How to Measure Thin Film Thickness: Profilometry, Ellipsometry and Interference

Thin-film thickness can be measured mechanically, as the height of a step, or optically, from interference fringes or the polarisation of reflected light. Each method needs a different sample and answers a slightly different question. This guide explains how stylus profilometry, AFM, interference and ellipsometry work, gives a worked fringe calculation, and sets out a workflow for reliable numbers.

In one paragraph: Step-height methods measure a physical height difference, but depend on step preparation, surface condition and measurement settings. Interference methods are non-contact; two adjacent fringes can estimate thickness using d = λ₁λ₂/[2n(λ₁ − λ₂)], assuming normal incidence and a known, near-constant refractive index [1][2]. Ellipsometry can measure very thin films and multilayers, but depends on the optical model [4]. Cross-check methods on the same sample and quantify repeatability [9].

1.  Choosing a Thickness Measurement Method

Spin-coated, dip-coated and slot-die-coated films are usually between a few nanometres and a few micrometres thick, and their thickness is the first number anyone asks for. There is no single best method: each technique measures something slightly different, needs a different sample and has different blind spots. The three families covered here are:

• Step-height methods (stylus profilometry and atomic force microscopy): a probe is scanned across an edge where the film has been removed, and the height of the step is the thickness.
• Interference methods (reflectance or transmittance spectra): light reflected from the top and bottom of a transparent film interferes, and the spacing of the fringes gives the optical thickness [1][2].
• Ellipsometry: the change in polarisation of reflected light is fitted with an optical model to give thickness and optical constants [4].

Qualitative comparison. Actual ranges and uncertainties depend on the instrument and the sample.

A practical rule: if the film is a new material, start with a step-height measurement because it does not need the refractive index; once the material is known, use an optical method for fast, non-contact routine checks, and cross-check the two on the same sample.

2.  Step-Height Methods: Stylus Profilometry and AFM

Making a good step

A step-height measurement is only as good as the step. Common ways to make one are:

• Masking before coating with a tape or a removable mask, so that part of the substrate stays bare;
• Scratching the film with a soft, sharp tool that cuts the film but not the substrate (works for soft organic films on glass or silicon, not for hard films on soft substrates);
• Wiping or dissolving a strip with a solvent swab, which leaves a step but can also leave a raised rim;
• Lithographic patterning or etching for the cleanest steps.

Whatever method is used, check that the substrate in the step is really bare and that the film next to the step is representative: edges, rims and the thick edge bead near a spin-coated substrate’s perimeter [10] are not. Edge bead is covered in Edge Bead Removal and Backside Rinse: EBR/BSR in Spin Processing.

Stylus profilometry

A stylus profilometer drags a fine diamond-tipped stylus across the surface under a small contact force and records its vertical position. The thickness is the height difference between the film and the bare substrate in the scan. Good practice:

• scan perpendicular to the step, long enough to include a flat region on both sides;
• level the profile on the substrate side, then average the film side over a flat region well away from the edge;
• repeat at several positions across the substrate, because spin-coated films can vary from centre to edge;
• use the lowest practical stylus force on soft films and inspect the scan track afterwards; a visible track means the film was deformed and the step height may be underestimated.

Atomic force microscopy

The atomic force microscope (AFM), introduced by Binnig, Quate and Gerber in 1986, measures surface topography using a probe that interacts with the sample [3]. The original paper reported sub-angstrom vertical resolution in preliminary measurements in air [3]. For thickness, an AFM scan across a step gives step-height information similar to a stylus scan, at higher lateral resolution but over a smaller area. AFM can be useful for thin films, small features and roughness measurements, but probe forces and imaging conditions must be chosen to avoid deforming soft or fragile films.

3.  Optical Interference: Thickness from Fringes

When light falls on a transparent film, part is reflected at the top surface and part at the film–substrate interface. The two reflected waves interfere, so the reflectance (or transmittance) spectrum shows a series of peaks and troughs, or fringes [1]. The fringe positions depend on the optical path length, the product of refractive index n and thickness d.

Thickness from two adjacent fringes

At normal incidence, two adjacent peaks (or two adjacent troughs) at wavelengths λ₁ > λ₂ differ by one interference order. If n is about the same at both wavelengths, the thickness follows as:

d = λ₁λ₂ / [2n(λ₁ − λ₂)]

Worked example. A film with n = 1.5 shows adjacent reflectance maxima at 600 nm and 500 nm. Then d = (600 × 500) / [2 × 1.5 × (600 − 500)] = 300,000 / 300 = 1,000 nm, or 1.0 µm. The example is illustrative; real films have dispersion (n changes with wavelength), so analysis software fits the whole spectrum with a dispersion model rather than using two points.

Three consequences follow from the formula:

• Thin films show few fringes. The thinner the film, the farther apart the fringes; a film much thinner than the wavelength may show no complete fringe in the visible range, and the analysis then relies on the amplitude of the spectrum rather than on fringe positions.
• Thickness and index are coupled. An error in n gives the same relative error in d. If n is not known, it has to be fitted or measured separately.
• The film must be optically simple. Strong absorption, rough interfaces or a graded film wash out the fringes.

For suitable films on transparent substrates measured in transmission, Swanepoel developed an envelope method that estimates thickness and optical constants from the transmission spectrum [2]. The achievable accuracy depends on the sample, the spectral data and the model assumptions.

4.  Spectroscopic Ellipsometry

Ellipsometry measures how the polarisation state of light changes on reflection from the sample, usually expressed as two angles, Ψ and Δ, measured over a range of wavelengths in spectroscopic ellipsometry (SE) [4]. Because the measurement is a ratio of two polarisations, it is less sensitive to the light intensity than a reflectance measurement, and it is sensitive to films much thinner than the wavelength [4]. The history and development of the method are reviewed by Aspnes [5].

The key point for users is that ellipsometry does not measure thickness directly. The software builds an optical model (substrate, each layer with its thickness and a dispersion model for its optical constants, perhaps a roughness layer) and adjusts the unknown parameters until the calculated Ψ and Δ match the measurement [4]. A good fit with a wrong model can still give a wrong thickness, so:

• start from the simplest model that fits, and add layers only when the data require them;
• check that the fitted optical constants are physically sensible;
• watch for correlation between thickness and optical constants, which is strongest in very thin absorbing films;
• check the model once against a step-height measurement on the same sample.

Ellipsometry pays off when the optical constants are needed anyway. In organic photovoltaics, for example, Pettersson and co-workers modelled photocurrent spectra by calculating the optical field inside the device from complex refractive indices and layer thicknesses obtained by spectroscopic ellipsometry [6]. The role of thickness in such stacks is discussed in Solution-Processed OLED and OPV Layers: Thickness Targets and a Lab Process Flow.

5.  A Workflow for Reliable Thickness Numbers

A thickness number is useful only when it is reproducible and when it is clear what it represents. The following workflow ties the methods together for a coating process.

1. Define the measurand. Dry film thickness after the final bake, at the substrate centre, or an average over a defined area? Write it down.
2. Calibrate the process. For spin coating, the final thickness falls approximately with the reciprocal square root of the spin speed [7]. Shaban and co-workers, for example, reported about 290 nm at 900 rpm and about 160 nm at 3000 rpm for their organic blend [8]. Coat at several speeds, measure each film and plot a spin curve; see Spin Speed vs Film Thickness: How to Build and Use a Spin Curve.
3. Cross-check methods. Measure at least one sample by a step-height method and by an optical method at comparable positions. Agreement supports consistency between the methods but does not independently validate every model assumption; disagreement can indicate an index error, roughness, a deformed step or different sampled regions.
4. Measure repeatability. In metrology, repeatability is the closeness of agreement between measurements under the same conditions, while reproducibility concerns measurements under changed conditions [9]. Repeat measurements without moving the sample, then repeat after reloading it. Use the observed spread and relevant uncertainty sources to decide whether a measured thickness difference is distinguishable from measurement variation.
5. Map the substrate. Measure at the centre and at several radii to see the thickness profile, not only a single point.
6. Record everything. Method, instrument settings, model or refractive index used, positions and number of repeats belong in the process log next to the coating parameters.

Process background is in Spin Coating Explained: Stages, Physics and Film Thickness, Soft Bake, Post-Bake and Annealing on a Hot Plate: What Each Step Does (films can shrink during baking, so measure after the final bake) and Solution-Processed Thin Films: Choosing Spin, Dip, Slot-Die or Blade Coating.

6.  Common Pitfalls

Summary of common issues; general measurement practice except where cited.

7.  Levon™ Spin Coaters and Hot Plate for Making the Films You Measure

Levon™ does not make thickness measurement instruments; the equipment below produces the films that are measured. Values are quoted from the Levon™ product pages; “Not stated” means the page does not give the value.

Specifications as published on the Levon™ product pages on 24 October 2026. The supplied configuration is confirmed in the quotation. “No load” is stated by the AC100-S page.

The EZ6-S page publishes film-thickness uniformity results for two specific photoresist formulations: on 4-inch wafers, within-wafer ±0.53% and wafer-to-wafer ±1.09% at 1350 nm; on 6-inch wafers, within-wafer ±1.20% and wafer-to-wafer ±1.89% at 4400 nm. The product page identifies both formulations. These are condition-specific results, not a guarantee for other films, speeds or processes.

How this connects to the guide: a spin curve (Section 5) needs films coated at several speeds with a repeatable program. The EZ4-S stores 5 programs of 5 steps and covers 100–9,999 rpm, the AC100-S stores 100 recipes of up to 100 steps, and the EZ6-S covers 100–12,000 rpm. Because films can change during baking, measure after the final bake; the HP6 hot plate reaches 250 °C with ±0.2 °C precision after the temperature is stable. These specifications describe the instruments; they do not establish the thickness or uniformity of a particular film. Questions about a specific configuration can be sent through the contact form.

8.  Keep Exploring the Thin-Film Coating and Metrology Knowledge Hub

Start here

• Spin Coating Explained: Stages, Physics and Film Thickness — stages and physics of spin coating
• Spin Speed vs Film Thickness: How to Build and Use a Spin Curve — building and using a spin curve
• Solution-Processed Thin Films: Choosing Spin, Dip, Slot-Die or Blade Coating — comparing coating methods

Related guides

• Soft Bake, Post-Bake and Annealing on a Hot Plate: What Each Step Does — baking and annealing before measurement
• Spin Coating Defects: Comets, Striations, Pinholes and Edge Beads Diagnosed — coating defects
• Edge Bead Removal and Backside Rinse: EBR/BSR in Spin Processing — edge bead and why to measure inside the edge
• Solution-Processed OLED and OPV Layers: Thickness Targets and a Lab Process Flow — thickness targets in OLED and OPV stacks

Links will be added as each article is published.

Equipment

• EZ4-S Spin Coater — 100–9,999 rpm, 5 programs
• AC100-S Programmable Spin Coater — 100 recipes
• EZ6-S Benchtop Spin Coater — published uniformity data
• HP6 Hot Plate — RT to 250 °C
• Levon™ Spin Coaters — all spin coaters

Questions about a specific instrument can be sent through the contact form.

9.  FAQ

10.  References