JP2617240B2 - Control method of acceleration energy in high frequency quadrupole accelerator - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a control method of acceleration energy in a high-frequency quadrupole accelerator, and particularly to a control method suitable for application to, for example, ion implantation into a semiconductor.

<Conventional technology> A radio frequency quadrupole accelerator (hereinafter, referred to as an RFQ accelerator) has a capability of accelerating a high-current ion beam to high energy under high transmittance, and in recent years, Applications in many fields are being studied.

FIG. 3 is a partial cross-sectional perspective view showing a conceptual structure of a 4-vane RFQ accelerator among RFQ accelerators. In addition,
Although the RFQ accelerator has modifications such as a four-rod type RFQ accelerator, the technical features are essentially the same as those of the four-vane type, so that the following description will be made by taking the four-vane type RFQ accelerator as an example.

Four electrodes 32a to 32d (hereinafter referred to as vanes 32a to 32d) are fixed in a cylindrical tank 30 having both ends closed by plates 31a and 31b, and these form a quadrupole cavity resonator. doing.

Each of the vanes 32a to 32d has a tank 30 at its tip.
The vanes opposing each other have peaks and valleys facing each other, and valleys and valleys face each other. The waveforms of the vanes 32a and 32c and the vanes 32b and 32d have a phase difference of 180 °. have. The period of each waveform gradually increases from the entrance to the exit.

When a high frequency is introduced into such a structure, opposing vanes are applied with a voltage in the same phase and adjacent vanes are applied with a voltage in the opposite phase and resonate.However, due to the above-described waveform, the vanes are surrounded by vanes 32a to 32d. An accelerating electric field is generated along the axis of the tank 30 in the space defined, and the charged particle beam incident thereon is accelerated under a predetermined acceleration energy while being converged.

Here, in order to make the acceleration energy of the particles accelerated by the RFQ accelerator variable, conventionally, instead of a complicated configuration in which the resonance frequency is made variable, the high-frequency voltage to be applied is changed according to the introduction speed of the charged particles. There has been proposed an RFQ accelerator having a configuration in which the acceleration energy of charged particles is changed by shifting a predetermined amount below a voltage value that satisfies a resonance condition based on the frequency of the high-frequency voltage and the cycle of the waveform of each electrode described above ( JP-A-2-27699).

<Problems to be Solved by the Invention> However, generally, in the RFQ accelerator, in a state where the resonance condition is satisfied, the phase of the incident charged particles is sequentially changed for each of the valleys and valleys forming the waveform formed on the vane serving as the acceleration electrode. It is aligned and accelerated by obtaining a predetermined energy for each peak and valley. In such a case, the degree of convergence of the phase of the charged particles accelerated as the peak and valley at the later stage of the vane increases, so that the parameters satisfying the resonance condition When only the high-frequency voltage is changed, particles that are not accelerated increase as much as the peaks and valleys at the later stage of the vane.

In other words, the effect on the acceleration of charged particles due to the decrease in the high-frequency voltage is small because the allowable range of the phase accompanying the acceleration is wider at the front stage of the vane, and the effect is greater at the latter stage because the allowable range is narrower and the high-frequency voltage decreases In this case, the final acceleration energy of the charged particles obtained first is the final acceleration energy obtained when the resonance condition is completely satisfied minus the energy added at the peak and valley of the last stage of the vane. .

Therefore, in the RFQ accelerator of the conventional configuration in which the high-frequency voltage to be applied is changed, the energy of the charged particles obtained when the high-frequency voltage is reduced becomes discrete with the energy interval added at the peak and the valley of the vane. Since particles having energies of a plurality of peaks are generated, only particles having desired acceleration energy cannot be obtained.

An object of the present invention is to provide a control method that can easily change the acceleration energy without changing the resonance frequency of the RFQ accelerator and obtain particles having a single-peak acceleration energy. .

<Means for Solving the Problems> A feature of the method for controlling acceleration energy in the RFQ accelerator according to the present invention is that a cavity having four electrodes disposed therein is surrounded by the tip of each electrode. The speed of the charged particles to be introduced into the space is determined by the frequency and voltage of the high-frequency voltage applied to the electrodes of this cavity, and the continuous acceleration of the charged particles based on the period of the waveform formed at the tip of each electrode. An object is to change the acceleration energy of the charged particles by increasing a predetermined amount from a speed satisfying the condition.

<Operation> With the resonance frequency and the high-frequency voltage kept constant, the same charged particles are changed in incident speed (energy).
FIG. 2 shows the experimental results when introduced into the RFQ accelerator (in the following, the incident speed is referred to as incident energy for the sake of description of the same charged particle).
Compared to the case where charged particles are introduced with incident energy that satisfies the condition of continuous beam acceleration determined by the cycle of the electrode waveform, the frequency of the high-frequency voltage, and the voltage (FIG. 2A),
The acceleration energy could be shifted downward by increasing the incident energy (FIG. 2 (b)).

At present, it is not exactly clear at this time how the incident energy is changed by changing the incident energy. But RFQ
In comparison with a high-frequency accelerator other than the accelerator, for example, a drift tube linac, the following assumption is made.

That is, in the drift tube linac,
It is a fact that when the energy of the incident beam is increased, the beam diverges from the condition of continuous acceleration of the beam. Where the drift tube linac and RF
The major difference in function from the Q accelerator lies in its beam focusing power. In the former, the beam converging force is obtained by an electrostatic or magnetic Q lens installed in the drift tube, and the converging force does not work outside the drift tube. In the latter case, on the other hand, the high-frequency voltage induced in the vane simultaneously converges and accelerates the particles, so that the particle beam is spatially continuous and always receives a strong convergence force.

Therefore, in RFQ acceleration, a beam that cannot satisfy the condition of continuous acceleration is accelerated to the end without diverging due to its strong convergence. However, since the continuous acceleration condition is not satisfied, it is estimated that the final acceleration energy shifts to a lower energy side than the design value, and the above-described result is obtained. In addition, since the incident speed of the particles is different from the energy that satisfies the resonance condition from the beginning of the acceleration process, the original continuous acceleration condition easily deviates from the time of introduction. For this reason, unlike the case where the high-frequency voltage is reduced, the acceleration conditions hardly deviate only in the later stage of the acceleration process, that is, at the peak and the valley of the vane at the rear end of the RFQ accelerator. Is obtained.

<Example> An example of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing a layout of an experimental apparatus for actually measuring the acceleration energy of particles by applying the present invention.

The ion source 1 ionizes a raw material to generate ions. The ion source 1 is provided with a positive potential by a DC power supply 2, and positive ions generated inside the ion source 1 are accelerated toward a lower potential extraction electrode 3.

Since the ions extracted in this manner include those other than the target positive ions, they are analyzed by the analysis magnet 4 and only the target ions are converted into the incident beam Bin _{in the} RFQ accelerator 5. To be introduced.

The RFQ accelerator 5, which is a cavity resonator, includes a high-frequency power source 6
By the high frequency voltage is applied, an electric field to accelerate the incident beam B _in is formed.

The beam accelerated and emitted by the RFQ accelerator 5, that is, the emitted beam B _out is subjected to energy analysis by the analysis magnet 7 at the next stage, and enters the Faraday cup 8.

An ammeter 9 is connected to the Faraday cup 8,
The beam current can be measured.

Then, the energy of the incident beam B _in the, DC power supply 2 by changing the potential applied to the ion source 1, and is configured so as to change.

With the above apparatus, an acceleration experiment of ¹¹ B ⁺ ions was performed. Note that BF ₃ was used as a source gas, and only ¹¹ B ⁺ ions ionized by the ion source 1 were analyzed by the magnet 4.
Guided into the RFQ accelerator 5.

Here, the RF power applied to the RFQ accelerator 5 is 18 kW, and the incident energy of the beam satisfying the continuous acceleration condition is 66 keV.

In the experiment, as the incident energy of the beam B _in, and incident energy 66keV satisfying the continuous acceleration condition is set to a higher than 85 keV, other conditions were exactly the same.

 FIG. 2 shows the results of the above experiment.

2 (a) and 2 (b) show the outgoing beam B _out
In the graph, the vertical axis represents the beam current incident on the Faraday cup 8, and the horizontal axis represents the current of the analysis magnet 7 (corresponding to the energy of the beam). That is, in the experiment, the current of the beam incident on the Faraday cup 8 was measured while continuously changing the current of the analysis magnet 7.

Figure 2 (a) is a case of a 66keV the incident energy of the beam B _in, FIG. (B) shows the case of a 85 keV.

As is apparent from this figure, to be larger than the value that satisfies the continuous accelerated conditions incident energy to RFQ accelerator 5 in the incident beam B _in, a single energy peak with an acceleration energy of the beam is shifted downward It has been found that a beam current having the same can be obtained. Accelerated particles having single peak energy were obtained because the incident speed of the particles was different from the energy that satisfies the resonance conditions from the beginning of the acceleration process. It is presumed that the acceleration condition hardly deviates for the first time at the later stage of the acceleration process as in the case where the high-frequency voltage is lowered, that is, at the mountain valley at the rear end of the vane of the RFQ accelerator.

<Effects of the Invention> As described above, according to the present invention, the acceleration energy of charged particles can be changed only by changing the incident speed of the charged particles to the RFQ accelerator, and the energy of a single peak can be changed. Is obtained.

This greatly expands the possibilities of applying RFQ accelerators to ion beam applications that require high energy, large current, and energy variability, such as ion implantation into semiconductors. It is expected to bring.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a layout of an experimental apparatus for actually measuring the acceleration energy of particles by applying the present invention, FIG. 2 is a graph showing the experimental results, and FIG. 3 is a partial cross section showing a conceptual structure of an RFQ accelerator. FIG. DESCRIPTION OF SYMBOLS 1 ... Ion source 2 ... DC power supply 3 ... Leader electrode 4 ... Analysis magnet 5 ... RFQ accelerator 6 ... High frequency power supply 7 ... Analysis magnet 8 ... Faraday cup 9 ... Ammeter

Claims (1)

(57) [Claims] 1. A high-frequency voltage of a predetermined frequency is applied to a cavity in which a cylindrical electrode is provided with four electrodes each having a waveform along the axial direction of the tank at the tip thereof. In the state in which the charged particles are guided at a predetermined speed into the space surrounded by the tip of the electrode in the state where the charged particles are accelerated, the speed of the charged particles to be introduced into the space is reduced. The frequency and voltage of the high-frequency voltage applied to the cavity resonator, and the speed that satisfies the condition for continuously accelerating the charged particles based on the period of the waveform of each electrode, are increased by a predetermined amount to accelerate the charged particles. A method for controlling acceleration energy in a high-frequency quadrupole accelerator characterized by changing energy.