In this investigation, a polarization multiplexing technique is applied to optical testing, especially shearing interferometers, low coherence interferometers and a clocking motion interpreter. Polarization multiplexing techniques possess the snapshot capability in interferometers and polarization imaging devices, and can enhance the measurement speed compared to the conventional optical techniques, which needs a timely-mannered procedure. Experimental results show the various application of the polarization multiplexing technique and give some insight to be used in optical testing area.
Deflectometry is a non-contact optical technique used for measuring specular free-form surfaces by projecting structured light using a display screen (LCD screen). While one-shot deflectometry has been extensively researched to develop reliable measurement methods, it is very challenging to measure and inspect complex surfaces using one-shot with low reflectivity because of obtaining phase information from a single, poor-quality complex pattern remains difficult. To address this, we propose a novel single-shot deflectometry approach that utilizes deep learning (DL) to measure complex surfaces accurately in a single shot. We employ DYnet++, a deep learning network model capable of retrieving phase information from single composite patterns. By comparing the results with the 16-step phase-shifting (PS) method, we validate the feasibility and effectiveness of our deep learning-based single-shot deflectometry, which offers potential applications in various industries.
A geometric phase component is very attractive in optical metrology because its meta-surface characteristic enables traditional optical systems to be more compact and multi-functional. In this presentation, we introduce two types of geometric phase components, i.e. a geometric phase lens and a polarization grating in surface figure metrology. Their features of polarized beam splitting and phase retardation play a role of wavefront shearing device, and simple shearing interferometers can be designed. We focus on the instrumentation of a radial shearing interferometer using a geometric phase lens and a lateral shearing interferometer based on a polarization grating. With the aid of a polarization pixelated CMOS camera, each interferometer can provide the phase map corresponding to the sheared wavefront as a snapshot measurement. In the experiment, various wavefronts generated by a deformable mirror and shapes of several mirrors were measured and compared with other commercial devices.
As a summary of of the authors' previous paper of Ref 1, we describe a new scheme of a Linnik interferometric configuration based on spectrally-resolved white-light interferometry for simultaneous measurement of top surface and its underlying film surfaces in multilayer film structure. Our proposed technique enables accurate measurements of the phase and reflectance over a large range of wavelengths using the iterative least-squares phase-shifting algorithm by suppressing critical phase shift errors, and it provides a better measurement result than conventional methods. To verify our method a complex multilayer film was prepared and we measured it, and compared with well-known conventional techniques. Comparison results show our new method successfully works well with high precision as same as existing methods.
In order to reduce the fabrication time of the diffractive optical elements (DOEs), a new process is proposed by combining the laser ablation phenomenon using the laser intensity in the conventional thermochemical process. The basic mechanism of the proposed method and experimental results are also presented. We confirmed the effect of reducing the movement distance of the stage for the production of the overall lithography when we made repetitive square patterns. The time reduction rate is drastically improved when the number of patterns is increased. Various patterns including rectangular, triangular, parallelogram, and diamond shape were fabricated by using the proposed method.
Contrary to conventional mechanical polishing methods using polyurethane or pitch tool, non-contact polishing
technique based on ion beam sputtering provides deterministic and ultra-precision surfacing at any given surfaces. Owing
to no contact between a tool and a workpiece, several issues related to tool wear and edge effects can be evitable.
Moreover, the atomic level sputtering makes it possible to obtain ultra-precision optical surfaces with a sub-nanometer
surface roughness. In this paper, we have simulated ion beam figuring process according to the characteristics of ion
beam and performed a simple test.
For the purpose of fabricating off-axis aspheric optics, we propose a 8-axis-polishing machine combined with a testing
tower whose height is about 9 m. The proposed polishing machine was designed and analysed by using a well-known
finite element method. The eight axes of the machine have a synchronized motion generated by a computer, and each
axis was calibrated by a heterodyne laser interferometer or an optical encoder. The maximum capability of the proposed
polishing machine is up to 2 m in diameter, and the maximum radius of curvature of the product (optics) is slightly over
7 m. After calibration, the maximum positioning error of the machine was less than 2 μm within a whole 2 m × 2 m area.
A typical fabrication result of a φ1.5 m concave mirror was also described in this manuscript.
Current commercial height profile measuring instruments, e.g. the confocal microscope and, the white light
interferometer, are widely used in both research institutes and industry. The systematic error of such instruments can be
the same order of magnitude as features on the surface to be measured, if care is not taken with calibration. Instrument
error in most cases depends on the surface slope. Thus, calibration of the instrument is important. The random ball test,
proposed by Parks et al, is a self calibration technique for transmission sphere calibration in phase shift interferometry.
The idea is, by measuring a collection of random patches on the surface of a sphere and then averaging the results, the
contributions from the ball go to zero leaving only the systematic biases due to the instrument. This paper shows it can
also be used to calibrate slope-dependent errors in profilometers such as the scanning white light interferometer (SWLI).
This will be demonstrated with both simulation and experimental results. For example, with a commercial SWLI
measurement with a 20X objective, our random ball test indicates that the height error can be as large as 250 nm at a
slope value of 2.9 degrees when using the envelope peak algorithm for analysis. Similarly, with a confocal microscope
measurement using a 50X objective, the height error can be as large as 800 nm at a slope value of 12.1 degrees. These
slope-dependent errors can be used to compensate future sloped-surface measurements.
Wavelength scanning interferometry based on a reflectometry model is proposed for measuring the absolute thickness
profile of a thin silicon wafer. A Fourier-based method of wavelength scanning interferometry is limited to thicker
wafers because of a tuning range limitation of the source. As an example, the minimum thickness measurable with the
conventional Fourier-based technique using a 4 nm-tunable (500 GHz) 1550 nm laser is approximately 170 μm. Our
proposed method enables an extension of thickness measurements with a reduction in systematic measurement error,
representing a significant advance. The so-called 'ripple-error' or 'fringe-bleed through' is much lower with a
reflectometry-based analysis compared to a Fourier-based analysis. Our method was verified by measuring and testing
several wafers with various thicknesses.
Wavelength scanning interferometry offers many advantages over traditional phase shifting interferometry, most
significantly the elimination of mechanical movement of the part/s for phase modulation by implementing a tunable light
source. Further, Fourier analysis on the interference time history enables this technique to accurately measure distances,
treating the distance between two optical surfaces as an interferometric cavity. We propose to evaluate the uncertainty in
the thickness measurement of a transparent cavity using a commercial Fizeau wavelength scanning interferometer. This
work follows the theory and measurement performed in a previous manuscript of measuring absolute distances of
opaque objects using a commercial wavelength scanning interferometer. The limits in measuring a cavity using the
commercial wavelength scanning interferometer depend on many factors such as temperature variations that affect the
test and reference cavity, uncertainty in the reference cavity calibration, tuning rate non-linearities, etc. In addition to an
analytical approach, a simulation is described to better understand the measurement process and the uncertainty
associated in measuring absolute distances (thickness) of cavities. Preliminary experimental results on the absolute
thickness of a transparent cavity are reported along with uncertainty sources.
The principle of angle-resolved reflectometry is exploited for thin-film thickness measurements. Within an optical
microscope equipped with a high NA objective, a sequence of quasi-monochromatic light of different wavelengths is
generated from a white-light source through spectral filters. Then for each wavelength, the reflectance intensity from the
thin-film sample is monitored on the back focal plane of the objective. This enables collection of reflectance with
varying incident angles. The film thickness is then uniquely determined by fitting the measured data to an ideal multi-reflection
model of thin films. This method can be readily extended to multi-layered film structures, finding applications
for industrial inspection of semiconductor devices and flat panel display products.
A dispersive method of white-light interferometry for measuring the tomographical thickness profile of a thin-film layer
through a Fourier-transform analysis of a spectrally-resolved interference signal is proposed. The refractive index is also
determined without prior knowledge of the geometrical thickness of the film layer. In contrast with standard white-light
scanning interferometry, dispersive white-light interferometry generates the spectral distribution of interferograms
directly by means of dispersive optics without mechanical depth scanning. Therefore, the proposed method in this paper
is well suited for in-line 3-D inspection of dielectric thin-film layers, particularly for the semiconductor and flat-panel
display industry, with high immunity to external vibration and high measurement speed.
Emerging possibility of applying white-light interferometry to the area of thin-film metrology is addressed. Emphasis is
given to explaining underlying spectrally-resolved interferometric principles of white-light interferometry for measuring
the top surface profile as well as the thickness of thin-film layers, which enables one to reconstruct the complete 3-D
tomographical view of the target surface coated with thin-film layers. Actual measurement results demonstrate that
white-light interferometry in either scanning or dispersive scheme is found well suited for high speed 3-D inspection of
dielectric thin-film layers deposited on semiconductor or glass substrates.
We describes a new scheme of dispersive white-light interferometer that is capable of measuring the thickness profile of thin-film layers, for which not only the top surface height profile but also the film thickness of the target surface should be measured at the same time. The interferometer is found useful particularly for in-situ inspection of micro-engineered surfaces such as liquid crystal displays, which requires for high-speed implementation of 3-D surface metrology.
Current technological issues arising to meet rapidly grwoing demand on 3-D measurements in the field of microelectronics packaging and integration are addressed with special emphasis on white-light interferometry. We first discuss the problem of phase change upon reflection which causes significant measurement errors unless properly compensated for measuring composite targets made of dissimilar materials. Next, an extended application of white-light interferometry is described with aims of measuring not only the surface height profile but also the thickness profile of target surfaces coated with transparent thin-film layers. Finally addressed is the dispersive white-light interferometry that draws much attention for high-speed implementation of surface metrology, which is found useful for in-situ inspection of micro-engineered surfaces.
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