Sopra GES 5E

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

Contents

1. Introduction
2. The ellipsometry in few words
3. Equipment description
4. How to use the system

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.

IV. How to use the system ↑