膜厚测试(反射与椭偏)

Reflection/Refraction of Light
Complex Index of Refraction
The complex index of refraction (N) is a combination of a real and imaginary number
N = n – jk
Single Wavelength Ellipsometer
SWE can measure very thin layer – a few tens of Angs thick. For in-situ analysis, this technique can measure a fraction of a monolayer of adsorbed gas. Both SWE and Reflectometery excel when the sample is a single layer on a substrate. One of two layers can be analyzed if all of the information is available for the other layer. In general, these two techniques suffer when the material is not well understood (i.e., a material with unknown composition) or when multiple layers are involved.
Spectroscopic Ellipsometer
• A broad-band light source (UV-VIS-NIR) is used • Angle of incidence is varied. • Delta & Psi Vs wavelength are measured – Values for n & k as function of λ are determined
Advantages of Reflectometry
High speed of measurements Precision measurements Smaller spot size possible These factors make “reflectometer” a desirable tool for the large number of measurements required for statistical process control on a patterned wafer. Non-contact, precise method needed for advanced critical thickness measurements
Reflectometer (Single Beam)
Principle of Operation
Simple single beam reflectometer without and with video imaging feature
Reflectometry Reflectance Measurements
Source Book
ISBN: 0-471-18172-2 Wiley & Sons, 1999
Reflectometry and Ellipsometry Introduction
Sensitive optical methods for determining the surface properties of materials and thin films. These techniques are used to measure the thickness of films of dielectric, semiconductors, and to a small extent conductors. The sample structure and the optical constants need to be known if the film thickness is to be determined.
Reflectance
The Reflectance (ℜ) is defined as the ratio of the intensity of the outgoing wave to the intensity of the incoming wave. The total reflection coefficients Rp and Rs are defined as the ratio of the amplitude of the outgoing wave to the amplitude of the incoming wave. The reflectance is the square of the magnitude of the total reflection coefficient, ℜp = ⎥ Rp⎥2 and ℜs = ⎥ Rs⎥2 Because the intensity (I) is proportional to the square of the amplitude of the wave.
Single-Wavelength Ellipsometer
• A monochromatic light source (VIS) is used • A fixed angle of incidence is used. • Delta-Psi trajectory is measured- n & k determined
where n is the index of refraction k is the extinction coefficient. In dielectric materials like glass, n is an inverse measure of the phase velocity in the materials.
The intensity of a beam of light (usually monochromatic) is measured before and after it reflects from the surface. Absolute Reflectance ( R ) = I incident / I reflected Difficult to measure the I of the light beam before it strikes the sample. Relative reflectance (R’) is measured. Relative Reflectance (R’) = I sample / I standard The R’ is the ratio of the reflected intensity from the unknown sample to the reflected intensity from the standard sample It represents the reflectance of the unknown samples relative to that of the standard sample.
Spectroscopic Ellipsometer
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Spectroscopic ellipsometry is useful for analyzing multiple layers and to determine the optical constant dispersion relationship. From these optical constants, microstructural information can be deduced. The degree of crystallinity of annealed amorphous silicon; The aluminum fraction in AlxGa1-xAs. The combination of SE and reflectometry on a single instrument provides the speed needed for metrology and the ability to analyze in depth when unknown materials are involved.
Lecture Notes
Reflectometry and Ellipsometry
R.A. Gunasekaran IfM: Room 114 Tel: 257-5115
Reflectometry and Ellipsometry Overview
Principles of Operation Polarization Phenomenon Applications Optical Properties of Materials and Thin films Instrumentation Basic Techniques - Different Approaches Refractive Index and Thickness Determination Supplementary Reading
Single Wavelength Ellipsometer
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n2 λ n2’ Substrate
n2 = n2’
Two different transparent materials which have the same index of refraction at a given wavelength cannot be distinguished with SWE. If they have the different n values at different wavelengths, it is possible to distinguish them easily. The amorphous silicon can be distinguished from crystalline Si.
n = c/v
k is a measure of how rapidly the intensity decays as the light passes through a given material, and it is related to absorption coefficient:
k = (λ/4π) α
Optical Interference Phenomenon
Natural phenomenon – provides a scale for measuring minute thickness of transparent bodies by their colors ///optics/lightandcolor/interference.html
Reflectometer Vs. EllipPrinciples
Reflectometer and Ellipsometer
Commercial Research Tools
Combined Reflectometer and Ellipsometer
Absorption Coefficient
In an absorbing medium, the decrease in intensity I per unit length z is proportional to the value of I. This is expressed as dI(z) / dz = -αI(z) The solution to this equation is I(z) = I0e-αz Where I0 is the intensity of light inside the material of interest. α is dependent on loss of intensity due to absorption only. (Note: loss of light due to the reflection at the interfaces does not contribute to the magnitude of α) k = (λ/4π)α