Interlocking proof mass for mems inertial sensing device
A microelectromechanical systems (MEMS) inertial sensing device having a large proof mass with interlocking tabs is disclosed. The interlocking tabs limit motion of the proof mass when subjected to large inertial forces. The interlocking tabs are formed around the periphery of the proof mass and interact with corresponding interlocking tabs formed in a frame to which the proof mass is tethered. The motion of the proof mass is limited by the interlocking tabs, which are formed during MEMS fabrication of the inertial sensing device rather than as part of the packaging process. With a large proof mass, the inertial sensing device can provide high sensitivity in low inertial force environments while the interlocking tabs protect the device when subjected to high inertial forces.
1. An inertial sensing device comprising:
a proof mass, the proof mass including:
N proof mass interlocking tabs formed around a periphery of the proof mass, where N is an integer greater than 1; and
N proof mass recesses formed around the periphery of the proof mass;
a frame, the frame including:
an orifice therethrough;
N frame interlocking tabs formed around a periphery of the orifice, each of the N frame interlocking tabs adapted to fit in a respective one of the N proof mass recesses;
N frame recesses formed around the periphery of the orifice, each of the N proof mass interlocking tabs adapted to fit in a respective one of the N frame recesses; and
a plurality of electrostatic pick-offs formed around the periphery of the orifice; and
a plurality of flexures, a first end of each of the plurality of flexures coupled to the proof mass, a second end of each of the plurality of flexures coupled to the frame, the plurality of flexures adapted to allow motion of the proof mass in a direction normal to a plane of the frame, the plurality of flexures adapted to inhibit motion of the proof mass in a direction parallel to the plane of the frame.
2. The inertial sensing device of claim 1 , wherein each of the proof mass and the frame includes:
a first doped semiconductor layer;
a first insulating layer adjacent the first doped semiconductor layer;
a first doped primary semiconductor substrate adjacent the first insulating layer;
a eutectic metal layer adjacent the first doped primary semiconductor substrate;
a second doped primary semiconductor substrate adjacent the eutectic metal layer;
a second insulating layer adjacent the second doped primary semiconductor substrate; and
a second doped semiconductor layer adjacent the second insulating layer.
3. The inertial sensing device of claim 2 , wherein each of the first doped semiconductor layer and the second doped semiconductor layer includes a doped crystalline or polycrystalline silicon layer having a thickness between approximately 20 μm and 30 μm.
4. The inertial sensing device of claim 2 , wherein each of the first insulating layer and the second insulating layer includes a silicon dioxide layer having a thickness between approximately 0.7 μm and 1.2 μm.
5. The inertial sensing device of claim 2 , wherein each of the first doped primary semiconductor substrate and the second doped primary semiconductor substrate includes a doped silicon substrate having a thickness between approximately 500 μm and 700 μm.
6. The inertial sensing device of claim 2 , wherein the eutectic metal layer includes an alloy of one of aluminum/germanium, gold/silicon, or copper/tin having a thickness less than a thickness of the first insulating layer.
7. The inertial sensing device of claim 2 , wherein each of the plurality of electrostatic pick-offs includes a doped crystalline or polycrystalline silicon layer having a thickness between approximately 20 μm and 30 μm.
8. The inertial sensing device of claim 2 , wherein each of the plurality of flexures includes a doped crystalline or polycrystalline silicon layer having a thickness between approximately 20 μm and 30 μm.
9. The inertial sensing device of claim 1 , wherein the proof mass is generally circular or polygonal with a diameter between approximately 5 mm and 8 mm.
10. The inertial sensing device of claim 1 , wherein Nis an integer between three and 48.
11. The inertial sensing device of claim 1 , wherein each of the N proof mass interlocking tabs and each of the N frame interlocking tabs extends between approximately 50 μm and 200 μm in the radial direction and extends between approximately 50 μm and 200 μm in the angular direction.
12. The inertial sensing device of claim 1 , wherein a trench between the proof mass and the frame has a width between approximately 60 μm and 100 μm.
13. The inertial sensing device of claim 1 , wherein each of the plurality of flexures has a width between approximately 100 μm and 500 μm.
14. The inertial sensing device of claim 1 ,
wherein a spacing between each of the plurality of flexures and the proof mass in the radial direction is between approximately 3 μm and 30 μm; and
wherein a spacing between each of the plurality of flexures and the frame in the radial direction is between approximately 3 μm and 30 μm.
15. The inertial sensing device of claim 1 , wherein each of the N proof mass interlocking tabs includes one or more dimples on a corresponding surface thereof, the one or more dimples adapted to minimize stiction between the N proof mass interlocking tabs and the N frame interlocking tabs.
16. The inertial sensing device of claim 1 , wherein each of the N frame interlocking tabs includes one or more dimples on a corresponding surface thereof, the one or more dimples adapted to minimize stiction between the N proof mass interlocking tabs and the N frame interlocking tabs.
17. The inertial sensing device of claim 1 further comprising one or more electrostatic actuators, the one or more electrostatic actuators adapted to levitate the proof mass within the frame.
18. The inertial sensing device of claim 17 wherein each of the one or more electrostatic actuators includes a comb drive.
19. The inertial sensing device of claim 17 wherein each of the plurality of flexures includes a corresponding one of the one or more electrostatic actuators.
20. A method for fabricating an inertial sensing device, the method comprising the steps of:
providing starting material, the starting material including a doped primary semiconductor substrate, an insulating layer adjacent the primary semiconductor substrate, and a doped semiconductor layer adjacent the insulating layer;
defining a plurality of flexures and a plurality of electrostatic pick-offs in the doped semiconductor layer;
defining a plurality of flexure contacts and a plurality of electrostatic pick-off contacts in a contact metal layer, the contact metal layer adjacent the doped semiconductor layer;
defining a eutectic metal layer, the eutectic metal layer adjacent the doped primary semiconductor substrate;
defining a proof mass and a frame by etching a trench through the doped primary semiconductor substrate, the proof mass including N proof mass interlocking tabs formed around a periphery of the proof mass where N is an integer greater than 1, and N proof mass recesses formed around the periphery of the proof mass, the frame including an orifice therethrough, N frame interlocking tabs formed around a periphery of the orifice, each of the N frame interlocking tabs adapted to fit in a respective one of the N proof mass recesses, N frame recesses formed around the periphery of the orifice, each of the N proof mass interlocking tabs adapted to fit in a respective one of the N frame recesses, and a plurality of electrostatic pick-offs formed around the periphery of the orifice;
bonding together a pair of the proof masses and a pair of the frames using the eutectic metal layer; and
etching a plurality of cavities in the insulating layer beneath the plurality of flexures and the plurality of electrostatic pick-offs.