EUV Optical Element Having Blister-Resistant Multilayer Cap

(19) TZZ Z¥_T

(11) EP 2 905 637 A1

(12) EUROPEAN PATENT APPLICATION

(43) Date of publication: (51) Int Cl.: 12.08.2015 Bulletin 2015/33 G02B 5/08 (2006.01) G21K 1/06 (2006.01)

(21) Application number: 14154265.4

(22) Date of filing: 07.02.2014

(84) Designated Contracting States: (72) Inventor: Böwering, Norbert AL AT BE BG CH CY CZ DE DK EE ES FI FR GB 5503 LN Veldhoven (NL) GR HR HU IE IS IT LI LT LU LV MC MK MT NL NO PL PT RO RS SE SI SK SM TR (74) Representative: Grünecker Patent- und Designated Extension States: Rechtsanwälte BA ME PartG mbB Leopoldstraße 4 (71) Applicant: ASML Netherlands B.V. 80802 München (DE) 5504 DR Veldhoven (NL)

(54) EUV optical element having blister-resistant multilayer cap

(57) A multilayer mirror having a cap with a multilayer the materials for the top layer, absorber layers, and spac- structure including a top layer and a series of bilayers er layers are chosen to resist blistering. each having an absorber layer and a spacer layer, where EP 2 905 637 A1

Printed by Jouve, 75001 PARIS (FR) 1 EP 2 905 637 A1 2

Description vacuum chamber with the plasma to collect and redirect the EUV radiation. The environment within the chamber FIELD is inimical to the optical element and so limits its useful lifetime, for example, by degrading its reflectivity. An op- [0001] The present disclosure relates to optical ele- 5 tical element within the environment may be exposed to ments designed to operate in environments in which they high energy ions or particles of target material. The par- are subject to contamination and wear. An example of ticles of target material can contaminate the optical ele- such an environment is the vacuum chamber of an ap- ment’s exposed surface. Particles of target material can paratus for generating extreme ultraviolet ("EUV") radi- also cause physical damage and localized heating of the ation from a plasma created through discharge or laser 10 MLM surface. The target materials may be particularly ablation of a target material. In this application, the optical reactive with a material making up at least one layer of elements are used, for example, to collect and direct the the optical element surface, e.g., molybdenum and sili- radiation for utilization outside of the vacuum chamber, con. Temperature stability, ion-implantation, and diffu- e.g., for semiconductor photolithography. sion problems may need to be addressed even with less 15 reactive target materials, e.g., tin, indium, or xenon. Blis- BACKGROUND tering of the MLM coating must also be avoided. [0008] There are techniques which may be employed [0002] Extreme ultraviolet radiation, e.g., electromag- to increase optical element lifetime despite these harsh netic radiation having wavelengths of around 50 nm or conditions. For example, a capping layer may be placed less (also sometimes referred to as soft x-rays), and in- 20 on the optical element to protect the surface of the optical cluding radiation at a wavelength of about 13.5 nm, can element. To make the capping layer more reflective it be used in photolithography processes to produce ex- may also have multiple layers spaced to increase reflec- tremely small features in substrates such as silicon wa- tivity at the wavelength of the radiation to be reflected. fers. [0009] Such multilayer capping layers are, however, [0003] Methods for generating EUV radiation include 25 themselves prone to damage through mechanisms such converting a target material from a liquid state into a plas- as hydrogen diffusion and blistering. In some systems ma state. The target material preferably includes at least H2 gas at pressures in the range of 0.5 to 3 mbar is used one element, e.g., xenon, lithium or tin, with one or more in the vacuum chamber for debris mitigation. In the ab- emission lines in the EUV range. In one such method, sence of a gas, at vacuum pressure, it would be difficult often termed laser produced plasma ("LPP"), the re-30 if not impossible to protect the collector adequately from quired plasma can be produced by using a laser beam target material debris ejected from the plasma. Hydrogen to irradiate a target material having the required line-emit- is relatively transparent to EUV radiation having a wave- ting element. length of about 13.5 nm and so is preferred to other can- [0004] One LPP technique involves generating a didate gases such as He, Ar or other gases which exhibit stream of target material droplets and irradiating at least 35 a higher absorption at about 13.5 nm. some of the droplets with laser radiation pulses. In more [0010] H2 gas is introduced into the vacuum chamber theoretical terms, LPP sources generate EUV radiation to slow down the energetic debris (ions, atoms, and clus- by depositing laser energy into a target material having ters) of target material created by the plasma. The debris at least one EUV emitting element, such as xenon (Xe), is slowed down by collisions with the gas molecules. For 40 tin (Sn), or lithium (Li), creating a highly ionized plasma this purpose a flow of H 2 gas is used which may also be with electron temperatures of several 10’s of eV. counter to the debris trajectory. This serves to reduce [0005] The energetic radiation generated during de- the damage of deposition, implantation, and sputtering excitation and recombination of these ions is emitted from target material on the optical coating of the collector. Us- the plasma in all directions. In one common arrangement, ing this method it is believed possible to slow down en- a near-normal-incidence mirror (often termed a "collector 45 ergetic particles with energies of several keV to a few mirror" or simply a "collector") is positioned to collect, tens of eV by the many gas collisions at these pressures direct, and, in some arrangements, focus the radiation over the distance between the plasma site and the col- to an intermediate location. The collected radiation may lector surface. then be relayed from the intermediate location to a set [0011] Another reason for introducing N2 gas into the of scanner optics and ultimately to a wafer. 50 vacuum chamber is to facilitate cleaning of the collector [0006] In the EUV portion of the spectrum it is generally surface. The EUV radiation generated by the plasma cre- regarded as necessary to use reflective optics for the ates hydrogen radicals by dissociating the H 2 molecules. collector. At the wavelengths involved, the collector is The hydrogen radicals in turn help to clean the collector advantageously implemented as a multi-layer mirror surface from target material deposits on the collector sur- ("MLM"). As its name implies, this MLM is generally made 55 face. For example, in the case of tin as the target material, up of alternating layers of material over a foundation or the hydrogen radicals participate in reactions on the col- substrate. lector surface that lead to the formation of volatile gase- [0007] The optical element must be placed within the ous stannane (SnH4) which can be pumped away. For

2 3 EP 2 905 637 A1 4 this chemical path to be efficient it is preferred that there and is not intended to identify key or critical elements of is a low H recombination rate (to form back H 2 molecules) all embodiments nor set limits on the scope of any or all on the collector surface so that the hydrogen radicals are embodiments. Its sole purpose is to present some con- available instead for attaching to the Sn to form SnH4. cepts of one or more embodiments in a simplified form Generally, a surface consisting of non-metallic com-5 as a prelude to the more detailed description that is pre- pounds like nitrides, carbides, borides and oxides has a sented later. lower H recombination rate as compared to a surface [0018] According to one aspect, there is provided a consisting of pure metals. multilayer mirror comprising a substrate, a multilayer [0012] The use of H2 gas, however, can have a nega- coating on the substrate, and a capping layer on the mul- tive effect on a coating applied to the collector by both 10 tilayer coating, in which the capping layer includes an the light hydrogen atoms and molecules on the coating. outermost layer comprising a material having a high re- It is believed that the hydrogen atoms are so small that sistance to target material deposition and a multilayer they can easily diffuse several layers deep into a collector structure positioned between the outermost layer and the configured as a multilayer mirror. Hydrogen can be im- substrate, the multilayer structure comprising a plurality planted if ion deceleration is insufficient and can also15 of bilayers, each of the bilayers comprising a spacer layer diffuse into the collector cap and layers of the multilayer including a material resistant to hydrogen diffusion and mirrorbeneath thecap. These phenomena mostseverely blistering and an absorber layer including a material re- affect outermost layers. sistant to ion penetration. [0013] Once atomic hydrogen invades the body of the [0019] The outermost layer may be a nitride or oxide multilayer mirror it can bond to Si, get trapped at layer 20 with high resistance to target material deposition, good boundaries and interfaces, or both. The magnitude of energy reduction for incident ions, and low secondary these effects depends on the dose and concentration of electron yield, such as ZrN, Si 3N4, YN, ZrO 2, Nb2O5, and hydrogen in these regions. If the hydrogen concentration TiO2. The spacer layers are preferably made from hydro- is above a certain threshold it can form bubbles of gas- gen-diffusion and blister-resistant materials such as ni- 25 eous hydrogen compounds, either recombining to 2 H trides, carbides, and borides. The absorber layers are molecules or perhaps also forming SiH4. This happens preferably made from suitable oxide, nitride or metal lay- most severely typically underneath the cap layer or in the ers which can reduce the penetration of incident ions. outermost Si layer. When a gas bubble starts to form Suitable materials for the nitride layers include Si 3N4 and there is a high probability that it will grow in the presence YN. Suitable materials for the carbide and boride layers 30 of additional hydrogen. If such bubbles do form then their include B4C, C, ZrC, and YB6. Suitable materials for the internal gas pressure will deform the layer above the bub- oxide layers include ZrO 2, TiO2, Ta2O5, and Nb 2O5. Suit- ble. The layer may then burst, thus releasing the gas, able materials for the metal layers include Mo 2C, Mo and leadingto theformation of blisters on thecoating, typically W. with a size of a few tens of nm. [0014] A blistered coating creates several problems. It 35 BRIEF DESCRIPTION OF THE DRAWINGS has a higher surface area and is more prone to degradation by oxidation and other contaminants and by dep- [0020] osition of target material. Due to higher absorption this generally leads to a reduction of EUV reflectance. A blis- FIG. 1 shows a schematic, not-to- scale, view of an tered coating also scatters more light due to higher rough- 40 overall broad conception for a laser-produced plas- ness and thus leads to significantly reduced EUV reflect- ma EUV radiation source system according to an ance, even though the undamaged layers below still con- aspect of the present invention. tribute to reflection of EUV light and even if the target FIG. 2 is a schematic, not-to-scale diagram of a cross material deposits are removed by cleaning. section of an EUV optical element with a multilayer [0015] In addition to these effects, hydrogen uptake 45 capping layer. and penetration can also lead to embrittlement of metal layers and thus cause layer degradation. DETAILED DESCRIPTION [0016] There thus is a need to exploit the advantages with respect to enhancing the EUV reflectance of using [0021] Various embodiments are now described with a multilayer capping layer while at the same time having 50 reference to the drawings, wherein like reference numer- a capping layer that is resistant to blistering. als are used to refer to like elements throughout. In the following description, for purposes of explanation, nu- SUMMARY merous specific details are set forth in order to promote a thorough understanding of one or more embodiments. [0017] The following presents a simplified summary of 55 It may be evident in some or all instances, however, that one or more embodiments in order to provide a basic any embodiment described below can be practiced with- understanding of the embodiments. This summary is not out adopting the specific design details described below. an extensive overview of all contemplated embodiments, In other instances, well-known structures and devices

3 5 EP 2 905 637 A1 6 are shown in block diagram form in order to facilitate de- system is itself advantageously a multilayer system com- scription of one or more embodiments. posed of several alternating spacer and absorber layers [0022] With initial reference to FIG. 1 there is shown a to provide enhanced EUV reflectance of the collector mir- schematic view of an exemplary EUV radiation source, ror coating (for example at 13.5 nm wavelength). Just as e.g., a laser produced plasma EUV radiation source 20 5 with the multilayer of the main (Mo/Si) coating of the col- according to one aspect of an embodiment of the present lector 30, the multilayered cap layer system also has to invention. As shown, the EUV radiation source 20 may have a graded design with the bilayer spacing matched include a pulsed or continuous laser source 22, which to the incidence angle as a function of the radius of the may for example be a pulsed gas discharge CO2 laser collector 30. source producing radiation at 10.6 mm. The pulsed gas 10 [0026] An example of an MLM collector 30 with a mul- discharge CO2 laser source may have DC or RF excita- tilayer cap is shown in FIG. 2 which is a cross section tion operating at high power and high pulse repetition though a portion of such a collector. As can be seen there, rate. the collector 30 includes a substrate 100. A multilayer [0023] The EUV radiation source 20 also includes a coating 110 is located on the substrate 30. The multilayer target delivery system 24 for delivering target material in 15 coating 110 is made up of alternating layers of material, the form of liquid droplets or a continuous liquid stream. for example, molybdenum and silicon, in a known fash- The target material may be made up of tin or a tin com- ion. Located on the multilayer coating 110 is a capping pound, although other materials could be used. The tar- layer 120 which is made up of an outermost layer 130 get material delivery system 24 introduces the target ma- and a series of repeating bilayers 140. Each of the bilay- terial into the interior of a chamber 26 to an irradiation 20 ers 140 preferably includes a spacer layer 150 and an region 28 where the target material may be irradiated to absorber layer 160. FIG. 2 shows an arrangement with produce plasma. In some cases, an electrical charge is five bilayers but one of ordinary skill in the art will readily placed on the target material to permit the target material appreciate that other numbers of bilayers may be used. to be steered toward or away from the irradiation region [0027] The purpose of the multilayer cap is to protect 28. It should be noted that as used herein an irradiation 25 the collector 30 without excessively decreasing the over- region is a region where target material irradiation may all reflectivity of the collector 30 at the wavelengths of occur, and is an irradiation region even at times when no interest, e.g., 13.5 nm. It is, however, preferable to select irradiation is actually occurring. materials for the layers within the multilayer cap that will [0024] Continuing with FIG. 1, the radiation source 20 resist blistering and hydrogen diffusion. For example, may also include one or more optical elements. In the 30 multilayered cap bilayers that include silicon such as a following discussion, a collector 30 is used as an example zirconium nitride/silicon (ZrN/Si) bilayer or a tungsten/sil- of such an optical element, but the discussion applies to icon (W/Si) bilayer may be prone to blistering. This is due other optical elements as well. The collector 30 may be to a hydrogen reaction within the Si layers where dangling a normal incidence reflector, for example, implemented bonds at the layer boundary react with hydrogen and in 35 as an MLM, that is, a silicon carbide (SiC) substrate coat- the bulk of the layer. The reaction can form SiH 4 (silane) ed with a molybdenum/silicon (Mo/Si) multilayer with ad- and hydrogen blisters inside of the silicon layers. Other ditional thin barrier layers, for example B4C, ZrC, Si3N4 bilayer combinations such as molybdenum/yttrium or C, deposited at each interface to effectively block ther- (Mo/Y) may not provide an effective barrier to hydrogen mally-induced interlayer diffusion. Other substrate mate- diffusion. rials, such as aluminum (Al) or silicon (Si), can also be 40 [0028] It is thus advantageous to provide for a cap layer used. The collector 30 may be in the form of a prolate system that protects the collector 30 coating against tar- ellipsoid, with an aperture to allow the laser radiation to get material (e.g., tin) deposition, hydrogen ion penetra- pass through and reach the irradiation region 28. The tion, hydrogen diffusion, and hydrogen or oxygen in- collector 30 may be, e.g., in the shape of a ellipsoid that duced blistering. has a first focus at the irradiation region 28 and a second 45 [0029] By choosing materials for the spacer layers of focus at a so-called intermediate point 40 (also called the the cap multilayer system in the form of suitable nitrides, intermediate focus 40) where the EUV radiation may be carbides, and borides (such as trisilicon tetranitride output from the EUV radiation source 20 and input to, (Si3N4), zirconium nitride (ZrN), silicon carbide (SiC), car- e.g., an integrated circuit lithography tool 50 which uses bon (C), yttrium nitride (YN), yttrium hexaboride (YB6), 50 the radiation, for example, to process a silicon wafer zirconium carbide (ZrC), silicon hexaboride (SiB6), and workpiece 52 in a known manner. The silicon wafer work- boron carbide (B4C)) hydrogen diffusion into the multi- piece 52 is then additionally processed in a known man- layer coating is reduced and reaction with hydrogen in ner to obtain an integrated circuit device. the spacer layers is reduced, leading to a resistance [0025] As described above, one of the technical chal- against the formation of hydrogen-induced blisters. By lenges in the design of an optical element such as the 55 choosing materials as absorber layers in the form of suit- collector 30 is extending its lifetime. One way to extend able oxide, nitride, or metal layers (such as tantalum pen- the lifetime of the collector 30 involves protecting it from toxide (Ta2O5), titanium dioxide (TiO 2),zirconium dioxide damage by using an outermost cap layer. The cap layer (ZrO2), niobium pentoxide (Nb 2O5), yttrium oxide (Y 2O3),

4 7 EP 2 905 637 A1 8 aluminum oxide (Al2O3), titanium-aluminum-oxynitride [0033] Besides these properties, the layer materials in (TiAION), ZrN, silicon nitride (SiN), titanium nitride (TiN), the cap layer also have to have good transparency to Mo, W, and Zr) the protection of the topmost layer against EUV radiation at 13.5 nm wavelength. tin deposition is increased and the protection against hy- [0034] Suitable materials for nitride layers include 5 drogen penetration and target material penetration and, Si3N4, ZrN, YN, SiN, NbN, TiN, and BN. in part, against hydrogen diffusion is increased. [0035] Suitable materials for carbide layers include

[0030] Referring again to FIG. 2, the topmost layer 130 SiC, B4C, C, and ZrC. of the cap 120 is preferably a nitride or oxide with high [0036] Suitable materials for boride layers include resistance to target material deposition. In effect, these ZrB2, NbB2, YB6, and SiB6. are preferably materials having a low recombination rate 10 [0037] Suitable materials for the oxide layers include for atomic hydrogen to enable a high formation rate of ZrO2, TiO2, Ta2O5, Nb2O5, Y2O3, Al2O3, and titanium- stannane. These would typically be materials having a aluminum-oxynitride (TiAION). hydrogen recombination coefficient in a range of about [0038] Suitable materials for the metal layers include -4 -3 10 to about 10 . Effectively this means the preferred Mo, W, and Mo2C. material exhibits a good tin cleaning rate since the H can 15 [0039] The presently preferred combinations of mate- react with Sn before it recombines to H 2. As an example, rials for the absorber/spacer bilayer include: Mo as the the metal stainless steel has a recombination coefficient material for the absorber and Si 3N4, YN, B4C, ZrC, C, or -3 of 2.2x10 . A preferred material for the topmost layer YB6 as the material for the spacer; W as the material for 130 of the cap 120 also preferably exhibits good energy the absorber and Si3N4, YN, B4C, ZrC, C, or YB6 as the 20 reduction for incident ions and low secondary electron material for the spacer; ZrO2 as the material for the ab- yield. Examples of materials having low recombination sorber and Si 3N4, YN, B 4C, ZrC, C, or YB 6 as the material coefficients, good energy reduction for incident ions, and for the spacer; Nb2O5 as the material for the absorber low secondary electron yield include ZrN, TiO2, Ta2O5, and Si3N4, YN, B4C, ZrC, C, or YB6 as the material for and ZrO2. the spacer; TiO2 as the material for the absorber and 25 [0031] The spacer layers are preferably made from hy- Si3N4, YN, B4C, ZrC, C, or YB6 as the material for the drogen-diffusion and blister-resistant materials such as spacer; and Mo2C as the material for the absorber and nitrides and carbides. The spacer layers are preferably Si3N4, YN, B4C, ZrC, C, or YB6 as the material for the grown amorphously to act as efficient barriers for hydro- spacer. gen diffusion. Some materials exhibit microcrystalline [0040] The above description includes examples of growth in thin layers. For such materials, hydrogen can 30 one or more embodiments. It is, of course, not possible diffuse more easily along grain boundaries in crystalline to describe every conceivable combination of compo- layers; therefore, amorphously grown layers and layers nents or methodologies for purposes of describing the with low defect densities are preferred as hydrogen bar- aforementioned embodiments, but one of ordinary skill riers. Carbides, borides and nitrides are perceived as in the art may recognize that many further combinations good hydrogen diffusion barrier layers. In general, ce- 35 and permutations of various embodiments are possible. ramics are considered good barriers for H diffusion. Also, Accordingly, the described embodiments are intended to the spacer layers are preferably made of a material that embrace all such alterations, modifications and varia- is relatively inert with respect to reactions with hydrogen. tions that fall within the spirit and scope of the appended For example, SiC (silicon carbide) has all bonds between claims. Furthermore,to the extentthat the term"includes" Si and C saturated and is thus less prone to blistering. 40 is used in either the detailed description or the claims, Yttrium nitride (YN) is a better barrier layer with respect such term is intended to be inclusive in a manner similar to hydrogen diffusion compared to pure yttrium which to the term "comprising" as "comprising" is construed shows micro-crystalline growth. when employed as a transitional word in a claim. Fur- [0032] The absorber layers are preferably made from thermore, although elements of the described aspects suitable oxide or metal layers which can reduce the pen- 45 and/or embodiments may be described or claimed in the etration of incident ions. In other words, the material for singular, the plural is contemplated unless limitation to the absorber layer preferably has relatively high stopping the singular is explicitly stated. Additionally, all or a por- power for impacting hydrogen ions. This implies a rela- tion of any aspect and/or embodiment may be utilized tively large preferred stopping cross section. It is pre- with all or a portion of any other aspect and/or embodi- ferred hydrogen ions having energy in the about 100 eV 50 ment, unless stated otherwise. energy should not be able to penetrate the material more than a few nanometers. ZrO2 is an example of such a material. As for metals, molybdenum is a preferred ma- Claims terial, and for some applications molybdenum carbide 55 (Mo2C) is preferred as the "metal" material because it 1. A multilayer mirror (30) comprising: has almost the same EUV reflectance as Mo but better growth properties and better properties with respect to a substrate (100); H diffusion. a multilayer coating (110) on the substrate; and

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a capping layer (120) on the multilayer coating comprises a metallic material. (110), the capping layer (120) comprising an outermost layer (130) comprising a material 13. A multilayer mirror as claimed in claim 12 wherein having a high resistance to target material dep- the metallic material comprises one of the group of 5 osition, and materials comprising Mo2C. Mo, and W. a multilayer structure positioned between the outermost layer and the substrate, the multilayer structure (130) comprising a plurality of bilayers (140), each of said bilayers (140) comprising a spacer layer (150) comprising a material re- 10 sistant to hydrogen diffusion and blistering; and an absorber layer (160) comprising a material resistant to ion penetration.

2. A multilayer mirror as claimed in claim 1 wherein the 15 outermost layer (130) comprises a first oxide material or a first nitride material.

3. A multilayer mirror as claimed in claim 2 wherein the first oxide material comprises one of the group of 20 materials comprising ZrO2, TiO2, and Nb2O5.

4. A multilayer mirror as claimed in claim 2 wherein the first nitride material comprises one of the group of materials comprising ZrN and YN. 25

5. A multilayer mirror as claimed in one of claims 1 to 4 wherein at least one of the spacer layers (150) comprises one of the group of materials comprising a second nitride material, a carbide material, and a 30 boride material.

6. A multilayer mirror as claimed in claim 5 wherein the second nitride material comprises Si3N4 or YN. 35 7. A multilayer mirror as claimed in claim 5 wherein the carbide material comprises one of the group of materials comprising B4C, C, and ZrC.

8. A multilayer mirror as claimed in claim 5 wherein the 40

boride material comprises YB6.

9. A multilayer mirror as claimed in one of claims 1 to 8, wherein at least one of the spacer layers (150) is grown amorphously to act as an efficient barrier45 against hydrogen diffusion.

10. A multilayer mirror as claimed in one of claims 1 to 9, wherein at least one of the absorber layers (160) comprises a second oxide material. 50

11. A multilayer mirror as claimed in claim 10 wherein the second oxide material comprises one of the

group of materials comprising ZrO2, TiO2, and 55 Nb2O5.

12. A multilayer mirror as claimed in one of claims 1 to 9, wherein at least one of the absorber layers (160)

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