Controlled Electronic and Magnetic Landscape in Self-Assembled Complex Oxide Heterostructures.

Complex oxide heterointerfaces contain a rich playground of novel physical properties and functionalities, which give rise to emerging technologies. Among designing and controlling the functional properties of complex oxide film heterostructures, vertically aligned nanostructure (VAN) films using a self-assembling bottom-up deposition method presents great promise in terms of structural flexibility and property tunability. Here, the bottom-up self-assembly is extended to a new approach using a mixture containing a 2Dlayer-by-layer film growth, followed by a 3D VAN film growth. In this work, the two-phase nanocomposite thin films are based on LaAlO3 :LaBO3 , grown on a lattice-mismatched SrTiO3001 (001) single crystal. The 2D-to-3D transient structural assembly is primarily controlled by the composition ratio, leading to the coexistence of multiple interfacial properties, 2D electron gas, and magnetic anisotropy. This approach provides multidimensional film heterostructures which enrich the emergent phenomena for multifunctional applications.

Atomically engineered interfaces yield extraordinary electrostriction.

Electrostriction is a property of dielectric materials whereby an applied electric field induces a mechanical deformation proportional to the square of that field. The magnitude of the effect is usually minuscule (<10-19 m2 V-2 for simple oxides). However, symmetry-breaking phenomena at the interfaces can offer an efficient strategy for the design of new properties1,2. Here we report an engineered electrostrictive effect via the epitaxial deposition of alternating layers of Gd2O3-doped CeO2 and Er2O3-stabilized δ-Bi2O3 with atomically controlled interfaces on NdGaO3 substrates. The value of the electrostriction coefficient achieved is 2.38 × 10-14 m2 V-2, exceeding the best known relaxor ferroelectrics by three orders of magnitude. Our theoretical calculations indicate that this greatly enhanced electrostriction arises from coherent strain imparted by interfacial lattice discontinuity. These artificial heterostructures open a new avenue for the design and manipulation of electrostrictive materials and devices for nano/micro actuation and cutting-edge sensors.

Al0.83Sc0.17N Contour-Mode Resonators With Electromechanical Coupling in Excess of 4.5.

In this paper, we demonstrate the fabrication of contour-mode resonators (CMRs) with Al0.83Sc0.17N as a piezoelectric layer. Moreover, we assess the electromechanical coupling and the maximum achieved quality factor from 150 to 500 MHz. In comparison to pure aluminum nitride (AlN) CMRs, our results show electromechanical coupling coefficients of more than a 2× factor higher at around 200 MHz. The highest quality factor is measured on a CMR operating at 388 MHz and is in excess of 1600. From the characterization of devices operating at different frequencies, material parameters of the Al0.83Sc0.17N are extracted such as the stiffness constant, the relative permittivity, and the piezoelectric constant. In particular, the reported d31 piezoelectric constant is equal to -3.9 pm/V. This represents a 2.25× improvement when compared to pure AlN. Finally, we report the first temperature compensation experimental results for Al0.83Sc0.17N CMRs. Our results show that about 1.5 [Formula: see text] of sputtered oxide, deposited on top of a released resonator, allows near zero temperature coefficient of frequency variation for CMRs operating up to 500 MHz.

Geometry of Chemical Beam Vapor Deposition System for Efficient Combinatorial Investigations of Thin Oxide Films: Deposited Film Properties versus Precursor Flow Simulations.

An innovative deposition system has been developed to construct complex material thin films from single-element precursors by chemical beam vapor deposition (CBVD). It relies on well distributed punctual sources that emit individually controlled precursor beams toward the substrate under high vacuum conditions combined with well designed cryo-panel surfaces that avoid secondary precursor sources. In this configuration the impinging flows of all precursors can be calculated at any substrate point considering the controlled angular distribution of the emitted beams and the ballistic trajectory of the molecules. The flow simulation is described in details. The major advantage of the deposition system is its ability to switch between several possible controlled combinatorial configurations, in which the substrate is exposed to a wide range of flow compositions from the different precursors, and a uniform configuration, in which the substrate is exposed to a homogeneous flow, even on large substrates, with high precursor use efficiency. Agreement between calculations and depositions carried out in various system configurations and for single, binary, or ternary oxides in mass transfer limited regime confirms that the distribution of incoming precursors on the substrate follows the theoretical models. Additionally, for some selected precursors and in some selected conditions, almost 100% of the precursor impinging on the substrate is incorporated to the deposit. The results of this work confirm the potentialities of CBVD both as a research tool to investigate efficiently deposition processes and as a fabrication tool to deposit on large surfaces.

Dimensional and Structural Control of Silica Aerogel Membranes for Miniaturized Motionless Gas Pumps.

With growing public interest in portable electronics such as micro fuel cells, micro gas total analysis systems, and portable medical devices, the need for miniaturized air pumps with minimal electrical power consumption is on the rise. Thus, the development and downsizing of next-generation thermal transpiration gas pumps has been investigated intensively during the last decades. Such a system relies on a mesoporous membrane that generates a thermomolecular pressure gradient under the action of an applied temperature bias. However, the development of highly miniaturized active membrane materials with tailored porosity and optimized pumping performance remains a major challenge. Here we report a systematic study on the manufacturing of aerogel membranes using an optimized, minimal-shrinkage sol-gel process, leading to low thermal conductivity and high air conductance. This combination of properties results in superior performance for miniaturized thermomolecular air pump applications. The engineering of such aerogel membranes, which implies pore structure control and chemical surface modification, requires both chemical processing know-how and a detailed understanding of the influence of the material properties on the spatial flow rate density. Optimal pumping performance was found for devices with integrated membranes with a density of 0.062 g cm(-3) and an average pore size of 142.0 nm. Benchmarking of such low-density hydrophobic active aerogel membranes gave an air flow rate density of 3.85 sccm·cm(-2) at an operating temperature of 400 °C. Such a silica aerogel membrane based system has shown more than 50% higher pumping performance when compared to conventional transpiration pump membrane materials as well as the ability to withstand higher operating temperatures (up to 440 °C). This study highlights new perspectives for the development of miniaturized thermal transpiration air pumps while offering insights into the fundamentals of molecular pumping in three-dimensional open-mesoporous materials.

Fine structuration of low-temperature co-fired ceramic (LTCC) microreactors.

The development of microreactors that operate under harsh conditions is always of great interest for many applications. Here we present a microfabrication process based on low-temperature co-fired ceramic (LTCC) technology for producing microreactors which are able to perform chemical processes at elevated temperature (>400 °C) and against concentrated harsh chemicals such as sodium hydroxide, sulfuric acid and hydrochloric acid. Various micro-scale cavities and/or fluidic channels were successfully fabricated in these microreactors using a set of combined and optimized LTCC manufacturing processes. Among them, it has been found that laser micromachining and multi-step low-pressure lamination are particularly critical to the fabrication and quality of these microreactors. Demonstration of LTCC microreactors with various embedded fluidic structures is illustrated with a number of examples, including micro-mixers for studies of exothermic reactions, multiple-injection microreactors for ionone production, and high-temperature microreactors for portable hydrogen generation.

Comparison of lead zirconate titanate thin films for microelectromechanical energy harvester with interdigitated and parallel plate electrodes.

Lead zirconate titanate (PZT) thin films on insulator- buffered silicon substrates with interdigitated electrodes (IDEs) have the potential to harvest more energy than parallel plate electrode (PPE) structures because the former exploit the longitudinal piezoelectric effect, which is about twice as high as the transverse piezoelectric effect used by PPE structures. In this work, both options are compared with respect to dielectric, ferroelectric, and piezoelectric properties, leakage currents, and figure of merit (FOM) for energy harvesting. The test samples were silicon beams with {100} PZT thin films in the case of the PPE geometry, and random PZT thin films for the IDE geometry. Both films were obtained by an identical sol-gel route. Almost the same dielectric constants were derived when the conformal mapping method was applied for the IDE capacitor to correct for the IDE geometry. The dielectric loss was smaller in the IDE case. The ferroelectric loops showed a higher saturation polarization, a higher coercive field, and less back-switching for the IDE case. The leakage current density of the IDE structure was measured to be about 4 orders of magnitude lower than that of the PPE structure. The best FOM of the IDE structures was 20% superior to that of the PPE structures while also having a voltage response that was ten times higher (12.9 mV/µ strain).

Admittance matrix of a trapezoidal piezoelectric heterogeneous bimorph.

Bimorph structures are a standard method for transforming the high force of piezoelectric materials into a large deflection. In micro electromechanical systems (MEMS) applications, it is preferable to use structures consisting of a passive substrate (usually silicon) and one or more piezoelectric layers on the top. Such structures are called heterogeneous bimorphs or enakemesomorphs. In some MEMS applications- for example, for use as acoustic transducers-it is desirable to arrange such heterogeneous bimorphs in a circular shape, which results in trapezoidal cantilever structures. In this paper, an analytic dynamic description of such actuators is obtained. The resulting model is proved to be compatible with existing models for heterogeneous bimorphs with constant width. A comparison to a finite element analysis model of an exemplary layout shows divergences wholly within the same range as found for published models for constant-width structures.

Measurement of effective piezoelectric coefficients of PZT thin films for energy harvesting application with interdigitated electrodes.

Interdigitated electrode (IDE) systems with lead zirconate titanate (PZT) thin films play an increasingly important role for two reasons: first, such a configuration generates higher voltages than parallel plate capacitor-type electrode (PPE) structures, and second, the application of an electric field leads to a compressive stress component in addition to the overall stress state, unlike a PPE structure, which results in tensile stress component. Because ceramics tend to crack at relatively moderate tensile stresses, this means that IDEs have a lower risk of cracking than PPEs. For these reasons, IDE systems are ideal for energy harvesting of vibration energy, and for actuators. Systematic investigations of PZT films with IDE systems have not yet been undertaken. In this work, we present results on the evaluation of the in-plane piezoelectric coefficients with IDE systems. Additionally, we also propose a simple and measurable figure of merit (FOM) to analyze and evaluate the relevant piezoelectric parameter for harvesting efficiency without the need to fabricate the energy harvesting device. Idealized effective coefficients e(IDE) and h(IDE) are derived, showing its composite nature with about one-third contribution of the transverse effect, and about two-thirds contribution of the longitudinal effect in the case of a PZT film deposited on a (100)-oriented silicon wafer with the in-plane electric field along one of the <011> Si directions. Randomly oriented 1-µm-thick PZT 53/47 film deposited by a sol-gel technique, was evaluated and yielded an effective coefficient e(IDE) of 15 C·m(-2). Our FOM is the product between effective e and h coefficient representing twice the electrical energy density stored in the piezoelectric film per unit strain deformation (both for IDE and PPE systems). Assuming homogeneous fields between the fingers, and neglecting the contribution from below the electrode fingers, the FOM for IDE structures with larger electrode gap is derived to be twice as large as for PPE structures, for PZT-5H properties. The experiments yielded an FOM of the IDE structures of 1.25 × 10(10) J/m(3) and 14 mV/µ strain.

Electro-mechanical coupling in shear-mode FBAR with piezoelectric modulated thin film.

The electro-mechanical coupling and resonance characteristics of a piezoelectric modulated thin film bulk acoustic wave resonator (PMBAR) is described quantitatively. The derived 1-D analytical model is in agreement with 2-D finite element modeling (FEM). The modeling shows that a pure shear mode excitation can be achieved and that coupling of parasitic Lamb waves is reduced in this design.

Tunable thin film bulk acoustic wave resonator based on Ba(x)Sr(1-x)TiO3 thin film.

A tunable membrane-type thin film bulk acoustic wave resonator (TFBAR) based on a Ba(0.3)Sr(0.7)TiO(3)(BST) thin film has been fabricated. The resonance and antiresonance frequencies of the device can be altered by applying a dc bias: both shift down with increasing dc electric field. The resonance and antiresonance frequencies showed a tuning of -2.4% and -0.6%, respectively, at a maximum dc electric field of 615 kV/cm. The electromechanical coupling factor of the device increased up to 4.4%. We demonstrate that the tuning of the TFBAR is nonhysteretic. The Q-factor of the device showed some variation with dc bias and is about 200. The tuning of the TFBAR is caused by the dc bias dependence of the sound velocity and the intrinsic electromechanical coupling factor of the BST layer. We apply our recently developed theory on the electrical tuning of dc bias induced acoustic resonances in paraelectric thin films to successfully model the tuning behavior of the TFBAR. The modeling enabled us to de-embed the intrinsic electromechanical properties of the BST thin film. We show that the mechanical load of our device does not significantly degrade the tuning performance of the BST layer. The performance of the TFBAR is compared with the available data on varactor tuned TFBARs.

High-Q AlN/SiO2 symmetric composite thin film bulk acoustic wave resonators.

High-Q, bulk acoustic wave composite resonators based on a symmetric layer sequence of SiO(2)-AlN-SiO(2) sandwiched between electrodes have been developed. Acoustic isolation was achieved by means of deep silicon etching to obtain membrane type thin film bulk acoustic wave resonators (TFBARs). Three different device versions were investigated. The SiO(2) film thicknesses were varied (0 nm, 70 nm, 310 nm, and 770 nm) while the piezoelectric AlN film had a constant thickness of 1.2 microm. The sputter-deposited AlN film grown on the amorphous, sputter-deposited SiO(2) layer exhibited a d(33,f) of 4.0 pm/V. Experimental results of quality factors (Q) and coupling coefficients (k(t)(2)) are in agreement with finite element calculations. A Q of 2000 is observed for the first harmonic of the 310 nm oxide devices. The most intense resonance of the 770 nm oxide device is the third harmonic reaching Q factors of 1450. The temperature drift reveals the impact of the SiO(2) layers, which is more pronounced on the first harmonic, reducing the TCF to 4 ppm/K for the 3rd harmonic of the 310 nm oxide devices.

Characterization of sol-gel Pb(Zr0.53Ti0.47O3) in thin film bulk acoustic resonators.

The behavior of {f111g}-textured Pb(Zr(0.53Ti0.47O3) (PZT) deposited by the sol-gel technique in thin film bulk acoustic resonators (TFBAR's) was investigated at a resonance frequency of about 1 GHz. The resonators were fabricated on Si wafers using deep silicon etching to create a membrane structure and using platinum as top and bottom electrodes. The best response of the resonators was observed at a bias voltage of -15 kV/cm with values of about 10% for the coupling constant and about 50 for the quality factor. This voltage corresponds to optimal values of piezoelectric constant d33 and dielectric constant measured as a function of the electric field. The influence of a bias voltage on the resonance frequency, antiresonance frequency, and coupling constant were observed. Both the resonance and antiresonance frequency show a hysteretic change with applied bias. This effect can be used to shift the whole band of a filter by applying a voltage. The TFBAR structure also allowed us to extract values for materials parameters of the PZT film. Dielectric, piezoelectric, and elastic properties of the f111g-textured PZT film are reported and compared to direct measurements and to literature values.

Piezoelectrics in micro and nanosystems: solutions for a wide range of applications.

Piezoelectric thin films have interesting prospects in a number of applications for which miniaturization is a driving force. Miniaturization means higher frequency, or higher resolution, or increased functionality. This paper gives a short review on piezoelectric thin films, their deposition processes, integration, properties and applications in microsystems (MEMS), concentrating on the most frequently investigated piezoelectric thin film materials.

Piezoelectric thin films: evaluation of electrical and electromechanical characteristics for MEMS devices.

We present a new measurement method to characterize piezoelectric thin films utilizing a four-point bending setup. In combination with a single- or a double-beam laser interferometer, this setup allows the determination of the effective transverse and longitudinal piezoelectric coefficients e31,f and d33,f, respectively. Additionally, the dielectric coefficient and the large signal electrical polarization are measured to add further important characteristics of the film. These data are essential for piezoelectric thin film process specification and the design and qualification of microelectromechanical systems devices.

A highly sensitive Pb(Zr,Ti)O3 thin film ultrasonic micro-sensor with a grooved diaphragm.

A highly sensitive piezoelectric ultrasonic micro-sensor with a grooved multilayer membrane was developed by a Si-based MEMS technique. The groove was located at one-quarter of the distance away from the edge of the membrane and opened into piezoelectric layer. The piezoelectric layer Pb(Zr,Ti)O(3) (PZT) was 2.2 microm thick and was prepared by a sol-gel method. The prepared PZT film was pure perovskite and showed a highly (100) textured structure. The sensitivity of the fabricated piezoelectric ultrasonic sensor without the groove structure was 100 microV/Pa. In comparison, the sensitivity of the ultrasonic sensor with the groove structure was about 500 microV/Pa, which is 5 times that without the groove structure. The diaphragm having grooves showed a corrugate-like structure that was formed by residual stress. The high sensitivity of the membrane with the grooved diaphragm is considered to relate to the corrugate-like structure.

DC bias-dependent shift of the resonance frequencies in BST thin film membranes.

Direct current (DC) bias-dependent acoustic resonance phenomena have been observed in micromachined tunable thin film capacitors based on Ba(0.3)Sr(0.7)TiO3 (BST) thin films. The antiresonance frequency is only weakly DC bias dependent, and the resonance frequency exhibits a much stronger dependence on the applied DC bias. The resonance frequency shifted by 1.2% for a frequency of about 6.7 GHz and an applied field of 667 KV/cm. At the same time the effective electromechanical coupling constant k(2)(t,eff) increased to 2.0%. The tuning of the resonance frequency depends on the tunability of the film permittivity and on the mechanical load on the piezoactive layer. The experimental observations correlate well with the theoretical predictions derived from the free energy P expansion using Landau theory.

Shear mode coupling and tilted grain growth of A1N thin films in BAW resonators.

Polycrystalline A1N thin films were deposited by RF reactive magnetron sputtering on Pt(111)/Ti electrode films. The substrates were tilted by an angle ranging from 40 degrees to 70 degrees with respect to the target normal. A low deposition temperature and a high sputter gas pressure were found ideal for tilted growth. The resulting grain tilt angle amounts to about half the substrate tilt angle. For coupling evaluation, 5 GHz solidly mounted resonator structures have been realized. The tilted grain A1N films exhibited a permittivity in the 9.5-10.5 range and loss tangent of 0.3%. Two shear modes as well as the longitudinal mode could be clearly identified. The coupling coefficient k2(eff) of the fundamental thickness shear mode (TS0) was found to be about 0.5%, which is compatible with a c-axis tilt of about 6 degrees.

Bandpass filters for 8 GHz using solidly mounted bulk acoustic wave resonators.

Frequency shift, design, and fabrication issues have been investigated for the realization of 8 GHz bandpass filters based on AlN thin film bulk acoustic wave resonators. Fabrication includes well-textured AIN thin films on Pt (111) electrodes and SiO2/AlN Bragg gratings for the solidly mounted resonators. The chosen ladder filter design requires the tuning of the shunt resonators with respect to the series one. For this purpose, mass loading of the shunt resonators with aluminum (Al) and SiO2 were studied. Design simulations showed that the channel bandwidth can be doubled by shifting more than the difference of resonance and antiresonance frequency. Bandpass filters at 8 GHz were successfully fabricated with -5.5 dB insertion loss, -26 dB out-of-band rejection, 99 MHz (1.2%) +/- 0.2 dB channel bandwidth, and 224 MHz (2.8%) 3 dB bandwidth. The group delay variations within any 30 MHz channel inside the channel bandwidth amounts to < 0.2 ns. Comparisons with simulation calculations and single resonator characteristics show that each pi-section includes a parasitic series resistance and inductance.