WO2009155275A1

WO2009155275A1 - Sample imaging with charged particles

Info

Publication number: WO2009155275A1
Application number: PCT/US2009/047478
Authority: WO
Inventors: John Morgan; Lawrence Scipioni; Dirk Aderhold; Christoph Riedesel; Rainer Knippelmeyer; Ulrich Mantz; Wolfgang Singer
Original assignee: Carl Zeiss Smt. Inc.
Priority date: 2008-06-20
Filing date: 2009-06-16
Publication date: 2009-12-23

Abstract

Disclosed herein are methods that include: (a) exposing a cross-sectional surface (5010) of a channel (5020) formed in a sample (5000) to particles from a charged particle source (5005) to cause a first plurality of particles (5035) to leave the cross-sectional surface (5010), and determining a position of a reference mark on the cross-sectional surface (5010) based on the first plurality of particles (5035); (b) registering a coordinate system of the charged particle source (5005) relative to the position of the reference mark; and (c) exposing the cross-sectional surface (5010) to additional particles from the charged particle source (5005) to cause a second plurality of particles to leave the cross-sectional surface, and forming multiple images of the cross-sectional surface (5010) based on the second plurality of particles. After formation of at least one of the multiple images, the coordinate system of the charged particle source (5005) is registered again relative to the position of the reference mark prior to forming the next one of the multiple images.

Description

SAMPLE IMAGING WITH CHARGED PARTICLES

TECHNICAL FIELD

This disclosure relates to exposing samples to charged particles, and in particular, to imaging samples with charged particles.

BACKGROUND

Samples can be exposed to charged particles for a variety of applications, including sample imaging. Charged particle beams can experience relative drift over time with respect to samples that are exposed to the particle beams.

SUMMARY

In general, in a first aspect, the disclosure features a method that includes: (a) exposing a cross-sectional surface of a channel formed in a sample to particles from a charged particle source to cause a first plurality of particles to leave the cross-sectional surface, and determining a position of a reference mark on the cross-sectional surface based on the first plurality of particles; (b) registering a coordinate system of the charged particle source relative to the position of the reference mark; and (c) exposing the cross-sectional surface to additional particles from the charged particle source to cause a second plurality of particles to leave the cross- sectional surface, and forming multiple images of the cross-sectional surface based on the second plurality of particles. After formation of at least one of the multiple images, the coordinate system of the charged particle source is registered again relative to the position of the reference mark prior to forming the next one of the multiple images.

Ln another aspect, the disclosure features a method that includes: (a) exposing a cross- sectional surface of a channel formed in a sample to particles from a first charged particle source to cause a first plurality of particles to leave the cross-sectional surface, and determining a position of a reference mark on the cross-sectional surface based on the first plurality of particles; (b) registering a coordinate system of a second charged particle source relative to the position of the reference mark; and (c) exposing the cross-sectional surface to particles from the second charged particle source to cause a second plurality of particles to leave the cross-sectional surface, and forming multiple images of the cross-sectional surface based on the second plurality of particles. After formation of at least one of the multiple images, the coordinate system of the second charged particle source is registered again relative to the position of the reference mark prior to forming the next one of the multiple images.

In a further aspect, the disclosure features a method that includes: (a) exposing a cross- sectional surface of a channel formed in a sample to particles from a charged particle source to cause a first plurality of particles to leave the cross-sectional surface, and determining a position of a reference mark on the cross-sectional surface based on the first plurality of particles; (b) registering a coordinate system of the charged particle source relative to the position of the reference mark; and (c) exposing the cross-sectional surface to additional particles from the charged particle source to cause a second plurality of particles to leave the cross-sectional surface, and forming an image of the cross-sectional surface based on the second plurality of particles. During formation of the image, the coordinate system of the charged particle source is registered again relative to the position of the reference mark.

Embodiments can include one or more of the following features. The charged particle source can be an ion beam source. Alternatively, or in addition, the charged particle source can be an electron beam source.

The particles from the charged particle source can include ions. The particles from the charged particle source can include noble gas ions. The particles from the charged particle source can include helium ions. The reference mark can include a defect in the cross-sectional surface. Alternatively, or in addition, the reference mark can include a mark formed in the cross-sectional surface by an ion beam. Prior to exposing the cross-sectional surface to particles from the charged particle source, the cross-sectional surface can be exposed to an ion beam that forms the reference mark in the cross-sectional surface. The ion beam can include gallium ions. Registering the coordinate system of the charged particle source can include determining a coordinate transformation between a coordinate system of the charged particle source and the position of the reference mark. The coordinate transformation can include a translation vector.

Determining the position of the reference mark can include forming a reference image of the cross-sectional surface based on the first plurality of particles, and locating the reference mark in the reference image. The reference image can be formed at a first magnification of the cross-sectional surface, and the multiple images can be formed at a second magnification of the cross-sectional surface larger than the first magnification. A field of view of the reference image can have a minimum dimension of at least one micron (e.g., at least five microns, at least ten microns). A maximum dimension of the field of view of the multiple images can be less than one micron (e.g., less than 500 nanometers, less than 100 nanometers). Registering the coordinate system of the charged particle source again relative to the position of the reference mark can include determining a coordinate transformation between a coordinate system of the charged particle source and the position of the reference mark.

The coordinate system of the charged particle source can be registered relative to the position of the reference mark prior to forming each one of the multiple images. The coordinate system of the charged particle source can be registered relative to the position of the reference mark during formation of at least one of the multiple images.

The coordinate system of the charged particle source can be registered relative to the position of the reference mark during formation of each of the multiple images.

The reference mark can be a first reference mark, and the method can include determining a position of a second reference mark on the cross-sectional surface based on the first plurality of particles. Registering the coordinate system of the charged particle source relative to the position of the reference mark can include determining an orientation angle of the coordinate system relative to the first and second reference marks. The second reference mark can include a defect in the cross-sectional surface. Alternatively, or in addition, the second reference mark can include a mark formed in the cross-sectional surface by an ion beam.

The method can include selecting a region of interest on the cross-sectional surface relative to the reference mark. The region of interest can be selected manually from a reference image that is formed based on the first plurality of particles. Alternatively, or in addition, the region of interest can be selected automatically based on reference information for the cross- sectional surface. The reference information can include one or more images of the cross- sectional surface. Alternatively, or in addition, the reference information can include design specifications for the cross-sectional surface.

The multiple images can correspond to the region of interest on the cross-sectional surface. The method can include determining a dimension of a feature on the cross-sectional surface relative to the reference mark. Determining the dimension can include determining a first edge position of the feature relative to the reference mark, determining a second edge position of the feature relative to the reference mark, and calculating a difference between the first and second edge positions to determine the distance. The coordinate system of the charged particle source can be registered relative to the position of the reference mark prior to determining the first edge position and again prior to determining the second edge position. The position of the reference mark can be determined prior to determining the first edge position and again prior to determining the second edge position.

The method can include correcting image distortion in at least one of the multiple images by determining positions of one or more features on the cross-sectional surface relative to the reference mark. The one or more features can include additional reference marks. Alternatively, or in addition, the one or more features can include edge positions of portions of the cross- sectional surface that are formed from different materials. The distortion can arise due to variations in magnification in the at least one of the multiple images.

The second plurality of particles can include secondary electrons. Alternatively, or in addition, the second plurality of particles can include scattered ions (e.g., helium ions). Alternatively, or in addition, the second plurality of particles can include secondary ions.

The method can include determining a position of an additional reference mark on the cross-sectional surface, and assigning an origin of a coordinate system of the cross-sectional surface to coincide with the position of the additional reference mark. The particles from the first charged particle source can include a first type of ions, and the particles from the second charged particle source can include a second type of ions different from the first type of ions.

The particles from the first charged particle source can include electrons and the particles from the second charged particle source can include helium ions. Registering the coordinate system of the second charged particle source can include determining a coordinate transformation between a coordinate system of the second charged particle source and the position of the reference mark. Registering the coordinate system of the second charged particle source again relative to the position of the reference mark can include determining a coordinate transformation between a coordinate system of the second charged particle source and the position of the reference mark. The coordinate system of the second charged particle source can be registered relative to the position of the reference mark prior to forming each one of the multiple images. The coordinate system of the second charged particle source can be registered relative to the position of the reference mark during formation of at least one of the multiple images. The coordinate system of the second charged particle source can be registered relative to the position of the reference mark during formation of each of the multiple images.

Registering the coordinate system of the second charged particle source relative to the position of the reference mark can include determining an orientation angle of the coordinate system relative to the first and second reference marks. The second plurality of particles can include secondary electrons and/or scattered ions

(e.g., helium ions) and/or secondary ions.

Forming the image of the cross-sectional surface can include storing partial image data in an electronic memory, registering the coordinate system of the charged particle source again relative to the position of the reference mark, retrieving the stored partial image data, and completing formation of the image of the surface.

The method can include forming a plurality of images of the cross-sectional surface, where during formation of each one of the plurality of images, the coordinate system of the charged particle source is registered again relative to the position of the reference mark.

The method can include forming a plurality of images of the cross-sectional surface, where during formation of more than one of the plurality of images, the coordinate system of the charged particle source is registered again relative to the position of the reference mark.

The coordinate system of the charged particle source can be registered again relative to the position of the reference mark after each one of a plurality of exposures of the cross-sectional surface to particles from the charged particle source, where each one of the plurality of exposures corresponds to a line scan of a portion of the cross-sectional surface.

The coordinate system of the charged particle source can be registered again relative to the position of the reference mark after each one of a plurality of exposures of the cross-sectional surface to particles from the charged particle source, where each one of the plurality of exposures corresponds to a point exposure of a portion of the cross-sectional surface. The coordinate system of the charged particle source can be registered again relative to the position of the reference mark after each one of a plurality of exposures of the cross-sectional surface to particles from the charged particle source, where each one of the plurality of exposures corresponds to a plurality of point exposures of a portion of the cross-sectional surface.

Embodiments can include one or more of the following advantages.

In some embodiments, registering the coordinate system of the charged particle source relative to a reference mark on a sample reduces positioning errors of a charged particle beam prior to sample imaging. Repeating the registering of the coordinate system of the charged particle source during sample imaging can reduce positioning errors of the charged particle beam that could introduce artifacts in sample images. hi certain embodiments, a first image of a sample can be obtained at relatively low resolution, and used to register a coordinate system of a charged particle source. The low resolution image can be obtained more quickly than a high resolution image of the same sample, and can provide adequate registration of the coordinate system without introducing significant delays in sample imaging. hi some embodiments, a first relatively low resolution image of sample can be used to select a region of interest, and then subsequent images of the region of interest can be obtained at higher resolution. By selecting one or more regions of interest based on low resolution images, the overall imaging process is faster than if the regions of interest were selected based on higher resolution images which take longer to obtain. hi certain embodiments, positions of multiple features on a sample surface can be determined relative to a reference mark on the sample, and the positions can be used to correct for image distortion. After correction for image distortions, sample images more faithfully represent the actual sample surface, and permit more accurate quantitative measurements of positions of features and distances between features on sample surfaces.

The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features and advantages will be apparent from the description, drawings, and claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing a system for imaging a sample that is exposed to a charged particle beam. FIG. 2 is a schematic diagram showing a sample that includes multiple sub-surface features.

FIG. 3 is a cross-sectional diagram of the sample in FIG. 2.

FIG. 4 is a flow chart that shows steps in a coordinate system registration protocol. FIG. 5 is a cross-sectional diagram of a sample that includes two reference marks.

FIG. 6 is a cross-sectional diagram of a sample that shows measurement of a dimension of a sub-surface feature of the sample.

FIG. 7 is a cross-sectional image of a sample that includes four reference marks used to correct for image distortion. FIG. 8 is a distorted cross-sectional image of the sample of FIG. 7.

FIG. 9 is a foreshortened cross-sectional image of the sample of FIG. 7.

FIG. 10 is a schematic diagram showing a sample imaging system that includes two charged particle beam sources.

FIG. 11 is a schematic diagram of an ion microscope system. FIG. 12 is a schematic diagram of a gas field ion source.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

During sample imaging with a charged particle beam, drift of the coordinate system of the charged particle beam with respect to the sample can lead to imaging artifacts such as blurring, subjecting quantitative measurements of sample features from images to potential inaccuracies. By registering (and, in some embodiments, re-registering) the coordinate system of the charged particle beam relative to one or more reference positions on the sample, the effects of drift can be mitigated. The following discussion focuses on imaging of cross-sectional surfaces of samples.

However, the methods and systems disclosed herein can also be applied to imaging of other (e.g., non-cross-sectional) sample surfaces. The methods and systems are generally applicable to all sample imaging schemes where drift of the coordinate system of a charged particle source (or of another type of source) is to be mitigated. This disclosure is divided into two sections. The first section discloses methods and systems for compensating drift of charged particle beams with respect to samples during sample imaging processes. The section portion discloses ion beam systems that can be used to expose and image samples.

I. Compensating Drift of Charged Particle Beams FIG. 1 shows a schematic diagram of a sample 5000 that is imaged by a charged particle beam 5015 from a charged particle source 5005. Charged particle source 5005 receives control signals and instructions from an electronic controller 5002. A channel 5020 is formed in sample 5000, and includes a cross-sectional surface 5010. Cross-sectional surface 5010 exposes interior portions of sample 5000 which are positioned below surface 5025. Charged particle beam 5015 is incident on cross-sectional surface 5010, and causes a plurality of particles 5035 to leave surface 5010. Particles 5035 are captured by detector 150, and one or more images of surface 5010 are formed based on the captured particles.

FIG. 2 shows a top view of sample 5000. Sample 5000 includes features 5110, 5120, and 5130. Feature 5120 is positioned at surface 5025 of sample 5000, and is visible in FIG. 2. Feature 5120 overlies sub-surface feature 5130, which is positioned below surface 5025 and is shown using dotted lines. Sample 5000 also includes another sub-surface feature 5110, which is positioned below surface 5025, and contacts but does not overlie feature 5130.

Channel 5020 can be formed in surface 5025 by one or more processes including, for example, ion or electron beam milling, etching, and/or sputtering. In some embodiments, channel 5020 is formed using a focused ion beam, such as a focused Ga ion beam.

Cross-sectional surface 5010 corresponds approximately to a surface formed by making a cut along section line A-A in FIG. 2. The cross-sectional surface 5010 exposed by sectioning along line A-A is shown in FIG. 3. Features 5110, 5120, and 5130 are shown in cross-section, with features 5110 and 5130 positioned entirely beneath surface 5025. Surface 5010 also includes a reference mark 5140 formed in the surface. Features 5110, 5120, and 5130 can be imaged by directing charged particle beam 5015 to be incident on surface 5010, and by detecting particles that leave surface 5010 in response to incident particle beam 5015.

To mitigate the effects of drift of charged particle beam 5015 relative to the positions of features 5110, 5120, and 5130 on cross-sectional surface 5010, the coordinate system of charged particle source 5005 can be registered relative to reference mark 5140 on surface 5010. FIG. 4 is a flow chart 5200 that shows various steps in a protocol for registering the coordinate system of charged particle source 5005 to reference mark 5140. In step 5210, a first image of surface 5010 is measured by detecting particles 5035 that leave surface 5010 during exposure of surface 5010 to charged particle beam 5015. Typically, the first image is obtained at a first magnification Mi of surface 5010, and can include the entire surface 5010, or only a portion thereof. In step 5220, reference mark 5140 on surface 5010 is located in the first image, and the coordinates of mark 5140 in the coordinate system of charged particle source 5005 are determined. This procedure corresponds to a first registration of the coordinate system of charged particle source 5005 relative to mark 5140.

Then, in step 5230, a second image of surface 5010 (or a portion thereof) is measured by directing charged particle beam 5015 to be incident on surface 5010, and detecting particles 5035 that leave surface 5010 under the influence of charged particle beam 5015. The second image can be a complete image of surface 5010, or an image of only a portion of surface 5010. Typically, the second image is obtained at a magnification M₂ of surface 5010.

After the second image has been obtained, the position of reference mark 5140 in the second image (e.g., in the coordinate system of charged particle source 5005) is determined in step 5240. If the position of reference mark 5140 determined in step 5240 differs from the position of reference mark 5140 determined in step 5220, then in step 5250, a coordinate transformation is calculated between the previous coordinates of the reference mark and the current coordinates of the reference mark. Typically, for example, the coordinate transformation includes a translation vector that corresponds to the displacement of the reference mark in the coordinate system of charged particle source 5005 over time. In effect, the coordinate transformation represents drift of charged particle source 5005 with respect to the nominally fixed positions of features on surface 5010.

In step 5260, if a non-zero coordinate transformation was calculated in step 5250, then the coordinate system of charged particle source 5005 is adjusted based on the calculated coordinate transformation. Adjustment of the coordinate system can include hardware-based and/or software-based corrections to the coordinate system. For example, in some embodiments, hardware-based corrections are applied by altering operating parameters of charged particle source 5005 to implement adjustments to the coordinate system. Operating parameters that can be altered include voltages applied to one or more charged particle optical elements (e.g., lenses, deflectors) of charged particle source 5005, for example. In certain embodiments, software-based corrections are applied by altering - in software - coordinate values that are reported by the charged particle source. For example, a displacement vector can be added electronically to coordinate positions reported by a electronic controller 5002 to account for the coordinate transformation calculated in step 5250. The electronically implemented displacement vector adjusts the reported position coordinates so that they correspond with the drift of charged particle source 5005 relative to surface 5010.

In some embodiments, both hardware- and software-based corrections can be applied to charged particle source 5005. The selection of which type of corrections to apply depends upon a number of factors including, for example, the ease with which hardware- and software-based alterations can be made to charged particle source 5005.

In step 5270, a decision is made as to whether sample imaging is complete. If imaging has been completed, then the protocol described by flow chart 5200 terminates at step 5280. However, if imaging is not yet complete (e.g., if only one image among a set of images has been acquired and/or if an image has only partially been acquired), then control returns to step 5230, where another image of surface 5010 (or a portion thereof) is obtained.

The re-registration procedure of the coordinate system of charged particle source 5005 relative to reference mark 5140, described by steps 5240, 5250, and 5260 of flow chart 5200, can be performed one or more times during sample imaging. For example, in some embodiments, only a single image at magnification M₂ is obtained and during measurement of the single image at magnification M₂, image measurement is interrupted one or more times to re-register the coordinate system of charged particle source 5005 relative to reference mark 5140. Between re- registrations, only a portion of the single image at magnification M₂ is acquired. Typically, for example, the acquired portion of the image (e.g., the cumulative acquired portion) is stored in an electronic memory while re-registration occurs. Thereafter, the acquired portion of the image is retrieved from memory, and image acquisition resumes.

In some embodiments, re-registration of the coordinate system of charged particle source 5005 is performed based not on one or more images acquired at magnification M₂, but on another image acquired at Mj. For example, following step 5230 in flow chart 5200 (and prior to step 5240), another image of surface 5010 (or a portion thereof) is acquired at magnification Mj. From the newly acquired image, the position coordinates of the reference mark are determined according to step 5240, and a coordinate transformation (e.g., translation vector) corresponding to the drift of charged particle source 5005 with respect to sample 5000 is determined, as in step 5250.

In certain embodiments, multiple images of surface 5010 are acquired, at least one of which is at magnification M₂. During acquisition of the multiple images, imaging can be interrupted following complete acquisition of at least some of the multiple images to re-register the coordinate system of charged particle source 5005 relative to reference mark 5140 prior to acquisition of the next of the multiple images. Alternatively, or in addition, imaging can be interrupted following partial acquisition of at least some of the multiple images to re-register the coordinate system of charged particle source 5005. For example, in some embodiments, the coordinate system of charged particle source

5005 can be re-registered after each one of a plurality of exposures of surface 5010 to charged particle beam 5015. Some or all of the plurality of exposures can correspond, for example, to a line scan of a portion of surface 5010. Alternatively, or in addition, some or all of the plurality of exposures can correspond to point exposures of portions of surface 5010 to charged particle beam 5015. Between some (or all) of the exposures of surface 5010 to charged particle beam 5015, the coordinate system of charged particle source 5005 can be re-registered relative to reference mark 5140 on surface 5010.

Reference mark 5140 can include one or more of a variety of different types of marks on surface 5010. For example, in some embodiments, reference mark 5140 corresponds to a defect or other imperfection on surface 5010 that is visible in images of surface 5010. hi certain embodiments, reference mark 5140 is applied to surface 5010 after surface 5010 has been exposed. For example, prior to initiating imaging of surface 5010, the surface can be exposed to a charged particle beam such as an ion beam (e.g., a gallium ion beam) and/or an electron beam to form one or more reference marks in surface 5010. Reference marks formed in surface 5010 can have a wide variety of shapes including, for example, points, lines, crosses, diamonds, and other recognizable structural shapes.

In some embodiments, the protocol shown in flow chart 5200 can include an additional step of establishing a coordinate system on surface 5010. Establishing the coordinate system can include assigning known coordinate values to a particular position on surface 5010. For example, following determination of the position coordinates of reference mark 5140, a new surface coordinate system can be established in which reference mark 5140 is assigned to a known location such as the origin, e.g., (0,0), of the surface coordinate system.

In general, one or more images can be formed at a magnification of M₂. hi some embodiments, M₂ is larger than Mi by a factor of 1.1 or more (e.g., by a factor of 1.3 or more, by a factor of 1.5 or more, by a factor of 2.0 or more, by a factor of 5.0 or more, by a factor of 10.0 or more, by a factor of 100 or more, by a factor of 1000 or more). In certain embodiments, M₂ is less than or equal to Mi.

In some embodiments, a field of view of the image measured at magnification Mi has a minimum dimension of at least one micron (e.g., at least 2 microns, at least three microns, at least five microns, at least seven microns, at least ten microns, at least 15 microns, at least 20 microns, at least 30 microns, at least 40 microns, at least 50 microns). In certain embodiments, a field of view of at least one of the images measured at magnification M₂ has a maximum dimension of less than one micron (e.g., less than 750 nanometers, less than 500 nanometers, less than 300 nanometers, less than 200 nanometers, less than 100 nanometers, less than 50 nanometers, less than 30 nanometers, less than 20 nanometers, less than 10 nanometers). The field of view of an image refers to the area of a sample surface that is imaged.

Typically, for example, magnification Mi is selected so that the resulting image of surface 5010 includes sufficient resolution to correct for a specified minimum amount of drift. For example, if drift correction of 2 nm or more is specified, the image at magnification Mi has a resolution of at least 2 nm in order to quantify drift displacements of the same order. As used herein, resolution refers to the size of the smallest feature that can be reliably measured from the image. A size of a feature is reliably measured if it can be determined to within an error of 10% or less of the actual size of the feature, and with a standard deviation in the measured size of less than 5% of the actual size of the feature, from ten images of the feature obtained under similar conditions, hi some embodiments, the resolution of the image at magnification Mi is 10 nm or less (e.g., 8 nm or less, 6 nm or less, 5 nm or less, 4 nm or less, 3 nm or less, 2 nm or less, 1 nm or less, 0.5 nm or less, 0.25 nm or less). hi some embodiments, registration is performed relative to more than one reference mark on surface 5010. For example, two reference marks can be located on surface 5010 and used to perform registration. Each of the two reference marks can be a defect or imperfection, for example, or can be formed by exposing surface 5010 to a charged particle beam or another type of beam.

FIG. 5 shows a cross-sectional view of a sample 5000 that includes three features 5110, 5120, and 5130, which are visible in cross-sectional surface 5010. Two reference marks 5140a and 5140b are positioned on surface 5010, and an imaginary line 5140c extends between the two reference marks. To perform registration, the positions of each of the two reference marks 5140a and 5140b are determined based on an image of surface 5010 that is derived from measurement of particles 5035 by detector 150. Typically, when two reference marks are used to perform registration, the registration includes two steps. A first step includes determination of a coordinate transformation (e.g., a translation vector) based on the position of one of the reference marks (e.g., reference mark 5140a), as discussed previously. The first step can also include establishing a surface coordinate system by assigning a known surface coordinate location to the position of one of the two reference marks, for example. A second step includes determination of an orientation angle of the coordinate system of charged particle source 5005 relative to imaginary line 5140c. The orientation angle can then be used to apply a rotational correction to the coordinate system of charged particle source 5005.

In some embodiments, surface 5010 can include three or more reference marks. The positions of two of the reference marks can be used to determine a displacement vector and an orientation angle of surface 5010 with respect to charged particle beam 5015, for example. A third reference mark can be used, for example, to establish a surface coordinate system: a known surface coordinate position can be assigned to a selected position - such as a position of the third reference mark - on surface 5010. Thereafter, all measurements performed on surface 5010 can be referenced to the surface coordinate system.

The region of surface 5010 that is imaged at magnification M₂ can correspond to a portion of surface 5010 that is selected based on the image of surface 5010 at magnification M₁. For example, in some embodiments, a portion of surface 5010 corresponding to a region of interest can be located relative to a reference mark and selected automatically by electronic controller 5002. For example, the electronic controller can evaluate reference information such as one or more images of surface 5010 and/or design specifications (e.g., CAD specifications) for sample 5000, and can then select the region of interest based on this information. In certain embodiments, a region of interest of surface 5010 can be selected manually by a system operator based on, for example, information such as one or more images of surface 5010 and/or design specifications for sample 5000.

In certain embodiments, reference mark 5140 will not be positioned in the region of surface 5010 that is imaged at magnification M₂ (e.g., in higher resolution images of surface 5010) and/or reference mark 5140 may fall outside a high resolution field of view of the imaging system. Accordingly, to re-register the coordinate system of charged particle source 5005 after a particular period of time, the field of view over which the higher resolution image is obtained (e.g., the one or more images at magnification M₂) can be enlarged so that reference mark 5140 appears within the field of view of the image. By obtaining at least one image at magnification M₂ in which reference mark 5140 appears, relative position information for reference mark 5140 can be determined, and re-registration of the coordinate system of charged particle source 5005 can be performed.

In some embodiments, in addition to (or as an alternative to) increasing the field of view at magnification M₂ to capture a higher resolution image that includes reference mark 5140, one or more lower resolution images (e.g., at magnification Mi) can be acquired in which reference mark 5140 appears. The lower resolution images can be acquired when re-registration of the coordinate system of charged particle source 5005 is desirable, and acquisition of images at magnification Mi can be interspersed with acquisition of images at magnification M₂. Relative position information regarding reference mark 5140 can be determined based on the lower resolution image(s) acquired at magnification Mi, and this information can be used to re-register the coordinate system of charged particle source 5005. Electronic control system 170 can configure the various components of the imaging system to obtain images at different fields of view and magnifications, as required.

In certain embodiments, for example, where high resolution images are used to re-register the coordinate system of charged particle source 5005, an exemplary re-registration sequence proceeds as follows. Initially, a low resolution image that includes reference mark 5140 is obtained, and analysis of position information for reference mark 5140 in the low resolution image is used to register the coordinate system of charged particle source 5005. Based on any one or more of the criteria discussed above, one or more regions of interest on surface 5010 of the sample are selected for high resolution imaging. The selected region(s) of interest may or may not include reference mark 5140. Images of the region(s) of interest are obtained at magnification M₂ and high resolution. For images of regions of interest that include reference mark 5140, position information for reference mark 5140 can be used to re-register the coordinate system of charged particle source 5005 as needed. However, for images of regions of interest that do not include reference mark 5140, when re-registration is needed, electronic control system 170 (or a human operator) configures a high resolution imaging mode of the imaging system to increase the field of view of the imaging system so that reference mark 5140 now appears within the field of view of the imaging system, even at high resolution. Then, one or more high resolution images of surface 5010 - including reference mark 5140 - are acquired by the system. The high resolution images that include reference mark 5140 are analyzed to determine position information for reference mark 5140, and the position information is compared to position information for reference mark 5140 derived from the initial low resolution image(s) that were obtained for surface 5010. On the basis of this comparison, the coordinate system of charged particle source 5005 can then be re-registered relative to reference mark 5140. Following re-registration of the particle source's coordinate system, one or more high resolution images of the region(s) of interest can be obtained (at any magnification, resolution, and/or field of view) and analyzed, and position information in the high resolution images can be expressed in the coordinate system of the initially-measured low resolution image(s).

In some embodiments, for example, where low resolution images are used to re-register the coordinate system of charged particle source 5005, an exemplary re-registration sequence proceeds as follows. Initially, a low resolution image that includes reference mark 5140 is obtained, and analysis of position information for reference mark 5140 in the low resolution image is used to register the coordinate system of charged particle source 5005. Based on any one or more of the criteria discussed above, one or more regions of interest on surface 5010 of the sample are selected for high resolution imaging. The selected region(s) of interest may or may not include reference mark 5140.

Images of the region(s) of interest are obtained at magnification M₂ and high resolution. Periodically, when re-registration of the coordinate system of charged particle source 5005 is needed, electronic control system 170 (or a human operator) configures the imaging system to return to low resolution imaging mode. The imaging system obtains one or more low resolution images of surface 5010 of the sample, where reference mark 5140 appears in the low resolution images. The position of reference mark 5140 in the newly acquired low resolution image(s) can be compared to the position of reference mark 5140 in the initial (or previous) low resolution image(s), and on the basis of this comparison, the coordinate system of charged particle source 5005 can be re-registered relative to reference mark 5140. Following re-registration of the particle source's coordinate system, one or more high resolution images of the region(s) of interest can be obtained (at any magnification, resolution, and/or field of view) and analyzed, and position information in the high resolution images can be expressed in the coordinate system of the initially-measured (or previously-measured) low resolution image(s). hi some embodiments, dimensions of one or more features on cross-sectional surface 5010 are determined relative to a reference mark. FIG. 6 shows a cross-sectional view of a sample 5000 that includes a cross-sectional surface 5010 in which three features 5110, 5120, and 5130, are visible. Reference mark 5140 is also visible on surface 5010. To measure the width of feature 5110 in the x-direction on surface 5010, the distance between the left edge of feature 5110 and reference mark 5140, L₁, is first measured, followed by measurement of the distance between the right edge of feature 5110 and reference mark 5140, L₂. The width of feature 5110 is then calculated as L₂ - L₁.

In certain embodiments, distances L₁ and L₂ can be determined from a single image of surface 5010 (or a portion thereof). For example, the coordinate system of charged particle source 5005 can be registered relative to reference mark 5140, and then an image of surface 5010 can be obtained and the distances L₁ and L₂ can subsequently be calculated from the image. In some embodiments, distances L₁ and L₂ can be determined from different images of surface 5010 (or a portion thereof). For example, the coordinate system of charged particle source 5005 can be registered relative to reference mark 5140, and then a first image of surface 5010 can be obtained. From the first image, distance L₁ can be measured. Then, the coordinate system of charged particle source 5005 can be re-registered relative to reference mark 5140, and a second image of surface 5010 can be obtained. From the second image, distance L₂ can be measured. Finally, the width of features 5110 can be calculated as the difference between Li and L₂.

In some embodiments, certain types of image distortion can be corrected by determining positions of one or more features on cross-sectional surface 5010 relative to reference mark 5140. For example, a common type of distortion that occurs in sample cross-section imaging is keystone distortion, which arises due to variations in sample magnification across the cross- sectional surface. When charged particle beam 5015 is incident at a non-normal angle relative to surface 5010, the path length traveled by particle beam 5015 to reach one portion of surface 5010 is, in general, different than the path length traveled to reach other portions of surface 5010. The path length differences for different portions of surface 5010 manifest as variations in magnification of different portions of surface 5010 in sample images. If the variations in path length (and therefore magnification) are sufficiently large, noticeable distortion of surface 5010 can be apparent in images of the surface.

Known positions of surface features can be used to correct images to reduce such distortions. FIG. 7 shows an image of cross-sectional surface 5010 of sample 5000. Surface 5010 includes four reference marks 5141 a, 5141 b, 5141 c, and 5141 d. The four reference marks have been applied to surface 5010 at known positions relative to one another (e.g., using a charged particle beam such as an ion beam). As a result, distances Di (between marks 5141a and 5141b) and D₂ (between marks 5141c and 514Id) are known to relatively high accuracy before surface 5010 is imaged. FIG. 8 shows an image of cross-sectional surface 5010 that is measured using charged particle beam 5015. Due to the keystone distortion discussed above, the image shown in FIG. 8 is foreshortened in the horizontal dimension. However, distances Dj and D₂ between reference marks 5141a and 5141b, and 5141c and 514 Id, respectively, can be measured from the image in FIG. 8. Because Di and D₂ are known to relatively high accuracy, measured values of Dj and D₂ derived from FIG. 8 can be used to correct the image (e.g., via calculation and application of a stretching factor) so that it more closely corresponds to the image shown in FIG. 7.

In some embodiments, as shown in FIGS. 7 and 8, reference marks can be applied to surface 5010 and used to correct for image distortions, hi certain embodiments, sub-surface features of known dimensions (e.g., distances between edge positions of features 5110, 5120, and/or 5130) can be used to correct for image distortions in a manner that is analogous to the method discussed above in connection with FIGS. 7 and 8. In selecting sub-surface features to correct for image distortion, it can be advantageous to select features that are formed from different materials where the image contrast between such features is relatively high.

In certain embodiments, angular projection foreshortening of images of cross-sectional surfaces can also be corrected using reference marks and/or sub-surface features of known dimensions. Angular projection foreshortening arises when charged particle beam 5015 is incident at non-normal incidence on surface 5010, and corresponds to the projection of distances in the plane of surface 5010 onto a plane normal to the propagation direction of charged particle beam 5015. The projection of distances results in foreshortening, and the extent of foreshortening increases as the angle of incidence of charged particle beam 5015 on surface 5010 increases.

To correct for angular projection foreshortening, reference marks and/or features of known dimensions in surface 5010 can be used. For example, with reference to FIG. 7, reference marks 5141a and 5141c can be positioned on surface 5010 so that distance D₃ between the marks is known to relatively high accuracy. When an image with angular projection foreshortening is obtained, as shown in FIG. 9, distance D₃ can be measured in FIG. 9 and compared to the known value ofD₃ to determine a transformation (e.g., a stretching factor) that can be applied to the image of FIG. 9 so that the image more closely corresponds to the image of FIG. 7. hi some embodiments, charged particle source 5005 is an ion beam source. For example, charged particle beam 5015 can include ions such as noble gas ions (e.g., helium ions, neon ions, krypton ions), hydrogen ions, oxygen ions, and various other types of ions. In certain embodiments, charged particle source 5005 is an electron beam source, and charged particle beam 5015 includes electrons. The methods and systems disclosed herein can generally be implemented as part of an ion microscope system or an electron microscope system.

In general, particles 5035 can include a wide variety of different types of particles that leave surface 5010 during exposure of the surface to charged particle beam 5015. For example, in some embodiments, particles 5035 can include electrons (e.g., secondary electrons that are generated in response to incident charged particle beam 5015 and/or backscattered electrons from charged particle beam 5015). hi certain embodiments, particles 5035 can include ions (e.g., scattered ions from charged particle beam 5015, and/or secondary ions from sample 5000). In some embodiments, particles 5035 can include neutral atoms (e.g., neutral atoms derived from charged particle beam 5015, and/or neutral atoms derived from sample 5000). In certain embodiments, particles 5035 can include multiple types of particles, including two or more of the different particle types discussed above.

When images of surface 5010 are formed based on scattered ions from charged particle beam 5015 (e.g., Rutherford backscatter imaging of surface 5010), image acquisition times can, in some embodiments, be relatively long, depending upon the abundance of backscattered ions. If the abundance of ions is relatively small, an extended acquisition time may be required to achieve a particular signal-to-noise ratio in measured images. By imaging at high resolution only a relatively small region of interest (rather than the entire surface 5010), image acquisition times can be reduced. In some embodiments, sample imaging systems can include more than one charged particle source. FIG. 10 is a schematic diagram showing a sample imaging system 5300 that includes two charged particle sources. A number of features of FIG. 1 are common to FIG. 10 as well, and have been labeled accordingly; these features have already been discussed in detail. However, in addition to first charged particle source 5005 which generates first charged particle beam 5015, system 5300 includes a second charged particle source 5007 which generates second charged particle beam 5017. As discussed previously in connection with FIG. 1, detector 150 is configured to detect particles 5035 that leave surface 5010 in response to incident first charged particle beam 5015. In some embodiments, detector 150 can also be configured to detect particles 5037 that leave surface 5010 in response to incident second charged particle beam 5017. hi certain embodiments, as shown in FIG. 10, system 5300 can include a second detector 157 configured to detect particles 5037.

In general, second charged particle source 5007 can include any of the features discussed above in connection with first charged particle source 5005. Second charged particle source 5007 can be an ion beam source or an electron beam source, and can generate second charged particle beam 5017 that includes electrons and/or ions (e.g., noble gas ions such as helium ions, neon ions, and krypton ions, and other types of ions such as hydrogen ions and oxygen ions).

In some embodiments, first and second charged particle beams 5015 and 5017 include at least one type of particle in common (e.g., two or more types of particles in common, three or more types of particles in common, four or more types of particles in common), hi certain embodiments, first and second charged particle beams 5015 and 5017 include no types of particles in common. hi some embodiments, first charged particle beam 5015 includes at least one type of particles not included in second charged particle beam 5017 (e.g., at least two types of particles not included in second charged particle beam 5017, at least three types of particles not included in second charged particle beam 5017, at least four types of particles not included in second charged particle beam 5017). In certain embodiments, first charged particle beam 5015 includes no types of particles that are not included in second charged particle beam 5017.

Similarly, in some embodiments, second charged particle beam 5017 includes at least one type of particles not included in first charged particle beam 5015 (e.g., at least two types of particles not included in first charged particle beam 5015, at least three types of particles not included in first charged particle beam 5015, at least four types of particles not included in first charged particle beam 5015). In certain embodiments, second charged particle beam 5017 includes no types of particles that are not included in first charged particle beam 5015. hi particular, in some embodiments, first charged particle beam 5015 can include electrons, and second charged particle beam 5017 can include ions, such as noble gas ions (e.g., helium ions).

First and second charged particle sources 5005 and 5007, respectively, are linked to a common electronic control system 5310. During operation, electronic control system 5310 instructs first charged particle source 5005 to direct first charged particle beam 5015 to be incident on surface 5010, causing particles 5035 to leave the surface and be detected by detector 150. Based on particles 5035, an image of surface 5010 at a relatively low magnification Mi is formed. One or more reference marks on surface 5010 are located in the low magnification image, and then the coordinate system of second charged particle source 5007 is registered relative to the one or more reference marks, as discussed above. Thereafter, second charged particle beam 5017 is incident on surface 5010, and one or more images at a magnification M₂ larger than Mi are formed based on particles 5037 that leave surface 5010 under the influence of second charged particle beam 5017. The coordinate system of second charged particle source 5007 can be re-registered relative to the one or more reference marks on surface 5010, either based on the locations of the one or more reference marks in images at magnification M₂ that are formed based on exposure of surface 5010 to second charged particle beam 5017, or based on another image of surface 5010 acquired at magnification Mi following further exposure of surface 5010 to first charged particle beam 5015.

In general, one or more images of surface 5010 (or a portion thereof) can be obtained at magnification M₂. hi particular, when multiple images at magnification M₂ are obtained, the coordinate system of second charged particle source 5007 can be re-registered relative to a reference mark after each of one or more of the multiple images are obtained, and prior to the next one of the images being obtained. In some embodiments, for example, the coordinate system of second charged particle source 5007 is re-registered relative to a reference mark after each of the multiple images are obtained, and prior to the next one of the images being obtained.

Alternatively, or in addition, the coordinate system of second charged particle source 5007 can be re-registered relative to a reference mark after a portion of at least one of the multiple images are obtained (e.g., after a portion of each of more than one of the multiple images are obtained), and prior to the next one of the images being obtained. In some embodiments, for example, the coordinate system of second charged particle source 5007 is reregistered relative to a reference mark after a portion of each of the multiple images is obtained, and prior to the next one of the images being obtained.

II. 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.

Typically, 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.

FIG. 11 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. During use, 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.

As shown in FIG. 12, 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. In general, 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. In general, as measured at surface 181 of sample 180, the current of ions in ion beam 192 increases monotonically as the pressure of the noble gas in system 100 increases. In certain embodiments, 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.

Optionally, gas source 110 can supply one or more gases in addition to the noble gas(es); an example of such a gas is nitrogen. Typically, while 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.

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. Electrically conductive tip 186 can be formed of various materials. In some embodiments, tip 186 is formed of a metal (e.g., tungsten (W), tantalum (Ta), indium (Ir), rhenium (Rh), niobium (Nb), platinum (Pt), molybdenum (Mo)). In certain embodiments, electrically conductive tip 186 can be formed of an alloy. In some embodiments, electrically conductive tip 186 can be formed of a different material (e.g., carbon (C)). During use, 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, and optional suppressor 188 is biased positively or negatively (e.g., from -5 kV to +5 kV) with respect to tip 186. Because 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. hi general, 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.

In general, ion optics 130 are configured to direct ion beam 192 onto surface 181 of sample 180. 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. Generally, ion optics 130 include a variety of electrostatic and other ion optical elements that are configured as desired. 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. For example, 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.

When ion beam 192 impinges on sample 180, a variety of different types of particles 194 can be produced. These particles include, for example, secondary electrons, Auger electrons, secondary ions, secondary neutral particles, primary neutral particles, scattered ions and photons (e.g., X-ray photons, IR photons, visible photons, UV photons). 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. 11, 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). As described in more detail below, in general, any number and configuration of detectors can be used in the microscope systems disclosed herein. In some embodiments, multiple detectors are used, and some of the multiple detectors are configured to measure different types of particles. In certain embodiments, 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). Optionally, combinations of such detector arrangements can be used. In general, the information measured by the detectors is used to determine information about sample 180. Typically, this information is determined by obtaining one or more images of sample 180. By rastering ion beam 192 across surface 181, 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. The operation of microscope system 100 is typically controlled via electronic control system 170. For example, 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. Optionally, one or more of these parameters may be manually controlled (e.g., via a user interface integral with electronic control system 170). Additionally or alternatively, 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. Typically, 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.

In certain embodiments, electronic control system 170 can be configured to control various properties of ion beam 192. For example, control system 170 can control a composition of ion beam 192 by regulating the flow of gases into gas field ion source 120. By adjusting various potentials in ion source 120 and ion optics 130, 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. In some embodiments, electronic control system 170 can be configured to control one or more additional particle beams. For example, in certain embodiments, 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. 11, 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. In general, 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. Ln some embodiments, 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 particle.

Detection systems and methods are generally disclosed, for example, in U.S. Patent Application Serial No. 11/600,711 entitled "ION SOURCES, SYSTEMS AND METHODS" by Billy W. Ward et al., filed on November 15, 2006, now published as U.S. Publication No. US 2007/0158558, the entire contents of which are incorporated herein by reference.

Computer Hardware and Software

In general, any of the analysis 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. However, the programs can be implemented in assembly or machine language, if desired. In any case, the language can be a compiled or interpreted language. Moreover, 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 analysis 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.

OTHER EMBODIMENTS

Other embodiments are in the claims.

Claims

WHAT IS CLAIMED IS:

1. A method, comprising: exposing a cross-sectional surface of a channel formed in a sample to particles from a charged particle source to cause a first plurality of particles to leave the cross-sectional surface, and determining a position of a reference mark on the cross-sectional surface based on the first plurality of particles; registering a coordinate system of the charged particle source relative to the position of the reference mark; and exposing the cross-sectional surface to additional particles from the charged particle source to cause a second plurality of particles to leave the cross-sectional surface, and forming multiple images of the cross-sectional surface based on the second plurality of particles, wherein after formation of at least one of the multiple images, the coordinate system of the charged particle source is registered again relative to the position of the reference mark prior to forming the next one of the multiple images.

2. The method of claim 1 , wherein the charged particle source is an ion beam source.

3. The method of claim 1 , wherein the charged particle source is an electron beam source.

4. The method of claim 1 , wherein the particles from the charged particle source comprise ions.

5. The method of claim 1 , wherein the particles from the charged particle source comprise noble gas ions.

6. The method of claim 1 , wherein the particles from the charged particle source comprise helium ions.

7. The method of claim 1, wherein the reference mark comprises a defect in the cross- sectional surface.

8. The method of claim 1 , wherein the reference mark comprises a mark formed in the cross-sectional surface by an ion beam.

9. The method of claim 8, wherein prior to exposing the cross-sectional surface to particles from the charged particle source, the cross-sectional surface is exposed to an ion beam that forms the reference mark in the cross-sectional surface.

10. The method of claim 9, wherein the ion beam comprises gallium ions.

11. The method of claim 1 , wherein registering the coordinate system of the charged particle source comprises determining a coordinate transformation between a coordinate system of the charged particle source and the position of the reference mark.

12. The method of claim 11 , wherein the coordinate transformation comprises a translation vector.

13. The method of claim 1 , wherein determining the position of the reference mark comprises forming a reference image of the cross-sectional surface based on the first plurality of particles, and locating the reference mark in the reference image.

14. The method of claim 13, wherein the reference image is formed at a first magnification of the cross-sectional surface, and the multiple images are formed at a second magnification of the cross-sectional surface larger than the first magnification.

15. The method of claim 13, wherein a field of view of the reference image has a minimum dimension of at least one micron.

16. The method of claim 15, wherein the minimum dimension of the field of view of the reference image is at least five microns.

17. The method of claim 15 , wherein the minimum dimension of the field of view of the reference image is at least ten microns.

18. The method of claim 1 , wherein a maximum dimension of the field of view of the multiple images is less than one micron.

19. The method of claim 18, wherein the maximum dimension of the field of view of the multiple images is less than 500 nanometers.

20. The method of claim 18, wherein the maximum dimension of the field of view of the multiple images is less than 100 nanometers.

21. The method of claim 1 , wherein registering the coordinate system of the charged particle source again relative to the position of the reference mark comprises determining a coordinate transformation between a coordinate system of the charged particle source and the position of the reference mark.

22. The method of claim 1, wherein the coordinate system of the charged particle source is registered relative to the position of the reference mark prior to forming each one of the multiple images.

23. The method of claim 1, wherein the coordinate system of the charged particle source is registered relative to the position of the reference mark during formation of at least one of the multiple images.

24. The method of claim 1 , wherein the coordinate system of the charged particle source is registered relative to the position of the reference mark during formation of each of the multiple images.

25. The method of claim 1, wherein the reference mark is a first reference mark, the method further comprising determining a position of a second reference mark on the cross-sectional surface based on the first plurality of particles.

26. The method of claim 25, wherein registering the coordinate system of the charged particle source relative to the position of the reference mark comprises determining an orientation angle of the coordinate system relative to the first and second reference marks.

27. The method of claim 25, wherein the second reference mark comprises a defect in the cross-sectional surface.

28. The method of claim 25, wherein the second reference mark comprises a mark formed in the cross-sectional surface by an ion beam.

29. The method of claim 1 , further comprising selecting a region of interest on the cross- sectional surface relative to the reference mark.

30. The method of claim 29, wherein the region of interest is selected manually from a reference image that is formed based on the first plurality of particles.

31. The method of claim 29, wherein the region of interest is selected automatically based on reference information for the cross-sectional surface.

32. The method of claim 31 , wherein the reference information comprises one or more images of the cross-sectional surface.

33. The method of claim 31 , wherein the reference information comprises design specifications for the cross-sectional surface.

34. The method of claim 29, wherein the multiple images correspond to the region of interest on the cross-sectional surface.

35. The method of claim 1, further comprising determining a dimension of a feature on the cross-sectional surface relative to the reference mark.

36. The method of claim 35, wherein determining the dimension comprises determining a first edge position of the feature relative to the reference mark, determining a second edge position of the feature relative to the reference mark, and calculating a difference between the first and second edge positions to determine the distance.

37. The method of claim 36, wherein the coordinate system of the charged particle source is registered relative to the position of the reference mark prior to determining the first edge position and again prior to determining the second edge position.

38. The method of claim 36, wherein the position of the reference mark is determined prior to determining the first edge position and again prior to determining the second edge position.

39. The method of claim 1, further comprising correcting image distortion in at least one of the multiple images by determining positions of one or more features on the cross-sectional surface relative to the reference mark.

40. The method of claim 39, wherein the one or more features comprise additional reference marks.

41. The method of claim 39, wherein the one or more features comprise edge positions of portions of the cross-sectional surface that are formed from different materials.

42. The method of claim 39, wherein the distortion arises due to variations in magnification in the at least one of the multiple images.

43. The method of claim 1, wherein the second plurality of particles comprises secondary electrons.

44. The method of claim 1, wherein the second plurality of particles comprises scattered ions.

45. The method of claim 1, wherein the second plurality of particles comprises secondary ions.

46. The method of claim 44, wherein the scattered ions comprise helium ions.

47. The method of claim 1 , further comprising determining a position of an additional reference mark on the cross-sectional surface, and assigning an origin of a coordinate system of the cross-sectional surface to coincide with the position of the additional reference mark.

48. A method, comprising: exposing a cross-sectional surface of a channel formed in a sample to particles from a first charged particle source to cause a first plurality of particles to leave the cross-sectional surface, and determining a position of a reference mark on the cross-sectional surface based on the first plurality of particles; registering a coordinate system of a second charged particle source relative to the position of the reference mark; and exposing the cross-sectional surface to particles from the second charged particle source to cause a second plurality of particles to leave the cross-sectional surface, and forming multiple images of the cross-sectional surface based on the second plurality of particles, wherein after formation of at least one of the multiple images, the coordinate system of the second charged particle source is registered again relative to the position of the reference mark prior to forming the next one of the multiple images.

49. The method of claim 48, wherein the first charged particle source is an ion source.

50. The method of claim 48, wherein the first charged particle source is an electron source.

51. The method of claim 48, wherein the second charged particle source is an ion source.

52. The method of claim 48, wherein the particles from the first charged particle source comprise a first type of ions, and the particles from the second charged particle source comprise a second type of ions different from the first type of ions.

53. The method of claim 48, wherein the particles from the second charged particle source comprise noble gas ions.

54. The method of claim 48, wherein the particles from the second charged particle source comprise helium ions.

55. The method of claim 48, wherein the particles from the first charged particle source comprise electrons and the particles from the second charged particle source comprise helium ions.

56. The method of claim 48, wherein the reference mark comprises a defect in the cross- sectional surface.

57. The method of claim 48, wherein the reference mark comprises a mark formed in the cross-sectional surface by an ion beam.

58. The method of claim 57, wherein prior to exposing the cross-sectional surface to particles from the first charged particle source, the cross-sectional surface is exposed to an ion beam that forms the reference mark in the cross-sectional surface.

59. The method of claim 48, wherein registering the coordinate system of the second charged particle source comprises determining a coordinate transformation between a coordinate system of the second charged particle source and the position of the reference mark.

60. The method of claim 48, wherein determining the position of the reference mark comprises forming a reference image of the cross-sectional surface based on the first plurality of particles, and locating the reference mark in the reference image.

61. The method of claim 60, wherein the reference image is formed at a first magnification of the cross-sectional surface, and the multiple images are formed at a second magnification of the cross-sectional surface larger than the first magnification.

62. The method of claim 48, wherein registering the coordinate system of the second charged particle source again relative to the position of the reference mark comprises determining a coordinate transformation between a coordinate system of the second charged particle source and the position of the reference mark.

63. The method of claim 48, wherein the coordinate system of the second charged particle source is registered relative to the position of the reference mark prior to forming each one of the multiple images.

64. The method of claim 48, wherein the coordinate system of the second charged particle source is registered relative to the position of the reference mark during formation of at least one of the multiple images.

65. The method of claim 48, wherein the coordinate system of the second charged particle source is registered relative to the position of the reference mark during formation of each of the multiple images.

66. The method of claim 48, wherein the reference mark is a first reference mark, the method further comprising determining a position of a second reference mark on the cross-sectional surface based on the first plurality of particles.

67. The method of claim 66, wherein registering the coordinate system of the second charged particle source relative to the position of the reference mark comprises determining an orientation angle of the coordinate system relative to the first and second reference marks.

68. The method of claim 48, further comprising selecting a region of interest on the cross- sectional surface relative to the reference mark.

69. The method of claim 68, wherein the region of interest is selected manually from a reference image that is formed based on the first plurality of particles.

70. The method of claim 68, wherein the region of interest is selected automatically based on reference information for the cross-sectional surface.

71. The method of claim 70, wherein the reference information comprises one or more images of the cross-sectional surface.

72. The method of claim 70, wherein the reference information comprises design specifications for the cross-sectional surface.

73. The method of claim 68, wherein the multiple images correspond to the region of interest on the cross-sectional surface.

74. The method of claim 48, further comprising determining a dimension of a feature on the cross-sectional surface relative to the reference mark.

75. The method of claim 74, wherein determining the dimension comprises determining a first edge position of the feature relative to the reference mark, determining a second edge position of the feature relative to the reference mark, and calculating a difference between the first and second edge positions to determine the distance.

76. The method of claim 75, wherein the coordinate system of the second charged particle source is registered relative to the position of the reference mark prior to determining the first edge position and again prior to determining the second edge position.

77. The method of claim 75, wherein the position of the reference mark is determined prior to determining the first edge position and again prior to determining the second edge position.

78. The method of claim 48, further comprising correcting image distortion in at least one of the multiple images by determining positions of one or more features on the cross-sectional surface relative to the reference mark.

79. The method of claim 48, wherein the second plurality of particles comprises secondary electrons.

80. The method of claim 48, wherein the second plurality of particles comprises scattered ions.

81. The method of claim 48, wherein the second plurality of particles comprises secondary ions.

82. The method of claim 80, wherein the scattered ions comprise helium ions.

83. The method of claim 48, further comprising determining a position of an additional reference mark on the cross-sectional surface, and assigning an origin of a coordinate system of the cross-sectional surface to coincide with the position of the additional reference mark.

84. A method, comprising: exposing a cross-sectional surface of a channel formed in a sample to particles from a charged particle source to cause a first plurality of particles to leave the cross-sectional surface, and determining a position of a reference mark on the cross-sectional surface based on the first plurality of particles; registering a coordinate system of the charged particle source relative to the position of the reference mark; and exposing the cross-sectional surface to additional particles from the charged particle source to cause a second plurality of particles to leave the cross-sectional surface, and forming an image of the cross-sectional surface based on the second plurality of particles, wherein during formation of the image, the coordinate system of the charged particle source is registered again relative to the position of the reference mark.

85. The method of claim 84, wherein forming the image of the cross-sectional surface comprises storing partial image data in an electronic memory, registering the coordinate system of the charged particle source again relative to the position of the reference mark, retrieving the stored partial image data, and completing formation of the image of the surface.

86. The method of claim 84, further comprising forming a plurality of images of the cross- sectional surface, wherein during formation of each one of the plurality of images, the coordinate system of the charged particle source is registered again relative to the position of the reference mark.

87. The method of claim 84, further comprising forming a plurality of images of the cross- sectional surface, wherein during formation of more than one of the plurality of images, the coordinate system of the charged particle source is registered again relative to the position of the reference mark.

88. The method of claim 84, wherein the coordinate system of the charged particle source is registered again relative to the position of the reference mark after each one of a plurality of exposures of the cross-sectional surface to particles from the charged particle source, wherein each one of the plurality of exposures corresponds to a line scan of a portion of the cross- sectional surface.

89. The method of claim 84, wherein the coordinate system of the charged particle source is registered again relative to the position of the reference mark after each one of a plurality of exposures of the cross-sectional surface to particles from the charged particle source, wherein each one of the plurality of exposures corresponds to a point exposure of a portion of the cross- sectional surface.

90. The method of claim 84, wherein the coordinate system of the charged particle source is registered again relative to the position of the reference mark after each one of a plurality of exposures of the cross-sectional surface to particles from the charged particle source, wherein each one of the plurality of exposures corresponds to a plurality of point exposures of a portion of the cross-sectional surface.

91. The method of claim 84, wherein the reference mark comprises a defect in the cross- sectional surface.

92. The method of claim 84, wherein the reference mark comprises a mark formed in the cross-sectional surface by an ion beam.

93. The method of claim 92, wherein prior to exposing the cross-sectional surface to particles from the charged particle source, the cross-sectional surface is exposed to an ion beam that forms the reference mark in the cross-sectional surface.

94. The method of claim 84, wherein registering the coordinate system of the charged particle source comprises determining a coordinate transformation between a coordinate system of the charged particle source and the position of the reference mark.

95. The method of claim 84, wherein determining the position of the reference mark comprises forming a reference image of the cross-sectional surface based on the first plurality of particles, and locating the reference mark in the reference image.

96. The method of claim 95, wherein the reference image is formed at a first magnification of the cross-sectional surface, and the image of the cross-sectional surface is formed at a second magnification of the cross-sectional surface larger than the first magnification.

97. The method of claim 84, wherein registering the coordinate system of the charged particle source again relative to the position of the reference mark comprises determining a coordinate transformation between a coordinate system of the charged particle source and the position of the reference mark.

98. The method of claim 84, wherein the reference mark is a first reference mark, the method further comprising determining a position of a second reference mark on the cross-sectional surface based on the first plurality of particles.

99. The method of claim 98, wherein registering the coordinate system of the charged particle source relative to the position of the reference mark comprises determining an orientation angle of the coordinate system relative to the first and second reference marks.

100. The method of claim 84, further comprising selecting a region of interest on the cross- sectional surface relative to the reference mark.

101. The method of claim 100, wherein the region of interest is selected manually from a reference image that is formed based on the first plurality of particles.

102. The method of claim 100, wherein the region of interest is selected automatically based on reference information for the cross-sectional surface.

103. The method of claim 102, wherein the reference information comprises one or more images of the cross-sectional surface.

104. The method of claim 102, wherein the reference information comprises design specifications for the cross-sectional surface.

105. The method of claim 100, wherein the image of the cross-sectional surface corresponds to the region of interest.

106. The method of claim 84, further comprising determining a dimension of a feature on the cross-sectional surface relative to the reference mark.

107. The method of claim 106, wherein determining the dimension comprises determining a first edge position of the feature relative to the reference mark, determining a second edge position of the feature relative to the reference mark, and calculating a difference between the first and second edge positions to determine the distance.

108. The method of claim 107, wherein the coordinate system of the charged particle source is registered relative to the position of the reference mark prior to determining the first edge position and again prior to determining the second edge position.

109. The method of claim 107, wherein the position of the reference mark is determined prior to determining the first edge position and again prior to determining the second edge position.

110. The method of claim 84, further comprising correcting image distortion in the image by determining positions of one or more features on the cross-sectional surface relative to the reference mark.

111. The method of claim 84, wherein the second plurality of particles comprises secondary electrons.

112. The method of claim 84, wherein the second plurality of particles comprises scattered ions.

1 13. The method of claim 84, wherein the second plurality of particles comprises secondary ions.

114. The method of claim 112, wherein the scattered ions comprise helium ions.

115. The method of claim 84, further comprising determining a position of an additional reference mark on the cross-sectional surface, and assigning an origin of a coordinate system of the cross-sectional surface to coincide with the position of the additional reference mark.