WO2011095209A1 - Microlithographic projection exposure system - Google Patents

Abstract

The invention relates to a microlithographic projection exposure system (600), comprising a lighting device (601) and a projection lens (602), wherein the lighting device (601) illuminates an object plane (OP) of the projection lens in the operation of the projection exposure system (600) and the projection lens images said object plane onto an image plane (W), wherein the illumination of the object plane is accomplished with linearly polarized light, the wavelength of which is less than 15 nanometers (nm), and wherein the projection lens has at least one pupil (110, 210, 310, 410, 510), wherein at least one boundary line of said pupil has a geometry that deviates from a circular shape.

Description

 Microlithographic projection exposure machine

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION The invention relates to a microlithographic projection exposure apparatus.

Background Art Microlithography is used to fabricate microstructured devices such as integrated circuits or LCDs. The microlithography process is carried out in a so-called projection exposure apparatus which has an illumination device and a projection objective. The image of a mask (= reticle) illuminated by means of the illumination device is hereby projected onto a substrate (eg a silicon wafer) coated with a photosensitive layer (photoresist) and arranged in the image plane of the projection objective in order to apply the mask structure to the photosensitive coating of the Transfer substrate. In EUV projected projection lenses, i. at wavelengths of e.g. about 13 nm or about 7 nm, mirrors are used as optical components for the imaging process, due to the lack of availability of suitable translucent refractive materials. As the numerical aperture of the systems increases, typically in systems with NA above about 0.4, problems associated with so-called "central obscuration" (i.e.

Shading in the central area of the pupil). Such a central obscuration is provided in order to be able to design the light feedthrough by means of a mirror breakthrough in such a way that even at high numerical apertures the beam angle can be kept small. However, such central obscurations have the disadvantage of light loss as well as a loss of imaging quality. The term "pupil plane" is understood in accordance with the usual terminology to mean a plane which represents the Fourier-transformed plane to a field plane (ie approximately the image plane or an intermediate image plane) in the field plane, identical angles in the pupil plane, so that in a pupil plane in each case principal rays, which impinge in a field plane at the same angle, cross each other.

Another well-known problem in high aperture projection exposure machines is the so-called vector effect which occurs in EUV systems due to the low wavelengths of e.g. 13 nm in the vicinity of one refractive index of the resist and thus no longer taking place in the resist refraction - already comes from a value of the numerical aperture of about 0.4 to bear. "Vector effect" describes the situation in which the vector of the electric field in the region of the image plane has different directions for different diffraction orders even with the same polarization state, which in turn results from the fact that the p-polarized components (= TM components) of the electric field strength vector are no longer parallel to each other, so that the imaging contrast is dependent on the polarization state.

Finally, there is also another problem in high-aperture EUV systems in the increasingly difficult optimization of the highly reflecting layers (HR layers) located on the reflecting mirrors, since the high beam angles to be captured by these HR layers (based on the surface normal of the respective mirror) to make optimization of the layer design increasingly difficult.

From WO 2004/046771 A1, inter alia, a projection objective with a non-round aperture stop is known in order to avoid a so-called corner shading, even if the distance between the aperture stop and the mirror is larger greater distance a simpler exchange of the aperture is to be enabled.

The use of non-rotationally symmetrical aperture diaphragms in catadioptric projection objectives (not designed for the EUV range) is also known from JP 2007 305821 A.

From US 5,863,712 u.a. A projection exposure method is known, in which a pupil filter with a first pupil function is exchanged between successive exposures for a pupil filter with a second pupil function, at least one of these pupil functions having an asymmetric transmission distribution.

From US 5,673,103 u.a. a microlithographic projection exposure apparatus is known which comprises a polarization control means and means for changing the direction of the opening area of a slit filter in synchronism with the polarization direction change set by the polarization control means.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a microlithographic projection exposure apparatus which at least largely avoids the problems described above or a loss of imaging quality and / or a loss of light in the EUV range, even at relatively high apertures.

This object is solved by the features of independent claim 1.

According to one aspect, the invention relates to a microlithographic projection exposure apparatus having an illumination device and a projection objective, wherein the illumination device is operated during operation of the projection exposure apparatus. ge illuminates an object plane of the projection lens and the projection lens images this object plane onto an image plane,

 - wherein the illumination of the object plane is carried out with linearly polarized light whose wavelength is less than 15 nanometers, and

 - wherein the projection lens has at least one pupil, wherein at least one boundary line of this pupil has a different geometry from a circular shape.

In this case, the at least one boundary line may be an outer boundary line or else an inner boundary line of the pupil. In particular, both the inner boundary line and the outer boundary line can have a geometry deviating from a circular shape.

As a criterion for the characteristic of the geometry deviating from a circular shape, the maximum deviation or the maximum distance from a best-fitting circular line can be used, this best-fitting circular line can be determined by the known method of least squares and wherein the distance in the radial direction relative to this circle is determined. This maximum deviation from the best fitting circular line in the radial direction according to the invention is preferably at least 1%, in particular at least 2%, and more particularly at least 5%.

The pupil can be the entrance pupil, the exit pupil or the system pupil.

A pupil of an imaging optic generally refers to all images of the aperture stop that delimits the imaging beam path. The levels in which these images come to rest are called pupil levels. However, since the images of the aperture diaphragm are not necessarily exactly planar, the planes that approximately correspond to these images are also generalized as pupil planes. The plane of the aperture stop itself is also referred to as the pupil plane. If the aperture diaphragm is not flat, then As with the images of the aperture diaphragm, the plane closest to the aperture diaphragm is called the pupil plane.

The entrance pupil of the imaging optics is understood to be the image of the aperature diaphragm which is formed when the aperture diaphragm is imaged through the part of the imaging optics which lies between the object plane and the aperture diaphragm. Correspondingly, the exit pupil is the image of the aperture diaphragm which results when the aperture diaphragm is imaged through the part of the imaging optics which lies between the image plane and the aperture diaphragm.

If the entrance pupil is a virtual image of the aperture diaphragm, that is to say that the entrance pupil plane lies in front of the object field, this is called a negative focal distance of the entrance pupil. In this case, the main rays to all object field points are as if they came from a point in front of the imaging path. The principal ray to each object point is defined as the connecting ray between the object point and the center of the entrance pupil. With a negative intercept of the entrance pupil, the main rays to all object points thus have a divergent ray path at the object field.

A shadowed or obscured exit pupil at a pixel is present if this pixel can not be reached by all the rays emanating from the associated object point within the aperture. So there is an area within the exit pupil that can not reach any rays from this pixel. This area is called pupil obscuration.

According to another approach, a pupil may also be described as the region in the imaging optical path of the imaging optical system in which individual beams emanating from the object field points intersect, which are associated with the same illumination angle relative to the main beams emanating from these object field points. The plane of the pupil can be designated as the plane in which the intersections of the individual rays lie according to the alternative pupil definition or the spatial distribution of these Intersections, which need not necessarily be exactly in one plane, come closest.

The present invention encompasses all solutions in which at least one pupil in the sense of at least one of the present definitions has a boundary line with a geometry deviating from a circular shape (in the sense of the above criterion).

By virtue of the fact that, according to the invention, the pupil has at least one boundary line with a geometry deviating from a circular shape (in the following, such a pupil is also referred to for short as a "non-circular pupil"), it can first be considered in connection with the problems explained above in connection with FIG The so-called central obscuration a geometrically advantageous situation are created, which allows even at larger apertures a "pass" the light at the relevant, arranged in a pupil plane or in the vicinity mirror, since this mirror in the remaining (ie not counting to the transmitting area ) Area may have a mirror breakthrough, by there eg has a hole or by the mirror is cut off there.

In other words, in a geometrically favorable manner close to the center of the pupil areas can arise which are not used anyway in the microlithographic imaging process and through which the light can be passed. The problems associated with a previously described central obscuration, such as loss of light and impairment of the imaging quality, are thereby avoided.

In particular, the pupil according to preferred embodiments of the invention, a cross-shaped, rectangular or strip-shaped or a substantially hourglass-shaped geometry (ie, a central tapered portion between two outer, relatively wider sections). The use of linearly polarized light in accordance with the invention in combination with the non-circular pupil also has the result that, especially in the case of a non-circular, eg rectangular, strip-shaped or cruciform transmitting region in conjunction with a defined direction of polarization of the light in FIG With regard to the vector effect, it is possible to achieve advantageous polarization distributions. Thus, for example, as will be explained in more detail below, at constant linear preferred direction of polarization in a sufficiently narrow, rectangular strip cutout, a so-called quasi-potential polarization distribution which is advantageous with respect to the vector effect results, at which the electric field strength vector is at least approximately perpendicular to that on the optical axis directed radius runs.

Another property of e.g. Rectangular or striped pupil is that dense line structures (i.e., high spatial frequency or narrow line spacing structures) can only be transmitted in one direction due to the required comparatively large diffraction angles. According to the invention, however, this "disadvantage" is deliberately accepted, which is based on the consideration that the imaging of high spatial frequency structures running in the corresponding vertical direction can also be achieved using the same, non-circular pupil only the (eg vertical) line grid must be rotated so that the lines to be imaged are horizontal with respect to the projection exposure machine ("machine system"), whereby at the same time the wafer is also rotated and the lithography process is carried out so that the "disadvantage" described above is simply reversed. can be done.

Another important advantage of using light with a defined direction of polarization preference is the design of the highly reflective HR layers on the mirrors, since compromises in layer optimization, which conventionally apply both with regard to p-polarized light and with respect to s-polarized light has to be done, are no longer necessary. In other words, one degree of freedom is used in layer optimization. because the HR layers only have to be optimized for a defined polarization direction.

Another essential aspect of the present invention is that the pupil deviating from the circular shape and created according to the invention is not necessarily achieved by a non-circular diaphragm, but rather by a beam cone designed in a defined manner or resulting from the system. Compared to a system with a round pupil shape, this also creates additional design options in which e.g. two pupil planes are arranged side by side and this summing only a little more beam cone space as a system with a single, but approximately shaped pupil.

In accordance with a preferred embodiment, the reticle or mask and the wafer are designed to be rotatable relative to the projection objective. Furthermore, the area exposed in one and the same exposure step ("the") preferably has a square geometry.

According to a further embodiment, a ("physical") aperture stop may also be provided for adjusting the non-circular pupil, whereby the shape and size of the pupil may be variable, in particular by variable design of the aperture stop A possible embodiment consists in the arrangement of four cutting edges displaceable perpendicularly to their straight boundary, which together form a rectangle, in which case a centered rectangle is preferred, so that opposite cutting edges are preferred According to one embodiment of the invention, the non-circular pupil has a cross-shaped geometry The cross-shaped geometry has the advantage that the above-described rotation of reticle and wafer in relation to the pro jection lens is no longer required, whereby the mechanical complexity of the structure is reduced.

According to a further embodiment, the pupil has a substantially hourglass-shaped geometry. For the purposes of the present application, "hourglass-shaped geometry" is understood as meaning a geometry with a tapered, ie relatively narrow central region between two outer, relatively broader regions This refinement enables a more moderate refinement of those structural features which are perpendicular to the preferred structure orientation In this case, if a dipole illumination setting is used, the illumination poles are located near the pupil edge.

Further embodiments of the invention are described in the description and the dependent claims. The invention will be explained in more detail with reference to embodiments shown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG.

Figures 1 and 2 are schematic representations for explaining the effect of different polarization distributions in a rectangular or strip-shaped area of the pupil plane;

Figure 3 is a schematic diagram illustrating the provision of additional design options enabled by the present invention; Figures 4 and 5 are schematic representations of non-circular pupil shapes for explaining further embodiments of the present invention; and Figure 6 is a schematic representation of a designed for EUV microlithographic projection exposure apparatus.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following, the effect of the inventive use of a non-circular pupil in combination with polarized light is first explained with reference to FIGS. 1 and 2.

1 shows a so-called radial polarization distribution, in which the direction of the electric field strength vector is oriented parallel to the radius directed onto the optical axis of the system (in the z-direction in the drawn coordinate system), wherein the direction of oscillation of the electric field strength vector is represented by double arrows is. In contrast, Fig. 2 shows a linear polarization distribution 200 with a constant, in the drawn coordinate system in the y direction extending polarization preferred direction, wherein the direction of vibration of the electric field strength vector is again represented by double arrows.

In a first exemplary embodiment, the non-circular pupil used in accordance with the invention now has a rectangular or strip-shaped geometry, this pupil, ie the transmissive region of the pupil plane, being denoted by 10 in FIG. 1 and 210 in FIG , 2, due to the set polarization distribution within this pupil 210, there is a so-called "quasi-tangential polarization distribution" in which the oscillation direction of the electric field strength vector is oriented at least approximately perpendicular to the radius directed towards the optical axis extending in the z direction This quasitangential polarization distribution succeeds on wafer level to avoid a negative influence of the already explained vector effect on the imaging result of the lithographic process, even at higher numerical apertures (of eg NA> 0.4). 3 illustrates how, due to the non-circular configuration of the pupil according to the invention (in the exemplary embodiment, a strip or rectangular shape is again selected), a geometrically advantageous situation is created which enables the light to pass through optically unused areas outside the optical axis, without having a central axis

Obscuration is required. With "320" or "330" areas are designated, which are available for additional (folded) beam paths or optical elements, since the pupil 310 is limited to a rectangular area, which extends in the embodiment in the x direction.

4 and 5 show further preferred embodiments of a non-circular pupil according to the invention.

In detail, FIG. 4 shows a pupil 410 with a cross-shaped geometry, which includes symmetrical to the optical axis extending in the z-direction in each case rectangular or strip-shaped sections 41 1-414. 5 shows a "hourglass-shaped" pupil 510 having a central region 51 1 which tapers relative to the two adjacent outer regions 512 and 513. Both in the embodiment of FIG. 4 and in that of FIG not available to the pupil 410 or 510, remaining ("white") areas available to provide there on about a mirror disposed in close proximity to the pupil plane in question mirror permitting the passage of light (hole or the like). 6 shows a schematic illustration of a microlithographic projection exposure apparatus 600 designed for operation in the EUV, in which the present invention can be implemented. The basic structure shown in FIG. 6 is known from WO 2006/1 17122 A1. The projection exposure apparatus 600 according to FIG. 6 has an illumination device 601 and a projection objective 602. In the light propagation direction of the, in particular, linearly polarized illumination light 604 emitted by a light source 603, the illumination device 601 comprises a collector 605, an NEN spectral filter 606, a diaphragm 607 in which an intermediate image IMI is generated, a first optical raster element 608 and a second optical raster element 609 from which the light impinges on further optical elements 610-612 on an object plane arranged in an object field OP. The light emanating from the object field enters the projection lens 602. In the exemplary embodiment 8, the projection objective 602 has mirrors M1-M8 in order to transmit the mask structure of a reticle arranged in the object plane OP to the photosensitive coating of a wafer W arranged in the image plane IP. In the region of the second mirror M2, an aperture diaphragm B is arranged. According to the present invention, as described above with reference to FIGS. 1 to 5, in the projection exposure apparatus 600, a non-circular pupil can be set. In particular, the aperture diaphragm B present in the region of the second mirror M2 can be designed such that it adjusts a non-circular pupil as described above.

While the invention has been described in terms of specific embodiments, numerous variations and alternative embodiments, e.g. by combination and / or exchange of features of individual embodiments. Accordingly, it will be understood by those skilled in the art that such variations and alternative embodiments are intended to be embraced by the present invention, and the scope of the invention is to be limited only in terms of the appended claims and their equivalents.

Claims

claims 1 . Microlithographic projection exposure apparatus with an illumination device and a projection objective, wherein the illumination device illuminates an object plane of the projection objective during operation of the projection exposure apparatus and the projection objective images this object plane onto an image plane,  Wherein the illumination of the object plane is with linearly polarized light whose wavelength is less than 15 nanometers (nm); and Wherein the projection lens has at least one pupil (1 10, 210, 310, 410, 510), wherein at least one boundary line of this pupil has a geometry deviating from a circular shape. 2. Microlithographic projection exposure apparatus according to claim 1, characterized in that the at least one pupil at least in sections has a rectangular, strip-shaped or cross-shaped geometry. 3. Microlithographic projection exposure apparatus according to claim 1 or 2, characterized in that the at least one pupil has at least one section tapering between two adjacent sections. 4. Microlithographic projection exposure apparatus according to one of claims 1 to 3, characterized in that the light in the at least one pupil (210) has an at least approximately tangential polarization distribution. 5. Microlithographic projection exposure apparatus according to one of the preceding claims, characterized in that a reticle arranged in the object plane and / or a wafer arranged in the image plane are rotatable relative to the projection objective. 6. Microlithographic projection exposure apparatus according to one of the preceding claims, characterized in that an at least one pupil (1 10, 210, 310, 410, 510) adjusting aperture diaphragm is provided. 7. Microlithographic projection exposure apparatus according to claim 6, characterized in that this aperture diaphragm for varying the shape and / or size of the pupil (1 10, 210, 310, 410, 510) is adjustable. 8. Microlithographic projection exposure apparatus according to one of the preceding claims, characterized in that the projection objective has a numerical aperture (NA) of at least 0.3, in particular of at least 0.4, more particularly of at least 0.5. 9. Microlithographic projection exposure apparatus according to one of the preceding claims, characterized in that the projection lens has at least one mirror with non-circular mirror surface. 10. Microlithographic projection exposure apparatus according to claim 9, characterized in that this mirror is arranged at least in the immediate vicinity of a pupil plane. 1 1. Microlithographic projection exposure apparatus according to one of the preceding claims, characterized in that the projection objective has an optical axis (OA) and at least one mirror which has a mirror aperture outside the optical axis (OA) permitting the passage of light. 12. Microlithographic projection exposure apparatus according to one of the preceding claims, characterized in that the projection lens has an optical axis (OA), wherein no Obskuration in the region of this optical axis (OA) is present. 13. Microlithographic projection exposure apparatus according to one of the preceding claims, characterized in that the projection lens has at least one mirror on which a highly reflective layer system is arranged, wherein the reflectivity of this  Layer system for one of two mutually perpendicular polarization states by a factor of at least 10, in particular of at least 15, more particularly of at least 20, greater than for the other of these polarization states. 14. Microlithographic projection exposure apparatus according to one of the preceding claims, characterized in that the projection objective has at least two pupil planes. 15. A microlithographic exposure method, wherein the illumination device illuminates an object plane of the projection objective during operation of the projection exposure device and the projection objective images this object plane onto an image plane,  Wherein the illumination of the object plane is with linearly polarized light whose wavelength is less than 15 nanometers (nm); and Wherein the projection lens has at least one pupil (1 10, 210, 310, 410, 510), wherein at least one boundary line of this pupil has a geometry deviating from a circular shape. 16. A microlithographic exposure method according to claim 15, wherein both a reticle arranged in the object plane and a wafer arranged in the image plane are rotated relative to the projection objective between two successive exposure steps. 17. Method for microlithographic production of microstructured components with the following steps: Providing a substrate on which at least partially a layer of a photosensitive material is applied; Providing a mask having structures to be imaged;  Providing a microlithographic projection exposure apparatus according to any one of claims 1 to 14; and  Projecting at least a portion of the mask onto a portion of the layer using the projection exposure apparatus. A microstructured device produced by a method according to claim 17.