WAVEFRONT CORRECTION ELEMENT FOR USE IN AN OPTICAL SYSTEM

Abstract

A wavefront correction element for use in an optical system, in particular in an optical system of a microlithographic projection exposure apparatus, includes a substrate (220, 230), an arrangement of electrically conductive conductor tracks (222, 232) provided on the substrate, wherein a wavefront of electromagnetic radiation incident on the wavefront correction element is manipulatable by electrical driving of the conductor tracks, and an insulating layer (221, 231), which electrically insulates the conductor tracks from one another, wherein the insulating layer has first regions and second regions, wherein the electrical breakdown strength of the insulating layer to withstand a breakdown of electrical charge through the insulating layer as far as the arrangement of conductor tracks is lower in the second regions than in the first regions by at least a factor of two.

Claims

1. A wavefront correction element for use in an optical system, comprising: a substrate; an arrangement of electrically conductive conductor tracks provided on the substrate and configured to be electrically driven to manipulate a wavefront of electromagnetic radiation incident on the wavefront correction element; and an insulating layer, which electrically insulates the conductor tracks from one another; wherein the insulating layer has first regions and second regions, wherein an electrical breakdown strength of the insulating layer to withstand a breakdown of electrical charge through the insulating layer as far as the arrangement of conductor tracks is lower in the second regions than in the first regions by at least a factor of two.

2. The wavefront correction element as claimed in claim 1, wherein the second regions have a reduced density in comparison with a density of the first regions.

3. The wavefront correction element as claimed in claim 1, wherein the insulating layer has channel-shaped defects extending as far as the arrangement of conductor tracks in the second regions.

4. A wavefront correction element for use in an optical system, comprising: a substrate; an arrangement of electrically conductive conductor tracks provided on the substrate and configured to be electrically driven to manipulate a wavefront of electromagnetic radiation incident on the wavefront correction element; and an insulating layer, which electrically insulates the conductor tracks from one another; wherein the insulating layer has channel-shaped defects extending as far as the arrangement of conductor tracks.

5. The wavefront correction element as claimed in claim 1, wherein the insulating layer comprises particles that disturb layer growth of the insulating layer.

6. The wavefront correction element as claimed in claim 5 and configured to operate at an operating wavelength, wherein the insulating layer comprises a first material and the particles comprise a second material, wherein refractive indices of the first material and of the second material differ from one another by a maximum of 10% at the operating wavelength.

7. The wavefront correction element as claimed in claim 6, wherein the second material is selected from the group consisting essentially of quartz glass (SiO.sub.2) and calcium fluoride (CaF.sub.2).

8. The wavefront correction element as claimed in claim 1, wherein the insulating layer comprises quartz glass (SiO.sub.2).

9. The wavefront correction element as claimed in claim 1 and configured as a transmissive optical element.

10. The wavefront correction element as claimed in claim 1 and configured as a reflective optical element.

11. The wavefront correction element as claimed in claim 1 and configured for an operating wavelength of less than 30 nm.

12. A method for producing a wavefront correction element, comprising: a) providing a substrate; b) applying an arrangement of electrically conductive conductor tracks on the substrate; and c) applying an insulating layer, which electrically insulates the conductor tracks from one another; wherein applying the insulating layer comprises providing the insulating layer with first regions and with second regions, and wherein an electrical breakdown strength of the insulating layer to withstand a breakdown of electrical charge through the insulating layer as far as the arrangement of conductor tracks is lower in the second regions than in the first regions by at least a factor of two.

13. The method as claimed in claim 12, wherein applying the insulating layer is carried out in a coating process in which the layer-forming particles have an energy of no more than 5 eV.

14. The method as claimed in claim 13, wherein the coating process is a plasma enhanced chemical vapor deposition process or a physical vapor deposition process.

15. The method as claimed in claim 12, further comprising applying particles that disturb a layer growth of the insulating layer before or during the process of applying the insulating layer.

16. The method as claimed in claim 15, further comprising providing the wavefront correction element in an optical system, wherein the insulating layer comprises a first material and the particles comprise a second material, wherein refractive indices of the first material and of the second material differ from one another by a maximum of 10% at an operating wavelength of the optical system.

17. An optical system of a microlithographic projection exposure apparatus, comprising at least one wavefront correction element which is embodied as claimed in claim 1.

18. An optical system of a microlithographic projection exposure apparatus, comprising at least one wavefront correction element which is produced by the method as claimed in claim 12.

19. A microlithographic projection exposure apparatus comprising an illumination device and a projection lens, wherein at least one of the illuminating device and the projection lens comprises a wavefront correction element as claimed in claim 1.

20. A microlithographic projection exposure apparatus comprising an illumination device and a projection lens, wherein at least one of the illuminating device and the projection lens comprises a wavefront correction element which is produced by the method as claimed in claim 12.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] FIG. 1 shows a schematic illustration of the possible construction of a microlithographic projection exposure apparatus designed for operation in the EUV;

[0046] FIGS. 2A-2C show schematic illustrations for explaining various embodiments of the invention, in which FIG. 2A shows a basic construction of a wavefront correction element, FIG. 2B shows an element with channel defects, and FIG. 2C shows an element with channel defects and layer-growth disturbing particles; and

[0047] FIGS. 3A and 3B show schematic illustrations for explaining a problem addressed by the invention, in which FIGS. 3A and 3B show wavefront correction elements exhibiting, respectively, accumulating electrical charge and electrical breakdown.

DETAILED DESCRIPTION

[0048] FIG. 1 shows a schematic illustration of an exemplary projection exposure apparatus 100 which is designed for operation in the EUV and in which the present invention can be realized.

[0049] According to FIG. 1, an illumination device of the projection exposure apparatus 100 comprises a field facet mirror 103 and a pupil facet mirror 104. The light from a light source unit comprising a plasma light source 101 and a collector mirror 102 is directed onto the field facet mirror 103. A first telescope mirror 105 and a second telescope mirror 106 are arranged in the light path downstream of the pupil facet mirror 104. A deflection mirror 107 operated with grazing incidence is arranged downstream in the light path and directs the radiation impinging on it onto an object field in the object plane of a projection lens, which is merely indicated in FIG. 1. At the location of the object field, a reflective structure-bearing mask 121 is arranged on a mask stage 120, said mask being imaged with the aid of a projection lens 150 into an image plane in which a substrate 161 coated with a light-sensitive layer (photoresist) is situated on a wafer stage 160.

[0050] During operation of the projection exposure apparatus 100, at any suitable location it is then possible to use a wavefront correction element according to the invention for correcting wavefront aberrations that occur, wherein possible configurations of this wavefront correction element are described below with reference to the schematic illustrations in FIGS. 2A-2C.

[0051] FIG. 2A firstly shows once again the basic construction of a wavefront correction element comprising conductor tracks 212 situated on a substrate 210 and electrically insulated from one another by an insulating layer 211, wherein the insulating layer 211 can be produced e.g. from quartz glass (SiO.sub.2) and the conductor tracks 212 can be produced e.g. from chromium (Cr). The wavefront of electromagnetic radiation incident on the wavefront correction element is manipulatable in a desired manner by the electrical driving of the conductor tracks 212, but in the absence of further measures, with increasing accumulation of electrical charges on the insulating layer 211 and thus as the electric field strength increases, the electrical breakdown already described in the introduction with reference to FIG. 3Band thus damage to the conductor tracks 212 and possibly to the electronic components connected theretocan ultimately occur.

[0052] According to the invention, then, the risk of destruction of the conductor tracks as a result of said electrical breakdown is reduced by virtue of the fact that, as a result of suitable configuration of the insulating layer, said electrical breakdown is not prevented per se, but is brought about already at a lower electric field strength or electrical voltage. Since the energy released during the electrical breakdown increases quadratically with the electrical voltage established, less energy is released during such a breakdown which takes place already at a significantly lower electrical voltage. This has the result that corresponding damage to insulating layer and/or conductor track is significantly reduced and possibly rendered negligible. Merely by way of example, the electrical voltage at which the electrical breakdown takes place can be reduced from a value of the order of magnitude of 1000 volts to e.g. of the order of magnitude of 100 volts on account of the configuration according to the invention.

[0053] In order, then, to achieve the above-described reduction of the electrical breakdown strength of the insulating layer, according to the invention the insulating layer is embodied regionally with channel-shaped defects extending as far as the arrangement of conductor tracks. In FIG. 2B and FIG. 2C, such channel-shaped defects are designated by 225 and 235, respectively, wherein moreover components which are analogous or substantially functionally identical to those in FIG. 2A are designated by reference signs increased by 10 and 20, respectively. The porosity of the insulating layer 221 and 231, respectively, that is provided by said channel-shaped defects 225, 235 can be achieved in various ways, as explained below.

[0054] In embodiments of the invention, the presence of the conductor tracks 222 in accordance with FIG. 2B can itself be used to produce said channel-shaped defects 225 during the process of producing the insulating layer 221, with use being made of the geometric shading effect brought about by the conductor tracks 222 and the attendant disturbance of the layer growth. For a geometric shading effect of the conductor tracks 222 that is as pronounced as possible, a coating process in which the layer-forming particles have an energy of a maximum of 5 eV, in particular a maximum of 1 eV, is preferably used for applying the insulating layer 221. Suitable low-energy coating processes are e.g. PECVD or PVD processes. In this case, moreover, the disturbance of the layer growth can be influenced by way of the geometric shape of the conductor tracks 222. Merely by way of example, said conductor tracks 222 can have dimensions of 100 nm*100 nm in cross section, wherein a comparatively higher and narrower configuration of the conductor tracks 222 can lead to a more pronounced shading effect and thus more highly pronounced channel-shaped defects 225.

[0055] Said channel-shaped defects 225 form (in a manner that is advantageous for an electrical breakdown occurring at even low electrical voltage) above the conductor tracks 222.

[0056] Besides the regions of reduced electrical breakdown strength provided by the channel-shaped defects 225 (which regions enable the above-described electrical breakdown at even lower electrical voltage, but in return also bring about a local impairment of the optical properties), the insulating layer 221 and 231, respectively, still has in the remaining regions a comparatively dense and substantially undisturbed, compact layer construction having a comparatively high electrical breakdown strength (with e.g. a small stray light and absorption effect in the case of a transmissive configuration of the wavefront correction element).

[0057] In further embodiments, additionally or alternately, as indicated in FIG. 2C, particles 233 can also be applied during the coating process in order to disturb the layer growth. In the case of a transmissive configuration of the wavefront correction element, a material (e.g. quartz glass (SiO.sub.2) or calcium fluoride (CaF.sub.2)) having a refractive index which corresponds to that of the insulating layer 231 or deviates only slightly therefrom is preferably chosen for the particles 233.

[0058] Even though the invention has been described on the basis of specific embodiments, numerous variations and alternative embodiments will be apparent to a person skilled in the art, for example by combination and/or exchange of features of individual embodiments. Accordingly, such variations and alternative embodiments are concomitantly encompassed by the present invention, and the scope of the invention is restricted only within the meaning of the accompanying patent claims and equivalents thereof