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euspen’s 19th International Conference &
Exhibition, Bilbao, ES, June 2019
www.euspen.eu
High-speed, Roll to Roll Coherence Scanning Interferometry in a laser texturing
process
C. Bermudez, P. Martínez, C. Cadevall, R. Artigas
Sensofar Tech SL, Barcelona, Spain
bermudez@sensofar.com
Abstract
Surface engineering is a crucial technology in many industry sectors. It consists of adding functionality to a surface by coating or
texturing it. Currently, coating technologies are mature and well established, but coatings suffer from limited durability and poor
sustainability. Laser surface texturing is nowadays emerging due to the latest improvements in high resolution, excellent repeatability
and keeping the process non-contact. However, associated costs with the laser texturing system and low throughput are still to
improve. In such a process, quality assurance is mandatory. A surface characterization procedure needs to take part of the full process
without impacting the cycle time. These premises can be attained using a non-contact, optical 3D profiler integrated with the laser
texturing machine. We have developed a miniaturized optical sensor head that integrates Coherence Scanning Interferometry (CSI)
as the measuring technique with interchangeable optics, allowing to adapt the lateral resolution to the laser texturing process.
Interferometric measurements are carried out using dual-wavelength illumination, granting measurements from 25 µm/s with 1 nm
of system noise to more than 200 µm/s with less than 100 nm of system noise. Reduced dimensions of the sensor head make possible
to integrate it inside the laser texturing machine without interfering with the laser texturing process, even parallelizing both texturing
and measurement. Together with feedback analysis and finite element modelling, a closed-loop manufacturing scheme is achieved.
In-process measurement, Interferometry, Microscope, Surface
1. Introduction
As a form of surface engineering, laser functional texturing is
a key enabling technology that is relevant to almost every
industrial sector, with applications ranging from anti-icing, self-
cleaning surfaces to wear reduction and biocompatibility
enhancement. It can offer significant benefits to manufacturers,
such as cost savings, improved product performance and faster
product development. However, the process is viewed by many
as complex and costly, which involve huge design of experiments
to get the desired surface texture.
To face such a challenge, a fast and accurate quality control
(QC) stage, as in most high precision processes is a must. In
advanced manufacturing, a perfect example would be Roll to
Roll (R2R) techniques, where high throughput and low cost are
required.
According to Industry 4.0 trend, QC can be integrated into the
manufacturing process giving feedback for close-loop
manufacturing and establishing real-time knowledge for a data
management system and modelling software. This is currently
being pursued by the SHARK project, depicted in Figure 1.
SHARK (Laser surface engineering for new and enhanced
functional performance with digitally enabled knowledge base)
project will unlock the potential for laser texturing for the
generation of functional surfaces by boosting the productivity,
efficiency and flexibility of the process.
The rest of the paper is organized as follows: in section 2, the
methodology is described. Section 3 shows the results on real
measurements implementing the explored method whereas
section 4 presents the conclusions.
Figure 1. Close-loop manufacturing scheme being pursued in the SHARK
project.
2. Method
The chosen technology that enables non-contact, high
accuracy and high speed measurement is Coherence Scanning
Interferometry (CSI) [1]. CSI features an extremely good vertical
resolution (down to 1 nm). However, it requires a dense Z scan
to extract the areal information. This is due to the short
coherence length of the white light source, intended to
maximize the measurement repeatability.
To overcome this limitation, a light source with narrower
bandwidth than white light has can be used to have a wider
correlogram, and then increase the scanning speed but still
getting enough measured data (Figure 2). This allows the sensor
to measure at higher speed (up to 9 times the original speed with
the same frame rate).
This technology has been implemented into a Sensofar
commercial 3D optical profiler using CSI and white light epi-
illumination (Figure 3). Green and white optical beams are mixed
through a beam-splitter. The measurement light source is
automatically enabled according to the selected speed factor.
White LED for 1X and 3X, Green for 5X, 7X, and 9X. Speed ranges
from 24 µm/s to 219 µm/s at 340 fps, VGA resolution.
Figure 2. White and Green LED correlograms. In red, acquisition points
for a single pixel, separated between them by λ/8 multiplied by the
speed factor: 1X (top) and 9X (bottom).
Figure 3. Dual LED arrangement: broadband (white) and monochromatic
(green) light sources for high- accuracy and high-speed measurements:
3D cut model (left), prototype (right).
3. Results
According to the measurement speeds described in the former
section (up to 219 µm/s), a 3D topography of a variety of typical
samples usually measured with interferometry can be obtained
in less than 2 seconds. An example of a laser texturing
application obtained with such a method appears in Figure 4.
Figure 4. Laser textured friction disc (courtesy of MAN Energy Solutions).
Lateral size of holes is in the order of 25 to 75 µm. Depths range from 10
to 40 µm. Obtained with a 50X Mirau 0.55 NA objective lens.
The repeatability of the system with both light sources at
typical speed (factor set to 1X) and maximum speed (9X) has
been studied with the calibration specimen Areal Irregular (AIR)
B40-P1F from the NPL. Its calibration values are depicted in
Table 1. Measurement settings, instrument noise obtained
according to the ISO method and statistical results of AIR B40-
P1F specimen are presented in Table 2.
Table 1 NPL AIR B40 calibration values.
Sa, Mean measured value
790.7 nm
Sa, expanded uncertainty
26.3 nm
Table 2 Measurement settings and results comparison for two different
measuring speeds.
Speed factor
1X
9X
Light source
White
Green
Objective lens
20X Mirau, 0.4NA
Speed
24 µm/s
219 µm/s
Instrument noise
9 nm
120 nm
AIR B40-P1F Average
815.25 nm
829.50 nm
AIR B40-P1F Std Dev
0.80 nm
3.95 nm
The results depict a noise increase proportional to the speed
rise, although it is still low to applications that demand speed
rather than precision. Values have been obtained without
vibration isolation, similar to a precision laser texturing
environmental conditions. With proper vibration isolation,
system noise can decrease down to 1 nm at 1X speed and below
100 nm at 9X speed.
In terms of accuracy, it can be calibrated as stated in ISO 25178
with an “Amplification Factor”. According to the results,
accuracy varies implying that possibly every measuring speed
must have a different amplification factor. We will study this
effect in future work.
4. Conclusions
A custom made, fast interferometer has been built for
measuring, with high speed, into a laser surface texturing, close-
loop application. The sensor, by the use of Coherence Scanning
Interferometry with white and green light sources, allows the
system to measure up to 219 µm/s with 120 nm of system noise.
A repeatability test with a standard calibration specimen has
been performed. Results point to calibrate each speed with a
different calibration (amplification factor), which will be
investigated in the future. The setup enables a roll to roll
application to obtain a single measurement in less than 2
seconds, and use such data to quickly readapt process
parameters or even re-texture the sample to meet the
specifications.
Acknowledgements
This project has received funding from the European Union’s
Horizon 2020 Framework Programme for research and
innovation under grant agreement no 768701.
References
[1] de Groot P 2011 Coherence Scanning Interferometry in “Optical
measurement of surface topography” (Springer), Berlin, 187-206.
[2] ISO 25178-700, Geometrical product specifications (GPS) —
Surface texture: Areal—700: Calibration, adjustment and
verification of areal topography measuring instruments.
