JP2001319612A - Direct imaging electron microscope - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a direct imaging electron microscope with small aberrations. SOLUTION: This is equipped with an electron gun 21, a condenser lens 22, a beam separator 23, a magnetic-field type objective lens 24, a specimen stage 25 that mounts a specimen 26 thereon, an intermediate lens 27, an energy analyzer 28, a projection lens 29 and a image detector 30; and constructed so that an electrostatic field is superposed with the lens magnetic field of the objective lens 24. The objective lens 24 consists of an upper yoke 31 connected with an upper magnetic pole 32 at the tip, a lower yoke 33 connected with a lower magnetic pole 34 at the tip, a cylindrical body 30 made of an insulating material, a coil 36 and a disk plate 38. The specimen 26 is kept connected to the ground potential and a positive voltage of +9 kV is kept applied to an upper yoke 35 of the objective lens 24.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a direct mapping electron microscope which irradiates a sample with electrons at a high accelerating voltage.

[0002]

2. Description of the Related Art When inspecting a pattern formed on a semiconductor wafer such as a silicon wafer for defects or foreign matters, an image of the pattern is obtained by scanning the wafer with light, and another image is obtained. The presence or absence of a pattern defect or the like is inspected by comparing the image of the same type of pattern acquired in the field of view or the image of the reference pattern prepared in advance.

[0003] However, such an optical microscope type inspection apparatus using light has a problem that a minute defect cannot be detected due to insufficient spatial resolution.

[0004] In recent years, an inspection apparatus using a scanning electron microscope has been developed. The inspection apparatus acquires an image of a pattern based on a secondary electron signal or the like detected by scanning the wafer with an electron beam, and prepares an image of the same type of pattern acquired in a different field of view or a previously prepared image. The presence or absence of a pattern defect is inspected by comparing the image with the reference pattern image. Needless to say, the inspection apparatus using such a scanning microscope uses electrons, so that the spatial resolution is much higher than that of the optical microscope inspection apparatus.

However, in such an inspection apparatus using a scanning microscope, it is necessary to obtain a pattern image at a high speed in order to obtain a practical inspection speed, and the S / N ratio of the image is sufficient. Since it must be high, a large beam current of 100 times (10 nA) or more compared to a normal scanning electron microscope is used. However, since the electron beam is narrowed down to a point and the sample is two-dimensionally scanned on the sample by the point-like electron beam, there is a natural limitation in increasing the speed.

[0006]

Recently, a Low Energy Electron Reflection Microscope (LEEM) using an electron beam but capable of obtaining an image at a high speed has been developed.
ope) has been developed. In such an apparatus, for example, as shown in FIG. 1, the primary electron beam from the electron gun 1 is accelerated at about 10 KV, focused by the irradiation system lens 2, further largely deflected by the separator 3,
The light is vertically incident on the sample 4 through the electrodes 5 and 6. Here, a predetermined voltage is applied to the sample 4 and the electrodes 5 and 6 from the power source 7, thereby forming an emission lens 8 by which the primary electron beam is decelerated and accelerated below 100 V. The light enters the sample 4 at a voltage. The irradiation lens system 2 has a sample 4
Deflecting coil for scanning the desired area of the target with the primary electron beam. The reflected electrons generated from the sample 4 by the incidence of the primary electron beam are accelerated by the emission lens 8, go straight through the separator 3 and enter the Wien filter 9, where the energy is separated and has a predetermined energy. Only backscattered electrons are sorted out. The selected reflected electrons are transmitted to the Wien filter 9.
And the image based on the reflected electrons is formed by the imaging lens system 10.
Thus, an image is formed on the screen 11 after being enlarged to a predetermined size, and an image is obtained by reflected electrons having a predetermined energy.

When such a low-acceleration reflection electron microscope is used as a pattern inspection apparatus to inspect a defect or the like of a pattern formed on a silicon wafer, the acceleration voltage of the electron beam incident on the silicon wafer is 100V. Because of the smaller voltage, the electrons do not go deeply into the sample to be inspected such as a silicon wafer. Therefore, the observation based on the electrons reflected and elastically scattered near the surface of the sample to be inspected (several atomic layers on the sample surface) Become. For this reason, the pattern inspection apparatus using the low-acceleration reflection electron microscope is affected by products formed by the interaction between the gas around the sample and the electron beam when the pressure on the surface of the sample to be inspected is high. . In order to avoid such effects, the sample to be inspected must be placed in an ultra-high vacuum, which makes it technically difficult to observe and costs considerably. It will be limited.

More recently, in order to solve such a problem, a direct mapping electron microscope which irradiates a sample with an electron beam having a high accelerating voltage (100 V or more) and displays an image based on reflected electrons from the sample has been developed. Has been proposed (Japanese Unexamined Patent Application Publication No.
-273610). In such an electron microscope, the electrons penetrate deeply into the sample to be inspected, so there is no need to place the sample to be inspected in an ultra-high vacuum. The effect is easy, inexpensive, and the number of observable samples is not limited.

The present invention relates to such a direct-mapping electron microscope, and an object of the present invention is to provide a novel direct-mapping electron microscope having less aberration.

[0010]

According to the present invention, there is provided a direct mapping electron microscope, comprising: an electron beam generating means; a magnetic field type objective lens for irradiating an electron beam from the electron beam generating means onto a sample; A beam separator that separates an electron beam from an electron beam generator that irradiates the sample with electrons generated from the sample, a lens system that forms a sample image by electrons generated from the sample and that has passed through the beam separator, and a lens system. In a direct mapping electron microscope provided with an image detector for detecting a formed sample image, an electrostatic lens field is superimposed on a lens magnetic field of the magnetic field type objective lens.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention realizes a direct-mapping electron microscope in which the aberration is reduced and the electron beam is decelerated in front of the sample so that the sample is not damaged by the irradiation of the electron beam. It was done.

For this purpose, the objective lens of the direct-mapping electron microscope is modified as follows.

For example, when observing the reflected electrons by direct mapping, the factor limiting the resolution is the chromatic aberration of the objective lens. Now, the chromatic aberration coefficient is Cc, the acceleration voltage of electrons passing through the objective lens is Eo, the energy width of electrons passing through the slit of the objective lens is ΔE, and the opening angle of the objective lens is β.
Then, the spatial resolution dc determined by the chromatic aberration is dc = C
c (ΔE / Eo) β.

If the acceleration voltage is assumed to be 1 KV, the assumed spatial resolution is assumed to be 3 nm, and the energy width is assumed to be 10 V, from the above equation, the chromatic aberration is 0.3 mm when the opening angle is 1 mrad. In order to realize such small chromatic aberration, it is difficult with a normal magnetic field type objective lens. The acceleration voltage value, the spatial resolution value, the energy width value, and the opening angle value are all values that can be normally used in an objective lens.

According to the present invention, chromatic aberration is reduced by providing means for superposing an electrostatic lens field on the lens magnetic field of the magnetic field type objective lens.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 shows a schematic example of the direct-mapping electron microscope of the present invention.

In the figure, reference numeral 21 denotes an electron gun, and 22 denotes a magnetic condenser lens. 23 is a beam separator, for example,
An E × B or Wien filter beam selector that superimposes an electric field (E) and a magnetic field (B) orthogonally is used. The Wien filter separator is used at a deflection angle larger than 45 °, in the illustrated example, 90 °.

Reference numeral 24 denotes an objective lens, 25 denotes a stage on which a sample 26 is mounted, and 27 denotes a magnetic field type intermediate lens. An energy analyzer 28 uses, for example, an omega filter. In this case, a filter having an extremely small secondary aberration such as an omega filter is used.
An energy filter image as good as EM or SEM can be obtained. 29 is a projection lens, 30 is an image detector, for example, a CCD camera is used.

It should be noted that the magnetic field type intermediate lens 27 can be disposed at least one of before and after the energy analyzer 28.

FIG. 3 schematically shows the objective lens 24 shown in FIG. In FIG. 3, an upper yoke 31 is connected to an upper magnetic pole 32 at a tip end thereof.
A lower yoke 33 is connected to a lower magnetic pole 34 at the tip. A cylinder 35 made of an insulating material is arranged between the upper yoke 31 and the lower yoke 33, and a coil 36 is arranged between the cylinder and the upper yoke 31.

A disk-shaped plate 38 having an electron beam passage hole 37 at the center is attached to the tip of the lower magnetic pole 34, and the upper yoke 31 and the plate 38
During this time, a voltage is applied from a power supply (not shown).

In the direct-mapping electron microscope having such a configuration, for example, the sample 26 is grounded, and a negative power of 1 KV is applied to the electron emission portion of the electron gun 21 from an acceleration power supply (not shown).

Then, the upper yoke 31 of the objective lens 24
A voltage of +9 KV is applied from a power source (not shown) to the upper yoke 31 while the plate 38 connected to the lower magnetic pole 34 is grounded. +9 KV is applied). still,
A voltage of +9 KV is applied to the condenser lens 22, the beam separator 23, and the magnetic field type intermediate lens 27 with respect to the ground.
3 and magnetic field type intermediate lens 27 plus 9 KV from ground
Float on top.

Incidentally, the energy analyzer 28, the projection lens 29 and the image detector 3 are all placed on the ground potential.

In such a state, the electrons generated from the electron gun 21 are accelerated at 1 KV and
Head to 2. 9 applied to the condenser lens 22
It is accelerated by KV, and acceleration energy of 10 KV is given. That is, when the electrons from the electron gun 21 enter the condenser lens 22, acceleration energy of 10 KV is given,
The beam enters the beam separator 23, where it is deflected in the direction of the sample 26 and travels toward the sample 26. Further, the electrons are focused by the objective lens 24 and irradiate the sample 25 at the ground potential perpendicularly. At this time, the electrons are decelerated to 1 KV before reaching the sample 25. In this way, the one from the electron gun 21
The electron beam accelerated by the acceleration voltage of KV is maintained at an acceleration energy of 10 KV between the condenser lens 22 and the objective lens 24, and is irradiated to the sample 25 at a reduced speed of 1 KV. FIG. 5 shows the relationship between the electron acceleration voltage Vr in the objective lens 24 and the chromatic aberration coefficient Cc. When the electron acceleration voltage in the objective lens 24 is 10 KV, the chromatic aberration is a very small value of about 0.3 mm. ing. As described above, the chromatic aberration of the objective lens is extremely reduced by forming the electrostatic lens field so as to overlap the lens magnetic field of the objective lens 24.

Electrons irradiated to the sample at a high acceleration enter the interior of the sample to some extent and then reflect, so that the reflected electrons fly out in the vertical direction of the sample 26. At this time, the reflected electrons have energy corresponding to a voltage of about 1 KV.

The reflected electrons are again accelerated to about 10 KV by the objective lens 24 and travel toward the beam separator 23. At this time, since the beam separator 23 is set so that the reflected electrons accelerated to around 10 KV go straight, the reflected electrons go straight through the beam separator 23 and enter the energy analyzer 28 via the intermediate lens 27. At this time, the reflected electrons are decelerated to about 1 KV. The energy of the reflected electrons is broadly distributed over a wide range from the energy of the incident electron beam to zero. Therefore, in this energy analyzer 28,
Among the energies of the reflected electrons distributed over a wide range, the energy of a certain narrow width is selected, and an image based on the electrons corresponding to the energy width can be obtained. Then, this image is projected onto an image detector 30 such as a CCD camera by a projection lens 29 and photographed.

FIG. 3 shows an electromagnetic field superimposed type objective lens.
Is not limited to For example, a structure as shown in FIG. 4 may be used. In FIG. 4, reference numeral 39 denotes a yoke, an upper magnetic pole 40 is connected to an inner end thereof, and a lower magnetic pole 41 is connected to an outer end thereof. The coil 42 is arranged inside the yoke 39. A cylindrical electrode 43 is disposed inside the yoke 39 with a space between the yoke 39 and a positive 9 KV from a power supply (not shown). I have.

In the above example, the condenser lens 22, the intermediate lens 27, the beam separator 23 and the objective lens 2
The upper yoke 31 or the cylindrical electrode 43 of FIG. 4 is arranged at a high potential of +9 KV with respect to the ground. However, in order to reduce aberration, the electrostatic lens is superimposed on the lens magnetic field in the objective lens 24. Since it is necessary to generate a field, a positive high voltage may be applied only to the upper yoke 31 or the cylindrical electrode 43 of the objective lens 24, and the others may be set to the ground potential. In this way, the production and control of the power supply becomes easier.

In the above-described example, the sample 26 is dropped to the ground. However, the sample 26 may be floated to a slightly negative high potential. In such a case, the potential of the electrode of the objective lens 24 with respect to the ground increases. So, as shown in FIG.
Further, the aberration can be reduced. However, if the floating high voltage value is large, a device for floating the entire sample stage 25 to a high voltage becomes large and operation becomes troublesome. Therefore, it is better to keep the value at about -1 KV.

In the above example, when the sample 26 is irradiated with an electron beam, secondary electrons having an energy of several volts to several tens of volts are generated together with reflected electrons having an energy of about 1 KV. Therefore, if the beam separator 23 is set so that such secondary electrons travel straight, the reflected electrons are deflected and collide with the wall of the beam separator 23 and are absorbed. However, the secondary electrons travel straight. Thus, a secondary electron image can be obtained. However, when a secondary electron image is to be obtained in this way, the sample 26 is floated to a negative high potential of about several hundred volts so that the energy of the secondary electrons is not lost, or The lens 29 is set at a higher potential than the sample 26.

In the example shown in FIG. 3, a plate 38 having an electron beam passage hole 37 is attached to the tip of the lower magnetic pole, and this plate is connected to the ground. Connect the side pole 34 to ground,
A high positive voltage may be applied to the upper yoke 31.

If a disk made of a magnetic material or a non-magnetic material having an electron beam passage hole in the center is attached to the tip of the lower yoke of the objective lens 24, the electric field generated in the objective lens will cause a sample to be generated. Can be prevented. When the disk is made of a magnetic material, it is possible to further prevent the magnetic field generated by the objective lens from affecting the sample.

An electric field,
A filter in which a magnetic field and an electric field are formed in this order along the electron beam optical axis may be used.

[Brief description of the drawings]

FIG. 1 shows a schematic example of a low acceleration reflection electron microscope.

FIG. 2 shows a schematic example of a direct mapping electron microscope of the present invention.

FIG. 3 shows a schematic example of an objective lens used in FIG.

FIG. 4 shows another schematic example of the objective lens used in FIG.

FIG. 5 shows chromatic aberration characteristics of the objective lens used in the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Electron gun 2 ... Irradiation lens system 3 ... Separator 4 ... Sample 5, 6 ... Electrode 7 ... Power supply 8 ... Emission lens 9 ... Wien filter 10 ... Projection lens 11 ... Screen 21 ... Screen 22 ... Condenser lens 23 ... Beam separator 24 ... Objective lens 25 ... Sample stage 26 ... Sample 27 ... Intermediate lens 28 ... Energy analyzer 29 ... Projection lens 30 ... Image detector 31 ... Upper yoke 32 ... Upper magnetic pole 33 ... Lower yoke 34 ... Lower magnetic pole 35 ... Cylinder 36 ... Coil 37 ... Electron beam passage hole 38 ... Plate 39 ... Yoke 40 ... Upper magnetic pole 41 ... Lower magnetic pole 42 ... Coil 43 ... Cylindrical electrode

Claims (11)

[Claims] An electron beam generating means, a magnetic field type objective lens for irradiating the sample with the electron beam from the electron beam generating means, an electron beam from the electron beam generating means disposed in front of the sample and irradiating the sample. A beam separator for separating electrons generated from the sample, a lens system for forming a sample image by electrons generated from the sample and passing through the beam separator, and an image detector for detecting a sample image formed by the lens system. A direct-mapping electron microscope, wherein an electrostatic lens field is superimposed on a lens magnetic field of the magnetic field-type objective lens. 2. An apparatus according to claim 1, wherein an electrode is arranged on an electron beam path of said magnetic field type objective lens, and an electrostatic lens field is generated by applying a positive voltage to said electrode to a sample. The direct-mapping electron microscope according to claim 1, wherein: 3. The magnetic field type objective lens according to claim 1, wherein an upper magnetic pole and a lower magnetic pole are insulated, and an electrostatic lens field is generated by applying a voltage between them. Direct mapping electron microscope as described. 4. The direct mapping electron microscope according to claim 1, wherein an acceleration voltage of the electron beam applied to the sample is 100 volts or more. 5. The filter according to claim 1, wherein the beam separator is an E × B in which an electric field and a magnetic field are superimposed orthogonally, or a filter or a Win filter in which an electric field, a magnetic field, and an electric field are formed in this order along the optical axis of the electron beam. 2. The direct mapping electron microscope according to claim 1, wherein the electron microscope is a beam separator. 6. The direct mapping electron microscope according to claim 1, wherein an energy analyzer is provided between said lens systems forming a sample image. 7. The objective lens is inserted between an upper yoke having an upper magnetic pole at a distal end, a lower yoke having a lower magnetic pole at a distal end, and an upper yoke and a lower yoke. An insulating member, a coil disposed between the upper yoke and the insulating member, and a plate attached to the lower magnetic pole and having an electron beam passage hole in a center portion, wherein a voltage is applied between the upper yoke and the plate. The direct-mapping electron microscope according to claim 1, wherein the direct-mapping electron microscope is adapted to be applied. 8. The objective lens is provided with an upper yoke having a distal end forming an upper magnetic pole, a lower yoke having a distal end forming a lower magnetic pole, and disposed between the upper yoke and the lower yoke. Consisting of a cylindrical electrode mounted inside the upper yoke with a space with respect to the yoke,
2. The method according to claim 1, wherein a voltage is applied to said electrode.
Direct mapping electron microscope as described. 9. The direct mapping electron microscope according to claim 1, wherein the electrons generated from the sample are reflected electrons. 10. The direct mapping electron microscope according to claim 1, wherein the electrons generated from the sample are secondary electrons. 11. The sample according to claim 9, wherein the sample has a negative potential.
Direct mapping electron microscope as described.