Patent Publication Number: US-7211754-B2

Title: Fluid-based switch, and method of making same

Description:
BACKGROUND 
     A fluid-based switch such as a liquid metal micro switch (LIMMS) comprises a switching fluid (e.g., mercury) that serves to electrically couple and decouple at least a pair of electrically conductive elements in response to forces that are applied to the switching fluid. Typically, the forces are applied to the switching fluid by means of an actuating fluid that is heated or pumped. 
     SUMMARY OF THE INVENTION 
     In one embodiment, a switch comprises first and second mated substrates that define therebetween a number of cavities. A plurality of electrically conductive elements extends to near at least a first of the cavities. A switching fluid is held within at least the first of the cavities and serves to electrically, but not physically, couple and decouple at least a pair of the electrically conductive elements, in response to forces that are applied to the switching fluid. A passivation layer covers at least a first of the electrically conductive elements and i) separates the first of the electrically conductive elements from at least the first of the cavities, and ii) is a dielectric for a capacitor formed between the first of the electrically conductive elements and the switching fluid. 
     In another embodiment, a method for forming a switch comprises depositing a plurality of electrically conductive elements on a first substrate. A passivation layer is then deposited on at least a first of the electrically conductive elements, and the first substrate is mated to a second substrate to seal a switching fluid in one or more cavities formed between the first and second substrates. The one or more cavities are sized to allow movement of the switching fluid between first and second states. The passivation layer i) separates the first of the electrically conductive elements from the one or more cavities, and ii) serves as a dielectric for a capacitor formed between the first of the electrically conductive elements and the switching fluid. 
     Other embodiments are also disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative embodiments of the invention are illustrated in the drawings, in which: 
         FIGS. 1–3  illustrate a first exemplary embodiment of a fluid-based switch; 
         FIG. 4  illustrates a schematic representation of the switch shown in  FIG. 4 ; 
         FIG. 5  illustrates an alternative positioning of a passivation layer shown in  FIG. 1 ; 
         FIG. 6  illustrates a schematic representation of the switch shown in  FIG. 5 ; 
         FIG. 7  illustrates a switch wherein wettable surfaces are formed by roughening portions of the switch&#39;s passivation layer; 
         FIG. 8  illustrates a switch wherein wettable surfaces are formed by layers of metal that are deposited on walls of the switch&#39;s switching fluid cavity; and 
         FIG. 9  illustrates an exemplary method for forming the switch shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1–3  illustrate a first exemplary embodiment of a fluid-based switch  100 . The switch  100  comprises first and second mated substrates  102 ,  104  that define therebetween a number of cavities  106 ,  108 ,  110 ,  112 ,  114 . Although five cavities  106 – 114  are shown in  FIG. 1 , it is envisioned that more or fewer cavities may be formed within the switch  100 . By way of example, the cavities are shown to comprise a switching fluid cavity  108 , a pair of actuating fluid cavities  106 ,  110 , and a pair of cavities  112 ,  114  that connect corresponding ones of the actuating fluid cavities  106 ,  110  to the switching fluid cavity  108 . A plan view of these cavities  106 – 114  is shown in  FIG. 2 . 
     Extending to near a first one or more of the cavities (and as best seen in  FIG. 3 ) is a plurality of electrically conductive elements  116 ,  118 ,  120 . Although the switch  100  is shown with three electrically conductive elements  116 – 120 , alternate switch embodiments may have different numbers of (two or more) electrically conductive elements. 
     A switching fluid  122  that is held within one or more of the cavities serves to couple and decouple at least a pair of the electrically conductive elements  116 – 120  in response to forces that are applied to the switching fluid  122 . By way of example, the switching fluid  122  may comprise a conductive liquid metal, such as mercury, gallium, sodium potassium or an alloy thereof. An actuating fluid  124  (e.g., an inert gas or liquid) held within one or more of the cavities may be used to apply the forces to the switching fluid  122 . 
     A cross-section of the switch  100 , illustrating the switching fluid  122  in relation to the electrically conductive elements  116 – 120 , is shown in  FIG. 3 . 
     The forces applied to the switching fluid  122  may result from pressure changes in the actuating fluid  124 . That is, the pressure changes in the actuating fluid  124  may impart pressure changes to the switching fluid  122 , thereby causing the switching fluid  122  to change form, move, part, etc. In  FIG. 1 , the pressure of the actuating fluid  124  held in cavity  106  applies a force to part the switching fluid  122  as illustrated. In this state, the rightmost ones of the switch&#39;s electrically conductive elements  118 ,  120  are coupled to one another. If the pressure of the actuating fluid  124  held in cavity  106  is relieved, and the pressure of the actuating fluid  124  held in cavity  110  is increased, the switching fluid  122  can be forced to part and merge so that electrically conductive elements  118  and  120  are decoupled and electrically conductive elements  116  and  118  are coupled. 
     By way of example, pressure changes in the actuating fluid  124  may be achieved by means of heating the actuating fluid  124  (e.g., by heaters  128 ,  130 ), or by means of piezoelectric pumping. The former is described in U.S. Pat. No. 6,323,447 of Kondoh et al. entitled “Electrical Contact Breaker Switch, Integrated Electrical Contact Breaker Switch, and Electrical Contact Switching Method”, which is hereby incorporated by reference for all that it discloses. The latter is described in U.S. Pat. No. 6,750,594 of Wong entitled “A Piezoelectrically Actuated Liquid Metal Switch”, which is also incorporated by reference for all that it discloses. Although the above referenced patents disclose the movement of a switching fluid by means of dual push/pull actuating fluid cavities, a single push/pull actuating fluid cavity might suffice if significant enough push/pull pressure changes could be imparted to a switching fluid from such a cavity. 
     Additional details concerning the construction and operation of a switch such as that which is illustrated in  FIGS. 1–3  may be found in the afore-mentioned patents of Kondoh et al. and Wong. 
     A feature of the switch  100  which has yet to be discussed is the passivation layer  126 . The passivation layer  126  covers at least a first of the electrically conductive elements  116 – 120 , and preferably covers all of the electrically conductive elements  116 – 120 . In this manner, the passivation layer  126  separates one or more of the electrically conductive elements  116 – 120  from the cavity  108  and serves as a dielectric for one or more capacitors formed between the electrically conductive elements  116 – 120  and the switching fluid  122 . 
     In  FIG. 5 , the passivation layer  502  covers the central conductive element  118  of the switch  500 . A schematic representation of this switch embodiment is shown in  FIG. 6 . One will note that, regardless of the state in which the switch  100  is placed, a capacitor  600  (formed as a result of the passivation layer  502 ) appears in the electrical path through the switch  100 . By choosing the material used to form the passivation layer  502 , and by controlling its thickness, the value of the capacitor  600  may be adjusted. Given that many radio frequency (RF) switching circuits have no need to pass direct current (DC), the capacitor  600  may be used as a DC block capacitor. 
       FIGS. 1–3  illustrate a switch embodiment  100  wherein a passivation layer  126  covers all of the electrically conductive elements  116 – 120 . In addition, the passivation layer  126  may be deposited between the electrically conductive elements  116 – 120  and may form a uniform continuous surface over the electrically conductive elements  116 – 120 . A schematic representation of this switch embodiment is shown in  FIG. 4 . In this circuit, two capacitors ( 400 / 402  or  402 / 404 ) appear in an electrical path through the switch  100  at any given moment. However, by choosing the material used to form the passivation layer  126 , and by controlling its thickness, the capacitors  400 – 404  may provide the same function as the single capacitor  600  ( FIG. 6 ). 
     One will note that the passivation layers  126 ,  502  shown in  FIGS. 3 &amp; 5  electrically, but not physically, couple the switching fluid  122  to the electrically conductive elements  116 – 120  that are covered by the passivation layers  126 ,  502 . When the passivation layer  126  is used to cover all of the electrically conductive elements  116 – 120 , the formation of alloys (e.g., amalgams) between the switching fluid  122  and electrically conductive elements  116 – 120  is prevented. Covering the electrically conductive elements  116 – 120  with the passivation layer  126  also tends to limit both oxidation and contamination of the electrically conductive elements  116 – 120  as a result of impurities in the switching and actuating fluids  122 ,  124 , as well as any stray gases (e.g., oxygen) that are trapped in the cavity  108 . Further, covering the electrically conductive elements  116 – 120  tends to limit contamination of the switching fluid  122  as a result of impurities in the electrically conductive elements  116 – 120  and the substrate  104 . 
     In prior fluid-based switches, the surface tension of the switching fluid  122 , as it wetted to the electrically conductive elements  116 – 120 , could sometimes lead to stiction that was difficult for the forces applied by the actuating fluid  124  to overcome. When this occurred, a switch did not switch properly. By covering one or more of the electrically conductive elements  116 – 120 , the passivation layers  126 ,  502  can mitigate the effects of stiction between the electrically conductive elements  116 – 120  and the switching fluid  122 . However, some amount of stiction is typically needed to keep a switch from inadvertently switching (e.g., due to bumps, drops and vibrations). 
     If a passivation layer  126 ,  502  eliminates too much stiction, stiction can be increased by providing a switch with a plurality of surfaces to which its switching fluid wets.  FIG. 7  illustrates a switch  700  wherein wettable surfaces  702 ,  704 ,  706  are formed by roughening portions of the passivation layer  126 .  FIG. 8  illustrates a switch  800  wherein wettable surfaces  802 ,  804 ,  806 ,  808 ,  810 ,  812 ,  814 ,  816  are formed by layers of metal that are deposited on walls of the cavity  108 . The layers of metal may be deposited in various locations, including “on” the passivation layer  126 , or on other walls of the cavity  108 , including its top, bottom, sides and ends. The layers of metal may comprise any metal to which a particular switching fluid  122  wets. However, one of the layers is preferably a metal that has a low (or no) probability of forming alloys with the switching fluid  122 . In this manner, the wettable surfaces  802 – 816  will not fully resolve into the switching fluid  122 . By way of example, the wettable surfaces  802 – 816  may comprise at least one of: iridium, rhodium, platinum and chromium. 
     The wettable surfaces  702 – 706  or  802 – 816  are preferably positioned over, and aligned with, the electrically conductive elements  116 – 120 . In this manner, the values of the capacitances formed by the passivation layer  126  and  502  can be more precisely controlled, and parasitic capacitance and other undesirable electrical phenomenon can be avoided. 
     By way of example, the passivation layers  126 ,  502  may comprise silicon dioxide, silicon nitride, silicon carbon, or polysilicon; and, in some cases, a passivation layer may comprise multiple layers of different materials. In one embodiment, the passivation layer is deposited using a chemical vapor deposition process. 
     In the past, it has been difficult to construct a fluid-based switch with conductive runners that extend from within to outside the switch&#39;s switching fluid cavity. This is because switching fluid  122  would normally wet to the conductive runners  116 – 120  and be drawn between the substrates  102 ,  104  during switch manufacture. However, in the switch  100 , the switching fluid  122  does not physically contact the conductive runners  116 – 120 . Furthermore, the passivation layer  126  may be selected so that it is not wettable by the switching fluid  122 . In this manner, the conductive runners  116 – 120  may extend from near the first of the cavities  108  to one or more exterior surfaces of the switch  100 , without the switching fluid  122  being drawn between the substrates  102 ,  104 . 
     A plurality of bonding pads  132 ,  134 ,  136  may be formed at ends of the conductive runners  116 – 120 . In some embodiments, the bonding pads  132 – 136  and/or conductive runners  116 – 120  as a whole, may be formed from a layer of titanium, on which a layer of platinum is deposited, on which a layer of gold is deposited. In alternate embodiments, the bonding pads  132 – 136  and/or conductive runners  116 – 120  may be formed from one or more other materials (or combinations of materials). 
       FIG. 9  illustrates an exemplary method for forming the switch  100 . The method comprises depositing  902  a plurality of electrically conductive elements  116 – 120  on a first substrate  104 . A passivation layer  126  is then deposited  904  on at least a first of the electrically conductive elements  118 . Thereafter, the first and second substrates  102 ,  104  are mated  906  to seal a switching fluid  122  in a cavity  108  formed between the first and second substrates  102 ,  104 . The cavity is sized to allow movement of the switching fluid  122  between first and second states. The passivation layer  126  1) separates the first of the electrically conductive elements  118  from the cavity  108 , and 2) serves as a dielectric for a capacitor formed between the first of the electrically conductive elements  118  and the switching fluid  122 .