Patent Publication Number: US-2022236646-A1

Title: Immersion exposure tool

Description:
BACKGROUND 
     Immersion lithography includes the use of an immersion exposure tool (e.g., an immersion scanner) to transfer a pattern (e.g., a semiconductor device pattern) to a substrate such as a semiconductor wafer. The immersion exposure tool may employ a high refractive index processing fluid between a lens and a substrate to be patterned to increase the numerical aperture of the lens, which may permit smaller structure sizes to be patterned onto the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIGS. 1 and 2  are diagrams of an example immersion exposure tool described herein. 
         FIG. 3  is a diagram of example components of an example device described herein. 
         FIG. 4  is a flowchart of an example process relating to forming a hydrophobic coating on a bottom lens of an immersion exposure tool. 
         FIG. 5  is a flowchart of an example process relating to operating an immersion exposure tool. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     The performance of an immersion exposure tool may depend on various operating parameters of the immersion exposure tool. As an example, the operating temperature of a bottom lens of the immersion exposure tool may fluctuate during operation of the immersion exposure tool based on the amount of processing fluid in contact with the bottom lens. In particular, the operating temperature of the bottom lens may decrease as the amount of processing fluid in contact the bottom lens increases, and the operating temperature of the bottom lens may increase as the amount of processing fluid in contact the bottom lens decreases. Fluctuations in operating temperature of the bottom lens may result in overlay variation during operation of the immersion exposure tool, which may cause the pattern to be transferred to shift during patterning of a substrate. This can result in misalignment of structures and/or layers on the substrate, which may reduce manufacturing yield, cause device failures, and/or lead to increased rework and repairs. 
     Some implementations described herein provide a bottom lens for an immersion exposure tool. The bottom lens includes a hydrophobic coating on the sidewalls of the bottom lens. A bottom portion of the bottom lens is not coated with the hydrophobic coating to maintain the optical performance of the bottom lens and to not distort the pattern that is to be transferred to a substrate. The hydrophobic coating may reduce the thermal instability of the bottom lens. This may reduce overlay variation during operation of the immersion exposure tool, which may increase manufacturing yield, decrease device failures, and/or decrease rework and repairs. 
       FIGS. 1 and 2  are diagrams illustrating an example immersion exposure tool  100  described herein. The immersion exposure tool  100  may be used in semiconductor immersion lithography to transfer device and/or layer patterns to a photoresist layer on a substrate  102 . The substrate  102  may include a semiconductor substrate, such as semiconductor wafer, having an elementary semiconductor material such as crystal silicon, polycrystalline silicon, amorphous silicon, germanium, or diamond, a compound semiconductor material such as silicon carbide or gallium arsenic, an alloy semiconductor material such as silicon germanium (SiGe), gallium arsenide phosphide (GaAsP), aluminum indium arsenide (AlInAs), aluminum gallium arsenide (AlGaAs), or gallium indium phosphorous (GaInP), or a combination thereof. The substrate  102  may be coated with a photoresist layer that is resistive to etch and/or ion implantation and is sensitive to electromagnetic radiation. 
     As shown in the cross-sectional view in  FIG. 1 , the immersion exposure tool  100  may include a processing chamber  104  configured to control, reduce, and/or prevent contaminants from reaching the substrate  102 . The immersion exposure tool  100  may include a wafer stage  106  in the processing chamber  104 . The wafer stage  106  may include a chuck, a platform, or another type of structure configured to support and secure the substrate  102 . The immersion exposure tool  100  may operate in various exposure modes, such as a step exposure mode, a scan exposure mode, or a step-and-scan exposure mode. Accordingly, the wafer stage  106  may be configured to provide various motions, such as transitional motion and/or rotational motion to support the various types of exposure modes. The substrate  102  may include a plurality of fields having integrated circuits defined therein for one or more dies. A field includes one or more circuit dies and a frame region at a boundary area. During a lithography exposure operation, the substrate  102  may be exposed one field at a time. For example, the immersion exposure tool  100  may scan an integrated circuit pattern to transfer the integrated circuit pattern to one field, and may then step to a next field (e.g., by moving the wafer stage  106 ) and may repeat the scanning until all of the fields of the substrate  102  are exhausted. 
     The immersion exposure tool  100  may include an exposure source (or radiation source)  108  that is configured to emit radiation  110  toward the wafer stage  106  (and thus, the substrate  102  located thereon). The exposure source  108  may include a light source that is capable of generating electromagnetic radiation at or approximately near a particular wavelength of light, such as UV light, deep UV (DUV) light, or extreme UV (EUV) light. In some implementations, the exposure source  108  may include a mercury lamp that is capable of generating a wavelength of electromagnetic radiation of approximately 436 nanometers or approximately 365 nanometers, a Krypton Fluoride (KrF) excimer laser that is capable of generating a wavelength of electromagnetic radiation of approximately 248 nanometers, an Argon Fluoride (ArF) excimer laser that is capable of generating a wavelength of electromagnetic radiation of approximately 193 nanometers, a Fluoride (F 2 ) excimer laser that is capable of generating a wavelength of electromagnetic radiation of approximately 157 nanometers, or another light source that is capable of generating a particular wavelength (e.g., below approximately 100 nanometers) of electromagnetic radiation. In some implementations, the light source is an EUV source having a wavelength of approximately 13.5 nanometers or less. 
     The immersion exposure tool  100  may include an illumination section  112 , which may include one or more condenser lenses, condenser mirrors, illumination slits, and/or other components to condense and direct the radiation  110  toward a photomask  114 . The photomask  114  is an apparatus that is used to transfer a pattern (e.g., a die layer pattern, an integrated circuit pattern, among other examples) to a wafer. The photomask  114  may include a lithography mask or a reticle on which a pattern is formed, a frame to which the lithography mask or reticle is attached, and a pellicle layer to protect the pattern from damage and dust that could otherwise cause defects in the transfer of the pattern to the substrate  102 . 
     The photomask  114  may be supported by a photomask stage  116 . The photomask stage  116  is configured and designed to secure the photomask  114 , for example, by a clamping mechanism such as vacuum chuck or e-chuck. The photomask stage  116  may be further designed to be operable to move for various actions, such as scanning, stepping, among other examples. During a lithography exposure operation, the photomask  114  may be secured on the photomask stage  116  and positioned such that an integrated circuit pattern (or a layer of a pattern) defined thereon may be transferred to or imaged on the photoresist layer coated on the substrate  102 . 
     The immersion exposure tool  100  may include a project lens  118  (or projection lens). The project lens  118  may have a single lens element or a plurality of lens elements configured to provide proper illumination to the photoresist layer on the substrate  102 . For example, the project lens  118  may increase or reduce the size of the pattern projected toward the substrate  102  to control the focus of the pattern on the substrate  102  and to control the size of the exposure field on the substrate  102 . 
     The immersion exposure tool  100  may include a bottom lens  120  between the project lens  118  and the wafer stage  106 . As described above, the immersion exposure tool  100  is configured to perform immersion lithography. Accordingly, a processing fluid  122  may be provided between the project lens  118  and the substrate  102  to increase the optical refractive index of the immersion exposure tool  100  and to enhance the optical resolution of the immersion exposure tool  100 . The bottom lens  120  may be configured to protect the project lens  118  from contamination by contaminants in the processing fluid  122  before, during, and/or after an exposure operation of the immersion exposure tool  100 . In this way, the bottom lens  120  may reduce the frequency of maintenance and cleaning of the project lens  118 . 
     The immersion exposure tool  100  may include an immersion hood  124 . The immersion hood  124  may be mounted to the bottom lens  120  and/or to another part of the immersion exposure tool  100 . The immersion hood  124  may include a ring-shaped structure having an open center such that the radiation  110  may travel through the bottom lens  120 , through the opening in the immersion hood  124 , and toward the substrate  102 . The area in the opening of the immersion hood  124  between the bottom lens  120  and the substrate  102  may be referred to as a processing area  126 . The immersion hood  124  may be configured to supply the processing fluid  122  to the processing area  126 , to contain the processing fluid  122  within the processing area  126  during an exposure operation of the immersion exposure tool  100 , and to remove or evacuate the processing fluid  122  from the processing area  126 . The radiation  110  may travel through the processing area  126  and the processing fluid  122  included therein during the exposure operation. The immersion hood  124  may be configured to continuously cycle the processing fluid  122  through the processing area  126  so that fresh processing fluid is continuously supplied to the processing area  126  when the immersion exposure tool  100  is in operation. 
     The processing fluid  122  may include an optically clear liquid having a relatively high refractive (e.g., compared to the refractive index of air) such as water, purified water, or deionized water. The increased refractive index (e.g.,  1 . 333  for water) relative to air may increase the angle of refraction of the radiation  110  directed toward the substrate  102 . The increased angle of refraction may cause the radiation  110  to be condensed into a smaller area, which enables the immersion exposure tool  100  to transfer smaller pattern sizes onto the photoresist layer of the substrate  102 . 
     A distance between the bottom lens  120  and the immersion hood  124  may be equal to or greater than approximately 30 microns. A height (or thickness) of the immersion hood  124  may be equal to or greater than approximately 30 millimeters. A distance between the immersion hood  124  and the wafer  102  may be equal to or greater than approximately 100 microns. 
       FIG. 2  shows a close-up cross-sectional view of a portion of the immersion exposure tool  100 , including the wafer stage  106 , the project lens  118 , the bottom lens  120 , the processing fluid  122 , the immersion hood  124 , and the processing area  126 . As shown in  FIG. 2 , the bottom lens  120  may include an upper portion  202  and a lower portion  204 . The upper portion  202  may include an approximately straight portion having approximately straight sidewalls  206  that connect to bottom walls  208  of the upper portion  202  at approximate right angles. 
     The lower portion  204  may include a tapered portion having tapered sidewalls  210  that taper down from the bottom walls  208  to a bottom surface  212  of the lower portion  204  (which is also the bottom surface of the bottom lens  120 ). The angle of the tapered sidewalls  210  may be based on an exposure field size range for the immersion exposure tool  100 , based on the size of one or more lens components included in the upper portion  202  and/or in the lower portion  204  of the bottom lens  120 , based on a size reduction in the pattern that is to be transferred to the substrate  102  as the pattern is carried by the radiation  110  from the project lens  118  through the bottom lens  120 , and/or based on another parameter. In some implementations, at least a portion of the bottom surface  212  includes an optically clear wall that permits the radiation  110  to travel through the bottom surface  212  of the bottom lens  120 . In some implementations, at least a portion of the bottom surface  212  includes a lens component of the bottom lens  120 . 
     The bottom lens  120  may absorb some of the radiation  110  traveling through the bottom lens  120  when the immersion exposure tool  100  is in operation. The energy of the absorbed radiation  110  is released into the bottom lens  120  as heat, which causes the temperature of the bottom lens  120  to increase. The thermal transfer of heat continues from the bottom lens  120  to the processing fluid  122 , which causes the temperature of the bottom lens  120  to decrease and the temperature of the processing fluid  122  to increase. The volume of processing fluid  122  in the processing area  126  may vary during an exposure operation and/or across multiple exposure operations. 
     The difference in volume of processing fluid  122  in the processing area  126  results in a greater amount of the bottom lens  120  being in contact with the processing fluid  122  in some cases (such as when a greater volume of the processing fluid  122  is in the processing area  126 ), while resulting in a lesser amount of the bottom lens  120  being in contact with the processing fluid  122  in other cases (such as when a lesser volume of the processing fluid  122  is in the processing area  126 ). When the bottom lens  120  is in contact with a greater volume of the processing fluid  122 , a greater amount of heat is thermally transferred from the bottom lens  120  to the processing fluid  122  through a larger surface area. This results in a greater amount of cooling of the bottom lens  120  and, thus, a lower temperature of the bottom lens  120 . Conversely, when the bottom lens  120  is in contact with a lesser volume of the processing fluid  122 , a lesser amount of heat is thermally transferred from the bottom lens  120  to the processing fluid  122  through a smaller surface area. This results in a lower amount of cooling of the bottom lens  120  and, thus, a higher temperature of the bottom lens  120 . 
     As shown in  FIG. 2 , a hydrophobic coating  214  may be included on the bottom lens  120  to prevent direct contact between the processing fluid  122  and at least a portion of the bottom lens  120 . The purpose of the hydrophobic coating  214  is to reduce, minimize, and/or eliminate the transfer of thermal energy (e.g., heat) between the bottom lens  120  and the processing fluid  122 . Reducing, minimizing, and/or eliminating the transfer of heat between the bottom lens  120  and the processing fluid  122  reduces, minimizes, and/or eliminates the temperature variation effect on the bottom lens  120  that might otherwise occur due to varying volumes of processing fluid  122  in the processing area  126 . In particular, the hydrophobic coating  214  may be formed of a hydrophobic material having a relatively high heat capacity. The relatively high heat capacity enables the hydrophobic coating  214  to absorb large amounts of heat without a change (e.g., an increase), or with a very slight change (e.g., +/−0.5% or +/−0.25%) in temperature of the hydrophobic coating  214 . Since the hydrophobic coating  214  is capable of absorbing large amounts of heat without increasing in temperature, little to no heat may be transferred from the bottom lens  120  to the processing fluid  122  through the hydrophobic coating  214 . Accordingly, the hydrophobic coating  214  provides thermal stabilization of the bottom lens  120  by resisting thermal transfer between the bottom lens  120  and the processing fluid  122 , regardless of the volume of processing fluid  122  in the processing area  126 . This may reduce overlay variation during operation of the immersion exposure tool  100  (e.g., exposure field-to-exposure field alignment variation, layer-to-layer alignment variation), may increase manufacturing yield of the immersion exposure tool  100 , may decrease device failures resulting from the immersion exposure tool  100 , may reduce downtime of the immersion exposure tool  100 , and/or may decrease rework and repairs resulting from the immersion exposure tool  100 . 
     As described above, the hydrophobic coating  214  may be formed of a hydrophobic material (e.g., to repel the processing fluid  122  from the bottom lens  120 ) having a relatively high heat capacity (e.g., to resist thermal transfer between the processing fluid  122  and the bottom lens  120 ). Examples of materials that may be used for the hydrophobic coating  214  include, but are not limited to, a fluorocarbon (C x F y ) coating, a hydrocarbon (C x H y ) coating, a silicon oxide (Si x O y ) coating, a hydrosilicon (Si x H y ) coating, a nitrogen oxide (N x O y ) coating, a silicon nitride (Si x N y ) coating, a zinc oxide (Zn x O y ) coating, a fluorine-doped silica (Si x O y F z ) coating, or a manganese oxide (Mn x O y ) coating. The hydrophobic coating  214  may be formed using one or more semiconductor processing tools such as a deposition tool. For example, the deposition tool may deposit the hydrophobic coating  214  on the bottom lens  120  using a chemical vapor deposition technique, a physical vapor deposition (e.g., sputter deposition) technique, an atomic layer deposition technique, or another deposition technique. 
     In some implementations, the hydrophobic coating  214  may be formed to a thickness that is equal to or approximately greater than 10 angstroms based on processing capabilities of forming the hydrophobic coating  214  and to provide a sufficient amount of thermal isolation between the bottom lens  120  and the processing fluid  122 . In some implementations, the thickness of the hydrophobic coating  214  may be based on various parameters, such as the estimated temperature increase of the bottom lens  120 , a heat capacity of the material used for the hydrophobic coating  214 , and/or another parameter. For example, a thicker hydrophobic coating  214  may be used in cases where a greater temperature increase of the bottom lens  120  is estimated and/or in cases where the heat capacity of the material used for the hydrophobic coating  214  is lower. As another example, a thinner hydrophobic coating  214  may be used in cases where a lesser temperature increase of the bottom lens  120  is estimated and/or in cases where the heat capacity of the material used for the hydrophobic coating  214  is greater. 
     The hydrophobic coating  214  may be formed and/or included on one or more sidewalls of the bottom lens  120 . For example, the hydrophobic coating  214  may be formed and/or included on the tapered sidewalls  210  of the lower portion  204 . In some implementations, the hydrophobic coating  214  is additionally formed and/or included on the straight sidewalls  206  of the upper portion  202  and/or the bottom walls  208  of the upper portion  202 . The hydrophobic coating  214  may be formed and/or included on at least a portion of the height (h) of the bottom lens  120  from the bottom surface  212 . In other words, the hydrophobic coating  214  may extend from the bottom of the tapered sidewalls  210  adjacent to the bottom surface  212 , along the length of the tapered sidewalls  210 , along at least a portion of the width of the bottom walls  208 , and/or along at least a portion of the length of the straight sidewalls  206  such that a least a portion of the height (h) of the bottom lens  120  is covered by the hydrophobic coating  214 . 
     The amount of the height (h) of the bottom lens  120  that is covered by the hydrophobic coating  214  (and thus, the portion of the tapered sidewalls  210 , the portion of the bottom walls  208 , and/or the portion of the straight sidewalls  206  that are covered by the hydrophobic coating  214 ) may be based on various parameters. In some implementations, the amount of the height (h) of the bottom lens  120  that is covered by the hydrophobic coating  214  is in a range of approximately 0.01 millimeter to approximately 1 millimeter of the height (h) of the bottom lens  120  to accommodate an estimated variation in volume of the processing fluid  122  in an exposure operation or across multiple exposure operations. In this way, the hydrophobic coating  214  is formed to at least a portion of the height (h) of the bottom lens  120  to prevent direct contact between the processing fluid  122  and the bottom lens  120  regardless of variation in the volume of the processing fluid  122  in the processing area  126 . 
     The hydrophobic coating  214  may be excluded or omitted from one or more surfaces of the bottom lens  120 . In particular, the hydrophobic coating  214  may be excluded or omitted from at least a portion of (or all of) the bottom surface  212 . In these examples, the deposition tool, when forming the hydrophobic coating  214  on the one or more sidewalls and/or the bottom walls  208 , may refrain from forming the hydrophobic coating  214  on at least a portion of (or all of) the bottom surface  212 . The hydrophobic coating  214  may be excluded or omitted from at least a portion of (or all of) the bottom surface  212  to prevent the hydrophobic coating  214  from blocking or reducing the intensity of the radiation  110  traveling through the bottom lens  120 . In this way, excluding or omitting the hydrophobic coating  214  from the bottom surface  212  permits the radiation  110  to travel through the bottom surface  212  and toward the substrate  102 . In some implementations, the hydrophobic coating  214  is excluded or omitted from the width (w) of the bottom surface  212 , which may be in a range of approximately 0.01 millimeter to approximately 33 millimeters for a 193 nanometer ArF excimer laser immersion exposure tool, to permit the radiation  110  to travel through the entire bottom surface  212 . 
     As indicated above,  FIGS. 1 and 2  are provided as one or more examples. Other examples may differ from what is described with regard to  FIGS. 1 and 2 . 
       FIG. 3  is a diagram of example components of a device  300  described herein. In some implementations, the immersion exposure tool  100  may include one or more devices  300  and/or one or more components of device  300 . As shown in  FIG. 3 , device  300  may include a bus  310 , a processor  320 , a memory  330 , a storage component  340 , an input component  350 , an output component  360 , and a communication component  370 . 
     Bus  310  includes a component that enables wired and/or wireless communication among the components of device  300 . Processor  320  includes a central processing unit, a graphics processing unit, a microprocessor, a controller, a microcontroller, a digital signal processor, a field-programmable gate array, an application-specific integrated circuit, and/or another type of processing component. Processor  320  is implemented in hardware, firmware, or a combination of hardware and software. In some implementations, processor  320  includes one or more processors capable of being programmed to perform a function. Memory  330  includes a random access memory, a read only memory, and/or another type of memory (e.g., a flash memory, a magnetic memory, and/or an optical memory). 
     Storage component  340  stores information and/or software related to the operation of device  300 . For example, storage component  340  may include a hard disk drive, a magnetic disk drive, an optical disk drive, a solid state disk drive, a compact disc, a digital versatile disc, and/or another type of non-transitory computer-readable medium. Input component  350  enables device  300  to receive input, such as user input and/or sensed inputs. For example, input component  350  may include a touch screen, a keyboard, a keypad, a mouse, a button, a microphone, a switch, a sensor, a global positioning system component, an accelerometer, a gyroscope, and/or an actuator. Output component  360  enables device  300  to provide output, such as via a display, a speaker, and/or one or more light-emitting diodes. Communication component  370  enables device  300  to communicate with other devices, such as via a wired connection and/or a wireless connection. For example, communication component  370  may include a receiver, a transmitter, a transceiver, a modem, a network interface card, and/or an antenna. 
     Device  300  may perform one or more processes described herein. For example, a non-transitory computer-readable medium (e.g., memory  330  and/or storage component  340 ) may store a set of instructions (e.g., one or more instructions, code, software code, and/or program code) for execution by processor  320 . Processor  320  may execute the set of instructions to perform one or more processes described herein. In some implementations, execution of the set of instructions, by one or more processors  320 , causes the one or more processors  320  and/or the device  300  to perform one or more processes described herein. In some implementations, hardwired circuitry may be used instead of or in combination with the instructions to perform one or more processes described herein. Thus, implementations described herein are not limited to any specific combination of hardware circuitry and software. 
     The number and arrangement of components shown in  FIG. 3  are provided as an example. Device  300  may include additional components, fewer components, different components, or differently arranged components than those shown in  FIG. 3 . Additionally, or alternatively, a set of components (e.g., one or more components) of device  300  may perform one or more functions described as being performed by another set of components of device  300 . 
       FIG. 4  is a flowchart of an example process  400  relating to forming a hydrophobic coating on a bottom lens of an immersion exposure tool. In some implementations, one or more process blocks of  FIG. 4  may be performed by a semiconductor processing tool. Additionally, or alternatively, one or more process blocks of  FIG. 4  may be performed by one or more components of device  300 , such as processor  320 , memory  330 , storage component  340 , input component  350 , output component  360 , and/or communication component  370 . 
     As shown in  FIG. 4 , process  400  may include forming a bottom lens for use in an immersion exposure tool (block  410 ). For example, the semiconductor processing tool may form the bottom lens  120  for use in the immersion exposure tool  100 , as described above. 
     As further shown in  FIG. 4 , process  400  may include forming a hydrophobic coating on one or more sidewalls of the bottom lens (block  420 ). For example, a semiconductor processing tool (e.g., a chemical vapor deposition (CVD) tool, a sputter deposition tool, or another type of semiconductor processing tool) may form the hydrophobic coating  214  on one or more sidewalls (e.g., sidewall  206 , sidewall  210 ) of the bottom lens  120 , as described above. 
     Process  400  may include additional implementations, such as any single implementation or any combination of implementations described below and/or in connection with one or more other processes described elsewhere herein. 
     In a first implementation, forming the hydrophobic coating  214  on the one or more sidewalls (e.g., sidewall  206 , sidewall  210 ) of the bottom lens  120  includes forming the hydrophobic coating  214  on at least a portion of the bottom lens  120  including a range of approximately 0.01 millimeter to approximately 1 millimeter of a height (h) of the bottom lens  120 . In a second implementation, alone or in combination with the first implementation, forming the hydrophobic coating  214  on the one or more sidewalls (e.g., sidewall  206 , sidewall  210 ) of the bottom lens  120  includes forming the hydrophobic coating  214  on a plurality of sidewalls (e.g., sidewall  206 , sidewall  210 ) of the bottom lens while refraining from forming the hydrophobic coating  214  on the bottom surface  212  of the bottom lens  120  to permit the radiation  110  of the immersion exposure tool  100  to pass through the bottom surface  212  of the bottom lens  120 . 
     In a third implementation, alone or in combination with one or more of the first and second implementations, a width of the bottom surface  212  of the bottom lens  120  is in a range of approximately 0.01 millimeter to approximately 33 millimeters. In a fourth implementation, alone or in combination with one or more of the first through third implementations, forming the hydrophobic coating  214  on the one or more sidewalls (e.g., sidewall  206 , sidewall  210 ) of the bottom lens  120  includes forming the hydrophobic coating  214  using a sputter deposition technique. In a fifth implementation, alone or in combination with one or more of the first through fourth implementations, forming the hydrophobic coating  214  on the one or more sidewalls (e.g., sidewall  206 , sidewall  210 ) of the bottom lens  120  includes forming the hydrophobic coating  214  using a CVD technique. 
     Although  FIG. 4  shows example blocks of process  400 , in some implementations, process  400  may include additional blocks, fewer blocks, different blocks, or differently arranged blocks than those depicted in  FIG. 4 . Additionally, or alternatively, two or more of the blocks of process  400  may be performed in parallel. 
       FIG. 5  is a flowchart of an example process  500  relating to operating an immersion exposure tool. In some implementations, one or more process blocks of  FIG. 5  may be performed by an immersion exposure tool (e.g., the immersion exposure tool  100 ). Additionally, or alternatively, one or more process blocks of  FIG. 5  may be performed by one or more components of device  300 , such as processor  320 , memory  330 , storage component  340 , input component  350 , output component  360 , and/or communication component  370 . 
     As shown in  FIG. 5 , process  500  may include placing a substrate (e.g., the substrate  102 ) on a wafer stage (e.g., the wafer stage  106 ) of an immersion exposure tool (e.g., the immersion exposure tool  100 ) (block  510 ). As further shown in  FIG. 5 , process  500  may include providing a processing fluid (e.g., the processing fluid  122 ) to a processing area (e.g., the processing area  126 ) of the immersion exposure tool (block  520 ). 
     As further shown in  FIG. 5 , process  500  may include exposing one or more fields of the substrate to radiation (e.g., the radiation  110 ) through a bottom lens (e.g., the bottom lens  12 ) of the immersion exposure tool to transfer a pattern to the one or more fields (block  530 ). As further shown in  FIG. 5 , process  500  may include using a hydrophobic coating (e.g., the hydrophobic coating  214 ), on the bottom lens and between the bottom lens and the processing fluid, to reduce temperature variation of the bottom lens during exposure of the one or more fields to the radiation (block  540 ). 
     Process  500  may include additional implementations, such as any single implementation or any combination of implementations described below and/or in connection with one or more other processes described elsewhere herein. 
     In a first implementation, the bottom lens includes a straight portion configured to be orientated toward a project lens of the immersion exposure tool and a tapered portion configured to be orientated toward a wafer stage associated with the immersion exposure tool. In a second implementation, alone or in combination with the first implementation, the hydrophobic coating is included on a sidewall of the tapered portion and not on a bottom surface of the tapered portion, and on a sidewall of the straight portion. In a third implementation, alone or in combination with one or more of the first or second implementations, process  500  includes exposing the one or more fields of the substrate to radiation through the bottom lens of the immersion exposure tool to transfer a second pattern to the one or more fields, where a hydrophobic coating on a bottom lens of the immersion exposure tool reduces overlay variation between transfer of the pattern to the one or more fields and transfer of the second pattern to the one or more fields. 
     In a fourth implementation, alone or in combination with one or more of the first through third implementations, the hydrophobic coating includes at least one of a nitrogen oxide (N x O y ) coating, a silicon nitride (Si x N y ) coating, a zinc oxide (Zn x O y ) coating, or a fluorine-doped silica (Si x O y F z ) coating. In a fifth implementation, alone or in combination with one or more of the first though fifth implementations, a thickness of the hydrophobic coating is equal to or greater than approximately 10 angstroms. In a sixth implementation, alone or in combination with one or more of the first through fifth implementations, the hydrophobic coating is on a first surface and not on a second surface of a tapered portion of the bottom lens. 
     Although  FIG. 5  shows example blocks of process  500 , in some implementations, process  500  may include additional blocks, fewer blocks, different blocks, or differently arranged blocks than those depicted in  FIG. 5 . Additionally, or alternatively, two or more of the blocks of process  500  may be performed in parallel. 
     In this way, a bottom lens for an immersion exposure tool includes a hydrophobic coating on sidewalls of the bottom lens. A bottom portion of the bottom lens is not coated with the hydrophobic coating to maintain the optical performance of the bottom lens and to not distort the pattern that is to be transferred to a substrate. The hydrophobic coating may reduce the thermal instability of the bottom lens. This may reduce overlay variation during operation of the immersion exposure tool, which may increase manufacturing yield, decrease device failures, and/or decrease rework and repairs. 
     As described in greater detail above, some implementations described herein provide a method. The method includes placing a substrate on a wafer stage of an immersion exposure tool. The method includes providing a processing fluid to processing area of the immersion exposure tool. The method includes exposing one or more fields of the substrate to radiation through a bottom lens of the immersion exposure tool and through the processing fluid to transfer a pattern to the one or more fields. The method includes using a hydrophobic coating, on a bottom lens of the immersion exposure tool and between the bottom lens and the processing fluid, to reduce temperature variation of the bottom lens during exposure of the one or more fields to the radiation. 
     As described in greater detail above, some implementations described herein provide an immersion exposure tool. The immersion exposure tool includes a project lens. The immersion exposure tool includes a bottom lens adjacent to the project lens. The immersion exposure tool includes an immersion hood around the bottom lens. The immersion exposure tool includes a hydrophobic coating on a sidewall of the bottom lens. 
     As described in greater detail above, some implementations described herein provide a bottom lens for use in an immersion exposure tool. The bottom lens includes a straight portion configured to be orientated toward a project lens of the immersion exposure tool. The bottom lens includes a tapered portion configured to be orientated toward a wafer stage associated with the immersion exposure tool. The bottom lens includes a hydrophobic coating on the tapered portion. 
     As described in greater detail above, some implementations described herein provide a method. The method includes forming a bottom lens for use in an immersion exposure tool. The method includes forming a hydrophobic coating on one or more sidewalls of the bottom lens. 
     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.