Departamento de Fsica Curso Electivo INTRODUCCIN A PELCULAS DELGADAS

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Departamento de Física - Curso ElectivoINTRODUCCI
ÓN A PELÍCULAS DELGADAS

• Física de películas delgadas
• Crecimiento de películas
• Caracterización de películas
• Propiedades de películas

Segundo Período 2003 Profesora María Elena Gómez
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Departamento de Física - Curso ElectivoINTRODUCCI
ÓN A PELÍCULAS DELGADAS

• I Física de películas delgadas
• 5. Física del Plasma

Segundo Período 2003 Profesora María Elena Gómez
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Física de películas delgadas
Departamento de Física Curso Electivo INTRODUCCIÓN
A PELÍCULAS DELGADAS

• 5. Física del plasma
• Definición de Plasma
• Electrostatics
• Electrodynamics
• Special case Dilute Plasmas
• Special Case Dense Plasmas
• Applications of plasmas
• Plasma Sources

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Crecimiento de películas
Departamento de Física Curso Electivo INTRODUCCIÓN
A PELÍCULAS DELGADAS

• References
• "Classical Electromagnetic Radiation" M. A. Heald
  and J. B. Marion
• "Foundations of Electromagnetic Theory" J. R.
  Reitz, F. J. Milford and R. W. Christy
• "Electromagnetic Fields and Waves" P. Lorrain, D.
  R. Corson, and R. Lorrain

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Plasma Fundamentals

• dilute ionized gas
• (often at high temperature)
• contains free electrons (light) and positive ions
  (heavy)
• excellent conductor with current mainly carried
  by electrons
• Usually we concentrate on the behavior of the
  electrons because they are more mobile
• Creating a plasma
• If we start with a gas of neutral atoms, we
  create a plasma by removing an electron from an
  atom, leaving a positive ion.
• Since 1 eV corresponds to about 11,600 K, it is
  typically not practical to achieve ionization by
  thermal processes. Instead we rely on electron
  collisions with atoms.
• electrons ionize by collision most effectively
  for energies around 100 eV

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Plasma Typical ionization energies
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Characterizing a plasma

• characterize by temperature (energy), electron
  density (Ne) and particle density or neutral atom
  density (N)
• cold plasma
• particle energy of a few eV
• typical of most thin film processes
• hot plasma
• particle energy of a few thousand eV
• typical of nuclear fusion and some astrophysics
• electron temperature often gt ion temperature
• - especially in dilute plasmas

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Characterizing a plasma

• density
• at pressure of 5 mTorr total particle density
  about 1013 particles/cm3
• weakly ionized ion density electron density is
  about 108 ions/cm3
• strongly ionized ion density electron density
  is about 1012 ions/cm3
• These parameters are often grouped as follows
• Debye length
• distance over which significant charge separation
  can occur
• ?D(cm) 743 (Te / Ne)1/2
• with density in electrons / cm3
• plasma frequency
• to be derived shortly
• ?p 56,548.67 Ne1/2
• with density in electrons / cm3
• critical degree of ionization
• if Ne / N is much greater than this critical
  degree of ionization, the plasma behaves as
  though it is fully ionized
• ?c 1.73 x 1012 ?eA Te2
• - where ?eA is the electron-atom collision
  cross section in cm2
• (typically 10-16 - 10-15)

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Characterizing a plasma Electrostatics

• charged particles in a constant Electric field
  (E) with no magnetic field (B 0)
• When subjected to Electric field
• charges redistribute
• themselves to shield
• the interior from the
• fields
• plasma region is field
• free and approximately
• neutral
• for T 2000 K and N 1018 electrons/meter3,
  sheath thickness is about 1 micrometer

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Characterizing a plasma Electrostatics (2)

• electrons in a constant Magnetic Field (B) with
  no electric field (E0)
• note electrons have a longer actual path length
  before reaching one side of the system gt more
  likely to ionize a neutral gas atom

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Characterizing a plasma Electrostatics (3)

• electrons in a uniform, constant E and B with E
  perpendicular to B
• motion has three components
• constant velocity vparallel in direction of B
• gyration about the B field lines
• constant drift velocity vd E/B perpendicular to
  E and B
• note vd does not
• depend on mass
• or charge - so all
• particles drift
• together

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Electrodynamics

• Apply an oscillator model to electrons in the
  plasma
• at low frequencies (lt50 kHz) ions and
  electrons both oscillate
• at high frequencies (gt50 kHz) heavy ions can
  not follow switching fields gt only electrons
  oscillate while ions are relatively stationary
• Examine forces on electrons
• driving force from varying E field
• no restoring force since electrons are not
  bound (spring constant 0) (not true if charge
  separation in plasma leads to electrostatic
  restoring forces)
• damping term, ?, from collisions (this could
  be large)
• ? collision frequency
• consider effects of electromagnetic wave on
  plasma (with no static fields)
• from F ma we can write down an equation of
  motion

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Electrodynamics equation of motion

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Electrodynamics Dilute Plasmas

• Dilute few collisions ? small (ltlt?)
• Two frequency regions
• ? gt?p dielectric domain - k real no
  attenuation of waves
• ? lt ? lt ?p evanescent domain - k imaginary
  -Waves attenuated

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Electrodynamics Dilute Plasmas

• When is a plasma dilute ?
• criteria ? ltlt ? to put numbers in ? 0.01 ?
• estimate collision frequency ? from kinetic
  theory of gasses

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Electrodynamics Dilute Plasmas

• When is a plasma dilute ?
• Most plasmas in our processes are not dilute
• reality check is the plasma frequency for these
  conditions greater than the incident frequency?

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Electrodynamics Dilute Plasmas

• reality check is the plasma frequency for these
  conditions greater than the incident frequency?

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Electrodynamics Dense Plasmas

• collisions between charged particles are common
• electrons and ions are in thermal equilibrium
• electrons and ions move together
• equilibrium theory formulation of plasmas
• particles maintain a Maxwell-Boltzmann velocity
  distribution
• kinetic properties and transport properties of
  particles can be calculated from this

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Applications of plasmas

• cleaning / etching of surfaces
• sputter deposition source
• bombardment during deposition to modify film
• activation of reactive gasses
• Plasma Sources
• plate electrodes
• Inductively coupled plasma (ICP)
• Helicon
• Electron cyclotron resonance (ECR)

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Plasma Sources

• Plate electrodes
• low plasma densities (109 - 1010 charged
  particles/cm3)
• common in sputter deposition
• discuss further during sputter deposition
• Inductively coupled plasma (ICP)
• high plasma densities (1011 - 1012 charged
  particles/cm3)
• operates well at lower gas densities (lt 50 mTorr)
  
• can be used up to atmospheric pressures (and
  beyond)
• couple RF energy inductively into plasma (lossy
  electrical conductor produces more efficient
  ionization !

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Plasma Sources

• Helicon
• high plasma densities (1011 - 1012 charged
  particles/cm3)
• operates well at lower gas densities (lt 50 mTorr)
• radiates RF energy into plasma for resonant
  absorption
• produces more efficient ionization
• Electron cyclotron resonance (ECR)
• high plasma densities (1012 - 1013 charged
  particles/cm3)
• operates well at lower gas densities (down to 0.1
  mTorr)
• couples microwave energy to electrons by matching
  frequency to electron gyration frequency
• ?c eB/me produces more efficient
  ionization
• control the plasma density with microwave power
  and gas pressure
• can also control ion species created (O2, O)

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Plasma Sources
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