Patent Publication Number: US-2023161273-A1

Title: Lithography system and operation method thereof

Description:
PRIORITY CLAIM AND CROSS-REFERENCE 
     This application is a Continuation application of U.S. application Ser. No. 16/410,426, filed on May 13, 2019, now U.S. Pat. No. 11,550,233, issued on Jan. 10, 2023, which claims priority to U.S. Provisional Application Ser. No. 62/718,936, filed on Aug. 14, 2018, which are herein incorporated by references. 
    
    
     BACKGROUND 
     Manufacturing of an integrated circuit (IC) has been driven by increasing the density of the IC formed in a semiconductor device. This is accomplished by implementing more aggressive design rules to allow a larger density of the IC device to be formed. Nonetheless, the increased density of IC devices, such as transistors, has also increased the complexity of processing semiconductor devices with decreased feature sizes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG.  1    illustrates a lithography system according to some embodiments of the present disclosure. 
         FIG.  2    is a flow chart of a process according to some embodiments of the present disclosure. 
         FIG.  3    illustrates the lithography system of  FIG.  1    with which the process of  FIG.  2    is implemented according to some embodiments of the present disclosure. 
         FIG.  4    illustrates a drawing of partial enlargement of the lithography system of  FIG.  3   . 
         FIG.  5    illustrates the lithography system of  FIG.  1    with which the process of  FIG.  2    is implemented according to some embodiments of the present disclosure. 
         FIG.  6    illustrates a lithography system according to some embodiments of the present disclosure. 
         FIG.  7    illustrates a drawing of partial enlargement of the lithography system during operation. 
         FIG.  8    illustrates a drawing of partial enlargement of a lithography system during operation according to some embodiments of the present disclosure. 
         FIG.  9    illustrates a drawing of partial enlargement of a lithography system during operation according to some embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Furthermore, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     The advanced lithography process, method, and materials described in the current disclosure can be used in many applications, including fin-type field effect transistors (FinFETs). For example, the fins may be patterned to produce a relatively close spacing between features, for which the above disclosure is well suited. In addition, spacers used in forming fins of FinFETs can be processed according to the above disclosure. 
       FIG.  1    illustrates a lithography system  100 A according to some embodiments of the present disclosure. The lithography system  100 A includes a chamber  102 , a collector  110 , a laser generator  120 , a droplet generator  130 , a droplet catcher  135 , an inlet port  140 , an outlet port  142 , optical reflectors  152 ,  154 ,  156 ,  158 ,  160 ,  162 ,  164 ,  166 ,  168 , a reticle  170 , a droplet deflector  180 , and controllers  190 ,  191  and  192 . The lithography system  100 A is an extreme ultraviolet (EUV) exposure tool that can perform an exposure operation for exposing a photoresist layer  302  coated on a wafer  300  within the chamber  102 . For example, the lithography system  100 A may include a stepper  104  disposed within the chamber  102 , and the wafer  300  on which the photoresist layer  302  is coated is mounted on the stepper  104 . The stepper  104  is movable in the chamber  102  and is configured to shift the wafer  300 , such that the wafer  300  can be shifted at a suitable position for the exposing. 
     The collector  110  is disposed within the chamber  102 . In some embodiments, the collector  110  is mounted to a support (not shown in  FIG.  1   ) that is a part of the lithography system  100 A. The collector  110  has a concave mirror surface  112 . The concave mirror surface  112  of collector  110  may have a focal point  114  and an axis of symmetry  116  which can serve as an optical axis of the collector  110 . In greater detail, the axis of symmetry  116  of the collector  110  connects a center  118  of the mirror surface  112  and the focal point  114 . 
     In some embodiments, the mirror surface  112  of the collector  110  can also be a multilayer reflector of any suitable structure and composition. The mirror surface  112  can include a distributed Bragg reflector formed from alternating layers of a high index of refraction material and a low index of refraction material. For example, the alternating layers can be Mo and Si or Mo and Be. In some embodiments, the mirror surface  112  includes more than 20 pairs of alternating layers. In some embodiments, the mirror surface  112  obtains a reflectivity greater than 60%. In some embodiments, the uppermost layer of the mirror surface  112  can be protected from oxidation by a capping layer, such as a layer of Ru. In some embodiments, the mirror surface  112  has an opening  113  through the center  118  of the mirror surface  112 , and the opening  113  can be provided to allow passage of a light beam propagated from a back side of the collector  110 . 
     The laser generator  120  is disposed within the chamber  102  and at the back side of the collector  110 , and thus the mirror surface  112  of the collector  110  faces away from the laser generator  120 . The laser generator  120  is configured to provide a laser beam. The laser generator  120  can be oriented such that the laser beam emitted from the laser generator  120  can go along the axis of symmetry  116  of the mirror surface  112 . Stated differently, the laser generator  120  is oriented such that an optical path  121  of the laser generator  120  is the same as the axis of symmetry  116  of the mirror surface  112 . The laser generator  120  is configured to generate a laser beam traveling along the optical path  121  of the laser generator  120  and aiming at an excitation zone  122  in front of the mirror surface  112  of the collector  110 . In some embodiments, the laser beam generated by the laser generator  120  is focused on the excitation zone  122 . In some embodiments, the excitation zone  122  may be between the center  118  and the focal point  114  of the mirror surface  112 . The laser generator  120  may emit a laser beam from the back side to a front side of the collector  110  through the opening  113  of the collector  110 . In some embodiments, the laser generator  120  includes a laser source, such as a pulse carbon dioxide (CO 2 ) laser source. 
     The droplet generator  130  and the droplet catcher  135  are disposed within the chamber  102  and on two opposite sides of the collector  110  (e.g., a left side and a right side of the collector  110 ). The droplet generator  130  is configured to provide droplets. The droplet generator  130  can be oriented such that the droplets shot from the droplet generator  130  can go along a droplet path  131  (i.e. an initial path for the droplets) through the excitation zone  122  (i.e., the position on which the laser beam generated by the laser generator  120  is focused on) in front of the mirror surface  112  of the collector  110 . Stated differently, the droplet path  131  intersects with the axis of symmetry  116  of the mirror surface  112  at the excitation zone  122 . Furthermore, because the optical path  121  of the laser generator  120  is the same as the axis of symmetry  116  of the mirror surface  112 , the droplet path  131  intersects with the optical path  121  of the laser generator  120  at the excitation zone  122  as well. As shown in  FIG.  1   , when the droplet deflector  180  is turned on, a rear segment  131   r  of the droplet path  131  will be non-parallel with a front segment  131   f  of the droplet path  131 . For example, the front segment  131   f  of the droplet path  131  is substantially linear and perpendicular to the outlet of the droplet generator  180 , but the rear segment  131   r  of the droplet path  131  is slightly tilted toward the collector  110  because droplets will be deflected by the droplet deflector  180 , which will be described in greater detail below. 
     The droplet catcher  135  is configured to catch the droplets from the droplet generator  130 . In some embodiments, example materials shot from the droplet generator  130  may include tin or other suitable material that can be used to generate EUV. In some embodiments, the pulses of the laser beam provided by the laser generator  120  and the droplet generating rate of the droplet generator  130  are controlled to be synchronized such that the droplets receive peak powers consistently from the laser pulses of the laser beam. In some embodiments, the laser generator  120  and the droplet generator  130  can be collectively operated to generate EUV light, and therefore the laser generator  120  in combination with the droplet generator  130  can serve as an EUV light source. 
     The inlet port  140  and the outlet port  142  pass sidewalls of the chamber  102  and are coupled to the inside of the chamber  102 . In some embodiments, the inlet port  140  and the outlet port  142  are configured to provide a continuous gas flow through the chamber  102  during the operation of the lithography system  100 A, so as to protect the collector  110  from contaminations, such as tin particles (e.g., tin debris). 
     The optical reflectors  152 ,  154 ,  156 ,  158 ,  160 ,  162 ,  164 ,  166 ,  168  are within the chamber  102  and are mounted to respective supports (not shown in  FIG.  1   ) that are parts of the lithography system  100 A. The optical reflectors  152 ,  154 ,  156  are optically coupled between the collector  110  and the reticle  170 , and the optical reflectors  158 ,  160 ,  162 ,  164 ,  166 ,  168  are optically coupled between the reticle  170  and the photoresist layer  302  on the wafer  300 . The optical reflector  152  is optically coupled to the collector  110 , and thus the mirror surface  112  of the collector  110  can reflect a light beam to the optical reflector  152 . Afterward, the light beam can be reflected from the optical reflector  152  to the reticle  170  through reflection by the optical reflectors  154 ,  156 . The optical reflector  158  is optically coupled to the reticle  170 , and thus the light beam can be reflected from the reticle  170  to the optical reflector  158 . Thereafter, the light beam can be reflected from the optical reflector  158  to the photoresist layer  302  through reflection by the optical reflectors  160 ,  162 ,  164 ,  166 , and  168 . 
     In some embodiments, the optical reflectors  152 ,  154 ,  156 ,  158 ,  160 ,  162 ,  164 ,  166 ,  168  are mirrors which respectively have reflection surfaces. In some embodiments, the optical reflectors  152 ,  154 ,  156 ,  158  can be multilayer structures that operate as distributed Braggs reflectors. The thickness of the layers can be optimized for each of the optical reflectors  152 ,  154 ,  156 ,  158  with respect to wavelength and angle of an incident light beam. In some embodiments, a first group of the optical reflectors  152 ,  154 ,  156 ,  158 ,  160 ,  162 ,  164 ,  166 ,  168  includes at least one concave mirror, and a second group of the optical reflectors  152 ,  154 ,  156 ,  158 ,  160 ,  162 ,  164 ,  166 ,  168  includes at least one convex mirror. 
     The reticle  170  can be used to impart the light beam with a pattern thereof so as to create a pattern in the photoresist layer  302 . It is noted that the pattern imparted to the light beam may not exactly correspond to the desired pattern in the wafer, for example if the pattern includes phase-shifting features. Generally, the pattern imparted to the light beam will correspond to a particular functional layer in a device being created in the wafer, such as an integrated circuit. In some embodiments, the reticle  170  may include a distributed Bragg reflector. In some embodiments, the reticle  170  may include phase shifting layers and/or absorber layers to define the pattern. In some embodiments, the reticle  170  is an absorberless phase-shifting mask. 
     The droplet deflector  180  is disposed within the chamber  102 , and the collector  110  and the droplet deflector  180  are on two opposite sides of the droplet path  131  of the droplet generator  130  (e.g., an upward side and a downward side of the droplet path  131  of the droplet generator  130 ). In some embodiments, the droplet deflector  180  provides an airflow toward the droplet path  131  to apply a force on the droplet path  131 . In some embodiments, the droplet deflector  180  applies sonic wave toward the droplet path  131  to apply a force on the droplet path. For example, the droplet deflector  180  is a wave generator providing a wave, such as, a sound wave (e.g., vibrations in pressure, particle of displacement, or particle propagation), a vibration wave, or combinations thereof. The droplet deflector  180  can be oriented such that the airflow or the wave provided from the droplet deflector  180  can travel along a traveling path  181  to a second position  182  in the droplet path  131  and between the droplet generator  130  and the excitation zone  122 . Stated differently, the traveling path  181  intersects with the droplet path  131  of the droplet generator  130  at the second position  182 , and thus provides the airflow or the wave along the traveling path  181  to the second position  182 . 
     The airflow or the wave provided by the droplet deflector  180  may deflect at least one droplet in the traveling path  181 . For example, in some embodiments, the droplet deflector  180  is a wave generator that can produce high intensity sound waves traveling along the traveling path  181 , and droplets passing through the traveling path  181  may be deflected by the pressure gradients of the high intensity sound waves. In some embodiments, the droplet deflector  180  can produce high intensity sound waves traveling along the traveling path  181  when the droplet generator  130  shoots droplets along the droplet path  131 , and thus the droplets at the second position  182  may be deflected by the pressure gradients of the high intensity sound waves, due to the intersection of the droplet path  131  and the traveling path  181 . In some other embodiments, the droplet deflector  180  can supply an airflow traveling along the traveling path  181  when the droplet generator  130  shoots droplets along the droplet path  131 , and thus the droplets at the second position  182  may be deflected by the pressure gradients of the airflow, due to the intersection of the droplet path  131  and the traveling path  181 . 
     The controller  190  is electrically connected the droplet generator  130  and is configured to trigger the droplet shooting operation of the droplet generator  130 . In some embodiments, the controller  190  can be configured to halt the droplet shooting operation of the droplet generator  130 . In some embodiments, after the droplet shooting operation of the droplet generator  130  is halted, the controller  190  can also be configured to resume the droplet shooting operation of the droplet generator  130 . 
     The controller  191  is electrically connected the laser generator  120  and is configured to trigger the laser emission operation of the laser generator  120 . In some embodiments, the controller  191  can be configured to halt the laser emission operation of the laser generator  120 . In some embodiments, after the laser emission operation of the laser generator  120  is halted, the controller  191  can also be configured to resume the laser emission operation of the laser generator  120 . 
     The controller  192  is electrically connected to the droplet deflector  180  and is configured to trigger the force applying operation of the droplet deflector  180 . In some embodiments, the controller  190  is configured to halt the force applying operation of the droplet deflector  180 . In some embodiments, after the force applying operation is halted, the controller  190  can be configured to resume the force applying operation of the droplet deflector  180 . 
     In some embodiments, the controllers  190  and  192  can be programmed such that the controller  190  can trigger the droplet shooting operation before the controller  192  triggers the force applying operation. In some embodiments, the controllers  190  and  192  can be programmed such that the controller  190  can trigger the droplet shooting operation after the controller  192  triggers the force applying operation. In some embodiments, the controllers  190  and  192  can be programmed such that the controller  190  can halt the droplet shooting operation before the controller  192  halts the force applying operation. 
     In some embodiments, the controllers  190  and  191  can be programmed such that controller  190  can trigger the droplet shooting operation before the controller  191  triggers the laser emission operation. In some embodiments, the controllers  190  and  191  can be programmed such that the controller  190  can trigger the droplet shooting operation after the controller  191  triggers the laser emission operation. In some embodiments, the controllers  190  and  191  can be programmed such that the controller  190  can halt the droplet shooting operation before the controller  191  halts the laser emission operation. 
     In some embodiments, the laser emission operation, the droplet shooting operation, and the force applying operation are synchronized. For example, the controllers  190 ,  191  and  192  can be programmed such that the controllers  190 ,  191  and  192  can synchronously trigger the droplet shooting operation, the laser emission operation, and the force applying operation. 
       FIG.  2    is a flow chart of a process  200  according to some embodiments of the present disclosure.  FIG.  3    illustrate the lithography system  100 A of  FIG.  1    with which the process  200  of  FIG.  2    is implemented according to some embodiments of the present disclosure.  FIG.  4    illustrates a drawing of partial enlargement of the lithography system  100 A of  FIG.  3   . The process  200  includes actions S 210 , S 220 , S 230 , S 240 , S 250 , and S 260 . The lithography system  100 A can be operated to expose a photoresist layer coated on a wafer by the process  200 . For example, as shown in  FIG.  3   , the wafer  300  on which the photoresist layer  302  is coated is mounted on the stepper  104  within the chamber  102 , and the lithography system  100 A is operated to expose the photoresist layer  302  coated on the wafer  300 . 
     The action S 210  is applying a force using the droplet deflector. For example, as shown in  FIGS.  3  and  4   , the controller  192  can trigger a force applying operation such that the droplet deflector  180  can produce a force along the traveling path  181  toward the second position  182 . In some embodiments, the force may be applied using a sound wave. In some embodiments, the sound wave has a frequency less than 20 Hz and thus can be referred to as an infrasound wave. In some embodiments, the sound wave has a frequency greater than 20000 Hz and thus can be referred to as an ultrasound wave. In some embodiments, the droplet deflector  180  can produce forces using other techniques, such as pressure wave, vibration wave, and/or electromagnetic wave. 
     The action S 220  is generating a laser beam from a laser generator. For example, as shown in  FIGS.  3  and  4   , the controller  190  can be programmed to trigger a laser emission operation such that the laser generator  120  can generate a laser beam  311 . As previously described, the laser generator  120  can be oriented such that the laser beam  311  emitted from the laser generator  120  can go along the axis of symmetry  116  of the mirror surface  112 , and the laser generator  120  is configured to generate the laser beam  311  aiming at the excitation zone  122  in front of the mirror surface  112  of the collector  110 . As such, the laser beam  311  can be sent to the excitation zone  122  in the axis of symmetry  116  of the mirror surface  112 . In some embodiments, the laser beam  311  generated by the laser generator  120  is propagated through the opening  113  and focused in the excitation zone  122 . 
     The action S 230  is generating fuel droplets by a droplet generator. For example, as shown in  FIGS.  3  and  4   , the controller  190  can trigger a droplet shooting operation such that fuel droplets  312  (e.g., droplets of molten tin) are generated by the droplet generator  130 . In some embodiments, the fuel droplets  312  may include tin or other suitable materials that can be used to generate EUV. The fuel droplets  312  are generated by the droplet generator  130  to form a stream of the fuel droplets  312  directed along the droplet path  131  and toward the excitation zone  122 . 
     Together with the fuel droplets  312  there may also be generated very small fuel fragments referred to as satellite droplets  314  that result from incomplete coalescence of the primary fuel droplets  312 , as illustrated in  FIG.  4   . By way of example, a fuel droplet may have a diameter of about 30-50 microns whereas a satellite droplet may have a diameter of 6-10 microns. The satellite droplets  314  may lead to a negative impact on the EUV generation. For example, if a satellite droplet  314  and a neighboring fuel droplet  312  travel along an undeflected droplet path  131 ′ into the excitation zone  122  together, the laser beam might excite the satellite droplet  314  and thus lead to a shock wave. This shock wave might accelerate the neighboring fuel droplet  312  to move away from the excitation zone  122 , which in turn would result in incomplete excitation of the fuel droplet  312 . 
     However, in some embodiments of the present disclosure, because the droplet deflector  180  applies a force along the traveling path  181  to the second position  182  between the excitation zone  122  and the droplet generator  130 , the fuel droplets  312  and the satellite droplets  314  can be deflected from the undeflected path  131 ′. Moreover, because the satellite droplets  314  have lighter weights than the fuel droplets  132 , the satellite droplets  314  can be deflected by greater distances than that the fuel droplets  132  are deflected by. Therefore, the fuel droplets  312  and the deflected satellite droplets  314  will move along different paths P 1  and P 2 . In greater detail, the path P 1  along which the deflected fuel droplets  312  move intersects with the excitation zone  122 , but the path P 2  along which the deflected satellite droplets  314  move does not intersect with the excitation zone  122 . Stated differently, the deflected fuel droplets  312  will pass through the excitation zone  122 , but the deflected satellite droplets  314  will not move into the excitation zone  122 . In this way, shock waves resulting from excitation of the satellite droplets  314  will be reduced, which in turn will prevent acceleration of the fuel droplets  312  in the excitation zone  122  resulting from the shock waves, thus preventing the fuel droplets  312  from incomplete excitation. Moreover, due to absence of accelerating the fuel droplets  312  by the shock waves, the fuel droplets  312  may pass through the excitation zone  122  at a substantially constant speed. 
     In some embodiments, the droplets (e.g., the fuel droplets  312  and the satellite droplets  314 ) are shot out by the droplet generator  130  after turning on the droplet deflector  180 , so that undeflected satellite droplets  314  can be reduced. The asynchronous turn-on operations of the droplet generator  130  and the droplet deflector  180  can be achieved by the individual controllers  190  and  192 . For example, the controller  190  triggers the droplet shooting operation after the controller  192  triggers the force applying operation. 
     In some embodiments, the droplet deflector  180  is turned off after stopping shooting the droplets (e.g., the fuel droplets  312  and the satellite droplets  314 ), so that undeflected satellite droplets  314  can be reduced. The asynchronous turn-off operations of the droplet generator  130  and the droplet deflector  180  can be achieved by the individual controllers  190  and  192 . For example, the controller  190  halts (i.e., stops) the droplet shooting operation before the controller  192  halts (i.e., stops) the force applying operation. 
     The deflected fuel droplets  312  goes toward the excitation zone  122  which is at the focus of the laser beam  311 , so that the fuel droplets  312  are vaporized by the laser beam  311  to form an EUV-generating plasma. For example, when the laser beam  311  is incident on a fuel droplet  312 T, the fuel droplet  312 T can be excited, so as to produce high-temperature plasma  316 . In some embodiments, the high-temperature plasma  316  may be referred to as a microplasma which can generate EUV light  138 , as shown in  FIG.  3   . In some embodiments, the lithography system  100 A produces EUV light  138  with a wavelength in the range from about 3 nm to about 15 nm, for example a wavelength of about 13.5 nm. 
     In some embodiments, the laser beam  311  is emitted from the laser generator  120  before turning on the droplet generator  130 , so that the fuel droplets  312  can be exited. The asynchronous turn-on operations of the droplet generator  130  and the laser generator  120  can be achieved by the individual controllers  190  and  191 . For example, the controller  190  triggers the droplet shooting operation after the controller  191  triggers the laser emission operation. 
     In some embodiments, the deflected fuel droplets  312  moving along the path P 1  may be caught by the droplet catcher  135 , and the deflection satellite droplets  314  moving along the path P 2  may be caught by another droplet catcher  136  which is separated from the droplet catcher  135 . The droplet catcher  135  is misaligned with the droplet generator  130  for catching the deflected primary droplets  312 . In some embodiments, the collector  110  is located between the droplet catchers  135  and  136 . For example, the satellite droplet catcher  136  is disposed behind the collector  110 , because the satellite droplets  314  are deflected toward a back side of the collector  110 . On the other hand, the fuel droplet catcher  135  is disposed in front of the collector  110 , because the fuel droplets  312  still move in front of the collector  110  even if they are deflected by the droplet deflector  180 . 
     In some embodiments, a vertical distance D 1  between an outlet of the droplet generator  130  and an entrance of the droplet catcher  135  is a first distance D 1 , and a vertical distance D 2  between the outlet of the droplet generator  130  and an entrance of the minor droplet catcher  136  is a second distance D 2  greater than the first distance D 1 . Either the first distance D 1  or second distance D 2  is less than or equal to about 30 cm (e.g., in a range from about 20 cm to about 30 cm). The first and second distances D 1  and D 2  are associated with deflections of the fuel droplets  312  and the satellite droplets  314 . For example, the fuel droplets  312  may have a diameter in a range from about 25 μm to about 33 μm and a mass in a range from about 7*10 −12  kg to about 8*10 −12  kg, and the satellite droplets  314  may have a diameter in a range from about 5 μm to about 7 μm and a mass in a range from about 2*10 −13  kg to about 3*10 −13  kg. In such example, The droplet deflector  180  applies a force  310  along the direction A 1  to the satellite droplets  314  in a range from about 2.5*10 −11  N to about 3*10-11 N, such that the satellite droplets  314  may be deflected by a desired second distance D 2  (e.g., about 30 cm), and the fuel droplets  312  may be deflected by a desired first distance D 1  (e.g., about 25 cm). Applying the force  310  may include generating a wave or an airflow along the direction A 1 . 
     After generating the EUV light  138  by exciting the fuel droplets  312 , the EUV light  138  is reflected by the mirror surface  112  of the collector  110  toward the optical reflector  152 , as shown in  FIG.  3   . In some embodiments, the EUV light  138  is widely scattered to produce the reflected EUV light. The collector  110  can gather the EUV light  138  and direct the EUV light  138  onto the optical reflector  152 . The EUV light  138  then can be reflected by the optical reflectors  152 ,  154 ,  156  in sequence and to the reticle  170  as illustrated in  FIG.  3   . The reticle  170  reflects the EUV light  138 , which in turn imparts the EUV light  138  with a pattern. 
     After using the optical reflectors  152 ,  154 ,  156  to reflect the EUV light  138  to the reticle  170 , a pattern is imparted to the EUV light  138 . Thereafter, the EUV light  138  imparted with the pattern is directed to the photoresist layer  302  coated on the wafer  300  by the optical reflectors  158 ,  160 ,  162 ,  164 ,  166 ,  168 . The lithography system  100 A thereby selectively exposes the photoresist layer  302  coated on the wafer  300  in the pattern defined by the reticle  170  (i.e., the pattern imparted to the EUV light  138 ). 
     The action S 240  is providing a continuous gas flow through a chamber. For example, as shown in  FIG.  3   , a gas flow including gases  320 A,  320 B,  320 C, and  320 D is provided to flow through the chamber  102 . The gas  320 A is the gas flow as it enters the chamber  102  through the inlet port  140 . The gas  320 B is the gas flow when it resides within the chamber  102 . The gas  320 C is a portion of the gas flow that is located in proximity to the mirror surface  112  of the collector  110 . The gas  320 D is gas flow as it leaves the chamber  102  through outlet port  142 . In some embodiments, the continuous gas flow is provided to flow through the chamber  102  in which the collector  110  and other components of the lithography system  100 A are enclosed. In some embodiments, concentration of contaminants in the gas  320 B can be reduced and the proportion of contaminants carried away with the outflow gas  320 D can be increased by raising the flow rate of the gas flow through the chamber  102 . 
     Following the actions S 210 , S 220 , S 230 , and S 240 , the process  200  continues with the action S 250  which is halting generating the laser beam and generating the fuel droplets. For example, as shown in  FIG.  5   , which illustrates the lithography system  100 A of  FIG.  1    with which the process  200  of  FIG.  2    is implemented according to some embodiments of the present disclosure, the controllers  190  and  191  can be programmed to halt the droplet shooting operation and the laser emission operation, and thus halting the generation of the EUV light. In some embodiments, the controllers  190 ,  191  and  192  are programmed such that the laser emission operation and the droplet shooting operation are halted before halting the force applying operation, so as to reduce variation in the EUV light. After halting the laser emission operation and the droplet shooting operation, the process  200  continues with the action S 260  which is halting the force. For example, the controller  192  is programmed to halt the force applying operation after halting the laser emission operation and the droplet shooting operation. 
     In the example configuration in  FIG.  5   , halting the force applying operation is performed after halting the laser emission operation and the droplet shooting operation, but is not limited thereto. In other embodiments, the force applying operation is halted before halting the laser emission operation and the droplet shooting operation. 
     After halting the laser emission operation and the droplet shooting operation, the controllers  190  and  191  can be programmed to resume the laser emission operation and the droplet shooting operation. Similarly, after halting the force applying operation, the controller  192  can be programmed to resume the force applying operation. In some embodiments, after halting the laser emission operation, the droplet shooting operation, and the force applying operation, the laser emission operation and the droplet shooting operation are resumed. The force applying operation can be resumed after resuming the laser emission operation and the droplet shooting operation. In some embodiments, after halting the laser emission operation and the droplet shooting operation and before halting the force applying operation, the laser emission operation and the droplet shooting operation can be resumed. 
       FIG.  6    illustrates a lithography system  100 B according to some embodiments of the present disclosure.  FIG.  7    illustrates a drawing of partial enlargement of the lithography system  100 B during operation. Many aspects of the lithography system  100 B are the same as or similar to those of the lithography system  100 A as previously described in  FIG.  1   . For example, the lithography system  100 B includes a chamber  102 , a collector  110 , a laser generator  120 , a droplet generator  130 , a droplet catcher  135 , an inlet port  140 , an outlet port  142 , optical reflectors  152 ,  154 ,  156 ,  158 ,  160 ,  162 ,  164 ,  166 ,  168 , a reticle  170 , a droplet deflector  180   a , and controllers  190 ,  192 , and the detailed explanation may be omitted. The lithography system  100 B is an EUV exposure tool that can perform an exposure operation for exposing a photoresist layer  302  coated on a wafer  300  within the chamber  102 . For example, the lithography system  100 B may include a stepper  104  disposed within the chamber  102 , and the wafer  300  on which the photoresist layer  302  is coated is mounted on the stepper  104 . The stepper  104  is movable in the chamber  102  and is configured to shift the wafer  300 , such that the wafer  300  can be shifted at a suitable position for the exposing. 
     Different from the lithography system  100 A, the collector  110  and the droplet deflector  180   a  of the lithography system  100 B are disposed on the same side of a droplet path  131  of the droplet generator  130 . In this way, the droplet deflector  180   a  can apply a force along a direction A 2  away from the collector, so as to deflect the satellite droplets  314  away from the collector  110 . As a result, the satellite droplet catcher  136   a  is disposed in front of the collector  110 , so as to catch the deflected satellite droplets  314 . In some embodiments, a distance between the droplet catcher  136   a  and the collector  110  along a direction parallel to the axis of symmetry  116  of the collector  110  is greater than a distance between the excitation zone  122  and the collector  110  along the direction. 
       FIG.  8    illustrates a drawing of partial enlargement of a lithography system  100 C during operation according to some embodiments of the present disclosure. Many aspects of the lithography system  100 C are the same as or similar to those of the lithography system  100 A as previously described in  FIG.  1   . For example, the lithography system  100 C includes a collector  110 , a laser generator  120 , a droplet generator  130 , a droplet catcher  135 , a droplet deflector  194 , and controllers  190 ,  192 , and the detailed explanation may be omitted. The lithography system  100 C is an EUV exposure tool that can perform an exposure operation, as previously described. 
     Different from the lithography system  100 A, the droplet deflector  194  can generate an electric field on the droplet path  131 , so as to deflect the fuel droplets  312  and the satellite droplets  314  from the undeflected path  131 ′ by a force resulted from an electric field, such as Coulomb force. In order to generate an electric field, the droplet deflector  194  may include a pair of electrode plates  196 A and  196 B which are disposed at opposite sides of the droplet path  131 . When a voltage difference is applied to the electrode plates  196 A and  196 B, an electric field in a direction substantially perpendicular to the droplet path  131  can be generated. For example, a positive voltage is applied to the electrode plate  196 A and a negative voltage is applied to the electrode plate  196 B, which in turn generates an electric field in a downward direction A 3 , such that the fuel droplets  312  and the satellite droplets  314  in the electric field can be deflected by a force  310  along the downward direction A 3 . In this way, as previously described, since the droplet deflector  194  can apply the force  310  along the downward direction A 3 , it makes the deflected fuel droplets  312  pass through the excitation zone  122  but make the deflected satellite droplets  314  not move into the excitation zone  122 . 
     In some embodiments, the controller  192  electrically connected to the droplet deflector  194  is configured to adjust a voltage difference applied into the electrode plates  196 A and  196 B, so as to vary an intensity of an electric field generated by the electrode plates  196 A and  196 B. In some embodiments, the electrode plates  196 A and  196 B are symmetric about the droplet path  131 . In other embodiments, the electrode plates  196 A and  196 B are asymmetric about the droplet path  131 . For example, the electrode plate  196 B may be further away from the droplet path  131  than the electrode plate  196 A, so as to avoid the fuel droplets  312  and the satellite droplets  314  from hitting the electrode plate  196 B. 
       FIG.  9    illustrates a drawing of partial enlargement of a lithography system  100 D during operation according to some embodiments of the present disclosure. Many aspects of the lithography system  100 D are the same as or similar to those of the lithography system  100 C as previously described in  FIG.  8   . For example, the lithography system  100 D includes a collector  110 , a laser generator  120 , a droplet generator  130 , a droplet catcher  135 , a droplet deflector  194   a , and controllers  190 ,  192 , and the detailed explanation may be omitted. The lithography system  100 D is an EUV exposure tool that can perform an exposure operation, as previously described. 
     Different from the lithography system  100 C, a voltage difference applied into the electrode plates  196 A and  196 B of the droplet deflector  194   a  is inverse to the voltage difference applied into the electrode plates  196 A and  196 B of the droplet deflector  194  as previously described in  FIG.  8   , and therefore the droplet deflector  194   a  can deflect the fuel droplets  312  and the satellite droplets  314  from the undeflected path  131 ′ by a force  310  along an upward direction A 4 . For example, a negative voltage is applied to the electrode plate  196 A and a positive voltage is applied to the electrode plate  196 B, which in turn generates an electric field in the upward direction A 4 , such that the fuel droplets  312  and the satellite droplets  314  in the electric field can be deflected by the force  310  along the downward direction A 4 . In this way, since the droplet deflector  194   a  can apply the force  310  along the direction A 4  away from the collector  110 , it can deflect the satellite droplets  314  away from the collector  110 . In some embodiments, the electrode plates  196 A and  196 B are symmetric about the droplet path  131 . In other embodiments, the electrode plates  196 A and  196 B are asymmetric about the droplet path  131 . For example, the electrode plate  196 A may be further away from the droplet path  131  than the electrode plate  196 B, so as to avoid the fuel droplets  312  and the satellite droplets  314  from hitting the electrode plate  196 A. 
     Based on the above discussions, it can be seen that the present disclosure offers advantages. It is understood, however, that other embodiments may offer additional advantages, and not all advantages are necessarily disclosed herein, and that no particular advantage is required for all embodiments. One advantage is that the satellite droplets can be deflected away from the excitation zone, which in turn will reduce shock waves resulting from excitation of the satellite droplets, which in turn will prevent acceleration of the fuel droplets in the excitation zone, thus preventing the fuel droplets from incomplete excitation. 
     According to various embodiments of the present disclosure, a method includes shooting a primary droplet and a satellite droplet from a droplet generator along a common initial direction; applying a force to the primary droplet and the satellite droplet, wherein after applying the force, the primary droplet has a first deflection toward a first direction different than the common initial direction, and the satellite droplet has a second deflection toward a second direction different than the common initial direction, wherein the second deflection of the satellite droplet is greater than the first deflection of the primary droplet; and generating an extreme ultraviolet (EUV) light using an excitation laser hitting the primary droplet with the first deflection. 
     According to various embodiments of the present disclosure, a method includes shooting a primary droplet and a satellite droplet along an initial droplet path; applying, using a droplet deflector, a force to the primary droplet and the satellite droplet, such that the primary droplet is directed toward an excitation zone in front of a collector, while the satellite droplet is directed to a first droplet catcher offset from the excitation zone, wherein along a direction vertical to the initial droplet path, the collector is closer to the droplet deflector than to the first droplet catcher; and generating an extreme ultraviolet (EUV) light using an excitation laser hitting the primary droplet but not hitting the satellite droplet. 
     According to various embodiments of the present disclosure, a method includes shooting, using a droplet generator, primary droplets and satellite droplets along an initial droplet path; applying a force to the primary droplets and the satellite droplets, such that the primary droplets are directed toward an excitation zone, while the satellite droplets are directed to a first droplet catcher offset from the excitation zone; generating an extreme ultraviolet (EUV) light using an excitation laser hitting the primary droplet at the excitation zone; and receiving the primary droplets by a second droplet catcher downstream of the excitation zone. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.