Abstract:
A chuck assembly for use in semiconductor processing equipment includes elements that permit reticle expansion and assembly misalignment without additional reticle deformation. Reticle expansion is allowed by flexible support elements that are positioned to move in the direction of expansion, but that also combine to provide the control necessary for processing. Misalignment is allowed by connections that attach the reticle securely and uniquely to the support elements despite some amount of imperfection in the reticle, or the connections themselves. Accounting in this way for expansion and misalignment prevents additional reticle distortion and thus improves the accuracy of the product.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to photolithography instruments used for patterning and processing substrates such as semiconductor chips and wafers. More specifically, the invention is concerned with reducing distortion of the reticle and the corresponding reticle pattern.  
         BACKGROUND OF THE INVENTION  
         [0002]    Lithography processes require positioning a reticle between an energy beam (typically electron or light) and the substrate chip, or wafer. The reticle must be held without slippage and in a way that does not cause distortion of the reticle pattern. This reticle is typically very thin. This thinness can cause the reticle to deform rather easily even though the reticle usually includes material reinforcing its perimeter, sometimes in the form of a ring secured to its perimeter. If a reticle deforms it can produce an imperfect image on the substrate that results in an imperfect final product.  
           [0003]    In modern lithography processes for exposing patterns on wafers and other substrates the reticle is moved at high speeds between discrete and precise positions to facilitate focusing the image on the substrate. This motion can generate dynamic reaction forces where the reticle is supported, leading to distortion of the reticle and, hence, distortion of the image focused on the substrate. It is therefore critical that the reticle be chucked in a manner that reduces or eliminates such reaction forces. The problem is complicated by the fact that lithography processes may occur in a clean room vacuum environment, rendering pneumatic chucks ineffective.  
           [0004]    Various attempts to address this difficulty have been less than successful. For example, reaction forces generated by the wafer (substrate) stage motion can be mechanically released to the floor (ground) by use of a frame member as described in U.S. Pat. No. 5,528,118 and published Japanese Patent Application Disclosure No. 8-166475. Additionally, reaction forces generated by the reticle (mask) stage motion can be mechanically released to the floor (ground) by use of a frame member as described in U.S. Pat. No. 5,874,820 and published Japanese Patent Application Disclosure No. 8-330224. These proposed solutions, however, have not provided optimum results.  
           [0005]    As the preceding discussion implies, a lithography process is a complex interaction of sensitive subsystems. The reticle and chuck assembly subsystem is sensitive to numerous inputs. Many things can contribute to the distortion of the reticle such as motion or vibration of the support structure for the reticle, or heat from an electron beam, which may cause thermal expansion. Prior art chuck assemblies can impart stresses where they clamp the reticle. In addition, such chuck assemblies that hold the reticle rigidly in all dimensions can compound deformation caused by thermal expansion by forcing the reticle to twist or bow as it expands.  
           [0006]    Other known methods of holding the reticle are often also not satisfactory solutions. Electrostatic methods of holding the reticle such as described in U.S. Pat. No. 5,532,903 to Kendall can be less secure than necessary considering the accelerations of 40 m/sec 2  or more that the reticle undergoes during the lithographic process. Vacuum methods of holding the reticle are ineffective in processes because typically the process itself is conducted in a vacuum.  
           [0007]    Thus, there is a continuing need in the art for a chucking assembly and method for the reticle retention that eliminates the stresses involved with mechanical clamping and allows for thermal expansion, while simultaneously holding the reticle with enough force that the reticle accelerations associated with the process do not degrade the final product.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention provides a chuck assembly for connecting a reticle to a stage while limiting distortion of the reticle and while still allowing movement with the stage. The chuck assembly includes a number of connector members secured to the stage where each member permits a seat to translate along a single axis in a plane parallel to the movement of the stage. The chuck assembly also includes protrusions that are secured to the reticle and that are also received by the seats on connector members. Using the protrusion and seat arrangement removes or reduces clamping forces as a source of reticle distortion. The interface between protrusion and seat also allows for some degree of misalignment while still providing the degree of retention and control necessary to position the reticle. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    The foregoing and other aspects and advantages will be better understood from the following detailed description of the preferred embodiment of the invention with reference to the drawings, in which:  
         [0010]    [0010]FIG. 1 is a side view of an alternate lithography exposure apparatus employing the present invention;  
         [0011]    [0011]FIG. 2 is a different side view of an alternate lithography exposure apparatus employing the present invention;  
         [0012]    [0012]FIG. 3 is a top view of a chuck assembly according to an embodiment of the present invention;  
         [0013]    [0013]FIG. 4 is a cross-sectional side view along line  4 - 4  in FIG. 3;  
         [0014]    [0014]FIG. 5 is a cross-sectional side view illustrating an embodiment of the reticleprotrusion sub-assembly according to the invention;  
         [0015]    [0015]FIG. 6 is a partial cross-sectional view of an alternative embodiment of the invention; and  
         [0016]    [0016]FIG. 7 is a schematic elevational depiction of the principal components of the optical system and associated control systems of a conventional divided-reticle electron beam microlithography apparatus according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    Referring to FIGS. 1 and 2, a lithography exposure apparatus  21  may be used to employ the present invention. In doing so exposure apparatus  21  transfers a pattern of an integrated circuit from reticle  10  onto semiconductor wafer  23 . According to the present invention, reticle  10  is mounted on a kinematic support structure, as described in greater detail below, in order to reduce or eliminate distortion of the reticle during the photolithography process.  
         [0018]    Apparatus frame  72  preferably is rigid and supports the components of the exposure apparatus, but can be varied to suit the design requirements for a particular application. Apparatus frame  72  generally supports reticle stage  76 , wafer stage  24 , lens assembly  78 , and illumination system  74 . Alternatively, for example, separate, individual structures (not shown) can be used to support wafer stage  24  and reticle stage  76 , illumination system  74 , and lens assembly  76 .  
         [0019]    Illumination system  74  includes an illumination source  84  and an illumination optical assembly  86 . Illumination source  84  emits an exposing beam of light energy. Optical assembly  86  guides the beam of light energy from illumination source  84  to lens assembly  78 . The beam illuminates selectively different portions of reticle  10  and exposes wafer  23 . In FIG. 1, illumination source  84  is illustrated as being supported above reticle stage  76 . Typically, however, illumination source  84  is secured to one of the sides of apparatus frame  72  and the energy beam from illumination source  84  is directed to above reticle stage  76  with illumination optical assembly  86 .  
         [0020]    Lens assembly  78  projects and/or focuses the light passing through reticle  10  to wafer  23 . Depending upon the design of apparatus  21 , lens assembly  78  can magnify or reduce the image illuminated on reticle  10 .  
         [0021]    Reticle stage  76  holds and precisely positions reticle  10  relative to lens assembly  78  and wafer  23 . Similarly, wafer stage  24  holds and positions wafer  23  with respect to the projected image of the illuminated portions of reticle  10 . In the embodiment illustrated in FIG. 1 and FIG. 2, wafer stage  24  and reticle stage  76  are positioned by shaft-type linear motors  30 . Depending upon the design, apparatus  21  may include additional servo drive units, linear motors and planar motors to move wafer stage  24  and reticle stage  76 , but other drive mechanisms may be employed.  
         [0022]    The basic device as described may be used in different types of lithography processes. For example, exposure apparatus  21  can be used in a scanning type photolithography system that exposes the pattern from reticle  10  onto wafer  23  with reticle  10  and wafer  23  moving synchronously. In a scanning type lithography process, reticle  10  is moved perpendicular to an optical axis of lens assembly  78  by reticle stage  76 , and wafer  23  is moved perpendicular to an optical axis of lens assembly  78  by wafer stage  24 . Scanning of reticle  10  and wafer  23  occurs while reticle  10  and wafer  23  are moving synchronously.  
         [0023]    Alternatively, exposure apparatus  21  may be employed in a step-and-repeat type photolithography system that exposes reticle  10  while reticle  10  and wafer  23  are stationary. In the step-and-repeat process, wafer  23  is in a constant position relative to reticle  10  and lens assembly  78  during the exposure of an individual field. Subsequently, between consecutive exposure steps, wafer  23  is consecutively moved by wafer stage  24  perpendicular to the optical axis of lens assembly  78  so that the next field of semiconductor wafer  23  is brought into position relative to lens assembly  78  and reticle  10  for exposure. Following this process, the images on reticle  10  are sequentially exposed onto the fields of wafer  23 .  
         [0024]    Referring now to FIG. 3, chuck assembly  100  according to the invention is illustrated. Chuck assembly  100  provides for connection of reticle  10  or similar objects to a frame such as stage  76 , while limiting reaction forces and distortion of the object which might otherwise result from movement of the stage. Chuck assembly  100  generally comprises a plurality of protrusions  102  received in seats  104 , which are supported by a pair of flexures  106 . As explained further below, this arrangement may be employed advantageously in other assemblies wherein an object other than a reticle must be restrained and distortion of that object must be addressed. For example, the present invention could be used in mechanisms experiencing less acceleration, which would allow the cone-angle of the present embodiment to be relaxed and thus reduce the friction of the protrusion in the seat, resulting in further reduction of object distortion. In a preferred embodiment three connector members may be equally spaced around the circular reticle.  
         [0025]    As shown in FIG. 5, reticle  10 , according to one embodiment of the invention, comprises reticle ring  11  secured to the top of reticle membrane  112 , or mask. Protrusions  102  are secured to the bottom of reticle membrane  112 . Protrusions  102  may be secured, for example, by being bonded to reticle membrane  112 . Alternatively, protrusions  102  may be bonded to, or incorporated directly into, reticle ring  11  and projected through openings in the reticle membrane  112 . Additionally, the positions of reticle ring  11  and reticle membrane  112  could be reversed allowing protrusions  102  to be bonded directly to reticle ring  11  without having to account for reticle membrane  112 .  
         [0026]    Protrusions  102  are preferably evenly distributed along the periphery of reticle membrane  112  and total three in number. In a preferred embodiment, hemispherical protrusions  102  are set into and are supported by seats  104  of conical cross-section. The interface between protrusion  102  and seat  104  permits some degree of misalignment while still providing the degree of control necessary to position the reticle. In an alternate preferred embodiment, the relative positions of seat  104  and protrusion  102  are reversed, with seat  104  being set into reticle  10 .  
         [0027]    Seats  104  are in turn connected to flexures  106 . Flexures  106  are members that limit each seat  104  to translation in its specific radial direction as indicated by arrows A, B and C. Flexures  106  are in turn supported by reticle stage  76  itself. Flexures may be, for example, a pair of flexible plates that permit bending only in one direction and resist rotation. Flexures may also be rigid members equipped with a mechanism that allows the seat to travel in a desired direction, but no other.  
         [0028]    It should be understood that the combined effect of limiting seats  104  to radial translations A, B, and C is that a reticle is allowed to undergo planar expansion without the resulting stresses that would occur if protrusions  102  were constrained in the direction of planar expansion. Without these stresses there is reduced opportunity for the reticle to deform. The combined effect of limiting seats  104  to radial travel A, B and C still allows the control necessary to use the chuck assembly to position the reticle.  
         [0029]    Similarly, the ability of the interface between protrusions  102  and seat  104  to permit some degree of misalignment works to minimize the effect of that misalignment on the reticle. For example, where a hypothetical localized deformation of the reticle would cause a misalignment, that deformation would not cause a general deformation of the reticle if the misalignment fell within the range permitted by the interface. This same hypothetical deformation could have caused a general deformation had, for example, a clamp been used.  
         [0030]    It should also be understood that it is preferable for flexures  106  perform their function with as little friction as possible. Frictionless travel in the direction allowed by the flexures  106  causes less stress to remain in the reticle  10  while still allowing the control necessary to position reticle  10 . In a preferred embodiment, flexures  106  are composed of twin, flat, members that are very flexible in only one direction. These members are then oriented in parallel to support seat  104 , yet constrain seat  104  to radial motion.  
         [0031]    As shown in FIG. 4, incident electron beam  116  is generally perpendicular to reticle membrane  112  and the direction of isolated translation (arrows A, B) allowed by flexures  106 . This side view clearly shows hemisphere-shaped protrusions  102  received in seats  104 . This arrangement permits seat  104  to receive protrusion  102  should reticle membrane  112  or reticle ring  11  be non-planar (deformed) in a way inducing misalignment. Other configurations of protrusions and seats that also provide these advantages may be used. For example, it is contemplated that seats  104  could be holes that receive cone-shaped protrusions  102 .  
         [0032]    Whatever configuration used should permit the reticle to move where it is in contact with the chuck assembly, but this movement must be limited. Translation of each contact point should be limited to account for in-plane radial distortion of the reticle, or account for misalignment of the seats and protrusions. According to a preferred embodiment, where the reticle is a planar circle and contact points are three in number and are evenly spaced about the periphery of the reticle, the allowed translation would typically be in the reticle plane along axes that originate at the contact points and intersect near the center of the reticle. This arrangement allows in-plane expansion and contraction of the reticle without causing net translation with respect to the reticle stage. If a chuck assembly should prevent such inplane expansion and contraction at the contact points, it would be contributing to stresses causing the reticle to bow or twist. Care should also be taken to design the chuck assembly so that in-plane accelerations caused by the photolithography process do not get converted into out-of-plane translation.  
         [0033]    A preferred embodiment of the invention contemplates meeting these requirements by setting protrusions  102  into conical seats  104  having a vertex angle of between about 15 to 45 degrees and more preferably about 20 to 40 degrees. The probability of out-of-plane translation decreases when the vertex increases, as does the ability of the protrusion to rotate relative to the seat. Thus, the vertex must be adjusted to account for reticle accelerations and degrees of expected distortion. Alternatively, seat  104  may be a cylindrical hole to provide line contact with the spherical protrusions.  
         [0034]    [0034]FIG. 6 illustrates a further alternative embodiment of the invention. As shown in FIG. 6, conduit  118  is provided within seat  104 , protrusion  102 , and reticle ring  11 . Conduit  118  permits gas or liquid to be circulated within the ring to control the temperature. The fluid flows in through one seat and exits via a different seat. Flexible hose  119  is provided as supply and return for the fluid. The return is provided by a separate conduit similar to that shown in FIG. 6, the only difference being the direction of flow. This flow provides control of the reticle temperature. Line contact  120  between the hemisphere-shaped protrusion  102  and conical seat  104  provides the seal, provided the pressure is kept low enough that it does not cause the protrusion to lift off the seat.  
         [0035]    In a preferred embodiment of the invention as described above chuck assembly  100  is employed in an electron beam lithography exposure apparatus  21  as is shown in FIG. 7. This figure depicts the type of exposure apparatus known as a conventional divided-reticle electron-beam microlithography system in which the illumination source is electron gun  1 . Difficulties encountered in electron beam lithography include heat build-up that can cause the deformation of reticle  10  that the present invention may alleviate.  
         [0036]    In this apparatus electron gun  1  emits electron beam EB that propagates along optical axis AX toward first and second condenser lenses  2 ,  3  respectively. The optical path for the electron beam is typically in a vacuum. The electron beam EB then passes through condenser lenses  2 ,  3  to form crossover image C.O. 1 . Crossover image C.O. 1  is located on optical axis AX at blanking aperture  7 .  
         [0037]    Beam-shaping aperture  4  is situated between second condenser lens  3  and blanking aperture  7 . Beam-shaping aperture  4  creates an opening that is sized and shaped to pass only the portion of electron beam EB necessary to illuminate a single exposure unit (“subfield”) of downstream reticle  10 . For example, if the subfields on reticle  10  are rectangular in shape (and each subfield is usually sized and shaped identically) then beam-shaping aperture  4  defines a corresponding rectangular opening as viewed axially. If the subfields on reticle  10  are square in shape and have an area of, for example, 1 mm 2 , then beam-shaping aperture  4  defines an opening that provides the electron beam, as seen by the reticle, with a square transverse profile where each side of the square is slightly greater than 1 mm. Collimating lens  9 , which is situated between blanking aperture  7  and reticle  10 , forms on reticle  10  an image of the opening defined by beam-shaping aperture  4 .  
         [0038]    The portion of electron beam EB propagating between electron gun  1  and reticle  10  is termed herein “illumination beam” IB. The portion of the electron-optical system comprising the lenses  2 ,  3 ,  9 , the apertures  4 ,  7 , blanking deflector  5 , and selection deflector  8 , is termed herein “illumination-optical system” IOS.  
         [0039]    Blanking deflector  5  is disposed between beam-shaping aperture  4  and blanking deflector  7 . During moments when no exposure is desired, blanking deflector  5  is energized and deflects illumination beam IB laterally so as to cause the entire illumination beam IB to be blocked by blanking aperture  7 . Selection deflector  8  is situated between blanking aperture  7  and collimating lens  9 . Selection deflector  8  deflects illumination beam IB mainly in the X-, or left-to-right direction (note the non-standard axes orientation shown in the figure) in a scanning manner. Collimating lens  9 , situated between selection deflector  8  and reticle  10 , collimates illumination beam IB before beam IB illuminates the desired subfield of reticle  10 . Scanning the illumination beam IB in this manner illuminates successive subfields on reticle  10  within the field of illumination-optical system IOS. Thus, an image of the opening defined by beam-shaping aperture  4  is focused on reticle  10 .  
         [0040]    In FIG. 7, only a single subfield (centered on optical axis AX) is shown. Actual reticle  10  extends outward in the X-Y plane and comprises many subfields. In any event, reticle  10  defines a pattern (chip pattern) for a single semiconductor device (“die”) to be formed on downstream substrate  23 , and each subfield defines a respective portion of the pattern.  
         [0041]    As noted above, illumination beam IB is deflected laterally to illuminate successive subfields situated within the field of the illumination-optical system. These multiple subfields in the field of illumination-optical system IOS, however, do not typically constitute the sum total of the surface of substrate  23 . Illuminating a subfield situated outside the field of illumination-optical system IOS requires moving reticle  10  relative to illumination-optical system IOS. To facilitate this, reticle  10  is mounted on reticle ring  11 , which is in turn affixed to chuck assembly  100 , as described above, which is movable in the X and Y directions.  
         [0042]    As illumination beam IB passes through the illuminated subfield on reticle  10 , electron beam EB becomes capable of forming an image of the illuminated subfield on substrate  23 . Electron beam EB is therefore termed “patterned beam” PB after it propagates through reticle  10 . The electron-optical system located between reticle  10  and substrate  23  is primarily concerned with projecting patterned beam PB onto the desired location on substrate  23 . That portion of the electron-optical system is therefore termed “projection-optical system” POS.  
         [0043]    Projection-optical system POS includes first and second projection lenses  15 ,  19 , respectively, that are typically configured as a “symmetric magnetic doublet” (SMD). First and second projection lenses  15 ,  19  operate in concert to form a reduced image of the illuminated reticle subfield on substrate  23 . This reduced image is smaller than, or demagnified relative to, the corresponding illuminated subfield of reticle  10  by a factor termed the “demagnification ratio.” The demagnification ratio is a factor such as 1/4 or 1/5. Projection-optical system POS also includes deflector  16 . Deflector  16  deflects patterned beam PB laterally to form the image of the illuminated subfield at the desired location on substrate  23 .  
         [0044]    The surface of substrate  23  (or “wafer”) is coated with an appropriate resist prior to exposure so that patterned beam PB imprints the demagnified image when it illuminates wafer  23 . The demagnified images of successively illuminated subfields form a complete die pattern on wafer  23  when all images are contiguous with each other (i.e., a “stitched” together) in the proper order and arrangement. Proper stitching is facilitated by mounting wafer  23  on wafer stage  24 , which is moved as required in the X and Y directions and by deflecting patterned beam PB for each image using deflector  16 .  
         [0045]    First projection lens  15  causes patterned beam PB to form crossover image C.O. 2  on the optical axis upstream of second projection lens  19 . At crossover image C.O. 2 , the axial distance between reticle  10  and wafer  23  is divided such that the axial distance from reticle  10  to crossover image C.O. 2 , divided by the axial distance from crossover image C.O. 2  to wafer  23 , is equal to the inverse of the demagnification ratio. Crossover image C.O. 2  is also the location, along optical axis AX, of contrast aperture  18 . Contrast aperture  18  blocks charged particles in patterned beam PB that were scattered previously by illumination beam IB from propagating to wafer  23 .  
         [0046]    Backscattered-electron (BSE) detector  22  is situated between second projection lens  19  and wafer  23 . BSE detector  22  detects backscattered electrons produced when patterned beam PB strikes certain regions (for example, alignment marks or analogous features) on wafer  23 . The positions of the alignment marks on wafer  23  are ascertained from characteristics of the BSE signal produced by BSE detector  22 . This yields the basic data concerning positions of the reticle and wafer, and alignments between wafer  23  and the electron-optical system or between wafer  23  and reticle  10 .  
         [0047]    Wafer  23  is preferably mounted on an electrostatic wafer chuck (not shown) that, in turn, is mounted on wafer stage  24 . Wafer stage  24  moves the wafer chuck (and thus wafer  23 ) in the X and Y directions. The various subfields of the chip pattern on reticle  10  can be exposed successively by synchronously moving (or “scanning”) reticle stage  76  (illustrated in greater detail in FIGS. 4, 5,  6 , and  7 ) and wafer stage  24  in opposite directions. The axis along which these scans are performed is perpendicular to the axis along which lateral beam scanning is performed using patterned beam PB. The respective positions of stages  76 ,  24  are determined accurately, in real time, using respective position sensors  12 ,  25  each employing one or more laser interferometers. Interferometers are typically used because extremely accurate position measurements are required to accurately stitch together the demagnified images.  
         [0048]    Lenses  2 ,  3 ,  9 ,  15 ,  19  and deflectors  5 ,  8 ,  16  are controlled by main controller (e.g., microprocessor)  31  via respective coil power supplies,  2   a ,  3   a ,  9   a ,  15   a ,  19   a ,  5   a ,  8   a ,  16   a . Also, reticle stage  76  and wafer stage  24  are controlled by main controller  31  via respective stage drivers  76   a ,  24   a , and position sensors  12 ,  25  are controlled by main controller  31  via respective interface units  12   a ,  25   a . Data from the BSE detector  22  are routed to main controller  31  via an interface  22   a . Main controller  31  determines stage-position control errors and corrects for these using deflector  16 , which facilitates accurate stitching.  
         [0049]    As will be appreciate by persons skilled in the art, in preferred embodiments discussed herein, it is assumed that the object held, e.g. reticle, is moving in a horizontal plane with gravity acting downward to prevent upward motion. It will be further appreciated that, if an embodiment of the invention is utilized with another orientation, a person skilled in the art may choose to include an additional restraining means, which may include a spring or clamp, to assist in holding the object.  
         [0050]    The use of the exposure apparatus described herein is not limited to a photolithography system for semiconductor manufacturing or to an electron beam exposure apparatus. The exposure apparatus, for example, can be used as an LCD photolithography system that exposes a liquid crystal display device pattern onto a rectangular glass plate or a photolithography system for manufacturing a thin film magnetic head. Furthermore, the exposure apparatus and chuck assembly can also be applied to a proximity photolithography system that exposes a reticle pattern by closely locating a reticle and a substrate without the use of a lens assembly. Additionally, an exposure apparatus utilizing a chuck assembly according to the invention can be used in other devices, including other semiconductor processing equipment, machine tools, metal cutting machines, and inspection machines.  
         [0051]    While the invention has been described in terms of its preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the scope of the appended claims.