Ion implantation is a process by which energetic impurity atoms can be introduced into a single crystal substrate in order to change its electronic properties.

Implantation is ordinarily carried out with ion energies in the 50 – 500 keV range.

Ion Implantation is an alternative to deposition diffusion and is used to produce a shallow surface region of dopant atoms deposited into a silicon wafer.

In this process a beam of impurity ions is accelerated to kinetic energies in the range of several tens of kV and is directed to the surface of the silicon.

As the impurity atoms enter the crystal, they give up their energy to the lattice in collisions and finally come to rest at some average penetration depth, called the projected range expressed in micro meters.

Depending on the impurity and its implantation energy, the range in a given semiconductor may vary from a few hundred angstroms to about 1micro meter.

Characteristics of Ion Implantation:

1. Process of implantation is carried out under high vacuum conditions, an inherently clean environment.
2. Process is usually carried at room temperature so wide variety of masks can be used for selecting doping.
3. Wide ranges of doses from 1011 to 1017 ions cm-2 are delivered to target. This provides more uniform surface coverage than diffusion.
4. This provides independent control of dose and penetration depth.
5. It can be used for introducing control amount of charge species in specific region of semiconductor. For instance, in MOS transistors, ion implantation can be used to accurately adjust the threshold voltage.
6. It is a non-equilibrium process, so that resulting carrier concentration is not limited by thermodynamic consideration.

Ion implantation process:

Basic requirements for ion implantation systems are ion sources & means for their extraction, acceleration and purification. This is followed by beam detection and scanning prior to impingement of the substrate.

1. Ion source:

   Ion source includes compounds of species desired and means of their ionization.

   Gaseous materials are more preferred over solid materials since they do not need vaporization chamber.

   Effectiveness of ion source is measured as magnitude of ion current delivered to the accelerometer and ultimately to target.

2. Accelerometer:

   High energy is given to ion beams by passing them through a long column containing biased, annular, ring electrodes.

   These electrodes establish accelerating potential.

   Output end of column should be at ground potential for safety. The beam energy determines projected range of ion.

3. Mass analyzer:

   Raw output from ion source from many species with contamination and hence purification of beam, to select desired implant species is important.

   It is carried out by means of mass analyzer that selects single species of interest.

   This can be done before or after the beam is accelerated.

   The mass separation technique allows handling variety of dopants in a single machine with freedom from contamination with each other.

   Most commonly employed technique uses a homogeneous field magnetic analyzer.

4. Beam scanner:

   These are required for uniform coverage and ability to handle large number of slices in a single pump down.

Advantages:

1. Mass separation techniques can be used to obtain highly pure form of impurity atoms.
2. Wide range of dopant doses.
3. It can be carried out at room temperature.
4. Provides independent control of dose and penetration depth.

Disadvantages:

1. Equipment’s are highly sophisticated and expensive.
2. Results in damage to conductor.
3. At high temp, annealing is needed.