CN116380577B - Method for processing semiconductor chip structure transmission sample by utilizing focused ion beam - Google Patents
Method for processing semiconductor chip structure transmission sample by utilizing focused ion beamInfo
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- CN116380577B CN116380577B CN202310215943.5A CN202310215943A CN116380577B CN 116380577 B CN116380577 B CN 116380577B CN 202310215943 A CN202310215943 A CN 202310215943A CN 116380577 B CN116380577 B CN 116380577B
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- G01N23/00—Investigating or analysing materials by the use of wave or particle radiation, e.g. X-rays or neutrons, not covered by groups G01N3/00 – G01N17/00, G01N21/00 or G01N22/00
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N1/00—Sampling; Preparing specimens for investigation
- G01N1/28—Preparing specimens for investigation including physical details of (bio-)chemical methods covered elsewhere, e.g. G01N33/50, C12Q
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Abstract
The application discloses a method for processing a transmission sample of a semiconductor chip structure by utilizing a focused ion beam, which is improved on the basis of a conventional transmission sample preparation flow by adopting a thinning flow, adopts a two-step cutting method and a clearance cutting method when cutting processing is carried out by adopting specific parameters, realizes uniform thickness processing of the semiconductor chip structure, particularly carries out optimal design on a cutting mode of a tungsten column structure, and provides various reference bases for judging the thickness of an observation sheet. Efficient transmissive sample preparation work of semiconductor chip structures can be achieved and high resolution transmissive images of critical device structures are obtained.
Description
Technical Field
The application relates to the technical field of sample preparation, in particular to a method for processing a transmission sample of a semiconductor chip structure by utilizing a focused ion beam.
Background
With the development of semiconductor manufacturing technology, the transistor structure process in the chip has been reduced to 7nm, and industrialization of the 3nm process is also steadily advancing. Whether the actual structure of the chip accords with the design structure or not can only be verified and tested by a microstructure characterization means of nanometer scale. This requires transmission electron microscope sample preparation and microstructure characterization at the nanoscale for semiconductor chips. The key of the whole test process is that the transmission electron microscope sample preparation for precisely positioning the semiconductor chip ensures that the target structure is in the range of the observation thin area, the thickness of the whole observation area is uniform, the complex internal structure of the semiconductor chip is ensured to be clearly presented, in addition, the observation thickness of a film layer with a specific structure is required to be smaller than 50nm, and the observed components and structure information are ensured to provide optimal reference data for process optimization.
The existing transmission sample preparation scheme adopting a focused ion beam microscope is designed aiming at a block sample with uniform materials. The method generally comprises the following five steps of (1) depositing a protective layer, namely depositing a layer of protective material with the thickness of 2 microns on the surface of a target processing area by using an electron beam or ion beam deposition method, (2) cutting a sheet, namely processing grooves with the depth of more than 10 microns on two sides of the target area to enable the target area to be a sheet with the thickness of 1-2 microns, (3) extracting and fixing, namely processing a cutting groove around the sheet, welding the sheet on a nanometer hand, transferring the sheet to a special supporting net for transmission electron microscope sample preparation, (4) roughly cutting, namely adopting an ion beam with the level of hundreds of pA to tens of pA under the voltage of 30kV to thin the sheet until the thickness of the sheet is 80-100 nanometers, and (5) finely cutting, namely adopting an ion beam with the thickness of 50-100pA under the voltage of 5kV to thin the sheet until the thickness of the sheet is less than 50 nm.
When the conventional transmission sample preparation process is adopted to prepare the sample of the semiconductor chip structure, the following defects exist:
First, the thin region of the semiconductor chip structure is not uniform in thickness. The semiconductor chip structure prepared by the standard flow sheet process generally comprises 4-6 layers of metal connecting layers, copper or aluminum is used as a metal material for electrode connection, a metal tungsten column is used as a filling material of an interlayer through pipe, silicon dioxide is used as an insulating material between circuits, and other various functional materials for forming a specific device structure exist. The different materials have larger sputtering rates, so that when the conventional transmission sample preparation method is adopted for processing, larger difference appears in the cut thickness of different areas, and uneven thickness exists in different material positions of the observation thin area, so that the final transmission electron microscope observation effect is affected.
Second, quantification criteria for determining the thickness of thin regions are lacking in the preparation of transmissive samples using focused ion beams. After the transmission sample is processed, whether the thickness of the thin area meets the requirement of transmission electron microscope observation or not can obtain high-quality high-resolution images, and the problem can only be judged by experience of a sample producer or can be finally concluded by the observation of a final transmission electron microscope. Conventional experience relies on the transparency of the thin region at low voltages, 5kv or 3kv, but for good and poor conductivity samples it is difficult to make accurate determinations by such basis.
Third, it is difficult to obtain high resolution images of the top structure of the tungsten pillars using conventional sampling methods. Since the sputtering rate of the gallium ion beam for cutting the tungsten pillar is lower than half the rate of cutting silicon (the sputtering rate of tungsten is 11.21 and the sputtering rate of silicon is 27.61), when the silicon substrate and the silicon oxide insulating material which are the main materials of the semiconductor chip are cut, only half the thickness of the tungsten pillar which is the filling pipe is cut, and thus the device structure made on the upper layer of the tungsten pillar cannot be sufficiently thinned. These structures do not give good quality high resolution images due to excessive thickness during final transmission electron microscopy. However, the device structure fabricated on these tungsten pillars is generally a novel device structure designed in the process technology development and scientific research and exploration work, and is the key point of technical progress.
Disclosure of Invention
The application provides a method for processing a transmission sample of a semiconductor chip structure by utilizing a focused ion beam, which can realize accurate positioning sample preparation of a 50nm scale, performs thinning processing of uniform thickness on heterogeneous materials in a vertical direction, and provides a reference standard for judging the thickness of a thin area so as to obtain a high-quality transmission electron microscope observation result.
The embodiment of the application provides a method for processing a semiconductor chip structure transmission sample by utilizing a focused ion beam, which comprises the following steps of obtaining a sample sheet of a semiconductor chip section, selecting a cutting parameter to perform rough cutting and fine cutting on the sample sheet for a plurality of times, and processing the sample sheet to a set thickness, wherein in rough cutting, when the thickness of the sample sheet is reduced pressure, ion beam voltage in cutting is reduced to continue cutting, in rough cutting and fine cutting, a two-step cutting method and a gap thinning method are utilized to perform cutting, a thickness judging method is utilized to evaluate whether the set thickness meets a preset requirement, and when the set thickness meets the preset requirement, the semiconductor chip structure transmission sample is obtained.
Optionally, in one embodiment of the present application, the multiple rough cutting includes a rough cutting step1 of processing the sample sheet to a first preset thickness with a first ion beam voltage and current at a first distance of the sample sheet from the center of the tungsten column, a first cutting angle, a first cutting depth being greater than a target processing depth, a rough cutting step 2 of processing the sample sheet to a second preset thickness with a second ion beam voltage and current at a second distance of the sample sheet from the center of the tungsten column, a second cutting angle, a second cutting depth being reduced by a gap reduction method in a second time, wherein the second cutting depth is the target processing depth, the second ion beam voltage and current are both smaller than the first ion beam voltage and current, the second distance is smaller than the first distance, a rough cutting step 3 of processing the sample sheet to a third preset thickness with a third ion beam voltage and current at a third distance of the sample sheet from the center of the tungsten column, a third ion beam voltage and current is reduced by a third distance of the third ion beam voltage and current, a fourth distance is reduced by a gap reduction method in a second time, the sample sheet thickness is processed to a third preset thickness, the fourth beam voltage and current are both less than the third beam voltage and current.
Optionally, in one embodiment of the present application, the multiple fine cutting includes a fine cutting step 1 of performing two-time thinning with a fifth ion beam voltage and current at a fifth distance of the sample sheet from the center of the tungsten column, a first cutting angle, and a third cutting depth, and performing thickness processing on the sample sheet to a fifth preset thickness by using the gap thinning method in the second thinning, wherein the fifth distance is smaller than the fourth distance, the fifth ion beam voltage and current are both smaller than the fourth ion beam voltage and current, the third cutting depth is smaller than a target processing depth, a fine cutting step 2 of performing two-time thinning with a sixth ion beam voltage and current at a sixth distance of the sample sheet from the center of the tungsten column, performing a fifth ion beam current to be smaller than the fifth preset thickness, performing overall cutting with a seventh ion beam current to be smaller than the fifth preset thickness, and performing a full-cut with a seventh ion beam voltage and current, and performing a full-cut with a fifth ion beam voltage and current setting, and a full cut with a full cut angle of the sample sheet being cut, and performing a full cut with a full cut angle of the fifth ion beam voltage and a full cut with a full cut angle being set up to a full cut angle, and a full cut with the ion beam voltage being set up to a full cut angle being performed in the fifth cut angle, the eighth beam current is less than the seventh beam current.
Optionally, in one embodiment of the present application, the rough cutting step 1 specifically includes setting a cutting position at a position 500 nm away from a center of the tungsten column, setting the first ion beam voltage and current to be 30KV and 80pA in a step cutting mode, tilting the sample sheet by 2 degrees relative to an incidence direction of the ion beam, increasing the first cutting depth by 2 micrometers based on the target processing depth, performing front thinning, performing back thinning, and processing the thickness of the sample sheet to 1 micrometer.
Optionally, in one embodiment of the present application, the rough cutting step 2 specifically includes setting a cutting position at 230 nm from the center of the tungsten column, setting the second ion beam voltage and current to 16KV and 50pA in a step cutting mode, tilting the sample sheet by 1.2 degrees relative to the incidence direction of the ion beam, performing a first thinning process, maintaining a position with a distance of 150-200nm outside the final plane of the first thinning process, setting a second thinning process area, performing step thinning with the same ion beam voltage and current, setting the thinning depth to a target value minus 1um, performing back thinning after front thinning, and processing the thickness of the sample sheet to 460 nm.
Optionally, in one embodiment of the present application, the rough cutting step 3 specifically includes setting a cutting position at a position 160nm away from the center of the tungsten column, setting the third ion beam voltage and current to 16KV and 11pA, thinning the sample sheet twice, performing front thinning and then performing back thinning, and processing the thickness of the sample sheet to 320 nm.
Optionally, in one embodiment of the present application, the rough cutting step 4 specifically includes setting a cutting position at a position 100nm away from the center of the tungsten column, and performing two thinning on the sample slice with the fourth ion beam voltage and current of 8KV and 12pA, and performing front thinning and then back thinning on the sample slice to a thickness of 200 nm.
Optionally, in one embodiment of the present application, the fine cutting step 1 specifically includes setting a cutting position at a position 50 nm away from the center of the tungsten column, setting the fifth ion beam voltage and current to 5KV and 15pA in a step cutting mode, tilting the sample sheet by 2 ° relative to the incidence direction of the ion beam, increasing the third cutting depth by 1 μm at the target processing depth for two times of thinning, and performing front thinning and back thinning for the second time of thinning by a gap thinning method, so as to process the thickness of the sample sheet to 100 nm.
Optionally, in one embodiment of the present application, the fine cutting step 2 specifically includes setting a cutting position at a position 25nm away from the center of the tungsten column, performing two thinning with a sixth ion beam voltage and current of 5KV and 9pA, performing front thinning and then performing back thinning, and processing the thickness of the sample sheet to 50 nm.
Optionally, in one embodiment of the present application, the fine cutting step 3 specifically includes adopting an integral cutting mode, setting the seventh ion beam voltage and current to be 2KV and 9pA, tilting the sample sheet by 3.5 degrees relative to the incidence direction of the ion beam, setting the cutting position to be the whole observation area visible under the field of view of the ion beam, setting the first cutting time to be 20s, performing front thinning, performing back thinning, and processing the thickness of the sample sheet to 30 nm.
Optionally, in one embodiment of the present application, the fine cutting step 4 specifically includes adopting a whole cutting mode, setting the eighth ion beam voltage and current to 2KV and 7pA, performing front thinning, performing back thinning, and processing the thickness of the sample sheet to 20 nm.
Alternatively, in one embodiment of the present application, the thickness determination method is selected from one or more of evaluating the thickness of the sample sheet during the dicing process using a tungsten column cross-sectional diameter, evaluating the thickness using a Pt deposition layer and an aluminum metal connection layer contrast in a secondary electron scan image, and evaluating the thickness using a morphology of the Pt deposition layer and a TiN electrode contrast in a scanning transmission electron microscopic image.
The method for processing the transmission sample of the semiconductor chip structure by utilizing the focused ion beam 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 mode of the tungsten column structure, and provides various reference bases for judging the thickness of the observation sheet. Thus, efficient transmissive sample preparation work of the semiconductor chip structure is achieved, and high resolution transmissive images of critical device structures are obtained.
Additional aspects and advantages of the application will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the application.
Drawings
The foregoing and/or additional aspects and advantages of the application will become apparent and readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a schematic cross-sectional view of a multi-layer chip structure;
FIG. 2 is a schematic view showing the projection of tungsten column during ion beam cutting;
FIG. 3 is a flow chart of a method for processing a transmissive sample of a semiconductor die structure using a focused ion beam in accordance with an embodiment of the present application;
FIG. 4 is a schematic diagram of a gap thinning method according to an embodiment of the present application;
FIG. 5 is a schematic diagram of test results of the effectiveness of the sample preparation method according to an embodiment of the present application;
FIG. 6 is a schematic view of determining the thickness of a thin cut region using the cross-sectional diameter of a tungsten pillar according to an embodiment of the present application;
FIG. 7 is an SEM image at 3kV provided according to an embodiment of the application;
FIG. 8 is a STEM image at 30kV provided in accordance with an embodiment of the present application;
FIG. 9 is a low-power diagram of a structure of a transmission electron microscope observation resistance change device according to an embodiment of the application;
fig. 10 is a high resolution view of a HfOx film in a resistive device structure according to an embodiment of the present application.
Detailed Description
Embodiments of the present application are described in detail below, examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to like or similar elements or elements having like or similar functions throughout. The embodiments described below by referring to the drawings are illustrative and intended to explain the present application and should not be construed as limiting the application.
The semiconductor chip structure includes a variety of complex structures and materials of different sputter rates as shown in fig. 1. In the conventional flow sheet process, first, a transistor structure of a bottom layer is manufactured on a silicon substrate, then connecting layers of different circuit structures are sequentially overlapped, the connecting layers are electrically connected through a through pipe, electric signal isolation is performed by adopting insulating electrolyte, and a special function device structure, such as a resistance change device layer shown in the figure, is processed at the top end of the through pipe. The device structure of each electrical function is realized by adopting materials with different properties, wherein a connecting layer is made of copper materials or aluminum materials, a connecting through pipe is filled with tungsten metal, an electrolyte is generally made of silicon dioxide, the final device structure with special functions can be made of various materials, and the constituting materials of the resistive random device shown in the example of fig. 1 are titanium nitride electrodes, a tantalum oxide oxygen storage layer and a hafnium oxide resistive random layer. When the microstructure test is carried out on the semiconductor chip structure, the microstructure of the special function device needs to be carefully characterized, so that an effective test result is obtained to assist in the process improvement design. However, the conventional sample preparation method can generate the phenomenon of protrusion of the tungsten column structure, as shown in fig. 2. Because the special functional structure is positioned vertically above the tungsten column, when the tungsten column is raised, the special functional structure cannot be cut by the ion beam sufficiently, so that the thickness of the final test sample is overlarge, and a high-resolution image of the special functional structure cannot be observed by the transmission electron microscope. The cutting method designed by the application can effectively remove the raised parts of the tungsten column and realize the uniform cutting of the cross section of the whole semiconductor chip structure.
Fig. 3 is a flow chart of a method for processing a transmissive sample of a semiconductor die structure using a focused ion beam in accordance with an embodiment of the present application.
As shown in fig. 1, the method for processing a semiconductor chip structure transmission sample by using a focused ion beam comprises the following steps:
step S101, a sample sheet of a semiconductor chip cross section is obtained.
Step S102, selecting cutting parameters to perform rough cutting and fine cutting on the sample sheet for a plurality of times, and processing the sample sheet to a set thickness, wherein in rough cutting, when the thickness of the sample sheet is a reduced pressure thickness, the ion beam voltage in cutting is reduced to continue cutting, and in rough cutting and fine cutting, the two-step cutting method and the gap thinning method are used for cutting.
The key point of the preparation method of the transmission sample designed by the embodiment of the application is that the convex part of the tungsten column is removed by adopting a gap thinning mode in the rough cutting process, as shown in fig. 4. The transmission sample preparation method designed by the embodiment of the application has the same difference with the conventional sample preparation method in that the sample preparation processing flows from the step 1 to the step 3 are carried out by adopting a consistent method, and the difference is in the processing methods and parameter selection of the step 4 and the step 5. There are two cutting modes of the focusing ion beam microscope, one is a step cutting mode and the other is a whole cutting mode. In the focused ion beam processing, the main parameters to be set include the selection of the cutting position, the ion beam voltage and beam current, the cutting mode, the cutting depth or cutting time, and the cutting angle.
The rough cutting process comprises four steps:
Rough cutting step 1, processing the sample sheet to a first preset thickness by adopting a stepping cutting mode at a first distance of the sample sheet from the center of the tungsten column and a first cutting angle and a first cutting depth by adopting a first ion beam voltage and current, wherein the first cutting depth is larger than the target processing depth.
Specifically, the cutting position is set at 500 nm from the center of the tungsten column, the voltage and current of the ion beam are set to 30KV and 80pA by adopting a stepping cutting mode, the slice is tilted for 2 degrees relative to the incidence direction of the ion beam, and the cutting depth is increased by 2 microns on the basis of the target depth. Front thinning is carried out firstly, then back thinning is carried out, and the thickness of the sample slice is processed to be 1 micrometer.
Rough cutting step 2, adopting a step cutting mode at a second distance of the sample sheet from the center of the tungsten column, carrying out twice thinning at a second ion beam voltage and current at a second cutting angle and a second cutting depth, and processing the thickness of the sample sheet to a second preset thickness by using a gap thinning method in the second thinning, wherein the second cutting depth is a target processing depth, the second ion beam voltage and current are smaller than the first ion beam voltage and current, and the second distance is smaller than the first distance.
Specifically, the cutting position is set at 230 nm from the center of the tungsten column, the voltage and current of the ion beam are set to 16KV and 50pA by adopting a stepping cutting mode, the slice is tilted by 1.2 degrees relative to the incidence direction of the ion beam, and the cutting depth is the target machining depth. In order to ensure that the thicknesses of the top and bottom of the observation area are uniform, the embodiment of the present application designs a gap thinning method, as shown in fig. 4, in which, outside the final plane of the first thinning process, the position of the 150-200nm interval is maintained, a second thinning process area is set, step thinning is performed by using the same ion beam voltage and current, and the thinning depth is set to be the target value minus 1um. Front thinning is firstly carried out, then back thinning is carried out, and the thickness of the sample slice is processed to 460 nanometers.
And a rough cutting step 3, namely carrying out twice thinning by adopting third ion beam voltage and current at a third distance from the sample sheet to the center of the tungsten column, and processing the thickness of the sample sheet to a third preset thickness by utilizing a gap thinning method in the second thinning, wherein the third distance is smaller than a second distance, and the second ion beam voltage and current are smaller than the first ion beam current.
Specifically, the cutting position is set at 160nm from the center of the tungsten column, the ion beam voltage and current are adopted to carry out thinning, the thinning method described in the steps is repeated for two times, and the second thinning method adopts a gap thinning method, so that the convex part of the tungsten column on the final thinning surface can be effectively removed, and the thickness uniformity of an observation thin area is ensured, as shown in fig. 4. Front thinning is firstly carried out, then back thinning is carried out, and the thickness of the sample slice is processed to 320 nanometers.
And 4, rough cutting, namely performing twice thinning on the sample sheet at a fourth distance from the center of the tungsten column by adopting fourth ion beam voltage and current, and processing the thickness of the sample sheet to a third preset thickness, wherein the fourth distance is smaller than the third distance, and the fourth ion beam voltage and current are smaller than the third ion beam voltage and current.
Specifically, the cutting position was set at a position 100nm away from the center of the tungsten column, thinning was performed using parameters of 8v for the ion beam voltage and current, and the thinning method described in the above steps was repeated twice. Front thinning is firstly carried out, then back thinning is carried out, and the thickness of the sample slice is processed to 200 nanometers.
The fine cutting process comprises four steps:
and a fine cutting step 1, namely adopting a stepping cutting mode at a fifth distance from the center of the tungsten column to the sample sheet, adopting a fifth ion beam voltage and current, adopting a first cutting angle, adopting a third cutting depth to carry out twice thinning, and adopting a gap thinning method to process the thickness of the sample sheet to a fifth preset thickness in the second thinning, wherein the fifth distance is smaller than a fourth distance, the fifth ion beam voltage and current are both smaller than the fourth ion beam voltage and current, and the third cutting depth is smaller than the target processing depth.
Specifically, the cutting position is arranged at a position 50 nanometers away from the center of the tungsten column, the voltage and the current of the ion beam are set to be 5KV and 15pA by adopting a stepping cutting mode, the slice is tilted for 2 degrees relative to the incidence direction of the ion beam, and the cutting depth is increased by 1 micrometer on the basis of the target depth. And the second thinning adopts a gap thinning method. Front thinning is firstly carried out, then back thinning is carried out, and the thickness of the sample slice is processed to 100 nanometers.
And a fine cutting step 2, namely carrying out twice thinning by adopting a sixth ion beam voltage and a sixth ion beam current at a sixth distance from the center of the tungsten column on the sample sheet, and processing the thickness of the sample sheet into a sixth preset thickness by adopting a gap thinning method in the second thinning, wherein the sixth distance is smaller than a fifth distance, and the sixth ion beam current is smaller than the fifth ion beam current.
Specifically, the cutting position is set at a position 25nm away from the center of the tungsten column, the thinning is carried out by adopting parameters of 5KV and 9pA of ion beam voltage and current, the thinning method described in the steps is repeated twice, front thinning is carried out firstly, then back thinning is carried out, and the thickness of the sample sheet is processed to 50 nanometers.
And 3, adopting an integral cutting mode, setting a seventh ion beam voltage and current, a third cutting angle, a first cutting time and a cutting position as all observation areas visible under the field of the ion beam, firstly carrying out front thinning, then carrying out back thinning, and processing the thickness of the sample sheet to a seventh preset thickness, wherein the seventh ion beam voltage is smaller than the sixth ion beam voltage.
Specifically, the whole cutting mode is adopted, the voltage and the current of the ion beam are set to be 2KV and 9pA, the slice is tilted for 3.5 degrees relative to the incidence direction of the ion beam, the cutting position is set to be the whole observation area visible under the field of view of the ion beam, and the cutting time is set to be 20s. Front thinning is firstly carried out, then back thinning is carried out, and the thickness of the sample slice is processed to 30 nanometers.
And 4, performing fine cutting, namely adopting an integral cutting mode, adopting an eighth ion beam voltage and current, performing front thinning, performing back thinning, and processing the thickness of the sample sheet to a set thickness, wherein the eighth ion beam current is smaller than the seventh ion beam current.
Specifically, the whole cutting mode is adopted, the voltage and the current of the ion beam are set to be 2KV and 7pA, the thinning method described in the steps is repeated, front thinning is carried out, back thinning is carried out, and the thickness of the sample sheet is processed to be 20 nanometers.
The test results shown in fig. 5 show the effectiveness of the sample preparation method of the embodiment of the present application. Fig. 5 a shows a chip structure processed by a conventional sample preparation method, wherein the tungsten pillar structure has a tendency to bulge from top to bottom, and a step is formed between the bottom position and the surrounding silicon dioxide material, as indicated by the arrow in the figure. By adopting the sample preparation method designed by the embodiment of the application, the bulge phenomenon of the tungsten column can be effectively reduced, as shown in b of fig. 5, the thickness of the tungsten column is kept consistent at the top and the bottom, and the final surface of ion beam cutting is uniformly pushed to the hole in the center of the tungsten column, as shown by the marked position of the white arrow.
Step S103, whether the set thickness meets the preset requirement is evaluated by using a thickness judging method, and when the set thickness meets the preset requirement, a transmission sample of the semiconductor chip structure is obtained.
During processing of a semiconductor chip structure transmissive sample using a focused ion beam, the thickness of the sample sheet is evaluated to determine if the requirements are met. The embodiment of the application provides three evaluation modes, namely, the thickness of a sample sheet is evaluated in the cutting process by using the section diameter of a tungsten column, the thickness evaluation is performed by using the contrast of a Pt deposition layer and an aluminum metal connecting layer in a secondary electron scanning image, and the thickness evaluation is performed by using the morphology of the Pt deposition layer and the contrast of a TiN electrode in a scanning transmission electron microscopic image. One or more of these may be selected for evaluation.
Specifically, the thickness of the cut thin region is determined by the diameter length of the tungsten column on the cut section. The cross-sectional diameter of the tungsten pillars has a fixed geometric relationship with the thickness of the thin regions, as shown in fig. 6. The diameter of the tungsten column is 350nm, the thickness of the cutting thin area is d, and the section diameter of the tungsten column isThe relationship of these three values satisfies the collusion law. After the rough cutting step 3, the thickness of the thin region is 320 nanometers, and the tungsten pillar structure is just exposed in cross section. After the rough cutting step 4, the thickness of the thin region was 200nm, and the cross-sectional diameter of the tungsten pillar was estimated to be 287 nm according to the geometric relationship shown in FIG. 6. After the fine cutting step 2, the thickness of the thin region was 50nm and the cross-sectional diameter of the tungsten pillar was 346 nm.
And judging the thickness of the thin region by using the contrast of the semiconductor chip structure in the low-voltage secondary electron scanning image. After finishing the fine cutting step 4, the thickness of the cut thin region is about 20-30 nm. The method of judging whether this thickness interval is reached is to set the electron gun voltage to 3kV and set the contrast of the secondary electron Scan (SEM) to an automatically adjusted contrast, at which time the deposited protective layer material Pt on the sample surface exhibits a transparent contrast, as shown in position 1 in fig. 7. The Al material of the metal layer connected to the bottom of the tungsten pillar also exhibits transparent contrast, as shown at position 2 in fig. 7.
And judging the thickness of the cut thin area by using the contrast of the scanning transmission electron microscopic image under high voltage. The method for judging whether the thickness interval of 20-30 nanometers is reached is that the voltage of an electron gun is set to be 30kV, the contrast of a scanning transmission electron microscopic image (STEM) is set to be the automatic adjustment contrast, and at the moment, the deposited protective layer material Pt on the surface of a sample presents clear granular morphology, as shown in a position 3 in fig. 8. The lining degree of the TiN electrode in the resistive random access device structure at the top end of the tungsten column is gray, and the preparation method of the design of the invention is that the following thinning parameters are adopted in the fine cutting processing of the step 5 as shown in the position 4 in fig. 8.
According to the method, in the process of preparing a transmission sample of a resistance variable device (TiN/TaOx/HfOx/TiN) on a tungsten column structure, the thickness of the sample is judged in real time, and a transmission electron microscope observation result as shown in fig. 9 and 10 is obtained. As shown in fig. 10, the lattice structure of the thinnest 8nm HfOx layer in the resistive device stack is clearly represented in the high resolution image, i.e., the white box marked areas in the fig. 9 view.
The preparation method of the transmission sample of the semiconductor chip structure designed by the embodiment of the application is purposefully improved on the basis of the conventional preparation flow of the transmission sample, can realize the preparation of the transmission sample of the semiconductor chip structure with the thickness of 20nm, and provides a judgment basis for the thickness of a cutting area in the sample preparation process. By adopting the sample preparation method provided by the embodiment of the application, the thin slice can be cut uniformly in thickness, the thickness of the sample meets the severe requirements of high-resolution transmission electron microscope observation, and the sample preparation efficiency and the quality of an observation result are improved. The sample preparation method provided by the embodiment of the application can be widely applied to semiconductor process research and microscopic test research of special devices, and meets the test requirements of the industry and academia in process monitoring and new device development research.
The sample preparation method designed by the design has the following advantages:
Firstly, uniform cutting of different materials can be realized on the cross section of the semiconductor chip, and a transmission electron microscope sample with uniform thickness is obtained. After rough cutting the sample to a thickness below 1 micron, a subsequent cutting pass was performed with an ion beam voltage of 16kV instead of the 30kV ion beam voltage used for conventional thinning. The 16kV voltage is selected, so that the difference of sputtering rates among different materials can be effectively reduced, and the thickness difference among different materials after cutting is reduced. In addition, the cutting processing of each parameter adopts a two-step cutting method, a gap cutting method is adopted to set a cutting position in the second cutting, a main cutting area is arranged in a raised tungsten column area with low sputtering rate, the tungsten column area is ensured to be sufficiently thinned, and a silicon dioxide area with high sputtering rate is protected from being excessively cut. In addition, in the second cutting process, the cutting depth is reduced by 1 micrometer, so that the structure of the bottom of the sheet can be sufficiently cut, the condition that the thickness of the bottom is higher than that of the top is avoided, and the uniformity of the thickness of a cutting thin area is ensured.
Secondly, the sample preparation efficiency of the ion beam microscope for preparing the semiconductor chip structure transmission sample can be improved, and the quality of the transmission electron microscope test image can be improved. Three methods of assessing the thickness of a transmissive sample of a semiconductor chip structure are presented. The first method utilizes the cross-sectional diameter of the tungsten column to evaluate the thickness during the cutting process, and can accurately control the cutting progress in the thickness range of 350 to 50 nanometers. The second method utilizes the thickness evaluation of the Pt deposition layer and the aluminum metal connection layer in the SEM image, and the third method utilizes the morphology of the Pt deposition layer and the TiN electrode lining in the STEM image to carry out thickness evaluation, so that the cutting progress can be accurately controlled in the thickness range from 50 nanometers to 20 nanometers. Whether the thickness of the transmission sample can reach the interval range directly determines the quality of the transmission electron microscope observation image. The evaluation method designed by the invention can improve the success rate of single sample preparation and provide guarantee for obtaining high-quality high-resolution images.
In the description of the present specification, a description referring to terms "one embodiment," "some embodiments," "examples," "specific examples," or "some examples," etc., means that a particular feature, structure, material, or characteristic described in connection with the embodiment or example is included in at least one embodiment or example of the present application. In this specification, schematic representations of the above terms are not necessarily directed to the same embodiment or example. Furthermore, the particular features, structures, materials, or characteristics described may be combined in any suitable manner in any one or N embodiments or examples. Furthermore, the different embodiments or examples described in this specification and the features of the different embodiments or examples may be combined and combined by those skilled in the art without contradiction.
Furthermore, the terms "first," "second," and the like, are used for descriptive purposes only and are not to be construed as indicating or implying a relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defining "a first" or "a second" may explicitly or implicitly include at least one such feature. In the description of the present application, "N" means at least two, for example, two, three, etc., unless specifically defined otherwise.
Any process or method descriptions in flow charts or otherwise described herein may be understood as representing modules, segments, or portions of code which include one or more N executable instructions for implementing specific logical functions or steps of the process, and further implementations are included within the scope of the preferred embodiment of the present application in which functions may be executed out of order from that shown or discussed, including substantially concurrently or in reverse order, depending on the functionality involved, as would be understood by those reasonably skilled in the art of the embodiments of the present application.
Claims (9)
1. A method for processing a transmissive sample of a semiconductor chip structure using a focused ion beam, comprising the steps of:
Obtaining a sample slice of a semiconductor chip section;
Selecting cutting parameters to perform rough cutting and fine cutting on the sample sheet for a plurality of times, and processing the sample sheet to a set thickness, wherein in rough cutting, when the thickness of the sample sheet is reduced pressure, the ion beam voltage in cutting is reduced to continue cutting, and in rough cutting and fine cutting, a two-step cutting method and a gap thinning method are used for cutting;
evaluating whether the set thickness meets a preset requirement or not by using a thickness judging method, and obtaining a semiconductor chip structure transmission sample when the set thickness meets the preset requirement;
the multiple rough cuts include:
Rough cutting step 1, processing the sample sheet to a first preset thickness at a first distance from the center of the tungsten column by adopting a stepping cutting mode according to a first ion beam voltage and current and a first cutting angle and a first cutting depth, wherein the first cutting depth is larger than a target processing depth;
Rough cutting step 2, adopting a step-by-step cutting mode, carrying out twice thinning with a second ion beam voltage and current at a second cutting angle and a second cutting depth, and processing the thickness of the sample sheet to a second preset thickness by using a gap thinning method in the second thinning, wherein the second cutting depth is the target processing depth, the second ion beam voltage and current are smaller than the first ion beam voltage and current, and the second distance is smaller than the first distance;
rough cutting step 3, adopting third ion beam voltage and current to carry out twice thinning at a third distance from the sample sheet to the center of the tungsten column, and processing the thickness of the sample sheet to a third preset thickness by utilizing the gap thinning method in the second thinning, wherein the third distance is smaller than the second distance, and the second ion beam voltage and current are smaller than the first ion beam current;
And 4, rough cutting, namely thinning the sample sheet at a fourth distance from the center of the tungsten column by adopting a fourth ion beam voltage and a fourth ion beam current, and processing the thickness of the sample sheet to a third preset thickness, wherein the fourth distance is smaller than the third distance, and the fourth ion beam voltage and the fourth ion beam current are smaller than the third ion beam voltage and the third ion beam current.
2. The method of claim 1, wherein the plurality of fine cuts comprises:
A step 1 of fine cutting, namely, at a fifth distance from the center of the tungsten column to the sample sheet, adopting a stepping cutting mode, carrying out twice thinning by adopting a fifth ion beam voltage and current at a first cutting angle and a third cutting depth, and adopting the gap thinning method to process the thickness of the sample sheet to a fifth preset thickness in the second thinning, wherein the fifth distance is smaller than the fourth distance, the fifth ion beam voltage and current are smaller than the fourth ion beam voltage and current, and the third cutting depth is smaller than a target processing depth;
A fine cutting step 2 of carrying out twice thinning by adopting a sixth ion beam voltage and a sixth ion beam current at a sixth distance from the center of the tungsten column on the sample sheet, and processing the thickness of the sample sheet into a sixth preset thickness by adopting the gap thinning method in the second thinning, wherein the sixth distance is smaller than the fifth distance, and the sixth ion beam current is smaller than the fifth ion beam current;
A fine cutting step 3 of adopting an integral cutting mode, setting a seventh ion beam voltage and current, a third cutting angle, a first cutting time and a cutting position as all observation areas visible under the field of the ion beam, firstly carrying out front thinning, then carrying out back thinning, and processing the thickness of the sample sheet to a seventh preset thickness, wherein the seventh ion beam voltage is smaller than the sixth ion beam voltage;
and a fine cutting step 4, namely adopting an integral cutting mode, adopting an eighth ion beam voltage and current, firstly carrying out front thinning, and then carrying out back thinning, and processing the thickness of the sample sheet to the set thickness, wherein the eighth ion beam current is smaller than the seventh ion beam current.
3. The method according to claim 2, wherein the rough cutting step 1 specifically comprises:
Setting a cutting position at a position 500 nanometers away from the center of a tungsten column, adopting a stepping cutting mode, setting the voltage and the current of the first ion beam to be 30KV and 80pA, tilting the sample sheet by 2 degrees relative to the incidence direction of the ion beam, increasing the first cutting depth by 2 micrometers on the basis of the target processing depth, performing front thinning, performing back thinning, and processing the thickness of the sample sheet to 1 micrometer;
The rough cutting step 2 specifically comprises the following steps:
setting a cutting position at 230 nanometers away from the center of a tungsten column, adopting a stepping cutting mode, setting the voltage and the current of a second ion beam to be 16KV and 50pA, tilting a sample sheet by 1.2 degrees relative to the incidence direction of the ion beam, performing first thinning processing on the sample sheet, keeping the position of a 150-200nm interval outside a final plane of the first thinning processing, setting a second thinning processing area, adopting the same ion beam voltage and current, performing stepping thinning, setting the thinning depth to be a target value minus 1um, performing front thinning, performing back thinning, and processing the thickness of the sample sheet to 460 nanometers.
4. A method according to claim 3, wherein the rough cutting step 3 comprises:
setting a cutting position at a position 160nm away from the center of the tungsten column, setting the third ion beam voltage and current to be 16KV and 11pA, thinning the sample sheet twice, adopting a gap thinning method for the second thinning, firstly thinning the front surface, then thinning the back surface, and processing the thickness of the sample sheet to 320 nanometers;
The rough cutting step 4 specifically comprises the following steps:
Setting the cutting position at a position 100nm away from the center of the tungsten column, wherein the fourth ion beam voltage and current are 8KV and 12pA, thinning the sample sheet twice, thinning the front side of the sample sheet for the second time by adopting a gap thinning method, thinning the back side of the sample sheet, and processing the thickness of the sample sheet to 200 nanometers.
5. The method according to claim 4, wherein the fine cutting step 1 specifically comprises:
Setting the cutting position at 50 nanometers away from the center of the tungsten column, adopting a stepping cutting mode, setting the voltage and the current of the fifth ion beam to be 5KV and 15pA, tilting the sample sheet by 2 degrees relative to the incidence direction of the ion beam, increasing the third cutting depth by 1 micrometer on the target processing depth for two times of thinning, adopting a gap thinning method for the second time of thinning, firstly carrying out front thinning, then carrying out back thinning, and processing the thickness of the sample sheet to 100 nanometers.
6. The method according to claim 5, wherein the fine cutting step 2 specifically comprises:
Setting the cutting position at a position 25nm away from the center of the tungsten column, carrying out thinning twice by using the voltage and the current of the sixth ion beam as 5KV and 9pA, carrying out front thinning firstly and then carrying out back thinning for the second thinning by adopting a gap thinning method, and processing the thickness of the sample sheet to 50 nanometers.
7. The method according to claim 6, wherein the fine cutting step 3 specifically comprises:
And adopting an integral cutting mode, setting the voltage and the current of the seventh ion beam to be 2KV and 9pA, tilting the sample sheet by 3.5 degrees relative to the incidence direction of the ion beam, setting the cutting position to be all the observation areas visible under the field of the ion beam, setting the first cutting time to be 20s, firstly carrying out front thinning, then carrying out back thinning, and processing the thickness of the sample sheet to 30 nanometers.
8. The method according to claim 7, wherein the fine cutting step 4 specifically comprises:
And adopting an integral cutting mode, setting the voltage and the current of the eighth ion beam to be 2KV and 7pA, firstly carrying out front thinning, then carrying out back thinning, and processing the thickness of the sample sheet to 20 nanometers.
9. The method of claim 1, wherein the thickness determination method is selected from one or more of evaluating the thickness of the sample sheet during the dicing process using tungsten column cross-sectional diameter, evaluating the thickness using Pt deposition layer and aluminum metal connection layer contrast in secondary electron scan images, evaluating the thickness using the morphology of Pt deposition layer and TiN electrode contrast in scanning transmission electron microscopy images.
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