Patent Description:
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of
radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.

Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.

A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (<NUM>): <MAT> where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k<NUM> is a process-dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (<NUM>) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k<NUM>.

In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of <NUM>-<NUM>, for example within the range of <NUM>-<NUM>. It has further been proposed that EUV radiation with a wavelength of less than <NUM> could be used, for example within the range of <NUM>-<NUM> such as <NUM> or <NUM>. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.

A lithographic apparatus includes a patterning device (e.g., a mask or a reticle). Radiation is provided through or reflected off the patterning device to form an image on a substrate. A pellicle may be provided to protect the patterning device from airborne particles and other forms of contamination. Contamination on the surface of the patterning device can cause manufacturing defects on the substrate.

Pellicles may also be provided for protecting optical components other than patterning devices. Pellicles may also be used to provide a passage for lithography radiation between regions of the lithography apparatus which are sealed from each other. Pellicles may also be used as filters. The pellicle may comprise a freestanding graphene membrane. A mask assembly may include the pellicle which protects a patterning device (e.g. a mask) from particle contamination. The pellicle may be supported by a pellicle frame, forming a pellicle assembly. The pellicle may be attached to the frame, for example by gluing a pellicle border region to the frame. The frame may be permanently or releasably attached to a patterning device. The freestanding graphene membrane may be formed by floating a thin film of graphene on a liquid surface and scooping the thin film onto a silicon frame. The quality of graphene membranes formed in this way has been found to be variable and difficult to control. Furthermore it is difficult to produce large graphene membranes reliably.

It has been found that the lifetime of pellicles comprising freestanding graphene membranes is limited.

It is desirable to improve consistency and control in methods of manufacturing pellicles using freestanding graphene membranes, improve the ability reliably to produce large pellicles using freestanding graphene membranes, or improve the lifetime of pellicles.

<CIT> discloses a graphene pellicle in a lithographic EUV apparatus. The pellicle is supported by a pellicle frame holding the pellicle at a fixed distance from the reticle. <CIT> discloses an X-ray radiation passage window for a radiation detector, which passage window may comprise a graphene-containing layer. <CIT> and <CIT> are both late published documents which are prior art under Art <NUM>(<NUM>) EPC. Both these documents seem to describe that graphene is one of the compounds usable in a heat radiation layer of a pellicle.

According to an aspect of the invention, there is provided a method of manufacturing a pellicle for a lithographic apparatus, comprising: depositing at least one graphene layer on a planar surface of a substrate, wherein the substrate comprises a first substrate portion and a second substrate portion; and removing the first substrate portion to form a freestanding membrane from the at least one graphene layer, the freestanding membrane being supported by the second substrate portion.

According to an aspect of the invention, there is provided a pellicle for a lithographic apparatus, comprising at least one graphene layer forming a freestanding membrane supported by a planar surface of a portion of a substrate on which the graphene layer was grown, said planar surface being located outside of the freestanding membrane when viewed in a direction perpendicular to the planar surface.

According to an aspect of the disclosure, there is provided a pellicle comprising a membrane bonded to a membrane support, wherein: the membrane comprises a graphene layer; and the membrane is bonded to and created on the membrane support with a thin film deposition process.

According to an aspect of the disclosure, there is provided a device manufacturing method comprising: using a patterning device to impart a pattern to a beam of radiation; using a pellicle comprising at least one graphene layer forming a freestanding membrane to protect the patterning device; and passing an electrical current through the at least one graphene layer to heat the at least one graphene layer.

According to an aspect of the disclosure, there is provided an apparatus for processing a pellicle, the pellicle comprising at least one graphene layer forming a freestanding membrane, the apparatus comprising: a current driving apparatus for driving an electrical current through the freestanding membrane to heat the at least one graphene layer.

According to an aspect of the disclosure, there is provided a method of processing a pellicle, the pellicle comprising at least one graphene layer forming a freestanding membrane, the method comprising driving an electrical current through the freestanding membrane to heat the freestanding membrane.

According to an aspect of the disclosure, there is provided a method of processing a pellicle, the pellicle comprising at least one graphene layer forming a freestanding membrane, the method comprising using electrochemical deposition to apply carbon to the at least one graphene layer.

According to an aspect of the disclosure, there is provided a method of manufacturing a pellicle for a lithographic apparatus, comprising: transferring at least one graphene layer from a surface of a liquid to a frame comprising an opening, thereby forming a freestanding membrane from the at least one graphene layer, the freestanding membrane spanning the opening and being supported by the frame, wherein a portion of the frame in contact with the at least one graphene layer is hydrophobic.

According to an aspect of the disclosure, there is provided a method of manufacturing a pellicle for a lithographic apparatus, comprising: transferring at least one graphene layer from a surface of a liquid to a frame comprising an opening, thereby forming a freestanding membrane from the at least one graphene layer, the freestanding membrane spanning the opening and being supported by the frame, wherein the liquid has a temperature in the range of <NUM>-<NUM> degrees Celsius during the transfer of the at least one graphene layer to the frame.

According to an aspect of the disclosure, there is provided a method of manufacturing a pellicle for a lithographic apparatus, comprising: transferring at least one graphene layer from a surface of a liquid to a frame comprising an opening, thereby forming a freestanding membrane from the at least one graphene layer, the freestanding membrane spanning the opening and being supported by the frame, wherein the liquid comprises water, an alcohol, and a further solvent that is not an alcohol.

According to an aspect of the invention, there is provided a pellicle comprising a freestanding membrane, the freestanding membrane comprising at least one layer of a two-dimensional material other than graphene.

According to an aspect of the invention, there are provided a pellicle assembly and a mask assembly comprising a freestanding membrane from the at least one graphene layer or another layer of a two-dimensional material.

According to an aspect of the invention, there is provided a method of manufacturing a pellicle for a lithographic apparatus, comprising: depositing at least one layer of a two-dimensional material on a planar surface of a substrate, wherein the substrate comprises a first substrate portion and a second substrate portion; and removing the first substrate portion to form a freestanding membrane from the at least one layer of a two-dimensional material, the freestanding membrane being supported by the second substrate portion.

<FIG> schematically depicts a lithographic apparatus <NUM> including a source collector module SO according to one embodiment of the invention. The apparatus <NUM> comprises:.

The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

The support structure MT holds the patterning device MA in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The support structure MT can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device MA. The support structure MT may be a frame or a table, for example, which may be fixed or movable as required. The support structure MT may ensure that the patterning device MA is at a desired position, for example with respect to the projection system PS.

The term "patterning device" should be broadly interpreted as referring to any device that can be used to impart a radiation beam B with a pattern in its cross-section such as to create a pattern in a target portion C of the substrate W. The pattern imparted to the radiation beam B may correspond to a particular functional layer in a device being created in the target portion C, such as an integrated circuit.

The patterning device MA may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable liquid-crystal display (LCD) panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam, which is reflected by the mirror matrix.

The projection system PS, like the illumination system IL, may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of a vacuum. It may be desired to use a vacuum for EUV radiation since other gases may absorb too much radiation. A vacuum environment may therefore be provided to the whole beam path with the aid of a vacuum wall and vacuum pumps.

As here depicted, the lithographic apparatus <NUM> is of a reflective type (e.g., employing a reflective mask).

The lithographic apparatus <NUM> may be of a type having two (dual stage) or more substrate tables WT (and/or two or more support structures MT). In such a "multiple stage" lithographic apparatus the additional substrate tables WT (and/or the additional support structures MT) may be used in parallel, or preparatory steps may be carried out on one or more substrate tables WT (and/or one or more support structures MT) while one or more other substrate tables WT (and/or one or more other support structures MT) are being used for exposure.

Referring to <FIG>, the illumination system IL receives an extreme ultraviolet radiation beam from the source collector module SO. Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has at least one element, e.g., xenon, lithium or tin, with one or more emission lines in the EUV range. In one such method, often termed laser produced plasma ("LPP") the required plasma can be produced by irradiating a fuel, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. The source collector module SO may be part of an EUV radiation system including a laser, not shown in <FIG>, for providing the laser beam exciting the fuel. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector, disposed in the source collector module. The laser and the source collector module SO may be separate entities, for example when a CO<NUM> laser is used to provide the laser beam for fuel excitation.

In such cases, the laser is not considered to form part of the lithographic apparatus <NUM> and the radiation beam B is passed from the laser to the source collector module SO with the aid of a beam delivery system comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the source collector module SO, for example when the source is a discharge produced plasma EUV generator, often termed as a DPP source.

The illumination system IL may comprise an adjuster for adjusting the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as □-outer and □-inner, respectively) of the intensity distribution in a pupil plane of the illumination system IL can be adjusted. In addition, the illumination system IL may comprise various other components, such as facetted field and pupil mirror devices. The illumination system IL may be used to condition the radiation beam B, to have a desired uniformity and intensity distribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask) MA, which is held on the support structure (e.g., mask table) MT, and is patterned by the patterning device MA. After being reflected from the patterning device (e.g., mask) MA, the radiation beam B passes through the projection system PS, which focuses the radiation beam B onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor PS2 (e.g., an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor PS1 can be used to accurately position the patterning device (e.g., mask) MA with respect to the path of the radiation beam B. The patterning device (e.g., mask) MA and the substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2.

A controller <NUM> controls the overall operations of the lithographic apparatus <NUM> and in particular performs an operation process described further below. Controller <NUM> can be embodied as a suitably-programmed general purpose computer comprising a central processing unit, volatile and non-volatile storage means, one or more input and output devices such as a keyboard and screen, one or more network connections and one or more interfaces to the various parts of the lithographic apparatus <NUM>. It will be appreciated that a one-to-one relationship between controlling computer and lithographic apparatus <NUM> is not necessary. In an embodiment of the invention one computer can control multiple lithographic apparatuses <NUM>. In an embodiment of the invention, multiple networked computers can be used to control one lithographic apparatus <NUM>. The controller <NUM> may also be configured to control one or more associated process devices and substrate handling devices in a lithocell or cluster of which the lithographic apparatus <NUM> forms a part. The controller <NUM> can also be configured to be subordinate to a supervisory control system of a lithocell or cluster and/or an overall control system of a fab.

<FIG> shows the lithographic apparatus <NUM> in more detail, including the source collector module SO, the illumination system IL, and the projection system PS. An EUV radiation emitting plasma <NUM> may be formed by a plasma source. EUV radiation may be produced by a gas or vapor, for example Xe gas, Li vapor or Sn vapor in which the radiation emitting plasma <NUM> is created to emit radiation in the EUV range of the electromagnetic spectrum. In an embodiment, a plasma of excited tin (Sn) is provided to produce EUV radiation.

The radiation emitted by the radiation emitting plasma <NUM> is passed from a source chamber <NUM> into a collector chamber <NUM>.

The collector chamber <NUM> may include a radiation collector CO. Radiation that traverses the radiation collector CO can be focused in a virtual source point IF. The virtual source point IF is commonly referred to as the intermediate focus, and the source collector module SO is arranged such that the virtual source point IF is located at or near an opening <NUM> in the enclosing structure <NUM>. The virtual source point IF is an image of the radiation emitting plasma <NUM>.

Subsequently the radiation traverses the illumination system IL, which may include a facetted field mirror device <NUM> and a facetted pupil mirror device <NUM> arranged to provide a desired angular distribution of the unpatterned beam <NUM>, at the patterning device MA, as well as a desired uniformity of radiation intensity at the patterning device MA. Upon reflection of the unpatterned beam <NUM> at the patterning device MA, held by the support structure MT, a patterned beam <NUM> is formed and the patterned beam <NUM> is imaged by the projection system PS via reflective elements <NUM>, <NUM> onto a substrate W held by the substrate table WT.

More elements than shown may generally be present in the illumination system IL and the projection system PS. Further, there may be more mirrors present than those shown in the Figures, for example there may be <NUM>- <NUM> additional reflective elements present in the projection system PS than shown in <FIG>.

Alternatively, the source collector module SO may be part of an LPP radiation system.

As depicted in <FIG>, in an embodiment the lithographic apparatus <NUM> comprises an illumination system IL and a projection system PS. The illumination system IL is configured to emit a radiation beam B. The projection system PS is separated from the substrate table WT by an intervening space. The projection system PS is configured to project a pattern imparted to the radiation beam B onto the substrate W. The pattern is for EUV radiation of the radiation beam B.

The space intervening between the projection system PS and the substrate table WT can be at least partially evacuated. The intervening space may be delimited at the location of the projection system PS by a solid surface from which the employed radiation is directed toward the substrate table WT.

In an embodiment the lithographic apparatus <NUM> comprises a dynamic gas lock. The dynamic gas lock comprises a pellicle <NUM>. In an embodiment the dynamic gas lock comprises a hollow part covered by a pellicle <NUM> located in the intervening space. The hollow part is situated around the path of the radiation. In an embodiment the lithographic apparatus <NUM> comprises a gas blower configured to flush the inside of the hollow part with a flow of gas. The radiation travels through the pellicle <NUM> before impinging on the substrate W.

In an embodiment the lithographic apparatus <NUM> comprises a pellicle <NUM>. As explained above, in an embodiment the pellicle <NUM> is for a dynamic gas lock. In this case the pellicle <NUM> functions as a filter for filtering DUV radiation. Additionally or alternatively, in an embodiment the pellicle <NUM> protects an optical element, for example a patterning device MA. The pellicle <NUM> of the present invention can be used for a dynamic gas lock or for protecting an optical element or for another purpose.

In an embodiment the pellicle <NUM> is configured to seal off the patterning device MA to protect the patterning device MA from airborne particles and other forms of contamination. Contamination on the surface of the patterning device MA can cause manufacturing defects on the substrate W. For example, in an embodiment the pellicle <NUM> is configured to reduce the likelihood that particles might migrate into a stepping field of the patterning device MA in the lithographic apparatus <NUM>.

If the patterning device MA is left unprotected, the contamination can require the patterning device MA to be cleaned or discarded. Cleaning the patterning device MA interrupts valuable manufacturing time and discarding the patterning device MA is costly. Replacing the patterning device MA also interrupts valuable manufacturing time.

In the embodiments described below references to upper/lower, up/down, top/bottom, above/below, etc. are made relative to the orientations on the page of the side sectional views. A front side of the pellicle faces upwards and a back side of the pellicle faces downwards. The substrate <NUM> is therefore always located on a back side of the at least one graphene layer <NUM> within this convention.

<FIG> schematically depict stages in a method of manufacture of a pellicle <NUM> according to an embodiment. The method comprises depositing at least one graphene layer <NUM> on a planar surface <NUM> of a substrate <NUM>. The substrate <NUM> may comprise a single layer or multiple layers of material. In an embodiment, the substrate <NUM> comprises a base layer <NUM> and one or more further layers <NUM> formed on top of the base layer <NUM>. In an embodiment, the base layer <NUM> comprises a silicon wafer. In other embodiments, the base layer <NUM> may be formed from other materials.

In an embodiment, the substrate <NUM> comprises a first substrate portion <NUM> and a second substrate portion <NUM>. The method of manufacture of the pellicle <NUM> comprises removing the first substrate portion <NUM> to form a freestanding membrane <NUM> from the at least one graphene layer <NUM>. The freestanding membrane <NUM> is supported by the second substrate portion <NUM>. In an embodiment the freestanding membrane <NUM> is at least <NUM>% transparent to EUV radiation used in an EUV lithographic apparatus such as <NUM> or <NUM> (e.g. <NUM>% transparent to radiation having a wavelength of <NUM> or <NUM>), optionally at least <NUM>% (e.g. <NUM>% transparent to radiation having a wavelength of <NUM> or <NUM>), optionally at least <NUM>% (e.g. <NUM>% transparent to radiation having a wavelength of <NUM> or <NUM>).

In the embodiments described below with reference to <FIG> the freestanding membrane <NUM> is formed exclusively from a portion of the at least one graphene layer <NUM>, optionally with a coating. Each of the embodiments, and other embodiments, can however be adapted so that the freestanding membrane <NUM> comprises a portion of the at least one graphene layer <NUM> in combination with an additional layer on an upper surface of the graphene layer <NUM> or an additional layer on a lower surface of the graphene layer <NUM>. An example of such an embodiment is shown schematically in <FIG>, where an additional layer <NUM> is formed on an upper surface of the at least one graphene layer <NUM> and an additional layer <NUM> is formed on a lower surface of the at least one graphene layer <NUM>. Such additional layers may be formed for example by stopping an etching process configured to remove a layer adjacent to the at least one graphene layer <NUM> before the layer has been completed removed. In the particular example shown in <FIG>, the additional layers <NUM> and <NUM> are formed by stopping the etching away of graphene-support layers <NUM> and <NUM> before the at least one graphene layer <NUM> is reached, thereby forming additional layers <NUM> and <NUM> from thin layers of the material forming the graphene-support layers <NUM> and <NUM>. Further details about the graphene-support layers <NUM> and <NUM> are given below. In other embodiments, the additional layers <NUM> and <NUM> may have a different composition. The additional layers <NUM> and <NUM> may provide additional mechanical support for the freestanding membrane <NUM>. The additional layers <NUM> and <NUM> are configured to be thin enough that the freestanding membrane <NUM> remains adequately transparent to radiation that is to be transmitted through the freestanding membrane <NUM> (e.g. EUV radiation, as described above).

It is understood in the field of pellicles that a freestanding membrane is to be distinguished from a mesh-supported membrane. A freestanding membrane spans freely over a continuous area without any supports positioned within the area (when viewed perpendicular to the freestanding membrane). A mesh-supported membrane, in contrast, is supported by a mesh positioned in the area over which the membrane spans (when viewed perpendicular to the membrane).

In an embodiment, the at least one graphene layer <NUM> consists of a single layer of graphene, a bilayer of graphene or more than two monolayers of graphene (e.g. between <NUM> and <NUM> layers of graphene, optionally between <NUM> and <NUM> layers of graphene). A single layer, or a small number of layers, of graphene provides good transparency, particularly where folds and other imperfections are minimized. Higher numbers of graphene layers are more robust. It has been found that <NUM> layers of graphene and above provides satisfactory rigidity in a range of embodiments. It has also been found that less than <NUM> layers of graphene provides satisfactory transparency in a range of embodiments (e.g. <NUM>% transmission of EUV radiation).

Graphene is understood to mean a one atom thick layer of graphite: a layer of sp2 bonded carbon atoms in a hexagonal or honeycomb lattice. Multiple layers of graphene are sometimes referred to as graphite, particularly where the number of layers is larger than about <NUM> layers. As the number of sheets of graphene increases the electronic structure becomes increasingly similar to, and eventually indistinguishable from, bulk graphite. Multiple layers of graphene (or graphite) are also sometimes referred to as graphite nanoplatelets or graphene nanoplatelets.

In an embodiment one or more of the layers in the at least one graphene layer <NUM> may comprise one or more layers of graphene derivatives, such as functionalized graphene or graphene with modifications, such as oxidized graphene, graphane, graphyne, fluorinated graphene, graphene bromide, graphene chloride, graphene iodide and graphene with other functionalities attached to the graphene. Graphene and graphene derivatives have in common that they are all membranes which have carbon sp2 bonded bases. The mechanical properties of graphene derivatives may be the same or similar to the mechanical properties of graphene, although the chemical properties may be different. Graphene fluoride may provide the advantage that it has bonds which are less susceptible than graphene bonds to breaking when illuminated by EUV radiation.

In an embodiment a coating is provided on the freestanding membrane <NUM>. The coating is configured to protect the at least one graphene layer <NUM> of the freestanding membrane <NUM>. The coating may provide one or more of thermal protection, mechanical protection, and chemical protection.

In the example shown in <FIG>, the freestanding membrane <NUM> comprises a portion of the at least one graphene layer <NUM> delimited by a boundary line <NUM> (see <FIG>) marking the edge of the first substrate portion <NUM>. The freestanding membrane <NUM> is thus formed from the portion of the at least one graphene layer <NUM> that was positioned over the first substrate portion <NUM>. The freestanding membrane <NUM> is thus not supported by any material positioned immediately below the freestanding membrane <NUM> (i.e. along a direction perpendicular to the planar surface <NUM> of the substrate <NUM>).

In an embodiment, the first substrate portion <NUM> is removed by selective etching of the substrate <NUM>. In an embodiment, an encapsulation layer or sacrificial layer is coated at least over a front and side surface of a stack comprising the at least one graphene layer <NUM> and the substrate <NUM> during the removal of the first substrate portion <NUM>. The encapsulation layer or sacrificial layer provides mechanical support to the stack during the processing to remove the first substrate portion <NUM>, which can involve relatively long etching steps. Covering of the side surface prevents unwanted ingress of etchant into the stack from the sides. The encapsulation layer or sacrificial layer may comprise any suitable material that is resistant to the processing steps (e.g. etching) needed to remove the first substrate portion <NUM>. In an embodiment the encapsulation layer or sacrificial layer comprises an organic polymer. In an embodiment the encapsulation layer or sacrificial layer comprises a poly(p-xylylene) polymer such as Parylene or ProTEK® type materials. In an embodiment the encapsulation layer or sacrificial layer comprises PMMA. In other embodiments the encapsulation or sacrificial material comprises an inorganic material, such as a metal layer. Examples of different encapsulation or sacrificial layers are mentioned below with reference to the detailed examples of <FIG>.

In an embodiment, the first substrate portion <NUM> comprises a continuous volume of material positioned underneath a portion of the at least one graphene layer <NUM> that will form the freestanding membrane <NUM>. In an embodiment, the first substrate portion <NUM> is surrounded by the second substrate portion <NUM> when viewed in a direction perpendicular to the planar surface <NUM> of the substrate <NUM> (i.e. in a vertical direction in the page in the orientation of the side sectional views in the figures). Configuring the second substrate portion <NUM> in this way helps to provide reliable and spatially homogeneous support to the freestanding membrane <NUM>. In such an embodiment, removal of the first substrate portion <NUM> forms a hole passing through the substrate <NUM> in a direction perpendicular to the planar surface <NUM>. The hole is spanned continuously (i.e. with no gaps) by the freestanding membrane <NUM>. A pellicle <NUM> formed in this way may be configured such that the freestanding membrane <NUM> spans continuously (i.e. with no gaps) across an optical element (e.g. patterning device MA) to be protected by the pellicle <NUM>.

The freestanding membrane <NUM> is supported by the second substrate portion <NUM>. In an embodiment the support is provided by adhesion of a portion of the at least one graphene layer <NUM> to the second substrate portion <NUM>. In the example shown in <FIG> the adhesion occurs in the region outside of the boundary line <NUM>. The freestanding membrane <NUM> is thus supported laterally via the portion of the at least one graphene layer that is positioned over the second substrate portion <NUM>.

The freestanding membrane <NUM> may remain substantially planar even after the first substrate portion <NUM> has been removed. Alternatively, the freestanding membrane <NUM> may sag under its own weight. The amount of sag may be controlled by changing a tension in the freestanding membrane <NUM>. The amount of sag that is acceptable will depend on the particular application of the pellicle <NUM>. In embodiments where the pellicle <NUM> protects an optical element, such as the patterning device MA, it may be desirable to arrange for the sag to be small enough to avoid contact between the pellicle <NUM> and the optical element. For example in one embodiment the pellicle <NUM> is positioned about <NUM> ± <NUM> from the patterning device MA and the tension in the freestanding membrane <NUM> is set so that a maximum sag in use will not exceed about <NUM> microns.

In an embodiment the freestanding membrane <NUM> has a surface area of at at least <NUM><NUM>, preferably at least <NUM><NUM>, preferably at least <NUM><NUM>, preferably at least <NUM><NUM>, preferably at least <NUM><NUM>, preferably at least <NUM><NUM>, when viewed in a direction perpendicular to the planar surface <NUM> of the substrate <NUM>. The minimum size of the freestanding membrane <NUM> will depend on the particular application in question and may be significantly larger than this value. Where the pellicle <NUM> is to protect an optical component, the freestanding membrane <NUM> will typically be configured to be at least as large as a cross-sectional area through which all radiation incident on the optical element, and/or all radiation leaving the optical element, passes.

Forming the freestanding membrane <NUM> using the above methods provides several benefits. High quality adhesion is achieved between the second substrate portion <NUM> and the at least one graphene layer <NUM> because the at least one graphene layer <NUM> remains on the surface on which it was originally deposited. The problems of folding, entrapment of gas bubbles and tearing of the graphene, which have been observed to occur when handling graphene films floating on liquids, are avoided. Tension in the freestanding membrane <NUM> can be controlled accurately and reliably. Variations in tension due to unpredictable adhesion and handling variations, which have been observed to occur when handling graphene films floating on liquids, are avoided. The techniques used in the method, including the depositing of graphene and processing of the substrate to selectively remove a part of the substrate, can be scaled up to allow larger freestanding membranes to be formed reliably.

<FIG> schematically depict initial stages in a method of manufacturing the pellicle <NUM> according to an embodiment. In this embodiment a base layer <NUM> comprising a silicon wafer is processed to form a silicon oxide layer <NUM> (SiO<NUM>) on an outer surface of the silicon wafer (<FIG>). The processing may comprise thermal processing.

In a subsequent step a graphene-support layer <NUM> is formed on an upper surface of the silicon oxide layer <NUM>. In an embodiment the graphene-support layer <NUM> comprises a layer of metal or a metal in silicized form. In an embodiment, the graphene-support layer <NUM> comprises one or more of the following: transition metals such as Mo, Ni, Ru, Pt, Cu, Ti, V, Zr, Nb, Hf, Ta, W, Cr, silicized Mo, silicized Ni, silicized Ru, silicized Pt, silicized Cu, silicized Ti, silicized V, silicized Zr, silicized Nb, silicized Hf, silicized Ta, silicized W, silicized Cr, carbide of Mo, carbide of Ni, carbide of Ru, carbide of Pt, carbide of Cu, carbide of Ti, carbide of V, carbide of Zr, carbide of Nb, carbide of Hf, carbide of Ta, carbide of W, carbide of Cr.

In this context, the reference to a silicized metal is understood to mean a layer of the metal covered by a layer of the metal silicide at a surface. It has been found that the metal silicide tends to have a lower melting point than the corresponding metal, which means that the graphene can be grown in conditions in which the metal part of the graphene-support layer is solid and the metal silicide part of the graphene-support layer is a liquid or liquid-like. The liquid or liquid-like surface provided by the metal silicide provides a very smooth surface for the graphene layer, thereby improving the quality of the graphene layer. Use of Mo or silicized Mo may be particularly desirable because it is possible to directly synthesize high quality multilayer graphene on Mo or silicized Mo using CVD. Multilayer graphene may be more robust that single layer graphene while still providing adequate transparency to radiation. Where Mo or silicized Mo is used a controllable and uniform thickness can be achieved by controlling the CVD process. The direct synthesis avoids the need to manually transfer multiple individual monolayers formed using other processes, for example using CVD on a graphene-support layer formed from Cu. The process of transferring the individual monolayers would tend to increase defectivity relative to direct formation without any transfer. Multilayer graphene can also be formed directly on graphene-support layers comprising Ni but the quality tends to be inferior in comparison to Mo or silicized Mo. For example a non-continuous layer comprising flakes may be formed.

The quality of graphene when growth by CVD may be largely influenced by the catalyst surface on which it grows, mostly because the grown graphene will follow the catalyst surface conformally. The catalyst surface may provide morphological changes at the high temperature required to grow graphene. Grain boundaries of the catalyst surface may occur and graphene may grow over surface grain boundaries sporadically. Reduction of the grain boundaries may be done by optimization for larger grain sizes, by influencing the growth rate dependence on crystal orientation by forming epitaxial layers or monocrystalline layers, by the improvement of layer thickness and layer thickness uniformity of CVD grown graphene and/or by improvement or changes in catalyst surface roughness. The catalyst surface can be optimized by optimization of gran sizes, which is influenced by temperature, growth time, internal stress and roughness. Epitaxial or monocrystalline surfaces may be formed by sputtering or CVD or any other PVD technique. A better quality graphene will improve imaging performance and the pellicle life time.

Transition metal carbides from metals in groups IVB, VB and VIB in the Periodic Table, such as the carbides of Mo, Ni, Ru, Pt, Cu, Ti, V, Zr, Nb, Hf, Ta, W, Cr mentioned above, exhibit catalytic activity which resembles that of noble metals. These catalysts are particularly active towards dehydrogenation and aromatization of hydrocarbons and therefore provide a particularly suitable support for synthesis of graphene. In practice, when graphene is grown on a nominally bare surface of a metal from group IVB, VB or VIB, it is expected that for some metals a layer of a carbide of the metal will be formed (e.g. a surface layer of the metal will be partially or completely converted to the carbide) initially as part of the process of forming the at least one graphene layer <NUM> on the graphene-support layer <NUM>. This is expected for example in the case of Mo due to the negative enthalpy of formation of Mo<NUM>C. For metals or processes where this does not occur, a separate process may be provided for forming the carbide on the metal prior to formation of the at least one graphene layer <NUM>. In either case, where it is expected that the at least one graphene layer <NUM> will be formed on a carbide layer, the process (e.g. CVD) for forming the at least one graphene layer <NUM> should be adapted to take the carbide layer into account. The carbide layer provides opportunities to pursue different strategies towards optimizing the growth of the at least one graphene layer <NUM>. For example, it is possible to control properties of the surface of the carbide to improve the formation of the at least one graphene layer <NUM>. Properties such as surface morphology, grain size and crystal orientation may be controlled for example. Account may be taken of the growth mechanism of the at least one graphene layer <NUM> on the carbide. The growth mechanism may involve for example growth from the bulk by either isothermal growth or segregation upon cooling or growth from the surface by chemisorption. The growth mechanism may involve epitaxial growth by direct deposition of the graphene with a desired crystal orientation. The overall thickness of the at least one graphene layer <NUM> may be controlled based on differences in diffusion coefficients versus crystal orientation.

In a variation on the above embodiments the step of forming the silicon oxide layer is omitted and the graphene-support layer <NUM> is formed directly on the base layer <NUM> (e.g. directly on a silicon wafer).

Due to the relatively low solid-solubility of C in Mo and the relatively high diffusion coefficient of Mo in C, the rate limiting step for growth of the at least one graphene layer <NUM> on Mo is the low solid-solubility. The low solid-solubility will limit the thickness of the at least one graphene layer <NUM> that can be efficiently grown directly on Mo. In an embodiment, the at least one graphene layer <NUM> is grown on silicized Mo (e.g. MoSi<NUM>). The solid-solubility of C in MoSi<NUM> is higher than the solid-solubility of C in Mo, thereby allowing the thickness of the at least one graphene layer <NUM> to be increased. In an embodiment, the silicized Mo (e.g. MoSi<NUM>) is provided in a tetragonal phase form. The tetragonal phase form provides a better epitaxial match with the at least one graphene layer <NUM> (the lattices of MoSi<NUM> and graphene are more similar than the lattices of Mo and graphene). Providing an improved epitaxial match will promote growth of an at least one graphene layer <NUM> with fewer defects and grain boundaries. In an embodiment, the graphene-support layer <NUM> comprises a layer of Mo and a layer of silicized Mo (e.g. MoSi<NUM>) grown on the layer of Mo. In an embodiment the layer of Mo has a thickness of <NUM>-<NUM> and the layer of silicized Mo (e.g. MoSi<NUM>) has a thickness of <NUM>-<NUM>. The layer of silicized Mo (e.g. MoSi<NUM>) may be grown by sputtering (or any other suitable physical or chemical deposition technique). In an embodiment an annealing step is performed to drive a phase transition of the grown layer of silicized Mo (e.g. MoSi<NUM>) from a hexagonal phase to the desired tetragonal phase. In an embodiment the annealing comprises heating the layer of silicized Mo (e.g. MoSi<NUM>) at a minimum temperature of <NUM> degrees C for a minimum time of <NUM> minutes. <FIG> depicts an example configuration in which a graphene-support layer <NUM> comprises a Mo layer 36A and a silicized Mo (e.g. MoSi<NUM>) layer 36B. The silicized Mo (e.g. MoSi<NUM>) layer 36B was grown directly on the Mo layer 36A and subsequently annealed as discussed above to provide the silicized Mo (e.g. MoSi<NUM>) in the tetragonal (epitaxially matching) phase.

In a subsequent step the at least one graphene layer <NUM> is formed on the graphene-support layer <NUM>. In an embodiment, the at least one graphene layer <NUM> is formed by chemical vapor deposition (CVD). The number of graphene layers <NUM> in the at least one graphene layer <NUM> may depend on the composition of the graphene-support layer <NUM>. For example, where the graphene-support layer <NUM> comprises Cu, CVD will typically produce a monolayer of graphene. CVD on Ni or Mo can produce multilayers. The resulting structure is shown in <FIG>.

In an embodiment the graphene-support layer <NUM> has a root mean squared roughness of less than <NUM>, optionally less than <NUM>, optionally less than <NUM>, optionally less than <NUM>. Increasing the smoothness of the graphene-support layer <NUM> reduces the risk of significant folding in, or other disruption to, the portion of the at least one graphene layer <NUM> that forms the freestanding membrane <NUM> when the underlying graphene-support layer <NUM> is removed. Increasing the smoothness will also tend to increase the tension in the freestanding membrane <NUM> because the surface area of the graphene will tend to be lower where it does not have to follow large irregularities in the surface on which it is deposited. Conversely, decreasing the smoothness will tend to decrease the tension in the freestanding membrane <NUM>. In an embodiment the degree of smoothness of the graphene-support layer <NUM> is selected to achieve a desired tension in the freestanding membrane <NUM> during use. Alternatively, one or both of thermal and chemical processing may be applied to the at least one graphene layer <NUM>, and/or to one or more surrounding layers, to achieve a desired tension in the freestanding membrane <NUM> during use.

In an embodiment, the substrate <NUM> comprises a base layer <NUM>, a first graphene-support layer <NUM> and a second graphene-support layer <NUM>. The at least one graphene layer <NUM> is formed on the first graphene-support layer <NUM> and the second graphene-support layer is formed on top of the at least one graphene layer <NUM>. The first graphene-support layer <NUM> and the second graphene-support layer <NUM> may have the same composition or a different composition.

Arranging for the first and second graphene-support layers <NUM> and <NUM> to have the same composition, for example both comprising Mo or silicized Mo, and/or the same thickness, may desirably balance capillary forces exerted on the at least one graphene layer <NUM> during wet etching steps.

Arranging for the first and second graphene-support layers <NUM> and <NUM> to have different compositions or thicknesses may be used to control a tension in the freestanding membrane <NUM> to be formed. For example, the second graphene-support layer <NUM>, which may still be present after the freestanding membrane <NUM> has been formed, may be selected to act as a control layer for controlling the tension. For example, the second graphene-support layer <NUM> may be formed from a material that can be processed to change a tension in the freestanding membrane <NUM>. For example, the material may shrink on heating and thereby pull the freestanding membrane <NUM> into a state of higher tension. Control layers are discussed in further detail below, particularly in relation to the embodiments discussed with reference to <FIG>.

In one embodiment the first graphene-support layer <NUM> comprises a metal or silicized metal and the second graphene-support layer <NUM> comprises hexagonal boron nitride. Hexagonal boron nitride is chemically more insert than graphene so a thin layer of the hexagonal boron nitride can be left on the at least one graphene layer <NUM> as a coating or additional layer to protect the graphene and/or act to reduce DUV reflection.

The combination of the first graphene-support layer <NUM> and the second graphene-support layer <NUM> protects the at least one graphene layer <NUM> during subsequent processing steps (e.g. preventing damage to the graphene or to the adhesion between the graphene and other layers), provides mechanical support to the at least one graphene layer <NUM> (e.g. facilitating handling), or both.

<FIG> depict example stages in a manufacturing method starting from the arrangement of <FIG> in the case where both of a first graphene-support layer <NUM> and a second graphene-support layer <NUM> are provided. In this example the second graphene-support layer <NUM> is formed by e-beam evaporation (or other deposition technique) to provide the arrangement shown in <FIG>. Subsequently, a further layer <NUM> is formed on the second graphene-support layer <NUM>. The further layer <NUM> may comprise one or more of the following: an adhesion layer, a plasma-enhanced chemical vapor deposition (PECVD) tetraethylorthosilicate (TEOS) layer, or a PECVD oxide layer. The further layer <NUM> provides further protection during subsequent processing steps. The further layer <NUM> provides predictable and therefore reliable adhesion with encapsulation layer or sacrificial layer <NUM> or further encapsulation layer or sacrificial layer <NUM> (e.g. Parylene). The further layer <NUM> may protect the graphene-support layer <NUM> from attack by etching steps, such as the O<NUM> barrel etch mentioned below. The further layer <NUM> may also increase the symmetry of the stack, thereby providing improved mechanical support for the at least one graphene layer <NUM>.

In a subsequent step the silicon oxide layer <NUM> on a lower surface of the base layer <NUM> is removed by etching to provide the structure shown in <FIG>.

In a subsequent step the structure is encapsulated in an encapsulation layer or sacrificial layer <NUM> (which may be referred to as an etch mask) to provide the structure shown in <FIG>. In an embodiment the encapsulation layer or sacrificial layer <NUM> comprises SixNy but other materials may also be used depending the etching processes to be used in subsequent steps. The encapsulation layer or sacrificial layer <NUM> should be resistant to at least a subset of etchants used in subsequent steps. In other embodiments the encapsulation layer or sacrificial layer <NUM> is omitted from the top of the stack.

In an alternative embodiment, schematically depicted in <FIG>, the step of forming the second graphene-support layer <NUM> is omitted. In this case the further layer <NUM> is formed directly on the at least one graphene layer <NUM>, as shown in <FIG> shows the result of applying the encapsulation layer or sacrificial layer <NUM>.

<FIG> depict example subsequent processing stages starting from the arrangement of <FIG>. The same processing could also be carried out starting from the arrangement of <FIG>.

To achieve the arrangement shown in <FIG>, the arrangement of <FIG> is photolithographically patterned and then processed to form windows <NUM> and <NUM> in the encapsulation layer or sacrificial layer <NUM> (e.g. by dry etching in SixNy). A further encapsulation layer or sacrificial layer <NUM> is then deposited around the resulting arrangement and processed (e.g. by dry etching or selective deposition) to open window <NUM>. The further encapsulation layer or sacrificial layer <NUM> may comprise a poly(p-xylylene) polymer such as Parylene or ProTEK® type materials for example.

In a subsequent step a KOH etch is used to remove the silicon forming the base layer <NUM>, thereby producing the arrangement shown in <FIG>. The presence of the further encapsulation layer or sacrificial layer <NUM> during this processing provides mechanical strength to facilitate handling and also acts to protect layers which are not being etched (e. g preventing damage to the at least one graphene layer <NUM> itself or damage to a quality of adhesion between the at least one graphene <NUM> and other layers).

In a subsequent step the further encapsulation layer or sacrificial layer <NUM> is removed to produce the arrangement shown in <FIG>. In an embodiment the further encapsulation layer or sacrificial layer <NUM> is removed using an O<NUM> barrel etch, Rie etch or other removal techniques.

In a subsequent step a portion of the further layer <NUM> within window <NUM> and a portion of the silicon oxide layer <NUM> in window <NUM> are removed to produce the arrangement shown in <FIG>. In an embodiment these layers are removed using a buffered oxide etch. In a subsequent step portions of the first and second graphene-support layers <NUM> and <NUM> are removed (via windows <NUM> and <NUM>) to leave a freestanding membrane <NUM>, as shown in <FIG>. In an embodiment, the first and second graphene-support layers <NUM> and <NUM> are removed using a metal etch.

<FIG> depict alternative subsequent processing stages starting from the arrangement of <FIG>. The same processing could also be carried out starting from the arrangement of <FIG>. The processing of <FIG> does not require the further encapsulation layer or sacrificial layer <NUM> (as used in the processing described above with reference to <FIG>). In the case where the further encapsulation layer or sacrificial layer <NUM> comprises Parylene, the processing which does not use this layer may be referred to as a Parylene-free processing flow.

To produce the arrangement shown in <FIG>, the arrangement of <FIG> is photolithographically patterned and then processed to form windows <NUM> and <NUM> in the encapsulation layer or sacrificial layer <NUM> (e.g. by dry etching in SixNy).

In a subsequent step a buffered oxide etch is used to remove a portion of the further layer <NUM> in window <NUM>. A KOH etch is used to remove a portion of the silicon forming the base layer <NUM> within window <NUM>, thereby producing the arrangement of <FIG>.

In a subsequent step portions of the first and second graphene-support layers <NUM> and <NUM> in windows <NUM> and <NUM> are removed to leave a freestanding membrane <NUM>, as shown in <FIG>. In an embodiment, the portions of the first and second graphene-support layers <NUM> and <NUM> are removed using a suitable etch.

The methods described above with reference to <FIG> are example embodiments in which a stack comprising the at least one graphene layer <NUM> is encapsulated with an encapsulation layer or sacrificial layer <NUM> over at least a front and a side surface of the stack during the removing of the first substrate portion <NUM>. In the particular examples shown, the stack comprises the base layer <NUM>, the silicon oxide layer <NUM>, the first graphene-support layer <NUM>, the at least one graphene layer <NUM> and the further layer <NUM> when starting from the arrangement of <FIG>. When starting from the arrangement of <FIG>, the stack further comprises the second graphene-support layer <NUM>. The first substrate portion <NUM> comprises the portions of the base layer <NUM>, silicon oxide layer <NUM> and first graphene-support layer <NUM> which are removed in order to form the freestanding membrane <NUM>, as shown for example in <FIG> and <FIG>. The encapsulation layer or sacrificial layer <NUM> protects the at least one graphene layer <NUM> from damage during the processing steps used to remove the first substrate portion <NUM> and form the freestanding membrane <NUM>. The layers provided above the at least one graphene layer <NUM> may also enhance mechanical rigidity of the stack, thereby facilitating safe handling of the stack during processing to remove the first substrate portion <NUM>.

<FIG> depict stages in an alternative embodiment. In this embodiment, a base layer <NUM> comprising a silicon wafer is processed to form a silicon oxide layer <NUM> (SiO<NUM>) on an outer surface of the silicon wafer (<FIG>). In a subsequent step a lower side of the stack is etched to remove the silicon oxide layer <NUM> on the lower side of the base layer <NUM>. In a subsequent step an encapsulation layer or sacrificial layer <NUM> is applied to produce the arrangement shown in <FIG>. The encapsulation layer or sacrificial layer <NUM> in this embodiment may comprise for example a PECVD nitride etch mask.

In a subsequent step the arrangement of <FIG> is photolithographically patterned and then processed to form windows <NUM> and <NUM> in the encapsulation layer or sacrificial layer <NUM> (e.g. by SixNy dry/wet etch), as shown in <FIG>.

In a subsequent step a graphene-support layer <NUM> is formed that fills window <NUM>. The graphene-support layer <NUM> may take any of the forms described above (e.g. comprising a metal or metal silicide).

In a subsequent step the at least one graphene layer <NUM> is formed on the graphene-support layer <NUM> to produce the arrangement shown in <FIG>. The at least one graphene layer <NUM> may take any of the forms described above (e.g. formed using CVD).

In a subsequent step a protection layer <NUM> is applied over the at least one graphene layer <NUM> to produce the arrangement shown in <FIG>. In an embodiment the protection layer <NUM> comprises PMMA or another organic material. PMMA can be applied (e.g. by spin coating) with minimum risk of disruption or damage to previously deposited layers (e.g. the at least one graphene-layer <NUM> or any other layers). PMMA is known to be compatible with graphene and various techniques are known for removing PMMA effectively without damaging graphene layers.

In a subsequent step a portion of the base layer <NUM>, silicon oxide layer <NUM> and graphene-support layer <NUM> in window <NUM> are removed in a region beneath the at least one graphene layer <NUM> to produce the arrangement shown in <FIG>. In an embodiment the removal is implemented using a dry/wet etch of SixNy following by a KOH etch.

In a subsequent step the protection layer <NUM> above the at least one graphene layer <NUM> is removed to leave a freestanding membrane <NUM>, as shown in <FIG>. In an embodiment the protection layer <NUM> is removed by thermal decomposition or by liquid/vapor solvation.

In an embodiment a control layer <NUM> is provided over a portion of the at least one graphene layer <NUM> outside of the freestanding membrane <NUM>. The control layer <NUM> can be used to control a tension in the freestanding membrane <NUM>. For example, in an embodiment the control layer <NUM> is processed (e.g. by heating or cooling) to cause a change in an internal structure of the control layer <NUM>. The change in the internal structure transfers forces to the freestanding membrane, thereby causing a change in tension in the freestanding membrane <NUM>. The change in the internal structure may be such as to persist after the processing (e.g. by heating or cooling) has stopped. In an embodiment, the control layer <NUM> is deposited on the at least one graphene layer <NUM> in such a way that the density of the layer is lower than an equilibrium bulk density. Subjecting such a layer to an external influence (e.g. by applying heat) can cause the layer to shrink so as to bring the density closer to the bulk density. This shrinking is an example of a change in the internal structure of the control layer <NUM> which would be effective in changing a tension in the freestanding membrane <NUM> (e.g. increasing the tension as the control layer <NUM> shrinks). In other embodiments the control layer <NUM> may be processed to change the tension in the freestanding membrane <NUM> by causing the control layer <NUM> to undergo a phase transition, by thinning the control layer <NUM>, for example by dry or wet etching, or by changing a chemical composition of the control layer <NUM>.

In an embodiment the tension in the freestanding membrane <NUM> is controlled so that the freestanding membrane <NUM> will remain sufficiently flat during use. If the tension in the freestanding membrane <NUM> is too low, the freestanding membrane <NUM> may be undesirably flappy, leading to excessive sagging, or wrinkled. Wrinkling may lead to non-uniform thickness of the freestanding membrane <NUM>. A freestanding membrane <NUM> that is loose or of non-uniform thickness can have poorer imaging properties. If the tension in the freestanding membrane <NUM> is too high, the freestanding membrane <NUM> can be brittle and more susceptible to breaking. Accordingly, it is desirable to control the tension in the freestanding membrane <NUM> to be within a target range at the manufacturing stage.

In an embodiment the tension in the freestanding membrane <NUM> is controlled at the manufacturing stage so that heat transferred to the freestanding membrane <NUM> during use (e.g. due to heating by absorption of lithography radiation) does not cause buckling or other deformation or breakage of the freestanding membrane <NUM>.

In an embodiment, the tension is controlled at the manufacturing stage so that expected heating of the freestanding membrane <NUM> during use leads to the tension in the freestanding membrane <NUM> reaching a desired range of values. For example, in the case where the heating in use raises the tension in the freestanding membrane <NUM>, the tension may be controlled at the manufacturing stage to be lower than the desired range of values by an amount which is such that the expected level of heating will cause the tension to rise to a value within the desired range of values.

In an embodiment, a method of manufacturing a pellicle <NUM> is provided which is particularly well adapted for providing a pellicle <NUM> with a control layer <NUM> for controlling a tension in the freestanding membrane <NUM>. <FIG> schematically depict stages in an example of such an embodiment.

In this embodiment a substrate <NUM> is provided which has a base layer <NUM> and a graphene-support layer <NUM>. The at least one graphene layer <NUM> is formed on the graphene-support layer <NUM>. <FIG> schematically depicts such an arrangement. The graphene-support layer <NUM> and base layer <NUM> may be formed according to any of the embodiments discussed above. The graphene-support layer <NUM> may comprise for example a metal layer or a metal silicide layer. The base layer <NUM> may comprise for example a silicon wafer. The at least one graphene layer <NUM> may be formed according to any of the embodiments discussed above. The at least one graphene layer <NUM> may be formed using CVD for example.

The method comprises removing a first portion <NUM> of the graphene-support layer <NUM> without removing a portion <NUM> of the at least one graphene layer <NUM> that was deposited on the first portion <NUM> of the graphene-support layer <NUM>. <FIG> schematically depict one way in which this may be achieved.

As shown in <FIG>, mask layers <NUM> are deposited on the front and back of the stack. The mask layers <NUM> are processed photolithographically so that the mask layers <NUM> cover selected regions on the front and back of the stack. In an embodiment, the selected region on the front of the stack contains the region in which the freestanding membrane <NUM> is to be formed, when viewed perpendicularly to the planar surface <NUM> of the substrate <NUM>. The selected region on the back of the stack is outside of the region in which the freestanding membrane <NUM> is to be formed, when viewed perpendicularly to the planar surface <NUM> of the substrate <NUM>.

In a subsequent step a region of the base layer <NUM> that is not protected by the mask layer <NUM> on the back of the stack is partially etched to produce the arrangement shown in <FIG>. In the case where the base layer <NUM> is formed from a silicon wafer a KOH etch may be used.

In a subsequent step a side etch (which may also be referred to as an undercut) is carried out to remove the first portion <NUM> of the graphene-support layer <NUM>. The first portion <NUM> to be removed is indicated in <FIG> by shading. The arrangement after removal is shown in <FIG>. After removal of the first portion <NUM> the previously overlying portion <NUM> of the at least one graphene layer <NUM> falls downwards and adheres to the previously underlying base layer <NUM>.

The method further comprises depositing a control layer <NUM> above the at least one graphene layer <NUM>. An example of the resulting arrangement is shown in <FIG>. The control layer <NUM> may be deposited for example using sputtering or evaporation (e.g. e-beam evaporation). In a subsequent step etching of the base layer <NUM> (e.g. using KOH etch) from the back of the stack is continued in order to penetrate through to the graphene-support layer <NUM>, thereby arriving at the arrangement shown in <FIG>.

The method further comprises removing a second portion of the graphene-support layer <NUM>. The removal of the second portion of the graphene-support layer <NUM> causes weakening or removal of adhesion between the at least one graphene layer <NUM> and layers which were positioned above the second portion of the graphene-support layer <NUM> (immediately prior to the removal of the second portion of the graphene-support layer <NUM>). <FIG> schematically depicts example processing. In this particular example the second portion of the graphene-support layer <NUM> consists of all of the remaining graphene-support layer <NUM>. Removal of the second portion of the graphene-support layer <NUM> therefore leads to complete removal of the graphene-support layer <NUM> in this embodiment.

The method further comprises lifting off the layers which were positioned above the second portion of the graphene-support layer <NUM>, thereby forming the freestanding membrane <NUM> as shown in <FIG>.

In an embodiment, the removing of either or both of the first portion <NUM> of the graphene-support layer <NUM> and the second portion of the graphene-support layer <NUM> is performed using side etching.

In an embodiment, a tension in the freestanding membrane <NUM> is controlled during manufacture of the pellicle by processing the substrate <NUM> on which the at least one graphene layer <NUM> is initially deposited. The processing of the substrate <NUM> may be performed prior to or after removal of the first substrate portion <NUM>. In an embodiment the processing of the substrate <NUM> comprises deforming the planar surface of the substrate <NUM> on which the at least one graphene layer <NUM> is initially formed. Example deformations are depicted schematically in <FIG>. In <FIG>, the substrate <NUM> has been processed to cause the substrate <NUM> to bow outwards on the side of the substrate <NUM> on which the at least one graphene layer <NUM> has been deposited. This applies a tensile force to the at least one graphene layer <NUM>. In <FIG>, the substrate <NUM> has been processed to cause the substrate <NUM> to bow inwards on the side of the substrate <NUM> on which the at least one graphene layer <NUM> has been deposited. This applies a compressive force to the at least one graphene layer <NUM>. The deformation of the substrate <NUM> may be performed in various ways. In one embodiment, the deformation is achieved by applying heating or cooling non-uniformly to the substrate <NUM>. The non-uniform heating or cooling causes a corresponding non-uniform thermal expansion or contraction, which can deform the substrate <NUM>.

When a pellicle comprising graphene is used in an EUV lithography apparatus, EUV photons, oxygen, hydrogen and/or water present near the pellicle can create defects in the graphene lattice. Defects may also be present due to intrinsic limitations in the processes used to deposit the graphene (e.g. CVD processes). Damage or intrinsic defects may reduce the mechanical robustness of the graphene and thereby increase the chance of pellicle failure. Defect free graphene is more robust against damage induced by EUV photons, oxygen, hydrogen and/or water. Undesirable etching away of carbon from the graphene will occur preferentially at defect sites. Reducing the number of defects will therefore reduce the extent and/or rate of undesirable etching. Reducing undesirable etching will help the pellicle maintain its transmissive properties and lateral imaging uniformity longer.

Amorphous carbon deposition is inherent to the use of EUV. This process is normally unwanted for pellicles because carbon reduces the pellicle transmission. However, for pellicles having a freestanding membrane comprising one or more layers of graphene, deposition of amorphous carbon on the pellicle surface can be used to repair inherently present defects or defects induced by EUV photons, oxygen, hydrogen and/or water. Conversion of amorphous carbon to graphene can be i) thermally activated, ii) catalytically activated, or iii) achieved by applying shear forces. Embodiments exploiting (i) and (ii) are described below.

In an embodiment, thermal activation is used. This approach may be particularly applicable for example where a freestanding membrane comprises at least one graphene layer and no capping layer. The temperature of a pellicle being used in a lithographic apparatus will depend on the particular operating parameters of the lithographic apparatus. Typically, it is expected that temperatures between about <NUM> and <NUM> will be reached in normal use for a <NUM> W source power. Such pellicle temperatures will only increase upon increasing source power if the thickness of the pellicle is not reduced concomitantly. For thermal activation of conversion of amorphous carbon to graphene pellicle temperatures of greater than <NUM> are preferred.

In an embodiment, a device manufacturing method is provided in which a pellicle <NUM> comprising at least one graphene layer <NUM> forming a freestanding membrane <NUM> is used to protect a patterning device MA. An electrical current is passed through the at least one graphene layer <NUM> to heat the at least one graphene layer <NUM>. The heating thermally activates conversion of amorphous carbon to single- or multilayer graphene, thereby effecting repair of defects or damage present in the at least one graphene layer <NUM>. The pellicle <NUM> is thereby at least partially repaired in-situ, improving average performance and longevity of the pellicle <NUM>.

In an embodiment, the at least one graphene layer <NUM> is heated to above <NUM>, optionally above <NUM>, optionally above <NUM>, optionally above <NUM>.

In an embodiment, a flow of material comprising a source of carbon may be provided onto the pellicle <NUM>. The flow may comprise one or both of the following: a flow of evaporated carbon (e.g. amorphous carbon); and a flow of a carbon based precursor gas. The carbon based precursor gas is a gas acting as a source of carbon (e.g. amorphous carbon). The carbon based precursor gas may comprise one or more of the following for example: methane (CH<NUM>) or acetylene (C<NUM>H<NUM>). Providing a flow of material comprising a source of carbon makes it possible to control the supply of carbon. Controlling the supply of carbon may be desirable for example to ensure uniformity of the repair process and/or to avoid excessive deposition of carbon (which may impair transmissivity through the pellicle). The flow of material may be applied during heating of the freestanding membrane. The provision of carbon is not limited to the above example methods. Carbon may be provided in any form.

In an embodiment, the pellicle <NUM> is provided with two or more conductive contact regions <NUM> positioned to allow an electrical current to be driven through the freestanding membrane <NUM> via the two or more conductive contact regions <NUM>. The two or more conductive contact regions <NUM> may be formed in direct contact with the at least one graphene layer <NUM>. An example of a pellicle <NUM> provided with conductive contact regions <NUM> is depicted in <FIG> in the case where the pellicle <NUM> is repaired offline. However, the pellicle <NUM> may also be configured in this manner for allowing heating to be applied while the pellicle <NUM> is in-situ within the lithography apparatus (e.g. protecting an optical element of the lithography apparatus, such as a patterning device). Fabrication of conductive (e.g. metallic) contact regions can easily be integrated into the fabrication process (e.g. where fabrication is CMOS/MEMS based).

In an embodiment, examples of which are shown in <FIG> and <FIG>, an apparatus <NUM> for processing (e.g. repairing) a pellicle <NUM> is provided. The apparatus <NUM> may be configured to operate offline or inline. When used offline, the apparatus <NUM> may be used to repair intrinsic defects in the at least one graphene layer before the pellicle <NUM> is first used in the lithographic apparatus. Alternatively or additionally, the apparatus <NUM> may be used to repair pellicles <NUM> after they have been damaged during use in the lithographic apparatus. The pellicle <NUM> comprises at least one graphene layer <NUM> forming a freestanding membrane <NUM>. The pellicle <NUM> may take any of the various forms disclosed elsewhere in this application, for example. The pellicle <NUM> may be obtainable or obtained by any of the methods disclosed in this application for example.

In an embodiment, as shown in <FIG>, the apparatus <NUM> comprises a current driving apparatus <NUM> for driving an electrical current through the freestanding membrane <NUM> to heat the freestanding membrane (and therefore also the at least one graphene layer <NUM> in the freestanding membrane <NUM>). The current driving apparatus <NUM> may comprise a power source of any type suitable for driving the required electrical current through the freestanding membrane <NUM>. The current driving apparatus <NUM> may comprise suitable leads and/or electrical connectors for connecting to the conductive contact regions <NUM>.

In an embodiment, the apparatus <NUM> comprises one or more supply ports <NUM>,<NUM> for applying a flow of material comprising a source of carbon (e.g. amorphous carbon) onto the pellicle <NUM>. The apparatus <NUM> may comprise suitable containers for storing the material comprising the source of carbon. Where the source of carbon comprises evaporated carbon, apparatus for evaporating carbon may be provided.

In an embodiment, the apparatus <NUM> further comprising an enclosure <NUM> for containing the pellicle <NUM> during repair of the pellicle <NUM>. The one or more supply ports <NUM>, <NUM> may, in such an embodiment, convey the flow of material comprising the source of carbon from the outside of the enclosure <NUM> to the inside of the enclosure <NUM>.

In an embodiment, as shown in <FIG>, the apparatus <NUM> is configured to use electrochemical deposition to apply carbon to the at least one graphene layer <NUM>. This may be achieved using an electrochemical cell. In the electrochemical cell the freestanding membrane <NUM> is immersed in a bath <NUM> containing a solution <NUM> of an electrolyte and/or carbon precursor. The freestanding membrane <NUM> acts as a working electrode. When an electrochemical potential is applied, a redox reaction on the surface of the freestanding membrane <NUM> will take place. The organic precursor forms carbon (by reduction or oxidation) and the carbon deposits on the surface of the freestanding membrane <NUM>. The freestanding membrane <NUM> is thus processed (e.g. repaired) as desired.

Many suitable configurations of electrochemical cell are available. In the example shown in <FIG>, the apparatus <NUM> comprises an electrochemical cell having three-electrodes: the freestanding membrane <NUM> (as a working electrode), a counter-electrode <NUM>, and a reference electrode <NUM>. The principle of operation of three-electrode electrochemical cells are well known in the art. Other types of electrochemical cell (e.g. two-electrode or four-electrode) may also be used.

Details for performing electrochemical carbon deposition in a general context may be found in the literature. These techniques may be used to process a pellicle according to the present method. Examples are given below.

In <NPL>et al. disclose use of various organic solvents as carbon precursor and investigated the influence of the carbon precursor (DMF, CH<NUM>CN etc.) on the sp<NUM>/sp<NUM> carbon ratio in the films obtained by electrodeposition on indium tin oxide.

In <NPL>et al. disclose the electrochemical growth of diamond-like carbon (DLC) coatings on substrates.

In many of the described techniques for electrochemical carbon deposition relatively large positive potentials are used (for example, in the region of 1000V), but there are also described techniques in which deposition is performed at lower voltages and at room temperature. In <NPL>et al. show that the current density at the defect sites and grain boundaries is higher thus enabling selective electrochemical deposition on these sites.

In <NPL>et al. showed that carbon nanotubes can be decorated with polymers obtained electrochemically. In <NPL>et al. disclose that graphene can be obtained electrochemically from graphene oxide.

In <CIT> an alternative method is described. Graphite is suspended in a solvent and doped with a Lewis or Brönsted acid to make the graphite sheets positively charged. A negative potential is then applied on a substrate so that the doped graphite migrates to the surface of the substrate forming graphene. The method is suitable for many kinds of substrates. In an embodiment the freestanding membrane <NUM> is used as the substrate. The method is thus used to deposit carbon on the at least one graphene layer <NUM> of the freestanding membrane <NUM>.

In <NPL>et al. disclose that doping of graphene with atoms with a lower electronegativity than carbon such as boron (B) can provide a distribution of positive charge on the graphene surface. In an embodiment, such doping is applied to graphene and a negative potential is applied to a freestanding membrane <NUM> to cause graphite from the graphene to migrate to the surface of the freestanding membrane <NUM>.

In an embodiment, a catalytically active metal which promotes the conversion of carbon to single- or multilayer graphene is provided within or in contact with the at least one graphene layer <NUM> of the pellicle <NUM>. The catalytic metal can be provided in any form, including for example one or more of the following: atoms, molecules, nanoparticles, vapor, or thin film. The catalytic metal can be provided at any stage of the manufacturing process. When present during use of the pellicle <NUM> or during processing to repair the pellicle <NUM> after use, the catalytically active metal may enable desirable conversion of carbon to single- or multilayer graphene to occur efficiently at lower temperatures than would be possible without the catalytically active metal. In an embodiment, the catalytically active metal is provided before or during the deposition of the at least one graphene layer <NUM>. The catalytically active metal may in this case improve the quality of the at least one graphene layer <NUM>. The catalytically active metal may reduce the number of defects present in the at least one graphene layer <NUM>. The catalytically active metal may be provided as a vapor. In this case, the requirement to etch away a metal film provided instead of the metal vapor, during manufacture of the pellicle <NUM>, may desirably be avoided. Alternatively or additionally, the use of a metal vapor instead of a metal film may improve the quality of the at least one graphene layer <NUM> by allowing improved optimization of grain sizes and/or surface morphology because graphene does not generally adhere conformally to a metal surface. In an embodiment the at least one graphene layer <NUM> is grown on a dielectric surface while catalytic activation is provided by a metal vapor. Optionally, the dielectric substrate is seeded prior to growth of the at least one graphene layer <NUM>. The seeding may be performed, for example, by deposition of a small exfoliated graphene flake on the dielectric.

In an embodiment, the catalytically active metal comprises a transition metal. In an embodiment, the catalytically active metal comprises one or more of Fe, Co, Ni, and Cu, but other materials could be used.

In an embodiment, an example of which is depicted schematically in <FIG>, the catalytically active metal is provided via formation of a layer <NUM>,<NUM> of the catalytically active metal on one or both sides of the at least one graphene layer <NUM>. In the particular example shown, the layer <NUM>,<NUM> is provided on both sides but this is not essential. The layer may be provided on the top side only or on the bottom side only.

Alternatively or additionally, in an embodiment, an example of which is depicted schematically in <FIG>, the catalytically active metal is provided via formation of a layer <NUM> of the catalytically active metal within the at least one graphene layer <NUM>.

Alternatively or additionally, in an embodiment, an example of which is depicted schematically in <FIG>, the catalytically active metal is provided via inclusion of nanoparticles <NUM> of the catalytically active metal within the at least one graphene layer <NUM>.

Alternatively or additionally, in an embodiment, the catalytically active metal is provided via doping of the graphene in the at least one graphene layer by atoms of the catalytically active metal.

In an embodiment, the catalytically active metal is provided by performing the depositing (e.g. by CVD) of the at least one graphene layer <NUM> in the presence of a vapor of the catalytically active metal.

In an embodiment, an example of which is depicted schematically in <FIG>, the freestanding membrane <NUM> is formed with a capping layer <NUM>,<NUM> on either or both sides of the at least one graphene layer <NUM>. In the particular example shown in <FIG>, the capping layer <NUM>,<NUM> is provided on both sides of the at least one graphene layer <NUM>. The capping layer <NUM>,<NUM> protects the at least one graphene layer <NUM> from chemical attack by radical species such as hydrogen, oxygen and hydroxyl radical species. Such radical species are likely to be present during scanning conditions and may cause degradation of the freestanding membrane <NUM> in the absence of the capping layer <NUM>,<NUM>. The inventors have performed experiments demonstrating for example the effects of exposure of graphite to a flux of hydrogen (H*) radicals. After <NUM> hours exposure in a hydrogen radical generator the number of holes seen in secondary electron images (SEM) was significantly greater than prior to the exposure.

In an embodiment the capping layer <NUM>,<NUM> comprises a metal or a metal oxide. Capping layers <NUM>,<NUM> formed from metal or metal oxide have been found to be particularly effective at protecting graphene. In an embodiment, the capping layer comprises one or more material selected from the following group: Ru, Mo, B, MoSi<NUM>, h-BN (hexagonal boron nitride), HfO<NUM>, ZrO<NUM>, Y<NUM>O<NUM>, Nb<NUM>O<NUM>, La<NUM>O<NUM>, and Al<NUM>O<NUM>. The metals Ru and Mo, the compounds MoSi<NUM> and h-BN, and the metal oxides HfO<NUM>, ZrO<NUM>, Y<NUM>O<NUM>, Nb<NUM>O<NUM>, La<NUM>O<NUM>, and Al<NUM>O<NUM> have been found to be particularly effective as capping layers <NUM>,<NUM>. Other high-k dielectric materials could also be used.

The capping layers <NUM>,<NUM> can be deposited using a variety of techniques, including for example physical vapor deposition (PVD), chemical vapor deposition (CVD), evaporation, or atomic layer deposition (ALD). The capping layers <NUM>,<NUM> should be relatively thin (of nanometer order) in order to minimize EUV transmission losses. The inventors have found that ALD is particularly effective for producing layers which are very thin and yet still fully closed.

In an embodiment, an example of which is shown in <FIG>, adhesion between the capping layer <NUM>,<NUM> and the at least one graphene layer <NUM> is improved by providing an adhesion layer <NUM>,<NUM> between the capping layer <NUM>,<NUM> and the at least one graphene layer <NUM>. In the absence of any adhesion layer, adhesion between graphene and materials coated on the graphene can be poor. It is possible to improve adhesion by creating hydrophilic -OH groups on the surface. Hydrophilic -OH groups on the surface allow good adhesion of oxides for example. It has been found however that creating hydrophilic -OH groups on the surface can compromise the electronic stability of graphene by disrupting the sp<NUM> bonded network. Compromising the electronic stability can cause atomic sites to be created which act is starting points for further defect generation.

In an embodiment the adhesion layer <NUM>,<NUM> is configured to reduce or avoid compromising of the electronic stability of the graphene. In an embodiment, the adhesion layer <NUM>,<NUM> comprises a material having sp<NUM>-bonded carbon and hydrophilic groups. The presence of the sp<NUM>-bonded carbon reduces or avoids compromising of the electronic stability of the graphene. The presence of the hydrophilic groups promotes good adhesion. In an embodiment the adhesion layer <NUM>,<NUM> comprises amorphous carbon (a-C). In an embodiment the amorphous carbon is partly oxidized. Partly oxidized amorphous carbon is expected to possess both sp<NUM>-bonded carbon and hydrophilic groups such as Cn-OH or Cn-COOH.

In embodiments described above a graphene-support layer <NUM> is provided. The graphene-support layer comprises one or more of the following: transition metals such as Mo, Ni, Ru, Pt, Cu, Ti, V, Zr, Nb, Hf, Ta, W, or their silicides, such as silicized Mo, silicized Ni, silicized Ru, silicized Pt, silicized Cu, silicized Ti, silicized V, silicized Zr, silicized Nb, silicized Hf, silicized Ta, silicized W. Due to the risk of contamination of processing apparatus, it is undesirable for some of these materials to be present when certain high temperature processing steps are being carried out. For example, it is undesirable for Mo or silicized Mo to be present during a low pressure chemical vapor deposition (LPCVD) process, which may typically be performed at around <NUM> degrees C. It has been found that such an LPCVD process can be used to form particularly effective encapsulation or sacrificial layers <NUM> of SixNy. PECVD may also be used to form an encapsulation layer or sacrificial layer <NUM> of SixNy but it has been found that pinholes in the encapsulation layer or sacrificial layer <NUM> can allow etchants (e.g. KOH) in subsequent wet etching steps to pass through the encapsulation layer or sacrificial layer <NUM>. It has further been found that adhesion to the at least one graphene layer <NUM> is poor, which limits the extent to which processing steps can be carried out after the at least one graphene layer <NUM> has been formed. It has further been found that stresses in the graphene-support layer <NUM>, particularly when comprising Mo or silicized Mo, can be changed by processing at high temperatures. Control of stresses in the graphene-support layer <NUM> is therefore made more complex by each high temperature processing step that is carried out while the graphene-support layer <NUM> is present. Process flows in which the above problems are avoided or reduced are described below with reference to <FIG>. In each process flow, a stack is provided in which a graphene-support layer <NUM> is formed after an encapsulation layer or sacrificial layer <NUM> is formed. Three alternative process flows are depicted respectively in <FIG>, <FIG> and <FIG>. Each process flow starts from a silicon wafer (with its native oxide) and produces a multi-layer structure in which a patterned encapsulation layer or sacrificial layer <NUM> is formed using LPCVD before a graphene-support layer <NUM> is formed and before the at least one graphene layer <NUM> is formed.

In the process flow of <FIG>, a base layer <NUM> comprising a silicon wafer (<FIG>) is processed to form a silicon oxide layer <NUM> (SiO<NUM>) on an outer surface of the silicon wafer (<FIG>). The silicon oxide layer <NUM> may be formed using thermal oxidation at about <NUM> degrees C, for example. In a subsequent step a lower side of the stack is etched to remove the silicon oxide layer <NUM> on the lower side of the base layer <NUM> (<FIG>). In a subsequent step an encapsulation layer or sacrificial layer <NUM> is applied to produce the arrangement shown in <FIG>. The encapsulation layer or sacrificial layer <NUM> in this embodiment comprises an LPCVD SixNy layer (deposited at around <NUM> degrees C for example). In a subsequent step the arrangement of <FIG> is photolithographically patterned and then processed to form windows <NUM> and <NUM> in the encapsulation layer or sacrificial layer <NUM> (e.g. by RIE), as shown in <FIG>. In a subsequent step a graphene-support layer <NUM> is formed that fills window <NUM>. The graphene-support layer <NUM> may take any of the forms described above (e.g. comprising a metal or metal silicide, for example Mo or silicized Mo). In an embodiment the graphene-support layer <NUM> comprises Mo deposited using CVD at <NUM> degrees C (or at a higher temperature, for example any temperature between room temperature and about <NUM> degrees C).

In the process flow of <FIG>, a base layer <NUM> comprising a silicon wafer (<FIG>) is processed to apply an encapsulation layer or sacrificial layer <NUM> around the base layer <NUM> (<FIG>). The encapsulation layer or sacrificial layer <NUM> in this embodiment comprises an LPCVD SixNy layer (deposited at around <NUM> degrees C for example). In a subsequent step the arrangement of <FIG> is photolithographically patterned and then processed to form a window <NUM> in the encapsulation layer or sacrificial layer <NUM> (e.g. by RIE), as shown in <FIG>. In a subsequent step a graphene-support layer <NUM> is formed on an upper side of the stack to provide the arrangement shown in <FIG>. The graphene-support layer <NUM> may take any of the forms described above (e.g. comprising a metal or metal silicide, for example Mo or silicized Mo). In an embodiment the graphene-support layer <NUM> comprises Mo deposited using CVD at <NUM> degrees C (or at a higher temperature, for example any temperature between room temperature and about <NUM> degrees C).

In the process flow of <FIG>, a base layer <NUM> comprising a silicon wafer (<FIG>) is processed to apply an encapsulation layer or sacrificial layer <NUM> around the base layer <NUM> (<FIG>). The encapsulation layer or sacrificial layer <NUM> in this embodiment comprises an LPCVD SixNy layer (deposited at around <NUM> degrees C for example). In a subsequent step the arrangement of <FIG> is photolithographically patterned and then processed to form a window <NUM> in the encapsulation layer or sacrificial layer <NUM> (e.g. by RIE). A TEOS layer <NUM> is then formed on an upper side of the stack, using PECVD or LPCVD at <NUM> degrees C for example, to provide the arrangement shown in <FIG>. In a subsequent step a graphene-support layer <NUM> is formed on an upper side of the stack to provide the arrangement shown in <FIG>. The graphene-support layer <NUM> may take any of the forms described above (e.g. comprising a metal or metal silicide, for example Mo or silicized Mo). In an embodiment the graphene-support layer <NUM> comprises Mo deposited using CVD at <NUM> degrees C (or at a higher temperature, for example any temperature between room emperature and about <NUM> degrees C).

<FIG> depict an example process flow to be performed subsequently to a process flow (such as the process flow of <FIG>, the process flow of <FIG>, or the process flow of <FIG>) that has provided the graphene-support layer <NUM> after forming an encapsulation layer or sacrificial layer <NUM> at an earlier stage using a high temperature LPCVD process (e.g. to form an LPCVD SixNy layer at around <NUM> degrees C).

In the particular example of <FIG> the process flow starts from the arrangement of <FIG>. The arrangement of <FIG> is processed to form the at least one graphene layer <NUM> on an upper surface of the graphene-support layer <NUM> (<FIG>). The at least one graphene layer <NUM> may take any of the forms described above (e.g. formed using CVD at a temperature of <NUM>-<NUM> degrees C). In a subsequent step, a further graphene-support layer <NUM> is formed on the at least one graphene layer <NUM> to produce the arrangement shown in <FIG>. In an embodiment the further graphene-support layer <NUM> has the same composition as the graphene-support layer <NUM> and is formed using the same methods. In an embodiment, both the graphene-support layer <NUM> and the further graphene-support layer <NUM> comprise Mo deposited using CVD at <NUM> degrees C (or at a higher temperature, for example any temperature between room temperature and about <NUM> degrees C). In a subsequent step the stack is encapsulated by a further encapsulation layer or sacrificial layer <NUM> to produce the arrangement shown in <FIG>. In an embodiment the further encapsulation layer or sacrificial layer <NUM> comprises Parylene deposited using CVD.

In a subsequent step, a KOH etch (or partial etching by deep RIE followed by a KOH etch) is used to remove a portion of the silicon forming the base layer <NUM> within window <NUM>, thereby producing the arrangement of <FIG>. In a subsequent step, the further encapsulation layer or sacrificial layer <NUM> is removed (e.g. in barrel etcher by O<NUM> microwave plasma) to produce the arrangement of <FIG>. In a subsequent step an oxide etch (e.g. BHF) is applied to the lower side of the stack to remove an exposed portion of the silicon oxide layer <NUM> underneath the graphene-support layer <NUM> (<FIG>). Finally, the graphene-support layer <NUM> and the further graphene-support layer <NUM> are removed (e.g. using a wet H<NUM>O<NUM> etch or a vapor etching process), to leave a freestanding membrane <NUM>, as shown in <FIG>.

In the embodiments described above with reference to <FIG>, a stress in the encapsulation layer or sacrificial layer <NUM>, in the graphene-support layer <NUM> and/or in the further graphene-support layer <NUM>, may be tuned during deposition, from tensile to compressive or from compressive to tensile, in order to control a stress in the freestanding membrane <NUM>.

In embodiments where the at least one graphene layer <NUM> is formed on a graphene-support layer <NUM>, the removing of the first substrate portion <NUM> to form the freestanding membrane <NUM> will comprise removing a portion of the graphene-support layer <NUM> underneath a portion of the at least one graphene layer <NUM> that is to form the freestanding membrane <NUM>. Removal of such a portion of the graphene-support layer <NUM> is described above with reference to the transitions between <FIG>, between <FIG>, between <FIG>, between <FIG>, and between <FIG>. It is possible to remove the portion of the graphene-support layer <NUM> using a wet etch. For example, Mo can be removed using a wet etch comprising hydrogen peroxide in water. The inventors have found, however, that wet etching can cause damage to the freestanding membrane <NUM> and reduce yield. The inventors have found that yield can be increased by using a vapor etching process instead of a wet etching process. The improvement in yield is believed to be due to a reduction or removal of capillary force effects, concentration gradient effects and Brownian motion effects, relative to wet etching. Reduction or removal of capillary force effects, concentration gradient effects and Brownian motion effects also facilitates upscaling of the method of manufacturing a pellicle to larger size pellicles.

Example apparatus <NUM> for removing a portion of a graphene-support layer <NUM> using a vapor etching process is depicted in <FIG>. In this example, a reservoir <NUM> containing a liquid to be vaporized (e.g. water) is heated to produce vapor (e.g. steam). A stack (such as is depicted in any of <FIG>, <FIG>, <FIG> and <FIG>) is positioned so that a portion of a graphene-support layer <NUM> is exposed to the vapor. The vapor is chosen so that the exposed portion of the graphene-support layer <NUM> is removed by vapor etching. The inventors have found this approach to be particularly applicable where the graphene-support layer <NUM> comprises Mo or silicized Mo (e.g. MoSi<NUM>) and the vapor comprises steam.

The above-described methods of manufacture, and other methods of manufacture, provide a pellicle <NUM> comprising a membrane bonded to a membrane support. In the context of the methods discussed above the membrane support is referred to as a second substrate portion <NUM>. In those methods the second substrate portion <NUM> is manufactured by removing a first substrate portion <NUM> from a substrate. However, it is not essential that the membrane support be formed in this way.

The membrane comprises a graphene layer (e.g. at least one graphene layer <NUM>, as described above). The membrane is bonding to and created on the membrane support with a thin film deposition process. The thin film deposition process may comprise chemical vapor deposition or another thin film deposition process. The bond between the membrane and the membrane support is an intrinsic bond between the membrane and the membrane support induced by the thin film deposition process of the membrane layer onto the membrane support. The bond may be an intrinsic bond between the graphene layer and the membrane support induced by a thin film deposition process of the graphene layer onto the membrane support. The intrinsic bonding has a bonding strength such that the membrane remains bonded to the membrane support under a gravity force acting on a membrane, optionally for all orientations of the pellicle relative to the direction of gravity.

In an alternative embodiment, as depicted schematically in <FIG> and not representative of the invention, a pellicle for a lithographic apparatus is manufactured by transferring at least one graphene layer <NUM> from a surface of a liquid <NUM> onto a frame <NUM>. The frame <NUM> comprises an opening <NUM> and a border region <NUM> surrounding the opening <NUM>. In <FIG>, the frame <NUM> is shown from the side so the opening <NUM> is not directly visible. A boundary of the opening <NUM> is depicted by broken lines. The opening <NUM> comprises a hole penetrating through from the right of the frame <NUM> to the left of the frame <NUM> in the orientation shown in <FIG>. After transfer to the frame <NUM>, the at least one graphene layer <NUM> spans the opening <NUM>, thereby forming a freestanding membrane <NUM>. In the embodiment shown in <FIG> the frame <NUM> is dipped into the liquid <NUM> in a direction perpendicular to the surface of the liquid <NUM> (i.e. vertically in the orientation shown) and then removed. Capillary and adhesion forces drag the at least one graphene layer <NUM> onto the frame <NUM>. Other arrangements are however possible. It is challenging to produce large freestanding membranes <NUM> with high yield. Due to the high aspect ratio of the freestanding membrane, surface tension and capillary force effects can cause tearing or rupturing of the freestanding membrane <NUM>. It is also difficult to ensure reliable adhesion between the at least one graphene layer <NUM> and the frame <NUM>.

In an embodiment not representative of the invention, the liquid <NUM> has a composition which reduces surface tension or capillary effects and thereby reduces the risk of tearing or rupture. In an embodiment, the liquid <NUM> comprises a mixture of water, an alcohol (e.g. ethanol at a concentration of less than <NUM>%), and a further solvent that is not alcohol (e.g. a ketone such as acetone, or acetonitrile). Preferably, the further solvent is selected such as to reduce the likelihood of, or prevent, the formation of a droplet of the liquid that completely spans the opening <NUM> in the frame <NUM>, after transfer of the at least one graphene layer <NUM> to the frame <NUM> (relative to case where the liquid comprises water and alcohol, e.g. ethanol at a concentration of less than <NUM>%, only). When the further solvent is not present and a droplet of the liquid that completely spans the opening <NUM> forms, breaking up of such a droplet can cause failure of the freestanding membrane <NUM> due to surface tension or capillary forces applied to the freestanding member <NUM>. In an embodiment the further solvent is fully miscible with water and/or a mixture of water and alcohol (e.g. ethanol at a concentration of less than <NUM>%). The further solvent is thereby able to form and disrupt hydrogen bonds in solution significantly. In an embodiment, the further solvent has a boiling point which is at least <NUM> degrees C (optionally at least <NUM> degrees C, optionally at least <NUM> degrees C) less than the boiling point of the liquid <NUM> without the further solvent. For example, in the case where the further solvent is acetone the boiling point will be about <NUM> degrees C, whereas the boiling point of a mixture of water and ethanol is typically in the range of <NUM>-<NUM> degrees C. Arranging for the boiling point of the further solvent to be significantly different in this manner promotes the formation of smaller droplets on the freestanding membrane <NUM>. Formation of smaller droplets will cause surface tension effects to be more local and therefore less likely to cause failure of the freestanding membrane <NUM>. A similar effect can be achieved for further solvent compositions which have a more similar boiling point to the water/ethanol mixture (e.g. a higher boiling point than acetone) but have a significantly different vapor pressure (e.g. lower than acetone). Acetonitrile is an example of such a further solvent composition.

In an embodiment not representative of the invention, adhesion of the at least one graphene layer <NUM> to the frame <NUM> is improved by configuring the frame <NUM> such that at least a portion of the frame <NUM> in contact with the at least one graphene layer <NUM> (the border region <NUM> in the example of <FIG>) is hydrophobic. In an embodiment, the hydrophobic portion is provided by forming at least a portion of the frame <NUM> from Si that has been treated to form Si-H at the surface (e.g. by immersion of the Si in an HF solution, for example a <NUM>% HF solution).

In an embodiment not representative of the invention, the transfer of the at least one graphene layer <NUM> to the frame <NUM> is performed while the liquid has a temperature in the range of <NUM>-<NUM> degrees Celsius, preferably <NUM>-<NUM> degrees Celsius, more preferably <NUM>-<NUM> degrees Celsius, more preferably <NUM>-<NUM> degrees Celsius, particularly <NUM>-<NUM> degrees Celsius or substantially at <NUM> degrees Celsius. It has been found that this modifies surface tension in such a way as to reduce the risk of surface tension or capillary effects causing failure of the freestanding membrane <NUM>.

In accordance with the invention, the method of manufacturing a pellicle is adapted so that the freestanding membrane <NUM> comprises a sequence of layers having different chemical compositions, wherein the sequence comprises the at least one graphene layer <NUM> and at least one layer of a two-dimensional material other than graphene <NUM>. Example arrangements are depicted in <FIG>.

A broad class of two-dimensional materials are available. When provided as a single layer, two-dimensional materials are sometimes referred to as 2D topological materials or single layer materials, and comprise a single layer of atoms. Layered combinations of different 2D materials are sometimes referred to as van der Waals heterostructures. Examples of 2D materials include graphene, graphyne, borophene, silicene, stanene, phosphorene, molybdenite, graphane, h-BN (hexagonal boron nitride), germanane, MXenes and transition metal dichalcogenides, including for example MoS<NUM>, MoSe<NUM> and WSe<NUM>. MXenes are layered transition metal carbides and carbonitrides with general formula of Mn+<NUM>XnTx, where M stands for early transition metal, X stands for carbon and/or nitrogen and Tx stands for surface terminations (mostly =O, -OH or -F).

The incorporation into the freestanding membrane <NUM> of one or more layers of a two-dimensional material other than graphene can provide various benefits.

Firstly, the one or more layers of a two-dimensional material other than graphene can be used to control (e.g. reduce) etching by radicals (such as H* and OH*) during use of the pellicle in lithography. The control of etching improves pellicle reliability and performance.

Secondly, the one or more layers of two-dimensional material other than graphene can provide additional mechanical strength to the freestanding membrane <NUM>. The additional mechanical strength improves pellicle robustness and lifetime. Phosphorene, an analog of graphene where every C atom is a P atom, can sustain tensile strain up to <NUM>% and is chemically inert. Phosphorene is particularly well suited to being incorporated into the freestanding membrane <NUM> to provide additional mechanical strength to the freestanding membrane <NUM>.

Thirdly, the one or more layers of a two-dimensional material other than graphene can improve the thermal properties of the freestanding membrane <NUM>. The improvement may comprise reducing a heat load on the freestanding membrane <NUM> during use, for example by improving DUV emission characteristics. h-BN is particularly well suited to this application. h-BN has a bandgap of about <NUM> eV, which allows DUV emission. h-BN is also chemically inert and thermally stable up to <NUM>. Furthermore, there is a good atomic lattice match between h-BN and other two-dimensional materials (including graphene), which favors epitaxial growth of stacks including graphene starting from a two-dimensional material such as h-BN.

<FIG> illustrate three different modes of incorporation of at least one layer of a two-dimensional material other than graphene <NUM> into the freestanding membrane <NUM>.

<FIG> depicts an arrangement in which a sequence of layers is provided that comprises an alternating sequence of at least one graphene layer <NUM> alternating with at least one layer of a two-dimensional material other than graphene <NUM>. Thus one or more layers of graphene <NUM> are followed by one or more layers of a different two-dimensional material <NUM> which are in turn followed by one or more layers of graphene <NUM>, etc. Arrangements of this type may protect the at least one graphene layer <NUM> from chemical attack from radicals, provide additional mechanical strength to the freestanding membrane <NUM> and/or improve the thermal properties of the freestanding membrane <NUM>.

<FIG> depicts an arrangement in which layers of two-dimensional material other than graphene are provided as capping layers on the outside of the freestanding membrane <NUM>. Arrangements of this type are particularly well suited to protecting the at least one graphene layer <NUM> from chemical attack from radicals.

<FIG> depicts an arrangement in which at least one layer of a two-dimensional material other than graphene <NUM> is sandwiched between at least one layer of graphene <NUM> on one side and at least one layer of graphene <NUM> on the other side. Arrangements of this type are particularly well suited to providing additional mechanical strength and/or controlling etching of the freestanding membrane <NUM> during use.

Each of the layers in the arrangements of <FIG> (and in other arrangements comprising graphene layers and layers of two-dimensional materials other than graphene) can be formed in a variety of different ways, including CVD, ALD, PVD or any other deposition technique suitable for the selected material.

In embodiments not according to the invention, the at least one graphene layer <NUM> may be replaced with at least one layer of a two-dimensional material other than graphene. A pellicle may thereby be provided for example that comprises a freestanding membrane <NUM> comprising at least one layer of a two-dimensional material other than graphene, and optionally containing no graphene. The at least one layer of a two-dimensional material other than graphene comprises at least one layer of one or more of the following: graphyne, borophene, silicene, stanene, phosphorene, molybdenite, graphane, h-BN, germanane, an MXene, a transition metal dichalcogenide, MoS<NUM>, MoSe<NUM>, WSe<NUM>.

In embodiments not according to the invention, the pellicle may be attached to a frame arranged to provide additional support to the free-standing membrane. The pellicle attached to the frame form a pellicle assembly. The pellicle assembly may be permanently or releasably attached to a patterning device, such as a lithographic mask, forming thereby a mask assembly.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, LCDs, thin-film magnetic heads, etc.. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed more than once, for example in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the various lacquer layers may be replaced by non-lacquer layers that perform the same function.

Claim 1:
A method of manufacturing a pellicle (<NUM>) for a lithographic apparatus (<NUM>), comprising:
depositing at least one graphene layer (<NUM>) on a planar surface of a substrate (<NUM>), wherein the substrate (<NUM>) comprises a base layer (<NUM>) and a graphene-support layer (<NUM>) , the at least one graphene layer (<NUM>) being deposited on the graphene-support layer (<NUM>); and wherein the substrate (<NUM>) comprises a first substrate portion (<NUM>) and a second substrate portion (<NUM>); and
removing the first substrate portion (<NUM>) to form a freestanding membrane (<NUM>) from the at least
one graphene layer (<NUM>), the freestanding membrane (<NUM>) being supported by the second substrate portion (<NUM>); and
wherein the freestanding membrane (<NUM>) comprises a sequence of layers having different chemical compositions, wherein the sequence of layers comprises the at least one graphene layer (<NUM>) and at least one layer (<NUM>) of a two-dimensional material other than graphene.