JP2011238387A - Electronic probe device using emitter array - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electronic probe device using an emitter array in which the probe current can be increased while keeping the diameter of a probe.SOLUTION: The electronic probe device comprises: an FE emitter array 1 arranged in the shape of a ring; a lens 2 which focuses a beam of the FE emitter array 1 to a diffraction surface; an iris 3 arranged on the diffraction surface; and an optical system 5 which irradiates a sample 6 with the reduced image of the diaphragm face of the iris 3. The electronic probe device is configured to increase a probe current while keeping the probe diameter of the device.

Description

  The present invention relates to an electron probe apparatus using an emitter array, and more particularly to an electron probe apparatus that can increase a probe current and correct spherical aberration.

  Field emission with minute conical emitters and micro cold cathodes formed in the immediate vicinity of the emitter and composed of control electrodes (gate electrodes) that draw out current from the emitter and have current control functions Cold cathodes are known. FIG. 5 is a diagram showing the structure of this type of field emission cold cathode.

  In the figure, 101 is a silicon substrate, 102 is a silicon oxide insulating layer, and a control electrode 103 is laminated on the insulating layer 102. A part of the insulating layer 102 and the control electrode 103 is removed to form a cavity 109, and an emitter 104 having a sharp tip is formed on the substrate 101 in the cavity 109. A micro cold cathode 107 is formed by the emitter 104, the control electrode 103, the control electrode 103, and a cavity 109 formed in the insulating layer 102. The micro cold cathode 107 is arranged in an array to form a cold cathode 108 having a planar electron emission region. Is formed.

The substrate 101 and the emitter 104 are electrically connected, and a predetermined voltage is applied between the emitter 104 and the gate electrode 103. The insulating layer 102 has a thickness of about 1 μm, the opening diameter of the gate electrode 103 is as narrow as about 1 μm, and the tip of the emitter 104 is made extremely sharp, about 10 nm. Therefore, a strong electric field is applied to the tip of the emitter 104. When this electric field is 2 to 5 × 10 ⁷ V / cm or more, electrons are emitted from the tip of the emitter 104.

  By arranging the micro cold cathodes having such a structure in an array on the substrate 101, a planar cathode that emits a large current is formed.

  In a conventional apparatus for forming an electron probe, a light source (electron source) produced by an electron gun is reduced by an electron lens system to form a final reduced image on a sample surface. The diameter of the probe to be formed determines the spatial resolution of the apparatus and should be as small as possible. On the other hand, the signal amount is determined by the current value of the probe, and the larger the better.

  Further, some recent apparatuses that form an electron probe improve the spatial resolution (or increase the probe current value) by correcting the aberration of the electron lens system. In such an apparatus, a complex multistage multipole lens system is used to correct aberrations.

  As a conventional device of this type, a technique describing a conventional field emitter and a new micro field emitter array is known (see, for example, Non-Patent Document 1). Also, the cathode electrode of a field emission emitter array cold cathode consisting of a two-stage gate structure consisting of a focusing electrode and a control electrode is divided into concentric rings and driven by a constant current source that can be turned on / off controlled by a digital circuit. An apparatus is known (see, for example, Patent Document 1).

  Also, a substrate and a plurality of sharp emitter assemblies each having an electron extraction electrode are formed on the substrate, and when the electrons emitted from the emitter assembly are focused by the electromagnetic lens, 2. Description of the Related Art An electron beam apparatus is known that controls emittance characteristics of emitted electrons so as to compensate for aberrations of an electron lens by a focusing electrode formed on the same plane as a surrounding electron extraction electrode (for example, Patent Document 2). reference).

  Also, a cathode that emits electrons, an electrode that focuses the emitted electrons to generate an electron beam and accelerates the electron beam, and the total current that forms the electron beam obtained by paraxial orbital approximation calculation method. Field emission with a magnet that forms a magnetic field parallel to the central axis of the cathode and the electrode using a cathode magnetic flux density that minimizes the maximum value of the beam radius of the electron beam that occupies a predetermined proportion of the current A cathode electron gun is known (see, for example, Patent Document 3).

Japanese Patent Laid-Open No. 9-219155 (paragraphs 0025 to 0031, FIGS. 1 to 3) JP-A-9-306338 (paragraphs 0032 to 0037, FIG. 2) JP 2000-200555 (paragraphs 0016 to 0026, FIGS. 1 to 3)

Junzo Ishikawa, “Recent Progress of Field Emissions and Expectations as an Electron Source”, Surface Science, 2002, Vol. 23, No. 1, pp 2-8

  In the conventional apparatus, the request to reduce the probe diameter is in a relationship (tradeoff) incompatible with the request to increase the probe current value. Naturally, if the probe diameter is large, it is easy to increase the current value. However, considering the case where the required probe diameter is specified in advance, the only way to increase the probe current is to increase the beam opening angle on the sample surface.

  The reason is as follows.

  The unit area on the emitter surface of the electron gun and the current value per unit solid angle, that is, the brightness of the electron gun, can be maintained at the same value on the sample surface, regardless of the lens system configuration from the emitter to the sample surface. Are known. This is called a luminance invariant. The luminance invariant means that the current value per unit area and unit solid angle on the sample surface is determined only by the properties of the emitter. Here, an electron gun using a field emission emitter has the highest luminance.

  Therefore, in order to increase the current with respect to the designated probe diameter, if the type of emitter to be used is determined, there is no other way but to widen the beam opening angle.

  However, the opening angle of the beam on the sample surface not only determines the probe current value but also determines the amount of aberration generated by the lens system at the same time. In the electron probe apparatus, the influence of spherical aberration of the objective lens (lens closest to the sample) is the largest, but the blurring of the beam due to spherical aberration increases in proportion to the cube of the opening angle. Therefore, in order to realize the required probe diameter, it is not allowed to increase the opening angle as much as desired, and the upper limit value of the opening angle is determined.

  As a result, the probe current value is limited according to the probe diameter. If the opening angle is too small, the diffraction aberration due to the wave nature of electrons increases, and the beam is blurred. Therefore, there is generally an optimum value for the beam opening angle.

  In addition, the aberration correction unit used in the current apparatus can correct spherical aberration and chromatic aberration at the same time, but the manufacturing and power supply operations are complicated, and the time required for optical adjustment is large. It becomes. Except for the case of low-energy electron beams, the primary factor that determines spatial resolution is spherical aberration, and chromatic aberration is not important. Therefore, a sufficient effect can be obtained by correcting only the spherical aberration.

  The present invention has been made in view of such a problem. First, it is intended to increase the probe current value while maintaining the probe diameter of the conventional apparatus, and secondly, the spherical aberration is appropriately corrected. An object of the present invention is to provide an electronic probe device that can be used.

  In order to solve the above problems, the present invention has the following configuration.

  (1) The invention according to claim 1 is an FE emitter array arranged in a ring shape, a lens for focusing the beam of the FE emitter array on a diffractive surface, a stop disposed on the diffractive surface, and the stop And an optical system for irradiating the sample with a reduced image of the diaphragm surface.

  (2) The invention according to claim 2 is characterized in that the FE emitter array arranged in a ring shape, a multi-emitter control power source for controlling the voltage applied to the FE emitter array for each array, and the beam of the FE emitter array are diffracted. It is characterized by comprising a lens for focusing on a surface, a stop arranged on the diffractive surface, and an optical system for irradiating the sample with a reduced image of the stop surface of the stop.

  (1) According to the first aspect of the present invention, the probe current value can be increased without changing the probe diameter by having the FE emitter array and concentrating the beam from each emitter array on the diffraction surface. .

  (2) According to the invention described in claim 2, the structure and the power supply control system are simpler than the conventional optical system for correcting aberrations, and the spherical aberration can be corrected by an easy operation.

It is a block diagram which shows one Example of this invention. It is explanatory drawing of the opening angle of the electron beam irradiated to a sample. It is explanatory drawing of the spherical aberration of a lens. It is a block diagram which shows the other Example of this invention. It is a figure which shows the structure of a field emission cold cathode.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a block diagram showing an embodiment of the present invention. In a normal electron gun, there is one emitter that emits electrons and is placed on the optical axis. In the present invention, an FE emitter array is used. In the figure, 1 is an FE emitter array. In this example, FE emitters of about several microns are arranged on a plane by using an integrated circuit technique.

  In the present invention, emitters arranged in a ring shape along the circumference centered on the optical axis are used. At present, the performance of the individual emitters is inferior to that of the conventional one, but a sufficient current can still be obtained by arranging several tens or more. Here, the lens 2 (LG) is used to create a diffractive surface of the light source (a surface in which the trajectories coming out from each point of the light source surface at the same angle are gathered at the same point), and the aperture 3 is placed at that position to form a data aperture Are formed on the sample 6. 4 is an image of the emitter, and 7 is a reduced image of the aperture.

  That is, the aperture of the diaphragm 3 is handled as a light source. After the diaphragm 3, a normal reduction lens system may be placed. In the figure, the reduction lens system is represented by one objective lens. The diameter of the diaphragm 3 is designed to simultaneously control the beam capture angle from each emitter.

  The difference between this optical system and the conventional optical system is that the current density impossible with a single emitter can be realized by concentrating the beam from each emitter on the diffraction surface. It is easier to understand this situation by considering the sample surface conjugate with the diaphragm surface.

  2A and 2B are explanatory diagrams of the opening angle of the electron beam irradiated on the sample. FIG. 2A shows the conventional case, and FIG. 2B shows the case of the present invention. In the figure, focusing on one point on the axis of the sample surface, a beam focused on the point is drawn. In (a), it is the beam opening angle Δα that determines the spherical aberration of the objective lens 5. It is said that the beam is blurred in proportion to the cube of the beam opening angle Δα.

  On the other hand, in the case of the present invention of (b), the opening angle Δα that determines the aberration is the same, but the solid angle of the beam viewed from one point on the sample surface increases, and the probe current increases by the ratio. As described above, the luminance of the sample surface is a constant value and does not change. However, if the irradiation area on the sample surface, that is, the probe diameter is the same, the probe current value is increased by the increase in the solid angle.

  In a conventional apparatus for forming an electron probe, spatial resolution has to be sacrificed particularly when current value is prioritized (SEM analysis mode, Auger electron spectroscopy, EMPA, etc.). According to the present invention, the probe current can be increased by the number of emitters without affecting the probe diameter.

  As described above, according to the present invention, in the apparatus for forming an electron probe, by using the optical system of the present invention, the current value can be increased while maintaining the conventional probe diameter.

  Next, correction of spherical aberration will be described. FIG. 3 is an explanatory diagram of the spherical aberration of the lens. When a point light source is imaged by a convex lens of an optical system, it is ideal that all the light rays emitted from the light source are collected at one point. However, when the lens has spherical aberration, blur occurs as shown in FIG. In the figure, the light beam incident on the lens 10 at a larger angle forms an image on the near side on the image side. At this time, the coefficient Cs indicating the magnitude of the spherical aberration is positive. In general, a position where light beams having a small angle converge is called a Gaussian image plane. 11 in the figure is a Gaussian image plane.

  In an optical lens, Cs of a convex lens can be positive or negative, but a normal axially symmetric electron lens is always positive and cannot be zero. That is, in the case of an electronic lens, the situation is as shown in FIG. This is due to the degree of freedom of the electromagnetic field that forms the electron lens. Since spherical aberration is a factor governing the spatial resolution of an electron microscope, correction of this aberration is a very important technique.

As can be seen from FIG. 3, Cs becomes positive when the focusing force is stronger than the ideal case on the outer side of the lens (location away from the optical axis). If the electrons entering the lens 10 have larger energy (velocity) as the angle increases, the effect is canceled and the spherical aberration is corrected. That is, an electron beam with a large angle is also formed on the Gaussian image plane 11. In a normal apparatus, since a monochromatic (ie, the same energy) electron beam generated by an electron gun is used, Cs is positive. However, if the energy can be adjusted for each angle, the electron beam focused by the lens 10 is An image is formed at one point, and the coefficient Cs can be made zero.

  For example, if there is a uniform magnetic field distribution in the optical axis direction, it acts as a lens, and its coefficient Cs is still positive, but the incident electron beam is not monochromatic ((b)), but the velocity component in the optical axis direction. Are the same ((c)), the coefficient Cs is strictly 0. In the case of a general electron lens, the coefficient Cs is not necessarily exactly 0 by the same method, but since it has the same tendency qualitatively, if the relationship between the incident angle and the energy is well set, the coefficient Cs can always be zero.

  FIG. 4 shows an optical system for setting the energy value for each angle. FIG. 4 is a block diagram showing another embodiment of the present invention. The same components as those in FIG. 1 are denoted by the same reference numerals. An array pattern in which FE (field emission) emitters 1A are arranged in a plane can be created, and if this is used, energy can be set as desired for each angle of the electron beam.

  In this case, since the relationship between the incident angle of the electron beam incident on the lens 2 and the energy depends on the lens system to be corrected, it is adjusted by the power supply system of the emitter. 15 in the figure is a multi-emitter control power source. Here, the emitter array 1A is provided with a focusing lens, and several tens to several hundreds are arranged.

  Here, it is ideal that the emitter arrays 1A are arranged concentrically to vary the cathode voltage (determining the energy of emitted electrons) for each radius from the center. However, since each emitter is very small, about several microns, the pattern itself may be linearly arranged in a square shape, for example. This is because the shape of electrons emitted from emitters arranged in a square shape and that of electrons emitted from concentric emitters do not change so much. Further, at present, the emitter array 1A is not stable in operation, and in many cases, it can be produced whether or not it is ignited by the emitter. However, even if there is such a variation, there is no problem in the present invention. This is because even if there is an emitter that does not ignite, the overall beam shape is not affected.

  The electron beam from the emitter array 1 </ b> A is focused by the lens 2, and the diaphragm 3 is placed at the focal point so that the image of the diaphragm 3 is formed on the sample 6. Here, the emitter array 1A to the diaphragm 3 constitute an electron gun unit for aberration correction according to the present invention. Although the electrons emitted from each emitter may spread, an emitter array of a type having a focusing lens mechanism inside each emitter has been developed, so if that type is adopted, the current at the sample position can be further increased. Can be increased.

  As shown in FIG. 4, the image of the array surface is placed in front of a lens system 5 (represented by one objective lens 5 in the figure) to be corrected. Here, the diameter of the probe on the surface of the sample 6 can be reduced by reducing the aperture diameter and increasing the reduction ratio of the lens system 5. However, since the current value decreases as the probe diameter is reduced, the specific arrangement and dimensions of the optical system depend on the purpose. The relationship between the emitter position and energy is set at 15 by the multi-emitter control power supply 15 so that the spherical aberration of all the lenses (in the figure, LG2 and OL5 combined) becomes zero. Specifically, the multi-emitter control power supply 15 may be adjusted so that the electron beam focused on the sample 6 is focused on one point. Specifically, the multi-emitter control power supply 15 is adjusted so that the sample image of the sample 6 displayed on a display device (not shown) is clearly displayed.

  According to this embodiment, the spherical aberration can be corrected by a simple operation with a simpler structure and power supply control system than the conventional optical system for correcting aberrations.

  The currently used aberration correction unit can correct only the third-order spherical aberration, and the correction effect can be obtained only when the beam opening angle is relatively small. In such a case, the performance of the apparatus is determined by the contribution of fifth-order spherical aberration. However, in the present invention, such an order is irrelevant, and any large opening angle can be corrected. Therefore, it is particularly effective for the purpose of obtaining an electron probe with a large current by widening the angle.

  As described above in detail, according to the present invention, by using the optical system of the present invention in an apparatus for forming an electron probe, the current value can be increased while maintaining the conventional probe diameter. Further, the structure and the power supply control system are simpler than the conventional optical system for correcting aberrations, and spherical aberration can be corrected by an easy operation. Conventionally, only third-order spherical aberration has been corrected, but in the present invention, all orders of spherical aberration are corrected. As a result, it is possible to realize an apparatus that enables higher resolution or a higher probe current value.

1 FE emitter array 2 Lens 3 Aperture 4 Emitter image 5 Multistage reduction lens system 6 Sample 7 Reduced image of the aperture

Claims (2)

An FE emitter array arranged in a ring;
A lens for focusing the beam of the FE emitter array on the diffraction surface;
A diaphragm arranged on the diffractive surface;
An optical system for irradiating the sample with a reduced image of the stop surface of the stop;
An electron probe device using an emitter array comprising: An FE emitter array arranged in a ring;
A multi-emitter control power source for controlling the applied voltage of the FE emitter array for each array;
A lens for focusing the beam of the FE emitter array on a diffraction surface;
A diaphragm arranged on the diffractive surface;
An optical system for irradiating the sample with a reduced image of the stop surface of the stop;
An electron probe device using an emitter array comprising:

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