Sample thickness estimation method and sample thickness estimation device

Abstract

To provide a sample thickness estimation method for estimating, with high accuracy, the thickness of a sample processed by the focused ion beam (FIB) method.SOLUTION: A sample thickness estimation method includes the steps of: manufacturing a plate-like sample and a rod-like sample on a sample fixation plate by the focused ion beam (FIB) method; fixing the sample fixation plate on which the plate-like sample is placed to a holder for a transmission electron microscope, and performing first measurement of the thickness tC of a single crystal layer of the plate-like sample from an image obtained by transmitting an electron beam through the plate-like sample; fixing the sample fixation plate on which the rod-like sample is placed to the holder, and measuring the thickness tD of a damage layer of the rod-like sample from an image obtained by transmitting the electron beam through the rod-like sample; and calculating the thickness t of the plate-like sample from the formula (1) by using the thickness tC of the single crystal layer and the thickness tD of the damage layer. In the first measurement, the plate-like sample is inclined so as to form an angle α in a first surface and an angle β in a second surface orthogonal to the first surface with respect to a transmission path of the electron beam, and measurement by the convergent-beam electron diffraction (CBED) method is executed.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method and an apparatus for estimating the thickness of a sample. [Background technology]

[0002] The single-crystal structure specimens observed or analyzed with a transmission electron microscope (TEM) or scanning transmission electron microscope (STEM) are small plate-shaped pieces with a thickness of about 100 nm. The specimens are prepared using the focused ion beam (FIB) method. The FIB method focuses ions accelerated to several tens of kV or less and causes them to collide with the specimen. The FIB method causes successive collisions between atoms near the surface of the specimen (collision cascade), which results in the release of atoms, and the surface of the specimen is scraped off (machined).

[0003] In samples obtained by the FIB method, a damage layer caused by collision cascades is formed on the side surface of a certain thickness in the depth direction from the surface of the sample. While the center of the sample maintains a single crystal structure, the side surface where the collision cascade occurred loses its original atomic arrangement and becomes amorphous or fine polycrystalline.

[0004] Observation of a sample using a transmission electron microscope or a scanning transmission electron microscope requires obtaining information on the thickness of the sample. Convergent-beam electron diffraction (CBED) is known as a method for measuring the thickness of a sample (Non-Patent Document 1), but the CBED method can only measure the thickness of the portion having a single crystal structure. When thickness measurement using the CBED method is applied to a sample obtained using the FIB method, the thickness of the damaged layer on the side surface is not included, so the evaluation of the obtained thickness information is underestimated. The thickness of the damaged layer can be thinned by performing finishing processing using an ion beam with a low acceleration voltage of several kV or less, but this requires high skill and a long time, and it is extremely difficult to confirm whether the thickness has become negligible. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Journal of the Japanese Society of Crystallography 44, 150-160 (2002) Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention has been made in consideration of the above circumstances, and has an object to provide a method and apparatus for estimating the thickness of a sample, which makes it possible to estimate with high accuracy the thickness of a sample prepared (processed) by the FIB method. [Means for solving the problem]

[0007] In order to solve the above problems, the present invention employs the following means.

[0008] (1) A method for estimating the thickness of a sample according to one aspect of the present invention includes a step of preparing a rod-shaped sample on a first sample fixing plate together with a plate-shaped sample to be subjected to thickness evaluation by using a focused ion beam method, a step of fixing the first sample fixing plate on which the plate-shaped sample is placed to a first holder for a transmission electron microscope, and a step of detecting a thickness t of a single crystal layer at the center of the plate-shaped sample from a first image obtained by transmitting an electron beam through the plate-shaped sample. C a first measurement step of measuring a thickness t of a damage layer formed on an outer periphery of the rod-shaped sample from a second image obtained by transmitting an electron beam through the rod-shaped sample, and D A second measurement step of measuring the thickness t C and the thickness of the damage layer t D and a thickness calculation step of calculating a thickness t of the plate-like sample by substituting the above equation (1) into the following equation, wherein in the first measurement step, the thickness direction of the plate-like sample is tilted relative to the transmission path of the electron beam so as to form an angle α in a first plane that includes the transmission path and an angle β in a second plane that includes the transmission path and is perpendicular to the first plane, and measurement is performed using a CBED method.

[0009]

number

[0010] (2) In the sample preparation step of the method for estimating the thickness of a sample described in (1) above, it is preferable that the plate-shaped sample and sample pieces for preparing the rod-shaped sample are placed on a second sample fixing plate and then shaped, and the second sample fixing plate is prepared in a state where it is fixed to a second holder for a focused ion beam irradiation device.

[0011] (3) In the method for estimating a thickness of a sample according to either of (1) or (2) above, it is preferable that the angle α and the angle β are in the ranges of −30 degrees or more and +30 degrees or less, respectively.

[0012] (4) In the method for estimating the thickness of a sample described in any one of (1) to (3) above, it is preferable that the plate-shaped sample and the rod-shaped sample contain any material selected from silicon, indium phosphide, gallium arsenide, aluminum, magnesium, and zinc.

[0013] (5) An apparatus for estimating the thickness of a sample according to one embodiment of the present invention includes a focused ion beam irradiation apparatus and a transmission electron microscope, wherein the focused ion beam irradiation apparatus is provided with a fixing means for fixing a plate-shaped sample and a rod-shaped sample on one side of a second sample fixing plate fixed to a second holder for the focused ion beam irradiation apparatus, and the transmission electron microscope is provided with a control means for controlling the position and orientation of the first sample fixing plate, which is fixed to a first holder for the transmission electron microscope and supports the plate-shaped sample and the rod-shaped sample. Effect of the Invention

[0014] According to the present invention, it is possible to provide a method and apparatus for estimating the thickness of a sample that has been fabricated (processed) by an FIB method, which makes it possible to estimate the thickness of the sample with high accuracy. [Brief description of the drawings]

[0015] [Figure 1]1A and 1B are diagrams illustrating a sample preparation process in a method for estimating a thickness of a sample according to an embodiment of the present invention, in which (a) is a diagram illustrating a preparation process of a plate-shaped sample, and (b) is a diagram illustrating a preparation process of a rod-shaped sample. [Diagram 2] FIG. 2 is a perspective view of a plate-shaped sample in the manufacturing process. [Diagram 3] FIG. 1 is a perspective view of a needle-shaped sample during the preparation process. [Figure 4] FIG. 11 is a diagram illustrating a first measurement step in the estimation method. [Diagram 5] 13(a) and 13(b) are plan views of the tilted first sample fixing plate in the first measurement step, viewed from two different directions in a plane perpendicular to the transmission direction of the electron beam. [Figure 6] 2 is an enlarged cross-sectional view of the plate-shaped sample, showing the vicinity of a portion through which an electron beam passes. [Figure 7] FIG. 11 is a diagram illustrating a second measurement step in the estimation method. [Figure 8] 1 is an enlarged cross-sectional view of a needle-shaped sample showing a region from its tip to a portion through which an electron beam passes. [Figure 9] 1 shows an image (top) and a brightness profile (bottom) of a cross section of a needle-shaped sample used in an example. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, a method and an apparatus for estimating the thickness of a sample according to an embodiment of the present invention will be described in detail with reference to the drawings. Note that the drawings used in the following description may show characteristic parts in an enlarged scale for the sake of convenience in order to make the characteristics easier to understand, and the dimensional ratios of each component may not necessarily be the same as in reality. In addition, the materials, dimensions, etc. exemplified in the following description are merely examples, and the present invention is not limited thereto, and may be appropriately modified and implemented within the scope of the present invention.

[0017] The "method of estimating thickness of a sample" according to one embodiment of the present invention is a method of estimating the thickness of a plate-shaped sample that is difficult to measure, and mainly includes a sample preparation step, a first measurement step, a second measurement step, and a thickness calculation step. The plate-shaped sample and the rod-shaped sample in the initial state have a single crystal structure or a crystal structure close to the single crystal structure. The plate-shaped sample and the rod-shaped sample are made of the same material, and the material includes any material selected from, for example, silicon, indium phosphide, gallium arsenide, aluminum, magnesium, zinc, etc.

[0018] The shape and size of the plate-shaped sample are not particularly limited, but in this embodiment, a rectangular parallelepiped sample having two main surfaces that are approximately rectangular and of equal size is used as an example. The shape and size of the rod-shaped sample are not particularly limited, but it is preferable that the rod-shaped sample has a shape that extends in one direction and is needle-shaped with one end pointed. In this embodiment, a needle-shaped sample is used as the rod-shaped sample.

[0019] 1(a) and (b) are diagrams for explaining the sample preparation process. In the sample preparation process, a plate-shaped sample 103 to be evaluated for thickness is prepared using a focused ion beam (FIB) method, as well as a rod-shaped sample 104 to be used for evaluating the thickness of the plate-shaped sample 103. Specifically, in an FIB device 101, sample pieces for preparing the plate-shaped sample 103 and the rod-shaped sample 104 are placed on one surface 102a of one sample fixing plate (second sample fixing plate) 102, and then shaped and processed, and the sample fixing plate 102 is fixed to a second holder (not shown) for a focused ion beam irradiation device. The plate-shaped sample 103 and the rod-shaped sample 104 can be fixed to the one surface 102a of the sample fixing plate 102 using, for example, a metal-containing gas. A metal-containing gas supplying means is used as the fixing means 114, and a focused ion beam I is irradiated while a metal-containing gas G is sprayed between the one surface 102a of the sample fixing plate 102 and the plate-shaped sample 103 or rod-shaped sample 104. As a result, a metal deposit is formed between the one surface 102a of the sample fixing plate 102 and the plate-shaped sample 103 or rod-shaped sample 104. The plate-shaped sample 103 and the rod-shaped sample 104 are fixed to the one surface 102a of the sample fixing plate 102 via the metal deposit. The one surface 102a of the sample fixing plate 102 is considered to have a tip surface that is smooth enough not to prevent the intrusion of the metal-containing gas G used for fixing.

[0020] In this state, first, the position of the sample fixing plate 102 is adjusted as shown in Fig. 1(a), and the plate-shaped sample 103 on the sample fixing plate 102 is irradiated with a focused ion beam I from an ion source 105 to process the surface layer of the plate-shaped sample 103. Next, as shown in Fig. 1(b), the position of the sample fixing plate 102 is adjusted, and the rod-shaped sample 104 on the sample fixing plate 102 is irradiated with a focused ion beam I from the ion source 105 to process the surface layer of the rod-shaped sample 104. Here, the case where the plate-shaped sample 103 and the rod-shaped sample 104 are produced in this order is illustrated, but the rod-shaped sample 104 and the plate-shaped sample 103 may also be produced in this order.

[0021] During irradiation with the focused ion beam I, the sample fixing plate 102 is tilted so that one side surface 103a of the plate-shaped sample 103 and the tip 104a of the needle-shaped sample 104 face the irradiated focused ion beam I. The state of the processed portions of the plate-shaped sample 103 and the rod-shaped sample 104 can be observed using a scanning electron microscope (SEM) 106. The position of the sample fixing plate 102 is configured so that it can be changed according to the position of the processing target.

[0022] 2 and 3 are perspective views of the plate-shaped sample 103 and the needle-shaped sample 104 in the preparation process, respectively. In both the plate-shaped sample 103 and the needle-shaped sample 104, the surface layer irradiated with the focused ion beam I becomes a damaged layer in which the atomic arrangement of the crystal structure is broken by collision cascades and the crystal structure is made amorphous or finely polycrystalline. Therefore, the prepared plate-shaped sample 103 is composed of a central single crystal layer 103C that maintains the single crystal structure and a damaged layer 103D formed on the outer periphery so as to cover the single crystal layer 103C. The prepared needle-shaped sample 104 is composed of a central single crystal layer 104C that maintains the single crystal structure and a damaged layer 104D formed on the outer periphery so as to cover the single crystal layer 104C.

[0023] 4 is a diagram for explaining the first measurement step. In the first measurement step, an image (first image) of the plate-shaped sample 103 is obtained by a convergent beam electron diffraction (CBED) method using a transmission electron microscope 107, and a thickness t c Measure.

[0024] The transmission electron microscope 107 is a commonly known transmission electron microscope (TEM, STEM), and mainly comprises a sample fixing plate (first sample fixing plate) 108 that supports a sample, an electron gun 109 that irradiates the sample with an electron beam E, an optical system (lens coil (electron lens), focusing lens aperture, etc.) 110 that adjusts the width of the electron beam E, and an output device 111 that detects the electron beam E that has passed through the sample and outputs the imaging state. Here, the sample fixing plate 108 is illustrated large in order to make it easier to understand the main configuration of the transmission electron microscope 107, but in reality, the sample fixing plate 108 and the first holder have a structure that does not impede the transmission of the electron beam.

[0025] The transmission electron microscope 107 preferably further includes a control means 112 for controlling the position and orientation of the sample fixing plate 108 supporting the plate-shaped sample 103 and the rod-shaped sample 104. The control means 112 has a function of moving the sample fixing plate 108 so that the sample is positioned at a position through which the electron beam E passes. The control means 112 also has a function of controlling the inclination angle of one surface 108a of the sample fixing plate with respect to the direction through which the electron beam E passes.

[0026] Before the electron beam E is transmitted through the plate-shaped sample 103, the plate-shaped sample 103 is placed on a sample fixing plate 108, the sample fixing plate 108 is fixed to a first holder (not shown) for a transmission electron microscope, and the position and orientation of the sample fixing plate 108 are adjusted. The plate-shaped sample 103 is placed on the sample fixing plate 108 so that the other side surface 103b of the plate-shaped sample 103 is fixed to and supported by one surface 108a of the sample fixing plate 108. The position and orientation of the sample fixing plate 108 are adjusted so that the electron beam E transmits obliquely through the plate-shaped sample 103, i.e., in a direction inclined with respect to the thickness direction T of the plate-shaped sample 103.

[0027] In other words, the position and orientation of the sample fixing plate 108 are adjusted to tilt the thickness direction T of the plate-shaped sample 103 so that the sample fixing plate 108 forms an angle α with respect to the transmission path of the electron beam E in a first plane (YZ plane) including the transmission path and an angle β with respect to the second plane (ZX plane) including the transmission path and perpendicular to the first plane. FIG. 5(b) is a diagram showing a state in which the sample fixing plate 108 is tilted by the angle α in the first plane. FIG. 5(a) is a diagram showing a state in which the sample fixing plate 108 is tilted by the angle β in the second plane. It is preferable that the tilt angles α and β are both -30 degrees or more and +30 degrees or less.

[0028] 6 is an enlarged cross-sectional view of the plate-shaped sample 103 near a portion through which the electron beam E passes. Due to the transmission of the electron beam E, only the overlapping portion of the single crystal layer 103C and the damaged layer 103D of the plate-shaped sample 103 can be visually recognized. From this image, it is not possible to visually recognize the thickness of the single crystal layer 103C, the damaged layer 103D, or the entire sample 103. Therefore, the thickness t C is measured by the CBED method. Since the CBED pattern is generated only by diffraction at the single crystal layer 103C, even if the damaged layer 103D is overlapped, the thickness t C can be measured.

[0029] 7 is a diagram for explaining the second measurement step. In the second measurement step, an image (second image) of the needle-shaped sample 104 is obtained using a transmission electron microscope 107, and the thickness t D Measure.

[0030] Before the electron beam E passes through the needle-shaped sample 104, the needle-shaped sample 104 is placed on the sample fixing plate 108, and the position and orientation of the sample fixing plate 108 are adjusted. The needle-shaped sample 104 is placed on the sample fixing plate 108 so that the rear end 104b of the needle-shaped sample 104 is fixed to and supported by one surface 108a of the sample fixing plate 108. The position and orientation of the sample fixing plate 108 are adjusted so that the electron beam E passes straight through the needle-shaped sample 104, i.e., in a direction perpendicular to the longitudinal direction L of the needle-shaped sample 104.

[0031] 8 is an enlarged view of a region from the tip 104a to the part through which the electron beam E passes, in a cross section of the needle-shaped sample 104 viewed in a plane from the transmission direction of the electron beam E. A protective layer 113 is formed on the tip 104a.

[0032] Since the damaged layer 104D is formed all around the needle-shaped sample 104, not only the overlapping portion of the single crystal layer 104C and the damaged layer 104D but also the portion where only the damaged layer 104D covers the single crystal layer 104C can be visually recognized as an image by the transmission of the electron beam E. The damaged layer 104D is formed at the tip 104A and the outer periphery 104B of the needle-shaped sample 104. From the obtained image of the needle-shaped sample, the thickness t D4 , t D5 Measure the total t D (=t D4 +t D5 ) is calculated.

[0033] The thickness t of the damage layer formed in the outer peripheral portion 104B D4 , t D5 is constant over the entire circumference of the needle-shaped sample 104, and this thickness is considered to be the same as the thickness of the damaged layer 103D formed on the surface of the plate-shaped sample 103 that has been subjected to FIB processing under the same conditions. Therefore, the thickness of the damaged layer 103D of the plate-shaped sample 103, which is difficult to measure, can be calculated by subtracting the thickness t D can be estimated from

[0034] Finally, in the thickness calculation step, the thickness t C , the thickness of the damaged layer obtained in the second measurement process t D By substituting these values ​​into the following formula (1), the true thickness t of the plate-shaped sample 103 can be calculated. Note that the order of the first measurement step and the second measurement step may be reversed from that described above, or both steps may be performed simultaneously.

[0035]

number

[0036] As described above, according to the method for estimating the thickness of a sample of this embodiment, the thickness t D Although it cannot be measured directly, it can be accurately estimated from the thickness of the damaged layer of a needle-shaped specimen prepared under the same conditions as the plate-shaped specimen. The estimated thickness of the damaged layer t D After correcting for the tilt angles α and β, the thickness t of the single crystal layer detected by the transmission electron microscope is C By adding this to the thickness of the FIB processed sample, the thickness can be estimated with high accuracy. EXAMPLES

[0037] The effects of the present invention will be more clearly understood by the following examples. Note that the present invention is not limited to the following examples, and can be modified as appropriate without departing from the spirit and scope of the present invention.

[0038] A silicon needle-shaped sample processed by the FIB method was observed using a transmission electron microscope. As shown in Figure 7, the sample fixing plate was tilted by 90 degrees, and an electron beam was transmitted through the needle-shaped sample in a direction perpendicular to its longitudinal direction. Figure 9 shows a bright-field image (top) of the needle-shaped sample corresponding to Figure 8 obtained by this observation with the incidence on the

[0112] zone axis, and the corresponding brightness profile (bottom). The horizontal axis of the brightness profile indicates the distance (nm) from one end (left end) in the short direction (Y direction) when viewed in a plan view from the transmission direction of the electron beam. The vertical axis of the brightness profile indicates brightness.

[0039] 9, it can be seen that the brightness changes discontinuously between the single crystal layer 104C and the damaged layer 104D, and that the brightness of the damaged layer 104D is four or more times that of the single crystal layer 104C. Therefore, the thicknesses of the single crystal layer 104C and the damaged layer 104D of the needle-shaped sample can be measured from the brightness profile.

[0040] Needle-shaped samples made of silicon, indium phosphide, and aluminum processed by the FIB method were prepared, and their thicknesses were obtained by two methods. First, as the method of Comparative Examples 1 to 3, only measurements were performed by the CBED method to obtain their thicknesses. Next, as the method of Examples 1 to 3, measurements were performed by the CBED method and by visual measurement using the brightness profile to obtain their thicknesses. Specifically, the thickness measured by the CBED method was added to the thickness measured by the CBED method, and the thickness of the damage layer visually observed in the brightness profile was added. The thickness measurement by the CBED method here was performed on the thickest part. The position of the thickest part was obtained by using the electron energy loss spectroscopy (EELS) method. The measurement results of Comparative Examples 1 to 3 and Examples 1 to 3 are shown in Table 1.

[0041] [Table 1]

[0042] In all samples, the thickness of the needle-shaped sample measured only by the CBED method is approximately the same as the thickness of the single crystal layer visible when tilted 90 degrees. From these results, it can be seen that the thickness measured only by the CBED method as in Comparative Examples 1 to 3 does not include the thickness of the damage layer. In contrast, it can be seen that the true thickness of the needle-shaped sample can be obtained by taking into account the thickness of the visible damage layer and adding it to the thickness measured by the CBED method as in Examples 1 to 3. It is considered that the thickness of the damage layer of the plate-shaped sample is approximately the same as the thickness of the damage layer of the needle-shaped sample formed under the same conditions as the plate-shaped sample. Therefore, although the thickness of the damage layer of the plate-shaped sample to be evaluated cannot be measured visually, the true thickness of the plate-shaped sample can be estimated with high accuracy by regarding the thickness of the damage layer of the needle-shaped sample as the damage layer of the plate-shaped sample and adding it to the thickness of the single crystal layer of the plate-shaped sample. [Explanation of symbols]

[0043] 101...FIB device 102: Sample fixing plate (second sample fixing plate) 103 Plate-shaped sample 103C: Single crystal layer of plate-shaped sample 103D Damage layer of plate specimen 104...Needle-shaped sample (rod-shaped sample) 104C: Single crystal layer of needle-shaped sample 104D: Damaged layer of needle-shaped specimen 105...Ion source 106 Scanning Electron Microscope 107 Transmission Electron Microscope 108: Sample fixing plate (first sample fixing plate) 109...Electron gun 110...Optical system 111 Output device 112... Control means 113...protective layer 114...Fixing means E...electron beam I Focused ion beam G Metal-containing gas

Claims

1. a sample preparation step of preparing a rod-shaped sample on a first sample fixing plate together with a plate-shaped sample to be subjected to thickness evaluation by using a focused ion beam method; The first sample fixing plate on which the plate-shaped sample is placed is fixed to a first holder for a transmission electron microscope, and a thickness t of the single crystal layer at the center of the plate-shaped sample is obtained from a first image obtained by transmitting an electron beam through the plate-shaped sample. Ｃ A first measurement step of measuring The first sample fixing plate on which the rod-shaped sample is placed is fixed to the first holder, and a thickness t of a damage layer formed on the outer periphery of the rod-shaped sample is obtained from a second image obtained by transmitting an electron beam through the rod-shaped sample. Ｄ A second measurement step of measuring The measured thickness t of the single crystal layer Ｃ and the thickness t of the damage layer Ｄ a thickness calculation step of calculating the thickness t of the plate-shaped sample by substituting the above into the following formula (1), A method for estimating the thickness of a sample, characterized in that in the first measurement step, the thickness direction of the plate-shaped sample is tilted relative to the transmission path of the electron beam so as to form an angle α in a first plane that includes the transmission path and an angle β in a second plane that includes the transmission path and is perpendicular to the first plane, and measurement is performed using a CBED method. [0010]

2. The method for estimating the thickness of a sample according to claim 1, characterized in that in the sample preparation process, sample pieces for preparing the plate-shaped sample and the rod-shaped sample are placed on a second sample fixing plate and then shaped, and the second sample fixing plate is prepared in a state where it is fixed to a second holder for a focused ion beam irradiation device.

3. 3. The method for estimating a thickness of a sample according to claim 1, wherein the angle α and the angle β are set in the ranges of -30 degrees or more and +30 degrees or less, respectively.

4. 3. The method for estimating a thickness of a sample according to claim 1, wherein the plate-shaped sample and the rod-shaped sample contain any material selected from the group consisting of silicon, indium phosphide, gallium arsenide, aluminum, magnesium, and zinc.

5. A focused ion beam irradiation device; a transmission electron microscope; the focused ion beam irradiation device includes a fixing means for fixing a plate-shaped sample and a rod-shaped sample on one surface of a second sample fixing plate fixed to a second holder for the focused ion beam irradiation device; The transmission electron microscope is fixed to a first holder for the transmission electron microscope, and is characterized in that it is equipped with a control means for controlling the position and orientation of a first sample fixing plate supporting the plate-shaped sample and the rod-shaped sample.

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