With the continued miniaturisation of mechanical and optical systems there is an increasing demand for high precision dimensional measurements on small parts. METAS combined a new probe head with a recently developed ultra precision CMM stage. The probe head with probing spheres in the diameter range of 0.1 mm to 1 mm has isotropic probing forces below 0.5 mN. Its unique parallel kinematic structure uses exclusively flexure hinges and is manufactured out of a single piece of aluminium. This structure blocks all rotational movements of the probing sphere and separates the 3D movement into three independent 1D displacements which are measured by inductive sensors. The repeatability for a single point probing is in the order of 5 nm.
This probe head was combined with an ultra precision micro-CMM, which is based on a development made at Philips CFT [1,2]. The micro-CMM features a 90 mm x 90 mm x 38 mm air bearing stage with interferometric position measurement at zero Abbe offset. At the reached level of precision, the shape deviation of the probing sphere becomes a major contribution to the uncertainty. Therefore a calibration method for spheres based on error separation techniques was implemented. The results of roundness measurements on 3 calibration spheres are presented.
On a commercial roundness measurement instrument METAS has implemented several modifications offering an up-grade of the measurement capabilities and a full understanding of the measurement process. The Talyrond 73 instrument is equipped with a rotating spindle with an oil-hydrostatic bearing. The modifications include an incremental encoder on the spindle, a vibration free DC motor with variable rotation speed for driving the spindle, new amplifier electronics with selectable gain for the LVDT probe, new software for data acquisition and evaluation, and a heat protection shield. The application of error separation techniques, the complete characterization of all relevant parameters and the high level of precision achieved allow the attribute "primary" be given to this roundness measuring machine.
An analysis of gauge block calibrations by mechanical comparison carried out at METAS with several sets of gauge blocks during several years has demonstrated very small variations. It is shown, that under optimum conditions with respect to laboratory environment and instrumental equipment as well as by following a suitable handling and measurement procedure, the contribution in the uncertainty budget added by the mechanical comparison process to the uncertainty of the interferometrically calibrated reference gauge blocks can be very small.
A photomask measuring instrument was developed and built at METAS. The instrument consists of an air bearing x-y stage for the positioning of the mask with a range of 400 mm by 300 mm, a digital video microscope system for the localization of the structures and a differential two axis plane mirror interferometer. The interferometric measurements are made with respect to an x-y reference mirror system made out of Zerodur. The uncertainty for 2D measurements is directly influenced by the shape of the reference system i.e. by the straightness and orthogonality of the mirrors. Through a precise characterization of the reference system its imperfections can be corrected numerically. An initial determination of the mirror shape was performed on a straightness measurement instrument consisting of a granite beam with an air bearing carriage and an inductive touch probe system. The method delivered initial flatness data with a high positional resolution but with some low order distortion due to a circular bending of the used granite beam which was induced by small temperature gradients. An in-situ calibration of the reference system on the photomask measuring instrument itself was used to improve these initial measurements. This second calibration was made by measurements of a 400 mm quartz line scale in axial and in two diagonal directions. By numerical simulation of these measurements of the x- and y- mirror shapes and the angle between the mirrors were determined. Circular and sinusoidal functions were used for the additional mirror form corrections, which were up to 40 nm for the y-axis and up to 140 nm for the x-axis. A final verification measurement showed that the agreement between the axial and the diagonal line scale measurements is now better than 10 nm.
The mechanical probing system is often one of the limiting factors in the calibration of length standards. It has been shown, that for highly accurate applications a particular effect, which is often not considered, has to be taken into account: a spherical probe on a stylus undergoes a small rotation due to the angular stylus deflection, which creates friction and potentially stick slip during the probing process and may thus lead to non-reproducible probing. A novel probe has been built which avoids this effect by an additional degree of freedom, providing a small vertical movement of the stylus. The probe is of a monolithic flexure hinge design with a ridge connection of the stylus and the mirror reflector for the plane mirror interferometer, which measures the displacement. Them measurement force, which is proportional to the deflection of the hinges, is measured with a capacitive probe. The probing procedure generates the force/deflection curve and allows for the measurement force to be extrapolated to zero. The presented test results show the system's capability for a probing accuracy in the nanometer range.
International measurement comparisons are important to demonstrate the technical competence of calibration laboratories, to experimentally confirm the measurement uncertainties and to demonstrate the equivalence of national measurement standards and traceability schemes in accreditation systems. The network of international comparison schemes in general and the specific issues of gauge block comparisons in particular are presented. An overview on different comparisons which were carried out in the past few years on a regional and a worldwide scale is given for both, gauge block measurements by interferometry and by comparison.
An instrument for the measurement of the thermal expansion coefficient near room temperature of gauge blocks and other samples of similar shape and size has been developed. The length dilation is measured by a differentia plane mirror interferometer. A special interference phase detection technique compensates for non-linearity errors caused by polarization mixing. In combination with an electronic phase meter this allows to achieve nanometer accuracy. Since the measurements are done in vacuum, no compensation for the refractive index of air has to be made. For samples with good thermal conductivity the slow heat exchange by thermal radiation allows for a small temperature gradient of the sample and a good stability in the thermal equilibrium. From the thermal expansion curve, measured in a temperature range typically between 10 degrees C and 30 degrees C, the linear and quadratic expansion coefficients are evaluated at 20 degrees C, the reference temperature for length. It is shown, that for the investigated gauge block materials the room temperature expansion can be very accurately described with two coefficients within a few parts in 109 per degree. A detailed analysis of the measurement uncertainty demonstrates the capability of the measurement instrument, which is confirmed by the results of an international comparison.