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Title:
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Current Mode Drive of Electrostatic Microactuators
Rafael Nadal Guardia
Co-Directors:
L. M. Castañer Muñoz
Angel Rodríguez Martínez
Director of the
Departament:
R. Alcubilla
1. Abstract
Several physical principles, electrostatic, magnetic, thermal, piezoelectric, shape
memory alloys, etc..., can be implemented in order to actuate microstructures. Nevertheless,
problems related to high power consumption, compatibility with standard CMOS processes
are still a bottleneck difficult to solve for many of theses principles. Concerning the
electrostatic principle, the generated electrostatic fields can produce large forces but generally
limited to short distances. When the actuator must work over larger distances, the generated
force will decrease. Only if higher voltage is used it will be possible to maintain a given force.
On the other hand, the low current consumption associated with electrostatic devices makes
them very attractive for highly efficient actuation. Electrostatic actuation gives the best
performance with the smallest power consumption and can be easily implemented in CMOS
technologies.
Nevertheless, when pull in events are carried out in voltage drive mode, for instance in
a micro-relay structure, current consumption increases at the end of the event, as pointed by
Castañer and Senturia. This Thesis has been addressed to investigate the energy requirements
during the pull in event when current instead of voltage is used. Not only advantages in power
consumption have been concluded, but also, new positioning techniques have been proposed.
In parallel plate actuators, assuming the lumped 1D model, it is well known that, in
voltage driving, the moveable electrode can be controlled between 0 and 1/3 of the gap. Only
applied voltages between 0 and the pull in value can be used. If higher values are applied, the
moveable electrode collapses towards the fixed one. When charge drive mode is
implemented, the voltage pull in point of instability can be overcome. Position control in the
whole gap is possible. Only parasitic capacitances in parallel with the actuator leads the
system to behave like a voltage driving scenario. It has been shown that this “ideal” behaviour
cannot be extrapolated to other structures, such as torsional actuators. In torsional actuators,
when charge drive mode is used, a pull in charge threshold is defined avoiding stable
operation for positions further 0.71max. A new set of equations has been proposed for
torsional actuators in charge drive mode. Thus, depending on the actuator, charge driving
must be studied to determine the range of stability. As it happens in parallel plate structures,
parasitic capacitances reduce the stable range towards the voltage drive mode limit.
Another important advantage of charge drive mode in front of voltage driving is found
when dynamic operation is studied. It has been shown that the transfer function of
displacement versus charge allows having a dynamic behaviour depending on the mechanical
parameters of the actuator. By means of optical methods, like interferometry the transfer
function of position respect to charge would be identified in each point of displacement. With
this technique, dependence of the mechanical parameters respect the remaining gap between
plates, can be studied. Thus, the predicted models for the damping coefficient, b, reported in
the literature would be easily validated. This Thesis has been focussed to study the static
behaviour proposing dynamic investigation as a future research work.
Based on bulk micromachining techniques of single crystal silicon, cantilever beam,
and bridge structures have been fabricated in the clean room of the Universitat Politècnica de
Catalunya. Cantilever beams with 600m, 1.5mm length and 250m, 500m width, have been
obtained with a final thickness of 5m. Bridge structures with 2mm length and 250m, 500m
width have been also fabricated with the same thickness. The final electrostatically actuated
structure has been fabricated bonding silicon and quartz, using 1% HF as bonding material.
The quartz substrate has been etched 4.8m to define the fixed electrode. A 1m aluminium
layer has been deposited on the patterned quartz defining an effective gap of 3.8m.
On the other hand, it has been also possible to have access to parallel plate actuators
from Infineon Technologies AG. In these actuators a final back sided micromachining step is
performed releasing a thin polysilicon moveable electrode. Characterisation of these devices
using C-V measurements, resonant frequency measurements, and other techniques, have been
carried out in order to model the actuator with the 1D lumped model.
In the proposed method of charge driving by means of current pulses, it has been
shown that precision of the injected charge can be critical in order to fix the moveable
electrode in a desired position inside the gap. Temperature stable CMOS currents sources are
mandatory in such applications. Reported work by Dr. Courtois and Dr. Bianchi showed that
it is possible to extend the temperature range of operation of standard CMOS technologies
when only small process modifications and proper design techniques are used. Extension of
the operating temperature range and control of the final slope have been achieved by design
techniques applied to a standard CMOS technology, AMS CYE 0.8m, using the available
lateral bipolar parasitic transistors. The proposed circuit has been fabricated and tested in a
high temperature chamber. A final range between –60°C and 160°C has been achieved with
very good precision on the designed current value.
Measurements in voltage drive mode of the actuator from Infineon Technologies have
been carried out showing that high values of the voltage source resistance can be used to find
an optimum between velocity and power consumption. New design techniques of
microactuators would be addressed to fabricate structures to be driven in the optimum point.
In order to implement the proposed method of current driving, high output resistance current
sources and sinks have been designed. Based on the cascode regulated current source, double
regulated structures have been fabricated in Alcatel Mietec HBimos 1.2 F technology. Energy
consumption measurements have been performed in current drive mode achieving the same
pull in time as the measured one in voltage drive mode. Comparison of both techniques has
shown, as predicted by simulations, that energy distribution during the pull in event is
different in both cases.
New techniques of controlling position of the moveable electrode have been proposed
and implemented. In contrast to previous works, the proposed method does not require special
geometry changes in the electrodes of the actuator, AC drive, or special closed loop control.
Based on the fabricated current sources and sinks, three different strategies have been
proposed. Two of them based on the - topology, that is, working in closed loop operation.
One of the proposed closed loop strategies has been used to fix positions before the voltage
pull in point; the other one, assisted by a series capacitance, has fixed positions after that
point. Measurements have been taken for both kinds of strategies. As predicted by
simulations, closed loop control based on a series capacitance tends to be unstable after
several cycles of operation.
An open loop strategy has been also implemented in which the charge is defined by
the amplitude and duration of the injected current pulse. In an ideal scenario, without any kind
of leakage currents, the moveable electrode would remain fixed in the desired position,
nevertheless, physical implementations must also consider these leakage currents.
Measurements of the leakage current due to the circuit implementation and the resistive losses
in the actuator have been taken, in order to be considered in the simulations. To return the
moveable electrode to the desired position, refresh cycles must be implemented, discharging
and charging the actuator periodically. Measurements have been taken fixing the electrode
after the voltage pull in point and showing that only parasitic capacitances limit the stable
range of operation.
The proposed techniques open a wide range of possibilities, not only when positioning
is required in the whole gap, but also in biasing of capacitive sensors. Biasing a capacitive
microphone in a post pull in distance, the sensitivity has been doubled respect the same DC
voltage corresponding to a different pre pull in displacement. Thus, new techniques based on
switched currents can be applied to read-out of capacitive sensors.
2. Future research work
Further investigation on the fabrication processes of cantilever beams and bridges with
the available technology in the clean room of the Universitat Politècnica de Catalunya should
be carried out. Study of stress in these structures is mandatory to be done comparing different
methods to obtain them. Thus, not only diffusions would be used to define the thickness of the
membrane, but also solutions based on epitaxial layers and SoI wafers would be studied.
Scaling down electrostatic actuators allows having structures with lower driving
currents. Measurements of power consumption would require a BICS (Built-In Current
Sensor) circuit in series with the actuator. Thus, accurate measurements of current could be
taken showing if the predicted behaviour is also valid when smaller actuators are used.
On the other hand, lowering the pull in voltage, it could be also possible to implement
the driving circuit without requiring non-high voltage CMOS technologies. Concerning to
reference circuits, further investigation could be carried out focussing on CMOS transistors,
working in weak inversion, as sensing devices of the temperature changes. The whole circuit
would be implemented, for instance, in AMS CYE 0.8m. Leakage currents will be smaller
and driving currents can be designed to properly work with values in the order of few
nanoAmper. The future work of research is addressed to drive smaller structures with smaller
driving currents, controlling smaller leakage currents.
It would be also interesting to implement optical measurement set-ups, based on
interfereometry, in order to measure the transient behaviour of the actuator when a certain
charge is injected. Accurate models of the actuator would be obtained studying the measured
transfer function of displacement respect to charge. It could be even possible to characterise
damping inside the gap for different positions of the moveable electrode.
Finally, limitations of switched current techniques applied to capacitive sensors should
be also studied. One of the most important parameters to be investigated will be the immunity
to noise. Only detailed research will determine the suitability of this method and will limit the
field of application.
Internal Reports Presented in Foreign Institutions
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Development of Analog Interfaces for a CMOS Compatible Temperature Sensor,
Internal Report presented at TIMA Laboratoire, Grenoble, France, December 1996
High Performance CMOS Current Sources for Drivable Electrostatic Microactuators,
Internal Report presented at “Mixed-Signal Group” of Texas A&M University, Texas,
USA, December 1997
Comparator Circuit for High Voltage Applications, Internal Report presented at
“Mixed-Signal Group” of Texas A&M University, Texas, USA, December 1997
Test Modes for The FPS3, Internal Report presented at “Sensor Group” of Infineon
Technologies München Perlach, Germany, December 1998
Design of Preamplification Stages for a Gyroscope, Internal Report presented at “Sensor
Group” of Infineon Technologies München Perlach, Germany, December 1998
Publications
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R. A. Bianchi, J. M. Karam, B. Courtois, R. Nadal, F. Presseq, S. Sifflet, "CMOS
Compatible Temperature Sensor with Digital Output for Wide Temperature Range
Applications". 5 THERMINIC Conference, Rome, October 3-6 1999.
A. Rodríguez, R. Nadal, L. Castañer, "Fabrication of Microrelays by Bulk
Micromachining and Silicon to Quartz Bonding". 1999 CDE, Madrid, June 1999.
R. A. Bianchi, J. M. Karam, B. Courtois, R. Nadal, F. Presseq, S. Sifflet, "CMOS
Compatible Temperature Sensor with Digital Output for Wide Temperature Range
Applications". Microelectronic Engineering, Accepted for Publication.
M. Handtmann, R. Aigner, R. Nadal, G. Wachutka, "Methodology of Macromodelling
Demonstrated on Force Feedback /-Architectures". MSM 2000, San Diego, March
27-29 2000.
L. Castañer, J. Pons, R. Nadal-Guardia, A. Rodríguez, “Analysis of the extended
operation range of electrostatic actuators by current-pulse drive”, Submited to Sensors and
Actuators, pending of acceptance.
R. Nadal, J. Pons, L. Castañer, A. Rodríguez, “Analysis of the Charge Loss Effects in
Switched Current Drive of Electrostatic Actuators”, Conf. Dispositivos Electrónicos,
CDE-01, Granada Spain, February 15-16 2001.
J. Pons, L. Castañer, A. Rodríguez, R. Nadal, “Analysis of the Stable Operation Range in
Pulsed Current Drive of Electrostatic Actuators”, Conf. Dispositivos Electrónicos,
CDE-01, Granada Spain, February 15-16 2001.
A.Rodríguez, J. Amirola, R. Nadal, L. Castañer, “Mechanical Coupling as Excitation of
Microelectromechanical Resonators”, Conf. Dispositivos Electrónicos, CDE-01, Granada
Spain, February 15-16 2001.
R. Nadal, A.M. Brosa, “Digital Signature-Based BIST with Ramp Stimuli for Analog
Circuits”, Proceedings of the Design, Automation and Test in Europe Conference, DATE
2001, Munich, Germany, March 12-16, 2001.
R. Nadal, R. Aigner, W. Nessler, M. Handtmann, "Control of Torsional Electrostatic
Actuators by Current Driving". ASDAM Conference, Smolenice Castle, Slovenia, October
16-18, 2000.
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R. Nadal, A. Dehé, R. Aigner, L. Castañer, “New Current Drive Method to Extend the
Stable Operation Range of Electrostatic Actuators: Experimental Verification”, pending of
acceptance in The 11th International Conference on Solid-State Sensors and Actuators,
Transducers’01/Eurosensors XV, Munich Germany, June 10-14, 2001.