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About the Spectroscopic Ellipsometry
About the Spectroscopic Ellipsometry
Spectroscopic Ellipsometry
Optical Metrology Basics
Transmission and reflection intensity
measurements acquire a single quantity at each wavelength (Intensity ratio). Fluctuations in lamp intensity or not collecting all of the beam can introduce error in the measurement.
Ellipsometry
measures the change in polarization state of light reflected from the surface of a sample. The measured values are expressed as psi and delta. These values are related to the ratio of Fresnel reflection coefficients, Rp and Rs for p- and s- polarized light, respectively. Because ellipsometry measures the ratio of two values, it can be highly accurate and very reproducible. The ratio is a complex number, thus it contains ��phase?information, delta, which makes the measurement very sensitive.
Ellipsometry Advantages
1. Measures a ratio of two values
Highly accurate and reproducible (even in low light levels)
No reference sample necessary
Not as susceptible to scatter, lamp or purge fluctuations
2. Measures a "phase"
Increased sensitivity, especially to ultrathin films (<10nm)
Provides TWO values at each wavelength (more information about sample)
3. Spectroscopic Ellipsometry (SE)
More information-more film properties
Data at wavelengths of interest (157nm, 193nm, 248nm, 1550nm, etc.)
4. Variable Angle Spectroscopic Ellipsometry (VASE)
New information (different path length), optimize sensitivi
SE Data Analysis Flow Chart
The optical constants define how light interacts with a material.
The real part or index of refraction, n , defines the phase velocity of light in material:
n = c / v ,
where n is the speed of light in the material and c is the speed of light in vacuum.
The imaginary part or extinction coefficient, k , is directly related to the absorption of a material.It is related to the absorption coefficient by :
a = 4p k /l,
where a is the absorption coefficient and l is the wavelength of light.
Optical constants vary with wavelength and temperature.
Optical constants determine reflected/transmitted intensities, phase change, and angle change.
Ellipsometry is Sensitive To ...
Motivation for VUV Ellipsometer : Need accurate optical constants at 157nm for candidate lithography thin films.
Layer thickness (single or multilayer)
Optical constants
Surface and interfacial roughness
Composition
Optical anisotropy/birefringence
Process conditions that change optical constants
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