Thickness Measurements of Opaque and Transparent Films or Coatings with WLI
Combine Film Thickness Metrology with WLI for Step Measurements and Through-Film Measurements
Accurate control over film thickness and uniformity is essential for throughput and performance in the automotive, aerospace, semiconductor, medical, and research industries. The modern standard for 3D, non-contact measurements of surface topography and texture is white light interferometry (WLI), also known as coherence scanning interferometry (CSI). It is the only method that provides film thickness, coating uniformity information, and roughness of both film and substrate over large areas, all in a single measurement that takes only a few seconds to obtain.
Contents include:
KEYWORDS: 3D Optical Profiler; 3D Surface Metrology; Profilometer; Application Note; White-Light Interferometry; Thick Films; Thin Films; Thickness Measurements
Introduction
Accurate control over film thickness and uniformity is essential for throughput and performance in the automotive, aerospace, semiconductor, medical, and research industries. The modern standard for 3D, non-contact measurements of surface topography and texture is white light interferometry (WLI), also known as coherence scanning interferometry (CSI). It is the only method that provides film thickness, coating uniformity information, and roughness of both film and substrate over large areas, all in a single measurement that takes only a few seconds to obtain. This application note describes film thickness metrology with WLI, particularly through-film measurements of transparent thick and thin films and opaque film step measurements.
Traditional Step Measurements for Thickness
The simplest method of determining thickness of opaque, non-transparent, or (sometimes) transparent films is by measuring a step from the top of the film to the bare substrate. The maximum measurable thickness is only limited by the profiler’s vertical scan length, which is up to 10 mm on Bruker WLI profilers. Minimum measurable thickness is 50–100 nm (below which most films are no longer opaque), depending on the film material. Automatic step measurement analysis with nanometer repeatability is a standard part of Bruker’s Vision64® analysis software.
Through-Film Measurements of Thickness
The thickness of semi-transparent films can alternatively be obtained by measuring through the film to the substrate, which provides meaningful film measurements anywhere on the sample. Interferometric objectives employed in WLI measurements develop the moiré fringe envelope when passed through focus. Using a special Vertical Scanning Interferometry (VSI) measurement mode, an interferometric objective is translated vertically such that the upper and lower film surfaces pass through focus, causing an interference fringe envelope pattern on each coating surface. For each scan location in the field of view, two sets of interference fringes are developed for each camera pixel as the objective moves downward: one corresponding to best focus at the top of the film, the second corresponding to the film/substrate interface, as shown in Figure 1.
Choice of objective lens dictates measurable film thickness, both due to light penetration and fringe envelope separation. Lower magnification objectives have a lower numerical aperture (NA), meaning more vertical light enters the film and penetrates deeper, enabling thicker films to be measured. On the thin film side, 50x and higher objectives have a shorter depth of focus and higher NA, enabling resolution of small distances between the two fringe sets of films less than 2 µm thick. The thinnest films will have overlapping fringe envelopes regardless of objective choice, necessitating a dedicated thin film data analysis discussed later.
Transparent Films .2 µm (Thick Films)
Thick Film Analysis, a Vision64 software option, determines thickness based on the fringes and separation of fringe envelopes. The analysis first determines the vertical center of each fringe envelope, then calculates the distance between their centers for each camera pixel. This distance is divided by the film’s index of refraction to determine an accurate thickness, as seen in Figure 3a.
Table 1 shows the results of ten measurement repeats for several semi-transparent films thicknesses. This data highlights the optical profiler’s excellent repeatability and accuracy over the 3–26 µm thickness range for various magnifications over large lateral areas.
For the analysis to determine thickness accurately, the film’s index of refraction must be well-known, homogenous, and usually measured independently by such means as reflectometry. If the index is not known, the step measurement method from the film surface to the substrate is used and the index of refraction can then be back-calculated and entered.
Transparent Films <2 µm (Thin Films)
For films thicknesses less than approximately 2 µm where the fringe envelopes are almost merged (see Figure 3b) causing difficulties to distinguish them, the thin film measurement mode was developed for these challenging applications. Thin Film is a fringe-signal modelling algorithm using a merit function, which measures the agreement from the measured data and a built fitting model. The fitting model, as visually seen in Figure 4, is created by a measurement model from the bare substrate after selecting magnification along with the film and material calibration models provided within Vision64 for many common film and substrate materials.
If a film model is not provided, customer-specific models can easily be constructed by creating a table with various n and k optical values for the wavelengths covered for the new coating material. Once proper thin film models are created for the materials to be measured, Vision64 can automatically measure and determine the film thickness with predetermined boundaries and has great correlation to calibrated thicknesses as seen in Table 2.
Advantages of WLI for Film Thickness Metrology
WLI can be used to measure a large variety of film types and thicknesses, with either the step method or through-film measurements, and has a number of significant benefits:
- With through-film measurements, WLI measures film thickness for every camera pixel in the field of view, highlighting variations in thickness and uniformity across an area up to 15 mm in a single, quick measurement.
- Topography of both the film and substrate surfaces is returned, giving the most comprehensive view of coating uniformity and surface textures.
- A white-light interferometer’s range extends to tens of micrometers using the thick film mode and 10 mm using the step measurement method.
- Optical profiling can characterize surfaces that many other techniques cannot reach, such as the inside of coated cylinders or the bottom of narrow trenches.
- Since WLI requires no sample preparation, the method offers non-contact, non-destructive analysis with immediate fast feedback.
Authors
Roger Posusta, Senior Marketing Application Specialist, Bruker (Roger.Posusta@bruker.com)
Sandra Bergmann, Product Line Manager, Bruker (Sandra.Bergmann@bruker.com)
Erica Erickson, Ph.D., Materials Science Writer, Bruker (Erica.Erickson@bruker.com)
©2024 Bruker Corporation. All rights reserved. Vision64 is a trademark of Bruker Corporation. All other trademarks are the property of their respective companies. AN583, Rev. A0.
