GB2209242A - Ion beam arrangement - Google Patents

Abstract

An ion beam arrangement including means for receiving the steady ion beam and for accelerating it and converting it into a sequence of discrete bunches of ions having a velocity distribution which is small compared to the mean velocity of the ions comprising a series of electrode tubes and drift tubes. The length of the electrode tubes and the acceleration gaps between them are chosen in relation to the alternating voltages applied to produce a particular and predetermined energy distribution in the bunched ion beam which emerges at the output end of the drift tube 27. Ions from an ion source (not shown) pass through earthed tube 1 and are accelerated or retarded by the field due to the HF signal applied to tube 2, and in long drift tube 3 ions of similar velocity are separated into discrete bunches. The separate bunches of ions arrive at drift tube 4 such that the phase of the HF signal applied thereto accelerates each bunch as a whole, the bunches then being fed to an accelerator having two stages 24, 25 to import the required energy distribution to each bunch. <IMAGE>

Description

ION BEAM ARRANGEMENT This invention relates to an ion beam arrangement and is specifically concerned with such an arrangement in which the ion beam produced thereby is in the form of a series of bunches or packets of energy. It is customary to generate ions in a continuous stream, and to direct them to the point at which they are required for utilisation. As the utilisation of the ions often depends on them having a sufficiently high energy, it is necessary for some applications for the ion beam to be passed through an accelerator which greatly increases their velocity.The acceleration of the ions is achieved by passing them through a strong electric field, but because the voltage necessary to impart very high energies to the ions can be very large indeed, it has been proposed to subject the ions to a high frequency alternating voltage so that they are given a series of accelerating "kicks" along the length of the accelerator.

Our co-pending patent application 8603585 relates to an ion beam arrangement in which an accelerator section of an ion beam arrangement is preceeded by an ion buncher, in which the net energy of an input steady ion beam is not altered appreciably but the beam is divided into a succession of discrete bunches of ions. This enables the bunches of ions to be accelerated in a following accelerator section to a relatively high energy but so that the emerging ion beam is substantially monoenergetic. In this way, a high energy ion beam can be generated very efficiently. For certain applications, however, a mono-energetic ion beam may not be preferred, and the present invention seeks to provide an improved ion beam arrangement.

According to this invention an ion beam arrangement includes means for receiving a steady ion beam and for converting it into a sequence of discrete bunches of ions having a velocity distribution which is small compared to the mean velocity of the ions; a plurality of drift tubes through which the bunches of ions pass in turn, the drift tubes forming part of an alternating voltage accelerator; and means for applying alternating voltages to each of said drift tubes, the physical properties of the drift tubes and the characteristics of the alternating voltages being such that the emergent bunches of ions have a predetermined energy distribution.

Generally, the number of drift tubes will be much fewer than in a conventional linear alternating voltage accelerator. In such a conventional accelerator a large number, typically fifty or more drift tubes are provided, in which the lengths of succeeding tubes progressively increase so that the length of each is proportional to the velocity of ions passing through it. This is not so with the present invention in which typically only four or so drift tubes are present. By careful choice of the lengths and spacing of the drift tubes, a combination of accelerations and de-accelerations of the ions produces a consistent and particular energy distribution within each bunch of ions, although in the output beam the bunches may merge.

For some applications of ion beam arrangements, it is desirable to utilise ions having different energies, for example in the fabrication of certain kinds of semiconductor devices. Previously it would have been necessary to subject a semiconductor material to a succession of ion beams, each beam having a different required narrow band of energies. The present invention enables this range of energies to be achieved in a single ion beam.

Preferably the ion buncher takes the form of means for receiving a steady ion beam having a predetermined mean velocity and for applying a high frequency alternating electric field to the beam, so as to speed up or to slow down ions by an amount which is small compared to the mean velocity of the ions to produce a velocity modulated ion beam; an elongate drift structure arranged to receive the velocity modulated beam and dimensioned such that the transit time of the beam through the drift region is in excess of a plurality of cycles of said high frequency so that at the output of the drift region structure the velocity modulated ion beam forms discrete bunches of ions.

Preferably again there are means between the ion buncher and the accelerator for receiving said discrete bunches of ions and for applying a high frequency alternating electric field to the bunches such that the ion bunches are accelerated in the same sense, the electric field applied to said bunches being substantially greater than the electric field applied to modulate the ion beam.

Although it is possible to generate a particular energy distribution by applying different alternating frequencies to the sequence of drift tubes which comprise the accelerator, it is preferred to apply the same frequency to each tube. The particular energy distribution then results from the relative lengths of each of these drift tubes and the spacing between them.

A correct combination of drift tubes lengths and spacing would in principle result in a required energy distribution at the output, even if an initial beam buncher were not used. However, the continuous ion beam, would be more difficult to control and would give rise to operational instabilities, in that undesired small voltage variations on the drift tubes could result in large energy variations in the output ion beam.

The invention is further described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a longitudinal section view of an ion beam arrangement, Figure 2 represents the energy distribution of an output ion beam with and without the use of an ion beam buncher, Figure 3 shows profiles of ion concentrations implanted into a semiconductor body for the ion beam energy distribution shown in Figure 3, Figure 4 represents another energy distribution of an output ion beam with and without the use of an ion beam buncher, and Figure 5 shows profiles of ion concentrations, implanted into a semiconductor body for the ion beam energy distribution shown in Figure 4.

Referring to Figure 1, a source of ions (not shown) produces a steady beam of ions in which the charge density is substantially constant and the mean energy of the ions varies only slightly from a nominal value. Such a steady ion beam can be produced in accordance with known techniques. In order to convert the steady ion beam into a sequence of ion bunches, the beam is passed through an ion buncher which consists of a first short earthed tube 1, an HF electrode tube 2 and a relatively long earthed drift tube 3 the interior of which represents an equipotential drift region. At the output of the drift tube 3 the ion bunches are passed towards a further HF electrode tube 4 which acts to accelerate them. The purpose of the tube 1 is to shield the ion source and any previous stages from the action of the HF signal on the HF electrode 2.The action of the HF signal applied by the electrode tube 2 to the ion beam is to set up an oscillating electric field which causes the ions to either be accelerated or retarded with respect to their initial velocity depending on their position relative to the electrode. Thus the ions entering the long drift tube 3 will have sightly differing velocities, and the amplitude of the HF electric field is chosen such that the velocity distribution is relatively small. The HF signal applied to the electrodes is in the range 2 MHz to 30 MHz. The length of the first tube 3 is chosen such that the transit time of the ions through the drift region is significantly longer than the repetition period of the HF frequency.

Thus typically, the length of the drift tube 3 is such that the ions take a time to travel from one end of the drift tube to the other in a time corresponding to approximately six cycles of the HF signal. During this time, the effect of the initial variation of velocity of the individual ions is to cause them to separate out into separate bunches. These separate and discrete bunches of ions are arranged to arrive at the gap of the HF drift tube 4 such that the phase of the HF signal thereon is in a sense which accelerates each bunch as a whole. The increased energy imparted by this electrode 4 is significantly greater than the energy distribution within a bunch.Whereas typically the AC voltage applied to electrode tube 4 is of the order of 50 KV, that applied to the electrode tube 2 via an attenuator 15 is of the order of only 221 KV, and the actual value is adjusted by means of the variable attenuator 15 so as to produce the best separation of the bunches when they arrive at the electrode tube 4. In this instance, the long drift tube 3 is provided with internal focussing elements 20, 21, 22, 23 which act to confine the ion beam to the central axis of the ion beam buncher. These focussing electrodes are electrostatic in nature and operate in well known manner.

Although relatively high voltages may be applied to these electrodes so as to cause transverse acceleration of the ions with respect to the longitudinal axis of the drift tube 3, they do not materially influence the longitudinal velocity of the ions.

The ion beam buncher feeds an accelerator having just two stages 24 and 25 and which are used to impart a required energy distribution to the ions making up each bunch. The lengths L of the drift tubes, and the acceleration gaps 1 between them do not increase progressively as in a conventional linear accelerator, but instead the lengths L and 1 are chosen to produce a particular and predetermined energy distribution within the bunched ion beam which emerges at the output end of drift tube 27.

Depending on the phase of the voltage when the ions enter a gap, the gap length and velocity of the ion, it will gain or loose energy. As the ion beam enters a different phase of the voltage cycle it will produce an energy distribution within the beam. The design may be optimised to produce a large energy distribution. This is achieved by designing the accelerator round a phase stable position in which an ion may initially enter each gap with a de-accelerating voltage across it or leave each gap as the voltage is rapidly decreasing. The largest energy distributions in the ion beam are achieved by choosing the phase stable position using phases outside of the first and third quadrants of the voltage cycle.However, ions entering during the second and fourth quadrants can lead to beam energy unstabilities; that is, a small voltage fluctuation results in a large change in energy distribution. Consequently, these areas should be avoided.

The beam buncher is used to concentrate the beam so that it enters the first gap at a particular phase of the voltage cycle. This may be optimised for the energy distribution required by varying the phase of the high frequency voltage on the beam buncher in relation to the high frequency voltage on the accelerator, and this is not a significant problem in the frequency range 2-30 MHz.

One example of an energy distribution which has been obtained is shown in Figure 2, using two accelerating gaps with a peak r.f.voltage of 50 kV, and an initial beam energy of 50 keV. The energy distribution obtained without using the beam buncher is also shown by way of comparison, and it will be seen that use of the beam buncher results in a higher overall energy and a relatively great energy variation against ion dose, ion dose representing the relative numbers of ions having a particular energy.In this instance the phase stable ions are accelerated before and after the peak of the high frequency voltage is reached, and the total ion implanted dosee is 1x1014 cm 2, Figure 3 shows the corresponding ion implantation that would occur with and without the beam buncher for the implantation of selenium ions into a body of gallium 14 -2 arsenide at a total dose of 1x10 cm 2 using the ion beam energy distribution shown in Figure 2. The implanted profile is assumed to be gaussian for a particular energy and the distributions for each energy have then been combined to form a summation of all the gaussian profiles.

Another example of an energy distribution is shown in Figure 4, using an ion beam arrangement having four accelerating gaps, with a peak voltage of 50 kV and an initial beam energy of 50 keV. As before, the values of L and M are selected to produce the required energy distribution, and the effect of not using the beam buncher is shown by way of comparison. In this instance, the phase stable ions are accelerated before and after the peak of the high frequency voltage in the first two gaps, but in the second and third gaps the ions experience a de-accelerating electric field before the r.f. phase is correct to accelerate the ions. The ion implanted dose 5x1012 -2 is 5X1012 cm 2 The corresponding implanted profiles are shown in Figure 5, both with and without the use of the ion beam bunchers, for the implantation of selenium ions into a semiconductor body of gallium arsenide at the total dose of 5x10 cm-2 using the ion beam energy distribution shown in Figure 4. An additional gaussian profile is included in this figure to demonstrate the change in the back edge of the profile.

Claims (5)

1. An ion beam arrangement including means for receiving the steady ion beam and for converting it into a sequence of discrete bunches of ions having a velocity distribution which is small compared to the mean velocity of the ions; a plurality of drift tubes through which the bunches of ions pass in turn, the drift tubes forming'part of an alternating voltage accelerator; and means for applying alternating voltages to each of said drift tubes, the physical properties of the drift tubes and the characteristics of the alternating voltages being such that the emergent bunches of ions have a predetermined energy distribution.

2. An arrangement as claimed in claim 1 and wherein the ion buncher takes the form of means for receiving a steady ion beam having a predetermined mean velocity and for applying a high frequency alternating electric field to the beam, so as to speed up or to slow down ions by an amount which is small compared to the mean velocity of the ions to produce a velocity modulated ion beam; an elongate drift structure arranged to receive the velocity modulated beam and dimensioned such that the transit time of the beam through the drift region is in excess of a plurality of cycles of said high frequency so that at the output of the drift region structure the velocity modulated ion beam forms discrete bunches of ions.

3. An arrangement as claimed in claim 1 or 2 and wherein there are means between the ion buncher and the accelerator for receiving said discrete bunches of ions and for applying a high frequency alternating electric field to the bunches such that the ion bunches are accelerated in the same sense, the electric field applied to said bunches being substantially greater than the electric field applied to modulate the ion beam.

4. An arrangement as claimed in any of the preceding claims and wherein the same frequency is applied to each of the drift tubes of the alternating voltage converter.

5. An ion beam arrangement substantially as illustrated in and described with reference to the accompanying drawings.