JPS643165Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a filament breakage detection circuit for detecting filament breakage in a charged particle beam device such as an electron microscope EM.

In charged particle beam devices such as electron microscopes (EM), a secondary charged particle beam (e.g. secondary electrons) is produced by heating a filament and converging thermionic electrons emitted from the filament with an electron lens to irradiate the sample and reflecting from the sample. A secondary charged particle beam image of the sample is obtained by capturing and converting it into an electrical signal. By the way, most conventional devices of this kind do not have a detection circuit for detecting filament breakage, and only special devices can detect filament breakage by detecting the current of the power source for controlling the filament. This is the extent of what was being done.

The filament used in this type of charged particle beam device is generally used in a high vacuum. Recently, there has been a trend toward high vacuum due to longer life of filaments, use of LaB6, and drier vacuum atmospheres. For this reason, once the vacuum is broken due to filament replacement, etc., it takes an extremely long time to raise the vacuum to a high vacuum state again.

By the way, in a charged particle beam device such as an electron microscope, when a charged particle beam is no longer emitted, it is necessary to determine whether the filament is broken, the filament is set incorrectly, or there is a circuit failure. In such a case, since the conventional apparatus is not provided with a filament breakage detection circuit as described above, it is necessary to once break the vacuum and perform a check, which takes a lot of time.

The present invention was developed in view of these points, and its purpose is to eliminate the defects of conventional devices and realize a filament breakage detection circuit for charged particle beam devices that can accurately detect filament breakage over a wide power range. It's about doing.

The present invention that achieves this objective detects filament breakage by comparing the load current of a switching transistor that supplies high voltage power to the filament with a reference value, and also changes the reference value in conjunction with the power supplied to the filament. This feature is characterized in that it allows the user to

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 is an electrical configuration diagram showing an embodiment of the present invention. In the figure, 100 is a high-voltage generator, power transmission isolation transformers T _I to T _N , and dividing resistors R _I that give equal potential to each of these isolation transformers.
~R _N , DC ignition rectifiers D ₁ and D ₂ provided on the secondary side of the final stage transformer T _N , power supply E _I for giving a negative potential to the filament F (described later), and the final stage dividing resistor R _N It consists of a variable bias resistor Rb connected to one end of the resistor. 200 is an electron gun (acceleration tube), Wehnelt electrode W, filament F
and an electron lens (not shown) that converges charged particles (electrons) emitted from the filament. The high voltage generator 100 and the electron gun 200 are connected by a high voltage cable CL.

Tr ₁ and Tr ₂ are switching transistors for transmitting high voltage power to the high voltage generator 100, and these transistors Tr ₁ and Tr ₂ are turned on and off in a complementary manner by a driving pulse applied to the base. There is. Further, the primary side of the transformer T _I is connected to the collector, and the voltage converted to alternating current is transmitted to the secondary side. Ra is a grounding resistor that is connected to the common emitter connection point of transistors Tr ₁ and Tr ₂ and converts changes in emitter current into voltage. E _B is a positive DC power supply for supplying power to the filament F via the high voltage generator 100, Tr ₃ is a voltage control transistor that receives E _B at its collector, and VR ₁ is a bias voltage applied to the base of transistor Tr ₃ . It is a variable resistance that gives By adjusting this variable resistor VR ₁ , the power supplied to the filament F can be controlled, and the power suitable for each type of filament can be selected.

The voltage corresponding to the load current taken out from the common emitter connection point of switching transistors Tr ₁ and Tr ₂ is generated by a series connection circuit (current detection circuit) of a parallel filter circuit consisting of a diode D ₃ , a resistor Rc, a resistor Rd, and a capacitor C ₁ . ) is converted to DC voltage. U ₁ receives the output of this current detection circuit at its non-inverting input terminal, and receives at its inverting input terminal the reference voltage created by dividing the output of transistor Tr ₃ with a series circuit of resistor Re and variable resistor VR ₂ . The first comparison circuit _U2 that detects filament breakage is a second comparison circuit that outputs a signal to prevent malfunction of the first comparison circuit. The output voltage of the transistor Tr ₃ is connected to the non-inverting input terminal of the second comparator circuit U ₂ through the resistor Rf.
The voltage divided by Rg is input, and the reference voltage Es is input to its inverting input terminal. Then, the output of the first comparator circuit _U1 is given to one input terminal of the NOR gate _G2 via the inverter _G1 ,
The output of the second comparator circuit U ₂ is a NOR gate as it is.
is given to the other input terminal of _G2 . Rh is the pull-up resistor connected to the output of gate G ₂ , D ₄
is a light emitting diode for filament breakage alarm driven by the output of gate _G2 .

The operation of the circuit configured in this way will be explained next.

The resistance value of the filament (load) is usually 0.5~
It requires a power of 5 to 15W at about 1Ω. This power is supplied from a switching circuit including transistors Tr ₁ and Tr ₂ , and the switching frequency is approximately 10 to 30 KHz. Now variable resistance
By adjusting Vr ₁ to increase or decrease the power supplied to the switching circuit, the transistor
The emitter currents of Tr ₁ and Tr ₂ also increase or decrease. FIG. 2 is a diagram showing the waveform of the emitter current of a transistor. The solid line shows the waveform when the power is small, and the broken line shows the waveform when the power is large. On the other hand, this emitter current (load current) is converted into voltage by a resistor Ra and rectified by a rectifier diode _D3 . FIG. 3 is an explanatory diagram showing emitter current characteristics rectified by diode _D3 . The horizontal axis shows the power supplied to the switching circuit, and the vertical axis shows the emitter current. Figure 3a shows the emitter current characteristics when the filament is normal. If the filament breaks during power transmission, the current will naturally stop flowing. This current change becomes a change in the current of the transmission transformer groups T _I to _TN and is transmitted to the switching transistors Tr ₁ , Tr ₂ , and the emitter current decreases. FIG. 3b shows the emitter current characteristics when the filament F is cut. Filament breakage can be detected using such changes in emitter current. That is, it is determined that the filament is broken when the emitter current is lower than the reference current as shown in c in FIG. 3. To make this judgment,
The reference voltage corresponding to the reference current is the first comparator circuit.
Applied from variable resistor VR ₂ to the inverting input terminal of U ₁ ,
A voltage corresponding to the emitter current is applied from the current detection circuit to the non-inverting input terminal. Therefore,
When the filament F is disconnected and the emitter current drops below the reference current, the first comparator circuit _U1 operates and its output goes to the "0" level, and the subsequent inverter _G1 changes the output to "1" and outputs the AND gate G. Enter ₂ .
At this time, the other input of AND gate G ₂ is “1”
At this time, the output of AND gate _G2 becomes "1" and lights up light emitting diode _D4 , indicating that the filament has run out. In this circuit, when variable resistor VR ₁ is adjusted to lower the power supplied to filament F, for example, the reference voltage input to the inverting input terminal of U ₁ of the first comparator circuit also decreases. Therefore, regardless of the magnitude of the supplied power, it is possible to always accurately detect filament breakage. Note that by adjusting the variable resistor VR ₂ , the trigger level position (indicated by the broken line in the figure) of the reference current shown in c in FIG. 3 can be adjusted.

By the way, as the power supplied to the switching circuit becomes extremely small, the difference in characteristics between a normal operating state and a broken filament state becomes smaller, making accurate detection difficult. Furthermore, it becomes necessary to determine whether or not power is being supplied to the filament F. In view of the above points, it is necessary to stop the detection operation below a certain power. A second comparator circuit U ₂ is used for this purpose. The reference value of power is the reference voltage Es
The actual power is applied to _the non-inverting input terminal of the second comparator circuit _U2 . When the power becomes smaller than the reference value, the output of the comparator circuit _U2 changes from "1" to "0" and closes the AND gate _G2 . As a result, the lighting operation of the light emitting diode _D4 is prohibited, and false alarms can be prevented.

In the above description, the change in load current is detected as a change in emitter current, but it is also possible to detect it as a change in collector current. However, in this case, the current is pulsed, so it needs to be smoothed.

As described above, according to the present invention, filament breakage can be detected by comparing the load current of the switching transistor with a reference value. Furthermore, since this reference value is changed in conjunction with changes in the power supplied to the filament, filament breakage can be accurately detected over a wide power range. Therefore, unlike conventional devices, there is no need to break the vacuum to check the device.

[Brief explanation of the drawing]

FIG. 1 is an electrical configuration diagram showing an embodiment of the present invention, FIG. 2 is a waveform diagram showing the emitter current waveform,
FIG. 3 is an explanatory diagram showing an emitter current waveform rectified by a diode. 100...High pressure generator, 200...Electron gun,
T ₁ - T _N ...Transformer, R _I - R _N , R _a - R _h ... Resistance, D ₁ - _{D 3} ... Diode, D ₄ ... Light emitting diode, W ... Wehnelt electrode, F ... Filament , E ₁ ... power supply, CL ... cable, Tr ₁ to _{Tr 3}
……Transistor, C ₁ ……Capacitor, Vr ₁ ~
Vr ₂ ... Variable resistance, E _B ... Power supply, U ₁ ~ _{U 2}
... Comparison circuit, Es ... Reference voltage source, G ₁ ... Inverter, G ₂ ... NOR gate.

Claims (1)

[Scope of utility model registration request] The filament breakage is detected by comparing the load current of a switching transistor that supplies high-voltage power to the filament with a reference value, and the reference value is changed in conjunction with the power supplied to the filament. Filament breakage detection circuit for charged particle beam equipment.