Nanoindentation hardness measurements using real-shape indenters: application to extremely hard and elastic materials

We study the technique of nanoindentation hardness measurement applied to extremely hard and elastic thin films. We do the study with the aid of Hertz’s solutions for elastic contacts. The effect of different apical angles in ideally sharp conical diamond indenters is analyzed. In addition, the blunt tip shape of practical diamond indenters is discussed. The area function of the tip of real indenters is deduced from experimental nanoindentation measurements performed with these indenters on fused quartz. Triangular-base pyramidal indenters with Berkovich and cube corner geometries are considered. Theoretical hardness values applying Hertz’s and Oliver and Pharr’s methods of analysis are obtained and compared with the experimental data deduced from nanoindentation measurements performed on very hard and elastic ta-C films. The theoretical analysis shows a necessary dependence of the calculated hardness values with the apical angle of the indenter in totally elastic materials and to some extent in elastoplastic materials. Moreover, when the indenter tip is blunt or when there are inaccuracies in the measured area function of the indenter tip, hardness values decrease for very small penetration depths. Besides, in these films, because of their very small thickness, measured hardness values also decrease for measurements with penetration depths larger than a fraction of film thickness, due to the effects of the softer substrate. Received: 13 June 2000 / Accepted: 21 June 2000 / Published online: 5 October 2000     

The structure and mechanical properties of thick rutile-TiO2 films using different coating treatments

The hardness and Young's modulus of thick rutile-TiO₂ films were determined using a continuous stiffness measurement (CSM) technique in this study. Pure rutile-TiO₂ nanopowders (TH₂O, TFeSO₄ and TCuSO₄) were prepared using a modified homogeneous-precipitation process at low temperature (MHPPLT) method. The TiO₂ films were prepared from sols using 3% (w/w) of the prepared-TiO₂ suspension solution coated onto silicon wafers. After dip-coating was completed, the coatings were further treated by natural air-drying, water-vapor exposure, and calcination, respectively. An ellipsometry with a monochromator was used to measure the thickness and refractive index of the TiO₂ films, and a scanning electron microscopy (SEM) to determine their morphology. Three coatings of TH₂O, TFeSO₄ and TCuSO₄ demonstrated their refractive indexes of around 1.60 under three treatments. Volumetric expansion and thickness of the coatings should influence their refractive index. Furthermore, the continuous stiffness measurement (CSM) technique was used to perform nanoindentation testing on the hardness and Young's modulus of prepared rutile-TiO₂ coatings. The mean hardness and Young's modulus of three coatings increased with preparation temperature. In addition, the TH₂O coatings demonstrated greater hardness and modulus than those of TFeSO₄ and TCuSO₄ coatings in the natural air-drying condition. Surface cracking observed on the calcinated TFeSO₄ should be the reason why an obvious decrease of the mean hardness and Young's modulus appeared. Finally, two mechanical properties and related nanoindentation depth of the coatings were discussed in detail.     

Effects of post-annealing on the structural and nanomechanical properties of Ga-doped ZnO thin films deposited on glass substrate by rf-magnetron sputtering

In this study, the effects of post-annealing on the structure, surface morphology and nanomechanical properties of ZnO thin films doped with a nominal concentration of 3 at.% Ga (ZnO:Ga) are investigated using X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM) and nanoindentation techniques. The ZnO:Ga thin films were deposited on the glass substrates at room temperature by radio frequency magnetron sputtering. Results revealed that the as-deposited ZnO:Ga thin films were polycrystalline albeit the low deposition temperature. Post-annealing carried out at 300, 400 and 500 °C, respectively, has resulted in progressive increase in both the average grain size and the surface roughness of the ZnO:Ga thin film, in addition to the improved thin films crystallinity. Moreover, the hardness and Young's modulus of ZnO:Ga thin films are measured by a Berkovich nanoindenter operated with the continuous contact stiffness measurements (CSM) option. The hardness and Young's modulus of ZnO:Ga thin films increased as the annealing temperature increased from 300 to 500 °C, with the best results being obtained at 500 °C.     

Berkovich nanoindentation and deformation mechanisms in GaN thin films

The deformation mechanisms of GaN thin films obtained by metal-organic chemical vapor deposition (MOCVD) method were studied using nanoindentation with a Berkovich diamond indenter, micro-Raman spectroscopy and the cross-sectional transmission electron microscopy (XTEM) techniques. Due to the sharpness of the tip of Berkovich indenter, the nanoindentation-induced deformation behaviors can be investigated at relatively lower load and, hence, may cover wider range of deformation-related phenomena over the same loading range. The load-displacement curves show the multiple “pop-ins” during nanoindentation loading. No evidence of nanoindentation-induced phase transformation and cracking patterns were found up to the maximum load of 300 mN, as revealed from the micro-Raman spectra and the scanning electron microscopy (SEM) observations within the mechanically deformed regions. In addition, XTEM observation performed near the cross-section of the indented area revealed that the primary deformation mechanism in GaN thin film is via propagation of dislocations on both basal and pyramidal planes. The continuous stiffness measurement (CSM) technique was used to determine the hardness and Young's modulus of GaN thin films. In addition, analysis of the load-displacement data reveals that the values of hardness and Young's modulus of GaN thin films are 19 ± 1 and 286 ± 25 GPa, respectively.     

High-throughput screening for biomedical applications in a Ti-Zr-Nb alloy system through masking co-sputtering

A method of co-sputtering deposition combined with physical masking was applied to the parallel preparation of a ternary Ti-NbZr system alloy. Sixteen independent specimens with varying compositions were obtained. Their microstructure, phase structure,Young's modulus, nanoindentation hardness, and electrochemical behavior in a phosphate buffer solution(PBS) were studied in detail. It was revealed that the Ti-Zr-Nb alloys possess a single BCC structure. As confirmed via nanoindentation tests, the Young's modulus of the specimens ranged from 80.3 to 94.8 GPa and the nanoindentation hardness ranged from 3.6 to 5.0 GPa.By optimizing the composition of the specimens, the Ti_(34)Zr_(52)Nb_(14) alloy was made to possess the lowest modulus in this work(76.5 GPa). Moreover, the Ti_(34)Zr_(52)Nb_(14) alloy showed excellent corrosion resistance in PBS without any tendency for pitting at anodic potentials up to 1 Vsce. These preliminary advantages offer the opportunity to explore new orthopedic implant alloys based on Ti-Zr-Nb alloys. Moreover, this work provides an effective method for the parallel preparation of biomedical alloys.     

Effect of material stiffness on hardness: A computational study based on model potentials

We investigate the dependence of the hardness of materials on their elastic stiffness. This is possible by constructing a series of model potentials of Morse type; starting with modelling natural Cu, the model potentials exhibit an increased elastic modulus, while keeping all other potential parameters (lattice constant, bond energy) unchanged. Using molecular-dynamics simulation, we perform nanoindentation experiments on these model crystals. We find that the crystal hardness scales with the elastic stiffness. Also the load drop, which is experienced when plasticity sets in, increases in proportion to the elastic stiffness, while the yield point, i.e. the indentation at which plasticity sets in, is independent of the elastic stiffness.     

A displacement sensing nanoindentation study of tribo-mechanical properties of the Ni-Co system

An analysis of the tribo-mechanical properties of the Ni-Co system, at the submicrometric contact scale, is conducted using displacement sensing nanoindentation. In particular, the influence of contact depth and surface finishing methods on the hardness, H, and Young's modulus, E, of the materials is analysed. Mechanically and electrolitically polished samples were tested with a conospherical indenter using a range of loads between 0.05 and 10 mN. It is shown that the hardness of these materials depends on the surface finishing method and increases with decreasing contact depth, while the Young's modulus is relatively insensitive to contact depth. Furthermore, sample polycrystallinity leads to a large scattering of hardness values in Co-rich samples and of Young's modulus values in Ni-rich ones. The combined parametric ratio H/E, which can be related to the tribological behaviour of the material, was found to be higher in samples with Co content larger than 80 wt.%.     

Calibrating a nanoindenter for very shallow depth indentation using equivalent contact radius

Nanoindenter tips are usually modelled as axisymmetric cones, with calibration involving finding a fitting function that relates contact area to contact depth. For accurate calibration of shallow depth indentation, this is not ideal because it means that deeper indents tend to dominate the fitting function. For an axisymmetric object, it is always possible to define an equivalent contact radius (which, in the case, of nanoindentation is linearly related to the reduced modulus) and to obtain a fitting function that relates this equivalent contact radius to indentation depth. The equivalent contact radius approach is used here to provide shallow depth calibration of a nanoindenter tip at three separate times. The advantage of the equivalent contact radius methodology is that it provides a clearer physical interpretation of the changes in tip shape than a conventional area-based fit. We also show that the minimum depth for a reliable hardness measurement is obtainable and increases as the tip blunts with age but that consistent measurements of very near surface elastic moduli can be made if the blunting of the tip over time is fully accounted for in the tip area function calibration.     

Correlation between structural and mechanical properties of PbTiO3 thin films grown by pulsed-laser deposition

Tetragonal lead titanate (PbTiO₃, PT) thin films are grown on (1 0 0) MgO substrate by pulsed-laser deposition (PLD) for expected applications in integrated optics. The realisation of outstanding and reliable devices into integrated circuits requires sufficient mechanical resistance despite that the obtained PT films display interesting waveguiding properties associated with low optical losses. Two mechanical properties characteristic of elasticity and hardness of PT films are studied. The elastic modulus (E or Young's modulus) and the hardness (H) are measured by the nanoindentation technique. These mechanical properties are correlated to the crystalline quality of PT/MgO thin films. The films show epitaxial relationship with the MgO substrate and the orientation of crystallites perpendicularly to the surface substrate may be the consequence of a growth process along c-axis, a-axis or both. Differences on curves plotting hardness and elastic modulus as a function of indentation depth are observed as the curves are less dispersed for the films mainly c-axis oriented.     

Elasticity and nanomechanical response of Aspergillus niger spores using atomic force microscopy

The elasticity and nanomechanical response of Aspergillus niger spores determined using atomic force microscopy (AFM) and nanoindentation are discussed. The force-displacement curve of the spore surfaces shows that the average surface roughness of spores was approximately 33 nm and that the adhesion force ranged from 9 to 28 nN. The Young's modulus of the A. niger spores ranged from 0.1 to 21.4 GPa and the hardness ranged from 0.01 to 0.17 GPa. The critical buckling load of the spore membrane is 290 μN.     

衍射法测量金属丝的杨氏模量

本文提出了一种新的测量金属丝杨氏模量的方法--夫琅禾费单缝衍射法.平行光遇到由金属丝被拉伸形变而形成的单缝,产生衍射,用CCD光强分布测量仪接收衍射光信号,用示波器定标方法测量衍射角,可以精确计算金属丝的杨氏模量.最后与普通光杠杆放大法比较,对可能引入的误差来源做了分析,结果证明,本方法测量精度更高、相对误差更小.     

Elastic stress field beneath an arbitrary axisymmetric punch

The nanoindentation test is commonly used for the local determination of mechanical properties (hardness, elastic modulus, etc.) and also to study the initial stages of plasticity (dislocation nucleation, dislocation interaction mechanisms) at the nanometre scale. In the latter case, the determination of the elastic stress field beneath the indenter is of primary interest. An analytical expression is derived for the elastic stress and strain fields beneath an axisymmetric punch. Most solutions, in the literature, are given for simple indenter shapes, such as flat, conical or spherical indenters. The complete solution proposed for an arbitrary indenter profile is described by a power law, in which the exponent can be an integer or not. The stress is given as the real part of complex analytical expressions.     

Nanoindentation study of Cu52Zr37Ti8In3 bulk metallic glass

In this paper, Cu₅₂Zr₃₇Ti₈In₃ bulk metallic glass has been studied by nanoindentation. Three different ways for nanoindentation tests were employed. Load-control nanoindentation was used to investigate the effect of the loading rate on the hardness and Young’s modulus. Young’s modulus of the specimen shows a loading rate dependence. The constant-load indentation creep measurement was performed. The creep data were fitted with the generalized Kelvin model, and the compliance spectrum and retardation spectrum were derived. Furthermore, the storage compliance and loss compliance were also discussed. For characterizing the change of the hardness and Young’s modulus with increasing indentation depth, depth-control nanoindentation analysis was conducted. The results show that both the hardness and Young’s modulus decrease with nanoindentation depth.     

Ab?initio molecular dynamics model for density, elastic properties and short range order of Co-Fe-Ta-B metallic glass thin films

Density, elastic modulus and the pair distribution function of Co-Fe-Ta-B metallic glasses were obtained by ab initio molecular dynamics simulations and measured for sputtered thin films using x-ray reflectivity, nanoindentation and x-ray diffraction using high energy photons. The computationally obtained density of 8.19 g cm(-3) for Co(43)Fe(20)Ta(5.5)B(31.5) and 8.42 g cm(-3) for Co(45.5)Fe(24)Ta(6)B(24.5), as well as the Young's moduli of 273 and 251 GPa, respectively, are consistent with our experiments and literature data. These data, together with the good agreement between the theoretical and the experimental pair distribution functions, indicate that the model established here is useful to describe the density, elasticity and short range order of Co-Fe-Ta-B metallic glass thin films. Irrespective of the investigated variation in chemical composition, (Co, Fe)-B cluster formation and Co-Fe interactions are identified by density-of-states analysis. Strong bonds within the structural units and between the metallic species may give rise to the comparatively large stiffness.     

Effect of Young's modulus evolution on residual stress measurement of thermal barrier coatings by X-ray diffraction

Subjected to thermal cycling, the apparent Young's modulus of air plasma-sprayed (APS) 8 wt.% Y₂O₃-stabilized ZrO₂ (8YSZ) thermal barrier coatings (TBCs) was measured by nanoindentation. Owing to the effects of sintering and porous microstructure, the apparent Young's modulus follows a Weibull distribution and changes from 50 to 93 GPa with an increase of thermal cycling. The evolution of residual stresses in the top coating of an 8YSZ TBC system was determined by X-ray diffraction (XRD). The residual stresses derived from the XRD data are well consistent with that obtained by the Vickers indention. It is shown that the evolution of Young's modulus plays an important role in improving the measurement precision of residual stresses in TBCs by XRD.     

Nanoindentation stress–strain curves as a method for thin-film complete mechanical characterization: application to nanometric CrN/Cr multilayer coatings

The mechanical behavior of CrN/Cr multilayer coatings deposited by rf magnetron sputtering has been investigated by nanoindentation measurements performed with indenters of different geometries. Nanoindentation stress–strain curves generated from these measurements allow us to characterize the complete mechanical behavior of these coatings in the elastic, elastoplastic, plastic and fracture deformation regimes. In particular, indentation measurements carried out with a 100-m-radius spherical indenter allowed us to study the elastic deformation regime and estimate the yield stress parameter through the initial indentation yielding point. The elastoplastic deformation regime has been studied using a 5-m-radius spherical indenter and the stationary yielding regime (fully plastic regime) has been investigated with a pyramidal indenter of Berkovich geometry. The use of a pyramidal cube-corner indenter allowed us to study coating fracture characteristics. Nanometric CrN/Cr multilayer structures as well as single CrN and Cr coatings have been characterized. The study has shown that multilayered coatings with period thicknesses less than 46 nm present values of yield stress, Youngs modulus, hardness and toughness higher than those for single-layer CrN and Cr coatings. PACS 62.20.Dc; 62.20.Qp; 68.60.Bs     

Nanoindentation on carbon thin films obtained from a C60 ion beam

Raman spectra, atomic force microscope (AFM) images, hardness (H) and Young's modulus (E) measurements were carried out in order to characterize carbon thin films obtained from a C₆₀ ion beam on silicon substrates at different deposition energies (from 100 up to 500 eV). The mechanical properties were studied via the nanoindentation technique. It has been observed by Raman spectroscopy and AFM that the microstructure presents significant changes for films deposited at energies close to 300 eV. However, these remarkable changes have not been noticeable on the mechanical properties: apparently H and E increase with higher deposition energy up to ∼11 and ∼116 GPa, respectively. These values are underestimated if the influence of the film roughness is not taken into account.     

Results of a joint auger/esca round robin sponsored by astm committee E-42 on surface analysis. Part II. Auger results

We report the results of a round robin involving kinetic-energy (KE) and relative-intensity measurements on high-purity samples of copper and gold by Auger-electron spectroscopy. These results were obtained using 28 different instruments or analyzers manufactured by four companies. We found that the spread in reported KE values ranged from 7 eV at a KE of 60 eV to 32 eV at a KE of ～2025 eV. The total spread in reported intensity ratios ranged from a factor of ～38 for the ～6O eV and ～92O eV peaks of Cu to a factor of $\text{~}\text{?}$120 for the ～70 eV and ～2025 eV peaks of Au. We have analyzed the observed trends in some detail. The systematic error of kinetic-energy measurements increases with kinetic energy for many instruments. Even though all instruments were adjusted with the use of 2 keV elastically scattered electrons, the spread in the reported positions of the ～2025 eV Au peak indicates that the instruments were not adequately calibrated. Examples of erratic response were found in the measurements of relative intensities; it was believed, though not proved, that the more extreme values of intensity ratios were associated with instrumental malfunctions or operator mistakes. As in the similar ESCA round robin (Part I), the spread in reported Auger kinetic energies and relative intensities demonstrates clearly the need for standards (e.g., calibration methods, operating procedures, and data analysis) to ensure that data of known accuracy can be obtained routinely. Until suitable standards are available, interested individuals may find it useful to compare measurements using their own Auger or ESCA instruments with the group results and the trends found in the round-robin results.We have conducted an extensive round robin consisting of AES measurements on high-purity samples of Cu and Au. Participants were asked to measure the kinetic energies and relative intensifies of designated Auger peaks under specified conditions. This round robin was conducted contemporaneously with a similar ESCA round robin, the results of which have already been published [1].The AES round robin had three principal objectives. First, it was intended to assess the overall accuracy of KE and relative-intensity measurements in a relatively straightforward AES measurement. An earlier round robin [2] with catalyst samples demonstrated substantial spreads of reported data, and it was believed that comparisons of data obtained for cleaned metallic samples should give a more accurate picture of the current state-of-the-art. With a larger number of participants in the present round robin than in the catalyst round robin, we in fact find a comparable spread in the raw data. The spread in the reported KE measurements is a function of kinetic energy, and ranges from 7 eV at a KE of 60 eV to 32 eV at a KE of 2 keV. The imprecision of the KE measurements is typically ～1–3 eV. The total spread in the reported intensity ratios ranges from a factor of ～38 for Cu (at an incident energy of 3 keV) to a factor of ～120 for Au (at the same incident energy). The imprecision of the intensity ratios is typically less than 10%.Second, it was desired to determine the variation of AES intensities as a Cu sample was displaced along the analyzer axis with respect to its optimum position. Many of the participants found maxima in the intensities of the Cu ～60 eV and ～920 eV peaks at or very close to that sample position found to be optimum for the 2 keV elastic peak. Other participants found intensity maxima at sample positions up to ～4 mm away from the 2 keV elastic-peak position; these participants found that when the sample was at the optimum position for the 2 keV elastic peak, the intensities of the ～60 eV and ～920 eV Cu peaks could be as low as 50% of the corresponding maximum peak intensities that were found when the sample was displaced.Third, it was intended to measure the Auger KE for the “adventitious” carbon that forms on initially clean samples in the ambient vacuum of each instrument. Few participants made this measurement, although it was observed that the spread of reported energies for the carbon Auger peak was comparable to that found for the low-energy (60–7OeV) peaks in Cu and Au.We have examined the KE and relative-intensity data in some detail. We found it useful to compute deviations of individual KE measurements from the median values of the reported measurements for the selected Auger peaks. These deviations were plotted as a function of kinetic energy and lines were drawn connecting data points obtained using the same instrument. Such plots can be regarded as “error functions” or “calibration curves” based on the use of the group median values as reference data. These curves indicate that for many instruments the error of KE measurements increases approximately linearly with KE (unlike the behavior found in similar plots for binding-energy measurements in the ESCA round robin, in which the error was often nearly constant or slowly varying with binding energy). The large spread (32 eV) in the reported positions of the Au M₅N_6,7N_6,7 peak at a KE of ～2024 eV was considered particularly significant, since most instruments were adjusted and aligned using elastically scattered electrons at an energy of 2 keV. This observation clearly indicates that the working KE scales of the instruments were not adequately calibrated using the elastic-peak method; this problem is believed to be due to the insufficient accuracy of the 2 kV power supplies or the voltmeters used to display the KE scales rather than to any intrinsic deficiencies in the use of the elastic-peak method. There were several examples, however, in which plots of the intensities of the ～60 eV and ～920 eV Cu peaks as a function of sample position had maxima for both peaks at a position different from that found optimum for the 2 keV elastic peak. These observations indicate that sample alignment by the elastic-peak method was not done with sufficient accuracy in some laboratories. Finally, while the imprecision in the locations of the elastic peak and of Auger peaks in the round robin was typically 1–3 eV, the overall inaccuracy of the KE measurement was usually substantially larger.Most participants found that ratios of peak heights for the low-energy and high-energy transitions in Cu and Au decreased slowly as the incident electron energy was increased from 3 keV to 8 keV. Some participants, however, obtained qualitatively different dependences on incident energy; these results were attributed to mistakes, instrument malfunctions, or to inadequate alignment. Our experience in the ESCA round robin indicated that operator mistakes or instrumental problems were responsible for most of the outliers in comparisons of measured intensity ratios. We suspect (although we have not proved) that the more extreme values of peak-height ratios in the AES round robin have a similar origin. The AES intensity data were analyzed to search for mechanisms that could account for the large range of reported intensity ratios. We considered several possible origins for the more extreme data values. First, we examined the reported peak-height ratios for Cu and Au, to search for possible variations of the instrumental transmission functions from their “ideal” values. Second, we considered whether relatively large amounts of residual surface carbon could account for the observed intensity ratios. Third, we tested whether the instruments which exhibited “non-ideal” behavior (probably because of significant stray magnetic fields or inadequate sample alignment) when the samples were translated parallel to the analyzer axis were also the ones which gave the more extreme peak-height ratios. Fourth, we investigated whether probable variations in the amplitude of the modulation voltage applied to the analyzers would modify significantly the ratios of the observed intensities. Fifth, we considered the effects of the differing energy resolutions of the analyzers in the round robin. Finally, we considered effects due to variations in surface roughness caused by the different ion-sputtering conditions used for initial cleaning of the samples. None of these factors alone could account for the more extreme variations of the peak-height ratios, although it is possible that some of these factors could affect certain specific instruments while a different combination of the factors could be important in other cases. Although we were unable to demonstrate conclusively the nature of instrumental artifacts or possible operator mistakes in the various intensity measurements, we believe that the spread in the reported intensity ratios is associated with specific measurement problems in particular individual laboratories. A variety of factors have been identified here to account for the more modest but nevertheless distressing range of intensity ratios (a factor of ～2 for Cu and of ～5 for Au) for the majority of the participants.The spreads in the energies and relative intensities of Auger and photoelectron peaks in this AES and the previous ESCA round robin indicate clearly that improved calibration and operating procedures are required for both Auger and ESCA measurements. Published data, for example, are of little value unless credible statements of accuracy can be associated with the numerical results. We hope that the standards needed for improved measurements can be developed by the ASTM Committee E-42 on Surface Analysis together with other interested parties.     

蛋白饲料原料粗蛋白含量近红外光谱模型转移研究

建立良好的蛋白饲料近红外光谱定量分析模型及实现在不同仪器间的模型共享,能极大提高模型的利用效率,满足饲料行业快速发展的需要。针对蛋白饲料原料粗蛋白含量近红外分析模型适用性问题,首次采用光谱差值转移、直接校正和分段直接校正法进行了三台不同类型的近红外光谱仪之间的模型转移研究。实验样品为四种蛋白饲料原料：玉米蛋白粉、菜粕、酒糟和鱼粉。实验仪器包括MATRIX-Ⅰ傅里叶变换型近红外光谱仪(主仪器),Spectrum 400傅里叶变换型近红外光谱仪(从仪器1)和SupNIR-2750光栅扫描型近红外光谱仪(从仪器2)。研究表明,同一样品体系在主仪器和从仪器2上所得光谱数据的差异性相对较小,且均与从仪器1所得光谱数据的差异性相对较大。除分段直接校正法对玉米蛋白粉从仪器2的预测结果无促进作用之外,其他模型的预测均方根误差和系统偏差均明显低于转移前。玉米蛋白粉、菜粕和酒糟样品采用三种方法转移后的模型预测相对分析误差(RPD)均大于3.0,预测效果良好。鱼粉样品模型转移后的预测RPD均大于2.5,预测效果较好。三种方法对于蛋白饲料原料不同仪器间的光谱差异进行了有效校正。该研究结果对于蛋白饲料品质近红外快速分析模型的广泛应用具有重要意义。     

Study of porosity and stiffness measurement of nanoporous aerogel films with wideband ultrasonic surface waves

Nanoporous aerogel films are of great scientific and commercial interest because of their outstanding potential for application to microelectronic interconnect, associated with low dielectric constant. Of the parameters which describe such aerogel films, density/porosity and stiffness are two of the most critical, but are difficult to measure. This paper shows how measurement of the dispersion of laser-generated surface acoustic wavepackets travelling on submicron-thick aerogel films on silicon substrates allows the density and Young's modulus to be extracted reliably. Particular attention is paid to accuracy of measurements and sensitivity to input data; and a method for extracting measurements on very thin films (<300 nm) is presented.