JPH08195181A - Scanning electron microscope - Google Patents

Abstract

PURPOSE: To remove the charge-up of a sample without limiting the primary function of an electron microscope. CONSTITUTION: The beam current value I1 of the electron beam radiated to a sample 7 and the current value I2 of the electron beam transmitting the sample 7 are detected, and the charge-up of the sample 7 is judged based on the detected current values I1 , I2 . When the sample is judged to have charge-up, the accelerating voltage Vo of a simplified electron gun 22 is obtained in response to the secondary electron emission ratio δ, the accelerating voltage Vo is applied to the filament 23 of the simplified electron gun 22, an electron beam is applied to the sample 7, and the charge-up of the sample 7 is removed.

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 (SEM) having a function of removing charge up of a sample.

[0002]

2. Description of the Related Art FIG. 4 is a block diagram of a scanning electron microscope. An electron gun 2 is provided on the microscope main body 1,
A first anode 3 and a second anode 4 for stopping the electron beam are provided downward from the electron gun 2, and a first electromagnetic lens (condenser lens 5) and a second electromagnetic lens (objective lens) 6 are arranged. ing.

A sample 7 is arranged below the microscope main body 1 with an angle of 45 degrees, for example. Note that this angle can be set arbitrarily. A secondary electron detector 8 is arranged near the sample 7. Although not shown, the microscope body 1 is provided with a deflection plate for scanning the electron beam on the sample 7 and a coil for alignment adjustment.

With this structure, when an electron beam is emitted from the electron gun 2, the electron beam is irradiated onto the sample 7. At this time, the electron beam is scanned by the deflection plate onto a desired area on the sample 7.

When the sample 7 is irradiated with this electron beam,
Secondary electrons are emitted from the sample 7. Secondary electron detector 8
Detects these secondary electrons as a current and outputs a detection signal corresponding to this current. However, by processing this detection signal, the surface shape of the sample 7 is measured as an image.

However, when the sample 7 is irradiated with an electron beam,
When the irradiation time becomes longer than the same scanning area, the scanning area of the sample 7 is charged, that is, charged up to the positive electrode “+” or the negative electrode “−”.

The polarity of this charge-up is determined by the acceleration voltage applied to the electron gun 2, for example, the acceleration voltage 1
Above kV, it is charged up to the negative electrode “−” as shown in FIG.

When the sample 7 is charged up to the positive electrode “+” or the negative electrode “−” in this way, the image quality of the surface of the sample 7 deteriorates, and in the worst case, the image of the sample surface does not appear.

When charge-up occurs in this way,
This charge-up is being removed. As the technique for removing the charge-up, for example, as a first method, a method of irradiating the back surface of the sample 7 with ions to mitigate the charge (Japanese Patent Laid-Open No. 15546/1990) or a second method of low vacuum is used. As used in the SEM, the sample chamber formed in the microscope main body 1 is placed in a low vacuum state, and there is a direction to neutralize the electrons charged on the surface of the sample 7 using the molecules charged by the electron beam.

As a third method for removing the charge-up, a method of easing the charge by alternately switching the acceleration voltage of the electron beam used for the measurement probe to a voltage having a polarity opposite to that of the voltage at the time of measurement. (Japanese Patent Application No. 4-12
845).

Further, as a method of preventing charge-up, there is a method of setting an accelerating voltage which does not cause charge-up by the sample 7 (Japanese Patent Laid-Open No. 62-69527). However, in the first method, since the back surface of the sample 7 is irradiated with ions, the sample 7 must be a thin film in order to neutralize the charge on the surface of the sample 7.

In the second method, the molecules after relaxation may cause contamination (sample contamination), and cannot be applied when the sample chamber is in a high vacuum. In the third method,
It is necessary to instantaneously align the axes of the lenses when switching the accelerating voltage, and there are many technically difficult aspects in the axis alignment of the lenses. Further, in the method of preventing charge-up, the acceleration voltage is fixed by the sample 7,
The accelerating voltage cannot be increased to take high resolution pictures.

[0013]

As described above, in order to remove the charge-up, the sample 7 must be a thin film, it cannot be applied when the sample chamber is in a high vacuum, and the lens cannot be used. There are many technically difficult aspects of axis alignment, and in order to prevent charge-up, the accelerating voltage is fixed and the accelerating voltage cannot be increased to take a high-resolution photograph. There was a limitation on the function of the electron microscope. Therefore, the present invention is
An object of the present invention is to provide a scanning electron microscope capable of removing charge-up of a sample without limiting the original function of the electron microscope.

[0014]

According to a first aspect of the present invention, a scanning type in which a sample is irradiated with an electron beam output from an electron gun and secondary electrons emitted from the sample are captured to measure a surface of the sample. In an electron microscope, a simple electron gun for removing charges accumulated in a sample, a beam current detecting means for detecting a beam current of an electron beam with which the sample is irradiated, and a current of an electron beam passing through the sample are detected. The transmission current detection means, the charge-up determination means for determining the charge-up of the sample based on at least the current values detected by these current detection means, and the beam current value and the secondary current detected by the beam detection means. Acceleration voltage calculation means for determining an acceleration voltage of a simple electron gun for removing charge-up of a sample according to a secondary electron emission ratio with a lower limit current value, and determination And a charge-up detection means for applying the acceleration voltage obtained by the acceleration-voltage calculation means to the simple electron gun when the charge-up determination is received from the stage. .

According to a second aspect of the present invention, the charge-up determination means determines the beam current of the electron beam with which the sample is irradiated by I
_{1. If} I _{2 is} the transmission current for detecting the current of the electron beam passing through the sample and I ₃ is the lower limit of the current value due to secondary electrons, | I ₁ − (I ₂ + I ₃ ) | When it becomes larger than the value, it is judged as charge-up.

According to the third aspect of the present invention, the charge-up determination means determines that the charge-up occurs when the signal level of the secondary electrons gradually increases or decreases. According to the fourth aspect, the accelerating voltage calculating means obtains the accelerating voltage of the simple electron gun for removing the charge-up of the sample from the curve showing the relationship between the accelerating voltage and the secondary electron emission ratio.

[0017]

According to the first aspect, when the sample is irradiated with the electron beam output from the electron gun and the secondary electrons emitted from the sample are captured to measure the surface of the sample, the sample is irradiated. The beam current of the electron beam is detected by the beam current detection means, and the current of the electron beam passing through the sample is detected by the transmission current detection means, and the charge-up determination means charges the sample up based on the detected current values. If it is determined that the charge-up has occurred, the acceleration voltage of the simple electron gun is obtained by the acceleration voltage calculation means according to the secondary electron emission ratio of the beam current value irradiated on the sample and the lower limit current value of the secondary current. The charge-up detection means applies the acceleration voltage to the simple electron gun to remove the charge-up of the sample.

According to claim 2, the beam current of the electron beam with which the sample is irradiated is I ₁ , the transmission current for detecting the current of the electron beam passing through the sample is I ₂ , and the lower limit value of the current value due to the secondary electrons. the case of the _{I 3, | I 1 - (} I 2 + I 3) | value is judged to charge-up when it becomes larger than a predetermined value.

According to the third aspect, when the signal level of the secondary electrons gradually increases or decreases, it is determined that the charge is up. According to the fourth aspect, the acceleration voltage of the simple electron gun for removing the charge-up of the sample is obtained from the curve showing the relationship between the acceleration voltage and the secondary electron emission ratio.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. The same parts as those in FIG. 4 are designated by the same reference numerals, and detailed description thereof will be omitted. FIG. 1 is a configuration diagram of a scanning electron microscope.

The main controller 10 has a function of controlling the entire scanning electron microscope. The main controller 10 controls the operation of the electron gun 2 in the microscope body 1 and also controls the first anode 3, the second anode 4 and the first electromagnetic lens 5. , And has a function of controlling the applied voltage to the second electromagnetic lens 6.

The secondary electron detector 8 includes an image holding circuit 11
Are connected, and the image of the processing result of the secondary electrons detected by the secondary electron detector 8, that is, the surface shape of the sample 7 is held as image data.

An observation CRT 13 is connected to the image holding circuit 11 via an image signal switching device 12.
On the other hand, a probe current detection diaphragm 20 is arranged below the second electromagnetic lens 6 in the microscope body 1. The probe current detection diaphragm 20 has a function of detecting the current of the electron beam with which the sample 7 is irradiated and outputting the detected current value.

A transmission current detector 21 is arranged below the sample 7. The transmission current detector 21 detects the current of the electron beam passing through the sample 7 and detects the detected current value I.
_It has the function of outputting ₂ .

The simple electron gun 2 is provided diagonally above the sample 7.
2 are provided. The simple electron gun 22 is for radiating an electron beam to the sample 7 to remove the charges charged up in the sample 7.

As shown in the enlarged view of FIG. 2, this simple electron gun 22 has a power source 24 connected to a coil-shaped filament 23, and a focusing electrode 25 and a deflection electrode on the traveling path of an electron beam emitted from the filament 23. The electrode 26 is arranged.

The computer 30 includes a probe current detection diaphragm 20.
Current value of the electron beam detected by, the current value transmitted by the sample 7 detected by the transmission current detector 21, and 2
Charge-up determination means for inputting current values due to secondary electrons detected by the secondary electron detector 8 and determining charge-up of the sample 7 based on these current values, current value irradiated to the sample 7 and secondary current It has each function as an acceleration voltage calculation means for obtaining the acceleration voltage of the simple electron gun 22 for removing the charge-up of the sample 7 according to the secondary electron emission ratio with the lower limit current value by.

More specifically, the charge-up determination means determines the beam current of the electron beam with which the sample 7 is irradiated by I ₁ ,
The transmission current for detecting the current of the electron beam passing through
_{2. When} the lower limit of the current value due to secondary electrons is I ₃ , it is determined that the charge-up occurs when the value of | I ₁ − (I ₂ + I ₃ ) | (1) is larger than a predetermined value. , Or has a function of determining charge-up when the signal level of secondary electrons at the same observation location detected by the secondary electron detector 8 gradually increases or decreases.

The beam current I ₁ of the electron beam with which the sample 7 is irradiated is the current value of the electron beam detected by the probe current detection diaphragm 20, the first electromagnetic lens 5 and the second electromagnetic lens 5.
It is obtained from the value of each lens current flowing through the electromagnetic lens 6.

The accelerating voltage calculating means is a secondary electron emission ratio δ δ = I ₃ / I ₁ (i) between the beam current I _{1 of the} electron beam with which the sample 7 is irradiated and the lower limit value I ₃ of the current due to the secondary current. According to 2), it has a function of obtaining the acceleration voltage of the simple electron gun 22 for removing the charge-up of the sample 7.

That is, this accelerating voltage calculating means stores in advance in the database a curve showing the relationship of the secondary electron emission ratio δ with respect to the accelerating voltage shown in FIG. This curve is
For example, a plurality of substances are stored for each of the substances A and B of the sample 7.

In this curve, the secondary electron emission ratio δ is δ
In the case of> 1, the sample 7 is charged up to the positive electrode “+”, and in the case of δ <1, the sample 7 is charged up to the negative electrode “−”.

Therefore, the accelerating voltage calculating means obtains the accelerating voltage Vo from the secondary electron emission ratio δ using a curve to charge up to the opposite polarity, and based on the width of the observation region and the accelerating voltage Vo at that time. It has a function of calculating each applied voltage of the focusing electrode 25 and the deflection electrode 26 for focusing the electron beam emitted from the simple electron gun 22 within a predetermined area on the surface of the sample 7.

The charge-up detector 31 is the computer 30.
When the charge-up determination is made by the computer 30, the acceleration voltage Vo obtained by the computer 30 is applied to the filament 23 of the simple electron gun 22, and the applied voltages of the focusing electrode 25 and the deflection electrode 26 obtained by the computer 30 are respectively applied. It has a function of applying to the focusing electrode 25 and the deflection electrode 26.

Next, the operation of the device configured as described above will be described. When an electron beam is emitted from the electron gun 2, the sample 7 is irradiated with this electron beam. At this time, the electron beam is scanned by the deflection plate onto a desired area on the sample 7.

When the sample 7 is irradiated with this electron beam,
Secondary electrons are emitted from the sample 7. Secondary electron detector 8
Detects these secondary electrons as a current and outputs a detection signal according to this current.

2 detected by the secondary electron detector 8
The secondary electron current is processed and held in the image holding circuit 11 as image data of the surface shape of the sample 7, and is sent to the observation CRT 13 via the image signal switcher 12.

In this state, the probe current detection diaphragm 20
Detects the current of the electron beam with which the sample 7 is irradiated and outputs the detected current value. The transmission current detector 21 detects the current of the electron beam passing through the sample 7 and outputs the detected current value I ₂ .

The charge-up determination means of the computer 30 is
The current of the electron beam applied to the sample 7 detected by the probe current detection diaphragm 20 is input, and the sample 7 is supplied to the sample 7 from the current value and the values of the lens currents flowing through the first electromagnetic lens 5 and the second electromagnetic lens 6. The beam current I ₁ of the irradiated electron beam is obtained.

The charge-up determination means inputs the beam current I ₁ and the current value I ₂ of the electron beam transmitted through the sample 7 detected by the transmission current detector 21,
When I ₃ is the lower limit of the current value due to secondary electrons, it is judged as charge-up when the value of | I ₁ − (I ₂ + I ₃ ) | becomes larger than the predetermined value in the above formula (1). To do.

The charge-up determination means determines that the charge-up occurs when the signal level of the secondary electrons at the same observation location detected by the secondary electron detector 8 gradually increases or decreases.

Therefore, the charge-up determination means determines whether the value of the above equation (1) is larger than a predetermined value or the signal level of the secondary electron shows an increase or a decrease. , Judgment that the sample 7 is charged up, and the charge-up determination signal J is sent to the charge-up detector 31.

When the charge-up is determined in this way, the accelerating voltage calculating means of the computer 30 has a secondary value of the beam current I _{1 of the} electron beam with which the sample 7 is irradiated and the lower limit value I ₃ of the current due to the secondary current. The electron emission ratio δ (= I ₃ / I ₁ ) is obtained, and the acceleration of the simple electron gun 22 for charging up the sample 7 in the opposite polarity from the curve of the relationship between the secondary electron emission ratio δ and the acceleration voltage Vo shown in FIG. Find the voltage Vo.

For example, if the accelerating voltage V ₁ with respect to the secondary electron emission ratio δ, the sample 7 is charged up to the negative electrode “−”, so that the simple electron is charged to the opposite polarity, that is, the positive electrode “+”. Acceleration voltage of gun 22, eg V ₂
Ask for.

The accelerating voltage calculating means focuses the electron beam emitted from the simple electron gun 22 on the basis of the width of the observation region and the accelerating voltage V ₂ at that time so as to focus the electron beam within a predetermined region on the surface of the sample 7. Each applied voltage of the electrode 25 and the deflection electrode 26 is calculated.

The charge-up detector 31 is the computer 30.
When the charge-up determination signal j is received from the computer 3,
Accelerating voltage V ₂ obtained by 0 receives each voltage applied to the focusing electrode 25 and deflection electrodes 26, the control signal of the acceleration voltage V ₂ and transmitted to the filament 23 of the simple electron gun 22, and the focusing electrode 25 and The control signals of the voltages applied to the deflection electrode 26 are sent to the focusing electrode 25 and the deflection electrode 26, respectively.

This simple electron gun 22 has a filament 23.
Accelerating voltage V ₂ is applied, and the voltage to the focusing electrode 25 and the deflecting electrode 26 is applied, thereby emitting an electron beam with respect.

This electron beam is focused by the focusing electrode 25, is deflected by the deflection electrode 26, and is irradiated onto a predetermined region on the sample 7. As a result, the charge-up of the sample 7 is removed.

The irradiation of the electron beam from the simple electron gun 22 is alternately performed with the observation of the sample 7 at regular intervals.
That is, the main controller 10 performs the switching operation of the image signal switch 12 at regular intervals to perform the sample 7 switching at regular intervals.
The observation image of is sent to the observation CRT 13.

Therefore, the observation CRT 13 is provided with the electron beam from the simple electron gun 22 just before the sample 7 is irradiated with the electron beam.
The next electronic image is displayed. Then, the electron beam from the simple electron gun 22 is repeatedly irradiated to the sample 7 at regular intervals until the charge-up of the sample 7 is removed.

As described above, in the above-described embodiment, the beam current value I _{1 of the} electron beam with which the sample 7 is irradiated, the sample 7
When the current value I ₂ of the electron beam passing through the sample is detected, the charge up of the sample 7 is judged based on the detected current values I ₁ and I ₂ , and when the charge up is judged, 2
Acceleration voltage Vo of the simple electron gun 22 according to the secondary electron emission ratio δ
Since the accelerating voltage Vo is applied to the filament 23 of the simple electron gun 22 to remove the charge-up of the sample 7, the sample 7 must be a thin film or the sample chamber is in a high vacuum. Is not applicable, there are many technically difficult aspects of the axis alignment of the lens, and the acceleration voltage of the electron gun 2 is fixed. Can use another simple electron gun 22 to remove charge-up of the sample 7.

Further, since the electron beam emitted from the simple electron gun 22 does not need to be scanned and the axis alignment with the electron beam emitted from the electron gun 2 is not necessary, the switching to the charge-up operation is short. Sample 7
You can reduce the time you can't observe.

Further, the charge-up judgment is made by | I ₁ − (I
₂ + I ₃ ) | becomes larger than a predetermined value, or the signal level of the secondary electrons gradually rises or falls, either one or both of them. Charge up can be detected reliably.

Further, since a curve showing the relationship between the accelerating voltage of various substances A, B and the like and the secondary electron emission ratio is stored, it can be applied to any kind of sample 7. Besides,
Even if the sample 7 is charged up to the positive electrode “+” or the negative electrode “−”, this charge-up can be removed.

The present invention is not limited to the above embodiment, but may be modified as follows. For example, the arrangement position of the simple electron gun 22 is not limited as long as the sample 7 is irradiated with the electron beam.
Also, the beam current I _{1 of the} electron beam with which the sample 7 is irradiated
Alternatively, a beam detector may be arranged on the traveling path of the electron beam for direct detection.

[0056]

As described in detail above, according to the present invention, it is possible to provide a scanning electron microscope capable of removing charge-up of a sample without limiting the original function of the electron microscope.
Further, according to the present invention, even if the sample is not a thin film, the sample chamber does not have to be in a high vacuum, there are not many technically difficult surfaces for axial alignment of the lens, and the acceleration voltage of the electron gun is fixed. It is possible to provide a scanning electron microscope capable of removing the charge-up of a sample by using a simple electron gun different from the original electron gun without restricting the function of the scanning electron microscope.

Furthermore, according to the present invention, it is possible to provide a scanning electron microscope which can switch to the charge-up operation in a short time, shorten the time during which the sample cannot be observed, and remove the charge-up of the sample.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of a scanning electron microscope according to the present invention.

FIG. 2 is an enlarged view showing the arrangement and configuration of a simple electron gun.

FIG. 3 is a diagram showing a relationship between a secondary electron emission ratio and an accelerating voltage.

FIG. 4 is a configuration diagram of a conventional electron microscope.

FIG. 5 is a diagram showing charge-up on a sample.

[Explanation of symbols]

1 ... Microscope main body, 2 ... Electron gun, 5 ... First electromagnetic lens, 6
... second electromagnetic lens, 7 ... sample, 8 ... secondary electron detector, 1
0 ... Main controller, 11 ... Image holding circuit, 12 ... Image signal switching device, 13 ... Observation CRT, 20 ... Probe current detection diaphragm, 21 ... Transmission current detector, 22 ... Simple electron gun, 23 ... Filament, 24 ... Power supply, 25 ... Focusing electrode, 26 ... Deflection electrode, 30 ... Calculator, 31 ... Charge-up detector.

Claims (4)

[Claims] 1. A scanning electron microscope which irradiates a sample with an electron beam output from an electron gun, captures secondary electrons emitted from the sample to measure the surface of the sample, and is charged up to the sample. A simple electron gun for removing electric charges, a beam current detecting means for detecting a beam current of an electron beam with which the sample is irradiated, a transmitted current detecting means for detecting a current of an electron beam passing through the sample, A charge-up determination unit that determines charge-up of the sample based on at least each current value detected by these current detection units, a beam current value detected by the beam detection unit, and a lower limit current value by the secondary current. Acceleration voltage calculating means for obtaining the acceleration voltage of the simple electron gun for removing the charge-up of the sample according to the secondary electron emission ratio of When subjected to charge-up judgment from the judging means,
A scanning electron microscope, comprising: a charge-up detection unit that applies the acceleration voltage calculated by the acceleration voltage calculation unit to the simple electron gun. 2. The charge-up determination means is a beam current of an electron beam with which a sample is irradiated, I ₁ , a transmission current for detecting a current of an electron beam passing through the sample is I ₂ , and a current value of secondary electrons _2. The scanning electron according to claim 1, wherein when the lower limit value is I ₃ , it is determined that the charge is up when the value of | I ₁ − (I ₂ + I ₃ ) | becomes larger than a predetermined value. microscope. 3. The scanning electron microscope according to claim 1, wherein the charge-up determination means determines that the charge-up occurs when the signal level of the secondary electrons gradually increases or decreases. 4. The accelerating voltage calculation means obtains an accelerating voltage of a simple electron gun for removing charge-up of a sample from a curve showing a relationship between the accelerating voltage and the secondary electron emission ratio. Scanning electron microscope.