Assessment of replication fidelity of optical microstructures by hot embossing

International audienceThis work reports the performance evaluation of low-cost hot embossing processes of fine three-dimensional microstructures, namely (i) cylindrical microlens array, (ii) plano-convex aspheric Fresnel lens, and (iii) pyramidal array structures. All those optical elements were replicated by employing a low-cost hot embossing tool in optical quality acrylic material. Scanning electron microscopy and optical interference microscopy techniques were used to quantitatively measure the obtained structures. In terms of the replication fidelity with respect to the mold counterpart, the resulting optical elements presented at least 96 % of dimensional fidelity at micro- and nanometer scale, including the structures with sharp edges present in the Fresnel lens. In terms of the surface optical quality, resulting root mean square surface roughness of at least one tenth of wavelength was obtained, considering applications in the visible range of spectrum. The results showed that even nanostructures generated by the material removal mechanisms during mold fabrication, such as crystal grain elastic recovery, were well replicated with differences in the range of few of nanometers which is within the vertical resolution of the employed optical interference technique

Surface amorphization in diamond turning of silicon crystal investigated by transmission electron microscopy

Silicon crystal exhibits a ductile regime during machining prior to the onset of fracture when appropriate cutting conditions are applied. The present study shows that the ductile regime is a result of a phase transformation which is indirectly evidenced by the amorphous phase detected in the machined surface. Transmission electron microscopy (TEM) planar view studies were successfully performed on monocrystalline silicon (1 0 0) single point diamond turned. TEM electron diffraction patterns show that the machined surface presents diffuse rings along with traces of crystalline material. This is attributed to crystalline silicon immersed in an amorphous matrix. Furthermore, only diffuse rings in the diffraction patterns of the ductile chip are detected, indicating that it is totally amorphous. © 2000 Elsevier Science B.V. All rights reserved

Brittle and ductile removal modes observed during diamond turning of carbon nanotube composites

International audienceUltraprecision diamond turning was used to evaluate the surface integrity of a carbon nanotube (CNT) composite as a function of the cutting conditions and the percentage of CNT in the epoxy matrix. The effects of cutting conditions on the chip morphology and surface roughness were analysed. The results showed that an increase in the percentage of CNT may influence the mechanism of material removal and consequently improve the quality of the machined surface. When smaller quantities of CNT (0.02 and 0.07 wt %) are present in the matrix, microcracks form within the cutting grooves (perpendicular to the cutting direction). This indicates that the amount of CNT on the epoxy matrix may have a direct influence on the mechanical properties of these materials. Chips removed from the CNT composite samples were analysed by scanning electron microscopy in order to correlate the material removal mechanism and the surface generation process. The area average surface roughness Sa was influenced by the material removal mechanism (Sa ranging from 0.28 to 1.1 μm)