Patent Publication Number: US-11644759-B2

Title: Droplet generator and method of servicing extreme ultraviolet radiation source apparatus

Description:
RELATED APPLICATION 
     This application is a Continuation Application of U.S. patent application Ser. No. 16/933,872 filed Jul. 20, 2020, now U.S. Pat. No. 11,029,613, which is a Continuation Application of U.S. patent application Ser. No. 16/404,235 filed May 6, 2019, now U.S. Pat. No. 10,719,020, which claims priority to U.S. Provisional Patent Application No. 62/692,565 filed on Jun. 29, 2018, the entire contents of each of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     As consumer devices have gotten smaller and smaller in response to consumer demand, the individual components of these devices have necessarily decreased in size as well. Semiconductor devices, which make up a major component of devices such as mobile phones, computer tablets, and the like, have been pressured to become smaller and smaller, with a corresponding pressure on the individual devices (e.g., transistors, resistors, capacitors, etc.) within the semiconductor devices to also be reduced in size. The decrease in size of devices has been met with advancements in semiconductor manufacturing techniques such as lithography. 
     For example, the wavelength of radiation used for lithography has decreased from ultraviolet to deep ultraviolet (DUV) and, more recently to extreme ultraviolet (EUV). Further decreases in component size require further improvements in resolution of lithography which are achievable using extreme ultraviolet lithography (EUVL). EUVL employs radiation having a wavelength of about 1-100 nm. 
     As the semiconductor industry has progressed into nanometer technology process nodes in pursuit of higher device density, higher performance, and lower costs, there have been challenges in reducing semiconductor feature size. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is best understood from the following detailed description when read with the accompanying figures. It is emphasized that, in accordance with the standard practice in the industry, various features are not drawn to scale and are used for illustration purposes only. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG.  1    shows an extreme ultraviolet lithography tool according to an embodiment of the disclosure. 
         FIG.  2    shows a schematic diagram of a droplet generator according to an embodiment of the disclosure. 
         FIG.  3    shows a detailed view of a droplet generator according to an embodiment of the disclosure. 
         FIG.  4    shows a detailed view of a droplet generator nozzle according to an embodiment of the disclosure. 
         FIGS.  5 A- 5 B  show schematic diagrams of generating a droplet by a droplet generator according to an embodiment of the disclosure. 
         FIG.  5 C  shows a schematic diagram of a method of cleaning a droplet generator according to an embodiment of the disclosure. 
         FIG.  6    shows a detailed view of a dry ice blasting assembly according to an embodiment of the disclosure. 
         FIG.  7    shows a detailed view of a dry ice blasting assembly according to another embodiment of the disclosure. 
         FIG.  8    shows a flow chart of a method of cleaning a droplet generator of an EUV radiation source apparatus according to an embodiment of the disclosure. 
         FIG.  9    shows a flow chart of a method of cleaning the droplet generator of the EUV radiation source apparatus according to another embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the following disclosure provides many different embodiments, or examples, for implementing different features of the disclosure. Specific embodiments or 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, dimensions of elements are not limited to the disclosed range or values, but may depend upon process conditions and/or desired properties of the device. Moreover, 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 interposing the first and second features, such that the first and second features may not be in direct contact. Various features may be arbitrarily drawn in different scales for simplicity and clarity. 
     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 device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. In addition, the term “made of” may mean either “comprising” or “consisting of.” 
     The present disclosure is generally related to extreme ultraviolet (EUV) lithography systems and methods. More particularly, it is related to extreme ultraviolet lithography (EUVL) tools and methods of servicing the tools. In an EUVL tool, a laser-produced plasma (LPP) generates extreme ultraviolet radiation which is used to image a photoresist coated substrate. In an EUV tool, an excitation laser heats metal (e.g., tin, lithium, etc.) target droplets in the LPP chamber to ionize the droplets to plasma which emits the EUV radiation. For reproducible generation of EUV radiation, the target droplets arriving at the focal point (also referred to herein as the “zone of excitation”) have to be substantially the same size and arrive at the zone of excitation at the same time as an excitation pulse from the excitation laser arrives. Thus, stable generation of target droplets that travel from the target droplet generator  115  to the zone of excitation at a uniform (or predictable) speed contributes to efficiency and stability of the LPP EUV radiation source. Any instability in the generation of target droplets can impact the EUVL tool performance, and in some cases, for example, if the nozzle  120  of the droplet generator  115  is clogged, the tool may have to be shut down to repair (e.g., unclog the nozzle  120 ) the droplet generator  115 . Additionally, when refilling the droplet generator  115 , there is a possibility of oxidation of tin, which can cause clogging of the nozzle  120 . In such cases of a clogged nozzle  120 , the entire droplet generator  115  needs to be changed, causing long downtime for the EUVL tool. Embodiments of the present disclosure provide for an apparatus and methods for cleaning and/or unclogging a droplet generator  115  without removing the droplet generator  115  from the EUVL tool. In other words, the presently disclosed embodiments enable in-line cleaning and/or unclogging of the droplet generator  115 . 
       FIG.  1    is a schematic view of an EUV lithography tool with a laser production plasma (LPP) based EUV radiation source, constructed in accordance with some embodiments of the present disclosure. The EUV lithography system includes an EUV radiation source  100  to generate EUV radiation, an exposure device  200 , such as a scanner, and an excitation laser source  300 . As shown in  FIG.  1   , in some embodiments, the EUV radiation source  100  and the exposure device  200  are installed on a main floor MF of a clean room, while the excitation laser source  300  is installed on a base floor BF located under the main floor. Each of the EUV radiation source  100  and the exposure device  200  are placed over pedestal plates PP 1  and PP 2  via dampers DP 1  and DP 2 , respectively. The EUV radiation source  100  and the exposure device  200  are coupled to each other by a coupling mechanism, which may include a focusing unit. 
     The EUV lithography tool is designed to expose a resist layer by EUV light (also interchangeably referred to herein as EUV radiation). The resist layer is a material sensitive to the EUV light. The EUV lithography system employs the EUV radiation source  100  to generate EUV light, such as EUV light having a wavelength ranging between about 1 nm and about 100 nm. In one particular example, the EUV radiation source  100  generates an EUV light with a wavelength centered at about 13.5 nm. In the present embodiment, the EUV radiation source  100  utilizes a mechanism of laser-produced plasma (LPP) to generate the EUV radiation. 
     The exposure device  200  includes various reflective optic components, such as convex/concave/flat mirrors, a mask holding mechanism including a mask stage, and wafer holding mechanism. The EUV radiation EUV generated by the EUV radiation source  100  is guided by the reflective optical components onto a mask secured on the mask stage. In some embodiments, the mask stage includes an electrostatic chuck (e-chuck) to secure the mask. 
     As used herein, the term “optic” is meant to be broadly construed to include, and not necessarily be limited to, one or more components which reflect and/or transmit and/or operate on incident light, and includes, but is not limited to, one or more lenses, windows, filters, wedges, prisms, grisms, gratings, transmission fibers, etalons, diffusers, homogenizers, detectors and other instrument components, apertures, axicons and mirrors including multi-layer mirrors, near-normal incidence mirrors, grazing incidence mirrors, specular reflectors, diffuse reflectors and combinations thereof. Moreover, unless otherwise specified, neither the term “optic”, as used herein, are meant to be limited to components which operate solely or to advantage within one or more specific wavelength range(s) such as at the EUV output light wavelength, the irradiation laser wavelength, a wavelength suitable for metrology or any other specific wavelength. 
     Because gas molecules absorb EUV light, the lithography system for the EUV lithography patterning is maintained in a vacuum or a low pressure environment to avoid EUV intensity loss. 
     In the present disclosure, the terms mask, photomask, and reticle are used interchangeably. In the present embodiment, the patterning optic is a reflective mask. In an embodiment, the reflective mask includes a substrate with a suitable material, such as a low thermal expansion material or fused quartz. In various examples, the material includes TiO 2  doped SiO 2 , or other suitable materials with low thermal expansion. The reflective mask includes multiple reflective multiple layers (ML) deposited on the substrate. The ML includes one or more 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 ML may include molybdenum-beryllium (Mo/Be) film pairs, or other suitable materials that are configured to highly reflect the EUV light. The mask may further include a capping layer, such as ruthenium (Ru), disposed on the ML for protection. The mask further includes an absorption layer, such as a tantalum boron nitride (TaBN) layer, deposited over the ML. The absorption layer is patterned to define a layer of an integrated circuit (IC). Alternatively, another reflective layer may be deposited over the ML and is patterned to define a layer of an integrated circuit, thereby forming an EUV phase shift mask. 
     The EUVL tool further includes other modules or is integrated with (or coupled with) other modules in some embodiments. 
     As shown in  FIG.  1   , the EUV radiation source  100  includes a target droplet generator  115  and a LPP collector  110 , enclosed by a chamber  105 . In various embodiments, the target droplet generator  115  includes a reservoir  150  (see  FIG.  3   ) to hold a source material and a nozzle  120  through which target droplets DP of the source material are supplied into the chamber  105 . The EUV radiation source  100  may further include a dry ice blasting assembly  1000  that includes a blasting member  1100  and an exhaust member  1200  selectively attachable to and extendable from the chamber  105 .  FIG.  1    illustrates an exemplary configuration of the dry ice blasting assembly  1000 . However, any appropriate configuration such as size, shape, and location with respect to the chamber  105  is contemplated and is not limited in this regard. 
     In some embodiments, the target droplets DP are droplets of tin (Sn), lithium (Li), or an alloy of Sn and Li. In some embodiments, the target droplets DP each have a diameter in a range from about 10 microns (μm) to about 100 μm. For example, in an embodiment, the target droplets DP are tin droplets, having a diameter of about 10 μm to about 100 μm. In other embodiments, the target droplets DP are tin droplets having a diameter of about 25 μm to about 50 μm. In some embodiments, the target droplets DP are supplied through the nozzle  120  at a rate in a range from about 50 droplets per second (i.e., an ejection-frequency of about 50 Hz) to about 50,000 droplets per second (i.e., an ejection-frequency of about 50 kHz). In some embodiments, the target droplets DP are supplied at an ejection-frequency of about 100 Hz to a about 25 kHz. In other embodiments, the target droplets DP are supplied at an ejection frequency of about 500 Hz to about 10 kHz. The target droplets DP are ejected through the nozzle  120  and into a zone of excitation ZE at a speed in a range of about 10 meters per second (m/s) to about 100 m/s in some embodiments. In some embodiments, the target droplets DP have a speed of about 10 m/s to about 75 m/s. In other embodiments, the target droplets have a speed of about 25 m/s to about 50 m/s. 
     Referring back to  FIG.  1   , an excitation laser LR 2  generated by the excitation laser source  300  is a pulse laser. The laser pulses LR 2  are generated by the excitation laser source  300 . The excitation laser source  300  may include a laser generator  310 , laser guide optics  320  and a focusing apparatus  330 . In some embodiments, the laser source  310  includes a carbon dioxide (CO 2 ) or a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser source with a wavelength in the infrared region of the electromagnetic spectrum. For example, the laser source  310  has a wavelength of 9.4 μm or 10.6 μm, in an embodiment. The laser light LR 1  generated by the laser generator  300  is guided by the laser guide optics  320  and focused into the excitation laser LR 2  by the focusing apparatus  330 , and then introduced into the EUV radiation source  100 . 
     In some embodiments, the excitation laser LR 2  includes a pre-heat laser and a main laser. In such embodiments, the pre-heat laser pulse (interchangeably referred to herein as the “pre-pulse) is used to heat (or pre-heat) a given target droplet to create a low-density target plume with multiple smaller droplets, which is subsequently heated (or reheated) by a pulse from the main laser, generating increased emission of EUV light. 
     In various embodiments, the pre-heat laser pulses have a spot size about 100 μm or less, and the main laser pulses have a spot size in a range of about 150 μm to about 300 μm. In some embodiments, the pre-heat laser and the main laser pulses have a pulse-duration in the range from about 10 ns to about 50 ns, and a pulse-frequency in the range from about 1 kHz to about 100 kHz. In various embodiments, the pre-heat laser and the main laser have an average power in the range from about 1 kilowatt (kW) to about 50 kW. The pulse-frequency of the excitation laser LR 2  is matched with the ejection-frequency of the target droplets DP in an embodiment. 
     The laser light LR 2  is directed through windows (or lenses) into the zone of excitation ZE. The windows adopt a suitable material substantially transparent to the laser beams. The generation of the pulse lasers is synchronized with the ejection of the target droplets DP through the nozzle  120 . As the target droplets move through the excitation zone, the pre-pulses heat the target droplets and transform them into low-density target plumes. A delay between the pre-pulse and the main pulse is controlled to allow the target plume to form and to expand to an optimal size and geometry. In various embodiments, the pre-pulse and the main pulse have the same pulse-duration and peak power. When the main pulse heats the target plume, a high-temperature plasma is generated. The plasma emits EUV radiation, which is collected by the collector mirror  110 . The collector  110  further reflects and focuses the EUV radiation for the lithography exposing processes performed through the exposure device  200 . The droplet catcher  125  is used for catching excessive target droplets. For example, some target droplets may be purposely missed by the laser pulses. 
     Referring back to  FIG.  1   , the collector  110  is designed with a proper coating material and shape to function as a mirror for EUV collection, reflection, and focusing. In some embodiments, the collector  110  is designed to have an ellipsoidal geometry. In some embodiments, the coating material of the collector  100  is similar to the reflective multilayer of the EUV mask. In some examples, the coating material of the collector  110  includes a ML (such as one or more Mo/Si film pairs) and may further include a capping layer (such as Ru) coated on the ML to substantially reflect the EUV light. In some embodiments, the collector  110  may further include a grating structure designed to effectively scatter the laser beam directed onto the collector  110 . For example, a silicon nitride layer is coated on the collector  110  and is patterned to have a grating pattern. 
     As shown in  FIG.  1   , in the present embodiment, a buffer gas is supplied from a first buffer gas supply  130  through the aperture in collector  110  by which the pulse laser is delivered to the tin droplets. In some embodiments, the buffer gas is H 2 , He, Ar, N 2  or another inert gas. In certain embodiments, H radicals generated by ionization of the H 2  buffer gas is used for cleaning purposes. The buffer gas can also be provided through one or more second buffer gas supplies  135  toward the collector  110  and/or around the edges of the collector  110 . Further, the chamber  105  includes one or more gas outlets  140  so that the buffer gas is exhausted outside the chamber  105 . 
     Hydrogen gas has low absorption to the EUV radiation. Hydrogen gas reaching the coating surface of the collector  110  reacts chemically with a metal of the droplet forming a hydride, e.g., metal hydride. When tin (Sn) is used as the droplet, stannane (SnH 4 ), which is a gaseous byproduct of the EUV generation process, is formed. The gaseous SnH 4  is then pumped out through the outlet  140 . 
       FIG.  2    illustrates the components of the droplet generator  115  in schematic format. As shown there, the droplet generator  115  includes a reservoir  150  holding a fluid  145 , e.g. molten tin, under pressure P. The reservoir  150  is formed with an orifice  155  allowing the pressurized fluid  145  to flow through the orifice  155  establishing a continuous stream which subsequently breaks into one or more droplets DP 1 , DP 2  exiting the nozzle  120 . 
     The target droplet generator  115  shown further includes a sub-system producing a disturbance in the fluid  145  having an electro-actuatable element  160  that is operably coupled with the fluid  145  and a signal generator  165  driving the electro-actuatable element  160  in some embodiments. In some embodiments, the electro-actuatable element  160  is a piezoelectric actuator that applies vibration to the fluid  145 . In some embodiments, the electro-actuatable element  160  is an ultrasonic transducer or a megasonic transducer. 
     A detailed cross section view of the droplet generator  115  according to an embodiment is shown in  FIG.  3   . The droplet generator  115  includes a reservoir  150  containing the molten metal  145  and nozzle  120  at the end of the reservoir  150 . 
     In some embodiments, the nozzle  120  is maintained at a certain temperature that is higher than the melting point of the source material. However, under certain conditions such as, for example, if the chamber  105  is vented for a service or if there is an unscheduled change in temperature of the chamber  105 , temperature of the nozzle  120  may be reduced to below the melting point of the source material, e.g., tin. When the nozzle  120  cools down, liquid source material may leak through the nozzle  120  because of particulate formation at the nozzle  120 . The leaked source material may be deposited on the collector  110  resulting in a reduction in the reflectivity of the collector  110 . This in turn results in the loss of stability and efficiency of the EUV radiation source  100 . In some cases, replacement of the collector  110  may be required, leading to unnecessary and avoidable expense as well as down-time for the entire lithography system. 
     In addition, if the chamber  105  is vented the molten source material may react with oxygen in the ambient resulting in the formation of metal oxide particulate contamination. For example, molten tin may react with oxygen forming tin oxide solid particles. The tin oxide particles can coat optical surfaces in the EUVL tool. The metal oxide particles may also clog the nozzle  120  interfering with subsequent droplet flow when the EUVL tool is restarted. 
       FIG.  4    shows a detailed view of a droplet generator nozzle  120  according to an embodiment of the disclosure. The outer body  190  of the nozzle  120  is made of a metal, such as titanium or stainless steel in some embodiments. The tip  195  of the nozzle  120 , where the droplets DP are generated, is constituted by a strong, non-fragile, material in some embodiments, for example a metal (e.g., titanium), a ceramic, silicon or a silicon based compound, such as silicon nitride. The tip  195  of the nozzle  120  is made of a material that can withstand the temperatures required to maintain the target metal in the molten state and not react with molten target metal  1020 . In some embodiments, the tip  195  of the nozzle  120  is made of silicon coated with silicon nitride. Such a tip  195  of the nozzle  120  is able to withstand high pressures within the nozzle  120 , and therefore, high gas pressures can be used to force the molten metal through the nozzle  120 . 
     An isolation valve  185  is located at the end of the nozzle  120 . The isolation valve  185  is open during operation of the droplet generator  115 . When maintenance or servicing of the radiation source  100  is required, the isolation valve  185  closes to seal the nozzle  120 . The chamber  105  of the EUV radiation source  100  is maintained under vacuum or low pressure during operation of the EUVL tool. Because EUV light is absorbed by most materials, including gases, it is necessary to operate the EUV tool under low pressure or vacuum to prevent loss of exposure light energy during imaging operations. 
     The vacuum chamber  105  may be opened when it is necessary to perform maintenance or service the EUVL tool. Exposing the vacuum chamber  105  to the ambient atmosphere introduces oxygen, which readily reacts with heated metals to form metal oxides. For example, the oxygen may react with molten tin in the nozzle  120  of the droplet generator  115  to form tin oxides, such as stannous oxide (SnO) and stannic oxide (SnO 2 ). In some embodiments, the molten tin is maintained at a temperature of about 250° C. At this temperature tin oxides are solid. Thus, any tin oxides that would form would precipitate out of the molten tin. When such cleaning is required, and in particular if the molten tin results in clogging of the droplet generator nozzle  120 , the droplet generator  115  may need to be removed from the EUVL tool. This can cause undesirable long downtimes. 
       FIG.  5 A  shows an exemplary stable generation of target droplets DP 1 , DP 2 . The target droplets travel from the target droplet generator  115  in the EUV radiation source  100  to the zone of excitation at a uniform (or predictable) speed that contributes to an efficiency and stability of the EUV radiation source  100 . The target droplets DP 1 , DP 2  are substantially the same size. As shown in  FIG.  5 B , the nozzle  120  of the droplet generator  115  may be clogged by a residual material  1022 , for example, molten target metal  1020 . The target droplets DP 1 , DP 2  are not the same size. In some embodiments, although the target droplets DP 1 , DP 2  are substantially the same size, the target droplets DP 1 , DP 2  may not arrive at the zone of excitation at a desired timing due to the residual material  1022 . 
     Referring to  FIG.  5 C , the EUV radiation source apparatus  100  according to the present disclosure includes a dry ice blasting assembly  1000  selectively attachable to and extendable from the chamber  105 . The dry ice blasting assembly  1000  includes the blasting member  1100  and the exhaust member  1200  selectively attachable to and extendable from the chamber  105 . The blasting member  1100  is configured to direct pressurized dry ice (CO 2 ) particles  1080  to the droplet generator  115  to clean the nozzle  120  and to remove the residual material  1022 . The exhaust member  1200  collects the residual material  1022  separated from the nozzle  120  of the droplet generator  115  and gaseous carbon dioxide  1088  sublimated from the solid dry ice particles  1082  through an exhaust line  1220 . 
     With respect to  FIG.  6   , in some embodiments, the dry ice blasting assembly  1000  includes a blasting air inlet  1120 , a blaster carbon dioxide (CO 2 ) inlet  1130 , a blasting mixer  1140  and a blasting nozzle  1150 . In a particular embodiment, the blasting nozzle  1150  further includes a pulsation insert  1160  and a directional insert  1170 . The pulsation insert  1160  is configured to generate a pulsation/oscillation of the pressurized air stream  1084  by inserting a mechanical device into the blasting nozzle  1150 . The directional insert  1170  is configured to change a two-dimensional direction and/or three-dimensional rotation of the pressurized air stream  1084  by inserting a mechanical device into the blasting nozzle  1150 . 
     Some embodiments of the dry ice blasting assembly  1000  further include an extendable positioner  1180 . The extendable positioner  1180  “pops-up” from the chamber  105  when needed and is substantially concealed within the chamber  105  when not in use. A controller  1500  selectively enables a telescopingly extendable portion  1182  of the extendable positioner  1180  in some embodiments. The telescopingly extendable portion  1182  includes a cylindrical body  1184  that is coaxially slideably received within the chamber  105  and has an inwardly projecting annular flange  1186  which bears against any appropriate type of sealing. 
     In certain embodiments, the extendable positioner  1180  is a 3-axis rotational device, and when it rotates in a direction, the blasting nozzle  1150  attached to the extendable positioner  1180  is moved to a cleaning position. 
     In some embodiments, the controller  1500  is configured to monitor residual material  1022  on the droplet generator by a monitoring device  1520 , adjust valves of the blasting pump when an amount of residual material  1022  in the droplet generator is more than a threshold amount, and regulate ejecting parameters of the dry ice particles by operating the blasting compressor and the blasting pump when the pressurized dry ice particles are ejected from the blasting nozzle. In some embodiments, the monitoring device is a camera. In some embodiments, the ejection of the pressurized dry ice particles from the blasting nozzle is stopped when the monitoring device detects the amount of the residual material on the droplet generator is below the threshold amount. Any appropriate controlling configuration regarding automatic and/or manual operation is contemplated and is not limited in this regard. 
     The cleaning position of the dry ice blasting assembly with respect to the nozzle  120  of the droplet generator  115  is programmed by the controller  1500  according to different cleaning modes. For example, the cleaning position may be programmed in a horizontal configuration of the chamber  105 . After positioning the blasting nozzle  1150  to the cleaning position (the horizontal configuration of the chamber  105 ), the extendable positioner  1180  stops moving. The dry ice particles  1082  then clean the droplet generator  115  until the end of a cleaning time  1070 . 
     As shown in  FIG.  7   , the dry ice blasting assembly  1000  further includes a supporting member  1300  that includes a blasting compressor  1320  and a blasting pump  1340 . The blasting compressor  1320  compresses a liquid form of carbon dioxide from the blaster carbon dioxide (CO 2 ) inlet  1130  into the solid dry ice particles  1082 , and pressurizes air taken in from the blasting air inlet  1120 . In some embodiments, the inlets  1120 ,  1130  and the blasting compressor  1320  are located outside of the chamber  105 . In some embodiments, the blasting compressor  1320  supplies liquefied nitrogen (LN 2 ) with a pressure of about 2,000 kPa to about 50,000 kPa to the dry ice transport port  1360 . In certain embodiments, the blasting compressor  1320  generates the dry ice particles  1082  with a density of about 1,000 g/cm3 to about 200,000 g/cm3. In such configuration, the dry ice particles  1082  impact the nozzle  120  of the droplet generator  115  at a pressure of range of about 1 kPa to about 1000 kPa, and clean the droplet generator  115 . In some embodiments, an ultrasonic generator may be used with the dry ice particles  1082  in a frequency of about 20 kHz to about 20 MHz. 
     In some embodiments, the dry ice blasting assembly  1000  further includes the blasting pump  1340  for mixing dry ice particles  1082  and the pressurized air stream  1084 , and for pressurizing the mixture  1086 . The pressurized dry ice particles  1080  are ejected from the dry ice blasting assembly  1000 , i.e., the blasting nozzle  1150 , and directed at the droplet generator nozzle  120 . In some embodiments, the flow rate of the pressurized air stream  1084  for the pressurized dry ice particles  1080  is in a range from about 0.5 liters per minute to about 500 liters per minute. Such pressurized dry ice particles  1080  impact the nozzle  120  of the droplet generator  115  at a pressure in a range from about 1 kPa to about 1000 kPa depending on the diameter of the blasting nozzle  1150 , the flow rate of the pressurized air stream  1084  for the pressurized dry ice particles  1080 , and a distance between the blasting nozzle  1150  and/or the droplet generator nozzle  120 . 
     In some embodiments, the blasting nozzle  1150  has a diameter in a range from about 10 μm to about 10 mm for dispensing pressurized dry ice particles  1080 , including the dry ice particles  1082  and the pressurized air stream  1084 . 
     The impact of the dry ice particles  1082  particles creates microscopic shock waves  1090  that help breakdown the tin oxide particulate clogging the droplet generator nozzle  120  in some embodiments. Additionally, because the dry ice particles  1082  immediately sublimate from solid to gas as it impacts the droplet generator nozzle  120 , additional solid particulate waste is not left behind. The gaseous carbon dioxide  1088  is relatively inert and does not react with any of the material of other components in the chamber  105 . Moreover, gaseous carbon dioxide  1088  can be easily removed from the chamber  105  along with the air as the chamber  105  is being evacuated. 
     An embodiment of the present disclosure provides a droplet generator  115  cleaning method, including: providing a selectively attachable dry ice blasting assembly  1000 . Ejecting the pressured dry ice particles  1080  toward the nozzle  120  of the droplet generator  115  at the pressure of range of about 1 kPa to about 1000 kPa during a cleaning operation wherein the pressured dry ice particles  1080  impact the nozzle  120  of the droplet generator  115 . At a pressure in the range of about 1 kPa to about 1000 kPa, the impact of the dry ice particles do not cause damage to the nozzle  120  of the droplet generator  115 , the temperature of the residual material  1022  adhered to the nozzle  120  impacted by the dry ice particles is rapidly lowered, the residual material  1022  is embrittled, and a crack is formed between the residual material  1022  and the droplet generator  115 . Next, a portion of the dry ice particles  1082  which subsequently impact the droplet generator  115  enters the crack  1024 , and the dry ice particles  1082  then rapidly sublimate into gas. Because the volume of the gaseous carbon dioxide  1088  is greater than an original volume of the dry ice particles  1082 , the gaseous carbon dioxide  1088  further enlarges the crack  1024  and reduces adhesion of the residual material  1022 . In some embodiments, as additional dry ice particles  1082  further impact the residual material, the residual material  1022  is separated from the nozzle  120  of the droplet generator  115 , thereby cleaning the nozzle  120  of the droplet generator  115 . 
     By using the foregoing method, the residual material  1022  clogging the nozzle can be effectively removed. In addition, the dry ice particles  1082  are not corrosive, and thus, would not corrode the droplet generator  115 . Moreover, the dry ice particles  1082  will rapidly sublimate into carbon dioxide gas after impact, and thus, a contamination medium will not be generated. Thus, without contamination or damaging the nozzle  120  of the droplet generator  115 , the droplet generator  115  is cleaned and maintenance and servicing time and/or cost are reduced in embodiments of the disclosure. 
     An exemplary cleaning procedure according to embodiments of the disclosure is as follows: firstly, the blasting air inlet  1120  transports the compressed air into the blasting mixer  1140 , and due to the compressed air, the liquid carbon dioxide is converted into the dry ice particles  1082 . The blasting mixer  1140  transports the dry ice particles  1082  via the dry ice transport port  1360  to the blasting nozzle  1150 . The blasting nozzle  1150  directs the pressured dry ice particles  1080  to the nozzle  120  of the droplet generator  115  at a pressure in the range of about 1 kPa to about 1000 kPa and a flow rate of the pressurized air stream  1084  for the pressurized dry ice particles  1080  in a range from about 0.5 liters per minute to about 500 liters per minute, to clean the droplet generator  115 . In this cleaning procedure, the blasting member  1100  directs the pressurized dry ice particles  1080  to the nozzle  120  of the droplet generator  115  and the microscopic shock waves are generated by the dry ice particles  1082  causing the residual material  1022  to be removed from the nozzle  120  of the droplet generator  115 . The exhaust member  1200  collects the residual material  1022  separated from the nozzle  120  of the droplet generator  115  and the gaseous carbon dioxide  1088  sublimated from the dry ice particles  1082  through an exhaust line  1220 . Thereby, the cleaning effect is further enhanced by the microscopic shock waves  1090 , and contamination inside the chamber  105  is reduced in some embodiments. 
     An embodiment of the disclosure, as shown in  FIG.  8    of a flow chart, is a method S 100  of cleaning an extreme ultra violet (EUV) radiation source apparatus. In operation S 110 , pressurized dry ice particles including dry ice particles and the pressurized air stream from the dry ice supporting member of the dry ice blasting assembly are formed. In operation S 120 , the pressurized dry ice particles are ejected through the blasting nozzle toward residual material at the nozzle of the target droplet generator. In operation S 130 , the residual material from the target droplet generator are removed. In operation S 140 , the residual material and sublimated gaseous carbon dioxide from the pressurized dry ice particles are collected, thereby the EUV radiation source apparatus is cleaned. 
     In another embodiment, the cleaning system for an extreme ultra violet (EUV) radiation source apparatus includes a target droplet generator for generating a metal droplet, a dry ice blasting assembly, and a chamber that encloses at least the target droplet generator and the dry ice blasting assembly. The dry ice blasting assembly comprises a blasting device, an exhausting device, and a supporting device. 
     As shown in the flow chart of  FIG.  9   , another embodiment of the disclosure is a method S 200  of cleaning an extreme ultra violet (EUV) radiation source apparatus. In operation S 210 , a target droplet generator for generating a metal droplet is provided within a chamber. In operation S 220 , the vacuum in the chamber  105  is removed to allow oxygen to enter the chamber. In operation S 230 , the oxygen within the chamber reacts with the residual material on the target droplet generator. In operation S 240 , a dry ice blasting assembly having a blasting nozzle and a dry ice supporting member is provided inside the chamber. In operation S 250 , pressurized dry ice particles including dry ice particles and the pressurized air stream from the dry ice supporting member are formed. In operation S 260 , the pressurized dry ice particles are ejected through the blasting nozzle toward the residual material at the nozzle of the target droplet generator. In operation S 270 , the residual material from the target droplet generator are removed. In operation S 280 , the residual material and sublimated gaseous carbon dioxide from the pressurized dry ice particles are collected. 
     Embodiments of the present disclosure provide the benefit of reducing downtime during maintenance and servicing of EUVL tools. The design of the cleaning system and dry ice blasting assembly allows for faster maintenance with reduced servicing time. The adaptation of the cleaning system allows an improved process resulting in reduced manpower required to perform the maintenance, and an increased output of conforming servicing items of the EUVL tools—both of which ultimately result in a cost-savings. As such, the EUVL tool is more efficiently used. However, it will be understood that not all advantages have been necessarily discussed herein, no particular advantage is required for all embodiments or examples, and other embodiments or examples may offer different advantages. 
     In a particular embodiment, an extreme ultra violet (EUV) radiation source apparatus includes a target droplet generator for generating a metal droplet, a dry ice blasting assembly, a chamber enclosing at least the target droplet generator and the dry ice blasting assembly. The EUV radiation source apparatus also includes a controller communicating with the dry ice blasting assembly and target droplet generator. The dry ice blasting assembly of the EUV radiation source apparatus is selectively attachable to and extendable from the chamber. The dry ice blasting assembly of the EUV radiation source apparatus also includes a blasting device, an exhaust device, and a supporting device. 
     An embodiment of the disclosure is a method of cleaning an extreme ultra violet (EUV) radiation source apparatus, in which the EUV radiation source apparatus comprises a target droplet generator for generating a metal droplet within a chamber, and a dry ice blasting assembly having a blasting nozzle disposed inside the chamber and a dry ice supporting member. The method includes forming pressurized dry ice particles including dry ice particles and a pressurized air stream from the dry ice supporting member of the dry ice blasting assembly, ejecting the pressurized dry ice particles through the blasting nozzle toward residual material at a nozzle of the target droplet generator, removing the residual material from the target droplet generator, and collecting the residual material and sublimated gaseous carbon dioxide from the pressurized dry ice particles. In an embodiment, the cleaning method includes positioning the blasting nozzle with respect to the residual material by an extendable positioner. In an embodiment, the cleaning method includes oscillating the pressure of the pressurized dry ice particles. In an embodiment, the cleaning method includes monitoring the residual material on the droplet generator, adjusting valves of the blasting pump when an amount of the residual material in the droplet generator is more than a threshold amount, and regulating the operating parameters of the blasting compressor and the blasting pump. In an embodiment, the cleaning method includes positioning a blasting member of the dry ice blasting assembly in the chamber. In an embodiment, the cleaning method includes positioning an exhaust member of the dry ice blasting assembly in the chamber. In an embodiment, the flow rate of the pressurized dry ice particles ejected through the blasting nozzle is in a range from 0.5 liters per minute to 500 liters per minute. In an embodiment, the pressure of the pressurized dry ice particles ejected through the blasting nozzle is in a range of 1 kPa to 1000 kPa at the nozzle. In an embodiment, the pressurized dry ice particles ejected through the blasting nozzle have a diameter in a range from about 10 μm to about 10 mm. 
     Another embodiment of the disclosure is a method of cleaning an extreme ultra violet (EUV) radiation source apparatus, includes providing a target droplet generator for generating a metal droplet within a chamber. The vacuum of the chamber is removed allowing residual material on the nozzle of the target droplet generator to react with oxygen. A dry ice blasting assembly having a blasting nozzle and a dry ice supporting member is provided inside the chamber. Pressurized dry ice particles including dry ice particles and a pressurized air stream from the dry ice supporting member are formed. The pressurized dry ice particles are ejected through the blasting nozzle toward the residual material at the nozzle of the target droplet generator. The residual material are removed from the target droplet generator, and collected the residual material and sublimated gaseous carbon dioxide from the pressurized dry ice particles. In an embodiment, the cleaning method includes regulating the cleaning using a controller configured to: monitor the residual material on the droplet generator, and compare an amount of the residual material in the droplet generator with a threshold amount to remove the residual material by the pressurized dry ice particles. In an embodiment, the cleaning method includes stopping the ejecting the pressurized dry ice particles when the amount of the residual material in the droplet generator is below the threshold amount. 
     Another embodiment of the disclosure is a cleaning system for an extreme ultra violet (EUV) radiation source apparatus that includes a target droplet generator for generating a metal droplet, a dry ice blasting assembly and a chamber enclosing at least the target droplet generator and the dry ice blasting assembly. In an embodiment, the dry ice blasting assembly of the cleaning system for the (EUV) radiation source apparatus includes a blasting device, an exhausting device, and a supporting device. In an embodiment, the dry ice blasting assembly of the cleaning system includes a monitoring device for monitoring residual material on the target droplet generator. In an embodiment, the blasting member of the cleaning system includes a blasting air inlet, a blaster carbon dioxide inlet, a blasting mixer and a blasting nozzle. In an embodiment, the blasting nozzle of the cleaning system includes a pulsation insert and a directional insert. In an embodiment, the supporting member of the cleaning system includes a blasting compressor. In an embodiment, the supporting member includes a blasting pump. In an embodiment, the blasting member includes an extendable positioner. In an embodiment, the cleaning system includes a controller configured to monitor residual material on a nozzle of the droplet generator, adjust valves of the blasting pump when an amount of the residual material in the droplet generator is more than a threshold amount, and regulate ejecting parameters of the blasting compressor and the blasting pump, when pressurized dry ice particles are ejected from the blasting nozzle. 
     Another embodiment of the disclosure is an extreme ultra violet (EUV) radiation source apparatus, including: a target droplet generator for generating a metal droplet, a dry ice blasting assembly, a chamber enclosing at least the target droplet generator and the dry ice blasting assembly, and a controller communicating with the dry ice blasting assembly and target droplet generator. The dry ice blasting assembly of the EUV radiation source is selectively attachable to and extendable from the chamber. The dry ice blasting assembly includes a blasting device, an exhaust device, and a supporting device. In an embodiment, the controller of the EUV radiation source is configured to monitor residual material in the droplet generator and adjust valves of the blasting pump when an amount of the residual material in the droplet generator is more than a threshold amount, and regulate ejecting parameters by operating the blasting compressor and the blasting pump when pressurized dry ice particles are ejected from a blasting nozzle. In an embodiment, the EUV radiation source includes a pulsation insert and a directional insert in the blasting nozzle. 
     The foregoing outlines features of several embodiments or examples 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 or examples 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.