Introduction
Ellipsometry is a powerful technique whereby the change in the polarization of light reflecting off a surface is analyzed to determine the optical and dielectric properties of a thin film. This analysis can yield information about layers that are thinner than the wavelength of the probing light itself, even down to a single atomic layer. Ellipsometry can probe the complex refractive index or dielectric function tensor, which gives access to fundamental physical parameters and is related to a variety of sample properties including thichkness, morphology, crystal quality, chemical composition, or electrical conductivity.
Unlike single-wavelength (laser) ellipsometry which uses a monochromatic light beam, spectroscopic ellipsometry (SE) employs broad band light sources which cover a certain spectral range in the infrared, visible or ultraviolet spectral region. SE in these regions studies the refractive index in the transparency or below-band-gap region and electronic properties such as band-to-band transitions or excitons. - Wikipedia
Equipment Information
The ellipsometer used is a J.A. Woollam Co., Inc vertical-variable angle spectroscopic ellipsometer (V-VASE).
Protocol
Turning On (after turning on, wait at least 10 min before use to allow for warming up)
- Turn on control box first – In Hardware window, click Initialize to sync the computer to the ellipsometer (key must be in computer to run WVase32 program)
- Press Light button on monochromator followed by Ignite to turn the light on
- Check to make sure appropriate fibre optic cable is attached – depends on light range/size of beam
- UV: 200nm - 2100nm (dead range from 1350nm – 1450nm due to water absorption)
- IR: 270nm – 2300nm
Turning Off (suggested to keep electronics warm by leaving on if using the ellipsometer frequently)
- Press Light button on monochromator to turn off light, press Light again but not Ignite to leave it off but powered
Applying Sample
- Ensure vacuum box is switched on
- Switch to appropriate vacuum port as labelled on the back. Slits and holes are used for large and small samples respectively
- Flip switch on ellipsometer from ‘vent’ to ‘vacuum’
- Use tweezers (pinch tight) through the sample stage groove to apply sample over the vacuum openings
Calibration (to make sure zero is known for the polarizer and analyzer - should be performed at start of every new testing session)
- Insert silicon calibration wafer (apply directly from spider case)
- Click on Align Sample – Calibrate System (default settings are OK)
- Calibration completed once "acquiring calibration data" progress reaches 100%
- Click on File – View Log to compare Ps, As values to previous calibrations
Aligning Sample (performed for every newly placed sample)
- In Hardware window, click Acquire Data – Alignment
- Carefully insert alignment detector
- Align red cross in centre of the four grey boxes on screen by adjusting alignment knobs (two black knobs) – cross will move around naturally but want it to be close to centre (X and Y values <1 ideally) – press Esc to exit once completed
- If alignment detector is way off (no or very small grey boxes) one must find the beam manually to get rough sample alignment first
- Remove alignment detector
- Align z-axis by adjusting silver knob to maximize the height of the red line on screen (it may gradually rise if the light is recently turned on) – press Esc to exit once completed
Acquiring Data (once sample is aligned)
- In Hardware window, click Acquire Data – Spectroscopic Scan
- Recommended options:
- 50 – 100nm intervals in wavelength
- 20 Revs/meas. – greater if noisy
- 3 angles: one below, equal to and above Brewster angle for substrate (73°, 76°, 79° for samples on a glass substrate)
- Dynamic Averaging (100 max)
- Save File every 15 min (forces to name file and comment initially)
- More Settings:
- Track Polarizer
- Auto Retarder if delta is expected to go above 180°
Data Analysis
To be updated