Spectrally controlled interferometry promises to increase accuracy at a reduced cost for radius of curvature measurements for spherical optics by removing the catseye position during measurement and exploiting the inherent ability to measure absolute distances without mechanical compensation.
High numerical aperture optical elements are relied on for the most demanding applications in optical imaging but pose a significant challenge for conventional metrology techniques. Laser Fizeau interferometers provide a flexible measurement platform for measuring spherical optics by offering a common path configuration to test spherical optics against a convex reference surface. However, in this configuration, traditional piezoelectric transducer (PZT) based phase shifters produce non-uniform phase shifts which vary across the aperture as the spherical reference surface is translated along the optical axis. While these errors are negligible for low numerical aperture optics, the phase shift errors quickly become significant for high numerical aperture optics. The phase shift nonuniformity results in fringe print through and phase ripple artifacts which limit overall accuracy of phase shifted interferometry (PSI) measurements. Spectrally controlled interferometry (SCI) is a method which produces localized, high contrast interference fringes in non-zero optical path length cavities through tailored control of the sources spectral distribution. In addition to fringe location, fringe phase is also controlled through spectrum manipulation without mechanical motion or compensation. As a consequence, the SCI method produces uniform, full-aperture phase shifts with a high degree of linearity regardless of numerical aperture; thus, phase shift errors associated with traditional PZTs can be eliminated. Furthermore, because SCI is a source driven method, it can easily be integrated with any Fizeau interferometer. In this paper, we present the fundamental background for SCI and the advantages of the method as they apply to the measurement of high numerical aperture spherical optics. Additionally, we compare PSI measurements between a traditional laser Fizeau interferometer with PZT based phase shifters and an SCI Fizeau interferometer. Existing methods to this problem are discussed and compared with the presented SCI method, as well.
Spectrally controlled interferometry (SCI) is a method which presents a host of advantages over traditional coherent and white light interferometry. As its name suggests, the source spectrum is precisely controlled to produce localized fringes whose location and phase are tunable. The approach has been demonstrated to produce accurate interferometric measurements of planar and spherical optics in the presence of detrimental back reflections over a large range of cavity sizes. Phase shifted measurements of single surfaces can be done without any means of mechanical phase shifting. Additionally, existing systems can be converted to be SCI compatible as the method is implemented entirely at the source level of the instrument. Previous demonstrations of this method have applied temporal phase shifting, but use of the SCI method does not preclude the use of alternative measurement techniques. While traditionally, SCI measurements are acquired by shifting the phase of the spectrum modulation function, here we present an alternative method for phase shifting via mean wavelength shift. It is a convenient extension of SCI because typically source parameters are already controlled electronically and shifting mean wavelength of the source adds no additional complication or modification to the existing hardware. By utilizing wavelength shifting novel architectures for instantaneous measurements become possible. In this paper we present two methods of instantaneous surface measurements: using carrier fringe approach and simultaneous PSI by mean wavelength shift. Various phase measurements of multiple surface cavities via both methods are presented to demonstrate the capability. Comparisons are made to traditional SCI and standard coherent phase shifting measurements. Limitations and sources of noise are addressed as well.
We present a new light source capable of locating interference fringes at an adjustable distance from the interferometer. The spectrum is electronically controlled in such a way that the fringes are limited to only one of the surfaces of the optics under test. With the new source it is straightforward, for example, to measure the parallel surfaces of thin glass plates and multiple surface cavities. Existing interferometers, as well as older systems, can be upgraded with this source.
Traditional methods of interferometry are widely used and accepted for simple measurement configurations, but measurement accuracy can decrease rapidly with increasing measurement complexity. For example, coherent interferometry struggles to achieve accurate and repeatable results with the presence of any additional feedback surface in the measurement cavity due to temporally coherent back reflections. Conversely, incoherent interferometers can isolate single surfaces for measurement but require more complex interferometer system designs. As a result, many of these systems are limited in their dynamic range of measurable cavity sizes and present considerable difficulties in the alignment process, increasing total measurement time. Both methods are inherently restricted by the intrinsic properties of their respective source.
Spectrally controlled interferometry (SCI) is a source driven method which inherits many advantages from both coherent and incoherent interferometry while evading typical limitations. The sources spectral properties are manipulated to produce a tunable coherence function in measurement space which allows control over the coherence envelope width, the fringe location, and the fringe phase. With this source realization, a host of measurement advantages which simplify measurement complexity and reduce total measurement time becomes available. One major application is the extinction of extraneous surface back reflections. Without any mechanical translation, realignment, or traditional piezoelectric transducers, front and back surfaces of planar optics can be isolated independently and complete phase shifting interferometric (PSI) measurements can be taken. Furthermore, because all control parameters are implemented at the source level, the spectrally controlled source is a good candidate for upgrading existing interferometer systems.
In this paper, we present the theoretical background for this source and the implications of the method. Additionally, a multiple surface cavity measurement is provided as a means of demonstrating the spectrally controlled sources capability to isolate individual cavities from detrimental back reflections across a large dynamic range of measurable cavity sizes without mechanical realignment. A discussion of the implementation benefits and practical details will be included. Limitations and comparisons to alternative methods will be addressed, as well.
Conventional interferometry is widely used to measure spherical and at surfaces with nanometer level precision but is plagued by back reflections. We describe a new method of isolating the measurement surface by controlling spectral properties of the source (Spectrally Controlled Interferometry - SCI). Using spectral modulation of the interferometer's source enables formation of localized fringes where the optical path difference is non-zero. As a consequence it becomes possible to form white-light like fringes in common path interferometers, such as the Fizeau. The proposed setup does not require mechanical phase shifting, resulting in simpler instruments and the ability to upgrade existing interferometers. Furthermore, it allows absolute measurement of distance, including radius of curvature of lenses in a single setup with possibility of improving the throughput and removing some modes of failure.
White-light Vertical Scanning Interferometry (VSI) is a well-established technique for retrieving the three-dimensional shape of small objects. It has the advantage of non-contact measurement with absolute depth resolution at nanometer level repeatability. The technique has proven to be very effective in measurements of microstructures such as MEMS devices, surface texture, roughness, etc. However, it can only measure areas as big as the field of view of the instrument, usually not more than 15 mm, or a stitching algorithm must be applied. This slows down the measurements and often can be a source of errors. In this paper we present a modification of the technique permitting measurements at higher speeds while retaining the overall accuracy and repeatability of VSI. In the presented method the object is scanned laterally in front of an instrument with a tilted coherence plane such that the data is acquired continuously eliminating the need for stitching for elongated objects. One of the advantages of the proposed system is possibility of faster scanning speed with the use of a high speed CCD arrays. In the paper we present the principle of the method along with an experimental confirmation.
The visualization and analysis methods for studying vibration modes of macro and micro scale objects with spectrally reflecting and scattering surfaces, developed at the Institute of Micromechanics and Photonics of the Warsaw University of Technology, are presented. Silicon technology prepared microspecimens (AFM cantilevers and active PZT micromembranes) are investigated using two-beam time-average interferometry. Vibration modes of flat and non-flat surface microelements are displayed using four and five-frame temporal phase stepping methods. The calculated contrast of vibrating object interferograms provides the information on the vibration amplitude. Scattering surface objects are studied by time-average fiber-optics digital speckle pattern interferometer (DSPI) with heterodyning. Sinusoidal phase modulation introduced at the object vibration frequency enables quantitative analysis of the amplitude and phase of sinusoidal type vibrations. Laser diode modulation and/or single mode fiber stretching is applied for that purpose. For low vibration amplitudes (a0 < l/20) the method using linear approximation of the zero order Bessel function provides an automatic analysis tool for quantitative estimation of the vibration modes.
Recently the lens fabrication technique is developed so fast that an aspherical surface is often used to achieve better imaging performance or reduce number of elements. Especially the popularity of micro-optics and miniature imaging system makes the use of aspherical optics very common. However the metrology of aspherical micro-optics has been disregarded and outpaced by the fabrication technique. It results in the lack of ignorance of metrology for aspherical micro-optics. This paper suggests the simple and cost-effective methodology for aspherical micro-optics by using computer generated hologram (CGH). Although the CGH technique is well-known and well-established technique for relatively larger aspherical optics, it is seldom used for micro-optics testing where there is higher demand of aspherical optics testing. By reporting the success of aspherical micro-optics testing in this paper, we confirm that CGH technique will play an important role to answer new demand of metrology.
White light interferometer (WLI) has become a common tool for measuring surfaces with large height range and/or roughness. Typically, the object is scanned through focus, thus varying the optical path difference (OPD) between the object and reference beams. The rate of the OPD change affects the quality and accuracy of the surface measurement. For high quality measurements a scanning device is often enhanced by a closed loop feedback while the scanning speed is assumed to be known and constant. In this paper we describe a white light interferometer that yields excellent results without requiring a high-end scanner. These results are achieved by embedding an additional interferometer with a long coherence length source that provides an interferometric reference signal that is used to monitor the motion of the scanner during each measurement in real time. The information about the scanner motion is then used in a WLI algorithm. This yields significant improvements in both the accuracy and repeatability of topography measurements.
White light phase shifting interferometric (WLPSI) techniques allow high precision shape measurement thanks to a combination of phase detection of the interference fringes and detection of the position of the fringe envelope. The WLPSI technique gives excellent results as long as ideal scanning and system aberration-free conditions exist. Ideal conditions rarely exist, however, and errors creep into measurements from a number of error sources. Scanner errors affect the measured phase of the object surface, and the finite size of the optical system and its aberrations cause a variation in the offset between the phase and coherence peak across the field of view of the system. This variation in turn causes unwanted 2π jumps in the phase portion of the measurement. This paper shows ways to overcome these challenges. We propose a real-time solution to correcting scanning position influence on measurement in WLPSI algorithm. In addition, we present adaptive phase shifting algorithms that avoid these jumps. Our overall technique is simple, very fast and yields highly precise and accurate results.
Interference microscopy is the primary tool used in the Data Storage industry for air bearing surface (ABS) and pole tip recession (PTR) measurements. ABS parameters include crown, camber and twist (CCT), and affect the behavior of the magnetic head as it flies above the spinning disk. PTR parameters describe the relative height of the writing poles and reading shield. The roadmap for several types of measurements will be discussed in this paper.
Veeco Metrology has designed, built and installed at the California Institute of Technology an interferometer for testing long radius optics for LIGO (Laser Interferometer Gravitational-Wave Observatory). Its accuracy is better than (lambda) /100 P-V for focus and astigmatism coefficients and (lambda) /1000 RMS for the residual surface. Its repeatability is better than (lambda) /4000 RMS with retrace error below 6 nm P-V with 4 fringes of tilt. ROC (radius of curvature) measurement error is less than 3%. In this paper we outline the requirements for the interferometer and discuss more challenging aspects of both the optical design and the alignment. Some measurement results are also presented.
The semiconductor industry is continually moving towards more complex processor designs. The new chips occupy bigger areas and surface properties of silicon wafers used in manufacturing, such as PV and RMS, become critical. One of most important characteristics of wafers is their site flatness, defined as height parameters over area occupied by the projected semiconductor chip. Smaller critical dimensions, shorter wavelength, and higher numerical aperture steppers impose more stringent requirements on PV and RMS of the site's profile. As the wafer goes through the manufacturing process, its value increases, so detecting defective sites is essential to lowering the production costs. To resolve this problem Veeco Process Metrology has designed the RTI 4100--a high performance laser Fizeau interferometer especially suited for inspection of site wafer flatness. High accuracy data taken y the instrument is analyzed by automated software package that performs evaluation of the user selectable sites and qualifies them using various user defined rejection criteria. In this paper, we present some aspects of the instruments' design and its measurement capabilities with interest to the semiconductor industry.
Demand for faster disk drives with bigger storage capacity calls for higher quality of aluminum disks used in their production. Accordingly, decreasing tolerances on disk flatness and tighter specifications on defect presence (pockets) must be met by aluminum bland suppliers. At the same time volume of production is growing requiring automated quality control at high speed. Veeco Process Metrology has developed high performance automated 95 mm aluminum blank disk tester based on Twymann-Green interferometer working a wavelength of 10.6 micrometer. It is a fully automated tool for high volume production with throughput of one disk per second. Lateral resolution of 0.6 mm in the disk plane allows flatness and shape testing as well as pocket detection. The use of long wavelength makes interferometer insensitive to environmental vibrations and dust. Vertical accuracy of instrument is 300 nm (PV) and repeatability is 130 nm (RMS). In this paper we present technical design issues and metrological capabilities of the device. The interferometer is supplied with new automated analysis software which automatically detects defects on the disk surface (pockets) as small as 0.5 micrometer in depth and performs shape categorization.
Requirements on wafer flatness, like most semiconductor specifications, are becoming increasingly tight, with greater accuracy and resolution needed for measurements. In addition to traditional peak-to-valley surface deviation and root-mean- square roughness measurements, it is desirable to measure the flatness of silicon wafers over a small area, or site flatness. This involves dividing the wafer into many sub- regions and calculating the surface statistics for these smaller regions in addition to the overall wafer statistics. Veeco Metrology has developed a high-resolution phase-shifting laser Fizeau interferometer for site flatness testing. The system is designed with 40 mm X 40 mm square field and a 1000 X 1000 pixel CCD camera. Features as small as 100 micrometer may be measured by the system with high resolution, repeatability, and accuracy. A motorized stage allows any region of the wafer to be measured by the system such that problem areas do not escape measurement. This paper discusses the overall system design and presents data from the wafer flatness tester developed by Veeco. Data on lateral resolution, vertical repeatability and accuracy are presented. In addition, the site flatness statistics of a silicon wafer measured by the instrument are given.
Two optical methods for obtaining the partial derivatives of in-plane and out-of-plane displacement fields in the in-plane
ESPSI configuration using temporal phase stepping for automatic analysis of fringe patterns are described. In the first method
lateral shear interferograms of object image fields generated by individual symmetrical illuminating beams are recorded
independently. The phases are calculated and their subtraction/addition gives required in-plane/out-of-plane displacement
derivatives, respectively. Phase stepping is readily performed in the setup based on fiber optics and modulated laser diode
illumination. In the second method two primary interferograms: a conventional in-plane displacement ESPI recording and one
with object images mutually laterally displaced are recorded. Similarly, software subtraction or addition is performed to separate
the out-of-plane displacement derivatives and double sensitivity in-plane displacement information.
Understanding and quantitative modeling of stress relaxation processes are very important for reliable and efficient design of the load resisting structures. In this paper a hybrid ESPI-FEM system for evaluation of time-dependent stress and strain characteristics in small components is presented. Based on results obtained by this method a long term prediction of material behavior can be done. Problems related to the coupling of results from the experiment to an FEM model are discussed and illustrated with reference to speckle interferometry stress analysis of a cantilever beam. Results show that for the conditions tested, stress level in the cantilever beam reduces by 10% of the level at the instant of the loading.
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.