Abstract:
This invention discloses a method for electronically decreasing the sensitivity of thin film movable micromachined layers to vibrations, accelerations, or rotations that would result in part or all of the movable layer being displaced in the direction of the film thickness. In addition, the disclosed method can also be used to remove some of the curvature introduced into thin film movable structures due to vertical stress gradients. Electronic stiffening is achieved by using position sensing and force feedback at one or more points on the movable micromachined structure. Precise servo control of Z axis height makes it possible to dramatically decrease the spacing between the movable MEMS layer or layers and fixed electrodes, which can lead to a dramatic increase in sensitivity and/or actuation force.

Description:
Prior art structures have been developed specifically for use in actuators. These actuators employ a structure similar to that shown in FIG.  2 . For example, it is known that an electrostatic linear motor can be fabricated. One major problem with these devices is that, as the actuation voltage on the structure is increased in order to move it in the X or Y planes, the undesired force in the Z direction pulls the movable structure down into contact with the electrodes below, thereby preventing any further motion. Existing research has focussed on methods of making the Z spring much stiffer than the X or Y spring. However, this typically requires that the springs have a very high aspect ratio, which is difficult to manufacture, resulting in a much more expensive MEMS device. 
     Because of the problems inherent with these types of devices, this invention is directed to a method for electrically flattening thin film movable mechanical structures and significantly improving their Z-axis mechanical stiffness, as well as compensating for any rotations about the X or Y axes. This method results in a significant reduction in manufacturing cost by making thin-film MEMS devices suitable for inertial sensing applications that would otherwise have required either thick-film MEMS devices or bulk silicon MEMS devices. 
     SUMMARY OF THE INVENTION 
     For purposes of this disclosure, the invention will be described in terms of a moveable MEMS structure that can be used as an accelerometer, however, the invention is equally applicable to any device having single or multiple moveable MEMS structures, located either beside each other or stacked along the Z axis. 
     It is well known to those of ordinary skill in the art that one way to detect motion of a moveable MEMS structure, is to apply a small amplitude high frequency periodic signal to it. The amplitude of this signal coupled onto adjacent stationary or moveable electrodes varies with the position of the MEMS device. The high frequency signal can either be imposed on the moveable MEMS structure, or onto nearby electrodes. In the case of the accelerometer example described herein, there are two sets of stationary interdigitated electrodes, which are finger-shaped, as well as a set of finger-shaped electrodes attached to the moveable MEMS structure. The capacitance between the finger-shaped electrodes on the movable MEMS structure and the nearby stationary finger-shaped electrodes can be measured by observing either the current, voltage or charge induced on those conductors by the high frequency signal applied to the movable MEMS structure. This is typically done by using a charge sensing amplifier, which is well known in the art. In this way, both the lateral position of the movable MEMS structure and its height along the Z axis above the finger-shaped electrodes can be measured, as the capacitance varies with the motion of the moveable MEMS structure. Measuring such capacitance variations in order to estimate the separation between conductors is well understood in the state of the art in MEMS. 
     To exemplify the invention, top and bottom electrodes have been added to the basic prior art structure illustrated in FIG. 4, thereby providing the capability of generating both upward and downward forces on the movable MEMS structure using purely attractive electrostatic forces. 
     The essence of the invention is as follows. A voltage is placed on the top electrode. This will generate an upward force on all parts of the movable structure. We can then apply a common voltage to the finger-shaped electrodes. This will generate a significant downward force on the movable MEMS structure. By sensing the Z height at one or many points on the movable MEMS structure and adjusting the common voltage on the finger-shaped electrodes, we can use feedback to keep the Z separation between the movable MEMS structure and the finger-shaped electrodes at a constant value. This will reject or servo out mechanical vibrations, accelerations, and rotations, even when the thin film itself is not stiff in the Z dimension. This method can be used to cancel out linear acceleration in the direction of the Z axis. Additionally, by varying the potential applied to the upper finger-shaped electrodes relative to the lower ones, the structure can also reject rotation about the X axis. 
     To reject rotations about the Y axis, electrodes under the movable MEMS structure have been added. Each of these electrodes is used to sense the separation between itself and the movable MEMS structure and to adjust the respective voltages in order to hold that separation constant. In this way, rotations about the Y axis will be removed by these feedback loops. 
     In addition to rejecting accelerations in the Z axis and rotations about the X and Y axes, the structure shown in FIG. 3 will also tend to flatten out the movable MEMS structure because it is sensing and servoing to a fixed value the Z axis height of the movable MEMS structure in several separate regions. 
     By applying a voltage difference between the fingers, a force can be generated in the X direction to cancel X acceleration. Note that the structure shown as an example of this invention cannot sense or cancel Y acceleration. To accomplish this would require the addition of horizontal fingers. 
     Currently in the prior art, cross axis sensitivity would have dramatically limited the applications and sensitivity that could have been achieved. However, by making use of this invention, extremely sensitive motion sensing MEMS devices can be manufactured using low cost thin-film fabrication techniques, and, in addition, these devices can have low Z axis sensitivity. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a prior art accelerometer structure; 
     FIG. 2 is a cross-section of a portion of the accelerometer MEMS structure shown in FIG. 1; 
     FIG. 3 is a schematic representation of the capacitance between various components of the structure of FIG. 1; 
     FIG. 4 is a second design of a prior art accelerometer structure; 
     FIG. 5 is a cross-section of a portion of the prior art structure of FIG. 4; 
     FIG. 6 is a schematic representation of the capacitance between various components of the structure of FIG. 4; 
     FIG. 7 shows a structure constructed according the preferred embodiment of the present invention; 
     FIG. 8 is a cross-sectional view through the main portion of the structure of FIG. 7; 
     FIG. 9 is a cross-sectional view through another portion of the structure of FIG. 7; and 
     FIG. 10 is a schematic representation of the capacitance between various components of the structure of FIG.  7 . 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a prior art accelerometer. In this prior art accelerometer, moveable mass  10  is free to move in the X axis direction, shown by the arrow. Moveable mass  10  is anchored through beams  11  at anchor points  14 . Electrode sets  16  and  18  have fingers attached thereto extending toward moveable mass  10 . Moveable mass  10  also has fingers  10   a  attached thereto, interdigitated between the fingers from electrode sets  16  and  18 . As shown schematically in FIG. 3, variable capacitances exist between moveable mass electrode fingers  10   a  and fingers  16   a  and  18   a  from electrode sets  16  and  18  respectively. As fingers  10   a  move in the X axis direction with moveable mass  10 , the capacitance between fingers  10   a  and fingers  16   a  will vary, and, the capacitance between fingers  10   a  and  18   a  will vary inversely. 
     In a second prior art design shown in FIG. 4, moveable mass  20  is once again allowed to move laterally in the X axis. Moveable mass  20  is anchored at anchor points  24  by beams  21 . Electrode sets  26  and  28  have fingers extending therefrom towards moveable mass  20 . However, in this design as opposed to the first prior art example, fingers  20   a  from moveable mass  20  reside in a plane above fingers  26   a  and  28   a  from electrode sets  26  and  28  respectively. The movement of moveable mass  20  in the direction of the X axis causes a difference in capacitance between fingers  20   a  and  28   a  and between fingers  20   a  and  26   a.    
     Two problems exist with the prior art designs. The first is that moveable masses  10  and  20  are able to move in the X direction but also are able to move along the Z axis and rotate about the X and Y axes. The accelerometer used to illustrate the present invention is able to servo out mechanical accelerations and vibrations in the Z direction, as well as rejecting rotations about the X and Y axes. 
     One of the major limitations of the design shown in FIG. 1 is that the sensitivity (i.e., the change in capacitance per unit change in X) is determined by the gap between the fingers. In FIG. 1, that gap is limited by photolithographic resolution and etch resolution. The prior art design of FIG. 4 overcomes this limitation by moving to a capacitor gap which is determined by the thickness of the sacrificial spacer layer use between electrodes  26   a  and  28   a  and moveable fingers  20   a.  Because this thickness can be much smaller that photolithographic resolution, higher sensitivity is achieved. 
     However, a major limitation of the structure shown in FIG. 4 is that a voltage difference is needed between electrodes  26   a  and  28   a  and moveable fingers  20   a  to sense the capacitance. This voltage difference generates a significant downward force on mass  20 . In addition, external Z acceleration can also generate a downward force on mass  20 . The usefulness of the FIG. 4 structure is limited by how stiff springs  21  can be made in the Z direction. 
     In the design of the present invention, an improvement has been made to the accelerometer shown in FIG.  4 . FIGS. 7-9 show the addition of top electrode  40  as well as the addition of electrodes  42  and  44  underneath the moveable mass. The top electrode may be composed of many deposited layers, but the layer nearest the moveable MEMS structure would preferably be a conductor. Lower electrodes  42  and  44  may have been deposited on top of an insulator or may be suspended. In the case of the example of the lateral accelerometer of the prior art and of the present invention, there is only one moveable MEMS structure, with a fixed layer of electrodes below and above it. Note that upper electrode  40  need not be limited to a single electrode. A plurality of electrodes could be used, for example, if multiple moveable masses are being sensed and controlled. Likewise, lower electrodes  42  and  44  can actually be any number of electrodes appropriate for the application. 
     In operation, moveable structure  30 , shown in FIGS. 7-9, would be held at a fixed potential and a small amplitude high frequency periodic signal would be impressed onto it. The frequency of this signal is typically chosen to be much higher than that to which the structure can respond mechanically. The capacitance between moveable mass  30  and electrode sets  36 ,  36 ′,  38  and  38 ′, or between moveable mass  30  and upper electrode  40  can be measured by observing either the current, voltage or charge induced on the conductors by the periodic signal applied to movable mass  30 . This is typically done by using a charge sensing amplifier or other method well known to those of ordinary skill in the art. One skilled in the art would realize that this could also be reversed, with different high frequency signals applied to each electrode and the charge sensing amplifier connected to mass  30 . Using these methods we can estimate both the lateral position of the moveable mass  30  and its Z height above electrodes  36 ,  36 ′,  38  and  38 ′ and their related fingers. This method of measuring the capacitance variations in order to estimate the separation between the conductors is also well known in the art. 
     The novelty in the present invention is to apply a voltage to the electrodes, both above and below moveable mass  30 , using capacitance position sensing and electrostatic force feedback to dampen or eliminate the unwanted motions of moveable mass  30 . A voltage is imposed on top electrode  40 . This voltage generates an upward electrostatic force on all parts of the movable structure, including moveable mass  30  and beams  31 . Additionally, a common voltage component is applied to electrodes  36 ,  36 ′,  38  and  38 ′. This generates a significant downward electrostatic force on moveable mass  30 . By sensing the Z height at one or multiple points on the structure as described above and adjusting the common voltage on electrodes  36 ,  36 ′,  38  and  38 ′, or on electrode  40 , we can use force feedback to keep the Z separation between moveable mass  30  and electrodes  36 ,  36 ′,  38  and  38 ′ at a constant value. This will reject or servo out mechanical vibrations, accelerations and rotations in the Z axis. Additionally, it is possible to measure the acceleration in the Z direction by determining how much force is necessary to keep movable mass  30  in position. 
     Note that if there is a single common voltage on electrodes  36  and  38  then this structure can only cancel out a linear acceleration in the Z direction. However, if the common potential applied to electrodes  36  and  38  is different from the common potential applied to electrodes  36 ′ and  38 ′, the feedback force on the top and bottom can be adjusted to keep the Z height of the comb fingers on both the top and the bottom constant, thereby damping or eliminating rotations about the X axis. 
     To reject rotations about the Y axis, electrodes  42  and  44  must be added. Both of these electrodes are used to sense the separation between moveable mass  30  and electrode  42  and  44  respectively and to adjust the respective voltage in order to hold the separation constant. In this way, rotation about the Y axis will also be damped or eliminated. Electrodes  42  and  44  may be used only to sense changes in capacitance, or also, by imposing a voltage thereon, to provide further downward electrostatic forces on moveable mass  30 . It is also possible to have multiple electrodes under moveable mass  30  instead of just electrodes  42  and  44 . For example, four separate electrodes could be placed below the four corners of moveable mass  30  to give greater sensitivity to the cancellation of rotations in the X and Y axes. 
     By rejecting Z accelerations and rotations about the X and Y axes, the structure shown in FIG. 3 will also tend to flatten out movable mass  30 , because it is sensing and servoing to a fixed value the Z height of the movable mass  30  in multiple separate regions. 
     The novelty of this invention lies in the addition of the top electrode  40  and bottom electrodes  42  and  44 , and the addition of electronics, the design of which is well understood in the art, to sense the changes in capacitance and to cancel out the accelerations in the Z direction and rotations about the X and Y axes by the application of variable voltages to the appropriate electrodes to provide electrostatic forces acting on moveable mass  30 . 
     This invention is applicable to any number of different types of structures having moveable MEMS members, and it is not restricted to structures having only one movable layer, nor to structures that are anchored. This concept can be applied to MEMS structures having multiple moveable layers and multiple fixed thin film layers, regardless of their application. Therefore, the actual scope of the invention is embodied in the claims which follow and is not intended to be limited by the examples of structures used herein.