Method of operating a dual beam device and corresponding apparatus

Abstract

A method of operating a charged particle beam device is provided. For adjusting a sample with respect to an intersection point of two beams of the dual beam device, a position for a reference point and topography information of the sample is used.

Description

[0001] METHOD OF OPERATING A DUAL BEAM DEVICE AND CORRESPONDING APPARATUS

[0002] TECHNICAL FIELD

[0003] The present application relates to methods of operating a dual beam device, for example for the inspection of samples like semiconductor samples or semiconductor wafers, and to corresponding apparatuses.

[0004] BACKGROUND

[0005] Semiconductor structures are amongst the finest man-made structures. Semiconductor manufacturing involves precise manipulation, e.g., lithography or etching, of materials such as silicon or oxide at very fine scales in the range of nm. A wafer made of a thin slice of silicon serves as the substrate for microelectronic devices containing semiconductor structures built in and upon the wafer. The semiconductor structures are constructed layer by layer using repeated processing steps that involve repeated chemical, mechanical, thermal and optical processes. Dimensions, shapes and placements of the semiconductor structures and patters are subject to several influences. For example, during the manufacturing of 3D-memory devices, the critical processes are currently etching and deposition. Other involved process steps such as the lithography exposure or implantation also can have an impact on the properties of the elements of the integrated circuits. Therefore, fabricated semiconductor structures suffer from rare and different imperfections. Devices for quantitative metrology, defect-detection or defect review are looking for these imperfections. These devices are not only required during Wafer fabrication. As this fabrication process is complicated and highly non-linear, optimization of production process parameters is difficult. As a remedy, an iteration scheme called process window qualification (PWQ) can be applied. In each iteration a test wafer is manufactured based on the currently best process parameters, with different dies of the wafer being exposed to different manufacturing conditions. By detecting and analyzing the test structures with devices for quantitative metrology and defect-detection, the best manufacturing process parameters can be selected. In this way, production process parameters can be tweaked towards optimality. Afterwards, a highly accurate quality control process and device for the metrology semiconductor structures in wafers is required.

[0006] Fabricated semiconductor structures are based on prior knowledge. The semiconductor structures are manufactured from a sequence of layers being parallel to a substrate. For example, in a logic type sample, metal lines are running parallel in metal layers or HAR (high aspect ratio) structures and metal vias run perpendicular to the metal layers. The angle between metal lines in different layers is either 0° or 90°. On the other hand, for 3D NAND type structures it is known that their cross-sections are circular on average. Furthermore, a semiconductor wafer has a diameter of 300 mm and consist of a plurality of several sites, so called dies, each comprising at least one integrated circuit pattern such as for example for a memory chip or for a processor chip. During fabrication, semiconductor wafers run through about 1000 process steps, and within the semiconductor wafer, about 100 and more parallel layers are formed, comprising the transistor layers, the layers of the middle of the line, and the interconnect layers and, in memory devices, a plurality of 3D arrays of memory cells.

[0007] The aspect ratio and the number of layers of integrated circuits constantly increases and the structures are growing into 3rd (vertical) dimension. The current height of the memory stacks is exceeding a dozen of microns. In contrast, the features size is becoming smaller. The minimum feature size or critical dimension is below 10nm, for example 7nm or 5nm, and is approaching feature sizes below 3 nm in near future. While the complexity and dimensions of the semiconductor structures are growing into the 3rd dimension, the lateral dimensions of integrated semiconductor structures are becoming smaller. Therefore, measuring the shape, dimensions and orientation of the features and patterns in 3D and their overlay with high precision becomes challenging. The lateral measurement resolution of charged particle systems is typically limited by the sampling raster of individual image points or dwell times per pixel on the sample, and the charged particle beam diameter. The sampling raster resolution can be set within the imaging system and can be adapted to the charged particle beam diameter on the sample. The typical raster resolution is 2nm or below, but the raster resolution limit can be reduced with no physical limitation. The charged particle beam diameter has a limited dimension, which depends on the charged particle beam operation conditions and lens. The beam resolution is limited by approximately half of the beam diameter. The lateral resolution can be below 2nm, for example even below 1nm. While this is an example of samples which may be analyzed or treated with techniques discussed herein, the techniques are also applicable to other samples than HAR structures.

[0008] A common way to generate 3D tomographic data from semiconductor samples on nm scale is the so-called slice and image approach obtained for example by a dual beam device. A slice- and image approach is described in WO 20201244795 A1. According to the method of the WO 20201244795 A1 , a 3D volume inspection is obtained at an inspection sample extracted from a semiconductor wafer. In another example, the slice and image method is applied under a slanted angle into the surface of a semiconductor wafer, as described in WO 2021 1 180600 A1. According to this method, a 3D volume image of an inspection volume is obtained by slicing and imaging a plurality of crosssection surfaces within the inspection volume. For a precise measurement, a large number N of cross-section surfaces in the inspection volume is generated, with the number N exceeding 100 or even more image slices. For example, in a volume with a lateral dimension of 5pm and a slicing distance of 5nm, 1000 slices are milled and imaged.

[0009] Such dual beam devices are generally adjusted such that the two beams, for example a focused ion beam (FIB) and a charged particle beam (CPB) like an electron beam for images intersect each other in an intersection point. For each site of the wafer to be examined, a corresponding point of the site on the surface of the sample is adjusted to be at the intersection point, and then the slicing and imaging is started. This correct setting is an important prerequisite for observation of for example an FIB milling process with the CPB imaging system.

[0010] In conventional approaches, the placing of the corresponding point of the sample at the position of the intersection point is performed by acquiring a charged particle beam image and a perspective corrected FIB image, registration of the two images, and moving the sample up and down according to some algorithm until the centers of both images coincide along an axis. This is a rather slow step which must be repeated at each sample site to be examined, unless the sample is an absolutely plane sample, which with real samples is seldom the case.

[0011] Therefore, there is a need for a faster adjustment of samples at different sample positions to be treated by a dual beam device. When a high number of sites is to be examined on the sample, as common for complex semiconductor structures or large wafers, this process is time consuming. Therefore, a need exists for faster possibilities for adjusting the sample correctly.

[0012] SUMMARY

[0013] According to some embodiments, a method and a dual beam device as defined in the independent claims are provided. The dependent claims define further embodiments.

[0014] The disclosure provides a method for adjusting a sample such that a certain point of the sample coincides with an intersection point of a dual beam device. This may then be used for an inspection as discussed in the background portion or for other kinds treating, processing or inspecting the sample.

[0015] According to an embodiment, a method for operating a dual beam device is provided, where a first charged particle beam is emitted to intersect a second charged particle beam in an intersection point, the method comprising: adjusting a position of a sample such that a reference point of a surface of the semiconductor sample coincides with the intersection point, and for a further point of the sample, adjusting the position of the sample based on the position of the sample at the reference point and topography information of the sample.

[0016] The adjusting of the position of the sample for the reference point may be made in a conventional manner, for example as described in the introductory portion using an image for the first charged particle beam and an image for the second charged particle beam and registering the two images. However, for the further point, the position at this reference point and topography information are used, i.e. information which describes the topography of the surface, in particular the height, of the sample. In this way, the adjusting for the further point may be performed automatically, in particular without action by an operator.

[0017] In some embodiments, for example, then the position of the sample in the further point may be adjusted in a direction essentially perpendicular to the surface of the sample based on a height difference between the sample at the reference point and the sample at the further point. This may be performed for a plurality of further points. Both at the reference point and at the further point (or the plurality of further points) then a corresponding treating of the sample, for example an inspection as described in the introductory portion, may be performed.

[0018] This may allow for a faster adjustment of the sample than using the conventional adjustment method described in the background for each point.

[0019] Essentially perpendicular to the surface may mean perpendicular to the surface (within tolerances of the dual beam device used) if the sample was a flat sample.

[0020] That the reference point or also a further point coincides with the intersection point is to be understood as coinciding within an adjustment precision of the dual beam device, i.e. some tolerances are possible. The adjustment of the further point may in particular be such that the position of the further point after the adjustment also coincides with the intersection point.

[0021] Regarding the tolerances mentioned above, in a dual-beam system there are both mechanical and electro-optical tolerances, with mechanical tolerances being usually larger. The adjustment may include a mechanical adjustment e.g. using a mechanical stage, an electro-optical adjustment using optics of beam systems for the dual beams, or both. For example, a first an adjustment with a mechanical stage may be performed until the two beams are closer apart than the mechanical tolerances. Then a final adjustment by beam shift of the two beams may be applied as static bias until the beams intersect in the intersection point within the electro-optical tolerances, usually better than 10 nm. This may in particular be perfomed during the adjustment for the reference point above.

[0022] In some embodiments, beam shifts in the plane formed by the nominal axes of the two beams can be used to compensate mechanical tolerances of the adjustment of the sample position by mechanical means like a stage, while beam shifts perpendicular to that plane are used to compensate for mechanical tolerances of the two beams, which could lead to a situation that the two beams are out of alignment and do not intersect in an intersection point. This adjustment by beam shift may be performed only for the reference point, or may also be performed also for the further points.

[0023] The sample may be a semiconductor sample, for example a semiconductor wafer. There are several ways how the topography information of the sample can be obtained. For example, the topography information may be obtained by measuring a surface topography of the sample in a profiling device for obtaining the topography. For example, e profilers for obtaining a topography are commercially available, which may be optical profilers (e.g. based on interferometry or focusing), tactile profilers, capacitive profilers or the like In this way, for example, while one sample is examined in the dual beam device, the topography information of a next sample to be treated in the dual beam device may be obtained in a profilometer in parallel, thus enhancing the overall throughput.

[0024] In other embodiments, the dual beam device itself may include the possibility to obtain the topography information. For example, the dual beam device may comprise a distance meter for measuring the distance between an emitter for one of the first charged particle beam or the second charged particle beam to the sample. Such a distance meter may be based on optical roundtrip times, or may include an autofocus device, for example for an imaging particle beam like an electron beam of a scanning electron microscope. In this way, a distance for the reference point may be obtained when adjusting the position of the sample such that the reference point coincides with the intersection point, and then a distance to the further point may be obtained when adjusting the further point, and the adjustment may be based on the distance difference.

[0025] In any case, the topography information may be stored in a memory, to be then used for the adjustment at the further point.ln this way, the topography information may be obtained in advance and then used for the adjustment.

[0026] The method may further comprise further adjusting the position for the further point based on comparison of a feature of an image captured by the dual beam device at the further point with an expected position of the feature.

[0027] The first charged particle beam may be an imaging beam of the dual beam device like a charged particle beam of a charged particle beam imaging system, for example an electron beam of a scanning electron microscope. The second charged particle beam may for example be a focused ion beam used for milling of the sample.

[0028] In another embodiment, a corresponding apparatus including a dual beam device or including a dual beam device and a profilometer may be provided. The above summary is merely intended to give a brief overview over some embodiments and is not to be construed as limiting.

[0029] BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Fig. 1 shows an example for a dual beam device in the form of a wafer inspection or metrology system for 3D volume inspection.

[0031] Fig. 2 is an illustration of a slice and image method of a volume inspection of a semiconductor wafer.

[0032] Fig. 3 is a flowchart illustrating a method according to an embodiment.

[0033] Figs. 4A and 4B illustrate charged particle beams of the dual beam device of Fig. 1.

[0034] Fig. 5 illustrates the target positioning of an intersection point of beams with respect to a sample.

[0035] Fig. 6 is a diagram illustrating some embodiments.

[0036] DETAILED DESCRIPTION

[0037] In the following, specific embodiments for illustrating techniques discussed herein will be discussed. These embodiments serve as examples only and are not to be construed as limiting. For example, while a dual beam device in the form of a wafer inspection device having a focus ion beam (FIB) and a charged particle beam (CPB) imaging system like a scanning electron microscope (SEM) will be used, also other dual beam devices, using first and second charged particle beams, may be used. Furthermore, while in embodiments discussed below a semiconductor wafer is used as an example for a sample, other kinds of samples, for example parts of (processed or unprocessed) semiconductor wafers or other kinds of samples, for example structures formed on glass substrates, may be used.

[0038] Throughout the figures and the description, same reference numerals are used to describe same or corresponding features or components. A coordinate system used herein is selected such that an ideal sample surface, for example a wafer surface 55 discussed below, coincides with the xy-plane, and a z direction is perpendicular thereto. “Ideal sample” refers to a sample which is perfectly flat, such that the surface indeed forms a plane. However, as will be discussed below, real samples exhibit some topography and are not exactly plane.

[0039] Variations, modifications or details described with respect to one of the embodiments of the figures are also applicable to other embodiments of figures and will therefore not be described repeatedly.

[0040] Recently, for the investigation of 3D inspection volumes in semiconductor wafers, a slice and imaging method has been proposed, which is applicable to inspection volumes inside a wafer. Thereby, a 3D volume image is generated at an inspection volume inside a wafer in the so called “wedge-cut” approach or wedge-cut geometry, without the need of a removal of a sample from the wafer. The slice and image method is applied to an inspection volume with dimensions of few pm, for example with a lateral extension of 5pm to 10pm in wafers with diameters of 200mm or 300mm. The lateral extension can also be larger and reach up to few 10ths of micrometers. A V-shaped groove or wedge is milled in the top surface of an integrated semiconductor wafer to make accessible a cross-section surface at an angle to the top surface. 3D volume images of inspection volumes are acquired at a limited number of inspection sites, for example representative sites of dies, for example at process control monitors (PCM), or at sites identified by other inspection tools. The slice and image method will destroy the wafer only locally, and other dies may still be used, or the wafer may still be used for further processing. The methods and inspection systems according to the 3D Volume image generation are described in more detail in WO 2021 1 180600 A1. An example of a wafer inspection system 1000 for 3D volume inspection is illustrated in Figure 1. The wafer inspection system 1000 is configured for a slice and imaging method under a wedge cut geometry with a dual beam device 1. For a wafer 8, several inspection sites, comprising inspection sites 6.1 and 6.2, are defined in a location map or inspection list generated from an inspection tool or from design information. The wafer 8 is placed on a wafer support table 15. The wafer support table 15 is mounted on a stage 155 with actuators and position control. Actuators and means for precision control for a wafer stage such as Laser interferometers are known in the art. A control unit 16 is configured to control the wafer stage 155 and to adjust an inspection site 6.1 of the wafer 8 at the intersection point 43 of the dual-beam device 1. The dual beam device 1 is comprising a FIB column 50 with a FIB optical axis 48 and a charged particle beam (CPB) imaging system 40 with optical axis 42. At the intersection point 43 of both optical axes of FIB and CPB imaging system, the wafer surface 55 is arranged at a slant angle GF to the FIB axis 48. FIB axis 48 and CPB imaging system axis 42 include an angle GFE, and the CPB imaging system axis forms an angle GE with the normal to the wafer surface 55 (assuming a plane surface). In the coordinate system of figure 1 , the normal to the wafer surface 55 is given by the z-axis. The focused ion beam (FIB) 51 is generated by the FIB-column 50 and is impinging under angle GF on the surface 55 of the wafer 8. Slanted cross-section surfaces are milled into the wafer by ion beam milling at the inspection site 6.1 under approximately the slant angle GF. In the example of figure 1, the slant angle GF is approximately 30°. The actual slant angle of the slanted cross-section surface can deviate from the slant angle GF by up to 1° to 4° due to the beam divergency of the focused ion beam, for example a Gallium-lon beam. With the charged particle beam imaging system 40, inclined under angle GE to the wafer normal, images of the milled surfaces are acquired. In the example of Figure 1, the angle GE is about 15°. However, other arrangements are possible as well, for example with GE = GF, such that the CPB imaging system axis 42 is perpendicular to the FIB axis 48, or GE = 0°, such that the CPB imaging system axis 42 is perpendicular to the wafer surface 55.

[0041] During imaging, a beam of charged particles 44 is scanned by a scanning unit of the charged particle beam imaging system 40 along a scan path over a cross-section surface of the wafer at inspection site 6.1 , and secondary particles as well as scattered particles are generated. Particle detector 17 collects at least some of the secondary particles and scattered particles and communicates the particle count with a control unit 19. Other detectors for other of interaction products may be present as well. Control unit 19 is in control of the charged particle beam imaging column 40, of FIB column 50 and connected to a control unit 16 to control the position of the wafer 8 mounted on the wafer support table 15 via the wafer stage 155. Control unit 19 communicates with operation control unit 2, which triggers placement and alignment for example of inspection site 6.1 of the wafer 8 at the intersection point 43 via wafer stage movement and triggers repeatedly operations of FIB milling, image acquisition and stage movements.

[0042] Each new intersection surface is milled by the FIB beam 51 , and imaged by the charged particle imaging beam 44, which is for example scanning electron beam or a Helium-lon- beam of a Helium ion microscope (HIM). In an example, the dual beam system comprises a first focused ion beam system 50 arranged at a first angle GF1 and a second focused ion column arranged at the second angle GF2, and the wafer is rotated between milling at the first angle GF1 and the second angle GF2, while imaging is performed by the imaging charged particle beam column 40, which is for example arranged perpendicular to the wafer surface 55.

[0043] While controllers 2, 16 and 19 are shown as separate blocks in Figure 1 , this is merely to illustrate the different control functionalities, and these controllers may be implemented as separate physical controllers, or two of all three of them may be implemented in a single physical controller (for example computer, microcontroller, etc.) Besides the functions described above, these controllers may also be used to implement the sample adjustment according to techniques discussed further below, or an additional controller may be provided to implement these adjustment functions.

[0044] Figure 2 illustrates the wedge cut geometry at the example of a 3D-memory stack. Figure 2 illustrates the situation, when the surface 52 is the new cross-section surface which was milled last by FIB 51. The cross-section surface 52 is scanned for example by SEM beam 44, which is in the example of Figure 2 arranged at normal incidence to the wafer surface 55, and a high-resolution cross-section image slice is generated. The cross-section surfaces 53.1...53.N are subsequently milled with a FIB beam 51 at an angle GF of approximately 30° to the wafer surface 9, but other angles GF, for example between GF = 20° and GF = 60° are possible as well. The cross-section image slice comprises first cross-section image features, formed by intersections with high aspect ratio (HAR) structures or vias (for example first cross-section image features of HAR-structures 4.1, 4.2, and 4.3) and second cross-section image features formed by intersections with layers L.1 ... L.M, which comprise for example SiO2, SiN- or Tungsten lines. Some of the lines are also called “word-lines”. The maximum number M of layers is typically more than 50, for example more than 100 or even more than 200. The HAR-structures and layers extend throughout most of the volume in the wafer but may comprise gaps. The HAR structures typically have diameters below 100nm, for example about 80nm, or for example 40nm. The cross-section image slices contain therefore first cross-section image features as intersections or cross-sections of the HAR structures at different depth (Z) at the respective xy-location. In case of vertical memory HAR structures of a cylindrical shape, the obtained first cross-sections image features are circular or elliptical structures at various depths determined by the locations of the structures on the sloped cross-section surface 52. The memory stack extends in the z-direction perpendicular to the wafer surface 55. The thickness d or minimum distances d between two adjacent cross-section image slices is adjusted to values typically in the order of few nm, for example 30nm, 20nm, 10nm, 5nm, 4nm or even less. Once a layer of material of predetermined thickness d is removed with FIB, a next cross-section surface 53. i... 53. J is exposed and accessible for imaging with the charged particle imaging beam 44. During repeated milling an imaging, a plurality of cross sections is formed and a plurality of cross section images are obtained, such that an inspection volume of size LX x LY x LZ is properly sampled and for example a 3D volume image can be generated. Thereby, the damage to the wafer is limited to the inspection volume plus a damaged volume in y-direction of length LYO. With an inspection depth LZ about 10pm, the additional damage volume in y-direction is typically limited to below 20pm.

[0045] In the following, scanning electron microscopy (SEM) will be used as an example for charged particle beam (CPB) imaging, and beam 44 will also be referred to as SEM beam.

[0046] In the process described above, the intersection point 43 is assumed to be on the surface 55 of wafer 8. For an ideally plane surface, the wafer 8 has to be adjusted in z-direction once such that the intersection point 43 coincides with the sample surface 55, and thereafter wafer 8 is moved in the xy plane. However, if a sample like wafer 8 is not flat, the adjustment has to be performed for each point or site to be examined, as described in the introductory portion.

[0047] Techniques discussed herein may offer a faster adjustment than the conventional approach of registering SEM images and FIB images for each point, as discussed in the introductory portion.

[0048] Fig. 3 illustrates a method according to an embodiment. As will be explained in more detail, the acts, steps and events shown in Fig. 3 are not necessarily performed in the order shown, but may for example also be performed in an interleaved or repetitive manner.

[0049] Generally, the method of Fig. 3 uses topography information, which may be stored in a memory provided in or connected to a controller, to adjust the sample position in a manner that for a point to be examined the intersection point between a first charged particle beam, for example SEM beam 44, and a second charged particle beam, for example FIB beam 51 , coincides with a sample surface also for an uneven topography of the sample. The general situation is illustrated in Figs. 4A and 4B.

[0050] Fig. 4A shows SEM beam 44 and FIB beam 51 intersecting in the intersection point 43 (within tolerances as explained above), in a z-y plane. An initial calibration of the dual beam device as shown is performed in a conventional manner, to ensure that the beams also intersect in the z-x plane, i.e. that Ax shown in Fig. 4B becomes zero.

[0051] For example, in some approaches during the initial calibration offsets between images taken by FIB beam 51 and SEM beam 44 in the x direction may be compensated at least to some extent by adjusting an inclination of FIB column 50, for example using adjustment screws. This typically allows an adjustment withing the pm range. Then, the beams may be shifted, e.g. by adjustment of FIB column 50 or by adjustment of SEM beam 44, to reach a coincidence in the x direction with a remaining tolerance in the order of e.g. 10- 20nm, which is acceptable for usual applications.

[0052] After this initial setup of the dual beam device, therefore the beams 44, 51 intersect at a z position z = z0..

[0053] The objective is to place a site to be examined, for example point 500 in Fig. 5, to essentially coincide with the intersection point 43 or, in other words, to place point 500 of wafer 8 to z = zO. In other words, after the initial calibration only an adjustment in the z direction is necessary.

[0054] As mentioned before, in case the sample is plane, this adjustment in the z direction may be made for one point only in the conventional manner described in the introductory portion, and then the sample is moved in the xy plane to examine other points. However, in practice, samples may have an uneven topography. For example, Fig. 6 shows the wafer 8 having some wafer bow, i.e. a dome like shape, which is a relevant practical example. Other topography issues may be caused by processing of the wafer. For example, such a dome like shape may be due to stress inherent in the wafer or due to stress caused by processing of the wafer, for example depositing layers having a lattice mismatch to the substrate.

[0055] Returning to Fig. 3, at 300 the method comprises obtaining topography information of the sample, for example height information for wafer 8 of Fig. 6 at a plurality of points or sites. The number of points for which the height information is obtained may depend on the accuracy required. For example, for highest accuracy, for every point to be examined, the height information is obtained by measurement. In other embodiments, height information may be obtained for a smaller number of points, and an interpolation may be made to obtain height information for other points not directly measured, for example by fitting a corresponding 2-dimensional manifold to the points measured. The point may be a point within a corresponding site to be examined, for example a center point of the respective site.

[0056] In some embodiments, the topography information may be obtained by a profilometer, which is shown as profiling device 61 in Fig. 1. In such a case, a wafer 8 may be measured in profiling device 61 to obtain a height map of the wafer, and then the wafer 8 may be processed in the dual beam device while a next wafer is measured in profiling device 61.

[0057] In other embodiments, obtaining the topography information may be performed directly within dual beam device 1. For example, charged particle beam (CPB) imaging system 40 may include an autofocus device 60 which may be used to perform a distance measurement to inspection site 6.1 , i.e. the point to be examined, for each point, such that a height map is obtained during the treating of wafer 8 by dual beam device 1. This is an example where step 300 is not performed in advance but interleaved with steps 301 and 302 explained below.

[0058] Besides an autofocus, also other distance measuring devices, for example optical devices based on interferometry or devices based on a roundtrip time of a emitted signal, may be used for distance measurement.

[0059] The two approaches, i.e. using profiling device 61 and device 60, may also be combined, and for example an average of the measurement may be performed, or one of the measurements may be used to check the validity of the other measurement. For example, if device 60 is an autofocus device, in some cases where there are no structures like edges visible an autofocus may have problems to determine the proper distance. If in such a case the distance determined by the autofocus differs significantly from the measurement by profiling device 61, the measurement may be repeated, or this particular point may be adjusted in a conventional manner. The topography information in this way may be obtained in advance prior to the further steps and stored in a memory as mentioned above, but may also be obtained automatically “on the fly” during or interleaved with the following steps.

[0060] At 301 of Fig. 3, the method comprises adjusting the sample position for a reference point, for example adjusting wafer 8 in the z-direction, such that the reference point coincides with the intersection point. In Fig. 6, for example, the height position in z direction for reference point 600 may be adjusted. This adjustment may be made in a conventional manner as explained in the introductory portion, i.e. by aligning an FIB image by beam 51 and an SEM image by beam 44. In another approach, a mark could be written with FIB beam 51 , and then this mark could be detected by SEM using SEM beam 44. Based on the detection, the mark can be centered in the SEM image. This approach may for example be used when the surface of the sample has no or only few features usable for image registration. The two approaches may also be combined, e.g. by using marks written by FIB beam 51 for image reg i strati on. A respective point (reference point or further point below) may be selected by moving the sample in the xy plane, for example by moving stage 155 in the xy plane.

[0061] At 302 of Fig. 3 for measurement at a further point 601 , the topography information is used. If the adjustment in the z-direction for example of wafer support table 15 as determined by the setting of stage 155 of Fig. 1 is ZR such that reference point 600 coincides with intersection point 43, the height of reference point 600 according to the topography information is h(xR,yR), wherein XR and yR are the coordinates of reference point 600 in the xy plane, and the height of further point 601 is h(x,y), where x,y are the coordinates of further point 601 in the xy plane, the z setting z, x, y for further point 601 to which the sample is adjusted is zxy= zR+(h(x,y) - h(xR,yR)).

[0062] This may be performed by a plurality of further points. In this way, the adjustment to one or more further points may be made faster than with the conventional approach where for each point (and not only for a reference point) an adjustment based on registering an SEM image and an FIB image is performed, and may be performed automatically. This applies regardless of whether the height h is obtained by topography information or by distance measuring using for example device 60. In case device 60 is used, a greater distance means a lower height in the above equation, such that it may also be written as zxy= zR-(d(xR,yR)-d(x,y)), where d(xR,yR) is the distance measured by device 60 for the reference point and d(x,y) is the distance measured at the further point. Note that as only height (or distance) differences are needed, the zero point (zero height or zero distance) may be set arbitrarily and is not needed for the calculation.

[0063] As mentioned above, while within tolerances some error in adjustment may happen, still in embodiments the focused ion beam 51 operates within the field of view of the SEM, which is sufficient.

[0064] In some embodiments, additionally, a fine correction may be performed based on images of the surfaces processed with the focus ion beam, i.e. images are captured as explained with reference to Fig. 2. This optional fine adjustment is performed at 303 in Fig. 3. When features are detected in the images such obtained and the position of such features deviates from expected positions (for example positions based on the design of the sample by more than a predefined threshold), this may indicate an error in positioning above an acceptable tolerance. In this case, the position may be aligned further until the found feature position matches with the expected feature position, and / or a “recalibration” may be performed, by choosing a new reference point and setting the z position for this new reference point in a conventional manner. For example, a SEM or other CPB image and a FIB image at the point may be generated for other purposes but then could be used to refine the adjustment beyond the accuracy of the forementioned method - especially if the method was applied to an interpolated height position rather than on an actually measured one.

Claims

Claims1. A method for operating a dual beam device, where a first charged particle beam (44) is emitted to intersect a second charged particle beam (51) in an intersection point (43), the method comprising: adjusting a position of a sample (8) such that a reference point (600) of a surface (55) of the sample (8) coincides with the intersection point (43), and for a further point (601) of the surface (55) of the sample (8), adjusting the position of the sample (8) based on the position of the sample (8) at the reference point (600) and topography information of the sample (8).

2. The method of claim 1 , wherein the adjusting for the further point is in a z-direction perpendicular to the surface (55), where the surface (55) essentially defines an xy- plane.

3. The method of claim 2, where the adjustment is based on a height difference between the reference point and the further point in the z-direction.

4. The method of any one of claims 1 to 3, wherein the adjusting for the further point is such that the further point essentially coincides with the intersection point.

5. The method of any one of claims 1 to 4, further comprising measuring the surface topography of the sample (8) in a profiling device (61 )to obtain the topography information.

6. The method of any one of claims 1 to 5, further comprising storing the surface topography of the sample in a memory, wherein the adjusting the position of the sample (8) based on the position of the sample (8) at the reference point (600) and topography information of the sample (8) is performed automatically based on the stored topography information.

7. The method of any one of claims 1 to 6, where the method further comprises measuring a distance of the surface (55) at the reference point (600) and the furtherpoint (601) to an emitter of one of the first charged particle beam (44) or second charged particle beam (51) to obtain the topography information.

8. The method of claim 7, where the measuring of the distance uses an autofocus device (60).

9. The method of any one of claims 1 to 8, wherein the sample (8) is a semiconductor sample.

10. The method of any one of claims 1 to 9, wherein the first charged particle beam (44) is a charged particle beam of an imaging system, and wherein the second charged particle beam (51) is a focused ion beam.

11. The method of any one of claims 1 to 10, further comprising further adjusting the position for the further point based on comparison of a feature of an image captured by the dual beam device at the further point with an expected position of the feature.

12. An apparatus, comprising: a dual beam device (1) configured to emit a first charged particle beam (44) to intersect a second charged particle beam (51) in an intersection point, an adjustable sample holder (55, 155), and a controller (2, 16, 19) configured to adjusti a position of a sample (8) such that a reference point (600) of a surface (55) of the sample (8) coincides with the intersection point (43), and for a further point (601) of the surface (55) of the sample (8), adjusting the position of the sample (8) based on the position of the sample (8) at the reference point (600) and topography information of the sample (8).

13. The apparatus of claim 12, further comprising a profiling device (61) for obtaining the topography information.

14. The apparatus of claim 12 or 13, further comprising a memory configured to store the surface topography of the sample, wherein the controller is configured to perform the adjusting the position of the sample (8) based on the position of the sample (8) at the reference point (600) and topography information of the sample (8) automatically based on the stored topography information.

15. The apparatus of any one of claims 12 to 14, further comprising a distance measuring device (60) in one of an emitter of the first charged particle beam (44) or second charged particle beam (51) to obtain the topography information.

16. The apparatus of any one of claims 12 to 15, configured to perform the method of any one of claims 1 to 11.

Images