Phase-shifting interferometry is the premier measurement technique for high-precision surface form metrology. Generally, the measurement cavity is illuminated with a stationary, on-axis, point-like source of coherent light situated at the interferometer source plane. We describe how to enable a variety of useful functions to aid interferometric metrology applications by dynamically moving the source point during a phase-shifting measurement using steering mirror technology. Called Dynamic Illumination, these motions describe trajectories that often take the form of geometric primitives like points, lines, arcs, circles, and spirals. A fixed point on the optical axis represents conventional operation, but measurements using different trajectories enable different functions. We highlight three exemplary functions in this paper; phase shifting, autofocus, and coherent noise reduction techniques.
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.
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.
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.
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 &mgr;m/mm. Keywords: Interferometry, optics, aspheres, metrology.
Here I describe results of a method for reducing the influence of vibrations in PSI using the spatial information in the
interference intensity images to achieve as much as 100X reduction in induced surface distortion for small-amplitude
vibrations. The interference patterns are first normalized for the distribution in illumination across the field of view by
measuring the empty-cavity intensity distribution of light reflected from the reference surface prior to measuring the
object. The technique then determines the phase increments between acquired frames by comparing the measured
interference intensity patterns, and then uses these increments to recalculate a vibration-corrected profile. This approach
does not require spatial carrier fringes and maintains full lateral sampling resolution. The method can be applied to any
PSI acquisition, and is compatible with most surface shapes encountered in optical testing, including flats, spheres and
mild aspheres. Unlike carrier-fringe techniques, as few as one or two interference fringes are sufficient for calculating the
phase increments.
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 <1-nm system error after calibration and 0.05-nm repeatability for both instruments working in a clean-room environment.
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.
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-(sigma) 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.
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.
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 lateral resolution 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 calculate the true phase increment between intensity measurements in the high spatial resolution data set and a generalized phase extraction algorithm then uses these phase increments when calculating the topographical phases for this set. The measured topography thereby benefits from the best qualities of both data sets, providing increased vibration immunity without sacrificing high spatial resolution.
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. 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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.