Apparatus For Producing A High Energy Beam Of Selected Metallic Ions

Abstract

A technique for producing a high energy beam of ions of a selected metal without having the selected metal initially in gaseous form and the application of the technique to treating solid bodies of material. An inert gas is ionized in the source chamber section of an accelerator. The exit channel section of the accelerator is made from a material which includes the preselected metal. A deflecting magnet is positioned along the path of the ion beam emerging from the accelerator. Included in the emerging beam are ions of the inert gas and ions of the preselected metal used in the exit channel. All ions except either those of the inert gas or those of the preselected metal are selectively discarded by regulating the current of the deflecting magnet. The undiscarded ions remaining in the beam are used to treat the solid body of material.

Description

This invention relates to ion beams. More particularly, this invention relates to a technique for producing high energy beams of metallic ions and the use of such beams in the treatment of solid bodies of materials, especially semiconductor materials.

An ion beam is essentially a beam of charged particles, other than electrons, all moving with substantially the same speed in a nearly common direction. Ion beams have been produced for some time now using devices known as accelerators. Ions are normally created in an accelerator by applying electrical energy to a selected gaseous material causing it to become ionized. In one form of accelerator, the ions so generated are then accelerated down a long tube where they emerge in the form of a high velocity beam. The portion of the accelerator in which the ions are generated is called the source chamber and the portion of the accelerator in which the ions are accelerated is called the accelerating column. The source is coupled to the accelerating column through a small elongated tubular element called the exit channel. Characteristically, the exit channel is made of aluminum. Although the emerging beam consists primarily of ions of the selected gas it may also contain ions of other materials, such as trace gases present in the source chamber and other parts of the system. However, by magnetic selection means well known in the art, all the residual ions can be and usually are deflected away from the beam so that the beam contains only ions of the selected gas initially supplied to the source chamber.

Ion beams are mainly used in the laboratory for research purposes. More recently, however, ion beams have been used as a technique for implanting energetic ions into various materials, such as semiconductors, in order to alter the electrical characteristics of the material. For example, phosphorous ions, generated by ionizing phosgene gas, have been successfully implanted into silicon type semiconductors. The technique does have limitations. One of these limitations is the requirement that the material whose ions are to be implanted must be available in gaseous form so that it can be ionized in the source chamber. Ion beams of many materials would be extremely useful in the treatment of semiconductors, as well as for other purposes, but hitherto have not been produceable because the material is not readily available in a gaseous state. Typical examples are zinc, lead and iron.

Accordingly, it is an object of this invention to provide a technique for producing ion beams. It is another object of this invention to provide a technique for producing an ion beam of a metallic material that is not readily volatile.

It is still another object of this invention to provide a technique for producing a beam of metal ions without reducing the metal to its gaseous form.

It is yet still another object of this invention to provide a technique for treating solid materials with ion beams.

It is another object of this invention to provide a technique for implanting metallic ions into solid materials.

It is still another object of this invention to provide a technique for implanting zinc ions into a gallium arsenide substrate.

The above and other objects are achieved according to this invention which is based on a discovery of appreciable ion currents in the output beam of an accelerator at mass-energy ratios corresponding to the material used in the fabrication of the exit channel. Hitherto, this phenomena was not realized. It is believed that these ions are created by sputtering of the exit channel. By simply adjusting the magnetic selection means, these ions rather than the ions of the selected gas can be retained in the beam. Thus, it is possible to produce a beam of ions of whatever material is used in the fabrication of the exit channel. One important advantage of this technique is that the material from which ions are desired need not be in gaseous form.

Accordingly, the invention involves producing an ion beam by using an accelerator in which the exit channel is made from a metallic material from which ions are desired. The source chamber section of the accelerator is filled with an ionizable gas. In using the ion beam to treat solid materials, the solid material is disposed in an evaporating chamber which is coupled to the accelerator. By regulating a deflection magnet located along the path of the ion beam, ions of either the selected gas or the exit channel material can be directed onto the solid material. The solid material is mounted in the evaporating chamber on a rotatable holder so that its surfaces can be positioned to receive either the ion beam or the evaporant.

Other features and attendant advantages of the invention will become apparent on reading the following detailed description and when taken in conjunction with the drawings in which:

FIG. 1 is a schematic view of an apparatus constructed according to this invention; and

FIG. 2 is a section view of a portion of the apparatus shown in FIG. 1, taken along axis 2-2.

Referring now to the drawing, there is shown in FIG. 1 an apparatus for practicing the invention.

The apparatus includes an accelerator 11 for producing an energetic beam of charged particles. The accelerator 11 comprises a source chamber 12, adapted to hold a quantity of gaseous material. The source chamber 12 may be in the form of an elongated quartz tube opened at one end and closed at the other end. A pair of RF electrodes 13 and 14 are mounted on the outside of the source chamber 12 and connected to an RF power supply (not shown) for supplying RF energy to the source chamber 12. An internally extending probe electrode 15 is mounted on the source chamber 12 at the closed end and connected to a DC power source (not shown). An electrically conductive plate member 16 is positioned at the open end of the source chamber 12 and is also connnected to the DC power source. The plate member 16 has a centrally located aperture. A small tubular metallic exit channel 17 is press fit into the central aperture of the plate member 16. The exit channel 17 is made from material including a metal of the type from which ions are desired. For example, if zinc ions are desired, the exit channel is made of zinc or a compound of materials including zinc. The plate member 16 is also provided with a passageway 18 through which the selected gaseous material (i.e. the source gas) is supplied to the source chamber 12. Probe electrode 15 and plate member 16 are connected to the DC power source in such a manner that the plate member 16 is at the higher voltage. A focusing solenoid 19 surrounds the source chamber 12 near the exit channel 17 to increase the ion current density in the region of the exit channel 17.

In operation, the source gas, which may be for example argon, is ionized by the application of RF energy. Because of the potential difference between probe electrode 15 and plate member 16 positive ions drift to the vicinity of the plate member 16.

A rubber gasket 20 is interposed between the plate member 16 and the source chamber 12 to provide a gastight seal between these two members. A focusing electrode 21 is positioned next to the plate member 16 for constraining the beam of particles passing though the exit channel 17 into a narrow stream. The focusing electrode 21 is also connected to the DC power source.

The accelerator 11 further includes an accelerating column 22 positioned to receive the charge particles passing through the exit channel 17. The accelerating column 22 is made-up of a plurality of electrically conductive sections 23 separated by electrically insulating spacer elements 24. A base plate 25 at ground potential is connected to the exit end of the accelerating column 22. Each of the conductive sections 23 is connected to a separate resistive element (not shown) which in turn is coupled to the DC power source, in a manner well known in the art, so that there is a uniform voltage drop from the high voltage end at the focusing electrode 21 to the low voltage end at the base plate 25. The base plate 25 is connected through a funnel shaped pipe section 26 to one leg of a T joint 27. A diffusion pump system 28 for evacuating the system is connected to another leg of the T joint 27. The third leg of the T joint 27 is connected to an elongated transport pipe 29 which in turn is connected to a vacuum chamber 30. A quadrapole focusing lens 31 surrounds the transport pipe 29 in order to constrain the ion beam. Ion selection means, which may be in the form of a deflecting magnet 32, is located in front of the vacuum chamber 30 for deflecting away from the beam ions of materials that are not wanted. The deflecting magnet 32 can be set, by adjusting the current, so as to retain in the beam ions of any of the various materials that are present in the beam. As can be seen in FIG. 2, a pair of evaporating boats 33 and 34 are disposed on the bottom of the vacuum chamber 30. A rotatably mounted substrate holder 35 is also located inside the vacuum chamber 30. The substrate holder 35 is adapted to rotate about a horizontal axis and can be set at any one of four positions so that each surface of a substrate S is facing either in the direction of the ion beam or in a direction to receive the evaporant from the evaporating boats 33 and 34. Thus, once the substrate S is mounted in the substrate holder 35, it can be treated either by ion bombardment or by evaporation.

In an actual example in which the technique of this invention was successfully practiced, a gallium arsenide substrate doped with tellurium was mounted on the substrate holder 35. One of the evaporating boats 33 was filled with N-type gold and the other evaporating boats 34 was filled with P-type lead. The source chamber 12 was filled with argon gas. The substrate holder 35 was set at a position in which one surface of the substrate S faced in the direction of the ion beam. The exit channel 17 was made of zinc. The current supplied to the deflecting magnet 32 was set so that all ions except those of zinc would be deflected away from the emerging beam. The accelerator 11 was turned on. After the desired amount of zinc ions was implanted in the substrate S, the accelerator 11 was turned off and substrate holder 35 was rotated 90.degree. so that the treated surface of the substrate S faced in the direction of the evaporating boats. The heater to the boat 34 containing P-type lead was energized and lead evaporated onto this surface. The heater was then turned off and the substrate holder 35 rotated 90.degree. so that the other surface of the substrate S was in position to receive the ion beam. The current supplied to the deflecting magnet 32 was set to deflect away from the beam all ions except argon ions. The accelerator 12 was turned on and this surface cleaned by bombarding it with argon ions. The accelerator 11 was then turned off and the substrate holder 35 rotated 90.degree. so that this surface faced in the direction of the evaporating boats. The heater to the boat 33 containing gold was then energized and gold evaporated onto this surface.

Additional features and advantages of the invention are described in an article entitled "Implantation of Zinc Into Gallium Arsenide," written by the inventors and appearing in the proceedings of The IEEE, Vol. 55, NO. 1, Jan. 1967, pgs. 125--126.

Obviously, many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that, within the scope of the appended claim, the invention may be practiced otherwise than as specifically described.

Claims

We claim:

1. Apparatus for treating solid materials with energetic ions of a predetermined metal comprising:

an accelerator having a source chamber adapted to be filled with a gaseous material, said chamber having an exit channel made of a material including said predetermined metal;

actuating means to ionize said gaseous material whereby there is produced a high energy beam of charged particles passing out of said source chamber and including ions of said predetermined metal from the exit channel;

deflecting means adjacent the path of said beam to separate the ions of said predetermined metal from the other particles in said beam;

an evacuated chamber connected to the accelerator for receiving the beam of ions of said predetermined metal; and

support means disposed in the evacuated chamber in the path of said beam for holding the solid material whereby ions of said predetermined metal from said exit channel impinge upon said material.