Ion accelerator and ion species selector

[57] ABSTRACT

The apparatus disclosed herein provides high energy positive ions, suitable for semiconductor doping, by projecting positive ions through an electron stripping gas at relatively low energy thereby to obtain positive ions which are multiply ionized or charged. Those ions which are raised to a preselected ionization level or state are segregated, and then accelerated by a relatively high accelerating voltage to achieve an energy suitable for ion implantation in a semiconductor matrix. Since the ions subjected to the relatively high accelerating voltage are multiply ionized, the energy imparted thereto, measured in electron volts, is substantially equal to an integer multiple of the accelerating voltage.

Claims, 1 Drawing Figure

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PATENTED J** 1513i4

3,786.359

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OJ

INVENTOR.

WILLIAM J. KING

BY

ATTORNEYS

3,786,359

ION ACCELERATOR AND ION SPECIES SELECTOR

BACKGROUND OF THE INVENTION

It has heretofore been proposed to manufacture various semiconductor devices by implanting ions in selected portions of a semiconductor material using an energetic ion beam, thereby to achieve a desired localized doping of the semiconductor material. However, to achieve the desired depths of implantation, quite high energies, e.g., up to a million electron volts, are required in certain application. Heretofore, it has been contemplated that such high energies would be provided by employing gas insulated potential sources such as accelerators of the type typically employed for scientific investigations. However, for the purpose of commerically producing semiconductor devices, such machines are much too inflexible, too difficult to service, and too limited in ion energy range for given machine size. Further, such machines are typically quite limited in the maximum available ion current.

Among the several objects of the present invention may be noted the provision of apparatus for providing energetic ions using a primary accelerating voltage which is only a fraction of the achieved ion energy; the provision of such apparatus which may be air insulated; the provision of such apparatus which will provide a substantial ion current; the provision of such apparatus which is relatively flexible and easy to service; the provision of such apparatus which is readily adaptable to semiconductor manufacture by ion implantation; the provision of such apparatus which is relatively simple and inexpensive. Other objects and features will be in part apparent and in part pointed out hereinafter.

SUMMARY OF THE INVENTION

Briefly, apparatus according to the present invention is adapted to provide high energy positive ions to a given ion utilization means, e.g., a means for presenting semiconductor materials to the ion beam for doping. Positive ions are projected by a relatively low accelerating voltage into a stripping canal. The stripping canal is supplied with a gas adapted to strip additional electrons from the positive ions projected through the canal thereby to increase the extent of ionization of at least a portion of those ions. Those ions which are raised to a particular multiple level of ionization are selected, e.g., by means of a suitable filter, and are then subjected to a relatively high accelerating voltage. The preacceleration selection is important if unnecessary beam tube current loading is to be avoided. Overloading of the beam tube by undesired ion species might preclude achieving useful currents of ions of the desired ionization state and species, since the latter may represent only a small fraction of the total ion current leaving the stripping canal. The energy thusly imparted to the selected ions is substantially proportional to the product of the relatively high accelerating voltage and the charge on the selected ions. Thus relatively high energies are imparted to the ions reaching the utilization means.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE is a diagrammatic illustration of an ion accelerator according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing, a source of positive ions is indicated at 11. For facilitating the use of this apparatus in semiconductor device manufacture, source 11 is preferably of a high current type such as a duoplasmatron. A stripping canal assembly is indicated at 13. As illustrated, the stripping canal assembly comprises a T-shaped tubular structure arranged to permit a beam of ions to pass through the cross bar of the T and to permit a stripping gas to be introduced into the beam path through the transverse tubular section. The ions emitted by source 11 are focused, e.g., by a so-called Einzel lens as indicated at 15, so that the ion stream or beam passes precisely through the canal in the cross bar of the assembly 13. A gas stripping medium is discussed for illustrative purposes only. Other stripping media (e.g., solid materials in form of thin diaphragms or fine liquid jet sprays) could also be used.

Ions emerging from the stripping canal pass through a second Einzel lens 17 for re-focusing and then through a so-called Wien filter as indicated at 19. As is understood by those skilled in the art, the Wien filter applies crossed magnetic and electric fields to the beam passing therethrough. By proper selection of the field strengths, the filter operates to deflect all ions passing therethrough except those which are of a particular selected species or degree of ionization. Those ions which are deflected are intercepted by a catcher 21, while ions of the selected species pass through the catcher 21 into a main accelerating tube 23. Catcher 21 may, for example, merely comprise an apertured plate.

A relatively low accelerating voltage, e.g., 10-40 kV, is normally applied to the ion stream prior to its entry into the main accelerating tube 23. To achieve higher stripping cross-sections, however, it may be desirable to raise this voltage to 100-150 kV, especially on machines where the total available voltage is higher, e.g., 300-600 kV. As is understood by those skilled in the art, the distribution of this potential between the ion source 11 and the catcher 21 will depend upon the configurations of the lens assemblies 15 and 17. Typically, a substantial portion of the field gradient will be applied immediately adjacent the source to provide adequate ion extraction and to impart appreciable energy to the ions before they reach the stripping canal, while the gradient in the vicinity of the stripping canal assembly 13 will be minimized to avoid the deflection of ions there. Since positive ions are to be accelerated, the source 11 will be positive with respect to the catcher 21 or the input end of the main accelerating tube 23.

A relatively large accelerating voltage, e.g., 100-600 kV, is applied across the length of the accelerating tube 23. While such a voltage is relatively high as compared with the voltage applied between the source and the input end of the tube 23, it is still within the range which can be achieved by air insulated voltage sources.

Assuming that a suitable gas, which normally will have an atomic weight similar to that of the desired ion species to maximize the stripping cross-section, is supplied to the stripping canal, additional electrons will be stripped from at least a portion of the positive ions projected through the canal so that these ions will then be multiply ionized, e.g., some of the ions passing through the stripping canal will become doubly or triply, etc., ionized. This is to be contrasted with a reversal in the

3,786,359 polarity of ionization to achieve twice the energy as in the so-called tandem machine configuration.

The parameters of the filter 19 are adjusted so that only ions of a particular species pass through the catcher 21 for further acceleration, e.g., only those ions which are triply ionized. Accordingly, current drawn from the high voltage source is not wasted in accelerating unwanted ions. Further, since the ions passed by catcher 21 are multiply ionized, the energy imparted to each such ion in transversing the main accelerating tube 23 will be substantially equal to an integer multiple of the accelerating voltage, e.g., 600, 900, 1,200, etc, keV in the case of a 300 kV accelerating voltage. Accordingly, relatively high energies are obtained even though the source voltage employed is below the range requiring gas insulation. In order to completely eliminate all particles other than the desired ion species emerging from the accelerating tube 23, the ion beam is preferably passed through the field of an analyzing magnet as indicated at 25. As is understood by those skilled in the art, such a magnetic field acts as a massenergy filter so that stray ions, e.g., such as may be generated by random collisions within the accelerating tube, are eliminated. The ion beam emitted from the magnet structure 25 is thus substantially spectrally pure and may be utilized in the production of semiconductor devices. A crossed-field filter could also be used for this purpose.

Means for utilizing the ion beam in this fashion are indicated generally at 27. Such means may, for example, comprise apparatus for presenting a substrate 29 of semiconductor material to the beam for irradiation, thereby to selectively dope the semiconductor lattice with the material the ions of which are provided by the source 11. The beam is made to uniformly cover the desired portion of the semiconductor surface by electrostatically scanning with vertical and horizontal plates, 30 and 31 respectively. Preferably the beam utilization means is maintained at ground potential to facilitate manipulation of semiconductor materials to be treated while the stripping canal and the ion source are maintained at relatively high positive potentials with respect to ground. A vacuum interlock mechanism may be provided to allow insertion and removal of substrate 29 without the necessity of shutting off the accelerator.

In view of the foregoing it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.