Method for detecting optical properties of a microscope, method for restoring the imaging quality of a microscope, and microscope suitable for carrying out the method

Abstract

The invention relates to a method for restoring the imaging quality of a high-performance microscope (22) which has an image sensor (36) and imaging optics (30) having a plurality of optical elements (L1 to L3) and images an object plane (32) onto the image sensor (36). A structure (60; 60a, 60b) is arranged in the object plane (32) at a first point in time at which the imaging quality of the microscope (22) meets the specifications. An image 1-1 of the structure is then recorded in a focused manner and a further image 1-2 is recorded in a defocused manner. After an optical element has been replaced or in order to ensure the imaging quality, the structure is arranged in the object plane (32) again at a later point in time. An image 2-1 of the structure is then recorded in a focused manner and compared with the image 1-1 in order to detect at least one first imaging error. This is followed by first measures for rough correction of the first imaging error. Subsequently, an image 2-2 of the structure is recorded in a defocused manner and compared with the image 2-2 in order to detect second imaging errors. Finally, second measures for correcting the second imaging errors are carried out. Alternatively, the images 1-1 and 1-2 can also be simulated on the basis of the optical design of the microscope.

Description

[0001] Method for detecting optical properties of a microscope, method for restoring the image quality of a microscope and microscope suitable for carrying out the method

[0002] BACKGROUND OF THE INVENTION

[0003] 1. Field of the invention

[0004] The invention relates to a method for detecting optical properties of a microscope and a method for restoring the imaging quality of a microscope after optical elements of the microscope have been replaced or after there are indications of a deterioration in the imaging quality. The microscope can, in particular, be a high-performance UV microscope, such as those used for the inspection of integrated circuits in the context of quality control. The invention also relates to a microscope suitable for carrying out the restoration method, which comprises a programmed control device with which the steps of the restoration method can be initiated.

[0005] 2. Description of the state of the art

[0006] As the structures in integrated circuits become ever smaller and now have a feature width of only a few nanometers, very powerful microscopes are required to detect manufacturing defects in the circuits as part of quality control. While standard light microscopes are sufficient to detect larger dust particles or scratches on exposed wafers, resolving smaller particles with diameters of only 10 nm or defects in the circuit structures requires imaging with light of shorter wavelengths. This can particularly involve light from the UV (ultraviolet, approximately 300 to 400 nm), DUV (deep ultraviolet, approximately 200 to 300 nm), or VUV (vacuum ultraviolet, approximately 100 to 200 nm) spectral ranges.

[0007] Since the transmittance of glass decreases sharply at wavelengths below approximately 320 nm, the glass normally used for the lenses in microscopes that use DUV light is replaced by other materials such as calcium fluoride. Such microscopes often contain one or more mirrors in addition to the lenses, as is known, for example, from WO 1999 / 008134 A1. At wavelengths below approximately 120 nm, even calcium fluoride is no longer sufficiently transmissive. Such microscopes must therefore be catoptric, i.e. they no longer contain lenses, but only mirrors. The substrates of the mirrors have complex multilayer coatings that reflect a large portion of the incident VUV light.

[0008] When such high-performance microscopes are used for quality control in the production of integrated circuits, they must be capable of inspecting large numbers of wafers in a short period of time. Wafer inspection systems equipped with such high-performance microscopes are capable of inspecting up to 100 wafers per hour. The wafers are automatically placed in wafer holders on a moving stage. The moving stage positions the wafer in such a way that all exposed circuits on the wafer can be inspected in a short period of time. Such wafer inspection systems are very expensive due to their complex optics and mechanics, and downtimes should therefore be as short as possible.

[0009] Due to the high demands on image quality, the optical elements of the imaging optics are adjusted during initial assembly in a complex process that requires numerous individual image quality measurements. Interferometric wavefront sensors are typically used for these measurements. These sensors are capable of precisely detecting the optical wavefronts and thus providing a complete picture of the image quality. Adjustment is performed using manipulators that are built into the microscopes and can move or tilt the optical elements using actuators. Wavefront measurement and adjustment typically alternate in a recursive process that can be very time-consuming.

[0010] When operating such high-performance microscopes, it often happens that individual optical elements need to be replaced after a certain period of time. This can be due to defects or signs of aging, but also due to the desire to be able to make certain adjustments to changing inspection tasks. Previously, such high-performance microscopes had to be returned to the manufacturer in such cases so that the microscope could be re-measured and adjusted. This leads to long downtimes and, in the worst case, to production losses in semiconductor manufacturing. The permanent integration of a wavefront sensor into a wafer inspection system is not feasible, primarily due to the high cost of such sensors.

[0011] SUMMARY OF THE INVENTION

[0012] The object of the invention is to provide a method for detecting the optical properties of a microscope, particularly suitable for the inspection of integrated circuits, which allows the optical state of the microscope to be easily detected in such a way that the detected state can be easily compared with another state of the microscope. The other state can be simulated or detected at a different time.

[0013] According to the invention, this object is achieved by a method having the following steps: a) providing a microscope, in particular a UV high-performance microscope for inspecting integrated circuits, which has an image sensor and imaging optics that contains a plurality of optical elements and is configured to image an object plane onto the image sensor; b) arranging at least one structure in the object plane; c) recording an image 1 -1 of the structure, wherein the structure is sharply imaged onto the image sensor; and d) recording an image 1 -2 of the structure, wherein the structure is blurred onto the image sensor.

[0014] The inventor recognized that the optical properties of a microscope can be very well captured by taking sharp and blurred images of a test structure. The blurrier the captured image, the more closely it approximates the intensity distribution in the pupil plane. This intensity distribution contains information about the angular distribution of light in the field planes. A synopsis of sharp and blurred images can thus yield a wealth of important information about the optical properties of the microscope. This information becomes particularly meaningful when the captured state is compared with another state of the microscope. The other state can be simulated or captured at a different time.If, for example, the simulated state is the state determined by the optical design of the microscope, imaging errors caused by manufacturing or aging can be detected.

[0015] It goes without saying that the two steps c) and d) can be interchanged.

[0016] Preferably, in step b), several structures are arranged in the object plane, spaced far enough apart that their partial images captured in steps c) and d) do not overlap on the image sensor. This allows the desired information to be captured at multiple field points. The more structures arranged on the test object and the more widely the structures are distributed across the entire imageable object field, the more complete the capture of the microscope's optical properties.

[0017] Furthermore, it is preferable to repeat step d) with a different defocus setting. This repetition can be performed just once or multiple times, resulting in a focus scale from which even more information about the optical properties can be derived.

[0018] A similar situation applies if step d) is repeated with a different illumination angle distribution. This repetition can be performed just once or multiple times, resulting in a graduated illumination angle distribution from which even more information about the optical properties can be derived. The illumination systems of high-quality UV high-performance microscopes usually allow the illumination angle distribution to be specifically adjusted within certain limits.

[0019] A further object of the invention is to provide a method by which the image quality of a microscope suitable in particular for the inspection of integrated circuits can be quickly restored after an optical element has been replaced or after a (possibly suspected) deterioration in image quality.

[0020] According to the invention, this object is achieved by a method having the following steps: aO) detecting the optical properties of the microscope by carrying out the steps a) to d) explained above; e) arranging the structure in the object plane at a later time; f) taking an image 2-1 of the structure, wherein the structure is imaged sharply onto the image sensor; g) comparing the images 1-1 and 2-1 in order to detect at least one first imaging error; h) carrying out first measures for correcting the at least one first imaging error detected in step g); i) taking an image 2-2 of the structure, wherein the arrangement is imaged blurred onto the image sensor; j) comparing the images 2-1 and 2-2 in order to detect at least one second imaging error; and k) carrying out second measures for correcting the at least one second imaging error detected in step j).

[0021] The invention is based on the realization that even without a wavefront sensor and with the usually already existing equipment of a high-performance microscope, namely an image sensor and manipulators for adjusting optical elements of the imaging optics, the image quality can be measured and, if necessary, improved by adjustment. This is achieved through a two-stage measuring and adjustment process, in which firstly dominant imaging errors (usually low-order aberrations) are measured with the image in focus and corrected if necessary. Subsequently, residual imaging errors (usually higher-order aberrations) are measured with the image in defocus and corrected if necessary. The measurement is carried out by comparing images of a sample, i.e.an array of one or more light-emitting dots or other structures, before the deterioration of image quality, with images of the pattern taken after the actual or assumed deterioration, once with the image in focus and once with the image out of focus. It has been shown that such a two-stage measurement and adjustment process can be used to very effectively measure and restore image quality.

[0022] In practice, the microscope is preferably measured at a first time shortly after initial adjustment and commissioning in the manner described above. This produces images taken with a focused image and images taken with a defocused image. This measurement is then repeated at a later second time. There may be a time between the two times when an optical element of the imaging optics is replaced, for example. However, the second time can also be specified by regularly scheduled monitoring. One option, for example, is to perform new measurements at fixed intervals, e.g. every 100 hours of operation or every 30 days. In this way, drift phenomena that could lead to a deterioration in the imaging properties can be detected early enough to prevent the microscope from falling out of specification and becoming unusable.Since the measurement only requires the use of a special object and the taking of two shots with different focuses, the downtimes are correspondingly short.

[0023] The at least one structure arranged in the object plane in step b) only needs to have the property that it can be imaged onto the image sensor with sufficient intensity and correspondingly high contrast. The structure can, for example, be self-luminous and formed by a miniature light source. Simple microscopes usually have an illumination unit with which objects are illuminated. In this case, the structure can be an opening in a pinhole.

[0024] Microscopes used to inspect exposed wafers are often equipped with an illumination unit that directs short-wavelength light as incident light (i.e. from the side of the microscope) onto the wafer. In most cases, it is possible to switch between bright-field and dark-field illumination by modifying the direction and / or angular distribution of the light emitted by the illumination unit. In this case, the structure can be reflected or scattered against an absorbing environment. In concrete terms, this can mean that in steps b) and e) a special wafer is used as the object to be imaged. This wafer has an absorbing coating that is only removed in the area of ​​the desired structure (e.g. a single dot or a reticle), so that incident light there is reflected or scattered.

[0025] Alternatively, a pinhole can be applied to a wafer. This corresponds to the solution described above, except that the permanently applied coating is replaced by a pinhole detachably applied to the wafer.

[0026] Preferably, in steps b) and e), not just a single structure but a plurality of spaced-apart structures are arranged in the object plane, so that the aberrations can be measured at multiple points distributed across the field. In this case, it is only necessary to ensure that the structures are sufficiently far apart that their blurred partial images captured in steps d) and i) do not overlap on the image sensor. Otherwise, it becomes difficult to reliably determine the image quality field-dependently by comparing images 1-2 and 2-2.

[0027] To create a blurred image in steps d) and i), an optical element of the imaging optics can be moved parallel to an optical axis of the imaging optics. In high-performance microscopes for wafer inspection, additional actuators are generally not required for this purpose, as the existing manipulators intended for adjustment purposes generally allow for sufficiently large travel distances.

[0028] The defocusing should be as strong as possible, as this is the only way to obtain sufficient information about the pupil. Ideally, the pupil is projected directly onto the image sensor, which is not achievable, but also not required. If the defocusing achieved with a single movement is insufficient, more than one optical element can be moved. If the travel distances possible with existing manipulators are too short, additional actuators must be provided.

[0029] Since the imaging optics are adjusted during microscope assembly and subsequently produce a nearly perfect image, any axial movement of an optical element with refractive power or a mirror with a curved surface causes defocusing. Preferably, the moved optical element is located in, or at least substantially in, a pupil plane of the imaging optics. This achieves field-independent defocusing, meaning that all points in the object plane are imaged with approximately the same degree of defocus.

[0030] Alternatively or additionally, in steps d) and i), an additional optical element can be introduced into the beam path of the imaging optics to create a blurred image. This element is not located in the beam path of the imaging optics in steps c) and f). In the simplest case, the additional optical element is a plane plate, which leads to a uniform axial displacement of the image plane. Other options for defocusing include axial displacement of the image sensor or the object plane.

[0031] Preferably, steps i) and j), and possibly also k), are repeated at least once with a different defocus setting. This yields additional images that can be mathematically compared to measure image quality. One particularly suitable approach is to create a type of focus echelon in which the defocus is changed at short intervals, and an image of the arrangement is captured each time.

[0032] It is also possible to perform steps i) and j), and possibly also k), at least once with a different illumination angle distribution. Images generated with different illumination angle distributions can also provide valuable information that can be used in the comparison in step j) to determine image quality.

[0033] The first corrective measures to be performed in step h) may, in particular, comprise the displacement of an optical element in a direction perpendicular to an optical axis of the imaging optics. This allows, in particular, the correction of misalignment (i.e., distortion). Low-order coma, which can be recognized in the images as a coma tail, can generally be corrected by combining displacements of several optical elements in different directions.

[0034] The second measures for correcting the residual errors, which are performed in step k), can also comprise the displacement of several optical elements in a direction perpendicular to an optical axis of the imaging optics and / or the tilting of at least one optical element, as is known per se in the prior art. More complex manipulators bend mirror surfaces or alter the wavefront through heat-induced changes in the refractive index.

[0035] If, as described above, a later condition of the microscope is compared with a delivery condition adjusted after initial assembly, subsequent corrective measures cannot result in a condition that is better than the delivery condition. The adjustment brought about by the corrective measures thus results in a condition that, while within the specifications, may not be optimal.

[0036] Imaging errors caused by manufacturing or aging can be measured and corrected particularly well if, instead of comparing two real states of the microscope, a measured real state is compared with a simulated state of the microscope, whereby the simulated state corresponds to the target properties of the microscope as it results from the optical design.

[0037] In this variant, images 1-1 and 1-2 are not captured using the image sensor, but are simulated based on the microscope's design data. The blur is preferably simulated in the same way as it will later be generated in reality, e.g., by simulated z-displacement of the object with the structures to be imaged.

[0038] Such a method for restoring the imaging quality of a microscope then comprises the following steps: a) Providing a microscope having an image sensor and imaging optics containing a plurality of optical elements and configured to image an object plane onto the image sensor; b) Simulating an image 1-1 of the hypothetical structure arranged in the object plane on the image sensor, based on the microscope's design data and a sharp image; c) Simulating an image 1-2 of the hypothetical structure arranged in the object plane on the image sensor, based on the microscope's design data and a blurred image; d) Arranging a real structure corresponding to the hypothetical structure in the object plane; e) Capturing an image 2-1 of the real structure, with the real structure being sharply imaged onto the image sensor;f) comparing images 1-1 and 2-1 in order to detect at least one first imaging error; g) performing first measures to correct the at least one first imaging error detected in step f); h) capturing an image 2-2 of the real structure, wherein the real structure is imaged blurred onto the image sensor; i) comparing images 2-1 and 2-2 in order to detect at least one second imaging error; and j) performing second measures to correct the at least one second imaging error detected in step i).

[0039] The object of the invention is further to provide a microscope suitable for carrying out the restoration process.

[0040] This object is achieved according to the invention by a microscope with an image sensor and imaging optics that contain a plurality of optical elements and are configured to image an object plane onto the image sensor, and with a programmed control device that is configured to trigger the following steps: a) Simulating an image 1-1 of a hypothetical structure arranged in the object plane on the image sensor, based on the design data of the microscope and a sharp image; b) Simulating an image 1-2 of the hypothetical structure arranged in the object plane on the image sensor, based on the design data of the microscope and a blurred image; c) Arranging a real structure that corresponds to the hypothetical structure in the object plane; d) Recording an image 2-1 of the real structure, wherein the real structure is sharply imaged onto the image sensor;e) comparing images 1-1 and 2-1 in order to detect at least one first imaging error; f) performing first measures to correct the at least one first imaging error detected in step e); g) capturing an image 2-2 of the real structure, wherein the real structure is imaged blurred onto the image sensor; h) comparing images 2-1 and 2-2 in order to detect at least one second imaging error; and i) performing second measures to correct the at least one second imaging error detected in step h).

[0041] The advantageous embodiments mentioned above for the method apply accordingly to the microscope.

[0042] BRIEF DESCRIPTION OF THE DRAWINGS

[0043] In the following, exemplary embodiments of the invention are explained in more detail with reference to the drawings. In these drawings:

[0044] Figure 1 shows a wafer inspection device with a microscope according to the invention according to an embodiment in a simplified perspective view;

[0045] Figure 2a shows the wafer inspection device in a schematic meridional section without a control device and display device, in which a plane plate is located in an inactive position outside the beam path; Figure 2b shows the wafer inspection device in a representation similar to Figure 2a, in which the plane plate is located in an active position in the beam path to produce a defocused image;

[0046] Figure 2c shows the wafer inspection device in a representation similar to Figure 2a, in which a lens has been moved perpendicular to the optical axis for the purpose of a rough correction;

[0047] Figure 3 is a flow chart to explain the method according to the invention;

[0048] Figure 4 is a plan view of an array of reflective points projected onto the image sensor for detecting aberrations;

[0049] Figure 5 shows an example image 1 -1 of a point taken with the adjusted microscope with a sharp image;

[0050] Figure 6 shows an example image 1 -2 of a point taken with the adjusted microscope with a blurred image;

[0051] Figure 7 shows an example image 2-1 of a point taken with the unadjusted microscope with a sharp image;

[0052] Figure 8 shows an example image of a point taken with the coarsely corrected microscope with sharp imaging; and

[0053] Figure 9 shows an example image 2-2 of a point taken with the coarsely corrected microscope with a blurred image.

[0054] DESCRIPTION OF PREFERRED EMBODIMENTS

[0055] 1. Structure of the wafer inspection device

[0056] Figure 1 shows a simplified perspective view of a wafer inspection device 10 with which integrated circuits 12 that were manufactured together on a wafer 14 can be inspected.

[0057] In the illustrated embodiment, the wafer inspection device 10 comprises a travel stage 16, which has a base 18 and a wafer holder 20 movable along the x- and y-directions relative to the base 18. The wafer holder 20 supports the wafer 14 with the integrated circuits 12 to be inspected. The wafer inspection device 10 further comprises a microscope 22, a control device 24, embodied here as a PC, and a display device 26, which may be, for example, a computer monitor connected to the PC. The enlarged image of an integrated circuit 12 or a portion thereof captured by the microscope 22 can be displayed on the display device 26.

[0058] Figure 2a shows the wafer inspection device 10 in a schematic meridional section. The control device 24 and the display device 26 are not shown again for the sake of simplicity.

[0059] The microscope 22 has a housing 28 and an imaging optics system 30 enclosed therein, which in the illustrated embodiment comprises three lenses L1, L2, and L3. The lens L1 images the surface of the wafer 14, which is arranged in the object plane 32 of the imaging optics system 30, into an intermediate image plane 34. The lenses L2 and L3 image the intermediate image of the wafer surface onto an image sensor 36, which is arranged in the image plane of the imaging optics system 30. The image sensor 36 captures the magnified image of the wafer surface and converts it into electrical image signals, which are processed by the control device 24 and converted into signals with which the display device 26 is controlled such that the desired section of the wafer surface can be viewed by an operator.

[0060] For simplicity, the imaging optics 30 are shown here with only three lenses L1 through L3. High-performance microscopes for wafer inspection devices 10 typically include considerably more lenses, curved mirrors, and / or other optical elements, as is known in the art.

[0061] The microscope 22 also has an illumination unit 38, which includes a UV light source 40 and illumination optics 42, which are indicated here only by a single lens. The illumination optics 42 are adjustable so that the UV light can be directed onto the wafer 14 with different angular distributions and from different directions. The UV light generated by the light source 40 is introduced into the beam path via an inclined coupling mirror 44 and strikes the wafer 14 from above, illuminating it with incident light.

[0062] Lenses L2 and L3 are equipped with manipulators 46-2 and 46-3, which allow the lenses L2 and L3 to be precisely moved in all three spatial directions x, y, and z. Manipulators 46-2 and 46-3 contain actuators controlled by control device 24.

[0063] The microscope 22 also includes a revolver holder 48, with which a light-transmitting plane plate 50 can be introduced into the beam path if necessary such that the parallel surfaces of the plane plate 50 are aligned perpendicular to the optical axis 52 of the microscope 22. For this purpose, the revolver holder 48 has a receptacle 54 for the plane plate 50 and a rotation axis 52, relative to which the receptacle 54 is rotatably mounted. By rotating the receptacle by 180°, caused by an actuator (not shown), the plane plate 50 can be moved from the inactive position shown in Figure 2a to an active position shown in Figure 2b, in which the plane plate 50 is located in the beam path.

[0064] 2. Procedure for restoring image quality

[0065] During operation of the inspection device 10, it may occur that individual lenses L1, L2, or L3 need to be replaced after some time. This can be due to defects or signs of aging, but also due to the desire to be able to adapt to specific inspection tasks. After replacement, the imaging optics 30 must be readjusted. The same applies if a check is to be performed regularly or if there is a noticeable deterioration in the imaging quality of the microscope 22 to determine whether the microscope still meets the specifications regarding imaging quality.

[0066] In the following, with reference to the flow chart in Figure 3, a method is described how the imaging quality of the microscope 22 can be restored in such constellations without having to install an expensive interferometric wavefront sensor.

[0067] The microscope 22 provided in a first step S1 is first measured at a first point in time at which the imaging quality of the microscope 22 meets the specifications. This point in time can be, for example, immediately after the initial adjustment of the imaging optics 30, which takes place during the manufacture of the microscope 22. During this measurement of the imaging quality, a structure is arranged in the object plane of the imaging optics 30 in a second step S2. An image of this structure is recorded by the image sensor 36 in a third step S3, wherein the structure is sharply imaged onto the image sensor 36, as is the case with the configuration shown in Figure 2a. Preferably, not just a single structure, but several structures are imaged. The structure can, for example, beThese can be reflective or scattering points 60, which can be arranged in a regular grid and are surrounded by a light-absorbing layer 62, as shown in a plan view in Figure 4. The points 60 are distributed over the entire imageable object field of the imaging optics 30. To produce the points 60, a wafer can, for example, be provided with a light-absorbing layer, with the desired arrangement of points 60 being defined lithographically in a small area of ​​the wafer. In a further process step, the coating is locally etched away, thereby exposing the partially reflective wafer surface in the area of ​​the points 60. If the UV light generated by the light source 40 strikes the wafer surface at the exposed points 60, it is partially reflected.The imaging optics 30 generates an image of the points 60 on the image sensor 36, as illustrated by the two beam paths shown in Figure 2a for an axial point 60a and an off-axis point 60b.

[0068] The image of the points 60 thus captured by the image sensor 36 and subsequently designated 1-1 is stored by the control device 26 and is required later, in a manner to be explained, to determine possible imaging errors of the imaging optics 30. Figure 5 shows a section of the image 1-1 in which the image of a single point 60 can be seen.

[0069] In a fourth step S4, which can also be carried out before the third step S3, another image of the points 60 is recorded, which is referred to below as image 1-2. However, for the recording of image 1-2, the points 60 are imaged out of focus onto the image sensor 36. A blurred image can be achieved by axially displacing the object plane, i.e. the traversing stage 16, and / or the image sensor 36 and / or one of the manipulable lenses L2, L3. If lenses are displaced, they should preferably be arranged in or near one of the two pupil planes P1, P2, because the defocusing is then field-independent. In the illustrated embodiment, for the purpose of defocusing, the plane plate 50 is rotated into the beam path with the aid of the revolver holder 48, as shown in Figure 2b.Inserting the plane plate 50 into the beam path causes the image plane to be axially displaced backward and thus move out of the plane of the image sensor 36. This results in a blurred image of the points 60 on the image sensor 36.

[0070] An example of a blurred image 1-2 of a single point captured by the image sensor 36 is shown in Figure 6. Different gray levels represent different intensities. Ideally, a so-called top-hat distribution results, in which the intensity is constant up to a limiting radius and then abruptly drops to zero. As can be seen, the defocused image 1-2 of a point 60 is not perfect, but it is still considered to be within the specifications of the microscope 22.

[0071] The image 1-2 taken in step S4 is also stored by the control device 24.

[0072] It is now assumed that a new measurement and, if necessary, readjustment of the imaging optics 30 is to be carried out for one of the reasons mentioned above (e.g. replacement of one of the lenses L1 to L3) in order to restore the original imaging quality.

[0073] For this purpose, in a fifth step S5, the same structure (i.e., points 60) from which images 1-1 and 1-2 were taken is again arranged in the object plane of the imaging optics 30. In a sixth step S6, a sharp image 2-1 of points 60 is acquired, as shown by way of example in Figure 7. It can be seen that the image of the point has a offset, i.e., it is laterally offset, which leads to distortion across the field. A coma tail 64 is also visible.

[0074] In order to detect one or more first aberrations, the image 2-1 recorded in the sixth step S6 is compared in a seventh step S7 with the image 1-1 serving as a reference. For this purpose, a simple difference calculation may be sufficient, from which aberrations such as offset and distortion can be easily derived. In an eighth step S8, initial measures are then carried out to at least roughly correct the first aberrations detected in step S7. Offset and low-order coma can often be corrected, for example, by moving one or more lenses L1 to L3. In the illustrated embodiment, the control device calculates, based on algorithms known per se, how the lenses L2 and L3 must be moved by the manipulators 46-2 and 46-3, respectively, in order to effect a rough correction of the first aberrations.A movement of lens L2 parallel to the x-direction is indicated schematically and greatly exaggerated in Figure 2c. Within the scope of this rough correction, an iterative process can also take place, in which the result of a first correction step is checked by taking another image, and if necessary, further corrective measures are carried out using manipulators 46-2, 46-3.

[0075] If, as assumed in the illustrated embodiment, the aberrations are determined not only at one point but at several points 60, a field-dependent correction requirement generally arises. The known correction algorithms then resort to optimization methods that can significantly reduce the aberrations averaged across the field.

[0076] Figure 8 shows an image taken after the coarse correction of a single point 60. It can be seen that the offset and the low-order coma were successfully corrected. However, as a comparison with image 1-1 shown in Figure 5 shows, the image of point 60 is still slightly broadened due to residual errors.

[0077] After completing the coarse correction, an image 2-2 of the points 60 is recorded in a ninth step S9, but again in the defocused mode shown in Figure 2b, in which the plane plate 50 is located in the beam path. Such an image 2-2 is shown as an example in Figure 9. The remaining second aberrations represent residual errors, which are determined in a tenth step S10 by comparison with the image 1-2 shown in Figure 6. For this purpose, for example, the difference image obtained from the two images 1-2 and 2-2 can be decomposed into Zernike polynomials by the control device 24, which enables direct assignment to aberrations and the use of known correction algorithms. The residual errors can be determined even more precisely if steps S9 and S10 are repeated one or more times with different defocusing and / or different illumination angle distributions.

[0078] The result of the correction algorithms is used by the control device 24 to perform second measures to correct the second aberrations in an eleventh step S11. For this purpose, the manipulators 46-2 and 46-3 are controlled again, which may also result in more complex movements or tilting of multiple lenses or other optical elements.

Claims

PATENT CLAIMS 1. A method for detecting optical properties of a microscope, comprising the following steps: a) providing a microscope (22) having an image sensor (36) and imaging optics (30) containing a plurality of optical elements (L1 to L3) and configured to image an object plane (32) onto the image sensor (36); b) arranging at least one structure (60; 60a, 60b) in the object plane (32); c) recording an image 1 -1 of the structure (60; 60a, 60b), wherein the structure is imaged sharply onto the image sensor (36); and d) recording an image 1 -2 of the structure (60; 60a, 60b), wherein the structure is imaged blurred onto the image sensor (36).

2. Method according to claim 1, wherein in step b) a plurality of structures (60; 60a, 60b) are arranged in the object plane (32) which are so far apart from each other that their partial images recorded in steps c) and d) do not overlap on the image sensor (36) 3. Method according to claim 1 or 2, wherein step d) is repeated with a differently set defocus.

4. Method according to one of the preceding claims, in which the microscope (22) has a reflected light illumination (38) for imaging reflective or scattering structures.

5. Method according to one of the preceding claims, in which steps c) and d) are repeated with a differently set illumination angle distribution.

6. A method for restoring the imaging quality of a microscope (22), comprising the following steps: aO) detecting the optical properties of the microscope (22) by performing steps a) to d) according to any one of the preceding claims; e) arranging the structure (60; 60a, 60b) in the object plane (32) at a later time; f) taking an image 2-1 of the structure (60; 60a, 60b), wherein the structure is imaged sharply onto the image sensor (36); g) comparing the images 1-1 and 2-1 in order to detect at least one first imaging error; h) carrying out first measures for correcting the at least one first imaging error detected in step g); i) taking an image 2-2 of the structure (60; 60a, 60b), wherein the structure is imaged blurred onto the image sensor (36); j) comparing the images 2-1 and 2-2 in order to detect at least one second imaging error; and k) carrying out second measures for correcting the at least one second imaging error detected in step j).

7. The method according to claim 6, wherein an optical element (L1 to L3) of the imaging optics (30) was replaced between steps d) and e).

8. Method according to one of claims 6 or 7 when referring back to claim 3, in which steps i) and j) are repeated with the differently set defocuses.

9. Method according to one of claims 6 to 9 when referring back to claim 5, in which steps i) and j) are repeated with the differently adjusted illumination angle distributions.

10. Method according to one of claims 6 to 9, wherein the first measures comprise the displacement of an optical element (L1 to L3) in a direction (x, y) perpendicular to an optical axis (52) of the imaging optics (30).

11. Method according to one of claims 6 to 10, wherein the second measures comprise the displacement of a plurality of optical elements (L1 to L3) in a direction perpendicular (x, y) to an optical axis (52) of the imaging optics (30) and / or the tilting of at least one optical element (L1 to L3).

12. A method for restoring the imaging quality of a microscope (22), comprising the following steps: a) providing a microscope (22) having an image sensor (36) and imaging optics (30) containing a plurality of optical elements (L1 to L3) and configured to image an object plane (32) onto the image sensor (36); b) simulating an image 1-1 of the hypothetical structure (60; 60a, 60b) arranged in the object plane (32) on the image sensor, using the design data of the microscope and a sharp image as a basis; c) simulating an image 1-2 of the hypothetical structure (60; 60a, 60b) arranged in the object plane (32) on the image sensor, using the design data of the microscope and a blurred image as a basis; d) arranging a real structure (60; 60a, 60b) corresponding to the hypothetical structure in the object plane (32); e) taking an image 2-1 of the real structure (60;60a, 60b), wherein the real structure is imaged sharply onto the image sensor (36); f) comparing images 1-1 and 2-1 to detect at least one first imaging error; g) carrying out first measures to correct the at least one first imaging error detected in step f); h) taking an image 2-2 of the real structure (60; 60a, 60b), wherein the real structure is imaged blurred onto the image sensor (36); i) comparing images 2-1 and 2-2 to detect at least one second imaging error; and; j) carrying out second measures to correct the at least one second imaging error detected in step i).

13. A microscope (22) with an image sensor (36) and with imaging optics (30) containing a plurality of optical elements (L1 to L3) and configured to image an object plane (32) onto the image sensor (36), and with a programmed control device (24) configured to trigger the following steps: a) simulating an image 1-1 of a hypothetical structure (60; 60a, 60b) arranged in the object plane (32) on the image sensor, using the design data of the microscope and a sharp image as a basis; b) simulating an image 1-2 of the hypothetical structure (60; 60a, 60b) arranged in the object plane (32) on the image sensor, using the design data of the microscope and a blurred image as a basis; c) arranging a real structure (60; 60a, 60b) corresponding to the hypothetical structure in the object plane (32); d) taking an image 2-1 of the real structure (60;60a, 60b), wherein the real structure is imaged sharply onto the image sensor (36); e) comparing images 1-1 and 2-1 to detect at least one first imaging error; f) performing first measures to correct the at least one first imaging error detected in step e); g) recording an image 2-2 of the real structure (60; 60a, 60b), wherein the real structure is imaged blurred onto the image sensor (36); h) comparing images 2-1 and 2-2 to detect at least one second imaging error; and i) performing second measures to correct the at least one second imaging error detected in step h).

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