TEM sample preparation procedure

Abstract

TEM sample preparation procedure, which includes: Placing a thin sample (21) on a sample holder (31) such that a side surface (21a) of the thin sample (21), which is closer to a desired observation target (32) within the thin sample (21) than another side surface of the thin sample (21), is opposite a focused ion beam column (2); Setting up a processing area (34, 35, 36, 37) which is to be subjected to etching by a focused ion beam (9) in order to form a thin-film section (21e) which contains the observation target (32) and has a thickness direction which is essentially parallel to a thickness direction (21d) from the thin sample (21) to a region of the side surface (21a) which adjoins the thin-film section (21e); and Performing the etching process on a section of the thin sample (21) extending from its side surface (21a) to its front surface (21b) by irradiating the processing area (34, 35, 36, 37) with the focused ion beam (9) from the focused ion beam column (2), wherein the processing area (34, 35, 36, 37) includes a first irradiation area (35, 37) on the front surface (21b) thereof and a second irradiation area (36, 34) on the thin-film section (21e) thereof, and wherein the amount of irradiation of the focused ion beam (9) in the second irradiation area (36, 34) is set higher than in the first irradiation area (35, 37).

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority from Japanese Patent Application No. 2012-027692, filed on February 10, 2012, the entire contents of which are incorporated herein by reference. TECHNICAL FIELD

[0002] Aspects of the present invention relate to a method for preparing a TEM sample by etching processing using a focused ion beam. BACKGROUND

[0003] In the prior art, transmission electron microscope (TEM) observation is known as a method for observing a micro-region in a sample for analyzing defects in a semiconductor device or for other purposes. In TEM observation, when preparing a sample to obtain a transmission electron image, it is necessary to prepare a TEM sample that has a thin film portion with a thickness sufficient to allow an electron beam to be transmitted through a portion thereof.

[0004] In recent years, a TEM sample preparation method using a focused ion beam has been used as a method for preparing a TEM sample. In this method, a peripheral portion of a sample is subjected to etching processing so that a portion containing a desired observation area remains within the sample. Then, the remaining portion is subjected to etching processing until the remaining portion has a thickness that allows an electron beam to be transmitted therethrough, thereby forming a thin film portion. In this way, a TEM sample can be prepared with the highest precision regarding the portion containing the desired observation area.

[0005] However, problems arose in that, as the thickness of the thin film portion decreases during TEM sample preparation, the thin film portion becomes curved due to internal stress. As a method to solve this problem, a technology for forming a slit in a curved portion has been proposed, thus relieving the thin film portion of stress (see JP 2000-35391 A).

[0006] In recent years, the size of device structures and defects as observation targets have become smaller. Accordingly, in TEM observation, in order to accurately observe only the observation target, it is necessary to prepare a TEM sample having a particularly small thin film portion, for example, with a thickness of 50 nm or less. In this case, a problem has arisen in that the thin film portion is bent and curved even if a slit is formed therein, as in the aforementioned prior art technology.

[0007] US 6,527,967 B1 relates to a method for producing a thin-walled sample for use in an electron microscope. An ion beam scan used to etch a sample block to form a thin-walled section is initiated from the outer periphery of two opposite sides of the sample block to be formed, one side at a time. Specifically, the ion beam is directed from the outer periphery of the sample block inward toward the center of the sample block, creating a thin wall in the inner part of the sample block.

[0008] US 2009 / 0 084 757 A1 relates to a method for providing uniform etching in an ion beam etching process. The method comprises providing a substrate, directing an ion beam onto a surface of the substrate, etching the surface of the substrate with the ion beam, and controlling the etching to achieve uniformity in the etching of the substrate. The control is performed as a function of at least one ion beam-based parameter selected from a plurality of ion beam-based parameters. OUTLINE

[0009] The present invention provides a TEM sample preparation method capable of preparing a TEM sample in which a thin film portion is not curved when preparing a TEM sample having a thin film portion having a small thickness.

[0010] According to one aspect of the present invention, there is provided a TEM sample preparation method, which includes: placing a thin sample on a sample holder such that a side surface of the thin sample, which is closer to a desired observation target within the thin sample than another side surface of the thin sample, faces a focused ion beam column; setting a processing region to be subjected to etching processing by a focused ion beam, so as to form a thin film portion containing the observation target and having a thickness direction substantially parallel to a thickness direction of the thin sample, to a region of the side surface adjacent to the thin film portion;and performing the etching processing on a portion of the thin sample extending from the side surface thereof to a front surface thereof by irradiating the processing region with the focused ion beam from the focused ion beam column, the processing region including a first irradiation region on the front surface thereof and a second irradiation region on the thin film portion thereof, and wherein an irradiation amount of the focused ion beam in the second irradiation region is set larger than in the first irradiation region.;

[0011] According to this method, etching is performed from the side surface of the thin sample that is closer to the observation target than the other side surface of the thin sample, and thus a thin film portion can be prepared with a smaller etching size compared to the case of performing etching from the other side surface opposite the side surface. Therefore, the etching size can be reduced, and thus the thin film portion can be firmly supported by a portion of the thin sample where etching was not performed. This can prevent the thin film portion from warping.

[0012] According to another aspect of the present invention, there is provided a TEM sample preparation method, which includes: placing a thin sample containing an observation target therein on a sample holder such that a side surface of the thin sample, which is closer to the desired observation target than another side surface of the thin sample, faces a focused ion beam column; setting a processing area to be subjected to etching processing by a focused ion beam to a portion of the side surface;and performing etching processing on a portion of the thin sample extending from the side surface thereof to a front surface thereof by irradiating the processing region with the focused ion beam from the focused ion beam column, thereby forming a thin film portion including the observation target and having a thickness direction substantially parallel to a thickness direction of the thin sample, wherein the processing region includes a first irradiation region on the front surface thereof and a second irradiation region on the thin film portion thereof, and wherein an irradiation amount of the focused ion beam in the second irradiation region is set larger than that in the first irradiation region;

[0013] According to the TEM sample preparation method of the aspects of the present invention, a TEM sample can be prepared without the thin film portion bending. SHORT DESCRIPTION OF THE DRAWING Fig. 1 is a structural diagram of a focused ion beam device according to an exemplary embodiment of the present invention; Fig. 2A and Fig. 2B are explanatory diagrams of a TEM sample preparation according to the exemplary embodiment of the present invention; Fig. 3A and Fig. 3B are explanatory diagrams of TEM sample preparation according to the exemplary embodiment of the present invention; Fig. 4 is an explanatory diagram of the TEM sample preparation according to the exemplary embodiment of the present invention; and Fig. 5 is a flowchart of TEM sample preparation according to the exemplary embodiment of the present invention. DETAILED DESCRIPTION

[0014] A TEM sample preparation method according to an exemplary embodiment of the present invention will be described below.

[0015] As in Fig. 1, a focused ion beam device for TEM sample preparation according to the exemplary embodiment includes an FIB column 2 and a sample chamber 3. The FIB column 2 is capable of irradiating a sample 7 accommodated in the sample chamber 3 with an ion beam 9.

[0016] The focused ion beam device further includes a secondary electron detector 4. The secondary electron detector 4 is capable of detecting secondary electrons generated by irradiation of the sample 7 with the ion beam 9.

[0017] The focused ion beam device further includes a sample stage 6 for placing the sample 7 thereon. The sample stage 6 can be tilted to change an angle of incidence of the ion beam 9 onto the sample 7. The movement of the sample stage 6 can be controlled by a sample stage control section 16.

[0018] The focused ion beam device further includes an FIB control section 13, an image generation section 14, and a display section 17. The FIB control section 13 controls the irradiation of the ion beam 9 from the FIB column 2. The image generation section 14 forms a SIM image based on a signal for scanning the ion beam 9, which is sent from the FIB control section 13, and a signal of the secondary electrons detected by the secondary electron detector 4. The display section 17 is capable of displaying the SIM image.

[0019] The focused ion beam device further includes an input section 10 and a control section 11. An operator inputs device control conditions, such as an irradiation condition of the ion beam 9, into the input section 10. The input section 10 transmits the input information to the control section 11. The control section 11 transmits a control signal to the FIB control section 13, the image generation section 14, the sample stage control section 16, or the display section 17, thereby controlling the operation of the device.

[0020] The control of the device will be described. For example, the operator sets an irradiation area of ​​the ion beam 9 based on the SIM image displayed on the display section 17. The operator inputs a processing frame for setting the irradiation area on the observation image displayed on the display section 17 via the input section 10. When the operator inputs an instruction to start processing into the input section 10, a signal indicating the irradiation area and a signal indicating the start of processing are transmitted from the control section 11 to the FIB control section 13, and the FIB control section 13 irradiates the ion beam 9 onto the specified irradiation area of ​​the sample 7. In this way, the irradiation area input by the operator can be irradiated with the ion beam 9.

[0021] In the TEM sample preparation method according to the exemplary embodiment, as shown in Fig. 2A, a wafer-shaped sample 7 is partially processed by the ion beam 9 to thereby prepare a thin sample 21. Fig. Figure 2B is an enlarged diagram of the thin sample 21 and its surroundings. The sample 7 is irradiated with the ion beam 9 to form a processing groove 22, while the thin sample 21 is left in place. In this step, the thickness of the thin sample 21 is equal to a thickness at which it is impossible to transmit an electron beam.

[0022] Next, a placement S1 of a thin sample in a flow chart of Fig. 5. The thin sample 21 is separated from the sample 7, and the thin sample 21 is placed on a sample holder 31, as shown in Fig. 3A. In this case, the thin sample 21 is placed on the sample holder 31 such that a desired observation target 32 ​​can be positioned on the side of the FIB column 2. In other words, the thin sample 21 is placed such that a side surface 21a thereof, which is closer to the observation target 32 ​​than another side surface thereof to the observation target 32, can be irradiated by the ion beam 9. The thin sample 21 is placed such that a thickness direction 21d of the thin sample 21 can be perpendicular to an irradiation direction 9a of the ion beam 9.

[0023] Next, a processing area setting S2 for irradiating the ion beam 9 is performed. In the processing area setting, a processing area is set on the SIM image displayed on the display section 17, and an irradiation amount (dose amount) of the ion beam 9 is set.

[0024] Fig. 3B is a SIM image 33 viewed from the irradiation direction 9a of the ion beam 9. Processing regions 34, 35, 36, and 37 are set on the SIM image 33. The processing regions 34 and 36 are set to sandwich the observation target 32 ​​on the side surface 21a. The longitudinal direction of the processing regions 34 and 36 is a direction perpendicular to the thickness direction of the thin sample 21. Then, the processing region 35 is set to a position in contact with the processing region 34 and a front surface 21b, and the processing region 37 is set to a position in contact with the processing region 36 and a back surface 21c.

[0025] In this case, the longitudinal length of the processing regions 34 and 36 is set to a length necessary for observing a TEM image of the observation target 32. The reason for this is that if the longitudinal length of the processing regions 34 and 36 is unnecessarily large, the thin sample 21 is subjected to more than necessary etching processing, with the result that the strength for supporting a thin film portion 21e is weakened, so that the thin film portion 21e bends.

[0026] The processing areas 34, 35, 36, and 37 are set such that the thin film portion 21e, which has a thickness direction parallel to the thickness direction 21d of the thin sample 21, remains. In this way, the thin film portion 21e can be exposed for observing the TEM image of the observation target 32 ​​without interrupting an optical path of the electron beam.

[0027] Then, the dosage amount per unit area of ​​the processing areas 34 and 36 by irradiation is adjusted to be twice to ten times the dosage amount per unit area of ​​the processing areas 35 and 37 by irradiation.

[0028] In general, the etching rate of a focused ion beam varies depending on the angle of incidence on the sample. The etching rate is twice as high when the focused ion beam enters the sample at an angle of incidence of approximately 0 degrees as when the focused ion beam enters the sample at an angle of incidence of approximately 45 degrees, and ten times higher than when the focused ion beam enters the sample at an angle of incidence of approximately 90 degrees.

[0029] In TEM sample preparation, etching processing is performed starting from the front surface 21b of the processing section 35 or from the back surface 21c of the processing section 37. In this case, the angle of incidence of the ion beam 9 onto the thin sample 21 is approximately 0 degrees. Then, as the processing progresses, starting from the front surface 21b of the processing section 35 toward the side of the processing section 34, the angle of incidence into the thin sample 21 changes in the range of 0 degrees to 45 degrees. This is because the angle of incidence of the ion beam 9 is determined by an angle with respect to a cross section formed by the processing, and the angle of the cross section changes during the processing.

[0030] On the other hand, in processing areas 34 and 36, the angle of incidence of the ion beam 9 on the thin sample 21 is approximately 90 degrees. Therefore, the angle of incidence of the ion beam 9 differs between processing areas 34 and 36 and processing areas 35 and 37, and therefore the etching rate differs. The respective dosage amounts are adjusted as described above so that the etching amount is substantially uniform in processing areas 34, 35, 36, and 37.

[0031] Next, an etching processing S3 is performed to irradiate the processing areas 34, 35, 36 and 37 by the ion beam 9. In this way, a TEM sample having a Fig. 4 shown form.

[0032] Accordingly, the thin film portion 21e containing the observation target 32 ​​can be exposed. Only a portion containing the observation target 32 ​​is thinned, and thus the thin film portion 21e can be firmly supported by a portion of the thin sample 21 on which the etching processing has not been performed. Thus, the thin film portion 21e is not curved even if the thickness of the thin film portion 21e is reduced.

[0033] Furthermore, the dosage amounts of the ion beam 9 are controlled to the processing areas 34, 35, 36, and 37, and thus a lower portion 21f formed by the etching processing assumes a surface substantially parallel to the side surface 21a. In this way, even in the case of TEM observation by irradiating the thin film portion 21e with the electron beam from a direction parallel to the thickness direction 21d of the thin sample 21, it is possible to form a shape such that the optical path of the electron beam is not interrupted, and the etching amount of the thin sample 21 can be minimized.

[0034] In the exemplary embodiment, the thickness of the thin sample 21 is 10 μm and the thickness of the thin film portion 21e is 50 nm. Even in the case of a TEM sample having the thin film portion 21e with a small thickness, the etching processing amount of the thin sample 21 can be suppressed, and thus the TEM sample can be prepared without the thin film portion 21e curving.

[0035] The present invention provides illustrative, non-limiting aspects as follows: (1) In a first aspect of the present invention, there is provided a TEM sample preparation method, which includes: placing a thin sample on a sample holder such that a first side surface of the thin sample, which is closer to a desired observation target within the thin sample than a second side surface of the thin sample to the observation target, faces a focused ion beam column; setting a processing region to be subjected to etching processing by a focused ion beam, so as to form a thin film portion containing the observation target and having a thickness direction substantially parallel to a thickness direction of the thin sample, to a region of the first side surface adjacent to the thin film portion;and performing the etching processing on a portion of the thin sample extending from the first side surface thereof to a front surface thereof by irradiating the processing area with the focused ion beam from the focused ion beam column;

[0036] Accordingly, etching is performed from the first side surface of the thin sample, which is closer to the observation target than the second side surface of the thin sample is to the observation target, and thus a thin film portion can be prepared with a smaller etching size compared to the case of performing etching from the second side surface opposite to the first side surface. Therefore, the etching size can be reduced, and thus the thin film portion can be firmly supported by a portion of the thin sample where etching was not performed. Thus, the thin film portion can be prevented from warping.

[0037] (2) In a second aspect of the present invention, there is provided a TEM sample preparation method according to the first aspect, wherein the processing region includes a first irradiation region on the front surface side thereof and a second irradiation region on the thin film portion side thereof, and wherein an irradiation amount of the focused ion beam in the second irradiation region is set larger than that in the first irradiation region.

[0038] Generally, the etching rate of a focused ion beam is higher when processing an edge portion than when processing a flat surface. Therefore, in the TEM sample preparation process, when the entire processing area is processed by the focused ion beam with the same irradiation amount, a portion in contact with the front surface corresponds to an edge portion, and this portion is etched to a large extent. In this case, the thin sample is subjected to more than necessary etching, resulting in a reduction in the strength for supporting the thin film portion and the warping of the thin film portion. To solve this problem, the irradiation amount of the portion in contact with the front surface is set small. Thus, a thin film portion can be prepared without etching the thin sample more than necessary.

[0039] (3) In a third aspect of the present invention, there is provided a TEM sample preparation method according to the second aspect, wherein the irradiation amount of the focused ion beam in the second irradiation region is set to be two to ten times higher than the irradiation amount of the focused ion beam in the first irradiation region.

[0040] (4) In a fourth aspect of the present invention, there is provided a TEM sample preparation method according to any one of the first to third aspects, wherein the etching processing includes forming a bottom surface which is in contact with the front surface and the thin film portion and is substantially parallel to the first side surface.

[0041] During TEM observation, the thin film portion is irradiated with an electron beam such that the electron beam is transmitted in a direction perpendicular to the thin film portion, that is, a thickness direction of the thin film portion. Accordingly, during TEM sample preparation, etching processing is performed such that the thin film portion is exposed without interrupting an optical path of the electron beam. In this case, since the bottom surface, which is in contact with the front surface and the thin film portion and is substantially parallel to the side surface, is formed, the electron beam is not interrupted during TEM observation, and further, the etching amount of the thin sample can be minimized. Thus, the thin film portion can be prevented from warping.

[0042] (5) In a fifth aspect of the present invention, there is provided a TEM sample preparation method, which includes: placing a thin sample containing an observation target therein on a sample holder such that a first side surface of the thin sample, which is closer to the desired observation target than a second side surface of the thin sample to the observation target, faces a focused ion beam column; setting a processing area to be subjected to etching processing by a focused ion beam to a region of the first side surface;and performing etching processing on a portion of the thin sample extending from the first side surface thereof to a front surface thereof by irradiating the processing area with the focused ion beam from the focused ion beam column, thereby forming a thin film portion including the observation target and having a thickness direction substantially parallel to a thickness direction of the thin sample;

Claims

[1] TEM sample preparation procedure, which includes: Placing a thin sample (21) on a sample holder (31) such that a side surface (21a) of the thin sample (21) which is closer to a desired observation target (32) within the thin sample (21) than another side surface of the thin sample (21) faces a focused ion beam column (2); Setting a processing area (34, 35, 36, 37) to be subjected to etching processing by a focused ion beam (9) so as to form a thin film portion (21e) containing the observation target (32) and having a thickness direction substantially parallel to a thickness direction (21d) of the thin sample (21), to a region of the side surface (21a) adjacent to the thin film portion (21e); and Performing the etching processing on a portion of the thin sample (21) extending from the side surface (21a) thereof to a front surface (21b) thereof by irradiating the processing area (34, 35, 36, 37) with the focused ion beam (9) from the focused ion beam column (2), wherein the processing region (34, 35, 36, 37) includes a first irradiation region (35, 37) on the front surface (21b) thereof and a second irradiation region (36, 34) on the thin film portion (21e) thereof, and wherein an irradiation amount of the focused ion beam (9) in the second irradiation region (36, 34) is set larger than in the first irradiation region (35, 37). [2] The TEM sample preparation method according to claim 1, wherein the irradiation amount of the focused ion beam (9) in the second irradiation region (36, 34) is set to be two to ten times higher than the irradiation amount of the focused ion beam (9) in the first irradiation region (35, 37). [3] The TEM sample preparation method according to claim 1 or 2, wherein the etching processing includes forming a bottom surface which communicates with the front surface (21b) and the thin film portion (21e) and is substantially parallel to the side surface (21a). [4] TEM sample preparation procedure, which includes: Placing a thin sample (21) containing an observation target (32) therein on a sample holder (31) such that a side surface (21a) of the thin sample (21) which is closer to the desired observation target (32) than another side surface of the thin sample (21) faces a focused ion beam column (2); Setting a processing area (34, 35, 36, 37) to be subjected to etching processing by a focused ion beam (9) on a region of the side surface (21a); ​​and Performing etching processing on a portion of the thin sample (21) extending from the side surface (21a) thereof to a front surface (21b) thereof by irradiating the processing area (34, 35, 36, 37) with the focused ion beam (9) from the focused ion beam column (2), thereby forming a thin film portion (21e) containing the observation target (32) and having a thickness direction substantially parallel to a thickness direction (21d) of the thin sample (21), wherein the processing region (34, 35, 36, 37) includes a first irradiation region (35, 37) on the front surface (21b) thereof and a second irradiation region (36, 34) on the thin film portion (21e) thereof, and wherein an irradiation amount of the focused ion beam (9) in the second irradiation region (36, 34) is set larger than in the first irradiation region (35, 37).

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