JPH05151927A - Scan type electron microscope and its observing method - Google Patents

Abstract

PURPOSE:To generate an observation image of high magnification and with high resolution by irradiating a specimen surface for a certain time with an electron beam at a lower magnification than the desired observation magnification prior to starting observing, and returning to the desired observation magnification. CONSTITUTION:First the desired observation magnification MO and the irradiation time at a lower magnification ML are set. Then the scanning width over the specimen surface with an electron probe 10 is varied by a magnification changing circuit 16 to change over the magnification from MO into ML. After irradiating with an electron beam for the set irradiation time at the magnification ML, the magnification is changed over from the ML to MO by a magnification control means 5 and the specimen 3 is irradiated with electron beam. An operator checks the observation image on a CRT 15 and executes if possible the observation and measuring length. This enables to generate charge balance temporarily and yields an observation image of high magnification and with high resolution.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning electron microscope and a method of observing the same, and in particular, in order to evaluate a semiconductor manufacturing process, a sample for a semiconductor device is irradiated with an electron beam to form a contact hole or a line pattern. The present invention relates to a scanning electron microscope suitable for observing or measuring a length and an observation method thereof.

[0002]

2. Description of the Related Art Conventionally, for observation or measurement of a submicron order (1 μm or less) contact hole or line pattern in a semiconductor device sample,
Scanning Electron Microscope
n Microscope; hereinafter referred to as SEM)
Is used.

Samples for semiconductor devices are generally Al and S.
It is constructed by laminating an electric insulator such as SiO2 or SiN on a conductor portion such as i. When such a semiconductor device sample is irradiated with an electron beam, the surface of the electrical insulator is positively or negatively charged.

Whether the surface of the insulator is positively or negatively charged depends on the energy of the irradiated electron beam, that is, the electron dose, and is positively charged when the electron dose is small and negatively charged when the electron dose is large. It is confirmed to do.

There is no problem when the surface of the insulator is positively charged, but when the surface of the insulator is negatively charged (hereinafter sometimes simply referred to as charge-up), secondary electrons generated from the bottom of the contact hole are generated. Is prevented by the negatively charged charges on the surface of the sample, and is not detected by the secondary electron detector. As a result, an abnormal contrast or a severely distorted image may occur in the SEM image. Therefore, conventionally, a so-called low acceleration SEM in which the energy of the primary electron beam with which the sample is irradiated is 1 keV or less has been used.

However, even with such a low accelerating voltage, if the amount of electron beams (electron beam density) irradiated per unit area of the sample increases, charge-up may occur and image defects may occur. .. In particular, in the case of ultra-fine processed semiconductor devices, image defects caused by charge-up are
This will seriously hinder the observation of contact holes and the measurement of lines and spaces. As a result, not only is it difficult to evaluate the semiconductor manufacturing process, but it is also a major obstacle to ensuring the quality of the semiconductor device itself. Therefore, in the prior art, in order to prevent charge-up, the accelerating voltage is made as low as possible and the probe current is made small.

[0007]

However, the above-mentioned prior art has the following problems. That is, when the accelerating voltage is lowered, the resolution is remarkably lowered due to chromatic aberration and the like, and it becomes difficult to observe at high magnification. Further, when the probe current decreases, the ratio of the secondary electron signal to the noise (S
/ N) is significantly reduced, the contrast as an SEM image is deteriorated, and it becomes difficult to observe at high magnification and high resolution.
In particular, in a semiconductor device manufactured by ultra-fine processing technology, a signal from a recess of a contact hole or a line pattern becomes weak, which is a great obstacle to fine observation and length measurement.

An object of the present invention is to solve the above-mentioned problems of the prior art and to eliminate image defects caused by charge-up of the sample surface so that image observation with a high SN ratio can be obtained. It is in.

[0009]

In order to achieve the above object, the present invention is characterized in that the following means are taken. (1) Before starting the observation, the sample surface is irradiated with an electron beam at a magnification lower than or higher than a desired observation magnification for a certain period of time, and then irradiated with an electron beam by returning to the desired observation magnification. At this time, a signal generated secondarily from the sample was captured to obtain an observation image. (2) An electrode for applying an electric field was provided on the sample surface. (3) The relative relationship between the probe current that does not easily charge up on the sample surface and the magnification was determined, and the probe current according to the desired observation magnification or the observation magnification according to the desired probe current was easily obtained.

[0010]

According to the above configuration (1), the charged state of the observation region can be temporarily balanced (charge balance). Therefore, if image observation is performed using this charge-balanced period, A clear observation image with a high SN ratio can be obtained.

According to the above configuration (2), the secondary electrons generated from the bottom of the contact hole are pulled up by the electric field, so that the secondary electron detector can detect the secondary electrons.

According to the above configuration (3), the sample, the magnification,
Also, it becomes possible to easily obtain the optimum observation condition according to the probe current.

[0013]

First, the basic concept of the present invention will be described.

As described above, the amount of electron beam irradiation per unit area is an important factor for observing the sample in a stable state without charge-up.

This electron beam dose IQ is given by the following equation (1).

IQ = (M ² × Ip × t) / S (1) where M: observation magnification Ip: probe current t: imaging time (= irradiation time) S: CRT scanning area where CRT scanning area Is determined by the size of the CRT, so the denominator is a constant. Therefore, if the probe current Ip and the imaging time (= irradiation time) t are constant,
The electron beam irradiation amount IQ is proportional to the square of the magnification M.

The state of charge-up on the surface of the sample varies depending on the composition of the conductor and the electrical insulator, but according to experiments by the inventors, charge-up occurs at the magnification M0 desired to be observed. Even if you can not observe well, you can reduce the magnification to a lower magnification ML and irradiate the electron beam, then return to the magnification M 0 you want to observe again and irradiate the electron beam, and charge up for a while. It was confirmed that a clear observation image with an excellent SN ratio could be obtained without any damage.

This is because even if the probe current Ip is constant, the electron beam irradiation area spreads at a low magnification ML, the electron beam irradiation amount per unit area is substantially reduced, and the sample surface is positively charged. After that, even if the magnification is returned to M0 and the state where the sample surface is easily negatively charged, since the sample surface is previously positively charged, the charge balance is temporarily changed from the positively charged state to the negatively charged state. It can be interpreted that it can be taken.

Therefore, if an observation image during this period without charge-up is stored in the image memory or photographed and recorded, a clear observation image with an excellent SN ratio can be obtained.

Further, according to the experiments conducted by the inventors, contrary to the above, even if the image cannot be observed well due to charge-up at the magnification M0 desired to be observed, the magnification is once increased to a higher magnification MH and the predetermined observation region is observed. Irradiate part of the
Then, it was confirmed that when the magnification was again returned to M0 and the observation was performed, a clear observation image with an excellent SN ratio was obtained without charge-up for a while, as described above.

Although such a phenomenon cannot be explained by the above interpretation, for example, since the contamination generated during irradiation with a high magnification MH adheres to the sample surface and the insulation property of the sample surface is deteriorated, It is presumed that even if it is returned to the magnification M0 and returned to the state of being easily negatively charged, it becomes difficult to be charged.

According to the present invention, the electron beam is previously irradiated on the sample surface at a magnification ML or a magnification MH lower than the magnification M0 originally desired to be observed as described above, and then the magnification M0 desired to be observed is returned to. It was made based on the experience that an excellent observation image can be obtained.

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a schematic view of an SEM which is an embodiment of the present invention. Here, the secondary electrons 4 generated from the sample 3 and passing through the objective lens magnetic pole hole 13 are directed above the objective lens 2. Detected by the secondary electron detector 1 arranged in
The structure of a (Through The Lens) type SEM is shown.

In the figure, an electron beam 7 emitted from a field emission type electron gun 6 composed of a field emission cathode 6a and an electrostatic lens 6b is passed through a condenser lens 8 and an objective lens 2 to a sample 3 on a sample stage 9. Is converged to an extremely thin electron probe 10 on the plane. Electronic probe 10
Is scanned on the sample surface by deflecting the electron beam 7 by the deflection coil 11 energized by the scanning power supply 12.

The secondary electrons 4 generated from the sample surface by the scanning of the electron probe 10 are caught by the magnetic field of the objective lens, pass through the magnetic pole hole 13, and are guided to the secondary electron detector 1. The signal detected by the secondary electron detector 1 is amplified by the amplifier circuit 14 and is used as a video signal in a cathode ray tube (Cathode).
e Ray Tube; hereinafter abbreviated as CRT) 15.

The magnification of the observation image is adjusted by changing the scanning width of the electronic probe 10 on the sample surface by the magnification changing circuit 16 and adjusting the ratio of this scanning width to the screen width on the CRT. The magnification control means 5 controls the magnification varying circuit 16 and the scanning power source 12 to make the magnification lower (ML) or higher than the magnification (M0) desired to be observed.
After irradiating the sample surface with an electron beam for a certain period of time, control is performed to return to the magnification (M0) normally desired for observation.

The control method by the magnification control means 5 may be such that the operator manually sets the magnification and the irradiation time (timer) for the sample which is easily charged up,
Alternatively, a pre-assembled menu may be selected and set by a software program.

FIG. 8 is a flow chart for explaining the observation method by the SEM having the above-mentioned structure.

In step S10, the desired observation magnification M0
To set. In step S11, the irradiation time t1 at the low magnification ML is set. In step S12, the magnification changing circuit 16 changes the scanning width of the electronic probe 10 on the sample surface, so that the magnification is switched from the observation magnification M0 to the low magnification ML.

In step S13, the electron beam is irradiated for the irradiation time t1 at the magnification ML, and thereafter, in step S14, the magnification is reduced by the magnification control means 5 to the low magnification M.
The observation magnification is switched from L to M0. Step S15
Then, the electron beam is irradiated onto the sample.

In step S16, the operator
If the observation or length measurement is possible, the process proceeds to step S17 to perform the observation or length measurement. If the observation or length measurement is difficult, the process returns to step S11 to repeat the above process.

FIG. 9 is a flowchart showing another observing method. Since the same processing is executed in the steps designated by the same reference numerals as those described above, the description thereof will be omitted.

In step S11a, the irradiation time t2 at the high magnification MH is set. In step S12a, the magnification changing circuit 16 changes the scanning width of the electronic probe 10 on the sample surface, so that the magnification changes from the observation magnification M0 to the high magnification M.
Switched to H.

In step S13a, the electron beam is irradiated at the magnification MH for the irradiation time t2, and thereafter, in step S14a, the magnification is switched from the low magnification MH to the observation magnification M0.

FIG. 10 is a flow chart showing still another observation method. Since the same processing is executed in the steps designated by the same reference numerals as those described above, the description thereof will be omitted.

In this embodiment, first, at high magnification MH, time t
The electron beam is irradiated for 3 times, then switched to the low magnification ML, and irradiated with the electron beam for time t4, and then the observation magnification M0
It is returned to and is observed or measured.

In each of the above observation modes, the scanning image may or may not be observed during the irradiation time at the low magnification ML and the high magnification MH.
Further, as a practical magnification, a low magnification ML = M0 / 50
˜M0 / 100, and high magnification MH ≧ 3M0 is desirable.

FIG. 2 is an SEM which is another embodiment of the present invention.
Is a schematic diagram of the above, and the same reference numerals as those used above represent the same or equivalent portions.

In the present embodiment, an electric field is applied to the sample surface to extract the secondary electrons 4 generated from the deep groove such as a contact hole and efficiently guide the secondary electrons 4 to the secondary electron detector 1. 18, the energy control electrode 21, and the electric field control means 17 for controlling these are further provided.

FIG. 3 is a cross-sectional view for explaining the structures of the electric field control electrode 18 and the energy control electrode 21 in detail.

The electric field control electrode 18 is concentrically attached to the magnetic pole hole 13 of the objective lens 2 so as to face the sample 3. A flat plate-shaped mesh 19 having an electron beam passage hole is attached to the upper portion of the electric field control electrode 18.

Although the mesh 19 is not always necessary,
It has the effect of averaging the kinetic energy of secondary electrons pulled up at an angle oblique to the central axis. In addition,
The mesh 19 may be hemispherical.

An electric field control voltage (VB1) 20 is applied to the electric field control electrode 18 from outside the vacuum. The value of the control voltage VB1 is lower than the accelerating voltage of the primary electron beam 7, and is set to a value that has a positive potential with respect to the sample 3. As a practical value, about 100 to 350 V is suitable.

Further, above the electric field control electrode 18,
A second control electrode (energy control electrode) 21 is provided. This has the role of efficiently selecting the electrons (secondary electrons and reflected electrons) that have passed through the electric field control electrode 18 by kinetic energy and efficiently guiding them to the secondary electron detector 1.

The energy control electrode 21 also has an electron beam passage hole sharing the optical axis (central axis) with the objective lens and a flat plate (or hemispherical) mesh 22. An energy control voltage (VB2) 23 is applied to the energy control electrode 21 from outside the vacuum. The value of the control voltage VB2 can be varied in the range of -20 to + 40V so that the energies of the secondary electrons and the reflected electrons can be selected. The control electrodes 18 and 21 are electrically insulated from the ground 25 by the insulator 24.

According to this embodiment, 2 generated from sample 3
Secondary electrons are positively pulled up by the electric field due to the control voltage VB1, pass through the electric field control electrode 18 while drawing a spiral orbit under the influence of the magnetic field of the objective lens, and are energy-selected by the energy control electrode 21 to the secondary electron detector 1. Guided efficiently.

FIG. 4 shows a so-called in-lens type scanning electron microscope in which the sample 3 is arranged in the pole piece gap of the objective lens 2 and the two control electrodes 18 and 21.
It is an example in the case of disposing. According to this embodiment,
The same effect as described above is achieved.

FIG. 7 shows an example in which the relationship between the control voltages VB1 and VB2 and the output of the secondary electron detector is measured with respect to the photoresist surface of the semiconductor element sample in the structure described with reference to FIG. The acceleration voltage at this time is 70
The probe current Ip is 0 V and 4 pA.

When the control voltage VB1 is increased at a magnification of 1,000 times, the peak of the output of the secondary electron detector becomes + the control voltage VB2.
It is understood that the surface potential of the sample is easier to control and the charge balance is easier to apply when the voltage is shifted to the side and the control voltage VB1 is applied than when it is not applied.

Further, according to the results of experiments conducted by the inventors, the contact hole surface of the semiconductor element made of SiO 2 and photoresist on the Si substrate is magnified M 0 = 50,000 using the SEM described with reference to FIG. 2. Magnification, ML = 1,000 times, MH = 150,000 times,
When the deep hole having an aspect ratio of 3 or more was observed under the condition of the probe current Ip = 3 pA, the shape and size of the bottom hole were clearly observed.

This sample has a sectional shape as shown in FIG. 5, for example. In the case of the conventional technique, the trench portion (b) of the photoresist is negatively charged and blocked by the strong negative electric field in this portion, so that secondary electrons generated from SiO2 and Si below cannot escape upward. , SiO2 and Si contours (d) and (c) cannot be observed. That is, it becomes impossible to measure the hole bottom dimension and confirm the residue at the hole bottom.

However, like the present embodiment, when the observation method described with reference to the flow charts of FIGS. 8 to 10 is adopted and the control voltages VB1 and VB2 are applied, a trench portion (a) is obtained. Charge balance occurs, and secondary electrons generated from SiO2 and Si are efficiently detected, so that an observation image with high contrast can be obtained.

If the observation method of the present invention is not adopted, the irradiation current per unit area (electron beam irradiation amount IQ) becomes large at the magnification (M0) desired to be observed normally, so that the charge balance is disturbed and the secondary electrons from the hole bottom are destroyed. Will not be obtained and the contrast will be lost.

If the observation magnification (M0) is lowered to obtain a high contrast, the hole diameter observable on the CRT becomes small. For example, the hole diameter is 0.5 μm
Then, at a magnification of 25,000, at most 12.5
It is only about mm, and it is difficult to observe the hole bottom precisely.

However, if the present invention is applied, as shown in FIG. 6, the boundary portions (d) and (c) of SiO 2 and Si are separated.
The contrast of can be obtained for a certain period of time. According to the experiments by the inventors, by adopting this observation method, the magnification (M0) that is normally desired to be observed can be improved by 2 times or more.

FIG. 11 shows an SE according to the third embodiment of the present invention.
It is the schematic of M, and the same code as the above represents the same or equivalent part.

In this embodiment, a probe current detection circuit 26 for detecting a probe current, an optimum magnification display circuit 27 for displaying a magnification range in which charge-up is unlikely to occur based on the value of the detected probe current, and a designated value. It is characterized in that it is provided with a probe current control circuit 28 that controls the probe current value so that charge-up does not easily occur with respect to the magnification.

The present embodiment focuses on the fact that the charge-up on the sample surface depends on the electron beam irradiation amount IQ, and the probe current detection circuit 26 has, for example, a Faraday cup shape on a part of the sample stage 9. What is necessary is just to provide the above thing so that the current value can be measured from the outside of the vacuum in the order of 1 μA to 0.5 pA.

The optimum magnification display circuit 27 displays the range of the optimum magnification for the detected probe current value on the console (not shown) or the CRT 15 in analog or digital form. Alternatively, a pre-programmed menu may display optimal probe currents and magnifications for various samples.

The probe current control circuit 28 is used when the probe current is changed in accordance with the magnification M0 to be actually observed, and the high voltage power supply control circuit 29 of the electron gun 6 is operated by an operation button (not shown). Operate and electron beam 7
Is controlled so that a predetermined probe current can be obtained.

According to this embodiment, since the probe current which is less likely to cause charge-up can be grasped with certainty, a clear observation image can be obtained, and there is an advantage that it can be used for sample damage and condition determination of the electron optical system.

FIG. 12 shows an SE according to the fourth embodiment of the present invention.
It is the schematic of M, and the same code as the above represents the same or equivalent part.

In this embodiment, images of different magnifications are subjected to the same CR.
A feature is that a simultaneous display control circuit 30 of different magnification images for simultaneously displaying on T15 is provided. The same CR
Instead of displaying on T, images of different magnifications may be displayed simultaneously on a plurality of CRTs.

The principle of the display control circuit 30 for simultaneously displaying different kinds of magnification images is described in, for example, Japanese Patent Publication No. 46-24459 and Japanese Patent Publication No. 5-5.
2-20819, or Japanese Patent Publication No. 51-3615
No. 0 publication.

As is clear from the basic concept of the present invention described above, according to this embodiment, the sample surface is positively charged by the scanning for low magnification observation, and the sample surface is charged by the scanning for high magnification observation. Since it is negatively charged, it is possible to observe in a state where the charge is always balanced.

FIG. 13 shows an SE according to the fifth embodiment of the present invention.
It is the schematic of M, and the same code as the above represents the same or equivalent part.

In this embodiment, a so-called out-lens type SEM in which the secondary electron detector 1 is arranged below the objective lens 2 is used, and the control electrode 1 described with reference to FIGS.
The feature is that 8 and 21 are provided.

Electric field control electrode 18 and energy control electrode 2
1 is attached to the sample stage 9 via an insulator 24. The present embodiment has an advantage that the strength of the applied electric field with respect to the sample surface does not change even if the working distance or the tilt angle of the sample stage is changed.

Of course, the present invention is not limited to the above-mentioned embodiments, and, for example, an electron detector in which the electron detector is arranged in the center of the optical axis directly above the sample or the objective lens, The same effect can be obtained even when applied to a scanning electron microscope composed of a channel plate or the like.

[0071]

As described above, according to the present invention, the following effects can be achieved. (1) Before starting the observation, the sample surface is irradiated with an electron beam at a magnification lower or higher than a desired magnification for a certain period of time,
After that, if the electron beam is radiated after returning to the desired observation magnification, the charge balance can be temporarily obtained. Therefore, if the image observation is performed using the space, an observation image with high magnification and high resolution can be obtained. .. (2) By applying an electric field to the sample surface, secondary electrons generated from the sample surface by electron beam irradiation can be efficiently guided to the secondary electron detector, so that a scan image with a good SN ratio is obtained. be able to. (3) Since the relative relationship between the probe current and the magnification at which the sample surface is less likely to be charged up was determined, the probe current according to the desired observation magnification or the observation magnification according to the desired probe current was easily obtained. Optimum observation conditions depending on the sample, the magnification, and the probe current can be easily obtained. (4) If scanning for low magnification and scanning for high magnification are performed at the same time, it is possible to always observe with a charge balance.

[Brief description of drawings]

FIG. 1 is a schematic view of a scanning electron microscope which is an embodiment of the present invention.

FIG. 2 is a schematic view of a scanning electron microscope which is a second embodiment of the present invention.

FIG. 3 is a partially enlarged view of FIG.

FIG. 4 is a modification of the second embodiment of the present invention.

FIG. 5 is a diagram for explaining the operation of the present invention.

FIG. 6 is a diagram for explaining the operation of the present invention.

FIG. 7 is a diagram for explaining the operation of the present invention.

FIG. 8 is a flow chart for explaining the operation of the present invention.

FIG. 9 is a flowchart for explaining the operation of the present invention.

FIG. 10 is a flow chart for explaining the operation of the present invention.

FIG. 11 is a schematic view of a scanning electron microscope which is a third embodiment of the present invention.

FIG. 12 is a schematic view of a scanning electron microscope according to a fourth embodiment of the present invention.

FIG. 13 is a schematic view of a scanning electron microscope which is a fifth embodiment of the present invention.

[Explanation of symbols]

1 ... Secondary electron detector, 2 ... Objective lens, 3 ... Sample, 4 ...
Secondary electrons, 5 ... Magnification control means, 6 ... Electron gun, 7 ... Electron beam, 9 ... Sample stage, 10 ... Electron probe 12, Scanning power supply, 13 ... Magnetic pole hole, 14 ... Signal amplification circuit, 15 ... Cathode ray tube, 16 ... Magnification variable circuit, 17 ... Electric field control means, 1
8 ... Electric field control electrode, 19 ... Mesh, 20 ... Electric field control voltage (VB1), 21 ... Energy control electrode, 22 ... Mesh, 23 ... Energy control voltage (VB2), 26 ... Probe current detection circuit, 27 ... Optimal magnification display Circuit, 28 ... Probe current control circuit, 30 ... Simultaneous different-magnification image display control circuit

Front page continued (72) Inventor Hiroshi Iwamoto 882 Ichimo Ichi, Katsuta-shi, Ibaraki Hitachi, Ltd. Naka factory (72) Inventor Tadashi Otaka 882 Ichimo, Katsuta-shi, Ibaraki Hitachi Ltd. Naka factory ( 72) Inventor Hideo Tokoro, 882 Ishi, Ichi, Katsuta-shi, Ibaraki Hitachi Ltd. Naka factory (72) Inventor, Ko Kota 832, Nagakubo, Horiguchi, Katsuta-shi, Ibaraki (2) Invention Kazumasa Tobita 1040 Ma, Ichi, Katsuta-shi, Ibaraki Hitachi Science Systems Co., Ltd.

Claims (11)

[Claims] 1. A scanning electron microscope which obtains an observation image by scanning an electron beam spot in an observation region on a sample and capturing a signal secondarily generated from the observation region to obtain a desired observation magnification (M0). First magnification setting means to be set, and a lower magnification (ML) and a higher magnification (M) than the observation magnification (M0).
Second magnification setting means for setting at least one of H), a first observation mode for observing an image at an observation magnification (M0) after irradiating an electron beam at the low magnification (ML) for a predetermined time, and the high magnification Observation magnification after irradiation with electron beam at (MH) for a predetermined time
After irradiating the electron beam with the second observation mode for observing at (M0) and the low magnification (ML) or the high magnification (MH) for a predetermined time, the electron beam is further irradiated with the other magnification. For the scheduled time, and then the observation magnification (M
And a means for observing an image in any of the third observation modes for observing in (0). 2. The scanning electron microscope according to claim 1, wherein an electrode having a positive potential with respect to the sample and providing an electric field to the sample surface is provided. 3. The apparatus according to claim 1, further comprising: a unit for detecting an electron dose applied to the sample, and a unit for displaying an appropriate value range of an observation magnification according to the detected electron dose. The scanning electron microscope according to claim 2. 4. The method according to claim 1, further comprising: a means for detecting an electron dose applied to the sample, and a control means for controlling the electron dose according to an observation magnification. Scanning electron microscope. 5. The scanning electron microscope according to claim 1, further comprising means for alternately changing the scanning range of the electron beam and displaying a low-magnification image and a high-magnification image at the same time. .. 6. A scanning electron microscope observing method for scanning an electron beam spot in an observing region on a sample and capturing a signal secondarily generated from the observing region to obtain an observed image, comprising at least a predetermined method. A method for observing a scanning electron microscope, which comprises irradiating an area including an observation area with a weak electron beam for a certain period of time and then irradiating the scheduled observation area with a strong electron beam to perform image observation. 7. An observing method of a scanning electron microscope, comprising: scanning an electron beam spot in an observation region on a sample, capturing a signal secondarily generated from the observation region to obtain an observation image, which is a scheduled observation. A method for observing a scanning electron microscope, which comprises irradiating a strong electron beam to at least a part of the region for a certain period of time and then irradiating a weak electron beam to the predetermined observation region to perform image observation. 8. An observing method of a scanning electron microscope, wherein an observing region on a sample is scanned with an electron beam spot, and a signal secondarily generated from the observing region is taken in to obtain an observed image. It is characterized in that an area including a planned observation area is irradiated with an electron beam for a certain period of time at a magnification (ML) lower than the magnification (M0), and then the desired observation area is observed at the desired observation magnification (M0). Observation method of scanning electron microscope. 9. A scanning electron microscope observing method for scanning an electron beam spot in an observation region on a sample and capturing a signal secondarily generated from the observation region to obtain an observation image, comprising a desired observation. It is characterized by irradiating a part of a predetermined observation region with an electron beam for a certain period of time at a magnification (MH) higher than the magnification (M0), and then observing the desired observation region at the desired observation magnification (M0). Observation method of scanning electron microscope. 10. A scanning electron microscope observing method for scanning an electron beam spot in an observation region on a sample and capturing a signal secondarily generated from the observation region to obtain an observation image, comprising a desired observation. After irradiating the area including the planned observation area with an electron beam for a certain period of time at a magnification (ML) lower than the magnification (M0), a part of the planned observation area with a magnification (MH) higher than the desired observation magnification (M0) A method for observing a scanning electron microscope, which comprises irradiating the subject with an electron beam for a certain period of time, and then observing a desired observation region at the desired observation magnification (M0). 11. An observing method of a scanning electron microscope, wherein an electron beam spot is scanned in an observation region on a sample, and a signal secondarily generated from the observation region is taken in to obtain an observation image, which is a desired observation. Area that includes the planned observation area at a magnification (MH) higher than the magnification (M0) and after a certain period of electron beam irradiation on a part of the planned observation area, at a magnification (ML) lower than the desired observation magnification (M0). A method for observing a scanning electron microscope, which comprises irradiating the subject with an electron beam for a certain period of time, and then observing a desired observation region at the desired observation magnification (M0).