Method for processing a transmission sample of a semiconductor chip structure by using a focused ion beam

Abstract

The present application discloses a method for processing a transmission sample of a semiconductor chip structure using a focused ion beam. Based on the conventional transmission sample preparation process, the thinning process is improved. When using specific parameters for cutting and processing, the two-step cutting method and the gap cutting method are adopted to achieve uniform thickness processing of the semiconductor chip structure. In particular, the cutting method for the tungsten pillar structure is optimized, and multiple reference bases for judging the thickness of the observed thin slice are provided. It can realize the efficient preparation of transmission samples of semiconductor chip structures and obtain high-resolution transmission images of key device structures.

Description

Technical Field

[0001] This application relates to the technical field of sample preparation, and particularly relates to a method for processing a transmission sample of a semiconductor chip structure by using a focused ion beam. Background Art

[0002] With the development of semiconductor manufacturing technology, the manufacturing process of transistor structures in chips has been reduced to 7 nm, and the industrialization of the 3 nm process is also steadily advancing. Whether the actual structure of the chip conforms to the designed structure can only be verified through microscopic structure characterization means at the nanoscale. This requires the preparation of transmission electron microscope samples for semiconductor chips and the microscopic structure characterization at the nanoscale. The key to the entire testing process lies in the accurate positioning of the transmission electron microscope sample preparation for the semiconductor chip to ensure that the target structure is within the range of the observation thin area; and the thickness of the entire observation area is uniform to ensure that the complex internal structure of the semiconductor chip is clearly presented; in addition, the observed thickness of the specific structure film layer needs to be less than 50 nm to ensure that the observed component and structure information provide the best reference data for process optimization.

[0003] The existing transmission sample preparation scheme using a focused ion beam microscope is designed for bulk samples with uniform materials. Generally, it includes the following five steps: (1) depositing a protective layer: depositing a protective material with a thickness of 2 microns on the surface of the target processing area by electron beam or ion beam deposition; (2) cutting a thin slice: processing grooves with a depth of more than 10 microns on both sides of the target area to make the target area a thin slice with a thickness of 1 - 2 um; (3) extracting and fixing: processing a cutting groove around the thin slice, welding the thin slice to a nano - hand, and then transferring it to a support grid dedicated for transmission electron microscope sample preparation; (4) rough cutting: thinning the thin slice by using an ion beam with a current level of several hundred pA to several tens of pA at a voltage of 30 kV until the thickness of the thin slice is 80 - 100 nanometers; (5) fine cutting: thinning the thin slice by using an ion beam current of 50 - 100 pA at a voltage of 5 kV to 1 kV until the thickness of the thin slice reaches below 50 nm.

[0004] When using the conventional transmission sample preparation process to process samples of semiconductor chip structures, the following defects exist:

[0005] First, the thickness of the thin region in the semiconductor chip structure is uneven. The semiconductor chip structure prepared by the standard chip manufacturing process usually contains 4-6 metal interconnection layers. Copper or aluminum is used as the metal material for electrode connection, tungsten metal pillars are used as the filling material for the interlayer through-tubes, and silicon dioxide is used as the insulating material between circuits. In addition, there are also various functional materials that make up specific device structures. There are significant differences in the sputtering rates between different materials. Therefore, when processed using conventional transmission sample preparation methods, there will be a large gap in the thickness of different regions being cut, resulting in uneven thickness of the observed thin region at different material positions and affecting the final transmission electron microscopy (TEM) observation effect.

[0006] Second, there is a lack of a quantitative standard for judging the thickness of the thin region during the process of preparing transmission samples using a focused ion beam. After the processing of the transmission sample is completed, whether the thickness of the thin region meets the requirements of TEM observation and whether high-quality high-resolution images can be obtained can only be judged by the experience of the sample maker or the final conclusion can only be obtained during the final TEM observation. The conventional experience is to judge based on the transmittance of the thin region at low voltages, 5 kV or 3 kV. However, it is very difficult to make an accurate judgment based on such a basis for samples with good and poor conductivity.

[0007] Third, it is very difficult to obtain high-resolution images of the top structure of tungsten pillars using conventional sample preparation methods. Since the sputtering rate of gallium ion beam cutting tungsten pillars is lower than half of the sputtering rate of cutting silicon (the sputtering rate of tungsten is 11.21 and the sputtering rate of silicon is 27.61), when the main material of the semiconductor chip, the silicon substrate, and the silicon dioxide insulating material are cut, only half of the thickness of the tungsten pillars used as filling through-tubes is cut, resulting in insufficient thinning of the device structures fabricated on the upper layer of the tungsten pillars. These structures cannot obtain high-quality high-resolution images due to excessive thickness during the final TEM observation. However, the device structures fabricated on these tungsten pillars are usually new device structures designed in the process technology development and scientific research exploration work and are the key to technological progress. Summary of the Invention

[0008] This application provides a method for processing transmission samples of semiconductor chip structures using a focused ion beam, which can achieve precise sample preparation at the 50 nm scale, perform thinning processing with uniform thickness on heterogeneous materials existing in the vertical direction, and provides a reference standard for judging the thickness of the thin region to obtain high-quality TEM observation results.

[0009] An embodiment of the present application provides a method for processing a transmission sample of a semiconductor chip structure by using a focused ion beam, including the following steps: obtaining a sample thin slice of a semiconductor chip cross-section; selecting cutting parameters to perform multiple rough cuts and multiple fine cuts on the sample thin slice to process the sample thin slice to a set thickness. Wherein, in the rough cut, when the thickness of the sample thin slice is the reduced pressure thickness, the ion beam voltage during cutting is reduced and cutting continues, and in both the rough cut and the fine cut, a two-step cutting method and a gap thinning method are used for cutting; using a thickness judgment method to evaluate whether the set thickness meets a preset requirement, and when the set thickness meets the preset requirement, a transmission sample of the semiconductor chip structure is obtained.

[0010] Optionally, in an embodiment of the present application, the multiple rough cuts include: rough cut step 1: at a first distance from the center of the tungsten column of the sample thin slice, adopting a step-by-step cutting mode, with a first ion beam voltage and current, a first cutting angle, and a first cutting depth, processing the sample thin slice to a first preset thickness, where the first cutting depth is greater than the target processing depth; rough cut step 2: at a second distance from the center of the tungsten column of the sample thin slice, adopting a step-by-step cutting mode, with a second ion beam voltage and current, a second cutting angle, and a second cutting depth for two thinning operations, and using the gap thinning method in the second thinning operation to process the thickness of the sample thin slice to a second preset thickness, where the second cutting depth is the target processing depth, and both the second ion beam voltage and current are less than the first ion beam voltage and current, and the second distance is less than the first distance; rough cut step 3: at a third distance from the center of the tungsten column of the sample thin slice, using a third ion beam voltage and current for two thinning operations, and using the gap thinning method in the second thinning operation to process the thickness of the sample thin slice to a third preset thickness, where the third distance is less than the second distance, and the second ion beam voltage and current are less than the first ion beam current; rough cut step 4: at a fourth distance from the center of the tungsten column of the sample thin slice, using a fourth ion beam voltage and current for two thinning operations to process the thickness of the sample thin slice to the third preset thickness, where the fourth distance is less than the third distance, and both the fourth ion beam voltage and current are less than the third ion beam voltage and current.

[0011] Optionally, in an embodiment of the present application, the multiple fine cuts include: Fine cut step 1: At a fifth distance from the center of the tungsten column of the sample thin slice, using a step-by-step cutting mode, with a fifth ion beam voltage and current, a first cutting angle, and a third cutting depth, perform two thinning operations. In the second thinning operation, use the gap thinning method to process the thickness of the sample thin slice to a fifth preset thickness. Wherein, the fifth distance is less than the fourth distance, the fifth ion beam voltage and current are both less than the fourth ion beam voltage and current, and the third cutting depth is less than the target processing depth; Fine cut step 2: At a sixth distance from the center of the tungsten column of the sample thin slice, perform two thinning operations with a sixth ion beam voltage and current. In the second thinning operation, use the gap thinning method to process the thickness of the sample thin slice to a sixth preset thickness. Wherein, the sixth distance is less than the fifth distance, and the sixth ion beam current is less than the fifth ion beam current; Fine cut step 3: Use an overall cutting mode, with a seventh ion beam voltage and current, a third cutting angle, a first cutting time, and the cutting position set to the entire observation area visible under the ion beam field of view. First perform front-side thinning, and then perform back-side thinning to process the thickness of the sample thin slice to a seventh preset thickness. Wherein, the seventh ion beam voltage is less than the sixth ion beam voltage; Fine cut step 4: Use an overall cutting mode, with an eighth ion beam voltage and current, first perform front-side thinning, and then perform back-side thinning to process the thickness of the sample thin slice to the set thickness. Wherein, the eighth ion beam current is less than the seventh ion beam current.

[0012] Optionally, in an embodiment of the present application, the rough cut step 1 specifically includes: Set the cutting position at 500 nanometers from the center of the tungsten column, use a step-by-step cutting mode, set the first ion beam voltage and current to 30 KV and 80 pA, tilt the sample thin slice 2 degrees relative to the ion beam incident direction, increase the first cutting depth by 2 micrometers on the basis of the target processing depth, perform front-side thinning first, and then perform back-side thinning to process the thickness of the sample thin slice to 1 micrometer.

[0013] Optionally, in an embodiment of the present application, the rough cut step 2 specifically includes: Set the cutting position at 230 nanometers from the center of the tungsten column, use a step-by-step cutting mode, set the second ion beam voltage and current to 16 KV and 50 pA, tilt the sample thin slice 1.2 degrees relative to the ion beam incident direction, the second cutting depth is the target processing depth, perform the first thinning process, at a position maintaining a distance of 150 - 200 nm outside the final plane of the first thinning process, set the second thinning process area, use the same ion beam voltage and current, perform step-by-step thinning, set the thinning depth to the target value minus 1 um, perform front-side thinning first, and then perform back-side thinning to process the thickness of the sample thin slice to 460 nanometers.

[0014] Optionally, in an embodiment of the present application, the rough cutting step 3 specifically includes: setting the cutting position at a position 160 nm away from the center of the tungsten column, setting the third ion beam voltage and current to 16 KV and 11 pA, thinning the sample wafer twice, using the gap thinning method for the second thinning, first thinning the front side and then the back side, and processing the thickness of the sample wafer to 320 nanometers.

[0015] Optionally, in an embodiment of the present application, the rough cutting step 4 specifically includes: setting the cutting position at a position 100 nm away from the center of the tungsten column, setting the fourth ion beam voltage and current to 8 KV and 12 pA, thinning the sample wafer twice, using the gap thinning method for the second thinning, first thinning the front side and then the back side, and processing the thickness of the sample wafer to 200 nanometers.

[0016] Optionally, in an embodiment of the present application, the fine cutting step 1 specifically includes: setting the cutting position at 50 nanometers away from the center of the tungsten column, adopting a step-by-step cutting mode, setting the fifth ion beam voltage and current to 5 KV and 15 pA, tilting the sample wafer 2 degrees relative to the ion beam incident direction, increasing the third cutting depth by 1 micrometer on the target processing depth for two thinnings, using the gap thinning method for the second thinning, first thinning the front side and then the back side, and processing the thickness of the sample wafer to 100 nanometers.

[0017] Optionally, in an embodiment of the present application, the fine cutting step 2 specifically includes: setting the cutting position at a position 25 nm away from the center of the tungsten column, setting the sixth ion beam voltage and current to 5 KV and 9 pA, performing two thinnings, using the gap thinning method for the second thinning, first thinning the front side and then the back side, and processing the thickness of the sample wafer to 50 nanometers.

[0018] Optionally, in an embodiment of the present application, the fine cutting step 3 specifically includes: adopting an overall cutting mode, setting the seventh ion beam voltage and current to 2 KV and 9 pA, tilting the sample wafer 3.5 degrees relative to the ion beam incident direction, setting the cutting position to the entire observation area visible under the ion beam field of view, setting the first cutting time to 20 s, first thinning the front side and then the back side, and processing the thickness of the sample wafer to 30 nanometers.

[0019] Optionally, in an embodiment of the present application, the fine cutting step 4 specifically includes: adopting an overall cutting mode, setting the eighth ion beam voltage and current to 2 KV and 7 pA, first thinning the front side and then the back side, and processing the thickness of the sample wafer to 20 nanometers.

[0020] Optionally, in an embodiment of the present application, the thickness determination method is selected from one or more of evaluating the thickness of the sample thin slice by using the cross-sectional diameter of the tungsten pillar during cutting processing, evaluating the thickness by using the contrast of the Pt deposition layer and the aluminum metal connection layer in the secondary electron scanning image, and evaluating the thickness by using the morphology of the Pt deposition layer and the contrast of the TiN electrode in the scanning transmission electron microscopy image.

[0021] The method for processing a transmission sample of a semiconductor chip structure by using a focused ion beam according to an embodiment of the present application improves the thinning process on the basis of the conventional transmission sample preparation process, realizes uniform thickness processing of the semiconductor chip structure, particularly optimizes the cutting method for the tungsten pillar structure, and provides multiple reference bases for judging the thickness of the observed thin slice. Thereby, efficient preparation work of the transmission sample of the semiconductor chip structure is realized, and high-resolution transmission images of key device structures are obtained.

[0022] Additional aspects and advantages of the present application will be given in part in the following description, become apparent in part from the following description, or be understood through the practice of the present application. Description of the Drawings

[0023] The above-mentioned and / or additional aspects and advantages of the present application will become apparent and be easily understood from the following description of the embodiments in conjunction with the drawings, where:

[0024] Figure 1 is a schematic cross-sectional view of a multi-layer chip structure;

[0025] Figure 2 is a schematic diagram of the tungsten pillar bulge phenomenon during ion beam cutting;

[0026] Figure 3 is a flowchart of a method for processing a transmission sample of a semiconductor chip structure by using a focused ion beam according to an embodiment of the present application;

[0027] Figure 4 is a schematic diagram of the gap thinning method according to an embodiment of the present application;

[0028] Figure 5 is a schematic diagram of the test result of the effectiveness of the sample preparation method according to an embodiment of the present application;

[0029] Figure 6 is a schematic diagram of judging the thickness of the cut thin area by using the cross-sectional diameter of the tungsten pillar according to an embodiment of the present application;

[0030] Figure 7 is an SEM image under 3 kV voltage according to an embodiment of the present application;

[0031] Figure 8STEM image at 30 kV voltage provided according to an embodiment of the present application;

[0032] Figure 9 Low-magnification view of the resistive switching device structure observed by a transmission electron microscope provided according to an embodiment of the present application;

[0033] Figure 10 High-resolution observation image of the HfOx thin film in the resistive switching device structure provided according to an embodiment of the present application. Detailed implementation manners

[0034] The embodiments of the present application will be described in detail below. Examples of the embodiments are shown in the accompanying drawings, where the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below by referring to the accompanying drawings are exemplary and are intended to explain the present application, and should not be construed as a limitation to the present application.

[0035] The semiconductor chip structure includes various complex structures and materials with different sputtering rates, such as Figure 1 shown. In the conventional chip manufacturing process, first, the underlying transistor structure is fabricated on a silicon substrate; then, connection layers with different circuit structures are sequentially stacked; the connection layers are electrically interconnected through vias, and an insulating electrolyte is used for electrical signal isolation; a special functional device structure, such as the resistive switching device layer shown in the figure, is processed on the top of the via. Each device structure with an electrical function is realized by materials with different properties: the connection layer is made of copper or aluminum, the connection via is filled with tungsten metal, the electrolyte generally uses silicon dioxide, and the final special functional device structure may be made of multiple materials, Figure 1 The constituent materials of an exemplary resistive switching device are a titanium nitride electrode stacked with a tantalum oxide oxygen storage layer and a hafnium oxide resistive switching layer. When performing microstructure testing on the semiconductor chip structure, it is necessary to perform detailed characterization on the special functional device structure to obtain effective test results to assist in process improvement design. However, when using the conventional sample preparation method, a phenomenon of tungsten pillar structure protrusion will occur, as Figure 2 shown. Since the special functional structure is located vertically above the tungsten pillar, when the tungsten pillar protrudes, the special functional structure cannot be sufficiently cut by the ion beam, resulting in an excessive thickness of the final test sample, and high-resolution images of the special functional structure cannot be observed by the transmission electron microscope. The cutting method designed in the present application can effectively remove the protruding part of the tungsten pillar and achieve uniform cutting of the cross-section of the overall semiconductor chip structure.

[0036] Figure 3 Flowchart of a method for processing a transmission sample of a semiconductor chip structure using a focused ion beam provided according to an embodiment of the present application.

[0037] As Figure 1 shown, the method for processing a transmission sample of a semiconductor chip structure using a focused ion beam includes the following steps:

[0038] Step S101: Obtain a sample thin slice of the cross-section of a semiconductor chip.

[0039] Step S102: Select cutting parameters to perform multiple rough cuts and multiple fine cuts on the sample thin slice, and process the sample thin slice to a set thickness. Among them, during the rough cut, when the thickness of the sample thin slice is the reduced pressure thickness, reduce the ion beam voltage during cutting and continue cutting. And during both the rough cut and the fine cut, use the two-step cutting method and the gap thinning method for cutting.

[0040] The key of the transmission sample preparation method designed in the embodiment of the present application lies in removing the protruding part of the tungsten column by means of gap thinning during the rough cut, as Figure 4 shown. The similarities and differences between the transmission sample preparation method designed in the embodiment of the present application and the conventional sample preparation method are as follows: The similarity is that the same method is used for the sample preparation process from Step 1 to Step 3. The difference lies in the processing methods and parameter selections of Step 4 and Step 5. There are two cutting modes of a focused ion beam microscope, one is the step-by-step cutting mode, and the other is the overall cutting mode. During the focused ion beam processing, the main parameters that need to be set include selecting the cutting position, ion beam voltage and beam current, cutting mode, cutting depth or cutting time, and cutting angle.

[0041] The rough cut process includes four steps:

[0042] Rough cut Step 1: At a first distance from the center of the tungsten column of the sample thin slice, adopt the step-by-step cutting mode, and process the sample thin slice to a first preset thickness with a first ion beam voltage and current, a first cutting angle, and a first cutting depth, where the first cutting depth is greater than the target processing depth.

[0043] Specifically, the cutting position is set at 500 nanometers from the center of the tungsten column. Adopt the step-by-step cutting mode. The ion beam voltage and current are set to 30 KV and 80 pA. The thin slice is tilted 2 degrees relative to the incident direction of the ion beam. The cutting depth is increased by 2 micrometers on the basis of the target depth. First, perform front-side thinning, and then perform back-side thinning to process the thickness of the sample thin slice to 1 micrometer.

[0044] Rough cut Step 2: At a second distance from the center of the tungsten column of the sample thin slice, adopt the step-by-step cutting mode, and perform two thinning operations with a second ion beam voltage and current, a second cutting angle, and a second cutting depth. During the second thinning, use the gap thinning method to process the thickness of the sample thin slice to a second preset thickness, where the second cutting depth is the target processing depth, and both the second ion beam voltage and current are less than the first ion beam voltage and current, and the second distance is less than the first distance.

[0045] Specifically, the cutting position is set at 230 nanometers from the center of the tungsten pillar. A step-by-step cutting mode is adopted. The ion beam voltage and current are set to 16 KV and 50 pA. The thin slice is tilted 1.2 degrees relative to the incident direction of the ion beam, and the cutting depth is the target processing depth. To ensure the uniform thickness at the top and bottom of the observation area, the embodiment of the present application designs a gap thinning method. As Figure 4 shown, outside the final plane of the first thinning process, a second thinning process area is set at a position maintaining a spacing of 150 - 200 nm. Using the same ion beam voltage and current, step-by-step thinning is performed, and the thinning depth is set to the target value minus 1 um. First, front-side thinning is carried out, and then back-side thinning is carried out to process the thickness of the sample thin slice to 460 nanometers.

[0046] Rough cutting step 3: At the third distance from the center of the tungsten pillar of the sample thin slice, two thinning operations are carried out using the third ion beam voltage and current. In the second thinning, the gap thinning method is used to process the thickness of the sample thin slice to the third preset thickness, where the third distance is less than the second distance, and the second ion beam voltage and current are less than the first ion beam current.

[0047] Specifically, the cutting position is set at a position 160 nm from the center of the tungsten pillar. Thinning is carried out with the parameters of ion beam voltage and current of 16 KV and 11 pA. The two thinning methods described in the above steps are repeated. The second thinning uses the gap thinning method, which can effectively remove the convex part of the tungsten pillar at the final thinning surface and ensure the thickness uniformity of the observation thin area. As Figure 4 shown. First, front-side thinning is carried out, and then back-side thinning is carried out to process the thickness of the sample thin slice to 320 nanometers.

[0048] Rough cutting step 4: At the fourth distance from the center of the tungsten pillar of the sample thin slice, two thinning operations are carried out using the fourth ion beam voltage and current to process the thickness of the sample thin slice to the third preset thickness, where the fourth distance is less than the third distance, and the fourth ion beam voltage and current are both less than the third ion beam voltage and current.

[0049] Specifically, the cutting position is set at a position 100 nm from the center of the tungsten pillar. Thinning is carried out with the parameters of ion beam voltage and current of 8 V and 12 pA. The two thinning methods described in the above steps are repeated. First, front-side thinning is carried out, and then back-side thinning is carried out to process the thickness of the sample thin slice to 200 nanometers.

[0050] The fine cutting process includes four steps:

[0051] Fine cutting step 1: At the fifth distance from the center of the tungsten column of the sample thin slice, adopt a step-by-step cutting mode, and perform two thinning operations with the fifth ion beam voltage and current, the first cutting angle, and the third cutting depth. In the second thinning operation, use the gap thinning method to process the thickness of the sample thin slice to the fifth preset thickness. Among them, the fifth distance is less than the fourth distance, both the fifth ion beam voltage and current are less than the fourth ion beam voltage and current, and the third cutting depth is less than the target processing depth.

[0052] Specifically, the cutting position is set at 50 nanometers from the center of the tungsten column. Adopt a step-by-step cutting mode. The ion beam voltage and current are set to 5 KV and 15 pA. The thin slice is tilted 2 degrees relative to the ion beam incident direction. The cutting depth is increased by 1 micron based on the target depth. The second thinning uses the gap thinning method. First, perform front-side thinning, and then perform back-side thinning to process the thickness of the sample thin slice to 100 nanometers.

[0053] Fine cutting step 2: At the sixth distance from the center of the tungsten column of the sample thin slice, perform two thinning operations with the sixth ion beam voltage and current. In the second thinning operation, use the gap thinning method to process the thickness of the sample thin slice to the sixth preset thickness. Among them, the sixth distance is less than the fifth distance, and the sixth ion beam current is less than the fifth ion beam current.

[0054] Specifically, the cutting position is set at a position 25 nm from the center of the tungsten column. Use the parameters of ion beam voltage and current of 5 KV and 9 pA for thinning. Repeat the two thinning methods described in the above steps. First, perform front-side thinning, and then perform back-side thinning to process the thickness of the sample thin slice to 50 nanometers.

[0055] Fine cutting step 3: Adopt an overall cutting mode, with the seventh ion beam voltage and current, the third cutting angle, and the first cutting time. The cutting position is set to the entire observation area visible under the ion beam field of view. First, perform front-side thinning, and then perform back-side thinning to process the thickness of the sample thin slice to the seventh preset thickness. Among them, the seventh ion beam voltage is less than the sixth ion beam voltage.

[0056] Specifically, adopt an overall cutting mode. The ion beam voltage and current are set to 2 KV and 9 pA. The thin slice is tilted 3.5 degrees relative to the ion beam incident direction. The cutting position is set to the entire observation area visible under the ion beam field of view. The cutting time is set to 20 s. First, perform front-side thinning, and then perform back-side thinning to process the thickness of the sample thin slice to 30 nanometers.

[0057] Fine cutting step 4: Adopt an overall cutting mode, with the eighth ion beam voltage and current. First, perform front-side thinning, and then perform back-side thinning to process the thickness of the sample thin slice to the set thickness. Among them, the eighth ion beam current is less than the seventh ion beam current.

[0058] Specifically, in the overall cutting mode, the ion beam voltage and current are set to 2 KV and 7 pA. Repeat the thinning method described in the above steps. First, perform front-side thinning, and then perform back-side thinning to process the sample thin slice to a thickness of 20 nanometers.

[0059] Figure 5 The test results shown demonstrate the effectiveness of the sample preparation method of the embodiments of the present application. Figure 5 As shown in a of, the chip structure processed by the conventional sample preparation method is shown. Among them, the tungsten pillar structure shows a convex trend from top to bottom, and a step is formed between the bottom position and the surrounding silica material, as indicated by the arrow in the figure. By using the sample preparation method designed in the embodiments of the present application, the convex phenomenon of the tungsten pillar can be effectively reduced, as Figure 5 shown in b of. The thickness of the tungsten pillar remains consistent at the top and bottom, and the final surface of the ion beam cutting uniformly advances to the hole in the center of the tungsten pillar, as indicated by the position marked by the white arrow.

[0060] In step S103, use the thickness judgment method to evaluate whether the set thickness meets the preset requirements. When the set thickness meets the preset requirements, a transmission sample of the semiconductor chip structure is obtained.

[0061] During the process of using a focused ion beam to process a transmission sample of a semiconductor chip structure, evaluate the thickness of the sample thin slice to determine whether it meets the requirements. The embodiments of the present application provide three evaluation methods, namely: evaluating the thickness of the sample thin slice by using the cross-sectional diameter of the tungsten pillar during the cutting process; evaluating the thickness by using the contrast of the Pt deposition layer and the aluminum metal connection layer in the secondary electron scanning image; evaluating the thickness by using the morphology of the Pt deposition layer and the contrast of the TiN electrode in the scanning transmission electron microscopy image. When evaluating, one or more of them can be selected.

[0062] Specifically, use the diameter length of the tungsten pillar on the cutting section to judge the thickness of the thinned area. There is a fixed geometric relationship between the cross-sectional diameter of the tungsten pillar and the thickness of the thinned area, as Figure 6 shown. The diameter of the tungsten pillar is 350 nm, the thickness of the thinned area is d, and the cross-sectional diameter of the tungsten pillar is . The relationship among these three values satisfies the Pythagorean theorem. After the third rough cutting step, the thickness of the thinned area is 320 nanometers, and at this time, the tungsten pillar structure just exposes the cross-section. After the fourth rough cutting step, the thickness of the thinned area is 200 nm. According to the Figure 6 shown geometric relationship, the cross-sectional diameter of the tungsten pillar can be estimated to be 287 nanometers. After the second fine cutting step, the thickness of the thinned area is 50 nm, and the cross-sectional diameter of the tungsten pillar is 346 nanometers.

[0063] Judging the thickness of the thin region by the contrast of the semiconductor chip structure in the low-voltage secondary electron scanning image. After completing the fine cutting step 4, the thickness of the cut thin region is about 20 - 30 nanometers. The method to judge whether this thickness range is reached is to set the electron gun voltage to 3 kV and set the contrast of the secondary electron scanning image (SEM) to automatic contrast adjustment. At this time, the deposited protective layer material Pt on the sample surface shows a transparent contrast, as shown in Figure 7 at position 1 therein. The metal layer Al material connected to the bottom of the tungsten column also shows a transparent contrast, as shown in Figure 7 at position 2 therein.

[0064] Judging the thickness of the cut thin region by the contrast of the scanning transmission electron microscopy image at high voltage. The method to judge whether the 20 - 30 nanometer thickness range is reached is to set the electron gun voltage to 30 kV and set the contrast of the scanning transmission electron microscopy image (STEM) to automatic contrast adjustment. At this time, the deposited protective layer material Pt on the sample surface shows a clear granular morphology, as shown in Figure 8 at position 3 therein. The contrast of the TiN electrode in the resistive switching device structure at the top of the tungsten column is gray, as shown in Figure 8 at position 4 therein. The preparation method designed by the present invention adopts the following thinning parameters in the fine cutting process of step 5.

[0065] According to the above method, during the process of preparing the transmission sample of the resistive switching device (TiN / TaOx / HfOx / TiN) on the tungsten column structure, the sample thickness was judged in real time, and the transmission electron microscopy observation results as shown in Figure 9 and 10 were obtained. As shown in Figure 10 , the lattice structure of the thinnest 8 nm HfOx layer in the resistive switching device stack was clearly presented in the high-resolution image, that is, Figure 9 the area marked by the white frame in the figure.

[0066] The method for preparing the transmission sample of the semiconductor chip structure designed in the embodiment of the present application makes targeted improvements on the basis of the conventional transmission sample preparation process, can realize the preparation of the transmission sample of the semiconductor chip structure with a thickness of 20 nm, and provides a basis for judging the thickness of the cutting area during the sample preparation process. By using the sample preparation method of the embodiment of the present application, a cut thin slice with uniform thickness can be realized, and the sample thickness meets the harsh requirements of high-resolution transmission electron microscopy observation, improving the sample preparation efficiency and the quality of the observation results. With the sample preparation method of the embodiment of the present application, it can be widely applied to semiconductor process research and microscopic test research of special devices, meeting the test requirements of the industrial and academic circles in process monitoring and new device development research.

[0067] The sample preparation method designed by this design has the following advantages:

[0068] First, uniform cutting of different materials can be achieved on the cross-section of a semiconductor chip to obtain a transmission electron microscope (TEM) sample with uniform thickness. After rough-cutting the sample thickness to less than 1 micron, a subsequent cutting process is carried out using an ion beam voltage of 16 kV, instead of the 30 kV ion beam voltage used in conventional thinning. Selecting a 16 kV voltage can effectively reduce the difference in sputtering rates between different materials and narrow the thickness difference between different materials after cutting. In addition, a two-step cutting method is used for the cutting process of each parameter. During the second cutting, the gap cutting method is used to set the cutting position, and the main cutting area is set in the raised tungsten column area with a low sputtering rate to ensure that the tungsten column area is sufficiently thinned and to protect the high-sputtering-rate silicon dioxide area from being over-cut. In addition, during the second cutting, a cutting depth reduced by 1 micron is adopted. Such a design can fully cut the structure at the bottom of the thin slice, avoid the situation where the bottom thickness is higher than the top thickness, and ensure the uniformity of the thickness of the cut thin area.

[0069] Second, the sample preparation efficiency of using an ion beam microscope to prepare a transmission sample of a semiconductor chip structure can be improved, and the quality of the TEM test image can be enhanced. Three methods for evaluating the thickness of a transmission sample of a semiconductor chip structure are proposed. The first method uses the diameter of the tungsten column cross-section to evaluate the thickness during the cutting process, and can precisely control the cutting progress in the thickness range from 350 nm to 50 nm. The second method uses the contrast of the Pt deposition layer and the aluminum metal connection layer in the SEM image to evaluate the thickness, and the third method uses the morphology of the Pt deposition layer and the contrast of the TiN electrode in the STEM image to evaluate the thickness, and can precisely control the cutting progress in the thickness range from 50 nm to 20 nm. Whether the thickness of the transmission sample can reach this range directly determines the quality of the TEM observation image. Using the evaluation method designed in the present invention can improve the success rate of single sample preparation and provide a guarantee for obtaining high-quality high-resolution images.

[0070] In the description of this specification, the descriptions referring to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or N embodiments or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0071] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include at least one such feature. In the description of the present application, the meaning of "N" is at least two, such as two, three, etc., unless otherwise specifically defined.

[0072] Any process or method description in a flowchart or otherwise described herein may be understood to represent a module, segment, or portion of code including one or more N executable instructions for implementing a customized logical function or process, and the scope of the preferred embodiments of the present application includes additional implementations in which functions may be performed in a substantially simultaneous manner or in a reverse order according to the functions involved, rather than in the order shown or discussed, which should be understood by those skilled in the art to which the embodiments of the present application pertain.

Claims

1. A method for processing a transmission sample of a semiconductor chip structure using a focused ion beam, characterized in that, It includes the following steps: Obtain a sample thin slice of the cross-section of a semiconductor chip; Select cutting parameters to perform multiple rough cuts and multiple fine cuts on the sample thin slice, and process the sample thin slice to a set thickness. Wherein, during the rough cut, when the thickness of the sample thin slice is the reduced pressure thickness, reduce the ion beam voltage during cutting and continue cutting, and in both the rough cut and the fine cut, use the two-step cutting method and the gap thinning method for cutting; Use a thickness judgment method to evaluate whether the set thickness meets the preset requirements. When the set thickness meets the preset requirements, obtain a transmission sample of the semiconductor chip structure; The multiple rough cuts include: Rough cut step 1: At a first distance from the center of the tungsten column of the sample thin slice, adopt a step-by-step cutting mode, and process the sample thin slice to a first preset thickness with a first ion beam voltage and current, a first cutting angle, and a first cutting depth, wherein the first cutting depth is greater than the target processing depth; Rough cut step 2: At a second distance from the center of the tungsten column of the sample thin slice, adopt a step-by-step cutting mode, and perform two thinning operations with a second ion beam voltage and current, a second cutting angle, and a second cutting depth. In the second thinning operation, use the gap thinning method to process the thickness of the sample thin slice to a second preset thickness, wherein the second cutting depth is the target processing depth, the second ion beam voltage and current are both less than the first ion beam voltage and current, and the second distance is less than the first distance; Rough cut step 3: At a third distance from the center of the tungsten column of the sample thin slice, perform two thinning operations with a third ion beam voltage and current. In the second thinning operation, use the gap thinning method to process the thickness of the sample thin slice to a third preset thickness, wherein the third distance is less than the second distance, and the second ion beam voltage and current are less than the first ion beam current; Rough cut step 4: At a fourth distance from the center of the tungsten column of the sample thin slice, perform two thinning operations with a fourth ion beam voltage and current, and process the thickness of the sample thin slice to the third preset thickness, wherein the fourth distance is less than the third distance, and the fourth ion beam voltage and current are both less than the third ion beam voltage and current.

2. The method according to claim 1, wherein The multiple fine cuts include: Fine cut step 1: At a fifth distance from the center of the tungsten column of the sample thin slice, adopt a step-by-step cutting mode, and perform two thinning operations with a fifth ion beam voltage and current, a first cutting angle, and a third cutting depth. In the second thinning operation, adopt the gap thinning method to process the thickness of the sample thin slice to a fifth preset thickness, wherein the fifth distance is less than the fourth distance, the fifth ion beam voltage and current are both less than the fourth ion beam voltage and current, and the third cutting depth is less than the target processing depth; Fine cut step 2: At a sixth distance from the center of the tungsten column of the sample thin slice, perform two thinning operations with a sixth ion beam voltage and current. In the second thinning operation, adopt the gap thinning method to process the thickness of the sample thin slice to a sixth preset thickness, wherein the sixth distance is less than the fifth distance, and the sixth ion beam current is less than the fifth ion beam current; Fine cutting step 3: Adopt the overall cutting mode. With the seventh ion beam voltage and current, the third cutting angle, the first cutting time, set the cutting position to the entire observation area visible under the ion beam field of view. First, perform front-side thinning, and then perform back-side thinning to process the thickness of the sample thin slice to the seventh preset thickness, where the seventh ion beam voltage is less than the sixth ion beam voltage; Fine cutting step 4: Adopt the overall cutting mode. With the eighth ion beam voltage and current, first perform front-side thinning, and then perform back-side thinning to process the thickness of the sample thin slice to the set thickness, where the eighth ion beam current is less than the seventh ion beam current.

3. The method according to claim 2, wherein The rough cutting step 1 specifically includes: Set the cutting position at 500 nanometers away from the center of the tungsten column. Adopt the step-by-step cutting mode. Set the first ion beam voltage and current to 30 KV and 80 pA. Tilt the sample thin slice 2 degrees relative to the ion beam incident direction. Increase the first cutting depth by 2 micrometers based on the target processing depth. After performing front-side thinning, then perform back-side thinning to process the thickness of the sample thin slice to 1 micrometer; The rough cutting step 2 specifically includes: Set the cutting position at 230 nanometers away from the center of the tungsten column. Adopt the step-by-step cutting mode. Set the second ion beam voltage and current to 16 KV and 50 pA. Tilt the sample thin slice 1.2 degrees relative to the ion beam incident direction. The second cutting depth is the target processing depth. Perform the first thinning process. At a position with a spacing of 150 - 200 nm outside the final plane of the first thinning process, set the second thinning processing area. Adopt the same ion beam voltage and current, and perform step-by-step thinning. Set the thinning depth to the target value minus 1 um. First, perform front-side thinning, and then perform back-side thinning to process the thickness of the sample thin slice to 460 nanometers.

4. The method according to claim 3, wherein The rough cutting step 3 specifically includes: Set the cutting position at 160 nm away from the center of the tungsten column. Set the third ion beam voltage and current to 16 KV and 11 pA. Perform two thinning operations on the sample thin slice. The second thinning uses the gap thinning method. First, perform front-side thinning, and then perform back-side thinning to process the thickness of the sample thin slice to 320 nanometers; The rough cutting step 4 specifically includes: Set the cutting position at 100 nm away from the center of the tungsten column. The fourth ion beam voltage and current are 8 KV and 12 pA. Perform two thinning operations on the sample thin slice. The second thinning uses the gap thinning method. First, perform front-side thinning, and then perform back-side thinning to process the thickness of the sample thin slice to 200 nanometers.

5. The method according to claim 4, characterized in that, The fine cutting step 1 specifically includes: Set the cutting position at 50 nanometers away from the center of the tungsten column. Adopt the step-by-step cutting mode. Set the fifth ion beam voltage and current to 5 KV and 15 pA. Tilt the sample thin slice 2 degrees relative to the ion beam incident direction. Increase the third cutting depth by 1 micrometer on the basis of the target processing depth and perform two thinning operations. The second thinning uses the gap thinning method. First, perform front-side thinning, and then perform back-side thinning to process the thickness of the sample thin slice to 100 nanometers.

6. The method according to claim 5, wherein The specific steps of the fine cutting step 2 include: Set the cutting position at a position 25 nm away from the center of the tungsten column. The voltage and current of the sixth ion beam are 5 KV and 9 pA. Perform two thinning operations. The second thinning operation uses the gap thinning method. First, perform front-side thinning, and then perform back-side thinning. Process the thickness of the sample thin sheet to 50 nanometers.

7. The method according to claim 6, characterized in that, The specific steps of the fine cutting step 3 include: Adopt the overall cutting mode. Set the voltage and current of the seventh ion beam to 2 KV and 9 pA. Tilt the sample thin sheet by 3.5 degrees relative to the incident direction of the ion beam. Set the cutting position to the entire observation area visible under the ion beam field of view. Set the first cutting time to 20 s. First, perform front-side thinning, and then perform back-side thinning. Process the thickness of the sample thin sheet to 30 nanometers.

8. The method according to claim 7, characterized in that The specific steps of the fine cutting step 4 include: Adopt the overall cutting mode. Set the voltage and current of the eighth ion beam to 2 KV and 7 pA. First, perform front-side thinning, and then perform back-side thinning. Process the thickness of the sample thin sheet to 20 nanometers.

9. The method according to claim 1, wherein The thickness judgment method is selected from one or more of evaluating the thickness of the sample thin sheet by using the cross-sectional diameter of the tungsten column during cutting processing, evaluating the thickness by using the contrast of the Pt deposition layer and the aluminum metal connection layer in the secondary electron scanning image, and evaluating the thickness by using the morphology of the Pt deposition layer and the contrast of the TiN electrode in the scanning transmission electron microscopy image.

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