Glass is a common material already employed in everyday applications, which has gained considerable interest for electronic components, due to its attractive electrical, physical, and chemical properties, as well as its prospects for a cost-efficient solution. Adhesion of thin metal film on glass is especially critical and bonding between glass and metal can broaden the applications of glass in many industrial areas. Numerous methods have been developed to assess the adhesion of thin films to substrates. Among these, acoustic waves and particularly ultra-high frequency acoustic waves, are highly sensitive to adhesion defects. At a poor interface, acoustic waves are much more reflected than expected and therefore much less transmitted. In this work, we use picosecond acoustics to measure both the thickness of metal film and the acoustic transmission coefficient at the interface with a glass substrate. Picosecond acoustics is an ultrafast laser technique that implements a nanoscale pulse-echo technique. By making measurements at various wavelengths, we detect subtle changes in the photo-acoustic response, which can be used to improve measurement accuracy. The methodology demonstrated here can be readily extended to any system comprising a metal film on a transparent substrate. This versatility allows for potential applications in various fields, including flexible electronics and space mirrors.

Colored Picosecond Acoustics (CPA) and Spectroscopic Ellipsometry (SE) are combined to measure elastic and thermoelastic properties of polymer thin-film resins deposited on 300 mm wafers. Film thickness and refractive index are measured using SE. Sound velocity and thickness are measured using CPA from the refractive index. Comparing the two thicknesses allows checking consistency between both approaches. The same combination is then applied at various temperatures from 19° to 180°C. As the sample is heated, both thickness and sound velocity change. By monitoring these contributions separately, the Temperature Coefficient on sound Velocity (TCV) and the Coefficient on Thermal Expansion are deduced. The protocol is applied to five industrial samples made of different thin-film resins currently used by microelectronic industry. Young’s modulus varies from resin to resin by up to 20%. TCV is large on each resin and varies from one resin to another up to 57%.

This paper presents some quantitative measurements of the adhesion energy of thin WTi films deposited on Si substrate. Two different techniques are applied to the same sample series. One is a mechanical test based on the analysis of spontaneously formed defects. The second is based on acoustic waves whose reflection at the interface between the thin-film and the substrate is sensitive to the adhesion. An excellent correlation is obtained between both approaches: the adhesion energy measured by buckles analysis and acoustic reflection coefficient measured by picosecond acoustics. The acoustic approach offers several advantages among which a non-destructive character, a compatibility with complex stacks and a sensitivity to detect adhesion anomaly even if no defect is formed.

Picosecond ultrasonics, which studies laser-induced high-frequency strain waves, is a reliable and versatile method for nondestructive materials' characterization. Strain waves are generated through a light interaction with charges and their subsequent relaxation, and these waves conceal a wealth of information on the material. However, strain waves are detected through their convolution with a sensitivity function, which blurs much of this information. Here, we show that the reflection of strain waves at a free surface leads to the appearance of a Fano resonance in the reflectivity spectrum, accompanied by a drastic increase in the detection bandwidth. We take advantage of this feature to provide a method for the reconstruction of strain waves. We apply it to unambiguously highlight the exact origin of the generation of coherent acoustic phonons in Stranski–Krastanov grown quantum dots, revealing that both the wetting layer and quantum dots are responsible for the generation. Our results will offer the possibility to understand better the interaction of light with charges and their interactions with the lattice.

In recent years, the manipulation of Fano resonances in the time domain has unlocked deep insights into a broad spectrum of systems' coherent dynamics. Here, inelastic scattering of light with coherent acoustic phonons is harnessed to achieve complex Fano resonances. The sudden change of phonon momentum during reflection leads to a transition from anti-Stokes to Stokes light scattering, producing two different resonances that interfere in the measurement process. We highlight the conditions necessary to achieve such interference, revealing an underlying symmetry between photons and phonons, and verify the theory experimentally. Then, we demonstrate the possibility to characterize energy and coherence losses at rough interfaces, thus providing a mechanism for nondestructive testing of interface quality. Our results describe numerous unexplained observations in ultrafast acoustics and can be generalized to the scattering of light with any waves.

Nanocrystalline materials exhibit properties that can differ substantially from those of their single crystal counterparts. As such, they provide ways to enhance and optimise their functionality for devices and applications. Here we report on the optical, mechanical and thermal properties of nanocrystalline silicon probed by means of optomechanical nanobeams to extract information of the dynamics of optical absorption, mechanical losses, heat generation and dissipation. The optomechanical nanobeams are fabricated using nanocrystalline films prepared by annealing amorphous silicon layers at different temperatures. The resulting crystallite sizes and the stress in the films can be controlled by the annealing temperature and time and, consequently, the properties of the films can be tuned relatively freely, as demonstrated here by means of electron microscopy and Raman scattering. We show that the nanocrystallite size and the volume fraction of the grain boundaries play a key role in the dissipation rates through non-linear optical and thermal processes. Promising optical (13000) and mechanical (1700) quality factors were found in the optomechanical cavity realised in the nanocrystalline Si resulting from annealing at 950 C. The enhanced absorption and recombination rates via the intra-gap states and the reduced thermal conductivity boost the potential to exploit these non-linear effects in applications, including NEMS, phonon lasing and chaos-based devices.

Nanocrystalline silicon is a material that shows highly interesting properties for both electronic and photonic applications. Recently, it has also been employed as the core material for building optomechanical systems, showing some novel features. In this work we provide insight in the optical, mechanical and thermal properties of nanocrystalline silicon as a material platform of optomechanical crystal cavities. The results of this work, extracted by means of a combination of complementary experimental techniques, can be useful to evaluate the potential benefits as well as disadvantages of this material highly relevant for the development of nano-opto-electro-mechanical systems (NOEMS). We show that the specific microscopic nature of the nanocrystalline material has a dominant effect in the optical and mechanical losses and in the thermal properties. More specifically, we find strong correlations between the measured parameters and the volume fraction of grain boundaries, which has been tuned by adjusting the annealing temperature of the layers.

This paper presents some applications of a recent technique so-called the Colored Picosecond Acoustics (APiC) to the characterization of complex stacks of thin films. The technique is a unique combination of optics and acoustics that implements an acoustic pulse-echo technique at the nanoscale using a tunable ultrafast laser. From the experimental point of view, it is a full optical setup, acoustics taking place in the sample only. Very high frequency acoustic waves (up to several hundreds of GHz) are emitted and detected using ultra-short laser pulses. The capabilities of the APiC technique are demonstrated on various thin-film samples made of metals, dielectrics and semiconductors. Ultra-high frequency acoustic waves are first used to assess the film thickness or to measure thin-film elasticity via the acoustic time-of-flight measurement. A great potential is the capability to detect adhesion defects at buried interface through an analysis of the acoustic reflection at the concerned interface. Acoustic mapping of the sample surface reveal, in non-destructive manner, weak points at the buried interface.

We present a method to perform elastic measurements on a thin-film as a function of frequency between a few GHz and a few hundred GHz. The technique is mainly based on Picosecond Acoustics (PA), which is an ultrafast optical technique that realizes pulse-echo measurements in the hypersonic range. Here, we combine gold layers serving as transducers and several opto-acoustic detection mechanisms to extend the PA technique to the lowest accessible frequencies (a few GHz) up to hundreds of GHz. We can therefore use the same technique on the same material to explore its elastic properties at a certain frequency over a very large frequency range. We have then applied this technique to explore the elastic properties of a lead zirconate titanate thin film from 3 to 80 GHz. We report a 9% increase in the longitudinal sound velocity above 20 GHz, which corresponds to a 19% increase in the C33 elastic modulus. We interpret such an observation as a direct evidence of ferroelastic domain wall relaxation.

Nowadays, the haptic effect is used and developed for many applications—particularly in the automotive industry, where the mechanical feedback induced by a haptic system enables the user to receive information while their attention is kept on the road and on driving. This article presents the development of a vibrotactile button based on printed piezoelectric polymer actuation. Firstly, the characterization of the electro-active polymer used as the actuator and the development of a model able to predict the electromechanical behavior of this device are summarized. Then, the design of circular membranes and their dynamic characterization are presented. Finally, this work is concluded with the construction of a fully functional demonstrator, integrating haptic buttons leading to a clear haptic sensation for the user.

We demonstrate the ability to construct wide-area spatial mappings of buried interfaces in thin film stacks in a non-destructive manner using two color picosecond acoustics. Along with the extraction of layer thicknesses and sound velocities from acoustic signals, the morphological information presented is a powerful demonstration of phonon imaging as a metrological tool. For a series of heterogeneous (polymer, metal, and semiconductor) thin film stacks that have been treated with a chemical procedure known to alter layer properties, the spatial mappings reveal changes to interior thicknesses and chemically modified surface features without the need to remove uppermost layers. These results compare well to atomic force microscopy scans showing that the technique provides a significant advantage to current characterization methods for industrially important device stacks.

Blistering of Al 2 O 3 /SiN X :H stacks has been regularly reported over the last decade. Despite several studies, it has not been possible to link blistering density and lifetime degradation. In this work we demonstrate the use APiC technique to probe the c-Si/Al 2 O 3 interface. It allows us to show that it might not be the delaminated surface fraction which rules the lifetime degradation but most likely the interface quality of non-delaminated areas. Blistering is the top of the iceberg; characterizing the interface with the high sensitivity provided by APiC is a breakthrough.

Photoresist degradation can occur during wet etching processes due to chemicals diffusion through the polymer. The adhesion of the resist is not guaranteed anymore and damage on the resist / material interface appears. This phenomenon is usually monitored by optical methods. However, invisible resist degradation cannot be detected and the physical nature of the resist / material modification remains unknown. A high-frequency acoustic echography method has been developed to overcome these problems and has been performed to study the apparition of blisters in a deep UV photoresist exposed to a Standard Clean 1 solution. This technique allows the quantitative detection of resist degradation even if blisters cannot be seen in the resist. It has also been found that gas pockets appear during blisters formation.

Wet etching in photoresist presence is commonly used in MEMS or integrated circuits manufacturing: metal gates [1], or gate oxides patterning [2]. Nonetheless the resist can’t stand a too long exposure to wet chemicals. Indeed, the liquids diffuse through the resist. Then the polymer enters a plasticizing sequence. It swells under the action of polar water and etchant molecules that break the cohesive hydrogen bonds between the polymer chains. The resist stress increases till fractures and blisters appears (figure 1). Hence these phenomena have been characterized by various acoustic means described hereby. First method consists in using a high-frequency echography principle [3]. A ZnO transducer, sputtered on the backside of the silicon wafer emits a 2GHz acoustic wave. The longitudinal wave reflection occurs at the interface between the silicon and the medium on the wafer frontside enabling its characterization by monitoring the transducer electrical impedance (figure 3). In the case of a silicon / air interface, the acoustic wave reflection is total. When air is replaced by water, the reflection is partial and the reflection coefficient value is 0.86. These reflection properties will be modified in the presence of a thin TiN layer coated with a 248nm deep UV resist. As the resist thickness (210 nm) is thin compared with the acoustic wavelengths in the different materials (micrometer range), reflections at each interface cannot be separated (silicon / resist and resist / upper medium) so only one total reflection occurs in the presence of air. Then, the reflection coefficient is measured with water on the resist. Without any resist damage, the reflection coefficient changes from 0.86 to 0.765. This latter value is then modulated by the resist blisters amount due to the modification of the mechanical properties of the silicon / resist interface. Measurements are performed on wafers exposed to SC1 (Standard Clean 1) solutions for different durations (figure 1). The reflection coefficient increases with the blisters apparition (figure 4). By determining the ratio of blisters area on optical microscope images of the resist, the reflection coefficient is theoretically calculated for different blisters mechanical properties (solids, liquids and gas). Experimental and theoretical values perfectly match in the case of gas. This result let us think that gas pockets appeared in the resist during blisters formation. The method sensitivity is excellent and mainly depends on the high contrast between gas and water acoustic impedances. A comparison with two other complementary technics will be made. First, picosecond acoustic will characterize the resist adhesion loss. Its principle is similar to a sonar. The acoustic wave into the sample is supplied by a tunable laser [4], and an echo is generated at each interface enabling its characterization. Finally the scanning acoustic microscopy will monitor the resist blister apparition with a thicker resist. This technic is commonly used to monitor wafer bonding voids in 3D integrated circuits assembly [5]. Conclusion Photoresist degradation occurs during long exposure to wet etching. Resist delamination from substrate and blisters appear after a certain contact duration. This latter is shorter with thin resist, which occurs more and more along the integrated circuits node evolution to maintain photolithography optical requirements. Three acoustic methods have been compared to monitor this degradation. Results show these methods are far better than optical microscopy to detect the resist degradation starting point. References [1] M. Foucaud, solid state phenomena, vol.195, pp58-61 [2] P. Garnier, solid State Phenomena, 2008; 134:71-74 [3] R. Dufour, Langmuir, 2013, 29(43) [4] A.Devos, Ultrasonics Symposium, 2006, pp 564-567 [5] H .Moriceau, Adv. Nat. Sci.: Nanosci. Nanotechnol. 2010 Figure 1

Picosecond acoustics is the perfect technique for measuring elastic properties at nanoscale. But there still are some limitations to reach in-plane properties of ultra-thin films related to optical detection of ultra-high frequency surface acoustic waves. Here, we demonstrate that it is possible to push the limits of the technique using an interferometric detection instead of a usual reflectometry scheme. The experimental observations are supported by a simple model which explains from where comes the previous limitations and why it is possible to overcome it using the interferometric setup. Thanks to those results, it is possible to excite and detect very high frequency surface acoustic waves confined in ultra-thin layer using conventional femtosecond laser and optical setup.

The tremendous development of tactile interface in many customers' applications such as Smartphone, tablet PC or touch pad leads industrials to study “haptic interfaces” or “touch screen” solutions. This technology is already used but with limitations such as high power consumption and limited feedback effect (simple vibration). PZT is a good candidate for many actuator applications due to its high piezoelectric coefficient. In particular, it can be used for haptic interfaces to create squeeze-film effect. It consists in changing the friction between the finger and a plate resonator. It provides high granularity level of haptic sensation (texture rendering), using low power consumption compared to existing solutions. We manufactured demonstrators using a generic technology. We proved the concept through electro-mechanical characterizations and haptic feedback effect was noticeable with one's finger on our thin-film PZT demonstrators. In this paper, we presented the characterization and post-simulation of PZT-actuated plates with various actuator configurations. Measurement results, in good agreement with simulation, indicate that 2 actuator columns separated by a wavelength allow obtaining the highest substrate displacement amplitudes.

The aim of this paper is to review the various laser-wavelength effects reported in the field of ultrafast acoustics (UA). First observed by chance in 1999, a wavelength change can indeed have a strong effect on the signal detected in UA. After the physical origin of the effect was clarified and from a systematic exploration we established that all the opto-acoustic mechanisms acting in UA are influenced by the laser-wavelength. From that we suggested original applications of UA to fundamental and applied physics. So emerged a new field, now referred as Colored Picosecond Acoustics or APiC.

In the present article, we report a study on the mechanical behaviour displayed by hydrogen atoms and pores in silicon nitride (SiN) films. A simple three-phase model is proposed to relate the physical properties (stiffness, film stress, mass density, etc.) of hydrogenated nanoporous SiN thin films to the volume fractions of hydrogen and pores. This model is then applied to experimental data extracted from films deposited by plasma enhanced chemical vapour deposition, where hydrogen content, stress, and mass densities range widely from 11% to 30%, −2.8 to 1.5 GPa, and 2.0 to 2.8 g/cm3, respectively. Starting from the conventional plotting of film's Young's modulus against film porosity, we first propose to correct the conventional calculation of porosity volume fraction with the hydrogen content, thus taking into account both hydrogen mass and concentration. The weight of this hydrogen-correction is found to evolve linearly with hydrogen concentration in tensile films (in accordance with a simple “mass correction” of the film density calculation), but a clear discontinuity is observed toward compressive stresses. Then, the effective volume occupied by hydrogen atoms is calculated taking account of the bond type (N-H or Si-H bonds), thus allowing a precise extraction of the hydrogen volume fraction. These calculations applied to tensile films show that both volume fractions of hydrogen and porosity are similar in magnitude and randomly distributed against Young's modulus. However, the expected linear dependence of the Young's modulus is clearly observed when both volume fractions are added. Finally, we show that the stiffer behaviour of compressive films cannot be only explained on the basis of this (hydrogen + porosity) volume fraction. Indeed this stiffness difference relies on a dual mechanical behaviour displayed by hydrogen atoms against the film stress state: while they participate to the stiffness in compressive films, hydrogen atoms mainly behave like pores in tensile films where they do not participate to the film stiffness.

We present a technique based on ultrafast acoustics which permits us to measure the electrical dependence of the elastic properties of a thin piezoelectric layer. Ultrafast acoustics offers a unique way of measuring elastic properties of thin-layer in a non-destructive way using ultrashort optical pulses. We apply this technique to a thin layer to which a dc voltage is simultaneously applied. Both the film thickness and the sound velocity are affected. The two effects can be separated by use of a semi-transparent top electrode. A demonstration is made on a thin aluminum nitride (AlN). From that the d33 piezoelectric coefficient and the stiffness variation induced by the bias in AlN are measured.