Impulsive Stimulated Thermal Scattering Research Articles

Revisiting impulsive stimulated thermal scattering in supercooled liquids: Relaxation of specific heat and thermal expansion.

Impulsive stimulated thermal scattering (ISTS) allows one to access the structural relaxation dynamics in supercooled molecular liquids on a time scale ranging from nanoseconds to milliseconds. Till now, a heuristic semi-empirical model has been commonly adopted to account for the ISTS signals. This model implicitly assumes that the relaxation of specific heat, C, and thermal expansion coefficient, γ, occur on the same time scale and accounts for them via a single stretched exponential. This work proposes two models that assume disentangled relaxations, respectively, based on the Debye and Havriliak-Negami assumptions for the relaxation spectrum and explicitly accounting for the relaxation of C and γ separately in the ISTS response. A theoretical analysis was conducted to test and compare the disentangled relaxation models against the stretched exponential. The former models were applied to rationalize the experimental ISTS signals acquired on supercooled glycerol. This allows us to simultaneously retrieve the frequency-dependent specific heat and thermal expansion up to the sub-100MHz frequency range and further to compare the fragility and time scale probed by thermal, mechanical, and dielectric susceptibilities.

Attenuation process of the longitudinal phonon mode in a TeO2 crystal in the 20-GHz range

We experimentally investigated the hypersonic attenuation process of a longitudinal mode (L-mode) sound wave in ${\mathrm{TeO}}_{2}$ from room temperature to a lower temperature using Brillouin scattering and impulsive stimulated thermal scattering (ISTS) measurements. For precise measurement of the Brillouin linewidth at low temperatures, whereby the mean free path of the phonon becomes longer than the sample length, it is indispensable that the phonon should propagate along the phonon-resonance direction. To figure out the suitable direction, we defined two indices characterizing a degree of phonon divergence and a purity of propagation direction. The best direction that we found from these indices is [110] direction in ${\mathrm{TeO}}_{2}$, and it was used to discuss the temperature and frequency dependences of Brillouin spectra. We extracted the temperature dependence of the attenuation rate of ${T}^{4}$ from the modulated Brillouin spectra due to the phonon resonance below Debye temperature. The frequency dependence ${\ensuremath{\omega}}^{1}$ of the hypersonic attenuation was also estimated from the polarization dependence of the Brillouin linewidth. Theoretically, it predicted that the L-mode phonon attenuation at low temperatures in ${\mathrm{TeO}}_{2}$ is a result of Herring's process, which shows the attenuation behavior of ${\ensuremath{\omega}}^{2}{T}^{3}$. The ${\ensuremath{\omega}}^{1}{T}^{4}$ dependence is not allowed in Herring's process but is allowed by the $L+L\ensuremath{\rightarrow}L$ process, which has been considered to be forbidden so far. We evaluated the thermal phonon lifetime using ISTS and established that it was finite even at 20 K, thereby allowing the $L+L\ensuremath{\rightarrow}L$ process. Therefore, we conclude that the $L+L\ensuremath{\rightarrow}L$ process dominates the attenuation of an L-mode phonon in ${\mathrm{TeO}}_{2}$ in the low-temperature region.

Determination of elastic properties of a MnO2 coating by surface acoustic wave velocity dispersion analysis

MnO2 is a material of interest in the development of high energy-density batteries, specifically as a coating material for internal 3D structures, thus ensuring rapid energy deployment. Its electrochemical properties have been mapped extensively, but there are, to the best of the authors' knowledge, no records of the elastic properties of thin film MnO2. Impulsive stimulated thermal scattering (ISTS), also known as the heterodyne diffraction or transient grating technique, was used to determine the Young's modulus (E) and porosity (ψ) of a 500 nm thick MnO2 coating on a Si(001) substrate. ISTS is an all optical method that is able to excite and detect surface acoustic waves (SAWs) on opaque samples. From the measured SAW velocity dispersion, the Young's modulus and porosity were determined to be E = 25 ± 1 GPa and ψ=42±1%, respectively. These values were confirmed by independent techniques and determined by a most-squares analysis of the carefully fitted SAW velocity dispersion. This study demonstrates the ability of the presented technique to determine the elastic parameters of a thin, porous film on an anisotropic substrate.

Thermomechanical properties of single crystals evaluated by impulsive stimulated thermal scattering technique

This paper describes the application of Impulse Stimulated Thermal Scattering (ISTS) for measurement of surface acoustic wave (SAW) velocity and thermal diffusion along a free surface of a strongly anisotropic material. The motivation for this work stems from the study of thermoelastic properties of individual phases of ferroelastics; experimental results were obtained on a single crystal in the austenitic phase a Cu-Al-Ni alloy (bcc single crystal having elastic anisotropy factor of about 12). The measured SAW velocities in specific directions are in a good agreement with the values calculated for the elastic constants obtained by other ultrasonic methods. Similarly, the evaluated thermal diffusivity coefficient (22 ± 2).10−6 m2/s of the austenite is consistent with the data in the literature. The proposed approach has also a potential for characterization of thin films grown on anisotropic substrates.

High frequency longitudinal and shear acoustic waves in glass-forming liquids

Three different picosecond ultrasonic techniques for longitudinal and transverse acoustic pulse generation have been combined with Impulsive Stimulated Thermal Scattering (ISTS) to probe structural relaxation dynamics in glycerol and DC704 (tetramethyl tetraphenyl trisiloxane) at megahertz and gigahertz frequencies (~ 50 MHz – 100 GHz) from below their respective glass transition temperatures up to 370 K.

Determination of second-order elastic constants of cyclotetramethylene tetranitramine (β-HMX) using impulsive stimulated thermal scattering

The second-order elastic constants for cyclotetramethylene tetranitramine (β-HMX) single crystals were determined using the impulsive stimulated thermal scattering (ISTS) method. Despite the low symmetry of these crystals, the complete set of 13 elastic constants were determined accurately from acoustic velocity measurements using samples cut parallel to three different crystal planes. Our acoustic velocities are consistent with the limited sound speed data available from ultrasonic measurements. However, significant differences are observed between the elastic constants determined from our experiments and those obtained previously using Brillouin scattering. Our results demonstrate the usefulness and efficiency of the ISTS method for determining the full set of elastic constants of low-symmetry molecular crystals, including energetic crystals.

Second-order elastic constants of pentaerythritol tetranitrate and cyclotrimethylene trinitramine using impulsive stimulated thermal scattering

Impulsive stimulated thermal scattering (ISTS) was used to determine the complete set of second-order elastic constants for pentaerythritol tetranitrate (PETN) and cyclotrimethylene trinitramine (RDX) single crystals. Despite the weak scattering efficiency of these materials, excellent signal quality was obtained by using an optical heterodyne detection approach. The elastic constants for PETN agree well with previous values obtained from ultrasonic velocity measurements. The elastic constants for RDX are consistent with previous values obtained from ultrasonic velocity measurements and from resonant ultrasound spectroscopy, but show significant differences with values obtained from Brillouin scattering data. The present results demonstrate that the ISTS method, with optical heterodyne detection, provides a useful and accurate approach for determining the elastic constants of energetic crystals.

Pulsed laser characterization of multicomponent polymer acoustic and mechanical properties in the sub-GHz regime

We investigated the acoustic properties in the sub-GHz frequency regime of a multilayer system comprising alternating 100-nm scale TiO2/poly(methyl methacrylate) (PMMA) layers through a laser photoacoustic method, impulsive stimulated thermal scattering (ISTS). The acoustic dispersion curves were determined, and the mechanical properties were extracted from the experimental results.

Shear properties of glycerol by interface wave laser ultrasonics

Acoustic waves at the interface between a polymeric solid and a glass-forming liquid are studied through impulsive stimulated thermal scattering (ISTS), a photoacoustic method. The transverse character of the interface waves provides a unique window into the shear relaxation dynamics of supercooled liquids. Real and imaginary parts of the shear modulus of glycerol are extracted as functions of temperature and acoustic frequency. Extension of ISTS to interface waves permits characterization of shear dynamics even under conditions of very strong acoustic damping which preclude more conventional ultrasonic methods.

Relaxation dynamics in glycerol studied by impulsive stimulated thermal scattering

Time resolved spectroscopy called impulsive stimulated thermal scattering (ISTS) has been carried out to investigate the dynamics of the liquid–glass transition in a supercooled polyalcohol, glycerol. The wave vector dependence of the time dependent density fluctuation has been detected by ISTS. It is found that the thermal relaxation time τ H related to thermal diffusion shows the remarkable anomaly at a temperature T hmax far above the glass transition temperature T g. Moreover, T hmax is found to depend on the wave vector. These findings are discussed in terms of phonon velocity and phonon mean free path.

Thin film photoacoustics: From the laboratory to the real world and back

Characterization of thin film anisotropic elastic moduli, thermal diffusivities, thickness, delamination, cure kinetics, residual stress, and other properties through impulsive stimulated thermal scattering (ISTS), a noncontact, nondestructive photoacoustic method, was demonstrated in the 1990s. The method, which involves two crossed ‘‘excitation’’ laser pulses used for generation of acoustic waves, a probe laser beam, and a diffracted signal beam whose time-dependent intensity is measured to reveal acoustic oscillations and damping as well as thermal diffusion, originally required large-frame lasers and painstaking optical alignment. A series of refinements resulted in a compact ISTS system including only miniaturized diode and diode-pumped lasers, using only wallplug electrical power, with automated tuning of the acoustic wavelength and frequency, and with no user-adjusted optical elements. Commercially available ISTS systems are now used routinely in the microelectronics industry for metal film thickness measurement, yielding angstrom precision with 1-s data acquisition times. The ISTS method will be reviewed. Several new advances in methodology and new applications and thin films and bulk materials will be discussed. Finally, the connection between ISTS and related time-resolved measurements of collective vibrations will be illustrated.

Impulsive stimulated thermal scattering study of structural relaxation in supercooled glycerol

Impulsive stimulated thermal scattering (ISTS) studies are conducted on glycerol in the supercooled liquid range. ISTS is used to measure the Debye–Waller factor, and to characterize the structural relaxation dynamics in the range from a few nanoseconds to hundreds of microseconds. The results are compared to predictions of mode-coupling theory (MCT), and suggest that inclusion of important microscopic degrees of freedom may be needed for MCT to successfully describe hydrogen-bonded liquids like glycerol.

Surface acoustic modes in thin films on anisotropic substrates

Propagation of surface acoustic modes on the (001) and (111) surfaces of Si coated by a thin isotropic overlayer is studied theoretically and experimentally. It is shown that when a surface acoustic wave (SAW) coexists with a pseudosurface wave (PSAW) of the uncoated substrate, the second-order acoustic mode of the film/substrate system originates from a PSAW and the first-order one from a SAW. The polarization pattern of either mode varies from Rayleigh type (saggital plane polarization) to Love type (horizontal polarization) depending on the propagation direction and the product of the wave vector q and film thickness d. It is also shown that the isolated off-symmetry pure mode point within the PSAW branch disappears at some critical qd value. Experimentally, surface acoustic modes of Ti-coated Si wafers are measured with the impulsive stimulated thermal scattering (ISTS) technique based on laser generation and detection of acoustic waves at a specified wave vector. ISTS data are shown to be determined by the surface elastodynamic Green function G13(ω,g). The measurements, only sensitive to Rayleigh-type waves, detect either the first- or the second-order mode, or both, depending on the qd and the observation angle.

Anisotropy of thermal relaxational mode in KHCO 3 studied by impulsive stimulated thermal scattering

The anisotropic behaviors of a thermal relaxational mode in KHCO 3 have been found by the impulsive stimulated thermal scattering (ISTS) experiment. The ISTS signal intensity at time origin I(0) of the mode with the wave vector q∥[0 1 0] is almost 1/100 of I(0) with q∥[4 0 −1 ]. The temperature dependence of I(0) of the former shows an anomaly but that of the latter does not. Moreover, the thermal diffusion constant D evaluated from the relaxation time τ of the mode is 2.1×10 −7 m 2 s −1 for q∥[0 1 0] and is 1.6×10 −7 m 2 s −1 for q∥[4 0 −1 ]. These anisotropic properties are successfully explained by the layer structure of KHCO 3 where the layers are stacked almost along [4 0 −1 ] .

High-precision film thickness determination using a laser-based ultrasonic technique

A noncontact and nondestructive laser-based acoustic technique called impulsive stimulated thermal scattering (ISTS) is used to measure thicknesses of metal films including Cu, Ta, W, Al, Ti, and others in single-layer and multilayer assemblies on silicon substrates. Other opaque film materials and substrates have also been examined. Thicknesses are determined with a repeatability of a few angstroms with data acquisition times of about 1 s. ISTS and conventional measurements (scanning electron microscopy, profilometry, and four-point electrical sheet resistance) are made on the same samples and the results are found to compare favorably.

Impulsive stimulated thermal scattering for sub-micron-thickness film characterization

Impulsive Stimulated Thermal Scattering (ISTS) is a noninvasive optical technique that can be used as a powerful tool for high frequency acoustic measurements at solid surfaces. In surface ISTS, two interfering picosecond laser pulses absorbed at the surface launch counterpropagating surface acoustic waves (SAWs) detected via diffraction of a probe laser beam. In this paper, we introduce surface ISTS measurements in the frequency domain using a Fabry-Perot interferometer to analyze the spectrum of the diffracted light. This spectrally separates the ISTS signal from light scattered by surface imperfections and enables us to extent the acoustic frequency range beyond 1 GHz. We demonstrate thickness and density determination for a gold film on silica from measured SAW velocity dispersion. The results obtained show the potential of the technique for mechanical characterization of films as thin as 0.1 μm or less.

Microelectronic Film Thickness Determination Using a Laserbased Ultrasonic Technique

AbstractA non-contact and non-destructive laser-based acoustic technique called impulsive stimulated thermal scattering (ISTS) is used to measure thicknesses of films in single-layer and multilayer assemblies such as W/Si, Ti/Si, and Ti/SiO2/Si structures. Thicknesses are determined to within a few percent accuracy with a laboratory version of the measurement apparatus, using conventional large-frame lasers and optics, and in a commercial prototype using compact diode-pumped and diode lasers and optics which all fit onto a 1-ft × 2-ft breadboard. ISTS and conventional measurements (profilometry, SEM, and 4-point resistance) are made on the same samples and the results are found to compare favorably.

Characterization of 6FDA-APBP polyimide films through impulsive stimulated thermal scattering

The elastic and thermal properties of 4,4′-(hexafluoroisopropylidene)-bis (phthalic anhydride)-4,4′-bis (4-aminophenoxy) biphenyl (6FDA-APBP) free-standing polyimide films (2–10 μm) were investigated using impulsive stimulated thermal scattering (ISTS), a non-contact optical method for thin film characterization. Shear and longitudinal velocities, Young's modulus, Poisson's ratio and the in-plane thermal diffusivity are reported.

Photoacoustic determination of the residual stress and transverse isotropic elastic moduli in thin films of the polyimide PMDA/ODA

We present results of experiments on thin films of the polyimide PMDA/ODA using the real-time impulsive stimulated thermal scattering (ISTS) technique. We determine the transverse isotropic modulus, and we describe a method which allows residual stress determination from ISTS data. The residual stress determined in this way is in good agreement with measurements made using other techniques.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dis-Bond Detection and the Possibility of Interfacial Stiffness Measurement with Real-Time Impulsive Stimulated Thermal Scattering

We describe a new, real-time, noninvasive method for dis-bond (delamination) detection which is based on a technique known as impulsive stimulated thermal scattering (ISTS). We first explain the ISTS technique and compare data from polyimide films tightly bound to silicon substrates with data from unsupported polyimide films. The observed differences in the data from these two cases are readily understandable and offer an unmistakable signature for delaminations. We demonstrate ISTS dis-bond detection by locating and mapping out randomly-distributed and spatially-fine regions of delamination in a polyimide film-silicon substrate system. Finally, we present two simple physical models of the interfacial region which comprehensively describe acoustic data from the tightly-bound and the unsupported samples. With insight from simulations using these models, we suggest how ISTS might be used to determine interfacial adhesion quality and we show how ISTS sensitivity to interfacial effects can be adjusted.