Mechanical Characterization Of Thin Films Research Articles

Impact of doping on the mechanical properties of conjugated polymers.

Conjugated polymers exhibit a unique portfolio of electrical and electrochemical behavior, which - paired with the mechanical properties that are typical for macromolecules - make them intriguing candidates for a wide range of application areas from wearable electronics to bioelectronics. However, the degree of oxidation or reduction of the polymer can strongly impact the mechanical response and thus must be considered when designing flexible or stretchable devices. This tutorial review first explores how the chain architecture, processing as well as the resulting nano- and microstructure impact the rheological and mechanical properties. In addition, different methods for the mechanical characterization of thin films and bulk materials such as fibers are summarized. Then, the review discusses how chemical and electrochemical doping alter the mechanical properties in terms of stiffness and ductility. Finally, the mechanical response of (doped) conjugated polymers is discussed in the context of (1) organic photovoltaics, representing thin-film devices with a relatively low charge-carrier density, (2) organic thermoelectrics, where chemical doping is used to realize thin films or bulk materials with a high doping level, and (3) organic electrochemical transistors, where electrochemical doping allows high charge-carrier densities to be reached, albeit accompanied by significant swelling. In the future, chemical and electrochemical doping may not only allow modulation and optimization of the electrical and electrochemical behavior of conjugated polymers, but also facilitate the design of materials with a tunable mechanical response.

Mechanical characterization of thin films via constant strain rate membrane deflection experiments

The mechanical behavior of freestanding metal thin films (gold, aluminum, and nickel-molybdenum-tungsten alloy) is characterized using micro-tensile testing and membrane deflection experiment (MDE). Micro-tensile and membrane samples are simultaneously fabricated on a single wafer to avoid microstructural differences. A constant strain rate MDE methodology is developed to directly compare each test method. Finite element analysis is performed to investigate potential sources of error in MDE. Specifically, the effects of in-plane and out-of-plane misalignment, tip blunting, sample dimension, and substrate deformation are studied, and physical insights gained from the analysis were applied to the experiments. By comparing with the micro-tensile tests, it is experimentally demonstrated that the stress-strain curve of freestanding metal thin films can be successfully measured via constant strain rate MDE. In particular, yield stress shows a good agreement with that obtained from micro-tensile tests, demonstrating the usefulness of applying the constant strain rate methodology in MDE testing. Furthermore, MDE enables residual stress measurements of the deposited films. Constant strain rate MDE developed in this study can serve as a useful technique for high-throughput measurement of in-plane mechanical properties of metal thin films.

A Combined Experimental and Numerical Study of the Effect of Surface Roughness on Nanoindentation

Gold and copper thin films are widely used in microelectromechanical system (MEMS) and nanoelectromechanical system (NEMS) devices. Nanoindentation has been developed in mechanical characterization of thin films in recent years. Several researchers have examined the effect of surface roughness on nanoindentation results. It is proved that the surface roughness has great importance in nanoindentation of thin films. In this paper, the surface topography of thin films is simulated using the extracted data from the atomic force microscopy (AFM) images. Nanoindentation on a rough surface is simulated using a three-dimensional finite-element model. The results are compared with the results of finite-element analysis on a smooth surface and the experimental results. The results revealed that the surface roughness plays a key role in nanoindentation of thin films, especially at low indentation depths. There was good compatibility between the results of finite-element simulation on the rough surface and those of experiments. It is observed that on rough films, at low indentation depths, the geometry of the location where the nanoindentation is performed is of major importance.

Effect of Relative Humidity on the Viscoelasticity of Thin Organic Films Studied by Contact Thermal Noise AFM.

Material scientists are in need of experimental techniques that facilitate a quantitative mechanical characterization of mesoscale materials and, therefore, their rational design. An example is that of thin organic films, as their performance often relates to their ability to withstand use without damage. The mechanical characterization of thin films has benefited from the emergence of the atomic force microscope (AFM). In this regard, it is of relevance that most soft materials are not elastic but viscoelastic instead. While most AFM operation modes and analysis procedures are suitable for elasticity studies, the use of AFM for quantitative viscoelastic characterizations is still a challenge. This is now an emerging topic due to recent developments in contact resonance AFM. The aim of this work was to further explore the potential of this technique by investigating its sensitivity to viscoelastic changes induced by environmental parameters, specifically humidity. Here, we show that by means of this experimental approach, it was possible to quantitatively monitor the influence of humidity on the viscoelasticity of two different thin and hydrophobic polyurethane coatings representative of those typically used to protect materials from processes like weathering and wear. The technique was sensitive even to the transition between the antiplasticizing and plasticizing effects of ambient humidity. Moreover, we showed that this was possible without the need of externally exciting the AFM cantilever or the sample, i.e., just by monitoring the Brownian motion of cantilevers, which significantly facilitates the implementation of this technique in any AFM setup.

Open-source micro-tensile testers via additive manufacturing for the mechanical characterization of thin films and papers.

The cost of specialized scientific equipment can be high and with limited funding resources, researchers and students are often unable to access or purchase the ideal equipment for their projects. In the fields of materials science and mechanical engineering, fundamental equipment such as tensile testing devices can cost tens to hundreds of thousands of dollars. While a research lab often has access to a large-scale testing machine suitable for conventional samples, loading devices for meso- and micro-scale samples for in-situ testing with the myriad of microscopy tools are often hard to source and cost prohibitive. Open-source software has allowed for great strides in the reduction of costs associated with software development and open-source hardware and additive manufacturing have the potential to similarly reduce the costs of scientific equipment and increase the accessibility of scientific research. To investigate the feasibility of open-source hardware, a micro-tensile tester was designed with a freely accessible computer-aided design package and manufactured with a desktop 3D-printer and off-the-shelf components. To our knowledge this is one of the first demonstrations of a tensile tester with additively manufactured components for scientific research. The capabilities of the tensile tester were demonstrated by investigating the mechanical properties of Graphene Oxide (GO) paper and thin films. A 3D printed tensile tester was successfully used in conjunction with an atomic force microscope to provide one of the first quantitative measurements of GO thin film buckling under compression. The tensile tester was also used in conjunction with an atomic force microscope to observe the change in surface topology of a GO paper in response to increasing tensile strain. No significant change in surface topology was observed in contrast to prior hypotheses from the literature. Based on this result obtained with the new open source tensile stage we propose an alternative hypothesis we term ‘superlamellae consolidation’ to explain the initial deformation of GO paper. The additively manufactured tensile tester tested represents cost savings of >99% compared to commercial solutions in its class and offers simple customization. However, continued development is needed for the tensile tester presented here to approach the technical specifications achievable with commercial solutions.

Comparison of three approaches to determine the projected area in contact from finite element Berkovich nanoindentation simulations in tungsten

Nanoindentation nowadays is a standard method for the mechanical characterization of thin films and small volumes of material. One of the most meaningful parameters determined in nanoindentation experiments and simulations is the hardness of the tested material. For its determination, the knowledge of the exact value of the projected area in contact between the indenter and the specimen is essential. Inaccurate results for the projected area will result in noticeable errors in hardness. The determination of this area in finite element (FE) nanoindentation simulations is challenging because it cannot be determined directly and phenomena like pile-up and geometric imperfections of the indenter have to be considered. Hence, a new method, namely the triangulation method has been developed. It provides a reliable way to determine the projected area in FE-simulations, even under the occurrence of material pile-up. It is based on the nodes in contact between the indenter and the specimen as well as on the coordinates of the nodes. With this information, a Delaunay triangulation and Alpha shapes can be used to calculate the projected area. The triangulation method is compared to two established methods, one following the Oliver-Pharr analysis and the other one based on the computed true area in contact between specimen and indenter. The three methods are applied to results from an elastic-plastic FE simulation. Bland-Altman plots are used to compare the results of the three methods and to validate the triangulation method.

Monitoring of stress–strain evolution in thin films by reflection anisotropy spectroscopy and synchrotron X-ray diffraction

With progressing miniaturization of modern electronic devices, interconnects become increasingly smaller. Additionally, as electronic devices move away from rigid substrates toward flexible ones, understanding their mechanical and structural stability is becoming crucial. In this work, a thorough mechanical characterization of copper thin films deposited on flexible substrates was performed with two techniques, namely well-established synchrotron X-ray diffraction (sXRD) and the rather new usage of reflectance anisotropy spectroscopy (RAS) for mechanical characterization of thin films. The comparison of these two techniques shows that RAS can be reliably used for the accurate and prompt yield stress measurements. The acquisition time of RAS is much faster than that of sXRD: 1 second per data point compared to several seconds per data point for sXRD experiments. Moreover, the signal-to-noise ratio of the RAS data is much higher than that of the sXRD. Our results show that yield stress of Cu films increases with the decrease in the film thickness, going from 352 MPa for a 500 nm films to 793 MPa for a 50 nm thick film. Microstructure analyses of the films by electron microscopy allowed correlation of the mechanical behavior of the films to their grain morphologies. We have shown that RAS can supplement sXRD measurements due to a faster acquisition rate which allowed us to analyze the creep behavior of our copper thin film at different strain rates.

Spherical indentation of a freestanding circular membrane revisited: Analytical solutions and experiments

Due to the development of novel micro-fabrication techniques to produce ultra-thin materials and increasing interest in thin biological membranes, in recent years, the mechanical characterization of thin films has received a significant amount of attention. To provide a more accurate solution for the relationship among contact radius, load and deflection, the fundamental and widely applicable problem of spherical indentation of a freestanding circular membrane have been revisited. The work presented here significantly extends the previous contributions by providing an exact analytical solution to the governing equations of Föppl–Hecky membrane indented by a frictionless spherical indenter. In this study, experiments of spherical indentation has been performed, and the exact analytical solution presented in this paper is compared against experimental data from existing literature as well as our own experimental results.

A measuring system for mechanical characterization of thin films based on a compact in situ micro-tensile tester and SEM moiré method

A measuring system for mechanical characterization of thin films based on a compact in situ micro-tensile tester and scanning electron microscope (SEM) moiré method is proposed. The load is exerted by the tensile tester and the full field strain is measured by SEM moiré method. The configuration of the tensile tester and the principle of SEM moiré method are introduced. In the tensile tester, a lever structure is designed to amplify the displacement imposed by lead–zirconate–titanate (PZT) actuator. The SEM moiré method is applied to measure the strain of the thin film, including both the average strain in the gage section and the local strain distribution at a specific region. As an application, the measuring system is applied to characterize the mechanical property of the free-standing aluminum thin film. The experimental results demonstrate the feasibility of the system and its good application potential for mechanical behavior analysis of film-like materials.

Mechanical Characterization of Thin Films by Use of Atomic Force Acoustic Microscopy

AbstractThe atomic force acoustic microscopy (AFAM) technique has been used to determine elastic properties of films with thicknesses decreasing from several hundreds of nanometers to several nanometers. It has been shown that metal films as thin as 50 nm can be characterized directly without the need to consider the influence of the substrate. For films with thicknesses ranging from about 30 to 50 nm, measurement parameters can be chosen such as to allow characterization of the elastic properties of either the film or the film–substrate interface. This attribute has been combined with the ability of the method to obtain qualitative stiffness images to show variations in the film–substrate adhesion. The AFAM technique has been also used to determine the indentation modulus of thin films of silicon oxide with thicknesses ranging from 7 to 28 nm. In this case, elastic properties of the substrate had to be considered. The examples of the applications of the AFAM method reported here for characterization of elastic properties of very thin films have shown that this technique has the lateral and depth resolution required to characterize the very thin films used nowadays in microelectronics industry.

Advances in phase‐sensitive acoustic microscopy studies of polymer blend films: annealing effects and micro‐elastic characterization of PS/PMMA blends

The unique phase-sensitive acoustic microscope is used for the structural and mechanical characterization of thin films of polystyrene/polymethylmethacrylate blends. The effect of annealing on blends of polystyrene/polymethylmethacrylate spin coated from different solvents unto a substrate is studied. Varying the solvents according to vapour pressure and spin coating at different speeds (for thickness variation) led to changes in phase domain distributions and overall structural properties before annealing. Annealing in vacuum at 190 degrees C for 48 h resulted in the elimination of solvent effects with all samples reverting to a similar morphology irrespective of common solvent and thickness. The Young's moduli at specific points on the film (E(polystyrene)= 3.4 +/- 0.3 GPa, E(polymethylmethacrylate)= 4.2 +/- 0.4 GPa) and over a given area (E(polystyrene/polymethylmethacrylate)= 3.9 +/- 0.4 GPa) were determined by combinatory use of the atomic force microscope and phase-sensitive acoustic microscope. These results demonstrate a minimally invasive method for the quantitative characterization of polymer blend films.

Acoustics and atomic force microscopy for the mechanical characterization of thin films

The science and technology of thin films require the development of nondestructive methods for their quantitative mechanical characterization with nanometric spatial resolution. High-frequency ultrasonic techniques--especially acoustic microscopy--and atomic force microscopy (AFM) have been demonstrated to represent versatile tools for developing such methods. In particular, in the last 15 years, the combination of AFM, which can probe the surface of a sample by applying ultralow loads (from micronewtons down to piconewtons) with a micromachined tip having an apex radius of a few nanometers, and ultrasonics techniques led researchers to develop some unique tools which allow one to perform not only spot measurements of the sample elastic modulus, but also to obtain both the qualitative imaging of mechanical properties and the quantitative mapping of the elastic modulus of the sample surface with nanometric lateral resolution. In the present review, firstly a brief overview of the main ultrasound-based techniques for thin film characterization is reported. Then, some of the ultrasonic AFM techniques are described, emphasizing their capability of retrieving maps of both the tip-sample contact stiffness and the sample elastic modulus. Although these techniques are less affected by the mechanical properties of the substrates than standard indentation tests, a method for the correction of the substrate effect in ultrathin films is reported in detail. Finally, by probing the mechanical properties of a small portion of the sample volume underneath the tip, we illustrate the techniques as tools for the qualitative and quantitative characterization of variations in the adhesion between a thin film and a buried interface, as well as for detecting subsurface defects, voids, cracks, and dislocations.

Superheterodyne techniques in laser ultrasonics.

Laser-based ultrasonics is a powerful tool for the mechanical characterization of thin films and nanostructures. In this paper, superheterodyne approaches to single-point and full-field detection of laser generated ultrasonic signals will be discussed. In these techniques, an amplitude modulated laser source is used to excite a narrow-bandwidth signal. The detection laser, incorporated into an interferometric detection scheme, is also amplitude or phase modulated at frequency that is offset from the generation laser modulation frequency by a fixed amount, serving as the local oscillator for superheterodyne detection. Introduction of the local oscillator allows for the optical down-conversion of the high-frequency intensity modulation associated with sample motion to a low and fixed intermediate frequency given by the difference between excitation and detection laser modulation frequencies. The primary benefit of using this approach is that the upper frequency bound is not dictated by the speed of the detection photodiode and electronic circuitry or, in the case of full-field detection, the frame rate of the CCD. Results are presented demonstrating single-point superheterodyne detection of gigahertz frequency bulk and surface acoustic waves using a low-frequency photodiode and full-field superheterodyne detection of megahertz vibrations of nanostructure arrays using a low frame rate camera.

Wafer-Scale Microtensile Testing of Thin Films

This paper reports on the mechanical characterization of thin films using the microtensile technique performed for the first time at the wafer scale. Multiple test structures are processed and sequentially measured on the same silicon substrate, thus eliminating delicate handling of individual samples. The current layout uses 26 test structures evenly distributed over a 4-in wafer, each of them carrying a microtensile specimen that bridges the gap between the fixed and movable parts of the micromachined wafer. A fully automated high-throughput setup makes possible the fast acquisition of data with statistical relevance for the reliable extraction of material properties. The technique was successfully applied to micrometer- and submicrometer-thick films. Two brittle materials, namely, polycrystalline silicon (poly-Si) obtained by low-pressure chemical vapor deposition and silicon nitride (SiNx) produced by plasma-enhanced chemical vapor deposition, and a ductile material, i.e., evaporated aluminum (Al), were characterized. The extraction of the Young's modulus E, tensile strength sigmau, mean tensile strength sigmatildeu, and Weibull modulus m is demonstrated. Young's moduli thus obtained for the poly-Si, SiNx, and Al films were 156.3plusmn 2.6, 112.2plusmn3.5, and 62.5plusmn 2.5 GPa, respectively. The SiNx layers, which have a mean tensile strength sigmatildeu of 2.084-0.177 +0.169 GPa and m=5.9-1.6 +1.8, are the strongest from the fracture point of view when compared to poly-Si with sigmatildeu=1.382-0.026 +0.023 GPa and m=17.3-3.2 +3.5 and Al with sigmatildeu=0.347plusmn0.013 GPa. In each case, the best estimate of the mean and the corresponding 90% confidence interval were evaluated using maximum likelihood estimation and the likelihood ratio method, respectively, on the basis of Gaussian and Weibull statistics.

Reliability of MEMS Materials: Mechanical Characterization of Thin-Films using the Wafer Scale Bulge Test and Improved Microtensile Techniques

AbstractThis paper reports on recent improvements of the bulge and microtensile techniques for the reliable extraction of material parameters such as the Young's modulus E, Poisson's ratio ν, plane strain modulusEps=E/(1–ν2), prestress σ0, fracture strength μ, Weibull modulusmand strain hardening coefficientsn, and on the direct comparison between the two methods. The bulge technique is extended to full wafer measurements enabling throughputs of data with statistical relevance whereas key improvements of a previous fabrication process of microtensile specimens lead now to much higher yields, approaching 100%. Both techniques are applied to an extensive set of materials, brittle and ductile, typically used in MEMS applications. These include thin films of silicon nitride, silicon oxide, polycrystalline silicon and aluminum deposited by techniques such as thermal oxidation, LPCVD, PECVD and PVD.

Strain and Stress Distribution in Vickers Indentation of Coated Materials

Depth sensing indentation equipment allows the mechanical properties of thin films to be easily determined, particularly the hardness and Young’s modulus. In order to minimize the influence of the substrate on the measured properties, the indentation depth must be limited to a small fraction of the film’s thickness. However, for very thin films, the determination of the contribution of the substrate and the film to the measured mechanical properties becomes a hard task, because both deform plastically. The numerical simulation of ultramicrohardness tests can be a helpful tool towards better understanding of the influence of the parameters involved in the mechanical characterization of thin films. For this purpose, a three-dimensional numerical simulation home code, HAFILM, was used to simulate ultramicrohardness tests on coated substrates. Materials with different Young’s modulus film/substrate ratios were tested. Analyses of strain and stress distributions for several indentation depth values were performed, in order to clarify the composite behaviour.

Mechanical Characterization of Thin Films by the Capacitance-Voltage Measurement of Microstructures

This paper presents a novel and high-precision technology for extracting the Young’s modulus of thin films through the capacitance-voltage (C-V) measurement of microstructures. An algorithm considering the electric-mechanical coupling effect and the distributed character of microstructures is developed for extracting the Young’s modulus through the C-V measurement of microstructures. The average error percentage of the extracted Young’s modulus of single-crystalline silicon is below 1% and the high precision and repeatability of the present methodology are verified. Since the driving and response signals are both electric, they could be accomplished using existing semiconductor testing equipments through probing on the bonding pads of devices. Because hardware replacement could be avoided, this methodology shows substantial advantage over other property-extraction methods for large-scale implementation in semiconductor or MEMS fabs.

Mechanical Characterization of Released Thin Films by Contact Loading

AbstractThe design of reliable micro electro-mechanical systems (MEMS) requires understanding of material properties of devices, especially for free-standing thin structures such as membranes, bridges, and cantilevers. The desired characterization system for obtaining mechanical properties of active materials often requires load control. However, there is no such device among the currently available tools for mechanical characterization of thin films. In this paper, a new technique, which is load-controlled and especially suitable for testing highly fragile free-standing structures, is presented. The instrument developed for this purpose has the capability of measuring both the static and dynamic mechanical response and can be used for electro∕magneto∕thermo mechanical characterization of actuators or active materials. The capabilities of the technique are demonstrated by studying the behavior of 75nm thick amorphous silicon nitride (Si3N4) membranes. Loading up to very large deflections shows excellent repeatability and complete elastic behavior without significant cracking or mechanical damage. These results indicate the stability of the developed instrument and its ability to avoid local or temporal stress concentration during the entire experimental process. Finite element simulations are used to extract the material properties such as Young’s modulus and residual stress of the membranes. These values for Si3N4 are in close agreement with values obtained using a different technique, as well as those found in the literature. Potential applications of this technique in studying functional thin film materials, such as shape memory alloys, are also discussed.

Photoisomerization of disperse red one in films of poly(methyl-methacrylate) at high pressure

Evanescent wave optics under high pressure is developed for the optical and mechanical characterization of thin films. Both the attenuated total reflection in the Kretschmann configuration and the grating coupling techniques are implemented. We apply total internal reflection and surface optical wave, e.g. surface plasmon spectroscopy, in order to gain information about the pressure dependence of the optical and mechanical properties of the pressure medium and the metal coupling layer. Optical modes guided in thin films allow us to characterize the pressure effects on these films. We show that hydrostatic pressure applied to thin polymer films increases their refractive index and decreases their thickness. The pressure increases the density of these films by a reduction of the free volume. As far as microscopic environments are concerned, we show that the molecular movement induced by photo- and thermal isomerization of an azo chromophore flexibly tethered to a poly(methyl-methacrylate) polymer is hindered by increasing the pressure.

Two interferometric methods for the mechanical characterization of thin films by bulging tests. Application to single crystal of silicon

Two optical methods are presented for the mechanical characterization of thin films, namely real time holographic interferometry and a fringe projection method called “contouring.” These two methods are coupled to the interferometry by the phase measurements, thus allowing the displacement field to be measured at all points on the membrane. We discuss the solutions retained in terms of their precision and sensitivity. These methods are then applied to membrane bulging tests, a type of test that is widely used in micro-mechanical studies. The measurements are performed on silicon single crystal and the results are compared to the solutions calculated by finite element methods. In both cases, the good agreement between theory and experiments allows the experimental apparatus to be validated.