Online proceedings - EDA Publishing Association

7-9 October 2009, Leuven, Belgium
coupled with each other using a frame to be driven in The suspension connecting proof masses m 1 and m 3 with
resonance electrothermally using a chevron-shaped the substrate via anchor is consisted of four double folded
thermal actuator along the x-axis. When
the gyroscope is
subjected to an angular rotation along the z-axis, Coriolis
force is induced in the sense direction along y-axis.
The general expression for the rotation induced Coriolis
flexures. These folded flexures
can be modeled as fixed-
to the axis of the
guided beams, deformed orthogonally
beam. The general expression
of the stiffness for such
flexure is:
force is 2Ω where is
the translational
velocity of the proof is mass in the rotating system and Ω
N ⎛
3
12EI
⎞ 2Etw
is the angular velocity vector. If both the drive and sense
k x =
=
n ⎜
3
mode have the same resonant frequencies, the Coriolis
L
⎟ 3
(1)
⎝ x ⎠ L x
force excites the system into resonancee causing the sense
electrodes along the sense y- direction
[6]. This motion
Where E is the Young’s Modulus, I=tw 3 /12 is the
creates a capacitance change due to change in the
second moment of inertia of the rectangular beam cross
electrode gap. This capacitance change is then detected by
section, t, L and w is the beam thickness, length and width
the CMOS capacitive interface circuit output of which is
respectively, N is the total number of the flexure
then conditioned using external electronics providing an
supporting the mass and n is
the number of folds per
electrical output proportional to the applied angular rate
flexure.
input [7].
Chevron actuator can be modeled as fixed-fixed beam if
we neglect the small angle change. For M number of
A. Suspension Design
beams of length of L c each, the stiffness of the Chevron
The complete suspension configuration of the system is shaped actuator can be calculated as [8]:
shown in Fig.2. Suspension flexures of the proposed
microgyroscope is designed in way that the all proof
⎡
3
192EII
⎤ ⎡16Etw
⎤
masses, m 1 , m 2 and m 3 moves together
under the driving
k
chev
= M ⎢ 3 ⎥ = M ⎢ 3 ⎥
force defining 1-DoF drive mode. The center mass m 2 is
⎢⎣ L c ⎥⎦ ⎢⎣ L c ⎥⎦
free to oscillate in the orthogonal sense direction under the
(2)
influence of rotation induced Coriolis force.
Where M is the The overalll stiffness of the system in
drive x-direction can be calculated by adding the
expression given in (1) and (2).
The suspension system of mass m 2 comprised of four
triple-folded flexure and stiffness in the sense y-direction
for the mass m 2 can also be
calculated by using the
expression given in (3).
k
y
=
N
n
⎛
⎜
⎝
3
12
EI 2Etw
=
3 3
L ⎟ L
y
⎞
⎠
y
(3)
Fig. 1. The layout of the electrothermally actuated microgyroscope
Fig. 2. The suspension system configuration
III. PROTOTYPE
FABRICATION
A prototype gyroscope is designed to be fabricated in
the Metal-Multi User MEMS Processes (MetalMUMPs)
[9] for the design concept verification. MetalMUMPs is a
low cost, commercially available, general purpose
electroplated nickel micromachining process for MEMS
devices. MetalMUMPs consists of a 20μm thick
electroplated nickel layer used
as the primary structural
material and electrical interconnect layer. A trench layer
in the silicon substrate can also be incorporated for
additional thermal and electrical isolation. Process
simulation of the prototype carried out in MEMSPro and
the device is shown in Fig. 3. Cross sectional views are
given in Fig. 4 (a) and (b) to illustrate different layer used
during prototype fabrication.
The process steps involved
in the fabrication of the
proposed microgyroscope using
MetalMUMPs are shown
in Fig. 5. The overall size of the device is 2.2mm ×
2.6mm. The movable parts of the MVG like Chevron
shaped actuator, proof masses and folded flexure are
defined using the 20µm thick nickel layer. A 25µm deep
trench is defined underneath the movable parts of the
MVG to provide electrical and
thermal isolation from the
silicon substrate. The anchors and fixed parts are formed
©EDA Publishing/THERMINIC 2009 41
ISBN: 978-2-35500-010-2