Development of a special CMM for dimensional metrology on microsystem components

INTRODUCTION Further miniaturization and modularization are current trends in microsystem technology, which require 3D-coordinate measurements to be performed with measurement uncertainties in the range of 0.1 μm. With conventional coordinate measuring machines sub-centimeter structures usually can be measured only optically, i.e. in two dimensions with measurement uncertainties of about 1 μm. Therefore activities were started to develop special measurement equipment which satisfies the described requirements. Moreover the equipment to be developed will also allow to perform measurements on other challenging objects, like e.g. small setting ring standards, small diameter wires etc. The PTB started a project to develop a special CMM for dimensional metrology on microsystem components with an uncertainty of < 0.1 μm. The measurement range will be 25 mm x 40 mm x 25 mm. The instrument is based on a commercial CMM with improved capabilities through the use of high resolution scales and optimised air bearings. The instrument will consist of an optical measurement system and two tactile 3D-micro-sensing systems. To improve the measurement uncertainty of the instrument the translational displacement and the guiding deviations of the CMM are measured by laser interferometry. At first an opto-tactile 3D-sensor with an optical fibre as a "probe pin" will be used. The sensor allows to use very small probing balls with diameters down to 25 μm and probing forces as low as 1 μN. 2D probing uncertainties of 0.15 μm were already obtained with this system [1]. The second tactile sensor to be used is based on a silicon boss-membrane with piezo resistive transducers [2]. First probing experiments have been carried out showing its resolution and 1Dreproducibility of the contacting points to be better than 10 nm. In parallel to the instrument development new precision diamond turned depth setting standards for topography measuring instruments with depths in the mm range have been developed [3]. The standards are made of OFHC copper and are covered with a wear-reducing nickel coating. The calibration uncertainty of the deepest grooves (currently 900 μm) amounts to 54 nm. Under good conditions, these standards allow to perform traceable measurements of topography measuring instruments with a maximum uncertainty of 78 nm for the range of 1 mm.