Woollam RC2

Contents

  1. Introduction
  2. The ellipsometry in few words
  3. Equipment description
  4. How to use the system
  5. CompleteEase Software and Ressources

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 

Routine Tests – Evolution of the light signal intensity

Average Signal measured during the Monday morning calibration. The measurement is done at 65° on a standard 25nm SiO2 on Si wafer from Woollam and the light filter is set at 50%.

IV. How to use the system↑

V. CompleteEase Software and Ressources↑

CompleteEase

Ressources