WO2016007502A1

WO2016007502A1 - Resonance enhanced diffraction grating

Info

Publication number: WO2016007502A1
Application number: PCT/US2015/039365
Authority: WO
Inventors: Douglas BUERKLE; Evgeni POPOV
Original assignee: Lumella, Inc.
Priority date: 2014-07-07
Filing date: 2015-07-07
Publication date: 2016-01-14

Abstract

The present invention is directed to a new class of readily manufactured, high-efficiency, high-dispersion and long lifetime diffraction gratings, preferably for use in the line-narrowing of excimer lasers such as ArF and KrF lasers. These gratings of the present invention are based on the unexpected discovery that high diffraction efficiency can be achieved by use of a metallized preferably symmetric groove profile such as a sinusoidal, triangular or lamellar profile and a thick (one or more layer) dielectric over-coating(s) thickness-optimized for the particular wavelength and groove profile and depth.

Description

RESONANCE ENHANCED

DIFFRACTION GRATING

FUELD OF THE INVENTION

[00001] The present invention is directed to a new class of readily manufactured, high-efficiency, high-dispersion and long lifetime diffraction gratings, preferably for use in the line-narrowing of excimer lasers such as ArF and KrF lasers. These gratings of the present invention are based on the unexpected discovery that high diffraction efficiency can be achieved by use of a metallized preferably symmetric groove profile such as a sinusoidal, triangular or lamellar profile and a thick (one or more layer) dielectric over-coating(s) thickness-optimized for the particular wavelength and groove profile and depth.

CROSS-REFERENCE TO RELATED APPLICATIONS

[00002] This application claims the benefit of prior-filed U.S. provisional application Serial Number 61/998,7 9, filed July 7, 2014. The contents of this earlier-filed application are herein incorporated in their entirety by reference.

BACKGROUND

[00003] High power excimer lasers such as ArF and KrF lasers are commonly used in high-resolution photolithography systems. These lasers work at deep UV (DUV) wavelengths, 193.3 nm and 248 nm respectively, and are the prevalent light sources for the production of semiconductor integrated circuits (ICs), a multi-billion dollar industry of worldwide importance.

[00004] The direct output of DUV lasers is not sufficient to produce IC features of the desired fine resolution, which instead requires that these lasers be spectrally narrowed by the use of optics. Line narrowing modules (LNMs) are designed for this purpose and are used in most production systems.

[00005] One of the critical components in a line-narrowing module (LNM) is a diffraction grating. The diffraction grating spatially separates incident laser light by wavelength, allowing one to construct a type of filter that only passes a very narrow band of wavelengths. Current excimer lasers require wavelength bandwidths as narrow as a few hundred femtometers (1 x 10^"6 nm) in order to enable the reliable production of integrated circuits with small feature sizes.

Gratings with very high dispersion are required to produce such narrow

bandwidths.

[00006] Diffraction gratings within LNMs are most often used in the Littrow condition, where the desired wavelength diffracts at the same angle as the incident light. Under the Littrow condition, the grating equation simplifies to ml = 2dsinq, where m = the diffraction order, I = the wavelength of interest, d = the grating pitch and q = the incident and diffracted angles.

[00007] The short wavelength and high dispersion requirements for this excimer application combined with the requirement of reasonable

manufacturability leads to the use of diffraction gratings designed for multi-order diffraction rather than diffraction at first order. Thus for example when q = 79 degrees and I = 193.3 nm a first order grating would require more than 10,000 grooves/mm (smaller than a 100 nm pitch), a practical impossibility to

manufacture. Therefore multi-order gratings are used instead.

[00008] The current state-of-the-art for gratings for LNM for excimer laser application is the use of high order gratings called "echelle" gratings (in French, "echelle" means stair or ladder, which is the cross-sectional profile of such gratings). Echelle gratings are precisely ruled, coarse gratings used at high angles of diffraction. Typical groove frequencies range between 20 grooves/mm and 316 grooves/mm. In order to maintain high efficiency, echelle gratings are designed such that the active groove facets are perpendicular to the angle of the incident light.

[00009] Echelle gratings suffer many drawbacks including that they are expensive to manufacture due to the requirement for precise mechanical burnishing of coarse grooves over large surfaces. This problem is commonly mitigated through the production of replicated gratings. The replication process enables copies of original ruled gratings to be cast in epoxy. The introduction of epoxies into the grating system helps the cost problem but leads to significant other shortcomings. It is difficult to maintain a sufficiently high fidelity groove profile during replication, which can lead to a significant reduction in diffraction efficiency. In addition, coating temperatures are limited to the glass transition temperature of the epoxy. This leads to porous coatings that can be penetrated by O2, H₂O and other contaminants, leading to accelerated aging. [000010] Because of the high intensity (fluence) of the laser light, DUV gratings such as echelle gratings are also subject to problems in terms of lifetime. For example, these gratings are typically coated with high reflectance aluminum and a thin dielectric overcoat to prevent oxidation and/or to moderately enhance efficiency. Nonetheless, high fluence DUV light eventually destroys these coatings and therefore the diffraction gratings, leading to replacement or reconditioning of the LNM and the expense and downtime associated with such repairs.

[000011] Lifetime problems in DUV gratings have been exacerbated as the semiconductor industry pushes DUV lasers to produce smaller features according to Moore's Law. The most advanced lasers are being operated at higher power levels and higher pulse rates, causing accelerated failures of optics in general, and diffraction gratings in particular. The industry is desperately seeking new solutions that extend the lifetime of these systems.

[000012] Since the limitations of echelle gratings within high fluence laser applications are known, inventors have proposed a variety of approaches for maximizing their performance and life. A list of such approaches includes: 1 ) US 6,067,197, which describes an echelle grating with enhanced efficiency at 193.3 nm and 248 nm simultaneously; 2) US 6,51 1 ,703, which describes a method for applying a protective overcoat to a replicated echelle which extends the grating lifetime; 3) US 7,561 ,61 1 , which describes a method for densifying dielectric overcoats applied to diffraction gratings and mirrors; and, 4) US 6,788,465, which describes an echelle grating with a compound groove profile which can be produced more economically than a classical echelle grating.

[000013] Unfortunately, such solutions for producing high performance, cost- effective gratings over narrow wavelength ranges all suffer significant

shortcomings. Therefore, there is a great need to provide either better echelle gratings or, most generally, gratings of whatever configuration (e.g., non-echelle gratings) that are easily manufactured and that have high diffraction efficiency into low orders of interest, in particular for DUV laser applications such as, e.g., excimer laser applications.

SUMMARY OF THE INVENTION

[000014] The present invention is directed to a ew class of readily manufactured, high-efficiency, high-dispersion and long lifetime diffraction gratings, preferably for use in the line-narrowing of excimer lasers such as ArF and KrF lasers. These gratings of the present invention are based on the unexpected discovery that high diffraction efficiency can be achieved by use of a metallized preferably symmetric groove profile such as a sinusoidal, triangular or lamellar profile and a thick (one or more layer) dielectric over-coating(s) thickness-optimized for the particular wavelength and groove profile and depth.

[000015] Thus in embodiment 1 the present invention is directed to a multi- order diffraction grating with enhanced efficiency of a desired diffraction order over a narrow wavelength range encompassing a wavelength of interest, comprising: a) a substrate of a defined groove profile; b) a metallic coating applied over the substrate; and, c) a thick dielectric coating applied over the metallic coating, where the thickness of the dielectric coating is optimized to simultaneously suppress multiple orders of disinterest, thereby enhancing the efficiency of the desired diffraction order.

[000016] In embodiment 2, the present invention is directed to the diffraction grating of embodiment 1 , where the wavelength of interest is selected from the group consisting of 193.3 nm and 248 nm.

[000017] In embodiment 3, the present invention is directed to the diffraction grating of embodiment 2, where the wavelength of interest is 193.3 nm.

[000018] In embodiment 4, the present invention is directed to the diffraction grating of embodiment 1 , where the thick dielectric coating is comprised of one or more layers of dielectric material.

[000019] In embodiment 5, the present invention is directed to the diffraction grating of embodiment 4, where the thick dielectric coating is comprised of one to three layers of dielectric material.

[000020] In embodiment 6, the present invention is directed to the diffraction grating of embodiment 4, where the thick dielectric coating is comprised of a single layer of dielectric material.

[000021] In embodiment 7, the present invention is directed to the diffraction grating of embodiment 4 or embodiment 5, where each of the one to three layers of dielectric material is independently optimized.

[000022] In embodiment 8, the present invention is directed to the diffraction grating of embodiment 4, where the total combined optical thickness of the one or more layers of dielectric material is greater than 50 nm, 75 nm, 100 nm, 125 nm, 150 nm, 175 nm, or 200 nm.

[000023] In embodiment 9, the present invention is directed to the diffraction grating of embodiment 4, where at least one layer of dielectric material is substantially thicker than ¼ wave relative to the wavelength of interest.

[000024] In embodiment 10, the present invention is directed to the diffraction grating of embodiment 2, where the thick dielectric coating comprises one or more layers of S1O2 with a total combined thickness of at least 150 nm.

[000025] In embodiment 1 1 , the present invention is directed to the diffraction grating of embodiment 1 , where the groove profile is selected from the group consisting of a symmetric groove profile and an asymmetric groove profile.

[000026] In embodiment 12, the present invention is directed to the diffraction grating of embodiment 1 , where the groove profile is selected from the group consisting of a sinusoidal groove profile, a trapezoidal groove profile, a lamellar profile and a triangular groove profile.

[000027] In embodiment 13, the present invention is directed to the diffraction grating of embodiment 1 , where the groove profile is a symmetric sinusoidal groove profile.

[000028] In embodiment 14, the present invention is directed to the diffraction grating of embodiment 13, where the wavelength of interest is 193.3 nm and the thick dielectric layer is a single dielectric layer.

[000029] In embodiment 15, the present invention is directed to the diffraction grating of embodiment 14, where the wavelength of interest is 193.3 nm and the thick dielectric layer is a single dielectric layer of thickness greater than 200 nm.

[000030] In embodiment 16, the present invention is directed to the diffraction grating of embodiment 1 , where the groove profile is a symmetric triangular groove profile, the wavelength of interest is 193.3 nm and the thick dielectric layer consists of two dielectric layers.

[000031] In embodiment 17, the present invention is directed to the diffraction grating of embodiment 1 , where the multi-order diffraction grating is optimized for TM.

[000032] In embodiment 18, the present invention is directed to the diffraction grating of embodiment 1 , where the multi-order diffraction grating is optimized for TE.

[000033] In embodiment 19, the present invention is directed to a method of manufacturing the diffraction grating of embodiment 1 , comprising: a) Patterning the defined groove profile of the diffraction grating, where this patterning also defines the groove height and groove pitch; b) Overcoating this patterned defined groove profile with a metallic coating; and, c) Overcoating the metallic coating with a thick dielectric coating, where the thickness of the dielectric coating is optimized to simultaneously suppress multiple orders of disinterest, thereby enhancing the efficiency of the desired diffraction order. BRIEF DESCRIPTION OF THE DRAWINGS

[000034] Applicants note that the figures are provided as non-limiting illustrative examples of various aspects of the present invention and that the present invention is explicitly not limited to only these illustrative examples.

[000035] Figure 1 - Echelie grating/groove. An ideal echelie groove profile

[000036] Figure 2 - Echelie efficiency curve, master MR228 from Richardson Gratings, 1 17.94 g/mm, m = 86, unpolarized efficiency. A representative efficiency curve for an echelie grating with a 79 degree blaze angle used in high order at 193.3 nm

[000037] Figure 3 - Holographic grating/sinusoidal Groove. A grating with a sinusoidal groove profile and reflective metal coating

[000038] Figure 4 - Efficiency of various orders as a function of groove depth, for aluminum coated sinusoidal grating. An efficiency curve for TM polarization at 193.3 nm as a function of groove depth for all orders propagating from the grating of Figure 3

[000039] Figure 5 - Holographic grating/dinusoidal groove, with thick dielectric coating. The grating of Figure 3, but with a thick dielectric coating 19 applied over the aluminum

[000040] Figure 6 - Efficiency of various orders as a function of groove depth for aluminum grating with a thick dielectric overcoat. An efficiency curve for TM polarization at 193.3 nm as a function of groove depth for all orders propagating from the grating of Figure 5. This shows a "blazing" of the 4^th order due to the optimized groove depth and thick dielectric coating [000041] Figure 7 - Efficiency at 193.3 nm as a function of groove depth and coating thickness, 2,539.1 g/mm, m = 4, TM efficiency. An efficiency contour map showing how polarized, TM efficiency varies at 193.3 nm as a function of groove height and the dielectric layer thickness for a 4^th order grating designed as shown in Figure 5.

[000042] Figure 8 - Zoomed version of Figure 4 around area of highest efficiency. A zoomed version of Figure 7 showing the area of interest where TM efficiency is highest

[000043] Figure 9 - Efficiency curve with: h = 125.5 nm, t = 246 nm, 2539.1 g/mm, m = 4, TM efficiency. An efficiency curve for a sinusoidal grating with the groove height and dielectric thickness optimized for 193.3 nm, TM polarization and 79 degree angle of incidence

[000044] Figure 10 - Efficiency at 193.3 nm as a function of groove depth and coating thickness, 3,385.5 g/mm, m = 3, TM efficiency. An efficiency contour map showing how polarized, TM efficiency varies at 193.3 nm as a function of groove height and the dielectric layer thickness for a 3^rd order grating designed as shown in Figure 5.

[000045] Figure 1 1 - Efficiency at 248 nm as a function of groove depth and coating thickness, 1 ,979.1 g/mm, m = 4, TM efficiency. An efficiency contour map showing how polarized, TM efficiency varies at 248 nm as a function of groove height and the dielectric layer thickness for a 4^th order grating designed as shown in Figure 5. [000046] Figure 12 - Silicon grating/triangular groove. A grating with a triangular groove profile and an aluminum coating with a thick dielectric coating comprised of two separate layers.

[000047] Figure 13 - Efficiency at 193.3 nm as a function of two coating thicknesses, 2,539.1 g/mm, m = 4, TM efficiency. An efficiency contour map showing how polarized, TM efficiency varies at 193.3 nm as a function of the thickness of the two dielectric layers for a 4^th order grating designed as shown in Figure 12. In this case the groove height is fixed at 278.6 nm.

[000048] Figure 14 - Holographic grating/sinusoidal groove, 2-layer coating. A grating with a sinusoidal groove profile and an aluminum coating with a thick dielectric coating comprised of two separate layers.

[000049] Figure 15 - Efficiency at 193.3 nm as a function of thickness t2 with t1 fixed at 50 nm, 2,539.1 g/mm, m = 4, TM efficiency. An efficiency contour map showing how polarized, TM efficiency varies at 193.3 nm as a function of the thickness of the outermost dielectric layer for a 4^th order grating designed as shown in Figure 14 and with an angle of incidence of 79 degrees. In this case the groove height is fixed at 50 nm.

[000050] Figure 16 - Efficiency at 193.3 nm as a function of thickness t2 with t1 fixed at 100 nm, 2,539.1 g/mm, m = 4, TM efficiency. An efficiency contour map showing how polarized, TM efficiency varies at 193.3 nm as a function of the thickness of the outermost dielectric layer for a 4^th order grating designed as shown in Figure 14 and with an angle of incidence of 79 degrees. In this case the groove height is fixed at 100 nm. [000051] Figure 17 - Efficiency at 193.3 nm as a function of thickness t2 with t1 fixed at 150 nm, 2,539.1 g/mm, m = 4, TM efficiency. An efficiency contour map showing how polarized, TM efficiency varies at 193.3 nm as a function of the thickness of the outermost dielectric layer for a 4^th order grating designed as shown in Figure 14 and with an angle of incidence of 79 degrees. In this case the groove height is fixed at 150 nm.

[000052] Figure 18 - Efficiency at 193.3 nm as a function of thickness t2 with t1 fixed at 200 nm, 2,539.1 g/mm, m = 4, TM efficiency. An efficiency contour map showing how polarized, TM efficiency varies at 193.3 nm as a function of the thickness of the outermost dielectric layer for a 4^th order grating designed as shown in Figure 14 and with an angle of incidence of 79 degrees. In this case the groove height is fixed at 200 nm.

DETAILED DESCRIPTION OF THE INVENTION

[000053] The present invention is directed to a new class of readily manufactured, high-efficiency, high-dispersion and long lifetime diffraction gratings, preferably for use in the line-narrowing of excimer lasers such as ArF and KrF lasers. These gratings of the present invention are based on the unexpected discovery that high diffraction efficiency can be achieved by use of a metallized preferably symmetric groove profile such as a sinusoidal, triangular or lamellar profile and a thick (one or more layer) dielectric over-coating(s) thickness-optimized for the particular wavelength and groove profile and depth. [000054] Thus as already discussed, line-narrowing modules (LNM) for use with, e.g., ArF and KrF lasers typically use stair-shaped "echelle" gratings, which have well-known problems including cost of manufacture and longevity, but which are commonly used despite their shortcomings because there has been no other alternative with superior performance/cost.

[000055] In the present invention, Applicants have unexpectedly discovered that easily-manufactured grating profiles such as sinusoidal, triangular or lamellar gratings may be used for high-efficiency, high-dispersion and long lifetime diffraction gratings when such profiles - whether asymmetric or (preferably) symmetric - are metallized and then coated with a thick dielectric layer or layers with thickness optimized for the particular wavelength and groove profile and groove depth of the diffraction grating being produced. As will be discussed below, while not limited by any particular principle of function, Applicants believe that based on optical theory they have developed/extended , the optimized dielectric layer(s) of the present invention act to simultaneously suppress multiple orders of disinterest, thereby enhancing the efficiency of the desired diffraction order over a narrow wavelength range.

[000056] Applicants discuss this and other aspects of the present invention in more detail below. Conventional (Thin) Dielectrics

[000057] At the outset, Applicants note that dielectric coatings are

conventionally used over a metallized surface in order to protect that surface against oxidation, e.g., to protect the metallized layer of an echelie grating. In these situations the dielectric layer is "thin," i.e., its optical thickness is

substantively equal to ¼ or ½ wavelength. Such thin dielectric coatings are not part of the present invention, and do not result in the exhibition of any of the advantageous properties of the thick dielectric coatings used here.

Exotic Muiti!ayer Dielectrics

[000058] Applicants also note that there have been a variety of attempts made to employ "exotic" multilayer dielectrics to improve diffraction grating characteristics, where the exoticism is in the use of many layers of dielectric material in order to obtain the desired effects.

[000059] Thus, for example, US 5,907,436 describes a multi-layer dielectric grating without an underlying metal and with many plane 1/4 dielectric layers and a corrugated grating structure created in the outermost layer. This type of grating promises very high performance but each must be precisely created as an original. This is currently cost-prohibitive for use in excimer lasers.

[000060] US 6,754,006 describes a plane aluminum substrate with multilayer dielectric grating structures deposited on its surface. This type of grating also must be precisely created as an original and is impractical to cost-effectively manufacture. [000061] US 6,958,859 attempted to solve several of the shortcomings of the previous exotic designs. It envisions a silicon substrate with grooves easily and precisely fabricated along the silicon crystal planes using conventional MEMS processing techniques. The silicon substrate is coated with multi-layer dielectrics, again with repeating stacks of high and low index materials. The thickness of the high and low index layers is the same in each repeated stack. This design solves many of the problems with the previous designs but is not ideal in that the silicon substrate has a coefficient to thermal expansion (CTE) higher than desired. Also, while the number of coating layers is limited, it remains challenging to conformally coat even nine layers, especially over a discontinuous groove profile.

[000062] Applicants further note that such multi-dielectric coatings have been used in non-echelle profile (non-stair-step) diffraction gratings. Thus for example Mashev and Popov (Optics Communications, Vol. 51 , No. 3, 1 September 1984, p. 131 -136) describes sinusoidal groove structures with a multilayered dielectric system of 2n layers with alternating high and low refractive indices, no aluminum base coating, and the dielectric layers limited to optical thicknesses of I/4

[000063] The exotic multilayer dielectric coatings provided in the above examples are different from the thick dielectric of the present invention, in that the present invention is based on the use of preferably a thick single dielectric layer or a thick dielectric comprising a few (preferably 3 or fewer) layers, the thickness of each being individually optimized. As will be discussed, one of ordinary skill in the art of making high efficiency diffraction gratings would expect more layers to be preferable to fewer layers, i.e., would not expect the favorable properties seen in the diffraction grating design of the present invention.

Thick Single- or Few-Layer Dielectrics of The Invention - Background

[000064] As already discussed, echelle gratings are based on a stair-step profile, and have many limitiations. In the present invention Applicants preferably use symmetric profiles such as, e.g., symmetric sinusoidal profiles, symmetric triangular profiles or symmetric lamellar profiles, since these profiles are easy to make. Unfortunately, while easy to make, diffraction gratings based on such profiles have low diffraction efficiencies, most significantly in the UV and DUV; the present invention is based on the unexpected discovery by Applicants that diffraction gratings with such profiles where the metallized grooves have been overcoated with a thick dielectric layer or a few dielectric layers (2-3) exhibit extremely high diffraction efficiencies approaching, e.g., 80%, when the thickness of the dielectric layer(s) is based on consideration of groove profile, depth, the angle of incidence and the desired wavelength of light for which the diffraction grating is to be used.

[000065] In order to better describe the present invention, Applicants first consider the properties of a standard ("classic") echelle grating as shown in Figure 1 . Specifically, Figure 1 shows a classic echelle grating with an ideal groove profile, a pitch d 1 , a blaze angle q 2 and an active grating facet 3. The grating is operating in the Littrow condition with the incident and diffracted light 4 impinging the active facet at angles from grating normal 5 which are

substantively equal and substantively the same as the grating blaze angle q 2.

[000066] Further with regard to standard echelle gratings, Figure 2 shows that an echelle grating such as that shown in Figure 1 exhibits a reasonably high diffraction efficiency of about 68% at around 192.7 nm at a high order of m = 86. Specifically, Figure 2 shows the measured unpolarized efficiency curve 6 under quasi-Littrow conditions of order 86 from an echelle replica made from

Richardson Grating master MR228. This grating is coated with aluminum and a thin protective dielectric (MgF₂) overcoat and is blazed near 193.3 nm, exhibiting peak efficiency 7 close to 68% at around 192.7 nm. Note that the efficiency at the desired wavelength 8 is reduced to approximately 55%. This difference between peak efficiency and efficiency at the desired wavelength is a common shortcoming of echelle gratings and can often be attributed to subtle

imperfections from an ideal groove profile.

[000067] Applicants next consider the lower performance of a sinusoidal profile diffraction grating without a thick dielectric coating. Sinusoidal gratings such as the grating of Figure 3 are commonly manufactured and used in applications where the highest efficiency is not required and/or where longer wavelengths are of interest, i.e., in non-DUV applications. These gratings are of interest because master gratings can be easily, quickly, and cost-effectively manufactured using interference lithography and other known techniques.

[000068] With regard to such sinusoidal gratings, Figure 3 shows a grating with a sinusoidal groove fabricated in epoxy, photoresist or some other medium 9. The grating has a groove height h 10 and a grating pitch d 11 and is coated with optically thick metal 12. The grating is fabricated onto a substrate 13, commonly a glass or composite material with low coefficient of thermal expansion (CTE). Although this figure does not show a thin, protective dielectric overcoat layer, such a layer may be optionally applied over the metal to prevent oxidation of the optically thick metal 12. Such oxidation is undesirable, as it leads to increased absorption and decreased diffraction efficiency of the grating, especially when operating at UV and DUV wavelengths.

[000069] Figure 4 shows the TM polarized diffraction efficiency of the various orders propagating from a sinusoidal grating at 193.3 nm with the grating pitch set to 393.84 nm. This pitch satisfies the Littrow condition for the 4^th order at 193.3 nm when the angle of incidence is fixed at 79 degrees according to the grating equation: ml = 2dsinq, where m = diffraction order, I = wavelength of interest, d = grating pitch, and q = the angle of incidence.

[000070] As Figure 4 demonstrates, there is no groove depth for this sinusoidal grating that permits the 4^th order diffraction efficiency 18 to exceed 32%. The energy is shared between all orders with localized peaks occurring for order 0 14, order 1 15, order 2 16, and order 3 17. These peaks never exceed 46% except for the specular, zero order 14.

Thick Single-Layer Dielectrics of The invention - Sinusoidal Profile

[000071] As discussed above, while a sinusoidal profile diffraction grating such as that shown in Figure 3 is easily fabricated, the diffraction efficiency of a grating with this profile is low - again, Figure 4 shows a maximum of only 32% efficiency for m = 4.

[000072] The present invention is based on the unexpected discovery by Applicants that a thick single dielectric layer or a thick dielectric comprising a few (preferably 2-3) layers drastically raises this low efficiency, e.g., to 80% for the conditions shown in Figure 6.

[000073] Thus Figure 5 shows the grating of Figure 3, but with a thick single dielectric coating 19 deposited on the surface, while Figure 6 shows the enormous increase in diffraction efficiency unexpectedly resulting from this embodiment of a thick dielectric shown in Figure 5.

[000074] Specifically, Figure 6 shows the TM polarized diffraction efficiency of the various orders propagating from the grating of Figure 5 when a 246 nm thick exemplary dielectric (S1O2) is deposited on the surface and the groove height is varied. It is surprising to note that diffraction efficiency near 80% can be achieved in the 4^th order 20 when the proper groove height and coating thickness are selected.

[000075] With regard to this unexpected behavior, it is commonly understood when light passes from a medium with one index of refraction into another, some light is reflected and some is transmitted (refracted) according to Fresnel's laws. Thin film dielectric coatings are generally used in the production of tenses, mirrors and filters. In each case, the coatings are designed to promote constructive or destructive interference at the various layer interfaces, thereby enhancing reflection or refraction. [000076] Multilayer coatings are typically made up of many dozens of layers and routinely of more than 100 individual layers designed with repeating high and low index layer pairs where individual layer optical thicknesses are equal to a fraction of a wavelength. High-reflectance (HR) dielectric coatings are commonly fabricated using quarter-wave thicknesses of alternately high and low refractive index materials. The peak reflectance depends on the ratio of refractive indexes as well as the number of layer pairs. Increasing either increases the reflectance.

[000077] For diffraction gratings, the same principles apply. As has been mentioned elsewhere, US 5,907,438 and US 6,754,006 and US 6,958,859 have each proposed dielectric coatings under or on top of corrugated grating structures. Maystre (Applied Optics, Vol. 19, No. 18, 15 September 1980, p. 3099-3102) and Mashev and Popov (Optics Communications, Vol. 51 , No. 3, 1 September 1984, p. 131 -138) have additionally proposed adding thin film dielectric coatings on top of corrugated grating structures. In each case, a high number of layers have been shown to maximize diffraction efficiency. In the case where the coatings are deposited on the surface of a corrugated structure there has been the need to trade-off the coating reflectivity (number of pairs) for manufacturability (the ability to deposit the coatings sufficiently conformal to the groove structures). More specifically, the number of layer pairs and

corresponding efficiency have been sacrificed in an attempt not to deform the underlying groove structure.

[000078] If is therefore unexpected that a simple, thick dielectric coating deposited on top of an easy-to-fabricate grating structure can produce efficiency matching the previously mentioned exotic designs. It is also unexpected that the addition of incremental layers does not substantively increase the diffraction efficiency as compared to a one or two layer design.

[000079] Returning now to the figures, in order to better visualize the relationship of diffraction efficiency to a combination of groove height and coating thickness, Figure 7 shows how the TM polarized diffraction efficiency varies as the groove height and coating thickness are varied for the grating of Figure 5. As this figure clearly shows, there is a range of combinations of groove height and dielectric thickness that results in a plateau of high efficiency - i.e., the

rectangular box 21 within the figure.

[000080] Figure 8 shows a zoomed view of Figure 7 detailing the area where the parameters can be selected to produce the highest diffraction efficiency. A peak TM efficiency of 78% can be achieved when the groove depth is selected and precisely controlled at 125.5 nm and a S1O2 coating is selected and precisely controlled to be 246 nm 22.

[000081] Figure 9 shows the TM efficiency curve for the optimized grating of Figures 5 and 8. In addition to a peak efficiency of 78%, note the broad nature of the TM efficiency curve, especially compared to the echelle of Figure 2. This broad efficiency curve leads to manufacturing tolerances which are much broader than in the case of an echelle grating.

[000082] Applicants note that the present invention is directed and "low multi- order" diffraction gratings, i.e., diffraction gratings where m is preferably 4 or less. In this regard, the previous figures provide a discussion of m = 4 situations, but Applicants explicitly contemplate other low multi-order m values. In this regard, Applicants note that Figure 10 shows the TM efficiency curve for a grating of Figure 5, but which is optimized to operate in the 3^rd order.

[000083] Applicants also note that the present invention is not confined to diffraction gratings optimized only for high diffraction efficiency at 193.3 nm. Thus for example Figure 11 shows the TM efficiency curve for a grating of Figure 5, but which is optimized to operate at 248 nm in the 4^th order.

[000084] Further with regard to wavelength, Applicants add that although the present invention is preferably directed towards DUV wavelengths, thick-coated gratings such as those of the present invention may also be designed for wavelengths outside DUV. In this regard, Applicants particularly contemplate short wavelengths, i.e., wavelengths of less than 300 nm.

Few-Layer Dielectrics of The invention - Triangular Profile

[000085] As already discussed, the present invention is directed to thick- coated diffraction gratings with a number of profiles, including (but not limited to) sinusoidal, triangular, lamellar, etc. The previous section discussed the use of a sinusoidal profile; in this section Applicants discuss the use of a triangular profile thick-coated diffraction grating, which is another non-limiting groove profile of the present invention. For this embodiment, Applicants discuss the use of a two- layer thick dielectric, which is a preferred embodiment for diffraction gratings of the present invention built on silicon. [000086] Figure 12 shows a grating with triangular grooves 25, an optically thick metal coating 26, a first dielectric coating layer 27, and an outermost dielectric coating layer 28. The two dielectric layers 26 and 27 together make up the thick dielectric layer. In this case, the groove height is fixed by the complete anisotropic etching of silicon grooves according to the relationship: h =

(d/2)^*tan(54.75), where d = grating pitch and 54.75 degrees = {1 1 1 } silicon crystal planes.

[000087] Figure 13 shows how the TM polarized diffraction efficiency varies as the thickness of the two dielectric layers are varied for the grating of Figure 12.

Few-Layer Dielectrics of The Invention -· Ssnosoidal Profile

[000088] In another non-limiting embodiment, the present invention is directed to a metallized sinusoidal groove diffraction grating overcoated with two dielectric layers. Thus Figure 14 shows a grating with sinusoidal grooves 30 of height h, an optically thick metal coating 31 , a first dielectric coating layer 32 with thickness t1 , and an outermost dielectric coating layer 33 with thickness t2. The two dielectric layers 32 and 33 together make up the thick dielectric layer.

[000089] Figure 15 shows how the TM polarized diffraction efficiency varies as a function of the outermost dielectric coating layer thickness and groove height with the thickness of the innermost dielectric layer fixed at 50 nm for the grating of Figure 14. [000090] Figure 16 shows how the TM polarized diffraction efficiency varies as a function of the outermost dielectric coating layer thickness and groove height with the thickness of the innermost dielectric layer fixed at 100 nm for the grating of Figure 14.

[000091] Figure 17 shows how the TM polarized diffraction efficiency varies as a function of the outermost dielectric coating layer thickness and groove height with the thickness of the innermost dielectric layer fixed at 150 nm for the grating of Figure 14.

[000092] Figure 18 shows how the TM polarized diffraction efficiency varies as a function of the outermost dielectric coating layer thickness and groove height with the thickness of the innermost dielectric layer fixed at 200 nm for the grating of Figure 14.

Other Embodiments of The Present Invention

[000093] Applicants note that the present invention encompasses other embodiments apart from those provided in the figures and in the examples above. A non-limiting list of embodiments contemplated includes (but is not limited to): 1 ) profile: sinusoidal, triangular, modified sinusoidal or triangular (e.g., with the peaks of the sinusoid or triangle flattened); 2) symmetry of profile:

preferably symmetric, but asymmetric profiles are explicitly contemplated; 3) thick dielectric coating(s): preferably between 1 -3 coatings, with a single thick coating being the most preferred embodiment, where the thickness(es) are determined as provided in the discussion above for the figures; 4) dielectric material: any material known to one or ordinary skill in the art of applying such dielectric layers to diffraction gratings, e.g., fluorides and oxides suitable for DUV coating

(transparent between 193 and 248 nm) such as MgF2, NaF, AIF3, SiO2, AI2O3, etc.; 5) wavelength of operation ("wavelength of interest") of the diffraction gratings of the present invention: preferably ArF (193.3 nm) or KrF (248 nm), but other wavelengths are explicitly contemplated, particularly short wavelengths (less than 300 nm); 6) diffraction order (m): preferably single-digit, and more preferably 4 or less; and, 7) "metallic coating": preferably a single coating, but the present invention also contemplates more than a single coating, i.e.,

contemplates at least one metallic coating.

CONVENTIONAL DEVICE MANUFACTURABILITY/COST DISADVANTAGES

[000094] In this section, Applicants briefly discuss issues regarding manufacturability.

[000095] As already discussed, diffraction gratings are commonly used to narrow spectral bandwidth, e.g., of excimer lasers in Deep Ultraviolet (DUV) lithography applications, with "echelle" gratings commonly used in such applications.

[000096] Unfortunately, echelle gratings suffer many drawbacks. Based on their extensive experience, Applicants consider these to include: 1 ) expense of manufacture due to the requirement for precise mechanical burnishing of large surfaces; 2) practical size limitations due to this same mechanical burnishing process - e.g., an "R8" grating (a grating with an angle of incidence equal to Tan^" ¹ 8 or 80.5 degrees) requires a 1 .85 degree steeper grating facet and a 20% increase in ruled length as compared to the state-of-the-art R5 echelle, so that it is impractical to generate this type of grating considering the associated expected increase in diamond tool wear; 3) energy (diffraction efficiency) is lost to orders of disinterest; 4) practical diffraction efficiency does not match theoretical efficiency due to unavoidable deviations from the ideal groove profile - - current echelle gratings rarely exceed 55% diffraction efficiency at 193 nm despite theoretical models predicting 70%; 5) a narrow free-spectral-range places very tight manufacturing tolerances on echelle gratings; and, 8) echelle gratings are difficult to coat because applying uniform protective or enhancing overcoats is difficult due to the groove geometry and due to the "rough" nature of the inactive grating facet.

[000097] In order to mitigate the expense of echelle gratings, epoxy replicas are commonly cast from master gratings and used in volume production. While these replicas help produce gratings at more reasonable costs, they also introduce additional problems to those listed above. Based on their extensive experience, Applicants consider these to include: 1 ) coating temperatures are limited to the glass transition temperature of the epoxy. The resulting "soft" coatings exacerbate the problem of coating echeiles by increasing the likelihood that O2, H₂0 and other contaminants may penetrate them; 2) it is difficult to maintain a high fidelity groove profile during replication, and very subtle changes to the grooves will lead to a significant reduction in diffraction efficiency; and, 3) release agents used in the replication process may serve to contaminate the replicated grating and/or other optics within the line narrowing module.

PRESENT INVENTION - NON-LIMITING THEORY

[000098] While not bound by any particular theory, Applicants believe the superior effects of the diffraction gratings of the present invention are achieved by simultaneously optimizing two resonant phenomena: Fabry-Perot resonances and waveguide mode excitation ("Resonant Effects"). This optimization requires consideration of: 1 ) groove profile; 2) groove depth; 3) wavelength (or wavelength range) of interest; 4) the angle of incidence; and, 5) dielectric thickness, where "dielectric" refers to one or more and preferably one to three dielectric layers.

[000099] The present invention is based on the optimization of the design of the thick-coated diffraction gratings of the invention. Techniques for optimizing the design of the grating structure are useful in the design of the dielectric layer or layers herein. Such optimization, or predictive, techniques— as applied to determine grating structure parameters— implement the vector formalism of electromagnetic theory (i.e., Maxwell's equations), and have been

commercialized in the form of software products such as, e.g., GSo!verTM available from Grating Solver Development Company, Allen, Tex. for grating structures.

[0000100] In the algorithms and techniques employed to run out the optimization of the grating and the thick dielectric iayer(s), various parameters are permuted numerous times and the results compiled. At the end of the exercise, energy maxima in the desired order for each permutation is reported until efficiencies are high enough to be commercially attractive, and in most instances better than devices that are optimized solely by the manipulation of grating structure parameters.

[0000101] With regard to the preceding discussions, Applicants note that, unless otherwise indicated, "a" refers to "at least one," i.e., "a" metallized coating should be taken as meaning "at least one" metallized coating, etc.

[0000102] The invention has been described in detail with particular reference to a presently preferred embodiment, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended embodiments, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.

Claims

What is Claimed is:

1 . A multi-order diffraction grating with enhanced efficiency of a desired diffraction order over a narrow wavelength range encompassing a wavelength of interest, comprising:

a. A substrate of a defined groove profile;

b. A metallic coating applied over the substrate; and,

c. A thick dielectric coating applied over the metallic coating, where the

thickness of the dielectric coating is optimized to simultaneously suppress multiple orders of disinterest, thereby enhancing the efficiency of the desired diffraction order.

2. The diffraction grating of claim 1 , where the wavelength of interest is selected from the group consisting of 193.3 nm and 248 nm.

3. The diffraction grating of claim 2, where the wavelength of interest is 193.3 nm.

4. The diffraction grating of claim 1 , where the thick dielectric coating is comprised of one or more layers of dielectric material.

5. The diffraction grating of claim 4, where the thick dielectric coating is comprised of one to three layers of dielectric material.

6. The diffraction grating of claim 4, where the thick dielectric coating is comprised of a single layer of dielectric material.

7. The diffraction grating of claim 4 or claim 5, where each of the one to three layers of dielectric material is independently optimized.

8. The diffraction grating of claim 4, where the total combined optical thickness of the one or more layers of dielectric material is greater than 50 nm, 75 nm, 100 nm, 125 nm, 150 nm, 175 nm, or 200 nm.

9. The diffraction grating of claim 4, where at least one layer of dielectric material is substantially thicker than ¼ wave relative to the wavelength of interest.

10. The diffraction grating of claim 2, where the thick dielectric coating comprises one or more layers of S1O2 with a total combined thickness of at least 150 nm.

1 1 . The diffraction grating of claim 1 , where the groove profile is selected from the group consisting of a symmetric groove profile and an asymmetric groove profile.

12. The diffraction grating of claim 1 , where the groove profile is selected from the group consisting of a sinusoidal groove profile, a trapezoidal groove profile, a lamellar profile and a triangular groove profile.

13. The diffraction grating of claim 1 , where the groove profile is a symmetric sinusoidal groove profile.

14. The diffraction grating of claim 13, where the wavelength of interest is 193.3 nm and the thick dielectric layer is a single dielectric layer.

15. The diffraction grating of claim 14, where the wavelength of interest is 193.3 nm and the thick dielectric layer is a single dielectric layer of thickness greater than 200 nm.

16. The diffraction grating of claim 1 , where the groove profile is a symmetric triangular groove profile, the wavelength of interest is 193.3 nm and the thick dielectric layer consists of two dielectric layers.

17. The diffraction grating of claim 1 , where the multi-order diffraction grating is optimized for TM.

18. The diffraction grating of claim 1 , where the multi-order diffraction grating is optimized for TE.

19. A method of manufacturing the diffraction grating of claim 1 , comprising: a. Patterning the defined groove profile of the diffraction grating, where this patterning also defines the groove height and groove pitch;

b. Overcoating this patterned defined groove profile with a metallic coating; and,

c. Overcoating the metallic coating with a thick dielectric coating, where the thickness of the dielectric coating is optimized to simultaneously suppress multiple orders of disinterest, thereby enhancing the efficiency of the desired diffraction order.