Patent Publication Number: US-2023161261-A1

Title: Optical assembly with coating and methods of use

Description:
PRIORITY CLAIM AND CROSS-REFERENCE 
     This application claims the benefit of U.S. Provisional Patent Application No. 63/283,088 filed Nov. 24, 2021, which is incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     The present disclosure relates to the field of ultraviolet and extreme ultraviolet lithography and to optical assemblies used in ultraviolet and extreme ultraviolet lithography. 
     In the semiconductor integrated circuit (IC) industry, technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of IC processing and manufacturing. 
     A photolithography process forms a patterned resist layer for various patterning processes, such as etching or ion implantation. The minimum feature size that may be patterned by way of such a lithography process is limited by the wavelength of the projected radiation source. Lithography machines have gone from using ultraviolet light with a wavelength of 365 nanometers to using deep ultraviolet (DUV) light including a krypton fluoride laser (KrF laser) of 248 nanometers and an argon fluoride laser (ArF laser) of 193 nanometers, and to using extreme ultraviolet (EUV) light of a wavelength of 13.5 nanometers, improving the resolution at every step. 
     In the photolithography process, a photomask (or mask) is used. The mask includes a substrate and a patterned layer that defines an integrated circuit to be transferred to a semiconductor substrate during the photolithography process. The mask is typically included with a pellicle assembly, collectively referred to as a mask system. The pellicle assembly includes a transparent thin membrane and a pellicle frame, where the membrane is mounted over the pellicle frame. The pellicle protects the mask from fallen particles and keeps the particles out of focus so that they do not produce a patterned image, which may cause defects in the patterned semiconductor substrate when the mask is being used. The membrane is typically stretched and mounted over the pellicle frame, and is attached to the pellicle frame by glue or other adhesives. An internal space may be formed by the mask, the membrane, and the pellicle frame. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic view of a lithography system in accordance with some embodiments of the present disclosure. 
         FIG.  2    is a schematic cross-sectional view of a mask for use in embodiments of the present disclosure. 
         FIGS.  3 A- 3 C  are a top view, a perspective view, and a cross-sectional view along line A-A′, respectively, of a mask pellicle system in accordance with some embodiments of the present disclosure. 
         FIGS.  4 A- 4 D  are schematic cross-sectional views of several pellicle membranes in accordance with embodiments described herein. 
         FIG.  5 A  is a schematic perspective view of a nanotube including a partially removed covering layer in accordance with embodiments of the present disclosure. 
         FIG.  5 B  is an illustration of a covering layer on an external surface and on an internal surface of a nanotube in accordance with embodiments of the present disclosure. 
         FIG.  5 C  is a schematic perspective view of a nanotube including a partially removed covering layer and an adhesion layer in accordance with embodiments of the present disclosure. 
         FIG.  6 A  is a schematic view of a nanotube bundle in accordance with embodiments of the present disclosure. 
         FIG.  6 B  is a cross-sectional view of a nanotube bundle including a covering layer in accordance with embodiments of the present disclosure. 
         FIGS.  7 A- 7 B  are schematic illustrations of uncoated nanotubes and nanotubes after deposition of a coating layer in accordance with embodiments of the present disclosure. 
         FIG.  8    is a flowchart illustrating a method according to an embodiment of the present disclosure. 
         FIG.  9    is a flowchart illustrating a method according to an embodiment of the present disclosure. 
         FIG.  10    is a schematic illustration of a system for applying a protective layer or a coating layer to a nanotube in accordance an embodiment of the present disclosure. 
         FIG.  11    is a flowchart illustrating a method according to an embodiment of the present disclosure. 
         FIG.  12    is a schematic illustration of a core-shell configuration of a nanotube structure useful in accordance with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, thicknesses and materials may be described for various layers and structures within an integrated circuit die. Specific dimensions and materials are given by way of example for various embodiments. Those of skill in the art will recognize, in light of the present disclosure, that other dimensions and materials can be used in many cases without departing from the scope of the present disclosure. 
     The following disclosure provides many different embodiments, or examples, for implementing different features of the described subject matter. Specific examples of components and arrangements are described below to simplify the present description. 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. 
     “Vertical direction” and “horizontal direction” are to be understood as indicating relative directions. Thus, the horizontal direction is to be understood as substantially perpendicular to the vertical direction and vice versa. Nevertheless, it is within the scope of the present disclosure that the described embodiments and aspects may be rotated in its entirety such that the dimension referred to as the vertical direction is oriented horizontally and, at the same time, the dimension referred to as the horizontal direction is oriented vertically. 
     In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the disclosure. However, one skilled in the art will understand that the disclosure may be practiced without these specific details. In other instances, well-known structures associated with electronic components and fabrication techniques have not been described in detail to avoid unnecessarily obscuring the descriptions of the embodiments of the present disclosure. 
     Unless the context requires otherwise, throughout the specification and claims that follow, the word “comprise” and variations thereof, such as “comprises” and “comprising,” are to be construed in an open, inclusive sense, that is, as “including, but not limited to.” 
     The use of ordinals such as first, second and third does not necessarily imply a ranked sense of order, but rather may only distinguish between multiple instances of an act or structure. 
     Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. 
     As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise. 
     Embodiments in accordance with the present disclosure provide optical assemblies suitable for transmitting UV or EUV radiation and protecting UV or EUV reflecting components of a lithography system. The optical assemblies exhibit desirable UV/EUV transmission levels and promote heat transfer from the optical assembly. The optical assemblies are also resistant to damage from exposure to gases such as hydrogen, oxygen and H+ gas. In some embodiments, the optical assemblies are lithography masks that include pellicles or are pellicles themselves. 
     The various advantages and purposes of embodiments in accordance with the present disclosure as described above and hereafter are achieved by providing, according to aspects of the present disclosure an optical assembly that includes the matrix of a plurality of nanotube bundles or a matrix of individual nanotubes. In some embodiments, a coating layer is provided that protects the transparent layer of the optical assembly from hydrogen and oxygen radicals to which the transparent layer may be exposed during EUV processing. In some embodiments, the nanotubes of the bundles or individual nanotubes have a core shell structure. In accordance with some embodiments, the individual nanotubes are coated with a coating layer to protect the nanotubes. In other embodiments, the nanotube bundles are coated with the coating layer; however, the individual nanotubes of the nanotube bundles are not individually coated with a coating layer. In other embodiments the individual nanotubes of the nanotube bundles are coated with a coating layer and the nanotube bundle is formed from such coated individual nanotubes. When the nanotubes have a core shell structure, the shell of the nanotubes is coated with an EUV transmissive protective coating layer. These optical assemblies are useful in methods for patterning materials on a semiconductor substrate. Such methods involve generating, in a UV lithography system, UV radiation. The UV radiation is passed through a coating layer of an optical assembly, e.g., a pellicle assembly. In some embodiments, the UV radiation that has passed through the coating layer is passed through a matrix of individual nanotubes or matrix of nanotube bundles. UV radiation that passes through the matrix of individual nanotubes or matrix of nanotube bundles is reflected from a mask and received at a semiconductor substrate. In accordance with other embodiments, the coating layer is applied to a transparent layer of a pellicle assembly that may or may not include nanotubes or bundles of nanotubes. The materials used for the coating layer protect the nanotube containing matrix or membrane, e.g., pellicle of an EUV mask from radicals such as oxygen or hydrogen radicals that are used in the EUV process. In some embodiments, the material of the coating layer is chosen such that EUV % transmittance through the material of the coating layer is relatively low which makes the material a good choice from the standpoint of being able to provide a relatively thicker coating for purposes of protecting the transparent layer and extending the life of the EUV mask. 
     Illustrated in  FIG.  1    is a schematic view of a lithography system  100 , in accordance with some embodiments. The lithography system  100  may also be generically referred to as a scanner that is operable to perform lithographic processes including exposure with a respective radiation source and in a particular exposure mode. In at least some of the present embodiments, the lithography system  100  includes an ultraviolet (UV) lithography system designed to expose a resist layer with UV radiation, i.e., UV light. Inasmuch, in various embodiments, the resist layer includes a material sensitive to the UV light (e.g., a UV resist). The lithography system  100  of  FIG.  1    includes a plurality of subsystems such as a radiation source  102 , an illuminator  104 , a mask stage  106  configured to receive a mask  108 , projection optics  110 , and a substrate stage  118  configured to receive a semiconductor substrate  116 . The following description of a UV photolithography system in accordance with embodiments of the present disclosure refers to extreme ultraviolet radiation as an example of ultraviolet radiation. Embodiments in accordance with the present disclosure are not limited to extreme ultraviolet radiation lithography systems. In other words, embodiments described with reference to extreme ultraviolet lithography systems include embodiments that utilize ultraviolet radiation. A general description of the operation of the lithography system  100  is as follows: EUV light from the radiation source  102  is directed toward the illuminator  104  (which includes a set of reflective mirrors) and is projected onto the reflective mask  108 . A reflected mask image is directed toward the projection optics  110 , which focuses the EUV light and projects the EUV light onto the semiconductor substrate  116  to expose a EUV resist layer deposited thereupon. Additionally, in various examples, each subsystem of the lithography system  100  may be housed in, and thus operate within, a high-vacuum environment, for example, to reduce atmospheric absorption of the EUV light. 
     In the embodiments described herein, the radiation source  102  may be used to generate the EUV light. In some embodiments, the radiation source  102  includes a plasma source, such as for example, a discharge produced plasma (DPP) or a laser produced plasma (LPP). In some examples, the EUV light may include light having a wavelength ranging from about 1 nm to about 100 nm. In one particular example, the radiation source  102  generates EUV light with a wavelength centered at about 13.5 nm. Accordingly, the radiation source  102  may also be referred to as an EUV radiation source  102 . In some embodiments, the radiation source  102  also includes a collector, which may be used to collect EUV light generated from the plasma source and to direct the collected EUV light toward imaging optics such as the illuminator  104 . 
     As described above, EUV light from the radiation source  102  is directed toward the illuminator  104 . In some embodiments, the illuminator  104  may include reflective optics (e.g., for the EUV lithography system  100 ), such as a single mirror or a mirror system having multiple mirrors in order to direct light from the radiation source  102  onto the mask stage  106 , and particularly to the mask  108  secured on the mask stage  106 . In some examples, the illuminator  104  may include a zone plate (not shown), for example, to improve focus of the EUV light. In some embodiments, the illuminator  104  may be configured to shape the EUV light passing there through in accordance with a particular pupil shape, and including for example, a dipole shape, a quadrapole shape, an annular shape, a single beam shape, a multiple beam shape, and/or a combination thereof. In some embodiments, the illuminator  104  is operable to configure the mirrors (i.e., of the illuminator  104 ) to provide a desired illumination to the mask  108 . In one example, the mirrors of the illuminator  104  are configurable to reflect EUV light to different illumination positions. In some embodiments, a stage (not shown) prior to the illuminator  104  may additionally include other configurable mirrors that may be used to direct the EUV light to different illumination positions within the mirrors of the illuminator  104 . In some embodiments, the illuminator  104  is configured to provide an on-axis illumination (ONI) to the mask  108 . In some embodiments, the illuminator  104  is configured to provide an off-axis illumination (OAI) to the mask  108 . It should be noted that the optics employed in the EUV lithography system  100 , and in particular optics used for the illuminator  104  and the projection optics  110 , may include mirrors having multilayer thin-film coatings known as Bragg reflectors. By way of example, such a multilayer thin-film coating may include alternating layers of Mo and Si, which provides for high reflectivity at EUV wavelengths (e.g., about 13 nm). 
     As discussed above, the lithography system  100  also includes the mask stage  106  configured to secure the mask  108  within the lithography system. Since the lithography system  100  may be housed in, and thus operate within, a high-vacuum environment, the mask stage  106  may include an electrostatic chuck (e-chuck) to secure the mask  108 . As with the optics of the EUV lithography system  100 , the mask  108  is also reflective. Details of the mask  108  are discussed in more detail below with reference to the example of  FIG.  2   . As illustrated in  FIG.  1   , light is reflected from the mask  108  and directed towards the projection optics  110 , which collects the EUV light reflected from the mask  108 . By way of example, the EUV light collected by the projection optics  110  (reflected from the mask  108 ) carries an image of the pattern defined by the mask  108 . In various embodiments, the projection optics  110  provides for imaging the pattern of the mask  108  onto the semiconductor substrate  116  secured on the substrate stage  118  of the lithography system  100 . In particular, in various embodiments, the projection optics  110  focuses the collected EUV light and projects the EUV light onto the semiconductor substrate  116  to expose an EUV resist layer deposited on the semiconductor substrate  116 . As described above, the projection optics  110  may include reflective optics, as used in EUV lithography systems such as the lithography system  100 . In some embodiments, the illuminator  104  and the projection optics  110  are collectively referred to as an optical module of the lithography system  100 . 
     As discussed above, the lithography system  100  also includes the substrate stage  118  to secure the semiconductor substrate  116  to be patterned. In various embodiments, the semiconductor substrate  116  includes a semiconductor wafer, such as a silicon wafer, germanium wafer, silicon-germanium wafer, III-V wafer, or other type of wafer. The semiconductor substrate  116  may be coated with a resist layer (e.g., an EUV resist layer) sensitive to EUV light. EUV resists may have stringent performance standards. For purposes of illustration, an EUV resist may be designed to provide at least around 22 nm resolution, at least around 2 nm line-width roughness (LWR), and with a sensitivity of at least around 15 mJ/cm2. In the embodiments described herein, the various subsystems of the lithography system  100 , including those described above, are integrated and are operable to perform lithography exposing processes including EUV lithography processes. To be sure, the lithography system  100  may further include other modules or subsystems which may be integrated with (or be coupled to) one or more of the subsystems or components described herein. 
     The lithography system may include other components and may have other alternatives. In some embodiments, the lithography system  100  may include a pupil phase modulator  112  to modulate an optical phase of the EUV light directed from the mask  108 , such that the light has a phase distribution along a projection pupil plane  114 . In some embodiments, the pupil phase modulator  112  includes a mechanism to tune the reflective mirrors of the projection optics  110  for phase modulation. For example, in some embodiments, the mirrors of the projection optics  110  are configurable to reflect the EUV light through the pupil phase modulator  112 , thereby modulating the phase of the light through the projection optics  110 . In some embodiments, the pupil phase modulator  112  utilizes a pupil filter placed on the projection pupil plane  114 . By way of example, the pupil filter may be employed to filter out specific spatial frequency components of the EUV light reflected from the mask  108 . In some embodiments, the pupil filter may serve as a phase pupil filter that modulates the phase distribution of the light directed through the projection optics  110 . 
     Referring to  FIG.  2   , the mask  108  includes a patterned image comprising one or more absorbers  208  having an anti-reflective coating (ARC) layer  210 . The one or more absorbers  208  and anti-reflective coating are on a multi-layer structure  204 , e.g., Mo—Si multi-layers, which is on a substrate  202 . Examples of the materials for the absorbers  208  include a tantalum nitride layer or a Ta x B y O z N u . Examples of materials for the antireflective coating layer include Ta x B y O z N u , an Hf x O y  layer or a Si x O y N z  layer. An example of a substrate  202  includes a low thermal expansion material substrate, such as TiO 2  doped SiO 2 . In the illustrated embodiment of  FIG.  2   , the multi-layer structure  204  is covered by a capping layer  206  and the backside of substrate  202  is covered with a backside coating layer  203 . An example of a material for capping layer  206  includes ruthenium. An example of a material for backside coating layer  203  includes chromium nitride. 
     As discussed above, the mask  108  is used to transfer circuit and/or device patterns onto a semiconductor wafer (e.g., the semiconductor substrate  116 ) by the lithography system  100 . To achieve a high fidelity pattern transfer from the patterned mask  108  to the semiconductor substrate  116 , the lithography process should be defect free. Unwanted particles, e.g., particles of Sn that are used to generate the EUV light in the radiation source  102  may be unintentionally deposited on the surface of the capping layer  206  and can result in degradation of lithographically transferred patterns if not removed. Particles may be introduced by any of a variety of methods besides as part of the EUV light generation, such as during an etching process, a cleaning process, and/or during handling of the EUV mask  108 . Accordingly, the mask  108  is integrated with a pellicle and is protected by the pellicle assembly. The mask and the pellicle assembly are collectively referred to as a mask-pellicle system. For example, during the lithography patterning process by the lithography system  100 , the mask-pellicle system is secured to the mask stage  106 . 
     With reference to  FIGS.  3 A,  3 B, and  3 C , illustrated therein is a top-view, a perspective view, and a cross-sectional view along line A-A′, respectively, of a mask pellicle system  300 . Referring to  FIGS.  3 A,  3 B, and  3 C , the mask pellicle system  300  and a method of using the same are described. While embodiments of the present disclosure are described with reference to a mask of an EUV photolithography system, it is understood that the embodiments of the present disclosure are useful with other UV or EUV reflecting components of a lithography system that reflect UV or EUV radiation. 
     The mask pellicle system  300  includes a mask  302 , a pellicle frame  304  and an optical assembly, e.g., membrane (or pellicle membrane)  306  integrated together through adhesive material layers  308  and  310 . As discussed above, the mask  302  also includes a patterned surface  314  used to pattern a semiconductor substrate by a lithographic process. In some embodiments, the mask  302  may be substantially the same as the mask  108 , discussed above. In the present embodiment, the mask  302  is integrated in the mask pellicle system  300  and is secured on the mask stage  106  collectively with the membrane  306  and the pellicle frame  304  during a lithography patterning process. 
     The membrane  306  is configured proximate to the mask  302  and is attached to the pellicle frame  304  through the adhesive layer  308 . Particularly, the membrane  306  is attached to the pellicle frame  304  through the adhesive material layer  308 . The mask  302  is further attached to the pellicle frame  304  through the adhesive material layer  310 . Thus, the mask  302 , the pellicle frame  304  and the membrane  306  are thus configured and integrated to enclose an internal space  312 . The patterned surface  314  of the mask  302  is enclosed in the internal space  312  and is therefore protected from contamination during a lithography patterning process, mask shipping, and mask handling. In the illustrated embodiment of  FIG.  3 C , pellicle frame  304  is provided with two vent holes  320 . Embodiments in accordance with the present disclosure can include only a single vent hole  320  or include more than two vent holes  320 . Vent holes serve to equalize air pressure between the open space bounded by the pellicle frame  304  and the pellicle membrane  306  and the environment outside the pellicle frame  304  and pellicle membrane  306 . Vent holes  320  can be provided with filters (not shown) configured to prevent particles from entering vent holes  320 . 
     The membrane  306  is made of a thin film transparent to the radiation beam used in a lithography patterning process, and furthermore has a thermal conductive surface. The membrane  306  is also configured proximate to the patterned surface  314  on the mask  302  as illustrated in  FIG.  3 C . In various embodiments, the membrane  306  includes a transparent material layer with a thermal conductive film on one surface (or both surfaces). 
       FIG.  4    is a cross-sectional view of the membrane  306 , constructed in accordance with some embodiments. The membrane  306  includes a transparent layer  402  or core material layer of one or more materials including silicon, such as polycrystalline silicon (poly-Si), amorphous silicon (a-Si), doped silicon (such as phosphorous doped silicon SiP or SiC) or a silicon-based compound, such as SiN or MoSi x N y  or combinations (SiN/MoSiN). Alternatively, the transparent layer  402  includes polymer, graphene, a carbon network membrane, carbon nanotubes, silicon carbon nanotubes or bundles of such nanotubes, boron nitride nanotubes or bundles of such nanotubes, carbon nanotube bundles, molybdenum disulfide nanotubes (MoS 2 ), bundles of molybdenum disulfide nanotubes, molybdenum diselenide nanotubes (MoSe 2 ), bundles of molybdenum diselenide nanotubes, tungsten disulfide nanotubes (WS 2 ), bundles of tungsten disulfide nanotubes, tungsten diselenide nanotubes (WSe 2 ), bundles of tungsten diselenide nanotubes or other suitable material. As used herein, nanotubes refers to single walled nanotubes, double wall nanotubes, multi-wall nanotubes including more than two walls and combinations of such nanotubes. 
     In other embodiments, the transparent layer  402  includes core-shell nanotubes.  FIG.  12    is a perspective view of a core-shell nanotube  1200  useful in accordance with embodiments of the present disclosure. A core-shell nanotube includes a core nanotube  1202 , e.g., a carbon nanotube, and a shell  1204  of a different material, e.g., a shell formed of nanotubes such as carbon nanotubes or non-carbon nanotubes or a shell formed of a 2D layer of carbon or non-carbon containing materials. In some embodiments, the non-carbon nanotubes are silicon carbide nanotubes, boron nitride nanotube, silicon carbide nanotube bundles, boron nitride nanotube bundles, molybdenum disulfide nanotubes (MoS 2 ), bundles of molybdenum disulfide nanotubes, molybdenum diselenide nanotubes (MoSe 2 ), bundles of molybdenum diselenide nanotubes, tungsten disulfide nanotubes (WS 2 ), bundles of tungsten disulfide nanotubes, tungsten diselenide nanotubes (WSe 2 ), bundles of tungsten diselenide nanotubes or other suitable material. Examples of materials for a 2D layer of non-carbon materials include silicon carbide, boron nitride, molybdenum disulfide (MoS 2 ), molybdenum diselenide (MoSe 2 ), tungsten disulfide (WS 2 ), tungsten diselenide (WSe 2 ) or other suitable material. 
     In some embodiments, such nanotubes are individually coated with materials described below or such bundles of nanotubes are coated with materials described below. In some embodiments, the membrane  306  is characterized by the absence of oxygen containing materials, e.g., SiO 2 . Membranes  306  without oxygen containing materials are less susceptible to degradation caused by H +  radicals that membranes  306  are exposed to during the photolithography process or during maintenance of the photolithography system. When a membrane  306  containing oxygen containing materials, such as SiO 2  is exposed to H+ radicals, peeling of coatings provided on the SiO 2  has been observed. The transparent layer  402  has a thickness with enough mechanical strength, but in some embodiments, not a thickness that degrades the transparency of the membrane to extreme ultraviolet radiation from the radiation source by more than 15% in some embodiments, more than 10% in some embodiments or more than 5% in some embodiments. In some examples, the transparent layer  402  has a thickness ranging between 30 nm and 50 nm. 
     In some embodiments, the membrane  306  includes a first coating layer  404  formed on the external surface  322  of the transparent layer  402  and a second coating layer  406  formed on the internal surface  324  of the transparent layer  402 . In  FIG.  4 A , the external surface  322  of the transparent layer  402  is its top surface and the internal surface  324  of the transparent layer  402  is its bottom surface. In accordance with the embodiment illustrated in  FIG.  4 A , the material of the coating layer  404  is the same as the material of the coating layer  406 . In other embodiments in accordance with  FIG.  4 A , the material of the coating layer  404  is different from the material of the coating layer  406 . In yet other embodiments in accordance with  FIG.  4 A , coating layer  404  comprises multiple layers of material. Similarly, in other embodiments, coating layer  406  comprises multiple layers of material. Such multiple layers can include the same material or different materials. In addition, the multiple layers of material making up coating layer  404  and/or coating layer  406  can be the same thickness or can be different thicknesses. In  FIG.  4 B , the coating layer  406  is provided only on the internal surface  324  and not the external surface  322 . In other embodiments in accordance with  FIG.  4 B , coating layer  406  comprises multiple layers of material. In addition, the multiple layers of material making up coating layer  406  in  FIG.  4 B  can be the same thickness or can be different thicknesses. In  FIG.  4 C , the coating layer  404  is provided only on the external surface  322  and not the internal surface  324 . In other embodiments in accordance with  FIG.  4 C , coating layer  404  comprises multiple layers of material. In addition, the multiple layers of material making up coating layer  404  in  FIG.  4 B  can be the same thickness or can be different thicknesses. The coating layer  404  or  406  protects the transparent layer  402  from attack, such as by chemicals and/or particles. In some embodiments, the coating layer  404  and  406  promotes heat transfer from the transparent layer  402 . In accordance with another embodiment of the present disclosure,  FIG.  4 D  illustrates an example where a first coating layer  408   a  is on transparent layer  402  and a second coating layer  408   b  is on first coating layer  408   a . In accordance with the embodiment of  FIG.  4 D , the material of the first coating layer  408   a  and the material of the second coating layer  408   b  may be the same or they may be different (for example:  408   a :MoSiN and  408   b :SiN2). 
     In accordance with some embodiments of the present disclosure, choice of a particular material for use as first coating layer  404  and/or second coating layer  406  should take into consideration a number of different factors, including how thick a layer of material is needed to provide a conformal coating, the scattering effect of the material on UV or EUV, transmission of the UV or EUV and reflection of the UV or EUV, absorption of the UV or EUV, resistance to desorption of oxygen and attack by ionized gases that come in contact with the coating layers, e.g., H+ gas. 
     For example, when there is a desire to minimize the amount of EUV light absorbed by the coating materials having similar EUV light absorption properties, materials which are susceptible to being applied as a thinner coat while providing a conformal coating are preferred over materials that require application of a thicker coat to provide a conformal coating. When there is a desire to maximize the amount of protection of the transparent layer  402  from oxygen or hydrogen radicals provided by the coating materials, thicker coats of the materials may be desired. In some embodiments, coating layers  404  or  406  have a thickness on the order of 1 to 10 nanometers. In some embodiments, coating layers  404  or  406  have a thickness on the order of 0.5 to 5 nm. 
     In some embodiments of the present disclosure, materials which scatter less of the EUV radiation directed at the mask are preferred over materials that scatter more of the same EUV radiation. Examples of materials useful as a coating layer in accordance with embodiments of the present disclosure include compounds that include non-metals such as boron or silicon. Examples of such types of compounds include boron nitride (BN) and silicon nitride (Si3N4). Ruthenium is not a suitable material for coating layers in accordance with some embodiments of the present disclosure because ruthenium exhibits a differential scattering cross-section of EUV radiation at zero degrees and 360 degrees that is about 6 times greater than the differential scattering cross-section of EUV radiation at zero degrees and 360 degrees for a transparent material coated with boron nitride or silicon nitride. Generally, a material with a lower index of refraction produces more scattering compared to a material with a higher index of refraction. Thus, when selecting a material for coating layers  404  and  406  based only on its index of refraction, a material having a higher index of refraction would be preferred over a material having a lower index of refraction. 
     Generally, a material with a higher extinction coefficient k, indicating a higher absorption of radiation, is less desirable than a material exhibiting a lower extinction coefficient k because the material with a higher extinction coefficient will transmit less UV or EUV. Thus, when selecting a material for coating layers  404  and  406  based only on its extinction coefficient k, a material having a lower extinction coefficient would be preferred over a material having a higher extinction coefficient. 
     Materials which transmit more of the UV or EUV radiation directed at the mask are preferred over materials that transmit less of the same UV or EUV radiation. For example, in some embodiments, materials that transmit 80% or more of the radiation directed at the mask are suitable. In other embodiments, materials that transmit 85% or more of the radiation directed at the mask are suitable. In yet other embodiments, materials that transmit 90% or more of the radiation directed at the mask are suitable. In other embodiments, materials that transmit 94% or more of the radiation directed at the mask are suitable. Materials that transmit more EUV or UV radiation can be applied as a thicker coating compared to materials that transmit less EUV or EUV radiation. A benefit of applying a thicker coating as opposed to a thinner coating is increased protection of the coated substrate from oxygen or hydrogen radicals. In accordance with some embodiments of the present disclosure, the ratio of the EUV % transmittance of a material used as a coating to the thickness in nanometers of the coating is below 40 and above 10. In other embodiments, this ratio is below 38 and above about 13. 
     Materials that reflect less of the radiation to be directed at the mask are preferred over materials that reflect more of the same EUV radiation. 
     Materials that absorb less of the EUV radiation to be directed at the mask are preferred over materials that absorb more of the same EUV radiation. 
     Materials that are more resistant to desorption of oxygen are preferred over materials that are less resistant to desorption of oxygen. 
     Materials that include higher valence oxides are less suitable as materials for coating layer  404  or  406  because they are susceptible to radiation stimulated desorption of oxygen initiated by creation of holes in shallow core levels. These resulting holes cause the coating layer to be more reactive with gas molecules the coating layer is exposed to during the photolithography process or maintenance processes as compared to the reactivity of a coating layer that does not include higher valence oxides. In accordance with embodiments of the present disclosure, materials that do not include higher valence oxides are preferred over materials that include higher valence oxides. 
     Particles, e.g., Sn particles, from the source of EUV radiation may fall on the pellicle surface. Removal of such particles is achieved by etching the pellicle surface with an ionized gas, such as H+. The ability of the ionized gas to etch the particles, e.g., Sn particles, from the pellicle surface depends in part on the difference in electronegativity between Sn and the material of the pellicle surface. Accordingly, selection of a material suitable for coating layers  404  and/or  406  takes into consideration the difference in the electronegativity between the particle to be etched, e.g., Sn particle having an electronegativity of 1.96, and the material of the coating layer. Materials having an electronegativity less than the electronegativity of the particle to be etched are preferred as the material for the coating layers compared to materials having an electronegativity greater than the electronegativity of the particle to be etched from the coating layer surface. In accordance with some embodiments, material suitable for coating layers  404  and/or  406  include materials having an electronegativity less than 1.96, for example materials having an electronegativity between 1.96 and −0.2. 
     Examples of materials useful for coating layers  404  and/or  406  taking into consideration one or more of the criteria described above are presented below. 
     In some embodiments, the coating layer  404  includes non-metal elements, such as B or Si or compounds that include non-metals, such as B or Si. In some embodiments coating layer  404  includes transition metals such as Zr, Nb or Mo or compounds that include transition metals, such as Zr, Nb or Mo. Examples of compounds containing non-metal or transition metal elements in accordance with the present disclosure include, non-metal silicides, non-metal carbides, non-metal nitrides, transition metal silicides, transition metal carbides, transition metal fluorides and transition metal nitrides. Generally, carbides and silicides have low EUV absorption properties thereby making them good candidates as coating layer materials, especially when coating layers of greater thickness are desired to protect the transparent layer  402  of the pellicle and extend the life of the pellicle. Examples of non-metals, non-metal silicides, non-metal carbides, non-metal nitrides, transition metals, transition metal silicides, transition metal carbides, transition metal fluorides and transition metal nitrides or compounds include boron (B), boron nitride (BN), boron silicon nitride (BNSi), boron carbide (B4C), boron silicon carbide (BCSi), silicon hexaboride (B6Si/borosilicide), silicon mononitride (SiN), silicon nitride (Si3N4), silicon dinitride (SiN2), silicon carbide (SiC), silicon carbon nitride (SiCxNy), niobium (Nb), niobium nitride (NbN), niobium monosilicide (NbSi), niobium silicide (NbSi2 and Nb5Si3), niobium silicon nitride (NbSiN), niobium titanium nitride (NbTixNy), niobium carbide (Nb4C3), zironcium nitride (ZrN), zirconium fluoride (ZrF4), zirconium silicide (ZrSi2), zirconium carbide (ZrC), yttrium nitride (YN), yttrium fluoride (YF), molybdenum (Mo), molybdenum nitride (MoN2), molybdenum carbide (Mo4C and Mo2C), molybdenum disilicide (MoSi2), molybdenum silicide (Mo5Si3), molybdenum silicon nitride (MoSixNy), ruthenium-niobium alloys (RuNb), ruthenium silicon nitride (RuSiN), titanium nitride (TiN), titanium carbon nitride (TiCxNy), hafnium nitride (HfNx), hafnium fluoride (HfF4), vanadium nitride (VN). Materials for coating layer  404  exclude materials that include higher valence oxides, such as TiO 2 , V 2 O 5 , ZrO 2 , Ta 2 O 5 , MoO 3 , WO 3 , CeO 2 , Er 2 O 3 , SiO 2 , Y 2 O 3 , Nb 2 O 5 , V 2 O 3  and HfO 2 . 
     In some embodiments, materials for coating layer  404  are selected from materials that do not include higher valence oxides, such as boron (B), boron silicon nitride (BNSi), silicon hexaboride (B6Si/borosilicide), silicon nitride (Si3N4), silicon dinitride (SiN2), niobium (Nb), niobium nitride (NbN), niobium monosilicide (NbSi), niobium silicide (NbSi2 and Nb5Si3), niobium silicon nitride (NbSiN), niobium titanium nitride (NbTixNy), niobium carbide (Nb4C3), zironcium nitride (ZrN), zirconium fluoride (ZrF4), zirconium silicide (ZrSi2), zirconium carbide (ZrC), yttrium nitride (YN), yttrium fluoride (YF), molybdenum (Mo), molybdenum nitride (MoN2), molybdenum disilicide (MoSi2), molybdenum silicide (Mo5Si3), molybdenum silicon nitride (MoSixNy), ruthenium-niobium alloys (RuNb), ruthenium silicon nitride (RuSiN), titanium nitride (TiN), titanium carbon nitride (TiCxNy), hafnium nitride (HfNx), hafnium fluoride (HfF4) or vanadium nitride (VN). 
     In some embodiments, materials for coating layer  404  are selected from materials that do not include ruthenium or molybdenum, such as boron (B), boron silicon nitride (BNSi), silicon nitride (Si3N4), silicon dinitride (SiN2), silicon hexaboride (B6Si/borosilicide), niobium (Nb), niobium nitride (NbN), niobium monosilicide (NbSi), niobium silicide (NbSi2 and Nb5Si3), niobium silicon nitride (NbSiN), niobium titanium nitride (NbTixNy), niobium carbide (Nb4C3), zironcium nitride (ZrN), zirconium fluoride (ZrF4), zirconium silicide (ZrSi2), zirconium carbide (ZrC), yttrium nitride (YN), yttrium fluoride (YF), titanium nitride (TiN), titanium carbon nitride (TiCxNy), hafnium nitride (HfNx), hafnium fluoride (HfF4) or vanadium nitride (VN). 
     In some embodiments, the coating layer  404  includes boron silicon nitride (BNSi), boron silicon carbide (BCSi), molybdenum carbide (Mo 4 C) or molybdenum carbide (Mo 2 C). 
     In some embodiments, the coating layer  404  includes one or more of the following silicides, zirconium silicide (ZrSi2), silicon hexaboride (B6Si/borosilicide), niobium silicide (NbSi2 and Nb5Si3), molybdenum disilicide (MoSi2) or molybdenum silicide (Mo5Si3). 
     In some embodiments, the coating layer  404  includes one or more of the following carbides, silicon carbide (SiC), molybdenum carbide (MoC, Mo4C and Mo2C), zirconium carbide (ZrC), niobium carbide (Nb4C3) or boron carbide (B4C). 
     The coating layer  404  is thin and does not degrade the transparency of the membrane  306  to UV or EUV light. In some examples, the thickness of the coating layer ( 404  and  406  as well if it is present) ranges between 1 nm and 10 nm. In other embodiments, the thickness of the coating layer  404  and  406  ranges between 0.5 and 5 nanometers. One example of a coating layer has a thickness of 5 nm with a variation of 10% or less. The coating layer may be formed by a suitable deposition technique, such as chemical vapor deposition (CVD), atomic layer deposition (ALD), plasma enhanced ALD (PEALD), physical vapor deposition (PVD), e-beam deposition, electrodeposition, electroless deposition or other suitable technique. 
     In accordance with some embodiments of the present disclosure, the coating layer  404  also serves as a thermal conductive layer which promotes the transfer of thermal energy from transparent layer  402  to the environment around the coating layer  404  and/or  406 . 
     In accordance with some embodiments of the present disclosure where the coating layer  404  and/or  406  is applied to a silicon containing transparent layer  402  or a silicon containing nanotube, the transparent layer  402  (core membrane) or silicon containing nanotube is treated prior to application of the coating layer  404  or coating layer  406  to produce minor defects in the surface of transparent layer  402  or silicon containing nanotube and/or to remove unwanted silicon dioxide. Producing minor defects in such surfaces and/or removing unwanted silicon dioxide improves the adhesion of coating layer  404  or coating layer  406  to the surfaces. Examples of suitable processes to treat the surfaces prior to application of coating layers  404  or  406  include nitrogen, oxygen, carbon fluoride or argon gas plasma treatment. The surfaces can be treated with the gas plasma using a combination of frequency, power, pressure and period of time sufficient to create minor defects in the surfaces which will improve adhesion of the coating layer and/or to remove unwanted silicon dioxide. 
     The mask pellicle system  300  also includes a pellicle frame  304  configured such that the membrane  306  can be attached and secured to the pellicle frame  304 . The pellicle frame  304  may be designed in various dimensions, shapes, and configurations. Among those and other alternatives, the pellicle frame  304  may have one single component or multiple components. The pellicle frame  304  includes a material with mechanical strength, and designed in a shape, dimensions, and a configuration so as to secure the membrane  306  properly across the pellicle frame  304 . In some embodiments, the pellicle frame  304  may be entirely formed by a porous material. 
     The pellicle frame  304  includes a porous material designed for ventilation and pressure balance since the pellicle-mask system  300  is in a vacuum environment when secured on the mask stage  106  during the lithography patterning process. As illustrated in  FIG.  3 C , the porous material of the pellicle frame  304  has connected-pore channels extending from an internal surface  316  to an external surface  318  of the pellicle frame  304  for ventilation. Alternatively as discussed above, pellicle frame  304  includes one or more vents  320  for ventilation and pressure balancing. 
     In accordance with another embodiment of the present disclosure, a transparent layer comprises a plurality of nanotubes forming a matrix or membrane of nanotubes, e.g., carbon nanotubes (CNT), boron-nitride nanotubes (BNNT), silicon carbide (SiC) nanotubes, molybdenum disulfide nanotubes (MoS 2 ), molybdenum diselenide nanotubes (MoSe 2 ), tungsten disulfide nanotubes (WS 2 ), tungsten diselenide nanotubes (WSe 2 ) or combinations thereof. In some embodiments, the transparent layer including the nanotubes is porous. Techniques for forming sheets of a plurality of nanotubes are known. Such sheets of nanotubes when treated in accordance with embodiments of the present disclosure form a combination of a transparent layer and a coating layer that are useful in optical assemblies, e.g., a pellicle membrane, in accordance with embodiments of the present disclosure. The following description proceeds with reference to a carbon nanotube; however, the description is equally applicable to nanotubes described herein which are not carbon nanotubes. 
     In accordance with embodiments of the present disclosure, the carbon nanotubes are single wall nanotubes or multi-walled nanotubes. In some embodiments, the nanotubes are carbon nanotubes. The nanotubes may be oriented nanotubes or they may be non-oriented nanotubes. The nanotubes may be individual, unbundled nanotubes or the nanotubes maybe be bundled individual nanotubes. Carbon nanotubes are susceptible to degradation from exposure to hydrogen gas or oxygen gas, such as the type employed during operation or maintenance of a photolithography system. In accordance with embodiments of the present disclosure, such carbon nanotubes are protected from such harmful gases by coating layers of the present disclosure. 
     Referring to  FIG.  5 A , an embodiment of the present disclosure includes an unbundled single wall carbon nanotube  500  including a coating layer  502  formed on outer surface  504  from one or more of the materials described above for coating layers  404  and/or  406 . As with prior described embodiments, coating layer  502  can comprise a single layer of the same material or can comprise multiple layers of the same material or multiple layers of different materials. In some embodiments, the nanotube  500  includes a core shell structure and the coating layer is one the shell of the core shell structure. Coating layer  502  has a thickness t. In some embodiments, t is 1 to 10 nanometers. In other embodiments, t is 0.5 to 5 nanometers.  FIG.  5 B  is an illustration of a cross-section of a coated individual carbon nanotube in accordance with embodiments of the present disclosure.  FIG.  5 B  illustrates how in accordance with some embodiments of the present disclosure, both the external surface  504  of carbon nanotube  500  is coated with coating material  502 . In other embodiments, both the external surface  504  of carbon nanotube  500  and the internal surface  506  of carbon nanotube  500  are coated with coating material  502 . 
     In accordance with some embodiments of the present disclosure, the surfaces of carbon nanotube  500  are treated prior to application of the material of coating layer  502  on outside surface  504  and inside surface  506  to modify, i.e., produce minor defects in, the surface of carbon nanotube  500  and/or to introduce functional groups, e.g., hydrophilic groups, to the surfaces of the carbon nanotube. Modifying the surfaces of carbon nanotube  500  improves the adhesion of coating layer  502  on the outer surface  504  or the inner surface  506  of carbon nanotube  500 . Examples of suitable processes to treat the surfaces of carbon nanotube  500  prior to application of coating layer  502  include nitrogen, oxygen, carbon fluoride or argon gas plasma treatment. In accordance with some embodiments, the surfaces of the carbon nanotube  500  are treated with the gas plasma using a combination of frequency, power, pressure and period of time sufficient to achieve the desired surface modifications to improve adhesion of coating layer  502  to the nanotube surfaces. In accordance with one embodiment, the carbon nanotube is treated with oxygen plasma at a frequency of about 13.6 MHz at a power of about 100-200 W and a pressure of about 1-200 mTorr. The length of time that the carbon nanotube is so treated is sufficient to provide the desired surface modifications without damaging the carbon nanotubes. 
     Referring to  FIG.  5 C , in other embodiments, the surfaces of the nanotubes  500 , e.g., carbon nanotubes, are coated with a layer  508  which promotes adhesion between the surface of the carbon nanotube  500  and the coating layer  502 . Such adhesion promoting materials are coated onto surfaces of the carbon nanotubes  500  by deposition processes such as ALD and PEALD. Examples of materials of layer  508  include amorphous carbon or other materials which can be deposited by ALD or PEALD processes and promote adhesion between the surfaces of the carbon nanotubes  500  and the coating layer  502 . In accordance with another embodiment of the present disclosure, layer  508  is a protective layer that serves to protect the nanotubes from degradation by a plasma of a PEALD process used to deposit the coating layer  502 . When a protective layer is formed on the nanotubes, it is formed by a first deposition process, for example, a thermal atomic layer deposition process in the absence of any plasma. Multiple cycles of the first deposition process can be employed in order to form multiple protective layers on the nanotubes. After one or more protective layers has been formed on the nanotubes, the coating layer  502  can be deposited using a second deposition process, for example, plasma enhanced atomic layer deposition (PEALD) techniques. Due to the presence of the protective layer, the nanotubes of the membrane are not damaged by the plasma of the PEALD process. Examples of materials useful as a protective layer include the same materials described above for forming coating layer  502 . 
     Referring to  FIG.  6 A , another embodiment of the utilization of nanotubes in an optical assembly, e.g., a pellicle membrane  306 , includes bundling multiple individual nanotubes. In  FIG.  6 A , seven nanotubes  600   a - 600   g  are bundled to form a nanotube bundle  602 . Techniques for bundling nanotubes to form a nanotube bundle  602  useful in embodiments in accordance with the present disclosure are known. In other embodiments, the number of nanotubes making up nanotube bundle  602  is less than seven or is greater than seven. Reducing the number of individual carbon nanotubes forming the nanotube bundle  602  reduces the index of refraction of the nanotube bundle  602 . As noted above, reducing the index of refraction has the effect of reducing the scattering of incident EUV radiation, which has the effect of increasing the transmission of incident EUV radiation. Thus, in some embodiments, a nanotube bundle including fewer individual nanotubes would be preferred over a nanotube bundle including a larger number of individual nanotubes. 
     Referring to  FIG.  6 B , in accordance with embodiments of the present disclosure, the nanotube bundle  602  is coated with or surrounded by a coating layer  604 . The description of materials useful for coating layers  404  and  406  above is applicable to the materials used for coating layer  604 . In the embodiment illustrated in  FIG.  6 B , the coating layer  604  is shown as surrounding the nanotube bundle  602  but is not covering all of the surfaces of the individual nanotubes  600   a - 600   g . In accordance with other embodiments, coating layer  604  coats more of the surfaces of the individual nanotubes  600   a  and  600   b  than is depicted in  FIG.  6 B . For example, coating layer  604  can coat the surfaces of nanotubes  600   a - 600   e  and  600   g  that are exposed on the exterior of the nanotube bundle  602 . In such embodiment, the outer surface of nanotube  600   f  is not coated with coating layer  604 . In other embodiments, the outer surface of each of the seven nanotubes  600   a - 600   g  are coated with the material making up coating layer  604 . In other embodiments, the transparent layer includes individual nanotubes that have been partially or completely coated with coating layer  604 , bundled to form nanotube bundle  602  and then the nanotube bundle is coated/surrounded with an additional layer of coating material  604 . In some embodiments, the coating layer  604  covers the entire surface of the nanotube upon which it resides; however, in other embodiments, the coating layer covers less than the entire surface of the nanotube upon which it resides. 
     In the embodiment of  FIG.  6 A and  6 B , the individual nanotubes are shown as being closely and tightly packed together. In other embodiments, the individual nanotubes may not be packed as closely and tightly as illustrated in  FIGS.  6 A and  6 B . For example, in other embodiments, portions of the outer surfaces of the nanotubes may not be in contact with each other. 
     In another embodiment, pellicle membrane  306  includes a plurality of nanotube bundles  602  and a plurality of individual unbundled nanotubes  500 . In this embodiment, the plurality of nanotube bundles  602  and the plurality of individual unbundled nanotubes  500  are coated in accordance with the description above. In addition, coating layers  402  and/or  406  can be formed on a membrane formed from the individual nanotubes and/or bundled nanotubes. 
     The description above regarding the selection of a particular material for coating layer  404  applies equally to the selection of a particular material for coating layer  604 . The description above regarding the layer  508  as an adhesion layer or a protective layer applies equally to the use of layer  508  in combination with the nanotube bundles of  FIGS.  6 A and  6 B  and the coating layer  604 . 
     Nanotube embodiments in accordance with  FIGS.  5 A,  5 B,  6 A and  6 B  provide a pellicle membrane  306  that transmits 85% or more of UV or EUV radiation incident on the transparent layer. In other embodiments, pellicle membrane  306  comprising coated nanotubes or coated nanotube bundles transmits 90% or more of UV or EUV radiation incident on the transparent layer. In some embodiments, pellicle membrane  306  comprising coated nanotubes or coated nanotube bundles transmits 94% or more of UV or EUV radiation incident on the transparent layer. 
       FIG.  7 A  is a schematic illustration of a matrix of nanotubes before deposition of a coating layer.  FIG.  7 B  is a schematic illustration of the matrix of nanotubes of  FIG.  7 A  after a coating layer has been applied. 
     Referring to  FIG.  8   , a method  800  in accordance with embodiments of the present disclosure for patterning a material on a semiconductor substrate in an EUV photolithography system is illustrated and described with reference to  FIGS.  1  and  4 A- 4 D . The method includes generating EUV radiation in step  820 . EUV radiation can be generated, for example, utilizing the radiation source  102  of  FIG.  1    as described above. The generated EUV radiation is passed through a coating layer of a pellicle membrane in step  830 . Examples of a coating layer includes coating layers  404 ,  406 ,  502  or  604 . Passing the generated EUV radiation though the coating layer is accomplished in accordance with the description above for doing the same. An example of a pellicle membrane is pellicle membrane  306 . At step  840 , the EUV radiation is passed through a transparent layer of the pellicle membrane. Examples of a transparent layer of the pellicle membrane include the transparent layer  402  described above with reference to  FIGS.  4 A- 4 C  or a pellicle membrane formed from the individual nanotubes of  FIG.  5 A and  5 B  or the bundles of nanotubes of  FIGS.  6 A and  6 B . Passing EUV radiation through a transparent layer of a pellicle membrane can be carried out in accordance with the description above. EUV radiation that is passed through the transparent layer of the pellicle membrane is reflected from a mask at step  850 . An example of a mask suitable for reflecting the EUV radiation is mask  108  described above. Mask  108  reflects EUV radiation as described above. At step  860 , EUV radiation is received at a semiconductor substrate. An example of a semiconductor substrate is semiconductor substrate  116  described above. 
     Referring to  FIG.  9   , a method  900  in accordance with embodiments of the present disclosure for patterning a material on a semiconductor substrate in an EUV photolithography system is illustrated and described below with reference to  FIGS.  1  and  4 A- 4 D . The method includes generating EUV radiation in step  920 . EUV radiation can be generated, for example, utilizing the radiation source  102  of  FIG.  1    as described above. The generated EUV radiation is passed through a coating layer of a pellicle membrane in step  930 . Examples of a coating layer include coating layers  404 ,  406 ,  502  or  604 . Passing the generated EUV radiation though the coating layer is accomplished in accordance with the description above for doing the same. An example of a pellicle membrane is pellicle membrane  306 . At step  940 , the EUV radiation is passed through a matrix of nanotubes of the pellicle membrane. Examples of nanotubes of the pellicle membrane include the nanotubes and nanotube bundles described above with reference to  FIGS.  5 A,  5 B,  6 A and  6 B . Passing EUV radiation through the matrix of nanotubes of a pellicle membrane can be carried out in accordance with the description above. EUV radiation that is passed through the matrix of nanotubes of the pellicle membrane is reflected from a mask at step  950 . An example of a mask suitable for reflecting the EUV radiation is mask  108  described above. Mask  108  reflects EUV radiation as described above. At step  962 , EUV radiation is received at a semiconductor substrate. An example of a semiconductor substrate is semiconductor substrate  116  described above. 
       FIG.  11    is a flowchart illustrating a method in accordance with the present disclosure for forming a protective, adhesion or coating layers on transparent layer  402  or a matrix of nanotubes formed into a membrane  960 . In method  1000  of  FIG.  11   , the transparent layer  402  or a matrix of nanotubes provided as a membrane  960  in step  1020 , e.g., on a frame or border  952  is supported, e.g., vertically, in a chamber  942  capable of carrying out a thermal ALD or CVD process and a plasma-enhanced ALD or CVD process. The transparent layer  402  or the frame  952  is supported within chamber  942  such that they have multiple freedoms of movement. For example in the embodiment illustrated in  FIG.  10   , frame  952  can be rotated around a vertical axis or it can be tilted around the horizontal axis. Embodiments in accordance with the present disclosure are not limited to rotating the frame around the vertical access or tilting it around the horizontal axis. In other embodiments, the frame has freedom of movement in addition to rotation around a vertical axis or tilting around the horizontal axis. Such rotation and tilting can be implemented during a thermal process and/or a plasma-enhanced process step to promote even coating of nanotubes of the membrane  960  with a coating layer, adhesion layer or protective layer. Conditions within the chamber are maintained to promote uniform deposition of the coating layer, adhesion layer or protective layer, e.g., temperatures in the range of 500 degrees Celsius to 1200 degrees Celsius. Such temperatures are provided by providing thermal energy from the chamber walls or heaters associated with the chamber walls. Embodiments in accordance with the present disclosure for forming protective, adhesion or coating layers on a matrix of nanotubes or transparent layer  402  are not limited to utilizing thermal PVD or CVD and plasma enhanced PVD or CVD. For example, such layers can be formed using ion beam deposition techniques. The description above regarding utilizing thermal PVD or CVD and plasma enhanced PVD or CVD also applies to the use of ion beam deposition. At step  1030 , a protective layer is formed on the transparent layer or nanotubes of the membrane by a thermal atomic layer deposition process. At step  1040 , a coating layer is formed over the protective layer by a plasma enhanced atomic layer deposition. The chamber  942  illustrated in  FIG.  10    is an example of a chamber where both thermal atomic layer deposition and plasma enhanced atomic layer deposition can be carried out. Method  1000  is not limited to utilizing a single chamber in which both thermal and plasma enhanced atomic layer deposition is carried out. For example, in other embodiments, the thermal deposition process can be carried out in one chamber and the plasma enhanced deposition can be carried out in another different chamber. 
     In one embodiment, the present disclosure describes an optical assembly, e.g., a pellicle assembly of an EUV mask, including a matrix of a plurality of nanotubes having a core shell structure. In such embodiment, a coating layer is provided on the matrix of nanotubes. The coating layer includes a compound selected from nonmetal carbides, nonmetal silicides, nonmetal fluorides, transition metal carbides, transition metal silicides or transition metal fluorides. 
     According to a second aspect of embodiments disclosed herein, there is provided a method that includes generating EUV radiation in an EUV lithography system. The EUV radiation is passed through a coating layer of a pellicle membrane. The coating layer includes one or more compound selected from nonmetal carbide compounds, nonmetal silicide compounds, transition metal carbide compounds and transition metal silicide compounds that exhibit a ratio of EUV % transmittance to thickness in nanometers that is between 40 and 10. EUV radiation that passes through the coating layer of the pellicle membrane is passed through a transparent layer of the pellicle membrane. The EUV radiation that has passed through the transparent layer of the pellicle membrane is reflected from a mask and received at a semiconductor substrate. 
     According to a third aspect of embodiments disclosed herein, there is provided a method that includes forming a matrix of nanotubes. In such embodiments, the nanotubes have a core shell structure and included protective layer on the shell of the core shell structure. A first coating layer is formed on the matrix of nanotubes by a first process and a second process forms a second coating layer on the first coating layer. In some embodiments, the nanotubes have a core shell structure. In other embodiments, the nanotubes do not have a core shell structure. In other embodiments, the nanotubes are individual nanotubes which are bundled and provide a plurality of nanotube bundles. 
     The various embodiments described above can be combined to provide further embodiments. Aspects of the embodiments can be modified, if necessary, to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
     These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.