Xavier Colonna de Lega

Determination of fringe order in white-light interference microscopy

Combining phase and coherence information for improved precision in white-light interference microscopy requires a robust strategy for dealing with the inconsistencies between these two types of information. We correct for these inconsistencies on every measurement by direct analysis of the difference map between the coherence and the phase profiles. The algorithm adapts to surface texture and noise level and dynamically compensates for optical aberrations, distortions, diffraction, and dispersion that would otherwise lead to incorrect fringe order. The same analysis also provides the absolute height data that are essential to relational measurements between disconnected surfaces.

Signal modeling for low-coherence height-scanning interference microscopy

We propose a computationally efficient theoretical model for low-coherence interferometric profilers that measure surface heights by scanning the optical path difference of the interferometer. The model incorporates both geometric and spectral effects by means of an incoherent superposition of ray bundles through the interferometer spanning a range of wavelengths, incident angles, and pupil plane coordinates. This superposition sum is efficiently performed in the frequency domain, followed by a Fourier transform to generate the desired simulated interference signal. Example applications include white-light interferometry, high-numerical-aperture microscopy with a near-monochromatic light source, and interference microscopy for thickness and topography analysis of thin-film structures and other complex surface features.

Optical interferometry for measurement of the geometric dimensions of industrial parts

We describe an instrument for the measurement of surface flatness, parallelism, and size (thickness) of plane-parallel parts in a single measurement to 1sigma gauge capability of 0.02, 0.03, and 0.06 microm, respectively. A low-coherence IR profiler viewing both sides of the part simultaneously, believed to be novel, accommodates a wide variety of industrial surface finishes, including machined, ground, or lapped parts, with a 75-mm field of view and 15,000 pixels per side. A heterodyne laser displacement gauge together with an integrated zeroing system allows for a range of part sizes from 0 to 100 mm.

Angle-resolved three-dimensional analysis of surface films by coherence scanning interferometry

The Fourier components of interference signals generated by scanning a high-numerical-aperture objective orthogonal to an object surface correspond to different angles of incidence on the surface. The phase and amplitude of these Fourier components relate to the structure of the object, including in particular the 3D topography and thickness profiles of thin-film layers.

Signal modeling for modern interference microscopes

A theoretical model of the optical system of an interference microscope includes both geometrical and spectral contributions to fringe contrast localization. An incoherent superposition of interference patterns over a range of wavelengths and pupil-plane coordinates predicts the frequency-domain portrait of the interference phenomenon. An inverse Fourier transform then provides simulated signals that correlate very closely to experimental data. The model is particularly useful for signal prediction, algorithm testing, uncertainty analysis and system characterization, including modern applications in thin film analysis and stroboscopic interferometry.

Optical Topography Measurement of Patterned Wafers

We model the measurement process in an interference microscope and derive libraries of signal signatures corresponding to various types of materials and thin film structures. These libraries allow calculating the top surface topography of patterned wafer features as well as the thickness of the underlying top transparent layer with nanometer height resolution. We show applications of the technique to the measurement of trenches etched in SiO2 films, post‐CMP STI patterns and line structures. The results of the optical profiler are in good agreement with AFM and ellipsometer measurements of nearby structures.

Interpreting interferometric height measurements using the instrument transfer function

Much of this paper has emphasized the precariousness of using a linear ITF for what is fundamentally a nonlinear process of encoding height into the phase of a complex wave amplitude. A more accurate model begins with an explicit calculation of this amplitude, then propagates the wavefront through the system to determine what the instrument will do.

Wavelet processing of interferometric signals and fringe patterns

The measurement of shape, displacement and deformations is often performed using interferometric methods, featuring nm to mm sensitivities and very high spatial and temporal resolutions. We first give a brief overview of interferometric techniques. Emphasis is laid on the wide purposes of these techniques. Then, we present a novel method using wavelet analysis to process live interference patterns. Further developments of the method are then presented. Finally, through two practical examples, we intend to highlight the interest of fringe processing by wavelet transform.

Transparent film profiling and analysis by interference microscopy

A white-light interferometer with new signal analysis techniques provides 3D top surface and thickness profiles of transparent films. With an additional change from conventional object imaging to pupil-plane imaging, the same instrument platform provides detailed properties of multilayer film stacks, including material optical properties. These capabilities complement conventional surface-topography measurements on the same platform, resulting in a highly flexible tool.

Valve cone measurement using white light interference microscopy in a spherical measurement geometry

We describe an interference microscope for valve cone angle, roundness, straightness, and waviness using scanning white light interferometry. Our unusual test geometry creates a spherical optical measurement surface centered at an optical datum point that represents the center of curvature of the mating valve needle or ball. A radial scan locates intersections of the measurement surface with respect to the datum point, and software reconstructs the conical sample surface by means of a calibrated mapping function. This flexible approach accommodates a wide range of cone angles and diameters without a change of optics. Experimental results show promising performance, including a 0.02-μm standard deviation for cone roundness, including sample removal and replacement.