Abstract:
An in-plane, closed-loop Micro Electro-Mechanical Systems (MEMS) accelerometer device with improved performance. An example MEMS device includes one or more components for generating a magnetic flux field perpendicular to a major plane of the device. The device includes substrates, a proof mass, spring elements that flexibly connect the proof mass to the substrate and constrain the proof mass to translate within the major plane of the device which corresponds to a major surface of the proof mass, a plurality of conductive traces located at a position on the proof mass proximate the magnetic flux field, a plurality of conductive springs, each of the springs are electrically connected to a corresponding one of the conductive traces, and a plurality of anchor pads connected to the substrate and one of the conductive springs.

Description:
PRIORITY CLAIM 
     This application is a Continuation Application of U.S. application Ser. No. 12/500,487 filed Jul. 9, 2009 which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Many accelerometers can be constructed on a single silicon-on-insulator (SOI) wafer leading to cost advantages over single-part-at-a-time construction methodologies. However, if a particular accelerometer includes electrical traces that form a coil applied to one side of a proof mass, then there is an issue of cost effectively constructing that device. Performance issues may arise because a coil trace that ends on the inside of a coil must loop over the coil in order to exit the proof mass. Also, devices formed in this manner may be susceptible to external magnetic fields, generate significant external flux leakage and may fail to meet flux requirements in order to servo the proof mass. 
     SUMMARY OF THE INVENTION 
     The present invention provides an in-plane Micro Electro-Mechanical Systems (MEMS) accelerometer device with improved performance. An example MEMS device includes one or more components for generating a magnetic flux field. The magnetic flux field being perpendicular to a major plane of the device. The device also includes a substrate, a proof mass, a spring element that flexibly connects the proof mass to the substrate for allowing motion of the proof mass in the major plane that corresponds to a major surface of the proof mass, a plurality of conductive coil traces located at a position on the proof mass proximate the magnetic flux field, a plurality of conductive springs, each of the springs being electrically connected to a corresponding one of the conductive coil traces, and a plurality of anchor pads connected to the substrate and one of the conductive springs. 
     The device also includes one or more sense combs having first tines located on the proof mass and opposing second tines attached to the substrate. 
     The device also includes damping combs having first tines located on the proof mass and opposing second tines attached to the substrate. The first damping comb tines are electrically isolated from the first sense comb tines. 
     In one aspect of the invention, each of the conductive springs includes two first legs having a first cross-sectional dimension, an elbow, and two second legs having a second cross-sectional dimension. The two second legs are connected between the elbow and one of the first legs and the second cross-sectional dimension is smaller than the first cross-sectional dimension. 
     In another aspect of the invention, isolation trenches directly connect to outer edges of the traces that are adjacent to other traces or proof mass material. 
     In still other aspects of the invention, the coil traces and sections of conductive springs include a plurality of slots. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
         FIG. 1-1  illustrates a cross-sectional view of translational mass in-plane Micro Electro-Mechanical Systems (MEMS) accelerometer formed in accordance with an embodiment of the present invention; 
         FIG. 1-2  illustrates a perspective view of the MEMS accelerometer shown in  FIG. 1-1 ; 
         FIG. 1-3  is a block diagram of a closed-loop accelerometer system using the MEMS accelerometer shown in  FIGS. 1-1  and  1 - 2 ; 
         FIG. 2-1  illustrates a top view of a device layer in an example MEMS accelerometer formed in accordance with an embodiment of the present invention; 
         FIG. 2-2  is a blow-up view of a portion of the MEMS accelerometer shown in  FIG. 2-1 ; 
         FIG. 3  illustrates a blow-up view of conductive springs and coil traces of the example MEMS accelerometer shown in  FIG. 2-1 ; 
         FIG. 4  illustrates a partial view of capacitive pick-off or position sense components for the MEMS accelerometer shown in  FIG. 2-1 ; 
         FIG. 5-1  illustrates a cross-sectional view of one of the conductive spring elements shown in  FIG. 3 ; 
         FIG. 5-2  illustrates a cross-sectional view of coil traces located on a proof mass of the example MEMS accelerometer shown in  FIG. 2-1 ; 
         FIG. 6  illustrates a partial top view of example coil traces used in the devices shown in FIGS.  1  and  2 - 1 ; and 
         FIGS. 7-1  through  7 - 9  are cross-sectional views illustrating an example process for creating the devices shown in FIGS.  1  and  2 - 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1-1  illustrates an x-ray cross section view and  FIG. 1-2  illustrates a perspective view of a translational mass in-plane Micro Electro-Mechanical Systems (MEMS) accelerometer  20  formed in accordance with an embodiment of the present invention. The accelerometer  20  includes a device layer  40  that is attached on each side to a substrate  25 ,  27 . Magnet pole-piece layer  22  and a magnetic return path layer  26  are attached to the substrates  25 ,  27 . A magnet  30  is sandwiched between each magnet pole-piece layer  22 . Each magnet pole-piece layer  22  includes a protrusion  31  that extends into the substrate  25  at a location that corresponds with coil traces located on a proof mass, which is an integral part of the device layer  40 . Each end of the magnetic return path layer  26  includes a protrusion  33  that extends into the substrate  27  at the location that corresponds with coils located on the proof mass. 
     As will be described in more detail below, the device layer  40  includes a proof mass that reacts to motion of the accelerometer  20  along the X axis. In closed loop operation, an electrical current is applied to the coil traces located on the proof mass creating a Lorentz force on the proof mass as the current passes through the magnetic field flux running between the magnet pole-piece layer  22  and the magnetic return path layer  26  at specific sections of the coil traces. The electrical current applied to the coil traces on the proof mass is adjusted so that the Lorentz force on the proof mass opposes and balances the acceleration inertial force on the proof mass there by forcing the proof mass to a null position consistent with a zero acceleration state. The amount of electrical current being applied is based on the position of the proof mass, which is based on a capacitance sensed by capacitive components formed between the movable proof mass and the fixed substrate  25  and  27 . 
       FIG. 1-3  is a block diagram illustrating a closed-loop system using the MEMS accelerometer  20  shown in  FIGS. 1-1  and  1 - 2 . The MEMS accelerometer  20  includes sense capacitors that generate a signal indicative of the location of the proof mass relative to a base substrate. The sense capacitors will be described in more detail below. This position signal is sent to a processor  10  that converts the position signal into a drive signal (electrical current) and a corresponding acceleration value, which is outputted via an output device (not shown). The drive signal is applied to a drive circuit  12  at the MEMS accelerometer  20 . The drive circuit  12  includes the coil traces located on the proof mass. A change in current applied to the coil traces causes a force to be applied to the proof mass as the current passes through the magnetic flux produced by the magnet pole-piece layer  22  and the magnetic return path layer  26 . This force should be equal and opposite to the sensed acceleration inertial force. 
       FIG. 2-1  illustrates a top view of a device layer  40  for an example translational mass in-plane MEMS accelerometer. The device layer  40  includes a proof mass  44  that is connected to a substrate (not shown) by outer support springs  48 - 1  and  48 - 2 . The springs  48 - 1  and  48 - 2  allow the proof mass  44  to move in the plane of the device layer  40  while constrained to move only in a single translation direction. Fabricated as an integral part of the proof mass  44  are a plurality of conductive coil traces  46 - 1 ,  46 - 2  and  46 - 3  shown on  FIG. 2-2 . The coil traces  46 - 1 ,  46 - 2  and  46 - 3  are electrically connected to respective inner support conductive springs  50 - 1 ,  50 - 2  and  50 - 3  shown on  FIG. 2-1 . Mounted on outer edges of the proof mass  44  is a damping comb trace  56  that includes a plurality of attached tines  58 , shown in  FIG. 2-2 , and opposing tines  59  attached to the substrate, shown in  FIG. 2-2 . The damping comb tines  58  and  59  may be used for electrostatic spring softening to reduce the natural frequency of the device. 
     The damping comb tines attached to the proof mass  44  are electrically isolated from the pick-off comb tines located on the proof mass  44 . The damping comb tines attached to the proof mass  44  are electrically connected to a flexure on one of the outer spring elements  48  and the pick-off comb tines  68  attached to the proof mass  44  are electrically connected to another flexure on the outer support spring  48 . 
     As shown in  FIG. 3 , each of the inner support conductive springs  50 - 1 ,  50 - 2  and  50 - 3  are physically and electrically attached to the respective coil trace  46 - 1 ,  46 - 2  and  46 - 3  and a respective anchor pad  60 - 1 ,  60 - 2  and  60 - 3 . The inner support conductive springs  50 - 1 ,  50 - 2  and  50 - 3  include narrow cross-sectional bending members interspersed with wider cross-sectional links, respective anchor pads  60 - 1 ,  60 - 2  and  60 - 3 , a corner element and ends of the coil traces  46 - 1 ,  46 - 2  and  46 - 3 . The narrow cross-sectional bending members minimize the stiffness of the conductive springs  50 - 1 ,  50 - 2  and  50 - 3 . Only the anchor pads  60 - 1 ,  60 - 2  and  60 - 3  are attached to the substrate. 
     The space  64 - 1  between the lengths of a particular one of the conductive springs and the space  64 - 2  around an exterior edge of the particular conductive springs are open all the way through the thickness of proof mass  44 . An insulator barrier in-fill  62  is located between each of the coil traces  46 - 1 ,  46 - 2  and  46 - 3  and the adjacent ground trace  98 . 
       FIG. 4  illustrates a partial top view of a portion of the pick-off comb region of the accelerometer shown in  FIG. 2-1 . Mounted on the inner region of the proof mass  44  is the pick-off comb trace  54  that includes a plurality of attached tines  68 . Opposing tines  70  and  74  are attached to the substrate and are interleaved with the pick-off tines  68 . In an optional configuration, a ground plane  76  is attached to the substrate between opposing stationary tines  70  and  74 . The ground plane  76  is intended to reduce parasitic capacitances. Interleaved tines  70  and  68  form one group of position sense capacitors and interleaved tines  74  and  68  form a second group of position sense capacitors. These two capacitor groups provide opposite changes in capacitance value, either increasing or decreasing, for a particular change in proof mass position. The position signal is then derived by taking the difference between the two capacitance groups. 
       FIG. 5-1  illustrates a cross-sectional view of one of the wider cross-sectional links of the conductive spring  50 - 1 . The conductive spring  50 - 1  includes solid areas of highly doped silicon  88 . The areas include a plurality of etched slots  86 . The slots  86  are used to help increase the electrical conductivity by allowing more doping of the link cross section of conductive spring  50 - 1 . This configuration is repeated in the other conductive springs  50 - 2  and  50 - 3 . 
       FIG. 5-2  illustrates a cross-sectional view of the coil trace  46 - 1 . The coil trace  46 - 1  includes an area of highly doped silicon  92 . The area of silicon  92  includes a plurality of etched slots  90 . The slots  90  also help increase electrical conductivity by allowing more doping of the electrical trace cross section. Adjacent to the coil trace  46 - 1  is In-fill  62 . The In-fill  62  is located between coil traces  46 - 1 ,  46 - 2  and  46 - 3  and is also located between coil traces  46 - 1 ,  46 - 2  and  46 - 3  and the adjacent ground trace  98  to provide electrical isolation from the pick-off trace  54  (shown in  FIGS. 2-2  and  3 ). The ground trace also provides electrical isolation between the pick-off comb trace  54  and the damping comb trace  56  (shown in  FIG. 2-2 ). In-fill is a dielectric material grown between the various conductive traces and provides electrical isolation while maintaining proof mass stiffness. One implementation of the In-fill includes a combination of oxide and nitride such that the compressive and tensile stresses created net out to zero to minimize warpage.  FIG. 6  shows a top view of the slots  90 . 
       FIGS. 7-1  through  7 - 9  are cross-sectional views of steps in an example process for forming a device such as is shown in  FIG. 2-1 . First, at  FIG. 7-1 , a silicon-on-insulator (SOI) wafer  100  having a silicon device layer  102  and a handle (e.g. silicon) layer  106  separated by an insulator layer  104 , such as silicon oxide. Next, as shown in  FIG. 7-2  isolation trenches and slots  110  are etched into the device layer  102 . Deep reactive ion etching (DRIE) is used to create the trenches and slots  110 . The trenches are comparable to the isolation barrier  62  shown in  FIG. 3 . Slots are comparable to the slots  90  shown in  FIGS. 5-2  and  6 . 
     Next, at  FIG. 7-3 , the exposed surfaces of the device layer  102  are doped in order to increase the electrical conductivity of exposed surfaces  114  of the device layer  102 . An example dopant is boron. 
     Next, at  FIG. 7-4 , In-fill is applied to the wafer  100 . In one example, the In-fill is oxide\nitride. The In-fill occupies the trenches and slots  110  and produces a layer on the top horizontal surface of the device layer  102 . 
     Next, as shown in  FIG. 7-5 , the wafer  100  is planarized to remove the In-fill that is located on the exterior horizontal surface of the device layer  102 , thereby leaving In-fill  120  in the previously open trenches and slots  110 . Next, contact bumps  124  are applied to the now exposed surface of the device layer  102 . Application of the bumps  124  is performed using a masking and metallization process. 
     Next at  FIG. 7-6 , a second DRIE process is performed in order to etch slots  128  for defining the proof mass, springs, damping combs and pick-off combs. 
     As shown in  FIG. 7-7 , a magnetic component  130 , comparable to the magnetic cover  22  shown in  FIG. 1 , is anodically bonded to the device layer  102  via the borosilicate glass component of the return path. This simultaneously serves to complete electrical connections via metallization bumps  124 . At  FIG. 7-8  both the handle layer  106  and the oxide layer  104  are removed. Then, at  FIG. 7-9 , a magnetic return path  136 , similar to the magnet return path  26  shown in  FIG. 1  is anodically bonded to an opposing surface of the device layer  102 . After this step, the wafer is ready for dicing for separation into individual components ready for mounting onto a circuit board or some other device. The present embodiment of the attach process has a top silicon element anodically bonded to the exposed oxide layer of the device around the periphery. The top silicon layer has recesses built into it using DRIE to accommodate extremities of the upper return path parts which allow the driving flux to intensify by minimizing the magnetic circuit air gap. The silicon/glass assembly is diced prior to the bonding of the magnetic circuit components. The bottom return path component is then aligned to the device and epoxy bonded. The magnet is aligned and bonded to the upper return path parts which are attached to either end of the magnet. This assembly is then aligned and epoxy bonded to the lower return path/device assembly. 
     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. 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.