Ion Implantation

• Technique used to introduce impurities into a silicon wafer
• Silicon wafers are placed in a vaccum chamber and are scanned by a beam of high energy dopant ions(borons for p-type and phosphorous for n-type)
• These ions are accelerated by energies between 20kv to 250 kv
• As the ions strike the silicon wafers ,they penetrate some small distance into the wafer
• The depth of penetration of any particular type of ion increases with increasing accelerating voltage

Properties of ion implantation

• The depth of penetration of any particular type of ion will increase with increasing accelerating voltage
• The penetration depth will generally be in the range of .1 to 1µm
  

Range distributions

Impurity Distribution of implanted ions:- The distribution of the implanted ions as a function of distance x from the silicon surface will be a Gaussian distribution ,given by

N(x)=Npexp[-(x-Rp)² /2ΔRp²]
x=distance into substrate from surface
Rp=projected range
ΔRp=standard deviation of the projected range
Np=peak concentration of implanted ions


An ion implantation impurity profile



The peak implanted ion concentration is related to the implantation dosage Q by
Np= Q/(2π)½ΔRp =.4[Q/ΔRp]

• The implantation dosage Q is the number of implanted ions per unit of surface area as given by such units as ions/cm2.
• The ion density drops off rapidly from the peak value with distance as measured from Rp in either direction

Annealing after implantation

• After the ions have been implanted they are lodged principally in interstitial positions in the silicon crystal structure and the surface region into which the implantation has taken place will be heavily damaged by the impact of the high energy ions
• The disarray of silicon atoms in the surface region is often to the extent that this region is no longer crystalline in structure but ,rather amorphous
• To restore this surface region back to well-ordered crystalline state and to allow the implanted ions to go into substitutional sites in the crystal structure , the wafer must be subjected to an annealing process
• The annealing process usually involves the heating of the wafers to some elevated temperature often in the range of 1000c for a suitable length of time such as 30 minutes
• Laser beam and electron beam annealing are also employed. In such annealing techniques only the surface region of the wafer is heated and recrystallized

Advantages

• Provides much more precise control over the density of dopants(Q) deposited into the wafer,and hence the sheet resistanse.
• In diffusion process, temperature has to be controlled over a large area inside the oven,whereas in ion implantation technique, accelerating potential and the beam current are electrically controlled from outside
• Possible to have very low values of dosage(Q) ,so that very large values of sheet resistance can be obtained
• Performed at low temperatures