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). The HS-190 monochromator is a Czerny-Turner Scanning Monochromator with a focal length of 160mm. A VB-400 Controle Module contains the power supply and motor drivers. An AutoRetarder is installed but its use is optional for each test. This device improves the analyzer's accuracy by changing the light entering the detector unit from linearly to circularly polarized.

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

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

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

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 on back of sample stage) – 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 - Spectroscopic Analysis (once sample is aligned)

• In Hardware window, click Acquire Data – Spectroscopic Scan
• Recommended options:

• 50 – 100A 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°

• Length: Testing one sample will take up to one hour (if AutoRetarder is used, 3 angles, 50nm wavelength intervals, 2000-10000nm)

Acquiring Data - Transmission Analysis (once sample is aligned)

• In Hardware window, click Acquire Data – R+T Data
• Baseline and data scans

• Must move slide stage out of way for baseline scan - do so by pushing base of stage back and wedging a thick object (beige sample holder) in between grey z-axis knob and stage

• Recommended options:

• 50 – 100A intervals in wavelength
• 20 Revs/meas. – greater if noisy
• FOR DIRECT TRANSMISSION: set angle from 0 to 0
• Set angles to match spectroscopic scan angles if combining transmission data to with ellipsometry data (eg. for determining material composition)
• Save File every 15 min (forces to name file and comment initially)

• Length: Testing one sample will take about 10-15 minutes

Data Analysis

To be updated