Online proceedings - EDA Publishing Association

11-13
May 2011, Aix-en-Provence, France
The basic idea consists to use a slow ramp-shaped signal
III. DESIGN AND SIMULATIONS
to control the electrostatic actuator and to analyze the so-
by the driver to
obtained actuation current (I act ) deliveredd
Smart drivers have been designed and fabricated using a
the actuator. To generate the driver HV control signal, the HV 0.35µm CMOS technology
from Austria MicroSystems
digital input (IN) is used to control the charge and discharge
(AMS). This process tolerates voltage drop across transistor
of an integrated capacitance (C ramp ) with two constant
channels (V DS ) up to 50V. Figure 3 (respectively 4) shows the
current generators (I charge and I discharge ). As a result, the
layout (resp. an image) of a control channel. Zone (1)
current buffer output (OUT) delivers a HV signal with a
represents the current source of the ramp module, (2) is a
constant voltage slope. The load is represented by the
current mirror copying a fraction of the current from the
MEMS device, that can be assumed to be a variable source and the circuit to control
the sign of the ramp (rise or
capacitance (C act ), the parasitic contribution which is
fall), (3) is the integrating MIM capacitor, C ramp, used to
capacitive (C p ), and a serial resistance (R s s) coming from the
generate a positive or negative voltage ramp, (4) is the current
routing or wire bonding. It is worth noting that both
source for common drain buffer (5), (6) and (7) are the NMOS
capacitors are connected in parallel and that the parasitic
and PMOS mirror respectively which copy and amplify the
capacitance is large compared to the actuator capacitance. actuation current for the diagnosis, while (8) are the voltage
The diagnosis section copies the actuation current limiters that prevent V I1 and V I2 to go beyond the low voltage
through a serial resistance (R 1 ) to obtain a voltage drop
supply (V DDA = 3.3V). Both rise and fall slopes of the ramp
proportional to the actuation current:
generator can be adjusted thanks
to a pair of external resistors
to allow a parametric study.
V
∂Vact
= n ⋅ R1
⋅ I
act
= n ⋅ R1
⋅ C
p
+ n ⋅ R ⋅V
∂t
I1
1
actt
∂
∂t
C act
= α + β
(1)
This voltage is composed of two terms; the first one (α)
represents the contribution of the parasitic capacitance
(assuming C act is small compared to C p p) and is constant
during the charge (respectively the discharge) of the
actuator, while the second (β) is proportional to capacitance
variations (assuming C p is bias-independent). The latter
term is null except during pull-in or pull-off events. Both
events correspond to a rapid change in the capacitance thus
producing a current peak. Finally, if the ramp is sufficiently
slow, pull-in and pull-off current spikes can be identified
out of the constant current contribution due to α. Finally, a
comparator is used to detect the spikes and to integrate the
actuation current (with capacitor C int ) for quantitative
evaluations. It allows measuring the total variation of
capacitance during the pull-in or the pull-ofoutput). Figure 2 illustrates the main signals: the absence of
events (Diag2
the pull-in and pull-off spikes implies a non-working
MEMS condition. From this diagnostic
“go-nogo”
information (Diag1 output) and the knownn previous state of
the switch the new state of the beam can be deduced. In this
paper, we concentrate on the analysis of the voltage drop in
R 1 (I 1 signal).
(7)
(5)
(2)
(1) (1) (4)
(8)
(3)
(6) (7)
Fig. 3. Layout of a smart-driver channel with a 760x550 µm 2 area occupied
in a HV 0.35µm CMOS technology.
Fig. 2. Main signals of the diagnosis unit: ramp-shaped actuation voltage
V act and actuation current image I 1. For a functional beam, two spikes can
be discriminated through the comparison thresholds.
Fig. 4. Microscope image
of a smart driver channel.
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