Abstract:
A microelectrical mechanical system (MEMS) microthruster array is disclosed. The MEMS microthruster array of the present invention can be used for maintaining inter satellite distance in small satellites. One microthruster array includes numerous microthruster propulsion cells, each having a vacuum enclosed explosive igniter disposed on one side by a breakable diaphragm and having a propellant-filled chamber on the opposite side of the diaphragm. Upon explosion of the explosive igniter, the first diaphragm breaks, which, together with the explosion of the explosive igniter, causes the propellant to expand rapidly, thereby providing exhaust gases which are ejected from an exterior face of the microthruster propulsion array, thereby providing a small unit of thrust.

Description:
This invention was made with Government support under Air Force contract number N00014-94-C-01115. The Government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention is related generally to micro electrical mechanical systems (MEMS). More specifically, the present invention is related to MEMS based microthruster arrays which find one application in satellite propulsion. 
     BACKGROUND OF THE INVENTION 
     Satellites orbiting the earth range in size from Sputnik to the Hubble Space Telescope and beyond. Attitude maintenance control has typically been carried out using conventional attitude control thrusters, for example, controllable compressed gas release thrusters. There has been interest in replacing single, large satellites with clusters of small satellites. Each satellite in a cluster may be as small as a deck of playing cards, but collectively the cluster could function as a single satellite having the diameter of the cluster. Building a cluster of small satellites may be cheaper and more versatile than building a single large satellite. In particular, one application may include arranging a number of small satellites as a sparse aperture radio wave antenna for imaging distant objects. Sparsely positioned small satellites may also function as different elements of an interferometer having a large aperture. 
     One difficulty with using numerous small satellites is controlling the inter satellite distance accurately. The satellite-to-satellite distance is preferably maintained within limits for many applications. Small satellites may require a very small rocket motor for maintaining the inter satellite distance. 
     Very small rocket motors have been difficult to design and/or manufacture. One proposed design uses cold gas thrusters, which can operate by opening gas valves intermittently. This requires a moderate amount of hardware, tubing, and valves, which are often difficult to scale down and do not scale down far enough to be of use in very small satellites. Digital propulsion rocket chips have been proposed and&lt;e prototypes designed. Some prototypes have arrays of individually addressable explosive pixels. Existing designs have drawbacks. One current problem is thermal fratricide. 
     Thermal fratricide presents at least two problems. First, an individually addressable and ignitable explosive element in an array requires ignition. Ignition typically requires heating an igniter element to a temperature sufficient to cause an explosion of a material disposed close to the igniter. As the igniter temperature increases, heat may be dissipated away from the igniter element, preventing the element from ever approaching a temperature sufficient to cause the explosion or combustion of a propellant or explosive. Many times, the igniter does generate sufficient heat, which is retained, causing the propellant to explode or vaporize. When the temperature is sufficiently high, the heat may be conducted into adjacent individually selectable and ignitable elements, causing them to explode as well. This thermal fratricide thus can cause the explosion of pixels adjacent to the pixel for which the only explosion is desired. 
     What would be desirable is a microthruster array which provides for individually addressable and selectable microthrusters that do not cause the unwanted ignition of adjacent microthruster cells. 
     SUMMARY OF THE INVENTION 
     The present invention provides a MEMS microthruster including a plurality of propulsion cells, wherein the propulsion cells can be deployed in an array, each cell being individually addressable and ignitable. In one embodiment, each propulsion cell has a first cavity having an explosive igniter disposed, and preferably suspended within the first cavity, and a second cavity separated from the first cavity by a first diaphragm. The first diaphragm is preferably cooperatively dimensioned together with the igniter to break the diaphragm after the explosion of the explosive igniter. In one embodiment, a propellant is disposed within the second cavity, where the second cavity is disposed on the opposite side of the first diaphragm from the first cavity. Upon the breakage of the first diaphragm, the propellant can expand rapidly in response to the igniter exploding through the first diaphragm, thereby causing the rapid expansion of the propellant within, and ultimately out of, the second cavity. In. a preferred microthruster cell, the explosive igniter is suspended within the first cavity, and substantially surrounded by a thermal insulator such as a vacuum. The individual propulsion cells are preferably individually selectable and ignitable. 
     In one embodiment, the propellant is provided as a single component disposed within the second cavity, and can be a plastic explosive such as a nitrocellulose or nitrocellulose acetate. In another embodiment, the propellant is provided as two components. The first propellant component can be a fuel and the second propellant component can be an oxidizer. The second cavity in this embodiment can be divided into a first portion and a second portion, separated therebetween by a second diaphragm. The first diaphragm can be broken by the explosive igniter which also breaks the second diaphragm, thereby causing mixing of the fuel and oxidizer. The fuel and oxidizer then can generate sufficient force to break a third diaphragm disposed toward the exterior of the microthruster, thereby allowing the propellant exhaust gas to be ejected from the microthruster cell. 
     In one illustrative embodiment, the first set of cavities is formed upon a silicon substrate. The silicon substrate may have supporting electronics formed in the top surface thereof. A silicon dioxide layer may then be grown on the top of the wafer and supporting electronics. To produce a suspended igniter element, a cavity may be etched into the silicon dioxide layer, and filled with a polymer or other sacrificial layer. A silicon nitride layer or the like may then be deposited over the silicon dioxide and sacrificial layer, and an electrically resistive layer may be put on top of the silicon nitride. The sacrificial layer may then be removed, leaving a suspended igniter structure. The igniter element is preferably a heatable metal resistor coated or otherwise coupled to an explosive compound. 
     In a preferred embodiment, the first cavity is filled with a thermal insulator, such as a vacuum. The first set of cavities can have a first diaphragm forming the ceiling of a cavity, which, in one embodiment, is formed of silicon nitride. The second set of the cavities can be formed on top of the silicon nitride diaphragm, and may have walls formed of silicon dioxide. In one embodiment, the top of the second cavity is open, and serves as an opening through which an explosive compound is packed into the second cavity. In another embodiment, a plastic explosive is poured into the second cavity through the top orifice and cured. In yet another embodiment, the second set of cavities is further capped by yet another silicon nitride diaphragm. 
     In one microthruster embodiment, the second cavity has a ceiling or second diaphragm separating the second cavity from a third cavity, or, alternatively, dividing the second cavity into a first portion and a second portion. In this arrangement, the first portion may be filled with a fuel and the second portion may be filled with an oxidizer. The explosion of the igniter can break both the first diaphragm and the second diaphragm, thereby allowing mixing of the fuel and oxidizer, which causes rapid expansion of the propellant formed by the combined oxidizer and fuel. In one embodiment, the top of the third cavity is capped by a silicon nitride third diaphragm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a highly diagrammatic, transverse, cross-sectional view of a set of first cavities or chambers having a suspended igniter within, and a second set of cavities or chambers having propellant within, separated from the first cavities by a first, breakable diaphragm; 
     FIG. 2 is a top view of the second cavities of FIG. 1; 
     FIG. 3 is a highly diagrammatic, transverse, cross-sectional view of a microthruster array having a set of first microthruster chambers separated from a set of second microthruster chambers by a first diaphragm, being further capped by a second diaphragm; 
     FIG. 4 is a highly diagrammatic, transverse, cross-sectional view of a microthruster array having a first set of igniter chambers disposed beneath a second fuel or oxidizer chamber separated from a third fuel or oxidizer chamber by a second diaphragm; 
     FIG. 5 is an electrical schematic of one igniter circuit suitable for use in the present invention; and 
     FIG. 6 is an exploded, perspective view of a microthruster array according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 illustrates a microthruster array  20  formed of numerous microthruster propulsion cells  21 , each having a set of first microthruster chamber or cavity  22  disposed beneath a second microthruster chamber or cavity  24 . First microthruster chambers  22 , in the embodiment illustrated, are defined by a set of first walls  26  which sit on top of a first floor or bottom wafer  28 . First cavity or chamber  22  also includes a suspended explosive igniter  32 . 
     In the embodiment illustrated, explosive igniter  32  is suspended, and disposed away from first floor wafer  28 . This helps thermally isolate the explosive igniter from wafer  28 . As illustrated in FIG. 1, explosive igniter  32  is preferably surrounded on a majority of its exterior surface by a thermal insulator  34 , such as a vacuum. The vacuum inhibits the dissipation of heat away from the explosive igniter, thereby allowing a faster build-up of heat within the igniter, and preventing the unwanted heating of adjacent propulsion cells. 
     A first diaphragm  30  forms a. top, or ceiling, of first cavity or chamber  22 . First diaphragm  30  is designed to be broken by the explosion of explosive igniter  32 . Second chambers  24  can be defined at the bottom by first diaphragm  30 , and on the side by a second set of walls  23 . Second chambers  24  are preferably substantially filled with a propellant. Second chambers  24  have an exterior orifice  25 , which may also be referred to as an exhaust orifice. In a preferred embodiment, second cavities  24  are filled with a propellant mixture  36 . Propellant mixture  36  is selected to be ignited by the explosion of explosive igniter  32  and the breakage of first diaphragm  30 . Upon the rapid expansion of propellant mixture  36 , exhaust propellant gases expand through exhaust opening or orifice  25 . 
     Wafer  28  may include a silicon wafer which is used as a substrate. The silicon substrate may have supporting electronics formed in the top surface thereof. A silicon dioxide layer may then be grown on the top of the silicon wafer. To produce a suspended igniter element, a cavity may be etched into the silicon dioxide layer, and filled with a poly or other sacrificial layer. A silicon nitride layer or the like may then be grown over the silicon dioxide and sacrificial layer. The sacrificial layer may then be removed, leaving a suspended igniter structure as shown in FIG.  1 . Other methods for forming a suspended igniter element are described in U.S. Pat. Nos. 6,046,485, and 5,600,148. 
     Explosive igniters  32  preferably includes a resistive metal or filament, which heats upon the application of current. In one embodiment, hafnium nitride is used to form the filament. The filament may be coated or otherwise coupled to an explosive material, such as a lead styphnate explosive, which explodes upon obtaining a sufficiently high temperature. 
     First cavity walls  26  are preferably formed of silicon dioxide, and are preferably about 10 microns thick, indicated by “T” in FIG.  1 . First cavities  22 , in one embodiment, are disposed on a center-to-center pitch of about 51 microns and have a width of about 41 microns, indicated by “W” in FIG.  1 . In one embodiment, second diaphragm  30  is formed of silicon nitride and is about 400 Angstroms thick. Second cavities or chambers  24  may have walls  23  formed of structural silicon, which may have a thickness of about 10 microns in some embodiments. In a preferred embodiment, the pitch of second cavities  24  is the same as the pitch of first cavities  22  over which the second cavities are coaxially disposed. 
     Second cavities  36  are preferably filled with a plastic explosive such as nitrocellulose acetate. As illustrated in FIG. 1, microthruster array  20  has generally a base plane or surface as defined by base wafer  28 , and an exhaust plane or surface, as defined by the tops of the second cavity walls  23 . In use, the exhaust gas from second cavities  24  generally is exhausted orthogonally to the exhaust plane along an exhaust axis. 
     Referring now to FIG. 2, microthruster array  20  is illustrated from a top view, illustrating microthruster cells  21  including second cavity walls  23  defining second cavities  24  therebetween, and having breakable first diaphragm  30  forming the floor of second cavities  24 . 
     Referring now to FIG. 3, another embodiment of the invention is illustrated in a microthruster device  100 . Microthruster device  100  is similar in some respects to microthruster device  20  illustrated in FIGS. 1-2, wherein similarly number elements are as previously described. In this embodiment, first cavities or chamber  122  have an explosive igniter  132  largely suspended away from wafer  28 . The second cavities  124  have a second diaphragm  135  disposed thereover. Second diaphragm  135  is formed of silicon nitride in some embodiments. 
     Referring now to FIG. 4, a microthruster device  200  is illustrated. Microthruster device  200  includes three layers of stacked or coaxially aligned propellant chambers or cavities. Microthruster device  200  includes a set of first propulsion chambers or cavities  122  defined between base wafer  28  and first diaphragm  30 . Second propulsion chambers or cavities  124  are defined between first diaphragm  30  and a second diaphragm  150  and are disposed between cavity walls  23 . A third set of propulsion chambers or cavities  126  is defined on the sides by a third set of cavity walls  123  and on the bottom by second diaphragm  150 . The third cavities  126  may have a top or exhaust face or orifice  134  for exhausting propellant gases. A third diaphragm  130  may cap the third propulsion chambers  126 , if desired. 
     The second cavities  124  may be filled with a fuel and the third cavities  126  may be filled with an oxidizer, or visa-versa. Together, the fuel and oxidizer may be considered a propellant. In yet another view of the invention, second cavities  124  and third cavities  126  together form a single cavity having a first portion  124  and a second portion  126 , separated therebetween by second diaphragm  150 . 
     In some embodiments of the invention, second cavities  124  are filled sufficiently full of material to guarantee that the detonation of explosive igniter  132  will cause breakage of both first diaphragm  30  and second diaphragm  150 , but not immediately breaking third diaphragm  130 . After mixing of fuel and oxidizer caused by the breakage of second diaphragm  150 , sufficient energy is generated to break third diaphragm  130 . In some embodiments, a smaller amount of void or gas space is left within second cavity  124 , relative to the void or gas space within third cavity  126 . In particular, second cavity  124  may have a sufficiently small void or gas space so as to insure that the transmission of the shock wave from detonated of explosive igniter  132  breaks through the second diaphragm  150 , while the third cavity  126  has a substantial amount of void or gas space to at least momentarily contain the shock wave caused by explosive igniter  132 . More specifically, the explosive shock wave caused by the detonation of explosive igniter  132  may break through first diaphragm  30  and second diaphragm  150 , but may be absorbed by third cavity  126 , acting as a shock absorber, thereby delaying the breakage of outer diaphragm  130 . 
     Referring now to FIG. 5, an electrical circuit  300  is illustrated, suitable for forming the addressing and igniting electronics for explosive igniter  33 , as previously discussed. Electrical circuit  300  includes an emitter resistor  302  for heating an explosive material to the point of explosion. Circuit  300  also has an address line  304 , a signal line  306 , a Vdd line  303 , and a Vss line  308 . The address line  304  may be used to select a particular row, and the signal line  306  may be used to select a particular column within an array of micro-thrusters cells. Referring specifically to FIG. 5, when the signal line  306  and the address line  304  are both high, transistor  305  is activated, pulling the gate of drive transistor  307  high. This turns on drive transistor  307 , which pulls current though emitter resistor  302 , This current heats the emitter resistor, causing the explosive igniter to explode, as described above. 
     Referring now to FIG. 6, microthruster device  200  of FIG. 4 is illustrated in an exploded, perspective view. The illustrative microthruster device  200  is formed upon substrate wafer  28 . A silicon dioxide layer is grown on the substrate wafer  28 . Cavities  122  are then etched into the silicon dioxide layer, to form first cavity walls  26 . Explosive igniter  132  is preferably suspended above the floor of the first cavities  122 , and may have a serpentine form, as previously described. Signal line  306  and pixel address line  304  may be used to active the explosive igniter  132 , as previously described in FIG.  5 . 
     Explosive igniter  132  is preferably formed of a serpentine-shaped resistor that is encapsulated in a sheet of silicon nitride. The combined resistor and encapsulating material may form a planar surface, and explosive materials such as lead styphnate may lie atop or below the resistor surface, awaiting heating to the point of explosion. First silicon nitride diaphragm  30 , which may be separately formed, may be affixed to the top surface of the first cavity walls  26 . Second cavity walls  23  extend upward from the first silicon nitride diaphragm  30 . In a preferred embodiment, first diaphragm  30  and second cavity walls  23  are formed together, and later added, atop first cavity walls  26 . Second diaphragm  150  may also be seen, having been previously affixed beneath a third cavity wall  123 , which defines third cavity  126  within. Disposed atop third cavity  126  is a third diaphragm  130 . 
     Conventional semiconductor manufacturing techniques may be used to make microthrusters according to the present invention in general, and microthruster propulsion cell  200  in particular. In one method of manufacturing, the illustrative microthruster device  200  is formed upon substrate wafer  28 . A silicon dioxide layer is grown on the substrate wafer  28 . Cavities  122  are then etched into the silicon dioxide layer, to form first cavity walls  26 . Explosive igniter  132  is preferably suspended above the floor of the first cavities  122 , and may have a serpentine form, as previously described. 
     Second cavity  124  may be formed by beginning with another wafer of structural silicon, and forming a layer of silicon nitride thereover. Conventional semiconductor manufacturing techniques may then be used to etch through the structural silicon from the back side of the wafer to the silicon nitride layer. This forms the second cavity  124 , bounded on the bottom by the silicon nitride diaphragm. A layer of aluminum may be provided before the silicon nitride layer to serve as an etch stop in the formation of the second cavity  124  through the structural silicon. Once formed, the substrate with the second cavities may be bonded to the top surface of the first cavity walls  26 , preferably using a solder or other bonding mechanism. Similar methods may be used in the manufacture of the third cavity. 
     In operation, the appropriate pixel may be addressed, and sufficient voltage applied, to heat the explosive igniter. The explosive igniter preferably heats to a temperature sufficient to ignite and explode the adjacent explosive material. In a preferred embodiment, the explosive igniter is surrounded on a majority of its surface by vacuum, thereby inhibiting the dissipation of heat as the heat is built up to the point of explosion. In one embodiment, the resistor emitter is heated to a temperature of about 290° C., at which point the adjacent lead styphnate explodes. A lead styphnate explosion can break the first diaphragm  30 , as well as the second diaphragm  150 , in embodiments having a second diaphragm. In this initial time period, third diaphragm  130  may be left unbroken. 
     The mixture of propellant components such as fuel and oxidizer within first cavity  124  and second cavity  126 , coupled together with the explosion of explosive igniter  132 , may serve to ignite the propellant, thereby causing the rapid expansion of propellant into exhaust gas, which breaks through third diaphragm  130 . In one embodiment utilizing plastic explosive, the plastic explosive has a rapid propagation for shock wave through the fuel, yet the burn rate of the fuel is relatively slow. The material is preferably selected such that the shock front propagates through the propellant more quickly than the burn products are created and expelled. In a preferred embodiment, the entire volume of heated propellant in a cavity is converted to gas instantaneously and is expelled as a gas. As previously discussed, adjacent cavities are preferably left untouched and available for future use. 
     One illustrative embodiment includes one quarter million (250,000) thrusters on a 1.3-inch by 1.3-inch silicon die. The thrusters themselves may be laid out on a series of 512 by 512 cell grids, each having a 51-micrometer by 51-micrometer pitch. Each thruster may have its own heater element, which can be coaxially aligned with a hollow, propellant filled cavity directly above it. The filaments are preferably built monolithically on top of space-qualified radiation hard electronics, such as Honeywell&#39;s RICMOS® electronics, such that each thruster is individually addressable and ignitable. 
     A preferred embodiment of the invention utilizes a small, for example one microgram, charge of thermally detonatable lead styphnate which explodes when heated to an auto ignition temperature of about 270° C. The lead styphnate preferably explodes, releasing a great deal of energy, but not a great deal of usable momentum. In the second stage of the reaction, the exploding lead styphnate breaks through the first diaphragm and ignites a nitrocellulose mixture in the second cavity, as previously described. 
     Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the invention. The invention&#39;s scope is, of course, defined in the language in which the appended claims are expressed.