Tae Bong Eom

Thickness and refractive index measurement of a silicon wafer based on an optical comb

We have proposed and demonstrated a novel method that can determine both the geometrical thickness and refractive index of a silicon wafer at the same time using an optical comb. The geometrical thickness and refractive index of a silicon wafer was determined from the optical thickness using phase information obtained in the spectral domain. In a feasibility test, the geometrical thickness and refractive index of a wafer were measured to be 334.85 microm and 3.50, respectively. The measurement uncertainty for the geometrical thickness was evaluated as 0.95 microm (k = 1) using a preliminary setup.

Measurement of refractive index and thickness of transparent plate by dual-wavelength interference.

We developed an accurate and efficient method for measuring the refractive indices of a transparent plate by analyzing the transmitted intensity versus angle of incidence. By using two different wavelengths, we resolved the 2pi-ambiguity inherent to the phase measurement involving a thick medium, leading to independent determination of the absolute index of refraction and the thickness with a relative uncertainty of 10(-5). The validity and the accuracy of our method were confirmed with a standard reference material. Furthermore, our method is insensitive to environmental perturbations, and simple to implement, compared to the conventional index measurement methods providing similar accuracy.

Measurement of microscope calibration standards in nanometrology using a metrological atomic force microscope

Microscope calibration standards in nanometrology were calibrated using a metrological atomic force microscope (metrological AFM) and the validity of calibrated values was shown. The metrological AFM was developed through the modification of a commercial AFM, which replaced the PZT tube scanner with flexure hinge scanners and displacement sensors. These modifications improved the traceability of measured values to metrological primary standards. The grating pitch and step height specimens, which are typical standard artefacts for the calibration of lateral and vertical magnifications of microscopes, were measured using the metrological AFM. The expanded uncertainties (k = 2) of calibrated values were estimated considering the characteristics of the calibration process and were less than 1 nm. The measurement results were compared with those obtained by other metrological methods or the certified values and their consistency was verified by checking the En numbers. These experimental results show that the metrological AFM can be used effectively for the measurements of microscope calibration standards in nanometrology.

A simple phase-encoding electronics for reducing the nonlinearity error of a heterodyne interferometer

A phase-encoding electronics capable of compensating for the nonlinearity error in a heterodyne laser interferometer is described. The system consists of the phase demodulating electronics and the nonlinearity compensating electronics. For phase demodulation, we use the phase-quadrature mixing technique. For nonlinearity compensation, the offsets, the amplitudes and the phase of two output signals from the demodulator are adjusted electrically so that their Lissajous figure is a circle. As a result, the correct phase can be obtained. An analysis of the nonlinearity in the heterodyne interferometer and the design of the phase-encoding electronics are presented. The experiment was performed in a Michelson-type interferometer using a transverse Zeeman stabilized He?Ne laser. We demonstrate that this method can encode the phase of a heterodyne interferometer with sub-nanometer accuracy.

A digital signal processing module for real-time compensation of nonlinearity in a homodyne interferometer using a field-programmable gate array

This note presents a digital signal processing module for the real-time nonlinearity compensation of a homodyne interferometer. The nonlinearity is corrected by using the parameter values describing two phase-quadrature signals, through simple arithmetic calculation of the quadrature signals at specific phases, which are multiples of π/4. A field-programmable gate array was employed for the real-time implementation of a processing module since it has reconfigurable input/output and high precision synchronization. The developed module has a minimum loop time of 4.4 µs and can compensate the nonlinearity error less than ±0.5 nm, which is comparable with the elliptical fitting method. We also proved the performance of the module by examining the convergence and the stability of parameter values under various operational conditions.

An optical absolute position measurement method using a phase-encoded single track binary code

We present a new absolute position measurement method using a single track binary code where an absolute position code is encoded by changing the phase of one binary state representation. It can be decoded efficiently using structural property of the binary code, and its sub-division is possible by detecting the relative positions of the binary state representation used for the absolute position encoding. Therefore, the absolute position encoding does not interfere with the sub-division process and so any pseudo-random sequence can be used as the absolute position code. Because the proposed method does not require additional sensing part for the sub-division, it can be realized with a simple configuration and efficient data processing. To verify and evaluate the proposed method, an absolute position measurement system was setup using a binary code scale, a microscopic imaging system, and a CCD camera. In the comparison results with a laser interferometer, the measurement system shows the resolution of less than 50 nm and the nonlinearity error of less than ±60 nm after compensation.

Vibration-insensitive measurement of thickness variation of glass panels using double-slit interferometry

A technique which can measure thickness variation of a moving glass plate in real-time with nanometric resolution is proposed. The technique is based on the double-slit interference of light. Owing to the nature of differential measurement scheme, the measurement system is immune to harsh environmental condition of a production line, and the measurement results are not affected by the swaying motion of the panel. With the preliminary experimental setup with scanning speed of 100 mm/s, the measurement repeatability was 3 nm for the waviness component of the thickness profile, filtered with a Gaussian filter with cutoff wavelength of 8 mm.

An interferometric Abbe-type comparator for the calibration of internal and external diameter standards

We developed an Abbe-type comparator using a laser interferometer and a linear variable differential transformer (LVDT) probe as displacement sensors, which can measure the diameter of ring and plug gauges up to 300 mm. The measurement system is configured according to the Abbe principle, and consists of translation stages, a laser interferometer, an LVDT probe and an electronic controller. The main translation stage is made by using a precision ceramic guide and air bearing pads, and is driven by a backlash-free lead screw and a micro-stepping motor. The laser interferometer measures the displacement of a moving mirror aligned with the probe coaxially. The environmental effect is corrected automatically during the measurement. The effective diameter of the probe ball is calibrated using a reference gauge block. The performance of each component was evaluated through experiments and the measurement uncertainty of the overall system was analyzed. We measured three diameter artifacts, which are 11.95 mm and 100 mm ring gauges and a 98.5 mm plug gauge, and compared the measured values with the calibrated ones. They were consistent with each other within 0.3 µm, which is less than the expanded measurement uncertainty (k = 2).

High speed phase shifting interferometry using injection locking of the laser frequency to the resonant modes of a confocal Fabry-Perot cavity

We present a high speed phase shifting interferometer which utilizes the self injection locking of a frequency tunable laser diode. 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. It provided equal phase steps to the interferograms which are ideal to be analyzed by the Carré algorithm. The phase step error was evaluated to be about 3 MHz which corresponds to 0.2 nm in length measurement. With this technique, profile measurements are insensitive to external vibration since four 640x480 pixels images can be acquired within 4 ms. Difference of two profile measurements, each made with and without vibration isolation, respectively, was evaluated to be 0.5 nm (rms).

A 50 m laser interferometer for automatic calibration of surveying tapes using wireless communication

A 50 m linear measuring interferometer, consisting of a precision laser interferometer, a 51 m long guide rail, a moving carriage, an optical microscope with a CCD camera and an image processor, is described here. The system is designed for the automatic calibration of surveying tapes. The carriage can move up to 50 m along the guide rail. The dc servo motor, which is fixed on the carriage, drives the carriage and its speed is controlled by a computer through wireless communication. The CCD camera captures the image of tape lines through the microscope fixed on the stage, and the image is wireless transferred to the image processor installed in the computer. The image processor calculates the deviation between the center of the line and the field-of-view of the CCD camera, and the laser interferometer measures the displacement of the carriage simultaneously. Finally, the intervals between lines are determined using the deviation and the reading of the laser interferometer. The calibration process is performed automatically after the installation of the tape. The estimated expanded uncertainty of the steel tape measurement is at the confidence level of approximately 95%.