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.
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.
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