JPH0547331A - Scanning electron microscope - Google Patents

Abstract

PURPOSE:To satisfactorily observe the bottom section of a hole having a high aspect ratio and formed on a sample face without applying strong magnetic field to the sample face. CONSTITUTION:In a scanning electron microscope provided with an objective lens 14 storing a sample 9 between an upper electrode 15a on the electron gun 1 side and a lower electrode 15b faced to the upper electrode 15a and detecting the secondary electrons from the sample 9 to observe the surface of the sample 9, the outer diameter phi2 of the lower electrode 15b is made smaller than the outer diameter phi1 of the upper electrode 15a on the electrode gun 1 side. The secondary electrons discharged nearly in the vertical direction from the bottom section of a hole 9a on the surface of the sample 9 are fed to a secondary electron detector 10 along the magnetic field diverged to the upper electrode 15a side.

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 capable of observing even the bottom of a hole having a high aspect ratio (depth / inner diameter).

[0002]

2. Description of the Related Art There are various objects to be observed by a scanning electron microscope. For example, a thickness of 1.5 μm coated on a silicon substrate.
The inner diameter formed on the resist thin film is about 0.3 μm
There are cases where it is desired to observe the state of the surface of the silicon substrate through a hole of about 0.8 μm. In order to clearly observe the bottom of the hole having such a high aspect ratio (depth / inner diameter), a scanning electron microscope using a so-called in-lens type objective lens is used.

FIG. 3 shows the structure of a scanning electron microscope using a conventional in-lens type objective lens. In FIG. 3, the electron beam emitted from the electron gun 1 about the optical axis AX is irradiated by the irradiation lens 2. It is focused once. The focused electron beam is diffused again, and then two-dimensionally or one-dimensionally scanned by the electron beam scanning device including the two-stage deflectors 3 and 4, and enters the in-lens type objective lens 5. The objective lens 5 narrows down the electron beam so that the beam diameter of the electron beam on the sample is minimized.

In the objective lens 5, reference numeral 6 denotes a substantially cylindrical yoke, and the optical axis A is provided on the yoke 6 side of the yoke 6.
A cylindrical upper pole 6a having an opening centered at X and an outer diameter of φ1 is formed, and a coil 7 is wound around the upper pole 6a so that the outer diameter is opposite to the upper pole 6a. The lower pole 8 also serving as a table made of a disk-shaped magnetic material of φ2 is arranged. The lower pole 8 is a base 12A made of a non-magnetic material.
This base 12A is placed on the bottom surface 6b of the yoke 6 via the vacuum seal material 12B.
And the lower pole 8 form a magnetic circuit. And
A sample 9 is supported on the lower electrode 8 which also serves as a table. The outer diameter φ2 of the lower pole 8 is set to a sufficiently large dimension so that the objective lens 5 operates even if the table is moved to an arbitrary position. Therefore, the outer diameter φ of the upper pole 6a
1 is set to be smaller than the outer diameter φ2 of the lower pole 8. The sample 9 can be relocated from the outside through a hole (not shown) formed in the outer wall of the yoke 6.

A disc-shaped secondary electron detector 10 having an opening centered on the optical axis AX is arranged at a position slightly away from the tip of the upper pole 6a. The secondary electron detector 1
An aperture aperture plate 11 having a small opening centered on the optical axis AX is arranged between 0 and the tip of the upper pole 6a. The secondary electron detector 10 is one in which a scintillator is applied to the surface of a glass plate and a high voltage of, for example, 5 to 20 kV is applied, and when the secondary electron emitted from the sample collides with the scintillator, light is emitted. This light is guided to the photoelectric conversion element via a light guide (not shown).
The aperture plate 11 is made of a refractory metal such as platinum or molybdenum, and limits the numerical aperture of the primary electron beam with which the sample 9 is irradiated to a predetermined value. Note that FIG.
In the example, the astigmatism correction device and the aligner are omitted for simplicity.

Since conventionally φ1 <φ2 is established, the magnetic field B formed between the upper pole 6a and the lower pole 8 is strong on the upper pole 6a side and on the lower pole 8 side (that is, the sample 9 side). It's getting weaker.
For example, it is assumed that a hole 9a having a high aspect ratio is formed on the surface of the sample 9. Then, when the electron beam is being scanned by the electron beam scanning device, the primary electrons are focused along the locus 13A connecting the opening of the aperture plate 11 and the bottom of the hole 9a from the electron gun 1 side. Then, secondary electrons are emitted from the bottom of the hole 9a.
If it can lead to the next electron detector 10, its hole 9a
The state of the bottom of the can be visualized and observed.

In this case, since the hole 9a has a high aspect ratio, when the magnetic field B is weak, the secondary electrons emitted from the bottom of the hole 9a have an initial velocity in the direction perpendicular to the optical axis AX. Most of the material is absorbed by the side wall of the hole 9a. In this regard, in the magnetic field B, an electron having a velocity component of vx in the plane perpendicular to the magnetic field vz in the direction of the magnetic field B progresses as it is in the direction of the magnetic field B at the velocity vz, and Radius r is vx in the vertical plane
It is known to have a cyclotron motion that is determined in proportion to / B. This means that the stronger the magnetic field B is, the more the electron makes cyclotron motion with a smaller radius, and the electron moves while spirally entwining along the magnetic field line (magnetic field vector).

Therefore, conventionally, the magnetic field B is made strong on the observation surface of the sample 9, and the secondary electrons generated at the bottom of the hole 9a are caused to undergo cyclotron motion having a diameter smaller than the diameter of the hole 9a. , For example, spiral locus 1 in FIG.
The secondary electrons were guided to the secondary electron detector 10 along 3B. According to this method, by strengthening the magnetic field B, most of the secondary electrons generated at the bottom of the hole 9a can be led out of the hole 9a.

On the other hand, the secondary electrons emitted from the bottom of the hole 9a to the electron gun 1 side substantially parallel to the optical axis AX can go out of the hole as they are without applying a high magnetic field.
However, since the magnetic field of the upper pole 6a is considerably stronger than that of the lower pole 8 in the related art, the secondary electrons thus emitted substantially parallel to the optical axis AX proceed in the direction of the upper pole 6a, The secondary electron detector 10 tended to be unable to capture.

[0010]

According to the conventional device,
By increasing the magnetic field B on the sample surface, the state of the bottom of the hole 9a having a high aspect ratio formed on the sample surface can be observed. In this case, if the diameter of the hole 9a becomes smaller, a very strong magnetic field must be applied to the sample surface in order to make the cyclotron motion with a diameter smaller than that diameter. However, there is a limit to the generation of a strong magnetic field due to a phenomenon such as saturation of a magnetic material,
The bottom of the hole whose diameter is smaller than a certain extent has the disadvantage that it cannot be observed well.

In view of the above point, the present invention provides a scanning electron microscope capable of satisfactorily observing the bottom of a high aspect ratio hole formed on a sample surface without applying a strong magnetic field to the sample surface. To aim.

[0012]

In the present invention, for example, as shown in FIG. 1, a sample (9) is provided between a magnetic pole (15a) on the side of an electron gun (1) and a counter electrode (15b) facing the magnetic pole. It has an objective lens (14) to be housed, and 2 from the sample (9)
In the scanning electron microscope which detects the secondary electrons and observes the surface of the sample (9), the magnetic pole (15) on the electron gun (1) side
The outer diameter φ2 of the counter electrode (15b) is smaller than the outer diameter φ1 of a).

Further, the present invention is such that the observation point (9a) of the sample (9) is displaced from the optical axis AX of the objective lens (14).

[0014]

According to the present invention, assuming that the hole (9a) is formed on the surface of the sample (9) as shown in FIG. 2, the hole (9a) is irradiated by the irradiation of primary electrons. Secondary electrons are generated at the bottom of the. Of these secondary electrons, most of the electrons emitted in the direction in which the angle with respect to the optical axis AX is large are absorbed by the side wall of the hole (9a), and the electrons are emitted outside the hole (9a). Since it does not appear, secondary electrons cannot be detected. However, the light is emitted in the direction of the optical axis AX or in the direction with a small angle with respect to the optical axis AX.
Secondary electrons can exit the hole (9a) without colliding with the side wall of the hole (9a).

The secondary electrons, which have come out of the hole (9a), travel toward the secondary electron detector (10) while moving around the magnetic field lines due to the cyclotron motion. In the present invention, the electron gun (1) of the objective lens (14)
Than the outer diameter φ1 of the magnetic pole (15a) on the side
Since the outer diameter φ2 of b) is smaller, the magnetic field is in a state of diverging from the sample (9) toward the magnetic pole (15a) on the electron gun (1) side. Therefore, the secondary electrons gradually take an orbit away from the optical axis AX. Therefore, by providing a secondary electron detector (10) at a position away from the optical axis AX, the secondary electron is The detector (1
0) can be captured.

Since the magnetic field diverges in the direction of the magnetic pole (15a) on the electron gun (1) side, the strength of the magnetic field at the position of the detector (10) is the surface of the sample (9). It is considerably smaller than the strength in. Therefore, the secondary electrons leave the magnetic field constraint and the detector (10)
It becomes easy to take a trajectory toward the direction of the electric field due to the positive voltage applied to the detector (10), and it becomes easy to enter the detector (10).

When the hole (9a) as the observation point of the sample (9) is near the optical axis AX of the objective lens (14), it is emitted from the bottom of the hole (9a). Two
The secondary electrons travel along the optical axis AX and are less likely to enter the detector (10). On the other hand, when the hole (9a) is displaced from the optical axis AX, the hole (9a)
The secondary electrons vertically emitted from the bottom of the hole take a trajectory along the magnetic field and easily enter the detector (10), so that the bottom of the hole (9a) can be observed well.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment of the present invention will now be described with reference to FIGS.
Let us explain with reference to. 1 and 2, those parts corresponding to those in FIG. 3 are designated by the same reference numerals, and a detailed description thereof will be omitted. FIG. 1 shows the entire configuration of the scanning electron microscope of this example. In FIG. 1, 14 is an in-lens type objective lens, which irradiates an electron beam emitted from the electron gun 1 along an optical axis AX. After the light beam is focused by the lens 2, it is deflected by the electron beam scanning device including the deflectors 3 and 4, and the objective lens 14 is irradiated therewith.

In the objective lens 14, 15 is a substantially cylindrical yoke having the optical axis AX as an axis.
A dish-shaped upper pole 15a having an outer diameter of φ1 is formed on the surface of the No. 5 on the electron gun 1 side. The upper pole 15a has a circular opening 15c with a diameter of φ3 about the optical axis AX. Forming the lower pole 15
The coil 16 is wound around b. In this example, the outer diameter φ1 of the bottom portion of the upper pole 15a is set to be larger than the outer diameter φ2 of the lower pole 15b. That is, φ1> φ
Since 2 is established, in the case of this example, the magnetic force lines (vectors of the magnetic field B) diverge from the lower pole 15b to the upper pole 15a. Since the strength of the magnetic field B is inversely proportional to the total area of the magnetic flux, in this example, the upper pole 15a is higher than the lower pole 15b side.
The magnetic field is weaker on the side.

Numerically, as an example, the outer diameter φ1 of the bottom surface of the upper pole 15a is about 10 to 30 mm, the inner diameter φ3 of the opening 15c is about 5 to 10 mm, and the outer diameter φ2 of the lower pole 15b is 5.
The gap between the upper electrode 15a and the lower electrode 15b is about 5 to 15 mm.

Also, the upper pole 15 is mounted on the coil 16 via movable tables 8A and 8B which are movable.
The sample 9 is installed at a position near the lower pole 15b between the a and the lower pole 15b, and the hole 1 formed in the side wall of the yoke 15
The position of the sample 9 is appropriately changed through 5c. A secondary electron detector 10 to which a high voltage is applied is arranged above the sample 9 at a position close to the upper pole 15a, and an aperture aperture is provided between the secondary electron detector 10 and the upper pole 15a. The plate 11 is arranged.

The operation of observing the bottom of a hole having a high aspect ratio in this example will be described. It is assumed that the observation target (viewing side point) in this example is the hole 9a formed on the surface of the sample 9. In this case, the primary electron beam reaches the bottom of the hole 9a along the locus 12 passing through the opening of the aperture plate 11 by passing a predetermined direct current through the deflectors 3 and 4. Then, an AC current is superposed on the deflectors 3 and 4, and the surface of the sample 9 is surface-scanned by the primary electron beam to obtain a plane image of a predetermined region surrounding the hole 9a.

The state of secondary electrons generated from the bottom of the hole 9a will be described with reference to FIG. In this case, since the outer diameter of the upper pole 15a is larger than the outer diameter of the lower pole 15b in this example, the line of magnetic force (vector of the magnetic field B) is lower pole 15b.
Is formed so as to diverge in the direction from the upper pole 15a to the upper pole 15a. Therefore, the secondary electrons emitted in a direction substantially perpendicular to the bottom of the hole 9a (that is, substantially parallel to the optical axis AX) are
For example, the light enters the secondary electron detector 10 while making a spiral motion based on the cyclotron motion along the locus 17. Particularly in this example, the magnetic field on the side of the upper pole 15a is weaker than that on the side of the lower pole 15b, so the secondary electrons emitted outside the hole 9a are
Without being bound by the magnetic field on the side of the upper pole 15a, the secondary electron detector 10 is attracted to the high potential applied to the secondary electron detector 10 and easily enters the detector 10.

Similarly, 2 generated from the bottom of the hole 9a
Among the secondary electrons, those having a relatively small inclination with respect to the optical axis AX and emitted to the outside after being reflected by the side wall of the hole 9a about once are the detectors along the locus 18 parallel to the magnetic lines of force, for example. It can be incident on 10. Returning to FIG. 1, in this example, the outer diameter φ1 of the upper pole 15a is set to the lower pole 15a.
When the outer diameter of b was set to about 3 times or more, the bottom of the hole 9a could be particularly well observed. In this case, the strength of the magnetic field on the secondary electron detector 10 is
Since it is about 1/10 of the strength of the magnetic field on the sample 9, if the strength of the magnetic field on the secondary electron detector 10 is set to about 1/10 or less of the strength on the sample 9. It will be possible to observe the bottom of the hole particularly well. The strength of the magnetic field may be that required for normally focusing the primary electron beam of a low acceleration voltage, and when the strength of the magnetic field at the position of the sample 9 is, for example, about 1000 gauss, When the outer diameter φ1 is about three times the outer diameter φ2, the strength of the magnetic field at the position of the secondary electron detector 10 is about 100 Gauss.

As described above, according to this example, the secondary electrons emitted from the bottom of the hole on the surface of the sample in a substantially vertical direction are positively captured, so that the strength of the magnetic field on the sample surface is increased. There is an advantage that the bottom of a hole having a high aspect ratio can be satisfactorily observed without making the height larger than usual.
Furthermore, even if the inner diameter of the hole is small, there are secondary electrons emitted from the bottom of the hole substantially vertically, so that the bottom of the hole can be observed without particularly increasing the magnetic field.

In FIG. 2, the hole 9a is shown for convenience of explanation.
Although the distance δ between the center of the optical axis and the optical axis AX is large, in reality, when the distance δ was about 100 μm, the bottom of the hole 9a could be brightly and satisfactorily observed. Conversely, when the hole 9a is located on the optical axis AX (that is, δ =
0), the bottom of the hole 9a became dark and difficult to observe. This is because in that case, the secondary electrons emitted from the bottom of the hole 9a substantially parallel to the optical axis AX proceed to the electron gun 1 without being captured by the secondary electron detector 10 as they are. It is thought to be from.

Further, in FIG. 1, the aperture aperture plate 11 is moved toward the electron gun 1 side and fixed, and the secondary electron detector 10 thereof is located at the intermediate position 1 between the deflector 4 and the upper pole 15a.
Even when it was provided in No. 9, the bottom of the hole 9a could be observed well. This is because the magnetic field is weak in the portion above the upper pole 15a, and the secondary electrons are easily attracted to the secondary electron detector by being attracted to the high-potential electric field created by the secondary electron detector. it is conceivable that.

As described above, the present invention is not limited to the above-mentioned embodiments, and various configurations can be adopted without departing from the gist of the present invention.

[0029]

According to the present invention, since the outer diameter of the counter electrode is smaller than the outer diameter of the magnetic pole on the electron gun side, a secondary electron detector is arranged near the magnetic pole on the electron gun side. in case of,
The magnetic field of the objective lens diverges from the sample surface toward the detector. Therefore, the secondary electrons emitted from the bottom of the hole on the surface of the sample substantially parallel to the optical axis of the objective lens are in a position apart from the optical axis while rotating along the magnetic field by the cyclotron motion. Since the light is incident on the secondary electron detector, there is an advantage that even if the aspect ratio of the hole is high, the bottom of the hole can be well observed in a bright state. In this case, the strength of the magnetic field may be such that it is necessary to normally focus the primary electron beam having a low acceleration voltage, and does not need to be particularly strong.

When the observation point of the sample is shifted from the optical axis of the objective lens, the secondary electrons emitted from the bottom of the hole on the surface of the sample substantially parallel to the optical axis are secondary electrons. Since the light easily enters the detector, there is an advantage that the bottom of the hole is more easily observed.

[Brief description of drawings]

FIG. 1 is a sectional view showing the configuration of an embodiment of a scanning electron microscope according to the present invention.

FIG. 2 is an enlarged view of a main part of FIG.

FIG. 3 is a cross-sectional view showing a configuration of a conventional scanning electron microscope.

[Explanation of symbols] 1 electron gun 2 irradiation lens 3, 4 deflector 9 sample 9a hole 10 secondary electron detector 11 aperture aperture plate 14 objective lens 15a upper pole 15b lower pole

Claims (2)

[Claims] 1. A scan for observing a surface of a sample by detecting an electron secondary from the sample by including an objective lens between the magnetic pole on the electron gun side and a counter electrode facing the magnetic pole. In the electron microscope, the scanning electron microscope is characterized in that the outer diameter of the counter electrode is smaller than the outer diameter of the magnetic pole on the electron gun side. 2. The scanning electron microscope according to claim 1, wherein the observation point of the sample is shifted from the optical axis of the objective lens.