J. Bryner

Measurement and simulation of the laser-based thermo-elastic excitation and propagation of acoustic pulses for thin film and MEMS inspection

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 Youngs 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.

Sub-wavelength optical diffraction and photoacoustic metrologies for the characterisation of nanoimprinted structures

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.

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 Youngs 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 Youngs modulus, suggesting that the nanoscale changes are due to interface effects.

4H-6 Guided Elastodynamic Waves in Radially Graded Cylindrical Structures

The three-dimensional time boundary value problem for axisymmetric elastic waves in radially graded full cylinders is solved numerically. Elastic waves are initiated with a linear-sweep-displacement signal multiplied by a Hanning window in the time domain. The displacement excitation is applied to the cross section at the end of the cylinder. Time shots of the displacement fields for various structural configurations and material gradients are presented and discussed. Due to axisymmetry the displacement field remains two-dimensional, consisting of a radial- and a longitudinal component. Material properties of aluminum and gold are chosen for the numerical simulations. The multimode nature of the guided waves propagating along the axis is analyzed with a two dimensional spectrum analysis method. The spectrum analysis in the space domain i.e. the longitudinal axis, and in the time domain leads to the projections of the dispersion curves into a plane formed by the real part of the wave number of the guided waves and the frequency. Influences of the radial material gradient on the shapes of the dispersion diagrams are presented

Mechanical characterization of Ta and TaN diffusion barrier layers using laser acoustics

The usage of copper, as interconnect metal in computer chips 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, shortpulse-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.

Physical properties of thin nanoimprint polymer films measured by photo-acoustic metrology

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 Youngs 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 Youngs 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 Youngs 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.

Laser acoustic characterization of Ta and TaN diffusion barriers beneath Cu layers

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.