Noncontact optical metrologies for Young’s modulus measurements of nanoporous low-k dielectric thin films

Brillouin light scattering (BLS) and picosecond laser ultrasonics (PLU) are two noncontact optical techniques that have garnered significant interest for thin film elastic constant measurements. PLU and BLS measurements were utilized to determine the elastic constants of 100 to 500 nm thick nanoporous low-k dielectric materials of significant interest for reducing capacitive delays in nanoelectronic interconnect circuits. PLU measurements with and without a metal acousto-optic transducer are described in detail and compared to previously reported BLS measurements. The values of Young’s modulus determined by both BLS and PLU were found to be in excellent agreement and consistent with nanoindentation measurements on thicker 2 micrometer films. While successful BLS measurements were achieved for films as thin as 100 nm, PLU measurements were limited to >∼200 nm thick films due to experimental constraints on observing acoustic pulses in thinner films. However, these results clearly demonstrate the capability of both BLS and PLU to determine the elastic constants of low-k dielectric materials at the desired thickness targets for future nanoelectronic interconnect technologies.