Comparison of measurement methods for microsystem components: application to microstructures made by the deep x-ray lithography process (x-ray LIGA)

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The LIGA (a German acronym for lithography, electroplating and molding) process using highly parallel x-rays permits the production of a microstructure with still unique characteristics: high aspect ratio, high accuracy, high perpendicularity and lower roughness of the side wall. From a marketing point of view, this qualitative description might suffice to attract users to the technology. Regarding widespread commercialization and standardization of x-ray LIGA products, our goal is to establish a rigorous dimensional metrology for which we need to understand and quantify uncertainty, which is the key to accuracy. We report on our metrological study using a coordinate measurement machine (CMM) equipped with a fibre probe (3D measurements) which will be compared to two versions of lateral top-view measurements (2D/surface measurements): an optical microscope provided with a micrometric table and a CMM with an image processing sensor; these two types of measurement methods being complementary. In fact, microsystem technology requires measurements to be performed with precision and accuracy within the range of 0.1 µm. In this paper, we present an analysis and a discussion of both types of measurement systems. The precision and reproducibility of the CMM (with fibre probe) during a two-year study will be exposed; a calibrated series part is being measured every time the machine is used. In this case, the CMM is used as a comparator. Its accuracy and the calibration of the ball diameter using an etalon (ceramic gage block) will be exposed. Furthermore, by taking into account the results obtained by the measurement system analysis (MSA), we will show the measurement's impact on the process by taking as an example the fabrication of mm gold gears for watch industry; a quantitative description of process reproducibility and of the influence of processing parameters influence will be possible in the future.