Impact of the substrate dependent polarity distribution in c-axis oriented AlN thin films on the etching behaviour and the piezoelectric properties

In this work, the influence of substrate properties on the polarization of highly c-axis oriented aluminium nitride (AlN) thin films and as a consequence, on the piezoelectric properties and the wet-chemical etching behaviour is investigated. Therefore, 620 nm thin AlN layers are simultaneously sputter-deposited under nominal unheated substrate conditions on silicon (Si) substrates or on those covered with a sputter-deposited titanium (Ti) film. After wet-chemically etching in a phosphorous acid based solution at 80 °C different residues of AlN remain. Wet-chemical etching of AlN films deposited on Ti results in a high film porosity. In contrast, AlN layers on Si are either hardly attacked or the complete thin film is removed except some remaining conical shaped residues. Furthermore, we demonstrate a change in the measured electro-mechanical properties with changing maximum deposition temperature caused by a self-heating effect of the substrate during the AlN deposition process. The change in piezoelectric properties and the differing etching behaviour is caused by a change in polarity within the AlN layer. These domains are visualized by piezoresponse force microscopy measurements, and are in good agreement with the observed etching results. For layers with mixed polarization, the absolute values of the piezoelectric constant d ₃₃ are reduced due to the counteraction of piezoelectric domains with opposite polarization.     

Study on microstructural, chemical and electrical properties of tantalum nitride thin films deposited by reactive direct current magnetron sputtering

The properties of tantalum nitride (TaN_x) thin films on silicon and low temperature co-fired ceramics based substrates were investigated with respect to their potential use for sensor elements operated under harsh environmental conditions. For deposition reactive direct current magnetron sputtering was applied at constant back pressure (=0.9 Pa) and plasma power (=1,000 W). In all experiments, the substrates were nominally unheated. The films were investigated electrically by four point probing. For morphological and chemical analyses, a large variety of techniques such as focussed ion beam, scanning electron microscopy, X-ray diffraction, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy and glow discharge optical emission spectroscopy were used. Only by combining all these techniques for analysing TaN_x films synthesised with varying nitrogen content in the deposition chamber can a proper evaluation of the microstructure and the chemical composition be done. Both the microstructure and the chemical composition are influenced strongly with a resulting effect on the electrical film properties.     

Modeling flexural plate wave devices

A lumped-parameter model is derived for flexural plate wave (FPW) devices which are rectangular plates or diaphragms with structural layers, a piezoelectric layer, and with interdigitated conducting combs for driving and sensing. This configuration is often used in micromechanical chemical sensors. The model is based an a closed-form solution of a resonating beam; however, the results are applicable to plates supported on four edges. The model gives a voltage or charge output from the sense combs as a function of voltage applied to the drive combs. The analysis predicts the response of the multiple plate modes to axial tensions and to comb finger dimensions and position relative to the diaphragm eigenfunctions. These models are much more detailed than those described in the literature on acoustic chemical sensors and are difficult to obtain by finite-element solutions. Frequency responses of FPW devices constructed from silicon with deposited aluminum nitride as the piezoelectric compared well with analytic results. The effects of boundary conditions on the plate's lateral edges are discussed in both the analysis and testing     

Sputtered thin film piezoelectric aluminum nitride as a functional MEMS material

A comprehensive study on the complete process for the fabrication of AlN-based MEMS sensors and actuators is presented. Varying the bias voltage during the reactive rf sputtering enables to adjust the stress level in AlN films in the range of about 2?GPa. For the first time the influence of the rf bias power on the whole set of piezoelectric parameters was investigated. It could be shown that the dielectric permittivity, dielectric loss, rocking curve width, effective longitudinal piezoelectric coefficient d _33,f and effective transverse piezoelectric coefficient e _31,f remained unchanged. Further it was observed that piezoelectric AlN films could be deposited at low process temperatures of only 200?°C. Moreover an increase in the e _31,f coefficient with thicker films could be stated. Finally cone formation during wet etching was observed and revealed a formation of {01–12} planes which exhibit a slow etching rate. The c-textured growth of AlN starts directly at the Pt interface.     

Design and optimization of cantilever based piezoelectric micro power generator for cardiac pacemaker

Piezoelectric micro-power generator (PMPG) converts mechanical vibration energy into electric energy via piezoelectric effects. In cardiac pace makers, the use of PMPG eliminates the need for a traditional lithium iodide battery replacement. In this paper we design and optimize PMPG that is able to harvest the mechanical movement of the heart beat to be converted into usable electrical power in frequency range 1–1.7 Hz. Eight control parameters are selected: which are proof mass material, piezoelectric material, proof mass length, proof mass thickness, piezoelectric layer width, piezoelectric layer thickness, silicon nitride layer width, silicon nitride layer thickness. Orthogonal arrays of Taguchi method for these eight parameters mentioned with three levels and signal-to-noise (S/N) ratio, and ANOVA analysis is studied to determine the optimum design. COMSOL Multiphysics ver. 4.2 is used in 18 different simulations. The maximum output power and highest efficiency designed at 1.2 Hz is equivalent to 72 beat per min. Both Taguchi and ANOVA confirms the same results of determining the parameter of having the most influence on the generated output power at 1.2 Hz in descending order: which are piezoelectric material of PZT-5A, proof mass length of 5 mm, piezoelectric layer thickness of 30 µm, proof mass thickness of 4 mm, piezoelectric layer width of 0.12 mm, silicon nitride layer width of 0.16 mm, silicon nitride layer thickness of 30 µm, and proof mass material of aluminum. Eigen frequency analysis for the first six modes of operation for PMPG frequencies are: 1.2 HZ, 5.4 Hz, 6.9 Hz, 29,7 Hz, 694.8 Hz, 708.3 Hz. The first mode of operation is selected as operation mode and shows that 93 % of PMPG’s total displacement and output power was produced in the range of 1–1.4 Hz, therefore PMPG can work when the heart rate between 60 and 84 bpm. Transient analysis performed at 1.2 Hz reaches the steady state before the first 10 cycles with output power density of 23.13 µW/cm³, which is suitable for powering cardiac pace maker.     

Unimorph and bimorph piezoelectric energy harvester stimulated by β-emitting radioisotopes: a modeling study

We use energy methods and lumped-element modelling in the analysis and optimization of a miniaturized aluminum nitride based piezoelectric energy harvester which utilizes tritiated silicon as an uninterrupted energy source. Tritiated silicon serves as a radioisotope source which emits energetic β particles that results in an electrostatic force between the radioactive source and collector that traps the emitted particles. The generated electrostatic force will drive the charging and actuating cycles of the piezoelectric cantilever leading to a continuous charge–discharge cycles, thus inducing vibrations in the piezoelectric cantilever. The energy generated from the piezoelectric thin-film is appropriately rectified and stored to provide continuous and uninterrupted electrical power to a low-power devices. The modelled results have been benchmarked against available experimental data for a unimorph piezoelectric harvester with very good agreement. Furthermore, the model was applied to a bimorph piezoelectric harvester, showing that the output power can be doubled in relation to a unimorph design. Moreover, the model accounts for the entire range of design and operating factors such as the ambient medium and associated damping losses, current leakage, and device scaling.     

Electro-mechanical behavior and direct piezoelectricity of cellulose electro-active paper

This study focuses on investigating the piezoelectric effects of cellulose-based electro-active paper (EAPap) using quasi-static direct piezoelectricity. Mechanical properties were investigated first and then electro-mechanical behavior was studied by applying electric field during the pulling test. In-plane piezoelectric charge constant (d₃₁) of EAPap was quantified by the quasi-static relation between induced charge and applied stress. Strong shear electro-mechanical coupling was observed and 45° sample provided the largest in-plane piezoelectric charge constant. The measured piezoelectric charge constant was in the range of 8–28.2 pC/N, which are similar to those of piezo polymer. Cellulose EAPap provides promising potential as biodegradable and cheap piezoelectric polymer material.     

Fabrication and characterization of PECVD silicon nitride for RF MEMS applications

The structural, optical and electrical properties of plasma enhanced chemical vapor deposited silicon nitride layers are investigated, which have been used as a dielectric layer during RF MEMS fabrication. During growth, the gas ratio (SiH₄/NH₃) is varied between 0.33 and 0.5 and pressure is varied between 400 and 700 mTorr while deposition time is kept constant. The results in the films show differing properties. The thicknesses of the resultant films are between 150 to 220 nm with different gas flow ratios and pressures whereas the deposition time was kept constant. A Bruggeman effective medium approximation is utilized to model the refractive index of the films. Reflectance measurements were carried out in the range of 210–250 nm. The refractive indexes of the films varied between 1.79 and 2.03, with a dielectric constant varying from 6.66 to 7.22. Capacitance voltage measurements yield a fixed dielectric charge value in the low ?10¹² cm^?2 while a breakdown voltage of 915 V μm^?1 is achieved for films grown at the lowest gas ratio and pressure. The quality of Si/Si_xN_y interface is also considered.     

多晶硅薄膜制备方法与压阻特性的研究

对磁控溅射和低压化学气相淀积(LPCVD)2种方法制备的多晶硅薄膜的电学和压阻特性进行了研究,并讨论了结晶化工艺对磁控溅射薄膜性质的影响。实验表明:LPCVD薄膜稳定性、重复性较好,应变系数可达到20以上;磁控溅射薄膜经适当结晶化工艺处理具有纳晶硅的结构特征,应变系数可达到80以上。利用扫描电镜(SEM)图片结合电阻率和应变系数的测试结果,讨论了2种方法制备出的多晶硅薄膜应用于压阻式力学量传感器的可行性。     

Temperature dependant dielectric breakdown of sputter-deposited AlN thin films using a time-zero approach

In MEMS (micro electromechanical system) devices, piezoelectric aluminum nitride (AlN) thin films are commonly used as functional material for sensing and actuating purposes. Additionally, AlN features excellent dielectric properties as well as a high chemical and thermal stability, making it also a good choice for passivation purposes for microelectronic devices. With those aspects and current trends towards minimization in mind, the dielectric reliability of thin AlN films is of utmost importance for the realization of advanced device concepts. In this study, we present results on the transversal dielectric strength of 100 nm AlN thin films deposited by dc magnetron sputtering. The dielectric strength is measured using a time-zero approach, using a fast voltage ramp to stress the film up to the point of breakdown. The measurements are performed at different device temperatures. In order to achieve statistical significance, at least 12 measurements are performed for each environment parameter set and the results are analyzed using the Weibull approach. Basically, lower breakdown fields are observed with increasing temperatures up to 300 °C with a characteristic breakdown field strength E ₀ following the relationship $\sqrt {E_{0} } \propto T$ as reported in literature for similar measurements performed at silicon nitride thin films. From the intersection of this linear behavior, the Poole–Frenkel (PF) barrier height ? _B is determined to 0.54 eV, which is reasonable for AlN thin films. The slope of this relation is similar to values reported for silicon nitride thin films. This allows an estimation of the breakdown field at higher temperatures by extrapolation. Leakage current measurements show a dominant PF type conduction mechanism, verifying the applicability of $\sqrt {E_{0} } \propto T$ . No breakdown occurs in negative field direction, which is attributed to the metal–insulator–semiconductor configuration of the sample and hence, the presence of a depletion layer forming in the n-doped silicon and dominating the leakage current behavior.     

Fluid damping in resonant flexural plate wave device

Fluid damping models are developed for resonant (standing wave) flexural plate wave (FPW) devices, which are rectangular plates or diaphragms with structural layers, a piezoelectric layer, and interdigitated conducting combs for driving and sensing. This configuration is often used in micromechanical chemical, biological, or nonvolatile residue sensors. Where much of the previous work on fluid effects in FPW devices focused on delay lines, this effort investigates resonant devices both analytically and experimentally. The fluid model is based on closed-form solution of a wide beam vibrating into a semi-infinite fluid volume and is mated directly into the beam equation. While the fluid's pressure versus wave motion solution has been reported previously, the application to the resonant FPW is mathematically rigorous and leads to a greater understanding of the FPW damping regimes. Frequency responses of FPW devices constructed from silicon with deposited piezoelectric aluminum nitride and operating in water and alcohol compared well with analytic results with some discrepancies noted.     

Etching behaviour of sputter-deposited aluminium nitride thin films in H3PO4 and KOH solutions

Aluminium nitride (AlN) reactively sputter deposited from an aluminium target is an interesting compound material due to its CMOS compatible fabrication process and its piezoelectric properties. For the implementation in micromachined sensors and actuators an appropriate patterning technique is needed to form elements made of AlN. Therefore, the influence of different sputtering conditions on the vertical etch rate of AlN thin films with a typical thickness of 600 nm is investigated in an etch mixture based on phosphoric acid. Under comparable conditions, such as temperature and concentration of the etchant, thin films with a high c-axis orientation are etched substantially slower compared to films with a low degree of (002) orientation. When a high c-axis orientation is present detailed analyses of the etched topographies reveal surface characteristics with a low porosity and hence, low roughness values. From temperature dependant etching experiments an activation energy of 800(± 30) meV is determined showing a reaction-controlled etching regime independent of sputter deposition conditions. For comparison, AlN films synthesized under the same conditions were etched in potassium hydroxide (KOH) at room temperature revealing comparable etching characteristics as a function of deposition parameters. Depending on the degree of (002) orientation the topography of the etched samples show a strong increase in surface roughness with time due to a selective etching behaviour between (002) and residual crystallographic planes.     

Fabrication of aluminium doped zinc oxide piezoelectric thin film on a silicon substrate for piezoelectric MEMS energy harvesters

Thin film piezoelectric materials play an essential role in micro electro mechanical system (MEMS) energy harvesting due to its low power requirement and high available energy densities. Non-ferroelectric piezoelectric materials such as ZnO and AlN are highly silicon compatible making it suitable for MEMS energy harvesters in self-powered microsystems. This work primarily describe the design, simulation and fabrication of aluminium doped zinc oxide (AZO) cantilever beam deposited on <100> silicon substrate. AZO was chosen due its high piezoelectric coupling coefficient, ease of deposition and excellent bonding with silicon substrate. Doping of ZnO with Al has improved the electrical properties, conductivity and thermal stability. The proposed design operates in transversal mode (d ₃₁ mode) which was structured as a parallel plated capacitor using Si/Al/AZO/Al layers. The highlight of this work is the successful design and fabrication of Al/AZO/Al on <100> silicon as the substrate to make the device CMOS compatible for electronic functionality integration. Design and finite element modeling was conducted using COMSOL? software to estimate the resonance frequency. RF Magnetron sputtering was chosen as the deposition method for aluminium and AZO. Material characterization was performed using X-ray diffraction and field emission scanning electron microscopy to evaluate the piezoelectric qualities, surface morphology and the cross section. The fabricated energy harvester generated 1.61?V open circuit output voltage at 7.77?MHz resonance frequency. The experimental results agreed with the simulation results. The measured output voltage is sufficient for low power wireless sensor nodes as an alternative power sources to traditional chemical batteries.     

Surface and Bulk-Silicon-Micromachined Optical Displacement Sensor Fabricated With the SwIFT-Lite™ Process

For many micromachined sensors such as microphones and accelerometers, optical displacement sensing may have advantages over capacitive sensing—offering potentially high-displacement sensitivity independent of sensor area and gap height. A particular diffraction-based optical displacement sensor design consisting of a sensing diaphragm suspended over a rigid grated electrode has demonstrated advantages in terms of sensitivity, integration and stability. When illuminated from the backside, this structure generates a zeroth and complementary higher order diffracted beams whose intensities are modulated by the diaphragm displacement with the sensitivity of a regular Michelson interferometer. In previous work, acoustic devices surface micromachined on quartz substrates using aluminum for the diaphragm and grating were presented and characterized. In this work, we present the fabrication and characterization of a surface- and bulk-silicon micromachined diffraction-based optical microphone structure fabricated with Sandia National Laboratories' SwIFT-Lite™ process, which uses silicon nitride and polysilicon structural materials that have been employed extensively in MEMS and form a more thermally matched material set robust against corrosion and fatigue. The process introduces a new design space to microscale optical displacement sensing, enabling large, soft structures with perforated back-plates ideal for microphone and inertial-sensor designs.1599     

Hydrogen gas sensing properties of Pt/Ta2O5 Schottky diodes based on Si and SiC substrates

We developed Pt/tantalum oxide (Ta₂O₅) Schottky diodes for hydrogen sensing applications. Thin layer (4 nm) of Ta₂O₅ was deposited on silicon (Si) and silicon carbide (SiC) substrates using the radio frequency sputtering technique. We compared the performance of these sensors at different temperatures of 100 °C and 150 °C. At these operating temperatures, the sensor based on SiC exhibited a larger sensitivity, whilst the sensor based on Si exhibited a faster response toward hydrogen gas. We discussed herein, the experimental results obtained for these Pt/Ta₂O₅ based Schottky diodes exhibited that they are promising candidates for hydrogen sensing applications.     

Electro-optical characterization of IC compatible microcantilevers

The aim of this work is to simulate and optically characterize the piezoelectric performance of complementary metal oxide semiconductor (CMOS) compatible microcantilevers based on aluminium nitride (AlN) and manufactured at room temperature. This study should facilitate the integration of piezoelectric micro-electro-mechanical systems (MEMS) such as microcantilevers, in CMOS technology. Besides compatibility with standard integrated circuit manufacturing procedures, low temperature processing also translates into higher throughput and, as a consequence, lower manufacturing costs. Thus, the use of the piezoelectric properties of AlN manufactured by reactive sputtering at room temperature is an important step towards the integration of this type of devices within future CMOS technology standards. To assess the reliability of our fabrication process, we have manufactured arrays of free-standing microcantilever beams of variable dimension and studied their piezoelectric performance. The characterization of the first out-of-plane modes of AlN-actuated piezoelectric microcantilevers has been carried out using two optical techniques: laser Doppler vibrometry (LDV) and white light interferometry (WLI). In order to actuate the cantilevers, a periodic chirp signal in certain frequency ranges was applied between the device electrodes. The nature of the different vibration modes detected has been studied and compared with that obtained by a finite element model based simulation (COMSOL Multiphysics), showing flexural as well as torsional modes. The correspondence between theoretical and experimental data is reasonably good, probing the viability of this high throughput and CMOS compatible fabrication process. To complete the study, X-ray diffraction as well as d₃₃ piezoelectric coefficient measurements were also carried out.     

Design and development of sub-micron scale specimens with electroplated structures for the microtensile testing of thin films

A novel designed microtensile specimen with electroplated structures is described here. It can be fit into a specially designed microtensile apparatus, which is capable of carrying out a series of tests on sub-micron scale freestanding thin films. Several thin films for microelectromechanical systems (MEMS) applications has been tested here including sputtered copper, gold, gold-chrome and tantalum nitride. All the metal specimens were fabricated by sputtering. For the tantalum nitride film samples, nitrogen gas was introduced into the chamber during the process of sputtering tantalum films on the silicon wafer. We have used copper, gold, 5% gold-chrome alloys and tantalum nitride thin films with thickness of 200–800 nm. The E values of the thin films tested here are consistent with the results from other measurement methods. The test results of metal specimens show the similar trend of the Hall-Petch prediction. However, the values of tantalum nitride thin films do not exhibit any systematic variation with respect to the thickness.     

微机械热电堆红外探测器的设计

阐述了微机械热电堆的设计原理、所用材料及主要结构,并研究了3×3阵列的微机械制造工艺,该热电堆结构支撑结构为氮化硅—氧化硅—氮化硅的复合介质膜。热电偶材料采用多晶硅和铝,热电偶采用并列排布的结构,对冷端覆盖了绝热层,用以提高热电堆的探测率。由于该制作工艺与标准IC工艺兼容,使得硅基热电堆红外探测器得到了越来越广泛的应用。     

Further examination of the Si KLL Auger line in silicon nitride thin films

The chemical bonding of Si in silicon nitride thin films has been studied using XPS. The kinetic energies of the Si KLL and Si LVV lines and the binding energies of the photoelectron lines along with their Auger parameters have been tabulated for sputter deposited, plasma deposited and CVD silicon nitride films. Characteristics of the silicon nitride thin films formed by N⁺₂ bombardment of Si have been discussed. Ar⁺ bombardment of the silicon nitride films is shown to cause reduction of the Si₃N₄ with greater amounts at the very near surface. The reduced species seem to be mostly substoichiometric Si₃N₄ rather than elemental Si. Analysis of the Si KLL line shapes shows variation in the amount of substoichiometric Si₃N₄ present for some of the silicon nitride films.     

E-beam lithography of nano-interdigital transducers on insulating and semiconducting substrates

This work describes the micro-fabrication process developed to manufacture nano-interdigital transducers (nano-IDTs) to be used in surface acoustic wave applications. The combination of electron-beam (e-beam) lithography and lift-off process is shown to be effective in fabricating IDT finger patterns with a line width below 100 nm and good yield. It is also shown how a very thin organic anti-static layer can be used to avoid charge accumulation on the resist layer during e-beam lithography, which is easy to occur on insulating piezoelectric substrates and results in e-beam deflection. However, it is also shown how the use of this anti-static layer is not required with the insulating piezoelectric layer resting on a semiconducting substrate such as highly doped silicon. The effect of the e-beam dose on insulating and semiconducting layers is also discussed.