Page 213 - MEMS Mechanical Sensors

202 Inertial Sensors

Drive and control electrodes
Supporting springs
Sense vibrating
mode
45°

Drive vibrating
mode
Vibrating ring
Figure 8.26 Vibrating ring structure gyroscope. Electrodes around the periphery of the ring are
used to electrostatically excite the ring into a resonance mode. A secondary mode at 45° is a
measure of the angular rate and is sensed capacitively. (After: [63].)

It is based on a ring supported by a number of semicircular springs and anchored
in the middle. The ring is excited to vibrate electrostatically in-plane, the vibration
having an elliptic shape. Any rotation about the axis normal to the ring structure
transfers energy to a secondary mode, which is 45° apart from the primary mode. In
other words, the antinodes of the primary flexural mode are located at the nodes of
the secondary flexural mode. Electrodes placed at these positions are used to capaci-
tively measure the amplitude of the secondary mode, which is proportional to the
angular rate to be measured. An obvious advantage of this design is the high degree
of symmetry of the sensing element. An early version was presented by Putty and
Najafi in 1994 [63]. It relied on a nickel electroplated ring structure, which was fab-
ricated on a wafer containing standard CMOS circuitry for the control and interface
electronics. Subsequently, this group presented more advanced versions of this
approach. Another electroplated ring gyroscope was presented by Sparks et al. [64],
which mainly improved the signal and interface circuitry. More recently, improved
designs have been reported based on a high aspect ratio ring made from polysilicon
[65, 66]. The fabrication relies on the deep reactive dry etching of 50- to 100-m-deep
trenches with near vertical sidewalls into a low-resistive silicon substrate. The
trenches are subsequently refilled with highly doped polysilicon over a sacrificial sili-
con dioxide layer. After various patterning and etching steps of the oxide and the
structural polysilicon, the sacrificial oxide is removed by a HF etch step to free the
ring structure and form the air gaps between the electrodes and the ring. The ring is
1.1 mm in diameter, the support post in the middle has a diameter of 120 µm, and
the width of the ring and support springs is 4 µm. Sixteen fixed electrodes are evenly
located around the periphery of the ring; they are 60 µm tall, 150 µm long, and are
separated from the ring by a 1.4-µm air gap. The fabrication technology has the
advantage that the height of the ring structure and the electrodes can be made in the
order of a hundred or more microns and the air gaps can be made in the submicron
range. This results in high values of capacitance for vibration measurements; thus,
the sensitivity is increased considerably. The fabrication process also allows large air
gaps, which can be used to excite the structure in the primary mode with high ampli-
tude, again resulting in higher sensitivity. There is, however, a trade-off between the
higher voltages required to electrostatically drive the ring using larger air gaps. Test