JP2000011936A - Electron beam optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electron beam optical system having high observation efficiency at low cost, while keeping a total optical path length and qualities related to observation such as an observation electron yield and resolution. SOLUTION: This system is provided with an irradiation means (an electron gun 1, illumination lenses 2, 3, a beam separator 4, an aperture restriction 5, a cathode lens 6) for irradiating an irradiating electron beam S on a sample surface 7, and an observation means (the cathode lens 6, the aperture restriction 5, the beam separator 4, an imaging lens front group 8, an imaging lens rear group 10) to image an observing electron beam K emitted from the sample surface 7 on an electron beam detection means 11, and is composed by forming a potential difference to accelerate the observing electron beam K between the sample surface 7 and an electrode 6a arranged at the nearest side of the observation means to the sample surface 7. In this case, a positive potential with respect to the ground potential is applied to the electrode 6a arranged at a side nearest the observation means to the sample surface 7.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron beam optical system for observing and inspecting a sample surface using an electron beam.

[0002]

2. Description of the Related Art Conventionally, an electron microscope using an electron beam has been widely used for observing and inspecting miniaturized and highly integrated semiconductor devices. In the electron microscope,
There is something called a low energy electron microscope (K.Tsun
o, Ultramicroscopy 55 (1994) 127-140 `` Simulation of a
Wien filter as beam separator in a low energy elec
tron microscope "). FIG. 3 briefly describes the low energy electron microscope. The electron beam (irradiation electron beam S) accelerated by the electron gun 1 to about 10 keV
After being shaped by the illumination lenses 2 and 3, the light enters the beam separator 4. The electron beam deflected by the beam separator 4 passes through the aperture stop 5 and then passes through the cathode lens 6 to irradiate the sample 7. Here, the cathode lens 6 is composed of three electrodes 6a, 6b, and 6c arranged in parallel with the optical axis direction, of which the central electrode 6b
Is applied to a negative potential, and the electrodes 6a and 6c at both ends are grounded electron lenses, so-called Einzel lenses. The potential of the central electrode 6b is a high potential of about -7 to -10 kV. On the other hand, the sample 7 is applied with a high voltage of about −10 kV, and forms an electric field between the cathode lens 6 and the first electrode 6 a disposed closest to the sample 7.
By this electric field, the irradiation electron beam S is decelerated to about 10 eV and enters the sample 7.

When a sample 7 is irradiated with an electron beam, secondary electrons, reflected electrons, backscattered electrons and the like are emitted from the sample. At least one of these electrons becomes the observation electron beam K. Here, the speed of the reflected electrons is 10 eV
It is about. Observation electron beam K emitted from sample 7
Is again accelerated to about 10 keV by an electric field formed between the sample 7 and the first electrode 6a of the cathode lens 6. Thereafter, the observation electron beam K passes through the other electrodes 6 b and 6 c of the cathode lens 6, further passes through the aperture stop 5, and enters the beam separator 4. Then, by satisfying the Wien condition, the observation electron beam K that has passed straight through the beam separator 4 passes through the imaging lenses 8 and 10, and then has an MCP (Micro Channel Platform).
The image is formed on the electron beam detector 11 as shown in e). here,
Similarly to the cathode lens 6, the illumination lenses 2, 3 and the imaging lenses 8, 10 are also Einzel lenses, and the central electrode has a high potential of about -5 to -10 kV.

[0004]

The above-mentioned conventional electron beam optical system has an advantage that chromatic aberration is reduced because the energy of an electron beam passing through an illumination lens, a cathode lens, and an imaging lens is high. Was. However, there were two major problems described below. One is that the cost of the electron beam optics is very high. That is, as described above, the illumination lens, the cathode lens, and the Einzel lens serving as the imaging lens all need to apply a high voltage, and therefore use a very expensive high-voltage power supply and a high-voltage type electrode. I was Here, when applying a relatively low pressure without applying a high pressure to each Einzel lens,
That is, when a low-cost power supply and electrodes are used, the focal length of each Einzel lens becomes longer, and the entire optical path length becomes longer accordingly. As described above, since the size of the electron beam optical system is increased, it is difficult to reduce the cost of the Einzel lens. Another problem is that it takes a long time to increase the pressure due to the application of a high pressure to the sample, that is, the observation efficiency is low. As described above, the sample is applied with a high pressure. However, if a high pressure is applied rapidly, the sample may be damaged and may be damaged. Therefore, the pressure of the sample is increased over time so as not to damage the sample.

[0005] These problems are even greater when secondary electrons are used as the observation electron beam than when reflected electrons are used as the observation electron beam. It is for the following reasons. Generally, reflected electrons are emitted from the sample in one direction, while secondary electrons are emitted isotropically from the sample. Therefore, in the case of secondary electrons, it is necessary to increase the amount of secondary electrons pulled from the sample to the cathode lens, that is, the so-called yield, in order to improve the observation accuracy (S / N). Therefore, the electric field formed between the sample and the first electrode of the cathode lens must be increased accordingly. As a result, the sample and the illumination lens, the cathode lens, and the imaging lens, which are Einzel lenses, are all subjected to high pressure, which has promoted the above two problems.

Here, in order to improve the accuracy of observation by secondary electrons, the inner diameter of the aperture stop is increased without increasing the electric field between the sample and the first electrode as described above. If you take the method of increasing the yield of secondary electrons,
In exchange for the above problem, another problem such as deterioration of aberration and reduction of resolution occurs. Therefore, the present invention
It is an object of the present invention to provide a low-cost, high-observation-efficiency electron beam optical system while maintaining the total optical path length and the quality related to observation such as observation electron yield and resolution.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problem. That is, when the reference numerals in the accompanying drawings are added in parentheses, the present invention provides an irradiation electron beam (S). ) On the sample surface (7);
An observation means for forming an electron beam for observation (K) emitted from the sample surface (7) on the electron beam detection means (11); a sample surface (7); and the most sample surface (7) of the observation means. In the electron beam optical system in which a potential difference for accelerating the observation electron beam (K) is provided between the electrode (6a) and the electrode (6a) arranged on the side, the electrode ( 6a) is an electron beam optical system characterized by applying a positive potential to a ground potential.

Further, according to the present invention, an irradiation electron beam (S) emitted from an irradiation beam source (1) is made incident on an optical path switching means (4) via an irradiation optical system (2, 3), and the light path switching means (4). ) Is passed through the objective optical system (6) to the sample surface (7) via the objective electron beam (S), and the observation electron beam (K) emitted from the sample surface (7) is passed through the objective optical system. The observation electron beam (K) is introduced into the optical path switching means (4) via (6), and guided by the optical path switching means (4) in a direction different from the direction reaching the irradiation source (1). The observation electron beam (K) after passing through (4) is incident on the electron beam detection means (11) via the imaging optical system (8, 10), and the sample surface (7) and the objective optical system ( Electrons having a potential difference for accelerating the observation electron beam (K) between the electrode (6a) disposed closest to the sample surface (7) in (6). In the optical system, an objective optical system (6)
An electron beam optical system characterized in that the electrode (6a) disposed closest to the sample surface (7) is applied with a positive potential with respect to the ground potential.

[0009]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 shows a first example of the electron beam optical system according to the present invention.
An example will be described. The irradiation electron beam S emitted from the electron gun 1 is shaped by the illumination lenses 2 and 3 and then enters the beam separator 4. Here, the irradiation electron beam S is accelerated to 5 keV. The illumination lenses 2 and 3 are Einzel lenses including three electrodes (not shown). The beam separator 4 is, for example, a Wien filter. Beam separator 4
The electron beam S for irradiation deflected to form a crossover image at the position of the aperture stop 5, passes through the cathode lens 6, and illuminates the sample 7 with Koehler illumination.

Here, the cathode lens 6 is composed of three electrodes: a first electrode 6a of the cathode lens, a second electrode 6b of the cathode lens, and a third electrode 6c of the cathode lens. The cathode electrode first electrode 6a is applied with +16 kV by the first electrode power supply 15a. The cathode lens second electrode 6b is connected to the second electrode power supply 15b by a negative voltage.
It is applied to 1.3 kV. The cathode lens third electrode 6c is grounded. As described above, the cathode lens 6 has a different configuration from the Einzel lens in which both electrodes are grounded. The sample 7 is applied with -4 kV by the sample power supply 14. Thus, an electric field (hereinafter, referred to as a first electric field) is formed between the sample 7 and the cathode lens first electrode 6a. This first
The irradiation electron beam S is decelerated to 1 keV by the electric field and is incident on the sample 7. Here, the electron gun 1, the illumination lenses 2, 3, the beam separator 4, the aperture stop 5, and the cathode lens 6 are irradiation means.

From the sample 7, secondary electrons having an energy of about 1 to 2 eV are emitted, and this is emitted by the observation electron beam K.
Used as The observation electron beam K is pulled up by the first electric field, and the cathode electrode first electrode 6
When passing through a, it is accelerated to about 20 keV. Observation electron beam K that has passed cathode electrode first electrode 6a
Are the cathode lens second electrode 6b and the cathode lens third electrode
After passing through the electrode 6c, the speed is reduced to 4 keV. The observation electron beam K that has passed through the cathode lens 6 passes through the aperture stop 5 and enters the beam separator 4. By satisfying the Wien condition, the observation electron beam K that has passed straight through the beam separator 4 passes through the front group 8 of the imaging lens, and then forms an intermediate image of the sample 7 once at the intermediate image forming position 9. Here, the imaging lens front group 8 is an Einzel lens, and the imaging lens front group first electrode 8a, the imaging lens front group second electrode 8b, and the imaging lens front group third electrode 8c.
And three electrodes. The first electrode 8a of the imaging lens front group and the third electrode of the imaging lens front group,
The electrode 8c is grounded, and the central electrode, ie, the second electrode 8b of the front lens group of the imaging lens, is applied with -1.8 kV.

Further, the observation electron beam K passing through the intermediate image forming position 9 passes through the rear group 10 of the imaging lens, and then forms an enlarged projection image of the sample 7 on the electron beam detector 11. Here, a cathode lens 6, an aperture stop 5, a beam separator 4, an imaging lens front group 8, and an imaging lens rear group 10
Are the observation means. Here, the imaging lens rear group 1
Reference numeral 0 denotes an Einzel lens, similar to the front lens group 8, and three first electrodes 10a, rear lens groups 10b, and third electrodes 10c. Electrodes. Then, the imaging lens rear group first electrode 10a and the imaging lens rear group third electrode 10c are grounded,
The voltage of -3.6 kV is applied to the second electrode 10b of the rear group of the imaging lens. As described above, in the first embodiment, since the applied potential of the sample 7 can be made relatively low, the time required for boosting the voltage can be shortened, and the observation efficiency can be improved. In addition to the power supply for the sample 14, the power supply for the illumination lenses 2 and 3, the power supply 17 for the front lens unit of the imaging lens, and the electric potential supplied from the power supply 18 for the rear unit of the imaging lens can be made relatively low. Can be. In addition, since the cathode lens first electrode 6a is applied with a positive potential, the cathode lens first electrode 6a is configured such that impurities as positive ions hardly adhere thereto.

In the first embodiment, an intermediate image is once formed by an optical system composed of the cathode lens 6 and the front lens unit 8 of the imaging lens, that is, a so-called double-sided telecentric optical system, and the intermediate image is formed after the imaging lens. The group 10 projects the image on the electron beam detector 11 in an enlarged manner.
Can be removed and the electron beam detector 11 can be arranged at the intermediate imaging position 9. Further, in the first embodiment, the case has been described where the double-sided telecentric optical system is configured by the cathode lens 6 and the imaging lens front group 8, but the double-sided telecentric optical system need not be configured. Also,
In the first embodiment, secondary electrons are used as the observation electron beam K, but reflected electrons or backscattered electrons may be used instead. At this time, the voltage applied to each lens may be changed.

Further, instead of the irradiation electron beam S and the observation electron beam K used in the first embodiment, an ion beam having a positive charge can be used. At that time, the cathode lens first electrode 6a is applied negatively with respect to the ground potential. In the first embodiment, an electrostatic electron lens composed of three electrodes is used as the cathode lens 6, but it is sufficient that the first electric field can be formed. Therefore, the cathode lens second electrode 6b and the cathode lens 3 electrode 6c
May be removed to form a structure including the cathode lens first electrode 6a and the magnetic field type lens.

Next, Table 1 shows simulation results of resolution and magnification by the electron beam optical system of the first embodiment. However, for the sake of simplicity, the simulation was performed with the resolution and magnification at the intermediate imaging position 9 by the cathode lens 6 and the imaging lens front group 8. The actual simulation is 2
This is based on the following finite element method. That is, the trajectories of the electron beam were obtained by obtaining the electrostatic fields in the cathode lens 6 and the imaging lens front group 8, respectively, and numerically solving the equations of motion in the electrostatic fields. Then, in order to confirm the effect of the first embodiment, a comparison was made with a conventional technique using an Einzel lens for the cathode lens 6.
With respect to the voltage applied to each lens in the prior art, the potential required to obtain the same resolution as in the first embodiment was estimated.

[0016]

[Table 1]

The above simulation was performed under the following conditions. Observation field: 200 μm × 200 μm Secondary electron yield: 1% of secondary electrons generated from the sample Secondary electron chromatic dispersion: 1 eV Overall length: 400 mm Furthermore, since chromatic aberration is mainly determined by the intensity of the first electric field, the first
The intensity of the first electric field in the example and the prior art was the same. Here, the secondary electron chromatic dispersion indicates the energy dispersion of the secondary electrons that are the observation electron beam K. Since electron beams having different energies pass through different trajectories, aberration occurs. This aberration is generally regarded as chromatic aberration in the electron beam optical system. The total length indicates the distance between the sample 7 and the intermediate imaging position 9.

The following effects can be confirmed from the simulation results of resolution and magnification shown in Table 1. First, in a case where the cathode lens 6 obtains the same secondary electron yield and achieves the same resolution, the conventional technique requires the sample 7-
While application of 20 kV is required, application of -4 kV is possible in the first embodiment. That is, the sample power supply 1
4 can reduce the boosting time. Second, the voltage applied to the second electrode 8b of the front lens unit of the imaging lens can be significantly reduced. That is, the cost of the front lens group power supply 17 can be reduced. On the other hand, since the potential of the cathode lens first electrode 6a according to the first embodiment is substantially equal to the potential of the cathode lens second electrode 6b according to the prior art, the cathode lens power supply 15 according to the first embodiment includes The cost can be made equivalent to that of the lens power supply 15. Here, the cathode lens second electrode 6b in the first embodiment has-
Although 1.3 kV needs to be applied, it is not necessary to add a power supply which does not exist in the related art. That is, the potential of the cathode lens second electrode 6b is low and can be supplied from the cathode lens power supply 15 common to the cathode lens first electrode 6a via a resistor or the like. It is almost equivalent to the prior art. Thus, the cost of the entire apparatus can be reduced.

Third, the magnification at the intermediate image position 9 can be made larger in the first embodiment than in the prior art. This is because, in the first embodiment, the energy of the electron beam is lower than that of the prior art, and the refractive index after emitting the cathode lens 6 is lower. As a result, when a two-sided telecentric optical system is configured by the cathode lens 6 and the imaging lens front group 8, the apparent rear focal length is reduced by that amount, and the magnification at the intermediate imaging position 9 is improved. Incidentally, the refractive index of the electron beam optical system can be generally estimated by the square root of the electron energy. Although the above simulation was performed at the intermediate imaging position 9 by the cathode lens 6 and the front lens group 8, the same result can be obtained by including the rear lens group 10 in this. . That is, the voltage applied to the rear group 10 of the imaging lens can be reduced without deteriorating the secondary electron yield and resolution and without increasing the distance between the sample 7 and the electron beam detector 11. Similar results can be obtained for the illumination lenses 2 and 3.

FIG. 2 shows a second embodiment of the electron beam optical system according to the present invention. The second embodiment has a configuration in which the beam separator 4 in the first embodiment is removed. In the second embodiment, after the irradiation electron beam S emitted from the electron gun 1 passes through the illumination lenses 2 and 3,
The light is directly incident on the sample 7. Thereafter, the process until the observation electron beam K emitted from the sample 7 reaches the electron beam detector 11 is the same as that of the first embodiment except that the observation electron beam K does not pass through the beam separator 4. Here, the electron gun 1 and the illumination lenses 2 and 3 are irradiation means. The cathode lens 6, the aperture stop 5, the front group 8 of the imaging lens, and the rear group 10 of the imaging lens constitute an observation unit. Also in the second embodiment, similarly to the first embodiment, since the applied potential of the sample 7 can be made relatively low, the time required for boosting the voltage can be shortened, and the observation efficiency can be improved. In addition to the power supply for the sample 14, the power supply for the illumination lenses 2 and 3, the power supply 17 for the front lens unit of the imaging lens, and the electric potential supplied from the power supply 18 for the rear unit of the imaging lens can be made relatively low. Can be.

[0021]

As described above, according to the present invention, it is possible to improve the observation magnification while maintaining the total optical path length, the observation electron yield and the resolution, and realize a low-cost and high observation efficiency electron beam optical system. Can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing an electron beam optical system according to a first embodiment of the present invention.

FIG. 2 is a view showing an electron beam optical system according to a second embodiment of the present invention.

FIG. 3 is a diagram showing a conventional electron beam optical system.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Electron gun 2, 3 ... Illumination lens 4 ... Beam separator 5 ... Aperture stop 6 ... Cathode lens 6a ... Cathode lens first electrode 6b ... Cathode lens second electrode 6c ... Cathode lens third electrode 7 ... Sample 8 ... Image formation Lens front group 8a: imaging lens front group first electrode 8b: imaging lens front group second electrode 8c: imaging lens front group third electrode 9: intermediate imaging position 10: imaging lens rear group 10a: imaging Lens rear group first electrode 10b ... imaging lens rear group second electrode 10c ... imaging lens rear group third electrode 11 ... electron beam detector 14 ... sample power supply 15 ... cathode lens power supply 15a ... first electrode power supply 15b ... Two-electrode power supply 17: Power supply for front group of imaging lens 18: Power supply for rear group of imaging lens S: Electron beam for irradiation K: Electron beam for observation

Claims (2)

[Claims] An irradiation means for irradiating the sample surface with an electron beam for irradiation; and an observation means for forming an image of the observation electron beam emitted from the sample surface on an electron beam detection means. In an electron beam optical system provided with a potential difference for accelerating the observation electron beam between the electrode disposed on the most sample side of the observation means, the electrode disposed on the most sample side of the observation means. An electron beam optical system, wherein the electron beam optical system is applied to a positive potential with respect to a ground potential. 2. An irradiation electron beam emitted from an irradiation source is made incident on an optical path switching means via an irradiation optical system, and the irradiation electron beam passing through the optical path switching means is projected onto a sample surface via an objective optical system. Incident, the observation electron beam emitted from the sample surface is incident on the optical path switching means via the objective optical system,
The observation electron beam is guided by the light path switching means in a direction different from the direction reaching the irradiation beam source, and the observation electron beam after passing through the light path switching means is detected by an electron beam through an imaging optical system. An electron beam optical system provided with a potential difference for accelerating the observation electron beam between the sample surface and an electrode disposed closest to the sample surface of the objective optical system. An electron beam optical system, wherein an electrode disposed closest to a sample surface of the system is applied with a positive potential with respect to a ground potential.