FIELD

[0001] The present disclosure relates to a method for calibrating deflectors of a charged particle beam device. More particularly, embodiments described herein relate to distortion calibration of the charged particle beam device, for example of an electron beam system. Further, a charged particle beam device for imaging a specimen is described.

BACKGROUND

[0002] In many applications, it is beneficial to inspect a specimen such as a substrate with structures formed thereon, particularly electronic or optoelectronic structures formed thereon. For example, the specimen may be inspected to monitor the quality of the specimen, particularly to detect defects which may occur during the processing of the substrates, e.g. during structuring or coating of the substrates.

[0003] In some applications, thin layers are deposited on a substrate, e.g. on a glass substrate or circuit board substrate. The substrate is typically coated in a vacuum chamber of a coating apparatus, particularly using a vapor deposition technique. Over the last years, electronic devices and particularly opto-electronic devices have been manufactured with increasing structure density. For example, for thin film transistor (TFT) displays, a high density TFT integration is beneficial. In spite of the increased number and density of structures within a device, the yield is to be increased and the manufacturing costs are to be reduced further.

[0004] During the manufacturing of electronic or opto-electronic devices such as displays, deposited structures of a specimen can be imaged to monitor the quality of the specimen. Imaging of the specimen can, for example, be carried out using a charged particle beam device. Deflectors of the charged particle beam device are conventionally calibrated using a calibration target with an array of marks arranged at a known distance of several millimeters. However, conventional calibration targets often require expensive, special masks to be produced and may further require a special production process. Further, conventional calibration targets may not be readily available, may not be inserted into a system at all times, may require storage space and/or may not be very accurate. Further, calibration of the deflectors based on such conventional calibration targets may be slow and/or inaccurate.

[0005] Accordingly, given the demand for improved quality, review and testing of displays or other specimens, there is a need for an improved method for calibrating the deflectors of a charged particle beam device with high calibration accuracy and/or high calibration speed.

SUMMARY

[0006] According to aspects of the disclosure, methods for calibrating deflectors of a charged particle beam device as well as charged particle beam devices for imaging a specimen are provided. Further aspects, advantages, and beneficial features are apparent from the dependent claims, the description, and the accompanying drawings.

[0007] According to one aspect, a method for calibrating deflectors of a charged particle beam device is provided, the deflectors deflecting a charged particle beam of the charged particle beam device based on device coordinates of the charged particle beam device. The method includes placing a specimen on a stage in a vacuum chamber, the specimen providing a periodic pattern, the periodic pattern including unit cell vectors in specimen coordinates of the specimen. The method includes acquiring a first image of a first region of the periodic pattern using the charged particle beam device. The method includes determining at least one first local distortion parameter based on the first image. The method further includes acquiring a second image of a second region of the periodic pattern, the second region being remote from the first region. The method includes determining at least one second local distortion parameter based on the second image. The method further includes determining a first vector in specimen coordinates between the first region and the second region based on the at least one first local distortion parameter and based on the at least one second local distortion parameter. The method includes calibrating the deflectors based on the first vector.

[0008] According to another aspect, a charged particle beam device for imaging a specimen is provided. The charged particle beam device includes a stage for arranging the specimen to be imaged, and deflectors for deflecting a charged particle beam of the charged particle beam device. The charged particle beam device further includes a computer-readable medium containing a program for calibrating the deflectors, which, when executed by a processor, performs a method according to embodiments described herein.

[0009] According to a further aspect, a method for calibrating deflectors, particularly a subdeflector and/or a main deflector, of a charged particle beam device is provided, the deflectors deflecting a charged particle beam of the charged particle beam device based on device coordinates of the charged particle beam device. The method includes placing a specimen on a stage in a vacuum chamber, the specimen providing a periodic pattern, the periodic pattern including unit cell vectors in specimen coordinates of the specimen. The method includes acquiring a first image of a first region of the periodic pattern using the charged particle beam device. The method further includes determining at least one first local distortion parameter based on the first image, wherein the at least one first local distortion parameter is determined based on a Fourier transform of the first image. Determining the at least one first local distortion parameter may include distribution fitting of at least one portion of the first image in frequency space, particularly distribution fitting based on a Gaussian distribution. The deflectors may be calibrated based on the at least one first local distortion parameter.

[0010] Embodiments are also directed at apparatuses for carrying out the disclosed methods and include apparatus parts for performing each described method aspect. The method aspects may be performed by way of hardware components, a computer programmed by appropriate software, by any combination of the two or in any other manner. Furthermore, embodiments according to the disclosure are also directed at methods for operating the described apparatus and a method for manufacturing the apparatuses and devices described herein. The methods for operating the described apparatus include method aspects for carrying out every function of the apparatus. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] A full and enabling disclosure to one of ordinary skill in the art is set forth in the remainder of the specification including reference to the accompanying drawings wherein:

[0012] FIG. 1 shows a charged particle beam device configured to be operated according to methods described herein;

[0013] FIG. 2 is a flow diagram illustrating a method for calibrating deflectors of a charged particle beam device according to embodiments described herein;

[0014] FIG. 3 schematically illustrates a periodic pattern provided by a specimen in specimen coordinates;

[0015] FIG. 4 shows a schematic illustration of an array of display pixels of a display used as a specimen for calibrating the charged particle beam device;

[0016] FIG. 5 shows a first image of the periodic pattern of FIG. 3 in device coordinates;

[0017] FIG. 6 illustrates a portion of a Fourier transform of an image of the periodic pattern;

[0018] FIG. 7 shows a distribution fit of a peak of the Fourier transform of FIG. 6;

[0019] FIG. 8 schematically illustrates a first image and a second image;

[0020] FIG. 9 shows a schematic illustration of a portion of the periodic pattern and vectors for determining a first vector according to embodiments described herein;

[0021] FIG. 10 illustrates grid positions in device coordinates for acquiring images of the periodic pattern;

[0022] FIG. 11 shows positions in specimen coordinates, the positions corresponding to the grid positions of FIG. 10;

[0023] FIG. 12 illustrates deflector digital/analogue (D/A) converter values in device coordinates for a plurality of grid positions in specimen coordinates;

[0024] FIG. 13 shows a calibration result based on calibration according to embodiments described herein; and

[0025] FIG. 14 illustrates a calibration result based on a calibration according to a previous calibration technique. DETAILED DESCRIPTION

[0026] Reference will now be made in detail to exemplary embodiments, one or more examples of which are illustrated in the figures. Each example is provided by way of explanation and is not meant as a limitation. For example, features illustrated or described as part of one embodiment can be used on or in conjunction with other embodiments to yield yet further embodiments. The intention is that the present disclosure includes such modifications and variations.

[0027] Within the following description of the drawings, same reference numbers refer to same components. Only the differences with respect to the individual embodiments are described. The structures shown in the drawings are not necessarily depicted true to scale but rather serve the better understanding of the embodiments.

[0028] FIG. 1 shows a charged particle beam device 100 configured to be operated according to methods described herein. The charged particle beam device 100 may include a scanning microscope 102, particularly a scanning electron microscope, with a beam source 110 configured to generate a charged particle beam 101, particularly an electron beam. The charged particle beam 101 can be directed along an optical axis A through a column 103 of the scanning microscope 102. An inner volume of the column 103 can be evacuated. The scanning microscope 102 may include beam influencing elements such as accelerators 115, decelerators, lens elements 120, deflectors 140 or other focusing or defocusing elements, beam correctors, beam separators, detectors and/or further elements provided for influencing the charged particle beam 101 propagating along the optical axis A.

[0029] The charged particle beam device 100 includes a stage 20 for arranging a specimen 10 thereon. The term “specimen” as used herein may relate to a substrate with one or more layers, features or electrical devices such as TFTs formed thereon. For example, the specimen may be a display or may include one or more displays. The specimen 10 may be placed on the stage 20 for calibrating the charged particle beam device 100, particularly the deflectors 140, and/or for imaging the specimen 10, e.g. for testing or reviewing the specimen 10. The stage 20 may be arranged in an imaging chamber 105 which can be evacuated in some embodiments. The stage 20 may be a movable stage. In particular, the stage 20 may be movable in a plane perpendicular to the optical axis A of the charged particle beam device 100 (also referred to herein as X-Y-plane). The stage 20 may be movable in a direction Z parallel to the optical axis A.

[0030] The scanning microscope 102 can include one or more focus lenses, e.g. one or more focus lenses of the lens elements 120. The one or more focus lenses may be configured to focus the charged particle beam 101 on the specimen 10 arranged on the stage 20. Secondary electrons or backscattered electrons (also referred to as “signal electrons”) are generated when the charged particle beam 101 impinges on the surface of the specimen 10. The signal electrons provide information on dimensions, spatial characteristics and/or other characteristics of features of the surface of the specimen 10. The signal electrons may be detected with a detector. By scanning the charged particle beam 101 over the surface of the specimen 10, e.g. with deflectors 140, and detecting the signal electrons as a function of generation position of the signal electrons, the surface of the specimen 10 or a region thereof can be imaged.

[0031] In embodiments, the deflectors 140 may be provided for positioning the charged particle beam 101 on the surface of the specimen 10 and/or for scanning the charged particle beam 101 over the surface of the specimen 10, e.g. in the X-direction and/or in the Y- direction. In some embodiments, the deflectors 140 include a main deflector 142 and a subdeflector 144. For example, the main deflector 142 may be a magnetic deflector. The subdeflector 144 may be an electrostatic deflector. In embodiments, the main deflector and/or the sub-deflector may each include two or more deflectors, particularly for deflecting the charged particle beam 101 in two or more different directions. According to some embodiments, the main deflector 142 may be configured for positioning the charged particle beam 101 at a specified position in a specified region on the specimen 10. The sub-deflector 144 or the main deflector 142 may scan the charged particle beam 101 over the specified region, e.g. for imaging the specified region or for testing electrical devices arranged within the specified region. For example, in FIG. 1, the charged particle beam 101 is deflected by the main deflector 142 to a first position 16 within a first region 14 of the specimen 10, the first region 14 spanning a first scanning range 18. The sub-deflector 144 or the main deflector 142 may scan the charged particle beam 101 over the first region 14 by scanning the charged particle beam 101 within the first scanning range 18. [0032] According to some embodiments, the charged particle beam device 100 is configured for reviewing and/or testing a specimen. For example, the specimen to be reviewed and/or tested may be a panel-leveling packing (PLP) substrate or an advanced packaging (AP) substrate, which may be tested contactlessly using a charged particle beam for identifying and characterizing defects such as shorts, opens, and/or leakages. In another example, the specimen may be a display including a substrate and a plurality of display pixels arranged thereon. The display pixels may be tested by applying a voltage pattern to the display pixels and then positioning the charged particle beam 101, particularly an electron beam, onto each display pixel. A voltage contrast of secondary electrons generated by the charged particle beam 101 impinging on the display pixel may be measured and evaluated. The more negative a display pixel is, the more secondary electrons are emitted. The more positive a display pixel is, the less secondary electrons are emitted. Defective display pixels are revealed by showing the wrong voltage in the voltage contrast image.

[0033] However, reviewing or testing of PLP substrates, AP substrates, displays or of other electronic or optoelectronic devices relies on accurate positioning of the charged particle beam 101 over the deflection area which can be scanned using the main deflector 142 and the sub-deflector 144. In some embodiments, the deflection area may be, e.g., larger than 25 cm ² , particularly larger than 100 cm ² or larger than 400 cm ² . To achieve accurate positioning of the charged particle beam 101 over the whole deflection area covered by the column 103, so called distortion calibrations may be performed for the deflectors 140. The distortion calibration may deliver a two-dimensional (2D) calibration function, which transforms deflector digital/analogue converter values (D/A converter values) into specimen coordinates over the whole deflection area - and vice versa.

[0034] As used herein, specimen coordinates can refer to coordinates of the specimen 10. Specifically, specimen coordinates may be expressed in a measure of length. Specimen coordinates may be specified particularly in length units such as micrometers or millimeters. Distances and directions in specimen coordinates may be indicative of real distances and real directions on the specimen 10. In embodiments, the charged particle beam device 100 may position the charged particle beam 101 based on internal coordinates of the charged particle beam device 100, which herein are referred to as “device coordinates”. In embodiments, the device coordinates may be or may correspond to the D/A converter values. Device coordinates may be expressed, e.g., in pixels of the charged particle beam device. For example, an object having a first shape in specimen coordinates may appear as a distorted, second shape in device coordinates in an image acquired by the charged particle beam device 100. A distortion of the first shape may include, e.g., stretching, rotating and/or tilting of the first shape. Further, the distortion may be different in different regions of the deflection area. The calibration function may account for such distortions and may provide accurate positioning of the charged particle beam 101 over the deflection area, particularly over the deflection area of the main deflector and/or the sub-deflector.

[0035] According to embodiments of the present disclosure, a method for calibrating deflectors 140 of a charged particle beam device 100 is provided, particularly for calibrating a sub-deflector and/or a main deflector of the charged particle beam device 100. FIG. 2 shows a flow diagram of a method 200 according to embodiments described herein. The charged particle beam device 100 may be configured to deflect the charged particle beam 101 of the charged particle beam device 100 based on device coordinates of the charged particle beam device 100.

[0036] In embodiments, the method 200 includes placing (box 210) a specimen 10 on a stage 20 in a vacuum chamber. The specimen 10 provides a periodic pattern 12. In some embodiments, the specimen 10 includes a substrate 11 and an array of electrical devices formed on the substrate 11. For example, the specimen 10 may be a display including a glass substrate and display pixels formed thereon, the display pixels particularly including TFTs. The periodic pattern 12 may be provided by applying a voltage pattern to the array of electrical devices. In particular, the periodic pattern 12 can be provided by applying a first voltage to a first group of the electrical devices, and by applying a second voltage to a second group of the electrical devices, the second voltage being different from the first voltage. Additionally or alternatively, the periodic pattern may be provided by periodically arranged structures on a substrate. In an image acquired using the charged particle beam device 100, the structures may show up with different material contrast or different topographical contrast. For example, the structures may include a lithographically produced, high- resolution pattern on a substrate, particularly a permanent pattern. More specifically, the structures of the pattern may not be electrically addressable. [0037] FIG. 3 shows a periodic pattern 12 of a specimen 10, wherein the specimen 10 includes a display. The display may include display pixels 410, as illustrated for instance in FIG. 4. The display pixels 410 shown in FIGS. 3 and 4 may be structurally similar or the same. The periodic pattern 12 may be provided by applying a voltage pattern to the display pixels 410 of the specimen 10. More specifically, a positive voltage may be applied to a first group 310 of display pixels, and a negative voltage may be applied to second group 320 of display pixels. In FIG. 3, the voltage pattern is applied according to a checker-board pattern. In further embodiments, the voltage pattern may be applied according to a different, periodic pattern.

[0038] In embodiments of the present disclosure, the periodic pattern 12 has unit cell vectors e, f in specimen coordinates x, y of the specimen 10, particularly unit cell vectors e, f of a primitive unit cell of the periodic pattern 12. For example, FIG. 3 shows unit cell vectors e, f of the periodic pattern 12 including a first unit cell vector e=(e _x , e _y ) and a second unit cell vector f=(f _x , fy) in specimen coordinates x, y. In embodiments, each of the unit cell vectors e, f of the periodic pattern 12 has a length of maximum 2 mm, particularly of maximum 1.5 mm or of maximum 1 mm. In some embodiments, each of the unit cell vectors e, f of the periodic pattern 12 may have a length of minimum 500 nm, particularly of minimum 1 micrometer or of minimum 5 micrometers.

[0039] According to embodiments, the method 200 includes acquiring (box 220) a first image 500 of a first region 14 of the periodic pattern 12 using the charged particle beam device 100. In particular, the charged particle beam 101 may be deflected to a first position 16 of the first region 14 using the main deflector 142. The first image 500 may be acquired by scanning the charged particle beam 101 over the first region 14. In some embodiments, the charged particle beam 101 may be scanned over the first region 14 by using the main deflector 142, particularly for calibrating the main deflector 142. In further embodiments, which can be combined with other embodiments described herein, the charged particle beam 101 may be scanned over the first region 14 by using the sub-deflector 144, particularly for calibrating the sub-deflector 144. In embodiments, the first image includes at least two unit cells of the periodic pattern 12, particularly at least four unit cells or at least eight unit cells. [0040] In some embodiments, particularly if the periodic pattern 12 is provided by applying a voltage pattern, the first image may be acquired by voltage contrast imaging. In voltage contrast imaging, a voltage contrast of secondary electrons generated by the charged particle beam 101 impinging on the periodic pattern 12 is measured and evaluated for acquiring an image such as the first image 500. For example, FIG. 5 illustrates a first image 500 of a first region 14 of the periodic pattern 12 shown in FIG. 3. The periodic pattern 12 provided by the voltage pattern can be observed by voltage contrast imaging of the display pixels 410. The more negative the voltage of a display pixel 410 is, the more secondary electrons are emitted. The more positive the voltage of a display pixel 410 is, the less secondary electrons are emitted. The first image 500 shows the periodic pattern 12 in device coordinates s, t, wherein the periodic pattern 12 appears distorted. In further embodiments, a first image of a periodic pattern may be acquired using other imaging techniques such as conventional SEM imaging, particularly without applying different voltages to the specimen. For example, conventional imaging may be used, if the periodic pattern is provided by structures on a substrate, wherein the structures are not electrically addressed or not electrically addressable.

[0041] In embodiments, the method 200 includes determining (box 230) at least one first local distortion parameter based on the first image 500. The at least one first local distortion parameter may be indicative of a distortion of the periodic pattern 12 in the first image 500. According to some embodiments, the at least one first local distortion parameter includes a first local distortion matrix U. The first local distortion matrix U may provide a local conversion between the specimen coordinates x, y and the device coordinates s, t, particularly for the first image 500. In the first image 500, the imaged periodic pattern may have image unit cell vectors u=(u _s , ut) and v=(v _s , vt), as illustrated for example in FIG. 5. In embodiments, the first local distortion matrix U may particularly provide the conversion between the unit cell vectors e, fin specimen coordinates x, y and the image unit cell vectors u, v in device coordinates s, t, for example according to equation 1. The unit cell vectors e, f of the periodic pattern 12 may be known, e.g. from the fabrication of the specimen 10. The image unit cell vectors u, v may be determined from the first image 500, particularly according to embodiments described herein. (equation 1) [0042] In some embodiments, the at least one first local distortion parameter is determined based on a Fourier transform of the first image 500. The Fourier transform may be a fast Fourier Transform. The Fourier transform can be a discrete Fourier transform. The Fourier transform can be a two-dimensional (2D) Fourier transform of the first image 500. For example, the Fourier transform may be a 2D fast Fourier transform. For example, the Fourier transform may be determined using image resolution N, using k, 1 = 0, 1 ... N-l, and using image grey levels g according to equation 2. (equation 2)

[0043] The periodic pattern 12 and particularly the image unit cell vectors u, v are reflected by peaks in the magnitude R := |F| of the Fourier transform F, more particularly by peaks at peak positions a, b with a = (ak, ai) and b = (bk, bi). In embodiments, the image unit cell vectors u, v may be determined based on the Fourier transform of the first image 500. Determining the image unit cell vectors u, v may include determining the peak positions a, b in frequency space of the Fourier transform of the first image 500. The peak positions a, b may be defined as vectors pointing to peaks 620 in the Fourier transform, particularly from the central peak 610 to two of the peaks 620 closest to the central peak 610. For example, FIG. 6 illustrates the magnitude R of the Fourier transform in frequency space coordinates k, 1. The Fourier transform includes a central peak 610. Each of the vectors corresponding to peak positions a, b points to one the peaks 620 closest to the central peak 610. For reasons of symmetry, it may be sufficient to determine two peak positions a, b.

[0044] According to some embodiments, determining the at least one first local distortion parameter includes distribution fitting of at least one portion of the first image 500 in frequency space. In particular, the peaks 620 closest to the central peak 610 of the Fourier transform of the first image 500 may be fitted using a distribution. In some embodiments, each peak 620 may be fitted by fitting the peak value 705 of the peak 620 and the neighboring values of the peak 620, e.g. the peak value 705 and the eight neighboring values 710. The maxima a ⁰ = b°) determined by the distribution fitting may be used as the peak positions a, b. The distribution fitting may particularly provide a more accurate determination of the peak positions a, b. [0045] In some embodiments, the distribution fitting is based on a Gaussian distribution, particularly a 2D Gaussian distribution. In particular, the peaks 620 may be fitted using a Gaussian distribution to determine the maxima a ⁰ , b° of the peaks 620. For example, FIG. 7 illustrates the peak value 705 corresponding to peak position a of FIG. 6 and eight neighboring values 710 of the peak value 705 fitted using a Gaussian distribution in frequency space. In particular, the 2D Gaussian distribution R(k, 1) may be fitted to the data values using parameters ci, C2, C3, ko and lo to determine the position of the maximum of peak value a=(ko, lo). For example, equation 3 provides a 2D Gaussian distribution. Parameter ci may be used for identifying images without periodic pattern, if the peak in the Fourier transform does not have a minimum height. (equation s)

[0046] According to embodiments, the image unit cell vectors u, v of the first image 500 may be determined based on the peak positions a, b, e.g. according to equations 4. For example, the unit cell vectors u, v illustrated in FIG. 5 may be determined based on the peak positions a, b determined in frequency space. (equations 4)

[0047] Based on the unit cell vectors e, f and based on the image unit cell vectors u, v, the first local distortion matrix U may be determined, particularly according to equation 5.

^{U =} (equation s)

[0048] The at least one first local distortion parameter, particularly the first local distortion matrix U, may be used for local distortion calibration. The local distortion calibration may be used for calibrating the sub-deflector 144, particularly for the first region 14.

[0049] For calibrating the deflectors 140, particularly the main deflector 142, for further regions of the deflection area of the main deflector 142 and the sub-deflector 144, particularly for the whole deflection area, a global distortion calibration may be performed. Performing a global distortion calibration may include performing additional local distortion calibrations for further regions of the periodic pattern in the deflection area. The global distortion calibration may further include determining vectors between the regions, for which a local distortion calibration has been performed.

[0050] In some embodiments, the method 200 includes acquiring (box 240) a second image 810 of a second region 910 of the periodic pattern 12. The first region 14 and the second region 910 are the regions of the periodic pattern 12 shown in the first image 500 and the second image 810, respectively. The second region 910 can be remote from the first region 14. In particular, the first region 14 and the second region may be disjoint regions of the periodic pattern 12. The first region 14 and the second region may not be overlapping regions and may not be adjoining regions of the periodic pattern 12. The charged particle beam 101 may be deflected to a second position of the second region 910 using the main deflector 142, the second position being different from the first position 16. The second image 810 may be acquired by scanning the charged particle beam 101 over the second region 910, particularly using the main deflector 142. In embodiments, the second image 810 includes at least two unit cells of the periodic pattern 12, particularly at least four unit cells or at least eight unit cells.

[0051] According to embodiments of the present disclosure, the method 200 includes determining (box 250) at least one second local distortion parameter based on the second image 810. The at least one second local distortion parameter may be determined based on the second image 810 analogously to the determination of the at least one first local distortion parameter based on the first image 500. In particular, the at least one second local distortion parameter may be determined based on a Fourier transform of the second image 810. Determining the at least one second local distortion parameter may include distribution fitting of at least one portion of the second image in frequency space, particularly using a Gaussian distribution.

[0052] In some embodiments, the at least one second local distortion parameter includes a second local distortion matrix U'. The second local distortion matrix U' may provide a local conversion between the specimen coordinates and the device coordinates, particularly for the second image 810 and the second region 910. [0053] According to some embodiments, the method 200 includes determining (box 260) a first vector d _g ' in specimen coordinates between the first region 14 and the second region 910 based on the at least one first local distortion parameter and based on the at least one second local distortion parameter. The first vector d _g ' may connect the first position 16, at which the first image 500 is acquired, to the second position, at which the second image 810 is acquired. In embodiments, the first vector d _g ' in specimen coordinates x, y may correspond to an image connecting vector d _g in device coordinates s, t, the image connecting vector d _g connecting the first image 500 and the second image 810. For example, the image connecting vector d _g may connect the center of the first image 500 to the center of the second image 810.

[0054] FIGS. 8 and 9 schematically illustrate the determination of the first vector d _g ' according to embodiments. FIG. 8 illustrates a first image 500 of a first region 14 of the periodic pattern 12, and a second image 810 of a second region 910 of the periodic pattern 12. An image connecting vector d _g connects the centers of the first image 500 and of the second image 810 in device coordinates s, t. In embodiments, the image connecting vector d _g is predetermined, or may be calculated from the first position and the second position in device coordinates s, t. FIG. 9 schematically shows the periodic pattern 12 in specimen coordinates x, y, the periodic pattern including the first region 14 and the second region 910. FIG. 9 shows a first vector d _g ' in specimen coordinates x, y, which corresponds to the image connecting vector d _g in device coordinates s, t.

[0055] According to embodiments, determining the first vector d _g ' includes determining an approximated first vector d^' based on the first local distortion matrix U, based on the second local distortion matrix U' and based on the image connecting vector d _g . In particular, the approximated first vector d^' may be determined based on an average of the first local distortion matrix U and the second local distortion matrix U', particularly according to equation 6. (equation 6)

[0056] In embodiments, the first vector d _g ' can further be determined more accurately than the approximated first vector d^'. In particular, distortion may change along the image connecting vector d _g . The first vector d _g ' may be determined more accurately based on the periodicity of the periodic pattern 12. In embodiments, the first vector d _g ' is further determined based on the unit cell vectors e, f of the periodic pattern 12. Determining the first vector dg' based on the periodic pattern 12 may particularly include determining shift vectors indicative of a shift or offset between the periodic pattern 12 and each of the first image 500 and the second image 810.

[0057] According to embodiments, the at least one first local distortion parameter includes a first shift vector si. The first shift vector si may be indicative of a first offset between the periodic pattern 12 and the first image 500. For example, the first shift vector si may be indicative of an offset between a pattern reference point of the periodic pattern 12 and an image reference point of the first image 500. The pattern reference point may be a reference point of a unit cell of the periodic pattern 12, particularly a characteristic point of the unit cell. For example, in FIG. 8, the pattern reference point is the comer of a rectangle in the checker-board pattern. The image reference point of the first image 500 may be, e.g. the center of the first image 500. In embodiments, the first shift vector si may be determined based on the Fourier transform of the first image 500. A shift s = (s _s , St) of an image (in device coordinates s, t) changes the phases (p of the Fourier transform F =: R e' ^1<p by Acp, particularly according to equation 7. (equation 7)

[0058] By defining that the phases (p of the Fourier transform F are caused by the shift s, shift s can be calculated from the phases (p for peak positions a, b in the Fourier transform. In particular, the shift s can be calculated from the phases (p (see equations 8 and 9) for peak positions a, b, wherein the maxima a ⁰ = bf) of the peaks 620 determined by distribution fitting in the frequency space are used as the peak positions a, b. The shift s may be calculated according to equation 10. cp(F(a ⁰ )) = (a ■ s _s + a? ■ s _t ) (equation 8) ) [0059] For example, a first shift vector si can be calculated as the shift s of the first image 500.

[0060] According to embodiments, the at least one second local distortion parameter includes a second shift vector S2. The second shift vector S2 may be indicative of a second shift or offset between the periodic pattern 12 and the second image 810. The second shift vector S2 may be calculated based on the second image 810 analogously to calculating the first shift vector si based on the first image 500. In particular, the second shift vector S2 can be calculated as the shift s of the second image 810.

[0061] The first shift vector si in device coordinates s, t may be converted to a converted first shift vector si' in specimen coordinates x, y, particularly based on the first local distortion matrix U by si'=U si. Similarly, the second shift vector S2 in device coordinates s, t may be converted to a converted second shift vector S2' in specimen coordinates x, y, particularly based on the second local distortion matrix U' by S2 -U' S2.

[0062] Vector addition of the first shift vector si, the image connecting vector d _g and the second shift vector S2 may provide a vector d _u in device coordinates s, t, wherein the vector d _u points from a pattern reference point of the first image 500 to another pattern reference point of the second image 810, as shown for example in FIG. 8. Similar to the vector d _u in device coordinates s, t, a reference vector d _u ' in specimen coordinates x, y connects two reference points of the periodic pattern 12, as shown e.g. in FIG. 9. The reference vector d _u -(dux', d _uy ') has to be an integer i, j multiple of the unit cell vectors e, f (see equation 11), since each of the converted first shift vector si' and the converted second shift vector S2' point to a pattern reference point of a unit cell.

[0063] An approximated reference vector d _u o' = dgo' + S2' - si' may be determined based on the approximated first vector dgo' and the shift vectors S2' and si'. As an approximation to the integers, non-integer numbers i _r and j _r may be determined based on the approximated reference vector d _u o', e.g. according to equation 12. (equation 12)

[0064] Next, integers i, j may be determined by rounding the non-integer numbers i _r and j _r , respectively. Based on the integers i, j corresponding to the reference vector d _u ', the first vector d _g ' can be determined by vector addition, particularly according to equation 13. dg = 1 ■ e + j ■ f + si — S2 (equation 13)

[0065] According to embodiments, the method includes calibrating (box 270) the deflectors based on the first vector d _g '. In embodiments, the first vector d _g ' provides the accurate relative position in specimen coordinates x, y of the second image 810 with respect to the first image 500. The first vector d _g ' and the corresponding image connecting vector d _g can be used to determine a relation between the specimen coordinates x, y and the device coordinates s, t. The determined relation may be used for example to determine a calibration function or a calibration lookup table. Further, one or more local distortion parameters can be interpolated for the regions between the first region 14 and the second region 910. In some embodiments, a local distortion matrix can be interpolated for any region between the first region 14 and the second region 910 based on the first local distortion matrix U and the second local distortion matrix U'.

[0066] In embodiments, the method 200 further includes determining a plurality of further vectors in specimen coordinates x, y based on further images of disjoint regions of the periodic pattern 12. According to embodiments, the first image 500, the second image 810 and the further images are acquired at grid positions 1010, particularly at grid positions 1010 in device coordinates s, t.

[0067] In embodiments, the distance between the first position of the first image and the second position of the second image, and/or the distance between grid positions may be chosen such that an absolute deviation of the non-integer numbers i _r and j _r from the corresponding integers i,j remains small, e.g. smaller than 0.3 or smaller than 0.2. At the occurrence of a larger absolute deviation, a smaller distance in device coordinates s, t between the positions may be chosen for the calibration. In some embodiments, the distance between the positions in specimen coordinates x, y may be greater than 1 mm, particularly greater than 2 mm or greater than 3 mm. For example, the distance may be approximately 5 mm.

[0068] Determining each further vector of the plurality of further vectors may include determining at least one further local distortion parameter based on one of the further images. The at least one further local distortion parameter for the further image may be determined analogously to the determination of the at least one first local distortion parameter for the first image 500. Based on the at least one further local distortion parameter and based on at least one previously determined local distortion parameter, the further vector of the plurality of further vectors may be determined. Each of the plurality of further vectors may be determined similarly to the determination of the first vector d _g '.

[0069] For example, a second vector of the plurality of further vectors may be determined according to the operations outlined with respect to boxes 240, 250, 260 of method 200. In particular, after determining the first vector d _g ', a third image may be acquired at a grid position 1010 remote from the second position. At least one third local distortion parameter may be determined based on the third image. The second vector may be determined based on the at least one second local distortion parameter associated with the second image 810, and further based on the at least one third local distortion parameter.

[0070] According to some embodiments, the first position for acquiring the first image may be a central position of the grid positions 1010. A subsequent image may be acquired at a neighboring grid position of the grid positions 1010. For example, the first image, the second image and the further images may be acquired following a square spiral path 1020 along the grid positions, particularly starting at the central position, as shown, e.g., in FIG. 10.

[0071] In embodiments, the method 200 includes calibrating the deflectors 140 based on the plurality of further vectors, particularly based on the first vector d _g ' and the plurality of further vectors. For example, FIG. 11 illustrates positions in specimen coordinates x, y corresponding to the grid positions 1010 shown in FIG. 10. The graphs according to FIGS. 10 and 11 allow for determining a calibration function or calibration lookup table relating the grid positions 1010 in device coordinates s, t (FIG. 10) to the corresponding positions 1110 in specimen coordinates x, y (FIG. 11) and vice versa. [0072] In particular, based on the relation between device coordinates s, t and specimen coordinates x, y, an inverse function of the distortion calibration function may be determined, which provides deflector D/A converter values in the device coordinates s, t as a function of specimen coordinates x, y. For example, FIG. 12 illustrates a plurality of D/A converter values in device coordinates s, t corresponding to a plurality of equally spaced grid positions in specimen coordinates x, y.

[0073] Further, local distortion matrices determined for the grid positions 1010 can be interpolated between the grid positions 1010. For example, FIG. 13 illustrates an example of a distortion calibration result based on calibration which used a display for providing the periodic pattern. In particular, FIG. 13 shows matrix elements Ui of local distortion matrices determined at grid positions 1010 and interpolated between the grid positions 1010. In contrast, FIG. 14 illustrates an example of matrix elements Ui determined according to a previous calibration technique based on a calibration with L-shaped marks spaced by a distance of approximately 5 mm. In particular, the deflector calibrations based on the local distortion matrices determined according to embodiments described herein (FIG. 13) may be more accurate and show less noise than deflector calibrations according to a previous calibration technique (FIG. 14). Main-deflector calibrations according to embodiments described herein may particularly provide a more reliable calibration. In particular, manual corrections may be reduced or avoided. Further, sub-deflector calibration may be executed approximately 10 times faster than with the previous calibration technique.

[0074] According to further embodiments, a method for calibrating a deflector 140, particularly a sub-deflector 144, of a charged particle beam device 100 is provided. The method may include any of the operations of method 200 described herein, particularly the operations described with respect to boxes 210, 220 and 230 of method 200. The method may further include calibrating the deflector 140, particularly the sub-deflector 144, based on the at least one first local distortion parameter. For example, the sub-deflector 144 may be calibrated based on a first local distortion matrix determined for an acquired first image 500.

[0075] In some embodiments, which can be combined with other embodiments, methods for calibrating one or more deflectors of a charged particle beam device may be partially or entirely automated. In particular, automated calibration and/or imaging may provide a high throughput and/or reduce costs.

[0076] According to some embodiments, the charged particle beam device may be used or configured for imaging, testing and/or reviewing packaging substrates such as PLP substrates or AP substrates. The complexity of packaging substrates has been increasing for years, with the aim of reducing the space requirements of semiconductor packages. For reducing the manufacturing costs, packaging techniques were proposed, such as 2.5D ICs, 3D-ICs, and wafer-level packaging (WLP), e.g. fan-out WLP. In WLP techniques, the integrated circuit is packaged before dicing. A “packaging substrate” as used herein relates to a packaging substrate configured for an advanced packaging technique, particularly an WLP-technique or a panel-level-packing (PLP) -technique.

[0077] “2.5D integrated circuits” (2.5D ICs) and “3D integrated circuits” (3D ICs) combine multiple dies in a single integrated package. Here, two or more dies are placed on a packaging substrate, e.g. on a silicon interposer or a panel-level-packaging substrate. In 2.5D ICs, the dies are placed on the packaging substrate side-by-side, whereas in 3D ICs at least some of the dies are placed on top of each other. The assembly can be packaged as a single component, which reduced costs and size as compared to a conventional 2D circuit board assembly.

[0078] A packaging substrate typically includes a plurality of device-to-device electrical interconnect paths for providing electrical connections between the chips or dies that are to be placed on the packaging substrate. The device-to-device electrical interconnect paths may extend through a body of the packaging substrate in a complex connection network, vertically (perpendicular to the surface of the packaging substrate) and/or horizontally (parallel to the surface of the packaging substrate) with end points (referred to herein as surface contact points) exposed at the surface of the packing substrate.

[0079] An advanced packaging (AP) substrate provides the device-to-device electrical interconnection paths on or within a wafer, such as a silicon wafer. For example, an AP substrate may include Through Silicon Vias (TSVs), e.g., provided in a silicon interposer, other conductor lines extending through the AP substrate. A panel-level-packaging substrate is provided from a compound material, for example material of a printed circuit board (PCB) or another compound material, including, for example ceramics and glass materials.

[0080] Panel-level-packaging substrates are manufactured that are configured for the integration of a plurality devices (e.g., chips/dies that may be heterogeneous, e.g. may have different sizes and configurations) in a single integrated package. Further, AP substrates may be combined on a PLP substrate. A panel-level substrate typically provides sites for a plurality of chips, dies, or AP substrates to be placed on a surface thereof, e.g. on one side thereof or on both sides thereof, as well as a plurality of device-to-device electrical interconnect paths extending through a body of the PLP substrate.

[0081] Notably, the size of a panel-level- substrate is not limited to the size of a wafer. For example, a panel-level-substrate may be rectangular or have another shape. Specifically, a panel-level-substrate may provide a surface area larger than the surface area of a typical wafer, e.g., 1000 cm ² or more. For example, the panel-level substrate may have a size of 30 cm x 30 cm or larger, 60 cm x 30 cm or larger, 60 cm x 60 cm or larger.

[0082] In some embodiments, the charged particle beam device is configured for imaging a specimen and/or for calibration using a specimen, the specimen including a large-area substrate for display manufacturing having a surface area of 1 m ² or more. The surface area may be from about 1.375 m ² (1100 mm x 1250 mm- GEN 5) to about 10 m ² , more specifically from about 2 m ² to about 10 m ² or even up to 12 m ² For instance, a substrate can be GEN 7.5, which corresponds to a surface area of about 4.39 m ² (1.95 m x 2.25 m), GEN 8.5, which corresponds to a surface area of about 5.7 m ² (2.2 m x 2.5 m), or even GEN 10.5, which corresponds to a surface area of about 10 m ² (2.95 m x 3.37 m). Even larger generations such as GEN 11 and GEN 12 can be implemented.

[0083] The specimen may include an inflexible substrate, e.g., a glass substrate or a glass plate, or a flexible substrate, such as a web or a foil or a thin glass sheet. The specimen may be a coated substrate, wherein one or more thin material layers or other features are deposited on the substrate, for example by a physical vapor deposition (PVD) process or a chemical vapor deposition process (CVD) or a lithographic process or an etch process. In particular, the specimen may include a substrate for display manufacturing and a plurality of electronic or optoelectronic devices formed thereon. The electronic or optoelectronic devices formed on the substrate are typically thin film devices including a stack of thin layers. For example, the specimen may be a substrate with an array of thin film transistors (TFTs) formed thereon, e.g. a thin film transistor based substrate.

[0084] According to embodiments of the present disclosure, a charged particle beam device 100 for imaging a specimen 10 includes a stage 20 for arranging the specimen 10 to be imaged. The charged particle beam device 100 may further include one or more deflectors 140 for deflecting a charged particle beam 101 of the charged particle beam device 100. In embodiments, the charged particle beam device 100 includes a computer-readable medium containing a program for calibrating the one or more deflectors 140, which, when executed by a processor, performs a method according to embodiments described herein, particularly a method for calibrating the one or more deflectors of the charged particle beam device 100. The charged particle beam device 100 may be configured for imaging a specimen 10 based on the calibration performed according to the methods described herein.

[0085] In some embodiments, the charged particle beam device 100 includes a controller 160, the controller 160 being connected to the charged particle beam device 100 (see, e.g., FIG. 1). The controller 160 of the charged particle beam device 100 may include a central processing unit (CPU), the computer-readable medium according to embodiments described herein and, for example, support circuits. To facilitate control of the charged particle beam device 100, the CPU may be one of any form of general purpose computer processor that can be used in an industrial setting for controlling various components and sub-processors. The computer-readable medium is coupled to the CPU. The computer readable medium or memory may be one or more readily available memory devices such as random access memory, read only memory, hard disk, or any other form of digital storage either local or remote. The support circuits may be coupled to the CPU for supporting the processor in a conventional manner. The support circuits include cache, power supplies, clock circuits, input/output circuitry and related subsystems, and the like. Instructions for calibrating the one or more deflectors 140 of the charged particle beam device 100 are generally stored in the computer-readable medium as a software routine typically known as a recipe. The software routine may also be stored and/or executed by a second CPU that is remotely located from the hardware being controlled by the CPU. The software routine, when executed by the CPU, transforms the general purpose computer into a specific purpose computer (controller) that controls the charged particle beam device 100, and can provide for calibrating the one or more deflectors 140 of the charged particle beam device 100 according to any of the embodiments of the present disclosure. Although the method of the present disclosure is discussed as being implemented as a software routine, some of the method operations that are disclosed therein may be performed in hardware as well as by the software controller. As such, the embodiments may be implemented in software as executed upon a computer system, and hardware as an application specific integrated circuit or other type of hardware implementation, or a combination of software and hardware. The controller 160 may execute or perform a method for calibrating one or more deflectors of a charged particle beam device according to embodiments described herein.

[0086] According to embodiments described herein, the methods of the present disclosure can be conducted using computer programs, software, computer software products and the interrelated controllers, which can have a CPU, a computer readable medium or memory, a user interface, and input and output devices being in communication with the corresponding components of the apparatus.

[0087] Methods according to embodiments described herein may be used for calibrating charged particle beam devices used in process control, e.g. in the production of packaging substrates such as PLP substrates or AP substrates, flat panels, displays, OLED devices such as OLED screens, TFT based substrates and/or other specimens including a plurality of electronic or optoelectronic devices formed thereon. Process control may include for example regular monitoring, imaging and/or defect review.

[0088] Embodiments of the present disclosure may advantageously provide a faster and/or more accurate calibration of one or more deflectors of a charged particle device such as a scanning electron microscope. Further, calibrations according to embodiments may be performed without conventional calibration plates, which may be expensive, may require storage space and/or may require special masks to be produced. For example, calibration according to embodiments may be performed using a specimen including a substrate with an accurate high resolution periodic pattern. Such specimens may be readily available or may be produced with an existing mask, e.g. a mask used for the fabrication of electronic or optoelectronic devices. In some embodiments, the specimens for calibration may be the specimens to be tested or reviewed, for example displays.

[0089] While the foregoing is directed to some embodiments, other and further embodiments may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.