Patent Publication Number: US-2023135538-A1

Title: Methods to improve mechanical properties of pellicle membrane

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application Ser. No. 63/274,642, filed on Nov. 2, 2021, which is incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     A photolithographic patterning process uses a reticle (i.e. photomask) that includes a desired mask pattern. The reticle may be a reflective mask or a transmission mask. In the process, ultraviolet light is reflected off the surface of the reticle (for a reflective mask) or transmitted through the reticle (for a transmission mask) to transfer the pattern to a photoresist on a semiconductor wafer. The minimum feature size of the pattern is limited by the light wavelength. Deep ultraviolet (UV) lithography uses a wavelength of 193 nm or 248 nm. Extreme ultraviolet (EUV) light, which spans wavelengths from 124 nanometers (nm) down to 10 nm, is currently being used to provide small minimum feature sizes. At such short wavelengths, particle contaminants on the photomask can cause defects in the transferred pattern. 
    
    
     
       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 cross-sectional view of an example reticle and pellicle assembly, in accordance with some embodiments. 
         FIGS.  2 A- 2 H  are side views of eight different embodiments of a multi-layer pellicle membrane.  FIG.  2 A  is a first embodiment.  FIG.  2 B  is a second embodiment.  FIG.  2 C  is a third embodiment.  FIG.  2 D  is a fourth embodiment.  FIG.  2 E  is a fifth embodiment.  FIG.  2 F  is a sixth embodiment.  FIG.  2 G  is a seventh embodiment.  FIG.  2 H  is an eighth embodiment. 
         FIG.  3    is an exploded view of a first embodiment of a pellicle membrane, pellicle membrane assembly, and pellicle assembly in accordance with some embodiments. 
         FIG.  4    is an exploded view of a second embodiment of a pellicle membrane, pellicle membrane assembly, and pellicle assembly in accordance with some embodiments. 
         FIG.  5    is an exploded view of a third embodiment of a pellicle membrane, pellicle membrane assembly, and pellicle assembly in accordance with some embodiments. 
         FIG.  6    is an exploded view of a fourth embodiment of a pellicle membrane, pellicle membrane assembly, and pellicle assembly in accordance with some embodiments. 
         FIG.  7 A- 7 D  are side views of different embodiments of a pellicle assembly attached to an EUV reticle.  FIG.  7 A  is a first embodiment,  FIG.  7 B  is a second embodiment,  FIG.  7 C  is a third embodiment, and  FIG.  7 D  is a fourth embodiment. 
         FIGS.  8 A- 8 C  are different views of a mounting frame, in accordance with some embodiments.  FIG.  8 A  is a plan cross-sectional view,  FIG.  8 B  is a first side view, and  FIG.  8 C  is a front side view. 
         FIG.  9    is a flow chart illustrating a first method for preparing a nanotube membrane layer, in accordance with some embodiments. 
         FIGS.  10 A- 10 D  are a set of diagrams illustrating the method of  FIG.  9   , in accordance with some embodiments.  FIG.  10 A  is a first diagram,  FIG.  10 B  is a second diagram,  FIG.  10 C  is a third diagram, and  FIG.  10 D  is a fourth diagram. 
         FIG.  11    is a flow chart illustrating a second method for preparing a nanotube membrane layer, in accordance with some embodiments. 
         FIG.  12    is a diagram illustrating a first embodiment of the method of  FIG.  11   , in accordance with some embodiments. 
         FIGS.  13 A- 13 C  are a set of diagrams illustrating a second embodiment of the method of  FIG.  11   , in accordance with some embodiments.  FIG.  13 A  is a first diagram,  FIG.  13 B  is a second diagram, and  FIG.  13 C  is a third diagram. 
         FIG.  14    is a flow chart illustrating a method for preparing a graphene membrane layer, in accordance with some embodiments. 
         FIGS.  15 A- 15 B  are a set of diagrams illustrating the method of  FIG.  14   , in accordance with some embodiments.  FIG.  15 A  is a first diagram, and  FIG.  15 B  is a second diagram. 
         FIG.  16    is a flow chart illustrating another method for preparing a multi-layer structure for a pellicle membrane, in accordance with some embodiments. 
         FIGS.  17 A- 17 C  are a set of diagrams illustrating the method of  FIG.  16   , in accordance with some embodiments.  FIG.  17 A  is a first diagram,  FIG.  17 B  is a second diagram, and  FIG.  17 C  is a third diagram. 
         FIG.  18    is a flow chart illustrating a method for preparing a multi-layer structure for a pellicle membrane, in accordance with some embodiments. 
         FIGS.  19 A- 19 C  are a set of diagrams illustrating the method of  FIG.  18   , in accordance with some embodiments.  FIG.  19 A  is a first diagram,  FIG.  19 B  is a second diagram, and  FIG.  19 C  is a third diagram. 
         FIGS.  20 A- 20 D  are a set of diagrams illustrating a method for coating an outer surface of a pellicle membrane, in accordance with some embodiments.  FIG.  20 A  is a first diagram,  FIG.  20 B  is a second diagram,  FIG.  20 C  is a third diagram, and  FIG.  20 D  is a fourth diagram. 
         FIG.  21    is a flow chart illustrating a fourth embodiment of a method for preparing a pellicle assembly, in accordance with some embodiments. 
         FIGS.  22 A- 22 D  is an illustration of one method for practicing the method of  FIG.  21   , in accordance with some embodiments.  FIG.  22 A  is a first diagram,  FIG.  22 B  is a second diagram,  FIG.  22 C  is a third diagram, and  FIG.  22 D  is a fourth diagram. 
         FIGS.  23 A- 23 B  are a set of diagrams illustrating an exemplary membrane stretching device, in accordance with some embodiments.  FIG.  23 A  is a top perspective view of the membrane stretching device.  FIG.  23 B  is a bottom perspective view of the membrane stretching device. 
         FIGS.  24 A- 24 D  are a set of side cross-sectional diagrams illustrating a first exemplary process of stretching a membrane with a membrane stretching device, in accordance with some embodiments.  FIG.  24 A  is a first diagram,  FIG.  24 B  is a second diagram,  FIG.  24 C  is a third diagram, and  FIG.  24 D  is a fourth diagram. 
         FIGS.  25 A- 25 C  are a set of side cross-sectional diagrams illustrating a second exemplary process of stretching a membrane with a membrane stretching device, in accordance with some further embodiments.  FIG.  25 A  is a first diagram,  FIG.  25 B  is a second diagram, and  FIG.  25 C  is a third diagram. 
         FIG.  26    is a flow chart illustrating a method for processing a semiconductor wafer substrate, in accordance with some embodiments. 
         FIGS.  27 A- 27 C  are diagrams illustrating some steps of the method for processing a semiconductor wafer substrate, in accordance with some embodiments. 
         FIG.  28    is an illustration of radiation raveling to the photomask, being reflected, and exposing a photoresist layer on a semiconductor wafer substrate, in accordance with some embodiments. 
     
    
    
     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. 
     Numerical values in the specification and claims of this application should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the stated value by less than the experimental error of conventional measurement technique of the type described in the present application to determine the value. All ranges disclosed herein are inclusive of the recited endpoint. 
     The present disclosure may refer to temperatures for certain method steps. It is noted that these references are usually to the temperature at which the heat source is set, and do not specifically refer to the temperature which must be attained by a particular material being exposed to the heat. 
     Photolithographic patterning processes use a reticle (i.e. photomask) that includes a desired mask pattern. The reticle may be a reflective mask or a transmission mask. In the process, ultraviolet light is reflected off the surface of the reticle (for a reflective mask) or transmitted through the reticle (for a transmission mask) to transfer the pattern to a photoresist on a semiconductor wafer. The exposed portion of the photoresist is photochemically modified. After the exposure, the resist is developed to define openings in the resist, and one or more semiconductor processing steps (e.g. etching, epitaxial layer deposition, metallization, et cetera) are performed which operate on those areas of the wafer surface exposed by the openings in the resist. After this semiconductor processing, the resist is removed by a suitable resist stripper or the like. 
     The minimum feature size of the pattern is limited by the light wavelength. Deep ultraviolet (UV) lithography, for example using a wavelength of 193 nm or 248 nm in some standard deep UV platforms, typically employs transmission masks and provides a smaller minimum feature size than lithography at longer wavelengths. Extreme ultraviolet (EUV) light, which spans wavelengths from 124 nanometers (nm) down to 10 nm, is currently being used to provide even smaller minimum feature size. At shorter wavelengths, particle contaminants on the reticle can cause defects in the transferred pattern. Thus, a pellicle assembly (or simply pellicle) is used to protect the reticle from such particles. The pellicle assembly includes a pellicle membrane which is attached to a mounting frame. The mounting frame supports the pellicle membrane over the reticle. Any contaminating particles which land on the pellicle membrane are thus kept out of the focal plane of the reticle, thus reducing or preventing defects in the transferred pattern. 
       FIG.  1    illustrates a cross-sectional view of an example reticle assembly  105  useful in lithography, according to some embodiments. The reticle assembly  105  includes a reticle  100  and a pellicle assembly  120 . The illustrative reticle  100  (also referred to in the art as a mask, photomask, or similar phraseology) is a reflective mask of a type commonly used in EUV lithography, and includes a substrate  102 , alternating reflective layers  104  and spacing layers  106 , a capping layer  108 , an EUV absorbing layer  110  that is patterned to define a mask pattern, an anti-reflective coating (ARC)  112 , and a conductive backside layer  114 . The illustrative reticle  100  is merely a nonlimiting example. More generally, pellicles as disclosed herein can be used with substantially any type of reflective or transmission reticle. As another example (not shown), the reticle may be a transmission reticle, in which case the substrate is transmissive for light at the wavelength at which the lithography is performed. In general, the reflective or transmissive reticle includes a substrate (e.g. substrate  102 ) and a mask pattern (e.g. absorbing layer  110 ) disposed on the substrate. As illustrated here, the pellicle assembly  120  includes a mounting frame  122 , an adhesive layer  124 , and a pellicle membrane  130 . In some non-limiting illustrative embodiments, the reticle and pellicle assembly are intended for use with EUV light wavelengths, for example from 124 nm to 10 nm, including about 13.5 nm. 
     In embodiments, the substrate  102  is made from a low thermal expansion material (LTEM), such as quartz or titania silicate glasses available from Corning under the trademark ULE. This reduces or prevents warping of the reticle due to absorption of energy and consequent heating. The reflective layers  104  and the spacing layers  106  cooperate to form a Bragg reflector for reflecting EUV light. In some embodiments, the reflective layers may comprise molybdenum (Mo). In some embodiments, the spacing layers may comprise silicon (Si). The capping layer  108  is used to protect the reflector formed from the reflective layers and the spacing layers, for example from oxidation. In some embodiments, the capping layer comprises ruthenium (Ru). The EUV absorbing layer  110  absorbs EUV wavelengths, and is patterned with the desired pattern. In some embodiments, the EUV absorbing layer comprises tantalum boron nitride. The anti-reflective coating (ARC)  112  further reduces reflection from the EUV absorbing layer. In some embodiments, the anti-reflective coating comprises oxidized tantalum boron nitride. The conductive backside layer  114  permits mounting of the illustrative reticle on an electrostatic chuck and temperature regulation of the mounted substrate  102 . In some embodiments, the conductive backside layer comprises chrome nitride. 
     The mounting frame  122  supports the pellicle membrane at a height sufficient to take the pellicle membrane  130  outside the focal plane of the lithography, e.g., several millimeters (mm) over the reticle in some nonlimiting illustrative embodiments. The mounting frame itself can be made from suitable materials such as anodized aluminum, stainless steel, plastic, silicon (Si), titanium, silicon dioxide, aluminum oxide (Al 2 O 3 ), or titanium dioxide (TiO 2 ). Vent holes may be present in the mounting frame for equalizing pressure on both sides of the pellicle membrane. 
     The adhesive layer  124  is used to secure the pellicle membrane to the mounting frame. Suitable adhesives may include a silicon, acrylic, epoxy, thermoplastic elastomer rubber, acrylic polymer or copolymer, or combinations thereof. In some embodiments, the adhesive can have a crystalline and/or amorphous structure. In some embodiments, the adhesive can have a glass transition temperature (Tg) that is above a maximum operating temperature of the photolithography system, to prevent the adhesive from exceeding the Tg during operation of the system. 
     The pellicle membrane  130  is usually stretched over the mounting frame to obtain a uniform and flat surface. However, sagging of the pellicle membrane can occur, causing the membrane to deflect significantly from the desired flat and uniform orientation. This deflection can affect the light that is being reflected from the reticle and the resulting transferred pattern. 
     In addition, reticles (and their protective pellicle assembly) are maintained in reticle pods for safety and protection during lithographic patterning and other processes. Current EUV lithography systems typically use a dual-pod configuration consisting of an inner metal pod under vacuum and an outer pod with access to the ambient environment. The inner pod is only opened when the pod is inside the tool. Pressure differences, gravity, and other external forces can cause the pellicle membrane to deflect or sag. If the pellicle membrane sags far enough to contact the inner surface of the inner metal pod in which the reticle is kept, contamination of the pellicle membrane can occur, or the pellicle membrane itself might break. 
     The present disclosure thus relates to pellicle membranes and methods for producing pellicle membranes that are intended to reduce deflection of the pellicle membrane while maintaining high transmittance of EUV light and the particle-protecting ability of the pellicle membrane. In particular, the pellicle membranes contain at least one layer made from nanotubes having a minimum length of at least 1,000 micrometers (μm) (1.0 mm). Put another way, all of the nanotubes in the layer have this minimum length. Such pellicle membranes have improved mechanical properties compared to those made with shorter nanotubes, in particular reduced deflection. 
     In some embodiments, the pellicle membrane is a single-layer structure. In other embodiments, the pellicle membrane is a multi-layer structure. In some embodiments, the layers of the multi-layer structure can be made of the same materials, and in other embodiments the layers of the multi-layer structure can be made of different materials selected for particular purposes and arranged in order as desired. For example, in some embodiments, the pellicle membrane may comprise one or more nanotube membrane layers and one or more graphene membrane layers. 
     The pellicle membrane can be attached to a border or to a suitably shaped mounting frame for mounting to the reticle. In some embodiments, a conformal coating is then applied to the outer surface of the pellicle membrane (which can be a single layer or a multi-layer structure). The coated pellicle membrane/border can then be affixed to a mounting frame (if needed) to form a pellicle assembly which can be mounted onto a reticle. 
     A combination of several low-density membrane layers can be used to obtain a pellicle membrane that has a combination of high transmittance, small pore size and a stiffness which minimizes any potential deflection. 
     In particular embodiments, the nanotube layer can be formed from single nanotubes, thin nanotube bundles, or thick nanotube bundles. A thin nanotube bundle is formed from two to 10 individual nanotubes wrapped around each other (i.e. helically). A thick nanotube bundle is formed from more than 10 individual nanotubes wrapped around each other. While there is no theoretical limit, in particular embodiments a thick nanotube bundle may be formed from a maximum of 20 nanotubes. It is noted that the single/individual nanotube may be a single-walled nanotube or a multi-walled nanotube. The walls of a multi-walled nanotube are arranged concentrically, not helically, and a multi-walled nanotube should not be considered a nanotube bundle. 
     A multi-layer pellicle membrane can be formed from any number of layers, and in any combination.  FIGS.  2 A- 2 H  are side views of eight different illustrative embodiments of a multi-layer pellicle membrane. In these figures, a conformal coating is not applied. 
       FIG.  2 A  depicts a first embodiment of a pellicle membrane  130  formed from two different layers. The top layer is a single nanotube layer  157 , and the bottom layer is a thick nanotube bundle layer  158 . The thick nanotube bundle layer is formed from thick nanotube bundles. 
       FIG.  2 B  depicts a second embodiment of a pellicle membrane  130  where the top layer is a thin bundle nanotube layer  159 , and the bottom layer is a thick nanotube bundle layer  158 . The thick nanotube bundle layer is formed from thin nanotube bundles. 
       FIG.  2 C  depicts a third embodiment of a pellicle membrane  130  where the top layer is a thick nanotube bundle layer  158 , and the bottom layer is a single nanotube layer  157 . 
       FIG.  2 D  depicts a fourth embodiment of a pellicle membrane  130  where the top layer is a thick bundle nanotube layer  158 , and the bottom layer is a thin bundle nanotube layer  159 . 
       FIG.  2 E  depicts a fifth embodiment of a pellicle membrane  130  formed from three different layers. The top layer is a first thick nanotube bundle layer  158 , the middle layer is a single nanotube layer  157 , and the bottom layer is a second thick nanotube bundle layer  158 . 
       FIG.  2 F  depicts a sixth embodiment of a pellicle membrane  130  where the top layer is a first thick nanotube bundle layer  158 , the middle layer is a thin bundle nanotube layer  159 , and the bottom layer is a second thick nanotube bundle layer  158 . 
       FIG.  2 G  depicts a seventh embodiment of a pellicle membrane  130  where the top layer is a thin bundle nanotube layer  159 , the middle layer is a single nanotube layer  157 , and the bottom layer is a thick nanotube bundle layer  158 . 
       FIG.  2 H  depicts an eighth embodiment of a pellicle membrane  130  where the top layer is a single nanotube layer  157 , the middle layer is a thin bundle nanotube layer  159 , and the bottom layer is a thick nanotube bundle layer  158 . 
     It is noted that the nanotube bundle layers are stiffer and minimize deflection more than a single nanotube layer. However, the nanotube bundle layers also have larger pore sizes compared to a single nanotube layer. Thus, it is particularly desirable in multi-layer pellicle membranes to use a combination of a single nanotube layer with a nanotube bundle layer. 
     Continuing,  FIGS.  3 - 6    show various embodiments of a pellicle membrane  130 , pellicle membrane assembly  170 , and pellicle assembly  120  according to the present disclosure. In these embodiments, a conformal coating is present. 
     In the first embodiment of  FIG.  3   , the pellicle membrane  130  is a multi-layer structure formed from a first nanotube membrane layer  150  and a second nanotube membrane layer  152 . As illustrated here, the first nanotube membrane layer  150  and the second nanotube membrane layer  152  are formed from randomly oriented nanotubes, and the two layers contact each other. In some embodiments, each nanotube membrane layer has a thickness of about 10 nm to about 100 nm. 
     Here, the second nanotube membrane layer is also considered the outer surface  132  of the pellicle membrane, to which a conformal coating is applied. The conformal coating may be considered to form the outermost layer  172  of the pellicle membrane. In some embodiments, the outermost layer has a thickness of about 1 nanometer (nm) to about 10 nm. The first nanotube membrane layer is also considered the inner surface  134  of the pellicle membrane and is attached to a border  128 . The border runs along the perimeter of the pellicle membrane. The border is also attached to a mounting frame  122 . 
     The combination of the outermost layer/conformal coating  172  and the pellicle membrane  130  together is referred to as a pellicle membrane assembly  170  herein. The combination of the pellicle membrane assembly  170 , border  128 , and mounting frame  122  is referred to herein as a pellicle assembly  120 . 
       FIG.  4    is an exploded view of a second embodiment of a pellicle membrane  130 , pellicle membrane assembly  170 , and pellicle assembly  120  according to the present disclosure. In contrast to  FIG.  3   , the first nanotube membrane layer  154  and the second nanotube membrane layer  156  are formed from directionally oriented nanotubes. In some embodiments, the directionally oriented nanotube membrane layers are aligned at an angle relative to each other. Here, the two nanotube membrane layers  154 ,  156  are aligned at 90° relative to each other. 
       FIG.  5    is an exploded view of a third embodiment of a pellicle membrane  130 , pellicle membrane assembly  170 , and pellicle assembly  120  according to the present disclosure. Here, the pellicle membrane is a multi-layer structure formed from a graphene membrane layer  160 , a first nanotube membrane layer  150 , and a second nanotube membrane layer  152 . The graphene membrane layer is also considered the outer surface  132  of the pellicle membrane. The graphene membrane layer may be, in some embodiments, a porous film or a continuous film without pores. The first nanotube membrane layer is also considered the inner surface  134  of the pellicle membrane and is attached to the border  128 . The first nanotube membrane layer  150  and the second nanotube membrane layer  152  are formed from randomly oriented nanotubes. In some embodiments, the graphene membrane layer  160 , the first nanotube membrane layer  150 , and the second nanotube membrane layer  152  directly contact each other. 
       FIG.  6    is an exploded view of a fourth embodiment of a pellicle membrane  130 , pellicle membrane assembly  170 , and pellicle assembly  120  according to the present disclosure. The pellicle membrane is similar to that of  FIG.  8   , except the first nanotube membrane layer  154  and the second nanotube membrane layer  156  are formed from directionally oriented nanotubes, as in the embodiment of  FIG.  7   . 
     In some different embodiments not illustrated, the graphene membrane layer  160  forms the inner surface  134  of the pellicle membrane and is attached to the border  128 . The second nanotube membrane layer would be considered the outer surface  132  of the pellicle membrane. Both nanotube membrane layers can be formed from randomly oriented nanotubes or directionally oriented nanotubes, and contact each other. 
     In other embodiments not illustrated, the graphene membrane layer  160  is located between the two nanotube membrane layers. The first nanotube membrane layer is also considered the inner surface  134  of the pellicle membrane and is attached to the border  128 . The second nanotube membrane layer is considered the outer surface  132  of the pellicle membrane. The first nanotube membrane layer  150  and the second nanotube membrane layer  152  are formed from randomly oriented nanotubes or directionally oriented nanotubes. 
       FIGS.  7 A- 7 D  are side views showing four different embodiments of a pellicle assembly attached to an EUV reticle. They differ from each other based on whether the pellicle membrane is attached to a mounting frame or a border, and on how the various components of the pellicle assembly are attached to each other 
     As illustrated in the first embodiment of  FIG.  7 A , the EUV reticle  100  includes a patterned image  107 . The pellicle assembly  120  includes the pellicle membrane  130  which is attached to border  128  via van der Waals forces. The border  128  is joined to the mounting frame  122  via adhesive layer  129  and protects the patterned image  107  from particle contaminants. As seen here, the mounting frame  122  can include vent holes  123 . The mounting frame  122  is joined to the reticle  100  via a mechanical attachment. 
     In the second embodiment of  FIG.  7 B , the pellicle membrane  130  is attached to border  128  via van der Waals forces. The border  128  is joined to the mounting frame  122  via a first adhesive layer  129 . The mounting frame  122  is joined to the reticle  100  via a second adhesive layer  141 . 
     In the third embodiment of  FIG.  7 C , the pellicle membrane  130  is attached to the mounting frame  122  via a first adhesive layer  129 . The mounting frame  122  is joined to the reticle  100  via a second adhesive layer  141 . 
     In the fourth embodiment of  FIG.  7 D , the pellicle membrane  130  is directly attached to mounting frame  122  via van der Waals forces. The mounting frame  122  is joined to the reticle  100  via a first adhesive layer  129 . 
       FIGS.  8 A- 8 C  are different views of the mounting frame  122 , according to some embodiments of the present disclosure.  FIG.  8 A  is a plan cross-sectional view in which the plane cuts through the vent holes  123 ,  FIG.  8 B  is a first side view, and  FIG.  8 C  is a front side view. Vent holes  123  are visible on all sides of the mounting frame. However, it is contemplated that vent holes may be present on only one, two, or three sides of the mounting frame. 
     Both the border and the mounting frame can each be made from suitable materials such as anodized aluminum, stainless steel, plastic, silicon (Si), titanium, silicon dioxide, aluminum oxide (Al 2 O 3 ), or titanium dioxide (TiO 2 ). As seen here, vent holes  123  may be present in the mounting frame  122  for equalizing pressure on both sides of the pellicle membrane. In some embodiments, the total area of the vent holes can range from zero to about 100 square millimeters (mm 2 ). It is noted that the pellicle membrane itself is relatively porous, and thus can provide the venting function itself. The vent holes can be spaced apart from each other as desired. 
     As described above, one or more layers of the pellicle membrane are formed from nanotubes. In some embodiments, the nanotubes can be carbon nanotubes (CNTs) or boron nitride nanotubes (BNNTs) or silicon carbide nanotubes (SiCNTs) or molybdenum disulfide nanotubes (MoS 2 NTs) or molybdenum diselenide (MoSe 2 NTs) or tungsten disulfide nanotubes (WS 2 NTs) or tungsten diselenide nanotubes (WSe 2 NTs). In some embodiments, the nanotubes can be single-wall nanotubes or multi-wall nanotubes. It is possible for multi-wall nanotubes to be made of different materials, for example a CNT inside a BNNT, or vice versa. In some embodiments, the nanotubes can be metallic or semiconducting or electrically insulating. The diameter of the individual nanotubes is not significant. However, the length of the individual nanotubes may be from about 1,000 μm to about 6 centimeters (cm). 
     The nanotubes may have different properties. For example, carbon nanotubes can have a Young&#39;s modulus of about 1.33 TPa; a maximum tensile strength of about 100 GPa; thermal conductivity of about 3,000 to about 40,000 W/mK; and be stable up to a temperature of about 400° C. in air. Boron nitride nanotubes can have a Young&#39;s modulus of about 1.18 TPa; a maximum tensile strength of about 30 GPa; thermal conductivity of about 3000 W/mK; and be stable up to a temperature of about 800° C. in air. 
     Generally, the nanotubes of each nanotube membrane layer can be randomly oriented or can be directionally oriented in a desired direction. The nanotube membrane layer(s), whether randomly oriented or directionally oriented, can be combined as desired. In some embodiments, the nanotube membrane layer(s) in the pellicle membrane are all randomly oriented. In some embodiments, the nanotube membrane layer(s) in the pellicle membrane are all directionally oriented. In these embodiments, the directionally oriented nanotube membrane layers are aligned at an angle relative to each other. That angle can be any angle between 0° and 180°, and for example may be 0°, 30°, 45°, 60°, 75°, 90°, 120°, 135°, 145°, 160°, or 180°. 
     In addition, in some embodiments, one or more layers of the pellicle membrane are formed from graphene or graphite. Such layers can provide more stiffness compared to layers formed from nanotubes. Graphite is made up of stacked graphene layers, and thus should be considered equivalent to graphene in this disclosure. In contrast to the nanotubes, graphene and graphite are in the shape of flat sheets or porous sheets. Graphene has a Young&#39;s modulus of approximately 1,000 GPa. 
     In some embodiments, the nanotube membrane layer(s), the graphene membrane layer(s), and the resulting pellicle membrane generally should not include any other materials. For example, the membranes should not contain any moisture or any other binders, metals, plastics, surfactants, acids, or other compounds that might have been present in precursor materials or used in prior processing steps. In some embodiments, each individual nanotube membrane layer can have a thickness ranging from about 10 nanometers (nm) to about 100 nm, although thicknesses outside this range are also contemplated. In some embodiments, each individual graphene membrane layer can have a thickness ranging from about 1 nm to about 10 nm, although thicknesses outside this range are also contemplated. 
       FIG.  9    is a flow chart illustrating some embodiments of methods for preparing a nanotube membrane layer. In step  200 , a suspension of nanotubes, such as carbon nanotubes or boron nitride nanotubes, is formed. The nanotubes may be suspended in water or some other suitable liquid. Other ingredients, such as surfactants, may also be present to disperse the nanotubes evenly. Ultrasonication may also be useful for even dispersion of the nanotubes. The nanotubes can then be filtered, treated, and/or cleaned as appropriate. For example, the surfactants can be removed through filtration after dispersion has been achieved. Mild acid, such as hydrochloric acid or nitric acid, may be used to remove particles such as amorphous carbon. 
     Next, in step  210 , an initial nanotube membrane is formed by depositing the suspension on a surface and separating the liquid from the nanotubes. For example, as illustrated here, the suspension can be poured through filter paper, such as a polytetrafluoroethylene (PTFE) membrane having a pore size of about 0.02 micrometers (μm) to about 1.2 μm. Suction can be applied to the opposite side of the filter paper to obtain a uniform dispersion of the nanotubes on the filter paper. 
     Finally, in step  220 , the initial nanotube membrane is processed to reduce its thickness and obtain the nanotube membrane layer. This can be done in multiple ways. In some embodiments, the thickness is decreased by applying compressive pressure (e.g. uniaxial compression) to the initial nanotube membrane, reducing the thickness of the initial nanotube membrane. In some embodiments, the compressive pressure applied is from about 0.1 bar to about 20 bar (about 0.01 MPa to about 2 MPa). In some embodiments, the compressive pressure is applied for a time period of about 1 minute to about 60 minutes. The degree of pressure can change during this time period. For example, the force can be increased gradually to a given force and then maintained at that force level. 
     In some other embodiments, the thickness is decreased by immersing the initial nanotube membrane in a solution, then running the solution through the initial nanotube membrane (for example by applying suction). It is noted that the resulting nanotube membrane layer is made from randomly oriented nanotubes. 
       FIGS.  10 A- 10 D  are a set of drawings illustrating the method of  FIG.  9   , in one embodiment.  FIG.  10 A  illustrates a suspension of nanotubes. The nanotubes  230  are suspended in a liquid  232  within a vessel  234 . In  FIG.  10 B , an initial nanotube membrane  240  is formed by depositing the suspension on a surface  236 , such as filter paper. 
     In  FIG.  10 C , a first method for processing the initial nanotube membrane to reduce its thickness and obtain the nanotube membrane layer is illustrated. Here, the initial nanotube membrane  240  and surface  236  are placed within a pressing machine  250 , which comprises a bolster plate  252  and a ram  254 . The initial nanotube membrane  240  is compressed between the bolster plate  252  and the ram  254  to obtain the nanotube membrane layer  260 . The nanotube membrane layer also has a higher density than the initial membrane, and the nanotube membrane layer is thinner than the initial membrane (i.e. reduced thickness). Without being bound by theory, it is believed that the deformation of the nanotube membrane layer introduced by the uniaxial compression is maintained after the compressive force is removed by van der Waals forces. In other words, the nanotube membrane layer does not return to its original thickness after the compressive force is removed. 
     In  FIG.  10 D , a second method for processing the initial nanotube membrane to reduce its thickness and obtain the nanotube membrane layer is illustrated. Here, the initial nanotube membrane  240  and surface  236  are immersed in a solution  238 . The solution is then run through the initial nanotube membrane to reduce its thickness. In some embodiments, the solvent in the solution is deionized water or an alcohol such as isopropyl alcohol (IPA). The resulting nanotube membrane layer  260  also has a higher density compared to the initial nanotube membrane. 
       FIG.  11    is a flow chart illustrating some additional embodiments of methods for preparing a nanotube membrane layer. In step  300 , nanotube fibers, such as fibers made from carbon nanotubes or boron nitride nanotubes, are produced. Next, in step  310 , an initial nanotube membrane is formed from the nanotube fibers. In some embodiments, this is done by arranging the fibers next to each other. Without being bound by theory, it is believed that the fibers are held together by van der Waals forces of sufficient strength to form the initial nanotube membrane. The initial nanotube membrane can be annealed. The annealing may occur at temperatures of about 1000° C. to about 2000° C. Finally, in step  320 , the initial nanotube membrane is processed to reduce its thickness and obtain the nanotube membrane layer. This can be done as previously described, for example by compression or immersion in solution. It is noted that the resulting nanotube membrane layer is made from directionally oriented nanotubes. 
     The nanotube membrane layer(s) and the graphene membrane layer(s) can be formed using several different fabrication processes. For example, such fabrication processes can include chemical vapor deposition (CVD) such as floating catalyst CVD or plasma-enhanced CVD; electrophoretic deposition; dispersal in a solution and concentration by removal of the solvent; vacuum filtration; and the like. 
     In some embodiments, nanotubes can be produced using water assisted, catalytic CVD. Generally, the nanotubes are produced in a reactor vessel, such as a 2-inch quartz reactor tube. The reactor vessel may be equipped with a heat source along its length in order to maintain a specified temperature inside the vessel. The temperature inside the vessel may range from about 500° C. to about 1000° C. Gaseous reactants, water, and a catalyst are introduced into the reaction vessel to grow the nanotubes. In some embodiments, the gaseous reactants may include argon, hydrogen, and/or ethylene. The partial pressure of argon may range from about 500 mmHg to about 600 mmHg. The partial pressure of hydrogen may range from about 10 mmHg to about 100 mmHg. The partial pressure of ethylene (as a carbon source) may range from about 50 mmHg to about 250 mmHg. The concentration of water within the reactor may range from zero to about 1000 ppm, or from about 200 ppm to about 700 ppm. In some embodiments, the catalyst may be an Iron-Gadolinium (Fe—Gd) alloy film, or another Fe-Lanthanide element alloy. Generally, lanthanide elements enhance the growth rate of nanotubes when used with a Fe catalyst. 
     In other embodiments, the nanotubes can be formed by direct spinning nanotubes from a floating catalyst CVD system. The direct spinning process begins by providing a reactor vessel. The reactor vessel may have a length of greater than five meters for an increased growth path, but can also be shorter. The reactor vessel may also be equipped with a heat source to ensure a specified temperature in the reactor vessel. The temperature inside the vessel may range from about 500° C. to about 1300° C. Usually ferrocene is introduced into the reactor vessel as both a carbon source and an iron catalyst along with hydrogen and sulfur (e.g. in the form of thiophene). Nanotubes are then grown in the vessel and form an aerogel that is then capable of being spun into a fiber. 
     In this regard, sulfur acts as a catalyst to improve the growth of the nanotubes. Sulfur acts as a promoter to enhance the addition of carbon atoms to the growing ends of graphene tubes. Sulfur also acts as a surfactant to encourage tube nucleation and thus prevent carbon encapsulation of the catalyst particle. Additionally, sulfur limits the rate at which the iron particles coarsen by collision. Further, sulphur prevents iron that is deposited on the reactor wall from nucleating and growing nanotubes. 
       FIG.  12    is a drawing illustrating a first embodiment of steps  300  and  310  of the method of  FIG.  11   . A reaction vessel  330  is illustrated, with a heat source  332  for heating materials passing through the reaction vessel. Reactants  334 , catalyst  336 , and carrier gas  338  enter the reaction vessel. Nucleation, growth, and aggregation of nanotubes in the form of an aerogel  344  occur, and the aerogel is then spun into fibers  339 . In some embodiments, this process occurs at temperatures of about 1100° C. to about 1300° C. This results in the nanotubes being directionally oriented (i.e., oriented in the same direction). An organic solvent is used for densification of the fibers. In some embodiments, the organic solvent can be acetone or an alcohol such as isopropyl alcohol. The fibers  339  are then deposited onto a treated filter paper or polymer  346 . Sucking pressure is applied to the treated filter paper  346 , and the treated filter paper  346  is rotated to ensure uniform fiber dispersion and obtain to form an initial nanotube membrane. 
       FIGS.  16 A- 16 C  are a set of drawings illustrating a second embodiment of the method of  FIG.  11   , in one embodiment.  FIG.  13 A  illustrates one embodiment of an apparatus that can be used to produce nanotube fibers using floating catalyst CVD. A reaction vessel  330  is illustrated, with a heat source  332  for heating materials passing through the reaction vessel. Reactants  334 , catalyst  336 , and carrier gas  338  enter the reaction vessel. Nucleation, growth, and aggregation of nanotubes in the form of an aerogel occur, and the aerogel is then spun into fibers  339 , which are collected here on a spool or reel. An organic solvent is used for densification of the fibers. In  FIG.  13 B , an initial nanotube membrane  240  is formed from the nanotube fibers. This can be done, for example, as described above. The carbon nanotubes in the initial nanotube membrane are directionally oriented. 
     In  FIG.  13 C , the initial nanotube membrane of  FIG.  12    or  FIG.  13 B  is processed to reduce its thickness and obtain the nanotube membrane layer. As illustrated here, the initial nanotube membrane  240  is supported by a surface  342 , which is placed within a pressing machine  250 , which comprises a bolster plate  252  and a ram  254 . The initial nanotube membrane  240  is compressed between the bolster plate  252  and the ram  254  to obtain the nanotube membrane layer  260 . 
       FIG.  14    is a flow chart illustrating one embodiment of a method for preparing a graphene membrane layer (which can be used in a multi-layer pellicle membrane). In step  400 , an initial graphene layer is formed by dispersing relatively small graphene flakes or sheets on a surface to obtain a relatively large initial membrane. The smaller flakes or sheets can be arranged so that pores of a desired size are present between the smaller flakes/sheets. It is noted that this initial membrane can fall apart easily, since the individual flakes/sheets are not strongly bound to each other. Finally, in step  410 , the initial graphene layer is processed to reduce its thickness and obtain the graphene membrane layer. In some embodiments, the thickness is reduced by applying compressive pressure (e.g. uniaxial compression) to the initial graphene layer, reducing the volume of the initial graphene layer. The resulting graphene membrane layer can then be affixed to the border or another membrane layer. 
       FIG.  15 A  and  FIG.  15 B  are a set of drawings illustrating the method of  FIG.  14   , in one embodiment. In  FIG.  15 A , an initial graphene layer  430  is formed by dispersing graphene flakes or sheets  432  on a surface  434 . As illustrated in  FIG.  15 B , the graphene membrane layer  160  is prepared by compressing the initial graphene layer  430  between the bolster plate  252  and the ram  254  of a pressing machine  250 . 
     In some embodiments, each initial nanotube or graphene membrane prior to processing has a thickness of at least 0.7 micron (700 nm), and the processing operates to reduce the thickness to produce an individual nanotube or graphene membrane layer having a thickness of 200 nm or less. In some embodiments, the initial membrane(s) may each have a thickness ranging from about 1 micrometer (μm) to about 10 μm. In some embodiments, the resulting nanotube or graphene membrane layer has a thickness of from about 10 nanometers (nm) to about 100 nm. At higher thicknesses, mechanical properties may change in undesirable ways. 
     After the membrane layer(s) for the pellicle membrane is/are made, the membrane layer(s) is/are then joined together to form the pellicle membrane. In some embodiments, the final pellicle membrane, which may be made up of one or more membrane layers, should have a thickness of from about 10 nanometers (nm) to about 100 nm, or from about 20 nm to about 70 nm. 
       FIG.  16    is a flow chart illustrating one embodiment of a method for preparing a multi-layer structure for a pellicle membrane. Very generally, in step  500 , the individual layers are stacked upon each other in their desired order. Next, in step  510 , pressure is applied to reduce the thickness and join the individual layers together to obtain the multi-layered pellicle membrane. Two or more individual layers can be joined together in this single compression step. Next, in step  520 , a mounting frame or border is placed adjacent to a surface of the pellicle membrane. Then, in step  530 , pressure is applied to affix the pellicle membrane to the mounting frame or border. In optional step  540 , the conformal coating is applied to the pellicle membrane. 
       FIGS.  17 A- 17 C  are a set of drawings illustrating some steps of the method of  FIG.  16   , in one embodiment. In  FIG.  17 A , two directionally oriented initial nanotube membranes  240  are illustrated. They can be placed at any angle relative to each other, ranging from 0° to 100° and any range in between, although the relative angle is not critical. In  FIG.  17 B , the two initial nanotube membranes  240  are concurrently compressed between the bolster plate  252  and the ram  254 , resulting in the multi-layer pellicle membrane  130 . In  FIG.  17 C , the border  128  is laid upon the pellicle membrane  130 , and compressive pressure is applied to join them together. A conformal coating is not applied in this set of drawings. 
       FIG.  18    is a flow chart illustrating another embodiment of a method for preparing a multi-layer structure for a pellicle membrane. Very generally, in step  600 , a border or mounting frame is placed adjacent to a surface of a first membrane layer. Next, in step  610 , pressure is applied to affix the first membrane layer to the border/frame. The first membrane layer and the border/frame remain attached via Van der Waals forces. If it is desired to make the pellicle membrane from more than one layer, then in step  620 , the border/frame and any already-attached membrane layer(s) are laid upon a surface of the additional membrane layer. The outermost already-attached membrane layer contacts the surface of the additional membrane layer. Next, in step  630 , pressure is applied again to affix the additional membrane layer to the already-attached membrane layer(s). Steps  620  and  630  can be repeated with additional membrane layers until the desired multi-layer structure of the pellicle membrane is assembled, and a pellicle assembly is obtained. In optional step  640 , the conformal coating is applied to the pellicle membrane. Finally, in step  650 , the pellicle assembly is disposed over a mask pattern on a reticle to form a reticle assembly. 
       FIGS.  19 A- 19 C  are a set of drawings illustrating some steps of the method of  FIG.  18   , in one embodiment. In  FIG.  19 A , a first nanotube membrane layer  150  is affixed to the border  128  through pressure applied by a pressing machine  250  comprising a bolster plate  252  and a ram  254 . In  FIG.  19 B , the border  128  and first nanotube membrane layer  150  (already attached to the border  128 ) are then laid upon a graphene membrane layer  160 . In  FIG.  19 C , pressure is again applied through the pressing machine  250  to attach the graphene membrane layer  160  to the first nanotube membrane layer  150 . A multi-layer pellicle membrane can thus be built up successively. It is noted that the thickness of the multi-layer pellicle membrane might vary slightly between the center of the pellicle membrane and the edges of the pellicle membrane where pressure has been used to attach the layers to the border. 
     If desired, a conformal coating can be applied to the outer surface of the pellicle membrane. This is illustrated in  FIGS.  20 A- 20 D , in one embodiment.  FIG.  20 A  shows the assembly of the border  128  and the pellicle membrane  130 , which is comprised of a nanotube membrane layer  150  and a graphene membrane layer  160 . The border is directly attached to the nanotube membrane layer  150 , which acts as the inner surface  134  of the pellicle membrane  130 . The graphene membrane layer  160  acts as the outer surface  132  of the pellicle membrane  130 . As seen in  FIG.  20 B , a coating is applied to the outer surface  132  of the pellicle membrane  130  to form the outermost layer  172 . It is noted that the coating is illustrated as also being applied to the sides of the pellicle membrane, and the coating can also end up on the border  128  due to the application process. In  FIG.  20 C , the coated pellicle membrane  130  and border  128  are then attached to a mounting frame  122 , for example through adhesive layer  129 , to form a pellicle assembly  120 . In  FIG.  20 D , the pellicle assembly  120  is mounted to the reticle  100  (having the desired mask pattern) by securing the frame to the reticle, with the pellicle membrane disposed over the mask pattern, to produce a final reticle assembly, such as that shown in  FIG.  1    by way of non-limiting illustrative example. 
     The conformal coating can be applied by conventional methods known in the art, such as spraying, dip coating, etc. It is desired that the conformal coating conforms to the exposed surfaces of the pellicle membrane, so that the pores which are present in the pellicle membrane remain present and are not filled by the conformal coating. Such exposed surfaces may be present in any or all of the different layers of a multi-layer pellicle membrane. In addition, the conformal coating will penetrate into the pellicle membrane, rather than being a single discrete layer upon the pellicle membrane. For example, when the conformal coating is applied to the pellicle membrane illustrated in  FIG.  5    as having a graphene membrane layer  160  and two nanotube membrane layers  150 ,  152 , it is expected that the sides of some nanotubes of the two nanotube membrane layers may also be covered with the conformal coating. 
     When applied, the conformal coating is intended to protect the pellicle membrane from damage that can occur due to heat and hydrogen plasma created during EUV exposure. Generally, the material used for the coating should have a low refractive index, i.e. should be as close to 1 as possible when measured at a wavelength of 13.5 nm. The material used for the coating should also have a low extinction coefficient at a wavelength of 13.5 nm. The extinction coefficient measures how easily the material can be penetrated by the wavelength. Desirably, the material used for the conformal coating has a transmittance (T%), when measured at an EUV wavelength of 13.5 nm, of greater than 90%, or of greater than 92%, or of greater than 94%, or of greater than 95%, when measured at a thickness of between 1 nanometer and 10 nanometers. This reduces EUV absorption by the conformal coating (permitting further downstream processing) while protecting the pellicle membrane. 
     In some embodiments, the coating comprises B, BN, B 4 C, B 2 O 3 , SiN, Si 3 N 4 , SiN 2 , SiC, SiC x N y , Nb, NbN, NbSi, NbSiN, Nb 2 O 5 , NbTi x N y , ZrN x , ZrY x O y , ZrF 4 , ZrSi 2 , YN, Y 2 O 3 , YF, Mo, Mo 2 N, MoSi, MoSi 2 , MoSiN, Ru, RuNb, RuSiN, TiN, TiC x N y , HfO 2 , HfN x , HfF 4 , or VN. In some embodiments, the outermost layer has a thickness of about 1 nanometer (nm) to about 10 nm. This thickness should be measured as the thickness of the coating on the individual components of each layer in the pellicle membrane, for example the thickness of the coating on a carbon nanotube. The coating may penetrate deeper into the pellicle membrane than this thickness. 
     Referring now to  FIGS.  8 A- 8 C  and  FIG.  20 B  together, it is noted that one significant distinction between the border  128  and the mounting frame  122  is that the mounting frame  122  includes vent holes  123 . These vent holes typically have very small diameters, which can be easily filled or plugged by the coating process illustrated in  FIG.  20 B . The use of a border  128  is more convenient for applying the conformal coating to the pellicle membrane, while also protecting the vent holes of the mounting frame. If desired, the use of the border can be omitted, with the pellicle membrane being attached directly to a mounting frame of suitable structure. For example, in some embodiments of such mounting frames, vent holes are present at the end of the mounting frame opposite the end to which the pellicle membrane is attached. Referring to  FIG.  7   , such a mounting frame could be envisioned as the combination of separate components  122 ,  129 , and  128 . 
       FIG.  21    is a flow chart illustrating yet another embodiment of a method for preparing a pellicle assembly. This method is applicable to single-layer or multi-layer membranes formed from nanotubes. In step  710 , one or more initial membranes is/are attached to a stretching frame. In step  720 , the initial membrane(s) is/are stretched to obtain an extended membrane. For example, the initial membrane(s) can be uniaxially stretched or biaxially stretched. In some embodiments, the initial membrane(s) is/are annealed during the stretching. The initial membrane(s) can be annealed at a temperature of about 200° C. to about 800° C. In other embodiments, the initial membrane(s) is/are heated at a temperature of about 200° C. to about 500° C. In some additional embodiments, the stretching is performed while an inert gas is flowed past or through the initial membrane(s). In some embodiments, the inert gas is pure nitrogen gas (N 2 ). These optional steps are indicated in step  722 . 
     In step  730 , a mounting frame is affixed to a portion of the extended membrane. The mounting frame has smaller dimensions (in length, or in width, or in both length and width) than the extended membrane, and thus surrounds a portion of the extended membrane. In step  740 , the mounting frame and the portion of the extended membrane are then separated from the remainder of the extended membrane to obtain the pellicle assembly. This can be done, for example, by cutting or other similar means. If desired, the annealing and/or inert gas flow can be maintained during these affixing and separating steps (i.e. either one or both of the annealing and inert gas flow), as indicated in step  742 . The portion of the extended membrane which is surrounded by the mounting frame can be considered the pellicle membrane. In this method, the initial membrane has a higher density than the final pellicle membrane. The final pellicle membrane is also thinner than the initial membrane(s). The resulting pellicle assembly can again then be attached to a reticle by securing the frame to the mask, with the pellicle membrane disposed over the mask pattern, to produce a final reticle with pellicle assembly, such as that shown in  FIG.  1   . 
       FIGS.  22 A- 22 D  illustrate one embodiment of the method of  FIG.  21   .  FIG.  22 A  shows a plan view, a length-side view, and a width-side view of the stretching frame without any membranes thereon. As illustrated here, the stretching frame  810  includes at least four side walls. Two adjacent side walls  820 ,  830  are fixed in place and can be made as one piece if desired. The other two side walls  840 ,  850  are mobile. As illustrated here, each mobile side wall is threaded upon two rods and fixed in place relative to each rod using nuts. For example, mobile side wall  840  is attached to rods  842  using nuts  844 , and one end of each rod  842  is attached to opposite side wall  820 . Similarly, mobile side wall  850  is attached to rods  852  using nuts  854 , and one end of each rod  852  is attached to opposite side wall  830 . As seen in the side views, rods  842  and rods  852  are on different planes along the height of the side walls. 
     In  FIG.  22 B , one or more initial membranes  310  is/are attached to each side wall of the stretching frame  810 . This can be accomplished, for example, using a suitable adhesive. 
     In  FIG.  22 C , the mobile side walls  840 ,  850  are extended along the X-axis and the Y-axis. As illustrated here, the mobile side walls  840 ,  850  are slid along the rods to a desired location, and the nuts are then tightened to hold them in place. This extension stretches the initial membrane(s) along two different axes, forming an extended membrane  870 . The extended membrane  870  also has a lower density than the initial membrane  310 , and the extended membrane  870  is thinner than the initial membrane  310 . 
     In  FIG.  22 D , a mounting frame  122  is affixed to a portion  872  of the extended membrane  870 . The mounting frame  122  has fixed dimensions and is attached to one side of the extended membrane  870 . The mounting frame  122  and the portion  872  of the extended membrane are then cut out of the extended membrane, or separated from the remainder of the extended membrane, to obtain the pellicle assembly  120 . 
       FIG.  23 A  and  FIG.  23 B  are two views depicting an exemplary embodiment of a membrane stretching table  900  which can also be used to stretch an initial nanotube membrane to obtain an extended membrane. Referring first to  FIG.  23 A , the table  900  includes a top  902  and four legs  904 , one in each corner of the top. If desired, the table can be made to be a self-leveling table using methods known in the art. 
     A large central recess  910  is present in the table top, away from the perimeter of the table top. Four smaller side recesses (relative in size to the central recess)  912 ,  914 ,  916 ,  918  are also present in the table top, one on each side of the table. A first x-axis side recess  912  is larger (in area) than a second x-axis side recess  914  on an opposite side of the table. A first y-axis side recess  916  is larger than a second y-axis side recess  918  on an opposite side of the table. 
     A first x-axis arm  922  extends from the first x-axis recess  912  towards and over the central recess  910 . A second x-axis arm  924  extends from the second x-axis recess  914  towards and over the central recess  910 . A first y-axis arm  926  extends from the first y-axis recess  916  towards and over the central recess  910 . A second y-axis arm  928  extends from the second y-axis recess  918  towards and over the central recess  910 . 
     The four arms  922 ,  924 ,  926 ,  928  are each joined to membrane stretching frame  930 . The membrane stretching frame is separable from the four arms, and has four sides. The four arms are each joined to a different side of the membrane holding frame. It is noted that as illustrated here, the central recess  910  is much larger than the membrane stretching frame  930 . It is contemplated that the table can be used to stretch membranes of varying starting sizes. 
     As illustrated here, second x-axis arm  924  and second y-axis arm  928  are stationary, while first x-axis arm  922  and first y-axis arm  926  are able to move along the x-axis and the y-axis, respectively. An x-axis caliper  942  is attached to the first x-axis arm  922  and a y-axis caliper  946  is attached to the first y-axis arm  926 , for measuring the amount of movement. It is also contemplated that in some embodiments, all of the arms  922 ,  924 ,  926 ,  928  could be mobile. Additional calipers may be used for measuring the degree of movement. 
     A vertical piston  950  is located within the central recess  910 , within the area bounded by the membrane stretching frame. As illustrated here, the head of the piston has a rectangular cross-section. In addition, as illustrated here, the piston is a concentric piston, with an annular outer head  952  and an inner head  954  which can be separately raised and lowered. It is also contemplated that the piston may have a single head, rather than an outer head and an inner head. A caliper (not shown) may also be present for measuring movement by the piston in the z-axis. Referring to  FIG.  23 B , the bottom of the piston  950  is visible. 
     It has been found that a nanotube membrane can be stretched up to 10 μm per 1 cm of unstretched length of the initial membrane along the x-, y-, or z-axis. If the membrane is stretched too much, its structure can be destroyed. Thus, the calipers  942 ,  946  are used to measure the degree of stretching. 
       FIGS.  24 A- 24 D  are a set of side cross-sectional views that depict a first exemplary process of stretching a membrane which can be performed using stretching table  900  to obtain a pellicle membrane. In this exemplary process, the initial membrane is stretched by movement in three directions to reduce its thickness and obtain the nanotube membrane. In addition, in this process, the nanotube membrane can be attached to a mounting frame or a border. 
     Starting with  FIG.  24 A , the initial membrane  960  is attached to stretching frame  930 . The piston  950  is illustrated here for reference. The distance d 0  represents the starting position of the stretching frame and the membrane before stretching, relative to the piston. Also mounted to the head of the piston is a mounting frame  970 . At this point, the mounting frame  970  does not contact the initial membrane  960 . 
     Next, in  FIG.  24 B , the initial membrane  960  is stretched by the stretching frame  930  along the x-axis and/or y-axis. This is indicated by distance d 1 , which is greater than distance d 0 . This stretching will also reduce the thickness of the initial membrane. The mounting frame  970  still does not contact the membrane  960 . 
     Moving to  FIG.  24 C , the piston  950  now moves upwards in the z-axis, so that the mounting frame  970  is affixed to the initial membrane, for example via van der Waals forces, to obtain the nanotube membrane  962 . The movement in the z-axis will impart shear forces to the nanotube membrane, which will cause some additional stretching of the nanotube membrane, although the majority of the stretching occurs in the x-axis and/or y-axis. 
     Lastly, as depicted in  FIG.  24 D , the mounting frame  970  and a portion of the nanotube membrane  962  is cut out and separated from the stretching frame  930  and the remainder of the nanotube membrane  962 . A pellicle assembly (still mounted to the piston) is the result. 
       FIGS.  25 A- 25 C  are a set of side cross-sectional views that depict a second exemplary process of stretching a pellicle membrane which can be performed using stretching table  900 . In this embodiment, the initial membrane is stretched by movement in only one direction. 
     Starting with  FIG.  25 A , again the initial membrane  960  is attached to stretching frame  930 . Distance d 0  represents the starting position of the stretching frame and the membrane before stretching, relative to the piston  950 . Also mounted to the head of the piston is a mounting frame  970 . 
     Next, in  FIG.  25 B , the piston  950  pushes upwards in the z-axis, with the mounting frame  970  being affixed to the initial membrane, for example via van der Waals forces, to obtain the nanotube membrane  962 . The initial membrane is not stretched by movement along the x-axis or the y-axis in this second exemplary process. 
     Finally, as depicted in  FIG.  25 C , the mounting frame  970  and a portion of the nanotube membrane  962  is separated from the stretching frame  930  and the remainder of the nanotube membrane  962 . A pellicle assembly (still mounted to the piston) is the result. Again, the degree of stretching of the membrane is greater in the first exemplary method of  FIGS.  24 A- 24 D  compared to the second exemplary method of  FIGS.  25 A- 25 C . 
     In one experiment using the table of  FIG.  23 A , an initial membrane of size 7 cm×7 cm prior to stretching had a deflection of 1325 μm. The initial membrane was then stretched an additional 26 μm in both the x-axis and the y-axis. Afterwards, the stretched membrane had a deflection of only 727 μm, which is an improvement of over 40%. 
       FIG.  26    is a flowchart illustrating a method for processing a semiconductor wafer substrate, in accordance with some embodiments.  FIGS.  27 A- 27 C  and  FIG.  28    illustrate some of the steps in this method. 
     In step  1010  of  FIG.  26   , a semiconductor wafer substrate is received. The substrate is placed in a fixed location within a photolithographic device. 
     In step  1020  of  FIG.  26   , a photoresist (PR) layer is formed on the semiconductor wafer substrate. The PR layer is typically deposited using spin-coating, though other methods known in the art can also be used. 
       FIG.  27 A  is a side cross-sectional view of the wafer substrate after step  1020 . Here, a metal layer  1120  is present on the semiconductor wafer substrate  1110 , and the photoresist layer  1130  is present upon the metal layer. 
     In step  1030  of  FIG.  26   , radiation is generated and directed at the photomask or reticle.  FIG.  28    is an illustrative schematic diagram, not drawn to scale, illustrating the various components of an extreme ultraviolet (EUV) lithography system. Generally, the EUV lithography system begins with an EUV light source  1140  that generates EUV light or radiation. Downstream of the EUV light source is an illumination stage  1150  in which the EUV light may be collected and focused as a beam, for example using field facet mirror  1152  that splits the beam into a plurality of light channels. These light channels can then directed using one or more relay mirrors  1154  onto the plane of the photomask. The photomask  1160  includes a pellicle membrane  1162 , through which the radiation passes before and/or after contacting the photomask. Downstream of the photomask  1160  is the projection optics module  1170 , which is configured for imaging the pattern of the photomask onto the semiconductor wafer substrate  1110 , such as a silicon wafer. The projection optics module  1170  may include refractive optics or reflective optics for carrying the image of the pattern defined by the photomask. Illustrative mirrors  1172 ,  1174  are shown. The lithography system can include other modules or be integrated with or coupled to other modules. 
     The photomask/reticle is patterned. In particular embodiments, the photomask is a reflective photomask. The photomask/reticle includes a pellicle membrane as described herein. The pellicle membrane comprises at least one nanotube membrane layer that contains nanotubes with a minimum length of 1,000 micrometers (μm). The radiation contacts the photomask/reticle, and is used to transfer the pattern thereon. 
     In step  1040  of  FIG.  26   , the PR layer is exposed to the patterned radiation reflected from the photomask. The exposed portion of the photoresist is photochemically modified. 
     In step  1050  of  FIG.  26   , the PR layer is developed, such that the pattern from the photomask is now made in the PR layer. This is illustrated in  FIG.  27 B . 
     In step  1060  of  FIG.  26   , the wafer substrate is etched to transfer the pattern to the substrate. This can be done, for example, using dry etching or wet etching. Referring now to  FIG.  27 C , trenches  1125  are now present in the metal layer  1120 . 
     In step  1070  of  FIG.  26   , the PR layer is removed. Further processing steps can then be performed. The method then ends. 
     Because the pellicle membrane is in the optical path between the reticle and the wafer upon which the transferred pattern is to be imaged, certain optical properties are desired for the pellicle membrane. For example, the pellicle membrane should have high transmittance (i.e. optically transparent) for EUV wavelengths, low reflectivity for EUV wavelengths, low non-uniformity, and low scattering. During exposure and regular operations, the pellicle membrane will be exposed to high temperatures, and so certain thermal properties are also desirable. For example, the pellicle membrane should have low thermal expansion, high thermal conductivity, and high thermal emissivity. The pellicle membrane should also have good mechanical properties, such as high stiffness (i.e. low sagging or deflection) and stability. The pellicle membranes of the present disclosure have combinations of these desired properties. 
     The methods described herein provide a pellicle membrane with an improved combination of EUV transmittance, pore size, stiffness, and service lifetime. The pellicle membranes of the present disclosure maintain high transmittance in the EUV wavelength range. This permits more light to reach the photomask for a given exposure energy and also reduces heat buildup in the pellicle membrane. In some embodiments, the pellicle membranes have a transmittance (T%), when measured at an EUV wavelength of 13.5 nm, of greater than 90%, or of greater than 95%, or of greater than 96%, or of greater than 97%. 
     One means by which the high transmittance is obtained is through the presence of pores in the pellicle membrane, since the pores do not reflect or absorb EUV wavelengths. The pellicle membranes of the present disclosure have an average pore size that is small enough to prevent particles from passing through the pellicle membrane and landing on the reticle/photomask. In some embodiments, the maximum pore size of the pores in the pellicle membranes is less than 30 nm in diameter (after the conformal coating has been applied). In this regard, a pore is considered to be any straight path that passes entirely through the pellicle membrane. Pores may be present due to spaces between nanotubes, or between the flakes/sheets of graphene or graphite. The pore size is the smallest diameter of this straight path (because a particle only has to be trapped before passing through the pellicle membrane, it does not have to be stopped at the outer surface of the pellicle membrane). In a multi-layer pellicle membrane, the pore size of the pellicle membrane is usually much smaller, because the pores of a given membrane layer do not align with the pores of another membrane layer. The pore size can be measured using conventional methods, for example by imaging the membrane and measuring the size of each pore. 
     The increased stiffness of the pellicle membrane minimizes any potential sagging or deflection that may occur over time. For example, the dimensions of the pellicle membrane (length and width) are on the order of about 100 millimeters. The pellicle membranes of the present disclosure may sag or deflect in the range of about 100 μm to about 300 μm under an applied pressure differential of two pascals (Pa). In embodiments, the pellicle membrane may have a thickness ranging from about 10 nanometers (nm) to about 100 nm, or from about 20 nm to about 70 nm. 
     The pellicle membranes of the present disclosure also have low reflectivity for EUV wavelengths. Again, this permits more light to reach the photomask for a given exposure energy and also reduces critical dimension (CD) error. In some embodiments, the pellicle membranes have a reflectivity (R%), when measured at an EUV wavelength of 13.5 nm, of 5% or less, or of 3% or less, or of 2% or less, or of 1% or less, or of 0.5% or less. 
     The pellicle membranes of the present disclosure also have low non-uniformity at EUV wavelengths, or in other words have high uniformity. This reduces local CD error that can otherwise occur. In some embodiments, the pellicle membranes have a non-uniformity (U%), when measured at an EUV wavelength of 13.5 nm, of 1% or less, or of 0.5% or less, or of 0.3% or less, or of 0.1% or less. 
     Some embodiments of the present disclosure thus relate to methods of forming a pellicle assembly. An initial membrane is provided, which is formed from nanotubes having a minimum length of 1,000 μm. The thickness of the initial membrane is reduced to obtain a first nanotube layer for a pellicle membrane, using methods described herein. 
     The pellicle membrane may be a single-layer structure or a multi-layer structure. The pellicle membrane is then affixed to a mounting frame to obtain the pellicle assembly. 
     Other embodiments of the present disclosure relate to methods for forming a reticle assembly. A pellicle membrane is affixed to a mounting frame to obtain a pellicle assembly. The pellicle membrane comprises at least one nanotube membrane layer containing nanotubes with a minimum length of 1,000 μm. The pellicle assembly is then disposed over a mask pattern on a reticle to form the reticle assembly. 
     Some other embodiments of the present disclosure relate to methods for producing a reticle assembly. A pellicle assembly is mounted over a mask pattern on a reticle. The pellicle assembly comprises a multi-layer pellicle membrane having a conformal coating on an outer surface thereof. At least one layer of the multi-layer pellicle membrane is a nanotube membrane layer containing nanotubes with a minimum length of 1,000 μm. 
     Some other embodiments of the present disclosure relate to devices for stretching pellicle membranes. The stretching device includes a table which includes a mounting frame and membrane stretching components. A membrane is affixed to a mounting frame which is connected to an x-axis and y-axis stretching component. On the underside of the table a z-axis stretching device is positioned such that a piston is flush with the underside of the membrane. 
     Finally, other embodiments of the present disclosure relate to pellicle membranes and pellicle assemblies. The pellicle membrane is a single-layer structure or a multi-layer structure, and comprises nanotubes having a minimum length of 1,000 μm. The pellicle membrane is affixed or attached to a mounting frame or border to form a pellicle assembly. In some embodiments, a conformal coating is present upon at least an outer surface of the pellicle membrane. 
     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.