Optical Profilometer: What It Measures, How It Works, and When Not to Use One
A 3D optical profilometer is a non-contact surface metrology instrument that measures roughness, step height and full 3D topography of a surface in one scan. TDM's published systems reach 0.1 µm resolution across a 300 × 150 × 70 mm range (SE310), or ≤0.1 nm RMS repeatability in phase-shift mode (SE320).
What an optical profilometer actually measures
Four outputs, four different process decisions:
- Roughness — the height variation of a surface, reported as 3D areal statistics rather than a single stylus-style Ra line.
- Step height — the difference between a film edge or an etched terrace and the substrate below it. The classic way to confirm a deposition or etch process without cross-sectioning.
- 3D topography and form — bumps, dents, coating beads, edge thinning, printed-line uniformity.
- Bow, warp and curvature — the shape a flat substrate took after processing.
What it does not do: read the thickness of a film that lies flat under a transparent top layer. With no edge to step over, that measurement belongs to spectral reflectometry or ellipsometry (section below).
Two ways to do it — and they are 1000× apart in Z
"Optical profilometer" is not one technology. TDM publishes two, and they answer different questions.
Light-section triangulation — SE310. A thin light sheet is projected onto the surface and a dual-telecentric imaging system reads its deflection. Published figures: 0.1 µm resolution, 1 scan/second full-area surface scanning, 300 × 150 × 70 mm measuring range (X, Y, Z), 12–160× continuous motorized zoom, and distortion-free imaging without perspective error. That last point is why it keeps working on steep slopes, machined edges, thick coatings and shiny or dark surfaces — where interferometry loses contrast.
White light interferometry + phase-shift interferometry — SE320. WLI locates the coherence envelope for coarse heights; PSI resolves sub-nanometre detail on smooth surfaces. Published figures: 200 × 200 mm XY fully automatic motorized stage with closed-loop grating encoders, 100 mm Z-axis travel for high-step measurement, RMS repeatability ≤0.1 nm in PSI mode with vertical resolution up to 0.01 nm, automatic switching among five objectives (5× / 10× / 20× / 50× / 100×), and a 5-megapixel industrial CCD camera (2592 × 1944).
Rule of thumb: rough, steep or tall features → light-section. Smooth, mirror-like, sub-nanometre steps → WLI/PSI. Buying the wrong one of these two is the most common and most expensive mistake in this equipment category.
One number people misread: 0.01 nm is the vertical resolution PSI achieves on a *surface height or step*, not a film-thickness range. If someone quotes this instrument as measuring 0.01 nm films, they are describing a different technique.
Optical profilometer vs spectroscopic reflectometer vs spectroscopic ellipsometer
| Optical profilometer (3D) | Spectroscopic reflectometer | Spectroscopic ellipsometer | |
|---|---|---|---|
| Answers | Roughness, step height, 3D form, warp | Thickness, plus T/R on flat films | Thickness with optical constants (n/k), multi-layer stacks |
| Contact | Non-contact | Non-contact | Non-contact |
| Needs a step or edge? | Yes for height-style readings | No | No |
| Published capability (TDM) | 0.1 µm and 300 × 150 × 70 mm (SE310); ≤0.1 nm RMS repeatability in PSI mode (SE320) | 4 nm–450 µm film thickness, repeatability ≤0.02 nm (SE200) | Reports n/k, band gap and PL on one bench, 5–1200 nm with ≤±1% repeatability (SE100) |
| Area / spot | Full-area scan, 300 × 150 mm (SE310) or 200 × 200 mm stage (SE320) | 1 µm spot at 100× to 100 µm at 5×, three apertures (SE200) | Model-dependent; micro-spot with mapping |
| Speed | 1 scan/second full-area (SE310) | Point readings, fast | Slower — angle/wavelength sweeps plus model fitting (TDM SE100 calculates in <10 s) |
| Best for | Machined edges, seed layers, coating beads, print uniformity, post-CMP profile | QC thickness on a known stack, micro-area checks on 4″–12″ wafers | Unknown optics, new materials, layer-by-layer stacks |
| Weak at | Buried layers with no edge; wet or very soft films | 3D shape, sub-nm step detail | Rough or scattering surfaces; needs a good optical model |
| TDM instrument | SE310 · SE320 · SE8000 | SE200 | SE100 |
Spectroscopic reflectometry and ellipsometry are covered in detail here: Spectroscopic Reflectometer · How to Measure Thin Film Thickness · Wafer Thickness Measurement.
Where an optical profilometer is the wrong tool
- A buried layer under a transparent top coat. No edge to step over — use reflectometry or ellipsometry.
- Wet or soft films. Slot-die, inkjet and photoresist in the wet state move under optical sectioning; measure wet film in situ at the coating head instead.
- Sub-nanometre film thickness on a smooth, unpatterned wafer. That is PSI/ellipsometry work, not profilometry.
- 100% line-speed inspection. Benchtop profilers sample; in-line metrology watches every pass.
Not sure between contact and optical? See stylus profilometer vs optical profilometer.
How to specify one before you ask for a price
Answer these five and any quote becomes comparable:
- Roughness or step height — which decision are you actually making?
- Surface finish — mirror, polished, machined, dark, porous, or wet?
- Feature size — the smallest step or defect you must resolve, and the area you must cover.
- Step height range — a 2 nm etch terrace and a 300 µm printed bead need different Z stacks.
- Point, map or full-area — one reading per sample is not the same claim as a 200 × 200 mm map.
Which TDM instrument fits your measurement
- 3D topography, roughness and step height on parts up to 300 × 150 mm, including steep or shiny surfaces → SE310 3D Optical Profiler
- Sub-nanometre steps on smooth surfaces, large-area mapping → SE320 3D Optical Profiler with Scan Platform
- 3D thickness of transparent and multi-layer films by imaging, with uniformity maps → SE8000 Imaging 3D Thin-Film Thickness Measuring Instrument
- Micro-area thickness with a 1 µm spot on a patterned wafer → SE200 Microscopic Thin Film Thickness Measurement Instrument
- Thickness with n/k, band gap and PL on one bench → SE100 Multi-functional Thin Film Measurement Instrument
Send us one sample. We run it on the light-section bench and the WLI/PSI bench and send back the 3D map, the roughness and step-height numbers, and which technology actually resolved your feature — including the honest answer "neither of these; use reflectometry".
Written by TDM Metrology Team · published 2026-11-10
Frequently Asked Questions
What is an optical profilometer?
A non-contact instrument that measures a surface's height point by point and rebuilds it in 3D, giving roughness, step height and topography without touching — or damaging — the sample. For when contact stylus scanning is still the better choice, see stylus profilometer vs optical profilometer (/news/guide/stylus-profilometer).
How does a 3D optical profilometer work?
TDM publishes two mechanisms. Light-section triangulation projects a thin light sheet and reads its deflection through a dual-telecentric objective (SE310: 0.1 µm resolution, 300 × 150 × 70 mm range, 12–160× motorized zoom). Interferometric systems read the white-light coherence envelope for coarse heights and add phase-shift analysis for sub-nanometre steps (SE320: RMS repeatability ≤0.1 nm in PSI mode, 200 × 200 mm motorized stage, 100 mm Z travel). Contact stylus instruments are a different category — compare them in the stylus profilometer guide (/news/guide/stylus-profilometer).
What is a profilometer used for?
Step height on a film edge or etched terrace, 3D roughness of polished and machined surfaces, coating beads and edge thinning, printed-line uniformity, warp and bow, and post-CMP profile checks.
Can an optical profilometer measure thin-film thickness?
Only where there is a step to measure — a film edge, a patterned terrace, an etched pit. A film lying flat under a transparent layer has no edge, so thickness then comes from spectral reflectometry (SE200: 4 nm–450 µm) or from fitting optical constants (SE100: n/k, 5–1200 nm). If your sample has no edge, profilometry is the wrong request.
How much does an optical profilometer cost?
TDM does not publish list prices, because the price follows three variables you control: which mechanism you need (light-section vs WLI/PSI), the Z and XY range, and the objective set. Ask for a sample test on your own part first — it settles all three, and it is free on our bench.
Related guides and products
- Stylus Profilometer vs Optical Profilometer
- SE310 3D Optical Profiler
- SE320 3D Optical Profiler with Scan Platform
- How to Measure Thin Film Thickness
- Spectroscopic Reflectometer Guide
- Spectroscopic Reflectometry vs Ellipsometry
