Spectroscopic Ellipsometry for Thin Film Metrology Guide
A thin film is defined by three numbers: how thick it is, how it bends light, and how it absorbs light. Get any one of them wrong — whether you are depositing a 5 nm gate oxide, a 500 nm perovskite absorber, or a 100 nm anti-reflection coating — and the device fails. Spectroscopic ellipsometry measures all three in a single non-contact, non-destructive scan. This guide explains what SE actually measures, how it compares to the other metrology techniques on your bench, and when it is the right tool versus when a faster technique like reflectometry will do the job.
What spectroscopic ellipsometry measures
Ellipsometry works by shining polarized light at a thin film and measuring how the polarization state changes upon reflection. Unlike reflectometry — which only measures how much light comes back — ellipsometry measures two parameters simultaneously at every wavelength:
Psi (Ψ). The amplitude ratio of p-polarized to s-polarized reflected light. This is primarily sensitive to film thickness.
Delta (Δ). The phase difference between p- and s-polarized light after reflection. This is exquisitely sensitive to the optical constants (n and k) of the film.
Because the measurement is a ratio (not an absolute intensity), ellipsometry is inherently more precise than reflectometry. It does not need a reference sample, and it is insensitive to source intensity drift.
What comes out of a spectroscopic ellipsometry measurement
| Output | Symbol | What it means for your process |
|---|---|---|
| Film thickness | d | Is the deposition hitting the target? |
| Refractive index | n | How the film bends light — sensitive to composition and density |
| Extinction coefficient | k | How the film absorbs light — directly linked to bandgap and defect states |
| Band gap | Eg | Derived from the k spectrum — critical for semiconductor and PV films |
| Surface roughness | — | Modeled via an effective medium approximation (EMA) layer |
Why spectroscopic (multi-wavelength) matters
Single-wavelength ellipsometry — the kind that uses a laser at 632.8 nm — can measure one film layer accurately if you already know n and k. But thin film stacks in real devices rarely cooperate: the n and k of a perovskite film deposited yesterday may differ slightly from last week's batch, and a transparent conductive oxide's optical constants depend on deposition conditions.
Spectroscopic ellipsometry solves this by measuring Ψ and Δ at hundreds of wavelengths simultaneously. With data spanning UV to near-IR (typically 200–1700 nm), the fitting algorithm has enough information to solve for thickness, n(λ), and k(λ) of each layer in a multi-layer stack — no assumptions needed about optical constants.
Ellipsometry vs. other thin film metrology techniques
| Technique | Measures | Thickness range | Accuracy | Multi-layer? | Reference needed? |
|---|---|---|---|---|---|
| Spectroscopic ellipsometry | d, n, k, Eg | 1 nm – 10 μm | ±0.1 nm or ±0.1% | Yes (2–5 layers) | No |
| Reflectometry | d (if n is known) | 10 nm – 50 μm | ±1 nm | Limited (2 layers) | Yes |
| Profilometry (stylus) | d (step height) | 10 nm – 1 mm | ±1 nm | No | Yes (bare substrate) |
| X-ray reflectivity (XRR) | d, density, roughness | 1 nm – 200 nm | ±0.1 nm | Yes | No |
| Photoluminescence (PL) | Eg, defect density | N/A | N/A | N/A | No |
XRR is more accurate for ultra-thin films below 200 nm, but it is slow (minutes per point) and only works on atomically flat surfaces. Ellipsometry is the practical sweet spot: fast enough for production, accurate enough for R&D, and rich enough in data to replace two or three single-purpose instruments.
Reflectometry vs Ellipsometry: Which Thin Film Measurement Technique Is Right for Your Process
If you measure thin films, you have two optical options on the bench: reflectometry and ellipsometry. They are frequently mentioned together — both are non-contact, both use light, both deliver thickness and optical constants. But the physics are fundamentally different, and so are the use cases where each excels.
How they measure — the physics difference
Reflectometry measures intensity. It shines light at the film and records how much comes back as a function of wavelength. The reflectance spectrum — a single number per wavelength — is fitted to a model to extract thickness and, in some configurations, n and k. It is fast, robust, and requires minimal optical alignment.
Ellipsometry measures polarization change. Instead of intensity, it tracks two parameters — Psi (Ψ, the amplitude ratio) and Delta (Δ, the phase shift) — at every wavelength. Because it measures a ratio rather than an absolute intensity, ellipsometry is inherently more precise and does not require a reference sample. That precision comes at the cost of more complex modeling and longer measurement time per point.
Precision vs speed — the trade-off that decides your workflow
Ellipsometry is the gold standard for accuracy. On transparent films below 100 nm — gate oxides, ALD barriers, anti-reflection coatings — ellipsometry can resolve thickness to sub-angstrom precision and simultaneously extract n and k with high confidence. Reflectometry on the same films will give you thickness within a few nanometers, which is often good enough for production QC but not for process development.
The trade-off is measurement speed. A single-point ellipsometry measurement takes seconds; a full-wafer mapping run takes minutes. Reflectometry, particularly when deployed as an in-line reflectometer with a broadband source, can measure hundreds of points per minute. If your bottleneck is throughput — not single-digit angstrom precision — reflectometry wins.
Where each technique belongs on your line
Reflectometry belongs on the factory floor. It tolerates vibration, needs minimal sample prep, and integrates directly into deposition chambers for real-time endpoint detection. TDM's In-Situ Thin Film Monitoring solution uses broadband reflectometry to track film growth layer by layer during sputtering, evaporation, and ALD — giving operators a live thickness readout without breaking vacuum.
Ellipsometry belongs in R&D and process qualification. When you are developing a new film stack, qualifying a new deposition tool, or troubleshooting a process shift, you need the full n/k dispersion curve and thickness uncertainty below 0.1 nm. TDM's SE100 Multi-functional Thin Film Measurement Instrument combines spectroscopic ellipsometry with reflectometry and transmission on one platform, and the Film Thickness Measurement product family is built for exactly this workflow.
Combine them — the production recipe that actually works
The two techniques are not competitors. High-volume fabs and coating houses routinely deploy reflectometry on the line for 100% inspection and ellipsometry in the lab for periodic verification and process tuning. TDM's product line is designed for this dual deployment: In-Situ products keep the line running, Film Thickness Measurement / Offline ellipsometers keep the process honest. If you are currently doing one without the other, that gap is your next yield improvement project.
The measurement workflow
- Mount the sample. Place the coated substrate on the stage. No special preparation is needed — the measurement is non-destructive and works on as-deposited films.
- Acquire the spectrum. The instrument scans Ψ and Δ across the full wavelength range (typically 200–1700 nm for TDM's benchtop instruments). This takes seconds to minutes depending on spectral resolution and signal-to-noise requirements.
- Build the optical model. The operator defines a layer stack model: substrate (e.g., glass or silicon), each film layer in sequence, and optionally a surface roughness layer using an effective medium approximation. Each layer needs a dispersion model — Cauchy for transparent films, Tauc-Lorentz or Cody-Lorentz for absorbing semiconductors, Drude for metals.
- Fit and extract. The fitting algorithm adjusts thickness and dispersion parameters to minimize the difference between measured and modeled Ψ and Δ. A good fit (MSE < 10) gives confidence that the model is physically meaningful.
- Validate. Cross-check the ellipsometry results against at least one independent measurement — profilometry for thickness, or a separate n/k measurement from literature — to confirm the model is not fitting noise.
TDM instruments for spectroscopic thin film metrology
SE100 Multi-functional Thin Film Measurement Instrument
The SE100 is TDM's benchtop workhorse for spectroscopic thin film characterization. It combines ellipsometry, reflectometry, and transmission spectroscopy in a single platform — so you get thickness, n, k, and band gap from one instrument without switching setups.
| Parameter | Specification |
|---|---|
| Measurement modes | Spectroscopic ellipsometry + reflectometry + transmission |
| Wavelength range | UV-Vis-NIR (published range) |
| Measurable outputs | Thickness, n(λ), k(λ), Eg, surface roughness (EMA) |
| Substrate types | Glass, silicon, sapphire, metals, flexible substrates |
| Application | R&D benchtop metrology and production QA sampling |
SE10 Portable PL Metrology Instrument
The SE10 complements ellipsometry with photoluminescence data — band gap, defect emission, and composition uniformity. While ellipsometry tells you the physical thickness and optical constants, PL tells you how the film will actually perform as a semiconductor. Together they give a complete picture: structure from SE, optoelectronic quality from PL.
When to use each
| What you need to know | Instrument |
|---|---|
| Thickness + n/k of a single film | SE100 (ellipsometry mode) |
| Thickness only (fast, n is known) | SE100 (reflectometry mode) |
| Band gap and defect emission | SE10 (PL mode) |
| Full stack: thickness + n/k + Eg | SE100 + SE10 |
Where spectroscopic ellipsometry guides still leave a gap
Many spectroscopic ellipsometry pages online are generic product landings. Few explain how to build an optical model or when to use a Tauc-Lorentz vs. Cody-Lorentz dispersion — the decisions that actually determine whether a fit is physically meaningful.
This guide is written for that workflow: choose SE when you need d, n, and k together; use reflectometry when thickness alone is enough and n is already known; add PL when you need optoelectronic quality alongside the optical stack.
Get a technical consultation
Tell us your film stack, substrate type, and which parameters you need to measure — our application engineers will recommend the right instrument configuration and measurement protocol.
- Free feasibility test on your sample
- Optical model development support
- On-site demo and installation
Explore the SE100 Multi-functional Thin Film Measurement Instrument, the SE10 Portable PL Metrology Instrument, the Offline Thin Film Measurement series, or contact TDM to submit an inquiry.
Frequently asked questions
How is spectroscopic ellipsometry different from reflectometry?
Reflectometry only measures how much light comes back (intensity). Ellipsometry measures the polarization change — both amplitude ratio (Ψ) and phase difference (Δ) — at every wavelength. Because it is a ratio measurement, it is inherently more precise and does not need a reference sample. More importantly, ellipsometry gives you n and k (refractive index and extinction coefficient), not just thickness.
What film thickness range can spectroscopic ellipsometry measure?
Spectroscopic ellipsometry can measure films from sub-nanometer (a few Å) to tens of microns, depending on the wavelength range and material. The lower limit is set by the sensitivity of Δ to ultra-thin films; the upper limit by the coherence length of the light source. For typical semiconductor and PV films (1 nm to 10 μm), SE is the gold standard.
Does ellipsometry require a reference sample?
No — this is one of its core advantages over reflectometry. Because ellipsometry measures a polarization ratio (not absolute intensity), it is self-referencing. It is also insensitive to source intensity drift, which means better long-term stability in production environments.
Can ellipsometry measure multi-layer stacks?
Yes, and this is where spectroscopic (multi-wavelength) ellipsometry pulls ahead of single-wavelength instruments. With data at hundreds of wavelengths, the fitting algorithm can simultaneously solve for thickness and n/k of each layer in a stack — typically 2–5 layers. Beyond that, fitting becomes increasingly sensitive to the quality of the initial optical model.
Which TDM instruments support spectroscopic ellipsometry?
The SE100 Multi-functional Thin Film Measurement Instrument is TDM's benchtop spectroscopic solution — it combines ellipsometry, reflectometry, and transmission spectroscopy in one platform. For portable applications, the SE10 provides PL metrology that complements ellipsometry data.
Related guides and products
- How to Measure Thin Film Thickness Guide
- In Situ Thin Film Monitoring — Real-Time Metrology Guide
- In-Situ Metrology solutions
- Offline Thin Film Measurement series
- SE100 Multi-functional Thin Film Measurement Instrument
- SE10 Portable PL Metrology Instrument
- Contact TDM
