Abstract:
For use in association with microlithography systems, reticle chambers and reticle cassettes are disclosed that provide ready access to and exchange of reticles for exposure as well as temperature control of the reticles. In an embodiment a vacuum reticle library is provided in a reticle-storage chamber. The vacuum reticle library includes a rack comprising multiple shelves for holding respective reticles at different respective elevations. One or more shelves comprises a fluid conduit through which is circulated a temperature-controlled fluid. By adjusting and controlling the temperature of the fluid (e.g., water) circulated to each shelf, the temperature of the respective reticles held on the shelves can be controlled and adjusted quickly. Similarly, an atmospheric-pressure reticle library can be provided in an atmospheric-pressure chamber containing a rack of multiple shelves on which respective reticle cassettes (containing reticles) can be stored. The cassettes and reticles can be identified by a bar code or the like that is read to avoid mixups during reticle transport and exchange.

Description:
FIELD  
         [0001]    This disclosure pertains to microlithography, which is a key technique used in the manufacture of microelectronic and other micro-devices such as semiconductor integrated circuits, displays, etc. More specifically, the disclosure pertains to microlithography in which a pattern, defined on a reticle or mask (generally termed “reticle” herein) is transfer-exposed by an energy beam from the reticle to a suitable substrate that is “sensitive” to the exposure in a way resulting in imprinting of an image of the pattern on the surface of the substrate. Even more specifically, the disclosure pertains to microlithography performed in a subatmospheric (“vacuum”) environment such as in a temperature-controlled vacuum chamber to and from which reticles are exchanged for exposure.  
         BACKGROUND  
         [0002]    [0002]FIG. 5 schematically depicts the structure of a conventional microlithography system employing an electron beam as a lithographic energy beam. A main unit  100  of the system includes an illumination-optical-system (IOS) lens column  101 , a reticle chamber  103 , a projection-optical-system (POS) lens column  105 , and a substrate chamber  107 . The IOS lens column  101  is a vacuum chamber that contains an electron gun (serving as the electron-beam source) and the illumination-optical system itself, including beam-shaping aperture, condenser lens(es), beam deflector(s), etc. (not detailed, but well understood in the art). The POS lens column  105  also is a vacuum chamber that is connected downstream of the IOS lens column  101  with the reticle chamber  103  therebetween. The POS lens column  105  contains the projection-optical system, comprising projection lens(es), beam deflector(s), correction coil(s), etc. (not shown, but well-understood in the art). Inside the reticle chamber  103  (also a vacuum chamber) is a reticle stage  104  that supports and positions the reticle for exposure. Inside the substrate chamber  107  (also a vacuum chamber but located downstream of the POS lens column  105 ) is a substrate stage  108  that supports and positions the substrate (e.g., resist-coated semiconductor wafer) for exposure.  
           [0003]    A reticle load-lock chamber  111  is connected to the reticle chamber  103  at the right in the figure. A vacuum-side transport robot  113  is provided inside the reticle load-lock chamber  111  as a reticle-exchange means. An atmosphere-side transport robot  115  and reticle stand  119  are situated at the right of the reticle load-lock chamber  111  in the figure. A reticle cassette  117  is placed on the reticle stand  119 . A reticle  120 , defining a pattern for use in microlithography, is situated inside the reticle cassette  117 .  
           [0004]    When exchanging reticles, the reticle  120  inside the reticle cassette  117  is placed manually on the reticle stand  119  by an operator. Upon receiving a reticle-load command from the operator, the atmosphere-side transport robot  115  removes the reticle  120  from the reticle cassette  117  and transports the reticle to inside the reticle load-lock chamber  111 . Then, the reticle load-lock chamber  111  is exhausted to a suitable vacuum level by a vacuum pump (not shown, but well-understood in the art). When the required vacuum level is reached inside the reticle load-lock chamber  111 , the vacuum-side transport robot  113  transports the reticle  120  from the reticle load-lock chamber  111  to inside the reticle chamber  103  and places the reticle on the reticle stage  104 . After the reticle  120  is placed on the reticle stage  104 , final reticle positioning and alignment are performed in preparation for exposure.  
           [0005]    The conventional device and operational sequence described above are subject to the following problems:  
           [0006]    (1) When exchanging reticles, the operator must place the reticle  120  inside the reticle cassette  117  on the reticle stand  119  each time. Hence, whenever microlithographic exposures are being performed using multiple different reticles, this manual operation consumes substantial time and effort.  
           [0007]    (2) When transporting a reticle  120  from inside the reticle cassette  117  to inside the reticle load-lock chamber  111 , the reticle moves from atmospheric pressure to a reduced-pressure atmosphere. This change is accompanied by a temperature change of the reticle. The reticle also experiences a significant temperature change when being used for exposure. These changes require substantial time for stabilization.  
         SUMMARY  
         [0008]    In view of the problems associated with conventional methods and apparatus, as summarized above, the present invention provides, inter alia, microlithographic-exposure methods and systems that achieve easy exchange of reticles while providing reticle-temperature control as required for rapid reticle exchange.  
           [0009]    According to one aspect of the invention, microlithography systems are provided for transfer-exposing a pattern, defined on a reticle, to a sensitive substrate. The transfer-exposure is performed under a condition including a subatmospheric pressure. An embodiment of such a system comprises a vacuum chamber in which the reticle is placed for exposure, and a reticle-storage chamber in communication with the vacuum chamber. The reticle-storage chamber and vacuum chamber collectively form a subatmospheric-pressure enclosure. The reticle-storage chamber comprises a rack configured to hold multiple reticles at the subatmospheric pressure.  
           [0010]    The system further can comprise a reticle stage located in the vacuum chamber, and a vacuum-side transport robot situated and configured to convey a reticle from the rack to the reticle stage or from the reticle stage to the rack. The multiple reticles can be held in at least one reticle cassette placed on the rack. In such a configuration the reticles in the reticle cassette desirably are accessible by the robot for loading and unloading of reticles directly to and from, respectively, the cassette without having to provide or actuate a gate valve or analogous appliance between the vacuum chamber and the reticle-storage chamber.  
           [0011]    Desirably, the rack is temperature-controlled, preferably in a manner by which the temperature of one or more reticles is changed and/or maintained at a desired value by conduction (for rapid attainment of a desired reticle temperature). To such end, the rack portion contacting a reticle directly or indirectly desirably comprises a hydraulic conduit through which a temperature-controlled fluid is circulated.  
           [0012]    The system further can comprise a load-lock chamber coupled to the reticle-storage chamber and configured for passage of a reticle from a higher-pressure environment to the reticle-storage chamber containing the subatmospheric pressure. This system further can comprise an atmospheric-pressure reticle “library” (assortment of one or more selectable reticles) situated so as to allow transfer of a reticle from the atmospheric-pressure reticle library to the load-lock chamber.  
           [0013]    The system further can comprise an atmosphere-side transport robot situated between the atmospheric-pressure reticle library and the load-lock chamber and configured to convey a reticle between the atmospheric-pressure reticle library and the load-lock chamber. The atmospheric-pressure reticle library can comprise a rack comprising multiple shelves each configured to hold at least one respective reticle cassette.  
           [0014]    The reticles and respective cassettes desirably bear respective identification symbols, wherein the system further comprises an identification-symbol reader situated and configured to read the respective identification symbols of a selected reticle and its respective cassette, and to confirm that the reticle is placed in its correct respective cassette. For example, the identification symbol is a bar code, and the identification-symbol reader is a bar-code reader. This system desirably further comprises an atmosphere-side transport robot situated between the atmospheric-pressure reticle library and the load-lock chamber. The atmosphere-side transport robot is configured to convey a reticle between the atmospheric-pressure reticle library and the load-lock chamber and to return the reticle to its respective cassette as determined by the identification-symbol reader.  
           [0015]    Another system embodiment comprises a vacuum chamber in which the reticle is placed for exposure, and multiple load-lock chambers coupled in parallel with the vacuum chamber. The load-lock chambers selectively are evacuated to the subatmospheric pressure and selectively brought into communication with the vacuum chamber (e.g., by opening a respective gate valve connecting the load-lock chamber to the vacuum chamber). Each load-lock chamber is configured to store at least one respective reticle, wherein the reticles are selectable for use in making a microlithographic exposure. Each load-lock chamber can contain a respective reticle cassette each configured to hold one or more respective reticles from which a desired reticle can be selected.  
           [0016]    Another aspect of the invention is set forth in the context of a microlithographic method in which a pattern, defined on a reticle, is transfer-exposed using an energy beam that passes, in a subatmospheric pressure environment in a vacuum chamber, from the reticle to a sensitive substrate. The aspect is directed to methods for providing and introducing a reticle to the vacuum chamber for use in making a lithographic exposure. An embodiment of the method comprises the step of coupling a reticle-storage chamber directly to the vacuum chamber. (Thus, the vacuum chamber and reticle-storage chamber can be evacuated to the subatmospheric pressure.) The method also comprises the steps of storing multiple reticles, that are selectable for exposure, in the reticle-storage chamber; selecting a reticle in the reticle-storage chamber; and conveying (e.g., using a vacuum-side transport robot) the selected reticle directly from the reticle-storage chamber to the vacuum chamber (e.g., to a reticle stage) for exposure. This conveyance is “direct” because it does not involve providing and/or actuating a gate valve between the reticle-storage chamber and the vacuum chamber. (A gate valve can be situated in this location for maintenance purposes. However, if present in this embodiment, the gate valve normally need not be actuated during transfer of reticles between the reticle-storage chamber and the vacuum chamber.)  
           [0017]    The reticles desirably are stored on a rack inside the reticle-storage chamber. The reticles further desirably are held in at least one reticle cassette placed on the rack. The conveying step can be performed using a vacuum-side transport robot, wherein the reticles in the reticle cassette are accessible by the robot for loading and unloading of reticles to and from, respectively, the cassette.  
           [0018]    The method further can comprise the step of controlling the temperature of the reticles stored in the reticle-storage chamber. In such a method the temperature-controlled reticles desirably are stored on at least one shelf of a rack located inside the reticle-storage chamber, and the temperature of the reticles is controlled by, e.g., circulating a temperature-controlled fluid through a conduit in the shelf. Thus, the temperature of the reticle(s) is controlled by conduction from the shelf to the reticle, which produces the desired reticle temperature by conduction.  
           [0019]    The method further can comprise the step of moving at least one reticle from a higher-pressure environment to the reticle-storage chamber. In such a method a reticle in an atmospheric-pressure reticle library is selected for use and conveyed from the atmospheric-pressure reticle library to the reticle-storage chamber. Selecting the reticle in the atmospheric-pressure reticle library desirably involves reading an identification symbol on the reticle. This reading step can be performed while the reticle is situated in the atmospheric-pressure reticle library, after the reticle has been removed from the atmospheric-pressure reticle library, and/or before the reticle is placed in the atmospheric-pressure reticle library. Furthermore, if the reticles are stored on a rack inside the reticle-storage chamber, the reticles can be held in at least one reticle cassette placed on the rack, and the reading step further can comprise reading an identification symbol (e.g., bar code) associated with the reticle cassette containing the selected reticle.  
           [0020]    In another method embodiment, multiple load-lock chambers are coupled in parallel to the vacuum chamber so as to allow the load-lock chambers selectively to be brought into communication with the vacuum chamber. At least one respective reticle, selectable for exposure, is stored in each of the load-lock chambers. A reticle, located in one of the load-lock chambers, is selected for exposure, and the selected reticle is conveyed to the vacuum chamber for exposure. Each load-lock chamber desirably contains a reticle cassette each configured to hold multiple respective reticles, wherein the conveying step desirably comprises removing the selected reticle from the respective reticle cassette.  
           [0021]    The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1(A) is a schematic elevational view of a first representative embodiment of a microlithography system including an “atmospheric-pressure” reticle library and a “vacuum” reticle library.  
         [0023]    [0023]FIG. 1(B) is an enlarged schematic elevational view of a temperature-controlled rack used in the vacuum reticle library of the system shown in FIG. 1(A).  
         [0024]    [0024]FIG. 2 is a schematic elevational view of a second representative embodiment of a microlithography system including an atmospheric-pressure reticle library and a vacuum reticle library.  
         [0025]    [0025]FIG. 3 is a schematic elevational view of the load-lock chamber of the embodiment of FIG. 2, showing a rack holding multiple reticle cassettes each capable of holding multiple respective reticles.  
         [0026]    [0026]FIG. 4 is a schematic elevational view of a representative embodiment of an electron-beam microlithography system of the step-and-repeat type that can include load-lock chambers and reticle cassettes as described herein.  
         [0027]    [0027]FIG. 5 is a schematic elevational view of a conventional microlithography system including a conventional load-lock chamber. 
     
    
     DETAILED DESCRIPTION  
       [0028]    The invention is described below in the context of representative embodiments that are not intended to be limiting in any way.  
         [0029]    First, the overall structure of an electron-beam microlithography system, as well as the general optical relationships of such a system, are depicted schematically in FIG. 4. The particular type of system depicted in FIG. 4 is a step-and-repeat type as known in the art, and the system is configured for use with load-lock chambers (and other peripherals) as described later below.  
         [0030]    An electron gun  1  is situated at the extreme upstream end of the system and generates an electron beam that propagates in a downstream direction (downward in the figure). The electron beam propagates in a generally axial direction through an illumination-optical system comprising a condenser lens  2  and an illumination lens  3 . The illumination-optical system shapes and directs the beam (termed an “illumination beam” IB) onto a selected region of the reticle  10 . In addition to the lenses  2 ,  3 , the illumination-optical system includes a beam-shaping aperture, a blanking deflector, a blanking aperture, an illumination-beam deflector, etc. (not detailed). The illumination beam IB, formed by the illumination-optical system, is moved in a scanning manner across successive regions of the reticle  10 , thereby illuminating exposure regions (“subfields”) of the reticle  10 , situated in the optical field of the illumination-optical system, in a sequential manner.  
         [0031]    As noted above, the reticle  10  defines a large number of exposure regions termed “subfields” each defining a respective portion of the overall pattern defined on the reticle. For exposure the reticle is mounted on a movable reticle stage  11 . Moving the reticle stage  11  in a plane perpendicular to the optical axis AX allows the subfields of the reticle to be illuminated selectively in a sequential manner. This exposure scheme is termed “divided-reticle” exposure and effectively achieves exposure of an entire pattern that covers a much greater area than the optical field of the illumination-optical system.  
         [0032]    Situated downstream of the reticle  10  is a projection-optical system comprising a first projection lens  15 , a second projection lens  19 , and deflectors  16 - 1  to  16 - 6 , the latter being used for aberration correction and image-position adjustment. The electron beam passing through a selected subfield of the reticle  10  is focused by the projection-optical system at a predetermined position on a sensitive substrate  23  (e.g., semiconductor wafer). So as to be imprinted with the projected image, the substrate  23  is coated with a material termed a “resist” that is sensitive to an exposure dose by the electron beam. Typically, the projection-optical system is a “reducing” system, by which is meant that the image as formed on the substrate  23  is smaller, usually by a factor termed a “demagnification factor” (e.g., ¼), than the corresponding pattern as defined on the reticle  10 .  
         [0033]    The electron beam forms a crossover C.O. at a point having an axial location, between the reticle  10  and substrate  23 , that is determined by the demagnification ratio. A contrast aperture  18  is situated at the crossover. The contrast aperture  18  blocks electrons of the beam that were scattered by passage through non-patterned regions of the reticle  10 , thereby preventing the scattered electrons from reaching the substrate  23 .  
         [0034]    The substrate  23  is mounted via an electrostatic chuck (not shown) to a substrate stage  24  that is movable in the XY direction. Each portion of a device pattern, extending wider than the optical field of the projection-optical system, can be exposed in a sequential manner by synchronously scanning the reticle stage  11  and substrate stage  24  in opposite directions.  
         [0035]    [0035]FIG. 1(A) schematically depicts the structure of a microlithography system according to a first representative embodiment. FIG. 1(B) depicts detail of a region of a rack  66  used for storing (under vacuum) a reticle library for the microlithography system. The system shown in FIG. 1(A) comprises an illumination-optical system (IOS) lens column  51  that contains the electron gun  1 , condenser lens  2 , illumination lens  3 , etc., summarized above with reference to FIG. 4. Downstream of the IOS lens column  51  is a reticle chamber  53  that contains a reticle stage  11 . Downstream of the reticle chamber  53  is a projection-optical system (POS) lens column  55  that contains the first projection lens  15 , second projection lens  19 , deflectors  16 , etc., summarized above with reference to FIG. 4. Downstream of the POS lens column  55  is a wafer chamber  57  that contains a wafer stage  24 . These columns  51 ,  55  and chambers  53 ,  57  are not necessarily separate, individual chambers; one or more of them can be contiguous with each other. The columns  51 ,  55  and chambers  53 ,  57  can be evacuated by one or more vacuum pumps (not shown but well-understood in the art) configured to evacuate respective individual columns and chambers or multiple columns and/or chambers.  
         [0036]    A vacuum-side transport robot  63  is situated in a right-hand (in the figure) extension  53   a  of the reticle chamber  53 . A vacuum reticle library  64  and reticle load-lock chamber  61  are connected to the extension  53   a  to the right (in the figure) of the robot  63 . The vacuum reticle library  64  is effectively a chamber containing an array of reticles  80  stored in a vacuum environment. A first gate valve  62  is interposed between the vacuum reticle library  64  and the reticle load-lock chamber  61 . A second gate valve  71  is provided at the entrance to the reticle load-lock chamber  61 . Inside the vacuum reticle library  64  is a rack  68  having, e.g., multiple shelves (four are shown) that support respective reticles  80  placed on them. In any event, the rack  68  is configured to hold multiple reticles  80 .  
         [0037]    The rack  68  desirably is temperature-controlled, preferably in a manner by which thermal exchange with one or more reticles  80  is by thermal conduction, which is rapid and efficient. To such end, as shown in FIG. 1(B), at least one shelf of the rack  68  includes a respective fluid conduit  68   a  connected to a fluid of which the temperature is controlled. The fluid circulates through the conduits  68   a . An exemplary temperature-control fluid is water. Thus, the vacuum reticle library  64  is configured to regulate the temperature of reticles  80  on the rack  68  by thermal conduction from the fluid to the reticles. The circulating fluid achieves rapid equilibration of the reticles  80  at the desired reticle temperature. In this manner, the temperature of a reticle  80  (that has experienced a temperature change when placed in the vacuum environment and/or that is destined for immediate use in making an exposure) placed on a shelf of the rack  68  can be stabilized quickly to a desired temperature. For example, the temperature of a reticle increases not only from being transported to a vacuum environment but also from being irradiated during use in lithographic exposure. This temperature increase can be predicted, and the temperature of the temperature-control fluid can be set accordingly so as to confer the predicted temperature to the reticles. The temperature of each shelf of the rack  68  need not be identical from shelf to shelf. Rather, the temperature of each shelf can be variable as desired.  
         [0038]    An atmospheric-pressure reticle library  66  is situated to the right (in the figure) of the reticle load-lock chamber  61 , with an atmosphere-side transport robot  65  interposed therebetween. A rack  69  having multiple shelves (four are shown) is provided in the atmospheric-pressure reticle library  66  so as to provide multiple reticles  80  that can be selected quickly for exposure. Each shelf supports a respective reticle cassette  67 . Multiple reticles  80  (which can be the same or different) desirably are stored in each reticle cassette  67  (or each cassette can hold as few as one reticle). To identify the contents of each cassette  67 , a respective bar code (as an exemplary identification symbol)  70  is applied to the cassette  67  and to each reticle  80  situated in the cassette  67 .  
         [0039]    In the context of using a bar code as a representative identification symbol, when transporting a reticle  80  to and from the atmospheric-pressure reticle library  66 , the respective bar code  70  on the reticle  80  and cassette  67  is read by a bar-code reader  59  so as to confirm that the proper reticle from the proper cassette is being transported. Since specific reticles  80  are stored in specific reticle cassettes  67 , each particular reticle desirably is returned to its original reticle cassette after use of the reticle. To ensure no mixups in this regard, the respective bar codes  70  of the reticle  80  and of the reticle cassette  67  are read and confirmed. After making such confirmation is the reticle returned to its cassette. By thus reading the bar code  70  and confirming the identity of the reticle  80 , the reticle is returned to its original reticle cassette without error. The bar codes  70  on the reticle  80  and reticle cassette  67  desirably are read in advance of reticle selection, wherein the read data are stored in a memory and recalled during the confirmation step.  
         [0040]    In other words, reticles desirably remain “paired” with their respective reticle cassettes, wherein, after a reticle is used for exposure it desirably is returned to its original cassette  67  (i.e., the cassette in which the reticle was stored prior to use). Upon reading and confirming the bar codes  70  on reticles  80  and cassettes  67 , the reticle is returned reliably to its proper cassette.  
         [0041]    In FIG. 1(A) the reticle libraries  64 ,  66  are configured desirably for vertical stacking of reticles  80  on a respective stationary rack. Alternatively, it is possible for the reticle libraries to be configured for use with a vertically movable transport robot or such that the reticle libraries  64 ,  66  themselves move vertically.  
         [0042]    Whenever a reticle  80  is being transported between the vacuum reticle library  64  and the atmospheric-pressure reticle library  66 , the operator can provide appropriate commands from a keyboard console or the like (not shown), or may operate appropriate controls (not shown) provided at or near the atmospheric-pressure reticle library  66 . If necessary, the bar code  70  applied to the reticle  80  can be read by a bar-code reader  58 .  
         [0043]    Pre-alignment of the reticle may be necessary until the reticle is transported onto the reticle stage  11 . This pre-alignment may be accomplished while transporting the reticle from the atmospheric-pressure reticle library  66  to the load-lock chamber  61 , before placing the reticle in the vacuum reticle library  64 , or while performing both operations.  
         [0044]    A second representative embodiment is depicted in FIG. 2, and FIG. 3 depicts an example of a load-lock chamber used with this embodiment. The microlithography system shown in FIG. 2 comprises multiple load-lock chambers  61 ′ instead of the vacuum reticle library  64  used in the first representative embodiment, coupled in parallel to the vacuum chamber  53 . Each load-lock chamber  61 ′ contains one or more respective reticles  80 . The operator selects whether the interior of each load-lock chamber  61 ′ is atmospheric pressure or vacuum, or a desired intermediate pressure. Each load-lock chamber  61 ′ can be brought selectively into communication with the vacuum chamber  53  by, e.g., opening a respective gate valve  62 ′. The use of multiple load-lock chambers  61 ′ arranged in this manner facilitates rapid and efficient movement of reticles  80  between the atmosphere side and the vacuum side. As shown in FIG. 3, a respective cassette  67  capable of holding one or more reticles  80  can be situated inside each load-lock chamber  61 ′.  
         [0045]    Reticle cassettes conventionally are made of a rigid plastic resin. If a reticle cassette is moved from an atmospheric-pressure environment to a vacuum environment, the resin tends to outgas, which can degrade the vacuum level. By storing the reticle cassettes  67  in the vacuum environment of the respective load-lock chambers  61 ′, outgassing proceeds and eventually is reduced substantially, thereby avoiding vacuum deterioration. Also, since multiple reticles  80  collectively are stored in the load-lock chambers  61 ′, it is unnecessary to perform outgassing evacuation each time a reticle is exchanged.  
         [0046]    Therefore, according to the embodiments described above, methods and devices are provided that provide easy reticle exchange as well as temperature control of the reticles, if desired.  
         [0047]    Whereas the invention has been described in connection with multiple representative embodiments, it will be understood that the invention is not limited to those embodiments. On the contrary, the invention is intended to encompass all modifications, alternatives, and equivalents as may be included within the spirit and scope of the invention, as defined by the appended claims.