Ion implantation is a another procedure to introduce impurity atoms to a silicon wafer.  It is like diffusion in its goal but has some advantages.  An ion implanter is a high-voltage particle accelerator that produces a beam of impurity atoms which can penetrate the surface of the wafer.  The basic parts are described in detail.

1. Ion Source.  The source operates at a high voltage and produces plasma containing the desired impurity. Solids can be sputtered to give a wide degree of choice in impurities.

2. Mass Spectrometer. A big magnet bends the ion beam through a right angle to select the desired impurity.  The select ion then passes through a small slit into the main accelerator column.  (Consult figure above.)

3. High-Voltage Accelerator. The accelerator adds up to 175 ke V of energy to accelerate the ions to their final velocity.  These are operated at high voltage so they are encased in a protective shield.

4.Scanning System. X and Y deflection plates scan the beam across the wafer to give uniform distribution and dose of impurity to the wafer.

5. Target Chamber. Silicon wafers are the target for impurity ions.  For safety  the target area is located near a ground and operates in a vacuum.

The advantages of ion implantation over diffusion are that the wafers can be maintained at low temperatures.  This prevents undesired spreading of the impurity.  Another advantage is its ability to use a wide variety of impurity “species”.

A production level ion implanter costs 2 to 4 million dollars.  However, the flexibility and control in adding impurities outweigh this cost.

When the ion enters the wafer it collides with atoms and interacts with the electrons in the silicon crystal.  Each nuclear or electronic interaction reduces the energy until it comes to rest with the target.

In most cases we want to only implant dopants on a selected area of the wafer.  “Windows” are opened on the barrier material whenever impurities are desired.

Summary

Ion implantation is a means for adding dopants to semiconductor material. Charged atoms (ions) of elements such as boron, phosphorus or arsenic are accelerated by an electric field into the semiconductor material. Especially useful for very shallow (<1µm) distributions of dopants in a semiconductor. Ion implantation is usually done at room temperature, with the resulting implantation-induced lattice damage removed by annealing at temperatures of approximately 700oC. This is more precise than diffusion doping.

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