Abstract:
An actuator has a pair of electromagnets and a curved magnetic member that is magnetically coupled to the electromagnets. The magnetic force acting on the curved magnetic member has less of a tendency to induce torque to the table than a straight magnetic member, because the geometry of the magnetic force between the curved magnetic member and electromagnets remain relatively unchanged over the permitted range of movement of the magnetic member.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates generally to a positioning apparatus and a positioning method for use in photolithography, among other applications. More particularly, the invention is directed to a positioning apparatus for a wafer table, the positioning apparatus including an electromagnetic bearing system which effectively provides fine position control of the wafer table. 
   2. Description of Related Art 
   Many applications, particularly in semiconductor manufacturing, require precision positioning of an object. They include scanning tunneling microscopy, optical inspection, and photolithography. During photolithography, a wafer table is commonly used to position a wafer precisely with respect to the photolithographic apparatus. 
   The positioning and alignment of the wafer table can be performed in various ways. In U.S. Pat. No. 5,294,854, the entire contents of which are incorporated by reference herein, an electromagnetic bearing system is used for fine position control of the wafer table. The electromagnetic bearing system used in this patent allows for positioning and movement control of the wafer table in multiple degrees of freedom. Specifically, the wafer table is moved into desired positions by applying different currents to electromagnets that interact with corresponding adjacent magnetic members that are attached to the wafer table. 
   The magnetic members disclosed in the &#39;854 patent have a rectangular shape (see FIG.  4 ). When the wafer table is moved horizontally (e.g., in the X- or Y-direction) and vertically (in the Z-direction), the wafer table sometimes becomes tilted. When the wafer table becomes tilted, the magnetic member becomes tilted with respect to the electromagnets, and the gap distance between the magnetic member and the electromagnets differs along the height of the magnetic member. The difference in the gap distances in turn causes the magnetic coupling across the magnetic member to change and a torque develops along the magnetic member. The resulting torque can affect the precision positioning of the wafer table, and so, to compensate for this torque, additional electrical current is applied to the electromagnets. 
   While the prior art attempts to solve the torque problem by adjusting the electrical current in the electromagnets, this can result in unnecessary wafer table flutter, an increase in the chance of inducing distortion to the wafer table, and undesirable heat in the electromagnet bearing and the attached members. Therefore, it is desirable to have an electromagnetic bearing system that can minimize the torque created along the magnetic member. 
   SUMMARY OF THE INVENTION 
   The present invention overcomes the drawbacks of the prior art by implementing an actuator device or a magnetic bearing, which permits relatively uniform magnetic coupling over the permitted range of movement of the magnetic member during electromagnetic actuation. 
   In one aspect of the invention, the actuator device comprises of a first member that includes at least one electromagnet and a second member that is magnetically coupled to the first member by a force generated by the at least one electromagnet, the second member including a curved outer periphery facing the at least one electromagnet generating the force. In another aspect of the invention, the actuator device comprises of a first member that includes at least one electromagnet and a second member that is magnetically coupled to the first member and movable relative to the first member by a force generated by the at least electromagnet, wherein a relative motion between the first member and the second member includes a relative movement in a first direction that differs from a second direction parallel with the direction of the force acting on the second member; and the force between the second member and the at least one electromagnet remains substantially same amount over the relative movement between the first member and the second member in the first direction. In a further aspect of the present invention, a lithography system is disclosed which deploys a stage system that incorporates the actuator device in accordance with the present invention. 
   In accordance with one application of this invention, a wafer table is provided with a surface that is positionally controllable in at least one degree of freedom (e.g. for photolithography). At least one magnetic member attached to the wafer table couples the wafer table to a wafer positioning stage via electromagnetic actuators that are attached to the wafer positioning stage. The electromagnetic actuation of the magnetic member controls the horizontal movement of the wafer table in at least one degree of freedom. The magnetic bearing system in accordance with this invention depends on the interaction of the magnetic member and the electromagnetic actuators for positioning the wafer table. The magnetic bearing system in accordance with the principles of this invention comprises of an electromagnetic actuator that are primarily a pair of electromagnetic cores and a magnetic member with curved surface. The electromagnetic core is typically an E-shaped laminated core, with wire coils wound around its center prong. Current is to flow through the coil to actuate this E-shaped electromagnetic actuator. The magnetic member is preferably of spherical shape, cylindrical or at least having a convex curved outer surface. The magnetic force acting on the curved magnetic member has less of a tendency to induce torque to the wafer table because the geometry of magnetic force between the curved magnetic member and electromagnetic actuator remain relatively unchanged over the permitted range of movement of the magnetic member. This provides for a relatively uniform magnetic coupling over the range of movement of the magnetic member, thus reducing torque generated across the magnetic member. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view illustrating the wafer positioning stage and table of a photolithography apparatus incorporating the electromagnetic bearing system in accordance with one embodiment of the present invention; 
       FIG. 2  is top view of the wafer table and wafer positioning stage; 
       FIG. 3  is a perspective view of an E-shaped electromagnetic actuator in accordance with one embodiment of the present invention; 
       FIG. 4  is a perspective view of a prior art I-shaped magnetic member; 
       FIG. 5  is a perspective view of an I-shaped magnetic member in accordance with one embodiment of the present invention; 
       FIG. 6  is a side view illustrating the orientation of the I-shaped magnetic member and the E-shaped electromagnetic actuator in a normal position, in accordance with one embodiment of the present invention; 
       FIG. 7  is a side view illustrating the orientation of the I-shaped magnetic member and the E-shaped electromagnetic actuator activated in the Y and Z positions in accordance with one embodiment of the present invention; 
       FIG. 8  is a schematic view illustrating a typical photolithography apparatus incorporating the electromagnetic bearing system in accordance with the principles of the present invention; 
       FIG. 9  is a block diagram of a general fabrication process for semiconductor devices; and 
       FIG. 10  is a detailed block diagram of fabricating semiconductor devices. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   This invention is described in a preferred embodiment in the following description with references to the following figures. While the invention is described in terms of best mode of achieving this invention&#39;s objectives, it will be appreciated by those skilled in the art that variations may be accomplished in view of these teachings without deviating from the spirit or scope of the invention. 
   The electronic bearing of the present invention can be implemented to control movements of an object in many types of systems. In particular, the present invention is used to control the movement of a wafer table in a photolithography system. The invention is applicable to a scanning type photolithography system (see, for example U.S. Pat. No. 5,473,410, the entire contents of which are incorporated by reference herein), which exposes a mask pattern by moving a mask and a substrate synchronously. It is also applicable to a step-and-repeat type photolithography system that exposes a mask pattern while a mask and a substrate are stationary and moves the substrate in successive steps for exposure. It is further applicable to a proximity photolithography system that exposes a mask pattern by closely locating a mask and a substrate without the use of a projection optical system. The use of a photolithography system need not be limited to a photolithography system in semiconductor manufacturing. For instance, it can be widely applied to an LCD photolithography system, which exposes a liquid crystal display device pattern onto a rectangular glass plate, and a photolithography system for manufacturing a thin film magnetic head. 
     FIG. 1  is a schematic view illustrating the electromagnetic bearing system in accordance with one embodiment of the present invention. As shown in the figure, a wafer table  1  is magnetically coupled to a wafer positioning stage  52  by pairs of electromagnetic actuators  10 ,  10 ′, electromagnetic members  12 , and voice coil motors  76 . The positioning mechanism for the wafer table  1  is similar to the one described in International Application no. PCT/US00/10831, entitled “Wafer Stage With Magnetic Bearings,” the contents of which are fully incorporated herein by reference. The positioning stage  52  provides small and precise movement of the wafer table  1  in the vertical plane (Z) and horizontal plane (X, Y). Voice coil motors  76  are used to control vertical movement because dynamic performance is not required (e.g., acceleration requirements are relatively low). To prevent overheating of the voice coil motors  76 , air bellows (not shown) are used to support the dead weight of the wafer table  1 . 
   The electromagnetic actuators or E-cores  10 ,  10 ′ are attached to the wafer positioning stage  52  in pairs. Preferably, two pairs of E-cores  10 ,  10 ′ are aligned parallel with the X-plane and one pair of the E-cores  10 ,  10 ′ are aligned parallel with the Y-plane, forming a triangular pattern as shown in FIG.  2 . Three electromagnetic members or I-cores  12  are attached to the wafer table  1 , preferably towards the outer periphery. The I-cores  12  are positioned such that they align with and rest between, each pair of E-cores  10 ,  10 ′. The E-cores  10 ,  10 ′ are assembled in pairs because they can only pull the I-core  12  in opposition. 
     FIG. 3  is a perspective view of the E-core. The E-core typically comprises of an E-shaped laminated core  30  made of a magnetic material, such as iron or Ni-Fe steel. Electrical magnetic wire  32  is wound around the center prong  34  forming a coil. 
     FIG. 5  is a perspective view of an I-core comprising of a cylindrically shaped magnetic material, preferably composed of the same material as the E-core. The shape of the I-core is not limited to the cylindrical shape, and may include, for example, circular shapes, spherical shapes, etc. The two sides of the I-core that face each E-core are in a shape that allows it to convex towards the E-cores. The overall size of the I-core is determined by the size of the E-core, but it is typically smaller than the E-core. The I-core must remain within the magnetic flux of the E-core. 
     FIG. 6  illustrates the position of one pair of E-cores  10 ,  10 ′ and an I-core  12  when the wafer table  1  is parallel with the wafer stage  52 . The I-core  12  is attached to the wafer table  1  such that the curved sides of the I-core  12  are adjacent to each E-core  10 ,  10 ′. Each E-core  10 ,  10 ′ and I-core  12  is separated by a gap  14 , which allows the I-core  12  to move feely between each E-core  10 ,  10 ′. The E-cores  10 ,  10 ′ are the variable reluctance actuating portions of the magnetic bearing and the reluctance varies with the distance defined by the air gap  14 , which also varies the flux and force applied to the I-core  12 . The attractive force between the E-core  10 ,  10 ′ and the I-core  12  is defined by:
 F=K (i/g) 2 , where         F is the attractive force, measured in Newtons;   K=an electromagnetic constant which is dependent upon the geometries of the E-core, I-core, and number of coil turns about the E-core  10 ,  10 ′   i=current through the E-core, measured in amperes; and   g=gap distance, measured in meters.       
     FIG. 7  illustrates the position of one pair of E-cores  10 ,  10 ′ and an I-core  12  when the wafer table  1  is moved in the Y and Z direction. Movement in the Z direction is accomplished through voice coil motors (not shown) and Y movement is accomplished by two pairs of E-cores  10 ,  10 ′, which are aligned parallel with the X direction of the wafer table  1 . When the two pairs of E-cores  10 ,  10 ′ are energized by an electrical current, a magnetic flux is produced and an attractive force on the I-core  12  occurs in accordance with the formula given, resulting+ in linear actuation in the Y direction. In this example, the Y movement is away from the outer periphery of the wafer stage, therefore the inner E-core  10 ′ is energized with a higher electrical current than the outer E-core  10 . This results in a differential magnetic flux having a force that draws the I-core  12  closer to the inner E-core  10 ′ than the outer E-core  10 . As mentioned above, the wafer table movement in the Z direction is accomplished through the activation of voice coil motors. 
   Although the I-core is now closer to the inner E-core  10 ′ and has also moved slightly upward, the curved sides of the I-core  12  help to maintain the magnetic force geometry between both pairs of E-cores  10 ,  10 ′. The size of the gap between the I-core  12  and the pair of E-cores  10 ,  10 ′ will change, resulting in a change in magnetic force acting on the I-core. However, the magnetic force will continue to act on the same I-core geometry (due to the curved sides). Therefore, the acting magnetic force has less of a tendency to induce torque to the I-core  12 . If the I-core in the present invention is replaced by the I-core in the prior art, the geometry of which is shown in  FIG. 4 , the geometry of the magnetic force acting on the I-core side will change. In the prior art case, the lower right side portions of the I-core will be closer to the inner E-core  10 ′ and the upper right side portions will be further away from the inner E-core  10 ′. Oppositely, the upper left side portions of the I-core will be closer to the outer E-core  10  and the lower left side portions will be further away from the outer E-core  10 . The end result is the introduction of torque to the I-core. 
     FIG. 8  is a schematic view illustrating a photolithography apparatus  40  incorporating a wafer positioning stage  52  that is driven by a planar motor and a wafer table  1  that is magnetically coupled to the wafer positioning stage  52  in accordance with the principles of the present invention. The planar motor drives the wafer positioning stage  52  by an electromagnetic force generated by magnets and corresponding armature coils arranged in two dimensions. A wafer  64  is held in place by a wafer chuck  74  which is attached to the wafer table  1 . The wafer positioning stage  52  is structured so that it can move in multiple (e.g. three to six) degrees of freedom under precision control by a drive control unit  60  and system controller  62 , and position the wafer  64  at a desired position and orientation relative to the projection optics  46 . 
   The wafer table  1  is levitated in the vertical plane by preferably three voice coil motors (not shown). At least three magnetic bearings (not shown) couple and move the wafer table  1  horizontally. The motor array of the wafer positioning stage  52  is supported by a base  70 . The reaction force generated by the wafer stage  52  motion can be mechanically released to the ground through a frame  66 , in accordance with the structure described in JP Hei 8-166475 and U.S. Pat. No. 5,528,118, the entire contents of which are incorporated by reference herein. 
   An illumination system  42  is supported by a frame  72 . The illumination system  42  projects a radiant energy (e.g. light) through a mask pattern on a reticle  68  that is supported by and scanned using a reticle stage  44 . The reaction force generated by motion of the reticle stage can be mechanically released to the ground through the isolator  54 , in accordance with the structures described in JP Hei 8-330224 and U.S. Pat. No. 5,874,820, the entire contents of which are incorporated by reference herein. The light is focused through a projection optical system  46  supported on a projection optics frame  50  and released to the ground through frame  54 . 
   The magnification of the projection optical system is not limited to a reduction system. It could be a  1 X or a magnification system. When far ultra-violet rays such as excimer laser is used, glass materials such as quartz and fluorite that transmit far ultra-violet rays should be used. When F 2  laser or X-ray is used, the optical system should be either catadioptric or refractive (the reticle should also be a reflective type). When an electron beam is used, electron optics should consist of lenses and deflectors, and the optical path for the electron beam should be in a vacuum. The light exposes the mask pattern onto a layer of photoresists on a wafer  64 . The light source for the photolithography system may be the g-line (436 nm), I-line (365 nm), KrF excimer laser (248 nm), ArF excimer laser (193 nm) and F 2  laser (157 nm). For certain lithography systems, charged particle beams such as X-ray and electron beam may be used. For instance, for electron beam lithography, thermionic emission type lanthanum hexaboride (LaB6) or tantalum (Ta) can be used as an electron gun. Further, for electron beam lithography, the structure could be such that either a mask is used or a pattern can be directly formed on a substrate without the use of a mask. 
   An interferometer  56  is supported on the projection optics frame  50  and detects the position of the wafer table  1  and outputs the information of the position of the wafer table  1  to the system controller  62 . A second interferometer  58  is supported on the reticle stage frame  48  and detects the position of the reticle stage  44  and outputs the information of the position to the system controller  62 . 
   There are a number of different types of lithographic devices in which the present invention may be deployed. For example, the exposure apparatus  40  can be used as scanning type photolithography system that exposes the pattern from the reticle onto the wafer with the reticle and wafer moving synchronously. In a scanning type lithographic device, the reticle is moved perpendicular to an optical axis of the projection optics  46  by the reticle stage assembly  44  and the wafer is moved perpendicular to an optical axis of the projection optics  46  by the wafer stage assembly ( 1 ,  52 ). Scanning of the reticle and the wafer occurs while the reticle and the wafer are moving synchronously. 
   Alternately, the exposure apparatus  40  can be a step-and-repeat type photolithography system that exposes the reticle while the reticle and the wafer are stationary. In the step and repeat process, the wafer is in a constant position relative to the reticle and the projection optics  46  during the exposure of an individual field. Subsequently, between consecutive exposure steps, the wafer is consecutively moved by the wafer stage perpendicular to the optical axis of the projection optics  46  so that the next field of the wafer  64  is brought into position relative to the projection optics and the reticle for exposure. Following this process, the images on the reticle are sequentially exposed onto the fields of the wafer so that the next field of the wafer is brought into position relative to the projection optics  46  and the reticle. 
   Further, the present invention can also be applied to a proximity photolithography system that exposes a mask pattern by closely locating a mask and a substrate without the use of a lens assembly. 
   The use of the exposure apparatus  40  provided herein is not limited to a photolithography system for semiconductor manufacturing. The exposure apparatus  40 , 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. 
   As described above, a photolithography system according to the above-described embodiments can be built by assembling various subsystems, including each element listed in the appended claims, in such a manner that prescribed mechanical accuracy, electrical accuracy and optical accuracy are maintained. In order to maintain the various accuracies, prior to and following assembly, every optical system is adjusted to achieve its optical accuracy. Similarly, every mechanical system and every electrical system are adjusted to achieve their respective mechanical and electrical accuracies. The process of assembling each subsystem into a photolithography includes mechanical interfaces, electrical circuit wiring connections and air pressure plumbing connections between each subsystem. Needless to say, there is also a process where each subsystem is assembled prior to assembling a photolithography system from the various subsystems. Once a photolithography system is assembled using the various subsystems, a total adjustment is performed to make sure that accuracy is maintained in the complete photolithography system. Additionally, it is desirable to manufacture an exposure system in a clean room where the temperature and cleanliness are controlled. 
   Further, semiconductor devices can be fabricated using the above described systems, by the process shown generally in FIG.  9 . In step  301  the device&#39;s function and performance characteristics are designed. Next, in step  302 , a mask (reticle) having a pattern is designed according to the previous designing step, and in a parallel step  303  a wafer is made from a silicon material. The mask pattern designed in step  302  is exposed onto the wafer from step  303  in step  304  by a photolithography system described hereinabove in accordance with the present invention. In step  305  the semiconductor device is assembled (including the dicing process, bonding process and packaging process), then finally the device is inspected in step  306 . 
     FIG. 10  illustrates a detailed flowchart example of the above-mentioned step  304  in the case of fabricating semiconductor devices. In  FIG. 10 , in step  311  (oxidation step), the wafer surface is oxidized. In step  312  (CVD step), an insulation film is formed on the wafer surface. In step  313  (electrode formation step), electrodes are formed on the wafer by vapor deposition. In step  314  (ion implantation step), ions are implanted in the wafer. The above-mentioned steps  311 - 314  form the preprocessing steps for wafers during wafer processing, and selection is made at each step according to processing requirements. 
   At each stage of wafer processing, when the above-mentioned preprocessing steps have been completed, the following post-processing steps are implemented. During post-processing, first, in step  315  (photoresist formation step), photoresist is applied to a wafer. Next, in step  316  (exposure step), the above-mentioned exposure device is used to transfer the circuit pattern of a mask (reticle) to a wafer. Then, in step  317  (developing step), the exposed wafer is developed, and in step  318  (etching step), parts other than residual photoresist (exposed material surface) are removed by etching. In step  319  (photoresist removal step), unnecessary photoresist remaining after etching is removed. 
   Multiple circuit patterns are formed by repetition of these preprocessing and post-processing steps. 
   In summary, the present invention provides a method of minimizing torque to the wafer table. The curved I-core provides a more uniform magnetic coupling between the pair of E-cores and I-cores. The two sides of the I-core that face the pair of E-cores are curved such that as the I-core moves in different directions, the magnetic force acting on the I-core remains generally unchanged. Although the gap between the I-core and E-core can change, the geometry of the I-core remains the same, reducing the tendency of torque applied on the I-core. 
   Although the invention has been described with reference to a wafer table supported by a wafer stage in a photolithography apparatus, the invention is also applicable to other forms of apparatus in which precision positioning and maintaining of an object is necessary. 
   While the present invention has been described with respect to the preferred embodiment in accordance therewith, it will be apparent to those in the skilled art that various modifications and improvements may be made without departing from the scope and spirit of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims.