Method for preparing lamella

A FIB is irradiated onto a sample to form a lamella whose upper side has a thickness of 100 nm or less and whose lower side has a thickness greater than that of the upper side. First and second measurement regions are set on an observation image of the lamella on the upper and lower sides, respectively, where the lamella is thin enough to transmit therethrough an EB. An EB is irradiated onto the first and second measurement regions and charged particles generated therefrom are detected, and a slant angle of one degree or smaller is calculated based on the detected amount of charged particles generated from the first and second measurement regions and the distance between the two regions. The lamella is slanted with respect to the FIB and then irradiated by the FIB by the calculated slant angle to uniformize the thickness of the lamella to a value of 100 nm or smaller.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lamella preparation method and a lamella preparation apparatus for preparing a lamella by etching processing using a focused ion beam.

2. Description of the Related Art

Conventionally, TEM observation is known as a method of observing a microscopic region in a sample for the purpose of analyzing a defect in a semiconductor device or the like. In the TEM observation, in order to obtain a transmission electron image, as preprocessing, it is necessary to process a sample into a lamella having a thickness through which an electron beam may transmit.

In recent years, as a method of preparing a lamella, a lamella preparation method using a focused ion beam is used. In this method, etching processing of peripheral portions of a sample is performed so as to leave a portion in the sample including a region of which observation is desired. Then, etching processing of the remaining portion is performed until the portion has a thickness through which an electron beam may transmit to prepare the lamella. This enables preparation of a lamella precisely including the region of which the observation is desired.

By the way, in the TEM observation, it is desired that the thickness be uniform throughout the lamella. When the thickness is nonuniform, the nonuniformity affects a TEM image and information of the defect and the effect of the nonuniformity in thickness cannot be separated, which causes the analysis to be difficult.

However, a focused ion beam has a certain beam density distribution due to the nature thereof, and thus, when a lamella is prepared by irradiating a focused ion beam from a surface side of a sample, the lamella is tapered, which means the thickness of the lamella is nonuniform.

As a method for solving such a problem, there is disclosed a method of preparing a lamella having a vertical thin wall by etching processing with the sample being slanted taking into consideration the beam density distribution of the focused ion beam (see Japanese Patent Application Laid-open No. Hei 4-76437). According to this method, a lamella having a uniform thickness irrespective of the beam density distribution of the focused ion beam may be prepared.

However, in recent years, as the density of semiconductor devices becomes higher and as the dimensions of a semiconductor device become smaller, the size of a defect which is a target of TEM observation also becomes smaller. In order to make the TEM observation of a microscopic defect with accuracy, it is necessary for the thickness of the lamella to be extremely small.

In conventional lamella preparation, in an apparatus including both a focused ion beam column and an electron beam column, SEM observation of both a front surface and a rear surface of a lamella is made to confirm the shape of the processed lamella. However, in the conventional method involving merely obtaining a SEM image and confirming nonuniformity in thickness by the contrast in the SEM image, when the thickness of the lamella is extremely small, the difference in contrast is small, and thus, it is difficult to confirm the nonuniformity in thickness.

Even if nonuniformity in thickness may be confirmed by the SEM observation, the slant angle of the sample stage necessary for processing for uniform thickness is unknown, and thus, an operator performs the processing while adjusting the slant angle. Such a method depends on the skill of the operator, and thus, it is difficult to secure a certain level of quality.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-mentioned circumstances, and the present invention provides a lamella preparation apparatus and a lamella preparation method capable of preparing a lamella having a uniform thickness without depending on the skill of an operator.

(1) According to an exemplary embodiment of the present invention, there is provided a lamella preparation apparatus for preparing a lamella by performing processing of a sample using a focused ion beam irradiated from a focused ion beam column, the lamella preparation apparatus including: a sample stage for mounting the lamella thereon; an electron beam column for irradiating an electron beam onto the lamella; a charged particle detector for detecting charged particles released from the lamella by irradiation of the electron beam; and a display unit for displaying an observation image of the lamella formed using a detection signal from the charged particle detector. The lamella preparation apparatus according to the exemplary embodiment of the present invention further includes: an input unit for setting a first measurement region on an upper side and a second measurement region on a lower side of the lamella in the observation image; and a slant angle calculating unit for calculating a slant angle of the lamella from a detected amount of the charged particles generated from the first measurement region and a detected amount of the charged particles generated from the second measurement region by irradiation of the electron beam and a distance between the first measurement region and the second measurement region.

This enables calculation of the slant angle of the lamella with accuracy. Therefore, by processing the lamella using an ion beam under a state in which a sample stage is slanted by the calculated slant angle with respect to the ion beam, a lamella having a uniform thickness may be prepared.

(2) According to the exemplary embodiment of the present invention, there is also provided a lamella preparation method of preparing a lamella by performing processing of a sample using a focused ion beam, the lamella preparation method including: irradiating an electron beam onto the lamella to form an observation image; setting a first measurement region on an upper side and a second measurement region on a lower side of the lamella in the observation image; irradiating the electron beam onto the first measurement region and the second measurement region and detecting charged particles generated there from; calculating a slant angle of the lamella from a detected amount of the charged particles generating from the first measurement region, a detected amount of the charged particles generated from the second measurement region, and a distance between the first measurement region and the second measurement region; slanting the lamella with respect to the focused ion beam by the calculated slant angle; and irradiating the focused ion beam onto the lamella to perform finish processing.

The present invention has an action and effect that the slant angle may be estimated, in particular, even when the slant angle of the lamella is small and a beam is irradiated from a direction perpendicular to a surface of the sample, that is, from a direction within an observation surface of the lamella, and thus, it is difficult to measure the slant angle from the obtained observation image.

According to the lamella preparation apparatus and the lamella preparation method of the exemplary embodiment of the present invention, a lamella having a uniform thickness may be prepared without depending on the skill of an operator.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A lamella preparation apparatus and a lamella preparation method according to an embodiment of the present invention are described in the following.

As illustrated inFIG. 1, the lamella preparation apparatus of this embodiment includes an EB column1, an FIB column2, and a sample chamber3. An electron beam8from the EB column1and an ion beam9from the FIB column2may be irradiated onto a sample7housed in the sample chamber3.

The lamella preparation apparatus further includes a secondary electron detector4and a reflected electron detector5as charged particle detectors. The secondary electron detector4may detect secondary electrons generated from the sample7by irradiation of the electron beam8or the ion beam9. The reflected electron detector5is provided in the EB column1. The reflected electron detector5may detect electrons reflected by the sample7as a result of irradiation of the electron beam8onto the sample7.

The lamella preparation apparatus further includes a sample stage6for mounting the sample7thereon. By slanting the sample stage6, the incident angle of the ion beam9on the sample7may be changed. The sample stage6is controlled by a sample stage control unit16.

The lamella preparation apparatus further includes an EB control unit12, an FIB control unit13, an image formation unit14, and a display unit17. The EB control unit12controls irradiation of the electron beam8from the EB column1. The FIB control unit13controls irradiation of the ion beam9from the FIB column2. The image formation unit14forms a reflected electron image from a signal for scanning the electron beam8of the EB control unit12and a signal of reflected electrons detected by the reflected electron detector5. The display unit17may display the reflected electron image. Further, the image formation unit14forms a SEM image from the signal for scanning the electron beam8of the EB control unit12and a signal of secondary electrons detected by the secondary electron detector4. The display unit17may display the SEM image. Further, the image formation unit14forms a SIM image from a signal for scanning the ion beam9of the FIB control unit13and the signal of secondary electrons detected by the secondary electron detector4. The display unit17may display the SIM image.

The lamella preparation apparatus further includes an input unit10and a control unit11. An operator inputs to the input unit10conditions with regard to control of the apparatus. The input unit10sends input information to the control unit11. The control unit11sends a control signal to the EB control unit12, the FIB control unit13, the image formation unit14, the sample stage control unit16, or the display unit17to control the apparatus.

With regard to control of the apparatus, an operator sets an irradiation region of the ion beam9based on, for example, an observation image such as a reflected electron image, a SEM image, or a SIM image displayed on the display unit17. An operator inputs using the input unit10a processing frame for setting the irradiation region on the observation image displayed on the display unit17. Further, an operator inputs an instruction to start processing to the input unit10. Then, a signal for indicating the irradiation region and a signal for starting processing are sent from the control unit11to the FIB control unit13, and the ion beam9is irradiated from the FIB control unit13onto the specified irradiation region of the sample7. This enables irradiation of the ion beam9onto the irradiation region specified by the operator.

As illustrated inFIG. 2A, the lamella preparation apparatus of this embodiment may prepare a lamella21by performing processing of a part of the sample7using the ion beam9.FIG. 2Bis an enlarged view of the vicinity of the lamella. The ion beam9is irradiated onto the sample7to form a processing groove22so as to leave the lamella21. At this stage, the lamella21has a thickness through which the electron beam8does not transmit. Further, as illustrated inFIG. 2C, by performing thinning processing of the lamella21using the ion beam9, the lamella21may have a thickness through which the electron beam8may transmit. In this case, a portion23which does not undergo the thinning processing has a thickness through which the electron beam8cannot transmit. Specifically, the lamella21having a thickness through which the electron beam8may transmit and a portion having a thickness through which the electron beam8cannot transmit may be formed in a part of the sample7.

By the way, as illustrated inFIG. 3A, the lamella21formed by the thinning processing using the ion beam9has a small thickness21bon an upper side which is the FIB column2side of the lamella21(on a distal end side of the lamella21) and a large thickness21con a lower side which is the sample stage6side (on a proximal end side of the lamella21). The reason for this is that the current density of the ion beam9has a Gaussian distribution, which affects the lamella21processed using the ion beam9to taper the lamella21.

In order to make high-resolution TEM observation, the thickness of the lamella21which is the target of the observation is desired to be uniform. Therefore, the sample stage6is slanted with respect to the ion beam9so as to prevent a tapered shape. However, when the slant angle of the sample stage6is large, as illustrated inFIG. 3B, the lamella31may have a reversed tapered shape in which a thickness31bon the upper side is large and a thickness31con the lower side is small.

In particular, when a lamella having a thickness of 100 nm or smaller is prepared in order to make the high-resolution TEM observation, for the purpose of causing the extent of the taper to be small, finish processing is performed using an ion beam with a small current amount so that the current density distribution of the ion beam is narrow. In this case, the slant angle of the tapered shape, that is, the slant angle of the lamella21is one degree or smaller, which is extremely small. Therefore, it is difficult to recognize the slant angle, and thus, conventionally, it follows that operation for slanting the sample stage6to adjust the angle of irradiation of the ion beam depends on the skill of an operator.

On the other hand, the lamella preparation apparatus of this embodiment includes a calculation unit15for calculating an optimum slant angle of the sample stage6, and thus, even when the slant angle is one degree or smaller, the sample stage6may be slanted to form an optimum slant angle and the lamella21having a uniform thickness may be prepared.

FIG. 4is a flow chart illustrating a lamella preparation method of this embodiment. First, the electron beam8is irradiated onto the lamella21formed by the thinning processing, and reflected electrons generated from a measurement region of the lamella21are detected (S1). Then, using the detected information, the slant angle of the sample stage6is calculated (S2). After that, the sample stage6is slanted so that the slant angle becomes a calculated angle (S3). Finally, the ion beam9is irradiated onto the lamella21to perform finish processing (S4). In this way, the lamella21having a uniform thickness may be prepared.

In the lamella preparation apparatus of this embodiment, reflected electrons are detected as follows. As illustrated inFIG. 5A, the lamella21having a thickness through which the electron beam8may transmit and a portion having a thickness through which the electron beam8cannot transmit are formed in a part of the sample7. In this case, the sample stage6is located so that the ion beam9is vertically incident on the surface of the sample7. Further, the sample7is located so that the electron beam8may scan and irradiate an observation surface21aof the lamella21.

FIG. 5Bis a SEM image obtained by scanning and irradiation of the observation surface21ausing the electron beam8. An operator sets measurement regions41and42on the upper side and the lower side, respectively, of the observation surface21abased on the SEM image. As shown, the measurement regions41,42are set on the observation surface21aat locations where the lamella has a thickness through which the electron beam8may transmit. Stated otherwise, the measurement regions41,42are located where the lamella is thin enough to transmit therethrough the electron beam8. Further, an operator sets a reference region43in the portion23which does not undergo the thinning processing. Then, the electron beam8is irradiated onto the measurement regions41and42and the reference region43and reflected electrons are detected.

FIG. 6is a graph showing the relationship between the film thickness and the amount of reflected electrons. InFIG. 6, a horizontal axis61denotes the film thickness while a vertical axis62denotes the amount of reflected electrons measured in the measurement region. When the film thickness is large, an amount of reflected electrons63almost does not change even when the film thickness changes. On the other hand, when the film thickness is small, as the film thickness becomes smaller, an amount of reflected electrons64becomes smaller. The reason is that, when the film thickness is small, an amount of electrons which transmit through the lamella21of the electron beam8irradiated onto the measurement region increases and the amount of reflected electrons decreases. The lamella preparation apparatus of this embodiment irradiates the electron beam8onto the measurement regions41and42and measures the amount of reflected electrons generated from each of the regions.

It is more desired to standardize the amounts of reflected electrons detected in the measurement regions41and42using the amount of reflected electrons detected in the reference region43. The reason is that the amount of reflected electrons depends on an amount of the irradiated electron beam8, and the current amount of the electron beam8may fluctuate every time the measurement is made. Therefore, in order to make an accurate measurement, the electron beam8is irradiated onto the measurement regions41and42and the reference region43, and the amounts of reflected electrons in the measurement regions41and42, respectively, are divided by the amount of reflected electrons in the reference region43having a thickness through which the electron beam8does not transmit. By using such figures, the measurement of the amounts of reflected electrons may be free from the effect of fluctuations in the amount of the irradiated electron beam8.

By obtaining in advance preliminary data with regard to the relationship between the amount of reflected electrons and the film thickness using a sample the thickness of which is known and comparing the data with the amounts of reflected electrons in the measurement regions41and42, the thicknesses of the measurement regions41and42may be estimated.

FIG. 7is a cross-sectional view of the lamella21. When a thickness71of the measurement region41and a thickness72of the measurement region42are the same, the amount of reflected electrons in the measurement region41and the amount of reflected electrons in the measurement region42are the same. When the lamella21has a tapered shape as illustrated inFIG. 7, the amount of reflected electrons in the measurement region41is smaller than the amount of reflected electrons in the measurement region42. From this difference and a distance73between the measurement region41and the measurement region42, the taper angle which is the slant angle of the lamella21may be calculated.

Specifically, as illustrated inFIG. 8A, the relationship between the distance73and a taper angle θ formed between a difference81between the thickness71and the thickness72and a line segment82of the tapered portion is expressed as tan θ=(difference81between thickness71and thickness72)/distance73, where the distance73between the measurement region41and the measurement region42is the distance between the center of the measurement region41and the center of the measurement region42.

For example, when the thickness71is 100 nm, the thickness72is 150 nm, and the distance73is 3 μm, it follows from the above relational expression that θ is 0.47 degrees.

As illustrated inFIG. 8B, by slanting the sample stage6with respect to the ion beam9by θ degrees, one surface of the lamella21may be set to be in parallel with the ion beam9. By performing processing of another surface of the lamella21using the ion beam9while maintaining this state, the lamella21having a uniform thickness may be prepared.

In the above-mentioned embodiment, an example in which reflected electrons are used is described, but, instead of the reflected electrons, secondary electrons generated from the measurement regions41and42and the reference region43may be used.