German
The
topic of my PhD is “Signal conditioning of smallest capacitive signals at
aF-range”. Supervisor is Prof. Buff (TU-Ilmenau,
Germany). My place of work is located at the research and predevelopment
department of the Robert Bosch GmbH (Stuttgart,
Germany), which is paying this project as well.
Calculating
the numbers of electrons, stored in a capacitor of 1aF capacitance on 1V, gives
as result exactly 6241 electrons. This shows, that this task is more like
counting electrons then real measuring.
In
deed, in this area the absolute value doesn’t matters. Rather it is of
interest, to get the unit, we are interested in. An ideal research object for
this topic is presented by micromechanical angular rate sensors. What here it
matters is, to win the angular rate back from the capacitive signal. The basics
are quickly explained, but beside the wanted measuring effect there are lots of
distortions, which have to be suppressed or compensated, in order to get the
angular rate signal. The aim is, to search and analyze new methods and
techniques for making a more noise free signal possible.
An
extract from a publication (IEEE
Spectrum July 1998 Volume 35 Number 7) is here presented. It shows a
basic principle of a micromechanical angular rate sensor. We will realize, that
for such a project different engineer areas are needed.
The Robert Bosch GmbH got this know-how and is presenting the top of angular rate sensor manufacturers for automotive applications. At present these sensors are used for ESP, navigation and roll over sensing. My work will help, to create more sensitive sensors, so that it will be possible to open even other application areas.
The micromechanical angular rate sensor has a
butterfly-shaped polysilicon rotor suspended above the substrate, free to
oscillate about the center tether. The rotor's perforations are a
necessary evil, needed to allow etching beneath the rotor during manufacturing.
Four interdigitated combs on the outer edge of the rotor drive it into resonant
oscillation. Electrical leads carry the driving signal to the combs and
the measurement signals from the detection electrodes below the rotor.
The rotor is driven into a resonant oscillation about
the z-axis. When the device package is rotated about the y-axis, the substrate
rotates about the same axis--the motion is transferred to the rotor through
elastic connections at the center. Obedient to the law of conservation of
angular momentum, the rotor tilts along the x-axis. This motion is sensed by the
electrodes below the rotor and converted into an electrical signal.
The oscillation of the tilting rotor is quantified by measuring the differential capacitance between it and the electrodes beneath. Capacitance measurements corresponding to rotor displacements of about the diameter of an atom can be attained.
The pictures are taken from: IEEE Spectrum July 1998 Volume 35 Number 7
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