Physical properties of thin nanoimprint polymer films measured by photoacoustic metrology

摘要

The implementation of nanoimprint lithography as a nanoscale manufacturing technique for features below 50 nm requires accurate values for the physical properties of the polymers, such as Young's modulus, used in this fabrication process. These affect the flow of polymer during imprinting, and determine the strength and stability of the polymer structures that are produced. Most physical parameter values used for nanoimprinting are taken from bulk measurements. However below 100 nm, physical properties can change significantly due to the increased importance of surface and interface effects, and the confinement of polymer molecules. It order to measure directly the physical properties of samples with very small dimensions the ultrashort laser pulse photoacoustic method has been applied to layers of poly(methyl methacrylate) of thicknesses from 586 to 11 nm, spin-coated onto silicon wafers. Acoustic speeds, calculated from time of flight and film thicknesses as measured by ellipsometry, were found to increase below approximately 80 nm, with an increase of 20% for a 13 nm sample, compared to the bulk value. This corresponds to an increase in Young's modulus of 44%. It was found that when a layer of Hexamethyldisilazane (HMDS) adhesion promoter was spin-coated onto the silicon wafer, before the polymer, there was a much smaller increase in Young's modulus, of approximately 21%, at 16 nm thickness, which indicates that the increase is due to chemical effects at the interface. The photoacoustic process is numerically modelled to ensure a full analysis of the recorded signal.