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Ion Implantation

Doping

The dominant doping method

A particle accelerator is used to accelerate a doping atom so that it can

penetrate a silicon crystal to a depth of several microns Excellent control of dose (cm-2)

Good control of implant depth with energy (KeV to MeV)

Repairing crystal damage and dopant activation requires annealing, whichcan cause dopant diffusion and loss of depth control.

Dopant ions

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Ion Implantation

Ion implanter

The ion implantation process is conducted in a vacuum chamber at very low pressure (10-4to 10-5torr).Large numbers of ions (typically 10

16

to 10

17

ions/cm

2

) bombard and penetrate a surface, interacting withthe substrate atoms immediately beneath the surface.Typical depth of ion penetration is a fraction of a micron.

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Annealing

After ion implantation, lattice damage to the crystal is repaired by heatingthe wafer at a moderate temperature for a few minutes. This process is

called annealing.

Furnace annealing takes minutes and causes too much diffusion of dopantsfor some applications.

Rapid thermal annealing (RTA), the wafer is heated to high temperature inseconds by a bank of heat lamps.

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Diffusion

Diffusionis atom movement along concentration gradients.

There are different mechanisms in which an atom moves within the crystal:

a) Interstitial diffusion, atoms jump from one interstitial site to another, whichis always available (small atoms, like sodium and lithium).

b)Substitutional/vacancy diffusion, necessitates that empty lattice site isavailable next to the diffusing atom (antimony and arsenic).

c) Interstitialcy mechanism, the self-interstitial atoms move to the lattice

sites, and kick the dopants to the interstitial sites, and from there they moveto the lattice sites (boron and phosphorus).

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A uniformly doped ingot is sliced intowafers.

An oxide film is then grown on thewafers.

The film is patterned and etched usingphotolithography exposing specificsections of the silicon.

The wafers are then spun with anopposite polarity doping sourceadhering only to the exposed areas.

The wafers are then heated in a furnace(800-1250oC) to drive the doping atomsinto the silicon.

Diffusion

Process

The doping material used can either be solid,

liquid or gaseous. In carrier gas diffusion doping, a carrier gas

carries the doping atoms to the silicon waferswhich been brought by radiant heat totemperatures of 1000C to 1285C.

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Comparison of Diffusion and Ion Implantation

Diffusion

cheaper and more simplisticmethod,

can only be performed from thesurface of the wafers.

dopants also diffuse unevenly,and interact with each otheraltering the diffusion rate.

Ion implantation

more expensive and complex. It does not require high

temperatures and also allowsfor greater control of dopantconcentration and profile.

anisotropic process andtherefore does not spread thedopant implant as much asdiffusion.

aids in the manufacture of self-aligned structures which

greatly improve theperformance of MOStransistors.

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Question??

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1. Sami Franssila, Introduction to Microfabrication John Wiley & Sons Ltd,2004.

2. http://www.daviddarling.info/encyclopedia/S/AE_silicon.html

3. Infrastructure -copyright, 1999

4. http://en.wikipedia.org/wiki/Photoresist

5. http://www.n2bio.com/surface-modification-technology/ion-implantation.php

6. http://www.oxford-instruments.com/

7. http://www.tpub.com/neets/book14/57d.htm

References

Thank you for attention