BITS Pilani

Sindhu S
BITS Pilani Dept of Physics, BITS Pilani, Pilani Campus
Dept of EEE, WILP Division, Bangalore
Pilani Campus
MELZG 611
IC Fabrication Technology
Lecture No.7
Date . 26/08/2023
 Four growth modes
• Layer by layer growth (Frank ‐ van der Merwe): film atoms more strongly bound to
substrate than to each other and/or fast diffusion
• Island growth (Volmer ‐ Weber): film atoms more strongly bound to each other
than to substrate and/or slow diffusion.
• Mixed growth (Stranski ‐ Krastanov): initially layer by layer then forms three
dimensional islands.

Thin film types based on crystallinity:

• Epitaxial (single‐crystalline, formed layer-by-layer, lattice match to substrate): no grain
boundaries, requires high temperatures and slow growth rate. High quality thin films such
as III‐V semiconductor films (e.g. GaAs) and complex oxides.
• Polycrystalline (island or mixed growth): lots of grain boundaries, e.g. most elemental
metals grown near room temperatures.
• Amorphous (island or mixed growth): no‐crystalline structures (yet with some short range
atomic ordering), no crystalline defects, e.g. common insulators such as amorphous SiO 2.
• Noble metals don’t bond (“wet”) to Si/SiO2 substrate, so tend to have island growth.
• Ag always form island (not continuous film); Au is better than Ag.
• Adhesion layer Ti or Cr can reduce island formation, but for Ag, surface is still very rough
• Here, higher adhesion is because Ti or Cr bond chemically to O in SiO 2.
 Effect of substrate temperature on the lateral grain size
100 Å thick Au films deposited at 100,
100oC
200, and 300℃ by vacuum evaporation

200oC

The small islands start coalescing with each other in

an attempt to reduce the surface area.
This tendency to form bigger islands is termed 300oC
agglomeration and is enhanced by increasing the
surface mobility of the adsorbed species, such as by
increasing the substrate temperature.
Except under special conditions, the crystallographic
orientation and the topographical details of different
islands are randomly distributed.
 Chemical Vapor Deposition (CVD)

CVD : deposit film through chemical

reaction and surface absorption.

CVD steps:
• Introduce reactive gases to the chamber.
• Activate gases (decomposition) by heat or plasma.
• Gas absorption by substrate surface .
• Reaction take place on substrate surface, film firmed.
• Transport of volatile byproducts away form substrate.
• Exhaust waste.
Chemical vapor deposition (CVD) systems

Atmospheric cold-wall system used

for deposition of epitaxial silicon.
(SiCl4 + 2H2  Si + 4HCl)

Low pressure hot-wall system used

for deposition of polycrystalline and
amorphous films, such as poly-silicon
and silicon dioxide.

Figure 9-4
 CVD advantages and disadvantages
(as compared to physical vapor deposition)
Advantages:
• High growth rates possible, good reproducibility.
• Can deposit materials which are hard to evaporate.
• Can grow epitaxial films. In this case also termed as “vapor phase epitaxy (VPE)”. For
instance, MOCVD (metal-organic CVD) is also called OMVPE (organo-metallic VPE).
• Generally better film quality, more conformal step coverage (see image below).
Disadvantages:
• High process temperatures.
• Complex processes, toxic and corrosive gasses.
• Film may not be pure (hydrogen incorporation…).
 Types of CVD reactions
• Thermal decomposition
AB(g) ---> A(s) + B(g)
Si deposition from Silane at 650oC: SiH4(g) → Si(s) + 2H2(g)
Ni(CO)4(g)  Ni(s) + 4CO(g) (180oC)
• Reduction (using H2)
AX(g) + H2(g)  A(s) + HX(g)
W deposition at 300oC: WF6(g) + 3H2(g)  W(s) + 6HF(g)
SiCl4(g) + 2H2(g)  Si(s) + 4HCl (1200oC)
• Oxidation (using O2)
AX(g) + O2(g)  AO(s) + [O]X(g)
SiO2 deposition from silane and oxygen at 450oC (lower temp than thermal
oxidation): SiH4(g) + O2(g) ---> SiO2(s) + 2H2(g)
2AlCl3(g) + 3H2(g) + 3CO2(g)  Al2O3 + 3CO + 6HCl (1000oC)
(O is more electronegative than Cl)
• Compound formation (using NH3 or H2O)
AX(g) +NH3(g)  AN(s) + HX(g) or AX(g) + H2O(g )  AO(s) + HX(g)
Deposit wear resistant film (BN) at 1100oC: BF3(g) + NH3(g)  BN(s) + 3HF(g)
(CH3)3Ga(g) + AsH3(g)  GaAs(s) + 3CH4 (650 – 750oC)
 Chemical reactions for silicon epitaxial growth

SiH 4  Si  2 H 2
SiCl4  H 2  SiHCl3  HCl
SiHCl3  H 2  SiH 2Cl2  HCl
T(K)

SiHCl3  SiCl2  HCl

SiH 2Cl2  SiCl2  H 2
SiH 2Cl2  Si  2 HCl
SiCl2  H 2  Si  HCl
Pressure of SiCl4 (atm) (HCl etches Si at high T, which is used
to prepare electronic grade Si)

Except SiH4 decomposition, ALL other reactions are reversible.

Which direction (etching of Si or growth of Si) to go depends on the partial
pressures of the reactants and temperature.

10
 CVD sources and substrates

• Types of sources
o Gasses (easiest)
o Volatile liquids
o Sublimable solids
o Combination
• Source materials should be
o Stable at room temperature
o Sufficiently volatile
o High enough partial pressure to get good growth rates
o Reaction temperature < melting point of substrate
o Produce desired element on substrate with easily removable
by-products
o Low toxicity
• Substrates
o Need to consider adsorption and surface reactions
o For example, WF6 deposits on Si but not on SiO2
 Types of CVD
APCVD (Atmospheric Pressure CVD), mass transport limited growth rate, leading to non-
uniform film thickness.
LPCVD (Low Pressure CVD)
• Low deposition rate limited by surface reaction, so uniform film thickness (many
wafers stacked vertically facing each other; in APCVD, wafers have to be laid
horizontally side by side.
• Gas pressures around 1-1000mTorr (lower P => higher diffusivity of gas to substrate)
• Better film uniformity & step coverage and fewer defects
• Process temperature 500°C
PECVD (Plasma Enhanced CVD)
• Plasma helps to break up gas molecules: high reactivity, able to process at lower
temperature and lower pressure (good for electronics on plastics).
• Pressure higher than in sputter deposition: more collision in gas phase, less ion
bombardment on substrate
• Can run in RF plasma mode: avoid charge buildup for insulators
• Film quality is poorer than LPCVD.
• Process temperature around 100 - 400°C.
MOCVD (Metal-organic CVD, also called OMVPE - organo metallic VPE), epitaxial growth
for many optoelectronic devices with III-V compounds for solar cells, lasers, LEDs,
photo-cathodes and quantum wells.
 Types of CVD

and epitaxy Si…

(can have high

deposition rate) (can be higher)

For R&D, PECVD is most popular, followed by LPCVD.

 Steps involved in a CVD process
Gas stream

1 7
2 6
Reaction rate may be limited by:
3 4 5 • Gas transport to/from surface.
Figure 9-5 Wafer
• Surface chemical reaction rate that
Susceptor
depends strongly on temperature.

1. Transport of reactants to the deposition region.

2. Transport of reactants from the main gas stream through the boundary layer to the wafer
surface.
3. Adsorption of reactants on the wafer surface.
4. Surface reactions, including: chemical decomposition or reaction, surface migration to
attachment sites (kinks and ledges); site incorporation; and other surface reactions
(emission and redeposition for example).
5. Desorption of byproducts.
6. Transport of byproducts through boundary layer.
7. Transport of byproducts away from the deposition region.
Steps 2-5 are most important for growth rate.
Steps 3-5 are closely related and can be grouped together as “surface reaction” processes.
 Derivation of film growth rate
(similar to/simpler than Deal-Grove model for thermal oxidation)
Boundary
layer
F1 = diffusion flux of reactant species to the wafer through the
CG
Gas Silicon boundary layer (step 2) = mass transfer flux
F1  hG C G  C S  (1)

F1 CS where hG is the mass transfer coefficient (in cm/sec).

F2 = flux
 of reactant consumed by the surface reaction (steps 3-
5) = surface reaction flux,
Figure 9-6 F2 F2  k SC S (2)

where kS is the surface reaction rate (in cm/sec).

In steady state:  F = F1 = F2 (3)

 k S 1
Equating Equations (1) and (2) leads to C S  C G 1   (4)
 hG 
F k h CG k h CT
The growth rate of the film is now given by v  S G  S G Y (5)
N k S  hG N k S  hG N
 in the film and Y is the mole fraction (partial
where N is the number of atoms per unit volume
pressure/total pressure) of the incorporating species, CT is total concentration of all molecules
in the gas phase . 
 Derivation of film growth rate (continued)
F k h CG k h CT
v  S G  S G Y (5)
N k S  hG N k S  hG N

a). If kS << hG, then we have the CT (6)

v k SY
surface reaction  controlled case: N

(b) If hG << kS, then we have the mass transfer, CT

v hG Y (7)
or gas phase diffusion, controlled case:  N


• ks increases with temperature.
(Arrhenius with EA depending on the particular
reaction, e.g. 1.6 eV for single crystal silicon
deposition).

• hG ≈ constant
(diffusion through boundary layer is insensitive
to temperature)
Higher T. Lower T.
 CVD film growth rate
Actually hG is not constant (depends on T)

Figure 9-8
Deposition rate vs. gas glow rate
Growth or deposition rate for silicon by
APCVD. The partial pressure of the reactant
gas is 0.8Torr (1atm=760Torr!!).
H2 is used as the carrier or diluent gas for
the solid curves.
For SiH4, using N2 carrier gas increases the
growth rate, because the carrier gas H2 is a
reaction product of SiH4 decomposition, thus
slowing down the reaction.
 Chemical Vapor Deposition (CVD) growth rate
 E 
• kS limited deposition is VERY temperature sensitive. k s  k 0 exp  a 
 kT 
• hG limited deposition is VERY geometry (boundary layer) sensitive.
• Si epitaxial deposition is often done at high T to get high quality single crystal
growth. It is then hG controlled, and horizontal reactor configuration is needed for
uniform film thickness across the wafer.
• When a high film quality is less critical (e.g. SiO2 for inter-connect dielectric),
deposition is done in reaction rate controlled regime (lower temperature). Then
one can greatly increase the throughput by stacking wafers vertically (for
research, usually 25 wafers per run; 100-200 for industry).
 Other factors affecting growth rate: thickness
of boundary layer and source gas depletion

Gas moves with the constant velocity U.

Boundary layer (caused by friction )
increases along the susceptor, so mass
transfer coefficient hG decreases.
Source gas also depletes (consumed by
chemical reaction) along the reactor.
Both decrease growth rate along the
chamber.
DG: diffusivity To compensate for this, one can:
: gas viscosity Should be C/y, • Use tilted susceptor.
: gas density since diffuse along
U: flow velocity y-direction • Use temperature gradient 5-25°C.
X: gas flow direction • Gas injectors along the tube.
• Use moving belt.

I don’t understand why not operates in the zone

where tube radius (independent of x).
 Mass transport in gas
• Two flow Regimes
o Molecular flow (diffusion in gas, particle transfer).
o Viscous flow (laminar & turbulent flow, moment transfer).
• Laminar flow is desired.
• In CVD growth rate model, it was assumed that mass
transport across the stagnant layer proceeds by diffusion.

 tube radius when x is large

Mass transport depends on:
Fundamental parameters Experimental parameters
Transport of reactants:
Reactant concentration Pressure
• Flow along x-direction.
Diffusivity Gas velocity • Diffusion along y-
Boundary layer thickness Temperature distribution direction. (anyway, no
Reactor geometry flow along y-direction)
Gas properties (viscosity . . .)
20
 Doping in CVD films

• Doping is usually done for epitaxial (thus single crystal) film during film growth.
• Dopant will be grown directly onto crystalline site (no need of dopant activation).
• Doping is realized by adding gas containing the dopant. Such as PH3, B2H6, AsH3 (all
gas phase at room temperature); or PCl3, BCl3, AsCl3 (all liquid at RT).
• They will go through: dissociation, lattice site incorporation, and burying of dopants
by other atoms in the film.
• The dopant concentration C: (P is partial pressure of he dopant species, and v growth
rate) C  P for low growth rates
i

Pi
C for high growth rates
v
• However, there is also unintentional doping process:
o Out-diffusion of dopant from heavily doped substrate into the epi-layer.
o Auto-doping 
– dopant from substrate diffuses into gas stream first, then back
into epi-layer.
 Auto-doping and out-diffusion in CVD film growth

Figure 9-11 Auto-doping processes in an epitaxial reactor. Illustrated are evaporation from 1)
the wafer front side; 2) the wafer backside or edges; 3) other wafers; and 4) the susceptor.

Out-diffusion:

Auto-doping:

Film thickness =vt  Dt = characteristic diffusion length.

That is, the growth is faster than diffusion after certain time t.
 Auto-doping and out-diffusion in CVD film growth

Here substrate is more

heavily doped than the
epitaxial layer.

Figure 9-12 Dopant profile in a Si epi-layer due to out-diffusion and auto-doping.

 Low Pressure Chemical Vapor Deposition (LPCVD)
Atmospheric pressure systems (APCVD) have major drawbacks:
• At high T, a horizontal configuration must be used (few wafers at a time).
• At low T, the deposition rate goes down and throughput is again low.
The fundamental reason (I think) for the low throughput of APCVD is that only a
small percentage of the gas is reactant gases, with the rest carrier/diluent gas.
Obviously, the solution is to operate at low pressure – LPCVD.
n
P0  T  1
In the mass transfer limited regime, hG  DG But DG  D0    (8)
S P  T0  Ptotal
• So as Ptotal goes down, DG and hence hG will go up.
• E.g. when pressure reduced from 1 atmosphere to
 1 Torr (760), hG increases by ~100 (because s
increases by only 5-7).
 Is always < tube radius.
/760, U, 
• Higher hG means higher T can be used while still ks <
hG (i.e. still in surface reaction controlled regime).
This is not one expects: lower pressure means less
reactants, so lower rate. But for APCVD, the reactant
gas is only a small portion of the total gas.
 Low Pressure Chemical Vapor Deposition (LPCVD)

• LPCVD reactors use: P = 0.25 – 2.0Torr, T = 500 – 900°C.

• Transport of reactants from gas phase to surface through boundary layer is still not
rate limiting (despite the high T), so wafers can be stacked vertically for high
throughput (100-200 wafers per run).
• Because LPCVD operates in reaction limited regime, it is VERY sensitive to temperature
and so temperature needs to be controlled closely (within +/- 1oC), so use hot walled
reactor for this precise control.
• Again, a 5-25oC temperature gradient is often created to offset source gas depletion
effects (or one can use distributed feeding).
• Requires no carrier gas, and low gas pressure reduces gas-phase reaction which causes
particle cluster that contaminants the wafer and system.
• Less auto-doping (at lower P), as out-diffused dopant gas pumped away quickly.
 Low Pressure Chemical Vapor Deposition (LPCVD)
Hot-wall Standup wafers
Exhaust
scrubber
Furnace - with resistance heaters
Trap

Vaccum
Pump
Possible disadvantages:
• For too low temperature, deposition rates may be too low, film quality decreases.
• Shadowing (less gas-phase collisions) due to directional diffusion to the surface, so
deterioration of the step coverage and filling.

Seems cold wall reactors also exist: cold wall reduce

deposition on walls, which leads to depletion of
deposition species and particle formation that may
flake off walls and fall on wafers.
Besides poorer temperature control than hot wall,
gas convection is another problem.
Cold-wall
 Plasma Enhanced CVD (PECVD)
RF power input

“Good” quality films (though

Electrode generally not as good as LP or
Cold-wall

APCVD films deposited at much

Plasma
higher T): energy supplied by
Wafers
plasma (i.e. ion bombardment of
Electrode
Heater
film) increases film density,
composition, and step coverage.
Gas inlet Gas outlet, pump
( SiH4, O2)

• Use RF-induced plasma to transfer energy into the reactant gases, forming radicals that is
very reactive. (RF: radio-frequency, typically 13.56MHz for PECVD)
• Low temperature process (<300oC), as thermal energy is less critical when RF energy exists.
• Used for depositing film on metals (Al…) and other materials that cannot sustain high
temperatures. (APCVD/LPCVD at such low temperatures gives increased porosity and poor
step coverage)
• Surface reaction limited deposition, thus substrate temperature control is important to
ensure uniformity.
• At low T, surface diffusion is slow, so one must supply kinetic energy for surface diffusion –
plasma (ion bombardment) provides that energy and enhances step coverage.
• Disadvantages: plasma damage, not pure film (often lots of H incorporated into film).
 PECVD process parameter
Substrate temperature (100-300oC, up to 1000oC PECVD available)
• Control by external heater, very little heating from PECVD process
Gas flow (10s to 100s sccm – standard cubic centimeter per minute)
• Higher flow rates can increase deposition rate and uniformity
Pressure (P  50mTorr – 5Torr )
• Changes the energy of ions reaching electrodes
• Can change deposition rate
• Increases pressure may lead to chemical reactions in the gas
• Effects also depend on gas concentration
Power (10s to 100s watts)
• Affects the number of electrons available for activation and the energy of those
electrons
• Increased power may lead to chemical reactions in gas
• Increased power increases deposition rate
Frequency (mostly 13.56MHz, same for plasma etching and sputter deposition)
• Changes plasma characteristics
• Changes ion bombardment characteristics
 High Density Plasma (HDP) CVD

Microwave
supply • High density plasma CVD gives dense layers (SiO2) at
(2.45 GHz)
magnetic coil low T (150 °C) and low P (1- 10 mTorr); T increases to
400°C by bombardment.
plasma
• Separate RF (gives substrate biasing for
bombardment) from plasma generation (electron
cyclotron resonance ECR and inductively coupled
plasma ICP).
gas inlet • Simultaneous deposition and sputtering/
wafer bombardment. Improved planarization and filling
due to preferential sputtering of sloped surface.
gas outlet, Mostly used for SiO2 deposition in backend
pump
RF
bias supply processes.
(13.56 MHz)
 Miscellaneous: selective deposition and laser CVD
Selective deposition:
• Especially important in microelectronics, surface Laser CVD
patterning and 3D-growth. (energy provided by laser)
• Reaction rate of precursor is limited on a non-growth
surface. E.g. deposition of Cu from (hfac)Cu(PMe3)
occur on Cu, Pt… but not on SiO2.
• Growth surface acts as co-reactant, and is selectively
consumed. E.g. Si reacts with WF6 or MoF6, while
reaction at SiO2 or Si3N4 is slower.
• A chemical reaction of a gaseous co-reactant occur on
the growth surface. E.g. H2 dissociation on a metal
surface, but not on SiO2 or metal oxide surfaces.

Tungsten spring
grown by laser CVD.
 CVD reactor types: quick summary

According to the LPCVD slides, APCVD growth rate should be lower,

• In APCVD reactive gas partial pressure could be set much higher than that in LPCVD.
• Its pressure could be much lower (by 10) than 1atm and is still called APCVD.
• Gas transport actually increases with T as T 3/2 (APCVD is usually done at higher T than LPCVD).
• When putting wafer side-by-side facing the gas, more exposed to gas, thus faster transport.