EUV optical element having blister-resistant multilayer cap

Abstract

A multilayer mirror having a cap with a multilayer structure including a top layer and a series of bilayers each having an absorber layer and a spacer layer, where the materials for the top layer, absorber layers, and spacer layers are chosen to resist blistering.

Claims

1. A multilayer mirror comprising: a substrate; a multilayer coating on the substrate; and a capping layer on the multilayer coating, the capping layer comprising an outermost layer comprising Nb2O5 or TiO2, and a multilayer structure positioned between the outermost layer and the multilayer coating, the multilayer structure comprising a plurality of bilayers, each of the bilayers comprising a spacer layer comprising a first material and an absorber layer comprising a second material different from the first material, one of the first material and the second material comprising ZrN.

2. The multilayer mirror as claimed in claim 1 wherein the other of the first material and the second material comprises B4C.

3. A multilayer mirror comprising: a substrate; a multilayer coating on the substrate; and a capping layer on the multilayer coating, the capping layer comprising: an outermost layer comprising Ta2O5 and a multilayer structure positioned between the outermost layer and the multilayer coating, the multilayer structure comprising a plurality of bilayers, each of said bilayers comprising a spacer layer comprising a spacer layer nitride material resistant to hydrogen diffusion and blistering and an absorber layer comprising an oxide material resistant to ion penetration.

4. A multilayer mirror comprising: a substrate; a multilayer coating on the substrate; and a capping layer on the multilayer coating, the capping layer comprising an outermost layer comprising ZrN, and a multilayer structure positioned between the outermost layer and the multilayer coating, the multilayer structure comprising a plurality of bilayers, each of the bilayers comprising a spacer layer comprising a first material and an absorber layer comprising a second material different from the first material, at least one of the bilayers comprising ZrN.

5. A multilayer mirror comprising: a substrate; a multilayer coating on the substrate; and a capping layer on the multilayer coating, the capping layer comprising an outermost layer comprising Nb2O5 or TiO2, and a multilayer structure positioned between the outermost layer and the multilayer coating, the multilayer structure comprising a plurality of bilayers, each of the bilayers comprising a spacer layer comprising a first material and an absorber layer comprising a second material different from the first material, one of the first material and the second material comprising ZrO2 and the other of the first material and the second material comprising ZrN.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic, not-to-scale, view of an overall broad conception for a laser-produced plasma EUV radiation source system according to an aspect of the present invention.

(2) FIG. 2 is a schematic, not-to-scale diagram of a cross section of an EUV optical element with a multilayer capping layer.

DETAILED DESCRIPTION

(3) Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to promote a thorough understanding of one or more embodiments. It may be evident in some or all instances, however, that any embodiment described below can be practiced without adopting the specific design details described below. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate description of one or more embodiments.

(4) With initial reference to FIG. 1 there is shown a schematic view of an exemplary EUV radiation source, e.g., a laser produced plasma EUV radiation source 20 according to one aspect of an embodiment of the present invention. As shown, the EUV radiation source 20 may include a pulsed or continuous laser source 22, which may for example be a pulsed gas discharge CO.sub.2 laser source producing radiation at 10.6 μm. The pulsed gas discharge CO.sub.2 laser source may have DC or RF excitation operating at high power and high pulse repetition rate.

(5) The EUV radiation source 20 also includes a target delivery system 24 for delivering target material in the form of liquid droplets or a continuous liquid stream. The target material may be made up of tin or a tin compound, although other materials could be used. The target material delivery system 24 introduces the target material into the interior of a chamber 26 to an irradiation region 28 where the target material may be irradiated to produce plasma. In some cases, an electrical charge is placed on the target material to permit the target material to be steered toward or away from the irradiation region 28. It should be noted that as used herein an irradiation region is a region where target material irradiation may occur, and is an irradiation region even at times when no irradiation is actually occurring.

(6) Continuing with FIG. 1, the radiation source 20 may also include one or more optical elements. In the following discussion, a collector 30 is used as an example of such an optical element, but the discussion applies to other optical elements as well. The collector 30 may be a normal incidence reflector, for example, implemented as an MLM, that is, a silicon carbide (SiC) substrate coated with a molybdenum/silicon (Mo/Si) multilayer with additional thin barrier layers, for example B.sub.4C, ZrC, Si.sub.3N.sub.4 or C, deposited at each interface to effectively block thermally-induced interlayer diffusion. Other substrate materials, such as aluminum (Al) or silicon (Si), can also be used. The collector 30 may be in the form of a prolate ellipsoid, with an aperture to allow the laser radiation to pass through and reach the irradiation region 28. The collector 30 may be, e.g., in the shape of a ellipsoid that has a first focus at the irradiation region 28 and a second focus at a so-called intermediate point 40 (also called the intermediate focus 40) where the EUV radiation may be output from the EUV radiation source 20 and input to, e.g., an integrated circuit lithography tool 50 which uses the radiation, for example, to process a silicon wafer workpiece 52 in a known manner. The silicon wafer workpiece 52 is then additionally processed in a known manner to obtain an integrated circuit device.

(7) As described above, one of the technical challenges in the design of an optical element such as the collector 30 is extending its lifetime. One way to extend the lifetime of the collector 30 involves protecting it from damage by using an outermost cap layer. The cap layer system is itself advantageously a multilayer system composed of several alternating spacer and absorber layers to provide enhanced EUV reflectance of the collector mirror coating (for example at 13.5 nm wavelength). Just as with the multilayer of the main (Mo/Si) coating of the collector 30, the multilayered cap layer system also has to have a graded design with the bilayer spacing matched to the incidence angle as a function of the radius of the collector 30.

(8) An example of an MLM collector 30 with a multilayer cap is shown in FIG. 2 which is a cross section though a portion of such a collector. As can be seen there, the collector 30 includes a substrate 100. A multilayer coating 110 is located on the substrate 30. The multilayer coating 110 is made up of alternating layers of material, for example, molybdenum and silicon, in a known fashion. Located on the multilayer coating 110 is a capping layer 120 which is made up of an outermost layer 130 and a series of repeating bilayers 140. Each of the bilayers 140 preferably includes a spacer layer 150 and an absorber layer 160. FIG. 2 shows an arrangement with five bilayers but one of ordinary skill in the art will readily appreciate that other numbers of bilayers may be used.

(9) The purpose of the multilayer cap is to protect the collector 30 without excessively decreasing the overall reflectivity of the collector 30 at the wavelengths of interest, e.g., 13.5 nm. It is, however, preferable to select materials for the layers within the multilayer cap that will resist blistering and hydrogen diffusion. For example, multilayered cap bilayers that include silicon such as a zirconium nitride/silicon (ZrN/Si) bilayer or a tungsten/silicon (W/Si) bilayer may be prone to blistering. This is due to a hydrogen reaction within the Si layers where dangling bonds at the layer boundary react with hydrogen and in the bulk of the layer. The reaction can form SiH.sub.4 (silane) and hydrogen blisters inside of the silicon layers. Other bilayer combinations such as molybdenum/yttrium (Mo/Y) may not provide an effective bather to hydrogen diffusion.

(10) It is thus advantageous to provide for a cap layer system that protects the collector 30 coating against target material (e.g., tin) deposition, hydrogen ion penetration, hydrogen diffusion, and hydrogen or oxygen induced blistering.

(11) By choosing materials for the spacer layers of the cap multilayer system in the form of suitable nitrides, carbides, and borides (such as trisilicon tetranitride (Si.sub.3N.sub.4), zirconium nitride (ZrN), silicon carbide (SiC), carbon (C), yttrium nitride (YN), yttrium hexaboride (YB.sub.6), zirconium carbide (ZrC), silicon hexaboride (SiB.sub.6), and boron carbide (B.sub.4C)) hydrogen diffusion into the multilayer coating is reduced and reaction with hydrogen in the spacer layers is reduced, leading to a resistance against the formation of hydrogen-induced blisters. By choosing materials as absorber layers in the form of suitable oxide, nitride, or metal layers (such as tantalum pentoxide (Ta.sub.2O.sub.5), titanium dioxide (TiO.sub.2), zirconium dioxide (ZrO.sub.2), niobium pentoxide (Nb.sub.2O.sub.5), yttrium oxide (Y.sub.2O.sub.3), aluminum oxide (Al.sub.2O.sub.3), titanium-aluminum-oxynitride (TiAlON), ZrN, silicon nitride (SiN), titanium nitride (TiN), Mo, W, and Zr) the protection of the topmost layer against tin deposition is increased and the protection against hydrogen penetration and target material penetration and, in part, against hydrogen diffusion is increased.

(12) Referring again to FIG. 2, the topmost layer 130 of the cap 120 is preferably a nitride or oxide with high resistance to target material deposition. In effect, these are preferably materials having a low recombination rate for atomic hydrogen to enable a high formation rate of stannane. These would typically be materials having a hydrogen recombination coefficient in a range of about 10.sup.−4 to about 10.sup.−3. Effectively this means the preferred material exhibits a good tin cleaning rate since the H can react with Sn before it recombines to H.sub.2. As an example, the metal stainless steel has a recombination coefficient of 2.2×10.sup.−3. A preferred material for the topmost layer 130 of the cap 120 also preferably exhibits good energy reduction for incident ions and low secondary electron yield. Examples of materials having low recombination coefficients, good energy reduction for incident ions, and low secondary electron yield include ZrN, TiO.sub.2, Ta.sub.2O.sub.5, and ZrO.sub.2.

(13) The spacer layers are preferably made from hydrogen-diffusion and blister-resistant materials such as nitrides and carbides. The spacer layers are preferably grown amorphously to act as efficient barriers for hydrogen diffusion. Some materials exhibit microcrystalline growth in thin layers. For such materials, hydrogen can diffuse more easily along grain boundaries in crystalline layers; therefore, amorphously grown layers and layers with low defect densities are preferred as hydrogen barriers. Carbides, borides and nitrides are perceived as good hydrogen diffusion barrier layers. In general, ceramics are considered good barriers for H diffusion. Also, the spacer layers are preferably made of a material that is relatively inert with respect to reactions with hydrogen. For example, SiC (silicon carbide) has all bonds between Si and C saturated and is thus less prone to blistering. Yttrium nitride (YN) is a better barrier layer with respect to hydrogen diffusion compared to pure yttrium which shows micro-crystalline growth.

(14) The absorber layers are preferably made from suitable oxide or metal layers which can reduce the penetration of incident ions. In other words, the material for the absorber layer preferably has relatively high stopping power for impacting hydrogen ions. This implies a relatively large preferred stopping cross section. It is preferred hydrogen ions having energy in the about 100 eV energy should not be able to penetrate the material more than a few nanometers. ZrO.sub.2 is an example of such a material. As for metals, molybdenum is a preferred material, and for some applications molybdenum carbide (Mo.sub.2C) is preferred as the “metal” material because it has almost the same EUV reflectance as Mo but better growth properties and better properties with respect to H diffusion.

(15) Besides these properties, the layer materials in the cap layer also have to have good transparency to EUV radiation at 13.5 nm wavelength.

(16) Suitable materials for nitride layers include Si.sub.3N.sub.4, ZrN, YN, SiN, NbN, TiN, and BN.

(17) Suitable materials for carbide layers include SiC, B.sub.4C, C, and ZrC.

(18) Suitable materials for boride layers include ZrB.sub.2, NbB.sub.2, YB.sub.6, and SiB.sub.6.

(19) Suitable materials for the oxide layers include ZrO.sub.2, TiO.sub.2, Ta.sub.2O.sub.5, Nb.sub.2O.sub.5, Y.sub.2O.sub.3, Al.sub.2O.sub.3, and titanium-aluminum-oxynitride (TiAlON).

(20) Suitable materials for the metal layers include Mo, W, and Mo.sub.2C.

(21) The presently preferred combinations of materials for the absorber/spacer bilayer include: Mo as the material for the absorber and Si.sub.3N.sub.4, YN, B.sub.4C, ZrC, C, or YB.sub.6 as the material for the spacer; W as the material for the absorber and Si.sub.3N.sub.4, YN, B.sub.4C, ZrC, C, or YB.sub.6 as the material for the spacer; ZrO.sub.2 as the material for the absorber and Si.sub.3N.sub.4, YN, B.sub.4C, ZrC, C, or YB.sub.6 as the material for the spacer; Nb.sub.2O.sub.5 as the material for the absorber and Si.sub.3N.sub.4, YN, B.sub.4C, ZrC, C, or YB.sub.6 as the material for the spacer; TiO.sub.2 as the material for the absorber and Si.sub.3N.sub.4, YN, B.sub.4C, ZrC, C, or YB.sub.6 as the material for the spacer; and Mo.sub.2C as the material for the absorber and Si.sub.3N.sub.4, YN, B.sub.4C, ZrC, C, or YB.sub.6 as the material for the spacer.

(22) The above description includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the aforementioned embodiments, but one of ordinary skill in the art may recognize that many further combinations and permutations of various embodiments are possible. Accordingly, the described embodiments are intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the term “includes” is used in either the detailed description or the claims, such term is intended to be inclusive in a manner similar to the term “comprising” as “comprising” is construed when employed as a transitional word in a claim. Furthermore, although elements of the described aspects and/or embodiments may be described or claimed in the singular, the plural is contemplated unless limitation to the singular is explicitly stated. Additionally, all or a portion of any aspect and/or embodiment may be utilized with all or a portion of any other aspect and/or embodiment, unless stated otherwise.