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.
For the interpretation of optical Pump-Probe Measurements on microstructures the wave propagation in anisotropic
3-D structures with arbitrary geometries is numerically calculated.
The laser acoustic Pump-Probe technique generates bulk waves in structures in a thermo-elastic way. This
method is well established for non-destructive measurements of thin films with an indepth resolution in the
order of 10 nm. The Pump-Probe technique can also be used for measurements, e.g. for quality inspection of
three-dimensional structures with arbitrary geometries, like MEMS components. For the interpretation of the
measurements it is necessary that the wave propagation in the specimen to be inspected can be calculated.
Here, the wave propagation for various geometries and materials is investigated. In the first part, the wave
propagation in isotropic axisymmetric structures is simulated with a 2-D finite difference formulation. The
numerical results are verified with measurements of macroscopic specimens. In a second step, the simulations
are extended to 3-D structures with orthotopic material properties. The implemented code allows the calculation
of the wave propagation for different orientations of the material axes (orientation of the orthotropic axes relative
to the geometry of the structure). Limits of the presented algorithm are discussed and future directions of the
on-going research project are presented.
The replacement of aluminum by copper as interconnect metal in computer chips was and still is driven by the necessity to enhance the current density thus enabling higher packaging densities, a fact that correlates directly with faster, smaller, and less energy consuming devices. The usage of copper, however, leads to new technological challenges which are caused by its mechanical properties on one hand side and by its tendency to migrate into dielectric and/or semiconducting layers on the other hand side. To prevent such diffusion processes, very thin layers consisting of tantalum and tantalum nitride or titanium and titanium nitride are deposited.
A non-contact, non-destructive, short-pulse-laser-acoustic method is used to determine the mechanical properties of the barrier layers and of the copper layer. Mechanical waves are excited and detected thermoelastically using laser pulses of 70 fs duration. For metals this leads to wavelengths of 10 to 20 nm and the corresponding frequencies amount to 0.3 to 0.6 THz. Thin film measurements of buried diffusion layers are provided and compared with Scanning Electron Microscopy measurements (SEM), Transmission Electron Microscopy (TEM), and Rutherford Backscattering Spectroscopy measurements (RBS). Results of a thermo-elasto-mechanical simulation are presented.
Current limits of the presented method are discussed and future directions of the on-going research project are presented.
Optical techniques for monitoring acoustic waves excited in thin films or micro-structures with ultrashort laser pulses are useful for the accurate and nondestructive evaluation as well as for the characterization of material properties. The pump-probe laser-based acoustic methods generate acoustic bulk waves in a thermo-elastic way by absorbing the pump laser pulses at the surface of the thin film. The acoustic waves are partly reflected at the interface of thin film and substrate. Back at the film surface the reflected acoustic wave causes a change of the optical reflection coefficient, which is measured by the probe laser pulse. One-dimensional, thermo-elastic models are developed to investigate the laser-based excitation and propagation of the longitudinal acoustic pulses in thin aluminium films. The change of the optical reflection coefficient is governed by the temperature distribution and the mechanical strain caused by the traveling acoustic pulse. The presented comparison of the simulation results of thin aluminium films with the pump-probe-measurements allows to determine film thickness or Young's modulus. Furthermore material properties like thermal conductivity and photoacoustic properties are estimated. The thermo-elastic modeling of the two-dimensional case and the resulting new possibility to use the pump-probe technique for the nondestructive evaluation of micro-structures is discussed. Further directions of the ongoing research project are presented.
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