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.
We report on the use of two original techniques for the quality evaluation of nanoimprint lithography with 50
nm feature size: sub-wavelength blazed diffraction gratings and photoacoustic metrology. Sub-wavelength diffraction
has been used to characterise nanoscale structures by studying the diffraction patterns of visible wavelengths of light
from gratings which are made up of features below the diffraction limit. Diffraction efficiencies of the diffracted orders
are related to the nanoscale line-widths, heights and defects of the gratings. A stamp of a sub-wavelength blazed grating
was fabricated by electron beam lithography and reactive ion etching in silicon and imprinted by NIL with different
tools. Measured diffraction efficiencies agree with those from finite difference time domain simulations and we
demonstrated the possibility to distinguish diffraction patterns from successfully imprinted gratings and those with a
defect. The photoacoustic method has been used for the first time to study nanoimprint polymers. Signals were obtained
from the top and bottom interfaces of polymer layers with aluminium and silicon, respectively, and thicknesses
calculated from the time of flight of the acoustic wave and modelling physical parameters of the polymers, agree well
with those measured by profilometry.