Abstract:
In accordance with the invention, a piezoelectrically actuated relay that switches and latches by means of a liquid metal is disclosed. The relay operates by means of a plurality of bending mode piezoelectric elements used to cause a pressure differential in a pair of fluid chambers. The piezoelectric elements act upon a membrane which in turn acts upon a fluid which fills the chambers. The differential pressure causes the liquid metal drop to overcome the surface tension forces that would hold the bulk of the liquid metal drop in contact with the contact pad or pads near the actuating piezoelectric element. The switch latches by means of surface tension and the liquid metal wetting to the contact pads.

Description:
BACKGROUND 
     Piezoelectric materials and magnetostrictive materials (collectively referred to below as “piezoelectric materials”) deform when an electric field or magnetic field is applied. Thus piezoelectric materials, when used as an actuator, are capable of controlling the relative position of two surfaces. 
     Piezoelectricity is the general term to describe the property exhibited by certain crystals of becoming electrically polarized when stress is applied to them. Quartz is a good example of a piezoelectric crystal. If stress is applied to such a crystal, it will develop an electric moment proportional to the applied stress. 
     This is the direct piezoelectric effect. Conversely, if it is placed in an electric field, a piezoelectric crystal changes its shape slightly. This is the inverse piezoelectric effect. 
     One of the most used piezoelectric materials is the aforementioned quartz. Piezoelectricity is also exhibited by ferroelectric crystals, e.g. tourmaline and Rochelle salt. These already have a spontaneous polarization, and the piezoelectric effect shows up in them as a change in this polarization. Other piezoelectric materials include certain ceramic materials and certain polymer materials. Since they are capable of controlling the relative position of two surfaces, piezoelectric materials have been used in the past as valve actuators and positional controls for microscopes. Piezoelectric materials, especially those of the ceramic type, are capable of generating a large amount of force. However, they are only capable of generating a small displacement when a large voltage is applied. In the case of piezoelectric ceramics, this displacement can be a maximum of 0.1% of the length of the material. Thus, piezoelectric materials have been used as valve actuators and positional controls for applications requiring small displacements. 
     Two methods of generating more displacement per unit of applied voltage include bimorph assemblies and stack assemblies. Bimorph assemblies have two piezoelectric ceramic materials bonded together and constrained by a rim at their edges, such that when a voltage is applied, one of the piezoelectric materials expands. The resulting stress causes the materials to form a dome. The displacement at the center of the dome is larger than the shrinkage orexpansion of the individual materials. However, constraining the rim of the bimorph assembly decreases the amount of available displacement. Moreover, the force generated by a bimorph assembly is significantly lower than the force that is generated by the shrinkage or expansion of the individual materials. 
     Stack assemblies contain multiple layers of piezoelectric materials interlaced with electrodes that are connected together. A voltage across the electrodes causes the stack to expand or contract. The displacements of the stack are equal to the sum of the displacements of the individual materials. Thus, to achieve reasonable displacement distances, a very high voltage or many layers are required. However, conventional stack actuators lose positional control due to the thermal expansion of the piezoelectric material and the material(s) on which the stack is mounted. 
     Due to the high strength, or stiffness, of piezoelectric material, it is capable of opening and closing against high forces, such as the force generated by a high pressure acting on a large surface area. Thus, the high strength of the piezoelectric material allows for the use of a large valve opening, which reduces the displacement or actuation necessary to open or close the valve. 
     With a conventional piezoelectrically actuated relay, the relay is “closed” by moving a mechanical part so that two electrode components come into electrical contact. The relay is “opened” by moving the mechanical part so that the electrode components are no longer in electrical contact. The electrical switching point corresponds to the contact between the electrode components of the solid electrodes. 
     Liquid metal micro switches have been developed that use liquid metal as the switching element and the expansion of a gas when heated to actuate the switching function. The liquid metal has some advantages over other micromachined technologies, such as the ability to switch relatively high power (approximately 100 mW) using metal-to-metal contacts without microwelding, the ability to carry this much power without overheating the switch mechanism and adversely affecting it, and the ability to latch the switching function. However, the use of a heated gas to actuate the switch has several disadvantages. It requires a relatively large amount of power to change the state of the switch, the heat generated by switching must be rejected effectively if the switch duty cycle is high, and the actuation speed is relatively slow, i.e., the maximum switching frequency is limited to several hundred Hertz. 
     SUMMARY 
     The present invention uses a piezoelectric method to actuate liquid metal switches. The actuator of the invention uses piezoelectric elements in a bending mode rather than in a shear mode. A piezoelectric driver in accordance with the invention is a capacitive device which sores energy rather than dissipating energy. As a result, power consumption is much lower, although the required voltages to drive it may be higher. Piezoelectric pumps may be used to pull as well as push, so there is a double-acting effect not available with an actuator that is driven solely by the pushing effect of expanding gas. Reduced switching time results from use of piezoelectric switches in accordance with the invention. 
     A piezoelectrically actuated liquid metal switch in accordance with the invention is comprised of a plurality of layers. Liquid metal is contained within a channel in one layer and contacts switch pads on a circuit substrate. The amount and location of the liquid metal in the channel is such that only two pads are connected at a time. The metal is movable so that it contacts the center pad and either end pad by creating an increase in pressure between the center pad and the first end pad such that the liquid metal breaks and part of it moves to connect to the other end pad. A stable configuration results due to the latching effect of the liquid metal as it wets to the pads and is held in place by surface tension. 
     An inert and electrically nonconductive liquid fills the remaining space in the switch. The pressure increase described above is generated by the motion of a piezoelectric pump or pumps. The type of pump of the invention utilized the bending action of piezoelectric elements on a membrane to create positive and negative volume changes. These actions may cause pressure decreases, as well as increases, to assist in moving the liquid metal. 
    
    
     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. 
     FIG. 1 shows a side view of the layers of a piezoelectric metal switch in accordance with the invention. 
     FIG. 2 shows a side cross section of a side view of the layers of a piezoelectric switch in accordance with the invention. 
     FIG. 3 shows a top level view of the substrate layer with the switch contacts. 
     FIG. 4A is a top view of the liquid metal channel layer. 
     FIG. 4B is a side-sectional view of the liquid metal layer. 
     FIG. 5A is a top view of the piezoelectric layer showing two sets of piezoelectric elements. 
     FIG. 5B shows a side-sectional view of the piezoelectric layer. 
     FIG. 6 shows a top view of the actuator fluid reservoir layer. 
     FIG. 7 shows an alternate side cross section of a side view of the layers of a piezoelectric switch in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a side view of an embodiment of the invention showing four layers of a relay  100 . The top layer  110  is an actuator fluid reservoir layer and acts as a reservoir for fluid used in the actuator. The second layer  120  is a piezoelectric layer which houses a piezoelectric switching mechanism. The third layer  130  is a liquid metal channel layer and houses a liquid metal used in the switching mechanism. The substrate layer  140  acts as a base and provides a common foundation for a plurality of circuit elements that may be present. 
     FIG. 2 shows a cross sectional view of an embodiment of an actuator  100  in accordance with the invention. FIG. 2 is also a cross sectional view of FIG.  1 . The actuator fluid reservoir layer  110  has a chamber  150  that houses a plurality of piezoelectric elements  160  utilized the relay  100 . The chamber  150  also contains a volume of actuator fluid. The actuator fluid is an inert, electrically nonconductive fluid. This fluid is preferably a low viscosity inert organic liquid such as a low molecular weight perfluorocarbon such as is found in the 3M line of Fluorinert products. It may alternatively consist of a light mineral or synthetic oil, for example. The piezoelectric elements  160  are grouped into two sets. It is understood by those skilled in the art that the grouping of the piezoelectric elements  160  is a function of the purpose of the actuator  100 . Accordingly, the grouping of the piezoelectric elements  160  may result in multiple sets equaling more than two. 
     Each set of piezoelectric elements  160  in FIG. 2 is attached to a membrane  170  which forms a portion of the top of piezoelectric layer  120 . In a preferred embodiment of the invention, the membranes  170  are constructed of metal. In other embodiments of the invention, the membranes  170  are constructed of a polymer. In still other embodiments of the invention, the membranes are constructed of any material that exhibits sufficient pliability to flex in response to bending of the piezoelectric elements  160 . The membranes  170  are bendable in either an upward or a downward fashion responsive to the piezoelectric elements  160 . 
     The embodiment of the invention shown in FIG. 2, the piezoelectric elements are shown as having been laminated on top, and above, of the piezoelectric layer  120   
     The membranes  170  also form a barrier between the piezoelectric elements  160  and an actuator fluid chamber  180  located in the piezoelectric layer  120 . Two actuator fluid chambers  180  are shown in FIG. 2 separated by a portion of the piezoelectric layer. The actuator fluid chambers  180  are filled with actuator fluid. A gap in the liquid metal layer  130  opposite each set of the piezoelectric elements  160  provides conduits between the fluid chambers  180  and the liquid metal layer  130 . The conduits allow fluid flow between the chambers  180  and the liquid metal layer  130 . 
     The liquid metal layer  130  comprises a liquid metal  190  which is contained within a channel  195  and a set of switch contact pads  200  located on the circuit substrate  140 . The space in the channel  195  which is not filled with liquid metal  190  is filled with the fluid. The liquid metal is inert and electrically conductive. The amount and location of the liquid metal  190  is such that only two pads  200  are connected at a time. The center pad  200  will always be contacted and either the left or rights pad  200 . In the embodiment of the invention shown in FIG. 2, the liquid metal  190  is in contact with the center pad  200  and the right pad  200 . The liquid metal  190  is moved to contact the left pad  200  by the bending action of the piezoelectric elements  160 . 
     Bending of the piezoelectric elements  160  causes either an increase or a decrease in chamber  180 . In the example shown in FIG. 2, the set of piezoelectric elements on the right bend downward to cause an increase in the right chamber  180 . The increase in pressure causes the liquid metal  190  to move leftward until it is contacting the center pad  200  and the left pad  200 . The pumping actions of the piezoelectric elements create either a positive or a negative volume, and pressure, change in chambers  180 . When the right set of piezoelectric elements  160  causes an increase in pressure—decreased volume—the left side can cause a decrease in pressure—increased volume—by bending upward. The opposite movements of the two sets of piezoelectric elements  160  assist in movement of the liquid metal  200 . 
     The piezoelectric elements  160  may be laminated to the membrane  170  or they may be deposited as thinfilm or thickfilm layers on the membrane  170 . FIG.  2  shows sets of five piezoelectric elements  160  on both the right and left side. It is understood by those skilled in the art that the number of piezoelectric elements  160  in each set is variable. As many as one to ten or more piezoelectric elements are possible depending only on the size of each element and the size of the application. The membrane is normally made of metal, although other materials are possible, such as polymers. 
     In a preferred embodiment of the invention, the liquid metal  190  is mercury. In an alternate preferred version of the invention, the liquid metal is an alloy containing gallium. 
     In operation, the switching mechanism of the invention operates by bending mode displacement of the piezoelectric elements  160 . An electric charge is applied to the piezoelectric elements  160  which causes the elements  160  to bend. As discussed above, the bending action of the piezoelectric elements can be on an individual basis—one set at a time—or in a cooperative manner—both sets together. Downward bending of the piezoelectric elements  160  of one of the sets causes an increase of pressure and decrease of volume in the chamber  180  directly below the downward bending set. This change in pressure/volume causes displacement of the moveable liquid metal  190 . To increase the effectiveness, the piezoelectric elements of the other set can bend upward at the same time. Reversing the bending motion of the piezoelectric elements  160  causes the liquid metal  190  to displace in the opposite direction. The piezoelectric elements  160  are relaxed, i.e. the electric charge is removed, once the liquid metal  190  has displaced. The liquid metal  190  wets to the contact pads  200  causing a latching effect. When the electric charge is removed from the piezoelectric elements  160 , the liquid does not return to its original position but remains wetted to the contact pad  200 . 
     FIG. 3 shows a top level view of the substrate layer  140  with the switch contacts  200 . The switch contacts  200  can be connected through the substrate  140  to solder balls (not shown) on the opposite side for the routing of signals. It is understood that there are alternatives to routing of signals. For instances, the signal routing can be place in the substrate layer  140 . It is also understood that the switch pads  200  in FIG. 2 are merely representative of the switch pads of the invention. Specifically, the substrate layer  140  and the switch pads  200  are not necessarily proportional to the switch pads and substrate layer in FIG.  3 . 
     FIG. 4A is a top view of the liquid metal channel layer  120 . The liquid metal layer  120  comprises the liquid metal channel  195  and a pair of through-holes  210  which act as the conduits for movement of liquid from the liquid metal channel  195  and the chamber  180  shown in FIG.  2 . FIG. 4B is a side-sectional view of the liquid metal layer  120  at the A—A point. The liquid metal channel  195  is shown connecting to the through-hole  210 . 
     FIG. 5A is a top view of the piezoelectric layer  120  showing two sets of piezoelectric elements  160 . The piezoelectric elements  160  are above the fluid chambers  180  and are affixed to the membrane  170 . The fluid chambers  180  connect to fluid flow restrictors  220 . The fluid flow restrictors  220  are conduits that connect to the fluid reservoir  150  shown in FIG.  2 . The fluid flow restrictor  220  is shown here for purposes of illustration only. It is understood by those skilled in the art that the restrictors  220  that connect the pumping chamber  180  with the fluid reservoir is small and assist in causing the pressure pulse to move the liquid metal by directing most of the fluid flow from the pumping action, of the piezoelectric elements  160  and membrane  170  into the channel  195  rather than into the fluid reservoir. 
     FIG. 5B shows a side-sectional view of the piezoelectric layer  120  at the point A—A. The piezoelectric elements  160  are affixed to the membrane  170  and above the chamber  180 . The chamber  180  connects to the fluid flow restrictor  220 . 
     FIG. 6 shows a top view of the actuator fluid reservoir layer  110  with the reservoir  150  and a fill port  230 . The fluid reservoir  150  is illustrated here as a single part in one embodiment of the invention. In an alternate embodiment of the invention, the fluid reservoir is made from multiple sections. The fluid reservoir  150  is a depository of the working fluid and has a compliant wall to keep pressure pulse interactions between pumping elements—crosstalk—to a minimum. The fluid reservoir  150  is filled after the switch assembly  100  has been assembled. The fill port  230  is sealed after the reservoir has been filled. 
     FIG. 7 shows an alternate embodiment of the invention wherein the fluid reservoir comprises multiple compartments  240 . The wall  250  separating the multiple compartments has a pressure relief port  260  which connects to both of the compartments  240  which equalizes the pressure between compartments  240 , and each of the compartments  240  has a compliant exterior wall which keeps pressure pulse interactions between pumping elements—crosstalk—to a minimum. 
     While only specific embodiments of the present invention have been described above, it will occur to a person skilled in the art that various modifications can be made within the scope of the appended claims.