Method for calibrating a scanning charged-particle microscope

Abstract

A method for calibrating a scanning charged particle microscope such as a scanning electron microscope (SEM) is provided. The method includes: dividing a wafer into a plurality of regions; preparing a pattern including a first periodic structure interlaced with a second periodic structure on each of the plurality of regions, the first periodic structure and the second periodic structure having an induced offset; determining an actual pitch of the first periodic structure and the second periodic structure, and thereby determining an actual induced offset on each of the plurality of regions; selecting a plurality of regions from the plurality of regions; measuring the pitch of the first periodic structure and the second periodic structure on each of the plurality of regions by means of a SEM; and performing linear calibration on the SEM based on the determination and the measurement.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. Application No. 62 / 787,100, filed on December 31, 2018, and U.S. Application No. 62 / 943,717, filed on December 4, 2019, which are incorporated herein by reference in their entirety. Technical Field

[0003] Apparatuses and methods consistent with the present disclosure relate generally to calibration techniques, and more particularly to methods and systems for calibrating charged particle beam inspection systems. Background Art

[0004] In the manufacturing process of integrated circuits (ICs), unfinished or completed circuit components are inspected to ensure that they are manufactured according to design and are free of defects. Inspection systems using optical microscopes or charged particle (e.g., electron) beam microscopes such as scanning electron microscopes (SEMs) can be used. For accurate measurements, especially for nanoscale circuit elements such as transistors, the settings of the SEM need to be calibrated from time to time.

[0005] When calibrating a SEM, a reference sample using a single pitch-based standard can be used. In this case, the reference sample can have a pattern of periodic lines with equal spacing between adjacent lines. In a standard calibration process, the pitch between adjacent lines of the same pattern can be measured by the SEM. However, these standard calibration processes have drawbacks and need to be improved. Summary of the invention

[0006] According to some embodiments of the present disclosure, a method for calibrating a scanning electron microscope (SEM) is provided. The method includes: providing a reference sample including a first periodic structure interlaced with a second periodic structure, each of the first periodic structure and the second periodic structure having a period along a measurement direction; measuring the offset between the first periodic structure and the second periodic structure by SEM; comparing the measured offset with the induced offset; and calibrating the SEM based on the comparison. The induced offset may include an offset intentionally introduced into the reference sample. The induced offset may also include an offset caused by an overlay error during the manufacturing process of the reference sample. The induced offset may be traceable to a laser interferometer. The offset measured by the SEM is based on an image of the reference sample obtained by the SEM, and the left and right edges of the first periodic structure and the second periodic structure on the image are asymmetric. The first periodic structure and the second periodic structure may be located on different layers of a wafer. Each of the first periodic structure and the second periodic structure includes a set of periodic lines.

[0007] The method may also include: tracking critical dimensions of the first periodic structure and the second periodic structure; and measuring the critical dimensions of the first periodic structure and the second periodic structure by means of a SEM, wherein calibrating the SEM further includes calibrating the SEM based on a correlation between the measured critical dimensions of the first periodic structure and the second periodic structure and the tracked critical dimensions.

[0008] The method may also include: tracking the line width of the first periodic structure or the second periodic structure; and measuring the line width of the first periodic structure or the second periodic structure by means of a SEM, wherein calibrating the SEM further includes calibrating the SEM based on a correlation between the measured line width and the tracked line width of the first periodic structure or the second periodic structure.

[0009] In the method, a first periodic structure includes a first group of lines extending in a first direction and a second group of lines connected to the first group of lines and extending in a second direction; and a second periodic structure includes a third group of lines extending in the first direction and a fourth group of lines connected to the third group of lines and extending in the second direction.

[0010] In the method, the first periodic structure may also include a plurality of first periodic substructures arranged on different regions of the reference sample, the second periodic structure may also include a plurality of second periodic substructures arranged on different regions of the reference sample, and each first periodic substructure in the plurality of first periodic substructures is paired with a corresponding second periodic substructure in the plurality of second periodic substructures to form a plurality of pairs of first periodic substructures and second periodic substructures.

[0011] According to some embodiments of the present disclosure, a method for calibrating a scanning electron microscope (SEM) is provided. The method may include: selecting multiple regions on a wafer; preparing a pattern including a first periodic structure interlaced with a second periodic structure on each of the multiple regions, the first periodic structure and the second periodic structure having an induced offset; tracking the pitch of the first periodic structure and the second periodic structure and the induced offset on each of the multiple regions; measuring the pitch of the first periodic structure and the second periodic structure on each of the multiple regions by SEM; and performing linear calibration on the SEM based on tracking and measurement. In the method, tracking the pitch of the first periodic structure and the second periodic structure may also include: tracking the pitch of the first periodic structure and the second periodic structure and the induced offset on each of the multiple regions by a laser interferometer. In the method, performing linear calibration on the SEM may also include: plotting the measured pitch in xy coordinates according to the tracked induced offset. The method may also include: applying a linear fit to the plot; obtaining the slope of the linear fit; comparing the slope with 1 or -1; and performing calibration based on the deviation of the slope from 1 or -1. The method may also include determining the coefficient of determination of the linear fit (R of the observed variance explained by the model / fit). 2 or fraction), root mean square deviation, or residual value range. In the method, the induced offsets of the plurality of regions may be different from each other. In the method, the plurality of regions on the wafer may be selected based on random numbers generated by a random number generator. In the method, each of the first periodic structure and the second periodic structure may further include a plurality of substructures extending periodically in two dimensions, and performing linear calibration on the SEM may further include performing linear calibration based on data acquired in the first direction and data acquired in the second direction, the second direction being substantially perpendicular to the first direction.

[0012] According to some embodiments of the present disclosure, a method for calibrating a scanning electron microscope (SEM) is provided. The method may include: preparing a pattern including a first periodic structure interlaced with a second periodic structure on each of a plurality of regions of a wafer, the first periodic structure and the second periodic structure having an induced offset; tracking the pitch of the first periodic structure and the second periodic structure and the induced offset on each of the plurality of regions; selecting a plurality of regions from the plurality of regions; measuring the pitch of the first periodic structure and the second periodic structure on the selected region by SEM; and performing linear calibration on the SEM. In the method, tracking the pitch of the first periodic structure and the second periodic structure may also include: tracking the pitch of the first periodic structure and the second periodic structure by a laser interferometer, thereby tracking the induced offset on each of the plurality of regions. In the method, performing linear calibration on the SEM may also include: plotting the measured pitch in xy coordinates according to the tracked induced offset. The method may also include: applying a linear fit to the plot; obtaining a slope of the linear fit; comparing the slope with 1 or -1; and performing calibration based on the deviation of the slope from 1 or -1. The method may also include determining at least one of a coefficient of determination, a root mean square deviation, or a range of residual values ​​for the linear fit.

[0013] According to some embodiments of the present disclosure, a system for calibrating a scanning electron microscope (SEM) is provided. The system may include: a reference sample including a first periodic structure interlaced with a second periodic structure, the first periodic structure and the second periodic structure having an induced offset; a laser interferometer having a circuit device configured to track the pitch and the induced offset between the first periodic structure and the second periodic structure; a scanning electron microscope configured to measure the pitch between the first group of periodic structures and the second group of periodic structures; and a controller having a circuit device configured to compare the measured pitch with the tracked pitch to perform calibration.

[0014] In the system, the first periodic structure and the second periodic structure include a plurality of first periodic structures, and each region has a pair of the first periodic structure and the second periodic structure.

[0015] In the system, the first periodic structure and the second periodic structure further include a plurality of second periodic structures, and each of the regions has a pair of the first periodic structure and the second periodic structure.

[0016] In the system, the controller can also be configured to: plot the measured pitch according to the tracked induced deviation in xy coordinates, apply a linear fit to the plotted data; obtain the slope of the linear fit; compare the slope to 1 or -1; and calibrate the SEM based on the deviation of the slope from 1 or -1.

[0017] According to some embodiments of the present disclosure, a reference sample for calibration is provided. The reference sample may include: a first periodic structure having a period along a measurement direction; and a second periodic structure having a period along the measurement direction and deviating from the first periodic structure, wherein the offset between the first periodic structure and the second periodic structure is an induced offset intentionally introduced into the reference sample. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1A is a block diagram illustrating an exemplary calibration system consistent with some embodiments of the present disclosure.

[0019] Figure 1B is a schematic diagram illustrating an exemplary electron beam inspection (EBI) system consistent with some embodiments of the present disclosure.

[0020] Figure 2 It is shown that some embodiments consistent with the present disclosure may be Figure 1B Schematic diagram of an exemplary electron beam tool that is a portion of an exemplary electron beam inspection (EBI) system.

[0021] Figure 3 is a schematic diagram showing a top view of a reference sample without the overlay error of a conventional system.

[0022] Figure 4A is a schematic diagram showing a top view of a reference sample with induced overlap consistent with some embodiments of the present disclosure, Figure 4B is a schematic diagram showing a cross-sectional view (line) of a reference sample consistent with some embodiments of the present disclosure and a waveform of a secondary electron signal measured by SEM, Figure 4C is a schematic diagram showing a cross-sectional view (extremely fine lines) of a reference sample consistent with some embodiments of the present disclosure and a waveform of a secondary electron signal measured by SEM.

[0023] Figure 4D is an exemplary top view of a reference sample using two periodic patterns and corresponding waveforms of secondary electron signals measured by SEM, consistent with embodiments of the present disclosure.

[0024] Figure 5 is a flow chart illustrating an exemplary method of calibrating a SEM consistent with some embodiments of the present disclosure.

[0025] Figure 6 is a flow chart illustrating another exemplary method of calibrating a SEM consistent with some embodiments of the present disclosure.

[0026] Figure 7 is a flow chart illustrating another exemplary method of calibrating a SEM consistent with some embodiments of the present disclosure.

[0027] Fig. 8A is a schematic diagram showing pitch variation and induced overlap variation in a reference sample consistent with some embodiments of the present disclosure, Figure 8B is a graph showing the relationship between pitch variation and overlap variation plotted in xy coordinates consistent with some embodiments of the present disclosure.

[0028] Fig. 9A is a flow chart illustrating another exemplary method of making a reference sample for calibrating a SEM, consistent with some embodiments of the present disclosure.

[0029] Fig. 9B is a flow chart illustrating another exemplary method of making a reference sample for calibrating a SEM, consistent with some embodiments of the present disclosure.

[0030] Fig. 10A is a graph showing an example of a linear calibration of a SEM consistent with some embodiments of the present disclosure, Fig. 10B It is consistent with some embodiments of the present disclosure. Fig. 9A Plot of the residual values ​​from the linear calibration in .

[0031] Fig.11A is a graph showing another example of linear calibration of a SEM consistent with some embodiments of the present disclosure, Fig. 11B It is consistent with some embodiments of the present disclosure. Fig.11A Plot of the residual values ​​from the linear calibration in .

[0032] Fig. 12A is a graph showing another example of linear calibration of a SEM consistent with some embodiments of the present disclosure, Fig. 12B It is consistent with some embodiments of the present disclosure. Fig. 12A Plot of the residual values ​​from the linear calibration in .

[0033] Fig.13A is a schematic diagram showing a top view of a reference sample having a two-dimensional line pattern without overlay error consistent with some embodiments of the present disclosure, Fig. 13B is a schematic diagram showing a top view of a reference sample having a two-dimensional line pattern with induced overlap, consistent with some embodiments of the present disclosure.

[0034] Fig.14A is a schematic diagram showing a top view of another reference sample having a two-dimensional line pattern without overlay errors consistent with some embodiments of the present disclosure, Fig. 14B is a schematic diagram showing a top view of another reference sample having a two-dimensional line pattern with induced overlap, consistent with some embodiments of the present disclosure.

[0035] Fig.15Ais a schematic diagram showing a top view of another reference sample having a two-dimensional line pattern without overlay errors consistent with some embodiments of the present disclosure, Fig. 15B is a diagram showing a top view of another reference sample having a two-dimensional line pattern with induced overlap, consistent with some embodiments of the present disclosure. DETAILED DESCRIPTION

[0036] Reference will now be made in detail to exemplary embodiments, examples of which are shown in the accompanying drawings. The following description refers to the accompanying drawings, and unless otherwise specified, the same numbers in different drawings represent the same or similar elements. The implementations set forth in the following description of the exemplary embodiments do not represent all implementations consistent with the present invention. Instead, they are merely examples of devices and methods consistent with the aspects related to the present invention described in the appended claims. For example, although some embodiments are described in the context of using a scanning electron microscope (SEM) to generate wafer images and defect detection, the present disclosure is not limited thereto. Other types of microscopes such as atomic force microscopes (AFM) and scanning probe microscopes (SPM) may also be similarly applied.

[0037] Increasing the computing power of electronic devices while reducing the physical size of the devices can be achieved by significantly increasing the packaging density of circuit components such as transistors, capacitors, diodes, etc. on IC chips. For example, in a smart phone, an IC chip (the size of a thumb) can include more than 2 billion transistors, each of which is less than 1 / 1000 the size of a human hair. Not surprisingly, semiconductor IC manufacturing is a complex process with hundreds of individual steps. Even an error in one step can significantly affect the functionality of the final product. Even a "fatal defect" can cause device failure. The goal of the manufacturing process is to improve the overall yield of the process. For example, in order to achieve a 75% yield for a 50-step process, the yield of each individual step must be greater than 99.4%, and if the yield of the individual step is 95%, the yield of the entire process will drop to 7%.

[0038] At various steps in the semiconductor manufacturing process, pattern defects may appear on at least one of the wafers, chips or masks, and may cause the manufactured semiconductor device to fail, thereby greatly reducing the yield. As semiconductor device sizes continue to become smaller and smaller (along with any defects), identifying defects becomes more challenging and costly. Currently, engineers in semiconductor production lines typically spend hours (sometimes even days) to identify the locations of small defects to minimize their impact on the final product.

[0039] Conventional optical inspection techniques are inefficient in inspecting small defects (e.g., nanoscale defects). Advanced electron beam inspection (EBI) tools, such as scanning electron microscopes (SEMs) with high resolution and large depth of focus, have been developed to meet the needs of the semiconductor industry. One function performed by an SEM is to measure critical dimensions (CD), such as measuring the distance between structures, measuring the size of structures, etc. (e.g., in Figure 4A In the manufacture of chips with billions of transistors and interconnects, being able to precisely control the size and spacing between structures such as interconnects, transistor gates, contacts, vias, etc. is very important to achieve target yields, because improperly controlled CD can lead to manufacturing defects such as shorts between adjacent lines, opens on a line, etc.

[0040] When scanning a structure(s) and measuring certain features of the structure(s), problems arise due to effects that cause asymmetry in the characterization of the structure(s). A SEM works by sending a beam of electrons to a sample, where the electrons interact with the sample and cause electrons (e.g., secondary electrons, backscattered electrons, etc.) to be emitted from the sample. A detector detects these electrons and measures the current waveform generated by the detected electrons. For example, in Figure 4B In the diagram, a light beam can be scanned from left to right through the structure shown in the lower figure, and a detector measures the resulting current waveform shown in the upper figure.

[0041] Although the structure being scanned is symmetrical (the left and right edges of the structure are mirror images of each other), the current waveform is asymmetrical, with the portion of the waveform corresponding to the left edge of the structure being very different from the mirror image of the portion of the waveform corresponding to the right edge of the structure. In this example, this asymmetry is not caused by any asymmetry in the structure being imaged by the SEM, but is due to other effects. For example, when scanning from left to right, the beam first encounters the left edge of the structure. As the electron beam moves through the structure from left to right, the charge from the electron beam begins to charge the left edge of the structure. This charging on the left side of the structure creates an electric field, which affects the electrons of the electron beam as it passes through the right side of the structure and results in a change in the electrons emitted when the SEM scans the right side of the structure. Effects such as these can cause the waveform measured by the detector to be asymmetrical, even if the structure being scanned is symmetrical.

[0042] This asymmetry in the waveform introduces errors into the measurement when attempting to measure the width of a structure (i.e., the distance from the left edge of the structure to the right edge of the structure). This asymmetry also introduces errors into the measurement of the distance between two structures, since this is a measurement from, for example, the right edge of the first structure to the left edge of the second structure. Interestingly, when measuring the distance between the left edge of the first structure and the left edge of the second structure, or between the right edge of the first structure and the right edge of the second structure, because the symmetrical edges are measured (e.g., from left edge to left edge, from right edge to right edge), the edge waveforms are similar and do not exhibit the above-mentioned waveform asymmetry. It is when the SEM measures asymmetrical edges (e.g., from left edge to right edge, from right edge to left edge) that the above-mentioned waveform asymmetry occurs.

[0043] If attempting to measure a CD involving an asymmetric measurement (i.e., from left edge to right edge or from right edge to left edge), the measurement accuracy will be improved if the error introduced in the CD measurement due to the asymmetric waveform described above can be characterized and used to adjust the measured CD to obtain a more accurate measurement (a measurement that takes into account the error introduced due to the asymmetric waveform).

[0044] Some disclosed embodiments enable characterization of the errors caused by the asymmetric waveform described above, which further enables more accurate CD measurements because the measured CD can be adjusted to account for the errors introduced due to the asymmetric waveform. The process of determining this error and adjusting the measurement to achieve a more accurate measurement can occur during the calibration phase of the SEM.

[0045] Calibration of the SEM can be performed using a reference sample having a periodic line pattern with a designed pitch. The periodic line pattern can be formed by selectively etching a material on a substrate, and the formed pitch is determined by a symmetric measurement technique and is considered as a "standard" pitch for calibration. During calibration, a reference sample is placed in the SEM; the pitch value of the reference sample is measured using the SEM; and the measured pitch is compared to the standard pitch to perform calibration.

[0046] But using a standard pitch for calibration is very limited. That is, the standard pitch standard uses only one set of lines with no variation in pitch, or only a very limited number of available pitches. In addition, pitch measurements use symmetrical measurements (e.g., from the left edge of a first line to the left edge of a second line adjacent to the first), while the SEM response to the left and right edges on a line in an image is likely to produce an asymmetrical waveform (e.g., Figure 4B, compare the symmetric geometry of the bottom graph to the asymmetric waveform of the top graph), which is probably due to non-ideal conditions (sample charging, slightly different local geometry). This can cause problems because as the left and right edges start to influence each other (e.g. Figure 4C The lines become smaller and it is more difficult to accurately determine the location of weaker edges (i.e., edges with intensities much lower than Figure 4C In addition, when applying conventional pitch measurement, many or all errors not directly related to the SEM image itself are cancelled out, leaving only magnification errors. The net result of conventional pitch measurement is that there is no traceable linear calibration of the actual measured CD, which determines the left and right edges of the line.

[0047] Embodiments of the present disclosure utilize CD linearity calibration by utilizing a double patterning process in conjunction with an induced critical dimension (induced CD) to create two sets of lines with different CD values. The difference between CD calibration and pitch calibration is that in pitch calibration, the same edge is always measured (whether it is the left edge or the right edge, there is no symmetry difference between the edges, i.e., symmetrical measurement), while in CD calibration, both the left edge and the right edge are measured (i.e., asymmetric measurement). The changes introduced in double patterning using CD linearity are more flexible, and more data points can be utilized to better calibrate the SEM. Double patterning changes can be induced by exposing and etching a first structure with a first mask, and then exposing and etching a second structure with a second mask, wherein the first mask and the second mask have an overlap characterized by a laser interferometer (resulting in an offset between a pair of corresponding first structures and second structures), so that the overlap is accurately known. For example, the SEM can measure the distance between the asymmetric edges of multiple pairs of corresponding first structures and second structures patterned on a wafer. Each pair of first structures and second structures can have a different offset and each offset can be characterized by an interferometer.

[0048] The disclosed embodiments provide a method for calibrating a SEM using a reference sample having a periodic pattern having an induced critical dimension (induced CD) intentionally introduced between the patterns. For example, the induced critical dimension may involve the use of two patterns, each having a different line width, or the induced critical dimension may involve an offset overlap (hereinafter referred to as "induced overlap"). A stage laser interferometer measures the position of a wafer stage (relative to a reference sample for calibration) with very high accuracy (using a mirror and a laser interferometer on the wafer stage). This can be used to offset (induced overlap) two patterns with higher accuracy. The stage laser interferometer is provided for dynamically determining the position of a stage holding a sample and automatically compensating for position errors. The stage laser interferometer may include a plurality of interferometer units configured to generate signals based on the position of the stage. The stage laser interferometer also includes a computing device, which may be configured to determine the position of the sample based on the signal, and in response to the determined position, provide instructions associated with the determined position through a control module to control the motor of the stage, or for controlling the motor to adjust the interferometer unit emitting a charged particle beam, or a combination thereof, to automatically compensate for the position error of the sample.

[0049] In addition, the disclosed embodiments provide a method for linear calibration of a SEM by forming multiple patterns with induced overlap in multiple areas across a reference sample and tracking the induced overlap of the multiple patterns. In addition, measured pitch / CD / overlap values ​​are obtained for the multiple patterns. Using the measured values ​​and the tracked induced overlap, a linear fit of the plot can be obtained. For example, Figure 4C The thin line shown, due to the asymmetric nature of CD measurement, there may no longer be a 1:1 relationship between the difference in actual CD and the difference in measured CD. By randomizing the layout of multiple patterns across the wafer, the system wafer fingerprint can be converted to noise (i.e., there is no correlation between the measured parameter and the signal caused by processing artifacts).

[0050] As used herein, unless specifically stated otherwise, the term "or" encompasses all possible combinations unless not feasible. For example, if it is stated that a database may include A or B, then unless otherwise explicitly stated or not feasible, the database may include A, or B, or A and B. As a second example, if it is stated that a database may include A, B, or C, then unless otherwise explicitly stated or not feasible, the database may include A, or B, or C, or A and B, or A and C, or B and C, or A and B and C.

[0051] Reference now Figure 1A , a block diagram of an exemplary calibration system consistent with some embodiments of the present disclosure is shown. Figure 1AAs shown, the calibration system 100 includes a first computer system 102a, an inspection system 114, a laser interferometer 112, and a reference sample manufacturing system 110. The computer system 102a also includes a processor 104a, a storage medium 106a, and a user interface 108a. The processor 104a may include multiple processors. The computer system 102a communicates with the reference sample manufacturing system 110 via wired or wireless communication.

[0052] Computer system 102a may include, but is not limited to, a personal computer, a workstation, a network computer, or any device having one or more processors. Storage medium 106a stores calibration instructions, and processor 104a is configured (via its circuit device) to execute the calibration instructions to control the calibration process. Processor 104a is configured to build a plurality of calibration knowledge files based on previous calibration results.

[0053] The user interface 108a includes a display configured to display a calibration reference sample image and an input device configured to transmit user commands to the computer system 102a. In some embodiments, the display displays measurements such as topological images and length measurements by using scientific instruments such as SEM and one or more interferometers. The display can be any type of computer output surface and projection mechanism for displaying information to computer users, including, but not limited to, cathode ray tubes (CRTs), liquid crystal displays (LCDs), light emitting diodes (LEDs), gas plasmas, touch screens or other image projection technologies. The input device can be any type of computer hardware device for providing data and control signals from an operator to the computer system 102a. The input device can include, but is not limited to, keyboards, mice, scanners, digital cameras, joysticks, trackballs, cursor direction keys, touch screen displays or audio / video commanders, etc., for communicating direction information and command selection to the processor, or for controlling the movement of a cursor on a display.

[0054] In some embodiments, the reference sample manufacturing system 110 includes a UV or EUV lithography system and an etching device to produce a reference sample. The laser interferometer 112 controls the accurate positioning of the wafer stage of the lithography system to induce accurate overlap between two lithography steps. The laser interferometer 112 is controlled by the computer system 102a. The user can operate the laser interferometer 112 by inputting parameters for measurement. In addition, the computer system 102a acquires the data determined by the laser interferometer 112 and stores the data in the storage medium 106a. In some embodiments, the user interface 108a can display the manufacturing or measurement parameters on a display. After measurement, the reference sample is transferred to the inspection system 114.

[0055] In the inspection system 114, a reference sample is placed on the SEM vacuum chamber. The SEM measures the pitch or critical dimension of the reference sample, and thereby further acquires the measured overlap. The user can operate the inspection system 114 by inputting parameters for measurement by the user interface 108b. In addition, in some embodiments, the user interface 108b displays the measurement results in a display. After the measurement, the SEM measurement values ​​of the CD, pitch, and overlap of the pattern in the reference sample are output to the processor 104b and stored in the storage medium 106b. The processor 104b further processes the data acquired from the SEM and transmits a calibration signal to the SEM.

[0056] Reference now Figure 1B , a schematic diagram of an exemplary electron beam inspection system included in the inspection system 114 is shown consistent with some embodiments of the present disclosure. Figure 1B As shown, the electron beam inspection system 120 includes a main chamber 122, a load / lock chamber 124, an electron beam tool 126, and an equipment front end module 128. The electron beam tool 126 is located in the main chamber 122. The equipment front end module 128 includes a first loading port 128a and a second loading port 128b. The equipment front end module 128 may include additional (one or more) loading ports. The first loading port 128a and the second loading port 128b receive a wafer box, which contains a wafer to be inspected (e.g., a semiconductor wafer or a wafer made of one or more other materials) or a sample (wafers and samples are collectively referred to as "wafers" hereinafter).

[0057] One or more robots (not shown) in the equipment front end module 128 transfer the wafer to the load / lock chamber 124. The load / lock chamber 124 is connected to a load / lock vacuum pump system (not shown), which removes gas molecules in the load / lock chamber 124 to reach a first pressure lower than atmospheric pressure. After reaching the first pressure, one or more robots (not shown) transfer the wafer from the load / lock chamber 124 to the main chamber 122. The main chamber 122 is connected to a main chamber vacuum pump system (not shown), which removes gas molecules in the main chamber 122 to reach a second pressure lower than the first pressure. After reaching the second pressure, the wafer is inspected by the electron beam tool 126.

[0058] The controller 129 is electrically connected to the electron beam inspection system 120. The controller 129 may be a computer configured to perform various controls of the electron beam inspection system 120. Figure 1B 106 is shown outside of a structure including the main chamber 122, the load / lock chamber 124, and the equipment front end module 128, but it should be understood that the controller 109 can be part of the structure.

[0059] Reference now Figure 2 , showing that some embodiments consistent with the present disclosure may be Figure 1B A schematic diagram of an exemplary electron beam tool of a portion of an exemplary electron beam tool 126 is shown. Figure 2 As shown, the electron beam tool 126 includes a motorized stage 400 and a wafer holder 402 supported by the motorized stage 400 to hold a wafer 403 to be inspected. The electron beam tool 126 also includes: an objective lens assembly 404, an electron detector 406 (which includes an electron sensor surface), an objective lens aperture 408, a condenser lens 410, a beam limiting aperture 412, a gun aperture 414, an anode 416, and a cathode 418. In some embodiments, the objective lens assembly 404 can include a modified swinging objective delayed immersion lens (SORIL) including a pole piece 404a, a control electrode 404b, a deflector 404c, and an excitation coil 404d. The electron beam tool 126 can also include an energy dispersive X-ray spectrometer (EDS) detector (not shown) to characterize the material on the wafer.

[0060] By applying a voltage between the anode 416 and the cathode 418, a primary electron beam 420 is emitted from the cathode 418. The primary electron beam 420 passes through the gun aperture 414 and the beam limiting aperture 412, both of which can determine the size of the electron beam entering the condenser lens 410 located below the beam limiting aperture 412. The condenser lens 410 focuses the primary electron beam 420 before the beam enters the objective lens aperture 408 to set the size of the electron beam before the electron beam enters the objective lens assembly 404. The deflector 404c deflects the primary electron beam 420 to facilitate beam scanning on the wafer. For example, during the scanning process, the deflector 404c can be controlled to sequentially deflect the primary electron beam 420 to different locations on the top surface of the wafer 403 at different points in time to provide data for image reconstruction of different portions of the wafer 403. In addition, the deflector 404c can also be controlled to deflect the primary electron beam 420 to different sides of the wafer 403 at different time points and at specific locations to provide data for stereoscopic image reconstruction of the wafer structure at that location. In addition, in some embodiments, the anode 416 and the cathode 418 can be configured to generate multiple primary electron beams 420, and the electron beam tool 126 can include multiple deflectors 404c to project multiple primary electron beams 420 to different parts / sides of the wafer at the same time to provide data for image reconstruction of different parts of the wafer 203.

[0061] The excitation coil 404d and the pole piece 404a generate a magnetic field starting from one end of the pole piece 404a and terminating at the other end of the pole piece 404a. A portion of the wafer 403 scanned by the primary electron beam 420 can be immersed in the magnetic field and can be charged, which in turn creates an electric field. The electric field reduces the energy of the primary electron beam 420 that strikes the wafer near the surface before the beam collides with the wafer. The control electrode 404b, which is electrically isolated from the pole piece 404a, controls the electric field on the wafer to prevent micro-arching of the wafer and ensure proper beam focusing.

[0062] Upon receiving the primary electron beam 420, a secondary electron beam 422 may be emitted from a portion of the wafer 403. The secondary electron beam 422 may form a beam spot on the surface of a sensor of the electron detector 406. In some embodiments, one or more detectors (e.g., Everhart-Thornley detectors) may be used to capture escaping secondary electrons using an electric field. The electron detector 406 may generate a signal (e.g., voltage, current, etc.) representing the intensity of the beam spot and provide the signal to a processing system (not shown). The intensity of the secondary electron beam 422 and the resulting beam spot may vary depending on the external structure or internal structure of the wafer 403. In addition, as described above, the primary electron beam 420 may be projected onto different locations on the top surface of the wafer to generate secondary electron beams 422 (and resulting beam spots) of different intensities. Therefore, by mapping the intensity of the beam spot to the position of the wafer 403, the processing system may reconstruct an image reflecting the internal structure or external structure of the wafer 403. Once the electron beam tool 126 acquires an image of the wafer, the wafer image may be transmitted to the computer system 102 (e.g., Figure 1A shown).

[0063] Reference now Figure 3 , shows a schematic diagram of a top view of a reference sample with no overlay error for a conventional system. Figure 3 In the embodiment of the present invention, the periodic line pattern A is interlaced with the periodic line pattern B along the measurement direction. The pattern A and the pattern B can be formed using a double patterning scheme during manufacturing. The double patterning scheme can involve using two separate lithography and etching steps (one for pattern A and the other for pattern B) on one layer. Each line in the pattern A has a line width W A And each line in pattern B has a line width W B And the line width W A and line width W B are intended to be the same. During manufacturing, the pitch PAB between the left edge of a line in pattern A and the left edge of a line in pattern B is used. Similarly, the pitch PBA between the left edge of a line in pattern B and the left edge of the subsequent line in pattern A is used. Figure 3In conventional systems of , the pitch PAB and the pitch PBA are the same. As described above, although the critical dimension (CD) between pattern A and pattern B may also be defined in the manufacturing stage, the CD is not used during the calibration process of the calibration system.

[0064] When calibrating a SEM, a reference sample is placed in the chamber of the SEM. The SEM measures the pitch of the reference sample.

[0065] But as device dimensions, along with any defects, become smaller and smaller, the pitch of the reference sample is reduced to an even smaller scale, for example, a few nanometers.

[0066] Reference now Figure 4A , a schematic diagram of a top view of a reference sample with induced overlap consistent with some embodiments of the present disclosure is shown. The reference sample with induced overlap can be obtained by a double patterning method used during the manufacture of the reference sample. During double patterning, a first pattern (such as pattern A) can be printed by lithography followed by etching. After printing the first pattern, new lithography and new etching are performed on the first pattern. When printing the second pattern (such as pattern B), the mask is aligned so that there is a predetermined offset between the first pattern and the second pattern. As described above, a laser interferometer can be used to help ensure the accuracy of pattern placement. The stage laser interferometer measures the position of the wafer stage (relative to the reference sample used for calibration) with very high accuracy (using mirrors and laser interferometers on the wafer stage). This can be used to offset (induce overlap) two patterns with higher accuracy.

[0067] like Figure 4A As shown, the periodic line pattern A is staggered with the periodic line pattern B along the measurement direction. Pattern A has a line width W A , pattern B has a line width W B Line width W A Can be combined with line width W B In some embodiments, pattern A and pattern B are not limited to linear shapes, and they can have any shape (e.g., FIG. 13A to FIG. 15B The overlap between pattern A and pattern B is obtained by subtracting P'BA from P'AB, thereby obtaining Figure 4A A non-zero value in .

[0068] exist Figure 4A In the example, the overall overlap of the reference sample includes intentionally induced overlap and natural overlap errors caused by processing. For example, the overlap or offset between pattern A and pattern B (e.g., Figure 4AThe "overlap" on the two patterns can be accurately controlled by a laser interferometer when the two patterns are manufactured (e.g., when they are exposed by a lithography tool). For example, the first tube die can be exposed with pattern A and pattern B offset by 20nm (called "induced offset" or "induced overlap"), the second tube die can be exposed with an induced offset of 15nm between pattern A and pattern B, and so on, where the offset is controlled by the laser interferometer of the lithography scanner. In addition, at the adjacent left edge (e.g., P' AB ) or the distance between adjacent right edges can be accurately characterized by SEM because charge accumulation does not affect the accuracy of the same edge measurement. The distance between adjacent left / right or right / left edges can be characterized by SEM, but the distance will be erroneous due to charge accumulation and the associated induced asymmetry in the image. By varying the offset between patterns (e.g., patterns A and B) and taking the above measurements, a trend can be determined that can be used to determine the error introduced in the CD measurement due to charge accumulation and the image asymmetry induced thereby (or other measurement-related errors). Note that an induced overlap can have a corresponding induced CD (e.g., as the induced overlap changes, the CD changes, and the resulting changed CD can be referred to as the induced CD).

[0069] For example, in Figure 4B and Figure 4C The top waveform represents the current detected by a detector that detects electrons emitted from the sample (e.g., secondary electrons, backscattered electrons, etc.). As the electron beam scans from left to right, charge begins to accumulate on the geometry (see e.g. Figure 4B and Figure 4C The geometry at the bottom). As charge accumulates, the surface of the geometry becomes more positively charged, resulting in fewer electrons being able to escape the geometry by being emitted from the right side of the geometry, as the electrons are attracted back into the geometry by the increased positive charge that accumulates on the surface as the electron beam scans from left to right. As a result, the current detected by the detector when the electron beam is at the right edge of the geometry is less than when the electron beam is at the left edge of the geometry (even though the edges are physical mirror images of each other), resulting in an error in the measured CD.

[0070] However, as previously discussed, by varying the offset between patterns (eg, patterns A and B) and taking the above measurements, trends can be determined that can be used to determine the error introduced in the CD measurement due to charge accumulation and the resulting induced image asymmetry.

[0071] In such Figure 2During SEM calibration of the electron beam tool 126 of FIG. 1 , a reference sample with a predetermined induced overlap is placed inside the chamber of the SEM. While the SEM measures pitch P'AB and pitch P'BA, it can also be used to measure the induced CD, in this case, the spacing between pattern A and pattern B. Unlike conventional systems that only measure pitch, the SEM measures both the left and right edges (i.e., an asymmetric measurement) to determine the induced overlap of the reference sample. Using CD linearity is more flexible with respect to variations introduced in double patterning, and more data points can be utilized to better calibrate the SEM. Therefore, using a linearity such as Figure 4A The calibration method for the reference sample shown is able to take into account natural overlay errors as well as asymmetries of the edges of the SEM image (e.g. due to asymmetric charge accumulation when the SEM scans, for example, from left to right, and the resulting effects due to charge accumulation on the image, such as Figure 4B and 4C ), resulting in a more accurate calibration result.

[0072] SEM asymmetric measurements measure the secondary electrons emitted from the sample as the electron beam is scanned across the sample. For well-spaced or shaped line patterns, i.e., where the left edge of a feature is a mirrored version of its adjacent right edge ( Figure 4B ), SEM images may show asymmetry, such as due to the asymmetric charge accumulation described above, which causes the image waveform to appear Figure 4B The waveform at the top. Figure 4B and Figure 4C The top waveform represents the current received by a detector that detects charged particles (eg, secondary electrons, backscattered electrons, etc.) emitted from a sample being scanned by the electron beam.

[0073] In some embodiments, the induced CD on the reference sample may correspond to the width of the pattern. Figure 4D As shown, the lines of one pattern may use a thinner width, while the lines of another pattern may have a thicker width. Although the left edge of the waveform affects the right edge of the waveform (e.g., Figure 4C As shown, conventional calibration systems have problems when calibrating with thinner wires, but embodiments of the present disclosure using induced CD allow for more accurate calibration of the SEM system.

[0074] Reference now Figure 5 , a flow chart showing an exemplary method of calibrating a SEM consistent with some embodiments of the present disclosure. Figure 5 In the method, the method comprises step S501, providing a reference sample, such as Figure 4A The reference sample shown includes a first periodic structure A interlaced with a second periodic structure B along a measurement direction. Figure 4AThere is an induced CD between the periodic structure A and the periodic structure B in Figure 4A The induced overlap corresponds to Figure 4A CD). Due to this induced overlap, a dual pitch method for calibrating the SEM is possible. It should be understood that although this method is for induced overlap, the method is equally applicable to induced line width, where one pattern uses thinner lines and the other uses thicker lines (e.g., Figure 4D shown).

[0075] The method further comprises step S502 of obtaining a traceable measurement associated with the interferometer, the measurement comprising a first periodic structure (eg, Figure 4A The first periodic structure A) and the second periodic structure (for example, Figure 4A The traceable measurement may be determined by a scanner, such as a laser interferometer based on a scanner that controls the induced CD or induced overlap. It will be appreciated that step S502 may be performed before S501 or after S503.

[0076] The method further comprises step S503 of measuring the reference sample at the first periodic structure (eg, Figure 4A The first periodic structure A) and the second periodic structure (for example, Figure 4A In step S504, the overlap or offset measured by the SEM is compared with the actual overlap or offset value tracked by the laser interferometer to perform calibration. By tracking the actual pitch using an asymmetric edge-sensitive laser interferometer, the calibration takes into account the natural overlap error and the asymmetry of the edges in the SEM image, thereby improving the accuracy of the calibration.

[0077] Reference now Figure 6 , a flow chart showing another exemplary method of calibrating a SEM consistent with some embodiments of the present disclosure. Figure 6 In the method, the method includes step S601, providing a reference sample, the reference sample including a first periodic structure having a period along a measurement direction (for example, Figure 4A A first periodic structure A) and a second periodic structure (eg, Figure 4A In some embodiments, there is an induced overlap of the second periodic structure relative to the first periodic structure (e.g., Figure 4A overlap).

[0078] This method involves something like Figure 5 Steps S502 to S504 are steps S602 to S604. For the sake of brevity, the description of steps S602 to S604 is omitted here. Figure 5 compared to, Figure 6 The method further comprises step S605 of obtaining traceable measurements associated with the manufactured interferometer, the measurements comprising induced CD. The induced CD may be a pattern (e.g., Figure 4D ) of the line width, or the first periodic structure (e.g., Figure 4A The first periodic structure A) and the second periodic structure (for example, Figure 4A The method further comprises step S606 of measuring CD using SEM. In step S607, the CD measured by SEM is compared with the tracked CD. Then, in step S608, calibration is performed based on the comparison of overlap and CD. It is understood that the execution of steps S602 to S604 may occur before, during or after the execution of steps S605 to S607. Figure 5 compared to, Figure 6 The method takes into account more parameters (ie, CD), further improving the accuracy of the calibration.

[0079] Reference now Figure 7 , a flow chart showing another exemplary method of calibrating a SEM consistent with some embodiments of the present disclosure. Figure 7 In the method, the method includes step S701, providing a reference sample, such as Figure 4A A reference sample comprising a first periodic structure (eg, Figure 4A A first periodic structure A) and a second periodic structure (eg, Figure 4A There is an induced overlap of the second periodic structure relative to the first periodic structure (for example, Figure 4A overlap).

[0080] This method includes something like Figure 6 The method further comprises steps S602 to S604 and steps S702 to S704. Figure 6 For the sake of brevity, the description of steps S702 to S704 and S705 to S707 is omitted here.

[0081] and Figure 6 compared to, Figure 7 The method further comprises step S708 of obtaining a traceable measurement associated with the interferometer, the measurement comprising the induced CD of the lines of the first periodic structure and the second periodic structure, that is, Figure 4A The line width W in A or line width W B .like Figure 4D As shown, the line widths of the two patterns may be different. The method further includes step S709, measuring the line width of the first periodic structure or the second periodic structure using a SEM. Then, in step S710, the measured width is compared with the tracked line width. Then, in step S711, calibration is performed based on the comparison of the overlap, CD and line width in steps S704, S707 and S710. It is understood that the execution of steps S702 to S704 may occur before, during or after the execution of steps S705 to S707 and S708 to S710. Figure 6 compared to, Figure 7 The method in takes additional factors (eg, line width) into account during calibration, thereby further improving the accuracy of calibration in various aspects.

[0082] Reference now Fig. 8A , showing a schematic diagram of pitch variation and induced overlap variation in a reference sample consistent with some embodiments of the present disclosure, and referring to Figure 8B , shows a graph of the relationship between pitch variation and overlap variation plotted in xy coordinates consistent with some embodiments of the present disclosure. Fig. 8A As shown, pattern B' has an induced overlap relative to pattern A. i . Induced overlap O i It can be determined by the difference between pitch P'BA and pitch P'AB that pattern B' is now shifted to the left to form pattern B". Pattern B" has an induced overlap O2 relative to pattern A. Therefore, the decrease in the overlap value (i.e., ΔO=O2-Oi) is the same as the increase in pitch from pattern B' to pattern B" (i.e., APB'A=PB"A-P'BA). In other words, the ratio between the change in pitch and the change in overlap is approximately equal to -1 (i.e., APB'A / AO≈-1). This relationship between the change in P'BA and the change in overlap O is as shown in Figure 8B shown.

[0083] exist Figure 8B In , each data point is plotted using the pitch value and the induced overlap value corresponding to the pitch value. In some embodiments, the data points (in this case, four data points) are acquired from four different patterns disposed on different regions of the wafer. A linear fit is then applied to the plotted data. In an ideal case, since the change in pitch is proportional to the change in induced overlap, Figure 8B The slope of the middle curve is AP B'A / AO, which is approximately equal to -1. Therefore, Figure 8BThe slope in represents the ideal operating state of the SEM, and the deviation of the slope from -1 represents the deviation of the SEM from the ideal operating state. Using this principle, a linear calibration can be performed on the SEM, i.e., the pitch values ​​are plotted according to the induced overlap, a linear fit is applied to the plotted data, the slope of the linear fit is obtained, and the SEM is calibrated until the slope of the linear fit is equal to -1, as described below.

[0084] Reference now Fig. 9A , a flowchart of an exemplary method of manufacturing a reference sample and calibrating a SEM consistent with some embodiments of the present disclosure is shown. It can be understood that a manufacturing system (e.g., reference sample manufacturing system 110 and laser interferometer 112) can be used to manufacture a reference sample in steps S901 to 903.

[0085] exist Fig. 9A In the method, step S901, a plurality of regions on a wafer are selected to be used as reference samples. In some embodiments, the regions are selected arbitrarily or randomly, such as based on a plurality of random numbers. In other embodiments, the regions are selected based on other criteria. In step S902, a pattern including a first periodic structure and a second periodic structure having an induced CD (e.g., induced overlap, induced line width) is prepared on each of the plurality of regions. In some embodiments, the first periodic structure and the second periodic structure are a set of periodic lines, such as Figure 4A Reference Figure 4A For example, pattern A and pattern B may be formed using a double patterning scheme during fabrication. The double patterning scheme may involve using two separate photolithography and etching steps (one for pattern A and another for pattern B) on one layer.

[0086] In some embodiments, each region in the plurality of regions has an induced overlap value different from the induced overlap values ​​of the other regions. In some embodiments, the induced overlap values ​​of the plurality of regions are randomly selected using a plurality of random numbers. In step S903, during the manufacture of the reference sample using the two sets of structures, the pitch of the first periodic structure and the second periodic structure on each region in the plurality of regions is tracked by a laser interferometer (such as a laser interferometer by a lithography scanner). The actual induced overlap value is further determined based on the pitch. In some embodiments, in addition to the pitch, the CD of the plurality of regions (e.g., Figure 4A The traceability data can be stored and used to calibrate the SEM.

[0087] After the reference sample is manufactured, the reference sample and traceability data can be provided to the SEM for calibration. In step S904, the pitch or CD of the plurality of regions is measured by the SEM to be calibrated. From the pitch of the plurality of regions measured by the SEM, the measured plurality of overlap values ​​can also be determined.

[0088] Then, in step S905, the pitch or CD or overlap values ​​of multiple regions measured by the SEM are plotted in xy coordinates according to the actual overlap values, which correspond to the values ​​of the laser interferometer tracked to the manufacturing system (e.g., scanner). A linear fit is applied to the plotted data, and the slope of the linear fit is obtained from the plotted data. Ideally, the slope of the linear fit is 1 (or -1) because the induced overlap decreases or increases linearly with the pitch or CD. The deviation of the slope from 1 or -1 represents the deviation of the SEM system from the ideal. Linear calibration can be performed by manipulating the parameters of the SEM (e.g., magnification or other measurement algorithms) until the slope of the linear fit is substantially equal to 1 or -1. By printing multiple patterns with different induced overlap values ​​on multiple regions across the wafer, the calibration takes into account non-uniformity (e.g., scanning distortion caused by the non-uniform electromagnetic field generated by the electromagnetic deflector). In addition, by randomizing the layout of multiple patterns on the reference sample, the overlay error caused by the manufacturing process can be reduced to the noise level. This is because some induced offsets (correlations between positions on the wafer) caused by manufacturing and the overlay error may be disrupted, thereby converting the overlay error into random noise that will not affect the slope in the linear fitting process and further improve the accuracy of the calibration.

[0089] Reference now Fig. 9B , a flowchart of another exemplary method of manufacturing a reference sample and calibrating a SEM consistent with some embodiments of the present disclosure is shown. It should be understood that a manufacturing system (e.g., reference sample manufacturing system 110 and laser interferometer 112) can be used to manufacture a reference sample in steps S911 to 913.

[0090] exist Fig. 9B In the method, the method includes step S911, dividing the wafer into a plurality of regions. In some embodiments, the regions may be equally divided based on a preset algorithm. In step S912, a pattern including a first periodic structure and a second periodic structure having an induced CD (e.g., induced overlap, induced line width) is prepared on each of the plurality of regions. In some embodiments, the first periodic structure and the second periodic structure are a set of periodic lines, such as Figure 4A In some embodiments, each region in the plurality of regions has an induced overlap value different from the induced overlap values ​​of the other regions. In some embodiments, the induced overlap values ​​of the plurality of regions are random numbers generated by a random number generator. Figure 4A For example, pattern A and pattern B may be formed using a double patterning scheme during fabrication. The double patterning scheme may involve using two separate photolithography and etching steps (one for pattern A and another for pattern B) on one layer.

[0091] In step S913, the pitch of the first periodic structure and the second periodic structure on each of the plurality of regions is tracked by a laser interferometer, and the actual induced offset value is determined according to the pitch. In some embodiments, in addition to the pitch, the CD of the plurality of regions is also tracked by a laser interferometer. The traceability data can be stored and can be used to calibrate the SEM.

[0092] After the reference sample is manufactured, the reference sample and traceability data can be provided to the SEM for calibration. In step S914, a plurality of regions are selected from the plurality of partitioned regions. In some embodiments, the plurality of regions are selected by the SEM. In some embodiments, the plurality of regions are manually selected by an operator of the SEM. After selection, the pitch or CD of the plurality of regions is measured by the SEM to be calibrated. From the pitch of the plurality of regions measured by the SEM, an overlap value of the plurality of measurements can also be determined.

[0093] Then, in step S915, the pitch, CD or induced overlap value of multiple regions measured by the SEM is plotted in xy coordinates according to the actual induced overlap value, which corresponds to the value of the laser interferometer tracked to the manufacturing system. A linear fit is applied to the plotted data, and the slope of the linear fit is obtained from the plotted data. Ideally, the slope of the linear fit is 1 (or -1) because the induced overlap decreases or increases linearly with the pitch. The deviation of the slope from 1 or -1 represents the deviation of the SEM system from the ideal. Linear calibration can be performed by manipulating the parameters of the SEM (e.g., magnification or other measurement algorithms) until the slope of the linear fit is substantially equal to 1 or -1. Again, by printing multiple patterns with different induced overlap values ​​on multiple regions across the wafer, the calibration takes into account non-uniformity (e.g., scanning distortion caused by the non-uniform electromagnetic field generated by the electromagnetic field deflector). In addition, by randomizing the layout of multiple patterns on the reference sample, the overlay error caused by the manufacturing process can be reduced to the noise level. This is because some induced offsets (correlations between positions on the wafer) caused by manufacturing and the overlay error may be disrupted, thereby converting the overlay error into random noise that will not affect the slope in the linear fitting process and further improve the accuracy of the calibration.

[0094] Reference now Fig. 10A , a diagram showing an example of a linear calibration of a SEM. Fig. 10A, which shows the induced overlap (nm) tracked by the laser interferometer on the x-axis ("Induced Overlap") and the pitch (nm) measured by the SEM on the y-axis ("Measured Pitch"). A linear fit is applied to the data points. The slope obtained is -1.00, indicating a good calibration result. In some embodiments, the slope can be between -1.1 and -0.9, depending on the requirements for calibration accuracy. The coefficient of determination (R of the observed variance explained by the model / fit) is 2 The linear fit (or score) is 1.00, indicating a good linear fit, and the root mean square deviation is 0.92, indicating that the variation in data points across the wafer is small and the calibration results are accurate. Fig. 10B It is shown that some embodiments of the present disclosure are consistent with Fig. 10A . The residual range is shown to be in the range between -2.0 nm and 2.5 nm, which is approximately within the noise level of the system, further indicating the quality of the calibration. When the slope of the linear fit deviates from 1 or -1 by a large amount, a linear calibration can be performed by manipulating the parameters of the SEM (e.g., magnification or other measurement algorithms) until the slope is substantially equal to 1 or -1. When the coefficient of determination, RMS deviation, or residual is too large, a calibration aimed at improving the uniformity of the SEM can be performed.

[0095] Reference now Fig.11A , another example of linear calibration of SEM. Fig.11A , which shows the induced overlap (nm) tracked by the laser interferometer on the x-axis ("Induced Overlap") and the critical dimension (nm) measured by the SEM on the y-axis ("Measured CD"). A linear fit is applied to the plotted data points and the slope of the linear fit is obtained. The slope obtained is -1.00, indicating that the calibration results are good. The coefficient of determination is 1.00, indicating that the differences in the linear fit are small, and the root mean square deviation is 1.06, indicating that the variation in the data points across the wafer is small and the calibration results are accurate. Fig. 11B It is consistent with some embodiments of the present disclosure. Fig.11A . The residual values ​​are between -2.2nm and 3.5nm, which is within the acceptable noise level of the system, further indicating the quality of the calibration. When the slope of the linear fit deviates from 1 or -1 by a large amount, a linear calibration can be performed by manipulating the parameters of the SEM (e.g., magnification or other measurement algorithms) until the slope is substantially equal to 1 or -1. When the coefficient of determination, RMS deviation, or residual is too large, a calibration aimed at improving the uniformity of the SEM can be performed.

[0096] Reference now Fig. 12A , another example of linear calibration of SEM. Fig. 12A, which shows the induced overlap (nm) tracked by the laser interferometer on the x-axis ("Induced Overlap") and the induced overlap (nm) measured by the SEM on the y-axis ("Measured Overlap"). A linear fit was applied to the plotted data points. The slope obtained was 1.01, indicating that the calibration results were good. The coefficient of determination was 0.99, indicating that the variance of the linear fit was small, and the root mean square deviation was 0.94, indicating that the variation of the data points across the wafer was small and the calibration results were accurate. Fig. 12B It is consistent with some embodiments of the present disclosure. Fig. 12A . The residual range is in the range between -2.2nm and 1.75nm, which is within the noise level of the system, further indicating the quality of the calibration. When the slope of the linear fit deviates from 1 or -1 by a large amount, a linear calibration can be performed by manipulating the parameters of the SEM (e.g., magnification or other measurement algorithms) until the slope is substantially equal to 1 or -1. When the coefficient of determination, RMS deviation, or residual is too large, a calibration aimed at improving the uniformity of the SEM can be performed.

[0097] Reference now Fig.13A , a schematic diagram showing a top view of a reference sample having a two-dimensional line pattern without overlay errors consistent with some embodiments of the present disclosure. Fig.13A The periodic line pattern in extends along the x and y directions. In the x direction, the periodic line pattern A is staggered with the periodic line pattern B with a critical dimension CDx. The pitch PAB,X is determined by the spacing from the left edge of a line in pattern A to the left edge of an adjacent line in pattern B in the measurement direction. Similarly, the pitch PBA,X is determined by the spacing from the left edge of a line in pattern B to the left edge of an adjacent line in pattern A in the measurement direction. The overlap between pattern A and pattern B is obtained by subtracting PBA,X from PAB,X, resulting in Fig.13A There is zero overlap in .

[0098] Similarly, in the y direction, periodic line pattern A is staggered with periodic line pattern B, with a critical dimension CDy. The pitch PAB.Y is determined by the spacing in the measurement direction from the left edge of a line in pattern A to the left edge of an adjacent line in pattern B. Similarly, the pitch PBA.Y is determined by the spacing in the measurement direction from the left edge of a line in pattern B to the left edge of an adjacent line in pattern A. The overlap between pattern A and pattern B is obtained by subtracting PBA.Y from PAB.Y, yielding Fig.13A There is zero overlap in . Similar to Figure 3 Configuration in Fig.13A The ideal case where the overlap is zero is depicted.

[0099] In such Figure 2During calibration of an SEM such as an electron beam tool 126 of the SEM, a reference sample is placed in the chamber of the SEM. The SEM measures the pitch PAB,X or CDx in the x-direction and the pitch PAB,Y or CDy in the y-direction of the reference sample. The pitches or CD in the x- and y-directions measured by the SEM are compared with the pitches or CD in the x- and y-directions tracked by an edge sensitive instrument (e.g., such as a laser interferometer) to perform calibration. By considering the pitches or CD in both the x- and y-directions, the accuracy of the calibration is further improved.

[0100] Reference now Fig. 13B , a schematic diagram showing a top view of a reference sample having a two-dimensional line pattern with induced overlap consistent with some embodiments of the present disclosure. Fig.13A compared to, Fig. 13B The reference sample in has an induced overlap in the x-direction and the y-direction, which induced overlap can be obtained, for example, by a double patterning method. In the x-direction, the periodic line pattern A is staggered with the periodic line pattern B, with a critical dimension CDx. The pitch PAB,X is determined by the spacing from the left edge of a line in pattern A to the left edge of an adjacent line in pattern B in the measurement direction. Similarly, the pitch PBA,X is determined by the spacing from the left edge of a line in pattern B to the left edge of an adjacent line in pattern A in the measurement direction. The overlap between pattern A and pattern B is obtained by subtracting P'BA,X from P'AB,X, resulting in Fig. 12B Non-zero overlap value in case of .

[0101] Similarly, in the y direction, periodic line pattern A is interlaced with periodic line pattern B with a critical dimension CDy. The pitch PAB,Y is determined by the spacing from the left edge of a line in pattern A to the left edge of an adjacent line in pattern B in the measurement direction. Similarly, the pitch PBA,Y is determined by the spacing from the left edge of a line in pattern B to the left edge of an adjacent line in pattern A in the measurement direction. The overlap between pattern A and pattern B is obtained by subtracting PBA,Y from PAB,Y, resulting in a non-zero overlap value. This method takes into account the variations in the x-axis and y-axis in addition to the natural overlap error in the calibration, further improving the accuracy of the calibration.

[0102] Reference now Fig.14A , showing a schematic diagram of a top view of another reference sample having a two-dimensional line pattern without overlay errors consistent with some embodiments of the present disclosure, and referring to Fig. 14B , a schematic diagram showing a top view of another reference sample having a two-dimensional line pattern with induced overlap, consistent with some embodiments of the present disclosure. Fig.14A and Fig. 14B The reference samples in the Fig.13A and Fig. 13BThe approximate diagonal direction of Fig.13A and Fig. 13B In this way, calibration can be performed taking into account different directions, further improving the accuracy of calibration.

[0103] Reference now Fig.15A , showing a schematic diagram of a top view of another reference sample having a two-dimensional line pattern without overlay errors consistent with some embodiments of the present disclosure, and referring to Fig. 15B , a schematic diagram showing a top view of another reference sample having a two-dimensional line pattern with induced overlap consistent with some embodiments of the present disclosure. Fig.15A In the embodiment, along a direction such as the y direction, the first periodic structure further includes a plurality of first periodic substructures A and a plurality of second periodic substructures B arranged on different regions such as the subsection 1 of the reference sample. The reference sample further includes a third periodic substructure A and a plurality of fourth periodic substructures B arranged along another direction such as the x direction, and the third periodic substructure A and the plurality of fourth periodic substructures B are arranged on different regions such as the subsection 2 of the reference sample. Fig. 15B In the embodiment of the present invention, the reference sample includes a plurality of first periodic substructures A arranged along the y direction and a plurality of second periodic substructures B having induced offsets relative to the first periodic substructures A, and the plurality of first periodic substructures A and the plurality of second periodic substructures B are arranged on different regions such as subsection 1 of the reference sample. The reference sample also includes a plurality of third periodic substructures A arranged along another direction such as the x direction and a plurality of fourth periodic substructures B having induced offsets relative to the third periodic substructures A, and the plurality of third periodic substructures A and the plurality of fourth periodic substructures B are arranged on different regions such as subsection 2 of the reference sample. Using the reference sample to calibrate the SEM provides a benefit, namely, for a wafer having a surface area, different separated regions of the wafer can be used to form a pattern to increase the number of data points and calibration accuracy. In addition, separate regions are less likely to affect other unconnected patterns. Fig.13A Fig. 15B These are just some examples of 2D reference sample patterns. Calibration using these 2D references is similar to calibration with 1-D reference samples (e.g. Figure 4A ). The difference is that in FIG. 13A to FIG. 15B In the embodiment of the present invention, measurement is additionally performed in another direction (eg, the y-axis direction).

[0104] The embodiments may be further described using the following first set of terms:

[0105] 1. A method for calibrating a scanning electron microscope (SEM), comprising:

[0106] providing a reference sample comprising a first periodic structure interlaced with a second periodic structure, each of the first periodic structure and the second periodic structure having a period along a measurement direction;

[0107] measuring, by the SEM, an offset between the first periodic structure and the second periodic structure;

[0108] comparing the measured offset to the induced offset; and

[0109] The SEM is calibrated based on the comparison.

[0110] 2. A method according to clause 1, wherein the induced offset comprises intentionally introducing an offset in the reference sample.

[0111] 3. The method of clause 2, wherein the induced offset further comprises an offset caused by an overlay error during a manufacturing process of the reference sample.

[0112] 4. A method according to any one of clauses 1 to 3, wherein the induced offset is trackable by a laser interferometer of a lithography scanner.

[0113] 5. A method according to any one of clauses 1 to 4, wherein the offset measured by the SEM is based on an image of the reference sample acquired by the SEM, and wherein the left edges and right edges of the first periodic structure and the second periodic structure on the image appear asymmetric.

[0114] 6. A method according to any one of clauses 1 to 5, wherein the first periodic structure and the second periodic structure are on different layers of a wafer.

[0115] 7. A method according to any one of clauses 1 to 6, wherein each of the first periodic structure and the second periodic structure comprises a set of periodic lines.

[0116] 8. The method according to any one of clauses 1 to 7, further comprising:

[0117] tracking critical dimensions of the first periodic structure and the second periodic structure; and

[0118] measuring the critical dimensions of the first periodic structure and the second periodic structure by the SEM,

[0119] Wherein calibrating the SEM further comprises calibrating the SEM based on a correlation between the measured critical dimension and the tracked critical dimension of the first periodic structure and the second periodic structure.

[0120] 9. The method according to any one of clauses 1 to 8, further comprising:

[0121] Tracking a line width of the first periodic structure or the second periodic structure; and

[0122] measuring the line width of the first periodic structure or the second periodic structure by the SEM,

[0123] Wherein calibrating the SEM further comprises calibrating the SEM based on a correlation between the measured linewidth and the tracked linewidth of the first periodic structure or the second periodic structure.

[0124] 10. The method according to clause 1, wherein:

[0125] The first periodic structure includes a first group of lines extending in a first direction and a second group of lines connected to the first group of lines and extending in a second direction; and

[0126] The second periodic structure includes a third group of lines extending in the first direction and a fourth group of lines connected to the third group of lines and extending in the second direction.

[0127] 11. The method according to clause 1, wherein:

[0128] The first periodic structure further includes a plurality of first periodic substructures arranged on different regions of the reference sample,

[0129] The second periodic structure further includes a plurality of second periodic substructures arranged on different regions of the reference sample, and

[0130] Each first periodic substructure of the plurality of first periodic substructures is paired with a corresponding second periodic substructure of the plurality of second periodic substructures to form a plurality of pairs of first periodic substructures and second periodic substructures.

[0131] 12. A method for calibrating a scanning electron microscope (SEM), comprising:

[0132] selecting a plurality of regions on a wafer;

[0133] preparing a pattern including a first periodic structure interlaced with a second periodic structure on each of the plurality of regions, the first periodic structure and the second periodic structure having an induced offset;

[0134] tracking the induced offset over a pitch of the first periodic structure and the second periodic structure or over each of the plurality of regions;

[0135] measuring the pitch of the first periodic structure and the second periodic structure in each of the plurality of regions by the SEM; and

[0136] A linear calibration of the SEM is performed based on the tracking and the measurements.

[0137] 13. The method of clause 12, wherein tracking the induced offset over the pitch or each of the plurality of regions of the first and second periodic structures further comprises:

[0138] The pitch of the first periodic structure and the second periodic structure or the induced offset in each of the plurality of regions is tracked by a laser interferometer.

[0139] 14. The method according to any of clauses 12 to 13, wherein performing the linear calibration on the SEM further comprises:

[0140] The measured pitch is plotted in xy coordinates against the tracked induced deflection.

[0141] 15. The method according to clause 14, further comprising:

[0142] applying a linear fit to the plot;

[0143] Obtaining the slope of the linear fit;

[0144] comparing the slope to 1 or -1; and

[0145] Calibration is performed based on the deviation of the slope from 1 or -1.

[0146] 16. The method according to clause 15, further comprising:

[0147] At least one of a coefficient of determination, a root mean square deviation, or a range of residual values ​​for the linear fit is determined.

[0148] 17. A method according to any one of clauses 12 to 16, wherein the induced offsets of the plurality of regions are different from one another.

[0149] 18. The method of any of clauses 12 to 17, wherein the plurality of regions on the wafer are selected based on a random number generated by a random number generator.

[0150] 19. A method according to any one of clauses 12 to 18, wherein:

[0151] Each of the first periodic structure and the second periodic structure further includes a plurality of substructures extending periodically in two dimensions, and

[0152] Performing the linear calibration on the SEM further includes performing a linear calibration based on data acquired in a first direction and data acquired in a second direction, the second direction being substantially perpendicular to the first direction.

[0153] 20. A method for calibrating a scanning electron microscope (SEM), comprising:

[0154] preparing a pattern including a first periodic structure interlaced with a second periodic structure on each of a plurality of regions of the wafer, the first periodic structure and the second periodic structure having an induced offset;

[0155] tracking a pitch of the first periodic structure and the second periodic structure and the induced offset in each of the plurality of regions;

[0156] selecting a plurality of regions from among the plurality of regions;

[0157] measuring the pitches of the first periodic structure and the second periodic structure on the selected area by the SEM; and

[0158] A linear calibration was performed on the SEM.

[0159] 21. The method of clause 20, wherein tracking the pitch of the first periodic structure and the second periodic structure further comprises:

[0160] The pitches of the first periodic structure and the second periodic structure are tracked by a laser interferometer, thereby tracking the induced offset in each of the plurality of regions.

[0161] 22. The method of any of clauses 20 to 21, wherein performing the linear calibration on the SEM further comprises:

[0162] The measured pitch is plotted in xy coordinates against the tracked induced deflection.

[0163] 23. The method according to clause 22, further comprising:

[0164] applying a linear fit to the plot;

[0165] Obtaining the slope of the linear fit;

[0166] comparing the slope to 1 or -1; and

[0167] Calibration is performed based on the deviation of the slope from 1 or -1.

[0168] 24. The method according to clause 23, further comprising:

[0169] At least one of a coefficient of determination, a root mean square deviation, or a range of residual values ​​for the linear fit is determined.

[0170] 25. A method according to any one of clauses 20 to 24, wherein the induced offsets of the plurality of regions are different from one another.

[0171] 26. The method of any one of clauses 20 to 25, wherein the plurality of regions on the wafer are automatically selected by the SEM.

[0172] 27. The method of any one of clauses 20 to 25, wherein the plurality of regions on the wafer are manually selected by an operator.

[0173] 28. A method according to any one of clauses 20 to 25, wherein:

[0174] Each of the first periodic structure and the second periodic structure further includes a plurality of substructures extending periodically in two dimensions, and

[0175] Performing the linear calibration on the SEM further includes performing a linear calibration based on data acquired in a first direction and data acquired in a second direction, the second direction being substantially perpendicular to the first direction.

[0176] 29. A method according to any one of clauses 20 to 28, wherein:

[0177] Tracking the pitch further includes tracking a critical dimension of the first periodic structure and the second periodic structure in each of the plurality of regions, and

[0178] Measuring the pitch further includes measuring the critical dimensions of the first periodic structure and the second periodic structure on each of the plurality of regions by the SEM.

[0179] 30. A system for calibrating a scanning electron microscope (SEM), comprising:

[0180] a wafer holder configured to hold a sample; and

[0181] A controller having circuitry configured to:

[0182] measuring a pitch between a first periodic structure and a second periodic structure of the sample, wherein at least one of the periodic structures has an induced offset, and wherein the first periodic structure is staggered with the second periodic structure;

[0183] comparing the measured pitch to the tracked pitches of the first periodic structure and the second periodic structure; and

[0184] The calibration is performed based on the comparison.

[0185] 31. The system of clause 30, wherein the controller has a circuit arrangement further configured to acquire an image of the sample having the first periodic structure and the second periodic structure.

[0186] 32. A system according to clause 30 or 31, wherein the first periodic structure comprises a plurality of first periodic structures and the second periodic structure comprises a plurality of second periodic structures.

[0187] 33. The system of clause 32, wherein the controller has circuitry further configured to measure a plurality of pitches between the plurality of first periodic structures and the plurality of second periodic structures.

[0188] 34. The system of clause 33, wherein the controller has circuitry further configured to:

[0189] plotting the measured pitch in xy coordinates based on the tracked plurality of induced deflections;

[0190] Apply a linear fit to the plotted data;

[0191] Obtaining the slope of the linear fit;

[0192] comparing the slope to 1 or -1; and

[0193] The SEM was calibrated based on the deviation of the slope from 1 or -1.

[0194] 35. A reference sample for calibration, comprising:

[0195] A first periodic structure having a period along a measuring direction; and

[0196] a second periodic structure having a period along the measuring direction and deviating from the first periodic structure,

[0197] The offset between the first periodic structure and the second periodic structure is an induced offset intentionally introduced into the reference sample.

[0198] 36. A method for characterizing measurement-related errors, the method comprising:

[0199] measuring distances between asymmetric edges of a plurality of corresponding pairs of first and second structures patterned on the sample, wherein each of the second structures has a different characteristic offset from a corresponding pair of first structures; and

[0200] An error associated with the measurement is determined based on the measured distance and the different feature offsets.

[0201] 37. The method of clause 36, wherein the asymmetric edge comprises a first edge of a first structure and an adjacent second edge on an opposite side of the first structure.

[0202] 38. The method of clause 37, wherein the asymmetric edge comprises a third edge of a second structure and a fourth edge of a third structure, wherein the third edge is adjacent to the fourth edge and the second structure is adjacent to the third structure.

[0203] The embodiments may be further described using the following second set of terms:

[0204] 1. A method for calibrating a scanning electron microscope (SEM), comprising:

[0205] providing a reference sample comprising a first periodic structure interlaced with a second periodic structure, each of the first periodic structure and the second periodic structure having a periodicity along a measurement direction;

[0206] measuring, by the SEM, an offset between the first periodic structure and the second periodic structure;

[0207] comparing the measured offset to the induced offset; and

[0208] The SEM is calibrated based on the comparison.

[0209] 2. A method according to clause 1, wherein the induced offset comprises intentionally introducing an offset in the reference sample.

[0210] 3. The method of clause 2, wherein the induced offset further comprises an offset caused by an overlay error during a manufacturing process of the reference sample.

[0211] 4. A method according to any one of clauses 1 to 3, wherein the induced deflection is trackable by a laser interferometer, or is obtained by tracking using a laser interferometer.

[0212] 5. A method according to any one of clauses 1 to 4, wherein the offset measured by the SEM is based on an image of the reference sample acquired by the SEM, and the left edge and right edge images of the first periodic structure and the second periodic structure are asymmetric.

[0213] 6. A method according to any one of clauses 1 to 5, wherein the first periodic structure and the second periodic structure are on different layers of a wafer.

[0214] 7. A method according to any one of clauses 1 to 6, wherein each of the first periodic structure and the second periodic structure comprises a set of periodic lines.

[0215] 8. The method according to any one of clauses 1 to 7, further comprising:

[0216] tracking critical dimensions of the first periodic structure and the second periodic structure; and

[0217] measuring the critical dimensions of the first periodic structure and the second periodic structure by the SEM,

[0218] Wherein calibrating the SEM further comprises calibrating the SEM based on a correlation between the measured critical dimensions and the tracked critical dimensions of the first periodic structure and the second periodic structure.

[0219] 9. The method according to any one of clauses 1 to 8, further comprising:

[0220] Tracking a line width of the first periodic structure or the second periodic structure; and

[0221] measuring the line width of the first periodic structure or the second periodic structure by the SEM,

[0222] Wherein calibrating the SEM further comprises calibrating the SEM based on a correlation between the measured linewidth and the tracked linewidth of the first periodic structure or the second periodic structure.

[0223] 10. The method according to clause 1, wherein:

[0224] The first periodic structure includes a first group of lines extending in a first direction and a second group of lines connected to the first group of lines and extending in a second direction; and

[0225] The second periodic structure includes a third group of lines extending in the first direction and a fourth group of lines connected to the third group of lines and extending in the second direction.

[0226] 11. The method according to clause 1, wherein:

[0227] The first periodic structure further includes a plurality of first periodic substructures arranged on different regions of the reference sample,

[0228] The second periodic structure further includes a plurality of second periodic substructures arranged on different regions of the reference sample, and

[0229] Each first periodic substructure of the plurality of first periodic substructures is paired with a corresponding second periodic substructure of the plurality of second periodic substructures to form a plurality of pairs of first periodic substructures and second periodic substructures.

[0230] 12. A method for calibrating a scanning electron microscope (SEM), comprising:

[0231] Selecting a plurality of arbitrary regions on the wafer;

[0232] Preparing a pattern including a first periodic structure interlaced with a second periodic structure on each of the plurality of arbitrary regions, the first periodic structure and the second periodic structure having an induced offset;

[0233] tracking the pitch of the first periodic structure and the second periodic structure and the induced offset at each arbitrary region of the plurality of arbitrary regions;

[0234] measuring the pitch of the first periodic structure and the second periodic structure in each arbitrary region of the plurality of arbitrary regions by the SEM; and

[0235] A linear calibration of the SEM is performed based on the tracking and the measurements.

[0236] 13. The method of clause 12, wherein tracking the pitch of the first periodic structure and the second periodic structure further comprises:

[0237] The pitches of the first periodic structure and the second periodic structure and the induced offset in each of the plurality of arbitrary regions are tracked by a laser interferometer.

[0238] 14. The method according to any of clauses 12 to 13, wherein performing the linear calibration on the SEM further comprises:

[0239] The measured pitch is plotted in xy coordinates against the tracked induced deflection.

[0240] 15. The method according to clause 14, further comprising:

[0241] applying a linear fit to the plot;

[0242] Obtaining the slope of the linear fit;

[0243] comparing the slope to 1 or -1; and

[0244] Calibration is performed based on the deviation of the slope from 1 or -1.

[0245] 16. The method according to clause 15, further comprising:

[0246] At least one of a variance, a root mean square deviation, or a range of residual values ​​of the linear fit is determined.

[0247] 17. A method according to any one of clauses 12 to 16, wherein the induced offsets of the plurality of arbitrary regions are different from each other.

[0248] 18. The method according to any one of clauses 12 to 17, wherein the plurality of arbitrary regions on the wafer are selected based on random numbers generated by a random number generator.

[0249] 19. A method according to any one of clauses 12 to 18, wherein:

[0250] Each of the first periodic structure and the second periodic structure further includes a plurality of substructures extending periodically in two dimensions, and

[0251] Performing the linear calibration on the SEM further includes performing a linear calibration based on data acquired in a first direction and data acquired in a second direction, the second direction being substantially perpendicular to the first direction.

[0252] 20. A method for calibrating a scanning electron microscope (SEM), comprising:

[0253] preparing a pattern including a first periodic structure interlaced with a second periodic structure on each of a plurality of regions of the wafer, the first periodic structure and the second periodic structure having an induced offset;

[0254] tracking a pitch of the first periodic structure and the second periodic structure and the induced offset in each of the plurality of regions;

[0255] selecting a plurality of arbitrary areas from the plurality of areas;

[0256] measuring the pitches of the first periodic structure and the second periodic structure on the selected arbitrary area by the SEM; and

[0257] A linear calibration was performed on the SEM.

[0258] 21. The method of clause 20, wherein tracking the pitch of the first periodic structure and the second periodic structure further comprises:

[0259] The pitches of the first periodic structure and the second periodic structure are tracked by a laser interferometer, thereby tracking the induced offset in each of the plurality of regions.

[0260] 22. The method of any of clauses 20 to 21, wherein performing the linear calibration on the SEM further comprises:

[0261] The measured pitch is plotted in xy coordinates against the tracked induced deflection.

[0262] 23. The method according to clause 22, further comprising:

[0263] applying a linear fit to the plot;

[0264] Obtaining the slope of the linear fit;

[0265] comparing the slope to 1 or -1; and

[0266] Calibration is performed based on the deviation of the slope from 1 or -1.

[0267] 24. The method according to clause 23, further comprising:

[0268] At least one of a variance, a root mean square deviation, or a range of residual values ​​of the linear fit is determined.

[0269] 25. A method according to any one of clauses 20 to 24, wherein the induced offsets of the plurality of regions are different from one another.

[0270] 26. The method of any one of clauses 20 to 25, wherein the plurality of arbitrary regions on the wafer are automatically selected by the SEM.

[0271] 27. The method according to any one of clauses 20 to 25, wherein the plurality of arbitrary regions on the wafer are manually selected by an operator.

[0272] 28. A method according to any one of clauses 20 to 25, wherein:

[0273] Each of the first periodic structure and the second periodic structure further includes a plurality of substructures extending periodically in two dimensions, and

[0274] Performing the linear calibration on the SEM further includes performing a linear calibration based on data acquired in a first direction and data acquired in a second direction, the second direction being substantially perpendicular to the first direction.

[0275] 29. A method according to any one of clauses 20 to 28, wherein:

[0276] Tracking the pitch further includes tracking a critical dimension of the first periodic structure and the second periodic structure at each of the plurality of arbitrary regions, and

[0277] Measuring the pitch further includes measuring the critical dimensions of the first periodic structure and the second periodic structure on each of the plurality of arbitrary regions by the SEM.

[0278] 30. A system for calibrating a scanning electron microscope (SEM), comprising:

[0279] a reference sample comprising a first periodic structure interlaced with a second periodic structure, the first periodic structure and the second periodic structure having an induced offset;

[0280] a laser interferometer having circuitry configured to track a pitch between the first periodic structure and the second periodic structure and the induced offset;

[0281] The scanning electron microscope is configured to measure the pitch between the first set of periodic structures and the second set of periodic structures; and

[0282] A controller has circuitry configured to compare the measured pitch to the tracked pitch to perform the calibration.

[0283] 31. The system of clause 30, wherein the first periodic structure and the second periodic structure comprise a plurality of first periodic structures, each of the arbitrary regions having a pair of the first periodic structure and the second periodic structure.

[0284] 32. The system according to clause 31, wherein the first periodic structure and the second periodic structure further include a plurality of second periodic structures, each of the arbitrary regions having a pair of the first periodic structure and the second periodic structure.

[0285] 33. The system of clause 31, wherein the controller is further configured to:

[0286] plotting the measured pitch in xy coordinates according to the tracked induced deflection;

[0287] Apply a linear fit to the plotted data;

[0288] Obtaining the slope of the linear fit;

[0289] comparing the slope to 1 or -1; and

[0290] The SEM was calibrated based on the deviation of the slope from 1 or -1.

[0291] 34. A reference sample for calibration, comprising:

[0292] A first periodic structure having a period along a measuring direction; and

[0293] a second periodic structure having a period along the measuring direction and deviating from the first periodic structure,

[0294] The offset between the first periodic structure and the second periodic structure is an induced offset intentionally introduced into the reference sample.

[0295] 35. A system for calibrating a scanning electron microscope (SEM), the system comprising:

[0296] a stage configured to support a reference sample comprising a first periodic structure interleaved with a second periodic structure, the first periodic structure having an induced offset therebetween;

[0297] Laser interferometer;

[0298] the SEM; and

[0299] The controller is configured as:

[0300] causing the laser interferometer to track the induced offset between the first periodic structure and the second periodic structure;

[0301] causing the scanning electron microscope to measure the induced offset between the first periodic structure and the second periodic structure; and

[0302] The measured induced offset is compared to the tracked induced offset to perform the calibration.

[0303] 36. The system of clause 35, wherein the induced offset comprises an offset predefined for the reference sample and an offset caused by an overlay error during a manufacturing process of the reference sample.

[0304] 37. A system according to clause 35, wherein the first periodic structure and the second periodic structure are located on different layers of the reference sample.

[0305] 38. A system according to clause 35, wherein the measured induced offset is acquired based on an image of the reference sample acquired by the SEM, and wherein left and right edges of the first periodic structure and the second periodic structure on the image are asymmetric.

[0306] The flow chart and block diagram in the accompanying drawings show the examples of the possible architecture, function and operation of the system, method and computer program product according to various embodiments.In this regard, each frame in the flow chart or block diagram can represent the module, segmentation or part of the code, which includes one or more executable instructions for realizing the specified logical function.It should also be noted that in some alternative implementations, the function marked in the frame may not appear in the order marked in the figure.For example, according to the function involved, the two frames shown in succession can actually be performed substantially at the same time, or these frames can sometimes be performed in reverse order.It should also be noted that each frame of the block diagram or flow chart description and the frame combination in the block diagram or flow chart description can be realized by a system based on special-purpose hardware that performs a specified function or action, or by a combination of special-purpose hardware and computer instructions.

[0307] It should be understood that the described embodiments are not mutually exclusive, and elements, components, materials, or steps described in conjunction with one exemplary embodiment may be combined with or removed from other embodiments in a suitable manner to achieve desired design goals.

[0308] References herein to "some embodiments" or "some exemplary embodiments" indicate that a particular feature, structure, or characteristic described in conjunction with the embodiment may be included in at least one embodiment. The appearance of the phrases "one embodiment," "some embodiments," or "some exemplary embodiments" in various places in the specification do not necessarily refer to the same embodiment, nor are separate or alternative embodiments necessarily mutually exclusive of other embodiments.

[0309] It should be understood that the steps of the example methods set forth herein do not necessarily need to be performed in the order described, and the order of the steps of such methods should be understood as merely examples. Likewise, in methods consistent with various embodiments, additional steps may be included in such methods, and certain steps may be omitted or combined.

[0310] As used in this application, the word "exemplary" is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "exemplary" is not necessarily to be construed as being preferred or advantageous over other aspects or designs. Instead, the use of the word is intended to present concepts in a concrete manner.

[0311] In addition, the articles "a" and "an" as used in this application and the appended claims should generally be construed to mean "one or more" unless specified otherwise or clear from context to be directed to a singular form.

[0312] Unless expressly stated otherwise, each numerical value and range should be interpreted as approximations as if the word "about" or "approximately" preceded the value or range of values.

[0313] The use of figure numbers or figure reference labels in the claims is intended to identify one or more possible embodiments of the claimed subject matter to facilitate interpretation of the claims. Such use should not be interpreted as necessarily limiting the scope of those claims to the embodiments shown in the corresponding figures.

[0314] Although elements in the following method claims, if any, are recited in a specific order and with corresponding labels, these elements are not necessarily intended to be limited to being implemented in only this specific order unless the claim recitation otherwise implies a specific order for implementing some or all of these elements.

[0315] It will be further understood that those skilled in the art may make various changes in the details, materials and arrangements of the components described and illustrated to explain the nature of the described embodiments without departing from the scope expressed in the appended claims.

Claims

1. A method for calibrating a scanning electron microscope (SEM), comprising: providing a reference sample comprising a first periodic structure interlaced with a second periodic structure, each of the first periodic structure and the second periodic structure having a period along a measurement direction; measuring, by the SEM, a shift in pitch between the first periodic structure and the second periodic structure; comparing the measured deviation of the pitch to an induced deviation of the pitch; as well as calibrating the SEM based on the comparison; Wherein the induced shift of the pitch comprises a shift predefined for the reference sample. 2 . The method of claim 1 , wherein the induced offset further comprises an offset caused by an overlay error during a manufacturing process of the reference sample. The method according to claim 1 , wherein the induced deviation is obtained by tracking using a laser interferometer.

4. The method of claim 1 , wherein the offset measured by the SEM is based on an image of the reference sample acquired by the SEM, and wherein left and right edges of the first and second periodic structures on the image are asymmetric. 5 . The method of claim 1 , wherein the first periodic structure and the second periodic structure are located on different layers of the reference sample. 6 . The method of claim 1 , wherein each of the first periodic structure and the second periodic structure comprises a set of periodic lines.

7. The method according to claim 1, further comprising: tracking critical dimensions of the first periodic structure and the second periodic structure; as well as measuring the critical dimensions of the first periodic structure and the second periodic structure by the SEM, Wherein calibrating the SEM further comprises calibrating the SEM based on a correlation between the measured critical dimension and the tracked critical dimension of the first periodic structure and the second periodic structure.

8. The method according to claim 1, further comprising: Tracking a line width of the first periodic structure or the second periodic structure; as well as measuring the line width of the first periodic structure or the second periodic structure by the SEM, Wherein calibrating the SEM further comprises calibrating the SEM based on a correlation between the measured linewidth and the tracked linewidth of the first periodic structure or the second periodic structure.

9. The method according to claim 1, wherein: The first periodic structure includes a first group of lines extending in a first direction and a second group of lines connected to the first group of lines and extending in a second direction; as well as The second periodic structure includes a third group of lines extending in the first direction and a fourth group of lines connected to the third group of lines and extending in the second direction.

10. The method of claim 1, wherein: The first periodic structure further includes a plurality of first periodic substructures disposed on different regions of the reference sample; The second periodic structure further includes a plurality of second periodic substructures disposed on different regions of the reference sample; as well as Each first periodic substructure of the plurality of first periodic substructures is paired with a corresponding second periodic substructure of the plurality of second periodic substructures to form a plurality of pairs of first periodic substructures and second periodic substructures for calibration.

11. A system for calibrating a scanning electron microscope (SEM), the system comprising: a stage configured to support a reference sample comprising a first periodic structure interleaved with a second periodic structure, the first periodic structure having an induced offset therebetween; Laser interferometer; said SEM; as well as The controller is configured as: causing the laser interferometer to track the induced shift in pitch between the first periodic structure and the second periodic structure; causing the scanning electron microscope to measure the induced shift in the pitch between the first periodic structure and the second periodic structure; as well as comparing the measured induced deviation of the pitch with the tracked induced deviation of the pitch to perform the calibration; Wherein the induced shift of the pitch comprises a shift predefined for the reference sample.

12. The system of claim 11, wherein the induced offset further comprises an offset caused by overlay errors during a manufacturing process of the reference sample.

13. The system of claim 11, wherein the first periodic structure and the second periodic structure are located on different layers of the reference sample.

14. The system of claim 11, wherein the measured induced shift is acquired based on an image of the reference sample acquired by the SEM, and wherein left and right edges of the first and second periodic structures on the image are asymmetric.

Images