Rationale
Film thickness measurement is a key quality control link in materials science, semiconductors, optical coatings, coating industry, and new energy. Its measurement principle is mainly based on different interaction mechanisms in physics, which can be divided into two categories: contact and non-contact. The core principle is to accurately calculate or derive thickness values by detecting differences in response between the film and the substrate or the film itself to specific physical signals.
For optically clear or translucent films, spectral reflection or ellipsometry is mainstream. When a beam of light with a wide spectrum or specific polarization state hits the surface of the film, it is reflected and interferes at the air-film and film-substrate interfaces. By analyzing the intensity, phase, or polarization state changes of the reflected spectrum through the detector, a mathematical model relationship with the film thickness can be established. For example, for a monolayer film, its optical thickness (nd) and the extreme point wavelength (λ) of the reflection spectrum satisfies: 2nd = mλ, where n is the refractive index, d is the physical thickness, and m is the interference order.
For opaque or metallic films, mechanical probe profilometry and eddy current/electromagnetic induction are common methods. The profiler measures the height difference directly by crossing the film step with a high-precision probe. The eddy current method uses the high-frequency electromagnetic field generated by the coil in the probe to induce eddy current in the conductive film, and its strength is affected by the thickness of the film, and the thickness can be inferred through calibration.
The X-ray fluorescence rule applies to thin films with known elements, and by measuring the intensity of the characteristic X-ray fluorescence of the film, the intensity of which is directly related to the amount of material (i.e., thickness) in the excitation zone.
Classification of measurement technology
Different measurement techniques correspond to different application scenarios, coating materials and accuracy requirements. The first step in selection is to understand the core characteristics and limitations of each technology.
| Technical name | Typical features and applicable scenarios |
| Spectral reflectometry | Suitable for transparent/translucent films. Fast speed and can measure multi-layer films. Sensitive to surface roughness. |
| ellipsometry | The accuracy is extremely high, and the film thickness and optical constant can be obtained at the same time. It is suitable for ultra-thin films and complex film systems. |
| Mechanical profiler | Direct contact measurement, steps need to be made. Measurable absolute thickness and may scratch the soft membrane. |
| eddy current/electromagnetic method | It is used for non-contact measurement of conductive film layers. Fast and suitable for metal plating or coating. |
| X-ray fluorescence | Used for films with known elemental composition. Non-destructive, the composition and thickness of the alloy can be measured. |
| Ultrasonic thickness measurement | Suitable for thicker coatings or less accessible to the back workpiece. Acoustic couplants are required. |
Selection considerations
Selecting a suitable film thickness gauge is a systematic project that requires a comprehensive evaluation of the following technical parameters and application conditions.
Film & Substrate Material:This is the deciding factor. The preferred optical method for transparent dielectric films; Metallic conductive films can be considered eddy current or XRF; For flexible polymer films, contact methods that may cause scratches should be avoided.
Thickness measurement range and accuracy:Clarify the order of magnitude (nanometer, micron or millimeter) and allowable error of the film thickness to be measured. Ellipsometers are good at nano to sub-micron levels; The spectral reflectometer has a wide range; Profilers and ultrasonics are suitable for thicknesses above microns.
Measurement speed and automation needs:In-line quality control requires high-speed, non-contact measurements (such as spectral reflections or eddy currents); Laboratory research and development may focus more on multifunctionality and high accuracy (such as ellipsometers), and can be used with automated platforms.
Sample Status and Environment:Consider sample size, shape (flat, curved), whether contact is allowed, whether a vacuum environment is required, etc. Large workpieces may require a handheld eddy current or ultrasonic thickness gauge.
Data Output & Analysis Software:The instrument's accompanying software should provide reliable model fitting, database support, and a report format that complies with relevant standards (e.g., ASTM, ISO), which is critical for traceability of results.
Application Scenarios and Standard References
In practical applications, measurement requirements are often associated with specific industry standards. For example, in the semiconductor industry, the measurement of oxide layer or photoresist thickness on silicon wafers often follows SEMI standards and is widely employed in spectral reflection or ellipsometry techniques. In the automotive industry, the thickness of body paint coatings is often measured using standards such as ISO 2808 using the eddy current method (metal matrix) or ultrasonic method (plastic matrix).
When selecting, it is recommended to first consult the materials testing standards prevalent in the industry, which usually recommend or specify applicable measurement methods, instrument accuracy requirements, and calibration procedures. This ensures that the measurement results are comparable and recognized within the industry.
Summary
There is no universal solution for the selection of film thickness gauge, and the core is to accurately match the measurement principle and technical characteristics of the instrument with the specific attributes, measurement objectives and production and development environment of the sample to be tested. It is recommended that users use representative samples for field testing or comparison experiments as much as possible to verify the performance of the instrument under actual conditions before making a decision. Through systematic principle understanding and prudent selection and evaluation, we can provide a reliable data cornerstone for quality control and scientific research.
References
Tompkins, H. G., & Irene, E. A. (2005). Handbook of Ellipsometry. William Andrew.
ASTM B568-98 (2018). Standard Test Method for Measurement of Coating Thickness by X-ray Spectrometry.
ISO 2808:2019. Paints and varnishes — Determination of film thickness.