CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of PCT Application No. US2011/040965 having an international filing date of Jun. 17, 2011 and entitled “METHOD FOR CREATING AND PACKAGING THREE DIMENSIONAL STACKS OF BIOCHIPS CONTAINING MICROELECTRO-MECHANICAL SYSTEMS”. PCT Application No. US2011/040965 claims priority to U.S. Provisional Application No. 61/356,515 filed on Jun. 18, 2010 and entitled “METHOD FOR CREATING AND PACKAGING THREE DIMENSIONAL STACKS OF BIOCHIPS CONTAINING MICROELECTRO-MECHANICAL SYSTEMS.” The entire contents of all the foregoing applications are hereby incorporated by reference. 
    
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made with government support under grant number R01NS055312-03-S1 awarded by NIH/NHGRI. The Government has certain rights in the invention. 
    
    
     BACKGROUND 
     Microelectro-mechanical systems (MEMS) relate to technologies based on an integration of mechanical elements, such as sensors and actuators, and/or electronics that are formed on a common substrate by microfabrication technology. MEMS components range in size from a few microns to a few millimeters. MEMS components are fabricated by microfabrication techniques that include techniques used to fabricate integrated circuits (IC) using IC process sequences (e.g., CMOS, Bipolar, or BICMOS processes). Integrated circuit microfabrication techniques have been used to create three dimensional arrays of electrical components. 
     Micromechanical components of MEMS systems are fabricated using “micromachining” processes that selectively etch away parts of the silicon wafer or add new structural layers to form the mechanical and electromechanical devices. By combining silicon based microelectronics and micromachining, MEMS technology creates systems and devices in a single chip. MEMS augments the computational ability of microelectronics with the sensing and control functions of microsensors and/or microactuators. 
     In recent years, advances have been made in the field of neurobiology. An important aspect of further advancement is observation of spatiotemporally distributed neural activity. MEMS technology has been applied to develop a self-anchoring MEMS intrafascicular neural electrode as disclosed by International Publication No. WO 2009/012502 A1, which is expressly incorporated herein by reference. 
     Several studies using animals have successfully investigated the use of movable microelectrodes that can be precisely positioned in the brain or can be moved in the event of neural-electrode interface failure. However, the size and weight of the movable microelectrodes are often large and interfere with or impair animal movement and/or behavior. Therefore, there is a need for a movable microelectrode device that can be integrated with advanced signal conditioning and control circuitry towards a fully autonomous microimplant in the brain. There remains a need for apparatus for sensing spatially distributed neural activity and for recording that activity. 
     BRIEF SUMMARY OF THE INVENTION 
     An aspect of the invention concerns three dimensional arrays of micro-components including microelectro-mechanical systems (MEMS), micro fluidics and micro-optical components. 
     Another aspect of the invention concerns a method for fabricating high density three dimensional arrays of micro components. 
     Yet another aspect of the invention resides in a three dimensional array of sensors that can he positioned within biological tissue to monitor biological signals within the volume of the sensor. 
     Still another aspect of the invention resides in a three dimensional array of active MEMS devices that provide stimulating signals to a brain. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a side cross section of a microsensor cluster according to the present invention. 
         FIG. 2  illustrates a section view of the top of the microsensor cluster as shown by  FIG. 1 . 
         FIG. 3  is an end view of the microsensor cluster as shown by  FIG. 1 . 
         FIG. 4  illustrates the assembly created by the first step of assembly of the microsensor cluster of  FIG. 1 . 
         FIG. 5  illustrates the assembly created by the second step of assembly of the microsensor cluster of  FIG. 1 . 
         FIG. 6  illustrates the assembly created by the third step of assembly of the microsensor cluster of  FIG. 1 . 
         FIG. 7  illustrates the assembly created by the fourth step of assembly of the microsensor cluster of  FIG. 1 . 
         FIG. 8  illustrates the assembly created by the fifth step of assembly of the microsensor cluster of  FIG. 1 . 
         FIG. 9  illustrates the assembly created by the sixth step of assembly of the microsensor cluster of  FIG. 1 . 
         FIG. 10  illustrates the assembly created by the seventh step of assembly of the microsensor cluster of  FIG. 1 . 
         FIG. 11  illustrates the assembly created by the eighth step of assembly of the microsensor cluster of  FIG. 1 . 
         FIG. 12  illustrates the assembly created by the ninth step of assembly of the microsensor cluster of  FIG. 1 . 
         FIG. 13  illustrates the assembly created by the tenth step of assembly of the microsensor cluster of  FIG. 1 . 
         FIG. 14  is an end view of a case in which four microsensor clusters according to the present invention are supported. 
         FIG. 15  is a cutaway side view of the case and microsensor clusters of  FIG. 14 . 
         FIG. 16  is a micrograph of a microactuator that can be used in a microsensor cluster according to the invention. 
         FIG. 17  shows the microactuator of  FIG. 16  with solder bumps applied. 
         FIG. 18  shows three actuators as shown by  FIG. 16  arranged in a cluster. 
         FIG. 19  shows components of a microactuator that may be incorporated in to a microsensor cluster according to the invention. 
         FIG. 20  depicts connection of a MEMS component to a substrate for use according to the invention. 
         FIG. 21  shows a MEMS component mounted to at substrate for use according to the invention. 
         FIG. 22  is a side view of a MEMS component and a connector both mounted to at substrate for use according to the invention. 
         FIG. 23  is a photograph of a MEMS component and a connector both mounted to a substrate. 
         FIG. 24  is a photograph of the assembly shown by  FIG. 23  with a protective sealant coating. 
         FIG. 25  is an SEM image of a MEMS chip mounted to a substrate with a seal material for use according to the invention. 
         FIG. 26  is a micrograph that shows a microactuator after assembly for use according to the invention. 
         FIG. 27  is a photograph that shows a leak test of a microactuator that is sealed for use as a neural sensor according to the invention. 
         FIG. 28  is an SEM image of a microactuator with sealant for use as a neural sensor according to the invention. 
         FIG. 29  is a micrograph of a microactuator having a retracted microelectrode. 
         FIG. 30  is a micrograph of a microactuator having an extended microelectrode. 
         FIG. 31  is a side view of assembly of a MEMS chip with a flexible interconnect. 
         FIG. 32  is a side view of a MEMS chip connected to a connector by a flexible interconnect. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention concerns clusters of microelectro-mechanical systems (MEMS) components. In particular, the invention concerns the configuration of a three dimensional stack of MEMS devices and a method of fabricating the stack. A specific application of the invention is a stack of movable microelectrodes that may be positioned within a brain so that the microelectrodes sense electrical impulses of single neurons and neuronal networks and transmit signals created by those electrical impulses for recording. 
     The present invention is described hereinafter by reference to the accompanying drawings that show embodiments of the invention and in which like numbers refer to like elements throughout. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are examples of the invention, which has the full scope indicated by the claims. 
       FIG. 1  illustrates a cross section of a microsensor cluster  10  according to the present invention. The cluster  10  includes seven MEMS active microactuators  12 . Each microactuator  12  is formed on one of seven silicon substrates  14 ,  16 ,  18 ,  20 ,  22 ,  24  and  26 . The silicon substrates  14 ,  16 ,  18 ,  20 ,  22 ,  24  and  26  each has a thickness that provides sufficient strength to support the microactuator  12  when assembled within the cluster  10  as described herein. The thickness of the silicon substrates may be one half millimeter thick. The substrates  14 ,  16 ,  18 ,  20 ,  22 ,  24  and  26  each have a leading edge  15 . The substrates  14 ,  16 ,  18 ,  20 ,  22 ,  24  and  26  are positioned generally parallel to each other and so that their leading edges  15  lie approximately in a plane. 
     A microactuator  12  is formed on each substrate  14 ,  16 ,  18 ,  20 ,  22 ,  24  and  26 . Each microactuator  12  extends on a substrate from the edge  15 . A plurality of microelectrodes  28  extend away from each microactuator  12  along the edge  15 . The microelectrodes  28  extend generally perpendicular to the edge  15  and generally perpendicular to the plane approximated by the edges  15  of the substrates  14 ,  16 ,  18 ,  20 ,  22 ,  24  and  26 . The microelectrodes  28  are movably mounted to the microactuators  12 . The microactuators  12  are constructed to support and to extend and retract the microelectrodes  28 . 
     The microsensor cluster  10  is positioned within a cover  32 . The cover  32  defines an interior and an opening  34 . The microcluster  10  is positioned within the interior of the cover  32  so that the edges  15  of the substrates  14 ,  16 ,  18 ,  20 ,  22 ,  24  and  26  are at the opening  34 . The microelectrodes  28  extend from the microactuators  12  at the opening  34 . A non-hermetic mesh encapsulation  38  is secured to the cover  32  to extend over the opening  34 . The mesh encapsulation  38  permits the microelectrodes  28  to extend therethrough. Preferably, the mesh is a composite of nylon mesh with silicon gel encapsulation as described by N. Jackson, S. Anand, M. Okandan and J. Muthuswamy, “Non-hermetic Encapsulation Materials for MEMS Based Movable Microelectrodes for Long-Term Implantation in the Brain,” IEEE/ASME J Microelectromech Syst, 18(6):1234-1245, 2009. 
     The microsensor cluster  10  includes a second level interconnect board  42  that is mounted to the cover  32 . The second level interconnect board  42  may be made of glass or polyimide and may be one half millimeter thick. The second level interconnect board  42  has connection pads  44  adjacent to the cover  32  at a location that is separated from the opening  34 . The connection pads  44  are electrically connected to the microelectrodes  28  and to the microactuators  12  as further described herein. Signals from the microelectrodes  28  are received by the connection pads  44  that are electrically connected to and the microelectrodes  28  and the microactuators  12  are controlled by signals provided to connection pads  44  that electrically connected to the microactuators  12 . Conductors  46  are electrically connected to the connection pads  44  and extend through the opening  36  in the cover  32  to an outer connect  48  at which electrical connections to the microelectrodes  28  and the microactuators  12  may be made. 
     The substrate  14  is mounted to the second level interconnect board  42  as will be described herein. The substrate  16  is mounted to the substrate  14  as will he described herein. The substrate  16  is spaced from the substrate  14  by approximately 60 micrometers. The microactuator  12  mounted to the substrate  14  is positioned on a surface of the substrate  14  that faces the substrate  16 . The microactuator  12  mounted to the substrate  16  is positioned on a surface of the substrate  16  that faces the substrate  14 . The microactuators  12  mounted to the substrates  14  and  16  are thereby positioned adjacent to and separated from each other within the space separating the substrates  14  and  16 . 
     The substrate  18  overlies and is mounted to the substrate  16  as will be described herein. The substrate  20  is mounted to the substrate  18  as will be described herein. The substrate  20  is spaced from the substrate  18  by approximately 60 micrometers. The microactuator  12  mounted to the substrate  20  is positioned on a surface of the substrate  20  that faces the substrate  18 . The microactuator  12  mounted to the substrate  18  is positioned on a surface of the substrate  18  that faces the substrate  20 . The microactuators  12  mounted to the substrates  16  and  18  are thereby separated from each other by the substrates  16  and  18  and by the bonding between them. The microactuators  12  mounted to the substrates  18  and  20  are thereby positioned adjacent to and separated from each other within the space separating the substrates  18  and  20 . 
     The substrate  22  overlies and is mounted to the substrate  20  as will be described herein. The substrate  24  is mounted to the substrate  22  as will be described herein. The substrate  24  is spaced from the substrate  22  by 60 micrometers. The microactuator  12  mounted to the substrate  24  is positioned on a surface of the substrate  24  that faces the substrate  22 . The microactuator  12  mounted to the substrate  22  is positioned on a surface of the substrate  22  that faces the substrate  24 . The microactuators  12  mounted to the substrates  20  and  22  are thereby separated from each other by the substrates  20  and  22  and the bonding between them. The microactuators  12  mounted to the substrates  22  and  24  are thereby positioned adjacent to and separated from each other within the space separating the substrates  22  and  24 . 
     The substrate  26  overlies and is mounted to the substrate  24  as will be described herein. The microactuator  12  mounted to the substrate  26  is positioned on a surface of the substrate  26  that faces away from the substrate  24 . The microactuators  12  mounted to the substrates  24  and  26  are thereby separated from each other by the substrates  24  and  26  and by the bonding between them. 
       FIG. 2  is a view of the microsensor cluster  10  from adjacent to substrate  26 . The cover  32  and mesh encapsulation  38  are shown in section. Three microelectrodes  28  are shown extending from the microactuators  12  at separated locations along the edge  15 . An underfill material  52  closes and hermetically seals the gap between substrate  24  and substrate  22  around the periphery of the substrate  24  except the periphery that lies along the leading edge  15 . Similarly, the underfill material  52  closes and hermetically seals the gap between substrate  20  and substrate  18  around the periphery of the substrate  20  except the periphery that lies along the leading edge  15 . The underfill material  52  also closes and hermetically seals the gap between substrate  16  and substrate  14  around the periphery of the substrate  16  except the periphery that lies along the leading edge  15 . As presently preferred, the underfill material is a silicon epoxy that is manufactured by DAP Products Inc. The underfill material  52  supports the substrates maintaining the separation between separated substrates. 
     As shown by  FIG. 2 , a plurality of connection pads  62  are positioned on an outward facing surface of the substrate  26  that faces oppositely from the substrate  24 . These connection pads  62  are electrically connected to the actuator  12  that is on the substrate  26  and the microelectrodes  28  that extend from that actuator  12 . Those electrical connections may comprise any electrical connection including conductive paths formed in the substrate  26  by integrated circuit (IC) processes. 
     A plurality of a plurality of connection pads  64  are positioned on a section of the substrate  22  that extends farther from the leading edge  15  than do substrates  24  and  26 . The pads  64  are positioned on a surface of the substrate  22  that faces oppositely from the substrate  20 . Connection pads  64  are electrically connected to the actuator  12  that is on the substrate  22  and the microelectrodes  28  that extend from that actuator  12 . Others of connection pads  64  are electrically connected to the actuator  12  that is on the substrate  24  and the microelectrodes  28  that extend from that actuator  12 . Those electrical connections include connections from the substrate  24  to the substrate  22  as described herein. 
     A plurality of connection pads  66  are positioned on a section of the substrate  18  that extend farther from the leading edge  15  than do substrates  20  and  22 . The pads  66  are positioned on a surface of the substrate  18  that faces oppositely from the substrate  16 . Connection pads  66  are electrically connected to the actuator  12  that is on the substrate  18  and the microelectrodes  28  that extend from that actuator  12 . Others of connection pads  66  are electrically connected to the actuator  12  that is on the substrate  20  and the microelectrodes  28  that extend from that actuator  12 . Those electrical connections include connections from the substrate  20  to the substrate  18  as described herein. 
     A plurality of connection pads  68  are positioned on a section of the substrate  14  that extend farther from the leading edge  15  than do substrates  16  and  18 . The pads  68  are positioned on a surface that faces oppositely from the interconnect board  42 . Connection pads  68  are electrically connected to the actuator  12  that is on the substrate  14  and the microelectrodes  28  that extend from that actuator  12 . Others of connection pads  68  are electrically connected to the actuator  12  that is on the substrate  16  and the microelectrodes  28  that extend from that actuator  12 . Those electrical connections include connections from the substrate  16  to the substrate  14  as described herein. 
     The connection pads can be aluminum or doped polysilicon and are fabricated along with the other microstructures on each substrate. In general, they can be made out of any conductive film. Typical industrial standard is copper or gold as these metal adhere well with solder paste. 
     As illustrated by  FIG. 2 , the substrates  26  and  24  have the same dimension along the leading edge  15 , the width, and the same dimension along the plane of the substrates  26  and  24  in the direction perpendicular to the leading edge, the length. For the embodiment illustrated, the width of substrates  26  and  24  is approximately 3 millimeters, and the length is approximately 5 millimeters. The substrates  22  and  20  both have a width of approximately 4 millimeters and a length of approximately 6.5 millimeters. The substrates  18  and  16  both have a width of approximately 5 millimeters and a length of approximately 8 millimeters. The substrate  14  has a width of approximately 6 millimeters and a length of approximately 9.5 millimeters. All substrates support a microactuator  12  to which forces are applied when the microelectrodes  28  are extended or retracted. 
       FIG. 3  shows the microsensor cluster  10  from the opening  34  in the cover  32 . As shown by  FIG. 3 , substrate  14  includes three mounting pads  114  on the surface facing substrate  16  on opposed sides in the width direction from the microactuator  12  mounted on the substrate  14 . The mounting pads  114  are spaced between the microactuator  12  and the extent of the substrate  14  in the width direction. Mounting pads  94  on the surface of substrate  16  are adjacent to and spaced from the mounting pads  114 . Solder  114  adheres to opposed pairs of mounting pads  114  and  94  and spaces the substrate  16  from the substrate  14 . Substrate  18  includes two mounting pads  92  on the surface facing substrate  18  on opposed sides in the width direction from the microactuator  12  mounted on the substrate  18 . The mounting pads  92  are spaced between the microactuator  12  and the extent of the substrate  18  in the width direction. Mounting pads  82  on the surface of substrate  20  are adjacent to and spaced from the mounting pads  92 . Solder  84  adheres to opposed pairs of mounting pads  82  and  92  and spaces the substrate  20  from the substrate  18 . One of the pads  64  on the surface of substrate  22  facing substrate  24  is located on each of the opposed sides in the width direction from the microactuator  12  mounted on the substrate  22 . Those pads  64  are spaced between the microactuator  12  and the extent of the substrate  22  in the width direction. Pads  74  on the surface of substrate  24  are adjacent to and spaced from the pads  64  that are outwardly adjacent to the microactuator  12  on the substrate  24 . Solder  76  adheres to opposed pairs of pads  64  and  74  and spaces the substrate  24  from the substrate  22 . 
     The substrates  14 ,  16 ,  18 ,  20 ,  22 ,  24  and  26  are mounted as described below to provide support the substrates to prevent deflection of the substrates and unacceptable movement of the microactuators  12  and microelectrodes  28 , and to avoid stress in the substrates that will damage or cause the substrate to fail. 
       FIG. 4  illustrates the assembly of the first step in assembling the microsensor cluster  10 . The substrate  14  is fabricated with the microactuator  12  and microelectrodes  28  on a surface of the substrate  14 , two rows of mounting pads  114  on that surface extending along the width direction of the substrate  14  and the pads  68  on that surface. The two rows of mounting pads  114  are spaced from each other along the length direction of the substrate  14  and spaced from the pads  68 . Signals from the microelectrodes  28  that extend from the microactuator  12  that is on the substrate  14  are received by connection pads  68  that are electrically connected to those microelectrodes  28 . The microactuator  12  that is on the substrate  14  is controlled by signals provided to connection pads  68  that are electrically connected to that microactuator  12 . The first step of assembly of microsensor cluster  10  comprises applying solder  104  to the pads  114  and solder  116  to pads  68 . Solder is composed of flux and solder particulates, with the size of 5 to 15 μm; as typically used in the industry a 50:50 composition by volume between solder particulates and flux. Solder particulate composition is determined by what is commercially available. 
       FIG. 5  illustrates the assembly created by the second step in assembling the microsensor cluster  10 . The substrate  16  is fabricated with the microactuator  12  and microelectrodes  28  on a surface of the substrate  16 , with two rows of mounting pads  94  extending along the width direction on that surface of the substrate  16  and with connection pads  118  on that surface. The two rows of mounting pads  94  are spaced from the leading edge  15  of the substrate  16  by the same distance that the rows of mounting pads  114  are spaced from the leading edge  15  of the substrate  14 . As shown by  FIG. 3 , the pads  94  are spaced along the width direction of the substrate  16  to be adjacent to the mounting pads  114  on the substrate  14  when the substrate  16  is positioned adjacent to the substrate  14 . The connection pads  118  on the substrate  16  are electrically connected to the microactuator  12  that is on the substrate  16  and to the microelectrodes  28  that extend from that microactuator  12 . Signals from the microelectrodes  28  may be received by the connection pads  118  that are electrically connected to and the microelectrodes  28 . The microactuator  12  on the substrate  16  may be controlled by signals provided to connection pads  118  that are electrically connected to that microactuator  12 . 
     The substrate  18  is fabricated with the microactuator  12  and microelectrodes  28  on a surface of the substrate  18 , a row of mounting pads  92  extends along the width direction on that surface of the substrate  18  and with connection pads  66  on that surface. The row of mounting pads  92  is separated from the leading edge  15  of the substrate  18  by the distance that separates the row of mounting pads  94  on the substrate  14  that is closest to the leading edge  15  of that substrate from the leading edge  15 . The connection pads  66  are electrically connected to the microactuator  12  that is on the substrate  18  and to the microelectrodes  28  that extend from that microactuator  12 . Signals from the microelectrodes  28  may be received by the connection pads  66  that are electrically connected to and the microelectrodes  28 . The microactuator  12  on the substrate  18  may be controlled by signals provided to connection pads  118  that are electrically connected to that microactuator  12 . 
     The substrates  16  and  18  are sized to have the same length and width. The second step of assembly of microsensor cluster  10  comprises positioning the substrates  16  and  18  adjacent to each other so that the leading edges  15  of the substrates  16  and  18  are adjacent to each other and a surface of each substrate that is opposed to the surface on which the microactuators  12  and pads are located abuts such an opposed surface of the other substrate. As shown by  FIG. 5 , in this position, the row of mounting pads  92  nearest the leading edges  15  is adjacent to the row of mounting pads  94  that is nearest those leading edges, the row of mounting pads  94  that is farther from the leading edges  15  and the row of connection pads  118  are adjacent to the connection pads  66 . The second step of assembly of microsensor cluster  10  further comprises eutectic bonding of the substrates  16  and  18  to each other at the abutting surfaces. 
       FIG. 6  illustrates the assembly of the third step in assembling the microsensor duster  10 . The bonded substrates  16  and  18  are positioned adjacent to the substrate  14  so that the leading edges  15  of the substrates  14 ,  16  and  18  lie approximately in a plane and the surface of the substrate  16  on which the microactuator  12  and pads  94  and  118  are positioned overlies the surface of the substrate  14  on which the microactuator  12  and pads  114  and  68  are positioned. The mounting pads  94  are positioned adjacent to the solder  104  on the mounting pads  114  and the connection pads  118  are positioned adjacent to the solder  116  on the connection pads  68 . The solder  104  and  116  bond the substrate  14  to the substrate  16  and reflow soldering creates a solder connection between mounting pads  94  and  104  and between connection pads  118  and  68 . 
     As presently preferred, solder used for this process is manufactured by Indium Corporation and is 63 Sn and 37 Pb solder. A slow reflow process is preferably used wherein the melting temperature is approached over 60 to 80 minutes. This slow heating allows flux to evaporate preventing contamination of the active MEMS components, microactuators  12  and microelectrodes  28 , by the flux which can interfere with functioning of the MEMS components. 
       FIG. 7  illustrates the assembly of the fourth step in assembling the microsensor cluster  10 . The substrate  20  is fabricated with the microactuator  12  and microelectrodes  28  on a surface of the substrate  20 , with a row of mounting pads  122  on that surface extending along the width direction of the substrate  20  and a row of connection pads  126  extending along the width direction of the substrate  20 . The row of mounting pads  122  is separated from the leading edge  15  of the substrate  20  by the same distance that the mounting pads  92  are separated from the leading edge  15  of the substrate  18 . The connection pads  126  are separated from the leading edge  15  of the substrate  20  by a distance that is approximately the distance that the connection pads  66  are separated from the leading edge  15  of the substrate  18 . The row of mounting pads  122  and the row of connection pads  126  are spaced from each other along the length direction of the substrate  20 . Signals from the microelectrodes  28  that extend from the microactuator  12  that is on the substrate  20  are received by connection pads  126  that are electrically connected to those microelectrodes  28  and the microactuator  12  that is on the substrate  20  is controlled by signals provided to connection pads  126  that are electrically connected to that microactuator  12 . The fourth step of assembly of microsensor cluster  10  comprises applying solder  124  to each of the mounting pads  122  and applying solder  128  to each of the connection pads  126 . 
       FIG. 8  illustrates the assembly of the fifth step in assembling the microsensor cluster  10 . The substrate  22  is fabricated with the microactuator  12  and microelectrodes  28  on a surface of the substrate  22  with connection pads  68  on that surface and separated from the leading edge  15  of the substrate  22 . The connection pads  68  are electrically connected to the microactuator  12  that is on the substrate  22  and to the microelectrodes  2 . 8  that extend from that microactuator  12 . Signals from the microelectrodes  28  may be received by the connection pads  68  that are electrically connected to and the microelectrodes  28 . The microactuators  12  on the substrate  22  may be controlled by signals provided to connection pads  68  that are electrically connected to that microactuator  12 . 
     The substrates  20  and  22  are sized to have the same length and width. The fifth step of assembly of microsensor cluster  10  comprises positioning the substrates  20  and  22  adjacent to each other so that a surface of each substrate that is opposed to the surface on which the microactuators  12  and pads are located abuts such an opposed surface of the other substrate. The fifth step of assembly of microsensor cluster  10  further comprises eutectic bonding of the substrates  20  and  22  to each other at the abutting surfaces. 
       FIG. 9  illustrates the assembly of the sixth step in assembling the microsensor cluster  10 . The bonded substrates  20  and  22  are positioned adjacent to the substrate  18  so that the leading edges  15  of the substrates  18 ,  20  and  22  lie approximately in a plane and the surface of the substrate  20  on which the microactuator  12  and pads  122  and  126  are positioned overlies the surface of the substrate  18  on which the microactuator  12  and pads  94  and  66  are positioned. The mounting pads  122  are positioned adjacent to the mounting pads  94  capturing the solder  124  therebetween and the connection pads  126  are positioned adjacent to connection pads  66  capturing the solder  128  therebetween. The substrate  20  is bonded to the substrate  18  by reflow soldering as described above. 
       FIG. 10  illustrates the assembly of the seventh step in assembling the microsensor cluster  10 . The substrate  24  is fabricated with the microactuator  12  and microelectrodes  28  on a surface of the substrate  24 , with a row of connection pads  74  extending along the width direction of the substrate  24 . The connection pads  74  are separated from the leading edge  15  of the substrate  24  by a distance that is approximately the distance that the connection pads  68  are separated from the leading edge  15  of the substrate  22 . Signals from the microelectrodes  28  that extend from the microactuator  12  that is on the substrate  24  are received by connection pads  74  that are electrically connected to those microelectrodes  28  and the microactuator  12  that is on the substrate  24  is controlled by signals provided to connection pads  74  that are electrically connected to that microactuator  12 . The seventh step of assembly of microsensor duster  10  comprises applying solder  76  to each of the connection pads  74 . 
       FIG. 11  illustrates the assembly of the eighth step in assembling the microsensor cluster  10 . The substrate  26  is fabricated with the microactuator  12  and microelectrodes  28  on a surface of the substrate  26  with connection pads  62  on that surface and separated from the leading edge  15  of the substrate  26 . The connection pads  66  are electrically connected to the microactuator  12  that is on the substrate  26  and to the microelectrodes  28  that extend from that microactuator  12 . Signals from the microelectrodes  28  may be received by the connection pads  66  that are electrically connected to and the microelectrodes  28 . The microactuator  12  on the substrate  22  may be controlled by signals provided to connection pads  66  that are electrically connected to that microactuator  12 . 
     The substrates  24  and  26  are sized to have the same length and width. The eighth step of assembly of microsensor cluster  10  comprises positioning the substrates  24  and  26  adjacent to each other so that a surface of each substrate that is opposed to the surface on which the microactuators  12  and connection pads are located abuts such an opposed surface of the other substrate. The eighth step of assembly of microsensor cluster  10  further comprises eutectic bonding of the substrates  24  and  26  to each other at the abutting surfaces. 
       FIG. 12  illustrates the assembly of the ninth step in assembling the microsensor cluster  10 . The bonded substrates  24  and  26  are positioned adjacent to the substrate  22  so that the leading edges  15  of the substrates  22 ,  24  and  26  lie approximately in a plane and the surface of the substrate  24  on which the microactuator  12  and connection pads  74  are positioned overlies the surface of the substrate  22  on which the microactuator  12  and connection pads  64  are positioned. The connection pads  74  are positioned adjacent to connection pads  64  capturing the solder  76  therebetween. The substrate  24  is bonded to the substrate  22  by reflow soldering as described above. The ninth step in assembling the microsensor cluster  10  further comprises applying the silicon epoxy underfill material  52  as described above. The silicon epoxy underfill material is allowed to dwell for 18 hours to assure that it is solid. 
       FIG. 13  illustrates the assembly of the tenth step in assembling the microsensor cluster  10 . The surface of the substrate  14  that faces oppositely from the surface on which the microactuator  12 , mounting pads  114  and connection pads  68  are located is positioned to overlie the interconnect board  42 . The substrate  14  is adhesively bonded to the interconnect board  42 . Wires  136  are then electrically connected to connection pads  62 ,  64 ,  66 ,  68  and  44  to provide electrical connection from the connection pads  44  to the microactuators  12  and the microelectrodes  28 . 
       FIGS. 14 and 15  show a case  132  supporting four microsensor clusters,  110 ,  120 ,  130  and  140  made in accordance with the invention. As shown, microsensor clusters may be mounted adjacent to each other to provide sensors to monitor larger regions than can be monitored by a single cluster. 
       FIG. 16  shows a micrograph of a 3 mm by 7 mm embodiment of a microactuator  12 . The micrograph of  FIG. 16  shows a mechanism  112  that includes an electrode  28  that is extended and retracted from the mechanism  112 . The microactuator and microelectrodes are fabricated in Sandia National Laboratories using the SUMMiT-V process, which is a 5 layer highly doped polysilicon surface micromachining process that is described by J. Muthuswamy, M. Okandan, A. Gilletti, M. Baker, and T. Jain, “An array of microactuated microelectrodes for monitoring single neuronal activity in rodents,” IEEE Trans Biomed Eng, 52:1470-1477, 2005. The microactuators have 4 thermal actuators and allow for bi-directional movement with &lt;10 μm resolution.  FIG. 16  is a SEM of a mechanism  112 . The electrode  28  is 50 μm wide and 4 μm thick and can move a maximum distance of 5 mm in steps of about 8.8 μm. The mechanism  112  has a first powl  114  and a second powl  116 . A move-up thermal actuator  172  causes powl  114  to translate by +X, 10 μm; when the current is off, Powl  114  elastically moves back into its original position while translates the electrode by −X, 10 μm through teeth engagement mechanism. A move-down thermal actuator unlocking mechanism  176  causes rachet  178 , which normally stops the +X translation of microelectrode  28 , to move in Y-direction in order to unlock the microelectrode  28 . A move-up unlocking mechanism thermal actuator  184  causes rachet  182 , which normally stop the −X translation of the electrode  28 , to move in Y-direction in order to unlock the electrode  28 . A move down thermal actuator  134  causes powl  116  to translate by −X, 10 μm; when the current is off, the arrow elastically moves back into its original position while translates the electrode by −X, 10 μm through teeth engagement mechanism. Translation guides  142  align and guide the microelectrode  28 . 
     Packaging and interconnects contribute significant additional weight to the microactuators  12 . The chip itself weighs about 0.18 g and packaging can add about ten times this weight. In an effort to miniaturize implantable devices MEMS technology is used to provide a packaging that is compact and light-weight. Flip-chip technique provides excellent solution with compact form factor. There exist special challenges to adapt this technique for MEMS devices: (a) Head-space for movement due to the presence of actuators, movable electrodes, (b) Contamination free process so that the moving parts are not obstructed (c) Semi-hermetic seal that allows for the movement of the microelectrodes outside the die as well as keeps blood and CSF fluids from entering the chip. Bumps of Ag epoxy bumps having a diameter of 50 μm diameter that avoid flux contamination are used.  FIG. 17  is a SEM of a mechanism  112  having Ag epoxy bumps  152 .  FIG. 18  shows a microactuator  12  after the bumping process is complete on the entire chip. 
     Assembly of an embodiment of microactuator  12  is illustrated by  FIGS. 19 through 24 .  FIG. 19  illustrates the kitting step. A glass substrate  154  is fabricated with the corresponding bond pads as the MEMS chip and interconnects. Au is deposited through thermal evaporation (200 μm) and patterned to form the flip-chip substrate. MEMS chip  156  with Ag epoxy bumps and TLI (Third Level Interconnect)  158  are assembled. Ag epoxy bumps are deposited on the MEMS chip  156  as illustrated by  FIG. 17 . Flip-chip connecting joins the 500 μm glass substrate  154  to the MEMS chip  156  as shown by  FIGS. 20 and 21 . TLI  157  is joined to the glass substrate  154  using Ag epoxy as illustrated by  FIG. 22 . As illustrated by  FIG. 23 , the assembly is sealed by a semi-hermetic and hermetic seal-chip with semi-hermetic seal on one edge and hermetic seal on the other three sides using non-flow silicone. A hard protection is provided by applying epoxy sealant to protect the package as shown by  FIG. 24 . 
       FIG. 25  shows the MEMS chip  156  mounted to a glass substrate  154  with the semi-hermetic and hermetic non-flow silicone seal  158 . The semi-hermetic seal is adjacent to three channels  162  within which the microelectrodes  28  move freely.  FIG. 26  is a micrograph that shows no flux contamination on the active MEMS structures after the flip chip process. The flip-chip technique creates an assembly that is 7 mm×9 mm and weighs approximately 0.5 g. 
       FIG. 27  shows a static pressure test of the assembly. The assembly was immersed in saline after the semi-hermetic seal was applied. The seal was shown to block fluid entrance at the maximum pressure of 80 cm of water, or about 7.8 kPa. At the pressure of 80 cm of water, trace of fluid leak start to leak into the channel. 
       FIGS. 28-30  show movement of microelectrodes  28  from the MEMS chip  156 .  FIG. 28  is a SEM of right and left microelectrodes  28  extending from the channels in the semi-hermetic seal  158 .  FIG. 29  is a micrograph showing the middle electrode  28  of MEMS chip  156  before actuation.  FIG. 30  is a micrograph showing the middle electrode  28  extended after actuation. Note that the spring that tethers the electrode to the bond-pad for electrical recording is also extended. 
     The force exerted on neural implants during insertion and removal of interconnects can be significant, and can eventually can lead to implant failure. In order to isolate these forces from the MEMS chip, a flexible parylene interconnect has been developed. These flexible interconnects makes the form factor of the package even smaller comparable to the actual chip itself. The flip chip technique described above can be adapted to bond the MEMS chip to parylene flexible substrate.  FIG. 31  shows a MEMS chip  156  with Ag epoxy bumps  162 , and a parylene substrate  166  is connected by flip-chip connection to the Ag epoxy bumps on the MEMS chip  156 .  FIG. 32  shows a complete packaged MEMS chip with flexible parylene interconnect  166  connecting the MEMS chip  156  to a connector  168 . 
     Apparatus according to the present invention is not limited to use with a particular instrument. The invention can be adapted to a variety of clusters of MEMS components. Possible applications include MEMS sensors, MEMS gyroscope, MEMS accelerometers.
Systems and methods of the present disclosure provide for three-dimensional stacks of microelectromechanical (MEMS) systems, such as sensors. The stacks may be encapsulated and sealed, and can be positioned within biological tissue, for example to monitor biological signals within the volume of the sensor, provide stimulating signals to a brain, and so forth.