REFLECTIVE OPTICAL ELEMENT

Description

[0001] The present invention relates to a reflective optical element for an operating wavelength in the DUV or VUV wavelength range, comprising a substrate, a dielectric layer system and a metallic coating between the substrate and the dielectric layer system, in which the dielectric layer system comprises at least one layer made of a material with a lower refractive index n1 at the operating wavelength, a layer made of a material with a higher refractive index n2 at the operating wavelength and a layer made of a material with a refractive index n3 at the operating wavelength with n1 <n3<n2, wobei an mindestens einem Übergang von einer Schicht mit einem niedrigeren Brechungsindex n1 zu einer Schicht mit einem höheren Brechungsindex n2 bzw. von einer Schicht mit einem höheren Brechungsindex n2 zu einer Schicht mit einem niedrigeren Brechungsindex n1 eine Schicht mit einem mittleren Brechungsindex n3 angeordnet ist.Furthermore, the invention relates to an optical system, a lithography device and a microscopy device having such a reflective optical element.

[0002] In microlithography, dielectric mirrors are used alongside lenses in optical systems using deep ultraviolet (DUV) or vacuum ultraviolet (VUV) radiation, particularly at wavelengths between 150 nm and 300 nm. The radiation source is often an excimer laser emitting in this wavelength range. Excimer lasers emitting at 193 nm or 248 nm are particularly common. US Pat. No. 5,850,309 discloses a reflective optical element comprising a substrate, a dielectric layer system, and a metallic coating between the substrate and the dielectric layer system. The metallic coating primarily serves for broadband reflection. The dielectric layer system serves to improve the properties of the reflective optical element.These include, for example, the reflectivity at the operating wavelength, the degree of polarization of the reflected radiation, and the resistance of the metallic coating to DUV radiation and other environmental influences. The dielectric layer system described in US Pat. No. 5,850,309 is constructed from alternating high- and low-refractive-index materials. It can be divided into sub-blocks, with different materials used as low- and high-refractive-index layers in different sub-blocks. This results in increased reflectivities at the operating wavelength—i.e., the wavelength of the radiation source for which the respective reflective optical element is optimized—while simultaneously providing increased laser resistance compared to reflective optical elements with a metallic coating and without a dielectric layer system.

[0003] WO 2015 / 039705 A1 discloses a multilayer mirror for the extreme ultraviolet wavelength range, which in one embodiment can comprise a multilayer system consisting of a repeating stack of a material with a high real part of the refractive index (H), a material with a low real part of the refractive index (L), and a material with an intermediate real part of the refractive index (IM), resulting in the layer sequence H-IM-L-IM-H-IM-L... This ensures that the mirror has a reflectivity maximum at exactly two wavelengths in the extreme ultraviolet wavelength range.

[0004] From US 2011 / 0206859 A1, a generic reflective optical element is known in which an amorphous layer of silicon dioxide or doped silicon dioxide is provided between at least two periods of fluoridic high- and low-refractive-index layers.

[0005] In JP 2006-227099 A, as in US 5,850,309, a reflective optical element for the UV wavelength range is described, which has a substrate, a dielectric layer system and a metallic coating between the substrate and the dielectric layer system.

[0006] US 2008 / 247044 A1 describes reflective optical elements for lithography for use in the visible wavelength range for the production of LCDs. These reflective optical elements do not have a metallic coating, but rather a dielectric multilayer coating designed to exhibit special polarization properties.

[0007] It is an object of the present invention to further develop the known reflective optical element.

[0008] This object is achieved by a reflective optical element for an operating wavelength between 150 nm and 300 nm in the DUV or VUV wavelength range, comprising a substrate, a dielectric layer system and a metallic coating between the substrate and the dielectric layer system, in which the dielectric layer system comprises a layer made of a material with a lower refractive index n1 at the operating wavelength and a layer made of a material with a higher refractive index n2 at the operating wavelength, wherein the dielectric layer system further comprises layers made of a material with a refractive index n3 at the operating wavelength with n1 <n3<n2,wherein a layer with the average refractive index n3 is arranged at transitions from a layer with the lower refractive index n1 to a layer with the higher refractive index n2 or from a layer with the higher refractive index n2 to a layer with a lower refractive index n1, such that the dielectric layer system has a four-layer sequence of (LMHM)m or (HMLM)m with L the layer of a material with a lower refractive index n1 at the operating wavelength, H the layer of a material with a higher refractive index n2 at the operating wavelength and M each the layer of a material with a refractive index n3 at the operating wavelength and wherein n1 <n3<n2 sowie m die Anzahl der Vierschichtenabfolge im dielektrischen Schichtsystem ist,wherein for an operating wavelength in the range between 240 nm and 300 nm, the at least one layer (L) is made of a material with a lower refractive index n1 from one or more materials from the group aluminum fluoride, cryolite, chiolite, lithium fluoride and magnesium fluoride, the at least one layer (H) is made of a material with a higher refractive index n2 from one or more materials from the group yttrium oxide, hafnium oxide, scandium oxide, zirconium oxide, aluminum nitride and synthetic diamond and the layers (M) are made of a material with an average refractive index n3 from one or more materials from the group barium fluoride, gadolinium fluoride, lanthanum fluoride, neodymium fluoride, dysprosium fluoride, aluminum oxide, yttrium fluoride,Ytterbium fluoride and silicon dioxide, and wherein for an operating wavelength in the range from 150 nm to 240 nm, the at least one layer (L) is made of a material with a lower refractive index n1 from one or more materials from the group aluminum fluoride, cryolite, chiolite, lithium fluoride and magnesium fluoride, the at least one layer (H) is made of a material with a higher refractive index n2 from one or more materials from the group neodymium fluoride, gadolinium fluoride, dysprosium fluoride, lanthanum fluoride and aluminum oxide, and the layers (M) are made of a material with the average refractive index n3 from one or more materials from the group magnesium fluoride, yttrium fluoride and silicon dioxide.

[0009] It has been found that providing at least one layer made of a material with an average refractive index n3 at the operating wavelength allows a reduction in the total number of individual layers of the dielectric layer system compared to the previously known reflective optical element while maintaining comparable properties. The layer materials for the dielectric layer system are selected, particularly with a view to increasing the reflectivity of the reflective optical element, such that the average refractive index n3 differs by at least 2% from the higher refractive index n1 and the lower refractive index n2.

[0010] This not only reduces production costs but also extends the lifetime of the reflective optical element by reducing the likelihood of stress or delamination, thus increasing mechanical stability. Furthermore, higher broadband performance can be achieved.

[0011] It should be noted that if the dielectric layer system includes more than one layer with a lower refractive index n1 or a higher refractive index n2, these are particularly preferably arranged alternately. Furthermore, it should be noted that if more than one layer is made of a material with a higher, lower, or medium refractive index, these can each be made of different materials, provided that the condition n1 <n3<n2 für jeweils benachbart angeordnete Schichten erfüllt ist. Um den Produktionsaufwand geringer zu halten, wird vorzugsweise jeweils immer nur ein Material verwendet.

[0012] With the proposed reflective optical element, the optical properties of the metallic coating in particular can be specifically and positively influenced. Furthermore, its stability against environmental influences and radiation damage can be increased with a reduced number of layers.

[0013] Preferably, the layer of the dielectric layer system furthest from the substrate is a layer made of a material with a medium refractive index n3, which further increases the reflectivity of the reflective optical element at the operating wavelength. Furthermore, this minimizes the electric field of the standing wave that forms during radiation reflection, thus reducing the emission of secondary electrons that could negatively impact the lifetime of the reflective optical element.

[0014] The dielectric layer system preferably comprises at least two layers of a material with an average refractive index n3. Even with two layers of a material with an average refractive index n3, a sufficient influence on the optical and other properties, for example, the broadband nature of the reflective optical element, can be achieved with a significantly reduced number of layers. With three, four, five, or more layers of a material with an average refractive index n3, a stronger influence on the optical and other properties of the reflective optical element can be achieved with a somewhat smaller number of layers.

[0015] Advantageously, the second closest layer of the dielectric layer system to the substrate is made of a material with an average refractive index of n3, particularly if the dielectric layer system comprises at least two layers of a material with an average refractive index of n3 and / or the layer of the dielectric layer system furthest from the substrate is a layer of the material with an average refractive index of n3. This can increase the radiation resistance of the metallic coating as well as its reflectivity.

[0016] The substrate is preferably made of quartz, titanium-doped quartz glass, calcium fluoride or glass ceramic.

[0017] Advantageously, the metallic coating comprises aluminum, an aluminum-silicon alloy, an aluminum-manganese alloy, an aluminum-silicon-manganese alloy, rhodium, or a combination thereof. Such metallic layers can lead to particularly broadband reflection or to high reflection despite broadband characteristics.

[0018] In a further aspect, the object is achieved by an optical system for a lithography device or a microscopy device, or by a lithography device or a microscopy device for an operating wavelength in the DUV or VUV wavelength range, with a reflective optical element as described. The microscopy devices can be, for example, wafer or mask inspection systems.

[0019] The present invention will be explained in more detail with reference to a preferred embodiment. Figure 1 shows a schematic diagram of a lithography device for the DUV or VUV wavelength range; Figure 2 shows a schematic diagram of a microscopy device for the DUV or VUV wavelength range; Figure 3 shows a schematic diagram of the structure of a reflective optical element suitable for use in lithography with DUV or VUV radiation; Figure 4 shows a schematic diagram of a first embodiment of the proposed reflective optical element; Figure 5 shows a schematic diagram of a second embodiment of the proposed reflective optical element.

[0020] Figure 1shows a schematic diagram of a lithography device 1 for the DUV or VUV wavelength range. The lithography device 1 has, as essential components, in particular two optical systems 12, 14: an illumination system 12 and a projection system 14, which in the present example are both designed as catadioptric systems. To carry out the lithography, a radiation source 10 is required, particularly preferably an excimer laser, which emits, for example, at 308 nm, 248 nm, 193 nm, or 157 nm and which can be an integral component of the lithography device 1. The radiation 11 emitted by the radiation source 10 is processed with the aid of the illumination system 12 such that a mask 13, also called a reticle, can be illuminated. For this purpose, the projection system 12 has at least one transmitting optical element and one reflective optical element.Representatively shown here are the lens 120, which focuses the radiation 11, for example, and the two mirrors 121, 122. In a known manner, a wide variety of transmitting, reflective and other optical elements can be combined with one another in any desired, even more complex, manner in the illumination system 12.

[0021] The mask 13 has a structure on its surface that is transferred to an element 15 to be exposed, for example, a wafer during the production of semiconductor components, using the projection system 14. In modifications, the mask 13 can also be designed as a reflective optical element.

[0022] The projection system 14 also has at least one transmitting optical element and one reflective optical element. In the example shown here, two mirrors 140, 141 and two transmitting optical elements 142, 143 are shown as representative elements, which serve, for example, in particular to reduce the structures on the mask 13 to the size desired for exposing the wafer 15. As with the exposure system 12, a wide variety of optical elements can be combined with one another in any desired manner in the projection system 14.

[0023] Figure 2 shows a schematic diagram of a microscopy device 2 for the DUV or VUV wavelength range. In the present example, it is designed as a wafer inspection system and includes an illumination system 16 and an imaging system 18, which are shown side by side in the present example for clarity and are both designed as a catadioptric system.

[0024] To perform the wafer inspection, a radiation source 163 is required, particularly preferably an excimer laser, which emits, for example, at 308 nm, 48 nm, 193 nm, or 157 nm and which can be an integral component of the microscopy device 2. The radiation 13 emitted by the radiation source 163 is processed with the aid of the illumination system 16 so that a wafer 17 can be illuminated. For this purpose, the projection system 12 has at least one transmitting optical element and one reflective optical element. Representatively shown here are the lens 160, which, for example, focuses the radiation 13, and the two mirrors 161, 162. In a known manner, a wide variety of transmitting, reflective, and other optical elements can be combined with one another in any desired, even more complex, manner in the illumination system 16.

[0025] The radiation 13' reflected from the surface of the wafer to be examined (in Figure 2 shown shifted in parallel) is guided by the imaging system 18 onto the detector 184 in such a way that structures on the surface of the wafer 17 are shown enlarged when they hit the detector, for example a spatially resolving area detector, for example based on a CCD (charge-coupled device) sensor.

[0026] The imaging system 18 also has at least one transmitting optical element and one reflective optical element. In the example shown here, two transmitting optical elements 180, 181 are shown as representative elements, which serve, for example, in particular to magnify the structures on the wafer 17, as well as two mirrors 182, 183. As with the illumination system 16, a wide variety of optical elements can be combined with one another in any known manner in the imaging system 18.

[0027] Both the mirrors 121, 122, 140, 141 and the mask 13 from Figure 1 as with mirrors 161, 162, 182, 183 from Figure 2It can be a reflective optical element, in particular for an operating wavelength in the range from 150 nm to 300 nm, which comprises a substrate, a dielectric layer system and a metallic coating between the substrate and the dielectric layer system, wherein the dielectric layer system comprises a layer of a material with a lower refractive index n1 at the operating wavelength, a layer of a material with a higher refractive index n2 at the operating wavelength and a layer of a material with a refractive index n3 at the operating wavelength with n1 <n3<n2, wobei Schichten mit einem niedrigerem Brechungsindex n1 und Schichten mit einem höheren Brechungsindex n2 abwechselnd angeordnet sind, wenn mehr als jeweils eine davon vorgesehen ist, und an mindestens einem Übergang von einer Schicht mit einem niedrigerem Brechungsindex n1 zu einer Schicht mit einem höherem Brechungsindex n2 bzw.a layer with an average refractive index n3 is arranged from a layer with a higher refractive index n2 to a layer with a lower refractive index n1.

[0028] In Figure 31 schematically illustrates the structure of a reflective optical element 20 suitable for use in lithography with DUV or VUV radiation. A metallic coating 24 is arranged on a substrate 22. The substrate 22 can be made of quartz, titanium-doped quartz glass, calcium fluoride, or glass ceramic, for example. The metallic coating 24 comprises, for example, aluminum, an aluminum-silicon alloy, an aluminum-manganese alloy, an aluminum-silicon-manganese alloy, rhodium, or a combination thereof and primarily serves as a broadband mirror. In order to protect the metallic coating 24, on the one hand, and to influence the properties of the radiation reflected by the reflective optical element 20, on the other hand, a dielectric layer system 26 is provided on the side of the metallic coating 24 facing away from the substrate 22, as a seal against the vacuum.

[0029] In an exemplary embodiment shown schematically in Figure 4 As shown, the dielectric layer system 26 has a four-layer sequence of (HMLM)m with L the layer of a material with a lower refractive index n1 at the operating wavelength, H the layer of a material with a higher refractive index n2 at the operating wavelength and M the layer of a material with a refractive index n3 at the operating wavelength, where n1 <n3<n2 sowie m die ganzzahlige Anzahl der Vierschichtenabfolge im dielektrischen Schichtsystem ist. Im hier dargestellten Beispiel ist m=1. Die Anzahl m kann insbesondere in Abhängigkeit von den gewünschten Eigenschaften des reflektiven optischen Elements beliebig gewählt werden.

[0030] Preferably, the layer 27 furthest from the substrate 22 or the layer 25 second closest to the substrate 22 is a layer M with an average refractive index n3. In Figure 4In the example shown, both the second closest layer 25 and the furthest layer 27 are made of a material with an average refractive index n3. Figure 4 The exemplary embodiment of a reflective optical element 20 proposed here has, in its dielectric layer system, in addition to the known layers H and L with higher and lower refractive indices n1, n2, two layers M with an average refractive index n3.

[0031] In a Figure 5 In the variant shown schematically, the dielectric layer system 26 can also have several four-layer sequences 28 of (LMHM)m. Figure 5In the example shown, the layer 27 furthest from the substrate 22 and the layer 25 second closest to the substrate 22 are layers M with an average refractive index n3. The dielectric layer system 26 has 2*m layers M. In further variants, at least one of the layers described in connection with Figure 4 or Figure 5The four-layer sequences 28 mentioned can be arranged directly on the metallic coating 24 and / or as the block of the dielectric layer system 26 which terminates in the vacuum or is furthest away from the substrate 22, and the remaining layers of the dielectric layer system can preferably be alternating H and L or L and H layers. One or more four-layer sequences 28 can also be arranged at other positions within the dielectric layer sequence. The materials for the L and H layers can always be one material each or, as described in US 5,850,309, different materials in blocks. The material for the M layers is selected depending on the materials of the adjacent H and L layers with the proviso that at the operating wavelength for which the reflective optical element is optimized, n1 <n3<n2 gilt.

[0032] According to the invention, in reflective optical elements for an operating wavelength in the range between 240 nm and 300 nm, the layer L is made of a material with a lower refractive index n1 from one or more materials from the group aluminum fluoride, cryolite, chiolite, lithium fluoride and magnesium fluoride, the layer H is made of a material with a higher refractive index n2 from one or more materials from the group yttrium oxide, hafnium oxide, scandium oxide, zirconium oxide, aluminum nitride and synthetic diamond and the layer M is made of a material with an average refractive index n3 from one or more materials from the group barium fluoride, gadolinium fluoride, lanthanum fluoride, neodymium fluoride, dysprosium fluoride, aluminum oxide, yttrium fluoride, ytterbium fluoride and silicon dioxide.

[0033] In reflective optical elements for an operating wavelength in the range from 150 nm to 240 nm, layer L is made of a material with a lower refractive index n1 from one or more materials from the group aluminum fluoride, cryolite, chiolite, lithium fluoride and magnesium fluoride, layer H is made of a material with a higher refractive index n2 from one or more materials from the group neodymium fluoride, gadolinium fluoride, dysprosium fluoride, lanthanum fluoride and aluminum oxide and layer M is made of a material with the average refractive index n3 from one or more materials from the group magnesium fluoride, yttrium fluoride and silicon dioxide.

[0034] The layer materials for the dielectric layer system are selected with a view in particular to increasing the reflectivity of the reflective optical element such that the average refractive index n3 differs by at least 2% from the higher refractive index n1 and the lower refractive index n2. Example

[0035] A reflective optical element made of a quartz substrate with a metallic aluminum coating features a dielectric layer system with five four-layer sequences of the LMHM type, i.e., 20 individual layers. It is designed for use with an excimer laser emitting at 193 nm.

[0036] The reflectivity of this reflective optical element, with a corresponding combination of high-, medium-, and low-refractive-index materials previously identified as suitable for operating wavelengths in the range of 150 nm to 240 nm, at an angle of incidence of 10° to the surface normal and unpolarized radiation, is above 95% for wavelengths between 190 nm and 215 nm, then decreases to values ​​between 95% and 90% up to 250 nm. Between 250 nm and 300 nm, the reflectivity still ranges between approximately 88% and just under 90%. A reflective optical element with comparable reflectivity and comparable properties, but without medium-refractive-index layers, has a dielectric layer system with a significantly larger number of individual layers.

[0037] Comparable results were obtained in variants with metallic coatings made of aluminum-silicon alloy, aluminum-manganese alloy, aluminum-silicon-manganese alloy, rhodium, or a combination thereof, or with aluminum, instead of aluminum. This also applies to variants optimized for excimer lasers emitting at other wavelengths, including materials suitable for operating wavelengths between 240 nm and 300 nm.

[0038] It should be noted that the reflective optical elements proposed here can exhibit a reflectivity of more than 90% even at a fixed wavelength of the incident radiation, but over an extended angle of incidence range of several 10°.

Claims

1. Reflective optical element for an operating wavelength of between 150 nm and 300 nm in the DUV or VUV wavelength range, comprising a substrate, a dielectric layer system and a metallic coating between the substrate and the dielectric layer system, wherein the dielectric layer system comprises at least respectively one layer composed of a material having a lower refractive index n1 at the operating wavelength, and also one layer composed of a material having a higher refractive index n2 at the operating wavelength, characterized in that the dielectric layer system furthermore comprises layers composed of a material having a refractive index n3 at the operating wavelength, where n1<n3<n2, wherein a layer having a medium refractive index n3 is arranged at transitions from a layer having a lower refractive index n1 to a layer having a higher refractive index n2 and / or from a layer having a higher refractive index n2 to a layer having a lower refractive index n1, in such a way that the dielectric layer system (26) comprises a four-layer sequence of (LMHM)m or (HMLM)m where L is the layer composed of a material having a lower refractive index n1 at the operating wavelength, H is the layer composed of a material having a higher refractive index n2 at the operating wavelength and M is in each case the layer composed of a material having a refractive index n3 at the operating wavelength, wherein n1<n3<n2, and m is the number of four-layer sequences in the dielectric layer system, wherein for an operating wavelength in the range of between 240 nm and 300 nm, the at least one layer (L) composed of a material having a lower refractive index n1 is composed of one or more materials of the group aluminium fluoride, cryolite, chiolite, lithium fluoride and magnesium fluoride, the at least one layer (H) composed of a material having a higher refractive index n2 is composed of one or more materials of the group yttrium oxide, hafnium oxide, scandium oxide, zirconium oxide, aluminium nitride and synthetic diamond, and the layers (M) composed of a material having a medium refractive index n3 are composed of one or more materials of the group barium fluoride, gadolinium fluoride, lanthanum fluoride, neodymium fluoride, dysprosium fluoride, aluminium oxide, yttrium fluoride, ytterbium fluoride and silicon dioxide, and wherein for an operating wavelength in the range of 150 nm to 240 nm, the at least one layer (L) composed of a material having a lower refractive index n1 is composed of one or more materials of the group aluminium fluoride, cryolite, chiolite, lithium fluoride and magnesium fluoride, the at least one layer (H) composed of a material having a higher refractive index n2 is composed of one or more materials of the group neodymium fluoride, gadolinium fluoride, dysprosium fluoride, lanthanum fluoride and aluminium oxide, and the layers (M) composed of a material having the medium refractive index n3 are composed of one or more materials of the group magnesium fluoride, yttrium fluoride and silicon dioxide.

2. Reflective optical element according to Claim 1, characterized in that the layer (27) of the dielectric layer system (26) which is the furthest away from the substrate (22) is a layer (M) composed of a material having a medium refractive index n3.

3. Reflective optical element according to Claim 1 or 2, characterized in that the layer (25) of the dielectric layer system (26) which is second closest to the substrate (22) is composed of a material (M) having a medium refractive index n3.

4. Reflective optical element according to any of Claims 1 to 3, characterized in that the metallic coating (24) comprises aluminium, an aluminium-silicon alloy, an aluminium-manganese alloy, an aluminium-silicon-manganese alloy, rhodium or a combination thereof.

5. Optical system for a lithography device or a microscopy device comprising a reflective optical element according to any of Claims 1 to 4.

6. Lithography device for an operating wavelength in the DUV or VUV wavelength range comprising a reflective optical element according to any of Claims 1 to 4.

7. Microscope device for an operating wavelength in the DUV or VUV wavelength range comprising a reflective optical element according to any of Claims 1 to 4.