GB2081501A

GB2081501A - Device for detecting secondary electrons in a scanning electron microscope

Info

Publication number: GB2081501A
Application number: GB8122416A
Authority: GB
Original assignee: Jeol Ltd; Nihon Denshi KK
Current assignee: Jeol Ltd
Priority date: 1979-06-28
Filing date: 1981-07-21
Publication date: 1982-02-17
Also published as: DE3126575A1; FR2488044A1; DE3126575C2; AU521225B2; JPS5730253A; GB2081501B; FR2488044B1; AU4851079A

Abstract

In a scanning electron microscope, a specimen 4 is inserted substantially centrally in a gap between the magnetic pole pieces of an objective lens 3 and the secondary electrons 6 from the specimen are detected by the detecting means 8 disposed upwardly of the objective lens. A pipe electrode 12 is incorporated along the optical axis 2 of the objective lens between the objective lens and the detecting means so that the primary electron beam irradiating the specimen is not adversely affected by the detecting means, and an outer pipe electrode 13 is located around the pipe electrode 12. Further, a mesh electrode 15 maintained at positive potential against the specimen is positioned between bottom ends of the pipe electrode 12 and the outer pipe electrode 13 so that almost all the secondary electrons from the specimen are attracted towards the detecting means. <IMAGE>

Description

SPECIFICATION Device for detecting secondary electrons in a scanning electron microscope The present invention relates to an improve ment in a device for detecting secondary electrons from a specimen in a scanning elec tron microscope.

Recent transmission type electron micro scopes are equipped with an additional device allowing observation of the scanning micro scope image with secondary electrons. With such a modified scanning electron micro scope, it is customary for the specimen to remain inserted in a gap between the mag netic pole pieces of an objective lens when observing the scanning image, as when ob serving a transmitted microscope image.

Reference will now be made to Fig. 1 of the accompanying drawings, which is a schematic cross-sectional drawing of a known electron microscope.

Referring to Fig. 1 there is shown at 1 a system for illuminating a specimen with an electron beam, the system comprising an elec tron gun for producing the primary electron beam along an optical axis 2 and a condenser lens for converging the electron beam. The specimen 4 is inserted substantially centrally in a gap between the magnetic pole pieces of an objective lens 3. The electron beam for iliuminating the specimen along the optical axis is focused on the specimen surface by a magnetic field generated in front of (towards the electron gun) the specimen.The objective lens magnetic field in front of the specimen thus acts as a final stage condenser lens, and also acts as the deflecting means together with a deflection coil 5 disposed above the upper magnetic pole piece of the objective lens so that the electron beam scans two dimensionally over a surface of the specimen, and further acts as the focusing means for focusing secondary electrons 6 emitted in all directions from the specimen surface towards the direction of the optical axis. A device for detecting the secondary electrons is disposed upwardly of the objective lens, and comprises a light pipe 7 with a scintillator attached to a front (towards optical axis) end thereof, a photomultiplier 8 positioned at the rear end of the light pipe, and others.The scintillator on the front end of the light pipe 7 is coated on its front (towards optical axis) face with a thin conductive layer. Such conductive layer and an accelerating ring electrode 9 therearound are held at a potential of the order of + 10 KV by a d.c. power supply 1 0. A shield sleeve 11 which is at a ground potential is disposed in surrounding relation to the front end of the light pipe.

With the arrangement shown in Fig. 1, the secondary electrons emitted from the speci men 4 have relatively low energy ranging from several eV to several tens of eV, and hence become focused progressively in the direction of the optical axis 2 as the secondary electrons follow a spiral path. The secondary electrons tend to divert from the optical axis 2 again as they move out of the magnetic field formed by the objective magnetic pole pieces.

The ring electrode 9, however, forms an electric field which extends above the objective lens and serves to accelerate the secondary electrons towards the scintillator. The secondary electrons as they hit the scintillator generate light, which is transmitted through the pipe 7 and converted by the photomultiplier 8 into electricai signals to be picked up. Since the primary electron beam which is focused onto the specimen has relatively high energy that is normally 20KeV or higher, the degree to which the electron beam is deflected by the ring electrode 9 is negligibly small.

It is preferred that the primary electron beam irradiating the specimen should have high energy so that it transmits through the specimen with an increased power of transmission. However, producing a scanning image with secondary electrons does not of necessity require such a high energy, and instead there are some instances where an electron beam with low energy is preferred to prevent the specimen from being damaged by electron beam irradiation. In such case, primary electron beam is used which is accelerated by low voltage, for example, by several KV and the magnetic field of the objective lens is set at low excitation.However, such low energy electron beam is then adversely affected or deflected largely by the electric field generated by the ring accelerating electrode 9, and secondary electrons are not sufficiently collected by the weak magnetic field intensity of the objective lens. As a result, a high quality scanning microscope image is not obtained.

The main object of the invention is to provide a device for obtaining secondary electron signals with sufficient intensity under the condition that a low energy electron beam irradiates the specimen positioned in the weak magnetic field of the objective lens.

Another object of the invention is to provide a device in which the electric field generated for accelerating the secondary electrons emitted from the specimen cannot deflect the low energy primary electron beam irradiating the specimen.

According to this invention, there is provided a device for detecting secondary electrons in a scanning electron microscope in which a primary electron beam emitted along an optical axis from an electron gun is directed onto a specimen placed substantially centrally in a gap between the magnetic pole pieces of an objective lens for emanating the secondary electrons, which are detected by a scintillator with an accelerating electrode and a photomultiplier that are located in a position displaced off the objective lens towards the electrode gun, the device having a pipe electrode disposed around the optical axis for preventing deflection of the primary electron beam, an outer pipe-shaped electrode around said pipe electrode, and an annular-shaped mesh electrode maintained at positive potential and positioned between the ends of said pipe electrode and said outer pipe-shaped electrode for attracting the secondary electrons from the specimen towards said scintillator.

The invention will further be described with reference to Figs. 2 and 3 of the accompanying drawings, of which: Figure 2 is a schematic drawing of an electron microscope embodying the invention; and Figure 3 is a schematic drawing of a modification of the microscope of Fig. 2.

In Fig. 2, like reference numerals are used to denote like parts appearing in Fig. 1, and designated at 1 2 is a slender and electrically conductive pipe electrode for preventing deflection of a primary electron beam accelerated with a low energy for irradiating a specimen. The pipe electrode 1 2 is maintained at ground potential and is surrounded by an outer pipe-shaped electrode 1 3. An annularshaped mesh electrode 1 5 is attached between the pipe electrode 1 2 and the outer electrode 1 3 at a lower position thereof by means of an insulating ring 18.The output, for example, approximately + 500 V, of a d.c. power supply 14 is applied to the outer electrode 13 and mesh electrode.15. The outer pipe-shaped electrode 1 3 has a hole 1 3a towards which the front end of the detecting device faces. With this embodiment, even if the intensity of the objective lens magnetic field is not high enough to collect secondary electrons, the secondary electrons are attracted by an accelerating electric field generated by the mesh electrode 1 5 upwardly into a region where they are subjected to influence by an accelerating electric field by the accelerating ring electrode 9 and enter the scintillator.The primary electron beam, even if accelerated by a low voltage, is prevented from being deflected by the accelerating electric field by the accelerating ring electrode 9 since the pipe electrode 12 surrounds the path of the primary electron beam which is within the reach of the accelerating electric field.

Fig. 3 shows another embodiment according to this invention, whose structure is different from that of Fig. 2. A pipe electrode 1 6 and an outer pipe-shaped electrode 1 7 are kept at the same ground potential as that of the shield pipe 11 for the light pipe 7. An annular-shaped mesh electrode 20 extends be tween an insulating ring 1 8 attached to the pipe electrode 1 6 at a lower portion thereof and an insulating ring 1 9 attached to the outer pipe-shaped electrode 1 7 at a lower portion thereof. The mesh electrode 20 is held at a potential of the order of several hundred volts by a d.c. power supply 14 to collect secondary electrons from a specimen to the same advantage as that of the device of Fig.

2. According to this embodiment, a detector is laterally inserted in and fixed to the outer pipe-shaped electrode 1 7.

Claims

1. A device for detecting secondary elect trons in a scanning electron microscope in which a primary electron beam emitted along an optical axis from an electron gun is directed onto a specimen placed substantially centrally in a gap between the magnetic pole pieces of an objective lens for emanating the secondary electrons, which are detected by a scintillator with an accelerating electrode and a photomultiplier that are located in a position displaced off the objective lens towards the electron gun, the device having a pipe electrode disposed around the optical axis for preventing deflection of the primary electron beam, an outer pipe-shaped electron around said pipe electrode, and an annular-shaped mesh electrode maintained at positive potential and positioned between the ends of said pipe electrode and said outer pipe-shaped electrode for attracting the secondary electrons from the specimen towards said scintilla tor.

2. A device for detecting secondary electrons in a scanning electron microscope as described in claim 1, wherein the said pipe electrode and said outer pipe-shaped electrode are maintained at earth potential.

3. A device for detecting secondary electrons in a scanning electron microscope as described in claim 1, wherein the said outer pipe-shaped electrode is maintained at the same potential as that of said mesh electrode.

4. A device for detecting secondary electrons in a scanning electron microscope substantially as herenbefore described with reference to Fig. 2 or Fig. 3 of the accompanying drawings.