Part 6 thin film depositoin

Fall 2008 EE 410/510:
Microfabrication and Semiconductor Processes
M W 12:45 PM – 2:20 PM
EB 239 Engineering Bldg.
Instructor: John D. Williams, Ph.D.
Assistant Professor of Electrical and Computer Engineering
Associate Director of the Nano and Micro Devices Center
University of Alabama in Huntsville
406 Optics Building
Huntsville, AL 35899
Phone: (256) 824-2898
Fax: (256) 824-2898
email: williams@eng.uah.edu

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JDW, Electrical and Computer Engineering,
UAHuntsville
2
Basic Vacuum Science
• 1 atm = 101 kPa= 760 torr
• Gass flow is measured in torr liters/ sec
• Flow rate for a vacuum, Q, is determined the difference in pressure on each end of
the system times the conductance across that system
• Conductance for a given tube diameter in a vacuum is
• Pumping speed of the system, Sp = Q/Pinlet. Pinlet = pressure at the pump inlet
• Net speed of the vacuum chamber is
• Time required to pump the system from an initial pressure is (V= chamber volume)

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Vacuum pumps• Rotary-vane (roughing pumps) pull vacuums
from atm to 10-3 torr
• Below 100 mtorr oil in the pump often leaks
back into the pump chamber. This is called
backstreaming and is eliminated by placing a
filter between the pump and the chamber.
• High vacuum pumps are required to operate
chambers under vacuums lower than 10-3 torr
(or vacuums of 10-2 torr for long time periods)
• High vacuums are generated by
– Turbomolecular pumps
• Series of high speed fans that pull molecules
through the spinning blades into a low vacuum
regime
– Diffusion pumps
– Cryo pumps
– Base pressures of 10-7 torr, below which there
are not enough molecules to effectively pull from
the vacuum
• Ultra high vacuum is obtained in high vacuum
conditions by adding an ion pump that
electrostatically captures ionized molecules in
the gas
– Requires outgassing of the vacuum by baking at
350-400oC after high vacuum pumping
– Reaches base pressures of 10-11 torr
Mechanical pump
Turbomolecular pump
http://www.lesker.com/newweb/index.cfm Drawings from Caltech Thin Film Lab no. 3 lab description

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Vacuum pumps (Diffusion Pumps)
Diffusion pump• Require rough vacuum to operate
• Heat oil which evaporates and is cooled along the height of the pump
• Hot oil “grabs” molecules and is condensed by cooling back down
into a liquid near the heat source.
• Oil is cooled by piping liquid nitrogen or cold water around the pump
http://www.lesker.com/newweb/index.cfm

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Vacuum Gauges
http://www.lesker.com/newweb/index.cfm

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High Vacuum System for Physical
Vapor Deposition
• Vacuum Chamber (in this
case a bell jar with a
metallic safety shield
around it)
• Valves
• Turbo pump
• Capacitive Gauge
• Mechanical Pump
• Question: Can you spot the
course vacuum gauge?
http://fie.engrng.pitt.edu/fie98/papers/1228b.pdf

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Thin Film Deposition
• Physical Vapor Deposition (PVD)
– Films formed by physical process of atom transport to the substrate in the gas phase
• Thermal evaporation
• E‐beam evaporation
• Sputtering
– DC, DC magnetron, RF
• Molecular Beam Epitaxy (MBE)
• Chemical Vapor Deposition (CVD)
– Chemically reacted materials at the substrate surface form thin film of product material
• Low‐Pressure CVD (LPVCD)
• Plasma‐Enhanced CVD (PECVD)
• Atmospheric‐Pressure CVD (APCVD)
• Metal‐Organic CVD (MOCVD)
• Laser Assisted CVD
• Atomic Layer Deposition (ALD)
• Combination processes
– Reactive Sputtering – material in gas phase reacts with oxygen, nitrogen, etc. to form oxide or nitride
film
– Electroplating – electrochemical reactions in the liquid or solid phase produce metallic or organo‐
metallic thin films on a surface

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General Characteristics
• Deposition Rate
• Film Uniformity
– Across the substrate
– Run to run
• Materials deposited by a particular method
– Metal, dielectric, polymer
• Film Quality
– Adhesion
– Stress
– Stoichiometry
– Density
– Grain size and orientation
– Breakdown voltage (dielectrics)
– Impurities
• Conformality (depends on technique and process)

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Thermal Evaporation
• Sufficient vacuum is pulled on a chamber such that the mean free path of
atoms in the chamber is greater than the distance between the source and
the target
• Mean free path, l, of a molecule in a gas is
• Source material is then heated until it evaporates from the surface
kb = boltzmans constant
T= temperature
D = diameter of the gas molecule
P = pressure of the chamber
http://www.mrsec.harvard.edu/education/ap298r2004/Erli%20chenFabrication%20II%20-%20Deposition-1.pdf http://www.lesker.com/newweb/index.cfm

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E‐beam Evaporation
• Same vacuum conditions as before
• Electrons are emitted from a metallic tip at 10KeV
• Their trajectory is bent using a strong magnetic field
• Electrons are smashed into a crucible containing the source
material
• Constant flow of electrons into material heats it until
evaporation takes place
• Virtually unlimited supply of source material for a single
deposition
• Electron bombardment heats very effectively allowing
deposition of very high temperature materials and
dielectrics
http://www.mrsec.harvard.edu/education/ap298r2004/Erli%20chenFabrication%20II%20-%20Deposition-1.pdf http://www.lesker.com/newweb/index.cfm

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• Evaporation Rate, R, is
– Constant =1/(2πkb)1/2
– M = molecular weight of the evaporant
– T = source temperature in Kelvin
– Pv(T)= capor pressure of the evaporatant in torr
Evaporation Rate from Souce
( ) 2/12
22
)(
)(coscos
10*513.3
MT
TP
r
R vϕθ
=
Of great importance is to note that this is a line of
sight method. Deposition rate depends directly on
the angle between the source and the target
Note: book uses F instead of R
http://www.mrsec.harvard.edu/education/ap298r2004/Erli%20chenFabrication%20II%20-%20Deposition-1.pdf

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• Deposition rate, dh/dt, is
– Ae = source surface area
– ρ = evaporant density
Deposition Rate on Surface
( ) 2/12
22
)(
)(coscos
10*513.3
MT
TPA
r
FA
dt
dh vee
ρ
ϕθ
ρ
==
Pe for Al at 900K is approx 10-10 torr
Which increases to roughly 0.5 at approx. 1500 K

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Deposition Uniformity
Across the Substrate

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Uniformity of Films as a Function of
Distance From the Source

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Comparing E‐beam and Thermal
Evaporation

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Sputter Deposition
• Physical deposition occurs when a
plasma is struck between a target and
a substrate in an inert gas such as (Ar)
• Average voltage placed across the
chamber is between 300 and 1000 V
• Unlike Plasma etching, the cathode
(positive electrode) is the target
• Substrate is the negative surface
(anode) which receives deposited ions
impinged on the surface
• Sputtering occurs at relatively high
pressures where the mean free path is
much smaller than distance between
target and substrate
• Deposition rate is inversely
proportional to both the path length
and the pressure of the system
• Instead deposition is driven by the
working voltage and the discharge
current (or ionic flux) across the
plasma
• Reactive Deposition occurs in the
presence of O2 or N2 plasmas
Sputter Targets

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Magnetron Sputtering
• Increases deposition rate by up to 100 times
• Lower chamber pressure by up to 100 times
• Magnetic field near cathode allow electrons to
hop near the surface increasing ion milling rate

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RF Sputtering
• Allows deposition of dielectrics
• Ions cannot follow switching at
frequencies greater than 1 MHz
and are accelerated toward
substrate
• Electrons follow voltage cycles
maintaining potential matching at
target and neutralizing positive
charge buildup that would
normally inhibit dielectric
deposition
• Reduced voltage between
electrodes may require higher
fields to be used

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Comparison of Evaporation and
Sputter Deposition
Nonconformal, Line of sight
process
Conformality Depends on
Plasma Conditions

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Theoretical Models for Thin Film
Growth

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Thermal Oxidation and Nitration
H20

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Fick’s First Law of Diffusion
• Fick’s 1st law of diffusion: any material that
is free to move will experience a net
redistribution in attempt to eliminate any
concentration gradient
– C = impurity concentration (mol/m3)
– D = diffusion coefficient (m2/s)
– J = net flux of material (mol/(m2*s)
– Flux moves from high to low
concentrations (i.e. negative sign)
x
txC
DJ
∂
∂
−=
),(
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛ −
−=
Tk
E
DTD
B
A
o exp)(
Cp Co

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Diffusive Oxide Growth
• For oxide growth, flux is assigned in three
regions, J1, J2, J3
x
txC
DJ
∂
∂
−=
),(
SiO2 SiGas
Cg Cs Co Ci
)(1
1 2
sgggas
B
g
sl
sg
O
CChJJ
Tk
Pg
V
n
C
t
CC
DJ
−==
==
−
≈
Oxygen molecules diffuse from
bulk to surface concentrations
Bulk gas concentration can be
estimated using ideal gas law
Flux is estimated using mass
transport coefficient, hg

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Diffusive Growth (2)
• Assuming that there are no sources or
sinks of oxygen at the surface, then the
flux of oxygen within the growing oxide
film is based simply on diffusion from the
surface
– tox is the oxide surface thickness
• The third flux is due to the reaction rage
of silicon with the oxygen concentration at
the Si/SiO2 interface
– k3= chemical rate constant
• Finally, under equilibrium conditions, all of
the individual fluxes must balance
321
33
2 2
JJJ
CkJ
t
CC
DJ
i
ox
io
O
==
=
−
=≈
SiO2 SiGas
Cg Cs Co Ci

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• Accounting for these three fluxes provides
two equations and three unknown
concentrations, Cs, Co, Ci. Solving to find
the growth rate requires one more
equation.
• Henry’s Law states that the concentration
of an adsorbed species at the solid of a
surface is proportional to the partial
pressure of the gas just above the solid
– H = Henry’s gas constant
• Now we have 3 eqns. and 3 unknowns
Diffusive Growth (3)
2
1
O
oxs
g
s
g
i
D
tk
h
k
HP
C
++
=
gBgo TCHkHPC ==
yielding,
SiO2 SiGas
Cg Cs Co Ci

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Oxide Growth Rate
• To obtain the growth rate, divide the
flux, by the number of mol of O2 per
unit volume SIO2: N1
• For Oxidations by molecular oxygen,
N1 is half the number density of SiO2,
or 2.2*1022 cm‐3
• Assuming that at time = zero, that the
oxide thickness is to, the solution of
the differential equation can be
written as
)(
2
τ+=+ tBAtt oxox
⎥
⎥
⎦
⎤
⎢
⎢
⎣
⎡
++
===
2
11
1
O
oxs
g
s
gox
D
tk
h
k
N
HP
dt
dt
N
J
R
2
)(4
/2
11
2
2
2
1
τ
τ
+++−
=
+
=
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+=
tBAA
t
B
Att
NDHPB
hk
DA
ox
oo
g
gs
SiO2 SiGas
Cg Cs Co Ci

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More Detail
• Most silicon oxidation is performed at
atmospheric pressure.
– ks<<hg
• Furthermore, growth rate is nearly independent of
the gas phase mass transport (and therefore
reactor geometry)
• Oxides are grown under both wet (H2O)and dry
(O2) conditions using the same equations.
However the following changes must be
accounted for:
– Diffusivity (D)
– Mass transport (hg)
– Reactivity (ks)
– Pressure (Pg)
– Number of molecules/unit volume (N)
• A and B both depend on Diffusivity and are
therefore both Arrhenhius functions
• τ is the time required for the initial oxide
thickness prior to the current growth process.
Thus, one must account for a growth time that
includes τ in order to account for the nonlinear
growth rate associated with prior oxidation when
growing any thermal oxide layer.
SiO2 SiGas
Cg Cs Co Ci
2
)(4
/2
11
2
2
2
1
τ
τ
+++−
=
+
=
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+=
tBAA
t
B
Att
NDHPB
hk
DA
ox
oo
g
gs

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Wet Vs. Dry Oxidation
• For sufficiently thin oxides,
one can neglect the
quadratic term in the
differential equation for
thickness, yielding
• For thick oxides, however
the linear term can be
neglected and
• Because of these two forms,
B/A is called the linear rate
coefficient
• B is called the parabolic rate
coefficient
• It is these two terms that
are commonly quoted for
oxidation
)( τ+= t
A
B
tox
)(
2
τ+= tBtox
Arrhenius plot for B(T) Arrhenius plot for B(T)/A(T)
Dry
Dry
Wet Wet

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Calculating Oxide Thickness
)(
2
τ+=+ tBAtt oxox
ti=
tox

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Oxide Thickness Chart
• Assuming no previous oxide on silicon
surface prior to growth, τ=0.
• Using the chart to calculate growth of
oxide assuming a prior layer
– Find initial thickness of oxide on chart at
the desired process temperature.
Determine the oxidation time required for
initial thickness and assign it the value τ.
– Determine process time required to
oxidize the sample to the desired
thickness.
– Subtract τ from time t to arrive at the
optimal process time required.
Table from Wolf and Tauber, Silicon Processing for the VLSI Era vol. II

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Chemical Vapor Deposition
CVD Reactor
Substrate
Continuous film
8) By‐product
removal
1) Mass transport of
reactants
By‐products
2) Film precursor
reactions
3) Diffusion of
gas molecules
4) Adsorption of
precursors
5) Precursor
diffusion into
substrate
6) Surface reactions
7) Desorption of
byproducts
Gas delivery
purge
Reference unknown at this time, Plumber Perhaps?

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CVD Growth
• Deal‐Grove Model using Fick’s First Law
• Equilibrium solution involves two cases
– Mass transfer to the surface from the gas
– Reaction kinetics at the surface
sSCKJ =2
x
txC
DJ
∂
∂
−=
),(
)(1
1
sgggas
B
g
sl
sg
CChJJ
Tk
Pg
V
n
C
t
CC
DJ
−==
==
−
≈
Mass transfer by diffusion Reaction Kinetics
gs
g
s
hk
C
C
JJ
/1
21
+
=
=
Equilibrium Condition
• As the value of the surface concentration
approaches Cg, hg>> ks and the deposition
process is surface reaction controlled
• When the surface concentration approaches
zero, then hg<<ks and the deposition is said to
be mass flow controlled.

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CVD Growth
• CVD Growth rate
• Concentration of the reactant in the gas phase is:
– Y is the mole fraction of the reaction species
– CT is the total number of molecules per cm3 in the gas
• Substitution yields
– Accurately predicting that the growth rate is proportional
to the mole fraction of the reacting species in gas phase
– And that the growth rate for any constant reactant mole
fraction, Y, is controlled by the mass flow of Y to the
surface and the reaction rate kinetics on that surface
• For surface‐reaction rate controlled deposition:
• For mass transfer controlled deposition:
1N
YC
hk
hk
R T
gs
gs
+
=
1
1/
N
C
hk
hk
NFR g
gs
gs
+
==
YCC Tg =
1/ NYkCR sT=
1/ NYhCR gT=
Continuous gas flow
Deposited film
Silicon substrate
Boundary layer
Diffusion of
reactants

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CVD Growth
• Chemical reactions are thermally activated and thus can
be represented using an Arrhenius type equation
• Mass transfer is relatively temperature insensitive and
depends primarily on gas flow conditions
• At low temperatures growth follows the exponential law
and the process is primarily dominated by reaction
kinetics
• At high temperatures, the growth reaction occurs so fast
that the system becomes governed by the amount of
reactants that flow across the surface. Under these
conditions, mass flow dominates the deposition process
and there is less control over the exact composition of
the reactant species
• This model is oversimplified b/c it does not consider the
flux of reaction products, but only the surface
concentration. More complicated models are required
to examine individual reactant fluxes to generate
variations on the growth material
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛ −
=
Tk
E
kk
B
a
os exp
gs hk >>
gs hk <<
Wolf and Tauber, Silicon Processing for the VLSI Era vol. II

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Flux Compensation
in Mass Transport
L
DD
h
L
dUL
x
L
dxx
L
dU
x
CC
DJ
Lg
s
g
g
L
s
L
ss
s
s
sg
2
Re3
Re3
2
3
2
)(
1
0
==
==
=
=
−
=
∫
δ
μ
δ
δδ
μ
δ
δ
Fick’s First Law
Boundary Layer
μ = viscosity
d = gas density
U = free stream velocity
ReL = Reynold’s number
ReL<2000 represents laminar flow
Mass transfer coefficient depends on
diffusivity of the gas, the Reynold’s
number, and the length of the flow
• Boundary layer theory
– Better calculation of hg
– Better representation of events near a surface
Turbulent Flow causes
convective rolls and changes in
thickness along the x direction
Wolf and Tauber, Silicon Processing for the VLSI Era vol. II

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Crystallographic Growth Dependence
• The deposition rate can depend strongly on the crystallographic orientation of
the substrate
• In GaAs, growth on (111) is 3x to 4x faster than on (100)
• Growth dependence on orientation is amplified at high temperatures where
epitaxial deposition becomes dominant
• There are several reasons for this:
the densities and geometric
arrangements of surface sites
the number and nature of
surface bonds
the chemical composition of the
surface - GaAs (111A)
vs.(111B)
the presence of surface
features such as steps, kinks,
ledges, vacancies, etc

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Three Major Types of CVD
• Atmospheric pressure chemical vapor deposition (APCVD)
– High deposition rate
– low uniformity
– Moderate film quality
• Low‐pressure chemical vapor deposition (LPCVD)
– Low pressure (0.1 to 1 torr)
– high film quality
– moderate deposition rate
– Low pressure operation removes many of the restrains of reactor design; problems with gas transport are
minimized
– Main problem is optimization of temperature profiles (usually resistance heating)
• Plasma‐enhanced chemical vapor deposition (PECVD)
– Plasma Activation allows for deposition at low temperatures and pressures
– Reactive gas species are formed by reactions in the plasma; since the electron temperature is ~100X the
gas temperature, PECVD creates reactive species that normally occur only at high temperatures
– High deposition rate
– Low quality
• APCVD and LPCVD involve elevated temperatures ranging from 500 0C to 800 0C. These temperatures are too
high for metals with low eutectic temperature with silicon, such as gold (380 0C) or aluminum (577 0C).
• PECVD has a part of their energy in the plasma; thus, lower substrate temperature is needed, typically on the
order of 100 0C to 300 0C.

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Continuous‐Processing APCVD
Reactors
WaferFilm
Reactant gas 2
Reactant gas 1
Inert separator
gas
(a) Gas‐injection type N2
Reactant gases
Heater
N2 N2 N2N2 N2
Wafer
(b) Plenum type

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• GaAs growth by CVD is often performed by chloride transport
• HCl reacts with Ga to form volatile GaCl, which is transported to the
substrate
• AsH3 thermally decomposes to from As4 and As2
• GaCl(g) + ¼As4(g) + ½H2(g) → GaAs(s) + HCl(g) is the simplest reaction,
but others are possible
High temperature APCVD reactor
for GaAs deposition
gas exhaust
Ga-source
heater
substrate
heater
HCl, H2
AsH3, H2
Ga source substrate
http://ecow.engr.wisc.edu/cgi-bin/get/msae/333/matyi/notes/30_cvd2_00.ppt#324,3,CVD reaction kinetics (5)

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LPCVD
(Ex. TEOS Oxide Deposition)
Pressure controller
Three‐zone
heater
Heater TEOS
N2 O2
Vacuum
pump
Gas flow controller
LPCVD
Furnace
Temp. controller
Computer terminal
operator interface
Furnace
microcontroller
Exhaust

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General Schematic of PECVD for Deposition of Oxides,
Nitrides, Silicon Oxynitride or Tungsten
Process gases
Gas flow controller
Pressure controller
Roughing
pump
Turbo
pump
Gas panel
RF generatorMatching
network
Microcontroller
operator Interface
Exhaust
Gas dispersion
screen
Electrodes
Gate valve
Gas flow
Deposited film
Silicon substrate
Reaction product
Diffusion of
reactantsInside the
PECVD Chamber
Typical PECVD
conditions:
Ar-gas at 100 mtorr
RF bias 600 - 1500 volts
250°C to 400°C (high
temp.
reactions can take place
on
temperature sensitive
materials

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Safety issues in CVD
Safety issues represent a major concern
in CVD!
• toxic gases → TLV (threshold limit value) per 8 hour day
– 0.3 ppm PH3
– 0.05 ppm AsH3 (500 ppm AsH3 lethal in < 2 minutes)
• flammable, corrosive, explosive, pyrophoric gases (SiH4)
• high pressure cylinders ‐‐ handling and transport; store in
ventilated cabinets
• piping ‐‐ seamless tube (all welded), double wall tubing, purging
and ventilation
• safety devices ‐‐ monitoring
• exhaust systems ‐‐ dilution and scrubbing

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• Polysilicon (pyrolysis of silane): SiH4 → Si + 2H2
– gas: 100% SiH4 at 0.2 to 1 torr gives a growth rate
of 10 nm/min but problems with gas phase
nucleation (sol.: dilute to 10% to 20% in H2 or N2)
– deposition temperature:
< 575°C  → amorphous
> 625°C  → columnar structure
700°C  → crystalline grains
>1100°C  → single crystal
– doped polysilicon: B2H6, PH3, AsH3
Silicon CVD Processes
1420oC 1100oC 1000oC
0.80.6
Poly- Region
0.7
10-2
1
102
104
103/T (K-1)
GrowthRate(μm/min)
Monocrystalline
Region
Bloem, J. Crsytal Growth, 50, 581 (1980).

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• Silicon Epitaxy
– 4 commercial methods: Silicon Tetrachloride (SiCl4), tricholorosilane (SiHCl3), dichlorosilane (SiH2Cl2), and
silane (SiH4)
– All have particularly desirable deposition conditions.
– Silicon Tetrachloride was the most widely used and studied
SiCl4 + 2H2 → Si +4HCl
– Demands for very thin silicon layers have moved processes toward SiH2Cl2, and SiH4
– Surface Reactions
SiCl4(ads)+H2(ads) SiHCl3(ads)+HCl(g)
SiHCl3(ads)+ H2(ads) SiH2Cl2(ads)+HCl(g)
SiH2Cl2(ads)+HCl(g) SiCl2(ads)+H2(ads)
SiHCl3(ads) SiCl2(ads)+HCl(g)
SiCl2(ads)+H2(ads) Si(s)+2HCl(g)
– SiCl4 1150‐1250oC  0.4‐1.5 μm/min  good selectivity
– SiHCl3 1100‐1200oC  0.4‐2.9 μm/min  easily reduced
– SiH2Cl2 1050‐1150oC  0.4‐3.0 μm/min  good epi. quality
– SiH4        950‐1050oC  0.2‐0.3 μm/min  (g) dep. problems
used for SOI
less out diffusion
heavy deposits on reactor walls
Silicon CVD Processes
1420oC 1100oC 1000oC
0.80.6
Poly- Region
0.710-2
1
102
104
103/T (K-1)
GrowthRate(μm/min)
Monocrystalline
Region
Bloem, J. Crsytal Growth, 50, 581 (1980).

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Common CVD Processes for MEMS
J. Micromech. Microeng. 6 (1996) 1–13. http://iopscience.iop.org/0960-1317/6/1/001/pdf?ejredirect=.iopscience

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Nitride Deposition:
Comparing LPVCD and PECVD
Currently below 200 MPa using ICP

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Color Charts for Oxide and Nitride
Deposits on Silicon
Use ellipsometer at known angles and wavelengths to determine film thickness by
measurement of the polarization of the light reflected back to the sensor
Similar calculations can be performed using interferometry

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Conformality Issues in CVD
• Consider a typical reaction:
SiH4 + O2 → SiO2 + 2H2
• Important variables:
– SiH4/O2
– total pressure
– substrate temp.
– dilutent gas
– topography
N2, Ar, 300-500°C
long mean free
path, reduced
pressure
short mean free
path, 1 atm
SiH4+O2 with
surface diffusion
little surface diffusion

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Semiconductor Process Example
Liner oxide
p Silicon substrate
p Epitaxial layer
n-well p-well
Trench CVD oxide
TEOS-O3
Trench fill by chemical vapor deposition
Nitride
-
+
LPCVD Oxide
LPCVD Nitride
APCVD

Part 6 thin film depositoin

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