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Ellipsometry vs Reflectometry: What They Measure, Thin-Film Sensitivity and Multilayers

Both methods shine light on a wafer and measure the reflection to find the thickness of thin films without contact or damage. Spectroscopic ellipsometry sends polarized light in at an oblique angle and measures, at each wavelength, how the reflection changes the polarization (Psi and Delta). Spectral reflectometry usually illuminates at normal incidence and measures the spectrum of reflected intensity (reflectance). Both fit an optical model to get the thickness, but ellipsometry also uses phase information, so it is stronger for films a few nanometers thick, multilayer stacks and optical constants, while reflectometry is simpler and faster and suits thickness control of thicker films.

AspectSpectroscopic ellipsometrySpectral reflectometry
What is measuredAmplitude ratio (tan Psi) and phase difference (Delta) between p- and s-polarized reflectionSpectrum of reflected intensity (reflectance)
IlluminationPolarized light at an oblique angle (typically about 60 to 75 degrees)Usually normal incidence; no polarization control needed
ResultsThickness and optical constants (refractive index n and extinction coefficient k) can be found togetherMainly thickness; known optical constants are usually assumed
Model dependenceStrong: values come from fitting a model of the film stack and its dispersionAlso model-based; for a thick single layer the fringe spacing gives the thickness fairly directly, but results depend on the assumed optical constants
Sensitivity to very thin films (a few nm or less)High: Delta resolves differences below 0.1 nm, so native oxides and gate dielectrics can be measuredLow: no interference fringes appear and the change in reflectance is small, so precision drops
Best thickness range (rough guide)From below 1 nm to a few micrometersFrom tens of nanometers to tens of micrometers
MultilayersStrong: two quantities per wavelength help separate the thickness and optical constants of each layerSeparating individual layers gets hard as layers are added
Tool and speedMore complex optics with polarizers; the oblique beam makes an elongated spot and each point takes a little longerSimple and fast, with an easily small spot; easy to build into other tools
Typical usesGate and high-k dielectrics, thin oxides and nitrides, multilayer thickness and optical constants, optical critical-dimension metrology (scatterometry)Resist and thick oxide or interlayer dielectric thickness, and thickness monitoring built into CMP and other tools

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

Spectroscopic ellipsometry
When you need accurate thickness of films a few nanometers thick or of multilayer stacks, or when you also need the optical constants. It suits films such as gate dielectrics, where a 0.1 nm difference matters.
Spectral reflectometry
When films are tens of nanometers thick or more and you want fast measurements at many points. Its simple optics also make it the usual choice for thickness monitoring built into deposition and CMP tools.

FAQ

What are Psi and Delta in ellipsometry?

They describe the ratio between the p- and s-polarized components of the reflected light: tan Psi is the amplitude ratio and Delta is the phase difference. Both change with film thickness and optical constants, so the thickness and refractive index are found by fitting an optical model to them.

Why is ellipsometry so good at very thin films?

The phase difference Delta changes clearly even for films a few nanometers thick or less. Because it measures a ratio of two polarization components, it is also insensitive to drifts in light-source intensity and needs no reference sample.

How does spectral reflectometry find thickness?

Light reflected from the top of the film interferes with light reflected from the interface below, so some wavelengths are strengthened and others weakened. Thicker films give more closely spaced fringes, so the thickness can be found from the reflectance spectrum. It is the same effect that makes oxide on silicon look colored.

How far can the measured thickness be trusted?

Both methods are indirect: they fit an optical model. If the assumed stack or optical constants are wrong, the result is wrong too. For a new film, the model is usually checked against a TEM cross-section or X-ray reflectivity (XRR) first.

Check your understanding

What is mainly measured by ellipsometry (spectroscopic ellipsometry)?

  1. AThe resistance when current is passed through the sample
  2. BThe thickness and refractive index of thin films, obtained from changes in the polarization of reflected light
  3. CThe mass spectrum of a sample sputtered by ions
Show answer and explanation

Answer: B. The thickness and refractive index of thin films, obtained from changes in the polarization of reflected light

Ellipsometry shines polarized light at an angle and measures the amplitude ratio (tanΨ) and phase difference (Δ) between p- and s-polarized reflections, fitting them to an optical model to extract film thickness and refractive index. Spectroscopic ellipsometry, which scans wavelength, can analyze multilayer stacks with repeatability better than 0.1 nm. Being non-contact and non-destructive, it is widely used for oxide thickness control. It does not measure resistance or mass spectra.

Why does oxide on silicon appear in different colors depending on its thickness?

  1. ABecause of the color of impurities in the oxide
  2. BBecause of interference between light reflected from the top surface of the oxide and from its bottom (the interface with silicon)
  3. CBecause the oxide itself emits light
Show answer and explanation

Answer: B. Because of interference between light reflected from the top surface of the oxide and from its bottom (the interface with silicon)

SiO2 is transparent, but light reflected from the film surface and from the silicon interface interferes, reinforcing particular wavelengths (colors) depending on thickness—roughly brown-purple near 100 nm, blue near 300 nm and greenish near 500 nm, with colors repeating periodically as thickness grows. Color thus indicates approximate thickness, once checked against color charts; precise measurement uses ellipsometry or spectral reflectometry.

Which technique directs X-rays at a very shallow angle and determines thin-film thickness and density from oscillations in reflectivity?

  1. ASIMS
  2. BX-ray reflectometry (XRR)
  3. CFour-point probe method
Show answer and explanation

Answer: B. X-ray reflectometry (XRR)

XRR measures reflectivity as the grazing incidence angle of X-rays is varied. X-rays reflected from the film surface and interfaces interfere, so the reflectivity oscillates; the period gives thickness, the critical angle for total reflection gives density, and the damping gives interface roughness. Because it requires no assumed optical constants, it is suited to films only a few nanometers thick. SIMS and four-point probe measure other quantities.

  1. Which method measures the thickness of a lightly doped epitaxial layer on a heavily doped substrate using infrared interference?
  2. What is a wafer called that is run through a process separately from products to check equipment condition or measure film thickness?

Practice Metrology and analysis questions

Key terms explained

Ellipsometry
A measurement method that shines polarized light on a sample and determines thin-film thickness and refractive index from the change in polarization on reflection. It is nondestructive.
Refractive index
The speed of light in vacuum divided by its speed in a material. At a wavelength of 1.55 µm, silicon is about 3.48 and oxide (SiO2) about 1.45; the larger this contrast, the more tightly light can be confined in a small waveguide.
Thermal oxidation
A method of forming an SiO2 film on silicon by heating it in oxygen or water vapor. There are dry and wet oxidation.
Native oxide
A thin oxide about 1 nm thick that forms naturally when silicon is exposed to air or water. Because it raises contact resistance and disturbs epitaxial growth, it is removed right before such steps.
Photoresist
A light-sensitive material whose solubility in developer changes when exposed. In positive resist the exposed areas dissolve; in negative resist they remain.
CMP
Chemical mechanical polishing. Planarizes the wafer surface by combining the chemical action of a slurry with mechanical polishing by a pad.
Monitor wafer
A wafer run through a process separately from products to check equipment or measure film thickness and particles. Also called a test wafer, it is often stripped, repolished and reused as a reclaimed wafer.

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