Methods of fabricating in-plane MEMS thermal actuators

Details Methods of fabricating in-plane MEMS thermal actuators Methods of fabricating in-plane MEMS thermal actuators

2001-02-06

2002-06-25

Niebling, John F. (Department: 2812)

Semiconductor device manufacturing: process

Making device or circuit responsive to nonelectrical signal

Thermally responsive

C438S052000, C310S307000, C337S003000, C337S333000

Reexamination Certificate

active

06410361

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to microelectromechanical actuators, and more particular to a thermal actuator having self-contained heating capabilities and providing in-plane actuation.
BACKGROUND OF THE INVENTION
Microelectromechanical structures (MEMS) 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, including microgears, micromotors, and other micromachined devices that are capable of motion or applying force. These MEMS devices can be employed in a variety of applications including hydraulic applications in which MEMS pumps or valves are utilized and optical applications that include MEMS light valves and shutters.
MEMS devices have relied upon various techniques to provide the force necessary to cause the desired motion within these microstructures. For example, cantilevers have been employed to transmit mechanical force in order to rotate micromachined springs and gears. In addition, some micromotors are driven by electromagnetic fields, while other micromachined structures are activated by piezoelectric or electrostatic forces. Recently, MEMS devices that are actuated by the controlled thermal expansion of an actuator or other MEMS component have been developed. For example, U.S. patent application Ser. Nos. 08/767,192; 08/936,598, and 08/965,277 are assigned to MCNC, the assignee of the present invention, and describe various types of thermally actuated MEMS devices. In addition, MEMS devices have been recently developed that include rotational connections to allow rotation with less torsional stress and lower applied force than found with torsion bar connections. For instance, U.S. patent application Ser. No. 08/719,711, also assigned to MCNC, describes various rotational MEMS connections. The contents of each of these applications are hereby incorporated by reference herein.
Thermally actuated MEMS devices that rely on thermal expansion of the actuator have recently been developed to provide for actuation in-plane, i.e. displacement along a plane generally parallel to the surface of the microelectronic substrate. However, these thermal actuators rely on external heating means to provide the thermal energy necessary to cause expansion in the materials comprising the actuator and the resulting actuation. These external heaters generally require larger amounts of voltage and higher operating temperatures to affect actuation. For examples of such thermally actuated MEMS devices see U.S. Pat. No. 5,881,198 entitled “Microactuator for Precisely Positioning an Optical Fiber and an Associated Method” issued Mar. 9, 1999, in the name of inventor Haake, and U.S. Pat. No. 5,602,955 entitled “Microactuator for Precisely Aligning an Optical Fiber and an Associated Fabrication Method” issued Feb. 11, 1997, in the name of inventor Haake.
As such, a need exists to provide MEMS thermal actuated devices that are capable of generating relatively large displacement, while operating at significantly lower temperatures (i.e. lower power consumption) and consuming less area on the surface of a microelectronic substrate. These attributes are especially desirable in a MEMS thermal actuated device that provides relatively in-plane, linear motion and affords the benefit of having a self-contained heating mechanism.
SUMMARY OF THE INVENTION
A MEMS thermal actuator device is therefore provided that is capable of providing linear displacement in a plane generally parallel to the surface of a substrate. Additionally, the MEMS thermal actuator of the present invention may provide for a self-contained heating mechanism that allows for the thermal actuator to be actuated using lower power consumption and lower operating temperatures.
The MEMS thermal actuator includes a microelectronic substrate having a first surface and an anchor structure affixed to the first surface. A composite beam extends from the anchor and overlies the first surface of the substrate. The composite beam is adapted for thermal actuation, such that it will controllably deflect along a predetermined path that extends substantially parallel to the first surface of the microelectronic substrate. In one embodiment the composite beam comprises two or more layers having materials that have correspondingly different thermal coefficients of expansion. As such, the layers will respond differently when thermal energy is supplied to the composite. An electrically conductive path may extend throughout the composite beam to effectuate thermal actuation.
In one embodiment of the invention a two layer composite beam comprises a first layer of a semiconductor material and a second layer of a metallic material. The semiconductor material may be selectively doped during fabrication so as to create a self-contained heating mechanism within the composite beam. The doped semiconductor region may afford a path of least resistance for electrical current. The doping process may also enhance the fabrication of contacts on the surface of the anchor. Additionally, the composite beam, which is characterized by a high aspect ratio in the z plane direction, may be constructed so that the first and second layers lie in planes that are generally perpendicular to the first surface of the microelectronic substrate. The vertical layer of the composite beam provides for deflection of the beam along a predetermined path that extends generally parallel to the surface of the microelectronic substrate.
In another embodiment of the invention, a MEMS thermal actuator includes two or more composite beams. The two or more composite beams may be disposed in an array or a ganged fashion, such that the multiple composite beams benefit from overall force multiplication. In one such embodiment, two composite beams are disposed on the surface of a microelectronic substrate such that the ends of the beam farthest from the anchor structure face one another. An interconnecting element is operably connected to the free ends of the composite beam. The interconnecting element is configured so as to impart flexibility when the two composite beams are actuated in unison. The flexible nature of the interconnecting element allows for the overall MEMS thermal actuator device to impart a greater distance of linear deflection.
In yet another multi composite beam embodiment, two composite beams are disposed on the surface of a microelectronic substrate such that the ends of the beam farthest from the anchor structure face one another and the beams are proximate a flexible beam structure. The flexible beam structure comprises a platform disposed between two or more anchors affixed to the substrate. One or more flexible beams operably connect the platform and the anchors. The platform is adjacent to the free ends of the composite beams such that thermal actuation of the composite beam causes the beams to operably contact the platform and deflect it in a generally linear fashion. The flexible beam structure that houses the platform compensates for variances that may occur in the thermal actuation of the composite beams.
The invention is also embodied in a method for fabricating the MEMS thermal actuators of the present invention. The method comprises affixing a first microelectronic substrate to a second microelectronic substrate. After the second substrate has been thinned to a predetermined thickness, a first portion of the MEMS thermal actuator construct is then defined in the second microelectronic substrate, including the first layer of a composite beam and a portion of the anchor structure. A doping process may be undertaken to define a path of least resistance in the first layer of the composite beam. A second layer is disposed on the first layer, the second layer and first layer comprising different materials that respond differently to thermal actuation. The variance in thermal coefficients of expansion causing the first and second layers of the composite beam to actuate the

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