JPH0610969B2 - Electron beam detector - Google Patents

Description

Detailed Description of the Invention a. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a secondary electron multiplier used in a secondary electron detection device in a scanning electron microscope.

B. 2. Description of the Related Art A secondary electron multiplier tube repeats the operation of accelerating secondary electrons between dynodes to which a high voltage is sequentially applied and making them enter the dynodes to generate more secondary electrons. The number of electrons increases, and a potential difference of several KV is given between the sample and the anode of the secondary electron multiplier, and this high voltage is required to input the output signal to the amplifier at the next stage. It is necessary to cut off the voltage and extract only the signal component. Capacitor coupling may be used when the signal to be extracted is an AC signal, but when the signal to be extracted is a video signal containing a component from a DC component to a considerably high frequency, as in a scanning electron microscope, Capacitor coupling cannot be used, and as shown in FIG. 2, the secondary electron multiplier 1 is provided between the anode 1a of the secondary electron multiplier 1 and the next-stage amplifier A.
In principle, the purpose can be achieved by inserting a DC power supply 2 that generates a voltage that is equal to the sum of the voltages of the DC power supplies E1 and E2 connected to it can. However, since this method actually uses a battery for the DC power supply 2, it has drawbacks such as a short battery life and a large size. It may be possible to use a floating power supply instead of a battery, but it is difficult to make an ideal floating power supply, so that there is a problem that the S / N ratio decreases. Therefore, various solutions have been proposed (for example, Japanese Patent Publication No. 57-32462). However, since the circuit configuration is complicated, it cannot be said to be a principle solution.

(C) Problems to be Solved by the Invention The present invention provides a simple circuit configuration in which the battery 2 shown in FIG. 2 is not used and the DC component of the signal can be taken out completely without being cut. .

(2) Means for solving the problem A voltage applied to the secondary electron multiplier is charged and held in a capacitor, and this capacitor is used instead of the power source 2 in FIG.

E action In a scanning image forming device such as a scanning electron microscope,
Since there is a blanking period between scans, the capacitor may be charged using this blanking period. Although this capacitor has the same position on the circuit as the capacitor in the capacitor coupling, the capacitor in the present invention functions as a part of the power source of the secondary electron multiplier, and the capacitor current in the secondary electron multiplier is used. It is fundamentally different from the coupling capacitor in that it is reversely charged. In other words, the fact that the reverse current is caused by the tube current means that the tube current flows from the amplifier in the subsequent stage toward the secondary electron multiplier, and the DC component of the signal is completely transmitted to the amplifier in the next stage. Is to say.

F. Embodiment FIG. 1 shows an embodiment of the present invention. 1 is a continuous dynode type secondary electron multiplier, and 1a is its anode. S
Is a sample, and E1 is a power source for applying a voltage for attracting and accelerating the secondary electrons emitted from the sample S toward the secondary electron multiplier 1. E2 is a power supply that gives a potential gradient from one end of the dynode 1b to the other end. The anode 1 a is connected to the DC power source 6 via the switch 7. The capacitor 8 is inserted between the anode 1a and the amplifier A at the next stage, and the diode 10 is inserted between the amplifier A and the capacitor. The switch 7 is off during the scanning period and is on during the retrace line period. The output voltage of the power supply 6 is the power supply E
1 to the sum of the voltages of E2, the dynode 1b to the anode 1a
Voltage for drawing electrons toward Is set to the value added.

Considering the state in which the switch 7 is turned on during the scan retrace period, the capacitor 8 is charged by the power supply 6, and the charging current at that time passes through the diode 9 and the diode 1
Since there is 0, this charging current is prevented from flowing into the amplifier A. Considering the case where the charging voltage of the capacitor 8 is 0, the high voltage of the power supply 6 is applied to the amplifier A at the moment when the switch 7 is turned on.
By 0, this voltage is cut off. Switch 7 is ON
During the period, the capacitor 8 is charged to the voltage of the power supply 6.
Then, the switch is turned off and the scanning period starts. During the scanning period, the charging voltage of the capacitor 8 is applied to the anode 1a, and the voltage is Therefore, all the electrons generated at the dynode are attracted to the anode 1a, and the corresponding amount of electrons are absorbed from the capacitor 8 to the anode 1a.
A current flows toward the capacitor 8 and the capacitor 8 is discharged (reversely charged). This current is the output current of the secondary electron multiplier 1 itself, which is covered by the discharge of the capacitor 8. Therefore, the output current of the secondary electron multiplier 1 from the amplifier A through the diode 10 and the capacitor 8 is eventually increased. As it flows, the output signal of the secondary electron multiplier is completely input to the amplifier A including the DC component.

FIG. 3 shows a configuration of capacitor coupling for comparing the capacitor 8 of the present invention with a capacitor coupled capacitor. Reference numeral 4 is a coupling capacitor, and a voltage source E for extracting electrons from the dynode is provided between the anode 1a and the dynode.
3 is connected. The current due to the electrons flowing into the anode 1a flows from the power source E3 toward the anode 1a as indicated by the arrow, and does not flow from the capacitor 4 toward the anode. Therefore, the capacitor 4 is a DC component in the output of the secondary electron multiplier. Is completely cut, and is fundamentally different from the capacitor 8 of the present invention.

In the case of the present invention, the secondary electron multiplier 1 is provided by the capacitor 8.
Although the anode current is supplied, it is preferable that the change in the charging voltage of the capacitor 8 during the scanning period is as small as possible. For this purpose, it is necessary to increase the capacity, but it is sufficient to set the voltage change to 1 V, the anode current to 0.1 μA, the scanning period to 1 second, and the capacity of the capacitor 8 to about 0.1 μF, and the scanning period to 1 second. In this case, the switch 7 does not have to be turned on and off for each horizontal scan, but can be turned on once for each scan of the sample surface to charge the capacitor 8. Since the amplification factor of the secondary electron multiplier changes depending on the dynode voltage, that is, the voltage of E2 and does not depend on the anode voltage, the above-mentioned influence of the change in the anode voltage can be practically ignored.

In the above-described embodiment, the charging period of the capacitor 8 is set to the emission electron non-detection period of the sample, but it is possible to make the required charging time sufficiently short by reducing the impedance of the charging circuit. , When the change of the signal current is slow compared to this charging time, continuous measurement ignoring the charging period is also possible. In other words, it means that the battery may be charged in a timely and instantaneous manner during the measurement. Since the diode 10 is inserted, the power supply 6 and the amplifier A are insulated from each other, and the output of the amplifier A becomes 0 for a moment during the charging period. Therefore, the present invention is not applied only to the scanning type image device. Further, as a modified embodiment of the present invention, the switch 7 is a diode that conducts from the power source 6 toward the anode 1a, and the power source 6 is a high voltage pulse generating circuit. You may make it apply to 1a.

G. Effect According to the present invention, it is possible to cut the high voltage of the secondary electron multiplier with an extremely simple circuit configuration, and further, the output signal including the DC component can be completely applied to the amplifier of the next stage, Since no battery is used, there is no problem of life.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an embodiment of the present invention, FIG. 2 is a circuit diagram of a conventional example, and FIG. 3 is a circuit diagram of a conventional AC signal detection circuit using capacitor coupling.

Claims (3)

[Claims] 1. A terminal of a capacitor is connected to the anode of a secondary electron multiplier, and the other terminal of the capacitor is connected to a next-stage amplifier through a diode that conducts toward the same capacitor. An electron beam detector in which a diode that conducts in the ground direction is connected to a capacitor, and a secondary electron multiplier driving high voltage is applied to the anode only during a non-measurement period. 2. The electron beam detector according to claim 1, wherein the means for applying a high voltage to the anode only during the non-measurement period is a high voltage DC power supply and a switch. 3. The electron beam detector according to claim 1, wherein the means for applying a high voltage to the anode only during the non-measurement period is a high voltage pulse generating means.