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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

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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)		backscattered and secondary electrons	Scanning Electron Microscopy
moderate energy electrons (5 keV)		secondary electrons	Auger Electron Spectroscopy
light (X-rays - 1 keV)		secondary electrons	X-ray Photoelectron Spectroscopy
low energy electrons (100 eV)		diffracted electrons	Low Energy Electron Diffraction
moderate energy electrons (5 keV)		diffracted electrons	Reflection High Energy Electron Diffraction
moderate-high energy ions (2-30 keV)		secondary ions	Secondary Ion Mass Spectrometry
polarized light (2 eV)		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

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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