Denis Montaner

Implementation of a fringe visibility based algorithm in coherence scanning interferometry for surface roughness measurement

Coherence scanning interferometry (CSI) is an optical profilometry technique that uses the scanning of white light interference fringes over the depth of the surface of a sample to measure the surface roughness. Many different types of algorithms have been proposed to determine the fringe envelope, such as peak fringe intensity detection, demodulation, centroid detection, FFT, wavelets and signal correlation. In this paper we present a very compact and efficient algorithm based on the measurement of the signal modulation using a second-order nonlinear filter derived from Teager-Kaiser methods and known as the five-sample adaptive (FSA) algorithm. We describe its implementation in a measuring system for static surface roughness measurement. Two envelope peak detection techniques are demonstrated. The first one, using second order spline fitting results in an axial sensitivity of 25 nm and is better adapted to rough samples. The second one, using local phase correction, gives nanometric axial sensitivity and is more appropriate for smooth samples. The choice of technique is important to minimize artifacts. Surface measurement results are given on a silicon wafer and a metallic contact on poly-Si and the results are compared with those from a commercial interferometer and AFM, demonstrating the robustness of the FSA algorithm.

Deep submicron 3D surface metrology for 300 mm wafer characterization using UV coherence microscopy

Abstract With the increase in diameter of Si wafers to 300 mm and above, and the decrease in minimum feature size to the deep submicron, there is a need to improve the lateral resolution of optical inspection techniques. Coherence microscopy normally uses a white light source in order to produce a narrow fringe envelope for use in surface relief measurement. In this paper we show that high intensity coherent UV light sources can also be used if a large numerical aperture objective is employed, in order to improve the lateral resolution. This forms the basis for a new metrology tool that could find many applications in the inspection of large Si wafers.

Teager-Kaiser energy and higher-order operators in white-light interference microscopy for surface shape measurement

In white-light interference microscopy, measurement of surface shape generally requires peak extraction of the fringe function envelope. In this paper the Teager-Kaiser energy and higher-order energy operators are proposed for efficient extraction of the fringe envelope. These energy operators are compared in terms of precision, robustness to noise, and subsampling. Flexible energy operators, depending on order and lag parameters, can be obtained. Results show that smoothing and interpolation of envelope approximation using spline model performs better than Gaussian-based approach.

Tomographic analysis of medium thickness transparent layers using white light scanning interferometry and XZ fringe image processing

Medium thickness transparent layers are becoming increasingly important in various fields of materials science such as in micro-electronics, nanotechnologies, polymer science, biomaterials and chemistry. Such layers vary from simple, transparent layers to those that are much more complex, containing heterogeneous materials and very rough interfaces and requiring new types of characterization techniques. In this paper we present the application of white light scanning interferometry to the structural tomography of such layers. Due to the complexity of the fringe signals along the optical axis, we have developed 2D signal processing techniques of the XZ images to improve the robustness to noise. Knowing that the measurements are prone to artifacts we have also developed a cautious approach to the extraction of pertinent information. Thus, using a manual point Z-scan investigation in an XY image, initial information of the quality of the fringe signals and the appropriate signal processing necessary can be obtained and provide initial structural information. Then, optimized image processing can be performed on the XZ images to provide tomographic cross sections of the layer. Applications of the technique are given on transparent and insulating layers used in electronics and micro-electronics, layers of hydroxyapatite (a biomaterial) and colloidal layers.

Towards real time 3D quantitative characterisation of in situ layer growth using white light interference microscopy

Quantitative 3D characterisation of layer growth or modification in situ in liquid systems is a challenge because of the changing morphology of the layer and the presence of the growth liquid. Because of the limited bandwidth of many surface profiling techniques, measurement of microscopic surface roughness is generally limited to that of static surfaces. The aim of the present work is to develop a new technique using high speed scanning interference microscopy combined with an adapted immersion head for use in liquid growth systems. For several years we have been developing a real time 3D surface analysis system based on a high speed camera and cabled logic processing, combined with continuous scanning white light interferometry. An optical measurement head is also being developed for use in liquid immersion conditions, with the view of measuring layer growth or modification in biomaterials. In this paper we report on the present status of the development of our second prototype 4D system and also of the optical immersion head for in situ measurements, describing the achievements made and the difficulties still to be overcome.

Local reflectance spectra measurements of surfaces using coherence scanning interferometry

Interference microscopy is a widely used technique in optical metrology for the characterization of materials and in particular for measuring the micro and nanotopography of surfaces. Depending on the processing applied to the interference signal, either topographic analysis of the sample can be carried out by identifying the envelope peak of the fringe signal, which leads to 3D surface imaging, or spectral analysis may be performed which gives spectroscopic measurements. By applying a Fourier transform to the interference fringes, information about the source spectrum, the spectral response of the optical system, and the reflectance spectrum of the surface at the origin of the interferogram can be obtained. By using a sample of known reflectivity for calibration, it is possible to extract the spectral signature of the entire system and therefore to deduce that of the surface of interest. In this paper, we first explain theoretically how to retrieve the reflectance information of a surface from the interferometric signal. Then, we present some results obtained by this means with a white light scanning Linnik interferometer on different kinds of samples (silicon, tin oxide (SnO2), indium tin oxide (ITO)). The initial results were slightly different from those obtained with a conventional optical spectrometer until averaged temporally and were improved even further when averaged spatially. We show that the reflectance of the surface can be calculated over the given wavelength range of the effective spectrum, which is defined as the source spectrum multiplied by the spectral response of the camera and the spectral transmissivity of the optical system. We thus demonstrate that local spectroscopic measurements can be carried out with an interference microscope and that they match well with those measured with an optical spectrometer model Lambda19 UV-VIS-NIR from Perkin Elmer. A simulation study is also presented in order to validate the method and to help identify the potential sources of errors in the spectroscopic analysis.

Depth-resolved local reflectance spectra measurements in full-field optical coherence tomography

Full-field optical coherence tomography (FF-OCT) is a widely used technique for applications such as biological imaging, optical metrology, and materials characterization, providing structural and spectral information. By spectral analysis of the backscattered light, the technique of spectroscopic-OCT enables the differentiation of structures having different spectral properties, but not the determination of their reflectance spectrum. For surface measurements, this can be achieved by applying a Fourier transform to the interferometric signals and using an accurate calibration of the optical system. An extension of this method is reported for local spectroscopic characterization of transparent samples and in particular for the determination of depth-resolved reflectance spectra of buried interfaces. The correct functioning of the method is demonstrated by comparing the results with those obtained using a program based on electromagnetic matrix methods for stratified media. Experimental measurements of spatial resolutions are provided to demonstrate the smallest structures that can be characterized.

Optimised 3D surface measurement of hydroxyapatite layers using adapted white light scanning interferometry

Biomineralization is intensively studied at present due to its importance in the formation of bones, teeth, cartilage, etc. Hydroxyapatite is one of the most common natural biomaterials and the primary structural component of bones and teeth. We have grown bio-like hydroxyapatite layers in-vitro on stainless steel, silicon and silica glass by using a biomimetic approach (immersion in a supersaturated aqueous solution resembling the ion composition of human blood plasma). Using classical techniques such as stylus profiling, AFM or SEM, it was found difficult, destructive or time-consuming to measure the topography, thickness and profile of the heterogeneous, thick and rough hydroxyapatite layers. White light scanning interferometry, on the other hand, has been found to be particularly useful for analyzing such bio-like layers, requiring no sample preparation and being rapid and non-destructive. The results have shown a typical layer thickness of up to 20 μm and a rms roughness of 4 μm. The hydroxyapatite presents nonetheless a challenge for this technique because of its semi-translucence, high roughness and the presence of cavities within its volume. This results in varying qualities of fringe pattern depending on the area, ranging from classical fringes on smooth surfaces, to complex speckle-like fringes on rough surfaces, to multiple fringe signals along the optical axis in the presence of buried layers. In certain configurations this can affect the measurement precision. In this paper we present the latest results for optimizing the measurement conditions in order to reduce such errors and to provide additional useful information concerning the layer.