JP3220997B2 - Secondary or backscattered electron detectors for scanning electron microscopes and similar devices - Google Patents

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 similar devices, and more particularly to a scanning electron microscope using a pulse counting type detector as a sample image signal detector for secondary electrons or the like.

[0002]

2. Description of the Related Art A scanning electron microscope performs two-dimensional scanning of a finely converged electron beam on a sample, detects and amplifies signals such as secondary electrons generated from the sample, and converts this signal as a luminance modulation signal to a cathode ray. The enlarged sample image is displayed on a tube or the like. As means for detecting the signal,
A combination of a scintillator (electron-light conversion element) that emits light by electron incidence and a photomultiplier tube (scintillator-photomultiplier type detector) is generally used, and the output current of the photomultiplier tube is used. Is amplified by an amplifier and directly converted into a luminance modulation signal of a cathode ray tube, or is once converted into a digital signal by an A / D converter through a digital circuit such as an image memory, and then analogized again by a D / A converter. As a signal, as a luminance modulation signal of a cathode ray tube,
An image was formed.

In this method, a signal is treated as a continuous amount (analog amount).
It is a collection of electrons as particles, and the magnitude of the signal is the number of electrons, so there is a method (pulse counting method) in which individual electrons are separated and detected separately and counted as a signal,
For detecting weak light, a method using a photomultiplier is used.

In the pulse counting method, since the incident electrons are separately detected, the detector, the amplifier, and the counting circuit need to operate at a speed higher than the interval between the incident electrons. The secondary electron signal of a scanning electron microscope is at least several pA, and 1 pA is 6,250,000 per second.
Since the number of electrons corresponds to the number of electrons, in order to count this without counting down, the above detection system needs an operation speed exceeding 100 MHz (considering that the generation of electrons is a stochastic process). Because of this, it was not easy to satisfy this, and the pulse counting method was not conventionally used. However, in recent years, a high-speed detection system that satisfies the above conditions has become possible, and the pulse counting method has been used in a scanning electron microscope.

The pulse counting method is described, for example, in the document “Secondary
Electron Counting Images in SEM, Proceedings of th
e XIIth International Congress for Electron Micros
copy, 1990, pp. 402-403 ”, compared to the method of processing as an analog signal,
(1) When the amount of secondary electrons is the same, the S / N ratio is improved several times, and data with less noise can be obtained.

(2) In order to obtain data having the same S / N ratio, the amount of secondary electrons may be small. That is, the amount of primary electrons irradiating the sample can be reduced, and the spatial resolution can be improved, charging, sample contamination can be reduced, and sample damage can be reduced.

This is an excellent method.

[0008]

When a pulse counting method using a scintillator-photomultiplier detector is used, individual electrons must be separated and counted.
However, increasing the amount of incident electrons means that
The separation is shorter, which exceeds the time resolution
Then, there is a problem that it does not operate normally .

The object of the present invention has such a problem.
Scanning electron microscope with pulse counting detector.
To secure both the quantity of incident electrons and the prevention of malfunction.
And there .

[0010]

To achieve the above object of the Invention The present pulse counting type scintillator is invention - the scanning electron microscope with a photomultiplier-type electron detector Contact
And disposed between the scintillator and the sample,
Electron suppression for controlling the amount of electrons incident on the scintillator
Controlling the voltage applied to the control electrode and the electron suppression electrode
A control circuit, the control circuit comprising an output of the detector.
The incident electron on the detector when exceeds a certain threshold
Is controlled to limit the incident electron
Is set to be less than the time resolution of the detector.
A scanning electron microscope is provided.

According to the above configuration, the amount of incident electrons is
In the range that does not exceed the time resolution of the scanning electron detector
In this case, the maximum amount of incident electrons can be
If it exceeds the range, the amount of incident light will be suppressed and
It can prevent the malfunction of the electronic counting type electronic detector.
You .

In the case of controlling the amount of electrons incident on the scintillator in accordance with the amount of primary electrons incident on the sample, a detector for detecting the amount of electrons incident on the sample is provided in the electron beam path. Then, a control signal corresponding to the output signal may be generated and input to the power supply. This control signal measures the output signal of the primary electron detector when the time resolution of the pulse counting type detector is exceeded, or the output signal when the scintillator is deteriorated more than necessary. If a voltage is generated in such a direction as to decrease the amount of electrons incident on the scintillator when it exceeds, the life of the scintillator will be significantly shortened, and the amount of electrons exceeding the time resolution of the detector will not be incident on the scintillator.

When the control signal is generated not according to the primary electron signal but according to the amount of secondary electrons generated from the sample, the control signal is generated from the output signal of the pulse counting type detector itself, The amount of electrons incident on the scintillator may be controlled.

[0014]

An embodiment of the present invention will be described below with reference to FIG. The scanning electron microscope uses an electron beam 2 generated by an electron gun 1.
Is narrowed by the converging lens 3 and the objective lens 7 to make the sample 8 narrower.
Irradiate on top. At the same time, it is two-dimensionally deflected by a deflection system 6 driven by a scanning circuit, and forms an image using an image signal of secondary electrons or the like generated from the sample as a luminance signal. In the present embodiment, the amount of electrons incident on the scintillator 11 is controlled based on the signal amount of primary electrons on the sample 8. Therefore, a diaphragm 4 is provided in the electron beam path for detecting the amount of primary electrons, and this output is amplified by a preamplifier 5.

The secondary electrons 9 generated from the sample are attracted by the voltage from the high voltage power supply 23 applied to the scintillator surface and collide with the scintillator 11. At this time, the secondary electron control electrode 10 provided between the sample and the scintillator
As described in the previous section, the amount of secondary electrons that reaches the scintillator changes depending on the voltage applied to.

When electrons collide with the scintillator, the scintillator emits light for a certain period of time. This light is taken out of the vacuum while being totally reflected by a rod-shaped light guide 12 whose surface is sufficiently polished. This light is converted into an electric signal by a photomultiplier tube 13 and further converted to a preamplifier 1.
Amplify by 4. Since the output of the preamplifier 14 contains noise, the noise is removed by the wave height discriminator 15 and the waveform is shaped for input to the counter. The counter 15 outputs a count value to the register 17 and resets it at a clock of a cycle of one pixel of the image formed by the scanning circuit 18 and starts counting the data of the next pixel. The data in the register 17 is converted into an analog signal by the D / A converter 20, further amplified by the amplifier 21, and applied to the grid of the cathode ray tube 22 to become a luminance signal. The one composed of these is called a pulse counting type detector.

The output of the preamplifier 5 proportional to the amount of primary electrons is input to a dead zone circuit 25. This dead zone circuit
As shown in FIG. 2, an output signal is not generated until the input signal reaches a certain reference voltage, and can be constituted by a general operational amplifier or the like. A signal of V1 is input from the reference voltage generator 26 to the dead zone circuit 25. As the voltage V1, the preamplifier output voltage of the primary electron detector corresponding to the amount of primary electrons when exceeding the time resolution of the pulse counting type detector is set in advance. Therefore, the output of the dead zone circuit 25 is not output until the voltage exceeds V1. However, when the output exceeds V1, a negative control signal is generated so as to reduce the number of secondary electrons incident on the scintillator 11. Until the output of the preamplifier 5 of the primary electron detector reaches V1, the voltage of the secondary electron control electrode is controlled by the control signal from the reference voltage generator 27 so that the secondary electrons incident on the scintillator 11 are not affected. The secondary electrons 9 set and generated are almost taken into the scintillator.

With the above configuration, a large amount of secondary electrons exceeding the time resolution of the pulse counting type detector will not be taken into the scintillator. If the voltage V1 is set to the upper limit of the range of the normal observation state, the life of the scintillator will not be significantly shortened.

In this embodiment, the amount of primary electrons incident on the sample is directly detected by the aperture plate, and the voltage to the secondary electron control electrode is controlled based on the signal. However, looking at the configuration of recent scanning electron microscopes, as shown in FIG. 1, the high voltage power supply 29 and the convergent lens power supply 30 are controlled by a microcomputer, and the acceleration voltage value, the radiation current value from the electron gun, and the convergent current Values can be read easily. Therefore, the current amount of primary electrons is obtained from the conditions of these electron optical systems,
It is also possible to directly control the input signal of the control power supply, that is, the voltage of the secondary electron control electrode from the microcomputer. According to this method, the dead zone circuit 25 and the adder 2
8. The reference voltage generators 26 and 27 become unnecessary, and the device can be realized more easily.

Next, an embodiment in which the amount of secondary electrons incident on the scintillator is controlled by the output of the pulse counting type detector itself will be described with reference to FIG. The pulse counting type detector that detects secondary electrons generated from a sample and obtains an image signal from its output is as described in FIG. Here, the output from the counter 16 is input to the digital comparator 31 at the same time as being input to the register.
The digital comparator 31 compares a preset set value with the output data of the counter, and generates a control signal when the output of the counter 16 exceeds the set value. With this control signal, the control power supply 24 changes the voltage of the secondary electron control electrode stepwise in the negative direction so that the output of the counter becomes equal to or less than the set value. The control power supply 24 is configured so that the voltage does not increase again even if the amount of secondary electrons decreases and the control signal from the comparator disappears. A control signal is input to control the amount of secondary electrons manually. With the above-described configuration, it is possible to realize a control in which the number of secondary electrons is reduced automatically and the number of secondary electrons is increased manually. As a result, no secondary electrons exceeding the set value enter the scintillator, and the same result as in the embodiment of FIG. 1 is obtained.

When the output value of the counter falls below a certain value, it is possible to automatically increase the amount of secondary electrons. However, in general, the output value of the counter greatly varies depending on the sample shape. Therefore, this method is not considered appropriate.

[0022]

According to the present invention, the amount of incident electrons can be secured and detected.
This makes it possible to prevent malfunction of the output unit .

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an example in which the amount of electrons incident on a scintillator is controlled according to the amount of primary electrons incident on a sample.

FIG. 2 is a diagram showing the relationship between input and output signals of a dead zone circuit 25 in FIG.

FIG. 3 shows an embodiment in which the amount of electrons incident on the scintillator is controlled by the output signal of the pulse counting type detector itself, and when secondary electrons are further reduced, automatic control is performed; FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Electron gun, 2 ... Electron beam, 3 ... Convergent lens, 4 ... Aperture plate, 5 ... Preamplifier, 6 ... Deflection system, 7 ... Objective lens, 8
... sample, 9 ... secondary electron, 10 ... secondary electron control electrode, 11
... Scintillator, 12 ... Light guide, 13 ... Photomultiplier tube, 14 ... Preamplifier, 15 ... Speaker discriminator, 16 ...
Counter, 17 register, 18 scanning circuit, 19 microcomputer, 20 D / A converter, 21
Amplifier 22, 22 cathode ray tube, 23 high voltage power supply, 24 control power supply, 25 dead zone circuit, 26 reference voltage generator, 2
7 ... Reference voltage generator, 28 ... Adder, 29 ... High voltage power supply, 30 ... Convergent lens power supply, 31 ... Digital comparator, 3
2. Control signal generator.

Continuation of the front page (72) Inventor Tetsuya Nishioka 882 Ma, Katsuta-shi, Ibaraki Pref. Naka Works, Hitachi, Ltd. (56) References JP-A-2-2968 (JP, A) JP-A-59-79970 (JP) (U, J) U.S.A. 49-88661 (JP, U) (58) Fields investigated (Int. Cl. ⁷ , DB name) H01J 37/244 H01J 37/28

Claims (2)

(57) [Claims] 1. A pulse counting scintillator - scanning electron microscope odor with a photomultiplier-type electron detector
And disposed between the scintillator and the sample,
Electronic suppression to control the amount of electrons incident on the scintillator
Electrode and a control for controlling the voltage applied to the electron suppression electrode.
A control circuit, wherein the control circuit outputs the output of the detector.
When a certain threshold is exceeded, electrons incident on the detector are
Control to limit the incident electrons.
Volume is set to be less than the time resolution of the detector
A scanning electron microscope . 2. The control circuit according to claim 1 , wherein the output value of the detector determines the threshold value.
When this is exceeded, the amount of electrons incident on the detector is reduced, and
That the reduction control is maintained even when the pressure falls below the threshold value.
A scanning electron microscope characterized by the following .