WO2010126958A1 - Imagerie de contraste de tension - Google Patents

Imagerie de contraste de tension Download PDF

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Publication number
WO2010126958A1
WO2010126958A1 PCT/US2010/032695 US2010032695W WO2010126958A1 WO 2010126958 A1 WO2010126958 A1 WO 2010126958A1 US 2010032695 W US2010032695 W US 2010032695W WO 2010126958 A1 WO2010126958 A1 WO 2010126958A1
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WIPO (PCT)
Prior art keywords
sample
charged particles
average
image
particles
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PCT/US2010/032695
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English (en)
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WO2010126958A4 (fr
WO2010126958A9 (fr
Inventor
Dave Ferranti
Xiong Liu
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Carl Zeiss Smt Inc.
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Publication of WO2010126958A1 publication Critical patent/WO2010126958A1/fr
Publication of WO2010126958A4 publication Critical patent/WO2010126958A4/fr
Publication of WO2010126958A9 publication Critical patent/WO2010126958A9/fr

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/26—Electron or ion microscopes; Electron or ion diffraction tubes
    • H01J37/28—Electron or ion microscopes; Electron or ion diffraction tubes with scanning beams
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
    • H01J37/26—Electron or ion microscopes; Electron or ion diffraction tubes
    • H01J37/261—Details
    • H01J37/263—Contrast, resolution or power of penetration
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
    • H01J2237/004—Charge control of objects or beams
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
    • H01J2237/245—Detection characterised by the variable being measured
    • H01J2237/24507—Intensity, dose or other characteristics of particle beams or electromagnetic radiation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
    • H01J2237/245—Detection characterised by the variable being measured
    • H01J2237/24592—Inspection and quality control of devices

Definitions

  • This disclosure relates to particle beams and sample imaging with particle beams.
  • Samples can be exposed to particle beams for a variety of applications, including sample imaging.
  • sample imaging differences in intensity measured in images of sample features are due, at least in part, to charge accumulation and dissipation in the features.
  • the systems and methods disclosed herein permit measurement of sample images with improved image contrast relative to images that are obtained under more common imaging conditions.
  • the measured sample images have improved image contrast, permitting discrimination and identification of various types of structures in the samples.
  • the images can be used to identify different types of doped structures (e.g., n-type structures, p-type structures).
  • the images can also be used to identify electrical pathways between circuit elements, including pathway errors (e.g., short-circuits and open circuits).
  • the energy of incident particles on a sample is reduced to fall within a specified range to enhance image contrast.
  • reducing the particle energy may lead to some loss of image resolution, this loss of resolution is typically not too severe.
  • a particular image contrast threshold can be selected for two different regions of the sample, and the particle energy can be adjusted, either manually or automatically, until the threshold is reached. This procedure can help to ensure that sample images of a certain minimum image contrast, at least between the selected regions, are obtained.
  • the disclosure features methods that include exposing a sample to a plurality of charged particles having an average energy of 20 keV or less, detecting particles that leave a surface of the sample in response to the plurality of charged particles, and forming an image of the sample based on the detected particles.
  • the disclosure features methods that include: (a) exposing first and second regions of a sample to a plurality of charged particles generated by a charged particle source, the plurality of charged particles having an average energy of 20 keV or less; (b) detecting particles that leave a surface of the sample in response to the plurality of charged particles; (c) forming an image of the sample based on the detected particles; (d) determining a difference between average intensities of portions of the image that correspond to the first and second regions; (e) comparing the determined difference to a target value of a difference between average intensities of the portions; (f) adjusting the charged particle source to produce charged particles having a lower average energy; and (g) repeating the exposing, detecting, forming, determining, comparing, and adjusting until the determined difference is less than the target value.
  • the disclosure features methods that include: (a) exposing a region of a sample to a plurality of charged particles generated by a charged particle source, the plurality of charged particles having an average energy of 20 keV or less; (b) detecting particles that leave a surface of the sample in response to the plurality of charged particles; (c) forming an image of the sample based on the detected particles; (d) determining an average intensity of a portion of the image that corresponds to the region; (e) comparing the average intensity to a target value of the average intensity of the portion; (f) adjusting the charged particle source to produce charged particles having a lower average energy; and (g) repeating the exposing, detecting, forming, determining, comparing, and adjusting until the average intensity is greater than the target value.
  • Embodiments of the methods can include one or more of the following features.
  • the methods can include selecting first and second regions of the image and determining a difference between an average intensity of the first region and an average intensity of the second region, where the difference in average intensity is larger than a difference in average intensity between two corresponding regions of an image formed by exposing the sample to a plurality of charged particles having an average energy greater than 20 keV.
  • a ratio of the difference between the average intensity of the first and second regions to the difference between the two corresponding regions can be 1.1 or more (e.g., 1.15 or more, 1.2 or more, 1.25 or more, 1.3 or more, 1.35 or more, 1.4 or more, 1.5 or more, 1.7 or more, 2.0 or more, 3.0 or more, or even more).
  • the plurality of charged particles can have an average energy of 15 keV or less. Alternatively, or in addition, the plurality of charged particles can have an average energy of between 10 keV and 20 keV.
  • the methods can include selecting the average energy of the plurality of charged particles based on a composition of the sample. Selecting the average energy can include retrieving reference information about the sample, and adjusting the average energy of the plurality of charged particles based on the reference information. Adjusting the average energy of the plurality of charged particles can include adjusting one or more electrical potentials applied to components in a charged particle column.
  • the methods can include: (a) selecting a target value of a difference between the average intensities of the first and second regions; (b) comparing the target value to the difference in average intensities of the first and second regions determined from the image; (c) adjusting the average energy of the plurality of charged particles; and (d) repeating the exposing, detecting, forming, determining, comparing, and adjusting until a difference between the target value and the difference in average intensities of the first and second regions determined from the image is less than a threshold value.
  • Adjusting the average energy of the plurality of charged particles can include adjusting one or more electrical potentials applied to components in a charged particle column.
  • An angle between a path of the plurality of charged particles and a normal to the sample surface at a position where the charged particles are incident on the sample can be 10 degrees or more (e.g., 30 degrees or more).
  • the methods can include, when the sample is exposed to the plurality of charged particles, applying an electrical potential difference to the sample.
  • a magnitude of the applied potential difference can be between 0 V and 700 V.
  • the methods can include adjusting both the average energy of the plurality of charged particles and an angle between a path of the plurality of charged particles and a normal to the sample surface at a position where the charged particles are incident on the sample.
  • the plurality of charged particles can include ions.
  • the plurality of charged particles can include noble gas ions.
  • the plurality of charged particles can include helium ions.
  • the methods can include generating the plurality of charged particles using a gas field ion source. The adjusting, exposing, detecting, forming, determining, and comparing can be repeated until the plurality of charged particles have an average energy between 10 keV and 20 keV (e.g., between 10 keV and 15 keV).
  • the adjusting can include adjusting the charged particle source to increase an angle between a path of the plurality of charged particles and a normal to the sample surface at a position where the charged particles are incident on the sample.
  • the charged particle source can be adjusted so that the angle is 10 degrees or more (e.g., 30 degrees or more).
  • the charged particle source can be a gas field ion source.
  • the region of the sample can include an element formed of a semiconductor material.
  • the charged particle source is adjusted so that the plurality of charged particles have an average energy of 15 keV or less (e.g., 10 keV or less).
  • FIG. 1 is a schematic diagram showing a sample with two different regions that have different average intensity values in images of the sample.
  • FIG. 2 is a schematic diagram showing a beam of charged particles that are incident on a surface of a sample.
  • FIG. 3 is an image of a device that was obtained by exposing the device to charged particles having an average energy of 30 keV.
  • FIG. 4 is an image of the device of FIG. 3 that was obtained by exposing the device to charged particles having an average energy of 10 keV.
  • FIG. 5A is an image of the device of FIG. 3 showing two features having different average intensity values.
  • FIG. 5B is a graph showing the measured image contrast between the two features of FIG. 5 A at different incident charged particle energies.
  • FIG. 6 is a schematic diagram showing exposure of a sample to a beam of charged particles at a non-normal angle of incidence.
  • FIG. 7 is an image of a device that was obtained by exposing the device to charged particles at normal incidence to the device.
  • FIG. 8 is an image of the device of FIG. 7 that was obtained by exposing the device to charged particles at a 30 degree angle of incidence.
  • FIG. 9 is a plot showing calculated helium ion penetration in a silicon sample at an average incident ion energy of 30 keV.
  • FIG. 10 is a plot showing calculated helium ion penetration in a silicon sample at an average incident ion energy of 10 keV.
  • FIG. 11 is a graph showing calculated helium ion penetration depth in silicon as a function of incident ion energy.
  • FIG. 12 is a schematic diagram of an ion microscope system.
  • FIG. 13 is a schematic diagram of a gas field ion source.
  • Voltage contrast in images of samples measured with charged particle beams arises due to differential charging and charge dissipation in different regions of the samples as they are exposed to charged particles. For example, when a feature of a sample that is formed of an electrically conductive material is exposed to positively charged particles, a portion of the charged particles are implanted within the feature, creating a temporary excess of positive charge. Because the material is conductive, the excess of positive charge is neutralized relatively quickly by the transport of negative charges to the exposure site. Surface fields that would otherwise have been created by the excess positive charges are therefore reduced, and particles such as secondary electrons are relatively free to leave the sample in response to the positively charged particles. As a result, the secondary electron flux from the sample feature is relatively large, and the feature has a relatively high intensity in sample images.
  • the differences in charge accumulation and dissipation among various features of a sample can be exploited to obtain secondary electron images of samples where intensity contrast among features and/or regions of the sample results, at least in part, from the differences.
  • the images exhibit contrast among sample features that are formed of different materials and among features whose charging behavior varies as a function of time.
  • Disclosed herein are methods and systems for increasing the intensity contrast between certain regions of a sample in secondary electron images of the sample that are obtained by exposing the sample to charged particles.
  • the regions can include, for example, different elements such as portions of integrated circuits formed in the samples.
  • the first part of this disclosure discusses these methods and systems.
  • the second part of this disclosure discusses helium ion beam systems and methods for sample imaging.
  • Modern electronic devices include features with maximum dimensions of 100 nm or less, and sometimes, even substantially less than 100 nm (e.g., 80 nm or less, 60 nm or less, 40 nm or less, 20 nm or less, 10 nm or less, 5 nm or less, 2 nm or less, 1 nm or less).
  • Heavy ions are therefore unsuitable for imaging such devices; heavy ions may cause damage to structures of such small size, and images formed by heavy ion exposure of such devices may not have sufficient resolution to properly image such small device features. That is, beams of heavy ions may not be focusable to a significantly small focal spot to enable accurate measurement of such small features.
  • Lighter charged particles such as helium ions can provide significantly improved images of these device features.
  • helium ions e.g., produced in a gas field ion source
  • implant in devices over a significantly more diffuse volume than heavier ions implant in devices over a significantly more diffuse volume than heavier ions.
  • sputtering due to incident helium ions is significantly reduced relative to sputtering by heavier ions, which helps to prevent device damage during exposure.
  • the sample is exposed to charged particles such as helium ions having an average energy in a range from about 25 keV to about 35 keV.
  • the average energy of a group of charged particles corresponds to the mean of the particles' individual kinetic energies at the position where the particles are incident on a sample. Charged particles in this average energy range can be focused to a small spot size at the sample surface, thereby ensuring relatively high resolution in the image.
  • Secondary electrons leave the surface of the sample in response to the incident charged particles.
  • the secondary electrons are detected by suitably configured detector. Typically, for example, the detector counts the number of secondary electrons emerging from an exposed region of the sample in a selected measurement time period.
  • the secondary electron count for the exposed region of the sample corresponds to a particular region of a sample image.
  • the intensity (e.g., the grey level in a monochrome image) of selected region corresponds to the secondary electron count, measured by the detector, when a corresponding region of the sample was exposed to the charged particles.
  • the secondary electron count can be measured as a function of position, and translated into a map of intensity as a function of position to construct an image of the sample.
  • an average difference in image intensity between two different regions of the sample is related to the flux of secondary electrons that leave each of the two regions when the regions are exposed to the incident charged particles.
  • Various factors influence the secondary electron flux including the material(s) of the sample. Different types of materials have different secondary electron yields when exposed to charged particles, permitting material discrimination based on the measured secondary electron flux. Further, as discussed above, materials having relatively high electrical conductivity tend to appear brighter in secondary electron images than materials having lower electrical conductivity due to excess charge dissipation; the greater the material's electrical conductivity, the faster the rate at which excess charge deposited by the charged particles can be neutralized via charge transport in the material.
  • the flux of secondary electrons from the sample surface can be relatively high.
  • the intensity of the sample in an image is therefore relatively high, as the intensity depends on the measured flux of secondary electrons leaving the sample surface.
  • sample images are used to accurately identify many sample features that are very small in size, including features that are less than 100 nm in size, as discussed above. Incident charged particles having an average energy between 25 keV and 35 keV allow sample images to be obtained at high resolution, permitting the discrimination of small sample features.
  • FIG. 1 shows an image of a sample 3000 that includes two different regions 3002 and 3004 formed of different materials. Each of regions 3002 and 3004 includes multiple image pixels.
  • Detector 3010 which is in electrical communication with electronic processor 3012, measures a secondary electron count from regions of sample 3000 including regions 3002 and 3004 and generates electronic signals with amplitudes that are related to the measured secondary electron counts.
  • Electronic processor 3012 receives the electronic signals and constructs the image of sample 3000 shown in FIG. 1.
  • each pixel in the image corresponds to a position on the sample that is exposed to charged particles 3006, and from which secondary electrons leave the sample.
  • the count of secondary electrons for each pixel is converted to an electronic signal, the amplitude of which corresponds to the intensity, or grey level, of the pixel in the constructed image.
  • the average intensity of the region corresponds to the mean of the intensities of all of the image pixels in the region.
  • Each of regions 3002 and 3004 has an average intensity in the image of sample 3000.
  • a difference in average intensity between regions 3002 and 3004 in the image of sample 3000 is denoted///.
  • AI can be increased by 5% or more (e.g., 10% or more, 15% or more, 20% or more, 25% or more, 30% or more, 35% or more, 40% or more, 45% or more, 50% or more, 60% or more, 70% or more, or even more).
  • a ratio of the average intensity of region 3002 to region 3004 is 1.05 or more (e.g., 1.1 or more, 1.15 or more, 1.2 or more, 1.25 or more, 1.3 or more, 1.35 or more, 1.4 or more, 1.45 or more, 1.5 or more, 1.6 or more, 1.7 or more, or even more).
  • the average incident charged particle energy is reduced to less than 25 keV.
  • some image resolution is lost by reducing the average charged particle energy, and so reducing the average charged particle energy would otherwise appear to be undesirable.
  • reducing the average charged particle energy also increases AI, offsetting the loss of image resolution for the purposes of imaging sample features, particularly small sample features.
  • the average intensity of the charged particles is reduced to less than 20 keV (e.g., between 5 and 20 keV, between 10 and 20 keV, between 10 and 18 keV, between 10 and 16 keV, between 10 and 15 keV, between 12 and 20 keV, between 14 and 20 keV, between 14 and 18 keV, between 14 and 16 keV).
  • 20 keV e.g., between 5 and 20 keV, between 10 and 20 keV, between 10 and 18 keV, between 10 and 16 keV, between 10 and 15 keV, between 12 and 20 keV, between 14 and 20 keV, between 14 and 18 keV, between 14 and 16 keV.
  • FIGS. 3 and 4 provide an example showing the effects of reducing the average energy of the incident charged particles on secondary electron images of the sample.
  • FIG. 3 shows an image of an electronic device that includes a plurality of relatively small features formed on a silicon substrate.
  • the device includes multiple features 3100, each having an intensity that is similar to an intensity of surrounding features such as 3110, for example.
  • the image in FIG. 3 was obtained by exposing the device to helium ions in a helium ion microscope and detecting secondary electrons leaving the surface of device in response to the incident helium ions.
  • the helium ions had an average energy of 30 keV.
  • FIG. 4 shows an image of the same device obtained with incident helium ions having an average energy of 10 keV.
  • the intensity contrast between features 3100 and features such as 3110 is enhanced relative to FIG. 3.
  • features 3100 are identified even more easily in FIG. 4 than in FIG. 3.
  • features 3100 and 3110 of the device are formed of the same material. Accordingly, by comparing the images in FIGS. 3 and 4, it is evident that features 3100 are connected to other sample features with different types of contacts than features 3110. Due to the different electrical contacts of these features, charge dissipation from features 3100 occurs more easily than from features 3110, so that features 3100 appear brighter in FIG. 4.
  • a sample when a sample includes different features that are formed of similar materials, it is expected that the different features will appear with similar average intensities in sample images. Reducing the average energy of the incident charged particles can lead to measured differences in the average intensities of the features in sample images. By comparing the different average intensities of the features, differences in the contacts of sample features to other sample elements can be identified. Thus, reducing the average energy of the incident charged particles can be used to reveal information about electrical contacts between different features of a sample.
  • FIG. 5A shows two particular features selected from the device shown in FIGS. 3 and 4.
  • the incident energy of the helium ions as illustrated in FIG. 5B, the image contrast between the two features can be increased from 13% at 25 keV to 35% at 10 keV.
  • image contrast can be increased by directing the charged particles to be incident on the sample at a non-normal angle.
  • charged particles 3006 are incident on sample 3000 at an angle ⁇ relative to a normal 3112 to the surface of the sample.
  • FIG. 7 shows an image of a device that includes multiple features 3120 and 3122, all of which have similar intensities. The image in FIG. 7 was obtained by exposing the device to helium ions in a helium ion microscope at normal incidence.
  • the intensities of features 3120 are different from the intensities of features 3122.
  • the differences in intensity between features 3120 and 3122 indicate that features 3120 and 3122 have different contacts to other sample elements. More generally, by varying the angle of incidence of the charged particles, differences in image intensity of different features in secondary electron images of a sample can be produced, and these differences provide information about electrical contacts between sample features and other elements of the sample.
  • the angle of incidence of the charged particles can generally be varied as desired to improve contrast in secondary electron images. For example, in certain embodiments, ⁇ can be 5 degrees or more (e.g., 10 degrees or more, 15 degrees or more, 20 degrees or more, 25 degrees or more, 30 degrees or more, 35 degrees or more, 40 degrees or more, 45 degrees or more, 50 degrees or more, 55 degrees or more, 60 degrees or more, 70 degrees or more, or even more).
  • an electrical potential difference can be applied to a sample to improve voltage contrast.
  • the applied potential difference may assist in charge dissipation in certain regions of the sample, for example.
  • voltage source 3014 is in electrical communication with sample 3000, and applies an electrical potential difference to sample 3000.
  • the electrical potential difference has a magnitude of between 0 V and 100 V (e.g., between 0 V and 80 V, between 0 V and 60 V, between 0 V and 40 V, between 0 V and 20 V, between 0 V and 10 V) relative to an electrical potential applied to an extraction electrode of an ion optical system that produces charged particles 3006.
  • FIG. 9 shows results of a simulation of helium ions incident on a silicon sample with an average energy of 30 keV. The ions penetrate into the silicon sample to a depth of 5000 Angstroms or more.
  • FIG. 10 shows a simulation performed for the same silicon sample exposed to incident helium ions with an average energy of 10 keV. Comparing FIGS. 9 and 10, the average penetration depth of the ions at 10 keV is considerably less than at 30 keV.
  • both the lateral spatial (e.g., perpendicular to the direction of incidence of the ions) and the longitudinal spatial (e.g., parallel to the direction of incidence of the ions) distributions of ion trajectories are wider at 30 keV than at 10 keV.
  • the increased penetration depth of incident helium ions as a function of incident energy on a silicon sample can be calculated; an example of such a calculation is shown in FIG. 11. In FIG. 11, the penetration depth of the incident ions scales almost linearly with the ion energy within the range of depths shown.
  • the incident charged particles e.g., helium ions
  • the deeper penetration by the charged particles creates temporary conducting channels that extend between the sample surface and structures - such as circuit elements - that are located below the surface of the sample.
  • the relative electrical isolation of features on the surface of the sample is thereby mitigated, to a certain extent, by the temporary channels.
  • images of the surface features show reduced contrast, because the channels provide pathways for charge transport away from the surface.
  • measured images will include background signal contributions from one or more sub-surface regions of the sample. These sub-surface contributions also reduce contrast in measured images.
  • contributions from sub-surface layers are visible in the image shown in FIG. 3, obtained by exposing the sample to 30 keV helium ions. Such contributions are reduced in the image shown in FIG. 4, obtained by exposing the sample to 10 keV helium ions.
  • sample images measured by exposing the sample to charged particles having average incident energies less than 20 keV typically show greater image contrast (e.g., greater differences in image intensity among particular regions of the sample) than images measured via exposure of the sample to charged particles having average incident energies greater than 20 keV.
  • the penetration depth of the charged particles can be changed. More specifically, at larger angles of incidence, the kinetic energy of the charged particles in a direction normal to the sample surface is reduced, leading to a smaller penetration depth and greater surface charge build-up in the sample. By increasing surface charge build-up in the sample, the secondary electron images at non-zero angles of incidence typically show greater image contrast than images obtained at normal incidence.
  • the particular average incident energy of the charged particles less than 20 keV can be selected according to one or more criteria. Typically, for example, the average energy of the incident charged particles is adjusted by changing one or more electrical potentials applied to components (e.g., ion optical components) in an ion column that is used to extract and/or guide the charged particles.
  • the average incident energy can be selected based on the composition of the sample.
  • Information about the penetration depth of charged particles into particular materials as a function of particle energy can be stored in a database such as a lookup table, for example.
  • a system operator referring to the database, can adjust the incident particle energy based on the composition of the sample.
  • electronic processor 3012 can be configured to automatically adjust the incident charged particle average energy according to the nature of the sample and information stored in the database.
  • the average penetration depth of charged particles into the sample can be determined analytically based on a model of particle interactions with the material of the sample.
  • An example of such a calculation is shown in FIG. 11.
  • a system operator and/or electronic processor 3012 can be configured to select a particular average particle energy based on a calculation of penetration depth.
  • a collection of information contains the penetration depth of charged particles as a function of charged particle primary energy and sample material for various charged particle primary energies and sample materials.
  • the collection may be in the form of two dimensional matrix. For example, each material can be listed in a different row, each primary energy can be listed in a different column, and the respective penetration depths can be listed in corresponding cells of the table.
  • An example of such a table is Table 1.
  • primary energies can be listed in a different rows, materials can be listed in a different columns, and the respective penetration depths can be listed in corresponding cells of the table.
  • a data collection is stored in a memory accessible to electronic processor 3012.
  • such a data collection can be in the form of a look up table readable by a system operator.
  • Such a data collection can be used by a system operator as follows.
  • the system operator has a prior knowledge about: 1) what materials of interest are expected to be present in a sample; and 2) the depth within the sample where each material of interest is expected to be located.
  • This information input to electronic processor 3012, and processor 3012 determines an "optimum" primary energy.
  • an "optimum" primary energy is determined for each material as that primary energy which has a penetration depth corresponding to the depth at which the material is expected to be present in the sample.
  • These "optimum" energies may be the same or different for the different materials.
  • various approaches may be used to then select the primary energy to be used to investigate the sample.
  • the average of the "optimum" energies determined for the different materials may be used to investigate the sample.
  • the largest of the "optimum” energies determined for the different materials may be used to investigate the sample.
  • the lowest of the "optimum” energies determined for the different materials may be used to investigate the sample.
  • electronic controller 3012 proposes this primary energy to the system operator, and the system operator may manually adjust the primary energy by, for example, manipulating the voltage controls (e.g., acceleration voltage) of the ion beam column, to achieve the primary energy.
  • the voltages of focusing lenses may also be manually manipulated to achieve proper focusing of the charged particle beam at the sample.
  • electronic controller 3012 automatically adjusts the system to obtain the primary energy to be used to investigate the sample
  • electronic controller 3012 automatically adjust the primary energy by, for example, manipulating the voltage controls of the ion beam column (e.g., acceleration voltage), to achieve the primary energy.
  • the voltages of focusing lenses may also be manually manipulated to achieve proper focusing of the charged particle beam at the sample.
  • the system operator can fine tune the primary energy to try to further improve the voltage contrast.
  • a data collection may further include an additional piece of information (additional matrix dimension) for each material and primary energy.
  • This additional information may be, for example, the bias voltage applied to the sample or portions of the sample.
  • the bias voltage applied to the sample or portions of the sample there exists a pair of values for the corresponding bias voltage and primary energy. Determining, for a group of materials of interest believed to be in a sample, a bias voltage that provides "optimum" voltage contrast then becomes mathematically a little bit more complicated, but the additional "dimension" of bias voltage provides an additional degree freedom for system adjustment when attempting to optimize voltage contrast.
  • the control system would not only propose or adjust a particular primary particle energy, but also propose or adjust a particular bias voltage to be applied to the sample or portions of the sample. It may be useful when the control software of the system allows the operator to make the choice as to whether to apply bias voltages to particular portions of the sample or not. The control software then can consider the operator's choice, and, when determining the primary energy and bias voltages, the control system only will consider a bias voltage different from a generally given sample voltage for such materials for which the operator has inputted that he will select a bias voltage different form the generally given sample voltage. The operator will have to make his choice and input the respective materials for which he, because of his a priori knowledge about the sample, is confident to be able to apply a respective bias voltage.
  • the average incident particle energy can be determined based on one or more features that appear in sample images. For example, referring to FIG. 3, a subsurface layer is visible in the secondary electron image of the sample that is generated by exposing the device to incident charged particles having energies of 30 keV. Detection of the sub-surface layer arises because of the relatively large penetration depth of the charged particles at energies of 30 keV, as shown in FIG. 9.
  • the sample image shown in FIG. 3 can be monitored, either manually on a display device by a system operator, or automatically by processor 3012.
  • the system operator or processor 3012 can be configured to reduce the average energy of the incident charged particles while measuring additional secondary electron images of the device, until the sub-surface layer no longer appears in the measured images.
  • the images obtained include information about only a relatively thin surface layer of the sample (and the sample features positioned within the layer).
  • the average incident particle energy can be selected based on a target intensity contrast between selected regions in an image of a sample.
  • an image of the sample can be obtained by exposing the sample to incident charged particles, as disclosed herein.
  • Two regions in the image can be selected either manually by a system operator, or automatically by processor 3012.
  • a measured image contrast value can then be calculated as the difference (e.g., percentage difference) between the average image intensities of the two regions.
  • the average incident particle energy can then be adjusted either manually or automatically.
  • another image of the sample can be obtained, and a measured image contrast value for the same two regions can be calculated.
  • Adjustment of the particle energy can be continued until a difference between the measured image contrast value and a target image contrast value (e.g., determined either manually by a system operator or automatically by processor 3012) is less than a tolerance value that can also be selected either manually or automatically.
  • a target image contrast value e.g., determined either manually by a system operator or automatically by processor 3012
  • the average energy of the charged particles can be adjusted until a difference between the average image intensities of the two regions is equal to or greater than the target image contrast value.
  • image contrast can be expressed in a number of ways.
  • image contrast can be expressed as a difference in average intensities between two regions in an image.
  • the image shown in FIG. 1 includes region 3002, which has an associated average intensity (e.g., the mean intensity of all image pixels in region 3002), and region 3004, which also has an associated average intensity (e.g., the mean intensity of all image pixels in region 3004).
  • the image contrast can be expressed as the difference between the average intensities of regions 3002 and 3004.
  • the difference between the average intensities can be expressed as a positive difference (e.g., by taking the absolute value of the difference).
  • the image contrast can be expressed as a percentage value.
  • the image contrast between regions 3002 and 3004 can be expressed as the difference between the average intensities of regions 3002 and 3004, divided by the average intensity of either region 3002 or 3004, depending upon which of the two regions is selected as a baseline value for comparative purposes.
  • the image contrast can be expressed as the difference between the average intensities of regions 3002 and 3004, divided by a baseline intensity value selected either automatically or by a system operator.
  • the baseline intensity value can, in some embodiments, be derived from a particular region of the image (e.g., a background region of the image).
  • the difference between the average intensities of regions 3002 and 3004 can be expressed as a positive difference (e.g., by taking the absolute value of the difference).
  • the various methods disclosed herein can also be implemented by adjusting the angle of incidence of the charged particles. That is, the angle of incidence of the charged particles can be adjusted to increase contrast among particular regions of the sample in secondary electron images. Moreover, the foregoing methods can also be implemented by adjusting both the average energy and the angle of incidence of the charged particles. Adjustment of the angle of incidence of the charged particles can be achieved by changing one or more electrical potentials applied to charged particle lens and/or deflector elements positioned within a charged particle column where the particles are generated, for example, and/or by tilting a stage on which the sample is positioned, relative to the charged particle column.
  • Ion Beam Systems This section discloses systems and methods for producing ion beams, and detecting particles including secondary electrons that leave a sample of interest due to exposure of the sample to an ion beam. The systems and methods can be used to obtain one or more images of the sample.
  • gas ion beams that are used to interrogate samples are produced in multipurpose microscope systems.
  • Microscope systems that use a gas field ion source to generate ions that can be used in sample analysis (e.g., imaging) are referred to as gas field ion microscopes.
  • a gas field ion source is a device that includes an electrically conductive tip (typically having an apex with 10 or fewer atoms) that can be used to ionize neutral gas species to generate ions (e.g., in the form of an ion beam) by bringing the neutral gas species into the vicinity of the electrically conductive tip (e.g., within a distance of about four to five angstroms) while applying a high positive potential (e.g., one kV or more relative to the extractor (see discussion below)) to the apex of the electrically conductive tip.
  • a high positive potential e.g., one kV or more relative to the extractor (see discussion below)
  • FIG. 12 shows a schematic diagram of a gas field ion microscope system 100 that includes a gas source 110, a gas field ion source 120, ion optics 130, a sample manipulator 140, a front-side detector 150, a back-side detector 160, and an electronic control system 170 (e.g., an electronic processor, such as a computer) electrically connected to various components of system 100 via communication lines 172a-172f.
  • a sample 180 is positioned in/on sample manipulator 140 between ion optics 130 and detectors 150, 160.
  • an ion beam 192 is directed through ion optics 130 to a surface 181 of sample 180, and particles 194 resulting from the interaction of ion beam 192 with sample 180 are measured by detectors 150 and/or 160.
  • gas field ion source 120 can be maintained at a pressure of approximately 10 "10 Torr.
  • the background pressure rises to approximately 10 "5 Torr.
  • Ion optics 130 are maintained at a background pressure of approximately 10 "8 Torr prior to the introduction of gas into gas field ion source 120.
  • the background pressure in ion optics 130 typically increase to Torr.
  • Sample 180 is positioned within a chamber that is typically maintained at a background pressure of approximately 10 ⁇ - " 6 Torr.
  • gas source 110 is configured to supply one or more gases 182 to gas field ion source 120.
  • gas source 110 can be configured to supply the gas(es) at a variety of purities, flow rates, pressures, and temperatures.
  • at least one of the gases supplied by gas source 110 is a noble gas (helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe)), and ions of the noble gas are desirably the primary constituent in ion beam 192.
  • the current of ions in ion beam 192 increases monotonically as the pressure of the noble gas in system 100 increases.
  • this relationship can be described by a power law where, for a certain range of noble gas pressures, the current increases generally in proportion to gas pressure.
  • the pressure of the noble gas is typically 10 "2 Torr or less (e.g., 10 "3 Torr or less, 10 "4 Torr or less), and/or 10 "7 Torr or more (e.g., 10 "6 Torr or more, 10 "5 Torr or more) adjacent the tip apex (see discussion below).
  • the He can be at least 99.99% pure (e.g., 99.995% pure, 99.999% pure, 99.9995% pure, 99.9999% pure).
  • other noble gases Ne gas, Ar gas, Kr gas, Xe gas
  • the purity of the gases is desirably high purity commercial grade.
  • gas source 110 can supply one or more gases in addition to the noble gas(es).
  • an example of such a gas is nitrogen.
  • the additional gas(es) can be present at levels above the level of impurities in the noble gas(es), the additional gas(es) still constitute minority components of the overall gas mixture introduced by gas source 110.
  • the overall gas mixture can include 20% or less (e.g., 15% or less, 12% or less) Ne, and/or 1% or more (e.g., 3% or more, 8% or more) Ne.
  • the overall gas mixture can include from 5% to 15% (e.g., from 8% to 12%, from 9% to 11%) Ne.
  • the overall gas mixture can include 1% or less (e.g., 0.5% or less, 0.1% or less) nitrogen, and/or 0.01% or more (e.g., 0.05% or more) nitrogen.
  • the overall gas mixture can include from 0.01% to 1% (e.g., from 0.05% to 0.5%, from 0.08 to 0.12%) nitrogen.
  • the additional gas(es) are mixed with the noble gas(es) before entering system 100 (e.g., via the use of a gas manifold that mixes the gases and then delivers the mixture into system 100 through a single inlet).
  • the additional gas(es) are not mixed with the noble gas(es) before entering system 100 (e.g., a separate inlet is used for inputting each gas into system 100, but the separate inlets are sufficiently close that the gases become mixed before interacting with any of the elements in gas field ion source 120).
  • Gas field ion source 120 is configured to receive the one or more gases 182 from gas source 110 and to produce gas ions from gas(es) 182.
  • Gas field ion source 120 includes an electrically conductive tip 186 with a tip apex 187, an extractor 190 and optionally a suppressor 188.
  • the distance from tip apex 187 to surface 181 of sample 180 is five cm or more (e.g., 10 cm or more, 15 cm or more, 20 cm or more, 25 cm or more), and/or 100 cm or less (e.g., 80 cm or less, 60 cm or less, 50 cm or less).
  • the distance from tip apex 187 to surface 181 of sample 180 is from five cm to 100 cm (e.g., from 25 cm to 75 cm, from 40 cm to 60 cm, from 45 cm to 55 cm).
  • Electrically conductive tip 186 can be formed of various materials.
  • tip 186 is formed of a metal (e.g., tungsten (W), tantalum (Ta), iridium (Ir), rhenium (Rh), niobium (Nb), platinum (Pt), molybdenum (Mo)).
  • electrically conductive tip 186 can be formed of an alloy.
  • electrically conductive tip 186 can be formed of a different material (e.g., carbon (C)).
  • tip 186 is biased positively (e.g., approximately 20 kV) with respect to extractor 190
  • extractor 190 is negatively or positively biased (e.g., from -20 kV to +50 kV) with respect to an external ground
  • optional suppressor 188 is biased positively or negatively
  • tip 186 is formed of an electrically conductive material, the electric field of tip 186 at tip apex 187 points outward from the surface of tip apex 187. Due to the shape of tip 186, the electric field is strongest in the vicinity of tip apex 187. The strength of the electric field of tip 186 can be adjusted, for example, by changing the positive voltage applied to tip 186. With this configuration, un-ionized gas atoms 182 supplied by gas source 110 are ionized and become positively-charged ions in the vicinity of tip apex 187.
  • the positively-charged ions are simultaneously repelled by positively charged tip 186 and attracted by negatively charged extractor 190 such that the positively-charged ions are directed from tip 186 into ion optics 130 as ion beam 192.
  • Suppressor 188 assists in controlling the overall electric field between tip 186 and extractor 190 and, therefore, the trajectories of the positively-charged ions from tip 186 to ion optics 130.
  • the overall electric field between tip 186 and extractor 190 can be adjusted to control the rate at which positively-charged ions are produced at tip apex 187, and the efficiency with which the positively-charged ions are transported from tip 186 to ion optics 130.
  • He ions can be produced as follows.
  • Gas field ion source 120 is configured so that the electric field of tip 186 in the vicinity of tip apex 187 exceeds the ionization field of the un-ionized He gas atoms 182, and tip 186 is maintained at a relatively low temperature.
  • the He atoms can be polarized by the electric field of the tip, producing a weakly attractive force between He atoms 182 and tip apex 187.
  • He atoms 182 may contact tip apex 187 and remain bound (e.g., physisorbed) thereto for some time.
  • the electric field is high enough to ionize He atoms 182 adsorbed onto tip apex 187, generating positively charged He ions (e.g., in the form of an ion beam).
  • ion optics 130 are configured to direct ion beam 192 onto surface 181 of sample 180. As described in more detail below, ion optics 130 can, for example, focus, collimate, deflect, accelerate, and/or decelerate ions in beam 192.
  • Ion optics 130 can also allow only a portion of the ions in ion beam 192 to pass through ion optics 130.
  • ion optics 130 include a variety of electrostatic and other ion optical elements that are configured as desired.
  • electrostatic deflectors By manipulating the electric field strengths of one or more components (e.g., electrostatic deflectors) in ion optics 130, He ion beam 192 can be scanned across surface 181 of sample 180.
  • ion optics 130 can include two deflectors that deflect ion beam 192 in two orthogonal directions. The deflectors can have varying electric field strengths such that ion beam 192 is rastered across a region of surface 181.
  • Detectors 150 and 160 are positioned and configured to each measure one or more different types of particles resulting from the interaction between He ion beam 192 and sample 180. As shown in FIG. 12, detector 150 is positioned to detect particles 194 that originate primarily from surface 181 of sample 180, and detector 160 is positioned to detect particles 194 that emerge primarily from surface 183 of sample 180 (e.g., transmitted particles).
  • any number and configuration of detectors can be used in the microscope systems disclosed herein.
  • multiple detectors are used, and some of the multiple detectors are configured to measure different types of particles.
  • the detectors are configured to provide different information about the same type of particle (e.g., energy of a particle, angular distribution of a given particle, total abundance of a given particle).
  • combinations of such detector arrangements can be used.
  • the information measured by the detectors is used to determine information about sample 180.
  • this information is determined by obtaining one or more images of sample 180.
  • pixel-by-pixel information about sample 180 can be obtained in discrete steps.
  • Detectors 150 and/or 160 can be configured to detect one or more different types of particles 194 at each pixel.
  • a pixel is a square, although in some embodiments, pixels can have different shapes (e.g., rectangular).
  • a pixel size, which corresponds to a length of a side of the pixel, can be, for example, from 100 pm to two ⁇ m (e.g., from one nm to one ⁇ m).
  • the location of adjacent pixels can be determined to within at least 200 pm (e.g., to within at least 100 pm, to within at least 75 pm, to within at least 50 pm).
  • the operator of the system can determine the location of the center of the beam spot to within at least 200 pm (e.g., to within at least 100 pm, to within at least 75 pm, to within at least 50 pm).
  • the field of view (FOV) of sample 180 is 200 nm or more (e.g., 500 nm or more, l ⁇ m or more, 50 ⁇ m or more, 100 ⁇ m or more, 500 ⁇ m or more, 1 mm or more, 1.5 mm or more), and/or 25 mm or less (15 mm or less, 10 mm or less, five mm or less).
  • the field of view refers to the area of a sample surface that is imaged by the ion microscope.
  • microscope system 100 is typically controlled via electronic control system 170.
  • electronic control system 170 can be configured to control the gas(es) supplied by gas source 110, the temperature of tip 186, the electrical potential of tip 186, the electrical potential of extractor 190, the electrical potential of suppressor 188, the settings of the components of ion optics 130, the position of sample manipulator 140, and/or the location and settings of detectors 150 and 160.
  • one or more of these parameters may be manually controlled (e.g., via a user interface integral with electronic control system 170).
  • electronic control system 170 can be used (e.g., via an electronic processor, such as a computer) to analyze the information collected by detectors 150 and 160 and to provide information about sample 180 (e.g., topography information, material constituent information, crystalline information, voltage contrast information, optical property information, magnetic information ), which can optionally be in the form of an image, a graph, a table, a spreadsheet, or the like.
  • electronic control system 170 includes a user interface that features a display or other kind of output device, an input device, and a storage medium.
  • Electronic control system 170 can also be configured to control operation of other devices in microscope system 100.
  • electronic control system 170 can control heating of the sample by controlling operation of a laser source that is configured to heat the sample.
  • control system 170 can control operation of an electron source that is configured to heat the sample.
  • control system 170 can control operation of a heating element (e.g., a resistive heating element) that can be used to heat the sample.
  • electronic control system 170 can be configured to control various properties of ion beam 192.
  • control system 170 can control a composition of ion beam 192 by regulating the flow of gases into gas field ion source 120.
  • control system 170 can control other properties of ion beam 192 such as the position of the ion beam on sample 180, and the average energy of the incident ions.
  • electronic control system 170 can be configured to control one or more additional particle beams.
  • one or more types of ion beam source and/or electron beam sources can be present.
  • Control system 170 can control each of the particle beam sources and their associated optical and electronic components.
  • Detectors 150 and 160 are depicted schematically in FIG. 12, with detector 150 positioned to detect particles from surface 181 of sample 180 (the surface on which the ion beam impinges), and detector 160 positioned to detect particles from surface 183 of sample 180.
  • a wide variety of different detectors can be employed in microscope system 200 to detect different particles, and a microscope system 200 can typically include any desired number of detectors.
  • the configuration of the various detector(s) can be selected in accordance with particles to be measured and the measurement conditions.
  • a spectrally resolved detector may be used.
  • Such detectors are capable of detecting particles of different energy and/or wavelength, and resolving the particles based on the energy and/or wavelength of each detected particles.
  • a spectrally resolved detector includes components capable of directing particles to different regions of the detector based on the energy and/or wavelength of the particle. Detection systems and methods are generally disclosed, for example, in U.S. Patent Application Serial No. 11/600,711.
  • detectors 150 and/or 160 can include any one or more of the following detector types: Everhart-Thornley (ET) detectors, which can be used to detect secondary electrons, ions, and/or neutral particles; microchannel plate detectors, which can be used to amplify a flux of secondary electrons, neutral atoms, or ions from a sample; conversion plates, which can be used to detect ions (e.g., scattered ions, secondary ions) from a sample or neutral particles (e.g., primary neutral He atoms) from the sample; channeltron detectors, which can be used to detect particles such as electrons, ions and neutral atoms leaving a sample; phosphor- based detectors, which include a thin layer of a phosphor material deposited atop a transparent substrate, and a photon detector such as a CCD camera, a PMT, or one or more diodes, and which can be used to detect electrons, ions and/or neutral particles from a sample; solid state detectors, which can be used
  • any of the methods described above can be implemented in computer hardware or software, or a combination of both.
  • the methods can be implemented in computer programs using standard programming techniques following the methods and figures described herein.
  • Program code is applied to input data to perform the functions described herein and generate output information.
  • the output information is applied to one or more output devices such as a display monitor.
  • Each program may be implemented in a high level procedural or object oriented programming language to communicate with a computer system.
  • the programs can be implemented in assembly or machine language, if desired. In any case, the language can be a compiled or interpreted language.
  • the program can run on dedicated integrated circuits preprogrammed for that purpose.
  • Each such computer program is preferably stored on a storage medium or device (e.g., ROM or magnetic diskette) readable by a general or special purpose programmable computer, for configuring and operating the computer when the storage media or device is read by the computer to perform the procedures described herein.
  • the computer program can also reside in cache or main memory during program execution.
  • the methods can also be implemented as a computer-readable storage medium, configured with a computer program, where the storage medium so configured causes a computer to operate in a specific and predefined manner to perform the functions described herein.

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  • Chemical & Material Sciences (AREA)
  • Analytical Chemistry (AREA)
  • Analysing Materials By The Use Of Radiation (AREA)

Abstract

L'invention porte sur des procédés et des systèmes qui comprennent l'exposition d'un échantillon à une pluralité de particules chargées ayant une énergie moyenne de 20 keV ou moins, la détection de particules qui quittent une surface de l'échantillon en réponse à la pluralité de particules chargées, et la formation d'une image de l'échantillon sur la base des particules détectées.
PCT/US2010/032695 2009-05-01 2010-04-28 Imagerie de contraste de tension WO2010126958A1 (fr)

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Citations (2)

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Publication number Priority date Publication date Assignee Title
US20060037182A1 (en) * 2003-02-28 2006-02-23 Roy Erwan L Method and apparatus for the improvement of material/voltage contrast
US20070138388A1 (en) * 2003-10-16 2007-06-21 Ward Billy W Ion sources, systems and methods

Patent Citations (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20060037182A1 (en) * 2003-02-28 2006-02-23 Roy Erwan L Method and apparatus for the improvement of material/voltage contrast
US20070138388A1 (en) * 2003-10-16 2007-06-21 Ward Billy W Ion sources, systems and methods

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