# Sopra GES 5E

Note: Only the spectrograph is fully calibrated (The spectrometer has to be considered as not operational).

## 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 compagny.

## 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.

### Source

• Xenon lamp 75 W – Wavelength range: From 185 nm to 2500 nm (UV – Visible light – NIR).
• The light coming from the lamp goes through:
1. A rotating polarizer to give a linearly polarized light.
2. Then a compensator (A quarterwave plate) to give a circular polarized light.

### Goniometer & Arms

The system is equipped with:

• Two arms : The polarizer arm & the analyser arm.
• High quality prisms made of MgF2.
• The analyser arm is connected to the spectrometer/spectrograph through a optical fiber.
• A motorized goniometer:
• 0.007 ° resolution.
• The incidence angle can be selected from 20° to 90° to be close to the optimal angle (Brewster angle – Material dependent)

### Spectrograph & Spectrometer

Two systems are available  for the analysis of signals after reflection of the light on the the surface of the material:

• A fast spectrograph:
• Main unit :
• UV-Visible 190 – 990 nm.
• 1024 x 64 pixels – CCD sensor.
• Bandwidth FWHM (Full Width Half Maximum) : less than 3 nm over the entire spectrum.
• Extension :
• NIR 900 – 1700 nm
• 256 pixels – Linear sensor.
• Bandwidth FWHM (Full Width Half Maximum) : less than 9 nm over the entire spectrum.
• A high resolution spectrometer:
• Main unit:
• UV-Visible 190 – 900 nm
• Network and Prism double stage monochromator.
• Photomultiplier tube (PMT).
• Modus operandi: photon counting.
• Spectral resolution: <0.5 nm.
• Extension:
• NIR 750 – 2000nm
• P-doped InGaAs diode.
• Spectral resolution: <3 nm.

### Spot Size

• Standard mode :
• Size : From 1 to 5 mm.
• Microspot mode:
• Automatic electropneumatic positioning of the microspot.
• Usable to get rid of the reflection on the backside of transparent substrate (Thicker than 300 um).
• Working spectral range with the microspot: 210 – 2000 nm.
• Size : Typically smaller than 365 µm x 1mm (Incidence angle and wavelength dependent).

### Compensator

• Automatic electropneumatic positioning of the compensator.
• Composed of  2 prisms and 1 mirror.
• Working spectral range with the compensator: 190 – 2000 nm.
• Adapted for the measurement of transparent layers deposited on transparent substrate.

### Table X-Y-Z

• XY stage mapping.
• X & Y resolution :  +/- 0.25µm.
• Motorized Z + Autofocus.