Abstract:
A micro-relay device is provided including a fluid non-conductor. A first substrate and a second substrate are bonded together. A channel is defined in at least one of the substrates, and has a liquid metal in the channel. Electrodes are spaced along the channel and selectively interconnectable by the liquid metal. An open via is defined in one of the substrates and contains the fluid non-conductor. A heater substrate includes a suspended heater element in fluid communication with the open via. The suspended heater element is operable to cause the fluid non-conductor to separate the liquid metal.

Description:
BACKGROUND  
         [0001]    1. Technical Field  
           [0002]    The present invention relates to an electrical micro-relay device and more specifically to a liquid metal micro-relay device.  
           [0003]    2. Background Art  
           [0004]    There are many different types of electrical micro-relay devices, and one popular type is the reed micro-relay, which is a small, mechanical contact type of electrical micro-relay device. A reed micro-relay has two reeds made of a magnetic alloy sealed in an inert gas inside a glass vessel surrounded by an electromagnetic driver coil. When current is not flowing in the coil, the tips of the reeds are biased to break contact and the device is switched off. When current is flowing in the coil, the tips of the reeds attract each other to make contact and the device is switched on.  
           [0005]    The reed micro-relay has problems related to large size and relatively short service life. As to the first problem, the reeds not only require a relatively large volume, but also do not perform well during high frequency switching due to their size and electromagnetic response. As to the second problem, the flexing of the reeds due to biasing and attraction causes mechanical fatigue, which can lead to breakage of the reeds after extended use.  
           [0006]    In the past, the reeds were tipped with contacts composed of rhodium, tungsten, or were plated with rhodium or gold for conductivity and electrical arcing resistance when making and breaking contact between the reeds. However, these contacts would fail over time. This problem with the contacts has been improved with one type of reed micro-relay called a “wet” relay. In a wet relay, a liquid metal, such as mercury, is used to make the contact. This solves the problem of contact failure, but the problem of mechanical fatigue of the reeds remained unsolved.  
           [0007]    In an effort to solve these problems, electrical micro-relay devices have been proposed that make use of the liquid metal in a channel between two micro-relay electrodes without the use of reeds. In the liquid metal devices, the liquid metal acts as the contact connecting the two micro-relay electrodes when the device is switched ON. The liquid metal is separated between the two micro-relay electrodes by a fluid non-conductor when the device is switched OFF. The fluid non-conductor is generally high purity nitrogen or other such inert gas.  
           [0008]    With regard to the size problem, the liquid metal devices afford a reduction in the size of an electrical micro-relay device since reeds are not required. Also, the use of the liquid metal affords longer service life and higher reliability.  
           [0009]    The liquid metal devices are generally manufactured by joining together two substrates with a heater in between to heat the gas. The gas expands to separate the liquid metal to open and close a circuit. Previously, the heaters were inline resistors patterned on one of the substrates between the two substrates. The substrates were of materials such as glass, quartz, and gallium arsenide upon which the heater material was deposited and etched. Since only isotropic etching could be used, the heater element would consist of surface wiring. The major drawback of surface wiring is that such wiring has poor high frequency characteristics, high connection resistance, and poor thermal transfer to the gas.  
           [0010]    More recently, suspended heaters have been developed. A suspended heater refers to a configuration in which the heating elements are positioned so that they can be surrounded all the way around by the gas.  
           [0011]    Generally, the suspended heaters are made by placing a heater material in a patterned shape on a sacrificial layer. The sacrificial layer is then etched away from under the heater material so that the heater material is suspended in space. The advantages of suspended heaters are that the gas heating efficiency is high and almost all of the heat that is generated by the heater is used to heat the gas because the surface area of the heater face that contacts the gas is large and the support areas are small. As a result, the transfer of heat to the support structure is minimized.  
           [0012]    The preferred method for manufacturing a suspended heater is to place the heater material on a silicon substrate and then to etch the silicon substrate by anisotropic etching to undercut the heater material.  
           [0013]    The problem with using silicon through out a micro-relay is that it is difficult to form multiple layer substrates with multiple layers of wiring.  
           [0014]    On the other hand, ceramic materials can be formed to provide multiple layers of wiring and surface wiring does not have to be used. Contact electrodes can be formed with connecting vias. This permits a low connection resistance and favorable high frequency characteristics. Unfortunately, the formation of a suspended heater on a ceramic substrate is problematic and so the heater element must be formed on the surface of the ceramic substrate. With the heater formed on the surface of the ceramic substrate, a considerable portion of the heat generated by the heater is transferred directly to the substrate so that the gas heating efficiency decreases substantially. As a result, it is difficult to obtain rapid switching at low power.  
           [0015]    Solutions to these problems have been long sought, but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art.  
         DISCLOSURE OF THE INVENTION  
         [0016]    The present invention provides a micro-relay device including a fluid non-conductor. A first substrate and a second substrate are bonded together. A channel is defined in at least one of the substrates, and has a liquid metal in the channel. Electrodes are spaced along the channel and selectively interconnectable by the liquid metal. An open via is defined in one of the substrates and contains the fluid non-conductor. A heater substrate includes a suspended heater element in fluid communication with the open via. The suspended heater element is operable to cause the fluid non-conductor to separate the liquid metal. The micro-relay device provides rapid switching at low power in a small package.  
           [0017]    Certain embodiments of the invention have other advantages in addition to or in place of those mentioned above. The advantages will become apparent to those skilled in the art from a reading of the following detailed description when taken with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    [0018]FIG. 1 is a bottom view of a liquid metal micro-relay in accordance with an embodiment of the present invention;  
         [0019]    [0019]FIG. 2 is a cross-section of the structure of FIG. 1 taken along line  2 - 2 ;  
         [0020]    [0020]FIG. 3A is a cross-section of the structure of FIG. 2 taken along line  3 A- 3 A;  
         [0021]    [0021]FIG. 3B is a cross-section of the structure of FIG. 3A taken along line  3 B- 3 B;  
         [0022]    [0022]FIG. 3C is a cross-section of the structure of FIG. 3A taken along line  3 C- 3 C;  
         [0023]    [0023]FIG. 4 is a bottom view of a liquid metal micro-relay in accordance with a further embodiment of the present invention;  
         [0024]    [0024]FIG. 5 is a cross-section of the structure of FIG. 4 taken along line  5 - 5 ;  
         [0025]    [0025]FIG. 6 is a bottom view of a liquid metal micro-relay in accordance with a still further embodiment of the present invention; and  
         [0026]    [0026]FIG. 7 is a cross-section of the structure of FIG. 6 taken along line  7 - 7 . 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]    Referring now to FIGS. 1 and 2, therein are shown a bottom view of a liquid metal micro-relay  100  and a cross-section of the structure of FIG. 1 taken along line  2 - 2 , both in accordance with an embodiment of the present invention.  
         [0028]    The liquid metal micro-relay  100  includes a bottom substrate  102  having heater substrates  104  and  106  bonded to its bottom surface by sealing resins  110  and  112 , respectively. The sealing resins  110  and  112  may be a Teflon® type resin or an epoxy resin, which provide an airtight bond between the heater substrates  104  and  106  and the bottom substrate  102 . The bottom substrate  102  is bonded in turn to a top substrate  108 .  
         [0029]    The term “horizontal” as used in herein is defined as a plane parallel to the major surface of a substrate, regardless of its orientation. Terms, such as “top”, “bottom”, “above”, “below”, “over”, and “under” are defined with respect to the horizontal plane.  
         [0030]    The bottom substrate  102  has a plurality of bonding pads  121  through  127  on its bottom horizontal surface for connection of electrical wires to the outside world. The bonding pads  121  through  128  are electrically conductive and connected to via conductors  131  through  138  in and extending at least partially through the bottom substrate  102 . The via conductors  133 ,  134 , and  135  form the contact electrodes for the liquid metal micro-relay  100 . The via conductors  131  through  138  can be of standard conductor materials such as copper or aluminum, and via conductors  131 ,  132 , and  136  through  138  may also be of a liquid metal since they are totally enclosed. Also, semiconductor device type vias of tungsten, tantalum, or titanium may also be formed.  
         [0031]    The bottom substrate  102  further has via conductors  141  through  144 , which also extend at least partially through the bottom substrate  102 . Further, the bottom substrate  102  has a pair of open vias  151  and  152  in the area of the heater substrates  104  and  106 , which extend through the bottom substrate  102 .  
         [0032]    Embedded in the bottom substrate  102  are conductors  161  through  164 . The conductor  161  connects the via conductors  131  and  141 , the conductor  162  connects the via conductors  132  and  142 , the conductor  163  connects the via conductors  136  and  143 , and the conductor  164  connects the via conductors  137  and  144 .  
         [0033]    The top substrate  108  contains a main channel  170  connected by subchannels  171  and  172  to the respective open vias  151  and  152  above the heater substrates  104  and  106 . The main channel  170  contains a liquid metal, such as mercury (Hg), separated into two parts, liquid metal  180 A and liquid metal  180 B by a fluid non-conductor  182 , such as high purity nitrogen or other such inert gas. The subchannels  171  and  172  are defined as being smaller than the main channel  170  so that the liquid metal does not enter the subchannels  171  and  172  but so that the fluid non-conductor  182  will. The subchannels  171  and  172  may also be formed in the bottom substrate  102 .  
         [0034]    A ground plane  185 , which is optional, may be in any position that permits impedance matching for high frequency signal transmission through the liquid metal micro-relay  100 . The ground plane  185  may be on the top substrate  108  or under the bottom substrate  102 . It may be above the main channel  170  or two separate ground planes may be positioned above and below the main channel  170 . The ground plane for purposes of illustration only is shown positioned in the bottom substrate  102  under the main channel  170 . The ground plane  185  is connected by the via conductor  138  to the bonding pad  128 .  
         [0035]    Referring now to FIGS. 3A through 3C, it may be seen that the heater substrates  104  and  106  have suspended heater elements  201  and  202 , respectively. In one embodiment, a polysilicon film with a thickness of 100 nm can be used as the suspended heater element; however, it is also possible to use a metal layer of a material such as platinum, nickel, or chrome as the heating element. In this latter case, it is necessary to coat the metal layer with a material, e.g., silicon oxide or silicon nitride, that does not react with the vapor of the liquid metal to avoid direct contact between the suspended heater element and the liquid metal.  
         [0036]    The heater substrates  104  and  106  have respective undercuts  204  and  205 , which separate the suspended heater elements  201  and  202  from the heater substrates  104  and  106 . This undercut can be manufactured by accurately controlled anisotropic etching, which allows for accurate regulation of the volume of the fluid non-conductor  182  surrounding the suspended heater elements  201  and  202 .  
         [0037]    The suspended heater elements  201  and  202  are further spaced away from the bottom substrate  102  and oriented by protrusions of the via conductors, as exemplified by the via conductors  143  and  144 , which extend from the bottom substrate  102  to separate the heater substrate  104  from the bottom substrate  102 . The heater substrate  106  is then held in place by the sealing resin  112 . To further precisely size the volume of the fluid non-conductor  182  all around the suspended heater elements  201  and  202 , the bottom substrate  102  is provided with reliefs  206  and  208  around the open vias  151  and  152 .  
         [0038]    In the present invention, the different substrates may be manufactured out of different materials such as silicon, glass, ceramic, or combinations thereof. The bottom substrate  102  of FIG. 2 is one example of a finished multilayer structure.  
         [0039]    In manufacturing substrates out of ceramic and glass, unfired materials, i.e., “green” or “raw” ceramics and glasses, are processed to make multilayer structures, which are machined and then fired. These materials have been used because of their mechanical integrity and ability to be incorporated with electrical circuitry. In some cases, they were used because of high temperature resistance, good high frequency signal characteristics, or good thermal coefficient properties.  
         [0040]    The multilayer ceramic manufacturing process consists of forming a slurry of ceramic and glass powders combined with thermoplastic organic binders and high pressure solvents. The slurry is doctor-bladed onto a carrier. After volatilization of the high vapor pressure solvents and removal from the carrier, a green ceramic tape is formed. The green ceramic tape generally has sufficient rigidity that it is self-supporting.  
         [0041]    A mechanical or laser operation may be used to form via holes, channels, recesses, or other structures in the green ceramic tape. Green ceramic is used at this point because it is softer than fired ceramic and thus easier to process by normal manufacturing tools for high volume manufacturing.  
         [0042]    For example, vias can easily be drilled, punched, or otherwise formed in the green ceramic tape. Similarly, other processes such as grinding and laser ablation are easily performed on the green ceramic tape to form channels or ducts. Various types of laser ablation can be used for patterning, such excimer lasing and YAG lasing. Using a laser allows fine structures to be formed but require more time.  
         [0043]    Thick-film printing techniques can be used to lay down conductor material on the green ceramic tape in the form of a fusible metal paste. The fusible metal paste can also fill the vias and channels or ducts to form conductor structures. These conductor structures allow the connection resistance to be low and permit impedance matching for high frequency signal transmission.  
         [0044]    A number of green ceramic tapes are placed on top of each other and aligned in multiple layers. Open vias extending through one or more layers can be provided with inserts to transmit the lamination force through unsupported regions from the top tape to the bottom tape.  
         [0045]    The green ceramic tapes are then compressed and fired.  
         [0046]    During the compression, the thermoplastic component (e.g., polyvinyl butyral) within the green layers flows and results in mutual adhesion of the green layers and conformation of the green layers around the pattern of metal paste. In addition to binding the individual green layers into a coherent green laminate structure, the lamination operation determines the density of the green laminate structure and thus the shrinkage during firing and the dimensional accuracy of the fired laminate structure. The green lamination should have a uniform density to prevent differential shrinkage during firing.  
         [0047]    A high temperature firing of the green laminate results in a volatilization of the organic components and sintering of the coherent green laminate structure into a monolithic ceramic. At the same time, the fusible metal paste fuses into an electrically and mechanically connected conductors, electrodes, and pads.  
         [0048]    By way of example, the lamination operation can impose a compressive stress of the order of 500 psi to 2,000 psi on the green laminate structure and the firing can be performed at an elevated temperature of approximately 75° C.  
         [0049]    In operation, by reference to FIG. 1, by applying a current across the bonding pads  121  and  122 , the heating element  201  of FIG. 2 is heated causing the gas above the heater substrate  102  to expand and move through the via  151  and the subchannel  171  to cause the liquid metal  180 A to separate with a center portion joining with the liquid metal  180 B. This opens the conductive connection between the bonding pad  123  and the bonding pad  124 , and closes the conductive connection between the bonding pad  124  and the bonding pad  125 .  
         [0050]    Conversely, applying a current across the bonding pads  126  and  127  heats the heating element  202  of FIG. 2 and causes the liquid metal  180 B to be separated to return the liquid metal micro-relay  100  to the position shown in FIG. 1.  
         [0051]    Referring now to FIG. 3A, therein is shown a structure of FIG. 2 along line  3 A- 3 A. The heater substrate  104  is shown with the suspended heater element  201  positioned above it. It may be seen that the suspended heater element  201  has a plurality of openings  301 - 1  through  301 -N.  
         [0052]    Referring now to FIG. 3B, therein is shown the structure of FIG. 3A taken along the line  3 B- 3 B. The heater substrate  104  has the suspended heater element  201  positioned above it and the heater substrate  104  has the undercut  204  so that the suspended heater element  201  is suspended in space.  
         [0053]    Referring now to FIG. 3C, therein is shown the structure of FIG. 3A taken along line  3 C- 3 C. The cross-section shows the openings  301 - 1  through  301 -N which would permit free flow of gases around the suspended heater element  201 .  
         [0054]    Referring now to FIGS. 4 and 5, therein are shown a bottom view of a liquid metal micro-relay  400  and a cross-section of the structure of FIG. 4 taken along line  5 - 5 , both in accordance with a further embodiment of the present invention.  
         [0055]    The liquid metal micro-relay  400  includes a bottom substrate  402  having heater substrates  404  and  406  bonded to its top surface by sealing resins  410  and  412 , respectively. The sealing resins  410  and  412  may be a Teflon® type resin or an epoxy resin between the heater substrates  404  and  406  and the bottom substrate  402 . The bottom substrate  402  is bonded in turn to a top substrate  408 .  
         [0056]    The bottom substrate  402  has a plurality of bonding pads  421  through  427  on its bottom horizontal surface for connection of electrical wires to the outside world. The bonding pads  421  through  427  are electrically conductive and connected to via conductors  431  through  437  in and extending at least partially through the bottom substrate  402 . The via conductors  433 ,  434 , and  435  form contact electrodes for the liquid metal micro-relay  400 .  
         [0057]    Further, the bottom substrate  402  has open vias  451  and  452  under the heater substrates  404  and  406  and open vias  453  and  454  under a main channel  470 . The open vias  451  and  453  are connected at the bottom by a subchannel  471  and the open vias  452  and  454  are connected at the bottom by a subchannel  472 . The subchannel  471  is covered at the bottom by a sealing plug  473  and the subchannel  472  is covered at the bottom by a sealing plug  474 . This structure is easily achievable through the use of a ceramic multilayer structure as described above.  
         [0058]    The top substrate  408  contains a main channel  470  connected by the subchannels  471  and  472  to respective open vias  451  and  452 . The main channel  470  contains a liquid metal, such as mercury (Hg), separated into two parts, liquid metal  480 A and liquid metal  480 B.  
         [0059]    In FIG. 5, it may be seen that the heater substrates  404  and  406  have suspended heater elements  501  and  502 , respectively. The heater substrates  404  and  406  have respective undercuts  504  and  505 , which separate the suspended heater elements  501  and  502  from the heater substrates  404  and  406 , respectively. The suspended heater elements  501  and  502  are further spaced away from the bottom substrate  402  by conductor pads, as exemplified by conductor pads  504  and  505  on the via conductors, as exemplified by the via conductors  436  and  437 , to separate the heater substrate  406 , which is then held in place by the sealing resin  412 . To further precisely size the volume of the fluid non-conductor  503  around the suspended heater elements  501  and  502 , the bottom substrate  402  is provided with reliefs  506  and  508 .  
         [0060]    The heater substrates  404  and  406  are respectively disposed in cavities  510  and  512  in the top substrate  408 . Since the top substrate  408  is bonded to the bottom substrate  402  by an airtight seal, the sealing resins  410  and  412  do not necessarily have to be airtight.  
         [0061]    In operation, by reference to FIG. 4, by applying a current across the bonding pads  421  and  422 , the suspended heating element  501  of FIG. 5 is heated causing the gas above the heater substrate  404  to expand and move through the via  451  and the subchannel  471  to cause the liquid metal  480 A to separate with a center portion joining with the liquid metal  480 B. This opens the conductive connection between the bonding pad  423  and the bonding pad  424 , and closing the conductive connection between the bonding pad  424  and the bonding pad  425 .  
         [0062]    Conversely, applying a current across the bonding pads  426  and  427  heats the suspended heating element  502  of FIG. 2 and causes the liquid metal  480 B to be separated to return the liquid metal micro-relay  400  to the position shown in FIG. 4.  
         [0063]    Referring now to FIGS. 6 and 7, therein are shown a bottom view of a liquid metal micro-relay  600  and a cross-section of the structure of FIG. 6 taken along line  7 - 7 , both in accordance with a still further embodiment of the present invention.  
         [0064]    The liquid metal micro-relay  600  includes a bottom substrate  602  and a top substrate  608 . The top substrate  608  may be glass and includes a lower layer  609  having heater substrates  604  and  606  bonded to its top surface by sealing resins  610  and  612 , respectively. The sealing resins  610  and  612  may be a Teflon® type resin or an epoxy resin. The bottom substrate  602  is bonded to the lower layer  609  of the top substrate  608 .  
         [0065]    The bottom substrate  602  has a plurality of bonding pads  621  through  627  on its bottom surface. The bonding pads  621  through  627  are electrically conductive and connected to via conductors  631  through  637  in and extending at least partially through the bottom substrate  602 . The via conductors  633 ,  634 , and  635  form contact electrodes for the liquid metal micro-relay  600 . The via conductors  631 ,  632 ,  636 , and  637  are respectively connected to countersunk regions  641 ,  642 ,  643 , and  644  in the lower layer  609 .  
         [0066]    Further, the lower layer  609  has countersunk regions, which form open vias  651  and  652  in the area of the heater substrates  604  and  606 . The lower layer  609  also contains a main channel  670 . The main channel  670  contains a liquid metal, such as mercury (Hg), separated into two parts, liquid metal  680 A and liquid metal  680 B. The main channel may also have top and bottom plating  690  and  691  (only the top plating  690  is shown).  
         [0067]    The bottom substrate  602  contains a pair of trenches, which form subchannels  671  and  672  from the open vias  651  and  652 , respectively, below the heater substrates  604  and  606  to the main channel  670 .  
         [0068]    In FIG. 7, it may be seen that the heater substrates  604  and  606  have attached suspended heater elements  701  and  702 , respectively. The heater substrates  604  and  606  have respective undercuts  704  and  705 , which cause the suspended heater elements  701  and  702  to be suspended away from the heater substrates  604  and  606 . The suspended heater elements  701  and  702  are further spaced away from the bottom substrate  602  by the sealing resins  610  and  612 .  
         [0069]    The heater substrates  604  and  606  are respectively disposed in cavities  710  and  712  in the top substrate  608 . Since the lower layer  609  of the top substrate  608  is bonded to the bottom substrate  602  by an airtight seal, the sealing resins  610  and  612  do not necessarily have to be airtight.  
         [0070]    The open bottom portion of the heater substrates  604  and  606  are open to the open vias  651  and  652  (with only the open via  651  shown) and connected by the subchannels  671  and  672  (with only the subchannel  671  shown) to the main channel  670 . The main channel  670  is shown with top and bottom plating  690  and  691 , respectively, adjacent the via conductors  633 ,  634 , and  635 . The top and bottom plating  690  and  691  are of metals with sufficient wetability to allow the liquid metal to conform to the shape of the main channel  670 . This prevents leakage of a fluid non-conductor  703  around the liquid metal so that the expansion force is transmitted to the liquid metal with high efficiency, and thus increases the reliability of the movement of the liquid metal so that the reliability of the switching operation can be increased.  
         [0071]    In operation, by reference to FIG. 6, by applying a current across the bonding pads  621  and  622 , the suspended heating element  701  of FIG. 7 is heated causing the fluid non-conductor  703  above the heater substrate  602  to expand and move through the via  651  and the subchannel  671  to cause the liquid metal  680 A to separate with the center section joining with liquid metal  680 B. This opens the conductive connection between the bonding pad  623  and the bonding pad  624 , and closes the conductive connection between the bonding pad  624  and the bonding pad  625 .  
         [0072]    Conversely, applying a current across the bonding pads  626  and  627  heats the suspended heating element  702  of FIG. 2 and causes the liquid metal  680 B to be separated to return the liquid metal micro-relay  600  to the position shown in FIG. 6.  
         [0073]    The present invention has been described with reference to examples in which the channel is provided or defined in the top substrate. However, the channel can alternatively be defined in the bottom substrate or in both the top and the bottom substrates. The via conductors, the open vias, conductors, electrodes, subchannels, and ground planes may similarly be formed or defined in the top and/or bottom substrates. Micro-relays in accordance with the present invention can be oriented differently from the examples shown.  
         [0074]    While the invention has been described in conjunction with specific embodiments, it is to be understood that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations that fall within the scope of the included claims. All matters hithertofore set forth or shown in the accompanying drawings are to be interpreted in an illustrative and non-limiting sense.