JPS5973840A - Objective lens for scanning electron microscope - Google Patents

Abstract

PURPOSE:To improve the secondary electron detection efficiency by fitting an air-core coil generating a magnetic field nearly offsetting the magnetic field leaking downward from a magnetic pole hole to a lower magnetic pole. CONSTITUTION:A thin air-core coil 9 is fitted symmetrically to the axis at the lower side of a lower magnetic pole 2 so as to generate a magnetic field offsetting the magnetic field leaked from the lower magnetic pole 2. This magnetic field (a) is generated in the opposite direction to a lens magnetic field and has a magnetic field distribution as shown in the diagram. According to this reverse magnetic field, the lens magnetic field with an original distribution as shown by a dotted line is corrected as shown by a solid line (b) and is barely generated under the lower magnetic pole 2. Since the air-core coil 9 is in contact with the lower magnetic pole 2 on one side, its magnetic resistance becomes one half of that when placed in a free space, thereby the strength of the generated magnetic field is doubled with the same exciting current. Accordingly, the secondary electron detection efficiency is improved, and the secondary electron sample with high resolution can be observed.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an objective lens for a scanning electron microscope that can significantly improve secondary electron detection efficiency.

In a scanning electron microscope, an electron beam generated by an electron gun is focused onto the sample surface using an objective lens, and various signals generated from the sample are detected by scanning the electron beam two-dimensionally using a deflection coil. The sample surface is observed by introducing the signal A as a brightness modulation signal 8 into a cathode ray tube synchronized with the scanning. In such a device, the sample (
The objective lens is generally asymmetrical because it is placed below the objective lens, and the hole diameter of the upper magnetic pole is 40 to 50 mm.
On the other hand, the hole diameter of the lower magnetic pole is 10 to 2'Qmm. Can lens aberration be reduced by increasing the diameter of the hole in the bottom magnetic pole? If the diameter of the hole in the bottom magnetic pole is made too small, the lens magnetic field leaking downward (toward the sample) through the hole J will increase. Therefore, on the platform where low-energy electrons (secondary electrons) from the sample form an image of the sample surface, the secondary electrons are restrained by the above-mentioned 1liil leakage magnetic field, resulting in a significant decrease in detection efficiency. (If the distance between the lower magnetic pole and the sample (hereinafter referred to as WD) is shortened, lens aberrations will decrease and resolution will improve, but the strength of the leakage magnetic field at the sample surface will increase, so The secondary electrons of are restrained by the magnetic field, −
The light is absorbed by the magnetic field or by the lens member located further above, and the detection efficiency is also reduced.

FIG. 1 is a cross-sectional view of an example of an objective lens of a conventional scanning electron microscope, and FIG. 2 shows the axial magnetic field distribution of the lens.

In the figure, 1 is the lower magnetic pole of the objective lens, 2 is the lower magnetic pole, and both magnetic poles are positively coupled by -1 and 3. Reference numeral 4 denotes an excitation coil, which is housed in an Element and supplies a predetermined current to a lens (
It is for forming the 6 realms. Reference numeral 5 denotes a sample, which is placed in the vicinity of or below the lower magnetic pole, and can be freely moved in a plane perpendicular to the electron beam axis by a known sample moving mechanism (not shown), 6 is a secondary electron detector C' such as a photomultiplier tube, which is installed in the horizontal direction of the sample. A high voltage is applied for this purpose.

Therefore, the electric field generated by the voltage generates 2
The next electron 7 is accelerated while being deflected as shown by the arrow, and collides with the scintillator of the detector. However, the second
As can be seen from the figure, the lens magnetic field traces its tail considerably below the lower tiIi pole, and when the peak value of the objective lens magnetic field is actually 500 Gauss, WD=Omm (lower magnetic pole) is approximately 100 Gauss, W[ )=about 30 force at 5mm, WD
= 1 On + 01, there is a leakage branch field of about 10 cous (r), and the restraining force of this magnetic field is strong, so despite the electric field il+ from the detector, the secondary electron σ part is as shown by arrow 8. Because it flies upwards, its detection efficiency is low.

However, the invention of the C1 tree aims to solve and overcome the above-mentioned drawbacks.
In the lens, which comprises an excitation coil for magnetically coupling the two magnetic poles and forming a lens magnetic field in the gap between the two, the sample is placed near or below the lower magnetic pole. An air-core coil that generates a magnetic field of 4 inches is attached to the lower pole to substantially cancel out the magnetic field leaking downward from the anode hole of the lower magnetic pole. Figure 3 is a cross-sectional view of one embodiment of the present invention, and the same components as in Figure 1 are indicated at j' and T.
are installed axially symmetrically, and this air-core coil generates a magnetic field that offsets the leakage magnetic field from the lower magnetic pole.

The magnetic field is generated in the opposite direction to the lens magnetic field, and the magnetic field is shifted to the right as shown in a in FIG. The four magnetic fluxes, which originally have a distribution like the dotted line, are modified, and almost no /υ is generated below the bottom magnetic pole.The air-core coil has one side in contact with the bottom magnetic pole, When placed in a free space, the strength of the magnetic field generated by C is approximately 218 minutes even with the same excitation current.

Now, if the number of turns of the coil is 40 turns, the supply current is 1△, the radius of the coil is Icm, and the I'F is 201nl,
The strength of the correction magnetic field is approximately 5 near W D = 1 m + J = J
Since the polarity of the correction magnetic field is opposite to that of the lens magnetic field, since WD = 5 mm, the polarity of the correction magnetic field is opposite to that of the lens magnetic field. This means that the leakage magnetic field is approximately equal to Canrel C.

Therefore, the secondary electrons generated from the sample are hardly restrained by the leakage magnetic field from the objective lens, and are attracted only by the electric field from the detector, greatly improving the detection efficiency. .

The supplementary i [(1! .But (, Ninha,
The power supply is varied and the sample surface is
Leakage of Rurusu j object 14i field and correction (・Ii world and X or abbreviation J, sea urchin adjusted to one line LU W D
It is possible to maintain the detection efficiency of secondary electrons regardless of the size of the secondary electrons.

By installing the air-core coil 9,
ZOi W D! ,) In order to effectively capture secondary electrons, it becomes possible to irradiate the sample with a thinner electron probe because the WD is shortened and the aberration of the single-object lens is reduced. Therefore, there is an effect that high-resolution secondary electron sample images can be observed in both C and Hatake.

[Brief explanation of the drawing]

Figure 1 is a sectional view of a conventional objective lens, Figure 2 is a diagram showing its axial magnetic field distribution, Figure 3 is a sectional view of the objective lens of the present invention, and Figure 4 is an axial magnetic field distribution of A. FIG. 1: tt+i pole, 2: lower f! & pole, 3: 3 j-ri, 4:
Excitation coil, 5: sample, 6: secondary electron detector, 7: secondary electron, 9: air-core coil. Patent applicant ■Representative of Hondenshi Co., Ltd. Right Ito

Claims (1)

[Claims] In a lens consisting of an upper magnetic pole, a lower magnetic pole, a yoke for magnetically coupling both magnetic poles, and an excitation coil for forming a lens rii & field in the gap between the two magnetic poles, the sample is placed near or below the lower magnetic pole. An objective lens for a scanning electron microscope, characterized in that an air-core coil is attached to the lower magnetic pole to generate a magnetic field having a value that substantially cancels out a magnetic field leaking downward from the magnetic pole hole of the lower magnetic pole.