Bi-directional, single material thermal actuator

Details Bi-directional, single material thermal actuator Bi-directional, single material thermal actuator

2001-06-28

2003-08-19

Schwartz, Jordan M. (Department: 2873)

Optical: systems and elements

Optical modulator

Light wave temporal modulation

C359S288000, C359S290000

Reexamination Certificate

active

06608714

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The invention relates to microelectromechanical structures (MEMS). In particular, this invention relates to bi-directional thermal actuators made using MEMS technology.
2. Description of Related Art
MEMS devices and other microengineered devices are presently being developed for a wide variety of applications in view of the size, cost and reliability advantages provided by these devices. Many different varieties of MEMS devices have been created that are capable of motion or applying force, including, for example, microgears and micromotors. These MEMS devices can be employed in a variety of applications including hydraulic applications in which MEMS pumps or valves are used and optical applications that include MEMS light valves and shutters.
The manipulation of micromachined structures for applications such as microassembly is a delicate task handled best by on-chip devices. One such device, known as a horizontal thermal actuator (HTA), can be used to physically move other parts on or off chip.
FIG. 1
shows a conventional HTA
1000
. The HTA
1000
is formed on an insulated substrate (not shown), typically a nitride insulated silicon substrate, and includes a first cantilever beam
1120
, a first anchor
1130
, a second cantilever beam
1150
and a second anchor
1160
. The first anchor
1130
anchors the first cantilever beam
1120
to the substrate at a proximal end
1122
of the first cantilever beam
1120
. The first cantilever beam
1120
has a distal end
1127
. The second anchor
1160
anchors the second cantilever beam
1150
to the substrate at a proximal end
1152
of the second cantilever beam
1150
. The second cantilever beam
1150
has a distal end
1157
. The first cantilever beam
1120
has a portion
1125
that is wider than the second cantilever beam
1150
. The distal end
1127
of the first cantilever beam
1120
is coupled to the distal end
1157
of the second cantilever beam
1150
. The first cantilever beam
1120
is electrically connected to ground at the proximal end
1122
of the first cantilever beam
1120
and the second cantilever beam
1150
is electrically connected to a current source
1170
at the proximal end
1152
of the second cantilever beam
1150
. The first cantilever beam
1120
is made from the same material as the second cantilever beam
1150
.
In operation, current is applied by the current source
1170
to the second cantilever beam
1150
. The current applied by the current source
1170
causes the second cantilever beam
1150
to heat up. The first cantilever beam
1120
also heats up very slightly, but only insignificantly. The second cantilever beam
1150
, being of smaller width than the first cantilever beam
1120
, has a higher current density than the first cantilever beam
1120
. The higher current density in the second cantilever beam
1150
causes the temperature of the second cantilever beam
1150
to increase much more rapidly than the first cantilever beam
1120
(thus, beam
1120
is referred to as a “cold arm”, whereas beam
1150
is referred to as a “hot arm”). Thus, beam
1150
heats up much faster than beam
1120
, which, in turn, causes the second cantilever beam
1150
to longitudinally expand relative to beam
1120
, and therefore move toward the first cantilever beam
1120
. As a result, the coupled ends
1127
and
1157
of the first cantilever beam
1120
and the second cantilever beam
1150
move in the direction of arrow A. When the current supplied by the current source
1170
is removed, the second cantilever beam
1150
quickly cools and returns to its original position, unless another object prevents it from moving back.
The conventional HTA is reliable and typically requires less than 5 volts, making it CMOS compatible. However, it can exert force in only one direction. In many applications, it is desirable to exert force in two opposite directions. In order to exert force in opposite directions, one could provide a first set of one or more HTAs that operate in a first direction, and a second set of one or more HTAs that operate in a second, opposite direction. However, this doubles the overall size and amount of material required. In addition, bi-directional actuators have been developed that use two different materials and multiple layers having different coefficients of thermal expansion, but such devices have exhibited bending in the off state. Further, the manufacturing process for these devices is cumbersome because of the need to accommodate two different types of materials.
SUMMARY OF THE INVENTION
One aspect of this invention provides a thermal actuator that can apply force to an object in two directions without using two different materials having different coefficients of thermal expansion.
Another aspect of this invention provides a bi-directional thermal actuator which can be incorporated into an array of such actuators to apply force in two directions without sacrificing valuable chip space.
Another aspect of this invention provides a bi-directional actuator that is easily manufactured.
The bi-directional actuator includes first and second “hot” arms (instead of a single “hot” arm), and a third arm, which is the “cold” arm. The bi-directional actuator preferably is made of a single material, for example, polysilicon, using a MEMS process such as, for example, surface micromachining.
According to one embodiment, a bi-directional actuator includes first, second and third arms, each being parallel to each other, and each having distal and proximal ends. The third arm is arranged between the first and second arms. The third arm has at least one portion between its proximal and distal ends with an in-plane width that is wider than an in-plane width of each of the first and second arms. The distal ends of the first, second and third arms are coupled together to form a distal end of the bi-directional thermal actuator. The proximal end of the third arm can be connected to ground, whereas the proximal ends of the first and second arms selectively have current applied thereto. The first, second and third arms preferably are made of the same material. When current is applied to the first arm, the distal end of the thermal actuator moves and applies force in a first direction. When current is applied to the second arm, the distal end of the thermal actuator moves and applies force in a direction opposite to the first direction.
These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention.


REFERENCES:
patent: 5870007 (1999-02-01), Carr et al.
patent: 6428173 (2002-08-01), Dhuler et al.
patent: 6438954 (2002-08-01), Goetz et al.
Wen-Chih Chen, Jerwei Hsieh, and Weileun Fang, “A Novel Single layer bi-directional out-of-plane Electrothermal Microactuator”, Published Jan. 2002 in Micro Electro Mechanical Systems, 2002. The Fifteenth International Conference on pp. 693-697.*
Martin Huja and Miroslav Husak, “Thermal Microactuators for Optical Purpose”, Published in Apr., 2001in Information Technology:Coding and Computing, 2001 Proceedings.*
Hergen Kapels, Robert Aigner, and Josef Binder, “Fracture Strength and Fatique of Polysilicon Determined by a Novel Thermal Actuator”, Published Jul. 2000 in Electron Devices, IEEE Transactions on pp. 1522-1528, vol. 47, Issue 7.

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