WO2001040875A1

WO2001040875A1 - Dual-stage lithography apparatus and method

Info

Publication number: WO2001040875A1
Application number: PCT/US2000/042336
Authority: WO
Inventors: Stephen Roux; Todd Joseph Bednarek
Original assignee: Silicon Valley Group, Inc.
Priority date: 1999-11-30
Filing date: 2000-11-29
Publication date: 2001-06-07
Also published as: AU3271801A

Abstract

A dual-stage lithography apparatus having increased throughput is disclosed. The apparatus is capable of simultaneous data collection and exposure functions. An embodiment of the disclosed apparatus (100) includes first and second substrate stages (110, 120), each with a corresponding load/unload and data collection stations (140, 150). The apparatus includes a projection optics station (160) located between the load/unload and data collection stations associated with each stage. In this way, data collection of a second stage occurs simultaneously with exposure of the first stage. Once both processes are completed, the two stages move linearly, allowing a simple and fast transition of stages below the exposure apparatus. Also disclosed is a method of performing lithography in which a first substrate is exposed while a second wafer is simultaneously aligned.

Description

Dual-Stage Lithography Apparatus and Method

Background of the Invention

Field of the Invention

This invention relates to an improved lithography apparatus. More specifically, this invention relates to a lithography apparatus capable of high throughput, as well as a method of performing lithography.

Related Art

Lithography is a process used to create features on the surface of substrates. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. While this description is written in terms of a semiconductor wafer for illustrative puφoses, one skilled in the art would recognize that this description also applies to other types of substrates known to those skilled in the art. During lithography, a wafer, which is disposed on a wafer stage, is exposed to an image projected onto the surface of the wafer by exposure optics located within a lithography apparatus. While exposure optics are used in the case of photolithography, a different type of exposure apparatus may be used depending on the particular application. For example, x-ray, ion, electron, or photon lithographies each may require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only.

The projected image produces changes in the characteristics of a layer, for example photoresist, deposited on the surface of the wafer. These changes correspond to the features projected onto the wafer during exposure. Subsequent to exposure, the layer can be etched to produce a patterned layer. The pattern corresponds to those features projected onto the wafer during exposure. This patterned layer is then used to remove exposed portions of underlying structural layers within the wafer, such as conductive, semiconductive. or insulative layers. This process is then repeated, together with other steps, until the desired features have been formed on the surface of the wafer.

As should be clear from the above discussion, the accurate location and size of features produced through lithography is directly related to the precision and accuracy of the image projected onto the wafer by the exposure optics. While the precision and accuracy of lithography tools are related to many factors, two factors stand out as being particularly important: alignment accuracy and optical precision.

Substrate alignment (sometimes referred to as characterization) is the process in which data is collected by an overhead alignment camera for the purposes of, for example, target mapping and wafer flatness mapping as well as other calibration functions. As used herein, the term data collection is meant to encompass all types of wafer alignment and calibration functions. Thus, while particular types of data collection are described, anything that aids the process of superimposing a subsequent layer more accurately onto the substrate is covered by the phrase data collection as used herein. This data is then used by the exposure optics to ensure that the projected images are accurately focused and projected onto the correct location of the wafer. Once aligned, the wafer is then exposed under the projection optics. The highly sophisticated projection optics include such components as may be necessary in, for example, step-and-scan type tools.

Step-and-scan technology works in conjunction with a projection optics system that has a narrow imaging field. This narrow imaging field is unable to project the entire reticle field onto the wafer at once and therefore requires that the wafer and reticle be simultaneously scanned across the imaging area to allow the full reticle pattern to be exposed on the wafer. Further, the wafer stage must be asynchronously stepped between field exposures to allow multiple copies of the reticle pattern to be exposed over the wafer surface. In this manner, the sharpness of the image projected onto the wafer is maximized. Through increases in both alignment precision and projection accuracy, today's lithography tools are capable of producing devices with ever decreasing minimum feature size. However, minimum feature size is but one measure of a lithography tool's utility. Another critical measure is throughput.

Throughput refers to the number of wafers per hour that can be processed by a lithography tool. The time needed to process a given wafer includes both overhead time and exposure time. Overhead time refers to the time needed to perform data collection as well as the loading and unloading of the wafers at the wafer stage. Exposure time refers to the time the wafer is located under the exposure optics.

In an effort to increase throughput, device manufacturers sometimes sacrifice precision. For example, a compromised wafer alignment strategy is often used. In such a strategy, the alignment time is lowered by making fewer measurements. The shorter time taken to measure alignment results in the process of exposure occurring sooner than if more alignment data were recorded. This strategy thus increases throughput. Such throughput increases, however, come at the cost of decreased accuracy. One method of increasing lithography tool throughput has been suggested by McEachern et al. in U.S. Patent No. 5,677,758 (hereafter "McEachern"). McEachern appears to describe a structure in which overhead time and exposure time can overlap through the use of lithography tool including a platen large enough to accommodate two substrate stages. The stages move in a rotating fashion about the platen so that one stage is subject to exposure while the other stage is subject to alignment. McEachern' s structure appears to include a single exposure station as well as a single load, unload, and align station. Thus, in the structure proposed by McEachern, it appears that if one stage ceases to function, the tool cannot be operated. That is, since both stages move in a circular pattern, if one stage stops it is in the way of the remaining stage, halting operation. An alternate method for increasing throughput has been suggested by Loopstra et al. in International Publication WO 98/40791 (hereafter "Loopstra"). Loopstra appears to describe a lithographic tool that includes first and second displacement units as well as first and second object holders. During operation, a wafer on the first obj ect holder undergoes alignment while a second wafer on the second object holder undergoes exposure. Once these steps are complete, the first and second displacement units operate to exchange the first and second object holders so that the wafer that was aligned in the previous step can be exposed while a new wafer is loaded onto the second object holder to undergo alignment. In this way, Loopstra' s structure appears to achieve an increase in throughput since alignment and exposure can overlap in time. However, as understood, Loopstra' s structure requires a time-consuming object holder exchange. Furthermore, it is uncertain whether Loopstra' s structure can operate with only one functioning displacement unit.

Summary of the Invention

The present invention increases lithography tool throughput while simultaneously increasing the volume of alignment data collected through the use of two substrate stages. Each substrate stage has associated load/unload and data collection stations. The load/unload and data collection stations are located on either side of an exposure station. The substrate stages are mounted on a common rail such that as a first stage moves away from the exposure station, a second stage can immediately move in to take its place under the exposure apparatus. Through this arrangement, use of the exposure apparatus is maximized. Because wafer data collection and exposure steps occur in parallel in the instant invention, the compromised wafer alignment strategies sometimes employed to increase throughput need not be used. In fact, the parallel nature of the instant invention allows for greater data collection without a corresponding decrease in throughput.

Furthermore, since the instant invention includes two data collection stations, the complicated wafer exchanges used by the arrangements discussed above under the heading "Related Art" are unnecessary. Furthermore, since each substrate stage works independently of the other substrate stage within its associated load/unload and data collection stations, the lithography tool is capable of operation with only one functioning substrate stage. This results in throughput levels comparable with conventional tools even when the tool of the instant invention has a non-functional substrate stage.

In one embodiment, a lithography apparatus according to the instant invention comprises an exposure station and a plurality of substrate stages, each of the substrate stages having an associated data collection station separate from a data collection station associated with other of the plurality of substrate stages.

Each of the plurality of substrate stages is movable from the associated data collection station to the exposure station.

During operation, each of the plurality of substrate stages is alternately moved from its associated data collection station to the exposure station such that data collection of a first of the plurality of substrate stages can occur at the same time a second of the plurality of substrate stages is undergoing exposure at the exposure station.

The lithography apparatus can further be characterized as including first and second data collection cameras disposed over first and third positions within the apparatus. The exposure apparatus being disposed over a second position within the lithography apparatus. The first and second substrate stages being movable from the first position to said second position and from the third position to the second position, respectively.

The disclosed lithography apparatus can further be characterized as comprising a rail, to which the first and second substrate stages are movably mounted. In order to optimize data collection accuracy without compromising throughput, the rail can be of a length such that each data collection station overlaps with the exposure station. Such an apparatus would include a stage controller capable of controlling correlated stage movement. Also disclosed is a method of performing lithography. In one embodiment of the disclosed method, a first substrate is aligned at a first location. Next, the first substrate is exposed at an exposure station. In the disclosed method, while the first substrate is being exposed, a second substrate is aligned at a second location. Once data collection of the second substrate is complete, the second substrate is exposed at the exposure station after the first substrate is moved away from the exposure station.

Brief Description of the Figures

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 pertinent art to make and use the invention. Like reference numbers refer to like elements within the different figures.

Figures 1 A and IB illustrate first and second views of an embodiment of the present invention.

Figure 2 is a diagram spatially illustrating the exposure apparatus and data collection stations of an embodiment of the present invention.

Figures 3A and 3B are flow charts illustrating steps in a lithography process according to the present invention.

Detailed Description of the Preferred Embodiments

While this description is written in terms of a semiconductor wafer for illustrative purposes, one skilled in the art would recognize, given this disclosure, that this description also applies to other types of substrates known to those skilled in the art. Such substrates can include those used in the manufacture of flat panel displays, circuit boards, various integrated circuits, and the like. A frequently used substrate for such applications is a semiconductor wafer. Figures 1 A and IB illustrate first and second views of an embodiment of the present invention. Data collection and exposure structure 100 includes a first wafer stage 1 10 and a second wafer stage 120. The first and second wafer stages are depicted in the figures as having wafers 1 1 1 and 121 mounted thereon. Wafer stage 1 10 is mounted via sub-stages 1 12 and 1 13 to rail 130. Sub-stage 1 13 is movably mounted to sub-stage 1 12 to permit stage movement in a direction perpendicular to rail 130. Though not shown, substages 1 12 & 1 13 can include components of a linear brushless motor of the type known to those skilled in the art to effectuate this movement. Motors 131 and 132 propel sub-stage 1 13 along the rail 130. Motors 131 and 132 can also be linear brushless motors of the type known to those skilled in the art. Likewise, wafer stage 120 is mounted to rail 130 via sub-stages 122 and 123. Motors 131 and 132 also propel sub-stage 123 along rail 130. As with sub-stages 112 & 113, an additional motor components are included within sub-stages 122 & 123 to effectuate stage movement in a direction perpendicular to rail 130. Furthermore, interferometers (not shown) are disposed within the structure to accurately determine the location of wafer stages 1 10 and 120 on rail 130 and along an axis perpendicular to 130. These interferometers work together with a control system, discussed below, to control stage movement.

In a preferred embodiment, wafer stages 1 10 and 120 can each comprise a precision stage. Such a precision stage is capable of small and highly accurate motions relative to the sub-stage on which the wafer stage is mounted. These motions may include up to six degrees of freedom. These degrees of motion include motion in directions parallel to an X, Y, and Z axis as well as pivotable motion about axes parallel to each of the X, Y, and Z axes. The implementation of such a precision stage within the instant invention is within the level of skill in the art given this disclosure. Data collection and exposure structure 100 works together with first and second data collection cameras 140 and 150, respectively. These cameras are mounted to a structure separate from the data collection and exposure apparatus. These data collection cameras are of the type known to those skilled in the art as being capable of data gathering for calibration functions such as wafer alignment target mapping and wafer flatness mapping. The first and second data collection cameras are mounted above regions referred to herein respectively as first and second data collection stations. The term data collection station is meant to refer to a region along rail 130 where wafer data collection occurs during operation and is not meant to be limited a single particular wafer stage location within the structure. The data collection station associated with each data collection camera is larger in area than its associated wafer stage since each wafer stage moves within its associated data collection station during the data collection process. Data collection cameras 140 and 150 communicate with the control system, discussed below.

Data collection and exposure structure 100 further works together with exposure apparatus 160. While exposure optics are used in the case of photolithography, a different type of exposure apparatus may be used depending on the particular application. For example, x-ray, ion, electron, or photon lithographies each may require a different exposure apparatus, as is known to those skilled in the art. The particular example of photolithography is discussed here for illustrative purposes only. Exposure optics 160 are mounted to the same structure, separate from the data collection and exposure apparatus, to which data collection cameras 140 and 150 are mounted, as discussed above. Exposure optics are of the type known to those skilled in the art as being capable of lithographic exposure functions. These exposure optics can include, for example, components and functionality for use in step-and-scan type tools as well as step and repeat tools where the full reticle field is exposed without scanning. Exposure optics 160 is disposed above a region referred to herein as the exposure station. The term exposure station is meant to refer to a region along rail 130 where wafer exposure occurs during operation and is not meant to be limited to a singe particular wafer stage location within the structure. The exposure station is larger in area than a single one of the wafer stages since the wafer stage being exposed moves within the exposure station during the wafer exposure process. The exposure station is located between the first and second data collection stations. While the first and second data collection stations are separated by the exposure station, each of the data collection stations can overlap with the exposure station. This is discussed more fully below in connection with Figure 2. Further included within the lithography apparatus of the instant invention, though not shown in the figures, is a load/unload robot. The load/unload robot can be located to the side of rail 130. The load/unload robot may include and extendable arm capable of loading a wafer, or other substrate, onto either of the first and second wafer stages when these stage are located within associated, respective, first and second load/unload stations. Alternatively, two load/unload robots may be included within the lithography apparatus, one for each of the associated wafer stages. As used herein, the term load/unload station is meant to refer to a location along the rail 130 where a wafer, or other substrate, can be loaded onto, and unloaded from, an associated wafer stage. The precise location of each of the first and second load/unload stations is not critical, as long as the they are located in regions reachable by the load/unload robot. Furthermore, the first and second load/unload stations can overlap partially, or completely, with the associated first and second data collection stations, discussed above.

Also located within the lithography apparatus of the instant invention, though not shown in the figures, is a control system. The control system receives location information from highly accurate sensors located within the apparatus.

In a preferred embodiment these sensors comprise interferometers, though one skilled in the art would recognize that other position feedback devices could be used without departing from the scope of this disclosure. For example, linear encoders could be used to provide location information. This location information is used by the control system to monitor the position on rail 130 where each wafer stage is located at any given moment. The control system further receives data from the first and second data collection cameras for performing the necessary alignment functions. During operation, the control system controls stage motion and can control precision stage motion if such precision stages are included in the apparatus. Likewise, the control system also controls the motors in the sub-stages that serve to move the stages in a direction perpendicular to the rail. The control syste further includes collision avoidance functions. Such collision avoidance functions are included because both wafer stages can occupy the same location along the rail at different times during operation, as discussed more fully below. The control system prevents collisions between the wafer stages as they move along the rail. The particular design and implementation of the control system is not critical and can be done in any number of ways which are within the level of ordinary skill in the art, given this disclosure. For example, the collision avoidance function can ensure a specific separation between the wafer stages during operation. Given this disclosure, one skilled in the art could determine an appropriate collision avoidance separation depending on the specific application.

As discussed above, interferometers can be used in conjunction with the control system to measure the precise location of the wafer stages at any given time. The interferometers use lasers to measures distances with high levels of accuracy and are of the type known to those skilled in the art. The error associated with these interferometers, while small, is related to the distance measured. At greater distances, the error increases. Thus, the alignment accuracy is related to the overall distances the stages travel during operation. This relationship is not limited to structures which include interferometers but exists whenever position error increase with the distance measured. Based on the above discussion, it is apparent that there can be a trade-off between alignment accuracy and stage mobility. A longer rail will provide greater room within which the stages can move. However, a longer rail may reduce the alignment accuracy of the lithography apparatus by increasing associated interferometer error. The instant inventors have discovered that a structure which includes a rail too short for complete stage movement freedom can be used together with a correlated stage movement technique in order to achieve optimal lithography apparatus performance. A spatial diagram of such an arrangement is shown in Figure 2.

Figure 2 is a diagram spatially illustrating the exposure and data collection stations of an embodiment of the present invention. The figure illustrates exposure apparatus 200. Below exposure apparatus 200 is a region corresponding to the exposure station 210. Below this region are regions corresponding to first and second data collection stations 230 and 240, respectively. Furthermore,

Figure 2 includes a representation of a typical substrate diameter 220. It should be noted that the instant invention is scalable to accommodate any desired substrate size or shape. For example, given this disclosure, one skilled in the art would understand that the instant invention would be equally advantageous for

100 mm, 150 mm, 200 mm, and 300 mm wafers. Returning to Figure 2 the data collection stations each overlap partially with the exposure station. Such an overlap means that the overall length of the rail used can be reduced by an amount corresponding the sum of the lengths of the overlapping regions. In other words, without overlapping the data collection stations with the exposure station, the rail would need to be as long as the sum of the lengths of the data collection and exposure stations. By reducing rail length through the use of overlapping regions, the effects of interferometer error can be reduced. However, shorter rail length means that collisions between stages are possible while a first stage is at its respective data collection station and a second stage is at the exposure station.

For example, as can be seen from Figure IB, in the preferred embodiment of the instant invention, a collision could occur while the stage subject to exposure remained within the exposure station and the stage subject to data collection remained at the data collection station. For example, if the first stage 1 10 were moved to the far right in order to expose a left edge region of the first wafer 111 while the second stage 120 moved to the far left in order to gather data at a right edge region of the second wafer 121, the two stages would collide. Such possible collisions are avoided through the use of correlated stage movement. Correlated stage movement is meant to refer to the movement of a first stage that is related to the location and movement of a second stage within the lithography apparatus. While the throughput improvements realized by the instant invention are achieved through simultaneous data collection and exposure operations, these improvements are possible without complete stage movement freedom. By correlating the movements of the stages, a shorter rail can be used. as discussed above. Stage movement during operation is correlated such that the two stages can move somewhat in step with each other along the length of the rail if this is necessary to avoid a collision. The stage located under its corresponding data collection camera is moved to accommodate the stage undergoing exposure thereby optimizing the exposure function which, in turn, increases throughput. Correlated stage movement is controlled by the control system discussed above.

Much like the collision avoidance function, the correlated stage movement function of the control system is within the level of skill in the art given this disclosure. Furthermore, the instant inventors have discovered that such correlation can take place without decreasing wafer throughput. Thus, a shorter rail can be used thereby increasing alignment precision while still benefitting from plural wafer stages.

Figure 3A is a process flow diagram illustrating the process upon initial start-up of the lithography apparatus according to an embodiment of the instant invention. In Figure 3A, the first and second wafer stages are referred to as stage A and stage B, respectively. Upon start-up, neither wafer stage is loaded with a wafer. Thus, during a first step 310 a wafer is loaded onto stage A. Once the first wafer is loaded, the load/unload robot can then load a second wafer onto stage B, while the first wafer undergoes data collection and is moved to the exposure station, as illustrated in a step 320. In the same step, data collection of the second wafer proceeds while the first wafer is moved to the exposure station. Once stage

A has been moved to the exposure station, wafer 1 can be exposed while data collection of the second wafer continues at stage B, as illustrated in a step 330. Once exposure of the first wafer and data collection of the second wafer is complete, stage A moves away from the exposure station, as illustrated in a step 340. In the same step, stage B moves to the exposure station. In a next step 350, the first wafer is unloaded from stage A and a third wafer is loaded onto stage A, while the second wafer undergoes exposure at the exposure station. In the same step, data collection of the third wafer begins while the second wafer remains at the exposure station. Once start-up is complete, and the lithography process in underway, wafers on stages A and B continue to be unloaded, loaded, and aligned while making efficient use of the exposure optics. Figure 3B is a process flow diagram illustrating a series of repeating steps that occur during a lithography process according to the instant invention. The steps illustrated in Figure 3B involve four wafers from within a continuing process numbered: n-2, n-1 , n, and n+1. In a first step 360, wafer n-1 is exposed on stage A while the previously exposed wafer, n-2, is unloaded from stage B. While the exposure of wafer n-1 is ongoing, the next wafer, n, is loaded onto stage B where data collection occurs. Once data collection of wafer n and exposure of wafer n- 1 is complete, stage A moves away from the exposure station while stage B moves to the exposure station, as illustrated in a step 370. In a next step 380, the second wafer, n-1 , is unloaded from stage A and the fourth wafer, n+1, is loaded onto stage A where data collection takes place. Meanwhile, in the same step, wafer n is exposed at the exposure station. Finally, in a last illustrated step 390, stage A is moved to the exposure station and stage B is moved away from the exposure station. Once this step is complete, there is a wafer, n+1 , located at the exposure station ready for exposure, and there is an already exposed wafer, n, located on stage B ready for unloading. This returns the process to the situation shown in the first step 360. The process thus continues with subsequent wafers.

As can be seen in Figure 3B, the step of exposing a wafer on one of the stages occurs simultaneously with the unloading of a previous wafer and the loading and data collection of a subsequent wafer on a second stage, step 360, for example. As can further be seen from the process flow of Figure 3B, if one stage is inoperative, the other stage can continue to move through the process. Either stage can move through the complete process so long as the other station is positioned away from the exposure station. This allows the lithography apparatus of the instant invention to operate with the same throughput as a conventional single-stage lithography apparatus.

As can further be seen from Figure 3B, lithography throughput can be increased over a conventional single-stage lithography apparatus by virtue of the parallel data collection and exposure capabilities of the instant invention. In a conventional structure, loading, data collection, exposure, and unloading occur in series. Accordingly, throughput time is based, in part, on the sum of the times required to complete these steps. As discussed above, data collection time is a function of the desired precision of a particular lithographic process. As precision increases, so to does data collection time. In a conventional lithographic apparatus, increased data collection time means decreased throughput. In the instant invention, throughput is not directly related to data collection time. Thus, as lithographic precision increases, the throughput improvement realized by the present invention will also increase.

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. For example, while the invention has been described in terms of a wafer, one skilled in the art would recognize that the instant invention could be applied to any type of substrate used in a lithography process. It will be understood by those skilled in the art that various changes in form and details can be made therein without departing from the spirit and scope of the invention as defined in the appended claims. 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.

Claims

What Is Claimed Is:

1 . A lithography apparatus, comprising: an exposure station; and a plurality of substrate stages, each of said substrate stages having an associated data collection station separate from an data collection station associated with other of said plurality of substrate stages.

2. The lithography apparatus of claim 1 , wherein said plurality of substrate stages comprise precision substrate stages.

3. The lithography apparatus of claim 1 , wherein each of said plurality of substrate stages is movable from said associated data collection station to said exposure station.

4. The lithography apparatus of claim 3, wherein during operation each of said plurality of substrate stages is alternately moved from said associated data collection station to said exposure station such that data collection of a first of said plurality of substrate stages can occur at the same time a second of said plurality of substrate stages is undergoing exposure at said exposure station.

5. The lithography apparatus of claim 1 , wherein each data collection station corresponding to a respective one of said substrate stages overlaps in area with said exposure station.

6. The lithography apparatus of claim 1 , wherein the apparatus is configured to permit a substrate-loading-robot having an extendable arm to load a first substrate onto said first substrate stage at a first load/unload station and a second substrate onto said second substrate stage at a second load/unload station.

7. The lithography apparatus of claim 6, wherein said first and second load/unload stations are located in a common plane and separated by a third location, said exposure station located at said third location.

8. The lithography apparatus of claim 5, wherein said first and second substrates are semiconductor wafers.

9. A lithography apparatus, comprising: a first data collection camera disposed over a first position within the apparatus; an exposure apparatus disposed over a second position within the apparatus; a second data collection camera disposed over a third position within the apparatus; a first substrate stage movable from said first position to said second position; and a second substrate stage movable form said third position to said second position.

10. The lithography apparatus of claim 9, wherein said exposure apparatus comprises exposure optics for use in a photolithography process.

1 1. The lithography apparatus of claim 9, further comprising a rail, said first and second substrate stages movably mounted to said rail.

12. The lithography apparatus of claim 1 1, said rail comprising a first data collection station corresponding to a region in which said first stage must have freedom of movement such that said first data collection camera can view a substantial entirety of a substrate loaded onto said first substrate stage; a second data collection station corresponding to a region in which said second stage must have freedom of movement such that said second data collection camera can view a substantial entirety of a substrate loaded onto said second substrate stage; and an exposure station corresponding to a region in which a stage subject to exposure must have freedom of movement such that said exposure apparatus can expose a substantial entirety of a substrate loaded onto said stage subject to exposure.

13. The lithography apparatus of claim 12, wherein said first data collection station overlaps with said exposure station.

14. The lithography apparatus of claim 13, wherein said second data collection station overlaps with said exposure station.

15. The lithography apparatus of claim 12, wherein: said first data collection station has a first length measured along the length of said rail; said second data collection station has a second length measured along the length of said rail; and said exposure station has a third length measured along the length of said rail; said rail having a total length less than the sum of said first, second and third lengths.

16. A method of performing lithography comprising the steps of:

(a) aligning a first substrate at a first data collection station;

(b) exposing said first substrate at an exposure station; and (c) aligning a second substrate at a second data collection station while performing step (b).

17. The method of claim 16, further comprising the step of exposing said second substrate at said exposure station after said first substrate is moved away from said exposure station.

18. The method of claim 16, wherein the first and second substrates are semiconductor wafers.