Improved accuracy in stereoscopic measurements using pre-deposited layers

Abstract

A method of determining a depth of a hole milled into a first region of a sample, the method comprising: positioning the sample in a processing chamber having a charged particle beam column; depositing a material directly over a top surface of the sample in a second region of the sample adjacent the first region; milling an aperture in the first region of the sample using a charged particle beam generated by the charged particle beam column, wherein the aperture abuts the material deposited over the top surface and includes a sidewall extending from a bottom surface of the aperture to an interface between the deposited material and the top surface of the sample; and calculating the depth of the hole based on a distance measurement between a first point along an interface between the material and the top surface and a second point along the bottom surface of the hole using a stereo measurement technique.

Description

[0001] Cross - reference to Related Applications

[0002] This application claims the benefit and priority of U.S. Patent Application No. 17 / 983,225, filed on November 8, 2022, entitled "PRECISION IN STEREOSCOPIC MEASUREMENTS USING A PRE - DEPOSITION LAYER", the entire content of which is incorporated herein by reference. Background Art

[0003] In the study of electronic materials and the processes for fabricating such materials into electronic structures, specimens of the electronic structures can be used for microscopy for purposes of failure analysis and device verification. For example, a specimen, such as a silicon wafer, including one or more electronic structures formed thereon, can be milled and analyzed using a focused ion beam (FIB) to study specific characteristics of the structures formed on the wafer.

[0004] (It is necessary to determine the precise depth of the milled hole in the sample. Scanning electron microscopy (SEM) techniques can be beneficially used to determine the depth of the milled hole. However, in some cases, during the milling process, the top surface of the sample may be partially milled away in the area around the milled hole. For example, in some FIB instruments, such as plasma - source FIB systems, the focused ion beam may have a relatively large "tail" around the spot, which can cause damage to the sample surface outside the direct area of beam focus. The damaged top surface may adversely affect the accuracy of thickness determination.

[0005] Accordingly, there is a need for improved methods for accurately determining the depth of milled holes in a sample. Summary of the Invention

[0006] The embodiments described herein provide improved systems and methods for measuring the depth of milled holes in a sample. In some embodiments, as part of a sample evaluation process, a focused ion beam (FIB) is used to mill a hole (e.g., a box, trench, or other structure milled into the sample) in the sample. As described above, in some cases, the top surface of the sample may be damaged by the milling process in the area around the hole. To ensure highly precise measurement of the hole depth, it is important that the measured distance is between the top surface that existed as a surface before the hole was milled and the bottom surface of the hole. Any damage to the top surface near the hole makes it difficult to accurately identify the precise position of the top surface.

[0007] Some embodiments disclosed herein can highly precisely measure the depth of a milled hole even if the surface of the sample is damaged during the milling process. To this end, in some embodiments, a small amount of material is deposited on the sample surface near the area to be milled before the milling process. This small deposition can be partially milled during the milling process, forming a boundary with high contrast between the deposited material and the original surface. Images of the hole along the sidewalls can be obtained from two different perspectives, and the high-contrast boundary can be used as the precise height of the original surface in the image. Using the known precise height of the original surface, the distance between the top surface of the sample and the bottom surface of the milled hole along the sidewall of the hole can be obtained, and the depth of the hole can be determined as described herein.

[0008] Although the disclosed embodiments can be used to measure holes milled into various different types of samples, some embodiments are particularly useful when measuring samples such as semiconductor wafers or similar specimens.

[0009] In some embodiments, a method for determining the depth of a hole milled into a first region of a sample is provided. The method can include: positioning the sample in a processing chamber having a charged particle beam column; depositing material directly above the top surface of the sample in a second region of the sample adjacent to the first region; milling a hole in the first region of the sample using a charged particle beam generated by the charged particle beam column, wherein the hole abuts the material deposited above the top surface and includes sidewalls extending from the bottom surface of the hole to the interface between the deposited material and the top surface of the sample; and calculating the depth of the hole using stereometric techniques based on a distance measurement between a first point along the interface between the material and the top surface and a second point along the bottom surface of the hole.

[0010] In some embodiments, a system for determining the depth of a hole milled into a first region of a sample is provided. The system can include: a vacuum chamber; a sample support configured to hold the sample in the vacuum chamber during the milling process; a charged particle beam column configured to direct a charged particle beam into the vacuum chamber; and a processor and a memory coupled to the processor. The memory can include a plurality of computer-readable instructions that, when executed by the processor, cause the system to: position the sample in a processing chamber having a charged particle beam column; deposit material directly above the top surface of the sample in a second region of the sample adjacent to the first region; mill a hole in the first region of the sample using a charged particle beam generated by the charged particle beam column; and calculate the drilling depth using stereometric techniques. The hole can abut the material deposited above the top surface and includes sidewalls extending from the bottom surface of the hole to the interface between the deposited material and the top surface of the sample. The depth of the hole can be calculated based on a distance measurement between a first point along the interface between the material and the top surface and a second point along the bottom surface of the hole.

[0011] In some embodiments, a non-transitory computer-readable memory storing instructions for determining the depth of a hole milled into a first region of a sample is provided. The instructions, when executed by a processor, may cause the milling to be performed by: positioning the sample in a processing chamber having a charged particle beam column; depositing a material directly above the top surface of the sample in a second region adjacent to the first region of the sample; milling a hole in the first region of the sample using a charged particle beam generated by the charged particle beam column, wherein the hole abuts the material deposited above the top surface and includes sidewalls extending from the bottom surface of the hole to an interface between the deposited material and the top surface of the sample; and calculating the depth of the hole using a stereometric technique based on a distance measurement between a first point along the interface between the material and the top surface and a second point along the bottom surface of the hole.

[0012] In some embodiments, the stereometric technique for calculating the depth of the hole may include: (i) obtaining a first image and a second image of the sidewalls from different perspectives; (ii) for each of the first image and the second image, measuring the distance between a first point along the interface between the material and the top surface and a second point along the bottom surface of the hole; and (iii) calculating the depth of the hole based on the first image, the second image, and the measured distances.

[0013] In some embodiments, the stereometric technique for calculating the depth of the hole may include: (i) obtaining a first image of a speckled sidewall, the first image being obtained from a first perspective associated with a first angle relative to the sample; (ii) using the first image to measure a first distance between a first point at the interface between the deposited material and the top surface of the sample on the sidewall and a second point corresponding to the bottom surface of the hole on the sidewall; (iii) obtaining a second image of the speckled sidewall, the second image being obtained from a second perspective associated with a second angle relative to the sample, wherein the first angle and the first perspective are different from the second angle and the second perspective; (iv) using the second image to measure a second distance between a first point corresponding to the interface between the deposited material and the top surface of the sample on the sidewall and a second point corresponding to the bottom surface of the hole on the sidewall, wherein the first point and the second point generally lie on a line extending vertically through the milled hole; and (v) using the first distance, the first angle associated with the first perspective, the second distance, and the second angle associated with the second perspective to determine the depth of the hole.

[0014] In various embodiments, the examples may include one or more of the following features. The charged particle beam column may be a focused ion beam (FIB) column, and the charged particle beam may be a focused ion beam. The deposited material may include: (i) injecting a deposition precursor gas into a second region of the sample; (ii) generating a focused ion beam using the focused ion beam column and focusing the ion beam within the second region of the sample; and (iii) scanning the focused particle beam across the second region of the sample to activate the molecules of the deposition gas that have attached to the surface of the sample in the deposition region and deposit material on the sample within the second region. The processing chamber may be a vacuum chamber including both a focused ion beam (FIB) column and a scanning electron microscope (SEM) column. The sample may be a semiconductor wafer. Each of the first image and the second image may be obtained using scanning electron microscope (SEM) techniques.

[0015] To better understand the nature and advantages of the present disclosure, reference should be made to the following description and the drawings. However, it should be understood that each drawing is provided for illustrative purposes only and is not intended to limit the scope of the present disclosure. Additionally, as a general rule, and unless it is apparent to the contrary from the specification, elements having the same reference numerals in different drawings are generally the same or at least similar in function or purpose. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1A is a simplified illustration of a sample evaluation system in accordance with some embodiments of the present disclosure;

[0017] Figure 1B is of an SEM column tilted in accordance with some embodiments Figure 1A simplified illustration of the sample evaluation system shown;

[0018] Figure 2 is a simplified cross-sectional view of a hole milled in a sample using a charged particle beam of an FIB system;

[0019] Figure 3 is a simplified illustration of a hole milled in a sample using a charged particle beam system that creates unwanted damage in the region adjacent to the hole;

[0020] Figures 4A to 7 is a simplified diagram depicting various views and aspects of a hole that can be measured using the stereometric techniques described herein;

[0021] Figure 8 is a simplified diagram illustrating how the vertical thickness of a buried layer can be calculated using stereometric techniques;

[0022] Figure 9is a flowchart depicting steps associated with a method for determining the depth of a milled hole in a sample according to some embodiments;

[0023] Figures 10A to 10D The figure is a simplified cross-sectional view of a sample according to some embodiments at different stages of being processed according to the method as Figure 9 shown;

[0024] Figures 11A to 11C is a simplified top view of a sample according to some embodiments having material deposited at a location adjacent to where a hole is to be milled; and

[0025] Figure 12 is a simplified illustration of a sample according to embodiments disclosed herein that may have structures or buried layers formed on and measured thereon. DETAILED DESCRIPTION

[0026] The embodiments described herein provide improved systems and methods for measuring the depth of a milled hole in a sample. In some embodiments, as part of a sample evaluation process, a focused ion beam (FIB) is used to mill a hole in the sample. In a typical milling process, the top surface of the sample may be damaged in the area surrounding the hole, making it difficult to determine the exact location of the original top surface. Some embodiments disclosed herein deposit a small amount of material above the surface of the sample next to the area to be milled prior to the milling process. This small deposit can be partially milled during the milling process, creating a high-contrast boundary between the deposited material and the original surface, thereby preserving the exact location of the original top surface of the sample. Images of the buried layer along the sidewalls can then be obtained from two different perspectives, and the high-contrast boundary can be used as the exact height of the original surface in the images. Using the known exact height of the original surface, the exact distance between the original top surface of the sample and the bottom surface of the milled hole along the sidewalls can be obtained, and the depth of the hole can be accurately determined as described herein.

[0027] As used herein, the term "hole" generally may refer to a cavity, trench, or other structure milled in a sample, where one or more surfaces of the hole are below the original surface of the sample prior to the milling operation.

[0028] Example focused ion beam (FIB) tool

[0029] To better understand and appreciate the present disclosure, first refer to Figure 1A , which is a simplified schematic diagram of a sample evaluation system 100 according to some embodiments of the present disclosure. The sample evaluation system 100 can be used for defect review and analysis of structures formed on a sample such as a semiconductor wafer, as well as other operations.

[0030] As Figure 1AAs shown, the sample evaluation system 100 may include a vacuum chamber 110, a focused ion beam (FIB) column 120, a scanning electron microscope (SEM) column 130, a sample support assembly 140, a gas injection nozzle 160, and secondary electron detectors 162, 164 (or in some embodiments, a secondary ion detector, or a combination of two detectors operating in parallel), as well as other components. The FIB column 120 and the SEM column 130 are connected to the vacuum chamber 110 such that charged particle beams generated by either charged particle column propagate through the vacuum environment formed within the vacuum chamber 110 before impinging on the sample 150. For example, the FIB column 120 is operable to generate a charged particle beam 122 and direct the charged particle beam 122 towards the sample 150 (sometimes referred to herein as the "object" or "specimen") to mill or otherwise process the sample. The SEM column 130 can generate an image of a portion of the sample 150 by irradiating the sample with a charged particle beam 132, detecting the particles emitted due to the irradiation, and generating a charged particle image based on the detected particles.

[0031] A sample 150, such as a semiconductor wafer, can be supported on the sample support assembly 140 within the vacuum chamber 110. The sample support assembly 140 can also move the sample area within the vacuum chamber 110 between the fields of view of the two charged particle columns 120 and 130 as needed for processing. For example, the FIB column 120 can be used to mill an area on the sample 150, and then the support assembly 140 can move the sample such that the SEM column 130 can image the milled area of the sample 150.

[0032] The FIB column 120 can mill the sample 150 (e.g., drill a hole therein) by irradiating the sample with one or more charged particle beams to form a cross-section or a hole. The FIB milling process generally involves positioning the specimen in the vacuum chamber 110 and emitting an ion beam towards the specimen to etch or mill away the material on the specimen. Common milling processes form a cross-section of the sample 150 and, if desired, can also smooth the cross-section. In some cases, the vacuum environment can be purged with a background gas, which is used to control the etching rate and other parameters. The accelerated ions can be generated from xenon, gallium, or other suitable elements and are typically accelerated towards the sample by a voltage in the range of 500 volts to 100,000 volts (and higher, typically falling in the range of 3,000 volts to 30,000 volts). Depending on the FIB instrument configuration and the specific application, the beam current is typically in the range of a few picoamperes to a few microamperes, and the pressure is typically controlled between 10 -10 to 10 -5 millibars.

[0033] During a milling operation, the charged particle beam 122 generated by the FIB column 120 propagates through the vacuum environment formed within the vacuum chamber 110 before impinging on the sample 150. Secondary electrons and ions 124 are generated in the collision of the ions with the sample and can be detected by the detector 162. The detected secondary electrons or ions 124 can be used to analyze the characteristics of the milled layer and structure, to determine the end point of the milling process, and / or to form an image.

[0034] During a particle imaging operation, the charged particle beam 132 generated by the SEM column 130 propagates through the vacuum environment formed within the vacuum chamber 110 before impinging on the sample 150. Secondary electrons 134 are generated in the collision of the electrons with the sample 150 and can be detected by the detector 164. The detected secondary electrons 134 can be used to form an image of the milled area and / or to analyze the characteristics of the milled layer and structure.

[0035] The particle imaging and milling processes each typically include raster scanning the charged particle beam (e.g., in a raster scan mode) back and forth at a constant rate over a specific area of the imaged or milled sample. As is known to those skilled in the art, one or more lenses (not shown) coupled to the charged particle column can effect the scan mode. The scanned area is typically a very small portion of the total area of the sample. For example, the sample can be a semiconductor wafer with a diameter of 200 or 300 mm, and each area scanned on the wafer can be a rectangular area having a width and / or length measured in microns or tens of microns.

[0036] During some processing operations, one or more gases can be delivered into the chamber 110 through the gas injection system 160. For simplicity of explanation, the gas injection system 160 is shown as a nozzle in FIG. 1, but it should be noted that the gas injection system 160 can include a gas reservoir, a gas source, valves, one or more inlets and one or more outlets, and other components. In some embodiments, the gas injection system 160 can be configured to deliver the gas to a local area of the sample 150 exposed to the charged particle beam rather than delivering the gas to the entire upper surface of the sample. For example, in some embodiments, the gas injection system 160 has a nozzle diameter measured in hundreds of microns (e.g., between 400 and 500 microns) and is configured to deliver the gas directly to a relatively small portion of the sample surface that includes the charged particle beam scan mode or collision zone. In various embodiments, a first gas injection system 160 can be configured to deliver the gas to the sample positioned under the FIB column 120, and a second gas injection system 160 (not shown) can be configured to deliver the gas to the sample positioned under the SEM column 130.

[0037] As shown in FIG. 1, system 100 may include one or more controllers, processors, or other hardware units 170 that control the operation of system 100 by executing computer instructions stored in one or more computer-readable memories 180, as is known to those of ordinary skill in the art. For example, the 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-updatable, and / or the like), magnetic disks, optical storage devices, or similar non-transitory computer-readable storage media.

[0038] Figure 1B A substrate inspection system 100 with an inclined SEM column 130 is shown. As explained more fully below, the SEM column 130 may be inclined relative to the surface of the sample 150 to obtain images from different angles (or from different perspectives) relative to the surface of the sample 150. Alternatively, in some embodiments, the support assembly 140 may be configured to tilt the sample 150 such that images may be obtained from different angles. The gas nozzle 160 and detectors 162, 164 are not shown in Figure 1B for purposes of illustration.

[0039] will be as Figure 1A and Figure 1B The inspection system 100 shown is provided as an example of a system that may be used with some of the embodiments described herein. It should be understood that the embodiments are not limited to inspection system 100, and other inspection systems may be used with some of the embodiments. Additionally, in some embodiments, an FIB tool may be used to mill holes in a sample, and a separate SEM tool may be used to obtain images of the holes.

[0040] Charged particle enhanced deposition process

[0041] Embodiments of the present disclosure may use a sample evaluation system 100 to deposit a material on a sample located on a support 140 by initiating a deposition process under a FIB column 120. As an example, in some embodiments, the FIB column 120 may be used in a deposition mode to initiate a focused ion beam enhanced deposition process. To this end, a deposition gas may be supplied to the sample 150 through a gas injection system 160, and the energy from the FIB column 120 may generate an ion beam 122. The cascade of impinging ions may in turn activate the deposition gas, resulting in the deposition of the material on the sample within the sample area scanned by the ion beam. Thus, the deposition that occurs according to these embodiments does not occur simultaneously over the entire surface of the processed sample or wafer. Instead, deposition occurs only in the general area where the ion beam (as a non-limiting example, for a xenon plasma, its diameter may be in the range of 0.5 to 25 microns) impinges on the wafer and when the ion beam scans these areas of the wafer. Thus, the deposition according to some embodiments may be performed with micron-scale resolution.

[0042] Example of a milled hole

[0043] Figure 2 is a simplified cross-sectional view of a sample 200 having a hole 210 milled therein using a charged particle beam of a FIB system according to an embodiment. The sample 200 may represent Figure 1A and Figure 1B the sample 150 discussed in. The sample 200 may include a top surface 212 of the sample 200 in which the hole 210 is milled. The hole 210 may then include a bottom surface 214 and sidewalls 216 that may extend between the sample surface 212 and the hole bottom 214 and, in some cases, may be inclined at an angle, as Figure 2 shown.

[0044] The hole 210 is shown in Figure 2 as a theoretical or ideal shape. In some actual milling processes, the top surface 212 may be damaged to some extent during the process of milling the hole 210. For illustration, refer to Figure 3 , which is a simplified cross-sectional view of a sample 300 similar to the sample 200 in which a hole 310 is milled. As Figure 3 shown, the hole 310 is milled from the upper surface 312 of the sample 300 and includes a bottom surface 314. The sidewalls 316 may extend between the sample surface 312 and the hole bottom 314 and may be inclined at an angle similar to that of the sidewalls 214 discussed above. As can be seen from the example provided in Figure 3 , the sidewalls 316 may have portions with a relatively constant inclination angle. However, unlike the hole sample 200, the sample 300 includes a damaged area 318 along one or more portions of the perimeter of the hole 310.

[0045] The depths of holes 210 and 310 can be measured stereoscopically, which is to measure the length between two points from two different perspectives. For example, when the first measurement is made from the top view perspective and the second measurement is made from an inclined perspective (e.g., a 45-degree inclination), the length difference between the two measurements is the vertical height of the measured slope (or, the depth of the milled hole).

[0046] To make such stereoscopic height measurements accurate, the positions of the two measured points need to be accurately identified. As Figure 3 shown, however, there can be a damaged area 318 at the surface 312 of the sample 300. During the milling process, the damaged area 318 can be formed, for example, along the outer periphery of the hole 310. As a non-limiting example and as described above, in some FIB instruments, such as a plasma source FIB system, the focused ion beam can have a relatively large "tail" around the spot, which can create the damaged area 318 on the sample surface. The surface of the damaged area 318 makes it difficult to accurately identify the top surface 312 of the sample 300 during the SEM imaging process, which in turn has an adverse effect on the accuracy in determining the depth of the hole 310.

[0047] Before explaining how the embodiments disclosed herein can accurately determine the depth of a milled hole even when there is damage (such as the damaged area 318) near the hole perimeter at the sample surface, an example of how to perform stereoscopic measurement is first given below according to some embodiments.

[0048] General concept - Stereometric measurement with a scanning electron microscope instrument

[0049] Stereoscopic measurement techniques can be used in conjunction with an imaging device, such as a scanning electron microscope (SEM), to determine the thickness or depth of different structures formed on a sample. One such technique for doing so is described in the co-assigned U.S. Patent Application No. 17 / 408,876, entitled "Analysis of Buried Layers of a Sample," filed on July 19, 2021. The 17 / 408,876 application is incorporated herein by reference in its entirety, but for convenience, the stereoscopic measurement technique set forth in the 17 / 408,876 application is briefly described below with reference to Figures 4A to 8 FIGs.

[0050] Figure 4A and Figure 4B are a simplified cross-sectional view and a top view, respectively, of an exemplary hole 400 milled in a sample 410. The shown hole 400 includes inclined sidewalls formed in the sample 410. In this example, the sample 410 includes a buried layer 420 having a composition different from that of the sample 410. The buried layer 420 includes an upper surface 422 and a lower surface 424, and the techniques discussed below can be used to determine the height (thickness) of the buried layer 420, i.e., the distance between the upper surface 422 and the lower surface 424.

[0051] When observing the hole 400 from different perspectives, the apparent thickness change of the buried layer 420 indicated by the distance between the top surface 422 and the bottom surface 424 of the buried layer 410. More specifically, the distance between the top surface 422 and the bottom surface 424 of the buried layer 420 increases as the tilt angle increases, reaches a maximum at a specific tilt angle depending on the sidewall slope, and then decreases as the tilt angle further increases. This is shown in the comparison between Figure 4B and Figure 4C where Figure 4C is the simplified illustration of the hole 400 as viewed from the tilt angle Figure 4A and Figure 4B shown. From this perspective, the buried layer 420 can be seen along the inclined sidewall of the hole 400. In some embodiments, the inclined viewing angle can be about 45° (e.g., within a few degrees) with respect to the surface of the sample 410.

[0052] The thickness (vertical thickness) of the buried layer 420 can be determined using the distance measured between the top surface and the bottom surface of the buried layer observed from different perspectives. Figure 5 is a simplified diagram (same inclined viewing angle as Figure 4C ) showing how, according to one embodiment, images of the buried layer 420 on the sidewall of the hole 410 in the sample 400 can be obtained from different perspectives. In the example described, a first image can be obtained from a first perspective 500a, and a second image can be obtained from a second perspective 500b. Any type of imaging device or technique that allows distance measurements to be obtained between points in the image can be used to obtain the images. Examples include optical or SEM devices and techniques. Then the images can be used to measure the distance between points on the upper and lower surfaces of the buried layer 420. It should be understood that the field of view of the imaging device can include the hole 410 (sometimes referred to herein as a spot) or a larger portion of the sample 400, rather than just the buried layer 420.

[0053] Figure 6A and Figure 6B are additional simplified cross-sectional views of the hole 400, depicting a first point 600a on the upper surface 422 of the buried layer 420 and a second point 600b on the lower surface 424 of the buried layer 420. The points 600a, 600b can be located at positions having some feature or features such that the points 600a, 600b are recognizable in images obtained from different perspectives.

[0054] In Figure 6A , the points 600a, 600b appear vertically aligned, as shown by the dashed line 610. However, Figure 6B relative to Figure 6ARotated 90°, and it shows that points 600a and 600b are vertically offset because they are located on the inclined sidewalls of the milling hole 400. In practice, one of the points 600a and 600b can be selected, for example, point 600a, and the distance between point 600a and another point on the lower surface 424 of the buried layer 420 that appears directly below point 600a can be measured. The point associated with the shortest distance can be identified as point 600b. This technique should provide two points that generally fall on the line 610 extending vertically through the buried layer 420, as Figure 6A shown.

[0055] Figure 7 is a simplified diagram showing some steps of using the measured distance between points 600a and 600b to determine the vertical thickness 710 of the buried layer 220. In this example, a first image of the buried layer 220 is obtained from a first perspective 700a, and a second image of the buried layer 220 is obtained from a second perspective 700b. The distance 710 between points 600a and 600b in each image can be determined using known measurement techniques that depend on the specific imaging device and measurement technique.

[0056] Figure 8 is a simplified diagram illustrating how the vertical thickness of a structure 800 (such as the buried layer 420) or the vertical depth of a hole can be determined according to some embodiments. For ease of illustration, the upper and lower surfaces of the structure 800 layer are represented by horizontal lines 810 and 820, respectively, in this figure. The horizontal lines are connected by a line 830 representing the sidewall of the structure 800, and the sidewall is inclined at an angle β with respect to the vertical direction. In this example, the first image of the sidewall is obtained from a first perspective 850a at a first tilt angle α1, and the second image is obtained from a second perspective 850b at a second tilt angle α2. The tilt angles α1 and α1 can be defined by the user and / or can be obtained from or determined by the imaging device. The vertical thickness (height or depth) of the structure 800 is represented by H.

[0057] When analyzing features from an inclined perspective, most conventional SEM imaging devices measure the distance projected onto a horizontal or vertical plane. As an example, in Figure 8 the distance projected onto the horizontal plane from the first perspective 850a is L1, and the distance projected onto the vertical plane from the first perspective is h1. Similarly, the distance projected onto the horizontal plane from the second perspective 850b is L2, and the distance projected onto the vertical plane from the second perspective is h2. According to some embodiments, these measured distances can be used together with the tilt angles α1 and α2 to determine the vertical thickness H of the buried layer 800 using any of the following equations:

[0058]

[0059] In some cases, the first viewing angle 850a can be approximately looking down (perpendicular to the surface of the sample), and the second viewing angle 850b can be approximately 45° with respect to the surface of the sample. In this configuration, the vertical thickness H of the buried layer can be simplified and determined using the following equation:

[0060] H = L2 - L1 (3)

[0061] Determining the depth of a milled hole at the surface of a sample

[0062] Although the above discussion describes using stereometric techniques to determine the thickness of the buried layer, the embodiments disclosed herein can use the stereometric techniques described above to determine the depth (i.e., height) of a milled hole in a sample. As described above with respect to Figure 3 In some cases, the portion of the top surface of the sample surrounding the milled hole poses a challenge to accurately determining the depth of the milled hole. As an example, this damage occurs when the focused ion beam has a relatively large "tail" or other features that cause some degree of damage to the top surface. Even when the top surface is damaged, the embodiments disclosed herein can accurately determine the depth of the hole using the techniques described below.

[0063] Some embodiments are capable of accurately identifying the top surface of a sample by depositing a small amount of material directly on the surface near the area to be milled. Even if a portion of the small deposit is partially milled during the milling process, the remaining deposit portion creates a high-contrast boundary between the deposit and the original surface, thereby accurately identifying the height of the original surface. Then, the points along the high-contrast boundary can be used in conjunction with the stereometric techniques described herein to determine the precise depth of the milled hole.

[0064] For illustration, reference is made to Figure 9 、 Figures 10A to 10D Figures and Figures 11A to 11C . Figure 9 is a flowchart depicting the steps associated with method 900 according to some embodiments disclosed herein. Figures 10A to 10D The figure is a simplified cross-sectional view of the sample 1000 at different stages of processing according to method 900. Additionally, Figures 11A to 11C is a simplified top view depicting different patterns according to different embodiments, in which materials are deposited on the sample 100 during the implementation of method 900.

[0065] As Figure 9 shown, method 900 begins by positioning the sample on a sample support in a chamber of a suitable evaluation system (step 910). For example, in some embodiments, step 910 includes positioning the sample 1000 ( Figure 10A) is positioned on the sample support 140 within the vacuum chamber 110 of the sample evaluation system 100. The sample 1000 can represent any one of the samples 150, 200, 300, or 400 discussed above. Then, the sample 1000 can be moved under the field of view of the focused ion beam column (step 920), and as Figure 10B shown, the material 1020 can be locally deposited at one or more locations, directly adjacent to the location where the hole 1010 (represented by the dashed line) is to be milled in the sample (step 930).

[0066] As will be understood by those skilled in the art, the material 1020 can be selected based on the composition of the sample 1000 or the uppermost layer of the sample 1000 to be milled. Generally, the material 1020 should have properties that exhibit a strong contrast between it and the sample 1000 (or the uppermost layer of the sample 1000).

[0067] The deposition of the material on the sample 1000 in step 930 can be carried out in a charged particle enhanced deposition process, such as the focused ion beam deposition process described above. For example, an appropriate deposition gas can be supplied to the sample 130 through the gas injection system 150, and the energy from the FIB column 120 can generate an ion beam 122. The cascade of impinging ions can in turn activate the deposition gas, resulting in the deposition of the material on the sample, which is located within the sample area scanned by the ion beam. Thus, the system 1000 can control the location where the material 1020 is deposited based on the location of the scan pattern used in step 930. As various non-limiting examples, the material 1020 can be deposited along a complete edge of the perimeter of the hole 1010 ( Figure 11A ), only along a portion of the edge ( Figure 11B ), or around most or the entire perimeter of the hole to be milled ( Figure 11C ).

[0068] After the material 1020 has been deposited, the hole 1010 can be milled in the upper surface 1012 of the sample 1000 (step 940). The milling process can scan the focused ion beam in the area of the sample that is directly adjacent to the location where the material 1020 is deposited. As Figure 10C shown, the hole 1010 can include a bottom surface 1014 and sidewalls 1016 that extend between the upper surface 1012 and the bottom surface 1014.

[0069] As described above, in some cases, the milling step 940 can produce a damaged area 1018 around part or all of the upper perimeter of the hole 1010. Even if the damaged area etches or mills away some portions of the material 1020 closest to the perimeter of the hole 1010, as long as some of the material 1020 remains, the interface (boundary) 1030 between the remaining material 1020 and the sample 1000 represents the original position of the upper surface 1012.

[0070] The first point 1032 along the interface 1030 and the second point 1034 located at the bottom of the hole 1010 can then be identified and used to calculate the depth of the hole 1010 according to the stereometric techniques discussed above and graphically depicted in Figure 10D . Returning reference to Figure 9 , the depth of the hole 1010 can be determined by measuring a first distance between the first point 1032 and the second point 1034 from a first perspective (step 950), and then measuring a second distance between the same two points 1032, 1034 from a second perspective (step 960). Once these two measurements have been made, the depth of the hole 1010 can be determined using the first distance, a first angle associated with the first perspective, the second distance, and a second angle associated with the second perspective, as discussed above with reference to Figure 8 (step 970).

[0071] Example of a sample to be milled and measured

[0072] As described above, embodiments of the present disclosure can be used to determine the depth of holes milled in a sample. Embodiments can be used to determine the depth of milled holes within many different types of samples, including electronic circuits formed on semiconductor structures, solar cells formed on polycrystalline or other substrates, nanostructures formed on various substrates, and the like. As a non-limiting example, Figure 12 is a simplified illustration of an area on a semiconductor wafer that can include a milled hole, the depth of which can be determined according to the embodiments described herein. Specifically, Figure 12 includes a top view of the wafer 1200 and two expanded views of a particular portion of the wafer 1200. The wafer 1200 can be, for example, a 150 mm, 200 mm, or 300 mm semiconductor wafer and can include a plurality of integrated circuits 1210 (52 in the example shown) formed thereon. The integrated circuits 1210 can be in an intermediate stage of manufacture, and the techniques described herein can be used to evaluate and analyze one or more regions 1220 of the integrated circuit.

[0073] The disclosed embodiments can analyze and evaluate the region 1220 by sequentially milling the material within the region where the milled hole is to be formed. The depth of the milled hole can then be determined as described above. When milling the hole, the milling process can mill the region 1220 by scanning the FIB back and forth within the region according to a raster pattern until the hole has been milled to the desired depth (with the desired slope). Even if the milling process inadvertently damages the area of the top surface of the sample 1200 surrounding the region 1220, the techniques described herein can be used to accurately determine the depth of the milled hole.

[0074] Additional embodiments

[0075] For purposes of explanation, the foregoing description uses specific terms to provide a thorough understanding of the described embodiments. However, it will be apparent to one of ordinary skill in the art that the specific details are not required in order to practice the described embodiments. Thus, for purposes of illustration and description, the foregoing description of the specific embodiments described herein is presented. The foregoing description is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. For example, while the above embodiments describe a focused ion column as part of a tool having a single charged particle column, in some embodiments, the focused ion beam column can be part of a SEM-FIB tool having a scanning electron microscope column and a focused ion beam column. Additionally, while various simplified diagrams of holes whose depth can be measured are discussed herein as examples, it should be understood that these examples are generally highly simplified diagrams for illustrative purposes only. The actual holes milled in a sample can have a morphology different from that depicted in the figures, and the embodiments described herein are not limited to any particular shape or morphology of the milled holes. Further, while the profile of a hole (e.g., hole 310) is often depicted as smooth in the included figures, it should be understood that the profile can be rough and jagged at the micro level without significantly affecting the depth measurement techniques described herein.

[0076] Additionally, while different embodiments of the present disclosure are disclosed above, the specific details of the particular embodiments can be combined in any suitable manner without departing from the spirit and scope of the embodiments of the present disclosure. Further, it will be apparent to one of ordinary skill in the art that many modifications and variations are possible in view of the above teachings. Thus, it is to be understood that the appended claims are intended to cover all such modifications and alterations that fall within the true spirit of the disclosed embodiments.

[0077] Moreover, any reference to a method in the foregoing description should, as necessary, be applied to a system capable of performing the method and, as necessary, to a computer program product storing instructions that, when executed, cause the method to be performed. Similarly, any reference to a system in the foregoing description should, with the necessary modifications, be applied to a method executable by the system and, with the necessary modifications, to a computer program product storing instructions executable by the system; and any reference to a computer program product in the description should, with the necessary modifications, be applied to a method that can be performed when executing the instructions stored in the computer program product and, with the necessary modifications, to a system configured to execute the instructions stored in the computer program product.

[0078] Moreover, in circumstances where the disclosed illustrated embodiments can be largely implemented using electronic components and circuits known to one of ordinary skill in the art, in order to understand and appreciate the disclosed basic concepts and in order not to obscure or distract from the disclosed teachings, these details are not explained in a greater extent than considered necessary above.

Claims

1. A method for determining the depth of a hole milled into a first region of a sample, the method comprising: Position the sample in a processing chamber having a charged particle beam column; Deposit material directly above the top surface of the sample in a second region of the sample adjacent to the first region; Mill the hole in the first region of the sample using a charged particle beam generated by the charged particle beam column, wherein the hole abuts the material deposited above the top surface and includes sidewalls extending from the bottom surface of the hole to an interface between the deposited material and the top surface of the sample; And Calculate the depth of the hole using a stereometric technique based on a distance measurement between a first point along the interface between the material and the top surface and a second point along the bottom surface of the hole.

2. The method for determining the depth of a hole milled into a first region of a sample according to claim 1, wherein using stereometric techniques to calculate the depth of the hole comprises: Obtain a first image and a second image of the sidewall at different perspectives; For each of the first image and the second image, measure the distance between a first point along the interface between the material and the top surface and a second point along the bottom surface of the hole; And Calculate the depth of the hole based on the first image, the second image, and the measured distances.

3. The method for determining the depth of a hole milled into a first region of a sample according to claim 2, wherein each of the first image and the second image is obtained using scanning electron microscope (SEM) techniques.

4. The method for determining the depth of a hole milled into a first region of a sample according to any one of claims 1 to 3, wherein the charged particle beam column is a focused ion beam (FIB) column, and the charged particle beam is a focused ion beam.

5. The method for determining the depth of a hole milled into a first region of a sample according to claim 4, wherein depositing the material comprises: Inject a deposition precursor gas into the second region of the sample; Generate a focused ion beam using the focused ion beam column and focus the ion beam within the second region of the sample; And Scan the focused particle beam across the second region of the sample to activate molecules of the deposition gas that have adhered to the sample surface in the deposition region and deposit material within the second region on the sample.

6. The method for determining the depth of a hole milled into a first region of a sample according to claim 1, wherein the stereometric techniques comprise: Obtain a first image of the sidewall of the hole, the first image being obtained from a first perspective associated with a first angle relative to the sample; Measure a first distance between a first point at the interface between the deposited material and the top surface of the sample on the sidewall and a second point corresponding to the bottom surface of the hole on the sidewall using the first image; Obtain a second image of the sidewall of the hole, the second image being obtained from a second perspective associated with a second angle relative to the sample, wherein the first angle and the first perspective are different from the second angle and the second perspective; Measure a second distance between the first point corresponding to the interface between the deposited material and the top surface of the sample on the sidewall and the second point corresponding to the bottom surface of the hole on the sidewall using the second image, wherein the first point and the second point generally lie on a line extending vertically through the milled hole; And Determine the depth of the hole using the first distance, the first angle associated with the first perspective, the second distance, and the second angle associated with the second perspective.

7. The method for determining the depth of a hole milled into a first region of a sample according to claim 6, wherein each of the first image and the second image is obtained using scanning electron microscope (SEM) techniques.

8. The method for determining the depth of a hole milled into a first region of a sample as claimed in claim 1, wherein the processing chamber is a vacuum chamber comprising both a focused ion beam (FIB) column and a scanning electron microscope (SEM) column.

9. The method for determining the depth of a hole milled into a first region of a sample as claimed in claim 1, wherein the sample is a semiconductor wafer.

10. A system for determining the depth of a hole milled into a first region of a sample, the system comprising: Vacuum chamber; Sample support configured to hold a sample within the vacuum chamber during a milling process; Charged particle beam column configured to direct a charged particle beam into the vacuum chamber; A processor and a memory coupled to the processor, the memory including a plurality of computer-readable instructions that, when executed by the processor, cause the system to: Position the sample in a processing chamber having a charged particle beam column; Deposit material directly above the top surface of the sample in a second region of the sample adjacent to the first region; Mill the hole in the first region of the sample using a charged particle beam generated by the charged particle beam column, wherein the hole is adjacent to the material deposited above the top surface and includes sidewalls extending from the bottom surface of the hole to an interface between the deposited material and the top surface of the sample; And Calculate the depth of the hole using stereometric techniques based on a distance measurement between a first point along the interface between the material and the top surface and a second point along the bottom surface of the hole.

11. The system for determining the depth of a hole milled into a first region of a sample as claimed in claim 10, wherein the stereometric technique for calculating the depth of the hole comprises: Obtain a first image and a second image of the sidewall from different perspectives; For each of the first image and the second image, measure the distance between a first point along the interface between the material and the top surface and a second point along the bottom surface of the hole; And Calculate the depth of the hole based on the first image, the second image, and the measured distance.

12. The system for determining the depth of a hole milled into a first region of a sample as claimed in claim 11, wherein each of the first image and the second image is obtained using scanning electron microscope (SEM) technique.

13. The system for determining the depth of a hole milled into a first region of a sample as claimed in claim 10, wherein the charged particle beam column is a focused ion beam (FIB) column, and the charged particle beam is a focused ion beam.

14. The system for determining the depth of a hole milled into a first region of a sample as claimed in claim 13, wherein depositing the material comprises: Inject a deposition precursor gas into the second region of the sample; Generate a focused ion beam using the focused ion beam column and focus the ion beam within the second region of the sample; And Scan the focused particle beam across the second region of the sample to activate molecules of the deposition gas that have adhered to the sample surface in the deposition region and deposit material within the second region of the sample.

15. The system for determining the depth of a hole milled into a first region of a sample as claimed in any one of claims 10 to 14, wherein the processing chamber is a vacuum chamber including both a focused ion beam (FIB) column and a scanning electron microscope (SEM) column.

16. A non - transitory computer - readable memory storing instructions for determining the depth of a hole milled into a first region of a sample by: Positioning the sample in a processing chamber having a charged particle beam column; Depositing a material directly above the top surface of the sample in a second region of the sample adjacent to the first region; Milling the hole in the first region of the sample using a charged particle beam generated by the charged particle beam column, wherein the hole abuts the material deposited above the top surface and includes sidewalls extending from the bottom surface of the hole to an interface between the deposited material and the top surface of the sample; and Calculating the depth of the hole using stereometric techniques based on a distance measurement between a first point along the interface between the material and the top surface and a second point along the bottom surface of the hole.

17. The non - transitory computer - readable memory storing instructions for determining the depth of a hole milled into a first region of a sample as claimed in claim 16, wherein the stereometric techniques for calculating the depth of the hole include: Obtain a first image and a second image of the sidewall from different perspectives; For each of the first image and the second image, measure the distance between a first point along the interface between the material and the top surface and a second point along the bottom surface of the hole; And Calculate the depth of the hole based on the first image, the second image, and the measured distance.

18. The non - transitory computer - readable memory storing instructions for determining the depth of a hole milled into a first region of a sample as claimed in claim 17, wherein each of the first image and the second image is obtained using scanning electron microscope (SEM) techniques.

19. The non - transitory computer - readable memory storing instructions for determining the depth of a hole milled into a first region of a sample as claimed in claim 16, wherein the charged particle beam column is a focused ion beam (FIB) column and the charged particle beam is a focused ion beam.

20. The non - transitory computer - readable memory storing instructions for determining the depth of a hole milled into a first region of a sample as claimed in any one of claims 16 to 19, wherein depositing the material includes: Inject a deposition precursor gas into the processing chamber at a location adjacent to the deposition region; Generate a focused ion beam using the focused ion beam column and focus the ion beam within the deposition region of the sample; And Scan the focused particle beam across the deposition region of the sample to activate molecules of the deposition gas that have adhered to the sample surface in the deposition region and deposit material within the deposition region of the sample.

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