[0001] This application claims the benefit of priority under 35 U.S.C. § 1 19 of U.S. Provisional Application Serial No. 61/934,351 filed on January 31 , 2014, the content of which is relied upon and incorporated herein by reference in its entirety.

FIELD

[0002] The present disclosure is directed to coated mirrors for deep ultraviolet (DUV) lithography applications, and in particular, to infrared transparent cold mirrors having high reflectance over a broad wavelength band in the UV-DUV region.

BACKGROUND

[0003] Laser produced plasma (LPP) has been extensively investigated as a potential high power light source for the Extreme Ultraviolet (EUV) lithography industry. Generally in LLP, a high power infrared laser is used to irradiate a metal or gas target to generate plasma emitting EUV radiation, and the radiation is then collected by an EUV collector mirror. Similar concepts have been used to develop broadband UV-VIS light sources, specifically laser-driven light sources (such as LDLS™, commercially available from Energetiq Technology Inc., Woburn, MA).

[0004] The LDLS concept is very similar to LPP as a source for EUV radiation. A focused high power infrared (IR) laser passes through a hole at the center of a collector mirror. This configuration is due to lack of an IR transparent and DUV- VIS high reflective mirror, or a UV-DUV expanded cold mirror. As an alternative to a conventional cold mirror, aluminum-based mirror have been used. However, the aluminum-based mirror is opaque in the IR range, and thus does not transmit light from the IR laser. As a result, a center hole in the mirror is commonly necessary in order to allow passage of light from the IR laser. In addition, the aluminum-based mirror has a low damage threshold which makes the aluminum-based mirror less compatible with high power light sources. [0005] Cold mirrors are frequently applied to reflectors that are located close to a source. Figure 1 A illustrates a cold mirror having an angle of incidence of 0 degrees. Figure IB, is a graph of reflectance versus wavelength for the cold mirror of Figure 1A, shows that the cold mirror has high reflectance at wavelengths of about 400 nm to about 650 nm when operating in the VIS-IR range. Similarly, Figure 1C illustrates a cold mirror having an angle of incidence of 45 degrees. Figure ID, which is a graph of reflectance versus wavelength for the cold mirror of Figure 1 C, shows that the cold mirror has high reflectance at wavelengths of about 400nm to about 650nm when operating in the VIS-IR range. Such cold mirrors are designed to reflect about 90% of visible light and to transmit about 80% of infrared radiation/energy (heat) and may be used to provide visible light for lighting applications, such as lighting for stadiums, projection lighting, studio settings, medical applications and others, while also reducing the amount of reflected heat.

[0006] Although a conventional cold mirror can be modified to provide high transmittance in the NIR range, the high reflective band in the DUV range of the mirror is limited. For example, as shown in Figure 2, the high reflective band in the DUV range for a conventional cold mirror operating at an angle of incidence of 0 degrees is about 15 nm. Similarly, as shown in Figure 3, the high reflective band in the DUV range for a conventional cold mirror operating at an angle of incidence of 45 degrees is about 10 nm.

SUMMARY

[0007] According to an embodiment of the present disclosure, an expanded cold mirror is provided. The mirror includes a substrate and a coating deposited on the substrate. The coating includes a first coating stack comprising at least one period of a low refractive index metal oxide coating layer and a high refractive index metal oxide coating layer, a second coating stack comprising at least one period of a low refractive index metal fluoride coating layer and a high refractive index metal oxide layer, and a third coating stack comprising at least one period of a low refractive index metal fluoride coating layer and a high refractive index metal fluoride coating layer.

[0008] Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments as described herein, including the detailed description which follows, the claims, as well as the appended drawings.

[0009] It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understanding the nature and character of the claims. The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the description serve to explain principles and operation of the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The disclosure will be understood more clearly from the following description and from the accompanying figures, given purely by way of non-limiting example, in which:

[0011] Figure 1 A is an illustration of a conventional cold mirror operating at an angle of incidence of 0 degrees;

[0012] Figure IB is a graph of reflectance and transmittance versus wavelength for the cold mirror of Figure 2A;

[0013] Figure 1 C is an illustration of a conventional cold mirror operating at an angle of incidence of 45 degrees;

[0014] Figure ID is a graph of reflectance and transmittance versus wavelength for the cold mirror of Figure 2C;

[0015] Figure 2 is a graph of reflectance versus wavelength for a conventional DUV cold mirror operating at an angle of incidence of 0 degrees;

[0016] Figure 3 is a graph of reflectance versus wavelength for a conventional DUV cold mirror operating at an angle of incidence of 45 degrees; [0017] Figure 4 is a schematic diagram of a cold mirror in accordance with embodiments of the present disclosure;

[0018] Figure 5 is a schematic diagram of a cold mirror in accordance with embodiments of the present disclosure;

[0019] Figure 6 is a schematic diagram of a cold mirror in accordance with embodiments of the present disclosure;

[0020] Figure 7 is a graph of reflectance versus wavelength for a cold mirror in accordance with embodiments of the present disclosure;

[0021] Figure 8 is a graph of reflectance versus wavelength for a cold mirror in accordance with embodiments of the present disclosure; and

[0022] Figure 9 is a graph of reflectance versus wavelength for a cold mirror in accordance with embodiments of the present disclosure.

DETAILED DESCRIPTION

[0023] Reference will now be made in detail to the present embodiment(s), an example(s) of which is/are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts.

[0024] The singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise. The endpoints of all ranges reciting the same characteristic are independently combinable and inclusive of the recited endpoint. All references are incorporated herein by reference.

[0025] As used herein, the following terms refer to different light ranges: near infrared (NIR) light is in the range of about 750nm to about 1400nm; visible (VIS) light is in the range of about 400nm to about 750nm; ultraviolet (UV) light is in the range of about 300nm to about 400nm; middle ultraviolet light is in the range of about 200nm to about 300nm; and deep ultraviolet (DUV) light is in the range of about 122nm to about 200nm. As used herein, the UV-DUV range is between about 122nm and about 400nm. Also as used herein, the term "cold mirror" refers to a mirror that reflects visible light while transmitting infrared light.

[0026] As used herein, the term "period" refers to a coating layer pair that repeats to form a periodical multilayer structure. Also as used herein, the term "stack" refers to a plurality of such layers. The periods are described throughout the disclosure using a low refractive index to high refractive index format such as "L ₀ /H ₀ ", "L _f /H ₀ " and "L H _f ", where L ₀ is a low refractive index metal oxide, H ₀ is a high refractive index metal oxide, L _f is a low refractive index metal fluoride, and ¾ is a high refractive index metal fluoride.

[0027] Embodiments of the present disclosure are directed to an IR transparent cold mirror having high reflectance over a broad wavelength band in the UV-DUV region, and to methods for forming at least one coating on the expanded cold mirror.

[0028] The expanded cold mirror disclosed herein includes a substrate. The substrate may be for example, but is not limited to, CaF ₂ , Si0 ₂ , F-Si0 ₂ , fused silica, for example HPFS® (commercially available from Corning Incorporated, Corning, NY) and quartz. The expanded cold mirror also includes at least one coating deposited on the substrate. The at least one coating includes at least one coating material. According to an embodiment of the present disclosure, the at least one coating may include at least five coating materials, wherein the at least five coating materials are deposited in at least three coating stacks.

[0029] Figure 4 is a schematic diagram of a cold mirror 10 in accordance with embodiments of the present disclosure. As shown, the cold mirror 10 includes a substrate 100 and a coating 1 10 deposited on the substrate 100. The coating 1 10 includes a first coating stack 200, a second coating stack 300 and a third coating stack 400. As shown, the first coating stack includes at least one period of a first layer 210 and a second layer 220. The second coating stack includes at least one period of a first layer 310 and a second layer 320. The third coating stack includes at least one period of a first layer 410 and a second layer 420. [0030] The first coating stack 200 is deposited on the substrate 100. The first coating stack 200 may include at least one period of a first layer 210 and a second layer 220. The first layer 210 may be a high refractive index metal oxide and the second layer 220 may be a low refractive index metal oxide. The high refractive index metal oxide may be, but is not limited to, Α1 ₂ 0 ₃ , Ta ₂ Os, Nb ₂ Os, Ti0 ₂ , Hf0 ₂ and Sc ₂ 0 ₃ . The low refractive index metal oxide may be, but is not limited to, Si0 ₂ , F-S1O2. N-doped Si0 ₂ and Al-doped Si0 ₂ . For example, at least one period may include Si0 ₂ /Hf0 ₂ , where Si0 ₂ is the low refractive index metal oxide and Hf0 ₂ is the high refractive index metal oxide. The at least one period may include other high and low refractive index metal oxide pairs, such as, but not limited to, a Si0 ₂ /Sc ₂ 0 ₃ . At least one period may also include Si0 ₂ /Ta ₂ 0 ₅ , Si0 ₂ /Nb ₂ 0 ₅ , or Si0 ₂ /Ti0 ₂ deposited on the substrate 100 and at least one second period of a Si0 ₂ /Hf0 ₂ or Si0 ₂ /Sc ₂ 0 ₃ deposited on the at least one first period. The first coating stack 200 provides to the at least one coating 1 10 high reflectance from the UV region to the VIS region. As used herein, the term "high reflectance" refers to a % reflectance greater than about 95%. For example, at an angle of incidence of 45 degrees, the first coating stack 200 has a high reflectance between about 250nm and about 760nm.

[0031] The second coating stack 300 is deposited over the first coating stack 200. The second coating stack 300 may include at least one period of a first layer 310 and a second layer 320. The first layer 310 may be a layer of a low refractive index metal fluoride and the second layer 320 may be a layer of a high refractive index metal oxide. The high refractive index metal oxide may be, but is not limited to, A1 ₂ 0 ₃ , Ta ₂ Os, Nb ₂ Os, Ti0 ₂ , Hf0 ₂ , and Sc ₂ 0 ₃ . The low refractive index metal fluoride may be, but is not limited to, A1F ₃ , MgF ₂ , CaF ₂ , BaF ₂ , SrF ₂ and Na ₃ AlF ₆ . For example, the at least one period may be A1F ₃ /A1 ₂ 0 ₃ , where A1F ₃ is the low refractive index metal fluoride and A1 ₂ 0 ₃ is the high refractive index metal oxide. The at least one period may include other low refractive index metal fluoride and high refractive index metal oxide pairs, such as, but not limited to, MgF ₂ /Al ₂ 0 ₃ or Na ₃ AlF ₆ /Al ₂ 0 ₃ . The second coating stack 300 further provides to the at least one coating 1 10 high reflectance in the UV region. For example, at an angle of incidence of 45 degrees, the second coating stack 300 deposited over the first coating stack 200 expands the high reflectance of between about 250nm and about 760nm for the first coating stack 200 to between about 200nm and about 760nm for the combination of the first coating stack 200 and the second coating stack 300.

[0032] The third coating stack 400 is deposited over the second coating stack 300. The third coating stack 400 may include at least one period of a first layer 410 and a second layer 420. The first layer 410 may be a high refractive index metal fluoride and the second layer 420 may be a low refractive index metal fluoride. The low refractive index metal fluoride may be, but is not limited to, A1F ₃ , MgF ₂ , CaF ₂ , BaF ₂ , SrF ₂ and Na ₃ AlF ₆ . The high refractive index metal fluoride may be, but is not limited to, a lanthanide metal fluoride such as LaF ₃ , GdF ₃ , PrF ₃ , NdF ₃ , PmF ₃ , SmF ₃ , EuF ₃ , DyF ₃ and HoF ₃ . For example, the at least one period may be AlF ₃ /GdF ₃ , where A1F ₃ is the low refractive index metal fluoride and GdF ₃ is the high refractive index metal fluoride. The at least one period may include other high and low refractive index metal fluoride pairs, such as, but not limited to, MgF ₂ /LaF ₃ , Na ₃ AlF ₆ /LaF ₃ , Na ₃ AlF ₆ /GdF ₃ or MgF ₂ /GdF ₃ . As shown in Figure 5, the third coating stack 400 may also include at least one intermediate layer 430 between periods of the third coating stack 400. The intermediate layer 430 may be fluoride doped silica (F— Si0 ₂ ) layer deposited to provide interface smoothing. In addition, as shown in Figure 6, the third coating stack 400 may also include a capping layer 440. The capping layer may be a F-Si0 ₂ layer deposited over the outermost layer of the third coating stack 400 as a top or layer to seal the at least one coating 1 10. The third coating stack 400 further provides to the at least one coating 1 10 high reflectance in the DUV region. For example, at an angle of incidence of 45 degrees, the third coating stack 400 deposited over the second coating stack 300 expands the high reflectance of between about 200nm and about 760nm for the first coating stack 200 and the second coating stack 300 to between about 180nm and about 760nm for the combination of the first coating stack 200, the second coating stack 300 and the third coating stack 400. The combination of the first coating stack 200, the second coating stack 300 and the third coating stack 400 also provides to the at least one coating 1 10 anti-reflective properties in the NIR region.

[0033] It should be appreciated that the different layers as illustrated in Figures 4-6 have described only some embodiments of the cold mirror disclosed herein for ease of understanding. It should be appreciated that the configuration of the layers in some or all of the periods of each of the coating stacks may be reversed. For example, the first layer 210 and the second layer 220 of the first coating stack 200 may be reversed such that the first layer 210 may be a low refractive index metal oxide and the second layer 220 may be a high refractive index metal oxide. Furthermore, it should be appreciated that the dashed lines of Figure 4-6 are meant to illustrate that each of the coating stacks may include more than one period.

[0034] While the reflectance of embodiments of the present disclosure have been discussed above with regards to an angle of incidence of 45 degrees, the cold mirror discussed herein also has a high reflectance from between about 250 nm and about 760 nm when the angle of incidence is 30 degrees, 40 degrees, or even 60 degrees, and a reflectance of greater than about 90% from between about 200 nm and about 760 nm when the angle of incidence is 30 degrees, 40 degrees, or even 60 degrees.

[0035] According to embodiments of the present disclosure, the broadness of the high reflective band may be adjusted from the VIS-DUV region to the DUV region based on different intended applications. Figure 7 is a graph of reflectance versus wavelength for a UV-DUV cold mirror having high reflectance between about 180 nm and about 290 nm at an angle of incidence of 45 degrees. Figure 8 is a graph of reflectance versus wavelength for a UV-DUV cold mirror having high reflectance between about 190 nm and about 300 nm at an angle of incidence of 30 degrees. Figure 9 is a graph of reflectance versus wavelength for a UV-DUV cold mirror having high reflectance between about 185 nm and about 270 nm at an angle of incidence of 60 degrees.

[0036] According to embodiments of the present disclosure, a method of forming the at least one coating 1 10 on a substrate 100 is provided. The method includes depositing the first coating stack 200 on the substrate 100. Depositing the first coating stack 200 on the substrate 100 includes depositing alternating period layers using an electron beam evaporation technique. Depositing the first coating stack 200 may further include using a modified, plasma-ion assisted, electron beam evaporation technique to form a first coating stack 200 that is dense and smooth. Depositing the first coating stack 200 on the substrate 100 may be completed at a temperature of between about 120°C and about 150°C. [0037] The method further includes depositing the second coating stack 300 on the first coating stack 200. Depositing the second coating stack 300 on the first coating stack 200 includes depositing alternating period layers using a thermal boat evaporation technique to deposit the fluoride layers and using an electron beam evaporation technique to deposit the oxide layers. Depositing the second coating stack 300 on the first coating stack 200 may be completed at a temperature sufficient to ensure densification of the second coating stack 300. The temperature may be between about 250°C to about 400°C, or may be about 280°C to about 320°C.

[0038] The method further includes depositing the third coating stack 400 on the second coating stack 300. Depositing the third coating stack 400 on the second coating stack 300 includes depositing alternating period layers using a thermal boat evaporation technique. For example, two thermal boats having a depth sufficient to restrict the angle of deposition may be used to deposit the alternating period layers. Depositing the third coating stack 400 on the second coating stack 300 may be completed at a temperature sufficient to ensure densification of the third coating stack 400. The temperature may be between about 250°C to about 400°C, or may be about 280°C to about 320°C.

[0039] Depositing the third coating stack 400 on the second coating stack 300 may further include depositing at least one intermediate layer 430 between periods of the alternating period layers, wherein the at least one intermediate layer 430 is a fluoride doped silica layer. In addition, depositing the third coating stack 400 on the second coating stack 300 may further include depositing a capping layer 440 over the outermost layer of the third coating stack 400, wherein the capping layer 440 is a fluoride doped silica layer. The at least one intermediate layer 430 and the capping layer 400 may be deposited using a modified, plasma-ion assisted, electron beam evaporation technique to form a coating surface that is dense and smooth.

[0040] Depositing oxide layers using an electron beam evaporation technique may include placing an inversed mask above the oxide evaporation source to restrict the angle of deposition. The shape of the inversed masks may also be chosen to improve coating uniformity. Similarly, depositing fluoride layers using a thermal boat evaporation technique may include providing a thermal boat having a depth sufficient to restrict the angle of deposition.

[0041] Different deposition temperatures as discussed herein are selected to minimize interface stresses between the different coating stacks. Thus, a relatively low deposition temperature may be used when depositing the first coating stack 200, and a relatively high deposition temperature may be used when depositing the second coating stack 300 and the third coating stack 400.

[0042] The cold mirrors as described herein provide a laser damage resistant design, having low loss properties and that are environmentally stable in the UV-DUV range. The mirrors described herein may be used with broadband VIS-DUV laser-driven light sources and with broadband UV-DUV laser-driven light sources. The mirrors transmit IR driven laser radiation for plasma generation and collect corresponding VIS-DUV emission. Further, cold mirrors as described herein may reduce design complexity and increase lifetime of the laser-driven light sources when compared to conventional aluminum-based mirrors.

[0043] While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.