Semiconductor Photodetectors

Transcription

Semiconductor Photodetectors
Chapter 6
Fabrication Technology of Optoelectronic Devices
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Introduction
Bulk Crystal Growth
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LEC method
HB method
FZ method
Epitaxy
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Fabrication Technology
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Chloride Transport
MOVPE
ALE
MBE
GSMBE and MOMBE
Diffusion
Ion Implantation
Photolithography
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LPE
VPE
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Wet Etching
Dry Etching
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Plasma etching
RIE
Ion Beam etching
Metallization
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Lift-off process
E-beam lithography
PVD
CVD
Dielectric Deposition
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Introduction to Fabrication Process of Optical Devices
a)
b)
c)
d)
e)
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Starting materials
→ growth of epitaxial layers
→ deposition of photoresists
→ deposition of metals
→ lift-off for formation of top contact
Mesa-etching
Formation of bottom contact (as in (a))
Mesa-etching for device isolation
→ deposition of dielectric passivation
Overlay metallization for interconnects
and bonding pad
→ deposition of dielectric passivation
→ substrate thinning and backhole
The semi-insulating substrate is used
for certain high-speed device (in which
both contacts are formed on the top
side ), while a conducting substrate is
useful for device that has top and
bottom contacts.
Typical processing for fabricating optoelectronic devices
on a semi-insulating substrate
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Bulk Crystal Growth
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Bulk crystal growth technology for III-V
compound semiconductors:
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Czochralski Crystal Puller
Liquid Encapsulated Czochralski (LEC) method
Horizontal Bridgmann (HB) method
Floating Zone (FZ) method
Main reasons for the difference progress between
Si and GaAs bulk crystal growth:
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The growth of a compound semiconductor is
always more difficult than that of a single-element
system
The demand from device manufacturers for large
area compound semiconductors is lacking. (2” or 3”
wafers are popularly used in opto-electronic
fabrication plants)
Synthesis of polycrystalline GaAs as the raw
materials to grow single-crystal GaAs:
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Ga is placed in a graphite boat and heated to 610 to
620 ℃, while As is placed in another graphite boat
and heated to 1240 to 1260 ℃ (slight above the
melting temperature of GaAs.
As is overpressure and will transport to the Ga melt.
When the melt cools, a high-quality polycrystalline
GaAs results.
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Liquid Encapsulated Czochralski (LEC) Technology
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To prevent decomposition of the melt due
during crystal growth due to the high vapor
pressure of As, the melt is encapsulated by a
layer of boron trioxide.
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The whole system is contained in a pressure
vessel, with an inert gas ambient providing an
overpressure (< 3 atm)to the melt.
The crucible is pyrolytic boron nitride (PBN),
instead of the quartz used in conventional Si
CZ method.
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B2O3, which is an inert liquid with a low melt
point and low vapor pressure.
→ liquid encapsulant
B2O3 can dissolve silicon dioxide (fused-silica)
as this material can be used several times over
and does not act as a source of intentional
silicon dopants.
The high dislocation defect density ( 104 to 105
cm-2) produced by this method is its its major
its major disadvantage, as compared to the HB
method.
Semi-insulating wafers are usually made by
this method.
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Horizontal Bridgmann Technology
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The assembly is placed inside a zoned heating
system.
A GaAs seed crystal is placed at one end of the
quartz boat, and molten GaAs is maintained in
the bulk of the boat region (T1 zone ~ 1250℃).
An arsenic overpressure is provided by the
elemental As source in the ampoule.
As the seed end of the melt moves out of the
T1 zone to the cooler T2 zone, it solidified into
single crystal GaAs.
The advantages of the HB method over the
LEC method:
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Dislocation densities are usually lower (13104 cm-2)
The ability to deliberately incorporate dopants
and give a conducting wafer.
The disadvantages of the HB method:
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The lack circular wafers of suitable size for IC
fabrication. (D-shaped boules are formed.)
It can not produce GaP wafers due to the high
melting point (1470 ℃ ) of this material. (quartz
boat is molten at this point)
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Floating Zone Technology
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The floating zone (FZ) method can be used to
grow very purity Si wafers.
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A high-purity polycrystalline rod with a seed
crystal at the bottom is held in a vertical
position and rotated.
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FZ degree crystal are used mainly for highpower, high-voltage devices, where highresistivity materials are required.
The rod is enclosed in a quartz envelope with
which an inert gas (Ar) is maintained.
During the operation, a small zone of the
crystal is kept molten (floating zone) by a RF
heater.
As the floating zone moves upward, a singlecrystal silicon freezes at the zone’s retreating
end and grows as an extension of the seed
crystal.
Materials with higher resistivities can be
obtained from the floating zone process than
from the CZ method.
Since no crucible is used in this method, there
is no contamination from the crucible.
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Summary of process for GaAs wafer production
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Epitaxy
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The word “epitaxy” comes from Greek terms: epi = upon, taxis = arrangement.
Bulk crystals can rarely be used for optimized optoelectronic devices for the
following reasons:
– Light emitters and photodetectors operating at different wavelengths are
required.
(This requires different semiconductors with different bandgaps.)
– Material quality for bulk crystals is generally lower than that of epitaxial
layers for compound semiconductors.
(However, the reverse is true for silicon.)
In the epitaxy process, the wafer (substrate) acts as not only a seed for crystal
growth, but also as physical support for the thin epilayer.
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Liquid Phase Epitaxy (LPE)
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LPE is the growth of epitaxial layers on
crystalline substrates by direct precipitation
from the liquid phase. (somewhat like the HB
method except its more transient growth time)
– LPE is the volume method used for
standard LED production today.
The LPE growth process:
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During operation while the system is heated to
the required temperature, the substrate is fully
covered by part of the graphite block.
When the target temperature is reached, the
substrate is moved under te first well and the
furnace temperature is lowered at a given rate.
To grow additional layers, the substrate can be
moved successively under the solution.
The wafer is moved out from under the solution
to terminate the process.
In LPE, the material to be grown must
dissolves in a solvent, and the solution must
melt at a temperature well below the melting
point of the substrate.
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Ga is most commonly used as the solvent for
the growth of GaAs.
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Melt-Back LPE Method
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This method was developed primarily for
green LEDs based on GaP:S/GaP:Zn junctions.
The growth process:
A GaP substrate (in contact with a Ga melt) is
heated and cooling subsequently to result in an
epitaxial layer. Three types of gases are
introduced into the hydrogen ambient:
– H2S, to provide S doping and result an nlayer.
– Zn vapor (from a solid source) to provide
a p-layer.
– NH3 during junction growth to introduce
an isoelectronic center based on nitrogen
to maximum the LED efficiency.
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Vapor Phase Epitaxy (VPE)
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Two types of VPE:
(i) Chloride transport
(ii) metal-organic VPE (MOVPE)
MOVPE, OMVPE (organo-metallic VPE)
MOCVD, OMCVD
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VPE (I) --- Chloride Transport
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Trichloride VPE
– In this system, trichloride AsCl3 (or
PCl3), which is liquid, is transferred to
the reaction chamber by bubbling
hydrogen through.
– An initial crust of GaAs is formed on
the Ga source as a result of the release
of arsenic from the incoming gases:
4 AsCl3 + 6 H2 → 12 HCl + As4
Hydride VPE
– In this system, hydrides such as arsine
(AsH3) and HCl are used.
– GaCl is generated from the Ga source
and As4 from the pyrolysis of arsine :
2 Ga + 2 HCl → 2 GaCl + H2
4 AsCl3 + 6 H2 → 12 HCl + As4
– GaAs is formed on the substrate by the
reaction:
GaCl + AsH3 → GaAs + HCl + H2
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VPE (II) --- MOVPE
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In the MOVPE process, the group III
compound is transported to the substrate
surface in as organic precursor, while arsine
(AsH3) or phosgine (PH3) are still used as the
carrier for the group V compound.
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The organic precursors are all liquid at room
temperature, so bubbling hydrogen system is
used.
A reaction occurs near the heated substrate
surface to form II-V compound directly from
the two carrier gases: (such as)
Ga(C2H5)3(g) + AsH3(g) → GaAs(s) + 3 C2H6(g)
MOVPE can produce multilayered structures
of excellent electrical and morphological
characteristics over a large area.
Tri-methyl-gallium: Ga(CH3)3 or TMGa
Tri-ethyl-gallium: Ga(C2H5)3 or TEGa
Tri-ethyl-indium: In(C2H5)3 or TEIn
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Various MOVPE Systems
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Atmospheric-pressure MOVPE
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Advantage: High growth rate
Problems: unwanted polymer formation,
autodoping, outdiffusion
Low-pressure MOVPE
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Atmospheric-pressure MOVPE System
Eliminating the above problems
The main stream of the MOVPE system
Photo-MOVPE
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Pyrolytical process
Photolytical process
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Low-Pressure MOVPE
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Comparison of VPE and LPE
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For large scale production of non-critical LEDs, the obvious choice is LPE.
The quality of the surface finish of LPE materials is poor.
The most important advantage of the VPE over LPE is the ability to grow
thin layers of heterostructure material on large area substrates.
The competition for the best quality growth technique is really between
MOVPE and MBE.
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Atomic Layer Epitaxy (ALE)
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The epitaxy process that can deposit atoms
one layer at a time.
MBE and MOVPE are both the basic ALE
systems.
The advantages of the ALE process:
– Accurate thickness control and
reproducibility.
– High quality surface finish, free of
many types of defects.
– The ability to dope selectively grown
epitaxial layers – this is useful for
heavily doped contact regions.
– Sidewall epitaxy, where a previous
fabricated multilayer epitaxial wafer
can have a different semiconductor
grown on the edge, giving threedimensional confinement.
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Molecular Beam Epitaxy (MBE)
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MBE is an epitaxial process involving the
reaction of one or more thermal beams of
atoms or molecules with a crystalline surface
under ultrahigh vacuum (UHV) conditions
(~10-10 Torr).
Separate effusion oven (made of pyrolytic
boron nitride) are used for Ga, As, and the
dopants.
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The temperature of each oven is adjusted to
give the desired evaporation rate.
High-temperature baking and low energy ion
beam sputter-clean process are used to clean
the surface of a substrate in-situ.
In-situ analysis includes a mass spectrometer
for ‘residual gas analysis’ (RGA) and
“reflection high energy electron diffraction”
(RHEED) to monitor the substrate and the
epilayer quality.
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Basic Growth Kinetics of MBE
1)
2)
3)
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Evaporation from the source and
molecular travel towards the substrate.
Absorption onto the substrate.
Surface migration and dissociation,
leading to incorporation onto the
substrate (growth).
During MBE process, the substrate
temperatures range from 400 to 900℃,
and the growth rates range from 0.001
and 0.3 m/min.
Because of the low-temperature process
and low-growth rate, many unique
doping profiles, alloy composition and
novel device structures (such as
superlattice) not obtainable from
conventional VPE and LPE can be
produced in MBE.
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Gas-Source MBE (GSMBE)
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The problem with III-V growth using solid
source MBE in that the flux of the group V
source is much greater that of group III
element.
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In the GSMBE process, solid source is used
for group III material, while hydride gas
(AsH3 or PH3) is used as the group V source.
During GSMBE, hydride decomposition
(AsH3 or PH3 → As2, P2, H2) is achieved by a
thermal cracker system.
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Hence, the source is rapid depleted, leading to
regular changes.
Beam flux variations with time, making it
difficult to control.
Source supply and control are external to the
MBE system, leading a much longer time
between source replenishment, and a more
easily control flux of group V atoms.
The GSMBE has a higher reproducibility
than a convention MBE system with solid
elemental source.
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Metal-Organic MBE (MOMBE)
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In the MOMBE process, gas sources are used
for both group III and group V (AsH3 or PH3),
with a reaction between TMGa or TEGa (and
TMIn or TEIn) on the substrate surface with
previous cracked As2 and P2.
MOMBE is also known as “chemical beam
epitaxy” (CBE).
Growth process of MOMBE:
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The beam of both gas molecules impinge
directly onto the heated substrate surface.
The group III alkyl dissociated to leave a free
group III atom on the surface and a desorbed
alkyl group.
Group V elements can then react readily with
the surface group III atoms.
MOMBE process uses the directional nature
of MBE to react gas molecules together (like
MOVPE), but only on the substrate surface
(unlike MOVPE).
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Advantages of MOMBE
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Advantages of MOMBE over MBE:
– Greater control over a source that will
produce many epilayers before needing
refurbishment
– Reproducibility and the ability to
produce very abrupt interface are
inherent in this system.
– No oval defects (surface defects)
– A single group III beam, ensuring
composition uniformity
Advantages of MOMBE over MOVPE:
– The use of RHEED and RGA to
monitor epilayer growth and gas
stoichiometry in situ.
– No flux pattern problems for large
area/multi-wafer applications.
– More abrupt interfaces due to the beam
nature of the growth.
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Fabrication Technology
(1) Doping: Diffusion, Ion Implantation
(2) Photolithography: Resist coating, Exposure, Development, Resist removal
Lift-off process
E-beam lithography
(3) Etching: Wet Etching (Isotropic and Anisotropic Etching, Crystallographic)
Dry Etching (Plasma etching, RIE, Ion beam etching)
(4) Metallization: PVD ( Thermal and E-Beam evaporation, Sputtering)
CVD (PECVD, Laser-induced CVD)
(5) Dielectric Deposition: PECVD, LPD (Liquid Phase Deposition)
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Lift-off Process
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In the GaAs process, chemical etching is
rarely used to pattern metals because:
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The lift-off technique is capable of high
resolution and is used extensively for
discrete devices (e.g. high-power MESFET,
optoelectronic devices).
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Many metal etchants will also etch GaAs.
GaAs processing tends to use multi-layered
metals, such as Au/Ge/Ni for n-type (Au/Zn
for p-type) ohmic contacts and Ti/Pt/Au for
rectifying or overlay metals, which are
difficult to etch.
However, it is not used as widely applicable
for VLSI, in which dry etching is the
preferred technique.
Lift-off process:
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(a) & (b) A positive resist is used to form the
resist pattern on the substrate.
(b) The film is deposited over the resist and
the substrate. The film thickness must be
smaller than that of the resist.
Those portions of the film on the resist are
removed by selectively dissolving the resist
layer in an appropriate liquid etchant so that
the overlying film is lifted off and removed.
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E-beam lithography
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The advantages of E-beam lithography:
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The generation of micron and submicron resist
geometries.
– Highly automated and precisely controlled
operation.
– Greater depth of focus (DOF) than that
available from optical lithography.
– Direct patterning on a wafer without using a
mask.
– Disadvantage: low throughput
Electron Resist: polymers
– Positive resist → higher resolution (but slow)
PMMA (poly-methyl-methacrylate)
PBS (poly-butene-1 sulfone)
– Negative resist → higher sensitivity (fast)
COP (poly-glycidyl-methacrylate-co-ethyl
acrylate)
 There is a trade-off between sensitivity and
resolution.
 In e-beam lithography, resolution is limited by
electron scattering, while is limited by
diffraction of light in optical lithography.
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Wet Etching
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Wet etching → involving chemicals in a
liquid
solution
Dry etching → involving reactive species
generated in a plasma
Wet chemical etching is used extensively in
semiconductor processing:
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Lapping and polishing of a sawed wafer,
chemical cleaning and scrubbing, delineating
pattern, opening window.
Isotropic etching
Anisotropic etching
Orientation-dependent etching
Crystallographic etching
Very few etchants are isotropic for GaAs.
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Most etchants give a polished surface on
arsenic face, but the gallium face tends to slow
crystallographic.
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Crystallographic Etching
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In diamond and zincblend lattices, the (111)plane is more closely packed than the (100)plane; therefore, the etching rate is expected
to be slower for the (111)-plane.
Orientation-dependent etching of Si:
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Etching of <100>-oriented Si through a
patterned SiO2 mask creates precise V-shaped
grooves, the edges being (111)-planes at an
angle of 54.7° from the (100)-plane.
If <110>-oriented Si is used, essentially
straight-walled grooves with sides of (111)planes can be formed.
Si
GaAs
Orientation-dependent etching of GaAs:
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The (111)-Ga planes usually have the slowest
etch rate, while the (111)-As planes have the
fastest.
When the mask window is aligned with the
<110>-axis, the etch profile is trapezoidal in
one direction and dovetailed in the other.
If the mask window is turned 45 with respect
to the <110>-direction, a straight-walled
groove is obtained.
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Dry Etching --- Plasma Etching & RIE
Plasma Etching vs. RIE
RIE System
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Reactive Ion Beam Etching (RIBE)
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Higher ion energy and lower operating
pressure than RIE are used in the ion-beam
etching (IBE) process.
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To obtained higher ion energy by altering the
relative areas of the two electrodes, an
accelerating grid is introduced between plasma
and substrates.
RIBE has a greater degree of anisotropy in
the etching than the conventional RIE.
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Because of the higher ion energy, material
removed by physical sputtering can contribute
to the etching process.
Ion Milling:
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A mechanism that is purely physical since an
inert gas is introduced.
No volatile species are produced, etching rate
are a function of mean angle.
There is no undercutting of any etch mask.
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Rapid Thermal CVD (RTCVD)
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