LAYER-FORMING METHOD, OPTICAL ELEMENT AND OPTICAL SYSTEM

Abstract

A method of forming a layer (3) on a substrate (2) made of a fluoridic material includes: depositing a coating material (9) on the substrate to form the layer and generating a plasma (12) to assist the deposition of the coating material. The plasma is formed from a gas mixture (14) containing a first gas (G) and a second gas (H), wherein the second gas has an ionization energy less than an ionization energy of the first gas, the first gas is a noble gas and the second gas is a further noble gas. An associated optical element includes: a substrate (2) composed of a fluoridic material, in particular a metal fluoride, wherein the substrate has a coating (18) having a layer (3) formed by the above method. An associated optical system, in particular for the DUV wavelength range, includes at least one such optical element.

Claims

1. A method of forming at least one layer on a substrate made of a fluoridic material, comprising: depositing at least one coating material on the substrate to form the layer, and generating a plasma to assist said depositing of the coating material, wherein the plasma is formed from a gas mixture containing a first gas and a second gas, wherein the second gas has an ionization energy less than an ionization energy of the first gas, wherein the first gas is a noble gas, and wherein the second gas is a further noble gas.

2. The method as claimed in claim 1, wherein the noble gas is Ar and the further noble gas is selected from the group consisting essentially of: Kr and Xe.

3. The method as claimed in claim 1, wherein the noble gas is Kr and the further noble gas is Xe.

4. The method as claimed in claim 1, wherein the noble gas is Ne and the further noble gas is selected from the group consisting essentially of: Ar, Kr and Xe.

5. The method as claimed in claim 1, wherein said generating comprises adding a third gas to the gas mixture.

6. The method as claimed in claim 5, wherein the third gas is selected from the group consisting essentially of: O.sub.2, N.sub.2, O.sub.3, N.sub.2O, H.sub.2O.sub.2, and fluorine-containing gases.

7. The method as claimed in claim 5, wherein the third gas is added to the gas mixture in a proportion of less than 2% by volume.

8. The method as claimed in claim 7, wherein the third gas is added to the gas mixture in a proportion of less than 0.1% by volume.

9. The method as claimed in claim 7, wherein the third gas is added to the gas mixture in a proportion of less than 0.001% by volume.

10. The method as claimed in claim 1, wherein the first gas and the second gas and/or the gas mixture are/is introduced via at least one gas inlet into a plasma source in which the plasma is generated.

11. The method as claimed in claim 1, wherein the gas mixture is formed by introducing the first gas via a gas inlet into a plasma source in which the plasma is generated, and in which the second gas is introduced into a vacuum chamber in which the substrate is disposed, or introducing the second gas via the gas inlet into the plasma source in which the plasma is generated, and in which the first gas is introduced into the vacuum chamber in which the substrate is disposed.

12. The method as claimed in claim 1, wherein a coating rate in depositing the coating material is less than 10.sup.−10 m/s.

13. The method as claimed in claim 1, wherein an active ion energy of ions present in the plasma is less than 100 eV.

14. The method as claimed in claim 13, wherein the active ion energy of the ions present in the plasma is between 45 eV and 100 eV.

15. The method as claimed in claim 1, wherein the substrate is a metal fluoride.

16. The method as claimed in claim 15, wherein the substrate is an alkaline earth metal fluoride.

17. An optical element comprising: a substrate composed of a fluoridic material, a coating on the substrate that comprises at least one layer formed by the method as claimed in claim 1.

18. The optical element as claimed in claim 17, wherein the substrate is composed of a metal fluoride.

19. An optical system, comprising: a radiation source, an illumination system, a mask, and a projection system, wherein the illumination system is configured to illuminate the mask with radiation from the radiation source, and the projection system is configured to project the radiation of the illuminated mask onto a wafer, and wherein at least one of the illumination system, the mask and the projection system comprises at least one optical element as claimed in claim 17.

20. The optical system as claimed in claim 17 and configured for operation in a deep-ultraviolet (DUV) wavelength range.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Working examples are shown in the schematic drawing and are explained in the description which follows. The figures show:

[0044] FIG. 1 a schematic diagram of a coating apparatus for coating of substrates,

[0045] FIG. 2 a schematic diagram of the first ionization energy of a plurality of chemical elements as a function of atomic number,

[0046] FIG. 3 schematic diagrams of a degree of compaction as a function of ion energy, coating rate and angle of vapor deposition,

[0047] FIG. 4 a schematic diagram of an example of an optical element having a coating that has been applied with the coating apparatus of FIG. 1, and

[0048] FIG. 5 a schematic diagram of an example of a DUV lithography system having the optical element of FIG. 4.

DETAILED DESCRIPTION

[0049] In the description of the drawings that follows, identical reference signs are used for components that are analogous or are the same or have the same or analogous function.

[0050] FIG. 1 shows a schematic of a coating apparatus 1 for forming or for depositing of layers—a layer 3 in the example shown—on a substrate 2. The coating apparatus 1 has a high-vacuum-tight recipient 4 surrounding a vacuum chamber 5. A turbomolecular pump 6 serves to evacuate the vacuum chamber 5. Disposed within the vacuum chamber 5 is a holding device 7 on which a plurality of substrates can be mounted, of which FIG. 1 shows just a single substrate 2. The holding device 7 is formed like a planetary system and has a main axis about which the substrates are rotated during coating, as indicated by an arrow in FIG. 1. The or a particular substrate 2 additionally rotates about its own axis of symmetry during coating, as likewise indicated by an arrow in FIG. 1. The substrate 2 in the example shown is a plane-parallel plate to which an antireflection coating is to be applied. The substrate 2 in the example shown is formed from calcium fluoride (CaF.sub.2), but may also be formed from a different material. The material of the substrate 2 may, for example, be another fluoridic material, for example a metal fluoride, in particular an alkaline earth metal fluoride.

[0051] Within the vacuum chamber 5 is disposed an electron beam evaporator 8 into which a coating material 9 has been introduced. During the deposition, the electron beam evaporator 8 generates an evaporation cone 10 in which the substrate 2 to be coated is disposed, such that the coating material 9 can be deposited on a surface of the substrate 2 to be coated that faces the electron beam evaporator 8. The coating material 9 in the example shown is SiO.sub.2, but it may also be another material, for example an oxidic or fluoridic material or an oxyfluoride.

[0052] Also disposed within the vacuum chamber 5 is a plasma source 11 that serves to generate a plasma 12, which is directed onto the surface of the substrate 2 to be coated in the form of a plasma jet. The plasma source 11 is a DC plasma source, as described in detail in the article [7]. The plasma source 11 has a rod-shaped cathode 13a surrounded by a ring-shaped anode 13b. An electrical field is generated between the cathode 13a and the anode 13b for generation of the plasma 12, in order to ionize a gas, more specifically a gas mixture 14, in an ionization space of the plasma source 11 and to produce the plasma 12 in this way. A coil (not shown) surrounds the anode 13b on its outside and generates an axial magnetic field in the ionization space.

[0053] As likewise apparent in FIG. 1, the gas mixture 14 is supplied to the plasma source 11 via a gas inlet 15, which, in the example shown, is disposed at the base of the ionization space of the plasma source 11. It will be apparent that the gas inlet 15 may also be positioned elsewhere in the plasma source 11 in order to supply the plasma source 11 with the gas mixture 14.

[0054] As likewise apparent in FIG. 1, the gas mixture 14 is formed in a mixing unit 16 which is likewise arranged outside the vacuum chamber 5. The mixing unit 16 has a valve arrangement in order to mix a first gas G present in a first gas reservoir with a second gas H present in a second gas reservoir. With the aid of the valve arrangement, it is optionally possible to adjust the mixing ratio of the two gases G, H, but this is not absolutely necessary. For the use described here, it is generally sufficient when the second gas H is present in the gas mixture 14 in a proportion of less than 10% by volume, of less than 1% by volume or of less than 0.1% by volume.

[0055] The gas mixture 14 is supplied to the plasma source 11 in order to generate chemoionization, i.e. transfer of excitation energy in the case of particle collisions between molecules or atoms of the first gas G and molecules or atoms of the second gas H. For the considerations that follow, it is assumed that the second gas H in the gas mixture 14 has a (first) ionization energy smaller than a (first) ionization energy of the first gas G. In this case, the Penning ionization can take place, for example, according to equations (2) and (3) given further up, meaning that, in the case of a collision, the excitation energy of the first gas G can be transferred to the second gas M such that the second gas M is ionized, or the two gases G, M may together form an ionized species, for example of the GM.sup.+ form.

[0056] In the example shown in FIG. 1, a third gas K is added to the gas mixture 14 including the two gases G, H. The third gas K can be added in the mixing unit 16, but it is also possible to supply the gas mixture 14 with the third gas K via a further gas inlet 15a in the plasma source 11, as shown in FIG. 1. The further gas inlet 15a is what is called a gas shower that brings the third gas K to the vicinity of, and causes it to emerge at, an outlet opening of the plasma source 11. The gas mixture 14 of the two gases G, H therefore forms at the exit of the plasma source 11 or at the exit of the ionization space.

[0057] The third gas K in the example shown is a fluoridic gas, but may also be another kind of gas, for example O.sub.2 or N.sub.2. In the present case, the proportion of the third gas K in the gas mixture 14 is less than 0.001% by volume. The proportion of the third gas K may alternatively be greater and may, for example, be less than 1% by volume or less than 2% by volume.

[0058] The fluoridic gas may, for example, be a gas selected from the group comprising: F.sub.2, CF.sub.4, SF.sub.6, xenon fluorides, for example XeF.sub.2, XeF.sub.4, XeF.sub.6, NF.sub.3, HF, BF.sub.3, CH.sub.3F, C.sub.2F.sub.4. The third, fluoridic gas K in the present case serves to generate a fluorine-containing atmosphere in the vacuum chamber 5. Fluorination of the coating material 9 is also possible with the aid of the fluoridic gas K in the deposition onto the substrate 2, for example in order to deposit fluorinated SiO.sub.2. It will be apparent that the third, fluoridic gas K can also serve for fluorination of other coating materials 9.

[0059] As an alternative to the supply of the gas mixture 14 via the (first) gas inlet 15, it is possible to feed in the first gas G to the plasma source 11 or the ionization space via the first gas inlet 15 and the second gas H via a second gas inlet (not shown in the image). In that case, the gas mixture 14 is formed only in the plasma source 11, more specifically in the ionization space.

[0060] Alternatively, the first gas G may be introduced into the plasma source 11 or into the ionization space, for example, via the first gas inlet 15, and the second gas H is introduced into the vacuum chamber 5 outside the plasma source 11. In that case, for example, the second gas H may be fed in via the further gas inlet 15a, which forms a gas shower, in the vicinity of the exit opening of the plasma source 11, as described further up in connection with the third gas K. In principle, the second gas H may alternatively be introduced into the vacuum chamber 5 elsewhere in order to form the gas mixture 14. It will be apparent that the roles of the first gas G and of the second gas H may also be exchanged.

[0061] The supply of the second gas H via the further gas inlet 15a is favorable in particular when the first gas G is a noble gas and the second gas H is a reactive gas, e.g. oxygen, ozone or a fluorine-containing gas. The supply of a reactive gas into the ionization space of the plasma source 11 would possibly result in damage to the components disposed therein. Supply of the second gas H via the further gas inlet 15a, which may, for example, be in ring-shaped form and have radial inlet openings, can avoid such damage. The gas mixture 14 may, for example, be a mixture of krypton (ionization energy 14 eV) and NF.sub.3 (ionization energy 13 eV) or xenon (ionization energy 12.1 eV) and tetrafluoroethylene C.sub.2F.sub.4 (ionization energy 10.1 eV), but it is also possible to use other kinds of gas mixtures 14.

[0062] The first gas G and the second gas N may also be noble gases, for which FIG. 2 shows the (first) ionization energy E.sup.+ (in eV) as a function of atomic number N. For the first ionization energy E.sup.+ of the noble gases, the order is as follows (from the greatest to the smallest ionization energy): He, Ne, Ar, Kr, Xe.

[0063] There are various options for the choice of the two noble gases G, H in the gas mixture 14: For example, the first gas G may be Ar, and the second gas H may be Kr or Xe. The use of a gas mixture 14 containing Ar is favorable since the plasma source 11 shown in FIG. 1 is designed for operation with Ar. In particular, a gas mixture of Ar and Kr has been found to be favorable.

[0064] Alternatively, it is possible that the first gas G in the gas mixture 14 is Ne and the second gas H in the gas mixture 14 is selected from the group comprising: Ar, Kr and Xe. If the second gas H is Ar, the result is the classic example of Penning ionization (E.sup.+(Ne)=16.5 eV, E.sup.+(Ar)=15.8 eV, i.e. E.sup.+(Ne)>E.sup.+(Ar)):

Ne*+Ar.fwdarw.Ar.sup.++e.sup.−+Ne  (4)

[0065] Alternatively, the gas mixture 14 may contain krypton as the first noble gas G and xenon as the second noble gas H. This is favorable since the degree of compaction of layer 3 is affected primarily by the transfer of momentum from the ions H.sup.+ present in the plasma 12 and not by the ion energy. Therefore, the use of heavier noble gases in the gas mixture 14 generally leads to greater compaction than is the case for lighter noble gases.

[0066] As apparent in FIG. 3, the degree of compaction D (in arbitrary units) also depends on other parameters, for example on the ion energy E, the rate of vapor deposition or coating R, and angle of vapor deposition β (cf. FIG. 1). The four diagrams shown in FIG. 3 differ by the ion energy E, which increases from left to right (E1 to E4). The diagrams shown in FIG. 3 each show the degree of compaction for four coating rates R1 to R4, increasing from left to right. For a respective coating rate R1 to R4, the dependence of the degree of compaction on the angle of vapor deposition β is shown. As apparent in FIG. 3, the degree of compaction decreases proceeding from a minimum angle of vapor deposition β.sub.min up to a maximum angle of vapor deposition β.sub.min, in most diagrams.

[0067] It has been found to be favorable when the coating rate R in the deposition of the coating material 9 is less than 10.sup.−10 m/s. This is favorable firstly because the energy per molecule EPM increases with decreasing coating rate R, as apparent from equation (1). This is likewise favorable because, at low coating rates R, the degree of compaction is essentially constant or independent of the vapor deposition angle. But the coating rate R chosen should not be too small in order to avoid introduction of any residual gases present in the vacuum chamber 5 into the deposited coating material 9.

[0068] As apparent, for example, from FIG. 1 of article [3], it is favorable for the degree of compaction when the active ion energy E is less than about 100 eV. Favorable values for the active ion energy E of the ions H.sup.+ present in the plasma 12 are within an interval between about 60 eV and about 100 eV.

[0069] FIG. 4 shows an optical element 17 to which all layers 3 of a coating 18 have been applied in the manner described further up in the coating apparatus 1 of FIG. 1. The design of the optical element 17 in the example shown in FIG. 4 is as described in the above citation U.S. Pat. No. 9,933,711 B2 or U.S. Ser. No. 10/642,167 B2. But it will be apparent that the design of the optical element 17 or coating 18 may also be different.

[0070] The optical element 17 has a substrate 2 composed of CaF.sub.2. A first layer 3a composed of a fluoride compound of low refractive index is applied to the substrate 2. The first layer 3a can be applied directly to the substrate 2, but it is also possible that an adhesion promoter layer or another kind of functional layer is applied between the substrate 2 and the first layer 3a. The coating 18 additionally comprises a layer system 19 disposed between the first layer 3a and a last layer 3d of the coating 18. The last layer 3d in the example shown is an oxide compound.

[0071] The layer system 19 in the example shown in FIG. 4 has two pairs of alternating layers 3b, 3c, which respectively include a fluoride compound and an oxide compound. The layer 3b adjacent to the first layer 3a in the layer system 19 includes an oxide compound; the layer 3c applied to the layer 3b includes a fluoride compound. The entire coating 18 therefore consists of an alternating sequence of three fluoridic and oxidic layers 3a-d. The material of the oxidic layers 3b, 3d may, for example, be SiO.sub.2, etc.; the material of the fluoridic layers 3a, 3c may, for example, be AlF.sub.3, MgF.sub.2, etc. As an alternative to the optical element 17 shown in FIG. 4, the coating 18 may also have an odd number of layers. In that case, the first layer and the last layer may, for example, be oxidic layers, such that an alternating sequence of oxidic layers and fluoridic layers within the coating 18 may be maintained.

[0072] The coating 18 shown in FIG. 4 serves as antireflection coating for avoidance of the reflection of DUV radiation at the surface of the substrate 2 to which the coating 18 is applied. It will be apparent that the substrate 2 may also have a corresponding coating 18 on the opposite side. The substrate 2 may also be formed from a material other than a fluoridic material, for example from quartz glass (SiO.sub.2).

[0073] FIG. 5 shows a schematic of an optical system 21 in the form of a DUV lithography apparatus at wavelengths of less than 250 nm, in particular for wavelengths in the range between 100 nm and 200 nm or 190 nm. The DUV lithography apparatus 21 has, as essential components, two optical systems in the form of an illumination system 22 and a projection system 23. For the performance of an exposure process, the DUV lithography apparatus 21 has a radiation source 24 which may, for example, be an excimer laser which emits radiation 25 at a wavelength in the DUV wavelength range of, for example, 193 nm, 157 nm or 126 nm and may be an integral part of the DUV lithography apparatus 21.

[0074] The radiation 25 emitted by the radiation source 24 is conditioned with the aid of the illumination system 22 such that a mask 26, also called a reticle, can be illuminated thereby. In the example shown in FIG. 5, the exposure system 22 has both transmitting and reflecting optical elements. In a representative manner, FIG. 5 shows a transmissive optical element 27, which focuses the radiation 25, and a reflective optical element 28, which deflects the radiation 25, for example. In a known manner, in the illumination system 22, a wide variety of transmitting, reflecting or other optical elements can be combined with one another in any manner, even in a more complex manner.

[0075] The mask 26 has, on its surface, a structure which is transferred to an optical element 29 to be exposed, for example a wafer, with the aid of the projection system 23 in the context of production of semiconductor components. In the example shown, the mask 26 is designed as a transmitting optical element. In alternative executions, the mask 26 may also be designed as a reflective optical element. The projection system 22 has at least one transmitting optical element in the example shown. The example shown illustrates, in a representative manner, two transmitting optical elements 30, 31, which serve, for example, to reduce the structures on the mask 26 to the size desired for the exposure of the wafer 29. In the case of the projection system 23 as well, it is possible for reflective optical elements among others to be provided, and for any optical elements to be combined with one another as desired in a known manner. It should be pointed out that optical arrangements without transmissive optical elements can also be used for DUV lithography.

[0076] The optical element 17 shown in FIG. 4 may, for example, be a transmitting optical element 27 of the exposure system 22, or the mask 26, or a transmitting optical element 30, 31 of the projection system 23 of the DUV lithography system 21.

[0077] As an alternative to the optical system 21 shown in FIG. 5 in the form of the DUV lithography system, it is also possible for another optical system, in particular for the DUV wavelength range, to include at least one optical element 17 as shown in FIG. 4. The optical system 21 may, for example, be a wafer inspection system or a mask inspection system. The excimer laser 24 may also comprise an optical element 17 comprising a substrate 2 with a coating 18 applied in the manner described above. The optical element 17 may, for example, be an exit window of a laser chamber.

[0078] In summary, in the manner described above, layers 3, 3a-d or a coating 18 may be deposited on a substrate 2 of an optical element 17, which ensures a high degree of compaction, combined with a long lifetime or high radiation resistance of the optical element 17 even in the case of irradiation at high radiation intensities.

REFERENCES

[0079] [1] U. Natura, S. Rix, M. Letz, L. Parthier, “Study of haze in 193 nm high dose irradiated CaF.sub.2 crystals”, Proc. SPIE 7504, Laser-Induced Damage in Optical Materials: 2009, 75041P (2009). [0080] [2] M. Bischoff, O. Stenzel, K. Friedrich, S. Wilbrandt, D. Gabler, S. Mewes, and N. Kaiser, “Plasma-assisted deposition of metal fluoride coatings and modeling the extinction coefficient of as-deposited single layers”, Appl. Opt. 50 (2011) C232-C238. [0081] [3] R. Kaneriya, R. R. Willey, K. Patel, “Improved Magnesium Fluoride Process by Ion-Assisted Deposition”, Proc. Soc. Vac. Coat. 53 (2010) 313-319. [0082] [4] J. Targove et al., “Ion-assisted deposition of lanthanum fluoride thin films”, Appl. Opt. 26 (1987) 3733-3737. [0083] [5] M. J. Shaw, “Penning ionization”, Contemporary Physics, 15 (1974), 445-464. [0084] [6] J. Targove and H. A. Macleod, “Verification of momentum transfer as the dominant densifying mechanism in ion-assisted deposition”, Appl. Opt. 27 (1988) 3779-3781. [0085] [7] J. Harhausen, R. P. Brinkmann, R. Foest, M. Hannemann, A. Ohl and B. Schroder, “On plasma ion beam formation in the Advanced Plasma Source”, Plasma Sources Sci. Technol. 21 (2012) 035012.