Section sample fixing method based on low-melting-point alloy

Abstract

The invention provides a section sample fixing method based on low-melting-point alloy, which comprises the following steps: providing a sample table which comprises a bottom plate and a side plate, and the side plate is vertically arranged on the surface of the bottom plate; molten low-melting-point alloy is placed on the surface of the bottom plate and the surfaces of the side plates, and alloy liquid drops are formed; providing a section sample, and attaching the section sample to the alloy liquid drops on the surface of the bottom plate and the surface of the side plate, so that the to-be-observed surface of the section sample faces upwards; and cooling the sample table until the alloy liquid drops are solidified so as to fix the section sample. In the application, aiming at the special fixing requirement of the chip cross section sample, alloy liquid drops are formed on the bottom surface and the side surface of the cross section sample, so that the inclination of the sample is fundamentally eliminated physically by utilizing the liquid phase self-balance effect during solidification of the alloy liquid drops, and lossless and stress-free firm fixing can be realized.

Description

Technical Field

[0001] This application relates to the field of semiconductor defect analysis, and in particular to a method for fixing cross-sectional samples based on low-melting-point alloys. Background Technology

[0002] In chip failure analysis, scanning electron microscopy (SEM) is a key instrument for analyzing the microstructure, structure, and composition of the target area. SEM observation of cross-sectional samples is an important part of this process.

[0003] When observing sample cross-sections, the sample needs to be stably and flatly fixed on the sample stage. Currently, the most common methods are to directly tighten the metal screws or use double-sided carbon tape for adhesion. However, since the cross-sectional dimensions of chips (usually along the thickness direction of the wafer) are generally small, the above methods have the problem of unstable fixation; moreover, the cross-sectional sample has high requirements for the parallelism of the surface to be observed. When using screws for fixation, the torque generated by tightening can easily cause the sample to tilt and warp at the nanometer or even micrometer level (although the tilt function in SEM can adjust the surface orientation, it is easy to cause image distortion); secondly, for fragile or tiny samples, the pressure of the screws can easily cause the sample to break or be damaged; while tape has the problems of uncontrollable adhesive strength and inconvenience in removing the sample, which may result in important samples not being preserved intact.

[0004] Therefore, developing a method for fixing SEM cross-sectional samples that is non-destructive, stress-free, and highly stable has become a pressing technical problem in this field.

[0005] It should be noted that the above introduction to the technical background is only for the purpose of providing a clear and complete explanation of the technical solutions of this application and facilitating understanding by those skilled in the art. It should not be assumed that these technical solutions are known to those skilled in the art simply because they have been described in the background section of this application. Summary of the Invention

[0006] The purpose of this invention is to provide a non-destructive, stress-free, and highly stable method for fixing SEM cross-sectional samples.

[0007] To address the aforementioned issues, a method for fixing cross-sectional samples based on low-melting-point alloys is provided below, comprising the following steps: S1. A sample stage is provided, the sample stage including a base plate and a side plate, the side plate being vertically disposed on the surface of the base plate; S2. Place molten low-melting-point alloy on the surface of the base plate and the surface of the side plate to form alloy droplets; S3. Provide a cross-sectional sample and place alloy droplets attached to the surface of the base plate and the surface of the side plate, so that the side of the cross-sectional sample to be observed faces upward; S4. Cool the sample stage until the alloy droplets solidify to fix the cross-sectional sample.

[0008] In this application, in response to the special fixing requirements of chip cross-section samples, alloy droplets are formed on the bottom and side surfaces of the cross-section sample. The liquid phase self-equilibrium effect during solidification is then utilized to physically eliminate the tilting of the sample and achieve a reliable fixation without damage or stress.

[0009] In step S2, molten low-melting-point alloy is dropped onto the adjacent surfaces of the base plate and the side plate in the sample stage to form alloy droplets.

[0010] The bottom surface of the cross-sectional sample is attached to the alloy droplet on the surface of the base plate, and the side surface of the cross-sectional sample is attached to the alloy droplet on the surface of the side plate; wherein the bottom surface of the cross-sectional sample is opposite to and parallel to the surface to be observed.

[0011] In step S2, the diameter of the alloy droplets formed on the base plate surface is not less than the length of the cross-sectional sample, and the height of the alloy droplets formed on the side plate surface is not greater than the height of the cross-sectional sample. This ensures a secure fixation while avoiding interference with the object under observation.

[0012] The low-melting-point alloy has a melting point between 50 °C and 100 °C. This ensures convenient heating and cooling, as well as relative stability of the sample state during observation.

[0013] The low-melting-point alloy is a Field alloy. It is lead- and cadmium-free, making it more environmentally friendly. It has excellent vacuum stability, does not release gas, and is suitable for the clean SEM vacuum environment.

[0014] In steps S2 and S3, the temperature of the sample stage is made higher than the melting point of the low-melting-point alloy.

[0015] In steps S2 and S3, the temperature of the sample stage is maintained at 75°C to 85°C. This ensures convenient heating and cooling, as well as relative stability of the sample state during observation.

[0016] In steps S2 and S3, the sample stage is placed on a heating stage so that the temperature of the sample stage is higher than the melting point of the low-melting-point alloy.

[0017] The sample stage is made of aluminum.

[0018] Compared with the prior art, the beneficial effects of the present invention mainly include the following: 1) The self-balancing effect of the liquid phase of the alloy droplets on the bottom and side surfaces of the cross-sectional sample during the cooling and solidification process is used for adaptive adjustment, thereby ensuring that the observation surface of the cross-sectional sample and the bottom plate surface of the sample stage remain relatively parallel, fundamentally eliminating sample tilting from a physical perspective; 2) After the alloy solidifies, it forms a rigid whole with the sample, eliminating sample tilting and damage caused by screw torque or colloidal shrinkage; 3) The fixing effect of the alloy material is extremely stable. After solidification, it forms a strong mechanical interlock with the sample and sample stage, with high fixing strength, and can withstand the vibration caused by electron beam scanning and sample stage movement; 4) The alloy material does not contain lead or cadmium, has excellent vacuum stability, does not release gas, and is suitable for the clean SEM vacuum environment; 5) The alloy material itself has good conductivity, which can meet the conductivity requirements for SEM observation and reduce the negative impact of the charge effect; 6) The sample can be completely removed by simple heating, achieving non-destructive disassembly. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the specific embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 The present invention provides a flowchart of a method for fixing cross-sectional samples based on low-melting-point alloys.

[0021] Figure 2 This invention provides a schematic diagram of a method for fixing cross-sectional samples based on low-melting-point alloys. Detailed Implementation

[0022] The foregoing and other technical contents, features, and effects of the present invention will be clearly presented in the following detailed description of a preferred embodiment with reference to the accompanying drawings. The directional terms mentioned in the following embodiments, such as up, down, left, right, front, or back, are merely for reference to the accompanying drawings. Therefore, the directional terms used are for illustrative purposes and not for limiting the present invention.

[0023] The embodiments of this application will now be described in detail with reference to the accompanying drawings. However, those skilled in the art will understand that many technical details have been provided in the embodiments of this application to facilitate a better understanding of the application. However, the technical solutions claimed in this application can be implemented even without these technical details and various variations and modifications based on the following embodiments.

[0024] The steps in the following embodiments do not correspond one-to-one with the contents of the invention.

[0025] Example 1 like Figure 1 The diagram shown is a flowchart of a method for fixing cross-sectional samples based on low-melting-point alloys according to an embodiment of the present invention. Figure 2 This invention provides a schematic diagram of a method for fixing cross-sectional samples based on low-melting-point alloys.

[0026] refer to Figure 1 and Figure 2 This invention provides a method for fixing cross-sectional samples based on low-melting-point alloys, the method comprising the following steps: Step 1: Provide the sample stage and cross-sectional sample; It is understood that the sample stage 1 is a structural component used to fix the sample to be tested and place it into the SEM for observation. Different sample stages 1 can be used to fix different shaped samples. The fixing between the sample stage 1 and the SEM can be adapted to the existing structure of different SEM equipment, which will not be elaborated on here.

[0027] In this application, in order to facilitate the fixation and observation of cross-sectional samples using this method, such as Figure 2 As shown, the provided sample stage 1 includes at least a receiving space consisting of a base plate 10 and a side plate 12, wherein the side plate 11 is vertically disposed on the surface of the base plate 10; the side plate 11 may include multiple side plates, and different plates 11 may be connected to each other or be independent of each other.

[0028] refer to Figure 2 As shown, the surface to be observed in cross-sectional sample 2 is generally a cross-section of a chip, and therefore it is usually sheet-like (the thickness dimension of a wafer is relatively small). It is understood that this method does not impose many restrictions on the preparation process of cross-sectional sample 2, but to ensure the SEM observation effect, it is usually necessary to ensure the flatness of the surface to be observed in cross-sectional sample 2. In this application, to ensure the fixation effect, the bottom surface of cross-sectional sample 2 can be set opposite to and parallel to the surface to be observed, while the bottom surface of cross-sectional sample 2 is set perpendicular to the surface to be observed.

[0029] Step 2: Add an alloy droplet to sample stage 1 to fix the cross-section of sample 2; In this application, a low-melting-point alloy is proposed as the material for the fixed-section sample 2. In this embodiment, Field alloy is used, with the specific composition of 32.5% bismuth (mass fraction), 51% indium, and 16.5% tin; the melting point of this alloy is 62 °C. In other embodiments, other low-melting-point alloys can be used, generally ensuring that their melting points are below 100 °C and above 50 °C, thereby ensuring convenient heating and cooling, and relatively stable state during observation. More preferably, the melting point of the alloy is below 70 °C and above 50 °C.

[0030] refer to Figure 2 As shown, the sample stage 1 is first placed on the heating stage 4. In this embodiment, the sample stage 1 is made of aluminum and can be heated to a preset temperature (at least higher than the melting point of the low-melting-point alloy) through heat conduction. Similarly, the heating stage 4 can also be used to heat the Field alloy above its melting point to obtain a molten low-melting-point alloy. In this embodiment, the preset temperature is 80 °C, meaning that both the sample stage 1 and the Field alloy on it are heated to 80 °C and kept stable by the heating stage 4; in other embodiments, the temperature can also be controlled between 75 °C and 85 °C.

[0031] Then, using a preheated dropper, a suitable amount of molten alloy liquid is drawn up and dripped onto the adjacent surfaces of the base plate 10 and side plate 11 in the sample stage 1, forming two alloy droplets 3 of suitable size (due to surface tension, the formation positions of the alloy droplets 3 can remain relatively fixed). To ensure the fixed effect and prevent the target position to be observed from being covered by the alloy droplets 3, the diameter of the alloy droplet formed on the surface of the base plate 10 can be no less than the length of the cross-sectional sample 2, and the height of the alloy droplet formed on the surface of the side plate 11 can be no greater than the height of the cross-sectional sample 2.

[0032] Step 3: Fix the cross-sectional sample using alloy droplets; refer to Figure 2 As shown, alloy droplets 3 attached to the surface of the base plate 10 and the side plate 11 of the cross-sectional sample 2 are placed with the observation surface of the cross-sectional sample 2 facing upwards; specifically, the bottom surface of the cross-sectional sample 2 is attached to the alloy droplets 3 on the surface of the base plate 10, and the side surface of the cross-sectional sample 2 is attached to the alloy droplets 3 on the surface of the side plate 11.

[0033] Step 4: Cool the sample stage to fix the cross-sectional sample.

[0034] Specifically, the sample stage 1 and the alloy droplets 3 on it can be cooled to ambient temperature by turning off the heating stage 4 or removing the sample stage 1 from the heating stage 4.

[0035] The sample stage 1 and the cross-sectional sample 2 on it can then be sent into the SEM for observation. After the observation is completed, the alloy can be easily melted and the sample can be completely removed by heating again without causing any damage to the sample.

[0036] Compared to existing technologies, the technical solution of this application utilizes the self-balancing effect of the liquid phase of the alloy droplets 3 on the bottom and side surfaces of the cross-sectional sample 2 during the cooling and solidification process to adaptively adjust and ensure that the surface to be observed of the cross-sectional sample 2 and the surface of the base plate 10 of the sample stage 1 remain relatively parallel, fundamentally eliminating sample tilting physically. After solidification, the alloy forms a rigid whole with the sample, eliminating sample tilting and damage caused by screw torque or colloidal shrinkage. The fixing effect of the alloy material is extremely stable, forming a strong mechanical interlock with the sample and sample stage after solidification, with high fixing strength, and can withstand vibrations caused by electron beam scanning and sample stage movement. This alloy material is lead- and cadmium-free, more environmentally friendly, has excellent vacuum stability, does not release gas, and is suitable for the clean SEM vacuum environment. The alloy material itself has good conductivity, which can meet the conductivity requirements for SEM observation and reduce the negative impact of the charge effect. The sample can be completely removed by simple heating, achieving non-destructive disassembly.

[0037] The common English terms or letters used in this invention for clarity of description are for illustrative purposes only and are not limiting interpretations or specific uses. They should not be used to limit the scope of protection of this invention based on their possible Chinese translations or specific letters.

[0038] It should also be noted that in this article, relational terms such as “first” and “second” are used only to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations.

Claims

1. A method for fixing cross-sectional samples based on low-melting-point alloys, characterized in that, Includes the following steps: S1. A sample stage is provided, the sample stage including a base plate and a side plate, the side plate being vertically disposed on the surface of the base plate; S2. Place molten low-melting-point alloy on the surface of the base plate and the surface of the side plate to form alloy droplets; S3. Provide a cross-sectional sample and place alloy droplets attached to the surface of the base plate and the surface of the side plate, so that the side of the cross-sectional sample to be observed faces upward; S4. Cool the sample stage until the alloy droplets solidify to fix the cross-sectional sample.

2. The method for fixing SEM cross-sectional samples based on low-melting-point alloys according to claim 2, characterized in that, In step S2, molten low-melting-point alloy is dropped onto the adjacent surfaces of the base plate and the side plate in the sample stage to form alloy droplets.

3. The method for fixing SEM cross-sectional samples based on low-melting-point alloys according to claim 2, characterized in that, The bottom surface of the cross-sectional sample is attached to the alloy droplet on the surface of the base plate, and the side surface of the cross-sectional sample is attached to the alloy droplet on the surface of the side plate. The bottom surface of the cross-sectional sample is positioned opposite to and parallel to the surface to be observed.

4. The method for fixing SEM cross-sectional samples based on low-melting-point alloys according to claim 2, characterized in that, In step S2, the diameter of the alloy droplets formed on the surface of the base plate is not less than the length of the cross-sectional sample, and the height of the alloy droplets formed on the surface of the side plate is not greater than the height of the cross-sectional sample.

5. The method for fixing SEM cross-sectional samples based on low-melting-point alloys according to claim 1, characterized in that, The melting point of the low-melting-point alloy is between 50 °C and 100 °C.

6. The method for fixing SEM cross-sectional samples based on low-melting-point alloys according to claim 1, characterized in that, The low-melting-point alloy is Field alloy.

7. The method for fixing SEM cross-sectional samples based on low-melting-point alloys according to claim 6, characterized in that, In steps S2 and S3, the temperature of the sample stage is made higher than the melting point of the low-melting-point alloy.

8. The method for fixing SEM cross-sectional samples based on low-melting-point alloys according to claim 7, characterized in that, In steps S2 and S3, the temperature of the sample stage is set to 75°C to 85°C.

9. A method for fixing SEM cross-sectional samples based on low-melting-point alloys according to claim 6, characterized in that, In steps S2 and S3, the sample stage is placed on a heating stage so that the temperature of the sample stage is higher than the melting point of the low-melting-point alloy.

10. A method for fixing SEM cross-sectional samples based on low-melting-point alloys according to claim 1, characterized in that, The sample stage is made of aluminum.

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