Power plants – Motor operated by expansion and/or contraction of a unit of... – Mass is a solid
Reexamination Certificate
2002-08-16
2004-02-17
Richter, Sheldon J. (Department: 3748)
Power plants
Motor operated by expansion and/or contraction of a unit of...
Mass is a solid
C060S528000, C310S306000, C310S309000
Reexamination Certificate
active
06691513
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention generally relates to micro-electro-mechanical systems (MEMS) and, in particular, to an improved electrothermal actuator for a micro-electro-mechanical device.
BACKGROUND OF THE INVENTION
Electrothermal actuators are used in micro-electro-mechanical devices to provide force to move elements of the micro-electro-mechanical device. Electrothermal actuators use ohmic heating (also referred to as Joule heating) to generate thermal expansion and movement. Electrothermal actuators are typically capable of providing lateral deflections of eight microns (8 &mgr;m) to ten microns (10 &mgr;m). A micron is one millionth of a meter. Electrothermal actuators typically require drive voltages of approximately five volts (5 v).
FIG. 1
illustrates a perspective view of a prior art thermal beam actuator
100
mounted on a dielectric substrate
110
. Micro-electro-mechanical systems (MEMS) technology is used to form thermal beam actuator
100
from a layer of polysilicon deposited on a dielectric substrate
110
such as silicon nitride. The components of thermal beam actuator
100
are formed from a common layer of polysilicon.
Thermal beam actuator
100
comprises first arm
120
and second arm
130
. First arm
120
and second arm
130
are joined together at one end with a rigid polysilicon mechanical link
140
. The end of thermal beam actuator
100
that comprises mechanical link
140
is able to move laterally and parallel to the surface of substrate
110
. This end of thermal beam actuator
100
is therefore referred to as the “free” end.
The other end of first arm
120
is coupled to anchor
150
and the other end of second arm
130
is coupled to anchor
160
. Anchor
150
and anchor
160
are in turn coupled to substrate
110
. This end of thermal beam actuator
100
is therefore referred to as the “fixed” end.
As shown in
FIG. 1
, thermal beam actuator
100
is formed having portions that define a gap
170
between first arm
120
and second arm
130
. Gap
170
is formed by an interior edge of first arm
120
and by an interior edge of second arm
130
. The width of gap
170
is determined by the width of mechanical link
140
. Air in gap
170
provides electrical insulation between first arm
120
and second arm
130
.
The width of second arm
130
is greater than the width of first arm
120
for most of the length of thermal beam actuator
100
. As shown in
FIG. 1
, thermal beam actuator
100
is formed having portions that define a flexure portion
180
of second arm
130
. Flexure portion
180
usually has a width that is the same width as first arm
120
. A first end of flexure portion
180
is attached to anchor
160
and a second end of flexure portion
180
is attached to the end of the wide portion of second arm
130
that is adjacent to flexure portion
180
.
Electric current (from an electrical source not shown in
FIG. 1
) may be passed through anchor
150
, through first arm
120
, through mechanical link
140
, through second arm
130
, through flexure portion
180
, through anchor
160
, and back to the electrical source. Alternatively, electric current (from an electrical source not shown in
FIG. 1
) may be passed through anchor
160
, through flexure portion
180
, through second arm
130
, through mechanical link
140
, through first arm
120
, through anchor
150
, and back to the electrical source.
Because the width of first arm
120
is narrower than the width of second arm
130
(with the exception of flexure portion
180
), the current density in first arm
120
will be greater than the current density in the wider portion of second arm
130
. The larger current density in first arm
120
causes first arm
120
to become hotter than second arm
130
. For this reason first arm
120
is sometimes referred to as a “hot” arm
120
and second arm
130
is sometimes referred to as a “cold” arm
130
. The higher level of heat in first arm
120
causes the thermal expansion of first arm
120
to be greater than the thermal expansion of second arm
130
.
Because first arm
120
and second arm
130
are joined at the free end of thermal beam actuator
100
by mechanical link
140
, the differential expansion of first arm
120
and second arm
130
causes the free end of thermal beam actuator
100
to move in an arc-like trajectory parallel to the surface of substrate
110
. When the electric current is switched off, the heating of first arm
120
and second arm
130
ceases. Then first arm
120
and second arm
130
cool down. As first arm
120
and second arm
130
cool down they return to their equilibrium positions.
The essential requirement for generating deflection in thermal beam actuator
100
is to have one arm expand more than the other arm. Prior art thermal beam actuators such as thermal beam actuator
100
are capable of producing lateral deflections (i.e., deflections parallel to the plane of substrate
110
) on the order of five microns (5.0 &mgr;m) with typical drive voltages that are less than seven volts (7.0 v).
FIG. 2
illustrates a schematic plan view of thermal beam actuator
100
. Anchor
150
is coupled to electrical connector
210
and anchor
160
is coupled to electrical connector
220
. Electrical connector
210
and electrical connector
220
are coupled to a source of electric current (not shown in FIG.
2
). Portions of the surface of second arm
130
adjacent to substrate
110
are formed into a plurality of support dimples
230
spaced along the length of second arm
130
. The plurality of support dimples
230
position second arm
130
above substrate
110
and serve as near frictionless bearings as second arm
130
moves laterally across the surface of substrate
110
. An exemplary placement of the plurality of support dimples
230
along second arm
130
is shown in FIG.
2
. Although the support dimples
230
are located under second arm
130
, they are shown in
FIG. 2
in solid outline (rather than in dotted outline) for clarity.
FIG. 3
illustrates a cross sectional view of thermal beam actuator
100
taken along line A—A of FIG.
2
.
FIG. 3
shows how second arm
130
is positioned above substrate
110
by the plurality of support dimples
230
.
Thermal beam actuator
100
may be constructed using the following typical dimensions. First arm
120
is one hundred ninety microns (190 &mgr;m) long, two microns (2 &mgr;m) wide, and two microns (2 &mgr;m) thick. Flexure portion
180
of second arm
130
is forty microns (40 &mgr;m) long, two microns (2 &mgr;m) wide, and two microns (2 &mgr;m) thick. The remaining portion of second arm
130
is one hundred fifty microns (150 &mgr;m) long, fifteen microns (15 &mgr;m) wide, and two microns (2 &mgr;m) thick. The width of gap
170
determined by mechanical link
140
is two microns (2 &mgr;m). Each support dimple
230
is five microns (5 &mgr;m) long, five microns (5 &mgr;m) wide, and one micron (1 &mgr;m) thick. Anchor
150
and anchor
160
are each fifteen microns (15 &mgr;m) long and fifteen microns (15 &mgr;m) wide. Electrical connector
210
and electrical connector
220
are each one hundred microns (100 &mgr;m) long and one hundred microns (100 &mgr;m) wide. These dimensions are exemplary. Other dimensions may be used to construct thermal beam actuator
100
.
As shown in FIG.
4
and in
FIG. 5
, thermal beam actuator
100
can be operated in two modes. In the basic “thermo-elastic” mode (illustrated in
FIG. 4
) electric current is passed through thermal beam actuator
100
from electrical connector
210
to electrical connector
220
(or vice versa). The higher current density in first arm
120
(the narrower hot arm) causes it to heat and expand more than second arm
130
(the wider cold arm). As previously explained, the differential expansion of first arm
120
and second arm
130
causes the free end of thermal beam actuator
100
to move in an arc about flexure portion
180
that is attached to anchor
160
. The deflected position of thermal beam actuator
100
is shown in dotted outline
410
in FIG.
4
. Switching off the e
PC Lens Corporation
Richter Sheldon J.
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