Patent Publication Number: US-11656544-B2

Title: Robust, high transmission pellicle for extreme ultraviolet lithography systems

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional application of U.S. patent application Ser. No. 16/885,126 filed May 27, 2020, now U.S. Pat. No. 11,314,169, issued on Apr. 26, 2022, which claims the benefit of U.S. Provisional Patent Application Ser. No. 62/928,230, filed Oct. 30, 2019, all of which are herein incorporated by reference in their entireties. 
    
    
     BACKGROUND 
     Extreme ultraviolet (EUV) lithography is an optical lithography technique in which the scanner uses light in the extreme ultraviolet region (e.g., spanning wavelengths of approximately one to one hundred nanometers). A light source is configured to emit EUV radiation. For instance, the light source may vaporize a molten metal such as tin into a highly ionized plasma that emits the EUV radiation. The EUV radiation is subsequently guided, using a series of optics (e.g., including multilayer mirrors), into the scanner. In the scanner, the EUV radiation is used to project a pattern, which is etched into a photomask, onto a silicon wafer. The EUV process can be used to fabricate a high resolution pattern of lines onto the silicon wafer, potentially at a scale of seven nanometers or beyond. 
    
    
     
       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 simplified schematic diagram of an example lithography system, according to examples of the present disclosure; 
         FIG.  2 A  is a cross sectional view of an example pellicle-photomask structure, according to examples of the present disclosure; 
         FIG.  2 B  is an isometric view of the example pellicle-photomask structure of  FIG.  2 A ; 
         FIG.  2 C  illustrates an example top membrane portion of the pellicle membrane of  FIGS.  2 A and  2 B ; 
         FIG.  2 D , for instance, illustrates a cross sectional view of an example carbon nanotube of the network of carbon nanotubes illustrated in  FIG.  2 C ; 
         FIG.  3    is a flow diagram illustrating a method  300  for assembling a pellicle for a lithography process, according to one example of the present disclosure; and 
         FIG.  4    illustrates a flowchart of a method of fabricating a semiconductor device according to at least one embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     In one example, the present disclosure provides a robust, high transmission pellicle for extreme ultraviolet (EUV) lithography systems. As discussed above, an EUV lithography system may use EUV radiation to project a pattern formed in a photomask onto a silicon wafer, and the pattern may be etched into the wafer. In some examples, a pellicle (e.g., a thin, transparent film or membrane) may be used to protect the photomask from contamination. For instance, particles may fall onto the surface of the photomask. When the scanner subsequently prints the photomask pattern onto the wafer, the particles may also print onto the wafer, resulting in defects in the pattern. However, a properly positioned pellicle can prevent the particles from falling onto the photomask. 
     Although pellicles can reduce photomask contamination, pellicles can also reduce the amount of EUV radiation that reaches the photomask. For instance, if the membrane of the pellicle is too thick, the membrane may absorb much of the EUV radiation before the EUV radiation can reach the photomask, which may in turn reduce the throughput of the EUV lithography system. Moreover, many common membrane materials are prone to mechanical deformation under the typical processing conditions of an EUV lithography system. For example, an EUV lithography system may operate at as high as 250 Watts for high volume manufacturing; under such conditions, the temperature of the pellicle membrane may reach up to 686 degrees Celsius, which is well over the melting point of many materials. As such, conventional pellicles may need to be replaced relatively frequently. 
     Examples of the present disclosure provide a robust, high transmission pellicle that is resistant to temperature- and pressure-induced deformation and that transmits a high percentage (e.g., greater than eighty-two percent, and in some examples greater than ninety percent) of radiation onto the photomask. In one example, the pellicle membrane comprises a carbon-based or silicon-based material, such as a transparent carbon nanotube film or a transparent silicon nanowire film, which is coated with a shell to ensure environmental stability. Thus, the pellicle membrane is mechanically robust while allowing for improved transmission of radiation. In further examples, the pellicle frame that supports the pellicle membrane over the photomask may include a vent structure that minimizes the pressure differential between the sides of the pellicle membrane (e.g., the side facing the photomask and the side facing away from the photomask). Thus, environmental factors that may contribute to deformation of the pellicle membrane may also be minimized. 
     Additional features can be added to the pellicle disclosed herein. Some of the features described below can also be replaced or eliminated for different examples. Although some examples disclosed below discuss operations that are performed in a particular order, these operations may be performed in other orders as well without departing from the scope of the present disclosure. 
     Moreover, the pellicle and methods disclosed herein may be deployed in a plurality of applications, including the fabrication of fin-type field effect transistors (finFETs). For instance, examples of the present disclosure may be well suited for patterning the fins of a finFET to produce a relatively close spacing between features. In further examples, spacers used in forming the fins of the finFET may be processed according to examples of the present disclosure. 
       FIG.  1    is a simplified schematic diagram of an example lithography system  100 , according to examples of the present disclosure. The lithography system  100  may also be referred to herein as a “scanner” that is operable to perform lithography exposing processes with respective radiation sources and exposure modes. 
     In one example, the lithography system  100  generally comprises a high-brightness light source  102 , an illuminator  104 , a mask stage  106 , a photomask  108 , a projection optics module  110 , and a substrate stage  112 . In some examples, the lithography system may include additional components that are not illustrated in  FIG.  1   . In further examples, one or more of the high-brightness light source  102 , the illuminator  104 , the mask stage  106 , the photomask  108 , the projection optics module  110 , and the substrate stage  112  may be omitted from the lithography system  100  or may be integrated into combined components. 
     The high-brightness light source  102  may be configured to emit radiation having wavelengths in the range of approximately one nanometer to 250 nanometers. In one particular example, the high-brightness light source  102  generates EUV light with a wavelength centered at approximately 13.5 nanometers; accordingly, in some examples, the high-brightness light source  102  may also be referred to as an “EUV light source.” However, it will be appreciated that the high-brightness light source  102  should not be limited to emitting EUV light. For instance, the high-brightness light source  102  may be utilized to perform any high-intensity photon emission from excited target material. 
     In one example, the term “approximately” is understood to mean +/−twenty percent of the stated value, and more typically +/−ten percent of the stated value, and more typically +/−five percent of the stated value, and more typically +/−three percent of the stated value, and more typically +/−two percent of the stated value, and more typically +/−one percent of the stated value, and even more typically +/−0.5 percent of the stated value. The stated value is therefore an approximate value. In the absence of any specific description, any stated value stated herein is approximate in accordance with the above definition. 
     In some examples (e.g., where the lithography system  100  is a UV lithography system), the illuminator  104  comprises various refractive optical components, such as a single lens or a lens system comprising multiple lenses (zone plates). In another example (e.g., where the lithography system  100  is an EUV lithography system), the illuminator  104  comprises various reflective optical components, such as a single mirror or a mirror system comprising multiple mirrors. The illuminator  104  may direct light from the high-brightness light source  102  onto the mask stage  106 , and more particularly onto the photomask  108  that is secured onto the mask stage  106 . In an example where the high-brightness light source  102  generates light in the EUV wavelength range, the illuminator  104  comprises reflective optics. 
     The mask stage  106  may be configured to secure the photomask  108 . In some examples, the mask stage  106  may include an electrostatic chuck (e-chuck) to secure the photomask  108 . This is because the gas molecules absorb EUV light, and the lithography system  100  for EUV lithography patterning is maintained in a vacuum environment to minimize EUV intensity loss. Herein, the terms “photomask,” “mask,” and “reticle” may be used interchangeably. In one example, the photomask  108  is a reflective mask. 
     In some examples, a pellicle  114  may be positioned over the photomask  108 , e.g., between the photomask  108  and the substrate stage  112 . The pellicle  114  may protect the photomask  108  from particles and may keep the particles out of focus, so that the particles do not produce an image (which may cause defects on a wafer during the lithography process). 
     The projection optics module  110  may be configured for imaging the pattern of the photomask  108  onto a semiconductor wafer  116  secured on the substrate stage  112 . In one example, the projection optics module  110  comprises refractive optics (such as for a UV lithography system). In another example, the projection optics module  110  comprises reflective optics (such as for an EUV lithography system). The light directed from the photomask  108 , carrying the image of the pattern defined on the photomask  108 , may be collected by the projection optics module  110 . The illuminator  104  and the projection optics module  110  may be collectively referred to as an “optical module” of the lithography system  100 . 
     In some examples, the semiconductor wafer  116  may be a bulk semiconductor wafer. For instance, the semiconductor wafer  116  may comprise a silicon wafer. The semiconductor wafer  116  may include silicon or another elementary semiconductor material, such as germanium. In some examples, the semiconductor wafer  116  may include a compound semiconductor. The compound semiconductor may include gallium arsenide, silicon carbide, indium arsenide, indium phosphide, another suitable material, or a combination thereof. 
     In some examples, the semiconductor wafer  116  includes a silicon-on-insulator (SOI) substrate. The SOI substrate may be fabricated using a separation by implantation of oxygen (SIMOX) process, a wafer bonding process, another applicable process, or a combination thereof. 
     In some examples, the semiconductor wafer  116  comprises an undoped substrate. However, in other examples, the semiconductor substrate  116  comprises a doped substrate, such as a p-type substrate or an n-type substrate. 
     In some examples, the semiconductor wafer  116  includes various doped regions (not shown) depending on the design requirements of the semiconductor device structure. The doped regions may include, for example, p-type wells and/or n-type wells. In some examples, the doped regions are doped with p-type dopants. For example, the doped regions may be doped with boron or boron fluoride. In other examples, the doped regions are doped with n-type dopants. For example, the doped regions may be doped with phosphor or arsenic. In some examples, some of the doped regions are p-doped and other doped regions are n-doped. 
     In some examples, an interconnection structure may be formed over the semiconductor wafer  116 . The interconnection structure may include multiple interlayer dielectric layers, including dielectric layers. The interconnection structure may also include multiple conductive features formed in the interlayer dielectric layers. The conductive features may include conductive lines, conductive vias, and/or conductive contacts. 
     In some examples, various device elements are formed in the semiconductor wafer  116 . Examples of the various device elements may include transistors (e.g., metal oxide semiconductor field effect transistors (MOSFETs), complementary metal oxide semiconductor (CMOS) transistors, bipolar junction transistors (BJTs), high-voltage transistors, high-frequency transistors, p-channel and/or n-channel field effect transistors (PFETs and/or NFETs), diodes, or other suitable elements. Various processes may be used to form the various device elements, including deposition, etching, implantation, photolithography, annealing, and/or other applicable processes. 
     The device elements may be interconnected through the interconnection structure over the semiconductor wafer  116  to form integrated circuit devices. The integrated circuit devices may include logic devices, memory devices (e.g., static random access memory (SRAM) devices), radio frequency (RF) devices, input/output (I/O) devices, system-on-chip (SoC) devices, image sensor devices, other applicable devices, or a combination thereof. 
     In some examples, the semiconductor wafer  116  may be coated with a resist layer that is sensitive to EUV light. Various components including those described above may be integrated together and may be operable to perform lithography exposing processes. 
       FIG.  2 A  is a cross sectional view of an example pellicle-photomask structure  200 , according to examples of the present disclosure.  FIG.  2 B  is an isometric view of the example pellicle-photomask structure  200  of  FIG.  2 A . As illustrated in  FIGS.  2 A and  2 B , the photomask  108  may include a mask substrate  202  and a mask pattern  204  positioned over the mask substrate  202 . 
     In some examples, the mask substrate  202  comprises a transparent substrate, such as fused silica that is relatively free of defects, borosilicate glass, soda-lime glass, calcium fluoride, low thermal expansion material, ultra-low thermal expansion material, or other applicable materials. The mask pattern  204  may be positioned over the mask substrate  202  as discussed above and may be designed according to the integrated circuit features to be formed over a semiconductor substrate (e.g., wafer  116  of  FIG.  1   ) during a lithography process. The mask pattern  204  may be formed by depositing a material layer and patterning the material layer to have one or more openings where beams of radiation may travel through without being absorbed, and one or more absorption areas which may completely or partially block the beams of radiation. 
     The mask pattern  204  may include metal, metal alloy, metal silicide, metal nitride, metal oxide, metal oxynitride, or other applicable materials. Examples of materials that may be used to form the mask pattern  204  may include, but are not limited to, Cr, Mo x Si y , Ta x Si y , Mo, Nb x O y , Ti, Ta, Cr x N y , MO x O y , MO x N y , Cr x O y , Ti x N y , Zr x N y , Ti x O y , Ta x N y , Ta x O y , Si x O y , Nb x N y , Zr x N y , Al x O y N z , Ta x B y O z , Ta x B y N z , Ag x O y , Ag x N y , Ni, Ni x O y , Ni x O y N z , and/or the like. The compound x/y/z ratio is not limited. 
     In some examples, the photomask  108  is an EUV mask. However, in other examples, the photomask  108  may be an optical mask. 
     As illustrated in  FIGS.  2 A and  2 B , the pellicle  114  may be positioned over the photomask  108 . In one example, the pellicle  114  includes a pellicle frame  206  that may be positioned over at least one of the mask substrate  202  and the mask pattern  204 . In one example, the pellicle frame  206  may be formed from Si, SiC, SiN, glass, a low coefficient of thermal expansion material (such as an Al alloy, a Ti alloy, Invar, Kovar, or the like), another suitable material, or a combination thereof. In some examples, suitable processes for forming the pellicle frame  206  may include machining processes, sintering processes, photochemical etching processes, other applicable processes, or a combination thereof. 
     In one example, the pellicle frame  206  may include a side portion  208  having an inside surface  210  and an outside surface  212 , where the inside surface  210  and the outside surface  212  are oriented on opposite sides of the side portion  208 . The pellicle frame  206  may further include a bottom surface  214  or base that connects the inside surface  210  and the outside surface  212 . 
     As illustrated in  FIGS.  2 A and  2 B , the pellicle-mask structure  200  may further include a vent structure  216  formed in the side portion  208  and extending from the inside surface  210  through to the outside surface  212 . In some examples, the vent structure  216  may comprise one or more apertures formed in the side portion  208  of the pellicle frame  206 . The apertures may take any shape, including circular apertures, rectangular apertures, slit-shaped apertures, other shapes, or any combination thereof. The apertures may allow for a flow of air through a portion of the pellicle-mask structure  200 , as discussed in further detail below. In some examples, the apertures may include filters to minimize passage of outside particles through the vent structure  216 . 
     In some examples, where the vent structure includes filters, the vent structure  216  may be formed together with the pellicle frame  206 . In some examples, the vent structure  216  may be formed using a photochemical etching process, another applicable process, or a combination thereof. 
     In other examples, again where the vent structure includes filters, the vent structure  216  and the pellicle frame  206  may be formed separately, and an opening (not shown) may be formed in the side portion  208  of the pellicle frame  206 . Afterwards, in some examples, the vent structure  216  may be placed into the opening in the side portion  208  of the pellicle frame  206 . The vent structure  216  may then be bonded to the pellicle frame  206 , e.g., by a brazing process, a direct diffusion bond process, a eutectic bonding process, another applicable process, or a combination thereof. 
     In some examples, the vent structure  216  may prevent the pellicle membrane from rupturing during the EUV lithography process, as discussed in further detail below. 
     As further illustrated in  FIGS.  2 A and  2 B , the pellicle-mask structure  200  may further include a pellicle frame adhesive  218  positioned between the pellicle frame  206  and the mask substrate  202 . 
     In some examples, the pellicle frame adhesive  218  may be formed from a crosslink type adhesive, a thermoplastic elastomer type adhesive, a polystyrene type adhesive, an acrylic type adhesive, a silicon-based adhesive, an epoxy type adhesive, or a combination thereof. 
     In some examples, a surface treatment may be performed on the pellicle frame  206  to enhance the adhesion of the pellicle frame  206  to the pellicle frame adhesive  218 . In some examples, the surface treatment may include an oxygen plasma treatment, another applicable treatment, or a combination thereof. However, in other examples, no surface treatment may be performed on the pellicle frame  206 . 
     The pellicle-mask structure  200  may further include a pellicle membrane adhesive  220  positioned over the pellicle frame  206 . In some examples, the pellicle membrane adhesive  220  may be formed from a thermoplastic elastomer type adhesive, a polystyrene type adhesive, an acrylic type adhesive, a silicon-based adhesive, an epoxy type adhesive, another suitable adhesive, or a combination thereof. In some examples, the pellicle membrane adhesive  220  may be formed from a material that is different from the material making up the pellicle frame adhesive  218 . 
     As further illustrated in  FIGS.  2 A and  2 B , the pellicle-mask structure  200  may further include a pellicle membrane  222  positioned over the pellicle frame  206  and the pellicle membrane adhesive  220 . As illustrated, the pellicle membrane adhesive  220  may be positioned between the pellicle membrane  222  and the pellicle frame  206 . 
     In some examples, the pellicle membrane  222  may include a border  224  positioned over the pellicle membrane adhesive  220  and a top membrane portion  226  positioned over the border  224 . In some examples, the border  224  may be formed from Si. In further examples, the border  224  may be formed from boron carbide, C, graphene, carbon nanotube, SiC, SiN, SiO 2 , SiON, Zr, Nb, Mo, Cd, Ru, Ti, Al, Mg, V, Hf, Ge, Mn, Cr, W, Ta, Ir, Zn, Cu, F, Co, Au, Pt, Sn, Ni, Te, Ag, another suitable material, an allotrope of any of these materials, or a combination thereof. The border  224  may mechanically support the top membrane portion  226  around the periphery of the top membrane portion  226 . The border  224  may, in turn, be mechanically supported by the pellicle frame  206  when the pellicle-mask structure  200  is fully assembled. That is, the pellicle frame  206  may mechanically support the border  224  and the top membrane portion  226  of the pellicle membrane  222  on the photomask  108 . 
     In one example, the vent structure  216  of the pellicle frame  206  may be formed so that at least one side portion  208  of the pellicle frame  206  includes one aperture formed in both the top of the side portion  208  (e.g., near the border  224 ) and another aperture formed in the bottom of the side portion  208  (e.g., near the mask pattern  204 ). 
     In some examples, the top membrane portion  226  may be formed from a transparent carbon-based film or a transparent silicon-based film, such as a carbon nanotube film or a silicon nanowire film. 
       FIG.  2 C , for instance, illustrates an example top membrane portion  226  of the pellicle membrane  222  of  FIGS.  2 A and  2 B . In the example illustrated in  FIG.  2 C , the top membrane portion  226  comprises a carbon nanotube film. In this example, the network of carbon nanotubes making up the carbon nanotube film may have a structure density of between 0.2 and one, depending on the desired percentage of radiation to be transmitted by the pellicle  114 . For instance, carbon nanotube films have been shown to achieve up to approximately ninety percent visible light transmittance. In general, a one nanometer tube thickness for the carbon nanotubes should translate into approximately 0.5 to one percent absorption for radiation in the extreme ultraviolet wavelengths. The precise structure density may be chosen to maximize EUV radiation transmission while minimizing passage of particles through the top membrane portion  226 . For instance, although a looser structure density may allow for greater EUV radiation transmission, the looser structure density may also allow particles to fall through to the photomask  108 . The carbon nanotube film may be formed by a roll-to-roll process, another suitable process, or any combination thereof. 
     In some examples, the carbon-based film or the silicon-based film may further be coated with a protective shell.  FIG.  2 D , for instance, illustrates a cross sectional view of an example carbon nanotube  230  of the network of carbon nanotubes illustrated in  FIG.  2 C . As illustrated, the carbon nanotube  230  is coated with a protective shell  232 . The protective shell  232  may comprise, for instance, Ru, Mo, Zr, B, Nb, MoSi, SiN, SiO, another suitable material, or a combination thereof. The protective shell  232  may have a thickness that is between 0.1 and ten nanometers. The material and the thickness of the protective shell  232  may be selected to provide high transmission of EUV radiation, to dissipate heat from the top membrane portion  226 , and to minimize dissipation with the film material (e.g., carbon or silicon). In one example, the protective shell  232  may be deposited onto the network of carbon nanotubes using atomic layer deposition, physical vapor deposition, chemical vapor deposition, wet chemical plating, another suitable process, or any combination thereof. 
     In some examples, the border  224  and/or the top membrane portion  226  may include multiple layers. In some examples, the pellicle membrane  222  may be formed using a substrate backside photo/etching process, another applicable process, or a combination thereof. 
     In some examples, the material of the border  224  is the same as the material of the top membrane portion  226 . In other examples, however, material of the border  224  is different from the material of the top membrane portion  226 . For instance, the border  224  may be formed from silicon, while the top membrane portion  226  comprises a carbon nanotube film. In some examples, the thickness of the top membrane portion  226  is between ten nanometers and one hundred nanometers. In more particular examples, the thickness of the top membrane portion  226  is between twenty and fifty nanometers. These ranges have been found to provide sufficient robustness to the top membrane portion  226 , while also providing high EUV transmission. In general, the thicker the top membrane portion  226 , the more robust the top membrane portion  226  will be; however, if the top membrane portion  226  is too thick, the percentage of EUV transmission may decrease. Thus, the disclosed ranges strike a balance between these two aims. 
     In some examples, either or both of the pellicle frame adhesive  218  and the pellicle membrane adhesive  220  may include heat-dissipating fillers. The heat-dissipating fillers may include, for example, aluminum nitride, boron nitride, aluminum oxide, magnesium oxide, silicon oxide, graphite, metal powder, ceramic powder, another suitable material, or a combination thereof. In some cases, the EUV lithography process may involve a high-energy light beam that penetrates the pellicle membrane  222 , causing the temperature of the pellicle membrane  222  to increase. The heat-dissipating fillers may help to dissipate the heat of the pellicle membrane  222  through the pellicle membrane adhesive  220 , to the pellicle frame  206 , to the pellicle frame adhesive  218 , to the mask  108  and the EUV lithography apparatus. Thus, the temperature of the pellicle membrane  222  may be decreased during EUV lithography processing, thereby reducing the likelihood of the pellicle membrane  222  rupturing. 
     As illustrated in  FIG.  2 A , the pellicle  114  and the mask  108  may form an enclosed inner volume  228  that is enclosed by the pellicle  114  and the mask  108 . The pellicle  114  and the mask  108  may separate the inner volume  228  from an outer environment  230 . The vent structure  216  may extend from the inner volume  228  through to the outer environment  230 . 
     In some examples, the EUV lithography process may be performed under extremely high vacuum conditions. In such conditions, a pellicle-mask structure lacking sufficient ventilation might rupture due to the pressure differential between the inner volume  228  (e.g., the side of the pellicle  114  facing the mask  108 ) and the outer environment  230  (e.g., the side of the pellicle  114  facing away from the mask  108 ). However, the pellicle-mask structure  200  of  FIGS.  2 A and  2 B  is less prone to rupture, since the vent structure  216  allows the pressure in the inner volume  228  to be balanced with the pressure in the outer environment  230  during an EUV lithography process. 
     In some examples, the pellicle frame  206  includes the side portions  208 , and the pellicle-mask structure  200  includes at least one vent structure  216  positioned in the side portion  208 . In some examples, as shown in  FIG.  2 B , the pellicle frame  206  may include four side portions  208 ; accordingly, the pellicle-mask structure  200  may include at least four corresponding vent structures  216 . In some examples, the four vent structures  216  may be positioned in the four side portions  208 , respectively. However, many variations and/or modifications can be made to the example shown in  FIG.  2 B . For instance, in some examples, the pellicle-mask structure  200  may include additional vent structures  216 . In some examples, two or more vent structures  216  may be formed in a single side portion  208 . In other examples, some of the side portions  208  may include no vent structure  216 . In further examples, a vent structure  216  may consume the entire area of a side portion  208 . 
     It will be appreciated that  FIG.  1    represents a simplified form of a lithography system  100 . In some examples, the lithography system  100  may include additional components that are not illustrated, such as additional optics, a plasma source, and other components. 
       FIG.  3    is a flow diagram illustrating a method  300  for assembling a pellicle for a lithography process, according to one example of the present disclosure. For instance, the method  300  may be performed to assemble the pellicle  114  illustrated in  FIGS.  1  and  2 A- 2 B . The method  300  may be performed using one or more different machines, under the control of controller or processor. 
     The method  300  begins in step  302 . In step  304 , a transparent carbon-based or silicon-based film may be constructed on a template substrate. The template substrate may comprise, for instance, polyvinyl alcohol (PVA), polystyrene (PS), polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), or a chemical vapor deposited poly(p-xylylene) polymer (e.g., parylene C). In one example, where the film is a carbon-based film, the carbon-based film is a carbon nanotube film. In a further example, the carbon nanotube film has a nanotube network structure density of between 0.2 and one. In another example, where the film is a silicon-based film, the silicon-base film is a silicon nanowire film. In one example, the carbon-based or silicon-based film may be constructed using a roll-to-roll process in a plasma environment, such as a plasma reacting chamber (in which the reacting gas may comprise, for instance, C x H y , H 2 , Ar, O 2 , another suitable gas, or any combination thereof). 
     In step  306 , the transparent carbon-based or silicon-based film may be transferred to a border, and the template substrate may be removed to make the carbon-based or silicon-based film freestanding. In one example, a dry transfer technique is used to transfer the carbon-based or silicon-based film from the template substrate to the border. In one example, the border is formed of silicon. The border may have a rectangular shape, as shown in  FIG.  2 B . 
     In step  308 , the transparent carbon-based or silicon-based film may be coated with a protective shell to ensure the environmental stability of the carbon-based or silicon-based film. In one example, the protective shell may comprise Ru, Mo, Zr, B, Nb, MoSi, SiN, SiO, another suitable material, or a combination thereof. The shell may have a thickness that is between 0.1 and ten nanometers. 
     In step  310 , the border and carbon-based or silicon-based film may be attached to a pellicle frame including a vent structure. The pellicle frame may have the same shape as the border (e.g., rectangular), as shown in  FIGS.  2 A and  2 B . In one example, the pellicle frame may be formed from Si, SiC, SiN, glass, a low coefficient of thermal expansion material (e.g., an Al alloy, a Ti alloy, Invar, Kovar, or the like), another suitable material, or any combination thereof. In a further example, the pellicle frame may include a vent structure, for instance as illustrated in  FIGS.  2 A and  2 B . That is, the vent structure may include apertures shaped like circles, rectangles, slits, other suitable shapes, or any combination thereof. 
     The method may end in step  312 . 
       FIG.  4    illustrates a flowchart of a method  400  of fabricating a semiconductor device according to at least one embodiment of the present disclosure. At least some steps of the method  400  may be performed via a controller of an EUV lithography system, such as the lithography system illustrated in  FIG.  1   . 
     While the method  400  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. 
     The method  400  begins in step  402 . In step  404 , an EUV light source may be activated to pattern a resist layer on a substrate (where the substrate may be a semiconductor wafer). For example, the EUV light source may be part of a lithography system such as the system illustrated in  FIG.  1    and discussed above. Thus, the EUV light source may generate light in extreme ultraviolet wavelengths (potentially by vaporizing droplets of metal into a highly ionized plasma). 
     In step  406 , light emitted by the EUV light source may be directed onto a photomask. The photomask may have a pattern etched into its surface, where the pattern is to be printed onto the resist layer on the substrate. 
     In step  408 , falling particles may be caught on a pellicle that is positioned over the photomask, in order to keep the photomask clear of the falling particles while the light is being directed onto the photomask. In one example, the pellicle includes a transparent carbon-based or silicon-based film coated with a protective shell that dissipates heat from the film. The pellicle may further include a frame supporting the film, where the frame includes at least one aperture to allow for a flow of air through a portion of the pellicle. For instance, the pellicle may be arranged as shown in  FIGS.  2 A- 2 D . The pellicle may be positioned to prevent particles from falling onto and contaminating the photomask. 
     In step  410 , the light that passes through the pellicle and photomask may be collected by a projection optics module and focused onto the resist layer, to pattern the resist layer. For instance, exposure of the resist layer to the light may cause features having a target pitch to be printed in the resist. 
     In one embodiment, the blocks  404 - 410  may be continuously repeated during operation of the EUV light source (e.g., for multiple layers of the substrate). At block  412 , the method  400  may end. 
     It should be noted that the methods  300  and  400  may be expanded to include additional steps or may be modified to include additional operations with respect to the steps outlined above. Furthermore, steps, blocks, functions or operations of the above described method  300  or the method  400  can be combined, separated, and/or performed in a different order from that described above, without departing from the examples of the present disclosure. 
     Thus, examples of the present disclosure provide a robust, high transmission pellicle that is resistant to temperature- and pressure-induced deformation and that transmits a high percentage (e.g., greater than eighty-two percent, and in some examples greater than ninety percent) of radiation onto the photomask. The pellicle of the present disclosure may be especially suitable for use in ultraviolet lithography systems, and more particularly in extreme ultraviolet lithography systems. 
     In one example, the present disclosure provides a pellicle that includes a membrane and a frame supporting the membrane. The membrane may be formed from at least one of a transparent carbon-based film and a transparent silicon based film. The at least one of the transparent carbon-based film and the transparent silicon based film may further be coated with a protective shell. The frame may include at least one aperture to allow for a flow of air through a portion of the pellicle. 
     In another example, an apparatus includes an extreme ultraviolet illumination source, an illuminator, a photomask, and a pellicle. The extreme ultraviolet illumination source is arranged to generate a beam of extreme ultraviolet illumination to pattern a resist layer on a substrate. The illuminator is arranged to direct the beam of extreme ultraviolet radiation onto a photomask. The photomask includes a pattern with which to pattern the resist layer. The pellicle includes a membrane and a frame supporting the membrane. The membrane may be formed from at least one of a transparent carbon-based film and a transparent silicon based film. The at least one of the transparent carbon-based film and the transparent silicon based film may further be coated with a protective shell. The frame may include at least one aperture to allow for a flow of air through a portion of the pellicle. 
     In another example, a method includes activating an extreme ultraviolet light source to pattern a resist layer on a substrate. The light emitted by the extreme ultraviolet light source is directed onto a photomask that includes a pellicle. The pellicle includes a membrane and a frame supporting the membrane. The membrane may be formed from at least one of a transparent carbon-based film and a transparent silicon based film. The at least one of the transparent carbon-based film and the transparent silicon based film may further be coated with a protective shell. The frame may include at least one aperture to allow for a flow of air through a portion of the pellicle. The light that passes through the pellicle and the photomask is collected and directed onto the resist layer. 
     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.