Dark frame correction for long-term acquisition

Abstract

The present invention provides a charged particle beam (CPB) imaging apparatus and method for acquiring sample images and compensating the images based on a dark reference frame. [Solution] The charged particle beam imaging system intermittently acquires dark frames while the CPB114 is blanking and updates the dark reference with the intermittently acquired dark frames. The dark reference is used to compensate for additive noise components in the acquired sample frames, in particular additive noise that increases during multiple image acquisitions over long periods of time.

Description

[Technical Field]

[0001] This disclosure relates to charged particle beam (CPB) imaging. [Background technology]

[0002] Some CPB imaging applications require long-duration acquisitions based on the acquisition of numerous sample images over extended periods, such as in tomographic electron microscopy. These sample images can be corrected for the presence of additive noise components (e.g., 1 / f and / or random telegraph signal (RTS) noise) by using a dark reference to compensate for the sample images. In some examples, the dark reference includes a combination of dark frames taken before or after the sample images are acquired. One difficulty with these conventional approaches is that the additive noise level changes during image acquisition, and the dark reference acquired before or after image acquisition does not reflect the actual noise during image acquisition. For relatively short exposures, this is usually not a problem, but for longer exposures (e.g., a 30-second tomographic run), such dark references are not suitable. Furthermore, in some cases, sample images are used for feature tracking, a process in which artificial features, such as gold markers or recognizable intrinsic sample features, are used as a basis for image alignment. When sample images are used for feature tracking, fixed pattern noise associated with additive noise components can be mishandled to indicate that the feature of interest is not shifted within the sample image. For these and other reasons, an alternative approach is needed. [Overview of the project]

[0003] This specification describes a charged particle beam (CPB) imaging apparatus and method for acquiring sample images and compensating the images based on a dark reference frame.

[0004] A CPB imaging system may include a CPB source operable to irradiate a sample with a CPB, a beam blanker operable to selectively direct the CPB onto a sample, and a detection system operable to acquire a sequence of frames comprising a first set of sample frames, a second set of sample frames, a first set of dark frames, and a second set of dark frames, wherein the first set of sample frames and the second set of sample frames are associated with the operation of the beam blanker to irradiate the sample with the CPB, and the first set of dark frames and the second set of dark frames are associated with the operation of the beam blanker to divert at least a portion of the CPB from the sample. The dark frames may be used to define a dark reference or to reduce detector noise.

[0005] Using a CPB imaging device may include: acquiring a first set of one or more sample frames associated with a sample; compensating the first set of one or more sample frames based on a first dark criterion to generate a first set of one or more compensated sample frames; acquiring a second set of one or more sample frames; acquiring one or more dark frames after acquiring the first set of one or more frames, wherein the one or more dark frames are acquired before, after, or both before and after acquiring the second set of one or more sample frames; updating the first dark criterion based on one or more acquired dark frames to create a second dark criterion; and compensating a second set of one or more frames based on the second dark criterion to generate a second set of one or more compensated sample frames.

[0006] The aforementioned and other purposes, features, and advantages of this disclosure will become more apparent from the following detailed description, which proceeds with reference to the attached drawings. [Brief explanation of the drawing]

[0007] [Figure 1] This figure shows a typical charged particle beam (CPB) imaging system configured to update the dark reference. [Figure 2A-2C] This figure shows the effect of noise, such as 1 / f noise, on image quality during long-term image acquisition. [Figure 2D] This figure shows an exemplary sequence demonstrating the acquisition of sample frames, dark frames, and the updating of the dark frame criterion. [Figure 3] This diagram shows typical dark frame correction methods where the dark frame standard is updated. [Figure 4A] This figure shows a typical dark frame correction method in which dark frames are acquired during the interpulse period. [Figure 4B] This figure shows a typical dark frame correction method in which dark frames are acquired during the interpulse period. [Figure 5A] This figure shows the acquisition of dark frames and image frames using a subset of detector pixels. [Figure 5B] This figure shows the acquisition of dark frames and image frames using a subset of detector pixels. [Figure 6] This figure shows a typical method for obtaining sample frames and dark frames using a subset of detector pixels. [Figure 7] This is a block diagram representing an exemplary computing system or operating environment that can implement various disclosed embodiments. [Modes for carrying out the invention]

[0008] This specification discloses methods and apparatus for charged particle beam (CPB) imaging, such as CPB tomography. In CPB tomography, the sample is typically positioned on a rotatable sample stage for repeated exposure to the CPB at multiple angles. The disclosed examples are generally described with reference to transmission electron microscopy for use in tomographic reconstruction, but other CPBs and other types of imaging can be used. Alternatively, the sample can be irradiated with X-rays and an X-ray-based image can be obtained.

[0009] According to several aspects of this disclosure, during CPB imaging, such as transmission electron microscopy (TEM), image correction is performed to convert the raw data from the detector into frames that can be used for further processing, such as “dark correction.” Dark correction can compensate for the frames for additive noise components (e.g., 1 / f and / or RTS noise) present in the detector data. To achieve this correction, several dark frames are typically collected before the sample is exposed to the CPB, and sometimes after exposure. These dark frames can be combined to create a dark criterion, which is then used to compensate the sample frame. While it is possible to create a dark criterion using a single dark frame, in some examples, multiple dark frames can be combined to create a dark criterion. In some examples, it is desirable to create an improved dark criterion by averaging multiple dark frames. In some examples, as can be understood from the following disclosure, each dark frame can be assigned a weighting coefficient (e.g., with respect to time and / or additive noise level), and the dark criterion includes a weighted average of the dark frames.

[0010] General Considerations As used herein, “column” or “optical 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, stigmeter, or other CPB optical elements. Using one or more such optical elements, a pulsed CPB can be generated that can be directed at a sample to provide pulsed exposure. Such pulsed exposure is generally called “stroboscopic” exposure, where the effective exposure time is short enough for a rotation of the sample to produce a suitable image, i.e., there is no blurring due to excessive movement. Suitable exposure times can correspond to rotations of less than 0.0001, 0.001, 0.01 degrees, or other angles. Any specification of the maximum allowable exposure duration may vary depending on the magnification of the image and the desired resolution. In stroboscope exposure, the CPB may have a continuous component in addition to the stroboscope component. In many practical embodiments, pulsed exposure is preferred to reduce the degradation of the sample produced by CPB exposure with a continuous component. The continuous component can contribute to undesirable sample changes without improving the tomographic image.

[0011] The term "image" is used herein to refer to an image displayed on a computer monitor or the like, or a digital or analog representation that can be used to generate an image to be displayed. Digital representations can be stored in various formats, such as JPEG, TIFF, or other formats. Image signals can be generated using an array detector or a single-element detector, along with a suitable scan of the sample. In the most practical example, the image generated by the detector is two-dimensional, such as a two-dimensional array of pixels. In some examples, the image may be three-dimensional. In some examples, the image may be a composite image composed of multiple images. In one example, the composite image may include a tomographic reconstruction of the sample.

[0012] The term “frame” is used herein to refer to data related to a detector and can be used as an image, to form an image, or to compensate for an image. As used herein, “sample frame” refers to a frame based on the exposure of a sample to a CPB, and “dark frame” refers to a frame acquired without (or with reduced exposure to) exposure to a CPB, typically by blocking or attenuating the CPB that would otherwise be directed at the sample. In its most practical example, a frame is a two-dimensional array of pixels, often acquired using an array detector, particularly in TEM. Often, a frame contains image data as two-dimensional data arranged in rows and columns. The term “subframe” may be used to describe a portion of a frame, for example, a one-dimensional line of pixels in a two-dimensional frame or other parts of a frame.

[0013] In some cases, the detector used to generate a sample frame includes an array of pixels, each pixel accumulating charge in response to exposure of the sample S to the CPB. This charge accumulation allows the sample frame to be acquired from the detector for at least a period related to the charge accumulation time constant even after the CPB has been blanked. Further, using such charge accumulation, the acquisition of a dark frame is preferably timed such that any charge associated with a previous CPB exposure is dissipated, for example, by being read out as part of the sample frame, read out and discarded, or decaying based on the device charge accumulation time constant. In some applications, a number of sample frames are acquired, and it is preferred that switching the CPB current delivered to the sample from a higher current to a lower current (or no current) be rapid. Switching the CPB current delivered to the sample from a higher current to a lower current is generally referred to herein as "blanking," and such blanking can generally be completed such that the CPB current in the sample is zero. More generally, blanking can refer to reducing the CPB current delivered to the sample from a value of I related to the acquisition of the sample frame to I / 4, I / 5, I / 10, I / 20, I / 50, or I / 100 or less. In the example, blanking is shown using an electrostatic beam deflector, but blanking can be provided by a magnetic beam deflector, a combination of an electrostatic beam deflector and a magnetic beam deflector, direct modulation of the CPB emission, focusing and defocusing using one or more CPB lenses, or other means.

[0014] As used in this application and the claims, the singular forms "a," "an," and "the" include the plural form unless the context clearly dictates otherwise. Additionally, the term "includes" means "comprises." Further, the term "coupled" does not exclude the presence of intermediate elements between the coupled items.

[0015] The systems, devices, and methods described in this specification should not be construed as being limiting in any way. Instead, this disclosure is directed to all novel and non-obvious features and aspects of the various disclosed embodiments, alone and in various combinations and sub-combinations with each other. The disclosed systems, methods, and devices are not limited to any specific aspect or feature or combination thereof, and the disclosed systems, methods, and devices do not require the presence of any one or more specific advantages or the solving of any problems. Any theory of operation is for the purpose of facilitating explanation, but the disclosed systems, methods, and devices are not limited to such theory of operation.

[0016] Some of the operations of the disclosed methods are described in a particular sequential order for convenience of presentation, but it should be understood that the specification format encompasses permutations unless a particular ordering is required by specific language set forth below. For example, the operations described sequentially may in some cases be permuted or performed simultaneously. Further, for simplicity, the accompanying drawings may not show the various ways in which the disclosed systems, methods, and devices may be used with other systems, methods, and devices. Additionally, the specification may use terms such as "produce" and "provide" to describe the disclosed methods. These terms are high-level abstractions of the actual operations performed. The actual operations corresponding to these terms will vary depending on the particular implementation and will be readily recognizable to those of ordinary skill in the art.

[0017] In some examples, values, procedures, or devices are referred to as "lowest," "best," "minimum," etc. Such descriptions are intended to indicate that a selection can be made from among a number of available functional alternatives, but it should be understood that such selection need not be better, smaller, or otherwise preferable to other selections.

[0018] Examples are explained by referring to directions such as "above," "below," "upper," and "lower." These terms are used for convenience and do not imply any specific spatial orientation.

[0019] Example 1 Referring to Figure 1, the CPB microscope or other CPB imaging system 100 includes a CPB source and an optical column 113 that can be operated to irradiate a sample S with a CPB 114. A beam blanking system 103 (also called a “beam blanker”) is positioned to interrupt, deflect, or otherwise modulate the CPB 114 at predetermined intervals. In some examples, the CPB source may comprise an electron beam source. In some examples, the beam blanking system 103 can be operated to intermittently interrupt the CPB so that dark frames or sample frames can be acquired. As used herein, “intermittent interruption” includes interruptions at any one of periodic time intervals, random time intervals, after the acquisition of a predetermined number of sample frames, and / or intervals determined based on the size of the noise magnitude in the dark frames, or any other time.

[0020] In some examples, the beam blanking system 103 includes an aperture defined in an aperture plate 118 that can be used to interrupt or attenuate the CPB 114 in response to the application of a deflection voltage from a deflection driver 130 to a beam deflector 132, such as a resonant beam deflector. The deflector driver 130 is operable to generate the deflection of the CPB 114. When the beam deflector 132 is actuated, the CPB 114 is deflected to form a deflected beam 116 that is blocked by the aperture plate 118. In some examples, the CPB 114 is completely blocked by the aperture plate 118. In some examples, the deflected beam 116 is deflected such that only a portion of the CPB 114 is blocked by the aperture plate 118, and as a result, the sample S is irradiated with a reduced CPB beam current. In some cases, the blanked portion of CPB 114 irradiates a smaller portion of sample S than the portion irradiated by the unblanked CPB, or irradiates the same portion but at a lower CPB current density.

[0021] During the time when the CPB 114 is not deflected by the beam blanking system 103, the detection system, comprising the detector 119, is operable to acquire a sequence of sample frames related to the exposure of the sample S to the CPB 114. When the CPB 114 is deflected to form a deflected beam 116, the detector 119 is operable to acquire one or more dark frames related to additive noise components, such as background noise. As described above, in some cases, charges related to sample irradiation that are accumulated by the detector 119 and not blanked are read out or otherwise removed before acquiring the dark frames. In the example in Figure 1, the deflected CPB 116 is completely blocked by the aperture plate 118, and the dark frames contain only additive noise components or other signal portions not based on CPB irradiation. In some examples, only the portion of the deflected beam is attenuated, and the dark frames contain both the sample-based portion and the additive noise component together. In another example, a deflected CPB is incident on the sample S for acquiring the sample frame, while an undeflected CPB is associated with acquiring the dark frame. In yet another example, the beam blanking system can be operated to provide corresponding deflections to generate both the dark frame and the sample frame.

[0022] The deflector driver 130 is generally coupled to a controller 110 that can initiate or adjust beam deflection. In some examples, the controller 110 may be programmed to blank the CPB 114 as described above. The controller 110 is coupled to the CPB source and optical column 113 and can generate the CPB 114 as pulsed CPB to provide stroboscope sample exposure. The detector 119 is arranged to receive charged particles or electromagnetic radiation generated in response to the CPB 114, such as scattered electrons, secondary electrons, X-rays, or other charged or neutral particles, or other electromagnetic radiation. The detector 119 is coupled to the controller 110 so that sample frames associated with CPB exposure and dark frames associated with attenuated (or no exposure) exposure are acquired using computer-executable instructions stored, for example, in part 120 of memory 121. Memory 121 also stores one or more sample frames, dark frames, dark references, and computer-executable instructions for determining the dark reference in part 122. Such sample frames, dark frames, and dark references can be stored, for example, along with a series of exposure times, sample tilt angles, or relative phase or time differences between acquisitions. In a typical example, dark frames acquired in a sequence are averaged to generate a dark reference, but a single dark frame may be selected as a dark reference used to compensate the sample frame by removing or reducing non-sample-related contributions to the sample frame in order to generate an image frame (sometimes called a "compensated image frame"). In some examples, compensation means reducing the magnitude of dark noise in the image frame by more than 80%, more than 90% in some examples, more than 95% in some examples, and more than 99% in some examples. Compensation generally relies on generating an image frame by subtracting an appropriate dark reference pixel by pixel from the sample frame, and the controller 110 can then combine the image frames to create a composite image of the sample S.In some examples, the sample stage 102 is configured to rotate the sample S around axis 101. The actual exposure time may be determined based on a predetermined rotation rate of the sample S, or based on a fixed or variable rotation speed of the sample stage 102. Alternatively, the controller 110 can communicate sample frames and dark frames to any location via a wired or wireless network for noise correction, tomography processing, reconstruction, and display.

[0023] Example 2 Figures 2A and 2B show examples of the first (Figure 2A) and last (Figure 2B) sample images in a 30-second acquisition using a fixed dark criterion to compensate for noise, where the fixed dark criterion was acquired before the acquisition of the first sample frame. Figure 2C shows the noise associated with Figures 2A and 2B. The stripe pattern seen in Figure 2B is caused by changes in noise (e.g., 1 / f noise) as the frame is acquired, and as a result, the compensation fails to reduce image artifacts and introduces them. The fixed dark criterion may become less useful in correcting sample frames as the acquisition progresses due to changes in additive or other noise components such as 1 / f noise and random telegraph signal (RTS) noise. The fixed dark criterion induces a fixed pattern in the compensated sample image sequence. In some examples, the sample image sequence can be used for feature tracking, thereby allowing artificial features such as gold markers or recognizable unique sample features to be used as a basis for image alignment and / or to determine sample drift. When sample images using inappropriate or outdated dark frames are used for tracking, fixed pattern noise associated with noise components can lead to inaccurate tracking, thereby failing to detect shifts in the sample or tracking noise features instead of sample features.

[0024] For short exposures (sometimes around 1–10 seconds), a dark criterion based on dark frames collected before acquisition may suffice (see, for example, the image shown in Figure 2A, which does not exhibit the “stripe pattern” described above). For longer exposures, such as during a 30-second tomography, the impact of using a fixed dark criterion based on dark frames collected before acquisition can be significant, as the dark criterion may induce a fixed pattern within the compensated image sequence, as shown by the vertical stripes in Figure 2B.

[0025] Figure 2C shows plots 204 and 206 of the standard deviation versus time t in dark frames obtained from an exemplary charged particle detector. Plot 204 shows data based on a fixed dark criterion acquired before sample frame acquisition when there is no sample contribution. (A fixed dark criterion is well suitable.) As time progresses from the first frame, the initial dark images become less relevant. The effect of the mismatch between the dark criterion and the noise level is expected to increase approximately as the square root of the elapsed time. Plot 206 shows data based on updating the dark criterion after every 32 sample frames. It can be seen that the effect of dark noise on the sample images is reduced when the dark criterion is updated intermittently.

[0026] Figure 2D shows representative sequences 210, 220, and 230 of acquired frames, including sample frame SF and dark frame DF, which are used to construct the associated dark reference DR and sample image SI. Representative sequence 210 of frames is associated with an initial dark reference DR0 constructed before the acquisition of any sample frame. The sample is the sample frame

[0027]

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[0029] Dark frames can be combined to create a dark reference DR, for example, by averaging frames within a set of dark frames or by selecting a representative dark frame from a set of dark frames. An updated dark reference can be constructed using a weighted average of dark frames or previous dark references, with dark frames or dark references further away in time from the sample frame being given less weight. In some examples, a spatial context can be used to construct the updated dark reference. In some examples, known spatial correlations may exist between different pixels, and these correlations can be used to obtain a low-noise dark reference more quickly. In some examples, depending on the exact camera chip layout, the dark noise is related to the digital-to-analog converter blocks as well as rows and columns, and there are known spatial correlations between different pixels that can be used to quickly obtain a low-noise dark reference. In some examples, it is possible to develop a model of the rate of change of the dark noise pattern, which can be used for better estimation. In some examples, this modeling is primarily based on 1 / f characteristics.

[0030] In some cases, the dark criterion DR is calculated by updating a previously stored dark criterion with dark frames from a subsequently acquired set of dark frames. The dark criterion can also be obtained by combining multiple dark criterions, typically associated with a set of consecutively acquired dark frames. Sample frames within any given set of sample frames are generally corrected using the same dark criterion, but are not required to be so. For example, a subset of the sample frame set acquired at the beginning of a set can use a previously established dark criterion, while a subset of frames acquired at the end of a set can use a later determined dark criterion that includes contributions from the associated subsequent set of dark frames.

[0031] Sample frames and dark criteria (based on dark frames) can be used to create a sample image SI by compensating a given set of one or more sample frames based on the corresponding dark criteria. For example, a set of sample frames SF 2 Sample frame

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[0035] In some cases, dark frames are acquired using a CPB that is attenuated but not completely blanked. For example, the CPB current I associated with sample irradiation to acquire a sample frame can be attenuated to (1-α)I during dark frame acquisition, where 0 ≤ α < 1. In the case of sample and dark frames related to common additive noise, the common noise is removed by subtracting the dark reference from the sample frame, but the resulting difference reduces the size of the sample image (SI), which is...

[0036]

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[0037] Typically, multiple sample images are obtained by compensating multiple sets of sample frames based on their respective dark frame criteria. Compensation may be performed in real-time and / or near real-time as the sample frames are acquired, which is preferable for tracking sample image features during acquisition. In other examples, the sample frames and dark frames are stored, and the sample image is generated at other times, such as after the acquisition of the sample and dark frames.

[0038] Example 3 Referring to Figure 3, a typical method 300 includes, in 302, acquiring one or more dark frames and constructing a dark reference (DR). In some examples, constructing the dark reference includes averaging one or more dark frames. In some examples, the dark frames are associated with the additive noise level present before any sample frame is acquired, or the initial dark reference DR0 may be defined as an array of zeros. In 304, the CPB is not blanked, and the sample is exposed to the CPB.

[0039] In 306, while the sample is exposed to the CPB, one or more frames associated with the sample (sample frames) are acquired until a pre-selected criterion is met. The pre-selected criterion may be a predetermined number of sample frames, a fixed time interval, or any other desired criterion. After the pre-selected criterion is met, in 308, the beam is blanked again, and in 310, a second set of one or more dark frames is acquired. The dark frames in the second set may be one or more dark frames of a set number. The dark criteria are then updated based on these acquired dark frames.

[0040] At step 312, it is determined whether the acquisition should be terminated. If the acquisition is to continue, the process returns to step 304. In this way, the dark criterion may be intermittently updated during the acquisition of sample frames, and as a result, the dark criterion may be used to generate image frames by effectively compensating for noise in the sample frames. Sample images can be generated during frame acquisition as shown in Figure 3, but they can also be generated upon completion of all or some acquisitions.

[0041] Example 4 Using pulsed exposure, dark frames and sample frames can be acquired without requiring additional beam blanking. Referring to Figure 4A, method 400 includes selecting the CPB pulse duration and repetition rate or other CPB pulse timing in 402. In 404, an initial dark reference is established, and in 406, one or more CPB pulses are applied and the corresponding sample frames are acquired. In 408, one or more dark frames are acquired within the inter-pulse period. Dark frame acquisition may require reading the accumulated charge in the detector based on the previous pulse exposure, or providing a delay for the accumulated charge to dissipate. In some cases, dark frames are acquired in each inter-pulse period after each CPB pulse and sample frame acquisition. Alternatively, dark frames can be acquired at a selected pulse period, and if necessary, the pulse repetition rate can be reduced to accommodate longer time periods for dark frame acquisition. In 410, a decision is made whether to acquire additional sample frames and dark frames, and if so, the method returns to 406. If acquisition is complete, the sample frames and dark frames are communicated in 412. In this approach, beam blanking is associated with pulsed CPB exposure, and compensation is performed after all sample and dark frames have been acquired.

[0042] In some cases, the CPB pulse is applied by pulse deflection of a constant beam. In some cases, the CPB pulse is applied by a pulsed beam. In some cases, the CPB is created by photoemission, and the beam pulse is the result of blanking a light beam.

[0043] Referring to Figure 4B, method 450 includes selecting the CPB pulse duration and repetition rate or other CPB pulse timing in 452. In 454, an initial dark criterion is established, and in 456, one or more CPB pulses are applied and corresponding sample frames are acquired. In 458, one or more dark frames are acquired within the inter-pulse period. In 460, the sample frames and dark frames are combined to produce a compensated image. In some examples, one or more dark frames may be acquired before the application of one or more CPB pulses and the corresponding acquisition of sample frames. In some examples, one or more dark frames may be acquired after the application of one or more CPB pulses and the corresponding acquisition of sample frames. In some examples, one or more dark frames may be acquired before and after the application of one or more CPB pulses and the corresponding acquisition of sample frames. In 462, it is decided whether to acquire additional sample frames and dark frames, and if so, the method returns to 456.

[0044] Example 5 In a typical application, sample frames and dark frames are acquired using a common set (usually virtually all) of pixels in the array detector, and these frames are acquired and read out from the array detector with different readout operations, so that both the sample frame and the dark frame can generally be referred to as a “full” frame. If the exposed portion of the sample is imaged onto a subset of detector array pixels, the readout of the full detector array includes a sample subframe corresponding to the exposed portion of the sample imaged on the subset, and a dark subframe corresponding to other detector array pixels (a “complementary” set or region, as used herein).

[0045] In examples where a sample image is acquired by sequentially exposing a sample region smaller than the entire region of interest (ROI) and mapped to a subset of the detector pixel region, sample subset frames and dark subset frames can be acquired in single and multiple acquisitions of different sample regions used to provide a full frame. Referring to Figures 5A and 5B, a subset 504 of pixels on the detector active surface 500 is shown as receiving radiation in response to irradiation of the corresponding sample region. A subset of pixels 502 is not exposed and can be used to acquire a dark subset frame. In Figure 5B, a subset 514 of pixels on the detector surface 500 is shown as receiving radiation in response to irradiation of the corresponding sample region. A subset of pixels 512 is not exposed and can be used to acquire a dark subset frame. By combining multiple sample subset frames, a sample frame associated with the entire detector active surface can be generated. By combining multiple dark subset frames, a dark frame associated with the entire detector active surface can be generated. Pixel subsets can have various shapes and sizes. In this approach, the CPB is limited to exposing only the portion of the sample being imaged to a subset of detector pixels. In this example, the CPB is blanked (or shaped) to illuminate only the selected area. As mentioned above, the CPB within the dark frame area does not need to be completely blocked.

[0046] Example 6 Referring to Figure 6, a typical method 600 includes, in 602, illuminating a selected portion of the sample region of interest (ROI), and in 604, imaging the selected portion into a subset of detector array pixels. In 606, a sample subset frame corresponding to the illuminated sample portion is acquired, and a dark subset frame corresponding to the complementary (unilluminated) sample portion is acquired. In 608, additional portions of the sample region can be selected for imaging, and the acquisition process can be repeated. When completed, the sample subset frames and dark subset frames are stitched together in 612 and 614 to form the sample frame and dark frame, respectively, and in 616, the sample frame is compensated based on the dark frame to form the sample image. A specific arrangement of sample and dark frame values ​​as corresponding arrays can be used, but other suitable mappings can also be used.

[0047] Example 7 Figure 7 and the following discussion are intended to provide a brief and general description of exemplary computing environments in which the disclosed techniques may be implemented. For example, one or more aspects of CPB instrument controllers and frame processing may be achieved in this computing environment, which may also be used to control beam blanking systems, to control charged particle microscope systems, and / or to perform any part of the methods disclosed above.

[0048] Although not mandatory, the disclosed technologies are personal computers. This disclosure describes computer executable instructions, such as program modules, that are executed by a computer (PC). Generally, a program module includes routines, programs, objects, components, data structures, etc., that perform a specific task or implement a specific abstract data type. Furthermore, the disclosed technology can be implemented using other computer system configurations, including handheld devices, tablets, multiprocessor systems, microprocessor-based or programmable consumer electronics, network PCs, minicomputers, mainframe computers, virtual machines, and containerized applications. The disclosed technology can also be practiced in a distributed computing environment where tasks are performed by remote processing devices linked over a communication network. In a distributed computing environment, program modules can reside in both local and remote memory storage devices. The system of this disclosure can control image acquisition and provide a user interface, and can also function as an image processor.

[0049] Referring to Figure 7, an exemplary system implementing the disclosed technology includes a general-purpose computing device in the form of an exemplary conventional PC 700, which includes one or more processing units 702, a system memory 704, and a system bus 706 that connects various system components, including the system memory 704, to the one or more processing units 702. The system bus 706 may be one of several types of bus structures, including a memory bus or memory controller, peripheral bus, and local bus, using any of various bus architectures. The exemplary system memory 704 may include read-only memory (ROM) and random access memory (RAM), and the ROM may store a basic input / output system (BIOS) containing basic routines that help transfer information between elements within the PC 700.

[0050] An exemplary PC 700 further includes one or more storage devices 730, such as hard disk drives, for reading from and writing to hard disks or solid-state drives that can be connected to a system bus 706 via a hard disk drive interface. Such devices and associated computer-readable media provide non-volatile storage for computer-readable instructions, data structures, program modules, and other data for the PC 700. Some program modules may be stored in the storage devices 730, including operating systems, multiple operating systems, virtual operating systems, one or more application programs, other program modules, and program data.

[0051] An exemplary PC700 may include various devices configured for a user interface. For example, a user may input commands and information to the PC700 through one or more input devices 740, such as a keyboard and a pointing device such as a mouse. For example, a user may input commands to start image acquisition and / or to start one or more methods disclosed herein. The input devices are often connected to one or more processing units 702 through a serial port interface coupled to the system bus 706, but may be connected by other interfaces such as a parallel port, Universal Serial Bus (USB), or wired or wireless network connection. A monitor 746 or other type of display device may also be connected to the system bus 706 through an interface such as a video adapter, and may display one or more images of a sample or specimen, for example, before, after, and / or during the implementation of one or more methods disclosed herein.

[0052] As shown in Figure 7, the memory 704 includes portions 771 and 772 for storing sample frames and dark frames, respectively; portion 773 for storing computer-executable instructions for sample frame compensation and image frame storage; and portion 774 for providing instructions for CPB control, blanking, frame acquisition, and other operations related to the disclosed method.

[0053] The PC700 may operate in a network environment using logical connections to one or more remote computers, such as remote computers 760. In some examples, one or more network or communication connections 750 are included for data acquisition and control, such as wired or wireless connections, as well as digital-to-analog converters and analog-to-digital converters. The remote computer 760 may be another PC, server, router, network PC, or peer device, or other common network node, and typically includes many or all of the above elements in relation to the PC700, except that only the memory storage device 762 is illustrated in Figure 7. The personal computer 700 and / or remote computer 760 are local area networks (LANs) and wide area networks (wide It may be connected to an area network (WAN). Such networking environments are common in offices, enterprise-wide computer networks, intranets, and the internet. The network connections shown are illustrative, and other means may be used to establish communication links between computers.

[0054] Example 8. CPB imaging for tomographic reconstruction The disclosed approach is particularly useful in CPB tomography due to the relatively long image acquisition process required to obtain the necessary number of images to be combined. In some examples of CPB tomography, a sequence of frames used for tomographic reconstruction is acquired using stroboscope illumination of a rotating sample at random angles or other sets of angles. The sample may rotate at a uniform angular velocity, and the random angle exposures may be established based on one or more sets of angle values ​​that can be generated as needed or acquired from a computer-readable storage device such as memory. The angles may be based on the phase associated with the rotation of the sample, a set of exposure times based on the rotation speed of the sample, randomly generated during image acquisition, or otherwise specified. The sample may be rotated at a fixed or variable speed and stroboscope illumination during rotation. The stroboscope illumination may be a fixed or variable pulse rate including random time-duration rotations. The illumination angle may be determined before, during, or after illumination, and the illumination angle may be a fixed or variable angular distribution including random angles. Dark frames can be acquired between stroboscope pulses or at times associated with sample rotation where sample frame acquisition is not required. As mentioned above, while compensating for the sample frame by subtracting the dark reference during dark frame acquisition still attenuates noise, the pulsed CPB radiation does not need to be completely attenuated because the magnitude of the signal frame is somewhat reduced due to the non-zero CPB irradiation of the sample. For samples rotating in one direction, dark frames can be acquired for rotations at fixed angles, such as angles where sample frames are not acquired or other arbitrary convenient angles.

[0055] In some examples, sample exposure is performed using a constant sample rotation speed for convenience to achieve uniformly spaced exposures, but non-uniform speeds such as monotonically increasing or decreasing speeds, or arbitrary increasing and decreasing rotation speeds, can be used. Uniform and constant rotation allows for acquisition of samples at random exposure angles with appropriate pulse rates or pulse intervals. As mentioned above, such random exposures can be based on random exposure times or rotation phases that can be stored or generated as needed. Alternatively, the sample rotation can be made variable, such as a random speed, and the exposure times can be separated by a constant delay.

[0056] Example 9. Additional Examples In addition to imaging for tomographic reconstruction, the disclosed approach may be useful for several other applications. For example, applications requiring long acquisitions relative to the rate of change of dark noise. In some examples, the application involves navigating or searching on a sample. In some examples, the application involves dynamic image processing of the sample. In some examples, the application requires very high doses relative to the dose per unit of time, which may be limited by either the CPB source intensity or camera characteristics and therefore accumulate over long periods.

[0057] In some examples, a subsequent set of one or more sample frames is acquired after a predetermined number of sample frames have been acquired in the preceding set of one or more sample frames. In some examples, any of the parameters may change depending on user input, presets, or other inputs. In some examples, the number of samples in the first and / or second sets of one or more samples is dynamic, and the frameset size and / or duration are adjusted on the fly during acquisition, for example, depending on data quality.

[0058] Disclosure clauses In consideration of the above-mentioned implementations of the subject matter to be disclosed, this application discloses the following additional provisions. Note that one feature of a single provision, or a combination of two or more features of that provision, is also included in the disclosure of this application, at its discretion, in combination with one or more further provisions and one or more features of further provisions.

[0059] Clause 1 is a method that includes taking a first set of one or more sample frames associated with a sample; compensating the first set of one or more sample frames based on a first dark criterion to produce a first set of one or more compensated sample frames; taking a second set of one or more sample frames; taking one or more dark frames after taking the first set of one or more frames, wherein the one or more dark frames are taken before, after, or both before and after taking the second set of one or more sample frames; updating the first dark criterion based on one or more acquired dark frames to create a second dark criterion; and compensating a second set of one or more frames based on the second dark criterion to produce a second set of one or more compensated sample frames.

[0060] Clause 2 includes the subject matter of Clause 1 and further specifies that each sample frame of a first set of one or more sample frames and a second set of one or more sample frames is obtained by detecting charged particles or electromagnetic radiation in response to irradiation of the sample with a charged particle beam (CPB).

[0061] Clause 3 further specifies that the subject matter described in any of Clauses 1 or 2 includes an electron beam.

[0062] Clause 4 further specifies that the subject matter described in any of Clauses 1 to 3 is included, and that a first set of one or more sample frames and a second set of one or more sample frames are taken at one or more inclination angles of the sample relative to the CPB.

[0063] Clause 5 further includes combining a first set of one or more compensated sample frames and a second set of one or more compensated sample frames to create a composite image of a sample that includes the subject matter described in any of Clauses 1 to 4.

[0064] Clause 6 further specifies that the subject matter described in any of Clauses 1 through 5 includes the composite image, and that the composite image includes tomographic reconstruction of the sample.

[0065] Clause 7 further includes rotating the sample sequentially while obtaining a first set of one or more sample frames and a second set of one or more sample frames, with respect to the subject matter described in any of Clauses 1 through 6.

[0066] Clause 8 includes the subject matter described in any of Clauses 1 through 9, and further specifies that each sample frame comprises a two-dimensional array of pixels, a first set of one or more sample frames, a second set of one or more sample frames, and one or more acquired dark frames.

[0067] Clause 9 further specifies that obtaining a second set of one or more sample frames, which includes the subject matter described in any of Clauses 1 through 8, is performed after obtaining a predetermined number of sample frames in the first set of one or more sample frames.

[0068] Clause 10 further specifies that obtaining a second set of one or more sample frames, which includes the subject matter described in any of Clauses 1 through 9, shall be performed at fixed time intervals after obtaining one or more sample frames from the first set of one or more sample frames.

[0069] Clause 11 includes, further, tracking sample features using a first set of one or more compensated sample frames and a second set of one or more compensated sample frames, with respect to the subject matter described in any of Clauses 1 through 10.

[0070] Clause 12 includes the subject matter described in any of Clauses 1 through 11, and involves obtaining a second set of one or more sample frames, followed by a second set of one or more dark frames, and then a third set of one or more sample frames. The further includes compensating for a third set of one or more sample frames based on a first set of one or more dark frames and at least one of a second set of one or more dark frames.

[0071] Clause 13. A charged particle beam (CPB) imaging apparatus may include a CPB source operable to irradiate a sample with a CPB, a beam blanker operable to selectively direct the CPB onto a sample, and a detection system operable to acquire a sequence of frames comprising a first set of sample frames, a second set of sample frames, a first set of dark frames, and a second set of dark frames, wherein the first set of sample frames and the second set of sample frames are associated with the operation of the beam blanker to irradiate the sample with a CPB, and the first set of dark frames and the second set of dark frames are associated with the operation of the beam blanker to divert at least a portion of the CPB from the sample.

[0072] Clause 14 includes the subject matter set forth in Clause 13, and further specifies that a beam blanker comprises a beam deflector and an aperture plate, the beam deflector selectively directs the CPB away from the aperture of the aperture plate.

[0073] Clause 15 further includes a controller configured to control a beam blanker to generate a first set of frames, a second set of frames, a first set of dark frames, and a second set of dark frames, which include the subject matter described in any of Clauses 13 to 14.

[0074] Clause 16 includes the subject matter set out in any of Clauses 13 to 15 and further specifies that the controller is configured to establish a first dark standard based on a first set of dark frames, update the first dark standard based on a second set of dark frames to create a second dark standard, compensate the first set of sample frames using the first dark standard to create a first set of compensated sample frames, and compensate the second set of frames using the second dark standard to create a second set of compensated sample frames.

[0075] Clause 17 includes the subject matter described in any of Clauses 13 to 16, and further specifies that the controller is capable of generating a composite image by combining a first set of compensated sample frames with a second set of compensated sample frames.

[0076] Clause 18 includes the subject matter set out in any of Clauses 13 to 17, and further specifies that the controller may operate to calculate the tomographic reconstruction of the sample by combining a first set of compensated sample frames with a second set of compensated sample frames.

[0077] Clause 19 includes the subject matter set out in any of Clauses 13 to 18, further specifying that a CPB source includes an electron beam source, and a CPB includes an electron beam.

[0078] Clause 20 is a charged particle beam (CPB) imaging method, To acquire one or more dark frames during each intermittent interruption, sample frame acquisition is intermittently interrupted based on the radiation received from the sample in response to the CPB. A charged particle beam (CPB) imaging method comprising establishing a sample image based on a sample frame and one or more dark frames acquired between each intermittent interruption.

[0079] Clause 21 includes the subject matter set out in Clause 20 and further stipulates that establishing a sample image involves compensating for multiple sets of sample frames based on each set of dark frames taken during associated intermittent interruptions, and that the sample image is established by combining the compensated sets of sample frames.

[0080] Clause 22 includes the subject matter described in any of Clauses 20 to 21, and further specifies that the sample frame is taken at one or more inclination angles of the sample relative to the CPB.

[0081] In consideration of the many possible embodiments to which the principles of the disclosed technology may be applied, it should be recognized that the illustrated clauses are merely preferred clauses and should not be construed as limitations.

Claims

1. It is a method, Obtaining a first set of one or more sample frames associated with a sample, Compensating a first set of one or more sample frames based on a first dark criterion to generate a first set of one or more compensated sample frames, Obtain a second set of one or more sample frames, After obtaining a first set of one or more frames, one or more dark frames are obtained, wherein the one or more dark frames are obtained before, after, or both before and after, the acquisition of a second set of one or more sample frames. To create a second dark criterion, the first dark criterion is updated based on one or more acquired dark frames, Compensating a second set of one or more frames based on the second dark criterion to generate a second set of one or more compensated sample frames, Methods that include...

2. The method according to claim 1, wherein each sample frame of the first set of one or more sample frames and the second set of one or more sample frames is obtained by detecting charged particles or electromagnetic radiation in response to irradiation of the sample with a charged particle beam (CPB).

3. The method according to claim 2, wherein the CPB includes an electron beam.

4. The method according to claim 2, wherein the first set of the one or more sample frames and the second set of the one or more sample frames are obtained at one or more inclination angles of the sample with respect to the CPB.

5. The method according to claim 4, further comprising combining a first set of one or more compensated sample frames and a second set of one or more compensated sample frames in order to create a composite image of the samples.

6. The method according to claim 5, wherein the composite image includes tomographic reconstruction of the sample.

7. The method according to claim 4, further comprising continuously rotating the sample while acquiring a first set of one or more sample frames and a second set of the one or more sample frames.

8. The method according to claim 1, wherein each sample frame of the one or more sample frames, the one or more sample frames, and the one or more acquired dark frames comprises a two-dimensional array of pixels.

9. The method according to claim 1, wherein acquiring a second set of the one or more sample frames is performed after acquiring a predetermined number of sample frames in the first set of the one or more sample frames.

10. The method according to claim 1, wherein the acquisition of the second set of one or more sample frames is performed at fixed time intervals after the acquisition of one or more sample frames from the first set of one or more sample frames.

11. The method according to claim 4, further comprising tracking sample features using a first set of one or more compensated sample frames and a second set of one or more compensated sample frames.

12. After obtaining a second set of one or more sample frames, obtain a second set of one or more dark frames, Obtain a third set of one or more sample frames, The method according to claim 4, further comprising compensating a third set of one or more sample frames based on at least one of a first set and a second set of one or more dark frames.

13. A charged particle beam imaging device (CPB imaging device), A CPB source capable of irradiating a sample with CPB, A beam blanker that can be operated to selectively direct the CPB towards the sample, A detection system operable to acquire a sequence of frames comprising, respectively, a first set of sample frames, a second set of sample frames, a first set of dark frames, and a second set of dark frames, wherein the first set of sample frames and the second set of sample frames are associated with the operation of the beam blanker for irradiating the sample with the CPB, and the first set of dark frames and the second set of dark frames are associated with the operation of the beam blanker for diverting at least a portion of the CPB from the sample; A CPB imaging device equipped with the following features.

14. The CPB imaging apparatus according to claim 13, wherein the beam blanker comprises a beam deflector and an aperture plate, and the beam deflector selectively directs the CPB away from the aperture of the aperture plate.

15. The CPB imaging apparatus according to claim 13, further comprising a controller configured to control the beam blanker to generate a first set of frames, a second set of frames, a first set of dark frames, and a second set of dark frames.

16. The aforementioned controller, A first dark criterion is established based on the first set of dark frames. To create a second dark criterion, the first dark criterion is updated based on a second set of dark frames. To create a first set of compensated sample frames, the first set of sample frames is compensated using the first dark criterion, The CPB imaging apparatus according to claim 15, configured to compensate a second set of frames using the second dark criterion in order to create a second set of compensated sample frames.

17. The CPB imaging apparatus according to claim 16, wherein the controller is operable to generate a composite image by combining a first set of compensated sample frames with a second set of compensated sample frames.

18. The CPB imaging apparatus according to claim 16, wherein the controller is further operable to calculate a tomographic reconstruction of the sample by combining a first set of the compensated sample frames with a second set of the compensated sample frames.

19. The CPB imaging apparatus according to claim 13, wherein the CPB source includes an electron beam source, and the CPB includes an electron beam.

20. A charged particle beam imaging method (CPB imaging method), To acquire one or more dark frames during each intermittent interruption, sample frame acquisition is intermittently interrupted based on radiation received from the sample in response to CPB, A sample image is established based on the sample frame and one or more of the dark frames acquired during each of the intermittent interruptions. A CPB imaging method, including the above.

21. The CPB imaging method according to claim 20, wherein establishing the sample image comprises compensating for a plurality of sets of sample frames based on each set of dark frames acquired during associated intermittent interruptions, and the sample image is established by combining the compensated sets of sample frames.

22. The CPB imaging method according to claim 20, wherein the sample frame is acquired at one or more inclination angles of the sample with respect to the CPB.

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