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A powerful technique for interference fringe analysis uses tunable light sources. Originally developed to solve the problem of phase shifting in large aperture systems, the technique has evolved to the simultaneous measurement of multiple surfaces and optical thickness of optical assemblies and components. Here we review the principles and current state of the art for swept-wavelength interferometry for optical testing, including recent advances in digital holographic refocusing and environmental robustness using model-based data analysis. Applications for swept-wavelength interferometry span the full optical metrology space; we provide examples of the measurement of glass substrates for rigid data storage drives to planar waveguides for augmented and mixed reality.

Optical interferometry is widely used for surface metrology, and phase-shifting interferometry (PSI) remains the “gold standard” for measuring surface form and texture. In the past, the environmental sensitivity of PSI relegated its use to dedicated metrology labs with well-controlled environments. As metrology requirements have expanded beyond the lab, advances in metrology techniques now enable high-precision optical testing in places previously considered inappropriate for interferometric techniques. A class of environmentally-robust methods preserve the gold-standard accuracy of PSI by measuring and accounting for rigid-body motions from vibration and large-scale airflow during the PSI acquisition. These methods, including model-based PSI, use a physical model of the interference to measure cavity rigid body motion and accommodate a wide variety of measurement geometries, surface shapes, surface departure, reflectivities, and environmental conditions without changing the user’s measurement process. For more extreme environments, spatial carrier methods analogous to off-axis holography extract the surface phase fast enough to “freeze” vibration and air turbulence in a single camera frame. The price paid for single-frame acquisition is a loss of lateral resolution as well as a modest increase in sensitivity to optical aberrations, requiring calibration for compensation. This paper will describe current techniques and methods, as well as provide examples and results for practical measurement scenarios in the manufacture of high-precision optical components and systems.

The functional behavior of sealing surfaces and bearings depends on the texture direction with respect to the axis of rotation of a machined part, known as the twist or lead angle. We present our lead angle measurement solution using interference microscopy, multi-axis staging, and advanced software for determining surface texture direction and cylinder rotation axis.

Optical fabrication relies on precision metrology over a wide range of lateral scales. Consequently, an important performance parameter for Fizeau interferometers is the instrument transfer function (ITF), which specifies the system response as a function of surface spatial frequency. Advances in test procedures, instruments and automated analysis techniques now enable reliable ITF characterization independent of many traditional sources of error. Results here show the ITF for a commercial 100-mm aperture interferometer with spatial frequency response ranging from 0 to 1500 cycles per aperture

Optical systems for immersive displays incorporate a range of optical components and assemblies that require precision non-contact metrology, including Fizeau interferometry of surface form, new techniques for aspheric microlenses, and interference microscopy for surface structure and texture analysis. Here we consider the problem of evaluating the parallelism and surface form deformation for stacked assemblies of multiple flat glass substrates. Similar structures are common for RGB planar waveguides with slanted sub-wavelength gratings acting as in- and out-couplers. In our experiments, we demonstrate the effectiveness of coherence scanning over a large aperture area using an 100-mm aperture white-light interferometer.

Advances in test procedures and analysis techniques now enable reliable instrument transfer function measurements to 1000 cycles per aperture independent of several traditional sources of uncertainty and operator error.

A general method of surface profiling with phase-shifting interferometry techniques uses iterative linear regression to fit the sequence of interferograms to a physical model of the cavity. The physical model incorporates all important cavity influences, including environmentally induced rigid-body motion, phase shifter miscalibrations, multiple interference, geometry-induced spatial phase-shift variations, and their cross-couplings. By incorporating an initial estimate of the surface profile and iteratively solving for space- and time-dependent variables separately, convergence is robust and rapid. The technique has no restriction on surface shape or departure.

The measurement of flat optical components often presents difficulties because the presence of parallel surfaces generates multiple reflections that confuse conventional laser-based interferometers. These same parts have increasingly demanding surface finish tolerances as technologies improve over time, further complicating the metrology task. Here we describe an interferometric optical system for high-accuracy noncontact evaluation of the form and texture of precision flat surfaces based on an equal-optical-path geometry that uses extended, broadband illumination to reduce the influence of speckle noise, multiple reflections, and coherent artifacts by a factor of 10 when compared to laser-based systems. Combined with a low-distortion, fixed-focus imaging system and 4-Mpixel camera, the 100 mm aperture instrument offers surface height resolutions of 0.1 nm over lateral spatial frequencies extending from 0.01 to 10 cycles/mm. The instrument is vibration resistant for production-line testing of flat optics such as glass hard disks for the data-storage industry and flat-panel-display substrates.

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.

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.

A new instrument for measurements of thin transparent flats incorporates a novel in-line normal-incidence equal path interferometer, and extended broad-band illumination to isolate the surface of interest while reducing coherent noise and artifacts. Incorporating a 4Mpix camera, matching high resolution imaging system and vibration robust design; the instrument satisfies the needs of current and future hard disk and pellicle metrology.

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.

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.

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 10-μm range in 5 s, and has a smooth surface repeatability of 0.5 nm.

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.

We demonstrate a simple way of increasing the data acquisition and processing speed in a scanning white-light interferometer for surface topography measurement. The method consists of undersampling interference data and processing the resultant sub-Nyquist interferograms in the frequency domain to create complete three-dimensional images. Experimental results on a 20-μm step height standard show a measurement repeatability of 10 nm.

I describe the principle of operation and performance of a fiber-based absolute distance measuring interferometer system with 60 independent simultaneous channels. The system was designed for demanding applications requiring passive, electrically immune sensors with an extremely long MTTF. In addition to providing better than 0.3nm measurement repeatability at 5KHz for all channels, the system demonstrated absolute distance uncertainty of less than 5nm over a 500 micron measurement range.

A general method for reducing the influence of vibrations in phase-shifting interferometry corrects the surface phase map through a spectral analysis of a “phase-error pattern,” a plot of the interference intensity versus the measured phase, for each phase-shifted image. The method is computationally fast, applicable to any phase-shifting algorithm and interferometer geometry, has few restrictions on surface shape, and unlike spatial Fourier methods, high density spatial carrier fringes are not required, although at least a fringe of phase departure is recommended. Over a 100× reduction in vibrationally induced surface distortion is achieved for small amplitude vibrations on real data.

An alternative to the conventional linear phase shift in optical testing interferometers is a sinusoidal phase shift, which has the benefit of relaxing requirements on the phase-shifting mechanism. We propose new phase-demodulation algorithms and provide sensitivity analyses to random noise, nonlinearity, vibrations and calibration error to demonstrate that sinusoidal phase shifting can be as robust and computationally efficient as the more established linear phase-shift techniques.

A new method for reducing the influence of vibrations in phase-shifting interferometry uses spatial information to achieve a 100X reduction in vibrationally induced surface distortion for small-amplitude vibrations. The technique does not require high density spatial carrier fringes and maintains full lateral sampling resolution. The principles of the technique are discussed and calculations highlight the capabilities, supported by real measurements under a variety of vibration conditions.

Spherical-reference objectives, retrace error correction and environmental (vibration) compensation are incorporated into a scanning white light microscope to enable high-precision measurements of precision optics over a mid-spatial frequency range from 1 and 500 mm-1. The complete metrology platform with automated calibration and positioning demonstrates a measurement repeatability of less than 65pm, while achieving a global uncertainty of less than 100pm for surfaces up to 450mm in size and aspheric departures up to 2 mm/mm. Keywords: Interferometry, optics, aspheres, metrology.

Two specially-designed visible-wavelength interferometers meet demanding performance requirements in the mid-spatial frequency regime for current and next generation free-form x-ray and EUV optics. A Fizeau phase shifting interferometer measures waviness in the spatial frequency range from 0.5 to 10 mm-1 and an interferometric microscope measures finer-scale deviations from 1 to 1000 mm-1. Uncertainty analysis and experimental work demonstrate

We report on characterization techniques for microstructures using white-light interference microscopy. Capabilities include surface profilometry, integrated profilometry and lateral metrology for full 3D characterization, defect detection, profilometry of thin film structures, stroboscopic interferometry of vibrating samples, and real-time profile snapshots of moving MEMS devices.

Combining spatial and time-domain processing in wave-front measuring interferometers is a powerful tool for reducing the influence of environmental disturbances on the measurement. Modern digital imagers and computing platforms eliminate most of the disadvantages typically cited against spatial processing techniques. The ability to perform both spatial and time-domain processing in a single instrument provides the greatest flexibility for precision metrology applications in both static and dynamic environments. Keywords: Interferometry, phase shifting, metrology, carrier fringe, vibration, turbulence. 1. INTRODUCTION Phase Shifting Interferometry 1 (PSI) has been the preferred measurement technique for high-precision wave-front measurements for over 30 years. The technique utilizes an electronic imaging system to store a sequence of interferograms with well-known cavity phase-shifts and a computer recovers the original wave-front phase by an analysis of the variation of intensity as a function of phase-shift. Unfortunately, PSI can perform poorly in the presence of environmental disturbances such as acoustical or mechanical vibrations or air turbulence. The main reason for the environmental sensitivity is that the measurement sequence takes time, so cavity optical path variations due to changing environmental conditions during the measurement alter the expected phase variation and produce errors in the wave-front determination. Different schemes to minimize environmental influences in PSI have been proposed and fit broadly into three categories; least-squares processing, time domain processing and space domain processing. Least-squares processing, described by Greivenkamp

Though phase-shifting interferometry (PSI) is widely regarded as the method of choice for precision measurements of the surface and bulk characteristics of optical components, conventional PSI suffers from severe distortions in the presence of multiple reflections caused from co-parallel surfaces. The optical and physical characteristics of hard pellicles used for 157nm lithography mean that they are essentially parallel plates and the use of standard PSI techniques to evaluate the optical quality of these components will suffer from these problems. We describe a measurement method called Fourier Transform phase-shifting interferometry (FTPSI) that can overcome the disadvantages of standard PSI by the use of wavelength tuning and special analysis techniques. The technique can measure several surfaces simultaneously without distortion from multiple interference effects and is applied to the measurement of mounted and unmounted pellicles. Additionally, bulk properties of the pellicle, such as index homogeneity, can be easily measured with high precision. By spectrally separating the interference produced by different surfaces in the cavity during a wavelength chirp, each surface is identified and measured individually. In this paper, we describe the technique and give examples of measurements of hard pellicles provided by International SEMATECH.

Fourier transform phase-shifting interferometry is further developed and applied to the absolute measurement of interferometer cavities. Using a widely tunable IR laser diode initially developed for telecom applications along with specific interferometer cavities, I apply this new capability to the measurement of absolute cavity lengths, where a 1- σ precision of 12.6 ppb is demonstrated and the technique was then used to determinate the absolute index and thickness of a transparent parallel plate.

I show how phase shifting interferometry can be extended to account for multiple interference effects using a Fourier based analysis technique combined with wavelength tuning and a particular four-surface interferometer geometry. The technique is demonstrated by simultaneously measuring both surface profiles, the optical thickness variation and index homogeneity of a parallel plate. In addition, unlike traditional phase shifting techniques, the linear component of the homogeneity can be measured with high precision. It is shown that a significant linear component exists in a commercially supplied flat.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We describe a high performance interferometer system which provides measurements of extremely large parts in production with a high throughput. The system has a clear aperture of 610 mm. Mechanical stability is addressed by phase shifting via wavelength tuning, which allows locking the interferometric cavity for increased stiffness. We describe in detail the special illumination and optical mounting systems and their impact on the optical configuration, phase shifting and detection systems and present results on large optical flats in both transmission and reflection.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A geometrically desensitized interferometer (GDI) uses two beams incident on the same sample area at different angles of incident to generate an interference pattern with an equivalence wavelength larger than the illumination wavelength. The instrument is well adapted to the metrology of both smooth and rough samples that are beyond the range of conventional interferometers, while providing more accuracy than conventional moire techniques. In this paper, we extend the capabilities of a GDI with an equivalent wavelength of 12.5 micron using coherence scanning in a manner similar to that of scanning white light interferometry. We also present new analysis techniques to accommodate speckle phenomena that can be more prominent in GDI than in white light interferometry. Our scanning GDI can rapidly characterize the surface flatness and relative heights of discontinuous surface features over large measurement volumes.

Geometrically desensitized interferometry uses two beams at different incident angles to generate an interference pattern with an equivalent wavelength of 5 to 20 micrometers. Unlike conventional interferometers, the fringe contrast is primarily a function of the optical geometry, rather than the spectral properties of the source light. In particular, a line-shaped source provides either a narrow or broad contrast envelope, depending on the orientation of the line with respect to the plane of incidence. This adjustable coherence depth is useful for initial system alignment, depth scanning and for separating surfaces of transparent flat parts.

A method to reduce the sensitivity of phase-shifting interferometry to external vibrations without sacrificing lateralresolution is described. The returning interferogram is amplitude split to form two series of interferograms; a fast sampled,low spatial resolution data set and a slow sampled, high spatial resolution data set. The fast sampled set is used to calculatethe true phase increment between intensity measurements in the high spatial resolution data set and a generalized phaseextraction algorithm then uses these phase increments when calculating the topographical phases for this set. The measuredtopography thereby benefits from the best qualities of both data sets, providing increased vibration immunity withoutsacrificing high spatial resolution.Keywords: Interferometry, vibration, phase-shifting. 1. INTRODUCTION Phase-shifting interferometry' (PSI) is a technique used to measure surface topography by precisely measuring thephase angles in interferograms generated from combining light reflected from a known reference surface and the test object.The process acquires a sequence of interferograms while the reference phase is adjusted in a controlled manner. Theinterferogram intensities are described by

Quality control in the production of rigid disk drives depends in part on accurate metrology of the read-write head. We review the established technologies for measuring the distance or flying height between the head and the rotating disk, and propose a new instrument based on polarization interferometry. The new instrument has excellent repeatability, high sensitivity at low flying height and in situ determination of the phase change on reflection.

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. 1,2 This analysis breaks the white light up into its constituent colors and makes it possible to apply multiplewavelength techniques to the problem of surface height measurement. In an ideal dispersion-corrected interferometer with a monochromatic source, the interferometric phase θ is related to the round-trip optical path difference x by

Instruments utilizing Phase Shifting Interferometry (PSI) principles are used extensively in precision surface metrology when a non-contact method of surface characterization is required. PSI entails making point to point measurements of the phase difference between a test and reference beam of an interferometer. These phase differences are assumed to be produced from small height changes in the test surface. If the reference surface profile is accurately known, the calculated height differences represent deviations from the reference profile and the test surface profile can be reconstructed therefrom.

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.

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.

We describe a method and algorithm for reducing the sensitivity of phase-shifting interferometry to external vibrations. Using an interline-transfer camera, a shutter, and a fast phase shifter, we acquire a series of paired interferograms in quadrature, with the pairs spaced to maximize residual phase-error cancellation. The rapid acquisition of quadrature pairs significantly improves resistance of interferometry to low-frequency vibrations.

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.