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Physics of Thin Films

PES 449 / PHYS 549


Thin Film Characterization - Chemical Techniques

 Ohring Chapter 6 Section 4


Auger Electron Spectroscopy (AES)

  • Uses
    • elemental composition (qualitative, quantitative)
    • some chemical state information
    • element mapping with Scanning Auger
    • surface sensitive (30Å)
    • depth profiling using Ar ion sputtering
  • Samples
    • conducting solids
      • metals, semiconductors, films
    • samples must be clean
    • no high vapor pressure materials
  • Limitations
    • does not detect H or He
    • detection limit about 1 at %
    • sampling region diameter: 100 Å - 1 mm
    • requires ultra-high vacuum
    • beam damage on some samples

    Auger electron emission process

    • Auger electron emission process

       

    Analyze the energies of the electrons emitted in the process

    • N(E) vs E graph

      note how Auger electrons are difficult to see

      small peaks on large background

      => take derivative of spectrum to accentuate the peaks

      Auger electron spectrum of stainless steel.

      (figure from http://www.mee-inc.com/sam.html)

    Analysis of Auger Electron spectra

    • Qualitative:
      • elements have peaks at specific energies

        look for "fingerprint" of each element

        note Cr, Fe, and Ni in stainless steel spectrum

      Quantitative:

      One relatively simple method is to use the major peak of each element which we identified.

      measure the peak to peak heights off of the figure using a ruler. The Cr data is distorted by overlappinig with the oxygen peak.

      need to know the relative sensitivities of Auger to each element which are available in graphs and tables.

      element
      height (mm)
      sensitivity
      Fe
      65
      0.21
      Cr
      30
      0.32
      Ni
      10
      0.27

      correct all of the heights for the instrument sensitivity factors.

      The concentrations of each element present in the sample can be found from:

      equation for composition

      The denominator is simply (65/0.21) + (30/0.32) + (10)/0.27) = 440.

      We now find the corrected concentrations for each element:

      Fe:

      (65/0.21) / 440 = 0.70

      Cr:

      (30/0.32) / 440 = 0.21

      Ni:

      (10/0.27) / 440 = 0.08

      We can compare these to the actual bulk concentrations which were: 0.702, 0.205, and 0.093.

      Analysis neglects

      • matrix effects of the host solid
        • variations in electron mfp, backscattering
      • changes in peak shape
      • surface roughness

    Depth Profiling

    • Use Ar ion sputtering to knock off material from surface.

      Do AES analysis at different depths.

    AES References

    • G. Ertl and J. Kuppers, "Low Energy Electrons and Surface Chemistry", (Verlag Chemie, Weinheim, 1974).
    • C. C. Chang, Surface Science 25, 53 (1971).
    • A. Joshi, L. E. Davis, and P. W. Palmberg, in "Methods of Surface Analysis" , A. W. Czanderna, ed. (Elsevier Scientific Publishing Co., Amsterdam, 1975) Ch. 5.
    • R. Weissmann and K. Muller, Surface Science Reports 1, 251 (1981).
    • L. Davis et al. , "Handbook of Auger Electron Spectroscopy" (Physical Electronics, Eden Prairie, MN, 1976).

    AES Web pages:


Energy Dispersive X-ray Analysis (EDAX, EDX)

  • Use:
    • elemental analysis of regions of micron dimensions
  • Samples:
    • most solids (non-conducting sample must be coated with conducting layer)

     

  • usually an attachment to SEM
  • electron beam excites X-rays which have energies which are characteristic of the element
    • energy analyze the X-rays
    • typical resolution of 150 eV can lead to peak overlap
    • quantitative analysis possible with computer
  • good spatial resolution for mapping
  • units that use Be windows can not analyze elements with atomic number at or below Na


Wavelength Dispersive X-ray Analysis (WDX)

Electron Microprobe

see EDAX (above)
  • attachement to SEM
  • different detector for X-ray analysis
    • better resolution of peaks
    • slower speed


X-ray Photoelectron Spectroscopy (XPS, ESCA)

  • Use:
    • elemental composition (qualitative, quantitative)
    • chemical state information
    • element mapping available with scanning XPS (poor resolution)
    • surface sensitive (30 Å)
    • depth profiling of surface region (100 Å) by angle resolved XPS
    • depth profiling using Ar ion sputtering
  • Samples:
    • solids
      • metals, semiconductors, some insulators
    • samples must be clean
    • no high vapor pressure materials
  • Limitations:
    • does not detect H or He
    • detection limit of about 1 at %
    • sampling region diameter: 30 microns - 10 mm
    • requires ultra-high vacuum
    • beam damage on some samples

    X-ray Photoelectron process

    • X-ray incident on material with energy = hn
    • Excites electrons to be ejected from atoms
    • Electrons close enough to surface (30Å) can escape from the material
    • electron kinetic energy = hn - binding energy
    • (typically plot data vs. binding energy rather than kinetic energy)

    Chemical State Information

    • small differences in binding energy can be detected
    • allows identification of different chemical states
      • elemental Si has a different binding energy than Si in SiO2
      • C bonded to C has a different binding energy than C double bonded to O
    • curve fitting of data - see handout on polymers

    Angle resolved XPS

    • depth profiling of near surface region can be done by rotating the sample

      limited mean free path changes depth from which photoelectrons are detected

      angle resolved XPS figure

    Web references:


Secondary Ion Mass Spectroscopy (SIMS)

  • Use:
    • elemental composition (qualitative / quantitative)
    • isotopic ratio information
    • surface sensitive (30 Å)
    • depth profile using ion sputtering
  • Samples:
    • solids
      • metals, semiconductors, insulators, films
    • samples must be clean
    • no high vapor pressure materials
  • Limitations:
    • detection limit about 1 ppb (varies with element)
    • sampling region diameter: 500 Å - 2 mm
    • requires ultra-high vacuum
    • ion impact damage

    SIMS schematic

    analyze the positive and/or negative ions

    plot number of ions vs. mass/charge ratio (need sensitivity factors for quantitative analysis)

    Web references:


Rutherford Backscattering Spectroscopy (RBS)

  • Use:
    • elemental composition depth profile (one micron deep)
    • film thickness
  • Samples:
    • solids (minimum sixe about 1 mm2
    • any non-volatile material
  • Limitations:
    • large beam size (1 mm) - poor lateral resolution
    • depth resolution > 20 Å
    • weak signal for low atomic number elements
    • poor mass resolution for high atomic number elements

      RBS schematic

      detect incident ions scattered from atoms in material

      Energy of scattered ions depends on

      • element (mass)
      • angle
      • location in solid

      RBS data for PtSi on Si

      from graph:

      • height --> concentration
      • width --> layer thickness
      • absolute energy value --> element and depth

    Web references:


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