Jae Wan Kim

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 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 compact system for simultaneous measurement of linear and angular displacements of nano-stages

We report on a novel compact interferometery system for measuring parasitic motions of a precision stage. It is a combination of a Michelson interferometer with an auto-collimator, of which full physical dimension is mere 70 mm x80 mm x35 mm (WxLxH) including optical components, photo-detectors, and electronic circuits. Since the beams, which measure displacement and angle, can be directed at the same position on the moving mirror, the system is applicable for testing small nano-stages where commercial interferometers are not able to be used. And thus, errors from nano-scale deformation of the moving mirror can be minimized. We find that the residual errors of linear and angular motion measurements are 2.5 nm in peak-to-peak and 0.2, respectively.

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 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%.

Calibration of two-dimensional nanometer gratings using optical diffractometer and metrological atomic force microscope

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.

An interferometric calibration system for various linear artefacts using active compensation of angular motion errors

A calibration system for linear-dimension artefacts was developed, which employed a multi-axis laser interferometer for direct metrological traceability and active compensation of angular motion errors. It can calibrate various end and line standards by changing probes (contact and optical probe). We designed the system as a moving probe type with a cantilever structure to reduce overall size and increase efficiency in calibration. A stage part including a two-axis tilt stage provides precise linear motion of a probing part over the range of 2000 mm with nanometric resolution. The three-axis interferometer measuring linear and rotational motions of the stage enables us to obtain probing position and compensate angular motion errors precisely. It was also arranged to minimize the Abbe offset, and so the Abbe error can be reduced remarkably combining the active compensation of angular motion errors. The overall system was installed in a temperature-controlled chamber to decrease thermal variation during measurements. The measurement uncertainty of the calibration system was analysed by considering the performance of the main components. We measured several long gauge blocks and a precision line scale, and compared the measured values with the reference ones and also checked their stabilities. Their deviations were less than 100 nm and existed within the expanded measurement uncertainty (k = 2).