Sensor with separate actuator and sense fingers

Abstract

A micromachined force sensor containing separate sensing and actuator structures. A member is suspended above the substrate so that it is movable along an axis in response to a force. The member includes a set of parallel sense fingers and a separate set of parallel force fingers. The sense fingers are positioned between fingers of two sense plates, to form a first differential capacitor, whose capacitance changes when the member moves in response to a force along the axis. The change in capacitance induces a sense signal on the member, which permits the measurement of the magnitude and duration of the force. The force fingers are positioned between fingers of two actuator plates, to form a second differential capacitor. The sense signal can be used to provide feedback to the second differential capacitor to generate different electrostatic forces between the force fingers and the two actuator plates, to offset the force applied along the preferred axis. Limit stops limit the movement of the member to less than the distance between the electrodes of the differential capacitors, to prevent contact between the electrodes. Additional fingers be positioned around the differential capacitors to minimize parasitic capacitances.

We claim: 
     
       1. A micromachined device comprising: a member;   a suspension connected to the member and to a substrate, so that the member is movable relative to the substrate in response to a force applied to the device along a predetermined axis;   a sense differential capacitor having a first sense electrode connected to the substrate, a second sense electrode connected to the substrate, and a third sense electrode connected to the member, the sense differential capacitor including a first sense capacitor and a second sense capacitor, wherein the first sense capacitor includes the first sense electrode and the third sense electrode and the second sense capacitor includes the second sense electrode and the third sense electrode, and wherein the capacitances of the first sense capacitor and the second sense capacitor are oppositely variable in response to the force; and   an actuator differential capacitor having a first actuator electrode connected to the substrate, a second actuator electrode connected to the substrate, and a third actuator electrode connected to the member, the actuator differential capacitor including a first actuator capacitor and a second actuator capacitor, wherein the first actuator capacitor includes the first actuator electrode and the third actuator electrode and the second actuator capacitor includes the second actuator electrode and the third actuator electrode, and wherein the capacitances of the first actuator capacitor and the second actuator capacitor are oppositely variable in response to the force;   wherein the first sense electrode forms a separate electric node from the first actuator electrode, a separate electric node from the second actuator electrode, and a separate electric node from the third actuator electrode.   
     
     
       2. A micromachined device comprising: a plurality of anchors connected to a substrate;   a member suspended above the substrate:   a plurality of suspension arms suspended above the substrate, with each of the plurality of suspension arms having a first end connected to the member and a second end connected to one of the plurality of anchors, such that the member is movable along an axis parallel to the substrate;   at least one sense capacitor cell, with each sense capacitor cell including a sense finger connected to the member, a first sense plate adjacent to a first side of the sense finger, and a second sense plate adjacent to a second side of the sense finger; and   at least one actuator capacitor cell, with each actuator capacitor cell including an actuator finger connected to the member, a first actuator plate adjacent to the first side of the actuator finger and a second actuator plate adjacent to the second side of the actuator finger,   wherein the at least one sense capacitor cell comprises a first differential capacitor and the at least one actuator capacitor cell comprises a second differential capacitor; and   wherein the first sense plate of each sense capacitor cell forms a separate electric node from the actuator finger of each actuator capacitor cell, a separate electric node from the first actuator plate of each actuator capacitor cell, and a separate electric node from the second actuator plate of each actuator capacitor cell.   
     
     
       3. A micromachined device comprising: a plurality of anchors connected to a substrate:   a member suspended above the substrate having at least one member finger extending from the member, the member finger having a left side and a right side;   a plurality of suspension arms suspended above the substrate, with each of the plurality of suspension arms having a first end connected to the member and a second end connected to one of the plurality of anchors so that the member is movable along a preferred axis parallel to the substrate;   at least one left finger parallel to the at least one member finger, with one of the at least one left lingers being adjacent to the left side of each member finger, and the at least one left finger forming a first capacitor with the at least one member finger;   at least one right finger parallel to the at least one member finger, with one of the at least one right fingers being adjacent to the right side of each member finger, at the at least one right finger forming a second capacitor with the at least one member finger;   a polysilicon guard ring suspended above the substrate and encircling the member, the plurality of suspension arms, the at least one left finger, and the at least one right finger;   a guard diffusion area diffused into the substrate under a portion of the guard ring; and   a polysilicon guard anchor mechanically and electrically connecting the guard ring to the guard diffusion area.   
     
     
       4. The device according to claim 1, wherein the distance between the first actuator electrode and the third actuator electrode changes in a first direction in response to the force and the distance between the second actuator electrode and the third actuator electrode changes in a second direction in response to the force, the second direction being opposite the first direction. 
     
     
       5. The device according to claim 4, further comprising means for coupling the first sense electrode to a first carrier signal and the second sense electrode to a second carrier signal of the same frequency as the first carrier signal, with the first carrier signal being 180 degrees out of phase with the second carrier signal. 
     
     
       6. A device according to claim 5, further comprising means for coupling the first actuator electrode to a first biasing signal having a first amplitude and the second actuator electrode to a second biasing signal having a second amplitude. 
     
     
       7. The device according to claim 6, wherein the first sense electrode, the second sense electrode, the third sense electrode, the first actuator electrode, the second actuator electrode, and the third actuator electrode are formed from polysilicon; and the connecting means includes a plurality of anchors connected to a silicon substrate and a plurality of suspension arms, with each of the plurality of suspension arms having a first end connected to one of the plurality of anchors and a second end connected to the member.   
     
     
       8. The device according to claim 7, wherein each of the first sense electrode, the second sense electrode, and the third sense electrode includes a plurality of parallel fingers, and with each of the plurality of fingers of the third sense electrode being interleaved between one of the plurality of fingers of the first sense electrode and one of the plurality of fingers of the second sense electrode. 
     
     
       9. A micromachined device comprising: a plurality of anchors connected to a substrate;   a member suspended above the substrate having at least one sense finger extending from the member, and at least one force finger extending from the member, parallel to the at least one sense finger;   a plurality of suspension arms suspended above the substrate, with each of the plurality of suspension arms having a first end connected to the member and a second end connected to one of the plurality of anchors, so that the member is movable along a preferred axis parallel to the substrate;   at least one first sense plate parallel to the at least one sense finger, with one of the at least one first sense plates being adjacent to a first side of each sense finger, and the at least one first sense plate forming a first sense capacitor with the at least one sense finger;   at least one second sense plate parallel to the at least one sense finger with one of the at least one second sense plates being adjacent to a second side of each sense finger, and the at least one second sense plate forming a second sense capacitor with the at least one sense finger;   at least one first actuator plate parallel to the at least one force finger, with one of the at least one first actuator plates being adjacent to the first side of each force finger, and the at least one first actuator plate forming a first actuator capacitor with the at least one force finger; and   at least one second actuator plate parallel to the at least one force finger, with one of the at least one second actuator plates being adjacent to the second side of each force linger, and the at least one second actuator plate forming a second actuator capacitor with the at least one force finger;   wherein the at least one first sense plate forms a separate electric node from the at least one force finger, a separate electric node from the at least one first actuator plate, and a separate electric node from the at least one second actuator plate.   
     
     
       10. The device according to claim 9, wherein the at least one sense finger includes a plurality of sense fingers, parallel to each other, the at least one first sense plate includes a plurality of first sense plates, and the at least one second sense plate includes a plurality of second sense plates. 
     
     
       11. The device according to claim 10, wherein the at least one force finger includes a plurality of force fingers, the at least one first actuator plate includes a plurality of first actuator plates, and the at least one second actuator plate includes a plurality of second actuator plates. 
     
     
       12. The device according to claim 11, further comprising a conductive bootstrap region diffused onto the substrate, below the member. 
     
     
       13. The device according to claim 12, further comprising: a first plurality of shield plates anchored to the bootstrap region and parallel to the plurality of force fingers, with one of the first plurality of shield plates being adjacent to the first side of each of the plurality of first actuator plates; and   a second plurality of shield plates anchored to the bootstrap region and parallel to the plurality of force fingers, with one of the second plurality of shield plates being adjacent to the second side of each of the plurality of second actuator plates.   
     
     
       14. The device according to claim 11, wherein at least one of the plurality of suspension arms includes a limit stop for limiting movement of the member along the predetermined axis. 
     
     
       15. The device according to claim 11, wherein each of the plurality of suspension arms includes an anchor segment having a first end connected to one of the plurality of anchors, a first suspension segment approximately perpendicular to the anchor segment and having a first end connected to a second end of the anchor segment, a second suspension segment having a first end connected to a second end of the first anchor segment, a third suspension segment approximately parallel to the first suspension segment and having a first end connected to a second end of the second suspension segment, and a member segment approximately perpendicular to the third suspension segment and having a first end connected to a second end of the third suspension segment and a second end connected to the member. 
     
     
       16. The device according to claim 15, wherein at least one of the plurality of suspension arms further includes a stop segment connected to the first suspension segment and extending from the first suspension segment toward the third suspension segment. 
     
     
       17. The device according to claim 15, wherein at least one of the plurality of suspension arms further includes a stop segment connected to the third suspension segment and extending from the third suspension segment toward the first suspension segment. 
     
     
       18. The device according to claim 11, further comprising: a polysilicon guard ring encircling the member, the plurality of suspension arms, the plurality of first sense plates, the plurality of second sense plates, the plurality of first actuator plates, and the plurality of second actuator plates;   a guard diffusion area diffused into the substrate under a portion of the guard ring; and   a polysilicon guard anchor connecting the guard ring to the guard diffusion area.   
     
     
       19. The device according to claim 9, further comprising: a polysilicon guard ring suspended above the substrate and encircling the at least one sense linger, the at least one first sense plate, and the at least one second sense plate;   a guard diffusion area diffused into the substrate under a portion of the guard ring; and   a polysilicon guard anchor mechanically and electrically connecting the guard ring to the guard diffusion area.   
     
     
       20. The device according to claim 2, wherein the at least one sense capacitor cell includes a plurality of sense capacitor cells, with all of the first sense plates being connected and all of the second sense plates connected. 
     
     
       21. The device according to claim 20, wherein the plurality of sense capacitor cells includes a plurality of end cells and at least one middle cell, with each end cell having an end side non-adjacent to another sense capacitor cell and a middle side adjacent to one of the at least one middle cells, and each middle cell being adjacent to two of the plurality of sense capacitor cells; the device further including a plurality of dummy sense plates, with one of the plurality of dummy sense plates being adjacent to the end side of each of the plurality of end cells.   
     
     
       22. A device according to claim 21, wherein the plurality of end cells includes at least one first end cell and at least one second end cell, with the end side of each first end cell being on the first side of such first end cell, the end side of each second end cell being on the second side of such second end cell, the plurality of dummy sense plates including at least one dummy first sense plate connected to the plurality of first sense plates and at least one dummy second sense plate connected to the plurality of second sense plates, the dummy sense plate adjacent to each first end cell being a dummy first sense plate, and the dummy sense plate adjacent to each second end cell being a dummy second sense plate. 
     
     
       23. The device according to claim 21, wherein the at least one middle cell includes a plurality of middle cells. 
     
     
       24. The device according to claim 23, wherein the at least one actuator capacitor cell includes a plurality of actuator capacitor cells, with all of the first actuator plates being connected and all of the second actuator plates being connected. 
     
     
       25. The device according to claim 12, further comprising at least one first shield plate anchored to the bootstrap region and parallel to the plurality of force fingers, wherein each of the first shield plates is adjacent to one of the plurality of first actuator plates, on an opposite side of such first actuator plate from a side of such actuator plate adjacent to one of the force fingers. 
     
     
       26. The device according to claim 25, further comprising at least one second shield plate anchored to the bootstrap region and parallel to the plurality of force fingers, wherein each of the second shield plates is adjacent to one of the plurality of second actuator plates, on an opposite side of such second actuator plate from a side of such actuator plate adjacent to one of the force fingers. 
     
     
       27. The device according to claim 9, wherein the member, including the at least one sense finger and the at least one force finger, forms a single electric node. 
     
     
       28. The device according to claim 9, wherein the at least one sense finger and the at least one force finger extend from the member perpendicular to the preferred axis. 
     
     
       29. The device according to claim 1, wherein the member, the third sense electrode, and the third actuator electrode form a single electric node. 
     
     
       30. The device according to claim 9, wherein the at least one second sense plate forms a separate electric node from the at least one force finger, a separate electric node from the at least one first actuator plate, and a separate electric node from the at least one second actuator plate. 
     
     
       31. The device according to claim 9, further comprising at least one first shield plate anchored to the bootstrap region and parallel to the at least one force finger, wherein each of the first shield plates is adjacent to one of the at least one first actuator plates, on an opposite side of such first actuator plate from a side of such actuator plate adjacent to one of the force fingers. 
     
     
       32. The device according to claim 31, further comprising at least one second shield plate anchored to the bootstrap region and parallel to the at least one force finger, wherein each of the second shield plates is adjacent to one of the at least one second actuator plates, on an opposite side of such second actuator plate from a side of such actuator plate adjacent to one of the force fingers. .Iadd. 
     
     
       33.  A micromachined device comprising: a plurality of anchors connected to a substrate;   a member suspended above the substrate having at least one member finger extending from the member, the member finger having a left side and a right side;   a plurality of suspension arms suspended above the substrate, with each of the plurality of suspension arms having a first end connected to the member and a second end connected to one of the plurality of anchors so that the member is movable along an axis parallel to the substrate;   at least one left finger parallel to the at least one member finger, with one of the at least one left fingers being adjacent to the left side of each member finger, and the at least one left finger forming a first capacitor with the at least one member finger;   at least one right finger parallel to the at least one member finger, with one of the at least one right fingers being adjacent to the right side of each member finger, and the at least one right finger forming a second capacitor with the at least one member finger; and   a conductive guard ring formed above and mechanically connected to the substrate, wherein the guard ring surrounds the member, the plurality of suspension arms, the at least one left finger, and the at least one right finger. .Iaddend..Iadd.34. The device according to claim 33, wherein the conductive guard ring includes polysilicon. .Iaddend..Iadd.35. The device according to claim 33, wherein the guard ring is electrically connected to the substrate. .Iaddend..Iadd.36. A micromachined device comprising:   a plurality of anchors connected to a substrate;   a member suspended above the substrate having at least one member finger extending from the member, the member finger having a left side and a right side;   a plurality of suspension arms suspended above the substrate, with each of the plurality of suspension arms having a first end connected to the member and a second end connected to one of the plurality of anchors so that the member is movable along an axis parallel to the substrate;   at least one left finger parallel to the at least one member finger, with one of the at least one left fingers being adjacent to the left side of each member finger, and the at least one left finger forming a first capacitor with the at least one member finger;   at least one right finger parallel to the at least one member finger, with one of the at least one right fingers being adjacent to the right side of each member finger, and the at least one right finger forming a second capacitor with the at least one member finger; and   a conductive guard ring surrounding the member, the at least one left finger, and the at least one right finger so as to electrically isolate the member, the at least one left finger, and the at least one right   
     
     
        finger. .Iaddend..Iadd.37.  The device according to claim 36, wherein the conductive guard ring is suspended above the substrate. 
     
     
        .Iaddend..Iadd.     A micromachined device comprising: a plurality of anchors connected to a substrate;   a member suspended above the substrate having at least one member finger extending from the member, the member finger having a left side and a right side;   a plurality of suspension arms suspended above the substrate, with each of the plurality of suspension arms having a first end connected to the member and a second end connected to one of the plurality of anchors so that the member is movable along an axis parallel to the substrate;   at least one left finger parallel to the at least one member finger, with one of the at least one left fingers being adjacent to the left side of each member finger, and the at least one left finger forming a first capacitor with the at least one member finger;   at least one right finger parallel to the at least one member finger, with one of the at least one right fingers being adjacent to the right side of each member finger, and the at least one right finger forming a second capacitor with the at least one member finger; and   a conductive guard ring formed above and mechanically connected to the substrate so as to electrically isolate the member, the at least one left finger, and the at least one right finger. .Iaddend..Iadd.39. The device according to claim 38, wherein the conductive guard ring includes polysilicon. .Iaddend.