Monday, October 16, 2006

Ion Implantation Process in Semiconductor Manufacturing

For performing ion implantation, atoms or molecules are ionized, accelerated in an electric field and implanted into the target material. A wide variety of combinations of target material and implanted ions are possible. The dose of the implanted ions can vary between 1011 and 1018 cm-2. Usually, the acceleration energy lies between several keV and several hundred keV, but with special equipment it is possible to work with energies up to several MeV. The range of the implanted ions in the substrate depends on the mass of the implanted ions, their energy, the mass of the substrate atoms, crystal structure and the direction of incidence. As an example, the mean range of 100 keV phosphorus ions in silicon is about 150 nm.
Main advantages of ion implantation (in comparison to diffusion) for the doping of semiconductors are:
Short process times, good homogeneity and reproducibility of the profiles
Exact control of the amount of implanted ions by integrating the current. This is of particular importance for low concentrations, e.g. for adjusting the threshold voltage of MOS transistors
Relatively low temperatures during the process
Different materials can be used for masking, e.g. oxide, nitride, metals, and resist
Implantation through thin layers (e.g. SiO2 or Si3N4) is possible
Low penetration depth of the implanted ions. This allows modification of thin areas near the surface with high concentration gradients
Sequences of implantation steps (with different energies and doses) allow optimization of the dopant profiles
There are also some disadvantages, such as:
Damage of the substrate is caused by the implanted ions
The change of material properties is restricted to the substrate domains close to the surface
Additional effects during or after implantation (such as channeling or diffusion) make it difficult to achieve very shallow profiles and to theoretically predict the exact profile shapes.


Post a Comment

<< Home