4.5.2 Molecular Ion Implantation

In order to reduce the channeling effect it is desirable to use ions with high masses, because they produce more point defects in the target material and enhance thereby the damage induced de-channelling. Contrary to that requirement boron, a very light ion, is the most preferable doping species to form p-type doped regions in silicon, due to its better electrical characteristics compared to other group three elements.

To overcome this problem molecular ions or atom clusters containing boron, like BF$_2$ or B$_{10}$H$_{14}$ are used for implanting p-type doped regions. Thereby boron profiles with a significantly reduced channeling tail can be generated.

An additional advantage of using molecular ions is that the depth of the implanted profiles can be significantly reduced, what is of prime importance for advanced IC technologies. Each ion implanter is characterized by a minimum energy limit. Therefore the minimal depth of the doped areas is restricted when implanting monomer ions. If an implantation is performed with molecular ions the implantation energy is shared by several atoms why the effective energy for one atom species and thereby the profile depth of this species is significantly lower. Additionally the dose rate is higher because more than one atom of one species is implanted at the same time. A higher dose rate results in shorter implantation times and thereby the production costs are lowered.

MCIMPL provides two methods for the simulation of molecular ions.

• Simplified Molecular Method
• Full Molecular Method

The user can switch between these methods by the command-line parameter simpleMolecule. Both methods have in common that the implantation energy is shared between the different atom species according to the masses of the atoms and the stoichiometry of the molecule. The portion of energy related to one atom of species $i$ is

$$\frac{E_i}{E} = \frac{M_i}{\sum\limits^{atomspecies}_{j}{N_j\cdot M_j}}$$ (4.6)

if $N_i$ is the number of atoms of species $i$ in the molecule and $M_i$ is its atomic mass. For example in case of BF$_2$ the boron atom carries $\frac{11}{49}$ of the total energy, while each fluorine atom contributes $\frac{19}{49}$ to the total energy.