Substrate Preparation Techniques

1
Substrate Preparation Techniques

• Lecture 7
• G.J. Mankey
• gmankey_at_mint.ua.edu

2
Types of Substrates

• Insulating and Conducting
• Amorphous and Single Crystal
• Native Oxide or Passivated
• Cleaved or Cut
• Mechanical Polished or Electropolished

3
Characterization of Substrates

• Optical Microscopy
• Atomic Force Microscopy
• Spectroscopic Ellipsometry
• Electron Diffraction
• Auger or X-ray Photoelectron Spectroscopy

4
Surfaces and Solvents

• Never touch the substrate surface without first
  determining that it must be cleaned or polished
  before use.
• If a solvent wash is necessary verify the purity
  of the solvent before use.
• Do not allow the solvent to form droplets and dry
  up on the surface--they will redeposit dirt on
  the surface.
• Remember--the best solvents always have dirt
  dissolved in them, so use them sparingly.

5
Surface Polishing

• For most applications, a surface with a mirror
  finish is best.
• Mechanical polishing should be performed with
  progressively finer diamond or alumina polishing
  compound.
• The substrate should be thoroughly washed between
  steps to avoid contamination the polishing
  compounds.
• Leave the polishing area cleaner than when you
  found it.

6
Electrochemical Polishing

• Refer to Tegart, Electrochemical Polishing
  Techniques for the proper chemicals and voltages.
• Carefully monitor the surface quality with a
  microscope at small time intervals.
• Test the polarities and voltages by polishing a
  small amount of a test material first.
• Always label the electropolishing mixture with
  the chemical formula and amounts of materials.

7
Passivated Surface

• A layer of adsorbate material on the surface that
  limits further oxidation or corrosion is called
  the passivation layer.
• The layer can be amorphous as in the case of
  SiOx on Si or crystalline as in the case of H-Si.
• If there are defects in the passivation layer, it
  will be less resistant to corrosion and it will
  degrade with time.

SiOx
H-Si
8
Special Techniques

• Irradiation with UV light for a day or two to
  "break up" hydrocarbon contaminants.
• CO2 "snow" a jet of frozen particles gently
  bombard the surface to remove contaminants.
• Plasma etching can be used to roughen a surface
  in a controlled manner.

9
Single Crystal Surfaces

• For thin film deposition only the ordering of the
  topmost layers matters.
• A layer of adsorbed gas or oxide on a single
  crystal generally prevents epitaxial
  growth--there are few exceptions to this rule.
• Oxide layers on metals usually cannot be removed
  by annealing alone.
• On Si the oxide must be heated to above 800ºC to
  remove it.
• In situ processing, Ar bombardment and/or
  annealing, is usually required to make a good
  clean single crystal surface.

10
Ar Bombardment and Annealing

• Bombarding the surface with an energetic beam
  (500-5000 eV) of Noble gas ions removes
  contaminated surface layers.
• A typical current density of a few microamps per
  square centimeter will etch a few atomic layers
  per minute.
• Some material is redeposited, so the efficiency
  of removing contaminant layers is reduced.
• Cycles of bombardment and annealing to about 2/3
  of the melting point produce a smooth, ordered
  surface.

11
Optical Microscopy

• Use the optical microscope to check for
  scratches, spots and dirt.
• Use polarized light with the analyzer adjusted to
  close to extinction to detect small particles and
  imperfections.
• Vary the color of the incident light with the
  filters to highlight different features.

12
Atomic Force Microscopy

• Characterize the substrate roughness before and
  after each process step.
• Tapping mode is generally the best method for
  this task.
• A good substrate should have a root mean square
  roughness near the limit of detection (1 nm).
• Take scans at different scan sizes to fully
  characterize roughness scale (0.1, 1, 10, 40
  micrometer).

13
Spectroscopic Ellipsometry

• For future determination of film thickness, you
  must fully characterize the substrate optical
  properties first.
• Take data in the entire wavelength range (240 -
  1100nm) to insure future applicability of
  material optical constants.
• Compare a few substrates produced with the same
  process to identify possible variations.

14
Electron Diffraction

• RHEED and LEED probe only the topmost layers.
• An electron diffraction pattern indicates the
  surface has a periodic atomic arrangement.
• The appearance of a diffraction pattern does not
  guarantee good crystallinity.
• Analyze the diffraction pattern according to spot
  to background intensity ratio, dependence of peak
  width on energy, and dependence of peak intensity
  on diffraction conditions.

15
Auger / XPS

• Characterize the chemical composition of
  surfaces.
• Always perform a high statistics scan of the
  energies for C, N, and O-- these are the main
  contaminants of most surfaces.
• Depth profiling using Ar bombardment can be used
  to identify the location of contaminants--surface
  or bulk?