Jacqueline Vollmann

Sensitivity improvement of a pump–probe set-up for thin film and microstructure metrology

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 deltaR caused by acoustic pulses, is rather small. A set-up having an estimated sensitivity deltaR/R of about 10(-5) has shown to be sufficient to detect up to the fifth 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 photodetection are implemented. Practical aspects like beam guiding, modulation techniques, beam focus minimization, and beam focus matching are discussed. Measurements for single- and multi-layer metallic films demanding higher sensitivity are presented.

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.

Ultrasonic wave propagation in focussing tips with arbitrary geometries

Pulsed laser acoustic experiments have the advantage of very high temporal resolution. However, the lateral resolution amounts to several wavelengths of light. To improve the lateral resolution a focussing tip in which the mechanical waves are focussed is introduced. The combination of high resolution in time and space domain leads to a new potential time resolved scanning probe method. Therefore several axi-symmetric structures are investigated numerically using a finite difference method. The ultrasonic wave propagation in different tips is discussed. By varying the geometry of the tip, the displacement at the sharp end is maximized. The numerically calculated results are verified experimentally on structures having macroscopic dimensions. Scaling effects are considered in order to translate the results into the microscopic scale where arbitrary geometries are much more challenging to implement.

Measurement of the bulk acoustic wave propagation in ultrathin membranes

The measurement of bulk acoustic waves (BAW) excited in thin films or microstructures with ultrashort laser pulses is a powerful method for accurate and nondestructive evaluation of material or geometrical properties. Optical techniques like the pump-probe laser-based acoustic method generate BAW in a thermoelastic way by absorbing the pump laser pulses at the surface of the specimen. The acoustic waves are partly reflected at any discontinuity of the acoustic impedance. Back at the surface the reflected acoustic pulses cause changes of the optical reflection coefficient, which are measured with the probe laser pulses. The measurement technique is explained for the case of an aluminium thin film on sapphire. The influence of the film thickness and the deposition method of the thin films on the bulk wave speed is shown. In the second part of the paper this technique is used for measuring the bulk wave propagation in very thin membranes. The BAW propagation in freestanding silicon-nitride aluminium multilayer membranes with total thickness in the order of several hundred nanometers is measured. The measurements of the freestanding membranes are compared with measurements of the supported case. The technique presented in this paper can also be applied for the characterization of material or geometrical properties of thin film BAW resonators. The advantage of the method lies in its nondestructive and noncontact approach, which is necessary for ultrathin and brittle structures.

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.

High-resolution measurement of thin metallic films and multilayers by femtosecond laser pulses

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.

Laser induced acoustic pulse propagation in submicron metallic thin films having variable sound velocities

Free surfaces as well as interfaces between two neighboring materials are often subjected to diffusion processes like oxidation or migration of atoms. Such processes smooth out the difference of the acoustic impedances leading to microstructures having gradually varying mechanical properties like density and Young’s modulus. In the one-dimensional case of a metallic thin-film multilayer, the speed of sound becomes a function of the spatial thickness variable. Depending on the ratio between the acoustic wave length and the thickness of the diffusion zone, bulk waves reaching the zone are either dominantly transmitted or dominantly reflected. Thus a continuum having a variable sound velocity can be considered as an acoustic filter. The partial differential wave equation is solved numerically for an assumed velocity-versus-propagation direction function and the results are discussed. A series of experimental results obtained by a pump–probe–laser-acoustic technique is presented. Mechanical waves are excited ...

Wave propagation in inhomogeneous media, phenomena and potential applications

Intermetallic diffusion leads to microstructures having gradually varying sound velocities. Such diffusion zones represent inhomogeneous wave guides. Depending on the ratio between the wave length and the thickness of the diffusion zone, bulk waves reaching the zone are dominantly transmitted or dominantly reflected. Thus a continuum having a variable sound velocity can be considered as an acoustic filter. The partial differential wave equation is solved numerically for various velocity-versus-propagation direction functions and the results are discussed. A first series of experimental results obtained by a short-pulse-laser-acoustic technique is presented. Mechanical waves are excited and detected using laser pulses of 70 fs duration at a wave length of 810 nm. An improved pump-probe set-up is developed in order to detect the acoustic echoes of very thin layers. Experimental results are compared with simulations and potential applications are proposed. Future directions of the on-going research project are discussed.

P1G-4 Characterization of Nanoimprinting Polymer Films Using Picosecond Ultrasonics

laser acoustic setup is used to determine mechanical properties of polymer thin films which are used for nanoimprinting. Until now mechanical properties like the Youngs Modulus or the Poisson Ratio are not well known for these polymers in such small dimensions (100 - 600 nm thickness). The polymer films are spincoated on a silicon wafer and covered with a thin aluminum layer for a better energy absorption of the laser pulses. The measurements are performed on a femtosecond laser pump- probe setup with a collinear beam guidance. This measurement method is contact-free and non-destructive. Mechanical waves are excited and detected thermoelastically using infrared laser pulses of approximately 80 fs duration. The entire experimental setup is simulated numerically: The heat distribution and wave excitation in the thin films caused by the laser pulse, the wave propagation, and the photoacoustic detection. Results of the simulation are shown and a short overview of the simulation procedure is given. With the simulation it is possible to interpret and assign the various measured wave pulses. The laser acoustic measurements are compared with profilometry measurements performed on the same thin film structures in order to quantify the mechanical properties of the polymer films.

P2F-4 Frequency Selective Wave Propagation in Graded Materials

Lets consider a mechanical stress pulse propagating in an elastic medium. When this pulse encounters a material or phase interface, which generally represents a change of the acoustic impedance, the pulse is split up into two parts. The first part is propagating further into the new material and the second part is reflected. The amplitude ratio of the reflected and the transmitted part is governed by the normalized difference of the acoustic impedance 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 filtering, damping, acoustic isolation, and/or spectrum analysis purposes or for quantitative analysis of interfaces. The effect is of growing importance for micro- and nanostructures since the relative size of the interface layers is generally larger than in macroscopic structures. In this work, a pulse propagating in a linear elastic graded material is investigated with one-dimensional simulations. The numerical scheme is based on the Finite-Difference Time-Domain method (FDTD). The validation of the numerical model is carried out by comparing the simulated pulse propagation stress history with an analytical solution based on Chiu et al. [1].