Filling void structures using high energy SEM deposition for uniform delayering

Abstract

A method for evaluating a region of a sample that includes an array of holes separated by solid portions, the method including: placing the sample in a vacuum chamber of an evaluation tool that includes a scanning electron microscope (SEM) column and a focused ion beam (FIB), injecting a deposition gas onto the sample, scanning a portion of the sample that includes a plurality of holes in the array of holes with a first charged particle beam to locally deposit material from the deposition gas into the plurality of holes in the scanned portion, and milling the portion of the sample that includes the plurality of holes where material has been locally deposited with the FIB column.

Description

[Technical field]

[0001] This application claims priority to U.S. Patent Application No. 16 / 789,348, filed February 12, 2020, the disclosure of which is incorporated herein by reference in its entirety for all purposes. [Background technology]

[0002] In the study of electronic materials and the processes for manufacturing electronic structures from electronic materials, specimens of electronic structures can be used for microscopic examination for failure analysis and device verification. For example, specimens of electronic structures such as silicon wafers can be analyzed with a scanning electron microscope (SEM) or a transmission electron microscope (TEM) to study certain characteristic features of the wafer. Such characteristic features may include the circuits fabricated and defects formed during the manufacturing process. The electron microscope is one of the most useful instruments for analyzing the microscopic structures of semiconductor devices.

[0003] In preparing specimens of electronic structures for electron microscopy examination, various polishing and milling processes can be used to reduce the structure until specific characteristic features are exposed. As device dimensions continue to shrink to levels below 0.5 μm, techniques for preparing specimens for electron microscopy study become more important. Due to the unacceptable resolution of optical microscopes, it is not possible to study the features of modern electronic structures using traditional methods of studying structures by optical microscopy.

[0004] Although TEM techniques can provide high-resolution images and detailed depictions of a specimen's internal structure sufficient to analyze devices with features smaller than 0.5 μm, these techniques are only effective for samples that are transparent to electrons. Thus, a fundamental requirement of a TEM sample is that it must be thin enough to be penetrated by the electron beam and thin enough to prevent multiple scattering, which causes image blurring. Thin samples extracted from wafers for TEM processing techniques can be fragile and subject to breakage or shattering. For these and other reasons, TEM imaging processes are impractical for some defect investigation and analysis operations.

[0005] A dual column system that includes both a scanning electron microscope and a focused ion beam (FIB) unit can generate high-resolution SEM images of localized areas of electronic structures formed on a sample, such as a semiconductor wafer. A typical dual column system includes an SEM column, a FIB column, a support element that supports the sample, and a vacuum chamber in which the sample is placed while being milled (by the FIB column) and imaged (by the SEM column).

[0006] Removing one or more selected layers (or portions of layers) to isolate structures on a sample is known as delayering, and this removal can be performed in a dual column system such as that described above. For example, delayering can be performed by (i) locating a location of interest to be milled to remove a thickness of material from the sample (the location of interest can be located by SEM navigation, and sometimes using an optical microscope), (ii) moving the sample (e.g., by a mechanical support element) so that it is positioned under the FIB unit, and (iii) milling the sample to remove the desired amount of material at the location of interest. This delayering process can include forming a hole in the sample (usually a hole with lateral and vertical dimensions of a few microns to tens of microns) to expose the material to be sampled at the bottom of the hole. Summary of the Invention [Problem to be solved by the invention]

[0007] When attempting to mill certain structures formed on a sample, uniform delayering of the structure can be problematic due to the geometry of the structure being milled. For example, in a device that contains an array of high aspect ratio channel holes or similar structures with solid portions (e.g., slits) between the holes, the areas of the channel holes will be milled faster than areas with solid portions. This makes accurate measurements in those areas difficult or even impossible. Therefore, improved milling and delayering techniques are desirable. [Means for solving the problem]

[0008] The embodiments of the present disclosure relate to improved methods and systems for removing a selected layer or layers (or portions of layers) of a sample containing features smaller than 0.5 μm by a delayering process. Even if the portion to be delayered includes an array of high aspect ratio channel holes with solid portions formed between the holes, or similar structures, embodiments of the present disclosure can be used to uniformly delayer such a portion of the sample. Although embodiments of the present disclosure can be used to delayer structures formed on a variety of different types of samples, some embodiments are particularly useful for delayering samples that are semiconductor wafers or similar specimens.

[0009] Some embodiments relate to a method of evaluating a region of a sample that includes an array of holes separated by solid portions, the method including placing the sample in a vacuum chamber of an evaluation tool that includes a scanning electron microscope (SEM) column and a focused ion beam (FIB), injecting a deposition gas onto the sample, scanning a portion of the sample that includes holes in the array of holes with a first charged particle beam to locally deposit material from the deposition gas into the holes in the scanned portion, and milling the portion of the sample that includes the holes with the locally deposited material using the FIB column.

[0010] The milling step can include scanning the ion beam over both the material deposited in the array of holes and the solid portions separating the holes, and can repeatedly delayer both the material in the array of holes and the solid portions separating the holes, and in some embodiments, the sample can be imaged by the SEM column after each iteration of the milling process removes a layer of the sample in the milled area.

[0011] In some embodiments, the material deposited during the scanning step can be deposited into the tops of the plurality of holes, and the milling step can mill the sample to a level exposing the bottom, unfilled portions of the plurality of holes, and the method further includes, after the milling step, repeating the jetting step and scanning step to deposit additional material into the unfilled portions of the plurality of holes, and then milling the portion of the sample including the plurality of holes with the additional material locally deposited thereon using the FIB column.

[0012] Some embodiments relate to a system for evaluating a region of a sample that includes an array of holes separated by solid portions. The system can include a vacuum chamber, a sample support configured to hold the sample within the vacuum chamber during a sample evaluation process, a SEM column configured to introduce a first charged particle beam into the vacuum chamber, a FIB column configured to introduce a second charged particle beam into the vacuum chamber, a gas supply system configured to inject a deposition gas onto the sample, and a processor and a computer readable memory coupled to the processor. The memory includes computer readable instructions that, when executed by the processor, cause the system to inject the deposition gas onto the sample, scan a portion of the sample that includes the holes in the array of holes with the charged particle beam to locally deposit material from the deposition gas into the holes in the scanned portion, and mill the portion of the sample that includes the holes where the material has been locally deposited with the FIB column.

[0013] Some embodiments relate to a non-transitory computer readable memory having stored thereon instructions for evaluating a region of a sample including an array of holes separated by solid portions, wherein the evaluating is performed by placing the sample in a vacuum chamber of an evaluation tool including a SEM column and a FIB column, injecting a deposition gas onto the sample, scanning a portion of the sample including a plurality of holes in the array of holes with a first charged particle beam to locally deposit material from the deposition gas into the plurality of holes in the scanned portion, and milling the portion of the sample including the plurality of holes where material has been locally deposited with the FIB column.

[0014] Various implementations of the embodiments described herein may include one or more of the following features: The first charged particle beam may be a high energy SEM beam generated by an SEM column, or the first charged particle beam may be generated using a FIB column operated in reverse bias mode. The jetting and scanning may be performed simultaneously or sequentially, and the jetting and scanning may be repeated multiple times across the sample at different areas of the sample to be delayered. The high energy SEM beam may have a power level of at least 15 keV. The sample may be a semiconductor wafer. Each hole of the plurality of holes may have a diameter less than 100 nm and a depth greater than 3 μm. In some implementations, the plurality of holes are contact holes for memory channels in a 3D-NAND structure, and in some implementations, the plurality of holes are holes in which a capacitor of a DRAM device may be formed.

[0015] Another embodiment relates to a method of evaluating a region of a sample that includes an array of holes separated by solid portions, the method including placing the sample on a support in a vacuum chamber of an evaluation tool that includes a SEM column and a FIB column, moving the sample into the chamber to a position under the field of view of the SEM column, injecting a deposition gas onto the sample and scanning a portion of the sample that includes holes in the array of holes with a high energy SEM beam from the SEM column to locally deposit material in the holes in the scanned portion, moving the sample into the chamber to a position under the field of view of the FIB column, and milling with the FIB column the portion of the sample that includes the holes with the locally deposited material.

[0016] For a fuller understanding of the nature and advantages of the present disclosure, reference should be made to the following description and accompanying drawings. It should be understood, however, that each of the figures is presented for illustrative purposes only and is not intended to define the boundaries of the scope of the present disclosure. Moreover, generally, unless otherwise clear to the contrary in this description, when elements in different figures use the same reference numerals, those elements generally have the same function or purpose, or at least a similar function or purpose. [Brief description of the drawings]

[0017] [Figure 1A] 1 is a simplified cross-sectional view of a semiconductor wafer including an array of high aspect ratio channel holes separated by solid portions that may be subjected to a milling operation as part of a delayering process. [Figure 1B] 2 is a simplified cross-sectional view of the semiconductor wafer shown in FIG. 1 after a milling operation has been performed on the wafer in accordance with the prior art. [Diagram 2] 1 is an SEM image showing the results of a FIB milling process through an array of high aspect ratio channel holes according to the prior art. [Figure 3A] 1 is a simplified diagram of a sample evaluation system according to some embodiments of the present disclosure. [Figure 3B]1 is a simplified schematic diagram of a sample evaluation system according to an additional embodiment of the present disclosure. [Figure 4] 1 is a flow diagram illustrating steps associated with a method for de-layering a sample, according to some embodiments of the present disclosure. [Figure 5A] 5A-5C are simplified cross-sectional views of a semiconductor wafer at different stages of the de-layering process shown in FIG. 4 according to some embodiments. [Figure 5B] 5A-5C are simplified cross-sectional views of a semiconductor wafer at different stages of the de-layering process shown in FIG. 4 according to some embodiments. [Figure 5C] 5A-5C are simplified cross-sectional views of a semiconductor wafer at different stages of the de-layering process shown in FIG. 4 according to some embodiments. [Figure 5D] 1 is a simplified diagram of an area on a semiconductor wafer that may be de-layered in accordance with some embodiments. [Figure 5E] 1A-1C are simplified cross-sectional views of a semiconductor wafer as different layers are sequentially milled away from the wafer in accordance with some embodiments. [Figure 6A] 1A-1D are simplified cross-sectional views of another semiconductor wafer at different stages of a de-layering process according to some embodiments. [Figure 6B] 1A-1D are simplified cross-sectional views of another semiconductor wafer at different stages of a de-layering process according to some embodiments. [Figure 6C] 1A-1D are simplified cross-sectional views of another semiconductor wafer at different stages of a de-layering process according to some embodiments. [Figure 6D] 1A-1D are simplified cross-sectional views of another semiconductor wafer at different stages of a de-layering process according to some embodiments. [Figure 7] 11 is a flow diagram illustrating steps associated with a method for delayering a sample according to an additional embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The embodiments of the present disclosure can delayer a portion of a sample that includes an array of holes with solid portions formed between the holes. Although the embodiments of the present disclosure can be used to delayer structures formed on a variety of different types of samples, some embodiments are particularly useful for delayering samples that include small feature sizes and / or large aspect ratio holes (e.g., holes with diameters of 100 nm or less and / or aspect ratios of 30:1 or more, 40:1 or more, or 60:1 or more) formed on semiconductor wafers or similar specimens. Non-limiting examples of small feature sizes and large aspect ratio holes that can be delayered according to the embodiments of the present disclosure include contact holes for memory channels in 3D-NAND devices and holes in which capacitors of DRAM devices can be formed.

[0019] As mentioned above, when a portion of a sample containing an array of high aspect ratio holes with solid portions (e.g., slits) between the holes is delayered using standard delayering techniques, the holes typically mill faster than the slits. The inventors believe that the non-uniform milling of such samples is due to sputtering through the walls.

[0020] For illustration, refer to Figures 1A and 1B, which are simplified cross-sectional views of a semiconductor wafer 100 including an array of high aspect ratio channel holes 110 separated by solid portions 120. In Figure 1A, a milling operation that delivers an equal ion dose to all areas of the wafer (e.g., the FIB spot dwells for the same amount of time at each location on the wafer during milling) is performed in two separate areas. A first milling position (represented by beam 130) is performed on the array of channel holes, and a second milling position (represented by beam 140) is performed on an area of ​​the semiconductor wafer 100 that does not include channel holes 110. The penetration of ions 150 in each area represents a real TRIM simulation.

[0021] 1A, the material sputtered by this milling operation is indicated by the arrows pointing away from each ion entry area 150. Due to sputtering through the walls, the channel holes are milled faster than the slits. As a result, this milling process can produce a non-uniform surface with a thin layer of redeposited material 160 formed from material sputtered through the walls, as shown in FIG. 1B.

[0022] Figure 2, an SEM image of the FIB milling process through an array of channel holes, illustrates this phenomenon. Specifically, in Figure 2, the wafer 200 was imaged at a 45 degree tilt, and the image shows the milled area 210 of the array of channel holes, but an area 215 of the wafer that also contains the array of holes has not been milled. As can be seen from Figure 2, the milled area 210 exhibits the non-uniform trough-shaped cross-section shown in Figure 1B, which adversely affects metrology results.

[0023] The embodiments of the present disclosure solve this problem by filling the array of holes with a material that prevents the above phenomenon and at the same time provides contrast in SEM imaging for hole metrology.

[0024] In some embodiments, the array of holes is filled by a deposition process under a high energy SEM in a dual column defect analysis system. One example of a system suitable for filling an array of holes according to embodiments of the present disclosure is shown in Figure 3A, which illustrates a simplified sample characterization system 300 according to some embodiments of the present disclosure. The sample characterization system 300 can be used, among other things, for defect investigation and analysis of structures formed on semiconductor wafers.

[0025] The system 300 can include a vacuum chamber 310, as well as a scanning electron microscope (SEM) column 320 and a focused ion beam (FIB) column 330. A support element 350 can support a sample 355 (e.g., a semiconductor wafer) within the chamber 310 during processing operations in which the sample 355 (sometimes referred to herein as an "object" or "specimen") is subjected to a charged particle beam from one of the FIB column or the SEM column, and can move the sample between the fields of view of the two columns 320 and 330 within the vacuum chamber 310 as required for processing.

[0026] For certain operations, one or more gases can be delivered to the sample being processed by the gas supply unit 360. For simplicity of illustration, the gas supply unit 360 is shown in FIG. 3A as a nozzle, but it should be noted that the gas supply unit 360 can include a gas reservoir, a gas source, a valve, one or more inlets, and one or more outlets, etc. In some embodiments, rather than delivering gas to the entire top surface of the sample, the gas supply unit 360 can be configured to deliver gas to an area of ​​the sample that is exposed to the scanning pattern of the charged particle beam. For example, in some embodiments, the gas supply unit 360 has a nozzle diameter of several hundred microns (e.g., between 400-500 microns) configured to deliver gas directly to a relatively small portion of the sample surface around the charged particle beam scanning pattern. In various embodiments, a first gas supply unit 360 can be configured to deliver gas to a sample disposed under the SEM column 320, and a second gas supply unit 360 can be configured to deliver gas to a sample disposed under the FIB column 330.

[0027] The SEM column 320 and the FIB column 330 are connected to the vacuum chamber 310 such that a charged particle beam generated by either of the charged particle columns propagates through an evacuated environment formed within the vacuum chamber 310 before impinging on the sample 355. The SEM column 320 can generate an image of a portion of the sample 355 by irradiating the sample with a charged particle beam, detecting particles emitted by the irradiation, and generating a charged particle image based on the detected particles. The FIB column 330 can mill the sample 355 (e.g., drill holes in the sample 355) to form a cross section by irradiating the sample with one or more charged particle beams, and can smooth the cross section. The cross section can include one or more first portions of a first material and one or more second portions of a second material. The cross section can also include additional portions of other materials. Advantageously, the smoothing operation includes utilizing a smaller acceleration voltage compared to milling the sample.

[0028] Typically, these particle imaging and milling processes involve scanning a charged particle beam back and forth (e.g., in a raster scan pattern) at a constant speed over a particular area of ​​the sample being imaged or milled, respectively. As known to those skilled in the art, this scanning pattern can be implemented by one or more lenses (not shown) coupled to the charged particle column. The scanned area is typically a very small portion of the total area of ​​the sample. For example, the sample may be a semiconductor wafer with a diameter of 200 or 300 mm, while each scanned area on the wafer may be a rectangular area having a width and / or length of a few μm or tens of μm.

[0029] In some embodiments, as shown in FIG. 3B, the defect analysis system 300 can include an illumination unit 370 and / or a gas blowing unit 380. When the system 300 includes the illumination unit 370, the system can perform gas-assisted etching (discussed below) by exposing a light-activated etchant gas to light generated by the illumination unit. To this end, the illumination unit 370 can include a light source 372 and a focusing optic 374. In some embodiments, the light source 372 and the focusing optic 374 can each be located within the vacuum chamber 310, and in other embodiments, the light source 372 and the focusing optic 374 can each be located outside the vacuum chamber. The light source 372 can be a monochromatic light source, a broadband light source, a pulsed light source, a continuous light source, a laser, a lamp (such as, but not limited to, a mercury lamp), and in some embodiments, the light source 372 can generate light at a wavelength not exceeding 200 nanometers. The focusing optics 374 can focus the light from the light source 372 onto an area of ​​the sample 355 being processed, which can include the cross-section, can include only a portion of the cross-section, or can be located near the cross-section. For example, the area can be located a few nanometers or a few μm from the cross-section. It should be noted that even when the light beam is focused on the cross-section, the light beam can pass through light-activated etchant gas that is not located near the cross-section. By focusing the light beam on the above-mentioned area, selective etching can be performed (mainly or exclusively) near the cross-section, while other portions of the wafer are not substantially (or even substantially) etched.

[0030] In addition to or instead of the gas supply unit 360, a gas spray unit 380 can be included, which can include the gas sources, reservoirs, valves, etc. discussed above with respect to the gas supply unit 360, and can further include a nozzle for spraying a gas (e.g., a deposition gas or an etchant gas) onto the sample to deposit material on the sample or etch a cross section to provide a fine topography as described below.

[0031] 3A and 3B, system 300 may include one or more controllers, processors, or other hardware units that control operation of system 300 by executing computer instructions stored in one or more computer-readable memories, as known to those skilled in the art. By way of example, this computer-readable memory may include solid-state memory (e.g., random access memory (RAM) and / or read-only memory (ROM), which may be programmable, flash-updateable, and / or the like), disk drives, optical storage devices, or the like non-transitory computer-readable storage media.

[0032] In addition to generating a charged particle image using the SEM column 320, the system 300 can deposit material on the sample and / or perform gas-assisted etching of the sample. The system 300 can perform gas-assisted etching of the cross-section, for example, to generate a topographical difference between at least one first portion and at least one second portion of the cross-section. To perform gas-assisted etching, the gas supply unit 360 (or the gas blowing unit 380) can supply a suitable etchant source gas to an area that may include the cross-section or an area that may be near the cross-section. In some examples, the etchant source gas can be activated by secondary electrons emerging from the surface of the sample during exposure of the sample to the charged particle beam from the SEM column, wherever a cascade of impinging electrons reaches. In some examples, the etchant source gas can be activated by secondary electrons emerging from the surface of the sample during exposure of the sample to the charged particle beam from the FIB column, wherever a cascade of impinging ions reaches. The gas supplied during the gas-assisted etching step can be a gas that is non-reactive or slightly reactive in the absence of the charged particle beam. Once activated, the etchant source gas becomes reactive and can etch different materials at different rates to create fine topography.

[0033] In some embodiments of the gas-assisted etching process according to the present disclosure, an electron beam having an energy level of several thousand electron volts (several keV) is used to generate secondary electrons that activate the etchant source gas. In other embodiments, it is advantageous to use a lower energy ion beam, for example an ion beam of about several hundred electron volts, to initiate the above-mentioned events that activate the etchant source gas.

[0034] Some embodiments of the present disclosure may use a dual column defect analysis system, such as system 300 discussed above, to fill an array of high aspect ratio holes (or similar structures) by initiating a deposition process under a high energy SEM in the system. To this end, a deposition gas may be supplied to the sample 355 by a gas supply unit 360 (or gas blowing unit 380), and energy from the SEM column 320 may generate secondary electrons. A cascade of impinging secondary electrons may then activate the deposition gas, resulting in material being deposited on the sample and in the array of holes localized to the area of ​​the sample where the SEM particle beam struck. Thus, deposition occurring according to such embodiments of the present disclosure does not occur simultaneously across the entire surface of the sample or wafer being processed. Instead, deposition occurs only in the general areas where the SEM particle beam strikes the wafer (which may have a diameter in the range of 0.5-10 nm) and as those areas of the wafer are scanned with the SEM particle beam. Thus, deposition according to some embodiments may be performed with a resolution on the order of nanometers.

[0035] This confined deposition process prevents the non-uniform milling described with respect to Figures 1A, 1B and 2, while filling the holes with a material that provides contrast in SEM imaging for hole metrology. For example, high energy SEM processes (tens of kV) can have an electron penetration depth of more than 1 μm. Thus, when scanning the above geometries, the secondary electron yield through the hole walls near the bottom is higher than that at the surface. As a result, deposition according to some embodiments of the present disclosure occurs faster in the holes when deposition gas molecules are present in the holes. The deposition material can then be filled into the holes, which allows a subsequent milling operation to uniformly mill the filled structure.

[0036] For illustration, reference is made to Figure 4, which is a flow diagram showing steps associated with method 400 according to some embodiments of the present disclosure, and Figures 5A-5C, which are simplified cross-sectional views of a semiconductor wafer 500 upon which steps of method 400 have been performed. Semiconductor wafer 500 may include an array of small feature size, large aspect ratio holes 510 formed within semiconductor wafer 500 and separated by solid portions or slits 520. Holes 510 and solid portions 520 may be similar or identical to holes 110 and slits 120 discussed above with respect to Figures 1A and 1B.

[0037] The first step of the method 400 may include moving the wafer 500 under the field of view of the SEM column (block 410). After the wafer is properly positioned, a deposition gas may be sprayed onto the wafer (block 420). As shown in FIG. 5A, the deposition gas may adhere to the surface of the wafer 500, including both the top surface 505 and the surface within the hole 515, as indicated by the gas layer 530. The deposition gas may be selected based on the material from which the hole 510 is formed. For example, the deposition gas may be selected to deposit (during block 430, discussed below) a material that has a similar milling rate as the material from which the hole 510 is formed (i.e., the material that constitutes the solid portion 520), but has a different contrast for imaging than the material of the portion 520. As various examples, the solid portion 520 may include carbon, silicon oxide, or other suitable material, and the deposition gas may be selected to deposit carbon, platinum, tungsten, cobalt, palladium, or a suitable material depending on the material of the portion 520. In certain instances where the material to be deposited is a metal, the deposition gas may include a large molecule containing a single atom of the metal to be deposited, for example, tungsten hexafluoride (WF6) or tungsten hexacarbonyl (W(CO)6) may be the deposition gas for tungsten, and trimethyl(methylcyclopentadienyl)platinum ((C5H4CH3)(CH3)3Pt) may be the deposition gas for platinum.

[0038] Next, while still injecting gas onto the wafer 500, the method 400 may include scanning (block 430) a portion of the wafer 500 that will form the hole 510 that will subsequently be milled in block 450 with an SEM charged particle beam 540. To ensure a high degree of lateral precision, the charged particle beam may be focused on the surface 505 of the wafer 500, and the scanning speed (i.e., beam speed is a combination of parameters including pixel size, dwell time and overlap, as will be understood by those skilled in the art) and the i-probe (current) of the particle beam control the deposition rate, which may be optimized for best results in terms of deposition quality in the hole. The energy level of the SEM charged particle beam 540 directed towards the wafer in block 430 may be selected based on the charged particle type (e.g., electrons from the SEM column) and the material to be penetrated, such that the beam penetrates a few μm below the surface 505 of the wafer, as shown in FIG. 5B, due to the penetration of the electrons 545. The entry of electrons 545 initiates reactions of reactive gas molecules that deposit solid material 550 in an array of holes localized to the area where the SEM particle beam strikes the wafer. This deposition occurs only where the SEM beam strikes the wafer. The amount of deposition is controlled by the time it takes to scan the injected gas with the SEM charged particle beam. Note that FIG. 5B shows wafer 500 at a point where SEM beam 540 is about to begin scanning area 554, and thus depositing material in the holes in area 554, after scanning area 552 with SEM beam 540 and depositing material in the holes in area 552.

[0039] After the SEM beam 540 has been completely scanned over the portion of the wafer where deposition is desired (e.g., all of the portions of the holes to be milled), the holes in those areas will be filled with material 550. The wafer can then be moved into the field of view of the FIB column (block 440), and the filled areas can be uniformly milled by the FIB column (block 450), as shown in Figure 5C. After this milling process is complete, the surface 505 has been milled down to a lower, relatively flat surface 505b.

[0040] In some embodiments, the milling process of block 450 can include multiple sub-steps. For example, in a first sub-step, the top layer of the delayering portion can be milled and removed. The sample can then be returned to the field of view of the SEM column and the milled area can be imaged. The sample can then be returned to the FIB column and subsequent layers of the same portion of the wafer can be milled. This process of removing a layer of the wafer in a particular area and imaging that area can be repeated multiple times, effectively carving a hole in the sample that gets deeper with each iteration. Data captured during the imaging portion of this process can be used to evaluate the milled and imaged portion, including, for example, creating a three-dimensional model of the milled area of ​​the sample.

[0041] For further illustration, reference is made to Figures 5D and 5E. Figure 5D is a simplified diagram of the semiconductor wafer 500 shown in Figures 5A-5C, and Figure 5E is a simplified cross-sectional diagram of the wafer 500 as multiple layers within a region 570 of the wafer 500 are delayered and analyzed. Figure 5D includes a top view of the wafer 500 and two close-up views of specific portions of the wafer 500. The wafer 500 may be, for example, a 200 mm or 300 mm semiconductor wafer and may include multiple (52 in the illustrated example) integrated circuits 560 formed on the wafer 500. The integrated circuits 560 may be at an intermediate stage of fabrication, and the delayering techniques described herein may be used to evaluate and analyze one or more regions 570 of the integrated circuits that include an array of high aspect ratio holes separated by solid portions. For example, close-up A of Figure 5D shows multiple regions 570 of one integrated circuit 560 that may be evaluated and analyzed according to the techniques described herein. Enlarged view B shows one of those regions 570, which includes the array of holes 510 and solid portion 520 discussed above with respect to Figures 5A-5C.

[0042] Some embodiments may analyze and evaluate region 570 by sequentially removing the top layers of the region by milling and imaging the milled region during the milling step (block 450). The milling process may mill region 570 by scanning the FIB beam back and forth over the region according to a raster pattern, such as scan pattern 580 shown in simplified form in close-up B of FIG. 5D, to remove the top portion of region 570. The removed portion may have a particular depth in the Z direction, and the portion may be removed in its entirety from region 570 in both the X and Y directions. For example, if region 570 is a square having a length and width of X μm, separate slices of X×X μm each having a depth of Z μm may be sequentially removed from region 570 during the milling process, with each layer having a removed square including material deposited in the holes according to method 400 and solid portions between the holes. Thus, as shown in FIG. 5E, a first milling sub-step can remove layer 590(1), which is effectively an X×X μm square, from region 570, and an image of the region from which layer 570(1) has been removed can be produced by the SEM column. A second milling sub-step can then remove layer 590(2), which is also effectively an X×X μm square, from region 570, and an image of the region from which layers 590(1) and 590(2) have been removed can be produced by the SEM column. Although FIG. 5E shows four iterations of the delayering process, embodiments can repeat the delayering process any number of times appropriate or necessary for a particular analysis and evaluation situation. Furthermore, instead of milling the same sized area (i.e., an X×X μm square in this example) with each iteration of the delayering process, in some embodiments, one or more subsequent iterations can progressively remove portions of the region that are smaller than the immediately preceding iteration.

[0043] Returning to Figure 4, embodiments of the present disclosure can be used to fill holes with very small feature sizes yet very large depths, and therefore very large aspect ratios. As non-limiting examples, various embodiments can fill holes at blocks 420, 430 with diameters smaller than 100 nm and depths greater than 3 μm, holes with diameters of 80 nm and depths between 3-5 μm, and holes with diameters between 70-80 nm and depths between 4-5 μm.

[0044] The energy level of the SEM charged particle beam should generally be high enough to achieve a high secondary electron yield far enough below the top surface of the wafer that the deposition material reaches the bottom of the holes being filled. In some embodiments, the SEM has an energy level of tens of keV (e.g., an energy level of 15 keV or more, an energy level of 40 keV or more, or an energy level of 30 keV or more). At such energy levels, the particle beam can penetrate more than 1 μm or 2-3 μm below the top surface of the wafer.

[0045] Some currently known dual column SEM / FIB systems have an upper limit on the energy level of the SEM beam, such as 30 keV. Because the energy level of the SEM beam determines how deep the beam will penetrate in a given sample, some embodiments may use a multi-step approach when filling holes that are too deep to be filled by the tool in one go. For example, if a particular tool can produce a maximum 30 keV SEM beam that penetrates to a depth of 3 μm into a particular sample, some embodiments of the present disclosure may use a multi-step deposition-milling repeat process to uniformly mill holes greater than 3 μm deep with small feature sizes and large aspect ratios. As an example, see FIGS. 6A-6D. FIGS. 6A-6D are simplified cross-sectional views of a wafer 600 having an array of approximately 6 μm deep holes 610 separated by solid portions 620.

[0046] To mill the hole 610, an embodiment of the present disclosure may deposit a first layer of material 650a into the hole using the techniques described above with respect to Figure 4 to fill the top 3 μm of the hole. After this initial deposition step, the deposited material 650a may extend from the top surface 605a to a depth of about 3 μm into the wafer 600. As shown in Figure 6A, the depth of the hole 610 is about 6 μm, so a portion 610b (e.g., about 3 μm) of the hole is not filled.

[0047] The wafer 600 can be milled to remove portions of the wafer containing material 650a, leaving holes 610 with a depth of about 3 μm from the milled top surface 605b, as shown in FIG. 6B. The steps of FIGS. 6A and 6B can then be repeated according to the method of FIG. 4 to fill the holes 610 and further mill the holes 610 down to the surface 610c, as shown in FIGS. 6C and 6D, respectively. As another example, holes as deep as 9 μm can be filled by repeating this deposition-milling step three times instead of just two. Thus, embodiments of the present disclosure can uniformly mill holes that are too deep to otherwise deposit deposition material all the way to the bottom of the holes.

[0048] In some embodiments, a technique similar to that discussed with respect to FIG. 4 can be used, except that the gas flow can be stopped before applying the charged SEM beam to the wafer. This allows the gas molecules at the top surface to detach while the molecules that migrated in the holes still remain, as the deep geometry in the holes results in a longer characteristic time for detachment. The energy level of the SEM beam can be selected so that the electrons penetrate deep into the bulk material between the holes (i.e., between the hole walls) to the desired depth of deposition. A sufficiently high energy level of the SEM beam can provide a higher secondary electron yield at the bottom of the holes rather than at the top. Additionally, the absence of deposition gas molecules at the top surface can help prevent fast deposition at the top of the holes, which could otherwise close off before filling the bottom of the holes. Depending on how long the gas remains in the hole after the gas flow is stopped, some embodiments using this technique can deposit material in holes in different locations on the wafer by repeating the sequence of introducing gas into the chamber, stopping the gas flow, and then exposing the wafer to the SEM beam to initiate deposition deep within the hole multiple times.

[0049] In additional embodiments where the ion beam source of the FIB column is a plasma source, the dual mode FIB can be used to both deposit material on a desired area of ​​the sample, including the hole, and subsequently mill the desired area as part of a delayering process. Refer to FIG. 7 for illustration. FIG. 7 is a flow diagram showing steps associated with a method 700 according to some embodiments of the present disclosure. As shown in FIG. 7, the method 700 begins with reversing the energy and extraction voltage of the plasma source of the FIB column (as well as other voltages that will be understood by those skilled in the art to be required to operate the FIB column in reverse bias mode) and moving the sample under the field of view of the FIB column (block 710). Operating the FIB column in reverse bias mode allows the FIB column to operate as a SEM column. A deposition gas can then be sprayed onto the sample (block 720), and the reverse biased FIB charged particle beam can be focused on the surface of the sample and scanned over a portion of the sample within the portion of the wafer where the hole is to be subsequently milled (block 730). The energy level of the reverse biased FIB charged particle beam directed towards the wafer in block 430 can be selected based on the charged particle type and the material being penetrated such that the beam penetrates several microns below the surface of the wafer. As an example, in some embodiments, the energy level used to generate the reverse biased FIB beam is greater than 15 keV. The deposition process performed in blocks 720 and 730 can be a process similar to the process described above with respect to blocks 420 and 430 of FIG. 4 and shown with respect to FIG. 5B.

[0050] After the reverse biased FIB charged particle beam has been completely scanned over the portion of the wafer where deposition is desired (e.g., all of the holes to be milled), the holes in those areas will be filled with deposition material. The FIB column can then be returned to normal mode (block 740) and the filled areas can be uniformly milled by the FIB column (block 750).

[0051] In other embodiments, a dual mode FIB can be used to both deposit material into desired areas of a sample, including holes, and subsequently mill the desired areas as part of a delayering process using the deposition-milling iteration process described with respect to FIG. 6. That is, in some embodiments, a dual mode FIB can be used to delayer structures that are too deep to be filled in a single deposition step. For example, a first layer of material can be deposited into holes to fill the top of the holes using the techniques described above with respect to blocks 720, 730 of FIG. 7. After this initial deposition step, the deposited material can extend from the top surface of the sample to an intermediate depth in the very deep holes, but not to the bottom of the holes. The sample can then be milled to remove portions of the sample that include the deposited material, filled in the remaining portions of the holes in the milled areas that were not filled with the deposited material during the first deposition step, and further milled, by repeating the deposition and milling one or more times according to the method of FIG. 7. This repeated deposition-milling process can provide an alternative method for uniformly de-layering holes that are too deep to otherwise allow deposition material to be deposited at the bottom of the hole in a single deposition step.

[0052] Any references herein above to a method should, mutatis mutandis, apply to a system capable of carrying out that method, and should, mutatis mutandis, apply to a computer program product having stored therein instructions which, when executed, perform the method. Similarly, any references herein above to a system should, mutatis mutandis, apply to a method which may be carried out by that system, and should, mutatis mutandis, apply to a computer program product having stored therein instructions which, when executed, perform the method. Any references herein above to a computer program product should, mutatis mutandis, apply to a method which may be carried out by that system, and should, mutatis mutandis, apply to a system configured to carry out the instructions stored in that computer program product.

[0053] In the above description, for purposes of explanation, certain terms have been used to provide a thorough understanding of the described embodiments. However, it will be apparent to one of ordinary skill in the art that those specific details are not required to practice the described embodiments. For example, while some specific embodiments of the present disclosure described above use an exemplary sample that includes an array of channel holes with small feature sizes and large aspect ratios separated by solid slits, the present disclosure is not limited to samples having only such geometry. The embodiments of the present disclosure can be equally beneficially applied to delayering samples having filled hole arrays with etched portions or slits between the filled holes. The embodiments can further be beneficially used on samples having very small feature sizes etched (e.g., trenches) or otherwise formed with high aspect ratios between solid portions of the sample. Furthermore, the embodiments of the present disclosure are not limited to delayering samples having holes (or other features) of a particular size or aspect ratio, and the embodiments of the present disclosure can be beneficially applied to delayering samples having holes or other features that are larger and / or shallower than the holes or other features discussed in detail herein.

[0054] Thus, the above descriptions of the specific embodiments described herein have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. Moreover, while different embodiments of the present disclosure have been disclosed above, the specific details of the specific embodiments can be combined as appropriate without departing from the spirit and scope of the embodiments of the present disclosure. Moreover, it will be apparent to one skilled in the art that many modifications and variations are possible in light of the above teachings.

[0055] Because most of the illustrated embodiments of the present disclosure can be implemented using electronic components and circuitry known to those skilled in the art, details of such electronic components and circuitry will not be described beyond the extent deemed necessary for an understanding and appreciation of the ideas underlying the present disclosure, as explained above, so as not to obscure or detract from the teachings of the present disclosure.

Claims

1. 1. A method for evaluating an area of ​​a sample containing a plurality of pores separated by solid portions, the method comprising: placing the sample in a vacuum chamber of an evaluation tool including a scanning electron microscope (SEM) column and a focused ion beam (FIB) column; locally depositing material in the plurality of pores by injecting a deposition gas onto the sample, and scanning a first charged particle beam over an area of ​​the sample including the plurality of pores separated by the solid portions to deposit material in the plurality of pores and fill the plurality of pores; and Then, milling the area of ​​the sample including the plurality of pores separated by the solid portions with the FIB column. Including, the solid portion is scanned by the first charged particle beam and electrons enter each of the plurality of holes, thereby reacting reactive gas molecules and depositing the material in the plurality of holes.

2. The method of claim 1 , wherein the first charged particle beam is a high energy SEM beam produced by the SEM column.

3. The method of claim 2 , wherein the ejecting and the scanning are performed simultaneously.

4. The method of claim 2 , wherein the high energy SEM beam has a power level of at least 15 keV.

5. 5. The method of claim 4, wherein the penetration depth of electrons produces a high secondary electron yield at a location far enough below a top surface of the sample that deposition material reaches the bottom of a number of the plurality of holes.

6. 3. The method of claim 2, wherein the jetting and scanning are performed sequentially, and the jetting and scanning are repeated multiple times across the sample with different areas of the sample being delayered.

7. 2. The method of claim 1, wherein the ion beam source of the FIB column is a plasma source, and the first charged particle beam is generated with the FIB column operated in a reverse bias mode.

8. the material deposited during the scanning step deposits material on top of the plurality of holes, and the milling step mills the sample to a level exposing lower, unfilled portions of the plurality of holes, the method further comprising: repeating the jetting and scanning steps after the milling step to deposit additional material in the unfilled portions of the plurality of holes; and thereafter, milling the portion of the sample including the plurality of holes having the additional material locally deposited thereon with the FIB column. The method of claim 1 , comprising:

9. The method of claim 1 , wherein the sample is a semiconductor wafer.

10. 10. The method of claim 9, wherein each pore of the plurality of pores has a diameter less than 100 nm and a depth greater than 3 μm.

11. 10. The method of claim 9, wherein the plurality of holes are contact holes for memory channels in a 3D-NAND structure.

12. 10. The method of claim 9, wherein the plurality of holes are holes in which a capacitor of a DRAM device can be formed.

13. 13. The method of any of claims 1 to 12, wherein milling the portion of the sample comprises scanning an ion beam over both the material deposited in the plurality of holes and the solid portions separating the holes to repeatedly delayer both the material in the plurality of holes and the solid portions separating the holes.

14. The method of claim 13 , wherein the sample is imaged by the SEM column after each layer is removed by the milling process.

15. 1. A system for evaluating a region of a sample including a plurality of pores separated by solid portions, the system comprising: A vacuum chamber; a sample support configured to hold a sample within said vacuum chamber during a sample evaluation process; a scanning electron microscope (SEM) column configured to introduce a first charged particle beam into the vacuum chamber; a focused ion beam (FIB) column configured to introduce a second beam of charged particles into the vacuum chamber; a gas supply system configured to inject a deposition gas onto the sample; a processor and a memory coupled to the processor; the memory includes a plurality of computer readable instructions that, when executed by the processor, locally depositing material within the plurality of holes by injecting a deposition gas onto the sample; scanning a charged particle beam over an area of ​​the sample including the plurality of pores separated by the solid portions to deposit material into the plurality of pores to fill the plurality of pores; and Then, milling the area of ​​the sample including the plurality of pores separated by the solid portions with the FIB column. executing the above on the system; the solid portion is scanned by the first charged particle beam and electrons enter each of the plurality of holes, thereby reacting reactive gas molecules and depositing the material in the plurality of holes.

16. The system of claim 15 , wherein the first charged particle beam is a high energy SEM beam produced by the SEM column.

17. 17. The system of claim 15 or 16, wherein the ion beam source of the FIB column is a plasma source, and the second charged particle beam is generated by the FIB column operated in a reverse bias mode.

18. 11. A non-transitory computer readable memory having stored thereon instructions for evaluating a region of a sample that includes a plurality of pores separated by solid portions, the evaluating comprising: placing the sample in a vacuum chamber of an evaluation tool including a scanning electron microscope (SEM) column and a focused ion beam (FIB) column; locally depositing material in the plurality of pores by injecting a deposition gas onto the sample; scanning a first charged particle beam over an area of ​​the sample including the plurality of pores separated by the solid portions to deposit material in the plurality of pores; and filling the plurality of pores; Then, milling the area of ​​the sample with the FIB column, the area including the pores separated by the solid portions. This is carried out by the solid portion is scanned by the first charged particle beam and electrons enter each of the plurality of holes, thereby reacting reactive gas molecules and depositing the material in the plurality of holes.

19. 20. The non-transitory computer readable memory of claim 18, wherein the first charged particle beam is a high energy SEM beam produced by the SEM column.

20. 20. The non-transitory computer readable memory of claim 18 or 19, wherein an ion beam source of the FIB column is a plasma source, and the first charged particle beam is generated by the FIB column operated in a reverse bias mode.

Images