Abstract:
Methods for forming an enclosed liquid metal (LM) drop inside a sealed cavity by formation of LM components as solid LM component layers and reaction of the solid LM component layers to form the LM drop. In some embodiments, the cavity has boundaries defined by layers or features of a microelectronics (e.g. VLSI-CMOS) or MEMS technology. In such embodiments, the methods comprise implementing an initial microelectronics or MEMS process to form the layers or features and the cavity, sequential or side by side formation of solid LM component layers in the cavity, sealing of the cavity to provide a closed space and reaction of the solid LM components to form a LM alloy in the general shape of a drop. In some embodiments, nanometric reaction barriers may be inserted between the solid LM component layers to lower the LM eutectic formation temperature.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to and hereby claims the priority benefit of commonly-owned and U.S. provisional patent applications No. 61/633,624 filed Feb. 15, 2012, 61/633,625 filed Feb. 15, 2012, 61/685,113 filed Mar. 12, 2012 and 61/685,886 filed Mar. 27, 2012, all of which are incorporated herein by reference in their entirety 
    
    
     FIELD 
     Embodiments disclosed herein relate in general to methods for forming miniaturized liquid metal (LM) drops in sealed cavities. 
     BACKGROUND 
     The use of liquid metals in various electrical or electromechanical devices is known. Larger devices using liquid metals include relays (e.g. mercury relays) and switches. Small LM drops are also known for their use in micro-electro-mechanical system (MEMS) devices, for example for RF and other switching and cooling devices. Such small LM drops are inserted (i.e. “injected”) into the respective devices in liquid form. The incorporation of LM components into microelectronic or MEMS devices would benefit from use of processes and tools already in use in these technologies, for example various thin film formation technologies and tools. Microelectronics and MEMS thin film formation (e.g. by deposition) results in solid films. There is therefore a need for, and it would be advantageous to have methods for forming small and even ultra-small (on the scale of microelectronic devices) LM drops which are compatible with known microelectronics and MEMS technologies. 
     SUMMARY 
     In various embodiments, there are provided methods for forming a liquid metal (LM) drop enclosed and sealed inside a small cavity. The LM drop is liquid at room temperature or under normal use conditions. The term “drop” is not meant to be limiting to a particular shape or size. In some embodiments, the drop may have a volume on the order of tens of cubic micrometer. In other embodiments, the drop volume may be on the order of hundreds or thousands of cubic micrometers. In yet other embodiment, the drop volume may be up to a few millions of cubic micrometers. The drop may be enclosed and sealed in a cavity of slightly larger volume (referred to herein as “sealed cavity” or “sealed volume”). In some embodiments where the drop has such a miniaturized volume, the cavity sealing it may be formed in a microelectronic or a micro-electromechanical system (MEMS) device (including glass based MEMS) or in a microelectronics based interposer for 2.5D and/or 3D multiple IC integration. 
     The sealed cavity may then have boundaries defined by layers or features of a microelectronics process or technology (such as VLSI-CMOS) or of a MEMS technology. “CMOS” is used henceforth as an exemplary (but in no way limiting) term for the process or technology, to simplify the description. In other embodiments, a sealed cavity may be formed by covering and sealing a LM drop formed on a flat surface. 
     In an embodiment, the method comprises implementing an initial CMOS process to form CMOS layers or features, sequential formation (e.g. by deposition) of LM component materials as solid phase films or layers in an appropriate cavity formed in the CMOS layers, sealing of the cavity to provide a closed space, and reaction of the solid LM components to form the LM alloy in the general shape of a drop. 
     In an embodiment there is provided a method for forming a LM drop sealed in a cavity comprising the steps of providing a substrate having a cavity formed therein, forming a stack of a plurality of solid LM component layers in the cavity, sealing the cavity to form a sealed volume and reacting the solid LM component layers to form a sealed LM drop. 
     In an embodiment, the solid LM component layers are formed sequentially. 
     In an embodiment, the solid LM component layers are formed side by side. 
     In an embodiment there is provided a method for forming a LM drop sealed in a cavity comprising the steps of providing a substrate having a cavity formed therein, forming a plurality of solid LM component layers in the cavity, sealing the cavity partially to form a partially sealed volume, reacting the solid LM component layers to form a sealed liquid metal drop; and sealing the cavity to form a fully sealed volume. 
     In an embodiment there is provided a method for forming a LM drop sealed in a cavity comprising the steps of providing a substrate, forming plurality of solid LM component layers in defined areas of the substrate, sealing the defined areas to form a sealed volume, and reacting the solid LM component layers to a sealed liquid metal drop. 
     In an embodiment, the step of sealing the cavity includes forming an inert layer which seals the cavity. 
     In some embodiments, the substrate includes a microelectronic layered structure and cavity is formed partially inside the layered structure. 
     In some embodiments, the substrate includes a microelectronic layered structure and cavity is formed fully inside the layered structure. 
     In some embodiments, the microelectronic layered structure includes CMOS layers. 
     In some embodiments, a wetting agent may be applied to cavity surfaces to affect the wetting angle and therefore contact properties of the LM drop. 
     In some embodiments, a nanometric thickness reaction barrier may be added between two solid LM components to increase the liquefaction temperature beyond the nominal eutectic temperature of the components. 
     In some embodiments, the cavity may be under vacuum or filled with an inert gas. 
     Exemplary materials which can be formed as solid layers and reacted to form a LM include (but are not limited to) Gallium (Ga), Indium (In) and Tin (Sn). Various additives such as Copper (Cu), Silver (Ag) and Bismuth (Bi) see e.g. U.S. Pat. No. 5,478,978 may be added to reduce the liquefaction temperature of alloys formed therebetween and/or to reduce the oxidation rate of the alloy. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting embodiments are herein described, by way of example only, with reference to the accompanying drawings, wherein: 
         FIG. 1  show in cross section an embodiment of a microelectronics device layered structure with a cavity formed therein: (A) covered with photo-resist; (B) with a first solid LM component layer formed inside the cavity; (C) with a second solid LM component layer formed inside the cavity; (D) with a third solid LM component layer formed inside the cavity; (E) with a first inert layer covering the cavity; (F) after a lift-off process which removes all layers thereupon except in the cavity area; (G) with a second inert layer covering the entire wafer and sealing the cavity; and (I) after liquefaction of the LM metal components to form a LM drop inside the sealed cavity; 
         FIG. 2  shows a flow chart describing schematically steps in a first method embodiment disclosed herein; 
         FIG. 3  show in cross section another embodiment of a microelectronics device layered structure with a cavity formed therein: (A) covered with photo-resist; (B) after a lift-off process which removes all layers thereupon except in the cavity area; (C) after liquefaction of the LM components; and (D) with a second inert layer covering the entire wafer and sealing the cavity. 
         FIG. 4  show in cross section another embodiment of a microelectronics device layered structure with a cavity formed therein: (A) with a first reaction barrier layer on top of a first solid LM component layer; (B) with a second reaction barrier layer on top of a second solid LM component layer; and (C) with a third solid LM component layer on top of the second reaction barrier layer of  FIG. 3B . 
         FIG. 5  show in cross section yet another embodiment of a microelectronics device layered structure with a cavity formed therein: (A) with three side-by-side electrodes on a first sacrificial layer inside the cavity; (B) with a first solid LM component layer formed on one electrode inside the cavity; (C) with a second solid LM component layer formed on a second electrode inside the cavity; (D) with a third solid LM component layer formed on a third electrode inside the cavity; (E) with a second sacrificial layer formed over the cavity; (F) with a first inert layer formed and patterned over the wafer; (G) with the first and second sacrificial layers removed; and (H) after formation of a second inert layer to seal the cavity; and (I) after liquefaction of the LM metal components to form a LM drop inside the sealed cavity; 
         FIG. 6  show in cross section an embodiment of a substrate (A) with LM layered components; (B) with an inert layer covering the LM layered components; and (C) after liquefaction of the LM metal components to form a LM drop inside the sealed cavity; 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A-1I  show schematically, in cross section, a wafer with a cavity at various process stages in an embodiment of a method for forming a LM drop in a sealed cavity disclosed herein.  FIG. 2  shows a flow chart listing the different process steps. The following description uses exemplarily a cross section of a microelectronics device structure formed using VLSI/CMOS technology, and In, Ga and Sn as exemplary LM components. Other embodiments may use device structures formed using other technologies and/or substrates such as GaAs, GaN, SiGe, Silicon on Insulator (SOI) or glass. The liquefaction temperatures of the three metals are 30 C (Ga), 156 C (In) and 231 C (Sn). The eutectic temperatures are 120 C for In—Sn, 20 C for Ga—Sn, 15 C for Ga—In and −19 C for a Ga—In—Sn alloy with minute amounts of additives, see below. 
     A microelectronics wafer is prepared with an open cavity.  FIG. 1A  is a cross section of a microelectronics device layered structure  100  on a wafer  102  with an open cavity  104 . The cavity depth is typically a few micrometers (“microns”). Layered structure  100  includes pre-processed metallic layers (e.g. layers  106 ,  108  and  110 ) and non-metallic layers (e.g. layers  112 ,  114  and  116 ). Cavity  104  is defined by a bottom plane  104   a , side planes  104   b ,  104   c  (each number referring to one side “wall” and one back or front wall of the cavity when looking at the cross section) and a top plane  134   d  ( FIG. 1I ) Note that the cavity need not have a rectangular cross section, and may have other cross section shapes. In an embodiment, cavity  104  is formed partially inside the layered structure. In other embodiments, for example as in structure  100 , the cavity may be formed entirely inside the layered structure, with cavity side planes  104   b ,  104   c  bound by CMOS metallic layers  106 - 110  and CMOS dielectric layers  112 - 116 . 
     In step  202 , the wafer is covered with a photo-resist (PR) layer  120  which, after patterning, masks all areas except the cavity and its close surroundings ( FIG. 1B ). In step  204 , a first LM component layer  122  (e.g. Ga) is formed in solid phase to a predetermined thickness, volume or weight on the wafer ( FIG. 1C ). “Formation” is henceforth used as a generic term for the physical deposition, chemical deposition, vapor deposition or electro-deposition of a solid LM component or of an insulating layer. The wafer is appropriately cooled so that the Ga formed inside the cavity forms a solid layer. Since the liquefaction temperature of Ga is 30 C, the formation can be performed advantageously at a temperature just below 30 C, for example 29 C. This removes the need to cool the substrate to a lower temperature. In step  206 , a second LM component layer  124  (e.g. In) is formed in solid phase to a predetermined thickness over the first layer ( FIG. 1D ). The second layer can formed at the highest temperature below a liquefaction temperature of the alloy formed by the first and second layers. Exemplarily for Ga—In, the liquefaction temperature is 15 C. Note that if the alloy Ga—In were to be formed as one solid material (in one step), its formation would require a temperature lower than 15 C in order to keep it solid on the wafer. Optionally, in step  208 , a third LM component  126  (e.g. Sn) is formed in solid phase to a predetermined thickness over the second layer ( FIG. 1E ). As with the first two layers, the separate formation of the Sn can be advantageously performed at the highest temperature below the liquefaction temperature of the alloy formed by the first and second, in this case 15 C. 
     Exemplarily, predetermined thicknesses (or volume or mass) of the Ga, In and Sn layers may be chosen such that they provide a weight LM component ratio of 68.5% Ga, 21.5% In and 10% Sn. Optionally, low amounts of additives such as Cu, Au or Bi may be added to lower the LM solidification temperature of the complete drop. These can be added by deposition from additional targets (or electroplating sources) or as part of the In or Sn deposition targets or sources. The three layers thickness could be designed to fill the cavity in height. In step  210 , a first inert layer  128  of a material such as spin-on-glass (SOG), silicon nitride (SiN), polysilicon, atomic layer deposited (ALD) titanium-oxide or molecular vapor deposited (MVD) alumina is formed over the wafer ( FIG. 1F ). In an embodiment, this layer covers the entire wafer. Alternatively, in another embodiment shown in  FIG. 3A , the inert layer does not cover the entire cavity. In step  212 , a lift-off process removes PR layer  120  and all layers formed thereon, leaving a cavity covered with first inert layer  128  and with solid LM component layers there within ( FIG. 1G  and  FIG. 3B ). In the case where the inert layer does not cover the entire cavity, a passage  336  is created between the outside environment and the inner side of the future cavity,  FIG. 3C . For the embodiment in  FIG. 1 , in step  214 , a second inert layer  130  is formed over the entire wafer, covering the first inert layer and sealing the cavity ( FIG. 1H ). In step  216 , the wafer is brought to room temperature or slightly above it. Various additives such as Cu, Ag and Bi, see e.g. U.S. Pat. No. 5,478,978 may be added to reduce the liquefaction temperature of alloys formed therebetween and/or to reduce the oxidation rate of the alloy. The solid Ga, In and Sn layers undergo a melting and alloying transformation to form a liquid metal “drop” with an eutectic temperature of about −19° C. The LM drop  132  is shown encapsulated in the sealed cavity  134  in  FIG. 1I . For the embodiment in  FIG. 3 , the solid LM components undergo liquefaction after step  212  ( FIG. 3C ), and the second inert layer is formed as in step  214  to seal the cavity ( FIG. 3D ). 
     In an embodiment, LM drop  132  has a volume smaller that the cavity volume and is mobile within the cavity. In an embodiment, the cavity may be filled with an inert gas. In an embodiment, the cavity may be evacuated (under vacuum). The formation of each layer above may be achieved using processes known in the art. For example, the Ga may be evaporated or sputtered in a vacuum chamber using for example a process described in Morley et al., J. Vac. Sci. Technol. A, 26 (4), pp. 581-586, 2008. Alternatively, the Ga may be electroplated from an appropriate solution over a metal electrode layer (e.g. W) previously formed inside the cavity, using exemplarily a process described in Kim et al., J. Mater. Res, 26 (18), 2011. The In layer may be evaporated or sputtered using a process described in Bae et al., J. Phys. Chem. B, 109 (7), pp. 2526-31, 2005. Alternatively, the In may be electroplated from an appropriate bath over one of the other LM layers (e.g. Ga) which serves as electrode, using exemplarily a process described in Tian et al, IEEE Electronic Components and Technology Conference, ECTC 2008. 27-30 May, 2008. The Sn layer may be evaporated or sputtered using exemplarily a process described in U.S. Pat. No. 5,776,790. Alternatively, the Sn layer may be electroplated from an appropriate solution over the first LM layer which serves as electrode, using exemplarily a process described in U.S. Pat. No. 4,027,055. 
     In the solid LM component layer formation steps above, care is taken to prevent oxidation of each component. Evaporation and sputtering processed performed in vacuum are advantageous in that they create very low oxidation levels for the LM components. In some embodiments using deposition (e.g. evaporation or sputtering) processes, all LM components may be formed using a single tool while keeping the process under vacuum. In embodiments using electroplating or a combination of electroplating and deposition (evaporation or sputtering), oxidized surfaces may be treated (e.g. by acid etch or ion sputtering) to remove surface oxides between LM component formations. Electroplating is advantageous in that it can form a solid layer inside a cavity which has an overhang, such as overhang  118  in  FIG. 1A . Another of its advantages is the ability to use the electroplated metals as the base for the next layer of electroplated metal. 
     The formation order of the LM components can be used advantageously to reduce the total oxidization of the liquid metal. Formation of Ga first, followed by either In or Sn, may reduce the total oxidation especially if the formation of the insulating layer is done in a different tool than the LM components tool, when the wafer is shifted between these tools exposed to the atmosphere. However, the order of the formation of the solid LM component layers may be changed from the one described above. For example, if all three components (Ga, In and Sn) are electroplated, then the order may be reversed so that Ga is first, Sn is second and In is third. In an exemplary process, the three layers are electroplated with a weight ratio of about 68% Ga, 21% In and 10% Sn, with 1% left for additional metals (additives). Exemplarily, the additional metals may be silver or copper in low amounts, and these may be also added by additional electroplating processes. 
     In an embodiment, the formations may be performed even closer to 30 C by adding a (or a few) nanometer-thick layer which acts as a reaction barrier between the LM components and therefore prevents the LM components from reacting at nominal eutectic temperatures. The addition of such reaction barriers is illustrated schematically in  FIGS. 4A-4C . In these figures,  410  and  420  indicate respectively a first and a second nanometric reaction barrier layer inserted between solid LM components  422 ,  424  and  426  (which parallel layers  122 ,  124  and  126  in  FIG. 1 ). Materials serving as nanometric reaction barriers may exemplarily be metals such as Ta or W which have high eutectic temperatures when in contact with the LM components. Alternatively, the reaction barrier materials may be ceramics such as Al 2 O 3  or Ta 2 O 5 . The reaction barriers may be formed using processes compatible with those used to form the solid LM component layers. 
     In an embodiment of the method shown in  FIG. 5 , the solid LM components may be formed (e.g. electroplated) not on top of each other inside a cavity but side by side. As in  FIG. 1 , the process starts with a microelectronics wafer being prepared with an open cavity.  FIG. 5A  is a cross section of a microelectronics device layered structure  500  on a wafer  502  with a cavity  504 . The microelectronics layers may be as in  FIG. 1  and are not shown or described in detail except where needed. The cavity includes three electrodes (e.g. made of tungsten)  506 ,  508  and  510  which are separated by sections of a first sacrificial layer  512  (e.g. SiO 2 ), creating 3 sub-cavities  514 ,  516  and  518 . The electrodes shape and area may differ in the different sub-cavities. Following a process similar to that described with reference to  FIG. 2 , a first LM component layer  520  (e.g. Ga) is formed or electroplated in solid phase to a predetermined thickness, volume or weight on first electrode  506  ( FIG. 5B ). Then, a second LM component layer  522  (e.g. In) is formed or electroplated in solid phase to a predetermined thickness over second electrode  508  ( FIG. 5C ). Then, a third LM component  524  (e.g. Sn) is formed in solid phase to a predetermined thickness over the third electrode  510  ( FIG. 5D ). A second sacrificial layer  526  (e.g. SiO 2 ) is added and patterned ( FIG. 5E ). Then, a first inert layer  528  is formed and patterned over the wafer ( FIG. 5F ). Sacrificial layers  512  and  526  are then removed (e.g. by chemical etch) through openings  530  in the inert layer, leaving the cavity covered with first inert layer  528  and with solid LM component layers there within ( FIG. 5G ). A second inert layer  532  is formed over the entire wafer, covering the first inert layer and sealing the cavity ( FIG. 5H ). The wafer is then brought to room temperature or slightly above it to liquefy the LM component layers into a LM drop  534  shown encapsulated in a sealed cavity  536  in  FIG. 5I . 
     In an embodiment of the method shown in  FIG. 6 , the solid LM components  602 ,  604 ,  606  ( FIG. 6A ) may be formed sequentially not inside a cavity but above the surface of a flat substrate, using a process known in the art or as described above. Then, an inert layer  610  is formed and patterned over the substrate and over the LM layered components ( FIG. 6B ), using a process known in the art or as described above in  FIG. 1 . The wafer is then brought to room temperature or slightly above it to liquefy the LM component layers into a LM drop  612  shown encapsulated in a sealed cavity  614  in  FIG. 6C . 
     In various embodiments, prior to the formation of LM component layers, the surfaces of the substrate and the cavity may be covered with “wetting” materials that modify the surface energy (not shown). Such materials may include Self Assembled Monolayers (SAM) of Alkoxy-silanes, Chloro-silanes, Fluoro-silanes, 11-hydroxy-1-undecanethiol or poly(dimethyl siloxane). The application process of such materials is well known. The addition of wetting materials allows control of the wetting angle of the LM drop with all surrounding surfaces and therefore controls the shape and location of the final LM drop inside the sealed cavity. 
     Exemplary uses of a LM drop sealed in a cavity formed in a microelectronic layered structure may be found in co-pending PCT patent application PCT/IB2012/053899 by the same inventors. 
     All patents, patent applications and publications mentioned in this specification are herein incorporated in their entirety by reference into the specification, to the same extent as if each individual patent, patent application or publication was specifically and individually indicated to be incorporated herein by reference. In addition, citation or identification of any reference in this application shall not be construed as an admission that such reference is available as prior art. 
     While this disclosure describes a limited number of embodiments, it will be appreciated that many variations, modifications and other applications of such embodiments may be made. The disclosure is to be understood as not limited by the specific embodiments described herein, but only by the scope of the appended claims.