Abstract
Nanocrystalline diamond (NCD) membranes are promising candidates for use as sensitive pressure sensors. NCD membranes are able to withstand harsh conditions and are easily fabricated on glass. In this study the sensitivity of heavily boron doped NCD (B:NCD) pressure sensors is evaluated with respect to different types of supporting glass substrates, doping levels and membrane sizes. Higher pressure sensing sensitivities are obtained for membranes on Corning Eagle 2000 glass, which have a better match in thermal expansion coefficient with diamond compared to those on Schott AF45 glass. In addition, it is shown that larger and more heavily doped membranes are more sensitive. After fabrication of the membranes, the stress in the B:NCD films is released by the emergence of wrinkles. A better match between the thermal expansion coefficient of the NCD layer and the underlying substrate results in less stress and a smaller amount of wrinkles as confirmed by Raman spectroscopy and 3D surface imaging.
Highlights
The current state-of-the-art piezoresistive sensors are mostly based on a non-piezoresistive membrane or diaphragm to which piezoresistive components are attached
Merging of the first generation wrinkles into the second by the formation of wrinklons is indicated by the dotted lines (Fig. 8A,B). We found that this wrinklon formation, i.e. transition of first into second generation wrinkles, takes place closer to the edge for the membranes on Schott AF45 (S-AF45) compared to those on Corning Eagle 2000 (CE2000) glass, which means that both the wrinkles and wrinklons are shorter on S-AF45 substrates
The sensitivity of the B:Nanocrystalline diamond (NCD) membranes fabricated for this work is related to the residual stress, which contains both growth and thermal stress, in the diamond film as deduced from the Raman peak location on B:NCD and undoped NCD membranes
Summary
The current state-of-the-art piezoresistive sensors are mostly based on a non-piezoresistive membrane or diaphragm to which piezoresistive components are attached. Piezoresistive boron-doped nanocrystalline diamond (B:NCD) films or structures have been attached to Si diaphragms[7,8]. Due to the high Youngs modulus and fracture strength of diamond much thinner films can be fabricated compared to other materials, which means that less pressure is required for a significant response, i.e. a higher value for the gauge factor is obtained. To investigate the influence of the size of the membranes, the boron doping level and of the substrate material, and - of its thermal expansion coefficient, on the gauge factor and sensitivity of www.nature.com/scientificreports/. The difference in thermal expansion coefficient causes a difference in stress in the diamond films upon cooling after growth, which is directly observable by the number of wrinkles present in the membranes and which influences their sensitivity to pressure differences. Laser scanning micrographs confirm that a higher number of wrinkles is present for glass with a higher expansion coefficient (S-AF45) and reveal that the first generation wrinkles transform into the generation closer to the membrane edge compared to glass with a lower expansion coefficient (CE2000)
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