JPH05174765A - Scanning electron microscope of environment control type - Google Patents

Abstract

PURPOSE:To enable observation to be carried out without electrostatically charging an insulative object even with an electron beam having a large acceleration voltage and prevent drop of the sensitivity of a sensor even though the specimen chamber gas pressure is reduced by furnishing a vacuum piping for gas exhaustion in the lower part of a hole-equipped magnetic pole. CONSTITUTION:A vacuum piping 5 for gas exhaustion is installed between the lens barrel part of a hole--equipped magnetic pole 1 and a specimen chamber 13, and thereby the gas having intruded into the piping 5 from the specimen chamber 13 via a pressure limiting opening 7 is exhausted quickly through the piping 5. Also the gas having intruded to the lens barrel part of the magnetic pole 1 from the said opening 7 via the piping 5 and another pressure limiting opening 6 is exhausted by a vacuum pump provided for the lens barrel part. This allows keeping well the degree of vacuum in the lens barrel part of magnetic lens and preventing the inside from being contaminated with the gas in the chamber 13. Provision of notch in the piping 5 eliminates existence of obstacle in the track for secondary electrons, which enables acquisition of a satisfactorily large sensing signal for secondary electron, and even if the gas pressure in the chamber 13 is sunk, a sensing signal of good signal-noise ratio can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an environment controlled scanning electron microscope (ESEM), and more particularly to a technique for improving the signal-to-noise ratio of a secondary electron detection signal in the ESEM and simplifying a vacuum exhaust device.

[0002]

2. Description of the Related Art Conventionally, an environment-controlled scanning electron microscope (ESEM) is known in which a low-pressure gas is introduced into a sample chamber and secondary electrons from the sample are observed through the low-pressure gas.
In such an ESEM, the pressure limiting opening (aperture) provided between the sample chamber and the lens barrel of the magnetic lens also serves as the secondary electron detector.

Further, the magnetic lens constituting the objective lens is divided into two magnetic poles, a sample is inserted between these two magnetic poles, and observation is performed with a magnetic field applied to the sample.
M is known as an SEM having an in-lens type objective lens.

Further, there is known an SEM in which a secondary electron detector is provided in the magnetic field of the objective lens.

[0005]

However, in the ESEM as described above, since the pressure limiting opening also serves as the secondary electron detector, if the gas pressure in the sample chamber is made too small, (the secondary electron detection from the sample is detected. (Distance to the detector) × (pressure of gas), that is, the PD product becomes small, and the sensitivity of the secondary electron detector becomes poor. On the contrary, when the gas pressure in the sample chamber is increased, there is a problem that the primary electron beam irradiated on the sample is scattered more and the number of finely scattered non-scattered electron beams for irradiating the sample is decreased.

In the conventional in-lens type SEM, the surface of the insulator is charged when the sample is observed, so that the accelerating voltage of the electron beam needs to be set to 1 KV or less, the chromatic aberration becomes large, and the electron beam becomes large. There was an inconvenience that I couldn't squeeze it too thin.

In view of the above problems in the conventional apparatus, an object of the present invention is that even an electron beam having a relatively high accelerating voltage can be observed without charging the insulator, and the gas pressure in the sample chamber can be greatly increased. It is an object of the present invention to provide an environment-controlled scanning electron microscope in which the sensitivity of the secondary electron detector does not decrease even if it is reduced to 1.

[0008]

In order to solve the above problems, the present invention irradiates an electron beam from an electron gun onto a sample arranged in a magnetic field of an objective lens to scan the sample, Between the sample and the magnetic pole located on the passage side of the electron beam among the magnetic poles of the objective lens, in an ESEM having an electron optical system for detecting the secondary electrons of the secondary electron detector through a gas of low pressure. It is characterized in that a pipe for differential exhaust is provided in the.

The pipe has an opening through which the electron beam passes, and is provided with a cutout so as not to block the space where the secondary electron detector is seen from the incident position of the electron beam on the sample. It is convenient to provide a notch.

Further, in order to observe the sample while inclining the sample, it is convenient to incline the tip end face.

[0011]

In the above structure, since the pipe for differential pumping is provided between the magnetic pole located on the electron beam passage side of the magnetic pole of the objective lens and the sample, the primary electron beam from the electron gun passes. The passage is kept in a high vacuum, while the passage through which the secondary electrons emitted from the sample pass is kept at a pressure convenient for secondary electron multiplication. Therefore, the signal-to-noise ratio (S / N) is prevented from lowering due to the scattering of the primary electron beam, and a sufficiently large detection signal is obtained from the secondary electron detector.

In this case, the pipe is provided with a notch so as not to block the space where the secondary electron detector is seen from the incident position of the electron beam on the sample.
The obstacle does not exist in the orbit of the secondary electron, and a sufficiently large secondary electron detection signal is obtained.

Further, in the above structure, since the secondary electron detector is located between the two magnetic poles, a strong magnetic field exists between the beam incident position on the sample and the secondary electron detector. .. Therefore, the secondary electron beam emitted from the beam incident point moves in a spiral orbit along these magnetic fields and does not go straight to the secondary electron detector. Therefore, the secondary electron detector travels equivalently over a long distance, which greatly increases the probability of collision with gas molecules. Therefore, in combination with the action of the piping for the differential exhaust, the normal ES
A large secondary electron detection signal can be obtained even in a gas having a lower pressure than EM.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a sectional view showing a structure in the vicinity of an objective lens of an environment-controlled scanning electron microscope (ESEM) according to one embodiment of the present invention. The apparatus shown in the figure has a perforated magnetic pole or upper pole 1 having a hole through which an electron beam passes, and a frustoconical non-perforated magnetic pole or lower pole 2 facing the perforated magnetic pole 1 and exciting coils 3a, 3a.
and an objective lens having b. Further, deflectors 4a and 4b are provided for scanning an electron beam from an electron gun (not shown) provided above the perforated magnetic pole 1. A central opening of the perforated magnetic pole 1 is a pressure limiting opening 6, and the pressure limiting opening 6 appropriately adjusts the atmospheric pressure difference between the magnetic lens barrel and the sample chamber or the like.

A sample 11 attracted by an electrostatic chuck 12 is arranged between the magnetic pole 1 with holes and the magnetic pole 2 without holes. This sample 11 can be rotated together with the electrostatic chuck 12 from a horizontal state to an angle of 60 degrees as shown by an imaginary line in FIG.

A vacuum pipe 5 is arranged between the perforated magnetic pole 1 and the sample 11 and communicates with a vacuum pump (not shown).

FIGS. 2A and 2B are a side view and a bottom view showing the detailed structure of such a vacuum pipe 5. As is apparent from these figures and FIG. 1, the pressure limiting opening 6 is provided on the surface of the tip end of the vacuum pipe 5 which is in contact with the perforated magnetic pole 1, and another surface is provided on the surface facing the sample 11. A pressure limiting opening 7 is provided. The pressure limiting opening 7 has a necessary and sufficient size so that observation at a low magnification is possible. The tip of the vacuum pipe 5 has an inclined tip surface so that the sample 11 can be inclined together with the electrostatic chuck 12. Further, secondary electron detectors 10 are provided on both side surfaces of the tip of the vacuum pipe 5. The tip of these secondary electron detectors 10, that is, the edge of the end facing the sample 11 is processed into a hemispherical shape in order to increase the discharge voltage. The vacuum pipe 5 is provided with a notch 15 so as not to block the space where the secondary electron detector 10 is seen from the beam incident point 14 on the sample 11.

A sample chamber 13 in which the sample 11 is arranged
Is filled with a gas such as water (H ₂ O) of about 0.5 Torr. An electron beam from an electron gun (not shown) passes through, and the lens barrel inside the perforated magnetic pole 1 in which the deflectors 4a, 4b and the like are arranged is evacuated by a vacuum pump (not shown).

In the ESEM having the above structure, the electron beam generated from the electron gun (not shown) passes through the field-scanning deflectors 4a and 4b, is limited through the pressure limiting opening 6, and is vacuum piping. 5 and another pressure limiting aperture 7 to enter the beam incident point 14 of the sample 11. The sample 11 is, for example, a semiconductor wafer,
It is attracted by the electrostatic chuck 12 and is flattened. In this case, the electron beam is converged by the magnetic flux 8 indicated by the dotted line between the magnetic pole 1 with holes and the magnetic pole 2 without holes, and is deflected by the deflectors 4a and 4b with the pressure limiting aperture 6 as the deflection center, and on the sample 11. To scan.

When the electron beam is incident on the beam incident point 14 of the sample 11 in this manner, secondary electrons are emitted from the beam incident point according to the surface state of the sample 11. The emitted secondary electrons are trapped in the magnetic flux 8 of the objective lens and make a curved motion as shown in the orbit 9, for example, and repeatedly collide with gas molecules, and the potential energy initially held moves little by little. The secondary electrons are multiplied by changing to the energy of and further ionizing gas molecules. Then, after the potential energy is almost lost, it is incident on the secondary electron detector 10. The secondary electrons generated on the way are also multiplied and enter the secondary electron detector 10.

Further, since the reflected electrons from the sample 11 have high energy, they are hardly bent by the magnetic lines of force of the magnetic lens, and the gas pressure in the sample chamber is low, so that they hardly collide with gas molecules and secondary electrons are generated. It hardly enters the detector.

In such an operation, the vacuum pipe 5 is interposed between the barrel of the magnetic pole with hole 1 and the sample chamber, and the gas that has entered the vacuum pipe 5 from the sample chamber through the pressure limiting opening 7. Is quickly exhausted through the vacuum pipe 5. Further, the gas that has entered the barrel of the magnetic pole with holes 1 from the pressure limiting opening 7 through the vacuum pipe 5 and the pressure limiting opening 6 is exhausted and removed by the vacuum pump for the barrel. Therefore, the degree of vacuum inside the barrel of the magnetic lens can be kept good, and the inside is prevented from being contaminated by the gas in the sample chamber. Furthermore, since the vacuum pipe 5 is present, the distance that the primary electrons that enter the sample 11 from the electron beam pass through the gas is shortened, and the collision of the primary electrons with the gas is reduced.

FIG. 3 shows an ES according to another embodiment of the present invention.
The structure in the vicinity of the objective lens of the EM is shown. In the figure (a), the left half is a cross-sectional view along the line OA in the figure (b) along the center line, and the right half is along the line OB. A cross-sectional view is shown. FIG. 3B is a view of the vacuum piping and the secondary electron detector as seen from below.

The apparatus shown in FIG. 3 is for the case where it is not necessary to tilt the sample, and has a magnetic lens having a magnetic pole with hole 21, a flat magnetic pole without hole 22 and an exciting coil 23.
A deflector 24a for scanning and deflecting an electron beam from an electron gun (not shown) is provided above the central opening of the perforated magnetic pole 21.
24b is provided. A sample 31 is fixed on the flat magnetic pole 22 without holes by an electrostatic chuck 32. At the central opening of the perforated magnetic pole 21, a vacuum pipe 25 located between the magnetic pole 21 and the sample 31 is provided. A pressure limiting opening 26 is formed on the surface of the vacuum pipe 25 which is in contact with the central opening of the perforated magnetic pole 21.
Is provided, and another pressure limiting opening 27 is provided on the surface of the vacuum pipe 25 facing the sample 31.

Further, two secondary electron detectors 30 are provided on both sides of the vacuum pipe 25 at symmetrical positions with respect to the line connecting the pressure limiting openings 26 and 27. Each secondary electron has a rounded end facing the beam incident position on the sample 31 of the detector 30. Further, the vacuum pipe 25 is provided with a notch 35 so as not to block the space where the secondary electron detector is seen from the beam incident position on the sample 31.

In the apparatus of FIG. 3, an electron beam output from an electron gun (not shown) passes through the deflectors 24a and 24b, and passes through the pressure limiting opening 26, the vacuum pipe 25, another pressure limiting opening 27 and the sample chamber 33. It is incident on the sample 31.
As a result, secondary electrons are emitted from the sample 31,
The secondary electron makes a curvilinear motion by the magnetic field 28 of the magnetic lens and enters the secondary electron detector 30 to obtain a detection signal.

Also in the embodiment of FIG. 3, the sample chamber 3
The gas that has entered the vacuum pipe 25 from 3 through the pressure limiting opening 27 is quickly exhausted by the vacuum pump communicating with the vacuum pipe 25. Further, gas that has entered the magnetic lens barrel through the pressure limiting opening 26 is also removed by the vacuum pump for the barrel. Therefore, the degree of vacuum inside the magnetic lens barrel is maintained well, and the inside of the barrel is kept in the sample chamber 3
Contamination by the gas of 3 is prevented.

[0028]

As described above, according to the present invention, since the vacuum pipe for exhausting gas is provided below the perforated magnetic pole, the degree of vacuum inside the objective lens barrel can be kept extremely good.
A gas such as H ₂ O gas does not enter the lens barrel from the sample chamber to contaminate the inside.

Further, since the vacuum pipe is provided with a notch in the space from which the secondary electron detector is seen from the beam incident point, there is no obstacle in the orbit of the secondary electrons, which is sufficiently large. A secondary electron detection signal is obtained. In addition, the secondary electrons emitted from the sample are trapped by the strong magnetic field of the objective lens and enter the secondary electron detector through a long curved orbit, so that the signal-to-noise ratio is good even if the gas pressure in the sample chamber is lowered. A secondary electron detection signal is obtained.

Further, the reflected electrons generated by the reflection of the primary electrons by the sample hardly enter the secondary electron detector, and therefore a secondary electron image with almost no reflected electron signal mixed therein can be obtained. Very thin films such as can also be observed.

Further, since the edge of the secondary electron detector facing the sample is rounded, a high electric field is not generated near the surface of the secondary electron detector, and a comparison is made even in the presence of a magnetic field and gas. The target discharge voltage becomes high, and a high voltage can be applied to the secondary electron detector. Therefore, the sensitivity of the secondary electron detector is increased. Since the two secondary electron detectors are provided at positions symmetrical with respect to the beam axis as in the previous embodiment, no harmful illumination effect occurs.

[Brief description of drawings]

FIG. 1 is a cross-sectional explanatory view showing a configuration near an objective lens of an environment-controlled scanning electron microscope according to a first embodiment of the present invention.

FIG. 2 is a partial side view (a) and a partial bottom view (b) showing a detailed structure near a vacuum pipe used in the apparatus of FIG.

FIG. 3 is a cross-sectional explanatory view (a) showing a structure in the vicinity of an objective lens of an environment-controlled scanning electron microscope according to a second embodiment of the present invention and a bottom view showing a structure of a vacuum pipe and a secondary electron detector. It is (b).

[Explanation of symbols]

 1, 21 Perforated magnetic pole 2, 22 Non-perforated magnetic pole 3a, 3b, 23 Objective lens exciting coil 4a, 4b, 24a, 24b Scan deflector 5,25 Exhaust vacuum pipe 6,7,26,27 Pressure limiting opening 8 , 28 Magnetic field lines 9,29 Secondary electron orbit 10,30 Secondary electron detector 11,31 Sample 12,32 Electrostatic chuck 13,33 Sample chamber 14 Primary beam incident point 15,35 Notch

Claims (4)

[Claims] 1. A sample arranged in a magnetic field of an objective lens is irradiated with an electron beam from an electron gun to scan, and secondary electrons from the sample are passed through a low-pressure gas by a secondary electron detector. An environment control type scanning electron microscope having an electron optical system for detecting, wherein a pipe for differential exhaust is provided between the sample and a magnetic pole located on a passage side of the electron beam among magnetic poles of the objective lens. An environment-controlled scanning electron microscope characterized in that 2. The environment-controlled scanning electron microscope according to claim 1, wherein the pipe has an opening through which the electron beam passes. 3. The pipe according to claim 1, wherein the pipe has a notch so as not to block a space in which the secondary electron detector is seen from the incident position of the electron beam on the sample. Environmentally controlled scanning electron microscope as described. 4. The environment control type scanning electron microscope according to claim 1, wherein the end surface of the pipe is inclined in order to incline the sample.