Aluminum nitride (AlN) is a promising material for sensor applications in harsh environments such as turbine exhausts or thermal power plants due to its piezoelectric properties, good thermal match to silicon and high temperature stability. Typically, the usage of piezoelectric materials in high temperature is limited by the Curie-temperature, the increase of the leakage current as well as by enhanced diffusion effects in the materials. In order to exploit the high temperature potential of AlN thin films, post deposition annealing experiments up to 1000°C in both oxygen and nitrogen gas atmospheres for 2 h were performed. X-ray diffraction measurements indicate that the thin films are chemically stable in a pure oxygen atmosphere for 2 h at annealing temperatures of up to 900°C. After a 2 h annealing step at 1000°C in pure oxygen. However, a 100 nm thin AlN film is completely oxidized. In contrast, the layer is stable up to 1000°C in pure nitrogen atmosphere. The surface topology changes significantly at annealing temperatures above 800°C independent of annealing atmosphere. The surface roughness is increased by about one order of magnitude compared to the "as deposited" state. This is predominantly attributed to recrystallization processes occurring during high temperature loading. Up to an annealing temperature of 700°C, a Poole-Frenkel conduction mechanism dominates the leakage current characteristics. Above, a mixture of different leakage current mechanisms is observed.
Highly c-axis orientated sputter deposited aluminium nitride (AlN) thin films are widely used as piezoelectric layers in micro-electro-mechanical systems (MEMS). Therefore, stable and reliable deposition and patterning of the AlN thin films in the fabrication process of such devices is of utmost importance. In this work, we study the wet chemical etching behavior of highly c-axis oriented AlN layers as well as the film-related residuals after the etching procedure. To investigate the impact of the underlying material on the quality of the AlN films they are either deposited on pure silicon (Si) substrates or on Si substrates covered with a sputter-deposited thin titanium (Ti) film. The 620 nm thin AlN layers are synthesized simultaneously onto both substrate types and subsequently wet-chemical etched in a phosphorous acid based etching solution at a temperature of 80°C. We demonstrate a significant difference in surface roughness of the untreated AlN films when sputter-deposited on Ti or pure Si. Furthermore, we analyze the piezoelectric properties of the deposited films. Although the XRD analyses indicate a high c-axis orientated wurtzite structure for all deposited films, the absolute value of the piezoelectric coefficients |d33| of AlN thin films synthesized on Ti are 0.4-4.3 pC/N, whereas corresponding values of 5.2-6 pC/N are determined at those deposited on pure Si substrates,. Finally, after wet chemically etching a porous, but homogeneous AlN microstructure is observed for samples synthesized onto Ti layers, whereas AlN layers deposited directly on Si substrate are either etched very inhomogenously or almost completely with some etch resistant pyramidal-shaped residues. This might be due to a local change in polarity within the AlN layer.
The stability of piezoelectric scandium aluminium nitride (ScxAl1-xN) thin films with x= 27% was investigated after post deposition annealings up to 1000°C. The ScxAl1-xN thin films targeted for applications in micro-electromechanical systems (MEMS) were deposited close to room-temperature applying DC magnetron sputtering. Varying deposition parameters yielded films with different microstructural properties and piezoelectric constants. Upon annealing, the crystalline quality of thin films with c-axis orientation increased, as found via characterization techniques such as X-ray diffractometry and fourier transform infrared absorbance measurements. Additionally, piezoelectric constants after annealing steps up to 1000°C are reported as obtained via a Berlincourt measurement principle. Furthermore, modifications in chemical composition during temperature loads up to 1000°C were recorded by thermal effusion measurements.
This research work presents the design, fabrication and characterization of micromachined piezoelectric energy harvester
stimulated by ambient random vibrations utilizing AlN as a piezoelectric material. The device design consists of a silicon
cantilever beam on which AlN is sandwiched between two electrodes and a silicon seismic mass at the end of the
cantilever beam. The generated electric power of the devices was experimentally measured at various acceleration levels.
A maximum power of 34 μW was obtained at an acceleration value of 2g for the device which measures 5.6 x 5.6 mm2.
Various unpackaged devices were tested and assessed in terms of the generated power and resonant frequency at various acceleration values.
In this work, the fabrication process of piezoelectric AlN cantilevers is presented. The cantilevers were electrically
characterized in a vacuum chamber offering the possibility to close-loop control the back pressure from atmospheric
conditions down to 5x10-3 mbar. The quality factor (Q factor) is an important figure of merit to evaluate the performance
of micro-resonators. In particular, two different modes were detected and analyzed. The first bending mode detected at
19.5 kHz has a quality factor of 470 at atmospheric pressure which increases continuously to 985 at 1x10-1 mbar. The
corresponding resonant frequency shifted from 19.500 kHz at atmospheric pressure to 19.573 kHz at 5 mbar. Below this
pressure level, the resonance frequency stays unaffected within the measurement accuracy.
The second bending mode detected at 117.264 kHz exhibits a quality factor of about 570 at atmospheric pressure
increasing continuously to 1275 at 1x10-1 mbar. In agreement with the other resonant frequency under investigation the
corresponding resonant frequency decreased from 117.264 kHz at atmospheric pressure to 117.630 kHz at 5 mbar.
In this study, an analogue Q-control circuit is presented, based on a digital feedback loop. In this approach, a self-actuated
and self-sensing piezoelectric microstructure is used in combination with a lock-in amplifier to extract the Q-control
feedback signal amplitude which is proportional to the piezoelectric current. In the next step, the DC-value
supported by the lock-in amplifier is multiplied with a sine signal on a custom-designed analogue circuit board
containing an analogue amplifier IC. This generated analogue feedback signal allows to drive a complete analogue
feedback loop enabling faster response of the Q-control and further faster measurements can be performed. Using this Q-factor
enhancement technique, the Q-factor was increased from 397 to about 5357 in air without driving the used Q-control
approach to its limits. These promising results will push further activities in measuring the viscosity of liquids in
the future.
In this work, we study the modes of vibration for two different families of aluminium nitride-actuated piezoelectric
microstructures: contour modes and flexure-actuated modes. For the contour modes, the structure vibrates at frequencies
determined by its edge dimensions whereas for the flexure-actuated modes a suspended structure is displaced by the
lateral bending of the flexures. We combine electrical and optical techniques to fully characterize the vibrating modes of
these types of in-plane MEMS structures. An electronic speckle pattern interferometry technique is used for a full 3D
detection of the movement of the structures. Quality factors as high as 5000 and motional resistance as low as 4 KOhm
were obtained for in-plane modes in air and a quality factor as high as 300 was obtained for an in-plane structure with
water on the top surface. This work shows the great flexibility in the selection of resonant modes in piezoelectric
resonators and actuators, implemented by a proper design of the electrode layout geometry.
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 was measured using a time-zero approach, where the film is stressed using
a fast voltage ramp up to the point of breakdown. The measurements were performed using different contact pad sizes,
different voltage ramping speeds and device temperatures, respectively. In order to achieve statistical significance, at
least 12 measurements were performed for each environment parameter set and the results analyzed using the Weibull
approach.
The results show, that the breakdown field in positive direction rises with the pad size, as expected. Furthermore, lower
breakdown fields with increasing temperatures up to 300°C are observed with the mean field to failure following an
exponential law typical for temperature activated processes. The activation energy was determined to 27 meV, allowing
an estimation of the breakdown field towards even higher temperatures. In negative field direction no breakdown
occurred, which is attributed to the metal-insulator-semiconductor configuration of the sample and hence, the larger
depletion layer forming in the silicon dominates the observed current behavior.
Piezoelectric energy microgenerators are devices that continuously generate electricity when they are subjected to
varying mechanical strain due to vibrations. They can generate electrical power up to 100 μW which can be used to drive
various sensing and actuating MEMS devices. Today, piezoelectric energy harvesters are considered autonomous and
reliable energy sources to actuate low power microdevices such as wireless sensor networks, indoor-outdoor monitoring,
facility management and biomedical applications. The advantages of piezoelectric energy harvesters including high
power density, moderate output power and CMOS compatible fabrication in particular with aluminum nitride (AlN) have
fuelled and motivated researchers to develop MEMS based energy harvesters. Recently, the use of AlN as a piezoelectric
material has increased fabrication compatibility, enabling the realization of smart integrated systems on chip which
include sensors, actuators and energy storage. Piezoelectric MEMS energy microgenerator is used to capture and
transform the available ambient mechanical vibrations into usable electric energy via resonant coupling in the thin film
piezoelectric material. Analysis and modeling of piezoelectric energy generators are very important aspects for improved
performance. Aluminum nitride as the piezoelectric material is sandwiched between two electrodes. The device design
includes a silicon cantilever on which the AlN film is deposited and which features a seismic mass at the end of the
cantilever. Beam theory and lumped modeling with circuit elements are applied for modeling and analysis of the device
operation at various acceleration values. The model shows good agreement with the experimental findings, thus giving
confidence in the model.
Low temperature co-fired ceramics (LTCC) has established as a widespread platform for advanced functional ceramic
devices in different applications, such as in the space and aviation sector, for micro machined sensors as well as in micro
fluidics. This is due to high reliability, excellent physical properties, especially in the high frequency range, and the
possibility to integrate passive components in the monolithic LTCC body, offering the potential for a high degree of
miniaturisation.
However, for further improvement of this technology and for an ongoing increase of the integration level, the realization
of miniaturized structures is of utmost importance. Therefore, novel techniques for micro-machining are required
providing channel structures and cavities inside the glass-ceramic body, enabling for further application scenarios. Those
techniques are punching, laser cutting and embossing.
One of the most limitations of LTCC is the poor thermal conductivity. Hence, the possibility to integrate channels
enables innovative active cooling approaches using fluidic media for heat critical devices. Doing so, a by far better
cooling effect can be achieved than by passive devices as heat spreaders or heat sinks. Furthermore, the realization of
mechanic devices as integrated pressure sensors for operation under harsh environmental conditions can be realized by
integrating the membrane directly into the ceramic body. Finally, for high power devices substantial improvement can be
provided by filling those channel structures with electrical conductive material, so that the resistivity can be decreased
drastically without affecting the topography of the ceramics.
Copper (Cu) is commonly used as metallization for a wide range of microelectronic devices. Typically, organic
circuit boards as well as ceramic and glass-ceramic substrates use galvanic deposited Cu films for this purpose.
However, due to a thickness of several microns the lateral resolution in the μm-region being required e.g. for
novel high frequency applications can not be guaranteed when applying this technology. Hence, sputter
deposition is envisaged for the realization of Cu thin films on glass, LTCC (low temperature co-fired ceramics)
and alumina substrates. The reliability of 300 nm thick Cu thin films is investigated under accelerated aging
conditions, utilizing a test structure which consists of 20 parallel lines stressed with current densities up to
1•10+6 A•cm-2 at temperatures between T= 100°C and 200°C. 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 of
Ea~ 0.75 eV is similar to other face centered cubic metals such as silver, the higher activation energies of about
Ea~ 1 eV on glass and alumina indicate a suppression of back diffusion especially at enhanced temperature
levels. Therefore, the overall electromigration resistance is lower compared to Ag. This effect is predominantly
caused by a stable oxide layer being formed at high temperatures acting as passivation layer.
The load-deflection (LD) method is a common and convenient procedure to extract the Young's modulus and
the internal tensile stress of thin-film diaphragms from measurements of the maximum transverse deflection to
a uniformly distributed load. This technique allows simultaneous determination of both parameters by fitting
a theoretical to an experimental LD characteristic. Consequently, a proper knowledge of such a theoretical
relationship is of utmost importance to obtain accurate values. We deduced a novel LD formula covering all
relevant elastomechanical bending and stretching effects. It enables an easy but still accurate extraction of the
Young's modulus and the internal tensile stress from LD measurements on circularly-shaped diaphragms. This LD
relationship was derived from an adaptation of Timoshenko's membrane bending theory, where the in-plane and
the out-of-plane deflections were approximated by a series expansion and a polynomial, respectively. Utilizing
the minimum total potential energy principle yielded an infinite-dimensional system of equations which was
solved analytically resulting in a compact closed-form solution. The flexibility of our approach is demonstrated
by extracting the Young's modulus and the internal tensile stress of three disparate diaphragm materials made
of either sputtered AlN, PECVD SixNy, or microfiltered carbon nanotubes (bucky paper).
In this work we report on the development of electrostatically actuated RF MEMS switches which are based on a one
sided clamped cantilever made of two layers of the same alloy of aluminum-silicon-copper. The switches are based on a
low-complexity design and are fabricated by conventional sputter deposition and wet etching techniques on oxidized
silicon substrates. Due to a well defined intrinsic stress gradient the cantilevers bend away from the substrate surface
after release. This deflection allows the combination of high open-state isolation with a moderate pull-in voltage and
with high restoring forces, which help to reduce sticking effects. The temperature behavior of the residual stress of each
single layer that are the basis for the switch is investigated up to 400°C. Thereby, the change in stress over temperature
as well as stress level in the as-deposited state is strongly dependent on deposition parameters. Furthermore, the change
of deflection is evaluated up to 400°C at cantilever-type test structures. Finally, the high frequency performance of the
switches was measured in the 23 to 36 GHz range showing good results for isolation and insertion loss.
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 cofired 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 high frequency application. Therefore, the reliability of Ag thin films is
investigated under accelerated aging conditions, utilizing test structure which consist of 5 parallel lines stressed with a
current density of 2.5.106 A/cm2 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 according to the overall resistance of
the test structure. Knowing the mean time to failure (MTF) and activation energy at elevated temperatures lifetime
predictions can be made when extrapolating for room temperature scenarios. Applying this approach, the highest value
of 6053 days is determined for Ag thin films on LTCC. When compared to Si/SiO2 and alumina substrates the poorer
performance originates from the microstructure of the films. On polycrystalline aluminum oxide Ag thin films exhibit
sharp discontinuities due to a pronounced graining originating from the substrate. This effect could limit the distance of
electromigration tracks.
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