Scanning electron microscope sample preparation method applied to wafer sample with poor conductivity

Abstract

The invention belongs to the technical field of scanning electron microscopes, and particularly relates to a scanning electron microscope sample preparation method applied to a wafer sample with poor conductivity, which comprises the following steps: cleaning the wafer sample; fixing the cleaned wafer sample on a loading disc of a spin coater, dropwise adding a conductive adhesive on the surface of the wafer sample, and uniformly coating the surface of the wafer sample with the conductive adhesive through spin coating of the spin coater to obtain a sample for observation of the scanning electron microscope; wherein the conductive adhesive is a polyaniline solution. By utilizing the method, the charge effect on the surface of the non-conductive wafer can be effectively eliminated, so that the non-conductive wafer has clear surface appearance under a scanning electron microscope, the initial state of the wafer sample can be reduced after the test is finished, and the original components and original properties of the sample are not changed, so that the subsequent process and test links are not influenced. In addition, the method has a smaller shielding effect on the CL light emitting amount of the wafer sample, and has a gain effect in a cathode fluorescence test link of the wafer sample.

Description

Technical Field

[0001] This invention belongs to the field of scanning electron microscopy (SEM) technology, and specifically relates to a method for preparing SEM samples for wafers with poor conductivity. Background Technology

[0002] Scanning electron microscopy (SEM) is a high-resolution electron microscope used to observe and analyze the surface morphology and microstructure of materials. SEM works by scanning the sample surface with an electron beam, exciting signals such as secondary electrons, backscattered electrons, and cathodoluminescence. These signals are collected and analyzed using different detectors to acquire images and data. SEM has wide applications in materials science, biology, geology, and other fields, including materials characterization and fundamental research. Cathodoluminescence (CL) refers to the photon signals in the ultraviolet, infrared, or visible light bands excited by an electron beam bombarding a material surface. The detection and processing of electron beam-excited fluorescence signals are often combined with SEM, enabling the combined study of morphological observation, structural and compositional analysis with electron beam-excited fluorescence spectroscopy.

[0003] Since the basic principle of scanning electron microscopy (SEM) is to scan the sample surface with an electron beam and interact with the sample, a primary prerequisite for SEM is that the sample must have good conductivity. Otherwise, a large number of injected electrons will accumulate locally or over a large area on the sample surface and cannot be guided away from the sample. The large accumulation of electrons will affect the electrostatic field in that area, thus affecting the detection and imaging of secondary electrons. This is reflected in the electron microscope image as very large differences in brightness, and the bright areas fluctuate drastically with the magnification. In addition, drift can be observed in the image at higher magnifications. This imaging problem caused by poor conductivity of the sample is called the "charge effect".

[0004] To eliminate the effects of the "charge effect," the principle is to improve the conductivity of the sample, allowing injected electrons to form a pathway and be led away from the sample surface. The traditional industry practice is to deposit a layer of gold or carbon film on the sample surface. Both of these components have good conductivity, and covering the surface of a poorly conductive sample helps to lead electrons away. However, the conductive layer formed in this way is very difficult to remove from the sample surface, making it almost an "irreversible" sample preparation method. Especially in the semiconductor industry, wafer production and processing involve a series of complex steps. For some poorly conductive substrate materials, this coating method can affect subsequent processing and testing. Furthermore, the gold or carbon film can block the CL signal, affecting the accuracy and actual effect of CL detection. Summary of the Invention

[0005] The purpose of this invention is to propose a scanning electron microscope (SEM) sample preparation method for wafers with poor conductivity. This method can effectively eliminate the "charge effect" on the surface of non-conductive wafers, resulting in a clear surface morphology under the SEM. Furthermore, after testing, the initial state of the wafer sample can be restored without altering its original composition or properties, thus avoiding impact on subsequent processes and testing. In addition, in SEM-CL testing, compared to traditional coating methods, this invention has less obstruction of the CL emission from the wafer sample, providing a gain effect in the cathodoluminescence testing stage.

[0006] Specifically, the present invention provides the following technical solution: A scanning electron microscope (SEM) sample preparation method for wafers with poor conductivity includes the following steps: The wafer sample is cleaned. The cleaned wafer sample is fixed on the tray of a spin coater, and then conductive adhesive is dropped onto the surface of the wafer sample. The conductive adhesive is then evenly coated onto the surface of the wafer sample by spin coating, resulting in a sample for scanning electron microscopy. The conductive adhesive is a polyaniline solution.

[0007] Research has found that polyaniline is particularly suitable for this invention compared to other conductive adhesives. It effectively eliminates the "charge effect" caused by poor conductivity in wafer samples, and can be removed simply by rinsing with water, restoring the sample to its pre-test state without affecting subsequent processing and testing. Furthermore, this invention unexpectedly discovered that in scanning electron microscopy-catholuminescence (SEM-CL) testing, polyaniline conductive adhesive has a smaller obstruction effect on the CL light emission of the wafer sample compared to traditional coatings, thus providing a gain effect in the cathodoluminescence testing of the wafer sample.

[0008] Preferably, the polyaniline content in the polyaniline solution is 1.5-2.5% by mass. If the polyaniline concentration is too high, it cannot spread well on the surface, thus affecting the subsequent spin coating process.

[0009] Preferably, the cleaning process includes: S1. Place the wafer sample in acetone and ultrasonically vibrate it for a period of time; S2. Remove the wafer sample from the acetone and blow it dry, then place it in isopropanol and sonicate it for a period of time. S3. Remove the wafer sample from the isopropanol and blow it dry, then place it in deionized water and sonicate it for a period of time. S4. Remove the wafer sample from the deionized water and dry it.

[0010] Preferably, when the wafer sample is 2 inches (diameter), the volume of the added conductive adhesive is 700~800uL; When the wafer sample is 3 inches, the volume of the added conductive adhesive is 1.5~2.0 mL; When the wafer sample is 4 inches, the volume of the added conductive adhesive is 2.5~3.5 mL.

[0011] The amount of conductive adhesive used in this invention is determined by the surface area of ​​the wafer sample. Too little or too much adhesive will affect the accuracy and actual effect of the detection.

[0012] Preferably, the spin coating process of the spin coater is as follows: spin coating for 4 to 6 seconds at 400 to 600 rpm, then spin coating for 50 to 70 seconds at 5000 to 7000 rpm, and finally spin coating for 4 to 6 seconds at 400 to 600 rpm.

[0013] The coating thickness is related to the spin coating speed and time. The study found that the above spin coating method yields a conductive adhesive coating with a suitable thickness and excellent conductivity, while also being easy to remove from the wafer sample surface.

[0014] The present invention also provides the application of the above-mentioned scanning electron microscope sample preparation method in scanning electron microscope-cathodic fluorescence (SEM-CL) testing.

[0015] The beneficial effects achieved by this invention are as follows: (1) The present invention provides a scanning electron microscope sample preparation method for wafer samples with poor conductivity. A specific conductive adhesive is spin-coated on the surface of the wafer sample, which can effectively eliminate the "charge effect" caused by poor conductivity of the wafer sample.

[0016] (2) The present invention provides a scanning electron microscope sample preparation method for wafer samples with poor conductivity. Compared with the traditional gold plating and carbon plating methods, it is reversible, easy to clean, and can restore the initial state of the sample without damaging the sample, without affecting the subsequent process and testing.

[0017] (3) The present invention provides a scanning electron microscope sample preparation method for wafer samples with poor conductivity. The application scope includes all application fields of scanning electron microscope (SE, BSE, CL, EDS, etc.). It is particularly suitable for scanning electron microscope-catholuminescence (SEM-CL) testing. This sample preparation method has a less obstructive effect on the light output of the sample than traditional metal coating and has an amplifying effect on the cathodoluminescence testing of wafer samples. Attached Figure Description

[0018] Figure 1The image shows a comparison of CL images of GaN samples prepared by scanning electron microscopy using different methods. The left image is the CL image of the untreated GaN sample, the middle image is the CL image of the GaN sample prepared by conventional gold sputtering, and the right image is the CL image of the GaN sample prepared by the method described in Example 1.

[0019] Figure 2 To compare the CL spectral intensities of GaN samples prepared by scanning electron microscopy using different methods; where "untreated" represents the CL spectral intensity of the untreated GaN sample, "gold sputtering" represents the CL spectral intensity of the GaN sample prepared by conventional gold sputtering, "coating with conductive adhesive" represents the CL spectral intensity of the GaN sample prepared by the method described in Example 1, and "cleaning the conductive adhesive" represents the CL spectral intensity of the GaN sample prepared by the method described in Example 1 and then cleaned with water to remove the conductive adhesive. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0021] In the following embodiments, the wafer samples used are 2-inch gallium nitride wafers; The conductive adhesive is a polyaniline solution derived from ALLRESIST AR-PC 5092.02 in Germany, wherein the mass content of polyaniline is 2% and the mass content of water is 98%.

[0022] Example 1 Example 1 provides a scanning electron microscope sample preparation method for wafer samples with poor conductivity. The specific steps are as follows: 1) Place the wafer sample in acetone and sonicate for 5 minutes.

[0023] 2) Remove the wafer sample from the acetone and dry it with a nitrogen gun, then place it in an isopropanol solution and sonicate for 5 minutes.

[0024] 3) Remove the wafer sample from the isopropanol and dry it with a nitrogen gun, then place it in deionized water and sonicate for 5 minutes.

[0025] 4) Remove the wafer sample from the deionized water and dry it with a nitrogen gun.

[0026] 5) Select a carrier tray slightly smaller than the wafer sample and mount it on the spin coater. Place the wafer sample face up on the carrier tray. Turn on the vacuum valve to create suction by drawing a vacuum on the back of the wafer sample, thus fixing the wafer sample onto the carrier tray.

[0027] 6) Use a pipette to evenly apply the conductive adhesive to the sample surface, ensuring it covers the entire surface. The amount of conductive adhesive used is 750 μL.

[0028] 7) Click the start button on the spin coater to turn on the spin coater mode. The spin coater will rotate at the set speed: Step 1: 500 rpm, 5s; Step 2: 6000 rpm, 60s; Step 3: 500 rpm, 5s. The high-speed centrifugal force will form a thin coating of uniform thickness on the sample surface of the conductive adhesive.

[0029] 8) Place the homogenized wafer sample in the scanning electron microscope sample chamber and scan the sample.

[0030] 9) After the scanning electron microscope test is completed, remove the sample from the electron microscope sample chamber, rinse the wafer sample surface with deionized water for 2 minutes, and then blow it dry with a nitrogen gun to remove the conductive adhesive coated on the sample surface. The sample can be restored to its pre-test state and will not affect the subsequent process and testing of the wafer sample.

[0031] In addition, the resistivity of the conductive adhesive layer is affected by the moisture content of the layer. When placed in the atmosphere, water vapor in the atmosphere will enter the conductive layer over time, causing the resistivity of the conductive layer to gradually increase. Therefore, scanning electron microscopy should be performed immediately after the conductive adhesive layer is spin-coated, otherwise the conductivity will be affected.

[0032] Figure 1 To compare the CL images of GaN samples prepared by scanning electron microscopy (SEM) using different methods, the SEM parameters used were: accelerating voltage 5 kV, beam current 1 nA, image acquisition resolution 2048*2048, and scanning speed 10 μs / pxl. Figure 1 It can be seen that the CL image signal intensity of the GaN sample after spin coating with conductive adhesive is significantly stronger than that after gold sputtering.

[0033] Figure 2 To compare the CL spectral intensities of GaN samples prepared by scanning electron microscopy using different methods, it can be seen that the peak value at 362 nm is: I 未处理 (8075)≈I 清洗导电胶 (7900) > I 涂导电胶 (6212) > I 喷金 (3100), that is, compared with the untreated GaN sample, the peak value of the emission peak at 362nm is reduced by 23% after spin coating of conductive adhesive and by 61% after gold sputtering. At the same time, after cleaning the conductive adhesive, the sample can be restored to the state before testing and will not affect the subsequent process and testing of the wafer sample.

[0034] Comparative Example 1 Compared with Example 1, the only difference is that the amount of conductive adhesive used in step 6) is 300 μL, and the spin coating speed of the spin coater in the second step of step 7) is changed from 6000 rpm and 60s to 3000 rpm and 30s.

[0035] The results, when scanned under an electron microscope, showed that the charging effect was more pronounced and the effect was worse than in Example 1.

[0036] Although the present invention has been described in detail above with general descriptions, specific embodiments, and experiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A scanning electron microscope (SEM) sample preparation method for wafers with poor conductivity, characterized in that, Includes the following steps: The wafer sample is cleaned. The cleaned wafer sample is fixed on the tray of a spin coater, and then conductive adhesive is dropped onto the surface of the wafer sample. The conductive adhesive is then evenly coated onto the surface of the wafer sample by spin coating, resulting in a sample for scanning electron microscopy. The conductive adhesive is a polyaniline solution.

2. The scanning electron microscope sample preparation method for wafers with poor conductivity according to claim 1, characterized in that, The polyaniline solution contains 1.5% to 2.5% polyaniline by mass.

3. A scanning electron microscope sample preparation method for wafers with poor conductivity according to claim 1 or 2, characterized in that, The cleaning process includes: S1. Place the wafer sample in acetone and ultrasonically vibrate it for a period of time; S2. Remove the wafer sample from the acetone and blow it dry, then place it in isopropanol and sonicate it for a period of time. S3. Remove the wafer sample from the isopropanol and blow it dry, then place it in deionized water and sonicate it for a period of time. S4. Remove the wafer sample from the deionized water and dry it.

4. A scanning electron microscope sample preparation method for wafers with poor conductivity according to claim 1 or 2, characterized in that, When the wafer sample is 2 inches, the volume of the added conductive adhesive is 700~800 uL; When the wafer sample is 3 inches, the volume of the added conductive adhesive is 1.5~2.0 mL; When the wafer sample is 4 inches, the volume of the added conductive adhesive is 2.5~3.5 mL.

5. A scanning electron microscope sample preparation method for wafers with poor conductivity according to claim 1 or 2, characterized in that, The specific process of spin coating with the spin coater is as follows: spin coating for 4 to 6 seconds at 400 to 600 rpm, then spin coating for 50 to 70 seconds at 5000 to 7000 rpm, and finally spin coating for 4 to 6 seconds at 400 to 600 rpm.

6. The application of the scanning electron microscope sample preparation method according to any one of claims 1-5 in scanning electron microscope-cathodic fluorescence testing.

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