Piezoelectric transducer (PZT) is often used in the field of precision measurement to realize the micro-positioning function, such as Fizeau interferometer. But due to its inherent severe hysteresis and nonlinear characteristics, the use of piezoelectric transducer is affected. A piezoelectric transducer control instrument was designed to measure the displacement characteristic curve of piezoelectric transducer. The instrument included the control system and data received system. The control system produces a triangular wave signal with adjustable amplitude and frequency, then calculated from received signal to drive the piezoelectric transducer. The output displacement information of piezoelectric transducer is collected by the signal acquisition circuit composed of displacement sensor and ADC analog-digital converter. The purpose of the single chip is collecting and processing displacement data. According to the experimental results, we can get the characteristics curve of the piezoelectric transducer, which support the basis analysis and available for the future experiment.
Interferometry repeatability is an important indicator for measuring instrumentation and test results. The precondition of improving the detection accuracy is to ensure high-precision repeatability, so high-precision repeatability is one of the most important indicators in precision detection. In the Fizeau interferometer, common light paths are used for the distances from the laser light source to the reference surface, from the reference surface to the detector CCD, and the reference beam and the test beam. However, the strict sense of the co-beam does not exist, and it is not absolute ideal that reference plane has high-precision surface. The test beam reflects from the measured surface will have a certain angular deviation from the reference beam. If so, it may make the reference beam and the test beam reach the CCD detector along different optical paths and generate return error accordingly. And finally it makes effects in sampling and wave surface reproduction. Therefore, using the Zygo GPI system, the 4D system and the H and L system to make research on the same optical platform for relative interferometry repeatability comparison, ensuring that the cavity length is the same, and the standard reference mirror and the tested mirror is the same. Measurements were repeated 50 times for zero-stripes, five-stripes, and ten-stripes. The PV values and RMS values of the 50 measurements make mean processing to reflect the measurement repeatability. Further repeatability error analysis is performed on the phase shift algorithm and PZT phase shifter.
We developed a new application on Microsoft Foundation Classes (MFC) to identify correct
transmission spheres (TS) for Spherical Surface Testing (SST). Spherical surfaces are important optical
surfaces, and the wide application and high production rate of spherical surfaces necessitates an accurate
and highly reliable measuring device. A Fizeau Interferometer is an appropriate tool for SST due to its subnanometer
accuracy. It measures the contour of a spherical surface using a common path, which is
insensitive to the surrounding circumstances. The Fizeau Interferometer transmits a wide laser beam,
creating interference fringes from re-converging light from the transmission sphere and the test surface. To
make a successful measurement, the application calculates and determines the appropriate transmission
sphere for the test surface. There are 3 main inputs from the test surfaces that are utilized to determine the
optimal sizes and F-numbers of the transmission spheres: (1) the curvatures (concave or convex), (2) the
Radii of Curvature (ROC), and (3) the aperture sizes. The application will firstly calculate the F-numbers
(i.e. ROC divided by aperture) of the test surface, secondly determine the correct aperture size of a convex
surface, thirdly verify that the ROC of the test surface must be shorter than the reference surface’s ROC of
the transmission sphere, and lastly calculate the percentage of area that the test surface will be measured.
However, the amount of interferometers and transmission spheres should be optimized when measuring
large spherical surfaces to avoid requiring a large amount of interferometers and transmission spheres for
each test surface. Current measuring practices involve tedious and potentially inaccurate calculations. This
smart application eliminates human calculation errors, optimizes the selection of transmission spheres
(including the least number required) and interferometer sizes, and increases efficiency.
Higher accuracy of optical surface measurement is needed with the rapid development of optical industry and technology, especially in the field of optical manufacture and optical metrology. Fizeau interferometery is widely recognized as one of the most important measurement techniques currently in use. Results of the Fizeau interferometeric testing contain the reference surface errors and test surface errors. The test accuracy is restricted by the error of reference surface. As a result, so-called absolute flatness testing technology must be used to eliminate the system error such as reference surface error and higher accuracy of the surface profile testing. In this paper, the theory formula of even and odd functions is deduced, and the method are programmed for simulation. Based on HOOL phase-shifting interferometer, experiments are done to achieve high accuracy flatness measurement. The experiment shows that with high-accuracy interferometer, absolute flatness testing technology can effectively calibrate reference surface and improve the accuracy of flatness surface testing. An important result of this experiment is that the accuracy of the test surface can be reached λ/50.
A conventional phase shifting interferometer is capable of measuring opaque surfaces with sub-nanometer precision.
However, it cannot be used to measure an object with multiple parallel reflective surfaces such as a transparent plate, a
glass disk, or an Extreme Ultraviolet Lithography (EUVL) mask blank. This is because the plane parallel reflective
surfaces generate multiple interferograms that are superimposed in the recording plane of the interferometer. Although
every individual interferogram is associated with phase information that is related to the height or thickness, the
conventional interferometer is not able to differentiate one surface from another. To measure these surfaces, we have
developed a method that integrates a Fizeau interferometer with a tunable light source and a weighted least-square
technique. The tunable light source controls the wavelength during the data acquisition process, producing phase shift
speeds that are proportional to the optical path difference (OPD). The weighted least-square signal processing
technique separates each surface from the others in an optimal manner. Thus the desired information, such as the front
surface height, back surface height, and relative optical thickness of a plane-parallel transparent glass plate are extracted
without multi-surface fringe print-through artifacts. In this paper we will present the method and demonstrate its
performance. The demonstrated surface height accuracy for EUVL mask blank substrates is 5 nm and the RMS
repeatability is <0.01 nm.
Phase shifting interferometry (PSI) is a highly accurate method for measuring the nanometer-scale relative surface height
of a semi-reflective test surface. PSI is effectively used in conjunction with Fizeau interferometers for optical testing,
hard disk inspection, and semiconductor wafer flatness. However, commonly-used PSI algorithms are unable to produce
an accurate phase measurement if more than one reflective surface is present in the Fizeau interferometer test cavity.
Examples of test parts that fall into this category include lithography mask blanks and their protective pellicles, and
plane parallel optical beam splitters. The plane parallel surfaces of these parts generate multiple interferograms that are
superimposed in the recording plane of the Fizeau interferometer. When using wavelength shifting in PSI the phase
shifting speed of each interferogram is proportional to the optical path difference (OPD) between the two reflective
surfaces. The proposed method is able to differentiate each underlying interferogram from each other in an optimal
manner. In this paper, we present a method for simultaneously measuring the multiple test surfaces of all underlying
interferograms from these superimposed interferograms through the use of a weighted least-square fitting technique.
The theoretical analysis of weighted least-square technique and the measurement results will be described in this paper.
As the semiconductor industry progresses down the roadmap to smaller device geometries, precise metrology of the
wafer nanotopography is increasingly needed by the wafer manufacturers and the semiconductor device manufacturers.
To meet the demands, we have built an instrument that integrates our patented and proprietary white light interferometer
with a motorized x-y stage. It measures height variation over the entire 200mm or 300mm wafer front surface with high
lateral resolution and sub-nanometer height precision. This is accomplished by stitching together multiple maps
obtained from a white light interferometer. Each individual map represents height variation over a part of the wafer
surface. These individual maps are properly positioned to cover the entire wafer surface with sufficient overlapping area.
With such properly arranged maps, we are able to produce an entire wafer surface by stitching them together. The map
stitching expands the field of view available to our patented low coherence interferometer. This approach makes it
possible to reveal more detail nanotopographic information over the entire wafer surface.
In this paper, we will present the map stitching approach for the measurements of 200mm and 300mm wafers, including
the theoretical foundations of stitching technique and the arrangements of individual measurements. We will also
demonstrate measurement results on various wafers.
The precision metrology of patterned wafer is increasingly demanded by the semiconductor device manufacturers. The
most common methods include scanning probe microscopy (SPM) techniques such as stylus profilometry and Atomic
Force Microscopy (AFM). These methods acquire data by contacting the surface over a sequence of one-dimensional
scans. While high lateral resolution can be achieved in this way, such processes are time-consuming and can have the
potential to deform the surface under test. An alternative non-contact interferometric method is presented here. The
method uses the white-light interferometry (WLI) to provide wafer topography quickly in a direct three-dimensional
format. The improved measurement throughput suggests that it is feasible to use this method for production monitoring.
Most commercial interferometers with WLI are capable of measuring opaque surfaces with sub-nanometer precision.
The described method extends this capability to determine the top surface topography of structured surfaces in the
presence of varying phase shifts on reflection. The phase shift on reflection may be due to the material properties of bulk
surfaces, single or multi-layer film stacks on a substrate, or other micro-structures on the wafer. Furthermore, this
method simultaneously or separately provides additional parameters of the test piece e.g. layer thickness and/or material
refractive index for film stacks, or line width and structure depth of micro-structures. The measurement results on
various types of the wafer surfaces will be presented in this paper.
Wafer shape and thickness variation are important parameters in the IC manufacturing process. The thickness variation,
also called flatness, enters the depth-of-focus budget of microlithography, and also affects film thickness uniformity in
the CMP processing. The shape mainly affects wafer handling, and may also require some depth-of-focus if the wafer
shape is not perfectly flattened by chucking. In the progression of technology nodes to smaller feature sizes, and hence
smaller depth-of-focus of the lithography tool, the requirement for the PV-flatness over stepper exposure sites is
becoming progressively tighter, and has reached 45nm for the next technology node of 45nm half pitch. Consequently, in
order to be gauge-capable the flatness metrology tool needs to provide a measurement precision of the order of 1nm.
Future technology nodes will require wafers with even better flatness and metrology tools with better measurement
precision. For the last several years the common capacitive tools for wafer dimensional metrology have been replaced by
interferometric tools with higher sensitivity and resolution. In the interferometric tools the front and back surface figure
of the wafer is measured simultaneously while the wafer is held vertically in its intrinsic shape. The thickness variation
and shape are then calculated from these single-sided maps. The wafer shape, and hence each wafer surface figure, can
be tens of microns, necessitating a huge dynamic range of the interferometer when considering the 1nm measurement
precision. Furthermore, wafers are very flexible, and hence very prone to vibrations as well as bending. This presentation
addresses these special requirements of interferometric wafer measurements, and discusses the system configuration and
measurement performance of WaferSightTM, KLA-Tencor's interferometric dimensional metrology tool for 300mm
wafers for current and future technology nodes.
According to industry standards (SEMI M43, Guide for Reporting Wafer Nanotopography), Nanotopography is the non- planar deviation of the whole front wafer surface within a spatial wavelength range of approximately 0.2 to 20 mm and within the fixed quality area (FQA). The need for precision metrology of wafer nanotopography is being actively addressed by interferometric technology. In this paper we present an approach to mapping the whole wafer front surface nanotopography using an engineered coherence interferometer. The interferometer acquires a whole wafer raw topography map. The raw map is then filtered to remove the long spatial wavelength, high amplitude shape contributions and reveal the nanotopography in the filtered map. Filtered maps can be quantitatively analyzed in a variety of ways to enable statistical process control (SPC) of nanotopography parameters. The importance of tracking these parameters for CMOS gate level processes at 180-nm critical dimension, and below, is examined.
To achieve a measurement with high spatial resolution over a large area, we stitch together multiple high spatial resolution maps obtained from an interferometric microscope with or without a motorized stage. These maps have a high height precision and a lateral resolution as high as 0.11 micron/pixel. They can be stitched together with acceptable accuracy as long as each of their adjacent areas has enough overlapping pixels. The stitching technique we have developed is based on a computed probability that gives a quantitative measure for the goodness of the chi-square fitting. It is insensitive to the piston and tilt changes of each map. It is also insensitive to the lateral shift and/or rotation between the overlapping maps. This technique is robust and flexible. It is able to give a satisfactory measurement without relying on the accuracy of the stage movement. The map stitching expands the field of view available to an objective lens, which makes it possible to measure a large area without losing high-slope surface features.
A new generalized algorithm for phase extraction from interferograms recorded sequentially at
different phase shifts is proposed. It is developed from the principle of least-square fining with a weight
function. This algorithm removes many of the restrictions imposed by conventional phase-measuring
algorithms, such as requiring a constant phase shift between consecutive interferograms. This algorithm
can estimate phase from a series of sample interferograms with arbitrary phase shift, as long as the phase
shifts between these interferograms are known. By introducing the proper weight function, the proposed
algorithm is able to estimate the phase with a high degree of tolerance to common error sources including
phase-shift mis-estimation and detector nonlinearity. Consequently, this technique may offer improved
phase estimation relative to unweighted generalized algorithms using the same number of intensity frames.
Theoretical analysis and results supported by numerical simulation and experiments are presented.
Keywords: phase shifting interferometry, optical metrology, phase-measuring algorithms.
An electronic speckle-pattern interferometer with a polarization phase-shift technique for the deformation measurement of a diffuse surface is proposed. A common-path optical phase-shift arrangement is
adopted in the interlerometer to improve the stability of the optical system. A polarization phase-shift technique is used to obtain precisely phase-shifted interferograms. A phase map of a fringe pattern, which is capable of distinguishing surface depressions from elevations, can be automatically and accurately obtained from four interferograms by the computer data processing. The numerical data representing the deformation over the entire field can be easily extracted from the phase map. An example of deformation measurement using this interferometer is presented.
KEYWORDS: Edge detection, Image processing, Digital signal processing, Cameras, Optical inspection, Optical testing, Convolution, Image enhancement, Signal to noise ratio, Digital image processing
A novel edge extraction method that employs an active defocusing technique is presented. The method is based on
the principle that a Laplacian of Gaussian(LOG) operator can be well approximated by a Difference of Gaussian(DOG)
operation.. While such operation can be done by digital processing, it is more effective to be conducted in a combination
of optical and digital processing techniques. In this edge extraction process, a focused image of object in scene is first
acquired. Image of the scene is then slightly defocused by changing the focal length of camera lens. Real time
subtraction is applied to the defocused and the previously acquired images. It produces a residual image that emphasizes
abrupt intensity variations, which are typical of edges in the image. An objective evaluation called edge index is
performed on the resulting image. Amount of defocusing is carefully adjusted according to this measurement so that a
desired edge image is generated. Boundaries of objects can then be obtained by further enhancement of the edge image.
Since this edge detection method is an optical-based process aided by digital processing, it is rather fast and less
expansive.
A new method, based on a combination of interferometric grating projection and 2D digital FFT-based phase measurement, is presented for 3D shape measurement and surface inspection. This technique can generate a grating pattern focusing everywhere in its illumination space; thus, it has infinity focus depth. This capability makes it possible to improve measurement sensitivity by increasing the projection angle without losing the grating focus, making it more suitable for surface inspection of a large object. The sinusoidal intensity distribution of the grating, the monotonic phase nature of the fringe pattern coded on the object surface, and the phase information spectra clustered in the frequency domain make data deduction by this technique not only possible, but more accurate.
A new technique for phase extraction of a fringe pattern is presented and verified in this paper. The technique makes use of the orthogonality of the trigonometric functions. It acquires the wrapped phase of a fringe pattern at a pixel point with the use of intensity values at that pixel point and its neighbors. It is capable of obtaining the unwrapped phase of a fringe pattern at every pixel point automatically provided that the phase of the fringe pattern is monotonically increasing. In contrast to the Fast Fourier transform (FFT) based phase measurement technique, this technique not only eliminates the difficulty of choosing a passing-band in the frequency domain but also provides great flexibility for processing an arbitrary number of pixel points at every row of the image.
A novel two-dimensional Fast Fourier Transformation (FFT) based phase measurement technique for fringe pattern analysis is presented in this paper. The technique uses two fringe patterns whose phase are slightly shifted to eliminate any phase ambiguity which exists in a fringe pattern whose phase is not monotonically increasing. It is capable of extracting an accurate phase distribution from a noisy fringe pattern without assigning fringe orders and interpreting data in the regions between fringes. The technique is free from error caused by the high frequency components, the variation of background intensity, and the modulation of amplitude on fringe patterns. This technique is particularly useful for processing the high-frequency noise corrupted sinusoidal fringe patterns.
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