Pellicle membrane

Abstract

A pellicle membrane for an EUV lithographic apparatus, the pellicle membrane comprising a first membrane layer forming a first external surface, a second membrane layer forming a second external surface substantially parallel to the first external surface, a dielectric layer positioned between the first and second external surfaces and wherein the first and second external surfaces are reflective to infra-red radiation, the dielectric layer has a dielectric layer thickness, the membrane has a membrane thickness measured between the first external surface and second external surface, and the dielectric layer thickness and membrane thickness are selected such that the emissivity of the pellicle membrane is greater than 0.5.

Description

PELLICLE MEMBRANECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23209991.1 which was filed on 15 November 2023, and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a pellicle membrane. The present invention has particular, but not exclusive, use in connection with EUV lithographic apparatuses and EUV lithographic tools. The present invention also relates to pellicle membrane assemblies and methods of manufacturing pellicle membranes and pellicle membrane apparatuses.BACKGROUND

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern at a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0004] To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which can be formed on the substrate. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within the range 4-20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] A pattern may be imparted to a radiation beam in a lithographic apparatus using a patterning device (e.g. a mask or reticle). Radiation is provided through or reflected off the patterning device to form an image on a substrate. Contamination on the surface of the patterning device can cause manufacturing defects on the substrate. A membrane assembly, also referred to as a pellicle, may be provided to protect the patterning device from airborne particles and other forms of contamination.

[0006] Pellicles may also be provided for protecting optical components other than patterning devices. Pellicles may also be used to provide a passage for lithographic radiation between regions of the lithography apparatus which are sealed from one another. Pellicles may also be used as filters, such as spectral purity filters or as part of a dynamic gas lock of a lithographic apparatus.

[0007] The use of pellicles in lithography is well-known and well-established. A pellicle in a lithographic apparatus is a membrane (also referred to as a pellicle membrane) which is located away from the patterning device and is out of the focal plane of a lithographic apparatus in use. As the pellicle is out of the focal plane of the lithographic apparatus, contamination particles which land on the pellicle are out of focus in the lithographic apparatus. Consequently, images of the contamination particles arenot projected onto the substrate. If the pellicle were not present, then a contamination particle which landed on the patterning device would be projected onto the substrate and would introduce a defect into the projected pattern.

[0008] 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 or membrane assembly. The pellicle may be attached to the frame, for example, by gluing or otherwise attaching a pellicle border region to the frame. The frame may be permanently or releasably attached to a patterning device.

[0009] It is beneficial for a pellicle membrane to be highly transmissive, for example having a single -pass EUV transmission (EUVT) of 90% or higher. It is also beneficial for a pellicle to be highly thermally emissive, for example having an emissivity close to or equal to 1 in the infrared range. It is beneficial for a pellicle membrane to have a high emissivity because a pellicle membrane with a higher emissivity can typically be used at a higher operating temperature without risking damage to or instability of the membrane, compared to a pellicle membrane with a lower emissivity. It is beneficial for a pellicle membrane to have mechanical stability so as to avoid damage to the pellicle during manufacture, storage, transportation and / or use. It is typically difficult to achieve optimal tranmissivity, emissivity and mechanical stability due to competing requirements and physical limitations of suitable materials.

[0010] It is desirable to provide a pellicle membrane and / or a pellicle assembly which has an improved transmissivity, emissivity and / or mechanical stability, or which addresses a problem associated with the prior art.SUMMARY

[0011] According to a first aspect there is provided a pellicle membrane for an EUV lithographic apparatus, wherein the pellicle membrane comprises a first membrane layer forming a first external surface, a second membrane layer forming a second external surface substantially parallel to the first external surface and a dielectric layer positioned between the first and second external surfaces. The first and second external surfaces are reflective to infra-red radiation. The dielectric layer has a dielectric layer thickness. The membrane has a membrane thickness measured between the first external surface and second external surface. The dielectric layer thickness and membrane thickness are selected such that the emissivity of the pellicle membrane is greater than 0.5.

[0012] The first and second external surfaces may be substantially parallel to eachother. The dielectric layer may be substantially parallel to the first and second external surfaces. The dielectric layer may be arranged such that it is not immediately adjacent to either the first or second external surfaces (that is, the dielectric layer does not form part of an external surface of the pellicle membrane). The dielectric layer may be encapsulated within the volume contained between the first and second external surfaces. The membrane thickness may be measured generally perpendicular to the surface ofthe first and second external surfaces. The pellicle membrane may comprise more than one dielectric layer and more than one first membrane layer and second membrane layer.

[0013] The emissivity may be an emissivity in the infra-red range. Emissivity in the infra-red range may be referred to as a thermal emissivity. The first and second membrane layers may be highly reflective (e.g. >50%, >80%, >90%, >95%, >99%) in the infra-red range.

[0014] Thin layers of some materials, for example materials with a high reflectivity in the infrared range such as metal or metallic materials, have a theoretical emissivity limit of 0.5. Dielectric materials may have a higher emissivity, up to 1, when the dielectric material is relatively thick (e.g. 1mm). By combining reflective membrane layers with a dielectric layer therebetween, an emissivity greater than 0.5 can be achieved for the membrane as a whole. That is, the emissivity of the pellicle membrane is greater than the theoretical emissivity limit of the membrane layers alone. This is because an enhanced electric field is generated in the interior volume of the membrane.

[0015] A high emissivity (e.g. greater than 0.5) is generally desirable for membranes EUV lithographic apparatuses because high emissivity membranes are more durable, experience less dewetting, and have a longer lifetime.

[0016] The pellicle membrane of the first aspect may be referred to as an emissive pellicle membrane or highly emissive pellicle membrane.

[0017] The dielectric layer thickness may be less than 10 nm, and optionally on the order of or less than 1 nm.

[0018] Thick dielectric materials may be unsuitable for pellicle membranes in an EUV lithographic apparatus because they have very low EUV transmission. Providing a thin dielectric layer results in a pellicle membrane with adequate EUV transmission for an EUV lithographic apparatus and an emissivity greater than 0.5. The dielectric layer thickness may additionally or alternatively be selected to be substantially smaller (e.g. at least an order of magnitude smaller) than the membrane thickness.

[0019] The membrane thickness may be less than 100 nm. Optionally, the membrane thickness may be on the order of or less than 10 nm.

[0020] Some materials (for example but not limited to metals and metallic materials) exhibit particularly high reflectivity in the infra-red range at thicknesses below 100 nm. By having a membrane thickness less than lOOnm a high infra-red reflectivity may be realized at the first and second external surfaces. Furthermore, a membrane thickness of less than 100 nm may be particularly suitable for EUV applications due to higher EUV transmission.

[0021] The first and second membrane layers may comprise a metal or metallic material.

[0022] As discussed above, metals and metallic material with layer thicknesses below 100 nm exhibit high infra-red reflectivity. As such, by having the first and second membrane layers comprise a metal or metallic material the infra-red reflectivity of the first and second external surfaces are realized.

[0023] The membrane layers may be referred to as metallic layers or metal layers. The membrane layers may comprise ruthenium. Preferably, the ruthenium membrane layers may have a total thicknessof 4.5 nm. Such a thickness provides beneficial optical and thermal properties, for example increased EUV transmission of the membrane at 13.5 nm. The metallic layers may additionally or alternatively comprise platinum or molybdenum or any other material reflective to infra-red radiation and capable of EUV transmission. The metallic layers may have a membrane layer thickness configured to provide minimal EUV reflectivity and maximum EUV transmissivity.

[0024] The dielectric layer may comprise a two-dimensional material.

[0025] The two-dimensional material may comprise graphene. The graphene may be single layer graphene. Alternatively, the graphene may have more than one layer, for example bilayer graphene, few layer graphene (e.g. up to 5 layers) or multilayer graphene (e.g. up to 10 layers).

[0026] Beneficially, graphene minimizes dewetting effects typically experienced by thin membrane layers, and especially experienced by thin membrane layers in high temperature environments such as a lithographic apparatus. Graphene also provides significant strengthening properties to the membrane.

[0027] The two-dimensional material may comprise a transition metal dichalcogenide. The transition metal dichalcogenide may be a monolayer transition metal dichalcogenide. The two- dimensional transition metal dichalcogenide (TMD) may comprise MoSi, MoSe, WSi or WSe.

[0028] The dielectric layer may comprise a plurality of one-dimensional materials. For example, the plurality of one-dimensional materials may be carbon nanotubes.

[0029] In other words, the dielectric layer may comprise a two dimensional material, such as graphene or a transition metal dichalcogenide (TMD), or a layer comprising a plurality of onedimensional materials, such as an arrangement of carbon nanotubes.

[0030] The pellicle membrane may further comprise one or more additional membrane layers and one or more additional dielectric layers positioned between the first and second external surfaces. The additional membrane layers and additional dielectric layers may be arranged such that the pellicle membrane has N+l membrane layers and N dielectric layers. Each dielectric layer may be encapsulated between adjacent membrane layers. Encapsulated between may be interpreted as positioned directly between, for example each surface of the dielectric layer being immediately adjacent a surface of an adjacent membrane layer.

[0031] The pellicle membrane may have N = 4 dielectric layers. A pellicle membrane with 4 layers of a dielectric material (e.g. single layer graphene) may provide a significantly increased emissivity.

[0032] The pellicle membrane may have N > 4 dielectric layers. For example, the pellicle membrane may have 10 dielectric layers.

[0033] According to a second aspect there is provided a composite pellicle membrane comprising the pellicle membrane of the first aspect and one or more additional layers. The one or more additional pellicle layers may comprise a core layer. The core layer may comprise silicon. The one or more additional pellicle layers may comprise one or more capping layers arranged between the core layer and the pellicle membrane. Additionally or alternatively the one or more additional pellicle layers maycomprise one or more capping layers arranged on a surface of the core layer distal to the pellicle membrane.

[0034] That is, the emissive pellicle membrane of the first aspect may be used as part of a larger composite pellicle membrane with additional layers. The additional layers may be used, for example, as strengthening or protective layers for increasing the strength of the overall pellicle membrane and / or extending the lifetime of the overall composite pellicle membrane. Where graphene is used, a strengthening layer may not be required as graphene provides significant strength benefits.

[0035] The additional layers may be provided to improve the thermal or optical behavior of the overall composite membrane. The additional membrane layers may beneficially comprise a material with a high EUV transmission.

[0036] The additional pellicle layers can be provided between external to the first and second external surfaces of the emissive pellicle membrane. As such, the first and second external surfaces can be encapsulated within other layers and become intermediate external surfaces within the composite pellicle membrane.

[0037] The core layer may comprise, for example, silicon or silicon nitride. The capping layers may comprise, for example, an oxide (e.g. a native surface oxide) or a plasma etch retardant material. The composite pellicle membrane may comprise multiple capping layers, each having a different composition, or multiple capping layers may share the same composition.

[0038] The composite pellicle membrane may further comprise one or more adhesion layers between any adjacent layers therein. The adhesion layers may encourage adhesion between adjacent layers.

[0039] In an example, the composite pellicle membrane may comprise ordered layers of: the emissive pellicle membrane, an adhesion layer, a first plasma etch retardant cap layer, a core layer, a second plasma etch retardant layer and a native oxide layer.

[0040] In this way, the emissive pellicle membrane may be incorporated into composite pellicle membranes already in use, thereby improving the pellicles already in use.

[0041] According to a third aspect there is provided a composite pellicle membrane comprising the pellicle membrane of the first aspect and one or more additional layers. The one or more additional pellicle layers may comprise a first capping layer adjacent the first external surface of the pellicle membrane and a second capping layer adjacent the second external surface of the pellicle membrane.

[0042] The capping layers may have the same properties of one or more of the capping layers and / or additional layers of the previous aspect. In this aspect, the emissive pellicle membrane of the first aspect forms a core layer.

[0043] According to a fourth aspect there is provided a composite pellicle membrane comprising the pellicle membrane or composite pellicle membrane of any preceding aspect and further comprising a border.

[0044] The border may comprise silicon. The border may be arranged at the edge of the pellicle membrane.

[0045] According to a fifth aspect there is provided a pellicle assembly comprising the pellicle membrane or composite pellicle membrane of any preceding aspect and a frame, the pellicle membrane or composite pellicle membrane being supported by the frame.

[0046] According to a sixth aspect there is provided a patterning device assembly comprising the pellicle assembly of the previous aspect mounted on a patterning device for a lithographic apparatus.

[0047] The patterning device may be referred to as a reticle. The patterning device assembly be also be referred to as a pelliclized reticle.

[0048] Alternatively, the pellicle membrane, composite pellicle membrane or pellicle assembly of the previous aspects can be used in other areas of a lithographic apparatus, for example as part of a dynamic gas lock.

[0049] According to a seventh aspect there is provided a method of fabricating a pellicle membrane for an EUV lithographic apparatus. The method comprises: creating a first stack by providing a dielectric layer on a first supporting structure, creating a second stack by providing a first membrane layer on a second supporting structure, positioning the first stack on the second stack such that the dielectric layer is adjacent the first membrane layer, removing the first supporting structure, providing a second membrane layer on the dielectric layer such that the dielectric layer is arranged between the first and second membrane layers and removing the second supporting structure. The dielectric layer has a dielectric layer thickness, the fabricated pellicle membrane has a membrane thickness measured between a first external surface formed by the first membrane layer and a second external surface formed by the second membrane layer, the first and second external surfaces are reflective to infra-red radiation and the dielectric layer thickness and membrane thickness are selected such that the emissivity of the pellicle membrane is greater than 0.5.

[0050] The dielectric layer may be a first dielectric layer. By positioning the first stack on the second stack such that the dielectric layer is directly adjacent the first metallic layer, the dielectric layer and first metallic layer are adhered. Additional steps may be used to ensure adherence (e.g. between the dielectric layer and the first and / or second membrane layers), for example by annealing . An anneal temperature may be selected which is a relatively low temperature, e.g. a temperature below which atom diffusion is known to begin to play a significant role.

[0051] The dielectric layer thickness may be less than 10 nm. The dielectric layer may comprise a two-dimensional material or a plurality of one-dimensional materials, the membrane thickness may be less than 100 nm. The first and / or second membrane layers may comprise a metal or metallic material.

[0052] Creating the first stack may comprise: providing the dielectric layer on an initial support, providing the first supporting structure on the dielectric layer, and removing the initial support.

[0053] The initial support may be a growth medium, for example a catalyst for CVD such as copper. The dielectric material may be grown on the initial support. The first supporting structure maybe a capping layer provided atop (e.g. directly adjacent to) the dielectric material. The first supporting structure may be, for example, a layer of PMMA or naphthalene.

[0054] The method may further comprise: creating a third stack by providing a second dielectric layer on a third supporting structure; after providing the second membrane layer and before removing the second supporting structure: positioning the third stack on the combined first and second stack such that the second dielectric layer is directly adjacent the second membrane layer; removing the third supporting structure; and providing a third membrane layer on the second dielectric layer such that the second dielectric layer is positioned between the second and third membrane layers.

[0055] The method may further comprise creating additional third stacks and repeating the steps of creating a third stack and positioning the third stack on a combined first and second stack with additional one or more membrane layers and dielectric layers, so as to form a pellicle membrane with N dielectric layers and N+l membrane layers.

[0056] N may equal 4. A pellicle membrane with 4 layers of a dielectric material (e.g. single layer graphene) may provide a significantly increased emissivity. N may be greater than 4. The fabricated pellicle membrane may have, for example, 10 dielectric layers.

[0057] According to an eighth aspect there is provided a method of fabricating a composite pellicle membrane, comprising: fabricating a pellicle membrane using the method of any of the seventh aspect, and providing one or more additional pellicle layers. The one or more additional pellicle layers may comprise: a core layer, optionally comprising silicon, and one or more capping layers arranged between the core layer and the pellicle membrane, and / or arranged on a surface of the core layer distal to the pellicle membrane; or a first capping layer adjacent the first external surface of the pellicle membrane and a second capping layer adjacent the second external surface of the pellicle membrane.

[0058] In this way, the emissive pellicle membrane may be incorporated into composite pellicle membranes already in use, thereby improving the pellicles already in use while enabling the use of existing fabrication lines.

[0059] According to a ninth aspect there is provided a membrane for dynamic gas lock for an EUV lithographic apparatus, wherein the membrane comprises: a first membrane layer forming a first external surface; a second membrane layer forming a second external surface substantially parallel to the first external surface; a dielectric layer positioned between the first and second external surfaces. The first and second external surfaces are reflective to infra-red radiation, the dielectric layer has a dielectric layer thickness, the membrane has a membrane thickness measured between the first external surface and second external surface, and the dielectric layer thickness and membrane thickness are selected such that the emissivity of the membrane is greater than 0.5.

[0060] The features described in respect of any of the aspects may be combined with the features described in respect of any of the other aspects of the present invention.

[0061] The present invention will now be described with reference to an EUV lithography apparatus. However, it will be appreciated that the present invention is not limited to EUV lithography and may be suitable for other types of lithography.BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source;Figure 2 depicts a pellicle membrane;Figure 3 A-C show the emissivity, EUV transmissivity and EUV reflectivity for a range of materials for various layer thicknesses;Figure 4 depicts a pellicle membrane with multiple dielectric layers;Figures 5 A-C depict composite pellicle membranes;Figure 6 depicts a method of manufacturing a pellicle membrane.DETAILED DESCRIPTION

[0063] Figure 1 shows a lithographic system comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[0064] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0065] After being thus conditioned, the EUV radiation beam B interacts with the patterning device MA. As a result of this interaction, a patterned EUV radiation beam B’ is generated.

[0066] A pellicle assembly 15 is depicted in the path of the radiation to protect the patterning device MA. The pellicle assembly 15 comprises a pellicle membrane 19 and a frame 17 which supports the pellicle membrane 19. The pellicle membrane 19 may be referred to as a membrane. The pellicle membrane 19 comprises a thin film that is substantially transparent to EUV radiation (although it will absorb a small amount of EUV radiation). The pellicle membrane 19 acts to protect the patterning device MA from particle contamination. The pellicle membrane 19 may be referred to simply as apellicle. It will be appreciated that the pellicle assembly 15 may be located in any required position and may be used to protect any elements of the lithographic apparatus e.g. one or more of the mirrors in the lithographic apparatus.

[0067] Whilst efforts may be made to maintain a clean environment inside the lithographic apparatus LA, particles may still be present inside the lithographic apparatus LA. In the absence of a pellicle 19, particles may be deposited onto the patterning device MA. Particles on the patterning device MA may disadvantageously affect the pattern that is imparted to the radiation beam B and therefore the pattern that is transferred to the substrate W. The pellicle 19 provides a barrier between the patterning device MA and the environment in the lithographic apparatus LA in order to prevent particles from being deposited on the patterning device MA.

[0068] In use, the pellicle 19 is positioned at a distance from the patterning device MA that is sufficient that any particles that are incident upon the surface of the pellicle 19 are not in the focal plane of the radiation beam B. This separation between the pellicle 19 and the patterning device MA, acts to reduce the extent to which any particles on the surface of the pellicle 19 impart a pattern to the radiation beam B. It will be appreciated that where a particle is present in the beam of radiation B, but at a position that is not in a focal plane of the beam of radiation B (i.e., not at the surface of the patterning device MA), then any image of the particle will not be in focus at the surface of the substrate W. In some embodiments, the separation between the pellicle 19 and the patterning device MA may, for example, be between 2 mm and 3mm (e.g. around 2.5 mm). In some embodiments, a separation between the pellicle 19 and the patterning device may be adjustable.

[0069] After the generation of the patterned EUV radiation beam B’, the projection system PS is configured to project the patterned EUV radiation beam B’ onto the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13,14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’ , thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors).

[0070] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0071] A relative vacuum, i.e. a small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL, and / or in the projection system PS.

[0072] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source that is capable of generating EUV radiation.

[0073] Figure 2 depicts a pellicle membrane 200. The pellicle membrane 200 comprises a first membrane layer 200A, a second membrane layer 200B, and a dielectric layer 201 of dielectric material 201 encapsulated between the first and second membrane layers. In general, the membrane layers 200A, 200B are made from a material and / or structure that is reflective in the infrared radiation range. The membrane layers may comprise or consist of a metal, a metallic material, or a metal-like material (e.g. a material which behaves similar to a metal, especially with regard to its interaction with infrared radiation). The membrane layers, when comprising metal, may be referred to as a metal layer or metallic layer.

[0074] With the membrane 200, a reflective interface is formed by the first membrane layer 200A at a top surface 210, referred to as a first external surface 210. Similarly, a reflective interface is formed by the second membrane layer 200B at a bottom surface 220, referred to as a second external surface 220. Top and bottom used herein are relative terms, relative to the orientation of Figure 2, but it should be understood that the membrane may be oriented differently.

[0075] The behaviour of the pellicle membrane 200 will now be described with reference to a specific example composition, although it should be understood that the pellicle membrane 200 may be implemented differently while still achieving the beneficial membrane properties described herein.

[0076] The dielectric layer 201 may be formed from single layer graphene. The first and second membrane layers 200A, 200B may be metallic, for example comprising ruthenium. The membrane 200 has a membrane thickness (e.g. measured generally perpendicularly) between the first and second external surfaces 210, 220. The membrane thickness is preferably less than 100 nm. Generally, bulk metals have a poor emissivity because their surfaces are highly reflective, with reflectivity at infrared wavelengths approaching 100%. In thin layers of metals (e.g. less than 100 nm), radiation reflects from the metal surfaces but also penetrates into the metal. As such, when radiation is incident upon an external surface of a metal layer in free space (e.g. in air or a vacuum) a portion of the radiation will initially reflect with the metal-air (or metal-vacuum) interface with a 180° phase shift. Another portion of the radiation, however, will penetrate into the layer. The penetrating radiation will reflect from the opposite (bottom) surface of the metal layer and travel back to the top surface, where some of the radiation will couple out from the top surface of the metal layer into the air (or vacuum). However, the coupled out radiation has not experienced a phase shift and so cancels out the initially reflected radiation due to superposition. As a result, the total reflection of the thin metal layer almost vanishes (i.e. is almost zero). The radiation which penetrates the metal layer can be absorbed, resulting in an increased emissivity of the thin metal layer compared to a bulk metal. Regardless, the theoretical maximum thermal infrared emissivity attainable for a metal layer is 0.5 (50%).

[0077] It has been realised that, by encapsulating a layer of dielectric material 201 between two thin films of metal (or another reflective material), the emissivity of the overall membrane may be increased above 0.5. Figure 2 illustrates some of the mechanisms behind why such a high emissivity may be achieved. Figure 2 shows an emission event 203 whereby light is emitted internally within the membrane layer 200A. The origin of said emission event 203 may be due to the previous absorption of radiation by the membranelayers 200A, where the radiation may have penetrated into to membrane layer as described above. Figure 2 depicts this emission event 203 arising from the first membrane layer 200A but such an event may similarly arise from the second membrane layer 200B and the same physical mechanisms would occur as detailed below.

[0078] The light emitted at the emission event 203 travels to the surface 210 of the pellicle membrane 200 where some is transmitted out of the membrane 200 whereas a portion is reflected back into the membrane 200 due to the reflectivity at the external surface 210. The reflected portion travels through the dielectric layer 201 during its propagation through the membrane 200 and further absorption and subsequently further emission events 204 occur. Five further emission events 204 are depicted in Figure 2 for simplicity, although in reality many more emission events will occur. Each emission event 203, 204 leads to additional radiation propagating through, and reflecting from the top and bottom surfaces of, the membrane 200.

[0079] The multiple reflections contribute to an enhanced (e.g. increased) electric field within the membrane 200. The enhanced electric field subsequently increases the emissivity of the dielectric layer 201. The enhanced electric field therefore increases the emissivity of the overall membrane 200. In particular, it increases the emissivity of the membrane 200 compared to the theoretical maximum of its constituent parts (e.g. the thin layers of metal). In the specific example where single layer graphene is used, the thermal emissivity of single layer graphene can be increased up to a theoretical maximum of 0.97 (i.e. 97%) when the graphene is positioned in an enhanced electric field. In non-ideal situations, a significant increase in thermal emissivity (but below the theoretical maximum), for example 0.84 (84%) has also been observed.

[0080] It is also beneficial for the dielectric layer 201 to have a high EUV transmissivity so as to reduce losses in the EUV lithographic apparatus and process. Single layer graphene is particularly suitable for this purpose, because it has an EUV transmission of 99.8%.

[0081] The dielectric layer 201 may be formed from other materials than graphene. A range of two-dimensional materials may be used, for example transition metal dichalcogenides (e.g. tungsten diselenide WSe2, tungsten disulphide WS2, molybdenum diselenide MoSe2, molybdenum disulphide M0S2, etc.). Alternatively, a layer of other low-dimensional materials may be used, for example a layer of carbon nanotubes (CNT). The dielectric material may also be doped, for example using doped graphene. Multilayer graphene may also be used (e.g. bilayer, more than 2 layers). Rather than a dielectric layer 201, a high-emission material layer may be used in its place. That is, the material forming the dielectric layer 201 need not strictly comprise a dielectric material, but rather share theproperties of high emissivity in an enhanced electric field so as to achieve the enhanced emissivity (>0.5) of this pellicle membrane. As such, the terms dielectric layer and high-emission layer may be used interchangeably.

[0082] Many metals are suitable for use as the membrane layers 200A, 200B, for example ruthenium, molybdenum, yttrium, gold, copper, silver or aluminium. Other materials may also be used, for example metal silicides such as, but not limited to, molybdenum silicide, niobium silicide, ruthenium silicide and zirconium silicide. In general, the membrane layers 200A, 200B should have a high reflectivity for infra-red radiation. The membrane layers 200A, 200B may also be selected to have high EUV transmission, for example greater than 90%.

[0083] It can be useful to select the membrane thickness (e.g. the thickness between the first and second external surfaces 210, 220) and the thicknesses of the various sub-layers (e.g. the membrane layers 200A, 200b and dielectric layer 201) so as to optimise thermal emissivity, EUV reflectivity and EUV transmission. Figures 3A-C show the emissivity, EUV reflectivity (EUVR) and EUV transmission (EUVT) for a number of materials which may be used in the pellicle membrane described herein. In particular, the materials are ruthenium, molybdenum, yttrium and graphite. It should be understood that the materials may be used in their pure form, or may be used as components of (or materials related to) other compositions (e.g. M0S2 comprises molybdenum and single layer graphene shares some characteristics with graphite).

[0084] Figure 3A shows the emissivity of the materials for various layer thicknesses between 0 and 100 nm, and it can be seen that none exceed an emissivity of 0.5. Graphite achieves the highest emissivity of 0.5 at a thickness of approximately 20 nm.

[0085] Figure 3B shows the EUVR at a wavelength of 13.5 nm for the stated materials for various layer thicknesses. It is typically desirable to minimize EUVR so as to avoid dose losses due to reflection. EUVR maxima occur at multiples of of the EUV wavelength and EUVR minima occur at multiples of 'A of the EUV wavelength. As such, it may be particularly beneficial to select a layer thickness of a multiple of Yi the EUV wavelength (i.e. 6.8 nm, 13.5 nm, 20.3 nm). At these thicknesses, the layer effectively acts as an anti-reflection layer. In general, of the displayed materials ruthenium has the highest reflectivity, with graphite and yttrium exhibiting low EUVR.

[0086] Figure 3C shows the EUVT at a wavelength of 13.5 nm for the stated materials for various layer thicknesses. It can be seen that EUVT reduces as the layer thickness increases for all materials, but that yttrium experiences the smallest reduction in EUVT as layer thickness increases.

[0087] All three metrics (emissivity in Figure 3A, EUVR in Figure 3B and EUVT in Figure 3C) can be considered in parallel to optimise material choice and layer thickness. An optimal candidate for the membrane layers 200A, 200B has been found to be ruthenium with a total thickness (e.g. a thickness of all of the membrane layers 200A, 200B cumulatively) of 4-5nm. Ruthenium at 4-5 nm provides an advantageous balance of acceptably high emissivity, EUVT and acceptably low EUVR. Additionally, it has been found that layers of ruthenium of this thickness do not significantly dewet at standard EUVlithographic temperatures (e.g. on the order of hundreds of degrees, e.g. 400-700°C). Of course, other layer thicknesses and other materials, whether discussed herein or otherwise, may be selected based on their known material properties. While not shown in Figures 3A-C, platinum may be a suitable candidate metal for the membrane layers. Other material properties may also be used to determine a material’s suitability, for example it may be beneficial to have structurally and / or chemically stable materials. Non-oxidising materials may be desirable, for example.

[0088] By improving the emissivity of a pellicle membrane, the operating temperature of the membrane can be reduced. For example, if the emissivity is higher then the membrane can emit more heat and therefore maintain a lower temperature. In this way, the membrane can withstand a higher intensity of EUV radiation without reaching a high enough temperature to experience significant damage. As such, higher powers of EUV radiation can be used, leading to increased throughput of the lithographic process, without risking increased pellicle damage. An example pellicle membrane with an emissivity of 0.45 may have an operating temperature of 500°C when illuminated with an EUV source with a power of 400W. A pellicle membrane as described herein, with an emissivity of 0.84, can be used with the same EUV source with a power of 400W but maintain a lower temperature of 400°C. At this lower operating temperature, the membrane will become damaged more slowly and hence have a longer lifetime compared to the pellicle membrane with an emissivity of 0.45. Alternatively, the pellicle membrane can be used with a higher power EUV source, for example a source with a power of 650W, before it reaches an operating temperature of 500°C.

[0089] The membrane thickness described with reference to Figures 3A-C should be understood to be the entire membrane thickness (i.e. the distance between the first and second external surfaces 210, 220). The membrane thickness can be referred to as the total membrane thickness.. The membrane thickness is therefore the combined thickness of the first and second membrane layers 200A, 200B and the thickness of dielectric layer 201. The total membrane thickness may, in some instances, be marginally greater than the exact combined thickness of each of the individual layers, for example due to gaps between each layer.

[0090] In an example, if the first and second membrane layers 200A, 200B have a total combined thickness of 4.5 nm and the dielectric layer 201 comprises single layer graphene (with a layer thickness of approximately 0.8nm), the total membrane thickness will be approximately 5.8 nm. Each of the first and second membrane layers 200A, 200B may have substantially the same thickness (e.g. both having a thickness of 2.25 nm) or they may have different layer thicknesses.

[0091] Additional dielectric layers can be introduced to further improve the pellicle membrane. Figure 4 depicts an example of a pellicle membrane 400 with multiple dielectric layers 401A-D. In this example, the membrane 400 still has a first external surface 410 and a second external surface 420, with a membrane thickness defined as the distance between these two external surfaces 410, 420. The membrane 400 comprises four dielectric layers 401A-D and five membrane layers 400A-E. Theproperties of the dielectric layers 401A-D and membrane layers 400A-E may be as described above with reference to Figures 2 and 3A-C.

[0092] In general, a membrane as described herein has N dielectric layers and N+l membrane layers, with the dielectric layers arranged between adjacent membrane layers. In all cases, two of the membrane layers form the external surfaces, but other membrane layers are contained in an interior volume of the membrane. In general, a membrane will have a total membrane thickness T approximately equal to T = YN + X(N+1), where Y is the dielectric layer thickness and X is the membrane layer thickness. Alternatively, one or all of the membrane layers may have a different membrane layer thickness and / or one or all of the dielectric layers may have a different dielectric layer thickness. In this case, the total membrane thickness may be equal to the sum of each of the component layer thicknesses.

[0093] In the example in Figure 4, N=4. It has been found that this arrangement is particularly beneficial for realising a pellicle membrane with a high emissivity. Other values of N can be used. Table 1 below depicts the EUVT, thermal emissivity and operating temperature for various values of N. in example membranes where the dielectric layers comprise single layer graphene and the membrane layers each comprise ruthenium. The ruthenium membrane layers have layer thicknesses such that the sum all ruthenium layer thicknesses equals approximately 4.5 nm. For example, where N = 1, each ruthenium layer has a layer thickness of 2.25 nm and the total membrane thickness T is approximately 4.5 nm + 1(0.8 nm) = 5.3 nm. Alternatively, where N = 4, the membrane has, in order, a first membrane layer with a first membrane layer thickness of 1.5 nm, a first dielectric layer, a second membrane layer with a second membrane layer thickness of 0.5 nm, a second dielectric layer, a third membrane layer with a third membrane layer thickness of 0.5 nm, a third dielectric layer, a fourth membrane layer with a fourth membrane layer thickness of 0.5 nm, a fourth dielectric layer, and a fifth dielectric layer with a fifth dielectric layer thickness of 1.5 nm. If the first, second, third and fourth dielectric layers comprise single layer graphene, the dielectric layer thicknesses would each be 0.8 nm. The resultant membrane would have a total membrane thickness of 7.7 nm.

[0094] Table 1: optical and thermal properties of pellicle membranes with various numbers of dielectric layers comprising single layer graphene:

[0095] In the examples presented in Table 1, each of the membranes may be implemented with an additional core layer. Such a core layer may be provided so as to provide strength. The core layer may comprise silicon and / or silicon nitride (SiN). It has been particularly found that a core layer can be beneficial for the membranes with fewer dielectric layers. However, when single layer graphene is used as the dielectric layer, the graphene may provide sufficient strength to the membrane that additional strengthening layers (e.g. using a core layer) is not required. Multiple layers of graphene may provide even more strength. As such, in some implementations a core layer can be provided when N < 4 and no core layer is provided when N > 4.

[0096] The pellicle membrane may be incorporated into a pellicle assembly, for example the pellicle assembly 15 described with reference to Figure 1. Such a pellicle assembly can have a border 57 which supports the pellicle membrane. The border 57 may comprise silicon. Figures 5A-5C depict example pellicle assemblies which include an emissivity pellicle membrane as described herein.

[0097] Figure 5A depicts a composite pellicle membrane 500 which comprises a pellicle membrane 501 as described above. That is, it comprises a pellicle membrane 501 comprising at least one dielectric layer and at least two membrane layers, which has an overall emissivity greater than 0.5. The composite pellicle membrane 500 also has a border 57. The border 57 takes the form of a border around the edge of the pellicle membrane 501. Figures 5A-5C are shown in cross-section and so the visible parts of the border 57 appear like two arms supporting distal ends of the pellicle membrane 501.

[0098] Figure 5B depicts a second composite pellicle membrane 510. The second composite pellicle membrane 510 comprises a pellicle membrane 501 as described above. The second composite pellicle membrane 510 also comprises capping layers 512, 513 and a core layer 511. The core layer 511 may comprise, for example silicon. The capping layers include two plasma etch resistant capping layers 512 and a native oxide layer 513. Core layers and capping layers are known in the art of pellicles and will not be described in great detail here. However, Figure 5B is included to demonstrate how the pellicle membrane 501 described herein can be incorporated into other pellicle membrane structures or pellicle assemblies, for example known pellicle assembly structures, to improve the thermal and / or structural properties thereof. The second composite pellicle membrane 510 also has a border 57.

[0099] Figure 5C depicts a third composite pellicle membrane 520. The third composite pellicle membrane 520 comprises a pellicle membrane 501 as described above with capping layers 522 arranged at its first and second external surfaces 521. As such, the ‘external surfaces’ 521 of the pelliclemembrane 501 now form intermediate external surfaces within the general volume of the third composite pellicle membrane 520. However, the external surfaces 521 still exhibit a high reflectivity (e.g. due to the optical characteristics of the interface between the pellicle membrane 501 and the capping layers 522) and so the required enhanced electric field is still experienced within the pellicle membrane 501. The third composite pellicle membrane 520 also has a border 57.[000100] Each composite pellicle membrane 500, 510, 520 comprises a pellicle membrane according to the present invention and one or more additional layers. The additional layers can be the capping layers, core layers, borders etc. Any other additional layers may also be introduced, for example for improving the thermal, optical, chemical, mechanical properties, or any other properties of the composite membrane and pellicle assembly.[000101] In addition to or alternative to the border 57, the pellicle membranes 500, may be supported by a frame, e.g. at the edges of the pellicle membrane. The frame may comprise silicon. A pellicle membrane comprising a frame may be referred to as a pellicle assembly. The frame can be attached (e.g. glued) to the composite pellicle membrane at the location of the border 57. Alternatively, a frame can be attached without the use of a border. For example, as described above the use of graphene in the pellicle membrane confers significant strength onto the membrane and so additional strength from a border may not be required.[000102] Figure 6 depicts a method of manufacturing a pellicle membrane as described herein. The example method is described with reference to a pellicle membrane comprising graphene dielectric layers. However, the teaching can be expanded to other types of dielectric layers.[000103] In a first step 601, a layer of graphene 601a is provided on an initial support 601b. The initial support 601b may be, for example, copper or another catalyst suitable for chemical vapour deposition of graphene. The graphene layer 601a (or other dielectric layer) may be deposited in a different manner, in which case another initial support may be suitable.[000104] In a second step 602, a first supporting structure 602a may be provided on the layer of graphene 601a. The first supporting structure 602a may be a protective capping layer. It may be formed from a layer of PMMA or naphthalene.[000105] In a third step 603, the initial support 601b is removed, for example via etching or mechanical removal. This leaves the graphene 601a on the first supporting layer 602a. The combination of the graphene and the first supporting layer may be referred to as a first stack.[000106] The first two or three steps 601, 602, 603 may be performed in isolation from the remaining steps, such that a supply of first stacks is available for use in the following method steps.[000107] In a fourth step 604, a second stack is created by providing a first membrane layer 604a on a second supporting structure 604b. The first membrane layer 604a may be, for example and as described above, a layer of metal or other reflective material.[000108] In a fifth step 605, the first stack is positioned on the second stack such that the layer of graphene 601a is deposited on the first membrane layer 604a.[000109] In a sixth step 606, the first supporting 601a structure is removed. In this way, the surface of the graphene layer 601a which is was previously covered by the first supporting structure 601a is uncovered.[000110] In a seventh step 607, a second membrane layer 607a is provided on the uncovered surface of the graphene layer 601a. That is, the second membrane layer 607a is provided adjacent the graphene layer 601a. The graphene layer 601a is now positioned between (encapsulated by) the first and second membrane layers 604a, 607a.[000111] Optionally, additional graphene layers and additional membrane layers can be added, for example by introducing additional stacks comprising a dielectric layer and an additional supporting structure and subsequently removing the additional supporting structure.[000112] In a final step 608, the second supporting structure 604b is removed. The fabricated membrane therefore comprises (at least) two membrane layers 604a, 607a, and one (or more) dielectric layers 604a between each two adjacent membrane layers. Two outer membrane layers each have one surface uncovered such that the external surfaces are formed.[000113] In this way, a pellicle membrane with N dielectric layers and N+l membrane layers can be fabricated. This manufacturing method is particularly robust when graphene is used, for example because of the mechanical strength of graphene. As such, the risk of damage to the pellicle at each stage is significantly reduced.[000114] In any of the above steps where one layer is deposited on another, the adhesion between these layers may be improved by adding an annealing (e.g. heating) step. Graphene prevents or delays the onset of thermal dewetting of any adjacent layers, particularly metallic layers, and provides strong adhesive properties and so reduces the need to encourage adhesion using other methods.[000115] Reference has been made herein to ‘high’ emissivity, high EUV transmissivity (EUVT) and high or low EUV reflectivity (EUVR). High may be considered to be greater than 50%, 80%, 90% for emissivity. High may be considered to be greater than 90%, 95% or 99% for EUV transmissivity. High EUV reflectivity may be considered to be greater than 4%. Low EUV reflectivity may therefore be considered to be greater than 4%. These values are provided as examples and the value which is deemed high or low in a particular instance may depend on the desired throughput of the apparatus and / or comparison to the state of the art.[000116] The membranes described herein can be used in applications other than in the pellicle. For example, the membranes may be used as part of a dynamic gas lock. Dynamic gas locks are particularly useful in EUV lithographic apparatuses.[000117] Composite pellicle membranes are described above. The term composite is intended to indicate that the pellicle membranes include additional layers. The term composite in this context does not mean that the materials used must be composite materials. For example, the additional layer(s) may comprise a single material e.g. a metal.[000118] 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. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.[000119] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.[000120] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.[000121] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. A pellicle membrane for an EUV lithographic apparatus, wherein the pellicle membrane comprises: a first membrane layer forming a first external surface; a second membrane layer forming a second external surface substantially parallel to the first external surface; a dielectric layer positioned between the first and second external surfaces; wherein the first and second external surfaces are reflective to infra-red radiation, the dielectric layer has a dielectric layer thickness, the membrane has a membrane thickness measured between the first external surface and second external surface, and the dielectric layer thickness and membrane thickness are selected such that the emissivity of the pellicle membrane is greater than 0.5.

2. The pellicle membrane of claim 1, wherein the dielectric layer thickness is less than 10 nm, and optionally on the order of or less than 1 nm.

3. The pellicle membrane of claim 1 or 2, wherein the membrane thickness is less than 100 nm.

4. The pellicle membrane of any preceding claim, wherein the first and second membrane layers comprise a metal or metallic material.

5. The pellicle membrane of any preceding claim, wherein the dielectric layer comprises a two- dimensional material.

6. The pellicle membrane of claim 5, wherein the two-dimensional material comprises graphene, and optionally wherein the graphene is single layer graphene.

7. The pellicle membrane of claim 5, wherein the two-dimensional material comprises a transition metal dichalcogenide and optionally wherein the transition metal dichalcogenide is a monolayer transition metal dichalcogenide.

8. The pellicle membrane of any of claims 1 to 4, wherein the dielectric layer comprises a plurality of one-dimensional materials.

9. The pellicle membrane of any preceding claim, further comprising one or more additional membrane layers and one or more additional dielectric layers positioned between the first and secondexternal surfaces, arranged such that the pellicle membrane has N+l membrane layers and N dielectric layers and wherein each dielectric layer is encapsulated between adjacent membrane layers.

10. The pellicle membrane of claim 9, wherein N = 4.

11. The pellicle membrane of claim 9, wherein N > 4.

12. The pellicle membrane of any preceding claim, wherein the membrane layers comprise Ruthenium.

13. A composite pellicle membrane comprising the pellicle membrane of any preceding claim and one or more additional layers, wherein the one or more additional pellicle layers comprise: a core layer, optionally comprising silicon; one or more capping layers arranged between the core layer and the pellicle membrane, and / or arranged on a surface of the core layer distal to the pellicle membrane.

14. A composite pellicle membrane comprising the pellicle membrane of any preceding claim and one or more additional layers, wherein the one or more additional pellicle layers comprise a first capping layer adjacent the first external surface of the pellicle membrane and a second capping layer adjacent the second external surface of the pellicle membrane.

15. A composite pellicle membrane comprising the pellicle membrane or composite pellicle membrane of any preceding claim and further comprising a border.

16. A pellicle assembly comprising the pellicle membrane or composite pellicle membrane of any preceding claim and a frame, the pellicle membrane or composite pellicle membrane being supported by the frame.

17. A patterning device assembly comprising the pellicle assembly of claim 16 mounted on a patterning device for a lithographic apparatus.

18. A method of fabricating a pellicle membrane for an EUV lithographic apparatus, the method comprising: creating a first stack by providing a dielectric layer on a first supporting structure; creating a second stack by providing a first membrane layer on a second supporting structure; positioning the first stack on the second stack such that the dielectric layer is adjacent the first membrane layer;removing the first supporting structure; providing a second membrane layer on the dielectric layer such that the dielectric layer is arranged between the first and second membrane layers; and removing the second supporting structure; wherein the dielectric layer has a dielectric layer thickness, the fabricated pellicle membrane has a membrane thickness measured between a first external surface formed by the first membrane layer and a second external surface formed by the second membrane layer, the first and second external surfaces are reflective to infra-red radiation and the dielectric layer thickness and membrane thickness are selected such that the emissivity of the pellicle membrane is greater than 0.5.

19. The method of claim 18, wherein: the dielectric layer thickness is less than 10 nm, and optionally wherein the dielectric layer comprises a two-dimensional material or a plurality of one-dimensional materials; and / or the membrane thickness is less than 100 nm, and optionally wherein the first and / or second membrane layers comprise a metal or metallic material;20. The method of claim 18 or 19, wherein creating the first stack comprises: providing the dielectric layer on an initial support; providing the first supporting structure on the dielectric layer; removing the initial support.

21. The method of any of claims 18 to 20, further comprising: creating a third stack by providing a second dielectric layer on a third supporting structure; after providing the second membrane layer and before removing the second supporting structure: positioning the third stack on the combined first and second stack such that the second dielectric layer is directly adjacent the second membrane layer; removing the third supporting structure; and providing a third membrane layer on the second dielectric layer such that the second dielectric layer is positioned between the second and third membrane layers.

22. The method of claim 19, further comprising creating additional third stacks and repeating the steps of claim 19 so as to form a pellicle membrane with N dielectric layers and N+l membrane layers.

23. The method of claim 22, wherein N = 4.

24. The method of claim 22, wherein N > 4.

25. A method of fabricating a composite pellicle membrane, comprising: fabricating a pellicle membrane using the method of any of claims 18 to 24; and providing one or more additional pellicle layers, wherein the one or more additional pellicle layers comprise: a core layer, optionally comprising silicon, and one or more capping layers arranged between the core layer and the pellicle membrane, and / or arranged on a surface of the core layer distal to the pellicle membrane; or a first capping layer adjacent the first external surface of the pellicle membrane and a second capping layer adjacent the second external surface of the pellicle membrane.

26. A membrane for dynamic gas lock for an EUV lithographic apparatus, wherein the membrane comprises: a first membrane layer forming a first external surface; a second membrane layer forming a second external surface substantially parallel to the first external surface; a dielectric layer positioned between the first and second external surfaces; wherein the first and second external surfaces are reflective to infra-red radiation, the dielectric layer has a dielectric layer thickness, the membrane has a membrane thickness measured between the first external surface and second external surface, and the dielectric layer thickness and membrane thickness are selected such that the emissivity of the membrane is greater than 0.5.

Images