JPH03230464A - Scanning type electron microscope - Google Patents

Abstract

PURPOSE:To perform good high/low-magnifying factor observation without changing the position of a sample by providing the first magnetic field type objective lens and the small second magnetic field type objective lens controlled independently respectively, and providing the secondary electron detecting device above the second lens. CONSTITUTION:An electron beam generated by an electron gun 12 and a focusing lens 13 scans a sample 5 via the first objective lens 22 or the second objective lens 27, and secondary electrons generated by the sample 5 are fed to the secondary electron detector 32 above the lens 27. The lens 22 of a conical section corresponding to an inclined sample 5a at the tip of an inner tube yoke 20 is controlled by a coil 16, the small lens 27 provided on a lens yoke 30 is controlled by a coil 26 independently respectively for the excitation current strength, and high-magnifying factor observation with a high magnifying factor and high resolution and high-visual field/low-magnifying factor observation with high resolution and little distortion are performed by the lenses 22, 27 respectively without changing the position of the sample 5.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a scanning electron microscope capable of high-resolution oblique observation of large-diameter samples such as large-diameter semiconductor wafers.

[Summary of the invention]

The present invention is a first magnetic field type objective lens for an electron microscope, in which one magnetic pole is an inner magnetic pole narrowly focused near the optical axis of the primary electron beam, and the other pole (outer magnetic pole) is separated from the optical axis. an objective lens (single-pole magnetic field type objective lens); and a second magnetic field type objective lens (or einzel an electrostatic type objective lens such as a magnetic field type lens) and a second type that detects secondary electrons above the small magnetic field type objective lens.
Equipped with a second electron detection device and a means for independently controlling and supplying the intensity of the first and second objective lenses, the intensity of the first objective lens is increased during high magnification observation, and the intensity of the first objective lens is increased during low magnification observation. By increasing the strength of the second objective lens, high-resolution observation can be performed during high-magnification observation without changing the position of the sample, and during low-magnification observation, high resolution and less distortion can be achieved at the outer periphery of the observation surface. This is a scanning electron microscope that allows observation of a wide field of view.

The same effect can be obtained even if the second objective lens is an electrostatic lens.

[Conventional technology]

In recent years, in the field of semiconductor development and manufacturing, there has been a demand for scanning electron microscopes (SEMs) that can observe large-diameter wafers at high magnification and resolution by tilting them at large angles, and that can also observe wide-field images at low magnification. .

Conventionally, Japanese Patent Laid-Open No. 57-mm835 discloses an objective lens for a scanning electron microscope as shown in FIG. 2. A lens yoke 1 having an inner cylinder yoke 1a forming upper and lower magnetic pole pieces 3.4 around the optical axis of the primary electron beam, a first coil 2 that excites the lens yoke, and a first coil 2 of the lens yoke. Additional yoke 7 integrated with upper cork 1b
is provided on the outside of the outer cylinder yoke IC, a third magnetic pole piece 8 is formed at the end thereof, and a second magnetic field is formed between the third magnetic pole piece 8 and the lower magnetic pole piece 4 to form a magnetic field. This is a magnetic field lens consisting of an excitation coil 9.

[Problem to be solved by the invention]

Generally, when observing a sample with high resolution and high magnification in a scanning electron microscope, the distance working distance W between the top surface of the objective lens and the sample is made small, and the chromatic aberration coefficient and spherical aberration coefficient are made small. Need to observe. Furthermore, when observing a sample at low magnification, it is necessary to have a large working distance. In the case of low-magnification observation, when the working distance is reduced, the deflection aberration of the scanning system and the off-axis aberration of the objective lens become large in low-magnification observation where the scanning area is wide, resulting in large distortion and peripheral blur in the peripheral part of the field of view. This is because that. This conventional example can be said to be a magnetic field type lens in which an additional yoke 7 and an excitation coil 9 are provided on the objective lens of a normal scanning electron microscope.

When performing high-magnification observation, if the excitation coil 9 is activated,
Since a strong magnetic field is generated at the sample position, chromatic aberration and spherical aberration are further reduced than in a normal objective lens, making it possible to obtain a higher resolution image. Even in this case, a smaller aberration coefficient can be obtained by reducing the working distance. However, when performing low-magnification observation, large distortions and peripheral blurring tend to occur more easily than with a normal objective lens, making low-magnification observation difficult. In this conventional example, when observing a sample at low magnification, it is possible to reduce distortion and blur by exciting only the excitation coil 2 and operating it as a normal objective lens. However, the magnetic field generated by the excitation coil 2 is between the upper magnetic pole piece 3 and the lower magnetic pole piece 4, and the magnetic pole piece 4 is also used as the excitation piece excited by the excitation coil 9.
When the sample is set at a small working distance, which is a condition for high-resolution observation, large distortions and peripheral blur occur as described above. In other words, it is necessary to make the working disc larger during low-magnification observation, and to make the working disc smaller during high-magnification observation, and the sample position must be shifted each time.

In addition, the secondary electrons emitted from the sample surface are restrained by the magnetic field formed between the upper and lower magnetic pole pieces 3.4 and between the lower magnetic pole surface and the third magnetic pole surface 8, and are directed upward into the inner cylinder yoke la. but,
Since there is no magnetic field inside the inner cylinder yoke la, the secondary electrons passing through the inner cylinder yoke la spread, and a part of them collide with the wall of the inner cylinder yoke la and are absorbed by the wall. Therefore, in the conventional example described above, if the secondary electron detector is placed above the objective lens, the secondary electron detection efficiency tends to decrease. It is also possible to place the secondary electron detector on the side of the sample as is done in ordinary scanning electron microscopes, but when the excitation coil 9 is activated, the secondary electrons from the sample are Due to the magnetic field generated near the sample, the electron beam is restrained on the optical axis and directed toward the hole in the lower pole piece 4, making it impossible to sufficiently detect secondary electrons.

Here, a screen 10 of a CRT which is a display means of an electron microscope is shown.
FIG. 3 shows an example of the relationship between the minimum magnification (MAG m1n) that allows the distortion around 0 mm' to be 1% or less and the distance 1 +u between the main surface of the objective lens and the sample surface. However, beam scanning is performed using a two-stage deflection system. From Fig. 3, when l is 10 fl, to obtain an image with distortion of 1% or less, 80
If an observation magnification of 2 times or more is required, and p is less than that, the minimum magnification (MAG m1n) increases even more rapidly. For example, when the excitation coil 9 shown in the conventional example is strongly excited, the distance l between the main surface of the objective lens and the surface of the sample becomes approximately a few centimeters or less, and in order to obtain an image with distortion of 1% or less, a magnification of 200 times or more is required. must be observed. In other words, from Figure 3, 20
It can be seen that if an observed image of twice the size or less is required with a distortion of 1% or less, p is required to be 3ON or more.

On the other hand, when l is increased, there is a problem that the resolution of the observed image tends to decrease. Figure 4 shows the resolution d and p when using tungsten hairpin thermionic technique at an accelerating voltage of IKV.
showed the relationship between When l is 6°n or more, the resolution d is 0.
It will be more than 2JIm. If the observation magnification is 1000x, the resolution on the observation screen is 0.2** (0.2x
1000) or more, the deterioration of image quality becomes noticeable.

In addition, Fig. 8 shows the lens magnetomotive force J of the magnetic field type objective lens necessary to focus the electron beam for p on the sample surface.
An example of the magnitude of /u I/2 (J is the ampere-turn of the coil, and U is the acceleration voltage of the primary electron beam) is shown below. In addition, the gap S between the lower magnetic pole and the upper magnetic pole is lQ+n, and the hole diameter B is 2.
It was set to 01. From the figure, it can be seen that the smaller p is, the larger the magnetomotive force becomes, and especially when l is less than Ion, the magnetomotive force increases rapidly. Since the amount of heat generated by the coil is approximately proportional to the reciprocal of the cross-sectional area of the coil and the square of the magnetomotive force J, it is understood that it is difficult to downsize the lens and reduce l.

When ρ is 30 mm or more, the electron beam can be focused on the sample surface with a relatively small magnetomotive force J, and it is relatively easy to downsize the lens.

[Means to solve the problem]

In order to solve the above-mentioned problems, the present invention provides a first lens yoke having an inner magnetic pole in an inner tube yoke portion and an outer magnetic pole in an outer tube yoke portion outside the inner tube; A first coil is provided to excite the yoke, and the top surface of the magnetic pole provided at the tip of the inner cylinder yoke part has a tapered conical shape connected to the top surface of the magnetic pole, and the tip of the outer magnetic pole is arranged so that the tip of the outer magnetic pole faces the sample. a first objective lens located on the same side as the top surface of the magnetic pole and formed apart from the top surface of the magnetic pole in the optical axis direction; a small second objective lens of magnetic field type or electrostatic type provided so as to hang thereon; and a secondary objective lens whose detection portion is located within the inner cylinder yoke and between the scanning system and the second objective lens. a charged particle detector, and means for independently controlling the supply of excitation current to the first coil and the second coil or the voltage applied to the second electrostatic objective lens. This is a scanning electron microscope characterized by:

In other words, it is a scanning electron microscope equipped with an objective lens for high-magnification observation and a small objective lens for low-magnification observation.

[Effect]

a first lens yoke having an inner magnetic pole on an inner cylinder yoke portion and an outer magnetic pole on an outer cylinder yoke portion outside the inner cylinder;
a first coil for exciting the lens yoke;
The outer magnetic pole has a tapered conical shape connected to the top surface of the magnetic pole provided at the tip of the inner cylinder yoke part, and the tip of the outer magnetic pole is on the same side as the top surface of the magnetic pole with respect to the sample. A first portion formed apart from the top surface of the magnetic pole in the optical axis direction.
an objective lens; a second lens yoke having an upper magnetic pole and a lower magnetic pole; A small second coil with a second coil
Since it is equipped with two independent objective lenses, it is possible to have a large working distance between the small second objective lens and the sample position without moving the sample position, making it easy to use when observing samples at low magnification. can also be observed in high resolution. Further, since the objective lens of the small tube 2 is provided directly below the secondary electron detector, a magnetic field is formed up to the vicinity of the secondary electron detector, so that secondary electrons can be detected efficiently.

[Example] FIG. 1 shows an example of the present invention. A primary electron beam (electron beam) 14 generated by the electron gun 12 provided below the electron gun 12 is focused by two focusing lenses 13.13. A scanning coil (scanning electrode can also be used) 1 is provided at the lower stage for scanning the focused electron beam 14 in the X and Y directions.
7.17 is provided in two stages. Furthermore, a first objective lens (1) having a shape that enables observation of a large-diameter sample 5 at a large angle and inclination is provided. In other words, the first objective lens)
1 has a conical shape, so that even if a large sample is tilted greatly (sample 5a), it will not come into contact with the first objective lens 22. The first objective lens 22 includes an inner cylinder yoke 20, an upper yoke 15 at its upper end, and an upper yoke 15.
There is an outer yoke 18 surrounding the inner yoke 20 on the outside, a conical yoke 19 on the lower end of the inner yoke 20, and a conical yoke 19 on the inside of the end of the conical yoke 19, which is aligned with the optical axis of the primary electron beam. A donut-shaped magnetic pole top surface 2 parallel to the plane perpendicular to the
1, outer cylinder cork 18, upper yoke 15, and inner cylinder yoke 20
a first (excitation) coil 16 wound around the
It consists of This is an example of a so-called unipolar magnetic field lens. When a current is applied to the first coil 16 and it is energized,
A lens magnetic field is formed below the top surface of the magnetic pole. Here, the first
When the excitation of the coil is increased, the focal length of the primary electron beam by the first objective lens H becomes shorter, and the aberration coefficient becomes extremely small. Note that the first excitation coil 16 of the first objective lens 22 and the outer magnetic pole surface 25 of the outer cylinder yoke 18 are in a position recessed from the magnetic pole top surface 21 toward the electron beam source side, and the inner cylinder yoke 20 is recessed toward the electron beam source side. Small diameter magnetic pole top surface 2 provided at the tip
1 and a conical yoke 19 connected thereto and having a tapered conical shape, it is possible to observe a large diameter wafer as a sample even if it is tilted greatly (sample 5a indicated by a dotted line). It has become.

Next, the small second objective lens, which is a major feature of the present invention, will be explained. The second objective lens 27
The second objective lens is provided substantially independently of the objective lens, has upper and lower ends serving as an upper magnetic pole 23 and a lower magnetic pole 24 on the optical axis side (inner diameter side), and has a hollow rotating body shape. It consists of a lens yoke 30 and the second coil 26 formed in the hollow space.

A part of the second objective lens mm may be placed at least in the conical yoke of the first objective lens H, and the remaining part may be housed in the inner cylinder yoke 20. When a current is passed through the second coil, a magnetic field is generated between the upper magnetic pole 23 and the lower magnetic pole 24 of the second lens yoke 30, which acts as a magnetic field lens for the electron beam. Small second objective lens 2
Since the lens 7 is provided independently of the first objective lens 22, the lens can be positioned relatively far from the surface of the sample 5 than in the conventional example. Therefore, the working distance can be lengthened without shifting the position of the sample 5, and as explained in the operation section, mainly by operating the second objective lens 27, it is possible to reduce distortion and peripheral blurring. Magnification observation is possible.

The gap S between the second objective lens 27 (between the upper magnetic pole 23 and the lower magnetic pole 24) is lo+u, and the upper magnetic pole 23 and the lower magnetic pole 2
The inner diameter B of 4 is 20IN, and the distance ML between the magnetic pole top surface 21 of the first objective lens H and the center of the gap of the second objective lens I↑ is 4
It has become 0.

The electron beam 14 generated from the electron gun 2 is passed through a two-stage focusing lens 1.
3.13 and passes through a scanning system 17.17. During high magnification observation, the first objective lens (single-pole magnetic field lens) main 1 is strongly excited, and the small second objective lens mm is weakly or zero excited (see Figure 5a). The electron beam 14 passing through the scanning electrode is focused on the surface of the sample 5 at a distance W from the top surface 21 of the magnetic pole mainly by the action of the first objective lens H.

During low magnification observation, the first objective lens) (is weakly excited,
Alternatively, the small second objective lens 27 is strongly excited with zero excitation (see FIG. 5b). At this time, the electron beam 14 is focused on the surface of the sample 5 mainly by the action of the second objective lens mm.

With the above configuration, when the distance W between the surface of the sample 5 and the magnetic pole top surface 21 of the first objective lens I) is as small as about 5 mm (the first objective lens 22 is mainly excited for high-magnification observation) Sometimes, the smaller W is, the smaller the aberration of the lens (and the more high-resolution images can be obtained), allowing low-magnification wide-field observation with less distortion and peripheral blurring, and the higher magnification observation. At times, high-resolution observations can be made.

The distance between the main surface of the small second objective lens I↑ and the magnetic pole top surface 21 of the first objective lens is desirably set to 30 to 60 mm. As explained in Figure 3, L is 301
If it is less than m, the distortion will be large on the outer periphery of the observation area during low magnification observation, and the observed image will not be good.If you want to perform low magnification observation with a good image, it is necessary to move the position of the sample 5 downward. I have to let it happen. Furthermore, as explained in FIG. 4, if L is 60 sn or more, distortion will be small in the outer periphery of the observation area during low magnification observation, but the resolution will be poor and the observed image will not be good. The magnetomotive force J of the second objective lens 1 unit that acts on the king during low magnification observation is (L-40
mn, S = 10 mm, B = 201, the acceleration voltage of the electron beam 14 is 5 KV, W -5 vaO: ) is about 350 ampere AT, and the second objective lens) 1 can be sufficiently miniaturized. can.

2 generated from the surface of the sample 5 by irradiation with the electron beam 14
The secondary electrons pass through the hole in the top surface of the magnetic pole 21, and when the second objective lens mm is excited, they are restrained on the optical axis of the electron beam 14 by the magnetic field and are placed above the second objective lens 1. It is detected by the secondary electron detector 32 installed in the. 2
The secondary electron detector 32 is located below the scanning system 17.17 and directly above the second objective mm. Further, the detection part of the secondary electron detector 32 is located inside the inner cylinder yoke 20, and the detection part (
A light guide connected to a scintillator (in FIG. 1) passes through the inner cylinder yoke 20 and is connected to a photomultiplier tube. It is also possible to form the secondary electron detection part in a donut shape with the optical axis 14 as an axis, and it is also possible to form it in a microchannel plate. The secondary electron detector 32 supplies the signal strength of the secondary electrons to a cathode ray tube (not shown) which is scanned by a scanning signal synchronized with the scanning signal of the scanning system, and displays a secondary electron beam image. .

The second objective lens mm does not require a large magnetomotive force as described above and can be sufficiently miniaturized.
The distance from to the detection part of the secondary electron detector 32 can be shortened, so even when the second objective lens mm is not excited, the secondary electrons collide with the inner wall of the second objective lens 27. It is possible to reach the detection section of the secondary electron detector 32 without having to do so. However, if the inner diameter of the second objective lens is small and the secondary electron capture efficiency decreases, the sixth objective lens
As shown in the figure, in addition to the gap between the upper and lower magnetic poles 23 and 24,
A new gap 33 is provided in the inner diameter part of the second lens yoke 30 to widen the magnetic field distribution on the optical axis of the primary electron fringe, and this magnetic field can focus the secondary electrons on the optical axis. Alternatively, as shown in FIG. 7, by providing an axially symmetrical coil (solenoid) 34 on the inner diameter of the second objective lens 27 and energizing it, the secondary electrons can be focused on the optical axis. Thereby, secondary electrons can be detected even more efficiently.

It is also conceivable to provide the secondary electron detector 32 between the second objective lens 27 and the first objective lens.
Since the magnetic flux density is high near the magnetic pole top surface 21 of the objective lens 1, if a through hole is provided near this area and the secondary electron detector 32 is installed, an axially asymmetrical magnetic field will be generated, which may easily cause axial defects. .

Incidentally, even if the second objective lens 27 is constituted by an electrostatic lens, the same effect of the present invention can be obtained.

〔Effect of the invention〕

As described above, according to the present invention, secondary electrons can be detected efficiently, large-diameter samples can be observed at large tilt angles, high-resolution images can be obtained during high-magnification observation, and high-resolution images can be obtained during low-magnification observation. , a wide-field image with high resolution and little distortion can be obtained. Furthermore, there is no need to shift the sample position during high-magnification observation or low-magnification observation, making operations easier and faster.

[Brief explanation of drawings]

Fig. 1 is a cross-sectional view showing an embodiment of the present invention, Fig. 2 is a cross-sectional view of a conventional objective lens, and Fig. 3 shows the minimum magnification (MAG+m1n) that allows distortion to be 1% or less. Distance between the main surface of the objective lens and the sample surface @ 7! A diagram showing the relationship with m,
Fig. 4 is a diagram showing the relationship between resolution d and l, Fig. 5a is a cross-sectional view showing the focusing state of the primary electron beam during high magnification observation, and Fig. 5
Figure b is a cross-sectional view showing the focusing state of the primary electron beam during low magnification observation, Figure 6 is a cross-sectional view showing another example of the second objective lens, and Figure 7 is a further view of the second objective lens. A sectional view showing another embodiment, FIG. 8 is a lens magnetomotive force J/ of a magnetic field type objective lens.
It is a diagram showing the relationship between U"" (J is the ampere-turn of the coil, and U is the acceleration voltage of the primary electron beam) and ρ. 5. Sample 12. Electron gun 13. Focusing lens 17. Scanning system 21. Top surface of magnetic pole ■. 1st objective lens mm. 2nd objective lens 32 Secondary electron detector or higher

Claims (1)

[Scope of Claims] (1) An electron beam narrowed by an objective lens is scanned two-dimensionally over the sample surface by an electron beam scanning system, and secondary electrons generated from each scanning point of the sample are In a scanning electron microscope that displays a secondary electron image by detecting it with a detector and supplying the secondary electron intensity to a cathode ray tube synchronized with the scanning, the inner tube yoke has a magnetic pole top surface and an outer side of the inner tube. a first lens yoke having an outer magnetic pole in an outer cylindrical yoke portion;
a first coil for exciting the lens yoke;
The inner cylindrical yoke portion has a magnetic pole top surface provided at the tip and a tapered conical shape connected to the magnetic pole top surface, and the outer magnetic pole tip is located on the same side of the sample as the magnetic pole top surface, and a first objective lens formed apart from the top surface of the magnetic pole in the optical axis direction; and a small second objective lens provided so that at least a portion thereof hangs over the conical portion of the inner cylinder yoke in the optical axis direction. a first coil of the first objective lens; a secondary charged particle detector having a detection portion within the inner cylinder yoke and between the scanning system and the second objective lens; 1. A scanning electron microscope characterized by comprising means for independently controlling the supply of an excitation current to the second objective lens and the supply of an applied current or voltage to the second objective lens. (2) The scanning electron microscope according to claim 1, wherein the small objective lens comprises a second lens yoke having an upper magnetic pole and a lower magnetic pole, and a second coil for exciting the second lens yoke. (3) The scanning electron microscope according to claim 1, wherein the small objective lens is an electrostatic lens. (4) The distance between the lens main surface of the small second objective lens and the magnetic pole top surface of the first objective lens is 30 mm to 60 mm.
The scanning electron microscope according to claim 1, wherein the scanning electron microscope is mm. (5) A solenoid is provided in the inner cylinder yoke between the top surface of the magnetic pole and the secondary electron detector for improving secondary electron capture efficiency around the electron beam optical axis. scanning electron microscope. (6) The scanning electron microscope according to claim 2, wherein the second objective lens comprises a second lens yoke having a plurality of magnetic pole gaps and a second coil for exciting the second lens yoke.