NL2024506A - Improved lithography methods - Google Patents

Abstract

The present invention relates to methods for reducing wafer load grid and flatness degradation of a substrate holder, such as a wafertable, of a lithographic apparatus. The present invention also relates to systems comprising lithography substrate holders, such as wafertables, with improved resistance to wafer load grid and flatness degradation, and to methods of fabricating devices, e. g. integrated circuits, using such systems. The present invention also relates to substrates, such as wafers, with backsides configured to protect substrate holders, such as wafertables, from wafer load grid and flatness degradation when used in lithography, and to methods of removing hydrophobic coatings from such substrates, such as wafers. The present invention has particular use in connection with lithographic apparatus for fabricating devices, for example integrated circuits.

Description

NL A 2024506

(2?) Aanvraagnummer: 2024506 © 2024506 © Α OCTROOIAANVRAAG (22) Aanvraag ingediend: 19 december 2019 (30) Voorrang:

(4?) Aanvraag ingeschreven:

januari 2020 (43) Aanvraag gepubliceerd:

januari 2020 (51) Int. Cl.:

G03F7/20 (2020.01) © Aanvrager(s):

ASML Netherlands B.V. te Veldhoven © Uitvinder(s):

Johan Franciscus Maria Beckers te Veldhoven Dylan John David Davies te Veldhoven Paulus Albertus van Hal te Veldhoven

Beatriz Seoane De La Cuesta te Veldhoven Lucas Henricus Johannes Stevens te Veldhoven

Siegfried Alexander Tromp te Veldhoven Satish Achanta te Veldhoven

Julien van Kuilenburg te Veldhoven © Gemachtigde:

ir. A.J. Maas te Veldhoven © IMPROVED LITHOGRAPHY METHODS (57) The present invention relates to methods for reducing wafer load grid and flatness degradation of a substrate holder, such as a wafertable, of a lithographic apparatus. The present invention also relates to systems comprising lithography substrate holders, such as wafertables, with improved resistance to wafer load grid and flatness degradation, and to methods of fabricating devices, e.g. integrated circuits, using such systems. The present invention also relates to substrates, such as wafers, with backsides configured to protect substrate holders, such as wafertables, from wafer load grid and flatness degradation when used in lithography, and to methods of removing hydrophobic coatings from such substrates, such as wafers. The present invention has particular use in connection with lithographic apparatus for fabricating devices, for example integrated circuits.

Deze publicatie komt overeen met de oorspronkelijk ingediende stukken.

IMPROVED LITHOGRAPHY METHODS

Field [0001] The present invention relates to lithographic apparatus, to methods of making such apparatus and to manufacturing devices, for example integrated circuits, using such apparatus.

Background to the invention [0002] 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 devices, for example integrated circuits (ICs). A lithographic apparatus may for example project a pattern from a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate.

[0003] The substrate in a lithographic apparatus is typically supported by a support mechanism/substrate holder. When the substrate is a silicon wafer (for example, during integrated circuit manufacture), the support mechanism/substrate holder is typically referred to as a wafertable. [0004] The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features that can be formed on that substrate. A lithographic apparatus that uses EUV radiation, that is electromagnetic radiation having a wavelength within the range 4-20 nm, may be used to form smaller features on a substrate than a lithographic apparatus using deep ultraviolet (DUV) radiation (for example with a wavelength of 193 nm).

[0005] As the size of features to be formed in a lithographic process reduces, the performance requirements on all aspects of the lithographic apparatus and materials become stricter.

[0006] Substrate holders used during device fabrication typically comprise burls, which reduce the influence of contamination by particles on the backside of the substrate.

[0007] Typical semiconductor substrates have a thin layer of silicon, for example SiO_x or Si_xN_y, on the backside of the substrate (e.g. the wafer backside), which can lead to the formation of Si-OH groups at the surface. This results in substrate backside surfaces that have high surface energies. Charges (i.e. protons) are able to migrate over the surface of such substrate backsides.

[0008] Before loading a substrate into the lithography apparatus, the backside of the substrate is cleaned with a H₂O rinse with scrubbing on the central part of the substrate to remove contaminant particles. The substrate is then spin dried. This wet cleaning step extends substrate holder cleaning intervals, increasing the throughput of lithographic apparatus.

[0009] Due to the high surface energy of the substrate backside, water adheres to the surface, leaving adsorbed monolayers of H₂O on the substrate backside even after spin drying. Due to the streaming potential, this water rinse procedure results in the formation of surface charges on the substrate backside. Upon loading the substrate onto the substrate holder, residual water is thus present between the substrate backside and the burls of the substrate holder.

[0010] Electrochemical corrosion of the burl material may occur and over time, certain burls may change height. This may lead to increased wafer load grid (WLG), uneven flatness degradation and a local angle focus problem as the image cannot be focussed properly on to the substrate. Thus, the substrate holder may no longer meet the strict performance requirements required for lithographic apparatus and must be replaced.

Summary of the invention [0011] In view of the above, there remains a need to develop improved methods for minimising wafer load grid (WLG) and substrate holder/wafertable flatness degradation in lithographic systems. There is also a need for systems and apparatus for lithographic processes comprising components resistant to WLG and to substrate holder/wafertable flatness degradation.

[0012] The present invention relates to a method protecting a substrate holder, the method comprising: a) applying a first self-assembled monolayer (SAM A) to the backside surface of a substrate intended for use with the substrate holder, wherein SAM A lowers the surface free energy of the substrate backside surface; and wherein SAM A is applied to the substrate backside surface by reacting the substrate backside surface with a first SAM precursor molecule; and/or b) applying a second self-assembled monolayer (SAM B) to the surface of at least a portion of the burls of the substrate holder, wherein SAM B lowers the surface free energy of the surface of the burls; and wherein SAM B is applied to the surface of the burls by reacting the surface of the burls with a second SAM precursor molecule.

[0013] The present invention also relates to an apparatus configured to apply the above method. [0014] The present invention also relates to a substrate holder with a surface comprising burls, wherein at least a portion of the burls comprise a self-assembled monolayer (SAM B), wherein SAM B is a hydrophobic SAM.

[0015] The present invention also relates to a substrate, wherein the backside of the substrate comprises a self-assembled monolayer (SAM A), wherein SAM A is a hydrophobic SAM.

[0016] The present invention also relates to a method comprising processing a substrate supported by a substrate holder, wherein the surface of the backside of the substrate comprises a first selfassembled monolayer (SAM A) and/or at least a portion of the burls of the substrate holder comprise a second self-assembled monolayer (SAM B), wherein SAM A and SAM B are hydrophobic SAMs. [0017] The present invention also relates to a method of fabricating a device, the method comprising applying the above method.

[0018] The present invention also relates to a method of removing a self-assembled monolayer SAM A from the backside of a substrate after exposure of the substrate to radiation in a lithographic apparatus, the method comprising liquid backside stripping and/or gas-phase stripping of the backside of the substrate.

[0019] Implementing such methods and apparatus reduces the frequency at which maintenance must be undertaken and substrate holders/wafertables must be replaced, increasing throughput of the lithographic apparatus and the efficiency of device production

Brief description of the drawings [0020] Figure 1 depicts a schematic illustration of a lithographic system comprising a lithographic apparatus and a radiation source.

[0021] Figure 2 depicts a schematic overview of a lithographic cell.

[0022] Figure 3 depicts a substrate on a substrate support in a store unit of a lithographic apparatus.

[0023] Figure 4 shows how the surface free energy of the substrate backside coating affects substrate holder WLG.

[0024] Figure 5 depicts a substrate holder flatness drift test for a substrate holder used with substrates without a SAM backside coating (bottom) and with a SAM backside coating (top). Values arc in nm per thousand substrate passes.

[0025] Figure 6 shows how the surface free energy of the substrate backside affects electrostatic clamp WLG during EUV lithography.

[0026] Figure 7 shows a method of application of a SAM precursor to the backside of a substrate. [0027] Figure 8 shows how the contact angle changes upon application of a SAM according to the present invention.

[0028] Figures 9 to 11 show methods of application of a SAM precursor to the burls of a substrate holder.

Detailed description of the invention [0029] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., EUV radiation or DUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a substrate table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support WT in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0030] In operation, the illumination system IL receives the radiation beam B from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and/or other types of optical components, or any combination thereof, for directing, shaping, and/or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0031] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and/or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and/or for other factors such as the use of an immersion liquid, the use of Hl·, or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.

[0032] Tire lithographic apparatus may be of a type wherein at least a portion of the substrate W may be covered by an immersion liquid having a relatively high refractive index, e.g., water, so as to fill an immersion space between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in US 6,952,253, which is incorporated herein by reference.

[0033] The lithographic apparatus may be of a type having two or more substrate supports WT (also named “dual stage”). In such “multiple stage” machine, the substrate supports WT may be used in parallel, and/or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W on the other substrate support WT is being used for exposing a pattern on the other substrate W.

[0034] In addition to the substrate support WT, the lithographic apparatus may comprise a measurement stage (not depicted in Fig. 1). The measurement stage is arranged to hold a sensor and/or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. Tire measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.

[0035] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system PMS, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks Pl, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[0036] In a lithographic apparatus it is necessary to position with great accuracy the upper surface of a substrate or wafer to be exposed in the plane of best focus of the aerial image of the pattern projected by the projection system. To achieve this, the substrate or wafer can be held on a substrate holder or wafertable. The surface of the substrate holder that supports the substrate can be provided with a plurality of burls whose distal ends can be coplanar in a nominal support plane. The burls, though numerous, may be small in cross-sectional area parallel to the support plane so that the total cross-sectional area of their distal ends is a few percent, e.g. less than 5%, of the surface area of the substrate. The gas pressure in the space between the substrate holder and the substrate may be reduced relative to the pressure above the substrate to create a force clamping the substrate to the substrate holder.

[0037] As shown in Figure 2 the lithographic apparatus LA may form part of a lithographic cell LC, also sometimes referred to as a lithocell or (litho)cluster, which often also includes apparatus to perform pre- and post-exposure processes on a substrate W. Conventionally these include spin coaters SC to deposit resist layers, developers DE to develop exposed resist, chill plates CH and bake plates BK, e.g. for conditioning the temperature of substrates W e.g. for conditioning solvents in the resist layers. A substrate handler, or robot. RO picks up substrates W from input/output ports I/O 1,1/02. moves them between the different process apparatus and delivers the substrates W to the loading bay LB of the lithographic apparatus LA. The devices in the lithocell, which are often also collectively referred to as the track, are typically under the control of a track control unit TCU that in itself may be controlled by a supervisory control system SCS, which may also control the lithographic apparatus LA, e.g. via lithography control unit LACU.

[0038] In an embodiment the lithographic apparatus LA comprises a store unit. The store unit may be part of a substrate handler that controls movement of the substrate through the lithographic apparatus LA. When a substrate W is entered into a lithographic apparatus LA, the substrate W is positioned first on the store unit. Subsequently, the substrate W is moved from the store unit, after which the substrate W is positioned on the substrate table WT for an exposure process. Hence, the substrate W is positioned on the store unit before it is moved onto the substrate table WT.

[0039] In an embodiment the store unit comprises a substrate support 20. Figure 3 depicts a substrate W on the substrate support 20. The substrate support 20 is configured to support the substrate W.

[0040] As shown in Figure 3, the substrate support 20 comprises a main body 21. The main body 21 has a plate-like shape and may be approximately the same shape as the substrate W. For example, when the substrate W is circular, the main body 21 may correspondingly be circular. However, the shape of the main body 21 is not particularly limited. The main body 21 has an upper surface which forms a base surface 23 of the substrate support 20. In an embodiment, the base surface 23 of the substrate support 20 is electrically conductive. In an embodiment the substrate support 20 comprises a coating for the base surface 23. In an embodiment the coating comprises a diamond-like carbon, a silicon carbide (e.g. silicon infiltrated silicon carbide) and/or chromium nitride.

[0041] A wafer as discussed herein is one example of a substrate that may be supported by a substrate holder. When the substrate is referred to as a wafer, the substrate holder is typically referred to as a wafertable.

[0042] Contact between the substrate backside and the burl material in the presence of water can lead to electrochemical corrosion of the burl material, for example the diamond-like carbon (DLC) coating of the burls. Over time, certain burls may change height, resulting in an uneven flatness degradation and a local angle focus problem as the image cannot be focussed properly on to the substrate. Thus, the substrate holder may no longer meet the strict performance requirements required for lithographic apparatus and must be replaced. Minimising the rate of burl corrosion is therefore key to maximising substrate holder life and efficiency of device production.

[0043] In addition to the above, during loading the substrate makes contact with certain points of the substrate holder, which can introduce displacement errors in the x and y directions. These errors have been observed to drift over time and are reflected in the WLG drift.

[0044] Excluding the wet substrate cleaning step results in no measurable flatness degradation after 1000 substrate passes in an accelerated test environment. Thus, reducing the quantity of water and/or surface charges present on the substrate backside following washing with water is key to minimising flatness degradation. The wet cleaning step cannot be removed entirely as this would increase the frequency of substrate holder cleaning, reducing throughput of the lithographic apparatus.

[0045] Wafer load grid (WLG) is also a serious issue for lithography substrate holders. Local damage of substrate holders in the x, y and/or z direction has a negative impact on the overlay (i.e. the exact positioning of the substrate with respect to the light source) at the nm level. Over time, WLG drifts to higher values, resulting in overlay issues and reducing the lifetime of the substrate holder. Minimising the rate of burl corrosion is also key to minimising the rate of drift of WLG.

[0046] WLG of electrostatic clamps (ESCs) used in extreme ultra-violet (EUV) lithography is also problematic, and limits the effective life-time of the ESC for high-volume device production using EUV lithography. Minimising the rate of burl corrosion is also important in minimising ESC WLG. [0047] One method of reducing burl reactivity is to apply a cathodic overpotential to the burls to prevent oxidation. Another method of reducing burl reactivity is to use an anodic overpotential to create a passivation layer, which seals the underlying surface and prevents further oxidation. However, passivation layers are only applicable for certain metallic substrates, where the corresponding metal oxide forms a closed oxide layer which is able to prevent further corrosion of the underlying metal layer. For many metals, the metal oxide is open and further corrosion of the underlying metal surface can occur. For some metal oxides, and carbon-based materials such as diamond-like carbon (DLC), the corrosion of the carbon-based layers results in dissolution of the oxide, both in solution and possibly in gaseous species such as CO2,1UO and CO.

[0048] Thus, neither applying a cathodic overpotential nor creating a passivation layer are generally applicable to all substrate holder materials.

[0049] The present invention arises from the surprising finding that it is possible to modify the properties of the substrate backside materials and/or the burls of the substrate holder using selfassembled monolayers (SAMs) to significantly decrease the rate of substrate holder flatness degradation and/or wafer load grid (WLG).

[0050] The substrate can be made of any semiconducting material known in the art that may be used to produce substrates. For example, the substrate may be a silicon wafer, a silicon carbide wafer, a gallium nitride wafer, a gallium arsenide wafer or an aluminium titanium carbide wafer. Preferably, the substrate is a silicon wafer or a silicon carbide wafer.

[0051] The substrate holder comprises one or more of DLC, diamond, graphite, SiSiC, SiC, Zerodur, AhOs.TiN, Lipocer, SST and/or CrN, and preferably comprises one or more of diamond-like carbon (DLC), diamond, graphite, SiSiC, SiC and/or CrN.

[0052] The term “self-assembled monolayer” (SAM) used herein refers to a molecular assembly formed spontaneously on a surface by adsorption of SAM precursor molecules onto the surface. SAM precursor molecules typically comprise a head-group and a tail-group. Head-groups of the SAM precursor molecules are able to chemisorb onto a substrate. As more head-groups chemisorb to the surface, the tail-groups begin to self-organise, pointing away from the surface until a monolayer is formed on the surface of the substrate. The “end-group” of a SAM is the final group of the tail-group i.e. the group of the SAM that is furthest away from the surface of the substrate to which the SAM is adsorbed.

[0053] Tire tail-group of the SAM can be saturated or unsaturated. When the tail-group is unsaturated, it can comprise a conjugated structure, for example a phenyl group and/or a heterocyclic group, including heterocyclic groups comprising S, N and/or O atoms, for example a thiophene group.

The tail-group of the SAM can also comprise one or more azo groups. SAMs comprising one or more azo groups can be manipulated with light such that the one or more azo groups form either the trans or cis configuration.

[0054] Preferably, the SAM is a hydrophobic SAM. In terms of contact angle with water, the surface coated by the SAM preferably has a contact angle with water of greater than about 50°, preferably greater than about 60°, preferably greater than about 70°, preferably greater than about 80°, and most preferably greater than about 90°. The surface coated by the SAM preferably has a surface free energy of less than about 70 mJ/m², preferably less than about 60 mJ/m², preferably less than about 50 J/m², preferably less than about 40 J/m², preferably less than about 30 J/m² and most preferably less than about 20 mJ/m².

[0055] The contact angle values disclosed herein are determined using the sessile drop method, with the contact angles measured using drop shape analysis (DSA).

[0056] The surface free energy values disclosed herein are calculated by applying the Owens, Wendt, Rabel and Kaelble (OWRK) method after determining the contact angle of two liquids on the surface using the sessile drop method.

[0057] Preferably, the end-group of the SAM comprises an end-group with a low surface-energy moiety such as an alkyl group or halogenated group, preferably a halogenated group selected from -F, -Cl, -Br, -I, -CXs. -CHX2 or CIFX. wherein each instance of X is independently selected from F, Cl,BrorI.

[0058] Preferably, the tail-group of the SAM comprises one or more low surface-energy substituents such as -F, -Cl, -Br, -I, -CX3, -CHX2 or -CH2X, wherein each instance of X is independently selected from F, Cl, Br or I.

[0059] The hydrophobicity of the SAM can thus be modified by altering the end- and tail-groups of the SAMs used in the present methods and systems. For example, incorporating low surface-energy substituents such as those disclosed above can increase the hydrophobicity of the SAM and further reduce the surface energy of the surface following coating with the SAM, leading to further improvements in the WLG and flatness degradation. Using SAMs comprising tail-groups comprising hydrophobic substituents such as those above also increases the strength intermolecular forces between neighbouring tail groups of the SAM. This promotes ordering of the tail groups perpendicular to the surface which the SAM covers.

[0060] In the present invention, it is preferable for the SAM to quickly order roughly perpendicularly to the surface (i.e. to ‘stand up’) when the SAM is applied to the surface. This may be achieved using any method known in the art. For example, short, rigid molecules may be used in which the tail-groups have minimal adsorption energy towards the surface. This ensures that the tailgroups do not lay flat against the surface, and instead point away from the surface i.e. the SAM is arranged roughly perpendicularly to the surface. Another method to achieve this peipendicular ordering is to use SAMs with long, hydrophobic tail-groups which promote strong intennolecular interactions to tail-groups of neighbouring tail-groups when the SAM is grafted onto the surface. Such SAMs are able to form ordered molecular assemblies, with tail-groups pointing away from the surface to which the head-group is adsorbed.

[0061] Due to these differing approaches, the length of the SAMs used in the prevent invention vary widely. The length of the SAMs used can be in the range of from about 0.2 nm to about 8 nm. Preferably, the SAM is longer than about 0.3 nm, preferably longer than about 0.4 nm and most preferably longer than about 0.5 nm. Preferably, the SAM is shorter than about 7 nm, preferably shorter than about 6 nm, preferably shorter than about 5 nm, preferably shorter than about 4.5 nm, preferably shorter than about 4 nm, preferably shorter than about 3.5 nm, preferably shorter than about 3 nm, preferably shorter than about 2.5 nm and preferably shorter than about 2 nm. In one embodiment, the SAM has a length in the range of from about 0.5 nm to about 3 nm.

[0062] In the present invention, SAM precursor molecules attach to the surface of the substrate backside and/or to the burls of the substrate holder. Classes of molecules suitable as SAM precursor molecules for forming the SAMs of the present invention include alkylthiols, azides, silanes and cyclic azasilanes, phosphonates, carboxylates, catechols, alkenes, alkynes.

[0063] Preferred classes of SAM precursor molecules for forming a SAM on the backside surface of a substrate are silanes (including cyclic azasilanes), alkenes and alkynes.

[0064] For example, silane molecules can bond to S1O2 on the surface of a silicon wafer backside to form a SAM on the surface of the silicon wafer backside.

[0065] Examples of silanes which may be used in the present invention as the SAM precursor molecules to form the SAM include heneicosatluorododecyltrichlorosilane, heptadecafluorodecyltrichlorosilane, poly(tetrafluoroethylene), octadecyltrichlorosilane, methyltrimethoxysilane, nonafluorohexyltrimethoxysilane, vinyltriethoxysilane, ethyltrimethoxysilane and propyltrimethoxysilane and hexamethyldisilazane (HMDS).

[0066] Further preferred classes of molecules for forming a SAM on the backside surface of a substrate are cyclic azasilanes. Cyclic azasilanes meet the requirements of ‘click chemistry’ (i.e. there is sufficient thermodynamic driving force for the reaction to enable deposition of the SAM on the surface at or near room temperature, and with no by-products). This enables rapid formation of the SAM on the surface of the substrate backside without affecting throughput of the lithographic apparatus.

[0067] Preferred classes of SAM precursor molecules for forming a SAM on the burls of the substrate holder are silanes, phosphonates alkenes, alkynes, carboxylates and catechols.

[0068] When the burls of the substrate holder comprise SiC, the SAM precursor molecules are preferably silanes, phosphonates, alkenes and/or alkynes and most preferably silanes, alkenes and/or alkynes.

[0069] When the burls of the substrate holder comprise DLC, the SAM precursor molecules are preferably silanes, alkenes and/or alkynes.

[0070] When the burls of the substrate holder comprise CrN, the SAM precursor molecules are preferably silanes, phosphonates, carboxylates, catechols, alkenes and alkynes and most preferably silanes and phosphonates.

[0071] The SAMs can be applied by any method known in the art, for example by vapour phase reaction or by wet chemical application. The SAM can also be contact printed onto the substrate backside surface or the burls of the substrate holder, as exemplified in D. Qin et al. “Soft Lithography for Micro- and Nanoscale Patterning ” Nature Protocol, 5, 2010. Pp. 491 -502 and Schreiber. “Structure and growth of self-assembling monolayers” Progress in Surface Science 65 (2000) 15125.

[0072] When the SAM is applied from the gas phase to the surface, it is preferable to use a SAM precursor molecule with a low vapour pressure, for example hexamethyldisilazane (HMDS).

[0073] When the SAM is applied from the liquid phase to the surface, larger SAM precursor molecules with longer chain-lengths and higher vapour pressures can be used. Increased Van der Waals interactions between the long carbon chain tail results in increased ordering of the SAM tail, which minimises the surface energy of the coated surface.

[0074] The SAM can be contact printed onto the substrate holder surface or substrate backside surface. Thus, the SAM can be applied to specific regions of the substrate holder or substrate backside surface if required.

[0075] In a first aspect of the invention, the SAM is applied to the backside of a substrate.

[0076] A SAM can be applied to the surface of the substrate backside to lower the surface energy. This reduces the quantity of water that adsorbs to the substrate backside and is retained following cleaning and spin-drying of the substrate before exposing the substrate to radiation in the lithography apparatus.

[0077] In addition to lowering the surface energy of the substrate backside surface and lowering water adsorption, the SAM also minimises the migration of charges (‘proton hopping’) across the surface of the substrate backside (see Malhijssen, S., et al., “Charge Trapping at the Dielectric of Organic Transistors Visualized in Real Time and Space, Adv. Mater., 2008, 20: 975-979). Instead, any charges are localised at specific sites of the substrate backside. Hence, the charges cannot migrate over the surface of the substrate backside and reach the contact points between the substrate backside and the burls of the substrate holder. Therefore, it is less likely that any charges are transferred to the substrate holder, and the substrate holder is protected from oxidation and flatness degradation.

[0078] Typically, a flatness degradation of approximately 3.3 nm is observed for 1000 substrate passes through a lithographic apparatus.

[0079] When using a substrate with a backside coated with a HMDS monolayer and cleaned using ΗΌ prior to exposure, flatness degradation is reduced by a factor greater than 10 times, as compared to using untreated substrates following wet substrate cleaning. Thus, using substrates comprising backsides coated with hydrophobic SAMs is shown to significantly reduce the rate of flatness degradation of substrate holders supporting such substrates.

[0080] As shown in Figures 4 and 5, wafer load grid (WLG) is also reduced as a result of applying SAMs as set out above to the substrate backside before loading the substrate onto the substrate holder. Using SAMs comprising end-groups and/or substituents providing a low surface energy (such as alkyl and halogenated groups and substituents), rather than high surface-energy groups (such as the Si-OH groups found on the surface of untreated substrate backsides) means that low-surface energy groups are in contact with the burl material. These low surface-energy groups cannot form H-bonds or chemical bonds to the burls. This lowers the work of adhesion and the shear strength while loading the substrate, resulting in a lower friction and tin improved WLG. This improved WLG is particularly important for sensitive substrate holders.

[0081] Applying SAM substrate backside coatings has also been observed to improve the WLG of electrostatic clamps in EUV lithography apparatus. Figure 6 shows that for a sensitive electrostatic clamp, the WLG can be significantly reduced by decreasing the surface free energy of the substrate backside from about 70 ml/m² to about 45 mJ/m².

[0082] In general, the lower the surface energy of the SAM on the surface of the substrate backside, the lower the friction between the substrate backside and the substrate holder/electrostatic clamp and the lower the WLG.

[0083] The SAM can be implemented to the substrate backside at any point during the substrateprocessing cycle.

[0084] In one embodiment, the SAM is applied to the backside of the substrate before the backside of the substrate is cleaned with water. When the modified substrate backside is exposed to water, a reduction in the quantity of H2O present on the surface is observed. Less water adsorbs to the surface of the substrate backside, resulting in a reduction of charges.

[0085] In another embodiment, the SAM is applied to the backside of the substrate after the backside of the substrate is cleaned with water. In this case, it is preferably to use a straight-chain silane as the SAM precursor molecule. The surface of the backside of the substrate contidns H2O and charges which may react with the SAM, resulting in a surface with low surface energy. In this case, some residual charged sites (i.e. protons), and water may be present after applying the SAM precursor molecule as it may be challenging to obtain monolayer coverage of the entire substrate backside. As set out above, the charges are generally immobilised due to the presence of the SAM. Hence, when the substrate contacts the substrate holder, the chance of the charge being localised at the point of contact with a burl is minimal, and so the charge is unlikely to transfer from the substrate backside to the substrate holder. Consequently, a reduction of the rate flatness degradation of the substrate holder is observed.

[0086] It is preferably to dry the substrate backside surface using a flow of Ni or dry air before the application of the SAM.

[0087] To maintain throughput of the lithographic apparatus during device production, it is preferable for the SAM to be incorporated rapidly onto the substrate backside, and preferably in sync with the production throughput of the lithographic apparatus. Preferably, the monolayer is formed in a time of less than about 1 minute, preferably less than about 50 seconds, preferably less than about 40 seconds, preferably less than about 30 seconds, preferably less than about 20 seconds, preferably less than about 15 seconds, preferably less than about 12 seconds, preferably less than about 9 seconds and most preferably less than about 7 seconds.

[0088] Preferably, the monolayer is formed under room temperature conditions.

[0089] When the SAM is applied from the gas phase to the surface, it is preferable to use highlyreactive molecules to form the monolayer on the surface of the substrate backside. SAM precursor molecules which are able to undergo ‘click chemistry’ (i.e. where there is sufficient thermodynamic driving force for the reaction to enable deposition of the SAM on the surface at or near room temperature, preferably with no by-products) are most preferred. Classes of molecules suitable for such reactions include alkylthiols, azides, acetylenes, silanes and cyclic azasilanes.

[0090] It is preferable that during SAM formation, no by-products (such as H2O or methanol) are produced.

[0091] Preferably, the SAM is applied by using a cyclic azasilane as a SAM precursor molecule, which reacts with the backside of the substrate to form the desired SAM.

[0092] The cyclic azasilane preferably has the formula:

R²

wherein R^l, R², R’ and R⁴ are independently selected from the group consisting of hydrogen, Ci-20 alkyl, Cj-20 haloalkyl, C2-20 akenyl, Ci-20 amino, C1.20 alkoxy, C1.20 alkylthio, C1-20 alkylene C^o aryl, Cö-óo aryloxy, Có-m arylthio, C2-20 alkynyl, €2-20 acyl, C2-20 acyloxy, C2-20 oxycarbonyl, carboxyl groups, hydroxyl groups, nitro groups and cyano groups;

wherein n is 1,23,4, 5, 6,7, 8,9 or 10; and wherein one or more H atoms in the structure can optionally be substituted with a F atom.

[0093] For example, the cyclic azasilane may be N-methyl-aza-2,2,4-trimethylsilacyclopentane, N(2-aminoethyl)-2,2,4-trimethyl-l-aza-2-silacyclopentane, N-allyl-aza-2,2-dimethoxysilacyclopentane, 2-2-dimethoxy-l,6,-diaza-2-silacyclooctane or N-butyl-2,2-dimethoxy-l,2-azasilolidine or 1-butyl1,2-azasilolidine.

[0094] Cyclic azasilanes are known to react with hydroxyl groups in a ring-opening reaction in the timescale of a few seconds. The reaction is driven by the increase in bond strength of the Si-0 bond formed (570 kJ/mol) as compared to the Si-N bond of the cyclic azasilane (410 kJ/mol), and the ring strain of the cyclic azasilane SAM precursor molecule.

[0095] In addition to being able to maintain the throughput of the lithographic apparatus, a further advantage of the use of cyclic azasilanes is that no reaction by-products are formed during the grafting reaction.

[0096] When a cyclic azasilane SAM precursor is used to form the SAM, it is preferable to form the SAM by directing a N2 flow saturated with the cyclic azasilane at the backside of the substrate. The cyclic azasilane reacts with hydroxyl groups of the native oxide layer on the substrate backside.

[0097] Preferably, a SAM layer of with a thickness in the range of about 0.5 nm to about 1.5 nm is formed.

[0098] The SAM precursor can be applied to the substrate backside in the scanner of the lithographic apparatus to form the SAM. Preferably, the SAM precursor is applied to the substrate backside in the store unit of the lithographic apparatus.

[0099] Preferably, the SAM precursor is applied to the substrate backside for a time of less than about 30 seconds, preferably less than about 25 seconds, preferably less than about 20 seconds, preferably less than about 15 seconds, preferably less than about 12 seconds, preferably less than about 9 seconds and most preferably less than about 7 seconds.

[0100] The SAM precursor (which is preferably a cyclic azasilane) can be applied to the substrate backside at various points to form the SAM. In one embodiment (as shown in Figure 7), the SAM precursor is applied to the burl tops as a dry-Na flow saturated with the SAM precursor (76) and then transferred to the substrate backside while loading the substrate (71) on to the substrate holder (75) using a chuck (72). In this embodiment, the SAM precursor distributes underneath the substrate while loading it onto the set point (74) of the substrate holder. The SAM precursor can be applied to the burl tops through pre-clamp holes (77) of the substrate holder (75). In another embodiment, the SAM precursor is applied to the substrate while the substrate is fully clamped by means of surface diffusion across the backside of the substrate.

[0101] As shown in Figure 8, cyclic azasilanes can be used to modify the surface energy of the substrate backside within 1 second at room temperature (i.e. using a ‘click-chemistry’ approach). This figure shows that the contact angle (measured with water) increases from about 5° up to about 58° after about 1 second of contact between a substrate backside surface and a N2 flow saturated in N n-butyl-aza-2,2-dimethoxysilacyclopentane. Over the same timescale, the surface energy of the substrate backside surface decreases from about 78 mJ/m² to about 52 mJ/m², This provides a decrease in the WLG of from about 6 nm to about 3.5 nm for a sensitive table, and from about 1.5 nm to about 0.6 nm for a sensitive clamp. The SAM formed also reduces the substrate holder flatness degradation of substrate holders used with such substrates by a factor of greater than 10 times.

[0102] After exposure of the substrate to radiation, it is preferable to remove the SAM from the backside of the substrate before conducting further processing steps.

[0103] The SAM is preferably removed using liquid backside stripping methods, or by gas-phase stripping which is directed toward the substrate backside, [0104] In one embodiment, liquid backside stripping is used to remove the SAM from the substrate backside. When the substrate backside has a silicon oxide surface, it is preferable to use hydrofluoric acid (HF), or more preferably buffered oxide etch solution. In this case, Si-F groups are formed on the silicon surface instead of reactive Si-OH groups. This ensures that the surface becomes hydrophobic, reducing the quantity of water that the surface adsorbs in any subsequent steps. When the substrate backside has a silicon nitride surface, it is preferable to use concentrated hot orthophosphoric acid (H3PO4) at a temperature in the range of from about 150 °C to about 180 °C. It is also preferable to perform a second clean with HF after cleaning with phosphoric acid, to remove any silicon oxide layers underneath and to make the surface hydrophobic. Alternatively, APM/SPM cleaning can be used to remove the SAM. First, a mixture of ammonium hydroxide with hydrogen peroxide and deionised water (APM) in a ratio of from about 1:1:5 to about 1:2:7 is applied to the surface of the substrate. This solution removes particles and organic contaminants, and also breaks the bonds of silane groups. Second, a mixture of hydrogen chloride, hydrogen peroxide and deionised water (SPM) in a ratio of from about 1:1:6 to about 1:2:8 is applied to the surface of the substrate. This solution removes metallic contaminants. These cleaning techniques are typically performed at from about 75 °C to about 85 °C for from about 1 to about 5 minutes.

[0105] Gas phase stripping may also be used to remove the SAM from the substrate backside. For example, gas phase stripping is preferable when liquid backside stripping is not compatible with the topside of the substrate. When gas phase stripping is used, it is preferably directed towards the backside of the substrate. As set out in Ju, L ei al. Cyclic Azasilanes as Volatile and Reactive Precursors for Atomic Layer Deposition of Silicon Dioxide”, Journal of Materials Chemistry C, 4, 4034-4039, 20.16, oxidation of SAMs with O3 at elevated temperature affords silanol groups and removes the SAMs in a timescale of about 30 seconds.

[0106] As set out above, both WLG and substrate flatness degradation are a result of contact between two surfaces - the substrate backside and the burls of the substrate holder. Thus, while reducing the surface energy and hydrophobicity of the substrate backside reduces the WLG and improves substrate flatness degradation, modulating the properties of the substrate holder is also beneficial.

[0107] Accordingly, in another aspect of the invention the SAMs set out above are applied to the burls of a substrate holder used in a lithographic apparatus. The SAMs may be applied using any of the SAM precursor molecules set out above.

[0108] The SAM can be applied to the burls of the substrate holder before entering the substrate holder into the machine, or during maintenance of the machine.

[0109] The SAM can be applied to the burls of the substrate holder following mechanochemical wear of the substrate holder burl material during normal use of the lithographic apparatus.

[0110] Applying SAMs to the surface of the burls of the substrate holder will also protect the substrate holder from contamination, for example from resist-like particles sticking to burl tops. This will improve the focus spot sensitivity of the apparatus.

[0111] In addition, applying a SAM to the surface of the burls provides additional advantages. For example, chemical oxidation resistance of the burls can be increased by tuning the properties of the SAM e.g. through modifying tail-group length and substituents.

[0112] As the monolayers disclosed herein are compressible under high loads (full clamp pressure), the impact of a monolayer of the flatness of the substrate holder surface is negligible.

[0113] When the burls of the substrate holder comprise SiC or SiSiC with a native oxide layer, silanes (for example, cyclic azasilane) can be used to apply the SAM to the surface of the substrate holder.

[0114] Silanes (including cyclic azasilanes) bind specifically to Si-containing surfaces containing oxides, such as SiC or SiiN.j. Thus, when forming a monolayer on a Si-based substrate holder surface using a silane as the SAM precursor, the silane will bind specifically to the Si-containing surface, enabling the monolayer to be replenished without build-up of materials on the substrate holder. Any excess SAM precursor is preferably removed from the substrate holder, for example by using an extraction system, for example a vacuum. In this case, the surface of the substrate holder can optionally be treated with an oxygen plasma before exposing the substrate holder surface to the silane SAM precursor molecules. This improves the site coverage during the SAM-forming reaction between the substrate holder surface and the SAM precursor molecules.

[0115] When the burls of the substrate holder comprise SiC, SiSiC or DLC, n-alkenes or alkynes can be used to apply a SAM to the surface of the substrate holder, for example as discussed on pages 6339 to 6345 of Pujari, S. P. et al. “Covalent Surface Modification of Oxide Surfaces”. Angew. Chem. hit. Ed., 2014, 53: 6322-6356. In this case, heat or light (for example, ultraviolet light) is used to activate the reaction. Optionally, a surface treatment with hydrogen plasma can be used to improve the site coverage during the reaction.

[0116] In one embodiment, as shown in Figure 9 the SAM is applied to the surface of the burls by bubbling N? or dry air (CDA/XCDA) through a solution comprising the SAM precursor molecule present at e.g. the clamp-free (CF) unit and supplied via pre-clamp holes (94) (or optionally the water extraction seal (WES) holes (96)) while a substrate (91) is present on or slightly above the substrate holder (92), optionally supported by one or more E-pins (93,95). In Figure 9, the E-pin is shown in the centre of the substrate. However, the one or more E-pins may also be at the outer edge of the substrate. The SAM precursor molecules can be applied such that the volume underneath the substrate acts as a closed reaction vessel. Lowering the substrate distributes the SAM precursor underneath the substrate. WES extraction may also be used to distribute the SAM precursor molecules over the substrate holder surface. It is possible to have a high overpressure in the preclamp area while using WES extraction so that the substrate forms into an umbrella shape, and burl tops are covered by the SAM precursor molecules. The SAM precursor molecules may also reach the burl tops by means of surface diffusion, even when the substrate is fully clamped.

[0117] Optionally, the substrate can be treated before applying the SAM precursor molecules to the substrate holder. For example, a substrate backside material or coating that does not contain silicon can be applied to the substrate to prevent the substrate backside from reacting with the SAM precursor molecules. Optionally, an umbrella-shaped substrate be used to provide a seal during application of the SAM and thus prevent diffusion of chemicals to the environment. When a SAM precursor is used which requires activation by UV light, such as a n-alkene SAM precursor, the substrate is preferably transparent to UV light.

[0118] In a second embodiment, as exemplified in Figure 10, the SAM is applied to the surface of the burls (102) by a device (103) moving over the burls (102). This method may be used when the substrate holder (101) is first installed, or after a substrate has been removed from the substrate holder during operation of the lithographic apparatus. Preferably, the device moves over the surface of the burls at a height of less than about 60 pm, preferably less than about 50 pm, preferably less than about 40 pm and most preferably less than about 30 pm above the surface of the burls. Preferably, gas containing the SAM precursor (105) is applied in the centre of the device and air extraction takes place at the edge of the device (104) to prevent unreacted SAM precursor from escaping to the environment. Optionally, a dry gas flow' of N₂ or dry air may be applied to pre-dry the substrate holder surface and reduce the risk of diffusion of the SAM precursor outside the contained volume. The device preferably uses a dual-seal layout as depicted in Figure 10, but may have one outer seal or no seals. Preferably, the application of the SAM to the substrate holder takes less than about 5 minutes, preferably less than about 4 minutes, preferably less than about 3 minutes, preferably less than about 2 minutes and most preferably about I minute.

[0119] In a third embodiment, as exemplified in Figure 11 the SAM is applied to the surface of the burls (112) by microcontact printing by liquid-phase stamping, for example by using a polydimethylsiloxane (PDMS) stamp (113). A thin layer of the SAM precursor (114) may be applied to the bottom of the stamp (113), before the stamp is applied to tops of the burls (112). After one or more applications of the PDMS stamp (113) to the surface of the burls (112), more SAM precursor (114) may be applied to the bottom of the stamp (113) and the process repeated until a SAM has formed on at least a portion of the burls (112) of the substrate holder (111). The stamp may include capillaries to feed the bottom of the stamp with SAM precursor. Optionally, the air may be removed from the surface of the substrate holder, for example by applying a vacuum, to remove any unreacted SAM precursor that does not react with the substrate holder surface. The unreacted SAM precursor may then be recycled back to the PDMS stamp.

[0120] Using this microcontact printing approach allows self-assembled monolayers to be applied locally. Thus, it is possible to obtain different interaction parameters (such as surface energy) at different positions of the substrate holder.

[0121] The apparatus used for each of these embodiments of applying the SAM to the substrate holder surface may also be used to supply other substances to the surface of the substrate holder. For example, a cleaning substance, and preferably ozone and/or oxygen plasma, may be applied to remove any existing monolayers on the surface of the substrate holder. This may be desirable if, for example, the existing monolayer has partially worn off, or lost some of its beneficial characteristics. A new monolayer can then be applied, and the beneficial characteristics restored. Alternatively, surface pretreatment substances, and preferably oxygen plasma, may be applied to the substrate holder in a surface pre-treatment step before applying the SAM precursor. In this way, the properties of the substrate holder surface can be tuned to ensure a higher surface coverage of the SAM following application of the SAM precursor, allowing the surface energy of the substrate holder to be minimised.

[0122] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of integrated circuits (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, liquid-crystal displays (LCDs), thin film magnetic heads, etc.

[0123] 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. Tirus 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 clauses set out below. Other aspects of the invention are set out as in the following numbered clauses:

1. A method of protecting a substrate holder, the method comprising:

a) applying a first self-assembled monolayer (SAM A) to the backside surface of a substrate intended for use with the substrate holder, wherein SAM A lowers the surface free energy of the substrate backside surface; and wherein SAM A is applied to the substrate backside surface by reacting the substrate backside surface with a first SAM precursor molecule; and/or

b) applying a second self-assembled monolayer (SAM B) to the surface of at least a portion of the burls of the substrate holder, wherein SAM B lowers the surface free energy of the surface of the burls; and wherein SAM B is applied to the surface of the burls by reacting the surface of the burls with a second SAM precursor molecule,

2. The method of clause 1., wherein the first SAM precursor molecule is selected from a silane, a phosphonate, a carboxylate, a catechol, an alkene or an alkyne, and/or the second SAM precursor molecule is selected from a silane, a phosphonate, an alkene, an alkyne, a carboxylate or a catechol.

3. The method of clause 1 or clause 2, wherein the first SAM precursor molecule is selected from a silane, an alkene or an alkyne, and/or the second SAM precursor molecule is selected from a silane, a phosphonate, an alkene or an alkyne.

4. The method of any one of clauses 1 to 3, wherein the first and/or second SAM precursor molecule is a cyclic azasilane.

5. The method of clause 4, wherein the cyclic azasilane has the formula:

R²

wherein R¹, R², R³ and R⁴ are independently selected from the group consisting of hydrogen, C1-20 alkyl, C]_20 haloalkyl, C2-20 akenyl, C1-20 amino, Ci-20 alkoxy, Ci-20 alkylthio, Ci .20 alkylene Cè-60 aryl, Ce-co aryloxy, Cs-eo arylthio, C2-20 alkynyl, C2-20 acyl, C2-20 acyloxy. C2-20 oxycarbonyl, carboxyl groups, hydroxyl groups, nitro groups and cyano groups; wherein n is 1, 2 3,4, 5, 6, 7, 8,9 or 10; and wherein one or more H atoms in the structure can optionally be substituted with a F atom.

6. The method of clause 4 or clause 5, wherein, the cyclic azasilane is N-methyl-aza-2,2,4trimethylsilacyclopentane, N-(2-aminoethyl)-2,2,4-trimethyl-1 -aza-2-silacyclopentane, Nallyl-aza-2,2-dimethoxysilacyclopentane, 2-2-dimethoxy-l,6,-diaza-2-silacyclooctane or N-butyl-2,2-dimethoxy-l,2-azasilolidine, N-n-butyl-aza-2,2-diinethoxysilacyclopentane or 1 -butyl-1,2-azasilolidine.

7. The method of any one of clauses 1 to 6, wherein the first and/or second SAM precursor molecule is applied by applied by vapour phase reaction, wet chemical application or contact printing.

8. The method of any one of clauses 1 to 7. wherein the first and/or second SAM precursor molecule is applied in a time of lower than about 12 seconds.

9. The method of any one of clauses 1 to 8, wherein the first and/or second SAM precursor molecule is applied using a gaseous flow comprising N2 and the first and/or second SAM precursor molecule.

10. The method of any one of clauses 1 to 9, wherein SAM A is applied to the substrate backside surface in a scanner of a lithographic apparatus.

11. The method of any one of clauses 1 to 10, wherein SAM A is applied to the substrate backside surface in a store unit of a lithographic apparatus.

12. The method of any one of clauses 1 to 11, wherein SAM A is applied to the substrate backside surface in a time of less than about 10 seconds.

13. The method of any one of clauses 1 to 12, wherein the backside surface of the substrate is cleaned with water before the application of SAM A to the substrate backside surface.

14. The method of any one of clauses 1 to 9, wherein the backside surface of the substrate is cleaned with water after the application of SAM A to the substrate backside surface.

15. The method of any one of clauses 1 to 14, wherein the substrate comprises one or more of silicon, silicon carbide, gallium nitride, gallium arsenide or aluminium titanium carbide.

16. The method of any one of clauses 1 to 15, wherein the second SAM precursor molecule is a silane, and wherein SAM B is applied to the burls of the substrate holder after treating the surface of the substrate holder with an oxygen plasma.

17. The method of any one of clauses 1 to 15, wherein the second SAM precursor molecule is a N-alkene, and wherein SAM B is applied to the burls of the substrate holder after treating the surface of the substrate holder with a hydrogen plasma.

18. The method of any one of clauses 1 to 17, wherein the burls of the substrate holder comprise one or more of diamond-like carbon (DLC), diamond, graphite, SiSiC, SiC, Zerodur, AbCh.TiN, Lipocer, SST and/or CrN.

19. The method of any one of clauses 1 to 18, wherein SAM B is applied to the burls of the substrate holder by applying a flow comprising the second SAM precursor molecule through the pre-clamp holes of the substrate holder.

20. The method of clause 19, wherein the flow additional comprises N2 and/or dry air,

21. The method of any one of clauses 1 to 18, wherein SAM B is applied to the burls of the substrate holder by a device moving over the burls.

22. The method of any one of clauses 1 to 18, wherein SAM B is applied to the burls of the substrate holder by liquid-phase stamping using a polydimethylsiloxane (PDMS) stamp.

23. The method of any one of clauses 1 to 22, wherein the reaction between the first SAM precursor and the substrate backside surface produces no by-products.

24. The method of any one of clauses 1 to 23, additionally comprising drying the substrate holder and/or the backside surface of the substrate using a flow of N2 or dry air before the application of SAM A and/or SAM B.

25. The method of any one of clauses 1 to 24, wherein the surface free energy of the backside surface of the substrate after application of SAM A is less than about 50 mJ/in², and/or the surface free energy of the burls of the substrate holder covered by SAM B is less than about 50 mJ/m² after application of SAM B.

26. An apparatus configured to apply the method of any one of clauses 1 to 25.

27. A substrate holder with a surface comprising burls, wherein at least a portion of the burls comprise a self-assembled monolayer (SAM B), wherein SAM B is a hydrophobic SAM.

28. The substrate holder of clause 27, wherein the surface free energy of the burls of the substrate holder covered by SAM B is less than about 50 mJ/m².

29. The substrate holder of clause 27 or clause 28, wherein SAM B comprises at least one silicon atom.

30. The substrate holder of any one of clauses 27 to 29, wherein the burls of the substrate holder comprise one or more of diamond-like carbon (DLC), diamond, graphite, SiSiC, SiC, Zerodur. ALOj.TiN, Lipocer, SST and/or CrN.

31. A substrate, wherein the backside of the substrate comprises a self-assembled monolayer (SAM A), wherein SAM A is a hydrophobic SAM.

32. The substrate of clause 31, wherein the surface free energy of the substrate backside surface covered with SAM A is less than about 50 mJ/m².

33. The substrate of clause 31 or clause 32, wherein the substrate comprises one or more of silicon, silicon carbide, gallium nitride, gallium arsenide or aluminium titanium carbide.

34. The substrate of any one of clauses 31 to 33, wherein SAM A comprises at least one silicon atom.

35. The substrate of any one of clauses 31 to 34, wherein SAM A comprises at least one nitrogen atom.

36. A method comprising processing a substrate supported by a substrate holder, wherein the surface of the backside of the substrate comprises a first self-assembled monolayer (SAM A) and/or at least a portion of the burls of the substrate holder comprise a second selfassembled monolayer (SAM B), wherein SAM A and SAM B are hydrophobic SAMs.

37. The method of clause 36, wherein the surface free energy of the substrate backside surface coated with SAM A is less than about 50 mJ/m².

38. The method of clause 36 or clause 37, wherein the surface free energy of the surface of the burls coated with SAM B is less than about 50 mJ/m².

39. The method of any one of clauses 36 to 38, wherein the burls of the substrate holder comprise one or more of diamond-like carbon (DLC), diamond, graphite, SiSiC, SiC, Zerodur, AhO;.TiN. Lipocer, SST and/or CrN.

40. The method of any one of clauses 36 to 39, wherein the substrate comprises one or more of silicon, silicon carbide, gallium nitride, gallium arsenide or aluminium titanium carbide.

41. The method of any one of clauses 36 to 40, wherein SAM A and/or SAM B comprises at least one silicon atom.

42. The method of any one of clauses 36 to 41, wherein SAM A and/or SAM B comprises at least one nitrogen atom.

43. A method of fabricating a device, the method comprising applying the method according to any one of clauses 36 to 42.

44. A method of removing a self-assembled monolayer SAM A from the backside of a substrate after exposure of the substrate to radiation in a lithographic apparatus, the method comprising liquid backside stripping and/or gas-phase stripping of the backside of the substrate.

45. The method of clause 44, wherein SAM A is removed from the backside of the substrate by gas-phase stripping with ozone.

Claims (3)

CONCLUSION A device adapted to illuminate a substrate. 1/7

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