8.1. microtech ion implant,1,2
DESCRIPTION
VLSITRANSCRIPT
Ion Implantation
Ion Implantation
Process by which energetic impurity atoms can be introduced into single crystal substrate in order to change its electronic properties.
Ion Implantation and Controlled Doping of Impurities
In this process the ions are accelerated to high energies and allowed to impact the silicon surfaces.
Because of inherent energy they penetrate into the lattice and are placed inside the silicon lattice.
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Advantages
Extremely accurate dose control
Tailor made and well controlled doping profile
large range-of doses-1011 to 1016/cm2
Low Temperature process
Wide choice of masking materials (Oxide, PR, Metal)
Clean environment (Mass separation, vacuum)
Non-Equilibrium process (conc. Excess of S.S. limit)
Ion Implantation
Disadvantages
• Highly sophisticated and costly.
• Damage to semiconductor.
• Dopant redistribution during Annealing
• Charging of insulating layers.
• Photoresist heating and hard to strip.
Ion Implantation System
V
V
Ionsource
Analyzing magnet
Pump
Resolvingaperature
Accelerator
Focus
Neutralbeam gate
Neutraltrap X & Y
scanplates
Wafer
Faraday cup
Q0-30keV
0-200keV
ION IMPLANTATION
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Dr. G. Eranna Integrated Circuit Fabrication Technology © CEERI Pilani
Ion Implanter
Implant Profiles
• At its heart ion implantation is a random process.
• High energy ions (1-1000 keV) bombard the substrate and lose energy through nuclear collisions and electronic drag forces.
Range R
Projected range RP
Vacuum Silicon
•The projected range describes the peak of the implanted profile
Implant Profiles
• Profiles can often be described by a Gaussian distribution, with a projected range and standard deviation. (200keV implants shown.)
€
C(x) =CP exp −x−RP( )2
2∆RP2
.
1021
1020
1017
1019
1018
Con
cent
ration
(cm
-3)
0 0.2 0.4 0.6 0.8 1Depth (µm)
SbAs P
B
0.606 CP∆RP
RP
€
Q = C x()−∞
∞∫ dx
€
Q=2π∆RP CP
(1)
(2)
or
where Q is the dose in ions cm-2 and is measured by the integrated beam current.
Implant Profiles
Dep
th (µ
m)
Implant Profiles
Stan
dard
Dev
iati
on (µ
m)
Implant Profiles
Ranges and standard deviation ∆Rp of dopants in randomly oriented silicon
Implant Profiles
• Monte Carlo simulations of the random trajectories of a group of ions implanted at a spot on the wafer show the 3-D spatial distribution of the ions. (1000 phosphorus ions at 35 keV.)
ImplantBeam
x
y
z
80
40
0
-40
050
-100-50
040
80120
Implant Profiles
• Side view shows depth distribution Rp and ∆Rp while the beam direction view shows the lateral straggle.
zBeam direction
80
40
0
-40
y
050100 -100-50
y
-40
0
40
80
xSide view
0 40 80 120
Implant Profiles
• The two-dimensional distribution near window edge is often assumed to be composed of just the product of the vertical and lateral distributions.
€
C x, y( )=Cvert x()exp − y2
2∆R⊥2
(3)
• Now consider what happens at a mask edge - if the mask is thick enough to block the implant, the lateral profile under the mask is determined by the lateral straggle.
• The description of the profile at the mask edge is given by a sum of point response functions with Gaussian form of Eq (3) above.
Masking Implants
• How thick does a mask have to be?
• For masking,
€
C* xm( )=CP* exp −
xm −RP*( )2
2∆RP* 2
≤CB
.
CP*
RP*
Depth
xm
C*(xm)CB
Con
centration
(4)
For an efficient masking, the thickness of the mask should be large enough that the tail of implant profile in Si is at some specified background conc.
Qp
Masking Implants
• Setting C*(xm)= CB and solving for the required mask thickness,
€
xm =RP* +∆RP
* 2ln CP*
CB
=RP
* +m∆RP* (5)
• The dose that penetrates the mask is given by
€
QP = Q
2π∆RP*
exp− x−RP*
2∆RP*
xm
∞∫
2
dx =Q2
erfc xm −RP*
2 ∆RP*
(6)
• Parameter m indicates that the mask thickness should be equal to the range + some multiple m times the standard deviation in masking material
Since the integral or sum of Gaussian function can be described as an error function
Profile : High tilt Angle
• Real structures may be even more complicated because mask edges are not vertical.• High tilt angle implant for halo doping to minimize short channel
effect
.
30 Degree Tilt
Dis
tanc
e (µ
m)
Distance (µm)
30Þ tilt
0 0.2 0.4 0.6 0.8 1.0
0.2
0
-0.2
-0.4
TSUPREM50Kev P
Profile Evolution During Annealing
• The only other profile we can calculate analytically is when the implanted Gaussian is shallow enough that it can be treated as a delta function and the subsequent anneal can be treated as a one-sided Gaussian. (Recall example in Dopant Diffusion )
Delta FunctionDose Q
(Initial Profile)
ImaginaryDelta Function
Dose Q
DiffusedGaussian
VirtualDiffusion
x0
€
C x,t( )= QπDt
exp − x2
4Dt
(8)
Profile Evolution During Annealing
• Comparing Eqn. (1) with the Diffusion Gaussian profile, we see that ∆Rp is equivalent to . ThusDt2
€
C x,t( )= Q
2π∆RP2 +2Dt( )
exp −x−RP( )2
2 ∆RP2 +2Dt( )
(7)
²R P
Implanted
After Diffusion
2Dt
Evolution of a Gaussian Profile after annealing
Profile Evolution During Annealing
• Real implanted profiles are more complex.
• Light ions backscatter to skew the profile up.
• Heavy ions scatter deeper.Co
ncen
trat
ion
(cm
-3)
Amorphous Silicon
Dr. G. Eranna Integrated Circuit Fabrication Technology © CEERI Pilani
• Gaussian distribution often match the central region of the implanted profiles .• B distribution is skewed toward surface resulting significant deviations in the tail of the distribution
Profile Evolution During Annealing
• 4 moment descriptions of these profiles are often used (with tabulated values for these moments).
Range:
€
RP =1Q
xC x()−∞
∞∫ dx
Std. Dev:
€
∆RP = 1Q
x−RP( )−∞
∞∫
2
C x()dx
Skewness:
€
γ=x −RP( )3 C x( )dx
−∞
∞∫
Q ∆RP3
Kurtosis:
€
β=x −RP( )4 C x( )dx
−∞
∞∫
Q ∆RP4
(9)
(10)
(11)
(12)
• Channeling can produce unexpectedly deep profiles
Implants in Real Silicon - Channeling• Channeled profile is composed of two portions. Main peak+
channeled part. Accurate modeling requires additional parameter to specify the ratio of the dose in main &channeled profile
• Note that the channeling decreases in the high dose implant (green curve) because damage blocks the channels.
B Implant: <100> Zero Tilt 35KeV
ION IMPLANTATION
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Non-Local and Local Electronic Stopping Dielectric Medium
Retarding E-field• Drag force caused by charged ion in “sea” of electrons (non-local electronic
stopping).
• Collisions with electrons around atoms transfers momentum and results in local electronic stopping.
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