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

• II Crecimiento de películas
• Vacuum and kinetic theory of gases
• Evaporation
• Sputtern deposition
• Cathodic arc deposition
• Molecular beam epitaxy
• Chemical vapor deposition

Segundo Período 2003 Profesora María Elena Gómez
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1. Vacuum and kinetic theory of gases
Departamento de Física Curso Electivo INTRODUCCIÓN
A PELÍCULAS DELGADAS

• Pressure and Vacuum
• Pressure Conversion Calculator
• Ideal Gas Law
• Kinetic Theory of Gasses - Gas Flow
• Kinetic Theory of Gasses - Interactions with
  surface
• Vapor Pressure

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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
• Composition of gas in vacuum chamber is very
  different from atmosphere
• pumps remove certain gasses preferentially

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Pressure and Vacuum
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Pressure and Vacuum
Composition of gas in vacuum chamber is very
different from atmosphere
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Ideal Gas Law

• much of vacuum technology can be understood from
  the ideal gas law
• the equation of state of an ideal gas
• Where P absolute pressure V volume N
  number of gas molecules kB 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 ?

• 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 ?

• m masa of the
• molecule
• Not surprising The hotter it is, the faster
  they move - The lighter they are, the faster they
  move
• How far does a molecule travel before it collides
  with another molecule ?
• ? mean free path d diameter of a molecule
• n number per unit volume

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Kinetic Theory of Gasses - Gas Flow

• mean free path
• For air at room temperature

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Gas Flow

• three regimes
• viscous flow
• intermediate (transition) flow
• molecular flow
• 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)

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Gas Flow (2)

• 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 ?
• ? collision rate of gas molecules
• ? 0.25 n vrms , where n number of molecules
  per unit volume vrms average velocity of a gas
  molecule
• In terms of magnitudes we can directly measure
• 
  M is the
  molecular
• 
  weight of the gas
• 
  molecule
• 
  
• Example chamber with nitrogen (N2),at room
  temperature (293 K) and a pressure of 1 x 10-7
  torr molecular weight M 28
• ? 3.88 x 1013 molecules/cm2-sec

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Interactions with surface (2)

• 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

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Vapor Pressure

• in equilibrium, a certain pressure ot atoms
  (vapor pressure) will exist above solid surfaces
• If you heat
• Zn to 200 C
• (476 K) the
• vapor
• pressure of
• it is 6 x 10-6
• torr

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