Wavelength-scanning Interferometer Research Articles

Surface quality control of thin SiN layer by optical measurements

Fiber optic interferometers have a wide range of applications including biological and chemical measurements. Nevertheless, in case of a reflective interferometer setup, standard silver mirrors cannot be used in every measurement, due to their chemical activity. In this work, we investigate the surface quality of a thin optical layer of silicon nitride (SiN) which can serve as an alternative material for silver mirrors. We present measurements carried out with a Fabry-Perot fiber optic interferometer working in a reflective mode. Measurement results allow us to determine the surface quality of the investigated layer. Full Text: PDF ReferencesK. Karpienko, M.S. Wróbel, M. Jedrzejewska-Szczerska, "Determination of refractive index dispersion using fiber-optic low-coherence Fabry-Perot interferometer: implementation and validation", Opt Express, 53, 077103 (2014). CrossRef Jedrzejewska-Szczerska M., Gnyba M., Kosmowski B. B. "Low-coherence fibre-optic interferometric sensors", Acta Phys. Pol. A 120, 621 (2011). CrossRef M. Jedrzejewska-Szczerska "Response of a new low-coherence Fabry-Perot sensor to hematocrit levels in human blood",Sensors 14(4), 6965 (2014). CrossRef M. Kosowska, D. Majchrowicz, K.J. Sankaran, M. Ficek, K. Haenen, M. Szczerska, "Doped Nanocrystalline Diamond Films as Reflective Layers for Fiber-Optic Sensors of Refractive Index of Liquids", Materials 12, 2124 (2019). CrossRef Shou-YiChang, Yi-Chung Huang, "Analyses of interface adhesion between porous SiO2 low-k film and SiC/SiN layers by nanoindentation and nanoscratch tests", Microelectron. Eng. 84(2), 319 (2007). CrossRef X. Wang, C. Wang, X. Shen, F. Sun, "Potential Material for Fabricating Optical Mirrors: Polished Diamond Coated Silicon Carbide". Appl. Opt. 56, 4113 (2017). CrossRef G. Coppola, P. Ferraro, M. Iodice, S. De Nicola, "Method for measuring the refractive index and the thickness of transparent plates with a lateral-shear, wavelength-scanning interferometer", Appl. Opt. 42, 3882 (2003). CrossRef H. Mäckel, R. Lüdemann, "Detailed study of the composition of hydrogenated SiNx layers for high-quality silicon surface passivation", J. Appl, Phys. 92, 2602 (2002). CrossRef N. Atman, M. Krzywinski, "Visualizing samples with box plots", Nat. Methods, 11(2), 119 (2014). CrossRef M. Vignesh, R. Balaji, "Data analysis using Box and Whisker Plot for Lung Cancer", International Conference on Innovations in Power and Advanced Computing Technologies,(2017). CrossRef

Single-shot phase detection in a speckle wavemeter for the measurement of femtometric wavelength change

An optical system with instantaneous phase detection is proposed for wavelength change detection in the femtometre scale. Phase measurements are achieved within a few tens of femtometres using a custom made Integrating Sphere (IS) with temperature control. The phase is detected in a single-shot of speckle patterns that are produced using light from a tunable laser after multiple reflections in an IS and combined with a reference beam. It is shown that the proposed optical system produces fringe patterns with phase that is as sensitive as a Wavelength Scanning Interferometer (WSI), with similar optical path difference corresponding to the diameter of the IS. The interference patterns produced by the optical setup show some areas of the interferogram with phase sensitivities that are analogous to the WSI interference, and other areas were vortex generation and annihilation appear independently. Depending on the phase spatial distribution and sensitivity, it is shown here that the phases without vortices can be used to identify particular wavelengths within the femtometre resolution, therefore the sub-femtometer resolution can be attributed to the vortices creation and annihilation.

RF plasma based selective modification of hydrophilic regions on super hydrophobic surface

Selective modification and regional alterations of the surface property have gained a great deal of attention to many engineers. In this paper, we present a simple, a cost-effective, and amendable reforming method for disparate patterns of hydrophilic regions on super-hydrophobic surfaces. Uniform super-hydrophobic layer (Contactangle; CA>150°, root mean square (RMS) roughness ∼0.28nm) can be formed using the atmospheric radio frequency (RF) plasma on top of the selective hydrophilic (CA∼70°, RMS roughness ∼0.34nm) patterns imprinted by electrohydrodynamic (EHD) jet printing technology with polar alcohols (butyl carbitol or ethanol). The wettability of the modified surface was investigated qualitatively utilizing scanning electron microscopy (SEM), atomic force microscopy (AFM), and wavelength scanning interferometer (WSI). Secondary ion mass spectroscopy (SIMS) analysis showed that the alcohol addiction reaction changed the types of radicals on the super-hydrophobic surface. The wettability was found to depend sensitively on chemical radicals on the surface, not on surface morphology (particle size and surface roughness). Furthermore, three different kinds of representative hydrophilic samples (polystyrene nano-particle aqueous solution, Salmonella bacteria medium, and poly(3,4-ethylenediocythiophene) ink) were tested for uniform deposition onto the desired hydrophilic regions. This simple strategy would have broad applications in various research fields that require selective deposition of target materials.

Three-dimensional step–height measurement using sinusoidal wavelength scanning interferometer with four-step phase-shift method

A sinusoidal wavelength scanning interferometer with the four-step phase-shift method is demonstrated for a three-dimensional profile measurement. The interference phase-shift signal generated by the sinusoidal wavelength scanning contains information about optical path difference covering a nm-μm scale structure. The interference phase-shift signal was obtained by the four-step phase-shifting method. The sinusoidal wavelength shifting with a frequency of approximately 180 Hz and tunable range of 5.7 nm was performed by the Littman–Metcalf external resonator-type tunable laser with a center of 772.1 nm. The full-field surface profile measurement was conducted by a charge coupled device image sensor with an accuracy on a nm scale. The surface profiles of gauge blocks with a step height up to about 10 μm were successfully measured with a surface profile irregularity of 3.8×10−2 μm. The deviations of two measured surfaces of upper and lower steps were estimated to be 1.6×10−2 μm and 3.1×10−2 μm, respectively.

Accelerated Surface Measurement Using Wavelength Scanning Interferometer with Compensation of Environmental Noise

The optical interferometry systems are widely used for surface metrology. However, the environmental noise and the data analysis approach can limit the measurement performance during in- process inspection. This paper introduces a wavelength scanning interferometer (WSI) for micro and nano-scale areal surface measurement. The WSI can measure large discontinuous surface profiles without phase ambiguity problems. The proposed WSI is immune to environmental noise using an active servo control system that serves as a phase compensating mechanism. Furthermore, a parallel CUDA programming model is presented as a solution to accelerate the computing analysis. The presented system can be used for on-line or in-process measurement on the shop floor.

Surface and thickness measurement of a transparent film using wavelength scanning interferometry

A wavelength scanning interferometer for measuring the surface and thickness of a transparent film has been studied. A halogen light source combined with an acousto-optic tuneable filter is used to generate a sequence of filtered light in a Linnik interferometer, which leads to a sequence of interferograms captured by a CCD camera. When a transparent thin film is measured, the reflection signals from both the top and bottom surfaces of the film will interfere with the reference signal. At the same time, the multiple reflection signals between the two film surfaces will also interfere with each other. Effective separation of the interference signals from each other is the key to achieving a successful measurement. By performing a frequency-domain analysis, these interference signals can be separated. An optimized Fourier transform method is used in the analysis. Measurements of the top and bottom surface finishes of the film, as well as the film thickness map, have been achieved. The film needs to be more than 3 µm in optical path length, and must transparent with no absorption of light. The film's refractive index needs to be known as a function of wavelength. In this paper, the theoretical analysis and simulation study of wavelength scanning interferometry for transparent film measurement is discussed. Experiments on thin film layers of Parylene N coated on a glass slide surface are studied and analyzed. Comparison study results with other contact and non-contact methods are also presented.

Measurement of the mechanical properties of granular packs by wavelength-scanning interferometry

A wavelength-scanning interferometer has been constructed to observe both the normal and in-plane displacements particle by particle at the base of a model granular pack. The pack comprised 25 000 steel beads supported by a thick glass substrate and was subjected to local disturbing forces on its upper surface. The system allows measurement of normal displacements of the beads to a precision of ca 0.1 nm, thereby providing highly accurate determination of contact forces while minimizing artefacts due to substrate and grain compliance. The probability distribution of the normalized contact force was found to be approximately independent of the applied load on the upper surface of the granular pack and has an exponential tail. The probability distributions of the normalized response force and lateral displacement have similar power-law tails. The interactions between contact forces and lateral displacements suggest that significant internal rearrangement occurs in the granular pack as the load is increased, and particle displacement plays an important role in the mechanics of the granular material.

Simultaneous measurement method of total and self-interference for the volumetric thickness-profilometer

A simultaneous measurement method for the total interference and self-interference of a sample is proposed. The proposed method is capable of making separate measurements of the thickness and surface profile of micro-patterned thin film. The system is an extension of a full-field wavelength scanning interferometer with a single acousto-optic tunable filter (AOTF) as a spectral imaging device. Separate measurements are realized via the polarization-sensitive diffraction of non-collinear acousto-optic interaction. That is, the diffraction angle of an AOTF is separated into different directions depending on the polarization state of the incident light. In so doing, the polarization states of a reference and a sample light were manipulated differently so that a single AOTF can generate the total interference and the self-interference signal in different directions simultaneously. Thus, a compact and light-efficient system is realized with an AOTF, a beam splitter and two CCDs. Thus, a compact system with light-efficiency of two to four times higher than the previously reported system is realized with an AOTF, a beam splitter and two CCDs. Details of the calibration procedures such as wavelength-frequency relation, image shift and registration between two CCDs are provided for the proposed setup. Experimental results are provided and compared to those using commercial equipment that demonstrate the efficiency of the proposed system for the high-speed measurements of the thickness and the surface profile of micro-patterned thin film.

High speed volumetric thickness profile measurement based on full-field wavelength scanning interferometer

A novel high speed volumetric thickness profilometry based on a wavelength scanning full-field interferometer and its signal processing algorithm is described for a thin film deposited on pattern structures. A specially designed Michelson interferometer with a blocking plate in the reference path enables us to measure the volumetric thickness profile by decoupling two variables, thickness and profile, which affect the total phase function phi(k). We show experimentally that the proposed method provides a much faster solution in obtaining the volumetric thickness profile data while maintaining the similar level of accurate measurement capability as that of the least square fitting method.

Extension of distance measurement range in a sinusoidal wavelength-scanning interferometer using a liquid-crystal wavelength filter with double feedback control

The optical path difference (OPD) and amplitude of a sinusoidal wavelength scanning (SWS) are controlled with a double feedback control system in an interferometer, so that a ruler marking every wavelength and a ruler with scales smaller than a wavelength are generated. These two rulers enable us to measure an OPD longer than a wavelength. A liquid-crystal Fabry-Perot interferometer (LC-FPI) is adopted as a wavelength-scanning device, and double sinusoidal phase modulation is incorporated in the SWS interferometer. Because of a high resolution of the LC-FPI, the upper limit of the measurement range can be extended to 280 microm by the use of the phase lock where the amplitude of the SWS is doubled in the feedback control. The ruler marking every wavelength is generated between 80 microm and 280 microm, and distances are measured with a high accuracy of the order of a nanometer in real time.

Multiple-surface testing by a wavelength-scanning interferometer for refractive index inhomogeneity measurement

Wavelength-scanning interferometers can separate the multiple interference signals generated by several reflecting surfaces in the frequency space. Optical thickness variations of a three-layer object and its top surface shape were measured simultaneously by a wavelength-scanning Fizeau interferometer. An efficient sampling window for the phase detection has been derived that can compensate for the frequency detuning of the interference signal and suppress the cross talk from noise components in other frequencies. From the experimental results, the refractive index variation of a glass plate was calculated with an accuracy of ∼1 ppm, which is equivalent to several tens of nanometers in optical thickness variations.

Thickness and Surface Profile Measurement by a Sinusoidal Wavelength-Scanning Interferometer

We propose a sinusoidal wavelength-scanning interferometer for measuring thickness and surface profile of a thin film. The interference signal contains phase modulation amplitude Z and phase $aL which are related to the positions and profiles of the reflecting surfaces, respectively. By reducing the difference between the detected signal and the estimated signal using the multidimensional nonlinear least-squares algorithm, we estimate values of Z and $aL. Experimental results show that the front and rear surfaces of a silica glass plate of 20 $mUm-thickness could be measured with an error less than 10 nm.

波長走査干渉計による多面干渉計測手法の開発

波長走査干渉計にて, 任意の高調波信号の位相を検出可能であり, 波長走査の非線形性などによる周波数変調誤差に対する補償能力を有する位相検出アルゴリズムを開発した. これにより, 複数の透明で両面が平行な層からなる積層試料の, 各界面の形状と各層の光学的厚さ変動を, 不確かさ1/10以下で同時に測定可能であることを示した.

Sinusoidal wavelength-scanning interferometer using an acousto-optic tunable filter for measurement of thickness and surface profile of a thin film

A sinusoidal wavelength-scanning interferometer for measuring thickness and surfaces profiles of a thin film has been proposed in which a superluminescent laser diode and an acousto-optic tunable filter are used. The interference signal contains an amplitude Zb of a time-varying phase and a constant phase alpha. Two values of an optical path difference (OPD) obtained from Zb and alpha, respectively, are combined to measure an OPD longer than a wavelength. The values of Zb and alpha are estimated by minimizing the difference between the detected signals and theoretical ones. From the estimated values, thickness and surface of a silicon dioxide film coated on an IC wafer with different thicknesses of 1 mum and 4 mum are measured with an error less than 5 nm.

Real-time measurement of one-dimensional step profile with a sinusoidal wavelength-scanning interferometer using double feedback control

A linear CCD image sensor is used to electrically scan a measuring point and measure 1-D step-profiles in real time with a sinusoidal wavelength-scanning (SWS) interferometer using double feedback control. In this interferometer, the optical path difference (OPD) and the amplitude of the SWS are controlled so that a ruler marking every wavelength and a ruler with scales smaller than a wavelength are generated. These two rulers enable us to measure an OPD longer than a wavelength in real time. Two different step profiles with step heights of 1 and 20 μm, respectively, are measured with a measurement error of less than 8 nm. Measuring time for one measuring point is 0.04 s.

Method for measuring the refractive index and the thickness of transparent plates with a lateral-shear, wavelength-scanning interferometer.

A new method for measuring simultaneously the thickness and the refractive index of a transparent plate is proposed. The method is based on a simple, variable lateral-shear, wavelength-scanning interferometer. To achieve highly accurate measurements of both refractive index n and thickness d we use several means to determine these two quantities. We finely tune a distributed-feedback diode laser light source to introduce a phase shift into the detected signal, whereas we make the sample rotate to produce variable lateral shearing. Phase shifting permits precise determination of the optical thickness, nd, whereas refractive index n is obtained from the retrieved phase of the overall interference signal for all incidence angles.

파장 주사 간섭계를 이용한 불연속면의 표면 형상 측정 알고리즘

파장 주사 간섭계를 이용한 불연속면의 표면 형상 측정 알고리즘

波長走査干渉計における信号処理

A high accuracy and non-destructive absolute surface profile measuring system based on wavelength scanning interferometer has been developed. This system combined a compact interferometer based on Michelson interferometer with an electronically tuned Ti:sapphire laser for wide range wavelength scanning. This system measures absolute profile of object up to measurement height range of 1.5 mm for area of 2.5 mm × 5 mm. The depth resolution by quantization is approximately 3 μm. For improvement of the system, signal processing methods such as laser intensity revision method, phase revision method and interpolation method are adapted. From laser intensity revision and phase revision by Hanning window, S/N ratio of peak spectrum detection improved from 12.0 to 25.4. Five types of interpolation methods are tested to improve depth resolution. Using variable width cos 2θ fitting method, the depth resolution improved up to 0.06 μm. Experiment of profile measurement of Japanese one yen coin is done, and the accuracy of measurement is approximately 2 μm. The experimental results show the signal processing methods are useful for the profile measuring system.

Suppression of Multiple-Beam Interference Noise in Testing an Optical-Parallel Plate by Wavelength-Scanning Interferometry

Measurements of optical parallel plates in a Fizeau interferometer lead to problems from multiple-beam interference noise unless the back surface of the plate has an additional treatment to prevent its reflection from reaching the detector. Such internal-reflection noise can be separated from the fundamental interference signal by its modulation frequency in a wavelength-scanning interferometer. However, to completely eliminate the noise, the air-gap distance between the testing and reference surfaces of the interferometer must be adjusted to a specific value, depending on the thickness and refractive index of the plate. This adjustment requires an undesirable change in the air gap, which is related to the phase modulation rate of the interferometer, and also requires additional information regarding the testing plate, which makes the measurement laborious. We show that if we add new symmetries to the sampling functions of the phase measuring algorithm, we can obtain significantly greater tolerance for the air-gap error. A new 19-sample algorithm allows a tolerance of ±23% while keeping the systematic errors at less than λ/200. The periodicity of the new algorithm and an integrating bucket technique also provide more options for the thickness of the testing plate, which guarantees an air-gap distance more independent of this thickness.

Gauge Block Measurement Using a Wavelength Scanning Interferometer

A gauge block measurement system that uses a wavelength scanning interferometer was developed. By using a wavelength scanning technique, we determined the integer part of the interference fringe order. From that integer part, we determined the absolute length of a gauge block using only a single light source (wavelength scanning laser diode), even when no information on the nominal length of the gauge block was available. Using the Fourier transform technique to analyze the interference fringe, we measured the excess fraction parts of the interference fringe order with a resolution of over 0.003, and then we determined the integer part of the fringe order in the test measurement of a 10 mm gauge block. It was estimated that we could determine the integer part when the nominal length was shorter than approximately 100 mm, which can be expanded by improving the frequency stability of the light source. The final uncertainty was similar to that obtained with a conventional system based on a multiple wavelength interferometer.