阅读说明：本技术 检查工具和确定检查工具的畸变的方法 (Inspection tool and method of determining distortion of inspection tool ) 是由 P·C·J·M·德洛伊杰 于 2018-12-19 设计创作，主要内容包括：描述了一种确定扫描电子显微镜的视场的畸变的方法。该方法可以包括：提供样本,该样本包括在第一方向上延伸的基本平行的多条线；沿着在扫描方向上延伸的相应的多个扫描轨迹来在样本的视场上执行多个扫描；扫描方向基本垂直于第一方向；检测由样本的扫描引起的样本的响应信号；基于响应信号来确定线的第一线段与该线的第二线段之间的距离,其中第一线段和第二线段中的每个线段与扫描轨迹相交；针对视场内的多个位置执行先前的步骤；以及基于在多个位置处确定的所述距离来确定视场上的畸变。(A method of determining distortion of a field of view of a scanning electron microscope is described. The method can comprise the following steps: providing a sample comprising a plurality of substantially parallel lines extending in a first direction; performing a plurality of scans over a field of view of the sample along a respective plurality of scan trajectories extending in a scan direction; the scanning direction is substantially perpendicular to the first direction; detecting a response signal of the sample caused by the scanning of the sample; determining a distance between a first line segment of a line and a second line segment of the line based on the response signal, wherein each of the first line segment and the second line segment intersects the scanning trajectory; performing the previous steps for a plurality of locations within the field of view; and determining distortion over the field of view based on the distances determined at the plurality of locations.)

1. A method of determining distortion of a field of view of a scanning electron microscope, comprising:

-providing a sample comprising a plurality of substantially parallel lines extending in a first direction;

-performing a plurality of scans over a field of view of the sample along a respective plurality of scan trajectories extending in a scan direction; the scanning direction is substantially perpendicular to the first direction;

-detecting a response signal of the sample caused by the scanning of the sample;

-determining a distance between a first line segment of a line and a second line segment of the line based on the response signal, wherein each of the first and second line segments intersects a plurality of scanning trajectories;

-performing the previous steps for a plurality of locations within the field of view, an

-determining a distortion over the field of view based on the distances determined at the plurality of locations.

2. The method of claim 1, wherein each line segment intersects a subset of the plurality of scans, the subset comprising a plurality of scan trajectories.

3. The method of claim 1 or 2, wherein the distance is a distance in the scan direction.

4. The method of claim 1 or 2, wherein the step of determining the distance comprises: a position of the second line segment relative to the first line segment in the scan direction is determined.

5. The method of claim 1 or 2, wherein the distance is determined by performing a gap measurement between the first line segment and the second line segment.

6. The method of any preceding claim, wherein the distortion comprises the relative position of the line segments.

7. The method according to any of the preceding claims, wherein the step of determining the distortion comprises: performing a one-dimensional or two-dimensional fit based on the determined distance.

8. The method of claim 7, wherein the one-dimensional or two-dimensional fit comprises a piecewise linear fit.

9. The method of claim 6, wherein the distortion comprises a piecewise linear curve based on the relative positions of the line segments.

10. The method of any preceding claim, wherein the distance is determined based on an edge measurement of the line segment.

11. An inspection tool comprising:

-a stage configured to receive a sample comprising a plurality of substantially parallel lines extending in a first direction;

-an electron beam source configured to generate an electron beam;

-a beam manipulator configured to direct electrons onto the sample;

-a detector configured to detect a response signal of the sample caused by an interaction of the electron beam with the sample;

-a control unit configured to control the beam manipulator to perform a plurality of scans over a field of view of the sample along a respective plurality of scanning trajectories extending in a scanning direction; the scan direction is substantially perpendicular to the first direction, wherein the control unit is further configured to perform, during the scan, the step of determining a distance between a first line segment of a line and a second line segment of the line based on the response signal of the sample for a plurality of locations within the field of view, wherein each of the first and second line segments intersects a plurality of scan trajectories, and the control unit is configured to determine a distortion over the field of view based on the distances determined at the plurality of locations.

12. The inspection tool of claim 11, wherein the control unit is configured to determine the distortion by performing a one-dimensional or two-dimensional fit based on the determined distance.

Technical Field

The invention relates to an inspection tool and a method of determining distortion of an inspection tool.

Background

A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. Lithographic apparatus can be used, for example, in the manufacture of Integrated Circuits (ICs). In such cases, a patterning device (which is alternatively referred to as a mask or a reticle) may be used to generate a circuit pattern to be formed on an individual layer of the IC. The pattern can be transferred onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer). The transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. Typically, a single substrate will contain a network of adjacent target portions that are successively patterned. A conventional lithographic apparatus includes: so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time; and so-called scanners, in which each target portion is irradiated by scanning the pattern through the radiation beam in a given direction (the "scanning" -direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. The pattern may also be transferred from the patterning device to the substrate by imprinting the pattern onto the substrate.

The radiation beam typically applied in a lithographic apparatus may be, for example, a DUV radiation beam (e.g. having a wavelength of 248nm or 193nm) or an EUV radiation beam (e.g. having a wavelength of 11nm or 13.5 nm).

The fabrication of integrated circuits may typically require stacking of multiple layers, thereby requiring the layers to be accurately aligned. Without such alignment, the required connections between the layers may be defective, resulting in integrated circuit failure.

Typically, one or more underlying layers of an integrated circuit will contain minimal structures, such as transistors or components thereof. The structure of the subsequent layers is usually large and enables the connection of the structure in the bottom layer to the outside world. For this reason, at the bottom of the integrated circuit, alignment of the two layers will be the most challenging.

To ensure that the circuit or circuit layer is patterned correctly, the substrate is often inspected using an inspection tool, such as an electron beam inspection tool.

One example of such an inspection tool is a high resolution SEM (scanning electron microscope), which is used, for example, to inspect the size of a pattern on a substrate. Such high resolution SEMs typically use electrons with energies between 200eV to 30keV, which are accelerated towards the surface of the substrate, diffuse at the surface of the substrate and generate new electrons (i.e. secondary electrons). Thus, secondary and/or backscattered electrons are emitted from the surface. These secondary and/or backscattered electrons may then be recorded by a detector. By scanning an area of the substrate with an electron beam, information about the surface structure of the substrate can be acquired.

It is desirable to improve the performance of electron beam inspection tools such as the high resolution SEMs currently available.

Disclosure of Invention

It is desirable to improve the performance of electron beam inspection tools.

According to an embodiment of the present invention, there is provided a method of determining distortion of a field of view of a scanning electron microscope, the method comprising:

-providing a sample comprising a plurality of substantially parallel lines extending in a first direction;

-performing a plurality of scans over the field of view of the sample along a respective plurality of scan trajectories extending in a scan direction; the scanning direction is substantially perpendicular to the first direction;

-detecting a response signal of the sample caused by the scanning of the sample;

-determining a distance between a first line segment of a line and a second line segment of the line based on the response signal, wherein each of the first and second line segments intersects the plurality of scanning trajectories;

-performing the previous steps for a plurality of locations within the field of view, an

-determining a distortion over the field of view based on the distances determined at the plurality of locations.

According to an embodiment of the present invention, there is provided an inspection tool including:

-a stage configured to receive a sample comprising a plurality of substantially parallel lines extending in a first direction;

-an electron beam source configured to generate an electron beam;

-a beam manipulator configured to direct electrons onto the sample;

-a detector configured to detect a response signal of the sample caused by an interaction of the electron beam with the sample;

-a control unit configured to control the beam manipulator to perform a plurality of scans over a field of view of the sample along a respective plurality of scan trajectories extending in a scan direction; the scanning direction is substantially perpendicular to the first direction, wherein the control unit is further configured to perform the step of determining a distance between a first line segment of a line and a second line segment of the line based on the response signal of the sample for a plurality of positions within a field of view during the scanning, wherein each of the first line segment and the second line segment intersects the plurality of scanning trajectories, and the control unit is configured to determine a distortion over the field of view based on said distances determined at the plurality of positions.

Drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

FIG. 1 depicts a lithographic apparatus according to an embodiment of the invention.

FIG. 2 depicts an inspection tool according to an embodiment of the invention.

Fig. 3 schematically depicts a top view and a cross-sectional view of a structure that may be inspected using an inspection tool according to the present invention.

Fig. 4 schematically depicts a plurality of scanning trajectories that may be applied for scanning a sample comprising a plurality of lines.

Fig. 5 schematically depicts a cross-sectional view of the structure of fig. 4 and signals that may be acquired during a scan of such a structure.

Fig. 6 and 7 schematically depict, in undistorted and distorted form, images that may be acquired by scanning the structure of fig. 4.

Fig. 8 schematically depicts a process of determining distortion over the entire field of view based on distance measurements.

Fig. 9 schematically shows a cross-sectional view of an inspection tool according to the present invention.

Detailed Description

FIG. 1 schematically depicts a lithographic apparatus according to one embodiment of the invention. The apparatus comprises: an illumination system (illuminator) IL configured to condition a radiation beam B (e.g. UV radiation or any other suitable radiation), a mask support structure (e.g. a mask table) MT constructed to support a patterning device (e.g. a mask) MA and connected to a first positioning device PM configured to accurately position the patterning device in accordance with certain parameters. The apparatus also includes a substrate table (e.g., a wafer table) WT or "substrate support" constructed to hold a substrate (e.g., a resist-coated wafer) W and connected to a second positioner PW configured to accurately position the substrate in accordance with certain parameters. The apparatus also includes a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., including one or more dies) of the substrate W.

The illumination system may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic or other types of optical components, or any combination thereof, for directing, shaping, or controlling radiation.

The mask support structure supports, i.e., bears the weight of, the patterning device. It holds the patterning device in a manner that depends on the orientation of the patterning device, the design of the lithographic apparatus, and other conditions, such as for example whether or not the patterning device is held in a vacuum environment. The mask support structure can use mechanical, vacuum, electrostatic or other clamping techniques to hold the patterning device. The mask support structure may be, for example, a frame or a table, which may be fixed or movable as required. The mask support structure may ensure that the patterning device is projected at a desired position, for example with respect to the projection system. Any use of the terms "reticle" or "mask" herein may be considered synonymous with the more general term "patterning device".

The term "patterning device" used herein should be broadly interpreted as referring to any device that can be used to impart a radiation beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the radiation beam may not exactly correspond to the desired pattern in the target portion of the substrate, for example when the pattern includes phase-shifting features or so called assist features. Generally, the pattern imparted to the radiation beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples of patterning devices include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include various mask types (such as binary, alternating phase-shift, and attenuated phase-shift), and various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions. The tilted mirrors impart a pattern in a radiation beam which is reflected by the mirror matrix.

The term "projection system" used herein should be broadly interpreted as encompassing any type of projection system, including refractive, reflective, catadioptric, magnetic, electromagnetic and electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term "projection lens" herein may be considered as synonymous with the more general term "projection system".

As here depicted, the apparatus is of a transmissive type (e.g. employing a transmissive mask). Alternatively, the apparatus may be of a reflective type (e.g. employing a programmable mirror array of a type as referred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) or more substrate tables or "substrate supports" (and/or two or more mask tables or "mask supports"). In such "multiple stage" machines the additional tables or supports may be used in parallel, or preparatory steps may be carried out on one or more tables or supports while one or more other tables or supports are being used for exposure.

The lithographic apparatus may also be of a type wherein: wherein at least a portion of the substrate W may be covered by a liquid having a relatively high refractive index, e.g. water, so as to fill a space between the projection system and the substrate. Immersion liquids may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques can be used to increase the numerical aperture of projection systems. The term "immersion" as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from a radiation source SO. The source and the lithographic apparatus may be separate entities, for example when the source is an excimer laser. In such cases, the source is not considered to form part of the lithographic apparatus and the radiation beam is passed from the source SO to the illuminator IL with the aid of a beam delivery system BD comprising, for example, suitable directing mirrors and/or a beam expander. In other cases the source may be an integral part of the lithographic apparatus, for example when the source is a mercury lamp. The source SO and the illuminator IL, together with the beam delivery system BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD configured to adjust the angular intensity distribution of the radiation beam. Generally, at least the outer and/or inner radial extent (commonly referred to as σ -outer and σ -inner, respectively) of the intensity distribution in a pupil plane of the illuminator can be adjusted. IN addition, the illuminator IL may include various other components, such as an integrator IN and a condenser CO. The illuminator may be used to condition the radiation beam, to have a desired uniformity and intensity distribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask MA), which is held on the mask support structure (e.g., mask table MT), and is patterned by the patterning device. After passing through the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and position sensor IF (e.g. an interferometric device, linear encoder or capacitive sensor), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in the path of the radiation beam B. Similarly, the first positioner PM and another position sensor (which is not explicitly depicted in fig. 1) can be used to accurately position the mask MA with respect to the path of the radiation beam B, e.g. after mechanical retrieval from a mask library, or during a scan. In general, movement of the mask table MT may be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which form part of the first positioner PM. Similarly, movement of the substrate table WT or "substrate support" may be realized using a long-stroke module and a short-stroke module, which form part of the second positioner PW. In the case of a stepper (as opposed to a scanner) the mask table MT may be connected to a short-stroke actuator only, or may be fixed. Mask MA may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2. Although the substrate alignment marks as shown occupy dedicated target portions, they may be located in spaces between target portions (these alignment marks are referred to as scribe-lane alignment marks). Similarly, in situations in which more than one die is provided on the mask MA, the mask alignment marks may be located between the dies.

The depicted apparatus may be used in at least one of the following modes:

1. in step mode, the mask table MT or "mask support" and the substrate table WT or "substrate support" are kept essentially stationary, while an entire pattern imparted to the radiation beam is projected onto a target portion C at one time (i.e. a single static exposure). The substrate table WT or "substrate support" is then shifted in the X and/or Y direction so that a different target portion C can be exposed. In step mode, the maximum size of the exposure field limits the size of the target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT or "mask support" and the substrate table WT or "substrate support" are scanned synchronously while a pattern imparted to the radiation beam is projected onto a target portion C (i.e. a single dynamic exposure). The velocity and direction of the substrate table WT or "substrate support" relative to the mask table MT or "mask support" may be determined by the (de-) magnification and image reversal characteristics of the projection system PS. In scan mode, the maximum size of the exposure field limits the width (in the non-scanning direction) of the target portion in a single dynamic exposure, whereas the length of the scanning motion determines the height (in the scanning direction) of the target portion.

3. In another mode, the mask table MT or "mask support" is kept essentially stationary holding a programmable patterning device, and the substrate table WT or "substrate support" is moved or scanned while a pattern imparted to the radiation beam is projected onto a target portion C. In this mode, generally, a pulsed radiation source is employed and the programmable patterning device is updated as required after each movement of the substrate table WT or "substrate support" during a scan, or in between successive radiation pulses. This mode of operation can be readily applied to maskless lithography that utilizes programmable patterning device, such as a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use or entirely different modes of use may also be employed.

In the embodiment shown, the lithographic apparatus further comprises an inspection tool IT according to the invention. Such an inspection tool IT is for example capable of determining characteristics of a structure, in particular of a buried structure present on or in a region of interest of a substrate W processed by the lithographic apparatus. In an embodiment, as will be discussed in more detail below, the inspection tool may include an electron beam source for inspecting the substrate.

In an embodiment, the second positioning device PW may be configured to position the substrate W within the working range of the inspection tool IT. In such embodiments, the inspection tool IT may for example be configured to determine properties of the mentioned structure, such as electrical properties, material properties and/or geometrical properties. In an embodiment, this information may then be provided to a control unit of the lithographic apparatus and used in the exposure process, for example by controlling one or more of the illumination system, the projection system or the positioning device based on the information.

In the illustrated embodiment, the lithographic apparatus may be configured to apply DUV radiation to the radiation beam. In this case, patterning device MA may be a transmissive patterning device, and projection system PS may comprise one or more lenses.

Alternatively, a lithographic apparatus according to the invention may be configured to apply EUV radiation to a radiation beam. In this case, patterning device MA may be a reflective patterning device, and projection system PS may comprise one or more mirrors. In such embodiments, the apparatus may comprise one or more vacuum chambers for housing the illumination system IL and/or the projection system PS.

According to an aspect of the invention, a lithographic apparatus may include an inspection tool according to the invention for in-line or off-line inspection of a substrate to be processed or already processed.

According to an aspect of the present invention, there is provided an inspection tool configured to inspect an object such as a semiconductor substrate. Fig. 2 schematically shows an embodiment of such an inspection tool 100. According to the present invention, the inspection tool 100 includes an electron beam source 110, also referred to as an e-beam source 110. Such electron beam sources 110 are generally known and may be applied in the present invention to project an electron beam 120 onto an area of an object 130 (e.g., a substrate). In the illustrated embodiment, object 130 is mounted to stage 132 by means of a clamping mechanism 134 (e.g., a vacuum clamp or an electrostatic clamp). The area of the object onto which the electron beam is projected may also be referred to as a sample. Such an electron beam source 110 may for example be used for generating an electron beam 120 with an energy in the range of 0.2keV to 100 keV. The electron beam source 110 may typically have one or more lenses to focus the electron beam 120 to a spot having a diameter of about 0.4 to 5 nm. In an embodiment, the electron beam source 110 may further include one or more scanning coils or deflection plates that may deflect the electron beam 120. By doing so, the electron beam 120 may be deflected, for example, along X and Y axes (perpendicular to the X and Z axes), with the XY plane parallel to the surface of the object, so that a region of the object may be scanned.

In an embodiment of the invention, the electron beam source is configured to project a plurality of electron beams onto a corresponding plurality of sub-areas of the region of interest. By doing so, the region of interest that can be examined or detected per unit time can be enlarged. Further, in embodiments of the present invention, the electron beam source may be configured to generate electron beams having different energy levels.

When such an electron beam 120 impinges on a surface, interactions on the surface and with materials below the surface will occur, resulting in the exposed surface emitting both radiation and electrons. Typically, when the electron beam 120 interacts with the sample, the electrons that make up the beam will lose energy by scattering and absorption within a tear drop-shaped volume, referred to as the interaction volume. The energy exchange between the electron beam and the sample typically results in a combination of:

secondary electron emission by inelastic scattering,

electron emission that is reflected or backscattered out of the interaction volume by elastic scattering interaction with the sample,

-emission of X-rays, and

electromagnetic radiation emission, for example ranging from deep UV to IR.

The latter electromagnetic radiation emission is commonly referred to as cathodoluminescence or CL light.

In an embodiment of the invention, the inspection tool 100 further comprises a detector 150 for detecting secondary electrons and a detector 151 for backscattering electrons emitted by the sample. In fig. 2, the arrows 140 indicate the emitted secondary or backscattered electrons.

In the illustrated embodiment, the inspection tool further comprises a control unit 170 or a processing unit, e.g. comprising a microprocessor, a computer or the like, for processing signals representing emitted secondary or backscattered electrons detected by the detectors 150 and 151.

In an embodiment, the control unit 170 comprises an input terminal 172 for receiving a signal 152 from the detector 150, 151, the signal 152 being representative of the detected emitted secondary or backscattered electrons.

In an embodiment, the control unit may further have an output terminal 174 for outputting the control signal 112 for controlling the electron beam source 110. In an embodiment, the control unit 170 may control the electron beam source 110 to project an electron beam 120 onto a region of interest of an object to be inspected (e.g., a semiconductor substrate).

In an embodiment, the control unit 170 may be configured to control the electron beam source 110 to scan the region of interest.

For example, the inspection tool schematically shown in fig. 2 may be applied, for example, to evaluate the performance of a lithographic apparatus as shown in fig. 1. Such an evaluation may, for example, involve determining the CD uniformity of the patterned structures on the substrate. As an example of such a patterned structure, a structure including a plurality of parallel lines may be mentioned. To evaluate the performance of a lithographic apparatus, as described above, a sample having a patterned structure may be scanned with an electron beam while detecting one or more occurring emissions due to interaction of the sample with the electron beam. Typically, a rectangular portion of the sample is scanned along multiple scan trajectories. The portion of the sample being scanned or the area covered by the multiple scan trajectories is generally referred to as the field of view of the inspection tool.

Fig. 3 schematically shows a top view (a) and a cross-sectional view (b) of a patterned structure 300 comprising a plurality of substantially parallel lines 310. Such a structure 300 may be acquired, for example, by exposing a resist layer to a radiation beam that has been patterned by a grating. Fig. 3 (b) shows a cross-sectional view of the structure 300 along the line a-a' shown in fig. 3 (a).

Thus, FIG. 3 will depict the desired structure after the exposure and development processes. To evaluate the performance of these processes, an inspection tool, such as an SEM, may be used to inspect or inspect the patterned structures actually acquired by these processes.

In such inspection tools, the sample is scanned over the field of view of the inspection tool. The scanning process typically involves performing a plurality of scans along a corresponding plurality of scan trajectories that are substantially parallel to each other and extend in a scan direction. Fig. 4 schematically illustrates a top view of a field of view 400 of an inspection tool covering a portion of a sample including portions of three substantially parallel lines 410 extending in the illustrated X-direction. Fig. 4 also shows several scanning trajectories 420 along which the electron beam propagates during scanning. During such scanning, the electron beam scans the sample along the indicated scan trajectory, e.g. in the scan direction 430, while, between scans along two adjacent scan trajectories, at the start of the next scan trajectory, the electron beam is switched off or blocked and repositioned, the repositioning being indicated by the dashed line 425.

Typically, the field of view of the inspection tool may cover an area of, for example, 2 microns by 2 microns or less. To scan the entire field of view, tens or hundreds of scans are typically performed along the scan direction to cover the field of view.

Fig. 5 schematically shows a cross-sectional view of the sample portion shown in fig. 4, and a signal S detected by the inspection tool when the electron beam E scans such a sample along a scanning direction (indicated as Y-direction).

It can be seen that when the scanning electron beam E scanning in the indicated Y direction encounters a line 410, a change in the detection signal can be observed during each transition encountered along the scanning direction. Such a transition (e.g., due to a change in geometry or a change in material) may then be used to identify the location of the changed geometry or material.

In the example shown, the presence of the line 410 on the scan trajectory followed results in a transition from the first signal level S1 to the second signal level S2, and back to the first signal level S1. Based on this variation of the signal S along the scanning trajectory, the position of the line, in particular the position of the edges 410.1 and 410.2 of the line, may for example be determined to occur at a position of the signal S that is for example midway between the first signal level and the second signal level.

By assessing where the edges of the lines occur based on the detected signals along the various scanning trajectories, an image of the field of view in which the edges of the lines of the sample can be found (i.e., where the transitions of the detected signals acquired by scanning along the scanning trajectories are found) can be obtained.

In this way, when scanning a sample as shown in fig. 4, an image may be acquired which allows determining where the edges of the line 410 are located, thereby allowing determining the position of the line and allowing evaluating the accuracy of the line, which may be considered as a method for evaluating the quality of the lithographic process.

Fig. 6 shows highly schematically a field of view 400 covering a portion of a sample in which three parallel lines 410 are present, wherein transitions are detected when scanning the sample along a scanning trajectory 420. On the right side of fig. 6, the transition is represented by a plurality of points 610, which correspond, for example, to positions at which the detector signal changes during scanning along the scanning track. As such, the location of the point or transition 610 may be considered to represent the location of the edge of the line 410. Transition 610 may also be referred to as an edge position measurement or an edge measurement. Based on the image shown on the right in fig. 6, various parameters may be determined to assess the quality of the process applied to generate the sample. Such parameters may include, for example, CD (critical dimension) and CD uniformity, LER (linear edge roughness), LWR (line with roughness), and pitch. As an example, for determining the LER, the determined position of the edge may for example be compared with an average of the determined positions. As another example, the CD at a particular location along a line may be determined as the distance between the detected edges of the line at that location, as indicated by arrow 630. By determining the CD at various locations along the line, the uniformity or variation of the CD may be determined. As yet another example, the pitch of a line, i.e., the distance between two adjacent lines, may also be determined based on the detected transition 610. To determine the spacing between two lines, for example, the distance between a first edge of a first line (i.e., the edge first encountered during scanning) and a first edge of a second line may be determined. In order to make the determined distance unaffected by LER, it is for example possible to determine an average position of the first edge of the first line (for example averaging over the line segments) and to determine an average position of the first edge of the second line (for example averaging over the line segments of the second line corresponding to the line segments of the first line). Referring to fig. 6, the spacing between the first and second lines may be determined, for example, by comparing an average position of an edge of the first line (e.g., obtained by averaging the determined transitions 610 observed in the portion 640 of the first line) with an average position of an edge of the second line (e.g., obtained by averaging the determined transitions 610 observed in the portion 650 of the second line). Such a position measurement comparing the position of an edge of a line with the position of an edge of an adjacent line is also referred to as a gap measurement. Thus, such a gap measurement results in determining the distance between two edges of adjacent lines in the scan direction (i.e., the direction perpendicular to the lines).

As will be appreciated by those skilled in the art, the accuracy with which parameters such as LER, LWR, or pitch can be determined can depend strongly on the accuracy with which the location of the transition 610 can be determined and the reliability of the measurement.

To improve the accuracy of the measurements, multiple scans of the field of view may be made and the acquired scans (also referred to as frames) of the field of view may be averaged.

As regards the reliability of the measurement, the following can be mentioned:

in order to properly evaluate the performance of a lithographic apparatus, it is necessary to rely on the following facts: the image of the patterned structure generated by the inspection tool (which image for example is indicative of the scanned structure) is an accurate representation of the actual structure patterned by the lithographic apparatus. Without being able to rely on such correspondence, it is not possible to assess whether any observed defects are caused by the actual patterning process performed by the lithographic apparatus, or whether the observed defects are caused by an incorrect imaging process performed by the inspection tool.

When inspecting a patterned structure comprising a plurality of substantially parallel lines, for example to assess Critical Dimension (CD) uniformity or other parameters in question, there may be a variety of reasons why an image of the patterned structure taken from an inspection tool may deviate from the actual geometry of the patterned structure. Typically, to improve measurement accuracy, an image of the patterned structure is acquired by scanning the region of interest multiple times and combining the acquired scans into one image. Within the meaning of the present invention, a single scan over a region of interest or field of view is referred to as a frame. Thus, an image of the region of interest is acquired by combining (e.g., averaging) a plurality of frames.

During scanning of the patterned structure using an electron beam, the patterned structure may be affected by the electron beam. As an example, when scanning a patterned structure comprising parallel lines made of resist, the lines will degrade with each scan. This phenomenon is called resist shrinkage.

Another cause of deviation between the actual patterned structure and the image of the structure taken by the SEM inspection tool is the charging of the structure by the applied electron beam. The specimen or sample being inspected may become charged as a result of the application of the electron beam. Due to this charging, the trajectory of the electron beam may be affected. As a result, the measurement process will be affected, resulting in deviations between the actual patterned structure and the measured characteristics (e.g. geometry) of the patterned structure, e.g. an image representation of the patterned structure taken by an inspection tool. As an example of such a deviation, a rotation of the image may be observed. In general, the inspection tool may be configured to correct for such rotation of the image.

Another cause of deviation between actual patterned structures and measurements of structures (e.g., an image representation of the structures taken by an SEM inspection tool) is a phenomenon known as distortion. In the case of, for example, multiple scans of a patterned structure comprising a plurality of substantially straight parallel lines, the acquired image of the patterned structure may be distorted; instead of parallel straight lines, the image representing the measured positions of the lines may show curves that do not need to be parallel. This distortion is believed to be caused by a.o. electromagnetic interference affecting the electron beam.

Fig. 7 schematically shows the effect of such a distorted pattern of lines. Fig. 7 schematically shows on the left side a field of view 400 covering a sample portion in which three parallel lines 410 are present. Fig. 7 shows highly schematically on the right the locations where transitions, for example representing edges of a line, are detected. It can be seen that during a scan in which scans are performed sequentially along the scan trajectory 420, a movement along the translated scan direction can be observed, as shown by point 610. It should be noted that this perceived distortion does not correspond to the actual distortion of the patterned lines. Thus, as will be understood by those skilled in the art, when the measured distortion pattern is used as a basis for evaluating certain parameters characterizing the performance of the lithographic process, the evaluation may be adversely affected. As an example, in case the measurement of the first edge of the third line, indicated by the curved dashed line 710, is to be used for determining the value of LER, the roughness of the line edge will be overestimated. However, other parameters such as CD values may be less affected.

However, it would be advantageous to take into account distortions affecting the measurements of the inspection tool used to assess the quality of the lithographic process.

According to an aspect of the invention, a method of determining distortion of an SEM inspection tool is presented.

Once the tool distortion is known, the known distortion may be applied, for example, to correct inspection data acquired by scanning other structures. As an example, the SEM inspection tool may be applied, for example, to evaluate Local Position Errors (LPEs) of, for example, contact holes. When the distortion or distortion pattern of the SEM tool is known, a more accurate assessment of the actual LPE can be obtained by correcting the distortion.

In embodiments of the invention, distortion is determined by determining the distance between line segments, for example based on edge measurements, for a plurality of line segments, for example a plurality of lines available within the field of view of the SEM. In case the distance between line segments of the same line, in particular the distance in the scanning direction, is determined, this may be considered as a measure for the distortion.

Ideally, it is desirable that the distance between two line segments of the same line in the scanning direction is zero. However, due to distortion, the distance considered in the scanning direction between two line segments (e.g., two adjacent line segments) may vary along the line.

Within the meaning of the present invention, a line segment refers to a portion of a line on a sample, wherein the line segment has been scanned by a plurality of scans along a plurality of scanning trajectories. Or, in other words, the line segments intersect or intersect the plurality of scanning trajectories. By way of example, where a sample is scanned through 100 scan trajectories (such as trajectory 420), for example, a line (such as line 410) may be subdivided into 10 line segments, each of which is scanned along 10 scan trajectories. By determining the distance between these line segments, the distortion of the field of view of the inspection tool can be evaluated. This is illustrated in more detail in fig. 8.

Fig. 8 schematically shows a distorted image of a sample comprising three lines in more detail, similar to the right part of fig. 7. In fig. 8, a dashed line 810 represents an edge measurement acquired by scanning a sample along a plurality of scanning trajectories. In the arrangement shown, the line is subdivided into 7 sections or segments represented by lines 820. It can be seen that each section or segment intersects or intersects approximately 7 scan trajectories, since each line section or segment contains approximately 7 measurement points per line and per edge.

In order to determine the distortion of the field of view 800 of the inspection tool, distance measurements are applied in the present invention. In particular, with reference to fig. 8, distance measurements may be applied between different segments of the line. In particular, as can be seen in fig. 8, the first edge of the first line (i.e. indicated by the leftmost dashed line 810) is subdivided into 6 parts or segments, each part or segment comprising 7 edge measurements. It can be seen that the 6 segments are not in line along the scan direction.

In an embodiment of the invention, the distance between two line segments is determined. As an example, a distance between an uppermost line segment of the first line featuring edge measurement a and an adjacent line segment of the first line featuring edge measurement B may be determined. Based on the determined distance, a displacement of the second segment in the scanning direction relative to the first segment may be derived, for example.

Since the distance between segments in the X-direction (i.e. along line 410) is generally known (since the total length of the field of view along the X-direction (denoted L) is known), the length of each segment is also known, and hence the distance between adjacent segments in the X-direction is also known, the line segments need to be shifted relative to each other in the scan direction if the determined distance between two segments (e.g. based on edge measurements characterizing these segments) will deviate from the expected distance. This is shown in fig. 8, where Ls is the (known) distance in the X direction between the third line segment (characterized by the edge measurement C) and the fourth line segment (characterized by the edge measurement D), and La is the actual distance between the segments, e.g. determined as the distance between the average of the edge measurements C and the average of the edge measurements D. Based on the expected distance (Ls) and the actual distance (La) between the segments, a displacement (Ld) along the scan direction may be determined, which may be considered a measure for the distortion of the field of view 800 at the location of the segment under examination.

In an embodiment, the distance between the selected line segments (e.g., the first line segment and the second line segment) may be obtained by performing a gap measurement as described above. Note that typically, gap measurements are performed between line segments of different lines, e.g. determining the spacing between the lines. However, by performing gap measurement for two segments of the same line, information about the relative positions of the segments in the scanning direction can be acquired.

In an embodiment, the displacement along the scan direction between adjacent line segments may be determined at a plurality of locations within the field of view. In this way, a map of the distortion over the entire field of view can be determined. In fig. 8, graph 850 shows the relative positions of 6 line segments along the scan direction (i.e., Y direction), with point 850.1 indicating the determined relative position of each segment.

In an embodiment, the line or distortion along the line perceived in the SEM image of the sample may be determined as a piecewise linear curve constructed using the determined relative positions of the segments of the line. This piecewise linear curve is shown in fig. 8 as graph 850.2.

When considering the entire array of edge measurements indicating where the edge of the line was detected, the distortion (e.g., represented by a piecewise linear curve) can be considered a low frequency approximation of the location of the edge. As will be understood by those skilled in the art, such low frequency curvature, which is caused by the disturbance and does not correspond to the actual curvature of the lines of the sample, may be used to correct the edge measurements, thereby at least partially eliminating the effect of the distortion on the process parameter (e.g., LER or LWR) to be evaluated.

As described above, by determining the distortion over the entire field of view, a two-dimensional representation of the distortion may be acquired. In order to obtain a substantially continuous, i.e. one-or two-dimensional, representation of the distortion over the field of view or along a line, other fitting or mapping techniques may be considered in addition to piecewise linear fitting. As an example, one-dimensional or two-dimensional polynomial fitting may also be considered.

When determining such a two-dimensional representation of the distortion, it may for example be used to correct the measurement of the scanned sample. As an example, in addition to the process parameters described above, SEM inspection tools may also be applied to evaluate the geometric characteristics of patterned structures such as contact holes, particularly the relative positions of such contact holes. Using the above method, when the above distortion has been determined in advance, the distortion of the determined relative position of the contact hole on the sample (e.g., based on the sample scan as described above) can be corrected. As a result, the position of the contact hole on the sample can be determined more accurately.

The method for determining distortion over the field of view of an inspection tool, such as an SEM inspection tool, as described above, may be implemented in an inspection tool according to the present invention. Embodiments of such an inspection tool may be characterized, for example, by the following features:

-a stage configured to receive a sample comprising a plurality of substantially parallel lines extending in a first direction;

-an electron beam source configured to generate an electron beam;

-a beam manipulator configured to direct electrons onto the sample;

-a detector configured to detect a response signal of the sample caused by an interaction of the electron beam with the sample;

-a control unit configured to control the beam manipulator to perform a plurality of scans over a field of view of the sample along a respective plurality of scan trajectories extending in a scan direction; the scanning direction is substantially perpendicular to the first direction, wherein the control unit is further configured to perform the step of determining a distance between a first line segment of a line and a second line segment of the line based on the response signal of the sample for a plurality of positions within a field of view during the scanning, wherein each of the first line segment and the second line segment intersects the plurality of scanning trajectories, and the control unit is configured to determine a distortion over the field of view based on said distances determined at the plurality of positions.

In such an embodiment, the control unit may for example comprise a processing unit and a memory unit, wherein the latter may for example be applied for storing the response signals received by the detector during scanning along a plurality of scanning trajectories, and wherein the former (i.e. the processing unit) is configured to process the indicated received response signals for determining the distortion.

Fig. 9 schematically shows a more detailed embodiment of an inspection tool 200 according to the present invention. The inspection tool 200 includes an electron beam source (referred to as an electron gun 210) and an imaging system 240.

The electron gun 210 includes an electron source 212, a suppressor electrode 214, an anode 216, a set of apertures 218, and a capacitor 220. The electron source 212 may be a schottky emitter or a modified schottky emitter as described above. By the positive charge of the anode 216, the electron beam 202 may be extracted and the electron beam 202 may be controlled by using an adjustable aperture 218, which adjustable aperture 218 may have a different aperture size to eliminate unwanted electron beams outside the aperture. To converge the electron beam 202, a condenser 220 is applied to the electron beam 202, which also provides magnification. The condenser 220 shown in fig. 2 may be, for example, an electrostatic lens that may condense the electron beam 202. Alternatively, the condenser 220 may be a magnetic lens.

The imaging system 240 may include, for example, a collar 248, a set of apertures 242, a detector 244, four sets of deflectors 250, 252, 254, and 256, a pair of coils 262, a yoke 260, and an electrode 270. The electrodes 270 serve to delay and deflect the electron beam 202 and further have an electrostatic lens function due to the combination of the upper pole piece and the sample 300. Further, the coil 262 and the yoke 260 are configured as a magnetic objective lens.

Deflectors 250 and 256 may be used to scan electron beam 202 to a large field of view, and deflectors 252 and 254 may be used to scan electron beam 202 to a small field of view. All deflectors 250, 252, 254 and 256 can control the scanning direction of the electron beam 202. Deflectors 250, 252, 254, and 256 may be electrostatic deflectors or magnetic deflectors. The opening of the magnetic yoke 260 faces the sample 300, which immerses the magnetic field in the sample 300. On the other hand, the electrode 270 is placed under the opening of the yoke 260, and thus the specimen 300 will not be damaged. To correct for chromatic aberration of the electron beam 202, the retarder 270, the sample 300, and the upper pole piece form a lens to eliminate chromatic aberration of the electron beam 202.

As mentioned above, the inspection tool may further comprise a control unit. In an embodiment, the control unit may control the electron gun 210 and the imaging system 240 simultaneously. The control unit may also be configured to process measurement data acquired during a scan of the field of view and determine distortion over the entire field of view or a portion thereof.

The embodiments may be further described using the following clauses:

1. a method of determining distortion of a field of view of a scanning electron microscope, comprising:

-providing a sample comprising a plurality of substantially parallel lines extending in a first direction;

-performing a plurality of scans over the field of view of the sample along a respective plurality of scan trajectories extending in a scan direction; the scanning direction is substantially perpendicular to the first direction;

-detecting a response signal of the sample caused by the scanning of the sample;

-determining a distance between a first line segment of a line and a second line segment of the line based on the response signal, wherein each of the first and second line segments intersects a plurality of scanning trajectories;

-performing the previous steps for a plurality of locations within the field of view, an

-determining a distortion over the field of view based on the distances determined at a plurality of locations.

2. The method of clause 1, wherein each line segment intersects a subset of the plurality of scans, the subset comprising a plurality of scan trajectories.

3. The method of clause 1 or 2, wherein the distance is a distance in the scan direction.

4. The method of clause 1 or 2, wherein the step of determining the distance comprises determining a position of the second line segment relative to the first line segment in the scan direction.

5. The method of clause 1 or 2, wherein the distance is determined by performing a gap measurement between the first line segment and the second line segment.

6. The method of any of the preceding clauses wherein the distortion comprises a relative position of the line segments.

7. The method of any of the preceding clauses wherein the step of determining the distortion comprises performing a one-dimensional or two-dimensional fit based on the determined distance.

8. The method of clause 7, wherein the one-dimensional or two-dimensional fit comprises a piecewise linear fit.

9. The method of clause 6, wherein the distortion comprises a piecewise linear curve based on the relative positions of the line segments.

10. The method of any of the preceding clauses wherein the distance is determined based on an edge measurement of the line segment.

11. An inspection tool comprising:

-a stage configured to receive a sample comprising a plurality of substantially parallel lines extending in a first direction;

-an electron beam source configured to generate an electron beam;

-a beam manipulator configured to direct electrons onto the sample;

-a detector configured to detect a response signal of the sample caused by an interaction of the electron beam with the sample;

-a control unit configured to control the beam manipulator to perform a plurality of scans over a field of view of the sample along a respective plurality of scan trajectories extending in a scan direction; the scan direction is substantially perpendicular to the first direction, wherein the control unit is further configured to perform the step of determining a distance between a first line segment of a line and a second line segment of the line based on the response signal of the sample for a plurality of locations within the field of view during the scan, wherein each of the first and second line segments intersects a plurality of scan trajectories, and the control unit is configured to determine a distortion over the field of view based on the distances determined at the plurality of locations.

12. The inspection tool of clause 11, wherein the control unit is configured to determine the distortion by performing a one-dimensional or two-dimensional fit based on the determined distance.

Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications, such as the manufacture of integrated optical systems, guidance and detection modes for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms "wafer" or "die" herein may be considered as synonymous with the more general terms "substrate" or "target portion", respectively. The substrate referred to herein may be processed, before or after exposure, in for example a track (a tool that typically applies a layer of resist to a substrate and develops the exposed resist), a metrology tool and/or an inspection tool. Where applicable, the disclosure herein may be applied to such and other substrate processing tools. Further, the substrate may be processed, for example, multiple times, in order to create a multi-layer IC, so that the term substrate used herein may also refer to a substrate that already contains multiple processed layers.

Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention may be used in other applications, for example imprint lithography, and where the context allows, is not limited to optical lithography. In imprint lithography, a topography in a patterning device defines the pattern created on a substrate. The topography of the patterning device can be pressed into a layer of resist provided to the substrate, and the resist can then be cured by applying electromagnetic radiation, heat, pressure or a combination thereof. After the resist is cured, the patterning device is moved out of the resist, leaving a pattern therein.

The terms "radiation" and "beam" used herein encompass all types of electromagnetic radiation, including Ultraviolet (UV) radiation (e.g. having a wavelength of or about 365, 355, 248, 193, 157 or 126nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range of 5-20 nm), as well as particle beams, such as ion beams or electron beams.

The term "lens", where the context allows, may refer to any one or combination of various types of optical components, including refractive, reflective, magnetic, electromagnetic and electrostatic optical components.

While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. For example, the present invention may take the form of: a computer program containing one or more sequences of machine-readable instructions describing a method as disclosed above, or a data storage medium (e.g. semiconductor memory, magnetic or optical disk) having such a computer program stored therein.

The above description is intended to be illustrative and not restrictive. Thus, it will be apparent to those skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.