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Transcript of © 2000 by Prentice Hall Upper Saddle River NJ ION IMPLANTATION Dr. Wanda Wosik ECE 6466, F2012...
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© 2000 by Prentice HallUpper Saddle River NJ
ION IMPLANTATION
Dr. Wanda WosikECE 6466, F2012
Chapter 8
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© 2000 by Prentice HallUpper Saddle River NJ
Basic Concepts
• Ion implantation is the dominant method of doping used today. In spite of creating enormous lattice damage it is favored because:
• Large range of doses - 1011 to 1016 /cm2 • Extremely accurate dose control • Essential for MOS VT control• Buried (retrograde) profiles are possible• Low temperature process• Wide choice of masking materials
• There are also some significant disadvantages:
• Damage to crystal.• Anomalous transiently enhanced diffusion (TED). upon annealing this damage.• Charging of insulating layers.
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A. Implant Profiles
• At its heart ion implantation is a random process.• High energy ions (1-1000keV) bombard the substrate and lose energy through nuclear collisions and electronic drag forces.
• Profiles can often be described by a Gaussian distribution, with a projected range and standard deviation. (200keV implants shown.)
(1)
(2)or
where Q is the dose in ions cm-2 and is measured by the integrated beam current.
Heavy atoms have smaller projected range and smaller spread = struggle Rp
Doses 1*1012 cm-2 to 1*1016 cm-2 used in MOS ICs
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Ranges and standard deviation ∆Rp of dopants in randomly oriented silicon.
Energy Dependence
Rp and Rp for dopants in Si.
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• 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.)• Side view (below) shows Rp and ∆Rp while the beam direction view shows the lateral straggle.
Rp =50 nm, Rp =20 nm
Lateral struggle R|
3D Distribution of P Implanted to Si
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• The two-dimensional distribution is often assumed to be composed of just the product of the vertical and lateral distributions.
(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. (35keV and 120keV As implants at the edge of a poly gate from Alvis et al.)
(Reprinted with permission of J. Vac. Science and Technology.)
• The description of the profile at the mask edge is given by a sum of point response Gaussian functions, which leads to an error function distribution under the mask. (See class notes on diffusion for a similar analysis.)
Lateral Implantation - Consequences for Devices
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B. Masking Implants
• How thick does a mask have to be?
• For masking,
(4)
• Calculating the required mask thickness,
(5)
• The dose that penetrates the mask is given by
(6)
Dose that penetrates the mask
Depends on mask material
Lateral struggle important in small devices
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Masking Layer in Ion ImplantationPhotoresist,
oxide mask Lateral struggle important in small devices
Dose that penetrates the mask
To stop ions:
Poly thickness
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Masking Efficiency• Mask edges tapered – thickness not large enough
• Tilted implantation (“halo”) – use numerical calculations ( ex. to decrease short channel effects in small devices)
Shadowing effect rotate or implant at 0 Deg.
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C. Profile Evolution During Annealing
• Comparing Eqn. (1) with the Gaussian profile from the last set of notes, we see that ∆Rp is equivalent to . Thus
(7)
• 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 Chapter 7 notes.)
(8)
or
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Implantation Followed by Annealing
Function rediffused
Annealing requires additional Dt terms added to C(x) Cp, depth , C(x) remains Gaussian.
Backscattering of light atoms. C(x) is Gaussian only near the peak.
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• Real implanted profiles are more complex.• Light ions backscatter to skew the profile up.• Heavy ions scatter deeper.
• 4 moment descriptions of these profiles are often used (with tabulated values for these moments).
Range:
Std. Dev:
Skewness:
Kurtosis:
(9)
(10)
(11)
(12)
• Real structures may be even more complicated because mask edges or implants are not vertical.
Arbitrary Distribution of Dopants
Pearson’s model good for amorphous (&fine grain poly-) silicon or for rotation and tilting that makes Si look like amorphous materials.
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D. Implants in Real Silicon - Channeling
• At least until it is damaged by the implant, Si is a crystalline material.• Channeling can produce unexpectedly deep profiles. • Screen oxides and tilting/rotating the wafer can minimize but not eliminate these effects. (7˚ tilt is common.)
• Sometimes a dual Pearson profile description is useful.• Note that the channeling decreases in the high dose implant (green curve) because damage blocks the channels.
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Channeling Effect
As two profiles
Dual-Pearson model gives the main profile and the channeled part. Dependence on dose: damage by higher doses decreases channeling. No channeling for As @ high doses
Parameters are tabulated (for simulators). Include scattering in multiple layers (also masks’ edges). IMPORTANT in small devices! Screen oxide decreases channeling. But watch for O knock-out.
<100>
Channeling not forward scattering
c-Si, B
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Channeling
P implantation at 4- keV and low dose Q<1013cm-2
8°
0°
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Two – Dimensional Distributions
Near the mask edge 2D distributions calculated by MC model should be the best – verification difficult due to measuring problems.
Phenomenological description of processes is insufficient for small devices.
Atomistic view in scattering
Thin oxide
Verification through SIMS
Poly-Si
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Manufacturing Methods and Equipment
Mass Analysis
Ion velocity
Mass Selection
AsH3 PH3 BF2 in 15% H2, all very toxic
mr
Gives mass separation
Integrate the current to determine the dose
For low E implant no acceleration
Neutral ions can be implanted (w/o deflection=center) but will not be measured in Dose (use trap) Ion beam heating T increases - keep it below 200 °C
Centrifugal force
Lorentz force
B++, B+, F+, BF, BF2
+
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High Energy Implants
Applications in fabrication of: wells (multiple implants give correct profiles ex. uniform or retrograde), buried oxides,
buried layers (MeV, large doses)! - replaces highly doped substrate with epi-layers
CMOS
UEB
UBE
0.7V
0.7V
p-n-p n-p-n
Decrease of Rsub - less latch-up
Thyristor structure
In latch-up
Future IC fabrication: implantation at high energy becomes more important - reduction of processing steps
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Ultralow Energy Implants
Required by shallow junctions in VLSI circuits (50 eV- B) - ions will land softly as in MBE
Extraction of ions from a plasma source ~ 30keV
Options:•Lowering the extraction voltage Vout the space charge limited current limits the dose (Child’s Law)
• J V1/2extd-2 ex. J2keV=1/4•J5keV
• Extraction at the final energy used in the newest implantors but not for high doses due to self limitation due to sputtering at the surface. Now 250 eV available,; 50 eV to come
• Deceleration (decel mode) more neutrals formed and implanted deeper that ions (doping nonuniformities)
• Transient Enhanced Diffusion (TED) present in the low energy Ion Implantation and {311} defects.
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Models and Simulations
• Rutherford(1911) - (He) backscattered due to collision with a + nucleus.
• Bohr- the nuclear energy loss due to + atoms cores and electronic loss due to free electrons decrease
Many contributors.
• Lindhard, Scharff and Schiott (1963) (LSS)
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Modeling of Range Statistics
• The total energy loss during an ion trajectory is given by the sum of nuclear and electronic losses (these can be treated independently).
(13)
(14)
A. Nuclear Stopping
• An incident ion scatters off the core charge on an atomic nucleus, modeled to first order by a screened Coulomb scattering potential.
(15)
• This potential is integrated along the path of the ion to calculate the scattering angle. (Look-up tables are often used in practice.) • Sn(E) in Eqn. (14) can be approximated as shown below where Z1, m1 = ion and Z2, m2 = substrate.
(16)Nuclear stopping power
The range
Scattering potential
Role of electrons in screening
Thomas Fermi model
Energy transferred
Head-on collision (max energy transferred)Z2, m2
Computers used to find R
Elastic collisions
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B. Non-Local and Local Electronic Stopping
• Drag force caused by charged ion in "sea" of electrons (non-local electronic stopping).
• Collisions with electrons around atoms transfer momentum and result in local electronic stopping.
(17)
• To first order, where
C. Total Stopping Power
• The critical energy Ec when the nuclear and electronic stopping are equal is
B: ≈ 17keVP: ≈ 150keVAs, Sb : > 500keV
• Thus at high energies, electronic stopping dominates; at low energy, nuclear stopping dominates.
Energy loss w/o the trajectory change
NonlocalLocal
Inelastic Collisions with electrons momentum transfer, small change of the trajectory.
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Damage Production
Ed=Displacement energy (for a Frenkel pair) 15eV large damage induced by Ion Implantation
30 keV As Rp 25mm
E decreases to Ed so that ions stop.
n= Number of displaced Si atoms
Dose – large damage!
Si Si
• Consider a 30keV arsenic ion, which has a range of 25 nm, traversing roughly 100 atomic planes.
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Time for the ion to stop
1 ion primary damage: defect clusters, dopant-defect complexes, I and V
Increment in damage
more recombination for heavy ions since damage is less dispersed than for light ions: B-0.1, P-0.4, As-0.6, BF2-0.6.
Damage in Implantation Time Scale in Implantation
• Molecular dynamics simulation of a 5keV Boron ion implanted into silicon [de la Rubia].• Note that some of the damage anneals out between 0.5 and 6 psec (point defects recombining).
Damage evolution (atomic interaction) stabilization @ lower concentrations due to local recombination
Damage related to dose and energy
Fraction of recombined defects (displaced atoms)
Damage accumulates in subsequent cascades and depends on existing N -local defects
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Damage in Implantation Including Amorphization
Damage is mainly due to nuclear energy losses : for B @ Rp. For As – mostly everywhere in the Dopant profile.
- Si forms @ large doses and spread wider with the increasing Q.
- Si forms @ low T of II (LN2) , @ RT or higher – recombination (in-situ annealing)
- Si is buried
Preamorphization eliminates the channeling effect
• Cross sectional TEM images of amorphous layer formation with increasing implant dose (300keV Si -> Si) [Rozgonyi]• Note that a buried amorphous layer forms first and a substantially higher dose is needed before the amorphous layer extends all the way to the surface.• These ideas suggest preamorphizing the substrate with a Si (or Ge) implant to prevent channeling when dopants are later implanted.
Critical energy for amorphization Ec~f(1021 keV)/cm3 (f=0.1-0.5 Si)
For 100keV As implantation D=6x1013cm-2
Si used for amorphization
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Damage Annealing - Solid Phase Epitaxy
• If the substrate is amorphous, it can regrow by SPE.• In the SPE region, all damage is repaired and dopants are activated onto substitutional sites. • Cross sectional TEM images of amorphous layer regrowth at 525˚C, from a 200keV, 6e15 cm-2 Sb implant.
• In the tail region, the material is not amorphized.• Damage beyond the a/c interface can nucleate stable, secondary defects and cause transient enhanced diffusion (TED).
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Damage Annealing (more)
Formation of End-of-Range (EOR) defects @ a/c interface in Si large damage after II @ the C-Si side but below the threshold for amorphization. Loops R= 10 nm grow to 20 nm in 1000 °C
Solid Phase Epitaxy
Furnace
850 °C
RTP
1000 °C
5 min
60 min
960 min
1 sec
60 sec
400 sec 1000 °C gives stable dislocation loops1100 °C/60 sec may be enough to remove the dislocation loops .
Loops in P-N junctions leakage
Optimize annealing: Short time, high T to limit dopant diffusion but remove defects
Optimize I2 : LN2 Ge 4*1014 cm-2 RT- 5*1014 cm-2 produces a-Si@ RT , EOR @ 100 nm depth =25 nm, 1010 cm-2 @ 900 °C/15 min
@ LN2 NO EOR!
{311}&loops
Heating by I-beam - defects harder to be remove
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Damage Annealing - “+1” ModelGoals: • Remove primary damage created by the implant and activate the dopants.
• Restore silicon lattice to its perfect crystalline state. • Restore the electron and hole mobility.• Do this without appreciable dopant redistribution.
• In regions where SPE does not take place (not amorphized), damage is removed by point defect recombination. Clusters of I recombine = dissolve @ the surface
• Bulk and surface recombination take place on a short time scale.
• "+1" I excess remains. These I coalesce into {311} defects which are stable for longer periods.• {311} defects anneal out in sec to min at moderate temperatures (800 - 1000˚C) but eject I TED.
Primary defects start to anneal at 400 °C all damage must be annealed with only +1 atom remaining. (+1 model)
Frenkel pairs
@ 900 °C, 5 sec 1011 cm-2 of {311}; not long=10 nm rods
then dissolve if below critical size or else grow
dislocation loops (stable) = extrinsic e. i. Si I planes on {111}
= secondary defects. (difficult to remove)
After 10-2s only “I”
Fast
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Solid State EpitaxyRegrowth from the C-Si acting as a seed (as in crystal growth
from melt)
@ 600 deg C, 50 nm/min <100>
20 nm/min <110>
2 nm/min <111>
2.3 eV is for Si-Si bond breaking
Dopants are active =substitutional position with very little diffusion.
But high T might be needed for EOR annealing.
Time
Regrowth rate
No defects=no diffusion enhancements
Regrowth 10x larger for highly doped regions
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Dopant Activation
• When the substrate is amorphous, SPE provides an ideal way of repairing the damage and activating dopants (except that EOR damage may remain).
• At lower implant doses, activation is much more complex because stable defects form.
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Dopant Activation – No PremorphizationLow T Annealing is enough for low doses – low primary damage can be easily annealed.
High doses – damage below amorphization secondary defects = difficult to anneal and requires high T 950-1050 ° C.
1. High initial activation, full activation is fast @ low T,
2. Low initial activation, traps anneal out, I compete with B for substitutional sites, I –B complexes
3. More damage so activation decreases with dose maintaining the same behavior.
(1)(2)
(3)
Full activation
Doses below amorphization High doses - high T required which causes more diffusion - in small devices unacceptable
Amorphization improves activation @ low T leading to 100% @ high TNote: very high doses may result in low activation (25%)
Secondary defects from
Carriers’ mobility increases with damage anneal
Increasing dose