Ion Implantation :

This is much superior method of doping a semiconductor. In this technique dopant atoms are ionized and accelarated through a chamber by the application of high electric field. These ions bombard the semiconductor surface and get implanted there and in this way the semiconductor is doped.

The ion implantation method has several advantages over diffusion that are :

(1) The sources can be made of high purity by in situ mass spectrometric technique.

(2) Deposition can be done by scanning the ion beam over a large surface area.

(3) Dopant accuracy is very high due to in situ dopant concentration monitoring techniques.

(4) By varying the ion dose doping concentration can be altered by orders of magnitude.

(5) Almost total elimination of the lateral spreading effect which is a huge problem for diffusion.

Though, there are many such advantages, there are also few short comings of this ion implantation technique these are :

(1) High initial establishment cost of the ion implantation set up.

(2) Severe damage to the semiconductor surface due to high energy ion bombardment which necessitates a post implantation annealing.

The annealing process is done typically at a temperature 350°-400°C for 10 to 30 min. and repairs these damages to a large extent by relocating dislodged atoms to their proper lattice sites. Recently a technique known as Rapid Thermal Annealing (RTA) is employed which is typically carried out for a few seconds at high temperature of the order of 1000°C-1100°C.

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