Optical profiling instruments utilizing phase-shifting interferometry offer one the opportunity to obtain a large amount of surface texture data quickly without damaging the measured surface. Unfortunately, the presence of dissimilar surface materials or even spatial variations in the subsurface morphology can result in spurious optical measurements. This paper presents equations that can be used to calculate the reflection of electromagnetic radiation from thin film structures. These equations are utilized to determine what minimum metal overcoat thickness will ensure accurate optical step height measurements. Interferometric and stylus- based measurements of three thin film steps are presented and discussed. An opaque metal overcoat is found to be essential to the accurate optical measurement of step heights.
A novel non-contact optical profiler described in this paper is designed and made for measuring the surface characteristics of optical parts. Measurements are based on a combination of an optical heterodyne technique and a precise phase measurement procedure without the need of a reference surface. A Zeeman-split He-Ne laser is employed as the light source which offers two common-path polarized beams. The frequency difference between the beams is 1.8 MHz. A special optical head is designed and fashioned as a beam splitter which contains a birefringent lens and an objective. The whole optical system is completely common- path. This allows the optical common-mode rejection technique to be applied in the system for minimizing the environmental effects in measurements such as air turbulence, vibrations and temperature variations. To keep the sample surface focused on the ordinary rays in the optical head, an astigmatic autofocus system is employed. A stepping micro-stepping system can move the optical head in the range of 25 mm with 0.1 micrometers resolution. A data acquisition system is made to control the auto-focus system, data receiving and analyses. This makes the measurement automatically while the sample is being scanned. The characteristics of the surface can be displayed on the computer screen. The theoretical and experimental analyses of the profiler are completed. The profiler measures samples with 1.1 angstroms height accuracy and 4 micrometers lateral resolution when a 40X objective is used in the optical head. The accuracy comparisons of the profiler with different objectives 5X, 10X, 20X, and 40X are shown in good agreement. The advantages of the present profiler are presented. Based on the autofocus system, the profiler optical system will be designed to mount on a large linear air-bearing slide, so that it is capable of scanning over a distance covering from 4 micrometers to 1 m.
A new optical method of rms surface roughness (Rq) measurement for diffusely-reflecting or scattering surfaces with Rq greater than 1 micron is disclosed, and its application to grit- blasted and plasma-sprayed surfaces described. Data acquisition by the 'multiple line shadow' (MLS) method over a small area (typically 4 mm2), image analysis, and Rq readout require at most 10 seconds, with standard deviations of 10% of mean value attainable. Measurements are made by hand-held or automated optical probe at the workpiece, with no necessity to remove the piece from its production jig. The MLS method is absolute; no calibrations are required. When automated, the MLS probe need not touch the nominally flat or large-radius workpiece surface. Minor probe and algorithm modifications allow non-contact measurement of step heights or coating thicknesses in the 1 - 30 mil range (.025 - .75 mm), also in times of 10 seconds or less at the workpiece.
With manufacturing tolerances of microelectronic and electro-optical devices being improved to well below a micron, it is becoming increasingly difficult to check both the form of surface features and the presence of defects in a routine manner. These defects may be material or process related. In this paper we demonstrate the advantages of combining advanced optical methods with digital image processing, to form what we call 'Nanoscopy' techniques, for sub- micron surface measurement metrology. We distinguish between two imaging techniques, one for high resolution analysis of near flat surfaces and small features and the other for shape measurement of multi-micron high components. These are combined in the same instrument. The first, Phase Stepping Microscopy (PSM), is illustrated with examples of surface roughness measurement, epilayer defect analysis and feature identification after chemical etching. The second, Peak Fringe Stepping Microscopy (PFSM), is presented with examples of shape measurement of an optical laser guide and a Hetero-structure Bipolar Transistor (HBT).
An Optical Surface Profiler (OSP130) has been developed for the metrology of master tooling used in the coin stamping process. The OSP130 measure, in a non-contacting manner, the surface relief of tools ranging in diameter from 10 mm to 300 mm. Rapid measurements are performed simultaneously on a large grid of equispaced points across the surface of the tool. From the relief data, many parameters such as the location of high and low features, volume of impression, background curvatures and various diameters can be quickly evaluated. The technique used is phase-shifting moire profilometry. A white light projector illuminates a periodic transmission grating which is then imaged onto the object surface. The light pattern on the object is viewed by a high resolution TV camera connected to a computer. The grating is shifted under computer control to a number of positions and corresponding intensity images of the deformed pattern on the object surface are stored in the computer. From the intensity images a phase map, representing the deformation of the periodic grating by the surface relief, is evaluated and compared with an undeformed pattern. This results in an accurate contour map of the surface relief with an uncertainty less than 1% of the relief excursion on the object. Details of the instrument and its use at the Royal Australian Mint are presented.
Several wavefront sensing techniques were tested against a common aberrated beam. The error on the beam was predominantly spherical aberration. The techniques included Shack- Hartmann Test, Lateral Shear interferometry, Point Diffraction Interferometry, and a Knife Edge Test. Beam calibration was accomplished using Fizeau Interferometry.
This paper describes a method for measuring the absolute flatness of flats. A function in a Cartesian coordinate system can be expressed as the sum of even-odd, odd-even, even-even, and odd-odd functions. Three flats are measured at eight orientations; one flat is rotated 180 degree(s), 90 degree(s), and 45 degree(s) with respect to another flat. From the measured results the even-odd and the odd-even functions of each flat are obtained first, then the even-even function is calculated. All three functions are exact. The odd-odd function is difficult to obtain. For the points on a circle centered at the origin, the odd-odd function has a period of 180 degree(s) and can be expressed as a Fourier sine series. The sum of one half of the Fourier sine series is obtained from the 90 degree(s) rotation group. The other half is further divided into two halves, and one of them is obtained from the 45 degree(s) rotation group. Thus, after each rotation, one half of the unknown components of the Fourier sine series of the odd-odd function is obtained. The flat is approximated by the sum of the first three functions and the known components of the odd-odd function. In the simulation, three flats (each is an OPD map obtained from a Fizeau interferometer) are reconstructed. The theoretical derivation and the simulating results are presented.
The ISO standard for optical drawing specification (ISO 10110) includes a part on surface form tolerances (figure error). Standardization of this specification provides a succinct and readily understood nomenclature which will be useful in industry, especially as it is intended to unify visual and digital evaluation. It is a peculiar standard in that it does not control surface form directly, but controls it by its effect in a particular test, interferometric comparison with a 'perfect' reference. The application of this standard to digital interferometry is discussed, including a review of the mechanics of this standard and some examples. ISO 10110-5 is vague on some points and these are discussed. These issues include spatial frequency sampling, specific algorithms for calculating some of these tolerances, measurement accuracy concerns, aspherics and guidelines for determining if digital interferometry is required.
Recent interest in using optical systems at shorter and shorter wavelengths for applications like UV photolithography and synchrotron focussing mirrors has put increasing pressure on optical metrologists to improve absolute methods of testing. Because optical metrologist do not have the luxury of possessing measuring instruments that are many times more accurate than what they want to measure, they are forced to make differential measurements between "reference" surfaces and the surface under test. However, even the (often) flat reference surface figure is not known to the accuracy now required by these new applications. This need for a more accurate knowledge of the reference surfaces has created a wide interest in the absolute calibration of flats.
An f/10.3 lens with 6 waves of spherical aberration was tested on a Fizeau interferometer using a standard ZYGO retro sphere. It was found that this configuration led to retrace error difficulties. A longer radius retro sphere yielded results in much better agreement with theory.
A technique for combining Newton fringe interferometry with scanning methods to characterize unique optical elements is discussed. The approach uses a converging beam as a probe and observes the Newton fringe pattern formed by the interaction between the front and rear surface reflections of the optic under test. The result is a direct measurement of the optical thickness profile of the component. The technique is well suited to the characterization of windows, domes, conformal windows, and mild corrector plates.
Projection moire techniques are useful tools for the determination of surface contour features. They can provide interferometric type fringe patterns showing regions of equal height on the surface. The difference between moire and interferometry appears in the spacing of the equal height contours lines. In an interferometric fringe pattern, the height difference between consecutive fringes is equal to the wavelength of the illumination source and is a constant over the entire pattern. For a projection moire contour pattern the height difference between consecutive fringes is determined by illumination geometry parameters and is not constant; it changes as a function of the object's out of plane height. This variation in fringe spacing causes a misrepresentation of the surface's shape if the pattern is analyzed as a traditional interferogram. This paper discusses the types of aberrations generated by this process and the dependence of aberrations on the geometry of the object.
This paper describes measurement algorithms and control procedures that can be effectively applied to the automation of an alignment procedure for paraboloidal optical mirrors. Interferometric alignment of an off-axis paraboloidal optical mirror can be a tedious and labor- intensive process. We review a previous solution to the alignment problem that employs a corner-cube retroreflector and present a method of automating this process using an imaging interferometer coupled to a processing system that controls a set of actuators. The relevant image-processing algorithms are described, and the actuator-control system is discussed. Methods of extending this solution to the automatic alignment of other aspheric optics are explored.
Testing profiles of aspheric surfaces at various stages of figuring is essential, and a simple algorithm for finding the surface profile is very helpful. In order to find the wave front fourth order spherical aberration, a new algorithm resorting to decenter and defocus is presented here.
Since its discovery, interferometry has grown into a powerful technique for the characterization and evaluation of optical surfaces. With the advent of phase-measurement algorithms, computers, and solid state detectors, it is now possible to acquire wavefront maps over a well sampled pupil in fractions of a second. Nevertheless, even though there have been significant advances, limitations of the technology should always be acknowledged. Emphasis in this paper is placed on reviewing basic interference, optical component, source, imaging, phase-shifting, detector, mechanical and acoustical stability, air turbulence, and sampling requirements for high accuracy and precision phase-shifting interferometry measurements. Limitations of the technology will be outlined, and where applicable, techniques developed to overcome limitations will be discussed and references sited.
The paper describes the present design of contact profile meters for evaluation of topography- roughness of machine part surfaces in the manufacturing processes, including automated ones. Considerable attention is paid to the surfaces with a difficult or no access for the contact profilometer. In order to scan these surfaces of machine parts an experimental measuring device of module conception has been designed that works on the light principle and utilizes elements of laser technique. Interesting original experimental results are given that have been obtained by both, contact and light procedure. They explain in greater detail the given problems that are attractive especially from the standpoint of perspective automated production.
A new heterodyne interferometer to measure the surface topography is presented. The internal-mirror He-Ne laser used here has very high frequency difference (500 - 600 MHz). The frequency of this laser is stabilized by an intelligent servo circuit based on thermal negative feedback. Its frequency stabilization is better than 10-8. Besides, an improved interferometer is analyzed which has large measurement scale and is able to measure 3-D surface topography. By using precise phase measuring technique, the system has a height sensitivity of the order of magnitude of angstroms. In addition, the system has good dynamic characteristics.
The development of an optical profilometer for determining surface profile roughness is a fairly pressing problem, as it is indicated by a considerable number of publications dealing with this subject. Analysis of the most advanced developments used in solving this problem shows two different concepts for creating such profilometer being described either by the field amplitude or phase scatter models. If a field amplitude scatter model can describe such a measuring system as BRDF, then the phase scatter model describes laser shift interferometers. The field amplitude measuring systems demonstrate a high precision in roughness determination but they need to be calibrated. For this reason it is difficult to use them to control the profile of the moving surfaces. The phase shift laser interferometers present a high accuracy but their spatial resolution limited by a size of the focused spot. As it was mentioned, they have practically reached the limit of their potentials and further progress might be made by a development and implementation of the new photodetectors with a better spatial resolution. Recent developments in the area of creating the new instruments for determination of the surface roughness presently increase an amount of software for the data processing, solving the new measuring problems, especially for the laser interferometric applications. In this paper the problems related to the creating a new amplitude-phase scatter model with the advantages of the above mentioned models are discussed with a demonstration the results of a developed appropriate software.