Laser-based Measurement System Research Articles

Characterizing deflectable microstructures via a high-resolution laser-based measurement system

We have developed a high-resolution, laser-based system capable of real-time angular deflection measurement and have used it to characterize non-invasively micromechanical actuators with reflective surfaces. The angular resolution of this system is better than 0.0005° and the measurement bandwidth is 100 kHz. The linear range can be as high as ± 1.5° from any bias angle up to ± 30°. Steady-state deflection measurements of electrostatic torsional aluminum actuators show the expected nonlinear dependence on input voltage, while small-signal frequency response measurements show the characteristic response of a dominant second-order system. Thermally sensitive resonance modes were seen in some devices. The statistical spread in device natural frequency is used as a quantifier for process variations while device lifetime is confirmed beyond one billion cycles by the absence of discernible natural frequency shifts after repeated cycling. The system has also been used to demonstrate optically-based closed-loop control as a precursor to an integrated capacitive solution.

High-precision reflectometer for submillimeter wavelengths

A high-precision reflectometer has been designed and implemented to measure directly the specular reflectance (R) of materials in the submillimeter (SM) region of the spectrum (300 GHz < ν < 3000 GHz). Previous laser-based measurement systems were limited to an uncertainty in R of ± 1.0% because of a number of issues such as lack of an absolute reflection standard, difficulties in the interchange of sample and standard in the laser beam, and instabilities in the laser system. We realized a SM reflection standard by ellipsometrically characterizing the complex index of refraction of high-purity, single-crystal silicon to a precision such that its SM reflectivity could be calculated to better than ±0.03%. To deal with alignment issues, a precision sample holder was designed and built to accommodate both sample and silicon reflection standard on an air-bearing rotary stage. The entire measurement system operated under computer control and included ratioing of the reflected signal to a reference laser signal, measured simultaneously, to help to eliminate short-term laser instabilities. Many such measurements taken rapidly in succession helped to eliminate the effects of both source and detector drift. A liquid-helium-cooled bolometer was modified with a large area detecting element to help to compensate for the slight residual misalignment between sample and reflection standard as they were positioned into and out of the laser beam. These modifications enabled the final measurement precision for R to be reduced to less than 0.1%. The major contribution to this uncertainty was the difficulty in precisely exchanging the positions of sample and standard into and out of the laser beam and was not due to laser or detector noise or instabilities. In other words, further averaging would not help to reduce this uncertainty. This order-of-magnitude improvement makes possible, for the first time to our knowledge, high-precision reflectance measurements of common metals such as copper, gold, aluminum, and chromium whose predicted reflectivities exceed 99% in the SM region. Furthermore, precise measurement of the high-frequency losses in high-temperature superconducting materials is now also possible. Measurements reported here of metals at a laser wavelength of λ = 513.01 μm (ν ≈ 584 GHz) indicate a slight discrepancy between experimental and theoretically predicted values, with measured results falling 0.1–0.3% below predicted values.

Laser-Based Solid-Matrix Fluorescence of Stereoisomeric Tetrols

A laser-based fluorescence measurement system was constructed and used to obtain solid-matrix fluorescence spectra from four stereoisomers. The four isomers were tetrols which are products from benzo(a)pyrene-DNA adducts. The laser-based system showed unique advantages in obtaining the fluorescence fine structure from the tetrols in comparison to non-laser-based commercial spectrofluorometers. A new gatewidth dependence method was developed with the laser system that can be used to characterize and identify the stereoisomers adsorbed on α-cyclodextrin/NaCl at room temperature. The basis of the new gatewidth method depends on different interactions of the analyte molecules with the solid matrix. The method developed has the potential for general analytical application for the identification and characterization of fluorescent stereoisomers and other fluorescent compounds.

In situ friction and wear measurements in integrated polysilicon mechanisms

We describe a series of experiments, in situ measurements and theoretical models designed to provide estimates of the coefficients of friction and the nature of wear in integrated polysilicon micromechanisms. A laser-based measurement system was used to monitor the motion of the rotating micromechanisms and steady-state speeds of up to 10 000 rps (600 000 rpm) were recorded. Estimates of the dynamic coefficient of friction were obtained by incorporating sampled motion profiles and derived speed and distance measures into a dynamic model which included both coulomb friction and viscous damping forces. From such an analysis, we estimated dynamic coefficients of friction for polysilicon on silicon ranging in value from 0.25 to 0.35. The results were reproducible on numerous components, across structures of identical and different geometries, produced on different wafers from the same fabrication lot. Life-cycles for some of these structures were determined under various operating conditions, with typical components surviving for almost one million cycles.

Forecasting compensatory control (FCC) of machining flatness

Forecasting Compensatory Control (FCC) was implemented to improve the flatness of three dimensional prismatic workpieces in an end-milling operation. The basic logic of the strategy is based on inprocess error measurement, stochastic modelling, and prediction and compensation of the cutting errors. The proposed FCC system consisted of a newly developed laser-based in-process flatness error measurement system, a hydraulic positioning servo system, and a digital microcomputer control system. Through the application of the FCC system to on-line cutting experiments, the workpiece flatness error was reduced by up to 80% during machining.

Analysis of Alignment Errors in a Laser-Based In-Process Cylindricity Measurement System

A new laser-based in-process measurement system has been developed to measure the cylindricity errors in boring operations. This paper presents a mathematical model of the alignment errors and examines the effects of each alignment error on the overall system’s accuracy. A compensation scheme for the alignment errors is suggested to assure the accuracy of the measurement.

Rapid Particle Size Analysis of Fly Ash with a Commercial Laser Diffraction Instrument

ABSTRACTParticle size distributions of a suite of fly ashes previously analyzed by aclassical sedimentary proced ure have been redetermined using a commercial laser-based particle size measurement system. For most of the fly ashes the concordance between the two sets of data was unexpectedly close, considering the different physical principles involved in the two types of measurement. Laser diffraction instruments (“laser sizers”) appear to have the potential for extremely rapid and accurate size distribution measurement for fly ashes to be used in commercial applications. Such determinations, easily carried out on a routine basis, could serve as important elements in future specifications for acceptance and quality assurance programs.