Patent Publication Number: US-9411242-B2

Title: Exposure apparatus and exposure method thereof

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
     This application claims the priority of Chinese patent application No. 201310069924.2, filed on Mar. 5, 2013, the entirety of which is incorporated herein by reference. 
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of semiconductor manufacturing technology and, more particularly, relates to exposure apparatus and exposure methods thereof. 
     BACKGROUND 
     Photolithography process is a very important process of the semiconductor manufacturing technology, which transfers patterns on a mask to a substrate by an exposure process. The photolithography process is a core step of the manufacturing of large scale integrations (LSIs). The complex and time-consuming photolithography process of the semiconductor manufacturing technology is mainly performed by corresponding exposure apparatus. Further, the development of the photolithography technology or the improvement of the exposure apparatus are mainly focused on three specifications including feature size, overlay resolution, and yield. 
     In the manufacturing of a semiconductor device, the photolithography process may include three main steps: changing wafers on the wafer stages; aligning the wafers on the wafer stages; and transferring patterns on the mask to the wafers. These three steps may be sequentially repeated on the same wafer stage. 
     Since the photolithography process is a key step of the semiconductor manufacturing process, how to improve the yield of an exposure apparatus in the practical manufacturing process has become a very important topic. Various exposure apparatuses with twin-stages have been developed in past a few years in order to further increase the yield of the exposure apparatuses. 
       FIG. 1  illustrates an existing exposure apparatus with twin-stages. The exposure apparatus includes a first stage  101  and a second stage  102  for holding wafers; an alignment detection unit  103  for detecting align marks on wafers and aligning wafers; a mask stage  107  for holding a mask  108 ; an optical projection unit  104  under the mask stage  107  for projecting light through the mask  108  on the wafers on the first stage  101  or the second stage  102  and performing an exposure on the wafers; and an illuminator  109  above the mask stage  107  for providing an exposure light. 
     An exposure process using the existing tool may include sequentially: aligning the first wafer  106 ; moving the first stage  101  under the optical projection unit  104 ; and performing an exposure on the first wafer  106  using the optical projection unit  106  which projects light through the mask  108  on the first wafer  106 . At the same time, a second wafer  105  may be installed on the second stage  102 , and moved under the alignment detection unit  103 . The alignment detection unit  103  may detect the alignment mark on the second wafer  105  on the second stage; and the second wafer  105  may be aligned. After the first wafer  106  is exposed, a new wafer may be installed on the first stage  101 , and the first stage  101  with the new wafer may be moved under the alignment detection unit  103 . The alignment detection unit  103  may align the new wafer. At the same time, the second stage  102  may be moved under the optical projection unit  104 , and the second wafer  105  may be exposed by the optical projection unit  104 . 
     However, even with such improvements, the exposure efficiency and the yield of the existing exposure apparatuses may still be relatively low. The disclosed methods and systems are directed to solve one or more problems set forth above and other problems. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     One aspect of the present disclosure includes a wafer alignment system for performing a unidirectional scan-exposure. The wafer alignment system includes a plurality of wafer stages successively moving from a first position to a second position of a base cyclically. The wafer alignment system also includes an encoder plate having a first opening and a second opening. Further, the wafer alignment system includes a plurality of encoder plate readers and a plurality of wafer stage fiducials on the wafer stages. Further, the wafer alignment system also includes an alignment detection unit above the first opening of the encoder plate. 
     Another aspect of the present disclosure includes a wafer alignment method. The wafer alignment method includes loading a wafer on each of a plurality of wafer stages successively; and moving one of the wafer stages loaded with the wafer to a first position successively. The wafer alignment method also includes zeroing the wafer at the first position using zeroing mark detection units to detect the zeroing marks on an encoder plate. Further, the wafer alignment method includes aligning and leveling the wafer stage with the wafer using the alignment detection unit to detect wafer stage fiducials. Further, the wafer alignment method also includes obtaining a position information of a to-be-exposed column of exposure regions by detecting alignment marks on the wafer using an alignment detection unit. 
     Other aspects of the present disclosure can be understood by those skilled in the art in light of the description, the claims, and the drawings of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an existing exposure apparatus and exposure process; 
         FIG. 2  illustrates scanning directions for each exposure regions of the existing exposure apparatus; 
         FIGS. 3-9  illustrate certain structures of an exemplary exposure apparatus consistent with the disclosed embodiments; and 
         FIGS. 10-17  illustrate certain steps of an exemplary exposure process consistent with the disclosed embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to exemplary embodiments of the invention, which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
       FIG. 2  illustrates scanning directions of the existing exposure apparatus for each exposure regions of a wafer, i.e., the direction of an arrow may refer to the scanning direction. For example, referring to  FIG. 1 , for each scanning, the stage (the first stage  101  or the second stage  102 ) may move along the scanning direction, and the mask stage  107  with the mask  108  may move along an opposite direction, and a first exposure region may be exposed. After exposing the first exposure region, a second exposure region may be exposed. 
     As shown in  FIG. 2 , the first wafer  106  may have a plurality of exposure regions. Referring  FIGS. 1-2 , the first stage  101  may move along a first direction to scan and expose a first exposure region  11 . Wherein the first direction is the direction of the arrow shown in first exposed region  11 . After exposing the first exposure region  11 , a second exposure region  12  adjacent to the first exposure region  11  may be exposed by moving the first stage  101  with a direction opposite to the first direction which may be referred as a second direction as shown in the second exposure region  12 . Scanning (or exposures) along the first direction and the second direction may be sequentially repeated until all the exposure regions of the first wafer  106  are exposed. 
     An accelerating process, a scanning and exposing process and a decelerating process may thus exist in every scanning (or exposure) when the existing exposure apparatus (or method) is used. The accelerating process and the decelerating process may be used to improve the accuracy and efficiency of changing exposure regions, and they may be unable to aid the actual exposure of wafers although occupying a portion of the total time of the exposure process. Thus, the accelerating process and the decelerating process may waste time, and may cause a relatively low yield in per unit time. 
       FIGS. 3-9  illustrate certain structures of an exemplary exposure apparatus consistent with the disclosed embodiments. 
       FIG. 3  illustrates an exemplary exposure apparatus. As shown in  FIG. 3 , the exposure apparatus includes a base (not shown). The base may provide a moving region for wafer stages. The base may be designed with any appropriate size and geometry. Certain damping system may also be included in the base to reduce vibration, or other kind of noises. Other necessary components may also be installed in the base to make it have desired functionalities. 
     The exposure apparatus also includes a stage group on the base which is used to hold, load and unload wafers. The stage group may have a plurality of wafer stages  301 . The plurality of wafer stages  301  may sequentially circularly move between a first position (pre-align position) and a second position (pre-exposure position). The stage group may be controlled by a close-loop control system, or an open-loop control system, etc. 
     Further, the exposure apparatus includes an alignment detection unit  302  above the first position  311  of the base. The alignment detection unit  302  may be used to detect stage fiducials on the wafer stages  301  at the first position. The alignment detection unit  302  may also be used to detect alignment marks on wafers on the wafer stages  301 , and align the wafers. Various sensors may be used in the alignment detection unit  302 , such as a laser sensor, an infrared sensor, or an position sensor, etc. 
     Further, the exposure apparatus also includes a cylindrical reticle system  305  above the second position of the base. The cylindrical reticle system  305  may be used to hold a cylindrical reticle  303 , and to cause the cylindrical reticle  303  to rotate along the center axis of the cylindrical reticle system  305 . 
     Further, the exposure system also includes an optical projection unit  309  between the cylindrical reticle stage  305  and the base. The optical projection unit  309  may project light onto exposure regions of the wafers on the wafer stages  301 , and perform exposure. After the wafer stages  301  move from the first position to the second position, an unidirectional scan may be performed along a scanning direction. The cylindrical reticle  303  may rotate along the center axis of the cylindrical reticle stage  305 , and the light through the cylindrical reticle  303  may be projected on the wafer on the wafer stage  301 . Thus, a column of exposure regions on the wafer along the scanning direction may be exposed. 
     Further, the exposure apparatus may also include a main control unit  300 . The main control unit  300  may communicate with the wafer stages  301 , the alignment detection unit  302 , the cylindrical reticle system  305 , and the optical projection unit  309 , etc. The main control unit  300  may be used to manage wafer alignment and exposure processes. The main control unit  300  may also be used to send out various kinds of commands, receive feedback information from various units, manage position information, and calculate lateral movement constants, rotation constants, and amplification constants of the wafer stages  301  and the cylindrical reticle  303 . 
     Further, the exposure apparatus may also include an encoder plate  306  having a first opening  311  and a second opening  312 . The encoder plate  306  may be used to align wafers, wafer stages  301  and cylindrical reticle  303 , etc. 
     Further, the exposure apparatus may also include a plurality of cables, such as charging cables  307  for charging a power source of the wafer stages  301 . Other cables may also be used for communications of different components. 
     Further, the exposure apparatus may also include an illuminator box  308 . The illuminator box  308  may have a light source inside. The light source may be used to expose the wafers. 
     Further, the exposure apparatus may also include a time fiducial unit (not shown). The time fiducial unit may be used to provide a fiducial time unit signal, and cause the collection of all signals of the wafer alignment and exposure processes to be related with time. 
     The detailed structures of the exposure apparatus are described below together with various illustrative drawings. 
       FIG. 4  and  FIG. 5  illustrates the detail structures of the cylindrical reticle system shown  305  in  FIG. 3 . 
     As shown in  FIGS. 4-5 , the cylindrical reticle system  305  includes a rectile stage frame  328 , a center shaft  329  at one end of the recticle stage frame  328 , a first bearing  304   a , a second bearing  304   b , reticle pre-alignment imaging sensors  341 , the cylindrical reticle  303 , and a control unit (not shown). The control unit may coincident with the main control unit  300 . 
     The reticle stage frame  328  may be used to support the cylindrical reticle  303  through the center shaft  329 . The reticle stage frame  328  may be connected with a fourth drive unit (not shown). The fourth drive unit may drive the reticle stage frame  328  to move along the x-axis direction, the y-axis direction, and the z direction. The fourth drive unit may also drive the reticle stage frame  328  to rotate around the x-axis direction, the y-axis direction, and the z direction. 
     The first bearing  304   a  and the second bearing  304   b  may all have a bearing outer ring, a bearing inner ring, a bearing cage and a plurality of rolling elements between the bearing outer ring and bearing inner ring and held by the bearing cage. The rolling elements may be balls and/or cylinders. 
     In certain other embodiments, the first bearing  304   a  and the second bearing  304   b  may be electromagnetic bearings. An electromagnetic bearing may include a bearing outer ring, a bearing inner ring, and coils between the bearing outer ring and the bearing inner ring. A rotation of the electromagnetic bearing may be precisely adjusted by varying the electric current distribution of the coils. 
     The bearing inner ring of the first bearing  304   a  may be fixed at an end of the center shaft  329  near the reticle stage frame  328 . The bearing inner ring of the second bearing  304   b  may be fixed at the other end of the center shaft  329  far from the reticle stage frame  328 . That is, the first bearing  304   a  is fixed at one end of the center shaft  329  near the reticle stage frame  328 ; and the second bearing  304   b  is fixed at the other end of the center shaft  329  far from the reticle stage frame  328 . 
     The cylindrical reticle  303  may be installed on the bearing outer rings of the first bearing  304   a  and the second bearing  304   b . That is, inner surfaces of the non-imaging areas  332  of the cylindrical reticle  303  may contact with the bearing outer rings of the first bearing  304   a  and the second bearing  304   b . When the first bearing  304   a  and the second bearing  304   b  rotate around the center shaft  329 , the cylindrical reticle  303  may also rotate around the center shaft extension tube  329 . The center shaft  329  may be equivalent to a center rotation axis of the cylindrical reticle  303 . 
     The bearing outer rings of the first bearing  304   a  and the second bearing  304   b  may have a plurality of first gripping holders  334  as shown in the inserted figure in  FIG. 4 . When the cylindrical reticle  303  moves along x direction, and the non-imaging areas  332  of the cylindrical reticle  303  harness on surfaces of the bearing outer rings of the first bearing  304   a  and the second bearing  304   b , a pressure may be generated on the inner surfaces of the non-imaging areas  332  of the cylindrical reticle  303  by the first gripping holders  334 , which may cause the cylindrical recticle to attach on the bearing outer rings more tightly. 
     In one embodiment, the first gripping holder  334  may have a protruding part and a driving part connecting with the protruding part. When the cylindrical reticle  303  is harnessed (or installed) on the surfaces of the bearing outer rings of the first bearing  304   a  and the second bearing  304   b , the driving part may drive the protruding part to move outwardly (far from the center shaft  329 ), and may cause the protruding part to contact with the inner surfaces of the non-imaging areas  332  of the cylindrical reticle  303 . Thus, a pressure may be formed on the inner surfaces of the non-imaging areas  332 . 
     When the cylindrical reticle  303  is uninstalled from the surfaces of the bearing outer rings of the first bearing  304   a  and the second bearing  304   b , the driving part may drive the protruding part to move inwardly (close to the center shaft  329 ), and cause the protruding part to separate from the inner surface of the non-imaging areas  332  of the cylindrical reticle  303 . Thus, the cylindrical reticle  303  may be removed from the bearing outer rings of the first bearing  304   a  and the second bearing  304   b.    
     The first gripping holders  334  may be distributed with equal angles on the bearing outer rings of the first bearing  304   a  and the second bearing  304   b . A number of the first gripping holders  334  on the first bearing  304   a  and a number of the first grip holders on the second bearing  304   b  may be equal. Thus, pressure on the inner surfaces of the non-imaging areas  332  of the cylindrical reticle  303  applied by the first gripping holders  334  may be equivalent. The deformation of the cylindrical reticle  303  may be prevented, and the installation accuracy of the cylindrical reticle  303  may be improved. 
     As shown in  FIG. 4 , the cylindrical reticle  303  may be a hollow cylinder. The cylindrical reticle  303  may include an imaging area  331  in the middle portion of the cylinder and two non-imaging areas  332  at both sides of the middle imaging area  331 . The imaging area  331  may have a plurality of reticle alignment marks  333 . 
     The imaging area  331  may at least have two groups of reticle alignment marks  333 , and each group of reticle alignment marks  333  may include two mask alignment marks. Four mask alignment marks  333  of the two groups may locate at four corners of the imaging area  331 . A connecting line of the centers of two mask alignment marks  333  of each group may be parallel to the center axis of the center shaft  329 . 
     Each of the reticle alignment marks  333  may have a fifth optical grating  336  parallel to a rotation direction of the cylindrical reticle  303 , and a sixth optical grating  337  vertical to the fifth optical grating  336 . A cross line of the symmetric center of the fifth grating  336  and the symmetric center of the sixth grating  337  may be parallel to the center axis of a center shaft extension tube  329 . Thus, subsequently described reticle alignment sensors/wafer stage fiducials on the wafer stages  301  (shown in  FIG. 3 ) may detect at least four groups of position information of the reticle alignment marks  333  by detecting at least two groups of reticle alignment marks  333 . A position relationship between the wafer stages  301  and the cylindrical reticle  303  may be formed. Further, the amplification coefficients of the cylindrical reticle  303  in X direction and Y direction, the rotation coefficients and the orthogonality coefficient may be calculated by a main control unit  300  using the four groups of position information. 
     In certain other embodiments, the imaging area  331  of the cylindrical reticle  303  may have a plurality of reticle exposure areas (may correspond to a plurality of exposure areas on a wafer). Each of the reticle exposure areas may have at least two groups of reticle alignment marks  333 . When an alignment process of the cylindrical reticle  303  is performed, reticle alignment sensors/wafer stage fiducials may detect the reticle alignment marks of each of reticle exposure areas. Thus, a relationship between the reticle exposure areas of the cylindrical reticle  303  and the corresponding exposed areas on the wafer may be formed, the patterns of the reticle exposure areas on the cylindrical reticle  303  may be transferred to the corresponding exposure areas on the wafer. 
     As shown in  FIG. 5 , the center shaft  329  is fixed at one side of the reticle stage frame  328 . The center shaft  329  is a hollow structure which may be used to place the illuminator box (may also refer as an exposure light source)  308  as shown in  FIG. 3 . The center shaft  329  may also have an illumination slit  335  (as shown in  FIG. 4 ) penetrating through the bottom. The illumination slit  335  may be parallel to the center axis of the center shaft extension tube  329 . Light illuminating from the illuminator box  308  may reach on the imaging area  331  of the cylindrical reticle  303  through the illumination slit  335 . Then light illuminating through the reticle  303  may be projected on the exposure areas of the wafer on the wafer stage  301 , and the wafer may be exposed. 
     Various light sources may be used as the illumination box  308 . In one embodiment, the illumination box  308  may be a KrF excimer laser (wavelength may be approximately 248 nm) or an ArF excimer laser (wavelength may be approximately 193 nm). The illumination box  308  may also be an F 2  laser (wavelength may be approximately 157 nm), or an extreme ultraviolet light source (wavelength is approximately 13.5 nm), etc. The light of the illumination box  308  may also be a glow discharge in ultraviolet region generated from a high pressure mercury light source (i line, or g line, etc). 
     A length of the illumination slit  335  (in the x-axis direction) may be equal to a width of the imaging area  331  of the cylindrical reticle  303  (in the x-axis direction). A width of the illumination slit  335  may be in a range of approximately 1 mm˜80 mm. 
     As shown in  FIG. 5 , reticle pre-alignment imaging sensors  341  may be installed above the cylindrical rectile  303 . The reticle pre-alignment imaging sensors  341  may be fixed on the reticle stage frame  328  by an extension part. The reticle pre-alignment imaging sensors  341  may be used to detect the reticle alignment marks  333  on the imaging area  331  of the cylindrical reticle  303 . The loading and rotation status of the cylindrical reticle  303  may be estimated by signals detected by the reticle pre-alignment imaging sensors  341 . 
     Further, as shown in  FIG. 5 , the reticle pre-alignment imaging sensors  341  may have two sub-detection units: the first sub detection unit  340   a  and the second sub detection unit  340   b . The sub detection units may include a linear array of photo detectors. The sub detection units may also include two-dimensional arrays of fast imaging detectors, such as charge coupled detectors, or CMOS imaging sensors, etc. 
     A distance between the first sub-detection unit  340   a  and the second sub detection unit  340   b  may equal to a distance between two reticle alignment marks  333  of each group of reticle alignment marks  333 . When the cylindrical mask  303  is installed on the bearing outer rings of the first bearing  304   a  and the second bearing  304   b , and the first bearing  304   a  and the second bearing  304   b  drive the cylindrical reticle  303  to rotate at least one cycle, the reticle pre-alignments imaging sensors  341  may detect at least two groups of reticle alignment marks  333 . At least four electrical signals changing with time may be obtained. 
     Therefore, the rotation status (rotation characteristics in a zy-coordinate plane) and the installation status (may include a position in the x-direction and a position in y-direction of the coordinate) of the cylindrical reticle  303  may be estimated by the delay of time, the repeatability of signals, the period of pulse signals and the width of the pulse signals which may be compared between the four electrical signals. 
     Further, the reticle pre-alignment imaging sensors  341  may also have a height detection unit (not shown). The height detection unit may be used to detect a concentricity of the cylindrical reticle  303  with respect to the first bearing  304   a  and the second bearing  304   b , and to send the concentricity bias information back to the first griping holders  334 . The first gripping holders  334  may adjust their protruding size of the protruding part to cause the cylindrical reticle  303  and the first bearing  304   a  and the second bearing  304   b  to be concentric. The height detection unit may detect the height by detecting a height of a reflection point of a light reflected by the surface of the cylindrical reticle  303 . The protruding size of the protruding part of the first gripping holders  334  may be adjusted by the driving parts of the first gripping holders  334 . 
     Further, the cylindrical reticle system may also include a first drive unit (not shown). The first drive unit may connect with the first bearing  304   a  and/or the second bearing  304   b . The first drive unit may be used to drive the first bearing  304   a  and the second bearing  304   b  to rotate around the center shaft  329 . 
     Further, the cylindrical reticle system may also include a second drive unit (not shown). The second drive unit may be used to install (along the positive direction of the x-axis) or uninstall (along the negative direction of the x-axis) the cylindrical reticle  303  onto the bearing outer rings of the first bearing  304   a  and the second bearing  304   b.    
     As shown in  FIG. 5 , additionally or optionally, a cylindrical lens unit  330  may be installed between the cylindrical reticle system and the optical projection unit  309 . The cylindrical lens unit  330  may be right underneath the imaging area  331  of the cylindrical reticle  303 . The cylindrical lens unit  330  may have a concave surface and a flat surface, and the flat surface may face the cylindrical reticle system. The cylindrical lens unit  330  may be used to calibrate a focus plane of the light passing through the cylindrical reticle  303 , transform a cylindrical focus plane passing through the cylindrical reticle  303  to a flat focus plane. Thus, the light passing through the cylindrical reticle  303  and the optical projection unit  309  may be precisely projected on the exposure areas of the wafer. 
     The cylindrical lens unit  330  may be a single lens, or a plurality of lenses. In one embodiment, the cylindrical lens unit  330  is a combination of a plurality of the lenses. 
     The optical projection unit  309  may be used to project the light passing through the cylindrical reticle  303  on the exposed area of the wafer on the wafer stage  301 . When a KrF excimer laser or an ArF excimer laser is used as the light source in the illuminator box  308 , the optical projection system  309  may be a refraction system, only made of optical refraction devices (such as lenses). When an F 2  laser is used as the light source in the illuminator box  308 , the optical projection system  309  may be a deflection/refraction system, made of optical refraction devices, optical reflection devices, or a combination thereof. 
     A projected length of the perimeter of the outer surface of the cylindrical reticle  303  projected by the cylindrical lens unit  330  and the optical projection unit  309  may be equal to a length of an exposure area on a wafer (along a scanning direction). That is, referring to  FIG. 3 , when the cylindrical reticle  303  rotates around the center shaft  329  for one cycle, at the same time, the wafer stage  301  scans an exposure area along the scanning direction (arrow direction shown in  FIG. 3 ), patterns on the imaging area  331  of the cylindrical reticle  303  may be completely transferred to an exposure area on the wafer. The cylindrical reticle  303  may continue rotating, and wafer stage  301  keeps scanning along the scanning direction, the patterns on the imaging area  331  of the cylindrical reticle  303  may be completely transferred to next exposure area on the wafer. 
     The rotating and scanning process may be repeated, a column of exposure areas distributed along the scanning direction may all be exposed. During the exposure process of the exposure areas, it may be unnecessary for the cylindrical reticle  303  to change a rotation direction, and for the wafer stages to change a scanning direction as well. Thus, when the exposure area is changed, it may be unnecessary to have an accelerating process or a decelerating process. When a series of exposure areas along the scanning direction are exposed continuously, the wafer stages may keep a constant speed. Therefore, the time for exposure processes may be significantly reduced, and the exposure efficiency of the exposure apparatus may be increased. 
     Alternatively, the projected length of the perimeter of the outer surface of the cylindrical reticle  303  projected by the cylindrical lens unit  330  and the optical projection unit  309  may be equal to a length of a plurality of exposure areas on a wafer (along a scanning direction). That is, when the cylindrical reticle  303  rotates around the center shaft  329  for one cycle, the wafer stage  301  scans on the plurality of exposure areas along the scanning direction at the same time. The exposure efficiency can be further increased. 
     In another embodiment, the cylindrical reticle system may include: a reticle stage frame; a center shaft fixed at one side of the reticle stage frame; a first bearing and a second bearing. The first bearing may be fixed on one end of the center shaft near the reticle stage frame. The first bearing and the second bearing may all have a bearing outer ring, a bearing inner ring, and a plurality of the rolling elements. The bearing inner rings of the first bearing and the second bearing may be fixed on the center shaft. Two side faces of the bearing outer ring of the first bearing and the bearing outer ring of the second bearing facing each other may have slots. 
     The cylindrical reticle system may also include a cylindrical reticle. The cylindrical reticle may be a hollow cylinder. The cylindrical reticle may have an imaging area in the middle and non-imaging areas at both ends of the imaging area. The non-imaging areas of the cylindrical reticle may be clamped in the slots of the side faces of the bearing outer rings of the first bearing and the second bearing. The center shaft may perforate the hollow part of the cylindrical reticle. When the first bearing and the second bearing rotate around the center shaft, the cylindrical reticle may also rotate around the center shaft. 
     Further, the cylindrical reticle system may also include a second drive unit. The second drive unit may be used to install the cylindrical reticle into the slots on the side surfaces of the bearing outer rings of the first bearing and the second bearing. The second drive unit may also be used to uninstall the cylindrical reticle from the slots on the side surfaces of the bearing outer rings of the first bearing and the second bearing. 
     The second bearing may be a detachable bearing. The second bearing may connect with a third drive unit. The third drive unite may be used to fix or remove the second bearing from the center shaft extension tube. In order to conveniently fix the second bearing, the center shaft extension tube may have a plurality of protruding parts distributing with equal angles. The inner surface on the bearing inner ring of the second bearing may have a plurality of the groves corresponding to the protruding parts. When the groves on the inner surface of the bearing inner ring of the second bearing grips the protruding parts on the center shaft extension tube, the second bearing may be fixed on the center shaft extension tube. 
     A process for installing the cylindrical reticle may include uninstalling the second bearing from the center shaft extension tube using the third drive unit; installing one non-imaging area of the cylindrical reticle into the slot of the bearing outer ring of the first bearing using the second driving unit; and installing the second bearing to cause the other non-imaging area of the cylindrical reticle to be clamped into the slot of the bearing outer ring of the second bearing. 
     In one embodiment, when the cylindrical reticle is installed, the cylindrical reticle may be unlikely to touch surfaces of the bearing outer rings of the first bearing and the second bearing, it may prevent the inner surface of the cylindrical reticle from being scratched by the first bearing and the second bearing. Further, the cylindrical reticle is installed between the first bearing and the second bearing, thus it may be convenient to connect the second drive unit with the first bearing and the second bearing. 
     The bearing outer rings of the first bearing and the second bearing may have plurality of second gripping holders. When the non-imaging areas of the cylindrical reticle are installed in the slots of the bearing outer rings of the first bearing and the second bearing, the second gripping holders may generate a pressure on the outer surface of the non-imaging areas of the cylindrical reticle, which may cause the cylindrical reticle to be fixed tightly. 
     The second gripping holders may distribute on the bearing outer rings of the first bearing and the second bearing with equal angles, thus a uniform pressure may be generated onto the outer surface of the non-imaging areas. The deformation of the cylindrical reticle may be prevented, and the installation accuracy of the cylindrical recticle may be improved. 
     The center shaft is hollow, and an illuminator box may be installed in the hollow part of the center shaft. The bottom of the hollow center shaft may have a slit penetrating through the center shaft. The slit may be parallel to the center axis of the center shaft. The light emitting from the illuminator box may irradiate on the imaging area of the cylindrical reticle through the slit, and may be projected on the exposure area of the wafer on the wafer stage by an optical projection unit. Thus, the wafer may be exposed. 
     Referring to  FIG. 3 , the wafer stages  301  may be driven by a magnetic suspension system to move along a scanning direction (the arrow direction) in the xy-plane. The wafer stages  301  may also move from a first position to a second position by the magnetic suspension system. The wafer stages  301  may move along the positive direction of the y-axis (the scanning direction), the negative direction of the y-axis, the positive direction of the x-axis, the negative direction of the x-axis, the positive direction of the z-axis, and the negative direction of the z-axis. 
     The magnetic suspension system may be a maglev planar motor. The stator of the maglev planar motor may be fixed on the top surface of the base. The rotor of the maglev planar motor may be fixed on the bottom surface of the wafer stages  301 . The maglev planar motor may be a permanent magnet dynamic maglev planar motor, a permanent magnet moving-iron maglev planar motor, or a maglev induction planar motor, etc. 
     The wafer stages  301  may sequentially move on the base. As shown in  FIG. 3 , when a plurality of wafer stages  301  carrying wafers are sequentially aligned at the first position, they may sequentially move to the second position. At the same time, the cylindrical reticle  303  may rotate around the center axis of the cylindrical reticle stage  305 . The light penetrating through the cylindrical reticle  303  may be projected on the wafer on one wafer stage  301 , thus a column of exposure regions along the scanning direction may be exposed. After exposing the column of the exposure regions, the wafer stage  301  may sequentially move from the second position to the first position to perform an alignment. The alignment and exposure processes may be repeated until the entire wafer are exposed. 
     When the wafer is exposed, it may unnecessarily need to change the scanning direction the wafer stages  301 . At the same time, it may unnecessarily need to change the rotation direction of the cylindrical reticle  303 . Thus, it may unnecessarily need to have a deceleration and an acceleration process during the process for exposing each exposure area, a total exposure time of each exposure region may be significantly reduced. The yield in per unit time of the exposure apparatus may be significantly increased. 
     The wafer stages  301  may have corresponding sub-control units (not shown). The sub-control units may be inside of the wafer stages  301 . The sub-control units may be used to control the position of the wafer stages  301 , the movement of the wafer stages  301 , and the alignment of the wafers. The sub-control units may communicate with the main control unit  300 , the magnetic suspension system, the alignment detection unit  302 , and the cylindrical reticle system. 
     In one embodiment, when performing an exposure process, the wafer stages  301  may scan along one direction of the y-axis with a pipeline mode. The cylindrical reticle  303  may rotate around the center axis of the cylindrical reticle system  305 , the light passing through the cylindrical reticle  303  may be projected on the wafer on the wafer stages  301 . Thus, a column of exposure areas on the wafer may be exposed. 
     Therefore, a number of the wafer stages  301  may be greater than, or equal to two, which may increase the number of wafers carried by the wafer stages  301 , and improve the efficiency of the exposure. In one embodiment, the number of the wafer stages  301  may be 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, or 25, etc. 
     If the number of the wafer stages is two, the sub control units on the wafer stages  301  may communicate with the main control unit  300 , the alignment detection unit  302  and the cylindrical reticle system, etc., with a wired connection. In certain other embodiments, when the number of the wafer stages  301  is greater than two, the sub-control units on the wafer stages  301  may communicate with the main control unit  300 , the alignment detection unit  302  and the cylindrical reticle system, etc., with a wireless communication method. Since the wafer stages  301  may move circularly on the base, the wireless communication method may avoid the difficulties of placing wires and the movement of the wafer stages  301  which may be required for a wired communication method. 
     The wireless communication method may include: a Bluetooth technology, a infrared data association technology, a wireless fidelity (Wi-Fi) technology, a wireless application protocol (WAP), an ultra wideband technology, or a near field communication (NFC) technology, etc. 
     In one embodiment, if a wireless communication technology is used for the wafer stages  301  to communicate with peripheral circuits, each of the wafer stages  301  may have a power storage unit (not shown) used to store the power for operating the wafer stages  301 . The power for operating the wafer stages  301  may include the power for moving the wafer stages  301 , the power for operating the sub control units, and the power for the communication between the wafer stages  301  and the peripheral circuits. The power storage unit may be fast charging battery packs, or super capacitors, which may be able to store a relatively large amount of power, and may be charged in a relative fast speed. 
     As shown in  FIG. 3 , in order to charge the power storage unit of the wafer stage  301 , the exposure apparatus may have the charging cable  307  and a charging port  310 . When the charging cable  307  is connected with the charging port  307 , the charging cable may charge the power storage unit of the wafer stage  301 . 
     If the number of the wafer stages  301  is two, the charging cable  307  and the charging port  310  may be connected with an immovable method. 
     If the number of the wafer stages  301  is equal to, or greater than two, the exposure apparatus may have a cable connection unit. The cable connection unit may be used to form a detachable connection between the charging cable  307  and the power storage unit on the wafer stages  301 . When the wafer stages  301  move to a certain position (such as the first position), a connection may be formed between the charging cable  307  and the charging port  310  by the cable connection unit, the charging cable  307  may charge the power storage unit. 
     When the charging cable  307  moves to a next position (such as the second position) with the wafer stages  301 , the charging cable  307  may disconnect with the charging port  310  by the cable connection unit, a charging process may be finished. When a plurality of wafer stages  301  moves cyclically, the difficulties for arranging wires may be effectively solved. The charging process may be performed during an exposure process. 
     In certain other embodiments, the charging process may be performed when the exposure apparatus is at a standby status. Specifically, when the wafer stages  301  are moving to a certain position, a connection may be formed between the charging cable  307  and the power storage unit on the wafer stage  301  by the cable connection unit, the power storage unit may be charged by the charging cable  307 , and the wafer stages  301  and the charging cable  307  may be kept static. After a certain period of time, the charging cable  307  may disconnect with the wafer stage  301  by the cable connection unit. 
     The cable connection unit may include a clamping unit (not shown) and a drive unit (not shown). The clamping unit may be used to clamp the charging cable  307 , and move the charging cable  307  in a micrometer scale. The drive unit may be used to cause the clamping unit to move along the x-axis, the y-axis and the z-axis, and to rotate around the x-axis, the y-axis and the z-axis. 
     In order to achieve an accuracy position of multiple wafer stages  301 , the exposure apparatus may have position detection units. Referring to  FIG. 3  and  FIG. 6 , the position detection unit may include an alignment detection system  302 , an encoder plate  306  between the optical projection unit  309  and the wafer stages  301 , and encoder plate readers  313  on the top surface of the wafer stages  301 . 
     As shown in  FIG. 6 , the top surface of the wafer stage  301  may have a wafer holding region  314  and a peripheral region  315  surrounding the wafer holding region  314 . The encoder plate readers  313  may be at the peripheral region  315  of the top surface of the wafer stage  301 . Each of the encoder plate readers  315  may have two encoder detector units  318 . Each of the encoder detector units  318  may have optical emission units (not shown), optical receiver units (not shown), and sub-recognition units (not shown), etc. 
     The optical emission units may be used to emit detection light. The optical receiver units may be used to receive the light reflected by the encoder plate  306 , and covert the light signal to an electric signal varying with time. The sub-recognition units may be used to perform a signal amplification process, a signal filtering process, a direction sensing process, a pulse width calculation process, and/or a data counting process, etc, thus the displacement of the wafer stage  310  may be found after a signal processing. 
     When a number of the encoder plate readers  315  is greater than one, each of the sub recognition units may send signals processed by amplifying or filtering, etc. to a main recognition unit (not shown). The main recognition unit may perform a direction sensing process, a pulse width calculation process, and/or a data counting process, etc., thus the displacement of the wafer stage  301  may be found. A direction of the light emitted from the optical emission unit may be inclined to the encoder plate  306 , that is, the irradiating direction of the emission light may have a certain angle with the normal of the encoder plate  306 , which may cause the reflected optical signal from the encoder plate  306  to have a relatively large amplitude. Further, the emission light on the two encoder detector units  318  may be both inclined to a direction opposite to the two encoder detector units  318 , which may cause the two encoder detector units  318  to have different phases when the wafer stage  301  moves along the z-axis. Thus, the displacement of the wafer stage  301  along the z-axis direction may be determined. 
     The number of the encoder plate readers  313  may be at least three. The three encoder plate readers  313  may be distributed at different positions of the peripheral region  315  of the wafer stage  301 . In one embodiment, the number of the encoder plate readers  313  is four. The four encoder plate readers  313  are at the four corners of the peripheral region  315  of the wafer stage  301 . A distance between each of the encoder plate readers  313  and the center of the wafer stage  301  may be equal or different. 
     Referring to  FIG. 6 , each of the encoder plate readers  313  may have a zeroing mark detection unit  317 . The zeroing mark detection unit  317  may be used to detect a zeroing mark on the encoder plate  306 . In one embodiment, the zeroing mark detection unit  317  may be an imaging alignment sensor. The zeroing mark detection unit  317  may include a light source (such as a halogen light), an optical imaging system, and a camera (such as a CCD camera), etc. 
     The zeroing mark detection unit  317  may irradiate the zeroing mark on the encoder plate  306  using a wide band light from the light source, the camera may receive the reflected signal from the zeroing mark using the optical imaging system. Thus, the zeroing mark may be imaged on the viewing filed of the camera. After a signal processing of the image of the zeroing mark on the viewing field of the camera, a position relationship between the zeroing marks and the center of the viewing field may be obtained. 
     The zeroing mark detection unit  317  may return the position relationship information to the sub-control unit on the wafer stage  301 , the wafer stage  301  may be driven to adjust until the zeroing mark appear at the center of the viewing field. A coarse position information (information obtained by the encoder plate readers  313 ) of the wafer stages  301  may be obtained, and a coarse alignment of the wafer stage  301  may be finished. 
     A pull-in range of the camera of the zeroing mark detection unit  317  may be approximately 100 μm˜300 μm. After the coarse alignment of the wafer stage  301 , a position accuracy of the wafer stage  301  may be in a range of approximately ±2 μm. The position accuracy may be sufficient for subsequent wafer alignments and cylindrical reticle alignments. 
     As shown in  FIG. 7 , the peripheral region  315  of the wafer stage  301  may also have reticle alignment sensors/wafer stage fiducials  339 . The reticle alignment sensors/wafer stage fiducials  339  may include reticle alignment sensors  338  and wafer stage fiducials  316 . A number of reticle alignment sensors/wafer stage fiducials  339  may be two or more. In one embodiment, the number of reticle alignment sensors/wafer stage fiducials  339  is two. The two reticle alignment sensors/wafer stage fiducials  339  may be distributed along a diagonal direction at two corners of the wafer stage. 
     The reticle alignment sensors  338  may be used to detect the reticle alignment marks  333  on the cylindrical reticle  303  shown in  FIG. 4 , and the position relationship between the cylindrical reticle  303  and the wafer stage  301  may be formed. Thus the position relationship between the cylindrical reticle  303  and the wafer on the wafer stage  301  may also be formed, an alignment process of the cylindrical reticle  303  may be finished. 
     The wafer stage fiducials  316  may be used for a precise positioning of the wafer stage  301 . When the zeroing mark detection unit  317  is at a position of the zeroing detection mark, and the coarse alignment is finished, the wafer stage  301  may move to cause the alignment detection unit  302  to detect the wafer stage fiducials  316  of the wafer stage  301 , a precise position information (information detected by the encoder plate readers  313 ) of the wafer stage  301  (or the wafer stage fiducials  316 ) may be obtained. The precise position information of the wafer stage  301  may be used as the zero position (origin) of the coordinate of the wafer stage  301 , thus a fine alignment of the wafer stage  301  may be achieved. In certain other embodiments, after the fine alignment of the wafer stage  301 , the information detected by the encoder detector units  318  (as shown in  FIG. 6 ) may be correspondingly reset, the information after the resetting process may be used as the zero position (origin) of the coordinate of the wafer stage  301 . 
     Further, as shown in  FIG. 7 , the wafer stage fiducials  316  may be at least two groups. The two groups of wafer stage fiducials  316  may distribute along the scanning direction (the y direction) in the peripheral region  315  of the wafer stage  301 . A connection line of the centers of the two groups of wafer stage fiducials  316  may be parallel to the y-axis. Each of the wafer stage fiducials  316  may have two wafer stage fiducials  316  distributing orthogonally on the wafer stage  301 . Each of the wafer stage fiducials  316  may have a plurality of first gratings  344  and a plurality of second grating  345 . The first gratings  344  and the second gratings  345  may distribute orthogonally. The first gratings  344  may have a plurality of first grating strips distributed along a first direction, the second gratings  345  may have a plurality of second grating strips distributed along a second direction, and the first grating strips may be perpendicular to the second grating grips. 
     When the wafer stage  301  moves under the alignment detection unit  302  (referring to  FIG. 2 ), a position information of four wafer stage fiducials  316  may be obtained by detecting the two groups of the wafer stage fiducials  316  using the alignment detection unit  302 . Lateral movement constants in x and y directions, amplify constants in x and y directions, rotation constants in xy plane and orthogonal constants of the wafer stage  301  may be obtained by calculating the position information performed by the main control unit  300 . 
     Further, referring to  FIG. 7 , a wafer on the wafer stage  301  may have a plurality of exposure regions. The exposure regions may be isolated by scribe lines. Alignment marks  348  on the wafer may locate in the scribe lines. The alignment marks  348  may have a plurality of third gratings  346  and a plurality of second gratings  347 . The first gratings  346  and the second gratings  347  may symmetrically distribute at both sides of a center line of the scribe lines. The first grating  347  may have a first angle  41  with the center line, and the second gratings  348  may have a second angle  42  with the center line. The first angle  41  may be equal to the second angle  42 . An angle of the first angle  41  and the second angle  42  may be in a range of approximately 0°˜90°. In one embodiment, the first angle  41  and the second angle  42  are both 45°. 
     The alignment detection unit  302  (as shown in  FIG. 3 ) may have a first sub-detection unit (not shown) to detect the wafer stage fiducials  316  on the wafer state  301  and the alignment marks  348  on the wafer. The first sub-detection unit may be a one-dimensional array of optoelectronic sensors. When a detection process is performed, an optical system of the alignment detection unit  302  may irradiate the stage fiducials  316  of the wafer stage  301  or the wafer align marks  348  of a wafer, reflected light from the stage fiducials  316  of the wafer stage  301  or the wafer align marks  348  of the wafer may be received by the one-dimensional array of optoelectronic sensors, electrical signals varying with time may be outputted by the one-dimensional array of optoelectronic sensors. Whether or not the wafer stage fiducials  316  or the wafer alignment marks  348  are detected may be determined by identifying a time delay, a width of pulses and intervals of the pulses of the electrical signals outputted from the one-dimensional array of optoelectronic sensors. 
     In a specific detection process, the wafer stage  301  may move to a pre-alignment position (the first position of the base) firstly, the first sub detection unit may detect the wafer stage fiducials  316  on the wafer stage  301  when the wafer stage  301  is moving along the y direction. Then the wafer stage  310  may continue moving along the y direction, the first sub detection unit may detect the wafer alignment marks on the wafer. It may be unnecessary for the wafer stage  301  to move repeatedly in the xy plane, thus wafer alignment time may be saved, and the output in per unit time may be improved. 
     The first sub detection unit may also be a two dimensional imaging sensor (such as a CCD imaging sensor). When the alignment detection unit  302  detects the wafer stage fiducials  316 , the optical system of the alignment detection unit  302  may irradiate the wafer stage fiducials  316 , reflected light from the wafer stage fiducial  316  may form an image in the view field of a camera of the alignment detection unit  302 . A position relationship information of the wafer stage fiducials  316  related to the center of the view field of the camera may be obtained after processing the image signals in the camera. The position relationship information may be sent back to the sub control unit of the wafer stage  301 , and the position of the wafer stage  301  may be adjusted by the sub control unit until the wafer stage fiducials  316  is imaged in the center region of the view field of the camera. 
     The alignment detection unit  302  may also include a second sub-detection unit. The second sub detection unit may be used to detect the leveling of the wafer stage  301 . 
     The reticle alignment sensors  338  may be at least three groups. Each group of the reticle alignment sensors  338  may have two reticle alignment sensors. A cross line between the two reticle alignment sensors may be parallel to the center line of the center shaft extension tube  329 . Each group of reticle alignment sensors  338  (two sensors) may be used to correspondingly detect two reticle alignment marks  333  on the cylindrical reticle  303  (shown in  FIG. 3 ). A distance between the centers of two sensors of each group of the reticle alignment sensors  338  may be equal to a distance between the centers of the two projected images on the wafer stage  301 . Wherein the projected images are images of the two reticle alignment marks  333  projected by the optical projection unit  309  (shown in  FIG. 3 ) of the exposure apparatus. Thus, the reticle alignment marks  333  may be precisely detected. 
     The reticle alignment sensors  338  may be a one-dimensional array of optoelectronic sensors  343 . As shown in  FIG. 7 , the one-dimensional array of optoelectronic sensors  343  may includes a plurality of the sub sensor units aligned along the x-axis. When light irradiates on the one-dimensional array of optoelectronic sensors  343 , the one-dimensional array of optoelectronic sensors  343  may send out electrical signals varying with time. 
     In one specific detection process, light from the illuminator box  308  (may also refer as the exposure light) may irradiate the reticle alignment marks  333  on the cylindrical reticle  303  (as shown in  FIG. 5 ), the light passing through the reticle alignment marks  333  and the optical projection unit  309  may be received by the one-dimensional array of optoelectronic sensors  343 , and electrical signals varying with time may be sent out by one-dimensional array of optoelectronic sensors  343 . Whether the reticle alignment marks  333  are detected may be judged by the time delay, the width of the pulse, and the intervals of the output electrical signals. 
     As shown in  FIG. 7 , in one embodiment, the reticle alignment sensors  338  are three groups including a first reticle alignment sensor  338   a , a second reticle alignment sensor  338   b , and a third reticle alignment sensor  338   c . Surfaces of the three groups of reticle alignment sensors  338  may be different. The surface of the first reticle alignment sensor  338   a  may have a standard height. The surface of the second reticle alignment sensor  338   b  may be lower than the surface of the first reticle alignment sensor  338   a . The surface of the third reticle alignment sensor  338   c  may be higher than the surface of the first reticle alignment sensor  338   a . When detecting the reticle alignment marks  333 , the first reticle alignment sensor  338   a , the second reticle alignment sensor  338   b , and the third reticle alignment sensor  338   c  may be at the exact focus plane and/or off the exact focus plane of the optical system (the optical projection unit  309 ), respectively. Thus, an exact focus distance may be found when using the reticle alignment sensors  338  to align the cylindrical reticle  303  through calculating the detected imaging contrast from the three sensors  338   a ,  338   b , and  338   c.    
     In one embodiment, when the wafer stages  301  move underneath the optical projection unit  309  (shown in  FIG. 3 ), and the reticle alignment marks  333  are detected, the first reticle alignment sensor  338   a  may be at the exact focus plane, the second reticle alignment sensor  338   a  may be at a negatively off focus plane, and the third reticle alignment sensor  338   c  may be at a positively off focus plane. According to the position information (may refer to a height along the z-direction), a height of the first reticle alignment sensor  338   a , and the relationship between the center of the optical system and the designed height of the base, an exact focus plane may be obtained. Other combination of the heights of the first reticle alignment sensor  338   a , the second reticle alignment sensor  338   b , and the third reticle alignment sensor  338   c  may also be used. 
     Specifically, when the reticle alignment marks  333 , the wafer stage  301  may firstly move to cause the first reticle alignment sensors  338   a  to move underneath the optical projection unit  309 , the exposure light may irradiate the reticle alignment marks  333  on the cylindrical reticle  303  through the illumination slit  335 . At the same time, the cylindrical reticle  303  may rotate at last one cycle around the center axis of the center shaft  329 , the light passing through the reticle alignment marks  333  may be received by the first reticle alignment sensor  338   a.    
     Then, the wafer stage  301  may continue to move along the scanning direction (may refer to the positive direction of the y-axis) to cause the second reticle alignment sensor  338   b  to move underneath the optical projection unit  309 . At the same time, the cylindrical reticle  303  may rotate at last one cycle around the center axis of the center shaft extension tube  329 , the light passing through the reticle alignment marks  333  may be received by the second reticle alignment sensor  338   b.    
     Further, the wafer stage  301  may continue to move along the scanning direction (may refer to the positive direction of the y-axis) to cause the third reticle alignment sensor  338   c  to move underneath the optical projection unit  309 . At the same time, the cylindrical reticle  303  may rotate at last one cycle around the center axis of the center shaft  329 , the light passing through the reticle alignment marks  333  may be received by the third reticle alignment sensor  338   c.    
     The wafer stage  301  may not move along the z direction when it move along the y-direction. The first reticle alignment sensor  338   a , the second reticle alignment sensor  338   b , and the third reticle alignment sensor  338   c  may obtain a plurality of electrical signals respectively after the above movements of the wafer stage  301 . By comparing the electrical signals obtained from the same reticle alignment marks  333  by the first reticle alignment sensor  338   a , the second reticle alignment sensor  338   b , and the third reticle alignment sensor  338   c , a height information of the reticle alignment sensors  338  corresponding to a maximum peak value of the electrical signals may be obtained. Further, according the height information, the position information of the wafer stage  301  (may refer to a height along the z direction) and the relationship between the center of the optical system and the designed height of the base, an exact focus plane may be obtained. 
     A height difference between the surface of the second reticle alignment sensor  338   b  and the first reticle alignment sensor  338   a  may be in a range of approximately 50 nm˜1000 nm. A height different between the surface of the third reticle alignment sensor  338   c  and the first reticle alignment sensor  338   a  may be in a range of approximately 50 nm˜1000 nm. Thus, a relatively high sensitivity for the reticle alignment sensors  333  to detect the exact focus plane may be obtained, and the obtained exact focus distance may be relatively accurate. 
     The reticle alignment detection unit  338  may be at an overlap region of an extended region of first exposure region of the wafer along the scanning direction (extended along the CD direction shown in  FIG. 7 ) and the peripheral region  315 . After the wafer stage  301  moves to a pre-exposure position, the reticle alignments marks  333  may be detected. After the reticle alignments marks  333  are detected, the wafer stage  301  may directly move along the scanning direction without moving along x direction and y direction repeatedly. A first column of exposure regions may be exposed, thus the time of the exposure process may be reduced. 
     Further, referring to  FIG. 3 , the encoder plate  306  may be fixed on the base (not shown) of the exposure apparatus, a width of the encoder plate  306  may be greater than a width of the wafer stage  301 ; and a length of the encoder plate  306  may be greater than a line distance between the alignment detection unit  302  and the optical exposure unit  309 . Thus, when the wafer stage  301  is at a first position, a second position, and or moves from the first position to the second position, an accuracy position information may always be obtained by a position detection unit consisting of the alignment detection unit  302  and the encoder plate  306 , and the accuracy position information may be obtained during an alignment process and an exposure process. Therefore, the exposure precision may be improved. When the wafer stage  301  moves on other positions of the base, other appropriate methods may be used to detect the position, such as guide rails, photoelectrical sensors, or interferometers, etc. 
     As shown in  FIG. 3 , the encoder plate  306  may have the first opening  311  (may refer as a metrology window) and the second opening  312  (may refer as an exposure window). A position of the first opening  311  may correspond to a position of the alignment detection unit  302 , i.e., the first opening  311  may be underneath the alignment detection unit  302 . The first opening  311  may be used as an optical pass between the alignment detection unit  302  and the wafer stage  301 . A position of the second opening  312  may correspond to a position of the optical projection unit  309 , i.e., the second opening  312  may be underneath the optical projection unit  309 . The second opening may be used as an optical pass between the optical projection unit  309  and the wafer stage  301 . 
     Further, as shown in  FIG. 8 , the encoder plate  306  may have a plurality of parallel equal lines  326  and spaces  325 . The lines  326  and the spaces  325  may form a reflective grating. The surface of the encoder plate  306  having the lines  326  and the spaces  325  may face the top surface of the wafer stage  301 . A distance between adjacent lines  326  and a distance between adjacent spaces  325  may be equal. 
     The encoder plate  306  may be made of glass, the lines  326  and the spaces  325  may be formed by etching the glass. In certain other embodiments, opaque strips may be formed on the transparent glass encoder plate  306  to form the lines  326  and the spaces  325 , that is the reflective grating. The opaque stripes may be made of any appropriate material, such as metal thin film, etc. 
     As shown in  FIG. 8 , in one embodiment, the encoder plate  306  may have a first sub grating plate  320  and a second sub grating plate  321 . The first sub grating plate  320  and the second sub grating plate  321  may be dissymmetrical. A symmetrical center line AB of the first sub grating plate  320  and the second sub grating plate  321  and the cross line between the alignment detection unit  302  and the center of the optical projection unit  309  may be on a same plane perpendicular to the base of the exposure apparatus. The symmetrical center line AB of the first sub grating plate  320  and the second sub grating plate  321  and a cross line between the center of the first sub grating plate  320  and the second sub grating plate  321  may be coincide. 
     In one embodiment, the lines  326  and the spaces  325  of the first sub grating plate  320  and the lines  326  and the spaces  325  of the second sub grating plate  321  may be dissymmetrical. The lines  326  of the first sub grating plate  320  may connect with the symmetrical center line AB, and have a first angle  32 . The lines  326  of the second sub grating plate  321  may connect with the symmetrical center line AB, and have a second angle  31 . The angle of the second angle  31  may be equal to the angle of the first angle  32 . Thus, the accuracy of the first sub grating plate  320  may be identical to the accuracy of the second sub grating plate  321 , a high accuracy of the position coordinate of the wafer stage  301  detected by the encoder plate readers  313  may be obtained. A moving direction and displacement of the wafer stage  301  may be obtained by the phase change, interval change, and pulse number of signals detected by the encoder plate readers  313 . 
     Angles of the first angle  32  and the second angle  31  may be in a range of approximately 5°˜85°. Specifically, the angle of the first angle  32  and the second angle may be 10°, 20°, 30°, 40°, 45°, 50°, 60°, 70°, or 80°, etc. When the wafer stage  301  moves under the first sub grating plate  320  and the second grating plate  321  along the x-axis, the y-axis or the z-axis, phase and interval changes between two electrical signals may be obtained by a single encoder detector unit  318 . In certain other embodiments, the phase and interval changes between a plurality signals may be obtained from a plurality of the encoder detector unit  318 . The moving direction and displacement of the wafer stage  301  may be determined and calculated by the obtained phase/interval changes. 
     In one embodiment, the number of the encoder plate readers  313  may be at least three; and each of the encoder plate readers  313  may include two encoder detector units  318 . The encoder plate readers  313 , the first sub grating plate  320  and the second sub grating plate  321  may form a detection system. Each of the encoder plate readers  313  may detect two degrees of freedom of the wafer stage  301 , including the x-direction (or the y-direction) and the z-direction. A detection of six degrees of freedom may be achieved using at least three encoder plate readers  313 . The six degrees of freedom may include the x-direction, the y-direction, the z-direction, the rotation x-direction, the rotation y-direction, and the rotation z-direction. The detection may be simple with a high accuracy, and the data processing may also be simple. 
     Referring to  FIG. 8 , when the encoder plate  306  and the encoder detector units  318  are used as a position detection unit, a detection accuracy may be in a range of approximately 1 nm˜10 nm. When the wafer stage  301  moves within the measurement range of the encoder plate  306 , the position error of the wafer stage  301  may be smaller than a range of approximately 1 nm˜100 nm. A precise position relationship between the wafer stage  301 , the wafer on the wafer stage  301 , and the cylindrical reticle  303  may be achieved, thus the exposure apparatus may have a relatively high accuracy. 
     Further, as shown in  FIG. 8 , the encoder plate  306  in the peripheral region of the first opening  311  may have zeroing marks  322 . The zeroing marks  322  may be used for a quick alignment and leveling capture. A number of the zeroing marks  322  may be equal to the number of the zeroing mark detection units  317 . A distribution of the zeroing marks  322  may be same as a distribution of the zeroing mark detection units  317  on the wafer stage  301 . That is, a pattern formed by center cross lines of the zeroing marks  322  on the encoder plate  306  may coincide with a pattern formed by center cross lines of the zeroing detection units  317  on the wafer stage  301 . Thus, an accurate positioning of the wafer stage  301  may be achieved. 
     In one embodiment, the number of the zeroing marks  322  is corresponding to the number of the zeroing mark detection units  317 . The number of the zeroing marks  322  may be four. As shown in  FIG. 8 , each of the zeroing marks  322  may have a plurality of first grating  323  distributed along the y-axis direction and a plurality of the second grating  324  distributed along the y-axis direction. 
       FIG. 9  illustrates an exemplary embodiment using a detection system consisting of four encoder plate readers  313 , the first sub grating plate  320  and the second sub grating plate  321  to detect a position of the wafer stage  301 . As shown in  FIG. 9 , the four encoder plate readers  313  may include a first encoder plate reader  313   a , a second encoder plate reader  313   b , a third encoder plate reader  313   c , and a fourth encoder plate reader  313   d . Distance between the center of the surface of the wafer stage  301  and the first encoder plate reader  313   a , the second encoder plate reader  313   b , the third encoder plate reader  313   c , and the fourth encoder plate reader  313   d  may be equal. The distance may refer as “r”. Cross lines of the centers of the four encoder plate reader may form a rectangle. 
     The x-axis coordinate of the wafer stage  301  obtained by the detection system may refer as C x . The y-axis coordinate of the wafer stage  301  obtained by the detection system may refer as C y . The z-axis coordinate of the wafer stage  301  obtained by the detection system may refer as C z . The x-axis rotation constant obtained by the detection system may refer as R x . The y-axis rotation constant obtained by the detection system may refer as R y . The z-axis rotation constant obtained by the detection system may refer as R z . Wherein: 
     
       
         
           
             
               Cx 
               = 
               
                 
                   
                     E 
                     2 
                   
                   + 
                   
                     E 
                     3 
                   
                   - 
                   
                     E 
                     1 
                   
                   - 
                   
                     E 
                     4 
                   
                 
                 
                   2 
                   ⁢ 
                   
                     2 
                   
                 
               
             
             ; 
           
         
       
       
         
           
             
               Cy 
               = 
               
                 - 
                 
                   
                     
                       E 
                       1 
                     
                     + 
                     
                       E 
                       2 
                     
                     + 
                     
                       E 
                       3 
                     
                     + 
                     
                       E 
                       4 
                     
                   
                   
                     2 
                     ⁢ 
                     
                       2 
                     
                   
                 
               
             
             ; 
           
         
       
       
         
           
             Cz 
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     Wherein: E 1 , E 2 , E 3 , and E 4  may refer to detected values obtained by the first encoder plate reader  313   a , the second encoder plate reader  313   b , the third encoder plate reader  313   c , and the fourth encoder plate reader  313   d  when the wafer stage  301  moves along the x-direction and/or the y-direction, respectively. E 1z , E 2z , E 3z , and E 4z  may refer to detected values obtained by the first encoder plate reader  313   a , the second encoder plate reader  313   b , the third encoder plate reader  313   c , and the fourth encoder plate reader  313   d  when the wafer stage  301  moves along the z-direction, respectively. S y  may refer to a vertical distance between the cross line of the centers of the first encoder plate reader  313   a  and the second encoder plate reader  313   b  and the center of the surface of the wafer stage  301 . S y  may refer to a vertical distance between the cross line of the centers of the first encoder plate reader  313   a  and the second encoder plate reader  313   d  and the center of the surface of the wafer stage  301 . The above equations may be correct when the two encoder plates are angled at 45 degrees to the symmetry axis and orthogonal to each other. 
       FIGS. 10-17  illustrate certain steps of an exemplary exposure process using the above disclosed exposure apparatus. For illustrative purposes, four wafer stages may be used to describe the exposure process in details. 
     As shown in  FIG. 10 , the exposure apparatus have four wafer stages including a first wafer stage  3011 , a second wafer stage  3012 , a third wafer stage  3013 , and a fourth wafer stage  3014 . A first wafer  1 , a second wafer  2 , a third wafer  3  and a fourth wafer  4  may be successively loaded on the first wafer stage  3011 , the second wafer stage  3012 , the third wafer stage  3013 , and the fourth wafer stage  3014 . 
     Before the exposure apparatus starts working, the first wafer stage  3011 , the second wafer stage  3012 , the third wafer stage  3013 , and the fourth wafer stage  3014  may distribute in queue on the base of the exposure apparatus. When the exposure process is started, exposure programs may be firstly installed in the exposure apparatus. Then four wafers may be loaded on the four wafer stages successively. The wafer stages loaded with wafers may successively move toward a first position to align the wafers at the first position. Before aligning the first wafer  1  on the first wafer stage  3011 , the second drive unit may install the cylindrical reticle  303  into the cylindrical reticle stage  305 . After installing the cylindrical reticle  303 , the cylindrical reticle  303  may rotate around the center shaft extension tube  329 ; and the pre-alignment imaging sensors  341  may detect the reticle alignment marks  333  and loading status and rotating status of the cylindrical reticle  303 . 
     When the first wafer stage  3011 , the second wafer stage  3012 , the third wafer stage  3013 , and the fourth wafer stage  3014  move in regions outside the encoder plate  306 , tracks, interferometers, or grating plates may be used to detect their positions. An accumulated position errors may be less than 100 μm. 
     Further, as shown in  FIG. 11 , the first wafer stage  3011  moves to the first position of the base, i.e., a pre-alignment position underneath the first opening  311  of the encoder plate  306 , the zeroing mark detection units  317  (shown in  FIG. 8 ) on the first wafer stage  3011  may detect the zeroing marks  322  on the encoder plate  306 , a coarse position information of the first wafer stage  3011  may be obtained, and a coarse alignment of the first wafer stage  3011  may be finished. 
     In one embodiment, if at least three zeroing mark detection units  317  have detected the corresponding zeroing marks  322 , the coarse alignment of the wafer stage  3011  may be finished. If less than three zero mark detection units  317  have detected the corresponding zeroing marks  322 , the coarse alignment may need to repeat. 
     After the coarse alignment of the wafer stage  3011  is finished, the wafer stage  3011  may quickly to move under the alignment detection unit  302  (referring to  FIG. 3 ), the alignment detection unit  302  may detect the wafer stage fiducials  316  on the first wafer stage  3011 , an accurate position information of the wafer stage  3011  (or wafer stage fiducials  316 ) may be obtained. At the same time, the accurate position information of the wafer stage  3011  may be used as a zero point (origin) of a position coordinate of the wafer stage  3011 , and a fine alignment of the wafer stage  3011  may be finished. 
     In certain other embodiments, after finishing the fine alignment of the wafer stage  3011 , the detected information of the encoder plate readers  313  on the wafer stage  3011  may be reset correspondingly, the reset zero information may be used as the zero (origin) of the position coordinate of the wafer stage  3011 . The main control unit  300  may save and calculate the displacement bias of the wafer stage  3011  between the fine alignment and the coarse alignment. 
     After the alignment detection units  302  detect the wafer stage fiducials  316  on the first wafer stage  3011 , the first wafer stage  3011  may move, the alignment detection unit  302  may detect alignment marks on the first wafer  1  loaded on the first wafer stage  3011 , the position information of the alignment marks on the first wafer  1  may be obtained. The main control unit  300  may convert the position information of the alignment marks on the first wafer  1  to a position coordinate using the position information of the wafer stage fiducials  316  as an origin, a position management of exposure regions on the wafer  1  may be achieved. The related position information and position coordinate may be saved in the main control unit  300 , and an alignment of the first wafer  1  may be finished. The main control unit  300  may save and calculate the displacement bias of the wafer stage  3011  between the fine alignment and the detection of the alignment marks on the first wafer  1 . 
     As shown in  FIG. 12 , after the alignment of the wafer  1 , the first wafer stage  3011  may move from the first position to the second position, a pre-exposure position underneath the second opening  312  of the encoder plate  306 . Then, the wafer stage  3011  may perform a unidirectional scan along a scanning direction, i.e., the positive direction of the y-axis. At the same time, the cylindrical reticle  303  may rotate around the center axis of the cylindrical reticle stage  305 . Light passing through the cylindrical reticle  303  may be projected on the first wafer  1  on the first wafer stage  3011  by the optical projection unit  309  (shown in  FIG. 3 ), thus a first column of exposed regions along the scanning direction may be exposed. At the same time, the second wafer stage  3012  may move to the first position of the base, and the second wafer  2  on the second wafer stage  3012  may be aligned. 
     Before exposing the first wafer  1 , an exact focus distance may be obtained by detecting the reticle alignment marks  333  on the cylindrical reticle  303  using the reticle alignment sensors  338  with different heights, a position relationship between the cylindrical reticle  303  an the first wafer stage  3011  may be formed. Thus, a position relationship between the cylindrical reticle  303  and the first wafer  1  may be formed as well, and an alignment of the cylindrical reticle  303  may be finished. The related position information and position coordinate may be saved in the main control unit  300 . 
     Referring to  FIG. 7 , since the reticle alignment sensors  338  may be at an overlapped region of an extended region of the first exposure region along the scanning direction (extends along the CD direction) and the peripheral region  315  of the top surface of the wafer stage  301 , after the first wafer stage  3011  moves to the pre-exposure region, the reticle alignment marks  333  may be detected. After detecting the reticle alignment marks  333 , the first wafer stage  3011  may unnecessarily move back and forth along the x-direction or the y-direction, thus the exposure time may be reduced. 
     When the first exposure region of the first wafer  1  is exposed, the wafer stage  3011  may keep the positive scanning direction of the y-axis, the rotation direction of the cylindrical reticle  303  around the center axis of the cylindrical reticle stage  305  may also keep same, thus after exposing the first exposure region of the first column of exposure regions, the scanning direction of the first wafer stage  3011  and the rotation direction of the cylindrical reticle  303  may unnecessarily change, a second exposure region of the first column of exposure regions may be exposed. Similarly, all the exposure regions of the first column may be exposed without changing the scanning direction of the first wafer stage  3011  and the rotation direction of the cylindrical reticle  303 . Thus, when the first column of exposure regions of the first wafer  1  are exposed, the wafer stage  3011  may unnecessarily need to accelerate or decelerate, the time for the exposure process may be significantly reduced. 
     Before exposing the first exposure region of the first column of exposure regions of the first wafer  1 , the first wafer stage  3011  may need an acceleration process. After exposing the last exposure region of the first column of exposure region, the first wafer stage  3011  may need a deceleration process. 
     In one embodiment, since the scanning direction of the first wafer stage  3011  may keep constant, on the basis providing a relatively high exposure resolution, the scanning speed may be increased. The scanning speed of the first wafer stage  3011  may be in a range of approximately 100 mm/s˜10 m/s, or higher. Physically, the scanning speed may have no limitation except the position detections and the feedback time. 
     The time for the cylindrical reticle  303  to rotate around the center axis of the cylindrical reticle stage  305  for one circle may equal to the time for the first wafer stage  3011  to move one exposure region along the scanning direction. Thus, patterns of the cylindrical reticle  303  may be completely transferred onto the exposure regions on the first wafer  1 , and an unidirectional step scanning exposure of the exposure regions on the first wafer  1  may be achieved. 
     In certain other embodiments, the time for the cylindrical reticle  303  to rotate around the center axis of the cylindrical reticle stage  305  for one circle may equal to the total time for the first wafer stage  3011  to move a plurality of exposure regions along the scanning direction. When the cylindrical reticle  303  rotate for one circle, a plurality of exposure regions of the first column of exposure region along the scanning direction may be exposed. 
     When the first column of exposure regions of the first wafer  1  on the first wafer stage  3011  are partially exposed, after finishing aligning the second wafer  2 , the second wafer stage  3012  may move from the first position to the second position or a nearby position on the base to wait for an exposure. The third wafer stage  3013  may move to the first position to align the third wafer  3 . Then, the fourth wafer stage  3014  may move toward the first position after loading the fourth wafer  4 . The alignment of the second wafer  2  is similar as the alignment of the first wafer  1 . 
     As shown in  FIG. 13 , when the exposure of the first column of exposure regions of the first wafer  1  on the first wafer stage  3011  is completed, the first wafer stage  3011  may continue to move along the positive direction of the y-axis, the second wafer stage  3012  may move to the second position. Then, the second wafer stage  3012  may perform an unidirectional scan along the scanning direction, i.e., the positive direction of the y-axis. At the same time, the cylindrical reticle  303  may rotate around the center axis of the cylindrical reticle stage  305 , the light passing through the cylindrical reticle  303  may be projected on the second wafer  2  loaded on the second wafer stage  3012  by the optical projection unit  309 . A first column of exposure regions of the second wafer  2  along the scanning direction may be exposed. At the same time, after aligning the third wafer  3 , the third wafer stage  3013  may move from the first position toward the second position. The fourth wafer stage  3014  move to the first position, and the fourth wafer  4  may be aligned. 
     Before exposing the second wafer  2 , an exact focus distance may be obtained by detecting the reticle alignment marks  333  on the cylindrical reticle  303  using the reticle alignment sensors  338  with different heights, a position relationship between the cylindrical reticle  303  and the second wafer stage  3012  may be formed. Thus, a position relationship between the cylindrical reticle  303  and the second wafer  2  may be formed, and an alignment of the cylindrical reticle  303  may be finished. The related position information and position coordinate may be saved in the main control unit  300 . 
     As shown in  FIG. 14 , the first wafer stage  3011  may move toward the first position. After the first column of exposure regions of the second wafer  2  are exposed along the scanning direction, the second wafer stage  3012  may move away from the second position and the third wafer stage  3013  may move to the second position. Then, the third wafer stage  3013  may perform a unidirectional scan along the scanning direction, i.e., the positive direction of the y-axis. At the same time, the cylindrical reticle  303  may rotate around the center axis of the cylindrical reticle stage  305 , and the light passing through the cylindrical reticle  303  may be projected on the third wafer  3  loaded on the third wafer stage  3013  by the optical projection unit  309 . A first column of exposure regions of the third wafer  3  along the scanning direction may be exposed. At the same time, after aligning the fourth wafer  4 , the fourth wafer stage  3014  may move from the first position toward the second position. 
     Before exposing the third wafer  3 , an exact focus distance may be obtained by detecting the reticle alignment marks  333  on the cylindrical reticle  303  using the reticle alignment sensors  338  with different heights, a position relationship between the cylindrical reticle  303  and the third wafer stage  3013  may be formed. Thus, a position relationship between the cylindrical reticle  303  and the third wafer  3  may be formed, and an alignment of the cylindrical reticle  303  may be finished. The related position information and position coordinate may be saved in the main control unit  300 . 
     As shown in  FIG. 15 , after the first column of exposure regions of the third wafer  3  are exposed, the first wafer stage  3011  may move to the first position. The first wafer  1  on the first wafer stage  3011  may be aligned. The fourth wafer stage  3014  may move to the second position. Then, the fourth wafer stage  3014  may perform a unidirectional scan along the scanning direction, i.e., the positive direction of the y-axis. At the same time, the cylindrical reticle  303  may rotate around the center axis of the cylindrical reticle stage  305 , and the light passing through the cylindrical reticle  303  may be projected on the fourth wafer  4  loaded on the fourth wafer stage  3014  by the optical projection unit  309 . A first column of exposure regions of the fourth wafer  4  along the scanning direction may be exposed. 
     Since the main control unit  300  may be used to save and calculate the displacement bias of the wafer stage  3011  between the fine alignment and the coarse alignment and the displacement bias of the wafer stage  3011  between the fine alignment and the detection of the alignment marks on the first wafer  1 , when the first wafer  1  on the first wafer stage  3011  is realigned, it may only need the zeroing mark detection unit  317  to detect the zeroing marks  322  on the encoder plate  306  and verify the position coordinate of the wafer stage  3011 . The wafer position information may be obtained by the main control unit  300  by using the position information of the first wafer stage  3011  and certain types of calculation. Thus, a position management of the first wafer  1  may be achieved, and the time for realignment of the first wafer  1  may be reduced. 
     In certain other embodiments, when the first wafer  1  is realigned, the alignment detection unit  302  may detect the wafer stage fiducials  339  on the first wafer stage  1  and the alignment marks on the first wafer  1 . 
     Further, as shown in  FIG. 16 , the first wafer stage  3011  may move from the first position to the second position, then the first wafer stage  3011  may perform an unidirectional scan along the scanning direction, i.e., the positive direction of the y-axis. At the same time, the cylindrical reticle  303  may rotate around the center axis of the cylindrical reticle stage  305 , and the light passing through the cylindrical reticle  303  may be projected on the first wafer  1  on the first wafer stage  3011  by the optical projection unit  309 . A second column of exposure regions of the first wafer  1  along the scanning direction may be exposed. At the same time, the second wafer stage  3012  may move to the second position of the base, the second wafer  2  on the second wafer stage  3012  may be aligned. 
     In one embodiment, since the position relationship between the cylindrical reticle  303  and the first wafer stage  3011  may be already formed when the first column exposure regions of the first wafer  1  are exposed, the reticle alignment marks  333  may unnecessarily need to be detected, a position relationship between the cylindrical reticle  303  and the first wafer stage  3011  for exposing the second column of exposure regions may be obtained by the main control unit  300  using the saved position relationship between the cylindrical reticle  303  and the first wafer stage  3011 , and the displacement of the first wafer stage  3011  moving along the x direction, i.e., a distance along the x-direction for the wafer stage  3011  to move one exposure region comparing with the first column of exposed regions after aligning the first wafer  1  and before exposing the second column of exposure regions, the exposure time may be effectively reduced. 
     In certain other embodiments, before exposing the second column of exposure regions on the first wafer  1 , the reticle alignment sensors  338  may detect the reticle alignment marks  333  on the cylindrical recticle  303 , the position relationship of the cylindrical reticle  303  and the first wafer stage  3011  may be obtained. 
     Further, as shown in  FIG. 17 , after the second column of exposure regions of the first wafer  1  are exposed, the first wafer stage  3011  may continue to move toward the first position of the base along the positive direction of the y-axis, and the second wafer stage  3012  may move to the second position. Then the second wafer stage  3012  may perform a unidirectional scan along the scanning direction, i.e., the positive direction of the y-axis. At the same time, the cylindrical reticle  303  may rotate around the center axis of the cylindrical reticle stage  305 , the light passing through the cylindrical reticle  303  may be projected on the second wafer  2  on the second wafer stage  3012  by the optical projection unit  309 . A second column of exposure regions of the second wafer  2  along the scanning direction may be exposed. At the same time, after the third wafer  3  is aligned, the third wafer stage  3013  may move from the first position toward the second position of the base. The fourth wafer stage  3014  may move to the first position  311 , and the fourth wafer  4  on the fourth wafer stage  3014  may be aligned. 
     The first wafer stage  3011 , the second wafer stage  3012 , the third wafer stage  3013 , and the fourth wafer stage  3014  may cyclically move on the base in a pipelined mode. The alignment and exposure processes may be repeated until all the exposure regions of the first wafer  1 , the second wafer  2 , the third wafer  3  and the fourth wafer  4  are exposed. When a certain wafer on one of the four wafer stages is completely exposed, a new wafer may be loaded. 
     The present exposure apparatus may have the cylindrical reticle  303  and a plurality of wafer stages  301 . The cylindrical reticle  303  may rotate around the center axis of the cylindrical reticle system  305 , one wafer stage  301  may perform a unidirectional scan along the scanning direction, the light passing through the cylindrical reticle  303  may be projected on a certain column of exposure regions on the wafers on the wafer stage  301 , thus the certain column of exposure regions may be exposed along the scanning direction. When the certain column of exposure areas are exposed, since the cylindrical reticle  303  may return to the origin after rotating one circle, after exposing the certain column of the exposed areas, the rotation direction of the reticle  303  may unnecessarily change, and the scanning direction of the wafer stage  301  may also unnecessarily change, an exposure of a next exposure region in the same column may be achieved. Therefore, an accelerating process and a decelerating process may unnecessarily need when exposure regions in a same column are exposed, the total time for an exposure process may significantly reduced, and the yield in per unit time of the exposure apparatus may be increased. 
     In one embodiment, using the cylindrical reticle  303  may have a similar reticle alignment process as other exposure apparatuses. However, the wafer exposure map may be changed. Each column of exposure regions is unidirectional scan-exposed during one scan instead of one exposure shot. The only time loss may be to scan back the wafer for next column exposure. The throughput gain may be still substantial: For example: for a two-wafer stage configuration (alignment and leveling may be done in parallel, and may unnecessarily be counted as a throughput overhead), currently, each exposure area may need 0.26 seconds for entire scan, including acceleration (0.10 sec), scan-exposure (0.06 sec), de-acceleration (0.10 sec). For a scan speed of 600 mm/s and 300 mm wafer, each column may only needs about 2 times (back scan to prepare for next column exposure) of (0.1 sec+0.5+0.1 sec), but it may include the exposure of up to 9 shots (average of 7 shots), the time gain is: 0.26×7−0.7×2=1.82−1.4=0.42 sec, or 30% throughput gain. If the scan speed may be further increased, for example, 2 msec, assuming the acceleration and de-acceleration may use the same time, the gain will be more: 0.22×7−0.36×2=1.54−0.72=0.82 sec, i.e., 114% yield gain. 
     For more wafer stages, the gain in yield may be more since during one exposed wafer stage&#39;s back scan, other wafers can be exposed. For example, in case of a 25 wafer stage systems, the total processing time for 25 wafers may be estimated as: [(0.1+0.54+0.1+0.06×2 (time lag between wafer stage to wafer stage))×12 (column)+2 (first column reticle alignment)]×25=308 seconds, i.e., the yield will be 3600/308×25=292 wph which is much higher than current, about 3600/[(0.1+0.06+0.1)×110]=126 wph. Wherein, “wph” may refer to wafer per hour. 
     In certain other embodiments, the exposure system may have one wafer stage  301 , the wafer stage  301  may unnecessarily go back the first position after the first column of the wafer is exposed. The wafer stage  301  may only need move to a next exposure region, and perform a unidirectional scan. At the same time, the cylindrical reticle  303  may rotate for one cycle, the next exposure region may be exposed. The exposure process may be repeated until the entire wafer is exposed. 
     The above detailed descriptions only illustrate certain exemplary embodiments of the present invention, and are not intended to limit the scope of the present invention. Those skilled in the art can understand the specification as whole and technical features in the various embodiments can be combined into other embodiments understandable to those persons of ordinary skill in the art. Any equivalent or modification thereof, without departing from the spirit and principle of the present invention, falls within the true scope of the present invention.