Multi Scale Micro and Nano Metrology for Advanced Precision Moulding Technologies

The technological revolution that has deeply influenced the manufacturing industry over the past two decades opened up new possibilities for the realisation of advanced micro and nano systems but, at the same time, traditional techniques for quality assurance became not adequate any longer, as the technology progressed.
The gap between the needs of the manufacturing industry and the well-organized structure of the dimensional and geometrical metrology appeared, above all, related to the methodologies and, also, to the instrumentation used to deal with the incessant scaling down of the critical dimensions of the novel micro and nano production.
Nowadays, design methodologies and concurrent tolerance guidelines are not yet available for advanced micro manufacture. Moreover, there are no shared methodologies that deals with the uncertainty evaluation of feature of size in the sub-millimetre scale.
On the other hand, a large choice of measurement equipment is now available but limitations in their use and of the instruments themselves are, in many cases, not completely understood, yet. In this context, the ambition of the PhD project was to develop and implement a complete metrological framework for advanced precision micro moulded products with micro/nano structured surfaces and micro/nano geometries, across several length scales.
Uncertainty evaluation and traceability, specification intervals formulation, assessment of the moulded parts replication and a deep investigation on the optical instruments currently available for micro/nano dimensional and geometrical measurements were all subjects of the research conducted during the three years of the PhD course of study and that were collected in this final work.
Traceability and uncertainty evaluation were dealt with the development of a comprehensive statistical methodology based on the well-known frequentist approach. It was successfully applied to dimensional and geometrical measurements in the micro/nano length scale.
A novel method was developed on purpose for the formulation of specification intervals. Based on the evaluation of the shrinkage uncertainty, it allows to discriminate between the shrinkage of 1D and 2D features and cope with the influence of length scale. The method was applied and validated in the specific case of a micro-powder injection moulding production. Nevertheless, it is of general validity for any moulding process in which the material undergoes a change in dimensions from the mould cavity, due to a phase transformation. In parallel to the formulation of specification intervals, an investigation of two instruments with two different working principle proved a mutual dependence between the quality of the measurement process and the quality of the production. The measurement process influenced the quality assurance, but the lack of quality of the parts influenced the measurement process.
The surface texture replication was investigated about the amplitude (Sa, Sq) and the slope (Sdq) and assessed by the replication fidelity, i.e., comparing the produced parts with the tool used to replicate the geometry and evaluating the measurement uncertainty. The evaluation included the repeatability and reproducibility of the production process, the amplitude and slope replication of the features on the surface, the evaluation of the uncertainty of the replication fidelity.
The investigation of optical instruments started with the processing of the data of an international comparison of surface texture measurements, in the sub-micrometre scale, by optical instruments, organised under the umbrella of the Scientific Technical Committee on ‘Surfaces’ (STC-S) of The International Academy for Production Engineering (CIRP). The comparison unveiled the state-of-the-art performance, in the sub-micrometre scale, of the three main microscopes working principle currently used in areal topography measurement (confocal microscopy, coherent scanning interferometry and focus variation microscopy). Results showed that agreement between optical instruments and reference measurements (by atomic force microscopy) could be reached to some extent, largely depending on the technology of the instruments used.
The limitations of the performance of the optical instruments were, also, inspected in specific cases that can arise during practical operation and that are becoming more and more common in modern micro and nano manufacturing. Several environmental sources were identified (thermal drifts, air conditioning system, stray light), which can introduce substantial environmental noise into the measurements, but, also, internal noise related to a prolonged use of an instrument.