Controller for a linear accelerator

Details Controller for a linear accelerator Controller for a linear accelerator

2000-03-27

2001-11-20

Brown, Glenn W. (Department: 2858)

Electric lamp and discharge devices: systems

High energy particle accelerator tube

Magnetic field acceleration means

C315S506000, C315S005410, C315S111010, C315S111610, C250S492200, C250S492210

Reexamination Certificate

active

06320334

ABSTRACT:

FIELD OF THE INVENTION
The invention is concerned with a controller for a linear accelerator, particularly but not exclusively for use in an ion implanter.
BACKGROUND OF THE INVENTION
A linear accelerator structure accelerates charged particles of a specific mass/charge ratio which are injected into the accelerator at a specific injection energy. Radio frequency (rf) linear accelerators have been known for many years from the field of nuclear physics where they have been employed to accelerate heavy ions. More recently, rf accelerators have been used in semiconductor wafer processing. Typically, a beam of ions of a required species (such as boron, phosphorous, arsenic or antimony) is produced and directed at a wafer so that the ions become implanted under the surface of that wafer. Although electrostatic acceleration systems are suitable for producing beams of singly charged ions of 200 keV or more, it has been recognised that the desirable characteristics (for certain applications) of relatively high beam current and relatively high beam energy can be achieved by including an rf accelerator in the ion implanter device.
The use of rf linear accelerators for implantation of ions into semiconductor wafers has been suggested at least since 1976 in “Upgrading of Single Stage Accelerators” by K. Bethge et al, pages 461-468, Proceedings of the Fourth Conference on the Scientific & Industrial Applications of Small Accelerators, North Texas State University, Oct. 27-29, 1976; and in “Heavy Ion Post-acceleration on the Heidelberg MP Tandem Accelerator”, edited by J. P. Wurm, Max Planck Institute for Nuclear Physics, Heidelberg, May 1976. U.S. Pat. No. 4,667,111 discloses an ion implanter incorporating a radio frequency linear accelerator to provide ultimate beam energies as high as 2 MeV or more.
As discussed by Glavish et al in “Production of high energy ion implanters using radio frequency acceleration”, Nuclear Instruments and Methods in Physics Research B21 (1987), at pages 264 to 269, it is necessary that each resonator in the rf accelerator be kept in precise tune and matched to its amplifier, for example by feedback control of a movable plate capacitor. The resonators tend to be sensitive to thermal and mechanical disturbances as they are part of highly tuned systems, with Q values between 1000 and 2000. It is also important that the amplitude and phase of the rf voltage at the acceleration electrode be controlled. In one arrangement, a signal from the inductive or capacitative probes associated with each cavity is compared with the desired phase and amplitude derived from a master oscillator via a precision phase shifter. Using a microprocessor, a “parameter set” for a given ion beam energy and species may be developed. Phase may be held to about 1° and amplitude to within 1%.
U.S. Pat. No. 5,801,488 also describes the control of an rf accelerating device. Here, a control unit determines the respective value is of phase and rf power, based upon a predetermined programmed algorithm, to obtain a target energy which is set by an operator. The controller adjusts the phase and amplitude under feedback control. In “The Development of a Beam Line using an RFQ and 3-Gap RF Accelerators for High Energy Ion Implanters”, presented by Fujisawa et al at IIT in Kyoto, Japan, Jun. 24, 1998, a personal computer is employed to control phase and amplitude to an RFQ and 3-gap rf beam line. Again, phase is controlled to around 1° and amplitude to around 0.5%.
It will thus be appreciated by those skilled in the art that the precision and stability of the system relies upon the ability to generate a signal, for each resonator, which has a precise phase and amplitude. It is also important that the relative phase shift between resonators is accurately maintained.
SUMMARY OF THE INVENTION
One object of the present invention is accordingly to stabilize and set the phase shift between signals as it fluctuates due to mechanical or thermal drift, for example. It is a further object to provide a technique for introducing an accurate chosen phase shift into a sinusoidal signal. Still a further object is to accurately determine and control the amplitude of such a signal.
In a first aspect of the invention, there is provided a controller for controlling a phase shift between a reference signal and a measured signal in an rf resonator having an rf power supply, the controller comprising an oscillator for providing a reference sinusoidal signal having a reference phase; a detector for generating a transduced signal from the rf resonator, the transduced signal having a detected phase; a phase shifter apparatus including a quadrature signal generator arranged to shift the phase of the reference sinusoidal signal by 90° relative to the reference phase so as to generate a reference cosinusoidal signal; a phase demand signal generator arranged to generate a first phase demand signal representing the sine of a desired angle of phase shift of the said reference sinusoidal signal plus a further 90° phase shift, and to generate a second phase demand signal representing the cosine of the said desired angle of phase shift plus a further 90° phase shift; a first multiplier arranged to multiply the said cosinusoidal reference signal with the first phase demand signal representing the sine of the desired angle of phase shift plus 90° to generate a first composite signal, and a second multiplier to multiply the said sinusoidal reference signal with the second phase demand signal representing the cosine of the desired angle of phase shift plus 90° to generate a second composite signal; and a summer arranged to sum the first and second composite signals to generate a phase shifter output signal which is a second sinusoidal signal that is shifted in phase relative to the reference phase of the reference sinusoidal signal by the said desired angle of phase shift plus 90°, the second sinusoidal signal being equivalent to a second cosinusoidal signal that is shifted in phase relative to the reference phase of the reference cosinusoidal signal by the said desired angle of phase shift; a second multiplier arranged to multiply the transduced signal with the second cosinusoidal signal and to generate a phase error signal having a dc component from the resultant product; and a processor arranged to generate a control signal on the basis of the dc component of the phase error signal, to control the output of the said power supply so as to minimize the dc phase error signal.
The controller of the present invention relies upon the trigonometrical identity
sin(&ohgr;t+b)=sin(&ohgr;t)cos(b)+cos(&ohgr;t)sin(b)
where the phase shift in the sinusoidal signal is represented by “b”.
Sin(b) and cos(b) are dc values which may be accurately generated. Thus, precise linear adjustment of the phase shift relative to a master oscillator may be provided. The phase angle “b”, may be continuously adjusted over a full 360° and with no discontinuity. The linearity and stability of the apparatus is also improved relative to the prior art.
It is desirable to ensure that the phase of the second sinusoidal signal (having a “demand phase” accurately determined using the trigonometrical function outlined above) is identical with the phase of the rf signal in the rf resonator which is obtained by the detector. When this is the case, the product of the second sinusoidal signal, shifted by exactly 90°, (so that it becomes the second cosinusoidal signal) and the transduced signal, should be zero. This principle can be used to provide a phase controller which uses the accurately determined phase shift as a reference to which the phase of the rf signal in the resonator cavity is locked via closed loop feedback. With this technique, phase can be controlled to about 0.5°. It will be appreciated that, instead of shifting the desired phase angle by 90° so as to produce, in effect, the second cosinusoidal signal, a quadrature signal of the transduced signal may instead be multiplied by a sinusoidal signal shifted by the chosen phase shift only (that is, not

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