Method and system for backside plan view lamella preparation

Abstract

Method and system for backside plan view lamella preparation. The backside of the plan view lamella is prepared from a sample extracted from a workpiece. The sample includes a plurality of device layers and a substrate layer. After removing at least a portion of the substrate layer covering the final device layer to obtain the sample surface, an electron beam is used to alternately scan a region of interest (ROI) associated with the sample surface and spontaneously etch the region of interest until the final device layer within the ROI is exposed. After the final device layer is exposed to obtain the backside of the plan view lamella, one or more device layers can be removed from the backside of the sample.

Description

Technical Field

[0001] The present invention relates generally to methods and systems for preparing sheeting and, more particularly, to processing the backside of a plan view sheeting. Background Art

[0002] Transmission electron microscopy (TEM) requires that the specimen be thin enough so that the electrons transmitted through the specimen can be used to form an image. One method of preparing TEM samples is to extract the sample by milling the workpiece using a focused ion beam (FIB). The extracted sample is then thinned from the front and back sides to form a thin TEM sample, i.e., a lamella. In order to perform failure analysis on microelectronic devices, such as those having 3D-NAND structures, a plan view lamella can be prepared. A plan view lamella has front and back surfaces extending parallel to the device layers and can be used to observe specific layers of a 3D-NAND structure.

[0003] Franco et al. disclose in US2018 / 0350558A1 a method for preparing a plan view lamella. In this method, the back side of a lifted sample is thinned using a FIB with or without an etching assist gas. However, the applicants recognize that in order to ensure that the back side of the lamella is parallel to the device layer, the back side surface of the sample formed before thinning using the FIB must be parallel to the device layer. One method of ensuring that the back side surface of the sample is parallel to the device layer is to use, for example, XeF 2 The etching gas spontaneously etches the back of the sample. 2 The silicon substrate can be selectively etched while retaining the device layer. However, the applicant has realized that when preparing samples with 3D-NAND structures, XeF 2 The polysilicon in the device layer may be over-etched. As a result, the etched surface may be uneven. In addition, the device layer adjacent to the substrate layer may be damaged during the etching process. Summary of the invention

[0004] In one embodiment, a method for processing a sample including at least a substrate layer and a device layer using a charged particle beam comprises: removing at least a portion of the substrate layer to obtain a sample surface; scanning a region of interest (ROI) associated with the sample surface using an electron beam; flowing a first gas toward the ROI to spontaneously etch the scanned ROI; and scanning the etched ROI using the electron beam in response to the device layer not being exposed in the ROI. In this manner, a plan view sheet having a flat back surface can be formed from the processed sample.

[0005] It should be understood that the above summary is provided to introduce some concepts further described in the detailed description in a simplified form. It is not meant to identify the key or essential features of the claimed subject matter. In addition, the claimed subject matter is not limited to embodiments that address any disadvantages noted above or in any part of this disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 A dual beam system according to some embodiments of the present invention is shown.

[0007] Figure 2 A method for forming the sheet is shown.

[0008] Figure 3A A workpiece having multiple device layers is shown.

[0009] Figure 3B A sample extracted from a workpiece is shown.

[0010] Figure 3C Extracted samples are shown.

[0011] Figure 4 A portion of the device layer of a 3D-NAND structure is shown.

[0012] Figure 5 is a flow chart for processing the reverse side of a sample extracted from a workpiece.

[0013] Figure 6 is an image of the back side of the sample with the final device layer exposed.

[0014] Figure 7 The signal received during the layer reduction process is shown.

[0015] Like reference numerals refer to corresponding parts throughout the several views of the drawings. DETAILED DESCRIPTION

[0016] The following description relates to systems and methods for preparing the backside of a plan view lamella for imaging in a transmission electron microscope (TEM). The preparation may be performed in Figure 1 The dual beam system shown has a first column for forming an ion beam and a second column for forming an electron beam.

[0017] Specific device layers of the workpiece can be inspected based on high-resolution TEM images of plan view slices prepared from the workpiece.In one example, a workpiece having a 3D-NAND structure may include dozens of layers of integrated circuit (IC) chips fabricated on a silicon substrate. Figure 44 is a TEM image showing a portion of a device layer of a 3D-NAND structure. An xy cross-section of multiple vertical structures can be observed in the TEM image. Each memory cell 400 includes concentric material layers including silicon dioxide, polysilicon, and silicon nitride. The vertical structures extend in the z direction. The device layers in the 3D-NAND structure can be separated from each other by spacer materials such as silicon dioxide. The measurements of the device structures (such as memory cells) in a particular device layer can be determined from the TEM image.

[0018] Plan view slices can be based on Figure 2 The sample is milled and extracted from the workpiece. Figure 3A-3B As shown in Figure 2, multiple device layers in the extracted sample extend parallel to the surface of the workpiece. Figure 3C As shown, it is a portion of the top surface of the workpiece. On the back side of the extracted sample, the final or bottom device layer is embedded under at least one substrate (such as a silicon substrate). In some examples, a spacer layer such as a silicon dioxide layer may be located between the substrate layer and the final device layer. There are no other device layers between the final device layer and the substrate layer. The material on the back and front sides of the extracted sample is removed to obtain a plan view slice with a specific device layer exposed from both the back and front sides within a region of interest (ROI). For example, in order to obtain the back side of the plan view slice, the material covering the final device layer (such as the substrate layer and the spacer layer) is removed to expose the final device layer. Then, one or more device layers can be removed from the back side of the sample by a layer reduction process.

[0019] In order to form a plan view lamella with a flat and large ROI, the exposed final device layer must be flat and parallel to the device layer. One method of removing the substrate is to use XeF 2 Spontaneous etching of the back side of the sample. 2 Silicon is etched selectively so that a portion of the final device layer remains after etching. However, it is difficult to control XeF 2 Etching exposure time: Short exposure time can cause under-etching of the silicon substrate; while long exposure time can over-etch some materials, such as polysilicon in the device layer. Under-etching can reduce the area of ​​the exposed final device layer, thereby reducing ROI. On the other hand, over-etching can cause unevenness and damage to the device layer. For example, the device layer close to the silicon substrate, such as the 0-5 device layer far away from the silicon substrate, cannot be observed.

[0020] Figure 5 A method for preparing the back side of a plan view lamella is shown in FIG. The back side of the sample can be thinned overall using a focused ion beam (FIB) and / or spontaneous etching using etching gases. When the exposed back side sample surface is close to the final device layer, the ROI relative to the back side of the sample is alternately scanned with a high current electron beam and the XeF2 Spontaneous etching. When XeF is removed from the lower chamber 2 Electron beam scanning is performed while XeF 2 Etching. The scan-etch sequence may be performed in multiple iterations until most of the final device layer within the ROI is exposed. Here, the final device layer is exposed when the device or device structure in the final device layer (e.g., a memory cell in a 3D-NAND structure) is exposed. In one example, the iteration may be terminated when the ratio between the area of ​​the final device layer exposed in the ROI and the total area of ​​the ROI is greater than a threshold ratio. In another example, the iteration may be terminated when the exposed final device layer is flat and flat. The duration of the spontaneous etching within each iteration may be constant and predetermined. The scan-etch iteration removes the material covering the final device layer on the back side of the sample. If there is a spacer layer between the substrate layer and the final device layer, both the substrate layer and the spacer layer are removed during the iteration. After exposing the final device layer, one or more device layers on the back side of the sample may be removed by a subtractive process to obtain the back side of the plan view wafer.

[0021] During the preparation of the back plane view lamella, an SEM image of the back side of the sample can be taken for monitoring the preparation process. The beam current used for SEM imaging is lower than the beam current used to scan the ROI in the scan-etch sequence. The beam current used for SEM imaging may be lower than 1nA, while the beam current used for ROI scanning may be higher than 1nA. In one example, the beam current used for SEM imaging is 100pA to 1nA, and the beam current used for ROI scanning is 2-4nA. In addition, in order to observe the sample surface, the beam energy used for SEM imaging may be lower than the beam energy used to scan the ROI in the scan-etch sequence. For example, the beam energy for SEM imaging is lower than 10kV (e.g., 2-10kV), while the beam current for ROI scanning is higher than 10kV (e.g., 10-20kV).

[0022] By scanning the ROI with a high current electron beam, the ROI can be deposited and / or doped with carbon. In some examples, a carbon source can be provided in the lower chamber. By using an electron beam to deposit / dope the ROI, a preferential etch rate between device structures in the device layer and materials in other layers (such as substrate layers and spacer layers) is achieved during the etching process immediately following the scan. For example, in XeF 2During etching, the silicon substrate is etched at a higher rate than the device structure. The preferential etching rate can be caused by the thin carbon layer preferentially deposited above the uneven device material of the device structure compared to the relatively uniform substrate layer during electron beam scanning. In addition, electron beam scanning can lead to enhanced adsorption of carbon materials onto the device layer. The intrinsic mechanism of this enhancement may be due to material differences, topological considerations, or a combination thereof. Due to the preferential etching rate, materials covering the final device layer (such as substrate layer and spacer layer) can be selectively removed during scan-etch iterations while retaining the device structure near the substrate layer. As a result, a large and flat final device layer is exposed in the ROI.

[0023] Figure 6 is a SEM image of the back side of a sample with exposed final device layers. The sample with exposed final device layers can be subtracted using a FIB with or without an etch assist gas to obtain a plan view of the back side of the lamella. Figure 2 The back surface of the sample formed by the method is flat and parallel to the device layer, which is due to the following Figure 7 The signal received during the backside subtraction process is shown.

[0024] Go to Figure 1 , Figure 1 A dual beam system 110 is shown. The dual beam system includes an ion beam that is vertical or tilted a few degrees relative to the plane of the workpiece and an electron beam that has an axis that is also tilted, for example, 52 degrees relative to the axis of the ion beam. In some embodiments, the ion beam and the electron beam are aligned so that the fields of view of the two beams overlap within a few microns. Both the ion beam and the electron beam can be used to image and / or process a sample.

[0025] The dual beam system 110 includes an ion column 111 for generating an ion beam. The ion column 111 includes an ion source 114, an extraction electrode 115, an electrostatic optical system 117, and an electrostatic deflection plate 120. The ion beam 118 generated by the ion source 114 is deflected by the electrostatic deflector 120 before irradiating a workpiece 122. The workpiece 122 is located on a movable XYZ stage 124 in a lower chamber 126. The lower chamber 126 can be evacuated using a turbomolecular and mechanical pumping system 168 under the control of a pumping controller 130. The vacuum system provides, for example, approximately 5×10 -8 Torr and 5×10 -4 When using etch assist, etch delay, or deposition precursor gases, the chamber background pressure can be increased, for example, to about 1×10 -5 Entrust.

[0026] A high voltage power supply 134 is connected to the ion source 114 and to appropriate electrodes in the ion column 111 for forming and directing downwardly an ion beam 118. A deflection controller and amplifier 136, which operates according to a prescribed pattern provided by a pattern generator 138, is coupled to the deflection plates 120, whereby the ion beam 118 can be controlled to trace a corresponding pattern on the workpiece 122. In some systems, the deflection plates are placed before the final lens.

[0027] The ion source 114 typically provides a metal ion beam of gallium, but other ion sources may also be used, such as multi-spike or other plasma ion sources. The ion source 114 is typically capable of focusing into a sub-tenth micron wide beam at the workpiece 122 for modifying the workpiece 122 by ion milling, enhanced etching, material deposition, or for imaging the workpiece 122. A charged particle multiplier 140 for detecting secondary ions - or, for example, a secondary electron detector 140 for detecting secondary electron emission for imaging - is connected to a signal processor 142, in which the signal from the charged particle multiplier 140 is amplified, converted to a digital signal, and signal processed. The resulting digital signal will display an image of the workpiece 122 on a monitor 144.

[0028] An electron column 141 is also provided with the dual electron beam system 110 together with a power supply and control unit 145. An electron beam 143 is emitted from the cathode 152 by applying a voltage between the cathode 152 and the anode 154. The electron beam 143 is focused to a fine point with the aid of a condenser lens 156 and an objective lens 158. The electron beam 143 is two-dimensionally scanned on the workpiece with the aid of a deflection coil 160. The operation of the condenser lens 156, the objective lens 158 and the deflection coil 160 is controlled by the power supply and control unit 145. When the electrons in the electron beam 143 hit the surface of the workpiece 122, secondary electrons and backscattered electrons are emitted. Accordingly, these electrons are detected by the SE detector 140 or the backscattered electron detector 162. The analog signal generated by the SE detector 140 or the backscattered electron detector 162 is amplified and converted into a digital brightness value by the signal processor unit 142. The resulting digital signal can be displayed on the monitor 144 as an image of the workpiece 122.

[0029] The micromanipulator 147 may include precision motors 148 to provide X, Y, Z, and theta control of a portion 149 located within the vacuum chamber. The micromanipulator 147 may be equipped with various end effectors for manipulating small objects, such as lifting a sample cut from the workpiece 122. In the embodiment described herein, the end effector is a thin probe 150.

[0030] The door 170 is opened for inserting the workpiece 122 onto the XY stage 124, which may be heated or cooled, and for maintaining the internal gas supply reservoir if used. The door is interlocked so that it cannot be opened if the system is under vacuum. A gas delivery system 146 extends into the lower chamber 126 for introducing and directing gaseous vapors to the workpiece 122.

[0031] The system controller 119 controls the operation of various parts of the dual beam system 110. Through the system controller 119, a user can cause the ion beam 118 or the electron beam 143 to scan in a desired manner by inputting commands into a user interface (not shown). The system controller 119 may also include a computer readable memory 121, and may control the dual beam system 110 according to data or computer readable instructions stored in the memory 121 to implement the methods described herein.

[0032] The above-mentioned apparatus and system can utilize high-precision beam placement methods for local navigation. In addition, it should be recognized that elements, aspects or embodiments can be implemented via computer hardware or software or a combination of the two. The method can be implemented in a computer program using standard programming techniques, including a computer-readable storage medium configured with a computer program, wherein the storage medium so configured, according to the methods and drawings described in this specification, enables the computer to operate in a specific and predefined manner. Each program can be implemented in a high-level procedural or object-oriented programming language to communicate with a computer system. However, if necessary, the program can be implemented in assembly language or machine language. In any case, the language can be a compiled language or an interpreted language. Moreover, the program can be run on a dedicated integrated circuit programmed for this purpose.

[0033] In addition, the methods may be implemented in any type of computing platform, including but not limited to personal computers, microcomputers, mainframes, workstations, networked or distributed computing environments, computer platforms separate from, integrated with, or in communication with charged particle tools or other imaging devices, and the like. Various aspects may be implemented in machine-readable code stored on a storage medium or device, whether removable or integrated with a computing platform, such as a hard disk, an optical read and / or write storage medium, a RAM, a ROM, and the like, such that the storage medium or device can be read by a programmable computer for configuring and operating the computer to perform the processes described herein when the computer reads the storage medium or device. Moreover, the machine-readable code or portions thereof may be transmitted over a wired or wireless network. When such media contain instructions or programs for implementing the above steps in conjunction with a microprocessor or other data processor, the embodiments described herein may include these and other various types of computer-readable storage media. When programmed according to the methods and techniques described herein, the embodiments may also include the computer itself.

[0034] The computer program can be applied to input data to perform the functions described herein, thereby transforming the input data to generate output data. The output information is applied to one or more output devices, such as a display monitor. In some embodiments, the transformed data can represent physical and tangible objects, including generating specific visual depictions of physical and tangible objects on a display.

[0035] As indicated, some embodiments may also utilize charged particle beams, such as ion beams or electron beams, so as to image the sample using the particle beam. Such charged particles used to image the sample may inherently interact with the sample, resulting in some degree of physical transformation. In addition, throughout this specification, discussions utilizing terms such as "calculate," "determine," "measure," "generate," "detect," "form," etc. also relate to the operation and process of a computer system or similar electronic device that manipulates data represented as physical quantities within the computer system and transforms it into other data similarly represented as physical quantities within the computer system or other information storage, transmission, or display device.

[0036] Ion beams and electron beams are described herein as examples of charged particle beams used to image or process a workpiece.Other charged particle beams may be used, such as a laser beam, or some other shape of ion beam, such as an ion beam from a liquid metal ion source.

[0037] Figure 2 Shows the use of Figure 1 A method 200 for preparing a plan view lamella using a dual beam system of a device. The plan view lamella is prepared for inspecting a device layer of a workpiece including a plurality of device layers. The device layer may have a 3D-NAND structure.

[0038] At 202, a sample is extracted from a workpiece. The sample may be first milled from the workpiece using a FIB and then removed using a micromanipulator (e.g., Figure 1 A micromanipulator 147) lifts / extracts the sample from the workpiece for further processing. Figures 3A-3C The process of sample extraction is shown.

[0039] exist Figure 3A-3B In , workpiece 310 is oriented such that the length and width extend in the xy plane, and the height extends along the z axis. The workpiece includes one or more device layers fabricated on a substrate 307. Substrate 307 may be silicon. The device layers extend in the xy plane. The device layers may be separated from each other by spacer layers. The spacer layers may be formed of silicon dioxide. In Figure 3A, three device layers (301-303) are shown as examples. For a 3D-NAND structure, the number of device layers may be tens of layers or more than one hundred layers. The top surface 308 of the workpiece 310 on the front side is parallel to the device layers. In one example, the final or bottom device layer 303 is in direct contact with the substrate 307. In another example, the final device layer 303 is separated from the substrate 307 by a spacer layer. The front side of the workpiece is indicated by arrow 305 (z direction), and the back side of the workpiece is indicated by arrow 306 (opposite to the z direction).

[0040] Figure 3B A method is shown for cutting a sample 330 from a workpiece 310. The workpiece 310 is first undercut from opposite directions by two intersecting ion beam cuts 321 and 322. The ion beam then cuts the sides 323 and 324. The sample 330 can be lifted from the workpiece 310 by a probe for further processing.

[0041] Figure 3C An extracted or lifted sample 330 is shown. Sample 330 includes multiple device layers (331-333) on the front side 305. The front surface of the extracted sample is a portion of the top surface 308 of the workpiece 310. Sample 330 includes at least one substrate layer 334 on the back side 306. The wedge-shaped substrate layer 334 must be removed to expose the final device layer. The sample with the exposed final device layer can also be subtracted to remove one or more device layers from the front and / or back side to obtain a plan view slice.

[0042] Transfer back Figure 2 At 204, the back side of the extracted sample is processed to form the back side of the plan view slice. The back side processing removes the wedge-shaped substrate layer (such as Figure 3C The substrate layer 334) and the final device layer adjacent to the substrate layer (such as Figure 3C The backside preparation of the wafer may also include subtracting one or more device layers from the backside of the sample using a FIB with or without an etch assist gas. Details of the backside processing are described in Figure 5 Described in.

[0043] At 206, the front side of the extracted sample is processed to form the front side of the plan view slice. The front side processing may include reducing or removing a predetermined number of device layers from the front side using the FIB. An etch assist gas may optionally be provided with the FIB. In one example, a predetermined number of device layers are removed to expose a specific device layer embedded in the sample using a facing FIB.

[0044] In some examples, step 206 is omitted and only the back side of the sample is processed to inspect the top device layer.

[0045] At 208, a high resolution TEM image of the plan view slice is acquired. Metrics of specific device layers can be determined based on the TEM image. The TEM image can be viewed at Figure 1 or alternatively acquired in a different TEM system.

[0046] In this way, the device layer in the plan view lamella is parallel to the back surface of the lamella and the front surface of the lamella. The plan view lamella provides a larger ROI in which a specific device layer can be imaged. In addition, the specific device layer can be a device layer close to the substrate layer.

[0047] Figure 5 A method 500 for processing the back side of a lifted sample to form the back side of a plan view lamella is shown. First, the lifted sample is thinned overall by milling and / or etching. When the exposed sample back side surface is close to the final device layer, a high current electron beam is used to alternately scan the sample back side within the ROI and spontaneously etch until most of the final device layer within the ROI is exposed. Then, one or more device layers may be optionally removed from the sample back side.

[0048] At 501, the back side of the sample is thinned overall to obtain the back side surface of the sample. The sample can be thinned overall using FIB milling and / or spontaneous etching. The overall thinning process can remove most of the wedge-shaped substrate layer (e.g., Figure 3C Steps 502-508 illustrate an example workflow for overall thinning of the back side of a sample.

[0049] At 502, the back side of the extracted sample is milled using FIB. Milling can be performed using an edge-on FIB, where the angle of the incident ion beam to the sample surface is less than 45 degrees. The back side of the sample can be milled using FIB with or without an etch assist gas. The milling process can be monitored based on an image of the back side of the sample. In one example, the image is formed by secondary electrons collected during milling. In another example, the image can be taken after the ion beam scans across the back surface of the sample. FIB milling can be terminated when the milling surface approaches the final device layer. For example, FIB milling is terminated when the milling surface is within a first threshold distance from the final device layer. In another example, FIB milling is terminated after removing a first thickness of substrate. The thickness of the first substrate can be determined based on the total thickness of the sample and the thickness of the device layer.

[0050] In 504, XeF 2 The XeF flows to the back of the sample to spontaneously etch the silicon substrate without the assistance of a charged particle beam. 2 The amount and / or amount used to make XeF 2The duration of the flow may be determined based on the second thickness of the substrate to be removed. The second thickness of the substrate to be removed may be estimated based on the thickness of the removed substrate, the total sample thickness, and the thickness of the device layer.

[0051] In 506, in making XeF 2 After flowing to the sample, an SEM image of the back surface of the sample is acquired to monitor the spontaneous etching process. SEM images can be taken with beam currents below 1 nA. For example, the beam current used for SEM imaging is 100 pA. When the chamber pressure is reduced and the XeF is removed from the lower chamber, 2 When , SEM images were taken.

[0052] At 508, method 500 determines whether the exposed sample backside is within a second threshold distance from the final device layer based on the image acquired at 506. At 502, the second threshold distance is less than the first threshold distance. In some examples, the second threshold distance may be zero. That is, due to the XeF 2 The spontaneous etching is terminated if the exposed back side of the sample is within a second threshold distance from the final device layer. Otherwise, a XeF 2 To further etch the back side of the sample.

[0053] At 510, method 500 checks whether the final device layer is exposed in the ROI associated with the back surface of the sample. The area of ​​the ROI can be smaller than the area of ​​the back surface of the sample. For example, Figure 6 As shown, the area of ​​ROI 601 is smaller than the cross-sectional area of ​​the workpiece in the xy plane. Therefore, the plan view lamella has at least one thick edge for supporting the thinned subtractive region of the lamella. In one example, the ROI can be determined based on an image of the back surface of the sample. The ROI can be an area of ​​the back surface of the sample that is non-uniform or uneven.

[0054] When the device or device structure of the final device layer is not covered by the substrate layer or the spacer layer, the final device layer within the ROI is exposed. In one example, when the final device layer A exposed in the ROI 器件层 Area and ROI A ROI When the ratio of the total area of ​​the final device layer A in the ROI is greater than the threshold ratio, the final device layer is exposed. 器件层 Area and ROIA ROIWhen the ratio between the total area of ​​the ROI and the total area of ​​the ROI is not greater than a threshold ratio, the final device layer is not exposed. The area of ​​the final device layer exposed in the ROI can be estimated based on an image of the back side of the sample recently acquired. The image can be a SEM image acquired using a low beam current, such as a beam current less than 1 nA. The image can be taken in step 506 or step 516. In one example, the threshold ratio is greater than 90%. In another example, the threshold ratio is greater than 95%. If the final device layer is exposed in the ROI, method 500 moves to 518 to further reduce one or more device layers from the back side of the sample. If the final device layer is not exposed in the ROI, method 500 moves to 512. The ROI is alternately scanned with a high current electron beam and self-etched until most of the final device layer is exposed.

[0055] In another example, when the final device layer within the ROI is flat, the final device layer is exposed. Flatness can be determined based on a recently acquired SEM image. For example, any changes in surface uniformity observed in the SEM image can indicate unevenness.

[0056] At 512, a ROI associated with the back surface of the sample is scanned using a high current electron beam. By scanning the ROI using an electron beam, carbon can be deposited or doped into the scanned area. At 506 or 516, the beam current used to scan the ROI is higher than the electron beam current used for SEM imaging. For example, the beam current used for ROI scanning is greater than 1nA, while the beam current used for imaging is less than 1nA. As an example, the electron beam energy used to scan the ROI is 10kV, and the beam current is 3.2nA. ​​At 506 and 516, the beam energy used to scan the ROI is higher than the beam energy used for SEM imaging. At 506 and 516, the duration for scanning the ROI is longer than the duration for acquiring a SEM image. In one example, the ROI is scanned without detecting scattered electrons. That is, no SEM image is formed by scanning the ROI using an electron beam. In another example, a SEM image of the ROI is acquired based on scattered electrons received during the ROI scanning.

[0057] At 514, the scanned ROI is spontaneously etched. The scanned ROI is etched using XeF 2 The ROI is spontaneously etched for a predetermined duration. The duration of the etching process can be determined by the interaction cross section of the XeF2 molecules with the substrate material. In one example, the ROI is scanned for one minute using a high current electron beam. The scanned ROI is then spontaneously etched for one minute. 2 The duration of the flow can be shorter than that of the XeF in 504 2 Duration of the flow.

[0058] At 516, an image including the etched ROI is taken. The image may be a SEM image acquired with parameters similar to the SEM image acquired at 506. For example, the electron beam current and energy are lower than the beam current and energy used for the ROI scan in 512. The beam energy is reduced to reduce the electron penetration depth, thereby exciting more surface electrons to better observe the etched surface.

[0059] At 518, the device layer within the ROI is optionally reduced using a FIB to remove a predetermined number of layers. The layer reduction process can be performed by scanning the ROI using a face-to-face FIB with or without an etching assist gas. The angle between the face-to-face FIB and the exposed sample surface within the ROI is greater than 45 degrees. The etching assist gas may be methyl nitroacetate (MNA) or a MNA-like gas. In one example, the etching assist gas includes methyl nitroacetate. In other examples, the etching assist gas is a combination of one or more of methyl acetate, ethyl acetate, ethyl nitroacetate, propyl acetate, propyl nitroacetate, nitroethyl acetate, methyl oxyacetate, or methyl oxyacetyl chloride. The layer reduction process can be monitored based on secondary electrons or level currents recorded during each FIB scan of the ROI.

[0060] In this way, a large area of ​​the final device layer can be exposed. Due to the selective etching of the carbon deposited / doped surface, the exposed final device layer is flat. In addition, damage to the device layer close to the substrate layer is prevented.

[0061] Figure 6 shows that after multiple scan-etch iterations ( Figure 5 601 ). An SEM image of the back side of a sample with the final device layers of the 3D-NAND structure exposed in ROI 601 after steps 510-514 of FIG. 601 . The pattern of the final device layers can be seen with high contrast in the large area of ​​the ROI. Arrow 602 points to an area close to the edge of the ROI 601 covered by the silicon substrate. Further layer reduction can be performed within the ROI 601 to remove one or more device layers.

[0062] Figure 7 A graph based on secondary electrons and the level current collected from the back of the sample during the layer reduction process is shown. The sample includes multiple device layers of a 3D-NAND structure. The device layers are removed using the FIB starting from the last level device layer. Grayscale curve 701 is the total grayscale level of the image formed by secondary electrons. The higher the grayscale level, the smaller the number of secondary electrons collected. Level current curve 702 is the total level current sensed during each scan of the FIB above the ROI.

[0063] Both the grayscale curve 701 and the level current curve 702 decrease from the first peak at the 4 second time point and then increase again from about the 60 second time point. The first peak at about the 4 second time point corresponds to the layer reduction of the final device layer. The second peak at about the 80 second time point corresponds to the layer reduction of the next to the final device layer. As the layer reduction process proceeds in the final device layer, curves 701 and 702 decrease from 4 seconds to 23 seconds. The low grayscale and level current from 23 seconds to 65 seconds indicate that the spacer layer of the 3D-NAND structure is being removed. Both curves rise smoothly from 60 seconds to 80 seconds to the second peak, indicating that the sample surface is flat and parallel to the device layer.

[0064] The technical effect of scanning the ROI with an electron beam before spontaneously etching the sample is to reduce the etch rate of at least a portion of the device layer and prevent over-etching of the final device layer. The technical effect of scanning the ROI with a high current electron beam is to utilize the carbon deposition / doping ROI. The technical effect of imaging the sample surface with a low energy electron beam during backside processing of the thin plate is to obtain images with low penetration depth. The technical effect of repeating the scan-etch process is to selectively etch the substrate layer while retaining the device in the device layer.

[0065] In one embodiment, a method for processing a sample including at least a substrate layer and a device layer using a charged particle beam, the method comprising: removing a portion of the substrate layer to obtain a sample surface; scanning a region of interest (ROI) associated with the sample surface using an electron beam; flowing a first gas to the ROI to spontaneously etch the scanned ROI; and scanning the etched ROI using an electron beam in response to the device layer not being exposed in the ROI. In a first example of the method, the method further comprises acquiring a sample image including the etched ROI, and determining that the device layer is not exposed in the ROI based on the sample image. A second example of the method optionally includes the first example, and further comprises, wherein the sample image is a scanning electron microscope (SEM) image, and the beam current of the electron beam used to scan the ROI is higher than the beam current used to acquire the sample image. A third example of the method optionally includes one or more of the first and second examples, and further comprises, wherein the sample image is a SEM image, and the beam energy of the electron beam used to scan the ROI is higher than the beam energy used to acquire the sample image. A fourth example of the method optionally includes one or more of the first to third examples, and further comprises, wherein the duration for scanning the ROI is longer than the duration for acquiring the sample image. The fifth example of the method optionally includes one or more of the first to fourth examples, and further includes, wherein determining that the device layer is not exposed in the ROI based on the sample image includes determining that the etched ROI is not flat based on the sample image. The sixth example of the method optionally includes one or more of the first to fifth examples, and further includes, wherein determining that the device layer is not exposed in the ROI based on the sample image includes determining that the ratio between the area of ​​the device layer exposed in the ROI and the area of ​​the ROI is not greater than a threshold ratio. The seventh example of the method optionally includes one or more of the first to sixth examples, and further includes, wherein scanning the ROI with an electron beam includes scanning the ROI with an electron beam so as to deposit or dope the ROI with carbon. The eighth example of the method optionally includes one or more of the first to seventh examples, and further includes, wherein flowing a first gas to the sample surface includes flowing the first gas for a predetermined first predetermined duration. The ninth example of the method optionally includes one or more of the first to eighth examples, and further includes, wherein removing a portion of the substrate layer includes flowing a second gas to the sample surface for a second duration to etch the sample surface, the second duration being longer than the first duration. A tenth example of the method optionally includes one or more of the first to ninth examples, and further includes wherein removing a portion of the substrate layer includes milling the substrate layer using a focused ion beam.

[0066] In one embodiment, a method for preparing a plan view lamella using a charged particle beam, the method comprising extracting a sample from a workpiece using a focused ion beam, the sample comprising at least a device layer and a substrate layer; removing a portion of the substrate layer from the back of the sample to obtain a sample surface; and alternately scanning a region of interest (ROI) associated with the sample surface using an electron beam and flowing a gas to the scanned ROI until the device layer is exposed within the ROI, wherein the scanned ROI is spontaneously etched by the gas. In a first example of the method, the method further comprises determining that the device layer is exposed within the ROI when a ratio between an area of ​​the device layer exposed in the ROI and an area of ​​the ROI is greater than a threshold ratio. A second example of the method optionally includes the first example, and further comprises, wherein the sample comprises a plurality of device layers, and the exposed device layer is a final device layer among the plurality of device layers; and the method further comprises: after the final device layer is exposed within the ROI, removing at least one device layer among the plurality of device layers from the back of the sample. A third example of the method optionally includes one or more of the first and second examples, and further comprises, forming a plan view lamella based on a sample having an exposed device layer; and imaging the plan view lamella with a transmission electron microscope. A fourth example of the method optionally includes one or more of the first to third examples, and further includes removing at least one device layer of the plurality of device layers from a front side of the sample, the front side being opposite to the back side.

[0067] In one embodiment, a system for processing a sample including at least a substrate layer and a device layer, the system includes a first column for forming a focused ion beam; a second column for forming an electron beam; a lower chamber coupled to both the first column and the second column; a gas supply system coupled to the lower chamber; and a controller having instructions stored in a non-transitory memory, the controller being configured to: remove a portion of the substrate layer to obtain a sample surface; scan a region of interest (ROI) associated with the sample surface with an electron beam; flow a gas to the ROI via the gas supply system to spontaneously etch the scanned ROI; and scan the etched ROI with an electron beam in response to the device layer not being exposed in the ROI. In a first example of the system, the system also includes instructions to further cut the sample from the workpiece using the focused ion beam before removing a portion of the substrate layer to obtain the sample surface. A second example of the system optionally includes the first example, and further includes, wherein the sample includes a plurality of device layers, the device layer is a final device layer adjacent to the substrate layer, and the controller is further configured to remove one or more device layers using a focused ion beam in the presence of an etching assist gas after exposing the final device layer in the ROI. A third example of the system optionally includes one or more of the first and second examples, and further includes, wherein the controller is further configured to flow a gas to the scanned ROI after scanning the etched ROI with an electron beam.

Claims

1. A method for processing a sample comprising at least a substrate layer and a device layer using a charged particle beam, comprising: removing at least a portion of the substrate layer to obtain a sample surface; Scanning a region of interest (ROI) associated with the sample surface using an electron beam; flowing a first gas toward the ROI to spontaneously etch the scanned ROI; as well as In response to the device layer not being exposed in the ROI, the etched ROI is scanned with the electron beam. 2 . The method of claim 1 , further comprising acquiring a sample image including the etched ROI, and determining based on the sample image that the device layer is not exposed in the ROI. 3 . The method according to claim 2 , wherein the sample image is a scanning electron microscope (SEM) image, and a beam current of the electron beam used to scan the ROI is higher than a beam current used to acquire the sample image. 4 . The method according to claim 2 , wherein the sample image is a SEM image, and a beam energy of the electron beam used for scanning the ROI is higher than a beam energy used for acquiring the sample image. The method of claim 2 , wherein a duration for scanning the ROI is longer than a duration for acquiring the sample image. 6 . The method of claim 2 , wherein determining based on the sample image that the device layer is not exposed in the ROI comprises determining based on the sample image that the etched ROI is not flat.

7. The method according to any one of claims 2 to 5, wherein determining based on the sample image that the device layer is not exposed in the ROI includes determining that a ratio between an area of ​​the device layer exposed in the ROI and an area of ​​the ROI is not greater than a threshold ratio. 8 . The method of claim 1 , wherein scanning the ROI with the electron beam comprises scanning the ROI with the electron beam to deposit or dope the ROI with carbon.

9. The method of claim 1 or 8, wherein flowing the first gas toward the sample surface comprises flowing the first gas for a predetermined first duration.

10. The method of claim 9, wherein removing at least a portion of the substrate layer comprises flowing a second gas toward the sample surface for a second duration to etch the sample surface, the second duration being longer than the first duration.

11. The method of claim 1, wherein removing at least a portion of the substrate layer comprises milling the substrate layer using a focused ion beam.

12. A method for preparing a plan view sheet using a charged particle beam, comprising: extracting a sample from a workpiece using a focused ion beam, the sample comprising at least a device layer and a substrate layer; removing at least a portion of the substrate layer from a back side of the sample to obtain a sample surface; and A region of interest ROI associated with the sample surface is alternately scanned with an electron beam and a gas is flowed toward the scanned ROI until the device layer is exposed within the ROI, wherein the scanned ROI is spontaneously etched by the gas. 13 . The method of claim 12 , further comprising determining that the device layer is exposed within the ROI in response to a ratio between an area of ​​the device layer exposed in the ROI and an area of ​​the ROI being greater than a threshold ratio.

14. A method according to claim 12 or 13, wherein the sample comprises multiple device layers, and the exposed device layer is a final device layer among the multiple device layers; and the method further comprises: after the final device layer is exposed within the ROI, removing at least one device layer among the multiple device layers from the back side of the sample.

15. The method of claim 12, further comprising: forming the plan view lamellae based on the sample with the device layer exposed; and imaging the plan view lamellae using a transmission electron microscope.

16. The method of claim 14, further comprising removing at least one device layer of the plurality of device layers from a front side of the sample, the front side being opposite to the back side.

17. A system for processing a sample comprising at least a substrate layer and a device layer, comprising: a first column for forming a focused ion beam; a second column for forming an electron beam; a lower chamber coupled to both the first column and the second column; A gas supply system coupled to the lower chamber ； as well as A controller having instructions stored in a non-transitory memory, the controller being configured to: removing at least a portion of the substrate layer to obtain a sample surface; Scanning a region of interest (ROI) associated with the sample surface using the electron beam; flowing a gas to the ROI via the gas supply system to spontaneously etch the scanned ROI; and The etched ROI is scanned with the electron beam in response to the device layer not being exposed in the ROI.

18. The system of claim 17, wherein the controller includes instructions such that the sample is further milled from a workpiece using the focused ion beam prior to removing the at least a portion of the substrate layer to obtain the sample surface.

19. The system of claim 17, wherein the sample comprises a plurality of device layers, the device layers being final device layers adjacent to the substrate layer, and the controller is further configured to remove one or more device layers using the focused ion beam in the presence of an etching assist gas after the final device layer in the ROI is exposed.

20. The system of any one of claims 17 to 19, wherein the controller is further configured to flow the gas toward the scanned ROI after scanning the etched ROI with the electron beam.

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