WO2017076694A1 - Multilayer reflector, method of manufacturing a multilayer reflector and lithographic apparatus - Google Patents

Abstract

A multilayer reflector for use in EUV lithography, for example, comprises alternating layers of Mo and Rb_xSi_y. The Rb_xSi_y and Mo interface is thermodynamically stable, reducing intermingling of the layers and preventing reduction in reflectivity. The Rb_xSi_y layer can be hydrogenated to form RbSiH₃. For the case of Mo/RbSiH₃ an interlayer of RbH between Mo and RbSiH₃ layers can be used. An Mo/RbH multilayer mirror is also useful.

Description

Multilayer Reflector, Method of Manufacturing a Multilayer

Reflector and Lithographic Apparatus

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001 ] This application claims priority of EP application 15192512.0 which was filed on November 2, 2015 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to multilayer reflectors for EUV or X-ray radiation, to methods of making such multi-layer reflectors and to lithographic apparatus using such multi-layer reflectors.

BACKGROUND

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

[0004] The wavelength of radiation used by a lithographic apparatus to project a pattern onto a substrate determines the minimum size of features which can be formed on that substrate. A lithographic apparatus which uses EUV radiation, being electromagnetic radiation having a wavelength within the range 5-20 nm, may be used to form smaller features on a substrate than a conventional lithographic apparatus (which may for example use electromagnetic radiation with a wavelength of 193 nm).

[0005] Collecting EUV radiation into a beam, directing it onto a patterning device (e.g. a mask) and projecting the patterned beam onto a substrate is difficult because it is not possible to make a refractive optical element for EUV radiation. Therefore these functions have to be performed using reflectors (i.e. mirrors). Even constructing a reflector for EUV radiation is difficult. The best available reflector for EUV radiation is a multi-layer reflector (also known as a distributed Bragg reflector) which comprises a large number of layers which alternate between a relatively high refractive index layer and a relatively low refractive index layer. Each period, consisting of a high refractive index layer and a low refractive index layer, has a thickness equal to half the wavelength (λ/2) of the radiation to be reflected so that there is constructive interference between the radiation reflected at the high to low refractive index boundaries. Such a multilayer reflector still does not achieve a particularly high reflectivity. [0006] Currently available multilayer reflectors for EUV using alternate layers of

Molybdenum (Mo) and Silicon (Si) can in theory achieve a reflectivity of 74.77% (number of periods = 100, period thickness = 6.9 nm, Mo-layer-to-period thickness ratio γ = 0.4). In practice however Mo/Si multilayer reflectors are known to suffer from three common imperfections: intermixing of Si and Mo at their interfaces, formation of molybdenum silicide interlayers, and roughening of the Mo/Si interface during the fabrication of multilayer reflectors. The combination of these effects reduces the reflectivity of Mo/Si multilayer reflectors to about 70% or less.

[0007] Since about 10 multilayer reflectors are used in series in a EUV lithography apparatus between the EUV light source and the substrate, a reflectivity of about 70% for each multilayer reflector results in less than 3% of the initially generated EUV radiation reaching the substrate, whilst the other 97% is absorbed in the multilayer reflectors and goes to waste. Since increasing the source power is difficult, the low proportion of the source output that actually reaches the substrate limits throughput of the lithographic apparatus.

[0008] The absorbed radiation, including infra-red radiation also emitted by the radiation source, can cause the temperature of the multilayer reflector to rise, which triggers further intermixing, silicide formation and roughening at the interface. This occurs because the molybdenum silicide phase is thermodynamically more stable than elemental Si and Mo. Therefore it is necessary to provide substantial cooling of the multilayer reflectors to avoid further degradation of their EUV reflectivity during their operation, and hence a reduction of their economic lifetime.

SUMMARY

[0009] It is an aim of the invention to provide an improved multilayer reflector.

[0010] According to the present invention, there is provided a multilayer reflector comprising a plurality of periods, each period comprising a low refractive index layer and a high refractive index layer, wherein: in at least one period the low refractive index layer comprises Mo and the high refractive index layer comprises a compound comprising a combination of at least two elements selected from the group consisting of Si, Rb, and H.

[0011] According to the present invention, there is provided a lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, the apparatus comprising at least one multilayer reflector as described above.

[0012] According to the present invention, there is provided a method of manufacturing a multilayer reflector comprising a plurality of periods, each period comprising a low refractive index layer and a high refractive index layer, the method comprising a physical vapour deposition process to form at least one high refractive index layer, the physical vapour deposition step using an evaporation target comprising Si and Rb BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

- Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source according to an embodiment of the invention;

Figure 2 depicts a radiation source according to an embodiment of the invention;

Figure 3 depicts a multilayer reflector according to an embodiment of the present invention.

- Figure 4 is a graph showing refractive index and absorption as a function of wavelengths for rubidium and silicon;

Figure 5 depicts refractive index difference and spectral absorbance sum of Mo and

Si;

Figure 6 depicts refractive index difference and spectral absorbance sum of Mo and Si with a chemical shift;

Figure 7 is a graph showing the phase boundary between RbSi and RbSiH₃ phases as a function of temperature and hydrogen gas pressure;

Figure 8 is a graph of calculated reflectance value vs wavelength for a multi-layer mirror of composition Mo/Rb_nA_m for various different compounds as A; and

- Figure 9 is a graph of calculated reflectance value vs wavelength for a series of 10 multi-layer mirrors of composition Mo//Rb_nA_m for various different compounds as A.

DETAILED DESCRIPTION

[0014] Figure 1 shows a lithographic system including a multilayer reflector according to one embodiment of the invention. The lithographic system comprises a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an extreme ultraviolet (EUV) radiation beam B. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS and a substrate table WT configured to support a substrate W. The illumination system IL is configured to condition the radiation beam B before it is incident upon the patterning device MA. The projection system is configured to project the radiation beam B (now patterned by the mask MA) onto the substrate W. The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus aligns the patterned radiation beam B with a pattern previously formed on the substrate W.

[0015] The radiation source SO, illumination system IL, and projection system PS may all be constructed and arranged such that they can be isolated from the external environment. A gas at a pressure below atmospheric pressure (e.g. hydrogen) may be provided in the radiation source SO. A vacuum may be provided in illumination system IL and/or the projection system PS. A small amount of gas (e.g. hydrogen) at a pressure well below atmospheric pressure may be provided in the illumination system IL and/or the projection system PS.

[0016] The radiation source SO shown in Figure 1 is of a type which may be referred to as a laser produced plasma (LPP) source). A laser 1 , which may for example be a C0₂ laser, is arranged to deposit energy via a laser beam 2 into a fuel, such as tin (Sn) which is provided from a fuel emitter 3. Although tin is referred to in the following description, any suitable fuel may be used. The fuel may for example be in liquid form, and may for example be a metal or alloy. The fuel emitter 3 may comprise a nozzle configured to direct tin, e.g. in the form of droplets, along a trajectory towards a plasma formation region 4. The laser beam 2 is incident upon the tin at the plasma formation region 4. The deposition of laser energy into the tin creates a plasma 7 at the plasma formation region 4. Radiation, including EUV radiation, is emitted from the plasma 7 during de-excitation and recombination of ions of the plasma.

[0017] The EUV radiation is collected and focused by a near normal incidence radiation collector 5 (sometimes referred to more generally as a normal incidence radiation collector). The collector 5 may have a multilayer structure (described further below) which is arranged to reflect EUV radiation (e.g. EUV radiation having a desired wavelength such as 13.5 nm). The collector 5 may have an elliptical configuration, having two ellipse focal points. A first focal point may be at the plasma formation region 4, and a second focal point may be at an intermediate focus 6, as discussed below.

[0018] The laser 1 may be separated from the radiation source SO. Where this is the case, the laser beam 2 may be passed from the laser 1 to the radiation source SO with the aid of a beam delivery system (not shown) comprising, for example, suitable directing mirrors and/or a beam expander, and/or other optics. The laser 1 and the radiation source SO may together be considered to be a radiation system.

[0019] Radiation that is reflected by the collector 5 forms a radiation beam B. The radiation beam B is focused at point 6 to form an image of the plasma formation region 4, which acts as a virtual radiation source for the illumination system IL. The point 6 at which the radiation beam B is focused may be referred to as the intermediate focus. The radiation source SO is arranged such that the intermediate focus 6 is located at or near to an opening 8 in an enclosing structure 9 of the radiation source.

[0020] The radiation beam B passes from the radiation source SO into the illumination system IL, which is configured to condition the radiation beam. The illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 1 1 . The faceted field mirror device 10 and faceted pupil mirror device 1 1 together provide the radiation beam B with a desired cross-sectional shape and a desired angular distribution. The radiation beam B passes from the illumination system IL and is incident upon the patterning device MA held by the support structure MT. The patterning device MA reflects and patterns the radiation beam B. The illumination system IL may include other mirrors or devices in addition to or instead of the faceted field mirror device 10 and faceted pupil mirror device 1 1 . The faceted field mirror device 10, faceted pupil mirror device 1 1 and other reflectors of the illumination system may have a multilayer structure as described further below.

[0021 ] Following reflection from the patterning device MA the patterned radiation beam B enters the projection system PS. The patterning device may include a reflector having a multilayer structure as described further below. The projection system comprises a plurality of mirrors which are configured to project the radiation beam B onto a substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the radiation beam, forming an image with features that are smaller than corresponding features on the patterning device MA. A reduction factor of 4 may for example be applied. Although the projection system PS has two mirrors in Figure 1 , the projection system may include any number of mirrors (e.g. six mirrors). The mirrors, and any other reflectors of the projection system PS, may have a multilayer structure as described further below.

[0022] Figure 2 shows a laser produced plasma (LPP) radiation source SO which has an alternative configuration to the radiation source shown in Figure 1 . The radiation source SO includes a fuel emitter 3 which is configured to deliver fuel to a plasma formation region 4. The fuel may for example be tin, although any suitable fuel may be used. A pre-pulse laser 16 emits a pre-pulse laser beam 17 which is incident upon the fuel. The pre-pulse laser beam 17 acts to preheat the fuel, thereby changing a property of the fuel such as its size and/or shape. A main laser 18 emits a main laser beam 19 which is incident upon the fuel after the pre-pulse laser beam 17. The main laser beam delivers energy to the fuel and thereby coverts the fuel into an EUV radiation emitting plasma 7.

[0023] A radiation collector 20, which may be a so-called grazing incidence collector, is configured to collect the EUV radiation and focus the EUV radiation at a point 6 which may be referred to as the intermediate focus. Thus, an image of the radiation emitting plasma 7 is formed at the intermediate focus 6. An enclosure structure 21 of the radiation source SO includes an opening 22 which is at or near to the intermediate focus 6. The EUV radiation passes through the opening 22 to an illumination system of a lithographic apparatus (e.g. of the form shown schematically in Figure 1 ).

[0024] The radiation collector 20 may be a nested collector, with a plurality of grazing incidence reflectors 23, 24 and 25 (e.g. as schematically depicted). The grazing incidence reflectors 23, 24 and 25 may be disposed axially symmetrically around an optical axis O. The illustrated radiation collector 20 is shown merely as an example, and other radiation collectors may be used.

[0025] A contamination trap 26 is located between the plasma formation region 4 and the radiation collector 20. The contamination trap 26 may for example be a rotating foil trap, or may be any other suitable form of contamination trap. In some embodiments the contamination trap 26 may be omitted.

[0026] An enclosure 21 of the radiation source SO includes a window 27 through which the pre-pulse laser beam 17 can pass to the plasma formation region 4, and a window 28 through which the main laser beam 19 can pass to the plasma formation region. A mirror 29 is used to direct the main laser beam 19 through an opening in the contamination trap 26 to the plasma formation region 4.

[0027] The radiation sources SO shown in Figures 1 and 2 may include components which are not illustrated. For example, a spectral filter may be provided in the radiation source. The spectral filter may be substantially transmissive for EUV radiation but substantially blocking for other wavelengths of radiation such as infrared radiation.

[0028] Figure 3 depicts a multilayer reflector 30 according to an embodiment of the present invention. The multilayer reflector 30 comprises a plurality of alternating high refractive index layers 32 (sometimes referred to as spacer layers) and low refractive index layers 34 (sometimes referred to as refracting layers). A pair of adjacent layers is referred to herein as a period. The thickness of a period is approximately equal to half the wavelength (λ/2) of the radiation that is desired to be reflected, e.g. 6.9 nm to reflect EUV radiation at 13.5 nm. The multilayer reflector functions as a distributed Bragg reflector with constructive interference between the radiation reflected at each boundary between a high refractive index layer and a low refractive index layer. The multilayer reflector may be formed on a substrate 38 and may be provided with a capping layer 36. Capping layer 36 can be formed of various known materials and helps to protect the multilayer reflector from chemical and physical damage.

[0029] In an embodiment of the invention the low refractive index layers are Mo and the high refractive index layers are silicon with at least one high refractive index layer having added rubidium (Rb). Desirably at least 50%, at least 80%, at least 90% or all of the high refractive index layers have added rubidium. In an embodiment in which not all of the high refractive index layers have rubidium it is preferable that the layers with rubidium are those closest to the incident surface. The added Rb increases both the reflectivity and the thermal stability of the multilayer reflector compared to a conventional Mo/Si multilayer reflector. Thus the invention replaces the silicon layers by rubidium silicide and the resulting multilayer reflectors have the general structure of Mo/Rb_xSi_y. The use of Rb leads to four notable advantages. [0030] Firstly, the solubility of Rb in Mo is predicted to be defect-controlled and can be assumed to be virtually zero (see W. Moffatt, The Handbook of Binary Phase Diagrams, Genium Pub. Corp., USA, 1984). On the other hand Rb is very reactive and makes a strong bond with Si. The enthalpy of formation for RbSi has not been reported in the literature at the time of writing but for the closely related compound of RbGe (which has a zintl crystal structure that is similar to RbSi) the value is -100 kJ/mole (see J. Sangster and A.D. Pelton, Journal of Phase Equilibria 18 (1997) 298). Note that the formation energy expressed here in units of kJ/mole of RbSi (or RbGe) 'molecules'. It is known that the zintl germanides of the alkali metals are slightly more stable than their silicide counterparts (see E. Hohman, Z. Anorg. Allg. Chem. 257 (1948) 1 13). In addition, the formation free energy for barium silicide (which also has a zintl crystal structure) has been reported to be about 80% of the barium germanide (see H. Peng et al Physics Letters A 374 (2010) 3797). So it would be safe to predict the formation energy of RbSi as -80 kJ/mole.

[0031 ] There are three known molybdenum silicide phases with the following formation enthalpies (H. Fujiwara, Y. Ueda , J. Alloys Compd. 441 (2007) 168) :

1 /4 Si + 3/4 Mo→ 1/4 Mo₃Si ΔΗ°_Γ = 1 /4 ΔΗ°, (Mo₃Si) = 1 /4 (-122.1 kJ/mole) = -30.5 kJ/mole of atoms in Mo_xSi_y

3/8 Si + 5/8 Mo→ 1/8 Mo₅Si₃ ΔΗ°_Γ = 1 /8 ΔΗ°, (Mo₅Si₃) = 1 /8 (-313.5 kJ/mole) = -39.2 kJ/mole of atoms in Mo_xSi_y

2/3 Si + 1/3 Mo→ 1/3 MoSi₂ ΔΗ°_Γ = 1 /3 ΔΗ°, (MoSi₂) = 1 /3 (-135.8 kJ/mole) = -45.3 kJ/mole of atoms in Mo_xSi_y

The mole coefficients are chosen such that there is one mole of atoms at either side of each reaction equation, and the reaction enthalpies are thus expressed here per mole of atoms in Mo_xSi_y. The negative enthalpy values show that all three molybdenum silicide phases are more stable than their elemental constituents. MoSi₂ has the most negative formation enthalpy per mole of atoms, and this is also the main and most stable silicide phase that forms in Mo/Si multilayer reflectors, causing significant sharpness loss at Mo/Si interfaces. However if we replace the Si with RbSi, the reaction enthalpies for the formation of molybdenum silicide phases change drastically:

1 I A RbSi + 3/4 Mo→ 1 I A Mo₃Si + 11 A Rb ΔΗ°_Γ = 1 /4 (-122.1 ) - 1 /4 (-80.0) = -10.5 kJ/mole of atoms in Mo_xSi_y

(65% reduction)

3/8 RbSi + 5/8 Mo→ 1/8 Mo₅Si₃ + 3/8 Rb ΔΗ°_Γ = 1 /8 (-313.5) - 3/8 (-80.0) = -9.2 kJ/mole of atoms in Mo_xSi_y

(76% reduction) 2/3 RbSi + 1 /3 Mo→ 1 /3 MoSi₂ + 2/3 Rb ΔΗ°_Γ = 1/3 (-135.8) - 2/3 (-80.0) = +8.0 kJ/mole of atoms in Mo_xSi_y

(full suppression).

[0032] These data show that the presence of Rb in the Si layer should suppress both the intermixing between Si and Mo and the formation of molybdenum silicide phases at each Si/Mo interface. More specifically, the driving force behind Mo₃Si, and Mo₅Si₃ formation in Mo/RbSi multilayer reflectors would be 65% and 76% lower than that in the Mo/Si system. At the same time, formation of the most problematic silicide, i.e. MoSi₂, would be totally suppressed, due to the positive enthalpy of the reaction. In other words, the presence of Rb would improve and protect the sharpness (and hence the reflectivity) of Mo/Rb_xSi_y interfaces thermodynamically. This would also imply a significant increase in the thermal/chemical stability and hence the lifetime of the multilayer reflectors.

[0033] Secondly, for reflecting EUV light, elemental Rb has superior optical properties in comparison to Si. This is mainly due to the lower spectral absorbance of Rb for 13.5 nm light, compared to Si, as shown in Figure 4. Theoretically Mo/Rb multilayer reflectors can reach a EUV reflectivity over 77%. Unfortunately Mo/Rb multilayer reflectors cannot be used in practice since Rb has a melting point of only 39.3°C. However all phases of rubidium silicide are expected to have melting points above 600°C (see Sangster et al cited above). In the EUV regime the optical properties of a compound, specifically the refractive index n, can be approximated to be a linear function of the individual constituents (see D. L. Windt, Computers in Physics 12 (1998) 360):

2 wm_ec² ∑jXjAj

where p is density, f₁ and f₂ are the atomic scattering factors, the sums range over each of the chemical elements that compose the compound, the x_j are the relative concentrations of each element, and the A, are the associated atomic densities; e, m_e , c, and N_a are the electron charge, the electron mass, the speed of light, and Avogadro's number, respectively.

[0034] So the refractive index of an Rb_xSi_y phase would be a function of its stoichiometry ratio and density. To give an example, the reflectivity for Mo/RbSi multilayer reflectors is predicted to be above 72.9% (number of bilayers = 1 00, bilayer thickness = 6.9 nm, Mo- layer-to-bilayer thickness ratio γ = 1 /3, interface roughness = 4 A, RbSi density = 2.72 gr/cm³ - see H. G. von Schnering, et al. Z. Kristallogr. NCS 220 (2005) 525). Note that this value is lower than the maximum theoretical reflectivity of normal Mo/Si multilayer reflectors

(74.77%). However as mentioned earlier, the thermodynamic instability of the Mo/Si system leads to a loss of interface sharpness and, hence, a loss of reflectivity down to -70%. In contrast, the thermodynamic stability of Mo/Rb_xSi_y should protect the sharpness of interfaces and prohibit the loss of reflectivity. Note, that the calculation of the above-mentioned 72.9% reflectivity of Mo/RbSi multilayer reflectors was based on a moderate estimate of the sharpness of the interfaces, as we have still assumed a roughness/diffuseness of 4 A (~3 monolayers) at all interfaces.

[0035] Thirdly, the reflectivity of the multilayer reflector is improved through "chemical shift". It is known that a chemical bond can shift the electronic energy levels of an atom. This effect is commonly known as "chemical shift" (CS).

[0036] Fig. 5 shows the refractive index difference (contrast) between Mo and Si (solid line), the sum of their spectral absorbances (dotted line) and the reflectivity of a Mo/Si multilayer mirror with 6.9 nm period, γ = 0.4 (hatched area). Qualitatively speaking, higher contrast and lower absorbance of a multilayer reflector for 13.5 nm wavelength light would lead to a higher reflectivity of the multilayer reflector for this wavelength. For radiation of lower energy, toward the L₃ absorption edge of the silicon (located at -99 eV), the absorbance sum of Mo and Si (dotted line) drops. This seems to imply that a Mo/Si multilayer reflector would have higher reflectivity for light energies closer to the L₃ edge. Unfortunately this is not the case as the Mo/Si contrast (solid line) also decreases for light energies closer to the L₃ edge. However by shifting the L₃ edge of silicon (corresponding to Si 2/f'³ electronic orbital) to lower energy levels the refractive index difference could be increased for the 13.5 nm wavelength, while keeping the total absorption almost constant (see Fig. 6, which corresponds to Figure 5 but with a hypothetical -4 eV chemical shift of the Si 2p orbital). This would lead to higher reflectivity for 13.5 nm radiation.

[0037] Since Rb has one of the lowest electronegativity values in the periodic table, silicon atoms in a Rb_xSi_y matrix (being an ionic solid) become negatively charged. This should push the electronic energy levels of silicon, including the Si 2p orbital, toward lower binding energies. It is anticipated that the change in energy levels will be reflected, at least to some extent, in a downward shift of the L₃ absorption edge. This is motivated by the fact that the chemical shift of a orbital with higher bonding energy (here the Si 2p orbital which is closer to the core) is in general larger than the chemical shift of an orbital with less bonding energy (here the unoccupied orbitals). The shift of L₃ edge should increase (the real part of) the refractive index of the Si in this silicide, while its absorption remains almost constant. The effective complex refractive index calculated by Eq. 1 does not take into account the beneficial effect of this chemical shift of the Si 2p level, caused by compound formation with Rb. The chemical shift of the Si 2p orbital would be a function of the oxidation state of the Si in the Rb_xSi_y, and the lattice structure of the silicide phase. X-ray photoelectron spectroscopy experiments or density functional theory calculations are needed to determine the precise values of the CS of the Si 2p level for a specific Rb_xSi_y film and the resulting shift of the L₃ absorption edge.

[0038] The exact quantification of the Si L-edge shift depends on the arrangement of atoms and the nature of bounds within the silicide. Qualitatively, the chemical shift of the absorption edges is proportional to the effective charge of the absorbing ions (see M.N. Ghatikar, B. D. Padalia, and R.M. Nayak, "Chemical shifts and effective charges in ternary and complex systems", J. Phys. C: Solid State Phys., 10 (1977) 4173-4180). The negative charge transfer to Si is expected to be strong for Rb_nSi_m as Rb has one of the lowest electronegativity values in the periodic table. Suchet's empirical rule approximates the effective charge of an ion q , in a binary compound as:

where z , r , and n represent the total number of electrons, ionic radius, and oxidation state of the ion. Here a and c subscripts stand for anion and cation. For RbSi, the predicted effective charges for Rb ( z_c = 37 , r = 1.48 A, n = +1) and Si ( z_a = 14 , r = 3.84 A, n = -1 ) are q_c ~ +0.774 and q_a ~ -0.774 electron per atom. Here for the r and r_a , Pauling ionic radii of Rb⁺¹ and Si^"1 have been used (see J.E. Huheey, E.A. Keiter, and R.L. Keiter in Inorganic Chemistry : Principles of Structure and Reactivity, 4th edition, HarperCollins, New York, USA, 1993. J.E. Huheey, Inorganic Chemistry: Principles of structure and reactivity, 3rd edition, Harper International, New York, 1983, ISBN 0-06-042987-9). From the experimental L-edge absorption data of SiC, Si₃N₄, Si0₂, and SiF₄ (see I. Waki and Y. Hirai, "The silicon L-edge photoabsorption spectrum of silicon carbide" J. Phys. : Condens. Matter 1 (1989) 6755-6762) the proportionality factor between L-edge chemical shift and effective charge is estimated to be 1 .24 eV per electron per atom (F† = 0.12 for the linear fit). This implies a ~ - 1 .0 eV chemical shift for the Si L-edge in RbSi.

[0039] It can be calculated that this chemical shift should improve the integral reflectivity of single multilayer reflector in 12.5-14.5 nm range by an additional amount of about 0.27% and the integral reflectivity of a series of 10 mirrors by 0.58% compared to the current state-of- the-art technology (i.e. 70.15% reflectivity of Mo/B₄C/Si/B₄C multilayer reflector). So, with the same EUV light source power, but using Mo/RbSi multilayer reflectors instead of Mo/Si multilayer reflectors, the throughput of an EUV lithography apparatus can increase by 60% or more.

[0040] Fourthly, the addition of Rb allows for further optimization of a multilayer reflector. For a multilayer reflector that is a combination of Mo and uniform Rb_xSi_y layers, the total reflectivity and thermal/chemical stability is a function of the Rb_xSi_y stoichiometry and the crystal structure of the silicide phase (e.g. Rb₄Si₄, Rb₆Si₄6, etc.). The silicide phase can be amorphous and consist of any stoichiometry ratio between Rb and Si. An additional possibility that can be optimized to get the highest reflectivity and stability is to use multiple silicide phases in an multilayer reflector, e.g. { Mo / Rb₄Si₄ / Rb₆Si₄₆ / Rb₄Si₄ }_n , { Mo / Rb₄Si₄ / Si / Rb₄Si₄ }_n , { Mo / Rb₆Si₄₆ / Si / Rb₆Si₄₆ }_n , etc. (where {.../.../.../...}„ denotes the layers of a period with n the number of periods). In such an embodiment, a high Rb-content-silicide interlayer is used as a diffusion barrier to protect a core silicon layer or a core silicide layer with a lower Rb content from coming in direct contact with Mo. This strategy can be useful since, even though Rb_xSi_y phases with higher Rb content would have higher stability in combination with Mo, they also tend to have higher densities in comparison to pure silicon or silicides with lower Rb content. So by having the Si (or low-Rb-concentration silicide) layers sandwiched between high-Rb concentration silicide interlayers, it is possible to obtain multilayer reflectors with an even more superior combination of reflectivity and stability.

[0041] To give an example, the maximum theoretical reflectivity for 13.5 nm light of a multilayer reflector with the structure {Mo [22.6 A] / B₄C [3.0 A] / Si [40.40 A] / B₄C [3.0 A]}₁₀₀ is predicted to be above 74.75% (assuming all interfaces to be perfectly sharp and flat). However by replacing the B₄C diffusion barriers with Rb₄Si₄, the reflectivity can be raised above 75.35%, which reflects the superior optical properties of Rb₄Si₄ compared to B₄C. This reflectivity is even higher than the maximum theoretical reflectivity of a {Mo [22 A] / Rb₄Si₄ [46 A]}₁₀₀ multilayer reflector, which is 74.74% (again assuming all interfaces to be perfectly sharp and flat and neglecting chemical shift). Nevertheless, {Mo / Rb₄Si₄ / Si / Rb₄Si₄} is expected to have a somewhat lower thermodynamic stability in comparison to {Mo / Rb₄Si₄} since the Rb in the Rb₄Si₄ interlayer may diffuse into the Si core layer, which would give the remaining Si in the interlayer the opportunity to diffuse toward Mo and form MoSi₂.

[0042] To summarize, the effects of adding rubidium to the Si layer of the Mo/Si multilayer reflectors include:

1 - Improving and protecting the sharpness of multilayer reflector interfaces

thermodynamically. This leads to:

a - Increasing the reflectivity of the multilayer reflector by suppressing the intermixing between Si and Mo and by suppressing the formation of MoSi₂ at the interface. b- Increasing the lifetime of the multilayer reflector at high temperatures due to enhanced thermal and chemical stability of the Mo/ Rb_xSi_y system.

2 - Increasing the reflectivity of Mo/Si multilayer reflectors by increasing the refractive index of Si in the RbSi layers as a result of the negative chemical shift of the Si 2p orbital caused by compound formation with Rb.

[0043] By using the rubidium additive in Mo/Si multilayer reflectors, the intensity of the EUV light that is transferred from the light source to the substrate - and hence the throughput of an EUV lithography apparatus - can be improved by at least 45%. (comparing the reported world record reflectivity of 70.15% for Mo/B₄C/Si/B₄C with the predicted reflectivity of 72.9% for Mo/RbSi/Mo multilayer mirror assuming number of bilayers = 100, bilayer thickness = 6.9 nm, Mo-layer-to-bilayer thickness ratio γ = 1 /3, interface

roughness/diffuseness = 4 A, RbSi density = 2.72 gr/cm³)

[0044] Different interlayers such as B₄C and carbon have been used as diffusion barriers between Si and Mo layers, in order to kinetically prohibit the formation of MoSi₂. In this approach, MoSi₂ still tends to form but only at a slower pace. The present invention is superior in the sense that the MoSi₂ formation is prohibited thermodynamically. Hence, there is no driving force for MoSi₂ formation. In contrast to Rb, which has superior optical properties to Si, both B and C lead to a slight degradation of the optical properties of the Si layer. The presence of Rb in the Si layers also alters the optical properties of the Si layer so that it leads to higher reflectivity, in addition to the beneficial, thermodynamic effect, described above.

[0045] According to a further embodiment of the invention, hydrogen is included in the high refractive index layers. The introduction of hydrogen to the high refractive index layer can reduce its density and increase the reflectivity of a multilayer reflector. The presence of Rb in the Si layer can stimulate the hydrogen uptake of the Si layer significantly. At room temperature, RbSi can take up hydrogen up to a concentration of 60 at% (atomic %). This is achieved by simply exposing the RbSi to hydrogen gas of only 40 mbar pressure. The resulting RbSiH₃ phase has a density of only 1 .84 gr/cm³ which is 20% less than that of pure Si (see W. S. Tang, J.-N Chotard, P. Raybaud, R. Janot, J. Phys. Chem. C, 1 18 (2014) 3409 ). This decrease in density (along with the other benefits of Rb) can lead to an increase in the reflectivity of a single Mo/RbSiH₃ multilayer reflector up to 76.1 % (number of bilayers = 100, bilayer thickness = 6.9 nm, Mo-layer-to-bilayer thickness ratio γ= 1 /3, interface roughness/diffuseness = 4 A).

[0046] Figure 7 shows the phase boundary between the RbSi and RbSiH₃ phases as a function of temperature and hydrogen gas pressure. At room temperature, a hydrogen gas pressure of only 40 mbar is enough to turn RbSi into RbSiH₃ and stabilize the hydride phase. This also implies that if there is a void or crack in the Mo/RbSiH₃ structure, the segregation and release of hydrogen gas into the void can lead to only 40 mbar hydrogen pressure inside the void. This corresponds to 4.0 KPa stress which is far less than the typical fracture strength of the solid materials involved and their interfaces, which are in the MPa- GPa range. So, while hydrogen-implanted silicon with 30 at.% H is prone to H segregation and blistering, the equilibrium pressure of H₂ in RbSiH₃ with 60 at.% H is too low to cause the nucleation and/or growth of voids and result in blistering damage. [0047] Whether a Mo/RbSi or a Mo/RbSiH₃ multilayer reflector is more suitable in a particular EUV lithography apparatus, depends on the operation temperature and on the activity of the H radicals around the multilayer reflector in that apparatus. If a H-radical generator is provided in a lithography apparatus (e.g. for Sn cleaning purposes), the enhanced H activity is expected to stabilize the RbSiH₃ phase. This would be beneficial, since a Mo/RbSiH₃ multilayer reflector would have significantly higher reflectivity compared to both Mo/RbSi and Mo/Si multilayer reflectors (76.1 %, compared to 72.9% and 70.15% respectively). Therefore, replacing 10 Mo/Si multilayer reflectors in a lithographic apparatus with Mo/RbSiH₃ multilayer reflectors would lead to a more than 125 % increase in the intensity of the EUV light on the substrate.

[0048] A Mo/RbSiH₃ multilayer reflector, according to an embodiment of the invention, for reflecting 13.5 nm EUV light has several advantages over traditional Mo/Si multilayer reflectors.

[0049] As discussed above, the presence of Rb increases the reflectivity of Mo/Si multilayer reflectors from 70.15% to 72.9% because: 1 ) added Rb atoms have optical properties superior to those of Si as spacer atoms between the Mo layers in a multilayer reflector; 2) the presence of Rb increases the refractive index of Si in the RbSi layers as a result of the negative chemical shift of the Si 2p orbital; and 3) the formation of rubidium silicide suppresses the intermixing between Si and Mo and the formation of MoSi₂ and, hence, protects the sharpness of all interfaces in the multilayer reflector. The latter property would also lead to an expected increase in the lifetime of the multilayer reflector at high temperatures due to the enhanced thermal and chemical stability of the Mo/Rb_xSi_y system in comparison to Mo/Si.

[0050] Three additional benefits can be achieved due to reduction of the density of the high refractive index layer via the uptake of hydrogen.

[0051 ] Firstly, the presence of Rb in the Si high refractive index layer stimulates hydrogen uptake up to 60 at.% via the spontaneous formation of the RbSiH₃ phase. The density of this hydride phase is -20% lower than that of pure Si, which raises the reflectivity of a complete multilayer reflector up to 76.1 %. This a significant improvement in comparison to the alternative approach of H-implantation of Si layers, which could lead to density reduction of 7 %, but is known not to raise the reflectivity, probably due to ion-bombardment-induced interface roughening (V. Rigato et al., Surf. Coat. Tech.174 -175 (2003) 40).

[0052] Secondly, the hydrogen uptake by the RbSi phase up to 60 at.% can be achieved by simply exposing the RbSi to hydrogen gas at a relatively low pressure. For the case of pure silicon the solubility of hydrogen is only 3-4 at.%. The only way to achieve a higher H- content in pure silicon is to force the H atoms into the Si layer using hydrogen ion implantation, with the problematic consequences, mentioned above. [0053] Thirdly, unlike the H-implanted Si layers, which are supersaturated and

thermodynamically unstable, the RbSiH₃ phase is stable at room temperature under only 40 mbar hydrogen pressure. This translates to a low H₂ pressure of merely 4.0 KPa inside potential voids and cracks in a multilayer reflector structure, which would be much too low to cause blistering damage in the multilayer reflector.

[0054] A further aspect of the invention provides a multilayer layer mirror comprising a multi-layer stack of Mo/RbH.

[0055] The reaction of elemental Rb with hydrogen gas leads to the formation of a RbH phase (NaCI crystal structure type), which has a density of 2.60 gr-cm^"3 (2600 kg/m³) and a decomposition temperature of 443 K. Similar to RbSi, RbH reacts strongly in contact with air (see D.R. Lide, Handbook of Chemistry and Physics (87 ed.), Boca Raton, FL: CRC Press, (1998) pp. 4-79).

Rb + ^H₂→RbH AH° = -52.3 ± 5.2 KJ AS° = -17.8 ± 1.8 JK ¹ (3)

Here AH° and AS° are the standard enthalpy and entropy of reaction respectively (298.15

K, 101 .3 kPa). The entropy terms in this reactions cannot be ignored as they involve the change of hydrogen from molecular form in the gas phase into atomic interstitial form in the solid phase. The standard entropy of formation of RbH is calculated from the standard molar entropies of Rb and RbH, i.e. S° = 76.8 JK 'mol ¹ and S° = 59 JK 'mol ¹ . The errors for the thermodynamic data relative to RbH were not reported. To be on the safe side we have assumed them to be 10 % of the standard enthalpy and entropy values.

[0056] To check the compatibility of RbH for EUV MLM applications it is important to check the equilibrium hydrogen pressure of RbH as a function of temperature. Every material unavoidably has numerous cavities or micro-cracks. If the equilibrium H₂ pressure at a given temperature is higher than the stress needed for the growth of cracks (fracture strength), the release of H₂ gas into these voids can lead to blistering and MLM failure. Using the standard enthalpy and entropy of formation of RbH it is possible to calculate the equilibrium pressure of hydrogen ( P_H ) gas as function of temperature ( 7):

Here R is the gas constant, and K is the molar coefficient of H₂ in the reaction. For the temperature range relevant for EUVL, P_Hi is less than 1 E-6 Pa. This value is far below the typical fracture strength in solids (MPa's to GPa's range). Therefore, from this aspect the application of RbH in MLMs appears to be safe. [0057] Moreover, a Mo/RbH MLM is not expected to undergo any intermixing, as nor Rb nor H form any compound with Mo, and their solubility in Mo is negligible. For this reason, we expect the intermixing at the Mo/RbH interface to be fully suppressed and the interface sharpness to be thermodynamically protected. This implies that the a of interfaces can be assumed to be at its minimum possible value (i.e. 2.0 A).

[0058] This can be confirmed by calculating reflectance values of Mo/RbH MLM with σ = 4.5 A (corresponding to only 50 % decrease of intermixing). The results are presented in Table 3 which gives calculated reflectance values (normal incidence, 13.5 nm wavelength light) for Mo/RbA (A = Si, H, and SiH₃) MLMs in comparison to Mo/Si, with and without a B₄C protective interlayer The effect of intermixing at interfaces is taken into account. For each Mo/Rb_nA_m MLM, two set of reflectance values are presented: one with low σ value for which the suppression of intermixing driving force are derived from thermodynamic analysis, and the other with higher σ values in which the suppression is assumed to be only half of first one. For each MLM, the d and Γ combination is optimized to reach the highest possible peak reflectance at 13.5 nm wavelength. For all cases N = 100 and σ for both substrate and top surface is assumed to be 2.0 A. For Mo/Si without interlayer (to which the other integral reflectance values are normalized), the integral reflectance in the 12.5-14.5 nm range for a single and 10 consecutive mirrors are 37.6 and 0.584 [% nm] respectively. See

supplementary information for the graphs of the reflectance peaks.

[0059] While for Mo/RbSi, Mo/RbH, and Mo/RbSiH₃ MLMs the reflectance values have an increasing order, their thermal stability is in descending order as RbSi, RbH, and RbSiH₃ decompose at 623, 443, and 401 K [34] respectively.

[0060] In the environment of EUVL machines, both RbH and RbSiH₃ phases are expected to be more stable with respect to the Rb and RbSi phases. Using RbSiH₃ or RbH layers in the MLM instead of Si can reduce or eliminate the hydrogen blistering problems which are common for current Mo/Si MLMs. The equilibrium H₂ pressures of RbH and RbSiH₃ (at the appropriate service temperatures) are rather low, and H diffusion in these phases are expected to be high (especially for RbSiH₃ due to the high concentration of H atoms, low hydrogenation free energies and its open crystal structure). This is an additional advantage as it opens the possibility that the excess H₂ pressure in the MLM cracks or extra H-atoms, which are forced into the MLM by H+ bombardment, can effectively be drained out of the MLM via the RbSiH₃ layers, hence preventing the development of blistering. On the other hand, for a pure Si layer the low H solubility and diffusion coefficient lead to accumulation of hydrogen and eventually to blistering.

[0061] According to an embodiment of the invention, it is proposed to add RbH interlayers to a multilayer mirror with a high refractive layer comprising Si and Rb. Such interlayers inhibit thermodynamically driven interlayer mixing. The RbH interlayer can have a thickness in the range of from 0.1 to 0.4 nm, desirably about 0.2 nm.

[0062] By implementing the proposed Mo/RbSi, Mo/RbH, and Mo/RbSiH₃ (the latter with RbH interlayers) MLMs or a combination of them, the total EUV throughput can be increased up to a factor 2 with respect to the current Mo/Si MLM solutions.

[0063] In addition, using RbH or RbSiH₃ phases may offer new possibilities to reduce or eliminate the hydrogen blistering problems which are common for current Mo/Si MLMs.

[0064] In an embodiment of the invention, the multilayer reflector is a reflective mask for patterning an EUV beam. A reflective mask for EUV comprises a multilayer reflector exactly as described above with a patterned absorber layer provided on the incident surface. The patterned absorber layer is designed to expose a radiation sensitive layer in such a way that a desired pattern can be transferred to a substrate. The pattern in the absorber layer does not necessarily directly correspond to the pattern transferred to the substrate. The pattern in the absorber layer may include non-printing optical proximity correction (OPC) features and/or variations in feature size (e.g. line width) to compensate for effects of development and pattern transfer steps.

[0065] A multilayer reflector according to an embodiment of the invention can be manufactured using conventional techniques, known for manufacture of Mo/Si multilayer reflectors. There is no change to the process of forming Mo layers. To form a Si layer containing Rb, a deposition technique, such as pulse laser deposition or sputtering deposition, using a target containing Si and Rb in the appropriate proportions, can be used. To hydrogenate the Rb_xSi_y layer, the deposition can be performed in a hydrogen atmosphere. Generally, no specific measures need to be taken to ensure adhesion between Mo layers and Rb_xSi_y layers; Van der Waals forces are sufficient.

[0066] In an embodiment, the invention may form part of a mask inspection apparatus. The mask inspection apparatus may use EUV radiation to illuminate a mask and use an imaging sensor to monitor radiation reflected from the mask. Images received by the imaging sensor are used to determine whether or not defects are present in the mask. The mask inspection apparatus may include multilayer reflectors of the type described above configured to receive EUV radiation from an EUV radiation source and form it into a radiation beam to be directed at a mask. The mask inspection apparatus may further include multilayer reflectors of the type described above configured to collect EUV radiation reflected from the mask and form an image of the mask at the imaging sensor. The mask inspection apparatus may include a processor configured to analyze the image of the mask at the imaging sensor, and to determine from that analysis whether any defects are present on the mask. The processor may further be configured to determine whether a detected mask defect will cause an unacceptable defect in images projected onto a substrate when the mask is used by a lithographic apparatus.

[0067] In an embodiment, the invention may form part of a metrology apparatus. The metrology apparatus may be used to measure alignment of a projected pattern formed in resist on a substrate relative to a pattern already present on the substrate. This

measurement of relative alignment may be referred to as overlay. The metrology apparatus may for example be located immediately adjacent to a lithographic apparatus and may be used to measure the overlay before the substrate (and the resist) has been processed. The metrology apparatus may use EUV radiation for increase resolution and hence may use multilayer reflectors of the type described above to form and direct an EUV radiation beam.

[0068] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.

[0069] The present invention can also be applied to multilayer reflectors that are used in other fields than EUV lithography, for example in x-ray optics for astronomical applications.

[0070] The term "EUV radiation" may be considered to encompass electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. EUV radiation may have a wavelength of less than 10 nm, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm.

[0071] Although Figures 1 and 2 depict the radiation source SO as a laser produced plasma LPP source, any suitable source may be used to generate EUV radiation. For example, EUV emitting plasma may be produced by using an electrical discharge to convert fuel (e.g. tin) to a plasma state. A radiation source of this type may be referred to as a discharge produced plasma (DPP) source. The electrical discharge may be generated by a power supply which may form part of the radiation source or may be a separate entity that is connected via an electrical connection to the radiation source SO.

[0072] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.

[0073] 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 multilayer reflector comprising a plurality of periods, each period comprising a low refractive index layer and a high refractive index layer, wherein: in at least one period the low refractive index layer comprises Mo and the high refractive index layer comprises a compound comprising a combination of at least two elements selected from the group consisting of Si, Rb, and H.

2. A multilayer reflector according to claim 1 wherein the high refractive index layer of at least one period comprises RbSi.

3. A multilayer reflector according to claim 1 wherein the high refractive index layer of at least one period comprises Rb₆Si₄₆.

4. A multilayer reflector according to claim 1 , 2 or 3, wherein the high refractive index layer of at least one period further comprises a rubidium-silicon hydride.

5. A multilayer reflector according to claim 4 wherein the high refractive index layer of at least one period comprises RbSiH₃.

6. A multilayer reflector according to claim 4 or 5 further comprising a RbH interlayer between the low refractive index layer and the high refractive index layer.

7. A multilayer reflector according to any one of the preceding claims wherein the high refractive index layer is amorphous.

8. A multilayer reflector according to claim 2 wherein the high refractive index layer is in a zintl phase.

9. A multilayer reflector according to any one of the preceding claims wherein the high refractive index layer of at least one period comprises, in order, a first sub-layer, a second sub-layer and a third sub-layer, wherein the first and third sub-layers have a higher content of Rb than the second sub-layer.

10. A multilayer reflector according to claim 1 wherein the high refractive index layer comprises RbH.

1 1 . A multilayer reflector according to any one of the preceding claims wherein the high refractive index layers of all the periods have essentially the same composition.

12. A multilayer reflector according to any one of the preceding claims further comprising at least one capping layer on a radiation receiving surface of the multilayer reflector.

13. A lithographic apparatus arranged to project a pattern from a patterning device onto a substrate, the apparatus comprising at least one multilayer reflector according to any one of the preceding claims.

14. A lithographic tool comprising at least one multilayer reflector according to any one of claims 1 to 12.

15. A mask for use in a lithographic apparatus, the mask comprising at least one multilayer reflector according to any one of claims 1 to 12 and a patterned absorber layer.

16. A method comprising projecting a patterned beam of radiation onto a substrate, wherein the patterned beam is directed or patterned using at least one multilayer reflector according to any one of claims 1 to 12.

17. A method of manufacturing a multilayer reflector comprising a plurality of periods, each period comprising a low refractive index layer and a high refractive index layer, the method comprising a physical vapour deposition process to form at least one high refractive index layer, the physical vapour deposition step using an evaporation target comprising Si and Rb.

18. A method according to claim 17 wherein the physical vapour deposition process comprises pulsed laser deposition.

19. A method according to claim 17 wherein the physical vapour deposition process comprises sputter deposition.

20. A method according to claim 17, 18 or 19 wherein the physical vapour deposition process is performed in a hydrogen atmosphere.