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The piezoelectric properties of wurtzite aluminium nitride (w-AlN) are enhanced by alloying with scandium (Sc), thus offering superior properties for applications in micro electro-mechanical systems devices. ScxAl1-xN thin films have been prepared by DC reactive magnetron sputtering on Si (100) substrates from a single target. When targeting a concentration range from x=0 up to x=0.15, the preparation conditions have been optimized by varying the Ar/N2 ratio in the sputtering gas. To incorporate an increasing Sc concentration, a higher Ar/N2 ratio has to be applied during the deposition process. Hence, the argon concentration in the sputtering gas becomes a crucial parameter for microstructure-related parameters. To determine phase purity, degree of c-axis orientation, lattice parameter, and grain size, the ScxAl1-xN thin films were investigated by techniques, such as scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. [ABSTRACT FROM AUTHOR]

In this paper, the electric field induced deformations of sputter-deposited piezoelectric aluminum nitride thin films sandwiched between electrodes on top of a silicon substrate are studied by numerical calculations and scanning laser interferometric measurements. In our calculations based on the finite element method, the results show the displacement of the top and bottom surfaces of both the thin film and the substrate, for either a free or a perfectly clamped structure. The confirmation that the bottom surface of the film is deformed reveals the limitations of techniques that only access the top surface, as well as the double-beam interferometric configuration, under specific conditions. In addition, the simulations demonstrate the dependence of the displacements on the size of the upper electrode and the contribution of the transverse piezoelectric coefficient d31 to the features of the displacement profiles. A laser scanning vibrometry technique was used to measure deformations on the top surface with subpicometer vertical resolution. By comparing the calculated and the experimental displacement profiles, an advanced approach is discussed to obtain accurate quantitative information of both coefficients d31 and d33. [ABSTRACT FROM AUTHOR]

Piezoelectric energy harvesting offers the possibility to make use of ambient vibrations most beneficially to feed low power sensor nodes. This paper demonstrates the fabrication and characterization of piezoelectric cantilevers with AlN and ScxAl1–xN (x = 27%) thin films for the evaluations of their energy harvesting performance. The characterization mainly focuses on the measurement of the output power at variable load resistance to achieve maximum power output. Superior properties of ScAlN thin films for energy harvesting compared with AlN films are confirmed. Furthermore, an analytical model is employed to extract material parameters for ScAlN. High piezoelectric coefficients \texte31=1.6 C/ \textm^2 and \textd31~=6.9 pm/V are determined for ScAlN. Finally, a proof mass is attached to further increase the output power at optimized load resistance. [2016-0145] [ABSTRACT FROM AUTHOR]

Scandium Aluminum Nitride thin films (ScAlN) are attracting more and more attention for micro-electromechanical systems (MEMS) because of significantly increased piezoelectric constants compared to pure AlN. This work provides a comprehensive study of thermal annealing effects on ScAlN (x = 27 %) films synthesized via DC magnetron sputter deposition at nominally unheated Silicon and Sapphire substrates. Compared to the 'as deposited' state increasing c-axis orientation and crystalline quality upon annealing up to 1000 °C of films with mixed crystallographic orientation is observed via X-ray diffraction and transmission electron microscopy based analyses. Also the piezoelectric coefficient d of ScAlN on Si shows increasing values at enhanced annealing temperatures. However, the improved piezoelectric properties are accompanied by both increased leakage currents and loss tangent values. [ABSTRACT FROM AUTHOR]

This paper investigates the performance of surface acoustic wave (SAW) devices consisting of reactively sputter deposited scandium doped aluminum nitride (ScxAl1-xN) thin films as piezoelectric layers on sapphire substrates for wireless sensor or for RF-MEMS applications. To investigate the influence of piezoelectric film thickness on the device properties, samples with thickness ranging from 500 nm up to 3000 nm are fabricated. S21 measurements and simulations demonstrate that the phase velocity is predominantly influenced by the mass density of the electrode material rather than by the thickness of the piezoelectric film. Additionally, the wave propagation direction is varied by rotating the interdigital transducer structures with respect to the crystal orientation of the substrate. The phase velocity is about 2.5% higher for a-direction compared to m-direction of the sapphire substrate, which is in excellent agreement with the difference in the anisotropic Young's modulus of the substrate corresponding to these directions. [ABSTRACT FROM AUTHOR]

In this work the influence of the film thickness and titanium as an adhesion promoter on the gauge factor of Platinum thin films used as strain gauges is investigated for different temperatures. Therefore strain gauges with varying bi-layer thickness (Ti/Pt: 50/1000 nm and 20/100 nm) are fabricated and evaluated at a custom-built gauge factor measuring setup. Up to 250°C no substantial difference for the two bi-layers can be found. Latest at 300°C the diffusion of titanium starts to heavily influence predominantly the 20/100 nm Ti/Pt bi-layer system. Moreover a different temperature dependent behavior of the gauge factor under tensile and compressive load is demonstrated. [ABSTRACT FROM AUTHOR]

Due to economic and environmental requirements there is a strong need both to increase the efficiency and to monitor the actual status of gas turbines, rocket engines and deep drilling systems. For these applications, micromachined pressure sensors based on a robust substrate material (e.g. sapphire) as well as strain gauges made of platinum for long-term stable operation are regarded as most promising to withstand harsh environments such as high temperature levels, aggressive media and/or high pressure loads. For pre-evaluation purposes, a novel, custom-built measurement set-up is presented allowing the determination of electro-mechanical thin film properties up to 850 °C. Key components of the measurement set-up are the one-sided clamped beam made of Al2O3 ceramics which is deflected by a quartz rod and a high precision encoder-controlled dc motor to drive the quartz rod. The specific arrangement of the infrared halogen heaters in combination with the gold coated quartz half shells ensures a high degree of temperature homogeneity along the beam axis. When exposed to tensile as well as compressive load conditions, the corresponding gauge factor values of 1 µm thick platinum thin films show a good comparison at room temperature and in the temperature range from 600 up to 850 °C where the effects originating from grain boundaries or from the film surfaces are negligible. Between 150 and 600 °C, however, a strong deviation in the gauge factor determination depending on the mechanical load condition is observed, which is attributed to the gliding of adjacent grains. [ABSTRACT FROM AUTHOR]

Doping of wurtzite aluminium nitride (AlN) with scandium (Sc) significantly enhances the piezoelectric properties of AlN. ScxAl1-xN thin films with different Sc concentrations (x = 0 to 0.15) were deposited by DC reactive magnetron sputtering. Infrared (IR) absorbance spectroscopy was applied to investigate the Sc concentration dependent shift of the IR active modes E1(TO) and A1(TO). These results are compared to ab initio simulations, being in excellent agreement with the experimental findings. In addition, IR spectroscopy is established as an economical and fast method to distinguish between thin films with a high degree of c-axis orientation and those exhibiting mixed orientations. [ABSTRACT FROM AUTHOR]

Silver (Ag) and copper (Cu) are regarded as advanced material for metallization systems in microelectronic devices because of their high electrical conductivity and enhanced electromigration resistance. Typically, organic circuit boards as well as ceramic and glass-ceramic substrates use galvanic deposited Cu films or screen-printed metallization for this purpose. When applying the latter approach, however, the lateral resolution in the μm-region being required e.g. for novel high frequency applications can not be guaranteed. Hence, sputter deposition is envisaged for the realization of thin film metallization systems. The reliability of 300 nm thick Cu and Ag thin films is comparatively investigated under accelerated aging conditions, utilizing a test structure which consists of parallel lines stressed with current densities up to 2.5 × 10A cm² at temperatures up to 300°C on Si/SiO, glass, LTCC (low temperature co-fired ceramics) and alumina substrates. To detect the degradation via the temporal characteristics of the current signal a constant voltage is applied according to the overall resistance of the test structure. Knowing the mean time to failure (MTF) and the activation energy at elevated temperatures conclusions on the migration mechanism can be drawn. Whereas on LTCC substrates the activation energy E is about 0.75 eV for both Ag and Cu thin films, the higher activation energies of about E ~ 1 eV measured for Cu on glass and alumina indicate a suppression of back diffusion especially at enhanced temperature levels. This effect is predominantly caused by a stable oxide layer which is formed at high temperatures and which acts as passivation layer. Therefore, the overall electromigration resistance is lower compared to Ag. [ABSTRACT FROM AUTHOR]

The purpose of this work is to study the performance of resonantly driven microcantilevers excited with a piezoelectric aluminium nitride thin film as a mass sensor for the detection of interactions in the field of immunosensing. Two cantilevers with different width (i.e. 300 and 200 μm), but constant length and thickness of 300 and 20 μm were considered. The figures of merit of these structures, such as the mass sensitivity and the limit of mass detection, were determined. According to the results obtained, a cantilever being 300 × 200 μm in size and a high frequency mode was the best combination to detect antibody/antigen interactions. The previous combination was applied to detect the affinity between rabbit immunoglobulin G (IgG) and the complementary anti-rabbit IgG produced in goat. The binding was registered as a shift of the resonance frequency of the mode under investigation and the correlation between the added mass and the measured shift is analysed. [ABSTRACT FROM AUTHOR]

In this study, the influence of post-deposition annealings (PDA) up to temperatures of TPDA=700°C on the room-temperature resistivity of e-beam evaporated titanium/platinum (Ti/Pt) bi-layers on low temperature co-fired (LTCC) substrates covered with a glass encapsulate is investigated. The thickness of the platinum top layer is varied between 24 and 95 nm (titanium film thickness: 5 nm) and between 23 and 90 nm (titanium film thickness: 15 nm), respectively. In the “as-deposited” state and up to post-deposition annealing temperatures of TPDA=450°C, the film resistivity is linearly correlated with the reciprocal value of the platinum film thickness according to the size effect. When applying, however, solely the Fuchs-Sondheimer model for evaluation, the effective mean free path for electrons is substantially above the value reported for crystalline platinum at room temperature. Compared to similar investigations on smooth Si/SiO2 substrates yielding interpretable results within this theoretical approach, this is due to the increase of the thickness-dependent fraction in film resistivity which is strongly affected by the enhanced LTCC/glass surface roughness. At TPDA>600°C, diffusion of titanium into the platinum top layer and the roughening of the LTCC/glass substrate dominate the electrical behavior, both causing an increase in film resistivity above average. In contrast to Si/SiO2 substrates, thermal induced grooving effects in the Pt top layer play a minor role as the temperature coefficients of expansion of metallization and glass-ceramic substrate match better and the effective temperature difference for stress generation is lower due a glass softening temperature of about 450°C. [ABSTRACT FROM AUTHOR]

Abstract: In this work, the influence of sputter conditions and film thickness on both the morphology and the resistivity of tantalum (Ta) thin films will be investigated. Glass wafers were used as substrates. With increasing back pressure level ranging from 0.3 to 6 Pa at a fixed plasma power of 100 W, the microstructure changes from a dense to an open porous characteristics. At a moderate back pressure level of 0.9 Pa, the stable α-phase is detected when the film thickness exceeds a value of about 1.3 µm. Besides the impact of phase composition, the microstructure influences in addition the electrical performance of the Ta films. Therefore, the room-temperature resistivity can be varied within two orders of magnitude by applying different parameters for film deposition. [Copyright &y& Elsevier]

In this study, the authors investigate the influence of thermal annealing up to temperatures of 700 °C on the room-temperature resistivity of electron-beam-evaporated titanium/platinum thin films. Oxidized silicon wafers are used as the substrate. The titanium has a fixed thickness of 5 nm and serves as an adhesion layer. The thickness of the platinum top is varied between 21 and 97 nm. Up to annealing temperatures of 450 °C, the film resistivity of the bilayer system is a linear correlation with the reciprocal platinum film thickness, as expected from the size effect. In this annealing regime, the change in intrinsic film stress dominates the electrical behavior. At annealing temperatures of 600 °C and above, however, the diffusion of titanium into the top layer leads to an enhanced increase in film resistivity, especially at low platinum thicknesses. The onset of plastic deformation of the platinum layer causes an additional effect that decreases the electrical conductivity above average. [ABSTRACT FROM AUTHOR]

Abstract: Due to their chemical inertness, hydrogenated amorphous silicon carbide (a-SiC:H) thin films exhibit excellent properties as passivation layers for micro-sensors and micro-actuators operating in harsh environmental applications. They are typically synthesized with a plasma-enhanced chemical vapor deposition process at moderate temperatures, providing an excellent backend compatibility. To demonstrate their suitability and their signal stability for hot film anemometers in high pressure loaded automotive applications, molybdenum and titanium/platinum micro-heaters without and with a-SiC:H passivation were operated in an oil medium for up to 380h at nominal thin film temperatures of 60 and 160°C. As expected, the drift of the electrical resistance increased with rising temperature. In the unpassivated case, the molybdenum metallization exhibited an increased long-term stability when operated as the flow sensitive thin film. A protection of the platinum based thin film element with a 1μm thick a-SiC:H layer reduced the drift in sensor resistance by more than one order of magnitude compared to a bare hot film probe. As a result of the tensile stress in the metallic thin film and a compressive stress in the a-SiC:H coating, a 30h pre-aging is recommended in order to stabilize the micro-heater structure. At a high-pressure hydraulic test bench, an a-SiC:H/Pt/Ti test structure on a low temperature co-fired ceramics substrate was exposed to the dynamic pressure fluctuations with a maximum value of 135MPa (1350bar) for 1h. No cracks or delaminations were found in the a-SiC:H surface passivation layer, as determined by optical inspection and scanning electron microscopy. [Copyright &y& Elsevier]

Abstract: In this study, the influence of substrate properties (e.g. roughness characteristics and chemical composition) on the electrical resistivity of evaporated molybdenum thin films is investigated as a function of varying parameters, such as film thickness (25–115 nm) and post-deposition annealing with temperatures up to T PDA =900 °C. A thermally oxidized silicon wafer with very low surface roughness was used as one substrate type. In contrast, a low temperature co-fired ceramics substrate with a glass encapsulant printed in thick film technology is the representative for rough surface morphology. The electrical resistivity follows the prediction of the size effect up to T PDA =600 °C independent of substrate nature. On the silicon-based substrate, the thickness-independent portion of the film resistivity ρ g in the “as deposited” state is about 29 times higher than the corresponding bulk value for a mono-crystalline sample. Thin films of this refractory metal on the SiO2/Si substrate exhibit an average grain size of 4.9 nm and a negative temperature coefficient of resistivity (TCR). On the glass/ceramic-based substrate, however, ρ g is half the value as compared to that obtained on the SiO2/Si substrate and the TCR is positive. [Copyright &y& Elsevier]

Aluminum nitride (AlN) is a material of significant importance in the field of micro electro-mechanical systems (MEMS) due to its piezoelectric properties and its use as passivation layer. This work provides a comprehensive study on the film thickness dependence of the temperature activated leakage current behavior of sputtered aluminum nitride thin films. The thickness ranges from 40 to 400 nm, providing insight into the electrical characteristic of AlN thin films for thickness values typically used in MEMS devices. The leakage current shows a highly symmetrical behavior for both positive and negative bias directions due to a tailored silicon substrate pre-treatment process. At low electric field strengths E ≤ 0.1 MV/cm, the leakage current is dominated by ohmic behavior, while for E ≥ 0.3 MV/cm, the leakage current is controlled by a Poole–Frenkel mechanism. Both the defect-related barrier height and the defect density for the Poole–Frenkel regime can be extracted from arranging the data in an Arrhenius configuration. The barrier height shows no significant influence of film thickness. However, the defect density correlates directly with the leakage current level of the film and thus dominates the electrical behavior. The defect density increases significantly with increasing film thickness. As the thin films were deposited under nominally unheated substrate conditions the substrate temperature increases with increasing sputter time. A simple model is proposed in order to provide a qualitative understanding of the observed effect by assuming a substrate temperature driven redistribution of defect states within the band gap of aluminum nitride thin films. [ABSTRACT FROM AUTHOR]

[Display omitted] • Sputter deposited piezoelectric Y 0.09 Al 0.91 N compound thin films with improved d 33. • Investigation of the sputter parameters power, pressure and gas composition. • Detailed investigation of the microstructure using SEM, XRD, HRTEM, SAED and EDX. Alloying aluminum nitride thin films with elements, such as scandium, is an established approach to increase piezoelectric coefficients, what is a strong request for future micro- and nano-machined, piezoelectric sensor and actuator applications. As an alternative to scandium, Y 0.09 Al 0.91 N thin films were reactively synthesized on (1 0 0) silicon substrates from an alloy target consisting of 15 at% yttrium and 85 at% aluminum to demonstrate the increase in piezoelectric coefficient d 33 . For this purpose, sputter parameters such as plasma power, gas composition and sputter pressure, have been varied to achieve a highly c-axis oriented crystalline microstructure. X-ray diffraction, transmission electron microscopy and energy dispersive X-ray analyses were performed to determine the degree of crystallinity and to analyze the elemental composition. Under optimized sputter conditions, a d 33 value of 7.79 pm/V is measured at Y 0.09 Al 0.91 N being in excellent agreement with density functional theory calculations predicting at this composition a value of 6.9 pm/V. [ABSTRACT FROM AUTHOR]

Abstract: Silver (Ag) is regarded as advanced material for metallization purposes in microelectronic devices because of its high conductivity and its enhanced electromigration resistance. Besides the typical use of silicon based substrate materials for device fabrication, thin film metallization on ceramic and glass–ceramic LTCC (low temperature co-fired ceramics) substrates gets more and more into focus as only thin film technology can provide the required lateral resolutions of structures in the μm-range needed for e.g. high frequency applications. Therefore, the reliability of Ag thin films is investigated under accelerated aging conditions, utilizing test structure which consists of 5 parallel lines stressed with current densities up to 1.5×107 Acm−2 at temperatures ranging from room-temperature up to 300°C. To detect the degradation via the temporal characteristics of the current signal a constant voltage is applied taking the overall resistance of the test structure into account. The mean time to failure of the Ag metallization substantially depends on the degree of (111)-orientation which, in turn, is strongly affected by the plasma power P P during deposition. Therefore, Ag thin films deposited at P P =1000W feature a 7 times higher reliability than those deposited at P P =100W. Due to the enhanced stability of grains being (111)-oriented in textured thin films the material transport predominantly occurs along grain boundaries, whereas in Ag films without a (111)-orientation volume-related diffusion effects dominate due to the lower stability of these grains. [Copyright &y& Elsevier]

Abstract: Aluminum oxide (Al2O3) thin films are synthesized by reactive d.c. magnetron sputter deposition on silicon substrates. The impact of varying plasma power P p (i.e. 400 to 1000 W) and of thin film temperatures T up to 540 °C on the electrical performance are evaluated, as these dielectric layers with a thickness of 450 nm are targeted as potential candidates for high temperature sensor applications. From 150 °C to 500 °C, the current–voltage measurements show a leakage current behavior according to the Poole–Frenkel electron emission with an activation energy of 1.16 eV. At T >500 °C, the conductivity increases above average, in respect to the extrapolated Poole–Frenkel behavior at T <500 °C, most probably due to the migration of charged ions, such as Ar+, incorporated into the film during deposition. Basically, samples synthesized at higher plasma levels show an enhanced electrical insulation behavior. This result is supported by measurements applying optical ellipsometry as well as by the determination of the wet chemical etching behavior in phosphoric-based acid at different bath temperatures. At higher plasma power, the refractive index shows a slight tendency to increase, staying, however, below the value of single-crystalline Al2O3. In contrast, the etch rate decreases by a factor of 1.5 at samples deposited at 1000 W when lowering the temperature of the etchant from 90 °C to 60 °C. These results indicate an enhanced film density at higher P p values as the microstructure of the Al2O3 films is X-ray amorphous independent of plasma power and post-deposition annealing temperatures up to 650 °C. [Copyright &y& Elsevier]

The flexoelectric effect describes the electromechanical coupling of a strain gradient to a polarization and vice versa. This effect scales linearly with permittivity and strain gradients can get very high for dimensions on the micro and nanoscale. Even though the flexoelectric effect can be best exploited within micro or nanoelectromechanical systems (M/NEMS) applications, it has not been established in today's M/NEMS device architectures as other transducer principles, like piezoelectricity. In this work, values of the converse flexoelectric coefficient for one of the most promising flexoelectric materials, titanium dioxide (TiO 2) are provided. The experimental results are based on a carefull characterization of IrO 2 /TiO 2 /IrO 2 cantilevers. Besides CMOS compatiblity TiO 2 is selected as functional thin film material as it offers a very high permittivity and shows no hysteresis or saturation effects as it is neither ferro- nor paraelectric. Additionally, it guarantees a low cost, lead-free realization and can be directly integrated in a standard silicon MEMS fabrication process by sputter deposition. In order to correctly determine the flexoelectric coefficient, other electromechanical coupling effects are considered and assessed. The flexoelectric coefficient is shown to be µ eff = 1.78 ± 0.16 nC m−1 at 10 kHz. The flexoelectric coupling constant with a value of 2.75 V is in good agreement with that theoretically predicted by Kogan's estimate of 3.14 V. Image, graphical abstract [ABSTRACT FROM AUTHOR]

• The electro-mechanical properties of a new composite material are investigated. • The influence of different annealing steps on the TCR of AlN/Pt multilayer is determined. • The impact of the post deposition annealing on the microstructure and chemical composition is studied using different measurement methods like TEM, EDX and XRD. • The gauge factor of a 100 nm thin AlN/Pt multilayer is presented up to 500 °C. • Compared to pure Pt a significant decrease in the TCR is achieved while obtaining a similar gauge factor. In this study, the electro-mechanical properties of multilayered thin films consisting of 10 bi-layers of 7 nm aluminum nitride (AlN) and 3 nm platinum (Pt) are investigated in the as deposited state and after different post deposition annealing steps. The multilayers are deposited using direct current magnetron sputtering on thermally oxidized silicon wafers or sapphire substrates and are annealed in Ar atmosphere at 800, 900 and 1000 °C up to 24 h. The electro-mechanical properties are characterized from room temperature up to 500 °C using Van-der-Pauw as well as gauge factor measurements. Furthermore, transmission electron microscopy and energy dispersive X-ray analyses are used to investigate the microstructure and the chemical composition of the multilayers before and after thermal loading. The influence of the annealing on the crystalline structure is examined by X-ray diffraction analyses. Annealing in this high temperature range causes an intermixture of the individual Pt and AlN sub-layers as well as a recrystallization of the Pt thin films. Annealing the multilayered thin film system at 900 °C for 1 h in Argon atmosphere results in a multilayer which is electrically stable up to 500 °C in air and which exhibits a 3 times lower temperature coefficient of resistance at a similar gauge factor when compared to pure Pt thin films. [ABSTRACT FROM AUTHOR]

Highlights • The gauge factor of differently fabricated Pt thin films is investigated up to 500 °C. • The influence of the adhesion promoter on the electro-mechanical properties is estimated. • The microstructure and chemical composition are studied using TEM and EDX measurements. • The results are in accordance with the Tellier-Tosser-Pichard model. Abstract In this study the influence of film thickness, adhesion promoters such as chromium (Cr) and rutile titanium dioxide (TiO 2), sputtering power and substrate temperature on the electro-mechanical properties of platinum (Pt) thin films is investigated. The bilayered samples consisting of an adhesion promoter and a Pt thin film are deposited using direct current magnetron sputtering on sapphire substrates und characterized by Van-der-Pauw as well as gauge factor measurements up to 500 °C in air. Additionally, transmission electron microscopy and energy dispersive X-ray spectroscopy analysis are performed to investigate the microstructure and the chemical composition of the thin films before and after temperature loading. No substantial impact of the Pt film thickness on the electro-mechanical properties is determined. The diffusion of Cr into the Pt thin film at elevated temperatures leads to a lower gauge factor with a higher scatter range. Furthermore, higher sputtering power and substrate heating during deposition lead to a higher crystallinity of the Pt thin film and thus promote higher gauge factors. [ABSTRACT FROM AUTHOR]

In the field of MEMS, aluminum nitride (AlN) in the wurtzite phase is an important functional material due to its piezoelectricity. To further enhance device performance, transition metal alloying of AlN is proposed to achieve a higher piezoelectric response. In this work, the effect of moderate chromium (Cr) doping (up to x = (5.0 ± 0.6) at.%) as well as the influence of changing deposition pressures and gas composition are investigated, focusing on the microstructure, morphology and piezoelectric response of sputter deposited Cr x Al 1 −x N thin films. X-ray diffraction analyses reveal a wurtzite type structure with highest degree in c-axis orientation for lowest deposition pressures and Cr concentrations. With increasing deposition pressure, a deterioration in crystal quality and the formation of other crystal orientations within the films are observed. The presence of crystallite orientations away from the 〈002〉 orientation is linked to the presence of randomly oriented, conical-shaped grains which are embedded in the fine grainy thin film surface topography as indicated by atomic force microscopy. By using piezo force microscopy, these grains are identified to cause a reduction in the effective piezoelectric response. Furthermore, we show that combining both the analysis of orientation distribution and surface morphology, is by far more reliable for estimating the piezoelectric response of thin films than the sole use of one method. [ABSTRACT FROM AUTHOR]

For an optimized performance of micro electromechanical systems (MEMS) double-clamped bridge-type resonators for mechanical strain sensing, a modified sputter process was developed which exploits the influence of varying sputter pressure during deposition on the intrinsic stress component of functional thin films. In detail, four different, polycrystalline aluminum nitride layers, synthesized with different sputter parameter sets were characterized related to their microstructure with techniques, such as X-ray diffraction, scanning electron microscopy and atomic force microscopy, respectively. Furthermore, the intrinsic thin film stress and the longitudinal piezoelectric coefficient were evaluated. The best performing layers were integrated in the fabrication process of two MEMS resonant strain sensor devices to study the stress-related impact on the resonant device performance. The initial static buckling of the sensor devices was studied by white light interferometric measurements, whereas the frequency response as a function of externally applied as well as intrinsic strain was analyzed by laser Doppler vibrometry. The behavior of the sensor devices was compared to theoretical predications and the influence of intrinsic thin film stress on the resonance frequency as a function of strain was studied. With a precise tailoring of the intrinsic film stress, a responsivity of ∼17000 is measured, representing an improvement by a factor of ∼5 compared to state-of-the-art resonant strain sensors. [Display omitted] • Stress tailoring of aluminum nitride through improved process methodology of sputter deposition. • Improved thin films show better microstructure, lower compressive stresses while maintaining high piezoelectric coefficients. • Stress engineering can increase responsivity of resonant microbridges used for strain sensing. • Stress tailored MEMS strain sensor fit better to theoretical model than their higher stressed counterpart. [ABSTRACT FROM AUTHOR]

Amorphous silicon nitride SiN x thin films in a thickness range of 40 to 500 nm were deposited onto (100) silicon wafers using DC magnetron sputtering. The biaxial stress of the films was found to be tuneable in the range of − 1300 MPa (compressive) to almost 0 by varying the plasma power density and the back pressure in the deposition chamber. The films were close to stoichiometric composition with x ranging from 1.27 to 1.34. The refractive index n decreases from 2.10 to 1.97 with increasing back pressure, indicating compositional changes in the thin films. This finding, however, was not confirmed by Fourier-Transform Infrared Spectroscopy measurements. On the other hand, lower wet chemical etch rates revealed a larger robustness of layers deposited at conditions where the mean kinetic energy of the sputtered particles is higher. Temperature dependent leakage current measurements using Au/Cr/SiN x /Si MIS (metal–insulator–semiconductor) structures between 25 and 300 °C have shown that ohmic and Poole–Frenkel conduction mechanisms dominate the leakage current behaviour at electrical fields ranging up to 0.5 MV/cm. The extracted physical parameters such as the corresponding activation energies were found to be mildly affected by the deposition parameters. [ABSTRACT FROM AUTHOR]

Piezoelectric scandium aluminium nitride (Sc x Al 1− x N) thin films offer a large potential for the application in micro electromechanical systems, as advantageous properties of pure AlN thin films are maintained, but combined with an increased piezoelectric actuation and sensing potential. Sc x Al 1− x N thin films with x = 27% have been prepared by DC reactive magnetron sputtering to find optimized deposition parameters to maximize the piezoelectric constants d 33 and d 31 . For the accurate and simultaneous measurement of these constants Laser Doppler Vibrometry has been applied and compared to finite element (FEM) simulations. The electrode design has been optimized to rotational symmetric structures enabling a 180° phase shifted excitation, so that a straight-forward comparison of experimental displacement curves with those obtained from FEM is feasible. [ABSTRACT FROM AUTHOR]

Highlights: [•] Amorphous hydrogenated silicon nitride thin films were deposited using the ICP-CVD technique. [•] Deposition rates range from 5.4 to 26.7nm/min. [•] Samples deposited with different N2/SiH4 ratios show substantial differences in mechanical, chemical and microstructural properties. [•] Drift behavior of the film stress is strongly related to the deposition parameters. [ABSTRACT FROM AUTHOR]

Abstract: Aluminum nitride (AlN) reactively sputter deposited from an aluminum target is an interesting compound material due to its CMOS compatible fabrication process and its piezoelectric properties. To obtain high piezoelectric coefficients it is a necessary pre-request to synthesize films with c-axis orientation. Besides the influence of sputter conditions on the microstructure of AlN thin films the condition of the substrate surface is another important factor of utmost importance. In this study, the influence of 350nm thick titanium metallization DC sputter-deposited on SiO2/Si substrates at varying back pressure levels b p,Ti in the range of 2×10−3 to 14×10−3 mbar on the c-axis orientation and the piezoelectric coefficients of 600nm thick AlN thin films is investigated. Besides the plasma power for Ti deposition (P p,Ti =100W) the parameters for AlN synthetization are fixed to P p =1000W and b p,AlN =4×10−3 mbar in 100% N2 atmosphere. Basically, the surface roughness of the Ti bottom layer is the dominating factor resulting either in a high degree of c-axis orientation (i.e. at low b p,Ti values) or in an amorphous AlN microstructure (i.e. at high b p,Ti values). Under low pressure conditions, a smooth and dense surface characteristics is achieved due to a higher kinetic energy associated with the adatoms what is especially important at nominally unheated substrate conditions. The piezoelectric coefficient d 33 decreases from 2.55 to 1.7pm −1 when increasing the titanium sputter pressure from 2×10−3 to 14×10−3 mbar. When decreasing the Ti film thickness to 60nm and hence, reducing the root mean square roughness by a factor of about 2, the intensity associated with the AlN (002) peak is increased by a factor of about 1.7 demonstrating the direct impact. Furthermore, the highest values for d 33 and d 31 (i.e. 3.15pmV−1 and −1.28pmV−1) are determined. [Copyright &y& Elsevier]

Abstract: In this study film properties of sputter-deposited tantalum nitride (TaN x ) thin layers are investigated focusing on the impact of substrate properties, varying nitrogen content for film synthetization as well as post-deposition annealings in the temperature range up to 500°C. For comparison, these investigations are done on low temperature co-fired ceramics and on silicon based substrates whereas the latter approach ensures defined and well-known surface properties. Furthermore, results on the phase evolution with high temperature annealings are presented showing a transformation of Ta4N to Ta2N in the temperature range between 350°C and 500°C. With increasing nitrogen content (i.e. nitrogen flow during film deposition) in the TaN x layers the topography shows first an increase in surface roughness, next a range where a smoothing of the surface characteristics is observed, and finally buckling and the existence of grain agglomerates. All these analyses are further evaluated with electrical measurements on the film resistivity and on the oxidation behaviour to gain deeper insight into material parameters relevant for micromachined devices which are operated under harsh environmental conditions. [Copyright &y& Elsevier]

• Very sharp and clean interfaces between polycrystalline Si and 4H-SiC are produced. • High boron concentrations lead to a larger grain size. • Polycrystalline silicon with very low surface roughness is produced. • Preferred silicon orientation depends on film thickness. The crystallization behavior of sputter-deposited amorphous silicon (a-Si) films on 4H-SiC by post-deposition annealing (PDA) is investigated. Film thicknesses between 100 and 1500 nm were selected. PDA was carried out at temperatures between 800 and 1100 °C. In addition, the influence of the annealing time and the heating rate is evaluated. Furthermore, the impact of high boron doping and the face of the SiC substrate was determined. A low-pressure chemical vapor deposition (LPCVD) Si film was deposited on the same substrate for reasons of comparison. The crystallinity of the Si films was investigated using X-ray diffractometry and transmission electron microscopy, whereas atomic force microscopy was applied to determine the surface roughness. The amorphous phase of the as-deposited Si thin films was crystallized into a polycrystalline microstructure applying different PDA conditions. A sharp and distinct interface to the 4H-SiC substrate without the presence of structural defects, amorphous regions or any indication for alloy formation was observed. No influence of the underlying SiC to stimulate the texture in the Si grain orientation could be found. Different PDA parameters showed an influence on the average grain size and the stress inside the grains. Highly boron-doped samples exhibit mean grain size values up to several hundreds of nanometers. Although starting with a poor film quality represented by an amorphous microstructure due to sputter deposition at low substrate temperatures, polycrystalline Si layers on 4H-SiC substrates are realized exhibiting a more than twenty times lower surface roughness compared to those synthesized with LPCVD. [ABSTRACT FROM AUTHOR]

• The influence of the AlN/Pt ratio on the electro-mechanical properties of AlN/Pt multilayer is investigated. • The impact of different annealing steps at 900 °C is studied using XRD mesurements. • It is shown, that the TCR can be tuned using different AlN/Pt ratios. • Significant changes in gauge factor are observed for varying AlN/Pt ratios. • Comparing to pure Pt thin films, a very high gauge factor of about 5 is observed for an AlN5/Pt5 multilayer at room temperature. In this work, the impact of the AlN/Pt ratio on the strain-sensitive properties of multilayered AlN/Pt thin films is investigated. These thin film systems consisting of 10 bi-layers of AlN and Pt each with a thickness of 3 nm AlN and 7 nm Pt or 5 nm AlN and 5 nm Pt are fabricated, evaluated and compared to those realized with a sequence of 7 nm AlN and 3 nm Pt thin bi-layers. The thin film systems are sputter-deposited on oxidized silicon wafers or sapphire substrates. The influence of different annealing steps at 900 °C up to 24 h in Argon (Ar) atmosphere on the electrical film resistivity and the temperature coefficient of the electrical resistance (TCR) is investigated for these different multilayers using Van-der-Pauw measurements up to 330 °C in air. Furthermore, the impact of the AlN/Pt-ratio on the gauge factor of the thermally stabilized multilayers is determined using a purpose-built measurement setup up to 500 °C in air. Transmission electron microscopy and X-ray diffraction analyses are utilized to examine the microstructure and the crystallographic phase composition of the multilayers before and after thermal loading. Annealing the different multilayers at 900 °C leads to diffusion effects between the AlN and Pt thin films and recrystallizations processes in the Pt sublayers depending on the individual sublayer thickness values. After thermal pre-conditioning at 900 °C for 1 h in Ar the samples were stable even in air up to 500 °C. Finally, it is shown that the TCR as well as the gauge factor of an AlN/Pt multilayer can be tuned through the AlN/Pt-ratio. The highest gauge factor with a value of 4.7 (±0.3) at room temperature is achieved with a multilayer consisting of 5 nm AlN and 5 nm Pt (pure Pt thin film on Cr adhesion promotor: 3.2), while the lowest linear TCR with a value of α = 7.4 ∙ 10−4 K-1 is measured for multilayers with 7 nm AlN and 3 nm Pt sublayers (pure Pt thin film on Cr adhesion promotor: 3.66 ∙ 10-3 K-1). [ABSTRACT FROM AUTHOR]