Abstract:
A diffractive optical MEMS device includes a plurality of parallel planar reflective surfaces and an actuator system. The actuator system is used to adjust the position of each of the planar reflectors in a direction perpendicular to the planar reflectors to change characteristics (e.g., phase, intensity, etc.) of light interacting with the device.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a lithographic apparatus and a device manufacturing method. 
   2. Related Art 
   A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., including part of, one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements which serve to generate the circuit pattern. 
   In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers and scanners. In steppers, each target portion is irradiated by exposing an entire pattern onto the target portion during one pass. In scanners, each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction. 
   A lithographic apparatus is known that uses patterning devices including an array of individually controllable elements (e.g., a grating light valve). In particular, a lithographic apparatus is known in which each of the individually controllable elements is a diffractive optical MEMS device. Each diffractive optical MEMS device can include of a plurality of reflective ribbons. Alternate ribbons can be deformed, relative to the remaining ribbons, such that the undeformed ribbons form a grating (e.g., a diffraction gratin). Accordingly, in the undeformed state the diffractive optical MEMS device functions as a plane reflector, reflecting incident light. In the deformed state, the diffractive optical MEMS device functions as a grating, and the incident light is diffracted. 
   Using an appropriate spatial filter, the undiffracted light (i.e., the reflected light from diffractive optical MEMS devices functioning as planar reflectors) can be filtered out of the beam of radiation returned from the array, leaving only the diffracted light to reach the substrate. In this manner, the beam is patterned according to the addressing pattern of the array of diffractive optical MEMS devices. Typically, the array is matrix-addressable, using suitable electronic means. 
   The use of diffractive optical MEMS devices is, however, limited because each device is only capable of controlling the intensity of radiation directed onto a portion of the substrate, and cannot adjust the phase of the radiation relative to the radiation from adjacent devices. 
   SUMMARY OF THE INVENTION 
   Embodiments of the present invention provide an array of individually controllable elements that can pattern a beam of radiation with both contrast and phase information. 
   An embodiment of the present invention provides a patterning array for patterning a beam of radiation including a plurality of individually controllable elements. The individually controllable element each include a row of substantially parallel planar reflectors and an actuator system for setting the position of the reflectors in an actuation direction substantially perpendicular to the reflectors. The actuator system can set a first group of one or more reflectors to a desired one of a first plurality of distances, in the actuation direction, from a base of the patterning array and the actuator system can set a second group of one or more reflectors, independently of the first group, to a desired one of a second plurality of distances, in the actuation direction, from the base. 
   Accordingly, by actuating the two groups of reflectors independently, it is possible to control not only the separation between the reflectors in a distance perpendicular to the reflectors, thus controlling whether radiation is reflected or diffracted (and hence the contrast of a patterned beam of radiation from which either reflected or diffracted radiation has been filtered), but also the position of all of the reflectors relative to the base of the patterning array. This enables control of the phase of the radiation reflected/diffracted from each individually controllable element relative to adjacent individually controllable elements. Therefore, such a patterning array can be used to pattern a beam of radiation with both contrast and phase information. 
   Preferably, alternate reflectors in the row are in the first group of reflectors, the remaining reflectors being in the second group. Therefore, by setting the first group of reflectors to be at a different distance from the base of the patterning array than the second group, a grating is formed that diffracts incident radiation. In a preferred arrangement, each individually controllable element includes a row of six planar reflectors, three in the first group and three in the second group. In a further preferred arrangement, the separation between adjacent reflectors in the row is substantially one quarter of the wavelength of the radiation in the beam to be patterned. Many other combinations are contemplated by the inventor. 
   The actuator system can be arranged so as to at least one of the first and second pluralities of distances to at least three different distances. Accordingly, the distance between the groups of reflectors and/or the distance of the reflectors from the base (in other words, at least one of the contrast control and the phase control) can be set to one of at least three settings. Therefore, in addition to providing contrast and phase control, the individually controllable element can provide intermediate levels of at least one of the phase and contrast control. Preferably, the individually controllable elements can provide multiple intermediate levels of both phase and contrast. This capability can be used to provide greater control of the pattern generated on the substrate. 
   The control of at least one of the first and second pluralities of distances may include provision for a continuous range of values. This, in turn, can provide continuous control of the phase and/or contrast generated by the individually controllable element. This can provide further enhancement to the control of the pattern generated on the substrate by the exposure. 
   The actuator system can be configured so as to adjust the position of each of the reflectors independently within at least one of the first and second groups. This may be desirable in order to increase the contrast available, provide further imaging effects and/or enhanced calibration control of the phase and contrast control for each individually controllable element. It will be appreciated, however, that independent control of each reflector may increase the complexity of the actuator system and/or the control system that controls it. 
   Another embodiment of the present invention provides a lithographic apparatus including an illumination system for supplying a projection beam of radiation, a patterning array as described above for patterning the projection beam, a substrate table for supporting a substrate, and a projection system for projecting the patterned beam onto a target portion of the substrate. 
   Preferably, the lithographic apparatus also includes an array of focusing elements, each of which focuses a portion of the beam of radiation from the illumination system onto the planar reflectors of one of the individually controllable elements in the patterning array. Therefore, the radiation is only directed onto the active parts of the individually controllable elements, namely those parts that can be adjusted to control the phase and contrast of the radiation. Accordingly, the control of the phase and contrast of the resultant patterned beam is improved. 
   Each of the focusing elements within the array of focusing elements may also collect the radiation that is reflected and/or diffracted from its associated individually controllable element and direct it into the projection system. Therefore, in the resulting patterned beam that is projected onto the substrate, radiation from adjacent individually controllable elements is projected adjacent to each other on the substrate. This would not be the case if the loosely-packed array of individually controllable elements was directly imaged onto the substrate. In the latter case, the radiation from adjacent individually controllable elements would be projected onto the substrate at non-abutting locations. Accordingly, the use of the array of focusing elements enables the phase control of adjacent individually controllable elements to be put to effective use. For example, it can be used to provide improved contrast in the aerial image, increasing the resolution that can be printed. 
   A still further embodiment of the present invention provides a device manufacturing method including the steps of providing a substrate, providing a projection beam of radiation using an illumination system, using a patterning array to impart the projection beam with a pattern in its cross-section, and projecting the patterned beam of radiation onto a target portion of the substrate. The patterning array includes a plurality of individually controllable elements that each include a row of substantially parallel planar reflectors and an actuator system for setting the position to the reflectors in an actuation direction substantially perpendicular to the reflectors. For each individually controllable element, the actuator system is used to set a first group of one or more reflectors to a desired one of a first plurality of distances, in the actuation direction, from a base of the patterning array; and to set a second group of one or more reflectors, independently of the first group, to a desired one of a second plurality of distances, in the actuation direction, from the base. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES 
     The accompanying drawings, which are incorporated herein and form a part of the specification, illustrate the present invention and, together with the description, further serve to explain the principles of the invention and to enable a person skilled in the pertinent art to make and use the invention. 
       FIG. 1  depicts a lithographic apparatus according to an embodiment of the invention. 
       FIG. 2  depicts an individually controllable element in a first state according to an embodiment of the present invention. 
       FIG. 3  depicts an individually controllable element in a second state according to an embodiment of the present invention. 
       FIG. 4  depicts an array of individually controllable elements according to an embodiment of the present invention. 
       FIGS. 5   a ,  5   b ,  5   c  and  5   d  depict, in cross-section, an individually controllable element according to an embodiment of the present invention in four different states. 
   

   The present invention will now be described with reference to the accompanying drawings. In the drawings, like reference numbers may indicate identical or functionally similar elements. 
   DETAILED DESCRIPTION OF THE INVENTION 
   The term “array of individually controllable elements” as here employed should be broadly interpreted as referring to any means that can be used to endow an incoming radiation beam with a patterned cross-section, so that a desired pattern can be created in a target portion of the substrate. The terms “light valve,” “grating light valve,” and “Spatial Light Modulator” (SLM) can also be used in this context. Examples of such patterning means may include the following. 
   It should be appreciated that where pre-biasing of features, optical proximity correction features, phase variation techniques and multiple exposure techniques are used, for example, the pattern “displayed” on the array of individually controllable elements may differ substantially from the pattern eventually transferred to a layer of or on the substrate. Similarly, the pattern eventually generated on the substrate may not correspond to the pattern formed at any one instant on the array of individually controllable elements. This may be the case in an arrangement in which the eventual pattern formed on each part of the substrate is built up over a given period of time or a given number of exposures during which the pattern on the array of individually controllable elements and/or the relative position of the substrate changes. 
   Although specific reference 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, 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) or a metrology or inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers. 
   The terms “radiation” and “beam” used herein encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g., having a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g., having a wavelength in the range of 5–20 nm), as well as particle beams, such as ion beams or electron beams. 
   The term “projection system” used herein should be broadly interpreted as encompassing various types of projection system, including refractive optical systems, reflective optical systems, and catadioptric optical systems, as appropriate for example for the exposure radiation being used, or for other factors such as the use of an immersion fluid or the use of a vacuum. Any use of the term “lens” herein may be considered as synonymous with the more general term “projection system.” 
   The illumination system may also encompass various types of optical components, including refractive, reflective, and catadioptric optical components for directing, shaping, or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens.” 
   The lithographic apparatus may be of a type having two (dual stage) or more substrate tables (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposure. 
   The lithographic apparatus may also be of a type wherein the substrate is immersed in a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the final element of the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the first element of the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. 
   Lithographic Projection Apparatus 
     FIG. 1  schematically depicts a lithographic projection apparatus  100  according to an embodiment of the invention. Apparatus  100  includes at least a radiation system  102  (e.g., EX, IL (e.g., AM, IN, CO, etc.) etc.), an array of individually controllable elements PPM  104 , an object table WT  106  (e.g., a substrate table), and a projection system (“lens”) PL  108 . 
   Radiation system  102  can be used for supplying a projection beam PB  110  of radiation (e.g., UV radiation), which in this particular case also comprises a radiation source LA  112 . 
   Array of individually controllable elements  104  (e.g., a programmable mirror array) can be used for applying a pattern to the projection beam  110 . In general, the position of the array of individually controllable elements  104  can be fixed relative to projection system  108 . However, in an alternative arrangement, array of individually controllable elements  104  may be connected to a positioning device (not shown) for accurately positioning it with respect to projection system  108 . As here depicted, individually controllable elements  104  are of a reflective type (e.g., has a reflective array of individually controllable elements). 
   Object table  106  can be provided with a substrate holder (not specifically shown) for holding a substrate W  114  (e.g., a resist-coated silicon wafer or glass substrate) and object table  106  can be connected to positioning device PW  116  for accurately positioning substrate  114  with respect to projection system  108 . 
   Projection system (e.g., a lens)  108  (e.g., a quartz and/or CaF 2  lens system or a catadioptric system comprising lens elements made from such materials, or a mirror system) can be used for projecting the patterned beam received from beam splitter  118  onto a target portion C  120  (e.g., one or more dies) of the substrate  114 . The projection system  108  may project an image of the array of individually controllable elements  104  onto the substrate  114 . Alternatively, the projection system  108  may project images of secondary sources for which the elements of the array of individually controllable elements  104  act as shutters. The projection system  108  may also comprise a micro lens array (MLA) to form the secondary sources and to project microspots onto the substrate  114 . 
   The source  112  (e.g., an excimer laser) can produce a beam of radiation  122 . This beam  122  is fed into an illumination system (illuminator) IL  124 , either directly or after having traversed conditioning device  126 , such as a beam expander Ex, for example. The illuminator  124  may comprise adjusting device AM  128  for setting the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in the beam  122 . In addition, it will generally comprise various other components, such as an integrator IN  130  and a condenser CO  132 . In this way, the beam  110  impinging on the array of individually controllable elements  104  has a desired uniformity and intensity distribution in its cross-section. 
   It should be noted, with regard to  FIG. 1 , that the source  112  may be within the housing of the lithographic projection apparatus  100  (as is often the case when the source  112  is a mercury lamp, for example). In alternative embodiments, source  112  may also be remote from the lithographic projection apparatus  100 . In this case, radiation beam  122  would be led into the apparatus  100  (e.g., with the aid of suitable directing mirrors). This latter scenario is often the case when the source  112  is an excimer laser. It is to be appreciated that both of these scenarios are contemplated within the scope of the present invention. 
   The beam  110  subsequently intercepts the array of individually controllable elements  104  after being directing using beam splitter  118 . Having been reflected by the array of individually controllable elements  104 , the beam  110  passes through the projection system  108 , which focuses the beam  110  onto a target portion  120  of the substrate  114 . 
   With the aid of the positioning device  116  (and optionally interferometric measuring device IF  134  on base plate BP  136  that receives interferometric beams  138  via beam splitter  140 ), the substrate table  106  can be moved accurately, so as to position different target portions  120  in the path of the beam  110 . Where used, the positioning device for the array of individually controllable elements  104  can be used to accurately correct the position of the array of individually controllable elements  104  with respect to the path of the beam  110 , e.g., during a scan. In general, movement of the object table  106  is realized with the aid of a long-stroke module (course positioning) and a short-stroke module (fine positioning), which are not explicitly depicted in  FIG. 1 . A similar system may also be used to position the array of individually controllable elements  104 . It will be appreciated that the projection beam  110  may alternatively/additionally be moveable while the object table  106  and/or the array of individually controllable elements  104  may have a fixed position to provide the required relative movement. 
   In an alternative configuration of the embodiment, the substrate table  106  may be fixed, with the substrate  114  being moveable over the substrate table  106 . Where this is done, the substrate table  106  is provided with a multitude of openings on a flat uppermost surface, gas being fed through the openings to provide a gas cushion which is capable of supporting the substrate  114 . This is conventionally referred to as an air bearing arrangement. The substrate  114  is moved over the substrate table  106  using one or more actuators (not shown), which are capable of accurately positioning the substrate  114  with respect to the path of the beam  110 . Alternatively, the substrate  114  may be moved over the substrate table  106  by selectively starting and stopping the passage of gas through the openings. 
   Although the lithography apparatus  100  according to the invention is herein described as being for exposing a resist on a substrate, it will be appreciated that the invention is not limited to this use and the apparatus  100  may be used to project a patterned projection beam  110  for use in resistless lithography. 
   The depicted apparatus  100  can be used in four preferred modes: 
   1. Step mode: the entire pattern on the array of individually controllable elements  104  is projected in one exposure (i.e., a single “flash”) onto a target portion  120 . The substrate table  106  is then moved in the x and/or y directions to a different position for a different target portion  120  to be exposed by the beam  110 . 
   2. Scan mode: essentially the same as step mode, except that a given target portion  120  is not exposed in a single flash. Instead, the array of individually controllable elements  104  is movable in a given direction (the so-called “scan direction”, e.g., the y direction) with a speed v, so that the projection beam  110  is caused to scan over the array of individually controllable elements  104 . Concurrently, the substrate table  106  is simultaneously moved in the same or opposite direction at a speed V=Mv, in which M is the magnification of the projection system  108 . In this manner, a relatively large target portion  120  can be exposed, without having to compromise on resolution. 
   3. Pulse mode: the array of individually controllable elements  104  is kept essentially stationary and the entire pattern is projected onto a target portion  120  of the substrate  114  using a pulsed radiation system  102 . The substrate table  106  is moved with an essentially constant speed such that the projection beam  110  is caused to scan a line across the substrate  106 . The pattern on the array of individually controllable elements  104  is updated as required between pulses of the radiation system  102  and the pulses are timed such that successive target portions  120  are exposed at the required locations on the substrate  114 . Consequently, the projection beam  110  can scan across the substrate  114  to expose the complete pattern for a strip of the substrate  114 . The process is repeated until the complete substrate  114  has been exposed line by line. 
   4. Continuous scan mode: essentially the same as pulse mode except that a substantially constant radiation system  102  is used and the pattern on the array of individually controllable elements  104  is updated as the projection beam  110  scans across the substrate  114  and exposes it. 
   Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed. 
   Exemplary Individually Controllable Element 
     FIG. 2  depicts an individually controllable element  10  according to an embodiment of the present invention. For example, in one embodiment a grating light valve can be used as individually controllable element  10 . A general overview of grating light valves can be found in U.S. Pat. No. 5,661,592, which his incorporated herein by reference in its entirety. It has a plurality of reflective surfaces  11 , 12 , 13 , 14 , 15 , 16  that are reflective to the radiation provided by the illumination system  102 . The reflective surfaces  11 – 16  can be elongate ribbons and can be arranged adjacent to one another to form a row. It will be appreciated, however, that shapes other than elongate ribbons may also be used. The reflective surfaces can be arranged to be substantially parallel to each other. In a first position, all of the reflective surfaces  11 – 16  lie within a same plane. Therefore, the individually controllable element  10  functions as a planar reflector for radiation that is incident on it. 
   Each of the individually controllable elements  10  has an associated actuator system (not shown) for adjusting the position of the ribbons.  FIG. 3  shows the individually controllable element  10  in a second state according to an embodiment of the present invention. In this embodiment, alternate reflectors  11 , 13 , 15  are displaced relative to the remaining reflectors  12 , 14 , 16 . Accordingly, the first group of reflective surfaces  11 , 13 , 15  lie within a first plane and a second group of reflective surfaces  12 , 14 , 16  lie in a second plane that is parallel to the first plane, but displaced from it in a direction perpendicular to the reflective surfaces. Accordingly, in the second state, the individually controllable element  10  functions as a grating (e.g., a diffraction grating) and radiation that is incident upon it is returned as diffracted radiation. 
   For example, if the separation between the first and second planes (in which the first and second groups of reflectors lie) in a direction perpendicular to the reflective surfaces is set to one quarter of the wavelength of the radiation generated by the illumination system  102 , then all the of the radiation incident on the individually controllable element can be diffracted. It is to be appreciated that other configurations are also contemplated within the scope of the present invention. 
   The lithographic projection apparatus is provided with a filter (not shown), for example an aperture within the projection system, that can prevent either the reflected radiation (i.e., zero order radiation) or the diffracted radiation (i.e., first order, or higher, radiation) from being projected onto the substrate  114  by the projection system  108 . Accordingly, by setting a selected set of the individually controllable elements  10  within a patterning array to be reflecting and others to be diffracting, a pattern is imparted to the beam of radiation  110  projected onto the substrate  114 . For example, various patterning arrays are taught in U.S. application Ser. No. 10/449,908, filed May 30, 2003, entitled “Maskless Lithography Systems and Methods Utilizing Spatial Light Modulator Arrays,” which is incorporated by reference in its entirety. 
   As shown in  FIGS. 2 and 3 , the displacement of one of the reflective surfaces may be achieved by deforming the shape of one of the ribbons. In one embodiment, when the actuation force applied to the ribbon is removed, the ribbon may naturally return to its undeformed state and consequently it will not require a return actuation force. It will be appreciated that, in such an arrangement, at least a part of each ribbon does not move when the reflective surface is displaced. Accordingly, only a portion of the individually controllable element  10  will be active (i.e., can be controlled). Furthermore, space may be required around each of the sets of ribbons in order to provide control circuitry (not shown) and/or other surfaces for each of the individually controllable elements  10 . Therefore, the array of individually controllable elements  10  may only include a relatively small active area. This is commonly referred to as a “loose-packed” configuration. 
   The lithographic apparatus may therefore be arranged as shown in  FIG. 4 . According to the embodiment of the present invention shown in  FIG. 4 , the array of individually controllable elements  20 , which correspond to controllable elements  10  of  FIGS. 2 and 3  (hereinafter both referred to as element  10 ), include active areas  21  (e.g., the moveable parts of the reflectors  11 – 16 ). An array  22  of focusing elements, such as an array of lenses  23 , is provided adjacent to the array of individually controllable elements  10 . Each focusing element  23  focuses a portion  24  of the beam of radiation from the illumination system  102  onto the active area  21  of a corresponding individually controllable element  10 . Preferably, each focusing element  23  also collects the radiation that is either reflected or diffracted from the active area  21  of each individually controllable element  10  and directs the collected radiation to the projection system  108 . Accordingly, the patterned beam of radiation is largely formed only from radiation that has been incident on the active areas  21  of the array of individually controllable elements  10  rather than the non-active areas that lie between the active areas  21 . 
   The reflectors  11 – 16  can be actuated by any device known to one or ordinary skill in the art. The actuation can be provided by electrostatic forces between the reflector and a static portion of the array of individually controllable elements  10 . However, the present invention is not limited to this. Alternatively, for example, each reflective surface (e.g.,  11 – 16 ) may be mounted on a piezo-electric actuator (not shown). 
     FIGS. 5   a ,  5   b ,  5   c  and  5   d  show, in cross-section, an individually controllable element  10  in four different states according to an embodiment of the present invention. 
     FIG. 5   a  shows the individually controllable element  10  in a first state, in which all of the reflective surfaces  31 , 32 , 33 , 34 , 35 , 36  are within a common plane (e.g., reflective surfaces  31 – 36  are substantially parallel planar reflectors) and are a given distance, in a direction perpendicular to that plane, from a base  30  of the array of individually controllable element  10 . Accordingly, the individually controllable element  10  functions as a planar reflector to incident radiation. 
   In a second state, shown in  FIG. 5   b , alternative reflective surfaces  32 , 34 , 36  are displaced relative to the remaining reflective surfaces  31 , 33 , 35 . Consequently, the individually controllable element  10  functions as a grating (e.g., a diffraction grating) and incident radiation is diffracted. 
     FIG. 5   c  shows the individually controllable element  10  in a third state. As with the element in the second state shown in  FIG. 5   b , alternate reflective surfaces  32 , 34 , 36  are displaced relative to the remaining reflective surfaces  31 , 33 , 35  in a direction perpendicular to the reflective surfaces. Therefore, as before, the element  10  functions as a grating and the incident radiation is diffracted. However, in comparison to the element  10  in the second state, all of the reflective surfaces ( 31 – 36 ) are further displaced in a direction perpendicular to the reflective surfaces. Accordingly, if a first individually controllable element  10  is set to the second state and an adjacent individually controllable element  10  is set to the third state, radiation incident on both individually controllable elements  10  will be diffracted, but there will be a phase difference in the diffracted radiation of one individually controllable element relative the other. 
     FIG. 5   d  shows an individually controllable element in a fourth state. In this case, all of the reflective surfaces  31 , 32 , 33 , 34 , 35 , 36  are within a single plane such that, as in the first state, shown in  FIG. 5   a , the individually controllable element  10  functions as a planar reflector reflecting incident radiation. However, all of the reflective surfaces ( 31 – 36 ) are displaced relative to their position when in the first state, shown in  FIG. 5   a . Therefore, if a first individually controllable element  10  is set to the first state and a second individually controllable element  10  is set to the fourth state, then both individually controllable elements  10  will act as planar reflectors, reflecting radiation, but there is a phase shift in the radiation reflected from the first individually controllable element  10  relative to the radiation reflected from the second individually controllable element  10 . 
   Therefore, by separately controlling the positions of the two groups of reflective surfaces, namely a first group with alternate reflective surfaces  32 , 34 , 36  and a second group with the remaining reflective surfaces  31 , 33 , 35 , it is possible to provide an individually controllable element  10  with both contrast and phase control. In turn, by setting selected sets of individually controllable elements  10  to one of the four states, the beam projected onto the substrate  114  is patterned with both contrast and phase information. 
   As shown in  FIGS. 5   a ,  5   b ,  5   c  and  5   d , a distance d 1  represents the distance between the planes of the two groups of reflective surfaces and a distance d 2  represents the distance of the grating/reflector formed by the reflective surfaces relative to a reference position. Therefore, setting the distance d 2  for an individually controllable element  10  effectively sets distance of the grating/reflector relative to the substrate  114 . Therefore, by setting d 2  for adjacent individually controllable elements to different values, a phase difference in the radiation reflected/diffracted from each to the substrate  114  can be provided. 
   For example, if the difference in d 2  values for adjacent individually controllable elements is set to a quarter of the wavelength of the radiation provided by the illumination system  102 , the radiation at the substrate  114  from one will be fully out of phase with the radiation from the other. If, however, the d 2  values are set to be the same, the radiation from each will be in phase with each other. It will be appreciated that by setting d 2  to intermediate values between a minimum and maximum value, intermediate phase shifts can be obtained. 
   As discussed above, setting d 1  to zero causes the individually controllable element  10  to function as a planar reflector. Setting d 1  to be, for example, a quarter of the wavelength of the radiation provided by the illumination system  102 , results in the individually controllable element  10  functioning as a pure grating, namely no zero order radiation being reflected. Accordingly, if, for example, the pupil of the projection system  108  is set to filter out all diffracted radiation (e.g., first order and higher radiation) then the intensity of the radiation directed into the projection system  108  from a given individually controllable element  10  will be a maximum when d 1  is zero (or set to a minimum). Conversely, if the projection system aperture is arranged such that the zero order radiation is not directed into the projection system  108 , but first order radiation is, then the radiation in the projection system  108  from the individually controllable element  10  will be a minimum when d 1  is set to zero and a maximum when d 1  is set to the quarter wavelength value. 
   It will be appreciated that when d 1  is set to intermediate values, some of the radiation incident on the individually controllable elements  10  will be reflected and some will be diffracted. Therefore, the intensity of the radiation from the individually controllable element  10  that passes through the projection system  108  and is projected onto the substrate  114  will likewise be at an intermediate value. Therefore, adjusting d 1  can be used to set the contrast of the individually controllable element  10 . It will further be appreciated that contrast control can also be achieved by varying d 1  through values other than from zero to a quarter wavelength of the radiation provided by the illumination system  102 . For example, a similar effect will be provided by controlling d 1  to vary between three quarters of the wavelength and one wavelength. 
   The actuator system may be arranged such that it can set the reflective surfaces to a limited number of predetermined positions. For example, the actuator system can nominally set each of d 1  and d 2  to one of two values. In this case the individually controllable element  10  may only be set to the four states shown in  FIGS. 5   a ,  5   b ,  5   c  and  5   d . These can correspond to the individually controllable element  10  being set to provide a maximum and a minimum level of radiation in the projection system  108  (namely on or off) and for providing radiation that is either in phase or out of phase with that of the adjacent individually controllable element  10 . Alternatively, the actuator system may be arranged such that it can provide a plurality of intermediate values of d 1  and/or d 2  providing a plurality of contrast levels and/or phase differences between adjacent individually controllable elements  10 . The actuator system can, furthermore, be arranged such that d 1  and d 2  can be set to a continuous range of values between a minimum and a maximum. 
   CONCLUSION 
   While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.