Patent Publication Number: US-2023152710-A1

Title: Method of operating semiconductor apparatus

Description:
RELATED APPLICATIONS 
     This present application is a divisional application of U.S. patent application Ser. No. 17/411,571, filed on Aug. 25, 2021, which is a divisional application of U.S. patent application Ser. No. 16/512,767, filed on Jul. 16, 2019, now U.S. Pat. No. 11,106,140, issued on Aug. 31, 2021, which is herein incorporated by reference. 
    
    
     BACKGROUND 
     In the manufacture of integrated circuits (IC), patterns representing different layers of the IC are fabricated using a series of reusable photomasks to transfer the design of each layer of the IC onto a semiconductor substrate during the manufacturing process in a photolithography process. These layers are built up using a sequence of processes and resulted in transistors and electrical circuits. However, as the IC sizes continue to shrink, meeting accuracy requirements as well as reliability in multiple layer fabrication has become increasingly more difficult. Photolithography uses an imaging system that directs radiation onto the photomask and then projects a shrunken image of the photomask onto a semiconductor wafer covered with photoresist. The radiation used in the photolithography may be at any suitable wavelength, with the resolution of the system increasing with decreasing wavelength. With the shrinkage in IC size, extreme ultraviolet (EUV) lithography becomes one of the technologies for smaller node device patterning. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG.  1    is a flowchart illustrating a method of operating a semiconductor apparatus in accordance with some embodiments of the present disclosure; 
         FIG.  2    is a schematic diagram illustrating a semiconductor apparatus used in a deep ultraviolet (DUV) lithography in accordance with some embodiments of the present disclosure; 
         FIG.  3    is a partial enlargement diagram illustrating the semiconductor apparatus shown in  FIG.  2    in accordance with some embodiments of the present disclosure; 
         FIGS.  4  and  5    are schematic diagrams illustrating the semiconductor apparatus shown in  FIG.  2    at various stages in accordance with some embodiments of the present disclosure; 
         FIG.  6    is a schematic diagram illustrating a semiconductor apparatus used in an extreme ultraviolet (EUV) lithography in accordance with some embodiments of the present disclosure; 
         FIG.  7    is a partial enlargement diagram illustrating the semiconductor apparatus shown in  FIG.  6    in accordance with some embodiments of the present disclosure; 
         FIGS.  8  and  9    are schematic diagrams illustrating the semiconductor apparatus shown in  FIG.  6    at various stages in accordance with some embodiments of the present disclosure; 
         FIG.  10    is a flowchart illustrating a method of operating a semiconductor apparatus in accordance with some embodiments of the present disclosure; 
         FIG.  11    is a schematic diagram illustrating a semiconductor apparatus used in a DUV lithography in accordance with some embodiments of the present disclosure; 
         FIG.  12    is a partial enlargement diagram illustrating the semiconductor apparatus shown in  FIG.  11    in accordance with some embodiments of the present disclosure; 
         FIGS.  13  and  14    are schematic diagrams illustrating the semiconductor apparatus shown in  FIG.  11    at various stages in accordance with some embodiments of the present disclosure; 
         FIG.  15    is a schematic diagram illustrating a semiconductor apparatus used in an EUV lithography in accordance with some embodiments of the present disclosure; 
         FIG.  16    is a partial enlargement diagram illustrating the semiconductor apparatus shown in  FIG.  15    in accordance with some embodiments of the present disclosure; 
         FIG.  17    is a partial enlargement diagram illustrating the semiconductor apparatus shown in  FIG.  15    in accordance with some embodiments of the present disclosure; and 
         FIGS.  18  and  19    are schematic diagrams illustrating the semiconductor apparatus shown in  FIG.  15    at various stages in accordance with 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 component or feature&#39;s relationship to another component(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. 
     As used herein, “around,” “about,” “substantially” or “approximately” shall generally mean within 20 percent, within 10 percent, or within 5 percent of a given value or range. Numerical quantities given herein are approximate; meaning that the term “around,” “about,” “substantially” or “approximately” can be inferred if not expressly stated. 
     The advanced lithography process, method, and materials described in the current disclosure can be used in many applications, including fin-type field effect transistors (FinFETs). For example, the fins may be patterned to produce a relatively close spacing between features, for which the above disclosure is well suited. In addition, spacers used in forming fins of FinFETs can be processed according to the above disclosure. 
     A reverse immersion lithography process (also referred to as a lithography process), method, and materials described in the current disclosure can be used in many applications. For example, the lithography process may be used in a deep ultraviolet (DUV) lithography or an extreme ultraviolet (EUV) lithography. 
     Regarding the DUV lithography and the EUV lithography, energy deposited onto the photomask during an exposure generates heat which causes thermal expansion and further induces critical dimension uniformity (CDU) error. Furthermore, heat generated may also bend the workpiece and make overlay error. Additionally, contaminants such as particles and residues may be introduced into the lithography apparatus and further contaminate the photomask and the semiconductor wafer to be processed therein. In an effort to adequately address the aforementioned issues, various semiconductor apparatuses and methods of operating the same for taking heat away from the photomask and removing particles and residues from the photomask surface are presented in accordance with various embodiments of the present disclosure as follows. 
       FIG.  1    is a flowchart illustrating a method of operating a semiconductor apparatus in accordance with some embodiments of the present disclosure. The method begins with block  10  in which a working fluid is driven to flow between a photomask and a fluid retaining structure and through a first slit of the fluid retaining structure, such that a boundary of the working fluid is confined between the photomask and the fluid retaining structure. The method continues with block  12  in which a light is generated to irradiate the photomask through a light transmission region of the fluid retaining structure. While the method is illustrated and described below as a series of acts or events, it will be appreciated that the illustrated ordering of such acts or events are not to be interpreted in a limiting sense. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein. In addition, not all illustrated acts may be required to implement one or more aspects or embodiments of the description herein. Further, one or more of the acts depicted herein may be carried out in one or more separate acts and/or phases. It is noted that each method presented in the present disclosure is merely an example and not intended to limit the present disclosure beyond what is explicitly recited in the claims. Additional operations may be provided before, during, and after each of the methods. Some operations described may be replaced, eliminated, or moved around for additional embodiments of the fabrication process. Additionally, for clarity and ease of explanation, some elements of the figures have been simplified. 
       FIG.  2    is a schematic diagram illustrating a semiconductor apparatus  100  used in a DUV lithography in accordance with some embodiments of the present disclosure. The semiconductor apparatus  100  is a reverse immersion lithography apparatus, such as a DUV lithography apparatus including a light source  110 , a wafer stage  120 , and an optical section  130 . The light source  110  may be any source able to produce radiation beam  112  (also referred to as light  112 ) in the DUV wavelength range. One example of a suitable light source  110  is a krypton fluoride excimer providing a krypton fluoride laser (KrF laser). As another example, a suitable light source  110  may be an argon fluoride excimer providing an argon fluoride laser (ArF laser). It is noted that the semiconductor apparatus  100  used in the DUV lithography is carried out in an air environment. 
     A photomask  140  may be introduced into the semiconductor apparatus  100  during a lithography process. The photomask  140  is a transmissive mask including a transparent substrate and a patterned absorption layer. The photomask  140  may be positioned in space by one or more than one holding fixtures. Various holding and locking mechanisms can be used for the purpose of holding the photomask  140  in a predetermined location. The holding fixtures are operable to move the photomask  140  relative to the semiconductor apparatus  100 . For example, the holding fixtures may be designed to be capable of moving the photomask  140  for scanning. The holding fixtures may include various components suitable to perform a precise movement. 
     A semiconductor wafer  122  (also referred to as a wafer  122 ) may be introduced into the semiconductor apparatus  100  during a lithography process. The wafer stage  120  is configured to hold the wafer  122  to be processed by the semiconductor apparatus  100  for patterning. The wafer stage  120  may be designed to be capable of moving the wafer  122  in a horizontal direction, such as a direction parallel to a side of the photomask  140 . The wafer  122  may be positioned on the wafer stage  120  under the optical section  130 . The optical section  130  may include one or more than one optical modules  132  placed at various positions within the semiconductor apparatus  100 . For example, as shown in  FIG.  2   , the optical section  130  includes two optical modules  132  with one of which being placed between the light source  110  and the photomask  140 , and the other of which being placed between the photomask  140  and the wafer stage  120 . The optical section  130  receives the light  112  from the light source  110 , modulates the light  112  by the image of the photomask  140 , and directs the light  112  to a resist layer on the wafer  122 . Each optical module  132  is designed to have a refractive mechanism in the semiconductor apparatus  100  used in the DUV lithography process. In some embodiments, the materials used for each optical module  132  may be chosen based on the wavelength of the light  112  used in a lithography process to minimize absorption and scattering. 
     The semiconductor apparatus  100  includes a fluid retaining structure  150  on the photomask  140 . The fluid retaining structure  150  is configured to hold a working fluid  160 , such as an immersion fluid. The working fluid  160  may be positioned between the photomask  140  and the fluid retaining structure  150 . In other words, the photomask  140  and the fluid retaining structure  150  form an immersion region configured to contain the working fluid  160 . The working fluid  160  may include water (water solution or de-ionized water) or other suitable fluid, but the present disclosure is not limited in this regard. In some embodiments, the fluid retaining structure  150  may be made of materials including metal, such as a metallic material, a plastic material with metal coated on its surface, a composite material with metal coated on its surface, or other suitable materials. The fluid retaining structure  150  may also be made of materials having low thermal expansion coefficient, such as a plastic material, a composite material, or other suitable materials. 
       FIG.  3    is a partial enlargement diagram illustrating the semiconductor apparatus  100  shown in  FIG.  2    in accordance with some embodiments of the present disclosure. The fluid retaining structure  150  has a first slit  152  therein. In block  10  of  FIG.  1   , the working fluid  160  is driven to flow between the photomask  140  and the fluid retaining structure  150  and through the first slit  152  of the fluid retaining structure  150 , such that a boundary  162  of the working fluid  160  is confined between the photomask  140  and the fluid retaining structure  150 . When the semiconductor apparatus  100  is in operation, the working fluid  160  is driven to flow between the photomask  140  and the fluid retaining structure  150 . The working fluid  160  can be recycled through the first slit  152  of the fluid retaining structure  150 . The first slit  152  serves as an outlet of the working fluid  160  to transfer the working fluid  160  into an additional apparatus, such as a cooling device, a filter, a pump, or combinations thereof. A negative pressure may be applied to a top end of the first slit  152  of the fluid retaining structure  150  to provide a pumping force to the working fluid  160 , such that the working fluid  160  is sucked by the negative pressure and flows through the first slit  152  of the fluid retaining structure  150  into the additional apparatus. The negative pressure may be provided by a pump or the like. After the working fluid  160  passes through the additional apparatus, the working fluid  160  may be driven back to flow between the photomask  140  and the fluid retaining structure  150 . For example, the working fluid  160  can be driven back by passing through other slit(s) or channel(s). In other words, the working fluid  160  is recycled from the first slit  152  of the fluid retaining structure  150  to be continuously reused within the semiconductor apparatus  100 . 
     In some embodiments, the fluid retaining structure  150  may further include a second slit  154  as a working gas inlet. A pipe  155  may be optionally disposed in the second slit  154 . A working gas  170  may include air, noble gas, other suitable gas, or combinations thereof, but the present disclosure is not limited in this regard. A positive pressure may be applied to a top end of the second slit  154  of the fluid retaining structure  150 , such that the working gas  170  is driven to flow toward the photomask  140  from the second slit  154  of the fluid retaining structure  150 . In some embodiments, the positive pressure may be provided by a pump or the like, and a value of the positive pressure is larger than about 1 atm. The working gas  170  through the second slit  154  and the working fluid  160  through the first slit  152  respectively flow in two opposite directions. For example, the working gas  170  through the second slit  154  flows toward the photomask  140  (e.g., a downward direction) while the working fluid  160  through the first slit  152  flows away from the photomask  140  (e.g., an upward direction). The working gas  170  contacts the boundary  162  of the working fluid  160  and serves as an “air knife” to confine the boundary  162  of the working fluid  160  between the photomask  140  and the fluid retaining structure  150 , such that the working fluid  160  may not easily escape from space that is between the photomask  140  and the fluid retaining structure  150 , and may be used repeatedly in cycles. In some embodiments, a position of the boundary  162  of the working fluid  160  can be controlled and determined by the negative pressure and the positive pressure respectively applied to the first slit  152  and the second slit  154 . For example, if the value of the negative pressure applied to the working fluid  160  in the first slit  152  is greater than the value of the positive pressure applied to the working gas  170  in the second slit  154 , the position of the boundary  162  of the working fluid  160  may be closer to the first slit  152  than the second slit  154 ; if the value of the negative pressure applied to the working fluid  160  in the first slit  152  is smaller than the value of the positive pressure applied to the working gas  170  in the second slit  154 , the position of the boundary  162  of the working fluid  160  may be closer to the second slit  154  than the first slit  152 . In some embodiments, the boundary  162  of the working fluid  160  may be substantially straight and substantially perpendicular to the photomask  140 . 
     Reference is made to  FIGS.  2  and  3   . The fluid retaining structure  150  may include a light transmission region  156 . In block  12  of  FIG.  1   , the light  112  is generated to irradiate the photomask  140  through the light transmission region  156  of the fluid retaining structure  150 . The light  112  generated by the light source  110  passes through the light transmission region  156 , and irradiates a portion of the photomask  140  exposed through the light transmission region  156  of the fluid retaining structure  150 . In some embodiments, the second slit  154  of the fluid retaining structure  150  is between the first slit  152  and the light transmission region  156  of the fluid retaining structure  150 . Additionally, a portion  158  of the fluid retaining structure  150  is between the first slit  152  and the second slit  154  of the fluid retaining structure  150 . Stated differently, the portion  158  of the fluid retaining structure  150  is between the first slit  152  and the light transmission region  156  of the fluid retaining structure  150 . The boundary  162  of the working fluid  160  is confined between the photomask  140  and the portion  158  of the fluid retaining structure  150 . Furthermore, the working gas  170  mentioned above flows through the second slit  154  of the fluid retaining structure  150  toward the photomask  140  and out of the light transmission region  156  of the fluid retaining structure  150 . 
     When the semiconductor apparatus  100  is in operation, the light  112  in the DUV wavelength range generated by the light source  110  irradiates a portion of the photomask  140  through the light transmission region  156  of the fluid retaining structure  150 , and is transmitted onto the wafer  122  to create a pattern. In the meanwhile, the working fluid  160  flows over the photomask  140  and is in contact with the photomask  140 . As a result, heat of the photomask  140  generated by the light  112  during an exposure of the lithography process can be taken away by the working fluid  160 . It is noted that a temperature of the working fluid  160  within the semiconductor apparatus  100  is lower than a temperature of the photomask  140  during the lithography process. The working fluid  160  is then sucked by a negative pressure and flows out of the first slit  152  of the fluid retaining structure  150  to be treated by a cooling process. The cooled-down working fluid  160  then flows back to space between the photomask  140  and the fluid retaining structure  150  to take heat away from the photomask  140  again. As a result, a working area of the photomask  140  exposed through the light transmission region  156  of the fluid retaining structure  150  is kept opened while other areas of the photomask  140  are immerged under the working fluid  160  for cooling. 
       FIGS.  4  and  5    are schematic diagrams illustrating the semiconductor apparatus  100  shown in  FIG.  2    at various stages in accordance with some embodiments of the present disclosure. When the semiconductor apparatus  100  is in operation, the photomask  140  is moved in a direction D (also referred to as a scanning direction) along a side of the fluid retaining structure  150  by holding fixtures. During an exposure, the fluid retaining structure  150  is fixed at a specific position. When the photomask  140  is moved in the scanning direction D, different portions of the photomask  140  are sequentially exposed through the light transmission region  156  to receive the light  112  such that patterns on different portions of the photomask  140  are sequentially transferred from the photomask  140  to corresponding target portions of the wafer  122 . 
     Furthermore, positions of the working fluid  160  are controlled based on positions of the photomask  140  when the photomask  140  is moved in the scanning direction D. For example, as shown in  FIG.  4   , an outer boundary  161  of the working fluid  160  is controlled to be substantially aligned with a boundary  141  of the photomask  140 , and an outer boundary  163  of the working fluid  160  is controlled to be substantially aligned with a boundary  153  of the fluid retaining structure  150 . After the photomask  140  is moved, as shown in  FIG.  5   , the outer boundary  161  of the working fluid  160  is controlled to be substantially aligned with a boundary  151  of the fluid retaining structure  150 , and the outer boundary  163  of the working fluid  160  is controlled to be substantially aligned with a boundary  143  of the photomask  140 . As a result of such a configuration, contact areas between the working fluid  160  and the photomask  140  can be increased to facilitate heat dissipation. In some embodiments, when the photomask  140  is at an initial position as shown in  FIG.  4   , an amount of the working fluid  160  at the right side of the light transmission region  156  is smaller than an amount of the working fluid  160  at the left side of the light transmission region  156 . As the photomask  140  moves in the scanning direction D to reach a final position as shown in  FIG.  5   , an amount of the working fluid  160  at the right side of the light transmission region  156  is larger than an amount of the working fluid  160  at the left side of the light transmission region  156 . 
       FIG.  6    is a schematic diagram illustrating a semiconductor apparatus  200  used in an EUV lithography in accordance with some embodiments of the present disclosure. The semiconductor apparatus  200  is a reverse immersion lithography apparatus, such as an EUV lithography apparatus. The semiconductor apparatus  200  includes a light source  210 , a wafer stage  220 , an optical section  230 , and a mask stage  242 . The light source  210  may be any source able to produce radiation beam  212  (also referred to as light  212 ) in the EUV wavelength range. One example of a suitable light source  210  is creating plasma when a laser illuminates a gas, such as a supersonic jet of xenon gas. As another example, a suitable light source  210  may be using bending magnets and undulators associated with synchrotrons. As a further example, a suitable light source  210  may be using discharge sources, which have the potential to provide adequate power in the desired wavelength range. EUV radiation is strongly absorbed in virtually all transmissive materials, including gases and glasses. To minimize unwanted absorption, the semiconductor apparatus  200  used in the EUV lithography is carried out in near vacuum. 
     A photomask  240  and a wafer  222  may be introduced into the semiconductor apparatus  200  during a lithography process, and the wafer stage  220  is configured to hold the wafer  222  to be processed by the semiconductor apparatus  200  for patterning. The wafer stage  220  may be designed to be capable of moving the wafer  222  in a horizontal direction, such as a direction parallel to a side of the photomask  240 . Since the photomask  240  herein is a reflective mask, the semiconductor apparatus  200  further includes the mask stage  242 . The photomask  240  is secured to the mask stage  242  by various holding and locking mechanisms. The mask stage  242  may be designed to be capable of moving the photomask  240  for scanning. Furthermore, the optical section  230  in the semiconductor apparatus  200  is reflective because of the absorption associated with EUV radiation. Accordingly, the optical section  230  includes reflection mirrors  232  that project radiation reflected from the mask stage  242  onto the wafer  222 . 
     The semiconductor apparatus  200  includes a fluid retaining structure  250  on a bottom of the photomask  240 . The fluid retaining structure  250  is configured to hold a working fluid  260 , such as an immersion fluid. The working fluid  260  may be positioned between the photomask  240  and the fluid retaining structure  250 . Stated differently, the photomask  240  is positioned between the mask stage  242  and the working fluid  260 . In comparison with the working fluid  160  shown in  FIG.  2   , the working fluid  260  may include materials with high vapor pressures and high cohesions, one example of a suitable working fluid  260  is mercury (Hg), in which the vapor pressure of mercury is about 2E-3 mmHg at about 26° C. . 
     The materials used for the working fluid  260  should be chosen based on characteristics of high vapor pressures to minimize the disruption of the vacuum environment to further ensure the environment being continuously maintained at near vacuum during a lithography process. 
       FIG.  7    is a partial enlargement diagram illustrating the semiconductor apparatus  200  shown in  FIG.  6    in accordance with some embodiments of the present disclosure. The fluid retaining structure  250  has a first slit  252  therein. In block  10  of  FIG.  1   , the working fluid  260  is driven to flow between the photomask  240  and the fluid retaining structure  250  and through a first slit  252  of the fluid retaining structure  250 , such that a boundary  262  of the working fluid  260  is confined between the photomask  240  and the fluid retaining structure  250 . When the semiconductor apparatus  200  is in operation, the working fluid  260  is driven to flow between the photomask  240  and the fluid retaining structure  250 . The first slit  252  serves as an outlet of the working fluid  260  to transfer the working fluid  260  into an additional apparatus, such as a cooling device, a filter, a pump, or combinations thereof. A negative pressure may be applied to a bottom end of the first slit  252  of the fluid retaining structure  250  to provide a pumping force to the working fluid  260 , such that the working fluid  260  is sucked by the negative pressure and flows through the first slit  252  into the additional apparatus. The negative pressure may be provided by a pump or the like. After the working fluid  260  passes through the additional apparatus, the working fluid  260  may be driven back to flow between the photomask  240  and the fluid retaining structure  250 . In other words, the working fluid  260  is recycled from the first slit  252  of the fluid retaining structure  250  to be continuously reused within the semiconductor apparatus  200 . 
     In comparison with the semiconductor apparatus  100  of  FIG.  3   , no working gas is introduced in the semiconductor apparatus  200  because the EUV lithography is performed in a near-vacuum environment. Therefore, the fluid retaining structure  250  has no second slit between the first slit  252  and a light transmission region  256 . That is to say, the boundary  262  of the working fluid  260  is confined between the photomask  240  and the fluid retaining structure  250  without a working gas. Instead, the boundary  262  of the working fluid  260  is confined by taking advantage of its characteristic of high cohesions and controlling a value of a negative pressure applied to the first slit  252  to suck the working fluid  260 . For example, if the value of the negative pressure of the working fluid  260  is larger, the position of the boundary  262  of the working fluid  260  may be closer to the first slit  252 ; if the value of the negative pressure of the working fluid  260  is smaller, the position of the boundary  262  of the working fluid  260  may be farther from the first slit  252 . 
     Reference is made to  FIGS.  6  and  7   . The fluid retaining structure  250  may include the light transmission region  256 . In block  12  of  FIG.  1   , the light  212  is generated to irradiate the photomask  240  through a light transmission region  256  of the fluid retaining structure  250 . The light  212  generated by the light source  210  passes through the light transmission region  256 , and irradiates a portion of the photomask  240  exposed through the light transmission region  256  of the fluid retaining structure  250 . In some embodiments, a portion  258  of the fluid retaining structure  250  is between the first slit  252  and the light transmission region  256  of the fluid retaining structure  250 . The boundary  262  of the working fluid  260  is confined between the portion  258  of the fluid retaining structure  250  and the photomask  240 . Furthermore, the boundary  262  of the working fluid  260  is confined between the first slit  252  and the light transmission region  256  of the fluid retaining structure  250 . 
     When the semiconductor apparatus  200  is in operation, the light  212  in the EUV wavelength range generated by the light source  210  irradiates a portion of the photomask  240  through the light transmission region  256  of the fluid retaining structure  250 , and is reflected onto the wafer  222  to create a pattern in a target portion of the wafer  222 . In the meanwhile, the working fluid  260  is driven to flow in cycles and in contact with the photomask  240 , such that heat of the photomask  240  generated by the light  212  during an exposure of the lithography process can be taken away by the working fluid  260 . In addition, heat of the photomask  240  generated by the light  212  may further be taken away by a heat conduction from the photomask  240  to the mask stage  242  in contact with the photomask  240 . 
       FIGS.  8  and  9    are schematic diagrams illustrating the semiconductor apparatus  200  shown in  FIG.  6    at various stages in accordance with some embodiments of the present disclosure. When the semiconductor apparatus  200  is in operation, the photomask  240  secured on the mask stage  242  is moved in a direction D (also referred to as a scanning direction) along a side of the fluid retaining structure  250 . As such, different portions of the photomask  240  are sequentially exposed through the light transmission region  256  to receive the light  212 , such that patterns on different portions of the photomask  240  are sequentially transferred from the photomask  240  to corresponding target portions of the wafer  222 . Similar to the aforementioned semiconductor apparatus  100 , positions of the working fluid  260  are controlled based on positions of the photomask  240  when the photomask  240  is moved in the scanning direction. 
       FIG.  10    is a flowchart illustrating a method of operating a semiconductor apparatus in accordance with some embodiments of the present disclosure. The method begins with block  20  in which a particle is removed on a photomask by an electrical field or an air flow, in which the electrical field or the air flow is formed by at least one covering structure adjacent to the photomask. The method continues with block  22  in which a light is generated to irradiate the photomask through a light transmission region of the covering structure. While the method is illustrated and described below as a series of acts or events, it will be appreciated that the illustrated ordering of such acts or events are not to be interpreted in a limiting sense. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein. In addition, not all illustrated acts may be required to implement one or more aspects or embodiments of the description herein. Further, one or more of the acts depicted herein may be carried out in one or more separate acts and/or phases. 
       FIG.  11    is a schematic diagram illustrating a semiconductor apparatus  300  used in a DUV lithography in accordance with some embodiments of the present disclosure. The semiconductor apparatus  300  is a reverse immersion lithography apparatus, such as a DUV lithography apparatus. Since some components of the semiconductor apparatus  300  illustrated in  FIG.  11    are similar to those corresponding components of the semiconductor apparatus  100  illustrated  FIG.  2   , descriptions for those similar components will not be repeated hereinafter. In comparison with the semiconductor apparatus  100 , a working fluid and a working gas are not used in the semiconductor apparatus  300 . Accordingly, a fluid retaining structure  350  herein is referred to as a “covering structure” since there&#39;s no working fluid to be retained. Furthermore, the covering structure  350  has no slit therein. In some embodiments, the semiconductor apparatus  300  includes two covering structures  350  respectively on two opposite sides  345  of a photomask  340 . In other words, the photomask  340  may be positioned between the two covering structures  350 . 
       FIG.  12    is a partial enlargement diagram illustrating the semiconductor apparatus  300  shown in  FIG.  11    in accordance with some embodiments of the present disclosure. In block  20  of  FIG.  10   , a particle P on the photomask  340  is removed by an electrical field , in which the electrical field is formed by at least one covering structure  350  adjacent to the photomask  340 . The particle(s) P herein may be charged particle(s) P such as fall-on particle(s) or chemical residue(s), but the present disclosure is not limited in this regard. In some embodiments, the covering structure  350  may be made of materials including metal, such as a metallic material, a plastic material with metal coated on its surface, a composite material with metal coated on its surface, or other suitable materials. 
     In some embodiments, a gap may be preserved between the covering structure  350  and the photomask  340  to ensure that the covering structure  350  is spaced apart from the photomask  340 . When the semiconductor apparatus  300  is in operation, at least one covering structure  350  may be charged to form charges  352  thereon to attract the particle(s) P from the photomask  340 . In some embodiments, a voltage may be applied to the covering structure  350 . Subsequently, an electric field may be generated in a peripheral area of the covering structure  350  through the applied voltage to further generate charges  352  on the covering structure  350 . In some embodiments, when the charges  352  formed on the covering structure  350  is electrically positive and the particle(s) P is electrically negative, the particle(s) P would be attracted onto the covering structure  350  through the electric field. In alternative embodiments, when the charges  352  formed on the covering structure  350  is electrically negative and the particle(s) P is electrically positive, the particle(s) P would be attracted onto the covering structure  350  through the electric field. 
     Reference is made to  FIGS.  11  and  12   . The covering structure  350  may include a light transmission region  356 . In block  22  of  FIG.  10   , a light  312  is generated to irradiate the photomask  340  through the light transmission region  356  of the covering structure  350 . The light  312  in the DUV wavelength range generated by the light source  310  passes through the light transmission region  356  of the covering structure  350 , and irradiates a portion of the photomask  340  exposed through the light transmission region  356  of the covering structure  350 . When the semiconductor apparatus  300  is in operation, the light  312  generated by the light source  310  irradiates a portion of the photomask  340  through the light transmission region  356  of the covering structure  350 , and is transmitted onto the wafer  322  to create a pattern. In the meanwhile, the charges  352  formed on the covering structure  350  removes the particle(s) P from the photomask  340  by the electrical field. As a result, a working area of the photomask  340  exposed through the light transmission region  356  of the covering structure  350  is kept opened while other areas of the photomask  340  are under a cleaning process. In other words, a lithography process may be carried out simultaneously with the cleaning process of the photomask  340 . 
       FIGS.  13  and  14    are schematic diagrams illustrating the semiconductor apparatus  300  shown in  FIG.  11    at various stages in accordance with some embodiments of the present disclosure. When the semiconductor apparatus  300  is in operation, the photomask  340  is moved in a direction D (also referred to as a scanning direction) along a side of the covering structure  350  to sequentially transfer patterns from the photomask  340  to the wafer  322 . In some embodiments, a number of the covering structure  350  is two, and the photomask  340  is moved between the two covering structures  350  such that particles on both sides of the photomask  340  can be removed during the lithography process. 
       FIG.  15    is a schematic diagram illustrating a semiconductor apparatus  400  used in an EUV lithography in accordance with some embodiments of the present disclosure. The semiconductor apparatus  400  is a reverse immersion lithography apparatus, such as an EUV lithography apparatus. In comparison with the aforementioned semiconductor apparatus  300 , the semiconductor apparatus  400  is used in an EUV lithography process. Furthermore, since the semiconductor apparatus  400  includes a mask stage  442  above a photomask  440 , one covering structure  450  is located below the mask stage  442  and the photomask  440 . In other words, the photomask  440  is positioned between the covering structure  450  and the mask stage  442 . 
       FIG.  16    is a partial enlargement diagram illustrating the semiconductor apparatus  400  shown in  FIG.  15    in accordance with some embodiments of the present disclosure. In block  20  of  FIG.  10   , a particle P on the photomask  440  is removed by the electrical field , in which the electrical field is formed by at least one covering structure  450  adjacent to the photomask  440 . The particle(s) P herein may be charged particle(s) P such as fall-on particle(s) or chemical residue(s), but the present disclosure is not limited in this regard. When the semiconductor apparatus  400  is in operation, the covering structure  450  may be charged to form charges  452  thereon to attract the charged particle(s) P from the photomask  440 . In some embodiments, a voltage may be applied to the covering structure  450  to further generate charges  452  on the covering structure  450 . In some embodiments, when the charges  452  formed on the covering structure  450  is electrically positive and the particle(s) P is electrically negative, the particle(s) P would be attracted onto the covering structure  450  through the electric field. In alternative embodiments, when the charges  452  formed on the covering structure  450  is electrically negative and the particle(s) P is electrically positive, the particle(s) P would be attracted onto the covering structure  450  through the electric field. 
       FIG.  17    is a partial enlargement diagram illustrating the semiconductor apparatus  400  shown in  FIG.  15    in accordance with some embodiments of the present disclosure. In block  22  of  FIG.  10   , a particle P on the photomask  440  may be alternatively removed by an air flow  454 , in which the air flow  454  is formed by at least one covering structure  450  adjacent to the photomask  440 . The particle(s) P herein may be charged particle(s) P or neutral particle(s) P, such as fall-on particle(s) or chemical residue(s), but the present disclosure is not limited in this regard. In some embodiments, the air flow  454  may be confined by the covering structure  450 . Accordingly, a confined air flow  454  may be formed from the covering structure  450  to the photomask  440 , such that the particle(s) P on the photomask  440  is detached from the photomask  440 . In alternative embodiments, the air flow  454  may be ionized by the covering structure  450 . Accordingly, an ionized air flow  454  may be formed from the covering structure  450  to the photomask  440  to achieve the aforementioned effects. It is noted that the air flow  454  is provided at a low pressure to ensure the environment being continuously maintained at near vacuum during a lithography process. 
     In other embodiments, particle(s) P on the photomask  440  may be removed by an applied magnetic field. The particle(s) P herein may be charged particle(s) P such as fall-on particle(s) or chemical residue(s), but the present disclosure is not limited in this regard. For example, the magnetic field is generated to be adjacent to the covering structure  450 , such that the particle(s) P is driven away from the photomask  440  due to Lorentz force (or electromagnetic force). More specifically, the Lorentz force is a combination of electric force and magnetic force exerted on a charged object due to electric field and magnetic field. In some embodiments, using the electrical field and/or the magnetic field can control and confine the ionized air flow  454  in a vacuum environment. 
     Reference is made to  FIGS.  15 - 17   . The covering structure  450  may include a light transmission region  456 . In block  22  of  FIG.  10   , a light  412  is generated to irradiate the photomask  440  through the light transmission region  456  of the covering structure  450 . When the semiconductor apparatus  400  is in operation, the light  412  in the EUV wavelength range generated by the light source  410  irradiates a portion of the photomask  440  through the light transmission region  456  of the covering structure  450 , and is reflected onto the wafer  422  to create a pattern. In the meanwhile, the charges  452  formed on the covering structure  450  removes the particle(s) P from the photomask  440  by the electric field or the air flow  454 . As a result, the lithography process herein may be carried out simultaneously with a cleaning process of the photomask  440 . 
       FIGS.  18  and  19    are schematic diagrams illustrating the semiconductor apparatus  400  shown in  FIG.  15    at various stages in accordance with some embodiments of the present disclosure. Similar to the aforementioned semiconductor apparatus  300 , when the semiconductor apparatus  400  is in operation, the photomask  440  below the mask stage  442  is moved in a direction D (also referred to as a scanning direction) along a side of the covering structure  450  to sequentially transfer patterns from the photomask  440  to the wafer  422 . 
     Based on the aforementioned descriptions, various advantages may be provided by the present disclosure. More specifically, since heat may be generated on the photomask during an exposure of the lithography process, thermal expansion may occurs and thereby inducing critical dimension uniformity (CDU) error. Furthermore, particle(s) on the photomask may result in photomask particulate contamination and thereby causing wafer yield loss. However, the present disclosure may take heat away from the photomask by driving the working fluid to flow between the photomask and the fluid retaining structure. Additionally, the present disclosure may also utilize electric field, air flow, or magnetic field within the semiconductor apparatus to attract or guide away the particle(s) from the photomask surface. As such, a working area of the photomask is kept opened while other areas of the photomask are under the cooling/cleaning process. Therefore, heat generated may be efficiently taken away and the particle(s) on the photomask may be efficiently removed such that wafer yield will be improved. 
     In some embodiments, a method includes driving a working fluid to flow between a photomask and a fluid retaining structure and through a first slit of the fluid retaining structure, such that a boundary of the working fluid is confined between the photomask and the fluid retaining structure; and generating a light to irradiate the photomask through a light transmission region of the fluid retaining structure. 
     In some embodiments, a method includes removing a particle on a photomask by an electrical field or an air flow, in which the electrical field or the air flow is formed by at least one covering structure adjacent to the photomask; and generating a light to irradiate the photomask through a light transmission region of the covering structure. 
     In some embodiments, an apparatus includes a photomask, a fluid retaining structure, a working fluid, and a light source. The fluid retaining structure is on the photomask and has a light transmission region and a first slit. The working fluid flows between the photomask and the fluid retaining structure and through the first slit, in which a boundary of the working fluid is confined between the photomask and a portion of the fluid retaining structure, and the portion is between the light transmission region and the first slit. The light source is configured to irradiate a portion of the photomask exposed through the light transmission region. 
     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.