Operation of an ion implanter

The main function of the ion implanter is to deliver a beam of ions of a particular type and energy to the surface of a silicon wafer. [1]

• A gas source feeds a small quantity of source gas such as BF3 into the ion source, where energetic electrons emitted from a hot filament ionize the dopant atoms.

• An extraction voltage, V kilowatts (kV) causes the charged ions to move out of the ion source into the analyzer.

• The magnetic field of the analyzer is chosen such that only ions with the desired charge to mass ratio can travel through without being blocked by the analyzer walls. A magnetic field B is applied normal to the path of ions with charge q and velocity v. The force on the ions is given by F=qvB and the ions move in a circular path of radius r and there is a centripetal force, F=M v²/r.

Therefore, qvB=Mv²/r [velocity v is unchanged]

And r = Mv/qB ------------------------------------(1)

Now for an ion accelerated through a potential V the velocity is given by:

Mv²/2 = qV , from where we get v as:

From (1) and (2) we get

For a given acceleration potential and magnetic flux density, the radius of the ion path is directly proportional to the square root of the mass-to-charge ratio.

Thus we get the desired dopant species.

• Surviving ions, which are the desired dopant species enter the acceleration tube where they are accelerated to the implantation energy as they move from high voltage to ground voltage.

· The beam is then scanned over the surface of the wafer using electrostatic deflection plates. The wafer is tilted with respect to the axis of the acceleration tube so that neutralized ions don’t hit the wafer.

· The silicon wafers serve as targets for the ion beam and the complete implanter system is operated under vacuum condition.[2]

Fig 1 Schematic drawing of a typical ion implanter[1]