Leslie L. Deck

High-speed noncontact profiler based on scanning white-light interferometry

We describe a system for fast three-dimensional profilometry, of both optically smooth and optically rough surfaces, based on scanning white-light techniques. The system utilizes an efficient algorithm to extract and save only the region of interference, substantially reducing both the acquisition and the analysis times. Rough and discontinuous surfaces can be profiled without the phase-ambiguity problems associated with conventional phase-shifting techniques. The system measures steps to 100 µm, scans a 10µLm range in 5 s, and has a smooth surface repeatability of 0.5 nm.

Surface Profiling by Analysis of White-light Interferograms in the Spatial Frequency Domain

Abstract We describe a scanning white-light interferometer for high-precision surface structure analysis. Interferograms for each of the image points in the field of view of the instrument are generated simultaneously by scanning the object in a direction perpendicular to the object surface, while recording detector data in digital memory. These interferograms are then transformed into the spatial frequency domain and the surface height for each point is obtained by examination of the complex phase as a function of frequency. The final step is the creation of a complete three-dimensional image constructed from the height data and corresponding image plane coordinates. The measurement repeatability is better than 0·5 nm r.m.s. for a surface height range of 100 μm.

Fourier-transform phase-shifting interferometry

Phase-shifting interferometry is a preferred technique for high-precision surface form measurements, but the difficulty in handling the intensity distortions from multiple-surface interference has limited the general use of the technique to interferometer cavities producing strict two-beam interference. I show how the capabilities of phase-shifting interferometry can be extended to address this problem using wavelength tuning techniques. The basic theory behind the technique is reviewed and applied specifically to the measurement of parallel plates, where surfaces, optical and physical thickness, and homogeneity are simultaneously obtained. Basic system requirements are derived, common error sources are discussed, and the results of the measurements are compared with theory and alternative measurement methods.

Interferometric back focal plane microellipsometry

We present a technique for ellipsometric analysis of materials with high lateral resolution. A Michelson-type phase-shifting interferometer measures the phase distribution in the back focal plane of a high numerical aperture objective. Local measurements of the ellipsometric parameter delta are performed over the entire spectrum of angles of incidence. We show that delta is to leading order linearly proportional to the phase change on reflection of normally incident light. We furthermore invert the Fresnel reflection equations and derive expressions for the real and imaginary parts of the refractive index as functions of the phase change on reflection and the reflectivity at normal incidence, both of which are measurable with the same apparatus. Hence we accomplish local measurements of the refractive indices of our samples. Determination of the phase change on reflection permits correction of interferometric topography measurements of heterogeneous specimens.

Numerical simulations of vibration in phase-shifting interferometry.

Computer simulations predict the expected rms measurement error in a phase-shifting interferometer in the presence of mechanical vibrations. The simulations involve a numerical resolution of a nonlinear mathematical model and are performed over a range of vibrational frequencies and amplitudes for three different phase-shift algorithms. Experimental research with an interference microscope and comparison with analytical solutions verify the numerical model.

VIBRATION-RESISTANT PHASE-SHIFTING INTERFEROMETRY

A method to reduce the sensitivity of phase-shifting interferometry to external vibrations is described. The returning interferogram is amplitude split to form two series of interferograms, taken simultaneously and with complementary properties, one with high temporal and low spatial resolution and the other with low temporal and high spatial resolution. The high-temporal-resolution data set is used to calculate the true phase increment between interferograms in the high-spatial-resolution data set, and a generalized phase-extraction algorithm then includes these phase increments when the topographical phases in the high-spatial-resolution data set are calculated. The measured topography thereby benefits from the best qualities of both data sets, providing increased vibration immunity without sacrificing high spatial resolution.

Two-color light-emitting-diode source for high-precision phase-shifting interferometry.

We describe a two-color light-emitting-diode source that provides intense, uniform, and highly stable illumination with a wavelength precision sufficient for multiwavelength phase-shifting interferometry applications. The source has feedback-stabilized intensity and junction temperature sensing for wavelength compensation. It has a theoretical mean time between failure of more than 30,000 h, and the two colors provide a step measurement capability of greater than 2.5 microm. Measurements made with this source in an interferometric microscope show subangstrom repeatability.

High-speed non-contact profiler based on scanning white light interferometry

Abstract We describe a new profiler for fast 3-dimensional measurements of both optically smooth and optically rough surfaces based on scanning white light techniques. The profiler utilizes an efficient algorithm to extract and save only the region of interference, substantially reducing both the acquisition and analysis time while measuring rougher surfaces and larger step heights than conventional phase shifting techniques. The instrument measures steps to 100 microns, scans a 10 micron range in 5 seconds and has a smooth surface repeatability of 0.5nm.

Polarization interferometer for measuring the flying height of magnetic read–write heads

Traditional optical flying-height testers use only the normal-incidence reflectivity of the interface between the read-write slider and a glass disk surrogate. We propose a tester that fully analyzes the complex amplitude ref lectivity of the interface, including the polarization-dependent complex phase. The new approach is more accurate and repeatable and has no loss of precision at zero flying height. Further, the same instrument directly measures the complex index of refraction for the slider material in situ, obviating the need for a separate metrology step with an ellipsometer.

Surface profiling by frequency-domain analysis of white light interferograms

Three-dimensional imaging interferometric microscopes have outstanding accuracy, provided of course that the test objects are sufficiently smooth and continuous. The present study shows that a white-light source and spatial-frequency domain analysis of the resulting interferograms can dramatically increase the range of application of interferometric surface profilers. This analysis breaks the white light up into its constituent colors and makes it possible to apply multiple-wavelength techniques to the problem of surface height measurement.