Films are widely used in electronic shielding, surface protection, etc. Since the spectral reflectance of the thick film layer has enough peaks and troughs, there are various ways to extract its thickness, such as the extreme value method, the envelope method, the Fourier transform and so on. However, because the theoretical model of the extreme value method ignores the extinction coefficient, the envelope method has a large extraction error, and the Fourier method ignores the dispersion characteristics of the refractive index, the measurement accuracy is limited for some precise scenario. In this paper, a new film thickness extracting strategy is proposed with genetic algorithm. At first, the measured spectral reflectance of the film is used to extract the extreme value points, and an extreme value sequence is obtained by dividing their corresponding wavelengths, and then the slope of the extreme value sequence is fitted with the slope of the extreme value sequence of the preset film thickness. When the two slops are consistent, the preset film thickness is the thickness to be measured. Theoretical analysis and measuring experiments are carried out to show a stability of about 0.05nm for a 8630.11nmfilm sample, verifying the feasibility and stability of the proposed strategy.
Thin films are used in many areas for protecting the base, isolating electrons, and so on. Based on the thin-film interference principle and the characteristics of Y-shaped fiber, this paper proposed a reflection objective with annular reflection. With the objective, incident light from the central fiber can be well received for high signal-to-noise ratio. By comparing with the prior-known spectral reflectance of the standard mirror, the absolute spectral reflectance of the thin-film can be achieved. Furthermore, according to the spectral reflectance of the thin-film, film thickness is acquired by the evolutionary genetic algorithm. In the algorithm, the thickness is taken as the independent variable, and the squared difference between the measured spectral reflectance and the preset theoretical spectral reflectance is set as the goal to carry out the global optimization. By testing different silicon-based silica films, the measurement results proved high feasibility and good global fitness. And the total measurement time of a single point is less than 1s. In conclusion, the thin-film measurement system based on the proposed reflection objective and inversion algorithm own excellent performance, which is hopeful for broad usage.
For the demand of line chromatic confocal displacement measurement, a new multi-channel compact spectrometer needs to be developed. In this paper, a microlens array-grating structure, in which one side is microlens array and the other side is blazed gating, is proposed for a miniature spectrometer. The microlens array-grating structure can be pressed in the form of "sandwich" by soft lithography using PDMS material. The micro lens array is a 10*10 mm square, consisting of 45*45 microlens elements, each with a diameter of 220 μm, and the outer diameters of the elements are closely connected with each other. The blazed grating is an equal period grating with a grating line density of 600 lines /mm and a blazed angle of 8°37’. By optimizing the design parameters, the image resolution of the spectrometer based on the grating-microlens array is within 1 nm in the wide band of 400 nm~700 nm. Different microlens grating surface elements have performance consistency with a standard deviation of about 0.5 nm, which can realize the function of arrayed micro-spectrometer.
As a fundamental geometric indicator, high precision roundness measurement is the basis evaluation index of cylindrical or spherical parts. In most roundness measurements, the rotation platforms are used to bring certain rotation error to the measurement result. Two-probe method is a typical roundness measurement strategy with error separation technique, coming from three-probe method with low cost, online integration, flexible installation, etc. We developed a roundness measurement system with three chromatic confocal displacement sensors with flexibility and high axial-resolution. As the measurement start, two sets of displacement data are achieved to take part in the frequency calculation. A typical cylindrical workpiece was measured for its roundness, which was very close with the measurement result by an ultra-precision roundness meter. In a word, the chromatic confocal roundness measurement system is feasible to provide high precision roundness with two-probe method.
Spectral reflectance plays an important role in the evaluation of the object surface, such as the color, the roughness. A new method is proposed to measure the spectral reflectance at a fixed incident angle. The chromatic confocal displacement sensor is modified with an annular stop to generate a hollow cone beam of incident light on the to-be measured sample surface. With self-reference strategy, the upper and lower envelops are derived from the axial scanning of the dispersion probe, whose minus is named as self-reference spectrum. Thus, the reflectance at every wavelength among the valid dispersion bandwidth can be achieved by comparing the self-reference spectra of the sample surface and a standard mirror with known spectral reflectance. With the proposed method and experimental setup, the spectral reflectance of several metal surfaces is obtained and compared with their theoretical spectral reflectance within ±0.04. It proves that the modified chromatic confocal can provide feasible results for the spectral reflectance measurement.
Thickness measurement for the optical thin films is very important for the industries of mechanics, printing, battery and so on. Infrared transmittance method is a very useful method to achieve the physical response, which is related to the film thickness according to the Lambert-Beer law. A detail measurement system is designed with two light paths of reflectance and transmittance, consisting of the light source, filter motor, beam splitter, paraboloid mirror, PbSe detector, and so on. An experimental setup is also established with two different infrared wavelengths by two different bandpass optical filters. The stability of the measurement system is tested to be only about 0.3% in 10 min. The experiments show the feasibility of the proposed double-wavelength infrared transmittance method for film thickness measurement.
Chromatic confocal displacement measurement is a newly developed technology for linear precision positioning with a sub-micron meter accuracy over a millimeter order range. In this technique, the chromatic spectrum of a white light source is divided by an optical dispersion system. The encoded spectrum disperses along the axial direction, with which the axial position can be detected by a spectrometer at a high precision. Thus, the optical dispersion system that determines the final measurement precision is required to own a large dispersion capacity and a small spherical aberration. A chromatic dispersion lens (CDL) module and a spatial-bandpass-filter (SBF) were proposed in this research to improve the dispersion performance. Based on the geometrical imaging principle, the CDL and SBF parameters were preliminarily designed and further tested by the optical design software ZEMAX. The testing results verified that with SBF and CDL, the spherical aberration was largely reduced and the imaging quality was greatly modified. The focus beam spot of the central wavelength 550 nm can be as small as 10μm. Under such a condition, the measurement error is no larger than ±0.7μm over a 1700μm measurement range.
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