Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
2001-06-19
2003-04-08
Phan, Trong (Department: 2818)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
C073S514320
Reexamination Certificate
active
06543286
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to electrical driving circuits. In particular, the invention relates to electrical driving circuits configured to drive an array of electrostatic actuators, for example, micro electromechanical systems used for optical switches.
BACKGROUND ART
The technology of micro electromechanical systems (MEMS) originates from technology developed over decades for the fabrication of silicon integrated circuits. MEMS permits the fabrication of large arrays of microactuators that can serve as mirrors, valves, pumps, etc. for a variety of applications. Although the invention is not so limited, an important application is an array of tiltable mirrors integrated in a single substrate and used for switching of a large number of optical beams. Each mirror is part of a separately controlled actuator. These actuators are typically electrostatic in nature and require actuation voltages near 100V to operate.
An example of one cell of an electrostatically controlled MEMS array is illustrated in plan view in FIG.
1
and in cross-sectional view in FIG.
2
. The cell is one of many such cells arranged typically in a two-dimensional array in a bonded structure including multiple levels of silicon and oxide layers. The cell includes a gimbal structure of an outer frame
110
twistably supported in a support structure
112
of the MEMS array through a first pair of torsion beams
114
extending along and twisting about a minor axis. The cell further includes a mirror plate
116
having a reflective surface
117
twistably supported on the outer frame
110
through a second pair of torsion beams
118
arranged along a major axis perpendicular to the minor axis and twisting thereabout. In the favored MEMS fabrication technique, the illustrated structure is integrally formed in an epitaxial (epi) layer of crystalline silicon. The process has been disclosed in US Provisional Application, Serial No. 60/260,749, filed Jan. 10, 2001, incorporated herein by reference in its entirety.
The structure is controllably tilted in two independent dimensions by a pair of electrodes
120
under the mirror plate
116
and another pair of electrodes
122
under the frame
110
. The electrodes
120
,
122
are symmetrically disposed as pairs across the axes of their respective torsion beams
18
,
114
. A pair of voltage signals V
A
, V
B
are applied to the two mirror electrodes
120
, and another pair of voltage signals are applied to the frame electrodes
122
while a common node voltage signal V
C
is applied to both the mirror plate
116
and the frame
110
. The driving circuitry for these and similar voltage signals is the central focus of this invention.
Horizontally extending air gaps
124
,
126
are formed respectively between the frame
110
and the support structure
112
and between the mirror plate
116
and the frame
110
and overlie a cavity or vertical gap
128
formed beneath the frame
110
and mirror plate
116
so that the two parts can rotate. The support structure
112
, the frame
110
, and the mirror plate
116
are driven by the common node voltage V
C
, and the frame
110
and mirror plate
116
form one set of plates for variable gap capacitors. Although
FIG. 2
illustrates the common node voltage V
C
being connected to the mirror plate
116
, in practice the electrical contact is made in the support structure
112
and electrical leads are formed on top of the torsion beams
114
,
118
to apply the common node voltage signal to both the frame
110
and the mirror plate
116
, which act as top electrodes. The electrodes
118
,
120
are formed at the bottom of the cavity
128
so the cavity forms the gap of the four capacitors, two between the bottom electrodes
118
and the frame
110
, and two between the bottom electrode
120
and mirror plate
116
.
The torsion beams
114
,
118
act as twist springs attempting to restore the outer frame
110
and the mirror plate
116
to neutral tilt positions. Any voltage applied across opposed electrodes exerts a positive force acting to overcome the torsion beams
114
,
118
and to close the variable gap between the electrodes. The force is approximately linearly proportional to the magnitude of the applied voltage, but non-linearities exist for large deflections. If an AC drive signal is applied well above the resonant frequency of the mechanical elements, the force is approximately linearly proportional to the root mean square (RMS) value of the AC signal. In practice, the precise voltages needed to achieve a particular tilt are experimentally determined.
Because the capacitors in the illustrated configuration are paired across the respective torsion beams
114
,
118
, the amount of tilt is determined by the difference of the RMS voltages applied to the two capacitors of the pair. The tilt can be controlled in either direction depending upon the sign of the difference between the two RMS voltages.
As shown in
FIG. 2
, the device has a large lower substrate region
130
and a thin upper MEMS region
132
, separated by a thin insulating oxide layer
134
but bonded together in a unitary structure. The tilting actuators are etched into the upper region, each actuator suspended over the cavity
128
by several tethers. The electrodes are patterened onto the substrate, which can be an application specific integrated circuit (ASIC), a ceramic plate, a printed wiring board, or some other substrate with conductors patterned on its surface. The actuators in the upper region form a single electrical node called the “common node”. Each actuator is suspended above four electrodes, each electrode being isolated from every other electrode. To cause the actuator to tilt in a specific direction, an electrostatic force is applied between the actuator and one or more of its electrodes by imposing a potential difference between the common node and the desired electrode. Each actuator has two pairs of complementary electrodes, one causing tilt along the major axis and the other causing tilt along the minor axis. Fabrication details are supplied in the aforementioned Provisional Application No. 60/260,749.
One drawback of electrostatic actuation used for this micromirror is a phenomenon known as “snapdown”. Because electrostatic force is inversely proportional to the distance between the electrodes, there comes an angle at which the attractive force increases very rapidly with greater electrode proximity. Beyond this angle, a small decrease in distance leads to an enormous increase in force, and the electronic control loop becomes unstable, causing the electrodes to snap together. With such an actuator in which the electrodes comprise a flat plate suspended over a cavity by small tethers, a rule of thumb states that the plate will begin to snap down at a deflection corresponding to approximately four ninths the depth of the cavity. Hence, in order to achieve a deflection of &thgr; at the end of the cantilever, the cavity must be approximately 2.25 &thgr; deep. Electrostatic MEMS mirror arrays have been used as video display drivers, but they operated at two voltage levels, zero and full snap-down. In contrast, the mirrors described above must be nearly continuously tiltable over a significant angular range.
Optical constraints determine the deflection distance requirement for the electrostatic micromirror. The RMS voltage level required for a given amount of deflection results from a combination of actuator size, tether spring constant, and cavity depth. The cavity depth required to avoid snapdown generally dictates the use of relatively high voltages, typically in excess of 40V, the upper limit for many standard IC processes. The generation of such voltages requires an electronic system composed of high-voltage (HV) semiconductor components, either off-the-shelf or customized, which are fabricated by specialized HV processes, such as the HVCMOS process available from Supertex, Inc.
The application for which the invention was developed requires a 12×40 array of micromirrors, and the mirrors must be independently til
Garverick Steven L.
Nagy Michael L.
Guenzer Charles
Movaz Networks, Inc.
Phan Trong
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