Optical element with an antireflection coating, projection objective, and exposure apparatus comprising such an element

Abstract

An optical element (14) transparent for radiation with a wavelength in the ultraviolet wavelength range below 250 nm, in particular at 193 nm, comprises a substrate (17) with a refractive index n.sub.s larger than 1.6, and an antireflection coating (16) formed on at least part of the surface of the substrate (17) between the substrate (17) and an ambient medium with a refractive index n.sub.A, preferably with n.sub.A=1.0. The antireflection coating (16) consists of a single layer of a material with a refractive index n.sub.L of about n.sub.L={square root over (n.sub.An.sub.S)}, in particular n.sub.L>1.3, and the optical thickness d.sub.L of the single layer is about /4. The optical element (14) is preferably part of a projection objective (5) in a microlithography projection exposure apparatus (1) and located adjacent to a light-sensitive substrate (10).

Claims

1. Optical element transparent for radiation with a wavelength in the ultraviolet wavelength range below 250 nm, comprising: a substrate with a refractive index n.sub.s no larger than 1.8, and an antireflection coating formed on at least part of the surface of the substrate between the substrate and an ambient medium with a refractive index n.sub.A, wherein the antireflection coating comprises low refractive index layers alternating with medium refractive index layers, such that a first low refractive index layer is provided adjacent to the substrate and at least one further low refractive index layer alternates with at least one medium refractive index layer, and such that the first low refractive index layer lies closer to the substrate than do any of the medium refractive index layers, wherein all the medium refractive index layers have optical thicknesses that differ from each other, and all the low refractive index layers have optical thicknesses that differ from each other, wherein the optical thickness of the first low refractive index layer is less than 0.45, where is 193 nm, wherein the optical thickness of a second low refractive index layer is between 0.26 and 0.34, where is 193 nm, and where the second low refractive index layer is arranged on the first low refractive index layer with a medium refractive index layer arranged between the first low refractive index layer and the second low refractive index layer, wherein the optical thicknesses of all the medium and low refractive index layers are such that reflection of the radiation is minimized, and wherein the substrate comprises a material selected from the group consisting of: barium fluoride (BaF.sub.2), silicon oxide (SiO.sub.2), and potassium chloride (KCl).

2. Optical element according to claim 1, wherein the medium refractive index layers have a refractive index between 1.58 and 1.8, and the low refractive index layers have a refractive index smaller than 1.58.

3. Optical element according to claim 1, wherein a material of the low refractive index layers is selected from the group consisting of: chiolithe (Na.sub.5AI.sub.3F.sub.14), cryolite (Na.sub.3AIF.sub.6), aluminium fluoride (AIF.sub.3), magnesium fluoride (MgF.sub.2), silicon oxide (SiO.sub.2), calcium fluoride (CaF.sub.2), lithium fluoride (LiF), sodium fluoride (NaF), and strontium fluoride (SrF.sub.2).

4. Optical element according to claim 1, wherein a material of the medium refractive index layers is selected from the group consisting of: gadolinium fluoride (GdF.sub.3), lanthanum fluoride (LaF.sub.3), erbium fluoride (ErF.sub.3), yttrium fluoride (YF.sub.3), neodymium fluoride (NdF.sub.3), dysprosium fluoride (DyF.sub.3), holmium fluoride (HoF.sub.3), scandium fluoride (ScF.sub.3), zirconium fluoride (ZrF.sub.4), ytterbium fluoride (YbF.sub.3), hafnium fluoride (HfF.sub.4), and thorium fluoride (ThF.sub.3).

5. Optical element according to claim 1, wherein the surface is a hemispherical surface.

6. Projection objective for imaging a structure onto a light-sensitive substrate, having at least one optical element according to claim 1.

7. Projection objective according to claim 6, wherein the optical element is located adjacent to the light-sensitive substrate.

8. Microlithography projection exposure apparatus with a projection objective according to claim 7, wherein an immersion liquid is disposed between the light-sensitive substrate and the optical element which is located adjacent to the light-sensitive substrate.

9. Microlithography projection exposure apparatus with a projection objective according to claim 6, wherein an immersion liquid is disposed between the light-sensitive substrate and the optical element which is located adjacent to the light-sensitive substrate.

10. Optical element transparent for radiation with a wavelength in the ultraviolet wavelength range below 250 nm, comprising: a plano-convex substrate with a refractive index n.sub.s no larger than 1.8 and with a convex radiation-entrance surface, and an antireflection coating formed on at least part of the convex surface of the substrate between the substrate and an ambient medium with a refractive index n.sub.A, wherein the antireflection coating comprises low refractive index layers alternating with medium refractive index layers, such that a first low refractive index layer is provided directly adjacent to the substrate and at least one further low refractive index layer alternates with at least one medium refractive index layer, and such that the first low refractive index layer lies closer to the substrate than do any of the medium refractive index layers, wherein all the medium refractive index layers have optical thicknesses that differ from each other, and all the low refractive index layers have optical thicknesses that differ from each other, wherein the optical thickness of the first low refractive index layer is less than 0.45, where is 193 nm, wherein the optical thickness of a second low refractive index layer is between 0.26 and 0.34, where is 193 nm, and where the second low refractive index layer is arranged on the first low refractive index layer with a medium refractive index layer arranged between the first low refractive index layer and the second low refractive index layer, wherein the optical thicknesses of all the medium and low refractive index layers are such that reflection of the radiation is minimized, and wherein the substrate comprises a material selected from the group consisting of: barium fluoride (BaF.sub.2), silicon oxide (SiO.sub.2), and potassium chloride (KCl).

11. The optical element according to claim 10, wherein the medium refractive index layers have a refractive index between 1.58 and 1.8, and the low refractive index layers have a refractive index smaller than 1.58.

12. The optical element according to claim 10, wherein a material of the low refractive index layers is selected from the group consisting of: chiolithe (Na.sub.5AI.sub.3F.sub.14), cryolite (Na.sub.3AIF.sub.6), aluminium fluoride (AIF.sub.3), magnesium fluoride (MgF.sub.2), silicon oxide (SiO.sub.2), calcium fluoride (CaF.sub.2), lithium fluoride (LiF), sodium fluoride (NaF), and strontium fluoride (SrF.sub.2).

13. The optical element according to claim 10, wherein a material of the medium refractive index layers is selected from the group consisting of: gadolinium fluoride (GdF.sub.3), lanthanum fluoride (LaF.sub.3), erbium fluoride (ErF.sub.3), yttrium fluoride (YF.sub.3), neodymium fluoride (NdF.sub.3), dysprosium fluoride (DyF.sub.3), holmium fluoride (HoF.sub.3), scandium fluoride (ScF.sub.3), zirconium fluoride (ZrF.sub.4), ytterbium fluoride (YbF.sub.3), hafnium fluoride (HfF.sub.4), and thorium fluoride (ThF.sub.3).

Description

DRAWING

(1) The schematic drawing shows an embodiment of the invention which is explained in the following description.

(2) FIG. 1 shows an embodiment of a microlithography projection exposure apparatus according to the invention with an projection objective having an optical element with an antireflection coating located adjacent to the light-sensitive substrate,

(3) FIG. 2A, 2B each show a diagram of reflectance (in %) in dependence of the angle of incidence (in ) calculated for an optical element with a substrate of crystalline SiO.sub.2 covered with a single layer of chiolithe (FIG. 2A), and with two layers made of chiolithe and lanthanum fluoride (FIG. 2B),

(4) FIG. 3 shows an analogous diagram for a substrate of magnesium oxide covered with three layers with decreasing refractive index being made of sapphire, lanthanum fluoride, and chiolithe,

(5) FIG. 4 shows an analogous diagram for a substrate of magnesium oxide covered with an arrangement of alternating medium and low refractive index layers made of lanthanum fluoride and magnesium fluoride, and

(6) FIG. 5A-D show embodiments of the optical element according to the invention with an antireflection coating with A) one, B) two, C) three, and D) six layers.

DETAILED DESCRIPTION

(7) FIG. 1 shows a schematic representation of a microlithography projection exposure apparatus 1 in the form of a wafer stepper which is provided for manufacturing highly integrated semiconductor devices through immersion lithography. The projection exposure apparatus 1 comprises an excimer laser 2 as a light source with an operating wavelength of 193 nm, other operating wavelengths, for example 248 nm also being possible. A downstream illuminating system 3 produces in its exit plane 4a large, sharply delimited, very homogeneously illuminated image field.

(8) Arranged downstream of the illuminating system 3 is a device 7 for holding and manipulating a mask 6 such that the latter lies in the object plane 4 of a projection objective 5 and can be moved in this plane for the purpose of scanning operation in a transverse direction 9. Following downstream of the plane 4, also designated as mask plane, is the projection objective 5, which projects an image of the mask on a reduced scale, for example the scale of 4:1 or 5:1 or 10:1, onto a wafer 10 covered by a photoresist layer. The wafer 10 serving as photosensitive substrate is arranged such that the flat substrate surface 11 with the photoresist layer substantially coincides with the image plane 12 of the projection objective 5. The wafer is held by a device 8 which comprises a scanner drive, in order to move the wafer synchronously with the mask 6 and parallel to the latter. The device 8 also comprises manipulators in order to move the wafer both in the z-direction parallel to the optical axis 13 of the projection objective 5, and in the x- and y-directions perpendicular to the said axis.

(9) As an optical element 14 which is located adjacent to the wafer 10, the projection objective 5 has a hemispherical plano-convex lens whose flat exit surface is the last optical surface of the projection objective 5 and is arranged at a working distance above the substrate surface 11. Between the optical element 14 and the substrate surface 11 an immersion liquid 15, e.g. water, is disposed, the optical element 14 being in contact with the immersion liquid 15 with its flat exit surface on the wafer side. The larger numerical aperture produced in this manner permits imaging of smaller structures with the exposure apparatus 1 than is possible with use of air or vacuum as medium between the projection objective 5 and the substrate 10. For this purpose, the bulk of the optical element 14 is made of a substrate 17 with a refractive index at or above 1.6.

(10) Suitable materials for the substrate 17 of the optical element 14 are given ordered by their refractive index in the following non-exhaustive table:

(11) TABLE-US-00003 TABLE 3 barium fluoride (BaF.sub.2) 1.57, crystalline SiO.sub.2 1.66, potassium chloride (KCl) 1.76, sodium chloride (NaCl) 1.83, spinel (MgAl.sub.2O.sub.4) 1.87, sapphire (Al.sub.2O.sub.3) 1.93, magnesium oxide (MgO) 2.0, yttrium aluminium garnet (Y.sub.3Al.sub.5O.sub.12) 2.0, germanium oxide (GeO.sub.2) 2.05, lutetium aluminium garnet (Lu.sub.3Al.sub.5O.sub.12) 2.14, calcium oxide (CaO) 2.70

(12) On the curved surface of the optical element 14 adjacent to the medium present inside of the projection objective 5, being either air or vacuum both with n.sub.A=1.0, an antireflection coating 16 is formed. The antireflection coating 16 is selected from one of four different types of coatings being described in detail below and shown in FIG. 5A-5D (the curved surface of the substrate 17 being represented as flat for the sake of simplicity), each consisting of one or more layers made of materials having an optical thickness and a refractive index being selected in dependence of the refractive index of the substrate material 17 of the optical element 14. Of course, other optical elements with a substrate having a high refractive index above 1.6 can be covered with such antireflection coatings as well in order to advantageously suppress reflections.

(13) To the flat exit surface of the optical element 14, an antireflection coating may be applied according to one of the types described below, yet adapted to the refractive index of the immersion liquid constituting the ambient medium. This antireflection coating may be covered by a further layer made of a material which is inert with respect to the immersion liquid 15. This further protective layer prevents damage of the antireflection coating and the underlying substrate due to a chemical attack by the immersion liquid. If the immersion liquid is water, the protective layer cab be made e.g. of SiO.sub.2 or Teflon.

(14) A first type of antireflection coating 16 for the internal surface of the substrate 17 consists of only a single layer 20a, as shown in FIG. 5A. This type is preferred when it is sufficient to reduce reflectance for small to medium angles of incidence only. In this case, the refractive index of the layer n.sub.L should be approximately equal to the square root of the refractive index n.sub.s of the substrate material 17 of the optical element 14 and the optical thickness of the layer should be about /4.

(15) In the diagram of FIG. 2A, the reflectance of an optical element made of a substrate of crystalline SiO.sub.2 (with refractive index n.sub.s=1.66) covered with such a single-layer antireflection coating consisting of chiolithe (n.sub.L=1.35) is shown in dependence of the angle of incidence. The three plots represented in FIG. 2A show the reflectance R.sub.S of the s-polarized radiation component, the reflectance R.sub.P of the p-polarized radiation component, and the average reflectance R.sub.U of the both polarization components. Similar results can be obtained when a substrate of lutetium aluminium garnet Lu.sub.3Al.sub.5O.sub.12 (with refractive index n.sub.S=2.14) is covered with a single layer of magnesium fluoride MgF.sub.2 (with refractive index n.sub.L=1.44).

(16) For crystalline SiO.sub.2, however, the ideal layer material according to the formula n.sub.L={square root over (n.sub.An.sub.S)} given above has a refractive index n.sub.L=1.29. As such a material with a sufficient durability when exposed to intense laser radiation is not available, it is convenient to use a second type of antireflection coating consisting of two layers with refractive indices being approximately related by n.sub.L1=n.sub.L2{square root over (n.sub.S)}, both layers having an optical thickness of about /4.

(17) FIG. 2B shows the reflectance in dependence of the incident angle for such a two-layer antireflection coating shown in FIG. 5B with the same substrate as FIG. 2A (crystalline SiO.sub.2), the material of the first layer 20a being lanthanum fluoride (LaF.sub.3) with refractive index n.sub.L1=1.7, the material of a second layer 20b being cryolite (Na.sub.3AlF.sub.6) with refractive index n.sub.L2=1.35. The layer with the higher refractive index (LaF.sub.3) is located adjacent to the substrate, whereas the layer with the smaller refractive index (Na.sub.5Al.sub.3F.sub.14) is located adjacent to the ambient medium.

(18) When comparing the reflectance plots of FIG. 2A and FIG. 2B, it is evident that the use of two layers instead of a single layer yields an improved suppression of reflectance for small to medium angles of incidence ranging from 0 to 45 and that the separation of polarization components for larger angles of incidence is reduced. Although showing in the present example only to a small extent, the two-layer type of antireflection coating may also be advantageously used to suppress reflectance for higher angles of incidence above 60

(19) Antireflection coatings with two layers are also applicable for substrate materials with a high refractive index (1.8 and above). For such materials, the above formula for the refractive indices can hardly be satisfied, as e.g. for MgF.sub.2 with n.sub.L2=1.35 as a second layer material, the refractive index of the first layer material would be ideally n.sub.L1=2.1. Therefore, the optical thickness of the layers is no longer chosen to be equal to /4. With layers of different optical thickness, the reflectance for large incident angles and polarization splitting can be improved. Moreover, a two-layer antireflection coating may be chosen for high-index substrate materials not only for optical, but also for mechanical reasons, e.g. for improving adhesion of layers or layer hardness, or as a diffusion barrier. For lutetium aluminium garnet as a substrate material, an antireflection coating with a first layer 20a having a high refractive index and an optical thickness between 0.04 and 0.15 and a second layer 20b having a low refractive index and an optical thickness between 0.20 and 0.3 is preferred, where the first layer 20a is preferably made of sapphire (Al.sub.2O.sub.3) and the second layer is preferably made of magnesium fluoride MgF.sub.2. Optical thickness is defined herein as full wave optical thickness, i.e. layer thickness in units of wavelength.

(20) A third type of antireflection coating which can be applied to the optical element 14 consists of at least three layers with refractive indices n.sub.Li, the refractive indices n.sub.Li of the layers decreasing with increasing distance from the substrate. This type of antireflection coating approximates an ideal antireflection coating with a refractive index decreasing continuously from the substrate to the ambient medium.

(21) An example of the reflectance of such an antireflection coating is shown in FIG. 3 for a substrate of magnesium oxide (MgO, n.sub.s=2.0) with a coating consisting of three layers 20a to 20c shown in FIG. 5C, the material of the first layer 20a adjacent to the substrate being sapphire (Al.sub.2O.sub.3, n.sub.L1=1.9, d.sub.L1=0.3), the material of a second layer superimposed over the first layer being lanthanum fluoride (LaF.sub.3, n.sub.L2=1.7, d.sub.L2=0.45), and the material of a third layer superimposed over the second layer being chiolithe (Na.sub.5Al.sub.3F.sub.14, n.sub.L3=1.35, d.sub.L3=0.3). The overall optical thickness of such a type of coating should be in an range between 0.6 to 3 in units of wavelength, which is the case for the present example with an overall optical thickness of 1.05. This type of antireflection, coating is preferably used for reducing reflectance at high angles of incidence.

(22) A fourth type of antireflection coating consists of high or medium refractive index layers alternating with low refractive index layers, wherein the optical thicknesses of all high or medium and low refractive index layers are chosen such that reflection of radiation, in particular for high angles of incidence, is minimized. The layer thicknesses are preferably optimized by numerical calculations and experiments. Suitable designs of antireflection coatings for the substrates of table 3 are shown in table 2, the layer thicknesses of all layers being different from each other. In table 2, the high refractive index layers have a refractive index larger than 1.8, the medium refractive index layers have a refractive index between 1.58 and 1.8, and the low refractive index layers have a refractive index smaller than 1.58, the number of layers varying between two and six.

(23) The optical performance of this type of antireflection coating especially with four or more layers is superior to that of the other coating designs, particularly for high angles of incidence. For a substrate of magnesium oxide, the reflectance of a design consisting of six layers 20a to 20f shown in FIG. 5D with layer thicknesses reproduced in the first column of table 2 is shown in FIG. 4. As a layer material for the medium and low refractive index layers, lanthanum fluoride with a refractive index of 1.7 and magnesium fluoride with a refractive index of 1.44 have been used, respectively. As can be seen from FIG. 4, the reflectance of radiation with high angles of incidence is strongly suppressed in the present example and especially the separation of the two polarization components is drastically reduced compared to the design shown in FIG. 3 for the same substrate material (MgO). Similar results can be achieved with a substrate of lutetium aluminium garnet Lu.sub.3Al.sub.5O.sub.12 being covered with antireflection coatings with four to six layers, each having a range of optical thicknesses as described in table 2.

(24) It is understood that the use of optical elements provided with the antireflection coatings as described above is not limited to microlithography exposure apparatuses, but any transparent optical element with a substrate having a high refractive index above 1.6 in the UV wavelength range below 250 nm may be covered with such an antireflection coating in order to efficiently suppress reflections. Also, if a first part of the surface of the optical element is in contact with a first ambient medium and a second part of the surface with another, an antireflection coating of one of the types described above being adapted to the refractive index of the corresponding ambient medium may be applied in each case.