Abstract:
Reticle and/or wafer stage interferometers are mounted to a supporting body that is separate from the body that supports the projection optical system of a lithography apparatus. This enables the size of the body supporting the projection optical system to be reduced so that it has more favorable dynamic characteristics.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. Provisional Application No. 60/725,662 filed Oct. 13, 2005. The disclosure of U.S. Provisional Application No. 60/725,662 is incorporated herein by reference in its entirety. 

   BACKGROUND 
   This invention relates to lithography apparatus and methods of performing lithographic exposure, commonly used to transfer a pattern onto a substrate in order to manufacture devices such as, for example, semiconductor devices, liquid crystal displays, etc. 
   Many current lithography apparatus have a large body structure that holds the projection lens, the metrology system and that supports the reticle stage and components of the illumination unit.  FIGS. 1 and 2  show such an apparatus. In particular, body  20 , which can be mounted on a base  100 , includes one or more platforms and a plurality of columns, and supports a projection optical system  60  and a reticle stage  80 . The metrology system also is mounted to the body  20 . The metrology system includes laser interferometers  30  for measuring the position of the wafer stage  70 , laser interferometers  40  for measuring the position of the reticle stage  80 , and other sensors such as auto-focus sensors for measuring the vertical position of the wafer and/or reticle and alignment microscopes, etc. (generally represented by  50 ). 
   The body  20  is designed to hold all of the metrology elements in a fixed position relative to the projection optical system  60 . Of course, some vibrations and distortions occur and cause degradation of the lithography apparatus performance. 
   One problem with existing lithography apparatus is that the body  20  is a relatively large structure, and therefore has undesirably low vibration frequencies. One contributing factor to the large size of the body is that the stage interferometers  30  and  40  are located relatively far from the projection optical system. Accordingly, when the interferometers are mounted to the same body that supports the projection optical system and other components of the lithography apparatus, that body becomes large. 
   SUMMARY 
   According to aspects of the invention, the reticle and/or wafer stage interferometers are mounted to a supporting body to define an interferometer unit that is separate from the body that supports the projection system. This enables the size of the body supporting the projection system to be reduced so that it has more favorable dynamic characteristics. 
   According to preferred embodiments, the interferometer unit is suspended. This is beneficial in that the interferometer unit can be isolated from background vibrations. 
   According to one aspect of the invention, a lithography apparatus includes a projection system, a stage for holding an object and an interferometer unit. The interferometer unit mounts an interferometer that emits a first measurement beam to the projection system and that emits a second measurement beam to the stage. A position of the stage relative to the projection system is determined from the first and second measurement beams. The interferometer unit is suspended from a support member and is movable relative to the projection system. 
   At least one suspension member is disposed between the interferometer unit and the support member so as to flexibly suspend the interferometer unit from the support member. The suspension member preferably is stiff in an axial direction and is flexible in directions orthogonal to the axial direction. However, according to some embodiments, the suspension member can be flexible in its axial direction. 
   According to some embodiments, the suspension member is a wire or a rod. The wire or rod preferably is rotatably attached to the interferometer unit. The point of attachment to the interferometer unit preferably is vertically above the center of gravity of the interferometer unit. In embodiments having three or more suspension members, the suspension members can be attached to the interferometer unit at a location that is in or below a horizontal plane containing the center of gravity of the interferometer unit. 
   The suspension member can be directly attached to the support member or it can be attached to the support member through a mounting device. The mounting device, which is disposed between the support member and the suspension member, has a stiffness in the axial direction that is less stiff than a stiffness of the suspension member in the axial direction. According to some embodiments, the mounting device includes a piston supported by gas or a vacuum so as to absorb vibrations in the axial direction. 
   The stage can be a reticle stage or a substrate stage, and thus the interferometer can measure the position of the reticle or the substrate stages. In some embodiments, the interferometer unit includes interferometers for the reticle stage and for the substrate stage. 
   In some embodiments, a plurality of interferometer units are separately suspended from a support member and measure the position of the stage relative to the projection system in different directions (for example, different orthogonal directions). Alternatively, the plurality of interferometer units can be attached to each other so that their positions are fixed relative to each other. 
   The support member can be a frame from which the interferometer unit(s) is/are suspended. The frame can also suspend the projection system and support the reticle stage. Alternatively, the frame supporting the interferometer unit(s) can be separate from the frame that supports the reticle stage and/or projection system. The frame can be mounted on vibration isolation mounts. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described in conjunction with the following drawings of exemplary embodiments in which like reference numerals designate like elements, and in which: 
       FIG. 1  is a simplified elevational view illustrating a conventional lithography apparatus; 
       FIG. 2  is a simplified perspective view of part of the body of the lithography apparatus in  FIG. 1 ; 
       FIG. 3  is an elevational view of a lithography apparatus to which aspects of the invention are applied; 
       FIG. 4  schematically illustrates an interferometer unit according to one embodiment of the invention; 
       FIG. 5  illustrates an interferometer unit according to a second embodiment of the invention; and 
       FIG. 6  illustrates an arrangement in which separate X and Y interferometer units are provided. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   In accordance with aspects of the invention, the interferometer system is supported separately from the projection system, and preferably separately from the other components of the lithography apparatus such as the wafer and the reticle stages. In the illustrated embodiments, one or more interferometer units holding components of the reticle and wafer interferometer systems are suspended from a support member (or frame) that also suspends the projection system. However, the invention also can be implemented by providing a separate support frame from which only the interferometer unit(s) is/are suspended. That is, a first support member or frame can be provided to support the projection system and/or reticle stage while a second support member or frame can be provided to support (by suspension) the interferometer unit(s). In addition, the invention can be applied to systems that support the projection system by means other than suspension. For example, the invention can be applied to systems in which the projection system is rigidly held to a support frame, as is well known, rather than being suspended. In such an arrangement, the interferometer unit(s), however, would be suspended from either the support member (or frame) that rigidly supports the projection system or from a separate support member (or frame). 
     FIG. 3  illustrates a lithography apparatus according to one embodiment of the invention. A support member or frame  25  is mounted to a base unit  100 , either directly or by vibration isolation mounts  104 . Vibration isolation mounts  104  can be passive or active devices. Passive vibration isolation mounts typically include resilient components made from rubber and/or including gas or other damping devices. Active vibration isolation mounts include a precisely controlled and actively driven member such as a voice-coil motor and/or gas-driven piston unit whose movement is controlled by controlling the supply of gas to opposite sides of a piston. 
   A wafer stage base  72  is mounted to the base unit  100 , for example, by active or passive vibration isolation mounts  74 . A wafer stage  70  that holds a wafer W is supported by the wafer stage base  72 . As is well known, the wafer stage  70  moves in the X and Y directions to expose multiple shot areas on the wafer to a pattern projected through projection system  60 . 
   Projection system  60  is suspended from the support member  25  by three suspension members  65  (only two are shown in  FIG. 3 ). The suspension members  65  can be wires or rods that are stiff in the Z direction but flexible in the X and Y directions. The projection system  60  can be supported, for example, in the manner described and shown in WO 2006/038952 published on Apr. 13, 2006. The disclosure of WO 2006/038952 is incorporated herein by reference in its entirety. Active or passive vibration isolation units can be provided between the support member  25  and each suspension member  65  to prevent Z-direction vibrations from transmitting to the suspension members  65  from the support member  25 . 
   A reticle stage base  85  is mounted on the support member  25 , for example, by passive or active vibration isolation mounts  84 . A movable reticle stage  80  holding a reticle R is controlled to move in the X and Y directions on the reticle stage base  85 . An illumination optical system (not shown) also is provided and can be entirely mounted on, or have components mounted on, the support member  25 . 
   An interferometer unit  200 , shown in  FIGS. 3 and 4 , is suspended from the support member  25  by a suspension member  250 . Member  250  is stiff in the Z direction but flexible in the X and Y directions. Member  250  can be a wire, rod, or beam, for example. A first end  252  of the flexible suspension member  250  is attached to the interferometer unit  200  at a position located above the center of gravity  275  of the interferometer unit  200 . The first end  252  of member  250  should be attached to the interferometer unit  200  in a manner that allows it to rotate freely relative to the interferometer unit  200 . This rotatable attachment of the flexible suspension member  250  to the interferometer unit  200  above the center of gravity  275  enables the interferometer unit  200  to hang freely with the desired orientation. For example, if the suspension member  250  is a flexible wire, the end of the wire can be rigidly attached to the interferometer unit  200  because the wire itself can bend or twist to act like a flexible joint. If the suspension member  250  is a beam or a rod that is relatively stiff in bending, then flexible joints should be provided, preferably at both ends of the suspension member  250 . Each flexible joint can be, for example, a universal joint, a ball joint, a ball-in-socket, etc. 
   In the embodiment of  FIG. 4 , the second, upper end of the flexible suspension member  250  is attached to an isolation member  300  that is supported by the support member  25 . In particular, the second end of flexible suspension member  250  is attached to a piston  310  of isolation member  300 . The isolation member  300  is filled with gas (or a vacuum) such that it has a low stiffness in the Z-direction. Therefore, isolation member  300  reduces or prevents Z-direction vibrations from being transmitted to the suspension member  250  (and thus to the interferometer unit  200 ) from the support member  25 . Isolation member  300  also provides the lifting force to support the weight of the interferometer unit  200 . Other examples of structures that can be used as isolation member  300  include: rubber or elastomer members, attractive or repulsive magnets (permanent magnets, electromagnets or a combination), mechanical springs (coil, leaf, etc.), or any combination of passive and active isolation devices. Isolation member  300  also can be provided at the other (lower) end of suspension member  250 . 
   In the  FIG. 4  embodiment, a reticle stage interferometer  220  and a wafer stage interferometer  210  are mounted to the interferometer unit  200 . The invention also could be implemented by attaching only one of the reticle stage or wafer stage interferometers to the interferometer unit  200 . 
   The reticle stage interferometer  220  emits a measurement beam  261  to the reticle stage  80  and a measurement beam  262  to the projection system  60  so that the position of the reticle stage  80  relative to the projection system  60  can be determined. This information then is used to control the movement of the reticle stage  80 . The wafer stage interferometer  210  emits a measurement beam  264  to the wafer stage  70  and a measurement beam  263  to the projection system  60 . Based on measurement beams  263  and  264 , the position of the wafer stage  70  relative to the projection system  60  can be determined similar to the way in which the position of the reticle stage  80  relative to the projection optical system  60  is determined. For simplicity of explanation, each measurement beam  261 - 264  is referred to in the singular; however, as is known, each beam  261 - 264  can be one or more beams depending on the number of axes measured. For example, each beam can include four or more beams, and measurements can be obtained in the X, Y, Z, θX, θY and θZ axes. 
     FIG. 4  illustrates beams  261 - 264  extending in a single direction. However, as is known, the position of the stages  70 / 80  relative to the projection system  60  usually is determined in both the X and Y directions. Thus,  FIG. 4  is merely a simplified diagram. The stage position preferably is measured in six degrees of freedom (X, Y, Z, θX, θY and θZ). The interferometer unit  200  could be L-shaped so as to hold reticle stage and wafer stage interferometers that emit beams in the X and Y directions. Alternatively, as shown in  FIG. 6 , separate interferometer units  200 X and  200 Y can be provided to obtain information in the X and Y directions.  FIG. 6  also shows (in phantom) an L-shaped bracket that can be provided to rigidly fix the interferometer units  200 X and  200 Y to each other. 
     FIG. 5  shows a second embodiment in which the flexible suspension member  250  is directly attached to the support member  25  without any vertical isolation piston or other structure between the members  25  and  250 . In this implementation, there is no vertical isolation between the support member  25  and the interferometer unit  200 . This implementation may be more appropriate for architectures in which the interferometer unit is supported by a member (or frame) separate from the member (or frame) that supports the reticle stage (and possibly the projection system), which tends to receive vibrations due to movement of the reticle stage. That separate support member (or frame) for the interferometer unit  200  preferably is vibrationally isolated from the ground by providing active or passive vibration isolation mounts between the support member (or frame) and the ground or base unit on which the support member (or frame) is mounted. 
   An alternative embodiment would be to incorporate the vertical compliance in the suspension member  250  itself. This could be done by using an axially flexible member such as a spring or elastic strap/band as the suspension member. 
     FIG. 6  illustrates an embodiment in which an X-direction interferometer unit  200 X and a Y-direction interferometer unit  200 Y are provided. Each interferometer unit is suspended in the manner that was described in conjunction with  FIG. 4 . The X-direction interferometer unit  200 X supports a reticle interferometer that emits a reticle stage beam  261 X and a projection system beam  262 X, whereas the Y-direction interferometer unit  200 Y supports a reticle stage interferometer that emits a Y-direction reticle stage beam  261 Y and a Y-direction projection system beam  262 Y. Using these beams, the position of the reticle stage  80  relative to the projection system  60  is determined in both the X and Y directions. Each of the beams  261 X,  261 Y,  262 X and  262 Y consists of one or more beams so as to measure position in one or more axes, preferably in X, Y, Z, θX, θY and θZ directions. The X-direction interferometer unit  200 X and the Y-direction interferometer unit  200 Y respectively emit wafer stage and projection system beams  263 X,  264 X,  263 Y and  264 Y (each consisting of one or more beams) so that the position of the wafer stage  70  relative to the projection system  60  can be determined in at least the X and Y directions, and preferably in X, Y, Z, θX, θY and θZ directions, similar to the manner in which determination is made for the reticle stage  80 . 
   A single suspension member can be provided for the, or each, interferometer unit, or each interferometer unit can be supported by more than one suspension member. For example, according to one embodiment, each interferometer unit is supported by three suspension members. If there are three or more suspension members for an interferometer unit, the point of attachment of the suspension members to the interferometer unit need not be above the interferometer unit center-of-gravity, but can be in or below the horizontal plane containing the interferometer unit center-of-gravity. 
   Attaching the X and Y interferometer units  200 X and  200 Y to each other, for example, with one or more L-shaped brackets (one is shown with phantom lines in  FIG. 6 ), keeps the units properly oriented relative to each other. In addition, suspending the assembly formed by the joined units with a third suspension member (also shown with phantom lines in  FIG. 6 ) reduces rotation of the interferometer unit assembly relative to the projection system  60 . When additional stabilization of the interferometer units is desired, actuators, preferably non-contact electromagnetic actuators such as voice-coil motors, for example, can be provided to prevent the interferometer unit(s) from moving excessively in the θX, θY and θZ directions. It is noted that the tendency of the interferometer unit(s) to move increases as the number of suspension members decreases (a single suspension member permitting more movement than arrangements using three or more suspension members) and as the flexibility of the suspension member(s) increases. Thus, if three or more suspension members are used, particularly if the suspension members are rods or beams, no actuators may be needed to further stabilize the interferometer unit(s). 
   The lithography apparatus can be a step-and-repeat apparatus that exposes the pattern onto the substrate while the substrate is stationary or it can be a scanning lithography apparatus that exposes the pattern onto the substrate while the substrate is moving. The lithography apparatus can use immersion technology in which an immersion liquid is disposed between the projection system and the substrate. 
   The lithography apparatus of the above-mentioned embodiments can be manufactured by incorporating and optically adjusting an illumination optical system composed of a plurality of lenses and a projection system into the main body of the lithography apparatus, and installing the reticle stage and the wafer stage composed of a plurality of mechanical parts to the main body of the lithography apparatus, connecting wires and pipes, and performing overall adjustment (electrical adjustment, operation check, etc.). Furthermore, it is preferable that manufacturing of the lithography apparatus is performed in a clean room with controlled temperature and cleanliness. 
   Furthermore, when a semiconductor device is manufactured by using the lithography apparatus of the above-described embodiments, the semiconductor device is manufactured by a step of designing a performance capability and function of the device, a step of manufacturing a reticle based on the designing step, a step of forming a wafer from a silicon material, a step of performing alignment by the lithography apparatus of the above-mentioned embodiment and exposing a pattern of the reticle onto a wafer, a step of forming a circuit pattern such as etching or the like, a step of assembling a device (including a dicing process, a bonding process, a packaging process), a step of testing, and the like. 
   This invention can be applied to a liquid crystal panel manufacturing exposure apparatus disclosed in, for example, International Publication No. WO 99/49504. Furthermore, this invention can be applied to a lithography apparatus using extreme ultraviolet light (EUV light) having a wavelength of several nm-100 nm as an exposure beam. 
   Furthermore, this invention is not limited to the application for the lithography apparatus for manufacturing a semiconductor device. For example, this invention can be applied to a lithography apparatus for manufacturing various devices such as a liquid crystal display element formed on a square-shaped glass plate, or a display device such as a plasma display or the like, or an imaging element (CCD), a micro-machine, a thin-film magnetic head, a DNA chip, or the like. Furthermore, this invention can be applied to a lithography process (lithography apparatus) in which a mask (photomask, reticle, or the like) having a mask pattern of various devices is formed by using a photolithographic process. 
   While the invention has been described with reference to preferred embodiments thereof, which are exemplary, it is to be understood that the invention is not limited to the preferred embodiments or constructions. The invention is intended to cover various modifications and arrangements. In addition, while the various elements of the preferred embodiments are shown in various combinations and configurations, that are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.