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Title:
IMPROVED EXTREME ULTRA VIOLET LIGHT SOURCE
Document Type and Number:
WIPO Patent Application WO/2015/108420
Kind Code:
A1
Abstract:
The present invention relates to an Extreme Ultra Violet light source, to a trap for use in such a light source, to a multilayer mirror for use in such a light source, to a grazing incident mirror for use in such a light source, and to a multi- layer for use in the multilayer mirror. Such a light source may for instance be used in lithography, such as used in semiconductor integrated circuit production.

Inventors:
MULDER FOKKO MARTEN (NL)
Application Number:
PCT/NL2015/050028
Publication Date:
July 23, 2015
Filing Date:
January 16, 2015
Export Citation:
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Assignee:
UNIV DELFT TECH (NL)
International Classes:
H05G2/00; G03F7/20
Foreign References:
DE102012202850A12013-08-29
US20090074962A12009-03-19
US20090001288A12009-01-01
Attorney, Agent or Firm:
VOGELS, Leonard Johan Paul (1017 XS Amsterdam, NL)
Download PDF:
Claims:
CLAIMS

1. An improved extreme ultraviolet light (EUV) source for use in nanolithography comprising

at least one vacuum chamber,

an emission source, wherein the emission source pro- duces EUV light upon activation,

a hydrogen source,

at least one multilayer mirror for reflecting and directing EUV light, the at least one multilayer mirror having interlayer interference and comprising a mirror top layer, the at least one multilayer mirror having an operating temperature of 273-375 °K,

at least one means for capturing particles of the emission source, the at least one means for capturing parti¬ cles having an operating temperature of 1000-1500 °K,

characterized in that

the means for capturing particles, and the at least one multilayer mirror top layer both comprise a material which material comprises at least one metal and has, at given bound¬ ary conditions, the boundary conditions comprising the operat- ing temperature Toper/

Rla) a negative enthalpy of MH formation (ΔΗ<0 mate¬ rial + hydrogen -» Metal-hydride ) and

Rib) a hydrogen equilibrium pressure at an operating temperature Toper of the material of 0.1 Pa-108 Pa, allowing ab- sorption of hydrogen.

2. A light source according to claim 1, wherein the means for capturing particles is a rotating foil trap which is made of an alloy or intermetallic compound comprising AaBtCc, wherein A is selected from Nb, Re, and Ta, B is selected from W, and Mo, and C is selected from V, Hf, Ti, Ir, Si, Ge, Mo, Os, Pt, Pd, Au, and Zr, ae [ 0, 75; 0, 99] , b [ 0, 01 ; 0, 25] , and C £ [0, 01; 0, 10] .

3. A light source according to any of the preceding claims, comprising at least one of a grazing incidence mirror having an operating temperature of 240-375 °K, the material has (R0) a melt temperature Tmeit of more than 100 K higher than an operating temperature (Tmeit > Toper + 100 K) , a layer thick¬ ness is from 0.5-30 nm, the emission source is laser excita- tion of Sn, a means for forming at least partially coherent EUV light, and a trap for capturing particles.

4. A light source according to any of the preceding claims, wherein boundary conditions further comprise a mathe- matical average of the plasma potential over time or maximum of the plasma potential, as measured by a Langmuir probe on a surface of a mirror or surface of means for capturing parti¬ cles.

5. A light source according to any of the preceding claims, wherein the hydrogen equilibrium pressure at given boundary conditions of the material is in a range from 1 Pa-10 Pa, and wherein

(R2) the hydrogen equilibrium pressure at given boundary conditions when the plasma is applied is in a range from 10 Pa-107 Pa.

6. A light source according to any of the preceding claims, wherein the at least one multilayer mirror comprises a sequence of layers ie[2;n], wherein a layer closer to a surface of the mirror, the surface reflecting EUV light, has a lower hydrogen equilibrium pressure at given boundary conditions of the material than a subsequent layer (Ρ[Η2]±(@Τ)< P[H2 ] i+i (@T) ) .

7. A light source according to any of the claims 2-7, wherein the rotating foil trap comprises a material with at least one of

(ROa) a hardness of > 4 Moh,

(ROb) a tensile strength of > 300 MPa, and

(ROc) an elastic modulus of > 100 GPa.

8. A light source according to any of the preceding claims, wherein a surface voltage compensator is provided for the at least one mirror and/or for the means for capturing particles .

9. A light source according to any of the claims 3-8, wherein the grazing incidence mirror comprises a layer of an alloy or intermetallic compound CcDdE e , wherein C is selected from Zr, Mo, Ta, Re, and Nb, D is selected from W, Rh, Mo, Ru, Zr, Nb, Sc, Ti, Mg, Ni, Co, Zn, Cu, Fe, Cr, Mn, Hf, La, Ta, Ir, Os, Pt, Pd, Au, and V, E is selected from Rh, Mo, Ru, Zr, Nb, Sc, Ti, Mg, Ni, Co, Zn, Cu, Fe, Cr, Mn, Hf, La, Ta, Ir, Os, Si, H, Re and V, ce [ 0 , 30 ; 0 , 95 ] , de [ 0 , 05 ; 0 , 70 ] , and ee [0, 00; 0, 35] .

10. A light source according to any of the preceding claims, wherein the at least one multilayer mirror comprises at least two layers,

wherein a first layer has a different density and thus EUV scattering density and refractive properties compared to a second layer, and

wherein a layer has metallic or non-metallic proper¬ ties.

11. A light source according to claim 10, wherein the metallic layer comprises an alloy or intermetallic compound FfGgJj, wherein F is selected from Zr, Mo, Pd, Re, and Nb, G is selected from Rh, Mo, Ru, Zr, Nb, Sc, Ti, Mg, Ni, Co, Zn, Cu, Fe, Cr, Mn, Hf, La, Ta, Ir, Os, N, Ir, Os, Pt, Pd, Au and V, J is selected from Rh, Mo, Ru, Zr, Nb, Sc, Ti, Mg, Ni, Co, Zn,

Cu, Fe, Cr, Mn, Hf, La, Ta, Ir, Os, Si, Ge, H, B, 0, Al, N, Re and V, f [0, 16; 0, 99] , ge [ 0, 05; 0, 45] , and j e [ 0 , 00 ; 0 , 70 ] , wherein the alloy or intermetallic compound has a EUV-light trans- mittance (@ 13 nm) of >0,7, and

wherein the non-metallic layer comprises a compound

¾LjMm where K is selected from Al, Si, B, L is selected from B, C, N, 0, Mg, Al, P, and M is selected from H, B, C, N, 0, Mg, Al, P, k<≡ [0, 25; 0, 99] , l e [ 0 , 0 ; 0 , 7 ] , me [ 0, 0 ; 0, 7 ] .

12. Rotating foil trap for use in a light source ac- cording to any of the preceding claims, wherein the rotating foil trap comprises a material which material comprises at least one metal and has, at given boundary conditions, the boundary conditions comprising operating temperature Toper of 1000-1500 °K,

Rla) a negative enthalpy of MH formation (ΔΗ<0 mate¬ rial + hydrogen -» Metal-hydride) and

Rib) a hydrogen equilibrium pressure of 0.1 Pa-108 Pa, allowing absorption of hydrogen.

13. Multilayer mirror or grazing incident mirror for use in a light source according to any of the claims 1-12, wherein the mirror comprises a sequence of layers ie[2;n], wherein a layer closer to a surface of the mirror, the surface reflecting EUV light, has a lower hydrogen equilibrium pressure at given boundary conditions, the boundary conditions comprising operating temperature Toper / than a subsequent layer (P [¾] i ( @T) < P [¾] i+i ( @T) ) , and optionally comprising a protec¬ tion layer.

14. Multilayer for use in multilayer mirror according to claims 13 comprising a sequence of layers ie[2;n], wherein a layer closer to a surface of the multilayer, the surface be¬ ing capable of reflecting light, such as EUV light, has a low¬ er hydrogen equilibrium pressure at given boundary conditions, the boundary conditions comprising operating temperature Toper of 273-375 °K, than a subsequent layer (Ρ[Η2]±(@Τ)<

P[H2 ] i+i(@T) ) .

Description:
FIELD OF THE INVENTION

The present invention relates to an Extreme Ultra Vi- olet light source, to a trap for use in such a light source, to a multilayer mirror for use in such a light source, to a grazing incident mirror for use in such a light source, and to a multilayer for use in the multilayer mirror. Such a light source may for instance be used in lithography, such as used in semiconductor integrated circuit production.

BACKGROUND OF THE INVENTION

An Extreme Ultra Violet light (EUVL) light source re ¬ lates to a complex system.

In terms of a source of radiation it is noted that neutral atoms or condensed matter cannot emit EUV radiation. For matter to emit it, ionization must take place first. In view of energy involved, EUV light can only be emitted by electrons which are bound to multicharged (M n+ ) positive ions. Such electrons are more tightly bound than typical valence electrons. A thermal production of multicharged positive ions is typically only possible in a hot dense plasma, which itself strongly absorbs EUV. As possible elements Xe and Sn are con ¬ sidered. The Xe or Sn plasma sources for EUV lithography may be discharge-produced or laser-produced. Discharge-produced plasma is made by lightning in e.g. a tin vapor. Laser- produced plasma is made by microscopic droplets of molten tin heated by powerful laser. Laser-produced plasma sources are considered to outperform discharge-produced plasma sources. An EUVL output power exceeding 100 W is a requirement for ena- bling sufficient throughput of a lithography system that uti ¬ lises EUV. While state-of-the-art 193 nm ArF excimer lasers offer intensities of 200 W/cm 2 , lasers for producing EUV- generating plasmas need to be much more intense, on the order of 10 10 W/cm 2 ; in other words very high energy levels per unit surface.

A further characteristic of the plasma-based EUV sources under development is that they are not coherent, un ¬ like lasers used for prior art optical lithography.

A further issue relates to dosing of EUV light and calibration of an EUV dosimeter, which is a nontrivial un- solved issue. The secondary electron number variability is a well-known root cause of noise in avalanche photodiodes.

It is noted that EUVL differs significantly from deep ultraviolet lithography. Unfortunately all matter absorbs EUV radiation. In order to overcome this problem at least partial ¬ ly EUV lithography needs to take place in a vacuum (or at least reduced pressure) . Optical elements used in lithography, including a photomask, must make use of advanced multilayers. These multilayers must be largely defect-free, in order to minimize side effects, such as absorption of light. An example of such a multilayer is a Mo/Si multilayer which reflects light by means of interlayer interference; unfortunately these mirrors will still absorb a significant amount of the incident light («30%) .

Typically EUVL systems contain a significant amount of mirrors, such as at least two condenser multilayer mirrors, six projection multilayer mirrors, and a multilayer object (mask) . A disadvantage of the present optics is that these ab ¬ sorb a large fraction («96%) of available EUV light; therefore a EUV source will need to be sufficiently bright amongst oth ¬ ers for compensating absorption losses. EUV source development has focused on plasmas generated by laser or discharge pulses. A mirror for collecting the light is unfortunately directly exposed to the plasma; it is therefore vulnerable to damage from the high-energy ions and other debris from the plasma.

This damage associated with the high-energy process of gener ¬ ating EUV radiation has hindered implementation of EUV light sources for lithography.

It is noted that various other issues are associated with use of EUVL in lithography, such as flare, heating per feature volume, and increased heating due to the vacuum envi ¬ ronment .

Heating is also a particularly serious issue for mul ¬ tilayer mirrors used, because, as EUV is absorbed within a thin distance from the surface, the heating density is higher. As a result, water cooling is expected to be used for the high heating load; however, the resulting vibration of cooling is a concern .

Also multilayer optics contamination is a concern. It is noted that as EUV is highly absorbed by all ma- terials, EUV optical components used inside a lithography tool are susceptible to damage, mainly manifest as observable abla ¬ tion. Such damage is a new concern specific to EUV lithogra ¬ phy, as conventional optical lithography systems use mainly transmissive components and electron beam lithography systems do not put any component in the way of electrons, although these electrons end up depositing energy in the exposed sample substrate .

A further problem is that elements used in an EUV light source tend to degrade. In an example thereof multi ¬ layers used tend to delaminate. Also elements are vulnerable to contamination, such as from the multicharged ions (e.g. Sn) in the EUV emitting plasma. Thereto in principle unwanted spe ¬ cies, such as hydrogen, are introduced, in order to capture the multicharged ions and also to chemically remove debris from the plasma condensed on the optical components (e.g. Sn) . However, hydrogen gas as well as ionised hydrogen in the plas ¬ ma also interacts with optical and construction elements used, which is in principle unwanted.

Various documents relate to EUV light sources.

DE 10 2012 202850 Al recites a method for optimizing a protective layer system for an EUV radiation reflecting multilayer system of an optical element, comprising various steps, wherein a topmost layer has a thickness of not more than 5 nm and a thickness of the further layer or of the further layers is greater than 5 nm. The invention relates to protecting a mirror from particles, typically Sn particle, in the EUV light source. It does consider the chemical stability or enthalpy of formation of the topmost layer itself, but not its stability and durability during operation, e.g. in a hot plasma comprising hydrogen. It further does not relate solu ¬ tions to capturing particles.

US2009001288 (Al) recites a lithographic apparatus that includes a radiation system constructed to provide a beam of radiation from radiation emitted by a radiation source. The radiation system includes a contaminant trap configured to trap material emanating from the radiation source. It does not relate to multilayer mirrors suited in an EUV system, nor to suitable materials for such a system.

It is observed that the material used in the above two patent applications are bound to suffer from material deg ¬ radation, due to given boundary conditions such as operating temperature, plasma, and presence of hydrogen. Further, the two documents do not provide any detail on layers, apart from highly protective and partly reflective characteristics. On the other hand, the generation of EUV-radiation per se may be applicable in the present invention.

The present invention therefore relates to an im ¬ proved EUV light source and further details thereof, which overcome one or more of the above disadvantages, without com ¬ promising functionality and advantages.

SUMMARY OF THE INVENTION

It is an object of the invention to overcome one or more limitations of EUV light sources of the prior art and at the very least to provide an alternative thereto. The above mentioned EUV light source per se could be used, as well as further source as mentioned e.g. in US 2007/0008517 Al, US 2005/0199830 Al .

In a first aspect, the invention relates to EUV light sources according to claim 1, comprising one or more improvements, amongst others relating to an improved hydrogen con ¬ trol/management/balance, e.g. an amount of hydrogen entering and leaving a component of the lights source is managed and controlled, a hydrogen amount in such a component is con- trolled within certain limits, a balance at given boundary conditions is establishes between hydrogen atoms entering and leaving, a hydrogen pressure is controlled within certain limits by a suitable choice of hydrogen affinity and hydrogen transport characteristics of the materials, interaction with other species such as Sn is controlled and managed, removal of other species such as Sn and other trace species is balanced and controlled, etc.

The present EUV light source preferably comprises enough mirrors for reflection and condensing, such as at least two condenser multilayer mirrors and six projection multilayer mirrors .

A mirror, such as a grazing incidence mirror, may comprise a supporting layer, such as a Mo layer. In view of Mo, it is preferred to use another material, such as Ni . It may further comprise one or more EUV-light reflecting layers. Another type of mirror (for use in reflecting higher angle up to perpendicular incidence rays) may also contain a series of further layers, such as at least one Mo-Si double layer.

It is preferred that the present material (and layer thereof) match with further layers being provided, such as the above one or more reflecting layers, and a supporting underground. In an example of a grazing incidence mirror, specifi ¬ cally the present material matches with a reflecting layer, e.g. in terms of lattice constant, option of forming an alloy therewith, etc.

For the present mirrors the material is preferably not absorbing EUV light, such as 50% or more is reflected by the (thin) layers. It is also preferably capable of absorbing, transferring and releasing hydrogen. The layer preferably does not expand more than a few percent, preferably less than 2% (at operating temperature and hydrogen pressure) .

It is noted that at least some of the present ele ¬ ments of the claims are somewhat functional of nature. Howev ¬ er, said functionality is considered in view of specific cir- cumstances, such as given boundary conditions. In view of the enthalpy of formation and hydrogen equilibrium pressure, as well as an average plasma potential, it is noted that these are taken at the given boundary conditions, e.g. operating temperature. For the rotating foil trap such an operating tem- perature is typically from 1000 -1500 K, and for the multi ¬ layer mirror and grazing incident mirror it typically is 273- 375 K and 240-375 K, respectively. The mirror typically are cooled, such as by water, ammonia or methanol. The hydrogen pressure is from about 5 Pa - 500 Pa, typically 10-400 Pa, whereas an average plasma potential ranges from 0.1-5 V, e.g. depending on an average plasma temperature. In view of the plasma potential it is noted that it has been found that a difference in average plasma potential at a surface of e.g. a mirror, of e.g. 0.1 V may have as a result a change in hydro- gen equilibrium pressure of 10 8 Pa, and a difference of 0.2 V may already result in a change in hydrogen equilibrium pres ¬ sure of 10 16 Pa. So despite modest changes in average poten ¬ tial, a relative huge change in hydrogen equilibrium pressure may occur. Of course exact equilibrium pressures depend on ma- terials, the (average) plasma potential operating on charge hydrogen species, as well as the release rate of neutral H2 from the materials surface.

In an example, the EUV produced by a pulsed laser ig ¬ nition source causes a time dependent plasma generation and intensity, resulting in a pulsed plasma potential interacting on the materials and components in contact with the plasma. The plasma potential varies rapidly in time. Using a so called Langmuir probe the effective plasma potential with respect to the wall can be measured during operation. As described above the plasma potential leads to insertion of charged H + and ¾ + ions in the materials that continues to raise the H content of the materials. Neutral ¾ can, however, form at the surface from the H adsorbed and can escape from the metal, not hin ¬ dered by the plasma potential but only obeying the van t Hoff relation above. The equilibrium hydrogen content is thus determined by an average plasma potential, which can be deter- mined with the Langmuir probe, and the materials hydrogen up ¬ take and release characteristics under the applied hydrogen pressures and the temperature.

The plasma potential is measured near a surface of a component of the EUV light source, such as the mirror. It may further be used to determine e.g. the temperature of a plasma. As a result the potential between plasma and the surface can be obtained. The technique is well known to a person skilled in the art. It is preferred to use a double or triple probe, in view of accuracy and dependence on position relative to the source. It is also preferred to use electrodes that can with ¬ stand high temperatures, such as platinum, tungsten, tantalum, or an alloy mentioned with respect to the rotating foil trap, of adequate thickness, such as a 10-100 μπι. in order not to disturb the plasma.

It is noted that the given boundary conditions can be established quite accurately, upon construction and use of a light source. As a result the characteristics of the present mirrors and trap can be adapted quite easy.

In this respect it is noted that in order to capture debris and particles, emitted by the emission source and plas ¬ ma, various measures are considered. One of these measures is to capture debris and particles by a trap, such as a rotating trap. A further option is to reduce particles and debris to relatively innocent species, such as by chemically reacting particles and debris. An example of such a chemical species reacting relates to hydrogen (¾) or protons, which e.g. react with Sn to form gaseous Sn¾ . Thereto a hydrogen source is pro ¬ vided .

The removal of debris from the pulsed light source, including condensed Sn may be assisted by the installation of a microwave assisted continuous plasma. This is considered to induce continuously present hydrogen plasma with a not so high plasma temperature and potential, which then can continuously remove the debris in addition to the very hot pulsed plasma from the pulsed EUV source itself. The hydrogen pressure in the vacuum chamber may then be reduced since the continuously present plasma can work effectively with lower hydrogen pres ¬ sure. The lower hydrogen pressure helps to reduce the hydrogen implantation during the high plasma potential pulses of the EUV source. To stop the debris from the EUV source preferably still a sufficient gas pressure is present; however, this may be a low pressure hydrogen or a low pressure mixture of hydro ¬ gen and argon.

The materials used in the present light source expe- rience a high dose of atomic H + and ¾ + implantation as a re ¬ sult of the hydrogen containing plasma. It has been found that such implantation leads to significant damage to the materials used. The damage leads over time to hydrogen embrittlement , blister formation and degradation of the specifications of the materials, and a reduced lifetime.

Present inventors have examined an origin of the dam ¬ age in the materials used. The results of these examinations are used to enable mitigation of damage by suitable materials modification, by modified operating methods, and/or by modi- fied design rules for the materials and components. Some spec ¬ ifications of components typically considered are given in the examples below.

It has been found that especially the prior art met ¬ als Ru and Mo (as relatively pure metals per se) are consid- ered relatively unsuited for use in the present EUV light source. Especially presence of hydrogen plasma is a concern. It has been found that hydrogen will build up a high hydrogen pressure in defects or at interfaces of the materials (c.q. layers), resulting in damaging of the material by hydrogen em- brittlement, strains and delamination . The effect of this dam- age of the materials is that these do not fulfil specifica ¬ tions any more.

When considering hydrogen and e.g. metals the following is considered.

The presence of an electrical potential on a conduc ¬ tive hydrogen and protons containing medium (the plasma) is considered to cause a change in the thermodynamic equilibrium hydrogen content inside a material, as is detailed below.

In general hydrogen reacts with metals in a reaction:

M + ½H 2 <→ MH

The equilibrium hydrogen gas pressure P¾ is considered to fol ¬ low the van t Hoff equation:

^P Hi _2AH f 2AS 0

P ref RT R with AH f the enthalpy of formation in kJ/mole H of the metal hydride, T the operating temperature, R the gas constant, and ASo the entropy change in the reaction above. The equilibrium pressures at different temperatures can experimentally be de- termined using a Sieverts instrument for measuring pressure composition isotherms. In such isothermal experiment the amount of hydrogen (composition) is varied in known amounts while measuring the resulting hydrogen pressures. The pres ¬ sures required to reach a certain composition is temperature dependent and is following the van' t Hoff relation above.

Electrochemical insertion of hydrogen in a metal is considered to follow the relation: where P ref is 10 5 Pa, AG the change in Gibbs free energy, EMH the energy of formation of a metal hydride, Eo the electric po ¬ tential, and F is the Faraday constant of 96485.34 C/mol.

These equations relate the alterations in hydrogen equilibrium pressure when an electrical potential is applied between the metal and a reference electrode with a proton conducting elec ¬ trolyte in between. As such the hydrogen equilibrium pressure can be determined at any given temperature. From the equations above it can be concluded that:

Rrin -E 0 ) = 2AH f - 2TAS 0

which indicates that under an applied potential the equilibri ¬ um pressure behaves as if the metal hydride has an altered AH f (assuming that ASo essentially remains constant) .

The plasma generates a plasma potential Φ that is considered to depend on the plasma electron temperature T e :

1

φ = kT e -

2 e

The resulting potential Φ = E MH - E 0 will thus drive the insertion of hydrogen in the metal facing the plasma.

The plasma is pulsed, leading to high pulses of the potential and intermediate return to (close to) zero potential difference. On average there results a negative potential how ¬ ever, that will on average alter the hydrogen equilibrium pressure and the hydrogen content in the material.

It has been found that the present material is capa ¬ ble to accept hydrogen inserted from the plasma, and it is ca ¬ pable of releasing it again to the plasma, at present condi ¬ tions of about 5 Pa and also at future conditions of 200 Pa hydrogen pressure. It has been found that as a result of the selection of the present material that if H would diffuse to an underlying material that cannot accept the H and/or there is a defect, the pressure in the defect will essentially not go above the ¾ equilibrium pressure; thereby damage caused by hydrogen pressure build-up is prevented. Such is further de- tailed in figure 2.

Typically characteristics of a metal are that they may oxidize, may form a cation, are an electrical conductor, etc. The present alloys and intermetallic compounds have an equilibrium pressure (Pa) for hydrogen absorption. Such an equilibrium pressure per se can be measured with means known to a person skilled in the art.

In table 1 various metals and hydrogen equilibrium pressures (Pa, @273 K) (as calculated/determined) are given. A typical method of determining such pressures is the so-called Sieverts method. The below example are given to get an impres ¬ sion of typical values. With the numbers below the equilibrium pressures at e.g. any given temperature can be determined, e.g. using the relations detailed in the present description.

P0273K [Pa] AH hydride [kJ/mol H 2 ]

FeTi 1.1*10 5 -14

LaNi 5 4*10 4 -15

Mg 1.1*10° -38

Ti 6.1*10 ~15 -71

Zr 1.2*10 ~23 -85

Ta 1*10 ~13 -35

Table 2. Enthalpy of metal hydride formation for some relevant metals in the EUV context . AH so i ut ion and AH hy dride refer to low concentration dissolved H in the metal ( -phase) and high concentration hydrides respectively (β-phase) .

AHhydride

[kJ/mol H] [kJ/mol

Nb -35 -40

Ta -35 -35

Mo +24 +52

W + 90 +22

Zr -60 -85

Pd -12 -19

Pt +25 +20

Ru +50 +55

V -32 -32

Ni -4 -5

It is noted that hydrogen is typically provided to remove the debris of the emission source, such as Sn. Specifi ¬ cally a thin layer of e.g. Sn is in operation deposited on e.g. a mirror; the thin layer can largely be removed by providing hydrogen, typically in a continuous mode. When using e.g. Sn, as is typically done, hydrogen seems a requirement for proper operation of an EUV light source, albeit at the consequence of the side-effects.

The present equilibrium pressure is taken such that at the one hand the present material can absorb hydrogen at given boundary conditions, and at the other hand can release hydrogen, for instance at room temperature. For practical purpose the various parameters men ¬ tioned can be used at an average value, e.g. an average oper ¬ ating temperature. Such is not optimal; however by carefully selecting various components and building in safeguards, e.g. a large difference in hydrogen equilibrium pressure between two subsequent layers, no further issues are to be expected.

As a result of the above selection of materials, in view of given boundary conditions, the present light source provides an extended life time, with no or limited degradation of components, such as rotating foil trap, multilayer mirrors and grazing incident mirror. Blister formation, delamination of layers and the like is largely prevented.

It is noted that a term as "on top" or "above" may relate to a sequence of e.g. layers, a first layer covering a second layer. The layer may also partly be on top, i.e. a first layer covering a part of a second layer. In view of the present application such terminology is mainly functional of nature .

The present invention provides a solution to one or more of the above mentioned problems and overcomes drawbacks of the prior art.

Advantages of the present description are detailed throughout the description.

DETAILED DESCRIPTION OF THE INVENTION

In a first aspect the present invention relates to an improved extreme ultraviolet light (EUV) source for use in nanolithography according to claim 1, wherein the trap, the optional grazing incidence mirror and the at least one multi ¬ layer mirror top layer comprise a material which material com- prises at least one metal and has

Rla) a negative enthalpy of MH formation (ΔΗ<0 mate ¬ rial + hydrogen -» Metal-hydride ) and

Rib) a hydrogen equilibrium pressure at an operating temperature T ope r of the material of 0.1 Pa-10 8 Pa, allowing ab- sorption of hydrogen. The pressure range may seem rather large for a single material; it should however be realized that if a stack of layers is used a gradual increase from hydrogen equi ¬ librium pressure from layer to layer can be made use of. Such is detailed below. Further, with the details and definitions presented throughout the description the person skilled in the art is capable of determining the given boundary conditions R0, Rl and R2 respectively. The fact that the boundary condi ¬ tions depend on other parameters, e.g. temperature, is at the most a slightly complicating factor for such a determination.

In an example of the present light source the bounda ¬ ry conditions further comprise an average plasma potential. The boundary conditions are taken into account, e.g. when de ¬ termining/calculating requirements for the present material, as measured by a Langmuir probe on a surface of a mirror or surface of a trap.

It is noted that plasma facing materials experience an electrochemical proton potential that also depends on a plasma temperature. Neutral ¾ is however released from the ma ¬ terials without interference from the plasma potential. As a result also the effective hydrogen equilibrium pressures will be influenced significantly by the presence of the plasma and the rate of neutral hydrogen release. This is considered a significant complication that is present, and in addition the plasma is pulsed in time. In a first order approximation it has been found sufficient to work with the time averaged plas ¬ ma potential as a driving force for the hydrogen equilibrium pressures. As an alternative approach a maximum of the plasma potential may be used. Or a combination may be taken. It is noted that the goal is to compensate for the plasma potential (being relatively small) and for the behaviour in time of the plasma potential. An exact understanding of the behaviour would of course be optimal; in practice a good approximation of the behaviour, and hence average/maximum values, allows for adequate compensation of the potential, and hence alleviate the side effects of the potential. This may however not be the case when the protons diffuse rapid enough between the pulses and react instantaneously to the varying potential; in this case an advanced approximation may be required.

In an example of the present light source the hydro- gen equilibrium pressure at given boundary conditions of the material is in a range from 1 Pa-10 7 Pa, preferably from 100 Pa-5*10 6 Pa, such as from 300 Pa-10 6 Pa. It has been found that the hydrogen equilibrium pressure is preferably not too small; sufficiently high to be above the applied hydrogen pressure in the light source, but also limited to some extent, in order to prevent e.g. blister formation. The hydrogen equilibrium pressure is preferably not too large, in order to allow (re-) ab ¬ sorption from high pressure defects.

In an example of the present light source (R2) the hydrogen equilibrium pressure at given boundary conditions when the plasma is applied is in a range from 10 Pa-10 7 Pa, preferably from 50 Pa-5*10 6 Pa, such as from 200 Pa-10 6 Pa. It is preferably somewhat higher than the hydrogen pressure of the light source, e.g. larger than 5 Pa for a prior art system and larger than 200 Pa for an advanced system. In the first case >20 Pa is preferred, in the second case > 300 Pa is pre ¬ ferred .

In an example of the present light source the at least one multilayer mirror comprises a sequence of layers ie[2;n], wherein a layer closer to a (plasma facing) surface of the mirror, the surface reflecting EUV light, has a lower hydrogen equilibrium pressure at given boundary conditions of the material than a subsequent layer (Ρ[Η 2 ]±(@Τ)< P [H 2 ] i+ i ( @T ) ) . The hydrogen equilibrium pressures of subsequent layers pref- erably differ by at least a factor 2, more preferably at least a factor 5, even more preferably at least a factor 10; i.e. P[H 2 ] i(@T) : P[H 2 ] i+ i(@T) <10. As accelerated protons coming from the plasma may be inserted up to a few layers deep in the mul ¬ tilayer mirror, these protons are inherently transferred back through a sequence of layers from a deeper layer (i+1) to a less deep layer (i), and revolve as hydrogen molecules in the vacuum chamber (see also fig. 2) . It has been found that this elegant solution mitigates problems caused by hydrogen absorp ¬ tion and high pressure hydrogen trapped in defects to a large extent, and at the same time allows hydrogen to be present in the plasma, in order to e.g. remove Sn particles and Sn from a surface of a mirror or remove trace oxide impurities.

In an example of the present light source (R0) a melt temperature T me i t of the material is more than 100 K higher than an operating temperature (T me it > T ope r + 100 K) . Such is in par ¬ ticular relevant for the rotating foil trap, as this trap is operated at relatively high temperatures. It is preferred that the melt temperature is more than 250 K higher, such as more than 400 K higher. It has been found that such a melt tempera- ture is sufficient to provide a durable light source. In an example of the present light source the rotat ¬ ing foil trap comprises a material with (mechanical proper ¬ ties)

(ROa) a hardness of > 4 Moh, preferably > 5 Moh, and/or

(ROb) a tensile strength of > 500 MPa, and/or

(ROc) an elastic modulus of > 100 GPa. The hardness can be determined as in mineralogy, using the so called Mohs scale. Likewise the Vickers hardness can be taken; it is pre- ferred to have a Vickers hardness of more than 800 MPa, pref ¬ erably more than 1000 MPa. The tensile strength is considered a maximum stress that a material can withstand while being stretched or pulled before failing or breaking. It can be measured with a tensometer under standard conditions, such as ASTM conditions. These conditions may vary somewhat, from ma ¬ terial to material. The elastic modulus relates to a mathemat ¬ ical description of an object or substance's tendency to be deformed elastically. Here specifically the Young's modulus (E) is considered. The Young's modulus (E) describes tensile elasticity, or the tendency of an object to deform along an axis when opposing forces are applied along that axis; it is defined as the ratio of tensile stress to tensile strain. It is often referred to simply as the elastic modulus.

Prior art systems use e.g. Mo or alloys thereof as a material, having for a defect free Mo material an elastic modulus of about 330 Gpa, a tensile strength of about 650 Mpa, and a Mohs value of 5.5, which values are considered good enough. It is noted that for a manufactured and processed foil trap there are normally imperfections because of the difficult workabil- ity of the Mo material. This reduces the actual strength of the prior art systems.

For the tensile strength a value of > 300 Mpa is pre ¬ ferred, preferably > 500 Mpa. For the elastic modulus a value of > 200 GPa is preferred.

In an example of the present light source a surface voltage compensator is provided for the at least one mirror and/or for the rotating foil trap, preferably a voltage com ¬ pensator comprising one or more of a surface potential detec ¬ tor, an oscillating voltage compensator, a surface temperature sensor, an ampere measurer, a power supply and a controller. The surface voltage compensator is adapted to compensate for a plasma induced (electrical) potential on a surface of the pre ¬ sent mirror or rotating foil trap. Such a compensation can be provided in relation to the plasma potential, being present in operation of the light source. It is preferred to compensate actively, in response to and to (partially) counteract a (var ¬ ying) plasma potential. Thereto a surface potential detector is provided, for determining a real time surface potential. Preferably also a surface temperature is determined real time, in order to compensate for a varying temperature, if present. Likewise an ampere measurer and a power supply are present. A controller may actively provide output, based on determined parameters, to the oscillating voltage compensator. The controller preferably comprises software for predicting changes in the potential, based on measurements.

For alloys and materials also combinations of ele ¬ ments are foreseen, e.g. in an AB alloy A and B could be more than one element.

In an example of the present light source the rotat- ing foil trap is made of an alloy or intermetallic compound comprising A^B^C c wherein A is selected from Nb, Re, and Ta, B is selected from W, and Mo, and C is selected from V, Hf, Ir, Si, Ge, Mo, Os, Pt, Pd, Au, Ti, and Zr, ae [ 0, 75; 0, 99] , and b<≡ [0, 01; 0, 25] , preferably be [ 0 , 02 ; 0 , 10 ] , and ce [ 0 , 01 ; 0 , 10 ] . Examples are Nbo.s5Wo.15Moo.05, Ta 0 . goW 0 . osHf 0 .02 ·

It is noted that a component A is selected such that it is not equal to a component B. It is found that especially Nb and Ta comprising materials are particularly suited. Albeit the Nb and Ta being somewhat less suited than Mo in terms of mechanical properties, the hydrogen behaviour is much better. Also a product such as rotating foil trap of Nb and Ta can be produced much easier. The Nb/Ta preferably comprises a small amount of alloying metals. It has been found that the second metal improves mechanical properties significantly. Preferred second metals are W, Zr, Mo, Hf, Ti and V, in particular W, Zr and V. Preferred alloys are NbZr, NbW and NbV, such as

NbVo.05Moo.05Zro.01, NbWo.i, and NbV 0 . 0 4, and TaZr, TaW and TaV, such as TaVo.05Moo.05Zro.01, TaWo.i, and TaV 0 .o4- The addition of a small amount of W has shown to reduce negative effects of Sn being present. Nb and Ta are specifically suitable in addressing hy- drogen control/management/balance .

Further examples of these materials relate to Nb with 15%W, 5%Mo, l%Zr) , having a recrystallization T of 1540 °C, a tensile strength of 0.51 GPa at 980 °C, and a density of 9.9 g/cm 3 ; Nb with 4%V, having a recrystallization T of 1175 °C, a tensile strength of 0.14 GPa at 1204 °C, and a density of 8.3 g/cm 3 ; and Ta with 8%W, 2%Hf, having a recrystallization T of 1540 °C, a tensile strength of 0.59 GPa at 980 °C, and a density of 16.7 g/cm 3 .

Especially Nb-V, NbW, TaW, and TaV alloys have been found to have very good workability and mechanical properties at higher operating temperatures, enabling thinner lamellae with closer spacings to be made, that in turn make the reduc ¬ tion of the rotation speed possible. Next to the lower rota- tion speed also the lower density of Nb compared to Mo reduces the forces exerted on the lamellae. These factors make that the smaller strength of the e.g. Nb-V alloy are even more than compensated. In the presence of hydrogen for these alloys it is of importance that their temperature is always kept near the normal high operating temperatures of the source, since then the equilibrium pressure is high and the hydrogen will not be absorbed.

In an example of the present light source the grazing incidence mirror comprises a layer of an alloy or intermetal- lie compound C c DdE e . The layer is preferably thick enough, such as 10-500 nm, preferably 100-300 nm, in order to stop protons entering from the plasma into the (underlying) layer. Herein C is selected from Zr, Mo, Ta, Re, and Nb, D is selected from W, Rh, Mo, Ru, Zr, Nb, Sc, Ti, Mg, Ni, Co, Zn, Cu, Fe, Cr, Mn, Hf, La, Ta, Ir, Os, Pt, Pd, Au, and V, E is selected from Rh, Mo, Ru, Zr, Nb, Sc, Ti, Mg, Ni, Co, Zn, Cu, Fe, Cr, Mn, Hf, La, Ta, Ir, Os, Si, H, Re and V, ce [ 0 , 30 ; 0 , 95 ] , de [ 0 , 05 ; 0 , 70 ] , and ee [ 0 , 00 ; 0 , 35 ] . It is noted that a component C is selected such that it is not equal to a component D or E, and a compo- nent D is selected such that it is not equal to a component E. Preferred compositions of the reflecting layer are ZrMoH, such as ZrMo2H 0 -o.i, ZrRuH, such as ZrRu2H 0 -o.i, and ZrRuPd, such as ZrRui.gPdo.i . The layer may be coated on a construction made out of Ni metal.

In an example of the present light source the at least one multilayer mirror comprises at least two layers, wherein a first layer has a lower EUV light scattering density, compared to a second layer with a higher EUV light scat ¬ tering density, and wherein a layer has metallic or non- metallic properties. It has been found that multi layers can reach a reflectivity of 60-70% of the EUV light.

In an example of the present light source the above high density layer comprises an alloy or intermetallic com ¬ pound F f G g J j , wherein F is selected from Zr, Mo, Pd, Ta, Re, and Nb, G is selected from W, Rh, Mo, Ru, Zr, Nb, Sc, Ti, Mg, Ni, Co, Zn, Cu, Fe, Cr, Mn, Hf, La, Ta, Ir, Os, N, Ir, Os, Pt, Pd, Au and V, J is selected from Rh, Mo, Ru, Zr, Nb, Sc, Ti, Mg, Ni, Co, Zn, Cu, Fe, Cr, Mn, Hf, La, Ta, Ir, Os, Si, Ge, H, B, 0, Al, N, Re and V, f [ 0 , 16 ; 0 , 99 ] , ge [ 0, 05; 0, 45] , and j e [ 0 , 00 ; 0 , 70 ] , wherein the alloy or intermetallic compound has a EUV-light transmittance (@ 13 nm) of >0,7. It is noted that a component F is selected such that it is not equal to a com ¬ ponent G or J, and a component G is selected such that it is not equal to a component J. The thin layers of the materials preferably have a sufficient EUV-light transmittance.

In view of hydrogen absorption behaviour the above mentioned metals are considered specifically. Preferred compo ¬ sitions of the high density layer are ZrMoH, such as Ζ ΓΜΟ 2 ΗΟ- o.i, ZrRuH, such as ZrRu 2 H 0 -o.i, and ZrRuPd, such as ZrRui.gPdo.i . Each individual layer may be adapted to requirements. Typical ¬ ly a layer thickness is from 1-20 nm, such as from 5-10 nm. For the deepest layers there will be no penetration of protons from the plasma anymore and the normally chosen Si layers can be used.

In an example of the present light source the low density layer comprises a compound ¾LjM m where K is selected from Si, Al, Mg, and B, L is selected from B, C, N, 0, Mg, Al, P, and M is selected from H, B, C, N, 0, Mg, Al, P,

k<≡ [0, 25; 0, 99] , le [ 0 , 0 ; 0 , 7 ] , me [ 0, 0 ; 0, 7 ] . It is noted that a component k is selected such that it is not equal to a compo ¬ nent L or M, and a component L is selected such that it is not equal to a component M. Preferred compositions are SiMgH, such as SiMg 0 .iH x , MgSiN, such as MgSiN 2 - x , SiMgAl, such as

SiMgo. 2 Al 0 .i, and MgAlO, such as MgAl 2 0 4 -x.

In an example of the present light source a layer thickness is from 0.5-30 nm. Each individual layer may be adapted to requirements. Typically a layer thickness is from 1-20 nm, such as from 5-10 nm. It is considered that for the deepest/deeper layers there will be no penetration of protons from the plasma anymore and Si layers can be used.

In an example the present light source comprises a means for forming at least partially coherent EUV light. Such means can be provided by controlling electro-magnetic charac ¬ teristics of the light source, by providing reflecting means, by providing resonance means for light, and combinations thereof .

In a second aspect the present invention relates to a foil trap for use in a light source according to the inven ¬ tion, wherein the rotating foil trap comprises a material which material comprises at least one metal and has, at given boundary conditions, the boundary conditions comprising operating temperature T oper of 1000-1500 °K,

Rla) a negative enthalpy of MH formation (ΔΗ<0 materi ¬ al + hydrogen -» Metal-hydride) and

Rib) a hydrogen equilibrium pressure of 0.1 Pa-10 8 Pa, allowing absorption of hydrogen. Advantages of the present ro ¬ tating foil trap are amongst others given throughout the de ¬ scription .

In a third aspect the present invention relates to a multilayer mirror or grazing incident mirror for use in a light source according to the invention, wherein the mirror comprises a sequence of layers ie[2;n], wherein a layer closer to a surface of the mirror, the surface reflecting EUV light, has a lower hydrogen equilibrium pressure at given boundary conditions, the boundary conditions comprising operating temperature T 0per , than a subsequent layer (Ρ[Η 2 ]±(@Τ)<

P [¾] i + i ( @T) ) . Advantages of the present mirrors are amongst others given throughout the description.

In a fourth aspect the present invention relates to a multilayer for use in multilayer mirror according to the invention comprising a sequence of layers ie[2;n], wherein a layer closer to a surface of the multilayer, the surface being capable of reflecting light, such as EUV light, has a lower hydrogen equilibrium pressure at given boundary conditions, the boundary conditions comprising operating temperature T oper of 273-375 °K, than a subsequent layer (P[H 2 ]i(@T)< P [H 2 ] i + i ( @ ) ) . Advantages of the present multilayer are amongst others given throughout the description. One of the advantages is that now a hydrogen adsorption behaviour of each individual layer can be adapted and controlled. As such the multilayer can be adapted easily to e.g. requirements of a light source, or any other application. As such the multilayer can be used to adapt and control absorption of hydrogen.

The invention will hereafter be further elucidated through the following examples which are exemplary and explanatory of nature and are not intended to be considered limiting of the invention. To the person skilled in the art it may be clear that many variants, being obvious or not, may be con ¬ ceivable falling within the scope of protection, defined by the present claims.

FIGURES

Figure 1 is a schematic representation of a multi ¬ layer, having layers i, i+1 and i+2.

Figure 2 shows movement of hydrogen.

DETAILED DESCRIPTION OF FIGURES

Figure 1 is a schematic representation of a multi ¬ layer mirror, having a top layer 1, and deeper layers i, i+1 and i+2. Therein an optional top layer (ZrN) is shown, having a thickness of 6.25 to 13.5 nm. The ZrN layer is followed by a 3.8 nm thick Mgo.1Al1.9O2.9 layer. Then a sequence of layers is provided, for instance having as layer i Pto.iPdo.9 (2.5 nm) , Mgo.05Al1.95O2.95 of 3.8 nm, and followed by further Pto.14Pdo.86 (2.5 nm) and Mgo.1Al1.9O2.9 (3.8 nm) layers. In the deeper layers a Mo-Si sequence of layers of e.g. 20 layers is provided, hav- ing a thickness of 2.5 nm and 3.8 nm, respectively. In an al ¬ ternative a large number of Si-Mo layer is provided, such as 50-100 layers, having a thickness between 2.8 and 4.5 nm. Such improves reflectance of the mirror. A supporting structure is provided .

In an example a sequence of alternating layers of denser and less dense materials are provided, such as 6.25 to 13.5 nm Pto.iPdo.sSio.i, 3.8nm Mgo.1Al1.9O2.9, 2.5 nm Pto.iPdo.9, 3.8 nm Mgo. 0 5Alo. 9 5O2, and further similar layers , and then Si/Mo layers. A thickness of the layers mentioned specifically is in the example in the same order of magnitude, or the same. In the figure also a voltage compensator 21 is shown schematically .

The top layer is preferably a high density layer con ¬ sisting of a well EUV light reflecting alloy that is not easi- ly chemically reduced by the hydrogen plasma and that does not easily chemically react with Sn or with trace impurities like oxygen, water or carbohydrates. It is noted that reduction by hydrogen plasma will for instance occur for transition metal oxides such as Nb oxides. In addition it should have good hy- drogen uptake and release characteristics and the hydrogen equilibrium pressure at the operating temperature higher than the applied hydrogen gas pressure. In general elements such as Pd and Nb have excellent H uptake and release characteristics, including the recombination of absorbed H atoms into ¾ mole- cules at their surface. To suppress the reaction over time with Sn, a limited content of Si, Ge or W can be incorporated. Specific examples that can function as top layer are alloys such as Pto. iPdo.85S10.05, Pto.iNbo.9, Ruo.iNbo.9. Such materials are known to exhibit sufficient hydrogen uptake and release char- acteristics, while the equilibrium pressures at a certain tem ¬ peratures can be tuned by the Pd or Nb content.

For deeper high density layers than the top layer the noble metal content does not need to be high (no oxidation or reduction risk) . In view of the hydrogen absorption behaviour the above mentioned alloys are considered with slightly de ¬ creasing contents of the Pd or Nb, which will increase the equilibrium pressure for hydrogen release. Alternatively also preferred compositions of the high density deeper layers are ZrMoH, such as ZrMo2H 0 -o.i, etc.

Figure 2. Hydrogen gas and hydrogen plasma in contact with metal parts from the EUV source. Absorbed hydrogen atoms H abs . can be introduced in the first metal from the gas phase, and via the direct insertion by the plasma (either protons H + or atomic Η·) . Depending on the characteristics of the metal hydrogen can be trapped in the metal and form destructively high pressures in defects. The present invention provides ma ¬ terials that under the operating conditions transport the H a b S . towards the plasma facing surface and via adsorbed hydrogen ( H a ds . ) states back to H 2 as a gas (H 2 , gas ) ·

In general metals are chosen that have a first so- called a phase having the original metal alloy structure with a limited amount of H a b S absorbed in it. This is the phase that one would like to have in the EUV source at all times. The hy ¬ drogen insertion by the plasma causes a relatively low H con- tent in these metals as the H transport and ¾ desorption is found to be rapid and any defect will not have higher internal hydrogen pressures than the limited equilibrium pressure; as a result they can absorb and desorb the hydrogen amounts provid ¬ ed without damage.

With reference to figure 1 and 2 and the description, where reference is made to a "metal" also an "alloy" or more general a "solid material" is envisaged as well.

In a further example the following mirror is designed. The topmost layer is a high optical density material and the subsequent layers are alternating in high and reduced optical density. The periodicity of the thickness of layers i and i+1 together equals 6.25 nm for a EUV wavelength of 13.5 nm under perpendicular incidence and reflection. 1 As described in 1 the thickness of them together vary depending on the angle of inclination and the wavelength according to the well-known Bragg relation. The equilibrium pressures for hydrogen release are as described: higher for deeper layers.

As an example for the multilayer mirror is given: Top layer ZrN ~ 6.25 nm to 13.5 nm

Mgo.1Al1.9O2.9 ~ 3.8 nm

Mgo.05Al1.95O2.95 ~ 3.8 nm

Mgo.04Al1.9eO2.96 ~ 3.8 nm

Mgo.02Al1.9sO2.98 ~ 3.8 nm

Mo ~ 2.5 nm

Si ~ 3.8 nm

Mo ~ 2.5 nm

Si - 3.8 nm

Mo and Si up to 60 layers in total. The ~ indicates that the thickness of the layers mentioned is within measurement accu ¬ racy as given. It is noted that these numerical values depend on incident angle of the EUV light and its wavelength. er example:

ZrN ~ 6.25 nm to 13.5 nm

Mgo.05Al1.95O2.95 ~ 3.8 nm

Mgo.04Al1.9eO2.96 ~ 3.8 nm

Pto.isPdo.82 ~ 2.5 nm

Mgo.02Al1.9sO2.98 ~ 3.8 nm

Mo ~ 2.5 nm

Si ~ 3.8 nm

Mo ~ 2.5 nm

Si ~ 3.8 nm

up to 70 layers in total.

In yet a further example:

Pt 0 . iPd 0 . sGeo.1 ~ 6.25 nm to 13.5 nm or

Pto. iPdo. eSio. 1 ~ 6.25 nm to 13.5 nm

Mgo.05Al1.95O2.95 ~ 3.8 nm

Mgo.04Al1.9eO2.96 ~ 3.8 nm

Pto.isPdo.82 ~ 2.5 nm

Mgo.02Al1.93O2.98 ~ 3.8 nm

Mo ~ 2.5 nm

Si ~ 3.8 nm

Mo ~ 2.5 nm

Si ~ 3.8 nm

i up to 80 layers in total.

In yet a further example:

Pto.osPdo.92 ~ 6.25 nm to 13.5 nm

Mgo.05Al1.95O2.95 ~ 3.8 nm

Mgo.04Al1.9eO2.96 ~ 3.8 nm

Pto.isNbo.s2 ~ 2.5 nm

Mgo.02Al1.9sO2.98 ~ 3.8 nm

Mo ~ 2.5 nm

Si ~ 3.8 nm Mo ~ 2.5 nm

Si ~ 3.8 nm

up to 90 layers in total

In yet a further example:

Mg 0 05A11.95O2.95 3. 8 nm

Mg 0 O4AI1.9602.96 3. 8 nm

Mg 0 O2All.9eO2.98 3. 8 nm

Mo 2. 5 nm

Si 3. 8 nm

Mo 2. 5 nm

Si 3. 8 nm

up to 100 layers in total

In yet a further example:

Mg 0 .iAli. 9 O 2 .9 3. 8 nm

Mgo.05Al1.95O2.95 3. 8 nm

Mgo.04All.9eO2.96 3. 8 nm

Mgo.02All.9so2.98 3. 8 nm

Mo 2. 5 nm

A1203 3. 8 nm

Mo 2. 5 nm

A1203 3. 8 nm

2O 3 up to 50 layers in total

In yet a further example:

Pto.08Pdo.92 6. 25 ' nm to 13.5 nm

MgSi 2 -xN x 3. 8 nm Mo 2.5 nm

Si 3.8 nm

Mo 2.5 nm

Si 3.8 nm

Mo and Si up to 60 layers in total

For the grazing incidence mirror the following examples are given:

For high temperature operation:

Top layer of ZrRu 2 of 10 nm having a hydrogen equilibrium pres- sure significantly higher than hydrogen pressure applied; op ¬ tional next layer of ZrMo 2 200 nm having a higher equilibrium pressure than top layer; and e.g. a Ni alloy construction.

For between 50 and 90 °C temperature operation:

Top layer of Pto.iPd of 20 nm; next layer of Pto. isPdo. sGeO .05 of 200 nm; and e.g. Ni alloy construction. For low operation temperatures and less small angles of incidence a multilayer mir ¬ ror as above but with much larger layer thicknesses following from application of Braggs law is an example. For the rotating foil trap the following examples are given:

Top layer: Nbo.gW of 200 nm; during operation ¾ equilibrium pressure significantly higher than applied ¾ pressure; next layer: Nbo. 8 5Wo. 15 of 200 nm; equilibrium pressure signifi ¬ cantly higher than top layer; subsequent layer: Nbo. soWo.15M00. 05 construction thickness; equilibrium pressure significantly higher than layer above.

In a further example:

Top layer: Tao.90W0. osHf0.02 of 50 nm; during operation H 2 equilibrium pressure significantly higher than applied ¾ pres ¬ sure; next layer: Tao.88Wo. 12 of 50 nm; equilibrium pressure sig- nificantly higher than top layer; subsequent layer:

Nbo. soWo.15M00. 05 construction thickness; equilibrium pressure significantly higher than layer above.

Figure 2 shows that the selected metals should have the appropriate hydrogen behavior, reflected in conditions Rla) and Rib) . If applied at higher temperature they should also have an adequate melting temperature, and further materi ¬ al conditions (R0), e.g. in order to withstand the harsh con ¬ ditions exerted on them.

The materials in the multilayer mirror, the grazing incidence mirror and the rotating foil trap are thus chosen in relation to the operating temperature of the individual compo ¬ nents and in relation to the range of hydrogen pressures that are applied during operation. For a certain material the enthalpy of formation determines which equilibrium pressure for hydrogen uptake the material has. The materials should always have equilibrium pressures for hydrogen uptake and release that are higher than the pressure that is applied; in this way only limited spontaneous hydrogen uptake from the gas can oc ¬ cur and hydrogen atoms inserted from the plasma can diffuse towards the plasma side of the material and will be released as ¾ . Note that cooling respectively heating is required to operate the mirrors at the appropriate temperatures. When the actual temperatures of the components are outside these de ¬ signed limits measures should be taken to keep them inside the required range. These measures also include removing the hy ¬ drogen pressure when the temperature of the rotating foil trap or the grazing incidence mirror becomes lower than the de ¬ signed lower limit temperature of the component since other ¬ wise the hydrogen density absorbed in the materials will start growing to too high levels, which could lead to deformation of the component .

EXAMPLES

1 Specifics of the components

RFT (Rotating Foil Trap)

For a RFT the Operating temperature is typically 1100

- 1500K, it is operated at a high rotation speed, typically thin lamellae of high strength are used. Prior art material are Mo based, with small additions of e.g. Ti-Zr.

Examples of present Material: Nbo.96Vo.o4· This alloy has a high enough hydrogen equilibrium pressure at the high operating temperatures. As a matter of precaution hydrogen preferably only reaches it at the high operating temperature and is removed when cooling down. Especially below 800K there should be no hydrogen gas in the system in contact with the RFT (and therefore also the EUV source is switched off) . Oth ¬ erwise the ¾ will be absorbed in the alloy and may deform it.

The Nb-based alloy has very good formability but is less strong than the currently used Mo. However, the lower density and the better formability enable the appropriate per- formance compared to current Mo. A further example is Nbo.95Zro.05 which takes up H under pressure forming (Nbo. 9 5Zro.05) HO .01.

MM (Multilayer Mirrors)

The T is water cooled, so not high. Prior art material relate to Mo and Si multilayers. These typically comprise a protection layer, such as a 10 nm ZrN toplayer. This should be EUV transparent while protecting multilayers from H implanta ¬ tion by having a high stopping power

GM (Grazing incidence Mirror)

Having a not very high operating T by cooling, typically a Ru toplayer for high EUV reflectivity, limited EUV absorption. A typical prior art material base is Mo.

Plasma

The plasma is typically a high T pulsed plasma, (e.g. 80 kHz), having fast decay between pulses, and a P(¾) of 5-500 Pa.

Substrate preparation

Thin films with a thickness of 10 nm were deposited at room temperature substrates in an ultrahigh-vacuum (UHV) DC/RF magnetron sputtering system (base pressure 10 ~8 mbar, Ar deposition pressure 0.003 mbar) .

Alloy thin films with a thickness of 5 nm were co- deposited in a similar DC magnetron sputter system using quartz substrates. On deposition the films are crystalline, which decays on cycling with hydrogen.