Abstract:
A switch comprises an input contact and at least one output contact, a single droplet of conductive liquid located in a channel, the droplet being in constant contact with the input contact, and a heater configured to heat a gas. The heated gas expands to cause the droplet to translate through the channel.

Description:
BACKGROUND OF THE INVENTION 
   Many different technologies have been developed for fabricating switches and relays for low frequency and high frequency switching applications. Many of these technologies rely on solid, mechanical contacts that are alternatively actuated from one position to another to make and break electrical contact. Unfortunately, mechanical switches that rely on solid-solid contact are prone to wear and are subject to a condition referred to as “fretting.” Fretting refers to erosion that occurs at the points of contact on surfaces. 
   To minimize mechanical damage imparted to switch and relay contacts, switches and relays have been fabricated using liquid metals to wet the movable mechanical structures to prevent solid to solid contact. It is also possible to move a volume a liquid metal, creating a switch without any solid moving parts. 
   A liquid metal microswitch is described in U.S. Pat. No. 6,559,420, assigned to the assignee of the present application, and hereby incorporaed by reference. The liquid metal microswitch in U.S. Pat. No. 6,559,420 uses gas pressure to divide one of two liquid metal switching elements to provide the switching function. For a SPDT (single pole, double throw) switch, one of the two liquid metal elements is always in contact with the input electrode and with one output electrode, and one liquid metal element is always in contact with the other output electrode (the isolated output electrode, also referred to as the isolated port). The application of pressure to the liquid metal that connects the input electrode to one of the output electrodes will toggle the switch to the other state, providing SPDT action. Unfortunately, using two elements of liquid metal causes the microswitch to be susceptible to capacitive coupling into the isolated port. Further, dividing one of the liquid metal elements of the microswitch frequently causes fragmentation of the liquid metal element through the formation of one or more microdroplets, also referred to as “satellites.” Microdroplets frequently form when one of the liquid metal elements is divided by the gas pressure. The microdroplets may enter the gas conduit through which actuating pressure is directed, clogging the conduit channel and reducing the amount of liquid metal available for switching. 
   SUMMARY OF THE INVENTION 
   In accordance with the invention a switch includes an input contact, at least one output contact, and a single droplet of conductive liquid located in a channel. The single droplet is in constant contact with the input contact. The switch also typically includes a heater configured to heat a gas. The heated gas expands causing the droplet to translate through the channel. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
       FIG. 1A  is a schematic diagram illustrating a micro circuit for a SPDT switch. 
       FIG. 1B  is a simplified cross-sectional view through section A—A of  FIG. 1A . 
       FIG. 2A  is a schematic diagram illustrating a cross-section of a portion of the liquid metal micro-switch taken through section B—B of  FIG. 1A . 
       FIG. 2B  is a schematic diagram illustrating a plan view of a portion of the main channel  120 . 
       FIG. 3  is a schematic diagram illustrating a portion of the main channel of  FIG. 1A . 
       FIG. 4A  is a plan view illustrating the feature of  FIG. 1A . 
       FIG. 4B  is a schematic diagram illustrating the feature in  FIG. 4A . 
   

   DETAILED DESCRIPTION 
   The embodiments in accordance with the invention described below can be used in any application where it is desirable to provide fast, reliable switching. While described below as switching a radio frequency (RF) signal, the architecture can be used for other switching applications, such as low frequency switching. 
     FIG. 1A  is a schematic diagram illustrating a micro circuit  100 . In this example, the micro circuit  100  can be a liquid metal micro-switch. The liquid metal micro-switch  100  is fabricated on a substrate  102  that may include one or more layers (not shown). For example, the substrate  102  can be partially covered with a dielectric material (not shown) and other material layers. The liquid metal micro-switch  100  can be a fabricated structure using, for example, thin film deposition techniques and/or thick film screening techniques which could comprise either single layer or multi-layer circuit substrates. 
   The liquid metal micro-switch  100  includes heaters  104  and  106 . The heater  104  resides within a cavity  107  and the heater  106  resides within a cavity  108 . The liquid metal micro-switch  100  also includes a cover, or cap, which is omitted from  FIG. 1A  The cavities  107  and  108  can be filled with a gas, which can be, for example, nitrogen (N 2 ) and which is illustrated using reference numeral  135 . The cavity  107  is coupled via a sub-channel  115  to a main channel  120 . Similarly, the cavity  108  is coupled via sub-channel  116  to the main channel  120 . The main channel  120  is partially filled with a single droplet  130  of liquid metal. The droplet  130  is sometimes referred to as a “slug.” The liquid metal, which is typically mercury, gallium alloy, or another liquid metal, is in constant contact with an input contact  121  and one of two output contacts  122  and  124 . 
   In this exemplary embodiment in accordance with the invention, a portion  151  of metallic material underlying the contact  122  extends past the periphery of the main channel  120  onto the substrate  102 . Similarly, a portion  152  of metallic material underlying the output contact  124  extends past the periphery of the main channel  120  onto the substrate  102 , and portions  154  and  156  of the metallic material underlying the input contact  121  extend past the periphery of the main channel  120  onto the substrate  102 . The metal portions  151 ,  152 ,  154  and  156  are generally covered by a dielectric, which is omitted from  FIG. 1A  for simplicity of illustration. Metallic material is also deposited, or otherwise applied to the substrate  102  approximately in regions  109 ,  111  and  112  to provide metal bonding capability to attach a cap, if desired. The cap, also referred to as a cover that defines walls and a roof, will be described below. Bonding the roof to the switch  100  may also be accomplished by anodic bonding, in which case the regions  109 ,  111  and  112  would include a layer of amorphous silicon. The output contacts  122  and  124  are preferably fabricated as small as possible to minimize the amount of energy used to separate the droplet  130  from the output contact  122  or from the output contact  124  when switching is desired. Further, minimizing the area of the contacts  121 ,  122  and  124  further improves electrical isolation among the contacts by minimizing the likelihood of capacitive coupling between the droplet  130  and the contact with which the droplet is not in physical contact. 
   The main channel  120  includes a feature  125  and a feature  126  as shown. The features  125  and  126  can be fabricated on the surface of the substrate  102  as, for example, islands that extend upward from the base of the main channel  120  and that contact the edge of the liquid metal droplet  130  as shown. These features  125  and  126  may also be defined as part of the cover that defines the sidewalls and roof of the channel  120 . The features  125  and  126  determine the at-rest position of the liquid metal droplet  130 . To effect movement of the liquid metal droplet  130  and therefore perform a switching function, one of the heaters  104  or  106  heats the gas  135  in the cavity  107  or  108  causing the gas  135  to expand and travel through one of the sub-channels  115  or  116 . The expanding gas  135  exerts pressure on the droplet  130 , causing the droplet  130  to translate through the main channel  120 . When the position of the droplet  130  is as shown in  FIG. 1A , the heater  104  heats the gas  135  in the cavity  107 , thus expanding and forcing the gas through the sub-channel  115  and around the feature  125  so that a relatively constant wall of pressure is exerted against the droplet  130 . The gas pressure thus exerted causes the droplet to move towards the output contact  124 . As will be described in greater detail below, the feature  125  and the feature  126  prevent the droplet  130  from extending past a definable point in the main channel  120 , but allow the droplet  130  to easily de-wet from the features  125  and  126  when movement of the droplet  130  is desired. 
   Further, because a single droplet  130  is used in the micro-switch  100 , the likelihood that the droplet  130  will fragment into microdroplets that may enter the sub-channels  115  and  116  is significantly reduced when compared to a switch in which the liquid metal droplet is divided into multiple segments to provide the switching action. 
   Although omitted for clarity in  FIG. 1A , the main channel  120  also includes one or more vents that are used to load the liquid metal into the main channel  120 . The vents can be sealed after the introduction of the liquid metal. 
   The main channel also includes one or more defined areas that include surfaces that can alter and define the contact angle between the droplet  130  and the main channel  120 . A contact angle, also referred to as a wetting angle, is formed where the droplet  130  meets the surface of the main channel  120 . The contact angle is measured at the point at which the surface, liquid and gas meet. The gas can be, in this example, nitrogen, or another gas that forms the atmosphere surrounding the droplet  130 . A high contact angle is formed when the droplet  130  contacts a surface that is referred to as relatively non-wetting, or less wettable. The wettability is generally a function of the material of the surface and the material from which the droplet  130  is formed, and is specifically related to the surface tension of the liquid. 
   Portions of the main channel  120  can be defined to be wetting, non-wetting, or to have an intermediate contact angle. For example, it may be desirable to make the portions of the main channel  120  that extends past the output contacts  122  and  124  to be less, or non-wetting to prevent the droplet  130  from entering these areas. Similarly, the portion of the main channel in the vicinity of the features  125  and  126  may be defined to create an intermediate contact angle between the droplet  130  and the main channel  120 . This will be described below. 
   The liquid metal micro-switch  100  also includes one or more gaskets, as shown using reference numerals  131 ,  132 ,  134 ,  136 ,  137  and  138 . The gaskets will be described in greater detail below. 
     FIG. 1B  is a simplified cross-sectional view through section A—A of  FIG. 1A . The substrate  102  supports the liquid metal droplet  130  approximately as shown. The droplet  130  is in contact with the input contact  121  and the output contact  122 , and rests against the feature  125 . When gas pressure is exerted through the sub-channel  115 , the gas  135  passes around and through portions of the feature  125 , exerting pressure on the droplet  130  and causing the droplet  130  to move toward the output contact  124 . In accordance with another aspect of the invention, and which will be described in greater detail below, portions of the surface  142  of the substrate  102  include a material or surface treatment designed to produce an intermediate contact angle between the droplet  130  and the surface  142 . An area of intermediate wettability forms an intermediate contact angle under the droplet and in the vicinity of, but not in contact with the input contact  121  and the output contacts  122  and  124 . In general, the contact angle between a conductive liquid and a surface with which it is in contact ranges between 0° and 180° and is dependent upon the material from which the droplet is formed, the material of the surface with which the droplet is in contact, and is specifically related to the surface tension of the liquid. A high contact angle is formed when the droplet contacts a surface that is referred to as relatively non-wetting, or less wettable. A more wettable surface corresponds to a lower contact angle than a less wettable surface. An intermediate contact angle is one that can be defined by selection of the material covering the surface on which the droplet is in contact and is generally an angle between the high contact angle and the low contact angle corresponding to the non-wetting and wetting surfaces, respectively. If the gas pressure exerted against the droplet causes the droplet  130  to overshoot the desired position, the intermediate contact angle helps cause the droplet  130  to return to the desired position in the vicinity of, and in contact with, the output contact  122  or  124 . The liquid metal micro-switch  100  also includes a cap  140 , thus encapsulating the droplet  130 . 
     FIG. 2A  is a schematic diagram  200  illustrating a cross-section of a portion of the liquid metal micro-switch  100  taken through section B—B of  FIG. 1A . An isolating dielectric layer  201  of, for example, silicon dioxide (SiO 2 ) or silicon nitride (SiN) is applied over the surface of the substrate  102 . Portions of the substrate  102  include a first metal layer  151  and a first selectively applied layer of dielectric  202  formed thereon. A second metal layer is formed over the first metal layer  151  and forms the portion of the output contact  122  that contacts the droplet  130 . The first selective dielectric layer  202  can be formed using, for example, SiO 2  or SiN. A second selectively applied dielectric layer  212  is formed over the first selective dielectric layer  202  and a portion of the second metal layer  122 . The second selective dielectric can be formed using, for example, SiO 2  or tantalum pentoxide (Ta 2 O 5 ). A layer of amorphous silicon is applied over the second selective dielectric layer  212  in the regions  111  and  109  to allow the cap  140  to be anodically bonded to the substrate  102 . Other methods of attaching the cap  140  are also possible and would influence the choice of material in the regions  109  and  111 . In accordance with this aspect of the invention, gasket portions  131  and  132  seal the main channel  120  against the cap  140 . The material from which the gasket portions  131  and  132  are formed can be a photo-definable polymer, such as, for example, polyimide. The gasket material eliminates leak paths for the pressurized gas, ensuring proper switch operation. 
     FIG. 2B  is a schematic diagram  250  illustrating a plan view of a portion of the main channel  120 . Portions of the surface  142  of the base of the main channel  120  are covered with the first metal layer  151 , the second selective dielectric  212  and the second metal layer, which forms the output contact  122 . The output contact  122  is fabricated from a metal material that is designed to contact the droplet  130  (not shown). The metal material of the output contact  122  is in electrical contact with the metal material of the first metal layer  151  ( FIG. 1A ). An opening  255  is created in the second selective dielectric layer  212  to expose the portion of the second metal layer that will be the output contact  122 . 
     FIG. 3  is a schematic diagram  300  illustrating a portion of the main channel  120  of  FIG. 1A . Much of the second selective dielectric  212  in the channel  120  is omitted from  FIG. 3  for clarity. The portion of the main channel  120  includes the feature  125  and also shows the droplet  130 . An intermediate wetting region  310  is illustrated approximately as shown in  FIG. 3  to assist in preventing the liquid metal droplet  130  from traversing past the output contact  122  and to reposition the droplet  130  over the output contact  122  should the gas pressure cause the droplet  130  to overshoot the output contact  122 . A similar intermediate wetting region would be provided in the vicinity of output contact  124  ( FIG. 1A ). 
   The main channel  120  also includes a non-wetting region  312  (the second selective dielectric layer  212 ) to further prevent the droplet  130  from entering non-wetting region  312  of the main channel  120 . The main channel  120  also includes a wetting region  314  (i.e., the input contact  121  of  FIG. 1A ). Although omitted for clarity, the surface of the cap  140  that contacts the droplet  130  may have a wetting pattern similar to the wetting pattern on the surface  142 . 
   Examples of features that define a wetting pattern and influence the contact angle formed by the droplet  130  with respect to the surface  142  include the type of material that covers the surface  142 , the patterning of a wetting material formed over a non-wetting surface, and microtexturing to alter the wettability of portions of the surface  142 , etc. 
     FIG. 4A  is a plan view illustrating the feature  125  of  FIG. 1A . The feature  125  includes sub-feature  402  and sub-feature  404 . The sub-features  402  and  404  can be formed in the main channel  120  ( FIG. 1A ) approximately as shown. The sub-feature  402  includes a point  406  and the sub-feature  404  includes a point  408 . The points  406  and  408  are designed to provide minimal contact with the droplet  130  ( FIG. 1A ). 
     FIG. 4B  is a schematic diagram illustrating the feature  125  in  FIG. 4A . In  FIG. 4B , the feature  125  is shown residing over the substrate  102  and under the cap  140 . The points  406  and  408  illustrate the portions of the feature  125  that would come into contact with the liquid metal droplet  130  as the liquid metal droplet  130  crosses either the RF output contact  122  or the RF output contact  124 . The pointed shape of the feature  125  would make it easy for the liquid metal droplet  130  to de-wet therefrom when gas pressure influences the liquid metal droplet  130  to translate in the direction away from the points  406  and  408 . The feature  125  can also be coated with a substance that alters the contact angle between the droplet  130  and the feature  125 . The feature  126  is similar to the feature  125 . 
   This disclosure describes illustrative embodiments in accordance with the invention in detail. However, it is to be understood that the invention defined by the appended claims is not limited by the embodiments described.