Patent Description:
This disclosure relates to the field of electron beam imaging in semiconductor manufacturing and, in particular, to multiple-electron-beam imaging for defect inspection.

The manufacture of integrated circuits (ICs) is a multi-step process carried out on a wafer or a mask, which can be generally referred to as a substrate. Multiple ICs are typically produced on each wafer and each IC can be inspected for defects. Defect inspection is one step of the manufacturing process of ICs. Inspection systems can detect defects that occur during the manufacturing process. Optical wafer/mask inspection systems have been conventionally used for wafer/mask inspection. High-resolution inspection systems also exist for substrate inspection. <CIT> and <CIT> show examples of multi-electron beam inspection devices.

Disclosed herein are aspects, features, elements, and implementations of methods, apparatuses, and systems for multiple-electron-beam ("multi-beam") imaging.

In an aspect, a method for imaging a surface of a substrate using a multi-beam imaging system in accordance with claim <NUM> is disclosed. Optional features are recited in the dependent claims <NUM> to <NUM>.

In another aspect, a system for imaging a surface of a substrate using multiple electron beamlets in accordance with claim <NUM> is disclosed. Optional features are recited in dependent claims <NUM>-<NUM>.

In semiconductor manufacturing, microchips, or integrated circuits (ICs) are fabricated on a wafer. The process of manufacturing ICs involves several phases, including a design phase, a manufacturing phase, and an inspection phase, for example. The design phase involves designing structures and arrangements of circuit elements for the ICs. The manufacturing phase can include multiple operations, such as, for example, lithography, etching, deposition, or chemical-mechanical planarization (CMP). In the manufacturing phase, during a "patterning" process, geometric feature (e.g., patterns) on a photomask (or "mask") or a reticle can be transferred to a surface of the wafer. The wafer with the transferred geometric features can be referred to as a "patterned wafer. " In the inspection phase, the manufactured ICs can be inspected for quality control.

During the manufacturing phase, defects can occur. For example, the wafer surface can include defects, or the mask can include defects that can be transferred to the wafer. Therefore, it is advantageous to inspect the wafer and/or the mask (e.g., at proper processing operations) for potential defects in the inspection phase. The results of the inspection can be used to improve or adjust the design, the manufacturing, the inspection phases, or any combination thereof. Without loss of generality, a "patterned substrate" (or simply a "substrate" in a context without confusion) can be used herein to mean a wafer, a mask, a reticle, or any structure with patterns on it.

As the manufacture of ICs strives for smaller sized elements to achieve higher density for performances, detecting defects of small sizes becomes a challenge in semiconductor manufacturing. Imaging techniques are typically used to inspect defects on a patterned substrate. High throughput inspection systems (e.g., optical inspection systems) can face challenges of having insufficient sensitivity to find defects (e.g., physical defects) as design rules shrink (e.g., below <NUM>). In addition, optical inspection systems can have insufficient capability of detecting electrical defects buried below the surface. A high-resolution inspection system, such as an Electron Beam Inspection (EBT) system or a charged particle beam imaging system, becomes more important in defect inspection, especially for electrical defects and tiny physical defects. However, EBI systems have insufficient throughput, which limits its popularity in use for in-line process monitoring and high-volume manufacturing in a semiconductor process.

To increase the throughput of EBI systems, multiple-electron-beam (or hereafter referred to as "multi-beam") imaging techniques are used. A multi-beam imaging system uses multiple electron beams (referred to as "electron beamlets" or simply "beamlets") to inspect the patterned substrate. For example, the beamlets can be generated by splitting a single electron beam (referred to as "e-beam") using a splitting apparatus or apparatuses. The beamlets can be focused into spots on an object plane. The beamlets can also be transferred by projection of an intermediate lens (or intermediate lenses) toward an objective lens (or objective lenses). The objective lens can focus the beamlets. The focused beamlets can be used as a probe on a substrate surface. The beamlets can be deflected (e.g., being simultaneously deflected in the same direction) by a deflection apparatus for performing a raster scan (e.g., a two-dimensional raster scan) on the substrate surface. The raster scan on the substrate surface can excite secondary electron beamlets, which can be used to construct an image or images. In this disclosure, the scope or range within which the beamlets can perform an imaging process is referred to as a main field of view ("main-FOV"), and the scope or range within which a single beamlet of the beamlets can perform the imaging process is referred to as a sub-field of view ("sub-FOV").

In this disclosure, implementations of a multi-beam imaging system and scan methods for the multi-beam imaging system are described. The described multi-beam imaging system can be used for substrate (e.g., wafer or mask) inspection with a high throughput in semiconductor manufacturing. The described multi-beam imaging system can work in a continuous scan mode for inspection. The described multi-beam imaging system can also work in a step-and-scan mode for inspection. In the continuous scan mode, the multi-beam imaging system can increase the inspection throughput by reducing the settling time of the substrate stage. In some implementations, the continuous scan mode can increase the throughput of the multi-beam imaging system by two orders of magnitude compared to the step-and-scan mode. In some implementations, a linearly-arranged array of beamlets (referred to as "linear beamlets") can be used in the described multi-beam imaging systems to perform a line scan of the substrate in the continuous scan mode. The linear beamlets can be generated by splitting a modified single e-beam through a beam-splitting device. For example, the beam-splitting device can have multiple apertures or holes (referred to as a "multi-aperture device"). The multi-aperture device can include multiple apertures or holes to allow an electron beam to pass through. For example, the multi-aperture device can include multiple linearly-aligned apertures. The multi-beam imaging system and methods of performing inspection using the same will be detailed in the following description.

<FIG> is a block diagram of a multi-beam imaging system <NUM> in accordance with implementations of this disclosure. The system <NUM> can include an apparatus such as a computing device, which can be implemented by any configuration of one or more computers, such as a microcomputer, a mainframe computer, a supercomputer, a general-purpose computer, a special-purpose/dedicated computer, an integrated computer, a database computer, a remote server computer, a personal computer, or a computing service provided by a computing service provider, e.g., a web host, or a cloud service provider. In some implementations, the computing device can be implemented in the form of multiple groups of computers that are at different geographic locations and can communicate with one another, such as by a network. While certain operations can be shared by multiple computers, in some implementations, different computers can be assigned for different operations. In some implementations, the system <NUM> can be implemented using general-purpose computers/processors with a computer program that, when executed, carries out any of the respective methods, algorithms and/or instructions described herein. In addition, for example, special purpose computers/processors can be utilized which can contain specialized hardware for carrying out any of the methods, algorithms, or instructions described herein.

The system <NUM> has an internal configuration of hardware including a processor <NUM> and a memory <NUM>. The processor <NUM> can be any type of device capable of manipulating or processing information. In some implementations, the processor <NUM> can include a central processing unit (CPU). In some implementations, the processor <NUM> can include a graphics processor (e.g., a graphics processing unit or GPU). Although the examples herein are described with a single processor as shown, advantages in speed and efficiency can be achieved using multiple processors. For example, the processor <NUM> can be distributed across multiple machines or devices (in some cases, each machine or device can have multiple processors) that can be coupled directly or connected to a network. The memory <NUM> can be any transitory or non-transitory device capable of storing codes and data that can be accessed by the processor (e.g., via a bus). For example, the memory <NUM> can be accessed by the processor <NUM> via a bus <NUM>. Although a single bus is shown in the system <NUM>, multiple buses can be utilized. The memory <NUM> herein can be a random-access memory device (RAM), a read-only memory device (ROM), an optical/magnetic disc, a hard drive, a solid-state drive, a flash drive, a security digital (SD) card, a memory stick, a compact flash (CF) card, or any combination of any suitable type of storage devices. In some implementations, the memory <NUM> (e.g., a network-based or cloud-based memory) can be distributed across multiple machines or devices. The memory <NUM> can store data <NUM>, an operating system <NUM>, and an application <NUM>. The data <NUM>. can be any data for processing (e.g., computerized data files or database records). The application <NUM>. can include programs that permit the processor <NUM> to implement instructions to perform functions described in this disclosure.

In some implementations, in addition to the processor <NUM> and the memory <NUM>, the system <NUM> can include a secondary (e.g., additional or external) storage device <NUM>. The secondary storage device <NUM> can provide additional storage capacity for high processing needs. The secondary storage device <NUM> can be a storage device in the form of any suitable transitory or non-transitory computer-readable media, such as a memory card, a hard disc drive, a solid-state drive, a flash drive, or an optical drive. Further, the secondary storage device <NUM> can be a component of the system <NUM> or can be a shared device that can be accessed via a network. In some implementations, the application <NUM> can be stored in whole or in part in the secondary storage device <NUM> and loaded into the memory <NUM>. For example, the secondary storage device <NUM> can be used for a database.

In some implementations, in addition to the processor <NUM> and the memory <NUM>, the system <NUM> can include an output device <NUM>. The output device <NUM> can be, for example, a display coupled to the system <NUM> for displaying graphics data. If the output device <NUM> is a display, for example, it can be a liquid crystal display (LCD), a cathode-ray tube (CRT), or any other output device capable of providing a visible output to an individual. The output device <NUM> can also be any device transmitting visual, acoustic. or tactile signals to a user, such as a touch-sensitive device (e.g., a touchscreen), a speaker, an earphone, a light-emitting diode (LED) indicator, or a vibration motor. In some cases, an output device can also function as an input device-a touch screen display configured to receive touch-based input, for example.

In some implementations, the output device <NUM> can also function as a communication device for transmitting signals and/or data. For example, the output device <NUM> can include a wired mean for transmitting signals or data from the system <NUM> to another device. For another example, the output device <NUM> can include a wireless transmitter using a protocol compatible with a wireless receiver to transmit signals from the system <NUM> to another device.

In some implementations, in addition to the processor <NUM> and the memory <NUM>, the system <NUM> can include an input device <NUM>. The input device <NUM> can be, for example, a keyboard, a numerical keypad, a mouse, a trackball, a microphone, a touch-sensitive device (e.g., a touchscreen), a sensor, or a gesture-sensitive input device. Any type of input device not requiring user intervention is also possible. For example, the input device <NUM> can be a communication device such as a wireless receiver operating according to any wireless protocol for receiving signals. The input device <NUM> can output signals or data, indicative of the inputs, to the system <NUM>, e.g., via the bus <NUM>.

In some implementations, in addition to the processor <NUM> and the memory <NUM>, the system <NUM> can optionally include a communication device <NUM> to communicate with another device. Optionally, the communication can be via a network <NUM>. The network <NUM> can include one or more communications networks of any suitable type in any combination, including, but not limited to, Bluetooth networks, infrared connections, near-field connections (NFC), wireless networks, wired networks, local area networks (LAN), wide area networks (WAN), virtual private network (VPN), cellular data networks, or the Internet. The communication device <NUM> can be implemented in various ways, such as a transponder/transceiver device, a modem, a router, a gateway, a circuit, a chip, a wired network adapter, a wireless network adapter, a Bluetooth adapter, an infrared adapter, an NFC adapter, a cellular network chip, or any suitable type of device in any combination that can communicate with the network <NUM>.

The system <NUM> can communicate with a wafer or reticle high-resolution inspection apparatus. For example, the system <NUM> can be coupled to one or more wafer or reticle inspection apparatus, such as an e-beam system or an optical system, which is configured to generate wafer or reticle inspection results.

The system <NUM> (and algorithms, methods, instructions etc. stored thereon and/or executed thereby) can be implemented as hardware modules, such as, for example, intellectual property (IP) cores, application-specific integrated circuits (ASICs), programmable logic arrays, optical processors, programmable logic controllers, microcode, firmware, microcontrollers, servers, microprocessors, digital signal processors, or any other suitable circuit. Further, portions of system <NUM> do not necessarily have to be implemented in the same nimmer.

According to implementations of this disclosure, an example multi-beam imaging system includes devices, components, or sub-systems for performing multi-beam imaging for a substrate on a stage. The multi-beam imaging system can include an electron optical system, a substrate stage, or relative control systems or units.

<FIG> is a diagram of an example multi-beam imaging system <NUM> according to implementations of this disclosure. For example, the system <NUM> can be included in or connected to the system <NUM> of <FIG>. The system <NUM> can also include the system <NUM> in <FIG>. The components or sub-systems of the system <NUM> are described as follows.

An electron source <NUM> is used to generate an electron beam ("primary beam"). For example, the electron beam can be a primary beam <NUM> as shown in <FIG>. The electron source <NUM> can be, for example, thermionic emitters, cold field emitters, or thermionic field emitters (e.g., Schottky-type emitters). The electron source <NUM> can include a single emitter or multiple emitters. In an implementation, the electron source <NUM> can be a thermionic field emitter, and electrons emitted by the field emitter can be extracted by an electrode set <NUM>. The electrode set <NUM> can include one or more electrodes or plates applied with voltages. The extraction electrodes included in the electrode set <NUM> can have apertures for the emitted electrons to pass through. In an implementation, the electrode set <NUM> can include a suppressor electrode plate <NUM> and an extractor electrode plate <NUM>. The suppressor electrode plate <NUM> can be applied with a suppressing voltage for suppressing a portion of electrons (e.g., unwanted scattered electrons) emitted from the electron source <NUM> to form the primary beam <NUM>. The extractor electrode plate <NUM> can be applied with an accelerating voltage for extracting the electrons in the primary beam <NUM> and accelerate them to a certain velocity.

In some implementations, the electrode set <NUM> can also include electrostatic lenses (e.g., by using more electrodes) that can modify (e.g., collimate or focus) the primary beam <NUM>. In another implementation, the electrode set <NUM> can include a single anode plate with an aperture placed downstream the electron source <NUM>. For example, the aperture of the single anode plate can have a diameter of <NUM> microns (µm).

Downstream the electron source <NUM> and the electrode set <NUM>, a multipole-field device <NUM> is placed. In this disclosure, "downstream" refers to a direction along or with the direction of the electron beam emitting away from the electron source <NUM>, and "upstream" refers to a direction against or opposite to the emitted electron beam. The multipole-field device <NUM> can include electric and/or magnetic devices that generate one or more multipole electric and/or magnetic fields to modify the shaper of the primary beam <NUM>. For example, through the multipole electric and/or magnetic field, the multipole-field device <NUM> can extend the primary beam <NUM> along with a specific direction and suppress it in another direction (e.g., orthogonal or perpendicular to the specific direction).

In some implementations, the electric and/or magnetic devices can include four, six, eight, ten, twelve, or any number of poles. Each multipole electric and/or magnetic field device can be respectively "excited" with different voltages or currents for controlling a parameter referred to as "excitation strength. " The excitation strength represents the ability to extend or suppress the cross-section of the e-beam (referred to as a "beam spot"). In this disclosure, "excitation" refers to a process of generating an electric or magnetic field using a voltage or an electric current, respectively. In multi-beam imaging systems using the step-and-scan mode, the beam spot of the primary beam <NUM> is modified to a substantially round shape before being split into beamlets. Round primary beams (or substantially round primary beams) can be used for generating multiple beamlets in many multi-beam imaging systems.

For optimization of the multi-beam imaging system in the continuous scan mode, the beam spot of the primary beam <NUM> is modified to an elliptical shape. For example, as shown in <FIG>, a shape <NUM> represents a profile of the beam spot of the primary beam <NUM>, and a shape <NUM> represents a profile of the beam spot of the primary beam <NUM> after being modified. The multipole-field device <NUM> is used to change the shape of the beam spot, from a round primary beam (e.g., the shape <NUM>) to an elliptical shape (e.g., the shape <NUM>) by stretching and suppressing the primary beam to nearly confine it in one direction.

In an implementation, a round primary beam is used to generate multiple beamlets. For example, a two-dimensional ("2D") multi-aperture device can be used to generate the multiple beamlets with the round primary beam. For another example, each aperture of the 2D multi-aperture device can be used to generate a beamlet of the generated beamlets. In another implementation, an elliptical primary beam is used to generate multiple beamlets.

The multipole-field device <NUM> can be a single-stage (e.g., a single electric or magnetic multipole unit) or multi-stage device (e.g., a series of electric and/or magnetic multipole units). In an implementation, the multipole-field device <NUM> can be a two-stage device. The first stage can be used to extend the primary beam <NUM> along one direction (referred to as "x-direction") while the second stage can be used to suppress the primary beam <NUM> along with another direction orthogonal to the x-direction (referred to as "y-direction"). For example, the multipole-field device <NUM> can include an octupole electrostatic assembly and/or a quadrupole electrostatic assembly. The shape and size of the beam spot of the primary beam <NUM> can also be controlled by adjusting the excitation strength, a relative distance between the multipole-field device <NUM> and a beam-splitting device <NUM>, or using a focusing device (not shown). For example, the electrode set <NUM> can function as the focusing device.

An electrostatic lens set <NUM> can be placed downstream the multipole-field device <NUM>. The electrostatic lens set <NUM> can include the beam-splitting device <NUM> and a set of single-aperture electrode plates. The beam-splitting device <NUM>. is used to generate multiple beamlets <NUM> by splitting the primary beam <NUM> (e.g., after modification and correction) projected onto it.

In some implementations, the beam-splitting device <NUM> can include one or more multi-aperture plates. The multi-aperture plates can have different implementations and/or parameters. In this disclosure, if an aperture of the multi-aperture plates does not have a straight-line profile, the smallest diameter or dimension of the aperture on a multi-aperture plate can be referred to as a "beam limiting size. " For different multi-aperture plates, the configurations can be different for the beam limiting sizes of each aperture in a multi-aperture plate or pitches between each aperture of the multi-aperture plate.

In an implementation, the beam-splitting device <NUM> can include multiple multi-aperture plates. The multi-aperture plates can be aligned with respect to apertures on them. For example, for a multi-aperture plate including an odd number of linearly-arranged apertures, the center aperture (e.g., the aperture in the middle of the linearly-arranged apertures) can be used as a reference position. For another example, for a multi-aperture plate including an even number of linearly-arranged apertures, the central axis (e.g., an axis penetrating through the center of the multi-aperture plate) of the multi-aperture plate can be used as a reference position. In addition to the center aperture or the central axial axis, other apertures in each of the multi-aperture plates can also be used as reference positions. When aligning the multi-aperture plates, a reference position can be selected for each multi-aperture plate, and the multi-aperture plates can be aligned with respect to the selected reference positions. Further, the multi-aperture plates can be aligned with each other in different orientations. For example, linearly-arranged apertures in each of the multi-aperture plates can be aligned in different orientations (e.g., x- and/or y-orientations). Also, the multiple multi-aperture plates can have different arrangements of the apertures (e.g., some multi-aperture plates with linearly-arranged apertures, some multi-aperture plates with non-linearly-arranged apertures).

In an implementation, the beam-splitting device <NUM> can include switchable multi-aperture plates. For example, the switchable multi-aperture plates can include a first multi-aperture plate with 2D-arranged (e.g., non-linearly-arranged) apertures in a first region, a second multi-aperture plate with one-dimensional ("1D") arranged (e.g., linearly-arranged) apertures in a second region, and a third multi-aperture plate with a single aperture in a third region. Other arrangements and combinations of the multiple multi-aperture plates are also possible. By switching between the switchable multi-aperture plates, the multi-beam imaging system can work in different imaging modes. For example, the multi-beam imaging system can switch to work in the step-and-scan mode, the continuous scan mode, and the single-beam mode using the first multi-aperture plate, the second multi-aperture plate, and the third multi-aperture plate, respectively.

In an implementation, the beamlets generated (e.g., split) by a first multi-aperture plate can be converging, and a second multi-aperture plate downstream can be configured to have a pitch smaller than the pitch of the first multi-aperture plate. In another implementation, the beamlets generated by a first multi-aperture plate can be diverging, and a second multi-aperture plate downstream can be configured to have a pitch larger than the pitch of the first multi-aperture plate. In another implementation, the beamlets generated by a first multi-aperture plate can be parallel, and a second multi-aperture plate downstream can be configured to have a pitch the same with the pitch of the first multi-aperture plate.

In an implementation, the beam-splitting device <NUM> can include at least one multi-aperture plate that further includes multiple apertures. For example, the multiple apertures in the multi-aperture plate can be linearly arranged along a straight line, as shown in <FIG>. For another example, the multiple apertures in the multi-aperture plate can be arranged along multiple parallel straight lines, as shown in <FIG>. For another example, the multiple apertures in the multi-aperture plate can be arranged in a first 2D array, as shown in <FIG>. For another example, the multiple apertures in the multi-aperture plate can be arranged in a second 2D array, as shown in <FIG>. The second 2D array in <FIG> shows a multi-aperture plate <NUM> with a <NUM>-aperture array. The second 2D array in <FIG> can have other numbers or arrangements for the apertures. For example, the number of the apertures of the multi-aperture plate <NUM> can be <NUM> · n<NUM>, in which n is a positive integer.

In <FIG>, the second 2D array of the apertures shows a cross-shaped profile or layout, which can be used to generate a 2D array of beamlets. The 2D array of beam lets can have a main-FOV covering a scanning section that can be joined (or "stitched") in a tile-fashion without causing duplicate or redundant scanning and/or stepping in the step-and-scan mode of the multi-beam imaging systems. The joining of the scanning sections covered by the 2D array of beam lets generated using the multi-aperture plate <NUM> will be shown and described in <FIG>.

In an implementation, for example, as shown in <FIG>, the multi-aperture plate <NUM> includes multiple linearly-arranged apertures <NUM>. After passing through the multi-aperture plate <NUM>, the primary beam <NUM> can form an array of linearly-arranged multiple beamlets (e.g., the beamlets <NUM>). In an implementation, the beam-splitting device <NUM> can be a multi-aperture plate with <NUM> linearly-arranged apertures, each having a diameter of <NUM> and a spacing of <NUM> in between. Other numbers and dimensions of the apertures on the beam-splitting device <NUM> are possible. The number of beamlets can be controlled using a focus device (e.g., an electrostatic immersion lens), which can variate the size of the beam spot of the primary beam <NUM>. For example, the electrostatic immersion lens can be placed downstream or upstream the suppressor electrode plate <NUM>.

The multipole-field device <NUM> is also used as an aberration corrector to correct aberrations of a round primary beam before generating the beamlets. The extent or level of the aberration correction applied by the multipole-field device <NUM> can be controlled. For example, the multipole-field device <NUM> can be controlled to minimize the aberrations. For another example, the multipole-field device <NUM> can be controlled to maintain a certain extent of aberrations, and downstream devices (e.g., the beam-splitting device <NUM>) can be used/controlled to further correct the remaining aberrations (e.g., by producing couiiter-abefrations with an opposite sign or in an opposite direction of the remaining aberrations) to obtain substantially complete cancellation of the aberrations.

In a step-and-scan mode, beam spot of the primary beam <NUM> is modified by the multipole-field device <NUM> to have an approximately round profile or shape. Generally, different numbers of beamlets can be generated by control the size of the beam spot. For example, as shown in <FIG>, the apertures <NUM> on the multi-aperture plate <NUM> can be covered by a round beam spot of the primary beam <NUM>. The round beam spot can be controlled to have different sizes, such as a first round beam spot <NUM> with a larger size and a second round beam spot <NUM> with a smaller size. In <FIG>, the first round beam spot <NUM> can generate more beamlets than the second round beam spot <NUM>. The size of the beam spots can be adjustable. Though the apertures <NUM> is shown as linearly arranged, they can be arranged in any form. For example, the apertures <NUM> can be arranged as shown in <FIG>.

In a continuous scan mode the beam spot of the primary beam <NUM> is modified by the multipole-field device <NUM> to have an elliptical profile. The elliptical-profile primary beam can be used to optimize performances of the continuous scan mode for the multi-beam imaging system. For example, as shown in <FIG>, the apertures <NUM> on the multi-aperture plate <NUM> can be covered by an elliptical beam spot <NUM> of the primary beam <NUM>. The second round beam spot <NUM> is also shown in <FIG> for comparison. In an implementation, the elliptical beam spot <NUM> can be adjusted to a size that is just large enough to cover the apertures <NUM> on the multi-aperture plate <NUM>.

In some implementations, by modifying the primary beam <NUM> into an elliptical shape, the multi-aperture plate <NUM> with linearly-arranged apertures can generate beamlets with higher beam density, which can further cause more effective use of beams. In another implementation, the primary beam <NUM> can be modified into other shapes in addition to an elliptical shape.

In some multi-beam imaging systems, the multiple apertures on the multi-aperture plate are arranged in two dimensions. For example, the multiple apertures can be arranged as a 2D array symmetric to the center axis of the primary beam <NUM>. Designs of the 2D arrays can include but are not limited to a square arrangement, a hexagon arrangement or a circular arrangement. With such multi-aperture plate configuration, a 2D beamlet array can be generated. In this disclosure, as an example, the multi-aperture plate (e.g., the multi-aperture plate <NUM>) is designed with linearly-arranged apertures (e.g., the apertures <NUM>). The linearly-arranged apertures on the plate can form an aperture array along a single line or multiple parallel lines. The longer side of the aperture array can also be aligned with the long axis of the ellipse beam spot (e.g., the elliptical beam spot <NUM>), thus all the linearly-arranged apertures can be covered by the elliptical beam spot projected on the multi-aperture plate. With such multi-aperture plate configuration, a 1D beamlet array can be generated. For example, the 1D beamlet array can be used for a line scan in the continuous scan mode of the multi-beam imaging system. The area of the substrate surface covered by one line scan is referred to as a "line" herein.

To optimize imaging properties of the beamlets <NUM>, electrostatic lenses or similar devices can be used to control the primary beam <NUM> and/or the beamlets <NUM>. For example, the electrostatic lens set <NUM> can include a first single-aperture electrode plate <NUM> placed upstream the beam-splitting device <NUM> and a second single-aperture electrode plate <NUM> placed downstream the beam-splitting device <NUM>. The first single-aperture electrode plate <NUM> and second single-aperture electrode plate <NUM> can be centered at a center axis of the primary beam <NUM>. In an implementation, the apertures of the first single-aperture electrode plate <NUM> and the second single-aperture electrode plate <NUM> can be larger than <NUM>. Other dimensions of the apertures on the single-aperture electrode plates <NUM> and <NUM> are possible. The first single-aperture electrode plate <NUM> and the second single-aperture electrode plate <NUM> can be used to generate a local electric field that determines an incident angle of the primary beam <NUM>. Each of the generated beamlets <NUM> can be further modified by the local electric field generated by the single-aperture electrode plates <NUM> and <NUM> by, for example, being converged, diverged, collimated, focused, and/or defocused.

In an implementation, with the beam-splitting device <NUM>, the first single-aperture electrode plate <NUM> and the second single-aperture electrode plate <NUM> are applied with different voltages to form an electrostatic lens. For example, the electrostatic lens can be used for collimating the beamlets <NUM> and focusing each beamlet thereof. For better performance, the primary beam <NUM> can be collimated before passing through the beam-splitting device <NUM>. For another example, the incident angle of the primary beam <NUM> can be adjusted by changing the voltages applied on the first single-aperture electrode plate <NUM> and the second single-aperture electrode plate <NUM>. For optimization, the incident angle can be adjusted to determine the brightness and reduce aberration for the beamlets <NUM>.

In the above implementation, the voltages of the first single-aperture electrode plate <NUM>, the second single-aperture electrode plate <NUM>, and the beam-splitting device <NUM> can be set so that each beamlet of the beamlets <NUM> can be individually focused on a plane downstream the electrostatic lens set <NUM>. The profile of each beamlet can be determined by the local electric field between the first single -aperture electrode plate <NUM>, the second single-aperture electrode plate <NUM>, and the beam-splitting device <NUM>. To optimize the imaging condition of multi-beam EBI, the beamlets <NUM> can also be slightly converged or collimated. For example, in an implementation, with an anode plate (e.g., the extractor electrode plate <NUM> or a single anode plate in the electrode set <NUM>) placed upstream the beam-splitting device <NUM>, voltages G, V<NUM>, V<NUM>, and V<NUM>, with G < V<NUM> < V<NUM> < V<NUM>, can be applied to the anode plate, the beam-splitting device <NUM>, the first single-aperture electrode plate <NUM>, and the second single-aperture electrode plate <NUM>, respectively. The values of those voltages are determined so that the primary beam <NUM> can be collimated before passing through the beam-splitting device <NUM> and each beamlet can focus individually while remaining parallel to each other as much as possible. The voltages G, V<NUM>, V<NUM>, and V<NUM> can be varied to other values. The size of the beam spot of the primary beam <NUM> on the beam-splitting device <NUM> can also be adjusted by tuning the aforementioned voltages. In an implementation, the beam-splitting device <NUM> can be configured as multi-aperture lenses by biasing the beam-splitting device to a voltage in a range from -<NUM> kV to <NUM> kV.

Due to a variety of factors (e.g., locations of the beamlets <NUM>, deformation of the beam spot, and/or non-uniformity of electric fields), beamlets <NUM> downstream the electrostatic lens set <NUM> can have aberrations. Aberrations existing in a multi-beam imaging system can include spherical aberration, chromatic aberration, astigmatism, and field curvatures. Spherical aberration and chromatic aberration occur mainly due to the non-uniformity of on- or off-axis focus conditions (e.g., local electric or magnetic fields of electrostatic lenses) for the electron beams. Astigmatism and field curvatures occur mainly due to the anisotropic asymmetry of the on- or off-axis focus conditions and off-axis electron beams. For example, one of the causes for the anisotropic asymmetry and the off-axis electron beams can be the elliptical deformation of the primary beam <NUM> modified by the multipole-field device <NUM>. The aberration can lead to degradation in imaging resolution of the multi-beam imaging systems.

In some implementations, an optional aberration corrector set including one or more aberration correctors (not shown) can be used in the system <NUM> to eliminate or reduce the aberration of the beamlets <NUM>. The optional aberration correctors can be placed upstream or downstream a focusing plane of the beamlets <NUM>. In some implementations, the system <NUM> can include a spherical aberration corrector, an astigmatism corrector, and/or a field curvature corrector.

In an implementation, the spherical aberration corrector can be one or more multipole-field devices upstream or downstream the focusing plane of the beamlets <NUM>. For example, the multipole-field device <NUM> can function as the spherical aberration corrector. For another example, the spherical aberration corrector can be a multiple (e.g., quadrupole or octupole) magnetic field device upstream the multi-aperture plate.

In an implementation, astigmatism and the field curvatures can be reduced via a specially designed multi-aperture plate. For example, the multi-aperture plate included in the beam-splitting device <NUM> can function as the specially designed multi-aperture plate.

In an implementation, the beam-splitting device <NUM> can include a two-layer multi-aperture plate 500A as shown in a cross-sectional diagram <FIG>. The multi-aperture plate 500A can include a first layer <NUM> facing the primary beam <NUM> (upstream) with a thickness T<NUM> and a second layer <NUM> downstream the first layer <NUM> with a thickness TL. The multi-aperture plate 500A can be fabricated with micromachining techniques. The first layer <NUM> can include apertures with a uniform size D<NUM> to split the primary beam <NUM>. D<NUM> can function as the beam limiting size that is used to limit the current of each outgoing beamlet. The second layer <NUM> can include apertures with different sizes, correspondingly aligned with the apertures of the first layer <NUM>. A size of an aperture in the second layer <NUM> decreases as its distance to a center axis <NUM> of the multi-aperture plate 500A increases. For example, in <FIG>, an aperture <NUM> with a size DL<NUM> has a first distance to the center axis <NUM> (e.g., zero distance-i.e., the aperture <NUM> is centered at the center axis <NUM>) of the multi-aperture plate 500A. An aperture <NUM> with a size DL1 and an aperture <NUM> with the size DL<NUM> have a second distance to the center axis <NUM> of the multi-aperture plate 500A. The first distance is smaller than the second distance, and DL<NUM> is greater than DL<NUM>. To keep off scattered electrons, the apertures of the second layer <NUM> all have larger sizes than the corresponding apertures of the first layer <NUM>. For example, as shown in <FIG>, it is DL<NUM> > D<NUM> and DL<NUM> > D<NUM>. With such configuration, beamlets outgoing from different apertures (e.g., the apertures <NUM>, <NUM>, and <NUM>) can have different foci and thus can focus on the same plane with reduced aberration (e.g., reduced astigmatism and field curvatures).

In another implementation, as shown in <FIG>, in addition to the first layer <NUM> and the second layer <NUM> in <FIG>, the multi-aperture plate 500B can further include a third layer <NUM> upstream the first layer <NUM> with a thickness TU. For example, the multi-aperture plate 500B can be placed upstream the focusing plane of the beamlets, using the third layer <NUM> to converge the beamlets incident to the first layer <NUM>. The third layer <NUM> can include apertures with different sizes, aligned with the corresponding apertures of the first layer <NUM>. Similar to the second layer <NUM>, a size of an aperture in the third layer <NUM> decreases as its distance to the center axis <NUM> of the multi-aperture plate 500B increases. For example, in <FIG>, an aperture <NUM> with a size DU<NUM> has a third distance to the center axis <NUM> (e.g., zero distance-i.e., the aperture <NUM> is centered at the center axis <NUM>) of the multi-aperture plate 500B. An aperture <NUM> with a size DU<NUM> and an aperture <NUM> with the size DU<NUM> have a fourth distance to the center axis <NUM> of the multi-aperture plate 500B. The third distance is smaller than the fourth distance and DU<NUM> is greater than DU<NUM>. In some implementations, the third and fourth distances can be equal to the first and second distances, respectively. To converge the beamlets incident to the first layer <NUM>, the apertures of the third layer <NUM> all have sizes larger than the corresponding apertures of the first layer <NUM> and smaller than the corresponding apertures of the second layer <NUM>. For example, as shown in <FIG>, the apertures <NUM>, <NUM>, and <NUM> can correspond to the apertures <NUM>, <NUM>, and <NUM>, respectively, with DL<NUM> > DU<NUM> > D<NUM> and DL<NUM> > DU<NUM> > D<NUM>.

Referring back to <FIG>, downstream the electrostatic lens set <NUM>, a projection lens set (or referred to as "intermediate lens set") <NUM> is used to project (e.g., converge or concentrate) the beamlets <NUM>. The projection lens set <NUM> can include one or more electric/magnetic projection lenses. In an implementation, the projection lens set <NUM> can include a magnetic condenser lens. Together with an objective lens set <NUM>, the projection lens set can magnify or minify the profile of the beamlets <NUM> projected onto a surface of a substrate <NUM> under inspection. For example, excitation strengths of the projection lens set <NUM> (e.g., the magnetic condenser lens) and the objective lens set <NUM> can be determined so that a spacing between each beamlet of the beamlets <NUM> is around <NUM> on the surface of the substrate <NUM>, and the sub-FOV of each beamlet is larger than the spacing of <NUM>. The spacing between beamlets <NUM> on the surface of the substrate <NUM> and the sub-FOV of each beamlet can be adjustable.

In an implementation, an optional aperture plate <NUM> can be placed downstream the projection lens set <NUM> to block scattered electrons. Downstream the projection lens set <NUM>, a deflector set <NUM> is used to drive the beamlets <NUM> to scan at least a portion (e.g., a section/strip of a care area) of the substrate <NUM>. A "care area" is an area on a wafer that is to be inspected. The deflector set <NUM> can include one or more scanning deflectors. The scanning direction of each scanning deflector can be adjustable. For example, the scanning directions can be perpendicular or skew-intersected. In an implementation, the deflector set <NUM> can be concentrically placed in the center of the objective lens set <NUM>.

The objective lens set <NUM> can focus the beamlets <NUM> on the surface of the substrate <NUM>. In an implementation, the objective lens set <NUM> can include a magnetic condenser lens. For example, the objective lens set <NUM> can focus the beamlets <NUM> onto a section/strip of the care area, each beamlet having a sub-FOV covering a sub-section of the section/strip. In an implementation, the objective lens set <NUM> can be an immersive objective lens with a booster <NUM> to converge the beamlets <NUM> in shorter foci. Using the immersive objective lens, the beamlets <NUM> can "immerse" in an electromagnetic field generated by the booster <NUM> and the substrate <NUM>. For example, the electromagnetic field can be generated by applying a voltage on the substrate <NUM> and the booster <NUM>, and the voltage of the booster <NUM> can be set higher than the voltage of the immersive objective lens.

A substrate stage <NUM> is used to carry the substrate <NUM>. The substrate stage <NUM> is controllable to move to expose different portions of the substrate <NUM> under the beamlets <NUM> for inspection. As aforementioned, there are two types of motion control modes for the substrate stage <NUM> corresponding to two image scan methods: the step-and-scan mode and the continuous scan mode. In the continuous scan mode, the substrate stage <NUM> can keep moving in a first direction (e.g., a horizontal direction or "x-direction") at a constant speed while the linearly-arranged beamlets can perform a line scan in a second direction (e.g., a vertical direction or "y-direction"). For example, the second direction can be approximately orthogonal to the first direction.

When the beamlets <NUM> hit the surface of the substrate <NUM>, the electrons can be scattered, such as in a direction against the incident beamlets <NUM>. Generally, the scattered electrons can be categorized into two groups: backscattering electrons (BSEs) scattered due to elastic collisions and secondary electrons (SEs) scattered due to inelastic collisions (e.g., ionization). The BSEs and SEs generated from the beamlets can form BSE beamlets and SE beamlets, respectively. In this disclosure, the BSE beamlets and SE beamlets can be collectively referred to as "scatter beamlets.

A Wien filter set <NUM> including at least one Wien filter can be used to deflect or bend the scatter beamlets <NUM> away from a center axis of the incident beamlets <NUM>, while keeping the incident beamlets <NUM> not bent. The scatter beamlets <NUM> are directed toward an off-axis (e.g., away from the center axis of the primary beam <NUM>) detector <NUM> to be captured. The detector <NUM> is a detector array including multiple detectors. An excitation strength of the Wien filter set <NUM> can be determined so that the scatter beamlets <NUM> can reach the surface of the detector <NUM>. In an implementation, the Wien filter set <NUM> can be concentrically placed in the center of the objective lens set <NUM>.

In an implementation, the Wien filter set <NUM> can be replaced by other types of multipole-field devices, such as, for example, E × B deflectors, in which E represents an electric field and B represents a magnetic field.

Corresponding to different detector setups, there can be at least two ways of providing Wien filter applications. A first application is to slightly deflect the scatter beamlets <NUM> via a weak excitation strength of the Wien filter set <NUM> (e.g., by setting a weak electric and/or magnetic field of the Wien filter set <NUM>) and to place the detector <NUM> adjacent to the center axis of the beamlets <NUM>. A second application is to deflect the scatter beamlets <NUM> by a large angle via a strong excitation strength of the Wien filter set <NUM> (e.g., by setting a strong electric and/or magnetic field of the Wien filter set <NUM>) and to place the detector <NUM> far from the center axis of the beamlets <NUM>. The first application can save space and reduce the total size of the system <NUM>. The second application can reduce interactions between the incident beamlets <NUM> and the scatter beamlets <NUM>, and there can be more space for an optional projection system for the scatter beamlets (not shown). In an implementation, the first application is used in the system <NUM>. In another implementation, the second application is used in the system <NUM>.

In an implementation, the objective lens set <NUM> can include at least one electrode for controlling an electric field on the surface of the substrate <NUM>. For example, a high voltage can be applied to provide an electric field (referred to as a "surface extraction field") to extract scattered electrons (e.g., BSEs or SEs) effectively to form the scatter beamlets <NUM>. For another example, the substrate <NUM> can be biased at a negative voltage with respect to grounded magnetic lens polepiece to provide the surface extraction field. For another example, a field strength of the surface extraction field can range from <NUM> V/mm to <NUM> V/mm.

The detector <NUM> is used to capture the scatter beamlets <NUM> and generate signals <NUM>. The signals <NUM> can be analog and/or digital signals, and can be further processed by an image processing system (not shown). The image processing system can receive and process the signals <NUM> to generate one or more images of the scanned substrate surface for inspection. In an implementation, the image processing system can generate and process images with high speed (e.g., with an image capture rate greater than or equal to <NUM>). For example, the image processing system can process the images using parallel computing. For another example, the image processing system can use CPUs and/or GPUs in the system <NUM> (e.g., the processor <NUM>) and a memory (e.g., the memory <NUM>) for processing. The image capture rate can be adjusted. When the system <NUM> works in the continuous scan mode, depending on data processing methods used by the image processing system, the generated images of all strips can be mosaicked for inspection, or the image of each strip can be preprocessed.

The detector <NUM> can be of various types, including but not limited to a microchannel plate (MCP), a silicon diode detector (SDD), an Everhart-Thornley (ET) detector, or a charge-coupled device (CCD) detector. In an implementation, the detector <NUM> can be a detector array that includes multiple detector units or regions, and each detector unit can detect a single scatter beamlet. For example, the detector units of the detector array can match the arrangement of the scatter beamlets <NUM> so that each scatter beamlet can be captured by one detector unit. In an implementation, a <NUM>-aperture plate is used as the beam-splitting device <NUM>, and, correspondingly, an SDD detector with <NUM> strip-shape detecting regions can be used. The SSD detector can be placed off-axis above the objective lens for the system <NUM> working in the continuous scan mode. The shape and dimension of the detector units can be varied as long as there is no crosstalk between the scatter beamlets <NUM> and each scatter beamlet can be detected.

In some implementations, optionally, there can be a projection system (not shown) upstream the detector <NUM> for optimizing imaging conditions on the detector surface. For example, the projection system can scale and project the scatter beamlets <NUM> to respective detector units (e.g., separate units or isolated units) of the detector <NUM>. The projection system can also eliminate or reduce aberration, deflection/displacement errors, and/or rotation errors of the scatter beamlets <NUM>. For example, the projection system can include a projection lens, a deflector, and/or a rotation corrector.

For moveable components of the system <NUM>, electronic control systems (not shown) can be used to drive and control them to function. For example, the electronic control systems can control at least one of the projection lens set <NUM>, the optional aperture plate <NUM>, the deflector set <NUM>, the objective lens set <NUM>, the booster <NUM>, the substrate stage <NUM>, the Wien filter set <NUM>, and/or the optional projection system (not shown) upstream the detector <NUM>. Based on the motion modes of the substrate stage <NUM>, parameters of the electronic control systems and other components of the system <NUM> can be adjusted for optimizing imaging conditions and the total throughput. For example, in the step-and-scan mode, 2D beam array is used, and the parameters of the electronic control system can be adjusted to optimize performance. The control strategies can also be adjusted to coordinate with the step-and-scan method. For another example, in the continuous scan mode, 1D beam array is used, and different designs and control strategies can be used corresponding to the 1D beamlet configuration. The parameters of the electronic control system for the continuous scan mode can be different from the parameters for the step-and-scan mode. For another example, in the continuous scan mode, the moving speed of the substrate stage <NUM> can be set to match the image capture rate of the image processing system (not shown) so that all pixels of the care area can be scanned. For example, the moving speed can be determined or optimized using a learning technique (e.g., a machine learning technique and/or a statistics-based learning technique). For another example, the moving speed can be adaptively determined for varying types of substrates, inspection conditions, defects, and/or aberrations. In an implementation, the electronic control system can deflect the beamlets <NUM> for scanning with high speed (e.g., with a scanning rate greater than or equal to <NUM>). The scanning rate can be adjustable.

It is understood that components or sub-systems of the system <NUM> as described herein is not limited to the aforementioned implementations or examples. More parts or components with various designs and/or functions can be added to the system <NUM> for function extensions or performance optimization.

For example, in an implementation, the system <NUM> can include the following combination: an electron source, at least one multipole-field device, at least one multi-aperture plate, at least one single-aperture electrode plate, at least one optional aberration corrector, at least one projection lens, an objective lens, at least one deflector, at least one Wien filter, a substrate stage, a detector or detector array, an image processing system, and at least one electronic control system.

For another example, in another implementation, the system <NUM> can include the following combination: a single electron emitter as an electron source, a set of an octupole/quadrupole electrostatic assembly as a multipole-field device, a <NUM>-aperture plate as a multi-aperture plate, two single-aperture electrode plates, a magnetic condenser lens as a projection lens, two electrostatic deflectors, a quadrupole Wien filter, an irmnersive objective lens with a booster, a substrate stage, a strip-arrayed SDD detector, a scattered-electron (e.g., BSEs or SEs) projection system, an image processing system, and control systems of moveable modules/components.

For another example, in another implementation, the system <NUM> can include: an electron source for generating a primary electron beam, a multipole-field device for shaping the primary electron beam, an electron lens for collimating the primary electron beam before entering a splitting device, at least one multi-aperture plate for splitting the primary electron beam into multiple beamlets and bringing each beamlet into focus on a plane in a downstream region, an electron lens for manipulating the foci of the multiple beamlets on an image plane after the splitting, a projection lens for projecting the foci of the multiple beamlets to the substrate, an objective lens for focusing the multiple beamlets into fine spots on the surface of the substrate, a deflector set comprising at least one deflector for scanning all of the multiple beamlets for exciting scattered electrons (e.g., BSEs or SEs), a stage for holding the substrate and for moving in a specific mode to position the substrate for primary beamlets scanning, a multipole-field device for deflecting scatter beamlets off-axis, a scattered-electron (e.g., BSEs or SEs) optics system for projecting and guiding the scatter beamlets towards an array of detectors, the array of detectors coupled to a signal process circuit for converting the scatter beamlets into electron signals, a processor for constructing, storing, or distributing images obtained from the array of detectors based on the electron signals, and a computer system for processing the images for a predefined application.

According to the invention, both the step-and-scan mode and the continuous scan mode are available and switchable for the system <NUM>. For performance optimization, various scan parameters (e.g., the image capture rate, the scanning rate, the beamlet shape and sizes, overlapping of neighboring FOVs of beamlets, or any other operational parameters of the multi-beam imaging system) can be applied for the step-and-scan mode and the continuous scan mode, respectively.

In some implementations, a multi-beam imaging system (e.g., the system <NUM>) can also work in a single-beam imaging mode in addition to the multi-beam imaging mode. For example, a multi-aperture plate of the multi-beam imaging system can switch between a multi-beam mode and a single-beam mode. In an implementation, the multi-aperture plate of the multi-beam imaging system can be movable using a moving mechanism (e.g., rotating).

As shown in <FIG>, a multi-aperture plate <NUM> can include multiple apertures <NUM> in a first region and a single aperture <NUM> in a second region. For example, the multiple apertures <NUM> and the single aperture <NUM> can have a distance <NUM> to each other. The multi-aperture plate <NUM> can switch the first and second region to be under a beam spot of the primary beam. The multiple apertures <NUM> are under the beam spot when the multi-aperture plate <NUM> is at a first position, while the single aperture <NUM> is under the beam spot when the multi-aperture plate <NUM> is at a second position. For example, the switching of the first and second positions can be done by rotating the multi-aperture plate <NUM>. When in the single-beam mode, a round beam spot <NUM> can be used, and the operational parameters of components of the multi-beam imaging system can be adjusted so that imaging conditions for the single-beam mode can be optimized. When in the multi-beam mode, an elliptical beam spot <NUM> (e.g., modified by the multipole-field device <NUM>) can be used, and the operational parameters of components of the multi-beam imaging system can be adjusted so that imaging conditions for the multi-beam mode can be optimized.

In an implementation, there can be more than one single aperture on the multi-aperture plate <NUM>. For example, there can be two or more single apertures with different diameters on the multi-aperture plate <NUM>. In another implementation, the multi-aperture plate of the multi-beam imaging system can be replaceable. For example, the multi-aperture plate <NUM> can be replaced with the multi-aperture plate <NUM>.

In this disclosure, scan methods for the aforementioned implementations of the multi-beam imaging system are also included. Detailed descriptions of the methods are provided as follows.

In some multi-beam imaging systems, the images of a given region of interest (ROI) or a care area can be captured in FOVs of the beamlets. For example, the images of the given region of ROI can be captured by scanning (e.g., raster scanning) a main-FOV of the beanilets. In an implementation, during the scanning of a FOV, a substrate stage (e.g., the substrate stage <NUM> in <FIG>) can be kept stationary at a first position, and at least one deflection unit (e.g., the deflector set <NUM> in <FIG>) can deflect the beamlets to scan a substrate (e.g., the substrate <NUM> in <FIG>) placed on the substrate stage. For example, the deflection unit can be excited and/or driven by raster-scan signals. In an implementation, all beamlets (e.g., all beamlets of the beamlets <NUM> in <FIG>) can scan (e.g., simultaneously scan) the substrate and a main-FOV image can be generated. The main-FOV image can include multiple sub-FOV images, each of the sub-FOV images formed by a beamlet of the beamlets. When the scanning of the main-FOV is completed, the substrate stage can move to a second position for a next scan (referred to as "stepping"). The stepping and scanning can be repeated until all care areas on the substrate are scanned and the inspection process is completed. This mode of inspection is generally referred to as a step-and-scan (or "step-and-repeat") mode. In some implementations, multiple beamlets can be used to inspect the substrate in the step-and-scan mode.

In some implementations, 2D beamlets can be used to inspect the substrate in the step-and-scan mode. For example, as shown in <FIG>, a multi-aperture plate <NUM> can include a 2D aperture array in a matrix arrangement. The 2D aperture array includes multiple apertures, including an aperture <NUM>. Multiple 2D beamlets (e.g., in the matrix arrangement) can be generated using the multi-aperture plate <NUM>. The 2D beamlets can have a main-FOV <NUM> on a substrate surface, which includes multiple sub-FOVs including a sub-FOV <NUM>. The sub-FOVs can correspond to respective individual beamlets of the 2D beamlets. For example, the sub-FOV <NUM> can correspond to an individual beamlet generated by the aperture <NUM>. In some implementations, the sub-FOV <NUM> and its generated image can be square or rectangular. An actual size of the sub-FOV <NUM> on the substrate surface can be slightly overlapped, connected (or "stitched") with, or separated from its neighboring sub-FOVs. In an implementation, the sub-FOV <NUM> can be square, and its physical size can be controlled so that all sub-FOVs of the main-FOV <NUM> on the substrate surface can be stitched with a neighboring stib-FOV, in which the main-FOV <NUM> can cover an actual size expected to be equal to a sum of its all sub-FOVs.

In some implementations, a care area of a patterned substrate can be a rectangular or square shape. In the step-and-scan mode, a main-FOV of the 2D beamlets can cover a first portion of the care area for scanning, and the substrate stage can step or move in a way such that the main-FOV can cover a second portion of the care area that is stitched with the first portion of the care area after stepping. This stepping and scanning process can be repeated until the entire care area is covered.

For example, as shown in <FIG>, a care area <NUM> is in a rectangular shape. For inspection of the care area <NUM>, multiple sections can be used, including a section <NUM>. The multiple sections can cover a region larger than or equal to the care area <NUM>. Each section can be covered by a main-FOV (e.g., the main-FOV <NUM>) of the 2D beamlets. In some implementations, based on the shape and size of the main-FOV <NUM>, the 2D beamlets can be generated using the multi-aperture plate <NUM> in <FIG>, in which the main-FOV <NUM> can cover the section <NUM>. In some other implementations, other shapes and configurations of multi-aperture plates can be used to generate the 2D beamlets to cover a section for inspecting the care area. In an implementation, as shown in <FIG>, the substrate stage can move in a way such that the main-FOV <NUM> can move in accordance with a stepping path (or sequence) <NUM>. Following the arrows of the stepping path <NUM> from a start point to an endpoint as shown in <FIG>, the main-FOV <NUM> can sequentially cover each section similar to the section <NUM> to inspect the care area <NUM>, until all of the care area <NUM> is covered. In some implementations, when the actually-inspected region is larger than the care area (e.g., the scenario shown in <FIG>,). the generated image can be filtered (or "cropped") to discard image portions outside the care area, in which only image portions corresponding to the care area will be processed for defect inspection or image measurement.

In some implementations, linearly-arranged (1D) beamlets can be used to inspect the substrate in the step-and-scan mode. For example, as shown in <FIG>, a multi-aperture plate <NUM> can include a linearly-arranged aperture array in a straight-line arrangement. The linearly-arranged aperture array includes multiple apertures, including an aperture <NUM>. Multiple linearly-arranged beamlets (e.g., in the straight-line arrangement) can be generated using the multi-aperture plate <NUM>. The linearly-arranged beamlets can have a main-FOV <NUM> on a substrate surface, which includes multiple sub-FOVs including a sub-FOV <NUM>. The sub-FOVs can correspond to respective individual beamlets of the linearly-arranged beamlets. For example, the sub-FOV <NUM> can correspond to an individual beamlet generated by the aperture <NUM>. In some implementations, the main-FOV <NUM> can be rectangular and its sub-FOVs (such as the sub-FOV <NUM>) can be square or rectangular. An actual size of the sub-FOV <NUM> on the substrate surface can be slightly overlapped, stitched with, or separated from its neighboring sub-FOVs. In some implementations, the quantity of the linearly-arranged beamlets can be greater than or equal to <NUM>. In an implementation, the quantity of the linearly-arranged beamlets can be in a range from <NUM> to <NUM>. In another implementation, the quantity of the linearly-arranged beamlets can be greater than <NUM>.

In some implementations, based on the shape and size of the main-FOV (e.g., the main-FOV <NUM>) of the linearly-arranged beamlets, a rectangular care area on a patterned substrate can be divided into sections for inspection in the step-and-scan mode. For example, as shown in <FIG>, a care area <NUM> is rectangular and divided into multiple sections for inspection. In <FIG>, the care area <NUM> is divided into <NUM> rectangular sections, including a section <NUM>. The section <NUM> is similar to other <NUM> divided sections in the care area <NUM>. Each of the rectangular sections can be covered by a main-FOV (e.g., the main-FOV <NUM>) of the linearly-arranged beamlets. In an implementation, the linearly-arranged beamlets can be generated using the multi-aperture plate <NUM> in <FIG>, and the main-FOV <NUM> can cover the section <NUM>.

In some implementations, the shape and size of the section <NUM> can be determined based on the quantity and arrangement of the beamlets and sub-FOV sizes of each beamlet. For example, based on the configuration of care area division as shown in <FIG>, beamlets for imaging the section <NUM> includes <NUM> linearly-arranged individual beamlets, including an individual beamlet <NUM>. A sub-FOV (e.g., the sub-FOV <NUM> in <FIG>) of each individual beamlet can be larger than, matching to, or smaller than its corresponding sub-section (e.g., the sub-section <NUM> in <FIG>). In <FIG>, the arrangement of the linearly-arranged beamlets including <NUM> individual beamlets with square sub-FOVs is chosen for ease of explanation of the implementation without causing any redundancy or ambiguity. Typically, the shape of a sub-FOV (e.g., the sub-FOV <NUM> in <FIG>) corresponding to the sub-section <NUM> can be rectangular or square.

During the step-and-scan mode, in an implementation, the section <NUM> can be scanned by all the beamlets with the main-FOV <NUM> in <FIG>. When the scan of the section <NUM> is finished, the substrate stage can step to a next section (e.g., an adjacent or non-adjacent section) followed by a next scanning. In an implementation, as shown in <FIG>, the path or sequence of the stage stepping can be set in accordance with a predetermined order, such as, for example, a stepping path <NUM> or any other path. As shown in <FIG>, following arrows of the stepping path <NUM>, the imaging scan first starts from a starting point of the stepping path <NUM> and then traversed through each of the <NUM> sections of the care area <NUM> for scanning until an endpoint. Although the substrate stage in <FIG> steps following the stepping path <NUM>, the scanning of each of the <NUM> sections can be performed in any combination of spatial orders or directions (e.g., the "x-direction" or "y-direction" as shown in <FIG>). In some implementations, each section of the care area can be scanned for once during the inspection. In another implementation, each section of the care area can be scanned for multiple times during the inspection.

Typically, the throughput of a multi-beam imaging system can be increased (in some cases, significantly increased) compared with single-beam systems. However, some multi-beam imaging systems using the step-and-scan mode still provide insufficient throughput for in-line application. A limiting factor for the step-and-scan mode is time for stage settling. The substrate stage typically vibrates after stepping. It takes time for the vibration to stop or attenuate to a certain extent before the next scanning can start. The vibration can cause degradation of imaging quality of the scanned section. In some multi-beam imaging systems working in the step-and-scan mode, the time for the substrate stage to settle between stepping ("settling time") can be long. Typically, in those systems, the settling time can be longer than (in some cases, by an order of magnitude) the time for scanning a section ("scanning time") of the care area. For example, for a pixel rate of <NUM>, the scanning time for a <NUM>×<NUM> image is slightly over <NUM> milliseconds (ms), while the stage stepping and settling time can be over <NUM> milliseconds (ms). The long settling time of the substrate stage can become a potential bottleneck to the inspection throughput of those multi-beam imaging systems.

The multi-beam imaging systems as described herein can work in a continuous scan mode (e.g., in addition to a step-and-scan mode) to further increase the inspection throughput. In the continuous scan mode, the substrate stage keeps moving in one direction at a constant speed while the e-beam or beamlets, driven by deflectors, can scan the care area, without interrupting the motion of the stage. For example, the e-beam or beamlets can be driven to perform a line scan on the care area. The trace of the line scan can be referred to as a "scan line" herein. Typically, there are two ways to drive the deflectors for raster scanning: (i) the scan line is perpendicular to the stage motion direction; (ii) the scan line is parallel to the stage motion direction.

In some implementations, the scan line can be perpendicular to the stage motion direction in the continuous mode of the multi-beam imaging system. For example, as shown in <FIG>, an e-beam or beamlets is performing a raster scan for a scanning region <NUM> of a patterned substrate while a substrate stage is moving at a constant speed VS along an x-axis. The e-beam or beamlets can be, for example, the individual beamlet <NUM> in <FIG>. The scanning region <NUM> can be a section (e.g., the section <NUM> in <FIG>) or a sub-section (e.g., the sub-section <NUM> in <FIG>) of a care area of the substrate.

<FIG> shows two line scan paths on the surface of the scanning region <NUM>, which correspond to two line scans performed by the beamlet: a first line scan path <NUM> and a second line scan path <NUM>. The line scan paths have a vertical direction from up to down along a y-axis. After one line scan is finished, the beamlet can moves, as shown by a resetting paths <NUM>, in a raster scanning manner to start a next line scan, the process of which can be repeated for multiple times (e.g., two times). After the multiple line scans having been performed, the beamlet can moves, as shown by a resetting path <NUM>, back to the starting point of the first-time line scan path to start the next set of multiple line scans. The area covered by each set of the multiple line scans can be referred to as a "frame," and the multiple line scans to cover a frame can be referred to as a "frame scan. " The direction of the frame scan can be perpendicular to the line scan.

For example, as shown <FIG>, a frame scan includes two line scarts-that is, the beamlet moves to perform a first frame scan along the line scan path <NUM>, the resetting path <NUM>, and the line scan path <NUM>, and then moves to start a next frame scan along the resetting path <NUM>. Although the frame scan in <FIG> includes only two line scans (or, the frame shown in <FIG> includes only two lines), any number of line scans can be included in a frame scan.

The beamlet can be driven by a deflector set to perform the line scans. The deflector set can include multiple deflectors along any direction. Each deflector can be applied with a scan signal (e.g., a voltage) for driving the beamlet. For example, as shown in <FIG>, line scans corresponding to the line scan paths <NUM> and <NUM> can be controlled or driven using a saw-teeth scan signal Vy. In some implementations, Vy can be a time variant voltage signal. For example, as shown in <FIG>, Vy can be a periodical voltage signal with a period TL. Each period of Vy includes a first portion (or "scanning portion") for controlling the beamlet to perform a line scan in a first direction and a second portion (or "resetting portion") for resetting the beamlet in a second direction (e.g., opposite to the first direction) to perform a next line scan. The "first" and "second" herein are for indicating purpose only, and not referring to the order of the portions of the voltage signal. For example, as shown in <FIG>, a period of Vy includes a first portion <NUM> and a second portion <NUM>. In an implementation, the first portion <NUM> can be used to drive the beamlet to move along the line scan path <NUM>, and the second portion <NUM> can be used to drive the beamlet to move along the resetting path <NUM> to position the beamlet at a starting point of the line scan path <NUM>. The first portion <NUM> has a steeper slope than the second portion <NUM>, which represents the beamlet moves in a slower speed when scanning (e.g., along the line scan path <NUM>) and moves in a faster speed when being reset (e.g., along the resetting path <NUM>) for performing the next line scan. A change of the direction of Vy (e.g., at a wave peak or a wave trough) represents a change of the direction of the driven beamlet. When Vy periodically changes, the beamlet can scan the scanning region <NUM> in a raster scanning manner. The period TL of Vy can then be set as equal to a time period between the starting or ending of two immediately consecutive line scans.

To perform the frame scan, as shown in <FIG>, the beamlet can further be controlled using an additional saw-teeth scan signal Vx. In some implementations, Vx can be a time variant voltage. For example, as shown in <FIG>, Vx can be a periodical voltage with a period TF. Similar to Vy, each period of Vx also includes a scanning portion and a resetting portion. When Vx periodically changes, the beamlet can correspondingly moves back to the starting point of the first line of the frame to start a next frame scan.

In some implementations, TF can be set equal to TL, in which each frame scan includes one line scan. In some implementations, TF can be greater than TL, in which each frame scan can include more than one line scan. When TF is greater than TL, the scanning portions of Vx can have a gentler slope than the scanning portions of Vy. For example, as shown in <FIG>, TF = <NUM>TL, and the slope of the scanning portions of Vx is one half of the slope of the scanning portions of Vy. The period TF of Vx can be set as equal to a time period between the starting or ending of two immediately consecutive frame scans. By controlling the values and the change patterns of Vx and Vy, the frame scan and line scans included therein can be performed in any manner, such as in different sizes of covered area, in any speed, or along any path. For example, in <FIG>, when Vx ≠ <NUM> and Vy = <NUM>, the beamlet is at a point <NUM>.

In the continuous scan mode, Vs can be set based on Vx and Vy. In some implementations, to avoid or reduce image distortion, Vs can be determined based on a physical size corresponding to a portion (e.g., a pixel) of the generated image and the number of lines included in one frame. A pixel of the generated image can correspond to a physical portion (referred to as a "physical pixel") of the frame scan performed on the substrate surface. The size of the physical pixel can be referred to as the "physical pixel size" or simply "pixel size. " The pixel size can depend on the physical size and the pixel dimension of the image. The pixel size can also be different in the horizontal and vertical directions. For example, if the physical size of the image is A × B (e.g., <NUM> × <NUM>) and the pixel dimension of the image is m × n (e.g., <NUM> pixels × <NUM> pixels), then the pixel size in the horizontal direction (Ph) is Ph = A/m (e.g., Ph = <NUM> / <NUM> = <NUM>), and the pixel size in vertical direction (Pv) is Pv = B/n (e.g., Pv = <NUM> / <NUM> = <NUM>). In some implementations, the pixel size of the generated image can be the same in the horizontal and the vertical directions, i.e., Ph = Pv = P.

A line scan can generate a line of pixels in the generated image (e.g., m image pixels), each pixel corresponding to a physical pixel having a pixel size P. In other words, the physical size (or length) covered by the line scan corresponding to the line of pixels is A = m × P. If the time needed for scanning a physical pixel is TP, then TL = m × TP. For a square frame scan, the frame scan can include m lines. In other words, the physical size (or area) covered by the frame scan is A × A, and the pixel dimension of the generated image of the frame scan is m × m. In some implementations, the pixel dimension m × m can be limited by image resolutions on the boundaries of the frame scan. The pixel size P (or the corresponding physical size A) can be limited by physical limitations or conditions of the system (e.g., optical aberrations).

For example, suppose the line scan is in vertical directions, in some implementations, a frame scan can cover a vertical physical line with a horizontal width on the substrate surface, which can generate a vertical line of image pixels of the scan image. In an implementation, a frame scan can cover a horizontal width of a vertical line of physical pixels (referred to as a "physical line"), each physical pixel with a pixel size P. When there are N lines in the frame and the line scan period is TL, Vs is in the horizontal direction and can be determined as Eq. (<NUM>): <MAT>.

In Eq. (<NUM>), the frame scan period TF = TL × N. In the time duration of TF, a frame scan including N line scans can be performed to cover the physical line, the results of which can be used to generate a line of pixels of the generated image. In other words, the physical line can be scanned for N times for generating the line of pixels in the scan image.

In an implementation, a frame can include one line (e.g., each frame scan covers a physical line), or N = <NUM>. In other words, the line scan is equivalent to the frame scan. In this implementation, when Vs = P/TL, the continuous scan can generate a strip-shaped image, and no physical line on the substrate surface is scanned for more than once to generate the strip-shaped image (i.e., the frame scan covers no overlapped physical line between consecutive frames).

In another implementation, a frame can include multiple lines (e.g., each frame scan covers multiple physical lines), or N > <NUM>. In this implementation, when Vs < P/TL, each physical line can be scanned for multiple times in a frame scan. For example, VS can be set as <MAT> (N =<NUM>, <NUM>, <NUM>, <NUM>,. Each physical line can be line scanned for N times in a frame scan, and each frame of a continuous scan (except the first and the last frame of the continuous scan) can be frame scanned for N times. For each line scan of the physical line, a line scan signal can be generated (e.g., a binary value, an integer value, or an RGB value), and the N line scan signals can be summed and averaged to generate an average signal for the physical line. The average signals of the line scans can be used to generate an average scan image.

For another example, when N = <NUM>k (k = <NUM>, <NUM>, <NUM>, <NUM>,. ) and Vs = P/(TL × N), each frame includes N lines (or, each frame scan includes N line scans). Each line can have a horizontal physical size of <MAT>. The N line scans of the frame scan can be labeled as line scan <NUM>, line scan <NUM>,. , line scan N. The frame scans cover a region with a horizontal width of <MAT> overlapped between consecutive frames. In this example, except for the first and last frame of the continuous scan, each of the <NUM>k lines in each frame can be scanned for N times. For example, the line scan <NUM> can be used to generate a first strip-shaped image, the line scan <NUM> can be used to generate a second strip-shaped image, and so on. A total of N strip-shaped images can be obtained. Because each of the N strip-shaped images can be shifted by <MAT> from its adjacent or neighboring strip-shaped image, the N strip-shaped images can cover an overall strip area larger than a strip area of a single strip-shaped image. For example, the overlapped portion of the N strip-shaped images can be used to generate a final image. For another example, image pixels of the N strip-shaped images can be matched with positions (e.g., a physical pixel) on the substrate surface. The matching can be either exact or with an ignorable shifting error. Image data of the image pixels corresponding to the same position of the substrate surface can be summed and averaged to generate average image data for that position. The average image data can be used to generate a final image, by which noise cancelation and signal-to-noise ratio can be improved.

For another example, when N = <NUM> and Vs = <NUM> P/TL, the frame scan can include two line scans: line scan <NUM> and line scan <NUM>. Between two consecutive frame scans, such as frame scan kth and frame scan (k+<NUM>)th, the line scan <NUM> of the frame scan kth and the line scan <NUM> of the frame scan (k+<NUM>)th can scan the same physical line. First pixels of the image generated from the line scan <NUM> and second pixels of the image generated from the line scan <NUM> can correspond to the same or almost the same (i.e., with ignorable shifting errors) physical positions of the physical line. By averaging the pixel data of the first and second pixels, an average image can be produced.

In some implementations, the scan line can be parallel to the stage motion direction in the continuous mode of the multi-beam imaging system. In these implementations, a frame scan including more than one line scans can be used to achieve a 2D scan. For example, as shown in <FIG>, a single electron beamlet is performing a raster scan for a scanning region <NUM> of a patterned substrate while a substrate stage is moving at a constant speed Vs along an x-axis. The e-beam or beamlets can be, for example, the individual beamlet <NUM> in <FIG>. The scanning region <NUM> can be a section or a sub-section of a care area of the substrate, such as, for example, the sub-section <NUM> in <FIG>. In <FIG>, multiple line scans performed by the beamlet are shown, including a line scan <NUM>. The line scans have a direction from left to right along an x-axis, parallel to Vs. In some implementations, for example, the scanning region <NUM> can be covered by a frame scan including <NUM> line scans as shown in <FIG>. Although <NUM> line scans are shown as an example in the frame scan, any number of line scans can be included in the frame scan, such as <NUM>, <NUM>, <NUM>, or any other number.

As shown in <FIG>, the line scans including the line scan <NUM> can be implemented and controlled using a saw-teeth scan signal <MAT> and an additional saw-teeth scan signal <MAT>. In some implementations, <MAT> and <MAT> can be time variant voltages along the x-direction and y-direction, respectively. For example, as shown in <FIG>, <MAT> can be a periodical voltage along the y-direction with a period TL (representing the time needed for a line scan) and <MAT> can be a periodical voltage along the y-direction with a period TF (representing the time needed for a frame scan). In some implementations, TF can be greater than or equal to TL. Similar to Vx and Vy in <FIG>, <MAT> and <MAT> can include scanning portions and resetting portions. In some implementations, the scanning portions of <MAT> can have a gentler slope than the scanning portion of <MAT>. <MAT> can drive the beamlet to move along the y-axis. For example, as shown in <FIG>, when Vx ≠ <NUM> and Vy = <NUM>, the beamlet can be centered at a point <NUM>.

Within each period of <MAT>, there is a first (scanning) portion and a second (resetting) portion. For example, the scanning portion of <MAT> can drive the beamlet to perform the line scan <NUM>, and the resetting portion of <MAT> can drive the beamlet to move along a resetting path <NUM> to position the beamlet to a starting point for a next line scan. When <MAT> periodically changes with time, the beamlet can scan the scanning region <NUM> from left to right. The period TL of <MAT> can then be equal to a total time for performing the line scan (e.g., the line scan <NUM>) plus resetting the beamlet (e.g., along the resetting path <NUM>) for a next line scan. When <MAT> periodically changes with time, the beamlet can traverse the scanning range <NUM> from up to down.

Because the substrate stage is moving, to keep an imaging area as a rectangular shape, a jump <MAT> as shown in <FIG> can be applied to <MAT> for shifting the starting point of the next line scan. In an implementation, half of the line scan capability can be reserved for the purpose of shifting the line scan starting points. For example, for square physical pixels, the line scan can cover m physical pixels, while the frame scan can include <NUM>m line scans. As shown in <FIG>, <MAT> has a period longer than <MAT>, representing a relatively slower scan rate. Each frame scan can shift from consecutive frame scans by a given dimension, such as by one physical pixel.

In an implementation, given the pixel size as P, the time for scanning a physical pixel as TP, a line scan covering m physical pixels, a frame scan including <NUM>m line scans, and the stage moving m physical pixels after the frame scan, to ideally stitch images generated from consecutive frame scans along the stage motion direction, the stage speed can be set as <MAT> in which TL = m × Tp.

For example, as shown in <FIG>, with Vs = <NUM> P/TL in the continuous scan mode, each frame scan can generate a rectangular segmented image, including segmented images <NUM>-<NUM>. A strip-shaped image <NUM> can be generated by stitching multiple consecutive segmented images. In some implementations, the strip-shaped image <NUM> can be a non-strip-shaped image. In some implementations, the consecutive segmented images can have overlapped portions (e.g., overlapping by several physical pixels, which can be determined by Vs). In some implementations, the strip-shaped image <NUM> can be generated by a non-stitching method.

For multi-beam imaging systems using linearly-arranged beamlets, the continuous scan mode for imaging or inspection can be made possible by moving a substrate stage in a constant speed in a direction (e.g., an x-direction) perpendicular to a direction (e.g., a y-direction) of the beamlets linearly arranged along. In some implementations, all beamlets can work in parallel to generate a strip-shaped image. For example, a width of the strip-shaped image can be determined by the quantity of the beamlets and a width of a line scan width associated with each beam. For another example, a length of the strip-shaped image can be determined by the care area or stage control units. With the stage settling time minimized, the inspection throughput can be greatly improved.

In an implementation, multi-beam imaging systems equipped with a linearly-arranged aperture array can work in the continuous scan mode. In another implementation, the multi-beam imaging systems can be selected to work in the continuous scan mode or the step-and-scan mode. For example, the multi-beam imaging systems can switch between the continuous scan mode and the step-and-scan mode. In another implementation, the multi-beam imaging sy stems can switch to use a single beam. For example, the multi-beam imaging systems can switch to use different beam-splitting devices to generate a single beam or multiple beamlets.

<FIG> shows an example care area <NUM> scanned by strip-shape sections ("strips") using multiple beamlets in the continuous scan mode. As shown in <FIG>, the care area <NUM> is divided into <NUM> parallel strips, including a strip <NUM>. The strip <NUM> is similar to the other <NUM> strips in the care area <NUM> and will be described hereinafter as an example for ease of explanation without causing any redundancy or ambiguity. It should be noted that the care area <NUM> can be divided into any number of strips based on the number of beamlets for scanning one strip and scan widths of each beamlet. In an implementation, as shown in <FIG>, the strip <NUM> is scanned by <NUM> linearly-arranged beamlets, the scanning region of each beamlet forming a corresponding sub-strip. The beamlets or sub-strips can be of any number in any configuration, depending on the number and configuration of the apertures in the multi-aperture plate. In an implementation, the strip <NUM> can be scanned by the <NUM> beamlets for generating <NUM> strip-shaped images. The <NUM> strip-shaped images can be stitched to generate a combined strip image for the strip <NUM>.

In an implementation, the combined scanning regions of the beamlets (e.g., the <NUM> beamlets) can be equal to the area of a strip (e.g., the strip <NUM>) for performing a full sampling (i.e., a <NUM>% coverage of the scanning region). In another implementation, the strip can be chosen to be smaller than the combined scanning regions of the beamlets for performing a percentage sampling (i.e., less than <NUM>%, coverage of the scanning region). In another implementation, the strip can be chosen to be larger than the combined scanning regions of the beamlets for performing an oversampling (i.e., more than <NUM>% coverage of the scanning region). The oversampling can be used, for example, when some defects are located at a boundary of the scan image and cannot be detected if full sampling is used due to alignment shift.

<FIG> shows a portion <NUM> of the strip <NUM> in an enlarged view. The portion <NUM> includes <NUM> sub-strips for <NUM> beamlets to scan, including a sub-strip <NUM>. The sub-strip <NUM> is similar to the other <NUM> sub-strips in the portion <NUM> and will be described hereinafter as an example for ease of explanation without causing any redundancy or ambiguity. In an implementation, the sub-strip <NUM> can be scanned by a beamlet <NUM> for generating a strip-shaped image. The strip-shaped image of the sub-strip <NUM> can be stitched with adjacent strip-shaped images generated by adjacent scanning beamlets.

In an implementation, the scanning region of the beamlet <NUM> can be equal to the area of the sub-strip <NUM> for performing the full sampling. In another implementation, the sub-strip <NUM> can be chosen to be smaller than the scanning region of the beamlet <NUM> for performing the percentage sampling. In another implementation, the sub-strip <NUM> can be chosen to be larger than the scanning region of the beamlet <NUM> for performing the oversampling.

In the continuous scan mode of multi-beam imaging system, in an implementation, the substrate stage carries the substrate to move at a constant speed in a direction <NUM>. While the substrate is moving, the beamlets can be controlled to scan the strip <NUM> along with a scanning path <NUM> (e.g., starting from the left end of the strip <NUM> and continued as a head-to-tail fashion). The e-beam scan for each strip (e.g., the strip <NUM>) of the care area <NUM> can generate a combined strip-shaped image. The combined strip-shaped image can be obtained by performing the scan in a way as shown and described in <FIG> in which the line scan is performed perpendicular to the scanning path <NUM> (i.e., in the y-direction shown in <FIG>), or in a way as shown and described in <FIG> in which the line scan is performed parallel to the scanning path <NUM> (i.e., in the x-direction shown in <FIG>). When the beamlets reached an end position of the strip <NUM> (e.g., when the beamlets reach the right end of the strip <NUM>), the substrate stage can move to an end of another strip (e.g., the right or left end of an adjacent or non-adjacent strip) to repeat the scanning procedure. For example, by following the scanning path <NUM>, the care area <NUM> can be scanned strip by strip in a continuous fashion, in which the need for stopping and settling the stage can be reduced.

In this disclosure, a method for imaging a surface of a substrate using a multi-beam imaging system is also provided. <FIG> is an example process <NUM> for multi-beam imaging using a multi-beam imaging system capable of working in a step-and-scan mode and a continuous scan mode. The process <NUM> can be implemented as software and/or hardware modules in the system <NUM> in <FIG> or the system <NUM> in <FIG>. For example, the process <NUM> can be implemented as modules included in the system <NUM> or the system <NUM> by one or more apparatuses. The process <NUM> includes operations <NUM>-<NUM> set forth as follows.

At operation <NUM>, an e-beam is modified using a multipole-field device. For example, the e-beam can be the primary beam <NUM> in <FIG>. and the multipole-field device can be the multipole-field device <NUM> in <FIG>.

In an implementation, the e-beam can be generated from an electron source. The e-beam can have an essentially round beam spot. For example, the electron source can be the electron source <NUM> in <FIG>. In some implementations, an electrode set (e.g., the electrode set <NUM> in <FIG>) can be used to extract, collimate, and/or focus the e-beam. The essentially round beam spot can be similar to a beam spot with the shape <NUM> in <FIG>.

In an implementation, the multipole-field device can extend the essentially round beam spot along a first direction aligned with the linearly-arranged apertures and suppress the essentially round beam spot along a second direction orthogonal to the first direction. The multipole-field device (e.g., the multipole-field device <NUM>) is used to change the shape of the beam spot, from a round primary beam (e.g., the shape <NUM> in <FIG>) to an elliptical shape (e.g., the shape <NUM> in <FIG>). In some implementations, the multiple-field device can further correct aberrations of the e-beam.

In an implementation, the multipole-field device can include one or more stages and each stage generates a multipole electric field and/or a multipole magnetic field. The number of the multipoles of the multipole electric/magnetic field can be <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, or any other number.

At operation <NUM>, beamlets are generated from the modified e-beam using a beam-splitting device. For example, the beam-splitting device can be the beam-splitting device <NUM> in <FIG>. In some implementations, the beam-splitting device can have any number of apertures in any configuration, such as shown in <FIG>. For example, the beam-splitting device can have linearly-arranged apertures (e.g., the multi-aperture plate <NUM> in <FIG>), in which the modified (or, in some cases, unmodified) e-beam can cover at least a portion of the apertures.

In some implementations, structures of the beam-splitting device can include different layers, such as the multi-aperture plates 500A in <FIG> or the multi-aperture plate 500B in <FIG>. The layers of the beam-splitting device can have different functions. For example, the first layer <NUM> can limit the beamlet size. For another example, the layer <NUM> can have different foci and thus focus beamlets generated at different plate positions onto the same plane with reduced aberration. For another example, the third layer <NUM> can converge the beamlets incident to the first layer <NUM>. It should be noted that to implement the same or similar functions, the structures of the beam-splitting device can use any number of layer designs in any configuration, profile, or dimensions.

In some implementations, the beam-splitting device can have predetermined sets (e.g., <NUM>, <NUM>, <NUM>, or any number) of apertures arranged on different regions of the beam-splitting device for different working modes. For example, the beam-splitting device can be a multi-aperture plate. The predetermined sets of apertures can include at least one of a single aperture, a one-dimensional aperture array (i.e., linearly-arranged apertures), or a two-dimensional aperture array. The sets of apertures are switchable, such as by switching the predetermined sets of apertures for use via a moving mechanism. The moving mechanism can be a rotating method for rotating the beam-splitting device. The moving mechanism can also be replacing the beam-splitting device.

For example, the multi-aperture plate <NUM> in <FIG> have sets of apertures (i.e., the multiple apertures <NUM> and the single aperture <NUM>). The multiple apertures <NUM> can be used in a multi-beam imaging mode, and the single aperture <NUM> can be used in a single-beam imaging mode. The multi-aperture plate <NUM> can switch between the sets of apertures (e.g., by rotating the multi-aperture plate <NUM> to expose different sets of apertures under coverage of the e-beam).

At operation <NUM>, the beamlets are driven to scan a region of the substrate surface. Foci of the beamlets can be projected onto the substrate. The beamlets can be driven using a deflector set. Electrons scattered from the region can form scatter beamlets and be deflected and received by a detector for generating signals. The signals can be processed by an image processing system for generating a scan image.

In an implementation, the beamlets can be projected onto the substrate using the projection lens set <NUM> and the objective lens set <NUM> in <FIG>. In some implementations, additional components can be used to reduce aberration of the beamlets and improve imaging conditions. The additional components can include aberration correctors as set forth in the previous description, the optional aperture plate <NUM>, additional electrostatic lenses (e.g., the first single-aperture electrode plate <NUM> and the second single-aperture electrode plate <NUM>), or any other suitable projection device.

In an implementation, the deflector set an array of deflectors (e.g., the deflector set <NUM> in <FIG>). The beamlets can be controlled by the deflector set to perform a raster scan for the substrate. In an implementation, the substrate is placed on a substrate stage controllable to move in a motion mode. The motion mode can include any combination of a step-and-scan mode and a continuous scan mode. In some implementations, when the substrate stage can move in at least two motion modes (e.g., the step-and-scan mode and the continuous scan mode), different motion modes can be selectable and switchable. For example, when the substrate stage is controlled to move in the step-and-scan mode, the beamlets can be driven to scan the region when the substrate stage settles. When the substrate stage is controlled to move in the continuous scan mode, the beamlets can be driven to scan the region when the substrate stage moves at a constant speed.

In an implementation, based on the motion mode of the substrate stage, operational parameters (e.g., the moving speed of the substrate stage) associated with the motion mode can be determined. For example, when the motion mode is selected as the step-and-scan mode or the continuous scanning mode, the operational parameters can be adjusted to optimize the corresponding motion mode.

When the substrate stage is controllable to move in the continuous scan mode, the moving speed is a constant speed of the substrate stage. The moving speed can be determined based at least on a ratio between a dimension of a sub-region (e.g., a physical pixel) of the scanned region (e.g., a pixel size of a physical pixel) on the substrate and a time duration of performing a line scan for the sub-region. An image pixel of the scan image can be generated from signals received based on electrons scattered from the sub-region. In some implementations, the moving speed can be determined further based on the number of line scans included in a frame scan. For example, the moving speed can be determined using Eq. (<NUM>).

In some implementations, in the continuous mode, a frame scan can be performed. When a frame scan includes multiple (e.g., N) line scans, each physical line of the frame can be scanned for N times. For each physical pixel of a physical line, N signals can be generated. The image pixel can be generated from average signal data of the physical pixel, which is generated by averaging the N signals.

In the continuous mode, the substrate stage moves at the constant speed in a stage motion direction. The line scans (e.g., included in a frame scan) can be performed in different directions with respect to the stage motion direction. For example, the line scans can be performed parallel to the stage motion direction. For another example, the line scans can be performed perpendicular to the stage motion direction.

In some implementations, for different motion modes of the substrate stage, different aperture arrays can be used. For example, a one-dimensional aperture array can be used for the continuous scan mode, a two-dimensional aperture array can be used for the step-and-scan mode, and a single aperture can be used for a single-beam scan mode.

In some implementations, the scatter beamlets can be deflected or bent by a deflecting device (e.g., the Wien filter set <NUM> in <FIG>). The deflected scatter beamlets can be off-axis (e.g., the scatter beamlets <NUM> in <FIG>).

In an implementation, the electron signal (e.g., the signal <NUM> in <FIG>) can be generated by the detector (e.g., the detector <NUM> in <FIG>) using the received deflected scatter beamlets. In some implementations, the detector <NUM> can be a detector array including multiple detectors.

At operation <NUM>, a scan image of the region of the substrate surface is determined for inspection based on the signals. For example, the image can be determined using the aforementioned image processing sy stem.

In this disclosure, a method for imaging a substrate using multi-beam systems is provided. <FIG> is an example process <NUM> for imaging a substrate using the multi-beam system. The process <NUM> can be implemented as software and/or hardware modules in the system <NUM> in <FIG> or the system <NUM> in <FIG>. For example, the process <NUM> can be implemented as modules included in the system <NUM> or the system <NUM> by one or more apparatuses. The process <NUM> includes operations <NUM>-<NUM>, which is set forth as follows.

At operation <NUM>, a primary e-beam is generated from an electron source.

At operation <NUM>, the primary e-beam is modified using a multipole-field device for beam shaping and aberration correction.

At operation <NUM>, the e-beam is collimated by an electrostatic lens for illuminating a beam splitting device. In some implementations, the operation <NUM> can be performed as a step prior to the operation <NUM> in the process <NUM>.

The implementations herein can be described in terms of functional block components and various processing steps. The disclosed processes and sequences can be performed alone or in any combination. Functional blocks can be realized by any number of hardware and/or software components that perform the specified functions. For example, the described implementations can employ various integrated circuit components, e.g., memory elements, processing elements, logic elements, look-up tables, and the like, which can carry out a variety of functions under the control of one or more microprocessors or other control devices. Similarly, where the elements of the described implementations are implemented using software programming or software elements the disclosure can be implemented with any programming or scripting languages such as C, C++, Java, assembler, or the like, with the various algorithms being implemented with any combination of data structures, objects, processes, routines, or other programming elements. Functional aspects can be implemented in algorithms that execute on one or more processors. Furthermore, the implementations of the disclosure could employ any number of techniques for electronics configuration, signal processing and/or control, data processing and the like. The steps of all methods described herein can be performable in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.

Aspects or portions of aspects of the above disclosure can take the form of a computer program product accessible from, for example, a computer-usable or computer-readable medium. A computer-usable or computer-readable medium can be any device that can, for example, tangibly contain, store, communicate, or transport a program or data structure for use by or in connection with any processor. The medium can be, for example, an electronic, magnetic, optical, electromagnetic, or a semiconductor device. Other suitable mediums are also available. Such computer-usable or computer-readable media can be referred to as non-transitory memory or media, and can include RAM or other volatile memory or storage devices that can change over time. A memory of a system described herein, unless otherwise specified, does not have to be physically contained by the system, but is one that can be accessed remotely by the system, and does not have to be contiguous with other memory that might be physically contained by the system.

In this disclosure, the terms "signal," "data," and "information" are used interchangeably. The use of "including" or "having" and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items.

The term "example" is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as "example" is not necessarily to be construed as preferred or advantageous over other aspects or designs. Rather, use of the word "example" is intended to present concepts in a concrete fashion.

In addition, the articles "a" and "an" as used in this disclosure and the appended claims should generally be construed to mean "one or more" unless specified otherwise or clear from context to be directed to a singular form. Moreover, use of the term "an aspect" or "one aspect" throughout is not intended to mean the same implementation or aspect unless described as such. Furthermore, recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein.

As used in this disclosure, the term "or" is intended to mean an inclusive "or" rather than an exclusive "or" for two or more elements it conjoins. That is unless specified otherwise, or clear from context, "X includes A or B" is intended to mean any of the natural inclusive permutations. In other words, if X includes A; X includes B; or X includes both A and B, then "X includes A or B" is satisfied under any of the foregoing instances. The term "and/or" used in this disclosure is intended to mean an "and" or an inclusive "or. " That is, unless specified otherwise, or clear from context, "X includes A, B, and/or C" is intended to mean X can include any combinations of A, B, and C. In other words, if X includes A; X includes B; X includes C; X includes both A and B; X includes both B and C; X includes both A and C; or X includes all A, B, and C, then "X includes A and/or B" is satisfied under any of the foregoing instances. Similarly, "X includes at least one of A, B, and C" is intended to be used as an equivalent of "X includes A, B, and/or C.

The aspects shown and described herein are illustrative examples of the disclosure and are not intended to otherwise limit the scope of the disclosure in any way. For the sake of brevity, electronics, control systems, software development and other functional aspects of the systems (and components of the individual operating components of the systems) cannot be described in detail. Furthermore, the connecting lines or connectors shown in the various figures presented are intended to represent exemplary functional relationships and/or physical or logical couplings between the various elements. Many alternative or additional functional relationships, physical connections or logical connections can be present in a practical device.

Claim 1:
A method for imaging a surface of a substrate using a multi-beam imaging system, comprising:
modifying an electron beam using a multipole-field device;
generating beamlets from the electron beam using a beam-splitting device having multiple apertures;
in response to projecting foci of the beamlets onto the surface, driving the beamlets using a deflector set to scan a region of the surface for receiving signals based on electrons scattered from the region; and
determining an image of the region for inspection based on the signals,
wherein the substrate is placed on a substrate stage controllable to move the substrate for scanning in at least one of a step-and-scan mode and a continuous scan mode, and wherein
when the substrate stage is controlled to move in the step-and-scan mode, the beamlets are driven to scan the region when the substrate stage settles, and
when the substrate stage is controlled to move in the continuous scan mode, the beamlets are driven to scan the region when the substrate stage moves at a constant speed in a stage motion direction,
characterized in that modifying the electron beam using the multipole-field device further comprises:
receiving the electron beam from an electron source, wherein a cross section of the electron beam has a round shape during the step-and-scan mode; and
modifying the electron beam using the multipole-field device for beam shaping and beam aberration correction, wherein the cross section of the electron beam is modified from the round shape during the step-and-scan mode into an elliptical shape during the continuous scan mode.