- The ellipsometry in few words
- Equipment description
- How to use the system
- CompleteEase Software
I. Introduction ↑
The spectroscopic ellipsometer is used to measure the thicknesses and the refractive indices of thin films (transparent or not).
The ellipsometer allows the study of properties of materials:
- Dielectric and semi-conductor films such as silicon dioxide, silicon nitride, polysilicon and amorphous silicon.
- Metallic films (Note: Generally, the film needs to be really thin)
- Organic layers such as carbon and photosensitive resists.
II. The ellipsometry in few words↑
The spectroscopic ellipsometry is an optical technique using the properties of polarized light, after its interaction with surfaces, to determine refractive indexes and thicknesses of thin films (from a few A to several um depending on material type).
When a linearly polarized light is directed onto the surface of a material with a certain angle of incidence, it can be expressed through its parallel (s-) and perpendicular (p-) components. After reflection on the surface, these components change resulting in an elliptically polarized light.
The ellipsometry uses this phenomenon to give an estimation of the thickness of a transition region between the substrate and the air by measuring the ratio Rp/Rs, the reflection coefficients of s- and p- components. It can be written as :
with Ψ and Δ related to the thickness and the refractive index of the film.
It is important to remind that: the ellipsometer does not directly measure the refractive index (n) or the thickness (t) of the thin film. An algorithm has to be used the resolve n and t based on values of Ψ and Δ measured.
For a given wavelength, Ψ plotted as a function of Δ results in a periodic curve1 for each value of the refractive index (if the material is transparent with k = 0).
The size and position of the curves are optical constants, nature of the substrate, and angle of incidence dependent. Therefore, for a given refractive index, each set Ψ / Δ represents a certain “periodic” thickness. It means that for a given refractive index, the thickness follows its specific and periodic ellipsometric curve.
The measurement in a spectral range cancels the ambiguity related to the use of a single wavelength.
1: For absorbent layers where k ≠ 0, the corresponding graphs are not periodic but have a spiral shape (the corresponding thickness values do not repeat with a constant period)
For more details, do not hesitate to visit Ellipsometry Tutorial from J.A. Woollam company.
III. Equipment description↑
The system accepts:
- Up to 200 mm wafers
- Chips with a surface for the measurement bigger than the cross section of the incident beam.
- Xenon Arc lamp 75W
- Wavelength range : From 210 nm to 2500 nm (UV – Visible light – NIR).
- Without focusing probes : 3 to 4 mm beam diameter
- With focusing probes : 120 microns beam diameter
- Motorized XY stage mapping
- Motorized Z and Autofocus
- Automatic Tilt Alignement
IV. How to use the system↑
V. CompleteEase Software↑
- The software manual can be consulted on the tool directly by clicking on F1 or in the Options tab.
- Short video tutorials can be found on J.A. Woollam website.