Abstract:
Two opposing substrate layers each having one or more recesses filled with magnetic material guide the flow of flux through a coil in a MEMS device layer to provide for closed-loop operation. Flux flows from one pole piece through the coil to a second pole piece. A method of making using lithographic etching techniques is also provided.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    A widely used Micro-Electro-Mechanical System/Sensor (MEMS) device for force detection and measurement employs a mechanical capacitive readout force accelerometer having a capacitive output proportional to the force applied. In one such accelerometer, one or more capacitors are formed on a proof mass suspended by one or more flexures. The proof mass is suspended in an instrument frame. A force applied to the proof mass along a particular axis will cause displacement of the proof mass relative to the frame. This displacement varies the capacitive output. The force applied to the proof mass is quantified by measuring a resultant change in capacitive output. Determining the applied force based on change in capacitance is known as open-loop operation. 
         [0002]    By contrast, a closed-loop accelerometer achieves higher performance by using a feedback loop to cancel the movement of the proof mass, thus keeping the mass nearly stationary. Ideally, the capacitance output stays constant in the closed-loop accelerometer. To maintain a constant output, a proportional reaction force is generated to oppose the motion of the proof mass displaced from the neutral or zero displacement point when the proof mass is subjected to an action (external) force. Instead of using the change in capacitance to determine the action force applied to the mass, the action force is determined based on the reaction force necessary to maintain the proof mass in the neutral position. An example of a conventional (non-MEMS) closed-loop, flexure-type accelerometer is the Q-FLEX® manufactured by Honeywell International, Inc. 
         [0003]    The reaction force applied to the proof mass can be generated using a magnetic field in combination with an electrical field. In such a device, the accuracy of the measurements outputted by the MEMS device depends on the properties of the magnetic field, in particular, the strength and uniformity of the magnetic field. 
       SUMMARY OF THE INVENTION 
       [0004]    The present invention relates generally to a magnetic circuit used in a MEMS device to facilitate closed-loop operation. More specifically, the present invention relates to using precisely formed pole pieces to guide magnetic flux through a coil in a MEMS device layer. 
         [0005]    An example system includes a first etched recess deposited with a high magnetic permeability material, a MEMS device layer containing an accelerometer including a coil and a proof mass, and a flux return. The return is also made of a high permeability material. Flux flows from the magnetic material, through the coil, and to the return. 
         [0006]    In accordance with further aspects of the invention, a second etched recess filled with a magnetic material is positioned adjacent to the return, and flux flows through the second etched recess to the return. 
         [0007]    In accordance with other aspects of the invention, the magnetic fill material is saturated. 
         [0008]    In accordance with still further aspects of the invention, the width of the coil is greater than the width of the etched recess. 
         [0009]    In accordance with yet another aspect of the invention, a magnet is positioned adjacent to the first etched recess. 
         [0010]    In accordance with still other aspects of the invention the system is made by packaging a MEMS device between two substrates, etching opposing recesses relative to the MEMS device in each substrate, adding a high magnetic permeability material to create pole pieces, attaching a magnet to the first substrate and pole piece, and attaching a return to the second substrate and pole piece. Anisotropic and/or isotropic etching techniques may be utilized to form the recesses. Various substrate materials may also be used in combination with various etching chemicals and/or other etching parameters to form recesses having different shapes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The foreign aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings. 
           [0012]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0013]      FIG. 1  is partial cross-sectional view showing a magnetic field focused across a MEMS layer formed in accordance with the current invention; 
           [0014]      FIG. 2  is a cross-sectional view of a MEMS coil and portions of a magnetic drive formed in accordance with the current invention; 
           [0015]      FIG. 3  is a cross-sectional view of a MEMS device layer and magnetic field components showing etched pole pieces formed in accordance with the current invention; 
           [0016]      FIG. 4  is a partial cross-sectional view of the MEMS device layer and magnetic field components oriented 90 degrees from the view shown in  FIG. 3 ; 
           [0017]      FIG. 5  is a partial cross-sectional view showing the MEMS device layer, handle, lid, and etched recesses formed in accordance with the present invention; and 
           [0018]      FIG. 6  is a top view of the center-mount configuration between a magnet and a lid in accordance with embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0019]      FIG. 1  is partial cross-sectional view showing a magnetic field  22  passing magnetic flux (flux) through a MEMS coil  44  created according to the principles of the current invention. The magnetic field  22  is substantially orthogonal to the coil  44 . As discussed below in more detail, an electrical field (not shown) is generated by passing current through the coil  44 . 
         [0020]    The magnetic field  22  is formed by magnetic field components  170 . The magnetic field components  170  include a magnet  164 , a first pole piece  28 , a lid  128 , a second pole piece  30 , and a handle  124 . The magnet  164  is attached to the lid  128 . The first pole piece  28  is in a first recess  130 , which has been etched into the lid  128 . The second pole piece  30  is in a second recess  122 , which has been etched into the handle  124 . The second pole piece  30  and the handle  124  are attached to a return  192 . The return  192  and the pole pieces  28 ,  30  include a magnetically conductive material having high permeability such as, but not limited to, Alloy 39 or Kovar. 
         [0021]    A MEMS device layer  60  is located between the handle  124  and the lid  128 . The MEMS device layer  60  is made primarily of silicon. The MEMS device layer  60  also includes the coil  44 . The coil  44  is made of electrically conductive material wrapped in a loop. The coil  44  contains a plurality of coil elements  129 . 
         [0022]    The pole pieces  28 ,  30  define a gap  20 . A portion of a MEMS device layer  60  is located within the gap  20 . Electrical current is provided to the coil  44  by a servo controller (not shown). The recesses  122 ,  130  (and pole pieces  28 ,  30 ) should be positioned so that a portion of the coil  44  containing multiple coil elements  129  resides within the gap  20 . 
         [0023]    Generally, magnetic flux flows in the path of least resistance. The handle  124  and lid  128  are made of a material having a substantially lower permeability (or higher resistance to flux) relative to the material of the pole pieces  28 ,  30 . In one embodiment, the lid  128  and handle  124  are formed from Pyrex 7740. Based on the permeability of the handle  124  and the lid  128  relative to the permeability of the pole pieces  28 ,  30 , flux is directed through the pole pieces  28 ,  30  rather than the handle  124  or lid  128 . 
         [0024]    It is desirable to pass the magnetic field  22  through the coil  44 . Accordingly, the pole pieces  28 ,  30  are positioned relative to the handle  124 , lid  128 , and coil  44  to direct the magnetic field  22  through the coil  44  from the magnet  164  into the return  192 . In the configuration shown in  FIG. 1 , a substantial magnetic flux generally flows from the magnet  164  through the first pole piece  28 , the coil  44  within the MEMS layer  60 , the second pole piece  30 , and into the return  192 . 
         [0025]    It is also desirable that the magnetic field  22  passing through the coil  44  is uniform. To increase the flux uniformity of the magnetic field  22 , the pole pieces  28 ,  30  may be partially or fully saturated by increasing the strength of the magnet  164  and/or decreasing the magnetic permeability of the pole pieces  28 ,  30 . 
         [0026]    To achieve uniform performance over temperatures the magnetic field components  170  are ideally coefficient of thermal expansion (CTE) matched to each other, subject to system/device constraints. Typically, the material of the magnet  164  and the material of the lid  128  are a poor CTE match to each other. As will be discussed in more detail below, the poor CTE match is partially mitigated by mounting the lid  128  to the center of the magnet  164 . 
         [0027]      FIG. 2  shows a portion of an electrostatic comb-drive, closed-loop, in-plane, micro-machined, accelerometer  25  located in the MEMS device layer  60  of the present invention. The accelerometer  25  includes the MEMS coil  44  and the proof mass  32 . MEMS coil  44  is attached to the proof mass  32 . Pole pieces  24 ,  28  are positioned adjacent to the MEMS device layer  60  above the coil  44 . The coil  44  has a width  64 . 
         [0028]    The accelerometer  25  includes two types of combs: stator combs  46  and shuttle combs  48 . The shuttle combs  48  are attached to the proof mass  32 . The position of the stator combs  46  is fixed, while the proof mass  32 , and shuttle combs  48  may translate relative to the stator  46  by means of suspension flexure  50 . The direction of translation is shown by arrows  66 . The stator  46  has a first tine  38 . The shuttle  48  has a second tine  36 . The tines  36 ,  38  function as electrodes. A capacitor is formed by a capacitive gap  40  between the first tine  38  and the second tine  36 . More than one capacitor can be constructed between the combs  46 ,  48 . Translation of the stator  46  relative to the shuttle  48  changes the value of the capacitor. For a device utilizing an open-loop design, the value of the capacitor may be used to determine the active force applied in the direction of translation of the proof mass  32 . 
         [0029]    In a device utilizing a closed-loop design, the proof mass  32  is maintained in a neutral position, i.e. zero displacement by feeding back the effect of the displacement. The action (external) force applied to the proof mass  32  is determined based on the reaction force necessary to maintain the proof mass  32  in a null position. 
         [0030]      FIG. 3  is a view of a configuration having two pair of pole pieces  24 ,  26 ,  28 ,  30 . Other embodiments using different numbers or placements of pole pieces are possible. However, two pair of pole pieces provides optimal performance for the cost. The pole pieces  24 ,  26 ,  28 , and  30  are substantially smaller in size than the magnet  164  and/or the return  192 . The pole pieces  24 ,  26 ,  28 , and  30  are formed by filling the recesses that have been etched in the handle  124  and lid  128 . The pole pieces  24 ,  26 ,  28 , and  30  have a width  62 . The magnet  164  has a south pole  162  and a north pole  166 . A plurality of arrows indicates the direction of flux  160 . The magnetic field  22  and the current traveling through the coil  44  generate a reaction force in opposition to any external acceleration acting on the proof mass  32 . The reaction force restores the proof mass  32  to its initial position and thereby implements a closed-loop design. Specifically, the reaction force acting on the proof mass  32  is the cross product of the magnetic field  22  and the electric field vector of the current within the coil  44 . 
         [0031]    Although a closed-loop design should minimize the movement of the coil  44  and proof mass  32  in response to any external acceleration, some movement of the coil  44  and proof mass  32  may occur, typically due to vibration. Movement of the coil  44  could cause a portion of the coil  44  to be displaced outside of the gap  20  defined by the pole pieces  28 ,  30 . Preferably, the MEMS device layer  60  is desensitized to movement of the proof mass  32  and coil  44  by configuring the width  62  of the pole pieces  28 ,  30  to be less than the width  64  of the coil  44 . Referring to  FIG. 2 , if the coil  44  does translate, an identical number of coil elements  129  will still be located within the magnetic field  22  formed in the gap  20 . The number of coil elements  129  being invariant allows the magnetic field  22  to interact with an E field having a substantially uniform density The uniform density in the E field allows for a substantially constant reaction force (E×B). 
         [0032]      FIG. 4  shows the components of  FIG. 3  from a view that has been rotated 90° on its vertical axis. Shown are the magnetic field components  170  and one coil element  129 . The coil element  129  of the coil  44  is between the lid  128  and the handle  124 . The pole piece  24  and the lid  128  are attached to the magnet  164 . Correspondingly, the pole piece  26  and the handle  124  are attached to the return  192 . 
         [0033]    The recesses of the foregoing invention are formed using lithographic etching. Lithographic etching provides a precisely formed, located, and relatively inexpensive means to form the recesses. To create a strong magnetic field  22  across the coil  44  the depth of recesses  130  and  122  should ideally be as close to the depth of the lid  128  or handle  124  as possible. In other words, wafer thicknesses  125  and  127  should be minimized. 
         [0034]    Different types of lithographic etching and substrate materials may be combined to form the recesses having various dimensions. Isotropic etching is etching that proceeds in all directions at approximately the same rate, while anisotropic etching is etching that proceeds more rapidly in one direction and slower in the other direction. The type of etching chemical, type of lithographic mask, etch duration, temperature, agitation, and chemical concentration, and type of substrate are some of the factors that determine the dimensions of an etch. Accordingly, it is understood that a chemical and a substrate material may be selected by one of ordinary skill in the art to etch a recess having a desired shape and size. Magnet size, lid and handle thickness, and coil width are factors that would influence the desired size and shape of a recess. 
         [0035]    Alternatively, dry etching or machining techniques could be used to form the recesses; however, wet etching is the most cost effective way to form the recesses with the least damage to the substrate material. Specifically, a Deep Reactive Ion Etching (DRIE) process (Bosch derived) could be used with an oxide stop module to prevent etching through a handle made of Si. Alternatively the same DRIE process, without need for the oxide stop module, could be used to etch a timed distance into a handle made of Si. 
         [0036]      FIG. 5  shows a cross-sectional view of the MEMS device layer  60 , the lid  128  and the handle  124 . Recesses  122 ,  130  have been etched into the lid  128  and the handle  124  using an isotropic etching process. The lid  128  is etched to the first wafer thickness  125 . The handle  124  is etched to the second wafer thickness  127 . Alloy 39, Kovar, or a similar permeable magnetically conductive fill material is deposited into the recesses  122 ,  130  by electroplating to form pole pieces. Other techniques can be utilized to deposit the fill material into the recesses. In an exemplary embodiment, slump molding and machining can be used to form pole pieces from fill material. The pole pieces are then inserted into the recesses. After the material has been deposited into the recesses, the handle  124  and/or lid  128  is masked and plated. Alternatively, the handle  124  and/or lid  128  could be plated and then planarized. The pole pieces  24 ,  28  and the lid  128  are attached to the magnet  164  and the pole pieces  26 ,  30  are attached to the handle  124 . The handle  124  and the pole pieces  26 ,  30  are attached to the return  192  using a compliant epoxy. 
         [0037]      FIG. 6  shows a detailed view of a center-mount configuration  200  for a lid or handle surface  206  relative to a return or magnet surface  208 . Specifically, the lid or handle surface  206  is positioned at the approximate center of the magnet or return surface  208  in order to achieve consistent performance across temperature ranges. There is a small out-of-plane offset between the surfaces  206 ,  208 . A compliant epoxy  202  is added to the gap to attach the return or magnet to surface  208 . When the epoxy  202  cures and shrinks, the return or magnet surface  208  is pulled into hard contact with the lid or handle surface  206 . The center-mount configuration  200  is especially useful when attaching the magnet  164  to the lid  128  because the magnet  164  typically does not have a CTE match to the lid  128  material. By comparison, the return  192  could be made from alloy 39, Kovar, or similar material with a good CTE match to the lid or handle surface  206  and could be bonded directly to the lid or handle surface  206  and the pole piece  30 . 
         [0038]    The various embodiments described above can be combined to provide further embodiments. U.S. Patent Application 2006/0163679 and U.S. Pat. Nos. 5,959,207, 5,763,779, and 5,111,694, are incorporated herein by reference in their entireties. Aspects can be modified, if necessary, to employ devices, features, and concepts of the various patents, applications and publications to provide yet further embodiments. 
         [0039]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. For example, recesses having different dimensions can be used. Correspondingly, pole pieces having different dimensions can be formed. Also, a second handle could be used instead of the lid  128 . Additionally, materials such as, but not limited to, silicon or Schott Borofloat could be utilized for the lid  128  or the handle  124 . Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.