JP2004264145A - Method for making transmission electron microscope observation specimen - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for making a transmission electron microscope observation specimen for satisfactory TEM observation by removing roughness on an amorphous layer or on an observed cross-section, these being formed by focused ion beam irradiation in manufacturing. <P>SOLUTION: The observation specimen is made out of an observation specimen substrate, the specimen having a structure made by together combining a columnar specimen body 3, a pyramid-shaped support body 4 projecting in a columnar shape vertically from the specimen body 3, and a thinned part 5, worked/formed to have a width including an observed area, thin-formed, and equipped with an observation face, on an end of the pyramid-shaped support body. The observation face is sputter-etched by irradiating the observation face with an ion beam at a low angle. This makes it possible to remove roughness on the amorphous layer or on the observed cross-section formed by the focused ion beam irradiation, to make the transmission electron microscope specimen with high quality, and to perform satisfactory TEM observation. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for preparing a transmission electron microscope observation sample by a focused ion beam (hereinafter, abbreviated as FIB) method, in which a detailed cross-sectional structure of a multilayer film including a metal layer, a semiconductor device, or the like is examined by a transmission electron microscope. (Transmission Electron Microscope: hereinafter referred to as TEM).
[0002]
[Prior art]
When observing the sample using a transmission electron microscope, the sample is mounted on a hole of about 3 mmφ formed in the sample folder using a C-shaped ring, and is irradiated with an electron beam. To be observed. For this purpose, as shown in FIG. 19, the transmission electron microscope observation sample 1 has a columnar sample body 3 which is a holding portion having a strength enough to be self-supporting, and a thinned portion having a thickness small enough to transmit an electron beam. It consists of five.
[0003]
Conventionally, such a sample has been thinned by etching the observation site of the sample using a focused ion beam (see Patent Documents 1 and 2).
Therefore, the production of a transmission electron microscope observation sample using this focused ion beam will be described below with reference to FIGS.
First, as shown in FIG. 13, a sample such as the sample substrate 111 in which the observation site 2 requiring the TEM observation exists in the fine pattern on the sample substrate 111 is prepared, and this is set to a dummy substrate larger than the sample substrate 111. 12 is bonded and fixed as flat as possible using a resin wax or the like. Next, as shown in FIG. 14, the projection 13 is formed by cutting the substrate in the longitudinal direction shallowly by straight-type blade dicing so that the observation site 2 remains. The thickness of the convex portion 13 is processed as thin as possible so as not to be damaged by chipping or the like in consideration of the FIB processing time in a later process. Next, as shown in FIGS. 15 and 16, a size (for example, a thickness of about 0.2 mm × a length of about 2 mm, which can be inserted into a column of an electron microscope (hereinafter referred to as an apparatus), and a height of the 17), the sample is deeply cut in the longitudinal direction of the substrate and then in the direction perpendicular to the longitudinal direction of the substrate to obtain a columnar sample having a convex cross section as shown in FIG. Next, the columnar sample having a convex cross section is attached to a reinforcing ring (for example, a C-shaped ring made of SUS), and as shown in FIG. 18, an appropriate thickness (for example, A thin TEM sample is prepared by exposing the thinned portion observation surface 18 to about 0.1 μm), and the observation electron beam 19 is inserted into the thinned portion observation surface 18 as shown in FIG. Irradiate and observe with TEM.
[0004]
[Patent Document 1] JP-A-7-318468 [Non-Patent Document] JP-A 10-221227 [0005]
[Problems to be solved by the invention]
However, in the conventional TEM sample manufacturing method, a damaged layer is formed on the thinned portion observation surface 18 by ion beam etching, and an oxide-based thin film superconducting device mainly composed of heavy elements, When thinning a semiconductor device having a complicated microstructure including a metal layer, the former is harder to etch as a heavy element, so the TEM image is generally unclear, and the latter is etched with a heavy element-based part that is hard to be etched. An etching rate difference is generated between the light-element-based portion and the interface, and a step is formed at the interface, and the thinned portion observation surface 18 is easily roughened, which is a major problem in the current TEM sample preparation and observation.
[0006]
Various factors have been considered for the formation of the damaged layer, and the most significant effect is the acceleration voltage of FIB irradiation. The acceleration voltage of the FIB is usually 30 kV, but when the Si crystal is thinned by the FIB of 30 kV, an amorphous damage layer of about 30 nm is formed on only one side. If the reduced thickness is about 0.1 μm, 60% of the thickness is covered with the damaged layer. Here, in order to examine the effect of the acceleration voltage on the sample in the FIB irradiation, the thinned portion observation surface 18 of the Si crystal thinned by the 30 kV FIB was further irradiated in parallel with the 25 kV FIB in parallel with the damage. It was found that about 20% of the layer was removed. This suggests that the damage layer can be removed by lowering the acceleration voltage. As described above, it has been clarified that the step at the interface due to the difference in etching rate can be reduced and a smooth observation cross section can be expected by using the technique of lowering the acceleration voltage.
Therefore, in order to obtain a smooth observation section while minimizing the roughness of the observation section while simultaneously removing the damaged crystalline layer to expose a sound crystalline portion as much as possible, it is necessary to use secondary polishing such as ion polishing with a low acceleration voltage. Processing is required, but there is a problem that it is difficult to perform secondary processing in a columnar cutout having a convex cross section as in the related art.
[0007]
The present invention has been completed on the basis of the knowledge on FIB irradiation obtained as a result of the above-described study, and removes a damaged layer formed by the influence of FIB irradiation and roughness of an observation cross section, thereby enabling good TEM observation. An object of the present invention is to provide a method for preparing a sample observed by a transmission electron microscope.
[0008]
[Means for Solving the Problems]
The present invention relates to a method for preparing a transmission electron microscope observation sample in which a region to be observed in a columnar sample having a projection at a tip is thinned to expose an observation surface.
A first step of cutting out the columnar sample from the sample substrate having the observed area so that the observed area remains;
A second step of irradiating the first ion beam from a direction perpendicular to the top surface of the projection to thin the projection to form a thinned portion;
And a third step of irradiating a second ion beam at a low irradiation angle onto the thinned portion plane serving as the observation surface.
[0009]
It is preferable that the columnar sample has a structure in which the sample main body, the convex portion, and a thinned portion formed on the convex portion are integrated.
[0010]
A focused gallium ion beam can be thinned as the first ion beam and argon ions can be thinned as the second ion beam.
[0011]
It is preferable that the acceleration voltage of the second ion beam is lower than that of the first ion beam. When the irradiation angle of the second ion beam to the thinned portion is α, it is preferable that the second ion beam be irradiated at an angle of 0 ° <α ≦ 30 ° (where α is the irradiation angle). Further, it is preferable that the second ion beam selectively irradiates the thinned portion.
[0012]
According to the method of the present invention, the column-shaped sample main body 3, the pyramid-shaped support 4 protruding in a column shape in the vertical direction of the column-shaped sample main body 3, and the observation region 6 at the tip of the pyramid-shaped support 4 are included. By adopting a structure in which the front end of the support 4 processed and shaped into a width and the thinned portion 5 including the thinned portion observation surface 18 formed by flake processing at the front end of the support 4 are integrated, the thinning by the ion beam is performed. Secondary processing such as ion polishing after the formation can be facilitated, the damaged layer due to FIB irradiation can be efficiently removed, and roughness of the observed cross section caused by the difference in constituent elements and the influence of the cross sectional structure can be reduced.
[0013]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 2 is a perspective view showing a base material for preparing a TEM observation sample according to the present invention. 3 to 6 are perspective views showing a first step of the present invention. FIG. 7 is a perspective view showing the shape of a sample manufactured by the first step of the present invention. FIG. 8 is a perspective view showing a second step of the present invention, and is a view showing the relationship between the thinned portion and the irradiation direction of the first ion beam. FIG. 9 is an embodiment of the present invention, in which (a) is a cross-sectional view showing a thinned state 1 and (b) is a plan view showing a thinned state 2. FIG. 10 is a perspective view showing a third step of the present invention, showing the relationship between the thinned observation cross section, the irradiation direction of the second ion beam, and the rotation of the sample. FIG. 11 is a diagram showing a relationship between a thinned observation cross section and an irradiation angle of the second ion beam in the embodiment of the present invention. FIG. 12 is a diagram showing the relationship between the observation sample of the present invention and the electron beam irradiation direction of the TEM.
[0014]
Hereinafter, the present embodiment will be described according to the sequential steps.
First, as shown in FIG. 2, a sample substrate 11 in which an observation site 2 requiring TEM observation is present in a fine pattern on the sample substrate 11 is prepared. Adhesively fix as flat as possible using Since the surface of this sample is easily affected by the FIB irradiation described later, a protective film having a thickness considering the influence of the FIB irradiation is formed on the surface of the substrate 1 in advance by a means such as a vacuum evaporation method according to the observation purpose. Is desirable.
[0015]
Next, as shown in FIG. 3, a tapered convex portion 13 is formed by cutting the substrate in the longitudinal direction by taper type blade dicing so that the observation site 2 remains. What should be noted here is the depth of cut in the longitudinal direction of the substrate and the thickness of the tip of the projection. The cutting depth in the longitudinal direction of the substrate varies depending on the sample form and properties, but it is necessary to secure a sufficient height in consideration of the irradiation efficiency of ion polishing in the subsequent process, so shaping in the range of tens to hundreds of μm Is desirable. The thickness of the tip of the convex portion needs to be shaped so as not to be damaged by chipping in consideration of the FIB processing time in the subsequent process. For example, the thickness is preferably 0.03 mm or less for a Si substrate, and 0.05 mm or less for a hard and brittle transparent substrate.
[0016]
Next, as shown in FIG. 4, the longitudinal direction of the substrate is deeply cut by straight-type blade dicing so as to have a thickness of about 0.2 mm or less around the observation site 2, and the sample substrate 11 is completely cut into a plurality.
[0017]
Next, as shown in FIG. 5, a part of the projection 13 is removed so that the observation site 2 remains, and the support 4 is formed. The shape and height of the support 4 are desirably a linear tapered shape as shown in FIG. 7, and the height is desirably shaped in the range of several tens to several hundreds of μm. For example, if the cutting depth in the above step (FIG. 3) is about 0.1 mm, the height of the support 4 is also adjusted to about 0.1 mm and shaped. An important point to note here is the width of the support 4 (the longitudinal direction of the base). The width of the support 4 is desirably shaped to the same width as the thinned portion 5 to be formed in the observation area 6 as shown in FIG. 8, but if such shaping is difficult, the second step described later is used. 9A, an unnecessary portion at the tip of the support 4 is removed to an appropriate depth according to the width of the thinned portion 5 as shown in FIG. 9A, or both ends of the thinned portion 5 as shown in FIG. It is more effective to remove wedges from the thinned portion observation surface 18 to an appropriate depth. The support 4 does not require strict processing accuracy as long as the support 4 can be shaped to a level at which the support 4 is not damaged. Next, as shown in FIG. 6, the longitudinal section of the base is deeply cut so as to have a length of about 2 mm or less that can be inserted into the apparatus barrel around the observation site 2, and completely cut off from the base. By the above steps, the columnar sample main body 3 as shown in FIG. 7, the pyramid-shaped support 4 protruding in the vertical direction of the columnar sample main body 3 in a branch shape, and the observation region 6 at the tip of the pyramid-shaped support 4 are formed. A columnar transmission electron microscope sample 1 having a structure in which the tip of the support 4 processed and shaped to include the width is integrated is obtained. Here, when it is difficult to confirm the position of the observation site 2 in the formation of the support 4, the protective film is formed in advance, and the periphery of the observation site 2 is marked by the FIB. If there is a risk of film peeling, the protective film is formed first, the four sides around the observation site 2 are cut to an appropriate depth by FIB, separated from the substrate, and then covered with a FIB CVD film to give an adhesive effect. By doing so, the observation site 2 can be easily identified, and sample damage, film peeling, and the like are reduced.
[0018]
Next, the columnar sample is bonded and fixed to a reinforcing ring (for example, a C-type ring made of SUS). Next, while being attached to the FIB-dedicated holder, it is inserted into the apparatus main chamber to perform predetermined apparatus and beam adjustment.
[0019]
Next, after a W protective film is formed on the region 6 to be observed by the CVD function of the FIB, FIB having an acceleration voltage of 30 kV is irradiated from above the region 6 to be observed as the first ion beam 14 as shown in FIG. The ion beam current is gradually reduced until the thickness reaches an observable thickness, and the thinned portion observation surface 18 is exposed as shown in FIG. In the case of a 200-400 kV class TEM, the thickness suitable for observation is about 0.1 μm although it depends on the acceleration voltage.
[0020]
Through the above steps, the columnar sample main body 3, the pyramid-shaped support 4 protruding in the vertical direction of the columnar sample main body 3 in a branch shape, and the end of the pyramid-shaped support 4 processed into a width including the observation area 6. A transmission electron microscope (TEM) observation sample 1 having a structure in which the shaped support 4 tip and the thinned portion 5 including the thinned portion observation surface 18 thinned and shaped at the support 4 tip are integrated is obtained. .
[0021]
Next, the thinned sample is taken out of the main chamber of the apparatus, and set so that the thinned portion observation surface 18 is oriented parallel to a dedicated ion polishing stage as shown in FIG. While rotating the sample at a low angle with respect to the thinned portion observation surface 18 from the longitudinal direction of the substrate with respect to the thinned portion observation surface 18, the argon ions at an acceleration voltage of 4 kV were adjusted so that they were selectively irradiated only to the thinned portion 5 and irradiated for several minutes. , And TEM samples. The third step using the second ion beam is desirably performed by an apparatus or system in which the FIB mechanism and the ion polishing mechanism are integrated. Here, the irradiation purpose of the second ion beam 15 will be described. The purpose is to remove a damaged layer reaching as much as about 30 nm on only one surface formed on the thinned portion observation surface 18 by the FIB irradiation of 30 kV to expose a sound crystalline portion, and to obtain a step at the interface due to a difference in etching rate. And to obtain a smooth thinned portion observation surface 18. To realize this, it is important that the former lower the acceleration voltage of the second ion beam 15 and that the latter lower the voltage and perform irradiation at a low angle. In the embodiment of the present invention, the argon ion whose acceleration voltage of the second ion beam 15 is adjusted to 4 kV which is sufficiently lower than that of the first ion beam 14 is used. The irradiation angle (α) 11 of the second ion beam 15 was set to 15 °. However, depending on the sample form and structure, it is necessary to finely adjust the acceleration voltage within the range of 0 to 10 kV and the irradiation angle (α) 11 of the argon ion beam within the range of 0 ° <α ≦ 30 °.
[0022]
The irradiation angle 11 of the second ion beam 15 will be described with reference to the drawings. FIG. 11 is a diagram showing a relationship between the thinned portion observation surface 18 and the irradiation angle 11 of the second ion beam 15 when the thinned portion 5 is viewed from directly above.
[0023]
According to this, when the ion beam is irradiated on the thinned portion observation surface 18, the side surface of the thinned portion (the side surface of the support) may be cut at the same time, but as shown in the figure, the irradiation angle (α) 11 And the sample rotation speed (R) are adjusted to 0 ° <α ≦ 30 ° and 2 ≦ R ≦ 10 rpm, respectively, and the thinning section 5 is rotated (usually counterclockwise) with the thinning section 5 as the sample rotation shaft 16. Irradiation on the side surface can be minimized. The reason why the second ion beam 15 is not irradiated from above the thinned portion 5 is that when irradiated at a low angle from above the observation section, the second ion beam 15 is gradually cut off from above.
Finally, the prepared TEM sample was inserted into the apparatus barrel, and the observation electron beam 19 was irradiated to the thinned portion observation surface 18 as shown in FIG. As a result, it was possible to more clearly observe the interface of a heavy metal layer such as an oxide-based superconducting device or a magnetic device and the thickness of an insulating layer such as a DRAM. On the other hand, in the conventional sample preparation method, damage on the Si crystal plane was remarkable, and the interface of the heavy metal layer was unclear.
[0024]
【The invention's effect】
As described above, according to the present invention, a column-shaped sample main body, a pyramid-shaped support that protrudes in a branch shape in the vertical direction of the main body, and a support that is processed and shaped into a width including a region to be observed at the pyramid-shaped support tip. By forming a structure in which the tip of the body and the thinned part including the observation cross section shaped into a slice at the tip of the support are integrated, secondary processing such as ion polishing becomes easy. Since the damage layer caused by the focused ion beam irradiation, which was an adverse effect, can be removed, and the roughness of the observation cross section caused by the difference in the constituent elements and the cross-sectional structure can be reduced, a high-quality TEM sample can be prepared and good TEM observation can be performed. Will be possible. As an example, the interface of a heavy metal layer such as an oxide-based superconducting device or a magnetic device and the thickness of an insulating layer of a semiconductor device could be more clearly observed.
[Brief description of the drawings]
FIG. 1 is a perspective view of a transmission electron microscope sample of the present invention. FIG. 2 is a perspective view showing a base material for producing the transmission electron microscope sample of the present invention. FIG. 3 is a perspective view showing a first step of the present invention. FIG. 4 is a perspective view showing a first step of the present invention. FIG. 5 is a perspective view showing a first step of the present invention. FIG. 6 is a perspective view showing a first step of the present invention. FIG. 8 is a perspective view showing a sample shape manufactured by one step. FIG. 8 is a perspective view showing a second step of the present invention. FIG. 9 is a cross section showing a first thinned state in an embodiment of the present invention. FIG. 10B is a plan view showing a second thinned state. FIG. 10 is a perspective view showing a third step of the present invention. FIG. 11 is a diagram showing the relationship between the thinned observation section and the second ion beam irradiation angle in the embodiment of the present invention. FIG. 13 is a perspective view showing a relationship between a sample and an electron beam of a transmission electron hoof microscope. FIG. 13 is a perspective view showing a substrate for producing a conventional transmission electron microscope observation sample. FIG. 14 is a conventional transmission electron microscope observation sample. FIG. 15 is a perspective view showing the next step of FIG. 14. FIG. 16 is a perspective view showing the next step of FIG. 15. FIG. 17 is a perspective view showing the next step of FIG. FIG. 18 is a perspective view showing the next step of FIG. 17; FIG. 19 is a perspective view showing the relationship between a conventional observation sample and an electron beam of a transmission electron microscope.
DESCRIPTION OF SYMBOLS 1 ... Transmission electron microscope sample 2 ... Observation part 3 ... Column-shaped sample main body 4 ... Supporter 5 ... Thinned part 6 ... Observation area 11 ... Sample base 12 ... Dummy substrate 13 ... Convex part 14 ... First ion beam 15 ... First 2 ion beam 16 ... sample rotation axis 17 ... irradiation angle 18 ... thinned part observation surface 19 ... observation electron beam

Claims (6)

In a method for preparing a transmission electron microscope observation sample in which a region to be observed in the columnar sample having a convex portion at the tip is thinned to expose an observation surface,
A first step of cutting out the columnar sample from the sample substrate having the observed area so that the observed area remains;
A second step of irradiating the first ion beam from a direction perpendicular to the top surface of the projection to thin the projection to form a thinned portion;
A third step of irradiating the thinned portion plane, which is the observation surface, with a second ion beam at a low irradiation angle. 2. The method for preparing a transmission electron microscope observation sample according to claim 1, wherein the columnar sample has a structure in which the sample body, the protrusion, and a thinned portion formed on the protrusion are integrated. The method for preparing a transmission electron microscope observation sample according to claim 1, wherein a focused gallium ion beam is thinned as the first ion beam and argon ions are thinned as the second ion beam. 2. The method for producing a transmission electron microscope observation sample according to claim 1, wherein the acceleration voltage of the second ion beam is lower than that of the first ion beam. The irradiation of the second ion beam at an angle of 0 ° <α ≦ 30 ° (here, α is an irradiation angle) when the irradiation angle is α with respect to the thinned portion. Method for preparing a sample observed with a transmission electron microscope. 2. The method according to claim 1, wherein the second ion beam is selectively applied to the thinned portion.

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