JPH0773841A - Scanning electron microscope and secondary electron detection system - Google Patents

Abstract

(57) [Abstract] [Purpose] A scanning electron microscope equipped with a pull-up electrode that always maintains a high secondary electron yield. [Structure] A primary electron beam 4 is focused on a sample 7 through a deflection coil and an objective lens 6 and irradiated. The secondary electrons 9 emitted from the sample 7 are wound around the magnetic flux of the objective lens 6 and ascend while spiraling due to the electric field generated by the lower pulling upper electrode 11a. Since the upper pulling upper electrode 11b is provided at a distance from the lower pulling upper electrode 11a and an electrostatic lens is formed between the two electrodes 11a and 11b, the secondary electrons 9 are focused by this electrostatic lens and the objective The secondary electron detector 10 always captures with a high secondary electron yield without colliding with the outer wall of the central opening of the lens 6 and being lost.

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 of a type in which secondary electrons generated when a sample is irradiated with primary electrons are detected by pulling them up in the incident direction of the primary electrons, and particularly to high resolution and high sensitivity. And a scanning electron microscope suitable for holding.

[0002]

2. Description of the Related Art FIG. 2 is a sectional view of a secondary electron pulling portion used in a secondary electron detection system of a scanning electron microscope. When the sample 7 is irradiated with primary electrons, CO is emitted from the surface of the sample 7.
Secondary electrons 9 angularly distributed in an S shape are emitted. In an area where the objective lens 6 makes a sufficiently strong magnetic field of the objective lens, the secondary electrons 9 are wound around the lines of magnetic force and rise, pass through the central aperture of the objective lens 6, and are placed above the secondary electron detector ( (Not shown).

By the way, since the magnetic field strength of the objective lens 6 is almost proportional to the square root of the acceleration voltage of the primary electrons, when the acceleration voltage drops to about 1 kV or lower, the magnetic field strength of the objective lens also drops. Moreover, when the sample 7 is installed away from the objective lens 6, the magnetic field strength of the objective lens on the sample surface is lowered. For this reason, the radius of gyration of the secondary electrons 9 becomes large, and the secondary electrons 9 cannot enter the central aperture of the objective lens 6.

In order to avoid this problem, the pull-up electrode 11 is installed in the central axis opening of the objective lens 6 and a positive voltage of about several hundred volts is applied to the space between the pull-up electrode 11 and the sample 7. An electric field is generated, and the secondary electrons 9 generated on the surface of the sample 7 are efficiently pulled upward and incident on the central aperture of the objective lens 6 (Tonomura: Electron Microscope Technology, Maruzen, p.179). (1989)).

On the other hand, in a scanning electron microscope which does not use the pull-up electrode 11, as described in, for example, JP-A-60-130044 and JP-A-3-230464,
An auxiliary electromagnetic lens is provided to prevent the secondary electrons from diverging in the region where the magnetic field of the objective lens is attenuated, so that the secondary electrons can efficiently reach the secondary electron detector.

[0006]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
The prior arts described in Japanese Patent No. 130044 and Japanese Patent Application Laid-Open No. 3-230464 have an electromagnetic lens for converging secondary electrons. However, the electromagnetic lens has a strong convergence even in the vicinity of the central axis through which the primary electrons pass, and there is a problem that even an auxiliary electromagnetic lens having only a weak magnetic field strength cannot reduce the influence on the primary electrons to zero. In addition, since the electromagnetic lens requires a coil, its size and shape cannot be arbitrarily set, and in particular, the secondary electron detection in which the central aperture of the objective lens 6 is narrow, as in a scanning electron microscope having a pull-up electrode. It is difficult to install in the system, and there is also a problem that the degree of freedom in design is narrowed.

It is an object of the present invention to provide a scanning electron microscope equipped with a pull-up electrode, which can always maintain a high secondary electron yield and has little influence on primary electrons.

[0008]

The above object is to irradiate a primary electron beam from a primary electron irradiation system onto a sample mounted in a magnetic field of an objective lens, and to draw secondary electrons generated from the sample along the optical axis. In a scanning electron microscope with a lifting electrode,
This is achieved by providing an electrostatic lens that focuses the secondary electrons in a position where the magnetic field and the electric field of the objective lens inside the pull-up electrode are attenuated and secondary electrons are diverged.

[0009]

[Function] Unlike the electromagnetic lens, the electrostatic lens does not need a coil and can be composed of only the electrode plate. Therefore, an electrostatic lens having an arbitrary shape can be formed even in a narrow area such as the central aperture of the objective lens. Further, the electrostatic lens has almost no effect on the primary electrons passing through the center of the lens, and focuses only the secondary electrons spreading in the direction away from the center.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 4 is a configuration diagram of a secondary electron detection system according to the first embodiment of the present invention. FIG. 3 is a diagram for explaining the converging action of this secondary electron detection system. The magnetic flux density distribution made by the objective lens 6 from the sample 7 to the vicinity of the pulling electrode 11 and the pulling electrode 1 are shown.
The potential distribution created by No. 1 is shown. The magnetic flux density of the objective lens 6 of the type having a gap on the lower surface facing the sample 7 is relatively strong from the sample surface to the incident portion of the central aperture of the objective lens 6, but is rapidly attenuated inside the central aperture. Therefore, the secondary electrons 9 start to diverge inside the central opening. In the conventional device, since the pulling-up electrode 11 having the same potential is installed in the entire center opening of the objective lens 6, it is not possible to prevent the divergence of the secondary electrons 9.

Therefore, in the present invention, the pulling-up electrode 11 is divided into the lower pulling-up electrode 11a and the upper pulling-up electrode 11b, and the electric field due to the ground potential of the objective lens 6 permeates through the gap between the two electrodes 11a and 11b. By doing so, a potential distribution as shown in the graph at the right end is formed. The region in which this permeation electric field exists becomes a focusing lens, and the secondary electrons 9 are focused. Since the strength of this focusing lens matches the pulling voltage of the secondary electrons in principle, the secondary electrons 9 can be focused without depending on the voltage of the pulling electrode 11.

The above is an example in which a single potential type electrostatic lens (Einzel lens) is used as the focusing lens.
It is also possible to use a bipotential type electrostatic lens.

FIG. 1 is an overall configuration diagram of a scanning electron microscope to which the secondary electron detection system shown in FIG. 4 is applied. Cathode 1 and 1
The primary electron beam 4 extracted from the cathode 1 by the extraction voltage V1 applied between the anode 2 and the anode 2 is accelerated by the acceleration voltage Vacc applied between the cathode 1 and the second anode 3 and is transferred to the lens system in the subsequent stage. proceed. The accelerated primary electron beam 4 is focused as a minute spot on the sample 7 by the lens action of the focusing lens 5 and the objective lens 6 controlled by the lens control power supply 16, and the scanning coils 8a and 8b two-dimensionally move on the sample 7. Scanned. The scanning signal supplied to the scanning coils 8a and 8b is controlled by the deflection control circuit 15 according to the observation magnification. The aperture angle of the primary electron beam 4 at the objective lens 6 is determined by a diaphragm 18 arranged below the focusing lens 5.

On the other hand, inside the central opening of the objective lens 6,
The lower pulling upper electrode 11a and the upper pulling upper electrode 11b are arranged along the optical axis across the region where the magnetic field of the objective lens is attenuated and the secondary electrons 9 start to diverge, and the ground electrode 14 is arranged further above the lower pulling electrode 11a.
The same voltage of about several hundreds of volts is applied to both pull-up electrodes 11a and 11b. The secondary electrons 9 generated from the sample 7 are accelerated by the pulling electric field generated by the lower pulling upper electrode 11a, and rise while making a spiral motion by the magnetic field of the objective lens 6. Subsequently, the secondary electrons 9 are focused by the electrostatic lens generated by the ground potential of the objective lens 6 penetrating in the central direction from between the pulling-up electrodes 11 a and 11 b, and at the central portion of the ground electrode 14. The image display device 1 is decelerated to energy and finally captured by the secondary electron detector 10 installed outside the optical axis.
6 displays an enlarged image of the sample 7.

In the single-potential type electrostatic lens of this embodiment, the energy of the accelerated secondary electrons 9 and the strength of the focusing lens are proportional to each other, so that the focusing is stable regardless of the voltage of the pull-up electrodes 11a and 11b. In addition to high performance, the same number of power supplies as before is sufficient, so there is almost no increase in cost. In addition, the electrode structure is extremely small, which is suitable for installation in the narrow aperture of the objective lens. Furthermore, since it is an osmotic electric field,
Since it is weak on the optical axis and strong on the periphery, only divergent secondary electrons can be focused while the influence on the primary electrons near the optical axis is small.

FIG. 5 is a block diagram of a secondary electron detection system according to the second embodiment of the present invention. In this embodiment, the pull-up electrode 11
Is composed of a triple funnel-shaped electrode 11a, 12, 11b. Here, the electrodes 11a and 11b are applied with the same potential of about 100 V, and the electrode 12 is normally set to the ground potential, but it is also possible to apply a variable voltage. Triple electrode 11
By inserting and fixing a funnel-shaped insulating material between a, 11b, 12 and the objective lens 6, high-precision assembly is possible.

FIG. 6 is a block diagram of a secondary electron detection system according to the third embodiment of the present invention. In this embodiment, the secondary electrons 9 are focused by using a bipotential type electrostatic lens. The pulling-up electrode is composed of the lower pulling-up electrode 11a, the upper pulling-up electrode 11b, and the lower ground electrode 12a. The lower pulling upper electrode 11a is installed so as to sandwich the magnetic pole of the objective lens 6, and the electric field is shielded by the lower ground electrode 12a except for the central portion. Upper electrode 11b
By adjusting the voltage of, the secondary electron 9 can be easily made into a substantially parallel beam and pulled up.

[0018]

According to the present invention, by providing the electrostatic lens for focusing the secondary electrons, the secondary lens always has a high secondary intensity regardless of the magnetic field strength of the objective lens, the pulling electrode voltage, and the distance between the objective lens and the sample. The electron yield can be maintained and high sensitivity can be realized. In addition, since the pull-up electrode voltage can be set arbitrarily, it is possible to increase the pull-up electrode voltage to 1 kV or more for high resolution, or to control the charge amount of the sample.

[Brief description of drawings]

FIG. 1 is an overall configuration diagram of a scanning electron microscope including a secondary electron detection system according to a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a conventional secondary electron detection system.

FIG. 3 is an explanatory diagram of a focusing action of the electrostatic lens of the secondary electron detection system according to the first exemplary embodiment of the present invention.

FIG. 4 is a configuration diagram of a secondary electron detection system according to the first embodiment of the present invention.

FIG. 5 is a configuration diagram of a secondary electron detection system according to a second embodiment of the present invention.

FIG. 6 is a configuration diagram of a secondary electron detection system according to a third embodiment of the present invention.

[Explanation of symbols]

1 ... Cathode, 2 ... First anode, 3 ... Second anode, 4 ... Primary electron beam, 5 ... Focusing lens, 6 ... Objective lens, 7 ... Sample, 8 ...
Scanning coil, 9 ... Secondary electron, 10 ... Secondary electron detector, 1
DESCRIPTION OF SYMBOLS 1 ... Pull-up electrode, 11a ... Bottom pull-up electrode, 11b ... Top pull-up electrode, 12 ... Intermediate ground electrode, 12a ... Bottom ground electrode, 14 ...
Ground electrode, 15 ... Magnification control circuit, 16 ... Lens control circuit, 17 ... Image display device, 18 ... Aperture.

Claims (10)

[Claims] 1. A pull-up electrode that irradiates a primary electron beam from a primary electron irradiation system to a sample mounted in a magnetic field of an objective lens to form an electric field that pulls secondary electrons generated from the sample along the optical axis. In the scanning electron microscope having, an electrostatic lens for converging secondary electrons is provided at a position where the objective lens magnetic field and the pulling electric field inside the pulling electrode are attenuated and secondary electrons are about to diverge. Scanning electron microscope. 2. The scanning electron according to claim 1, wherein the electrostatic lens of a single potential type is constituted by a ground potential of an objective lens leaking from a pair of upper and lower pulling electrodes separated from each other. microscope. 3. A scanning electron microscope according to claim 1, wherein a single potential type electrostatic lens is constituted by triple funnel-shaped electrodes. 4. The scanning electron microscope according to claim 1, wherein a bipotential electrostatic lens is configured by applying different voltages to a pair of upper and lower separated pull-up electrodes. 5. A scanning electron microscope in which a primary electron beam is focused through a central opening of an objective lens to irradiate a sample, and secondary electrons generated from the sample are captured by a secondary electron detector through the central opening, A configuration is provided in which a pulling-up electrode that pulls up secondary electrons is provided in the central opening, and the pulling-up electrode is divided into two in the pulling direction of the secondary electrons to form an electrostatic lens that focuses the secondary electrons at the divided portions. A scanning electron microscope characterized in that. 6. A scanning electron microscope in which a primary electron beam is focused through a central opening of an objective lens to irradiate a sample, and secondary electrons generated from the sample are captured by a secondary electron detector through the central opening, A configuration is provided in which a pull-up electrode that pulls up secondary electrons is provided in the central opening, an electrostatic lens that converges the pulled secondary electrons is provided, and the pull-up electrode is used as an electrode that constitutes the electrostatic lens. A scanning electron microscope characterized in that 7. An objective lens having a central aperture for focusing primary electrons to irradiate a sample and taking in secondary electrons generated from the sample through the central aperture, and a voltage for pulling the secondary electrons into the central aperture. In a secondary electron detection system including a pull-up electrode for applying a secondary electron and a secondary electron detector for trapping secondary electrons taken in through the central opening, a secondary electron pulled into the central opening by the pull-up electrode. The secondary electron detection system is characterized in that an electrostatic lens is provided at a position where the secondary electrons are converged and then captured by the secondary electron detector. 8. The electrostatic lens according to claim 7,
Secondary electron detection, characterized in that the pull-up electrode is formed by an electric field in which the ground potential of the objective lens penetrates through the gap formed when the pull-up electrode is divided into two in the direction along the central axis of the central opening. system. 9. The secondary electron detection system according to claim 7, wherein the electrostatic lens is a single potential type electrostatic lens or a bipotential type electrostatic lens. 10. An objective lens in which an electromagnetic coil is wound around a core having a funnel-shaped central opening, and a secondary electron generated from the sample when a primary electron beam converged through the central opening is irradiated to the sample. A pull-up electrode for generating an electric field for pulling an electron beam to the central opening, the pull-up electrode being electrically insulated from the objective lens and fitted in the central opening,
A secondary electron, comprising: an electrode electrically insulated from the pull-up electrode and fitted to the pull-up electrode, the electrode for an electrostatic lens forming an electrostatic lens in the central opening. Detection system.