Artur G. Olszak

High-precision shape measurement by white-light interferometry with real-time scanner error correction

White-light interferometric techniques allow high-precision shape measurement of objects with discontinuous structures by detecting the peak of the coherence envelope. These techniques assume a specific change in the optical path difference (OPD) between the interfering beams; however, the scanning device effecting that change often introduces OPD errors that are carried over to the measurements. We present a technique for measuring OPD changes from the collected interference fringes during each measurement. Information about the scan is directly fed into the algorithm, which compensates for the errors, resulting in improved measurement accuracy. The method corrects not only the scanner errors but also slowly varying vibrations. In addition, this technique can be easily adapted to any existing low-coherence interferometer because no large data storage or postprocessing is required.

High-stability white-light interferometry with reference signal for real-time correction of scanning errors

White-light interferometry (WLI) for object topography measurements relies on an accurate rate of change of the optical path difference (OPD) between the object and reference beams. However, the motion of the scanner realizing the OPD change is not perfect, and scanning errors directly impact measurement accuracy. We describe a white-light interferometer that is capable of accounting for these errors and correcting them in real time through the monitoring of the scanner motion. This monitoring is achieved by embedding an additional high-coherence interferometer into the system. Besides allowing for monitoring the scanning progress, this system also provides the means to automatically calibrate the WLI. Significant improvements both in the accuracy and repeatability are demonstrated experimentally. Data analysis is discussed as well.

Lateral scanning white-light interferometer

White-light vertical scanning interferometry is a well-established technique for retrieving the three-dimensional shapes of small objects, but it can measure only areas as big as the field of view of the instrument. For bigger fields a stitching algorithm must be applied, which often can be a source of errors. A technique in which the object is scanned laterally in front of an instrument with a tilted coherence plane is described. It permits measurements at higher speeds while measurement accuracy is retained and eliminates the need for stitching in one direction. Experimental confirmation is provided.

White light interferometry with reference signal

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.

Challenges in white-light phase-shifting interferometry

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.

Design of an interferometer for the measurement of long radius optics

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.

Spectrally controlled interferometry

Optical interferometers are typically categorized by their source type into incoherent (white-light) and coherent (laser). Both approaches provide adequate solutions for many measurement applications, offer unique advantages, and suffer distinct limitations. A novel interferometry method, spectrally controlled interferometry, is presented, which successfully merges many advantages from both categories while bypassing some of the limitations. The relationship between measurement accuracy and fringe stability as a function of fundamental control parameters is explored. Surface measurements of common optical components are presented, and method specific noise sources and measurement accuracy are assessed as well.

Improved fast white-light scanning profilometer

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.

Laser Fizeau interferometer for silicon wafer site flatness testing

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.

Towards instantaneous spectrally controlled interferometry

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.