JP2010040461A - Scanning electron microscope - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a scanning electron microscope efficiently detecting secondary electrons even when varying a retarding voltage. <P>SOLUTION: This scanning electron microscope controlling the magnitude of an electric field includes an electron source, an objective lens to focus a primary electron beam generated from the electron source, a stage for placing a sample, a plurality of detectors to detect secondary electrons and reflected electrons generated from the sample by irradiation of the primary electron beam, and a power supply to impress a voltage on the stage. The electron microscope is also equipped with at least two conversion electrodes, and a deflector to generate an electric field and a magnetic field. In this scanning electron microscope, the magnitude of the negative voltage impressed on the stage is increased, the magnitude of the electric field generated by the deflector is increased, and the magnitude of the electric field is controlled so that the secondary electrons generated from the sample pass through a hole provided in the first conversion electrode close to the sample side, and collide with the second conversion electrode close to the electron source side. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a scanning electron microscope, and more particularly, to a scanning electron microscope having a high-efficiency detection system having a signal discrimination function in ultra-low irradiation voltage and high-resolution observation.

  A scanning electron microscope accelerates electrons emitted from an electron source and converges them with an electrostatic lens or a magnetic lens to form a thin electron beam (primary electron beam), which is then scanned with a scanning deflector. A device that obtains a two-dimensional scanned image by scanning a sample to be observed, detecting a secondary signal generated from the sample by the irradiated primary electron beam, and recording the signal intensity in synchronization with the primary electron beam scanning. It is.

  Electrons with a wide energy distribution are generated from the sample by irradiation with the primary electron beam. Part of the primary electron beam incident on the sample is elastically scattered by atoms near the sample surface. This is called a backscattered electron (BSE) and has a high energy up to the same energy as the primary electron beam. Reflected electrons are suitable for observing the composition distribution because the intensity and scattering direction vary depending on the composition of the sample.

  A part of the primary electron beam interacts with atoms in the sample, and electrons in the sample gain kinetic energy and are emitted from the sample. This is called a secondary electron (SE) and has an energy of 50 eV or less, and on average about 10 eV. Since secondary electrons are difficult to escape from the deep part of the sample, it is considered that they mainly have information on the sample surface, which is suitable for observation of the surface shape and sample potential distribution.

  In recent years, observation at an extremely low acceleration voltage of 1 kV or less has been increasingly used for the purpose of reducing damage to a sample due to electron beam irradiation and acquiring the outermost surface information.

  However, when passing through the lens with a low acceleration voltage, the resolution is reduced due to chromatic aberration caused by variations in electron beam energy. Therefore, in many cases, a deceleration method (also referred to as a retarding method) that converges the primary electron beam by superimposing an electric field that decelerates the primary electron beam in addition to the magnetic lens is used.

  The deceleration method is achieved by, for example, creating an electrostatic lens on the optical axis by applying a negative voltage (retarding voltage) to the sample stage on which the sample is mounted, and decelerating the primary electron beam with this electrostatic lens. The

  This decelerating method is characterized in that SE and BSE radiated from the sample are accelerated by the electrostatic lens used for decelerating the primary electron beam.

  Patent Documents 1 and 2 disclose a technique for obtaining an SEM image by separating secondary electrons and reflected electrons in ultra-low acceleration observation using a deceleration method in which a negative voltage is applied to a sample.

  Patent Document 1 discloses a method of obtaining a SEM image by providing a deflector between a sample and a scanning device, and providing an opening at a separation position of the trajectory of secondary electrons and reflected electrons, for example. Yes.

  Patent Document 2 discloses a method of separately detecting secondary electrons and reflected electrons accelerated by a decelerating electric field of a primary electron beam and then decelerating with an electromagnetic field applied in a direction perpendicular to the optical axis.

JP-A-11-67139 WO99 / 46798

  According to the technique disclosed in the above-mentioned document, it is possible to discriminate and detect SE / BSE in ultra-low acceleration observation using a deceleration method in which a negative voltage is applied to a sample, but there are the following problems.

  Patent Document 1 assumes a scanning electron microscope used for process management during semiconductor manufacturing, and relates to contact hole observation and line-and-space line width measurement. Therefore, since secondary electrons and reflected electrons from the sample are limited to the shape of the semiconductor processing, it is assumed that they are emitted from the sample in a certain range in the optical axis direction (in the embodiment, an example within 20 degrees is shown). Yes. Accordingly, when used for purposes other than the sample observation of a semiconductor having a high aspect ratio, there is a problem that orbits of secondary electrons and reflected electrons are not separated as described in the patent.

  Further, in Patent Document 2, the use of a mesh is indispensable for discriminating signal electrons having a small energy difference, and there is a problem that the detection efficiency must be sacrificed.

  In recent years, there has been a need to increase the strength of the electrostatic lens used for decelerating the primary electron beam, that is, to increase the magnitude of the negative voltage applied to the sample stage in ultra-low acceleration observation using the deceleration method. is there. In such a case, there is a problem that the trajectory of the secondary electrons changes due to the change of the optical conditions and does not enter the detector. The above Patent Documents 1 and 2 cannot solve this problem.

  In view of the above problems, the present invention is to provide a scanning electron microscope capable of efficiently detecting secondary electrons even when the retarding voltage is changed.

  In order to achieve the above-mentioned problems, an electron source, an objective lens that focuses the primary electron beam generated from the electron source onto the sample, a stage on which the sample is mounted, and generated from the sample by irradiation with the primary electron beam In a scanning electron microscope having a plurality of detectors for detecting secondary electrons and reflected electrons and a power source for applying a voltage to the stage, the electron microscope generates at least two conversion electrodes, and an electric field and a magnetic field. A deflector for increasing the magnitude of the negative voltage applied to the stage, and increasing the magnitude of the electric field generated by the deflector. Among these, the magnitude of the electric field is controlled so as to pass through a hole provided in the first conversion electrode close to the sample side and collide with the second conversion electrode close to the electron source side among the conversion electrodes. To provide a scanning electron microscope.

  In addition, an electron source, an objective lens that focuses the primary electron beam generated from the electron source on the sample, a stage on which the sample is mounted, and secondary electrons and reflected electrons generated from the sample by irradiation of the primary electron beam In a scanning electron microscope having a plurality of detectors for detection and a power source for applying a voltage to the stage, the electron microscope includes at least two conversion electrodes, and a deflector for generating an electric field and a magnetic field, Of the conversion electrodes, the size of the hole provided in the first conversion electrode close to the sample side is larger than the opening angle of the primary electron beam, and among the conversion electrodes, the second conversion electrode close to the electron source side The scanning electron microscope is characterized in that the provided hole is smaller than the off-axis amount of the secondary electrons deflected by the deflector.

  According to the above configuration, secondary electrons can be efficiently detected even when the retarding voltage is changed.

  Further, when observing a sample with an extremely low irradiation voltage and high resolution, it becomes possible to separately detect secondary electrons and reflected electrons among secondary signals generated by irradiation with a primary electron beam.

  FIG. 1 shows a schematic diagram of an embodiment of the present invention. Uses an objective lens (semi-in lens) in which the lens gap of the objective lens 4 is opened downward, and uses a deceleration method that applies a negative voltage to the sample to decelerate the primary electron beam between the objective lens and the sample. The present invention will be described with reference to a scanning electron microscope.

  In this example, an example of a semi-in-lens type objective lens is shown, but the shape of the lens is not limited to the semi-in-lens type, and the effects of the invention are the same.

  The primary electron beam 2 generated from the electron source 1 is scanned by deflectors 3 a and 3 b arranged in two stages, and converged on the sample 6 by the objective lens 4 and the deceleration voltage application mechanism 5. Since the objective lens is at the ground potential, the primary electron beam 2 is decelerated by the electric field generated between the objective lens and the sample by the deceleration voltage application mechanism 5, so that when the sample is irradiated, it passes through the objective lens. It is slower than. By irradiation with the primary electron beam 2, secondary electrons 20 and reflected electrons 21 are generated from the sample 6.

  The reflected electrons 21 are substantially equal to the energy when the sample is irradiated with the primary electron beam.

  In the present embodiment, the case where the irradiation voltage (Vi) is set to 100 eV and the voltage (Vr, hereinafter referred to as retarding voltage) applied to the sample by the deceleration voltage applying mechanism 5 is set to −1500 V will be described. The effect of the invention is the same even if the ding voltage is changed.

  Since the reflected electrons 21 are accelerated by the electric field generated between the objective lens and the sample, the reflected electrons 21 have energy of 1600 eV when passing above the objective lens. At this time, the speed of the reflected electrons 21 is the sum of the speed in the beam optical axis direction due to acceleration by the electric field generated between the objective lens and the sample and the speed when generated on the sample. Accordingly, the velocity component in the direction perpendicular to the beam optical axis only maintains the velocity component in the direction perpendicular to the beam optical axis among the velocities generated on the sample, and its maximum value is a velocity equivalent to 100 eV.

  On the other hand, the velocity component in the direction parallel to the optical axis of the beam at that time is a velocity corresponding to the retarding voltage. Therefore, the maximum opening angle by the velocity addition is 100/1500 = 0.067 rad (about 3.8 ° in this example). ) The reflected electrons 21 travel in the direction of the electron source 1 and reach the lower detection electrode 7.

  A part of the reflected electrons 21 collides with the lower detection electrode 7. Regenerated secondary electrons 22 are emitted from the surface of the lower detection electrode 7 by the energy of the reflected electron collision. The regenerated secondary electrons 22 are moved to the porous electrode 11a side by the magnetic field generated by the magnetic field deflection coils 10a and 10b and the electromagnetic field in which the electric field generated by the porous electrode 11a and the counter electrode 11b which are electrostatic deflection electrodes is superimposed. Led. The secondary electron detector 12 disposed outside the porous electrode 11a has a positive high voltage applied to the front surface. The regenerated secondary electrons 22 are accelerated through the hole of the porous electrode 11 a by the electric field caused by the voltage applied to the front surface of the secondary electron detector 12, and are captured by the secondary electron detector 12.

  In this way, the secondary electron detector 12 detects a signal caused by the reflected electrons 21. Signals detected by the secondary electron detector 12 and signals from other detectors (not shown) are selected or synthesized by the signal processing means 13 and displayed on the display device 15.

  In the magnetic field deflection coils 10a and 10b, the porous electrode 11a and the counter electrode 11b, an electric field and a magnetic field are applied orthogonally so as to guide the regenerated secondary electrons 22 without affecting the trajectory of the primary electron beam 2 in the direction of the detector. (This deflector using the orthogonal electromagnetic field is hereinafter abbreviated as ExB). The application method of the orthogonal electromagnetic field that does not affect the primary electron beam 2 trajectory is described in detail, for example, in the cited document 1.

  When an electric field Ex is generated by applying a potential difference of ± Ve (V) between the porous electrode 11a and the counter electrode 11b, the electron beam 104 having a velocity Vz by the acceleration voltage Vacc causes the electrostatic force and the Lorentz force F1 = −e ( Ex + Vz · By). The coil current amount for generating the voltage Ve or By (a magnetic field orthogonal to the electric field Ex) is adjusted so as to satisfy F1 = 0.

At this time, the reflected electrons 21 and the secondary electrons 20 have angles θb = Ex · L / Vacc (Equation 1)
And θs = Ex · L (1 + √ (Vr / Vacc)) / (2Vr) (Formula 2)
Only deflected. Here, L is the electrode length.

On the other hand, since the electrostatic force and the Lorentz force are balanced and controlled so that F1 = 0, the primary electron beam 2 goes straight without receiving a deflection action. That is,
F1 = −e (Ex + Vz · By) = 0 (Formula 3)
ExB can be operated without affecting the primary electron beam if the control is satisfied.

  Consider the deflection angle in this embodiment by substituting specific numerical values. When Vr = 1500V, Vacc = Vr + Vi = 1600V, Ex = 500V / m, L = 20 mm, θb = 0.063 rad (about 3.6 °) and θs = 0.0066 rad (about 0.37 °). .

  The energy of the secondary electrons 20 is 50 eV or less and is said to be about 3 eV on average. The scattering intensity distribution is assumed to be proportional to cos θ. θ is an angle formed with a line perpendicular to the sample surface. The secondary electrons 20 are accelerated by the electric field generated between the objective lens and the sample. The secondary electrons 20 have an energy of 1500 eV to 1550 eV above the objective lens, and on average about 1503 eV. At this time, the opening angle of the secondary electrons 20 is 3/1500 = 0.002 rad (about 0.11 °). The opening angle 0.002 rad is equal to or less than the opening angle of the primary electron beam 2, and the hole for passing the primary electron beam 2 provided in the center of the lower detection electrode 7 is designed to be larger than the distribution diameter of the secondary electrons 20. . Accordingly, the secondary electrons 20 travel further upward without colliding with the lower detection electrode 7.

  At this time, the secondary electrons 20 are deflected by ExB. The deflection angle can be calculated by (Equation 2), and is 0.0063 rad in this example. After the orbital center is deflected by 0.0063 rad by ExB, the secondary electrons 20 are guided toward the electron source 1. Most of the secondary electrons 20 collide with the upper detection electrode 8.

  The secondary electrons 20 are accelerated by an electric field obtained by decelerating the primary electrons, and have an energy of about 1503 eV when colliding with the upper detection electrode 8. Therefore, the regenerated secondary electrons 23 are emitted from the surface of the upper detection electrode 8 due to the energy of the secondary electron 20 collision. The secondary electron detector 14 has a positive high voltage applied to the front surface. The regenerated secondary electrons 23 are accelerated by the electric field caused by the voltage applied to the front surface of the secondary electron detector 14 and captured by the secondary electron detector 14. In this way, the secondary electron detector 14 detects a signal caused by the secondary electrons 20.

In this embodiment, the ratio of the secondary electrons 20 colliding with the upper detection electrode 8 will be examined by substituting specific numerical values. In addition to the premise of calculating the deflection angle first, the distance from the sample 6 to the lower detection electrode 7 is 50 mm, and the distance from the lower detection electrode 7 to the upper detection electrode 8 is 100 mm. The diameter of the hole for passing the primary electron beam 2 formed in the center of the upper detection electrode 8 is 1 mm. The center position of the secondary electrons 20 is separated from the optical axis of the primary electron beam 2 by a distance given by the product of the deflection angle θ _S = 0.0066 rad and the distance from the center position of ExB to the upper detection electrode 8. . That is, in this example, it is 0.72 mm away from the optical axis of the primary electron beam 2. The radius of the distribution range of the secondary electrons 20 is calculated by the product of the opening angle 0.002 rad and the distance from the sample 6 to the upper detection electrode 8 of 150 mm. That is, the secondary electrons 20 are only distributed within a radius of 0.3 mm.

  FIG. 3 shows the positional relationship between the collision position of the secondary electrons 20 to the upper detection electrode 8 and the holes for passing the primary electron beam 2. Since most of the secondary electrons fall within about 3 eV in FIG. 3, most of the secondary electrons 20 collide with the upper detection electrode 8, and the secondary electrons 23 are regenerated on the electrodes, and the secondary electrons 20. The secondary electron detector 14 detects the signal resulting from the above with high efficiency. In this way, by designing the hole diameter for passing the primary electron beam 2 provided in the central portion of the upper detection electrode 8 to be smaller than the off-axis amount of the secondary electrons, highly efficient secondary electron detection can be performed.

  A part of the reflected electrons 21 also collides with the upper detection electrode 8, and similarly, a signal caused by the reflected electrons 21 is detected by the secondary electron detector 14. Most of the signals are signals caused by the secondary electrons 20. . There are two reasons for this.

  The first point is that the generation amount of the secondary electrons 20 is larger than that of the reflected electrons 21.

  The second point is that the diameter of the reflected electron 21 beam when the reflected electron 21 reaches the upper detection electrode 8 is larger than the diameter of the secondary electron 20 beam. The rate of collision with an internal structure (not shown) and reaching the upper detection electrode 8 is not high. In the numerical substitution in this example, the radius of the distribution of the reflected electrons 21 on the lower detection electrode 7 is 3.3 mm, and the radius of the distribution range on the upper detection electrode 8 is 10 mm.

  Signals detected by the secondary electron detector 14 and signals from other detectors (not shown) are selected or synthesized by the signal processing means 13 and displayed on the display device 15.

  That is, a scanning type that can detect secondary electrons and reflected electrons in a discriminating manner by providing a reflective electrode in two upper and lower stages and a deflector between the sample and the upper reflective electrode, and can detect secondary electrons with high efficiency. An electron microscope can be constructed.

  In this embodiment, an example of detecting signal electron information using a combination of a reflective electrode and a secondary electron detector is shown. Instead of this combination, a scintillator is provided at the position of the reflective electrode, and light emission in the scintillator is performed with a light guide. The same effect can be expected even when the structure is changed to a photomultiplier tube. The same effect can be expected even if the configuration is such that the signal electrons are directly detected by a semiconductor detector having a thin surface barrier instead of the combination of the reflective electrode and the secondary electron detector.

  The ExB detector is also used as a deflector for pulling the secondary electrons 22 regenerated on the lower detection electrode 7, but the present invention is configured if the installation position is between the sample 6 and the upper detection electrode 8. Is possible.

  FIG. 4 is a diagram related to an embodiment in which the electric field generated in ExB is increased with the retarding voltage.

  In the electron microscope having the same arrangement as the embodiment shown in FIG. 1, when the retarding voltage is changed to −2500 V, the secondary electrons 20 have a radius centered at a position 0.43 mm away from the optical axis of the primary electron beam 2. It is distributed within the range of 0.18 mm. Since this is almost located in the hole (diameter 1 mm) for passing through the primary electron beam 2 in the center of the upper detection electrode 8, the secondary electrons 20 hardly collide with the upper detection electrode 8.

  4 shows the relationship between the deflection electric field and the deflection magnetic field for the current control method. In the figure, when the retarding voltage is increased, the magnetic field generated by the deflection coil is changed while the voltage applied to the ExB deflection electrode is kept constant. At this time, the change angle of the secondary electrons and the reflected electrons is shown in the lower column of the left column in FIG. As can be seen from this figure, the deflection angle changes as the retarding voltage increases. That is, the secondary electrons pass through the primary electron beam passage hole and do not collide with the upper detection electrode.

  That is, in the embodiment shown in FIG. 1, when the retarding voltage is increased, it is necessary to review the dimensions of the internal structure in order to maintain the secondary electron detection efficiency. In this example, the secondary electron detection efficiency is maintained even when the retarding voltage is increased without reexamining the dimensions of the internal structure.

  In this example, ExB is controlled so as to maintain the deflection angle θs due to ExB of the secondary electrons 20 with respect to an increase in the retarding voltage. At the same time, the control conditions for satisfying the condition that does not affect the trajectory of the primary electron beam 2 (Expression 3) are shown in (Expressions 4 and 5).

Ex = θs · Vr / L (Formula 4)
By = Ex / Vz (Formula 5)
Based on this control condition, the electric field strength and magnetic field strength of ExB when the electron microscope having the same arrangement as in the embodiment shown in FIG. 1 is controlled are shown in the upper column of FIG.

  In this embodiment, as shown in the lower diagram of the center column in FIG. 4, even if the retarding voltage is increased, the deflection angle of the secondary electrons is substantially constant, so that the secondary electron detection efficiency can be kept high. Although FIG. 3 shows an example in which the electric field strength is controlled in proportion to the retarding voltage, the electric field strength of ExB may be controlled step by step so as not to fall below this deflection angle. Furthermore, the same effect can be expected for the changes added to the embodiment shown in FIG.

  The upper diagram in the column on the right side of FIG. 4 is a diagram relating to an embodiment in which the electric field generated by ExB is increased when the retarding voltage is a certain level or higher. In the embodiment shown in the upper diagram of the center column in FIG. 4, when the retarding voltage is small, the electric field strength for guiding the secondary electrons 22 regenerated at the lower detection electrode 7 to the secondary electron detector 12 is not sufficient. Therefore, when the retarding voltage is equal to or lower than a predetermined voltage, the electric field intensity of ExB is controlled to be constant, and when it is higher than that, the same control as in the embodiment shown in the upper column of FIG. 4 is performed. Based on this control condition, the electric field strength and magnetic field strength of ExB when the electron microscope having the same arrangement as in the embodiment shown in FIG. 1 is controlled are shown in the upper column of FIG.

  In this embodiment, as shown in the lower diagram in the right column of FIG. 4, in the region where the voltage is changed, the secondary electron deflection angle is almost constant even if the retarding voltage is increased. High efficiency can be maintained. Further, even when the retarding voltage is low, there is an advantage that the backscattered electron detection efficiency does not decrease. Similar effects can be expected for the modified examples added to the embodiment shown in FIG. 1 such as stepwise control of the ExB electric field intensity and the configuration of the detector.

  As described above, according to the embodiment apparatus of the present invention, a scanning electron microscope having a high-efficiency detection system having a signal discrimination function can be configured even under extremely low irradiation voltage conditions using the deceleration method.

1 is a configuration example of a scanning electron microscope according to the present invention. The figure which showed the deflection | deviation operation | movement of the secondary electron 20 by ExB. The figure which showed the collision range of the secondary electron 20 to the upper detection electrode 8. FIG. An embodiment in which the electric field generated in ExB is changed in accordance with the retarding voltage.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Electron source 2 Primary electron beam 3a, 3b Deflector 4 Objective lens 5 Deceleration voltage application mechanism 6 Sample 7 Lower detection electrode 8 Upper detection electrode 10a, 10b Magnetic field deflection coil 11a Porous electrode 11b Counter electrode 12, 14 Secondary electron detector 13 Signal Processing Unit 15 Display Device 20 Secondary Electron 21 Reflected Electron 22 Secondary Electron Regenerated due to Reflected Electron from Sample 23 Secondary Electron Regenerated due to Secondary Electron from Sample

Claims (5)

An electron source,
An objective lens for focusing the primary electron beam generated from the electron source on the sample;
A stage on which the sample is mounted;
A plurality of detectors for detecting secondary electrons and reflected electrons generated from the sample by irradiation of the primary electron beam;
A power supply for applying a voltage to the stage;
In a scanning electron microscope having
The electron microscope includes at least two conversion electrodes and a deflector that generates an electric field and a magnetic field,
While increasing the magnitude of the negative voltage applied to the stage, increasing the magnitude of the electric field generated by the deflector,
Secondary electrons generated from the sample pass through holes provided in the first conversion electrode close to the sample side among the conversion electrodes, and the second conversion electrode close to the electron source side among the conversion electrodes. A scanning electron microscope characterized by controlling the magnitude of the electric field so as to collide with the electron beam. The scanning electron microscope of claim 1,
A scanning electron microscope comprising: a region where the magnitude of an electric field generated by the deflector is not changed and a region where the magnitude is changed according to the magnitude of a negative voltage applied to the stage. The scanning electron microscope according to claim 1 or 2,
The scanning electron microscope according to claim 1, wherein the deflector is arranged closer to the sample side than the second conversion electrode. In claim 1,
The scanning electron microscope characterized in that the magnitude of the magnetic field generated by the deflector is changed in a predetermined relationship with the magnitude of the electric field. An electron source,
An objective lens for focusing the primary electron beam generated from the electron source on the sample;
A stage on which the sample is mounted;
A plurality of detectors for detecting secondary electrons and reflected electrons generated from the sample by irradiation of the primary electron beam;
A power supply for applying a voltage to the stage;
In a scanning electron microscope having
The electron microscope includes at least two conversion electrodes and a deflector that generates an electric field and a magnetic field,
Of the conversion electrodes, the size of the hole provided in the first conversion electrode close to the sample side is larger than the opening angle of the primary electron beam,
A scanning electron microscope characterized in that a hole provided in a second conversion electrode close to the electron source side among the conversion electrodes is smaller than an off-axis amount of secondary electrons deflected by the deflector.

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