![Page 1: Dean P. Neikirk © 1999, last update February 1, 20161 Dept. of ECE, Univ. of Texas at Austin Silicon Oxides: SiO 2 Uses: –diffusion masks –surface passivation](https://reader031.vdocuments.mx/reader031/viewer/2022012914/5a4d1b197f8b9ab0599929c8/html5/thumbnails/1.jpg)
Dean P. Neikirk © 1999, last update May 3, 2023 1 Dept. of ECE, Univ. of Texas at Austin
Silicon Oxides: SiO2
• Uses:– diffusion masks– surface passivation– gate insulator (MOSFET)– isolation, insulation
• Formation:– grown / “native”
• thermal: “highest” quality• anodization
– deposited:• C V D, evaporate, sputter
• vitreous silica: material is a GLASS under “normal” circumstances
– can also find “crystal quartz” in nature• m.p. 1732˚ C; glass is “unstable” below
1710˚ C– BUT devitrification rate (i.e.
crystallization) below 1000˚ C negligiblenetwork former
hydroxyl group
network modifier
silicon
bridging oxygen
non-bridging oxygen
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Dean P. Neikirk © 1999, last update May 3, 2023 2 Dept. of ECE, Univ. of Texas at Austin
• in dry (<< 20 ppm H2O) oxygen– Si + O2 SiO2
– once an oxide is formed, how does this chemical reaction continue?
• does the oxygen go “in” or the silicon go “out”?• density / formula differences
– SiO2 = 2.25 gm/cm3 , GMW = 60– Si = 2.3 gm/cm3 , GMW = 28– oxide d thick consumes
a layer 0.44d thick of Si
Growth of SiO2 from Si
0.44dd SiO2
• “bare” silicon in air is “always” covered with about 15-20 Å of oxide, upper limit of ~ 40 Å– it is possible to prepare a hydrogen terminated Si surface to
retard this “native” oxide formation
original silicon surface
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Dean P. Neikirk © 1999, last update May 3, 2023 3 Dept. of ECE, Univ. of Texas at Austin
“Wet” oxidation of Si
• overall reaction is– Si + 2 H2O SiO2 + H2
• proposed process– H2O + Si-O-Si Si-OH + Si-OH– diffusion of hydroxyl complex to SiO2 -Si interface
Si - OH Si - O - Si+ Si - Si + H2
Si - O H Si - O - Si
• this results in a more open oxide, with lower density, weaker structure, than dry oxide – wet ≈ 2 . 15 gm / cm3
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Dean P. Neikirk © 1999, last update May 3, 2023 4 Dept. of ECE, Univ. of Texas at Austin
gas oxide silicon
x
Oxide growth kinetics
• basic model is the Grove and Deal Model– supply of oxidizer is limited by diffusion through oxide to
growth interface• Fick’s First Law: flux j D Noxidizer
x
N x
N0 N1x
conc
entr
atio
n
moving growth interfaceN0
N1
• simplest approximation:
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Dean P. Neikirk © 1999, last update May 3, 2023 5 Dept. of ECE, Univ. of Texas at Austin
Oxidizer concentration gradient and flux
• N0 is limited by the solid solubility limit of the oxidizer in the oxide!– N0
O2 ~ 5 x 1016 cm-3 @ 1000˚ C– N0
H2O ~ 3 x 1019 cm-3 @ 1000˚ C• flux of oxidizer j’ at SiO2 / Si interface consumed to form new oxide
j' kN1
j' steadystate
j kN1 D N0 N1
x
j DN0
x Dk
– k is the chemical reaction rate constant• in steady state, flux in must equal flux consumed
solve for N1, sub back into
flux eq
conc
entr
atio
n
gas oxide silicon
moving growth
interface
x
N0
N1
xNND
xNDj oxidizer 10
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Dean P. Neikirk © 1999, last update May 3, 2023 6 Dept. of ECE, Univ. of Texas at Austin
Relation between flux and interface position
• flux: #oxidizer molecules crossing interface per unit area per unit time– # cm-2 sec-1
• rate of change of interface position:dx / dt (interface velocity)
– cm sec-1
• n: # of oxidizer molecules per unit volume of oxide:
nSiO2 NAGMWSiO2
2 for H2O1 for O2
2.25 1022 cm 3 2 for H2O1 for O2
d xd t
jn
DN0 nx D k
– # cm-3
• then relation is just
– now integrate with appropriate initial condition
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Dean P. Neikirk © 1999, last update May 3, 2023 7 Dept. of ECE, Univ. of Texas at Austin
Grove and Deal relation
• setting – 2D/k = A
• function of what’s diffusing, what it’s diffusing in, and what it reacts with
– 2DN0/n = B• function of what’s diffusing and what it’s diffusing in
– initial condition x (t = 0) = xi
• integration gives
1
B4At1
2Atx 2
xi 2 Axi
B
– where represents an “offset” time to account for any oxide present at t = 0
LiveMath
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Dean P. Neikirk © 1999, last update May 3, 2023 8 Dept. of ECE, Univ. of Texas at Austin
– thickness is linearly increasing with time• characteristic of a reaction rate limited process
– B/A is the “linear rate constant”
t A2 4B
Limiting behavior of Grove & Deal oxidation model
nkN
kD2
nND2
AB 00
• “short times”
• linear rate constant depends on– reaction rate between oxidizer and silicon (k) AND – solid solubility of oxidizer in oxide (N0)– temperature dependence mainly from reaction rate
x t A2 1
tA2 4B 1
x t
A2 1 1
2 tA2 4B
1
BAt
1
B4At1
2Atx 2
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Dean P. Neikirk © 1999, last update May 3, 2023 9 Dept. of ECE, Univ. of Texas at Austin
Rate constants and Arrhenius plots
• thermally activated process– i.e., process must thermally
overcome an energy barrier
kTE
o
A
eyy
• plot log(y) vs 1/T– if process has the simple
thermally-activated behavior you will get a straight line!
0.000 2
0.000 4
0.000 6
0.000 8
400 600 800 1x10+003 1.2x10+003 1.4x10+003
0.001 0.001 5 0.002 0.002 5 0.003
0
0
temperature
y
1/[temperature]
log[y]
EA = 1eV
EA = 0.5eV
EA = 1eV
EA = 0.5eV
LiveMath
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Dean P. Neikirk © 1999, last update May 3, 2023 10 Dept. of ECE, Univ. of Texas at Austin
Rate constant behavior: linear rate constant
– linear rate constant depends on
• reaction rate between oxidizer and silicon (k)
– is orientation dependent
– temperature dependence mainly from energy required to break Si-Si bond
– AND • solid solubility of oxidizer
in oxide (N0)
BA
2DN0n
2Dk
N0 kn
H20 (640 Torr)EA = 2.05 eV
dry O2EA = 2.0 eV
(111) Si
(111) Si (100) Si
(100) Si
Temperature 1000/T (K-1) Li
near
r ate
con
stan
t B/A
( m
/hou
r)
0.6 0.7 0.8 0.9 1.0 1.1 10-4
10-3
10-2
10-1
1
10 1200C 1000 C 800 C
adapted from Ghandhi
B A 111 B A 100
1.68
plane availablebonds
(100) 6.8 x1014cm-2
(111) 11.7 x1014cm-2
ratio 1.7 (from Sze, 2nd ed., p. 110)
tABtx
1
B4At1
2Atx 2
t A2 4B
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Dean P. Neikirk © 1999, last update May 3, 2023 11 Dept. of ECE, Univ. of Texas at Austin
– dependence is “parabolic”: (thickness)2 time• characteristic of a diffusion limited process
– B is the “parabolic rate constant”
t A2 4B
Limiting behavior of Grove & Deal oxidation model
B 2DN0
n
• “long times”
• parabolic rate constant depends on– diffusivity of oxidizer in oxide (D) AND – solid solubility of oxidizer in oxide (N0)– temperature dependence mainly from diffusivity
x t A2 1
tA2 4B 1
x t
A2
tA2 4B
B t
2
A tx t 1 12 A 4B
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Dean P. Neikirk © 1999, last update May 3, 2023 12 Dept. of ECE, Univ. of Texas at Austin
Rate constant behavior: parabolic rate constant
• parabolic rate constant depends on– diffusivity of oxidizer in oxide (D)
and solid solubility of oxidizer in oxide (N0)
adapted from Ghandhi
B 2DN0
n
Solid solubility@ 1000˚CH2O 3 x 1019cm-3
O2 5 x 1016cm-3
H20 (76 0 T orr )
dry O2
Temperatur e 1000/T ( K-1 ) Pa
rabo
lic ra
te c
onst
ant B
( m
2 /ho
ur)
0.7 0.8 0.9 1.0 1.1 1.2 10 -4
10 -3
10 -2
10 -1
1
1200
°C 10
00°C
800°
C 11
00°C
900°
C 70
0°C
600°
C
EA = 1 .2 4 eV
EA = 0 .7 1 eV
• is NOT orientation dependent• IS oxidizer dependent• temperature dependence mainly from
diffusivity of oxidizer in oxide
tBtx
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Dean P. Neikirk © 1999, last update May 3, 2023 13 Dept. of ECE, Univ. of Texas at Austin
Effect of Si doping on oxidation kinetics
• boron– k = Cox / CSi ~ 3– dopants accumulate in oxide
conc
entr
atio
n
oxide silicon
k < 1, slow SiO2 diffuser
conc
entr
atio
n
oxide silicon
k > 1, slow SiO2 diffuser
• phosphorus– k = Cox / CSi ~ 0.1– dopants “pile-up” at silicon surface
• little effect on linear rate constant B/A ( = Nok / n)
• can increase parabolic rate constant B ( = 2DNo / n )
– really only significant for Nboron > ~1020 cm-3
• little effect on parabolic rate constant B• increases linear rate constant B/A
– again, really only significant for Nphosphorus > ~1020 cm-3
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Dean P. Neikirk © 1999, last update May 3, 2023 14 Dept. of ECE, Univ. of Texas at Austin
100
101
10-1
10-2 100 101 10-1
1100°C
1050°C
1150°C
Time (hours)
Oxi
de th
ick n
ess
(mic
rons
)
(100) siliconsteam
1000°C
950°C
900°C
800°C
100
10-1
10-2 100 101 10-1
1100°C
1000°C
1200°C
Time (hours)
Oxi
de th
ick n
ess
(mic
rons
)
(100) silicondry
900°C
Oxidation thicknesses
• dry oxidation
• wet oxidation– 640 Torr partial pressure is typical
(vapor pressure over liquid water @ 95˚C)
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Dean P. Neikirk © 1999, last update May 3, 2023 15 Dept. of ECE, Univ. of Texas at Austin
Pressure Effects on Oxidation
• grow thick oxides at reduced time / temperature product
– use elevated pressures to increase concentration of oxidizer in oxide
• for steam, both B and B/A ~ linear with pressure
Oxi
de T
hick
ness
(m
icro
ns)
Time (hour)0.2 1 10
0.1
1
20 atmPyrogenic steam900 º C
10 atm
5 atm
1 atm(111)(100)
adapted from Sze, 2nd, p. 122
• rule of thumb: constant growth rate, if for each increase of 1 atm pressure, temperature is reduced ~ 30˚ C.
– pressures up to 25 atm have been used (commercial systems: HiPOx, FOX)
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Dean P. Neikirk © 1999, last update May 3, 2023 16 Dept. of ECE, Univ. of Texas at Austin
Oxidation isolation
• how do you insulate (isolate) one device from another?– what about using an oxide?
• try growing it first since it’ll be long, high temp
silicon
oxide
oxidation
• now etch clear regions to build devices in
, etch
devices
• it doesn’t do any good!!
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Dean P. Neikirk © 1999, last update May 3, 2023 17 Dept. of ECE, Univ. of Texas at Austin
Masking of oxidation and isolation techniques
• would like to form thick oxide between device regions with minimum step heights– mask oxidation using material with low water diffusivity / solubility:
Local Oxidation of Silicon (LOCOS) process• Si3N4 (silicon nitride)
silicon
padnitride mask
before oxidation after oxidation
SiO2
• induces high stress, must place “pad layer” below to prevent dislocations
– for oxide pad layer, tpad ~ 0.25 tnitride– do have lateral diffusion at mask edge, produces “bird’s beak”
• lateral encroachment ≈ oxide thickness• can reduce using more complex “pads”
– SiO2 / poly / nitride (~15nm/50nm/150nm) helps
bird’s beak
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Dean P. Neikirk © 1999, last update May 3, 2023 18 Dept. of ECE, Univ. of Texas at Austin
Use of SiO2 as a diffusion mask• mask against boron, phosphorus, arsenic diffusion
– most impurities transported to slice surface as an oxide• P2O5, B2O3
– reacts with oxide to form mixed phosphosilcate (or borosilicate) glass– reaction continues until full thickness of masking oxide is converted
• doping of underlying Si commences
simple parabolic relationship:
phosphorus:
boron:
xm 2tm
1.710 7 e
1.46eVkT cm2
sec
xm 2tm
4.910 5 e
2.80eVkT cm2
sec
adapted fromNicollian and Brews, p. 732
10-3
10-2
10-1
100
101
101 102 103
Oxi
deM
ask
Thic
knes
s(m
icro
ns)
Time (minutes)
phosphorus
boron
1200° C1100° C1000° C900° C
1200° C
1100° C
1000° C
900° C
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Dean P. Neikirk © 1999, last update May 3, 2023 19 Dept. of ECE, Univ. of Texas at Austin
Other problems in oxidation: MOS threshold stability
• for an MIS structure– if ionized impurities are
in the insulator, what happens?
Metal Oxide Silicon
imagecharge
-
-
-
fixed surfacestates
imagecharge
mobile ionized impurities inoxide (sodium), Qm
----------
++++++++++
• threshold voltage of MOS device is critically dependent on the location and amount of mobile ionic contamination in SiO2 gate insulator– stability can be adversely
affected
– if the ionized impurities are close to the metal, they are screened, and the silicon surface remains “unchanged”
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Dean P. Neikirk © 1999, last update May 3, 2023 20 Dept. of ECE, Univ. of Texas at Austin
Bias-Temperature-Stress (BTS) technique
– apply field of approximately 1MV/cm between capacitor metallization and substrate
– heat sample to ~200o C to increase diffusion rate of Na+
Metal Oxide Silicon
E 1MV/cm
T 200°C
-
-
-
mobile ionized impurities in oxide (sodium), Qm
++++++++++
• allows evaluation of contamination levels
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Dean P. Neikirk © 1999, last update May 3, 2023 21 Dept. of ECE, Univ. of Texas at Austin
BTS impurity drift
– apply field of approximately 1MV/cm across gate insulator
– heat sample to ~200o C to increase diffusion rate of Na+
• can cause surface inversion!!
• process is reversible!
image charge
Metal Oxide Silicon
-
-
-
mobile ionized impurities in oxide (sodium), Qm
+
+++++
++++
-
-
-
fixed surface states
image charge
-
--
----
external electric
field
• most of the impurities have drifted to near the silicon surface• screening now due to
electrons at silicon surface
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Dean P. Neikirk © 1999, last update May 3, 2023 22 Dept. of ECE, Univ. of Texas at Austin
C-V measurements
• measure C-V curves before and after BTS• Qm Cox x Vt
• Qm / toxAcapq• this occurs even at room temperature!
– threshold voltage shift Vt can be MANY volts!
CT
Cox
before BTSafter BTS Qm / Cox
p-type substrate
voltage (metal wrt substrate)
capa
cita
nce
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Dean P. Neikirk © 1999, last update May 3, 2023 23 Dept. of ECE, Univ. of Texas at Austin
Other problems in oxidation: oxidation induced stacking faults
• typically ~95% of all stacking faults are OSFs• essentially an “extra” (111) plane
– density can vary from ~0 to 107 / cm2
• highly dependent on process (high temperature) history– “size” (length) at surface can be many microns
• growth is related to presence of excess “unoxidized” Si at Si-SiO2 interface
• heterogeneous coalescence of excess Si on nucleation centers produce OSFs– any process that produces an excess of Si vacancies will
inhibit OSF formation/growth
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Dean P. Neikirk © 1999, last update May 3, 2023 24 Dept. of ECE, Univ. of Texas at Austin
Chlorine Gettering in SiO2
• effects of chlorine during oxidation– reduction of OSFs– increased dielectric breakdown strength– improved minority carrier lifetimes– improved MOS threshold stability
• when oxide is grown in presence of chlorine the Cl- getters ionic contaminants such as Na+
• Cl in gas stream reacts with Na diffusing from the furnace walls• Cl- is incorporated into the grown oxide near the silicon interface
(~20nm from Si) and can capture mobile sodium preventing threshhold instabilities
• efficiency of chlorine gettering can be evaluated by alternately drifting the Na+ back and forth from metal to silicon side of the test capacitor
• only effective for oxidation temperature above ~1100˚ C
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Dean P. Neikirk © 1999, last update May 3, 2023 25 Dept. of ECE, Univ. of Texas at Austin
Restrictions on the use of Cl Gettering
• chlorine injection techniques– mix HCl in O2 gas stream
• must be VERY dry or severe corrosion problems can occur• produces large quantities of H2O in furnace: 4HCl + O2 2Cl2 + 2H2O
– mix trichloroethylene (TCE) / trichloroethane (TCA) in O2 gas stream• produces less water:
C2HCl3 + 2O2 HCl +Cl2 + 2CO2 • for TCE must have large excess of O2 present to prevent carbon
deposits– under low temperatures and low oxygen conditions can form
phosgene COCl2 • only effective if oxidation temperature ≥ 1100˚ C
– large Cl concentrations, very high temperatures, or very long oxidation times can cause rough oxides
– very high Cl can cause blisters and separation of SiO2 from Si
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Dean P. Neikirk © 1999, last update May 3, 2023 26 Dept. of ECE, Univ. of Texas at Austin
Gate Oxide thickness trends
0.1
1
10
10
100
1000
1970 1975 1980 1985 1990 1995 2000 2005
Min
imum
feat
ure
(mic
rons
)
Oxi
de th
ickn
ess
(Ang
stro
ms)
year (dram: intro or production)
1K4K
16K
64K256K
1M4M
64M256M
16M
1G 16G
• MOS oxide thickness scales with gate length– circa 1998 thickness was ~ 4nm (64M and 256M DRAM technology)
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Dean P. Neikirk © 1999, last update May 3, 2023 27 Dept. of ECE, Univ. of Texas at Austin
Oxidation thicknesses: Grove & Deal calculations
0.01
0.1
1
10
0.01 0.1 1 10 100
oxid
e th
ickn
ess
(mic
rons
)
wet oxidation time (hours)
(100) Si
1100ºC
900ºC
1000ºC
• wet oxidation– 640 Torr partial pressure– fits very well to measurement
100
101
10-1
10-2 100 101 10-1
1100°C
1050°C
1150°C
Time (hours)
Oxi
de th
ick n
ess
(mic
rons
)
(100) siliconsteam
1000°C
950°C
900°C
800°C
calculation measurement
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Dean P. Neikirk © 1999, last update May 3, 2023 28 Dept. of ECE, Univ. of Texas at Austin
Problems with the simple Grove/Deal model of oxidation
– Arrhenius plots of linear and parabolic rate constants are not straight at low temperatures (T < 900o C).
– dry oxidation growth curves do not extrapolate back to zero oxide thickness at zero time
T (°C) A (m) B (m2/hr)
B/A (m/hr)
(hr) xi (nm)
1200 0.05 0.720 14.4 0 0
1100 0.11 0.51 4.64 0 0
1000 0.226 0.287 1.27 0 0
Wet
(640 Torr H2O) 920 0.5 0.203 0.406 0 0
1200 0.04 0.045 1.12 0.027 20
1100 0.09 0.027 0.30 0.076 19
1000 0.165 0.0117 0.071 0.37 23
920 0.235 0.0049 0.02 1.4 26
Dry
(760 Torr O2)
700 ? ? ~2.6 x10-3
81
– must use non-physical boundary condition of either an offset
time or xi200 Å in order to fit data
0.01
0.1
1
0.1 1 10
Dry
Oxi
datio
n Th
ickn
ess
(µm
)time (hours)
1100ºC
1000ºC
900ºC
(100) Si
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Dean P. Neikirk © 1999, last update May 3, 2023 29 Dept. of ECE, Univ. of Texas at Austin
Possible models to explain “rapid” initial growth
• micropores and intrinsic stress in low temperature thin oxides– micropores:
• ~10 Å about 100 Å apart• can visualize as small irregularities that mask oxidation, adjacent
regions grow up around "holes"• diffusion of O2 down pores can fit rapid initial growth stage and
curvature of Arrhenius plots
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Dean P. Neikirk © 1999, last update May 3, 2023 30 Dept. of ECE, Univ. of Texas at Austin
– experiments with external applied fields imply O2 is charged during oxidation:
• O2 O2- + h+
– because of mobility differences have ambipolar diffusion effects:
- - - - - - - - - - -
+ + + + + + + + + + +
~D
oxide
silicongas O2-
hole
Field-Enhanced Diffusion
– range of effect is approximately the Debye length D
• D 1/√N– ~150-200 Å for O2 in SiO2
– ~5 Å for H2O in SiO2
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Dean P. Neikirk © 1999, last update May 3, 2023 31 Dept. of ECE, Univ. of Texas at Austin
Thin oxide growth• thin oxides can be grown controllably
– use reduced pressure• usually dry O2 oxidation• need “pressure” (1 atms = 3x1019) near solid solubility limit (5x1016 @
1000˚C)• ~ 10-3 atms (0.25 - 2 Torr used)
adapted from Sze, 2nd ed, p. 116.
(100) silicon, dry oxygen
Oxidation time (minutes)
Oxi
de t
hick
ness
(nm
)
103 10-1 101 100 102
102
100
101
800°C
850°C 900°C 950°C 1000°C
– use low temperature!!!– intrinsic stress in low temperature-
grown oxides:• low temp oxides tend to have higher
density• not thermal expansion stress
– at high temp viscosity of SiO2 low enough to allow plastic flow
– at low temp viscosity is too high
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Dean P. Neikirk © 1999, last update May 3, 2023 32 Dept. of ECE, Univ. of Texas at Austin
ITRS roadmap (2000) requirements(http://public.itrs.net/Files/2001ITRS/Home.htm )
• 2005 projections require lgate ~ 65nm, “effective oxide thickness” ~ 1-1.5nm
– “EOT” :
– problem: excessive leakage current and boron penetration for oxide thicknesses < 1.5nm
– alternative “high k” dielectrics
“technology node” (nm) year min gate
length (nm)
equivalent gate oxide
thickness (nm)
130 2002 85-90 1.5-1.9
90 2005 65 1.0-1.5
60 2008 45 0.8-1.2
40 2011 32 0.6-0.8
2
rphysical
r SiO
EOT t
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Dean P. Neikirk © 1999, last update May 3, 2023 33 Dept. of ECE, Univ. of Texas at Austin
Diffusion Mechanisms• probability of movement
– substitutional impurity vacancy
interstitial impurity
interstitialcymechanism
, o ~ 1013 - 1014 sec-1
– Emove ~ 0.6 - 1.2 eV• T = 300K: ~ 1 jump
per minute• T = 1300K: ~ 109
jumps per sec
– Emove ~ 3 - 4 eV• T = 300K: ~ 1 jump
per 1030 - 1040 years!• T = 1300K: ~ few
jumps per sec
4oe E kT
• interstitial diffusers
• substitutional diffusers