JP3014986B2 - Scanning electron microscope - Google Patents

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 for obtaining a scanned image of a sample surface by scanning an electron beam spot on the surface of the sample, and more particularly to obtaining a scanned image having a high spatial resolution in a low acceleration voltage region. The present invention relates to a scanning electron microscope that can be used.

[0002]

2. Description of the Related Art Conventionally, it has been used for observing or measuring a contact hole or a line pattern of a submicron order (1 μm or less) in a semiconductor device sample.
A scanning electron microscope is used.

In a scanning electron microscope, an electron beam emitted from a heating type or field emission type electron source is scanned on a sample, and signals (secondary electrons and reflected electrons) obtained secondarily are detected. By using the secondary signal as the luminance modulation input of the cathode ray tube which is scanned in synchronization with the electron beam scanning, the scanned image (SEM
Image). In a general scanning electron microscope, electrons emitted from an electron source are accelerated between an electron source to which a negative potential is applied and an anode at a ground potential, and are irradiated on a test sample at a ground potential.

In recent years, scanning electron microscopes have been used in semiconductor manufacturing processes or inspection processes after completion (for example, inspection of electrical operation by electron beams). To be able to do so, a high resolution of 10 nm or less has been required at a low acceleration voltage of 1000 V or less.

That is, a sample for a semiconductor device is generally formed by laminating an electrical insulator such as SiO 2 or SiN on a conductor such as Al or Si. When such a semiconductor device sample is irradiated with an electron beam, the surface of the electric insulator becomes negatively charged (hereinafter, sometimes simply referred to as charge-up), and the trajectory of secondary electrons emitted changes, The orbit of the electron beam itself changes. As a result, abnormal contrast occurs in the SEM image or severe distortion occurs.

[0006] The image failure caused by such charge-up seriously hinders observation of contact holes and length measurement of lines and spaces, so that not only is it difficult to evaluate a semiconductor manufacturing process but also the quality of a semiconductor device itself. Is a major obstacle in securing Therefore, conventionally, the energy of the primary electron beam applied to the sample is 1K.
A so-called low acceleration SEM having an eV or less has been used.

However, the above-mentioned prior art has the following problems. That is, when the acceleration voltage is reduced, the resolution is significantly reduced due to the chromatic aberration caused by the energy variation of the electron beam, and observation at high magnification becomes difficult. Also,
When the electron current decreases, the ratio of the secondary signal to the noise (S /
N) is significantly reduced, the contrast as an SEM image is deteriorated, and observation with high magnification and high resolution becomes difficult. In particular, in a semiconductor device or the like manufactured by an ultra-fine processing technique, a signal generated from a concave portion such as a contact hole or a line pattern is weak, which is a major obstacle in performing fine observation and length measurement.

[0008] As a method for solving such a problem, for example, the eye triple eye, the ninth Annual Symposium on Electron Ion and Laser Beam Technology, pp. 176 to 186 (IEEE 9th Annual Symposium on) Electron, Ion
and Laser Technology), the acceleration voltage between the electron source and the anode at the ground potential is set high, and a deceleration electric field is generated between the objective lens at the ground potential and the test sample to which the negative potential is applied, so that There has been proposed a scanning electron microscope in which the irradiation electron beam is decelerated to finally set a relatively low acceleration voltage, thereby reducing chromatic aberration and preventing charge-up.

[0009]

In the above-mentioned prior art, a sample which is vulnerable to electric shock, such as a semiconductor device, may be destroyed by a sharp change in potential. There is a problem that it is difficult to handle at the time of sample exchange and the like, for example, it is necessary to carefully turn on / off the potential.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems of the prior art, and to provide a deceleration electric field between an objective lens and a sample in a scanning electron microscope having a structure in which a deceleration electric field is generated. Is to prevent

[0011]

In order to achieve the above object, according to the present invention, a primary electron beam emitted from an electron source is scanned on a sample and scanned based on a secondary signal generated from the sample. A scanning electron microscope for obtaining an image, a deceleration electric field generating means for generating a deceleration electric field for the primary electron beam between the sample and the objective lens, a control electrode disposed between the sample and the objective lens, and the control electrode And a voltage applying means for applying a predetermined voltage, and the predetermined voltage is adjusted to a value such that a potential difference between the sample and the control electrode is smaller than a potential difference between the sample and the objective lens.

According to the above configuration, the electric field generated between the objective lens and the sample is alleviated by the control electrode, so that damage to the element can be prevented and the handling thereof becomes extremely easy.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic configuration diagram of a scanning electron microscope system to which the present invention is applied. The system includes a scanning electron microscope main body 100 and a sample exchange mechanism 200. FIG. 2 is a diagram showing a configuration of the scanning electron microscope main body 100 of FIG. First, a deceleration electric field which is a prerequisite technology of the present invention will be described with reference to FIGS.

In FIG. 2, a cathode 1, an extraction electrode 2, and an anode 3 constitute a field emission type electron gun. An extraction voltage 4 is applied between the cathode 1 and the extraction electrode 2, and the cathode 1 is accelerated. Voltage 5 is applied. Electron beam 20 emitted from cathode 1
a is further accelerated by the voltage applied between the extraction electrode 2 and the anode 3 at the ground potential. Therefore, the energy (acceleration voltage) of the electron beam passing through the anode 3 matches the acceleration voltage 5. Further, a negative superimposed voltage 6 is applied to the sample 18 via the sample holder 21, and a deceleration electric field is formed between the objective lens 17 and the sample 18, so that the electron beam applied to the sample 18 is accelerated. The voltage is a voltage obtained by subtracting the superimposed voltage 6 from the acceleration voltage 5.

Electron beam 20b accelerated through anode 3
Is converged on the sample 18 by the condenser lens 7 and the objective lens 17. The electron beam that has passed through the objective lens 17 is decelerated by the deceleration electric field formed between the objective lens 17 and the sample 18, and is substantially equal to the acceleration voltage 5 minus the superimposed voltage 6. To reach.

The divergence angle of the electron beam at the objective lens 17 is determined by the stop 8 arranged below the condenser lens 7. Centering of the aperture 8 is performed by operating the adjustment knob 10. The accelerated electron beam 20b is deflected by the upper scanning coil 11 and the lower scanning coil 12, and the sample 1
8, on the convergent electron beam 20 decelerated by the deceleration electric field
c is raster-scanned. In this system, the scanning coil has a two-stage configuration so that the scanned electron beam always passes through the center of the objective lens 17.

The sample 18 is fixed by a sample holder 21, and the sample holder 21 is mounted on a sample stage 19, which can be adjusted in position such as horizontal adjustment, via an insulating table 9. A superimposed voltage 6 is applied to the sample holder 21. Secondary electrons 24 generated from the sample 18 by being irradiated with the decelerated electron beam 20c are accelerated by the deceleration electric field created between the objective lens 17 and the sample 18, are attracted into the objective lens 17, and are further absorbed by the objective lens 17. Under the influence of the 17 magnetic field, it rises while spiraling.

The secondary electrons 24 passing through the objective lens 17
Is a suction electrode 13 provided outside the electron beam path between the objective lens 17 and the lower scanning coil 12 to which a positive potential is applied.
And is accelerated by the scintillator 14 to which 10 kV (positive potential) is applied, and the scintillator is illuminated. The emitted light is guided to a photomultiplier tube 16 by a light guide 15 and is converted into an electric signal. The output of the photomultiplier 16 is further amplified and becomes the luminance modulation input of the cathode ray tube, but is not shown here.

According to the scanning electron microscope having such a configuration, the energy of the electron beam (electron beam 20b) passing through the condenser lens 7, the diaphragm 8, and the objective lens 17 is changed to the final stage electron beam (electron beam 20c). , The chromatic aberration was improved, and high resolution was obtained. Moreover,
Since the primary electron beam applied to the sample is decelerated to have low energy, the charge-up of the sample is also eliminated.

Specifically, the beam diameter, which was 15 nm when only the acceleration voltage (500 V) was applied, was improved to 10 nm by adding the acceleration voltage (1000 V) and the superimposed voltage 6 (500 V). Was.

In FIG. 1, a field emission cathode 1, an extraction electrode 2, an anode 3, a condenser lens 7, an objective lens 1
7, sample 18, sample holder 21, insulating table 9, sample stay
Components such as the jig 19 are housed in a vacuum housing 61.
The illustration of the vacuum exhaust system is omitted.

Here, when a negative voltage is applied to the sample 18, it is necessary to avoid exchanging the sample by the sample exchange mechanism 77 and setting the vacuum housing 61 to the atmosphere. In other words, the superimposed voltage 6 may be applied only when the electron beam is being scanned on the sample 18.

Therefore, in the system to which the present invention is applied, the switch S, which is a preparatory operation for mounting and replacing a sample, is used.
A first condition in which 1 is closed and an acceleration voltage 5 is applied;
The second condition that both the valves G1 and G2 provided between the cathode 1 and the sample 18 are open, and the valve G3 through which the sample exchange mechanism 77 passes to place the sample 18 on the sample stage 19 The switch S2 is closed and the control for applying the superimposed voltage 6 to the sample 18 is performed only when all of the third condition that is closed is satisfied.

The sample holder 21 and the sample stage 19
Are electrically connected to each other via a discharge resistor R. When the switch S2 is opened, the electric charge charged in the sample 18 has a constant time constant via the sample holder 21, the discharge resistor R, and the sample stage 19. Then, the discharge is quickly performed, and the potential of the sample 18 decreases.

It should be noted that the accelerating voltage 5 can be applied under the condition that the vacuum around the cathode 1 is equal to or higher than the set value, or the valves G1 and G are set only when the vacuum of the vacuum housing 61 is equal to or higher than the set value.
It goes without saying that a normal sequence in which 2 is opened is set.

Further, in the above-described system, it has been described that the superimposed voltage 6 is applied when all of the above three conditions are satisfied, but one or two of them are applied.
The switch S2 may be closed when the two conditions are satisfied.

FIG. 3 is a block diagram of a scanning electron microscope according to the first embodiment of the present invention. The same reference numerals as those described above denote the same or equivalent parts. The present embodiment is characterized in that an inconvenient sample can be observed when a strong electric field is applied.

In a semiconductor integrated circuit, an element may be damaged by a strong electric field. In order to solve such problems,
In the present embodiment, a control electrode 39 is provided between the objective lens 17 and the sample 18. The voltage applied to the control electrode 39 is set such that the potential difference between the control electrode 39 and the sample or sample stage to which the negative potential is applied is smaller than the potential difference between the sample or sample stage and the objective lens 17. Are adjusted by the voltage control unit 40.

According to this embodiment, the electric field generated between the objective lens 17 and the sample 18 is alleviated by the control electrode 39, so that damage to the element can be prevented.

FIG. 4 is a block diagram of a main part of a scanning electron microscope according to a second embodiment of the present invention. The same reference numerals as those described above denote the same or equivalent parts.

In the first embodiment, the secondary electrons 24
Is taken out of the electron path by the suction electrode 13 and detected. In this embodiment, however, the channel plate detector 2 is used.
6 in that secondary electrons are detected.

In the figure, a disc-shaped channel plate main body 25 having a center hole 33 is provided between the objective lens 17 and the lower scanning coil 12. The diameter of the central hole 33 is set so that the electron beam 20b deflected by the scanning coil 12 does not collide. A mesh 34 is provided below the channel plate main body 25.

In such a configuration, the accelerated electron beam 20b passes through the central hole 33 of the channel plate, is converged by the objective lens 17, and is irradiated onto the sample 18. The secondary electrons 24 generated in the sample 18 are
The light passes through the mesh 34 placed on the entire surface while diverging, and enters the channel plate 25. The secondary electrons 24 incident on the channel plate 25 are amplified by the amplified voltage 2 applied to both ends of the channel plate 25.
It is accelerated and amplified at 8. The amplified electrons 27 are further accelerated by the anode voltage 29 and are captured by the anode 37.

After the captured secondary electrons are amplified by the amplifier 30, they are converted into light 32 by the light conversion circuit 31. Light 32
This is because the amplifier 30 is floating with the amplified voltage 28 of the channel plate 25 and the like. The light 32 is converted again into an electric signal by the electric conversion circuit 35 of the ground potential, and is used as a luminance modulation signal of the scanned image. In this method, not only secondary electrons but also reflected electrons can be detected. As is evident, the present embodiment also achieves the same effects as described above.

FIG. 5 is a block diagram of a main part of a scanning electron microscope according to a third embodiment of the present invention. The same reference numerals as those described above denote the same or equivalent parts. The present embodiment is characterized in that a desired secondary signal can be selectively detected.

In the figure, a filter voltage 36 capable of arbitrarily controlling the potential is applied to the channel plate 25. For example, if the filter voltage 36 is set to a negative voltage of about 10 volts more than the superimposed voltage 6, of the secondary electrons and reflected electrons emitted from the sample, the secondary electrons will be between the channel plate 25 and the mesh 33. It is possible to selectively detect only the backscattered electrons with high energy which are turned back by the reverse electric field created in the above.

It is also possible to know the potential of the sample by measuring the limit filter voltage 36 that repels secondary electrons. By adding such a function, it is possible to perform a function test on a completed semiconductor device. Will be able to

FIG. 6 is a block diagram of a scanning electron microscope according to a fourth embodiment of the present invention. The same reference numerals as those described above denote the same or equivalent parts.

In each of the above-described four embodiments, the secondary signal is deflected by using the electric field and detected by the detector.
The present embodiment is characterized in that a secondary signal is deflected using a magnetic field and an electric field.

When the acceleration voltage increases and the deceleration ratio of the electron beam in the final stage due to the deceleration electric field decreases, the energy difference between the primary electron beam 20b irradiated on the sample and the secondary electrons 24 emitted from the sample decreases. So the secondary electrons 24
When a relatively large electric field E is generated by the suction electrode 13 in order to attract the primary electrons 20b by the electric field E,
Is also bent.

The present embodiment is made to solve such a problem, and focuses on the fact that the deflection direction of the electron beam due to the magnetic field differs depending on the traveling direction of the electron beam. And a magnetic field B that complements the amount of deflection of the secondary electrons 24 is generated.

That is, in this embodiment, the primary electron beam 20
The magnetic field B is generated so that b is deflected in the direction opposite to the direction of deflection by the electric field E generated by the suction electrode 13. Accordingly, by appropriately controlling the intensity of the magnetic field B, the deflection of the electron beam 20b due to the electric field E is canceled.

On the other hand, with respect to the secondary electrons 24, the direction of deflection by the magnetic field B and the direction of deflection by the electric field E are the same, so that the amount of deflection of the secondary electrons is increased and the detection of the secondary electrons is easy. become.

FIG. 7 is a block diagram of a scanning electron microscope according to a fifth embodiment of the present invention. The same reference numerals as those described above denote the same or equivalent parts. The present embodiment is characterized in that a secondary signal is detected using a single crystal scintillator.

In the same figure, single crystal scintillator 55
For example, an opening 5 for obliquely cutting a cylindrical YAG single crystal and allowing the primary electrons 20b to pass through the cut surface.
7, the tip of which is coated with a conductive thin film 56 of metal, carbon, or the like, to which a ground potential is applied.

In the present embodiment, the crossover 58 of the primary electron beam 20 b formed by the condenser lens 12 is set near the opening 57, and the crossover 59 of the secondary electron 24 by the objective lens 17 is transmitted from the opening 57. Try to come to a remote location. By doing so, the primary electron beam 2
0b is not blocked by the opening 57, so that the secondary electrons 24 can be detected efficiently.

In the above-described fifth embodiment, both the light-emitting portion of the scintillator and the light guide are described as being composed of a YAG single crystal. However, only the light-emitting portion for detecting secondary electrons is formed of a YAG single crystal. The other parts may be made of a transparent member such as glass or resin.

FIG. 8 is a block diagram of a main part of a scanning electron microscope according to a sixth embodiment of the present invention, and the same reference numerals as those described above denote the same or equivalent parts.

In this embodiment, the anode constituting the electron gun is omitted, and the electron beam is accelerated by the extraction voltage 4 applied between the extraction electrode 2 and the cathode 1 which are grounded. The electrons accelerated by the extraction voltage 4 are converged by the objective lens 17 and travel toward the sample 18. The positive electrode side of the acceleration voltage 5 is connected to the sample 18. At this time, since the extraction voltage 4 is higher than the acceleration voltage 5 in the low acceleration voltage region, the electron beam is decelerated between the objective lens 17 and the sample 18 and reaches the sample. In a typical example using a field emission cathode, the extraction voltage is 3 kV and the acceleration voltage is 1 kV. According to such a configuration, since the anode and the superimposed voltage source are omitted, the configuration is simplified.

[0050]

As described in detail above, according to the present invention,
By applying a negative potential to the sample and generating a deceleration electric field between the sample and the objective lens, the electron beam applied to the sample is decelerated to reduce chromatic aberration and prevent charge-up. A control electrode is provided between the objective lens and the sample, and a potential difference between the sample or the sample stage and the control electrode is smaller than a potential difference between the sample or the sample stage and the objective lens. Since the negative voltage is applied, the electric field generated between the objective lens and the sample is alleviated by the control electrode, and damage to the element can be prevented.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a scanning electron microscope system to which the present invention is applied.

FIG. 2 is a configuration diagram of a scanning electron microscope unit of FIG.

FIG. 3 is a configuration diagram of a scanning electron microscope according to the first embodiment of the present invention.

FIG. 4 is a configuration diagram of a scanning electron microscope according to a second embodiment of the present invention.

FIG. 5 is a configuration diagram of a scanning electron microscope according to a third embodiment of the present invention.

FIG. 6 is a configuration diagram of a scanning electron microscope according to a fourth embodiment of the present invention.

FIG. 7 is a configuration diagram of a scanning electron microscope according to a fifth embodiment of the present invention.

FIG. 8 is a configuration diagram of a scanning electron microscope according to a sixth embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Cathode, 2 ... Extraction electrode, 3 ... Anode, 4 ... Extraction voltage, 5
... acceleration voltage, 6: superimposed voltage, 7: condenser lens, 8
... Aperture, 9 ... Insulation base, 10 ... Adjustment knob, 11 ... Upper scan coil, 12 ... Lower scan coil, 13 ... Suction electrode, 14 ...
Scintillator, 15: light guide, 16: photomultiplier,
17 ... objective lens, 18 ... sample, 19 ... sample stage,
Reference numeral 20: primary electron beam, 21: sample holder, 24: secondary electron, 25: channel plate, 28: amplification voltage, 29
... Anode voltage, 30 ... Amplifier, 31 ... Optical conversion circuit, 3
3: Central hole, 34: mesh, 35: electric conversion circuit, 3
6 filter voltage, 37 anode, 39 control electrode,
40: voltage controller, 55: single crystal scintillator, 56:
Conductive thin film, 61 ... Case, 77 ... Sample exchange mechanism

──────────────────────────────────────────────────続 き Continued on the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01J 37/28 H01J 37/20 H01J 37/244 H01J 37/248

Claims (6)

(57) [Claims] A scanning electron microscope that scans a primary electron beam emitted from an electron source on a sample and obtains a scanning image based on a secondary signal generated from the sample. Means for generating a deceleration electric field generated between the objective lens and a secondary signal detection means disposed closer to the electron source than the objective lens;
An output device, a control electrode disposed between the sample and the objective lens, and voltage applying means for applying a predetermined voltage to the control electrode , wherein the predetermined voltage is such that the potential of the control electrode is the potential of the objective lens. Potential and
A scanning electron microscope characterized in that the value is a predetermined value between the potential of the sample and the potential of the sample . 2. The method according to claim 1, wherein the primary electron beam emitted from the electron source is used as a sample.
Scan on the basis of the secondary signal generated from the sample
In a scanning electron microscope for obtaining a scanning image, the sample and / or a sample holder on which the sample is placed are negatively charged.
Means for applying a second voltage, and a secondary signal detector disposed closer to the electron source than the objective lens.
An output device, a control electrode disposed between the sample and the objective lens, and voltage applying means for applying a predetermined voltage to the control electrode.
And Bei, the predetermined voltage, the potential of the control electrode and the potential of the objective lens
Characterized in that the value is a predetermined value between the potential of the sample and
Scanning electron microscope. 3. The means for applying a negative voltage includes a sample holder for fixing a sample, a sample stage for mounting the sample holder via an insulating table, and a means for applying a negative voltage to the sample holder. Specially
3. The scanning electron microscope according to claim 2, wherein: 4. The secondary signal is detected by a scintillator provided outside the electron beam path, and between the scintillator and the electron beam path, a suction electrode supplied with a potential for suctioning the secondary signal is provided. claims 1, characterized in that the installed to scanning electron microscope according to any one of the three. 5. An aperture serving as an electron beam path , wherein the secondary signal is an aperture.
The scanning electron microscope according to any one of claims 1 to 3, wherein the light is detected by a detector having a detection surface having: Wherein said secondary signal is of claims 1, characterized in that it is detected by the single crystal scintillator 3
The scanning electron microscope according to any one of the above.