Rotating sample holder for random angle sampling in tomography.

Abstract

To provide a rotating sample holder for random angle sampling in tomography.SOLUTION: A sample holder retains a sample and can continuously rotate the sample in a single direction while the sample is exposed to a charged particle beam (CPB) or other radiation source. Typically, the CPB is strobed to produce a series of CPB images at random or arbitrary angles of rotation. The sample holder can rotate two or more complete revolutions of the sample. The CPB images are used in tomographic reconstruction and, in some cases, relative rotation angles are used in the reconstruction, without input of an absolute rotation angle.SELECTED DRAWING: Figure 1A

Description

[Technical Field]

[0001] The present disclosure relates to electron tomography. [Background technology]

[0002] Tomographic imaging electron microscopy is based on acquiring sample images at multiple exposure angles. These angles are typically set by advancing the sample through a series of ascending angles. In other instances, angles are set by tilting the sample back and forth. One problem with these traditional approaches is the limited range of tilt angles available in conventional electron microscopes. Furthermore, it can be difficult to accurately set angles, especially in back-and-forth tilt protocols, given the successive large angle movements and changes in angle direction. Back-and-forth tilts can also be time-consuming, requiring multiple starts and stops. Furthermore, traditional ascending angle sampling methods are not optimal for radiation-sensitive samples, and it can be beneficial to first sample angles that result in minimal sample tilt. For these and other reasons, an alternative approach is needed. Summary of the Invention

[0003] Random angle sampling in tomographic acquisitions as disclosed herein can be beneficial to reconstruction quality and can enable reconstruction of dynamically deforming samples. In exemplary embodiments disclosed herein, stroboscopic charged particle beam (CPB) exposure of a rotating sample holder enables random angle sampling in tomographic acquisitions. The sample can be rotated at a constant or variable angular velocity, and the sample angle can be selected by selection of exposure time. As used herein, exposure duration is generally selected such that sample rotation during exposure does not produce unacceptable image blurring, and such exposures are referred to herein as "stroboscopic" exposures.

[0004] A typical method includes rotating the sample in one direction, the rotation being continuous, with the sample rotating through multiple revolutions, and all angles of the sample's complete revolution being accessible. The sample is illuminated with multiple electron beam pulses at a pulse rate and while the sample is rotating. Alternatively, the sample can be irradiated with X-rays. In response to the illumination, multiple images of the sample are acquired, each image acquired with the sample at a different relative angle relative to at least one of the other acquired images, and the relative angle of the sample in each acquired image is determined. In some examples, determining the relative angle of the sample is performed in conjunction with or after each acquisition of the images. In some embodiments, the absolute angle of the sample in each acquired image is determined during tomographic reconstruction of the sample based on a tomographic reconstruction of the sample. As used herein, absolute angle refers to the orientation angle of the sample relative to a fixed reference. In some examples, determining the relative angle of the sample in each acquired image includes reading an encoder coupled to a rotatable sample holder during acquisition, the rotatable sample holder rotating the sample. As used herein, the relative angle between a first projection image acquired at a first angle and a second projection image acquired at a second angle is defined as the rotation required to apply to the sample to move the sample from the first projection angle (i.e., first projection direction) to the second projection angle (i.e., second projection direction). In representative embodiments, determining the absolute angle of the sample in each acquired image is based on encoder readings. In other embodiments, determining the relative angle of the sample in each acquired image includes determining the relative angle based on a reconstruction or rotation time of the sample. The pulse rate can be variable, such as changing per revolution of the sample or after multiple revolutions, or the pulse rate can be changed after each full rotation of the sample. In some embodiments, the pulse rate increases or decreases after each full rotation of the sample, and the rotation occurs at a constant or variable rate.

[0005] An exemplary apparatus includes a rotatable sample holder operable to continuously rotate a sample in one direction through multiple revolutions, such that all angles of a complete revolution of the sample are accessible. An electron beam source operable to irradiate the sample with multiple electron beam pulses at a pulse rate and while the sample is rotating. A detection system operable to acquire multiple images of the sample corresponding to the multiple electron beam pulses. The detection system may include an electron detector positioned to receive electron pulse portions responsive to the electron beam illumination of the sample, with each image acquired with the sample at a different relative angle relative to at least one of the other acquired images. In some embodiments, the controller is configured to determine the relative or absolute angle of the sample in each acquired image. In some examples, an encoder is coupled to the rotatable sample holder during image acquisition. According to exemplary examples, the pulse rate of the electron beam source is variable using random, non-constant, or Poisson-distributed intervals, and the electron beam source may be operable to generate electron beam pulses at the varying pulse rate after each full rotation of the sample.

[0006] At least one computer-readable medium containing processor-executable instructions is configured to control an electron beam system to continuously rotate a sample on a rotatable sample holder in one direction through multiple revolutions, such that all angles of a complete revolution of the sample holder disposed on the rotatable sample holder are accessible. While the sample is rotating, the sample can be illuminated using an electron beam source having multiple electron beam pulses at a pulse rate. Multiple images of the sample are acquired corresponding to each of the multiple electron beam pulses and a reconstruction of the sample based on the acquired multiple images.

[0007] In some embodiments, a method includes unidirectionally rotating a sample through an angular range including at least one full rotational revolution. During the unidirectional rotation, the rotating sample is illuminated with a stroboscope at multiple angles, and a sample image is acquired at each of the corresponding multiple angles. In some cases, a tomographic image of the sample is generated based on the sample image. According to some embodiments, a relative or absolute rotation angle associated with the sample image is determined before, after, or during acquisition of the sample image. In some cases, the relative and / or absolute sample angle associated with the sample image is determined based on the tomographic reconstruction. In further embodiments, the sample is fixed to a rotatable sample holder, and the relative and / or absolute rotation angle is determined using an encoder coupled to the rotatable sample holder. In some embodiments, the rotating sample is illuminated with a stroboscope at a variable pulse rate, a fixed pulse rate, or random time intervals, such as Poisson-distributed intervals. In further embodiments, the stroboscopic exposure of the rotating sample is at a variable pulse rate determined based on the angle or number of rotations of the rotating sample. According to an exemplary embodiment, the sample is rotated in one direction over an angular range that includes multiple full rotations.

[0008] An exemplary apparatus includes a switchable charged particle beam (CPB) source and a rotatable sample stage operable to rotate a sample through at least one complete revolution. A controller is coupled to the CPB source and the rotatable sample stage and configured to expose a sample disposed on the rotatable sample stage at a plurality of angles during a unidirectional rotation of the sample, the plurality of exposure angles being in a range greater than 360°. In some examples, the controller is coupled to activate the CPB source at a plurality of angles during the unidirectional rotation of the sample to stroboscopically expose the sample to the CPB. In other examples, the controller is coupled to blank the CPB so that the sample is stroboscopically exposed to the CPB at a plurality of angles during the unidirectional rotation of the sample. According to some embodiments, a rotary encoder is coupled to the rotatable sample stage and provides at least one of a relative rotation angle and an absolute rotation angle for each of the exposure angles. In some examples, the controller establishes the stroboscopic exposure at a fixed or variable rate or at random times. In further examples, at least one CPB detector is coupled to detect radiation generated in response to the CPB exposure of the sample and to generate a corresponding tomographic image. In an embodiment, the controller is coupled to generate a tomographic reconstruction corresponding to the sample based on the tomographic image and the plurality of angles, and to generate estimates of the plurality of angles during the tomographic reconstruction.

[0009] The electron beam device includes a controller arranged to be coupled to the electron beam source. At least one computer-readable medium is coupled to the controller and includes controller-executable instructions for causing the controller to direct the electron beam source to stroboscopically illuminate the sample at a plurality of optional angles during unidirectional rotation of the sample and acquire a sample image at each of the corresponding plurality of angles. According to some embodiments, the at least one computer-readable medium further includes controller-executable instructions for reconstructing a tomographic image of the sample based on the sample image and determining a relative and / or absolute rotation angle associated with the sample image. In some embodiments, the relative and / or absolute rotation angle associated with the sample image is determined after acquisition of the sample image or is determined based on the tomographic reconstruction. In additional embodiments, the at least one computer-readable medium further includes controller-executable instructions for determining the relative and / or absolute rotation angle using an encoder coupled to the rotatable sample holder. In other exemplary embodiments, the controller is coupled to direct the electron beam source to stroboscopically illuminate at a plurality of arbitrary angles during a unidirectional rotation of the sample or at a variable pulse rate based on random time intervals, such as Poisson distribution intervals, where the angular range of the arbitrary angles includes a plurality of full rotations.

[0010] The foregoing and other features of the disclosed technology will become more apparent from the following detailed description, which proceeds with reference to the accompanying drawings. [Brief explanation of the drawings]

[0011] [Figure 1A] A representative CPB microscope including a rotatable sample stage is shown. [Figure 1B] Figure 1A shows a representative sample rotation obtained using CPB microscopy. [Figure 1C] Figure 1A shows a representative sample rotation obtained using CPB microscopy. [Figure 1D] Figure 1A shows a representative sample rotation obtained using CPB microscopy. [Figure 1E] A representative needle or column geometry sample arranged for rotation is shown. [Figure 2] Another representative system for acquiring random-angle CPB images for tomography using a continuously rotatable sample stage is shown. [Figure 3] A representative system for acquiring random-angle CPB images for tomography using a continuously rotatable sample stage is shown. [Figure 4] We present a representative method for generating tomographic reconstructions using images acquired with random-angle stroboscopic exposures of a rotating sample. [Figure 5] 1 shows a representative method for acquiring sample images based on random exposure times as a function of sample rotation speed. [Figure 6] A representative system for acquiring CPB images using a sample stage rotatable by a DC motor is shown. [Figure 7] 10 illustrates an alternative method for acquiring CPB images for tomographic reconstruction. [Figure 8] 1 illustrates a representative computing environment for image acquisition and analysis for tomographic reconstruction. DETAILED DESCRIPTION OF THE INVENTION

[0012] Disclosed herein are methods and apparatus for charged particle tomography. Typically, a sample is positioned on a rotatable sample stage for repeated exposure to a charged particle beam (CPB). While the disclosed embodiments are generally described with reference to transmission electron microscopy, other CPBs can be used. Alternatively, the sample can be illuminated with X-rays, and X-ray-based images can be acquired. In some embodiments, stroboscopic illumination of the rotating sample at a random angle or other series of angles is used to acquire a series of images used in tomographic reconstruction. The sample can be rotated at a uniform angular velocity, and the random angle exposure can be established based on one or more series of angle values ​​that can be generated as needed or retrieved from a computer-readable storage device such as a memory. The angles can be based on a phase associated with the rotation of the sample, a series of exposure times based on the rotation rate of the sample, randomly generated during image acquisition, or otherwise specified. The sample can be rotated at a fixed or variable speed and illuminated with a stroboscope during rotation. The stroboscopic illumination can be at a fixed or variable rate, including random time duration rotations. The illumination angle can be determined before, during, or after illumination, and can have a fixed or variable angular distribution, including random angles.

[0013] As used herein, "column" generally refers to one or more CPB optical elements or a combination of elements such as a CPB light source, CPB lens, CPB deflector, CPB aperture, stigmator, or other CPB optical elements. One or more such optical elements can be used to generate pulsed CPB that can be directed at a sample to provide a pulsed exposure. Such pulsed exposures are commonly referred to as "stroboscopic" exposures, indicating that the effective exposure time is short enough relative to the rotation of the sample that a suitable image is generated, i.e., free of blurring due to excessive motion. Suitable exposure times can correspond to rotations of less than 0.0001, 0.001, 0.01 degrees, or other angles. The specification of any allowable maximum exposure duration may vary depending on the magnification and desired resolution of the image. In the disclosed embodiments, a CPB or CPB column is energized to generate a stroboscopic CPB exposure; however, continuous CPB can be used with stroboscopic detection, i.e., detection of charged particles or electromagnetic radiation generated in response to the CPB exposure and received during the detection time frame (referred to herein as "stroboscopic" detection). In stroboscopic exposures, CPB may have a continuum component in addition to the stroboscopic component. In many practical applications, pulsed exposures are preferred to reduce sample degradation produced by CPB exposures that contain a continuum component. The continuum component can contribute to undesirable sample changes without improving the tomographic image.

[0014] In some embodiments, sample images are acquired using exposures at multiple angles, such as random angles or random angle differences. As used herein, random or randomly selected refers to values ​​that are unevenly spaced and can be selected using a random or pseudorandom number generator, or can otherwise approximate a randomly selected value. It will be understood that any set of such random values ​​can generally be selected using a so-called pseudorandom number generator. One or more sets of random values ​​can be used, with different sets containing different values ​​and / or different numbers of values. A value associated with an angle, exposure time, or phase can be determined based on a corresponding set of random numbers, and the random numbers in the set are processed to establish the corresponding angle, exposure time, or phase. For example, N random numbers R between 0 and 1 can be selected. i If a set of angles is obtained, then i is πR i radian, 2πR i Radians, or more generally, AπR i The angle α can be chosen in radians, where Aπ radians is the total angle range used. i can be specified as positive and negative, and for any particular angle α i The exposure at can include multiple rotations, i.e., α i is the rotation angle in radians modulo-2π or modulo-π. The exposure time, relative time, phase, and relative phase can similarly be specified based on a set of random numbers. A random number generator can also be used to generate values ​​on the fly as needed. In some implementations, the selected or generated values ​​are used for image reconstruction, and each image in the set is associated with a respective random number having a specific value.

[0015] In some embodiments, for convenience, sample exposure is performed using a constant sample rotation speed, although non-uniform speeds, such as monotonically increasing or decreasing speeds, or any increasing and decreasing rate, can be used. With uniform, constant rotation, the sample is acquired at random exposure angles. As noted above, such random exposures can be based on random exposure times or rotation phases, which can be stored or generated as needed. Alternatively, the sample can be rotated at a variable speed, such as a random speed, and exposure times can be separated by a fixed delay.

[0016] In some embodiments, the rotation speed is constant or variable, and the stroboscopic exposure can have different pulse distributions, such as random, fixed at a variable rate, or a combination thereof. The relative and / or absolute exposure angles can be determined after acquisition of some or all of the images.

[0017] Example 1 1A, a CPB system 100 includes a CPB emitter 102, which may include a field emitter 104 or other emission source, that generates CPB from an emitter tip 106. The CPB current can be controlled by one or more of the voltages applied to the field emitter 104 or the emitter tip 106, as provided by an emitter driver 107. A suppressor electrode 108 is typically disposed around the field emitter 104 to suppress stray charged particle emissions, and an extractor electrode 110 is disposed to establish a voltage across the emitter tip 106 to induce a selected CPB current. A beam current driver 112 is coupled to the suppressor electrode 108 and the extractor electrode 110. As shown in FIG. 1A, the suppressor electrode 108, the extractor electrode 110, and any or all of the field emitter 104 or emitter tip 106 can be controlled using an emitter driver 107 or a beam current driver 112 to generate pulsed or other variable CPBs, allowing the sample S to be stroboscopically exposed. One or more additional beam apertures can be positioned along the CPB system axis 119 and used to control the CPB, for example, by blocking the CPB except at predetermined exposure times. For example, an aperture defined in an aperture plate 118 can be used to block or attenuate the CPB in response to application of a deflection voltage from a deflection driver 120 to a beam deflector 122, such as a resonant beam deflector. When the beam deflector 122 is activated, the CPB 126 is deflected by the aperture plate 118 so that it is blocked. Additional apertures and deflectors can be provided but are not shown in FIG. 1A. 1A embodiment can also provide beam modulation using a gun lens, although such a lens is not shown. In addition to CPB modulation provided by driving one or more CPB lenses, deflectors, aperture plates, or other CPB optical elements, pulsed CPB emissions can be generated in response to illumination of a suitable target with a pulsed or modulated light beam(s).Such CPB can be further modulated using CPB optical elements as required.

[0018] The sample S is positioned on a sample stage 130 that can rotate about an axis 134 to a desired angle α in response to actuation of a motor or other mechanism 132. In some embodiments, all angles of the sample may be accessible due to continuous rotation of the sample. While all angles are accessible, some angles may be avoided due to limited data collection, depending on the sample geometry. For example, angles that present a surface of the sample that requires the CPB to penetrate a relatively long portion of the sample may be undesirable. Typically, the sample is adjusted through multiple angles, and corresponding stroboscopic (pulsed) electron beam emissions are applied to generate charged particles (e.g., scattered CPB fraction, secondary electrons) or electromagnetic radiation (e.g., X-rays), which are received by a detector 140 to generate corresponding images. These images can then be processed for tomography. A controller 142 is coupled to generate the stroboscopic CPB emissions and position the sample S at multiple angles relative to the axis 119. The angles can be random and include one or more complete rotations of the sample S about the axis 119. A controller typically controls the pulse timing and pulse distribution to obtain an image at a desired angle.

[0019] 1B-1D illustrate unidirectional rotation of the sample S to provide an arbitrary rotation. In these examples, an initial, near-normal incidence exposure can be used to reduce any sample degradation from this exposure. For convenience, the effective rotation angle α is considered positive for clockwise rotation of the sample S less than π / 2, and the effective rotation angle α is considered negative for counterclockwise rotation of the sample S less than π / 2. For convenience, the effective rotation angle is referenced to the CPB system axis 119. Arbitrary sample rotations starting at any initial rotation angle can be achieved using continuous clockwise or counterclockwise rotations. As shown in FIG. 1B, the sample S is positioned so that the CPB system axis 119 and the sample surface normal 129 are substantially parallel, i.e., α = 0. In FIG. 1C, the sample S is rotated clockwise such that the sample S is positioned at an angle α > 0 relative to the CPB system axis 119. In FIG. 1D, the sample S is further rotated clockwise by an angle 2π−|α| such that the sample S is positioned at an angle α<0 with respect to the CPB system axis 119. Additional rotation angles can be obtained using additional full rotational revolutions of the sample S without changing the direction of rotation. Using clockwise or counterclockwise rotation, any set of sample rotation angles α, . . . ,α N and any particular angle can be used for the initial exposure. All of these rotation angles can be provided using unidirectional rotation as needed.

[0020] Although sample S in Figures 1A-1D is shown as a lamella, other shapes can be used. As shown in Figure 1E, sample S' can have the shape of a column, pillar, or needle and can be rotated in a similar manner.

[0021] In some embodiments, images associated with smaller tilt angles are acquired before acquiring images at larger tilts. For example, exposures at a series of small tilts (both clockwise and counterclockwise relative to the CPB system axes) can be acquired first, and some exposures may require a sample rotation in one direction greater than 360 degrees or multiple orbits. The tilt angle α is generally acquired modulo 360 degrees. Exposures can then be made at larger tilts, and such exposures may require a sample rotation in one direction greater than 360 degrees.

[0022] Example 2 Referring to FIG. 2 , a CPB microscope or other CPB imaging system 200 includes a sample stage 202 arranged to rotate a sample S about an axis 201. In a typical embodiment, such rotation is continuous, and the rotation angle can be as large as 180 degrees, 360 degrees, 720 degrees, or other values. A stage driver 206 is operable to generate such rotation of the sample stage 202. The sample stage 202 and / or stage driver 206 are coupled to a rotary encoder 208, which enables the specimen rotation angle to be determined. The stage driver 206 and rotary encoder 208 are generally coupled to a controller 210, which can initiate or adjust the stage rotation. The controller 210 is also coupled to a memory 212, which stores a series of values ​​defining each stroboscopic exposure. Such values ​​can be stored, for example, as a series of exposure times, sample tilt angles, or relative phases or time differences between exposures. In some examples, the actual exposure time is calculated based on a predetermined rotation of the sample or based on a fixed or variable rotation speed of the sample stage 202.

[0023] The controller 210 is coupled to the CPB source and / or column 213 to generate the pulsed CPB 214 based on the stored set of values. The detector 218 is positioned to receive charged particles or electromagnetic radiation generated in response to the pulsed CPB 214, such as scattered electrons, secondary electrons, x-rays, or other charged or neutral particles or other electromagnetic radiation. The detector 218 is coupled to the controller 210 such that images associated with the pulsed exposures can be stored for tomographic processing using computer-executable instructions stored in memory 220. Alternatively, the controller 210 can communicate the images via a wired or wireless network to any location for tomographic processing, reconstruction, and display.

[0024] Example 3 Referring to FIG. 3, a CPB imaging system 300 includes a sample stage 302 configured to receive a sample S. A stage driver 304 is coupled to the sample stage 302 to generate sample rotation about an axis 301. In most cases, translational motion of the sample S along with the sample stage 302 is provided, although the translational components are not shown in detail in FIG. 3. The stage driver 304 can generate sample rotation at a fixed frequency f. This frequency can be set using one or more user or controller inputs to the stage driver 304, but can also be fixed internally. In most cases, the start of sample rotation is provided by a user input in preparation for image acquisition. As shown, the stage driver 304 can provide an indication of the fixed frequency f at one or more outputs. A pulse sequencer 310 is coupled to a CPB source or column 312 to generate stroboscopic exposures of the sample S at a series of sample angles or angular differences. The pulse sequencer 310 can also output data values ​​for the time or other indication of the relative time of the stroboscopic exposure. Detector 314 receives radiation responsive to the stroboscopic exposure and generates a corresponding sample image that can be output for remote or local processing to generate a tomographic image. As shown in Figure 3, the sample angle is not measured and the reconstruction is based on a series of angles or angle differences.

[0025] Example 4 Referring to FIG. 4, an exemplary method 400 includes, at 402, selecting a pulse distribution, where the pulse distribution is distributed over sample angles α 1 , . . . , α N, where N is a positive integer. The pulse distribution can be specified directly as an exposure time, an exposure rate or a combination of rates, or as a random time. The exposure time can be directly related to the angle as a relative phase to the sample rotation, or can be specified in any other convenient way. Typically, the angle is random. At 404, a stroboscopic image is acquired for each pulse of the pulse distribution using unidirectional sample rotation. In some cases, the pulse distribution is specified for sample angles α1, . . . ,α N , which may be specified modulo-2π, modulo-π, or otherwise (in radians) and may be achieved using one or more rotations greater than 90 degrees, 180 degrees, or 360 degrees. The stroboscopic images for each of the pulses are then processed at 406 to generate a tomographic reconstruction.

[0026] Example 5 Referring to FIG. 5, method 500 includes establishing a sample rotation rate f(t) at 502. Typically, this rate is constant, i.e., f(t) = constant, although any variable rotation rate can be used. Generally, a unipolar rate is preferred (f(t) > 0 rad / sec) so that the sample rotates in a single direction during exposure. Unidirectional motion reduces or eliminates rotation artifacts associated with changes in direction and allows for more rapid sample rotation, thus enabling the imaging of the sample at continuously more rapidly changing (relative) angles. At 504, an exposure time / pulse distribution is selected. Exposure times can be established using scaled random numbers to generate random exposure times. For a fixed or variable f(t), sample exposure times can be uniformly spaced, or can be based on a combination of uniform and random intervals, or some other distribution. Random rotation can also be used, but is not shown in FIG. 5 because it is generally more convenient to maintain a more or less constant rotation rate during exposure. At 506, a stroboscopic exposure is used to obtain an image for each of the exposure times, and the images can be communicated for tomographic processing or storage at 508. In some cases, the associated exposure angle is not communicated with the image, but typically each image and the associated angle is communicated.

[0027] Example 6 Referring to FIG. 6, a CPB imaging system 600 includes a sample stage 602 configured to hold a sample S. The sample stage 602 is coupled to a DC motor 604 such that the sample S can be rotated by any angle α about an axis 606 that is non-parallel (typically perpendicular) to a CPB exposure axis 608. An encoder 610 provides an indication of the rotation angle to a controller 612 coupled to a CPB source or column 614 that provides stroboscopic exposure of the sample S to pulsed CPB 615. A detector 616 is positioned to receive electromagnetic radiation or charged particles 620 from the sample S in response to exposure to the pulsed CPB 615. For example, X-rays, secondary electrons, scattered electrons, or other scattered, reflected, or diffracted charged particles can be generated. The detector 616 is coupled to the controller 612 and provides image data to the controller 612. The controller 612 can be configured to process the received images, direct the received images for remote processing, control the CPB source / column 614, the DC motor 604, the sample stage 602, and receive rotational data from the encoder 610.

[0028] Example 7 7, a method 700 of acquiring images for tomographic processing includes establishing an initial rotation angle and rotation frequency at 702. At 704, a counter I is initialized, where I is a positive integer, and at 706, samples are taken at time T I At 708, the image obtained in response to the exposure is stored, and at 710, it is determined whether to obtain an additional exposure. If so, at 712, a counter I is incremented and the exposure and image storage are repeated. Once the exposure is complete, at 714, the acquired image is communicated for tomographic reconstruction.

[0029] Example 8 FIG. 8 and the following discussion are intended to provide a brief, general description of an exemplary computing environment in which the disclosed technology may be implemented. Although not required, the disclosed technology will be described in the general context of computer-executable instructions, such as program modules, being executed by a personal computer (PC). Generally, program modules include routines, programs, objects, components, data structures, etc., that perform particular tasks or implement particular abstract data types. Furthermore, the disclosed technology can be implemented with other computer system configurations, including handheld devices, multiprocessor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, etc. The disclosed technology can also be practiced in distributed computing environments where tasks are performed by remote processing devices that are linked through a communications network. In a distributed computing environment, program modules can be located in both local and remote memory storage devices.

[0030] 8, an exemplary system implementing the disclosed technology includes a general-purpose computing device in the form of an exemplary conventional PC 800 including one or more processing units 802, a system memory 804, and a system bus 806 coupling the one or more processing units 802 to various system components including the system memory 804. The system bus 806 may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures. The exemplary system memory 804 includes read-only memory (ROM) 808 and random access memory (RAM) 810. Stored on the ROM 808 is a basic input / output system (BIOS) 812 containing the basic routines that help to transfer information between elements within the PC 800.

[0031] The exemplary PC 800 further includes one or more storage devices 830, such as a hard disk drive for reading from and writing to a hard disk, a magnetic disk drive for reading from and writing to a removable magnetic disk, and an optical disk drive for reading from and writing to a removable optical disk (such as a CD-ROM or other optical media). Such storage devices may be connected to the system bus 806 by a hard disk drive interface, a magnetic disk drive interface, and an optical drive interface, respectively. The drives and associated computer-readable media provide non-volatile storage of computer-readable instructions, data structures, program modules, and other data for the PC 800. Other types of computer-readable media can store data accessible by a PC and can also be used in the exemplary operating environment, such as magnetic cassettes, flash memory cards, digital video disks, CDs, DVDs, RAM, ROM, etc.

[0032] Several program modules may be stored on storage device 830, including an operating system, one or more application programs, other program modules, and program data. A user may enter commands and information into PC 800 through one or more input devices 840, such as a keyboard and a pointing device, such as a mouse. Other input devices may include a digital camera, microphone, joystick, game pad, satellite receiver, scanner, or the like. These and other input devices are often connected to the one or more processing units 802 through a serial port interface coupled to system bus 806, but may also be connected by other interfaces, such as a parallel port, game port, or universal serial bus (USB). A monitor 846 or other type of display device is also connected to system bus 806 via an interface, such as a video adapter. Other peripheral output devices, such as speakers and a printer (not shown), may be included.

[0033] The PC 800 can operate in a networked environment using logical connections to one or more remote computers, such as a remote computer 860. In some examples, one or more network or communication connections 850 are included. The remote computer 860 may be another PC, a server, a router, a network PC, or a peer device or other common network node, and typically includes many or all of the elements described above relative to the PC 800, although only memory storage device 862 is illustrated in FIG. 8 . The personal computer 800 and / or the remote computer 860 can be connected to logical local area networks (LANs) and wide area networks (WANs). Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets, and the Internet.

[0034] When used in a LAN networking environment, the PC 800 is connected to the LAN via a network interface. When used in a WAN networking environment, the PC 800 typically includes a modem or other means for establishing communications over the WAN, such as the Internet. In a networked environment, program modules depicted relative to the personal computer 800, or portions thereof, may be stored in the remote memory storage device or elsewhere on the LAN or WAN. The network connections shown are exemplary and other means of establishing a communications link between the computers may be used.

[0035] 8, memory 810 includes portions 804A, 806B, and 806C that store computer-executable instructions for generating random sequences (or storing one or more such sequences), image data, and stage and CPB system control, respectively. Communication with the CBP system or components associated with the CPB system is provided using one or more analog-to-digital converters (ADCs) 870 or one or more digital-to-analog converters (DACs) 871.

[0036] General Considerations As used in this application and the claims, the singular forms "a," "an," and "the" include the plural forms unless the context clearly dictates otherwise. Additionally, the term "comprises" means "comprises." Furthermore, the term "coupled" does not exclude the presence of intermediate elements between the coupled items.

[0037] The systems, devices, and methods described herein should not be construed as limiting in any way. Instead, the present disclosure is directed to all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and subcombinations with each other. The disclosed systems, methods, and devices are not limited to any particular aspect or feature or combination thereof, nor do the disclosed systems, methods, and devices require that any one or more particular advantages be present or problems be solved. While any theory of operation is for ease of explanation, the disclosed systems, methods, and devices are not limited to such theory of operation.

[0038] Although some operations of the disclosed methods are described in a particular order for convenience of presentation, it should be understood that this description style encompasses reordering unless a specific order is required by specific terminology described below. For example, operations described in order may, in some cases, be reordered or performed simultaneously. Furthermore, for simplicity, the accompanying figures may not show the various ways in which the disclosed systems, methods, and apparatuses can be used with other systems, methods, and apparatuses. In addition, the description sometimes uses terms such as "generate" and "provide" to describe the disclosed methods. These terms are high-level abstractions of actual operations that are performed. The actual operations corresponding to these terms will vary depending on the particular implementation and will be readily discernible to those skilled in the art.

[0039] In some instances, values, procedures, or devices are referred to as "lowest," "best," "smallest," etc. Such descriptions are intended to indicate that a selection from among many functional options may be made, and it will be understood that no such selection is necessarily better, lesser, or otherwise preferred than other options.

[0040] The examples are described with reference to directions indicated as "above," "below," "upper," "lower," etc. These terms are used for convenience of description and do not imply any particular spatial orientation.

[0041] The term "image" is used herein to refer to an image displayed, such as on a computer monitor, or a digital or analog representation that can be used to generate a displayed image. The digital representation can be stored in a variety of formats, such as JPEG, TIFF, or other formats. The image signal can be generated using an array detector or a single element detector, along with a suitable scan of the sample. In most practical instances, the image is two-dimensional.

[0042] Various types of motors and actuators can be used to rotate the sample stage, including DC motors, stepper motors, rotary piezoelectric motors, AC motors, or other devices. The angle of rotation can be detected using an optical encoder, a magnetic encoder, or other devices. Continuous rotation refers to rotation that can proceed during image acquisition. For example, the sequence of applying a drive signal to a stepper motor to generate rotation can continue during image acquisition, with no wait time required at the selected imaging angle. Although a stepper motor is used, the rotation is considered continuous. In other embodiments, a continuous (i.e., non-stepping) motor can freely rotate the sample. Images can be acquired at random times. In some embodiments, random time intervals are selected based on a Poisson distribution. In this case, the minimum time interval Δt and the mean time interval Δt are

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[0043] In some cases, the angular range is coarsely sampled and then refined. For example, a 20° angular step is used, followed by a 5° step (skipping previously sampled angles), then a 1° step (skipping previously sampled angles). In another example, called "dose symmetry," acquisition begins at a sample tilt of 0°, followed by angles of +°2, -2°, +4°, -4°, +6°, -6°, etc. In yet other embodiments, one or more angles can be selected using on-the-fly reconstruction, which can detect which angles contain the most useful information.

[0044] In view of the many possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated embodiments are preferred examples only and should not be construed as limiting the scope of the disclosure. We therefore claim all that comes within the scope and spirit of the appended claims.

Claims

1. 1. A method comprising: rotating the sample to each of a first plurality of absolute angles defined by a first inter-angle step size; irradiating the sample with a respective electron beam pulse after each rotation while acquiring a respective image of the sample in response to said irradiation; rotating the sample to each of a second plurality of absolute angles defined by a second inter-angle step size, the second plurality of absolute angles not coinciding with the first plurality of absolute angles; irradiating the sample with a respective electron beam pulse after each rotation through the second plurality of absolute angles that does not coincide with the first plurality of absolute angles, while acquiring a respective image of the sample in response to the irradiation; computing a tomographic reconstruction of one of the samples based on both images acquired while illuminating the sample after rotating the sample to each of the first plurality of absolute angles and images acquired while illuminating the sample after rotating the sample to each of the second plurality of absolute angles; A method comprising:

2. The method of claim 1 , wherein all rotations are in the same direction.

3. The method of claim 1 , wherein the second inter-angle step size is smaller than the first inter-angle step size.

4. The method of claim 3 , wherein the first inter-angle step size is an integer multiple of the second inter-angle step size.

5. The method of claim 1 , wherein each image is obtained by detecting charged particles or electromagnetic radiation emitted from the sample in response to the respective illumination.

6. The method of claim 1 , wherein each rotation to an absolute angle is determined by an encoder.

7. 1. An apparatus comprising: a rotatable sample holder; an electron beam source; a detection system operable to acquire an image of the sample in response to illumination of the sample by the electron beam source; one or more electronic controllers, computers, or processors electrically coupled to the rotatable sample holder, the electron beam source, and the detection system; causing the rotatable sample holder to rotate the sample on the rotatable sample holder through each of a first plurality of absolute angles defined by a first inter-angle step size; after each rotation, causing the electron beam source to irradiate the sample with a respective electron beam pulse while simultaneously causing the detection system to acquire a respective image of the sample in response to the respective irradiation; causing the rotatable sample holder to rotate the sample to each of a second plurality of absolute angles defined by a second inter-angle step size, the second plurality of absolute angles not coinciding with an absolute angle of the first plurality of absolute angles; after each rotation through the second plurality of absolute angles that does not coincide with the first plurality of absolute angles, causing the electron beam source to irradiate the sample with a respective electron beam pulse while simultaneously causing the detection system to acquire a respective image of the sample in response to the respective irradiation; computing a tomographic reconstruction of the sample based on both images acquired while illuminating the sample after rotating the sample to each of the first plurality of absolute angles and images acquired while illuminating the sample after rotating the sample to each of the second plurality of absolute angles; comprising processor-executable instructions operable to one or more electronic controllers, computers, or processors; An apparatus comprising:

8. 8. The apparatus of claim 7, wherein each image is obtained by the detection system detecting charged particles or electromagnetic radiation emitted from the sample in response to the respective illumination.

9. 8. The apparatus of claim 7, further comprising an encoder coupled to the rotatable sample holder and electrically coupled to the one or more electronic controllers, computers, or processors, the one or more electronic controllers, computers, or processors operable to read the encoder.

Images