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Physics of Thin Films

PES 449 / PHYS 549


Thin Film Characterization - Overview

 Ohring Chapter 6 Section 1


What do we want to know ?

How do we find this out ?

What does the sample look like ?

  • on a macroscopic scale
  • on a microscopic scale
  • on an atomic scale
  • optical microscopy
  • scanning electron microscopy (SEM)
  • transmission electron microscopy (TEM)
  • scanning probe microscopies (STM, AFM ...)

What is the structure of the sample ?

  • internal structure
  • density
  • microscopic and atomic scales
  • X-ray diffraction (XRD)
  • stylus profilometry
  • quartz crystal monitors (QCM)
  • ellipsometry
  • low energy electron diffraction (LEED)
  • reflection high energy electron diffraction (RHEED)

What is the sample made of ?

  • elemental composition
  • impurities
  • chemical states
  • Auger Electron Spectroscopy (AES)
  • Energy Dispersive Analysis of X-rays (EDAX)
  • X-ray Photoelectron Spectroscopy (XPS)
  • Secondary Ion Mass Spectrometry (SIMS)
  • Rutherford Backscattering (RBS)

What are the optical properties of the sample ?

  • refractive index, absorption
  • dielectric properties
  • as a function of wavelength
  • ellipsometry

What are the electrical properties of the sample?

  • device properties - not covered here
  • material properties
    • resistance / conductance
    • capacitance
  • resistance - four point probe
  • capacitance

What are the magnetic properties of the sample ?

  • hysteresis loops
  • magneto-optical Kerr effect (MOKE)
  • ferromagnetic resonance (FMR)

What are the mechanical properties of the sample ?

  • internal stress in films / substrates
  • friction
  • adhesion
  • stress curvature measurements
  • pin on disk friction test
  • adhesion tests


Characterization fundamentals

Available probes
  • light (electromagnetic radiation)
  • electrons
  • ions (and nuclei and protons)
  • neutrons
  • neutral atoms
  • "touching" the sample = forces

Need to develop techniques using these probes that answer the questions asked above.

IN

DETECT
Technique

high energy electrons (30 keV)

SEM drawing

backscattered and secondary electrons

Scanning Electron Microscopy

moderate energy electrons (5 keV)

AES drawing

secondary electrons

Auger Electron Spectroscopy

light (X-rays - 1 keV)

XPS drawing

secondary electrons

X-ray Photoelectron Spectroscopy

low energy electrons (100 eV)

LEED drawing

diffracted electrons

Low Energy Electron Diffraction

moderate energy electrons (5 keV)

RHEED drawing

diffracted electrons

Reflection High Energy Electron Diffraction

moderate-high energy ions (2-30 keV)

SIMS drawing

secondary ions

Secondary Ion Mass Spectrometry

polarized light (2 eV)

ellipsometry drawing

polarized light

ellipsometry

Two general types of techniques

  • "counting" techniques
    • How much ? or How many ?
      • intensity
      • force
      • numbers of particles
      • polarization
  • "spectroscopy" techniques
    • Distribution by
      • energy
      • wavelength, frequency
      • mass
      • position
    • need to consider instrument sensitivity variations


Surface vs. Bulk properties

CAUTION: Surface (top 5-10 atomic layers) properties are often different than bulk properties.

Know what you want to measure and what you are measuring !!

Why are surfaces different from the bulk ?

  • Bonding and mechanical properties
    • surface atoms have no bonds to atoms above them
    • can get surface reconstructions
  • Electrons bounce off the surface as well as off other atoms
    • electrons travel 10's to 100's of Ångstroms before scattering off an atom
    • if close to surface, may scatter sooner from surface
    • changes electrical properties in surface region
  • Chemistry can be different at surface
    • atoms from bulk may segregate to surface to minimize free energy
    • impurities from outside environment may be present


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Thomas M. Christensen