Abstract:
The present invention provides a switch suitable for efficient microfabrication. The switch elements are disposed in several layers. Various embodiments provide various switching capabilities and operational characteristics. The switches can be protected by suitable packaging, and can be efficiently fabricated in groups or arrays.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application 60/658,902, “Micro-Miniaturized RF Switch,” filed Mar. 4, 2005, incorporated herein by reference, and U.S. provisional application 60/658,957, “Micro-Miniaturized Safing Device,” filed Mar. 4, 2005, incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of miniaturized devices, and more specifically relates to the fields of switches and safing devices. 
     BACKGROUND OF THE INVENTION 
     Switching Devices. Micromechanical devices (sometimes known as MEMS devices) have been known for many years, and various switch designs have been proposed using MEMS technology. However, the designs presently available still have shortcomings. For example, none has proven suitable for switching high power radio frequency signals (e.g., 5 W of RF power at 0.1-6 GHz). It is generally considered essential to obtain a large contact force for reliable high-power switches, and this can only be done currently using thermal actuation. Cronos (later JDS Uniphase) developed a thermal actuation switch beginning in 1999 with low insertion loss and high isolation at 0.1-6 GHz [RF MEMS: Theory, Design and Technology, John Wiley and Sons, February 2003; R. Wood, R. Mahadevan, V. Dhuler, B. Dudley, A. Cowen, E. Hill, and K. Markus, MEMS microrelays, Mechatronics, Vol. 8, pp. 535-547, 1998]. This switch resulted in about 1 mN of contact force per contact, used a pure gold contact, and was tested up to 25 W for 50 million cycles in a tunable 50 MHz filter by the Raytheon group with no failures [R. D. Streeter, C. A. Hall, R. Wood, and R. Madadevan, VHF highpower tunable RF bandpass filter using microelectromechanical (MEM) microrelays, Int. J. RF Microwave CAE, Vol. 11, No. 5, pp. 261-275, 2001; Charles A. Hall, R. Carl Luetzelschwab, Robert D. Streeter, and John H. VanPatten, “A 25 Watt RF MEM-tuned VHF Bandpass Filter,” IEEE Int. Microwave Symp., pp. 503-506, June 2003]. However, the switch consumed 250 mW of continuous DC power for operation, and the tunable filter with 8 actuated switches on average required 2 Watts of DC control power. The University of California, Davis, improved the Cronos design by using a more efficient thermal actuator and dropped the drive power from 250 mW to 60-70 mW for a 0.5 mN of contact force [Y. Wang, Z. Li, D. T. McCormick, and N. C. Tien, Low-voltage lateral-contact microrelays for RF applications, in 15th IEEE International Conference on Micro-Electro-Mechanical Systems, January 2002, pp. 645-648]. While an improvement over the previous design, this was still not acceptable for phased arrays and complicated switch networks. The Cronos switch was not used by the DoD or commercial community due to its high control power, but it demonstrated that acceptable switch performance can be obtained with 1-2 mN of contact force per contact. 
     Some designs reduce the required control power with a latching switch. In a latching switch, the control power is activated for only 0.3-3 milliseconds. This can be suitable for slow scanning phased arrays on unmanned air vehicles or in satellite systems. A latching switch also keeps its state if the power is temporarily lost (or purposely removed), which can be a great advantage in set-and-forget systems such as large switch networks for automated testing of defense and commercial systems, or in satellite applications with large pipe-line switch networks. A principal component of many latching switch designs is a bi-stable spring and actuation mechanism. A switch by Magfusion (formerly Microlab) is rated to 10 mA only for 10 million cycles [RF MEMS: Theory, Design and Technology, John Wiley and Sons, February 2003, M. Ruan, J. Shen, and C. B. Wheeler, Latching Micromagnetic Relays, IEEE J. Microelectromech. Systems, Vol. 10, pp. 511-517, December 2001. Also, see www.magfusion.com] since it has low contact forces, of the order of 0.1 mN and uses a gold contact. Thermal latching switches switches by Michigan (and MIT) have not yet seen commercial acceptance [Long Que, Kabir Udeshi, Jaehyun Park, and Yogesh B. Gianchandani, “A BI-STABLE ELECTRO-THERMAL RF SWITCH FOR HIGH POWER APPLICATIONS,” IEEE Conf. on Micro-electro-mechanical Systems, pp. 797-800, January 2004; J. Qiu, J. H. Lang, A. H. Slocum, R. Strümpler, “A High-Current Electrothermal Bistable MEMS Relay,” MEMS&#39;03, pp. 64-67, 2003]. Latching-type switches are generally quite large due to the bi-stable spring used, and therefore are not generally suited for high microwave or mm-wave operation. 
     Another set of RF MEMS switches include the Radant MEMS metal-contact switch with electrostatic actuation [S. Majumder, J. Lampen, R. Morrison and J. Maciel, “A Packaged, High-Lifetime Ohmic MEMS RF Switch,” IEEE MTT-S Int. Microwave Symp., pp. 1935-1938, June 2003], and the Raytheon capacitive switch [RF MEMS: Theory, Design and Technology, John Wiley and Sons, February 2003], also with electrostatic actuation. Both are very small, have been taken to mm-wave frequencies, and have been tested for at least 20 Billion cycles and in some cases to 100 Billion cycles. However, the Radant switch results in 0.1 mN of contact forces and cannot handle 5 W of RF power, and the Raytheon capacitive switch is not suitable for 0.1-6 GHz applications. 
     Current switch designs suffer from various shortcomings, which have so far precluded development of a high-power latching RF MEMS switch. 
     Safing Devices. In order to prevent an energetic material used in a rocket motor, warhead, explosive separation device or other similar device, collectively sometimes referred to as “target devices”, from being unintentionally operated during handling, flight or in any circumstance that could produce an extreme hazard to personnel or facilities, a “safing device” is customarily incorporated in the firing control circuit for the foregoing devices as a safety measure. These generically fall into two categories: “arm/fire” and “safe and arm”. The arm/fire device electrically and/or mechanically interrupts the “ignition train” to the target device so as to prevent accidental operation. The arm/fire device includes a mechanism that permits the target device to be armed, ready to fire, only while electrical power is being applied to the target device. When that electrical power is removed, signifying the target device is disarmed, the mechanism of the arm/fire device returns to a safe position, interrupting the path of the ignition train. 
     The safe and arm device is of similar purpose, and is a variation of the arm/fire device. The mechanism of the safe and arm device enables the target device, such as the rocket motor, warhead and the like, earlier mentioned, to remain armed, even after electrical power is removed. The device may be returned to a “safe” position only by applying (or reapplying) electrical power. The safe and arm device is commonly used to initiate a system destruct in the event of a test failure, for launch vehicle separation and for rocket motor stage separation during flight. Typically, the safe and arm device uses a pyrotechnic output which may be either a subsonic pressure wave or which may be a flame front and supersonic shock wave or detonation to transfer energy to another pyrotechnic device (and serves as the trigger of the latter device). 
     Existing safety devices are typically of the size of a person&#39;s fist, and possess a noticeable weight of several pounds. Although MEMS and other microfabrication technologies have been brought to bear on such safing devices, it has been primarily in the area of the ignition device that initiates the ignition train or in only a portion of the mechanism. There are currently no completely microfabricated safing devices available. Microfabrication of a safing device can allow significant reduction of weight, volume and cost. Reduction of weight and volume of those devices can allow corresponding increases in weight and/or volume of payload and propulsion systems resulting in increased range and capability of a weapon system. Reduced size and cost can allow the safing of small munitions or sub-munitions that are currently not provided with safing systems. 
     SUMMARY OF THE INVENTION 
     The present invention provides a switch having a base layer, a moveable member layer substantially parallel to the base layer, and first and second terminals. Motion of the moveable member parallel to the base layer opens and closes an electrical connection between the first and second terminals. Embodiments of the present invention comprise a third terminal, with an electrical connection between the first terminal and either the second or third terminal established by motion of the moveable member. Embodiments also comprise fourth terminals, with motion of the moveable member completing an electrical connection between the first and second terminals, or completing an electrical connection between the third and fourth terminals. 
     Embodiments of the present invention provide contacts mounted with the moveable member, such that motion of the moveable member moves the contacts into electrical communication with each other. The contacts can also move substantially parallel to the base layer, and can be disposed in the moveable member layer or in another layer. Embodiments of the present invention comprise a bistable moveable member, such that, once moved to a configuration that either opens or closes a particular electrical connection, the moveable member will remain in that configuration until external energy is applied. The bistability is provided in some embodiments by a flexure having buckled states, or a beam or beams mounted with the moveable member. 
     The force desired to move the moveable member can be provided by one or more electrostatic actuators, comb drives, electrostatic actuators, thermal actuators, piezoelectric actuators, pneumatic actuators, or other actuators suitable for the forces desired and the desired assembly process. Embodiments of the present invention also provide for isolation between the actuation and the switched circuit, for example by an insulating layer disposed between a layer containing the switched circuit and a layer containing an electromagnetic actuator. Embodiments of the present invention can comprise a plurality of switched disposed on a single substrate, or stacked together. Separator structures and lids can be used in some embodiments to protect the switch from external influences such as dust or debris. Vias through the base layer can be used to allow convenient external electrical connection. 
     Advantages and novel features will become apparent to those skilled in the art upon examination of the following description or may be learned by practice of the invention. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an illustration of an example embodiment of an SPST (single pole single toggle) electromagnetic switch realized in four layers. 
         FIG. 2  is an exploded view of the top two layers of an example embodiment of an SPST switch. 
         FIG. 3  is an illustration of an example embodiment of an SPST switch. 
         FIG. 4  is an illustration of an example embodiment of an SPST switch. 
         FIG. 5  is an illustration of an example embodiment of an SPST switch. 
         FIG. 6  is an illustration of an example embodiment of a three contact switch. 
         FIG. 7  is an exploded view of an example embodiment of a three contact switch with the top layer separated from the bottom three layers. 
         FIG. 8  is an illustration of electrical paths in an example embodiment of a three contact switch. 
         FIG. 9  is an illustration of an example embodiment of a three contact switch. 
         FIG. 10  is an exploded view of an example embodiment of a basic SPST switch showing vias in the lower substrate layer. 
         FIG. 11  is an illustration of one embodiment of a basic SPST switch showing electrical connection of an electromagnetic coil to vias in the lower substrate layer. 
         FIG. 12  is an exploded view from the bottom of one embodiment of a packaged basic SPST switch showing the addition of a top cover layer and border features in the second and third layers. 
         FIG. 13  is an exploded view from the top of one embodiment of a packaged basic SPST switch with the top cover layer separated from the lower 4 layers. 
         FIG. 14  is a bottom view of a packaged basic SPST switch showing the addition of solder bumps for electrical connection. 
         FIG. 15  is a view of one embodiment of a 4×8 array of SPST switches residing on a common substrate. 
         FIG. 16  is an exploded view of a 4×8 array of SPST switches with the top cover removed from the lower 4 array layers. 
         FIG. 17  is an exploded view from the bottom of a 4×8 array of SPST switches showing solder bump connection extending from the lower layer. 
         FIG. 18  is a view from the bottom of one embodiment of a packaged 4×8 SPST switch array. 
         FIG. 19  is a view of the upper three layers (of four in total) of one embodiment of a micro-miniaturized safing device. 
         FIG. 20  is an exploded view of an example embodiment of a micro-miniaturized safing device. 
         FIG. 21  is a detailed view of the bottom surface of the upper housing layer. 
         FIG. 22  is a detailed view of the shutter layer with the shutter in the “safe” mode. 
         FIG. 23  is a detailed view of the flexure and damping structure of the shutter layer. 
         FIG. 24(   a ) is a detailed view of the magnetic circuit elements of the shutter layer with the shutter in “safe” mode. 
         FIG. 24(   b ) is a detailed view of the magnetic circuit elements of the shutter layer with the shutter in “armed” mode. 
         FIG. 25  is a detailed view of the upper surface of the lower housing layer. 
         FIG. 26  is a view of the upper surface of the initiator layer. 
         FIG. 27  is a perspective view of an example embodiment of the bistable acceleration shutter in the closed state. 
         FIG. 28  is a perspective view of an example embodiment of the bistable acceleration shutter in the open state. 
         FIG. 29  is a perspective view of an example embodiment of the bistable acceleration shutter in the closed state. 
         FIG. 30  is a perspective view of an example embodiment of the bistable acceleration shutter in the closed state. 
         FIG. 31  is a view of an acceleration shutter with accompanying spacers. 
         FIG. 32  is an exploded view of an example clamping assembly for holding an-acceleration shutter. 
         FIG. 33  is a perspective view of an example dual acceleration enabled shutter. 
         FIG. 34  is a perspective view of an example embodiment of a dual acceleration enabled shutter. 
         FIG. 35(   a - h ) are illustrations of an example switch embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Example Switch Embodiments 
     The present invention comprises a number of embodiments of switches that provide desirable performance characteristics and are suitable for efficient microfabrication. Some embodiments of the present invention provide one or more of the following advantages over previous approaches: electromagnetically actuated; self-latching, requiring no quiescent DC power; Low voltage (&lt;2 V) and low current (&lt;40 mA) actuation; capable of high contact forces (1-2 mN per contact); capable of high RF power handling (at least 5 W); extremely linear with very low intermodulation products; low sensitivity to temperature, shock, acceleration, and aging; easy to package in hermetic and near hermetic conditions; capable of very high isolation for 0.1-6 GHz applications. 
       FIG. 1  is a perspective view of a single pole single toggle (SPST) switch embodiment of the present invention. A substrate  100  can comprise an electrically insulating material, and provides a base layer for the switch. Electrically conducting input  302  and output  312  pads mount with the substrate such that the pads are electrically isolated from each other. An armature or movable member  308  is disposed in a second layer, and mounts with a supporting spring  304  cantilevered from the input pad  302  such that the movable member is able to move substantially in the plane of the second layer, parallel to the base layer. An electrically conductive contact spring  306  mounts with the movable member  308 . First and second magnetic poles  314 ,  316  and a magnetic core  402  can all comprise soft ferromagnetic material. The poles  314 ,  316  and core  402  become magnetized when coil  304  is energized with electrical current. The current induces a magnetic field in the movable member  308  and the gaps formed between the movable member  308  and the magnetic poles  318 ,  320 . The magnetic field creates an attractive force between the movable member  308  and the magnetic poles  314 ,  316 , urging the movable member  308  closer to or in contact with the poles. The motion of the movable member  308  also causes motion of the connected contact spring  306  in a manner to close the electrical contact gap  310  and make electrical connection between electrical pads  302 ,  312  through the armature spring  304  and the contact spring  306 . The spring elements can be formed such that their width is substantially less than their height to provide lower stiffness in the direction of actuation (parallel to the plane of the base layer). 
       FIG. 2  is an exploded view of an embodiment like that described in connection with  FIG. 1 . In  FIG. 2 , a spacing layer  202  is disposed between the base layer  100  and the moveable member layer. The spacing layer  202  provides a mechanical gap between the substrate or base layer  100  and the moveable member layer. The spacing layer material can be either electrically insulating or electrically conductive depending on the type of packaging used and the method of providing a conductive path from the electrical pads  302 ,  312  to external connections. 
     The example embodiment of  FIG. 1  can accommodate various other arrangements of electrical pads.  FIG. 3  is an illustration of an example embodiment using a similar electromechanical arrangement as the example of  FIG. 1  but with a different electrical arrangement. Two electrical contact pads  320  and  322  mount with the base layer near a tip  332  of an electrical contact spring  334 . The tip  332  can be separated from the contact pads  320 ,  322  by gaps  324 ,  325 . An anchor pad  326  supports the moveable member  330  and a spring  328 . When the switch is closed, an electrically conductive path is provided from one contact pad  320  through the tip  332  to the other contact pad  322 . 
       FIG. 4  is an illustration of another example embodiment using a similar electromechanical arrangement as the example of  FIG. 1  but with a different electrical arrangement. An electrical contact  340  is disposed between a first contact pad  342  and the armature  346 . Closure of the switch forms an electrical path from the first contact pad  342  to a second contact pad  344  through a cantilevered support spring  348 . 
       FIG. 5  is an illustration of another example embodiment. The arrangement of the elements is similar to that described in connection with  FIG. 1 . The armature  350  and magnetic poles  352 ,  354  in the example of  FIG. 5  are shaped differently than those of the example of  FIG. 1 . Tailoring the geometry of the magnetic path can allow operational characteristics such as the relationship between force and armature displacement to be adjusted, e.g., to beneficially match a desired current drive or electrical contact force adjustment. 
       FIG. 6  is an illustration of another example embodiment. The example of  FIG. 6  has first  360 , second  362 , and third  364  contact pads disposed in a second layer substantially parallel to a base layer. Energizing a first coil  368  urges an armature  366  to move substantially parallel to the base layer such that a contact spring  374 , mounted with or formed as part of the armature  366 , contacts the second contact pad  362 , forming an electrical circuit between the first  360  and second  362  contact pads. Energizing a second coil  370  urges the armature  366  to move substantially parallel to the base layer such that the contact spring  374 , mounted with or formed as part of the armature  366 , contacts the third contact pad  364 , forming an electrical circuit between the first  360  and third  364  contact pads. 
       FIGS. 7 ,  8 , and  9  are views of an extended topology of a switch like those described before. The switch can be described as comprising a plurality of substantially parallel layers: a base layer, an electrical layer, an insulating layer, and a moveable member layer. Those skilled in the art will appreciate combinations of layers or disposition of elements into different or additional layers. The electrically insulating layer  380 , comprising for example glass, ceramic or plastic material, can isolate the electrical paths and contacts in the electrical layer from the magnetic paths in the moveable member layer. The switch also comprises a bistable spring  382  which can maintain electrical contact in one state without requiring continuous application of current. The switch thus provides a latching single pole double toggle switch (SPDT) which can maintain electrical contact between two electrical paths without the continuous application of current to electromagnets  387 ,  388 . Energizing (e.g., by applying a current to) a first coil  387  urges an armature  389  to move the bistable spring  382  and a contactor  392  such that the contactor  392  electrically connects the electrical paths  384  and  385 . Energizing (e.g., by applying a current to) a second coil  388  urges the armature  389  to move the bistable spring  382  and a contactor  392  such that the contactor  392  electrically connects the electrical paths  384  and  386 . The contactor  392  can be mechanically coupled to the armature  389  and the bistable spring  382  with an insulator  381 . Anchors  390 ,  391  of the bistable spring  382  can be mounted directly on the insulating layer. The electrical paths, including the contactor  392 , are thus electrically isolated from the armature  389 , discouraging coupling of the electrical paths  384 ,  385 ,  386  to the armature  389 , attached supporting spring  382 , attached anchors  390 ,  391  and magnetic cores  393 ,  394 . 
       FIG. 10  is an exploded view of an example embodiment with electrical vias provided for external electrical connection. The switch in the figure reflects one of the examples described previously; the external electrical connections can be used with many embodiments. Electrical vias comprising paths of good electrically conducting material  102 ,  103 ,  104 ,  105  extend through an electrically insulating substrate  101 . The vias provide electrical connection to the switch contacts  302 ,  312  and electromagnetic coil wire  410  as shown on the substrate  110  in  FIG. 11 . 
       FIG. 12  is an exploded view of a switch like those described before, integrated with a covering to protect the switch mechanism from external environments. Borders  204 ,  395  and a cover  500  mount with the base layer  102  to provide a protective environment for the switch elements such as the coil  304 . Arrangement of the borders and cover in layers, similar to the switch element layers, makes the entire assembly suitable for wafer scale packaging. The cover  500  in this example embodiment comprises a lip  501  which provides additional clearance of the cover over the coil  304 .  FIG. 13  is another illustration of the example, with the borders  204 ,  395  attached to the substrate or base layer prior to attachment of the cover  500 . In  FIG. 14 , solder bumps  600  have been added to the external side of the base layer to provide for convenient external electrical connection to the switch elements, for example by mounting on a conventional printed circuit board. 
       FIG. 15  is an illustration of a substrate or base layer  120  with multiple switches mounted thereon. The layered structure of the switches can allow simultaneous fabrication of the relays on the substrate.  FIG. 16  is an illustration of a multiple switch substrate  120  with borders  205 ,  396  and corresponding cover  502  suitable for protecting the switches.  FIG. 17 and 18  are illustrations of a multiple switch substrate, packaged with borders and cover, and with solder bumps disposed on the external side of the base layer to provide for convenient external electrical connection to the switch elements, for example by mounting on a conventional printed circuit board. 
     Example Switch Embodiment 
       FIG. 35(   a - h ) are schematic illustrations of an example embodiment of the present invention. The example embodiment comprises a SPDT (single-pole double throw) switch, and comprises a bi-stable mechanical spring with a pair of variable reluctance magnetic actuators. The two magnetic actuators act to switch a common RF port to two stable states after which a DC control power is not required to maintain contact. Each stable state results in a high contact force between the common RF port and the output ports. 
     The example SPDT topology comprises of 4 layers and is depicted in  FIG. 35(   a,b ). Typical dimensions for the device are: switch length=3.5 mm (spring anchor-spring anchor), width=3.2 mm (outer coil edge-outer coil edge), height=0.9 mm (top of substrate to top of coil). The four layers, from the bottom up, are:  501 , substrate layer;  502 , RF layer;  503 , isolation layer; and  504 , electromagnetic actuation layer.  FIG. 35(   a ) depicts all four layers, while  FIG. 35(   b ) provides an exploded view of the upper three layers, all of which can be micro-fabricated. Also shown in the figures are plastic (PMMA) assembly pins that can be press fit into the components during assembly. Alternatively, the layers can be bonded together without the use of press fit pins. 
     The substrate layer, approximately 0.5 mm thick, can comprise commercial glass, and forms the bottom layer of what will become the package. The RF layer in the example comprises a deep x-ray lithography-defined copper layer of approximately 250 micrometer thickness and includes signal lines, a ground plane, RF contacts, wiring for electromagnetic coils, and a perimeter for the sealed package cover. A bottom view of this layer, with substrate and electromagnetic actuation layers removed, is shown in  FIG. 35(   c ). The locations of the plastic pins that affix this layer to the next are shown. The two output paths (Ports  1  and  2 ) are widely separated to provide isolation and both the input and output lines are 300-500 microns wide to minimize transmission-line losses. The dimensions of the CPW lines have been chosen to result in a 50 O t-line. Low loss is further enhanced by both the inherently smooth surface (15 nm roughness) of the copper layer which is provided by the micro-fabrication process, as well as by a gold coating to reduce oxidization and provide enhanced contact performance. The copper can be first sputtered with TiW to insure good adhesion, and then sputtered with gold. An additional layer of gold can be optionally plated over the sputtered layers. 
     Although this example embodiment of the switch is a CPW (co-planar waveguide) design, in another embodiment it uses microstrip transmission lines. Virtually nothing changes in the design of the microstrip embodiment, except the removal of the CPW ground. In this second embodiment, an RF ground can be electroplated on the bottom of the substrate layer (e.g., glass wafer, layer  1 ). The remainder of this description focuses on the CPW embodiment. 
     The dielectric isolation layer, approximately 100 to 250 micrometers thick, is fabricated in this example embodiment from deep x-ray lithography-patterned PMMA (plexiglass) due to the relative ease with which it can be implemented. Glass can also be used for the isolation layer. The isolation layer isolates the RF circuit from the magnetic circuit by providing a large dielectric spacer, and can be easily seen in the exploded view of  FIG. 35(   b ). The PMMA layer has reasonably low dielectric loss at 0.1-6 GHz and does not increase the loss of the CPW lines. 
     The electro-magnetic actuation layer is shown in  FIG. 35(   d,e ).  FIG. 35(   d ) shows a top view of the electromagnetic actuation layer alone, while  FIG. 35(   e ) shows the geometric relationship between the features in the electromagnetic actuation layer and the RF layer. An important aspect of the example switch which both generates the high contact forces and creates the bi-stability of the switch is the double beam bi-stable flexure shown in  FIG. 35(   d ). 
     The electromagnetic actuation layer is approximately 250 micrometers thick, and comprises a deep x-ray lithography patterned and electroformed nickel/iron alloy material, e.g. 78 Permalloy, which provides a soft ferromagnetic path to isolate magnetic flux and is also an excellent spring material. Two electromagnetic coils provide the driving magnetic field, and together with their pole faces and respective plungers attached to the spring comprise two separate magnetic circuits. A magnetic flux density of approximately 0.7 Tesla (78 Permalloy saturates at 1.0 Tesla) can be maintained in the working air gap which yields an equivalent pressure of about 30 PSI. Operation into two working gaps of approximately 30×250 micrometer yields a plunger force of several milliNewtons. This force can be further enhanced by using multiple poles. 
     The example embodiment can be assembled with a series of press fit steps. The castellated press fit interface between the coil mandrels and the rest of the two stationary magnetic circuits is also shown in  FIG. 35(   d ). By energizing one coil or the other, the holding force of the spring is overcome and the device switches states. Once in the new switched position, the force of the springs maintains the contact until the time to switch back, which occurs when the opposite coil is momentarily energized. 
     The RF layer contacts, which are attached to the moving pole piece through the PMMA pins and the isolation layer, are thereby switched between the two RF paths. Because all structures and press fit pins can be lithographically patterned with deep x-ray lithography, 0.25 micron precision is readily achieved and all relative alignments are correspondingly accurate. This also helps insure good switch performance both by the precise positioning of the plunger relative to the air gaps, as well as by the proper positioning of the moving contact relative to the fixed contacts. 
     Example Safing Device Embodiments 
     Safing device embodiments according to the present invention can provide a fully integrated micro-miniature device and method for initiating the ignition process for a rocket motor, warhead, explosive separation device or other similar device that relies on energetic materials while simultaneously providing a mechanism for mechanically safing the device. In one embodiment the device operates as a safe and arm device, while in another it operates as an arm/fire device. There are also several embodiments of a micro-fabricated initiation device integral to the ignition device. 
     In an example embodiment, an ignition device comprises four micro-fabricated layers. The upper three are shown in  FIG. 19 ; all four are shown in  FIG. 20 . These layers comprise: a first or “upper housing” layer ( 1102 ) providing a portion of the housing for the shutter mechanism and a mounting interface for a secondary or high explosive or for other mechanical interface; a second or “shutter” layer ( 1104 ) incorporating the physical safing mechanism that provides for interruption of the ignition train; a third or “lower housing” layer ( 1106 ) that protects and houses the shutter mechanism from below and also provides an interface into the fourth, or “initiator” layer ( 1208 ) that contains the initiating pyrotechnic as well as the electrical interfaces to the device. An electric coil ( 1110 ) is an integral part of the shutter layer and is wound around a mandrel contained within that layer but extends into cut-outs in the upper and lower housing layers. 
       FIG. 20  is an exploded view of the ignition device showing all four micro-fabricated layers in more detail. The first layer incorporates a central aperture ( 1202 ) which provides access to the secondary or high energy explosive that follows the ignition device in the overall ignition chain. The first layer incorporates a cut-out ( 1204 ) to accommodate the coil ( 1110 ).  FIG. 21  is a view of the lower surface of the first layer and shows bond pads that provide mounting points for the shutter/flexure and damping means ( 1302 ,  1302 ′), the magnetic circuit elements ( 1304 ,  1304 ′), the spacer ring ( 1306 ), and the shutter stop ( 1308 ) all of which are contained within the shutter layer. These bond pads also space the shutter/flexure and damping mechanisms away from the lower surface of the first layer so that neither the shutter nor the damping means are directly in contact with the first layer. The first layer can be fabricated from Permalloy, a Ni—Fe alloy. 
     The second layer, as shown in isolation in  FIG. 22 , incorporates a spacer ring ( 1422 ), the shutter ( 1424 ) and integral flexure structure ( 1426 ,  1426 ′), the shutter damping stop ( 1434 ), a magnetic circuit component consisting of a wound coil ( 1110 ) with a core that extends beyond the coil ( 1428 ,  1428 ′) and a damping means. In an example embodiment the damping means consists of two opposing springs ( 1430 ,  1430 ′) that attach to the base of the flexure, contact the shutter from opposite sides, and eliminate any tendency of the flexure structure to vibrate or otherwise execute unwanted lateral motion. The flexure mounting points ( 1432 ,  1432 ′) are, during assembly, bonded to the bond pads ( 1302 ,  1302 ′) contained within the first layer, and thus neither the shutter nor the damping means is in contact with the first layer but is separated by the thickness of the bond pads. The design of the flexure mechanism is such that the shutter can move freely in the lateral directions as required to cover and to expose the aperture through which the pyrotechnic energy is transferred, but is constrained with respect to motion in the vertical direction so that it does not rub or otherwise contact the first or third layers of the assembly. The flexure is a bi-stable design, for example a doubly folded design. This is clearly shown in  FIG. 23  which is a detail illustration of the flexure ( 1426 ) and damping spring ( 1430 ) and their relationship to the flexure mounting point ( 1432 ). 
     Shown in detail  FIG. 24(   a ), the magnetic circuit element comprises an electrical coil ( 1110 ) wound around a ferromagnetic core that extends beyond the coil material ( 1428 ,  1428 ′) with a gap ( 1602 ) into which a portion of the shutter ( 1606 ) may move freely and without physical contact between the shutter and the ferromagnetic core. The shutter ( 1424 ) and its constituent elements ( 1606 ,  1608 ,  1608 ′) are also fabricated of a ferromagnetic material. In one embodiment permalloy is used for the shutter and flexure as well as the core. This provides for strength, flexibility, ferromagnetic properties, and ease of microfabrication. Features ( 1604 ,  1604 ′) show the bond line between two independently microfabricated elements of the shutter layer. 
       FIG. 24(   a ) shows the shutter in safe mode, with the magnetic circuit not energized and the shutter not drawn into the gap ( 1602 ) in the magnetic circuit. In this position the shutter aperture ( 1610 ) is not aligned with either the aperture ( 1202 ) in the upper housing layer or the aperture ( FIG. 25 , item  710 ) in the lower housing layer. Thus the passage of energetic material from the initiator to the secondary or high explosive is blocked.  FIG. 24(   b ) shows the shutter in armed mode with the shutter drawn into the gap and the shutter stops, ( 1608 ,  1608 ′) up against a portion of the core of the coil that extends beyond the coil and forms the gap ( 1602 ). In this position, the shutter aperture ( 1610 ) is aligned with both the apertures in the upper and lower housing layers ( 1202 ) and ( 1710 ) respectively so that energetic material may be transferred from the initiator to the secondary of high explosive. 
     An isolated top view of the third layer ( 1106 ) is presented in  FIG. 25 . The third layer incorporates bond pads for the shutter/flexure component and damping means ( 1702 ,  1702 ′), the shutter stop ( 1712 ), the magnetic circuit elements ( 1704 ,  1704 ′), and the spacer ring ( 1706 ). These bond pads are identical in shape and functionality to those in the first layer. There is similarly a cutout ( 1708 ) in third layer to accommodate the coil. The aperture in the central portion of the third layer ( 1710 ) is smaller than the corresponding aperture in the first layer. 
     An isolated view of the fourth layer ( 1268 ) is presented in  FIG. 26 . This layer contains electrical bond pads ( 1904 ,  1804 ′) for the coil that drives the magnetic circuit, bond pads ( 1802 ,  1802 ′) for the electrical interface to the initiator, and the initiator itself consisting of charge sleeve ( 806 ) and butterfly bridge wire chip ( 1808 ). 
     In another embodiment, the initiator employs a microfabricated bridge wire integral to the charge sleeve. In yet another embodiment the flexure design is such that once the shutter has been moved into the armed mode, the spring forces continue to keep the shutter in the armed mode even if power is removed from the coil rather than return the shutter to the safe mode. This provides a latching mode of operation and is useful for an arm/fire device. 
     Operation. In use, energetic material is placed in the charge sleeve ( 1806 ) and electrical bond pads for both the initiator ( 1802 ,  1802 ′) and the magnetic circuit coil ( 1804 ,  1804 ′) are attached to external sources of electrical power. If no power is applied to the coil, the flexure structure ( 1426 ,  1426 ′) maintains the shutter ( 1424 ) in the “safe” mode, with the permalloy shutter fully blocking the path between the aperture in layer one ( 1202 ) and the aperture in layer three ( 1710 ).  FIG. 24(   a ) shows the shutter in the “safe” mode. In “safe” mode, even if the initiator is fired, the energetic material will not exit the aperture ( 1202 ) in layer one. 
     If electrical power is applied to the coil, the magnetic circuit is energized and the shutter is drawn in towards the coil.  FIG. 24(   b ) shows the shutter in “armed” mode, with the aperture in the shutter aligned with the apertures in layers one and three so that energetic material may pass from the initiator material in the charge sleeve to the secondary or high explosive material that interfaces with the invention by means of the aperture ( 1202 ) in the first layer. After the shutter has been moved to “arm” mode, the initiator material contained within the charge sleeve ( 1806 ) may be ignited via the initiator electrical interface ( 1802 , 1802 ′). Energetic material then freely passes from the initiator to the secondary or high explosive. 
     The design of the flexure is such that there is a restoring force that, if power is removed from the coil, will return the shutter to the “safe” mode. The function of the shutter damping stop ( 1434 ) is to help eliminate any tendency for the shutter to oscillate or vibrate when it thereby returns to “safe” mode. The function of the damping features ( 1430 ,  1430 ′) is not only to help eliminate any tendency for the shutter to oscillate or vibrate when it returns from armed to “safe” mode, but also to eliminate any tendency for the shutter to vibrate from the “safe” to the “armed” mode in the event of deployment in a mechanically noisy and shock prone environment. 
     Method of Making. One example method of building the microfabricated layers and elements of the micro-miniaturized safing device is described here. Alternative methods will be readily apparent to one skilled in the arts of precision fabrication, micro-fabrication and LIGA (LIGA is a German acronym which stands for lithography, electroplating, and molding) processing. The fabrication of the electrical circuit board and the means for winding the electrical coil are readily apparent to one skilled in the art. 
     In an example embodiment the invention can be microfabricated using a planar fabrication process, with each of the top three layers (upper housing, shutter and lower housing) microfabricated independently and then bonded together to form an integrated three layer shutter structure. The fourth layer, which contains a mix of micro fabricated and conventional elements, is assembled separately. The energetic material for the initiator is then loaded into the charge sleeve, and only then is the lower layer bonded to the integrated three layer shutter structure to complete the building of the device. This method of building isolates the energetic material from any microfabrication processes. 
     The upper and lower housing layers can be fabricated in the same fashion. Using conventional LIGA and Deep X-Ray lithographic technology, a substrate can be prepared with a plating base, photoresist, and is patterned in the shape of the top of the upper housing structure (or bottom of the lower housing structure) using x-ray lithography. The photoresist is developed and permalloy plated into the pattern. The remaining photoresist can be stripped, and copper or other sacrificial material is plated and effectively replaces the photoresist that was stripped. The wafer can be planarized so that the plated permalloy structure is revealed and forms the basis for a new substrate. Photoresist is applied and the bond pad features are patterned into the photoresist. The photoresist is developed and permalloy is plated into the pattern and the structure is again planarized. The remaining photoresist is stripped and the sacrificial material is removed leaving a wafer containing complete upper and/or lower housing layers. 
     The shutter layer can be fabricated in two parts and then assembled. Shutter assemblies can be microfabricated in permalloy using conventional deep x-ray lithographic processes, except that the core of the coil and the extensions ( 1428 ,  1428 ′) are not incorporated into this initial fabrication process. Rather the coil cores can be separately fabricated, wound, and then press fit and/or bonded into the body of the shutter structure. This bond line is revealed as features ( 1604 ,  1604 ′) in the completed shutter layer and can be easily seen in  FIG. 24(   a ). 
     The upper housing, shutter, and lower housing layers are then bonded using one of many methods that are known to those skilled in the arts. This results in a complete and integrated three layer shutter structure as described before. Then the charge sleeve can be microfabricated using conventional LIGA processing and is affixed to a miniature circuit board that comprises the main structure on the initiator layer. The assembly of the fourth layer, the initiator layer, and the bonding of that layer to the integrated three layer structure is then obvious to one skilled in the arts. 
     Example Acceleration Shutter Embodiments 
       FIG. 27  is an illustration of an example embodiment of a bi-stable shutter mechanism that reacts to an acceleration threshold. A center proof mass  902  is retained by the bi-stable spring element  901  that is in turn supported by an outer frame  900 . The entire mechanism can be fabricated from a high yield strength metal. The proof mass  902  can be sized to be sensitive to a certain acceleration threshold in conjunction with the bi-stable spring element  901  so that when an acceleration of the mechanism is experienced which is greater than this threshold, the proof mass and spring will be forced to the other bi-stable state of the spring mass mechanism. Thus, in  FIG. 28 , the proof mass  902 , which in this case is intended as a shutter, has experienced an acceleration above the threshold acceleration and is now positioned in the second bi-stable state. The movement of the proof mass  902  as shown in  FIG. 28  which can be a shutter has now permitted an “open-state” to occur, for example.  FIG. 29  shows another embodiment of the acceleration sensitive shutter whereby the proof mass is supported by a single beam  905  rather than a dual beam as in  FIG. 27 . 
     In order to prevent motion of the proof mass  907  back to the original state after an acceleration threshold has been experienced,  FIG. 30  shows a clamping mechanism to latch the proof mass. Consisting of a barb  909  and clamps  910 ,  911 , the clamping mechanism will latch the proof mass into the second bistable state and prevent it from releasing back to the previous state even if a negative acceleration is experienced which would have otherwise caused the return of the proof mass and spring back to their original state. 
       FIG. 31  shows an exploded view of an example embodiment that provides for mounting of the acceleration sensitive shutter by providing spacers  912 ,  913  located on either side of the shuttle  914 . One means to further mount the mechanism is shown in  FIG. 32  where a clamping interface consisting of a top clamp  915  which is aligned over pins  916  and clamps the acceleration shutter mechanism between the top clamp  915  and lower clamp  917 . Alignment holes  918 ,  919  are additionally provided in the acceleration shutter in order to align the acceleration shutter axis with the axis of the clamp. Thus, a pin can be inserted through an alignment hole in the top clamp  920 , an alignment hole in the acceleration shutter  919  and an alignment hole in the bottom clamp  921 . Alternatively, a flat  922  can be provided in the acceleration shutter frame  900  which allows alignment to the acceleration axis. A bolt hole  923  is shown which permits fixed attachment to another body. 
     Another example embodiment of the acceleration threshold shutter is shown in  FIG. 33  where a cantilever  932  with proof mass  933  is interlocked  934  into the proof mass  930  of a bi-stable acceleration shutter. The spring  932  can be fabricated to allow preferential motion in the direction of acceleration axis  1  so that when a certain acceleration is experienced in this direction, the proof mass  933  moves out of the plane of the mechanism thereby unlocking itself from the bi-stable acceleration shutter proof mass  930  and allowing it to move into its second stable state when it experiences an acceleration greater than the threshold acceleration in the direction of acceleration axis  2 . 
       FIG. 34  shows another example embodiment, comprising a multi-directionally sensitive shutter mechanism whereby the proof mass  941  of a first acceleration threshold shutter is attached to a blocking bar  945  which in its initial state prevents the motion of a second acceleration threshold shutter with proof mass  942 . The entire mechanism is supported by a common frame  940 . When a sufficient acceleration is experienced along acceleration axis  1  to move proof mass  941  to its second bi-stable state, the locking bar  945  is moved to allow the motion of proof mass  942  with its barb  946  into the clamp  947  when it experiences an acceleration above its threshold value along acceleration axis  2 . 
     The particular sizes and equipment discussed above are cited merely to illustrate particular embodiments of the invention. It is contemplated that the use of the invention may involve components having different sizes and characteristics. It is intended that the scope of the invention be defined by the claims appended hereto.