Hydrogenated amorphous SiC (a-SiC:H) is an attractive material for MEMS applications where high robustness or operation in harsh environments is targeted. In previous publications, it was demonstrated, that the properties of a-SiC:H thin films can be tailored over a wide range by changing the auxiliary table excitation power of a dual plasma source deposition process using an inductively coupled plasma-enhanced chemical vapour deposition system. In this work, the annealing behavior of dual plasma source deposited a-SiC:H thin films under argon atmosphere is investigated by using Fourier transform infrared (FT-IR) spectroscopy for chemical analysis. All investigated layers show a decrease of hydrogen containing bonds (X-Hx) and an increase of Si-C bonds with increasing annealing temperature in the FT-IR spectrum. This behaviour is directly linked to the effusion of hydrogen from the thin films at elevated temperatures. In addition, films deposited at higher auxiliary plasma power show more X-Hx and less Si-C bonds, indicating a higher hydrogen amount in those films. All layers shrink with increasing annealing temperature due to the effusion of hydrogen with a stronger shrink at higher PT values caused by the increased hydrogen amount. This shrink also leads to a densification of the thin films.
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.
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.
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).
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.
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