Abstract:
A system and method in accordance with the present invention provides for a low cost, bulk micromachined accelerometer integrated with electronics. The accelerometer can also be integrated with rate sensors that operate in a vacuum environment. The quality factor of the resonances is suppressed by adding dampers. Acceleration sensing in each axis is achieved by separate structures where the motion of the proof mass affects the value of sense capacitors differentially. Two structures are used per axis to enable full bridge measurements to further reduce the mechanical noise, immunity to power supply changes and cross axis coupling. To reduce the sensitivity to packaging and temperature changes, each mechanical structure is anchored to a single anchor pillar bonded to the top cover.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to motion sensing devices and more specifically to accelerometers that are utilized in integrated circuits. 
     BACKGROUND OF THE INVENTION 
     Accelerometers are widely used for motion sensing applications. Conventionally, an accelerometer consists of a suspended proof mass and a means of measuring the proof mass displacement with respect to the reference frame. Recent advances in microelectronics technology enabled fabrication of accelerometers with integrated electronics in volume production. One of the first applications of these integrated micromachined sensors was in airbag deployment for automobiles (Analog Device&#39;s XL50). 
     The first accelerometer products that were fabricated using MEMS technology were introduced by large corporations such as Analog Devices, ST and Bosch who had already infrastructure to produce integrated circuits. Integrated circuit fabrication mostly involves depositing several dielectric layers and selectively etching these layers. Therefore the first MEMS accelerometers were fabricated using the same techniques due to the ease of integration with electronics and compatibility with existing CMOS manufacturing equipment. 
     Building mechanical structures on silicon wafer based on the deposition and etching of different structural layers is called surface micromachining. In surface micromachining usually a sacrificial layer is deposited on a substrate followed by a deposition of mechanical layer where the moving parts of the accelerometer are going to be defined. The moving parts are later released by selectively removing the sacrificial layer. This method has many shortcomings for building low cost and high performance accelerometers. For example, there are contradicting requirements over the area (cost) of the accelerometer and the noise performance. The Brownian noise level of the accelerometer is proportional to the size of the proof mass. In surface micromachining, the proof mass height is determined by the deposited film thickness which is usually limited to less than 10 microns. Therefore, building heavy proof masses requires relatively large area which in return increases the cost. 
     Surface micromachining also necessitates complex fabrication steps. Depositing thick films which are required for low accelerometer noise is a very sophisticated process. Moreover, non-uniformity of the deposited films and large variation of the material properties have negative impact on the process yield and cost. Controlling stress level in the film is another issue which needs to be dealt with otherwise undesired curling of the released structures may occur. In addition, moveable parts released by using sacrificial wet etching may suffer from the stiction problem if their mechanical properties are not selected properly. Stiction can be avoided by fabricating structures with high spring constants. But this adversely affects the sensitivity of the accelerometer where the sensitivity is inversely proportional to the resonant frequency. Therefore, stiction problem limits the accelerometer sensitivity. 
     In addition to above described technical difficulties, surface micromachining tools are not readily available to small companies. Most of the required equipment can only be supported by a complicated infrastructure that only large companies can afford. This sets a very high barrier for small start-up companies that want to enter the accelerometer market. Surface micromachining is not a feasible solution for companies which do not have access to the expensive fabrication equipment. 
     Bulk micromachining, on the other hand, overcomes most of the technical difficulties of surface micromachining as well as it provides a viable solution for fabless semiconductor MEMS companies. In contrast to surface micromachining, bulk micromachining defines structures by selectively etching the substrate. Since the height of the structures is defined in the substrate, it is possible to build accelerometers with increased height and reduced foot print without the complexities associated with building structures using deposited layers. Increased mass in a small foot print provides fabricating accelerometer with better noise performance at a reduced cost. In addition, bulk micromachining techniques are readily available through MEMS foundaries. Bulk micromachined devices can easily be built on off the shelf SOI (silicon on insulator) substrates. 
     Another important process step for fabricating low cost MEMS device is the integration of mechanical parts with the electronics. To address this need “Nasiri-Fabrication” platform was introduced previously (U.S. Pat. No. 7,104,129, entitled “Vertically integrated MEMS structure with electronics in a hermetically sealed cavity”). This fabrication process makes use of bulk micromachining and readily allows for the wafer level integration of the MEMS substrate and the electronics (CMOS) substrate. In addition to integration, this method encapsulates the mechanical parts in a low pressure hermetically sealed chamber that protects the MEMS device against adverse effect of environment such as humidity. In summary, use of bulk micromachining and water scale MEM-CMOS integration result in low cost and high performance accelerometers. This patent describes a novel accelerometer design that uses bulk silicon machining and Nasiri-Fabrication integration solution. 
     There is a need for a small low cost high performance accelerometer. The present invention addresses such a need. 
     SUMMARY OF THE INVENTION 
     An accelerometer that is fabricated by bulk micromachining and integrated with CMOS wafer by Nasiri-Fabrication process is disclosed. The accelerometer comprises an actuator substrate; a reference substrate; and at least one moving proof mass on the actuator substrate. The proof mass is anchored to a cover plate at a single point, on the reference substrate with at least one flexure spring. The accelerometer also includes at least one electrode coupled to each proof mass, wherein a capacitance of the electrode increases and decreases with the motion of the other proof mass. 
     In one embodiment, a process is utilized that provides a low pressure cavity for hermetic protection of the moving parts from moisture and as such. Hence added features in the design are to allow for squeeze film dampening in order for the accelerometer to operate properly. The quality factor of the accelerometer resonances is suppressed by these dampers. The accelerometer can also be integrated with gyroscopes, that operate in a low pressure environment, without a need for separately encapsulating the sensors in different chambers. 
     Acceleration sensing in each axis is achieved by separate structures where the motion of the proof mass affects the value of sense capacitor. Two structures can be used per axis to enable full bridge measurements to further reduce the mechanical noise, susceptibility to power supply changes, cross axis coupling and the complexity of the sense electronics. To reduce the sensitivity to packaging and temperature changes, each mechanical structure is anchored to a single anchor pillar bonded to the top cover. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a top view of a portion of a MEMS assembly according to an embodiment. 
         FIG. 1B  illustrates an accelerometer in accordance with the present invention. 
         FIG. 1C  shows the cross section AA′ of Y-accelerometer. 
         FIGS. 2A and 2B  illustrate one structure for an X-axis accelerometer and one structure for a Y-axis accelerometer, respectively in accordance with the present invention. 
         FIG. 2C  shows bumps along the dampers of an accelerometer to increase the damping effect. 
         FIGS. 3A and 3B  illustrate a top view and a side view, respectively of a Z axis accelerometer in accordance with the present invention. 
         FIG. 4  illustrates a circuit which is electrode driven for sensing a change in capacitance in an accelerometer in accordance with the present invention. 
         FIG. 5  illustrates a circuit which is proof mass driven for sensing a change in capacitance in an accelerometer in accordance with the present invention. 
         FIG. 6  shows an accelerometer with a combined X (or Y) axis and Z axis in accordance with the present invention. 
         FIG. 7  shows an accelerometer in accordance with the present invention where all axes combined. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention relates generally to motion sensing devices and more specifically to accelerometers utilized in integrated circuits. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiments and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. 
     A method and system in accordance with the present invention relates to the accelerometers that are fabricated using silicon micromachining methods that have been described in U.S. Pat. No. 6,892,575, entitled “X-Y axis dual-mass tuning fork gyroscope with vertically integrated electronics and wafer-scale hermetic packaging”, issued May 17, 2005, and assigned to the assignee of the present application; and Published Application No. US2005/0170656, entitled “Vertical Integration of a Mems Structure with Electronics in a Hermetically Sealed Cavity,” filed Feb. 2, 2004, and assigned to the assignee of the present application, both of which are incorporated by reference in their entirety herein. 
       FIG. 1A  shows a top view of a portion of a MEMS assembly according to an embodiment. A MEMS device element  102  is positioned within a substantially planar frame  100 . Device element  102  can be a proof mass for an inertial sensor, such as an accelerometer or a gyroscope. Alternatively, device element  102  can be a movable element such as a mirror for optical applications, or a movable element within an RF device such as a switch or a resonator. In the example of  FIG. 1 , device element  102  is flexibly connected to frame  100  with flexures  104 . Flexures  104  can be either rotational flexures, permitting rotation about an axis, or translational flexures, permitting linear motion in a particular direction. Frame  100  and device element  102  can be regarded as being included in a MEMS subassembly. More generally, a wide variety of vertical MEMS devices, including inertial sensors such as gyroscopes and accelerometers, optical devices, and RF devices, have a MEMS subassembly having a substantially planar frame and at least one MEMS device element (such as a proof mass) within the frame. 
     Accordingly, these references disclose that the proof masses of the accelerometers are defined on a MEMS wafer that will be attached to a CMOS wafer to form a vacuum sealed cavity. The MEMS wafer is composed of two layers, actuator layer and cover layer. The MEMS structures are fabricated on the actuator layer that is bonded to the cover layer which has cavities over the moving parts.  FIG. 1B  shows a typical cross section of the accelerometer. The proof mass  102  is attached to an anchor  103  which is connected to a top cover plate  109 . The anchor  103  provides the electrical connection to the proof mass  102 . The anchor can be connected to the sense substrate  107  under the anchor  103  as shown in  FIG. 1B  or the electrical connections can be made through another structure between the anchor  103  and the substrate  107 . 
     The previously mentioned “Nasiri-Fabrication” process described in U.S. Pat. No. 7,104,129 provides a low pressure cavity for hermetic protection of the moving parts from moisture and as such. Hence added features in the design are to allow for squeeze film dampening in order for the accelerometer to operate properly. The quality factor of the accelerometer resonances is suppressed by these dampers. The accelerometer can also be integrated with gyroscopes, that operate in a low pressure environment, without a need for separately encapsulating the sensors in different chambers. 
     An accelerometer in accordance with the present invention uses three different structures for each axis and may use one or more proof masses per axis.  FIG. 1C  illustrates an accelerometer with six proof masses  200   a ,  200   b ,  300   a ,  300   b ,  400   a  and  400   b  in accordance with the present invention. As is seen, proof masses  200   a  and  200   b  are for the X-axis accelerometer, proof masses  300   a  and  300   b  are for the Y-axis accelerometer and proof masses  400   a  and  400   b  are for the Z-axis accelerometer. The following will discuss the features of the above described structures. 
     Structures for X-Axis Accelerometer and Y-Axis Accelerometer 
     The structures for proving an X-axis accelerometer and a Y-axis accelerometer are very similar to each other. To describe these two types of accelerometers in more detail refer now to the following description in conjunction with the accompanying Figures.  FIGS. 2A and 2B  illustrate one structure  250  for an X-axis accelerometer and one structure  350  for a Y-axis accelerometer, respectively, in accordance with the present invention. The structures  250  and  350  can be utilized interchangeably to enable the best use of the available area. Each of the x-axis accelerometer and y-axis accelerometer would utilize one or more of these structures to measure acceleration in two directions. 
     Each of the accelerometers  250  and  350  comprise a proof mass  200  and  300 , flexural springs  202  and  302  and dampers  204  and  304 . The mechanical parts are fabricated for example, by using deep reactive ion etching (DRIE) of a low resistivity single crystal silicon wafer. The proof mass  200  and  300  is attached to an anchor  206  and  306  through the flexural springs  202  and  302  as shown in  FIGS. 2A and 2B . The anchors  206  and  306  are attached to an upper cover (not shown) for example by diffusion bonding and are attached to the CMOS die (not shown), for example, by eutectic bonding. An oxide layer electrically insulates the anchor  206  and  306  and therefore the proof mass  200  and  300  from the cover but eutectic bond between the anchor and the CMOS electronics enables the electrical connections to the proof mass  200  and  300 . The proof masses  200  and  300  can be also connected to the substrate electrically through a soft spring. The main difference between the structure  250  and the structure  350  is that structure  250  uses a two folded flexural springs  209  whereas structure  350  uses four folded flexural springs  302 . By increasing the number of folds one can reduce the length of the springs while keeping the spring constant the same. This allows combining sensors with different aspect ratios on the substrate for efficient use of the surface area. 
     In a preferred embodiment, the motion of the proof mass  200  and  300  is sensed by parallel plate electrodes  208   a ,  208   b  and  308   a ,  308   b  that are placed perpendicular to the motion of the proof mass  200  and  300 . The electrodes  208   a ,  208   b  and  308   a ,  308   b  are held fixed by means of diffusion and eutectic bond. Each pair of electrodes  208   a ,  208   b  and  308   a ,  308   b  is used to enable differential capacitance sensing. The capacitance sensing can be implemented utilizing a bridge configuration. This feature will be described in detail later in this specification. 
     The dampers  204  and  304  attached to the proof mass  200  and  300  and the anchor  206  and  306  introduce damping to lower the quality factor of the resonance. The dampers  204  and  304  allow for the proper operation of the accelerometers in a low pressure vacuum environment which is required for the rate sensor operation. 
     It is desirable to fabricate dampers with very small gaps in between to increase the squeezed film damping. However, fabrication (usually DRIE) limitations dictate the minimum gap that can be achieved over a long distance.  FIG. 2C  shows bumps along the dampers to increase the damping effect. The bumps along the dampers reduce the gap in between without pushing the limits of the DRIE process. 
     The over range limiters (stoppers)  210  and  310  are provided in the sense direction since in this direction the springs have the lowest spring constants. The over range limit in the other directions can be either provided by other sets of stoppers or by making the spring constants very high. As shown in  FIGS. 2A and 2B , a single anchor  206  and  306  point connects the structures to the substrate. Therefore, the effect of relative stress between the package and the substrate is minimized on the performance of the accelerometer. Finally, self test electrodes  212  and  312  are also incorporated into the accelerometers as shown in  FIGS. 2A and 2B . 
     Structures for Z-Axis Accelerometer 
       FIGS. 3A and 3B  illustrate a top view and a side view, respectively of a Z axis accelerometer  450  in accordance with the present invention. The figure illustrates usage of two proof masses per axis, but only one structure can also be used for acceleration detection in Z-direction. Both structures of the accelerometer are depicted in the figures unlike  FIGS. 2A and 2B  where only one of the structures for the accelerometers is depicted per axis. For the Z-axis accelerometer  450 , the two structures are packed such that to make the best use of available chip area. A lever arm for each structure  410   a  and  410   b  pivoted at the center is attached to three proof mass systems  401   a ,  403   a ,  405   a  and  401   b ,  403   b  and  405   b . The total weight on the each side of the lever arm  410   a  and  410   b  is different. When there is acceleration in the Z direction this results in a bending of the lever arm around the pivot point. The electrodes  408   a  and  408   b  placed under the outer most proof mass systems  401   a ,  405   a ,  401   b  and  405   b  on the CMOS reference substrate  409  enable differential measurement. The center proof mass  403   a  and  403   b  adds more weight to one side of the lever arm. 
     To increase the damping further, one can place a thin film under proof masses  403   a  and  403   b  reducing the gap between the CMOS die and the proof mass. The placement and the shape of the proof masses are chosen such that the two accelerometers can be fit in a very small area as shown  FIG. 3A . The spring constant of the Z-axis accelerometer is also low in Y-axis direction. This makes it sensitive to the Y-axis acceleration. If there is no process variation along the thickness direction or if the side walls are perfectly 90 degrees this does not cause any cross-axis sensitivity since the capacitance will not change with translational motion provided that the electrodes on the CMOS are smaller than the proof masses. However, imperfections in the fabrication usually result in some coupling from Y axis to Z axis. But since there are two structures used to detect Z-motion in a full bridge configuration, this coupling will be canceled. To describe the detection of the change in capacitance utilizing the structures of  FIGS. 2A ,  2 B,  3 A and  3 B, refer now to the following description in conjunction with the accompanying figures. 
     Electronic Detection 
     The capacitance change of the accelerometers described above can be detected by various circuits. Here, only full bridge circuits are described but other means of capacitive detection such as pseudo bridge, half bridge can also be employed.  FIG. 4  illustrates a circuit  500  which is electrode driven for sensing a change in capacitance in an accelerometer in accordance with the present invention. As is seen an AC voltage  502  is applied to the sense electrodes  804   a  and  504   b  and the output voltage is detected off the proof masses  508   a  and  508   b  utilizing an operational amplifier  506 . When there is no acceleration the output voltage is zero. Acceleration of the proof masses  508   a  and  508   b  gives rise to an AC voltage at the operational amplifier output which can be demodulated later. 
       FIG. 5  illustrates a circuit  600  which is proof mass driven for sensing a change in capacitance in an accelerometer in accordance with the present invention. The advantage of the circuit of  FIG. 5  is that the AC drive does not cause any motion on the proof masses  508   a ′ and  508   b ′. The detection is determined by the amount of parasitic capacitances  604   a  and  604   b . There is a parasitic capacitance between the proof mass and the ground as well as between the electrodes and the ground. The operational amplifier  506 ′ is connected to the nodes where the parasitic capacitances  604   a  and  604   b  are lower. 
     Description of Variations and Alternate Embodiments 
     The structures of  FIGS. 2A ,  2 B,  3 A and  3 B can be combined to further reduce the size of the accelerometer  650 .  FIG. 6  shows a combined X (or Y) axis and Z axis accelerometer. For example, one can easily combine Y axis and Z axis by just adding electrodes  652  to the sides of the structure for the z-axis accelerometer. Since these structures are built using bulk micromachining the thickness is high allowing relatively high capacitance in the thickness direction. The accelerometer shown in  FIG. 6  can be also be used in X direction by just rotating it. 
       FIG. 7  shows another accelerometer  750  where all axes are combined. This allows very efficient use of chip area. The accelerometer shown in  FIG. 7  combines all the proof masses. X-axis and y-axis acceleration sense electrodes  701  and  703  are similar to the structures shown in  FIGS. 2A and 2B . For the axis z-axis, the center proof mass moves out of plane while bending the edge proof masses around the pivot points  702  and  704 . This allows differential capacitance detection for the z-axis measurement. 
     Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.