WO2023155078A1

WO2023155078A1 - Method of in-line inspection of a substrate, scanning electron microscope, and computer-readable medium

Info

Publication number: WO2023155078A1
Application number: PCT/CN2022/076535
Authority: WO
Inventors: Nikolai Knaub; Meng LIN; Robert TRAUNER; Bernhard SCHÜLER; Kulpreet Singh VIRDI; Bernhard G. Mueller
Original assignee: Applied Materials, Inc.; Meng LIN
Priority date: 2022-02-16
Filing date: 2022-02-16
Publication date: 2023-08-24
Also published as: TW202347398A

Abstract

A method of in-line inspection of a substrate is described. The method includes scanning a primary electron beam (105) over a first surface portion of a substrate, the substrate being a large area substrate with a grain structure, particularly an LTPS structure; detecting, with a first detector segment (231) and with a second detector segment (232) of a segmented secondary electron detector (130), secondary electron signals during the scanning of the primary electron beam (105) over the first surface portion, wherein the first detector segment (231) provides first intensity data (I _E (x, y) ) and the second detector segment (232) provides second intensity data (I _W (x, y) ); calculating a surface steepness of the substrate (10) in an x-direction based on differences between the first intensity data (I _E (x, y) ) and the second intensity data (I _W (x, y) ); and determining a topography (T) of the first surface portion based on the surface steepness in the x-direction. Further provided is a scanning electron microscope configured to perform such a method.

Description

METHOD OF IN-LINE INSPECTION OF A SUBSTRATE, SCANNING ELECTRON MICROSCOPE, AND COMPUTER-READABLE MEDIUM

FIELD

The present disclosure relates to LTPS layer qualification on large area display substrates with a scanning electron microscope having a multi-perspective secondary electron detector as well as to methods for in-line inspection of large area substrates having an LTPS structure formed thereon. Specifically, the present disclosure relates to a method of in-line inspection of a substrate, a scanning electron microscope, and a computer-readable medium.

BACKGROUND

In many applications, deposition of thin layers on a substrate, e.g. on a glass substrate is desired. Conventionally, the substrates are coated in different chambers of a coating apparatus. For some applications, the substrates are coated in a vacuum system using a vapor deposition technique. Over the last years, the price of electronic devices and particularly opto-electronic devices has been reduced significantly. Further, the pixel density in displays has continuously increased. For TFT displays, a high density TFT integration is desired. The yield is attempted to be increased and the manufacturing costs are attempted to be reduced in spite of the increased number of thin-film transistors (TFT) per unit area or per volume area of a display.

One aspect for increasing the pixel density is the utilization of LTPS-TFTs (LTPS = Low Temperature Poly Silicon) , which can be used, e.g., in LCD or AMOLED displays. During manufacture of a LTPS-TFT, the gate electrode can be used as a mask for doping of the contact area of the active layer to the source and the drain of the transistor. The quality of the self-aligned doping can determine the yield of the manufacturing process. Accordingly, it would be beneficial to improve and control the manufacturing process. Yet, also other self-aligned doping applications, i.e. other than manufacturing of a LTPS-TFT, can benefit from an improved manufacturing process.

An LTPS structure on a substrate can be provided by depositing an amorphous silicon material on a substrate surface, e.g. at a temperature of around 500℃, i.e. a comparatively low temperature. The amorphous silicon can then be thermally annealed, e.g. at a temperature above 900℃, for example with an excimer laser through ELA annealing (Excimer Laser Annealing) , for crystallizing the amorphous silicon material. The amorphous silicon is transformed to polycrystalline silicon (p-Si) by the irradiation. Polycrystalline is advantageous in terms of electron mobility and in terms of a high integration density of circuits. Specifically, the polycrystalline has larger grains that yield a better electron mobility for TFTs due to reduced scattering from grain boundaries.

It is beneficial to monitor the ELA process during the LPTS manufacture closely, because the ELA process is critical and yield relevant. Specifically, the LTPS protrusion heights and LTPS surface roughness have an affect on the electron mobility of the LTPS structure. An in-line monitoring and inspection of LTPS structures during and (directly) after manufacture of the LTPS structure would therefore be beneficial.

The standard approach for inspecting an LTPS structure is atomic force microscopy (AFM) . However, AFM is a destructive method, since the substrate is broken into small samples before being inspected. Further, it typically takes several days to obtain the inspection results, such that AFM is not suitable for an in-line monitoring process.

Alternatively, the inspection of a substrate can be carried out by an optical system. However, the LTPS grain structure, grain sizes and topography of the grains at the grain edges are difficult to review using optical systems, since the grain size may be below the optical resolution, making the grains invisible for the optical system. An inspection of small portions of substrates has also been carried out using charged particle beam devices combined with surface etching. The surface etching enhances the contrast of the grain boundaries, but involves breaking the glass substrate, so that small pieces of the substrate are inspected instead of the substrate as a whole. Therefore, it is impossible to continue processing of the substrate, e.g. for checking the impact of the grain structure on the final product, after the inspection of the substrate.

Other inspection methods rely on a scanning electron microscope with a segmented electron detector for generating a topographic image of the sample, e.g. through shadow measurements. However, it has not yet been possible to conduct reliable and repeatable quantitative height measurements in an in-line inspection process on an LTPS structure.

Accordingly, given the increasing demands on the quality of displays on large area substrates, there is a need for an improved method for in-line inspection of large area substrates, i.e. a non-destructive inspection method that allows a further processing of the substrate after the inspection in a processing system.

SUMMARY

According to the present disclosure, a method of in-line inspection of a substrate and a scanning electron microscope utilizing such a method are provided. Further aspects, benefits, and features of the present disclosure are apparent from the claims, the description, and the accompanying drawings.

According to one aspect, a method of in-line inspection of a substrate is provided. The method includes: scanning a primary electron beam of a scanning electron microscope over a first surface portion of a substrate, the substrate being a large area substrate with a grain structure; detecting, with a first detector segment and a second detector segment of a segmented secondary electron detector, secondary electrons signals during the scanning of the primary electron beam over the first surface portion; wherein the first detector segment provides first intensity data and the second detector segment provides second intensity data; calculating a surface steepness of the substrate in an x-direction based on differences between the first intensity data and the second intensity data; and determining a topography of the first surface portion based on the surface steepness in the x-direction.

Particularly, a topography is generated and/or displayed that includes a height map of the grain structure being present on or being manufactured on the first surface portion of the substrate. Optionally, several surface portions of the substrate can be subsequently inspected, and a topography of a predetermined sub-area of the substrate or of the whole substrate can be determined. The topography may include quantitative height values of the grain structure. In some embodiments, which can be combined with other embodiments described herein, the grain structure is or includes an LTPS structure. The grain structure may form a surface layer of the substrate.

In some embodiments, a surface steepness in a y-direction is further calculated based on differences among intensity data from third and fourth detector segments, and the topography is determined based on the surface steepness in the x-direction and the surface steepness in the y-direction.

According to a further aspect, a scanning electron microscope for in-line inspection of a substrate is provided. The scanning electron microscope includes a vacuum chamber; an electron source for providing a primary electron beam propagating along an optical axis; a substrate support for arranging a substrate thereon in the vacuum chamber, the substrate being a large area substrate with a grain structure; a scan deflector for scanning the primary electron beam over a first surface portion of the substrate; a segmented secondary electron detector with a first detector segment and a second detector segment for detecting secondary electron signals during the scanning of the primary electron beam over the first surface portion, wherein the first detector segment is configured to provide first intensity data and the second detector segment is configured to provide second intensity data; and a data processing unit. The data processing unit stores instructions that, when executed, cause a processor to calculate a surface steepness of the substrate in an x-direction based on differences between the first intensity data and the second intensity data, and to determine a topography of the first surface portion based on the surface steepness in the x-direction.

Further, a computer readable medium is provided having instructions stored thereon that, when executed, cause a scanning electron microscope to perform a method of in-line inspection of a substrate as described herein.

Embodiments are also directed at apparatuses for carrying out the disclosed methods and include apparatus parts for performing each described method aspect. Method aspects may be performed by way of hardware components, a computer programmed by appropriate software, by any combination of the two or in any other manner. Furthermore, embodiments according to the disclosure are also directed at methods for operating the described apparatuses. The methods for operating the described apparatus include method aspects for carrying out the functions of the apparatus. Embodiments are also directed at large area substrates with a grain structure, particularly an LTPS structure, and displays including LTPS-TFTs having undergone processing according to the methods described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted that the appended drawings illustrate only typical embodiments and are therefore not to be considered limiting of the scope, for the disclosure may admit to other equally effective embodiments.

FIG. 1 shows a schematic view of a scanning electron microscope configured for the inspection methods according to embodiments described herein;

FIG. 2 shows a detection arrangement with a segmented secondary electron detector configured for the inspection methods described herein;

FIG. 3 shows a flow diagram illustrating calculations made for providing a topography according to the methods described herein;

FIG. 4A shows four images taken by four detector segments of a segmented secondary electron detector in accordance with the methods described herein;

FIG. 4B shows a topography being a quantitative height map of an LTPS structure on a first portion of a substrate which is provided according to methods described herein;

FIG. 5 shows a flow diagram illustrating a method of in-line inspection of a substrate according to embodiments described herein; and

FIG. 6 shows a flow diagram illustrating a method of in-line inspection of a substrate according to embodiments described herein.

DETAILED DESCRIPTION

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

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

The term “substrate” as used herein embraces both inflexible substrates, e.g., a glass substrate, or a glass plate, and flexible substrates, such as a web or a foil. The substrate may be a coated substrate, wherein one or more thin layers of materials are coated or deposited on the substrate, for example by a physical vapor deposition (PVD) process or a chemical vapor deposition process (CVD) . An LTPS structure or another grain structure is provided as a surface layer of the substrate. The LTPS structure can be generated by irradiating an amorphous silicon material, e.g., with a laser, particularly via an ELA process, as explained above.

An LTPS structure is a specific 3-dimensional structure that typically comprises a plurality of grains of generally similar size, height and appearance. The grain size is typically below the resolution achievable by optical inspection systems. Grain boundaries are typically arranged between adjacent grains, the grain boundaries protruding from the substrate in a height direction (also referred to herein as a z-direction) . For monitoring the LTPS manufacturing process, detailed information about the grain characteristics is beneficial. According to embodiments described herein, the generally similar appearance of the grains of an LTPS structure allows the provision of a simplified inspection process with a scanning electron microscope providing topographic information about the LTPS structure, and particularly providing quantitative height information about the grain boundaries.

Embodiments described herein relate to large area substrates, in particular large area substrates for the display market. According to some embodiments, large area substrates or respective substrate supports may have a size of at least 1 m ². A major surface area of the substrate or essentially the whole surface area of the substrate surface may be covered by the grain structure, e.g. 1 m ² or more. The substrate size may be from about 1.375 m ² (1100 mm x 1250 mm–Gen 5) to about 9 m ², more specifically from about 2 m ² to about 9 m ² or even up to 12 m ². For instance, a large area substrate or substrate support can be GEN 5, which corresponds to about 1.375 m ² substrates (1.1 m x 1.25 m) , GEN 7.5, which corresponds to about 4.39 m ² substrates (1.95 m x 2.25 m) , GEN 8.5, which corresponds to about 5.7 m ² substrates (2.2 m x 2.5 m) , or even GEN 10, which corresponds to about 9 m ² substrates (2.88 m × 3130 m) . Even larger generations such as GEN 11 and GEN 12 and corresponding substrate areas can similarly be implemented.

According to embodiments described herein, which can be combined with other embodiments, a primary electron beam is generated by an electron source, is directed toward the substrate with beam influencing elements, such as deflectors and lenses, and impinges on a surface of the substrate, which leads to the generation of signal electrons, particularly secondary and/or backscattered electrons. Whereas backscattered electrons are electrons of the primary electron beam reflected or elastically scattered by the substrate, secondary electrons are typically emitted from the atoms of the substrate surface upon impingement of the primary electron beam. Unlike backscattered electrons (BSEs) , secondary electrons (SEs) have a comparatively low energy when leaving the substrate, typically an energy below 1 keV, such as 100 eV or less. In contrast, BSEs typically have the impinging beams’ energies, due to elastic scattering, above 1 keV, such as 10 keV or more, or even 50 keV or more. Since SEs are moving so slowly, SEs can be attracted toward an SE detector that is on a positive potential from a wide electron collection area, which considerably increases the signals strength as compared to BSE signals, such that high-contrast and low-noise images can be generated based on SE signals.

Imaging based on BSE signals typically requires high electron landing energies on the substrate. However, high-energy primary electrons impinging on a substrate may negatively affect the substrate surface or may even damage the substrate surface. Therefore, imaging predominantly based on BSEs is less suitable for an in-line inspection process, in which the inspected substrate is to be further processed after the inspection. Embodiments described herein provide a method of in-line inspection and imaging based on secondary electron signals, which provide high-contrast images and enable the usage of reduced landing energies, which can avoid substrate damage. Specifically, according to embodiments described herein, the primary electron beam may impinge on the substrate surface with a landing energy of 2 keV or less, particularly 1 keV or less.

FIG. 1 shows a scanning electron microscope 100 configured for inspection methods described herein. A primary electron beam 105 (also simply referred to herein as an “electron beam” ) may be generated by an electron source 112. Further beam shaping means like a suppressor, an extractor, and/or an anode may be provided in a gun chamber 110 of the scanning electron microscope 100. The electron beam may be aligned to a beam limiting aperture, which may be dimensioned to shape the beam, i.e. the beam limiting aperture may block a portion of the electron beam. The electron source 112 can, for example, include a thermal field emitter (TFE) . The gun chamber 110 may be evacuated to a pressure of 10 ^-8 mbar or less, particularly 10 ^-9 mbar or less.

The scanning electron microscope 100 may include a vacuum chamber 101 that houses the electron-optical elements that are configured for influencing the primary electron beam 105 and for focusing the electron beam on the substrate surface. Particularly, a condenser lens 123 may be provided in the vacuum chamber 101 for collimating the electron beam. Further electron-optical elements 126 can be provided in the vacuum chamber 101. The further electron-optical elements 126 can be selected from the group consisting of: a stigmator, correction elements for chromatic and/or spherical aberrations, and alignment deflectors for aligning the electron beam to an optical axis A of scanning electron microscope.

The primary electron beam 105 can be focused on the substrate 10 by an objective lens 140. The substrate 10 is positioned at a substrate position on a substrate support 150 that may be movable. Upon impingement of the electron beam on the substrate 10, secondary electrons are generated which can be detected by a secondary electron detector 130.

According to embodiments described herein, the secondary electron detector 130 is a segmented detector that includes several detector segments. FIG. 1 shows a segmented secondary electron detector 130 having a first detector segment 231 and a second detector segment 232, however, it is to be understood that the segmented secondary electron detector 130 may include more than two detector segments, particularly four detector segments (as is depicted in FIG. 2) . Specifically, the secondary electron detector 130 may be a four-quadrant electron detector for SEM image formation. The secondary electron detector 130 may be a four-quadrant solid state electron detector for SE signal detection and image formation based on the detected SE signal. The detector segments may respectively include a scintillator segment configured to transform the secondary electrons into photons.

Each detector segment of the secondary electron detector 130 can separately detect a respective secondary electron signal during the scanning of the primary electron beam 105 over the substrate surface, and each detector segment can separately provide intensity data based on the respective secondary electron signal. A separate image can be generated and/or displayed based on the intensity data from each detector segment, which allows image formation from two or more perspectives, particularly from four perspectives, based on the detected SE signals, enabling topographic contrast measurements. According to embodiments described herein, multi-perspective images allow the determination of a topography of the substrate, including a height information.

A two-stage deflection system (not shown) can be provided, for example downstream of the condenser lens 123. As shown in FIG. 1, the objective lens 140 can have a magnetic lens component having

pole pieces

142 and 146 and having a coil 144. The objective lens 140 can focus the primary electron beam 105 on the substrate 10. Further, an upper electrode 152 and a lower electrode 154 can form an electrostatic lens component of the objective lens 140.

A scan deflector 170 for scanning the electron beam over the surface of the substrate can be provided. The scan deflector 170 can, for example, be an electrostatic scanning deflector, which may be configured for high pixel rates.

The lower electrode 154 may be connected to a voltage supply (not shown) . The lower electrode 154 can be configured as a deceleration electrode, i.e. as a retarding field lens component, of the objective lens 140. Herein, the lower electrode 154 may be set at an electric potential to provide a landing energy of the charged particles on the substrate of 2 keV or below, e.g. 500 eV or less. As exemplarily illustrated in FIG. 1, the substrate support 150 can be set to a ground potential.

According to some embodiments, which can be combined with other embodiments described herein, the deceleration of the primary electron beam 105 can be provided in the vicinity of the substrate 10, for example in or behind the objective lens, or a combination thereof. A deceleration can be provided by the lower electrode 154, i. e., a retarding field electrode. A deceleration can, e.g., be provided by an electrostatic lens component of the objective lens. Additionally or alternatively, a retarding bias voltage can be applied to the substrate and/or the substrate support, in order to provide a retarding field lens component. The objective lens can be an electrostatic-magnetic compound lens having, e.g., an axial gap or a radial gap, or the objective lens can be an electrostatic retarding field lens.

An advantage of having a landing energy of 2keV or below, particularly a landing energy of 1 keV or below, is that the primary electron beam impinging on the substrate generates a stronger SE signal of the topmost layer compared to high-energy electron beams. Since layers, e.g. a LTPS structure, deposited on the substrate may be thin and since high-energy electrons penetrate deeply into the substrate, i.e. below the LTPS structure, only few electrons may generate a signal that contains information about the LTPS structure. In contrast, low-energy electrons, such as electrons having a landing energy of 2keV or below, penetrate into a shallow region of the substrate only and thus provide more information about the LTPS structure. Accordingly, an improved topography of the LTPS structure can be provided.

For high-resolution applications it is beneficial to provide a landing energy of e.g. 2 keV or below, such as 1 keV or below, while having a high-energy electron beam in the column, for example with a beam energy of 10 keV or above, such as 30 keV or above. Embodiments may include a deceleration above the substrate 10, e.g. within the objective lens and/or between the objective lens and the substrate 10, of a factor of 5 or more, such as a factor of 10 or more. For other applications, a low landing energy of 2 keV or below may also be provided without a deceleration, e.g. in the event the beam energy within the column is not above 2 keV.

In the following, a method of in-line inspection of a substrate having a grain structure, particularly an LTPS structure, in accordance with embodiments described herein will be explained. The in-line inspection method enables the determination of a topography of at least a first surface portion of the substrate. A topography includes spatial information of the grain structure in three dimensions, particularly including quantitative height information about protrusions of the grain structure.

The substrate 10 that is a large area substrate with a grain structure (i. e., a low temperature polysilicon structure) is arranged on the substrate support 150 in the vacuum chamber 101. The primary electron beam 105 is scanned over a first surface portion of the substrate, particularly in a 2-dimensional scanning pattern, e.g. in a raster-scanning pattern. The first surface portion may correspond to a surface area of the substrate that can be scanned with the electron beam without moving the substrate, i. e., the first surface portion may correspond to a “field of view” of the scanning electron microscope (also referred to as one “frame” ) . For example, the first area may have, in the substrate plane, a width (W) between 5 μm and 20 μm and a length (L) between 5 μm and 20 μm, as it is schematically depicted in FIG. 4A. Several frames can be subsequently scanned for generating a topography of a larger surface area of the substrate or of the whole substrate. Typically, the LTPS structure covers a major part of the first substrate area. In particular, the LTPS structure covers a major part of or the large-area substrate.

During the scanning over the first surface portion, secondary electron signals are detected with a first detector segment 231 and with a second detector segment 232 of the segmented secondary electron detector 130. The first detector segment 231 and the second detector segment 232 may be located radially opposed to each other in an x-direction in relation to the optical axis A, as it is schematically depicted in FIG. 1. Specifically, the first detector segment 231 may detect a first SE signal and the second detector segment 232 may detect a second SE signal during the scanning over the first surface portion.

The secondary electron signals may be processed by the secondary electron detector 130 to provide the respective intensity data. Specifically, the first SE signal may be processed by a first signal processing unit 235 for providing the first intensity data (I _E (x, y) ) , and the second SE signal may be processed by a second signal processing unit 236 for providing the second intensity data (I _W (x, y) ) .

Processing may include any one or more of: amplifying the SE signal, e.g. with a photomultiplier tube and/or another amplifier, for example in order to obtain intensity data that provide a predetermined contrast; adding an offset or shift to the SE signal, e.g. in order to obtain intensity data providing a predetermined brightness; digitalizing the SE signal, e.g. with an A/D converter, such that the intensity data can be processed further and/or displayed with a computer; discretization of the SE signal, e.g. in relation to a predetermined detector scale (corresponding to a grey scale of the detector images) , such that the intensity data can be shown as an image having said grey scale. Processing may particularly include transferring raw secondary electron signals to digital intensity data that can be displayed as an image that shows the relevant features of the LTPS structure without saturation. In some embodiments, the intensity data are displayable as an image of the first surface area having a predetermined grey scale.

For example, in some embodiments, the SE signals are detected with scintillator segments 136, the generated light signals are guided with a respective light guide 134 to a photo multiplier or another signal detection element, are transformed into an electric signal, are amplified and are digitalized by a respective

signal processing unit

235, 236 for providing the respective intensity data that can then be displayed as an image of the first surface area. The intensity data provided by the detector segments of the segmented secondary electron detector can then be forwarded to a data processing unit 240, e.g., to a computer, as it is schematically depicted in FIG. 1.

The first detector segment 231 is configured to provide the first intensity data (I _E (x, y) ) based on the first SE signal, and the second detector segment 232 is configured to provide the second intensity data (I _W (x, y) ) based on the second SE signal. Herein, the subscript “E” stands for intensity data from an “eastern” detector segment, and the subscript “W” stands for intensity data from a “western” detector segment.

According to the embodiments described herein, a surface steepness of the substrate in the x-direction is calculated based on differences between the first intensity data and the second intensity data. The x-direction is a direction parallel to the plane of the substrate, particularly corresponding to the direction in which the first detector segment 231 and the second detector segment 232 are opposing each other, as it is depicted in FIG. 1. In some embodiments, the surface steepness in the x-direction is determined at a plurality of pixel points of the first substrate area, particularly as a two-dimensional array of x-gradient values evenly distributed over the first substrate area.

Thereafter, the topography of the first surface portion is determined based on the calculated surface steepness in the x-direction.

In some embodiments, which can be combined with other embodiments described herein, the secondary electron detector 130 comprises at least four detector segments, as it is schematically depicted in FIG. 2. Specifically, the secondary electron detector 130 may be a quad-detector that includes a first detector segment 231, a second detector segment 232, a third detector segment 233 and a fourth detector segment 234.

The first and second detector segments may be located radially opposed to each other in relation to the optical axis A, and the third and fourth detector segments may be located radially opposed to each other in relation to an optical axis. The first and second detector segments may be opposed to each other in the x-direction, such that the surface steepness in the x-direction can be determined based on the intensity data from the first and second detector segments. The third and fourth detector segments may be opposed to each other in the y-direction, such that the surface steepness in the y-direction can be determined based on the intensity data from the third and fourth detector segments. The x-direction and the y-direction may be parallel to the plane of the substrate and may be perpendicular to each other.

Specifically, as depicted in FIG. 2, the detector segments may be arranged to provide an essentially annular segmented detection surface that surrounds the optical axis A, e.g. an annular segmented scintillator surface 136. An opening 201 for the primary electron beam may be arranged centrally in the annular segmented detection surface, such that the primary electron beam 105 can propagate through the opening 201 toward the substrate. The secondary electron detector 130 may be arranged surrounding the optical axis A at a position upstream of the objective lens 140 and downstream of the condenser lens 123 (see FIG. 1) .

The method of in-line inspection may further include detecting secondary electron signals with the third detector segment 233 and the fourth detector segment 234. Specifically the third detector segment 233 may detect a third SE signal and the fourth detector segment 234 may detect a fourth SE signal during the scanning over the first surface portion. The third and fourth secondary electron signals are then processed by the secondary electron detector 130 to provide respective intensity data. Specifically, the third SE signal may be processed to provide third intensity data (I _N (x, y) ) , for example by a third signal processing unit 237, and the fourth SE signal may be processed to provide fourth intensity data (I _S (x, y) ) , e.g. by a fourth signal processing unit 238. Herein, the subscript “N” stands for intensity data from a “northern” detector segment, and the subscript “S” stands for intensity data from a “southern” detector segment.

A surface steepness of the substrate in the y-direction can be calculated based on differences between the third intensity data (I _N (x, y) ) from the third detector segment 233 and the fourth intensity data (I _N (x, y) ) from the fourth detector segment 234. The y-direction is a direction parallel to the plane of the substrate, particularly corresponding to the direction in which the third detector segment 233 and the fourth detector segment 234 are opposite each other, as depicted in FIG. 2.

The topography of the first surface portion can be determined based on the calculated surface steepness in the x-direction and the calculated surface steepness in the y-direction. Specifically, a quantitative height map of the first surface region can be determined and/or displayed based on the surface steepness in the x-direction and the surface steepness in the y-direction.

FIG. 3 shows a diagram exemplarily illustrating calculations made for providing a topography of a substrate portion according to methods described herein, utilizing the intensity data provided by a total of four detector segments of a quad detector.

In stage (i) of FIG. 3, a surface steepness in the x-direction (i. e., an x-gradient value) at a specific pixel point x _i is calculated based on the difference between the first intensity data (I _E (x _i) ) from the first detector segment 231 and the second intensity data (I _W (x _i) ) from the second detector segment 232 at the respective pixel point, as follows:

In stage (ii) of FIG. 3, a surface steepness in the y-direction (i. e., an y-gradient value) at a specific pixel point y _i is calculated based on the difference between the third intensity data (I _N (y _i) ) from the third detector segment 233 and the fourth intensity data (I _S (y _i) ) from the fourth detector segment 234 at the respective pixel point, as follows:

In stage (iii) of FIG. 3, the topography of the first surface portion as a function of x-position (at a constant position y _i in the y-direction) can be determined based on the surface steepness values in x-and y-directions as follow:

Herein, z (x, y) represents a height value of the substrate at the position (x, y) . Hence, the topography can be determined as a height map of the first surface portion. Since the secondary electron signals may actually be detected at a finite number of substrate positions (corresponding to a finite number of pixel points) , the intensity data may be defined only at said finite number of pixel points. Hence, the above integrals may stand for sums over a discrete number of values, and the functions herein may be defined at a discrete number of pixel points, respectively.

Stage (iv) of FIG. 3 shows a graphical representation of the topography. The height function z (x, y _i) of stage (iii) , i.e. the substrate height as a function of x-position at a specific y-position, is depicted at several positions y _i in the y-direction, resulting in a three-dimensional surface topography of the first surface portion.

According to some embodiments described herein, the topography is determined by an integration (or sum) in the x-and y-directions over the surface steepness in the x-direction and the surface steepness in the y-direction, for example as illustrated by the formula of FIG. 3 (iii) .

The methods described herein may include a preceding stage of conducting a calibration measurement on a calibration substrate with a height structure having a known topography for determining a scaling factor. In particular, the calibration substrate may have a known height map expressed in quantitative height values as a function of position on the calibration substrate. The scaling factor can then be used for determining a quantitatively correct topography of substrates to be inspected (i. e., having an unknown LTPS structure) based on the surface steepness. In particular, if a substrate with an unknown LTPS structure is inspected, the integral of in FIG. 3 (iii) can be multiplied with the scaling factor, in order to obtain quantitative height values of the substrate, for example height values expressed in nanometers as a function of substrate position (x, y) . Accordingly, a quantitative topography or quantitative height map of the substrate or of surface portions thereof can be provided.

The calibration measurement can be conducted as follows: The calibration substrate can be inspected according to the methods described herein, and a topography of the calibration substrate can be determined. The determined topography of the calibration substrate can be compared with the known topography of the calibration substrate for determining the scaling factor. The scaling factor may be a multiplicator value that is to be multiplied with the determined topography for obtaining the known topography of the calibration substrate.

According to some embodiments, which can be combined with other embodiments described herein, the topography includes a (quantitative) height map of the first surface portion of the substrate, particularly a height map including height values of the LTPS structure relative to a predetermined height level and provided as a 2-dimensional array or function of the position (x, y) of the substrate. The height map can be generated and/or displayed on a screen. Further calculations and/or statistics can be carried out based on the height map.

According to some embodiments described herein, a quantitative height map of an LTPS structure can be generated in a repeatable way. In other words, repeated measurements of the same surface area of the substrate reliably yield the same height map of the LTPS structure, which has not been possible before based on secondary electron signals in a reliable way. Previous topographic imaging methods relied on a determination of peak heights based on the length of shadows in one or more topographic images, wherein the length of the shadows could be calibrated to a height of peaks of the substrate surface. However, the repeatability of such measurements is limited. Other known topographic imaging methods rely on backscattered electron signals. Such methods are not beneficial for an in-line inspection of a substrate that is to be processed further after the inspection.

Topographic measurements that rely on electron signals from different detector segments can be impaired or negatively affected, if the secondary electron detector (automatically) adjusts the brightness and/or the contrast of the secondary electron signals for providing the intensity data. Specifically, conventional secondary electron detectors typically suitably adjust a brightness setting and a contrast setting in a way such that the provided intensity data, if displayed as an image, allow an easy and convenient visual inspection of the substrate. However, a brightness adjustment, e.g., if done automatically, constitutes an offset or shift on the detected SE signals that may distort the measurements and may make quantitative measurements non-repeatable. In particular, an automatic brightness adjustment conducted on an SE signal in a detector segment may shift the provided intensity data in a non-repeatable way.

Therefore, according to some embodiments described herein, the detected electron signals are processed without a brightness adjustment, particularly without any signal shift or signal offset for adjusting the brightness, for providing the intensity data. Specifically, a first electron signal detected by the first detector segment is processed without a signal shift or signal offset due to a brightness adjustment for providing the first intensity data, and/or a second electron signal detected by the second detector segment is processed without a signal shift or signal offset due to a brightness adjustment for providing the second intensity data. Also the SE signals from the third and fourth detector segments may be processed similarly, i.e. without any brightness adjustment.

In particular, all detector segments of the segmented secondary electron detector may have a predetermined (and constant) brightness setting during the in-line inspection methods described herein, particularly during in-line inspection of several surface portions of the substrate. In some implementations, all detector segments may have a brightness setting of zero, which corresponds to “no shift” (or “no offset” ) of the detected SE signals for providing the intensity data. Specifically, the first surface portion and further surface portions of the substrate may be subsequently inspected with the predetermined and constant brightness setting of the detector segments. In some embodiments, a brightness adjustment of the detector segments may be completely switched off during in-line inspection.

Switching off a brightness adjustment of the detector segments avoids a falsification of the SE signals by signal offsets that would make a repeatable determination of quantitative height values difficult or impossible.

According to some embodiments, which can be combined with other embodiments described herein, a contrast adjustment is applied to the SE signals for providing the intensity data, respectively. The contrast adjustment may be applied such as to avoid a saturation of the intensity data, particularly with an aim to provide image data that are displayable without saturation and showing the features of interest of the LTPS structure. In particular, the contrast adjustment may lead to intensity data from each detector that can be respectively displayed as an image of the first surface area having the predetermined grey scale, as it is depicted in FIG. 4A, such that the grains of the LTPS structure are clearly visible without saturation.

In particular, a first contrast adjustment may be applied to a first SE signal detected by the first detector segment for providing the first intensity data, and/or a second contrast adjustment may be applied to a second electron signal detected by the second detector segment for providing the second intensity data. Similarly, contrast adjustments may also be applied to the third and fourth SE signals detected by the third and fourth detector segments, respectively.

In some embodiments, which can be combined with other embodiments described herein, the contrast adjustment may be conducted such that an intensity histogram of the intensity data from a respective detector segment has a pre-defined mean value and/or has a mean value in a pre-defined range in relation to a predetermined detector scale (corresponding to a grey scale of the images provided by the detector) . For example, the intensity histogram of the first intensity data (I _E (x, y) ) (over the first surface portion of the substrate) may have substantially a first pre-defined mean value, and/or the intensity histogram of the second intensity data (I _W (x, y) ) (over the first surface portion of the substrate) may have substantially a second pre-defined mean value on a grey scale (corresponding to a detector scale, e.g. 0-255 possibly intensity values for an 8-bit detector) . The first pre-defined mean value may correspond to the second pre-defined mean value.

In some embodiments, the method may further include a preceding stage of contrast balancing for determining first and second contrast adjustment settings for the first and second contrast adjustments. The preceding stage of contrast balancing may be conducted after focusing the primary electron beam on the first surface portion of the substrate and before taking the actual SE measurements for the inspection. The preceding stage of contrast balancing may include the determination of a respective contrast adjustment setting for each of the detector segments, for example a respective amplification factor for amplifying the respective SE signal in the respective detector segment. The preceding stage of contrast balancing may be conducted as an auto-contrast function respectively carried out separately for each detector segment.

In some embodiments, which can be combined with other embodiments described herein, the method may include: setting a pre-defined mean value and/or a pre-defined range around a pre-defined mean value (on a detector scale, e.g. among values from 0 to 255 for an 8-bit detector) ; conducting the preceding stage of contrast balancing based on the pre-defined mean value and/or based on the pre-defined range for determining the first and second contrast adjustment settings; and applying the first contrast adjustment with the first contrast adjustment setting to the first electron signal for providing the first intensity data, and applying the second contrast adjustment with the second contrast adjustment setting to the second electron signal for providing the second intensity data. Similarly, third and fourth contrast adjustment settings may be determined for the third and fourth detector segments and applied to the first and fourth detector segments for providing the third and fourth intensity data.

Accordingly, the intensity data provided by each detector segment during inspection are such that the respective intensity histogram has a pre-defined mean value (or a predefined range around a pre-defined mean value) , such that the features of interest of the LTPS structure will be well visible and contained in the intensity data without a risk of saturation.

In particular, the pre-defined mean value and/or the pre-defined range around a pre-defined mean value on the detector scale (corresponding to a grey scale of the images provided by the detector segments) is set to be the same for all detector segments (231, 232, 233, 234) of the segmented secondary electron detector (130) .

FIG. 4A shows four images of the first surface portion of the substrate taken by the four detector segments of a segmented secondary electron detector in accordance with the methods described herein. The image on the top left shows the intensity data (I _E (x, y) ) provided by the first detector segment, the image on the bottom right shows the intensity data (I _W (x, y) ) provided by the second detector segment, the image on the top right shows the intensity data (I _N (x, y) ) provided by the third detector segment, and the image on the bottom left shows the intensity data (I _S (x, y) ) provided by the fourth detector segment, respectively as a function of (x, y) -position on the substrate. The first surface area of the substrate is respectively shown that may correspond to the “field of view” of the scanning electron microscope. Further, the intensity histograms (here exemplarily of an 8-bit detector, i.e. of a detector having a detector scale from 0 to 255 corresponding to a grey scale of 256 grey values) of the intensity data from the first detector segment and from the fourth detector segment are exemplarily shown on the left side.

In the depicted example, a pre-defined mean value of about 90 (90+/-10%) with respect to the detector scale (0 to 255) has been set by a user for each detector segment. This means that the intensity data are provided based on the SE signals such that the resulting intensity histograms have a mean value of (about) 90, respectively, e.g. 90+/-10%. In particular, a respective contrast adjustment is separately applied to the SE signals detected by each detector segment based on a respective predetermined contrast adjustment setting, such that the intensity histograms of the intensity data have a pre-set distribution, particularly a pre-defined mean value. The mean value should preferably be set to lie in a central region of the detector scale, for example approximately half of a full-scale value (here: 256/2 = about 128) of the detector segment, such that there will most probably not be any saturation. The pre-defined mean value can be set to be the same for each detector segment.

In some embodiments, the full-scale value of the detector that may correspond to the number of grey values of the grey scale in the images, may be 2 ⁿ, n being an integer of six or more, for example eight, i.e. the detector may have an n-bit A/D converter, such as an 8-bit A/D converter. The pre-defined value for the contrast balancing may be set to be in a range of ( (2 ⁿ/2) –0, 2*2 ⁿ) to ( (2 ⁿ/2) + 0, 2*2 ⁿ) in relation to the detector scale. For example, if n=8, the pre-defined value may be in a range from 70 to 150, for example 90 as shown in FIG. 4A. Alternatively or additionally, a range can be set around a pre-defined mean value, e.g. a range of +/-10%around a pre-defined mean value. For example, a range of 10%can be set around a pre-defined mean value of 90, which leads to intensity data having a mean value in a range from 90-10%to 90+10%.

The pre-defined mean value and/or the pre-defined range can be set to be the same for each detector segment. Pre-setting a same mean value and/or range of the intensity histograms is possible according to embodiments described herein, because an LTPS structure is imaged. An LTPS structure looks generally similar, i.e. has generally similar gradient values, height values and grain sizes, in the whole first substrate portion, particularly over the whole substrate surface, and even from LTPS substrate to LTPS substrate. This means that the images and hence a specific contrast setting for an auto-balancing function may be suitable from substrate portion to substrate portion, and even from substrate to substrate, while still allowing repeatable height measurements, and saturation can be avoided.

Accordingly, a simple and convenient method of in-line inspection of LTPS structures based on SE signals that at the same time provides accurate and repeatable quantitative height values is provided according to the embodiments described herein.

FIG. 4B shows a topography T being a quantitative height map of an LTPS structure arranged in a first surface portion of the substrate. The height map is determined based on the intensity data from the four detector segments depicted in FIG. 4A, particularly through integration or summation as illustrated in FIG. 3. As it is depicted in FIG. 4B, the height of protrusions of the LTPS structure on a sub-micron scale can be repeatably determined, and the height may depend on the μm/pixel resolution. For example, protrusion heights can be determined with a resolution of 10 nm or less, particularly 5 nm or less, or even 1 nm or less.

According to some embodiments, which can be combined with other embodiments described herein, the LTPS structure has a grain structure with grain boundaries, and heights of the grain boundaries of the grain structure are provided by the topography, particularly with a height resolution of 10 nm or less. The grain boundaries of an LTPS structure typically have a height of 50 nm or less. In particular, heights of peaks of a boundary of grains of the grain structure are determined in quantitative values.

The method may further include verifying and/or adjusting process parameters of a manufacturing method of the LTPS structure based on the topography. Specifically, characteristics of the grain structure and/or statistics of the parameters of the grain structure can be used to verify process parameters of a method of manufacturing the LTPS film. A feedback to the manufacturing process of the LTPS structure can be provided. For example, an LTPS-TFT process can be controlled by electron beam review (EBR) according to embodiments described herein.

According to embodiments described herein, a grain structure can be described by the size of the grains, the shape of the grains, the distribution of the grains, the area of the grains, and the like. Said parameters can be evaluated with statistical analysis methods with respect to one or more of the parameters. For example, a characteristic of grains of the grain structure can be determined as an arithmetic mean value, as a quadratic mean value, as a weighted mean value, as a minimum value, as a maximum value and/or as a median value.

According to embodiments, which can be combined with other embodiments described herein, the topography can be used by a software algorithm, e.g., for analyzing the grain structure. A calculation of grain structure characteristics may also include a watershed algorithm. A calculation based upon the topography can provide at least one characteristic of grains of the grain structure selected from: an area of one or more grains of the grain structure, a circumference of one or more grains of the grain structure, a minimum size of grains of the grain structure, a maximum size of grains of the grain structure, a size of grains of the grain structure along a predetermined direction, and the height of peaks of a boundary of grains of the grain structure. Specifically, a two (or more) channel detector, such as a four-channel detector is used to image the LTPS structure from two or more, e.g. four different perspectives, respectively. The two or more, e.g. four perspectives give surface information to detect and evaluate the size, uniformity, local distribution and all statistics for the parameters describing the thin film with the grain structure, i.e. an LTPS structure, in quantitative and repeatable values.

According to some embodiments, which can be combined with other embodiments described herein, algorithms for identifying characteristics of the grain structure or statistics on parameters of the grain structure of the thin-film can be applied to the height map.

According to embodiments described herein, the area of grains of the grain structure, the circumference of grains of the grain structure, and/or one or more sizes of the grains of the grain structure can be measured. For example, the grains having a size of about 100 nm to 500 nm can be measured. According to some embodiments, which can be combined with other embodiments described herein, the field of view, which can be measured by scanning the primary electron beam over the first substrate portion, can have a size of, e.g., up to 10 μm.

According to some implementations, the inspection according to embodiments described herein may be conducted in-line as an intermediate action during display manufacture. For example, the vacuum chamber of the scanning electron microscope can be provided in-line with another, previous and/or subsequent, testing or processing chamber or procedure. Due to the low energies of the charged particle beam of 2 keV or below on the substrate, the structures provided on the substrate are not negatively affected. Accordingly, the substrate can be provided for further processing in the display manufacturing fab. As understood herein, the number of substrates to be tested can be 10%to 100%of an entire number of substrates in the fab for display manufacturing. Accordingly, the scanning electron microscope can be arranged as an in-line tool, even though without necessarily testing 100%of the substrates in the production line.

FIG. 5 shows a flow diagram illustrating a method of in-line inspection of a substrate according to embodiments described herein.

In box 602, a primary electron beam of a scanning electron microscope is scanned over a first surface portion of a large area substrate.

In box 604, SE signals are detected with a segmented secondary electron detector during the scanning of the primary electron beam over the first surface portion, particularly with first to fourth detector segments of the secondary electron detector, in particular simultaneously. The four detector segments may be arranged pairwise around the optical axis of the scanning electron microscope, i.e. a first segment pair being an eastern and a western detector segment for x-gradient determination, and a second segment pair being a northern and a southern detector segment for y-gradient determination.

In box 606, first to fourth intensity data are provided by the first to fourth detector segments based on the respective SE signal. In some embodiments, providing the intensity data does not include any brightness adjustment of the SE signals, but does include applying a contrast adjustment to the respective SE signal. The contrast adjustment can be applied to each SE signal based on a respective contrast adjustment setting determined at a preceding stage of contrast balancing. The contrast adjustment can be applied such that an intensity histogram of each intensity data has a pre-defined mean value (or a pre-defined range around a pre-defined mean value) .

In box 608, a surface steepness of the substrate, particularly in x-and y-directions, respectively, is calculated based on differences between the first and second intensity data and based on differences between the third and fourth intensity data.

In box 610, a topography of the first surface portion is determined based on the calculated surface steepness, particularly based on the surface steepness in the x-and y-directions. The topography may include a height map of the first surface portion.

Several surface portions of the substrate can be subsequently inspected, and a height map of a predetermined sub-area of the substrate or of the whole substrate can be generated and/or displayed.

In box 702, an LTPS substrate may be placed on a substrate stage of a scanning electron microscope, and the substrate stage may be moved to a specific position, such that a primary electron beam can be directed onto a first surface portion of the substrate, where an LTPS structure is provided.

In box 704, the scanning electron microscope may conduct autofocusing on the first surface portion of the substrate.

In box 706, the scanning electron microscope may conduct an automatic contrast balancing for determining a contrast adjustment setting for each of the detector segments.

In box 708, SE signals are detected by the detector segments and respective intensity data are provided by the detector segments, in accordance with

boxes

604 and 606 of FIG. 5.

In box 710, the topography of the first surface portion of the substrate is determined based on the intensity data of the detector segments, in accordance with

boxes

608 and 610 of FIG. 5. The topography may be calibrated by multiplication with a scaling factor, such that the topography contains quantitative height values of the substrate (the multiplication with the scaling factor may be implemented in the data processing unit 240, such as to be automatically conducted) .

In box 712, 3D-metrology statistics may be conducted based on the topography. For example, characteristics of grains of the LTPS structure may be determined based on the topography.

In box 714, feedback may be given to an LTPS manufacturing process based on the conducted statistics. For example, manufacturing parameters of an ELA process can be adapted based on the determined grain characteristics.

According to embodiments described herein, a topography of an LTPS substrate can be determined based on SE topographic imaging, wherein a stable contrast balancing is provided while simultaneously keeping a constant brightness value of all detector segments of the segmented SE detector. This enables quantitative height measurements with a high repeatability. Further, the calibration with a scaling factor determined with a calibration substrate enables the measurement of a well-defined sample height.

Whereas embodiments specifically relate to the inspection of a large-area substrate with an LTPS structure, the present disclosure is not limited thereto. For example, a substrate with another three-dimensional structure provided at a surface thereof may be likewise inspected. The methods described herein are particularly suitable for inspecting large-area substrates having a surface structure that includes a plurality of grains, i. e., a grain structure, wherein the grains may have a generally similar structure, e.g. similar heights and/or similar sizes. Therefore, references to an “LTPS structure” herein likewise apply to another grain structure being inspected.

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

Claims

A method of in-line inspection of a substrate, comprising:

scanning a primary electron beam (105) over a first surface portion of a substrate, the substrate being a large area substrate with a grain structure;

detecting, with a first detector segment (231) and a second detector segment (232) of a segmented secondary electron detector (130) , secondary electron signals during the scanning of the primary electron beam (105) over the first surface portion, wherein the first detector segment (231) provides first intensity data (I _E (x, y) ) and the second detector segment (232) provides second intensity data (I _W (x, y) ) ;

calculating a surface steepness of the substrate (10) in an x-direction based on differences between the first intensity data (I _E (x, y) ) and the second intensity data (I _W (x, y) ) ; and

determining a topography (T) of the first surface portion based on the surface steepness in the x-direction.
The method of claim 1, wherein secondary electron signals are further detected by a third detector segment (233) and a fourth detector segment (234) of the segmented secondary electron detector (130) , and the third detector segment (233) provides third intensity data (I _N (x, y) ) and the fourth detector segment provides fourth intensity data (I _S (x, y) ) ;

a surface steepness of the substrate in a y-direction is calculated based on differences between the third intensity data (I _N (x, y) ) and the fourth intensity data (I _S (x, y) ) ; and

the topography (T) of the first surface portion is determined based on the surface steepness in the x-direction and the surface steepness in the y-direction.
The method of claim 2, wherein the first and second detector segments (231, 232) are located radially opposed to each other in the x-direction, and the third and fourth detector segments (233, 234) are located radially opposed to each other in the y-direction.
The method of claim 2 or 3, wherein the topography (T) is determined via an integration or sum in the x-direction and in the y-direction over the surface steepness in the x-direction and the surface steepness in the y-direction.
The method of any of claims 1 to 4, wherein a first electron signal detected by the first detector segment is processed without a signal shift or signal offset due to a brightness adjustment for providing the first intensity data, and/or wherein a second electron signal detected by the second detector segment is processed without a signal shift or signal offset due to a brightness adjustment for providing the second intensity data.
The method of any of claims 1 to 5, wherein all the detector segments (231, 232, 233, 234) of the segmented secondary electron detector (130) have a predetermined and constant brightness setting during in-line inspection of several surface portions of the substrate, more particularly a brightness setting of zero, respectively.
The method of any of claims 1 to 6, wherein a first contrast adjustment is applied to a first electron signal detected by the first detector segment for providing the first intensity data, and/or wherein a second contrast adjustment is applied to a second electron signal detected by the second detector segment for providing the second intensity data.
The method according to claim 7, further comprising a preceding stage of contrast balancing for determining first and second contrast adjustment settings for the first and second contrast adjustments.
The method of claim 7 or 8, wherein the first and second contrast adjustments are applied such that an intensity histogram of the first intensity data (I _E (x, y) ) and an intensity histogram of the second intensity data (I _W (x, y) ) have a respective pre-defined mean value and/or a mean value in a pre-defined range on a detector scale.
The method of claims 8 and 9, further comprising

setting the pre-defined mean value and/or the pre-defined range;

conducting the preceding stage of contrast balancing based on the pre-defined mean value and/or based on the pre-defined range for determining the first and second contrast adjustment settings; and

applying the first contrast adjustment with the first contrast adjustment setting to the first electron signal, and applying the second contrast adjustment with the second contrast adjustment setting to the second electron signal.
The method of claim 9 or 10, wherein the pre-defined mean value and/or the pre-defined range is set to be the same for all detector segments (231, 232, 233, 234) of the segmented secondary electron detector (130) .
The method of any of claims 9 to 11, wherein

(i) the detector scale has a total of 2 ⁿ values, and the pre-defined mean value is in a range from ( (2 ⁿ/2) –0, 2*2 ⁿ) to ( (2 ⁿ/2) + 0, 2*2 ⁿ) , and/or

(ii) wherein the pre-defined range is a range of +/-10%or less around a pre-defined mean value defined as in (i) .
The method of any of claims 1 to 12, wherein the topography (T) includes a height map (z (x, y) ) of the first surface portion with quantitative height values of the grain structure relative to a predetermined height level provided as a 2-dimensional array or function.
The method of any of claims 1 to 13, further comprising a preceding stage of conducting a calibration measurement on a calibration substrate with a height structure having a known topography for determining a scaling factor that is used for determining the topography (T) .
The method of any of claims 1 to 14, wherein the grain structure is an LTPS structure with grain boundaries, and heights of the grain boundaries are provided by the topography.
The method of any of claims 1 to 15, further comprising verifying and/or adjusting process parameters of a manufacturing method of the grain structure based on the topography.
The method of any of claims 1 to 16, further comprising determining, from the topography (T) , at least one characteristic of grains of the grain structure selected from: an area of one or more grains, a circumference of one or more grains, a minimum size of the grains, a maximum size of the grains, a dimension of one or more grains along a predetermined direction, and a height of grain boundaries of one or more grains.
The method of claim 17, wherein the at least one characteristic of the grains is determined as an arithmetic mean value, as a quadratic mean value, as a weighted mean value, as a minimum value, as a maximum value, and/or as a median value.
A scanning electron microscope (100) for in-line inspection of a substrate (10) , comprising

a vacuum chamber (101) ;

an electron source (112) for providing a primary electron beam (105) propagating along an optical axis (A) ;

a substrate support (150) for arranging a large area substrate with a grain structure thereon in the vacuum chamber;

a scan deflector (170) for scanning the primary electron beam over a first surface portion of the substrate;

a segmented secondary electron detector (130) with a first detector segment (231) and a second detector segment (232) for detecting secondary electron signals during the scanning of the primary electron beam over the first surface portion, wherein the first detector segment (231) is configured to provide first intensity data (I _E (x, y) ) and the second detector segment (232) is configured to provide second intensity data (I _W (x, y) ) ; and

a data processing unit (240) storing instructions that, when executed, cause a processor to:

calculate a surface steepness of the substrate in an x-direction based on differences between the first intensity data (I _E (x, y) ) and the second intensity data (I _W (x, y) ) ; and

determine a topography (T) of the first surface portion based on the surface steepness in the x-direction.
A computer-readable medium having instructions stored thereon that, when executed, cause a scanning electron microscope to perform the method of in-line inspection of a substrate of any of claims 1 to 18.