Scanning electron microscope

A scanning electron microscope comprising an objective lens for forming lens magnetic field on the sample side, and observing the image of the sample after detecting the secondary electrons from the sample on the upper side of the objective lens is disclosed. The accelerating electrode is arranged along the electron beam passage of the objective lens, and an positive potential is applied thereto. The electric field correction electrode is disposed outside the accelerating electrode or to the sample side. A negative potential is applied to the electric field correction electrode. An image observation with high resolution also is realized even when the sample is inclined.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention is related to a scanning electron microscope, and 
particularly to a scanning electron microscope having a mechanism for 
including. 
2. Description of the Prior Art 
As a method for obtaining a high resolution image by a scanning electron 
microscope, conventionally have been known a method of so-called an inlens 
type wherein a sample is disposed between magnetic poles of an objective 
lens and a method wherein an outer size of a magnetic pole in the 
objective lens is made smaller than an inner size of the objective lens, 
and the lens magnetic field is generated on the side of the sample by 
protruding an under surface of the inner magnetic pole to the sample side 
as same as the under surface of the outer magnetic pole or more. 
As is described in U.S. Pat. No. 4,713,543, also known is a method for 
reducing an aberration of the objective lens 
wherein a high positive potential is applied by arranging electrodes in 
axisymmetry on an electron beam passage from an electron beam gun to the 
objective lens, and wherein energy of a primary electrons beam at the time 
of passing the objective lens is made higher than another energy (final 
accelerating voltage) at the time of reaching the sample. In the above 
method, a secondary electrons generated from the sample is wound up by the 
magnetic field of the objective lens or is accelerated by an applied 
voltage of the electrode arranged in the electron beam passage of the 
objective lens, and as a result, the secondary electrons moves to the 
upper part of the objective lens (the electronic source side). Therefore, 
it is necessary to detect the secondary electrons on the upper part of the 
objective lens. Especially, in the objective lens generating the lens 
magnetic field on the sample side, the secondary electrons that moves to 
the upper part of the objective lens emanates when the magnetic field of 
the objective lens extincts. 
To detect such a secondary electrons efficiently, the electrode 
(accelerating electrode) for accelerating the secondary electrons is 
arranged to the primary electrons beam passage of the magnetic pole of 
objective lens, and a positive voltage is applied. However, in the above 
prior art, an electric field occurs between the sample and the 
accelerating electrode by the positive voltage applied to the electrode 
(accelerating electrode) that accelerates the secondary electrons, or by 
another positive voltage applied to the electrode (accelerating electrode) 
that accelerates the primary electrons beam for the purpose of reducing 
the aberration. 
When the sample is not inclined, the electric field between the 
accelerating electrode and the sample becomes axisymmetric. Consequently, 
due to the effects of the electric field and the magnetic field of the 
objective lens, the secondary electrons occurred from the sample is 
efficiently guided to a secondary electrons detector on the upper part of 
the objective lens. When the sample is inclined, a distance between the 
accelerating electrode and the sample changes and places of a strong 
electric field and a weak one occur. Therefore, the secondary electrons is 
deflected in the direction that orthogonalizes an optic axis. As a result, 
there is a fault that the secondary electrons generated from the sample 
does not reach the secondary electrons detector arranged on the upper part 
of the objective lens by colliding with a wall on the way of passing the 
objective lens. This as asymmetric field becomes a cause of the 
aberration. When the electrode is arranged along the electron beam passage 
of the objective lens magnetic pole, a diameter of the lower part of the 
objective lens (horizontal part) becomes large. Therefore, when the sample 
is inclined by wide angles high angle, the distance (working distance) 
between the under surface of the objective lens and the sample become 
long, so that a resolution power reduces. On the other hand, an arranging 
the electrode to the lower part of the objective lens and an application 
of a negative voltage to this electrode are disclosed in Japanese Patent 
Granted Print No. 60-9543 for example. Because the purpose of the above 
method is energy discrimination of the secondary electrons, it is 
necessary to make the voltage of the electrode 0 (zero) v in the image 
observation of ordinary secondary electrons without energy discrimination, 
the emanation of the secondary electrons after the magnetic field of the 
objective lens extincts cannot be repressed. 
SUMMARY OF THE INVENTION 
A main object of this invention is to provide a scanning electron 
microscope, which detects a secondary electrons occurred from a sample on 
the upper part of an objective lens, that realizes a short working 
distance having a high resolution observation without reducing an 
efficiency of the secondary electrons detection even if the sample is 
inclined. 
Another object of this invention is to realize an automatic high resolution 
observation even when such an observation condition of the sample as an 
accelerating voltage of a secondary electrons beam or inclination angle is 
changeable. 
A main feature of this invention is that an electrode for correcting an 
electric field is disposed outside the accelerating electrode of the 
secondary electrons close to the sample or a sample side, and a negative 
voltage is applied to this electrode for correcting the electric field. 
Another feature of this invention is to automatically change the voltage to 
be applied to the above electrode in response to the voltage applied to 
the accelerating electrode of the above secondary electrons or the 
inclined angle in the above sample. 
The above objects, features, other objects and features of this invention 
are explained according to following examples.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Before explaining a concrete constitution of this invention, the principle 
of this invention is explained. When a sample is inclined, a deflection of 
a secondary electrons by an electric field generated between an 
accelerating electrode to which a positive voltage is applied and the 
sample is offset by the deflection effect of the electric field occurred 
between the electric field correction electrode, which is arranged on the 
same axis with accelerating electrode, a negative voltage and the sample. 
Therefore, the secondary electrons is not deflected between the electrode 
and the sample, and the secondary electrons can enter the accelerating 
electrode without deflecting. As the internal electric potential of the 
accelerating electrode is axisymmetric, the secondary electrons entered 
into the accelerating electrode is accelerated without deflecting in the 
orthogonal direction of the optic axis. Therefore, the secondary electrons 
can be detected in the same state as the sample is not inclined. 
FIGS. 2a to 2c explain the above by the change of an image pickup sight of 
a scanning electron microscope. A conventional scanning electron 
microscope is shown in FIG. 2a which has the accelerating electrode, but 
does not have the electric field correction electrode of this invention, 
wherein image pickup sight 80 when the inclination angel of the sample is 
zero, and light area 81 is in the sight center. When the sample is 
inclined in this state, light area 81 is deviated from the center of sight 
80 as is shown by an arrow in FIG. 2b. This is because the electric field 
between the accelerating electrode and the sample becomes nonaxisymmetric 
by inclining the sample so that the secondary electrons occurred from the 
sample is deflected by the nonobjective electric field. However, when both 
the electric field correction electrode and the accelerating electrode are 
disposed and the negative voltage is applied to the electric field 
correction electrode, light area 81 that has deviated to the corner of the 
sight moves toward the sight center as shown by arrow in FIG. 2c. When the 
negative voltage is set to a proper value, light area 81 can return to the 
sight center again, so that the sample can be observed under the similar 
condition of FIG. 2a. 
In the case where the electric field correction electrode is provided, even 
if the sample is inclined, an electric potential distribution close to a 
beam irradiation point of the sample does not become asymmetrical so that 
there is an advantage that an astigmatism about a primary electrons beam 
is repressed. A deflection action of the secondary electrons produced at 
the time of the sample inclination depends on the applied voltage of the 
accelerating electrode closest to the sample. When the negative voltage 
applied to the electric field correction electrode is responsive to the 
applied voltage of the accelerating electrode, the voltages are controlled 
so that a predetermined relationship (relationship to offset the 
deflection action of the secondary electrons) comes into existence. By the 
above relationship, highly efficient secondary electrons detection can be 
done without deflection action by the secondary electrons at the time of 
the sample inclination, even though the applied voltage of the 
accelerating electrode is changed according to an accelerating voltage or 
working distance. 
When the accelerating electrode closest to the sample among the electrodes 
is constituted by a magnetic body so as to function as a magnetic pole of 
an objective lens, a lower diameter of the objective lens can be made 
smaller, and high resolution power is obtained because of shortening the 
working distance at the time of the sample inclination. 
This invention is explained in detail by a concrete example as follows. 
FIG. 1 is a structure diagram of an example of the scanning electron 
microscope according to the present invention. Voltage V1 is applied 
between cathode 1 and first anode 2 from high-voltage control power source 
40 controlled by micro processor (CPU) 50, and a fixed emission current is 
drawn from cathode 1. Accelerating voltage Vacc is applied between cathode 
1 and secondary anode 3 from a high-voltage control power source 
controlled by CPU 50, and primary electrons beam 4 emitted from cathode 1 
is accelerated to voltage Vacc. Primary electrons beam moves towards the 
lens system in the later step. Primary electrons beam 4 is focused onto 
sample 7 as a minute spot by focusing lens 5 and objective lens 6 that are 
controlled in lens control power 16, and primary electrons beam 4 is 
scanned two-dimensionally with deflection coil 8 on the surface of sample 
7. The scanning signal of deflection coil 8 is controlled by deflection 
control circuit 15 according to an observation magnification. The focusing 
angle (beam divergence angle) of the primary electrons beam is decided by 
throttle 14 of the objective lens. Sample stage 70 that supports sample 7 
is provided with functions for moving the sample in a horizontal direction 
and for inclining the sample. 
Axisymmetric accelerating electrodes 10a and 10b are arranged on an 
electron beam passage of objective lens 6, to which positive voltages Va1 
and Va2 are applied by voltage control power sources 31a and 31b. After 
primary electrons beam 4 is further accelerated by voltages Va1 and Va2 
applied to accelerating electrodes 10a and 10b, primary electrons beam 4 
is decelerated to former energy (Vacc) between objective lens 6 and sample 
7 and is irradiated to sample 7. The magnetic pole of objective lens 6 has 
a hole diameter of an outer magnetic pole which is larger than that of an 
inner magnetic pole and the tip of the inner magnetic pole is more 
protruded to the sample than that of the outer magnetic pole, so as to 
generate lens magnetic field 100 at the sample side. When positive 
voltages Va1 and Va2 are applied to accelerating electrodes 10a and 10b, 
respectively, primary electrons beam 4 passes through objective lens 
magnetic field 100 with energy higher than Vacc, so that the lens 
aberration is reduced. 
Secondary electrons 9 generated from sample 7 by irradiating electron beam 
4 is trapped by the magnetic field of objective lens 6 and moves towards 
the upper side of the objective lens. After being accelerated by 
accelerating electrode 10b, the electron is focused by the action of an 
electrostatic lens formed in the gap between accelerating electrode 10a 
and accelerating electrode 10b. Therefore, secondary electrons 9 does not 
emanate in the area where not influenced by the magnetic field of the 
objective lens, and secondary electrons 9 is detected by secondary 
electrons detector 20 after moving towards the upper part of objective 
lens 6. The electrostatic lens can be formed in the boundary area even 
when either voltage Va1 or Va2 is higher or both are equal. Voltages Va1 
and Va2 are set suitably according to the values of accelerating voltage 
Vacc and the value of the working distance. The signal of secondary 
electrons detector 20 is taken into image memory 51 through signal 
amplifier 17 and is displayed as an expansion image of the sample on image 
display 18. Each control circuit controlling the components of the 
electronic optic system and the control power source are 
central-controlled by above CPU 50. 
Axisymmetric electric field correction electrode 11 is arranged to the 
outside of accelerating electrode 10b provided to objective lens 6 and is 
applied with negative voltage Vb by control power 32. Voltage Vb of 
electric field correction electrode 11 is set to the value so as to offset 
the deflection action resulted from accelerating electrode 10b when sample 
7 is inclined. The relationship between Vb and Va2 that offsets the 
deflection action of this secondary electrons can be in advance determined 
by experiments or simulation and can be set by control CPU 50 to fulfill 
the relationship. 
The action of electric field correction electrode 11 is explained according 
to FIG. 3, FIG. 4 and FIG. 5. 
FIG. 3 shows a cross section of the objective lens at the left side and the 
electric potential distribution on the optic axis o at the right. The 
electric potential of the position of sample 7 is 0 (zero). The electric 
potential on the optic axis o close to sample 7 changes according to 
voltage Vb applied to electric field correction electrode 11. The area 
where the electric potential gradient exists acts as the electrostatic 
lens, and the area conducts the action that focuses the secondary 
electrons occurred from sample 7. Curve a in the figure shows the electric 
potential distribution when there is no electric field correction 
electrode 11. The electric potential distribution on the optic axis o 
changes as shown by curves b and c as the absolute value of negative 
voltage Vb applied to electric field correction electrode 11 increases. 
Curve b shows the electric potential distribution when the value of 
voltage Vb is proper and curve c the electric potential distribution when 
the absolute value of voltage Vb is too large, respectively. Because the 
area where the electric potential becomes negative is formed on the optic 
axis o in electric potential distribution c, the secondary electrons 
occurred from the sample cannot be pumped up into objective lens 6, 
therefore the secondary electrons cannot be detected. 
FIG. 4 shows an equipotential lines at the time of the sample inclination 
without electric field correction electrode 11. As is shown in FIG. 4, 
when there is no electric field correction electrode 11, equipotential 
lines 101 at the time of the sample inclination becomes asymmetrical 
because the equipotential lines protrudes in the inclined direction of 
sample 7. Therefore, secondary electrons 9 occurred from sample 7 is not 
only accelerated in the direction of the optic axis, but also receives a 
deflection action in the orthogonal direction (direction of sample 
inclination) of the optic axis. As a result, almost all of secondary 
electrons 9 collide with inner wall of accelerating electrodes 10a and 
10b, and do not reach secondary electron detector 20. 
FIG. 5 shows equipotential lines 101 at the time of the sample inclination, 
wherein electric field correction electrode 11 is arranged outside 
accelerating electrode 10b and negative voltage Vb is applied. As negative 
voltage Vb is applied to electric field correction electrode 11, a 
direction of the secondary electrons deflected by negative voltage Vb at 
the time of the sample inclination is opposite to a direction of 
accelerating electrode 10b deflected by positive voltage Va2. 
Consequently, by selecting the value of voltage Vb so as to offset the 
deflection action between the secondary electrons of negative voltage Vb 
and secondary electrons of positive voltage Va2, the axisymmetry of the 
electric potential distribution of the sample vicinity of both electrodes 
is improved as shown in FIG. 5. Secondary electrons 9 move towards the 
upper part of objective lens 6 and is detected efficiently by secondary 
electron detector 20 without being deflected in the direction that 
secondary electrons 9 orthogonalize the optic axis. 
FIG. 6 is a diagram showing an example of the applied voltage to each 
electrode, wherein the vertical axis shows the applied voltage of the 
electrode and the horizontal axis shows sample inclination angle. FIG. 6 
also shows the relationship between voltage Va1, Va2 and Vb that are 
applied to each of electrodes 10a, 10b and 11 and the sample inclination 
angle when accelerating voltage Vacc is 1 kV. It also shows the 
relationship between voltage V'a1, V'a2 and V'b that are applied to each 
of electrodes 10a, 10b and 11 and the sample inclination angle when 
acceleration voltage Vacc is 10 kV. While voltages Vb and V'b are 
negative, the values are shown by absolute values for simplification in 
FIG. 6. When accelerating voltage Vacc is 1 kV, a magnetic field intensity 
of the objective lens that is necessary for focusing the primary electrons 
beam is small and the magnetic field intensity leading the secondary 
electrons, which occurred from the sample, to the secondary electron 
accelerating electrode of the objective lens is also weak. Therefore, 
applied voltage Va2 of secondary electron accelerating electrode 10b 
closest to the sample needs to set to the value higher than value V'a2 to 
compensate for the weakness of the magnetic field of objective lens 6 when 
the accelerating voltage is 10 kV. That is, voltage Vb of electric field 
correction electrode 11 is set to be high when accelerating voltage Vacc 
is 1 kV, but is set to low value V'b when accelerating voltage Vacc is 10 
kV. 
Since optimum value of the applied voltage of secondary electron 
accelerating electrode 10b depends on both the distance between sample 7 
and accelerating electrode 10b and the strength of the magnetic field 
between them, the optimum condition is found out for every accelerating 
voltage Vacc and the working distance. On the other hand, applied voltage 
Vb of electric field correction electrode 11 depends on applied voltage 
Va2 of accelerating electrode 10b closest to sample 7, and after deciding 
applied voltage Va2 to accelerating electrode 10b, the optimum value is 
determined so as to avoid the loss of the secondary electrons due to the 
sample inclination. As applied voltage Va1 to other accelerating electrode 
10a that are not close to the sample does not change in accordance with 
accelerating voltage Vacc and the working distance, applied voltage Va1 
can be made a constant value. This is because the energy of secondary 
electrons 9 does not depend on accelerating voltage Vacc or the working 
distance. 
In the case where the sample inclination angle is in the range from 0 
(zero) to around 45.degree., even if applied voltage Vb to electric field 
correction electrode 11 is made constant, the secondary electrons occurred 
from the sample is efficiently guided to secondary electron detector 20 so 
that a high resolution image can be obtained. However, when further 
enlarging the sample inclination, it is useful for highly efficient 
detection of the secondary electrons to control voltage Vb applied to 
electric field correction electrode 11 according to the sample inclination 
angle, as is shown in FIG. 6. 
The relationships described above can be obtained by experiments or 
numerical value simulations and can be memorized in the form of relational 
expressions or of tables. The relationships include: 
(1) Optimum value Va2 for accelerating voltage Vacc and the working 
distance; 
(2) Relationship between Va2 and Vb to offset the deflection action of the 
secondary electrons at the time of the sample inclination; and 
(3) Optimum voltage Vb applied to the large sample inclination angle. CPU 
50 most suitably controls power 31b and 32 by referring to those 
relational expressions or tables. 
Next, explained is another example of the accelerating electrode and the 
electric field correction electrode that are arranged along the electron 
beam passage of the objective lens. 
FIG. 7 is an example of accelerating electrodes 10a and 10b that are 
overlapped partially. In this case, it is necessary to make a difference 
between applied voltage Va1 to accelerating electrode 10a and applied 
voltage Va2 to accelerating electrode 10b to form the electrostatic lens 
at the boundary between both accelerating electrodes 10a and 10b. 
FIG. 8 is an example wherein one accelerating electrode 10 is disposed 
along the electron beam passage of objective lens 6. In this case, 
electric field correction electrode 11 is positioned below the inner 
magnetic pole of objective lens 6. In case of this example, there is a 
limit of the function that suppresses an emanation of the secondary 
electrons, the range of corresponding accelerating voltage Vacc or the 
working distance is limited. In this example, the flexibility of 
adjustment is limited as compared with a case that 2 or more accelerating 
electrodes are disposed, but a high resolution observation can be done in 
the state that the sample is inclined as same as other examples. 
FIG. 9 is an example of accelerating electrode 10b constituted by a 
magnetic body. This electrode 10b of magnetic body is connected 
magnetically with inner magnetic path 6a of objective lens 6, and it 
functions as the inner magnetic pole of objective lens 6. Electric field 
correction electrode 11 is arranged so as not to disturb the inclination 
of the sample at outer electrode 10b of magnetic body. According to this 
example, the under surface of accelerating electrode 10b becomes the lower 
surface of the objective lens, and the high resolution observation can be 
done in a short working distance even if the sample is inclined. As shown 
in FIG. 10, magnetic pole 10b of magnetic body can be made a structure 
that covers an upper part with inner magnetic path 6a for improving the 
assembling precision of objective lens 6 to inner magnetic path 6a. 
According to this invention, the short working distance can be realized 
even if the sample is inclined by wide angles, and a lens aberration can 
be made small. Since a high detection efficiency of the secondary 
electrons is acquired, the high resolution observation can be done by 
inclining the sample by wide angles.