dean p. neikirk © 1999, last update february 4, 20161 dept. of ece, univ. of texas at austin ion...
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Dean P. Neikirk © 1999, last update February 4, Dept. of ECE, Univ. of Texas at Austin sample surface Energy loss and ion stopping –where dE/dx is the energy loss rate R p = projected range R p = straggle R = lateral straggle RpRp RR RpRp incident ions Note that actual stopping is statistical in natureTRANSCRIPT
Dean P. Neikirk © 1999, last update May 3, 2023 1 Dept. of ECE, Univ. of Texas at Austin
Ion Implantation
• alternative to diffusion for the introduction of dopants• essentially a “physical” process, rather than “chemical”• advantages:
– mass separation allows wide varies of dopants – dose control:
• diffusion 5 - 10%• implantation:
– 1011 - 10 17 ions / cm2 ± 1 %– 1014 - 1021 ions / cm3
– low temperature – tailored doping profiles
Dean P. Neikirk © 1999, last update May 3, 2023 2 Dept. of ECE, Univ. of Texas at Austin
Ion Implantation
• high voltage particle accelerator– electrostatic and mechanical scanning
• dose uniformity critically dependent on scan uniformity– “dose” monitored by measuring current flowing through wafer
from: Sze, VLSI Technology, 2nd edition, p. 348.
wafer target position
wafer feedbeam line & end station
vacuum pumps
Faraday cage
y scan plates
x scan plates
acceleration tubes
gas sourceion power supply
ion source
source vac pump
ion beam
analyzer magnet resolving
aperture
Dean P. Neikirk © 1999, last update May 3, 2023 3 Dept. of ECE, Univ. of Texas at Austin
sample surface
Energy loss and ion stopping
– where• dE/dx is the energy loss rate• Rp = projected range• Rp = straggle• R = lateral straggle
E
dxdEEdR
0
RpR
Rp
incident ions
• Note that actual stopping is statistical in nature
Dean P. Neikirk © 1999, last update May 3, 2023 4 Dept. of ECE, Univ. of Texas at Austin
Nuclear Stopping: (dE/dx)n
• “billiard ball” collisions– can use classical mechanics to describe energy exchange between
incident ion and target particle • must consider
– Coulombic interaction between + charged ion and + nuclei in target – charge screening of target nuclei by their electron charge clouds – damage due to displaced target atoms
energyener
gy lo
ss ra
te
short interaction time
efficient electron screening
Dean P. Neikirk © 1999, last update May 3, 2023 5 Dept. of ECE, Univ. of Texas at Austin
Electronic Stopping• energy lost to electronic
excitation of target atoms– similar to stopping in a
viscous medium, is ion velocity ( E )
• total stopping power – very roughly CONSTANT
vs energy if nuclear dominates
– E if electronic stopping dominates
• to “lowest order” the range is approximately
10
100
1000
2000
10 100 1000
Ener
gy L
oss
(keV
/mic
ron)
Energy (keV)
Bnuclear
Belctrnc
Pnuclear
Asnuclear
Aselctrnc
Pelctrnc
RdEdE dx
0
E
nuclear: dEconstant
0
E energy
electronic: dEE
0
E energy
Dean P. Neikirk © 1999, last update May 3, 2023 6 Dept. of ECE, Univ. of Texas at Austin
Implant profiles• to lowest order implant profile is approximately gaussian
N x Qo2 Rp
exp 12
x RpRp
2
RdEdE dx
0
E
nuclear: dEconstant
0
E energy
electronic: dEE
0
E energy
Rp 23Rp
MionMtargetMionMtarget
, Minamu
mi 1Qo
x Rp i N x dx
m0 1Qo
N x dx
1
m1 1Qo
x Rp 1N x dx
Rp
m2 1Qo
x Rp 2 N x dx
Rp 2
– works well for moderate energies near the peak– essentially uses only first two moments of the distribution
Dean P. Neikirk © 1999, last update May 3, 2023 7 Dept. of ECE, Univ. of Texas at Austin
Summary of Projected Ranges in SiliconRange (m)energy (keV)ion
10keV 30keV 100keV 300keVapproxrange/MeV
boronB11 0.04 0.11 0.307 0.66 ~3.1
phos.P31 0.015 0.04 0.135 0.406 ~1.1
arsenicAs75 0.011 0.023 0.068 0.19 ~0.58
0.01
0.1
1
10 100 1000
Ion
Proj
ecte
d R
ange
(mic
rons
)
Incident Energy (keV)
boron
phos.
arsenic
Sb
0.001
0.01
0.1
1
10 100 1000
Ion
Stra
ggle
(del
ta R
p) (m
icro
ns)
Incident Energy (keV)
boron
phos.
As
Sb
Dean P. Neikirk © 1999, last update May 3, 2023 8 Dept. of ECE, Univ. of Texas at Austin
Pearson IV distribution
• four moments: Rp, Rp, and– skewness of profile 1
• 1 < 0 implant “heavy” to surface side of peak
– light species• 1 > 0 implant
“heavy” to deep side of peak
– heavy species– kurtosis
• large kurtosis flatter “top”
adapted from: Sze, VLSI Technology, 2nd edition, p. 335.
Con
c ent
ratio
n (
atom
s / c
m3 )
0.2 0.4 0.8 0 0.6 1.0 1.2 1.8 2.0 1.6 1.4 1017
1018
1019
1020
1021
Depth (m)
Boron in silicon
30 keV 100 keV 300 keV 800 keV
measured4-moment fitgaussian
Dean P. Neikirk © 1999, last update May 3, 2023 9 Dept. of ECE, Univ. of Texas at Austin
Ion stopping in other materials
• for masking want “all” the ions to stop in the mask– oxide and photoresist
– densities of oxide and resist are similar, so are stopping powers– do have to be careful about (unintentional) heating of mask material during
implant
thickness for 99.99% blocking
energy: 50keV 100keV 500keV
ion / material B11 / photoresist 0.4m 0.6m 1.8mB11 / oxide 0.35m 0.6m
P31 / photoresist 0.2m 0.35m 1.5mP31 / oxide 0.13m 0.22m
Dean P. Neikirk © 1999, last update May 3, 2023 10 Dept. of ECE, Univ. of Texas at Austin
Ion channeling
• if ions align with a “channel” in a crystal the number of collisions drops dramatically
– significant increase in penetration distance for channeled ions
– very sensitive to direction
(110) direction
• many implants are done at an angle (~7˚)
– shadowing at mask edges
adapted from: Ghandhi, 1st edition, p. 237.
Con
cent
ratio
n(#
/cm
3 )1015
1016
1017
0 0.2 0.8 1.00.60.4 1.2 1.4
channeled
Depth (m)
channeled <100>1° off <100>2° off <100>"random" 7.4° off <100>
random
TA = 850°C150keV BoronQo = 7x1011 cm-2
Dean P. Neikirk © 1999, last update May 3, 2023 11 Dept. of ECE, Univ. of Texas at Austin
Disorder production during implantation
• first 10-13 sec:– ion comes to rest
• next 10-12 sec:– thermal equilibrium established
• next 10-9 sec:– non - stable crystal disorder relaxes via local diffusion
• very roughly, 103 - 104 lattice atoms displaced for each implanted ion.
Dean P. Neikirk © 1999, last update May 3, 2023 12 Dept. of ECE, Univ. of Texas at Austin
Damage during ion implant
• light atom damage (B11)– initially mostly electronic stopping, followed by nuclear
stopping, at low energies • generates buried damage peak
• heavy atoms (P31 or As75)– initially large amounts of nuclear stopping
• generates broad peak with large surface damage• typically damage peaks at about 0.75 Rp
adapted from: Sze, 2nd edition, p. 341.
amorphous region
surface
1016cm-25x10154x10152x10151.5x1015dose: 1015cm-2
Si self-implant, 300keV, Tsubstrate = 300 K
Dean P. Neikirk © 1999, last update May 3, 2023 13 Dept. of ECE, Univ. of Texas at Austin
Amorphization during implant
• damage can be so large that crystal order is completely destroyed in implanted layer
– temperature, mass, & dose dependent
adapted from: Sze, 2nd edition, p. 343.
Temperature 1000/T (K-1)
Crit
ical
Dos
e ( c
m-2
)
2 8 10 6 4 1013
1014
1015
1016
1017
1018
Temperature (ûC)
600 100 -50 0 -100 -150 300
B11
P31
Sb122
• boron, ~30˚C: Qo ~ 1017 cm-2
• phosphorus, ~30˚C: Qo ~ 1015 cm-2
• antimony, ~30˚C: Qo ~ 1014 cm-2
Dean P. Neikirk © 1999, last update May 3, 2023 14 Dept. of ECE, Univ. of Texas at Austin
Implant activation: light atoms• as-implanted samples have carrier concentration << implanted
impurity concentration• electrical activation requires annealing
adapted from: Sze, 2nd edition, p. 357.
1020
1017
1018
1019
Con
cent
ratio
n (#
/cm
-3)
boron, 70keVQo = 1015 cm-2anneal time = 35 min
boron
free carriers
Tanneal = 800°C
0.6 0.4 0.8 1.0 0.2 Depth (microns)
Con
c ent
ratio
n (#
/cm
-3)
0.6 0.4 0.8 1.0 0.2 Depth (microns)
1016
1017
1018
1019
boron free carriers
Tanneal = 900°C
Dean P. Neikirk © 1999, last update May 3, 2023 15 Dept. of ECE, Univ. of Texas at Austin
Shee
t car
rier c
onc e
ntra
tion
/ do s
e
400 500 600 700 800 900 1000 10-2
10-1
1
Anneal Temperature (°C)
Boron, 150keVTsub = 25°Cannealing time = 30min
Annealing and defects: boron
• initial effect: impurities onto substitutional sites
adapted from: Sze, 2nd edition, p. 358.
Qo = 2 x 1015
Qo = 2.5 x 1014
Qo = 8 x 1012
point defects collect to form line defects
gross defect removal,
dopants to sub. sites
annealing of line defects
• high temp, high dose: line defects annealing out
– quite stable– need ~900-1000° C to
remove completely
• mid-range temp, high dose: point defects “collect” into line defects
– fairly stable, efficient carrier traps
Dean P. Neikirk © 1999, last update May 3, 2023 16 Dept. of ECE, Univ. of Texas at Austin
Shee
t car
rier c
once
ntra
tion
/ dos
e
400 500 600 700 800 900 0
0.2
0.4
0.6
0.8
1
Anneal Temperature (°C)
phosphorus, 280keVTsub = 25°Cannealing time = 30min
Implant activation: heavy atoms
• amorphization can strongly influence temperature dependence of activation / anneal
adapted from: Sze, 2nd edition, p. 359.
low
dos
e
Qo = 3 x 1012
6 x 1013
1 x 1014
med
ium
dos
e
3 x 1014 high
dos
e
1 x 1015
5 x 1015
– amorphous for phosphorus dose ~ 3x1014 cm-2
– solid phase epitaxial re-growth occurs at T ~ 600˚C
Dean P. Neikirk © 1999, last update May 3, 2023 17 Dept. of ECE, Univ. of Texas at Austin
• implant profile:
Diffusion of ion implants
2
21exp
2 p
p
p
implanto
RRx
R
QxN
2
2)gaussianhalf(
2exp2
2exp,
tDx
tD
Q
tDx
tD
QtxN
implanto
o
DtRRx
DtR
QxNp
p
p
o
221exp
222
2
2
• recall limited source diffusion profile
– so √Dt is analogous to ∆Rp
• to include effects of diffusion add the two contributions:
Dean P. Neikirk © 1999, last update May 3, 2023 18 Dept. of ECE, Univ. of Texas at Austin
Diffusion of ion implants
• how big are the Dt products?– temps: 600˚ C 1000˚ C– D’s: 9x10-18 cm2/sec 2.5x10-11cm2/sec– t’s : ~ 1000 sec (~17 min)– Diffusion lengths ~ 0.3 Å ~ 500Å
• Rapid thermal annealing– to reduce diffusion, must reduce Dt products
• critical parameter in anneal is temperature, not time• rapid heating requires high power density, “low” thermal mass
Dean P. Neikirk © 1999, last update May 3, 2023 19 Dept. of ECE, Univ. of Texas at Austin
Ion implant systems
• sources– usually gas source: BF3, BCl3, PH3, AsH3, SiCl4
• to keep boron shallow ionize BF3 without disassociation of molecule
– ionization sources• arc discharge• oven, hot filament
• machine classifications– medium current machines (threshold adjusts, Qo < 1014 cm-2)
• ~2 mA, ~200 kV• electrically scanned, ± 2˚ incident angle variation• ~10 sec to implant, few sec wafer handling time
– high current machines (source/drains, Qo > 1014 cm-2)• > 5 mA• mechanically scanned• can produce 1015 dose over 150mm wafer in ~6 sec to implant• wafer heating potential problem
Dean P. Neikirk © 1999, last update May 3, 2023 20 Dept. of ECE, Univ. of Texas at Austin
Ion Implanter
• high current implanter example: Varian SHC-80
images from http://www.varian.com/seb/shc80/shc80-1.html
Dean P. Neikirk © 1999, last update May 3, 2023 21 Dept. of ECE, Univ. of Texas at Austin
Applications of Ion Implantation
• high precision, high resistance resistor fabrication– diffusion ≤ 180 Ω /square ± 10%– implantation ≤ 4 kΩ / square ± 1%
• MOS applications– p-well formation in CMOS: precise,low dose control ~ 1- 5 x
1012 cm- 2
– threshold voltage adjustment:• not possible to control threshold voltage with sufficient accuracy
via oxidation process control • critical post-gate growth adjustment possible by controlled dose
implant:• ∆Vt ~ q•dose / (oxide capacitance per unit area)
– “self-aligned” MOSFET fabrication
Dean P. Neikirk © 1999, last update May 3, 2023 22 Dept. of ECE, Univ. of Texas at Austin
Tailored MOSFET source/drain doping
• alignment between source/drain doping and gate is criticalfield ox
gate oxsource contact drain contact
poly
PR
• what about using the gate itself as mask?
PR
high dose implant
deposit and pattern poly
too much overlap, too much gate/source, gate/drain capacitance
no overlap: can’t “complete” channel
poly
PRdeposit and pattern poly
Dean P. Neikirk © 1999, last update May 3, 2023 23 Dept. of ECE, Univ. of Texas at Austin
Tailored MOSFET source/drain doping
• alignment between source/drain doping and gate is critical– if no overlap, can’t
“complete” channel– if too much overlap,
have too much gate/source, gate/drain capacitance
PR
field ox gate ox
source contact drain contact
high dose implant
poly
PR
poly deposit, pattern
final source-drain implant
gate
light dose implant
• use gate itself as mask!• lightly-doped drain
(LDD) device
Dean P. Neikirk © 1999, last update May 3, 2023 24 Dept. of ECE, Univ. of Texas at Austin
Self-aligned LDD process
• “fully” self-aligned– “side-wall spacers”
can be formed many ways
• actual oxidation of poly gate
• cvd deposition / etch back
deposit oxide
polyoxide
CVD oxide
polyoxide
lighlty doped source-drain
light dose implant
etch back
source-drain contacts
side-wallspacers
high dose implant
Dean P. Neikirk © 1999, last update May 3, 2023 25 Dept. of ECE, Univ. of Texas at Austin
Silicon “on insulator”: Separation by IMplanted OXygen (SIMOX)
• high dose / energy oxygen implant
– dose ~ mid 1017 to 1018 cm-2
– energy ~ several hundred keV
– after anneal forms buried layer of SiO2
http://www.egg.or.jp/MSIL/english/semicon/simox-e.html
Dean P. Neikirk © 1999, last update May 3, 2023 26 Dept. of ECE, Univ. of Texas at Austin
Layer characterization
• need to characterize the layers produced thus far– oxides
• thickness• dielectric constant / index of refraction
– doped layers• junction depth• dopant concentrations• electrical resistance / carrier concentration