Application Note | Transparent Film Thickness
Transparent Film on Transparent Substrate Measurement Using 3D Profilometry
Thickness Two Ways and Roughness of a Clear Film on Glass with One Instrument
Research & Experimental Testing
Craig Leising
Visual Design & Editorial
Andrew Shore
Introduction
A single instrument that can measure both the thickness and roughness of thin transparent material on a transparent substrate is crucial both in development and quality control. This can prove challenging because traditional stylus-based profilometers have difficulty accurately measuring a soft film, while other optical techniques have difficulty measuring a thin transparent film on a transparent substrate. There are other instruments capable of measuring the thickness of this material combination, but they do not have the ability to measure roughness.
Importance of Surface Metrology Inspection for Quality Control
Thickness and roughness of transparent films of this type are critical, as they are typically intended to be optically transparent. If a material is too thick or its surface roughness is too high, the optical properties of the material will vary. Ensuring quality control of such parameters relies heavily upon quantifiable, reproducible and reliable inspection. The NANOVEA 3D Non-Contact Profilometers utilize chromatic confocal technology with unmatched capability for transparent-on-transparent applications. Where other techniques fail to provide reliable data due to probe contact, surface variation, angle, absorption or reflectivity, NANOVEA Profilometers succeed.
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Measurement Objective
The NANOVEA PS50 is used to measure the step height thickness, optical thickness and roughness of a thin transparent film on a transparent glass substrate. Step height is obtained by measuring an area of the film and an area where the substrate is exposed for the relative height difference, while optical thickness is measured using the PS50’s capability of measuring through the transparent film and detecting reflections from both the top surface of the film and the substrate simultaneously.
NANOVEA PS50 Compact
Optical Profilometer
Results
The 3D profile and false color height map show the film edge with the exposed glass substrate beside it.
| ISO 25178 height parameter | Value |
|---|---|
| Sa | 0.691 µm |
| Sq | 0.894 µm |
| Ssk | 0.484 |
| Sku | 4.174 |
| Sp | 4.8 µm |
| Sv | 4.024 µm |
| Sz | 8.823 µm |
Table 1: Roughness results of the transparent film.
Conclusion
Both the step height thickness measurement and the direct thickness measurement (measurement through the transparent film) show similar thickness calculations of 57 µm, confirming the accuracy of both methods of thickness measurement. Step height can be used when an area of the substrate is exposed, and direct thickness measurement can be used when there is no area of substrate exposed (the refractive index must be known). These two methods prove useful in both development phases and quality control applications.
The results above clearly illustrate the NANOVEA PS50’s ability to precisely measure thin transparent films. With the PS50 it is also possible to make these types of measurements on transparent or opaque films on transparent or opaque substrates, among many others.
Frequently Asked Questions About Transparent Film Measurement
Why is transparent-on-transparent hard to measure?
Stylus profilometers struggle with soft films and many optical techniques struggle with a thin clear film on a clear substrate; other instruments can measure the thickness but not the roughness.
How was the film thickness measured?
Two ways: as a step height between the film and an exposed area of substrate, and directly through the film by detecting reflections from the film surface and the substrate at the same time.
Did the two methods agree?
Yes. Both gave a thickness of about 57 µm; the step height calculation returned 57.293 µm.
When should each method be used?
Step height when an area of substrate is exposed; direct thickness when no substrate is exposed, provided the refractive index is known.
How rough was the film?
Sa 0.691 µm and Sq 0.894 µm (ISO 25178), with Sz 8.823 µm.
Why do thickness and roughness matter for clear films?
These films are intended to be optically transparent; if they are too thick or too rough, their optical properties change.
