Abstract:
A lithography system for processing a substrate is disclosed. The lithography system includes a stage for moving the substrate relative to a beam. The lithography system further includes a chuck for securely holding the substrate during stage movement. The lithography system additionally includes a support assembly for holding the chuck in a fixed position relative to the stage while accommodating for deformations in either the chuck or the stage during processing so as to precisely locate the substrate relative to the stage and to reduce external stresses that cause substrate distortions.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to a lithography system. More particularly, the present invention relates to improved techniques for supporting and holding a reticle. 
     Lithography systems used in the manufacture of integrated circuits and related devices have been around for some time. Such systems have proven extremely effective in the precise manufacturing and formation of very small details in the product. In most lithography systems, a circuit image is written on a wafer by projecting a beam through a patterned reticle. By way of example, optical lithography systems and electron beam projection systems, which tend to produce finer geometries than optical lithography systems, have been widely used to reproduce a circuit image on a silicon wafer. In optical lithography systems, a beam of light is used to scan the surface of the reticle. In electron beam projection lithography systems, a beam of electrons is used to scan the surface of the reticle. 
     Electron beam projection lithography systems typically include an illuminator for directing electron beams of finite area through patterns resident on the surface of a reticle, a stage for moving the reticle relative to the beam, a chuck for supporting the reticle relative to the stage, and a projector for projecting the transmitted electron beam (e.g., the pattern of electrons passing through the reticle) onto the surface of a wafer. In order to process the wafer, the stage is moved along a linear scan path while the electron beams are swept orthogonally to the linear path so that all or any selected part of the patterned reticle is scanned. Although only a small portion of the reticle is imaged at any one time, the surface of the reticle is sequentially exposed to electron beams, allowing a pattern to be built up on the wafer. 
     In general, the beam sweep is in a direction parallel to the Y-axis as viewed at the reticle, and the linear scan path is in a direction parallel to the X-axis as viewed at the reticle. More particularly, as the beams are swept, the stage carrying the reticle is typically moved back and forth in the X direction while being incremented in the Y-direction at the end of each traversal so that the beam sweeps along a substantially serpentine path across a predetermined area of the reticle. The predetermined area may correspond to a single identified sub area, a plurality of identified sub areas or the entire reticle. Furthermore, the chuck is required to hold the reticle in place while the stage is moved. In most cases, a large clamping force is needed to overcome the forces generated by the high acceleration (e.g., 4 g) associated with the stage. If the holding force is not sufficient, then the reticle may peel away or shift from the chuck during high accelerations. 
     Electron beam projection lithography systems generally require precise tolerances in order to achieve finer geometries. For example, because an electron beam projection lithography system generally determines reticle position relative to the stage position, the system must be capable of precisely locating the reticle relative to the stage. In general, the reticle must effectively not be allowed to change position with respect to the stage. As should be appreciated, reticle misalignment tends to cause errors in projecting the reticle pattern onto the wafer surface, especially with the extreme level of accuracy that is sought in electron beam projection lithography. 
     Unfortunately, reticle misalignment can be encountered when external stresses are induced on the reticle or other related structures such as the chuck or stage. Such stresses may be caused by mechanical distortion of the reticle or chuck to which the reticle is mounted, or by differential thermal expansion or contraction between the reticle and the chuck. With regards to differential thermal expansion or contraction, stresses may be transmitted to the reticle when the reticle expands or contracts while the chuck remains static or when the chuck expands or contracts while the reticle remains static. In most cases, a high intensity electron beam tends to raise the temperature of the reticle during processing thus making the reticle expand. By way of example, the reticle may bow if the chuck tries to keep its original dimension while the reticle is trying to expand. Alternatively, if the stress is too high then the reticle may slip from its original position. This change in height or position may adversely affect the projected pattern. Moreover, even if the chuck complied to the expansion and contraction of the reticle, stresses may be transmitted to the reticle and/or chuck when the reticle expands or contracts and the supporting structure remains static. 
     Furthermore, the throughput associated with electron beam projection systems has generally been limited, due at least in part to the fact that electron beam systems operate in a vacuum. Also, within electron beam projection systems, the implementation of a step and scan configuration may be difficult. Specifically, implementing a step and scan configuration with respect to a stage which scans reticles, e.g., a reticle stage, is difficult, as electron beam projection systems have specific requirements which are not requirements for typical optical lithography systems. By way of example, an electron beam projection system generally must operate in a high vacuum environment. Further, an electron beam projection system may not include moving magnets, as moving magnets cause the magnetic field associated with the electron beam projection system to change. An electron beam projection system also may not having moving iron structures, due to the fact that moving iron dynamically alters the static magnetic fields around an electron beam lens. Finally, an electron beam projection system may not have metal parts which move such that eddy currents are generated in static magnetic fields with concomitant additional varying magnetic fields. 
     Therefore, what is needed is a method and an apparatus for enabling reticles to be precisely and stablely held within an electron beam projection lithography system. 
     SUMMARY OF THE INVENTION 
     The invention relates, in one embodiment, to a lithography system for processing a substrate. The lithography system includes a stage for moving the substrate relative to a beam. The lithography system further includes a chuck for securely holding the substrate during stage movement. The lithography system additionally includes a support assembly for holding the chuck in a fixed position relative to the stage while accommodating for deformations in either the chuck or the stage during processing. 
     The invention relates, in another embodiment, to a support assembly for holding a chuck in a fixed position relative to a stage while allowing some plasticity of the chuck during processing. The support assembly includes a plurality of flexures, each of which has one end attached to the stage and an opposite end attached to the chuck. Additionally, the plurality of flexures work together to restrain the chuck from lateral, vertical and rotational movements while allowing some expansion or contraction of the chuck relative to the stage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements and in which: 
     FIG. 1 illustrates a simplified diagram of an electron beam projection lithography system, in accordance with one embodiment of the present invention. 
     FIG. 2 shows a top view of a reticle during an electron beam scan, in accordance with one embodiment of the present invention. 
     FIG. 3 shows a top view of a stage, in accordance with one embodiment of the present invention. 
     FIG. 4 illustrates a perspective view of the stage shown in FIG. 3, in accordance with one embodiment of the present invention. 
     FIG. 5 shows a top view of a kinematic support assembly, in accordance with one embodiment of the present invention. 
     FIG. 6 shows a side view of a kinematic support assembly, in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order not to unnecessarily obscure the present invention. 
     The invention generally pertains to an electron beam projection lithography system. More particularly, the invention pertains to an improved method and apparatus for mounting a reticle to a reticle stage. One aspect of the invention relates to precisely locating the reticle relative to the reticle stage. Another aspect of the invention relates to reducing external stresses that typically cause reticle distortion. 
     In accordance with one embodiment of the present invention, there is provided a lithography system that includes a stage, a chuck and a kinematic support assembly. The stage is configured to move the reticle relative to a beam such as for example an electron beam, and the chuck is configured to securely hold the reticle during stage movement to prevent reticle shifts. In one implementation, the chuck is arranged to be pliable so as to reduce stresses caused by a constrained and deforming reticle, i.e., a thermally expanding or contracting reticle. Furthermore, the kinematic support assembly is configured to hold the chuck in a fixed position relative to the stage, while permitting some plasticity of the chuck during processing so as to reduce stresses caused by a constrained and deforming chuck, i.e., a thermally expanding or contracting chuck. 
     Embodiments of the invention are discussed below with reference to FIGS. 1-6. However, those skilled in the art will readily appreciate that the detailed description given herein with respect to these figures is for explanatory purposes as the invention extends beyond these limited embodiments. 
     FIG. 1 is a simplified diagram of an electron beam projection lithography system  10 , in accordance with one embodiment of the present invention. The lithography system  10  is arranged for writing a circuit image on the surface of a wafer  12  by projecting a beam through a patterned reticle  14 . The dimensions of various components are exaggerated to better illustrate the components of this embodiment. As shown, the lithography system  10  includes an illuminator  16 , a reticle stage  18 , a projector  20  and a wafer stage  22 . In general, the illuminator  16  both generates an electron beam and directs the electron beam to the surface of the reticle  14 , and the projector  16  both collects transmitted electron beams and projects the transmitted electron beam to the surface of the wafer  12 . Although not shown, the illuminator  16  typically includes an electron source and an illumination lens assembly that work together to make the electron beam incident on the reticle and to sweep the electron beam in the Y-direction. In addition, the projector  20  typically includes a projection lens assembly for reducing the size of the transmitted electron beam so as to form the electronic circuit of final size onto the wafer  12 . 
     Furthermore, the reticle stage  18  is positioned within a gap  24  defined between the illuminator  16  and the projector  20 , and the wafer stage  22  is positioned below the projector  20 . Both stages are arranged for moving within a single plane and relative to the axis  25 . That is, the stages move in both the x and y directions. The reticle stage  18  is configured to move the reticle  14  so that all or any selected part of the reticle surface  15  is scanned by the electron beam. Although not shown, the reticle stage  18  may be arranged to accommodate multiple reticles, e.g., two or three reticles, which include complementary patterns of an entire chip circuit which is to be formed on the wafer  12 . The wafer stage  22 , on the other hand, is configured to move the wafer  12  so that the scanned reticle surface  15  is printed on all or any selected part of the wafer surface  13 . More particularly, electrons that pass through the reticle  14  during reticle stage  18  movement are projected onto the wafer  12  during wafer stage  22  movement such that a pattern defined by the reticle  14  is formed on the wafer  12 . In most cases, the wafer stage  22  moves in an opposite direction relative to the reticle stage  18 . In one embodiment, the stages  18  and  22  are arranged to move in a serpentine fashion. For example, the stage scans in one direction, e.g., along the X-axis (into and out of the page), and steps in another direction, e.g., along the Y-axis. 
     A representative stage used in electron beam projection lithography systems is described in co-pending U.S. patent application Ser. No. 60/226,409 to Watson et al., which is titled, “Cantilever reticle Stage for Electron Beam Projection Lithography System,” and which is herein incorporated by reference. 
     Referring to FIG. 2, a top view of the reticle  14  is shown during a typical scan. As mentioned, a beam sweep  26  is directed by the illuminator (not shown in FIG. 2) in a direction parallel to the Y-axis as viewed at the reticle surface  15 . As the beam is swept, the stage (not shown in FIG. 2) carrying the reticle  14  moves back and forth in the direction of the X-axis while being incremented in the Y-direction at the end of each traverse so that the electron beam is caused to sweep along a substantially serpentine path  27  across a predetermined area  28  of the reticle  14 . The predetermined area  28  may correspond to a single identified sub area (e.g., a single stripe), a plurality of identified sub areas (e.g., a plurality of stripes) or the entire reticle  14 . In the embodiment shown, the reticle  14  has two scanning stripes  30 . Alternatively, it should be noted that beam sweep and stage movements may be reversed such that the beam sweep is directed in an X direction, and the stage moves back and forth in the Y direction while being incremented in the X direction. 
     In order to scan the predetermined area  28 , the X stroke  31  of the reticle stage along the X-axis may be relatively large. By way of example, in some systems, the size of the stroke along the X-axis may be in the range of approximately 400 to 700 mm. On the other hand, each incremental Y stroke  33  of the reticle stage along the Y-axis may be relatively small, while the overall Y stroke  35  of the reticle stage along the Y-axis may be relatively large. By way of example, in some systems, the size of the incremental Y stroke along the Y-axis may be up to about 30 mm, and the overal Y stroke may be up to about 180 mm. It should be appreciated, however, that these sizes are not a limitation, and that the size of the stroke may vary according to the specific needs of each system. The size of the strokes are typically dependent upon the size and the configuration of the reticles. Specifically, each reticle that is supported by the reticle stage may have multiple scanning stripes, thereby effectively requiring that the stroke of the retcile stage be sufficient to cover each of the scanning stripes. 
     Referring now to FIGS.  1  and  3 - 6 , the reticle stage  18  will be described in greater detail. The reticle stage  18  generally includes a stage table  32 , a reticle chuck  34  and a kinematic support flexure  36 . As shown, the reticle chuck  34  is structurally coupled to the stage table  32  via the kinematic support assembly  36 . By kinematic, it is meant that the support assembly is allowed to move in order to adapt to external forces in a non-hysteretic way. Generally speaking, the stage table  32  provides a moving structure for scanning the reticle  14 , the chuck  34  provides a means for securely holding the reticle  14 , and the kinematic support assembly  36  provides a means for supporting the chuck  34  in a fixed position relative to the stage table  32 , while permitting some plasticity (e.g., expansion or contraction) of the chuck  34  during processing. By allowing some plasticity, stresses induced on the reticle  14  are reduced and the reticle  14  can be precisely located relative to the stage table  32 . 
     With regards to the stage table  32 , the stage table  32  may be in the form of a plate or cantilever, which includes an opening  38  that is sized to accommodate the reticle  14  and the reticle chuck  34 . Typically, the opening  38  is aligned such that the reticle  14  may be aligned in a scanning direction, i.e., along an X-axis, to enable travel along Y-axis to be substantially minimized. In most cases, the opening  38  is a circular opening. However, it should be noted that this may vary according to the specific needs of each system. For example, the opening may be oriented in a substantially square pattern. Moreover, the reticle table  32  may be formed from substantially any non-metallic material which has acceptable outgassing characteristics for a relatively high vacuum. By way of example, reticle table  32  may be formed as a ceramic structure. The formation of reticle table  32  from ceramic enables reticle table  32  to move within an electron beam projection system without significantly affecting the magnetic fields associated with the electron beam projection system. Although the reticle table  32  is shown with only one opening  38 , the table may generally include any number of openings, as for example two openings. The number of openings may be determined, for instance, on the size of the pattern area and the number of patterns needed to fit a chip on a wafer. 
     In general, at least one or two sides and a front edge of the reticle table may include mirrored surfaces, i.e., reticle stage mirrors. By way of example, a first mirror may be positioned on a first side  35  of the table  32  and a second mirror may be positioned on a second side  37  of the table  32 . As shown, the second side  37  is orthogonal to the first side  35 . It should be noted that this is not a limitation and that the position of the mirrors may vary according to the specific design of each system. The mirrored surfaces enable laser interferometer beams to be substantially reflected off of the sides and front edge to enable positioning measurements to be made with respect to the reticle table. For example, the front edge may be used to facilitate the measurement of a linear position of stage in x-direction as well as measurements of a rotational position of stage about y-axis and z-axis, i.e., an angle θy and an angle θz, respectively. Similarly, the sides may be used to measure a linear position of stage along y-axis, and rotational positions about z-axis, i.e., an angle θz, and x-axis, i.e., angle θx. 
     With regards to the reticle chuck  34 , the reticle chuck  34  is configured to support and hold the reticle  14  during processing. More particularly, the chuck  34  is required to hold the reticle  14  in place while the stage  18  is moved such that the position of the reticle  14  is known throughout processing. That is, if the chuck position is known, then the reticle position should be known as well. In general, a large clamping force is needed to overcome the forces generated by the stage&#39;s high acceleration (e.g., up to approximately 4 g). If the holding force is not sufficient then the reticle  14  may peel away or shift from the chuck  34  during high accelerations. It should be appreciated, however, that too high of a clamping force may cause mechanical distortion of the reticle  14 . Both reticle shifts and mechanical distortion may adversely effect the transferred reticle pattern. 
     The chuck  34  generally includes a top surface  40 , an outer peripheral surface  42 , an inner peripheral surface  44  and a bottom surface  46 . The top surface  40  provides a surface where the reticle  14  rests and the inner peripheral surface  44  defines an opening  48  which is sized to accommodate the pattern (or predetermined area in FIG. 2) disposed on the reticle  14 . That is, the opening  48  is sized to permit the transmitted electron beam to pass therethrough. In most embodiments, the reticle chuck  34  is an annular ring having a circular opening  48 . However, it should be noted that this may vary according to the specific needs of each system. For example, the opening (or the outer periphery) may be oriented in a substantially square or rectangular pattern. As should be appreciated, the reticle pattern is typically formed as a square or rectangle. Furthermore, the outer periphery of the chuck  34  generally coincides with outer periphery of the reticle  14  such that the top surface of the chuck  34  is fully covered by the reticle  14  when the reticle is disposed on the chuck  34  for processing. In an alternate embodiment, the outer periphery of the chuck  34  may extend past the outer periphery of the reticle  14 . 
     In one embodiment, the reticle chuck  34  represents an ESC (electrostatic) chuck, which secures the reticle  14  to the chuck&#39;s surface by electrostatic force. In one implementation, the top surface  40  is enlarged, at least in part, to provide a greater surface area for clamping. By way of example, the opening  48  may be made to match the minimum required space for projecting a beam through the reticle so as to increase the area of the top surface  40 . It is generally believed that the greater the area, the greater the holding force. By increasing the holding force, the stage can move faster (without reticle shifts) thus increasing productivity. In addition, because of the enlarged surface area there is less clamping distortion. Although an ESC chuck is described, it should be noted that this is not a limitation and that other forms of providing a clamping force may be used. For example, a mechanical chuck may also be used to secure the reticle to the chuck&#39;s surface. Electrostatic chucks may be preferred over mechanical clamping methods because mechanical clamping methods are generally more complex and may cause reticle distortion. Furthermore, if the chuck is used in a system other than electron beam projection lithography systems (processes wafers in a vacuum chamber) then a vacuum chuck may be used. 
     In general, the reticle is preferably maintained at a constant and uniform temperature. The electron beam, however, tends to heat the reticle to some extent, which means there is some thermal expansion and contraction taking place. As mentioned, stresses may transmitted to the reticle when the reticle thermally expands or contracts and the chuck remains static. For instance, the reticle may bow if the chuck tries to keep its original dimension during reticle expansion or contraction or if the stress is to high then the reticle may slip from its held position during reticle expansion or contraction. Both bowing and slippage may lead to circuit transfer problems. For example, bowing increases or decreases the height of certain portions on the reticle and therefore the projected pattern may be distorted. In addition, reticle slippage often changes the position of the reticle and thus the projected pattern may be misaligned. Moreover, it would be difficult for the system to determine where the reticle is relative to the moving stage. 
     Therefore, in accordance with one aspect of the invention, the chuck is arranged to be pliable so as to reduce stresses caused by a constrained and deforming reticle, i.e., a thermally expanding or contracting reticle. In one embodiment, the chuck  34  is configured to expand and retract at substantially the same rate as the reticle such that the reticle and the chuck expand and retract together. That is, the chuck is configured to go through essentially the same thermal distortion as the reticle. This can be accomplished in a variety of ways. In one implementation, for example, the chuck can be formed from a material that has a similar coefficient of thermal expansion as the reticle. In another implementation, the chuck can be formed from a material that is similar to the reticle. In most cases, the reticle is formed from silicon and therefore, in one embodiment the chuck is also formed from silicon. It should be noted, however, that the chuck could also be formed from a ceramic having the same coefficient of thermal expansion as the silicon reticle. Furthermore, in cases, where the reticle is formed from another material, the chuck could also be formed from this material, or from a material with a similar coefficient of thermal expansion. As should be appreciated, by allowing the chuck to expand and contract with the reticle, stresses induced between the reticle and the chuck are generally reduced. As such, reticle distortions, i.e., bowing and/or reticle misalignment such as slips, can be substantially prevented. 
     Even if the chuck and reticle are formed from the same material, or from materials with substantially similar coefficients of thermal expansion, they may still expand or retract at different rates. By way of example, thermal contact between the adjacent surfaces of the chuck and reticle may be poor due to the fact that they contact at discrete points (e.g., local points of contact). Poor thermal contact typically leads to poor heat transfer and therefore there may be a temperature difference between the reticle and the chuck. This temperature difference may cause the reticle and chuck to expand at different rates. By way of example, the reticle and chuck may have a temperature differential of between about 1 to about 3 degrees ° C. 
     Therefore, in accordance with another aspect of the invention, the dimensions of the chuck are arranged to reduce stress induced distortions of the reticle. By way of example, the reticle/chuck interface may not be planar and therefore when the reticle is secured to the chuck (e.g., electrostatic clamping), the reticle may distort to overcome the non planar interface. The non-planar interface may be due to a non planar chuck or reticle or particle(s) located between the chuck and reticle. In one embodiment, the chuck is made relatively thin (as compared to prior art chucks) to overcome these type of distortions. By making the chuck thin, the chuck bends rather than the reticle during clamping in order to maintain a secure hold on the reticle. As such, the reticle remains relatively unchanged and therefore reticle distortions associated with a bent reticle are substantially reduced. It has generally been observed that a chuck having a thickness less than about 10 mm, and more particularly between about 1 mm and about 3 mm may be used. 
     In accordance with yet another aspect of the invention, a mask support ring may be provided to reduce reticle distortion. The mask support ring, designated  50  in FIG. 1, is often an annular ring, which is permanently or rigidly attached to the upper surface of the reticle  14  (thereby forming a single unit). In one example, the mask support ring  50  may be glued to the upper surface of the reticle  14 . An opening  52  in the ring  50  is arranged to allow the passage of the electron beam therethrough so that the ring  50  does not interfere with the reticle pattern. In general, the outer periphery of the ring  50  corresponds to the outer periphery of the reticle  14 . In one example, ring dimensions of approximately 220 mm OD and approximately 185 mm ID may be used for approximately a 200 mm OD reticle. The thickness of the ring  50  is typically configured to control the amount of reticle distortion, e.g., in plane distortion and/or out of plane distortion. By way of example, for a reticle having a thickness of about 0.75 mm, the ring may have a thickness between about 0 mm to about 10 mm. It has generally been found that for a thicker ring, e.g., 10 mm, a thinner chuck, e.g., 3 mm helps to reduce distortions. 
     With regards to the kinematic support assembly  36 , the kinematic support assembly  36  is arranged to support the chuck  34  relative to the stage table  32 . More particularly, the kinematic support assembly  36  provides a non-hysteretic way of supporting the chuck  34  relative to the stage table  32 . By non-hysteretic, it is meant, for example, that the kinematic support assembly  36  may be altered by an external agent while having the ability to return to its original non-altered configuration when the altering agent is removed. By way of example, the external agent may be deforming chuck such as a thermally expanding or contracting chuck or a deforming stage such as a thermally expanding or contracting stage. Accordingly, the chuck is held in manner that does not distort the chuck. That is, the chuck is free to expand or contract (radially) substantially without causing distortion, as for example, distortion created by a constrained expanding chuck. As should be appreciated, stress induced on the reticle is reduced by substantially eliminating chuck distortions. Furthermore, the kinematic support assembly  36  allows the chuck  34  and the reticle  14  to expand together, which reduces reticle bowing and reticle slippage. 
     To elaborate further, the kinematic support assembly  36  includes a plurality of kinematic flexures  55  that are substantially rigidly attached to both the chuck  34  and the stage table  32 . The size, material, number, and position of the flexures  55  are arranged to both vertically and tangentially restrain the chuck  34  so as to effectively prevent up/down, sideways (e.g., lateral) and rotational movements of the chuck  34 . In addition, the flexures  55  are arranged to allow the chuck  34  to move radially, as for example during thermal expansion or contraction of the chuck. By way of example, when the chuck  34  expands or contracts the flexures  55  bend in the radial direction. A contracting chuck  34  tends to pull the flexures  55  in an inward radial direction, and an expanding chuck  34  tends to push the flexures in an outward radial direction. Again, the kinematic support assembly  36  is typically non-hysteretic, and therefore, the flexures  55  are arranged to bend back to their original position when the thermal deformation is removed. 
     Described another way, the flexures  55  hold the chuck  34  in space in 6 degrees of freedom, while allowing freedom of radial expansion and contraction, so that the position of at least one point on the reticle is known relative to the stage. The concept of DOF (degrees of freedom) refers to the number of independent coordinates required to define its position. As is generally well known, a rigid body in three dimensions has six degrees of freedom. For example, 3 linear positions, e.g., represented by points along the x-axis, y-axis and z-axis, and 3 rotational positions represented by the angles θx, θy and θz, which are the rotational positions of the rigid body about the x-axis, y-axis and z-axis respectively. As such, the chuck  34 , which is supported by the flexures  55 , represents a rigid body which for the purpose of kinematic analysis is incapable of moving linearly in the x, y and z directions or rotationally about the x, y, and z axis. 
     Accordingly, the kinematic support assembly  36  is configured to hold the chuck in a fixed position relative to the stage so that the reticle is precisely located relative to the stage and more particularly the measuring system, e.g., interferometer and mirrors. As should be appreciated, the interferometer and mirrors are used to determine the position of the stage so that the circuit image can be precisely written to the wafer. 
     In the embodiment shown, a top portion of the flexure  55  is structurally attached to the stage table  32  and a bottom portion of the flexure  55  is structurally attached to the chuck  34 . The attachment may be made in any suitable manner. By way of example, the attachment s can be made with a bolt, screw, adhesive glue and/or the like. In one implementation, titanium screws are used to attach the flexure  55  to the stage table  32 . Titanium yields less of an impact on the magnetic fields of the electron beam projection lithography system than many metals due to its relatively poor electrical conductivity. In another implementation, a glue such as epoxy is used to attach the flexure  55  to the chuck  34 . A spacer  57  may also be provided between the flexure  55  and the stage table  32  so as to create a clearance  59  between the flexures  55  and the stage table  32 . As should be appreciated, the clearance  59  allows for flexure movement, as for example during thermal expansion of the chuck  34 . The spacer  57  may or may not be coupled to the flexures  55  or the stage table  32 . That is, in some embodiments, the spacer  57  may be an integral part of the flexure  55  or the stage table  32 , and in other embodiments, the spacer  57  may be a free body that is disposed between the flexure  55  and stage table  32 . 
     Furthermore, the radial compliant flexures  55  are generally positioned on an outside edge  42  of the chuck  34  and an inside edge  39  of the stage table  34  (e.g., inside the opening  38 ). By placing the flexures  55  on the outside edge  42  of the chuck  34 , the flexures  55  can move to compensate for the radially expanding or contracting chuck  34 . In addition, the flexures  55  are tangentially coupled to the outside edge  42  of the chuck  34  so as to provide DOF rigidity. As shown, the flexures  55  are typically vertical members, which are parallel to both the inside of the opening  38  and the outside edge  42  of the chuck  34 . It should be appreciated, however, that this is not a limitation and the position of the flexures  55  may vary according to the specific design of each assembly. 
     To elaborate further, the kinematic support assembly  36  generally includes multiple, e.g., three, kinematic flexures  55  that are spaced apart along the outer periphery of the chuck  34 . The three flexures  55  work together to provide both vertical and tangential rigidity while allowing the chuck  34  the ability to expand or contract. In general, it can be said that each of the three flexures  55  takes up about two degrees of freedom, however, more specifically, the vectorial combination of the flexures  55  essentially prevents the chuck  34  from moving relative to the stage table  32 . As is well known in the art, three points define a plane and therefore its preferable to have three flexures  55  holding the chuck  34 . Three flexures  55  tend to eliminate out of plane distortions associated with other multiple arrangements. That is, the three flexured assembly maintains a planar support structure so that the chuck  34  and more particularly the reticle  14  remain substantially planar. In most cases, each flexure  55  is spaced an equal distance apart from one another along the chuck perimeter. In some cases, however, it may be necessary to space the flexures  55  at different distances, for example, to overcome a physical limitation of the lithography system. As such, an angle  60  between any two of the flexures  55  is configured to be between about 80 and about 140 degrees. By way of example, an angle of about 120 degrees between all three flexures  55  may be used. In one implementation, the flexures  55  are separated by 90 degrees, 135 degrees and 135 degrees, respectfully. As should be appreciated, as the angle  60  is decreased between any two flexures  55 , the planar and lateral support of the entire assembly decreases, i.e., the three flexures begins to perform like two flexures when the angle between them becomes small. In general, the shallower the angle  60 , the more rigid and less flexible the flexures  55  have to be configured. 
     As mentioned, moving iron structures may alter the static magnetic fields around an electron beam lens and moving metal parts may generate eddy currents in the static magnetic fields. Therefore, in most cases, the flexures  55  are formed from a suitable non-conducting or semi-conducting material. The material is also preferably formed from a ductile material that is flexible so that the flexure  55  can bend without fracture. A flexible material allows the flexures  55  to be compliant with respect to the expanding and contracting chuck  34 . In addition, the material also preferably has properties that allow it to remain rigid so as to keep the chuck  34  in a fixed position relative to the stage table  32 . In one embodiment, the flexures  55  are formed from zirconia. It should be noted, however, that this is not a limitation and that the material may vary according to the specific design of each system. For example, flexures  55  formed from a ceramic such as silicon may also be used. Other materials such as metals can also be used if the electrical conductivity is low. Moreover, if the kinematic support assembly is used in systems other than electron beam projection lithography systems then the flexures  55  may be formed from conducting materials. By way of example, if the assembly is used in standard optical lithography systems then the flexures  55  may be formed from stainless steel, spring steel, berylium-copper and the like. 
     Moreover, the size of the flexures  55  is dependent on many factors. For one, the size of the flexure  55  generally varies according to the position, material and number of the flexures. As shown in FIG. 5 and 6, the flexures  55  are rectangular plates having a length  62 , a width  64  and a thickness  66 . It is generally believed that the greater the width  64 , the greater the rigidity in the direction (e.g. tangential) orthogonal to the bending direction (e.g., radial). It is also generally believed that the greater the thickness  66 , the greater the rigidity in the bending direction (e.g., radial). Furthermore, it is also generally believed that the greater the length  62 , the greater the flexibility (lose some stiffness). All three properties can be balanced, along with the position, material and number to produce a kinematic support assembly  36  for holding the chuck in a precise and known position relative to the stage. In one embodiment, the flexures  55  are configured with more width and less thickness to provide the desired rigidity and bending. 
     While this invention has been described in terms of several preferred embodiments, there are alterations, permutations, and equivalents, which fall within the scope of this invention. For example, although the present invention was described in context of an electron beam projection lithography system, it should be understood that this is not a limitation and that other systems such as optical lithography systems may be used. It should also be noted that there are many alternative ways of implementing the methods and apparatuses of the present invention. By way of example, although the above-described embodiments are well suited for reticles, the invention is not limited to use with reticles. For example, the invention may be used to hold other types of substrates such as wafers, photomasks and the like. In addition, although the flexures are shown as vertical hanging structures, they could also be positioned at different angles or be support structures that are coupled from beneath the chuck. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations, and equivalents as fall within the true spirit and scope of the present invention.