A time-of-flight measurement-based three-dimensional (3D) profiler system employing a lightweight scanning system is demonstrated. To reduce the weight of the scanning system, and thereby achieve faster scanning speeds, two Fresnel prism sheets were employed as the scanning optics and installed to work as a pair of Risley prisms. Each Fresnel prism sheet has a diameter of 102 mm and mass of 15 g, which is about 12 times lighter than ordinary bulky prism. By scanning the laser beam with the developed scanning system, a 3D point cloud image of a target object located 8 m away could be successfully obtained. The image distortion was removable by correcting six geometrical parameters of the scanner using a simple optimization algorithm. It was confirmed by the experiment that once the distortion has been corrected, it is valid for other scanning speeds (and trajectories), enabling 3D profile measurements that do not require postprocessing of measured data. Measurement results for a standard target composed of square extrusions were in good agreement with the reference values, with deviations of <1 mm.
The uncertainties of measuring the geometrical thickness and refractive index of silicon wafers were evaluated. Both quantities of the geometrical thickness and refractive index were obtained using the previously proposed method based on spectral domain interferometry using the optical comb of a femtosecond pulse laser. The primary uncertainty factor was derived from the determination process of the optical path differences (OPDs) including the phase calculation, measurement repeatability, refractive index of air, and wavelength variation. The uncertainty for the phase calculation contains a Fourier transform in order to obtain the dominant periodic signal as well as an inverse Fourier transform with windowed filtering in order to calculate the phase value of the interference signal. The uncertainty for the measurement repeatability was estimated using the standard deviation of the measured optical path differences. During the experiments, the uncertainty of the refractive index of air should be considered for wavelengths in air because light travels through air. Because the optical path difference was determined based on the wavelength in use, the variation of the wavelength could also contribute to the overall measurement uncertainty. In addition, the uncertainty of the wavelength depends on the wavelength measurement accuracy of the sampling device, i.e. the optical spectrum analyzer. In this paper, the details on the uncertainty components are discussed, and future research for improving the performance of the measurement system is also proposed based on the uncertainty evaluation.
We describe a method to simultaneously measure both thickness profile and refractive index distribution of a silicon wafer based on a lateral scanning of the wafer itself. By using dispersive interferometer principle based on a broadband source, which is a femtosecond pulse laser with 100 nm spectral bandwidth, both thickness profile and refractive index distribution can be measured at the same time using a single scanning operation along a lateral direction. The proposed measurement system was tested using an approximately 90 mm range with a 0.2 mm step along the center-line, except for the rim area in a ϕ100 silicon wafer. As a result, the thickness profile was determined to have a wedge-like shape with an approximately 2 μm difference at an averaged thickness of 478.03 μm. Also, the mean value of the refractive index distribution was 3.603, with an rms value of about 0.001. In addition, the measurement uncertainty of the thickness profile was evaluated by considering two uncertainty components that are related to the scanning operation, like the yaw motion of the motorized stage and the long-term stability of an optical path difference in an air path. The measurement reliability of both the thickness profile and refractive index distribution can be increased through several methods such as an analysis of the correlation between the thickness profile and the refractive index distribution and a comparative measurement using a contact-type method; these potential methods are the subject of our future work.
We have designed and constructed the calibration system of line standards such as tape and rule for the secondary
calibration laboratories. The system consists of the main body with linear stage and linear encoder, the optical
microscope with digital camera, and the computer.
The base of the system is a aluminum profile with 2.9 m length, 0.09 m height and 0.18 m width. The linear stage and
the linear encoder are fixed on the aluminum profile. The micro-stage driven by micrometer is fixed on the carriage of
the long linear stage, and the optical microscope with digital camera and the tablet PC are on the this stage. The linear
encoder counts the moving distance of the linear stage with resolution of 1 μm and its counting value is transferred to the
tablet PC. The image of the scale mark of the tape is captured by the CCD camera of optical microscope and transferred
to the PC through USB interface. The computer automatically determines the center of the scale mark by image
processing technique and at the same time reads the moving distance of the linear stage. As a result, the computer can
calculate the interval between the scale marks of the tape. In order to achieve the high accuracy, the linear encoder
should be calibrated using the laser interferometer or the rigid steel rule. This calibration data of the linear encoder is
stored at the computer and the computer corrects the reading value of the linear encoder.
In order to determine the center of the scale mark, we use three different algorithms. First, the image profile over
specified threshold level is fitted in even order polynomial and the axis of the polynomial is used as the center of the line.
Second, the left side and right side areas at the center of the image profile are calculated so that two areas are same.
Third, the left and right edges of the image profile are determined at every intensity level of the image and the center of
the graduation is calculated as an average of the centers of the left and right edges at all intensity levels.
The system can measure the line standards up to 2.5 m. The expanded uncertainty for the tape calibration is U = [(0.04)2+ (0.015•L)2]1/2 mm, where L is measured length of the tape or rule in meters. At this system, the long distance
measuring instruments such as ultrasonic distance meter or laser displacement sensor can be also calibrated.
A laser radar (LADAR) system with a Geiger mode avalanche photodiode (GAPD) is used extensively due to its high
detection sensitivity. However, this system requires a certain amount of time to receive subsequent signals after detecting
the previous one. This dead time, usually 10 ns to 10 μs, is determined by the material composition of the detector and
the design of the quenching circuits. Therefore, when we measure objects in close proximity to other objects along the
optical axis using the LADAR system with GAPD, it is difficult to separate them clearly owing to the dead time problem.
One example for that is a case of hidden objects behind partially transparent blinds. In this paper, we suggested a
modified LADAR system with GAPD to remove the dead time problem by adopting an additional linear mode avalanche
photodiode (LAPD) as a complementary detector. Because the LAPD does not have dead time while still maintaining
relatively low detection sensitivity, the proposed system can measure an object placed within the dead time with high
detection sensitivity. Light is emitted from the pulsed laser of a light source and is delivered into a fast photodiode to
generate a start signal. Most of laser pulses are directed onto the target and scattered from the surfaces of targets. The
scattered light in the field-of-view of the system is divided by a polarizing beam splitter, after which it becomes incident
to two different types of APDs, the GAPD and the LAPD. The GAPD receives the signals from the target with high
sensitivity, and the signals scattered in the dead time zone are then detected by the LAPD. The obtained signals are
analyzed at the same time. In this way, the signals scattered from objects placed within the dead time can be
distinguished clearly.
We propose a microscopic system which could be applied to three-dimensional surface profile measurement. In the
system, a two-dimensional pinhole array is imaged onto the surface under measurement by an objective lens. These spots
act as discrete object points which are then imaged to the CCD chip by the microscope which contains two orthogonal
cylindrical lenses. Due to the astigmatism of the two cylindrical lenses, the shape of the image of object points on the
CCD camera becomes oval unless the object point is located at a position which satisfies the best imaging condition. By
calculating the focus error signal using the intensities measured at a group of CCD cells, the information on the distance
of the corresponding object point could be found out.
The basic concept of the system was checked by computer simulation on the point spread function of various object
points. A preliminary measurement system which consists of the same optical components used in the computer
simulation has been set up for verification of the idea. Since this system requires only one image to analyze the surface
profile, it is a one-shot measurement system, and is insensitive to environmental noises such as mechanical vibration.
We developed a gauge block interferometer which utilizes the frequency tunable laser diodes as both light sources and
phase shifters of a phase shifting interferometer. By using a confocal Fabry-Perot cavity made of ultra low expansion
glass, and linearly modulating the laser diode current, the laser frequency could be injection locked to the resonant
modes of the Fabry-Perot cavity consecutively. These equal spaced frequencies produce equally phase shifted
interferometric images which are ideal to be analyzed by the Carré algorithm. Two frequency scanning lasers at the
wavelengths of 636 nm and 657 nm are used as light sources for the gauge block interferometer. The system takes only
10 ms for a single measurement which acquires two sets of four equally phase shifted images with 640×480 pixels in
size. Central lengths of gauge blocks are measured by using the phase shifting interferometry and exact fraction method.
The performance of the high speed interferometer could be checked by comparing the measurement results on the same
gauge block made by two different methods. Two results agreed well within the measurement uncertainty.
This paper presents the design and fabrication of a precision dual level stage composing a dimensional metrological
system for large range surface topography, such as mask patterns for lithography, fine artifacts on a semi-conductor
wafer and micro roughness on a large specular surface. The stage was configured as dual level, a fine stage on a coarse
stage, to obtain large moving range and high resolution simultaneously. In the design of the coarse stage, we focused on
a simple structure with low profile to achieve insensitivity to vibration and high accuracy. Therefore, a high quality flat
surface plate was used as the reference plane of the coarse stage's movement, instead of a conventional simple stacking
of two long stroke one-axis stages. The surface plate also has a role of metrological frame for very low thermal
expansion coefficient and its size is 800 mm × 800 mm. The coarse stage is guided horizontally by a cross structure with
two precision straight bars perpendicularly linked and vertically by the surface plate. The sliding pads made of PTFE are
used to guarantee the smooth motion of the coarse stage for both horizontal and vertical directions. The fine stage fixed
on the coarse stage generates five-axis fine motion, such as two-axis in-plane translation, one-axis in-plane and two-axis
out-of-plane rotation. The fine stage is composed of flexure guided structures and actuated by five PZTs. The developed
dual level stage can achieve a large range of 200 mm × 200 mm and a nanometric resolution simultaneously. Its
movement is monitored and controlled using a five-axis laser interferometer system to be applied to a dimensional
metrology having direct meter-traceability.
A total integrated scattering (TIS) system consisting of an integrating sphere has been developed in KRISS for the
purpose of measuring the effective roughness amplitude of gauge blocks and platens, which are necessary for the
correction of phase shift due to roughness difference between gauge block and platen, in the calibration of gauge blocks
by optical interferometry. Details on the TIS system and its calibration by using two different methods are described. The
uncertainty of the effective roughness amplitude measurement by using the TIS system is evaluated to be 2 nm (k=1).
The pitch and orthogonality of two-dimensional (2D) gratings have been calibrated by using an optical diffractometer (OD) and a metrological atomic force microscope (MAFM). Gratings are commonly used as a magnification standard for a scanning probe microscope (SPM) and a scanning electron microscope (SEM). Thus, to establish the meter-traceability in nano-metrology using SPM/SEM, it is important to certify the pitch and orthogonality of 2D gratings accurately. ODs and MAFMs are generally used as effective metrological instruments for the calibration of gratings in nanometer range. Since two methods have different metrological characteristics, they give complementary information for each other. ODs can measure only mean pitch value of grating with very low uncertainty, but MAFMs can obtain individual pitch value and local profile as well as mean pitch value, although they have higher uncertainty. Two kinds of 2D gratings, each with the nominal pitch of 700 nm and 1000 nm, were measured, and the uncertainties of calibrated values were evaluated. We also investigated the contribution of each uncertainty source to the combined standard uncertainty, and discussed the causes of main ones. The expanded uncertainties (k = 2) of calibrated pitch values were less than 0.05 nm and 0.5 nm for the OD and the MAFM, and the calibration results were coincident with each other within the expanded uncertainty of the MAFM.
This paper describes a fringe scanning Fourier transform method to automatically measure the fractional interference order in gauge block interferometry. The advantages of the proposed method are presented with the comparison of measurement results between the existing Fourier transform method and the fringe scanning Fourier Transform method. The configuration of automatic gauge block measuring system is also described, where the proposed method is applied. The standard uncertainty evaluation of the fractional interference order measurement with this method is given.
A compact linear and angular displacement measurement device was developed by combining a Michelson interferometer in Twyman-Green configuration and an autocollimator to characterize the movement of a precision stage. A precision stage usually has 6 degrees of freedom of motion (3 linear and 3 angular displacements) due to the parasitic motions, thus linear and angular displacement should be measured simultaneously for the complete evaluation of precision stage. A Michelson interferometer and an autocollimator are typical devices for measuring linear and angular displacement respectively. By controlling the polarization of reflected beam from the moving mirror of the interferometer, some parts of light are retro-reflected to the light source and the reflected beam can be used for angle measurement. Because the interferometer and the autocollimator have the same optic axis, the linear and angular displacements are measured at the same position of the moving mirror, and the moving mirror can be easily and precisely aligned to be orthogonal to the optic axis by monitoring the autocollimator's signal. A single mode polarization maintaining optical fiber is used to deliver the laser beam to the device, and all components except the moving mirror are fixed with bonding to achieve high thermal and mechanical stability. The autocollimator part was designed to have the angular resolution of 0.1" and the measurement range of 60". The nonlinearity error of interferometer was minimized by trimming the gain and offset of the photodiode signals.
KEYWORDS: Photons, Signal detection, Holography, Scattering, Sensors, Signal processing, Two photon imaging, Quantum cryptography, Quantum computing, Quantum information
Principle of quantum holographic imaging is explained. While one of the entangled twin photons interacts with an object, the other one carries the holographic image of the object. While this scheme does not work for simple scattering objects, it works for absorbing objects with selective coincidence detection. Generalization of this scheme to state-transforming objects is also proposed.
We proposed a novel interferometer, which can measure the 3-dimensional coordinates of freely moving objects in space. The interferometer is composed of two fiber ends and three photo detectors. Frequency stabilized laser is incident into an optical fiber, and then split by the fiber beam splitter into two fiber ends. One fiber is used as the fixed reference arm, and the other as the movable test arm of the interferometer. By using servo control on one arm, the phase difference of the laser beams being emitted from the two fiber ends is forced to be kept constant. Beams from each fiber end are combined at the photo detectors on which interference fringes are formed. The displacement of the distances of the moving fiber end (MFE) from the three detectors can be measured by analyzing the interference fringe. From the measured distances of the MFE from each detector, the 3-dimensional coordinates of the MFE are obtained in real time by using the triangulation. Since the wavefront of the beam from the MFE is spherical, the displacement of the MFE is not restricted to a specific direction, which requires no guiding rail for the moving object. This interferometer could be applied to situations where 3-dimensional coordinates of freely moving objects are being measured with high precision.
A phase encoding electronics capable of compensating the nonlinearity in a heterodyne laser interferometer is described. The system consists of the phase demodulating electronics and the nonlinearity compensating electronics. For phase demodulation, the device uses the phase-quadrature mixing technique. And, for nonlinearity compensation, the offsets, the amplitudes and the phase of two output signals from demodulator are adjusted electrically so that their Lissajous figure can be a circle. As a result, the correct phase can be obtained. The brief analysis of nonlinearity in heterodyne interferometer and the design of the phase encoding electronics are presented. The test was performed in a Michelson-type interferometer using a transverse Zeeman stabilized He-Ne laser. The experiment demonstrates that this method can encode the phase of the heterodyne interferometer with sub-nanometer accuracy.
An x-ray vision can be a unique method to monitor in real time and analyze the motion of mechanical parts which are invisible from outside. Our problem is to identify the pose, i.e. the position and orientation of an object from x-ray projection images. It is assumed here that the x-ray imaging conditions that include the relative coordinates of the x-ray source and the image plane are predetermined and the object geometry is known. In this situation, an x-ray image of an object at a given pose can be estimated computationally by using a priori known x-ray projection image model. It is based on the assumption that a pose of an object can be determined uniquely to a given x-ray projection image. Thus, once we have the numerical model of x-ray imaging process, x-ray image of the known object at any pose could be estimated. Then, among these estimated images, the best matched image could be searched and found. When adequate features in the images are available instead of the image itself, the problem becomes easier and simpler. In this work, for simplicity, only polyhedral objects are considered whose image features consist of corner points and edge lines in their projection images. Based on the corner points and lines found in the images, the best-matched pose of a polyhedral object can be determined. To achieve this, we propose an adequate and efficient image processing algorithm to extract the features of objects in x-ray images. The performance of the algorithms is discussed in detail including the limitations of the method. To evaluate the performance of the proposed method a series of simulation studies is carried out for various imaging conditions.
12 Ever since surface-mounting technology for printed circuit board (PCB) assembly processes has been developed, electrical products continuously tend toward the miniaturization of components, with denser packing of its boards. With the increasing necessity for reliable PCB product, there has been a considerable demand for high speed, high precision vision system to place the electric parts on PCB automatically. To recognize the electric parts with high accuracy and reliability, illumination condition is instrumental to acquisition of part images. In this paper, a versatile lighting is developed which utilizes three different types of illuminating methods: direct, indirect, and back-light illumination.
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