WO2021122065A1 - 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

IMPROVED LITHOGRAPHY METHODS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 19217942.2 which was filed on December 19, 2019 and EP application 20185473.4 which was filed on July 13, 2020 which are incorporated herein in its entirety by reference.

FIELD

[0002] 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

[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 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.

[0004] 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. [0005] 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).

[0006] 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.

[0007] Substrate holders used during device fabrication typically comprise burls, which reduce the influence of contamination by particles on the backside of the substrate.

[0008] 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.

[0009] Before loading a substrate into the lithography apparatus, the backside of the substrate is cleaned with a ¾0 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.

[0010] Due to the high surface energy of the substrate backside, water adheres to the surface, leaving adsorbed monolayers of ¾0 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.

[0011] 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

[0012] 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.

[0013] 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.

[0014] The present invention also relates to an apparatus configured to apply the above method. [0015] 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.

[0016] 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.

[0017] 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 self- assembled 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. [0018] The present invention also relates to a method of fabricating a device, the method comprising applying the above method.

[0019] 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.

[0020] 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

[0021] Figure 1 depicts a schematic illustration of a lithographic system comprising a lithographic apparatus and a radiation source.

[0022] Figure 2 depicts a schematic overview of a lithographic cell.

[0023] Figure 3 depicts a substrate on a substrate support in a lithographic apparatus.

[0024] Figure 4 shows how the surface free energy of the substrate backside coating affects substrate holder WFG.

[0025] 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 are in nm per thousand substrate passes.

[0026] Figure 6 shows how the surface free energy of the substrate backside affects electrostatic clamp WFG during EUV lithography.

[0027] Figure 7 shows a method of application of a SAM precursor to the backside of a substrate. [0028] Figure 8 shows how the contact angle changes upon application of a SAM according to the present invention.

[0029] Figures 9 to 11 show methods of application of a SAM precursor to the burls of a substrate holder.

DETAIFED DESCRIPTION OF THE INVENTION

[0030] Figure 1 schematically depicts a lithographic apparatus FA. The lithographic apparatus includes an illumination system (also referred to as illuminator) IF 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.

[0031] 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.

[0032] 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 ¾, 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.

[0033] The 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.

[0034] 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.

[0035] 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. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS. [0036] 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 PI, P2.

Although the substrate alignment marks PI, P2 as illustrated occupy dedicated target portions, they may be located in spaces between target portions. Substrate alignment marks PI, P2 are known as scribe-lane alignment marks when these are located between the target portions C.

[0037] 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.

[0038] 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/Ol, 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.

[0039] 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.

[0040] In an embodiment the store unit comprises a substrate support 20. In an embodiment, the substrate support 20 is arranged in the lithographic apparatus LA as the substrate table WT to support the substrate W during, for example, the exposure process. Figure 3 depicts a substrate W on the substrate support 20. The substrate support 20 is configured to support the substrate W.

[0041] 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. The base surface 23 of the substrate support 20 comprises a plurality of burls 22. 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.

[0042] 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.

[0043] Contact between the substrate backside 25 and the burl material at the top of the burl 24 in the presence of water, for example, at the gap 26 between the substrate W and the base surface 23, can lead to electrochemical corrosion of the burl material, for example the diamond-like carbon (DLC) coating of the burls 22. 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 W. Thus, the substrate holder 20 may no longer meet the strict performance requirements required for lithographic apparatus LA and must be replaced. Minimising the rate of burl corrosion is therefore key to maximising substrate holder life and efficiency of device production.

[0044] In addition to the above, during loading the substrate W makes contact with certain points of the substrate holder 20, which can introduce displacement errors in the x and y directions. These errors have been observed to drift over time and are reflected in a wafer load grid (WLG) drift.

[0045] Excluding the wet substrate cleaning step results in no measurable flatness degradation after, for example, 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 LA.

[0046] 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. [0047] 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. [0048] 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 , the corrosion of the carbon-based layers results in dissolution of the oxide, both in solution and possibly in gaseous species such as CO2, ¾0 and CO.

[0049] Thus, neither applying a cathodic overpotential nor creating a passivation layer are generally applicable to all substrate holder materials.

[0050] 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 self- assembled monolayers (SAMs) to significantly decrease the rate of substrate holder flatness degradation and or wafer load grid.

[0051] 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.

[0052] The substrate holder comprises one or more of DLC, diamond, graphite, SiSiC, SiC, Zerodur, AEO_B,TIN, Lipocer, SST and/or CrN, and preferably comprises one or more of DLC, diamond, graphite, SiSiC, SiC and or CrN.

[0053] 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.

[0054] The 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.

[0055] 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.

[0056] The contact angle values disclosed herein are determined using the sessile drop method, with the contact angles measured using drop shape analysis (DSA).

[0057] 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.

[0058] 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, -CX3, -CHX2 or -CH2X, wherein each instance of X is independently selected from F, Cl, Br or I.

[0059] 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.

[0060] 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.

[0061] 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 tail- groups 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 perpendicular ordering is to use SAMs with long, hydrophobic tail-groups which promote strong intermolecular 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.

[0062] 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.

[0063] 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 silanes, silazanes, phosphonates, carboxylates, catechols, alkenes, alkynes.

[0064] Preferred classes of SAM precursor molecules for forming a SAM on the backside surface of a substrate are silanes (including cyclic silanes), silazanes, alkenes and alkynes.

[0065] 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.

[0066] Examples of silanes and silazanes which may be used in the present invention as the SAM precursor molecules to form the SAM include heneicosafluorododecyltrichlorosilane, heptadecafluorodecyltrichlorosilane, poly(tetrafluoroethylene), octadecyltrichlorosilane (OTS), methyltrimethoxysilane, nonafluorohexyltrimethoxysilane, vinyltriethoxysilane, ethyltrimethoxysilane, propyltrimethoxysilane, and hexamethyldisilazane (HMDS).

[0067] Further preferred classes of molecules for forming a SAM on the backside surface of a substrate are cyclic silanes, for example, cyclic azasilane. Cyclic silanes 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.

[0068] Preferred classes of SAM precursor molecules for forming a SAM on one or more burls of the substrate holder are silanes, silazanes, phosphonates alkenes, alkynes, carboxylates and catechols. [0069] When the burls of the substrate holder comprise SiC, the SAM precursor molecules are preferably silanes, silazanes, phosphonates, alkenes and/or alkynes and most preferably silanes, silazanes, alkenes and or alkynes.

[0070] When the burls of the substrate holder comprise DLC, the SAM precursor molecules are preferably silanes, silazanes, alkenes and or alkynes.

[0071] When the burls of the substrate holder comprise CrN, the SAM precursor molecules are preferably silanes, silazanes, phosphonates, carboxylates, catechols, alkenes and alkynes and most preferably silanes, silazanes, and phosphonates.

[0072] 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) 151- 25.

[0073] 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, a silazane, e.g., hexamethyldisilazane (HMDS).

[0074] 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.

[0075] The SAM can be contact printed onto the substrate holder surface (for example, onto a surface of substrate support 20) or onto substrate backside surface (for example, onto substrate backside 25). Thus, the SAM can be applied to specific regions of the substrate holder or substrate backside surface if required.

[0076] In a first aspect of the invention, the SAM is applied to the backside surface of a substrate W.

[0077] According to an embodiment, in reference to Figure 3, an atmosphere 27 between the substrate backside 25 and the base surface 23 may comprise a gas mixture comprising a SAM. The gas mixture may be conditioned by the supply and extraction channels 28 provided at the substrate support 20. Herewith, a SAM may be applied to the backside of the substrate W. For example, the gas mixture may comprise nitrogen gas and a SAM. The gas mixture may be nitrogen gas saturated with the SAM. The substrate support 20 may comprise a plurality of supply and extraction channels 28 to provide the gas mixture evenly at the substrate backside.

[0078] A SAM may 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.

[0079] 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 Mathijssen, 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, i.e., charge immobilization.

Hence, the charges cannot migrate over the surface of the substrate backside and reach the contact points between the substrate backside (for example, substrate backside 25) and the burls (for example, the burls 22) of the substrate holder (for example, the substrate support 20). 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.

[0080] Typically, a flatness degradation of approximately 3.3 nm is observed for 1000 or more substrate passes through a lithographic apparatus.

[0081] When using a substrate with a backside coated with a HMDS monolayer and cleaned using H2O 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.

[0082] 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 an improved WLG. This improved WLG is particularly important for sensitive substrate holders.

[0083] 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 mJ/m² to about 45 mJ/m² .

[0084] 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.

[0085] The SAM can be implemented to the substrate backside at any point during the substrate processing cycle.

[0086] 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 ¾0 present on the surface is observed. Less water adsorbs to the surface of the substrate backside, resulting in a reduction of charges.

[0087] 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 contains ¾0 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.

[0088] It is preferably to dry the substrate backside surface using a flow of N2 or dry air before the application of the SAM.

[0089] 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.

[0090] Preferably, the monolayer is formed under room temperature conditions.

[0091] When the SAM is applied from the gas phase to the surface, it is preferable to use highly - reactive 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, cyclic silanes, and silazanes,.

[0092] It is preferable that during SAM formation, no by-products (such as ¾0 or methanol) are produced.

[0093] In an embodiment, the SAM is applied by using a cyclic silane, for example a cyclic azasilane, as a SAM precursor molecule, which reacts with the backside of the substrate to form the desired SAM.

[0094] The cyclic azasilane typically has the formula:

wherein R¹, R², R³ and R⁴ are independently selected from the group consisting of hydrogen, Ci-20 alkyl, Ci-20 haloalkyl, C2-20 akenyl, Ci-20 amino, Ci-20 alkoxy, Ci-20 alkylthio, Ci-20 alkylene Ce-eo aryl, Ce-60 aryloxy, Ce-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, 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 Fluor (F) atom. [0095] 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 -butyl- 1, 2-azasilolidine.

[0096] 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-O 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.

[0097] 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.

[0098] When, for example, 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.

[0099] Cyclic azasilanes possess a Nitrogen (N) atom that, after grafting, may potentially form hydrogen bonds with the wafer surface, forcing the layers to lay down. This may result in a higher friction, and thus in a higher WLG. Further, in a similar way, the exposed N-atom may also react with the countersurface, i.e., with the substrate support WT, upon wafer loading.

[0100] In addition to cyclic azasilane, cyclic thiasilanes may also be used as precursor that reacts with the hydroxyl groups of a silicon surface. Cyclic thiasilane comprises an Sulphur (S) atom, as the heteroatom, belonging to group 16 of the periodic table, having electronic configuration ns2np4. This means that in the cyclic thiasilane, the S-atom will form two bonds, one with the Si and the other with one C-atom, having 2 pairs of (so-called) lone electrons. The result is that a SH-group is exposed upon the ring opening reaction with no room to expose a more hydrophobic and inert carbon tail at the outermost surface in one step, or tune the properties of the surface.

[0101] Other known cyclic silanes contain Selenium (Se) or Tellurium (Te) as the ring heteroatom, which may induce a similar side-effect as the cyclic thiasilanes.

[0102] It will be appreciated by the skilled person, that the inventors noticed that cyclic silanes comprising an element of the group 13 or group 14 of the periodic table as the ring heteroatom may also be used as SAM precursor molecules, which may be beneficial to avoid or to mitigate the potential side-effects of cyclic azasilane and cyclic thiasilane. For example, the heteroatom may be Boron (B), Carbon (C), Silicon (Si), or Germanium (Ge). Also for these cyclic silanes, the reaction is driven by the increase in bond strength of the Si-O bond formed (570 kJ/mol) as compared to the Si-B bond (317 kJ/mol), the Si-C bond (447 kJ/mol), and the Si-Si bond (310 kJ/mol) of the cyclic silane, and the ring strain of the cyclic silane SAM precursor molecule. Hence, the ring opening reaction is thermodynamically driven, which would speed up the reaction together with the ring strain release. The molecule may have different number of C atoms (4, 5, 6, etc. membered ring), which can be used to tune the molecule reactivity.

[0103] The different electronic configuration of elements in groups 13 and 14: ns2npl and ns2np2, respectively, allows for the grafting of organic molecules with a carbon-based organic tail (exposed at the outermost surface) and (potentially) facilitates the implementation of branched molecules. Also, the lower electronegativity, compared to N, may be exploited to prevent the formation of H-bonds with the hydroxyl groups at the wafer surface.

[0104] As set out above, the SAM precursor may be a cyclic silane comprising a heteroatom selected from the elements belonging to group 13, 14, 15, or 16 of the periodic table. For example, the cyclic silane comprise a heteroatom that is one of B, C, Si, Ge, N, S, Se, and Te.

[0105] According to an embodiment, a SAM layer of with a thickness in the range of about 0.5 nm to about 1.5 nm is formed.

[0106] 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 (for example, the lithographic apparatus LA as illustrated in Figure 1). [0107] 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.

[0108] The SAM precursor, which is may be a cyclic silane or a silazane, 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-N2 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 one or more E-pins 72. In this embodiment, the SAM precursor distributes underneath the substrate while loading it onto the set point 74 of the substrate holder 75. 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.

[0109] As shown in Figure 8, cyclic silanes 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 as formed also reduces the substrate holder flatness degradation of substrate holders used with such substrates by a factor of greater than 10 times.

[0110] After exposure of the substrate to radiation during the exposure process, it is preferable to remove the SAM from the backside of the substrate before conducting further processing steps.

[0111] The SAM is preferably removed using liquid backside stripping methods, or by gas-phase stripping, which is directed toward the substrate backside.

[0112] 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.

[0113] 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 et al. “Cyclic Azasilanes as Volatile and Reactive Precursors for Atomic Layer Deposition of Silicon Dioxide Journal of Materials Chemistry C, 4, 4034-4039, 2016, oxidation of SAMs with (¾ at elevated temperature affords silanol groups and removes the SAMs in a timescale of about 30 seconds.

[0114] Cyclic silanes are highly reactive organic molecules able to react with the hydroxyl groups of wafer backside surfaces through a click-chemistry reaction. However, while the whole trick is to choose highly reactive molecules, this may also translate in a reduced stability. In particular, cyclic silanes are highly reactive with water, resulting in the ring opening reaction of the silane molecule. Further, the contact of cyclic silane with water should be avoided in any process step, prior to the grafting, and the water monolayers and/or water droplets that may be present at the wafer backside will alter the chemical reaction, potentially affecting for instance the layer uniformity or reaction time. [0115] Instead of using a cyclic silane, a silazane may be selected as the SAM precursor. Silazanes are well known in the semiconductor industry as a photoresist adhesion promoter in the form of HMDS (hexamethyldisilazane). HMDS is commonly applied at the wafer top side, prior to spin coating the photoresist. Although HMDS coating results in a contact angle of approximately 70 degrees, fluorinated silazanes reach contact angles up to 100 degrees upon grafting on silicon oxides surfaces. This means that fluorinated silazanes have the potential of overcome some of the cyclic silane’ s possible drawbacks. Also, this type of chemistry is less water sensitive, facilitating the application of the coating from an implementation point of view.

[0116] Silazanes can be applied for a fast change of the surface free energy of the wafer backside surface at room temperature, without surface pre-treatment of the substrate. Silazanes, as HMDS, react with the wafer surface through hydrolysis and condensation reactions with the hydroxyl (OH) groups present at the native oxide layer at the wafer backside.

[0117] Different types of silazanes may be used. The silazane may be selected from fluorinated silazanes. The silazane may also contain other functional groups, such as benzene rings.

[0118] In addition to changing (or modifying) the surface free energy by means of applying SAMs at a surface, a (temporarily) nano-roughness at the surface can be realized by providing molecules of different length. Nano-roughness may be achieved by applying a SAM layer comprising at least two molecular species of different length to at least one of the surfaces of interest (the substrate backside, the burl top area), a nano-roughness is created. From literature, it is known that by increasing the nano-roughness, the effective work of adhesion is reduced, as discussed on page 156 of C.M. Mate and R.W .Carpick “Tribology on the small scale, 2nd edition, Oxford University Press Applying such a nano-roughness layer may be beneficial to reduce the WLG.

[0119] In embodiment, a relatively long molecular species is applied with a length in the range of 1 to 4 nm, and a relatively short molecular species in the range of 0.2 to 0.5 nm is applied at the surface, for example the substrate backside 25. Preferably, both relative long and relatively short molecular species are applied at the surface at the same time in order to realize a heterogenous surface resulting in a random surface roughness. Preferably these molecules are mixed upfront and deposited from the vapor phase onto the surface. Alternative deposition technologies (from solution) are also possible. [0120] Alternatively, relative long and relatively short molecular species are applied at the surface sequentially. For example, starting with the long molecular species with a certain surface coverage, after which the short molecules fill the open gaps. The SAMs with varying height (comprising at least two molecular species of different length) may be provided at a specified location by, for example, imprinting using soft conformable imprint lithography.

[0121] In general, the length and the ratios for the molecular species may be tuned in order to obtain the desired surface roughness at the surface of interest.

[0122] Alternatively, by combining the relatively long molecular species (1-4 nm) with low sites of bare Si substrate a surface with nano-roughness may be formed. This may, for example, be realized by formation of the well-known standing-up phase by OctadecylTrichloroSilane (OTS) SAMs by the formation of island growth (domains). The size of the domains may be tuned by the deposition, whereas the height of the domains is self-limiting to the length of OTS molecule (i.e., a linear C18 silane), being approximately 2.6 nm in length.

[0123] In order to know the length and ratio of the molecular entities needed, the nano-roughness of a surface can be calculated according to Sa = 1/n * åabs(z), with Sa the surface roughness, n the amount/ratio of the molecular species, and z the height (or length) of the molecular species. The highest roughness is therefore achieved for a 50:50 ratio of molecules.

In that case, Sa is equal to half of the difference in length of the two molecule species. For example, with OTS (C18H37SiC13 ) with a length of 2.6 nm at a bare Si02 surface with a coverage of 50%, a surface roughness of Sa = 1.3 nm may be obtained. Upon addition of HMDS (with a length of 0.3 nm) at the bare Si02 areas, to cover the additional 50%, the Sa slightly reduces to 1.15 nm.

[0124] In addition, the nano-roughness may be formed by ordered structures. By a self-assembly process using controlled dimerization or oligomerization an increased (temporary) nano-roughness can be obtained. The controlled process may result in the formation of circular-like shaped islands with a diameter of tens of nanometres, dash-like islands with a length of tens of nanometres, or ring- shaped nano-structures. For example, a linear N-butylsilane may be organized in ring-shaped structures with a height of approximately 8 nm.

[0125] An advantage is that these silanes can be grafted at room temperature within a few seconds. Although, the molecules are chemically bonded to the wafer backside surface during the grafting procedure, resulting in a fixed monolayer comprising one or more different molecular species, the nano-roughness monolayer coating should be removed or stripped after exposure of the wafer in the lithographic apparatus. The removal may be performed by a dry cleaning method, which is preferred above a wet chemical etching by, for example, HF. For this purpose, a standard ozone or oxygen plasma may be used.

[0126] 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.

[0127] As said above, a self-assembled monolayer may limit the lateral charge mobility and may result in the trapping of charges. But, one can imagine that the charges still left could still play a role in the oxidation reaction. To reduce the effect of charges on the substrate backside even further, a SAM with a redox active site may be used. In this way, the charges on the substrate backside can be transferred to this redox site and be neutralized. Additionally, the advantage of the SAM is that if still some residual charges are left, they will be immobilized by the SAM. This may be obtained by using anti-oxidants or inhibitors at a desired location. The SAM molecule can be designed in such a way, that besides the fast grafting at room temperature, also a functionality in the backbone is added, which takes care of the charges on the substrate backside as it acts as an anti-oxidant. The type of inhibitor can be tuned on desire by organic chemistry. List of inhibitors that can be incorporated are reported by T.G. Harvey et al., in “The effect of inhibitor structure on the corrosion ofAA2024 and AA7075” , Corrosion Science, 53(6), 2184-2190, 2011. Several structural components strongly inhibited corrosion, including the thiol group; positions para- and ortho- to a carboxylate on a monoaromatic ring; and substitution of N for C in an aromatic ring where it may form a co-ordinating site with a carbonyl or another nitrogen. Also 6-amino-2-mercaptobenzothiazole, 4,5-diamino-2,6- dimercaptopyrimidine, and compounds having the C(SH)=S grouping are very active corrosion inhibitors. By incorporated one of these structures into the SAM structure, the charged substrate backside can be locally inhibited and providing a redox active functionality. Hence, resulting in lower wear of the substrate holder due to the reduction of the corrosion processes between the substrate backside and the substrate holder. [0128] Accordingly, in another aspect of the invention the SAMs set out above are applied to the burls of a substrate holder 20 used in a lithographic apparatus LA. The SAMs may be applied using any of the SAM precursor molecules set out above.

[0129] 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.

[0130] 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.

[0131] 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.

[0132] 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.

[0133] 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.

[0134] 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.

[0135] According to another aspect of the invention, the SAMs set out above may be applied to at least a portion of a surface of an optical sensor. For example, the position sensor arranged in the lithographic apparatus LA, as illustrated in Figure 1, may be (at least partially) coated with a coating comprising SAMs.

[0136] In another embodiment, a SAM layer may be applied to at least a portion of the second positioner PW, with reference to Figure 1.

[0137] The SAM coating (layer) applied to the position sensor and/or the second positioner PW may be provided as a protective layer.

[0138] It will be appreciated by the skilled person that the SAM coating may be fluid-phobic coating, herewith changing the surface energy with respect to the contacting fluid. For example, the SAM coating is a hydrophobic coating.

[0139] Silanes (including cyclic silanes) bind specifically to Si-containing surfaces containing oxides, such as SiC or S13N4. 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.

[0140] 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. Int. 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.

[0141] In one embodiment, as shown in Figure 9 the SAM is applied to the surface of the burls by bubbling N2 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 7, 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 96 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 pre clamp area while using WES extraction 96 so that the substrate 71, 91 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 91 is fully clamped.

[0142] Optionally, the substrate 91 can be treated before applying the SAM precursor molecules to the substrate holder 92. 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.

[0143] 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 101 during operation of the lithographic apparatus LA. Preferably, the device 103 moves over the surface of the burls 102 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 102. Preferably, gas containing the SAM precursor 105 is applied in the centre of the device 103 and air extraction 104 takes place at the edge of the device 103 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 103 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 101 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 1 minute.

[0144] 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 113 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 113 may include capillaries to feed the bottom of the stamp 113 with SAM precursor. Optionally, the air may be removed from the surface of the substrate holder 111, 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 113.

[0145] 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 111.

[0146] The apparatus used for each of these embodiments of applying the SAM to the substrate holder surface, for example the surface of substrate holder 111, 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 pre -treatment 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.

[0147] 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. The invention may also be applied in an additive manufacturing apparatus, wherein the substrate support is arranged to support a three-dimensional model. The substrate support arranged in the additive manufacturing apparatus may not have burls and the SAM layer is provide at a surface of the substrate support that supports the model. Possible other applications include the manufacture of integrated optical systems, rapid prototypes, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin film magnetic heads, etc.

[0148] Aspects of the invention are set out in the clauses below.

1. A method of protecting a substrate holder, the method comprising: 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 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 silazane, a phosphonate, a carboxylate, a catechol, an alkene or an alkyne, and or the second SAM precursor molecule is selected from a silane, silazane, a phosphonate, an alkene, an alkyne, a carboxylate or a catechol.

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

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

6. The method of clause 4 or 5, wherein the cyclic silane is a cyclic azasilane having the formula:

wherein Rl, R2, R3 and R4 are independently selected from the group consisting of hydrogen, Cl -20 alkyl, Cl -20 haloalkyl, C2-20 akenyl, Cl -20 amino, Cl -20 alkoxy, Cl -20 alkylthio, Cl -20 alkylene C6-60 aryl, C6-60 aryloxy, C6-60 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.

7. The method of clause 5 or 6, wherein, the cyclic azasilane is 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, N-n-butyl-aza-2,2-dimethoxysilacyclopentane or 1 -butyl- 1,2- azasilolidine.

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

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

10. The method of any one of clauses 1 to 9, 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.

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

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

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

15. The method of any one of clauses 1 to 10, wherein the backside surface of the substrate is cleaned with water after the application of SAM A to the substrate backside surface.

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

17. The method of any one of clauses 1 to 16, 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.

18. The method of any one of clauses 1 to 16, 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.

19. The method of any one of clauses 1 to 18, wherein the burls of the substrate holder comprise one or more of diamond-like carbon (DLC), diamond, graphite, SiSiC, SiC, Zerodur, A1203,TiN, Lipocer, SST and or CrN. 20. The method of any one of clauses 1 to 19, 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.

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

22. The method of any one of clauses 1 to 19, wherein SAM B is applied to the burls of the substrate holder by a device moving over the burls.

23. The method of any one of clauses 1 to 19, wherein SAM B is applied to the burls of the substrate holder by liquid-phase stamping using a polydimethylsiloxane (PDMS) stamp.

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

25. The method of any one of clauses 1 to 24, 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.

26. The method of any one of clauses 1 to 25, wherein the surface free energy of the backside surface of the substrate after application of SAM A is less than about 50 mJ/m2, 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.

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

28. 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.

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

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

31. The substrate holder of any one of clauses 28 to 30, wherein the burls of the substrate holder comprise one or more of diamond-like carbon (DLC), diamond, graphite, SiSiC, SiC, Zerodur, A1203,TiN, Lipocer, SST and or CrN.

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

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

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

35. The substrate of any one of clauses 32 to 34, wherein SAM A comprises at least one silicon atom.

36. The substrate of any one of clauses 32 to 35, wherein SAM A comprises at least one nitrogen atom. 37. 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 self-assembled monolayer (SAM B), wherein SAM A and SAM B are hydrophobic SAMs.

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

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

40. The method of any one of clauses 37 to 39, wherein the burls of the substrate holder comprise one or more of diamond-like carbon (DLC), diamond, graphite, SiSiC, SiC, Zerodur, A1203,TiN, Lipocer, SST and or CrN.

41. The method of any one of clauses 37 to 40, wherein the substrate comprises one or more of silicon, silicon carbide, gallium nitride, gallium arsenide or aluminium titanium carbide.

42. The method of any one of clauses 37 to 41, wherein SAM A and/or SAM B comprises at least one silicon atom.

43. The method of any one of clauses 37 to 42, wherein SAM A and/or SAM B comprises at least one nitrogen atom.

44. A method of fabricating a device, the method comprising applying the method according to any one of clauses 37 to 43.

45. 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.

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

47. The method of clause 4, wherein the cyclic silane comprises a heteroatom selected from the elements belonging to group 13, 14, 15, or 16 of the periodic table.

48. The method of clause 47, wherein the heteroatom is one of B, C, Si, Ge, N, S, Se, and Te.

49. An optical sensor comprising a sensor surface, wherein at least a portion of the sensor surface is coated with a coating comprising a self-assembled monolayer, provided as a protective layer.

50. A substrate holder with a surface comprising burls, wherein at least a portion of the burls comprises a self-assembled monolayer (SAM), wherein the SAM comprises at least a first molecular species and a second molecular species, with the first and second molecular species having different length.

51. The substrate holder according clause 50, wherein the first molecular species and the second molecular species are provided to the burls at the same time. 52. The substrate holder according clause 50, wherein the first molecular species and the second molecular species are provided to the burls sequentially.

53. The substrate holder according clause 50, wherein the SAM is provided to the burls by imprinting.

54. A substrate for use in a lithographic apparatus, wherein at least a portion of a substrate backside surface is provided with a self-assembled monolayer (SAM), wherein the SAM comprises at least first molecular species and a second molecular species, with the first and second molecular species having different length.

55. The substrate according clause 54, wherein the first molecular species and the second molecular species are provided to the substrate backside surface at the same time. 56. The substrate according clause 54, wherein the first molecular species and the second molecular species are provided to the substrate backside surface sequentially.

57. The substrate according to clause 54, wherein the SAM is provided to the substrate backside surface by imprinting.

58. The method of any one of clauses 1 to 26 and 37 to 48, wherein the first self-assembled monolayer (SAM A) comprises an inhibitor to provide redox active functionality.

59. The substrate according to clause 54 to 57, wherein the self-assembled monolayer comprises an inhibitor to provide redox active functionality.

[0149] 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

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 claim 1, wherein the first SAM precursor molecule is selected from a silane, a silazane, a phosphonate, a carboxylate, a catechol, an alkene or an alkyne, and or the second SAM precursor molecule is selected from a silane, a silazane, a phosphonate, an alkene, an alkyne, a carboxylate or a catechol.

3. The method of claim 1, wherein at least one of the first SAM precursor molecule and the second SAM precursor molecule is a cyclic silane.

4. The method of claim 3, wherein the cyclic silane is a cyclic azasilane having the formula:

wherein R¹, R², R³ and R⁴ are independently selected from the group consisting of hydrogen, Ci-₂₀ alkyl, Ci-₂₀ haloalkyl, C_2-20 akenyl, Ci-₂₀ amino, Ci-₂₀ alkoxy, Ci-₂₀ alkylthio, Ci-₂₀ alkylene Ce-eo aryl, Ce-eo aryloxy, Ce-eo arylthio, C_2-20 alkynyl, C_2-20 acyl, C_2-20 acyloxy, C_2-20 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.

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

6. The method of any one of claims 1 to 5, wherein the first and/or second SAM precursor molecule is applied by applied by vapour phase reaction, wet chemical application or contact printing.

7. The method of any one of claims 1 to 6, wherein the backside surface of the substrate is cleaned with water before the application of SAM A to the substrate backside surface.

8. The method of any one of claims 1 to 7, wherein the backside surface of the substrate is cleaned with water after the application of SAM A to the substrate backside surface.

9. The method of any one of claims 1 to 8, wherein SAM A is applied to the substrate backside surface in a scanner of a lithographic apparatus.

10. The method of any one of claims 1 to 8, wherein SAM A is applied to the substrate backside surface in a store unit of a lithographic apparatus.

11. The method of claim 1 or 2, 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.

12. The method of claim 1 or 2, 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.

13. The method of any one of claims 1 to 12, 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.

14. The method of any one of claims 1 to 5, wherein SAM B is applied to the burls of the substrate holder by a device moving over the burls.

15. An apparatus configured to apply the method of any one of claims 1 to 14.