Patent Publication Number: US-2023164901-A1

Title: Method for using radiation source apparatus

Description:
PRIORITY CLAIM AND CROSS-REFERENCE 
     This application is a continuation of U.S. patent application Ser. No. 17/369,740, filed Jul. 7, 2021, now U.S. Pat. No. 11,553,581, issued on Jan. 10, 2023, which claims priority to U.S. Provisional Application Ser. No. 63/163,433, filed Mar. 19, 2021, the entirety of which is incorporated by reference herein in their entireties. 
    
    
     BACKGROUND 
     Photolithography is a process by which a reticle having a pattern is irradiated with light to transfer the pattern onto a photosensitive material overlying a semiconductor substrate. Over the history of the semiconductor industry, smaller integrated chip minimum features sizes have been achieved by reducing the exposure wavelength of optical lithography radiation sources to improve photolithography resolution. Extreme ultraviolet (EUV) lithography, which uses extreme ultraviolet (EUV) light, is a promising next-generation lithography solution for emerging technology nodes. 
    
    
     
       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. 
         FIGS.  1 A and  1 B  are flow charts of a method for using an EUV radiation source according to some embodiments of the present disclosure. 
         FIGS.  2 ,  3 A,  4 ,  5 A- 5 B,  6 , and  7    illustrates a method for using an EUV radiation source at different stages according to some embodiments of the present disclosure. 
         FIG.  3 B  illustrates a enlarge view of a droplet generator according to some embodiments of the present disclosure. 
         FIG.  5 C  is schematic view of a portion of an EUV radiation source and a robot arm according to some embodiments of the present disclosure. 
         FIG.  5 D  illustrates a movement of the robot arm in three-dimensional space according to some embodiments of the present disclosure. 
         FIGS.  8  and  9    are schematic views of a robot arm holding an extraction tube according to some embodiments of the present disclosure. 
         FIG.  10    is schematic view of a portion of an EUV radiation source and a robot arm according to some embodiments of the present disclosure. 
         FIG.  11    is schematic view of a portion of an EUV radiation source and a robot arm according to some embodiments of the present disclosure. 
         FIG.  12    is schematic view of a portion of an EUV radiation source and a robot arm according to some embodiments of the present disclosure. 
         FIGS.  13 - 14    illustrates a method for using an EUV radiation source at different stages according to some embodiments of the present disclosure. 
         FIG.  15    is a schematic view of robot arms used to move a droplet generator and move a cleaning device according to some embodiments of the present disclosure. 
         FIG.  16    is a schematic view of a lithography system 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. 
     Further, 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. 
     An extreme ultraviolet (EUV) photolithography system uses extreme ultraviolet radiation. One method of producing the extreme ultraviolet radiation is to emit a laser to droplets of tin. As the tin droplets are produced into the EUV radiation source vessel, the laser hits the tin droplets and heats the tin droplets to a critical temperature that causes atoms of tin to shed their electrons and become a plasma of ionized tin droplets. The ionized tin droplets emit photons, which is collected by a collector and provided as EUV radiation to an optical lithography system. The collector is designed with suitable coating material and shape, functioning as a mirror for EUV collection, reflection, and focus. 
     In some embodiments, the plasma of ionized tin droplets may cool down and become liquids and small particles, which are respectively referred to as drips and drops, and may be collectively referred to as debris. The debris may deposit onto a surface of the collector, thereby causing contamination thereon. Over time, the reflectivity of the collector degrades due to debris accumulation and other factors such as ion damages, oxidation, and blistering. In some cases, in order to keep EUV radiation power and productivity, the collector is moved out for cleaning tin contaminant, which may take lots of time. In some embodiments of the present disclosure, an in-line cleaning process is performed for cleaning the surface of the collector without moving the collector, thereby prolonging the lifetime of the collector and gains lots of tool time for productivity. 
       FIGS.  1 A and  1 B  are flow charts of a method  100  for using an EUV radiation source according to some embodiments of the present disclosure. The method  100  may include steps  102 - 118 . At step  102 , a droplet generator is assembled onto a port of a vessel. Subsequently, a lithography exposure process LEP including steps  104  and  106  is performed. For example, at step  104 , the droplet generator ejects a target droplet (e.g., tin droplet) into the vessel. At step  106 , a laser is emitted onto the target droplet for producing extreme ultraviolet (EUV) light out of the vessel. At step  108 , an intensity of EUV light is measured. At step  110 , a clogging condition of a nozzle in the droplet generator is detected. At step  112 , the droplet generator is disassembled from port of a vessel. At step  114 , a determination whether to clean the collector is made according to the measured intensity of EUV light. At step  116 , if the determination determines that the collector requires cleaning, a robot arm is inserted into the vessel through the port. At step  116 , the collector is cleaned by using a cleaning device held by the robot arm. At step  118 , a cone structure over the vessel is detected by a detector held by the robot arm. At step  120 , the robot arm is moved away from the vessel. At step  122 , a droplet generator is assembled onto the port. It is understood that additional steps may be provided before, during, and after the steps  102 - 124  shown by  FIGS.  1 A and  1 B , and some of the steps described below can be replaced or eliminated for additional embodiments of the method. The order of the operations/processes may be interchangeable. 
       FIGS.  2 ,  3 A,  4 ,  5 A- 5 C, and  6    illustrates a method  100  for using an EUV radiation source  200  at different stages according to some embodiments of the present disclosure. Referring to  FIGS.  1 A and  2   , the method  100  begins at step  102  where the droplet generator  240  is assembled onto the port  210 G of a vessel  210  of the EUV radiation source  200 . 
     In some embodiments, the EUV radiation source  200  may be optically coupling with a scanner (i.e., a lithography system  900  as shown in  FIG.  16   ). The EUV radiation source  200  may include a vessel  210 , a laser source  220 , a collector  230 , a bellow assembly BW, a target droplet generator  240 , a droplet catcher  250 , and a lower cone structure  260 . In some embodiments, the vessel  210  has a cover  212  surrounding itself ventilation. The cover  212  may be configured around the collector  230 . The laser source  220  may be at a bottom side of the vessel  210  and below the collector  230 . In some embodiments, the bellow assembly BW is configured for receiving the droplet generator  240 , and has an end connected to a port  210 G of the vessel  210 . The droplet catcher  250  may be installed opposite the port  210 G of the vessel  210 . In some embodiments, the lower cone structure  260  has a cone shape with its wide base integrated with the cover  212  and its narrow top section facing the scanner (i.e., the lithography system  900 ). The cone shape of the lower cone structure  260  tapers toward an exit aperture  210 O of the vessel  210 . The radiation source  200  may further include an intermediate focus (IF)-cap module  290  out of the exit aperture  210 O, and the IF-cap module  290  is configured to provide intermediate focus to the EUV radiation EL. 
     The droplet generator  240  may include a reservoir  242  and a nozzle  244  connected to the reservoir  242 . In some embodiments, the reservoir  242  may contain a suitable fuel material TD that is capable of creating a radiation in the EUV range when being converted to a plasma state. For example, the fuel material TD may include water, tin, lithium, xenon, or the like. In some embodiments, the element tin can be pure tin (Sn); a tin compound, for example, SnBr 4 , SnBr 2 , SnH 4 ; a tin alloy, for example, tin-gallium alloys, tin-indium alloys, tin-indium-gallium alloys, or any other suitable tin-containing material. In some embodiments, by assembling the droplet generator  240  onto the port  210 G of the vessel  210 , the space in the vessel  210  is closed. 
     Referring to  FIGS.  1 A and  3 A , the method proceeds to the step  104 , where the droplet generator  240  ejects droplets of the fuel material TD into the space in the vessel  210  of the radiation source  200 . In the present embodiments, the fuel material TD contained in the reservoir  242  is forced out of the reservoir  242  and the nozzle  244 , thereby generating the droplets of the fuel material TD through the nozzle  244 . The fuel material TD may be delivered in the form of liquid droplets, a liquid stream, solid particles or clusters, solid particles contained within liquid droplets or solid particles contained within a liquid stream. In some embodiments, the droplet generator  240  may release the droplet of the fuel material TD substantially along a X direction. 
     Still referring to  FIGS.  1 A and  3 A , the method proceeds to the step  106 , a laser beam LB is impinged onto the droplet of the fuel material TD for producing EUV light EL out of the vessel  210 . In some embodiments, the EUV light EL has a wavelength ranging between about 1 nm and about 100 nm. In certain examples, the EUV light EL has a wavelength range centered at about 13.5 nm. In some embodiments, the laser source  220  may include a carbon dioxide (CO 2 ) laser source, a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser source, or another suitable laser source to generate a laser beam LB. The laser beam LB is directed through an output window OW integrated with the collector  230 . The output window OW adopts a suitable material that is substantially transparent to the laser beam LB. The laser beam LB is directed to heating the fuel material TD, such as tin droplets, thereby generating high-temperature plasma (e.g., ionized tin droplets) which further produces the EUV light EL. The pulses of the laser source  220  and the droplet ejecting rate of the droplet generator  240  are controlled to be synchronized, such that the fuel material TD receives peak power consistently from the laser pulses of the laser source  220 . In some embodiments, the EUV radiation source  200  employs a laser produced plasma (LPP) mechanism to generate plasma and further generate EUV light EL from the plasma. In some alternative embodiments, the radiation source  200  may employ a dual LPP mechanism in which the laser source  220  is a cluster of multiple laser sources. 
     In some embodiments, the laser beam LB may or may not hit every droplet of the fuel material TD. For example, some droplets of the fuel material TD may be purposely missed by the laser beam LB. In the present embodiments, the droplet catcher  250  is installed opposite the target droplet generator  240  and in the direction of the movement of the droplet of the fuel material TD. The droplet catcher  250  is configured to catch any droplets of the fuel material TD that are missed by the laser beam LB. 
     The collector  230  may collect the EUV light EL, and reflect and focus the EUV light EL to the lithography system  900 , thereby performing a lithography processes, which is illustrated in  FIG.  16   . The collector  230  is designed with suitable coating material and shape, functioning as a mirror for EUV collection, reflection, and focus. In some examples, the coating material of the collector  230  includes a reflective multi-layer (such as a plurality of Mo/Si film pairs) and may further include a capping layer (such as Ru) coated on the reflective multi-layer to substantially reflect the EUV light. An optic axis of the collector  230  may be along a Z direction, which is orthogonal to the X direction that the droplet generator  240  generates the droplet of the fuel material TD substantially along, such that the EUV radiation source  200  may emits the EUV light EL substantially along the Z direction. In some examples, the collector  230  is designed to have an ellipsoidal geometry. Through steps  104  and  106 , the EUV radiation source  200  may emit EUV light EL, thereby performing one or more lithography processes on one or more semiconductor wafers. When EUV radiation source  200  emits EUV light EL, the space in the vessel  210  (e.g., the space surrounded the cover  212 ) is maintained in a vacuum environment since the air absorbs the EUV radiation. 
     In some embodiments, the radiation source  200  may include a shroud SR below the port  210 G. The shroud SR may be made of suitable material, such as ceramics. The shroud SR may extend substantially along the X direction that the droplet generator  240  generates the droplet of the fuel material TD substantially along. The shroud SR may obscure some unshaped fuel material TD (e.g., not in the form of droplet) released from the droplet generator  240 , thereby protecting the collector  230  from being contaminated by the unshaped fuel material TD. The shroud SR may have an end fixed to a top side of the collector  230 . In some embodiments, a length of the shroud SR may be less than a radius of a top side of the collector  230 , such that the shroud SR may not obscure a plasma-formation point C 1  (interchangeably referred to as zone of excitation where droplets are excited by laser) where the laser beam LB hits the droplet of the fuel material TD. For example, the target droplet of the fuel material TD may be ejected from the droplet generator  240  to a zone of excitation (i.e., plasma-formation point C 1 ) in front of the collector  230 , and the laser beam LB is emitted toward the zone of excitation (i.e., plasma-formation point C 1 ), such that the target droplet of the fuel material TD is heated by the excitation laser to generate EUV radiation. In some embodiments, the plasma-formation point C 1  may substantially locate at the optic axis of the collector  230  along the Z direction. 
     In some cases, ionized tin droplets may cool down and become liquids and small particles, which are collectively referred to as debris. Some of the debris (referred to as debris PD hereinafter) may deposit onto a surface of the collector  230 , thereby causing contamination thereon. The reflectivity of the collector degrades due to the debris accumulation and other factors such as ion damages, oxidation, and blistering, such that an intensity of the EUV light EL in the lithography process (referring to  FIGS.  3 A and  16   ) decreases, which in turn will lower the productivity of the lithography process (referring to  FIGS.  3 A and  16   ). In some embodiments, during or after the lithography process (referring to  FIGS.  3 A and  16   ), the method proceeds to the step  108  where an intensity of EUV light is measured by suitable light intensity sensors (e.g., the light intensity sensors  962  and  964  in  FIG.  16   ). Suitable inspection and calculation methods may be performed to calculate the intensity of the EUV light EL, so as to infer a condition of the contamination on the collector  230 . 
     Contaminations in the fuel material TD may result in clogging (i.e., at least partial blocking) of the nozzle  244 , which may impose a lifetime limit on the nozzle. In some embodiments, during the lithography process (referring to  FIGS.  3 A and  16   ), a clogging condition of the nozzle  244  of the droplet generator  240  is detected. For example, referring to  FIG.  3 B , a sensor  244 S of the nozzle  244  may detect a pressure in a capillary tube  244 C of the nozzle  244 , and a controller PC electrically connected with the sensor  244 S may receive a signal from the sensor  244 S in response to the detected pressure. The controller PC may determine whether to perform a maintenance process according to the signal from the sensor  244 S. For example, if the pressure measured by the sensor  244 S is too high (e.g., higher than a determined value, such as 400 psi), the nozzle may be damaged so that tin clog at its head, and the maintenance process would be conducted for replacing the droplet generator  240  with a new one. Referring to  FIGS.  1 A and  4   , the method  100  proceeds to the step  112 , where the droplet generator  240  (referring to  FIG.  3 A ) is disassembled from the port  210 G of the vessel  210  of the EUV radiation source  200 . For example, the port  210 G is free of a droplet generator. In some embodiments, prior to the maintenance process (e.g., the disassembling the droplet generator  240  from the port  210 G of the vessel  210 ), the droplet generator  240  stops ejecting the target droplet of the fuel material TD. In other words, the droplet generator  240  may be turned off prior to the maintenance process (e.g., the disassembling the droplet generator  240  from the port  210 G of the vessel  210 ). In some embodiments, disassembling the droplet generator  240  from the port  210 G would break the vacuum in the vessel  210 , and elements of the EUV radiation source  200  may stop operating. For example, the laser source  220  may be turned off prior to the maintenance process (e.g., the disassembling the droplet generator  240  from the port  210 G of the vessel  210 ), such that the laser beam LB may not emit into the vessel  210  during or after disassembling the droplet generator  240  from the port  210 G. 
     The controller PC may be a part of an overall EUV radiation source  200 . The robot controller PC may include electronic memory and one or more electronic processors configured to execute programming instructions stored in the electronic memory. In some embodiments, the robot controller PC may include processors, central processing units (CPU), multi-processors, distributed processing systems, application specific integrated circuits (ASIC), or the like. 
     The method  100  proceeds to the step  114  (referring to  FIG.  3 A ), where a determination whether to clean the collector is made according to the measured intensity of EUV light. In some embodiments, a controller (e.g., the controller PC) may be electrically connected with the light intensity sensors  962  and  964  (referring to  FIG.  16   ). The controller PC may receive signal from the light intensity sensors  962  and  964  (referring to  FIG.  16   ) and determine whether to perform a clean process according to the signals from the light intensity sensors  962  and  964  (referring to  FIG.  16   ). In some embodiments, the measured intensity of EUV light EL is compared with a reference value measured when the collector  230  is clean. If the detected intensity of EUV light EL is lower than the reference value by more than a certain percentage (e.g., about 10% to about 50%), the method  100  proceeds to steps  116 - 118 , where a cleaning process is performed to the collector  230 . In other words, the maintenance process of the droplet generator is performed with the cleaning process. If the detected intensity of EUV light EL is lower than the reference value by less than the certain percentage (e.g., about 10% to about 50%), the method  100  skips the steps  116 - 122  and proceeds to step  124 , where a new droplet generator is assembled onto the port  210 G of the vessel  210  without cleaning the collector  230 . In other words, the maintenance process of the droplet generator may be performed without the cleaning process. 
     Reference is made to  FIGS.  1 B and  5 A- 5 C .  FIG.  5 A  illustrates a schematic side view of the EUV radiation source  200  along a direction Y according to some embodiments of the present disclosure.  FIG.  5 B  illustrates a schematic side view of the EUV radiation source  200  of  FIG.  5 A  along the direction X, in which the direction X, Y, and Z are orthogonal to each other.  FIG.  5 C  is a schematic view of a portion of an EUV radiation source  200  and a robot arm  300  according to some embodiments of the present disclosure. If the determination determines that the collector  230  requires cleaning, the method  100  proceeds to the step  116 , where a robot arm  300  is inserted into the vessel  210  through the port  210 G. In the present embodiments, the robot arm  300  may hold a cleaning device  410  (referring to  FIG.  5 C ). By using the cleaning device  410 , the method  100  may proceed to the step  118 , where an in-line cleaning process is performed to the collector. In some embodiments, at steps  116  and  118 , the collector  230  may be cleaned during the maintenance process without moving the collector  230 , which is referred to as in-line cleaning process. The in-line cleaning process can extend the lifetime of the collector and gains lots of tool time for productivity. For clear illustration, the EUV radiation source  200  in  FIG.  5 B  is illustrated as being tilted for optically coupling with the scanner (i.e., the lithography system  900 ). For example, the direction Z that the EUV light EL emits along is inclined with respect to a direction of gravity (i.e., a direction Gin  FIG.  5 B ). 
     In some embodiments, the robot arm  300  may include an extending arm portion  312  and a movable arm portion  314 , in which the movable arm portion  314  can rotate with respect to the extending arm portion  312 . In some embodiments, the movable arm portion  314  may also be referred to an arm segment coupled to the extending arm portion  312 . In some embodiments, the extending arm portion  312  may include one or more arm segments connected in sequence. For example, a front end  314 E 1  of the movable arm portion  314  of the robot arm  300  can be moved from a first position in the vessel  210  to a second position in the vessel  210 . In some embodiments, when the movable arm portion  314  is moved, the extending arm portion  312  is held static. For example, the back end  314 E 2  of the movable arm portion  314  may remain at the same position when the front end  314 E 1  of the movable arm portion  314  is moved from the first position to the second position. 
     In some embodiments of the present disclosure, for preventing the movable arm portion  314  from hitting the shroud SR, the extending arm portion  312  of the robot arm  300  is longer than the shroud SR and extends further than the shroud SR does. In other words, the extending arm portion  312  of the robot arm  300  may extend beyond the shroud SR. For example, a length of the extending arm portion  312  of the robot arm  300  may be greater than the radius of a top side of the collector  230 . Through the configuration, the extending arm portion  312  of the robot arm  300  may extend beyond the plasma-formation point C 1 , and the back end  314 E 2  of the movable arm portion  314  is not located at the plasma-formation point C 1 . In some alternative embodiments, the extending arm portion  312  of the robot arm  300  may extend further than the shroud SR does, but not extend beyond the plasma-formation point C 1 . In some embodiments, the back end  314 E 2  of the movable arm portion  314  may be located at the plasma-formation point C 1 . 
     Referring to  FIG.  5 C , the cleaning device  410  is used to clean the debris PD (referring to  FIGS.  5 A and  5 B ) accumulated on the surface of the collector  230 . The cleaning device  410  may include an extraction tube  412  clamping on the robot arm  300  and a pump  414  connected to the extraction tube  412  and external to the vessel  210 . The pump  414  may draw gas from a vessel  210  connected with the extraction tube  412 . In some embodiments, the extraction tube  412  may be referred to as a vacuum tube. In some embodiments, the pump  414  may be referred to as a vacuum pump. The robot arm  300  with the extraction tube  412  is inserted into an EUV vessel  210  through the port  210 G, such that the collector  230  can be cleaned without being removed. 
     In some embodiments, the extraction tube  412  may include a first portion  4122  and a second portion  4124  continuously connected to the first portion  4122 , and the first and second portions  4122  and  4124  are respectively fixed to the extending arm portion  312  and the movable arm portion  314  of the robot arm  300 . Though the configuration, the first and second portions  4122  and  4124  of the extraction tube  412  can be moved in a way similar to the movement of the extending arm portion  312  and the movable arm portion  314  of the robot arm  300 . In some embodiments, by moving/rotating the movable arm portion  314  of the robot arm  300 , the second portion  4124  can be moved or rotated with respect to the first portion  4122 . For example, a front end FE of the second portion  4124  of the extraction tube  412  can be moved from a first position in the vessel  210  to a second position in the vessel  210 . In some embodiments, an opening  412 O of the extraction tube  412  may be mounted on the front end  314 E 1  of the movable arm portion  314  of the robot arm  300 . In some embodiments, by rotating the portion  314  of the robot arm  300 , the opening  412 O of the extraction tube  412  is moved to suitable position to provide a vacuuming suction force to the surface of the collector  230 . By vacuuming the surface of the collector  230 , to the cleaning device  410  may remove the debris PD (referring to  FIGS.  5 A and  5 B ), thereby cleaning the collector  230 . 
     Referring to  FIG.  5 C , in some embodiments, the robot arm  300  may include one or more joints connected between the arm portions. For example, the robot arm  300  include a joint  316  connected between the extending arm portion  312  and the movable arm portion  314 . The joint  316  allows the rotation of the movable arm portion  314  relative to the extending arm portion  312 . For example, the joint  316  may include an adaptor connected between the extending arm portion  312  and the movable arm portion  314 .  FIG.  5 D  illustrates a movement of the robot arm  300  in three-dimensional space with axes in X, Y, Z direction according to some embodiments of the present disclosure. Referring to  FIG.  5 D , the movable arm portion  314  of the robot arm  300  can be rotated with a variable altitude angle θ with respect to the X-Y plane and/or a variable azimuth angle φ with about the Z direction (illustrated with respect to the X-Z plane). For example, the azimuth angle φ can be in a range of about 0 degree to about 360 degrees. For example, the altitude angle θ can be in a range of about 0 degree to about 360 degrees, or about 0 degree to about 180 degrees. Through the configuration, the robot arm  300  can achieve two-degree-of-freedom for 360-degree cleaning. By adjusting the azimuth angle φ and/or altitude angle θ, as viewed from top, the front end  314 E 1  of the movable arm portion  314  is moved or rotated with respect to the back end  314 E 2  of the movable arm portion  314 . 
     In some embodiments, when cleaning the surface of the collector  230 , the front end  314 E 1  of the movable arm portion  314  may move to a position between the point C 1  (e.g., X-Y plane at point C 1 ) and the collector  230 . For example, when cleaning the surface of the collector  230 , the altitude angle θ of the movable arm portion  314  with respect to the extending arm portion  312  may be in a range from about 0 degree to about 180 degrees. Through the configuration, the front end  314 E 1  of the movable arm portion  314  may be close to the collector  230 , thereby easily vacuuming debris on the surface of the collector  230 . 
     Referring back to  FIGS.  5 A- 5 C , in some embodiments, the robot arm  300  may include a drive assembly  318  coupled to the joint  316  of the robot arm  300 , the drive assembly  318  may have a plurality of drive elements (e.g., rotational motors) for rotation. In some embodiments, the drive assembly  318  may include at least two drive elements (e.g., rotational motors) coupled to the robot arm  300  for providing at least two degrees of freedom to the movable arm portion  314 . As discussed above, the two degrees of freedom of movement may include the rotation of the movable arm portion  314  about the Z direction (e.g., having variable azimuth angle φ with respect to the X-Z plane in  FIG.  5 D ) and the rotation of the movable arm portion  314  with respect to the X-Y plane (e.g., having variable altitude angle θ with respect to the X-Y plane in  FIG.  5 D ). The drive assembly  318  may be electrically connected to a robot controller  320 , which may control the rotation of the joint  316  to move the movable arm portion  314 . 
     In some alternative embodiments, the drive assembly  318  may include one drive element (e.g., a rotational motor) coupled to the robot arm  300  for providing one degree of freedom to the movable arm portion  314 . In some examples, the one degree of freedom of movement may include the rotation of the movable arm portion  314  about the Z direction (e.g., having variable azimuth angle φ with respect to the X-Z plane in  FIG.  5 D ). In some alternative examples, the one degree of freedom of movement may include the rotation of the movable arm portion  314  with respect to the X-Y plane (e.g., having variable altitude angle θ with respect to the X-Y plane in  FIG.  5 D ). 
     In still some alternative embodiments, the drive assembly  318  may include more than two drive elements (e.g., rotational motors) coupled to the robot arm  300  for providing more than two degrees of freedom to the movable arm portion  314 . For example, the drive assembly  318  may include three drive elements (e.g., rotational motors) for providing three degrees of freedom to the movable arm portion  314 . Four or five degrees of freedom may also be applicable for the movement of the movable arm portion  314  in some embodiments. In the present embodiments, the drive elements of drive assembly  318  are illustrated at an external portion of the robot arm  300 . In some other embodiments, one or more drive elements of drive assembly  318  may be directly mounted on the corresponding joint  316  of the robot arm  300  and coupled to the joint  316  for allowing the movement of the movable arm portion  314 . 
     The robot controller  320  may be a part of an overall EUV radiation source  200 , of which the robot arm  300  is a part. The robot controller  320  may include electronic memory and one or more electronic processors configured to execute programming instructions stored in the electronic memory, which may involve a program controlling the rotation of the joint  316  and the movement of the robot arm  300 . In some embodiments, the robot controller  320  may include processors, central processing units (CPU), multi-processors, distributed processing systems, application specific integrated circuits (ASIC), or the like. 
     Referring to  FIG.  5 C , in some further embodiments, the robot arm  300  may optionally hold a detector  510 . Also, by using the detector  510 , the method  100  may proceed to the step  120 , where a condition of the lower cone structure  260  (referring to  FIGS.  5 A and  5 B ) is detected. The detector  510  may be used for detecting a tin contamination on the lower cone structure  260  (referring to  FIGS.  5 A and  5 B ) in images or videos. For example, the detector  510  can be an image detector (e.g., camera) including plural image sensors. In some embodiments, a detector  510  may be mounted on the movable arm portion  314  of the robot arm  300  by suitable fixing elements (e.g., clamps). Through the configuration, by rotating the movable arm portion  314  of the robot arm, the detector  510  may be moved to suitable position to inspect the lower cone structure  260 . The images or videos may be used as a reference to understand the extent of tin contamination in the vessel  210 . 
     In some embodiments, when detecting a condition of tin contamination on the lower cone structure, the movable arm portion  314  holding the detector  510  can be rotated with the azimuth angle φ (referring to  FIG.  5 D ) in a range of about 0 degree to about 360 degrees, and the altitude angle θ (referring to  FIG.  5 D ) in a range of about 0 degree to about 360 degrees. In some embodiments, when detecting a condition of tin contamination on the lower cone structure, the front end  314 E 1  of the movable arm portion  314  (or the detector  510 ) may move to a position between the point C 1  (e.g., X-Y plane at point C 1 ) and the exit aperture  210 O. That is, in these embodiments, the front end  314 E 1  of the movable arm portion  314  (or the detector  510 ) may be far away from the collector  230  when detecting a condition of tin contamination on the lower cone structure, not close to the collector  230  as cleaning the surface of the collector  230 . In some other embodiments, when detecting a condition of tin contamination on the lower cone structure, the front end  314 E 1  of the movable arm portion  314  (or the detector  510 ) may move to a position between the point C 1  (e.g., X-Y plane at point C 1 ) and the collector  230 . That is, in these embodiments, the front end  314 E 1  of the movable arm portion  314  (or the detector  510 ) may be close to the collector  230  as detecting a condition of tin contamination on the lower cone structure. 
     In some embodiments, the front end  314 E 1  of the movable arm portion  314  can be moved from a first position in the vessel  210  to a second position in the vessel  210 , in which the second position is different from the first position. For example, the azimuth angle φ (referring to  FIG.  5 D ) of the movable arm portion  314  with the front end  314 E 1  at the first position is different from that of the movable arm portion  314  with the front end  314 E 1  at of the second position. Alternatively, for example, the altitude angle θ (referring to  FIG.  5 D ) of the movable arm portion  314  with the front end  314 E 1  at the first position is different from that of the movable arm portion  314  with the front end  314 E 1  at of the second position. The detector  510  at the front end  314 E 1  of the movable arm portion  314  may capture a first image of the lower cone structure  260  at the first position in the vessel  210 , and capture a second image of the lower cone structure  260  at the second position in the vessel  210 . The first and second images of the lower cone structure  260  may show more detail information about the condition of tin contamination on the lower cone structure  260 . 
     In the present embodiments, the extraction tube  412  and the detector  510  may be mounted on the same movable arm portion  314  of the robot arm  300 . In some other embodiments, the extraction tube  412  and the detector  510  may be mounted on the different portions of the robot arm  300 . In some embodiments, the detection of the lower cone structure  260  (i.e., the step  120  in  FIG.  1 B ) may be performed before, during, or after the in-line cleaning process (i.e., the step  118  in  FIG.  1 B ). In some embodiments, the detection of the lower cone structure  260  (i.e., the step  120  in  FIG.  1 B ) may be omitted. 
     In some embodiments, the radiation source  200  may include a supporting structure SE for supporting the collector  230 . The supporting structure SE may include a rigid frame or box accommodating some auxiliary modules, such as a cooling module for controlling a temperature of laser source  220 . In some embodiments, during the in-line cleaning process and/or detecting a condition of the lower cone structure  260  (referring to  FIGS.  5 A and  5 B ), the collector  230  is kept supported by the supporting structure SE. In other word, the collector  230  is not moved out of EUV radiation source  200  during the in-line cleaning process and/or detecting a condition of the lower cone structure  260  (referring to  FIGS.  5 A and  5 B ). 
     Referring back to  FIG.  5 A , in some embodiments, the extending arm portion  312  has a first arm segment  312   a  in the vessel  210  and a second arm segment  312   b  external to the vessel  210 . In some embodiments, the first arm segment  312   a  is substantially parallel with the X direction, and the second arm segment  312   b  extends along a longitudinal direction of the bellow assembly BW. In some embodiments, for fitting the configuration of the bellow assembly BW and the vessel  210 , a longitudinal direction of the first arm segment  312   a  may tilt with respect to a longitudinal direction of the second arm segment  312   b . In some further embodiments, the first arm segment  312   a  may be moved or rotated with respect to the second arm segment  312   b , such that an angle between the longitudinal directions of the first arm segment  312   a  and the second arm segment  312   b  can be adjusted for fitting the configuration of the bellow assembly BW and the vessel  210 . In some other embodiments, the angle between the longitudinal directions of the first arm segment  312   a  and the second arm segment  312   b  can be fixed and not adjustable. 
     In some embodiments, prior to inserting the robot arm  300  into the vessel  210  through the bellow assembly BW, the front end  314 E 1  of the movable arm portion  314  may be rotated to a suitable position, thereby shrinking a size of a cross-section of the robot arm  300 . Through the configuration, the robot arm  300  can be moved into the vessel  210  through the bellow assembly BW, without damaging the port  210 G or the bellow assembly BW. For example, the movable arm portion  314  may be substantially aligned with the extending arm portion  312 , or rotated to have a small angle with the extending arm portion  312 , such that the robot arm  300  can be easily moved through the port  210 G and the bellow assembly BW. 
     Referring to  FIGS.  1 B and  6   , the method  100  proceeds to the step  122 , where the robot arm  300  is drawn out from the vessel  210  and the bellow assembly BW. For example, the port  212 G is free of the robot arm  300 . In some embodiments, prior to drawing the robot arm  300  out of the vessel  210  and the bellow assembly BW, the front end  314 E 1  of the movable arm portion  314  may be rotated to a suitable position, thereby shrinking a size of a cross-section of the robot arm  300 . Through the configuration, he robot arm  300  can be moved out from the vessel  210  and the bellow assembly BW, without damaging the port  210 G or the bellow assembly BW. For example, the movable arm portion  314  be substantially aligned with the extending arm portion  312 , or rotated to have a small angle with the extending arm portion  312 , such that the robot arm  300  can be easily moved through the port  210 G and the bellow assembly BW. 
     Referring to  FIGS.  1 B and  7   , the method  100  proceeds to the step  124 , where a droplet generator  240 ′ is assembled onto the port  210 G. In some embodiments, the droplet generator  240 ′ may be different from the droplet generator  240  of  FIG.  2   . For example, in some embodiments, the nozzle  244  of the droplet generator  240  of  FIG.  2    is clogged, the nozzle  244  of the droplet generator  240 ′ is free of being clogged. Subsequently, the EUV radiation source  200  may emit EUV light EL as illustrated in  FIG.  3 A  by repeating steps  104  and  106 . 
     In some embodiments of the present disclosure, an in-line cleaning process is performed to clean the surface of the collector  230  for extending the lifetime of the collector. The in-line cleaning process can be performed without moving the collector  230  out of the vessel  210 , thereby saving swap cost and gaining lots of tool time for productivity. For example, during the steps  112 - 124  of the methods  100 , the collector  230  is kept supported by the supporting structure SE ( FIG.  5 C ), and not moved out of EUV radiation source  200 . 
     In the present embodiments, the cleaning process to the collector  230  is illustrated as in-line cleaning process, which is performed during replacing the droplet generator  240  with a new one. In some alternative embodiments, the cleaning process to the collector  230  may be performed alone, not along with the maintenance process for replacing the droplet generator  240  with a new one. For example, in  FIG.  7   , a used droplet generator  240  (as illustrated in  FIG.  2   ) may be assembled back to the port  210 G after cleaning the collector  230 . Also, a method for detecting a condition of the lower cone structure  260  using the detector  510  may be performed alone, not along with the maintenance process for replacing the droplet generator  240  with a new one. For example, in  FIG.  7   , a used droplet generator  240  (as illustrated in  FIG.  2   ) may be assembled back to the port  210 G after detecting a condition of the lower cone structure  260 . 
     Reference is made back to  FIGS.  2 - 3 A . In some embodiments, the radiation source  200  may include an intermediate focus (IF)-cap module  290  configured to provide intermediate focus to the EUV radiation EL. The collector  230  may focus the EUV light EL generated by the plasma toward the IF-cap module  290 . The IF-cap module  290  is located between the EUV radiation source vessel  210  and the scanner (i.e., the lithography system  100 ) including optical elements configured to direct the EUV light EL to a workpiece (e.g., a semiconductor substrate). In some embodiments, the IF-cap module  290  may comprise a cone shaped aperture configured to provide for separation of pressures between the EUV radiation source vessel  210  and the scanner (i.e., the lithography system  100 ). In some embodiments, the IF-cap module  290  may extend into the scanner (i.e., the lithography system  100 ). The radiation source  200  may include a horizontal obscuration bar OB designed and configured to obscure the laser beam LB, thereby preventing the laser beam LB emits out of the vessel  210  through the exit aperture  210 O. The horizontal obscuration bar OB may have an end fixed to a lower side of the cone structure  260 . 
     In some embodiments, the cover  212  is made of a suitable solid material, such as stainless steel. The cover  212  of the vessel  210  may collects debris. For example, the cover  212  may include a plurality of vanes  212   a , which are spaced around the cone-shaped cover  212 . In some embodiments, the radiation source  200  further includes a heating unit disposed around part of the cover  212 . The heating unit functions to maintain the temperature inside the cover  212  above a melting point of the debris so that the debris does not solidify on the inner surface of the cover  212 . When the debris vapor comes in contact with the vanes, it may condense into a liquid form and flow into a lower section of the cover  212 . The lower section of the cover  212  may provide holes  212 H (referring to  FIG.  5 C ) for draining the debris liquid out of the cover  212 , for example, to a fuel container  400  (referring to  FIG.  5 B ). The fuel container  400  can collect liquid debris. In some embodiment, the tilt of the EUV radiation source  200  is designed such that the fuel container  400  is at a lower position than positions of the droplet generator  240  and the droplet catcher  250 , which may also facilitate tin collection. 
     In some embodiments, the radiation source  200  further includes a gas flow mechanism, including a gas supply module  270 , an exhaust system  280 , and various pipelines for integrating the gas flow mechanism with the collector  240 . The gas supply module  270  is configured to provide a gas GA into the vessel  210  and particularly into a space proximate the reflective surface of the collector  230 . In some embodiments, the gas GA is hydrogen gas, which has less absorption to the EUV radiation. The gas GA is provided for various protection functions, which include effectively protecting the collector  230  from the contaminations by tin particles. Other suitable gas may be alternatively or additionally used. The gas GA may be introduced into the collector  240  through openings (or gaps) near the output window OW through one or more gas pipelines. 
     In some embodiments, the exhaust system  280  includes one or more exhaust lines  282  and one or more pumps  284 . The exhaust line  282  may be connected to the wall of the vessel  210  for receiving the exhaust. In some embodiments, the cover  212  is designed to have a cone shape with its wide base integrated with the collector  240  and its narrow top section facing the illuminator  910 . To further these embodiments, the exhaust line  282  is connected to the cover  212  at its top section. The pump  284  draws airflow from the vessel  210  into the exhaust line  282  for effectively pumping out the gas GA. The gas GA may also function to carry some debris away from the collector  230  and the cover  212  and into the exhaust system  280 . In some embodiments, the exhaust system  280  may include a gas outlet structure  286  disposed at the entrance of the exhaust line  282 . The gas outlet structure  286  may be a scrubber, which may scrub gas vapors or dilute the exiting gas before the gas is released out of the vessel  210 . 
     In some embodiments, the radiation source  200  may include plural fuel receiving elements FR on inner sidewalls of the vessel  210 . For example, a first portion FR 1  of the fuel receiving elements FR near the cover  212  may include the vanes (e.g., the vanes  212   a  in  FIG.  5 C ), a second portion FR 2  of the fuel receiving elements FR near the gas outlet structure  286  may include plural vanes, a third portion FR 3  of the fuel receiving elements FR near the lower cone structure  260  may include plural vanes, and a fourth portion FR 4  of the fuel receiving elements FR near the IF-cap module  290  may include suitable coating layers. The shapes and densities of the vanes of the first to third portions FR 1 -FR 3  may be different from each other. For example, a density of the vanes of the second portion FR 2  is greater than a density of the vanes of the first portion FR 1  (e.g., the vanes  212   a  in  FIG.  5 C ). In some embodiments, the vanes of the third portion FR 3  may be spiral that laterally guides the liquid debris. The receiving elements FR may receive the liquid debris and direct the liquid debris to flow into the fuel container  400  (referring to  FIG.  5 B ). 
       FIG.  8    is schematic view of a robot arm  300  holding an extraction tube  412  according to some embodiments of the present disclosure. In some embodiments, the arm portion  312  and/or  314  of the robot arm  300  includes a cavity  300 O, and the extraction tube  412  may be disposed in the cavity  300 O. The cavity  300 O is illustrated as a trench in the present embodiments. In some other embodiments, the cavity  300 O may be a space enclosed by solid walls of the robot arm  300 , and the solid walls of the robot arm  300  may surround the extraction tube  412 . The detector  510  may be located on an outer lateral side of the arm portion  312  and/or  314  of the robot arm  300 , and having an image receiving surface  510 S facing upward, thereby detecting a condition of the lower cone structure  260  (referring to  FIGS.  5 A and  5 B ). For example, the detector  510  may be located on the arm portion  312  and/or  314  of the robot arm  300 . In some embodiments, the extraction tube  412  and the detector  510  may be fixed to the arm portion  312  and/or  314  of the robot arm  300  by suitable fixing elements (e.g., clamps). In some embodiments, one of the extraction tube  412  and the detector  510  may be omitted. 
       FIG.  9    is schematic view of a robot arm  300  holding an extraction tube  412  according to some embodiments of the present disclosure. The present embodiments are similar to the embodiments of  FIG.  8   , except that the extraction tube  412  and the detector  510  may be located on outer side(s) of the arm portion  312  and/or  314  of the robot arm  300 . For example, the extraction tube  412  and the detector  510  may be respectively located on opposite outer lateral sides of the arm portion  312  and/or  314  of the robot arm  300 . Alternatively, the extraction tube  412  and the detector  510  may be respectively located on an outer lateral side and an outer top side of the arm portion  312  and/or  314  of the robot arm  300 . In the present embodiments, the arm portion  312  and/or  314  of the robot arm  300  may be free of the cavity  300 O shown in  FIG.  8   . In some alternative embodiments, the arm portion  312  and/or  314  of the robot arm  300  may has the cavity  300 O shown in  FIG.  8   . In some embodiments, one of the extraction tube  412  and the detector  510  may be omitted. 
       FIG.  10    is a schematic view of a portion of an EUV radiation source  200  and a robot arm  300  according to some embodiments of the present disclosure. The present embodiments are similar to the embodiments of  FIG.  5 C , except that the detector  510  may be omitted. In the present embodiments, the step  120  in  FIG.  1 B  may be omitted. Other details of the present embodiments are similar to those illustrated in  FIG.  5 C , and therefore not repeated herein. 
       FIG.  11    is a schematic view of a portion of an EUV radiation source  200  and a robot arm  300  according to some embodiments of the present disclosure. The present embodiments are similar to those of  FIG.  5 C , except that the extraction tube  412  and the detector  510  may be mounted on two different movable arm portions  314  of the robot arm  300 . The two movable arm portions  314  of the robot arm  300  can be independently moved or rotated, such that the opening  412 O of the extraction tube  412  can be moved to a first position to clean the collector  230 , and the detector  510  can be moved to second position different from the first position to inspect the lower cone structure  260 . Other details of the present embodiments are similar to those illustrated in  FIG.  5 C , and therefore not repeated herein. 
       FIG.  12    is a schematic view of a portion of an EUV radiation source  200  and a robot arm  300  according to some embodiments of the present disclosure. The present embodiments are similar to those of  FIG.  5 C , except that the cleaning device  410  (e.g., the extraction tube  412  and pump  414 ) may be omitted. In the present embodiments, the step  118  (i.e., the in-line cleaning process) in  FIG.  1 B  may be omitted. Other details of the present embodiments are similar to those illustrated in  FIG.  5 C , and therefore not repeated herein. 
       FIGS.  13 - 14    illustrates a method for using an EUV radiation source at different stages according to some embodiments of the present disclosure. The present embodiments are similar to the embodiments of  FIGS.  2 - 7   , except that in the present embodiments, the robot arm  300  is inserted into the port  210 G when the bellow assembly BW is tuned to have a short length compared to the length of the bellow assembly BW that holds a droplet generator therein. In some embodiments, the bellow assembly BW is a flexible tube having a first end and a second end that can be moved relative to the first end to vary the length thereof. For example, the bellow assembly BW may have a motor to move the second end of the bellow assembly BW relative to the first end of the bellow assembly BW. 
     Referring to  FIG.  13   , after disassembling the droplet generator  240  from the port  210 G of the vessel  210  (referring to  FIG.  4   ), the bellow assembly BW is tuned to have a short length. For example, the bellow assembly BW is folded, thereby reducing a length thereof. In the present embodiments, the short length of the folded bellow assembly BW is less than the length of the bellow assembly BW that holds a droplet generator therein (as shown in  FIG.  7   ). 
     Subsequently, referring to  FIG.  14   , the robot arm  300  is inserted into the port  210 G through the folded bellow assembly BW. In the present embodiments, by folding the bellow assembly BW, the first arm segment  312   a  and the second arm segment  312   b  can be designed without fitting the configuration of the bellow assembly BW and the vessel  210 . For example, a longitudinal direction of the first arm segment  312   a  may be parallel with or tilting with respect to a longitudinal direction of the second arm segment  312   b . In some embodiments, the angle between the longitudinal directions of the first arm segment  312   a  and the second arm segment  312   b  can be fixed and not adjustable. 
     The robot arm  300  may be used in the in-line cleaning process or detecting the condition of the cone structure  260  as illustrated in the embodiments of  FIGS.  2 - 7   . After the in-line cleaning process and/or detecting the condition of the cone structure  260 , the robot arm  300  is drawn out from the vessel  210 , and then the bellow assembly BW may be tuned to have a long length (as the length in  FIG.  6   ). For example, the bellow assembly BW is unfolded, thereby increasing a length thereof. A droplet generator (as shown in  FIG.  7   ) may then be mounted onto the unfolded bellow assembly BW having the long length. Other details of the present embodiments are similar to those of the embodiments of  FIGS.  2 - 7   , and therefore not repeated herein. 
       FIG.  15    is a schematic view of robot arms used to move a droplet generator and move a cleaning device according to some embodiments of the present disclosure. In some embodiments, an exemplary robot arm  600  may be employed to automatedly move the droplet generator  240 . For example, the robot arm  600  may be used to assembling the droplet generator  240  onto the vessel  210  (referring to  FIGS.  2 - 7   ), or/and dissembling the droplet generator  240  from the vessel  210  (referring to  FIGS.  2 - 7   ). 
     In some embodiments, the robot arm  600  includes a rotatable base  610 , a rotatable arm  620 , a rotatable forearm  630 , a rotatable wrist member  640 , a gripper  650  and a robot controller  660 . Rotations of the base  610 , the arm  620 , the forearm  630  and the wrist member  640  are controlled by the robot controller  660  in such a way that the gripper  650  can be moved in a three-dimensional manner. For example, the base  610  is rotatable about an axis A 1 , the arm  620  is connected to the base  610  through a rotational joint or a pivotal joint in such a way that the arm  620  is rotatable about an axis A 2  perpendicular to the axis A 1 . The forearm  630  is connected to the arm  620  through a rotational joint or a pivotal joint in such a way that the forearm  630  is rotatable about an axis A 3  parallel with the axis A 1 . The wrist member  640  is connected to the forearm  630  through a rotational joint or a pivotal joint in such a way that the wrist member  640  is rotatable about an axis A 4  perpendicular to the axes A 1 -A 3 . The gripper  650  is connected to an end of the wrist member  640  farthest from the forearm  630 , so that the gripper  650  can be moved in a three-dimensional manner by using rotational motions performed by the base  610 , the arm  620 , the forearm  630  and the wrist member  640 . 
     As a result, in the maintenance process, the gripper  650  can be moved to grip the droplet generator  240  and then disassemble the droplet generator  240  from the port  212 G of the vessel  210  (referring to  FIGS.  3 A and  4   ). On the other hand, in the maintenance process, the gripper  650  gripping the droplet generator  240 ′ can be moved back to the vessel  210  and then assemble the droplet generator  240 ′ to the port  212 G of the vessel  210  (referring to  FIG.  7   ). 
     Although the embodiments depicted in  FIG.  15    use the robot arm  600  to automatedly move the droplet generator  240  (and/or the droplet generator  240 ′ in  FIG.  7   ), in some other embodiments the droplet generator  240  (and/or the droplet generator  240 ′ in  FIG.  7   ) can be moved by one or more experienced human users, for example, technicians and/or engineers. In such embodiments, the experienced human user may manually hold and move the droplet generator  240  (and/or the droplet generator  240 ′ in  FIG.  7   ) after the droplet generator cools down. 
     In some embodiments, the robot controller  320  (referring to  FIG.  5 A ) and the robot controller  660  are programmed to using the robot arm  600  to disassemble the droplet generator  240  from the vessel  210  (referring to  FIGS.  3 A and  4   ), inserting the robot arm  300  into the vessel  210  (referring to  FIGS.  5 A- 5 C ), cleaning the collector  230  using the robot arm  300  (referring to  FIGS.  5 A- 5 C ), withdrawing the robot arm  300  from the vessel  210  (referring to  FIG.  6   ), and assembling the droplet generator  240 ′ onto the vessel  210  (referring to  FIG.  7   ) in sequence. For example, in some embodiments, the robot controller  320  (referring to  FIGS.  5 A- 5 C ) may control the robot arm  300  to move the cleaning device  410  into the vessel  210  and clean the collector  230 , in response to the droplet generator being moved away from the vessel  210  (referring to  FIGS.  3 A and  4   ) by the robot arm  600  under the control of the robot controller  660 . In some embodiments, the robot arm  600  and the robot arm  300  are independently controlled. In other words, the robot arm  300  is free from control by the robot controller  660 , and the robot arm  600  is free from control by the robot controller  320  (referring to  FIG.  5 A ). 
     The robot controller  660  may be a part of an overall EUV radiation source  200 , of which the robot arm  600  is a part. The robot controller  660  may include electronic memory and one or more electronic processors configured to execute programming instructions stored in the electronic memory, which may involve a program controlling the movement of the robot arm  300 . In some embodiments, the robot controller  660  may include processors, central processing units (CPU), multi-processors, distributed processing systems, application specific integrated circuits (ASIC), or the like. In some embodiments, the robot controller  660 , the robot controller  320  (referring to  FIG.  5 A ), and the controller PC (referring to  FIG.  4   ) are in a same processor. In some other embodiments, the robot controller  660 , the robot controller  320  (referring to  FIG.  5 A ), and the controller PC (referring to  FIG.  4   ) are in different individual processors, respectively. 
       FIG.  16    is a schematic view of a lithography system  900  according to some embodiments of the present disclosure. The lithography system  900  may also be referred to as a scanner that is operable to perform lithography exposing processes with respective radiation source and exposure mode. In some embodiments, the lithography system  900  is an extreme ultraviolet (EUV) lithography system designed to expose a resist layer by EUV light EL. The resist layer is a material sensitive to the EUV light. In some embodiments, the EUV lithography system  900  employs the radiation source  200  to generate EUV light EL. 
     The lithography system  900  also employs an illuminator  910 . In some embodiments, the illuminator  910  includes various reflective optics such as a single mirror or a mirror system having multiple mirrors in order to direct the EUV light EL from the radiation source  200  onto a mask stage  920 , particularly to a mask  930  secured on the mask stage  920 . 
     The lithography system  900  also includes the mask stage  920  configured to secure the mask  930 . In some embodiments, the mask stage  920  includes an electrostatic chuck (e-chuck) used to secure the mask  930 . In this context, the terms mask, photomask, and reticle are used interchangeably. In the present embodiments, the lithography system  900  is an EUV lithography system, and the mask  930  is a reflective mask. One exemplary structure of the mask  930  includes a substrate with a low thermal expansion material (LTEM). For example, the LTEM may include TiO 2  doped SiO2, or other suitable materials with low thermal expansion. The mask  930  includes a reflective multi-layer deposited on the substrate. The reflective multi-layer includes plural film pairs, such as molybdenum-silicon (Mo/Si) film pairs (e.g., a layer of molybdenum above or below a layer of silicon in each film pair). Alternatively, the reflective multi-layer may include molybdenum-beryllium (Mo/Be) film pairs, or other suitable materials that are configurable to highly reflect the EUV light EL. The mask  930  may further include a capping layer, such as ruthenium (Ru), disposed on the reflective multi-layer for protection. The mask  18  further includes an absorption layer, such as a tantalum boron nitride (TaBN) layer, deposited over the reflective multi-layer. The absorption layer is patterned to define a layer of an integrated circuit (IC). The mask  930  may have other structures or configurations in various embodiments. 
     The lithography system  900  also includes a projection optics module (or projection optics box (POB))  940  for imaging the pattern of the mask  930  onto a semiconductor substrate W secured on a substrate stage (or wafer stage)  950  of the lithography system  900 . The POB  940  includes reflective optics in the present embodiments. The light EL that is directed from the mask  930  and carries the image of the pattern defined on the mask  930  is collected by the POB  940 . The illuminator  910  and the POB  940  may be collectively referred to as an optical module of the lithography system  900 . 
     In the present embodiments, the semiconductor substrate W is a semiconductor wafer, such as a silicon wafer or other type of wafer to be patterned. The semiconductor substrate W is coated with a resist layer sensitive to the EUV light EL in the present embodiments. Various components including those described above are integrated together and are operable to perform lithography exposing processes. 
     In the present embodiments, the lithography system  900  further include plural light intensity sensors. For example, a light intensity sensor  962  is installed near the mask stage  920  and a light intensity sensor  964  is installed on the substrate stage  950 . In some embodiments, an image sensor, such as a camera, may be installed on the substrate stage  950 . In some embodiments, as illustrated previously, a controller (e.g., the controller PC) may be electrically connected with the light intensity sensors  962  and  964 . 
     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 an in-line cleaning process is performed to clean the surface of the collector, thereby extending the lifetime of the collector. Another advantage is that the in-line cleaning process for cleaning the surface of the collector can be performed without moving the collector, thereby saving swap cost and gaining lots of tool time for productivity. Still another advantage is that a robot arm holding an image detector may be inserted into the EUV vessel, thereby detecting a condition of a cone structure at a top portion. 
     According to some embodiments of the present disclosure, a method for using an extreme ultraviolet radiation source is provided. The method includes performing a lithography process using an extreme ultraviolet (EUV) radiation source; after the lithography processes, inserting an extraction tube into a vessel of the EUV radiation source; and cleaning a collector of the EUV radiation source by using the extraction tube. 
     According to some embodiments of the present disclosure, a method includes performing a lithography process using an EUV radiation source; after the lithography processes, moving an image sensor into a vessel of the EUV radiation source; and detecting a tin contaminant on a cone structure at a top portion of the vessel by the image sensor. 
     According to some embodiments of the present disclosure, a method includes generating an EUV radiation in a vessel; directing, by using a collector at a bottom side of the vessel, the EUV radiation to an exit aperture of the vessel; measuring a light intensity of the EUV radiation; comparing the light intensity of the EUV radiation with a reference value; and according to a comparison result, providing a vacuuming suction force to a surface of the collector, wherein the collector is at the bottom side of the vessel. 
     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.