Absolute Thickness Measurements Research Articles

Self-Calibrated Absolute Thickness Measurement of Opaque Specimen Based on Differential White Light Interferometry

In this article, we present a self-calibrated absolute thickness measurement method for opaque specimens based on fiber-optic white light interferometry. A new type of self-reference probe, which is composed of a fiber ferrule, an integrated beam splitter, and a gradient index lens, is presented. In our setup, two probes of this kind are fixed facing each other, and the distance between them can be measured. Then we insert a specimen between the probes and measure the distance between the specimen surfaces and probes on both sides. This way, a self-calibrated measurement of specimen thickness can be realized without the traditional calibration process using a standard thickness sample. Thus, the uncertainty component introduced by the standard thickness sample can be eliminated. The performance of the proposed method is evaluated by measuring standard thickness samples calibrated by the National Institute of Metrology of China, and the results show a good agreement with the nominal values. Also, we analyzed several key uncertainty components, as well as the reliability of our method. The calculated extended measurement uncertainty (k = 2) is 0.06 μm for Cr specimen with a nominal thickness of 10.56, and 0.08 μm for Ni specimen with a nominal thickness of 31.46 μm.

Absolute thickness measurement of pyrolytic graphite spheroids by STEM-EELS

This paper studies the absolute thickness measurement of pyrolytic graphite spheroids (GSs) by using STEM-EELS mode with log-ratio method and Kramers-Kroning (K-K) method, taking the measured thickness from TEM image as reference that is the diameter of GSs ranging from 60 to 250nm. The effect of collection semi-angle (β) on thickness measurement has been investigated. It is found that in general the thickness obtained by K-K analysis with surface effect corrected shows the best accuracy, followed by K-K sum rule and then log-ratio method for the three different collection semi-angles of 12.4, 17.3 and 21.1mrad applied. Of these angles, the smallest one gives an overestimated result and the largest one gives an underestimated result, whereas between the two, the angle of 17.3mrad that is about 2x convergence semi-angle (9.0mrad) is identified as more appropriate for K-K analysis. The surface-scattering correction, inelastic mean free path of GS and effect of refractive index n on thickness measurement for different β angles are also investigated. Moreover, the optical property deduced from the data collected at the center of graphite spheroid, which is related to its microstructure, is characterized by K-K analysis.

A program to analyse optical coherence tomography images of the ciliary muscle.

To describe and validate bespoke software designed to extract morphometric data from ciliary muscle Visante Anterior Segment Optical Coherence Tomography (AS-OCT) images. Initially, to ensure the software was capable of appropriately applying tiered refractive index corrections and accurately measuring orthogonal and oblique parameters, 5 sets of custom-made rigid gas-permeable lenses aligned to simulate the sclera and ciliary muscle were imaged by the Visante AS-OCT and were analysed by the software. Human temporal ciliary muscle data from 50 participants extracted via the internal Visante AS-OCT caliper method and the software were compared. The repeatability of the software was also investigated by imaging the temporal ciliary muscle of 10 participants on 2 occasions. The mean difference between the software and the absolute thickness measurements of the rigid gas-permeable lenses were not statistically significantly different from 0 (t=-1.458, p=0.151). Good correspondence was observed between human ciliary muscle measurements obtained by the software and the internal Visante AS-OCT calipers (maximum thickness t=-0.864, p=0.392, total length t=0.860, p=0.394). The software extracted highly repeatable ciliary muscle measurements (variability ≤6% of mean value). The bespoke software is capable of extracting accurate and repeatable ciliary muscle measurements and is suitable for analysing large data sets.

Thickness determination of few-layer hexagonal boron nitride films by scanning electron microscopy and Auger electron spectroscopy

We assess scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) for thickness measurements on few-layer hexagonal boron nitride (h-BN), the layered dielectric of choice for integration with graphene and other two-dimensional materials. Observations on h-BN islands with large, atomically flat terraces show that the secondary electron intensity in SEM reflects monolayer height changes in films up to least 10 atomic layers thickness. From a quantitative analysis of AES data, the energy-dependent electron escape depth in h-BN films is deduced. The results show that AES is suitable for absolute thickness measurements of few-layer h-BN of 1 to 6 layers.

Measurements for Thickness Distribution of Molecularly Thin Lubricant Film by using Ellipsometric Microscopy

A quantitative measurement method for two dimensional thickness distribution of molecularly thin lubricant films on magnetic disks is presented. Visualization of a molecularly thin film is an extremely useful technique for the investigation of replenishment and retention of thin lubricant films. We demonstrated ellipsometric microscopy using a white light source could provide improvement of signal to noise ratio of images in our previous works. However, measurement of absolute thickness of films has not been established, although the relative thickness can be obtained by intensity of images. In this report, method for measuring the absolute thickness of films has been presented. In this method, by using the relationship between the polarizer angle and the film thickness, the thickness measurement with a sub nm accuracy has been demonstrated.

Thin coating characterization by Rayleigh waves: an analytical model based on normal-mode theory.

The acousto-optic technique to evaluate coated substrates, presented by Devolder et al. [Appl. Phys. Lett. 68, 1732 (1996)], uses the phase dependence of the reflected beam on the coating parameters. In this theoretical paper, a fully analytical model, based on normal-mode theory, is applied to this problem. It gives a transparent expression for the phase shift between the reflected and the incident field due to the presence of thin coatings (coating thickness much smaller than wavelength sound). The known numerical results from Fourier simulations, i.e. the phase dependency on the coating parameters and the advantages of using higher frequencies and wider beams, are reproduced. However, thanks to the analytical nature of the new theory, additional results are obtained. The calculation time is reduced by two orders of magnitude, which is important for the inverse problem. A technique for absolute thickness measurements is proposed. Measuring more to the right of the second reflected maximum increases the phase sensitivity. The versatility of the normal-mode approach is illustrated by applying it to other nondestructive testing (NDT) applications, such as hardness determination and adhesion testing, which are briefly discussed.

Magnetic measurements of component thickness in structures composed of nonmagnetic coating and ferromagnetic base

The paper describes a technique and an instrument for simultaneous absolute thickness measurements of components in structures composed of a nonmagnetic coating and a ferromagnetic base. The technique is based on measurements of the normal magnetic induction component of the secondary field at two points and comparison of these measurements with similar parameters of reference samples.

Applications and techniques of thickness measurement for sections and thin films

The thickness of a specimen to be examined in the electron microscope is a significant parameter for many applications. However, due to the lack of ideal methodology for absolute thickness measurement, most quantitative analyses are performed using approaches which are more convenient. For example, theoretical factors, indirect relative thickness or ratio methods are used to eliminate the necessity to obtain the absolute thickness. These alternatives are theoretically valid. However, experimental verification using standards with known thicknesses are still imperative. Since absolute thickness remains indispensable, continuous development and improvement of techniques for thickness measurement are still much needed. The purpose of this study is to critically survey the importance of absolute thickness measurement and assess the available methods for thickness determination.Knowledge of specimen thickness can be applied for both qualitative and quantitative purposes. Specimen thickness can be used to study imaging properties in different electron microscopy modes. Monitoring of film thickness is, for example, essential when coating specimens for high resolution secondary electron imaging, preparing multilayer specimens in material sciences or preparing ultrathin support films.

X-ray Pendellösung in garnet epitaxial layers

X-ray Pendellosung interference has been observed in reflection in garnet epitaxial layers. The reflection case differs from the well-studied transmission case in that the two interfering wave fields are generated one at the front surface and one at the back surface of the layer. Absolute thickness measurements of the layer are obtained by recording the interference maxima as a function of the specimen orientation and “fringe topographs” reveal local bending of the layer lattice planes.

Analysis of surface layers by light ion back-scattering and sputtering combined with Auger Electron Spectroscopy (AES)

The determination of the thickness and composition of thin layers up to several micrometres is of great importance in many applications. We have investigated the characteristics of two major methods in this field: (1) the back-scattering of protons and helium ions in the energy range 0.060–2.5 MeV, and (2) sputtering combined with AES. The non-destructive back-scattering technique gives quantitative thickness measurements of the high mass components (Nb, Cu) with an accuracy to about 10% of the total thickness. The depth resolution is between 100 and 200 Å near the surface. Its increase with depth depends on the energy of the primary ions, e.g. it increases by a factor of two roughly every 4000 Å in the case of 2 MeV He+ in Al. The destructive sputtering and AES technique has the advantage that the depth distribution of nearly all components (i.e. including impurities like C and O) can be determined with high depth resolution, especially in surface layers. If the excitation cross section and escape depth of the Auger electrons are taken into account, relative concentrations can be estimated. Non-uniform sputtering may cause serious errors. For absolute thickness measurements the method can be calibrated by the back-scattering method. Sputtering yields measured on Cu and oxygen-contaminated Nb are in rough agreement with the values in the literature. The depth resolution of sputtering combined with AES is approximately 50 Å near the surface and its increase with depth depends strongly on the target composition.

Continuous Incremental Thickness Measurements of Non- Conductive Cable Sheath

A method has been recently developed for measuring thickness variations of a non-conductive cable sheathing, extruded over a grounded metal jacket. The method translates direct capacitance increments, sensed by probes sliding on the surface of the sheath, into thickness increments. The accuracy of the system based on this method is sufficiently high that the electrical error, which is of the order of a few thousandths of a μμF, can be disregarded. Experimental data indicate that accuracy of the new system for absolute thickness measurements of homogeneous samples in stationary conditions is of the order of 0.002‖. The error caused by translating capacitance to thickness depends on manufacturing elements and process tolerances, and can be evaluated on a statistical basis. Thus incremental measurements of the cable sheath thickness on the production line yield accuracies of the order of 0.003‖. Application of this method to absolute sheath thickness measurements involves assumptions directly related to calibration and manufacturing process control. These aspects are rather extraneous to a measuring system per se, and, therefore, are not within the scope of this paper.