JPH0234413B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention aims to stabilize an ion beam.
Regarding EHD type field ion beam generator.

The emitter used in an EHD (Electro Hydro Dynamics) type field ion source (hereinafter referred to as an EHD type ion source) is a tungsten wire needle whose tip is electropolished to a diameter of approximately 0.5 μm and is wetted with a liquid metal (such as gallium). When a DC voltage of several kilovolts is applied between this emitter and the extractor, the liquid metal is drawn out to the needle tip by a strong electric field formed at the tip of the tungsten needle that forms the emitter. The liquid metal at the tip of the needle forms a conical protrusion called a Taylor Cone due to the strong electric field. An electric field concentrates at the tip of this conical protrusion, and the liquid metal at the tip of the needle evaporates due to the electric field.
For example, it is extracted as gallium ions.

By the way, EHD using emitsuta like this
In the type ion source, a feedback circuit is provided to improve the stability of the ion beam. Figure 1 is a connection diagram showing the electrical configuration of this type of conventional device.
In the figure, 1 is an emitter from which the ion beam is emitted, and E ₁ is a heating power source. Here, E ₁
Although direct current is used for this purpose, alternating current power may also be used. 2 is an extractor (Extactor),
3 is a cathode, and the extractor 2 and cathode 3 are provided with small holes through which the ion beam passes. Reference numeral 4 represents an ion beam detection aperture for detecting the amount of ion beam emitted from the emitter 1, and reference numeral 5 represents a current-voltage conversion circuit that converts the detection current of the aperture 4 into a voltage. This conversion circuit 5 is composed of an operational amplifier _U1 and a resistor _R1 . 6 is a setting circuit for setting the ion beam current, which includes a differential amplifier U ₂ that receives the output of the conversion circuit 5 at its negative input terminal, and a variable voltage source that applies a setting voltage to the positive input terminal of this differential amplifier U ₂ .
It consists of E ₂ . A control unit 7 receives the output of the setting circuit 6 and the output of the DC power supply 8, and controls the voltage of the DC power supply 8 and applies it to the high frequency oscillator 9 so that the output of the setting circuit 6 becomes zero. It controls the output amplitude. The output of the high frequency oscillator 9 is supplied to a rectifier circuit 10 via a high frequency transformer T and AC transmission capacitors C ₁ and C ₂ . The rectifier circuit 10 is composed of a diode bridge, and a smoothing capacitor _C3 and a dummy resistor _R2 are connected to both ends of the bridge. Moreover, the positive output terminal of the rectifier circuit 10 is
It is connected to the acceleration power source E ₃ and the emitter 1, and its negative output end is connected to the extractor 2. Also, a resistor R ₄ is connected between the setting circuit 6 and the control section 7.
The control side of resistor R ₄ is connected to capacitor C ₄ ,
Grounded through resistor ₅ .

The operation of the conventional EHD type ion source having such a configuration will be explained next. Ions are emitted from the emitter 1 heated by the heating power source E ₁ , and the accelerating power source
It is accelerated by the action of E ₃ and moves towards the cathode 3 side. Therefore, a current corresponding to the amount of ion beam flows from the aperture 4 to the conversion circuit 5, where it is converted into a voltage signal and input to the setting circuit 6. Since the differential amplifier U ₂ in the setting circuit 6 is given the setting voltage E ₂ as a reference value, the setting circuit 6
A signal obtained by amplifying the deviation voltage is outputted to the control unit 7, and the control unit 7 that receives this signal outputs the signal from the DC power supply 8.
The output voltage is controlled and sent to the high frequency oscillator 9. The high-frequency oscillator 9 supplies an output voltage corresponding to the output of the voltage control section 7 to a rectifier circuit 10 via capacitors C ₁ and C ₂ , and the rectifier circuit 10 converts it into a DC voltage. and add between extractor 2. Therefore, the potential of the extractor 2 is held at a potential (extraction voltage) that is lower than the positive potential of the accelerating power source E ₃ by the above DC voltage, and the ion beam current is controlled by changing the above DC voltage. I'm starting to be able to do it. In the case of the device shown in FIG. 1, a negative voltage composed of the converter circuit 5, setting circuit 6, control section 7, oscillator 9, rectifier circuit 10 _, etc. is set so that the output voltage of the converter circuit 5 is equal to the set voltage of the variable power source E2. A feedback circuit operates to keep the ion beam current constant.

In the feedback circuit configured in this manner, a delay element composed of R ₄ , R ₅ , C _{4 ,} etc. is provided to prevent oscillation of the circuit. However, when the time constants of all elements are constant, the circuit becomes stable once the constants are determined, but in the case of a transient state where a Taylor cone is generated as in an EHD type ion source, the emitter The relationship between the extraction voltage between the extractor 1 and the extractor 2 and the ion beam current is non-linear. In other words, as shown in Figure 2a, if the extraction voltage is gradually increased over time (t), the Taylor cone will change as shown in Figure 2b.
As shown in , it is formed with a steep rise at a certain extraction voltage. On the other hand, when a Taylor cone is formed, the ion beam current is at the second level at the moment of formation.
As shown in Figure c, it rises steeply, then decreases once and then increases again. This phenomenon is
This is a unique phenomenon of the EHD type ion source; in the absence of an extraction electric field, liquid metal moves toward the root of the needle tip, and when an extraction electric field is applied, the liquid metal moves to the needle tip and stays there. A Taylor cone is formed. As a result, the ion beam current increases to a certain value, but the amount of liquid metal that remains at the tip of the needle decreases due to the flow resistance of the liquid metal being transferred on the needle surface. Therefore, although the ion beam current decreases temporarily,
Eventually, the Taylor cone formed at the tip of the needle becomes balanced, and the ion beam current also becomes balanced. The time for this process does not depend on the rising speed of the extraction voltage and has a certain constant time ΔT. When an EHD type ion source with such transient characteristics is controlled by the feedback circuit shown in Figure 1, there is a positive feedback region ΔT', so the emission current is low (for example, 2 μA or less) close to the critical extraction voltage at which a Taylor cone is formed. ) had the disadvantage that the ion beam current became intermittent, exhibited an oscillating state, and became unstable, as shown in FIG. 2d.

The present invention was devised in view of the above points, and is used to emit ion beams in an EHD type field ion beam generator equipped with a feedback circuit that controls the potential between the emitter and the extractor so that the ion beam current is constant. The present invention is characterized in that it includes a delay circuit that delays the response of the pre-feedback circuit for an arbitrary period of time at the start, and a means for disconnecting the delay circuit from the feedback circuit after the elapse of the time period.

An embodiment of the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 3 is a connection diagram showing the electrical configuration of one embodiment of the present invention. In the figure, the same parts as in FIG. 1 are given the same numbers and their explanations will be omitted. The ion beam generator shown in this embodiment has a relay 11, a variable resistor R6, and a capacitor _C5 connected in series to the feedback resistor _R3 of the setting circuit 6 shown in FIG. ₃ in the feedback circuit of the device shown in FIG. The connected circuits are connected in parallel, and the relay 12 is further connected in parallel to the capacitor _C5 . Here, resistor R ₆ , capacitor
C ₅ , relay 11, and relay 12 are delay circuits that delay the response of the feedback circuit at the start of initial radiation of the ion beam for a time considerably longer than the time during which the ion beam current changes non-linearly with respect to the extraction voltage at the start. Yes, relay 11 connects the delay circuit consisting of resistor R ₆ and capacitor C ₅ after the specified time has elapsed.
It serves as a switch to disconnect from the feedback circuit.

In a device configured as above, for example, resistor R ₆
Feedback resistance R stands for the resistance value of ₃ .
Set it to 1/10, and make capacitor _C5 sufficiently large. First, the relay 11 is turned on and the relay 12 is turned off to start emitting the ion beam. In this state, it is connected in parallel to the feedback resistor _R3 connected in series, and this state is maintained for several seconds. next,
The relay 11 is turned off and the relay 12 is turned on. As a result, the feedback resistance of the setting circuit 6 is only _R3 , and the delay circuit formed by _R6 and _C5 is separated from the feedback circuit. By performing a series of these operations simultaneously with the initial ion beam emission, and by setting R ₆ and C ₅ to appropriate values for the feedback resistance R ₃ , the non-linear region of the emission when the Taylor cone is generated can be The relationship between the time delay ΔT and the response time delay Δt of the feedback circuit formed by the delay circuit is Δt≫ΔT.

That is, during the ΔT period when the Taylor cone is generated and the ion beam current is generated, the response of the negative feedback loop is delayed by Δt, and the delay has a time ΔT≪Δt such that the positive feedback region that occurs during the ΔT period can be ignored. By adding a circuit, it is possible to suppress ion beam oscillation when a Taylor cone is generated. Therefore, even if the device is started with a low ion beam current like the conventional device, it does not exhibit a so-called oscillation state.
Furthermore, the delay circuit formed by R ₄ , C ₄ , and R ₅ is used to operate the feedback circuit accurately and stably in a steady state (during ion beam emission), and is used not only during initial ion beam emission but also during ion beam emission. Also, a stable ion beam current can always be obtained. Therefore, if this ion beam generator is used in a micro-focus ion beam device, a stable ion probe can be obtained.

As described in detail above, according to the present invention, which includes a delay circuit that delays the response of the feedback circuit for an arbitrary period of time during the initial emission of the ion beam, and a means for separating the delay circuit from the feedback circuit after the elapse of the time period, stable We provide an EHD type feed ion beam generator.

[Brief explanation of drawings]

Figure 1 is a connection diagram showing the electrical configuration of a conventional device;
FIGS. 2a, b, c, and d are diagrams for explaining a conventional device, and FIG. 3 is a connection diagram showing the electrical configuration of an embodiment of the device of the present invention. 1: emitter, 2: extractor, : cathode, 4: aperture, 5: current-voltage conversion circuit,
6: Setting circuit, 7: Control unit, 8: DC power supply, 9:
High frequency oscillator, 10: Rectifier circuit, 11, 12: Relay.

Claims (1)

[Claims] 1 In an EHD type field ion beam generator equipped with a feedback circuit that controls the potential between the emitter and extractor so that the ion beam current is constant, a delay that delays the response of the feedback circuit for an arbitrary period of time at the start of ion beam emission. circuit and
An EHD type field ion beam generator, comprising means for separating the delay circuit from the feedback circuit after the elapse of the time period.