Patterned sapphire substrates (PSS) wafers are used in LED manufacturing to enhance the luminous conversion of LED
chips. The most critical characteristics in PSS wafers are height, width, pitch and shape of the pattern. The common way
to measure these characteristics is by using surface electron microscope (SEM). White light interferometry is capable to
measure dimension with nanometer accuracy and it is suitable for measuring the characteristics of PSS wafers. One of
the difficulties in measuring PSS wafers is the aspect ratio and density of the features. The high aspect ratio combined
with dense pattern spacing diffracts incoming lights and reduces the accuracy of the white light interferometry
measurement. In this paper, a method to improve the capability of white light interferometry for measuring PSS wafers
by choosing the appropriate wavelength and microscope objective with high numerical aperture. The technique is proven
to be effective for reducing the batwing effect in edges of the feature and improves measurement accuracy for PSS
wafers with circular features of 1.95 um in height and diameters, and 700 nm spacing between the features. Repeatability
of the measurement is up to 5 nm for height measurement and 20 nm for pitch measurement.
In this research, new nano-scale measurement methodology based on spectrally-resolved chromatic confocal interferometry (SRCCI) was successfully developed by employing integration of chromatic confocal sectioning and spectrally-resolve white light interferometry (SRWLI) for microscopic three dimensional surface profilometry. The proposed chromatic confocal method (CCM) using a broad band while light in combination with a specially designed chromatic dispersion objective is capable of simultaneously acquiring multiple images at a large range of object depths to perform surface 3-D reconstruction by single image shot without vertical scanning and correspondingly achieving a high measurement depth range up to hundreds of micrometers. A Linnik-type interferometric configuration based on spectrally resolved white light interferometry is developed and integrated with the CCM to simultaneously achieve nanoscale axis resolution for the detection point. The white-light interferograms acquired at the exit plane of the spectrometer possess a continuous variation of wavelength along the chromaticity axis, in which the light intensity reaches to its peak when the optical path difference equals to zero between two optical arms. To examine the measurement accuracy of the developed system, a pre-calibrated accurate step height target with a total step height of 10.10 μm was measured. The experimental result shows that the maximum measurement error was verified to be less than 0.3% of the overall measuring height.
KEYWORDS: Confocal microscopy, Point spread functions, Colorimetry, Deconvolution, Calibration, Signal processing, Near field optics, Sensors, Convolution, Spectroscopy
In this research, novel deconvolution methodology is proposed to resolve the lateral and axial cross-talk problems encountered in line-scanning chromatic confocal surface profilometry. The strategy integrates chromatic confocal principle, infinitive microscopic optics and deconvolution theory to resolve the entangled cross-talk problem in microscopic confocal measurement, so the measuring resolution can be greatly enhanced from the level of the traditional line-scanning up to the one achieved by generally traditional point-type confocal measurement. To overcome the problem, this research analyzes the physical phenomenon of optical near field using photonic spectrum analyses for establishing relationship between the light expansion and propagation depth, as well as light wavelength. In the confocal image, acquired spectrum intensity can be regarded as the convolution between the ideal signal from objects and the point spread function (PSF) of incident light. By employing spectrum analyses, important calibrated characteristics of the PSF along both of the lateral and depth directions can be carefully established. By using the individual PSF for its corresponding wavelength detected at its matching focal depth, the proposed deconvolution method has been proved effective theoretically and experimentally in greatly minimizing the full width half maximum (FWHM) of the depth response curve by more than 25 times, thus significantly improving the accuracy and repeatability of microscopic surface profilometry.
Full-field chromatic confocal surface profilometry employing a digital micromirror device (DMD) for spatial correspondence is proposed to minimize lateral cross-talks between individual detection sensors. Although full-field chromatic confocal profilometry is capable of enhancing measurement efficiency by completely removing time-consuming vertical scanning operation, its vertical measurement resolution and accuracy are still severely affected by the potential sensor lateral cross-talk problem. To overcome this critical bottleneck, a DMD-based chromatic confocal method is developed by employing a specially-designed objective for chromatic light dispersion, and a DMD for lateral pixel correspondence and scanning, thereby reducing the lateral cross-talk influence. Using the chromatic objective, the incident light is dispersed according to a pre-designed detection range of several hundred micrometers, and a full-field reflected light is captured by a three-chip color camera for multi color detection. Using this method, the full width half maximum of the depth response curve can be significantly sharpened, thus improving the vertical measurement resolution and repeatability of the depth detection. From our preliminary experimental evaluation, it is verified that the ±3σ repeatability of the height measurement can be kept within 2% of the overall measurement range.
In the research, full-field chromatic confocal surface profilometry employing digital micro-mirror device (DMD) for
spatial correspondence is proposed to minimize lateral cross talks between individual detection sensors. Although fullfield
chromatic confocal profilometry is capable of enhancing measurement efficiency by completely removing timeconsuming
vertical scanning operation, its vertical measurement resolution and accuracy are still severely affected by the
potential sensor cross talk problem. To overcome this critical bottleneck, a DMD-based chromatic confocal method is
developed by employing a specially-designed objective for chromatic light dispersion and a DMD for lateral pixel
correspondence and scanning. Using the chromatic objective, the incident light is dispersed according to a pre-designed
detection range from a few micrometers to several millimeters and a full-field reflected light is captured by a three-chip
color camera for multi color detection. Using this method, the full width half maximum (FWHM) of the depth response
curve can be significantly sharpened, thus improving the vertical measurement resolution and repeatability of the depth
detection. From our preliminary experimental evaluation, it is verified that the ±3σ repeatability of the height
measurement can be kept within 2% of the overall measurement range.
In the article, an in-situ 3-D microscopic surface profilometer employing novel lateral confocal scanning principle, also
called V-scan lateral confocal microscopy (VLCM), was developed to achieve in-field measurement with an effective
vibration-resistance capability. The developed methodology combines digital structured fringe projection, lateral
confocal scanning, shape from focus (SFF) and anti-vibration technique to perform lateral scanning for in-situ 3-D
surface measurement. For microstructures having low reflectivity and high-slope surfaces to be measured within in-field
process environment, it has been recognized as a great challenge for achieving accurate 3-D surface inspection. To
overcome this, the presented method employing a new lateral confocal scanning strategy in combining a Z-axis vertical
scanning with a horizontal X-axis scanning simultaneously, in which the scan pattern is similar to a V-shape. Meanwhile,
to detect potential environmental vibration, a laser fiber interferometric positioning sensor based on heterodyne
interferometry is employed to detect potential vibratory displacement between the optical probe and a tested surface for
minimizing environment disturbance encountered in a real factory. A depth response curve is constructed by a series of
images detected from successive depths during the V-scan lateral scanning. Potential vibration errors can be effectively
detected by a fiber optic interometric positioning sensor and compensated simultaneously. A standard step-height target
and several industrial V-groove microstructures have been measured to attest the measurement accuracy and feasibility
of the developed approach. From the experimental results, it is confirmed that the depth resolution can reach 0.1 μm and
the maximum measurement error can be controlled within 3% of the overall measuring height.
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