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CN-116959943-B - Method for determining aberration of charged particle beam and charged particle beam system

CN116959943BCN 116959943 BCN116959943 BCN 116959943BCN-116959943-B

Abstract

A method of determining aberrations of a charged particle beam (11) focused by a focusing lens (120) having a given Numerical Aperture (NA) onto a sample (10) in a charged particle beam system is described. The method includes (a)) simulating one or more beam cross-sections at one or more first defocus settings for each of two or more different values of a first beam aberration coefficient (C 1 ) in a set of beam aberration coefficients (C 1...n ) based at least on a given Numerical Aperture (NA) to provide a plurality of first simulated beam cross-sections, (b)) extracting two or more values of a first aberration characteristic (& gt 1 ) associated with the first beam aberration coefficient (C 1 ) from the plurality of first simulated beam cross-sections, (c.) determining a first correlation between the first beam aberration coefficient (C 1 ) and the first aberration characteristic (& gt 1 ) based on the values of the first beam aberration characteristic (C 1 ), and (d)) capturing one or more images of the sample at the one or more first defocus settings to provide one or more captured images, and retrieving one or more retrieved beam cross-sections from the one or more captured images, (e)) retrieving the first cross-section from the one or more first defocus settings and retrieving the values of the first aberration coefficient (C 1 ) based on the values of the first aberration coefficient and the values of the first aberration characteristic.

Inventors

  • D. Enberg
  • J. Broyle

Assignees

  • ICT半导体集成电路测试有限公司

Dates

Publication Date
20260505
Application Date
20230323
Priority Date
20220427

Claims (20)

  1. 1. A method of determining aberrations of a charged particle beam focused by a focusing lens having a given numerical aperture onto a sample in a charged particle beam system, the method comprising: (a.) simulating one or more beam cross-sections at one or more first defocus settings for each of two or more different values of a first beam aberration coefficient of a set of beam aberration coefficients based at least on the given numerical aperture to provide a plurality of first simulated beam cross-sections; extracting two or more values of a first aberration characteristic related to the first beam aberration coefficient from the plurality of first simulated beam cross-sections; (c.) determining a first correlation between the first beam aberration coefficient and the first aberration characteristic; (d.) taking one or more images of the sample at the one or more first defocus settings or at one or more second defocus settings to provide one or more taken images and retrieving one or more retrieved beam cross-sections from the one or more taken images; (e.) extracting retrieved values of the first aberration characteristic from the one or more retrieved beam cross-sections, and (F.) determining an actual value of the first beam aberration coefficient based on the first correlation and based on the retrieved value of the first aberration characteristic.
  2. 2. The method of claim 1, wherein the one or more first defocus settings comprise one or more defocus distances from a beam focus of the charged particle beam such that in (a.) the one or more beam cross-sections are simulated at the one or more defocus distances from the beam focus, and in (d.) the one or more images of the sample are taken while the sample is arranged at the one or more defocus distances from the beam focus.
  3. 3. The method of claim 1, wherein (a.) comprises simulating an over-focused beam cross-section and an under-focused beam cross-section for each of the two or more different values of the first beam aberration coefficient, and (b.) comprises extracting each of the two or more values of the first aberration characteristic from one of the over-focused beam cross-section and the corresponding under-focused beam cross-section.
  4. 4. The method of claim 1, wherein (d.) comprises capturing an undershot image of the sample and an overshot image of the sample, and retrieving an undershot retrieved beam cross-section from the undershot image and an overshot retrieved beam cross-section from the overshot image, and (e.) comprises extracting the retrieved value of the first aberration characteristic from the undershot retrieved beam cross-section and the overshot retrieved beam cross-section.
  5. 5. The method of claim 1, wherein in (b.), one or more radial line profiles are determined from each of the plurality of first analog beam cross-sections, and the two or more values of the first aberration characteristic are calculated based on the radial line profiles.
  6. 6. The method of claim 5, wherein one or more of a beam width value, an asymmetry value, and a curvature value are retrieved from each of the one or more radial line profiles, and the two or more values of the first aberration characteristic are calculated therefrom.
  7. 7. The method of claim 1, wherein the first beam aberration coefficient is selected from the group of beam aberration coefficients comprising defocus (C df ), third-order spherical aberration (C s ), first-order astigmatism (a 1 ), second-order astigmatism (a 2 ), third-order astigmatism (a 3 ), second-order coma (B 2 ), third-order star-delta (S 3 ), and one or more chromatic aberrations (C c ).
  8. 8. The method of claim 1, wherein the first correlation between the first beam aberration coefficient and the first aberration characteristic is a substantially linear correlation characterized by a first scale coefficient, and (c.) comprises determining the first scale coefficient.
  9. 9. The method of claim 1, wherein in (d.) retrieving the one or more retrieved beam cross-sections from the one or more images comprises dividing the one or more images in fourier space by a focused image of the sample in fourier space, or alternatively wherein the one or more retrieved beam cross-sections are retrieved based on an inverse convolution in real space.
  10. 10. The method of claim 1, further comprising determining a second correlation between a second beam aberration coefficient of the set of beam aberration coefficients and a second aberration characteristic.
  11. 11. The method of claim 10, wherein the second correlation is determined based on a plurality of second simulated beam cross-sections simulated for different values of the second beam aberration coefficient.
  12. 12. The method of claim 10, further comprising: Extracting a retrieved value of the second aberration characteristic from the one or more retrieved beam cross-sections, and An actual value of the second beam aberration coefficient is determined based on the second correlation and based on the retrieved value of the second aberration characteristic.
  13. 13. The method of claim 1, further comprising storing the first correlation in a memory of the charged particle beam system.
  14. 14. The method of claim 13, wherein the first correlation is a substantially linear correlation characterized by a first scale factor stored in the memory.
  15. 15. The method of claim 1, wherein in (c.), at least two first correlations between the first beam aberration coefficients and the first aberration characteristics are determined, each first correlation based on an associated first defocus setting; In (d.), the one or more images of the sample are taken at the one or more second defocus settings different from the one or more first defocus settings, and In (f.), the actual value of the first beam aberration coefficient is determined based on the at least two first correlations and based on the retrieved value of the first aberration characteristic.
  16. 16. The method of claim 1, wherein the one or more first defocus settings comprise one or more beam landing energies of the charged particle beam that vary from a focused beam landing energy such that in (a.) the one or more beam cross-sections are defocus simulated at the one or more beam landing energies and in (d.) the one or more images of the sample are defocus captured at the one or more beam landing energies.
  17. 17. The method of claim 1, further comprising setting an aberration correction intensity of one or more beam aberration correctors based on the actual value of the first beam aberration coefficient.
  18. 18. A method of determining aberrations of a charged particle beam focused by a focusing lens having a given numerical aperture onto a sample in a charged particle beam system, the method comprising: (a.) simulating one or more beam cross-sections at one or more first defocus settings for each of two or more different values of the first beam aberration coefficients based at least on the given numerical aperture to provide a plurality of first simulated beam cross-sections; (b.) extracting two or more values of a first aberration characteristic associated with the first beam aberration coefficient from the plurality of first simulated beam cross-sections, and (C.) determining a first correlation between the first beam aberration coefficient and the first aberration characteristic for determining an actual value of the first beam aberration coefficient.
  19. 19. A charged particle beam system comprising: A charged particle source for emitting a charged particle beam propagating along an optical axis; A sample platform; A focusing lens for focusing a charged particle beam having a given numerical aperture onto a sample placed on the sample stage; a charged particle detector for detecting signal particles emitted from the sample, and A processing unit and a memory storing a first correlation between a first beam aberration coefficient and a first aberration characteristic, the first aberration characteristic being a parameter having a predetermined relation to the first beam aberration coefficient and extractable from one or more defocused beam cross-sections, Wherein the memory stores instructions that, when executed by the processing unit, cause the charged particle beam system to: Retrieving one or more retrieved beam cross-sections from one or more images taken at one or more defocus settings; extracting a retrieved value of the first aberration characteristic from the one or more retrieved beam cross-sections, and An actual value of the first beam aberration coefficient is determined based on the first correlation and based on the retrieved value of the first aberration characteristic.
  20. 20. The charged particle beam system of claim 19, further comprising one or more aberration correctors configured to compensate for aberrations of the charged particle beam based on the actual value of the first beam aberration coefficient.

Description

Method for determining aberration of charged particle beam and charged particle beam system Technical Field Embodiments described herein relate to methods of determining aberrations of a charged particle beam in a charged particle beam system, for example in an electron microscope, in particular in a Scanning Electron Microscope (SEM). In particular, actual values of one or more beam aberration coefficients may be determined, facilitating correction of beam aberration and improving resolution. More particularly, embodiments described herein relate to methods of determining a beam aberration coefficient of a charged particle beam focused to a sample by a focusing lens having a given numerical aperture in a charged particle beam system, which helps to provide an aberration corrected charged particle beam. Embodiments also relate to charged particle beam systems for inspecting and/or imaging a sample, the charged particle beam systems configured for use in any of the methods described herein. Background Modern semiconductor technology places high demands on sample structuring and detection at the nanoscale and even sub-nanoscale. Micro-scale and nano-scale process control, inspection or structuring is typically performed using a charged particle beam (e.g., an electron beam) that is generated, shaped, deflected and focused in a charged particle beam system such as an electron microscope or electron beam pattern generator. For inspection purposes, charged particle beams provide a higher spatial resolution than, for example, photon beams. Inspection equipment, such as Scanning Electron Microscopes (SEMs), using charged particle beams have many functions in a number of industrial fields, including but not limited to electronic circuit inspection, exposure systems for photolithography, inspection systems, defect inspection tools, and test systems for integrated circuits. In such a particle beam system, a beamlet probe with a high current density may be used. For example, in the case of SEM, the primary electron beam generates signal particles, such as Secondary Electrons (SE) and/or backscattered electrons (BSE), that can be used to image and/or inspect the sample. However, it is challenging to reliably inspect and/or image samples with good resolution using charged particle beam systems, as charged particle beams often suffer from beam aberrations that limit the achievable resolution. In a typical charged particle beam system, an aberration corrector is provided for at least partially compensating for aberrations of the charged particle beam, such as spherical aberration, astigmatism and/or chromatic aberration. The aberration corrected charged particle beam may provide a smaller detection focal length than an uncorrected beam, thereby providing better resolution. However, it is challenging to properly adjust the settings of aberration correctors that may have a large number of controls to satisfactorily correct the beam aberrations, as the beam aberrations present in the system are generally unknown. Several methods for determining or correcting aberrations of charged particle beams in electron microscopes have been described. For example, "Aberration correction and its automatic control IN SCANNING electron microscopes (aberration correction in scanning electron microscopes and automatic control thereof)" (hereinafter referred to as "Uno") published by unoo (Uno) et al in journal of optical and electron optics international journal of academic paper (Optik-International Journal for Light and Electron Optics), 116 (9) pages 438 to 448 describes a method of determining the correlation between the so-called "digital aberration" and the field strength of a specific multistage aberration corrector, which is adapted to compensate for such "digital aberration". However, the aberration correction method of UNO is time consuming, since multiple measurements have to be made to determine the mentioned correlation, and the use of the determined correlation is in turn limited to one specific multistage aberration corrector, which is adapted to generate the corresponding field strength. In view of the above, it would be advantageous to provide a method for quickly and reliably determining the aberration of a charged particle beam focused by a focusing lens in a charged particle beam system, in particular for determining the actual value of the beam aberration coefficient, i.e. the absolute value of the beam aberration coefficient. Furthermore, it would be beneficial to provide a charged particle beam system for inspecting and/or imaging a sample, which is configured for operation according to any of the methods described herein. The determined actual aberration value may be used, for example, for correcting the charged particle beam and/or for checking the beam spot shape of the actual charged particle beam, for example for comparing the performance between different charged particle beam tools. Disclosure of Invention In vi