WO2009030390A1

WO2009030390A1 - Device and method for xuv microscopy

Info

Publication number: WO2009030390A1
Application number: PCT/EP2008/006950
Authority: WO
Inventors: Larissa Juschkin; Klaus Bergmann; Willi Neff
Original assignee: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.; Rheinisch Westfälische Technische Hochschule Aachen
Priority date: 2007-09-04
Filing date: 2008-08-25
Publication date: 2009-03-12
Also published as: DE102007041939A1

Abstract

The invention relates to an optical imaging system comprising a first magnification step, by means of which an intermediate image of at least one partial region of an object can be generated by means of XUV radiation emitted by the object while achieving a first magnification, at least a second magnification step and a detector which is sensitive to XUV radiation. According to the invention, the second magnification step ensures that the intermediate image can be projected onto the detector while achieving a second magnification, and the second magnification step comprises a zone plate.

Description

Patent Application:

Apparatus and method for XUV microscopy

Applicant: Fraunhofer-Gesellschaft for the promotion of applied research e.V.

Technical application

The invention relates to an optical imaging system and to a method for performing scattered light measurements and images in the XUV. The preferred field of application is the semiconductor industry, namely the examination of wafers, masks or mask blanks, in particular EUV masks for EUV lithography, with respect to defects.

State of the art

Extremely ultraviolet radiation microscopy, due to the short wavelength and very effective interaction with matter, allows the detection of all types of defects and structures on surfaces on a scale of magnitude down to a few tens of nm. Besides this good potential resolving power, microscopy has the other in this wavelength range Advantage that a significantly higher working distance can be achieved with a comparatively small numerical aperture than in the visible or UV wavelength range. On an elaborate sample preparation to create a sufficient surface smoothness, which is otherwise necessary in many cases, thus can generally be dispensed with in principle.

Therefore, EUV microscopy could in principle be well suited for the measurement or defect detection of EUV masks for EUV lithography. For industrial use, the fastest possible scanning of large areas in relation to the presence of small defects imprintable on the image in the resist on the wafer is on the order of about 30 nm. In order to be able to use commercially available XUV-sensitive detectors, for example CCD cameras, whose pixel sizes are typically approximately 13 μm, an enlargement with a factor of the order of 1000 must be achieved. Other smaller pixel detectors (eg, MCP or CMOS cameras) have significant disadvantages due to their low fill factor and low efficiency (they require a scintillator converter layer).

From the field of X-ray microscopy, more specifically microscopy with soft X-radiation (water window), microscopes are known which use (Fresnel) zone plates for magnification, as a source of radiation synchrotron radiation sources are used. These microscopes can be magnified by a factor of 1000. However, the use of zone plates requires a particularly high monochromaticity of the radiation (spectral width Δλ / λ of less than 0.5%) in order to achieve a sufficiently sharp image, since the focal length of the zone plate depends on the wavelength. Given that the existing EUV light sources have low luminous intensity compared to the synchrotron and are relatively broadband, monochromatization of the radiation, which would cause an additional reduction in luminous intensity, would obviously be problematic.

In addition, the resolution of about 30 nm to be achieved for the stated purpose requires that the zone plate have several hundred zones and that the smallest be about 30 nm wide. At the same time, the thickness of the substrate material of the zone plate should not be more than a few hundred nm, in order to ensure a sufficient transmission in the EUV. The known microscopes which use a zone plate are used almost exclusively in the area of soft X-radiation and can therefore have a significantly greater thickness. A zone plate with the specified specifications, which can also be used for the EUV range, is therefore very expensive and has a very short service life. Although reflective zone plates are more mechanically robust, they have disadvantageous imaging properties.

In addition, zone plates typically have a small numerical aperture; accordingly, with zone plates a very short working distance is required be set to investigate object, about a few tenths of a millimeter. On the one hand, this can lead to technical complications in sensitive samples. On the other hand, the object field and thus the achievable Untersuchungsbzw. Scanning speed very low. Zone plates for scattered light measurements in the EUV therefore, despite their high magnification, do not appear to be suitable for the intended applications, such as defect detection on EUV masks.

With a recently introduced EUV microscope (L. Juschkin, K. Bergmann, W. Neff, R. Lebert: "EUV Microscopy for Defect Inspection", 2006 EUVL Symposium, 15-18 October 2006, Barcelona, Spain) can scatter light measurements be carried out in the EUV, where by the use of a multi-layer coated Schwarzschild lens as both collecting and imaging optics and an EUV-sensitive detector and information about the location and size of the defects can be obtained. The advantage of a lens based on multilayer mirrors is a comparatively large numerical aperture (in the EUV up to about 0.5). Therefore, a relatively high spatial triggering can be achieved. In addition, such lenses have a comparatively large spectral bandwidth and can therefore use a comparatively large proportion of the light of the generally relatively broadband EUV radiation source.

For reasonably compact systems, however, the magnification technically feasible with a Schwarzschild objective is only in the range of about 20%.

50 (higher magnifications would lead to extremely large, unsuitable for many applications sizes of the microscope due to the large working distance). Structures on the order of 10 or a few 10 nm are thus increased to a size of about 1 micron or less. The usual detectors used in the XUV or EUV, v.a. CCD cameras, on the other hand, have pixel sizes of typically 13 μm, so the resolution of such a detector is not matched to that of the objective. The resolution of the system is therefore limited by the detector and is several 100 nm.

The difficulty in achieving sufficient magnification in XUV is circumvented in alternative prior art XUV microscopes by first converting to other types or wavelengths of radiation. So it is known (M. Booth et al .: "High-resolution EUV SPIE 5751, 78-89, 2005), the XUV radiation with the help of a scintillator first into visible radiation and then to increase them by conventional means., However, the disadvantage is the low Efficiency of such a microscope, which is several orders of magnitude lower, than with a direct detection of the XUV radiation, so that only a low throughput can be achieved, so that a fast scanning with such a system is not possible In addition, a vacuum separation is necessary, whereby the object size of the Nachvergrößerungsstufe can not fall below a certain value, so that a compact design of the imaging system does not seem possible.

Furthermore, it is known (K. Kinoshita et al.: "Electronic zooming TV readout system for an X-ray microscope") to convert the short-wave radiation into electrons and then enlarge this electron image corresponding to a lens in the electron microscope. Also against this solution speaks the low efficiency in the conversion. In addition, such a system requires particularly complex vacuum conditions and is relatively bulky and expensive.

The invention has for its object to provide a comparatively compact and inexpensive and for carrying out scattered light measurements in XUV, especially in EUV, suitable optical imaging system and a corresponding method, whereby a particularly high resolution can be achieved and which for the detection of defects and structures Surfaces of samples, in particular of EUV masks for EUV lithography, is suitable, and in particular makes it possible to scan large areas for the presence of small defects comparatively quickly.

Presentation of the invention

The solution to this technical problem is provided by an optical imaging system and a method according to the independent claims. Advantageous embodiments and further developments are indicated by the dependent claims or can be taken from the following description and the exemplary embodiments.

According to the invention, it has been recognized that the technical problem can be solved by an optical imaging system which has a first magnification stage with which an intermediate image of at least one subarea of an object can be obtained by obtaining a first magnification by means of the object

XUV radiation can be generated, and at least one second magnification stage, and a detector sensitive to XUV radiation, wherein the second magnification stage, the intermediate image to obtain a second magnification

Magnification on the detector is mapped, and wherein the second

Magnification level includes a zone plate.

In the invention is an imaging system (microscope) in the spectral range of the soft X-ray or extreme ultraviolet radiation in the spectral range of about 0.1 nm to 100 nm (short XUV radiation). A preferred field of application is the investigation of EUV masks for EUV lithography; In this regard, the invention is preferably used as a microscope in the EUV in the range of 10 to 20 nm, in particular 13.5 nm and is described below for this application. However, it is not limited to this, but basically usable for the entire XUV spectral range mentioned.

According to the invention, the magnification in imaging the sample is achieved in two stages. In this case, the second magnification stage comprises a zone plate, the second magnification stage may be formed in particular by exactly one zone plate. In fact, according to the invention, it has been recognized that the known disadvantages of a zone plate can be avoided if there is no direct imaging by means of the zone plate, but this merely forms the second stage of a two-stage magnification system. In this case, a zone plate can be used, which only comparatively low requirements in terms of resolution and the magnification factor corresponds. In contrast to known fields of application for zone plates of the prior art, the possibilities here are the zone plates offer in principle or for which they are known and usually represent the reason for using a zone plate, thus according to the invention just not exhausted.

Was already achieved by the first magnification stage, a first moderate magnification, preferably with a factor between 10 and 50, so sufficient for the second magnification level a spatial resolution in the order of several 100 nm to about 1 micron. For this purpose, a comparatively "simple" zone plate can be used. Thus, the width of the outermost zone, which is closely correlated with the resolution of the zone plate, may be at least an order of magnitude greater than a magnification corresponding to a one-step magnification realized alone with a zone plate. It is preferably at least 100 nm. Similarly, the number of zones can be limited to at most 300, preferably even at most 100. With the zone plate used according to the invention, an enlargement with a factor between 10 and 100 is preferably achieved. Due to the enlargement already achieved by the first magnification stage, such a moderate magnification or subsequent magnification through the zone plate suffices to achieve an overall magnification which makes it possible to use commercially available CCD detectors with pixel sizes on the order of 10 to 25 μm. The overall magnification achieved by the first and second magnification steps has a factor of 100 to 5,000, preferably a factor of 100 to 2,000. Such a "simple" zone plate with the specifications mentioned can be produced at a much lower production cost and price than a zone plate containing the Requirements of magnification with a factor of up to 1000 at a resolution of a few 10 nm "alone" met. In addition, because of the greater distance to mechanical components, it has a longer service life (in the case of a single-stage enlargement with a zone plate, this would have to be positioned very close to the sample to be traversed). Overall, the two-step arrangement with zone plate thus offers considerable economic advantages.

In comparison to a subsequent magnification in the visible (with previous corresponding conversion of the radiation, eg by means of converter crystals), the use of the zone plate achieves a significantly higher efficiency. So For zone plates in the XUV range, an efficiency of typically at least 5 - 10% can be assumed, up to 30% appear in the future.

Due to the moderate magnification of the first magnification stage with the large working distance and the small object distance of the zone plate (the distance between the zone plate and the image of the first

Magnification level is in the range of only a few 100 microns) can be in

Remaining - unlike a one-step enlargement with a

Schwarzschild lens similar magnification or at a Nachvergrößerung in the visible - even achieve a relatively compact size of the microscope. This is preferably less than 1.5 m.

The first magnification stage is preferably a multi-layer mirror-based objective. Such has the advantage that it may have a comparatively large numerical aperture (especially, for example, against a zone plate); this is here (based on a central wavelength of 13.5 nm) at least 0.1, but preferably at least 0.3. This achieves a high spatial resolution. In addition, such a lens provides a comparatively large spectral bandwidth, so that a relatively large proportion of the light of the XUV radiation source is used and thus a particularly efficient use of the existing radiation is possible. Incidentally, such an objective also spatially detects a particularly high proportion of the radiation, so that a comparatively high signal strength can therefore also be achieved. As a result, a sufficiently good imaging of the structures to be examined by means of a single radiation pulse (typically with a pulse duration of approximately 100 ns) is possible with the device according to the invention.

Such a lens can be realized for example by a Schwarzschild lens. The Schwarzschild lens consists of two multilayer coated mirrors, namely a spherical convex primary mirror and a spherical concave secondary mirror. The radiation to be detected, ie in the present case the scattered radiation emanating from the sample, is reflected by the spherical concave secondary mirror and guided onto the spherically convex primary mirror. This is located between sample and the Secondary mirror on the optical axis defined by the secondary mirror. This is typically substantially perpendicular to the usually flat sample. From the primary mirror, the radiation is passed through a located on the longitudinal axis opening in the secondary mirror to the viewer or zone plate and detector. With such an objective, a particularly high numerical aperture of typically up to 0.5 can be achieved. The objective allows imaging of radiation with a spectral width Δλ / λ of approximately 4%, ie, at a central wavelength of 13.5 nm, for example, a spectral width of +/- 0.2 to 0.3 nm can be realized. A Schwarzschild lens is also characterized by being free of chromatic aberrations. In addition, it has a high mechanical stability, since it comprises only two optical elements and these are held in a low-stress and mechanically stable manner via solid-state joints. A correction of the adjustment state is possible under operating conditions. Incidentally, a Schwarzschild lens has a comparatively long life.

With such an objective, a large working distance, e.g. 1 cm or even several cm, between the sample and the objective (i.e., the primary mirror of the objective) or a large object field, which opens the possibility for a fast scanning of a sample. In addition, the arrangement is thus relatively insensitive to the surface smoothness; Thus, no elaborate sample preparation is required.

Such a first magnification stage preferably allows an enlargement with a factor of between 10 and 50. Such a comparatively moderate magnification can be realized with a comparatively compact system.

According to the invention, an intermediate image is produced with the first magnification stage to obtain a first magnification. This is present as an image with a distance to the first magnification, which corresponds to their image size. It could be imaged onto it with a detector positioned at that location (yielding only the first magnification). For re-enlargement, instead the zone plate is brought into the vicinity of this point, namely at a distance to this point which corresponds to the object distance of the zone plate, positioned. In simple terms, the zone plate is placed so that the intermediate image and thus the object is sharply imaged onto the detector.

It is optimal if the numerical aperture of the zone plate is adapted to the image-side numerical aperture of the first magnification stage, or the Schwarzschild objective.

With the imaging system according to the invention, in particular the arrangement of the beam path of the radiation acting on the object and the first magnification stage, scattered light measurements in the dark field can be performed. Measurements in the dark field mode are particularly advantageous because the specularly reflected light does not contribute to the signal. An O-order shutter in front of the lens shadows the direct light in the beam path and ensures that only the light scattered from surface defects (e.g., particles on a thin film) is collected and imaged. This means that only the light scattered at defects is detected against a "zero signal". This increases the contrast and sensitivity of the system for small defects. Depending on the opening angle of the illumination beam bundle, the primary mirror of the Schwarzschild objective can occasionally act as the Ote order aperture.

The sources of radiation are those sources which have a high intensity in the XUV, v. A. in the range of 10 to 20 nm, such as a gas discharge lamp (having a high average power), in particular, a xenon or a tin gas discharge lamp, an x-ray tube (which is comparatively compact) can provide continuous radiation and stable can be operated) or a laser-induced plasma (high brilliance, small swelling volume). But it is also possible to use a synchrotron radiation source. The XUV radiation sources preferably have a greater spectral bandwidth than the multilayer mirrors of the first magnification stage (this is usually realized by conventional radiation sources). In order to achieve a sufficient signal strength so that a sufficiently fast scanning of samples is possible, the radiation source should be within the spectral bandwidth of the first

Magnification level have the highest possible intensity. Since the zone plate according to the invention must meet comparatively low requirements in terms of spatial resolution and magnification, a zone plate can be used, which tolerates a comparatively high spectral bandwidth (namely, a value of Δλ / λ between 2 and 4%) and still allows a sharp image. Preferably, the spectral bandwidth tolerated by the zone plate is adapted to the spectral bandwidth of the first magnification stage, specifically that of the Schwarzschild objective. The broadbandity of the entire system is thus predetermined or largely determined by the multilayer mirror, ie the Schwarzschild objective. This can occasionally be dispensed with an additional element for monochromatization of the radiation generated by the XUV radiation source. However, the use of an additional filter to suppress undesired visible, DUV and UV radiation is generally advantageous.

In this way, a particularly efficient use of the XUV radiation and particularly high signal strengths can be achieved when imaging the objects. With the device according to the invention, therefore, a single pulse already suffices for the imaging of the structures to be examined.

The radiation generated by the XUV radiation source is directed to the object by means of one or more guidance means. Like the mirrors of the microscope, the guide or guides have a high quality coating. Preferably, a collector, in particular a grazing incidence collector, is used as the guide means. In such a case, the incident EUV radiation forms a very small angle of incidence with the collector surface. While such a grazing incidence requires a relatively large distance (a few tens of centimeters or more) between the source and the collector focus. On the other hand, such a collector also has a particularly high transmission and high mechanical stability. The collector is used to focus the XUV radiation on the object to be examined. The imaging system is preferably adjusted so that the focus diameter (which depends on the collector and the source size) on the size of the object field of the Schwarzschild lens (which is in the range of a few 100 microns) is adjusted.

In an advantageous embodiment, a deflecting mirror is used as a further guide means. It is arranged downstream of the collector in the beam path and is arranged and aligned between the first magnification stage and the object in such a way that the radiation is incident on the object on the longitudinal axis of the imaging system or the first magnification stage and thus substantially perpendicular to the object. This arrangement is particularly suitable for the scattered light diagnosis in the dark field. The specularly reflected light is not detected by the aperture of the lens. At the same time, the deflecting mirror serves as the Ote-order diaphragm for the dark field measurements. Due to the large working distance between object and lens of at least 1 cm, which is granted by the first magnification stage, such an arrangement is feasible and unproblematic. In this embodiment, the microscope is operated in reflection, i. the imaging system detects radiation scattered substantially at the sample surface. Also possible is the operation of the microscope in transmission. In this case, the (usually flat) sample is irradiated by the imaging system opposite side. The detected radiation scattered by or in the sample has thus passed through the sample.

Incidentally, the sample can also be illuminated in grazing incidence. In the case of dark field measurements, however, the arrangement of the beam path and the first magnification stage must ensure that no direct reflections are detected but only the light scattered at (surface) defects is imaged and detected. Advantage of this mode is that almost all materials have a very high reflectivity (80-90%) with grazing incidence of EUV radiation.

The preferred detector is a CCD camera (because of its high quantum efficiency, this represents an almost ideal detector) or a direct XUV CMOS camera. With these is a quick scanning of

Samples possible. Other two-dimensional detectors, such as an MCP or PEEM are also usable. In addition, the two-stage arrangement of the arrangement basically offers the possibility of quickly and easily switching between a rough and a fine examination. This is particularly advantageous when large areas have to be scanned, as in defect detection on EUV masks. In an advantageous embodiment of the invention, a coarse and fast scan is performed with low resolution in the context of a coarse examination, wherein only the first, but not additionally the second magnification stage is used, that is, there is no Nachvergrößerung by the zone plate instead. The position of the zone plate can be changed for this purpose, ie the zone plate can be moved from a position in the beam path of the scattered radiation to a position outside the scattered radiation (eg pivoted) and vice versa. In this case, means are provided which ensure a reversibility of this change between the two positions with high accuracy.

Upon locating a defect, the zone plate may then be moved to the position in the optical path of the microscope, i. e.g. be swung. In this arrangement, the defect can then be detected with high spatial resolution. To focus the image with pivoted-in zone plate, the sample can be moved in the vertical direction (detection axis), namely the distance to the Schwarzschild objective or to the detector can be reduced or changed.

Alternatively, the detector itself (possibly also together with the zone plate) can be moved in the vertical direction with the same effect. The variation of the magnification of the Schwarzschild lens is to be considered according to the changes of the working and detector distance. For positioning the sample (or the detector) a known positioning unit is present.

The detailed investigation can also be referred to as the main process step. In terms of method, the invention thus consists in a method for imaging an object on a detector by means of XUV radiation with a main method step, in which a first enlargement stage is used

Intermediate image of at least a portion of the object is generated to obtain a first magnification by means of XUV radiation emanating from the object, and wherein in a second zone comprising a zone plate Magnification stage of this intermediate image is mapped with a second magnification on a detector.

This main process step is preferably preceded by a pre-process step in the form of the described coarse examination, which is used for coarse scanning of the object. Preferably, depending on a pre-process step, a selection is made of a subregion of the object for which a main process step is carried out. The image taken in the pre-process step has a magnification and resolution that is sufficient to at least not overlook potential defects of a size of about 30 nm. For large parts of the object, therefore, no main process step must be performed. Only those subareas for which the presence of a relevant defect could not be completely ruled out with the pre-procedural step are investigated in a high-resolution manner with a main process step. This leads to a considerable reduction of the examination time and thus to considerable cost advantages.

A stationary installation of the zone plate is also possible. In this case, the zone plate is arranged in the beam path that it only a part of the intermediate image on the detector, or on only a part of the detector, preferably a part in the center of the detector, images. The image is formed on the remaining part of the detector only via the first magnification stage, ie this image is formed by the scattered radiation emitted by the sample, which is guided past the zone plate directly from the first magnification stage to the detector. The detector thus has two subregions which detect differently resolved and enlarged images of the object. Preferably, in the region of the detector (ie, for example, of the CCD chip) which is not covered by the image of the zone plate enlargement, the intensity is reduced by an absorption filter. Thus, within the scope of the detector dynamics, both the less bright, but enlarged and better resolved image, and the less resolved dark field image of the defects only enlarged by the first magnification step can be simultaneously detected and displayed, thereby avoiding overexposure. The absorption filter thus adapts the light signals received from the two sections of the detector to one another. In this embodiment with Stationary zone plates should have higher exposure times than when scanning without the zone plate acting as a "magnifying glass". A narrow area around the zone plate should be covered with a shutter to prevent overlapping of the images. There are therefore also "dead zones" between the only small and the greatly enlarged areas of the sample, which are not imaged on the detector. However, these can be visualized by lateral sampling of the sample.

The advantage of the stationary installation of the zone plate is that it can be changed very quickly and flexibly between the coarse and the fine examination, since these are carried out in principle simultaneously. In order to investigate, in a high-resolution manner, a region of the sample with a potential defect of larger magnification determined in the course of the coarse examination of the coarse examination, the sample is only to be moved laterally relative to the microscope or the entire optical assembly until the radiation comes out of this region also detected by the zone plate and is imaged by this on the detector, and to focus.

All previously mentioned embodiments have been described without loss of generality only for the use of a single zone plate. However, it is also possible to use several zone plates. For example, e.g. conceivable to use several consecutively arranged zone plates. The

Imaging system would then have more than two magnification levels, or put another way, the second magnification would then include multiple zone plates. In this way, an even greater enlargement can be realized or the enlargement by a zone plate to be achieved for a given overall magnification can be reduced.

However, a particularly preferred embodiment consists in that the second magnification stage has a plurality of zone plates arranged in a plane (perpendicular to the detection axis). The detector may then comprise a plurality of separate portions, each portion being associated with a zone plate and detecting a post-magnification through that zone plate. With a typical object field (10 - 100 μm) of a zone plate, a clever arrangement of several zone plates can be a multiple of this Diameter of the intermediate image are imaged simultaneously, ie up to several 100 microns to a few millimeters in diameter.

Brief description of the drawings

The present invention will be explained in more detail below with reference to exemplary embodiments in conjunction with the drawings without limiting the scope of protection specified by the claims. Hereby show:

Fig. 1: an illustration of the device according to the invention

Fig. 2 is a table with calculated zone plate specifications for two embodiments of the invention

3a shows a representation of two rays emanating from spaced points on the object

3b: an enlarged view of a detail of Fig. 3a

Ways to carry out the invention

Fig. 1 shows an inventive EUV microscope. As XUV radiation source 6, it has a plasma of a xenon gas discharge source, a grazing incidence collector 7 and a deflection mirror 8 for guiding and focusing the EUV radiation 10 onto the sample 3. The first magnification stage is characterized by a multilayer-coated (Mo / Si Multilayer) Schwarzschild Lens 1 given. This has a spherical primary convex mirror 1 ^Λ and a spherical concave secondary mirror 1 ^" An EUV CCD camera functions as detector 4. The microscope operates in dark field mode in this example. An Ote-order aperture 9 (here the back of the mirror 1 ^* or the deflection mirror 8) in front of the lens 1 shadows the direct light in the beam path and ensures that only on defects on the surface of the object 3 (eg particles on a wafer) scattered light 11 is collected and imaged - a light spot on the detector 4 indicates the defect. This increases the contrast and sensitivity of the system for small defects. Incidentally, to suppress visible DUV and UV radiation, a filter (not shown), for example a 200 nm thin Zr foil, can be introduced into the beam path, eg near the sample.

A region around the intermediate image 5 around is highlighted by a circle, the position of the intermediate image 5 (location of the image of Schwarzschildobjektivs 1) additionally indicated by a dashed line. The illustration, which is blurred in the arrangement shown, can be made sharper by moving the detector 4 in the direction of the position of the intermediate image 5 or the sample 3 in the direction of the objective 1 (indicated by an arrow). In addition to this illustrated beam path, the beam path resulting after swiveling in of the zone plate 2 is shown as a sketch. The zone plate 2 is located at the distance of the object distance from the position of the intermediate image 5, wherein the numerical apertures of the Schwarzschild objective 1 and the zone plate are adapted to each other.

Typical dimensions for a Schwarzschild lens with a magnification of about 20 are given in the following example. The diameter of the concave secondary mirror is 52 mm with a radius of curvature of 100 mm. The convex primary mirror has a diameter of 10.6 mm and a radius of curvature of 35 mm.

Fig. 2 shows a table in which, for two different arrangements (designated V1 and V2), the location and the specifications of the zone plate 2 are calculated and each indicated in a column. In both cases, a construction with a Schwarzschild objective 1 and a wavelength of 13.5 nm is assumed. In the first example, the spectral width (Δλ / λ) of the Schwarzschild objective 1 is 3.0%, in the second it is 4.0%. Moreover, in the first example, a distance d (= b + g) between the position of the intermediate image 5 and the CCD camera 4 of 10.00 mm and a magnification V of 20, in the second example d = 30.00 mm and V = 10 is specified. M

The numerical apertures of Schwarzschild objective 1 (image side) and the zone plate 2 should be adapted as much as possible. With these specifications, the values given for the image width b, object distance g, focal length f, number of zones N, width of the outermost zone .DELTA.r, diameter D, numerical aperture NA ₁ resolution RES, depth of field DOF and radius of the first zone r1 of the respective Zone plate 2.

To a rough examination, so a quick scan with low resolution, in which only the first magnification level 1 is used, so the image plane of Schwarzschildobjektivs 1 is focused directly on the detector 4, to a fine examination, in addition, the zone plate 2 is used In the second example, according to the specification, the distance between the detector 4 and the image plane of the Schwarzschild objective 1 is to be increased by 30 mm (by moving the detector 4 or the sample 3). At the same time, a zone plate 2 with the specified parameters at the distance of the object's distance to the position of the intermediate image 5 (d = 2.7 mm) is to be introduced. In this way, a post-enlargement by a factor of 10 can be achieved in this example.

FIG. 3a shows a representation, which is not true to scale, of beam paths of two beams emanating at a small distance from the object and of the optical elements of the microscope, i. a Schwarzschild lens 1 and a

Zone plate 2. One beam is shown with solid lines, the second with dashed lines (the beams are hardly distinguishable in this Fig. 3a, however, due to the dimensions). The total distance between object 3 and detector screen 4 in this calculated example is 686.5 mm and is composed of the distance between object 3 and intermediate image 5 of 613.3 mm and the distance between intermediate image 5 and the image on detector 4 of 73.2 mm. In this example, a total magnification of 393 is realized by the two magnification levels 1, 2, the Schwarzschild objective 1 achieves a magnification of 20.73, the zone plate 2 a magnification of 18.95. In Fig. 3b, a detail of Fig. 3a is shown enlarged (not to scale), namely the rear, the intermediate image 5, the zone plate 2 and the detector 4 containing portion of the beam path or the microscope.

3b further shows results of a simulation calculation for the imaging of two test objects (in each case letter F, shown in the object plane in FIG. 3a) with different height scales of 1 μm (left) and of 100 nm (right) to illustrate the mode of action of the post-magnification with the zone plate and the picture quality.

LIST OF REFERENCE NUMBERS

1 first magnification level

2 second magnification level

3 object 4 detector

5 intermediate picture

6 XUV radiation source

7 collector

8 deflecting mirror 9 aperture

10 impinging XUV radiation

11 scattered radiation

Claims

claims

1. An optical imaging system comprising a first magnification stage (1), with an intermediate image (5) of at least a portion of an object (3) to obtain a first magnification by means of the object (3) outgoing XUV radiation is generated, at least a second magnification stage (2), as well as a XUV radiation sensitive detector (4), wherein the second magnification stage (2), the intermediate image (5) while obtaining a second magnification on the detector (4) can be imaged, and wherein the second magnification stage (2 ) comprises a zone plate (2).

2. An optical imaging system according to claim 1, characterized in that the imaging optical system (1) comprises one or more multilayer-coated optical elements.

3. Optical imaging system according to one of claims 1 to 2, characterized in that the first magnification stage (1) has a numerical aperture of at least 0.1, preferably at least 0.3.

4. An optical imaging system according to any one of claims 1 to 3, characterized in that the first magnification stage (1) is formed so that the first magnification has a factor between 10 and 50.

5. An optical imaging system according to any one of claims 1 to 4, characterized in that the first magnification stage (1) is a Schwarzschildobjektiv.

6. An optical imaging system according to any one of claims 1 to 5, characterized in that the second magnification stage (2) is formed so that the second magnification has a factor of at least 10, preferably between 20 and 100.

7. An optical imaging system according to any one of claims 1 to 6, characterized in that with the first (1) and second Magnification stage (2) together achievable total magnification has a factor between 100 and 1000.

8. Optical imaging system according to one of claims 1 to 7, characterized in that the zone plate (2) between the first magnification stage (1) and the detector (4) is arranged such that the object (3) on the detector (4) is sharply imageable.

9. An optical imaging system according to any one of claims 1 to 8, characterized in that the width of the outermost zone of the zone plate (2) is at least 50 nm, preferably at least 100 nm.

10. Optical imaging system according to one of claims 1 to 9, characterized in that the zone plate (2) has at most 300 zones, preferably at most 100 zones.

11. An optical imaging system according to any one of claims 1 to 10, characterized in that the zone plate (2) can be reversibly moved between positions within and outside the beam path.

12. An optical imaging system according to any one of claims 1 to 11, characterized in that the second magnification stage comprises a plurality of zone plates (2).

13. Optical imaging system according to one of claims 1 to 12, characterized in that the detector (4) is a two-dimensional detector, preferably a CCD or a CMOS camera or an MCP.

14. Optical imaging system according to one of claims 1 to 13, with an XUV radiation source (6) and with at least one guide means (7, 8) for guiding the radiation (10) emitted by the XUV radiation source (6) to the object (3 ).

15. An optical imaging system according to claim 14, characterized in that the XUV radiation source (6) is a gas discharge lamp, in particular a xenon or tin Gasentladunslampe, an X-ray tube or a laser-induced plasma.

16. An optical imaging system according to any one of claims 14 to 15, characterized in that the at least one guide means (7,8) is coated multilayer.

17. Optical imaging system according to one of claims 14 to 16, characterized in that by a guide means (7) focusing of the XUV radiation source (6) emitted XUV radiation (10) on the object (3) is effected.

18. An optical imaging system according to any one of claims 14 to 17, characterized in that a guide means is a collector (7), in particular a grazing incidence collector.

19. An optical imaging system according to any one of claims 14 to 18, characterized in that the beam path of the object (3) acting on XUV radiation (10) and the first magnification stage (1) are arranged so that the imaging system is operable in the dark field mode.

20. An optical imaging system according to any one of claims 14 to 19, characterized in that a guide means is a deflection mirror (8).

21. An optical imaging system according to claim 20, characterized in that the deflection mirror (8) between the object (3) and the first magnification stage (1) is arranged.

22. An optical imaging system according to any one of claims 1 to 21, characterized in that the XUV radiation (10) substantially perpendicular to the object (3).

23. Use of the optical imaging system according to one of the preceding claims for detecting defects, in particular defects with a dimension of less than 100 nm, on masks, in particular EUV masks for EUV lithography.

24. A method for imaging an object (3) on a detector (4) by means of XUV radiation with a main process step, in which in a first magnification stage (1) an intermediate image (5) of at least one subregion of the object (3) to produce a first magnification by means of XUV radiation emanating from the object (3) and in a second magnification stage (2) comprising a zone plate (2) this intermediate image (5) to obtain a second magnification is imaged on a detector (4).

25. The method as claimed in claim 24, characterized in that the main method step is preceded by at least one pre-process step in which only the first magnification stage (1), but not the second magnification stage (2), is used to image the object (3) onto the detector (4) becomes.

26. The method according to claim 25, characterized in that in the pre-process step, the distance between the first magnification stage (1) and the object (3) and / or the detector (4) is adjusted so that a sharp intermediate image (5) on the detector (4) is generated.

27. The method according to any one of claims 25 to 26, characterized in that a pre-process step for coarse scanning of the object (3) is used.

28. The method according to any one of claims 25 to 27, characterized in that depending on a Vorverfahrensschritt a selection of a portion of the object (3) is made, for which a main process step is performed.

29. The method according to any one of claims 24 to 28, characterized in that the distance between the first magnification stage (1) and the object (3) is more than 1cm.

30. The method according to any one of claims 24 to 29, characterized in that it is at the of the object (3) outgoing XUV radiation to the application of the object (3) with XUV radiation (10) generated stray radiation (11) is.

31. The method according to claim 30, characterized in that the zeroth diffraction order of the scattered radiation (11) does not reach the first magnification stage (1).

32. Method according to one of claims 24 to 31, characterized in that radiation is incident on a first subregion of the detector (4) which has passed through both the first (1) and the second magnification stage (2) and to a second subregion of the Detector (4) radiation is incident, which has passed through only the first (1), but not the second magnification stage (2).

33. Method according to claim 32, characterized in that the radiation incident on the second subarea of the detector (4) reaches it attenuated due to a filter.