Abstract:
A Micro Electro-Mechanical Systems (MEMS) thermal switch. The switch includes a FET having a source and drain in a substrate and a beam isolated from the substrate, wherein the beam is a monolithic beam. The beam is positioned over the source and the drain and spaced by a predefined gap. When the thermal set point is reached, the beam moves to electrically connect the source to the drain.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation in part of a co-pending U.S. patent application Ser. No. 10/371,572, filed Feb. 21, 2003, the complete disclosure of which is hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     Conventional thermal switches use bi or trimetallic disks for performing the switching process. These thermal switches include a metal-to-metal contact that results in microwelding, arching, and oxidization that can cause the switch to prematurely fail. Also, these thermal switches cannot be reduced below a certain size limit and thus, have limited applicability. Further, these thermal switches include a number of parts that require costly manual construction. The set point of these thermal switches is determined by the material and geometry of the thermal disk used and cannot be adjusted after construction. Therefore, these thermal switch set points cannot be adjusted once the switch is fabricated.  
         [0003]     Therefore, there exists a need for an easy-to-produce thermal switch with an adjustable set point that can be efficiently manufactured.  
       SUMMARY  
       [0004]     The present invention provides a Micro Electro-Mechanical Systems (MEMS) thermal switch. The switch includes a Field Effect Transistor (FET) having a source and drain in a substrate and a beam isolated from the substrate. The beam is positioned over the source and the drain and spaced by a predefined gap. When the thermal set point is reached, the beam moves to electrically connect the source to the drain.  
         [0005]     In one aspect of the invention, a voltage source applies a voltage potential to the beam. The voltage source is adjusted in order to attain an electrostatic force between the beam and the substrate, thereby adjusting one or more of a thermal set point for the switch or hysterisis of the switch.  
         [0006]     In another aspect of the invention, the beam is a monolithic beam. In yet another aspect, the beam is arched concave or convex relative the source and the drain. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]     The preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings.  
         [0008]      FIG. 1A  illustrates a perspective view of a single beam embodiment of the present invention;  
         [0009]      FIG. 1B  illustrates a cross-sectional view of the single beam thermal switch of  FIG. 1A ;  
         [0010]      FIG. 2  illustrates a cross-sectional view of a second embodiment of a single beam thermal switch;  
         [0011]      FIG. 3  illustrates a single bimetallic beam thermal switch formed in accordance with the present invention;  
         [0012]     FIGS.  4 A-F illustrate an example process of fabricating the thermal switch shown in  FIG. 3 ; and  
         [0013]      FIG. 5  illustrates an H-beam thermal switch formed in accordance with the present invention; and  
         [0014]      FIG. 6  illustrates a circuit for controlling set point and hysterisis of the thermal switch as shown in  FIGS. 1A, 2 , and  3 .  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]     The present invention is a Micro Electro-Mechanical Systems (MEMS) thermal switch with electrostatic control.  FIG. 1A  illustrates a perspective view of a single beam MEMS thermal switch  20 . The thermal switch  20  includes a beam  24  that is arched over a source  26  and a drain  28  that are created within a silicon substrate  30 .  
         [0016]      FIG. 1B  illustrates a cross-sectional view of the thermal switch  20  along a longitudinal axis of the beam  24 . The source  26  and drain  28  are embedded within silicon substrate  30 . The silicon substrate  30  is suitably a silicon wafer. Layered on top of the source  26  and the drain  28  is a gate oxide layer  32 . The beam  24  is attached at its ends to insulator mounts  34 . The insulator mounts  34  are attached to the gate oxide layer  32  on opposite sides of the source  26  and the drain  28  in order to allow the beam  24  to arch over the source  26  and the drain  28 . In one embodiment, the beam  24  includes a monolithic beam. A monolithic beam is a single metal beam.  
         [0017]     In another embodiment, the beam  24  includes a bimetallic beam. A bimetallic beam is composed of two different metals that expand differently when bonded together. In one aspect, the beam  24  is suitably a bimetallic beam that includes a first metal on one side of the beam  24  and a second metal on the other side of the beam  24 . The first and second metals have different thermal expansion rates, thereby causing motion of the beam  24  in a direction towards the source  26  and drain  28  at a predefined temperature. The predefined temperature that causes the motion is called the set point of the thermal switch  20 . When the set point is reached, the beam  24  flexes to make contact with the source  26  and drain  28 , thereby electrically connecting the source  26  and the drain  28  and turning the switch  20  on  
         [0018]     As temperature increases, the monolithic beam expands while the position of insulator mounts  34  does not expand as rapidly. This produces stress in the monolithic beam due to the thermal expansion because both ends are fixed. The stressed beam  24  bends (or flips). The direction of the bends depends on the boundary condition of the beam.  FIGS. 2 and 3  show a beam design that forces the beam to flip down as the beam expands. The beams in  FIGS. 2 and 3  have curved ends to flip down as the beam expands.  
         [0019]      FIG. 2  illustrates another single beam thermal switch  60 . The switch  60  includes a beam  64  mounted to insulator mounts  66 . The insulator mounts  66  are oxide or any other insulating material. The insulator mounts  66  are mounted to a silicon substrate  70 . A source  72  and a drain  74  are imbedded adjacent to each other within the substrate  70 . The beam  64  is convex relative to the source  72  and the drain  74 . A gap  78  exists between the beam  64  and the source  72  and the drain  74 . As the temperature around the switch  60  increases, the beam  64  tries to expand but cannot because of the connection to the silicon substrate  70 . Thus, the beam  64  flexes to make contact with the source  72  and the drain  74 , thereby turning the switch  60  on. Not shown is a small layer of gate oxide that covers the source  104  and the drain  105 . The gate oxide acts as an insulator and prevents an electrical short between the beam  64  and the substrate  70 .  
         [0020]     There is gate oxide deposited on the source  72  and drain  74 . When the biased beam touches the gate oxide, the source  72  and drain  74  are electrically connected (or shorted).  
         [0021]      FIG. 3  illustrates a switch  80  similar in construction to the switch  60 , however, the switch  80  includes a beam  82  that is a bimetallic beam. The bimetallic beam  82  of the switch  80  allows for more aggressive motion towards or away from the source and drain embedded within the substrate than motion of the beam  64  of the switch  60 . Not shown is a small layer of oxide that covers the source and drain. In another embodiment, the switch  60  includes a beam  82  that is a monolithic beam.  
         [0022]     FIGS.  4 A-F illustrate the fabrication steps for creating the switch  80 . As shown in  FIG. 4A , a silicon substrate  100  or a single crystal silicon wafer is provided with P-type doping (e.g., Boron). It can be appreciated that the silicon substrate can be N-type doped. A photoresist layer  102  is applied to the silicon substrate and is then etched according to a mask for a source  104  and drain  105 . Next, ion implantation occurs through the etched out portions of the photoresist  102  into the substrate  100  using an N-type matter, such as phosphorous. It can be appreciated that if the silicon wafer was N-type, the implantation would be with P-type matter. The photoresist layer  102  is then removed.  
         [0023]     As shown in  FIG. 4B , an oxide layer is applied to the silicon substrate  100  and etched according to a predefined mask. The predefined mask allows removal of oxide in order to create insulating mounts  106  for the mounting of a beam. Not shown is a small layer of gate oxide that covers the source  104  and drain  105 . In one embodiment, the small layer of gate oxide is grown after the creation of the insulating mounts  106 .  
         [0024]     As shown in  FIG. 4C , a sacrificial material layer  110  is applied over the insulating posts  106  and the silicon substrate  100 . The sacrificial material layer  110  is then etched according to a predefined mask in order to define a gap that is to exist between a beam and the source  104  (not shown) and drain  105  (not shown). A non-limiting example of the sacrificial material used in the sacrificial material layer  110  is titanium or any other material that can be removed without removing other material.  
         [0025]     As shown in  FIG. 4D , a first beam layer  112  is applied, masked, and etched on top of the sacrificial material layer  110 . The first beam layer  112  can be aluminum, oxide, nitride, polysilicon, tungsten or any of a number of other materials.  
         [0026]     Next, as shown in  FIG. 4E , a second beam layer  120  is applied over the insulating mounts  106 , the sacrificial layer  110 , and the first beam layer  112 . The second beam layer  120  is etched according to a predefined mask. The second beam layer  120  can be chromium, polysilicon, or another material that has a coefficient of expansion different than the first beam layer  112 .  
         [0027]     Finally, at  FIG. 4F , the sacrificial material layer  110  is removed, thereby creating a gap  126  between the beam that includes beam layers  112  and  120  and the source  104  (not shown) and drain  105  (not shown).  
         [0028]      FIG. 5  illustrates a top view of an H-beam thermal switch  200 . The H-beam thermal switch  200  includes a source  204 , a drain  206  and an H-beam  208 . The H-beam  208  includes four mounting pads  212  and that mount to insulating pads (not shown) that attach to a silicon substrate  214 . The source  204  and the drain  206  are embedded within the silicon substrate  214 . The H-beam  208  includes two parallel beams  220  and  222 . The first beam  220  connects to securing pads  212   a  and  212   b  and connects to the second beam  222  securing pads  212   c  and  212   d . A cross-beam  230  connects the beams  220  and  222  to each other at approximately their mid-points. The cross-beam  230  is preferably sized larger than ends of each of the source  204  and drain  206 . When the thermal switch  200  has reached its set point, the H-beam  208  flexes causing the cross-beam  230  to come in contact with portions of the source  204  and the drain  206 , thereby closing the circuit.  
         [0029]      FIG. 6  illustrates a control circuit  240 . The circuit  240  includes a voltage supply  250  that provides a voltage potential to the beams in any one of the embodiments shown in  FIGS. 1A, 2 , and  3 . The voltage source  250  is adjustable. By adjusting the voltage source  250  (i.e., the voltage potential on the beam), one can adjust an electrostatic force that is created between the beam and the substrate, because the substrate acts as ground. By adjusting the electrostatic force, the set point for each of the switches and the hysterisis can be increased or decreased.  
         [0030]     While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment.