Background of the Invention Ion implantation using cluster ions of the atomic species to be implanted has been discussed. In particular, reference is made to Decaborane Ion Implantation, by Perel et al, published Ion Implantation Technology-2000, pages 304 to 307.

Implanting decaborane ions can be used to implant individual boron atoms at very low energies, such as needed for the formation of ultra shallow junctions in semiconductor devices. The individual boron atoms in an ion beam of decaborane have an energy of approximately 1/11 of the energy of the molecular ions so that an ion beam energy of 2 keV gives rise to an individual boron ion implant energy of only about 200 eV.

In practice, a molecular ion beam extracted from a decaborane ion source typically contains molecular ions of BH with a range of masses. This range of masses arises because the individual molecules in the beam will have a varying number of hydrogen atoms.

Also, a variation in mass arises because boron occurs naturally in two isotopes: mass 10 having a natural abundance of about 19% and mass 11 having a natural abundance of about 81%.

In the above referred article by Perel et al, a typical spectrum of the masses of BxHx molecular ions is illustrated. It is also said that the energy difference per boron atom implanted over a range of ten neighbouring peaks of the spectrum, each spaced apart by a single mass unit, is only about 8.6%.

However, the reference suggests that such an energy range is not a problem in typical applications.

Summary of the Invention The present invention provides a method of ion implantation comprising the steps of producing an ion beam including ions each containing a predetermined plurality of atoms of a species to be implanted. The atomic species to be implanted is selected to be one occurring naturally in two primary isotopes, with the least abundant of these primary isotopes having a lower natural abundance which is at least 1% of the total and the most abundant of the primary isotopes having a higher natural abundance. In performing the method the atomic species contained in the ions of the beam are isotopically enriched so that one isotope has an enriched abundance which is greater than the above mentioned higher natural abundance. The method further includes mass selecting from the beam the ions containing said plurality of said atoms and directing the mass selected ions at a target for implantation therein.

By using an enriched source of the atomic species for producing the ion beam containing ions having a plurality of atoms of the species with two naturally occurring isotopes, the spread of masses in the resulting ion beam can be significantly reduced. This can provide increased beam current of desired species for implantation at the wafer, reduced erosion of beam line components and reduced contamination.

In a particular example, the atomic species may be boron which is isotopically enriched to increase the abundance of the isotope"B. Preferably the enriched abundance of"B is at least 99% and the ions each contain ten boron atoms.

The invention also contemplates an ion implanter provided with a source of isotopically enriched cluster ions.

Brief Description of the Drawings An example of the present invention will now be described with reference to the accompanying drawing which is a schematic representation of an ion implanter embodying the invention.

The ion implanter illustrated comprises an ion source 10, a mass selection magnet 11, a mass selection slit 12 and a process chamber 13. In the ion source 10, a plasma is formed in an arc chamber 14. Ions of the plasma are extracted from the arc chamber through a slit 15, by means of an extraction potential applied to an extraction electrode 16. An extracted ion beam 17 passes through a further shield electrode 17 which is typically held at the potential of the mass selector 11. The mass selector 11 comprises a sector magnet so that ions of different masses (or more strictly momenta) are spatially separated as they travel around the sector towards the mass selection slit 12. The mass selection slit 12 is arranged to permit only ions of a selected desired mass or range of masses to travel onwards into the process chamber 13 for implantation in a wafer 18 held on a wafer holder 19.

Normally, the ion beam 20 arriving at the wafer 18 in the process chamber 13 has a cross-sectional area which is smaller than the area of the wafer 18 and an arrangement is provided for mutually scanning the wafer holder 19 relative to the ion beam 20 to ensure all parts of the wafer are uniformly implanted.

The scanning arrangement may include mechanical scanning by means of a scanning arm 21 shown schematically and in part in the drawing. Other scanning arrangements may include hybrid arrangements in which the ion beam 20 is scanned, e. g. electromagnetically or electrostatically, in one plane, and the wafer holder 19 is mechanically scanned through the plane of the scanned beam. Two

dimensional beam scanning arrangements may also be used with embodiments of this invention.

It should be noted that many variations in the structure and design of the ion implanter may be employed in different embodiments of the present invention. Furthermore, the details of the design and operation of the ion source 10 are not important in the performance of the present invention, except insofar as they relate to the feed material used for the arc chamber 14 and other arrangements which promotes the production in the arc chamber of desired cluster ions (or ions with multiple atoms of the implant species).

In the illustrated example, an oven 22 contains an amount of solid decaborane (B1oH14) which has been prepared using enriched boron so that the enriched abundance of 1lB is higher than the natural abundance of about 81%.

Known techniques may be used for providing the enriched boron. One technique is to centrifuge gaseous BF3. Before centrifuging, multiple BF3 molecules are reacted with an adduct to form a large composite molecule, so that there will be then a larger range of masses of BF3 composite molecules resulting from the differing numbers of the two isotopes of B in the composite molecules. Multiple centrifuging iterations are performed, each time selecting the higher (or possibly the lower) mass fraction of composite molecules, dissociating the composite molecules in the selected fraction to individual BF3 molecules and then reacting them again to reform large molecules, repeating the centrifuging and again selecting the appropriate higher (or lower) fraction. With repeated iterations, the purity of a particular isotope (llB for selecting higher mass fractions and 1°B for selecting lower mass fractions) is increased to a desired level of enrichment. The

enriched BF3 is then used to make enriched decaborane in ways known in the art.

United States Patent No. 6086837 to Cowan et al discloses a medical therapeutic use of enriched decaborane in which at least about 90% of the boron atoms are 1°B. The patent also discloses a method for the production of the decaborane from the enriched boron. In this patent, the decaborane comprises primarily 1°B atoms. In the present invention, it will be easier and preferable to use decaborane in which the"B abundance is increased and enriched abundance levels for"B of 99% or higher are achievable using known techniques.

With naturally occurring boron, a molecule containing Blo typically will have masses associated with the boron atoms of between 100 and 110 in the following proportions: 100 (6E-6%), 101 (3E-4%), 102 (5E-3%), 103 (6E-2%), 104 (4E-1%), 105 (2. 2%), 106 (7. 7%), 107 (18. 8%), 108 (30.1%), 109 (28. 5%) and 110 (12.2%).

If the boron is enriched so that the abundance of "B is 99%, then the abundance of mass 110 is over 90%.

When the implant apparatus illustrated in the drawing is run using 99% l1B enriched decaborane as feed material, any spread in the masses of the beam ions is almost entirely due to the beam ions containing differing numbers of hydrogen atoms. If a beam could be generated which is predominantly B1oH then most of the boron in the beam could be transported for implantation in the wafer 18 when setting the mass selection slit 12 wide enough only to pass a single mass 124. This can be particularly useful in avoiding contamination of the implanted wafer 18 with other materials which may have a closely similar mass to the value 124 (1lBloHl4) for example In

at mass 115 or Sb at masses 121 or 123.

Even if the beam produced from the ion source 10 contains molecules with different numbers of hydrogen atoms, the overall range of masses within the beam is still considerably reduced when using the enriched boron.

Importantly, because more of the decaborane beam current is concentrated over a narrower range of masses, a greater proportion of the decaborane beam can be transmitted through the beam line and in particular the mass selection slit 12 to the target wafer. This reduces the amount of erosion of beam apertures along the beam line, and the mass resolution slit MRS in particular. This is particularly important as the mass of decaborane ions and their resulting sputter efficiency can produce considerable erosion of beam line elements, which decreases the lifetime of consumable elements within the beam line and increases beam line deposition and particulates on implanted wafers.

Although the example described herein is particularly concerned with beams of decaborane ions, or more generally molecular ions comprising ten boron atoms with a number of hydrogen atoms, the method and apparatus of the invention can also be used to improve the performance when producing beams of ions containing multiple atoms of other species for implantation. The techniques may also be used for cluster ion beams. Whenever the atomic species to be implanted occurs naturally in two or more isotopes, improved performance can be obtained by using an enriched form of this species so that the range of masses in the ion beam produced is reduced.

In recent years, use of rare earth borides have been used to provide boron molecular ions (see for example U. S. Patent No. 5861630). Molecular clusters with, for example, twelve boron atoms are produced.

Isotopic enrichment of the boron has significant advantages in such applications.