Techniques for imaging low duty cycle signals using scanning electron microscopes

Abstract

Techniques for imaging low duty cycle signals using a scanning electron microscope. Systems, components, methods, and algorithms for generating differential data are described. A computer-implemented method includes directing a first pulse of charged particles toward a sample. The method may include generating first detector data based at least in part on an interaction between the charged particles of the first pulse and the sample. The method may include directing a second pulse of charged particles toward the sample. The method may include generating second detector data based at least in part on an interaction between the charged particles of the second pulse and the sample. The method may also include generating differential data using the first detector data and the second detector data, wherein the differential data describes a change between an "on" state of the sample and an "off" state of the sample.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to U.S. non-provisional application No. 18 / 456,246, filed on August 25, 2023, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] Embodiments of the present disclosure relate to electronic test systems and algorithms and methods for their operation. In particular, some embodiments relate to techniques for integrated circuit testing. Background Art

[0004] Integrated circuit (IC) testing involves the measurement of individual transistors or groups of transistors of a semiconductor wafer or wafer portion (e.g., a sliced ​​wafer), referred to as a "device under test" or DUT. A typical IC test regime involves applying a periodic signal to a circuit under a variety of operating conditions for which the circuit has been designed to operate, and determining at what point the circuit fails to operate to specification. As an IC approaches failure, the duty cycle tends to deviate from the nominal 50% duty cycle, corresponding to a signal where the "on" time equals the "off" time. Such deviations from the nominal 50% duty cycle mean that the average power supplied to the circuit under observation is reduced, and measurements of duty cycle can be used to better understand the performance of the IC device under test, especially as the IC device begins to fail.

[0005] Typical methods for measuring electrical activity on a DUT include directing a charged particle beam at the DUT, measuring the time-varying signal strength at multiple points of the waveform across a test pattern, reconstructing the waveform, and using frequency domain information to detect DUT activity at a given clock frequency. Unfortunately, as the duty cycle deviates from a value of 50%, the signal-to-noise performance degrades. Such performance loss is at least partially due to the sampling mechanism of pulsed electronic imaging, which more heavily weights any state corresponding to a larger proportion of the waveform, especially when the test waveform is a square wave signal. Therefore, there is a need for electronic imaging techniques that improve the signal-to-noise ratio as the duty cycle approaches 0% or 100% during IC testing. Summary of the invention

[0006] Systems, components, methods, and algorithms for generating differential data are described. In a first aspect, a computer-implemented method includes directing a first pulse of charged particles toward a sample. The method may include generating first detector data based at least in part on an interaction between the charged particles of the first pulse and the sample. The method may include directing a second pulse of charged particles toward the sample. The method may include generating second detector data based at least in part on an interaction between the charged particles of the second pulse and the sample. The method may also include generating differential data using the first detector data and the second detector data, wherein the differential data describes a change between an "on" state of the sample and an "off" state of the sample.

[0007] In some embodiments, the first pulse and the second pulse are separated in time by a value of about A*P. A may be a half integer. The value "P" may be the period of the periodic electrical signal applied to the sample measured in units of time. The value of A may be greater than or approximately equal to the smallest half integer for which A*P is longer than the temporal resolution of the system generating the first detector data and the second detector data.

[0008] In some embodiments, the method further comprises: generating a charged particle beam using a charged particle source; operating a beam blanker using a periodic pulse signal to apply a pulse to the charged particle beam, the pulse signal having a frequency "f" and a phase "f". and modifying the phase of the pulse signal so that the "on" value of the pulse signal coincides in time with the "on" value of the electrical signal applied to the sample. Modifying the phase of the pulse signal may include: increasing the phase of the pulse signal by a differential of a set of phase values. A set of detector data is generated, including corresponding "on" data and corresponding "off" data for at least a subset of phase values; a set of difference data is generated using the set of detector data; and a phase offset is determined using the set of difference data. The phase offset may correspond to a discontinuity in an average value of the set of difference data.

[0009] In some embodiments, a first pulse of charged particles and a second pulse of charged particles can be generated using a scanning electron microscope in an imaging mode. The differential data can include a differential image of the surface of the sample. The first pulse of charged particles and the second pulse of charged particles can be generated using a scanning electron microscope in a fixed-point mode. The differential data can include a differential signal of amplitude relative to time.

[0010] In some embodiments, the first detector data corresponds to an "on" state of the sample. The second detector data may correspond to an "off" state of the sample. Generating differential data may include: generating a first set of detector data for the "on" state of the sample, including the first detector data; generating a second set of detector data for the "off" state of the sample, including the second detector data; and defining the differential data as the difference between the first set of detector data and the second set of detector data.

[0011] In some embodiments, the sample includes an integrated circuit. The charged particles can be directed to a region of the integrated circuit that includes a conductive feature.

[0012] In some embodiments, the differential data may include a secondary electron image.

[0013] In a second aspect, a system includes a charged particle source. The charged particle source may be configured to generate charged particle pulses. The system may include a control circuit operably coupled to the charged particle source. The system may also include one or more non-transitory machine-readable storage media operably coupled to the control circuit and storing instructions that, when executed, cause the system to perform the operations of the aforementioned aspects.

[0014] The terms and expressions that have been adopted are used as terms of description rather than limitation, and when such terms and expressions are used, it is not intended to exclude any equivalents of the features shown and described or portions thereof, but it should be recognized that various modifications are possible within the scope of the subject matter claimed. Therefore, it should be understood that although the subject matter claimed by the present disclosure has been specifically disclosed through embodiments and optional features, those skilled in the art may make modifications and changes to the concepts disclosed herein, and such modifications and changes are considered to be within the scope of the present disclosure as defined by the appended claims. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The foregoing aspects of the present disclosure and many of the attendant advantages will become more readily appreciated when the following detailed description is taken in conjunction with the accompanying drawings.

[0016] Figure 1 is a schematic diagram illustrating an exemplary integrated circuit testing system according to some embodiments of the present disclosure.

[0017] Figure 2 is a flow chart illustrating an exemplary process for generating a differential image of a device under test according to some embodiments of the present disclosure.

[0018] Figure 3 is a schematic diagram illustrating an exemplary image processing method for generating a differential image according to some embodiments of the present disclosure.

[0019] FIG. 4A to FIG. 4G is a schematic diagram illustrating exemplary integrated circuit test data including voltage data and a differential image according to some embodiments of the present disclosure.

[0020] Figure 5 is a graph illustrating exemplary average differential data as a function of differential phase differential according to some embodiments of the present disclosure.

[0021] In the accompanying drawings, unless otherwise specified, the same reference numerals represent the same components in various views. Where appropriate, not all instances of components need to be labeled to reduce clutter in the drawings. The drawings are not necessarily drawn to scale, with emphasis instead on illustrating the principles described. DETAILED DESCRIPTION

[0022] Although exemplary embodiments have been illustrated and described, it will be appreciated that various changes may be made therein without departing from the spirit and scope of the present disclosure. In the following paragraphs, embodiments of analytical instrument systems, components, and methods for generating differential data are described. To simplify the description, embodiments of the present disclosure focus on integrated circuit characterization and fault analysis. The embodiments are not limited to such instruments, but are contemplated as analytical instrument systems configured to analyze microscopic components, for which electrical activity measurements and / or mechanical activity measurements measured by techniques including secondary electron detection may be transformed into the frequency domain. In an illustrative example, a MEMs oscillator system having a part oscillating at a characteristic frequency of approximately 100 MHz or a period of approximately 10 ns may benefit from the techniques described herein, for which the surface of the oscillator appears relatively bright in a first conformation and relatively dim in a second conformation.

[0023] Integrated circuit testing involves the measurement of individual transistors or groups of transistors of a semiconductor wafer or wafer portion (e.g., a sliced ​​wafer), referred to as a "device under test," or DUT. Testing typically involves applying one or more voltage signals to the integrated circuit components of the DUT while measuring the DUT's response to the signals. During testing, the signals may be varied to simulate out-of-spec or other challenging conditions as a method of stress testing the DUT with the intent of inducing failure of the DUT while generating measurement data. The measurement data can then be used to better understand the performance characteristics of the DUT, including whether the wafer from which the DUT was removed meets one or more quality criteria.

[0024] A typical IC test regime involves applying a periodic signal (e.g., a clock signal) to a circuit and monitoring the behavior of the circuit as the operating voltage of the circuit varies around a nominal value. The power consumption of the circuit may decrease as the operating voltage decreases, which may be advantageous for low-power applications such as mobile computing, however, the circuit performance may also degrade below the nominal operating voltage. Below a threshold operating voltage, the circuit may stop operating. An illustrative example exists in propagating a clock signal through a circuit. At a nominal operating voltage, the circuit is able to propagate a clock signal through the circuit at a 50% duty cycle, corresponding to a clock signal that is logically high or "on" 50% of the time. As the operating voltage deviates from the nominal voltage, the duty cycle of the clock signal may also deviate from the target, i.e., 50%. Techniques for detecting such periodic clock signals that are independent of the duty cycle of the clock provide the advantages of redundant and non-contact measurements, which have relatively little impact on the system being measured compared to direct measurement techniques (e.g., probe circuits).

[0025] Typical methods for measuring electrical activity on a DUT involve directing an electronic beam at the DUT, measuring the signal strength over time, and using frequency domain information to detect DUT activity at a given clock frequency. Unfortunately, as the duty cycle deviates from the 50% value, the signal-to-noise performance degrades. During IC test, as the duty cycle approaches 0 or 100%, imaging techniques that already have relatively low signal-to-noise detectability for the nominal 50% duty cycle may not be able to fully detect the clock signal. Therefore, there is a need for electronic imaging techniques that improve the signal-to-noise ratio when the duty cycle approaches 0% or 100% during IC test.

[0026] To this end, embodiments of the present disclosure include systems, methods, algorithms, and non-transitory media storing machine-readable instructions for generating differential data describing electrical activity of integrated circuit components. In an illustrative example, a method may include directing a first pulse of charged particles toward a sample, generating first detector data based at least in part on an interaction between the charged particles of the first pulse and the sample, directing a second pulse of charged particles toward the sample, generating second detector data based at least in part on an interaction between the charged particles of the second pulse and the sample, and generating differential data using the first detector data and the second detector data, wherein the differential data describes a change between an "on" state of the sample and an "off" state of the sample.

[0027] Figure 1is a schematic diagram illustrating an exemplary integrated circuit (IC) test system 100 according to some embodiments of the present disclosure. The exemplary system 100 includes an instrument 105, an instrument computing device (IPC) 110, and a client computing device 115 that are operably coupled to each other via one or more networks 120. The exemplary system 100 is configured to interrogate an IC device, referred to as a device under test (DUT) 125, using a test component 130 that is electrically coupled to components of the DUT 125 via a controller (also referred to as a test fixture 135). By applying time-varying electronic signals to components of the DUT 125 (referred to as a "test loop"), performance characteristics of circuit components of the DUT 125 can be derived as part of a quality control and failure analysis technique for ICs manufactured according to a given IC design.

[0028] The instrument 105 includes a test section 140 in which a test assembly 130 is disposed, including a DUT 125 and electronic components for driving a test circuit (e.g., a test device), a vacuum component for isolating the DUT 125 from the atmosphere, and a thermal management system for removing heat from the DUT 125 during testing. Coupled to the test section 140 is a charged particle column 145. The charged particle column 145 can be an ion beam (e.g., a focused ion beam (FIB)) column or an electron beam column (e.g., as part of a scanning electron microscope). In some embodiments, the instrument 105 includes a FIB column and an electron beam column, wherein one of the charged particle sources is coupled to the test section 140 at an angle to the charged particle column 145.

[0029] The charged particle column 145 can generate a charged particle beam 147 and can focus the charged particle beam 147 onto the region 127 of the DUT 125. The interaction of the charged particle beam 147 with the DUT 125 generates one or more detectable signals, which can be received by one or more detectors 155 operably coupled to the test section 140 and configured to generate detector data based at least in part on the measurement results of the signals. In an illustrative example, the detector 155 can include a secondary electron detector, a backscattered electron detector, a photon detector, an imaging sensor (e.g., a CCD), etc. Compared to a typical scanning electron microscope (SEM), the test section 140 can omit sample manipulation tools, such as interlocks, sample stages, etc., at least in part because the DUT 125 can be removably coupled to the test assembly 130, which can be disposed on a stage, bracket, or other holding assembly that enables electronic coupling and thermal coupling (e.g., coupling to the test device 135) with the test section 140. The charged particle beam 147 may be directed toward the DUT 125 using various operating modes including, but not limited to, an imaging mode, a line scan mode, a fixed-point mode, and / or a pulse mode.

[0030] To this end, the charged particle column 145 may include a beam blanker 150 disposed in the column and configured to apply an electric field and / or a magnetic field to the path of the entire charged particle beam 147. For example, control electronics 151 operably coupled to the beam blanker 150 may apply a time-varying voltage to electrodes of the beam blanker 150 such that the electric field may reversibly deflect the charged particles 147 into the beam blanker. Operating the beam blanker may allow the charged particle column 145 to direct the charged particle pulse toward the DUT 125, as described with reference to FIG. Figures 2 to 5 Described in more detail. In some embodiments, the pulse includes as few as 1 charged particle to about 1000 charged particles, including portions and subranges thereof. For example, the pulse may include 3 charged particles, 5 charged particles, 10 charged particles, 15 charged particles, etc. In some embodiments, the pulse may extend over multiple cycles of the test signal, so that the technology of the present disclosure may include sampling from multiple points in a set of detector data that corresponds to a relatively long time period relative to the period of the test signal.

[0031] In some embodiments, the test assembly 130 is electrically coupled to components of the test section 140 via a coupling 165, through which one or more test cards 170 may be driven. The test cards 170 may encode test loop schemes and may interact with the DUT 125 to input and output signals to the DUT 125 and relay signals to other components of the exemplary system 100 (e.g., the client PC 115 and / or the IPC 110).

[0032] Computing devices 110 and 115 may be general purpose machines (e.g., laptops, tablet computers, smart phones, servers, etc.) configured to operate or otherwise interact with instrument 105. Instrument 105, in turn, may include electronic components that form part of a dedicated computing device, including control circuits configured to drive test loops, operate test assembly 130, control electron beam column 145, and operate vacuum systems and thermal management systems. IPC 110 may be a machine provided with software that is configured to interface with instrument 105 and allow a user of instrument 105 to perform testing of DUT 125. Similarly, client pc 115 may be configured to control one or more systems of instrument 105 (e.g., via IPC 110 and / or by interfacing with instrument 105 via network 120) to perform testing of DUT 125.

[0033] In some embodiments, the instrument 105, IPC 110, and / or client PC 115 are in separate physical locations and are coupled via a network 120 and / or by other means, such as a direct connection or by a wireless connection (e.g., near-field radio). The network 120 may include a public network (e.g., the Internet) and / or a private network (e.g., an intranet or a local area network). In some embodiments, the IPC 110 and / or client PC 115 are configured to operate the instrument autonomously (e.g., without human intervention) or semi-autonomously (e.g., with limited human intervention, such as initiating a test, identifying a sample, and / or confirming an automated analysis result). In this manner, the exemplary system 100 may be configured to operate autonomously with manual control and / or autonomously as part of a scalable IC characterization system for automated testing of ICs.

[0034] The exemplary system 100 may include additional and / or alternative components in addition to those illustrated. For example, the instrument 105 may be operably coupled to one or more external components such as a signal generator, a data acquisition system, a power supply system, thermal management, etc. Such components may be housed, for example, in a cabinet physically separate from the instrument 105, but may be operably coupled to the charged particle column 145, the test section 140, the detector 155, etc., via electrical and / or fluid handling connections.

[0035] Figure 2 is a flowchart illustrating an example process for generating a differential image of an area of ​​a device under test according to some embodiments of the present disclosure. One or more operations of the example process 200 may be performed by a computer system in communication with additional systems, including but not limited to a characterization system, a network infrastructure, a database, and a user interface device. In some embodiments, reference Figure 2 At least a subset of the described operations are automatically performed (e.g., without human involvement) or pseudo-automatically performed (e.g., initiated by a human or with limited human intervention). In the illustrative example, the operations for applying a test signal, directing a charged particle pulse toward the DUT (e.g., Figure 1 DUT 125) and operations to generate detector data, wherein the system (e.g., Figure 1 The exemplary system 100 of FIG. 1 is configured to generate visualization data ( FIG. 100 ) that presents one or more forms of differential data for interpretation by a human user. Figures 3 to 5, example shown). Although the exemplary process 200 is described as a sequence of operations, it should be understood that at least some of the operations may be omitted, repeated, parallelized, combined, and / or reordered. In some embodiments, additional operations are performed before and / or after the operations of the exemplary process 200, which are omitted for clarity of explanation, such as operations for calibration of the electron source, consistency and aberration correction of the electron beam, introduction of a DUT sample into the vacuum system, calibrating the system, etc. In another example, a test loop may be initiated through which a test pattern of a time-varying voltage signal is applied to an integrated circuit component as part of determining one or more failure modes of the DUT. Referring to the exemplary process 200, the operation of the instrument may be combined with a test component (e.g., Figure 1 The operation of the test assembly 130) is coordinated as part of generating differential data describing the electrical activity of the IC components of the DUT, from which duty cycle information and other data can be derived.

[0036] At operation 205, the exemplary process 200 includes directing a pulse of charged particles toward a DUT (eg, Figure 1 DUT 125). The charged particles (e.g., ions, electrons) may be used, at least in part, in a beam blanker (e.g., Figure 1 Operation 205 may be coordinated with a test signal, such as reference signal 204. Figures 3 to 4G In more detail, a charged particle pulse is incident on a region of the DUT (e.g., Figure 1 on area 127).

[0037] In this manner, at operation 210, one or more detectors (e.g., Figure 1 The detector 155) is capable of generating detector data based at least in part on the interaction between the pulsed charged particles and the DUT. In some embodiments, the detector data includes secondary electron detector data, which can be generated by collecting secondary electrons re-emitted from the surface of the DUT in response to the charged particle pulse (e.g., using Everhart-Thornley, other scintillator detectors, or semiconductor detectors). In the absence of a beam being constrained by a particular physical mechanism, the probability of a secondary electron being re-emitted in an electrically active region may be higher than in an electrically inactive region, so that detector data describing an active region of the DUT may appear relatively bright in the detector data. In the case of an electronic image, the relative brightness may look like a lighter coloration (e.g., a bright area or a saturated area of ​​an image), while for a spectrum, line scan, or fixed-point signal, the relative brightness may correspond to a higher average signal amplitude (e.g., varying over time or position).

[0038] In some embodiments, operations 205 and / or 210 may include operating the charged particle source in a mode referred to as "CW" or continuous wave, in which the charged particle beam is continuously emitted for a period of time long enough to extend over multiple cycles of the test signal. In CW operation, detector data may be sampled at high and low points corresponding to the "on" and "off" states, and differential data may be generated based on the sampled detector data. In such cases, sampling errors may occur when the frequency of the clock signal is higher than the cutoff frequency of the detector. With clock signals typically in the hundreds of MHz, for some typical secondary electron detectors, the detector bandwidth may be approximately 5 MHz. Therefore, secondary electron detectors having a bandwidth on the order of the clock signal or higher may be used to directly generate differential data by sampling the detector data and performing a differential operation on the sampled data.

[0039] At operation 215, exemplary process 200 includes directing another pulse of charged particles toward the DUT (eg, Figure 1 As described with reference to operation 205, the charged particles (e.g., ions, electrons) may be used, at least in part, in a beam blanker (e.g., Figure 1 Operation 215 may be coordinated with a test signal, such as reference signal 214. Figures 3 to 4G In more detail, a charged particle pulse is incident on a region of the DUT (e.g., Figure 1 In this manner, at operation 220, one or more detectors can generate detector data based at least in part on the interaction between the charged particles of the pulse and the DUT. In contrast to operation 210, the detector data describing the inactive region of the DUT may appear relatively dim in the detector data relative to the signal generated based on the active region of the DUT.

[0040] In some embodiments, operations 205 and 210 are repeated in multiple iterations as part of spatially scanning the charged particle beam with multiple pulses over at least a portion of the surface of the DUT. These iterations can be coordinated with multiple instances of the region of interest of the DUT being electrically active, so that the detector data generated in the multiple iterations of operation 210 can be used to generate spatial information about the active region of the DUT. In this way, the "on" state of the region of the DUT can be sampled to produce a "positive" image. Similarly, operations 215 and 220 can be repeated in multiple iterations as part of spatially scanning the charged particle beam with multiple pulses over at least the same portion or similar portions of the surface of the DUT. These iterations can be coordinated with multiple instances of the region of interest of the DUT being electrically inactive, so that the detector data generated in the multiple iterations of operation 210 can be used to generate spatial information about the active region of the DUT. In this way, the "off" state of the region of the DUT can be sampled to produce a "negative" image. Line scan or fixed-point mode data can be used to achieve similar purposes, for example, spatially scanning a pulsed beam across a linear region of the DUT and generating one-dimensional detector data showing signal amplitude that varies with beam position. However, in some embodiments, a reference image of a region of the DUT is used, so that operations 215 and 220 are omitted.

[0041] The test loop may include a drive pattern having a frequency on the order of 1 GHz to 10 GHz, corresponding to a period on the order of approximately 1 nanosecond or less. Physical limitations governing the response time of the charged particle microscopy system may limit the temporal resolution of the detector data such that successive iterations of operations 205 and 210 may be separated in time by an offset that is approximately an integer multiple of the period such that the detector data generated upon successive iterations of operation 210 are temporally resolved. For example, where the period of the test pattern is approximately 3 ns, and the temporal resolution of the charged particle system and detector is approximately 100 ns, the integer multiple may be 50, which results in a pulse period equal to 50 times the DUT period, or 150 ns. In this manner, the first iteration of operations 205 and 210 may be performed at an initial time t o =0, the second iteration is performed at t1 =150 ns, the third iteration is performed at t2 =300 ns, etc. In this way, each data point is resolved in time and samples a consistent point in the test pattern. Similarly, successive iterations of operation 215 and operation 220 may be separated in time by an offset of approximately an integer multiple of the period so that the detector data generated in successive iterations of operation 220 are resolved in time.

[0042] In some embodiments, operations 205 to 220 are parallelized so that the exemplary process 200 repeats multiple iterations including operations 205 to 220. In this way, the scanning pattern is programmed so that for both operations 205 and 215, the beam control circuit maintains the focus of the charged particle beam at a given position on the surface of the DUT, for which detector data is generated at operations 210 and 220, after which the beam is controlled to different positions on the surface of the DUT (e.g., by raster mode, line scanning, etc.). In order to accommodate the temporal resolution of the system, operations 205 and 215 and successive iterations of operations 210 and 220 may be offset in time by half-integer multiples of the period of the test pattern. In this case, a half-integer is a number obtained by dividing an odd integer by half. In the case where a half-integer is represented by a variable "A", the offset may be approximately A*P, where "P" is the period of the test pattern. As described above, the value of A may allow successive detector data points to be resolved in time, separated in time by approximately A*P. The value of A can vary in some cases, for example, as a method of adjusting the sampling rate and reducing the total charged particle dose reaching the surface of the DUT. To this end, A can be defined so that A is the smallest half-integer that makes A*P longer than the temporal resolution of the system and / or detector used to generate the detector data. In some embodiments, the value of A can be detuned so that A varies with time and is different from (e.g., greater than) the above positions. However, advantageously, defining A in the manner described above allows the beam blanker to operate with a consistent pulse frequency and period rather than a time-varying non-periodic signal, thereby reducing the complexity of coordinating the exemplary process 200 with the test loop applied to the DUT.

[0043] At operation 225, the exemplary process 200 includes generating differential data. Figures 3 to 4G Described in more detail, the detector data generated at operations 210 and 220 may be used to generate differential data to describe changes between an "on" state of a region of the DUT and an "off" state of a region of the DUT. Operation 225 may include performing one or more transformations on the detector data, such as adding data in a pixel-by-pixel manner, subtracting data in a pixel-by-pixel manner, normalizing, baseline shifting, etc. In this manner, the differential data may reveal individual components and / or groups of components of the DUT that are active at corresponding points in the test loop.

[0044] Figure 3 is a schematic diagram illustrating an exemplary image processing technique 300 for generating a differential image 305 according to some embodiments of the present disclosure. Generating the differential image 305 includes generating a DUT in a valid state (e.g., Figure 1125) of the DUT 125 (referred to as the first image 320) and the image of the region 310 of the DUT in an invalid state (referred to as the second image 330), as shown in reference Figure 2 As described, and generating differential data. In the case of image data, the differential data can be a differential image 305. In the case where the detector data includes line scan data or point pattern data, the differential data can be spectral data or include spectral data. Similarly, the image data can be sampled to produce a differential spectrum showing relative amplitudes that vary with position.

[0045] Region 310 includes a plurality of features 315, which may be integrated circuit components, conductive regions of a surface, or other features. During an active "on" portion of the test pattern, one or more features 315 may be electrically activated such that the first image includes one or more relatively bright regions 325 as compared to those regions corresponding to relatively dim inactive features 315. In its simplest form, exemplary technique 300 next subtracts second image 330 from first image 320 to generate difference image 305. However, in some cases, one or more pre-processing operations may be performed on the first image data and / or the second image data. For example, first image 320 and second image 330 may be baseline corrected, normalized to the peak intensity value of first image 320, and other operations that may improve the signal-to-noise ratio of difference image 305.

[0046] To further illustrate the concept of a differential image, a reference image of region 310 is shown for use in generating differential image 305 using second image 330, resulting in a negative case of a blank image. Similarly, portions of first image 320 that do not include bright regions 325 may produce blank portions of differential image 305. In contrast, relatively bright regions 325 of first image 320 may reveal active features 335 in differential image 305.

[0047] Advantageously, exemplary technique 300 is relatively insensitive to the duty cycle of the DUT, at least in part because first image 320 is generated during the "on" state and second image 330 is generated during the "off" state. In this manner, valid features 335 are revealed in differential image 305 even when the duty cycle deviates from 50%.

[0048] FIG. 4A to FIG. 4G is a schematic diagram illustrating exemplary integrated circuit test data including voltage data and a differential image according to some embodiments of the present disclosure. Figure 4A , Figure 4B , Figure 4D and Figure 4F Indicates a voltage signal corresponding to a test mode using a square wave signal. Figure 4C , Figure 4E and Figure 4G represents a difference image (e.g., Figure 3 The difference image 305) is shown in FIG. Figures 2 to 3 Describe in more detail.

[0049] Figure 4A 4 is an exemplary test mode using a square wave voltage signal characterized by a period 400. In integrated circuit testing, the period 400 may be on the order of 1 nanosecond to 100 nanoseconds, as determined by the IC manufacturer's test plan. In this way, the exemplary process 200 may be coordinated with any test mode. Figure 4A Duty cycles that deviate from 50% are also illustrated. Although not drawn to scale, Figure 4A The duty cycle of the exemplary test pattern in corresponds to a value between 10% and 30%. However, the technology of the present disclosure is not limited to this range and is relatively insensitive to the duty cycle compared to conventional technologies. Figure 4A , Figure 4B , Figure 4D and Figure 4F In each of FIG. 4 , a sampling period 405 is shown spanning multiple values ​​of period 400 , as indicated by the parallel lines intersecting the horizontal axis of time.

[0050] Figure 4B The sampling condition where the sampling period 405 coincides with two invalid "off" states of the DUT is illustrated. Figure 4C As illustrated, the resulting difference image is a blank image 410. In contrast, Figure 4D The sampling period 405 is illustrated as sampling conditions that correspond to the valid "on" state and invalid "off" state of the DUT. A phase offset 415 is also shown, It may be used to synchronize the sampling period 405 with the active "on" state of the DUT using the test mode signal.

[0051] In this manner, the processes and techniques of the present disclosure (e.g., Figure 2 The exemplary process 200 of FIG. 4 may include operations for aligning charged particle pulses and detector data with valid states of the DUT, thereby allowing the system to correct for duty cycle changes during testing. In some embodiments, the phase offset 415 may be achieved by incrementally differentiating the phase of the pulse signal. to determine, and detector data and / or differential data (eg, relative to a reference image) may be generated to determine consistent points. Figure 5 In more detail, the value of the phase offset 415 can be determined based at least in part on an average value of the differential data. The method can be repeated during the course of the test loop so that differential data can be generated in a manner that adapts to the change in duty cycle, such as Figures 4F to 4GAs illustrated, a second phase offset 425 is applied to the sampling period to correct for the duty cycle reduction.

[0052] Figure 5 4 , the asynchrony between the test pattern period (e.g., period 400 of FIG. 4 ) and the sampling period (e.g., sampling period 405 of FIG. 4 ) causes pulses to occur during the invalid state of the DUT, causing the system of the present disclosure (e.g., Figure 1 4 ). To this end, a phase offset (e.g., offset 415 of FIG. 4 ) may be determined to align the sampling period with the test period. Aligning the two periods may include incrementally increasing the phase offset of the sampling period while generating detector data, and determining an average value of the detector data and / or the differential data. As the phase offset reaches a value that aligns the sampling period with the test period, and in the case where the system samples a valid “on” state and an invalid “off” state, the average value of the detector data and the differential data may be increased. Figure 5 The data shown are normalized to generalize the description of the disclosed techniques to different test systems. In this way, the techniques described herein can be applied to various periodic systems where the duty cycle may affect the quality of the signal-to-noise ratio.

[0053] exist Figure 5 In the exemplary data shown, the phase shift corresponds to the differential phase shift Between the eighth and ninth increments, a discontinuity in the averaged data is observed, from which a phase offset can be discerned. In some embodiments, when the test pattern is a smooth function or a sawtooth function and when the DUT responds to a smooth voltage input in a continuous manner, a relatively smooth transition is observed in the differential data. In such cases, the phase offset can be determined based at least in part on the estimation of the stationary point in the data. In some embodiments, this technique can be used to replicate the test pattern itself. For example, by incrementing the phase offset over a complete cycle of the test pattern, the average differential data can be used to determine the frequency and / or shape of the test pattern.

[0054] In the foregoing description, various embodiments have been described. For the purpose of explanation, specific configurations and details have been set forth in order to provide a thorough understanding of the embodiments. However, it will also be apparent to those skilled in the art that these embodiments may be practiced without these specific details. In addition, in order not to obscure the described embodiments, well-known features may have been omitted or simplified. Although the exemplary embodiments described herein focus on integrated circuit test systems, and specifically on scanning electron microscope systems, these are non-limiting exemplary embodiments.

[0055] Some embodiments of the present disclosure include a system having one or more data processors and / or logic circuits. In some embodiments, the system includes a non-transitory computer-readable storage medium containing instructions, which, when executed on one or more data processors and / or logic circuits, cause one or more data processors and / or logic circuits to perform part or all of one or more methods disclosed herein and / or part or all of one or more processes and workflows. Some embodiments of the present disclosure include a computer program product tangibly embodied in a non-transitory machine-readable storage medium, the computer program product including instructions configured to cause one or more data processors and / or logic circuits to perform part or all of one or more methods disclosed herein and / or part or all of one or more processes.

[0056] The terms and expressions employed herein are used as terms of description rather than limitation, and when such terms and expressions are used, it is not intended to exclude any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the claims. Therefore, it should be understood that while the present disclosure includes specific embodiments and optional features, modifications and changes to the concepts disclosed herein may be made by those skilled in the art, and such modifications and changes are deemed to be within the scope of the appended claims.

[0057] When a term is used without explicit definition, it is understood that unless the term has a special and / or particular meaning in the field of charged particle microscopy systems or other related fields, the term refers to the ordinary meaning of the word. The term "about" or "substantially" is used to indicate a deviation from the stated property, wherein the deviation has little or no effect on the corresponding function, property or attribute of the described structure. In the illustrated example where one dimensional parameter is described as "substantially equal to" another dimensional parameter, the term "substantially" is intended to reflect that the two parameters being compared may be unequal within permissible limits (such as manufacturing tolerances or confidence intervals inherent to system operation). Similarly, in the case where a geometric parameter (such as alignment or angular orientation) is described as "approximately" perpendicular, "substantially" perpendicular, or "substantially" parallel, the term "about" or "substantially" is intended to reflect that the alignment or angular orientation may differ from the precisely stated condition (e.g., not precisely perpendicular) within permissible limits. For numerical values ​​(such as diameter, length, width, etc.), the term "about" may be understood to describe a deviation of up to ±10% from the stated value. For example, a dimension of "approximately 10 mm" may describe a dimension of 9 mm to 11 mm, and a time of the order of 1 ns may mean a time of 0.1 ns to 10 ns.

[0058] This description provides exemplary embodiments, and is not intended to limit the scope, applicability or configuration of the present disclosure. On the contrary, the subsequent description of the exemplary embodiments will provide enabling descriptions for realizing various embodiments to those skilled in the art. It should be understood that, without departing from the spirit and scope set forth in the appended claims, various changes may be made to the functions and arrangements of the elements. Specific details are given in the description to provide a thorough understanding of the embodiments. However, it should be understood that the embodiments may be put into practice without these specific details. For example, the specific system components, systems, processes and other elements of the present disclosure may be shown in schematic form, or omitted from the diagram, so as to avoid obscuring the embodiments due to unnecessary details. In other cases, well-known circuits, processes, components, structures and / or technologies may be shown without unnecessary details.

Claims

1. A computer-implemented method, the method comprising: directing a first pulse of charged particles toward the sample; generating first detector data based at least in part on an interaction between the charged particles of the first pulse and the sample; directing a second pulse of charged particles toward the sample; generating second detector data based at least in part on an interaction between the charged particles of the second pulse and the sample; as well as Differential data is generated using the first detector data and the second detector data, wherein the differential data describes a change between an “on” state of the sample and an “off” state of the sample.

2. The computer-implemented method of claim 1 , wherein the first pulse and the second pulse are separated in time by a value of approximately A*P, where A is a half-integer, and where the value "P" is the period of the periodic electrical signal applied to the sample measured in units of time.

3. The computer-implemented method of claim 2, wherein the value of A is greater than or approximately equal to a smallest half-integer for which A*P is longer than a temporal resolution of a system generating the first detector data and the second detector data.

4. The computer-implemented method of claim 1 , further comprising: generating a charged particle beam using a charged particle source; A beam blanker is operated using a periodic pulse signal to pulse the charged particle beam, the pulse signal having a frequency "f" and a phase "φ" and modifying the phase of the pulse signal so that an "on" value of the pulse signal coincides in time with an "on" value of an electrical signal applied to the sample.

5. The computer-implemented method of claim 4 , wherein modifying the phase of the pulse signal comprises: Increasing the phase of the pulse signal by a difference "Δφ" of a set of phase values; generating a set of detector data, the set of detector data comprising respective "on" data and respective "off" data for at least a subset of the phase values; generating a set of differential data using the set of detector data; as well as A phase offset is determined using the set of differential data, the phase offset corresponding to a discontinuity in a mean value of the set of differential data.

6. The computer-implemented method of claim 1, wherein the first pulse of charged particles and the second pulse of charged particles are generated using a scanning electron microscope in an imaging mode, and wherein the differential data comprises a differential image of a surface of the sample.

7. A computer-implemented method according to claim 1, wherein the first pulse of charged particles and the second pulse of charged particles are generated using a scanning electron microscope in a fixed-point mode, and wherein the differential data includes a differential signal of amplitude with respect to time.

8. The computer-implemented method of claim 1 , wherein the first detector data corresponds to the “on” state of the sample, wherein the second detector data corresponds to the “off” state of the sample, and wherein generating the differential data comprises: generating a first set of detector data for the “on” state of the sample, including the first detector data; generating a second set of detector data for the “off” state of the sample, including the second detector data; as well as The differential data is defined as the difference between the first set of detector data and the second set of detector data.

9. The computer-implemented method of claim 1, wherein the sample comprises an integrated circuit, and wherein the charged particles are directed to a region of the integrated circuit that comprises a conductive feature.

10. The computer-implemented method of claim 1, wherein the differential data comprises secondary electron image data.

11. A system, comprising: a charged particle source configured to generate a charged particle pulse; a control circuit operatively coupled to the charged particle source; and one or more non-transitory machine-readable storage media operably coupled to the control circuitry and storing instructions that, when executed, cause the system to perform operations comprising: directing a first pulse of charged particles toward the sample; generating first detector data based at least in part on an interaction between the charged particles of the first pulse and the sample; directing a second pulse of charged particles toward the sample; generating second detector data based at least in part on an interaction between the charged particles of the second pulse and the sample; and Differential data is generated using the first detector data and the second detector data, wherein the differential data describes a change between an “on” state of the sample and an “off” state of the sample.

12. The system of claim 11, wherein the first pulse and the second pulse are separated in time by a value of approximately A*P, where A is a half-integer, and wherein the value "P" is the period of the periodic electrical signal applied to the sample measured in units of time.

13. The system of claim 12, wherein the value of A is greater than or approximately equal to a smallest half-integer for which A*P is longer than a temporal resolution of a system generating the first detector data and the second detector data.

14. The system of claim 11, wherein the operations further comprise: generating a charged particle beam using the charged particle source; A beam blanker is operated using a periodic pulse signal to pulse the charged particle beam, the pulse signal having a frequency "f" and a phase "φ" and modifying the phase of the pulse signal so that an "on" value of the pulse signal coincides in time with an "on" value of an electrical signal applied to the sample.

15. The system of claim 14, wherein modifying the phase of the pulse signal comprises: Increasing the phase of the pulse signal by a difference "Δφ" of a set of phase values; generating a set of detector data, the set of detector data comprising respective "on" data and respective "off" data for at least a subset of the phase values; generating a set of difference data using the set of detector data; as well as A phase offset is determined using the set of difference data, the phase offset corresponding to a discontinuity in a mean value of the set of difference data.

16. The system of claim 11, wherein the first pulse of charged particles and the second pulse of charged particles are generated using a scanning electron microscope in an imaging mode, and wherein the differential data comprises a differential image of a surface of the sample.

17. The system of claim 11, wherein the first pulse of charged particles and the second pulse of charged particles are generated using a scanning electron microscope in a fixed-point mode, and wherein the differential data comprises a differential signal of amplitude with respect to time.

18. The system of claim 11, wherein the first detector data corresponds to the “on” state of the sample, wherein the second detector data corresponds to the “off” state of the sample, and wherein generating the differential data comprises: generating a first set of detector data for the “on” state of the sample, including the first detector data; generating a second set of detector data for the “off” state of the sample, including the second detector data; as well as The differential data is defined as the difference between the first set of detector data and the second set of detector data.

19. The system of claim 11, wherein the sample comprises an integrated circuit, and wherein the charged particles are directed to a region of the integrated circuit comprising a conductive feature.

20. The system of claim 11, wherein the differential data comprises secondary electron image data.

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