JP2001243904A - Scanning electron microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a scanning electron microscope which produces scanning image with high space resolution at a low-acceleration voltage area. SOLUTION: A top magnetic pole of an objective lens is electrically insulated against the part of the lens other than that for the top magnetic lens, and a scanning electron microscope with positive voltage applied accelerating a primary electron beam post-deflection is provided.

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 two-dimensional scanning image representing the shape or composition of the surface of a sample by scanning the surface of the sample to be inspected with an electron beam. The present invention relates to a scanning electron microscope suitable for obtaining a high-resolution scanning image.

[0002]

2. Description of the Related Art In a scanning electron microscope, electrons emitted from a heating type or field emission type electron source are accelerated and converted into a thin electron beam (primary electron beam) by an electrostatic or magnetic lens. Using a scanning deflector to scan over the sample to be observed, detect secondary signals such as secondary electrons or reflected electrons secondary from the sample by irradiating the primary electron beam, and scan the detected signal intensity with the primary electron beam A two-dimensional scanned image is obtained by using the luminance modulation input of the cathode ray tube being scanned in synchronization with the above. 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 an electron beam is scanned on a test sample at a ground potential.

As a result of the use of scanning electron microscopes in the process of semiconductor device fabrication or inspection after completion (for example, dimension measurement by an electron beam or inspection of electrical operation),
A high resolution of 10 nm or less has been required at a low accelerating voltage of 1000 V or less for observing an insulator without charging.

[0004]

A factor that hinders high resolution in a low acceleration voltage region is blurring of an electron beam due to chromatic aberration caused by a variation in energy of an electron beam emitted from an electron source. In a scanning electron microscope with a low accelerating voltage, to reduce the blur due to this chromatic aberration,
A field emission type electron source having a small variation in energy of an emitted electron beam is mainly used. However, even with a field emission type electron source, the spatial resolution at 500 volts is limited to 10 to 15 nm, and cannot satisfy the user's requirements.

As a solution to this problem, the acceleration of the primary electron beam between the electron source and the anode at the ground potential is set to a voltage value higher than the final acceleration voltage, and the negative potential and the objective lens at the ground potential are applied. There is a method to set the final low acceleration voltage by decelerating the primary electrons between the inspected specimens (see: I Triple E, 9th Annual Symposium on Electron Ion and Laser
Technology Proceedings, 176-186, IE
EE 9th Annual Symposium on Electron, Ionand Laser
Technology).

Although the effect of this method has already been confirmed by experiments, it is difficult to detect secondary electrons being drawn into the mirror by the deceleration electric field because a high voltage is applied to the sample. Because of the need for a highly sensitive sample stage, there have been few examples employed in commercially available devices.

[0007]

SUMMARY OF THE INVENTION The present invention provides a breakthrough for the aforementioned problems. In the present invention, an electron source, a scanning deflector for scanning a primary electron beam generated from the electron source on a sample, an objective lens for converging the primary electron beam, and an electron generated from the sample by irradiation of the primary electron beam. In the scanning electron microscope having a detector for detecting, the upper magnetic pole of the objective lens is electrically insulated from the rest of the objective lens other than the upper magnetic pole, and a positive voltage is applied. A scanning electron microscope.

As described above, the upper magnetic pole of the objective lens is electrically insulated from the rest of the objective lens, and the subsequent stage acceleration voltage is applied thereto, so that the acceleration voltage of the primary electron beam when passing through the objective lens is finally reduced. It can be higher than the accelerating voltage, and because the upper magnetic pole also serves as the accelerating cylinder,
The center of the magnetic lens and the center of the electrostatic lens can be easily realized.

[0009]

FIG. 1 is a schematic view of an embodiment of a scanning electron microscope according to the present invention. When an extraction voltage 3 is applied between the field emission cathode 1 and the extraction electrode 2, emitted electrons 4 are emitted. The emitted electrons 4 are further accelerated (or decelerated) between the extraction electrode 2 and the anode 5 at the ground potential. The energy (acceleration voltage) of the electron beam passing through the anode 5 coincides with the electron gun acceleration voltage 6. In the present invention, the primary electron beam 7 having passed through the anode 5 is further accelerated by an acceleration cylinder 9 provided through the objective lens 8.
The energy of the electron beam when passing through the objective lens 8 is the sum of the electron gun acceleration voltage 6 and the post-stage acceleration voltage 10 applied to the acceleration cylinder 9. The post-accelerated primary electron beam 11 is decelerated by the negative superimposed voltage 13 applied to the sample 12 to a desired acceleration voltage. The actual acceleration voltage in this method is a difference between the electron gun acceleration voltage 6 and the superimposed voltage 13 irrespective of the post-stage acceleration voltage 10.

The primary electron beam 7 passing through the anode 5 is supplied to a condenser lens 14, an upper scanning deflector 15, and a lower scanning deflector 1
After being subjected to the scanning deflection in 6, the accelerating cylinder 9 provided in the path of the objective lens 8 is further accelerated by the post-stage accelerating voltage 10. The primary electron beam 11 accelerated in the latter stage is narrowed down on the sample 12 by the objective lens 8. The primary electron beam 11 that has passed through the objective lens 8 is decelerated by the deceleration electric field 17 created between the objective lens 8 and the sample 12, and reaches the sample 12.

According to this configuration, the acceleration voltage of the primary electrons when passing through the objective lens 8 is higher than the final acceleration voltage. As a result, compared with the case where the primary electron beam of the final acceleration voltage is passed through the objective lens 8, the chromatic aberration at the objective lens is reduced, and a finer electron beam (high resolution) can be obtained. The divergence angle of the primary electron beam of the objective lens 8 is determined by a stop 18 placed below the condenser lens 14. Centering of the aperture 18 is performed by the adjustment knob 19. Although the mechanical adjustment is performed in the figure, an electrostatic or magnetic field deflector may be provided before and after the stop 18 to deflect the electron beam for adjustment.

The electron beam narrowed down by the objective lens 8 is scanned on the sample 12 by the upper scanning deflector 15 and the lower scanning deflector 16. At this time, the deflection of the upper scanning deflector 15 and the lower scanning deflector 16 is performed. The direction and intensity are adjusted so that the scanned electron beam always passes through the center of the objective lens 8. Sample 1
2 is fixed on the sample holder 20 to which the superimposed voltage 13 is applied. The sample holder 20 is placed on a sample stage 22 via an insulating table 21 so that the horizontal position can be adjusted.

When the primary electron beam 11 irradiates the sample 12, secondary electrons 23 are generated. Objective lens 8 and sample 1
Since the deceleration electric field 17 formed between the two works as an accelerating electric field for the secondary electrons 23, the deceleration electric field 17 is attracted into the passage of the objective lens 8 and rises while receiving a lens action by the magnetic field of the objective lens 8. The secondary electrons 23 that have passed through the objective lens 8 are attracted by the transverse electric field of the suction electrode 24 placed between the objective lens 8 and the lower scanning deflector 16, and after passing through the mesh of the suction electrode 24, 10 kV ( (Positive potential) is accelerated by the scintillator 25 to which the scintillator 25 is applied.
The emitted light is guided to a photomultiplier tube 27 by a light guide 26 and is converted into an electric signal. The output of the photomultiplier tube 27 is further amplified and becomes the luminance modulation input of a cathode ray tube (not shown).

The feature of this configuration is that the condenser lens 1
4. The accelerating voltage of the electron beam when passing through the aperture 18 and the objective lens 8 is higher than the final energy. In particular, when passing through the objective lens 8 which controls chromatic aberration, further post-stage acceleration is applied. It is. In a typical example, an electron gun acceleration voltage: 1000 volts, a post-stage acceleration voltage: 1000 volts, a negative superimposed voltage on the sample 12: 50
At 0 volts, the actual acceleration voltage is 500 volts. When passing through the objective lens 8, the chromatic aberration is reduced to about 50% because the voltage is 2000 volts. When the acceleration voltage is set to 500 volts, the beam diameter (resolution) is reduced from 15 nm to 7 nm. .

In the above-described embodiment, the secondary electrons 23 are taken out of the electron path by the suction electrode 24 and detected. In this method, as the superimposed voltage 13 increases, the energy of the secondary electrons 23 increases. Therefore, it is necessary to increase the voltage applied to the suction electrode 24 accordingly. As a result, there is a problem that the primary electron beam (the primary electron beam 7 that has passed through the anode 5) is also deflected.

The embodiment using the reflector shown in FIG. 2 solves the above-mentioned problem and enables highly efficient detection. In this embodiment, a reflection plate 29 having a central hole 28 is provided in the electron beam path.

The surface of the reflection plate is coated with a material such as gold, silver, and platinum, which easily generates secondary electrons by electron irradiation. The primary electron beam 7 that has passed through the anode 5 passes through the central hole 28 of the reflector 29 and then enters the accelerating cylinder 9.
The diameter of the central hole 28 is set so that the electron beam deflected by the scanning deflectors 15 and 16 does not collide with the reflecting plate 29. The secondary electrons 23 generated in the sample 12 and accelerated by the superimposed voltage 13 pass through the acceleration cylinder 9 while diverging due to the lens action of the objective lens 8, and collide with the back surface of the reflection plate 29.

Although the trajectory is different from that of the secondary electrons, the reflected electrons generated in the sample 12 also collide with the back surface of the reflecting plate 29.

Secondary electrons 30 formed on the back surface of the reflection plate 29
Is attracted by the electric field of the attracting electrode 24, and is converted into an electric signal via the scintillator 25, the light guide 26, and the photoelectron amplifier 27 as in FIG. The feature of this method is that even if the superimposed voltage 13 applied to the sample is high and the acceleration of the secondary electrons 23 is high, the only thing that is detected is that the reflector 2 is not accelerated.
Since the secondary electrons 30 are made by the step 9, the voltage applied to the suction electrode 24 may be low. Therefore, the influence of the electric field generated by the suction electrode 24 on the primary electron beam 7 passing through the anode 5 can be reduced. Here, the scintillator 25 is used to detect the attracted secondary electrons, but an electron detection amplifier such as a channel plate or a multi-channel plate may be used.

FIG. 3 shows the secondary electrons 3 formed by the reflector 29.
This is an example in which a magnetic field B is applied orthogonally to an electric field E that attracts 0. With this structure, the deflection of the primary electron beam due to the attraction electric field E described above can be corrected. That is,
The deflection of the primary electron beam 7 passing through the anode 5 is corrected by the deflection by the magnetic field B. Here, 31 'and 31 "are electric field deflection electrodes for generating an attractive electric field, and 31" is meshed so that the secondary electrons 30 can pass therethrough. 32 'and 32 "are orthogonal magnetic field deflection coils (coils 3 for generating a magnetic field B).
2 ', 32 "are indicated symbolically in the figure. The magnetic field B generated by the orthogonal magnetic field deflection coils 32', 32" is orthogonal to the electric field E, and the intensity of the magnetic field B is increased by the accelerated electron beam. 7 is adjusted so as to cancel the deflection due to the electric field E received. In this embodiment, the orthogonal magnetic field deflection coil 32
However, if two orthogonal magnetic field deflection coils are arranged at an angle, it is possible to strictly adjust the degree of orthogonality with the electric field by adjusting the current flowing through each set of coils. it can. Needless to say, even if the direction of the electric field is adjusted by using two sets of electric field deflection electrodes instead of two sets of the orthogonal magnetic field deflection coils 32, the degree of orthogonality between the electric field and the magnetic field can be strictly adjusted.

The secondary signal detection method using the reflector 29 shown in FIGS. 2 and 3 works effectively even when the accelerating cylinder 9 is not provided or when the accelerating cylinder 9 is grounded.

FIG. 4 shows the secondary signal detector as the upper scanning deflector 1
5 shows an embodiment provided above. In the figure, a secondary signal detector is provided between the upper scanning deflector 15 and the diaphragm 18.
As in FIG. 2, a central hole 28 is provided in the reflecting plate 29. However, since the primary electron beam has not yet been scanned and deflected, the size of the central hole 28 limits the aperture angle of the minimum primary electron beam. The diameter may be the same as the diameter of the stop 18 to be stopped. In the embodiment shown, a reflector 29 having a central hole 28 having a diameter of 0.1 mm is provided below the diaphragm 18. The diaphragm 18 and the reflection plate 29 can be shared.

When the reflection plate 29 is provided below the scanning deflector, the diameter of the central hole 28 is set to a size that does not cause the deflected electron beam to collide. Comparing the size of the central hole 28 with a typical example, when installed below, 3
A size of up to 4 mm is required, but it may be 0.1 mm or less when installed above. As described above, when the reflector is provided above the scanning deflector, the central hole of the reflector can be made sufficiently small, so that the efficiency of capturing the secondary electrons by the reflector is improved.

In the embodiment of FIG. 4, the sample 12 is located in the pole gap of the objective lens 8. This arrangement reduces the chromatic aberration coefficient of the objective lens 8 and has a shape pursuing higher resolution. The sample stage 22 is also provided in the objective lens 8.

FIG. 5 shows an embodiment in which secondary signal detectors are provided at both positions above and below the scanning deflector. An upper detector 33 is provided on the upper scanning deflector 15, and a lower detector 34 is provided between the lower scanning deflector 16 and the acceleration cylinder 9. Upper detector 3
As shown in FIGS. 3 and 4, the lower detector 3 and the lower detector 34 respectively include reflectors 29a and 29b and electric field deflection electrodes 31a and 31a.
1b, orthogonal magnetic field deflection coils 32a and 32b, scintillators 25a and 25b, light guides 26a and 26b, and photomultiplier tubes 27a and 27b.

In this embodiment, the reflection plate 2 of the lower detector 34
Secondary electrons or reflected electrons that have passed through the central hole 28b of 9b can be detected by the upper detector 33. Upper detector 33
The secondary signal detected by the detector 3 contains a large amount of secondary electrons and reflected electrons emitted from the sample 12 in the vertical direction.
An image having a contrast different from that of No. 4 is obtained. For example,
In the inspection of the contact hole in the semiconductor device manufacturing process, when the lower detector 34 is used, an image in which the contact hole portion is emphasized from the surroundings is obtained, and the upper detector 33
By using, a fine image of the bottom of the contact hole can be obtained. Further, by calculating the signals of the two detectors 33 and 34, it is also possible to create a contrast that emphasizes the characteristics of the sample.

Whether the scanned image is formed by the output of the upper detector or the lower detector can be selected by the operator, but may be automatically selected under predetermined conditions. For example, when the observation magnification is 2000 times or less, the lower detector 34 is selected,
For higher magnifications, the upper detector 33 is selected. Also,
The selection may be made according to the sample to be observed. In this case, a procedure such as inputting the type of the sample to be observed to the apparatus is performed. For example, when the observation of the contact hole of the semiconductor element is input, the upper detector 33 that emphasizes the inside of the hole is automatically selected, and when the resist on the surface is observed, the lower detector 34 is selected.

In the embodiment shown in FIG. 4 or FIG. 5, even if the accelerating cylinder 9 is removed or the accelerating cylinder 9 is grounded, the effect is large and sufficiently practical.

FIG. 6 shows an embodiment in which a secondary signal is detected using a multi-channel plate detector. The multi-channel plate 35 has a disk shape and has a central hole 28 through which a primary electron beam passes. A mesh 37 is provided below the multi-channel plate 35 and is grounded. In such a configuration, the primary electron beam 7 that has passed through the anode 5 passes through the central hole 28 of the microchannel plate, and is then converged by the objective lens and irradiated onto the sample. The secondary electrons 23 generated in the sample pass through the mesh 37 and enter the channel plate 35. The secondary electrons 23 incident on the channel plate 35 are accelerated and amplified by an amplification voltage 38 applied to both ends of the channel plate 35. The amplified electrons 39 are further accelerated by the anode voltage 40 and captured by the anode 41. After the captured secondary electron signal is amplified by the amplifier 42, it is converted into an optical signal 44 by the optical conversion circuit 43. The conversion into the optical signal 44 is performed when the amplifier 42 is operated by the channel plate
This is because the floating voltage occurs at the amplified voltage 38 of No. 5. The light signal 44 is converted again into an electric signal by the electric conversion circuit 45 of the ground potential, and is used as a luminance modulation signal of the scanned image.

Here, the anode 41 is divided into two parts or four parts.
It is also possible to obtain information on the emission direction of the secondary electrons 23 as division. In this case, the amplifier 42, the optical conversion circuit 43,
Needless to say, the number of the electric conversion circuits 45 required is equivalent to the division, and the signal processing for calculating the divided signals is performed.

FIG. 7 shows an embodiment in which a secondary signal is detected using a single crystal scintillator. In FIG. 7, a single crystal scintillator 46 is, for example, a cylinder-shaped YAG single crystal that is cut obliquely, and an opening 47 for allowing a primary electron beam to pass through the cut surface is provided. Alternatively, a conductive thin film 48 of carbon or the like is coated, and the conductive thin film 48 is grounded. The light emitted by the secondary electrons 23 generated from the sample 12 irradiating the scintillator 46 is reflected at an oblique portion, guided to the photomultiplier tube 27 by a light guide constituted by a cylindrical portion, and detected and amplified. . In the present embodiment, both the light emitting portion and the light guide of the scintillator 46 are described as being composed of a YAG single crystal. However, only the light emitting portion for detecting secondary electrons is made of a YAG single crystal or a phosphor, and the light guide is formed. You may make it comprise a transparent body, such as glass and resin.

A control method for efficiently detecting a secondary signal will be described with reference to FIG. Secondary signal (for example, secondary electron) 2
3 passes through the magnetic field of the objective lens 8 and receives a lens action, so that a secondary electron crossover 49 is formed. if,
When the secondary electrons are focused on the opening 47 of the scintillator 46 by the lens action, most of the secondary electrons pass through the opening 47 and cannot be detected. Therefore, the focus is adjusted so as to focus on the front and rear of the reflector, thereby improving the detection efficiency. In the embodiment, the post-stage acceleration voltage and the superimposed voltage applied to the sample are controlled so that the focal position of the secondary electrons does not change when the acceleration voltage (substantial acceleration voltage) is changed.

The focal length of the magnetic lens is represented by a variable I · N / V1 /, where I is the current flowing through the lens coil, N is the number of turns of the coil, and V is the acceleration voltage of electrons passing through the lens magnetic field.
It is a function of 2. The acceleration voltage when the primary electrons pass through the lens magnetic field is (Vo + Vb) when Vo is the electron gun acceleration voltage and Vb is the post-stage acceleration voltage applied to the acceleration cylinder. Since the sample position (focal length) is constant, I ·
N / (Vo + Vb) 1/2 always takes a constant value (= a). The accelerating voltage when the secondary electrons pass through the lens magnetic field is (Vr + Vb) when the superimposed voltage applied to the sample is Vr.
And the variable IN / V1 / 2 is represented by the following equation.

I / N / V1 / 2 = a (Vo + Vb) 1/2 / (Vr + Vb) 1/2 = a ｛1+ (Vb / Vo)｝ 1/2 / ｛(Vr / Vo) + (Vb / Vo) )｝ 1/2 From this equation, if the Vr / Vo and Vb / Vo ratios are controlled to be constant, the focal position of the secondary electrons will be constant. That is, V
r / Vo, Vb / Vo ratios are kept constant, and post-stage acceleration voltage Vb
By controlling the superimposed voltage Vr of the sample, the focal position of the secondary electrons can be controlled to be constant without depending on the acceleration voltage (substantial acceleration voltage).

FIG. 8 shows an example in which a control electrode for controlling the electric field applied to the sample surface is added. Objective lens 8 and sample 1
2 is provided with a control electrode 36 to which a control voltage 50
Is applied. The control electrode 36 has a hole through which an electron beam passes. The control electrode 36 controls the electric field intensity applied to the surface of the sample 12 between the acceleration cylinder 9 and the sample 12. This configuration is effective when it is inconvenient if a strong electric field is applied to the sample. For example, there is a case where there is a problem of element damage due to a strong electric field of a wafer on which a semiconductor integrated circuit is formed.

This is effective for the problem of electrical non-contact with the sample holder 20 when the periphery of the wafer is covered with an oxide film. More specifically, when the side surface and the back surface of the sample (wafer) are covered with an insulator, electrical connection for lettering cannot be performed. The sample (wafer) 12 is in an electric field created between the sample holder 20 and the objective lens 8, and when there is no control electrode, the superimposed voltage 13 applied to the sample holder 20 and the potential of the objective lens 8 at the ground potential This is because normal observation cannot be performed because only an intermediate potential is applied.

Further, by setting the potential of the control electrode 36 to the same potential as the potential of the sample holder 20 to which the superimposed voltage 13 is applied or to a positive potential several tens of volts higher than that of the sample holder 20, damage to the element and the wafer Floating from a potential can be prevented. In this case, the control electrode 36 has a sufficient size to always cover the sample (wafer).

FIG. 9 is a diagram showing an example in which a control electrode is added.

A control electrode 60 having an opening 59 through which a primary electron beam passes is provided above the sample (wafer) 12, and the same voltage as the superimposed voltage 13 applied to the sample holder 20 is applied to the control electrode 60. When the control electrode 60 having the same potential as the sample holder 20 is placed on the sample (wafer) 12, the wafer is surrounded by a metal having the same potential, and the wafer has the same potential as the potential of the surrounding metal. Strictly speaking, the intrusion of the electric field from the opening 59 through which the primary electron beam 7 passed through the anode 5 causes an error with the potential of the metal. This error is roughly the ratio of the area of the sample (wafer) 12 to the area of the opening 59. For example, if the wafer is 8 inches and the diameter of the opening 59 is 10 mm, the area ratio is 1/400 and the potential error is 1%, which is a sufficiently small value.

With the above configuration, the same potential as the potential of the metal surrounding the wafer can be applied to the wafer.

Thus, even if the front and back surfaces of the wafer are covered with the insulating film and the electrical connection with the sample stage cannot be made, the voltage for lettering can be applied. It becomes possible.

In this embodiment, at least a portion of the sample holder 20 located below the sample (wafer) 12 is formed of a conductor for applying the superimposed voltage 13, so that the wafer is formed as described above. It will be surrounded by metals of the same potential. The sample holder itself may be a conductor, or a conductor may be inserted into the sample holder.

FIG. 10 shows another example in which a control electrode is added.
It is an example.

A control electrode 60 is provided between the sample (wafer) 12 and the objective lens 8, and the same voltage as the superimposed voltage 13 applied to the sample holder 20 is applied to the control electrode 60. As a result, the sample (wafer) 12 is surrounded by the sample holder 20 and the control electrode 60 to which the same voltage is applied. Even if the sample (wafer) 12 is covered with the insulating film as described above, 13 can be applied to the sample (wafer).

The opening 59 of the control electrode 60 is usually circular, but may be other than circular. The size of the opening 59 is set so as not to disturb the visual field to be observed. In this embodiment, the size of the opening 59 is 4 mm in diameter. Since the distance between the control electrode 60 and the sample (wafer) 12 is 1 mm, there is a visual field with a diameter of 4 mm. Further, since the deceleration electric field reaches the wafer through the opening 59, the secondary electrons can be efficiently lifted onto the objective lens 8. When the aperture diameter is reduced, the deceleration electric field does not reach the sample (wafer) 12, but the wafer is tilted,
If the sample has irregularities, such conditions are better.
The occurrence of astigmatism and the displacement of the visual field can be reduced.

A stage 22 is provided for observing an arbitrary place on the sample (wafer) 12. In this case, when a position largely deviated from the center point of the sample (wafer) 12 is to be observed, the sample (wafer) 12 needs to be largely moved. At this time, the sample (wafer)
When 12 comes off the control electrode 60, the sample (wafer)
12 changes, and it becomes impossible to apply a constant lettering voltage.

In order to cope with this situation, in this embodiment, the control electrodes are formed along the movement trajectory of the sample (wafer) 12. With this configuration, a constant lettering voltage can be applied even when the position of the sample (wafer) 12 is changed by the stage 22, and further, element destruction due to an electric field generated between the objective lens 8 and the sample (wafer) 12 can be prevented.

In this embodiment, it is desirable to dispose a control electrode having a size larger than the moving range of the wafer. Specifically, the diameter of the control electrode for observing the entire surface of the 8-inch wafer is 400 mm in diameter. No matter how the wafer is moved by such a configuration,
The voltage applied to the wafer can be kept constant.

In this embodiment, the control electrode is a plate-like electrode. However, the control electrode may be formed in a mesh shape, in which a large number of holes or slits are formed, to improve the evacuation performance. In this case, the hole diameter and the slit width are desirably smaller than the distance between the wafer and the control electrode.

In FIG. 10, the primary electrons 63c are
When the sample (wafer) 12 is irradiated with the light passing through the zero opening 59, secondary electrons 62 are generated. Secondary electrons generated 6
2 is inversely accelerated by the deceleration electric field with respect to the primary electrons 63c and guided above the objective lens 8. At this time, since the lens acts as a lens due to the magnetic field of the objective lens 8, it is guided onto the objective lens 8 while making a focal point as shown in the figure.

The guided secondary electrons 62 collide with the reflecting plate 29 and generate secondary electrons 30. The secondary electrons 30 are deflected by the electric field generated by the negative electrode deflecting electrode 31 'and the positive electrode deflecting electrode 31 "placed opposite to each other. The deflecting electrode 31" is made of a mesh. Therefore, the secondary electrons 30 pass through the mesh and are detected by the scintillator 25. Deflection coils 32 "and 32 'generate a magnetic field orthogonal to the electric field generated by the deflection electrodes 31' and 31" and cancel the deflection action of the primary electron beam 63b by the electric field generated by the deflection electrodes 31 'and 31 ". ing.

Although not shown, it is also possible to reduce contamination caused by scanning the sample with the primary electron beam 63c by cooling the control electrode 64.

FIG. 11 shows still another example in which a control electrode is added.

The components such as the field emission cathode 1, the extraction electrode 2, the anode 5, the condenser lens 14, the objective lens 8, the sample 12, the sample holder 20, the insulating base 21, and the sample stage 22 are housed in a vacuum housing 66. I have. The illustration of the vacuum exhaust system is omitted.

Here, when a negative superimposed voltage is applied to the sample 12, it is necessary to avoid a sample exchange operation by the sample exchange mechanism 67 and to keep the inside of the vacuum housing 66 at atmospheric pressure. In other words, the superimposed voltage 13 may be applied only when scanning the sample 12 with the electron beam.

Therefore, in this embodiment, the switch 68, which is a preparatory operation for mounting and replacing the sample, is closed and the acceleration voltage 6
Is applied, the second condition in which both the valve 69 and the valve 70 provided between the field emission cathode 1 and the sample 12 are open, and the sample exchange mechanism 67
71 that passes through the sample stage 22 for loading
Only when all of the third conditions that are closed are met
Control is performed such that the switch 72 is closed and the superimposed voltage 13 is applied to the sample 12.

The sample holder 20 and the sample stage 22
Are electrically connected via a discharge resistor 73, and when the switch 72 is opened, the charges charged in the sample 12 are transferred to the sample holder 20, the discharge resistor 73, and the sample stage 22.
(The sample stage 22 is grounded.) The discharge is quickly performed under a constant time constant, and the potential of the sample 12 is lowered. The discharge resistor may be built in the power supply of the superimposed voltage 13.

Under the condition that the vacuum around the field emission cathode 1 is equal to or lower than the set value, it is possible to apply an acceleration voltage from the anode 5 and, when the vacuum of the vacuum housing 66 is equal to or higher than the set value. It goes without saying that a sequence in which only the valves 69 and 70 are opened is incorporated.

In this embodiment, the superimposition voltage 13 is applied when all of the above three conditions are satisfied. However, one or two of these conditions are satisfied. The switch 72 may be closed when it is pressed.

FIG. 12 shows still another example in which a control electrode is added, which is applied to a scanning electron microscope having a sample stage 22 capable of tilting a sample. In this embodiment, the control electrode 76 is attached so as to cover the upper surface of the sample 75. Also, depending on the viewpoint, it can be said that they are arranged along the shape of the objective lens. The shape of the objective lens is formed so as not to hinder the movement of the sample 12. In the case of an apparatus having a tilting device as shown in FIG. 12, the objective lens has a sharp shape toward the sample 12. By forming the control electrodes along the objective lens formed under such conditions, it is possible to arrange the control electrodes without hindering the movement of the sample.

In this case, the sample (wafer) 12 is surrounded by the sample holder 20 and the control electrode 76 regardless of the position and the tilt angle of the sample (wafer) 12. According to this configuration, no electric field is generated on the surface of the sample (wafer) 12. 20a shows a state where the sample (wafer) 12 is inclined. Reference numeral 74 denotes a tilting mechanism incorporated in the sample stage 22. In this embodiment, the diameter of the opening 65 of the control electrode 76 may be smaller than the distance between the opening 65 and the sample 12 so that the electric field created between the control electrode 76 and the sample (wafer) 12 does not change when tilted. desirable. Note that the voltage applied to the control electrode 76 is
By setting the positive potential to several tens of volts higher than the voltage applied to, the detection efficiency of secondary electrons can be improved.

At this time, for the purpose of applying a voltage for lettering to the sample, a desired electric potential is considered in consideration of the effect of a composite electric field based on the voltage applied to the sample and the voltage applied to the control electrode. It is desirable to set the voltage applied to each of the sample and the control electrode so that is applied to the sample.

It is also possible to increase the aperture diameter and apply an electric field to the sample 12 for attracting secondary electrons. In this case, the observation position shifts due to the tilt, but the tilt angle and the amount of the shift are measured in advance to deflect the electron beam. Alternatively, this shift can be eliminated by performing a correction such as moving the sample stage 22 horizontally. The control electrode 76 in this embodiment is made of a non-magnetic material so as not to affect the characteristics of the objective lens 8.

In this embodiment, the control electrodes are arranged so as to cover the inside of the sample chamber, but it is not always necessary to arrange them in this way. That is, it is sufficient that the sample is formed at least along the moving range of the sample. Even with such a configuration, the sample is surrounded by the sample holder and the control electrode. Although the sample holder has been described as a conductor in the present invention, for example, the conductor may be arranged above or below the sample holder. In the case of the embodiment described above, by forming the conductor larger than the sample, the sample (wafer) is almost surrounded by the control electrode and the conductor, and it becomes possible to apply a constant lettering voltage. I have.

FIG. 13 shows still another example in which a control electrode is added. In this example, the control electrode is not grounded between the objective lens 8 and the sample 12, but is opposed to the sample 12 in the objective lens 8 including the excitation coil 78, the upper magnetic path 77, and the lower magnetic path 79. The lower magnetic path 79 at the position is electrically insulated from the upper magnetic path 77, and the superimposed voltage 13 is applied thereto. The potential applied to the lower magnetic path 79 can be made more positive than the sample 12 to efficiently guide secondary electrons onto the objective lens 8.

FIG. 14 is a diagram for explaining an electron beam scanning deflector combining an electric field and a magnetic field. When a secondary electron detector is provided on the scanning deflector, secondary electrons generated in the sample are deflected by the scanning deflector when passing through the scanning deflector. For this reason, the deflection of the secondary electrons also increases at a low magnification when the scanning deflection angle of the electron beam increases, and the electron beam may collide with the inner wall of the electron beam path and become undetectable. The present embodiment has solved this problem. The scanning deflector comprises eight-pole electrostatic deflectors 51a to 51h and magnetic deflectors 52a to 52d.

Now, considering the deflection in the x-axis direction,
A positive electric potential is applied to the electrodes 51h, 51a, and 51b and a negative electric potential is applied to 51d, 51e, and 51f in the eight-electrode electrostatic deflector to generate a deflection electric field Ex. Here, as shown in FIG. 14, a potential of a magnitude Vx is applied to the electrodes 51a and 51e, and the first electrode 51h, 51b, 51d and 51f on both sides thereof.
A potential of / 21/2 is applied. This is a well-known method for producing a uniform electric field. At the same time as the electric field, the current Ix is supplied to the coils 52a and 52c of the magnetic field deflector 52.
To create a magnetic field Bx in a direction orthogonal to the electric field Ex as shown. The electric field Ex and the magnetic field Bx cancel the deflection of the secondary electrons coming from below, and work to reinforce the primary electrons from above.

Deflection θ (S) for secondary electrons coming from below
Is the difference between the deflection θ (B) due to the magnetic field and the deflection θ (E) due to the electric field, as in the following equation.

Θ (S) = θ (B) −θ (E) = L / 8 · Ex / Vr− (e / 2m) 1 / 2BxL / Vr1
Here, L is the working distance of the electric and magnetic fields, e and m are the charge and mass of the electrons, respectively, and Vr is the acceleration voltage when the secondary electrons pass through the scanning deflector. If the ratio of Ex to Bx is given by the following equation, secondary electrons coming from below will not be deflected.

Bx / Ex = (2 m / e) 1/2/8 Vr1 / 2 On the other hand, regarding the deflection of the primary electrons, the electric field deflection is added to the magnetic field deflection to obtain the following equation. In the equation, Vo is an electron gun acceleration voltage.

Θ (o) = θ (B) + θ (E) = (e / 2m) 1/2 BxL / Vo1 / 2 + L / 8 · Ex / V
o1 / 2 Accordingly, the deflection angle θ (o) under the condition that the secondary electrons are not deflected is as follows.

Θ (o) = (e / 2m) 1/2 B × L ｛1+
(Vr / Vo) 1/2｝ / Vo1 / 2 Deflection in the x-axis direction has been described above. The deflection in the y-axis direction is performed in the same manner. That is, the potential of the electrode 51c is set to Vy, and the potentials of the electrodes 51b and 51d are set to Vy / 21 /
2, the potential of the electrode 51g is set to −Vy, and the electrodes 51f, 5
The potential of 1h is set to -Vy / 21/2 to generate a deflection electric field Ey in the y-axis direction. At the same time, the coil 52 of the magnetic field deflector
A current Iy is supplied to b and 52d to generate a magnetic field By orthogonal to the electric field Ey. As described above, the electric field Ey and the magnetic field By are sized to cancel the deflection of the secondary electrons coming from below and reinforce the primary electrons coming from above.

In practice, deflection is performed by combining deflection in the x-axis direction and deflection in the y-axis direction. Therefore, as shown in FIG. 14, the potential of each deflection electrode is the sum of the deflection potential in the x-axis direction and the deflection potential in the y-axis direction.
In an actual apparatus, this deflector has two stages, an upper scanning deflector and a lower scanning deflector, so that deflected primary electrons pass through the center of the objective lens. Then, by changing the potentials Vx, Vy of the deflecting electrodes and the deflecting coil currents Ix, Iy with time while maintaining the above relationship, a desired primary electron beam scanning on the sample is realized.

Next, the lens center of the magnetic field type objective lens,
The relationship between the center of the electrostatic lens formed between the accelerating cylinder and the sample will be described. FIG. 15A is a diagram illustrating a problem when the center CB of the magnetic field type objective lens 8 and the center CE of the electrostatic lens formed between the acceleration cylinder 9 and the sample 12 do not match. In this case, the primary electron beam 11 accelerated in the latter stage is deflected so as to pass through the center CB of the magnetic lens, but passes through with a distance d from the center CE of the electrostatic lens. When the shift amount d increases, spherical aberration is added to the lens function of the electrostatic lens, and the scanned image is distorted.

FIG. 15B shows an example in which the lens center CB of the magnetic field type objective lens 8 and the lens center CE of the electrostatic lens formed between the acceleration cylinder 9 and the sample 12 are matched. In the present embodiment, the upper magnetic pole 53 of the objective lens 8 is projected so as to face the sample 12, and the sample 12 on which the electrostatic lens is formed is formed.
The center of both lenses is matched by creating a magnetic field between the lens and the acceleration cylinder 9. As a result, since the primary electron beam 11 accelerated in the subsequent stage is not affected by the lens function of the electrostatic lens, a scanned image without distortion can be obtained.

FIG. 16 shows the structure of the objective lens 8 that more effectively realizes the coincidence of the centers of the magnetic lens and the electrostatic lens. In the embodiments described so far, the acceleration cylinder 9 is inserted inside the electron beam path of the objective lens 8. in this case,
If the axial centers of the electrostatic lens formed by the accelerating cylinder and the magnetic lens formed by the objective lens deviate from each other, the resolution is reduced. Therefore, it is necessary to precisely match the mechanical centers of the two. This embodiment focuses on this point, and the upper magnetic pole 53 of the objective lens 8 is used.
Is projected to the end level of the lower magnetic pole 54 to face the sample 12. Further, the upper magnetic pole 53 is electrically insulated from the rest of the objective lens by the insulating plate 55, and the
0 is applied.

According to the present embodiment, since the upper magnetic pole which determines the lens center of the objective lens 8 and the post-acceleration electrode are also used, the above-described displacement between the electrostatic lens and the magnetic field lens may occur. Absent. The upper magnetic pole 5 of the magnetic lens
Since the sample 3 directly faces the sample 12 and the post-acceleration voltage is applied to the sample 3, not only the axial center but also the positions of the lens centers of the electrostatic lens and the magnetic field lens can be matched.

[0078]

According to the present invention, the accelerating voltage of the primary electron beam when passing through the objective lens can be made higher than the final accelerating voltage, and the beam blur due to the aberration generated in the objective lens can be reduced. In addition, the positions of the centers of the electrostatic lens and the magnetic field lens can be matched.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of one embodiment of the present invention.

FIG. 2 is an explanatory diagram of an embodiment using a reflector for detecting a secondary signal.

FIG. 3 is an explanatory view of an embodiment in which a reflector is used for detecting a secondary signal, and the deflection of primary electrons is prevented by using an electric field and a magnetic field orthogonal to the attraction of secondary electrons.

FIG. 4 is a schematic diagram of an embodiment in which a secondary signal detector is placed above a scanning deflector.

FIG. 5 is a schematic diagram of an embodiment in which secondary signal detectors are provided at two locations above and below a scanning deflector.

FIG. 6 is an explanatory diagram of an embodiment using a channel plate for detecting a secondary signal.

FIG. 7 is an explanatory diagram of an embodiment using a scintillator for detecting a secondary signal.

FIG. 8 is a schematic diagram of an embodiment in which the intensity of an electric field applied to a sample is controlled.

FIG. 9 is a diagram showing an example of an embodiment in which a control electrode is provided on a sample.

FIG. 10 is a diagram showing an example of a charged particle microscope provided with a control electrode.

FIG. 11 is a diagram showing another example of a charged particle microscope provided with a control electrode.

FIG. 12 is a diagram showing an example of a charged particle microscope in which a control electrode is formed along the inner surface of a sample chamber.

FIG. 13 is a diagram showing an example in which a part of the objective lens magnetic path is used as a control electrode.

FIG. 14 is an explanatory diagram of an embodiment in which a scanning deflector is a combination of an electric field and a magnetic field.

FIG. 15 is an explanatory view of an embodiment in which the lens centers of a magnetic field lens and an electrostatic lens are matched.

FIG. 16 is an explanatory view of an embodiment in which the magnetic pole of the magnetic field lens and the acceleration cylinder are shared.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Field emission cathode, 2 ... Extraction electrode, 3 ... Extraction voltage, 4 ...
Emitted electron, 5 ... Anode, 6 ... Electron gun acceleration voltage, 7 ... Anode 5
, An objective lens, 9 an accelerating cylinder, 10 a post-acceleration voltage, 11 a post-accelerated primary electron beam, 12 a sample, 13 a superimposed voltage, 14 a condenser lens, 15 on Scanning deflector, 16: lower scanning deflector, 17: deceleration electric field, 18: stop, 19: adjustment knob,
Reference Signs List 20: sample holder, 21: insulating table, 22: sample stage, 23: secondary electron, 24: suction electrode, 25: scintillator, 26: light guide, 27: photomultiplier tube, 28
... Center hole, 29 ... Reflector, 30 ... Secondary electron made of reflector, 31 ... Electric field deflection electrode, 32 ... Orthogonal magnetic field deflection coil, 33 ... Top detector, 34 ... Bottom detector, 35 ... Channel plate Main body, 36: control electrode, 37: mesh, 3
9 Amplified electrons, 40 Anode voltage, 41 Anode, 42 Amplifier, 43 Optical conversion circuit, 44 Optical signal, 45 Electrical conversion circuit, 46 Single crystal scintillator,
47 ... opening, 48 ... conductive coating, 49 ... secondary electron crossover, 50 ... control voltage, 51a-51h ... electrostatic deflection electrode, 52a-52d ... magnetic field deflection coil, 53 ...
Upper magnetic pole, 54 ... Lower magnetic pole, 55 ... Insulating plate.

Claims (2)

[Claims] An electron source, a scanning deflector for scanning a primary electron beam generated from the electron source on a sample, an objective lens for converging the primary electron beam, and an electron generated from the sample by irradiation of the primary electron beam. Wherein the upper magnetic pole of the objective lens is electrically insulated from the rest of the objective lens other than the upper magnetic pole, and that a positive voltage is applied. Scanning electron microscope characterized by: 2. A scanning electron microscope according to claim 1, further comprising means for applying a negative voltage to said sample.