Abstract:
An apparatus ( 10 ) for microlithographic projection exposure, which includes: an optical system ( 18 ) for imaging mask structures ( 16 ) onto a surface ( 21 ) of a substrate ( 20 ) by projecting the mask structures ( 16 ) with imaging radiation ( 13 ) onto an exposure area of the substrate surface, and various structure defining a measurement beam path ( 36 ) for guiding measurement radiation ( 34 ). The measurement beam path ( 36 ) extends within the optical system ( 18 ) such that the measurement radiation ( 34 ) impinges on a measurement area on the substrate surface that is offset from the exposure area.

Description:
This is a Continuation of U.S. application Ser. No. 12/895,747, which was filed on Sep. 30, 2010, which is a Continuation of International Application PCT/EP2009/002302, with an international filing date of Mar. 30, 2009, which was published under PCT Article 21(2) in German. The complete disclosures of both these applications are incorporated into the present application by reference. The present application claims the priority of German Patent Application 10 2008 017 645.1 and of U.S. Provisional Patent Application 61/072,980, both filed on Apr. 4, 2008, and the entire contents of which are also incorporated into the present application by reference. 
    
    
     FIELD OF AND BACKGROUND OF THE INVENTION 
     The invention relates to an apparatus for microlithographic projection exposure and to a method for determining a property of an arrangement which comprises an apparatus for microlithographic projection exposure and a substrate disposed in an exposure position of the apparatus. The invention further relates to an apparatus for inspecting a surface of a substrate and to a method for determining a property of an arrangement which comprises this type of inspection apparatus and a substrate. These types of inspection apparatuses include microscopes and optical inspection systems as used e.g. for the inspection of lithography masks or for the inspection of exposed wafers. Furthermore, they include optical systems for the calibration of mask patterning systems—so-called “registration units”—with which position marks can be measured with great precision on a lithography mask. 
     For the high precision imaging of micro- or nanostructures with the aid of a lithography exposure system it is important to know the position and the topography or the surface properties of the substrate to be exposed, e.g., in the form of a so-called wafer. In order to establish the position, focus sensors, for example, are used which, in direct proximity to a substrate table, guide a measurement signal such that it practically grazes the substrate plane and is then recaptured. In order to measure the substrate topography, measuring optics set up in parallel to the projection optics are often also used. This type of measuring optics is also called a “twin stage”. This type of measuring optics set up in parallel is associated with increased complexity because additional optics and also an additional displacement stage are required. 
     With regard to the use of these types of metrology systems the difficulty arises, in particular for lithography exposure systems which are operated in the EUV wavelength range (wavelength range of extreme ultraviolet radiation, e.g. 13.4 nm) that the working space of these systems on the substrate side is determined by the penultimate mirror in the beam path. From the optical point of view, it is advantageous if this space can be chosen to be particularly small. However, a very small working space only leaves a small amount of or even no installation space for a conventional focus sensor. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention to provide an apparatus and a method of the type specified above with which the aforementioned problems are addressed and/or resolved, and in particular with which a position of the substrate in relation to an imaging direction of the optical system can be determined with a smaller, or even the smallest possible, working space between the optical system and the substrate. 
     SUMMARY OF THE INVENTION 
     According to one formulation of the invention, an apparatus for microlithographic projection exposure is provided which has an optical system in the form of projection optics for imaging mask structures onto a surface of a substrate by projecting the mask structures with imaging radiation. The optical system is configured to operate in the EUV and/or higher frequency wavelength range, i.e. for wavelengths in the EUV range and/or for smaller wavelengths. This apparatus further has a measurement beam path for guiding measurement radiation. The measurement beam path extends within the optical system such that at least two optical elements of the optical system are included in the measurement path beam, and the measurement radiation only partially passes through the optical system during operation of the apparatus. 
     According to a further formulation of the invention, a method is provided for determining a property of an arrangement which comprises an apparatus for microlithographic projection exposure and a substrate disposed in an exposure position of the apparatus. Here the apparatus comprises an optical system for imaging mask structures onto the surface of the substrate by projecting the mask structures using imaging radiation in the EUV and/or higher frequency wavelength range. The method includes: guiding measurement radiation within the optical system such that at least two optical elements of the optical system are included in the measurement path beam, and the measurement radiation only partially passes through the optical system, and determining a property of the arrangement from the measurement radiation. 
     In other words, there is created within the optical system a beam path for measurement radiation with which a property of an arrangement which comprises the apparatus for microlithographic projection exposure and a substrate disposed in an exposure position can be determined. The optical system serves to image mask structures onto a surface of a substrate by projecting the mask structures with imaging radiation, and can also be called a projection objective of the projection exposure system. The optical system is configured to operate with imaging radiation in the EUV and/or higher frequency wavelength range. The EUV wavelength range is identified as the range below 100 nm, in particular the range between 5 nm and 20 nm. In particular, the optical system can be configured to operate with a wavelength of 13.5 nm or 6.9 nm. The configuration of the optical system to the aforementioned wavelength range generally requires the implementation of the optical system with purely reflective optical elements, and so as a so-called catoptric projection objective, and the provision of corresponding reflective coatings. 
     The measurement beam path extends within the optical system such that at least two optical elements of the optical system are included in the measurement beam path, and the measurement radiation only partially passes through the optical system during operation of the apparatus. It is to be understood from this that not all of the optical elements are included in the measurement beam path. An optical element can, for example, be included in the measurement beam path if the measurement radiation is reflected on the respective optical element. Another form of inclusion of an optical element in the form of a mirror on the measurement beam path can consist of the measurement radiation passing through an opening in this mirror. In other words, the measurement beam path only extends in sections within the optical system, and so there exists at least one optical element which is not included in the measurement beam path in the sense specified above. 
     According to one exemplary embodiment, the position of the substrate surface for at least one point on the substrate surface is measured as regards the imaging direction. If the optical system has an optical axis, which is the case when using rotationally symmetrical optical elements, the at least one point on the substrate surface is measured, in particular as regards its axial position, relative to the optical axis. The axial position is understood to be the position with regard to a coordinate axis which extends in the direction of the optical axis of the optical system. 
     The position of the substrate surface is then be determined by reflection of the measurement radiation on the substrate surface and subsequent analysis of the reflected measurement radiation. Due to the fact that the measurement radiation radiates from within the optical system onto the substrate surface, one can dispense with attaching measuring elements between the last element of the optical system and the substrate. In this way the working space between the reflective optical element lying closest to the substrate and the substrate can be kept very small. In the case where the measurement radiation is not reflected on the substrate, but rather e.g. on a mask having the mask patterns, corresponding advantages with regard to the working space between the reflective optical element lying closest to the mask and the mask are achieved. These or similar effects can be achieved correspondingly if it is not the position on the substrate surface, but generally a property of the arrangement that is determined. 
     In one embodiment, the measurement beam path is configured to measure, during operation of the apparatus, at least one point of the substrate surface as regards its position with the measurement radiation guided by the measurement beam path. In a further embodiment, the measurement beam path is configured to measure, during operation of the apparatus, the at least one point of the substrate surface as regards its position in the imaging direction of the optical system with the measurement radiation. Furthermore, the apparatus preferably has an analysis device which is configured to determine the position of the point of the substrate surface from the measurement radiation after the latter interacts with the substrate. 
     Generally, the apparatus according to one embodiment has an analysis device which is configured to determine a property of an arrangement of the measurement radiation and which includes the apparatus for projection exposure and the substrate. 
     According to one embodiment the measurement beam path is configured such that the measurement radiation is reflected on a surface of the substrate during operation of the apparatus. In this case the axial position on the substrate surface can then be determined from the measurement radiation reflected on the substrate surface. This can happen in different ways. For example, the measurement radiation reflected on the substrate surface can be determined with radiation moved past the optical system and passing back to the same radiation source as the measurement radiation, as described for example in US 2007/0080281 A1. Furthermore, one can, for example, fall back on the measuring principle described in U.S. Pat. No. 5,268,744 wherein displacement of the substrate surface in the z direction brings about displacement of the measurement beam on a detector surface. A further measuring principle of distance measurement is described in DE4109484C2. 
     In a further embodiment, the measurement beam path is configured to measure, during operation of the apparatus, the at least one point of the substrate surface as regards its position lateral to the imaging direction of the optical system using the measurement radiation guided by the measurement beam path. In this way the position of the at least one point is measured in the lateral direction. This can happen in particular with the aid of adjustment marks. In the simplest case just one adjustment mark, which is located on the substrate, is required. This adjustment mark is imaged onto a detector with the measurement radiation. The lateral position of the adjustment mark is taken from the position of the imaged adjustment mark, and from this the lateral position of the substrate, and so the so-called alignment of the substrate can be determined. 
     In addition to the adjustment mark on the substrate, a further adjustment mark can also be provided as a reference, e.g. on a reference mirror. This type of reference mark can be disposed upstream or downstream of the substrate in the measurement beam path. In the case of the upstream positioning the measurement radiation strikes the adjustment mark on the substrate “pre-patterned”, and in the case of the downstream positioning the measurement radiation patterned by the substrate mark is imaged onto the reference mark. In both cases an image, which includes the relative positions of the reference mark and the substrate mark, can be recorded by a downstream detector. From this the lateral position of the substrate mark relative to the reference mark can be determined. An example of this type of position determination using substrate and reference marks is the Moiré method. The Moiré measuring method, known in principle to the person skilled in the art, utilises the Moiré effect with which long-period brightness modulations are generated by overlaying two line grids the grid constants of which deviate only slightly from one another. By analysing the pattern produced, a relative displacement of the two grids in relation to one another can be determined with a high degree of precision. The Moiré analysis is advantageously implemented with the two-dimensional intensity pattern determined with the spatially resolving surface sensor. 
     For apparatuses for microlithographic projection exposure, in particular for methods which require high-precision lateral overlay of structures, and so have high “overlay” requirements, as is the case e.g. with double-patterning methods, this type of lateral position measurement is important. A measurement of the lateral position is also relevant to the inspection apparatuses described below. For these types of apparatus, also called “registration units”, it is made possible to take a lateral position measurement, e.g. for a so-called “pre-alignment”, i.e. a type of rough positioning. In one embodiment the high-precision measurement of the adjustment marks is then not implemented using the measurement beam path, but a different beam path, e.g. the imaging beam path. 
     In a further embodiment, the apparatus for microlithographic projection exposure has a substrate table displaceable laterally to the imaging direction and a control device. The control device is configured to control the apparatus such that during operation of the apparatus the at least one point of the substrate surface is measured at two different points in time as regards its position lateral to the imaging direction, and the apparatus is set up to determine from this a lateral displacement speed or a so-called “scan speed” of the substrate table. The substrate is held by the substrate table which can also be called the “wafer stage”. 
     In a further embodiment, the measurement beam path is configured such that during operation of the apparatus the measurement radiation is reflected on a surface of the substrate. 
     In a further embodiment, the apparatus has a measurement radiation source which generates the measurement radiation with at least two different wavelengths. Furthermore, the apparatus comprises a wavelength-resolving radiation detector which is configured to measure the respective intensity of the measurement radiation following reflection on the substrate for each of the at least two different wavelengths, and to determine from this the temperature on the substrate surface. This takes place in the same way as temperature determination with an infrared thermometer. 
     In a further embodiment, the mask structures to be imaged are disposed on a mask, and the measurement beam path is configured such that during operation of the apparatus the measurement radiation is reflected on a surface of the mask facing towards the optical system. 
     In a further embodiment, the apparatus comprises a mask table that can be displaced laterally to the imaging direction and a control device which is configured to control the apparatus for microlithographic projection exposure such that during operation of the apparatus the at least one point of the substrate surface is measured as regards its position lateral to the imaging direction at two different points in time, the apparatus being set up to determine from this a lateral displacement speed of the mask table. 
     In a further embodiment, the apparatus comprises a measurement radiation source which generates the measurement radiation with at least two different wavelengths. Furthermore, the apparatus includes a wavelength-resolving radiation detector which is configured to measure the respective intensity of the measurement radiation following reflection on the mask surface for the at least two different wavelengths, and from this to determine the temperature on the substrate surface. 
     In a further embodiment, the apparatus is configured to determine, during operation of the apparatus, a reduction in intensity of the measurement radiation upon passing through the optical system, and to determine from this a concentration of a gas contained in the optical system. This gas surrounds the optical elements of the optical system. This can also be a residual gas of an optical system operated in a vacuum. 
     In a further embodiment, the apparatus further comprises an optical injecting element which is provided for injecting the measurement radiation into the optical system. Moreover, an optical extracting element, which is provided for extracting the measurement radiation from the optical system, can be provided. For this purpose two deflection mirrors can be provided which are configured to reflect the measurement radiation. The deflection mirrors are advantageously independent of the at least one reflective optical element of the optical system. In one embodiment the first deflection mirror is positioned such that it steers the measurement radiation onto one of the reflective optical elements or, in the embodiment in which one of the reflective optical elements has an opening, onto this opening. 
     In a further embodiment, the measurement beam path is configured such that, during operation of the apparatus, the measurement radiation is reflected on the surface of the substrate or the surface of the mask between injection into the optical system and extraction from the optical system. In this way it is possible to determine an axial position of the substrate on the reflection point relative to the optical system. 
     In a further embodiment, the optical system has at least one reflective optical element, and the measurement beam path extends within the optical system such that during operation of the apparatus the measurement radiation is reflected on the at least one reflective optical element. According to a further variation the measurement beam path extends within the optical system such that during operation of the apparatus the measurement radiation passes through an opening in the at least one reflective optical element. 
     In a further embodiment, the apparatus further comprises a substrate plane into which the mask structures are imaged during operation of the apparatus, the measurement beam path being configured such that the measurement radiation is focussed onto the substrate plane. 
     In a further embodiment, an obscuration aperture is disposed in a pupil plane of the optical system. This type of obscuration aperture is also called a shading aperture. This can be created, for example, using a non-reflective coating with regard to radiation with the wavelength of the imaging radiation. This type of obscuration aperture is often used to avoid a high degree of light loss in the beam path of the imaging radiation because in an obscured system smaller angles of incidence can be achieved, as described, for example, in WO 2006/069725 A1. An obscuration aperture often blocks the imaging radiation in a central region of the beam cross-section. By positioning an obscuration aperture in a pupil plane area-independent obscuration of the pupil can be achieved. This type of optical system with an obscured pupil benefits particularly in the context of the invention because the size of the obscuration can be smaller the smaller the working space between the optical system and the substrate. 
     In a further embodiment, the at least one reflective optical element has an opening, and the measurement beam path extends through this opening. The at least one reflective optical element having the opening is downstream in relation to the obscuration aperture in the beam path of the imaging radiation, the opening being disposed in a region of the reflective optical element which is at least partially shaded from the imaging radiation by the obscuration aperture. Therefore, the imaging characteristics of the optical system are not negatively effected by the measurement radiation. 
     In a further embodiment, an obscuration aperture is disposed in a first pupil plane, the optical system has a further pupil plane downstream in relation to the obscuration aperture in the imaging beam path which is shaded in some regions by the obscuration aperture, and the measurement beam path extends at least once through the further pupil plane, at least partially in the shaded region. The further pupil plane can be disposed, for example, between the penultimate and the last reflective optical element of the optical system before the substrate. Preferably the measurement radiation passes through the further pupil plane twice. 
     In a further embodiment, the measurement beam path extends at least once through a pupil plane of the optical system. 
     In a further embodiment, at least one of the reflective optical elements has an opening, and the measurement beam path extends through this opening. In this embodiment the opening is disposed in a central region of the at least one reflective optical element. By positioning the opening in the central region of the reflective optical element, the opening has, if it has any effect at all, a symmetrical effect upon the imaging beam path in the optical system. In this way imaging errors when imaging the mask structures with the optical system are avoided. 
     In a further embodiment, the optical system comprises at least two reflective optical elements, and the measurement beam path extends through both openings. In one embodiment the reflective optical elements having the respective openings are disposed in a high-aperture part of the optical system, in particular these are the last two reflective optical elements of the optical system. 
     In a further embodiment, the at least one reflective optical element has an opening, and the measurement beam path is configured such that during operation of the apparatus the measurement radiation passes through the opening and is reflected on at least one other reflective optical element of the optical system. In this way, aspects and benefits associated with the invention can be realised through a variety of optical designs for the optical system. 
     In a further embodiment, the measurement beam path is configured such that during operation of the apparatus the measurement radiation is reflected on the at least one reflective optical element, the latter having a peripheral region disposed outside of the beam path of the imaging radiation, and the measurement beam path being configured such that the measurement radiation is reflected on the peripheral region. This peripheral region can, for example, have a reflective coating matched especially to the wavelength of the measurement radiation. Therefore, greatly differing wavelengths can be used for the imaging radiation and the measurement radiation. 
     In a further embodiment, the measurement beam path is configured such that the measurement radiation is reflected twice on the at least one reflective optical element. Preferably the measurement radiation is reflected once prior to reflection on the substrate, and once after this on the respective reflective optical element. Therefore, the measurement radiation reflected on the substrate can also be guided through the interior of the optical system to a detector device so that no further optical element need be disposed between the last optical element and the substrate plane. Therefore, the working space of the optical system can remain unaffected by the detection of the measurement radiation. 
     In a further embodiment, the optical system comprises a number of optical elements, and the measurement beam path is configured such that during operation of the apparatus the measurement radiation is reflected on at least two, in particular on three, four, five, six, etc. of the reflective optical elements. In one embodiment the reflection on the reflective optical elements takes place before the measurement radiation strikes the substrate, and then the measurement radiation is reflected once again on said reflective optical elements. 
     In a further embodiment, none of the reflective optical elements of the optical system has an opening in the optically used region of an optical area. Therefore, the optical areas of the reflective optical elements are all designed to be continuous. Therefore, within the optical system the measurement radiation is either reflected on a respective reflective optical element or runs past it. 
     In a further embodiment, the measurement beam path is configured such that the relative position of at least two points on the surface of the substrate, in particular the topography of at least one section of the surface of the substrate, can be measured with the measurement radiation guided therein. In other words, a number of points on the surface of the substrate are measured as regards their relative axial position in relation to one another with the measurement radiation. From this the surface properties of the substrate surface can be determined. From the surface properties determined matching of the exposure conditions with regard to the focus setting for the imaging of the mask structures onto the substrate surface can in turn take place. 
     In a further embodiment, at a given time during operation of the apparatus a limited area on the substrate is exposed with the imaging radiation, and the measurement beam path is configured such that at the time of exposing the substrate the measurement radiation is directed onto the exposed area. Therefore, the position measurement on the surface of the substrate takes place simultaneously during exposure of the respective substrate area. Any possible errors in the position caused by a scanning movement of the substrate can be eliminated by the simultaneous measurement. The exposed area on the substrate at any given time can in the case of a projection exposure system in the form of a so-called stepper be e.g. the whole exposed field on the wafer, or in the case of a projection exposure system in the form of a so-called scanner, be a slot-shaped area illuminated by the exposure slot. 
     In a further embodiment, the apparatus is configured as a scanner wherein during operation a slot-shaped area on the substrate is exposed by an exposure beam, the substrate being moved relative to the exposure beam so that the exposed area is displaced on the substrate. In this embodiment the measurement beam path is configured such that the measurement radiation is directed towards a section of the substrate which during exposure operation of the exposed area runs ahead and/or runs behind. In the case where the measurement radiation is directed towards the section running ahead of the exposed area there is the advantage that prior to exposure of the section of the substrate “sampled” in advance using the measurement radiation, the focus settings required for exposure are already provided. Therefore, there is sufficient lead time in order to establish an optimal focus setting, for example by mechanical manipulations, for the exposure of the substrate section. In the case of “subsequent measurement” of the substrate surface the measurement result can serve, for example, to verify the imaging settings in retrospect or also for quality control of the exposure. 
     In a further embodiment, the apparatus is configured to determine the position of the at least one point of the substrate surface while at the same time imaging the mask structures onto the substrate surface. In this case the measurement of the substrate surface during the exposure operation takes place in real time. 
     According to another aspect of the invention, furthermore, an apparatus for microlithographic projection exposure is provided comprising: an optical system for imaging mask structures onto a surface of a substrate by projecting the mask structures in an imaging direction with imaging radiation, the optical system having at least one reflective optical element and a measurement beam path for guiding measurement radiation, the measurement radiation serving to measure at least one point of the substrate surface as regards its position in the projection direction, and the measurement beam path being configured such that during operation of the apparatus the measurement radiation is reflected on at least one of the reflective optical elements. 
     Moreover, an apparatus for microlithographic projection exposure is provided comprising: an optical system for imaging mask structures onto a surface of a substrate by projecting the mask structures with imaging radiation, the optical system having at least one reflective optical element. Furthermore, the optical system comprises a measurement beam path for guiding measurement radiation, the measurement beam path extending, in particular in sections, within the optical system such that during operation of the apparatus the measurement radiation is reflected on the at least one reflective optical element and, furthermore, on the surface of the substrate. 
     The reflective optical element is in particular an optical element generating the imaging beam path. In other words, in this case the reflective optical element is positioned within the optical system such that it reflects the imaging radiation onto the surface of the substrate when imaging the mask structures. It is therefore an imaging optical element of the optical system with regard to the imaging of the mask structures. 
     Moreover, an apparatus for microlithographic projection exposure is provided. This apparatus comprises: an optical system for imaging mask structures onto a surface of a substrate by projecting the mask structures with imaging radiation, the optical system having at least one reflective optical element. Furthermore, the apparatus comprises a measurement beam path for guiding measurement radiation, the measurement beam path extending, at least in sections, within the optical system such that during operation of the apparatus the measurement radiation passes through an opening in the at least one reflective optical element and, furthermore, is reflected on the surface of the substrate. In the present case the optical system can be configured as a catadioptric system which, in addition to the at least one reflective optical element, comprises at least one refractive or diffractive optical element, but also as a catoptric system. 
     Furthermore, according to another aspect of the invention, a method is provided for determining a property of an arrangement which comprises an apparatus for microlithographic projection exposure and a substrate disposed in an exposure position of the apparatus. The apparatus comprises an optical system for imaging mask structures onto the surface of the substrate by projecting the mask structures in an imaging direction using imaging radiation. The optical system has at least one reflective optical element, and the method comprises the steps: guiding measurement radiation within the optical system such that the measurement radiation is reflected on the at least one reflective optical element or passes through an opening in the at least one of the reflective optical elements, reflecting the measurement radiation on the surface of the substrate, and determining a property of the arrangement from the reflected measurement radiation. 
     Moreover, an apparatus for microlithographic projection exposure is provided which comprises: an optical system for imaging mask structures onto a surface of a substrate by projecting the mask structures in an imaging direction with imaging radiation, the optical system having at least one reflective optical element. The apparatus further comprises a measurement beam path for guiding measurement radiation, which serves to measure at least one point of the substrate surface as regards its position in the imaging direction. Here at least one of the reflective optical elements has an opening, and the measurement beam path extends through this opening. 
     Furthermore, a method is provided for determining a position on a surface of a substrate which is disposed in an exposure position in an apparatus for microlithographic projection exposure. The apparatus has an optical system here for imaging mask structures onto the surface of the substrate by projecting the mask structures using imaging radiation. The optical system has at least one reflective optical element, and the method according to one formulation, comprises: guiding measurement radiation, in sections, within the optical system such that the measurement radiation is reflected on the at least one reflective optical element or passes through an opening in the at least one reflective optical element, reflecting the measurement radiation on at least one point on the surface of the substrate, and determining a position of the at least one point of the substrate surface relative to the imaging direction from the reflected measurement radiation. Preferably said opening in the reflective optical element is designed such that the imaging of the mask structures onto the surface of the substrate is not or is only slightly affected by the imaging radiation through the opening. In the case where the optical system has a partially obscured pupil, the opening can be positioned in an obscured region of the imaging beam path. 
     As already mentioned above, the apparatus can be configured as a projection exposure system for microlithography, in particular as an EUV projection exposure system. 
     Furthermore, according to yet another formulation of the invention, an apparatus for inspecting a surface of a substrate is provided comprising: an optical system for imaging at least one section of a surface of a substrate to be inspected into a detection plane by imaging the section with imaging radiation, the optical system having at least one reflective optical element, and a measurement beam path for guiding measurement radiation, the measurement beam path extending, in sections, within the optical system such that during operation of the apparatus the measurement radiation is reflected on the reflective optical element or passes through an opening in the reflective optical element. 
     Moreover, according to a further formulation of the invention, an apparatus for inspecting a surface of a substrate is provided comprising: an optical system for imaging at least one section of a surface of a substrate to be inspected into a detection plane by imaging the section in an imaging direction with imaging radiation, the optical system being configured to operate in the EUV and/or higher frequency wavelength range, and a measurement beam path for guiding measurement radiation, the measurement beam path extending within the optical system such that the measurement radiation only partially passes through the optical system during operation of the apparatus. Therefore, the substrate can be a semiconductor wafer, a lithography mask or in general an object to be inspected. 
     Moreover, a method for determining a property of an arrangement is provided which comprises an inspection apparatus and a substrate which is disposed in an inspection position in the inspection apparatus, the apparatus having an optical system for imaging at least one section of a surface of the substrate to be inspected into a detection plane by imaging the section using imaging radiation in the EUV and/or higher frequency wavelength range, the optical system comprising at least one reflective optical element, and the method comprising the steps: guiding measurement radiation within the optical system such that the measurement radiation only partially passes through the optical system, and determining a property of the arrangement from the measurement radiation. 
     Moreover, an apparatus for inspecting a surface of a substrate is provided comprising: an optical system for imaging at least one section of a surface of a substrate to be inspected into a detection plane by imaging the section in an imaging direction with imaging radiation, the optical system having at least one reflective optical element and a measurement beam path for guiding measurement radiation, the measurement beam path extending, in particular in sections, within the optical system such that during operation of the apparatus the measurement radiation is reflected on the at least one reflective optical element, or passes through an opening in the at least one reflective optical element. 
     According to one embodiment of this inspection apparatus, the latter has an analysis device which is configured to determine a property of an arrangement comprising the apparatus for projection exposure and the substrate from the measurement radiation. This type of property that can be determined with the analysis device can be e.g. the position of a point of the substrate surface. 
     Moreover, a method for determining a property of an arrangement is provided which comprises an inspection apparatus and a substrate which is disposed in an inspection position in the inspection apparatus, the apparatus having an optical system for imaging at least one section of a surface of the substrate to be inspected into a detection plane by imaging the section using imaging radiation, the optical system having at least one reflective optical element, and the method comprising the steps: guiding measurement radiation within the optical system such that the measurement radiation is reflected on the at least one reflective optical element, or passes through an opening in the at least one reflective optical element, reflecting the measurement radiation on the surface of the substrate, and determining a property of the arrangement from the reflected measurement radiation. 
     Moreover, according to a further aspect of the invention, a method is provided for determining a position on a surface of a substrate which is disposed in an inspection position in an inspection apparatus, the apparatus having an optical system for imaging at least a section of a surface of the substrate to be inspected into a detection plane by imaging the section in an imaging direction using imaging radiation, the optical system having at least one reflective optical element, and the method comprising: guiding measurement radiation, in sections, within the optical system such that the measurement radiation is reflected on the at least one reflective optical element, or passes through an opening in the at least one of the reflective optical elements, reflecting the measurement radiation on at least one point on the surface of the substrate, and determining a position of the at least one point of the substrate surface relative to the imaging direction from the reflected measurement radiation. 
     According to one embodiment, the inspection apparatus is configured as a microscope. In a further embodiment, the inspection apparatus is configured as an optical inspection system for inspecting substrates exposed by a microlithographic projection exposure system. Moreover, in a further embodiment the apparatus is in the form of an optical inspection system for inspecting masks for microlithography. Therefore, the inspection apparatus can be configured to calibrate mask structuring systems, and so as a so-called “registration unit” wherein position marks on a lithography mask are measured with great precision. From this measurement conclusions can be drawn regarding the writing accuracy of mask structures disposed on the lithography mask and which are intended for imaging onto a wafer. 
     The features specified in relation to the embodiments of the apparatus for microlithographic projection exposure listed above can be applied correspondingly to the apparatus for inspecting a surface of a substrate. 
     Furthermore, the features specified in relation to the embodiments of the apparatus summarized above can be applied correspondingly to the methods summarized, and vice versa. The resulting embodiments of the method are therefore hereby incorporated by this reference into the present disclosure. Furthermore, the aspects and advantages summarized above in relation to the embodiments of the apparatus therefore also apply to the corresponding embodiments of the method and vice versa. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, exemplary embodiments of an apparatus according to the invention for microlithographic projection exposure are discussed in greater detail with reference to the attached schematic drawings. These show as follows: 
         FIG. 1  a schematic side view of a first embodiment of an apparatus for microlithographic projection exposure in a first illustration plane with a greatly schematically illustrated imaging beam path and a measurement beam path, 
         FIG. 2  a detailed illustration of the imaging beam path according to  FIG. 1 , 
         FIG. 3  the imaging beam path in the schematised illustration according to  FIG. 1  with an individual beam serving to illustrate the precise course of the beam, 
         FIG. 4  a sectional view of the imaging beam path according to  FIGS. 1 to 3  in a second illustration plane rotated by 90°, 
         FIG. 5  a top view of a reflective optical element of the apparatus for microlithographic projection exposure, 
         FIG. 6  a sectional view of a second embodiment of the apparatus for microlithographic projection exposure in the first illustration plane, 
         FIG. 7  a sectional view of a third embodiment of the apparatus for microlithographic projection exposure in the first illustration plane, with which only the imaging beam path is drawn in, 
         FIG. 8  the apparatus according to  FIG. 7 , with which only one measurement beam path is drawn in, 
         FIG. 9  the imaging beam path according to  FIG. 7  in the second illustration plane, 
         FIG. 10  a top view of a substrate exposed by one of the apparatuses illustrated above in order to illustrate the exposure process, 
         FIG. 11  a sectional view of a further embodiment of the apparatus for microlithographic projection exposure in the first illustration plane with which only one measurement beam path is drawn in, and 
         FIG. 12  a sectional view of an embodiment of an apparatus for inspecting a surface of a substrate. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     In the exemplary embodiments described below any elements which are functionally or structurally similar to one another are provided as far as possible with the same or similar reference numbers. Therefore, in order to understand the features of the individual elements of a specific exemplary embodiment, reference should be made to the description of other exemplary embodiments or to the more general description of the invention. 
       FIG. 1  shows a first embodiment of an apparatus  10  for microlithographic projection exposure in the form of an EUV projection exposure system in a sectional view in the x-z plane according to the coordinate system drawn in in the figure. As shown in the drawing, the apparatus  10  can be designed as a so-called step and scan exposure system, called “scanner” for short, or also as a so-called “stepper”. 
     The apparatus  10  comprises an illumination system  12  which radiates illumination radiation  13  in the EUV wavelength range (extreme ultraviolet radiation, e.g. with a wavelength of 13.4 nm) onto a mask  14  with mask patterns  16  positioned over the latter. The mask  14  is held by a mask table  11  in the form of a so-called “reticle stage”. 
     The apparatus  10  further comprises an optical system  18  in the form of projection optics for imaging the mask structures  16  onto a surface  21  of a substrate  20  in the form of a wafer disposed in a substrate plane  19 . The apparatus  10  further comprises a substrate table  20   a  in the form of a so-called “wafer stage” by which the substrate  20  is held. As already mentioned above, according to the embodiment shown the apparatus is configured as a so-called “scanner”. During the exposure of an area on the substrate  20 , both the mask table  11  and the substrate table  20   a  are displaced at different speeds in the y direction according to the coordinate system shown in  FIG. 1 . 
     The optical system  18  comprises purely reflective optical elements  22  in the form of mirrors. Therefore, the optical system  18  can also be called a catoptric projection objective. The imaging of the mask structures  16  onto the surface  21  of the substrate  20  is implemented by projecting the mask patterns in an imaging direction  17   a . The imaging direction  17   a  extends in the direction of a reference axis  17  of the optical system  18 , which according to  FIG. 1  extends in the z direction. In the case where reflective optical elements  22  are rotationally symmetric, the reference axis  17  corresponds to the optical axis of the optical system. 
     The illumination radiation  13  is transformed by the mask  14  into imaging radiation  15 . The imaging radiation  15  passes through the imaging beam path  24  in the optical system  18  which is shown as an outline in  FIG. 1  in order to provide a clearer illustration.  FIG. 2  shows the imaging beam path  24  with a plurality of individual beams contained therein. As a further illustration of the path of the imaging radiation  15  in the optical system  18  an exemplary individual beam  25  of the imaging radiation  15  is shown in  FIG. 3 . As is evident from this, the imaging radiation  15  is reflected on the individual reflective optical elements  22 - 1  to  22 - 6 , one after the other, in the imaging beam path  24 . Here the elements  22 - 1  and  22 - 2  are respectively a concave mirror, element  22 - 3  is a convex mirror, element  22 - 4  is once again a concave mirror, element  22 - 5  is a convex mirror, and element  22 - 6  is in turn a concave mirror. 
     The optical system  18  has a first pupil plane  28  disposed between the reflective optical elements  22 - 3  and  22 - 4 . Disposed in the first pupil plane  28  there is in a central region of the beam cross-section of the imaging radiation  15  an obscuration aperture  29 , also called a shading aperture. The obscuration aperture  29  brings about area-dependent obscuration of the pupil, and is made of a material, or has a coating, which does not reflect any radiation with the exposure wavelength in the EUV wavelength range. The material substantially absorbs the incoming radiation with this wavelength. 
     The reflective optical elements  22 - 5  and  22 - 6  downstream in the beam path  24  are disposed in a high aperture part of the optical system  18 , and each have an opening  26  in the form of a central hole through the respective reflective optical surface  27 .  FIG. 5  shows this type of opening  26  in an exemplary reflective optical element  22 . As shown in  FIG. 5 , the opening  26  can be in the shape of a circular disc or also have other forms. The opening  26  is disposed in a region of the reflective optical elements  22 - 5  and  22 - 6  which is at least partially shaded by the obscuration aperture  29 . This type of obscuration aperture  29  can serve to prevent a high degree of light loss in the illumination beam path  24 , as described, for example, in WO 2006/069725. 
     Furthermore, the apparatus  10  has a measurement radiation source  32  for generating measurement radiation  34 . The measurement radiation  34  can have a different wavelength than the imaging radiation  15 , e.g. a wavelength in the visible wavelength range, in particular e.g. 632.8 nm, in the UV wavelength range, in particular the DUV wavelength range, e.g. 248 nm, the VUV wavelength range, e.g. 193 nm, or also in the infrared range. The measurement radiation  34  is injected into the optical system  18  by a first deflection mirror  38  in the form of an injecting mirror or injecting element. The injection takes place such that the measurement beam path  36  initially passes through the opening  26  in the reflective optical element  22 - 6  and then through the opening  26  in the reflective optical element  22 - 5 . 
     The measurement radiation  34  is then reflected on the surface  21  of the substrate  20  and passes through the openings  26  in the optical elements  22 - 5  and  22 - 6  once again. Here the measurement beam path  36  passes twice through a second pupil plane  30  conjugated to the first pupil plane  28  in a region shaded by the obscuration aperture  29 . After passing through the openings  26  in the elements  22 - 5  and  22 - 6  once again, the measurement radiation  34  is steered by a second deflection mirror  40  in the form of an extracting mirror or extracting element onto a detector system  42 . Since the measurement radiation  34  neither passes through nor is reflected on a series of optical elements of the optical system  18 , in particular the optical elements  22 - 1  and  22 - 2 , the measurement radiation  34  only partially passes through the optical system  18 . 
     Using the detector system  42 , the point or a number of points on the substrate surface  21  at which the measurement radiation  34  was reflected is/are measured as regards its/their position in the imaging direction  17   a . For this purpose the detector system  42  comprises a detector and an analysis device  45  which determines the axial position of the point or the points on the substrate surface  21  to be measured from the signal recorded by the detector. Since the imaging direction  17   a  according to  FIG. 1  extends parallel to the z coordinate axis, the z coordinate of the point or the respective z coordinates of a number of points is/are determined. This can happen, for example, by overlaying the reflected measurement radiation  34  with radiation from the measurement radiation source  32  moved past the optical system  18 , as described, for example, in US 2007/0080281 A1. Alternatively, the measuring principle described in U.S. Pat. No. 5,268,744 can be used with which the displacement of the substrate surface  21  in the z direction also leads to a displacement of the striking location of the measurement radiation on a detector surface in the detector system  42 . Alternatively, the measuring system described in DE 4109484 C2 can also be used to analyse the reflected measurement radiation  34 .  FIG. 4  shows the imaging beam path  24  according to  FIGS. 1 to 3  in the y-z plane. Using the measurement radiation  34 , the topography of the substrate surface  21 , and so the relative position of a number of points on the substrate surface  21  in relation to one another, can also be determined. 
     Furthermore, the measurement radiation  34  can also be used to determine the lateral position of an adjustment mark disposed on the substrate surface  21 , and so be used for the alignment of the substrate  20  before it is exposed. As already explained in greater detail above, for this purpose the adjustment mark can be imaged directly onto the detector system  42 . Alternatively, in addition a reference mark can be disposed in the measurement beam path  36 , for example on a reference mirror. One possibility for determining the lateral position is the Moiré measuring method known in principle to the person skilled in the art. 
       FIG. 6  shows the apparatus  10  in sections in a second embodiment in a sectional view in the x-z plane. The apparatus  10  according to  FIG. 6  differs from the apparatus  10  according to  FIG. 1  only in the configuration of the measurement beam path  36 . The measurement beam path  36  according to  FIG. 6  is configured such that after injection into the optical system  18  the measurement radiation  34  is initially reflected by the first deflection mirror  38  on the reflective optical element  22 - 6 . The reflection takes place here on a peripheral region  23  of the reflective optical element  22 - 6 . This peripheral region  23  can be disposed outside or inside the imaging beam path  15 , and it can also partially overlap with the imaging beam path  15 . 
     The measurement radiation  34  reflected on the peripheral region  23  then passes through the opening  26  in the reflective element  22 - 5 , is reflected on the substrate surface  21 , passes through the opening  26  in the reflective element  22 - 5  once again, and is then reflected again on the peripheral region  23  of the reflective element  22 - 6 . The repeated reflection takes place on an opposing region of the peripheral region  23  with regard to the first reflection relative to the reference axis  17  of the optical system  18 . The measurement radiation  34  is then steered by the second deflection mirror  40  onto the detector system  42 . 
       FIGS. 7 to 9  show a third embodiment of an apparatus  10  for microlithographic projection exposure. In order to provide a better illustration, in  FIGS. 7 and 9  respectively only the imaging beam path  24 , and in  FIG. 8  only the measurement beam path  36  is shown.  FIGS. 7 and 8  show the embodiment in a sectional view in the x-z plane, and  FIG. 9  in a sectional view in the y-z plane. 
     Unlike the previously described optical systems  18 , the optical system  18  according to  FIGS. 7 to 9  does not have an obscuration aperture  29 . As is evident from  FIG. 9 , the imaging radiation  15  is guided past the side of individual respective optical elements  22 - 1  to  22 - 6  before or after reflection on the optical elements in question without penetrating a reflective optical surface. Therefore, unlike the previous embodiments, the reflective optical elements  22 - 1  to  22 - 6  do not have an opening  26 . 
     As shown by  FIG. 8 , the measurement beam path  36  also extends such that the measurement radiation  34  is either reflected on the reflective optical elements  22 - 1  to  22 - 6  or runs past them. Therefore, in this embodiment the measurement radiation  34  does not pass through an opening in a reflective optical element  22 . Following injection into the optical system  18  via the first deflection mirror  38 , the measurement radiation  34  is reflected, one after the other, on the reflective optical elements  22 - 3  in the form of a convex mirror,  22 - 4  also in the form of a convex mirror,  22 - 5  also in the form of a convex mirror, and  22 - 6 , which is also in the form of a convex mirror. The measurement radiation  34  then passes onto the substrate surface  21  and passes through the optical system  18  by reflection on the aforementioned elements in the opposite sequence before it is steered onto the detector system  42  by the second deflection mirror  40 . 
       FIG. 10  shows a top view of the substrate  20  in the form of a wafer with an area  43  to be exposed drawn in as an example. In the case where the apparatus  10  is designed as a “scanner”, the exposure beam radiating onto the substrate is formed such that the latter exposes a slot-shaped area  44  at a given point in time. This slot-shaped area can be rectangular or also in the shape of a ring segment. During scan operation the substrate  20  is scanned in the y direction  50  relative to the fixed optical system  18 . The effect of this is that the slot-shaped exposed area  44  is effectively scanned in the opposite direction  52 . 
     In all of the embodiments of the apparatus  10  described above, the measurement beam path  36  can be configured in three different variations. In a first variation the measurement radiation  34  is directed towards a point or a region within the slot-shaped exposed area  44  at the time when it is exposed. Therefore, a simultaneous measurement of the surface properties of the substrate surface  21  is taken during the scanning process. In a second variation, the measurement radiation  34  is directed at a section  46  of the substrate  20  running ahead of the exposed area  44 . In a third variation the measurement radiation  34  is, in contrast, directed at section  48  of the substrate  20  running behind the exposed area  44 . 
     In a further embodiment of the apparatus  10 , a point on the substrate surface  21  is measured, as regards its position lateral to the imaging direction  17   a , at two different times during the scanning movement of the substrate  20 . From this the scanning speed and the lateral displacement speed of the substrate  20  is determined. 
     In a further embodiment, of the apparatus  10 , the measurement radiation  34  generated by the measurement radiation source  32  has at least two different wavelengths. The radiation detector  42  is wavelength-resolving and determines the respective intensity of the measurement radiation  34  following reflection on the substrate surface  21  for the at least two different wavelengths. From the radiation intensities the temperature of the substrate surface  21  is then determined in the same way as with the function of an infrared thermometer. 
     In a further embodiment of the apparatus  10 , during operation of the apparatus  10 , a reduction in intensity of the measurement radiation  34  upon passing through the optical system  18  is determined, and from this a concentration of a gas contained in the optical system  18  is determined. 
       FIG. 11  shows a further embodiment of the apparatus  10  for microlithographic projection exposure. With this embodiment, the measurement beam path  36  is formed such that during operation of the apparatus  10  the measurement radiation  34  is reflected on a surface  14   a  of the mask  14  facing towards the optical system  18 . Therefore, for example, the position of a point of the mask surface  14   a  or also the topography of the mask surface  14   a  can be determined. Also, other properties of the apparatus specified above with regard to the embodiment with which the measurement radiation  34  is reflected on the substrate  20  can be determined. 
       FIG. 12  shows an embodiment of an apparatus  110  for inspecting a surface of a substrate  20 . This can be a microscope or an optical inspection system, e.g. for the inspection of lithography masks or for the inspection of exposed wafers. The apparatus can also be configured to calibrate mask shaping systems, and so as a so-called “registration unit” with which position marks on a lithography mask are measured with high precision. From this measurement conclusions can be drawn regarding the writing precision of mask structures disposed on the lithography mask and intended to image into a wafer. 
     The substrate  20  can therefore be a semiconductor wafer, a lithography mask or generally an object to be inspected. The apparatus  110  only differs from the previously described apparatuses  10  for microlithographic projection exposure in that the imaging takes place in the opposite imaging direction  17   a . The substrate  20  is illuminated by illumination radiation  113  radiated at an angle. The imaging radiation  15  remitted from the surface  21  of the substrate  20  passes through the imaging beam path  24  of the optical system  18  in the opposite direction in comparison to the apparatus  10 . A section of the surface  21  to be inspected is imaged onto a detection surface  158  of a detection device  154  disposed in a detection plane  156  and is thereby detected. 
     The apparatus  110  comprises a measurement beam path  36  formed in the same way as the measurement beam path  36  according to  FIG. 1 . The embodiments shown in  FIGS. 2 to 5  can be correspondingly applied to the apparatus  110  in the embodiment according to  FIG. 12 . Furthermore, the apparatus  110  can be realized in further embodiments similar to the apparatuses  110  according to  FIGS. 6 to 9 . 
     The above description of the preferred embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures and methods disclosed. The applicant seeks, therefore, to cover all such changes and modifications as fall within the spirit and scope of the invention, as defined by the appended claims, and equivalents thereof.