Techniques for detecting probe landing in integrated circuit testing systems

Abstract

To provide methods for detecting a contact between a probe tip and a sample surface.SOLUTION: A method can include dithering a probe in a plane substantially parallel with a surface by using a periodic motion having a dither frequency. The method can include directing a beam of charged particles toward a region of the probe. The method can include generating detector data describing the periodic motion of the probe based at least in part on an interaction between the beam of charged particles and the region of the probe. The method can include displacing the probe toward the surface. The method can also include detecting a contact between the probe and the surface using the detector data.SELECTED DRAWING: Figure 5

Description

[Technical field]

[0001] (CROSS REFERENCE TO RELATED APPLICATIONS) This application claims priority to U.S. Provisional Patent Application No. 18 / 333,136, filed June 12, 2023, the disclosure of which is incorporated by reference in its entirety herein.

[0002] FIELD OF THEINVENTION SUMMARY OF THE DISCLOSURE Embodiments of the present disclosure are directed to electronic test systems, and algorithms and methods for their operation. In particular, some embodiments are directed to techniques for integrated circuit testing. [Background technology]

[0003] Integrated circuit (IC) testing involves the measurement of individual transistors or groups of transistors on a semiconductor wafer or wafer section (e.g., a diced wafer) referred to as the "device under test" or DUT. Typically, a probe is positioned in contact with the integrated circuit element and used to interrogate the DUT with a time-varying electrical signal. As the feature density and structural complexity of integrated circuits increase, placing a probe on a particular IC element involves precise positioning of a nanoscale probe tip and the development of nanoscale information about the probe tip position in three dimensions. Such precise positioning is a significant challenge for "open loop" positioning systems, where the position of the probe tip is known to within tolerances. Inaccurate positioning of the probe tip makes contact with a surface difficult to predict.

[0004] Accurate positioning of a probe tip on a conductive contact at the nanometer scale, corresponding to the characteristic feature sizes of current CMOS nodes, presents significant challenges. For example, the surface of a typical DUT contains many visually similar features. For at least this reason, the probe's contour, and in particular the probe tip, can be difficult to distinguish against the background. As a result, it is impractical to deploy open-loop positioning at the scale at which test platforms are deployed while using imaging-based contact detection methods. Thus, a need exists for improved probe tip localization and contact detection for IC test systems. Summary of the Invention

[0005] Systems, methods, and techniques for detecting contact between a probe tip and a sample surface. In one aspect, the method can include dithering the probe in a plane substantially parallel to the surface using a periodic motion having a dither frequency. The method can include directing a beam of charged particles toward a region of the probe. The method can include generating detector data describing the periodic motion of the probe based at least in part on an interaction between the beam of charged particles and the region of the probe. The method can include displacing the probe toward the surface. The method can also include detecting contact between the probe and the surface using the detector data.

[0006] The method may further include determining an approximate location of the tip of the probe within the region of the probe. Determining the approximate location of the tip of the probe may include generating an electronic image of the region of the probe and using the electronic image to determine the approximate location of the tip of the probe. Directing the beam may include scanning the beam of charged particles in a line across the region of the probe using a line scan. The mathematical product of the dither frequency and the dwell time of the beam on a point in the line may be equal to or greater than one. The dither frequency may be approximately equal to a resonant frequency of the probe.

[0007] The method may further include determining an approximate orientation of the probe and generating identifier information for the probe based at least in part on the orientation. The sample may include an integrated circuit having contact pads, and contact is formed between the probe and the contact pads. Producing detector data may include generating filtered data using a lock-in amplifier tuned to a dither frequency. Detecting contact may correspond to an attenuation of an output of the lock-in amplifier at the dither frequency.

[0008] The method may further include dithering the probe over a set of motion frequencies, directing a beam of charged particles toward the probe, generating detector data describing a response of the probe to the motion frequencies based at least in part on an interaction between the beam of charged particles and the probe, estimating a resonant frequency of the probe using the detector data, and selecting the dither frequency to be approximately equal to the resonant frequency. Directing the beam of charged particles may include scanning the beam over the surface in a scan pattern. Estimating the resonant frequency may include isolating the resonant frequency using a lock-in amplifier. The set of motion frequencies may include a component frequency of 100 kHz.

[0009] Detecting the contact can include determining a phase of a frequency component in the detector data, the frequency component being at about the dither frequency. Detecting the contact can include aligning the phase of the frequency component to a phase of the drive signal of the periodic motion and detecting a change in the frequency component or a change in the phase of the frequency component.

[0010] The probe may be a first probe, the periodic motion may be a first periodic motion, the dither frequency may be a first dither frequency, and the system may further include a second probe. The method may further include dithering the second probe in a plane substantially parallel to the surface using a second periodic motion having a second dither frequency near a resonant frequency of the second probe. The method may further include directing a beam of charged particles toward a region of the second probe. The method may further include generating detector data describing the periodic motion of the second probe based at least in part on an interaction between the beam of charged particles and a region of the probe. The method may further include displacing the second probe toward the surface. The method may further include detecting contact between the second probe and the surface using the detector data. The first dither frequency and the second dither frequency may be different.

[0011] In one aspect, one or more non-transitory machine-readable storage media storing instructions that, when executed by a machine, cause the machine to perform the operations of the method of the preceding aspect.

[0012] In one aspect, a system for testing integrated circuit components includes a probe, a charged particle beam system, a detector oriented toward the probe and configured to detect particles generated by an interaction of the charged particle with the probe, control circuitry operatively coupled to the probe, the charged particle beam system, and the detector and configured to operate the probe, and one or more non-transitory machine-readable storage media storing instructions that, when executed by the system, cause the system to perform operations of the preceding aspects of the method and media. The charged particle beam system can be a scanning electron microscope column and the detector is a secondary electron detector or a backscattered electron detector.

[0013] The embodiments of the present disclosure also include systems, components, and methods according to the above aspects. The terms and expressions used are used as terms of description and not as terms of limitation, and in using such terms and expressions, there is no intention to exclude any equivalents of the features or portions thereof shown and described, but it is understood that various modifications are possible within the scope of the claimed subject matter. Thus, although the subject matter claimed herein has been specifically disclosed by embodiments and optional features, it should be understood that modifications and variations of the concepts disclosed herein may be made by those skilled in the art, and such modifications and variations are considered to be within the scope of the present disclosure as defined by the appended claims. [Brief description of the drawings]

[0014] The foregoing aspects and many of the attendant advantages of the present disclosure will become more readily appreciated as the same become better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, in which: [Figure 1] 1 is a schematic diagram illustrating an example integrated circuit testing system according to some embodiments of the present disclosure. [Diagram 2] 1 is a schematic diagram illustrating, in a plan view, an exemplary integrated circuit test probe according to some embodiments of the present disclosure. [Diagram 3] 1 is a schematic diagram illustrating an exemplary integrated circuit test probe in a side view, according to some embodiments of the present disclosure. [Figure 4] 1 is a schematic diagram illustrating an example integrated circuit testing system including multiple probes according to some embodiments of the present disclosure. [Diagram 5] FIG. 1 is a block flow diagram illustrating an exemplary process for detecting contact between a probe tip and a surface, according to some embodiments of the present disclosure. [Figure 6A] 1 is a composite diagram illustrating an example SEM image of an integrated circuit testing system above a DUT surface according to some embodiments of the present disclosure. [Figure 6B]1 is a composite diagram illustrating an example SEM image of an integrated circuit testing system in contact with a DUT surface according to some embodiments of the present disclosure. [Figure 6C] FIG. 2 is a composite diagram illustrating an example lock-in amplifier output image of an integrated circuit testing system above a DUT surface in accordance with some embodiments of the present disclosure. [Figure 6D] 1 is a graph illustrating an example lock-in amplifier output spectrum of an integrated circuit testing system above a DUT surface in accordance with some embodiments of the present disclosure. [Figure 6E] FIG. 2 is a composite diagram illustrating an example lock-in amplifier output image of an integrated circuit testing system in contact with a DUT surface in accordance with some embodiments of the present disclosure. [Figure 6F] 1 is a graph illustrating an example lock-in amplifier output spectrum of an integrated circuit testing system in contact with a DUT surface in accordance with some embodiments of the present disclosure.

[0015] In the drawings, like reference numbers refer to like parts throughout the various views unless otherwise specified. Not every instance of an element is necessarily labeled, so as to reduce clutter in the drawings where appropriate. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles being described. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Although illustrative embodiments have been illustrated and described, it will be understood that various modifications can be made in the present disclosure without departing from the spirit and scope of the present disclosure. In the following paragraphs, embodiments of analytical instrument systems, components, and methods for probing tip position and detecting contact between a probe tip and a surface are described. For simplicity of explanation, the embodiments of the present disclosure focus on integrated circuit characterization and failure analysis. The embodiments are not limited to such instruments, but rather are contemplated for analytical instrument systems configured for locating and positioning nanoscale electromechanical elements. In one illustrative example, nanoscale composites such as quantum dots, MEMS devices, or other nanostructured materials can benefit from improved localization of nanoscale electromechanical components that are otherwise difficult to position with suitable precision.

[0017] Embodiments of the present disclosure include systems, methods, algorithms, and non-transitory media storing machine-readable instructions for locating and detecting contact between a probe tip and a sample surface. Technical advantages of the present disclosure include improved detection of contact between a nanoscale probe tip and a surface without detailed information about the sample surface location, as well as improved performance of integrated circuit evaluation platforms.

[0018] Integrated circuit testing involves the measurement of individual transistors or groups of transistors on a semiconductor wafer or wafer section (e.g., a diced wafer) referred to as a "device under test" or DUT. Typically, probes are positioned in contact with the integrated circuit elements and used to interrogate the DUT with periodic, aperiodic, and / or direct current electrical signals. One or more probes are used as inputs and one or more probes are used as outputs to measure the IC element's response to the signals. As the feature density and structural complexity of integrated circuits increases, positioning a probe on a particular IC element involves nanoscale information about the probe tip's nanoscale geometry and probe tip location. Such precise positioning is a significant challenge for "closed loop" positioning systems (e.g., aligned piezoelectric actuators). While technically possible, precise positioning of the probe tip on a conductive contact at the nanometer scale corresponding to the characteristic feature sizes of current CMOS nodes may increase the complexity of the control system to an extent that it is impractical at the scale at which test platforms are deployed. For example, a closed-loop fine control system based at least in part on uncertainties in sample topography and sample height measurements can still rely on feedback mechanisms using techniques of the present disclosure.

[0019] Instead of complex closed-loop systems, embodiments of the present disclosure leverage the availability of electromechanical systems (e.g., arbitrary waveform generators, piezoelectric actuators, etc.) to produce motion of the probe that is attenuated or eliminated upon contact with the sample surface. Compared to existing techniques, embodiments of the present disclosure improve such detection based at least in part on isolating motion at or near a characteristic frequency of the probe motion and revealing the effect of contact with the surface on the probe. As an illustrative example, contact between the probe tip and the surface can be detected at least in part by an image of the probe whose frequency domain components have been filtered to isolate the periodic motion of the probe. Such filtering can be used to generate an image that isolates individual probes in a field of view that includes multiple probes, removing vibrations of other frequencies and removing background (e.g., substantially stationary) information. Upon contact, the periodic motion of the probe ceases, resulting in the disappearance of the probe from the image. As described in more detail with reference to Figures 1-6, embodiments of the present disclosure enable a probe tip to be precisely positioned and lowered onto a nanoscale integrated circuit feature without detailed knowledge of the exact location of the IC's surface or the geometry of the probe.

[0020] 1 is a schematic diagram illustrating an example integrated circuit (IC) system 100 according to some embodiments of the present disclosure. The example system 100 includes an instrument 105, an instrument computing device (IPC) 110, and a client computing device 115 operatively interconnected via one or more networks 120. The example system 100 is configured to interrogate an IC device, referred to as a device under test (DUT) 125, using a probe assembly 130, also referred to as a test rig, that is electronically coupled to components of the DUT 125 via a controller. Through application of time-varying electronic signals, referred to as "test loops," to the components of the DUT 125, performance characteristics of the circuit components of the DUT 125 can be derived as part of quality control and failure analysis techniques for ICs fabricated according to a given IC design.

[0021] The instrument 105 includes a test section 135 in which is disposed a DUT 125 and a probe assembly 130, which includes electronic components for driving a test loop (e.g., a test rig), vacuum components for isolating the DUT 125 from the atmosphere, and a thermal management system for removing heat from the DUT 125 during testing. A charged particle column 140 and a vacuum chamber 145 are coupled to the test section 135. The charged particle column 140 can be an ion beam (e.g., a focused ion beam (FIB)) column or an electron beam column. In some embodiments, the instrument 105 includes a FIB column and an electron beam column, and one of the charged particle sources is coupled to the vacuum chamber at an angle to the charged particle column.

[0022] The electron beam column can generate a beam of electrons 147 and focus the beam of electrons 147 onto the DUT 125. Interaction of the beam of electrons 147 with the DUT 125 produces one or more detectable signals that can be received by one or more detectors 150 operatively coupled to the vacuum chamber 145 and configured to generate detector data based at least in part on measurements of the signals. In one illustrative example, the detectors 150 can include secondary electron detectors, backscattered electron detectors, photon detectors, imaging sensors (e.g., CCDs), and the like. In another example, the probe can obscure a bright spot on the DUT 125 surface such that a signal can result from the probe to periodically reveal the bright spot and periodically block the beam 147 from reaching the surface. In contrast to a typical scanning electron microscope (SEM), the vacuum chamber 145 may omit sample manipulation tools such as interlocks, sample stages, etc., at least in part because the DUT 125 may be removably coupled to a probe assembly 130, which may be disposed on a stage, cradle, or other holding assembly that provides electronic and thermal coupling with the test section (e.g., coupled to a test rig). The beam of electrons 147 may be directed towards the DUT 125 using a variety of operating modes, including but not limited to an imaging mode, a line scan mode, and a spot mode, as described in more detail with reference to FIGS. 2-6.

[0023] The probe assembly 130 can include individually addressable probes 155 that are movable in three spatial dimensions (labeled "x, y, z" Cartesian axes) by electromechanical actuators 160. In this manner, a probe tip (labeled 210 with reference to FIGS. 2-4) can be displaced toward a location on the surface of the DUT 125 with nanometer-scale precision. In some embodiments, the probe assembly 130 is electronically coupled to components of the test section 135 via couplings 165 and 170, such that the actuators 160 can be driven (e.g., with a drive signal) and an output signal can be generated.

[0024] Based at least in part on the challenges associated with calibrating a reference position of the DUT 125 surface (e.g., defining z=0 to be the surface of the DUT 125), the vertical displacement of the probe 155 can be measured with reference to a calibration displacement (e.g., a return or "zero" displacement) that is independent of the type and position of the DUT 125. To that end, the exemplary system 100 can implement open-loop positioning of the probe 155, which refers to an approach in which the position of the probe 155 is known with reference to a calibration displacement adjusted by any subsequent displacements (e.g., a starting point in three-dimensional coordinates to which each subsequent displacement is added as a vector of three displacement values). In some cases, the lateral (e.g., "xy") position of the probe 155 relative to the surface of the DUT 125 can be determined using detector data, such as a secondary electron image generated by directing a beam of electrons 147 to an area of ​​the DUT 125 or one or more of the probes 155.

[0025] The computing devices 110 and 115 can be general-purpose machines (e.g., laptops, tablets, smartphones, servers, etc.) configured to operate or otherwise interact with the instrument 105. The instrument 105, in turn, can include electronic components forming part of a dedicated computing device, including control circuitry configured to drive the test loop, actuate the probe assembly 130, control the electron beam column 140, and operate the vacuum system and the thermal management system. The IPC 110 can be a machine with software configured to interface with the instrument 105 and enable a user of the instrument 105 to perform testing of the DUT 125. Similarly, the client pc 115 can be configured to control one or more systems of the instrument 105 (e.g., by interfacing with the instrument 105 via the IPC 110 and / or via the network 120) to perform testing of the DUT 125. In some embodiments, the instrument 105, the IPC 110, and / or the client PC 115 are in separate physical locations and are coupled via the network 120 and / or by other means such as a direct connection or by a wireless connection (e.g., short-range wireless). The network 120 can 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 the 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 tests, identifying samples, and / or confirming automated analysis results). In this manner, the exemplary system 100 can be configured to operate autonomously and / or with human control as part of a scalable IC characterization system for automated testing of ICs.

[0026] 2 is a schematic diagram illustrating in plan view an exemplary integrated circuit test probe 200, according to some embodiments of the present disclosure. Probe 200 is an example of probe 155 of FIG. 1. Probe 200 includes a probe arm 205 and a probe tip 210. Probe 200 also includes an electromechanical actuator 215 (e.g., actuator 160 of FIG. 1). Probe 200 may be operatively coupled to control circuitry 220 (e.g., via couplings 165 and 170 of FIG. 1) as described with reference to FIG. 1.

[0027] The probe arm 205 and probe tip 210 can be fabricated from a shaped, pointed conductive wire. As illustrated in FIGS. 1 and 3, the probe tip 210 can be oriented at an angle relative to the probe arm 205 such that the tip 210 contacts the surface 225 of the DUT 125 within the target region 230. The probe tip 210 can taper to a terminal surface having characteristic dimensions on the same scale (e.g., on the order of 1-100 nm) as one or more features 235 of the DUT 125. The features 235 can be conductive and electronically coupled to one or more IC components (e.g., transistors, diodes, etc.). For example, the features 235 can include conductive contact pads formed in one or more layers of the IC that can be used to mechanically and / or electrically couple the probe 205 to the IC component.

[0028] The probe tip 210 may be brought into contact with the surface 225 at a feature 235 to electronically couple the control circuitry 220 to the DUT 125. This coupling may enable the probe 200 to apply at least a portion of a test signal to the DUT 125 through the feature 235. In some embodiments, multiple probes 200 are deployed to be brought into contact with multiple respective features 235 on the surface 225 to simultaneously apply the test signal and measure the output signal. An exemplary configuration having multiple probes 200 is described with reference to FIG.

[0029] The control circuitry 220 can drive the actuator 215 in one or more directions, for example, by encoding the motion of the probe 200 as a vector of displacement values. The motion can include a linear motion component (e.g., linear displacement in one or more directions) and / or a periodic motion component that is used to dither the probe 200. In this context, "dither" motion refers to a periodic motion of the probe arm 205 and / or the probe tip 210 that is characterized by a frequency and amplitude, where the amplitude, measured as a displacement of the probe tip 210 in a given dimension, can be on the order of the size of the feature 235, whether large or small.

[0030] In an open loop positioning system, the displacement vector can be used to track the position of the probe tip 210 within a tolerance of about 10%. In this manner, determining whether contact between the probe tip 210 and the surface 225 has occurred can be prone to error. Similarly, lifting the probe tip 210 off the surface (e.g., breaking contact) can be prone to error. Moving the probe tip 210 from one feature 235 to another feature 235 while the probe tip 210 is in contact with the surface 225 can damage the DUT 125 and / or the probe tip 210. As the size of the features 235 continues to shrink to the 1-10 nm scale, additional and / or alternative contact detection techniques are required. Advantageously, the techniques described herein allow for detection of contact between the probe tip and a surface independent of the position of the probe tip 210 on the surface (e.g., without requiring electrical contact with a precise point).

[0031] As part of detecting contact between the probe tip 210 and the surface 225, the techniques of the present disclosure can include dithering the probe arm 205 and the probe tip 210. In the context of the present disclosure, the term "dithering" is used to refer to a periodic and / or oscillatory motion, or dithering motion 240. In an illustrative example, the dithering motion 240 can be generated by applying a drive signal to the electromechanical actuator 215. The drive signal can be a sinusoidal or other periodic signal having a frequency and phase referred to as a dither frequency. The dithering motion 240 can be substantially transverse to the orientation of the probe arm 205 (e.g., transverse motion) or can be substantially aligned with the orientation of the probe arm 205 (e.g., longitudinal motion). As part of increasing the detectability of the dithering motion 240, the probe 200 can be dithered in a plane substantially parallel to the surface 225 of the DUT 125 and / or in a direction substantially perpendicular to the beam of charged particles 147.

[0032] Advantageously, the lateral motion can stimulate resonant and / or harmonic vibration modes of the probe arm to enhance the amplitude of the dithering motion 240. To that end, the dither frequency can be near the resonant frequency of the probe 200. In general, the equation for the resonant frequency of a cantilever under lateral motion is:

[0033]

number

[0034] To that end, the techniques of the present disclosure include determining a resonant frequency of the probe 200. Determining the resonant frequency of the probe 200 can include dithering the probe 200 over a set of motion frequencies. In some cases, the set of motion frequencies can be determined based at least in part on an estimate determined using the above equation. In some cases, the response of the probe 200 to a sweep of motion frequencies can be measured using a charged particle microscope. For example, a beam of charged particles can be directed toward the probe 200 (e.g., the probe arm 205 and / or the probe tip 210). The detector data can include, in the case of an SEM, a secondary electron image, a backscattered electron image, a line scan signal, a current absorbed by the probe 200 from the beam, and / or intensity data as a function of time. The detector data can be filtered by frequency to isolate frequency components corresponding to a given motion frequency. In this manner, the detector data can describe the response of the probe 200 to motion frequencies based at least in part on the interaction between the beam of charged particles and the probe 200. By determining the response of the probe 200 to each of the set of motion frequencies, it may be possible to estimate the resonant frequency of the probe 200 from the detector data. The dither frequency may be selected to be approximately equal to the resonant frequency. In some embodiments, the dither frequency may be a fraction or multiple of the resonant frequency. For example, the dither frequency may be selected to stimulate a harmonic vibration mode of the probe 200 rather than the first resonant mode (n=1 in the above equation). Similarly, the dither frequency may be selected to be different from the resonant frequency, for example, based at least in part on the performance constraints of the electromechanical actuator 215. If the dithering motion 240 produces a detectable signal in the detector data, then a touch may be detected without stimulating a resonant mode of the probe 200.

[0035] In some embodiments, the beam of charged particles may be scanned across an area of ​​the probe 200 (e.g., the probe arm 205 and / or the probe tip 210) using a line scan 245. A line scan may be a scan pattern defined by a vector connecting two ends across an area of ​​the probe 200. The beam may be repeatedly scanned over the probe such that the line scan 245 is characterized by a cycle time. The cycle time, excluding the time between successive line scans 245, may include the average dwell time of the beam over a given point on the surface 225. The dwell time may be determined at least in part based on a dither frequency such that a frequency component of the dithering motion 240 is captured by the beam. In some embodiments, the dither frequency (seconds) may be determined based on the dither frequency (seconds). -1 The mathematical product of the distance (having units of 100 kHz) and the dwell time of the beam on a point in the line scan 245, such as the dwell time of the beam on a point on the probe surface (having units of seconds) is equal to or greater than 1 (a unitless value). In some embodiments, the points in the line scan 245 may coincide with features of the probe 200 or the surface 225 of the DUT 125. In an illustrative example, for a dither frequency of about 100 kHz, the dwell time may be equal to or greater than about 10 μsec. With respect to frequency ranges, above a dither frequency of about 10 MHz, problems arise from electromagnetic effects such as conductance and / or impedance induced losses in the coupling (e.g., coupling 170 of FIG. 1) and electromechanical wear of the actuator (e.g., actuator 215). In contrast, below a dither frequency of about 50 kHz, the acquisition time for each probe position may reduce throughput and make the probing technique impractical on a large scale. In some cases, the cycle time of the line scan 245 is from about 20 ms to about 50 ms, which corresponds to a spot size of from about 1.2 nm to about 50 nm (including subranges, fractions, and interpolations thereof).

[0036] Figure 3 is a schematic diagram illustrating in side view the example integrated circuit test probe 200 of Figure 2 in accordance with some embodiments of the present disclosure. Figure 3 shows the probe arm 205, the probe tip 210, the actuator 215, the control circuitry 220, the surface 225 of the DUT 125, and other components including an imaging system 305 (e.g., an electron microscope, a focused ion beam source, and / or an optical microscope), a vacuum chamber 310 (e.g., an example of the vacuum chamber 145 of Figure 1), and one or more sensors 315 operably coupled to the probe 200, as described in more detail with reference to Figure 2.

[0037] In FIG. 3, the surface 225 is shown recessed into the DUT 125 to reflect that the features 235 are typically formed inside the DUT 125 and exposed by selectively removing material from the DUT 125 (e.g., by ion beam milling or other etching process). The probe tip 210 is shown at a displacement 320 from the surface 225. In some embodiments, an exemplary process of the present disclosure includes dithering the probe tip 210 and directing an electron beam onto at least a portion of the probe tip 210 as described with reference to FIG. 2 while simultaneously displacing the probe tip 210 toward the surface 225. The control circuitry 220 can include one or more electronic signal filters and other components to process signals generated by the detector 150. For example, the filters can include one or more lock-in amplifiers, bandpass filters, lowpass and / or highpass filters, etc. configured to isolate frequency components of the detector data resulting from the dithering motion 240. As the displacement 320 decreases as the probe tip 210 approaches the surface 225, the response signal generated as an output of the detector 150 and / or the filter may be stored, processed, and / or used by the control circuitry 220 to detect contact of the probe tip 210 with the surface 225. In the example of a lock-in amplifier, the control circuitry may be configured (e.g., with analog and / or digital components) to transform the time series of intensity values ​​generated by the detector 150 during the scan pattern (e.g., line scan 245 of FIG. 2) into frequency space and filter out frequency components that deviate from the dither frequency. In some cases, the deviation may be more than 0.1%, more than 0.5%, more than 1%, more than 2%, more than 3%, more than 4%, more than 5%, more than 10%, or more than 25% (including fractions and interpolations thereof).

[0038] The beam of charged particles 147 can be directed towards a region of the probe 200, which may include the probe tip 210, as an approach to detect the region of the probe 200 having the largest magnitude of dithering motion 240. In addition to and / or instead of line scanning and imaging modes, the beam of charged particles 147 can be focused on the edge of the probe arm 205 or the probe tip 210 in spot mode. The amplitude of the dithering motion 240 and the relative dimensions of the beam spot and the probe width (e.g., at or near the probe tip 210) can be such that during each period of the dithering motion 240, the region of the probe is traversed twice by the beam of charged particles 147. In this way, the frequency of interest for filtering can be approximately twice the dither frequency. Advantageously, a relatively higher sample frequency can enhance isolation of the characteristic signal of the dithering motion 240 from other sources of periodic noise (e.g., out-of-band acoustic and / or vibration noise, EMI, etc.), such as that which may result from electromagnetic interference from vacuum components (e.g., turbo pump rotation), DUT test signals, line frequencies, etc. Additionally and / or alternatively, the beam of charged particles 147 can be defocused to a size comparable to the probe tip dimensions (e.g., about 10-100 nm in characteristic dimensions). In this way, the exposure of the DUT surface 125 to the charged particles can also be reduced (e.g., the flux of charged particles is reduced on a particle number per area basis) while still incorporating the motion of the probe into the detector data. Advantageously, this approach allows for contact detection with reduced or eliminated beam-induced contamination or damage.

[0039] FIG. 4 is a schematic diagram illustrating an exemplary integrated circuit test system 400 including multiple probes, according to some embodiments of the present disclosure. The probes of FIG. 4 are examples of the probes 155 and 200 of FIGS. 1-3. The system 400 illustrates ten individual probes, each contacting a respective feature 235. For example, a first probe tip 210-1 can contact a first feature 235-1, and a second probe tip 210-2 can contact a second feature 235-2. Systems of the present disclosure can accommodate multiple probes, each individually employing techniques described herein to detect proximity and / or contact with a surface of a DUT. A probe assembly (e.g., probe assembly 130 of FIG. 1) can accommodate 1 to 12 probes, including subranges and interpolations thereof.

[0040] In some embodiments, each probe 200 is driven at a respective dither frequency to induce a respective dither motion 240. For example, a first probe can be driven using a first dither motion 240-1 at a first dither frequency, a second probe can be driven using a second dither motion 240-2 at a second dither frequency, and a third probe can be driven using a third dither motion 240-3 at a third dither frequency. The dither frequencies can be the same or different. If the filtering method can resolve the different frequencies within the tolerance of the resonance response of the probe 200, then each dither frequency can be distinguishable and resonate. In some cases, the first dither frequency can be approximately equal to a first resonant mode, the second dither frequency can be approximately equal to a second resonant mode, and so on. In some cases, the dither frequencies can be fractions or multiples of each other. In some cases, the dither frequencies can be determined to reduce crosstalk induced by the transmission of vibrations through a probe assembly (e.g., probe assembly 130 of FIG. 1). In one illustrative example, the respective dither frequencies can be selected to have irregular spacing that interferes with vibrations from propagating between the probes.

[0041] Advantageously, applying different dither frequencies to each probe may allow a single scan pattern 405 to distinguish between probes within a common set of detector data based on frequency. For example, the frequency content of the detector data may be filtered to separate the different respective dither frequencies, thereby detecting contact of each respective probe with the surface of the DUT in parallel. In contrast, for a single dither frequency, the scan pattern 405 may be labeled with position information that may be cross-referenced to identify a given probe. Such an approach may be limited by signal quality, as the change in signal strength when a single probe makes contact may be relatively small relative to the signals generated by the remaining probes. Similarly, the time resolution of the data may limit the ability to distinguish between probes that make contact with the surface of the DUT within a given period of time. As will be described in more detail with reference to Figures 2 and 5, the scan pattern is illustrated as a substantially circular scan, but may be or include one or more linear segments that may be continuous (e.g., a polygon) or discontinuous (e.g., a set of linear segments).

[0042] In some embodiments, the beam can be defocused to extend over the surface of the DUT 125 to capture motion information of multiple probes in the detector data. In this way, the detector data can include frequency domain information for each of the respective dither frequencies such that the frequency domain information of the probes is removed from the detector data when the probes contact the surface of the DUT. Advantageously, this approach protects the surface by reducing area-specific exposure of the surface to charged particles (e.g., for a given fluence, the flux is lower when the beam is defocused over a larger portion of the surface), while allowing multiple probes to be monitored without individually locating the probes by associating each of the probes with a respective dither frequency that is identifiable from the detector data.

[0043] FIG. 5 is a block flow diagram illustrating an example process 500 for detecting contact between a probe tip and a surface, according to some embodiments of the present disclosure. One or more operations of the example process 500 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, at least a subset of the operations described with reference to FIG. 5 are performed automatically (e.g., without human involvement) or pseudo-automatically (e.g., with human initiation or limited human intervention). In one illustrative example, the operations for positioning the probe tip, displacing the probe tip, dithering the probe, and generating detector data may be performed automatically, and the system 100 is configured to generate visualization data indicative of one or more response signals for interpretation by a human user (an example is shown in FIG. 6). Although the example process 500 is described as a series of operations, it is understood that at least some of the operations may be omitted, repeated, and / or reordered. In some embodiments, the operations of the exemplary process 500 are preceded and / or followed by additional operations that are omitted for clarity of explanation, such as operations for calibrating the electron source, aligning and correcting aberrations of the electron beam, initializing the probe position, etc.

[0044] In operation 505, the exemplary process 500 includes dithering the probe (e.g., probe 200 of FIG. 2). Dithering the probe can include generating a motion (e.g., dithering motion 240 of FIGS. 2 and 4) that is at least partially orthogonal to the orientation of the probe. As described in more detail with reference to FIG. 2, the dithering motion can include a periodic component characterized by a dithering frequency. The dithering motion can be generated by applying a periodic drive signal to an electromechanical actuator (e.g., actuator 160 of FIG. 1) that also moves the probe relative to the surface. In some embodiments, the probe includes one or more actuators for generating the dithering motion that are distinct from the electromechanical actuators used to move the probe. For example, the actuators for dithering can be designed to resonate near the dither frequency such that the energy used to drive the dithering motion can be relatively low compared to the energy used to drive the electromechanical actuators used to position the probe.

[0045] In some embodiments, operation 505 includes dithering multiple probes simultaneously. As an approach to distinguish between different probes, as described in more detail with reference to FIG. 4, the probes may be dithered at their respective dithering frequencies or phases. To that end, operation 505 may include dithering a first probe / probe tip (e.g., first probe tip 210-1) at a first dither frequency and dithering a second probe / probe tip (e.g., second probe tip 210-2) at a second dither frequency that may be different from the first dither frequency. Similarly, for any number of probes (e.g., 4, 6, 8, 10, 12 probes, or more), each probe may be dithered at its respective frequency. See FIGS. 2-4 for a description of dither frequencies. By synchronizing the detector signal processor to the beam scan pattern, multiple probes may be driven at substantially the same dither frequency, such that a probe may use the same resonant or near-resonant dither frequency phase-shifted relative to another probe using a resonant or near-resonant dither frequency. Detecting the phase of the dither motion of the probe allows for differentiation between multiple probes as well as detection of contact of a given probe with the surface of the DUT. For example, detecting contact may include determining the phase of the probe captured in the detector data, locking the phase of an appropriate frequency component near the dither frequency in the detector data to the phase of the drive signal, and detecting changes in amplitude and / or phase of the frequency component relative to the drive signal as the probe approaches and / or contacts the DUT surface.

[0046] In operation 510, the exemplary process 500 includes directing a beam of charged particles (e.g., beam of electrons 147 of FIG. 1) toward a region of the probe. As described in more detail with reference to FIGS. 1-3, the beam of charged particles can be a beam of electrons and / or ions. In an imaging mode, the beam can be directed over multiple probes and DUTs using a raster pattern such that the frame rate in the imaging mode is compatible with sampling the dithering motion of the probe. For example, the frame rate can be greater than about half the dither frequency to allow the dithering motion of the probe to be detectable in a sequence of images of the probe. In another example, an image can be generated by integrating the time information at each of a set of raster positions over a period of time. In this manner, detector data is generated for multiple points at each of a set of raster positions and used to generate an image of the probe contour without applying an edge detection algorithm. In one illustrative example, the probe can be dithered at approximately 91 kHz and a beam of electrons can be used to sample the dithered motion of the probe at a sampling frequency of 1 MHz for 100 data points, corresponding to a dwell time of 100 μsec at each position. The detector data at each position is processed (e.g., by conversion to the frequency domain and filtering) which can be used to isolate the 91 kHz mode. The magnitude of the 91 kHz mode can be used as the intensity value for each pixel in the image, thereby reproducing at least a portion of the circumference of the probe tip.

[0047] In some embodiments, directing the beam of charged particles includes scanning the beam in a scan pattern across the surface of the DUT. As described in more detail with reference to FIG. 4, the rotational pattern may correspond to a circular motion, a polygonal motion, etc., such that the beam traverses one or more probes as one or more line scans. In the illustrative example of FIG. 4, the beam traverses ten probes (e.g., probe 205 of FIG. 4) in a circular motion such that the beam passes over each probe in a line. In some cases, the scan pattern corresponds to a polygon having a number of sides equal to the number of probes on which the exemplary process 500 is performed. Although FIG. 4 includes ten probes, systems of the present disclosure may include more or fewer probes, including four probes, six probes, eight probes, twelve probes, etc., including interpolations thereof. In another example, the scan pattern may include a plurality of discontinuous line scans (e.g., line scan 245 of FIG. 2), each oriented to pass the beam over a respective probe area. In some cases, each line scan of the scan pattern may be substantially orthogonal to a respective probe. Advantageously, using a scan pattern including multiple discontinuous line scans can reduce the overall flux of electrons incident on the surface of the DUT, thereby limiting degradation of surface features 235 (e.g., conductive contact pads, dielectric regions of an IC, etc.) caused by beam-induced contamination.

[0048] The beam may be focused on a region of the probe and used in line scan and / or spot modes, as described in more detail with reference to Figures 2 and 3. In this manner, motion of the probe may be detected and used to identify transitions from free vibration to fixed vibration. The region of the probe may include the probe tip. Advantageously, directing the beam towards the probe tip may improve detection of probe contact with a surface (e.g., surface 225 of DUT 125 in Figure 2) by isolating a portion of the probe that exhibits the greatest range of motion. To that end, the exemplary process 500 may include determining an approximate location of the tip of the probe. Determining the approximate location of the tip of the probe may include generating one or more electronic images of the probe and using the electronic images to determine the approximate location of the tip of the probe. Determining the approximate location of the tip of the probe may include applying automatic and / or pseudo-automatic geometric fitting methods to the image and / or contour of the probe in the filtered frequency data. For example, the location of the probe tip may be approximated by fitting line segments to each edge of a tapered portion of the probe such that the convergence points of the line segments represent the location of the probe tip. In some embodiments, the position of the probe tip can be approximated by repeating a line scan over two or more locations of the tapered region of the probe. The probe tip can be located using an estimate of the width of the probe in each line scan and an estimate of the distance between the line scans. In some cases, the edges of the probe can be identified in the line scans, which can be used to identify the leading and trailing edges of the probe, as described in more detail with reference to FIG. 6D.

[0049] In operation 515, the exemplary process 500 includes generating detector data. As described in more detail with reference to FIGS. 2 and 3 and in the examples of FIGS. 6A-6F, the detector data can include one or more modalities describing the interaction of a beam of charged particles with a region of the probe. For example, the detector data can include a series of images generated using a secondary electron detector or a backscattered electron detector. In another example, the detector data can include a vector of intensity values ​​(proportional to the flux of charged particles incident on the detector surface) and corresponding position values ​​(e.g., related to an origin). In spot mode, the detector data can include a time series vector of intensity values, where each element of the vector is separated by a time step corresponding to the sampling frequency of the detector (e.g., detector 150 of FIG. 1).

[0050] Operation 515 may include generating filtered data to isolate one or more frequency components of the charged particle detector. For example, the output signal from the detector may be filtered using a lock-in amplifier (e.g., as part of the control circuit 220 of FIGS. 2 and 3), using a bandpass filter configured to isolate the dither frequency, and / or using other filtering techniques adapted to selectively remove periodic components of the detector signal that are attributable to sources other than the dithering motion of the probe. To that end, the intensity values ​​in the detector data may be transformed into frequency space (e.g., using a Fourier transform, a fast Fourier transform, etc.) to facilitate filtering by frequency. In this manner, the detector data may include a sequence of frequency-space intensity data for a set of time points such that the intensity data may be represented as a function of frequency and time.

[0051] In operation 520, the exemplary process 500 includes displacing the probe toward the surface. Displacing the probe tip can include engaging one or more individually addressable actuators (e.g., electromechanical actuator 160 of FIG. 1). In this manner, operation 510 can include engaging one or more actuators such that a vector describing the displacement of the probe tip is non-zero in a dimension substantially perpendicular to the surface of the sample and approximately equal to zero in other dimensions. In the example of a three-coordinate displacement vector, operation 510 can include applying a vector (X,Y,Z)=(0,0,z), where "z" is a non-zero number. In some embodiments, the xy coordinates (or their equivalents) are determined with reference to an image of the sample surface.

[0052] Operation 520 can include applying one or more displacement rates to the z-motion of the probe tip. For example, a first "coarse" motion rate can be greater than a second "fine" motion rate. In some embodiments, operation 510 includes reducing the displacement rate in the z-axis from a "coarse" rate to a "fine" rate when the estimated displacement of the probe tip (e.g., displacement 320 in FIG. 3) is between about 100 nm and about 1000 nm (including subranges, fractions, and interpolations thereof). If the tolerance for displacement is about 10%, reducing the displacement rate at the threshold distance can reduce the risk of damage to the probe and / or sample surface due to undesired contact.

[0053] At operation 525, the example process 500 includes detecting contact between the probe tip and the surface of the DUT. As described in more detail with reference to FIGS. 6A-6F, detecting contact may include one or more approaches using the detector data generated in operation 515. For example, if generating detector data includes generating filtered data using a lock-in amplifier tuned to a dither frequency, detecting contact may correspond to an attenuation of the output of the lock-in amplifier at the dither frequency. In some cases, detecting contact may include comparing a value of the lock-in output to a threshold at which the probe tip is deemed to be in contact with the surface of the DUT.

[0054] Example 1: Landing a probe tip on a device surface 6A-6F present exemplary data from an experimental embodiment of the present disclosure. FIGS. 6A and 6B are composites showing exemplary SEM images of an integrated circuit test system on a device surface (e.g., surface 225 of FIG. 2) and in contact with the device surface, respectively. FIGS. 6C and 6E are composites showing exemplary lock-in amplifier output images of an integrated circuit test system on a DUT surface and in contact with the DUT surface, respectively. FIGS. 6D and 6F are graphs showing exemplary lock-in amplifier output spectra of an integrated circuit test system on a DUT surface and in contact with the DUT surface, respectively. The images and data presented in FIGS. 6A-6F are illustrative examples and are not intended to limit the scope of the present disclosure. Instead, the images and data are presented to demonstrate the improved contact detection provided by the techniques of the present disclosure.

[0055] As illustrated in Figures 6A and 6B, the environment of the probe tip 210 may include a relatively high density of visual information, such as multiple features 235, surface particles, shadows, charged regions, and edges that together obscure the shape, contour, and / or shadow of the probe tip 210. As a result, visually determining contact between the probe tip 210 and the surface 225 of the DUT 125 may be difficult. In some cases, the probe tip 210 may have a rough or otherwise irregular surface that may be formed by surface buildup (e.g., carbon contamination or other deposits) and / or as a result of the fabrication process, which may further complicate visually determining contact with the DUT surface 225. As an illustrative example, Figure 6B shows little visually discernible difference from Figure 6A, but nevertheless represents the probe tip 210 in contact with the surface 225.

[0056] The image 605 of FIG. 6C can be generated at least in part from the SEM data by filtering frequency information around the dither frequency and / or multiples of the dither frequency. For example, pixel-by-pixel frequency information from a series of SEM images can be sampled by generating time series data on the intensity of a given pixel. By determining frequency spatial information (e.g., by digital signal processing techniques such as Fourier transform techniques), one can isolate the dithering motion 240 of the probe tip 210 relative to the surface 225 of the DUT 125, which does not vibrate at the same frequency or frequencies. As shown, the filtered frequency information generates a relief image of the probe tip 210 against a background of any average noise. Comparable spectral data 610 can be generated using the line scan 245 and is represented in the spectrum of FIG. 6D, where the edge 615 of the probe tip 210 is indicated in the intensity data by local maxima (e.g., peaks and troughs) in the spectral data. In FIG. 6D, the spectrum is shown with electron beam shadows resulting from the orientation of the detector (e.g., detector 150 of FIG. 1) relative to the probe tip 210. In this context, "shadow" refers to the tendency for secondary electron signals from surfaces facing toward the detector to be brighter relative to signals from surfaces facing away from the detector. Thus, the spectral data 610 includes a peak at the leading edge 615-1 that is brighter than any average background "A" and a peak at the trailing edge 615-2 that is darker than the background.

[0057] In some embodiments, an electron beam (e.g., beam of charged particles 147 in FIG. 1) may be directed at the edge 615 of the probe tip 210, e.g., in spot mode, such that a dithering motion 240 of the probe tip repeatedly brings the edge 615 of the probe tip under the beam spot. In this manner, the frequency of the intensity data may be a multiple of the dither frequency and the magnitude may be a scalar value generated as a function of time, the magnitude of the value indicating whether the probe tip 210 is in contact with the surface 225 of the DUT 125. This approach is also described with reference to FIGS. 2-5. With respect to the spectral data 610, the spot mode information may be understood as a vector of intensity values ​​of the edge 615, where each element of the vector is the filtered frequency information at a given time.

[0058] 6E and 6F present a second image 620 of frequency data at about the dither frequency or a multiple of the dither frequency and corresponding spectral data 625. In contrast to the image 605 and the spectral data 610, the probe tip 210 is absent in the second image 620, and the spectral data 625 shows a slight shadow of the probe tip 210 near the trailing edge 615-2. Based at least in part on the fixation of the probe tip 210 to the surface 225 upon contact, free vibration of the probe tip 210 at about the dither frequency and / or a multiple of the dither frequency has been substantially eliminated, as shown by the absence of the peak 610 in the spectral data 625. In some cases, some vibration from the dithering motion may remain, such that detecting contact may include comparing the intensity of the peak 610 intensity value in the image 605, spectrum 610, or scalar value as a function of time to a threshold below which the probe is considered to be in contact with the surface. In some embodiments, the probe signal may be used to determine whether a conductive coupling is established.

[0059] Various embodiments have been described in the preceding specification. For purposes of explanation, specific configurations and details have been set forth to provide a thorough understanding of the embodiments. However, it will be apparent to one skilled in the art that the embodiments may be practiced without the specific details. Furthermore, well-known features may be omitted or simplified so as not to obscure the described embodiments. Although the illustrative embodiments described herein focus on integrated circuit testing systems, and in particular on multi-probe systems, these are meant to be non-limiting illustrative embodiments. The embodiments of the present disclosure are not limited to such embodiments, but rather are intended to address analytical instrument systems capable of analyzing a wide range of material samples, such as microbiological samples (e.g., for physiological measurements) and / or nanostructured samples, with the probes of the present disclosure.

[0060] Some embodiments of the present disclosure include a system including one or more data processors and / or logic circuitry. In some embodiments, the system includes a non-transitory computer-readable storage medium including instructions that, when executed on the one or more data processors and / or logic circuitry, cause the one or more data processors and / or logic circuitry to perform some or all of one or more methods and / or some or all of one or more processes or workflows disclosed herein. Some embodiments of the present disclosure include a computer program product tangibly embodied in a non-transitory machine-readable storage medium including instructions configured to cause one or more data processors and / or logic circuitry to perform some or all of one or more methods and / or some or all of one or more processes disclosed herein.

[0061] The terms and expressions used are used as terms of description and not as terms of limitation, and in using such terms and expressions, there is no intention to exclude any equivalents of the features or portions thereof shown and described, but it is understood that various modifications are possible within the scope of the claims. Thus, although the present disclosure includes specific embodiments and optional features, it should be understood that modifications and variations of the concepts disclosed herein can be made by those skilled in the art, and such modifications and variations are considered to be within the scope of the appended claims.

[0062] When a term is used without an explicit definition, it is understood that the ordinary meaning of the word is intended unless the term has a special and / or specific meaning in the field of charged particle microscope systems or other related fields. The term "about" or "substantially" is used to indicate a deviation from a described characteristic, within which there is little or no effect on the corresponding function, characteristic, or attribute of the described structure. In an illustrative example where a 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 not be equal within acceptable limits, such as manufacturing tolerances or confidence intervals inherent in the operation of the system. Similarly, when a geometric parameter such as alignment or angular orientation is described as "about" 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 strictly described condition (e.g., not strictly perpendicular) within acceptable limits. With respect to numerical values ​​such as diameter, length, width, etc., the term "about" can be understood to describe a deviation of up to ±10% from the described value. For example, a dimension of "approximately 10 mm" can describe a dimension between 9 mm and 11 mm.

[0063] This specification provides exemplary embodiments and is not intended to limit the scope, applicability, or configuration of the present disclosure. Rather, the description following the exemplary embodiments provides those skilled in the art with an enabling description to realize various embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the spirit and scope as set forth in the appended claims. Specific details are given herein to provide a thorough understanding of the embodiments. However, it will be understood that the embodiments may be practiced without these specific details. For example, specific system components, systems, processes, and other elements of the present disclosure may be shown in schematic form or omitted from the illustrated diagrams so as not to obscure the embodiments in unnecessary detail. In other cases, well-known circuits, processes, components, structures, and / or techniques may be shown without unnecessary detail.

Claims

1. 1. A computer-implemented method for detecting contact between a probe tip and a sample surface, the method comprising: dithering the probe in a plane substantially parallel to the surface using a periodic motion having a dither frequency; directing a beam of charged particles toward a region of the probe; generating detector data describing the periodic motion of the probe based at least in part on an interaction between the beam of charged particles and the region of the probe; displacing the probe towards the surface; and detecting contact between the probe and the surface using the detector data.

2. The computer-implemented method of claim 1 , further comprising determining an approximate location of the tip of the probe within the region of the probe.

3. Determining the approximate location of the tip of the probe generating detector data describing the region of the probe; and determining the approximate location of the tip of the probe using the detector data.

4. The computer-implemented method of claim 1 , wherein directing the beam comprises scanning the beam of charged particles in a line across the region of the probe using a line scan.

5. The computer-implemented method of claim 4 , wherein a mathematical product of the dither frequency and the dwell time of the beam on a point in the line is greater than or equal to one.

6. determining an approximate orientation of the probe; The computer-implemented method of claim 1 , further comprising: generating identifier information for the probe based at least in part on the orientation.

7. 2. The computer-implemented method of claim 1, wherein the sample includes an integrated circuit having contact pads, the contacts being formed between the probes and the contact pads.

8. 2. The computer-implemented method of claim 1, wherein generating the detector data includes generating filtered data using a lock-in amplifier tuned to the dither frequency, and detecting the contact corresponds to an attenuation of an output of the lock-in amplifier at the dither frequency.

9. dithering the probe over a set of motion frequencies; directing the beam of charged particles towards the probe; generating detector data describing a response of the probe to the motional frequency based at least in part on the interaction between the beam of charged particles and the probe; using the detector data to estimate a resonant frequency of the probe; The computer-implemented method of claim 1 , further comprising: selecting the dither frequency to be approximately equal to the resonant frequency.

10. 10. The computer-implemented method of claim 9, wherein directing the beam of charged particles comprises scanning the beam over the surface in a scan pattern.

11. 10. The computer-implemented method of claim 9, wherein estimating the resonant frequency comprises isolating the resonant frequency using a lock-in amplifier.

12. 10. The computer-implemented method of claim 9, wherein the set of motion frequencies includes a component frequency of 100 kHz.

13. One or more non-transitory machine-readable storage media storing instructions that, when executed by a machine, cause the machine to perform operations, the operations including: dithering the probe in a plane substantially parallel to the surface using a periodic motion having a dither frequency; directing a beam of charged particles toward a region of the probe; generating detector data describing the periodic motion of the probe based at least in part on an interaction between the beam of charged particles and the region of the probe; displacing the probe towards the surface; and detecting contact between the probe and the surface using the detector data.

14. 14. The one or more media of claim 13, wherein generating the detector data includes generating filtered data using a lock-in amplifier tuned to the dither frequency, and detecting the contact corresponds to an attenuation of the output of the lock-in amplifier at the dither frequency.

15. The one or more media of claim 13 , wherein the dither frequency is approximately equal to a resonant frequency of the probe.

16. 16. The medium of claim 13 or 15, wherein the mathematical product of the dither frequency and the dwell time of the beam on a point is 1 or greater.

17. 1. A system for testing integrated circuit components, comprising: A probe; a charged particle beam system; a detector oriented toward the probe and configured to detect particles generated by interaction of a charged particle with the probe; a control circuit operably coupled to the probe, the charged particle beam system, and the detector and configured to operate the probe; One or more non-transitory machine-readable storage media storing instructions that, when executed by the system, cause the system to perform operations, the operations including: dithering the probe in a plane substantially parallel to the surface using a periodic motion having a dither frequency near a resonant frequency of the probe; directing the beam of charged particles towards a region of the probe; generating detector data describing the periodic motion of the probe based at least in part on an interaction between the beam of charged particles and the region of the probe; displacing the probe towards the surface; and detecting contact between the probe and the surface using the detector data.

18. 20. The system of claim 17, wherein the charged particle beam system is a scanning electron microscope column and the detector is a secondary electron detector or a backscattered electron detector.

19. Detecting the contact comprises: determining phase and frequency components in the detector data due to the periodic motion of the probe; aligning the phase component to a phase of a drive signal for the periodic motion; and detecting a change in the frequency content or the phase content of the detector data relative to the drive signal.

20. the probe is a first probe, the periodic motion is a first periodic motion, the dither frequency is a first dither frequency, the system further comprises a second probe, and the operation comprises: dithering the second probe in the plane substantially parallel to the surface using a second periodic motion having a second dither frequency near a resonant frequency of the second probe; directing the beam of charged particles toward a region of the second probe; generating detector data describing the periodic motion of the second probe based at least in part on an interaction between the beam of charged particles and the region of the probe; displacing the second probe towards the surface; The system of claim 17 , further comprising: detecting contact between the second probe and the surface using the detector data.

21. 21. The system of claim 20, wherein the first dither frequency and the second dither frequency are unequal.

Images