Low noise and high stability current source

Abstract

A charged-particle inspection apparatus includes a charged-particle beam source, a lens assembly that influences a charged particle beam, and a current source circuit that produces a driving signal applied to the lens. The current source circuit includes a voltage controlled current source (VCCS), a DC feedback component, an AC feedback component, and a summing component. The VCCS controls a driving signal to the lens assembly based on correction signals from each of the DC feedback component and the AC feedback component. Together, the correction signals compensate for both noise and drift in the driving signal.

Description

LOW NOISE AND HIGH STABILITY CURRENT SOURCECROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of International application PCT / CN2024 / 141581which was filed on December 23, 2024 and which is incorporated herein in its entirety by reference.TECHNICAL FIELD

[0002] The description herein generally relates to improving image resolution, and more particularly, to using a low noise and high stability current for improving the image resolution.BACKGROUND

[0003] In manufacturing processes of integrated circuits (ICs), unfinished or finished circuit components are inspected to ensure that they are manufactured according to design and are free of defects. Inspection systems utilizing optical microscopes or charged particle (e.g., electron) beam microscopes, such as a scanning electron microscope (SEM) can be employed. As the physical sizes of IC components continue to shrink, accuracy and yield in defect detection becomes more and more important. However, image resolution and throughput of inspection tools struggle to keep pace with ever-decreasing feature size of IC components. The accuracy, resolution and throughput of such inspection tools may be limited by the lack of stable control of the objective lens assembly that meet desired requirements.SUMMARY

[0004] Embodiments of the present disclosure provide a system and method for improving stability and reducing noise of an objective lens driving signal in a charged-particle inspection apparatus.

[0005] In some embodiments, a charged-particle inspection apparatus is disclosed. The apparatus may comprise a charged-particle beam source. The charged-particle beam source may be configured to generate a primary charged-particle beam for sample scanning. The apparatus may further comprise a lens assembly. The lens assembly may be configured to influence the charged particle beam. The apparatus may further comprise a current source circuit. The current source circuit may be configured to produce a driving signal for applying to the lens assembly. The current source circuit may comprise a voltage controlled current source (VCCS). The VCCS may comprise first, second, third, and fourth terminals. The first and second terminals may be configured to control current that flows between the third and fourth terminals. The VCCS may be configured to output a driving signal at the third terminal to a lens assembly. The VCCS may be further configured to control the driving signal proportionally to an input signal received at the first terminal and a first correction signal received at the second terminal. The current source circuit may further comprise a DC feedback component. The DC feedback component may be configured to provide the first correction signal based on a drift-sense resistor. The current source circuit may further comprise an AC feedback component. The AC feedback component may be configured to provide a second correction signal based on a noise-sense resistor and the drift-sense resistor. The current source circuit may further comprise a summing component. The summing component may be configured to combine a setpoint signal and the second correction signal to provide the input signal.

[0006] In some embodiments, a method for controlling a current source circuit for driving a lens assembly is disclosed. The method may comprise receiving an input signal from a current setting source. The method may further comprise acquiring a first correction signal from a DC feedback component. The method may further comprise acquiring a second correction signal from an AC feedback component. The method may further comprise generating, using a summing component, a control signal based on a combination of the input signal and the second correction signal. The method may further comprise generating a driving signal to drive the lens assembly based on the control signal and the first correction signal.

[0007] In some embodiments, a current source circuit is disclosed. The current source circuit may comprise a voltage controlled current source (VCCS). The VCCS may comprise first, second, third, and fourth terminals. The first and second terminals may be configured to control current that flows between the third and fourth terminals. The VCCS may be configured to output a driving signal at the third terminal to a lens assembly. The VCCS may be further configured to control the driving signal proportionally to an input signal received at the first terminal and a first correction signal received at the second terminal. The current source circuit may further comprise a DC feedback component. The DC feedback component may be configured to provide the first correction signal based on a driftsense resistor. The current source circuit may further comprise an AC feedback component. The AC feedback component may be configured to provide a second correction signal based on a noise-sense resistor and the drift-sense resistor. The current source circuit may further comprise a summing component. The summing component may be configured to combine a setpoint signal and the second correction signal to provide the input signal.

[0008] Other advantages of the embodiments of the present disclosure will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of the present invention.BRIEF DESCRIPTION OF FIGURES

[0009] The above and other aspects of the present disclosure will become more apparent from the description of exemplary embodiments, taken in conjunction with the accompanying drawings.

[0010] FIG. 1 is a schematic diagram illustrating an example charged-particle beam inspection system, consistent with embodiments of the present disclosure.

[0011] FIG.2 is a schematic diagram illustrating an example multi-beam tool that can be a part of the example charged-particle beam inspection system of FIG. 1, consistent with embodiments of the present disclosure.

[0012] FIG.3 is a cross-sectional view of an example magnetic objective lens assembly, consistent with embodiments of the present disclosure.

[0013] FIG.4 provides example SEM images, consistent with embodiments of the present disclosure.

[0014] FIG.5 is a schematic diagram of an example single feedback current source.

[0015] FIG.6 is a schematic diagram of an example voltage controlled current source.

[0016] FIG.7A is a schematic diagram of an example double feedback current source design, consistent with embodiments of the present disclosure.

[0017] FIG.7B is a schematic diagram of an example double feedback current source circuit design, consistent with embodiments of the present disclosure.

[0018] FIG.8 is a flow chart illustrating an example method for obtaining a low noise and high stability source current, consistent with embodiments of the present disclosure.DETAILED DESCRIPTION

[0019] Reference will now be made in detail to exemplary embodiments, examples of which are illustrated in the accompanying drawings. The following description refers to the accompanying drawings in which the same numbers in different drawings represent the same or similar elements unless otherwise represented. The implementations set forth in the following description of exemplary embodiments do not represent all implementations. Instead, they are merely examples of apparatuses and methods consistent with aspects related to the disclosed embodiments as recited in the appended claims. For example, although some embodiments are described in the context of utilizing electron beams, the disclosure is not so limited. Other types of charged particle beams (e.g., including protons, ions, muons, or any other particle carrying electric charges) may be similarly applied. Furthermore, other imaging systems may be used, such as optical imaging, photon detection, x-ray detection, ion detection, etc.

[0020] Electronic devices are constructed of circuits formed on a piece of semiconductor material called a substrate. The semiconductor material may include, for example, silicon, gallium arsenide, indium phosphide, or silicon germanium, or the like. Many circuits may be formed together on the same piece of silicon and are called integrated circuits or ICs. The size of these circuits has decreased dramatically so that many more of them can be fit on the substrate. For example, an IC chip in a smartphone can be as small as a thumbnail and yet may include over 2 billion transistors, the size of each transistor being less than 1 / 1000th the size of a human hair.

[0021] Making these ICs with extremely small structures or components is a complex, timeconsuming, and expensive process, often involving hundreds of individual steps. Errors in even onestep have the potential to result in defects in the finished IC, rendering it useless. Thus, one goal of the manufacturing process is to avoid such defects to maximize the number of functional ICs made in the process; that is, to improve the overall yield of the process.

[0022] One component of improving yield is monitoring the chip-making process to ensure that it is producing a sufficient number of functional integrated circuits. One way to monitor the process is to inspect the chip circuit structures at various stages of their formation. Inspection can be carried out using a scanning charged-particle microscope (“SCPM”). For example, an SCPM may be a scanning electron microscope (SEM). A SCPM can be used to image these extremely small structures, in effect, taking a “picture” of the structures of the wafer. The image can be used to determine if the structure was formed properly in the proper location. If the structure is defective, then the process can be adjusted, so the defect is less likely to recur.

[0023] As the physical sizes of IC components continue to shrink, defect detection accuracy becomes more important. Accordingly, precise control of charged particle beams to targeted positions on a sample has become critical to meet the higher demand for accurate inspection and metrology. In a charged-particle beam imaging or inspection system, such as a SCPM, an objective lens may be used to focus the charged-particle beam onto the wafer and to produce an initial magnification of an image. In the objective lens, the charged-particle beam may be focused on the wafer using an electromagnetic lens having electromagnetic coils (e.g., the coils being inside a ferromagnetic base). The electromagnetic coils act like lenses for the electron beam. An electric current through the electromagnetic coils may generate a magnetic field that can bend the paths of the electrons. This bending focuses or defocuses the charged-particle beam. Varying the electric current through the electromagnetic coils may adjust the strength and direction of the magnetic field produced. By varying the current in the coils, the strength of the magnetic field changes. Increasing the current strengthens the magnetic field, allowing the lens to focus the beam more tightly. Conversely, decreasing the current can defocus the beam. The electromagnetic coils may sometimes also be used to steer the beam across the wafer. By controlling the electric current in pairs of electromagnetic coils, the electron beam can be moved horizontally and vertically, allowing it to scan the surface of the wafer. Accordingly, the objective lens is a key component in imaging systems, and precise control of the objective lens can provide images having better resolution.

[0024] The quality of a control signal to the objective lens can be compromised by two primary issues: noise and drift. Noise is characterized by unwanted random variations or fluctuations in current that can disrupt the desired signal. This noise in the control signal can induce instability in the objective lens, consequently resulting in a scanned image of lower resolution. For instance, noise can be introduced into objective lens driver circuits due to variations in the power supply voltage. Such current noise may lead to variations in the strength of the objective lens, thereby causing instability in the electron beam focus. A specific example of this is ripple voltage, a type of variation in powersupply voltage that can cause periodic variation in the output voltage of a power supply due to incomplete suppression of the alternating current (AC) input.

[0025] On the other hand, drift in current refers to the slow, gradual change in current over time, potentially caused by variations in the supply voltage to the control signal. Consequently, a control signal's low noise and high stability current source becomes a crucial factor in its quality. This, in turn, enables the production of higher-resolution images necessary for the fabrication of small IC components.

[0026] In order to eliminate or reduce errors introduced by unwanted noise and large drifts across the control signal for the objective lens, various efforts have been made. For example, a voltage-controlled current source (VCCS) DC feedback loop is a configuration in electronic circuits where the output current is controlled by an input voltage, and feedback is used to maintain stability and accuracy. In a VCCS DC feedback loop, the value of a sense resistor in the feedback loop will influence the current noise and drift in current. However, the current drift and the resistance of the sense resistor are proportional, and the current noise and the resistance of the sense resistor are inversely proportional. Therefore, increasing the resistance of the sense resistor will beneficially lower current noise but detrimentally increase current drift. Decreasing the resistance of the sense resistor will beneficially lower current drift but detrimentally increase current noise.

[0027] The demand for a more accurate inspection systems is ever increasing as the physical sizes of IC components continue to shrink and defect detection accuracy becomes more important. Methods and systems that can more accurately and precisely manipulate the optical lens are thus desired.

[0028] According to some embodiments of the present disclosure, the current source of the control signal for the objective lens may be more precisely controlled using dual feedback to reduce noise while also reducing drift. For instance, the dual feedback loop may include a DC feedback loop and an AC feedback loop to improve both the drift performance and noise performance of the current source, which can lead to improved image resolution and stability.

[0029] Relative dimensions of components in drawings may be exaggerated for clarity. Within the following description of drawings, the same or like reference numbers refer to the same or like components or entities, and only the differences with respect to the individual embodiments are described. Other objects and advantages of the disclosure may be realized by the elements and combinations as set forth in the embodiments discussed herein. However, embodiments of the present disclosure are not necessarily required to achieve such exemplary objects or advantages, and some embodiments may not achieve any of the stated objects or advantages.

[0030] Without limiting the scope of the present disclosure, some embodiments may be described in the context of providing scanning deflection systems and scanning deflection methods in systems utilizing electron beams (“e-beams”). Some scanning deflection systems may use electric fields to influence a charged particle beam. However, the disclosure is not so limited. Other types of charged particle beams may be similarly applied. For example, systems and methods may be applicable withoptics, photons, x-rays, and ions, etc. Deflection may be used to scan a beam over a surface in, for example, cathode ray tubes (CRTs), lithography machines, scanning charged particle microscopes (SCPMs), or other analytical instruments. While some embodiments are discussed with reference to deflection systems that use electric fields to influence a beam, deflection may also be achieved with magnetic fields, for example.

[0031] As used herein, unless specifically stated otherwise, the term “or” encompasses all possible combinations, except where infeasible. For example, if it is stated that a component includes A or B, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or A and B. As a second example, if it is stated that a component includes A, B, or C, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or C, or A and B, or A and C, or B and C, or A and B and C. Expressions such as “at least one of’ do not necessarily modify an entirety of a following list and do not necessarily modify each member of the list, such that “at least one of A, B, and C” should be understood as including only one of A, only one of B, only one of C, or any combination of A, B, and C. The phrase “one of A and B” or “any one of A and B” shall be interpreted in the broadest sense to include one of A, or one of B.

[0032] FIG. 1 illustrates an example electron beam inspection (EBI) system 100 consistent with embodiments of the present disclosure. EBI system 100 may be used for imaging. As shown in FIG.1, EBI system 100 includes a main chamber 101, a load / lock chamber 102, a beam tool 104, and an equipment front end module (EFEM) 106. Beam tool 104 is located within main chamber 101. EFEM 106 includes a first loading port 106a and a second loading port 106b. EFEM 106 may include additional loading port(s). First loading port 106a and second loading port 106b receive wafer front opening unified pods (FOUPs) that contain wafers (e.g., semiconductor wafers or wafers made of other material(s)) or samples to be inspected (wafers and samples may be used interchangeably). A “lot” is a plurality of wafers that may be loaded for processing as a batch.

[0033] One or more robotic arms (not shown) in EFEM 106 may transport the wafers to load / lock chamber 102. Load / lock chamber 102 is connected to a load / lock vacuum pump system (not shown) which removes gas molecules in load / lock chamber 102 to reach a first pressure below the atmospheric pressure. After reaching the first pressure, one or more robotic arms (not shown) may transport the wafer from load / lock chamber 102 to main chamber 101. Main chamber 101 is connected to a main chamber vacuum pump system (not shown) which removes gas molecules in main chamber 101 to reach a second pressure below the first pressure. After reaching the second pressure, the wafer is subject to inspection by beam tool 104. Beam tool 104 may be a single-beam system or a multi-beam system.

[0034] A controller 109 is electronically connected to beam tool 104. Controller 109 may be a computer configured to execute various controls of EBI system 100. While controller 109 is shown in FIG. 1 as being outside of the structure that includes main chamber 101, load / lock chamber 102, and EFEM 106, it is appreciated that controller 109 may be a part of the structure.

[0035] In some embodiments, controller 109 may include one or more processors (not shown). A processor may be a generic or specific electronic device capable of manipulating or processing information. For example, the processor may include any combination of any number of a central processing unit (or “CPU”), a graphics processing unit (or “GPU”), an optical processor, a programmable logic controllers, a microcontroller, a microprocessor, a digital signal processor, an intellectual property (IP) core, a Programmable Logic Array (PLA), a Programmable Array Logic (PAL), a Generic Array Logic (GAL), a Complex Programmable Logic Device (CPLD), a Field-Programmable Gate Array (FPGA), a System On Chip (SoC), an Application-Specific Integrated Circuit (ASIC), and any type circuit capable of data processing. The processor may also be a virtual processor that includes one or more processors distributed across multiple machines or devices coupled via a network.

[0036] In some embodiments, controller 109 may further include one or more memories (not shown). A memory may be a generic or specific electronic device capable of storing codes and data accessible by the processor (e.g., via a bus). For example, the memory may include any combination of any number of a random-access memory (RAM), a read-only memory (ROM), an optical disc, a magnetic disk, 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 type of storage device. The codes and data may include an operating system (OS) and one or more application programs (or “apps”) for specific tasks. The memory may also be a virtual memory that includes one or more memories distributed across multiple machines or devices coupled via a network.

[0037] FIG.2 illustrates a schematic diagram of an example multi -beam tool 104 (also referred to herein as apparatus 104) and an image processing system 290 that may be configured for use in EBI system 100 (FIG. 1), consistent with embodiments of the present disclosure.

[0038] Beam tool 104 comprises a charged-particle source 202, a gun aperture 204, a condenser lens 206, a primary charged-particle beam 210 emitted from charged-particle source 202, a source conversion unit 212, a plurality of beamlets 214, 216, and 218 of primary charged-particle beam 210, a primary projection optical system 220, a motorized wafer stage 280, a wafer holder 282, multiple secondary charged-particle beams 236, 238, and 240, a secondary optical system 242, and a charged-particle detection device 244. Primary projection optical system 220 can comprise a beam separator 222, a deflection scanning unit 226, and an objective lens 228. Charged-particle detection device 244 can comprise detection sub-regions 246, 248, and 250.

[0039] Charged-particle source 202, gun aperture 204, condenser lens 206, source conversion unit 212, beam separator 222, deflection scanning unit 226, and objective lens 228 can be aligned with a primary optical axis 260 of apparatus 104. Secondary optical system 242 and charged-particle detection device 244 can be aligned with a secondary optical axis 252 of apparatus 104.

[0040] Charged-particle source 202 can emit one or more charged particles, such as electrons, protons, ions, muons, or any other particle carrying electric charges. In some embodiments, charged-particle source 202 may be an electron source. For example, charged-particle source 202 may include a cathode, an extractor, or an anode, wherein primary electrons can be emitted from the cathode and extracted or accelerated to form primary charged-particle beam 210 (in this case, a primary electron beam) with a crossover (virtual or real) 208. For ease of explanation without causing ambiguity, electrons are used as examples in some of the descriptions herein. However, it should be noted that any charged particle may be used in any embodiment of this disclosure, not limited to electrons. Primary charged-particle beam 210 can be visualized as being emitted from crossover 208. Gun aperture 204 can block off peripheral charged particles of primary charged-particle beam 210 to reduce Coulomb effect. The Coulomb effect may cause an increase in size of probe spots.

[0041] Source conversion unit 212 can comprise an array of image-forming elements and an array of beam-limit apertures. The array of image-forming elements can comprise an array of micro-deflectors or micro-lenses. The array of image-forming elements can form a plurality of parallel images (virtual or real) of crossover 208 with a plurality of beamlets 214, 216, and 218 of primary charged-particle beam 210. The array of beam-limit apertures can limit the plurality of beamlets 214, 216, and 218. While three beamlets 214, 216, and 218 are shown in FIG. 2, embodiments of the present disclosure are not so limited. For example, in some embodiments, the apparatus 104 may be configured to generate a first number of beamlets. In some embodiments, the first number of beamlets may be in a range from 1 to 1000. In some embodiments, the first number of beamlets may be in a range from 200-500. In an exemplary embodiment, an apparatus 104 may generate 400 beamlets.

[0042] Condenser lens 206 can focus primary charged-particle beam 210. The electric currents of beamlets 214, 216, and 218 downstream of source conversion unit 212 can be varied by adjusting the focusing power of condenser lens 206 or by changing the radial sizes of the corresponding beam-limit apertures within the array of beam-limit apertures. Objective lens 228 can focus beamlets 214, 216, and 218 onto a wafer 230 for imaging, and can form a plurality of probe spots 270, 272, and 274 on a surface of wafer 230.

[0043] Beam separator 222 can be a beam separator of Wien filter type generating an electrostatic dipole field and a magnetic dipole field. In some embodiments, if they are applied, the force exerted by the electrostatic dipole field on a charged particle (e.g., an electron) of beamlets 214, 216, and 218 can be substantially equal in magnitude and opposite in a direction to the force exerted on the charged particle by magnetic dipole field. Beamlets 214, 216, and 218 can, therefore, pass straight through beam separator 222 with zero deflection angle. However, the total dispersion of beamlets 214, 216, and 218 generated by beam separator 222 can also be non-zero. Beam separator 222 can separate secondary charged-particle beams 236, 238, and 240 from beamlets 214, 216, and 218 and direct secondary charged-particle beams 236, 238, and 240 towards secondary optical system 242.

[0044] Deflection scanning unit 226 can deflect beamlets 214, 216, and 218 to scan probe spots 270, 272, and 274 over a surface area of wafer 230. In response to the incidence of beamlets 214, 216, and 218 at probe spots 270, 272, and 274, secondary charged-particle beams 236, 238, and 240 may beemitted from wafer 230. Secondary charged-particle beams 236, 238, and 240 may comprise charged particles (e.g., electrons) with a distribution of energies. For example, secondary charged-particle beams 236, 238, and 240 may be secondary electron beams including secondary electrons (energies < 50 eV) and backscattered electrons (energies between 50 eV and landing energies of beamlets 214, 216, and 218). Secondary optical system 242 can focus secondary charged-particle beams 236, 238, and 240 onto detection sub-regions 246, 248, and 250 of charged-particle detection device 244. Detection sub-regions 246, 248, and 250 may be configured to detect corresponding secondary charged-particle beams 236, 238, and 240 and generate corresponding signals (e.g., voltage, current, or the like) used to reconstruct an SCPM image of structures on or underneath the surface area of wafer 230.

[0045] The generated signals may represent intensities of secondary charged-particle beams 236, 238, and 240 and may be provided to image processing system 290 that is in communication with charged-particle detection device 244, primary projection optical system 220, and motorized wafer stage 280. The movement speed of motorized wafer stage 280 may be synchronized and coordinated with the beam deflections controlled by deflection scanning unit 226, such that the movement of the scan probe spots (e.g., scan probe spots 270, 272, and 274) may orderly cover regions of interests on the wafer 230. The parameters of such synchronization and coordination may be adjusted to adapt to different materials of wafer 230. For example, different materials of wafer 230 may have different resistance-capacitance characteristics that may cause different signal sensitivities to the movement of the scan probe spots.

[0046] The intensity of secondary charged-particle beams 236, 238, and 240 may vary according to the external or internal structure of wafer 230, and thus may indicate whether wafer 230 includes defects. Moreover, as discussed above, beamlets 214, 216, and 218 may be projected onto different locations of the top surface of wafer 230, or different sides of local structures of wafer 230, to generate secondary charged-particle beams 236, 238, and 240 that may have different intensities. Therefore, by mapping the intensity of secondary charged-particle beams 236, 238, and 240 with the areas of wafer 230, image processing system 290 may reconstruct an image that reflects the characteristics of internal or external structures of wafer 230.

[0047] In some embodiments, image processing system 290 may include an image acquirer 292, a storage 294, and a controller 296. Image acquirer 292 may comprise one or more processors. For example, image acquirer 292 may comprise a computer, server, mainframe host, terminals, personal computer, any kind of mobile computing devices, or the like, or a combination thereof. Image acquirer 292 may be communicatively coupled to charged-particle detection device 244 of beam tool 104 through a medium such as an electric conductor, optical fiber cable, portable storage media, IR, Bluetooth, internet, wireless network, wireless radio, or a combination thereof. In some embodiments, image acquirer 292 may receive a signal from charged-particle detection device 244 and may construct an image. Image acquirer 292 may thus acquire SCPM images of wafer 230. Image acquirer292 may also perform various post-processing functions, such as generating contours, superimposing indicators on an acquired image, or the like. Image acquirer 292 may be configured to perform adjustments of brightness and contrast of acquired images. In some embodiments, storage 294 may be a storage medium such as a hard disk, flash drive, cloud storage, random access memory (RAM), other types of computer-readable memory, or the like. Storage 294 may be coupled with image acquirer 292 and may be used for saving scanned raw image data as original images, and postprocessed images. Image acquirer 292 and storage 294 may be connected to controller 296. In some embodiments, image acquirer 292, storage 294, and controller 296 may be integrated together as one control unit.

[0048] In some embodiments, image acquirer 292 may acquire one or more SCPM images of a wafer based on an imaging signal received from charged-particle detection device 244. An imaging signal may correspond to a scanning operation for conducting charged particle imaging. An acquired image may be a single image comprising a plurality of imaging areas. The single image may be stored in storage 294. The single image may be an original image that may be divided into a plurality of regions. Each of the regions may comprise one imaging area containing a feature of wafer 230. The acquired images may comprise multiple images of a single imaging area of wafer 230 sampled multiple times over a time sequence. The multiple images may be stored in storage 294. In some embodiments, image processing system 290 may be configured to perform image processing steps with the multiple images of the same location of wafer 230.

[0049] In some embodiments, image processing system 290 may include measurement circuits (e.g., analog-to-digital converters) to obtain a distribution of the detected secondary charged particles (e.g., secondary electrons). The charged-particle distribution data collected during a detection time window, in combination with corresponding scan path data of beamlets 214, 216, and 218 incident on the wafer surface, can be used to reconstruct images of the wafer structures under inspection. The reconstructed images can be used to reveal various features of the internal or external structures of wafer 230, and thereby can be used to reveal any defects that may exist in the wafer.

[0050] In some embodiments, the charged particles may be electrons. When electrons of primary charged-particle beam 210 are projected onto a surface of wafer 230 (e.g., probe spots 270, 272, and 274), the electrons of primary charged-particle beam 210 may penetrate the surface of wafer 230 for a certain depth, interacting with particles of wafer 230. Some electrons of primary charged-particle beam 210 may elastically interact with (e.g., in the form of elastic scattering or collision) the materials of wafer 230 and may be reflected or recoiled out of the surface of wafer 230. An elastic interaction conserves the total kinetic energies of the bodies (e.g., electrons of primary charged-particle beam 210) of the interaction, in which the kinetic energy of the interacting bodies does not convert to other forms of energy (e.g., heat, electromagnetic energy, or the like). Such reflected electrons generated from elastic interaction may be referred to as backscattered electrons (BSEs). Some electrons of primary charged-particle beam 210 may inelastically interact with (e.g., in the form of inelasticscattering or collision) the materials of wafer 230. An inelastic interaction does not conserve the total kinetic energies of the bodies of the interaction, in which some or all of the kinetic energy of the interacting bodies convert to other forms of energy. For example, through the inelastic interaction, the kinetic energy of some electrons of primary charged-particle beam 210 may cause electron excitation and transition of atoms of the materials. Such inelastic interaction may also generate electrons exiting the surface of wafer 230, which may be referred to as secondary electrons (SEs). Yield or emission rates of BSEs and SEs depend on, e.g., the material under inspection and the landing energy of the electrons of primary charged-particle beam 210 landing on the surface of the material, among others. The energy of the electrons of primary charged-particle beam 210 may be imparted in part by its acceleration voltage (e.g., the acceleration voltage between the anode and cathode of charged-particle source 202 in FIG.2). The quantity of BSEs and SEs may be more or fewer (or even the same) than the injected electrons of primary charged-particle beam 210.

[0051] According to some embodiments, the images generated by SEM may be used for defect inspection. For example, a generated image capturing a test device region of a wafer may be compared with a reference image capturing the same test device region. The reference image may be predetermined (e.g., by simulation) and include no known defect. If a difference between the generated image and the reference image exceeds a tolerance level, a potential defect may be identified. For another example, the SEM may scan multiple regions of the wafer, each region including a test device region designed as the same, and generate multiple images capturing those test device regions as manufactured. The multiple images may be compared with each other. If a difference between the multiple images exceeds a tolerance level, a potential defect may be identified.

[0052] While FIG. 2 illustrated a multi-beam inspection tool of EBI system 100, it is appreciated that the embodiments of the present disclosure can include EBI system 100 having a single-beam inspection tool that uses an objective lens consistent with the examples described herein.

[0053] FIG.3 is a cross-sectional view of an example magnetic objective lens assembly of an inspection apparatus, consistent with embodiments of the present disclosure. A magnetic field objective lens assembly 300 of FIG.3 may be a part of EBI system 100 of FIG. 1 or electron beam tool 104 of FIG.2. In some embodiments, magnetic field objective lens assembly 300 can generate a system magnetic field of EBI system 100 of FIG. 1 or electron beam tool 104 during operation. As shown in FIG. 3, magnetic field objective lens assembly 300 can include an objective lens 328 (e.g., objective lens 228 of FIG.2) and objective lens control source 380. Objective lens assembly 328, in some embodiments, may be a modified SORIL lens, which includes a pole piece 328a, an exciting coil 328b, a control electrode 328c, and a deflector unit 326. Deflector unit 326 may include deflectors 326a and 326b, which may be disposed within the magnetic field of magnetic field objective lens assembly 300 or may be implemented in a deflection scanning unit (e.g., deflection scanning unit 226 of FIG.2). Deflectors 326a and 326b may be configured to dynamically deflect electron beam 216 to scan a desired area on the surface of a wafer 230.

[0054] In a detection or imaging process, electron beam 216 may be focused into probe spot 272 by the magnetic field objective lens assembly 300 and impinge onto the surface of wafer 230. Objective lens control source 380 may be a voltage controlled current source (VCCS). Objective lens control source 380 may pass driving current 390 through exciting coil 328b.

[0055] In some embodiments, exciting coil 328b can be an electromagnetic coil configured to adjust the focus of a single or multiple charged-particle beams. Exciting coil 328b may comprise an electrical conductor, such as a wire in the shape of a coil, spiral, helix, etc. Driving current 390 may be passed through exciting coil 328b to produce a circular magnetic field around the conductor. In a coil configuration, the electrical conductor may be wound multiple times to increase the magnetic field density. Exciting coil 328b may comprise an electrical conductor including, but not limited to, copper, aluminum, silver, etc. In some embodiments, the electrical conductor wire, such as copper wire, may be encapsulated with an insulating material.

[0056] The magnetic field may focus and direct electron beam 216 onto a desired area on the surface of a wafer 230. Objective lens control source 380 may be a variable current source, where adjusting driving current 390 through exciting coil 328b may vary the strength of the magnetic field generated. In some embodiments, driving current 390 may be used to adjust the focus of beam 216 on wafer 230. For example, a higher driving current 390 may increase the magnetic field strength, which may produce a shorter focal length and a tighter focus probe spot 272. For example, a lower driving current 390 current may reduce the magnetic field strength, which may increase the focal length, which may allow for a wider probe spot 272 or changing the depth of field. In some embodiments, adjusting driving current 390 through exciting coil 328b may be configured to steer the electron beam. Exciting coil 328b may be comprised of a plurality of exciting coils 328b- 1 through 328b-n, where n can be any integer. For example, by applying differential currents to a plurality of exciting coils 328b- 1 through 328b-n, beam 216 may be configured to dynamically direct electron beam 216 to scan a desired area on the surface of wafer 230. The dynamic direction of an electron beam 216 may cause a desired area or a desired region of interest to be scanned iteratively, for example in a raster scan pattern.

[0057] The plurality of exciting coils 328b- 1 through 328b-n may be controlled together to steer the direction of the beam 216 such that steering the direction of beam 216 in a two-dimensional plane is possible. Undesirable beam drift may refer to the unintended movement or misalignment of electron beam 216 as it passes through objective lens 328. Beam drift may significantly affect imaging quality, resolution, and accuracy in SEM operations. Beam drift may occur when electron beam 216 does not remain focused on probe spot 272. This may cause image distortion, blurriness, or loss of detail in the scanned image. For example, in FIG.4, beam image comparison 400 shows a clear sample image 410 compared to a distorted sample image 420. In clear sample image 410, clear scan lines 412 have been captured by the SEM. In distorted sample image 420, blurred scan line 422 represents an unfocusedimage captured by an SEM in which the objective lens, for example, electron beam 216 was less focused, leading to image 420 having a lower resolution.

[0058] Reference is now made to FIG. 5, which illustrates a configuration of a single feedback current source circuit 500 associated with an objective lens assembly 528. As illustrated herein, single feedback current source circuit 500 may comprise current setting source 510, DC feedback component 520, voltage controlled current source (VCCS) 580, and a shunt resistor 526.

[0059] Current setting source 510 may be a stable and adjustable current source, which may include a linear current source. Current setting source 510 may be driven by a system controller, e.g., controller 109 of FIG. 1. Current setting source 510 may control and output a setpoint signal 514, where the setpoint signal 514 may provide a signal that represents the desired current to objective lens assembly 528. Current setting source 510 may send setpoint signal 514 to VCCS 580.

[0060] VCCS 580 may be configured to receive setpoint signal 514 from current setting source 510 and DC feedback signal 522 of DC feedback component 520. VCCS 580 may control a driving signal 590 based on setpoint signal 514 and DC feedback signal 522. Driving signal 590 may be received by objective lens assembly 528. Driving signal 590 may comprise a driving current sent to objective lens assembly 528, where the driving current may drive one or more exciting coils, as described above.

[0061] VCCS 580 may comprise a first terminal 580a, a second terminal 580b, and a third terminal 580c. The first terminal 580a may be configured to receive setpoint signal 514. Setpoint signal 514 corresponds with a voltage applied to first terminal 580a. The second terminal 580b may be configured to receive DC feedback signal 522. The third terminal 580c may be configured to output driving signal 590.

[0062] VCCS 580 may regulate driving signal 590 based on setpoint signal 514 from current setting source 510 and DC feedback signal 522 from DC feedback component 520. VCCS 580 may regulate driving signal 590 based on the voltage across the first terminal 580a and the second terminal 580b. VCCS 580 may regulate driving signal 590 as a driving current that is proportional to the voltage across the first terminal 580a and the second terminal 580b.

[0063] VCCS 580 may comprise a fourth terminal 580d, which may output source signal 518. Source signal 518 is provided across shunt resistor 526 to a reference 582, which may be a ground. The current of source signal 518 flowing through shunt resistor 526 is proportional to voltage applied to second terminal 580b (further explained below). Accordingly, VCCS 580 may be configured to maintain the voltage across the first terminal 580a and second terminal 580b to be equal to zero by adjusting source signal 518.

[0064] DC feedback component 520 may be a wire that receives DC feedback signal 522, where DC feedback signal 522 may be or correspond to output source signal 518. DC feedback signal 522 may be derived from the voltage drop from the fourth terminal 580d across shunt resistor 526 to reference 582. DC feedback signal 522 may correspond to the voltage applied at second terminal 580b. The voltage applied at the second terminal 580b may be equal to the voltage applied at the fourth terminal580d. The voltage applied at the second terminal 580b and fourth terminal 580d may be proportional to a noise component in source signal 518. Thus, DC feedback signal 522 may represent the noise component in source signal 518. Because VCCS 580 is configured to regulate driving signal 590 based on DC feedback signal 522 and by regulating source signal 518, any noise in source signal 518 may result in noise in driving signal 590, which may result in instability of the objective lens 528, as described above. Therefore, the voltage across the first terminal 580a and the second terminal 580b may be proportional to a noise component in driving signal 590. DC feedback signal 522 may also represent the drift in source signal 518. Therefore, any drift in source signal 518 may result in drive in driving signal 590, which may result in defocusing of the objective lens 528, as described above.

[0065] An exemplary operation of single feedback current source circuit 500 is now described. For example, 1 volt (IV) is applied to first terminal 580a. Because VCCS 580 may be configured to maintain the voltage across the first terminal 580a and the second terminal 580b to be equal to zero, VCCS 580 will produce a voltage applied at the second terminal 580b equal to -1 volt (-1V). This means that the voltage applied at the fourth terminal 580d will be equal to -1 volt (-1V). VCCS 580 can regulate source signal 518 in order to produce the necessary applied voltage. Shunt resistor 526 can produce a voltage drop in accordance with Ohms law, where Ohms law is understood to be voltage = current * resistance (V = I*R). Therefore, if shunt resistor 526 has a resistance of 1 Ohm (Q), IV drop voltage will be produced if source signal 518 has a current of 1 amp flowing through shunt resistor 526. Because VCCS 580 may regulate driving signal 590 proportionally to voltage across the first terminal 580a and the second terminal 580b, VCCS 580 may regulate driving signal 590 based on DC feedback signal 522.

[0066] In exemplary operation, Ohm’s law gives that V = I*R, where V is voltage, I is current, and R is resistance. Therefore, in exemplary operation, for a constant voltage (Vset) applied to the first terminal 580a by setpoint signal 514, when shunt resistor 526 has a high resistance value, i.e., Rs is large, Ioutis small, relatively. In this way, a higher resistance of shunt resistor 526 (Rs) may result in a more stable driving signal 590. At the same time, however, a higher resistance shunt resistor 526 may result in a greater drift in driving signal 590. Drift in driving signal 590 may be related to power dissipation through shunt resistor 526, which is proportional to the resistance of shunt resistor 526. Power dissipation through shunt resistor 526 is the conversion of electrical energy into heat as source signal 518 passes through shunt resistor 526. Increasing the resistance of shunt resistor 526 may make it harder for electricity to move through, thereby raising the power dissipation in the signal DC feedback signal 522 and resulting in increased drift in driving signal 590.

[0067] As a second example, for a constant voltage (Vset) applied to the first terminal 580a by setpoint signal 514, when shunt resistor 526 has a low resistance value, i.e., Rs is small, Ioutis large, relatively. A lower resistance shunt resistor 526 may reduce the power dissipation in the signal DC feedback signal 522 and result in a smaller drift in driving signal 590. At the same time, however, alower resistance shunt resistor 526 may not adequately compensate for current noise in DC feedback signal 522, resulting in a less stable driving signal 590.

[0068] Thus, it may be said that the undesirable noise in driving signal 590 may be inversely related to the resistance of shunt resistor 526. That is to say, a higher resistance of shunt resistor 526 may result in a more stable driving signal 590, i.e., less noise in driving signal 590, and a lower resistance of shunt resistor 526 may result in less stable driving signal 590. At the same time, drift in driving signal 590 is proportionally related to heat dissipation of shunt resistor 526, which is proportionally related to the resistance value of shunt resistor 526. Therefore, drift in driving signal 590 is proportionally related to the resistance value of shunt resistor 526. That is to say, a higher resistance of shunt resistor 526 may result in higher drift in driving signal 590, and a lower resistance of shunt resistor 526 may result in a lower drift in driving signal 590. Therefore, for example, a higher resistance of shunt resistor 526 may be selected to increase stability in driving signal 590 but result in undesirable drift in driving signal 590. As a second example, a lower resistance of shunt resistor 526 may reduce drift in driving signal 590 but result in instability in driving signal 590. This may be known as the resistor value tradeoff.

[0069] Reference is now made to FIG. 6, which illustrates an exemplary configuration voltage control system 600 having a VCCS 680. VCCS 680 may comprise a first terminal 680a configured to receive a driving signal 614. VCCS 680 may comprise a second terminal 680b configured to receive a feedback signal 622.

[0070] First terminal 680a may be connected to a non-inverting input 682a of op amp 682. Second terminal 680b may be connected to an inverting input 682b of op amp 682. Non-inverting input 682a is typically associated with the positive terminal, and the inverting input 682b is typically associated with the negative terminal. Op amp 682 may comprise an output 682c, which may be configured to output a signal 616.

[0071] In exemplary operation, op amp 682 may amplify the voltage difference between noninverting input 682a and inverting input 682b. In a negative feedback configuration, a portion of the voltage of output 682c is fed back to the inverting input 682b. Because the feedback to the inverting input 682b may oppose the non-inverting input 682a, any increase in voltage at output 682c may result in a corresponding decrease in the voltage difference at non-inverting input 682a and inverting input 682b, thus stabilizing the op amp output 682c.

[0072] Op amp output 682c may be coupled to a metal-oxide-semiconductor field-effect transistor (MOSFET) 684 to provide signal 616. MOSFET 684 may be a transistor that uses an N-type semiconductor as the channel for conduction. MOSFET 684 may include a gate 684a, a source 684b, and a drain 684c. Gate 684a may be coupled to op amp output 682c to receive signal 616. Gate 684a may be a control terminal configured to modulate the flow of current between source 684b and drain 684c. Gate 684a may comprise a conductive material (e.g., metal or polysilicon). Gate 684a may be separated from a channel formed between gate 684a and source 684b and drain 684c by a thin layer ofinsulating material (not pictured), for example silicon dioxide (SiO ). The insulating layer allows gate 684a to control a current through the channel without direct electrical contact with either of the source 684b or drain 684c. A threshold voltage may be a minimum gate-to-source voltage required to create a conductive channel. MOSFET 684 may operate in an enhancement mode or a depletion mode.

[0073] In enhancement mode, when a positive voltage may be applied to gate 684a relative to source 684b (above the threshold voltage), an N-type channel forms, allowing current to flow from drain 684c and source 684b. In depletion mode, a negative voltage may be applied to gate 684a. This negative voltage may deplete the channel of carriers, which may turn the MOSFET off. Drain 684c may be connected to a third terminal 680c of VCCS 680, where the third terminal 680c may output a driving signal 690. Source 684b may be connected to a fourth terminal 680d of VCCS 680, where the fourth terminal 680d may output a source signal 618.

[0074] In exemplary operation, op amp 682 may maintain the desired relationship between the voltage applied to gate 684a via signal 616, where signal 616 is based on the voltage between the first terminal 680a and the second terminal 680b, and the current flowing through MOSFET 684 between source 684b and drain 684c. Accordingly, MOSFET 684 may act as a current source, where the current output by drain 684c serves as driving signal 690 and can be regulated by the voltage applied to gate 684a by signal 616. Thus, VCCS 680 may regulate driving signal 690, which is output from third terminal 680c, based on the voltage between first terminal 680a and second terminal 680b

[0075] Reference is now made to FIG. 7A, which is a schematic diagram of an exemplary configuration of double feedback current source circuit 700, consistent with embodiments of the present disclosure. Double feedback current source circuit 700 may be associated with an objective lens assembly (e.g., objective lens assembly 328 of FIG. 3). Double feedback current source circuit 700 may comprise two feedback loops to correct for both current noise and current drift, consistent with embodiments of the present disclosure. As illustrated herein, double feedback current source circuit 700 may comprise current setting source 710, a DC feedback component 720, an AC feedback component 730, a noise-sense resistor 736, a drift-sense resistor 726, and a VCCS 780 (e.g., VCCS 680 of FIG.6).

[0076] According to some embodiments of the present disclosure, current setting source 710 may be a stable and adjustable current source, which may include a linear current source. Current setting source 710 may be driven by a system controller, e.g., controller 109 of FIG. 1. Current setting source 710 may control and output a setpoint signal 714, where the setpoint signal 714 represent the desired current to an objective lens assembly 728. Current setting source 710 may output setpoint signal 714 to VCCS 780.

[0077] According to some embodiments of the present disclosure, VCCS 780 may be a current source, as described above, which is configured to regulate a driving signal 790 based on an augmented setpoint signal 714-1 and a DC feedback signal 722. VCCS 780 may comprise a first terminal 780a configured to receive augmented setpoint signal 714-1 and a second terminal 780bconfigured to receive DC feedback signal 722. VCCS 780 may comprise a third terminal 780c configured to output driving signal 790.

[0078] In some embodiments, VCCS 780 may output driving signal 790 to an objective lens 728 (e.g., objective lens 328 of FIG. 3). Driving signal 790 may comprise a driving current. It is to be appreciated that, although not illustrated, double feedback current source circuit 700 may include a plurality of driving signals 790 to a plurality of components of objective lens 728, e.g., with reference to FIG. 3, to pole piece 328a, exciting coil 328b or the plurality of exciting coils 328b- 1 through 328b-n, control electrode 328c, and / or deflector unit 326. It is to be appreciated that, although not illustrated, double feedback current source circuit 700 may include other components and circuitry such as variable gain amplifiers, timing circuits, capacitors, resistors, etc. as appropriately needed to provide sufficient current to the objective lens (e.g., to exciting coils 328b of FIG.3).

[0079] VCCS 780 may be configured to regulate driving signal 790 based on augmented setpoint signal 714-1 and DC feedback signal 722. More particularly, VCCS 780 may be configured to regulate a current output from third terminal 780c as driving signal 790 based on the voltage across first terminal 780a that receives augmented setpoint signal 714-1 and second terminal 780b that receives DC feedback signal 522. VCCS 780 may regulate driving signal 790 as a driving current that is proportional to the voltage across first terminal 780a and second terminal 780b.

[0080] In some embodiments, VCCS 780 may comprise components of VCCS 680, such as, for example, components including op amp 682 and N-channel MOSFET 684. A gain of VCCS 780 may be defined in terms of amplitude of current output from the third terminal 780c as driving signal 790 for a given voltage across first terminal 780a by augmented setpoint signal 714-1 and second terminal 780b by DC feedback signal 722. As an example, the gain of VCCS 780 may be about 100 decibels.

[0081] According to some embodiments of the present disclosure, VCCS 780 may be configured to output a source signal 718 from a fourth terminal 780d. VCCS 780 may also be configured to maintain the voltage across the first terminal 780a and the second terminal 780b to be equal to zero by adjusting source signal 718.

[0082] In some embodiments, source signal 718 may be configured to flow from the fourth terminal 780d through noise-sense resistor 736 and through drift-sense resistor 726 to reference 782 (e.g., ground). Noise-sense resistor 736 may have a greater resistance than drift-sense resistor 726.

[0083] In some embodiments, DC feedback component 720 may be configured to regulate driving signal 790 with lower drift than source signal 718. In some embodiments, AC feedback component 730 may be configured to regulate driving signal 790 with low noise.

[0084] Regarding DC feedback component 720, in some embodiments, DC feedback signal 722 may be a signal derived from source signal 718. More particularly, DC feedback signal 722 may be derived from the voltage drop across drift-sense resistor 726 to reference 782. DC feedback signal 722 may be derived from the voltage applied at second terminal 780b. Because drift-sense resistor 726 may have a relatively low resistance value compared to noise-sense resistor 736, DC feedback signal722 may represent the drift component in source signal 718. The relationship between the source signal 718 (Uris), the current of driving signal 790 (I790) and the resistance of the drift-sense resistor 726 (R726 may be represented as follows:[ / 7i8 oc I790* R726

[0085] The relationship between the DC feedback signal 722 (U722), the current of driving signal 790 (I790), the resistance of the drift-sense resistor 726 (R726 and the resistance of the noise-sense resistor 736 (R736 may be represented as follows:U722K^790 * (^726 + ^736)

[0086] Because the resistance of the noise-sense resistor 736 is higher, noise-sense resistor 736 will cause more drift through heat dissipation. Thus, the DC feedback signal 722 (U722) will have less drift than source signal 718 ( zis).

[0087] Because VCCS is configured to regulate driving signal 790 based on DC feedback signal 722 and by regulating source signal 718, any drift in source signal 718 may result in drift in driving signal 790, which may result in defocusing of the objective lens 728, as described above. However, because VCCS 780 is configured to control driving signal 790 based on DC feedback signal 722, VCCS 780 may compensate for drift in driving signal 790 based on DC feedback signal 722.

[0088] Following is an example of VCCS 780 configured to control driving signal 790 based on DC feedback component 720, consistent with embodiments of the present disclosure. In some embodiments, a gain of VCCS 780 (Gvccs) may be defined in terms of the magnitude of current output from third terminal 780c as driving signal 790 for a given voltage across first terminal 780a as setpoint signal 714 and second terminal 780b as DC feedback signal 722.

[0089] As an example, the gain of VCCS 780 may be very large, e.g., about 40 decibels, about 50 decibels, about 60 decibels, about 70 decibels, about 80 decibels, about 90 decibels, or about 100 decibels. In some embodiments, DC feedback component 720 may be a wire. In some embodiments, a gain of DC feedback component 720 (GDC) may be 1. In some embodiments, a gain of DC feedback component 720 (GDC may be greater than 1.

[0090] Therefore, a current of driving signal 790 (lout) may be proportional to a setting signal (Uset) applied to the first terminal 780a by setpoint signal 714, a gain of the VCCS 780 (Gvccs), a gain of the DC feedback component 720 (GDC), a voltage (VDC) applied to the second terminal 780b by DC feedback signal 722. The voltage (VDC) applied to the second terminal 780b by DC feedback signal 722 may be represented as:DC=lout *

[0091] The DC feedback component drift correction relationship may be represented as:I outK(Uset ~ lout * Rs* DC) * ^VCCS

[0092] By solving for Iout, the DC feedback component drift correction relationship can be expressed as:

[0093] When the value of Rs* GDC* Gvccsis much greater than 1, for example when Gvccs = 100, the relationship may be further simplified as:

[0094] Therefore, in an exemplary operation, for a constant setting signal (Uset) applied to the first terminal 780a by setpoint signal 714, when shunt resistor 726 has a high resistance value, i.e., Rs is large, Ioutis small, relatively. In this way, a higher resistance of shunt resistor 726 (Rs) may result in a more stable driving signal 790. At the same time, however, a higher resistance shunt resistor 526 may result in a greater drift in driving signal 790. Drift in driving signal 790 may be related to power dissipation through shunt resistor 726, which is proportional to the resistance of shunt resistor 726. Power dissipation through shunt resistor 726 is the conversion of electrical energy into heat as source signal 718 passes through shunt resistor 726. Increasing the resistance of shunt resistor 726 may make it harder for electricity to move through, thereby raising the power dissipation in the signal DC feedback signal 722 and resulting in increased drift in driving signal 790.

[0095] As a second example, for a constant setting signal (Uset) applied to the first terminal 780a by setpoint signal 514, when shunt resistor 526 has a low resistance value, i.e., Rs is small, Ioutis large, relatively. A lower resistance shunt resistor 526 may reduce the power dissipation in the signal DC feedback signal 522 and result in a smaller drift in driving signal 790.

[0096] Thus, it may be said that drift in driving signal 790 is proportionally related to heat dissipation of shunt resistor 726, which is proportionally related to the resistance value of shunt resistor 726. Therefore, drift in driving signal 790 is proportionally related to the resistance value of shunt resistor 726. That is to say, a lower resistance of shunt resistor 726 may result in a lower drift in driving signal 570. As an example, a lower resistance of shunt resistor 726 may reduce drift in driving signal 790.

[0097] In some embodiments, the voltage applied at second terminal 780b (VDC) by DC feedback signal 722 may be equal to a second voltage (Vn2) plus a drift-correction voltage (Vd), where the driftcorrection voltage (Vd) may be equal to the current of the driving signal 790 (lout) times the resistance of the drift-sense resistor 726 (Rd). The second voltage (Vn2) may be substantially equal to the setpoint voltage (Vset). This may be represented as:VDC= Vn2+ VdVd out * RdDC Vn2 + lout * Rd

[0098] Regarding the AC feedback component 730, in some embodiments, an AC feedback signal 732 may be a signal derived from source signal 718. Particularly, AC feedback signal 732 may be derived from the voltage drop from fourth terminal 780d across noise-sense resistor 736 and driftsense resistor 726 to reference 782. More particularly, AC refence signal 734 may be derived from the voltage at reference 782, where reference 782 may be ground. AC feedback component 730 may receive AC feedback signal 732 and AC reference signal 734. AC feedback component 730 may output AC feedback input 738 based on AC feedback signal 732 and AC reference signal 734. AC feedback input 738 may pass through a first capacitor 738-c. It is to be appreciated that, although not illustrated, AC feedback component 730 may include other components and circuitry such as variable gain amplifiers, timing circuits, capacitors, resistors, etc. as appropriately needed to provide sufficient AC current regulation.

[0099] In some embodiments, a summing component 740 may receive setpoint signal 714 and AC feedback input 738. Summing component 740 may output augmented setpoint signal 714-1 to first terminal 780a. Augmented setpoint signal 714-1 may be derived from the voltage applied to first terminal 780a. Augmented setpoint signal 714-1 may represent a noise component in source signal 718. Because noise-sense resistor 736 may have a relatively high resistance value compared to driftsense resistor 726, and because AC feedback input 738 is based on the voltage drop across both noisesense resistor 736 and drift-sense resistor 726, AC feedback input 738 may represent the noise component in source signal 718. Because VCCS 780 is configured to control driving signal 790 based on augmented setpoint signal 714-1 (which is based on AC feedback input 738), VCCS 780 may compensate for noise in driving signal 790 based on AC feedback input 738, thereby increasing the stability of driving signal 790. As discussed below, the relationship between the gain of the DC feedback component 720 and the AC feedback component 730 and the relationship between the resistance of noise-sense resistor 736 and drift-sense resistor 726 are configured to reduce the noise of driving signal 790 and the drift of driving signal 790.

[0100] In some embodiments, a gain of AC feedback component 730 (GAC) may be defined in terms of the AC feedback input 738 compared to AC feedback signal 732 and AC reference signal 734. ACfeedback component 730 may amplify the voltage across AC feedback signal 732 and AC reference signal 734 to produce AC feedback input 738. AC feedback signal 732 may be derived from a voltage applied to fourth terminal 780d, as discussed below with respect to FIG.7B. In some embodiments, DC feedback component may be a wire. In some embodiments, a gain of DC feedback component 720 (GDC) may be 1. In some embodiments, a gain of DC feedback component 720 (GDC) may be greater than 1. In some embodiments, the gain of AC feedback component 730 may be much larger than the gain of DC feedback component 720.

[0101] In some embodiments, the gain of AC feedback component 730 being much larger than the gain of DC feedback component 720 is configured to enable double feedback current source circuit 700 to produce low noise and low drift in driving signal 790. Because AC feedback component 730 may be configured to provide a much larger gain than the gain the DC feedback component 720 is configured to provide for certain AC frequencies, the noise of driving signal 790 in these certain AC frequencies may be proportional to the voltage noise of AC feedback component 730 divided by the sum of the resistance of noise-sense resistor 736 and the resistance of drift-sense resistor 726. As discussed above, because the resistance of noise-sense resistor is relatively large, the noise of driving signal 790 is relatively small. In some embodiments, a voltage (Vni) may be applied by setpoint signal 714, where the setpoint voltage (Vni) represents a first correction component in the setpoint signal. In some embodiments, the setpoint voltage (Vni) represents the input noise.

[0102] In some embodiments, a current of driving signal 790 (Im) may be proportional to the gain of the VCCS 780 (Gvccs), the voltage (Vni) applied by setpoint signal 714, the AC feedback voltage (VAC from AC feedback component 730, the gain of AC feedback component 730 (GAC), DC feedback voltage (VDC) applied to the second terminal 780b as DC feedback signal 722, and the gain of DC feedback component 720 (GDC - In some embodiments, the sum of the voltage (Vni) applied by setpoint signal 714 and the AC feedback voltage (VAC) from AC feedback component 730 times the gain of AC feedback component 730 (GAC) is equal to the voltage applied at first terminal 780a by augmented setpoint signal 714-1. The resulting double-feedback relationship may be represented as:InxKGAC * AC + ^ni—GDC* kC) * Gvccs

[0103] The first correction component may be a noise component. The noise component may be related to a noise component of driving signal 790. In some embodiments, an AC feedback voltage (VAC) may be applied to first terminal 780a as augmented setpoint signal 714-1 and may be equal to the gain of the AC component (GAC) times the current of the driving signal 790 (Inx) times the sum of the resistance of the noise-sense resistor 736 (Rn) and the resistance of the drift-sense resistor 726 (Rd). This may be represented as:^AC —AC * Inx* (fin + ^d)

[0104] Consistent with some embodiments of the present disclosure, for a range of AC frequencies, noise can be reduced proportionally to the resistance of the noise-sense resistor 736 and the drift-sense resistor 726. In some embodiments, the current of driving signal 790 (Im) may be proportional to the gain of AC feedback component 730 (GAC), the voltage (Vni) applied by setpoint signal 714, the resistance of the noise-sense resistor 736 (Rn), the resistance of the drift-sense resistor 726 (Rd), the gain of DC feedback component 720 (GDC), the second voltage (V„2) , and the gain of VCCS 780 (Gvccs)- Consistent with some embodiments of the present disclosure, the relationship between the driving signal 790 (Im) and the voltage (Vni) applied by setpoint signal 714 is in alternate current domain, enabling nose reduction in the double feedback current source circuit 700. The AC component relationship with the driving current of the driving signal (Inx) may be represented as:

[0105] By solving for the current of driving signal 790 (Inx), the AC component 730 noise correction relationship may be expressed as:

[0106] In some embodiments, the value of Gvccs* (Rn+ Rd* GACis much greater than Gvccs* Rd* GDC, for example when GAC is much greater than GDC, and Gvccs* (Rn+ Rd* GACis much greater than 1, for example when Gvccs = 100. In some embodiments, Vnlmay be substantially equal to Vn2. In some embodiments, Vnl* GACmay be much greater than Vn2* GDC, for example when GAC is much greater than GDC- In such embodiments, the AC component 730 noise correction relationship may be simplified as:Vni‘nxKr, , nKn +Kd

[0107] Thus, as discussed above, in some embodiments, the gain of AC feedback component 730 being much larger than the gain of DC feedback component 720 is configured to enable double feedback current source circuit 700 to produce low noise and low drift in driving signal 790. Because AC feedback component 730 may be configured to provide a much larger gain than the gain the DC feedback component 720 is configured to provide for certain AC frequencies, the noise of driving signal 790 in these certain AC frequencies may be proportional to the voltage noise of AC feedback component 730 divided by the sum of the resistance of noise-sense resistor 736 and the resistance ofdrift-sense resistor 726. As discussed above, because the resistance of noise-sense resistor is relatively large, the noise of driving signal 790 is relatively small.

[0108] Therefore, according to some embodiments of the present disclosure, double feedback current source circuit 700 may correct for both signal noise and signal drift without the resistor value tradeoff, as described above with respect to FIG. 5.

[0109] As discussed above, a higher resistance may result in a lower amount of undesirable noise in a feedback loop and thereby reduce the noise in driving signal 790. A lower resistance resistor may result in a lower amount of undesirable drift and reduce the drift in driving signal 790. Therefore, a relatively higher resistance of noise-sense resistor 736 may result in less noise in driving signal 790, because VCCS 780 may compensate for noise in source signal 718 based on AC feedback input 738. A relatively lower resistance of drift-sense resistor 736 may result in less drift in driving signal 790 because VCCS 780 may compensate for drift in source signal 718 based on DC feedback signal 722. Therefore, in some embodiments, the DC feedback component 720 may provide feedback to VCCS 780 to provide a correction signal for undesirable drift. In some embodiments, the AC feedback component 730 may provide a correction signal for undesirable noise. By utilizing both DC feedback component 720 and AC feedback component 730, double feedback current source circuit 700 may deliver a low noise, high stability driving signal 790 to objective lens 728.

[0110] Reference is now made to FIG. 7B, which is a schematic diagram of an exemplary configuration of the AC feedback component 730 of double feedback current source circuit 700, consistent with embodiments of the present disclosure.

[0111] According to some embodiments of the present disclosure, AC feedback signal 732 may be derived from the voltage drop from the fourth terminal 780d across noise-sense resistor 736 and driftsense resistor 726 to reference 782. AC feedback signal 732 may pass through a first AC feedback loop capacitor 730-cl and a first AC feedback loop resistor 730-rl before being applied at a noninverting input of an AC feedback loop op amp 730-ol. A first portion 732-1 of AC feedback signal 732, having passed through the first AC feedback loop capacitor 730-cl, may pass through a second AC feedback loop resistor 730-r2. AC reference signal 734 and the first portion 732-1 of AC feedback signal 732 may be applied at an inverting input of the AC feedback loop op amp 730-ol.

[0112] According to some embodiments of the present disclosure, AC feedback loop op amp 730-ol may output pre-signal AC feedback 738-1 based on an amplified voltage difference between the non-inverting input and the inverting input. Thus, AC feedback loop op amp 730-ol may contribute to a gain of AC feedback component 730, where the gain of AC feedback component 730 may represent an amplified voltage different between a voltage applied at the non-inverting input of AC feedback loop op amp 730-ol by AC feedback signal 732 and voltage applied at the inverting input of AC feedback loop op amp 730-ol by AC reference signal 734 and the first portion 732-1 of AC feedback signal 732.

[0113] A second portion 732-2 of AC feedback signal 732, having passed through the first AC feedback loop capacitor 730-cl and the first AC feedback loop resistor 730-rl, may pass through a third AC feedback loop resistor 730-r3 and a second AC feedback loop capacitor 730-c2. A second portion 732-2 of AC feedback signal 732 may join pre-signal AC feedback 738-1 to form AC feedback input 738, which is received at the summing component 740 as described above.

[0114] Reference is now made to FIG. 8, which is a flow chart illustrating an example method 800 for obtaining a low noise and high stability driving signal, consistent with embodiments of the present disclosure. For illustrative purposes, method 800 will be described with respect to double feedback current source circuit 700 shown in FIGs. 7A and 7B.

[0115] In step S810, a setpoint signal may be determined. The setpoint signal may provide a signal that represents the desired driving current to an objective lens assembly, for example, objective lens 328 in FIG.3.

[0116] In step S820, a drift suppression signal may be determined from a DC feedback component (e.g., DC feedback component 720 in FIG. 7A). The drift suppression signal may be based on a voltage drop across a drift-sense resistor (e.g., drift-sense resistor 726 in FIG.7A). The drift suppression signal may represent a drift component in a source signal (e.g., source signal 718 in FIG.7A) from a VCCS (e.g., VCCS 780 in FIG.7A).

[0117] In step S830, a noise suppression signal may be determined from an AC feedback component (e.g., AC feedback component 730 in FIG.7A and 7B). The noise suppression signal may be based on a voltage across a noise-sense resistor (e.g., noise-sense resistor 736 in FIG. 7A) and the drift-sense resistor (e.g., drift-sense resistor 726 in FIG. 7A). The noise suppression signal may represent a noise component in a source signal (e.g., source signal 718 in FIG. 7A) from the VCCS (e.g., VCCS 780 in FIG.7A). In some embodiments, the resistance of the noise-sense resistor may be greater than the resistance of the drift-sense resistor.

[0118] In step S840, a control signal may be generated based on the noise suppression signal and the setpoint signal. The control signal (e.g., augmented setpoint signal 714-1 in FIG.7A) may be generated with a summing component (e.g., summing component 740 in FIG. 7A), where the summing component adds the noise suppression signal to the input signal. Thus, the control signal may represent the input signal having been augmented with the noise suppression signal to compensate for unwanted noise.

[0119] In step S850, a driving signal (e.g., driving signal 790 in FIG. 7A) may be generated by a voltage-controlled current source (e.g., VCCS 780 in FIG. 6) based on the drift suppression signal and the control signal. In some embodiments, the VCCS may comprise an op amp (e.g., op amp 682 in FIG. 6) and a MOSFET (e.g., MOSFET 684 in FIG.6). The op amp may output a signal (e.g., signal 616 in FIG. 6) to ensure that the difference in voltage across the non-inverting input (e.g., applied by the control signal) and the inverting input (e.g., applied by the drift suppression signal) is zero. In some embodiments, the signal (e.g., signal 616 in FIG. 6) will turn on the MOSFET toachieve the required negative feedback path across the inverting input. A load (e.g., objective lens 728 in FIG. 7A) may be connected to the driving signal. In some embodiments, because the driving signal is based on the noise suppression signal and the drift suppression signal, the driving signal is generated with low noise and high stability.

[0120] The embodiments may further be described using the following clauses:1. A current source circuit, comprising: a voltage controlled current source (VCCS) having first, second, third, and fourth terminals, the first and second terminals configured to control current that flows between the third and fourth terminals, wherein the VCCS is configured to output a driving signal at the third terminal to a lens assembly, and wherein the VCCS is configured to control the driving signal proportionally to an input signal received at the first terminal and a first correction signal received at the second terminal; a DC feedback component configured to provide the first correction signal based on a drift-sense resistor; an AC feedback component configured to provide a second correction signal based on a noise-sense resistor and the drift-sense resistor; and a summing component configured to combine a setpoint signal and the second correction signal to provide the input signal.2. The current source circuit of clause 1, wherein the noise-sense resistor and the drift-sense resistor are coupled in series between the fourth terminal and a reference.3. The current source circuit of any one of clauses 1 and 2, wherein the noise-sense resistor is coupled to the fourth terminal of the VCCS and the drift-sense resistor is coupled in series between the noisesense resistor and the reference.4. The current source circuit of any one of clauses 1-3, wherein the first correction signal is derived from a voltage drop across the drift-sense resistor.5. The current source circuit of any one of clauses 1-4, wherein the second correction signal is derived from a voltage drop over the drift-sense resistor and the noise-sense resistor to the reference.6. The current source circuit of any one of clauses 2-5, wherein the reference is a ground.7. The current source circuit of any one of clauses 1-6, wherein: a resistance of the noise-sense resistor is larger than a resistance of the drift-sense resistor.8. The current source circuit of any one of clauses 1-7, wherein the lens assembly is an objective lens assembly.9. The current source circuit of any one of clauses 1-8, wherein the AC feedback component further comprises: a first operational amplifier configured to receive an AC feedback signal applied at a noninverting input of the first operational amplifier and an AC reference signal applied at a inverting input of the first operational amplifier, wherein the AC feedback signal is derived from a voltage drop across the noise-sense resistor and the AC reference signal is derived from the reference and a first portion of the AC feedback signal; wherein the first operational amplifier is configured to output the second correction signal proportional to a second portion of the AC feedback signal and a voltageacross the non-inverting input and the inverting input such that the second correction signal compensates for noise in the driving signal.10. The current source circuit of any one of clauses 1-9, wherein the VCCS further comprises: a second operational amplifier configured to amplify a signal based on a difference between the input signal and the second correction signal; and a metal-oxide-semiconductor field-effect transistor (MOSFET) configured to output the driving signal based on the amplified signal.11. The current source circuit of any one of clauses 1-10, wherein the VCCS is configured to control the driving signal proportionally to a voltage applied to the first terminal by the input signal divided by a sum of the resistance of the noise-sense resistor and the resistance of the drift-sense resistor. 12. The current source circuit of any one of clauses 1-11, wherein: the DC feedback component is configured to provide a first gain; the AC feedback component is configured to provide a second gain; and the VCCS is configured to provide a third gain and to control the driving signal proportionally to the first gain, the second gain, and the third gain.13. A charged-particle inspection apparatus, comprising: a charged-particle beam source configured to generate a primary charged-particle beam for sample scanning; a lens assembly configured to influence the charged particle beam; and a current source circuit configured to produce a driving signal for applying to the lens assembly, wherein the current source circuit comprises: a voltage controlled current source (VCCS) having first, second, third, and fourth terminals, the first and second terminals configured to control current that flows between the third and fourth terminals, wherein the VCCS is configured to output a driving signal at the third terminal to the lens assembly, and wherein the VCCS is configured to control the driving signal proportionally to an input signal received at the first terminal and a first correction signal received at the second terminal; a DC feedback component configured to provide the first correction signal based on a drift-sense resistor; an AC feedback component configured to provide a second correction signal based on a noise-sense resistor and the drift- sense resistor; and a summing component configured to combine a setpoint signal and the second correction signal to provide the input signal.14. The charged-particle inspection apparatus of clause 13, wherein the noise-sense resistor and the drift-sense resistor are coupled in series between the fourth terminal and a reference15. The charged-particle inspection apparatus of any one of clauses 13 and 14, wherein the noisesense resistor is coupled to the fourth terminal of the VCCS and the drift-sense resistor is coupled in series between the noise-sense resistor and the reference.16. The charged-particle inspection apparatus of any one clauses of 13-15, wherein the first correction signal is derived from a voltage drop across the drift-sense resistor.17. The charged-particle inspection apparatus of any one of clauses 13-16, wherein the second correction signal is derived from a voltage drop over the drift-sense resistor and the noise-sense resistor to the reference.18. The charged-particle inspection apparatus of any one of clauses 13-17, wherein the reference is a ground.19. The charged-particle inspection apparatus of any one of clauses 13-18, wherein: a resistance of the noise-sense resistor is larger than a resistance of the drift-sense resistor.20. The charged-particle inspection apparatus of any one of clauses 13-19, wherein the lens assembly is an objective lens assembly.21. The charged-particle inspection apparatus of any one of clauses 13-20, wherein the AC feedback component further comprises: a first operational amplifier configured to receive an AC feedback signal applied at a non-inverting input of the first operational amplifier and an AC reference signal applied at an inverting input of the first operational amplifier, wherein the AC feedback signal is derived a from voltage drop over the noise-sense resistor and the AC reference signal is derived from the reference and a first portion of the AC feedback signal; wherein the first operational amplifier is configured to output the second correction signal proportional to a second portion of the AC feedback signal and a voltage across the non-inverting input and the inverting input such that the second correction signal compensates for noise in the driving signal.22. The charged-particle inspection apparatus of any one of clauses 13-21, wherein the VCCS further comprises: a second operational amplifier configured to amplify a signal based on a difference between the input signal and the second correction signal; and a metal-oxide-semiconductor fieldeffect transistor (MOSFET) configured to output the driving signal based on the amplified signal. 23. The charged-particle inspection apparatus of any one of clauses 13-22, wherein the VCCS is configured to control the driving signal proportionally to a voltage applied to the first terminal by the input signal divided by a sum of the resistance of the noise-sense resistor and the resistance of the drift-sense resistor.24. The charged-particle inspection apparatus of any one of clauses 13-23, further comprising: the DC feedback component is configured to provide a first gain; the AC feedback component is configured to provide a second gain; and the VCCS is configured to provide a third gain and to control the driving signal proportionally to the first gain, the second gain, and the third gain.25. A method for controlling a current source circuit for driving a lens assembly, the method comprising: receiving an input signal from a current setting source; acquiring a first correction signal from a DC feedback component; acquiring a second correction signal from an AC feedback component; generating, using a summing component, a control signal based on a combination of the input signal and the second correction signal; and generating a driving signal to drive the lens assembly based on the control signal and the first correction signal.26. The method of clause 25, further comprising: applying, as a first voltage, the control signal to a first terminal of a voltage controlled current source (VCCS); applying, as a second voltage, the first correction signal to a second terminal of the VCCS; and generating, by the VCCS, the driving signal in proportion to a difference between the first voltage and the second voltage.27. The method of any one of clauses 25 and 26, wherein the generating of the driving signal further comprises: compensating for a noise component of the driving signal and a drift component of the driving signal; wherein, the first correction signal comprises the drift component of the driving signal; and the second correction signal comprises the noise component of the driving signal.28. The method of any one of clauses 25-27, wherein acquiring the first correction signal from a DC feedback component further comprises receiving a DC feedback signal based on a drift-sense resistor; and acquiring the second correction signal from an AC feedback component further comprises receiving an AC feedback signal based on the drift-sense resistor and a noise-sense resistor; wherein a resistance of the drift-sense resistor is lower than a lower resistance of the noise-sense resistor.29. The method of any one of clauses 25-28, further comprising applying the driving signal to the lens assembly, wherein the lens assembly is configured to control an objective lens.

[0121] Block diagrams in the figures may illustrate the architecture, functionality, and operation of possible implementations of systems, methods, and computer hardware or software products according to various exemplary embodiments of the present disclosure. In this regard, each block in a schematic diagram may represent certain arithmetical or logical operation processing that may be implemented using hardware such as an electronic circuit. Blocks may also represent a module, segment, or portion of code that comprises one or more executable instructions for implementing the specified logical functions. It should be understood that in some alternative implementations, functions indicated in a block may occur out of the order noted in the figures. For example, two blocks shown in succession may be executed or implemented substantially concurrently, or two blocks may sometimes be executed in reverse order, depending upon the functionality involved. Some blocks may also be omitted. It should also be understood that each block of the block diagrams, and combination of the blocks, may be implemented by special purpose hardware -based systems that perform the specified functions or acts, or by combinations of special purpose hardware and computer instructions.

[0122] It will be appreciated that the embodiments of the present disclosure are not limited to the exact construction that has been described above and illustrated in the accompanying drawings, and that various modifications and changes may be made without departing from the scope thereof. The present disclosure has been described in connection with various embodiments, other embodiments will be apparent to those skilled in the art from consideration of the specification and practice of the technology disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

Claims

CLAIMS1. A charged-particle inspection apparatus, comprising:a charged-particle beam source configured to generate a primary charged-particle beam for sample scanning;a lens assembly configured to influence the charged particle beam; anda current source circuit configured to produce a driving signal for applying to the lens assembly, wherein the current source circuit comprises:a voltage controlled current source (VCCS) having first, second, third, and fourth terminals, the first and second terminals configured to control current that flows between the third and fourth terminals, wherein the VCCS is configured to output a driving signal at the third terminal to the lens assembly, and wherein the VCCS is configured to control the driving signal proportionally to an input signal received at the first terminal and a first correction signal received at the second terminal;a DC feedback component configured to provide the first correction signal based on a drift- sense resistor;an AC feedback component configured to provide a second correction signal based on a noise-sense resistor and the drift-sense resistor; anda summing component configured to combine a setpoint signal and the second correction signal to provide the input signal.

2. The charged-particle inspection apparatus of claim 1, wherein the noise-sense resistor and the drift-sense resistor are coupled in series between the fourth terminal and a reference.

3. The charged-particle inspection apparatus of claim 2, wherein the noise-sense resistor is coupled to the fourth terminal of the VCCS and the drift-sense resistor is coupled in series between the noise-sense resistor and the reference.

4. The charged-particle inspection apparatus of claim 1, wherein the first correction signal is derived from a voltage drop across the drift-sense resistor.

5. The charged-particle inspection apparatus of claim 2, wherein the second correction signal is derived from a voltage drop over the drift-sense resistor and the noise-sense resistor to the reference.

6. The charged-particle inspection apparatus of claim 2, wherein the reference is a ground.

7. The charged-particle inspection apparatus of claim 1, wherein a resistance of the noise-sense resistor is larger than a resistance of the drift-sense resistor.

8. The charged-particle inspection apparatus of claim 1, wherein the lens assembly is an objective lens assembly.

9. The charged-particle inspection apparatus of claim 1, wherein the AC feedback component further comprises:a first operational amplifier configured to receive an AC feedback signal applied at a noninverting input of the first operational amplifier and an AC reference signal applied at an inverting input of the first operational amplifier, wherein the AC feedback signal is derived a from voltage drop over the noise-sense resistor and the AC reference signal is derived from the reference and a first portion of the AC feedback signal;wherein the first operational amplifier is configured to output the second correction signal proportional to a second portion of the AC feedback signal and a voltage across the non-inverting input and the inverting input such that the second correction signal compensates for noise in the driving signal.

10. The charged-particle inspection apparatus of claim 1, wherein the VCCS further comprises:a second operational amplifier configured to amplify a signal based on a difference between the input signal and the second correction signal; anda metal-oxide-semiconductor field-effect transistor (MOSFET) configured to output the driving signal based on the amplified signal.

11. The charged-particle inspection apparatus of claim 1, wherein the VCCS is configured to control the driving signal proportionally to a voltage applied to the first terminal by the input signal divided by a sum of the resistance of the noise-sense resistor and the resistance of the drift-sense resistor.

12. The charged-particle inspection apparatus of claim 1, wherein:the DC feedback component is configured to provide a first gain;the AC feedback component is configured to provide a second gain; andthe VCCS is configured to provide a third gain and to control the driving signal proportionally to the first gain, the second gain, and the third gain.

13. A method for controlling a current source circuit for driving a lens assembly, the method comprising:receiving an input signal from a current setting source;acquiring a first correction signal from a DC feedback component;acquiring a second correction signal from an AC feedback component;generating, using a summing component, a control signal based on a combination of the input signal and the second correction signal; andgenerating a driving signal to drive the lens assembly based on the control signal and the first correction signal.

14. The method of claim 13, wherein the generating of the driving signal further comprises: compensating for a noise component of the driving signal and a drift component of the driving signal; wherein,the first correction signal comprises the drift component of the driving signal; and the second correction signal comprises the noise component of the driving signal.

15. A current source circuit, comprising:a voltage controlled current source (VCCS) having first, second, third, and fourth terminals, the first and second terminals configured to control current that flows between the third and fourth terminals,wherein the VCCS is configured to output a driving signal at the third terminal to a lens assembly, andwherein the VCCS is configured to control the driving signal proportionally to an input signal received at the first terminal and a first correction signal received at the second terminal;a DC feedback component configured to provide the first correction signal based on a driftsense resistor;an AC feedback component configured to provide a second correction signal based on a noise-sense resistor and the drift-sense resistor; anda summing component configured to combine a setpoint signal and the second correction signal to provide the input signal.

Images