it moves beneath the metroframe  13 . Exposure of the second substrate  12  continues. 
   As shown in  FIG. 4I , the first chuck  4  now returns in the y-direction along the second carousel lane  22  towards the loader  19  and unloader  20 . Exposure of the second substrate  12  continues. 
   As shown in  FIG. 4J , when the first chuck  4  reaches the unloader  20 , the unloader  20  is lowered so that the top of the unloader  20  is level with the top of the first chuck  4 . The first substrate  11  is then transferred from the first chuck  4  to the unloader  20 . From there it is transferred off the apparatus using a robot (not shown). 
   A third substrate  23  is then introduced onto the loader  19  as shown in  FIG. 4K , in a manner analogous to that shown in  FIG. 4A . The loader  19  is then lowered so that the top of the loader  19  is level with the top of the first chuck  4 . The third substrate  23  is then transferred to the first chuck  4  and the cycle begins again, with the first chuck  4  carrying the third substrate  23  to the metroframe  13  while the second substrate  12  is exposed. 
     FIG. 5  shows part of the apparatus of  FIG. 2 , according to one embodiment of the present invention. 
   Consideration must be given to the y-separation K of the chucks  4 ,  5  when they are both in the first carousel lane  21 , as shown in  FIG. 5 . In  FIG. 4F  the second chuck  5  moves under the metroframe  13  for metrology to begin even as exposure on the first substrate  11  is completed. This is the point at which the chucks are closest together and they are separated by K min , which is about 50 mm, for example. 
   At the point shown in  FIG. 4G , the first chuck  4  should be sufficiently far away from the second chuck  5  so as to move into the second carousel lane  22  as shown in  FIG. 4H  without impeding the second chuck  5  as it moves under the metroframe  13 . 
   Thus, it can be seen that at least one substrate is under the metroframe  13  being exposed substantially all (or very nearly all) of the time. 
   In one example, it is possible to improve the efficiency of the apparatus using an additional metrology system (not shown) in the second carousel lane  22 . This allows for some preparatory metrology to be performed on the second substrate  12  before the second chuck  5  is moved to the first carousel lane  21 . In one example, the additional metrology system can include, for example, a level sensor. 
   In the embodiment described above, it can be seen that the chucks  4 ,  5  essentially follow a circular path. The substrates  11 ,  12 ,  23  are loaded and unloaded from the same side of the apparatus (the left hand end in  FIGS. 2-6 ). Such an apparatus is known as a “front load/unload” apparatus. 
     FIGS. 6 and 7  show a plan view and a side view, respectively, of a lithographic apparatus, according to one embodiment of the present invention. These figures show an “inline” apparatus that allows substrates to be loaded into the apparatus from one end of the apparatus and unloaded at the other end. 
   The apparatus shown in  FIGS. 6 and 7  is similar to that of  FIGS. 2 ,  3 ,  4 , and  5  and is represented at a stage corresponding to the stage shown in  FIG. 4K . As in  FIG. 4K , the first substrate  11  has been exposed and unloaded from the first chuck  4  to the unloader  20 . The second substrate  12  is in the process of being exposed. A third substrate  23  has been placed on the loader  19  by an external robot. 
   The apparatus of  FIGS. 6 and 7  differs from  FIGS. 2 ,  3 , and  4  in that it also comprises an additional unloader  24  ( FIG. 6 ) having a fixed z-position. Rather than unload the first substrate  11  from the unloader  20  out of the apparatus, the unloader  20  is raised to the level of the additional unloader  24  and the first substrate  11  is transferred from the unloader  20  to the additional unloader  24 . The first substrate  11  is then unloaded from the apparatus from the additional unloader  24 . This extra step takes place in the second carousel lane  22 . 
   It is typically more difficult to unload directly from one of the chucks to an unloader at the right hand end of the apparatus because of the projecting nature of the metrology features  9 ,  10 . The substrates  11 ,  12 ,  23  are thus optionally loaded onto and unloaded from the chucks  4 ,  5  from the left hand side, because of the presence of these metrology features. 
   Because the loader  19 , unloader  20  and additional unloader  24  are all located in the second carousel lane  22 , the length of the apparatus does not need to be increased to accommodate the “inline” loading and unloading of the apparatus. For inline apparatuses having a single chuck, the placement of the unloader  24  at the rear of the machine results in the relatively very long machine (approximately three times the length of the chuck). Inline apparatuses having two chucks are approximately twice the length of the chuck. Furthermore, the length of a “two chuck” front load/unload apparatus is equal to or longer than that of a “single chuck” front load/unload apparatus. Therefore, the “footprint,” i.e., the area of the base frame, of front load/unload and inline apparatuses having two chucks is comparable with apparatuses with a single chuck. Typical substrates have widths and lengths of up to and above about 2.5 m. The width of an apparatus having two chucks (and two carousel lanes) is twice as large as that of an apparatus having a single chuck. Because single chuck inline apparatuses are so long, the total footprint of a two chuck inline apparatus is approximately 50% larger than that of a single chuck inline apparatus. 
   While specific embodiments of the invention have been described above, it will be appreciated that the invention can be practiced otherwise than as described. The description is not intended to limit the invention. For example, the embodiments described all relate to maskless lithography, in which an array of individually controllable elements is used to impart a pattern to the beam. However, it will be appreciated that the invention can equally well be applied to a lithographic apparatus operating with a fixed mask or reticle in place of the array of controllable elements. 
   Furthermore, the chucks  4 ,  5  have been described as being mounted via SS actuators  8  on LS frames  6 ,  7 , themselves mounted on an LS magnet plate  3  using a planar drive. It will be appreciated that there are many possible ways to mount the chucks on the base frame to provide the necessary two or three degrees of freedom. In one example, a planar drive can be used which has a stationary coil and moving magnets, which reduces the problem of cabling in a carousel. In another example, the LS frames can be supported in the z-direction by bearings, such as fluid or air bearings or ball bearings, or by an electromagnetic force, and provided with standard LS actuators for movement in the x-y direction. In a further example, the chucks can be mounted on the base plate via a single stroke drive. 
   In one example, the chucks can be driven by one-dimensional “intra-lane” and “inter-lane” drives. The intra-lane drives push the chucks along the carousel lanes in the ±y-direction, and the inter-lane drives push the chucks between the carousel lanes in the ±x-direction. This provides another solution to the cabling problem by allowing the chucks to move in a circle when the drives do not. 
   In one example, although the loader  19  and unloader  20  are shown in  FIGS. 2-7  as being located one above the other, a number of other arrangements can be envisaged. Either or both of these components can be located anywhere in the second carousel lane  22 , allowing substrates to be loaded and/or unloaded from a variety of locations. 
   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.