Using ultrashort pulsed optical excitation and interferometric detection, we image ultrahigh- frequency surface acoustic
waves on two-dimensional (2D) phononic crystals in the time domain. The samples consist of a square lattice of airfilled
holes etched in a silicon substrate. Good agreement with time-domain finite element numerical simulations is
obtained. The dispersion relation is derived and stop bands are revealed by means of Fourier transforms. The wave fields
corresponding to acoustic eigenmodes at specific frequencies are also presented.
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.
When a mechanical stress pulse, which is propagating in an elastic medium, encounters a material- or phase interface, which generally represents a change of the acoustic impedance, it is split up into a part which propagates further into the new material and another part which is reflected. The amplitude ratio of the reflected and the transmitted part is governed by the normalized difference of the acoustic impedances only, provided that the impedance change is a pure step function in space. If the acoustic impedance change is broadened spatially, the ratio of the transmitted and reflected part becomes frequency dependent and the effect can therefore be used for filter-, damping-, acoustic isolation-, and/or spectrum analysis purposes or for a quantitative analysis of the interface. The effect is of growing importance for micro- and nanostructures since the relative size of interface layers is generally larger than in macroscopic structures. Oxidation or diffusion processes might lead to 'smooth' acoustic interface layers which are characterized by gradually varying mechanical properties like density, Young's- and shear moduli, which need to be quantified by nondestructive in-depth profiling methods.
Surfaces as well as interfaces between two neighboring materials are often subjected to various diffusion processes. Such diffusion processes like oxidation or migration of atoms of neighboring materials can cause layers having gradually varying mechanical properties -- like density, Young’s modulus, or shear modulus -- perpendicular to the surface or interface. The growing miniaturization of MEMS devices enlarges the relative size of these layers and thus enhances the importance of phenomena occurring at such material or phase interfaces thus demanding a detailed quantification of its mechanical properties. In this investigation particular interest is drawn on the question how the propagation characteristics of bulk acoustic waves are affected by diffusion layers. The reflection and transmission behavior of bulk acoustic waves encountering a continuum having a spatially dependent sound velocity is discussed based on numerical simulations as well as on experimental verifications. In contrast to previous work done in this field in which diffusion effects are generally considered as undesirable phenomena, the deliberate realization of microstructures having well defined gradually varying material properties in one or more dimensions represents a goal of this investigation. For metallic thin film multi layers thermally induced diffusion processes have shown to be an easy and reliable technique for the realization of layered structures having continuously varying mechanical properties within several 10 nanometers. Among the experimental methods suitable for the in-depth profiling of submicron metallic thin films providing resolutions of several nanometers, are short pulse laser acoustic methods, Rutherford Backscattering Spectroscopy (RBS), and Glow Discharge Optical Emission Spectroscopy (GDOES). Short pulse laser acoustic methods and Rutherford Backscattering Spectroscopy (RBS) have the advantage to be nondestructive. The short pulse laser acoustic method is described in detail and RBS measurements are presented for verification purposes. Finally potential engineering applications like micro-machined spectrum analyzers, acoustic isolation layers, and band pass filters, operating at very high frequencies are presented.
Thin membranes are part of numerous microelectromechanical systems (MEMS) like sensors and bulk acoustic wave filters for example. In most applications the material properties of the membranes are key parameters for the correct working of the MEMS devices. Measuring bulk acoustic waves excited in MEMS-structures with ultra-short laser pulses is a powerful method for accurate and non-destructive evaluation as well as for the characterization of material properties. The laser based acoustic method generates acoustic bulk waves in a thermo-elastic way by absorbing the pump laser pulses at the surface of the MEMS-structure. The propagating acoustic pulses are partly reflected at any discontinuity of the acoustic impedance. Back at the surface the partly reflected acoustic pulses cause changes of the optical reflection coefficient, which are measured with the probe laser pulses. This technique is used for measuring the bulk wave propagation in very thin membranes. The bulk acoustic wave propagation in freestanding aluminium-silicon nitride multi-layer membranes with total thickness in the order 500 nanometers is measured and discussed. Furthermore comparisons of measurements on freestanding and supported membranes and of thermo-elastic models are presented. The measured results are used for the estimation of the Moduli of the aluminium-silicon nitride multi-layer. The technique presented in this work can also be applied for the characterization of material or geometrical properties of other components of MEMS like ultrasonic reflection layers and cantilevers. The advantage of the method lies in its non-destructive and non-contact approach, which is crucial for very thin and brittle structures.
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.
This investigation deals with various new aspects of the sensitivity improvement of a pump-probe laser based acoustic method. A short laser pulse is used to excite a mechanical pulse thermo-elastically. Echoes of these mechanical pulses reaching the surface are causing a slight change of the optical reflectivity. The surface reflectivity is scanned versus time with a probe pulse. Thus the time of flight of the acoustic pulse is measured. The quantity to be measured i.e. the optical reflectivity change DR caused by acoustic pulses, is rather small. A set-up having an estimated sensitivity DR/R of about 10(superscript -5 has shown to be sufficient to detect up to the 5th echo in a 50 nm aluminum film on sapphire substrate. A key challenge is the reduction of optical and electrical cross talk between the excitation and the detection. Therefore the concepts of double-frequency modulation, cross-polarization, and balanced-photo-detection are implemented. Practical aspects like beam guiding, modulation techniques, beam focus-minimization, beam focus-matching, and the variation of the pump-probe power ratio are discussed. Measurements for single and multi-layer metallic films demanding higher sensitivity are presented.
The increased use of micromechanical devices demands the miniaturization of corresponding testing- and evaluation methods. The well known scanning probe methods (SPM) have very high lateral resolution. Pulsed laser acoustic experiments on the other hand have the advantage of very high temporal resolution, whereas the lateral resolution is limited by the fact that the minimal spot size to which a laser pulse can be optically focused amounts to several wavelengths of light. The mechanical wavelength of acoustic waves excited by a ultra short laser pulse amounts to 10 to 20 nm. In contrast the spot size of the laser pulse is three orders of magnitude larger. The presented approach to improve the lateral resolution is the combination of the conventional scanning probe methods with pulsed laser acoustic methods. The introduction of a micro-opto-mechanical focusing tip in which the mechanical waves are focused leads to a new potential time resolved scanning probe technique. An elastodynamic finite difference method has been developed to investigate the ultrasonic wave propagation in the tip numerically. The mechanical wave propagation for a conical tip geometry is discussed. The numerically calculated results are verified by experiments with structures having macroscopic dimensions. Scaling effects and restrictions due to the pulsed laser experiment are considered in order to translate the results into the microscopic scale.