J. Nanophoton. 7, 073094 (2013)

To reap the benefits of recent improvements in transistor technology, it will be important to increase the speed of interconnects by employing materials with lower dielectric constants (k). So far this has been achieved by increasing the free volume of the constituent materials, but this has the unfortunate side effect of reducing the device's thermal and mechanical properties. There is thus a need for non-contact, non-destructive metrologies capable of assessing the mechanical properties of low-k dielectric thin films. Nano-indentation measurements are the current industry standard, but their accuracy decreases markedly as film thicknesses drop below 100 nm. Now, Brian Daly and co-workers in the USA have shown that two non-contact optical techniques — Brillouin light scattering and picosecond laser ultrasonics — have the potential to replace nano-indention measurements. When measuring the Young's moduli of films thicker than 2 μm, they found excellent agreement between nano-indention measurements and the two optical techniques. They also showed that Brillouin light scattering can be used to measure films as thin as 100 nm. The combination of the two approaches has the potential to determine the elastic constants of low-k dielectrics at the desired thickness targets for future nanoelectronic interconnect technologies.