Octadecaborane

Octadecaborane(22)
Other names octadecaborane; octadecaboron doicosahydride; octodecaborane; n-Octadecaborane; i-Octadecaborane
Identifiers
CAS number	21107-56-2
Properties
Molecular formula	B18H22
Molar mass	216.4 g/mol
Appearance	White to off white powder
Density	1.012 g/cm3
Melting point	110–115 °C
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Octadecaborane is an inorganic compound, a borane with chemical formula B₁₈H₂₂. It is formed by oxidation of either the B₉H₁₂⁻ anion or the B₂₀H₁₈²⁻ anion. Ions, typically the positive ion B₁₈H₂₂⁺, can be used as a P type dopant through ion implantation for the manufacture of semiconductors. The molecule is ionized and accelerated, more or less intact (B₁₈H_x⁺), to the necessary energy to cause it to embed in the substrate to the desired depth. Once the octadecaborane molecule has been implanted, it breaks apart and most of the hydrogen atoms diffuse and escape the substrate leaving behind the boron atoms.

Transporting octadecaborane, as opposed to atomic boron, or the more popular molecule borondiflouride (BF₂⁺), in the low energy range (< 5 keV) provides two notable advantages. The larger molecule leverages the first law of thermodynamics in that the effective energy of each boron atom in the molecule shares a fraction of the total energy of the molecule. Since ion beam become exceptionally more difficult to transport as energy approaches 0eV due to like charge repulsion of the atoms, a larger mass molecule can be transported at higher energies (and therefor higher speed) allowing less time for repulsion with the same effective atomic energy.

The second advantage to transporting octadecaborane over atomic boron or BF₂ is the increased flux of atomic boron. The molecular ion flux for a given energy is similar to an atomic ion flux at the same energy. Therefor, the total number of boron atom flux for a given ion flux is the simple multiple of atoms in the molecule. Comparing octadecaborane to atomic boron, we get eighteen times the boron flux when using the molecule. This equates to higher productivity in a manufacturing environment.

Drawbacks to transporting larger molecules is a reduction in the maximum energy, or depth, that the molecule can be implanted. Ion implantation equipment function much like a mass spectrum analyzer, utilizing a magnet to filter and segregate atomic species. Larger or heavier molecules and atoms require stronger magnets to bend the beam for filtering. The extreme size of the octadecaborane molecule places great demands on the magnets of the equipment making high energy operation difficult if not impractical.

References

• http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=220984
• http://www.atmi.com/documents/Octadecaborane.pdf, MSDS