BACKGROUND OF THE INVENTION  
         [0001]    An example of a liquid conductor-based switch device is disclosed by Jonathon Simon et al. in  A Liquid - Filled Microrelay with a Moving Mercury Drop,  6 IEEE J. OF MICROELECTROMECHANICAL SYSTEMS, 208-216. The disclosed switch device has a pair of cavities that are adjacent each other and connected by a communicating portion. Non-conductive liquid material is trapped inside the cavities. A drop of mercury is located in the communicating portion. A pair of terminals, which are disposed opposite each other, is also provided at the communicating portion. The mercury drop forms an electrical path in conjunction with the terminals.  
           [0002]    A heater is provided in each of the pair of cavities. The heater can be turned on to heat the inside of one of the cavities and vaporize the non-conductive liquid material. The vapor forms a bubble inside the cavity. The heating raises the pressure inside the cavity, causing the non-conductive liquid material to push the mercury drop toward the other cavity. As a result of the movement of the mercury drop, an electrical path that is normally in a connected or “on” state is put in a disconnected or “off” state. Conversely, movement of the mercury drop can put an electrical path that is normally in a disconnected state into a connected state.  
           [0003]    In this switch design, the non-conductive liquid material cannot be kept in a stable state that is suitable for operation. For example, operation can become unstable when a bubble is unexpectedly generated, such as by a non-uniform change in temperature, and the vapor that makes up the bubble moves undesirably between the cavities. Also, the disclosed switch device does not switch smoothly between the connected and disconnected states.  
         SUMMARY OF THE INVENTION  
         [0004]    In one aspect of the invention, a switch device comprises first and second cavities, a passage extending between the first and second cavities, a conductive liquid located in the passage and movable in the passage, an actuating liquid enclosed in each of the first and second cavities and covering inner surfaces of the first and second cavities, the actuating liquid being either an insulator or having low conductivity, and an actuating gas enclosed in each of the first and second cavities and existing as a bubble in each of the first and second cavities, the actuating gas being either an insulator or having low conductivity. In response to heating of the first cavity, part of the actuating liquid in the first cavity vaporizes and the actuating gas bubble in the first cavity expands, which causes part of the actuating liquid to be expelled out of the first cavity and the conductive liquid to move in the passage such that an electrical path that includes the conductive liquid changes from one of a connected and a disconnected state to the other of a connected state and a disconnected state. The first cavity includes a constriction element shaped to constrain the expansion of the actuating gas bubble in the first cavity.  
           [0005]    In another aspect of the invention, a method for switching an electrical path in a switch device having first and second cavities, the first cavity including a constriction element, a passage extending between the first and second cavities, a conductive liquid located in the passage and movable in the passage, an actuating liquid enclosed in each of the first and second cavities and covering inner surfaces of the first and second cavities, the actuating liquid being either an insulator or having low conductivity, an actuating gas enclosed in each of the first and second cavities and existing as a bubble in each of the first and second cavities, the actuating gas being either an insulator or having low conductivity. The method includes vaporizing part of the actuating liquid in the first cavity and expanding the actuating gas bubble in the first cavity in response to heating of the first cavity. The expansion of the gas bubble in the first cavity is constrained by the shape of the constriction element. Part of the actuating liquid is expelled from the first cavity in response to the expansion of the actuating gas bubble in the first cavity. The conductive liquid moves in response to the expulsion of part of the actuating liquid from the first cavity, which puts an electrical path that includes the conductive liquid from one of a connected and a disconnected state to the other of a connected state and a disconnected state. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]    [0006]FIG. 1 is a perspective view of a simplified structure of a switch device consistent with the invention;  
         [0007]    [0007]FIG. 2 is a simplified plan view of the structure of the passage extending between the pair of cavities shown in FIG. 1;  
         [0008]    [0008]FIG. 3 is a cross-sectional view of one of the cavities shown in FIG. 1, in which the boundary between the liquid phase portion and vapor phase portion is indicated with a solid line for a normal state, and with a broken line for a state of elevated pressure in the vapor phase portion;  
         [0009]    [0009]FIG. 4 is a perspective view of a heater for application to the cavity of FIG. 1;  
         [0010]    [0010]FIGS. 5A and 5B are plan views of the top and bottom, respectively, of a glass substrate or sheet used in another switch device consistent with the invention;  
         [0011]    [0011]FIGS. 6A and 6B are plan views of the top and bottom, respectively, of a glass substrate or sheet used in another switch device consistent the invention;  
         [0012]    [0012]FIGS. 7A and 7B are plan views of another switch device consistent with the invention;  
         [0013]    [0013]FIG. 7C is a cross section along the line  7 C- 7 C in FIG. 7B; and  
         [0014]    [0014]FIGS. 8A and 8B are perspective views of a simplified structure of another switch device consistent with the invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0015]    Switch devices in accordance with various aspects of the present invention will now be described through reference to the appended figures.  
         [0016]    In FIGS. 1 and 2, a switch device  10  in a first aspect of the invention has a pair of cavities  11  and  12  and an elongate passage  13 , which extends between the cavities  11  and  12  to enable the cavities to communicate with each other. An actuating gas  21  and an actuating liquid  22  are enclosed in each of the cavities  11  and  12 . The actuating gas  21  and actuating liquid  22  are preferably maintained in a state of equilibrium within the cavities  11  and  12 .  
         [0017]    The actuating liquid  22  is preferably a material capable of wetting glass and having a surface tension Γ of less than 7.5×10 −2  N/m. The actuating liquid  22  may be selected from among liquids that can be easily vaporized by a heater or other form of heat stimulation. For example, the actuating liquid  22  may comprise Freon (a trademark and product E. I. Du Pont de Nemours and Company Corporation), methanol, ethanol, ethyl bromide, acetone, cyclohexane, or other material with similar qualities.  
         [0018]    The actuating gas  21  may either comprise the same material as the actuating liquid  22  in its vapor phase, or comprise a mixture of the actuating liquid  22  with another gas. As shown in FIG. 3, the actuating gas  21  occupies the majority of the volume of the cavities  11  and  12 , while the actuating liquid  22  covers the inner surfaces  19  of the cavities  11  and  12 . The cavities  11  and  12  are preferably small enough to enable the actuating liquid  22  to cover the inner surfaces  19  of the cavities  11  and  12  by its own surface tension without being affected by gravity. As a result, the actuating gas  21  exists as a bubble in each of the cavities  11  and  12 . The bubble improves the reliability of the operation of the switch device  10 , as will be discussed in detail below.  
         [0019]    Referring specifically to FIG. 1, the passage  13  has a narrower width than the cavities  11  and  12 . A drop  23  of an electrically-conductive liquid is located in the passage  13 . As shown by the direction of arrow A in FIG. 2, the drop  23  of conductive liquid can move in the lengthwise direction of the passage  13 . The lengthwise direction of the passage  13  will be called the communicating direction. As shown in FIG. 2, terminals  15  and  16  are located on opposite sides of the passage  13  part-way along the length of the passage  13 . The drop  23  of conductive liquid may be positioned along the length of the passage  13  at a location where it electrically connects the terminals  15  and  16 . It is preferable for the conductive liquid constituting drop  23  to be a liquid metal, such as gallium, mercury, or an alloy that includes gallium, such as GaInSn, GaInSnAg, GaInSnBi, or GaInSnAgBi.  
         [0020]    As shown in FIG. 4, a heater  17  is located inside the cavity  11 . The heater  17  is shown located at the bottom of the cavity  11 , but may be located on another of the sides of the cavity instead. Another heater with the same construction may also be provided inside the cavity  12 . The heater  17  serves to heat and vaporize the actuating liquid  22  inside the cavities  11  and  12 . The current that flows to the heater  17  for heating may be pulsed. The internal pressure of the cavity  11  is increased by energizing the heater  17  inside the cavity  11  and vaporizing part of the actuating liquid  22 . The elevated internal pressure of the cavity  11  causes the drop  23  of conductive liquid to move along the length of the passage  13  toward the cavity  12 . As a result of its movement, the drop  23  moves out of contact with either or both of the terminals  15  and  16 . The movement of drop  23  opens the electrical circuit formed in a normal state of the switch device  10  by the drop  23  contacting the terminals  15  and  16  and puts the circuit in a disconnected state. Conversely, by turning off the heater  17  in the cavity  11  or by energizing a heater (not shown) in the cavity  12 , the drop  23  of conductive liquid can be moved in the opposite direction into contact with the terminals  15  and  16  to restore the normally-connected state of the electrical circuit.  
         [0021]    As shown in FIG. 4, the heater  17  may be composed of two heating elements that extend parallel to each other. Grooves  18  that extend parallel to the heater  17  and store additional actuating liquid  22  may also be formed. The actuating liquid  22  fills the grooves  18  through capillary action. As a result, even though the actuating gas  21  fills the majority of the volume of the cavity  11 , the actuating liquid  22  can be effectively heated by the heater  17 , and the efficiency of vaporization can be improved. The amount of actuating liquid  22  stored in the grooves  18  can be regulated by suitably selecting the depth and width of the grooves  18 . By regulating the amount of actuating liquid  22  stored in the grooves  18 , the amount of actuating liquid  22  vaporized in a specific time will not exceed a specified maximum even if power to the heater  17  is accidentally left on. As a result, there is no danger of damage to the device in such a situation. The grooves  18  can also be formed in the step of forming grooves  138  and  247  illustrated in FIGS. 5B and 6B, respectively.  
         [0022]    As described above, the actuating liquid  22  collects along the edges and in the corners of the cavities  11  and  12 , and the actuating gas  21  is located on the inside of the cavities  11  and  12 . The cavities  11  and  12  preferably have a substantially rectangular cross section. As shown in FIG. 3, the boundary  24  between the actuating gas  21  and the actuating liquid  22  is aspherical. A boundary portion  24   a  of the boundary, which extends parallel to the inner surfaces  19  of the cavities  11  and  12 , is a portion in which deformation of the boundary in response to an increase in pressure of the actuating gas  21  is restricted by the inner surfaces  19 . However, a boundary portion  24   b , which corresponds to the comers of the rectangular inner surfaces  19 , is not significantly restricted by the inner surfaces  19 .  
         [0023]    When heat is generated by the heater  17  with the boundary  24  in the state shown by the solid line in FIG. 3, part of the actuating liquid  22  vaporizes, and the pressure of the actuating gas  21  increases. The increased pressure primarily deforms the boundary portion  24   b  outwards, as indicated by the broken line  25  in FIG. 3. The increased pressure expels part of the actuating liquid  22  out of the cavity  11  to move the drop  23  of conductive liquid along the passage  13 , as described above. Although not shown in the figures, the volume of the actuating gas  21  inside the actuating liquid  22  is reduced when no heat is applied to the cavity. By providing a bubble of sufficient volume in the one of the cavities  11  and  12  that is not heated, excessive accumulation of the actuating liquid  22  is prevented, and the movement of the drop  23  is smoother.  
         [0024]    As heat increases the pressure inside the cavity  11  or  12 , the bubble of actuating gas  21  expands and the boundary portion  24   b  is deformed so that its radius of curvature decreases. The surface tension force on the surface of the actuating gas bubble increases approximately proportionally to the decrease in the radius of curvature of the boundary portion  24   b . The increased surface tension force resists further expansion of the actuating gas bubble, and limits the expulsion of the actuating liquid  22  into the passage  13 .  
         [0025]    Even when the heater  17  is not energized, heat from the environment may heat the actuating gas  21 . When such environmental heating occurs, the resulting increase in the pressure of the actuating gas  21  will deform the boundary portion  24   b  more than the boundary portion  24   a . Deforming the boundary portion  24   b  will increase the surface tension force on the surface of the actuating gas bubble.  
         [0026]    The increasing surface tension force on the surface of the actuating gas bubble constrains further expansion of the gas bubble in one of the cavities  11  and  12  subject to heating, and limits the expulsion of the actuating liquid  22  from the cavity subject to heating into the passage  13 . As a result, the switch device  10  according to the invention is highly stable and resists accidental changes in the connection state.  
         [0027]    [0027]FIGS. 5A and 5B show the glass substrates that form part of a switch device of a second aspect of the invention. FIGS. 5A and 5B show a top and a bottom glass substrate, respectively. In this aspect of the invention, as well as other aspects discussed below, specific structures are disclosed that facilitate manufacturing of the switch device. Since the switch device in these other aspects of the invention operates in the same manner as the switch device of the first aspect of the invention, the operation of the switch device in these other aspects of the invention will not be discussed.  
         [0028]    The switch device of the second aspect of the present invention may be manufactured by using the two glass substrates  110  and  120  shown in FIGS. 5A and 5B, respectively, and laying one of them on top of the other. An actuating liquid, an actuating gas, and a conductive liquid (each not shown), which act in the same way as in the first aspect of the present invention, are trapped in channels formed in the glass substrates  110  and  120 . These materials and the steps of manufacturing the switch device will be discussed in detail below.  
         [0029]    In a first manufacturing step, the glass substrate  110 , shown in FIG. 5A, is etched, such as by sandblasting, to form depressions approximately 150 μm deep. The depressions constitute cavities  131  and  132  and a passage  133 , corresponding to cavities  11  and  12  and passage  13  of the switch device  10  described above with reference to FIG. 1. The total length of the cavities  131  and  132  and the passage  133  is approximately 1.05 mm, and the total width of the cavities  131  and  132  is approximately 0.30 mm. Two rectangular chambers  141  and  142  formed in the passage  133  hold the conductive liquid in one of two stable location states and ensure the proper switching connection between the conductive liquid and the electrical traces  134 . Specifically, in the completed switch device, the conductive liquid can be latched in either of the chambers  141  and  142 . The conductive liquid connects a different electrical circuit path when located in each of the chambers  141  and  142 .  
         [0030]    In a second step, electrical traces  134  and  135 , heaters  136 , and grooves  137  and  138  are formed in and on the glass substrate  120 . The electrical traces  134  serve to form an electrical path in conjunction with the conductive liquid, and the electrical traces  135  serve to connect the heaters  136  to power sources. The electrical traces  134  and  135  and the heater  136  may be formed by known conductive film formation and patterning methods. The electrical traces  134  and  135  may be formed by patterning a tungsten film, while the heaters  136  may be formed by patterning a tantalum nitride film, for example.  
         [0031]    The groove  137  disposed parallel to the long edges of the substrate  120  and located to communicate with the passage  133  when the switch device is assembled enables the actuating liquid to move through the passage  133  when the conductive liquid is disposed in the passage  133  in the completed switch device. The grooves  138  provide a space adjacent to the heater  136  into which the actuating liquid enters to raise the efficiency of thermal transfer from the heater  136  to the actuating liquid. The groove  137  is not necessarily needed to move the actuating liquid through the passage  133  as long as the conductive liquid can be moved smoothly. This is because there are gaps between the inner surface of the passage  133  and the surface of the conductive drop that produce a similar effect. The grooves  137  and  138  may be formed simultaneously by reactive ion etching, for example. Rather than being formed in the glass substrate  120 , the groove  138  may be formed by patterning the tantalum nitride film having a thickness of approximately 10 μm that also constitutes the heater  136 .  
         [0032]    In a third step, the two glass substrates  110  and  120  are assembled with the conductive liquid, the actuating liquid, and the actuating gas trapped between them. More specifically, the glass substrate  110  is first arranged with the cavities  131  and  132  and the passage  133  facing up. Then, 6.5×10 6  μm 3  of the actuating liquid and actuating gas, such as Freon, is divided roughly in half and a dispenser is used put the portions of actuating liquid into the cavities  131  and  132 . By using a material such as Freon, which has good wettability with respect to the glass substrate  110 , as the actuating liquid, a suitable quantity of the material is retained in the cavities  131  and  132 . Additionally, 2×10 6  μm 3  of the conductive liquid, such as gallium, is placed in drops along the portion of the glass substrate  120  corresponding to the passage  133  in the glass substrate  110 . Because the glass substrate  120  is not wetted by the gallium, the surface tension of the gallium causes the form of the drops to be nearly spherical. It is also possible to use mercury instead of gallium.  
         [0033]    Next, the glass substrate  110  is turned over and positioned relative to the glass substrate  120 . The two substrates are then pressed together. As the glass substrate  110  is turned over, it faces downward, but since the Freon has good wettability, the Freon is retained in the cavities  131  and  132 . The gallium drops are held in the passage  133  of the substrate  110  by pressure. Epoxy resin is then applied around the edges of the glass substrate  110 , and the glass substrate  110  is fixed to the glass substrate  120  to complete the switch device.  
         [0034]    Assembly is preferably performed in a way that excludes gas other than Freon vapor from the cavities  11  and  12 . The glass substrate  120  is preferably selected by taking into account its wettability by Freon. If the Freon does not spreadably wet the surface of the tungsten nitride heaters, then the required wettability can be obtained by forming a thin film of silicon oxide over the tantalum nitride.  
         [0035]    [0035]FIGS. 6A and 6B are diagrams of the glass substrates used in a switch device of a third aspect of the invention. FIG. 6A and FIG. 6B show the top and bottom glass substrate, respectively. This aspect of the invention is a variation of the second aspect of the invention.  
         [0036]    In this aspect of the invention, a switch device is also completed by putting the two glass substrates  210  and  220  together and trapping the actuating liquid, actuating gas, and conductive liquid between them. In particular, the cavities  231  and  232  are shaped to maintain a stable bubble state in an extremely low surface tension liquid even with liquid materials that will not spreadably wet surfaces of the cavities  231  and  232 . As a result, it is unnecessary for the actuating liquid to exhibit spreadable wetting, which makes the selection of the actuating liquid easier. The groove  246 , which eases the flow of the actuating liquid, extends all the way to the heaters  245  and includes at either end a number of branch grooves  247  interleaved with the heater  245 . Electrical traces  243  and the heaters  245  may be formed from nickel films with a thickness of 1 μm, and are formed to be interleaved with the branch grooves  247 . This structure for the branch grooves  247  and the heater  245  provides effective thermal conduction from the heater  245  to the actuating liquid  
         [0037]    When the switch device is assembled, the actuating liquid  251  that can be vaporized so as to pool as a contiguous mass in the approximate center of the passage  233 , as indicated by the broken lines FIG. 6A, and a substantially equal amount of actuating gas  252  is placed in the two cavities  231  and  232 . Although not depicted in FIGS. 6A and 6B, a conductive liquid, such as mercury, gallium, or an alloy that includes gallium, is disposed in the passage  233 . The conductive material is able to move in the same manner as described above, and can be latched in either of first and second chambers  234  and  235  provided along the passage  233 , just as in the second aspect of the present invention.  
         [0038]    The gas material that forms bubbles in the cavities  231  and  232  in the initial state may be nitrogen gas at approximately 0.2 atm. As discussed above, the liquid material  251  is placed as a contiguous mass in the center of the passage  233 . However, since the groove  247 , which is part of the groove  246 , extends up to the proximity of the heater  245 , the actuating liquid  251  flows to the proximity of the heater  245  through capillary action. This effectively brings about the vaporization of the actuating liquid. The groove  246  does not necessarily have to continue to the center if the movement of the mercury, gallium, or other conductive liquid is sufficiently smooth.  
         [0039]    [0039]FIGS. 7A, 7B and  7 C show a switch device  300  in a fourth aspect of the invention. FIGS. 7A and 7B are plan views of the completed switch device, and FIG. 7C is a cross section along the line  7 C- 7 C in FIG. 7B. As shown in FIG. 7C, the switch device  300  is also manufactured by assembling two glass substrates  371  and  372 . The switch device  300  includes a pair of cavities  321  and  322 , and an elongate passage  330  that extends between these cavities. The passage  330  includes first, second, and third chambers  331 ,  332 , and  333 .  
         [0040]    In the initial state, a conductive liquid  350 , which may be mercury, gallium or an alloy that includes gallium, is placed as a contiguous mass in the passage  330  to form an approximately T-shape extending into the first and second chambers  331  and  332  from the center of the passage  330 . As shown in FIG. 7A, electrical traces  343  are located in each of the first and second chambers  331  and  332 . The conductive liquid  350  acts to electrically connect the electrical traces  343  located in the chambers  331  and  332 . The cavities  321  and  322  are similar to the cavities  11  and  12  described above.  
         [0041]    If heat is applied to the cavity  321 , part of the actuating liquid vaporizes and raises the internal pressure of the cavity  321 . This rise in the internal pressure of the cavity  321  causes the actuating liquid to move part of the conductive liquid  350  toward the cavity  322 , enter the third chamber  333 , and be latched therein. As a result, the conductive liquid  350  is separated into two portions, with the conductive liquid  350  located in the passage  330  being separated from the conductive liquid  350  located in the first and second chambers  331  and  332 . This separation of the conductive liquid  350  puts the electrical trace  343  in a disconnected state. The state shown in FIG. 7B can be restored by applying heat to the cavity  322 . The actuating liquid and actuating gas in the cavities  321  and  322  are maintained in a normal stable state, as described above.  
         [0042]    Band-shaped nickel films  361   a  and  361   b  are located opposite one another on the surface of the substrates  371  and  372  at some point along the passage  330 . After being put together, the two glass substrates  371  and  372  are bonded with epoxy resin  390 . A slight gap may be left between the nickel films  361   a  and  361   b , or a tight fit with no gap may be produced. The tight fit with no gap is preferable for the more effective action of the pressure. Effective operation of the switch device  300  is ensured when the conductive liquid has sufficiently good wettability with respect to nickel.  
         [0043]    Switch devices described above in the various aspects of the present invention are merely examples, and do not limit the present invention, which can be variously modified by a person skilled in the art. For example, it is also possible to manufacture more than one switch device on a single glass substrate, and a plurality of glass substrates can be laminated to create a switch device with a multilayer structure. In the former case in particular, a plurality of cavities can be radially linked to a single cavity, as shown in FIG. 8A, or a plurality of cavities can be concatenated.  
         [0044]    As shown in FIG. 8A, a switch device  400  includes a cavity  411  linked to a cavity  412  by a passage  433  and a cavity  413  linked to the cavity  412  by a passage  434 . If the cavity  412  is heated, the state of the electrical paths, which include traces  443  and  444  disposed along the passages  433  and  434 , respectively, are switched from being connected to disconnected, or vise versa.  
         [0045]    Furthermore, a plurality of cavities  411 - 413  may be linked to one another by a communicating portion located between them, as shown in FIG. 8B. In this case, the communicating portion can have a substantially radial structure or a branched structure, as shown by the passages  433  and  434  in the switch device  400  of FIG. 8B. A conductive liquid, such as a liquid metal, can be placed at an intersecting location so as to close off all of the passages or to close off the middle of all of the passages in this structure. In FIG. 8B, the electrical paths, which include traces  443  and  444  disposed along the passages  433  and  434 , respectively, are switched between connected and disconnected states by heating the cavity  412 .  
         [0046]    Other materials can also be used in place of a glass substrate. Furthermore, in addition to Freon, the vaporizable actuating liquid may be other halogen-based materials, or alcohols, acetone, and other such materials.  
         [0047]    The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light in the above teachings or may be acquired from practice of the invention. The embodiment was chosen and described in order to explain the principles of the invention and as practical application to enable one skilled in the art to use the invention in various embodiments and with various modifications suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claim appended hereto and their equivalents.
The switch device includes first and second cavities, a passage extending between the cavities, a conductive liquid located in the passage and movable therein, a conductive path that includes the conductive liquid, an actuating liquid enclosed in each of the first and second cavities and covering the inner surfaces thereof and an actuating gas enclosed in each of the first and second cavities and existing as a bubble therein. At least one of the cavities includes a constriction element shaped to constrain the expansion of the actuating gas bubble in the cavity. This limits expulsion of the actuating liquid into the passage and movement of the conductive liquid along the passage.