Vapor Deposition VD

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• Vapor Deposition (VD)
• Vapor deposition refers to any process in which
  materials in a vapor state are condensed through
  condensation, chemical reaction, or conversion to
  form a solid material. These processes are used
  to form coatings to alter the mechanical,
  electrical, thermal, optical, corrosion
  resistance, and wear properties of the
  substrates. They are also used to form
  free-standing bodies, films, and fibers and to
  infiltrate fabric to form composite materials.
  Vapor deposition processes usually take place
  within a vacuum chamber. There are two categories
  of vapor deposition processes
• Physical vapor deposition (PVD)
• Chemical vapor deposition (CVD)
• In PVD processes, the workpiece is subjected to
  plasma bombardment. In CVD processes, thermal
  energy heats the gases in the coating chamber and
  drives the deposition reaction.

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• Physical Vapor Deposition (PVD)
• Physical vapor deposition methods are clean, dry
  vacuum deposition methods in which the coating is
  deposited over the entire object simultaneously,
  rather than in localized areas. All reactive PVD
  hard coating processes combine
• A method for depositing the metal
• Combination with an active gas, such as nitrogen,
  oxygen, or methane
• Plasma bombardment of the substrate to ensure a
  dense, hard coating.
• PVD methods differ in the means for producing the
  metal vapor and the details of plasma creation.
  The primary PVD methods are ion plating, ion
  implantation, sputtering, and laser surface
  alloying.

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• Chemical Vapor Deposition (CVD)
• CVD is a widely used method for depositing thin
  films of a large variety of materials.
  Applications of CVD range from the fabrication of
  microelectronic devices to the deposition of
  protective coatings. In a typical CVD process,
  reactant gases (often diluted in a carrier gas)
  at room temperature enter the reaction chamber.
  The gas mixture is heated as it approaches the
  deposition surface, heated radiatively or placed
  upon a heated substrate. Depending on the process
  and operating conditions, the reactant gases may
  undergo homogeneous chemical reactions in the
  vapor phase before striking the surface. There is
  a great variety of chemical vapor deposition
  processes such as
• atmospheric pressure chemical vapor deposition
  (APCVD) low pressure chemical vapor deposition
  (LPCVD) plasma assisted (enhanced) chemical
  vapor deposition (PACVD, PECVD) PECVD Process -
  Institute for Semiconductor Electronics
  photochemical vapor deposition (PCVD) laser
  chemical vapor deposition (LCVD) metal-organic
  chemical vapor deposition (MOCVD) MOCVD
  Definitions - MOCVD.com chemical beam epitaxy
  (CBE) chemical vapor infiltration (CVI)

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• The PVD process advantages versus the CVD
  process
• The PVD process is conducted at lower
  temperatures (180oC to 500oC). The low PVD
  processing temperatures mean that nearly all tool
  materials can be coated without concern for
  softening or distortion.
• The PVD process does not use any hazardous
  materials which makes the process environmentally
  friendly.
• The PVD process is highly energy  efficient.
• The high degree of ion energy ensures excellent
  adhesion.

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The Cathodic Arc Vapor Deposition

• Physical Vapor Deposition (PVD)
• The PVD process is defined as the creation of
  vapors in a vacuum from solid material sources
  and their subsequent condensation onto a
  substrate. The Cathodic Arc Vapor Deposition
  method, in this method the source metal is
  simultaneously evaporated in microscopically
  small areas and the vapor particles are ionized
  and accelerated all in one single work stage.

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CVD
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Sputtering
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• Pulsed-laser deposition (PLD) has gained a great
  deal of attention in the past few years for its
  ease of use and success in depositing materials
  of complex stoichiometry. PLD was the first
  technique used to successfully deposit a
  superconducting YBa2Cu3O7-d thin film. Since that
  time, many materials that are normally difficult
  to deposit by other methods, especially
  multi-element oxides, have been successfully
  deposited by PLD.
• The main advantage of PLD derives from the laser
  material removal mechanism PLD relies on a
  photon interaction to create an ejected plume of
  material from any target. The vapor (plume) is
  collected on a substrate placed a short distance
  from the target. Though the actual physical
  processes of material removal are quite complex,
  one can consider the ejection of material to
  occur due to rapid explosion of the target
  surface due to superheating. Unlike thermal
  evaporation, which produces a vapor composition
  dependent on the vapor pressures of elements in
  the target material, the laser-induced expulsion
  produces a plume of material with stoichiometry
  similar to the target. It is generally easier to
  obtain the desired film stoichiometry for
  multi-element materials using PLD than with other
  deposition technologies.

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• Main Advantages of PLDThe main advantages of
  Pulsed Laser Deposition are
• conceptually simple a laser beam vaporizes a
  target surface, producing a film with the same
  composition as the target.
• versatile many materials can be deposited in a
  wide variety of gases over a broad range of gas
  pressures.
• cost-effective one laser can serve many vacuum
  systems.
• fast high quality samples can be grown reliably
  in 10 or 15 minutes.
• scalable as complex oxides move toward volume
  production.

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• Mechanisms of PLDThe principle of pulsed laser
  deposition, in contrast to the simplicity of the
  system set-up, is a very complex physical
  phenomenon. It does not only involve the physical
  process of the laser-material interaction of the
  impact of high-power pulsed radiation on solid
  target, but also the formation plasma plume with
  high energetic species and even the transfer of
  the ablated material through the plasma plume
  onto the heated substrate surface. Thus the thin
  film formation process in PLD generally can be
  divided into the following four stages. 1. Laser
  radiation interaction with the target 2. Dynamic
  of the ablation materials 3. Deposition of the
  ablation materials with the substrate 4.
  Nucleation and growth of a thin film on the
  substrate surface
• Each stage in PLD is critical to the formation of
  quality epitaxial crystalline, stoichiometric,
  uniform and small surface roughness thin film.

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• In the first stage, the laser beam is focused
  onto the surface of the target. At sufficiently
  high flux densities and short pulse duration, all
  elements in the target are rapidly heated up to
  their evaporation temperature. Materials are
  dissociated from the target surface and ablated
  out with stoichiometry as in the target. The
  instantaneous ablation rate is highly dependent
  on the fluences of the laser shining on the
  target. The ablation mechanisms involve many
  complex physical phenomena such as collisional,
  thermal, and electronic excitation, exfoliation
  and hydrodynamics.
• During the second stage the emitted materials
  tend to move towards the substrate according to
  the laws of gas-dynamic and show the forward
  peaking phenomenon. R. K. Singh reported that the
  spatial thickness varied as a function of cos q.
  The spot size of the laser and the plasma
  temperature have significant effects on the
  deposited film uniformity. The target-to-substrate
  distance is another parameter that governs the
  angular spread of the ablated materials. Hanabusa
  also found that a mask placed close to the
  substrate could reduce the spreading.
• The third stage is important to determine the
  quality of thin film. The ejected high-energy
  species impinge onto the substrate surface and
  may induce various type of damage to the
  substrate. The mechanism of the interaction is
  illustrated in the following figure. These
  energetic species sputter some of the surface
  atoms and a collision region is formed between
  the incident flow and the sputtered atoms. Film
  grows after a thermalized region is formed. The
  region serves as a source for condensation of
  particles. When the condensation rate is higher
  than the rate of particles supplied by the
  sputtering, thermal equilibrium condition can be
  reached quickly and film grows on the substrate
  surface at the expenses of the direct flow of the
  ablation particles and the thermal equilibrium
  obtained.

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• Nucleation-and-growth of crystalline films
  depends on many factors such as the density,
  energy, ionization degree, and the type of the
  condensing material, as well as the temperature
  and the physico-chemical properties of the
  substrate. The two main thermodynamic parameters
  for the growth mechanism are the substrate
  temperature T and the supersaturation Dm. They
  can be related by the following equationDm kT
  ln(R/Re)where k is the Boltzmann constant, R is
  the actual deposition rate, and Re is the
  equilibrium value at the temperature T.The
  nucleation process depends on the interfacial
  energies between the three phases present -
  substrate, the condensing material and the vapor.
  The minimum-energy shape of a nucleus is like a
  cap. The critical size of the nucleus depending
  on the driving force, i.e. the deposition rate
  and the substrate temperature. For the large
  nuclei, a characteristic of small
  supersaturation, they create isolate patches
  (islands) of the film on the substrate which
  subsequently grow and coalesce together. As the
  supersaturation increases, the critical nucleus
  shrinks until its height reaches on atomic
  diameter and its shape is that of a
  two-dimensional layer. For large supersaturation,
  the layer-by-layer nucleation will happen for
  incompletely wetted foreign substrates.The
  crystalline film growth depends on the surface
  mobility of the adatom (vapour atoms). Normally,
  the adatom will diffuse through several atomic
  distances before sticking to a stable position
  within the newly formed film. The surface
  temperature of the substrate determines the
  adatom's surface diffusion ability. High
  temperature favours rapid and defect free crystal
  growth, whereas low temperature or large
  supersaturation crystal growth may be overwhelmed
  by energetic particle impingement, resulting in
  disordered or even amorphous structures.Metev and
  Veiko suggested that the N99, the mean thickness
  at which the growing, thin and discontinuous film
  reaches continuity is given by the formulaN99
  A(1/R)1/3 exp (-1/T),where R is the deposition
  rate (supersaturation related) and T is the
  temperature of the substrate and A is a constant
  related to the materials.In the PLD process, due
  to the short laser pulsed duration (10 ns) and
  hence the small temporal spread (lt10 ms) of the
  ablated materials, the deposition rate can be
  enormous (10 mm/s). Consequently a
  layer-by-layer nucleation is favoured and
  ultra-thin and smooth film can be produced. In
  addition the rapid deposition of the energetic
  ablation species helps to raise the substrate
  surface temperature. In this respect PLD tends to
  demand a lower substrate temperature for
  crystalline film growth.Last edited04/28/2001

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Typical steps in making thin films emission of
particles from source ( heat, high voltage . .
.) transport of particles to substrate (free vs.
directed) condensation of particles on substrate
(how do they condense ?)   Simple model
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• Kinetic Theory of Gasses
• Pressure and Vacuum
• Many thin film processes involve vacuum.
• "vacuum" lower molecular density than in our
  atmosphere
• results in a lower pressure of gas - so typically
  measure this
• MANY different units are commonly used.
• Ideal Gas Law
• much of vacuum technology can be understood from
  the ideal gas law
• more correctly the equation of state of an ideal
  gas
• PV NkT
• where
• P absolute pressure
• V volume
• N number of gas molecules
• k Boltzmann's constant
• T gas temperature (in K)

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• Kinetic Theory of Gasses - Gas Flow
• Assumptions
• Gasses are composed of a very large number of
  very small particles.
• "very small" gt very small compared to the
  distance between particles
• Particles are always moving rapidly in a straight
  line.
• Particles exert no forces except during
  collisions.
• Freeze other molecules and examine motion of one
  molecule

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What is the distribution of velocities
? determine most properties from this Maxwell
velocity distribution
higher T shifts curve to right broadens and
lowers it lighter mass shifts curve to right
broadens and lowers it
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• How fast are the molecules moving ?
•  
• k Boltzmann's constant
• T temperature of the gas (K)
• m mass of the molecule
•  
• Not surprising
• The hotter it is, the faster they move.
• The lighter they are, the faster they move.

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•  At room temperature

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• How far does a molecule travel before it collides
  with another molecule ?
• l mean free path
• d diameter of a molecule
• n number per unit volume
• For air at room temperature, the mean free path
  can be expressed as

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• Gas Flow
• three regimes
• viscous flow
• mean free path ltlt size of the system (D)
• gas - gas collisions dominate
• molecules "drag" one another along in the flow
• when D(cm) P (Torr) gt 0.5
• for air at room temperature
• intermediate (transition) flow
• mean free path comparable to size of system (D)
• complicated flow
• molecular flow
• mean free path gtgt size of system
• gas - wall collisions dominate
• molecules move independently of one another
• when D(cm) P (Torr) lt 0.005
• for air at room temperature

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• Kinetic Theory of Gasses - Interactions with
  surface
• How many gas molecules collide with a surface
  each second ?
• F 0.25 n vrms
• F collision rate of gas molecules
• n number of molecules per unit volume
• vrms average velocity of a gas molecule
• In terms of things we can directly measure
• F will be in molecules/ cm2 - sec
• P is the pressure in torr
• M is the molecular weight of the gas molecule
• T is the temperature in K
• For example
• Nitrogen (N2) has a molecular weight M 28. If
  we have a chamber with nitrogen at room
  temperature (293 K) and a pressure of
• 1 x 10-7 torr
• F 3.88 x 1013 molecules/cm2 - sec

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• How long does it take to form a single complete
  layer of gas on a surface ?
• tm time to form a monolayer (in seconds)
• n number of molecules per unit volume
• vrms average velocity of the molecules
• d diameter of a molecule
• For air at room temperature, we can express this
  as
• tm 1.86 x 10-6 / P
• where P is the pressure in torr.

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• Vapor Pressure
• in equilibrium, a certain pressure ot atoms
  (vapor pressure) will exist above solid surfaces

Do not make high vacuum chambers out of Zinc. If
you heat it to 200 C (476 K) the vapor pressure
of Zn is 6 x 10-6 torr.
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• Physical Vapor Deposition
• Evaporation
• Ion Plating
• Sputter Deposition
• Process
• source material -gt gaseous state
• transport source atoms to substrate
• deposit atoms on substrate

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Evaporation Overview
1. Atoms to gas state heat source until Pvapor gt
10-4 torr some sources sublime from solid, others
evaporate from liquid compounds may break apart
produce films with different stoichiometry SiO2
--gt SiO2-x metal alloy sources do not give same
alloy in film components evaporate independently
based on each separate vapor pressure could try
to adjust source composition. BUT composition of
alloy source changes with time
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Describe evaporation rate (flux) from kinetic
theory

• where
• Pvap vapor pressure (Torr)
• M molecular weight
• cm2 gt area of source
• can convert this to mass flux

at Pvap 10-2 torr, mass flux 10-4 grams/cm2
sec
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• 2. Transport to surface
• line of sight deposition
• want to avoid collisions in gas
• long mean free path
• good vacuum
• let h source to substrate distance
• for h of 10 - 100 cm, want P lt 10-5 torr
• bigger h gt lower P
• Particles have energies comparable to evaporation
  temperature
• 1000 C is about 0.2 eV
• distribution of evaporant
• depends on geometry of source
• consider 2 geometries
• Point Source

q tilt of dAS from radial direction projection
of dAS onto sphere of radius r dAScosq dMS
mass hitting dAS Me total
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distribution depends on r and q
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• Surface Source
• For many materials, this is equivalent to Knudsen
  cell

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• if directions are random, only dAS cosq / 4¹r2
  are headed in right direction
• integrate over time and source
• now distribution depends on horizontal position
  as well
• Experimentally observe

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3. Deposition onto substrate Consider film
thickness and purity THICKNESS since dM/dAs
depends on r, q, f, so does film thickness (d)
consider flat substrate, perpendicular to source
surface source
surface source has slightly poorer thickness
uniformity
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• better uniformity
• decrease sample size (l)
• increase distance to substrate (h)
• need bigger chamber
• need better vacuum
• wastes evaporant
• use multiple sources
• move substrate during deposition
• use rotating mask to reduce evaporant near center

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• FILM PURITY
• PROBLEM contamination from source materials
• SOLUTION use pure materials (99.99999)
• PROBLEM contamination from source or substrate
  heaters
• SOLUTION use materials with low diffusion
• see tables of crucibles for each material
• online tables
• http//www.lesker.com/mfiles/m_tech_deposition_tec
  hniques.html

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SOLUTION better vacuum higher deposition
rate note P and Tg are not independent