JPH0227699A - Acceleration energy control method for high-frequency quadruple-electrode accelerator - Google Patents

Abstract

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

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for controlling acceleration energy in a high frequency quadrupole accelerator, and particularly to a control method suitable for application to, for example, ion implantation into semiconductors.

<Conventional technology> Radio frequency quadruple accelerator
A quadrupole accelerator (hereinafter referred to as an RFQ accelerator) has the ability to accelerate a large current ion beam to high energy with high transmittance, and has recently been researched for applications in many fields.

FIG. 9 is a partially sectional perspective view showing the conceptual structure of the RFQ accelerator.

Four electrodes 92a to 92d (hereinafter referred to as vane 9
2a to 92d) are fixed thereto, forming a quadrupole cavity.

Each of the vanes 92a to 92d has a waveform formed along the axial direction of the tank 90 at its tip, and the vanes facing each other have peaks of the waveform.

The valleys face each other, and the waveforms of the vanes 92a, 92c and the vanes 92b, 92d have a phase difference of 180°. Further, the period of each waveform gradually becomes longer from the entrance to the exit.

When a high frequency is introduced into such a structure, voltage is applied to opposing vanes in the same phase and adjacent vanes in opposite phases, causing resonance. However, due to the presence of the above-mentioned waveform, An accelerating electric field along the axis of the tank 90 is generated in the space where the accelerating electric field is formed, and the charged particle beam incident thereon is focused and accelerated to a predetermined acceleration energy.

<Problem to be solved by the invention> By the way, since the RFQ accelerator is a so-called high-frequency accelerator that accelerates charged particles using a high-frequency resonator, the acceleration energy cannot be made variable unless the resonance frequency is made variable. (Nuclear Ins
truments and Method in Rh
ics Research B21 (1987) P-
P218-223. H,F, Glavish.

Radio-Frequency Linear Ac
celerators for Jonlaplant
ers”).

In other words, in a high-frequency accelerator, the incident speed (energy) of particles, the arrangement position of the electrodes (the arrangement pitch in a drift chain accelerator, the period of the vane waveform in an RFQ accelerator), and the applied high-frequency voltage value are closely related to each other. For example, even if the high-frequency voltage value is changed, the acceleration energy remains constant as long as other factors are constant.In fact, if the high-frequency voltage value is significantly reduced, the charged particles cannot be accelerated at all. It is said that it will disappear. It is generally explained that the reason for this is that when the high frequency voltage is lowered, the speed of charged particles in the accelerating electric field becomes slower, and the resonance conditions are no longer satisfied in relation to the period of the electrode waveform.

On the other hand, in many application fields such as ion implantation equipment in semiconductor manufacturing processes, variability of acceleration energy is an essential performance requirement. Therefore, in order to obtain this acceleration energy variability, a measure has been proposed in which a complicated external resonator or the like is added to the RFQ accelerator to make the resonant frequency variable. (For example, JP-A-60-115199).

However, with the method of obtaining acceleration energy variability by making the resonant frequency variable, not only is it necessary to add complex mechanisms and devices to the RFQ accelerator, but also the frequency of the high-frequency power supply for supplying high-frequency power must be changed. There are many problems in terms of high cost and maintenance, such as the need to make it variable.

An object of the present invention is to provide a control method that can easily change the acceleration energy of an RFQ accelerator without changing its resonance frequency.

<Means for Solving the Problems> The feature of the method for controlling acceleration energy in an RFQ accelerator of the present invention is that the high frequency voltage to be applied to the electrodes is controlled by the speed at which charged particles are introduced into the RFQ accelerator and the speed of this high frequency voltage. The purpose of this method is to change the acceleration energy of charged particles by shifting a predetermined amount downward from a voltage value that satisfies resonance conditions based on the frequency and the period of the waveform formed on the electrode.

Note that the high frequency voltage in this specification refers to the voltage amplitude value of the high frequency to be applied.

Figures 3 to 3 show examples of experimental results in which the same charged particles were introduced into the RFQ accelerator under the same incident conditions and only the high-frequency voltage (vane voltage) was changed without changing the resonance frequency. 6
As shown in the figure. Compared to the case where a vane voltage that satisfies the resonance conditions of the RFQ accelerator is applied (Figure 3), by shifting the vane voltage downward, it was possible to change the acceleration energy downward (Figures 4 to 6). figure).

At present, it is not exactly clear how particles are actually affected and their energy changes by shifting the vane voltage downward.

However, in comparison with high frequency accelerators other than RFQ accelerators, such as drift tube liners, the following assumption holds true.

That is, it is true that in a drift tube linac, if the accelerating high-frequency voltage is lower than the design value, the beam will not be able to satisfy the resonance condition and most of the beam will be lost due to divergence or the like. Here, the drift tube linac and R
The major functional difference from the FQ accelerator lies in its beam focusing power. In the former case, the beam focusing force is obtained by an electrostatic or magnetic Q lens installed inside the drift tube, and the focusing force does not work outside the drift tube.

On the other hand, in the latter case, the high-frequency voltage induced in the vane converges and accelerates the particles at the same time, so that the particle beam is spatially continuous and always receives a strong convergence force.

Therefore, in RFQ acceleration, even a beam that cannot satisfy the resonance condition is accelerated to the end without diverging due to its strong converging force. However, since the resonance condition is not satisfied, the final acceleration energy shifts to a lower energy side than the designed value, and it is presumed that the above-mentioned result is obtained.

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

FIG. 1 is a professional diagram showing the layout of an experimental device for actually measuring the acceleration energy of particles by applying the present invention, and the part surrounded by the broken line is the RFQ accelerator.

Ions generated in the ion source 1 are accelerated to a predetermined energy by a DC high voltage power supply 2.

The ion beam B generated by this is transmitted through the electrostatic Q lens 3
2 analysis magnet 4. Furthermore, it passes through an electrostatic Q lens 5, becomes a beam 1 containing only target ions, and is guided into an RFQ accelerator 6.

The RFQ accelerator 6 includes a high frequency power source 7 for applying a high frequency voltage to its vanes, and a voltage detector 8 for detecting the voltage actually applied to the accelerator 6. A control circuit 9 is provided which inputs the detection signal and a setting signal from the voltage setting device 10 to control the output voltage of the high frequency power supply 7.

The high frequency power supply 7 is configured by, for example, a crystal oscillator and a power amplifier, and the control circuit 9 feeds back a detection signal from, for example, a voltage detector 8 to a setting signal from a voltage setting device 10, and based on the difference signal, the above-mentioned It has a circuit configuration that changes the amplification degree of the power amplifier. This results in
The high frequency voltage applied to the vanes of the RFQ accelerator 6 is controlled to a voltage value set by a voltage setting device 10.

Now, in order to measure the energy spectrum of the ions accelerated and emitted from the RFQ accelerator 6, a Rutherford Backscatter spectroscope was installed at the exit of the RFQ accelerator 6.
tering 5pectroscopy, hereinafter referred to as RB
) was installed. The RBS consists of a target 11 and an energy detector 12. Ions emitted from the RFQ accelerator 6 are scattered by the target 11 and enter the energy detector 12, where they generate an amount of charge according to the acceleration energy. By determining the amount of charge for each particle, the energy spectrum can be measured. The target 11 used was carbon with 100 to 200 gold deposited, and the energy detector 12 was a surface barrier type particle energy detector.

Using the above set amplifier, the high frequency voltage applied to the vane of the RFQ accelerator 6 was changed, and the results of measuring the energy spectrum of the particles emitted from the RFQ accelerator H6 were measured in the third
It is shown in FIGS. Note that in these figures, the horizontal axis represents energy, and the vertical axis represents the number of detected particle counts.

Figures 3 to 6 show experimental results for 14N particles. That is, in this experiment, 14N+ was generated from N2 gas in the ion source 1, accelerated to 84 keV, which is the injection condition for the RFQ accelerator 6, and introduced into the accelerator 6. The resonant frequency of the RFQ accelerator 6 is 70.30
It was kept constant at 0 MHz.

The design voltage value of the RFQ accelerator 6 under these conditions, that is, the high frequency voltage that satisfies the resonance condition determined by the ion incidence speed, the high frequency frequency, and the vane waveform of the RFQ accelerator 6 is approximately 54.8 kV.

Figure 3 shows the design voltage value of the RFQ accelerator 6 as it is (100
%), a single peak occurred at the designed energy value. Note that the presence of counts at energy values other than the peak is mainly caused by multiple scattered particles with low energy that occur during scattering by the target 11, measurement noise, and the like.

FIG. 4, FIG. 5, and FIG. 6 show the measurement results when the high frequency voltage was set to 87%, 84%, and 78% of the design voltage value, respectively. As is clear from these figures, one or more peaks were generated in the energy region lower than the peak in FIG. 3 due to the downward shift of the high frequency voltage and depending on the amount of shift. This clearly shows that the energy of the ions changes depending on the voltage applied to the vane.

FIGS. 7 and 8 show experimental results for 11134 particles. In this experiment, 1113- was generated from BF2 gas in the ion source 1, accelerated to 66 keV, and introduced to the RFQ accelerator 6. The resonant frequency of the RFQ accelerator 6 is 70.340MH2. The design voltage of the RFQ accelerator 6 under this condition is approximately 43 kV.

Figure 7 shows the 100% design voltage, and Figure 8 shows the 80% design voltage.
The measurement results are shown when a voltage of 8% is applied.

In this experiment as well, application of 100% voltage showed a single peak at the designed energy value, but by shifting the voltage downward, another peak was shown at an energy lower than that peak, and the high frequency voltage It has been demonstrated that the particle acceleration energy can be changed by shifting downward.

FIG. 2 is a block diagram showing the main structure of an apparatus to which the present invention is applied, and the same parts as in FIG. 1 are denoted by the same numbers. As is clear from the experimental examples described above, RFQ
If the high frequency voltage applied to the vanes of the accelerator 6 is shifted downward, multiple energy peaks may occur. Therefore, in this application example, an analysis magnet 20 is disposed at the exit of the RFQ accelerator 6, and is configured to select only ions having a desired energy and guide them in the target direction.

This configuration is extremely effective, for example, when the present invention is used for ion implantation into semiconductors.

<Effects of the Invention> As explained above, according to the present invention, the acceleration energy of charged particles can be changed simply by shifting the high-frequency voltage applied to the vane, which is superior to the conventional method of changing the resonance frequency. This makes it very easy to achieve energy variability. This greatly expands the possibility of applying RFQ accelerators to ion beam applications that require high energy, large current, and energy variability, such as ion implantation into semiconductors, and represents an innovative breakthrough in this application field. It is expected that this will bring about

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing the layout of an experimental device that actually measured the acceleration energy of particles by applying the present invention. FIG. 2 is a block diagram showing the main part configuration of the device to which the present invention is applied. FIG. 8 is a graph showing the experimental results using the apparatus shown in FIG. 1, and FIG. 9 is a partially sectional perspective view showing the conceptual structure of the RFQ accelerator. 1... 2... 3.5... 4... 6... 7... 8... 9... 10...

Claims (1)

[Claims] A high-frequency voltage of a predetermined frequency is applied to a cavity resonator in which four electrodes each having a waveform along the axial direction of the tank are formed at the tip of the cylindrical tank, causing resonance.
In a device that accelerates charged particles by guiding them at a predetermined speed into a space surrounded by the tips of the electrodes, the high frequency voltage to be applied is determined by the introduction speed of the charged particles and this high frequency voltage. Acceleration in a high-frequency quadrupole accelerator characterized by changing the acceleration energy of charged particles by shifting a predetermined amount downward from a voltage value that satisfies resonance conditions based on the frequency of and the period of the waveform of each electrode. How to control energy.