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The ion implantation allows for greater flexibility in obtaining doped layers?

lufer17

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I know that when implementing ionic using a dopant (example aletorio boron ) in the machine that makes the ionic implantation on top of the silicon material, it ends up shooting inserted dopants on top of the silicon. I know implementing ionico has greater flexibility, particularly if shallow doping depths and low doping densities are required.

My doubts:
1- Greater flexibility: greater diversity of materials that can be doped?
2- depth controlled, but can get doped layers?
 

dick_freebird

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With ion implantation you have control over the
initial depth of the dose; for deposited dopant
you can only have Z=0 initially. With ion implant
you can have Zpk=-0.1 or -1u depending on
accelerating voltage and ion charge (triply ionized
boron picks up 3X the energy from the accelerating
voltage, as single-ionized, so 3X range). You can
"stack" implants to get a desired profile prior to
drive / activation and could get a quasi-uniform
region with multiple shots of varying peak-location
and just enough drive to activate fully, not push far.

Deposited base is something I like because it retains
a higher surface concentration, leading to better
low-current beta (lightly doped surface steals base
current from the high-gain vertical region under the
emitter, a "regressive tax"; highly doped surface drives
more of injection to the "sweet spot").

Implants also have better dose control and you can
be sure how much of it ends up -in- the silicon vs
-on- the silicon, to be lost in subsequent strip / etch
process steps. Dep & difuse, you just slather it on
and count on the old-timey charts of dose@depth
vs time@temp to figure the profile. It worked, but
its control attributes do not satisfy people in modern
fabs, or support super narrow base, or allow tailoring
of base doping profile (e.g. it's tough to get a
retrograde base which would help base transport
(tF being a key component of beta and fT) and
Early voltage (stiffer base-bottom minimizes collector
depletion region push-in on the base side).
 

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