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Thin Film Epitaxy
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IntroductionEpitaxy comes from Greek words:
Epi: upon
Taxis: orderedEpitaxial growth: single crystal growth of a material in which a substrate
serve as a seed
2 types of epitaxy:
Homoepitaxy material is grown epitaxially on a substrate of thesame material. E.g. grow of Si on Si substrate
Heteroepitaxy a layer grown on a chemically different substrate.E.g. Si growth on sapphire
Similar crystal structures of the layer and the substrate, BUT
The shift of composition causes difference in lattice parameters
Limit the ability to produce epitaxial layers of dissimilar materials
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Film deposited on a
oriented wafer
orientation
The presence of SiO2Layer cause depositing
atoms have no
structurepolysilicon
Epitaxial and polysilicon film growth
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Applications of epitaxial layersDiscrete and power devices
Integrated circuits
Epitaxy for MOS devices
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1. Discrete and power devicesTechnology change: junction transistors diffused planar
structure
Requires a material structure that are not achieved by diffusion ofdopants from the surface
Si epitaxy was developed to enhance the electricalperformance of discrete bipolar transistors
Breakdown voltage of the discrete transistor was limited bythe field avalanche breakdown of the substrate materialUse higher resistivity substrates produced higher breakdown
voltages but increased collector series resistance
Structure needed: thin, lightly doped and single crystallayer of high perfection upon more heavily doped SisubstrateBut, the use of a more heavily doped substrate reduces the
collector series resistance while the base-collector breakdownvoltage is governed by the lighter doping in the near surfaceregion
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Epitaxial deposition of a lightly doped P+ epitaxial layer on
a N+ substrate make the desired properties are
achievable
Epitaxial grows also allows accurate control of doping
levels and advantages which arises from a generally low
oxygen and carbon levels in epitaxial layer
Epitaxial technique was developed to 2 and 3 layers
epitaxial structure
For lightly doped area of collector
Based region was also grown epitaxially
E.g. of multilayer structures: Si-Controlled Rectifier (SCR),
Triac, high voltage or high power discrete products
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Mesa discrete transistor fabricated in an epitaxial
layer on a heavily doped N+ substrate
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Transistors
Diodes
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2. Integrated circuit (IC)
Development of planar bipolar IC caused the requirement for
devices built on the same substrate to be electrically isolated
The use of opposite typed substrate and epitaxial layer met partof the requirement
Device isolation was completed by the diffusion of isolation
region through the epitaxial layer to contact the substrate
between active areas In planar bipolar circuits, common to employ a heavily doped
diffused (or implanted) region under the transistor
Usually called buried layer or DUF for diffusion under film
The buried layer
serves to lower the lateral series resistance between collector area below
the emitter and the collector contact
produce uniform planar operation of the emitter, avoiding current crowding
which leads to hot spots near edges of the emitter
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Integrated circuits
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(a) A junction isolated bipolardevice fabricated as part of an
integrated circuit using a buried
layer subcollector and a lightly
doped n-epitaxial layer
(b) An N-Well CMOS structure
fabricated in a lightly doped p-epitaxial layer
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3. Epitaxy for MOS devices
Unipolar devices such as junction field-effect
transistors (JFETs), VMOS, DRAMs technologyalso use epitaxial structures
VLSI CMOS (complimentary metal-oxide-
semiconductor) devices have been built in thin(3-8 micron) lightly doped epitaxial layers on
heavily doped substrates of the same type (N or
P) That epitaxial structure reduces the latch up of high
density CMOS IC by reducing the unwanted
interaction of closely spaced devices
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Advantages of epitaxy
Ability to place a lightly oppositely doped
region over a heavily doped region Ability to contour and tailor the doping
profile in ways not possible using diffusionor implantation alone
Provide a layer of oxygen free material
that is also contained low carbon
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Techniques for silicon epitaxy
1. Chemical Vapour Deposition (CVD)
2. Molecular Beam Epitaxy (MBE)3. Liquid Phase Epitaxy (LPE)
4. Solid phase regrowth
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1. Chemical Vapour Deposition (CVD)
The most common technique in Si epitaxy
In the CVD technique Si substrate is heated in a chamber: sufficient heat to
allow the depositing Si atoms to move into position to
Reactive Si containing gaseous compounds areintroduced
Gaseous react on the hot surface of the substrate and
deposit a Si layer The deposit will take on Si substrate structure if the
substrate is atomically clean and the temperature is
sufficient for atoms to have surface mobility
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Schematic drawing of a simple horizontal flow, cold
wall, CVD reactor
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CVD reactions
1. Pyrolysis: chemical reaction is driven by heat alone, e.g. silanedecomposes with heating
SiH4 Si + 2H2
2. Reduction: chemical reaction by reacting a molecule withhydrogen, e.g. silicon tetrachloride- reduction in hydrogen ambient
to form solid silicon
SiCl4 + 2H2 Si + 4HCl
3. Oxidation: chemical reaction of an atom or molecule withoxygen, e.g. SiH4 decomposes at lower temperatureSiH4 + O2 SiO2 + 2H2
4. Nitridation: chemical process of forming silicon nitride byexposing Si wafer to nitrogen at high temperature e.g. SiH2Cl2readily decomposes at 1050C
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CVD
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CVD film growth steps
1. Nucleation
Dependent on substrate quality
Occurs at first few atoms or molecules deposit on a surface
2. Nuclei growth
Atoms or molecules form islands that grow into larger islands
3. Island coalescence The islands spread , and coalescing into a continuous film
This is the transition stage of the film growth, thickness several
hundreds Angstroms
Transition region film possesses different chemical and
physical properties for thicker bulk film
4. Bulk growth
Bulk growth begins after transition film is formed
CVD film growth steps
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CVD film growth steps
Types of film
structure
Amorphous
Polycrystalline
Single crystal
Basic CVD subsystem
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2. Molecular Bean Epitaxy (MBE)
Uses an evaporation method
MBE is carried out at a lower temperature than 1000-
1200C (typical CVD temperature)
Reduces outdiffusion of local areas of dopant diffused
into substrates and reduce autodoping which is
unintentionally transfer of dopant into epitaxial layer
MBE is favourable
preparation of sub-micron thickness epitaxial layers or
high frequency devices requiring hyper-abrupt transition in the
doping concentration between the epitaxial layer and the
substrate
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In MBE, Si and dopant(s) are evaporated in an ultra high vacuum (UHV)
chamber
The evaporated atoms are transported at relatively high velocity
in a straight line from the source to the substrate They condense on the low temperature substrate
The condensed atoms of Si or dopant will diffuse on the surfaceuntil they reach a low energy site that they fit well the atomic
structure of the surface The adatom then bonds in that low energy site, extending the
underlying crystal by a vapour to solid phase crystal growth
Usual temperature range of the substrate is 400-800C. Higher
than 800C is possible but it will increase outdiffusion or lateraldiffusion of dopants in the substrate
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Schematic drawing of a molecular beam
epitaxial system
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Insitu cleaning of the substrate Can be done by high temperature bake at 1000-1250C for
several minutes under high vacuum to decompose the nativesurface oxide and to remove other surface contaminants
Other technique is by using a low energy beam of inert gas tosputter clean the substrate
Difficult to remove carbon but will decrease at the surface bydiffusion into the substrate during short anneal at 800-900C
Wider range of dopants for MBE than CVD epitaxy: Typical dopants: Antimony, Sb (N-type), aluminum, Al or gallium
(Ga) for P-type
N-type dopant: As and P, evaporate rapidly even at 200C.
Difficult to control P-type dopant: Boron, evaporate slowly even at 1300C
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Schematic drawing of a multiple chamber MBE
system
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MBE Equipment
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Liquid Phase Epitaxy (LPE)
LPE technique is widely used for preparation of epitaxiallayers on compound semiconductors and for magnetic
bubble memory films on garnet substrate In films growth by LPE from solution melts, low cooling
rates, when the surface reaction (growth)
Kinetics are rapid compare to the mass transport of Si tothe seed, epitaxial layer thickness will vary in proportionto the temperature drop
Increase cooling rates, mass transport rate will increase
and the growth rate will increase with cooling rate untilgrowth rate becomes limited by surface reaction kinetics
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Growth rate increases with
cooling rate up to about 1
degree/min while growth rate
above 2 degree/min occurred
under kinetically limited
conditions
LPE growth rate increasing with
cooling rate up to about 1 micron per
minute
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Schematic drawing of a typical silicon liquid
phase epitaxy (LPE)
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Schematic of fabrication steps in the fabrication vertical field effect transistors by etch and LPE refill techniques
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4. Solid Phase Re-growth
i. Re-growth of amorphous layers
Surface layers subjected to high dose ionimplants are in amorphous structure due to theheavy damage inflicted on the lattice as theenergetic ions are absorbed
Annealing above 600C amorphous layerre-crystallize
Re-crystallisation occurs from interface moves
toward the surface and results in solid phaseepitaxial re-growth
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ii. Re-crystallisation of thin films
Involves re-crystallisation of a deposited amorphous orpolysilicon film
Si film is deposited on a Si substrate or more commonly SiO2
heated using a strip heater passed over the surface or by a
scanned pulsed laser to crystallise the film to single crystal orlarge grain polysilicon
This fabrication technique is used to produce a stacked n-channel
device in re-crystallised polysilicon on a thermally grown ordeposited oxide
Oriented epitaxial growth can be obtained by making series of
holes in the oxide to allow points of contact between the
underlying substrate and the deposited polysilicon The contact points become seeds areas for establishing re-
growth orientation
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Re-crystallisation solid phase
epitaxy using a moving strip heater
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A stacked MOS structure over an
insulating oxide fabricated in a re-
crystallised polysilicon layer
S f
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Structure and defects in epitaxial layer
Surface morphology of Silicon epitaxial deposits isaffected by growth and substrate parameters
Growth parameters: Temperature
Pressure
Concentration of Si containing gas
Cl : H2 ratio
Substrates parameters Substrate orientation
Defects in the substrate Contaminants on the surface of the substrate
T i l d f t i it i l l
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Typical defects in epitaxial layers
1. Substrate orientation effects
2. Spikes and epitaxial stacking faults
3. Hillocks and pyramids in epitaxial layers
4. Dislocations and slip
5. Microprecipitates (S-pits)
1 S b t t i t ti ff t
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1. Substrate orientation effect
Growth of smooth epitaxial films can be obtained on
(100) and (110) oriented Si substrates
Epitaxial growth on substrate surface on oriented on(111) plane results in facetted alligator skin surface
(111) surfaces contain no atomic steps to provide a density of
growth sites Without atomic steps, the growth produces pyramids and
terraces
Misorientation of the surface by
0.5 degreeintroduces a sufficient density of steps for growth of
smooth planar films
2 S ik d it i l t ki f lt
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2. Spikes and epitaxial stacking faults
i. Growth spike
Originate from Si particle on the surface not removed
by the pre-epitaxial cleaning process
Si Chips may expose faster growing crystal planes
than the plane of the substrate
Chips nucleate and produce polysilicon nodule. The
chips then protrude above the substrates surface into
a region of richer supply of gaseous reactants
Results in nodule grows at 2-10 times the rate ofepitaxial film on the substrate.
May be removed mechanically before the next step
but will leave a region unusable for functionalmaterials
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ii. Epitaxial stacking faults
Crystallographic in nature and arise from defects in
atomic arrangement during film growth
Could result from an extra atomic layer (extrinsic fault)
or a missing atomic layer (intrinsic fault) along {111}
type plane
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Epitaxial growth spike Stacking fault on Si
3 Hillocks and pyramids in epitaxial layers
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3. Hillocks and pyramids in epitaxial layers
Hillocks: Small oval mounds on the surface of the
epitaxial
Pyramids: Faceted regions on the epitaxial surface
Density of hillocks and pyramids is dependent on
growth parameters such as type and concentration of
Si source and deposition temperature
4 Dislocations and slip
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4. Dislocations and slip
Non-uniform heating of a substrate results in non-uniform thermal
expansion of the substrate which produces elastic stresses
The thermal stress can cause bowing which may lift the edge ofthe substrate away from the substrate in response to the thermal
stress
At lower temperature (< 900C) the yield point of the Si lattice is
sufficiently high that the substrate behaves elastically. Duringcooling, the thermal stress is removed and the substrate returns
to its original shape
If the stress exceeds a critical values, the substrate will yield
plastically occurs due to generation and motion of dislocationswhich are atomic level line defects which glide along slip planes
of the crystal
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The passage of one dislocation offsets the material above andbelow the slip plane by a unit known as Bergers vector of thedislocations
Dislocations normally propagate from near the edge of thesubstrate (highest stress), and glide towards the centre of thesubstrate and produce plastic deformation of the substrate whichrelieves the thermal stress
Dislocation motions is slow because dislocation moves to aregion of lower shear stress
The continuous slow motion of the dislocations produces creepdeformation of the crystal
Device impact from slip normally comes from rapid pipediffusion of dopant along the core of the dislocations
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Typical wafer edge slip as a
result of excessive within
wafer temperature
gradients during heating or
during epitaxial film growth
Crystal slip
5 Microprecipitates (S pits)
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5. Microprecipitates (S-pits)
Microprecipitates may come from metallicelements such as copper, nickel, iron and
chromium This is due to their solubility in Si at high
temperatures and fast diffusion rates through
the Si The metal contaminants may exist in thestarting substrates or being pick up duringhandling in the loading operation or from metalparts or susceptors within the epitaxial reactoritself
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