Abstract:
In one embodiment, the present invention is directed to a method of fabricating a micro-mechanical latching device, comprising: depositing a structural layer in a fabrication plane, wherein the first structural layer possesses a topography; depositing a sacrificial layer adjacent to the first layer such that the sacrificial layer conforms to the topography of the first layer; depositing a second structural layer that conforms to the topography of the first layer; removing the sacrificial layer; and using at least the first structural layer and second structural layer to fabricate the micro-mechanical latching device.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention is directed to a system and method for latching a micro-structure and a process for fabricating a micro-latching structure. 
     2. Background 
     Various micro-mechanical systems and micro-electromechanical systems (MEMs) are known in the art to perform various mechanical tasks. For example, Ken Goldman and Mehran Mehregany disclose a temperature memory sensor that utilizes a micro-latching mechanism in their article, “A Novel Micromechanical Temperature Memory Sensor,” presented at The International Conference on Solid-State Sensors and Actuators, Eurosensors IX, Stockholm, Sweden, (Jun. 25‥29, 1995). The memory sensor utilizes two separately micro-machined semiconductor layers. The semiconductor layers are positioned parallel to each other with one layer slightly overlapping the other. The memory sensor utilizes bimorph actuation to latch the two layers. Specifically, when the temperature of the sensor exceeds a predetermined temperature by a sufficient amount, the bimetallic effect causes deflection of one of the layers with respect to the other. The deflection causes one layer to be latched due to the overlapping portion of the other layer. Accordingly, the latched layer cannot return to its original position, when the memory sensor device returns to the predetermined temperature. 
     The memory sensor configuration is useful for temperature sensing applications. However, this configuration is not appreciably useful for other applications for various reasons. For example, the latching occurs in the same direction as bimetallic layer movement. Moreover, the memory sensor configuration only provides two possible states. Also, the memory sensor does not efficiently utilize area associated with the device. 
     As another example, U.S. Pat. No. 6,130,464 to Carr discloses a latching structure implemented within a micro-accelerometer. The micro-accelerometer includes a mass disposed on a cantilever. In response to acceleration, the mass exerts force on the cantilever causing it to deflect and to retract laterally over a positioned notch. The positioned cantilever is prevented from retracting due to the notch and is, therefore, latched into its rest position. The positioned cantilever may also be released from the latched position by application of current to create a thermal gradient. Additionally, the micro-accelerometer may be implemented in an in-plane configuration or in an out-of-plane configuration. 
     The cantilever and notch configuration of Carr is useful for accelerometer applications. However, this configuration is not appreciably useful for other applications for various reasons. For example, the latching occurs within the plane of movement of the cantilever. The disclosed cantilever and notch configuration imposes an essentially linear configuration on the device. Moreover, a very limited number of latched positions are possible according to the disclosed cantilever and notch configuration. Additionally, the disclosed cantilever and notch approach does not efficiently utilize area associated with the device. 
     Another micro-latching device is disclosed by Martin Hoffinan, Peter Kopka, and Edgar Voges in “Lensless Latching-Type Fiber Switches Using Silicon Micromachined Actuators,” 25th Optical Fiber Communication Conference, OFC 2000, Baltimore, Maryland, USA, Technical Digest, Thursday, Mar. 9, 2000, p. 250-252. In the fiber switching device, the optical fibers are positioned within “V-grooves” and moved into position using a bi-stable actuator. The latching mechanism presented is attributed to the bi-stable actuator that takes advantage of thermally buckled cantilevers. Since the actuator has two low-energy states it can be actuated into one of the low-energy positions and will remain in that position. The latch disclosed does not fasten, or mechanically connect, the fibers into position. Rather, the mechanics holding the fibers in position are dependent on the stiffness of the bi-stable actuator. Further, the bi-stable latch device does not efficiently utilize area associated with the device. 
     Another micro-latching device utilizes electrostatic force to selectively latch the device into its latched position. In general, electrostatic latches provide two plates to create a capacitor. Additionally, the two plates are held apart by structure that possesses mechanical stiffness (e.g., the structure provides a spring force). Charge is provided to each plate by creating a potential difference. Additionally, a mass may be associated with one of the plates. When the mass is accelerated, the mass exerts a force against the plate causing it to be translated. If the plate is translated toward the other plate, the electrostatic force between the plates increases. When the electrostatic force becomes greater than the force provided by the mechanical stiffness of the separation structure, the plates remain in the latched position. 
     Electrostatic latching also possesses several disadvantages. First, electrostatic latching is not a “power-off” latching mechanism. Specifically, when the potential difference between the capacitive plates is removed, the electrostatic force is removed and the device becomes unlatched. Additionally, the geometry of electrostatic latching devices is limited. Moreover, electrostatic latching devices do not efficiently utilize space. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to a system and method of latching a micro-device. In embodiments of the present invention, the micro-mechanical latch comprises at least two layers. The two layers are disposed according to a defined topography. Specifically, one of the layers may possess, for example, a recess or void. The second layer possesses a protrusion that conforms to the recess or void. When the second layer is co-located with the first layer such that the protrusion superimposes the recess or void, the second layer becomes latched. Specifically, the second layer is prevented from being translated, because the first layer is operable to transmit mechanical force. 
     Embodiments of the present invention may be fabricated utilizing known semiconductor processing technology. In accordance with embodiments of the present invention, a first layer is provided or grown. A hole, recess, or other suitable feature is cut or etched into the first layer utilizing any suitable micro-machining technique. A sacrificial layer is created or deposited to cover the first layer. Additionally, the sacrificial layer possesses sufficiently minimal thickness to avoid completely filling the hole, recess, or other topological structure of the first layer. A third layer is then deposited over the sacrificial layer. Accordingly, conformal deposition causes the third layer to possess a complementary topography. The sacrificial layer is removed by, for example, utilizing an etching solution. The first and third layers are thereby released and may be translated with respect to each other. The first and third layers may be latched by positioning the layers to associate the complementary topography of the third layer with the hole, recess, or other topological structure of the first layer. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
     FIGS.1A-1F depict exemplary arrangements of layers to illustrate fabrication of a micro-latching structure according to embodiments of the present invention; 
     FIGS. 2A and 2B are isometric views of the arrangement of layers depicted in FIGS. 1A and 1E according to embodiments of the present invention; 
     FIG. 3 depicts an arrangement of layers with one layer possessing a hole or recess with walls that are angled or tapered according to embodiments of the present invention; 
     FIG. 4A depicts a cross-sectional view of an embodiment of the present invention that utilizes three layers to implement a micro-latching mechanism; 
     FIG. 4B depicts a rotator device that includes a micro-latching mechanism according to embodiments of the present invention; 
     FIG. 5 depicts another rotator device that includes a micro-latching mechanism according to embodiments of the present invention; 
     FIG. 6 depicts another arrangement of layers include a stop structure associated with a micro-latching mechanism according to embodiments of the present invention; and 
     FIG. 7 depicts a thermal actuator device including a micro-latching mechanism according to embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1A-1F depict exemplary arrangements of layers to illustrate fabrication of a micro-latching structure according to embodiments of the present invention. FIG. 1A depicts layer  101  which has been deposited on a substrate. Layer  101  is the underlying structural layer that will be utilized, in part, to form a micro-latching device. Layer  101  may be provided or grown according to any suitable micro-fabrication technique. Suitable fabrication techniques include semiconductor processing techniques such as chemical vapor deposition (CVD). Layer  101  may comprise any suitable material, including, but not limited to, polycrystalline silicon. Layer  101  may remain attached to the substrate or may be eventually removed from substrate as desired. 
     FIG. 1B depicts layer  101  after modification of layer  101  to possess a topography. In embodiments of the present invention, recess  104  may be provided to define the topography. Alternatively, a block or protrusion may be utilized to define the topography. 
     In FIG. 1C, layer  102  is deposited adjacent to layer  101 . Layer  102  is a sacrifical layer or release layer. Layer  102  may be deposited on layer  101  utilizing suitable micro-fabrication techniques that are known in the art. Layer  102  may comprise any suitable material that possesses a relatively high etch rate in comparison to the etch rates of layers  101  and  103 . For example, and without limitation, layer  102  may comprise phosphosilicate glass (PSG) or silicon dioxide. Layer  102  conforms to, but does not obscure, recess  104  of layer of layer  101 .Layer  102  may be fabricated to possess a thickness of approximately 0.75 microns. 
     In FIG. 1D, layer  103  is deposited as a second structural layer. Layer  103  is adjacent to layer  102 . Layer  103  may also comprise any suitable material including, but not limited to, polycrystalline silicon. Layer  103  may be deposited utilizing any suitable micro-fabrication technique. Layer  103  may be fabricated to possess a thickness of approximately 1.5 microns. When layer  103  is deposited, it conforms to the topography defined by layers  101  and  102 . Specifically, the topography of layer  103  possesses protrusion  105  that is complementary to recess  104  of layer  101 . 
     Since layer  102  is a sacrificial layer, it may be removed by appropriate etching techniques. For example, layer  102  may be etched away by utilizing a hydrofluoric acid (HF). FIG. 1E depicts a cross-section after removal of layer  102 . After being released, layers  101  and  102  are no longer mechanically coupled and may be moved relative to each other. As shown in FIG. 1E, layers  101  and  102  may be translated with respect to each other by, for example, an actuator. The actuation direction is perpendicular to the planes of fabrication of layers  101  and  103 . However, if layers  101  and  102  are positioned such that protrusion  105  of layer  103  superimposes recess  104 , the layers are in a latched position. Specifically, mechanical force is transmitted in a direction in the plane of fabrication Accordingly, translation of layers  101  and  103 , with respect to each other, does not occur in force transmission direction when layers  101  and  103  are latched. 
     It shall be appreciated that the materials and thickness described above for and  103  are merely exemplary. It shall be appreciated that any number of suitable materials may be utilized for layers  101 ,  102 , and  103 . Moreover, the thickness of layers  101 ,  102 , and  103  may be varied as desired, so long as each thickness is appropriate for a selected micro-fabrication technique and permits conformal deposition. 
     FIG. 1F depicts micro-latching device  100  implemented using, in part, layers  101  and  103 . Micro-latching device further comprises actuator  106  that is operable to actuate layer  103  relative to layer  101 . For example, an electrostatic force may be applied via actuator  106 . Upon application of the electrostatic force, protrusion  105  may be positioned such that it protrudes into recess  104 . In this position, layers  103  and  101  are latched. When the electrostatic force from actuator  106  is removed, layer  103  may be disengaged and translated relative to layer  101  by actuation means  107  (e.g., a thermal bimorph). 
     FIG. 2A depicts an isometric view of layers  101 ,  102 , and  103  that corresponds to the cross-sectional view of FIG.  1 D. FIG. 2B depicts an isometric view of layers  101  and  103  that corresponds to the cross-sectional view of FIG.  1 E. FIGS. 2A and 2B also depict the force transmission direction associated with the layers. 
     FIGS. 1B,  2 A, and  2 B depict recess  104  of layer  101  with walls that are approximately parallel to each other. However, the present invention is not so limited. FIG. 3 depicts an embodiment of the present invention where layer  101  possesses recess  104  with walls that are angled or tapered. Specifically, recess  104  may be more narrow at its “top.” Recess  104  may be implemented in this manner by utilizing micro-machining techniques that are known in the art. Additionally, protrusion  105  may also be more narrow at its “top” due to the conformal deposition. By shaping recess  104  in this manner, layers  101  and  103  may remain latched unless layer  103  is centered while being translated in the actuation direction. 
     It shall be appreciated that the present invention is not limited to any specific number or arrangement of layers. Embodiments of the present invention may utilize any number of layers. For example, FIG. 4A depicts a cross-sectional view of an embodiment of the present invention that utilizes three layers to implement a micro-latching mechanism. Device  400  comprises two blocks  401  disposed on substrate  404 . Blocks  401  are disposed in association to create recess  104 . Layer  402  is immediately adjacent to blocks  401 . Layer  402  possesses a topography that is complementary to the topography defined by blocks  401 . Specifically, layer  402  comprises protrusion  105  that corresponds to recess  104 . Likewise, layer  403  is adjacent to layer  402  and possesses a topography that is complementary to the topography defined by blocks  401 . Also, device  400  may be implemented utilizing the layer deposition and etching techniques as discussed above with respect to FIGS. 1A and 1B. 
     FIG. 4B depicts an implementation of exemplary rotator device  450  utilizing a plurality of layers as shown in the cross-sectional view of FIG.  4 A. Rotator device  450  comprises layer  402 . Layer  402  comprises a complementary topography including a plurality of protrusions  105 . Also, layer  402  is implemented as a ring with protrusions  105  disposed on an interior surface of the ring. Rotator device  450  further comprises layer  403  that also comprises a complementary topography (e.g., protrusions  105 ). Layer  403  is implemented as a mechanical cantilever (or “arm” ) to actuate layer  402 . For example, an electrostatic force may be applied causing layer  403  to engage layer  402 , thereby latching the layers. Then, by moving layer  403  in the actuation direction (i.e., by angularly displacing it), layer  403  may be operable to translate layer  402  via mechanical communication. 
     FIG. 5 depicts another rotator device  500 . Rotator device  500  is substantially similar to rotator device  450 . However, rotator device  500  is implemented utilizing two layers as depicted in FIGS. 2A and 2B. In this case, layer  101  is implemented as a ring with recesses  104  disposed on an interior surface of the ring. Layer  103  is shaped as a cantilever (or “arm” ) to actuate layer  101 . Specifically, protrusions  105  may be engaged in selected recesses  104  of layer  101  to thereby latch the layers. Layer  103  may be translated, thereby causing layer  101  to rotate. 
     FIG. 6 depicts another embodiment of the present invention. FIG. 6 depicts stop  601  on substrate  404 . Structural layer  602  is disposed above stop  601  Structural layer  602  comprises recess  104 . Structural layer  603  possesses a topography that is complementary to the topography of structural layer  602 . Specifically, structural layer  603  possesses protrusion  105  that also latches structural layer  602  with structural layer  603  when protrusion  105  is positioned in recess  104 . Additionally, protrusion  105  may rest against stop  601  in the latched position. Stop  601  may be utilized to limit the amount of distance that protrusion  105  extends into recess  104  in the latched state. 
     FIG. 7 depicts exemplary device  700  according to another embodiment of the present invention. Device  700  includes layers  101  and  103 . Layer  101  is implemented as a linear bar with a plurality of recesses  104 . Layer  103  is oriented parallel to structure of layer  101 . Further, layer  103  possesses a topography that is complementary to layer  101 . Specifically, layer  103  may be latched by placing protrusions  105  in one or ones of recesses  104 . In this embodiment, layer  103  may be latched or unlatched by moving layer  101  with a suitable actuation force. Moreover, thermal bimorph actuator device  701  moves layer  101  relative to layer  103  in a direction that is perpendicular to the plane of actuation. Additionally or alternatively, an electrostatic force may be utilized to move layer  103  relative to layer  101  within the plane of fabrication if desired. 
     Device  700  may be repositioned by unlatching layer  103  from layer  101 . For example, an electro-thermal force may be applied to layer  101 , by applying current through actuator devices  701 , to cause layer  101  to be pulled down toward the grounded substrate (not shown) until protrusions  105  are no longer engaged with ones of recesses  104 . After unlatching, another thermal bimorph actuator (implemented via structural layer  103 ) may be utilized. A current may be applied to the other thermal bimorph actuator causing deflection of the thermal bimorph. The deflection causes layer  103  to move relative to layer  101 . After layer  103  has been moved, layer  101  may be placed in the latched position by removing the electro-thermal force applied to layer  101 . Additionally, it shall be appreciated that by utilizing this configuration, the components of device  700  remains in a latched position in a powered-off state. Accordingly, power consumption is not required to retain embodiments of the present invention in latched states. 
     Although embodiments of the present invention have described the initially fabricated layer as possessing a hole, recess, or void, it shall be appreciated that the present invention is not so limited. Specifically, embodiments of the present invention may dispose a single protrusion or block on the initially fabricated layer. The second structural layer may be conformally deposited so as to possess a complementary hole, recess, or void to latch over the protrusion or block of the initially fabricated layer. 
     Embodiments of the present invention may provide several advantages. First, embodiments of the present invention may be implemented utilizing any suitable geometry. Specifically, embodiments of the present invention are not limited to linear designs. Embodiments of the present invention enable a greater actuation density (as a function of area and/or volume) than existing latching mechanisms permit by, for example, fabricating the conformal latching mechanism as part of an actuator structure. Accordingly, embodiments may implement any number of advantageous applications on the micro-scale, including but not limited to, rotators and gear assemblies. Additionally, embodiments of the present invention facilitate “power-off” latching of micro-mechanical structures. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.