Alignment method in microsample sampling

Abstract

In addition to reducing the alignment time during micro-sample sampling, this method improves alignment accuracy. [Solution] A FIB device 100 is provided that has the function of measuring length based on a secondary electron image obtained from a secondary electron detector 8. With the stage 5 tilted by an inclination angle θ, the width W of the attachment surface a of the mesh 4 is measured, and the minute sample 1 held by the probe needle 2 is brought closer to the mesh 4 while observing the secondary electron image until the tip of the minute sample 1 overlaps with the mesh, thereby estimating the distance d between the minute sample 1 and the attachment surface 4a of the mesh 4 to be within the range of 0 to W / tanθ, and alignment is performed based on this distance d.

Description

Technical Field

[0001] The present invention relates to an alignment method in micro-sample sampling.

Background Art

[0002] In micro-sample sampling, generally referred to as Micro Sampling (registered trademark), the stage of a focused ion beam apparatus (hereinafter referred to as a FIB apparatus) is tilted, and while observing a secondary electron image (SIM image) with a secondary electron detection device, a probe needle is manipulated at the micron order level.

[0003] In micro-sample sampling, in the process of cutting out a micro-sample from the base material, the probe needle is brought into contact with the micro-sample and lifted. Then, in the process of attaching the micro-sample to the mesh (mounting base of the micro-sample) in the next stage, the micro-sample held by the probe needle is brought into contact with the mesh and attached.

[0004] The micro-sample is a sample piece for transmission electron microscope (hereinafter referred to as TEM) observation and has a size of several tens of μm. This micro-sample is mounted on a transmission electron microscope in a state of being attached to a mesh.

[0005] In Patent Document 1, in the alignment process (alignment process) in the process of cutting out a micro-sample and attaching it to a mesh, for the purpose of shortening the working time and improving the alignment accuracy, based on the parallax between the probe needle and the micro-sample in the secondary electron image, alignment of the probe needle and the micro-sample is performed. Similarly, alignment of the probe needle and the mesh is performed based on such parallax.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

[0007] However, the alignment process described above remains the biggest challenge in micro-sample sampling, as even a slight misalignment could cause the micro-sample to fall. [Means for solving the problem]

[0008] The present invention provides a FIB (Fiber-Injection Bomb) apparatus for micro-sample sampling alignment, comprising: a probe needle capable of contacting, lifting, and holding a micro-sample; a mesh to which the micro-sample held on the probe needle can be attached; a tiltable stage on which the mesh is mounted; a charged particle gun for irradiating the micro-sample, probe needle, and mesh with a charged particle beam; and a secondary electron detector for detecting secondary electrons generated from the micro-sample, probe needle, and mesh, and having a function to perform length measurement based on a secondary electron image obtained from the secondary electron detector. In aligning a microscopic sample held on a probe needle with a mesh, the width W of the mesh attachment surface is measured with the stage tilted at an angle θ, and the microscopic sample is brought closer to the mesh while observing a secondary electron image until the tip of the microscopic sample held on the probe needle overlaps with the mesh. The distance between the microscopic sample and the mesh is estimated to be within the range of 0 to W / tanθ, and alignment is performed based on this distance. [Effects of the Invention]

[0009] According to the present invention, in addition to shortening the alignment work time in micro-sample sampling, alignment accuracy can be improved. [Brief explanation of the drawing]

[0010] [Figure 1] This figure shows the overall configuration of the FIB device according to the present invention. [Figure 2] This is a perspective view showing a microsample, probe needle, and mesh. [Figure 3] This is a diagram illustrating the alignment method according to the present invention. [Figure 4] This is a secondary electron image of the mesh. [Figure 5] This is a secondary electron image showing the attachment of a minute sample to a mesh. [Figure 6] This diagram shows a standard method for measuring the length of a cross-section. [Figure 7] This diagram shows the method for deriving the formula for calculating the measured length. [Figure 8] This figure shows a minute sample being measured. [Figure 9] This figure shows specific examples of length measurement. [Figure 10] This figure shows examples of length measurement. [Figure 11] This is a diagram showing a simulation of probing. [Figure 12] This figure shows the approach of a minute sample to the mesh. [Modes for carrying out the invention]

[0011] <Overall configuration of the FIB device> First, the overall configuration of the FIB apparatus 100 capable of implementing the alignment method for micro-sample sampling of the present invention will be explained with reference to Figure 1. As shown in the figure, a probe needle 2 capable of contacting, lifting, and holding a micro-sample 1 cut from a base material BO is positioned in the chamber CH. The probe needle 2 is controlled by a drive device 3 to move in the three-dimensional XYZ directions within the chamber CH. In Figure 1, the X direction corresponds to the front-back direction, the Y direction to the left-right direction, and the Z direction to the up-down direction.

[0012] At the bottom of the chamber CH, there is a stage 5 on which the base material BO and the mesh 4 (the mounting base for the micro-sample 1) can be moved. The stage 5 is configured to be capable of moving in the X, Y, and Z directions, rotating within the XY plane, and tilting from the horizontal direction. The base material BO is placed on the upper surface of the stage 5. The mesh 4 is mounted on the upper surface of the mesh mounting portion 6. This mesh mounting portion 6 is attached to one end of the stage 5 and is integrated with the stage 5, and can be regarded as a part of the stage 5.

[0013] Also, an ion gun 7 that irradiates an ion beam onto the micro-sample 1, the probe needle 2, the mesh 4, etc. is located at the upper part of the chamber CH and is arranged above the stage 5. A secondary electron detector 8 that detects secondary electrons generated from the micro-sample 1, the probe needle 2, the mesh 4, etc. is arranged on the side surface of the chamber CH.

[0014] The secondary electron detector 8 is arranged on the side surface of the chamber CH, but the observation direction of the secondary electron image obtained from the secondary electron detector 8 is in the vertical direction (the downward direction as seen from the ion gun 7). This is because the path of the secondary electrons is curved by a magnetic field and guided to the secondary electron detector 8. And the FIB device 100 has a function of measuring the length of these objects based on the intensity distribution of the secondary electrons excited from the mesh 4, etc.

[0015] <The alignment method of the present invention> Next, based on FIGS. 2 to 4, the alignment method of the present invention will be described. As shown in FIG. 2, the probe needle 2 is brought into contact with the micro-sample 1, and in the state where the micro-sample 1 is adhered with the adhesive 9, it is lifted up. Next, the micro-sample 1 held by the probe needle 2 is brought into contact with and adhered to the pasting surface 4a on the upper surface of the rectangular parallelepiped mesh 4.

[0016] At this time, alignment (positioning) between the micro-sample 1 and the mesh 4 is necessary. In the present invention, by estimating the distance d between the micro-sample 1 and the pasting surface 4a of the mesh 4 using the length measurement function of the FIB device 100, the working time of alignment is shortened and the alignment accuracy is improved.

[0017] As shown in FIG. 3, with the stage 5 inclined by the inclination angle θ, the width W of the attachment surface 4a of the mesh 4 is measured. Then, while observing the secondary electron image, the micro-sample 2 is brought close to the mesh 1 until the tip (lower end) of the micro-sample 1 held by the probe needle 2 overlaps with the mesh 4 and the attachment surface 4a. In this state, the distance d between the micro-sample 1 and the attachment surface 4a of the mesh 4 can be estimated within the range of 0 to W / tanθ.

[0018] This is because, as shown in FIG. 3, if the tip of the micro-sample 1 is placed within the right triangle with vertices A, B, and C, the micro-sample 1 can be brought into contact with the attachment surface 4a of the mesh 4. Here, the vertices B and C of the right triangle are the upper end points of the mesh 4, and the vertex A is the intersection of the vertical plane passing through the vertex B and the extension of the side surface of the mesh 4. If the length of the side AC of the right triangle is L, then L = W / tanθ.

[0019] Hereinafter, the alignment method of the present invention will be described by taking the case of θ = 60°, W = 4.84μm, and L = 2.794μm as an example. FIG. 4 is a secondary electron image of the mesh 4. FIG. 5 is a secondary electron image showing the attachment of the micro-sample 1 to the mesh 4, where (a) is an image before the micro-sample 1 held by the probe needle 2 contacts the mesh 4, and (b) is an image after the micro-sample 1 contacts the mesh 4.

[0020] First, using the alignment method using parallax described in Patent Document 1, the micro-sample 1 is brought close to about 3μm on the mesh 4 by the coarse movement of the probe needle 2 (for example, the pulse mode of the piezo element) (see FIG. 5(a)).

[0021] Next, using the length measurement function of the FIB apparatus 100, as described above, the distance d between the micro-sample 1 and the mesh 4 can be estimated. The distance d when the tip of the micro-sample 1 approaches until it almost overlaps with the attachment surface 4a of the mesh 4 is within the range of 0 to 2.794μm, but in this example, it is 2μm or less (see FIG. 3).

[0022] Since the range of the fine movement of the probe needle 2 (for example, the linear mode of the piezoelectric element) is about ±2 μm, by bringing the minute sample 1 as close to the mesh 4 as possible with coarse movement, the minute sample 1 can be easily brought into contact with the attachment surface 4a of the mesh 4 with fine movement, thereby improving the success rate of mesh attachment (see Figure 5(b)).

[0023] <Explanation of the principle for realizing the present invention> The principle for realizing the alignment method of the present invention described above will be explained below. (1)Technical background In electron microscopes, such as SEM (Scanning Electron Microscope), an image of a sample is obtained by capturing secondary electrons and backscattered electrons excited from the sample into the detector (these are called secondary electron images and backscattered electron images), and various information can be obtained using this electron beam image.

[0024] One of the pieces of information needed is length measurement. In particular, in cross-sectional analysis, a length measurement function is required to observe the cross-section by tilting the sample, measure the distance between two points in the vertical direction of the cross-section (measure the difference in coordinate values ​​between the two points), and calculate the distance (line width) between the two points from the observation magnification. At this time, since the cross-sectional image is compressed vertically due to the tilt observation, a method of measuring the line width by making a correction according to the tilt angle is commonly used.

[0025] For example, the line width d of the cross section is calculated by applying corrections such as multiplying by 2 when the inclination angle θ is 30°, by 1.414 when it is 45°, and by 1.155 when it is 60° (see Figure 6). In this method, it is necessary to calculate the line width d of the cross section for each inclination angle θ using the following formula. d = l / cos(π / 2-θ) (2) Development of measurement technology In recent years, methods have been developed that allow observation of cross-sectional and three-dimensional shapes using multiple backscattered electron detectors. These methods have been commercialized primarily as backscattered electron images and surface relief images, and the measurement principles have also been made public. Furthermore, a method for measuring length from secondary electron intensity using multiple secondary electron detectors (secondary electron length measurement method) has also been developed. However, it currently has many drawbacks and is not yet complete as a practical product, so electron microscope manufacturers have not disclosed its principles or specific details. The present inventors used a FIB device that experimentally incorporated the secondary electron length measurement method to analyze its incomplete functionality and investigate methods for practical application.

[0026] This required constructing a theory and geometric model of the secondary electronic measurement method. Specifically, we derived the basic principle of a measurement method that does not require line width correction for each of the above-mentioned inclination angles θ. This basic principle is based on the following geometric model and mathematical theory (see Figure 7).

[0027] In Figure 7, the upper triangle corresponds to a cross-section with an inclination angle θ, and the relationship L = W / tanθ is derived, where L is the length of the cross-section. The lower triangle corresponds to a plane with an inclination angle θ, and the equation L = W / tanθ is derived, where W is the length of the inclined plane. However, care must be taken when using the formula L = W / tanθ, and an example of its actual application is shown below.

[0028] (3) Specific examples The length-measuring function developed by the electron microscope manufacturer also appeared to be a technology similar to the calculation formula L=W / tanθ, so the inventor verified this theory using a FIB device equipped with multiple secondary electron detectors.

[0029] Figure 8 shows an example of a micro-sample 1 of a trench-type power MOSFET attached to TEM mesh 4. The depth of the trench-type gate electrode and the thickness of mesh 4 are known to be 2.6 μm and 4.8 μm, respectively. Applying the above theory, the micro-sample 1 was tilted at three different angles θ: 30°, 45°, and 60°, and its length was measured.

[0030] (4) Applications of measured values In the case of a 60° incline, the trench gate depth and mesh 4 thickness are calculated to be 2.563 μm and 4.840 μm, respectively. This calculation yields values ​​(secondary values) that differ from the desired length measurement values.

[0031] For cross-sections, L = 2.563 μm (desired value) and W = 4.440 μm (secondary value) (see Figure 10(a)). For planes, W = 4.840 μm (desired value) and L = 2.794 μm (secondary value) (see Figure 10(b)). Figure 10(c) shows the geometric model of the measurement.

[0032] In the formula L = W / tanθ, L on the cross-section represents the longitudinal length of the cross-section, and W on the plane represents the longitudinal length of the plane. Using this formula, length can be measured without correcting for the angle θ. The W on the cross-section and L on the plane, which are calculated secondarily, are thought to represent physical quantities related to secondary electron intensity, and we attempted to clarify their geometric meanings and utilize them.

[0033] In the process of analyzing this length measurement function, we repeatedly performed simulations in which we used probe needle 2 to microsample a minute sample 1 and bring it closer to mesh 4, with W on the plane being the thickness of mesh 4. During these simulations, we noticed the range in which probe needle 2 appeared to overlap with mesh 4.

[0034] In a 30° inclined plane, the right-angled triangular region formed by W (=4.873) and L (=8.440) corresponds to the space in which the probe needle 2 can contact the mesh 4 (see Figure 11(a)). In a 45° inclined plane, the right-angled triangular region formed by W (=4.837) and L (=4.837) corresponds to the space in which the probe needle 2 can contact the mesh 4 (see Figure 11(b)). In a 60° inclined plane, the right-angled triangular region formed by W (=4.840) and L (=2.794) corresponds to the space in which the probe needle 2 can contact the mesh 4 (see Figure 11(c)).

[0035] The dashed arrows in Figure 11 indicate the range in which the probe needle 2 appears to overlap with the mesh 4. If the probe needle 2 appears outside this range, it cannot contact the mesh 4. In other words, the area of ​​the right triangle formed by W and L is the space in which the probe needle 2 can contact the mesh 4.

[0036] The inventor created a geometric model during the analysis and development of the length-measuring function of an electron microscope and theoretically derived the following principle of length measurement: L = W / tanθ The probing simulation performed in Figure 11 revealed that the right-angled triangular region formed by W and L represents the space in which the probe needle 2 can contact the mesh 4. Applying this principle to attaching a microscopic sample 1 to a mesh 4, it becomes possible to bring the microscopic sample 1 close to the mesh 4 without worry by observing the secondary electron image of the sample 1 overlapping the mesh 4.

[0037] For example, if mesh 4 is tilted at 60°, it is a plane with θ=60°, so W=4.840μm and L=2.794μm. Microsample 1 can only come into contact with mesh 4 when it enters the right-angled triangular region (4.840*2.794μm) formed by W and L (see Figure 12).

[0038] The meaning of the calculated numerical value (secondary value) of secondary electron intensity is unclear, but it can be used as a geometric dimension in applications. In this example, the meaning of L = 2.794 μm is clear. Measurement information (the value of L on a plane) can now be used in microsampling systems. [Explanation of Symbols]

[0039] 1. Microsample 2 probe needles 3. Drive unit 4 mesh 4a Adhesive surface 5 stages 6 Mesh mounting section 7 Ion gun 8 Secondary electron detector 9 Adhesive 100 FIB equipment BO base material CH Chamber

Claims

[Claim 1] A probe needle capable of contacting, lifting, and holding a minute sample, a mesh to which the minute sample held by the probe needle can be attached, and a tiltable stage on which the mesh is mounted. The present invention provides a FIB (Fixed Ion Blast) apparatus comprising a charged particle gun that irradiates a minute sample, probe needle, and mesh with a charged particle beam, and a secondary electron detector that detects secondary electrons generated from the minute sample, probe needle, and mesh, and having a function to perform length measurement based on a secondary electron image obtained from the secondary electron detector. In the alignment of a microsample held by a probe needle and a mesh, With the stage tilted at an angle θ, measure the width W of the surface on which the mesh is attached. An alignment method for sampling minute samples, characterized by estimating the distance between the minute sample and the mesh within the range of 0 to W / tanθ by bringing the minute sample closer to the mesh while observing a secondary electron image until the tip of the minute sample held on the probe needle overlaps with the mesh, and performing alignment based on this distance.

Images