Physics of Thin Films

Thin Film Characterization - Structural Techniques

Ohring Chapter 6 Section 2 and 3.4

Crystalline Structure: Diffraction Techniques

wavelength of X-rays is around 1 Å => can diffract from crystal lattices

Uses
- determine crystal structure and lattice spacings
- identify phases present
- measure crystal defects
Samples
- must by crystalline
- need a minimum of about 0.001 gram
- sampling depth is 10 - 100 µm
Diffraction (reflection): constructive interference only occurs at some incident angles
- l = 2d sin q
  We know the wavelength, we measure the angle at which we see diffraction and find the lattice spacing.
  
  $diffraction of reflected beam$
Diffraction of transmitted beam
- note: get diffraction from many sets of planes as you rotate
  - => complicated sets of peaks
X-rays penetrate many microns into sample. => hard to see thin films (mainly see substrate)
- Use glancing angle primary beam without sample rotation for thin films.

see handouts

see handouts

electro-magnetic sensors detect the vertical motion of the stylus as it is moved horizontally across the sample.

measure

film thickness (step height)
- changes of 200 Å to 65 µm
- vertical resolution of about 10 Å
roughness
- horizontal resolution depends on tip radius
problem: stylus penetrates soft films

use piezoelectric properties of quartz crystal

put electrodes on thin (0.3 mm) disk of quartz
alternating voltage deforms crystal periodically (vibrates)
resonant vibration frequency between 5 and 6 MHz
deposition of film on crystal shifts frequency to lower values
relate film thickness to frequency shift IF film density is known
crystal can be used for up to about 50,000 Å of material
caution: stress and heat can also change vibration frequency