Nano-scale effects on Young's modulus of nanoimprint polymers measured by photoacoustic metrology

The ultrashort laser pulse photoacoustic method has been used to characterise the physical properties of spin-coated PMMA layers onto Si wafers with thicknesses from 586 to 13 nm, . Acoustic speeds of polymer films were derived from the measured time of flight of acoustic waves in polymers and from their calculated visco-elastic properties. A 12% increase, in comparison to the PMMA bulk value, of the acoustic speeds was measured for polymer films with thicknesses below 80 nm, which corresponds to an increase in Young's modulus of 26%. In addition, we found that adding a hexamethyldisilazane primer monolayer between the polymer film and the Si substrate lessen the increase in Young's modulus, suggesting that the nanoscale changes are due to interface effects.