JPH08329875A - Scanning electron microscope and its sample image display method - Google Patents

Abstract

(57) [Summary] [Object] An object of the present invention is to specify information in the depth direction of a sample and display a three-dimensional sample structure only by observing the sample from above with a scanning electron microscope. . [Configuration] Secondary electron images obtained by electron beam irradiation with a plurality of incident energies are respectively captured, and the amount of blurring in the vicinity of the internal structure is calculated from the difference images. By referring to the table of the relationship between the blur amount and the depth that is obtained in advance, the depth that is the same as the blur amount obtained from the difference image is obtained. The three-dimensional structure of the sample is displayed by the depth information and the secondary electron image. [Effect] A three-dimensional structure can be displayed by non-destructive observation of a sample.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning electron microscope, and particularly, in observing a sample structure such as a semiconductor integrated circuit,
The present invention relates to a sample image display technology that provides a non-destructive display of a three-dimensional structure.

[0002]

2. Description of the Related Art In the field of observing the fine structure of a sample,
A sample image display device such as a scanning electron microscope using an electron beam of several hundreds eV or more is used. In a scanning electron microscope, generally, secondary electrons generated by the interaction between incident primary electrons and a sample are detected to form a sample image. Since this displays the shape of the sample viewed from the side where the electron beam is applied, the information in the depth direction of the sample appears as the contrast of the image, but the depth of internal structures of the sample cannot be measured. It is possible.

Regarding display of such a stereoscopic image / cross-sectional image in the depth direction, Japanese Patent Application Laid-Open No. 5-290786 discloses a secondary electron image (resulting from the interaction of the particle beam irradiated on the sample with the sample again). In a scanning sample image display device in which a secondary electron image caused by backscattered electrons is a main image signal, a method of displaying based on images of two or more different incident energies is described. Here, as shown in FIG. 2, the difference in the penetration depth of the electron beam into the sample depending on the incident energy is utilized, and a difference image between the image when the low energy electron beam is irradiated and the image when the high energy electron beam is irradiated. The information in the depth direction is obtained from the 3D image.

[0004]

However, in the prior art disclosed in the above-mentioned Japanese Patent Application Laid-Open No. 5-290786, in order to specify the depth of the internal structure of the sample, the penetration depth of the electron beam is It is necessary to change within the range including the depth of the internal structure. Therefore, there is a problem that a large number of images with various values of incident energy have to be taken.

For example, in FIG. 2, an image A obtained with an electron beam 1a having an incident energy having a penetration depth d in the sample and an image A obtained with an electron beam 1b having an incident energy having a penetration depth d '. It can be seen from 'that the position of the internal structure 2b is deeper than d and shallower than d'. But to identify its position,
By irradiating several electron beams having an incident energy value between the electron beam 1a and the electron beam 1b, an electron beam having an incident energy that reaches the internal structure 2b is found, and the internal structure is determined from the penetration depth of the electron beam. The position of the object 2b must be specified.

Further, in the prior art described in the above-mentioned Japanese Patent Laid-Open No. 5-290786, since it is based on the secondary electron image caused by the backscattered electrons, it is generated by the interaction between the electron beam and the sample immediately after irradiation. It is not possible to obtain the depth of the surface step difference in which the secondary electron image (the secondary electron image caused by the primary electrons) becomes the main signal.

Further, when the thickness of the internal structure is different as shown in FIG. 2, the thickness cannot be specified only from the difference image of electron beams having different incident energies.

[0008]

Means for Solving the Problems The above-mentioned problems are caused by a difference between a plurality of (for each incident energy) secondary electron images (or two-dimensional electron detection data) obtained by sequentially changing the incident energy of an electron beam with which a sample is irradiated. From this, the degree of blurring (blur amount) due to scattering of the electron beam incident on the sample near the internal structure of the sample is obtained, and data showing the relationship between the depth in the sample and the blur amount of the secondary electron image obtained in advance It is solved by calculating the depth of the structure from.

Therefore, in the present invention, the sample holding means for holding the sample, the electron beam irradiating means for irradiating the sample while accelerating the electron beam and irradiating the sample while scanning, and the electron beam In a scanning electron microscope comprising a detection means for detecting secondary electrons generated by acting on a sample and an image display means for forming and displaying an image (sample image) based on the intensity of the secondary electrons detected by the detection means, The beam irradiating means includes an accelerating voltage changing means for changing the accelerating voltage setting value of the electron beam, and the image displaying means includes the above-mentioned electron beam irradiating means for irradiating the sample with the electron beam (scanning) And the difference in intensity gradient (that is, the amount of blur) of the secondary electrons detected by electron beam scanning for each accelerating voltage setting value with respect to the electron beam scanning direction Ability to form and display images To have.

Further, in the present invention, an electron beam scanning step of irradiating a sample with an electron beam while scanning and detecting secondary electrons generated by the action of the electron beam with the sample, and the sample based on the detection signal of the secondary electron In a sample image displaying method of a scanning electron microscope, which comprises a sample image displaying step of forming and displaying an image, the electron beam scanning step is performed by changing the incident energy of the electron beam on the sample and setting the incident energy set value for each set. The secondary electron detection data is repeatedly captured to the sample, and in the sample image display step, the spread value of the electron beam in the sample (ie, the spread value of the electron beam in the sample is determined based on the difference between the secondary electron detection data having different incident energy set values of the electron beam. , The value that determines the amount of blurring), and on the other hand, the spread value of the electron beam calculated by referring to the correspondence data between the depth information in the sample and the spread of the electrons that entered the sample in advance. versus 3D of the sample from the depth information based on the difference between the secondary electron detection data obtained by electron beam irradiation with different incident energies and the secondary electron detection data for each incident energy set value. Try to form and display an image of the structure. Corresponding data between the depth information in the sample and the spread of the electrons incident on the sample are the tailing amount (electron beam) of the secondary electron detection signal intensity obtained when the sample having a known step structure is scanned with the electron beam. It may be calculated from the change gradient with respect to the scanning direction), or may be calculated for a sample including an internal structure whose thickness is known.

[0011]

Function The incident electron spreads in the sample due to scattering. Due to the spread of the incident electron beam, part of the electron beam applied to a position other than directly above the internal structure is also scattered by the internal structure. This appears as a blur of the secondary electron intensity.

Since the degree of spread of the incident electrons varies depending on the incident energy, the blur amount of the secondary electron intensity also depends on the incident energy. For example, as shown in FIG. 3, the depth d
Consider irradiation with an electron beam 1a having a spread of r at r and an electron beam 1b having a spread r at a depth d smaller than r because the incident energy is higher than that of the electron beam 1a. In the secondary electron intensity B as a result of irradiation with the electron beam 1a, tailing (a tendency that the intensity gradually increases as the scanning position of the electron beam approaches the internal structure 2) appears from a position away from the internal structure 2. . With only the secondary electron intensity B, this tailing may depend on the shape of the sample and other internal structures (those other than 2). However, in the secondary electron intensity B ′ of the irradiation result of the electron beam 1b, the amount of tailing is small (the intensity change is sharp), so the tailing of the secondary electron intensity B is caused by the internal structure 2 of the incident electron beam. Blurring due to scattering by light (so-called blur)
It can be seen that it is. Using this blur amount, the depth of the internal structure 2 can be specified by referring to a table (corresponding data) of the blur amounts for the respective depths of the electron beams 1a and 1b which are obtained in advance.

Further, the depth of the step-shaped sample can also be obtained from the trailing amount of the secondary electron intensity. For example, in FIG.
In (1), the flat part is the part 5 where the electron beam is applied.
Only becomes the secondary electron emission surface. However, as shown in FIG.
When the irradiation position approaches the step portion as in (3), the step sidewall also becomes the secondary electron emission surface 5 ′. For this reason, the secondary electron intensity in FIG. On the lower side of the step, the secondary electron emission surface is blocked by the step, so the secondary electron detection intensity is reduced. This amount of blur is
If the step depth is different, it changes as shown in FIG. The depth of the given sample can be specified by referring to the table of the blur amount for each step depth obtained in advance from this blur amount. Again, it is specified by comparing the secondary electron images obtained by electron beam irradiation with different incident energies whether the obtained secondary electron intensity tailing is due to the shape or structure of the sample or due to blurring. .

FIG. 13 schematically shows an example of the method of observing a sample image according to the present invention which utilizes the behavior of the incident electron beam inside the sample. In this figure, the internal structure a is present at a shallow position with respect to the electron beam irradiation (scanning) surface of the sample, and the internal structure b is present at a deep position. On the other hand, the image of a is emitted (dense) by the secondary electron beam stronger than that of b. Further, when the accelerating voltage of the scanning electron beam (that is, the incident energy to the sample) is lowered (the image on the left), a thin and blurred region stands out along the contour of the internal structure. This area is the above-mentioned blur. Here, comparing the low accelerating voltage image and the high accelerating voltage image by taking the blurred area of the internal structure b as an example,
It can be seen that the steepness of the change gradient of the secondary electron detection intensity with respect to the scanning direction of the electrons irradiated on the sample surface depends on the acceleration voltage. On the other hand, the amount of blurring of b that is thicker in the depth direction is larger than that of the internal structure a. Therefore, in the present invention, the depth and thickness from the sample surface of the structure existing inside the sample are calculated from the images with different accelerating voltages of the scanning electron beams (or secondary electron detection data), and the three-dimensional shape of the sample is calculated based on this. Construct a structural image (schematic diagram as shown above).

Here, the data of the blur amount for each depth is obtained by an electron beam irradiation experiment using a sample having an internal structure whose depth position and thickness are known or a step-shaped sample whose height is known, The internal structure or step structure of the sample is calculated by a virtual computer simulation.

In the computer simulation, the spread amount of the primary electron may be obtained for each depth from the trajectory of the primary electron and used as the blur amount of the secondary electron image.

[0017]

EXAMPLES The present invention will be described below with reference to Examples 1 to 4.

In each of the embodiments, the scanning electron microscope shown in FIG. 14 was used, so the structure thereof will be described first. The main body of the scanning electron microscope is provided with an electron beam irradiation means (electron beam source 21, electron beam extraction electrode 2) provided inside the vacuum casing 20.
2, an electron beam accelerating lens 23, an electron beam converging lens 24, an electron beam deflecting electrode 25, an objective lens 26, and a means for holding a sample 27 (sample holder 28, tilting mechanism 29,
(Comprising a position control mechanism 30) and a sample 27 of an irradiation electron beam
It is composed of detection means (electron detector) 31 for detecting secondary electrons generated by incidence on the. In addition to this, Sample 2
7 There is a detector 32 for detecting electromagnetic waves (X-rays, etc.) generated from the surface. Around the main body, a signal amplifier 33 of the electron detector 31, a controller 34 of the electron beam deflection electrode 25, an electron beam acceleration voltage controller 35 (from the electron source 21 to the electron beam accelerating lens 2).
The control of the applied voltage up to 3) is provided. An electronic computer 36 is provided with the function of the image display means, and this electronic computer has a control signal from the electron beam deflection electrode controller 34 and the electronic detector 3.
In response to the secondary electron signal from 1, the two data are associated and a secondary electron image (or secondary electron detection data) is created and stored. Further, the electronic computer 36 controls the electron beam accelerating voltage controller 35 to set the accelerating voltage of the scanning electron beam to a desired value and to scan the electron beam on the surface of the sample 27 for each accelerating voltage set value. It also has a function of sending a command signal to the controller 34, fetching secondary electron detection data for each electron beam scan, and constructing a three-dimensional structure of the sample from these secondary electron detection data (for each acceleration voltage setting value). .

<Embodiment 1> In Embodiment 1, a case will be described in which the depth position of an internal structure is determined nondestructively. In the sample structure, the SOG 8 covers the W wiring 9 on the Si substrate 10 as shown in FIG. 7. The thickness of this SOG8 is 30k
It is specified by observing from above the sample with electron beams of eV and 200 keV. The processing procedure is shown in FIG.

First, the table 6 of the relationship between the blur amount and the depth in FIG. 1 was obtained by computer simulation. 30 ke
Assuming a V electron beam and calculating the secondary electron intensity of the W wiring edge under various SOG film thicknesses, a data table of the relationship between the blur amount t and the SOG film thickness s is obtained as shown in FIG. It was

Next, by the processing procedure 7a in FIG. 1, the observation sample was irradiated with electron beams of 30 keV and 200 keV to capture a secondary electron image. The contrast was adjusted according to the processing procedure 7b, and the secondary electron intensities of the respective incident energies became as shown in D and D ′ of FIG. The trailing edge t1 that appears in the secondary electron image D of 30 keV does not appear in the secondary electron image D ′ of 200 keV. That is, this is not due to the W wiring shape but due to the spread of the 30 keV electron beam in the SOG film. With an electron beam of 200 keV, there is almost no blurring, so the peak of D '(ie, position 0.0
The “shoulder” in μm is considered to be the W wiring end. In the processing procedure 7c, 30 from the difference between the secondary electron intensities D and D '
A blur amount of 0.5 μm at the W wiring position at keV was obtained. As a result of referring to the previously obtained table 6 (FIG. 8) of the relationship between the blur amount and the depth by the processing procedure 7d, the blur amount is 0.
It was found that the SOG film thickness was 1.0 μm when the thickness was 5 μm. A stereoscopic image of the sample was displayed using the depth direction information and the secondary electron images D and D '(procedure 7e).

<Embodiment 2> In Embodiment 2, a description will be given of a case where the depth of the step of the Si substrate is obtained by non-destructive observation only from above.

First, a table 6 showing the relationship between the amount of blur and the depth was prepared. Height from 0.1 μm to 2.0 μm 0.1
A Si substrate having a step that changes by μm was prepared.
This Si substrate was scanned with electron beams having incident energies of 30 keV and 50 keV to obtain secondary electron intensities. Figure 4 (4)
A value from the peak value of the secondary electron intensity to the position where the differential value is 0 is defined as the blur amount, and a data table showing the relationship between the step depth s and the blur amount t as shown in FIG. 6 was created.

Similar to the first embodiment, the processing procedure shown in FIG.
The Si substrate whose step depth was unknown was observed nondestructively. The secondary electron intensities obtained by scanning the upper surface of the sample with electron beams of 30 keV and 50 keV were contrast-matched at the flat portion, and the result was as shown in FIG. 30 keV
Since the trailing edge of the secondary electron intensity C is smaller in the secondary electron intensity C ′ in the case of 50 keV, it is understood that it is not due to the unevenness of the sample structure or the shape of the internal structure.

The difference in the amount of tailing between the two secondary electron intensities C and C'was found to be 0.25 μm. Referring to the table of blur amount t and depth s of FIG. 6, the difference in blur amount is 0.25 μm when the depth is 0.6 μm.
That is, the step depth of this sample was found to be 0.6 μm. It was possible to obtain a stereoscopic image display of the sample using the data of this depth.

<Embodiment 3> In Embodiment 3, a case will be described in which the film thickness of the poly-Si 11 covering the W wiring 9 is specified as shown in FIG.

Since the sample surface is poly-Si, the spread of the incident electrons in the sample is considered to be almost the same as that of Si. Therefore, as the table 6 of the relationship between the blur amount and the depth, the table obtained in the second embodiment is used.

The difference in the amount of tailing of the secondary electron intensities obtained by scanning the upper surface of the sample with electron beams of 30 keV and 50 keV in the same manner as in Example 2 was 0.5 μm. Figure 6
It was found that the depth was 0.8 μm by referring to the difference in blur between the table of the blur amount t of 2 and the blur of the table of depth s of 0.5 μm.

<Embodiment 4> In Embodiment 4, a case will be described in which the thickness of the W wiring 9 is specified by observing the same sample as in Embodiment 1.

The relationship table 6 between the amount of blur and the depth was obtained by computer simulation in which the thickness of W was variously changed.

The sample was irradiated with electron beams of 30, 50, and 200 keV, and the secondary electron intensities E, E ', and E "shown in Fig. 12 were obtained at the respective energies. There was almost no blur with the electron beam of 200 keV. Considering this, the W edge position was obtained with the secondary electron intensity E ″. As a result of obtaining the tailing amounts of the secondary electron intensities E and E ′ from the edges, they were 0.4 μm and 0.2 μm, respectively. From the relationship table 6 between the blur amount and the depth obtained earlier, 0.5 μm at 30 keV and 0.3 μ at 50 keV
When referring to the W thickness that is the blur amount of m, the depth is 1.0
It was found that the W wiring was 0.5 μm thick at the position of μm.

[0032]

According to the present invention, information in the depth direction of a sample can be obtained nondestructively.

[0033]

[Brief description of drawings]

FIG. 1 is a flowchart showing a processing procedure of the present invention.

FIG. 2 is a conceptual diagram of stereoscopic image processing in a conventional technique.

FIG. 3 is a conceptual diagram illustrating secondary electron intensity blurring amounts of electron beams having different incident energies.

FIG. 4 is a diagram for explaining blurring of secondary electron intensity due to secondary electron emission from a step side wall.

FIG. 5 is a diagram illustrating changes in step depth and blur amount.

FIG. 6 is a graph showing the amount of blurring of electron beams of 30 keV and 50 keV for each depth of Si.

FIG. 7 is a cross-sectional view of observation samples of Example 1 and Example 4.

FIG. 8 is a graph showing a blur amount of a 30 keV electron beam for each depth of SOG.

FIG. 9 is a diagram showing secondary electron intensities when scanned with electron beams of 30 keV and 200 keV.

FIG. 10 is a diagram showing secondary electron intensities when scanned with electron beams of 30 keV and 50 keV.

FIG. 11 is a cross-sectional view of an observation sample of Example 3.

FIG. 12 is a diagram showing secondary electron intensities when scanned with electron beams of 30 keV, 50 keV, and 200 keV.

FIG. 13 is a diagram schematically illustrating a sample image observation method according to the present invention.

FIG. 14 is a schematic configuration diagram of a scanning electron microscope used in Examples 1 to 4.

[Explanation of symbols]

1, 1a, 1b ... Scanning electron beam, 2, 2a, 2b ... Internal structure, 3 ... Electron beam penetration area, 4 ... Sample, 5 ... Secondary electron emitting surface, 6 ... Table, 7a to 7d ... Processing procedure, 8 ... SO
G, 9 ... W wiring, 10 ... Si substrate, 11 ... Poly Si.

Claims (4)

[Claims] 1. A sample holding means for holding a sample, an electron beam irradiating means including an electron source for radiating an electron beam and irradiating while accelerating the electron beam while scanning the sample, and the electron beam Detection means for detecting secondary electrons generated by interacting with the sample;
The electron beam irradiating means has an accelerating voltage changing means for changing the accelerating voltage setting value of the electron beam, the image displaying means forming and displaying an image based on the intensity of the secondary electrons detected by the detecting means. The image display means controls the electron beam irradiation means to irradiate the sample while scanning the electron beam for each acceleration voltage setting value, and is detected by the electron beam scanning for each acceleration voltage setting value. A scanning electron microscope, wherein an image of the sample is formed and displayed based on a difference in intensity gradient of secondary electrons with respect to the electron beam scanning direction. 2. An electron beam scanning step of irradiating a sample with an electron beam while scanning and detecting secondary electrons generated by the electron beam acting on the sample, and a sample image based on a detection signal of the secondary electron. Forming and displaying a sample image displaying step, wherein the electron beam scanning step is performed by changing and setting the incident energy of the electron beam to the sample and capturing secondary electron detection data for each incident energy set value. Repeatedly, in the sample image displaying step, the spread value of the electron beam in the sample is calculated based on the difference between the secondary electron detection data having different incident energy set values, and the sample depth information obtained in advance is calculated. Determining in-sample depth information corresponding to the calculated spread value of the electron beam by referring to the correspondence data with the spread of electrons incident in the sample,
A method for displaying a sample image, which comprises forming and displaying an image of a three-dimensional structure of a sample from depth information based on a difference between the secondary electron detection data and secondary electron detection data for each incident energy setting value. . 3. Corresponding data of the depth information in the sample and the spread of the electrons incident on the sample are the secondary electron detection signal intensity obtained when the sample having a known step structure is scanned with an electron beam. 3. The method according to claim 2, which is obtained from the amount of tailing.
The method for displaying a sample image as described in 1. 4. The correspondence data between the depth information in the sample and the spread of the electrons incident on the sample are obtained for a sample including an internal structure having a known thickness.
The method for displaying a sample image as described in 1.