l6 Ion Implantation
Transcript of l6 Ion Implantation
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ION-IMPLANTATION
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BASICS
Ionized impurity atoms areaccelerated by anelectrostatic field and
made to strike the surfaceof wafer
By measuring ion currentand adjusting electrostaticfield, the penetrationdepth and dose can becontrolled
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SYSTEM SET-UP
Consists of:
Ion source,Acceleration
tube, MassAnalyzer,End station
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MASS ANALYZER The ion beam is passed through a magnetic sector that
selects a particular ionic species An ion of mass Mand charge qmoving at a velocity vin a
circular path will experience a force:
According to the mass, the trajectory will bedifferent a slit is used at the appropriateplace to extract desired species
The kinetic energy of the ion is given by its extractionvoltage:
rMv
qB B
M
q
V 1
2 extE qV Mv vqV
M
extext1
2
22
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Ions are accelerated after mass separation.
Acceleration done in vacuum to avoid collisions.
Neutral species undesirable as they cannot be
deflected by electrostatic potential duringscanning will get implanted near center ofwafer.
Each species will have a different mass.
Neutral species are removed by deflecting theions (or bending the ion beam) so that the neutralswill not respond to the electrostatic field and willtravel straight and strike a beam stop.
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BASIC ARRANGEMENT:
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Vertical projected range The total distance traveled
by the ion before it stops iscalled range R.
Rp is the average vertical
distance for a uniformbeam= vertical projectedrange
Energy loss due to twomechanisms Interaction of incidentions with electrons of
target atom Interaction with lattice
ions
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Implantation Profile
Total distance traversed by ion is called Range, R. The average penetration depth of the ions is called
projected range Rp. Due to a distribution, there is a statistical fluctuation along
the direction of the projected range, called straggle, Rp.
The statistical fluctuation perpendicular to incidentdirection is called lateral straggle, R.
RpR
Ion Beam
Rpx
z
y
Log (ionConcentration)
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TARGET-ION INTERACTION:
Energy loss due to two mechanisms Electronic stopping interaction with
electron cloud around target atom Collisions with electrons around atoms
transfers momentum and results in localelectronic stopping
Nuclear stopping interaction withnucleus of target
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Nuclear stopping
Collision with nuclei of the lattice atoms
Scattered significantly
Causes crystal structure damage.
Electronic stopping
Collision with electrons of the lattice atoms
Incident ion path is almost unchanged Energy transfer is very small
Crystal structure damage is negligible
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THEORY OF STOPPINGMECHANISM:
Total rate of energy loss = sum of energy lossper unit length due to each mechanism
Total projected range is given by
[ dE/dx ]total = Sn(E) + Se(E)
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Stopping profile:
Ec: Critical energy
dE/dx : Energy lost
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Peak concentration:
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Dopant concentration can be expressedas a function of depth:
Where is the dose.
2
2
2exp
2 p
p
pR
RxR
xN
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Ion channeling: When the ion direction is such
that it orients itself along majorcrystallographic direction, ionstravel a great distance beforestopping
Results in Deep junctions For each species such as B, P,
etc, there is a critical anglewhen this begins
Avoided by implanting at smallangle ~ 7 tilt
By using a thin screening layer ofPR or SiO2
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Implant damage:
Light ions and heavy ions cause different
type of damage profiles
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Annealing: Removes implant damage Restores crystalline structure Activates dopants Done usually at 850 1000C in N2 ambient Long times (>30min) help remove defects
completely But cause significant dopant diffusion
Secondary defects start forming at
about 500-600C
Removed at 850 1000C
Amorphous layer recrystallizes when
annealed at 600C for 30min (called
solid phase epitaxy)
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RTA
High temperature attained in short time.
Two methods
Array of lamps
Laser
High throughput
No change in implantation profile.
2
2
( )
2( 2 )2
( , )2 ( 2 )
x Rp
Rp DtN x tRp Dt
e