Patent Publication Number: US-2023133576-A1

Title: Systems and methods for uniform cooling of electromagnetic coil

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to the provisional patent application filed Oct. 29, 2021 and assigned U.S. App. No. 63/273,155, the disclosure of which is hereby incorporated by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure relates to electron beam systems and, more particularly, to cooling systems for electromagnetic coils. 
     BACKGROUND OF THE DISCLOSURE 
     Evolution of the semiconductor manufacturing industry is placing greater demands on yield management and, in particular, on metrology and inspection systems. Critical dimensions continue to shrink, yet the industry needs to decrease time for achieving high-yield, high-value production. Minimizing the total time from detecting a yield problem to fixing it determines the return-on-investment for a semiconductor manufacturer. 
     Fabricating semiconductor devices, such as logic and memory devices, typically includes processing a semiconductor wafer or an EUV mask using a large number of fabrication processes to form various features and multiple levels of the semiconductor devices. For example, lithography is a semiconductor fabrication process that involves transferring a pattern from a reticle to a photoresist arranged on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing (CMP), etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated in an arrangement on a single semiconductor wafer that are separated into individual semiconductor devices. 
     Inspection processes are used at various steps during semiconductor manufacturing to detect defects on wafers to promote higher yield in the manufacturing process and, thus, higher profits. Inspection has always been an important part of fabricating semiconductor devices such as integrated circuits (ICs). However, as the dimensions of semiconductor devices decrease, inspection becomes even more important to the successful manufacture of acceptable semiconductor devices because smaller defects can cause the devices to fail. For instance, as the dimensions of semiconductor devices decrease, detection of defects of decreasing size has become necessary because even relatively small defects may cause unwanted aberrations in the semiconductor devices. 
     In electron beam systems, one or more elements may be placed in the electron beam path to focus the beam toward a target. For example, the beam may pass through an electromagnetic coil. When the coil is powered on, a resulting magnetic field in the aperture of the coil focuses passing electrons into a narrow beam. However, powering the coil also generates heat in coil. When heat is transferred to the housing surrounding the coil and to other optical components in the system, it can change of alignment and symmetry due to thermal expansion of the components. This can result in loss of calibration, image distortion, and drift. 
     Existing methods seek to reduce heat transfer from the coil to other components by cooling the coil. For example, a copper plate brazed with copper tubes may be bonded to the coil, and a cooling fluid may be pumped through the tubes to cool the copper plate and thereby cool the coil. However, these methods result in non-uniform cooling of the coil because: (1) the copper plate is only disposed on one side of the coil (i.e., the side of the coil closest to the copper plate is cooler than the side farthest from the copper plate); and (2) the temperature of the cooling fluid at the outlet is greater than the temperature at the inlet (i.e., the portion of the coil near the inlet is cooler than the portion near the outlet). Non-uniform cooling may be less effective at reducing heat transfer to the housing and mitigating adverse effects on the system. 
     Therefore, what is needed is an improved method of cooling an electromagnetic coil. 
     BRIEF SUMMARY OF THE DISCLOSURE 
     An embodiment of the present disclosure provides a system. The system may comprise an electromagnetic coil, a cooling structure, and a cooling fluid source. The cooling structure may surround the entirety of the perimeter of the electromagnetic coil. The cooling structure may comprise a first cooling channel and a second cooling channel arranged alternately about the electromagnetic coil. The cooling fluid source may be configured to deliver a first cooling fluid to the first cooling channel. The cooling fluid source may be configured to deliver a second cooling fluid to the second cooling channel. The first cooling fluid and the second cooling fluid may cool the electromagnetic coil. 
     According to an embodiment of the present disclosure, the system may further comprise an electron beam source. The electron beam source may be configured to generate an electron beam. The electron beam may be directed through an aperture of the electromagnetic coil toward a target. The system may further comprise a voltage source. The voltage source may be configured to power the electromagnetic coil, which may generate an electromagnetic field in the aperture and may focus the electron beam passing through the aperture. 
     According to an embodiment of the present disclosure, the first cooling channel and the second cooling channel may be defined by separate tubing circuits wrapped around the entirety of the perimeter of the electromagnetic coil. 
     According to an embodiment of the present disclosure, the first cooling channel and the second cooling channel may be defined by separate channel circuits in an integrated solid structure surrounding the entirety of the perimeter of the electromagnetic coil. 
     According to an embodiment of the present disclosure, the electromagnetic coil may have a ring shape comprising a top surface, a bottom surface, an inner surface, and an outer surface. The cooling structure may be disposed on each of the top surface, the bottom surface, the inner surface, and the outer surface. 
     According to an embodiment of the present disclosure, the first cooling channel and the second cooling channel may be arranged alternately about each surface of the electromagnetic coil in a single layer. 
     According to an embodiment of the present disclosure, the cooling structure may further comprise a first inlet and a first outlet in fluid communication with the first cooling channel. The cooling structure may further comprise a second inlet and a second outlet in fluid communication with the second cooling channel. The first inlet may be disposed adjacent to the second outlet. The second inlet may be disposed adjacent to the first outlet. 
     According to an embodiment of the present disclosure, the first cooling fluid may travel through the first cooling channel in a first direction and the second cooling fluid may travel through the second cooling channel in a second direction. The second direction may be opposite to the first direction. 
     According to an embodiment of the present disclosure, the cooling fluid source may comprise water. 
     According to an embodiment of the present disclosure, the first cooling channel and the second cooling channel may have the same cross-sectional area and effective length. 
     According to an embodiment of the present disclosure, the cooling structure may encase the electromagnetic coil. 
     According to an embodiment of the present disclosure, the electromagnetic coil and the cooling structure may be disposed within a housing. 
     An embodiment of the present disclosure provides a method. The method may comprise powering, via a voltage source, an electromagnetic coil. The method may further comprise delivering, via a cooling fluid source, a first cooling fluid and a second cooling fluid to a cooling structure surrounding the entirety of the perimeter of the electromagnetic coil. The cooling structure may comprise a first cooling channel and a second cooling channel arranged alternately about the electromagnetic coil. The first cooling fluid may be delivered to the first cooling channel. The second cooling fluid may be delivered to the second cooling channel. The first cooling fluid and the second cooling fluid may uniformly cool the electromagnetic coil. 
     According to an embodiment of the present disclosure, the method may further comprise generating, via an electron beam source, an electron beam. The method may further comprise directing the electron beam through an aperture of the electromagnetic coil toward a target. An electromagnetic field generated by the electromagnetic coil may focus the electron beam passing through the aperture. 
     According to an embodiment of the present disclosure, the method may further comprise delivering, via a first inlet, the first cooling fluid to the first cooling channel. The first cooling fluid may exist the first cooling channel at a first outlet. The method may further comprise delivering, via a second inlet, the second cooling fluid to the second cooling channel. The second cooling fluid may exit the second cooling channel at a second outlet. The first inlet may be disposed in the cooling structure adjacent to the second outlet, and the second inlet may be disposed in the cooling structure adjacent to the first outlet. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of the nature and objects of the disclosure, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG.  1 A  is a cross-sectional block diagram of an apparatus according to an embodiment of the present disclosure; 
         FIG.  1 B  is a top view of a coil assembly according to an embodiment of the present disclosure; 
         FIG.  2    is a perspective view of a cooling structure according to an embodiment of the present disclosure; 
         FIG.  3 A  is a cross-sectional view of a cooling structure according to an embodiment of the present disclosure; 
         FIG.  3 B  is a cross-sectional view of a cooling structure according to another embodiment of the present disclosure; 
         FIG.  4 A  a top view of a cooling structure according to an embodiment of the present disclosure; 
         FIG.  4 B  is a side view of a cooling structure according to an embodiment of the present disclosure; 
         FIG.  5 A  is a flow chart of a method according to an embodiment of the present disclosure; 
         FIG.  5 B  is a flow chart of a method according to another embodiment of the present disclosure; and 
         FIG.  6    is a block diagram of a system according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE DISCLOSURE 
     Although claimed subject matter will be described in terms of certain embodiments, other embodiments, including embodiments that do not provide all of the benefits and features set forth herein, are also within the scope of this disclosure. Various structural, logical, process step, and electronic changes may be made without departing from the scope of the disclosure. Accordingly, the scope of the disclosure is defined only by reference to the appended claims. 
     As shown in  FIG.  1 A , an embodiment of the present disclosure provides an apparatus  100 . The apparatus  100  may comprise an electromagnetic coil  110 . The electromagnetic coil  110  may have a ring shape (shown in  FIG.  1 B ), defining an aperture  111 . When connected to a voltage source  150 , an electromagnetic field may be generated in the aperture  111 . The strength of the electromagnetic field may be controlled by the voltage source  150  by adjusting the voltage or current applied to the electromagnetic coil  110 . For example, the voltage source  150  may supply a power of 120 to 300 watts to the electromagnetic coil  110 . In a particular example, the power may be 160 watts. When connected to a voltage source  150 , the electromagnetic coil may  110  also generate heat. For example, the temperature of the electromagnetic coil  110  may be up to 110° C. In some examples, the temperature of the electromagnetic coil  110  may be up to 300° C., depending on the type of wire used. 
     The apparatus  100  may further comprise an electron beam source  140 . The electron beam source  140  configured to generate an electron beam  141 . The electron beam  141  may be directed through the aperture  111  of the electromagnetic coil  110  toward a target  142 . The target  142  may be disposed on a stage  143 . When the electron beam  141  passes through the aperture  111 , the electromagnetic field generated by the electromagnetic coil  110  may focus the electron beam  141 . When performing inspection processes, the electron beam source  140  and the voltage source  150  may cooperate to power the electromagnetic coil  110  and improve accuracy of the measurements of the target  142  by focusing the beam  141 . 
     The apparatus  100  may further comprise a cooling structure  120 . The cooling structure  120  may surround the entirety of the perimeter of the electromagnetic coil  110 . The cooling structure  120  may include a first cooling channel  121  and a second cooling channel  122 . 
     The apparatus  100  may further comprise a cooling fluid source  130 . The cooling fluid source  130  may be configured to deliver a first cooling fluid  131  to the first cooling channel  121 . The cooling fluid source  130  may also be configured to deliver a second cooling fluid  132  to the second cooling channel  122 . In an instance, the first cooling fluid  131  and the second cooling fluid  132  may be the same. By delivering the first cooling fluid  131  and the second cooling fluid  132  to the cooling structure  120 , the electromagnetic coil  110  may be cooled to prevent heat transfer to other components. The cooling fluid source  130  may comprise water, mixtures of water with other liquids (e.g., ethylene glycol) a fluorinated liquid (e.g. FLUORINERT™ FC-77), or other refrigerants. For example, the cooling fluid source  130  may comprise water at a temperature of 15 to 25° C. If the temperature of the cooling fluid is too high (e.g., greater than 25° C.), the temperature of the electromagnetic coil  110  may exceed the maximum temperature of the wire, causing damage to the electromagnetic coil  110 . If the temperature of the cooling fluid is too low (e.g., less than 15° C.), the coolant plumbing connected to the cooling fluid source  130  may cause water vapor in the work environment to condense. Accumulation of condensate may cause electrical shorts in parts of the tool. The cooling fluid source  130  may deliver the first cooling fluid  131  and the second cooling fluid  132  at a rate of 0.3 to 1.0 L/min. For example, the rate may be 0.5 L/min. The rate of delivery may be controlled by a controllable fluid pump. The rate may be dependent on the size of the tubing and the required heat dissipation. When the rate is too high (e.g., greater than 1.0 L/min), turbulent flow may occur, which may cause vibrations in the tool, thereby degrading images produced by the tool. In an instance, the cooling fluid source  130  may include a single reservoir configured to deliver the first cooling fluid  131  and the second cooling fluid  132  to the cooling structure  120 . In another instance, the cooling fluid source  130  may include separate reservoirs configured to deliver the first cooling fluid  131  and the second cooling fluid  132  to the cooling structure  120 . 
     As shown in  FIG.  2   , the cooling structure  120  may encase the electromagnetic coil  110 . For example, the cooling structure  120  may cover all surfaces of the electromagnetic coil  110 . Compared to existing designs which only cool one surface of the electromagnetic coil  110 , the cooling structure  120  may provide more uniform cooling to the electromagnetic coil  110 . 
     In an embodiment shown in  FIG.  3 A , the cooling structure  120  may include separate tubing circuits which define the first cooling channel  121  and the second cooling channel  122 . The tubing circuits may be a flexible material, such as plastic, which are wrapped around the entirety of the perimeter of the electromagnetic coil  110 . Flexible tubing circuits may be able to thermally expand with less stress compared to other materials. Plastic or other insulating materials may provide better isolation of heat generated by the electromagnetic coil  110 . The tubing circuits may also be metal or other materials. 
     In an embodiment shown in  FIG.  3 B , the cooling structure  120  may include an integrated solid structure  123 , in which the first cooling channel  121  and the second cooling channel  122  are defined. The integrated solid structure  123  may be a 3D-printed structure which surrounds the entirety of the perimeter of the electromagnetic coil  110 . The integrated solid structure  123  may be metal, plastic, or other materials. The integrated solid structure  123  may be fabricated in two or more parts which, when assembled, surround the electromagnetic coil  110 . 
     As shown in  FIGS.  3 A and  3 B , the electromagnetic coil  110  may include a top surface  112 , a bottom surface  113 , an inner surface  114 , and an outer surface  115 . The cooling structure  120  may be disposed on each side of the electromagnetic coil  110 . For example, the first cooling channel  121  and the second cooling channel  122  may be disposed on each of the top surface  112 , the bottom surface  113 , the inner surface  114 , and the outer surface  115  of the electromagnetic coil  110 . The inner surface  114  may be more proximate the electron beam  141  in  FIG.  1    than the outer surface  115 . In some embodiments, the cooling structure  120  may be disposed on only some sides of the electromagnetic coil  110 . For example, the first cooling channel  121  and the second cooling channel  122  may be disposed on the top surface  112 , the bottom surface  113 , and the outer surface  115  of the electromagnetic coil  110 . 
     The first cooling channel  121  and the second cooling channel  122  may define a single layer. In some embodiments, the first cooling channel  121  and the second cooling channel  122  may define multiple layers. The first cooling channel  121  and the second cooling channel  122  may be arranged alternately. For example, each portion of the first cooling channel  121  may be disposed between two portions of the second cooling channel  122 . In this way, the first cooling channel  121  and the second cooling channel  122  may be arranged side-by-side as they are wrapped around the entirety of the perimeter of the electromagnetic coil  110 . 
     Referring to  FIG.  2   , the cooling structure  120  may further comprise a first inlet  121   a  and a first outlet  121   b  in fluid communication with the first cooling channel  121 . The first cooling fluid  131  may be delivered to the first cooling channel  121  via the first inlet  121   a.  The first cooling fluid  131  may exit the first cooling channel  121  via the first outlet  121   b.  The travel distance of the first cooling fluid  131  in the first cooling channel  121  from the first inlet  121   a  to the first outlet  121   b  may define an effective length of the first cooling channel  121 . 
     The cooling structure  120  may further comprise a second inlet  122   a  and a second outlet  122   b  in fluid communication with the second cooling channel  122 . The second cooling fluid  132  may be delivered to the second cooling channel  122  via the second inlet  122   a.  The second cooling fluid  132  may exit the second cooling channel  122  via the second outlet  122   b.  The travel distance of the second cooling fluid  132  in the second cooling channel  122  from the second inlet  122   a  to the second outlet  122   b  may define an effective length of the second cooling channel  122 . 
     According to an embodiment of the present disclosure, the effective length of the first cooling channel  121  and the effective length of the second cooling channel  122  may be the same. The cross-sectional area of the first cooling channel  121  and the cross-sectional area of the second cooling channel  122  may be the same. For example, the first cooling channel  121  and the second cooling channel  122  may have an inner diameter of 2.5 to 5 mm. In a particular example, the inner diameter may be 3.0 mm. The diameter may depend on the available space for the electromagnetic coil  110  and the heat that needs to be dissipated. With the first cooling channel  121  and the second cooling channel  122  having the same cross-sectional area and effective length, cooling of the electromagnetic coil  110  with the first cooling fluid  131  and the second cooling fluid  132  may be similar, thereby more uniformly cooling the electromagnetic coil  110  compared to existing designs. 
     The first inlet  121   a  may be disposed adjacent to the second outlet  122   b,  and the second inlet  122   a  may be disposed adjacent to the first outlet  121   b.  For example, as shown in  FIG.  4 A , the first inlet  121   a,  the first outlet  121   b,  the second inlet  122   a,  and the second outlet  122   b  may be arranged in a cluster in the cooling structure  120 . The cluster may be arranged on any side of the electromagnetic coil  110 . For example, the cluster may be arranged on the top surface  112 , the bottom surface  113 , the inner surface  114 , or the outer surface  115  of the electromagnetic coil  110 . It can be understood that the temperature of the first cooling fluid  131  may increase as it travels through the first cooling channel  121 , and the temperature of the second cooling fluid  132  may increase as it travels through the second cooling channel  122  as heat is transferred from the electromagnetic coil. Thus, by arranging the first inlet  121   a  adjacent to the second outlet  122   b  and the second inlet  122   a  adjacent to the first outlet  121   b,  the cooling structure  120  may more uniformly cool the electromagnetic coil  110  compared to existing designs. 
     The first cooling fluid  131  may travel through the first cooling channel  121  in a first direction. The second cooling fluid  132  may travel through the second cooling channel  122  in a second direction. As shown in  FIGS.  4 A and  4 B , the first direction may be opposite to the second direction. The cooling fluid source  130  may deliver the first cooling fluid  131  and the second cooling fluid  132  at similar or different rates. By delivering the first cooling fluid  131  and the second cooling fluid  132  in opposite directions, the cooling structure  120  may more uniformly cool the electromagnetic coil  110  compared to existing designs. 
     The apparatus  100  may further comprise a housing  160 . The electromagnetic coil  110  and the cooling structure  120  may be disposed within the housing  160  to define a coil assembly  101 , shown in cross-sectional view in  FIG.  1 A  and in top view in  FIG.  1 B . The coil assembly  101  may be used as a gun lens, an objective lens, or another optical component in an electron beam system. A thermally-conductive potting compound may be filled in the coil assembly  101  between the electromagnetic coil  110  and the cooling structure  120 . The thermally-conductive potting compound may facilitate heat transfer from the electromagnetic coil  110  to the cooling structure  120 . In some implementations, a thermally-insulating potting compound may be filled in the coil assembly  101  between the cooling structure  120  and the housing  160 . The thermally-insulating potting compound may reduce heat transfer between the cooling structure  120  and the housing  160 . Using these two types of potting compound may be beneficial for applications that need to minimize heat transfer to the housing  160 . With the coil assembly  101 , the cooling structure  120  may uniformly cool the electromagnetic coil  110  and may insulate the housing  160  from heat generated by the coil  110 . For example, the temperature of the housing  160  may vary by 3° C. or less when the electromagnetic coil  110  is powered on using the cooling structure  120 . In an instance, the temperature of the housing  160  may vary by 0.2° C. or less when the electromagnetic coil  110  is powered on using the cooling structure  120 . For some temperature-sensitive applications, the housing temperature variation may be ±0.1° C. For less demanding applications, the housing temperature variation may be ±1.0° C. Thus, the coil assembly  101  may reduce heat transfer to other components of the apparatus  100 , resulting in improved alignment and symmetry, which may require less frequent calibration and may prevent image distortion and drift. 
     With the apparatus  100 , more uniform cooling of the electromagnetic coil  110  may be achieved compared to existing designs due to the delivery of the first cooling fluid  131  and the second cooling fluid  132  to the cooling structure  120  surrounding the entirety of the perimeter of the electromagnetic coil  110 . 
     Another embodiment of the present disclosure provides a method  200 . The method may be performed using the apparatus  100 . As shown in  FIG.  5 A , the method may comprise the following steps. 
     At step  210 , an electron beam is generated. The electron beam may be generated by an electron beam source, such as a cathode source or an emitter tip. 
     At step  220 , an electromagnetic coil is powered. The electromagnetic coil may have a ring shape, defining an aperture. The electromagnetic coil may include a top surface, a bottom surface, an inner surface, and an outer surface. The electromagnetic coil may be powered by a voltage source. Powering the electromagnetic coil may generate an electromagnetic field in the aperture of the electromagnetic coil. Powering the electromagnetic coil may also generate heat. When performing inspection processes, the powered supplied by the voltage source may differ depending on the application and system parameters. 
     At step  230 , the electron beam is directed through the aperture of the electromagnetic coil toward a target. The target may be disposed on a stage. The electromagnetic field in the aperture of the electromagnetic coil may focus the electron beam toward the target. Focusing the electron beam toward the target may improve accuracy of measurements. 
     At step  240 , a first cooling fluid and a second cooling fluid are delivered to a cooling structure surrounding the entirety of the perimeter of the electromagnetic coil. The first cooling fluid and the second cooling fluid may be delivered by a cooling fluid source. The first cooling fluid and the second cooling fluid may be water. The cooling structure may include a first cooling channel and a second cooling channel. By delivering the first cooling fluid and the second cooling fluid to the cooling structure, the electromagnetic coil may be cooled to prevent heat transfer to other components. 
     The cooling structure may encase the electromagnetic coil. For example, the cooling structure may cover all surfaces of the electromagnetic coil, including the top surface, the bottom surface, the inner surface, and the outer surface of the electromagnetic coil. Compared to existing designs which only cool one surface of the electromagnetic coil, the cooling structure may provide more uniform cooling to the electromagnetic coil. 
     In an instance, the cooling structure may include separate tubing circuits which define the first cooling channel and the second cooling channel. The tubing circuits may be a flexible material, such as plastic, which are wrapped around the entirety of the perimeter of the electromagnetic coil. Flexible tubing circuits may be able to thermally expand with less stress compared to other materials. Plastic or other insulating materials may provide better isolation of heat generated by the electromagnetic coil. The tubing circuits may also be metal or other materials. 
     In an instance, the cooling structure may include an integrated solid structure, in which the first cooling channel and the second cooling channel are defined. The integrated solid structure may be a 3D-printed structure which surrounds the entirety of the perimeter of the electromagnetic coil. The integrated solid structure may be metal, plastic, or other materials. The integrated solid structure may be fabricated in two or more parts which, when assembled, surround the electromagnetic coil. 
     The first cooling channel and the second cooling channel may define a single layer. 
     In some embodiments, the first cooling channel and the second cooling channel may define multiple layers. The first cooling channel and the second cooling channel may be arranged alternately. For example, each portion of the first cooling channel may be disposed between two portions of the second cooling channel. In this way, the first cooling channel and the second cooling channel may be arranged side-by-side as they are wrapped around the entirety of the perimeter of the electromagnetic coil. 
     Referring to  FIG.  5 B , step  240  make comprise the following steps (performed in any order or simultaneously). 
     At step  241 , the first cooling fluid is delivered via a first inlet to the first cooling channel of the cooling structure. The first cooling fluid may exit the first cooling channel via a first outlet. The travel distance of the first cooling fluid in the first cooling channel from the first inlet to the first outlet may define an effective length of the first cooling channel. 
     At step  242 , the second cooling fluid is delivered via a second inlet to the second cooling channel of the cooling structure. The second cooling fluid may exit the second cooling channel via a second outlet. The travel distance of the second cooling fluid in the second cooling channel from the second inlet to the second outlet may define an effective length of the second cooling channel. 
     The effective length of the first cooling channel and the effective length of the second cooling channel may be the same. The cross-sectional area of the first cooling channel and the cross-sectional area of the second cooling channel may be the same. With the first cooling channel and the second cooling channel having the same cross-sectional area and effective length, cooling of the electromagnetic coil with the first cooling fluid and the second cooling fluid may be similar, thereby more uniformly cooling the electromagnetic coil compared to existing designs. 
     The first inlet may be disposed adjacent to the second outlet, and the second inlet may be disposed adjacent to the first outlet. For example, the first inlet, the first outlet, the second inlet, and the second outlet may be arranged in a cluster in the cooling structure. The cluster may be arranged on any side of the electromagnetic coil. For example, the cluster may be arranged on the top surface, the bottom surface, the inner surface, or the outer surface of the electromagnetic coil. It can be understood that the temperature of the first cooling fluid may increase as it travels through the first cooling channel, and the temperature of the second cooling fluid may increase as it travels through the second cooling channel. Thus, by arranging the first inlet adjacent to the second outlet and the second inlet adjacent to the first outlet, the cooling structure may more uniformly cool the electromagnetic coil compared to existing designs. 
     The first cooling fluid may travel through the first cooling channel in a first direction. The second cooling fluid may travel through the second cooling channel in a second direction. The first direction may be opposite to the second direction. The cooling fluid source may deliver the first cooling fluid and the second cooling fluid at similar or different rates. By delivering the first cooling fluid and the second cooling fluid in opposite directions, the cooling structure may more uniformly cool the electromagnetic coil compared to existing designs. 
     With the method  200 , more uniform cooling of the electromagnetic coil may be achieved compared to existing designs due to the delivery of the first cooling fluid and the second cooling fluid to the cooling structure surrounding the entirety of the perimeter of the electromagnetic coil. 
     Another embodiment of the present disclosure provides a system  300 . As shown in  FIG.  6   , the system  300  includes a wafer inspection tool (which includes the electron column  301 ) configured to generate images of a wafer  304 . 
     The wafer inspection tool includes an output acquisition subsystem that includes at least an energy source and a detector. The output acquisition subsystem may be an electron beam-based output acquisition subsystem. For example, in one embodiment, the energy directed to the wafer  304  includes electrons, and the energy detected from the wafer  304  includes electrons. In this manner, the energy source may be an electron beam source. In one such embodiment shown in FIG. 
       6 , the output acquisition subsystem includes electron column  301 , which is coupled to computer subsystem  302 . A stage  310  may hold the wafer  304 . 
     As also shown in  FIG.  6   , the electron column  301  includes an electron beam source  303  configured to generate electrons that are focused to wafer  304  by one or more elements  305 . The electron beam source  303  may include, for example, a cathode source or emitter tip. The one or more elements  305  may include, for example, a gun lens, an anode, a beam limiting aperture, a gate valve, a beam current selection aperture, an objective lens, and a scanning subsystem, all of which may include any such suitable elements known in the art. In particular, the one or more elements  305  may include the electromagnetic coil  110  and the cooling structure  120 . 
     Electrons returned from the wafer  304  (e.g., secondary electrons) may be focused by one or more elements  306  to detector  307 . One or more elements  306  may include, for example, a scanning subsystem, which may be the same scanning subsystem included in element(s)  305 . 
     The electron column  301  also may include any other suitable elements known in the art. 
     Although the electron column  301  is shown in  FIG.  6    as being configured such that the electrons are directed to the wafer  304  at an oblique angle of incidence and are scattered from the wafer  304  at another oblique angle, the electron beam may be directed to and scattered from the wafer  304  at any suitable angles. In addition, the electron beam-based output acquisition subsystem may be configured to use multiple modes to generate images of the wafer  304  (e.g., with different illumination angles, collection angles, etc.). The multiple modes of the electron beam-based output acquisition subsystem may be different in any image generation parameters of the output acquisition subsystem. 
     Computer subsystem  302  may be coupled to detector  307  as described above. The detector  307  may detect electrons returned from the surface of the wafer  304  thereby forming electron beam images of the wafer  304 . The electron beam images may include any suitable electron beam images. Computer subsystem  302  may be configured to perform any of the functions described herein using the output of the detector  307  and/or the electron beam images. Computer subsystem  302  may be configured to perform any additional step(s) described herein. A system  300  that includes the output acquisition subsystem shown in  FIG.  6    may be further configured as described herein. 
     It is noted that  FIG.  6    is provided herein to generally illustrate a configuration of an electron beam-based output acquisition subsystem that may be used in the embodiments described herein. The electron beam-based output acquisition subsystem configuration described herein may be altered to optimize the performance of the output acquisition subsystem as is normally performed when designing a commercial output acquisition system. In addition, the systems described herein may be implemented using an existing system (e.g., by adding functionality described herein to an existing system). For some such systems, the methods described herein may be provided as optional functionality of the system (e.g., in addition to other functionality of the system). Alternatively, the system described herein may be designed as a completely new system. 
     Although the system  300  is described above as being an electron beam system, embodiments disclosed herein also can be used in an ion beam system. Such system may be configured as shown in  FIG.  6    except that the electron beam source may be replaced with any suitable ion beam source known in the art. In addition, the embodiments disclosed herein may be any other suitable ion beam-based systems such as those included in commercially available focused ion beam (FIB) systems, helium ion microscopy (HIM) systems, and secondary ion mass spectroscopy (SIMS) systems. 
     The computer subsystem  302  includes a processor  308  and an electronic data storage unit  309 . The processor  308  may include a microprocessor, a microcontroller, or other devices. 
     The computer subsystem  302  may be coupled to the components of the system  300  in any suitable manner (e.g., via one or more transmission media, which may include wired and/or wireless transmission media) such that the processor  308  can receive output. The processor  308  may be configured to perform a number of functions using the output. The wafer inspection tool can receive instructions or other information from the processor  308 . The processor  308  and/or the electronic data storage unit  309  optionally may be in electronic communication with another wafer inspection tool, a wafer metrology tool, or a wafer review tool (not illustrated) to receive additional information or send instructions. 
     The processor  308  is in electronic communication with the wafer inspection tool, such as the detector  307 . The processor  308  may be configured to process images generated using measurements from the detector  307 . For example, the processor may perform embodiments of the method  200 . 
     The computer subsystem  302 , other system(s), or other subsystem(s) described herein may be part of various systems, including a personal computer system, image computer, mainframe computer system, workstation, network appliance, interne appliance, or other device. The subsystem(s) or system(s) may also include any suitable processor known in the art, such as a parallel processor. In addition, the subsystem(s) or system(s) may include a platform with high-speed processing and software, either as a standalone or a networked tool. 
     The processor  308  and electronic data storage unit  309  may be disposed in or otherwise part of the system  300  or another device. In an example, the processor  308  and electronic data storage unit  309  may be part of a standalone control unit or in a centralized quality control unit. Multiple processors  308  or electronic data storage units  309  may be used. 
     The processor  308  may be implemented in practice by any combination of hardware, software, and firmware. Also, its functions as described herein may be performed by one unit, or divided up among different components, each of which may be implemented in turn by any combination of hardware, software and firmware. Program code or instructions for the processor  308  to implement various methods and functions may be stored in readable storage media, such as a memory in the electronic data storage unit  309  or other memory. 
     If the system  300  includes more than one computer subsystem  302 , then the different subsystems may be coupled to each other such that images, data, information, instructions, etc. can be sent between the subsystems. For example, one subsystem may be coupled to additional subsystem(s) by any suitable transmission media, which may include any suitable wired and/or wireless transmission media known in the art. Two or more of such subsystems may also be effectively coupled by a shared computer-readable storage medium (not shown). 
     The processor  308  may be configured to perform a number of functions using the output of the system  300  or other output. For instance, the processor  308  may be configured to send the output to an electronic data storage unit  309  or another storage medium. The processor  308  may be further configured as described herein. For example, the processor  308  can be used to control pumps for the fluid flow in the cooling structure  120 . 
     The processor  308  may be communicatively coupled to any of the various components or sub-systems of system  300  in any manner known in the art. Moreover, the processor  308  may be configured to receive and/or acquire data or information from other systems (e.g., inspection results from an inspection system such as a review tool, a remote database including design data and the like) by a transmission medium that may include wired and/or wireless portions. In this manner, the transmission medium may serve as a data link between the processor  308  and other subsystems of the system  300  or systems external to system  300 . 
     Various steps, functions, and/or operations of system  300  and the methods disclosed herein are carried out by one or more of the following: electronic circuits, logic gates, multiplexers, programmable logic devices, ASICs, analog or digital controls/switches, microcontrollers, or computing systems. Program instructions implementing methods such as those described herein may be transmitted over or stored on carrier medium. The carrier medium may include a storage medium such as a read-only memory, a random access memory, a magnetic or optical disk, a non-volatile memory, a solid state memory, a magnetic tape, and the like. A carrier medium may include a transmission medium such as a wire, cable, or wireless transmission link. For instance, the various steps described throughout the present disclosure may be carried out by a single processor  308  (or computer subsystem  302 ) or, alternatively, multiple processors  308  (or multiple computer subsystems  302 ). Moreover, different sub-systems of the system  300  may include one or more computing or logic systems. Therefore, the above description should not be interpreted as a limitation on the present disclosure but merely an illustration. 
     Although the present disclosure has been described with respect to one or more particular embodiments, it will be understood that other embodiments of the present disclosure may be made without departing from the scope of the present disclosure. Hence, the present disclosure is deemed limited only by the appended claims and the reasonable interpretation thereof.