Abstract:
This invention relates to an apparatus for making highly sensitive differential measurements of acceleration. The vibration sensor includes the use of moveable gate field effect transistors to sense the motion of a cantilever beam relative to the motion sensed by a reference structure, it also includes an actuator element formed by a pair of electrodes actuating electrostatically on the beam. A feedback control loop is also included for force balance operation resulting in a very wide dynamic range for the sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of application Ser. No. 08/821,042, filed Mar. 20, 1997, now U.S. Pat. No. 5,874,675, issued Feb. 23, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of accelerometers, and particularly to enhancing the high frequency capabilities of accelerometers, and to differential measurement capabilities of accelerometers. 
     BACKGROUND OF THE INVENTION 
     The concept of a moveable gate Field Effect Transistor (FET) has been extensively studied and reported in literature. A number of devices have been disclosed that make use of a moveable gate FET for building accelerometers. 
     Force balance feedback control of vibration sensors has been used in seismometers and in accelerometers for attaining increased bandwidth and dynamic range. A number of devices using this approach have been routinely demonstrated and its theory of operation is well understood. Several seismometer and accelerometer manufacturers base their designs on this principle. 
     The fabrication of a silicon accelerometer using wafer bonding techniques is disclosed in great detail in U.S. Pat. No. 5,095,752 and No. 5,417,312. In these invention disclosures a relatively large mass made of silicon is encapsulated in a cavity formed by electrodes made out of glass on silicon. The accelerometer is operated using an active feedback loop, in which control voltages are applied to the upper and lower electrodes. The displacement of the free mass under acceleration requires compensation through changes in the voltage applied between the upper and lower electrodes and the moving mass. 
     Utilizing the FET concept, the feedback bulk silicon micromachined accelerometer disclosed in U.S. Pat. No. 5,205,171 makes use of a feedback loop and at least one pair of dual electrodes acting as capacitive transducers. Differential sensing of the beam-gate capacitance variations is used to generate a null feedback signal used to modulate the voltage applied to electrodes in order to prevent them from moving. Based on the similar concept of using a FET, a device with acceleration dependent gain is disclosed in U.S. Pat. No. 5,103,279 and a device that uses a piezoelectric device to generate voltage for the gate is disclosed in U.S. Pat. No. 4,873,871. 
     The type of accelerometers mentioned above have good sensitivity at low frequencies but limited sensitivity at high frequencies, although they result in higher bandwidth devices than open loop devices. In addition, due to mechanical and electronic manufacturing variations from one accelerometer to the next, the process of calibrating any given accelerometer to detect vibration as differentiated against a “zero” vibration level is difficult without means to establish such a differential reading. 
     One fundamental problem of implementing a vibration sensor is that most of the time the fabrication process used to implement the sensor is incompatible with the most common processes used to implement the standard electronics associated with the sensor. To solve this incompatibility, it is often preferred to fabricate the sensor in a separate die from the electronic circuitry. This type of multi-die implementation, however, results in higher costs and lower yield since a more complex multi-die packaging is needed. In some cases cost considerations might advise the use of a monolithic (single-dye) implementation in which the sensor and accompanying electronics reside in the same die. 
     OBJECTS OF THE INVENTION 
     It is an object of the invention disclosed herein to provide an accelerometer featuring a force balanced feedback loop in which the electrodes are used as actuating elements in the control loop and the sensing element utilizes a FET having a moveable gate. Variations in the geometric configuration of the gate can also be used to increase capacitance and hence sensitivity. This approach enables increased sensitivity and greater maximum signal range resulting in an increased bandwidth and dynamic range typical of a force balance system. Furthermore, the disclosed accelerometer is compatible with microelectromechanical systems processing. 
     It is also an object of the invention to provide said accelerometer with an associated mechanical and electrical reference to allow true calibration of vibration measurements. 
     It is also an object of the invention to provide a means of overcoming the usual incompatibilities between sensor fabrication and an electronic device fabrication processes. 
     SUMMARY OF THE INVENTION 
     The present invention provides a high sensitivity wideband MEMS (Microelectromechanical Systems) acceleration sensor of the force balanced type based on the use of a closed feedback loop for attaining maximum bandwidth for the sensed variables. The vibration sensor operates through the action of forces applied electrostatically between a pair of fixed electrodes and the suspended mass configured to form a closed feedback loop. The most useful form of control is negative feedback which is intended to keep the mass in a nearly fixed position with respect to the electrodes and substrate, making the suspension appear more stiff and increasing the natural frequency. 
     The operation of the wideband vibration sensor is based on a feedback loop which includes a moveable gate FET otherwise known as a MGT (Moveable Gate Transistor) as the sensor element. The MGT offers the best means for detecting out-of-plane oscillations in a very small area, since it relies on capacitance per unit area (as opposed to total capacitance). 
     An important embodiment of the invention includes an associated mechanical and electrical reference structure to allow for differential measurements with the accelerometer. The use of a reference allows calibration of the sensor to an electronic and mechanical “zero” of vibration. This simplified measurement approach eliminates costly precision tuning of individual units and allows for cost effective, automated self-calibration during operation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The features of the invention believed to be novel are set forth in the appended claims. The invention, however, together with further objects and advantages thereof, may be best understood by references to the following description taken in conjunction with the accompanying drawing in which: 
     FIG. 1 is an isometric view of the preferred embodiment of the wideband vibration sensor invention. The figure shows the primary components of the sensor, the electrode pair (upper and lower feedback electrodes), the MGT, and the reference structure. 
     FIG. 2 is a cross-sectional side view detailing the area of the MGT and the electrode pair. 
     FIG. 3 is an electrical circuit diagram of the integrated moveable gate transistor and a reference transistor of the reference structure. 
     FIG. 4 is a top view of an alternative embodiment of the channel, source, and drain geometry. 
     FIG. 5 is a top view of an alternative embodiment of the invention incorporating the channel, source, and drain geometry of FIG.  4 . 
     FIG. 6 is a top view of an alternative configuration of the gate geometry, specified as a coil. 
     FIG. 7 is a top view of an alternative configuration of the gate geometry, specified as a diaphragm. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     A preferred embodiment of the present invention is depicted in FIG.  1 . Referring to FIG. 1, vibration sensor  100  comprises a base  110  with a cavity  112  and a cavity floor  114  in a central region of base  110 . A moveable suspended mass  118  such as the illustrated cantilever beam, thinner than the depth of the cavity, projects horizontally from a top surface of one end of base  110 , partially extending over central cavity  112 . An electrically-conductive suspended mass (cantilever beam) contact pad  120  is positioned on the top surface of moveable suspended mass  118 . A gate region  116 , is defined as the free end of moveable suspended mass  118  immediately above a channel  122 , i.e., as that region of moveable suspended mass  118  that is sufficiently proximate to said channel  122  so as to electromagnetically interact with said channel  122  in a substantial manner. An external power means is connected to cantilever beam contact pad  120  in order to deliver specified voltages to gate region  116  of moveable suspended mass  118 , as subsequently described. 
     On cavity floor  114 , beneath gate region  116  of moveable suspended mass  118 , is channel  122 . Extending from one side of channel  122 , substantially perpendicular to the direction of moveable suspended mass  118 , is a source implant  124 . An electrically-conductive source implant contact pad  126  is located on source implant  124 . Extending from the substantially opposite side of channel  122 , is a drain implant  128 . An electrically-conductive drain implant contact pad  130  is located on drain implant  128 . An external power means is connected to source implant contact pad  126  to supply current to source implant  124 . Similarly, a means of sensing current flow is connected to drain implant contact pad  130  in order to sense the modulated current flow output from drain implant  128 . 
     A lower feedback electrode  132  is superficially buried in cavity floor  114  beneath moveable suspended mass, e.g., cantilever beam  118 . In the preferred embodiment of the invention, lower feedback electrode  132  is positioned generally perpendicular to and beneath a central region of moveable suspended mass  118 , as shown in FIG.  1 . Positioned on lower feedback electrode  132  is a lower feedback electrode contact pad  134 , which is used to supply a feedback voltage to lower feedback electrode  132 , as subsequently described. 
     Similarly, an upper feedback electrode  136  is positioned above the central region of moveable suspended mass  118 . Upper feedback electrode  136  is preferably positioned directly above and parallel to lower feedback electrode  132 . In the preferred embodiment of the invention, upper feedback electrode  136 , similarly to lower feedback electrode  132 , is substantially perpendicular to and above the central region of moveable suspended mass  118 . Upper feedback electrode  136  bridges across the width of cavity  112 , and is secured to base  110 . Positioned on upper feedback electrode  136 , is an upper feedback electrode contact pad  138 , which is used to supply a feedback voltage to upper feedback electrode  136 , as subsequently described. 
     A reference structure  140  is positioned substantially adjacent to and in close proximity to vibration sensor  100 . The purpose of reference structure  140  is to provide a means of achieving differential measurements that eliminate the need for precise process control and costly post-fabrication calibration of individual units. It allows for measurement of the static equilibrium current flow inherent in the sensor, by providing a current flow from a static suspended reference mass  146  that represents “zero” vibration. This is then subtracted from current flows detected through moveable suspended mass  118  to obtain a true calibration of readings from moveable suspended mass  118 . 
     Reference structure  140  is supported by base  110  and includes a secondary cavity  142  and a secondary cavity floor  144 . Static suspended reference mass  146  is doubly anchored on each end as shown, and is substantially similar in width and thickness to moveable suspended mass  118 . Static suspended reference mass  146  is suspended across secondary cavity  142  above secondary cavity floor  144 , at substantially the same height above secondary cavity floor  144  as moveable suspended mass  118  is above cavity floor  114 . Static suspended reference mass  146  is supported on both ends by base  110 . An electrically conductive reference mass contact pad  148  is positioned on the top surface of static suspended reference mass  146 . 
     On secondary cavity floor  144 , beneath reference gate region  160  of static suspended reference mass  146 , is a reference channel  150 . Extending from one side of reference channel  150 , substantially perpendicular to the direction of static suspended reference mass  146 , is a reference source implant  152 . An electrically-conductive reference source implant contact pad  154  is located on reference source implant  152 . Extending from the substantially opposite side of reference channel  150 , is a reference drain implant  156 . An electrically-conductive reference drain implant contact pad  158  is located on reference drain implant  156 . External power means are connected to reference source implant contact pad  154  in order to supply current to reference source implant  152 . Similarly, current flow sensing means are connected to reference drain implant contact pad  158  to sense the equilibrium current flow output from reference drain implant  156 . To be able to obtain measurements of a comparable nature, reference channel  150 , reference source implant  152 , and reference drain implant  156  are of substantially the same size and geometric configuration as channel  122 , source implant  124 , and drain implant  128 . 
     Referring to FIG. 2, the functional aspects of sensor  100  are described as follows. An airgap  260  is defined as the clearance between gate region  116  and channel  122 . As a voltage is applied to gate region  116  of moveable suspended mass  118  via cantilever beam contact pad  120 , charge accumulates in channel  122  between source implant  124  and drain implant  128 . At the operating bias voltage, inversion occurs in channel  122 , and current flows between source implant  124  and drain implant  128 . The operating bias voltage applied to both vibration sensor  100  and reference structure  140  are equal at all times, so that measurements taken from each can be appropriately compared. 
     The bias voltage applied to gate region  116  (via pad  120 ), source implant  124  (via pad  126 ), and drain implant  128  (via pad  130 ) are adjustable and are set to values that enable a good signal to noise ratio. The vibration of moveable suspended mass  118  modulates the current through channel  122 . Therefore the measured current flow through channel  122 , relative to a reference current flow through reference channel  150 , corresponds to and varies with the increase and decrease in distance between channel  122  and gate region  116  of moveable suspended mass  118 . When the distance is decreased such that channel  122  and gate region  116  are closer together, the current flow through channel  122  increases. When the distance is increased such that channel  122  and gate region  116  are further apart, the current flow through channel  122  decreases. Over a period of time, the source-drain current will resemble the modulation of the conductivity of channel  122 , and therefore will represent the position of moveable suspended mass  118  relative to entire wideband vibration sensor  100 . 
     FIG. 3 shows the electrical circuit diagram that represents the operation, current flow and points of measurement of the present invention. An input  364  consists of the application of a steady state voltage equally to a moveable gate transistor  366  (comprising channel  122 , source  124  and drain  128  of FIG. 1) and a reference transistor  370  (comprising reference channel  150 , reference source  152 , and reference drain  156  of FIG.  1 ). Similarly, a gate bias voltage  365  is equally applied to the gate regions of the circuit. Current flowing through moveable gate transistor  366  is measured at a MGT current sensing circuit  368 ; and current flowing through reference transistor  370  is measured at a reference current sensing circuit  372 . The voltage differential is provided at an output  374 . This circuit diagram provides the basic differential calibration functionality according to the invention. This diagram can easily be extended and generalized by someone of ordinary skill to include multiple moveable gate transistors, combined in series, parallel or other algorithmic combination needed to satisfy the requirements of the geometry of the sensor as seen in the alternative embodiments presented below. 
     A force balanced feedback control loop is then introduced that uses the positional information extracted from the source-drain current to dynamically restore the beam to its undeflected position. This mode of operation widens the dynamic range, provides a greater maximum signal range, and increases the sensitivity of the vibration sensor. A means of signal processing is located adjacent to the wideband vibration sensor with the control electronics. This feedback control loop provides a means of lossless dynamic damping to the system. 
     In particular, when the source-drain current is sensed to be lower than the reference value, which is indicative of an increased distance between gate region  116  and channel  122 , the control loop will increase the voltage applied to lower feedback electrode contact pad  134 . The voltage applied to lower feedback electrode  132  is enough to increase the electric field between moveable suspended mass  118  and lower feedback electrode  132 , producing an electrostatic restoring force which overrides any acceleration in the opposite direction. Similarly, when the control loop senses and detects that the source-drain current is higher than the reference value, which is indicative of a decreased distance between gate region  116  and channel  122 , the control loop will increase the voltage applied to upper feedback electrode contact pad  138 . This voltage applied to upper feedback electrode  136  will increase the electric field between moveable suspended mass  118  and upper feedback electrode  136 , to produce a restoring force attracting moveable suspended mass  118  towards upper electrode  136 . 
     Referring back to FIG. 1, reference structure  140  is used to simplify the measurement process by determining the aforementioned reference current flow at equilibrium to compare to the source drain current modulation generated by vibrations of moveable suspended mass  118  about the equilibrium current. The current flow across reference channel  150 , from reference source implant  152  to reference drain implant  156 , is the reference current flow. This reference current flow is measured identically to the source drain current measured across channel  122 . The difference in the two current measurements is related to the amount moveable suspended mass  118  is displaced from the equilibrium of static suspended reference mass  146 . It is this differential measurement that provides a localized calibration to the sensor, thereby eliminating costly tuning and calibration manufacturing procedures. 
     The position, d, of moveable suspended mass  118  relative to the channel surface at equilibrium can be described as function, f, of the source-drain current i sd  and the reference current i r  as follows: 
     
       
           d=f ( i   sd ( t )− i   r )  
       
     
     Since i r  is constant, the velocity, v, can be described as a function, g, of the derivative of the source-drain current with respect to time as follows: 
     
       
           v=g ( i   sd ( t )− i   r   , di   sd   /dt )  
       
     
     Therefore the acceleration, a, can be described as a function, h, of the second derivative of the source-drain current with respect to time squared as follows: 
     
       
           a=h ( i   sd ( t )− i   r   , di   sd   /dt, d   2   i   sd   /dt   2 )  
       
     
     In the preferred embodiment, referring to FIG. 2, base  110  is made of single crystal silicon, and moveable suspended mass  118  is made of polysilicon. On top of and completely covering channel  122 , is a thermally grown oxide layer  262 . 
     In the preferred embodiment, the transducer is an n-channel field effect device, meaning channel  122  and base  110  primarily comprise p-type silicon, while source implant  124  and drain implant  128  primarily comprise n-type silicon. Alternative embodiments of the present invention include the transducer comprising a p-channel field effect device, where channel  122  and base  110  primarily comprise n-type silicon, while source implant  124  and drain implant  128  primarily comprise p-type silicon. 
     The preferred embodiment of the present invention can be used to obtain standard acceleration measurements. For acceleration measurements, the disclosed wideband vibration sensor is precisely mounted in a standard packaging means such as a chip carrier. The preferred embodiment of the disclosed invention assumes hybrid packaging in which the control electronics are packaged in a separate but adjacent module of similar size considerations to the sensor package. 
     Alternative embodiments of the present invention include variations in the position and number of both lower feedback electrode  132  and upper feedback electrode  136 . When the electrodes are generally perpendicular to the direction of moveable suspended mass  118 , it is possible to place them in locations above and below other areas than the central region of moveable suspended mass  118 . The repositioning of the electrode pair in this manner affects the voltage required to deflect or exert the balancing force on moveable suspended mass  118 . To achieve the same deflection of moveable suspended mass  118 , a higher voltage is needed if the electrode pair is placed in close proximity to the base of moveable suspended mass  118 , than if the electrode pair is placed closer to the free end (i.e. gate region  116 ) of moveable suspended mass  118 . 
     An additional alternative embodiment of the present invention includes multiple electrode pairs distributed along the length of moveable suspended mass  118 . Multiple electrode pairs distributed along the length of moveable suspended mass  118  improve the controllability of moveable suspended mass  118 . Additional alternative embodiments of the present invention include aligning the sensing elements, i.e., source implant  124 , drain implant  128  and channel  122 , with the extended direction of moveable suspended mass  118 ; in other words, rotating these elements approximately 90 degrees from their position in the preferred embodiment of this invention. This rotation would allow for greater flexibility in the design of the transistor. 
     In the discussion to follow, when used in connection with channel  122  and reference channel  150 , the term “length” refers to the distance between source  124  and drain  128  and between reference source  152  and reference drain  156 , while the term “width” refers the overall span over which source  124  and drain  128 , as well as reference source  152  and reference drain  156 , face one another across respective channels  122  and  150 . It is to be noted that as illustrated in FIG. 1, in these terms, the “lengths” of channels  122  and  150  are geometrically smaller than their “widths.” 
     In these terms, the gain achieved by transistors generally is related to the lengths of their channels,  122  and  150  herein. A shorter channel length coupled with a larger channel width allows for increased capacitance and increased gain. In the configuration of FIG. 1, the widths of channels  122  and  150  are limited by the widths of moveable suspended mass  118  and static suspended reference mass  146 . Alternative embodiment configurations can be used which present greater opportunity to maintain a small channel length while increasing channel width, to thereby increase the capacitance and gain of the transistor without sacrificing other parameters. 
     One such alternative embodiment for increasing the sensitivity of the sensor by optimizing the ratio of the channel width to the channel length and by increasing the capacitance by ensuring the gate is close to and completely covering the channel region is illustrated in FIG.  4 . FIG. 4 shows an alternative channel, source, and drain configuration that can be implemented to increase the sensitivity of the sensor by increasing the channel width while the channel length remains optimally small. An alternative source implant  476  is configured with multiple fingerlike extensions that interdigitate with an alternative drain implant  478  with similar fingerlike extensions. An alternative channel  480  runs between alternative source  476  and alternative drain  478 . 
     FIG. 5 shows the implementation of this alternative channel, source, and drain geometry with an associated alternative gate geometry. As described in the preferred embodiment, moveable suspended mass  118  is supported by base  110  over cavity  112 . Upper electrode  136  and lower electrode  132  are also similarly positioned. In FIG. 5, a widened alternative gate region  582  is positioned over alternative source  476 , alternative drain  478 , and alternative channel  480 . The operation is the same but the sensitivity is significantly enhanced due to the increased channel width while the channel length remains optimally small, which increases the forward transconductance of the sensor. 
     To maintain comparable measurements, if an alternative transistor and gate configuration such as that shown in FIGS. 4 and 5 is used in connection with moveable suspended mass  118  (and vibration sensor  100  generally), a similar alternative transistor configuration should also be used in connection with static suspended reference mass  146  (and reference structure  140  generally). It is understood that FIG. 4 illustrates one particular approach to providing a higher-gain transistor for use in accordance with the invention, but that any other high-gain transistor configuration that may be known or apparent to someone of ordinary skill, when used in accordance with the invention, is considered to be within the scope of this disclosure and its associated claims. In this and related embodiments, a particular selection of moveable suspended mass  118  and static suspended reference mass  146  is accompanied by a suitably-compatible gate region  116  and reference gate region  160 , such as the widened alternative gate region  582  to accompany alternative source  476 , alternative drain  478 , and alternative channel  480 . 
     Yet another alternative embodiment of the present invention is a configuration for direct capacitance sensing measurements instead of the current capacitance measurements heretofore described. In order to accomplish this the capacitance in the gate area must be significantly increased. This can be achieved using one or both of the following approaches. First, the cantilever beam, or more generally moveable suspended mass  118 , can be made significantly wider. Second, one may use a plurality of cantilever beams, or more generally moveable suspended masses  118 , substantially in parallel with one another. 
     Another alternative embodiment of the present invention uses alternative geometries of moveable suspended mass  118  aside from a cantilever beam. Exemplary alternative geometries include coils and diaphragms. FIGS. 6 and 7 show a top view of these alternative suspended masses, and thus corresponding alternative gate geometries, respectively. In these alternative geometries, it is understood that the geometry of the movable suspended mass  118  is directly related to the vibratory motion to be detected, the configuration of the transistors, the current flow determination algorithms, and the configuration of the electrodes. More complex geometries require more anchors to base  110  than the cantilever beam described above, but in no way indicate a static reference such as static suspended reference mass  146 . Similarly, complex geometries require multiple transistors to determine vibratory motion and are combined in unique algorithms specific to the geometries to arrive at a total source-drain current to be compared to a reference. 
     FIG. 6 shows an alternative embodiment in which the suspended mass is a coil. This geometry is beneficial in applications where complex vibrations are to be measured since the device is bistable. As shown in FIG. 6, base  110  supports a coil suspended mass  684  over cavity  112 . Source implants  124 , drain implants  128 , and channels  122  are optimally placed under coil suspended mass  684 . Similarly, lower electrodes  132  and upper electrodes  136  are positioned to provide feedback control. Coil suspended mass  684  is doubly anchored, similarly to static suspended reference mass  146 . However, unlike with static suspended reference mass  146 , the geometry of this configuration lends to a rocking motion of coil  684  when subjected to vibrations. Multiple transistors are used to decipher the resulting complex motion according to a more generalized application of FIG. 3 to multiple transistors. 
     FIG. 7 shows an alternative embodiment in which the suspended mass is a diaphragm, or more specifically, a torsional diaphragm. This geometry would be beneficial in an application in which low frequency vibrations are to be measured, since the footprint of the sensor can be relatively large. As shown in FIG. 7, base  110  supports a diaphragm suspended mass  786  over cavity  112 . Source implants  124 , drain implants  128 , and channels  122  are optimally placed under diaphragm suspended mass  786 , for example, as shown. Similarly, lower electrodes  132  and upper electrodes  136  are positioned to provide feedback control, for example, as shown. Note from this embodiment, that upper  136  and lower  132  feedback electrodes do not necessary need to be elongated members substantially parallel and coextensive with one another. Any configuration suitable for applying appropriate negative feedback control is acceptable according to the invention. Similarly to FIG. 6, this also uses multiple transistors, and the current flowing through each of the several channels  122  are be combined using a suitable algorithm to arrive at a total source drain current to be compared to the reference current, again generalizing FIG. 3 to multiple transistors. 
     Yet another alternative embodiment of the present invention can be realized referring back to FIG.  1 . If reference structure  140  is implemented independently of vibration sensor  100 , it can serve as a generic transistor for electronic circuitry that solves the fabrication incompatibility problem described in the background. More specifically, reference structure  140  comprises the basic elements of a generic transistor such as reference gate  160 , reference channel  150 , reference source  152  and reference drain  156 . Implemented independently, reference structure  140  can serve as a generic transistor in a variety of electronic circuits and provide a means of overcoming the usual incompatibilities between sensor fabrication processes and electronic device fabrication process. 
     While only certain preferred features of the invention have been illustrated and described, many modifications, changes and substitutions will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.