WO2013029893A2

WO2013029893A2 - Lithographic system, method of controlling a lithographic apparatus and device manufacturing method

Info

Publication number: WO2013029893A2
Application number: PCT/EP2012/064755
Authority: WO
Inventors: Pieter De Jager; Arno Bleeker; Arie Den Boef; Erik Loopstra; Nitesh PANDEY
Original assignee: Asml Netherlands B.V.
Priority date: 2011-08-30
Filing date: 2012-07-27
Publication date: 2013-03-07
Also published as: TWI490665B; JP5793248B2; KR101558445B1; WO2013029893A3; KR20140041847A; NL2009239A; TW201316131A; JP2014527724A

Abstract

A lithographic system has a lithographic apparatus, an inspection system and a controller. The lithographic apparatus includes a projection system configured to project a radiation beam onto a layer of material on or above a substrate. The inspection system is configured to inspect a pattern formed on the substrate. The pattern is formed on the substrate by application of the radiation beam. The controller is configured to control the lithographic apparatus to form a pattern based on data from an inspection by the inspection system of a previously exposed pattern.

Description

LITHOGRAPHIC SYSTEM, METHOD OF CONTROLLING A LITHOGRAPHIC APPARATUS AND DEVICE MANUFACTURING METHOD

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of US provisional application 61 /529,064, which was filed on August 30, 201 1 and which is incorporated herein in its entirety by reference. This application also claims the benefit of US provisional application

61 /546,801 , which was filed on October 13, 201 1 and which is incorporated herein in its entirety by reference. And also claims the benefit of US provisional application

61 /651 ,449, which was filed on May 24, 2012 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a lithographic system, a method of controlling a lithographic apparatus and a method for manufacturing a device.

BACKGROUND

[0003] A lithographic apparatus is a machine that applies a desired pattern onto a substrate or part of a substrate. A lithographic apparatus may be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays and other devices or structures having fine features. In a conventional lithographic apparatus, a patterning device, which may be referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, flat panel display, or other device).

[0004] Instead of a circuit pattern, the patterning device may be used to generate other patterns, for example a color filter pattern, or a matrix of dots. Instead of a conventional mask, the patterning device may comprise a patterning array that comprises an array of individually controllable elements that generate the circuit or other applicable pattern. An advantage of such a "maskless" system compared to a conventional mask-based system is that the pattern can be provided and/or changed more quickly and for less cost. [0005] Thus, a maskless system includes a programmable patterning device (e.g., a spatial light modulator, a contrast device, etc.). The programmable patterning device is programmed (e.g., electronically or optically) to form the desired patterned beam using the array of individually controllable elements. Types of programmable patterning devices include micro-mirror arrays, liquid crystal display (LCD) arrays, grating light valve arrays, arrays of self-emissive contrast devices and the like.

SUMMARY

[0006] A maskless lithographic apparatus may be provided with, for example, an optical column capable of creating a pattern on a target portion of a substrate. The optical column may, for example, be provided with a self-emissive contrast device configured to emit a beam and a projection system configured to project at least a portion of the beam toward the target portion. The apparatus may be provided with an actuator system to move the substrate with respect to the optical column or a part thereof. Thereby, the beam may be moved with respect to the substrate. By switching "on" or "off" the self-emissive contrast device during the movement, a pattern on the substrate may be created. The pattern may be transferred on (part of) the substrate (e.g. silicon wafer or a glass plate), e.g. via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate, or by local deposition of droplets of material (e.g., metal).

[0007] In a lithographic process, the devices produced by the lithographic apparatus should be of sufficient quality. Defects in the product can correspond to discrepancies between the pattern that is intended to be created and the pattern that is actually created on the target portion of the substrate. In some cases (e.g. manufacture of flat panel displays), these defects may cause an undesirable intensity variation across the screen. If the product is of insufficient quality, there is a reduction in yield of the manufacturing process.

[0008] It is therefore desirable, for example, to provide a lithographic system capable of manufacturing devices with fewer defects.

[0009] According to an embodiment of the invention, there is provided a lithographic system comprising a lithographic apparatus, an inspection system and a controller. The lithographic apparatus comprises a projection system configured to project at least one radiation beam onto a layer of material on or above a substrate. The inspection system is configured to inspect a pattern formed on the substrate. The pattern is formed on the substrate by application of the at least one radiation beam. The controller is configured to control the lithographic apparatus to form a pattern based on an inspection by the inspection system of a previously exposed pattern.

[0010] According to an embodiment of the invention, there is provided a method of controlling a lithographic apparatus. The method comprises projecting at least one radiation beam onto a layer of material on or above a substrate, inspecting a pattern formed on the substrate, wherein the pattern is formed on the substrate by application of the at least one radiation beam, and controlling the lithographic apparatus to form a pattern based on an inspection of a previously exposed pattern.

BRI EF DESCRI PTION OF THE DRAWINGS

[0011] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which :

[0012] - Figure 1 depicts a part of a lithographic apparatus according to an

embodiment of the invention ;

[0013] - Figure 2 depicts a top view of a part of a lithographic system according to an embodiment of the invention ;

[0014] - Figure 3 depicts a highly schematic, perspective view of a part of a lithographic apparatus according to an embodiment of the invention ;

[0015] - Figure 4 depicts a schematic top view of projections by the lithographic apparatus according to Figure 3 onto a substrate according to an embodiment of the invention ;

[0016] - Figure 5 depicts a top view of a part of a lithographic system according to an embodiment of the invention ;

[0017] - Figure 6 depicts a side view of a material deposition apparatus and process;

[0018] - Figure 7 depicts a plan view of an inspection process according to an embodiment of the invention ; [0019] - Figure 8 depicts an inspection system according to an embodiment of the invention;

[0020] - Figure 9 depicts an alignment sensor according to an embodiment of the invention; and

[0021] - Figure 10 depicts a top view of a part of a lithographic system according to an embodiment of the invention.

DETAILED DESCRIPTION

[0022] Figure 1 schematically depicts a schematic cross-sectional side view of a part of a lithographic apparatus. In this embodiment, the lithographic apparatus has individually controllable elements substantially stationary in the X-Y plane as discussed further below although it need not be the case. The lithographic apparatus 1 comprises a substrate table 2 to hold a substrate, and a positioning device 3 to move the substrate table 2 in up to 6 degrees of freedom. The substrate may be a resist-coated substrate. In an embodiment, the substrate is a wafer. In an embodiment, the substrate is a polygonal (e.g. rectangular) substrate. In an embodiment, the substrate is a glass plate. In an embodiment, the substrate is a plastic substrate. In an embodiment, the substrate is a foil. In an embodiment, the lithographic apparatus is suitable for roll-to-roll manufacturing.

[0023] The lithographic apparatus 1 further comprises a plurality of individually controllable self-emissive contrast devices 4 configured to emit a plurality of beams. In an embodiment, the self-emissive contrast device 4 is a radiation emitting diode, such as a light emitting diode (LED), an organic LED (OLED), a polymer LED (PLED), or a laser diode (e.g., a solid state laser diode). In an embodiment, each of the individually controllable elements 4 is a blue-violet laser diode (e.g., Sanyo model no. DL-3146- 151 ). Such diodes may be supplied by companies such as Sanyo, Nichia, Osram, and Nitride. In an embodiment, the diode emits UV radiation, e.g., having a wavelength of about 365 nm or about 405 nm. In an embodiment, the diode can provide an output power selected from the range of 0.5 - 200 mW. In an embodiment, the size of laser diode (naked die) is selected from the range of 100 - 800 micrometers. In an

embodiment, the laser diode has an emission area selected from the range of 0.5 - 5 micrometers². In an embodiment, the laser diode has a divergence angle selected from the range of 5 - 44 degrees. In an embodiment, the diodes have a configuration (e.g., emission area, divergence angle, output power, etc.) to provide a total brightness more than or equal to about 6.4 x 10⁸ W/(m².sr).

[0024] The self-emissive contrast devices 4 are arranged on a frame 5 and may extend along the Y-direction and/or the X direction. While one frame 5 is shown, the lithographic apparatus may have a plurality of frames 5 as shown in Figure 2. Further arranged on the frame 5 is lens 12. Frame 5 and thus self-emissive contrast device 4 and lens 12 are substantially stationary in the X-Y plane. Frame 5, self-emissive contrast device 4 and lens 12 may be moved in the Z-direction by actuator 7.

Alternatively or additionally, lens 12 may be moved in the Z-direction by an actuator related to this particular lens. Optionally, each lens 12 may be provided with an actuator.

[0025] The self-emissive contrast device 4 may be configured to emit a beam and the projection system 12, 14 and 18 may be configured to project the beam onto a target portion of the substrate. The self-emissive contrast device 4 and the projection system form an optical column. The lithographic apparatus 1 may comprise an actuator (e.g. motor) 1 1 to move the optical column or a part thereof with respect to the substrate. Frame 8 with arranged thereon field lens 14 and imaging lens 18 may be rotatable with the actuator. A combination of field lens 14 and imaging lens 18 forms movable optics 9. In use, the frame 8 rotates about its own axis 10, for example, in the directions shown by the arrows in FIG. 2. The frame 8 is rotated about the axis 10 using an actuator e.g. motor 1 1 . Further, the frame 8 may be moved in a Z direction by motor 7 so that the movable optics 9 may be displaced relative to the substrate table 2.

[0026] An aperture structure 13 having an aperture therein may be located above lens

12 between the lens 12 and the self-emissive contrast device 4. The aperture structure

13 can limit diffraction effects of the lens 12, the associated self-emissive contrast device 4, and/or of an adjacent lens 12 / self-emissive contrast device 4.

[0027] The depicted apparatus may be used by rotating the frame 8 and

simultaneously moving the substrate on the substrate table 2 underneath the optical column. The self-emissive contrast device 4 can emit a beam through the lenses 12, 14, and 18 when the lenses are substantially aligned with each other. By moving the lenses 14 and 18, the image of the beam on the substrate is scanned over a portion of the substrate. By simultaneously moving the substrate on the substrate table 2 underneath the optical column, the portion of the substrate which is subjected to an image of the self-emissive contrast device 4 is also moving. By switching the self-emissive contrast device 4 "on" and "off" (e.g., having no output or output below a threshold when it is "off" and having an output above a threshold when it is "on") at high speed under control of a controller, controlling the rotation of the optical column or part thereof, controlling the intensity of the self-emissive contrast device 4, and controlling the speed of the substrate, a desired pattern can be imaged in the layer of material on the substrate. The pattern formed may be, for example a latent image formed in a layer of photoresist on the substrate, or formed from droplets of material (e.g., metal) locally deposited by application of the radiation beams on the substrate.

[0028] Figure 2 depicts a schematic top view of the lithographic apparatus of Figure 1 having self-emissive contrast devices 4. Like the lithographic apparatus 1 shown in Figure 1 , the lithographic apparatus 1 comprises a substrate table 2 to hold a substrate 17, a positioning device 3 to move the substrate table 2 in up to 6 degrees of freedom, an alignment/level sensor 19 to determine alignment between the self-emissive contrast device 4 and the substrate 17, and to determine whether the substrate 17 is at level with respect to the projection of the self-emissive contrast device 4. As depicted the substrate 17 has a rectangular shape, however also or alternatively round substrates may be processed.

[0029] The self-emissive contrast device 4 is arranged on a frame 15. The self- emissive contrast device 4 may be a radiation emitting diode, e.g., a laser diode, for instance a blue-violet laser diode. As shown in Figure 2, the self-emissive contrast devices 4 may be arranged into an array 21 extending in the X-Y plane.

[0030] The array 21 may be an elongate line. In an embodiment, the array 21 may be a single dimensional array of self-emissive contrast devices 4. In an embodiment, the array 21 may be a two dimensional array of self-emissive contrast device 4.

[0031] A rotating frame 8 may be provided which may be rotating in a direction depicted by the arrow. The rotating frame may be provided with lenses 14, 18 (show in Figure 1 ) to provide an image of each of the self-emissive contrast devices 4. The apparatus may be provided with an actuator to rotate the optical column comprising the frame 8 and the lenses 14, 18 with respect to the substrate.

[0032] Figure 3 depicts a highly schematic, perspective view of the rotating frame 8 provided with lenses 14, 18 at its perimeter. A plurality of beams, in this example 10 beams, are incident onto one of the lenses and projected onto a target portion of the substrate 17 held by the substrate table 2. In an embodiment, the plurality of beams are arranged in a straight line. The rotatable frame is rotatable about axis 10 by means of an actuator (not shown). As a result of the rotation of the rotatable frame 8, the beams will be incident on successive lenses 14, 18 (field lens 14 and imaging lens 18) and will, incident on each successive lens, be deflected thereby so as to travel along a part of the surface of the substrate 17, as will be explained in more detail with reference to Fig. 4. In an embodiment, each beam is generated by a respective source, i.e. a self- emissive contrast device, e.g. a laser diode (not shown in Figure 3). In the arrangement depicted in Figure 3, the beams are deflected and brought together by a segmented mirror 30 in order to reduce a distance between the beams, to thereby enable a larger number of beams to be projected through the same lens and to achieve resolution requirements to be discussed below.

[0033] As the rotatable frame rotates, the beams are incident on successive lenses and, each time a lens is irradiated by the beams, the places where the beam is incident on a surface of the lens, moves. Since the beams are projected on the substrate differently (with e.g. a different deflection) depending on the place of incidence of the beams on the lens, the beams (when reaching the substrate) will make a scanning movement with each passage of a following lens. This principle is further explained with reference to Figure 4. Figure 4 depicts a highly schematic top view of a part of the rotatable frame 8. A first set of beams is denoted by B1 , a second set of beams is denoted by B2 and a third set of beams is denoted by B3. Each set of beams is projected through a respective lens set 14, 18 of the rotatable frame 8. As the rotatable frame 8 rotates, the beams B1 are projected onto the substrate 17 in a scanning movement, thereby scanning area A14. Similarly, beams B2 scan area A24 and beams B3 scan area A34. At the same time of the rotation of the rotatable frame 8 by a corresponding actuator, the substrate 17 and substrate table are moved in the direction D (which may be along the X axis as depicted in Figure 2), thereby being substantially perpendicular to the scanning direction of the beams in the area's A14, A24, A34. As a result of the movement in direction D by a second actuator (e.g. a movement of the substrate table by a corresponding substrate table motor), successive scans of the beams when being projected by successive lenses of the rotatable frame 8, are projected so as to substantially abut each other, resulting in substantially abutting areas A1 1 , A12, A13, A14 (areas A1 1 , A12, A13 being previously scanned and A14 being currently scanned as shown in Figure 4) for each successive scan of beams B1 , resulting in substantially abutting areas A21 , A22, A23 and A24 (areas A21 , A22, A23 being previously scanned and A24 being currently scanned as shown in Figure 4) for each successive scan of beams B2, and resulting in substantially abutting areas A31 , A32, A33 and A34 (areas A31 , A32, A33 being previously scanned and A34 being currently scanned as shown in Figure 4) for each successive scan of beams B3.

Thereby, the areas A1 , A2 and A3 of the substrate surface may be covered with a movement of the substrate in the direction D while rotating the rotatable frame 8. The projecting of multiple beams through a same lens allows processing of a whole substrate in a shorter timeframe (at a same rotating speed of the rotatable frame 8), since for each passing of a lens, a plurality of beams scan the substrate with each lens, thereby allowing increased displacement in the direction D for successive scans.

Viewed differently, for a given processing time, the rotating speed of the rotatable frame may be reduced when multiple beams are projected onto the substrate via a same lens, thereby possibly reducing effects such as deformation of the rotatable frame, wear, vibrations, turbulence, etc. due to high rotating speed. In an embodiment, the plurality of beams are arranged at an angle to the tangent of the rotation of the lenses 14, 18 as shown in Figure 4. In an embodiment, the plurality of beams are arranged such that each beam overlaps or abuts a scanning path of an adjacent beam.

[0034] A further effect of the aspect that multiple beams are projected at a time by the same lens, may be found in relaxation of tolerances. Due to tolerances of the lenses (positioning, optical projection, etc), positions of successive areas A1 1 , A12, A13, A14 (and/or of areas A21 , A22, A23 and A24 and/or of areas A31 , A32, A33 and A34) may show some degree of positioning inaccuracy in respect of each other. Therefore, some degree of overlap between successive areas A1 1 , A12, A13, A14 may be required. In case of for example 10% of one beam as overlap, a processing speed would thereby be reduced by a same factor of 10% in case of a single beam at a time through a same lens. In a situation where there are 5 or more beams projected through a same lens at a time, the same overlap of 10% (similarly referring to one beam example above) would be provided for every 5 or more projected lines, hence reducing a total overlap by a factor of approximately 5 or more to 2% or less, thereby having a significantly lower effect on overall processing speed. Similarly, projecting at least 10 beams may reduce a total overlap by approximately a factor of 10. Thus, effects of tolerances on processing time of a substrate may be reduced by the feature that multiple beams are projected at a time by the same lens. In addition or alternatively, more overlap (hence a larger tolerance band) may be allowed, as the effects thereof on processing are low given that multiple beams are projected at a time by the same lens.

[0035] Alternatively or in addition to projecting multiple beams via a same lens at a time, interlacing techniques could be used, which however may require a comparably more stringent matching between the lenses. Thus, the at least two beams projected onto the substrate at a time via the same one of the lenses have a mutual spacing, and the lithographic apparatus may be arranged to operate the second actuator so as to move the substrate with respect to the optical column to have a following projection of the beam to be projected in the spacing.

[0036] In order to reduce a distance between successive beams in a group in the direction D (thereby e.g. achieving a higher resolution in the direction D), the beams may be arranged diagonally in respect of each other, in respect of the direction D. The spacing may be further reduced by providing a segmented mirror 30 in the optical path, each segment to reflect a respective one of the beams, the segments being arranged so as to reduce a spacing between the beams as reflected by the mirrors in respect of a spacing between the beams as incident on the mirrors. Such effect may also be achieved by a plurality of optical fibers, each of the beams being incident on a respective one of the fibers, the fibers being arranged so as to reduce along an optical path a spacing between the beams downstream of the optical fibers in respect of a spacing between the beams upstream of the optical fibers.

[0037] Further, such effect may be achieved using an integrated optical waveguide circuit having a plurality of inputs, each for receiving a respective one of the beams. The integrated optical waveguide circuit is arranged so as to reduce along an optical path a spacing between the beams downstream of the integrated optical waveguide circuit in respect of a spacing between the beams upstream of the integrated optical waveguide circuit.

[0038] In an embodiment a lithographic system comprises a lithographic apparatus 1 , an inspection system 40 and a controller 500. The lithographic apparatus 1 comprises a projection system 12, 14, 18 configured to project a plurality of radiation beams onto a layer of material on or above a substrate 17.

[0039] In an embodiment the projection system is configured to project a plurality of radiation beams. The controller 500 may be configured to control the angular separation between at least two of the plurality of radiation beams such that the plurality of radiation beams form a pattern on the substrate 17.

[0040] In an embodiment the projection system is configured to project a single radiation beam that is patterned. For example, in an embodiment the lithographic system comprises a programmable patterning device comprising a spatial light modulator configured to provide a patterned radiation beam.

[0041] In an embodiment the projection system projects the at least one radiation beam onto a layer of photoresist on a substrate 17 so as to form a latent image in the layer of photoresist. The latent image may be visible to a camera before further processing of the substrate 17 takes place.

[0042] In an embodiment the projection system projects the at least one radiation beam onto a layer of material above the substrate 17 so as to cause local deposition of droplets of the material (e.g. metal) by a laser induced material transfer.

[0043] Referring to FIG. 6, the physical mechanism of laser induced material transfer is depicted. In an embodiment, a radiation beam 200 is focused through a substantially transparent material 202 (e.g., glass) at an intensity below the plasma breakdown of the material 202. Surface heat absorption occurs on a donor material layer 204 (e.g., a metal film) overlying the material 202. The heat absorption causes melting of the donor material 204. Further, the heating causes an induced pressure gradient in a forward direction leading to forward acceleration of a donor material droplet 206 from the donor material layer 204 and thus from the donor structure (e.g., plate) 208. Thus, the donor material droplet 206 is released from the donor material layer 204 and is moved (with or without the aid of gravity) toward and onto the substrate 17. By pointing the beam 200 on the appropriate position on the donor plate 208, a donor material pattern can be deposited on the substrate 17. In an embodiment, the beam is focused on the donor material layer 204.

[0044] In an embodiment, one or more short pulses are used to cause the transfer of the donor material. In an embodiment, the pulses may be a few picoseconds or femtoseconds long to obtain quasi one dimensional forward heat and mass transfer of molten material. Such short pulses facilitate little to no lateral heat flow in the material layer 204 and thus little or no thermal load on the donor structure 208. The short pulses enable rapid melting and forward acceleration of the material (e.g., vaporized material, such as metal, would lose its forward directionality leading to a splattering deposition). The short pulses enable heating of the material to just above the heating temperature but below the vaporization temperature. For example, for aluminum, a temperature of about 900 to 1000 degrees Celsius is desirable.

[0045] In an embodiment, through the use of a laser pulse, an amount of material (e.g., metal) is transferred from the donor structure 208 to the substrate 17 in the form of 100-1000 nm droplets. In an embodiment, the donor material comprises or consists essentially of a metal. In an embodiment, the metal is aluminum. In an embodiment, the material layer 204 is in the form a film. In an embodiment, the film is attached to another body or layer. As discussed above, the body or layer may be a glass.

[0046] Both in an embodiment in which the radiation beams expose a photoresist layer on the substrate 17 and in an embodiment in which the radiation beams are used for local deposition of droplets of material (e.g. metal) on the substrate 17, a pattern is formed on the substrate by direct application of the at least one radiation beam. No further operation, such as development operation, is required in order to make the pattern visible such that it can be imaged by a camera, for example, or other suitably configured inspection system

[0047] The inspection system 40 is configured to inspect such a pattern formed on the substrate 17, namely a pattern formed on the substrate 17 directly by the application of the at least one radiation beam. In an embodiment the inspection system 40 is configured to capture an image of the pattern formed on the substrate 17. In an embodiment the inspection system 40 comprises a camera array. In an embodiment the inspected pattern is a latent image in a layer of photoresist on the substrate 17, or formed from droplets of material (e.g. metal) locally deposited on the substrate 17.

[0048] The controller 500 is configured to control the lithographic apparatus 1 to form a pattern based on data from by the inspection system 40 resulting from the inspection of a previously exposed pattern. Information (e.g. one or more defects and/or

inaccuracies) about a pattern formed by the lithographic apparatus 1 can be taken into account when the lithographic apparatus 1 forms a subsequent version of that pattern in a subsequent exposure operation. Additionally or alternatively, information (e.g. one or more defects and/or inaccuracies) about a pattern formed by the lithographic apparatus 1 can be taken into account when the lithographic apparatus 1 forms a subsequent version of a different pattern in a subsequent exposure operation. For example, if from the inspection of the pattern it seems that the intensity of the radiation was too low at a certain section of the substrate, e.g. relating to a single one of the optical columns, this information may be taken into account in forming the subsequent different pattern, for example by increasing the intensity of the radiation of this optical column.

[0049] An example of the operation of the lithographic system is provided. The lithographic apparatus forms a pattern on the substrate 17. The inspection apparatus 40 inspects the pattern. The inspection system 40 outputs the inspection information to the controller 500. The controller 500 controls the lithographic apparatus 1 to form a pattern based on the inspection information. The lithographic apparatus 1 can form an iteration of a pattern that at least partially corrects for a defect or inaccuracy in the pattern that it forms. The controller 500 is configured to use the information about the previous iteration of the exposed pattern as a feedback in order to improve the quality of the pattern formed in a subsequent exposure operation. [0050] The improvement of the formation of the pattern by the lithographic apparatus 1 can improve the accuracy of the devices manufactured by the lithographic apparatus 1 . This can increase the percentage of devices that are sufficiently accurate. This can increase the yield of the device manufacturing method using the lithographic apparatus 1 .

[0051] In particular, in the context of manufacturing a flat panel display, the screen should provide a uniform appearance to the viewer. This can be particularly difficult due to variations between the large numbers of optical elements (e.g. lenses) used in the lithographic apparatus 1 . These undesirable variations lead to the pattern formed by the lithographic apparatus 1 on the substrate 17 straying undesirably from the intended form of the pattern. The inaccuracy of the pattern formed by the lithographic apparatus 1 that results from characteristics of the lithographic apparatus 1 are known collectively as the fingerprint of the lithographic apparatus 1 . According to an embodiment of the present invention, the yield can be improved by correcting for the fingerprint of the lithographic apparatus 1 .

[0052] The inspection system 40 can inspect the pattern substantially immediately after the radiation beams have been projected onto the layer of material (e.g., substantially immediately after the exposure operation). The inspection system 40 may be configured to inspect the pattern before further operation, such as a development operation, that substantially changes the nature of the pattern is performed on the substrate 17. The pattern is formed directly by the application of the radiation beams.

[0053] Hence a difference between a lithographic system according to an embodiment and previous lithographic systems is that the inspection system 40 inspects a pattern that is formed by application of the radiation beams. In previous lithographic systems, a metrology system may be used to measure parameters of a layer of an IC, for example. However, the layer of the IC is formed by development and etching of the photoresist, rather than being directly formed by the radiation beams.

[0054] In an EUV lithography or immersion lithography, for example, the radiation beams produce an invisible pattern in the photoresist on the substrate. A photoresist, such as an l-line photoresist, may be used for the fabrication of LCDs, for example. The photoresist may be chemically amplified to increase the sensitivity to the radiation beams. In such processes, the radiation beams only release H⁺ from the photoresist. The chemical bonds of the polymer chains are broken only during the post exposure bake during which the H⁺ attacks the polymer chains at elevated temperature. The release of H⁺ only is hardly visible.

[0055] The direct result of the application of the radiation beams directly after exposure is more visible according to an embodiment of the present invention. The reason is that in an embodiment of the present invention, the radiation beams break chemical bonds directly in polymer chains in the photoresist resulting in a local shrinkage of the photoresist, which is more visible than release of H⁺. In an embodiment of the present invention, the at least one radiation beam (e.g. laser radiation) is used to release droplets of material from a donor plate directly above the substrate 17. Those droplets fall on the substrate 17 resulting in material patterning, which is more visible than release of H⁺.

[0056] In an embodiment the inspection system 40 comprises at least one slit camera. Other types and shapes of camera may be used as part of the inspection system 40. However, use of the slit camera enables the inspection system 40 to inspect the entire pattern efficiently even when the exposed pattern and/or the substrate is relatively large. In an embodiment the at least one slit camera extends substantially perpendicularly to a scanning direction (the X-direction in Figure 2 and Figure 5) of the substrate within the lithographic apparatus. This maximizes the area of the substrate 17 that can be inspected by the slit camera.

[0057] In an embodiment the inspection system 40 comprises a single slit camera that extends along at least the width of the target portion of the substrate 17 (where the pattern is formed) in the direction perpendicular to the scanning direction. In an embodiment the inspection system 40 comprises a plurality of slit cameras that combined extend along at least the width of the target portion of the substrate 17 in the direction perpendicular to the scanning direction. The slit cameras may overlap each other in the direction perpendicular to the scanning direction such that every part of the target portion must pass directly under at least one of the slit cameras during an exposure operation. As depicted in Figure 2 and Figure 5, the inspection system 40 may comprise three slit cameras. However, the number of slit cameras comprised in the inspection system 40 is not particularly limited.

[0058] In an embodiment the inspection system 40 inspects the pattern by using a time delay and integration charge-coupled device (CCD). Use of the time delay and integration CCD may enable high resolution imaging of the pattern formed in the layer on the substrate 17. This high resolution imaging may be possible even with a relatively high speed of the substrate 17 relative to the inspection system 40 during scanning movements. In an embodiment the inspection system 40 comprises at least one camera that measures marks on the substrate 17 that indicate line widths in the exposed pattern.

[0059] In order to correct and/or compensate for the fingerprint, the fingerprint of the lithographic apparatus 1 may be measured. In an embodiment the fingerprint is at least partly determined by a comparison between the inspected pattern and the target pattern. The target pattern is the intended form of the pattern, namely the pattern that would be inspected by the inspection system 40 in an ideal case. In an embodiment the controller 500 is configured to control the lithographic apparatus 1 to form the pattern based on a comparison between the inspection and the target pattern. In an

embodiment the inspection system 40 is configured to perform the comparison and provide comparison data to the controller 500. In an embodiment the controller 500 is configured to perform the comparison and generate comparison data.

[0060] The comparison may comprise comparing one or more specific parameters of the inspection of the pattern and the target pattern. In an embodiment the comparison is based on the identification of one or more selected from: a defect of the pattern, a line width deviation of the pattern, a placement deviation of the pattern and/or a side wall angle deviation of the pattern. Any or all of these particular parameters can be used individually or in combination with each other. In an embodiment the controller 500 is configured to control the lithographic apparatus 1 to form the pattern so as to correct for, or reduce, a defect and/or an inaccuracy of the pattern.

[0061] The comparison of the inspected pattern and the target pattern can be used to provide a map of the substrate 17 showing the one or more deviations of the pattern from the target pattern and/or the extent of the deviation so as to determine the fingerprint. For example, in an embodiment the inspection system 40 or the controller is configured to generate a map of the substrate 17 showing line width deviation of the pattern from the target pattern so as to determine the fingerprint. In an embodiment, for example, pattern placement deviation is obtained resulting in a map of the substrate 17 with the pattern placement deviation to determine the fingerprint. In an embodiment, for example, sidewall angle deviation is obtained resulting in a map of the substrate 17 with the sidewall angle deviation to determine the fingerprint.

[0062] The fingerprint of the lithographic apparatus 1 can be measured using one or more of the parameters as described above. A repetitive part of a measured fingerprint can be at least partly corrected and/or compensated for by controlling the lithographic apparatus 1 so as to adjust the pattern that is formed in subsequent exposure operations.

[0063] The controller 500 may control various parameters of the lithographic apparatus so as to affect the pattern that is formed on subsequent substrates. In an embodiment the controller 500 is configured to control the intensity of each of the plurality of radiation beams based on the data from the inspection. In an embodiment the self-emissive contrast devices 4 provide the radiation beams. The controller 500 may be configured to control the self-emissive contrast devices 4 so as to vary the output intensity of one or more of the self-emissive contrast devices 4 from a value nominally used to form a desired pattern depending on the data from the inspection. The intensity may be varied so as to at least partially correct for the fingerprint of the lithographic apparatus 1 , determined as above.

[0064] In an embodiment the controller 500 is configured to control the timing of providing each of the plurality of radiation beams based on data from the inspection. The timing of providing the plurality of radiation beams can be controlled by controlling the timing of switching on and switching off one or more of self-emissive contrast devices 4. In an embodiment the controller 500 is configured to switch on and switch off one or more self-emissive contrast devices 4 at a timing that depends on data from the inspection as well as desired pattern data.

[0065] Controlling the intensity of the radiation beams and/or the timing of providing the radiation beams for correcting for the measured fingerprint may be particularly advantageous in the context of a lithographic apparatus 1 in which the radiation beams are used to expose a layer of photoresist on the substrate 17. As mentioned above, in an embodiment, the radiation beams may be used to control formation of droplets of material locally on the substrate 17. In this case it may be particularly advantageous for the angular separation between the radiation beams to be controlled so as to correct for the fingerprint. In an embodiment the controller 500 is configured to control the angular separation between at least two of the plurality of radiation beams based on the inspection.

[0066] Regardless of whether the radiation beams are used to expose a layer of photoresist, or are used to locally deposit droplets of material on the substrate 17, or used in another image formation process, any or all of: the intensity of the radiation beams, the timing of providing the radiation beams and/or the angular separation between the radiation beams can be controlled by the controller 500 based on data from the inspection so as to correct for the measured fingerprint.

[0067] In an embodiment, it is possible for the inspection system 40 to inspect the exposed pattern while the lithographic apparatus 1 that formed the exposed pattern is on-line. It is not necessary to cease operation of the lithographic apparatus 1 in order to inspect the pattern. Accordingly, the controller 500 can adjust the lithographic apparatus 1 in-line so as to correct for the fingerprint without having to cease operations of the lithographic apparatus 1 .

[0068] In an embodiment the lithographic apparatus 1 is configured to operate substantially continuously during a time period after formation of the pattern that is inspected and before the controller 500 controls the lithographic apparatus 1 to form the pattern. During this time period, the lithographic apparatus 1 may perform an exposure operation on one or more substrates. The repeating cycle of inspecting a previous pattern and controlling the lithographic apparatus 1 to form a pattern based on the inspection can be repeated during continuous operation of the lithographic apparatus 1 . The pattern formed by the lithographic apparatus 1 on a series of substrates can be iteratively corrected via this process.

[0069] As depicted in Figure 2 and in Figure 5, in an embodiment, the inspection system 40 is attached to or part of the lithographic apparatus 1 . In other words, the lithographic apparatus 1 may comprise the inspection system 40. However, in an embodiment the inspection system 40 is separate from the lithographic apparatus 1 . Similarly for the controller 500, in an embodiment the controller 500 is attached to the lithographic apparatus 1 . However, in an embodiment the controller 500 is separate from the lithographic apparatus 1 .

[0070] It may be advantageous for the inspection system 40 and the controller 500 to be part of the lithographic apparatus 1 because this means that a single lithographic apparatus 1 may be capable of iteratively correcting for its own fingerprint so as to improve the accuracy of the pattern formed by the lithographic apparatus 1 on successive substrates.

[0071] As explained above, the substrate 17 may move relative to the projection system during an exposure operation of the lithographic apparatus 1 . In an embodiment the lithographic apparatus 1 is configured to form the pattern on the substrate 17 during a single forward scanning movement of the substrate 17 relative to the projection system. In the embodiment depicted in Figure 2 and in the embodiment depicted in Figure 5, the single forward scanning movement comprises the substrate 17 moving in the positive X-direction relative to the projection system. In another embodiment the projection system may move over the substrate 17, with the substrate 17 remaining substantially stationary.

[0072] In an embodiment the lithographic system is configured such that the inspection system 40 inspects the pattern during a single backward scanning movement of the substrate 17 relative to the projection system (i.e. the negative X-direction in Figure 2 and Figure 5) after the formation of the pattern on the substrate. Hence, the inspection system 40 can inspect the pattern on the substrate 17 during the transportation of the substrate 17 back to, for example, a load/unload position.

[0073] However, this need not be the case. In an embodiment the lithographic system is configured such that the inspection system 40 inspects the exposed pattern during the single forward scanning movement of the substrate 17 relative to the projection system. As depicted in Figure 2 the inspection system 40 may be downstream of the projection system with respect to the movement of the substrate 17 relative to the projection system. The inspection system 40 can inspect the pattern substantially immediately after the pattern has been formed on the substrate 17.

[0074] In an embodiment the data from the inspection may be used for subsequent exposure operations of the lithography apparatus 1 to which the inspection system 40 is attached. Once the inspection system 40 has inspected the exposed pattern, the inspection system 40 can output the data from the inspection to the controller 500, which may optionally be part of the same lithographic apparatus 1 , as described above. The controller 500 can then control that lithographic apparatus 1 so as to correct for the fingerprint in subsequent exposure operations of the same pattern on subsequent substrates.

[0075] In an embodiment, the inspection may be used for subsequent exposure operations of a lithography apparatus that is different from the lithography apparatus 1 to which the inspection system 40 is attached. In an embodiment, the inspection system 40 may not be attached to any lithographic apparatus. In an embodiment the inspection system 40 is attached to a tool that is not a lithographic apparatus, or may be separate from any other tool.

[0076] An embodiment of the present invention can be used in the context of a series of lithographic apparatuses that are used in combination with each other to manufacture a device. For example, each lithographic apparatus in the series may be used to form a particular pattern for a particular layer of the substrate 17. Once a pattern has been exposed for a particular layer by one lithographic apparatus in the series, the substrate 17 is transported to the next lithographic apparatus in the series.

[0077] The inspection system 40 may inspect a pattern formed by a first lithographic apparatus before a second lithographic apparatus forms a subsequent (e.g. overlying) layer. In an embodiment the inspection system 40 is configured to output the inspection information to the controller 500. The controller 500 may be configured to control the first lithographic apparatus 1 to form a pattern based on data from the inspection. Hence the information can be fed to the first lithographic apparatus 1 that formed the inspected pattern. The information can then be taken into account by the controller 500 to improve the quality of the visible pattern formed by the first lithographic apparatus on

subsequent substrates. [0078] The inspection system 40 may be attached to the second lithographic apparatus, or may be separate from both the first lithographic apparatus and the second lithographic apparatus, e.g. the inspection system 40 may be a stand alone apparatus.

[0079] As depicted in Figure 2 and Figure 5, in an embodiment the lithographic apparatus 1 comprises an isolated frame 15 to which the projection system is attached. The inspection system 40 may be attached to the same isolated frame 15. This may substantially fix the position of the inspection system 40 relative to the projection system. As such the system may inspect the exposed pattern at a consistent timing after the exposed pattern is formed. An isolated frame is a frame in which each part of the frame is fixed relative to all other parts of the frame. The isolated frame is not fixed relative to other parts of the apparatus in a way that does not allow the other parts of the apparatus to move relative to the frame.

[0080] In an embodiment the lithographic apparatus 1 comprises at least one alignment sensor 19 configured to measure a position of the substrate 17. The controller 500 may be configured to control the projection system based on the measured position of the substrate 17 such that the projection system forms the visible pattern on a target position on the substrate 17. In an embodiment the lithographic apparatus 1 comprises an isolated frame 15 to which the alignment sensor 19 is attached. The inspection system 40 may be attached to the same isolated frame 15. The projection system may be attached to the same isolated frame 15. Alternatively, the alignment sensor 19 and the inspection system 40 may be attached to the same isolated frame, with the projection system attached to a different frame.

[0081] In an embodiment the lithographic apparatus 1 comprises two inspection systems 40, 41 . Each inspection system 40, 41 is configured to inspect a pattern formed on the substrate 17, wherein the pattern is formed on the substrate 17 by application of the radiation beams. One of the inspection systems 41 may be upstream or one side of the projection system and the other inspection system 40 may be downstream or on the other side of the projection system with respect to movement of the substrate 17 relative to the projection system. Figure 5 depicts such an embodiment that has two inspection systems 40, 41 . [0082] Each inspection system 40, 41 may be configured to inspect an entire exposed pattern across the full width of the target portion of the substrate 17. The upstream inspection system 41 that is positioned upstream of the projection system is configured to inspect the exposed pattern during a backward scanning movement. This is because during the forward scanning movement, the exposed pattern has not yet been formed on the substrate when the substrate 17 passes under the upstream inspection system 41 .

[0083] In an embodiment, an upstream inspection system 41 is configured to inspect the exposed pattern of a previous layer on the substrate 17 prior to the projection system directing the radiation beams to form the subsequent layer. This use of the upstream inspection system 41 may be particularly advantageous in the context of a series of lithographic apparatuses. The upstream inspection system 41 may be attached to the second lithographic apparatus in the example of a series of lithographic

apparatuses provided above. The upstream inspection system 41 is configured to measure the fingerprint of the first lithographic apparatus. The information can be output to the controller 500 that controls the first lithographic apparatus so that the fingerprint can be accommodated for in subsequent substrates.

[0084] By using the above-described lithographic system and method of controlling a lithographic apparatus, the pixel-to-pixel uniformity of manufactured devices such as flat-panel displays may be improved. Pixel-to-pixel non-uniformity can be caused by, for example, slowly varying exposure conditions leading to a variation in the critical dimension (CD) of the pattern features between exposure operations. According to an embodiment of the invention, the inspection system 40 inspects a pattern formed on the substrate almost instantaneously after it has been formed. This allows any desired correction to be performed rapidly for subsequent formation of that pattern.

[0085] The pattern may comprise a plurality of spots written on the substrate 17. For example, there may be one written spot corresponding to each radiation beam that is projected onto and scanned along the substrate 17 by the projection system. Figure 7 depicts a schematic plan view of the substrate 17 having one, exemplary, written spot 70. The written spot 70 corresponds to a radiation beam that scans along the substrate 17 in the X-direction at a velocity V_Sp. [0086] In an embodiment at the same time as the radiation beam is scanned along the substrate 1 7, the substrate 1 7 moves relative to another component of the lithographic apparatus 1 at a linear velocity V_SB in the Y-direction. In an embodiment, V_SB is less than V_Sp. For example, in an embodiment V_SP is in the range of from about 1 0 m/s to about 1 00 m/s. In an embodiment, V_SB is in the range of from about 1 0 mm/s to about 1 00 mm/s, for example about 1 7 mm/s.

[0087] It is desirable to measure the pattern formed from the written spots such as written spot 70 on the substrate 1 7. However, this is made difficult by the varying surface below the pattern. For example, there may be another pattern below the pattern that is to be measured. The two different patterns may correspond to two different layers on the same substrate 1 7. A method of measuring the pattern is described below.

[0088] In an embodiment, before the written spot 70 is formed on the substrate 1 7, an image of the substrate 1 7 is measured to produce a first measurement. The first measurement is stored in a memory of, for example, the lithographic apparatus 1 . The first measurement is representative of the substrate immediately before the pattern is formed. In an embodiment, the inspection system 40 projects a first readout radiation beam onto the substrate 1 7. In Figure 7, the first readout radiation beam corresponds to first read spot 71 .

[0089] After the first measurement, the pattern is formed on the substrate 1 7. For example, the projection system projects a radiation beam onto the substrate 1 7 so as to form the written spot 70.

[0090] After the written spot 70 has been formed on the substrate 1 7, one or more optical properties of the portion of the substrate 1 7 on which the written spot 70 is formed may change. For example, there may be a change in the refractive index, the absorption and/or the thickness of the substrate 1 7.

[0091] After the written spot 70 has been formed on the substrate 1 7, an image of the substrate 1 7 is measured again to produce a second measurement. For example, the inspection system 40 may project a second readout radiation beam onto the substrate 1 7. In Figure 7, the second readout radiation beam corresponds to the second read spot 72. The second measurement may be stored in a memory of, for example, the lithographic apparatus 1 . [0092] The written spot 70, the first read spot 71 and the second read spot 72 are at substantially the same position on the substrate 17. In Figure 7 they appear to be at different positions because the radiation beam that forms written spot 70, the first readout radiation beam and the second readout radiation beam have optical paths that are spatially different from each other. However, the spots are formed at different timings such that the spots are formed at substantially the same point on the substrate 17, which moves during the operation.

[0093] The controller 500 is configured to compare the second measurement with the first measurement. For example, the first measurement may be subtracted from the second measurement. The result of the comparison is representative of the pattern formed on the substrate 17 (e.g. the written spot 70). The controller of the inspection system may be the same as the controller 500 of the lithographic apparatus 1 .

[0094] There is a time delay between the projection of the first read spot 71 on the substrate 17 and the projection of the second read spot 72 on the substrate 17. By storing the first measurement for the duration of this time delay, the comparison between the first measurement and the second measurement can be made

conveniently.

[0095] By comparing the first measurement with the second measurement, common or systematic aberrations shared by the first measurement and the second measurement may, at least to an extent, cancel each other out. In an embodiment, the properties of the readout radiation beam used to form the second read spot 72 are substantially the same as the properties of the readout radiation beam used to form the first read spot 71 . For example, in an embodiment the size and/or wavelength of the second readout radiation beam is substantially the same as the size and/or wavelength of the first readout radiation beam.

[0096] The pattern formed on the substrate 17 comprises a plurality of written spots including written spot 70. During an exposure operation, the timing at which the written spot 70 is formed depends on the position at which the written spot 70 is to be formed on the substrate 17. Hence, during exposure, the plurality of written spots are formed on the substrate 17 in a chronological sequence. The first read spot 71 is projected onto the portion of the substrate 17 corresponding to the written spot 70 before the written spot 70 is formed. The second read spot 72 is projected onto the portion of the substrate 17 that corresponds to the written spot 70 after the written spot 70 has been formed.

[0097] As depicted in Figure 7, in an embodiment the first readout radiation beam, the radiation beam that forms the written spot 70, and the second readout radiation beam are formed in a line at the substrate level that is substantially straight. However, it is not necessary for this to be the case. The spatial positions of the radiation beams that form the first read spot 71 and the second read spot 72 relative to the radiation beam that forms the written spot 70 should be known. This enables the data corresponding to the first measurement and the second measurement to be linked together correctly for the comparison.

[0098] Provided that the two read spots 71 , 72 are projected before and after the written spot 70 is formed, there is no particular limitation on the relative positions of the corresponding radiation beams. Of course, the position at which each of the three radiation beams is incident on the substrate 17 is the same position because of the relative movements of the substrate 17 and the scanning radiation beam.

[0099] In Figure 7, the first read spot 71 and the second read spot 72 are formed in a line with the written spot 70. The direction of the line is substantially in the direction of the scanning radiation beam that forms the written spot 70. As mentioned above, the scanning speed V_SP may be greater than the substrate speed V_SB- In an embodiment, the first read spot 71 and/or the second read spot 72 is/are formed in a straight line with the written spot 70 substantially in a direction corresponding to the substrate scanning direction. In this case, for a given distance between the read spot 71 , 72 and the written spot 70, there is a greater time delay.

[00100] It may be advantageous to have a greater time delay if a time period is required before the radiation beam that forms the written spot 70 effects a change in a property (e.g., optical property) of the substrate 17. For example, in an embodiment, the pattern is a latent image formed in a layer of photoresist. In this case, a time period is required for the chemical reaction to occur in the photoresist so that an optical property of the photoresist changes such that the written spot 70 is measurable. [00101] In Figure 7, the distance between the first read spot 71 and the written spot 70 is the same as the distance between the second read spot 72 and the written spot 70. However, this need not be the case. In an embodiment, the distance between the first read spot 71 and the written spot 70 is greater than or less than the distance between the second read spot 72 and the written spot 70. This distance between the radiation beams at substrate level is representative of the time delay between the points in time at which each radiation beam is incident on the target position of the substrate 17.

[00102] A property, such as the intensity and/or wavelength, of the first readout radiation beam corresponding to the first read spot 71 and the second readout radiation beam corresponding to the second read spot 72 is selected such that those beams themselves do not alter a property of the substrate 17 applicable to image formation. In other words, the readout radiation beams do not themselves form an image or detectable pattern on the substrate 17.

[00103] In an embodiment, the pattern to be measured is a latent image in a layer of photoresist. The photoresist may comprise an 1-line photoresist that is sensitive to radiation that has a wavelength within the range of from about 350 nm to about 450 nm, and insensitive to radiation that has a wavelength that is greater than about 500 nm. The radiation beam that corresponds to the written spot 70 may have a wavelength in the range to which the photoresist is sensitive. The radiation beam that corresponds to the written spot 70 may have a wavelength of about 405 nm. In an embodiment, the readout radiation beams have a wavelength to which the photoresist is insensitive. For example, the readout radiation beams may have a wavelength of about 633 nm.

[00104] Figure 8 depicts an example of an inspection system 40 and part of a projection system according to an embodiment of the present invention. The system comprises a write beam output 80 configured to project a radiation beam onto a layer of material on or above the substrate 17. The system may comprise a radiation source, such as a laser, connected to the output 80. The system further comprises a read beam output 83. The system may comprise a radiation source, such as a laser, connected to the output 83. The two readout radiation beams may be provided by different radiation sources. In the embodiment depicted in Figure 8, the two readout radiation beams are provided by the same radiation source. The read beam output 83 is configured to project the first and second readout radiation beams onto the substrate 17.

[00105] In an embodiment the read beam output 83 projects a readout radiation beam that has a different property from the radiation beam projected by the write beam output 80. In an embodiment, the intensity of the readout radiation beams is less than the intensity of the write radiation beam projected by the write output 80.

[00106] In an embodiment the system comprises a dichroic beam splitter 85 configured to transmit a high proportion of the radiation beam projected by the write beam output 80. The dichroic beam splitter 85 may be configured to reflect a high proportion of the readout radiation beam projected by the read beam output 83. The radiation beam and the readout radiation beams are focused by a focusing system 84, such as at least one lens.

[00107] The radiation beam and the readout radiation beams are redirected (e.g., reflected) from the pattern formed on the substrate 17. The intensity of each redirected readout radiation beam is measured so as to produce each of a first measurement and a second measurement. The intensity is representative of the change in a property (e.g., optical property) of the substrate 17 due to the formed pattern.

[00108] Depending on the wavelength of the readout radiation beam used, a phase change and/or a change in the level of absorption at the substrate may be measured. In an embodiment, the wavelength of the readout radiation beam is selected such that the redirected readout radiation beam is sensitive to both phase change and change in the level of absorption. A phase change is possible due to multiple reflections in a layer of photoresist, for example. This phase change leads to a variation in the intensity of the redirected readout radiation beam.

[00109] As depicted in Figure 8, the redirected first readout radiation beam may be detected by a first photodiode 81 . The redirected second readout radiation beam may be detected by a second photodiode 82. In Figure 8, the first readout radiation beam is represented by a dashed line. The second readout radiation beam is represented by a dotted line. The radiation beam that corresponds to the written spot 70 is represented by a solid line. [00110] As depicted in Figure 8, the redirected readout radiation beams may pass through a phase grating 86. One or more other or additional types of radiation beam splitter may be used instead of the phase grating. In an embodiment, the distance between each read spot 71 , 72 and the written spot 70 is at least as great as the diffraction-limited spot size on the substrate 17. This allows the different spots on the substrate 17 to be separated from each other when they are measured. However, the spacing between the spots should be less than the size of the field of view of the imaging optics. The distance between the spots may be, for example, within the range of about 10 μιτι to about 20 μιτι, and optionally about 12 μιη.

[00111] In an embodiment, the read beam output 83 is fiber-coupled to the first photodiode 81 and/or the second photodiode 82. In an embodiment, the phase grating 86 is a rectangular grating having a duty cycle of about 50%. In an embodiment, the phase grating 86 is dimensioned such that the phase grating 86 has maximum efficiency for the readout radiation beams in the higher orders (greater than the zeroth order) and a minimum efficiency for the 0^th order. In an embodiment, the pitch of the phase grating is chosen to be about the same as the separation between the radiation beams at the substrate level.

[00112] As mentioned above, in an embodiment the wavelength of the readout radiation beams is different from the wavelength of the radiation beam corresponding to the written spot 70. This may prevent the readout radiation beams from forming an image or a detectable pattern on the substrate 17. However, in an embodiment, the wavelength of the readout radiation beams is similar to or the same as the radiation beam that forms the written spot 70. In this case, the intensity of the readout radiation beams may be less than the intensity of the radiation beam used for exposure. The intensity of the readout radiation beams may be sufficiently low such that they do not form a detectable pattern on the substrate 17.

[00113] In the case that the wavelength of the readout radiation beams is the same as the wavelength of the radiation beam that forms the written spot 70, the dichroic beam splitter 85 depicted in Figure 8 may not be used. Instead, a polarizing beam splitter in combination with a quarter-waveplate may be used to separate the different radiation beams. [00114] In an embodiment, substantially the whole of each of the redirected readout radiation beams is detected so as to make the first measurement and the second measurement. This method of detecting the whole of the beam is termed bright-field detection. However, dark-field detection may instead or additionally be used. In the case of dark-field detection, only the redirected readout radiation beam that corresponds to radiation that has been scattered outside of the central illumination area is detected. The radiation scattered inside the central illumination area is not detected. The radiation scattered inside the central illumination area may be prevented from being detected by blocking. By using such dark-field detection, the smallest detectable variation in a property (e.g., an optical property) of the substrate 17 may be lowered.

[00115] In an embodiment the inspection system 40 comprises a beam profile detector 86, as depicted in Figure 8, for example. In an embodiment the beam profile detector 86 is configured to detect a profile of the first read out radiation beam. In an embodiment the beam profile detector 86 is configured to detect a profile of the second read out radiation beam.

[00116] The beam profile detector 86 is configured to measure the intensity of the first read out radiation beam and/or the second read out radiation beam as it varies over time. For example, the beam profile detector 86 may be used to determine that the intensity of the radiation beams initially rises quickly and subsequently decays slowly over time.

[00117] In an embodiment the controller of the inspection system is configured to control timing of the projection of the first radiation beam and/or the second read out radiation beam based on the profile detected by the beam profile detector 86. By taking into account the profile detected by the beam profile detector 86 it is possible to increase the accuracy of the timing of providing the radiation beams.

[00118] For example, it may be that there is a latent delay between controlling the source 80 to produce the radiation beams and the intensity of the radiation beams being sufficiently high as to be an effective radiation beam. By taking into account the beam profile detected by the beam profile detector 86, it is possible to reduce the effect of such latent delay by, for example, controlling the source 80 to project the radiation beams correspondingly earlier. The timing of providing the radiation beams from the source 80 can be synchronized with the system clock of the lithographic apparatus.

[00119] Figure 9 depicts a part of an alignment system according to an embodiment of the present invention. In an embodiment the lithographic system comprises an alignment system. The alignment system is configured to measure a position, or a change in position, of the substrate 17 as the substrate 17 moves relative to the projection system during a pattern formation operation of the lithographic apparatus 1 . The alignment system comprises a plurality of alignment sensors 19.

[00120] In an embodiment an alignment sensor 19 comprises a radiation output (e.g., source) 90 configured to project a radiation beam onto the substrate 17 and/or onto the substrate table 2. The radiation beam illuminates a part of the substrate 17 or substrate table 2. In particular, the radiation beam may be to illuminate a fiduciary marker 102 on the substrate table 2, for example. In an embodiment the alignment sensor 19 comprises at least one lens 94 configured to focus the radiation beam onto the substrate 17 and/or the substrate table 2.

[00121] In an embodiment the alignment sensor 19 comprises a beam splitter 95. In an embodiment the beam splitter 95 is a polarizing beam splitter. The beam splitter 95 is configured to split the radiation beam provided by the radiation output 90. The beam splitter 95 splits the radiation beam into two separate beams. One of the beams is directed towards the substrate 17 and/or substrate table 2, for example, for illuminating. The other beam is directed towards a beam profile detector 93, which will be described below. In an embodiment the beam splitter 95 splits the radiation beam into two substantially equal beams.

[00122] In an embodiment the alignment sensor 19 comprises a beam profile detector 93. The beam profile detector 93 is configured to detect a profile of the radiation beam. The beam profile detector 93 measures the varying intensity of the radiation beam over time.

[00123] In an embodiment the controller 500 is configured to control timing of the projection of the radiation beam by the radiation output 90 based on the profile of the radiation beam detected by the beam profile detector 93. This is to increase the synchronization of the effective radiation beam with the system clock of the lithographic apparatus and/or with an image detector 91 .

[00124] In an embodiment the alignment sensor 19 comprises an image detector 91 . The image detector 91 may be, for example, a type of camera. In an embodiment the alignment sensor 19 comprises at least one lens 92 configured to focus the radiation beam, having been reflected from the substrate 17 and/or substrate table 2, onto the image detector 91 . The image detector 91 images a fiduciary marker 102, for example, of the substrate 17 and/or substrate table 2.

[00125] In an embodiment the radiation output 90 comprises, or is connected to, a light emitting diode. In an embodiment the radiation output emits a radiation beam that has a wavelength of 625 nm. In an embodiment the controller 500 controls the radiation source of the radiation output 90 so as to produce pulses of radiation, wherein each pulse has a duration of about 20 με.

[00126] The timing of the pulse produced by the radiation source determines the position of the image within the field of view of the image detector 91 . It is desirable for the timing to be controlled precisely. Short pulse times are desirable, particularly for imaging a moving target. For example the alignment sensor 19 may be used to image one or more fiduciary markers 102 on a substrate table 2 as the substrate table 2 moves. The effective pulse duration can be further shortened by inducing a delay between the triggering of the radiation source and the triggering of the image detector 91 . This can be used as an indirect method of shortening the pulse.

[00127] In an embodiment the controller 500 is configured to control the pattern formation operation based on alignment data from the alignment system measured during the pattern formation operation, such that the lithographic apparatus forms the pattern on a target position on the substrate 17.

[00128] Accordingly, misalignment of the substrate 17 can be taken into account on the fly. The pattern formation operation can be adjusted to take into account misalignment of the substrate 17 on the substrate table 2 during the same pattern formation operation in which the misalignment is detected. The pattern formation operation can be adjusted by controlling the timing of the operation of the self-emissive contrast device 4, for example. [00129] As depicted in Figure 2, in an embodiment the alignment sensors 19 are provided on the same frame 15 that comprises the self-emissive contrast devices 4. In an embodiment the alignment system comprises a plurality of alignment sensors 19 attached to an isolated metrology frame 101 . In an embodiment the isolated metrology frame 101 is isolated from the frame 15 to which the self-emissive contrast devices 4 are attached. The isolated metrology frame 101 may be arranged such that the alignment sensors 19 are configured to image one or more fiduciary markers 102 positioned at a peripheral part of the substrate table 2 in the Y-direction.

[00130] In an embodiment the alignment system comprises a plurality of alignment sensors 19 arranged in a row extending in the X-direction. As the substrate table 2 moves under the isolated metrology frame 101 , each fiduciary marker 102 passes under the plurality of alignment sensors 19 in the row. In an embodiment the alignment system comprises two rows, each row comprising a plurality of alignment sensors arranged in the X-direction.

[00131] By providing two rows of alignment sensors 19, the alignment of the substrate 17 can be measured independently at two sides of the substrate table 2. This is advantageous because the alignment could be different at one side of the substrate table 2 to the other side of the substrate table 2. This is particularly the case for large sizes of substrates, such as 3 m x 3 m. In an embodiment the alignment system comprises at least 10, optionally at least 50, and optionally at least 100, alignment sensors 19 arranged in a row in the X-direction. In an embodiment the alignment sensors 19 are evenly spaced along the row.

[00132] The arrangement of alignment sensors allows each fiduciary marker 102 to be imaged by the plurality of alignment sensors 19 in the row. In an embodiment the controller 500 is configured to control imaging of a fiduciary marker 102 on the substrate table 2 at a first time, and imaging of a fiduciary marker 102 on the substrate 17 at a second time.

[00133] In an embodiment the controller 500 is configured to compare the image of the fiduciary marker 102 at the first time with the image of the fiduciary marker 102 at the second time so as to form alignment data. The alignment data indicates misalignment of the substrate 17 on the substrate table 2. [00134] In an embodiment the alignment data comprises a positional shift between the image of the fiduciary marker 102 at the first time and the image of the fiduciary marker 102 at the second time. The two images are compared so as to determine misalignment of the substrate 17 or substrate table 2. The two images can be correlated with each other to find the shift in the marker position. The two images may be recorded by different image detectors 91 . Desirably each imaging detector 91 is substantially the same as all of the other image detectors 91 so as to reduce any discrepancies in the imaging method itself. The correlation between the two images may be performed by a known technique for image correlation. For example, a linear correlation or a parabolic peak fitting method may be used or other image analysis and pattern recognition algorithm such as maximization of mutual information. The image analysis may include a digital image enhancement method such as denoising, edge enhancement and/or background subtraction. The profile detected by the detector 93 may also be taken into account while performing image processing operations on the images obtained by the image detector 91 .

[00135] In an embodiment the controller 500 is configured to interpolate the image of the fiduciary marker 102 at the first time and the image of the fiduciary marker 102 at the second time. In an embodiment the interpolation is done before performing the comparison between the two images. This is to determine the positional shift between the images to sub-pixel accuracy. A known technique may be used for performing the interpolation.

[00136] An exemplary method for taking into account the alignment of the substrate 17 when forming a pattern is described below. The system clock for the lithographic apparatus is monitored. When a certain (e.g., pre-determined) pulse time set point is reached, the controller 500 controls the radiation output 90 to project a radiation beam for measurement. When the pulse time set point is reached, the controller 500 also controls the image detector 91 to record an image from the radiation beam having been reflected back from the surface of the substrate table 2 and/or substrate 17.

[00137] The timing of triggering of the output of the radiation from the radiation output 90 and the timing of the triggering of the image detector 91 are synchronized based on the beam profile of the radiation beam as detected by the beam profile detector 93. [00138] An image of the fiduciary marker 102 at which the radiation beam was directed is acquired by the image detector 91 . The image was recorded at a first time. The image recorded at the first time is stored.

[00139] Subsequently, as the substrate table 2 moves, the fiduciary marker 102 moves from the range of one alignment sensor 19 to the range of another alignment sensor 19. An image of the fiduciary marker 102 is acquired at a second time. The image acquired at the second time is stored.

[00140] The two images are correlated. This may be done using a known technique. Optionally, the detected positional shift in the images is resolved to sub-pixel accuracy.

[00141] The alignment data comprising the positional shift is fed into the controlling of the data path so that a necessary adjustment can be made to the image formation operation.

[00142] In an embodiment a large number, for example at least 10, or at least 50 or at least 100, images of the same fiduciary marker 102 may be acquired and recorded. In an embodiment each subsequent image is correlated to the first image recorded at the first time. However, this need not necessarily be the case. For example, each image may be correlated with the directly preceding image, or any other preceding image. Alignment data corresponding to a plurality of different fiduciary markers 102 may be taken into account using this method.

[00143] In accordance with a device manufacturing method, a device, such as a display, integrated circuit or any other item may be manufactured from the substrate on which the pattern has been projected.

[00144] Further embodiments according to the invention are provided in below numbered clauses:

1 . A lithographic system comprising:

a lithographic apparatus comprising a projection system configured to project a radiation beam onto a layer of material on or above a substrate;

an inspection system configured to inspect a pattern formed on the substrate, wherein the pattern is formed on the substrate by application of the radiation beam; and a controller configured to control the formation of a pattern by the lithographic apparatus based on data from the inspection system relating to an inspection of a previously exposed pattern.

2. The lithographic system of clause 1 , wherein the inspection system is configured to inspect a latent image formed in a layer of photoresist on the substrate.

3. The lithographic system of clause 1 , wherein the inspection system is configured to inspect a pattern formed by droplets of the material on the substrate.

4. The lithographic system of any of the preceding clauses, wherein the controller is configured to control the lithographic apparatus to form the pattern based on a comparison between the inspection and a target pattern.

5. The lithographic system of clause 4, wherein the comparison is based on an identification of one or more selected from the following: a defect of the inspected pattern, a line width deviation of the inspected pattern, a placement deviation of the inspected pattern and/or a side wall angle deviation of the inspected pattern.

6. The lithographic system of any of the preceding clauses, wherein the controller is configured to control the intensity used for each of a plurality of radiation beams based on the data from the inspection.

7. The lithographic system of any of the preceding clauses, wherein the controller is configured to control the timing of providing a radiation beam based on the data from the inspection.

8. The lithographic system of any of the preceding clauses, wherein the projection system is configured to project a plurality of radiation beams and the controller is configured to control the angular separation between at least two of the plurality of radiation beams based on the data from the inspection. 9. The lithographic system of any of the preceding clauses, wherein the lithographic apparatus is configured to operate to project the radiation beam onto the layer of material and/or another layer of material substantially continuously during a time period after formation of the inspected pattern and before the control of the lithographic apparatus to form a pattern is based on the data from the inspection.

10. The lithographic system of any of the preceding clauses, wherein the inspection system comprises a camera array.

1 1 . The lithographic system of any of clauses 1 to 10, wherein the inspection system comprises:

a controller configured to control projection of a first readout radiation beam onto the substrate before the pattern is formed on the substrate, and projection of a second readout radiation beam onto the substrate after the pattern is formed on the substrate; and

a detector configured to detect the first readout radiation beam redirected by the substrate, and to detect the second readout radiation beam redirected by the substrate, wherein the controller of the inspection system is configured to compare the detection of the first readout radiation beam redirected by the substrate with the detection of the second readout radiation beam redirected by the substrate so as to form inspection data.

12. The lithographic system of clause 1 1 , wherein the controller of the inspection system is configured to control projection of the first readout radiation beam and the second readout radiation beam onto substantially the same position on the substrate as the pattern that is formed on the substrate by application of the radiation beam.

13. The lithographic system of clause 1 1 or clause 12, wherein the controller of the inspection system is configured to control a radiation source such that each of the first readout radiation beam and the second readout radiation beam has a lower intensity than the radiation beam.

14. The lithographic system of any of clauses 1 1 to 13, wherein the controller of the inspection system is configured to control a radiation source such that a wavelength of each of the first readout radiation beam and the second readout radiation beam is different from a wavelength of the radiation beam.

15. The lithographic system of clause 14, wherein:

the layer of material is a layer of photoresist; and

the controller of the inspection system is configured to control the radiation source such that a wavelength of each of the first readout radiation beam and the second readout radiation beam is outside of the range of wavelengths to which the layer of photoresist is sensitive.

16. The lithographic system of clause 13 or clause 14, wherein the inspection system comprises a dichroic beamsplitter configured to separate the radiation beam projected by the projection system from the first readout radiation beam and the second readout radiation beam.

17. The lithographic system of any of clauses 1 1 to 16, wherein:

the controller of the inspection system is configured to control a single radiation source to project the first readout radiation beam and the second readout radiation beam; and

the inspection system comprises a radiation beam splitter configured to split a radiation beam projected by the single radiation source into the first readout radiation beam and the second readout radiation beam. 18. The lithographic system of any of clauses 1 1 to 17, wherein the inspection system comprises a beam profile detector configured to detect a profile of the first readout radiation beam and/or the second readout radiation beam.

19. The lithographic system of clause 18, wherein the controller of the inspection system is configured to control timing of the projection of the first readout radiation beam and/or the second readout radiation beam based on the profile of the first readout radiation beam and/or the second readout radiation beam detected by the beam profile detector.

20. The lithographic system of any of the preceding clauses, wherein the inspection system is attached to the lithographic apparatus and/or the lithographic apparatus comprises the controller.

21 . The lithographic system of clause 20, wherein the controller is configured to control the lithographic apparatus to form the pattern during a single forward scanning movement of the substrate relative to the projection system, and the lithographic system is configured such that the inspection system inspects the pattern during a single backward scanning movement of the substrate relative to the projection system.

22. The lithographic system of clause 20 or clause 21 , wherein the data from the inspection is used for a subsequent exposure operation of the lithographic apparatus to which the inspection system is attached.

23. The lithographic system of any of clauses 20 to 22, wherein the data from the inspection is used for a subsequent exposure operation of a lithographic apparatus that is different from the lithographic apparatus to which the inspection system is attached.

24. The lithographic system of any of clauses 20 to 23, wherein the lithographic apparatus comprises an isolated frame to which the projection system is attached and the inspection system is attached to the isolated frame.

25. The lithographic system of any of clauses 20 to 24, wherein the inspection system is downstream of the projection system with respect to movement of a substrate relative to the projection system during a process to project the radiation beam onto the layer of material.

26. The lithographic system of any of clauses 20 to 25, wherein:

the lithographic apparatus comprises an alignment sensor configured to measure a position of the substrate; and

the controller is configured to control the lithographic apparatus, based on the measured position of the substrate, such that the lithographic apparatus forms the pattern on a target position on the substrate; and

the lithographic apparatus comprises an isolated frame to which the alignment sensor is attached and the inspection system is attached to the isolated frame.

27. The lithographic system of clause 26, wherein the projection system is attached to the isolated frame.

28. The lithographic system of any of the preceding clauses, wherein the lithographic apparatus comprises two inspection systems, each configured to inspect an exposed pattern on a substrate, wherein one of the inspection systems is upstream of the projection system and the other inspection system is downstream of the projection system with respect to movement of a substrate relative to the projection system during a process to project the radiation beam onto the layer of material.

29. The lithographic system of any of the preceding clauses, wherein the lithographic apparatus comprises a programmable patterning device, configured to provide the radiation beam. 30. The lithographic system of clause 29, wherein at least part of the projection system is configured to move with respect to the programmable patterning device during exposure of the substrate.

31 . The lithographic system of clause 29 or clause 30, wherein the lithographic apparatus comprises an actuator configured to cause at least part of the projection system to rotate relative to the programmable patterning device in a plane substantially perpendicular to the optical axis of the projection system.

32. The lithographic system of any of clauses 1 to 31 , comprising:

an alignment system configured to measure a position, or a change in position, of the substrate as the substrate moves relative to the projection system during a pattern formation operation of the lithographic apparatus; and

a controller configured to control the pattern formation operation based on alignment data from the alignment system measured during the pattern formation operation, such that the lithographic apparatus forms the pattern on a target position on the substrate.

33. The lithographic system of clause 32, wherein the alignment system comprises a plurality of alignment sensors attached to an isolated metrology frame.

34. The lithographic system of clause 32 or clause 33, wherein the alignment system comprises an alignment controller configured to control imaging of a fiduciary marker on the substrate or substrate table at a first time, and imaging of the fiduciary marker on the substrate or substrate table at a second time, wherein the controller of the alignment system is configured to compare the image of the fiduciary marker at the first time with the image of the fiduciary marker at the second time so as to form alignment data.

35. The lithographic system of clause 34, wherein the alignment data indicates a positional shift between the image of the fiduciary marker at the first time and the image of the fiduciary marker at the second time.

36. The lithographic system of clause 35, wherein the controller of the alignment system is configured to interpolate the image of the fiduciary marker at the first time and the image of the fiduciary marker at the second time before performing the comparison so as to determine the shift to sub-pixel accuracy.

37. The lithographic system of any of clauses 32 to 36, wherein the inspection system comprises:

a radiation output configured to project a radiation beam to measure a position, or a change in position, of the substrate or substrate table; and

a beam profile detector configured to detect a profile of the radiation beam.

38. The lithographic system of clause 37, wherein a controller of the inspection system is configured to control timing of the projection of the radiation beam based on the profile of the radiation beam detected by the beam profile detector.

39. The lithographic system of clause 37 or clause 38, wherein the inspection system comprises a beam splitter configured to divert part of a beam to detect the fiduciary marker to the beam profile detector.

40. A method of controlling a lithographic apparatus, the method comprising:

projecting a radiation beam onto a layer of material on or above a substrate; inspecting a pattern formed on the substrate, wherein the pattern is formed on the substrate by application of the radiation beam; and

controlling the lithographic apparatus to form a pattern based on data from an inspection of a previously exposed pattern.

41 . A device manufacturing method comprising:

the method of controlling a lithographic apparatus according to clause 40; and using the lithographic apparatus to form a pattern on a substrate as part of a process to manufacture a device.

[00145] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative

applications, any use of the terms "wafer" or "die" herein may be considered as synonymous with the more general terms "substrate" or "target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multilayer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

[00146] The term "lens", where the context allows, may refer to any one of various types of optical components, including refractive, diffractive, reflective, magnetic, electromagnetic and electrostatic optical components or combinations thereof.

[00147] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the embodiments of the invention may take the form of a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein. Further, the machine readable instruction may be embodied in two or more computer programs. The two or more computer programs may be stored on one or more different memories and/or data storage media. [00148] The descriptions above are intended to be illustrative, not limiting. Thus, it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS:

1 . A lithographic system comprising:

2. The lithographic system of claim 1 , wherein the inspection system is configured to inspect a pattern formed by droplets of the material on the substrate.

3. The lithographic system of any of the preceding claims, wherein the projection system is configured to project a plurality of radiation beams and the controller is configured to control the angular separation between at least two of the plurality of radiation beams based on the data from the inspection.

4. The lithographic system of any of claims 1 to 3, wherein the inspection system comprises:

5. The lithographic system of claim 4, wherein the controller of the inspection system is configured to control projection of the first readout radiation beam and the second readout radiation beam onto substantially the same position on the substrate as the pattern that is formed on the substrate by application of the radiation beam.

6. The lithographic system of any of the preceding claims, wherein the inspection system is attached to the lithographic apparatus and/or the lithographic apparatus comprises the controller.

7. The lithographic system of claim 6, wherein the controller is configured to control the lithographic apparatus to form the pattern during a single forward scanning movement of the substrate relative to the projection system, and the lithographic system is configured such that the inspection system inspects the pattern during a single backward scanning movement of the substrate relative to the projection system.

8. The lithographic system of any of the preceding claims, wherein the lithographic apparatus comprises two inspection systems, each configured to inspect an exposed pattern on a substrate, wherein one of the inspection systems is upstream of the projection system and the other inspection system is downstream of the projection system with respect to movement of a substrate relative to the projection system during a process to project the radiation beam onto the layer of material.

9. The lithographic system of any of the preceding claims, wherein the lithographic apparatus comprises a programmable patterning device, configured to provide the radiation beam.

10. The lithographic system of claim 9, wherein the lithographic apparatus comprises an actuator configured to cause at least part of the projection system to rotate relative to the programmable patterning device in a plane substantially perpendicular to the optical axis of the projection system.

1 1 . The lithographic system of any of claims 1 to 10, comprising:

12. The lithographic system of claim 1 1 , wherein the alignment system comprises an alignment controller configured to control imaging of a fiduciary marker on the substrate or substrate table at a first time, and imaging of the fiduciary marker on the substrate or substrate table at a second time, wherein the controller of the alignment system is configured to compare the image of the fiduciary marker at the first time with the image of the fiduciary marker at the second time so as to form alignment data.

13. The lithographic system of claim 12, wherein the alignment data indicates a positional shift between the image of the fiduciary marker at the first time and the image of the fiduciary marker at the second time.

14. A method of controlling a lithographic apparatus, the method comprising:

projecting a radiation beam onto a layer of material on or above a substrate;

inspecting a pattern formed on the substrate, wherein the pattern is formed on the substrate by application of the radiation beam; and

15. A device manufacturing method comprising:

the method of controlling a lithographic apparatus as claimed in claim 40; and using the lithographic apparatus to form a pattern on a substrate as part of a process to manufacture a device.