Quadrature laser interferometer for in-line thickness measurement of glass panels using a current modulation technique.

A thickness measurement system is proposed for in-line inspection of thickness variation of flat glass panels. Multi-reflection on the surfaces of glass panel generates an interference signal whose phase is proportional to the thickness of the glass panel. For accurate and stable calculation of the phase value, we obtain quadrature interference signals using a current modulation technique. The proposed system can measure a thickness profile with high speed and nanometric resolution, and obtain higher accuracy through real-time nonlinear error compensation. The thickness profile, measured by a transmissive-type experimental setup, coincided with a comparative result obtained using a contact-type thickness measurement system within the range of ±40 nm. The standard deviations of the measured thickness profiles and their waviness components were less than 3 nm with a scanning speed of 300 mm/s.

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.

Note: Nonlinearity error compensated absolute planar position measurement using a two-dimensional phase-encoded binary grating.

This Note presents a new absolute planar position measurement method using a two-dimensional phase-encoded binary grating and a sub-division process where nonlinearity error is compensated inherently. Two orthogonally accumulated intensity profiles of the image of the binary grating are analyzed separately to obtain the absolute position values in each axis. The nonlinearity error caused by the non-ideal sinusoidal signals in the intensity profile is compensated by modifying the configuration of the absolute position binary code and shift-averaging the intensity profile. Using an experimental setup, we measured a circular trajectory of 100 nm radius, and compared the measurement result with that of a laser interferometer. Applying the proposed compensation method, the nonlinearity error was reduced to less than 15 nm.

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.

Note: high precision angle generator using multiple ultrasonic motors and a self-calibratable encoder.

We present an angle generator with high resolution and accuracy, which uses multiple ultrasonic motors and a self-calibratable encoder. A cylindrical air bearing guides a rotational motion, and the ultrasonic motors achieve high resolution over the full circle range with a simple configuration. The self-calibratable encoder can compensate the scale error of a divided circle (signal period: 20") effectively by applying the equal-division-averaged method. The angle generator configures a position feedback control loop using the readout of the encoder. By combining the ac and dc operation mode, the angle generator produced stepwise angular motion with 0.005" resolution. We also evaluated the performance of the angle generator using a precision angle encoder and an autocollimator. The expanded uncertainty (k = 2) in the angle generation was estimated less than 0.03", which included the calibrated scale error and the nonlinearity error.

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.

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).