Scanning electron microscope

Details Scanning electron microscope Scanning electron microscope

2001-07-27

2003-07-29

Lee, John R. (Department: 2881)

Radiant energy

Inspection of solids or liquids by charged particles

Electron probe type

C250S3960ML

Reexamination Certificate

active

06600156

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a scanning electron microscope equipped with an objective lens that incorporates a detector.
2. Description of the Related Art
In a scanning electron microscope, as an electron beam hits a specimen, secondary electrons, backscattered electrons, X-rays, and cathodoluminescence are produced, and these are detected. In a scanning electron microscope, the shape of the objective lens is an important factor that determines the instrumental resolution. Today, an objective lens for developing a magnetic field lens below the lower ends of inner and outer polepieces (i.e., toward the specimen) is widely used as an objective lens for achieving high resolution.
FIG. 2
shows the main portion of a scanning electron microscope equipped with this type of objective lens. In this figure, the objective lens is indicated by numeral
1
and consists of an inner polepiece
2
, an outer polepiece
3
, a yoke
7
, and a coil
8
. The objective lens is so designed that the magnetic field reaches a specimen
4
.
Bores H
1
and H
2
are formed in the inner polepiece
2
and outer polepiece
3
, respectively, of the objective lens
1
. A secondary electron detector
5
is mounted in these holes. A positive voltage is applied to the front end of the secondary electron detector
5
to attract secondary electrons.
Secondary electrons se produced by illumination of the electron beam on the specimen
4
are directed upward through the objective lens
1
by the magnetic field produced by the objective lens
1
. The secondary electrons se are accelerated by the action of an electric field formed ahead of the front end of the secondary electron detector
5
, and are detected by the secondary electron detector
5
.
Electron beam impingement on the specimen
4
also produces backscattered electrons be, which are detected by a backscattered electron detector
6
mounted below the polepieces of the objective lens
1
, the detector
6
consisting of a semiconductor device. Usually, this backscattered electron detector
6
assumes a doughnut-like form and is centrally provided with an electron beam passage bore H
0
.
Where the specimen is subjected to an elemental analysis or other analysis, an X-ray analytical instrument is interfaced with the scanning electron microscope, and X-rays produced as a result of electron beam impingement on the specimen are detected. In this case, the X-ray detector of the X-ray analytical instrument is mounted on the sidewall of the specimen chamber between the objective lens
1
and the specimen
4
such that the X-ray detecting surface faces toward the optical axis O. Furthermore, the specimen is tilted such that the specimen surface faces the X-ray detector, in order that X-rays emanating from the specimen are efficiently detected by the X-ray detector.
The above-described objective lens
1
has the built-in secondary electron detector
5
. Therefore, in a scanning electron microscope equipped with such an objective lens, the secondary electron detector is not installed between the objective lens
1
and the specimen
4
. Consequently, the distance, or the working distance (WD), between the objective lens
1
and the specimen
4
can be set small. Hence, the aberration coefficient of the objective lens
1
can be made small. As a result, a high-resolution secondary electron image can be obtained.
On the other hand, backscattered electrons have much higher energies than secondary electrons and, therefore, backscattered electrons traveling at angles greater than a given angle with respect to the optical axis O bounce off the objective lens field as shown in FIG.
3
(
a
) and thus travel at greater angles. These backscattered electrons are indicated by be
2
. Those of the backscattered electrons which are traveling within the given angle are restricted by the objective lens field and pulled upward beyond the center of the objective lens
1
along the optical axis O. These backscattered electrons are indicated by be
1
.
Accordingly, backscattered electrons incident on the backscattered electron detector
6
are small in quantity. In consequence, the backscattered electron image is a coarse image of insufficient brightness.
Therefore, if the specimen
4
is moved downward (i.e., the working distance is increased) as shown in FIG.
3
(
b
), the effect of the objective lens field on the electrons backscattered out of the specimen
4
weakens. Therefore, most of the electrons be backscattered out of the specimen
4
and traveling at angles less than a given angle with respect to the optical axis O impinge on the backscattered electron detector
6
. The result is that a backscattered electron image of high brightness can be obtained.
However, where secondary electron images and backscattered electron images should be alternately obtained under optimum conditions from the same field of view to make a structural analysis of the sample or for other purpose, if a secondary electron image is obtained, it is necessary to set the working distance shorter to establish high-resolution conditions. If a backscattered electron image is obtained, it is necessary to set the working distance greater to enhance the detection efficiency and establish high-brightness conditions. That is, the working distance needs to be increased and reduced alternately by adjusting the height of the specimen stage (not shown) taken along the Z-axis. Consequently, the operability is poor. Furthermore, the magnification is varied when the working distance is varied. Therefore, the magnification needs to be corrected when either image is obtained.
In addition, the thickness of the backscattered electron detector
6
is normally about 3 to 5 mm. Since this detector is mounted to the undersides of the polepieces of the objective lens
1
as shown in
FIG. 2
, the working distance (WD) cannot be decreased.
Moreover, where X-rays from the specimen are detected by an X-ray detector, the specimen must be tilted. This makes it impossible to reduce the working distance. For this reason, the objective lens used as a high-resolution objective lens cannot exhibit its performance fully.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a scanning electron microscope capable of obtaining a high-resolution secondary electron image and a high-brightness backscattered electron image from the same field of view and of performing high-sensitivity X-ray analysis.
The present invention provides a scanning electron microscope comprising: an electron gun for emitting an electron beam; a system of condenser lenses for focusing the electron beam emitted from the electron gun; a scanning means for scanning a specimen with the electron beam emitted from the electron gun; an objective lens having inner and outer magnetic polepieces to form a magnetic field lens below the lower ends of the polepieces; and a backscattered electron detector for detecting electrons backscattered out of the specimen. The inner and outer magnetic polepieces are provided with bores via which the backscattered electron detector can be withdrawably inserted into the electron beam path within the objective lens.
The present invention also provides a scanning electron microscope comprising: an electron gun for emitting an electron beam; a system of condenser lenses for focusing the electron beam emitted from the electron gun; a scanning means for scanning a specimen with the electron beam emitted from the electron gun; an objective lens having inner and outer magnetic polepieces to form a magnetic field lens below the lower ends of the polepieces; and an X-ray detector for detecting X-rays emitted from the specimen. The inner and outer magnetic polepieces are provided with bores via which the X-ray detector can be withdrawably inserted into the electron beam path within the objective lens.
In addition, the present invention provides a scanning electron microscope comprising: an electron gun for emitting an electron beam; a system of condenser lenses for focusing the ele

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