Imaging confocal microscopy (ICM) and focus variation (FV) are two of the most used technologies for 3D surface metrology. Both methods rely on the depth of focus of the microscope objective, which depends on its numerical aperture and wavelength of the light source to compute an optical section. In this paper we study how several methods of structured illumination microscopy affect the metrological characteristics of an areal optical profiler. We study the effect of the projection of different structured patterns, the sectioning algorithms, and the use of high and low frequency components onto the optically sectioned image. We characterized their performance in terms of system noise, instrument transfer function and metrological characteristics such as roughness parameters and step height values.
Most 3D metrological microscopes used today require a scanning through the optical axis, which is time consuming. The common techniques are Coherence Scanning Interferometry (CSI), Imaging Confocal Microscopy (ICM), and Focus Variation (FV). If one technique is good for smooth surfaces, it is not for rough ones, while the good for rough is too noisy for smooth ones. Additionally, high local slopes are also dependent on the scattering properties of the surface, making the Numerical Aperture of the objective the most important property of the microscope. Imaging Confocal Microscopy is the best compromise in terms of surface application range (from smooth to rough), high local slopes on shiny surfaces, highest numerical aperture and highest possible magnification. Unfortunately, any kind of Confocal microscope today (laser scan, disc scan or microdisplay scan) requires an in-plane scanning to build up the confocal image in addition to the vertical scan, increasing the total measuring time in comparison to CSI and FV. This is against the needs of quality control in production environments, where scanning speed must be as short as possible. In this paper, we use a Microdisplay Scanning Microscope for obtaining the confocal image only relying on a single image per plane. We use a structured illumination to project a desired pattern onto the surface with a very well-defined frequency and direction. By means of the Hilbert transform, we digitally shift the projected pattern one or many times to recover the bright field and the optical sectioned images. This new method reduces significantly the measurement time, simplifies the overall cost of the system and eliminates the maintenance of scanning devices, while maintaining the optical sectioning properties of each plane. We also studied the performance of the resulting topography in terms of system noise, accuracy, repeatability and fidelity of the surface using different methods to obtain the confocal image. Finally, we also compared the results with true confocal results and with other techniques that require a single image per plane, such as Active illumination Focus Variation (AiFV).
Focus Variation (FV) has been successfully employed for the three-dimensional measurement of rough surfaces. The technique relies on scanning the sample under inspection across the depth of focus of a high numerical aperture microscope objective, while computing the local contrast of its surface. Only those samples with sufficient texture will provide a usable axial response to compute its height location, limiting the application of Focus Variation to optically rough surfaces. Active illumination Focus Variation (AiFV) introduces an artificial texture on the field diaphragm position which is superimposed onto the surface. The benefit is a usable axial response, even when scanning an optically smooth surface, while minimizing the evaluation window of the focus operator close to the spatial autocorrelation length of the artificial texture. In this paper, we show the development of an Active illumination Focus Variation on an existing confocal microscope using Microdisplay Scanning technology. We analyzed the performance of AiFV on smooth surfaces with low frequency components, such as traceable Step Height or Type B2 roughness standards. Higher frequency samples such as random direction roughness standards or high-resolution targets are affected by the lateral resolution loss inherent on the AiFV technique. In this paper, we compare the lateral resolution limit of AiFV and Confocal Microscopy with the use of a Siemens Star specimen for a range of microscope objectives with numerical apertures from 0.3 to 0.95. Its influence on the computed ISO 25178 parameters on random surfaces is shown.
KEYWORDS: Calibration, Standards development, 3D metrology, Solids, 3D image processing, Neodymium, Time metrology, Mirrors, 3D imaging standards, Metrology
Two kinds of 3D label free Bio-Transfer-Standards (BTS) have been further developed at the University of Helsinki (UH). The first one, NanoRuler, is a staircase BTS featuring eight fatty acid bilayers which allows vertical calibration in the range of 5 to 40 nm. The second one, NanoStar, is a V-shaped BTS featuring two 5 nm tall bilayers that overlap at 10° angle. This standard enables the determination of the Instrument Transfer Function (ITF). A stability test was conducted on the BTSs, during which the standards were stored in laboratory conditions, and were profiled each week. Profiling was done using a custom-built Scanning White Light Interferometer (SWLI). The stability of NanoStar was ± 0.3 nm, and of NanoRuler ± 0.5 nm to ± 2.5 nm. The BTSs maintained their specified properties for at least six months and therefore allow vertical calibration and ITF determination. In addition, changes in surface morphology of one NanoRuler subjected to water immersion are presented. This paper reports intermediate findings during an ongoing stability test that will run for 24 months.
Imaging Confocal Microscopes (ICM) are highly used for the assessment of three-dimensional measurement of technical surfaces. The benefit of an ICM in comparison to an interferometer is the use of high numerical aperture microscope objectives, which allows retrieving signal from high slope regions of a surface. When measuring a flat sample, such as a high-quality mirror, all ICM’s show a complex shape of low frequencies instead of a uniform flat result. Such shape, obtained from a λ/10, Sa < 0.5 nm calibration mirror is used as a reference for being subtracted from all the measurements, according to ISO 25178-607. This is true and valid only for those surfaces that have small slopes. When measuring surfaces with varying local slopes or tilted with respect to the calibration, the flatness error calibration is no longer valid, leaving what is called the residual flatness error.
In this paper we show that the residual flatness error on a reference sphere measured with a 10X can make the measurement of the radius to have up to 10% error. We analyzed the sources that generate this effect and proposed a method to correct it: we measured a tilted mirror with several angles and characterized the flatness error as a function of the distance to the optical axis, and the tilt angle. New measurements take into account such characterization by assessing the local slopes. We tested the method on calibrated reference spheres and proved to provide correct measurements. We also analyzed this behavior in Laser Scan as well on Microdisplay Scan confocal microscopes.
A V-shape Bio-Transfer-Standard (V-BTS), developed and produced at the University of Helsinki (UH), was measured in two laboratories. In comparison to Siemens Star calibration specimens, the V-BTS performs better at high lateral frequencies close to the diffraction limit of the optical instrument. This permits determining of the Instrument Transfer Function (ITF). The V-BTS features two lipid bilayer steps that partly overlap each other at an angle of 20°, with an average height of 4.6 ± 0.1 nm. The Round Robin (RR) test aims to determine whether the V-BTS and the developed application protocol work with different optical profilers in different laboratories. First the artefact was measured at Sensofar-Tech, S.L. using an S-neox profiler working in Phase Shifting Interferometry mode. Then V-BTS was measured at UH using a custom-built Scanning White Light Interferometer. All measurements done by four different operators at the two laboratories have a range or standard deviation of ±0.1 nm which agrees with the theoretical estimates and with measurements done using an atomic force microscope and with a surface plasmon resonance based instrument. The RR results show the applicability of the V-BTS for calibration and for ITF characterization of 3D optical profilers.
Confocal microscopes are widely used for areal measurements thanks to its good height resolution and the capability to
measure high local slopes. For the measurement of large areas while keeping few nm of system noise, it is needed to use
high numerical aperture objectives, move the sample in the XY plane and stitch several fields together to cover the
required surface. On cylindrical surfaces a rotational stage is used to measure fields along the round surface and stitch
them in order to obtain a complete 3D measurement. The required amount of fields depends on the microscope’s
magnification, as well as on the cylinder diameter. However, for small diameters, if the local shape reaches slopes not
suitable for the objective under use, the active field of the camera has to be reduced, leading to an increase of the
required number of fields to be measured and stitched. In this paper we show a new approach for areal measurements of
cylindrical surfaces that uses a rotational stage in combination with a slit projection confocal arrangement and a highspeed
camera. An unrolled confocal image of the cylinder surface is built by rotating the sample and calculating the
confocal intensity in the centre of the slit using a gradient algorithm. A set of 360º confocal images can be obtained at
different heights of the sample relative to the sensor and used to calculate an unrolled areal measure of the cylinder. This
method has several advantages over the conventional one such as no stitching required, or reduced measurement time. In
addition, the result shows less residual flatness error since the surface lies flat in the measurement direction in
comparison to field measures where the highest slope regions will show field distortion and non-constant sampling. We
have also studied the influence on the areal measurements of wobble and run-out introduced by the clamping mechanism
and the rotational axis.
Stent quality control is a highly critical process. Cardiovascular stents have to be inspected 100% so as no defective stent is implanted in a human body. However, this visual control is currently performed manually and every stent could need tenths of minutes to be inspected. In this paper, a novel optical inspection system is presented. By the combination of a high numerical aperture (NA) optical system, a rotational stage and a line-scan camera, unrolled sections of the outer and inner surfaces of the stent are obtained and image-processed at high speed. Defects appearing in those surfaces and also in the edges are extremely contrasted due to the shadowing effect of the high NA illumination and acquisition approach. Therefore by means of morphological operations and a sensitivity parameter, defects are detected. Based on a trained defect library, a binary classifier sorts each kind of defect through a set of scoring vectors, providing the quality operator with all the required information to finally take a decision. We expect this new approach to make defect detection completely objective and to dramatically reduce the time and cost of stent quality control stage.
A stair case height Bio-Transfer-Standard (BTS), developed and produced at the University of Helsinki (UH), was measured in two laboratories. The Round Robin test aims to determine whether BTS works with different optical profilers in different laboratories. First the artefact was measured at UH using a custom-built Scanning White Light Interferometer. Then BTS was measured at Sensofar-Tech, S.L. using an S-neox-type interferometer working either in Phase Shifting Interferometry mode or in Imaging Confocal Microscopy mode. To remove the influence of system calibration, a method featuring sample shifting and measurement subtraction was used. The BTS features eight lipid bilayer steps that each are 4.6 ± 0.1 nm tall on average. All 30 measurements done by four different operators at the two laboratories agree to within 0.1 nm which agrees with theoretical estimates and with measurements done using a surface plasmon resonance technique. The Round Robin results show the applicability of the newly developed bio-imaging transfer standard for calibrating 3D optical profilers.
Stent quality control is a critical process. Coronary stents have to be inspected 100% so no defective stent is implanted into a human body. We have developed a high numerical aperture optical stent inspection system able to acquire both 2D and 3D images. Combining a rotational stage, an area camera with line-scan capability and a triple illumination arrangement, unrolled sections of the outer, inner, and sidewalls surfaces are obtained with high resolution. During stent inspection, surface roughness and coating thickness uniformity is of high interest. Due to the non-planar shape of the surface of the stents, the thickness values of the coating need to be corrected with the 3D surface local slopes. A theoretical model and a simulation are proposed, and a measurement with white light interferometry is shown. Confocal and spectroscopic reflectometry showed to be limited in this application due to stent surface roughness. Due to the high numerical aperture of the optical system, only certain parts of the stent are in focus, which is a problem for defect detection, specifically on the sidewalls. In order to obtain fully focused 2D images, an extended depth of field algorithm has been implemented. A comparison between pixel variance and Laplacian filtering is shown. To recover the stack image, two different methods are proposed: maximum projection and weighted intensity. Finally, we also discuss the implementation of the processing algorithms in both the CPU and GPU, targeting real-time 2-Million pixel image acquisition at 50 frames per second.
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