Method for preparing a sample for an electron microscope

Abstract

The present invention provides a method for preparing a sample for an electron microscope. The method includes the following steps: providing an initial sample; forming a protective layer in a region to be detected of the initial sample; cutting the initial sample along the periphery of the protective layer to obtain a sample piece with one end connected to the initial sample; taking out the sample piece; thinning the sample piece by bombarding with an ion beam, and the ion beam has an inclination angle. The above technical solution can effectively reduce the bombardment of the electron beam on the sample piece while ensuring the thinning efficiency by thinning the sample piece under a low voltage and suppressing the dispersion problem by setting the inclination angle of the ion beam, thereby reducing the possibility of generating holes and improving the imaging quality. During the process of manufacturing a sample for an electron microscope, multi-step thinning is further implemented, and the voltage and current of each step are adjusted to improve the processing rate and also reduce the damage to the sample piece.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor manufacturing, and particularly to a method for preparing a sample for an electron microscope. Background Art

[0002] In the semiconductor manufacturing industry, there are various detection devices, among which the electron microscope is an important tool for detecting the morphology and size of thin films during the manufacturing process. Commonly used electron microscopes include TEM (transmission electron microscope) and SEM (scanning electron microscope). The working principle of TEM is to place the sample to be detected in the TEM observation chamber, irradiate the sample with an electron beam accelerated by high voltage, magnify and project the sample morphology onto the screen, take pictures, and then perform analysis. A prominent advantage of TEM is its high resolution, which can observe the morphology and size of extremely thin films. TEM is a very important analytical tool in materials science research, and can perform morphology analysis, structure analysis and composition analysis of samples, and has extremely wide and increasingly important applications in the field of integrated circuit analysis. And using focused ion beam (FIB) to prepare samples is the most main TEM sample preparation method in the semiconductor field.

[0003] In the prior art, using focused ion beam (FIB) to prepare samples is the most main TEM sample preparation method in the semiconductor field. However, the conventional method of using FIB to prepare TEM samples will cause defects in semiconductor materials, and the materials will spontaneously form holes, which will affect the imaging quality. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to provide a method for preparing a sample for an electron microscope, so as to solve the problem that holes are formed in the material when using FIB to prepare the electron microscope sample.

[0005] To solve the above problems, the present invention provides a method for preparing a sample for an electron microscope, including the following steps: providing an initial sample; forming a protective layer in the area to be detected of the initial sample; cutting the initial sample along the periphery of the protective layer to obtain a sample piece with one end connected to the initial sample; taking out the sample piece; thinning the sample piece by bombarding it with an ion beam, and the ion beam has an inclination angle.

[0006] The above technical solution thins the sample wafer under low voltage and suppresses the dispersion problem by setting the inclination angle of the ion beam. It can effectively reduce the bombardment of the sample wafer by the electron beam while ensuring the thinning efficiency, thereby reducing the possibility of hole generation and improving the imaging quality. During the process of manufacturing the sample for the electron microscope, multi-step thinning is further implemented, and the voltage and current of each step are adjusted to improve the processing rate and also reduce the damage to the sample wafer. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The Figure 1 figure shows a schematic diagram of the implementation steps of the specific implementation manner of the method for preparing a sample for an electron microscope according to the present invention.

[0008] The Figures 2A to 2E figure shows a schematic process flow diagram of the specific implementation manner of the method for preparing a sample for an electron microscope according to the present invention.

[0009] The Figure 3 figure shows a schematic diagram of the implementation steps of cutting the initial sample along the periphery of the protective layer in the specific implementation manner of the method for preparing a sample for an electron microscope according to the present invention.

[0010] The Figures 4A to 4C figure shows a sectional view of the process flow diagram of cutting the initial sample along the periphery of the protective layer in the specific implementation manner of the method for preparing a sample for an electron microscope according to the present invention.

[0011] The Figure 5 figure shows a schematic diagram of the implementation steps of taking out the sample wafer in the specific implementation manner of the method for preparing a sample for an electron microscope according to the present invention.

[0012] The Figures 6A to 6C figure shows a process flow diagram of taking out the sample wafer in the specific implementation manner of the method for preparing a sample for an electron microscope according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] The following provides a detailed description of the specific implementation manner of the method for preparing a sample for an electron microscope provided by the present invention with reference to the accompanying drawings.

[0014] The Figure 1The figure shows a schematic diagram of the implementation steps of the specific implementation manner of the method for preparing a sample for an electron microscope according to the present invention, including the following steps: Step S11, providing an initial sample; Step S12, forming a protective layer in the area to be detected of the initial sample; Step S13, cutting the initial sample along the periphery of the protective layer to obtain a sample piece with one end connected to the initial sample; Step S14, taking out the sample piece; Step S15, performing rough thinning on the sample piece; Step S16, performing voltage-reducing fine thinning on the sample piece to obtain a sample for an electron microscope.

[0015] As shown in the Figure 2A figure, referring to Step S11, an initial sample 20 is provided. The initial sample 20 is a test sample for TEM or other electron microscopes and can be any crystal such as single crystal silicon, gallium nitride, etc.

[0016] As shown in the Figure 2B figure, referring to Step S12, a protective layer 21 is formed in the area to be detected of the initial sample 20. The protective layer is formed by ion beam deposition. The tilt angle of the initial sample 20 is 50 degrees to 54 degrees; the voltage used is 15 kV to 20 kV, and the current is 90 pA to 0.26 nA; the length of the protective layer is 8 μm to 10 μm, and the width and thickness are 2 μm to 3 μm respectively. As a specific implementation manner, the protective layer 21 is formed by an ion beam processing process. For example, under the conditions of a voltage of 16 kV and a current of 0.2 nA, the initial sample 20 is tilted 52 degrees and inserted into the Pt gas, so that the ion beam is perpendicular to the initial sample 20, and Pt is deposited on the area to be detected. The length of the deposition area is 10 μm, and the width and thickness are 2 μm, that is, the Pt protective layer 21 can be formed. In other specific implementation manners, the protective layer 21 can also be a deposited W protective layer, the voltage can also be 15 kV or 20 kV, the current can also be 90 pA or 0.26 nA, the length of the deposition area can also be 8 μm or 9 μm, and the width and thickness can also be 3 μm. In this step, the method of tilting the initial sample 20 can also be used to obtain the angle between the thinning surface and the ion beam. The initial angle between the ion beam and the sample stage is 52°, and the tilt angle of the initial sample 20 can be set to 50 degrees to 54 degrees. By adjusting the angle by ±2°, the error of ion beam deposition can be reduced. And, the step of forming the protective layer 21 on the surface of the initial sample 20 can be assisted by introducing Pt gas.

[0017] As shown in the Figure 2C figure, referring to Step S13, the initial sample 20 is cut along the periphery of the protective layer 21 to obtain a sample piece 22 with one end connected to the initial sample 20. As a specific implementation manner, referring to the Figure 3As shown, further cutting the initial sample 20 along both sides of the protective layer 21 further includes the following steps: Step S31, grooving the initial sample 20 along both sides of the long side of the protective layer 21; Step S32, trimming the initial sample 20 along both sides of the long side of the protective layer 21; Step S33, performing a U-shaped cut on the initial sample 20 along one side of the wide side of the protective layer 21 to ensure that the bottom of the initial sample 20 is truncated.

[0018] Attached Figure 4A to attached Figure 4C Shown is a schematic diagram of the implementation process of the above optional method, and it is drawn as a cross-sectional view along the AA direction in attached Figure 2C figure.

[0019] Attached Figure 4A As shown, referring to Step S31, groove the initial sample 20 along both sides of the long side of the protective layer 21. The inclination angle of the initial sample 20 is 50 degrees to 54 degrees; the grooving voltage is 15 kV to 20 kV, and the current is 4.4 nA to 23 nA; the length of the sample area after grooving is 18 μm to 20 μm, the width is 9 μm to 10 μm, and the depth is 10 μm to 12 μm. As a specific implementation manner, under the conditions of a voltage of 16 kV and a current of 20 nA, the initial sample 20 maintains the inclination angle unchanged, and the initial sample 20 is grooved along both sides of the protective layer 21. After grooving, the length of the sample in the area to be detected is 18 μm, the width is 9 μm, and the depth is 10 μm. In other specific implementation manners, the voltage can also be 15 kV or 20 kV, the current can also be 4.4 nA or 23 nA, the length of the sample in the area to be detected after grooving can also be 20 μm, the width can also be 10 μm, and the depth can also be 12 μm. Increasing the current during grooving can increase the processing rate and shorten the processing time.

[0020] Attached Figure 4BAs shown, referring to step S32, trim the initial sample 20 along both sides of the long side of the protective layer 21. Adjust the inclination angle of the initial sample 20 by 1 degree to 3 degrees in two directions respectively; the trimming voltage is 6 kV to 7 kV, and the current is 2 nA to 9.1 nA; after trimming, the length of the sample area is 18 μm to 20 μm, the width is 2 μm to 3 μm, and the depth is 10 μm to 12 μm. As a specific implementation manner, adjust the inclination angles of the initial sample 20 to 50 degrees and 54 degrees respectively, that is, adjust the inclination angle by 2 degrees in each of the two directions, and under the conditions of a voltage of 6 kV and a current of 3 nA, trim the area to be detected of the initial sample 20 along both sides of the long side of the protective layer 21. After trimming, the width of the area to be detected of the initial sample 20 is reduced to 2 μm, and the length and depth remain unchanged. In other specific implementation manners, the adjustment of the inclination angle of the initial sample 20 can also be 1 degree or 3 degrees, the voltage can also be 7 kV, and the current can also be 2 nA or 9.1 nA; after trimming, the width of the area to be detected of the initial sample 20 can also be reduced to 3 μm. When performing fine trimming, a small-angle fine adjustment of the inclination angle of the initial sample 20 can, to a certain extent, correct the error of the current angle, reduce the current, and improve the quality of trimming.

[0021] As shown in the Figure 4C figure, referring to step S33, perform a U-shaped cutting on the initial sample 20 along one side of the wide side of the protective layer 21 to ensure that the bottom of the initial sample 20 is truncated. The inclination angle of the initial sample 20 is less than 1 degree; the cutting voltage is 15 kV to 20 kV, and the current is 2.4 nA to 5 nA. As a specific implementation manner, adjust the inclination angle of the initial sample 20 to 0 degree, and under the conditions of a voltage of 16 kV and a current of 2.4 nA, perform a U-shaped cutting on the initial sample 20. In other specific implementation manners, the voltage can also be 15 kV or 20 kV, and the current can also be 3 nA or 5 nA. As a specific implementation manner, after cutting is completed, rotate the initial sample 20 by 35 degrees, and observe whether the bottom is truncated during the rotation process. In other specific implementation manners, the initial sample 20 can also be rotated by 10 degrees or 30 degrees. If it is impossible to determine that the bottom has been truncated, then adjust the inclination angle of the initial sample 20 to 54 degrees again, and trim the bottom to ensure that the bottom of the initial sample 20 is truncated. For multi-component crystal materials, some unstable components are likely to precipitate during FIB bombardment, and crystals with low crystal structure / atomic stability are prone to this during FIB bombardment. For example, InGaN materials will cause defects due to the precipitation of In during FIB ion bombardment. It has been found through research that this is due to the high activity and low melting point of these components. Reducing the voltage can avoid the above problems.

[0022] After the above steps are completed, the obtained sample is as shown in the Figure 2C figure, and its cross-sectional structure along the AA direction is asFigure 4C as shown

[0023] As shown in the appended Figure 2D figure, referring to step S14, the sample slice 22 is taken out. As a specific implementation manner, referring to the appended Figure 5 figure, taking out the sample slice 22 further includes the following steps: step S51, connecting the cut end of the sample slice 22 and the initial sample 20 to the probe 23; step S52, cutting off the end of the sample slice 22 connected to the initial sample 20 from the initial sample 20; step S53, raising the probe 23 to separate the sample slice 22 from the initial sample 20; step S34, connecting the taken-out sample slice 22 to the copper mesh 24; step S55, separating the sample slice 22 connected to the copper mesh 24 from the probe 23.

[0024] As shown in the appended Figure 6A figure, referring to step S51, the cut end of the sample slice 22 and the initial sample 20 are connected to the probe 23. The inclination angle of the initial sample 20 is less than 1 degree; the voltage is 25 kV to 30 kV, and the current is 41 pA to 270 pA. As a specific implementation manner, under the conditions of a voltage of 30 kV and a current of 200 pA, the inclination angle of the initial sample 20 is adjusted to 0 degree, and Pt gas is introduced. On the side where the sample slice 22 and the initial sample 20 are cut off, the protective layer 21 is welded to the probe 23. In other specific implementation manners, the voltage can also be 25 kV, the current can also be 41 pA or 270 pA, and the inclination angle of the initial sample 20 can also be 1 degree. Since connecting the cut end of the sample slice 22 and the initial sample 20 to the probe 23 is the processing of the non-observation area, a high voltage is used to improve the resolution and focusing.

[0025] As shown in the appended Figure 6B figure, referring to step S52, the end of the sample slice 22 connected to the initial sample 20 is cut off from the initial sample 20. As a specific implementation manner, after the sample slice 22 is welded to the probe 23, keeping the inclination angle of the initial sample 20 unchanged, under the conditions of a voltage of 30 kV and a current of 0.5 nA, the end of the sample slice 22 connected to the initial sample 20 is cut off from the initial sample 20. In other specific implementation manners, the voltage can also be 25 kV, the current can also be 1 nA or 2.4 nA. Since cutting off the end of the sample slice 22 connected to the initial sample 20 from the initial sample 20 is the processing of the non-observation area, a high voltage is used to improve the resolution and focusing.

[0026] As shown in the appended Figure 6CAs shown, continue to refer to step S53, raise the probe 23 to separate the sample wafer 22 from the initial sample 20. As a specific implementation, keep the tilt angle of the initial sample 20 unchanged, and slowly move the sample wafer 22 away from the initial sample 20 under the conditions of a voltage of 30 kV and a current of 20 pA. In other specific implementations, the voltage can also be 25 kV and the current can also be 25 pA. Since observation is required during the change, the smaller the current during observation, the less damage, but if it is too small, the signal-to-noise ratio will be relatively weak. Therefore, the current needs to be continuously adjusted.

[0027] Continue to refer to step S54 and step S55, connect the taken-out sample wafer 22 to the copper mesh 24 and separate it from the probe 23. As a specific implementation, find the position of the copper mesh 24 in the electron beam and ion beam, and under the conditions of a voltage of 30 kV and a current of 0.26 nA, use gas Pt deposition to weld the sample wafer 22 to the copper mesh 24. In other specific implementations, the voltage can also be 25 kV and the current can also be 0.3 nA. After welding, separate the sample wafer 22 from the probe 23 and withdraw the probe 23.

[0028] After the above steps are completed, the sample wafer 22 separated from the initial sample 20 and fixed to the copper mesh 24 is obtained, as shown in the appendix Figure 2D as shown.

[0029] Step S15, perform rough thinning on the sample wafer 22. The voltage for rough thinning is 2 kV to 15 kV, and the tilt angle range of the ion beam is ±5°. The thinning current in this step is 0.04 nA to 0.90 nA, and the depth is 6 μm to 10 μm. As a specific implementation, under the conditions of a voltage of 16 kV and a current of 0.9 nA, perform rough thinning on the sample wafer 22, with a thinning width of 8 μm and a target thickness of 1 μm. In other specific implementations, the voltage can also be 15 kV or 20 kV, the current can also be 0.04 nA or 0.5 nA, the thinning width can also be 6 μm or 10 μm, and the target thickness can also be 2 μm. Generally, it is considered that although reducing the voltage will reduce the voids in the sample wafer 22, it will cause the problem of ion beam dispersion, which is not conducive to thinning. Therefore, the normal voltage will not be lower than 30 kV. However, actual operation has found that when the voltage is below 16 kV, the defect of ion beam dispersion can be compensated by changing the angle of the ion beam from the initial setting perpendicular to the surface of the sample wafer 22 to an angle between ±1.5°; when the voltage is reduced to 5 kV, the corresponding ion beam is tilted ±3°; when the voltage is reduced to 2 kV, the ion beam dispersion is more serious, and at this time the ion beam is tilted ±5°. Thinning at low energy can be achieved through the cooperation of ion beam tilting.

[0030] Step S16: Perform voltage-reducing fine thinning on the sample wafer 22 to obtain a sample for an electron microscope. The voltage for fine thinning is 1 kV to 5 kV, the current is 20 pA to 40 pA, the width of fine thinning is less than the width of rough thinning, and the fine thinning time is 30 s to 50 s. The thinned sample is as shown in Figure 2E the attached figure.

[0031] As a specific implementation manner, reduce the voltage to 5 kV, reduce the current to 30 pA, perform fine thinning for 30 s, and the width of fine thinning is 5 μm, which is less than the width of rough thinning. In other specific implementation manners, the voltage can also be reduced to 4 kV, the current can also be reduced to 20 pA or 40 pA, the fine thinning duration can also be 50 s, and the fine thinning width can also be 6 μm. After the fine thinning of the sample wafer 22 is completed, a sample for an electron microscope is obtained. Reducing the voltage and current during fine thinning can effectively reduce the bombardment of the sample wafer 22 by the electron beam, thereby reducing the possibility of hole generation and improving the imaging quality. The voltage in this step can be reduced to a minimum of 1 kV; the target thickness of the obtained sample wafer 22 is 50 nm - 100 nm. When the target thickness is 50 nm - 100 nm, the thickness of the amorphous layer is less than or equal to 30% of the total thickness. The amorphous layer is a defect caused by the bombardment of the material layer by the ion beam (such as Ga ions) in FIB. The thicker the amorphous layer, the more difficult it is to observe the information of the material itself in the prepared TEM sample.

[0032] The above technical solution refines the thinning process by performing two-step thinning on the sample wafer 22. In other specific implementation manners, it can also be simplified to one-step thinning. In order to obtain a better thinning effect, the voltage can also be gradually reduced, and the tilt angle of the ion beam can be correspondingly set to perform more steps of thinning. Reducing the voltage and current during gradual thinning and cooperating with the tilt of the ion beam by an angle can simultaneously suppress the ion beam dispersion phenomenon and reduce the bombardment of the sample wafer 22 by the electron beam, thereby reducing the possibility of hole generation and improving the imaging quality. And adjust the voltage and current of each step during the process of manufacturing a sample for an electron microscope to improve the processing rate and also reduce the damage to the sample wafer 22.

[0033] The above are only the preferred implementation manners of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can still be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing a sample for an electron microscope, characterized in that, It includes the following steps: Provide an initial sample; Form a protective layer in the area to be detected of the initial sample; Cut the initial sample along the periphery of the protective layer to obtain a sample piece with one end connected to the initial sample; Take out the sample piece and connect it to a copper mesh; Perform multiple thinning operations on the sample piece fixed to the copper mesh by ion beam bombardment; wherein, the ion beam has an inclination angle, the thinning voltage is 1 kV to 16 kV, the inclination angle range of the ion beam is ±5°, and the voltage and current are gradually reduced during the multiple thinning processes; Wherein, when the thinning voltage is 16 kV, the inclination angle range of the ion beam is ±1.5°; when the thinning voltage is reduced to 5 kV, the inclination angle range of the ion beam is ±3°; when the thinning voltage is reduced to 2 kV, the inclination angle range of the ion beam is ±5°.

2. The method according to claim 1, characterized in that, The step of performing multiple thinning operations on the sample piece by ion beam bombardment further includes: Perform rough thinning on the sample piece by ion beam bombardment; Perform voltage-reducing fine thinning on the sample piece by ion beam bombardment.

3. The method according to claim 2, wherein In the step of performing rough thinning on the sample piece, the rough thinning voltage is 2 kV to 16 kV, the thinning current is 0.04 nA to 0.90 nA, and the depth is 6 μm to 10 μm.

4. The method according to claim 2, wherein In the step of performing voltage-reducing fine thinning on the sample piece, the fine thinning voltage is 1 kV to 5 kV, the current is 20 pA to 40 pA, and the fine thinning time is 30 s to 50 s.

5. The method according to claim 1, characterized in that, Cutting the initial sample along the periphery of the protective layer further includes the following steps: Groove the initial sample along both sides of the long side of the protective layer; Trim the initial sample along both sides of the long side of the protective layer; Perform U-shaped cutting on the initial sample along one side of the wide side of the protective layer to ensure that the bottom of the initial sample is truncated.

6. The method according to claim 1, wherein In the step of forming a protective layer in the area to be detected of the initial sample, form the protective layer by ion beam deposition, the inclination angle of the sample is 50 degrees to 54 degrees; the voltage used is 15 kV to 20 kV, the current is 90 pA to 0.26 nA; the length of the protective layer is 8 μm to 10 μm, and the width and thickness are 2 μm to 3 μm respectively.

7. The method according to claim 5, wherein In the step of grooving the initial sample along both sides of the long side of the protective layer, the inclination angle of the sample is 50 degrees to 54 degrees; the grooving voltage is 15 kV to 20 kV, the current is 4.4 nA to 23 nA; the length of the sample area after grooving is 18 μm to 20 μm, the width is 9 μm to 10 μm, and the depth is 10 μm to 12 μm.

8. The method according to claim 5, characterized in that In the step of trimming the initial sample along both sides of the long side of the protective layer, adjust the inclination angle of the sample by 1 degree to 3 degrees in two directions respectively; the trimming voltage is 6 kV to 7 kV, the current is 2 nA to 9.1 nA; the length of the sample area after trimming is 18 μm to 20 μm, the width is 2 μm to 3 μm, and the depth is 10 μm to 12 μm.

9. The method according to claim 5, wherein In the step of performing U-shaped cutting on the initial sample along one side of the wide side of the protective layer, the inclination angle of the sample is less than 1 degree; the cutting voltage is 15 kV to 20 kV, the current is 2.4 nA to 5 nA.

10. The method according to claim 1, wherein Taking out the sample piece and connecting it to the copper mesh further includes the following steps: Connecting one end of the sample piece truncated from the initial sample to the probe; Truncating one end of the sample piece connected to the initial sample from the initial sample; Raising the probe to separate the sample piece from the initial sample; Connecting the taken-out sample piece to the copper mesh; Separating the sample piece connected to the copper mesh from the probe.

11. The method according to claim 10, wherein In the step of connecting one end of the sample piece truncated from the initial sample to the probe, the inclination angle of the initial sample is less than 1 degree; the voltage is 25 kV to 30 kV, and the current is 41 pA to 270 pA.

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