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# Physics of Thin Films

## Thin Film Characterization - Optical Techniques

Ohring Chapter 6 Section 2.2

### Ellipsometry

• Use
• dielectric/optical properties of materials
• thickness of thin films
• porosity and roughness information
• can be used during deposition or after
• Samples
• solids
• metals, semiconducors, insulators, films
• liquids
• samples must be flat and reasonably specularly reflecting
• Limitations
• usually visible wavelengths (350 - 700 nm)
• some IR and near-UV ellipsometry is done
• sampling region diameter: 30 microns - 10 mm
• depth of sampling depends on absorption of light in sample
• typically 100 Å or more

• Use change of polarization of light reflected from a sample

Polarization of Light

• polarization is described by the relative amplitude and phase of the Electric field components in the direction perpendicular (s) and parallel (p) to the plane of incidence

We often use these to define reflection coefficients:

rp= Erp / Eip and rs = Ers / Eis

In ellipsometry we define two other parameters (psi and delta):

Experimental parameters

• instrument parameters
• angle of incidence
• variable angle ellipsometry
• wavelength of light
• spectroscopic ellipsometry
• material parameters
• film thickness
• film structure
• temperature

Data analysis

• ellipsometry data depends on index of refraction of ambient medium (air), film(s) and substrate and thickness of film(s).

index is generally complex (real index and absorption) N = n - ik

single measurement of one film on substrate:

• assume index of ambient and substrate are known

three unknown parameters (n, k, d of film)

two measurable parameters (psi and delta)

need to vary experimental parameters to obtain enough data to solve

Ellipsometry Models

• Most analysis uses two types of models:
• layer model (as described above)
• assume homogeneous layers can be used to describe sample
• effective medium approximations (EMA)
• describes how physical properties change the optical properties
• use for porosity, roughness

Use measured data and layer model to determine optical properties and thickness of layers.

Then use EMA to determine porosity and roughness of layers.

### Interferometry

use interference of beams of light with different optical paths to determine film thickness

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© Thomas M. Christensen