JPH08264149A - Secondary electron detecting apparatus for scanning electron microscope - Google Patents

Abstract

PURPOSE: To effectively detect secondary electrons generated from a specimen even if the electrons get off the restriction owing to the magnetic field of an objective lens by accelerating the secondary electrons emitted from the specimen by an accelerating tube in which the voltage becomes higher as the distance from the specimen becomes longer and detecting the secondary electrons. CONSTITUTION: While being narrowed down by an objective lens having magnetic poles 1, 2, an electron beam 5 is irradiated to a specimen 3, an electron microscopic image is formed in the specimen 3, and corresponding secondary electrons are emitted. The secondary electrons are accelerated upward by a cylindrical accelerating tube 6 which penetrates the magnetic field formed by the magnets 1, 2 and the axis of the object lens and in which voltage becomes higher as the distance from the specimen 3 becomes longer. Consequently, even if the secondary electrons emitted from the specimen get off the restriction by the objective lens, the secondary electrons can be collected by a detector 9 and the secondary electrons can be detected effectively.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a secondary electron detector of a scanning electron microscope for detecting secondary electrons generated from a sample.

Recently, in a scanning electron microscope, a surface of a non-conductive sample, for example, a semiconductor is irradiated with a finely focused electron beam for scanning, and secondary electrons generated at that time are collected to collect a secondary electron image. Is displayed on the screen and observed. At this time, since the electron beam on the non-conductive sample is charged and a high-quality secondary electron image with high resolution cannot be observed, the accelerating voltage of the electron beam is low, for example, accelerating to an accelerating voltage of 2 KV or less. It is desired to collect secondary electrons efficiently even if the sample is irradiated with an electron beam.

[0003]

2. Description of the Related Art Conventionally, in a scanning electron microscope, as shown in FIG. 2, as a secondary electron detector for irradiating a sample with a finely focused electron beam and efficiently detecting secondary electrons generated at that time. There is something.

FIG. 2 shows an explanatory view of the prior art. In FIG. 2, the upper magnetic pole 21 and the lower magnetic pole 22 of the objective lens are for forming a magnetic field distribution for focusing the electron beam 25 on the sample 23.

The sample 23 is a sample to be observed by scanning the electron beam to collect secondary electrons generated at that time and displaying a secondary electron image. The axis 24 is an axis of an objective lens or the like.

The electron beam 25 is an electron beam generated by an electron gun (not shown), which is focused by an objective lens, narrowed down, and scanned on the sample 23.

The lens electrode 26 has a cylindrical shape provided inside the objective lens and is held by the lens electrode 28 and applies a positive voltage. Detector 2
Reference numeral 9 is a detector that detects secondary electrons.

The electrode 30 is applied to the sample 23 by applying a negative voltage.
Do not let secondary electrons pass through from the detector 29
To be collected in. Next, the collection of secondary electrons will be briefly described.

(1) An electron beam 25 generated and accelerated by an electron gun (not shown) is narrowed down by the magnetic field distribution formed by the upper magnetic pole 21 and the lower magnetic pole 22 of the objective lens to scan the sample 23. To do. At this time, the correction coil 33 corrects the electron beam 25 so that the electron beam 25 is narrowed down, and the deflection coil (not shown) scans the sample 23.

(2) When the electron beam 25 scans the sample 23 in (1), secondary electrons are emitted from the sample 23. The emitted secondary electrons are emitted in the upper direction while rotating in the magnetic field of the objective lens. On this occasion,
The positive voltage is applied to the lens electrode 26, which is provided in a cylindrical shape around the axis 4, and is drawn upward.

(3) 2 pulled out upward in (2)
Secondary electrons are blocked from passing by the anode 30 to which a negative voltage is applied, and are guided toward the detector 29 to be captured and detected.

[0012]

As described above, according to the conventional configuration of FIG. 2, the cylindrical lens electrode 26 is provided inside the objective lens to apply a positive voltage, and the magnetic field of the objective lens is applied. Due to the confinement action of the secondary electrons and the action of applying a positive voltage to the lens electrode 26 and accelerating in the upward direction, the secondary electrons emitted from the sample 23 are captured and detected by the detector 29 very efficiently. I was able to.

Generally, the resolution of an electron microscope depends on the acceleration voltage, and the higher the acceleration voltage is, the higher the resolution is. In order to obtain a relatively high resolution with a low acceleration voltage, the working distance (focal length) of the objective lens must be reduced. When the working distance is reduced, the gap between the sample 23 and the lower magnetic pole 22 of the objective lens becomes small, and it becomes difficult to extract and detect secondary electrons from this gap. The detection is performed from the upper side of the objective lens, and at this time, a positive voltage is applied to the cylindrical lens electrode 26 to accelerate the secondary electrons emitted from the sample 23 for efficient detection. .

However, the 2 emitted from the sample 23 of FIG.
The trajectories of secondary electrons have various initial velocities and directions. A considerable part of it collides with the lens electrode 26.
Particularly, in the case of the sample 23 such as a semiconductor, the sample 2
It is necessary to observe the secondary electron image in the state where the voltage Vb is applied to No. 3, and when this voltage Vb is applied, the voltage of the secondary electron is increased by the amount of the voltage Vb, and escape from the constraint by the magnetic field of the objective lens. It collides with the lens electrode 26,
There is a problem that the detector 29 cannot be reached and the secondary electron collection efficiency is reduced, and a secondary electron image with high resolution cannot be obtained.

Particularly, the sample 23 is scanned with an electron beam accelerated at a low acceleration voltage, for example, 1400 V, and secondary electrons generated at that time are restrained by the magnetic field of the objective lens while being applied with a positive voltage applied to the lens electrode 26. Guide in the direction and detector 29
In the case of collecting the sample 23, if a large voltage Vb = −600 V is applied to the sample 23, the incident voltage of the electron beam on the sample 23 is 1400−600 = 800.
V, the secondary electron emitted at this time is 600 V, which is 75% of the voltage (energy) of the primary electron,
There is a problem that the magnetic field of the objective lens cannot be expected to be restrained, the collision with the inner wall of the cylindrical lens electrode 26 may occur, and the secondary electron cannot be detected without reaching the detector 29.

The present invention solves these problems.
A cylindrical accelerating tube is provided around the axis of the objective lens to apply an accelerating voltage and to form a secondary electron emitting substance on the inner wall, so that the secondary electrons emitted from the sample collide with the inner wall of the accelerating tube. Also emits secondary electrons and accelerates the secondary electrons toward the detector by the accelerating voltage so that the secondary electrons generated from the sample can be efficiently detected even if they are out of the constraint of the magnetic field of the objective lens. Has an aim.

[0017]

[Means for Solving the Problems] Means for solving the problems will be described with reference to FIG. In FIG. 1, the electron beam 5 is used to irradiate the sample 3 after being narrowed down by an objective lens.

The sample 3 is a sample to be scanned by irradiating the electron beam 5. The accelerating tube 6 has a cylindrical semi-conductive tube shape and forms an accelerating voltage.

The lens electrodes 7 and 8 apply a voltage to the acceleration tube 6. The detector 9 detects secondary electrons. The electrode 10 returns the secondary electrons so that they do not pass therethrough, and applies a negative voltage.

[0020]

According to the present invention, as shown in FIG.
The accelerating voltage is applied to the accelerating tube 6 by the lens electrodes 7 and 8 provided on the upper and lower portions of the sample, and the electron beam 5 is narrowed down by the objective lens to irradiate the sample 3 for scanning and the secondary generated at that time. The electrons are gradually accelerated in the direction from the sample 3 to the detector 9 by the acceleration voltage of the accelerating tube 6 so that the electrons are efficiently collected by the detector 9.

At this time, a substance that easily generates secondary electrons is formed on the inner wall of the accelerating tube 6, and the secondary electrons emitted from the sample 3 deviate from the constraint of the magnetic field of the objective lens, so that the inner wall of the accelerating tube 6 is removed. When secondary electrons collide with, secondary electrons are further accelerated by the accelerating voltage of the accelerating tube 6 so that the secondary electrons are accelerated toward the detector 9. There is.

Further, an electrode 10 to which a negative voltage is applied is provided,
The accelerated secondary electrons are prevented from passing through without being captured by the detector 9 so that they are efficiently collected by the detector 9.

Therefore, a cylindrical accelerating tube 6 is provided around the axis of the objective lens to apply an accelerating voltage and to form a secondary electron emitting substance on the inner wall, so that the secondary electrons emitted from the sample 3 are accelerated. Even when it collides with the inner wall of the tube 6, secondary electrons are emitted and the secondary electrons are accelerated toward the detector 9 by the acceleration voltage, so that the secondary electrons generated from the sample 3 are restrained by the magnetic field of the objective lens. It became possible to detect efficiently even if it came off.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the configuration and operation of an embodiment of the present invention will be described in detail with reference to FIG. FIG. 1 shows a block diagram of an embodiment of the present invention.

In FIG. 1, the upper magnetic pole 1 of the objective lens
The lower magnetic pole 2 guides a magnetic field generated when a current is passed through a coil (not shown) to form a lens magnetic field on the shaft 4.

The sample 3 is a sample for scanning while being irradiated with the electron beam 5 that is narrowed down, collecting secondary electrons generated and subjecting them to brightness modulation or the like to generate a so-called secondary electron image on the screen. Yes, for example, a semiconductor wafer.

The electron beam 5 is an electron beam generated by an electron gun (not shown). The accelerating tube 6 has a cylindrical shape and forms an accelerating voltage. A voltage is applied between the upper lens electrode 8 and the lower lens electrode 7 to generate an accelerating voltage that gradually changes. To form. The accelerating tube 6 is semi-conductive and has an inner wall formed of a material that easily generates secondary electrons or a material that easily generates secondary electrons is formed on the inner wall by coating, vapor deposition, a vapor phase method, or the like. .

The detector 9 detects secondary electrons emitted from the sample 3. The electrode 10 applies a negative voltage to prevent secondary electrons accelerated by the accelerating tube 6 from passing through the position of the detector 9 and pushes them toward the detector 9.

The orbit 11 is the orbit of secondary electrons emitted from the sample 3. In this trajectory 11, when the radius of the spiral motion due to the magnetic field on the axis of the objective lens becomes large and collides with the inner wall of the accelerating tube 6 as shown in the figure, the inner wall is a substance having good secondary electron generation efficiency. Therefore, secondary electrons are further generated, and the generated secondary electrons are further accelerated by the accelerating voltage of the accelerating tube 6 as shown in FIG. It is possible to detect secondary electrons generated from the sample 3 extremely efficiently by traveling toward the detector 9 as shown in the orbit 12 by repeating generation of secondary electrons. .

The correction coil 13 receives the electron beam 5 from the sample 3
The correction is performed so that the aperture is finely narrowed, and the so-called astigmatism of the objective lens is corrected. Next, FIG.
The operation of the above configuration will be described in detail.

(1) A positive voltage V2 is applied to the upper lens electrode 8 of the semiconductive accelerating tube 6 and a positive voltage V1 is applied to the lower lens electrode 7. At this time, positive voltage V1 <positive voltage V2
The acceleration voltage is formed in the acceleration tube 6. For example, when the positive voltage V1 = + 30V and the positive voltage V2 = + 80V (ground potential), the acceleration voltage is formed from the sample 3 toward the detector 9. This positive voltage is actually
The lens electrodes 7 and 8 are detected by the detector 9 most efficiently in accordance with the acceleration voltage of the electron beam 5 and the voltage Vb applied to the sample 3 so that an optimum secondary electron image can be obtained.
The positive voltages V1 and V2 to be applied to the above are experimentally obtained and set, or set to optimum values while observing the secondary electron image.

(2) An appropriate negative potential is applied to the electrode 10 so that the secondary electrons accelerated by the accelerating tube 6 do not pass through the detector 9. (3) Scan the electron beam 5 generated by an electron gun (not shown) in a state where the electron beam 5 is narrowed down and irradiated onto the sample 3.

(4) Secondary electrons generated by irradiating the sample 3 with the electron beam 5 in (3) are accelerated upward by the accelerating voltage of the accelerating tube 6, and the secondary electrons restrain the magnetic field of the objective lens. When the inner wall of the accelerating tube 6 is deviated from and the secondary wall of the accelerating tube 6 collides against the inner wall of the accelerating tube 6, a new secondary electron is generated from the inner wall. The secondary electrons are repeatedly guided toward the detector 9.

(5) The secondary electrons induced in (4) are made to collide with the detector 9 to detect the secondary electrons. At this time, the secondary electrons that are about to pass through the detector 9 are pushed back by the electrode 10 to which a negative voltage is applied and detected by the detector 9.

As described above, the secondary electrons emitted from the sample 3 deviate from the constraint of the magnetic field of the objective lens, and the acceleration tube 6
Since the inner wall is formed of a substance that easily generates secondary electrons even when it collides with the inner wall of the detector, new secondary electrons are generated, and the secondary electrons are further detected by the acceleration voltage of the accelerating tube 6 by the detector 9
Therefore, the secondary electrons can be guided to the detector 9 and detected very efficiently as a result.

Next, the case where the voltage Vb is applied to the sample 3 will be described. (1) When the voltage Vb of the sample 3 is set to several tens% or more of the acceleration voltage Va of the electron beam 5, for example: -acceleration voltage Va = 1400V-voltage Vb = -600V, the electron beam 5 which is a primary electron The voltage difference (energy) incident on the sample 3 is 1400-600 = 80.
It becomes 0V. The voltage (energy) of the secondary electrons emitted from the sample 3 is 600 V, which is 600 ÷ 80 of that of the primary electrons.
0 × 100 = 75%. When the voltage of the secondary electrons is as high as 75% as compared with the voltage of the primary electrons in this way, the radius of the constraint by the helical motion of the magnetic field of the objective lens in FIG. 1 becomes large and the collision occurs with the inner wall of the accelerating tube 6. However, in the present invention, new secondary electrons are emitted even when they collide with the inner wall of the acceleration tube 6, and
The secondary electrons generated from the sample 3 can be guided to the detector 9 extremely efficiently by repeating the acceleration toward the detector 9 by the accelerating voltage from the accelerating tube 6.

[0037]

As described above, according to the present invention,
A cylindrical accelerating tube 6 is provided around the axis of the objective lens to apply an accelerating voltage and to form a secondary electron emitting substance on the inner wall.
Even if the secondary electrons emitted from the sample 3 collide with the inner wall of the accelerating tube 6, the secondary electrons are emitted and the secondary electrons are accelerated toward the detector 9 by the acceleration voltage. , The secondary electrons generated from the sample 3 can be efficiently detected even if they are out of the constraint due to the magnetic field of the objective lens.

[Brief description of drawings]

FIG. 1 is a configuration diagram of an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a conventional technique.

[Explanation of symbols]

 1, 2: Magnetic pole of objective lens 3: Sample 5: Electron beam 6: Accelerator tube 7: 8: Lens electrode 9: Detector 10: Electrode 11, 12: Orbit of secondary electron 13: Correction coil V1, V2: Acceleration Positive voltage applied to lens electrode of tube Vb: Voltage applied to sample

Claims (4)

[Claims] 1. An objective lens for irradiating a sample with a finely focused electron beam, a cylindrical piercing the axis of the objective lens, and an accelerating tube having a voltage that increases as the distance from the sample increases. A secondary electron detector of a scanning electron microscope provided with a detector for detecting secondary electrons that have passed through. 2. The accelerating tube is made semi-conductive, and a voltage is applied across the accelerating tube to form an accelerating voltage.
Secondary electron detector of the described scanning electron microscope. 3. The inner wall of the accelerating tube is made of a material that easily generates secondary electrons.
Secondary electron detector of the described scanning electron microscope. 4. An electrode for applying a negative voltage for blocking secondary electrons that pass without colliding with the detector for detecting the secondary electrons and colliding with the detector. The secondary electron detector of the scanning electron microscope according to claim 1.