Application Note | Film Thickness
Transparent Film Thickness by 3D Non-Contact Profilometry
Thickness and Uniformity of CD and Blu-ray Disc Cover Layers in Thickness Mode
Research & Experimental Testing
Duanjie Li
Visual Design & Editorial
Andrew Shore
Introduction
Transparent thin films and overcoats are extensively applied in industries such as optics, semiconductor, and solar cells to achieve specific functions. Effective and precise measurement of the thickness, metrology and quality of the transparent films becomes vital to ensure proper function and performance of the coated device.
Traditional stylus-based profilometers determine the surface morphology of the coatings by sliding in contact across the measured surface, which may create minor scratches on the delicate and sensitive optical device. Moreover, such a technique is impossible to measure the thickness of the film that covers the whole coated surface. The NANOVEA 3D Non-Contact Profilometers utilize chromatic confocal technology with unmatched capability to provide a comprehensive analysis of transparent coatings.
Importance of Coating Inspection for Quality Control
Surface finish on both the top and bottom surfaces of the transparent film as well as its thickness and uniformity is critical for product quality and performance. For example, in CD, DVD and Blu-Ray Disc (BD) production, precise control of the thickness and uniformity of the transparent cover and space layers plays an important role in avoiding focus errors of the laser. Improper injection molding process during CD and BD production may lead to stress-induced birefringence and unreliable data reading. An accurate thickness measurement of the transparent film ensures reliable product inspection and quality control.
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Measurement Objective
The optical thickness and uniformity of the transparent coatings on CD and BD were measured and compared using the NANOVEA PS50 optical profilometer. We would like to showcase the capability of the NANOVEA optical profilometer in measuring the thickness and homogeneity of the transparent films.
NANOVEA PS50 Compact
Optical Profilometer
Results and Discussion
Fig. 2 compares the spot size, wavelength and layer architecture of CD and Blu-ray disc (BD) [1]. Compared to CD, the BD technology decreases the wavelength from 780 to 405 nm, increases the numerical aperture from 0.60 to 0.85, and makes the cover layer thinner from ~1.1 to ~0.1 mm, resulting in a reduction of the pit size from 600 to 130 nm, and of the track pitch from 1.6 µm to 320 nm. Such an improvement substantially increases the storing capacity of BD to 25 GB per layer, compared to 700 MB for CD. The transparent cover coating plays a vital role in data reading and scratch protection for the recording layer beneath. In this study, the thickness and uniformity of the cover layer of CD and BD are measured using the NANOVEA PS50 profilometer in Thickness Mode.
The optical thickness and uniformity of the transparent cover coatings on CD and BD were obtained by subtracting the measured profiles of the surface and bottom of the cover coatings. The refractive index n of the polycarbonate cover layer is 1.585. The thickness of only the transparent film with a known uniform refractive index can be measured at this stage.
The 3D View of the optical thickness and uniformity of the transparent coatings on CD and BD are compared in Fig. 3. The BD shows a more uniform cover coating thickness compared to CD. This can be attributed to their difference in the physical structure. BD uses a coating only ~0.1 mm thick, compared to ~1.1 mm for CD. The thinner protective coating of BD causes less diffusion of the blue laser travelling through it during data reading, allowing for greater track density and hence higher capacity. Such diffusion of the light contributes to the higher thickness variation measured for the thicker CD coating in this test.
Fig. 4 confirms the above conclusion. It can be observed in the Abbott-Firestone curves that the thickness of the CD and BD cover coatings are about 1125.0 and 102.3 µm, respectively, in agreement with the diagram of the layer architecture shown in Fig. 2. The thickness of the CD cover layer varies from ~1121.5 to ~1128.5 µm (~7 µm difference), compared to a range from ~101.9 to ~102.5 µm (~0.6 µm difference) for BD. This is also reflected in the height variation parameters such as the Sa value, where Sa are 0.842 and 0.081 µm, respectively, for CD and BD cover layers. The 2D thickness profiles shown in Fig. 5 also confirm the better quality of the BD cover layer.
| ISO 25178 parameter | CD | BD |
|---|---|---|
| Sa (µm) | 0.842 | 0.081 |
| Sq (µm) | 1.756 | 0.099 |
| Sp (µm) | 5.017 | 7.624 |
| Sv (µm) | 1125.161 | 1.557 |
| Sz (µm) | 1130.177 | 9.180 |
| Ssk | −410.069 | 0.949 |
| Sku | 262763.139 | 68.170 |
Table 1: Areal parameters of the cover layer thickness maps (from Fig. 4).
Fig. 5: 2D thickness profiles of CD and BD cover layers.
| ISO 4287 roughness (Gaussian filter, 0.762 mm) | CD | BD |
|---|---|---|
| Ra (µm) | 0.646 | 0.032 |
| Rz (µm) | 4.257 | 0.239 |
Table 2: Roughness of the thickness profiles (from Fig. 5).
Conclusion
The thickness and uniformity of the CD and Blu-ray disc transparent cover coatings are measured using the NANOVEA PS50 non-contact profilometer, showcasing the accuracy of this technique. Such a measurement provides a useful tool in both development phases and quality control of the homogeneous transparent coatings used in a variety of industries, including optics, semiconductor, solar cells and many others. The analysis software provides measurements such as coating thickness, uniformity and roughness, enabling users to have a comprehensive understanding of the coating properties.
The data shown here represents only a portion of the calculations available in the analysis software. NANOVEA Profilometers measure virtually any surface in fields including Semiconductor, Microelectronics, Solar, Fiber Optics, Automotive, Aerospace, Metallurgy, Machining, Coatings, Pharmaceutical, Biomedical, Environmental and many others.
References
Blu-ray Disc, Wikipedia, http://en.wikipedia.org/wiki/Blu-ray_Disc
Frequently Asked Questions About Transparent Film Thickness Measurement
How can a profilometer measure a transparent film’s thickness?
In thickness mode, the chromatic confocal pen detects both the top and the bottom surface of the transparent layer; subtracting the two profiles gives the optical thickness, corrected with the film’s refractive index (1.585 for polycarbonate).
Why not use a stylus profilometer?
A stylus slides in contact and can scratch delicate optical surfaces, and it cannot measure the thickness of a film that covers the whole surface.
How thick were the CD and Blu-ray cover layers?
About 1125.0 µm for the CD and 102.3 µm for the Blu-ray Disc, in agreement with the nominal ~1.1 mm and ~0.1 mm layer architectures.
Which cover layer was more uniform?
The Blu-ray layer varied from ~101.9 to ~102.5 µm (~0.6 µm) against ~1121.5 to ~1128.5 µm (~7 µm) for the CD; Sa was 0.081 µm for BD and 0.842 µm for CD.
Why does cover layer uniformity matter?
Precise thickness and uniformity of the cover and spacer layers avoid laser focus errors, and improper injection molding can cause stress-induced birefringence and unreliable data reading.
What are the limits of the technique?
At this stage only a transparent film with a known, uniform refractive index can be measured.
