JPH02126548A - Filament heating device - Google Patents

Abstract

(57) [Abstract] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to a filament heating device that can stabilize the probe current, etc., particularly in a short time, in a thermal-filled electron gun or a Schottky emission electron gun such as an electron microscope. .

[Conventional technology] Thermionic emission used in electron microscopes, etc.! In the No. 42 electron gun, the electron gun is used under conditions where the emission current is saturated, so even if the filament current changes somewhat, the emission current hardly changes.

On the other hand, thermal field electron guns and shotguns
In the emission electron gun, the emitter is heated to about 1800 degrees, which is lower than the heating temperature of the thermionic emission electron gun, to emit electrons.When the filament temperature changes, the emission current changes, and the probe The current will change. Therefore, in conventional thermal field electron guns and Schottky emission electron guns, in order to keep the filament temperature constant, the current flowing through the filament is detected, and the filament heating current is controlled back based on this detection signal. A constant current is supplied to the

[Problem to be solved by the invention] In such an electron gun, the temperature of the emitter can be maintained constant by controlling the heating current of the filament at a constant level after a period of time has elapsed to reach thermal equilibrium around the emitter. , thereby also keeping the probe current constant. However, if the filament current is kept constant, the electrodes around the emitter will not be sufficiently warmed up immediately after the thermal field electron gun or Schottky emission electron gun starts operating, so the emitter will become unstable. The temperature does not reach the target temperature easily, and therefore, it takes a considerable amount of time to bring the current emitted from the electron gun to the target current value, as shown in FIG.

The present invention solves these conventional problems and makes it possible to maintain a constant probe current once the temperature reaches the peak temperature, and to achieve the target in a short time after starting the operation of the thermal field electron gun or Schottky emission electron gun. The object of the present invention is to provide a filament heating device capable of reaching a probe current of .

[Means for Solving the Problems] Therefore, the present invention provides a device that is equipped with a power source for supplying a constant heating current to the filament of an electron gun, in which the heating current supplied to the filament is used to start heating the filament. It is characterized by the addition of a correction circuit that initially supplies a current larger than the constant heating current and then gradually attenuates the heating current.

[Example] Embodiments of the present invention will be described in detail below based on the drawings.

FIG. 1 is a schematic block diagram showing one embodiment of the present invention, FIG. 2 is a diagram for explaining a correction circuit, and FIGS.
The figure is a diagram for explaining the operation of the apparatus of this embodiment.

In FIG. 1, 1 is a filament of, for example, a Schottky emission electron gun, and 2 is a filament heating setting circuit for determining the value of the heating current supplied to the filament 1. A signal v1 for determining the heating current value from the filament heating setting circuit 2 is sent to the impulse amplifier via the adding circuit 3. The output of the differential amplifier is converted into a high frequency signal by a high frequency conversion circuit 4. 5 is a booster circuit for transmitting the high frequency output from the high frequency conversion circuit 4 to the high voltage side; the output from the booster circuit 5 is converted to direct current by the DC conversion circuit 6, and a heating current is supplied to the filament 1. Ru. Reference numeral 7 denotes a current detection circuit for detecting the heating current supplied to the filament 1. A signal representing the heating current from this current detection circuit 7 is transmitted to a differential amplifier on the low voltage side and used as a feedback signal. Reference numeral 8 denotes a correction circuit for correcting the heating current supplied to the filament 1 so that the heating current is larger than the normal heating current at the initial stage of heating and then gradually attenuated.The correction signal from the correction circuit 8 is added The circuit 3 adds it to the signal V1 for determining the heating current value. FIG. 2 shows a specific example of the above-mentioned correction circuit 8, etc., and the correction circuit 8 has, for example, 100
It consists of a circuit in which a μF capacitor C and, for example, a 20 MΩ high resistance R are connected in series.

In the filament heating circuit configured in this manner, when a filament power source (not shown) is turned on, the correction voltage VO of the correction circuit 8 is charged to the capacitor C through the high resistance R. Here, the voltage at point A is VOfl-exp (-t
/RC)l.

Here, if the filament setting voltage is 1, the output V2 of the adder circuit 3 is Vl - VOtl - eXI) (-t/RC) l.

Based on the output of the adder circuit 3 having such characteristics, the high frequency conversion circuit 4. Boost circuit 5. Since a current is supplied to the filament 1 via the DC conversion circuit 6, a heating current having an attenuation characteristic as shown, for example, by curve A in FIG. 3 is supplied to the filament 1.

Therefore, in the device configured in this way, the circuit is configured to supply a relatively large current to heat the filament at the initial stage of heating, and then gradually attenuate to a constant heating current value, so the heating time is relatively short. The temperature of the electrode around the emitter and the electrode introduction terminal rises over time. Therefore, since a heating current having such characteristics is supplied to the filament 1, the total emission current It and the probe current 1p have characteristics as shown in the curves of FIG. 4. Therefore, compared to the conventional case, the total emission current 1t, L, and therefore the probe current Ip are stabilized in a short time.

In the above embodiments, the present invention is applied to a Schottky emission electron gun, but similar effects can be obtained even when the present invention is applied to a thermal field electron gun.

Furthermore, in the embodiments described above, an analog circuit is used to create a signal for instructing the filament current, but the signal may be created digitally.

[Effects of the Invention] As is clear from the above description, according to the present invention, in an apparatus equipped with a power source for supplying a constant heating current to the filament of an electron gun, the heating current supplied to the filament is Since we added a correction circuit that supplies a current larger than the constant heating current at the initial stage and then gradually attenuates it, it is possible to bring the probe current to the target current within a short time and then maintain it constant.

[Brief explanation of the drawing]

FIG. 1 is a schematic block diagram showing one embodiment of the present invention, and FIG.
This figure is a diagram for explaining the correction circuit, FIGS. 3 and 4 are diagrams for explaining the operation of this embodiment, and FIG. 5 is a diagram for explaining the prior art. 1; Filament, 2: Filament heating setting circuit, 3
: Adder circuit, 4: High frequency conversion circuit, 5: Boost circuit, 6:
DC conversion circuit, 7: 1u current detection circuit, 8: Correction circuit, D
:Differential amplifier.

Claims (1)

[Claims] In a device equipped with a power supply for supplying a constant heating current to the filament of an electron gun, the heating current supplied to the filament is initially set to be larger than the constant heating current when heating the filament is started, and then A filament heating device characterized by adding a correction circuit that gradually attenuates the heating current.