by Dr. Philippe Robert
Philippe Robert, Patrice Rey, Patrice,
Arnaud Walther, Guillaume Jourdan and Mylène Savoye, CEA-LETI
Corresponding author: firstname.lastname@example.org
We are presenting a novel approach for very low cost 6D inertial sensor. This
concept is based on the idea to mix on same device MEMS and NEMS technologies.
The MEMS part is used for the mass to keep sufficient inertial force, and the
NEMS is used as a very sensitive sub-µm suspended stress gage. This concept
allows both in-plane and out-of-plane acceleration or Coriolis force detection
on a same device. It is also compatible with differential detection to reduce
Technological realization and first characterizations of 3-axis accelerometer
and 3-axis gyrometer have been achieved and will be detailed in the presentation.
On the basis of these results, one provides a footprint smaller than 3.5 mm2
for the integration of 3 axis accelerometer and 3 axis gyrometer on the same
chip. To our knowledge, this level of integration and miniaturization has never
The MEMS market growth comes mainly from consumer market (cell phone, game,
…). For this market, a very strong pressure is exerted on MEMS manufacturers.
Typically 5 to 15% of cost reduction is expected each year for these components.
At the end, a simple optimization of design and process will be insufficient
and then a technological breakthrough is clearly expected to drastically miniaturize
the MEMS sensors.
Nevertheless, this size reduction has major impacts on inertial sensor, in
particular with regard to the performance: Reducing the seismic mass has a direct
impact on the sensitivity, and lowers the nominal capacitance, with consequences
on signal to noise ratio. To overcome these limitations, a new concept is proposed
mixing micro and nanoscale structures, thus named M&NEMS. The basic idea
is to combine on a same device a thick MEMS layer for the inertial mass, with
a thin and narrow NEMS part to realize a suspended strain gage. A high sensitivity
can be obtained due to the very high stress concentration induced by the very
small cross-section of the silicon nanowire gage and also by the lever arm effect
of the accelerometers and gyrometers designs (see Fig. 1). The two thicknesses
of the M&NEMS approach offer also the ability to have on a same chip an
in-plane and out-of-plane detection of the inertial mass movement (see Fig.
2). It means that with this concept and technology, inertial sensors can be
integrated in less than 1 mm2 for 3D-accelerometer and less than
2.5 mm2 for the 3D-gyrometer.
Concept of the M&NEMS accelerometer: In-plane acceleration causes
the mass to rotate around the rotating shaft that applies an axial stress
in the NEMS suspended gage. This stress is amplified by a lever arm
effect induced by the design (amplification x30), and also by the very
small section of the gage increased by thinned suspended gage (magnification
Concept of the M&NEMS out-plane accelerometer. In that configuration,
a vertical acceleration causes the mass to rotate around the hinges.
This rotation applies an axial stress in the NEMS suspended gage (as
the gage is thinner than the mass). As for the in-plane case, this stress
is amplified by a lever arm effect.
An example of in-plane accelerometer and X-axis gyrometer are shown in Fig.
3 and Fig. 5.
SEM view of a in-plane accelerometer
Electrical characterization of a 50g accelerometer (relative variation
of the gage resistance vs acceleration)
SEM view of a Z-axis gyrometer
A focus on the gage lets clearly appear the MEMS inertial mass of 15µm
thick, and the sub-µm gage that has a section of 0.25x0.25µm2.
The 6 mask levels of the M&NEMS accelero and gyro technology will be detailed
in the presentation (Fig. 7). This process is based on a SOI technology where
the NEMS part is manufactured in the thin silicon active layer. The MEMS part
is defined within a 15µm silicon epitaxial layer. The electrical characterizations
of these two kinds of sensors are still in progress, but so far, all the measured
parameters are in perfect agreement with the simulations.
Q-factor measurement on the Z-gyro
M&NEMS accelerometer process flow
- The sensitivity (Fig. 4), linearity and thermal drift for the accelerometer;
- The drive and sense resonant frequencies, Q-factors (Fig. 6), thermal and
pressure behavior for the gyrometer.
Concerning the gyrometer, the sensitivity of the nano-gages is such that it
can work in an open-loop mode. An operation in rough vacuum packaging (without
getter) seems also very likely.
New designs and technological runs are in progress to go further in the development
of this concept, in particular to integrate in the same flow a 3D magnetometer
and a pressure sensor. The goal is to achieve at the end the demonstration of
an IMU sensor module with nine-degrees-of-freedom (3-axis accelerometer + 3-axis
gyrometer + 3-axis magnetometer) and a pressure sensor integrated on a same
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