# Woollam RC2

## 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↑

### Substrate size

The system accepts:

• Up to 200 mm wafers
• Chips with a surface for the measurement bigger than the cross section of the incident beam.

### Light Source

• Xenon Arc lamp 75W
• Wavelength range : From 210 nm to 2500 nm (UV – Visible light – NIR).

### Spot Size

• Without focusing probes : 3 to 4 mm beam diameter
• With focusing probes : 120 microns beam diameter

### Table X-Y-Z

• Motorized XY stage mapping
• Motorized Z and Autofocus
• Automatic Tilt Alignement