Low-density diamondlike amorphous carbon at nanostructured metal-diamond interfaces

Next-generation nanoelectronic, energy, and quantum technologies require increasingly stringent thermal, optical, mechanical, and electrical properties of component materials, often surpassing the limits of widely used materials such as silicon. Diamond, an ultrawide bandgap semiconductor, is a promising material for these applications because of its very high stiffness, thermal conductivity, and electron mobility. However, incorporating diamond into devices that require high-quality metal-diamond interfaces is challenging. In this work, we use a suite of electron microscopy measurements to reveal an ultrathin amorphous carbon layer that emerges at metal-diamond interfaces after electron beam lithography. Using extreme ultraviolet scatterometry, we nondestructively determine lower bounds on the layer's Young's modulus and thermal conductivity, which at $>230\phantom{\rule{4pt}{0ex}}\mathrm{GPa}$ and $>1.1$ W/($\mathrm{m}\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) are indicative of a diamondlike form of amorphous carbon with high ${sp}^{3}$ bonding. However, extreme ultraviolet coherent diffractive imaging reflectometry and energy-dispersive x-ray spectroscopy measurements indicate a low and likely inhomogeneous density in the range of $1--2\phantom{\rule{4pt}{0ex}}{\mathrm{g}/\mathrm{cm}}^{3}$. The low density of such a stiff and conductive layer could indicate that it contains nanometer-scale voids or atomic-scale vacancies. The appearance of this unusual layer illustrates the nanofabrication challenges for diamond and highlights the need for better techniques to characterize surfaces and interfaces in nanoscale devices.