JPH08321273A - Objective lens of electromagnetic field superposed type - Google Patents

Abstract

PURPOSE: To lessen the spherical aberration and chromatic aberration and accomplish a high resolution objective lens of electromagnetic field superposed type. CONSTITUTION: When current is fed to a coil 1, a magnetic field in axial symmetry is produced inside a magnetic field lens. The obtained flux density distribution on the axis will have a peak around the bottom of the magnetic pole 11 of a magnetic substance. Therefore, the lens main surface as magnetic field lens approaches the specimen, and a small aberration is generated. When a high voltage is impressed on a high resistance film 15 from a power supply 16, a minute current flows from the top to bottom of the film 15, and a voltage effect is generated in the film longitudinal direction. Because the shape and thickness of the film 15 are designed symmetrical about the optical axis, the isopotential plane will be in axial symmetry.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high resolution electromagnetic field superimposing type objective lens which is optimal for use in an objective lens of a scanning electron microscope or the like.

[0002]

2. Description of the Related Art In recent years, with the miniaturization and complexity of semiconductor elements, it becomes necessary to observe large and flat objects such as wafers and masks from all directions for shape inspection in the manufacturing process of these elements. ing. As an objective lens for a scanning electron microscope which meets this purpose, the present inventor has put the lens described in Japanese Patent Application Laid-Open No. 1-298566 into practical use and has already obtained a certain result.

The invention of this prior application is shown in FIG. In the figure, 1 is a conical bobbin having an electron beam passage hole P, 2 is a coil, 3 is a yoke, 4 is a flat sample such as a wafer, and a sample 4
Are inclined along the yoke 3. With this configuration, the number of turns of the coil 2 wound over the conical bobbin 1 over several layers is changed for each layer,
As a result, the distribution of the number of windings along the axis of the coil is configured to be larger on the electron beam emitting side of the objective lens than on the electron beam incident side. With this configuration, the lens main surface and the magnetic field peak can be brought closer to the sample 4.

The configuration shown in FIG. 2 is a modification of the configuration shown in FIG. 1, and the same numbers as those in FIG. 1 indicate the same components. In the configuration of FIG. 2, a funnel-shaped bobbin 5 is provided as the bobbin. Then, the number of turns of the coil 2 wound over several layers on the outer side (hatched portion) of the funnel-shaped bobbin 5 is changed for each layer so that the coil outer surface formed by the coil 2 is formed into a substantially conical shape. I am configuring. Therefore, the distribution of the number of windings along the axis of the coil 2 gradually decreases toward the electron beam incident side and the electron beam outgoing side of the objective lens.

In the case of the axial magnetic field distribution of the objective lens shown in FIG. 2, compared with the axial magnetic field distribution of the objective lens of FIG. 1, the lens main surface and the magnetic field peak are closer to the sample 4, so that the lens performance is further improved. improves.

The above-described objective lens shown in FIGS. 1 and 2 has an outer shape of a truncated cone with an apex angle of 60 °.
This is because the sample can be tilted up to 60 °, and the winding distribution of the coil is increased on the top surface of the truncated cone and decreased toward the bottom surface. With this configuration,
The main surface of the objective lens was made as close to the top surface of the truncated cone as possible. As is well known, the aberration of the magnetic field type lens becomes smaller as the lens main surface is closer to the sample.
A small aberration can be achieved even when tilted at 0 °.

However, the improvement in the degree of integration of semiconductors has progressed rapidly, and the scanning electron microscope has been required to further improve the resolution. FIG. 3 shows a cross-sectional view of a recently proposed electromagnetic field superimposing type objective lens, which is a superposition of a magnetic field lens and an electric field lens (deceleration bipotential type).

In FIG. 3, 6 is a lower yoke of the magnetic lens, 7 is an upper yoke, and 8 is a coil. One electrode 9 of the electric field lens is arranged inside the upper yoke 7 of the magnetic field lens, and the electrode 9 is formed in a conical tube shape. Further, the other electrode 10 of the electric field lens is provided at the tip portion of the lower yoke 6, and a voltage is applied between the electrodes 9 and 10 (not shown).

FIG. 3 shows a cross section of the objective lens. In reality, the cross section has a bilaterally symmetrical shape. The magnetic lens and the equipotential lines of the magnetic field are shown.

This lens is (4 + α) m from the tip of the electrode.
At the m position, the sample can be tilted up to 50 °. Note that this α is a mechanical clearance so that the sample does not come into contact with the outer wall of the lens when the sample is tilted by 50 °.
In this lens, for example, when the working distance WD is 4 mm and the reduction ratio is Vi / Vo = 1/17, spherical aberration Cs = 3.7 mm and chromatic aberration Cc =
It is said that 1.8 mm was obtained. Note that Vi is the acceleration voltage of the electron beam (= potential of the sample seen from the emitter), Vo
Is an intermediate acceleration voltage (emitter reference).

[0011]

The above-mentioned aberration value of the objective lens having the configuration of FIG. 3 is about 1/10 that of the lens shown in FIGS. 1 and 2, which is very high performance. It has the drawbacks described in. First of all, the sample is 50 °
It can only be tilted to a certain degree. The larger the tiltable angle of the sample, the more information obtained from the scanning electron microscope observation, and it is considered to be insufficient at 50 °, and about 60 ° is desirable.

Secondly, the sample cannot be redesigned to be tilted up to 60 ° for the following four reasons. If the inner diameter of the upper magnetic pole 7 and the tip position are kept the same when the lower magnetic pole 6 has an apex angle of 60 °, the lower magnetic pole 6 and the lower magnetic pole 6 come too close to each other, and magnetic saturation occurs at a relatively low excitation. Therefore, the maximum usable acceleration voltage is inevitably low.

For the above reason, if the tip position is raised upward, magnetic saturation can be avoided, but the main surface of the magnetic lens moves upward, and the performance as a superposed field lens deteriorates.

For the above reason, if the inner diameter of the upper magnetic pole 7 is made small, magnetic saturation can be avoided, but this time it will come close to the high voltage electrode and discharge will easily occur, which is not practical. If the inner diameter of the electrode is reduced,
Discharge can be avoided, but the lens performance will deteriorate significantly.

For the above reason, the discharge can be avoided by lowering the upper electrode voltage Vo, but the reduction ratio Vi /
Vo becomes large and the performance is significantly deteriorated. Third, the performance of the magnetic field lens alone is worse than that of the lens shown in FIGS. In the lens shown in FIG. 3, the ground voltage (Vo-Vi) of the high voltage pipe is constant because of the withstand voltage of the electrodes. Therefore, when the acceleration voltage Vi is increased, the reduction ratio Vi / Vo increases. For example, Vi = 50
When 0V and Vo-Vi = 8kV, Vi / Vo = 1 /
17, but when Vi = 15 kV, Vi / Vo =
It increases to 15/23 = 1 / 1.53. In that case, the performance of the superposed field lens is almost equal to that of the magnetic field alone lens. The magnetic field lens of FIG. 3 has a traditional shape before the appearance of the lens of the shapes of FIG. 1 and FIG. 2 and is not a pursuit of maximum performance in a 50 ° tilted state as a magnetic field lens. Of course.

FIG. 4 shows the high voltage pipe ground voltage (Vo-V).
i) is fixed at 8 kV and Vi is changed from 0.5 kV to 15 kV
It is a graph of Cs and Cc when varied up to. Although two curves are drawn for both Cs and Cc in this graph, the curve A corresponds to the case where the sample is placed at the position where the cone vertex and the optical axis intersect (working distance WD = 4 mm), and the curve B Corresponds to when the sample is lowered by 2 mm from that position.

In the figure, the alternate long and short dash line indicates the values of Cs and Cc at the 60-degree tiltable position (position including mechanical clearance) in the lens having the configuration of FIG. As can be seen from this graph, when the acceleration voltage is 4.4 kV or higher,
Cs is worse than the lens of FIG. This is because the performance of the magnetic field lens to be superimposed is poor.

The present invention has been made in view of the above circumstances, and an object thereof is to realize a high resolution electromagnetic field superimposing type objective lens by reducing the values of Cs and Cc.

[0019]

According to a first aspect of the present invention, there is provided a magnetic pole having a substantially conical shape, a coil arranged inside the magnetic pole, an insulator arranged inside the coil, and an electron beam. A high resistance body is provided inside the insulator so as to surround the passage, and a voltage is applied to both ends of the high resistance body.

The invention of claim 2 is characterized in that, in the electromagnetic field superimposing type objective lens of claim 1, the high resistance is formed in a film shape. According to a third aspect of the present invention, in the electromagnetic field superimposing type objective lens according to the first aspect, the high resistance body is formed in a pipe shape.

According to a fourth aspect of the present invention, in the electromagnetic field superposition type objective lens according to the first aspect, a large number of conductive pipes are arranged inside the high resistance body. Claim 5
The present invention is characterized in that, in the electromagnetic field superimposing type objective lens described in claims 1 to 4, a material for heat dissipation is provided between the coil and the high resistance body.

According to a sixth aspect of the present invention, a magnetic pole formed in a substantially conical shape, a coil arranged inside this magnetic pole, an insulator arranged inside the coil, and a ring-shaped inside the insulator. It is characterized in that it is provided with a member in which a high resistance body and a conductor are laminated, and a voltage is applied to both ends of this member.

According to a seventh aspect of the present invention, a magnetic pole is formed in a substantially conical shape, a coil is arranged inside the magnetic pole, an insulator is arranged inside the coil, and a large number of rings are arranged inside the insulator. It is characterized in that a metal is sandwiched between the ring-shaped high resistance bodies, and a voltage is applied to both ends of the member.

[0024]

According to the invention of claim 1, the magnetic pole is formed in a substantially conical shape, the coil is arranged inside the magnetic pole, the insulator is arranged inside the coil, and the magnetic field is insulated so as to surround the electron beam passage. It has a high-resistor arranged inside the body, and is configured to apply a voltage across the high-resistor so that the electrostatic lens action occurs on the sample side from the magnetic flux density peak position of the magnetic lens. .

According to a second aspect of the invention, in the electromagnetic field superimposing type objective lens according to the first aspect, the high resistance body is formed in a film shape. According to a third aspect of the present invention, in the electromagnetic field superimposing type objective lens according to the first aspect, the high resistance body is formed in a pipe shape.

According to a fourth aspect of the present invention, in the electromagnetic field superimposing type objective lens according to the first aspect, a large number of conductive pipes are arranged inside the high resistance body, and an equipotential surface due to nonuniformity of the high resistance body. I corrected the disturbance.

According to a fifth aspect of the present invention, in the electromagnetic field superimposing type objective lens according to the first to fourth aspects, a material for radiating heat is provided between the coil and the high resistance element to efficiently dissipate heat generated by the coil.

According to a sixth aspect of the present invention, the magnetic pole is formed in a substantially conical shape, the coil is arranged inside the magnetic pole, the insulator is arranged inside the coil, and the ring-shaped inside the insulator. A member in which a high resistance member and a conductor are laminated is provided, and a voltage is applied to both ends of this member so that the electrostatic lens action occurs on the sample side from the magnetic flux density peak position of the magnetic field lens. .

According to a seventh aspect of the present invention, a magnetic pole is formed in a substantially conical shape, a coil is arranged inside the magnetic pole, an insulator is arranged inside the coil, and a large number of rings are arranged inside the insulator. It is equipped with a member in which a ring-shaped high resistance body is laminated, and a metal is sandwiched between the ring-shaped high resistance bodies, and a voltage is applied to both ends of the member. The magnetic flux density of the lens is set to occur on the sample side from the peak position.

[0030]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 5 shows an electromagnetic field superimposing type objective lens according to the present invention, and 11 is a magnetic pole formed of a conical magnetic body. The conical magnetic pole 11 is formed thin, and the coil 12 is wound in a thin shape along the inner wall portion of the magnetic pole 11. Inside the coil 12, a heat radiating member 13 made of copper or the like that also serves as a winding frame of the coil 12 and a heat radiating path is arranged. These magnetic poles 11,
The coil 12 and the heat dissipation member 13 form a conical magnetic field lens.

A hollow insulating member 14 formed of ceramic or the like is provided inside the heat dissipation member 13 coaxially with the conical magnetic field lens. Inside the insulating member 14, a film 15 of a high resistance material such as silicon carbide is formed. A voltage is applied from a power source 16 between both ends of the film 15. As a result, a high voltage is applied to the upper end (electron gun side) of the high resistance film 15, and a ground potential or a small voltage is applied to the lower end (sample side) of the high resistance film 15.

The upper inner wall portion 17 of the insulating member 14
Has been subjected to a metallizing treatment, and the insulating member 1
An electrode 18 connected to the high resistance film 15 is provided at the lower end of 4, and the metallized portion 17 and the electrode 15 are actually connected to the power supply 16. The operation of such a configuration will be described below.

When a current is passed through the coil 12, an axially symmetric magnetic field is generated inside the magnetic lens. Obtained on-axis magnetic flux density (B
The z) distribution has a peak near the lower end of the magnetic pole 11 as shown in FIG. Therefore, the lens main surface as the magnetic field lens approaches the sample (arranged in the lower part of the lens), and a smaller aberration is obtained as compared with the lens of FIG. 3, for example.

When a high voltage is applied to the high resistance film 15 from the power source 16, a minute current flows from the upper end of the high resistance film 15 toward the lower end thereof, and the high resistance film longitudinal direction (the direction of the optical axis of the electron beam). ) Voltage drop occurs. At this time, since the shape and thickness of the high resistance film 15 are made symmetrical with respect to the optical axis, the equipotential surface is axially symmetrical. FIG. 7 shows equipotential lines (FIG. 7A) in the internal space of the high resistance film 15 and the axial potential distribution (FIG. 7A).
(A)) is shown.

In the configuration of FIG. 5, the integrand function Rcs in the integration formula of the axial magnetic flux density B, the axial potential V, the electron trajectories T and Cs, Cc when an electric field and a magnetic field are simultaneously excited.
An example of (z) and Rcc (z) is shown in FIG. In the example of FIG. 8, the ground voltage V1 applied to the upper end of the high resistance film 15 is
Is 7 kV, and the ground voltage V applied to the lower end of the high resistance film 15
2 is 0V, and the acceleration voltage Vi is calculated for an electron of 500V.

In this figure, Z = 0 represents the position where the sample can be tilted at 60 ° including the mechanical clearance. In addition,
The ampere turn (AT) required at this time is about 500 AT
Met. The values of Cs and Cc obtained in this example are
The values are 8.0 mm and 3.6 mm, respectively, which are equivalent to the values when the lens acceleration voltage Vi = 0.5 kV and α = + 2 in FIG.

Here, the axial magnetic flux density B of the lens of FIG.
FIG. 9 shows integrand functions Rcs (z) and Rcc (z) in the integration formula of the on-axis potential V, the electron orbit T, and Cs and Cc. As can be seen from this figure, the electric field is generated only between the electrodes at the tip of the lens, and the first half of this electric field acts as a diverging lens. Due to this action, the orbit of the electron once becomes large in the radial direction, and then is focused on the sample by the focusing action in the latter half.

On the other hand, the potential distribution of the lens of FIG. 5 is distributed over a wide range of the Z axis, as can be seen from FIG. The orbital divergence of this lens is extremely small and almost unrecognizable in FIG. As we all know,
Since the integral formula of the spherical aberration of the electron lens contains a component proportional to the fourth power of the orbital radius, the high resistance pipe type electrostatic lens with a small orbit divergence action is more spherical than the gap type electrostatic lens. Aberration is small.

If the upper pole and the lower pole are separated by a gap type, it is possible to widen the existence range of the electric field, but this is not the case in practice. That is, there is a magnetic lens (the same potential as the lower pole potential) around the gap, and most of the potential drop occurs near the tip of the upper pole. Therefore, lifting the upper pole to widen the gap is substantially the same as moving the entire electric field lens away from the sample, which significantly deteriorates the performance.

As can be seen from FIGS. 8 and 9, the focusing action by the electrostatic lens occurs only after the primary electrons are sufficiently decelerated. That is, the electrode 18 in FIG.
In the vicinity of the electrode 10. Since this position is set closer to the sample than the peak position of the magnetic flux density, the focal length of the compound lens is shortened and the aberration coefficient is greatly improved.

FIG. 10 shows another embodiment of the present invention. The same or similar components as those of the embodiment of FIG. 5 are designated by the same reference numerals, and detailed description thereof will be omitted. In this embodiment, a cylindrical high resistance pipe 20 is arranged inside the insulating member 14 instead of the high resistance film. Further, an electrode 21 formed by metallizing a metal electrode or the inner surface of the insulator is provided on the upper inside of the insulating member 14.

As the high resistance pipe 20, a high resistance body having a specific resistance in the range of about 10 ⁶ Ωcm to 10 ⁹ Ωcm,
For example, Si, SiC, CaZrO ₃ or the like is used. These high-resistivity substances are stable even in air, it is difficult to form an oxide insulating layer, and even if scattered electrons collide,
It has the characteristic of not being charged.

The embodiment shown in FIG. 10 has an advantage that it is easy to manufacture because only the straight tubular portion is formed by the high resistance pipe 20. Of course, this advantage similarly occurs when a high resistance film is used instead of the high resistance pipe 20. In the configuration of FIG. 10, the strongest electric field is between A and B in the figure.
A homogeneous insulator occupies a space between B, which means that the withstand voltage is higher than the vacuum gap. Therefore, a larger voltage can be applied to a narrower space than when a vacuum gap is used. / Vo A small lens becomes practical.

FIG. 11 shows another embodiment of the present invention. In this embodiment, a plurality of electric field correction metal rings 22 are arranged in parallel inside the high resistance body 20. The high-resistance pipe 20 may have a partially different thickness, so that the electric field may not be accurately formed in axial symmetry, but in this embodiment, the metal pipes arranged in parallel in axial symmetry provide high resistance. Distortion of the equipotential surface due to non-uniformity of the body is corrected.
When the metal ring 22 is formed, a large number of thin metal rings may be fitted inside the high resistance pipe 20, and then the metal rings may be separated at a constant pitch.

FIG. 12 also shows another embodiment of the present invention. In this embodiment, the high resistance ring 23 and the metal ring 24 are stacked and arranged inside the insulating member 14, and as a result, the high resistance film 15 and the high resistance film 15 in the embodiment of FIG.
The same effect as the high resistance pipe 20 in the above embodiment is obtained.

FIG. 13 also shows another embodiment of the present invention. In this embodiment, the high resistance ring 25 is laminated inside the insulating member 14, but one side or both sides of each ring is metalized 26.

[0047]

As described above, the electromagnetic field superposition type objective lens according to the invention of claim 1 has a magnetic pole formed in a substantially conical shape, a coil arranged inside the magnetic pole, and an inside of the coil. And a high-resistor arranged inside the insulator so as to surround the electron beam passage, and is configured to apply a voltage across the high-resistor, and the electrostatic lens function is provided. Occurs on the sample side from the magnetic flux density peak position of the magnetic field lens. As a result, Cs and Cc
The values of and can be made extremely small, and a high-resolution objective lens can be provided.

According to the second aspect of the invention, in the electromagnetic field superposition type objective lens of the first aspect, since the high resistance body is formed in a film shape, the same effect as that of the first aspect of the invention can be obtained. According to the invention of claim 3, in the electromagnetic field superposition type objective lens of claim 1, since the high resistance body is formed in a pipe shape, in addition to the effect of the invention of claim 1, there is an effect of easy manufacture.

According to a fourth aspect of the present invention, in the electromagnetic field superposition type objective lens according to the first aspect, a large number of conductive pipes are arranged inside the high resistance body, and an equipotential surface due to nonuniformity of the high resistance body. I corrected the disturbance.

According to a fifth aspect of the present invention, in the electromagnetic field superimposing type objective lens according to the first to fourth aspects, a heat radiation material is provided between the coil and the high resistance element to efficiently dissipate heat generated by the coil. Prevented the effect of heat.

According to a sixth aspect of the present invention, a magnetic pole formed into a substantially conical shape, a coil arranged inside the magnetic pole, an insulator arranged inside the coil, and a ring-shaped inside the insulator. A member in which a high resistance member and a conductor are laminated is provided, and a voltage is applied to both ends of this member so that the electrostatic lens action occurs on the sample side from the magnetic flux density peak position of the magnetic field lens. . As a result, Cs and Cc
The values of and can be made extremely small, and a high-resolution objective lens can be provided.

According to a seventh aspect of the present invention, a magnetic pole formed in a substantially conical shape, a coil arranged inside the magnetic pole, an insulator arranged inside the coil, and a large number of rings inside the insulator. It is equipped with a member in which a ring-shaped high resistance body is laminated, and a metal is sandwiched between the ring-shaped high resistance bodies, and a voltage is applied to both ends of the member. The magnetic flux density of the lens is set to occur on the sample side from the peak position. As a result, the values of Cs and Cc can be made extremely small, and a high resolution objective lens can be provided.

[Brief description of drawings]

FIG. 1 is a diagram showing a conventional conical objective lens.

FIG. 2 is a diagram showing a conventional conical objective lens.

FIG. 3 is a diagram showing a conventional electromagnetic field superposition type objective lens.

FIG. 4 shows spherical aberration Cs and coma aberration Cc according to acceleration voltage.
FIG.

FIG. 5 is a diagram showing an example of an electromagnetic field superimposing type objective lens according to the present invention.

6 is a diagram showing an on-axis magnetic flux density of the objective lens in FIG.

FIG. 7 is a diagram showing equipotential lines and axial potentials in the high-resistor internal space in the lens of FIG.

8 is an axial magnetic flux density B, an axial potential V of the lens of FIG.
It is a figure which shows the integrand functions R (Cs) and R (Cc) in the integration formula of the electron trajectories T and Cs and Cc.

9 is an axial magnetic flux density B, an axial potential V of the lens of FIG.
It is a figure which shows the integrand R in the integration formula of the electron trajectories T and Cs, Cc.

FIG. 10 is a diagram showing another embodiment of the present invention.

FIG. 11 is a diagram showing another embodiment of the present invention.

FIG. 12 is a diagram showing another embodiment of the present invention.

FIG. 13 is a diagram showing another embodiment of the present invention.

[Explanation of symbols]

 11 Magnetic Pole 12 Coil 13 Heat Dissipating Member 14 Insulating Member 15 High Resistance Film 16 Power Supply 17 Metallization Processing Section 18 Electrode

Claims (7)

[Claims] 1. A magnetic pole having a substantially conical shape,
A coil arranged inside the magnetic pole, an insulator arranged inside the coil, and a high resistance body arranged inside the insulator so as to surround the electron beam passage are provided. An electromagnetic field superposition type objective lens configured to apply a voltage to the. 2. The electromagnetic field superposition type objective lens according to claim 1, wherein the high resistance body is formed in a film shape. 3. The electromagnetic field superposition type objective lens according to claim 1, wherein the high resistance body is formed in a pipe shape. 4. The electromagnetic field superposition type objective lens according to claim 1, wherein a large number of conductive pipes are arranged inside the high resistance body. 5. The electromagnetic field superposition type objective lens according to claim 1, wherein a material for heat dissipation is provided between the coil and the high resistance body. 6. A magnetic pole having a substantially conical shape,
A coil arranged inside the magnetic pole, an insulator arranged inside the coil, and a member formed by laminating a ring-shaped high resistance body and a conductor inside the insulator are provided. An electromagnetic field superposition type objective lens configured to apply a voltage to the. 7. A magnetic pole having a substantially conical shape,
The coil disposed inside the magnetic pole, the insulator disposed inside the coil, and the member formed by laminating a large number of ring-shaped high resistance bodies inside the insulator are provided. An electromagnetic field superposition type objective lens configured such that a metal is sandwiched between resistors and a voltage is applied to both ends of the member.