# Patent

## Title
Sensor design and process

## Abstract

An accelerometer (305) for measuring seismic data. The accelerometer (305) includes an integrated vent hole for use during a vacuum sealing process and a balanced metal pattern for reducing cap wafer bowing. The accelerometer (305) also includes a top cap press frame recess (405) and a bottom cap press frame recess (420) for isolating bonding pressures to specified regions of the accelerometer (305). The accelerometer (305) is vacuum-sealed and includes a balanced metal pattern (730) to prevent degradation of the performance of the accelerometer (305). A dicing process is performed on the accelerometer (305) to isolate the electrical leads of the accelerometer (305). The accelerometer (305) further includes overshock protection bumpers (720) and patterned metal electrodes to reduce stiction during the operation of the accelerometer (305).

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional application of U.S. patent application Ser. No. 09/936,640 filed on Apr. 9, 2002, which is a national phase application of international application PCT/US00/40039 filed on Mar. 16, 2000, which claims priority from U.S. provisional patent application 60/125,076 filed on Mar. 17, 1999. The entire contents of each referenced application are hereby incorporated herein by reference.


BACKGROUND OF THE INVENTION
This invention relates generally to an accelerometer and more particularly to a method of fabricating an accelerometer that reduces bowing of wafer bodies and facilitates the creation of a low-pressure environment within the accelerometer.
Accelerometers are used to detect and record environmental data. In particular, accelerometers are often used in seismic applications to gather seismic data. Existing accelerometers suffer from a number of limitations. These limitations include bowing of cap wafers due to thermal induced stress and an inability to achieve a large dynamic range due to the effects of Brownian noise, both of which adversely affect the quality of data acquired by these accelerometers. Many existing accelerometers fail to provide a large dynamic range, sub micro-G level sensitivity, high shock withstanding, and good cross-axis rejection in a cost-effective technology.
The present invention is directed to overcoming one or more of the limitations of the existing accelerometers.
SUMMARY
According to one embodiment of the present invention, an accelerometer is provided that includes a measurement mass for detecting acceleration, including a housing having a cavity, a spring mass assembly positioned within the cavity, and one or more metal electrode patterns coupled to the spring mass assembly, a top cap wafer coupled to the measurement mass, including a top capacitor electrode, a top cap balanced metal pattern, a top cap press frame recess, and top cap overshock bumpers, and a bottom cap wafer coupled to the measurement mass, including a bottom capacitor electrode, a bottom cap balanced metal pattern, a bottom cap press frame recess, and bottom cap overshock bumpers.
According to another embodiment of the present invention, a method of fabricating an accelerometer is provided that includes fabricating a measurement mass for detecting acceleration that includes a housing having a cavity, and a spring mass assembly positioned within the cavity, fabricating a top cap wafer, fabricating a bottom cap wafer, vertically stacking the measurement mass, the top cap wafer, and the bottom cap wafer in an approximately parallel manner, bonding the top cap wafer to a side of the measurement mass using a bonding process, bonding the bottom cap wafer to another side of the measurement mass using the bonding process, and making one or more dicing cuts at predetermined locations on the accelerometer.
According to another embodiment of the present invention, a method of bonding an accelerometer is provided that includes fabricating a measurement mass that includes a housing having a cavity, a spring mass assembly positioned within the cavity, and one or more bond rings coupled to the housing, fabricating a top cap wafer that includes a top bond ring and a top cap press frame recess, fabricating a bottom cap wafer that includes a bottom bond ring and a bottom cap press frame recess, vertically stacking the measurement mass, the top cap wafer, and the bottom cap wafer in an approximately parallel manner, bonding the top cap wafer to a side of the measurement mass using a bonding process, and bonding the bottom cap wafer to another side of the measurement mass using the bonding process.
According to another embodiment of the present invention, a method of shaping a wafer to create components for a sensor is provided that includes applying a protective layer to the wafer, patterning the protective layer to create an area of exposure, applying one or more etching agents to the area of exposure to remove the protective layer within the area of exposure, applying one or more etching agents to the area exposure to shape the wafer into a housing, a measurement mass, and one or more springs, and maintaining the etch-stop layer on the springs.
According to another embodiment of the present invention, a sensor for measuring data is provided that includes a measurement mass assembly including a housing, a measurement mass including one or more electrodes, and a plurality of springs for coupling the measurement mass to the housing, a top cap wafer coupled to the measurement mass assembly including a top cap overshock bumper pattern designed to reduce stiction within the sensor, and a bottom cap wafer coupled to the measurement mass assembly including a bottom cap overshock bumper pattern designed to reduce stiction within the sensor.
According to another embodiment of the present invention, a metal electrode pattern for use in a sensor is provided that includes a metal electrode including a stiction-reducing pattern.
According to another embodiment of the present invention, a method of creating a stiction-reducing metal electrode pattern for use within a sensor is provided that includes etching a surface pattern onto a surface of the sensor, and applying a metal layer to the surface of the sensor including the surface pattern, and molding the metal layer to create the stiction-reducing metal electrode pattern.
According to another embodiment of the present invention, a method of creating a metal electrode pattern including reduced-thickness recesses for reducing stiction between the metal electrode pattern and overshock bumpers within an accelerometer is provided that includes creating a lower metal electrode pattern layer, applying an upper metal electrode pattern layer on top of the lower metal electrode pattern layer, and selectively removing one or more portions of the upper metal electrode pattern layer to create the reduced-thickness recesses and expose the underlying lower metal electrode pattern layer within the metal electrode pattern.
According to another embodiment of the present invention, a method of creating a metal electrode pattern including cavities for reducing stiction between the metal electrode pattern and overshock bumpers within an accelerometer is provided that includes creating a lower metal electrode pattern layer, applying an upper metal electrode pattern layer on top of the lower metal electrode pattern layer, and selectively removing one or more portions of the upper metal electrode pattern layer and the lower metal electrode pattern layer to create the cavities within the metal electrode pattern.
The present embodiments of the invention provide an accelerometer for providing reliable data measurements. The accelerometer is vacuum-sealed and includes a balanced metal pattern to prevent degradation of the performance of the accelerometer. A dicing process is performed on the accelerometer to isolate the electrical leads of the accelerometer. The accelerometer further includes overshock protection bumpers and patterned metal electrodes to reduce stiction during the operation of the accelerometer.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an embodiment of a system used to acquire environmental data measurements.
FIG. 2 illustrates an embodiment of sensors and cabling used within the system of FIG. 1.
FIG. 3a is a cross-sectional side view of the positioning of an accelerometer within the sensor of FIG. 1.
FIG. 3b is a cross-sectional top view of the positioning of an accelerometer within the sensor of FIG. 1.
FIG. 4 illustrates a top perspective view of an embodiment of the accelerometer of FIG. 3a. 
FIG. 5 illustrates a bottom perspective view of the accelerometer of FIG. 4.
FIG. 6 illustrates a cross-sectional view of the accelerometer of FIG. 4.
FIG. 7a illustrates a cross-sectional view of, a top cap wafer of the accelerometer of FIG. 4.
FIG. 7b illustrates a top view of the top cap wafer of FIG. 7a. 
FIG. 7c illustrates a bottom view of the top cap wafer of FIG. 7a. 
FIG. 7d illustrates an embodiment of an arrangement of overshock bumpers on the top cap wafer of FIG. 7a. 
FIG. 7e illustrates an embodiment of an alternative arrangement of the overshock bumpers of FIG. 7d. 
FIG. 7f illustrates an embodiment of an alternative arrangement of the overshock bumpers of FIG. 7d. 
FIG. 7g illustrates an embodiment of an alternative arrangement of the overshock bumpers of FIG. 7d. 
FIG. 7h illustrates an embodiment of an alternative arrangement of the overshock bumpers of FIG. 7d. 
FIG. 7i illustrates an embodiment of an alternative arrangement of the overshock bumpers of FIG. 7d. 
FIG. 7j illustrates an embodiment of an alternative arrangement of the overshock bumpers of FIG. 7d. 
FIG. 7k illustrates an embodiment of an alternative arrangement of the overshock bumpers of FIG. 7d. 
FIG. 7l illustrates an embodiment of an alternative arrangement of the overshock bumpers of FIG. 7d. 
FIG. 8a illustrates a cross-sectional view of a bottom cap wafer of the accelerometer of FIG. 4.
FIG. 8b illustrates a bottom view of the bottom cap wafer of FIG. 8a. 
FIG. 8c illustrates a top view of the bottom cap wafer of FIG. 8a 
FIG. 9a illustrates a cross-sectional view of a mass wafer pair of the accelerometer of FIG. 4.
FIG. 9aa illustrates a cross-sectional view of a top cap overshock bumper and a patterned metal electrode within the accelerometer of FIG. 6.
FIG. 9ab illustrates a cross-sectional view of a bottom cap overshock bumper and a patterned metal electrode within the accelerometer of FIG. 6.
FIG. 9ac illustrates an embodiment of metal electrodes including reduced-thickness recesses within the accelerometer of FIG. 6.
FIG. 9ad illustrates an embodiment of metal electrodes including cavities within the accelerometer of FIG. 6.
FIG. 9b is a top view of a top mass half of the mass wafer pair of FIG. 9aFIG. 9c is a bottom view of the top mass half of FIG. 9b. 
FIG. 9d is a bottom perspective view of the top mass half of FIG. 9c. 
FIG. 9e is a bottom view of a bottom mass half of the mass wafer pair of FIG. 9a 
FIG. 9f is a top view of the bottom mass half of FIG. 9e. 
FIG. 9g is a top perspective view of the bottom mass half of FIG. 9e. 
FIG. 10 is a flowchart of a fabrication process for the accelerometer of FIG. 4.
FIG. 11a illustrates an embodiment of the two starting cap wafers of FIG. 10.
FIG. 11b illustrates a cross-sectional view of a top cap wafer and a bottom cap wafer resulting from the cap wafer process of FIG. 10.
FIG. 11c illustrates an embodiment of the starting mass wafers of FIG. 10.
FIG. 11d illustrates a top view of an embodiment of a photomask outline including corner compensation structures applied to the starting mass wafers during the mass wafer process of FIG. 10.
FIG. 11e illustrates a bottom view of the top starting mass wafer after an etching phase of the mass wafer process of FIG. 10.
FIG. 11f illustrates a cross-sectional view of the top starting mass wafer and the bottom starting mass wafer after an etching phase of the mass wafer process of FIG. 10.
FIG. 11g illustrates a cross-sectional view of a bonded mass wafer pair during the mass wafer process of FIG. 10.
FIG. 11h illustrates a cross-sectional view of the bonded mass wafer pair of FIG. 11g including electrodes and bond rings.
FIG. 11ha illustrates an embodiment of a metal electrode including a patterned surface on an upper surface of the mass wafer pair of FIG. 9a. 
FIG. 11hb illustrates an embodiment of a metal electrode including a patterned surface on a lower surface of the mass wafer pair of FIG. 9a. 
FIG. 11hc illustrates an embodiment of a patterned surface on the mass wafer pair of FIG. 9a. 
FIG. 11hd illustrates an alternative embodiment of the patterned surface of FIG. 11hc. 
FIG. 11he illustrates an alternative embodiment of the patterned surface of FIG. 11hc. 
FIG. 11hf illustrates an alternative embodiment of the patterned surface of FIG. 11hc. 
FIG. 11hg illustrates an alternative embodiment of the patterned surface of FIG. 11hc. 
FIG. 11hh illustrates an alternative embodiment of the patterned surface of FIG. 11hc. 
FIG. 11hi illustrates an alternative embodiment of the patterned surface of FIG. 11hc. 
FIG. 11hj illustrates an alternative embodiment of the patterned surface of FIG. 11hc. 
FIG. 11i illustrates a cross-sectional view of the bonded mass wafer pair of FIG. 11h including springs.
FIG. 11j illustrates a cross-sectional view of an accelerometer after the bonding process of FIG. 10.
FIG. 12a is a side view illustrating the relative positioning of dicing cuts on the accelerometer die of FIG. 6.
FIG. 12b is an illustration of the accelerometer die after the dicing cuts of FIG. 12a have been completed.
FIG. 12c is an illustration of an embodiment of the accelerometer of FIG. 12b after an integrated passage has been exposed.
FIG. 13 is an illustration of an embodiment of the accelerometer of FIG. 12c packaged within a housing.



DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS
Referring initially to FIG. 1, a preferred embodiment of a system 100 designed to record data measurements is illustrated. The system 100 preferably includes one or more sensors 105, a controller 110, and cabling 115.
Within the system 100, the sensors 105 are used to detect data measurements. In a preferred embodiment, the system 100 is used in seismic applications to record seismic data measurements. The sensors 105 may be any number of conventional commercially available sensors, such as, for example, a geophone, a hydrophone, or an accelerometer. In a preferred embodiment, each of the sensors 105 is an accelerometer.
The controller 110 is used to monitor and control the sensors 105. The controller 110 is preferably coupled to the sensors 105 by the cabling 115. The controller 110 may be any number of conventional commercially available controllers suitable for controlling the sensors 105, such as, for example, a seismic data acquisition device, a PID controller, or a microcontroller. In a preferred embodiment, the controller 110 is a seismic data acquisition device.
The cabling 115 couples the sensors 105 and the controller 110. The cabling 115 may be any cabling suitable for transmitting information between the sensors 105 and controller 110, such as, for example, wire or fiber optics. In a preferred embodiment, the cabling 115 is a wire.
Referring to FIG. 2, a preferred embodiment of the alignment of the sensors 105 and the cabling 115 within the system 100 is illustrated. The sensors 105 and the cabling 115 may be aligned linearly or non-linearly. In a preferred embodiment, the sensors 105 and cabling 115 are aligned linearly.
The sensors 105 may include any number of conventional commercially available components suitable for creating a sensor. Referring to FIGS. 3a and 3b, in a preferred embodiment, the sensors 105 include one or more accelerometers 305, and a housing 315 having a cavity 320. In another preferred embodiment, the sensors 105 further include a measurement device 310. In a preferred embodiment, the sensors 105 each include three accelerometers 305. The accelerometers 305 are preferably placed in the cavity 320 within the housing 315 of the sensor 105. The accelerometers 305 may be coupled to the measurement device 310, or may operate independently within the sensor 105. In a preferred embodiment, the accelerometers 305 operate independently within the sensor 105. The measurement device 310 may be any number of conventional commercially available devices suitable for coupling with the accelerometer 305 to create a sensor 105, such as, for example, a geophone or a hydrophone. In a preferred embodiment, the measurement device 310 is a hydrophone.
The accelerometer 305 may include any number of components suitable for forming an accelerometer. Referring to FIGS. 4, 5, and 6, in a preferred embodiment, the accelerometer 305 includes a top cap wafer 405, a top measurement mass half 410, a bottom measurement mass half 415, and a bottom cap wafer 420. The operation of the accelerometer 305 is preferably provided substantially as described in U.S. Pat. No. 5,852,242, the disclosure of which is incorporated herein by reference.
The top cap wafer 405 may include any number of conventional commercially available components suitable for forming a top cap wafer. In a preferred embodiment, as illustrated in FIGS. 7a, 7b, 7c, 7d, 7e, 7f, 7g, 7h, 7i, 7j, 7k, and 7l, the top cap wafer 405 includes a top cap wafer body 406, an upper surface 407, a bottom surface 408, a top capacitor electrode 705, a top bond ring 707, a top bond oxide ring 710, a top cap parasitic groove 715, top cap overshock bumpers 720, a top cap press frame recess 725, a top cap balanced metal pattern 730, and a top cap contact pad 735.
The top cap wafer body 406 may be fabricated from any number of conventional commercially available materials suitable for creating a cap wafer body, such as, for example, glass, quartz, ceramic, or silicon. In a preferred embodiment, the top cap wafer body 406 is made of silicon.
The top capacitor electrode 705 is preferably used for the time-based multiplexing of electrical signals from an external circuit, the operation of which is substantially as described in U.S. patent application Ser. No. 09/936,630, filed on Sep. 14, 2001, the disclosure of which is incorporated herein by reference. The top capacitor electrode 705 is preferably located on the bottom surface 408 of the top cap wafer body 406, within an area circumscribed by the top cap parasitic groove 715. In a preferred embodiment, as illustrated in FIG. 7c, the top capacitor electrode 705 includes slots 706 into which the top cap overshock bumpers 720 are fabricated. The top capacitor electrode 705 may be fabricated from any number of conductive materials suitable for creating an electrode, such as, for example, metals, silicides, or doped semiconductors. In a preferred embodiment, the top capacitor electrode 705 is fabricated from a combination of gold and titanium. In a preferred embodiment, the combination of gold and titanium includes a layer of gold located on top of a layer of titanium. The layer of titanium preferably improves the adhesion of the gold to silicon and silicon dioxide.
The top bond ring 707 and the top bond oxide ring 710 preferably bond the top cap wafer 405 to the top measurement mass half 410 and help establish a narrow gap between the top capacitor electrode 705 and an electrode located on an upper surface of the top measurement mass half 410. The top bond oxide ring 710 preferably provides electrical isolation between the top cap wafer 405 and the top measurement mass half 410. The top bond ring 707 and the top bond oxide ring 710 are preferably located on the bottom surface 408 of the top cap wafer body 406. The top bond ring 707 may be fabricated from any number of materials suitable for making a bond ring, such as, for example, gold, silver, or aluminum. In a preferred embodiment, the top bond ring 707 is fabricated from a combination of gold and titanium. In a preferred embodiment, the combination of gold and titanium includes a layer of gold located on top of a layer of titanium. The layer of titanium preferably improves the adhesion of the gold to silicon and silicon dioxide. The bond ring 707 may have any dimensions suitable for use within the accelerometer 305. In a preferred embodiment, as illustrated in FIG. 7a, the bond ring 707 has a width d1 that is smaller than the width of the top cap press frame recess 725. In a preferred embodiment, the bond ring 707 extends below the top cap overshock bumpers 720 by a distance d2. The top bond oxide ring 710 may be fabricated from any number of conventional commercially available materials suitable for making a bond oxide ring, such as, for example, silicon dioxide or dielectrics. In a preferred embodiment, the top bond oxide ring 710 is fabricated from silicon dioxide.
The top cap parasitic groove 715 preferably minimizes the coupling of electrostatic feedback of an external close-loop circuit to springs included in the top measurement mass half 410. The top cap parasitic groove 715 preferably is a groove within the bottom surface 408 of the top cap wafer body 406. The top cap parasitic groove 715 preferably circumscribes the top capacitor electrode 705 and is surrounded by the top bond oxide ring 710. The top cap parasitic groove 715 may include any dimensions suitable for creating an adequate parasitic groove. In a preferred embodiment, the top cap parasitic groove 715 measures greater than about 5 μm in depth and has a width wider than the width of the springs within the top measurement mass half 410.
The top cap overshock bumpers 720 preferably provide out-of-plane shock protection to the top measurement mass half 410. The top cap overshock bumpers 720 are preferably located on the bottom surface 408 of the top cap wafer body 406, and are exposed through the cutouts 706 in the top capacitor electrode 705. The top cap overshock bumpers 720 may be fabricated from any number of conventional commercially available materials suitable for creating overshock bumpers, such as, for example, silicon dioxide or dielectrics. In a preferred embodiment, the top cap overshock bumpers 720 are made of silicon dioxide. In a preferred embodiment, as illustrated in FIG. 7a, the top cap overshock bumpers 720 have a width w1. The top cap wafer 405 may include any number of top cap overshock bumpers 720. The design and layout of the top cap overshock bumpers 720 may be affected by any number of factors. In a preferred embodiment, the design and layout of the top cap overshock bumpers 720 balances the need for shock protection with the need for minimal stiction between the top cap overshock bumpers 720 and a metal electrode pattern 910 located on the top measurement mass half 410. Stiction occurs when the top cap overshock bumpers 720 stick to the metal electrode pattern 910 on the top measurement mass half 410 during the operation of the accelerometer 305. The stiction between the top cap overshock bumpers 720 and the metal electrode pattern located on the top measurement mass half 410 may be caused by any number of sources, such as, for example, imprinting of the top cap overshock bumpers 720 onto the metal electrode pattern 910 located on the top measurement mass half 410, Van Der Waals forces, electrostatic forces, surface residues resulting from the fabrication of the accelerometer 305, or package-induced stresses. In a preferred embodiment, as illustrated in FIG. 7d, the top cap wafer 405 includes four bumpers. In an alternative embodiment, as illustrated in FIG. 7e, the top cap wafer 405 includes five top cap overshock bumpers 720. In an alternative embodiment, as illustrated in FIG. 7f, the top cap wafer 405 includes eight geometrically arranged top cap overshock bumpers 720. In an alternative embodiment, as illustrated in FIG. 7g, the top cap wafer 405 includes nine geometrically arranged top cap overshock bumpers 720. In an alternative embodiment, as illustrated in FIG. 7h, the top cap wafer 405 includes nine top cap overshock bumpers 720 arranged in three linear, parallel rows with each row having three bumpers 720. In an alternative embodiment, as illustrated in FIG. 7i, the top cap wafer 405 includes thirteen geometrically arranged top cap overshock bumpers 720. In an alternative embodiment, as illustrated in FIG. 7j, the top cap wafer 405 includes forty nine top cap overshock bumpers 720. In an alternative embodiment, as illustrated in FIGS. 7k and 7l, the top cap wafer 405 includes a plurality of geometrically arranged top cap overshock bumpers 720 in the shape of circles and ridges.
The top cap press frame recess 725 is preferably located on the upper surface 407 of the top cap wafer body 406 between the top cap balanced metal pattern 730 and the top cap contact pad 735. The top cap press frame recess 725 preferably ensures that bond forces applied during a bonding process are localized to the top bond oxide ring 710 region. By localizing bond forces to the top bond oxide ring 710 region rather than to the region of the narrow gap between the top capacitor electrode 705 and the electrode located on an upper surface of the top measurement mass half 410, the narrow gap between the electrodes is maintained. The top cap press frame recess 725 may be formed using any number of processing steps suitable for forming a press frame recess such as, for example, silicon etching. In a preferred embodiment, the top cap press frame recess 725 is etched into the upper surface 407 of the top cap wafer body 406. The top cap press frame recess 725 may include any dimensions suitable for creating a press frame recess. In a preferred embodiment, the top cap press frame recess 725 measures greater than about 20 μm in depth, and has a width wider than the width d1 of the bond ring 707.
The top cap contact pad 735 is preferably located on the upper surface 407 of the top cap wafer body 406. The top cap contact pad 735 is preferably available for wire bonding. The top cap contact pad 735 may include any number of conventional commercially available materials suitable for creating a contact pad such as, for example, gold, aluminum, or silver. In a preferred embodiment, the top cap contact pad 735 is made of gold. In another preferred embodiment, the top cap contact pad 735 is made of a combination of gold and titanium. In a preferred embodiment, the combination of gold and titanium includes a layer of gold located on top of a layer of titanium. The layer of titanium preferably improves the adhesion of the gold to silicon and silicon dioxide.
The top cap balanced metal pattern 730 is used to minimize bowing of the top cap wafer body 406. Bowing of the top cap wafer body 406 is undesirable because it has an adverse effect on the performance of the accelerometer 305.
Bowing of the top cap wafer body 406 typically results from thermal coefficient of expansion (TCE) differences between the material of the top cap wafer body 406 and the metal of the top capacitor electrode 705. In a preferred embodiment, the material of the top cap wafer body 406 is silicon. In a preferred embodiment, the top cap balanced metal pattern 730 is approximately identical in pattern and thickness to the top capacitor electrode 705 and is placed within the top cap press frame recess 725, substantially opposite the top capacitor electrode 705. In a preferred embodiment, the top cap balanced metal pattern 730 includes cutouts 731 to offset the cutouts 705 in the top capacitor electrode 705. This alignment preferably creates a balanced metal/silicon/metal sandwich that helps minimize the TCE mismatch effects on accelerometer 305 performance.
The bottom cap wafer 420 may include any number of conventional commercially available components suitable for forming a bottom cap wafer. In a preferred embodiment, as illustrated in FIGS. 8a, 8b, and 8c, the bottom cap wafer 420 includes a bottom cap wafer body 421, an upper surface 423, a bottom surface 422, a bottom capacitor electrode 805, a bottom bond ring 807, a bottom bond oxide ring 810, a bottom cap parasitic groove 815, bottom cap overshock bumpers 820, a bottom cap press frame recess 825, a bottom cap balanced metal pattern 830, a bottom cap contact pad 835, and an extended cap solder attach (ECSA) metal bond pad 840.
The bottom cap wafer body 421 may be fabricated from any number of conventional commercially available materials suitable for creating a cap wafer body such as, for example, glass, quartz, ceramic, or silicon. In a preferred embodiment, the bottom cap wafer body 421 is made of silicon.
The bottom capacitor electrode 805 is preferably used for the time-based multiplexing of electrical signals from an external circuit, the operation of which is substantially as described in U.S. patent application Ser. No. 09/936,630, filed on Sep. 14, 2001, the disclosure of which is incorporated herein by reference. The bottom capacitor electrode 805 is preferably located on the upper surface 423 of the bottom cap wafer body 421, within an area circumscribed by the bottom cap parasitic groove 815. In a preferred embodiment, as illustrated in FIG. 8c, the bottom capacitor electrode 805 includes cutouts 806 into which the bottom cap overshock bumpers 820 are fabricated. The bottom capacitor electrode 805 may be fabricated using any number of conductive materials suitable for creating an electrode such as, for example, metals, silicides, or doped semiconductors. In a preferred embodiment, the bottom capacitor electrode 805 is fabricated from a combination of gold and titanium. In a preferred embodiment, the combination of gold and titanium includes a layer of gold located on top of a layer of titanium. The layer of titanium preferably improves the adhesion of the gold to silicon and silicon dioxide.
The bottom bond ring 807 and the bottom bond oxide ring 810 preferably bond the bottom cap wafer 420 to the bottom measurement mass half 415 and help establish a narrow gap between the bottom capacitor electrode 805 and an electrode located on a lower surface of the bottom measurement mass half 415. The bottom bond oxide ring 810 preferably provides electrical isolation between the bottom cap wafer 420 and the bottom measurement mass half 415. The bottom bond ring 807 and the bottom bond oxide ring 810 are preferably located on the upper surface 423 of the bottom cap wafer body 421. The bottom bond ring 807 may be fabricated from any number of materials suitable for making a bond ring such as, for example, aluminum, silver, or gold. In a preferred embodiment, the bottom bond ring 807 is fabricated from a combination of gold and titanium. In a preferred embodiment, the combination of gold and titanium includes a layer of gold located on top of a layer of titanium. The layer of titanium preferably improves the adhesion of the gold to silicon and silicon dioxide. In a preferred embodiment, the bond ring 807 has a width d4 that is smaller than the width of the bottom cap press frame recess 825. In a preferred embodiment, the bond ring 807 extends beyond the bottom cap overshock bumpers 820 by a distance d3. The bottom bond oxide ring 810 may include any number of conventional commercially available materials suitable for making a bond oxide ring such as, for example, dielectrics. In a preferred embodiment, the bottom bond oxide ring 810 is fabricated from silicon dioxide.
The bottom cap parasitic groove 815 preferably minimizes the coupling of electrostatic feedback of an external close-loop circuit to springs included in the bottom measurement mass half 415. The bottom cap parasitic groove 815 preferably is a groove within the upper surface 423 of the bottom cap wafer body 421. The bottom cap parasitic groove 815 preferably circumscribes the bottom capacitor electrode 805, and is surrounded by the bottom bond oxide ring 810. The bottom cap parasitic groove 815 may include any dimensions suitable for creating an adequate parasitic groove. In a preferred embodiment, the bottom cap parasitic groove 815 measures greater than about 5 μm in depth and has a width wider than the width of the springs within the bottom measurement mass half 415.
The bottom cap overshock bumpers 820 preferably provide out-of-plane shock protection to the bottom measurement mass half 415. The bottom cap overshock bumpers 820 are preferably located on the upper surface 423 of the bottom cap wafer body 421, and are exposed through the cutouts 806 in the bottom capacitor electrode 805. The bottom cap overshock bumpers 820 may be fabricated from any number of conventional commercially available materials suitable for creating overshock bumpers, such as, for example, dielectrics or silicon dioxide. In a preferred embodiment, the bottom cap overshock bumpers 820 are made of silicon dioxide. In a preferred embodiment, the bottom cap overshock bumpers 820 have a width w2. The bottom cap wafer 420 may include any number of bottom cap overshock bumpers 820. The design and layout of the bottom cap overshock bumpers 820 may be affected by any number of factors. In a preferred embodiment, the design and layout of the bottom cap overshock bumpers 820 balances the need for good shock protection with the need for minimal stiction between the bottom cap overshock bumpers 820 and a metal electrode pattern 915 located on the bottom measurement mass half 415. Stiction occurs when the bottom cap overshock bumpers 820 stick to the metal electrode pattern 915 on the bottom measurement mass half 415 during the operation of the accelerometer 305. The stiction between the bottom cap overshock bumpers 820 and the metal electrode pattern located on the bottom measurement mass half 415 may be caused by any number of sources, such as, for example, imprinting of the bottom cap overshock bumpers 820 onto the metal electrode pattern 915 located on the bottom measurement mass half 415, Van Der Waals forces, electrostatic forces, surface residues resulting from the manufacture of the accelerometer 305, or package-induced stresses. In a preferred embodiment, the number of bottom cap overshock bumpers 820 on the bottom cap wafer 420 equals the number of top cap overshock bumpers 720 on the top cap wafer 405, the variations of which are illustrated in FIGS. 7d, 7e, 7f, 7g, 7h, 7i, 7j, 7k, and 7l. 
The bottom cap press frame recess 825 is preferably located on the bottom surface 422 of the bottom cap wafer body 421 between the bottom cap balanced metal pattern 830 and the outer edge of the bottom surface 422. The bottom cap press frame recess 825 ensures that bond forces applied during a bonding process are localized to the bottom bond oxide ring 810 region. By localizing bond forces to the bottom bond oxide ring 810 region rather than to the region of the narrow gap between the bottom capacitor electrode 805 and the electrode located on an bottom surface of the bottom measurement mass half 415, the narrow gap between the electrodes is maintained. The bottom cap press frame recess 825 may formed using any number of processing steps suitable for forming a press frame recess such as, for example, silicon etching. In a preferred embodiment, the bottom cap press frame recess 825 is etched into the bottom surface 422 of the bottom cap wafer body 421. The bottom cap press frame recess 825 may include any dimensions suitable for creating a press frame recess. In a preferred embodiment, the bottom cap press frame recess 825 measures greater than about 20 μm in height and has a width wider than the width d4 of the bond ring 807.
The bottom cap contact pad 835 is preferably located on the bottom surface 422 of the bottom cap wafer body 421. The bottom cap contact pad 835 is preferably available for wafer probing. The bottom cap contact pad 835 may include any number of conventional commercially available materials suitable for creating a contact pad such as, for example, gold, aluminum, or silver. In a preferred embodiment, the bottom cap contact pad 835 is fabricated from a combination of gold and titanium. In a preferred embodiment, the combination of gold and titanium includes a layer of gold located on top of a layer of titanium. The layer of titanium preferably improves the adhesion of the gold to silicon and silicon dioxide.
The bottom cap balanced metal pattern 830 is used to minimize bowing of the bottom cap wafer body 421. Bowing of the bottom cap wafer body 421 is undesirable because it has an adverse effect on the performance of the accelerometer 305. Bowing of the bottom cap wafer body 421 typically results from thermal coefficient of expansion (TCE) differences between the material that makes up the bottom cap wafer body 421 and the metal of the bottom capacitor electrode 805. In a preferred embodiment, the material that makes up the bottom cap wafer body 406 is silicon. In a preferred embodiment, the bottom cap balanced metal pattern 830 is approximately identical in pattern and thickness to the bottom capacitor electrode 805 and is placed within the bottom cap press frame recess 825, substantially opposite the bottom capacitor electrode 805. As illustrated in FIG. 8b, the bottom cap balanced metal pattern 830 preferably includes cutouts 831 designed to offset the cutouts 806 in the bottom capacitor electrode 805. This alignment preferably creates a balanced metal/silicon/metal sandwich that helps minimize the TCE mismatch effects on accelerometer 305 performance.
The ECSA metal bond pad 840 is preferably available for conductive die-attach to an external package into which the accelerometer 305 is placed. The operation of the ECSA metal bond pad 840 is preferably as described in U.S. patent application Ser. No. 09/914,421, filed on Mar. 15, 2000, the disclosure of which is incorporated herein by reference.
The top measurement mass half 410 may include any number of conventional commercially available materials suitable for creating a measurement mass half. In a preferred embodiment, as illustrated in FIGS. 9a, 9aa, 9ac, 9ad, 9b, 9c, and 9d, the top measurement mass half 410 includes an upper surface 411, a lower surface 412, one or more springs 905, a top measurement mass 906, a housing 907, the metal electrode pattern 910, a bond ring 920, and a top mass contact pad 930. In another preferred embodiment, the top measurement mass half 410 further includes a groove 940.
The springs 905 preferably couple the top measurement mass 906 to the housing 907 and provide a conductive path between the top measurement mass 906 and the housing 907. The springs 905 may be fabricated from any number of conventional commercially available materials suitable for creating springs such as, for example, quartz, metals, or silicon. In a preferred embodiment, the springs 905 are made of silicon, and are micromachined out of the top measurement mass half 410 wafer. The springs 911 are preferably designed to maintain cross-axis rejection while providing lateral shock protection for the top measurement mass 906. The springs 905 are preferably linear L-shaped springs, the design of which is described in U.S. Pat. Nos. 5,652,384 and 5,777,226, the disclosures of which are incorporated herein by reference.
The top measurement mass 906 is used to detect measurement data. The top measurement mass 906 may be used in any application in which its use is suitable. In a preferred embodiment, the top measurement mass 906 is used in seismic applications to detect acceleration. The top measurement mass 906 is preferably coupled to the housing 907 by the springs 905. The top measurement mass 906 may be fabricated from any number of conventional commercially available materials suitable for creating a measurement mass such as, for example, metals, quartz, or silicon. In a preferred embodiment, the top measurement mass 906 is made of silicon, and is micromachined out of the top measurement mass half 410 wafer.
The housing 907 surrounds the top measurement mass 906 and is coupled to the top measurement mass 906 by the springs 905. The housing 907 may be fabricated from any number of conventional commercially available materials suitable for creating a housing such as, for example, metals, quartz, or silicon. In a preferred embodiment, the housing 907 is fabricated from silicon, and is micromachined out of the top measurement mass half 410 wafer.
The metal electrode pattern 910 is used for the time-based multiplexing of electrical signals from an external circuit. In a preferred embodiment, the metal electrode pattern 910 includes a single electrode. In a preferred embodiment, the metal electrode pattern 910 is located on the upper surface 411 of the top measurement mass half 410, on top of the top measurement mass 906. The metal electrode pattern 910 may include any number of conventional commercially available materials suitable for creating an electrode pattern such as, for example, aluminum, silver, or gold. In a preferred embodiment, the metal electrode pattern 910 is fabricated from a combination of gold and titanium. In a preferred embodiment, the combination of gold and titanium includes a layer of gold located on top of a layer of titanium. The layer of titanium preferably improves the adhesion of the gold to silicon and silicon dioxide. The metal electrode pattern 910 may be of any size or shape suitable for forming an electrode pattern such as, for example, circular, square, or rectangular. The metal electrode pattern 910 is preferably substantially identical in size and shape to the top capacitor electrode 705. In an alternative embodiment, the metal electrode pattern 910 is substantially equal in thickness to the bond ring 920. In a preferred embodiment, the thicknesses of the metal electrode pattern 910 and the bond ring 920 are smaller than the thickness of the top bond ring 707. The difference in thickness between the metal electrode pattern 910, the bond ring 920, and the top bond ring 707 preferably reduces stiction between the top cap overshock bumpers 720 and the metal electrode pattern 910 during the operation of the accelerometer 305 by reducing the imprinting of the top cap overshock bumpers 720 on the metal electrode pattern 910. In another preferred embodiment, as illustrated in FIG. 9aa, the metal electrode pattern 910 includes one or more patterns 960a designed to minimize stiction between the top cap overshock bumpers 720 and the metal electrode pattern 910 during the operation of the accelerometer 305. The patterns 960a may include any shape suitable for reducing stiction within the accelerometer 305. The patterns 960a in the metal electrode pattern 910 preferably reduce stiction between the top cap overshock bumpers 720 and the metal electrode pattern 910 by minimizing the surface area of the region of intimate contact between the top cap overshock bumpers 720 and the metal electrode pattern 910. In another preferred embodiment, as illustrated in FIG. 9ac, the metal electrode pattern 910 includes one or more reduced-thickness recesses 970a at areas in which the top cap overshock bumpers 720 come in contact with the metal electrode pattern 910. The reduced-thickness recesses 970a in the metal electrode pattern 910 are preferably designed to reduce stiction between the top cap overshock bumpers 720 and the metal electrode pattern 910. The reduced-thickness recesses 970a may be formed using any suitable method for forming reduced-thickness recesses in the metal electrode pattern 910. In a preferred embodiment, the reduced-thickness recesses 970a are formed by removing the gold layer from the metal electrode pattern 910 to expose the underlying titanium layer. The reduced-thickness recesses 970a may have any shape suitable for reducing stiction within the accelerometer 305. In a preferred embodiment, the reduced-thickness recesses 970a are wider than the width w1 of the top cap overshock bumpers 720, and are located on the metal electrode pattern 910 at areas in which the top cap overshock bumpers 720 come in contact with the metal electrode pattern 910. The reduced-thickness recesses 970a in the metal electrode pattern 910 preferably reduce stiction between the top cap overshock bumpers 720 and the metal electrode pattern 910 by reducing the amount of imprinting in the metal electrode pattern 910 that occurs when the top cap overshock bumpers 720 come in contact with the metal electrode pattern 910. In another preferred embodiment, as illustrated in FIG. 9ad, the metal electrode pattern 910 includes one or more cavities 980a The cavities 980a in the metal electrode pattern 910 are preferably designed to eliminate stiction between the top cap overshock bumpers 720 and the metal electrode pattern 910. The cavities 980a may be formed using any suitable method for forming cavities in the metal electrode pattern 910. In a preferred embodiment, the cavities 980a are formed by selectively removing the gold layer and the titanium layer from the metal electrode pattern 910 to expose the underlying top measurement mass half 410.
The cavities 980a may have any shape suitable for reducing stiction within the accelerometer 305. In a preferred embodiment, the cavities 980a are wider than the width w1 of the top cap overshock bumpers 720, and are located on the metal electrode pattern 910 at areas in which the top cap overshock bumpers 720 come in contact with the metal electrode pattern 910. The cavities 980a in the metal electrode pattern 910 preferably reduce stiction between the top cap overshock bumpers 720 and the metal electrode pattern 910 by eliminating imprinting in the metal electrode pattern 910 that occurs when the top cap overshock bumpers 720 come in contact with the metal electrode pattern 910. The operation of the metal electrode pattern 910 is substantially as that described in U.S. patent application Ser. No. 09/936,630, filed on Sep. 14, 2001, the disclosure of which is incorporated herein by reference.
The bond ring 920 facilitates bonding of the top measurement mass half 410 to the top cap wafer 405. The bond ring 920 may include any number of conventional commercially available materials suitable for creating a bond ring such as, for example, gold, aluminum, or silver. In a preferred embodiment, the bond ring 920 is fabricated from a combination of gold and titanium. In a preferred embodiment, the combination of gold and titanium includes a layer of gold located on top of a layer of titanium. The layer of titanium preferably improves the adhesion of the gold to silicon and silicon dioxide. The bond ring 920 is preferably located on the upper surface 411 of the top measurement mass half 410, adjacent to the inner edge of the housing 907.
The top mass contact pad 930 is preferably used to make electrical contact to the top measurement mass half 410. The top mass contact pad 930 may be located anywhere on the upper surface 411 of the housing 907. In a preferred embodiment, the top mass contact pad 930 is located on the outer edge of the upper surface 411 of the housing 907, away from the metal electrode pattern 910. The top mass contact pad 930 may be fabricated from any materials suitable for creating a contact pad such as, for example, silver, aluminum, or gold. In a preferred embodiment, the top mass contact pad 930 is made of a combination of gold and titanium. In a preferred embodiment, the combination of gold and titanium includes a layer of gold located on top of a layer of titanium. The layer of titanium preferably improves the adhesion of the gold to silicon and silicon dioxide. The top mass contact pad 930 may include any dimensions suitable for creating a contact pad. In a preferred embodiment, the top mass contact pad 930 is sufficiently large for enabling a conventional wire bond.
The groove 940 is preferably located on the lower surface 412 of the housing 907 and extends from the outer edge of the housing 907 to the inner edge of the housing 907. The groove 940 preferably forms a passage 950 when the top measurement mass half 410 is bonded to the bottom measurement mass half 415. The passage 950 is preferably used to remove air from a cavity within the accelerometer 305, creating a vacuum or a low-pressure environment within the accelerometer 305 when the accelerometer 305 is sealed within a vacuum package. The groove 940 may be shaped in any way suitable for creating a passage for venting air. In a preferred embodiment, the groove 940 is V-shaped. In a preferred embodiment, the groove 940 is designed to allow for the fluidic flow of air from within the accelerometer 305 during a vacuum pump-down. The top measurement mass half 410 may include any number of grooves 940. In a preferred embodiment, the top measurement mass half 410 includes two grooves 940. In an alternative embodiment, the top measurement mass half 410 includes one groove 940. In an alternative embodiment, the top measurement mass half 410 includes a plurality of grooves 940. In an alternative embodiment, the top measurement mass half 410 includes no groove 940. The shape of the groove 940 may be affected by any number of factors. In a preferred embodiment, the groove 940 is designed to achieve an optimal pumpdown time for air passing through the passage 950. The conductance of air through the passage 950 is preferably given by: 



  
    
      
        
          C
          =
          
            
              8
              
                3
                ⁢
                
                  π
                
              
            
            ⁢
            
              
                (
                
                  
                    2
                    ⁢
                    kT
                  
                  m
                
                )
              
              
                1
                /
                2
              
            
            ⁢
            
              (
              
                
                  A
                  2
                
                BL
              
              )
            
          
        
        ,
      
    
    
      
        (
        1
        )
      
    
  




where:

    
    
        C=the conductance of the passage 950,
        k=Boltzman's constant,
        T=absolute temperature,
        m=mass of gas atom,
        A=cross-sectional area of the passage 950,
        B=periphery of the cross-sectional area of the passage 950, and
        L=the length of the passage 950.
    
    


The dimensions of the passage 950, such as the length L, the cross-sectional area A, and the periphery B, are preferably designed to optimize the conductance of air through the passage 950. In a preferred embodiment, the optimal conductance C through the passage 950 produces an optimal pumpdown time for removing air from within the accelerometer 305. The pumpdown time is the amount of time it takes to remove enough air from within the accelerometer 305 to achieve the desired pressure within the accelerometer 305. The pumpdown time is preferably given by: 



  
    
      
        
          t
          ≈
          
            
              
                (
                
                  V
                  S
                
                )
              
              ⁡
              
                [
                
                  1
                  +
                  
                    S
                    /
                    C
                  
                
                ]
              
            
            ⁢
            
              ln
              ⁡
              
                (
                
                  
                    Pi
                    -
                    Pu
                  
                  
                    P
                    -
                    Pu
                  
                
                )
              
            
          
        
        ,
      
    
    
      
        (
        2
        )
      
    
  




where:

    
    
        t=pumpdown time,
        V=volume of the internal cavities within the accelerometer 305,
        S=speed of a vacuum pump used to remove air from the accelerometer 305,
        C=conductance of the passage 950 from equation (1),
        Pi=initial pressure within the accelerometer 305 (typically 1 atm),
        P=desired pressure within the accelerometer 305,
        Pu=(1+S/C)*Po, and
        Po=lowest pressure of the pump.
    
    


The bottom measurement mass half 415 may be fabricated from any number of conventional commercially available materials suitable for creating a measurement half. In a preferred embodiment, as illustrated in FIGS. 9a, 9ab, 9ac, 9ad, 9e, 9f, and 9g, the bottom measurement mass half 415 includes an upper surface 417, a lower surface 416, one or more springs 911, a bottom measurement mass 912, a housing 913, the metal electrode pattern 915, a bond ring 925, a bottom mass contact pad 935, and a groove 945.
The springs 911 preferably couple the bottom measurement mass 912 to the housing 913 and provide a conductive path between the bottom measurement mass 912 and the housing 913. The springs 911 may be fabricated from any number of conventional commercially available materials suitable for creating springs such as, for example, metals, quartz, polysilicon, or silicon. In a preferred embodiment, the springs 911 are made of silicon, and are micromachined out of the bottom measurement mass half 415 wafer. The springs 911 are preferably designed to maintain cross-axis rejection while providing lateral shock protection for the bottom measurement mass 912. The springs 911 are preferably linear L-shaped springs, the design of which is described in U.S. Pat. Nos. 5,652,384 and 5,777,226, the disclosures of which are incorporated herein by reference.
The bottom measurement mass 912 is used to detect measurement data. The bottom measurement mass 912 may be used in any application in which its use is suitable. In a preferred embodiment, the bottom measurement mass 912 is used in seismic applications to detect acceleration forces. The bottom measurement mass 912 is preferably coupled to the housing 913 by the springs 911. The bottom measurement mass 912 may be fabricated from any material suitable for creating a measurement mass such as, for example, silicon or quartz. In a preferred embodiment, the bottom measurement mass 912 is made of silicon, and is micromachined out of the bottom measurement mass half 415 wafer.
The housing 913 surrounds the bottom measurement mass 912 and is coupled to the bottom measurement mass 912 by the springs 911. The housing 913 may be fabricated from any material suitable for creating a housing such as, for example, quartz or silicon. In a preferred embodiment, the housing 913 is fabricated from silicon, and is micromachined out of the bottom measurement mass half 415 wafer.
The metal electrode pattern 915 is used for the time-based multiplexing of electrical signals from an external circuit. In a preferred embodiment, the metal electrode pattern 915 includes a single electrode. In a preferred embodiment, the metal electrode pattern 915 is located on the lower surface 416 of the bottom measurement mass half 415, on a surface of the bottom measurement mass 912. The metal electrode pattern 915 may include any number of conventional commercially available materials suitable for creating an electrode pattern such as, for example, silver, aluminum, or gold. In a preferred embodiment, the metal electrode pattern 915 is made of a combination of gold and titanium. In a preferred embodiment, the combination of gold and titanium includes a layer of gold located on top of a layer of titanium. The layer of titanium preferably improves the adhesion of the gold to silicon and silicon dioxide. The metal electrode pattern 915 may be of any size or shape suitable for forming an electrode pattern such as, for example, circular, square, or rectangular. The metal electrode pattern 915 is preferably identical in size and shape to the bottom capacitor electrode 805. In a preferred embodiment, the metal electrode pattern 915 is substantially equal in thickness to the bond ring 925. In a preferred embodiment, the thicknesses of the metal electrode pattern 915 and the bond ring 925 are smaller than the thickness of the bottom bond ring 807. The differences in thickness between the metal electrode pattern 915, the bond ring 925, and the bottom bond ring 807 preferably reduces stiction between the bottom cap overshock bumpers 820 and the metal electrode pattern 915 during the operation of the accelerometer 305 by reducing the imprinting of the bottom cap overshock bumpers 820 on the metal electrode pattern 915. In another preferred embodiment, as illustrated in FIG. 9ab, the metal electrode pattern 915 includes one or more patterns 960b designed to minimize stiction between the bottom cap overshock bumpers 820 and the metal electrode pattern 915 during the operation of the accelerometer 305. The patterns 960b in the metal electrode pattern 915 preferably reduce stiction between the bottom cap overshock bumpers 820 and the metal electrode pattern 915 by minimizing the surface area of the region of intimate contact between the bottom cap overshock bumpers 820 and the metal electrode pattern 915. In another preferred embodiment, as illustrated in FIG. 9ac, the metal electrode pattern 915 includes one or more reduced-thickness recesses 970b at areas in which the bottom cap overshock bumpers 820 come in contact with the metal electrode pattern 915. The reduced-thickness recesses 970b in the metal electrode pattern 915 are preferably designed to reduce stiction between the bottom cap overshock bumpers 820 and the metal electrode pattern 915. The reduced-thickness recesses 970b may be formed using any suitable method for forming reduced-thickness recesses in the metal electrode pattern 915. In a preferred embodiment, the reduced-thickness recesses 970b are formed by removing the gold layer from the metal electrode pattern 915 to expose the underlying titanium layer. The reduced-thickness recesses 970b may have any shape suitable for reducing stiction within the accelerometer 305. In a preferred embodiment, the reduced-thickness recesses 970b are wider than the width w2 of the bottom cap overshock bumpers 820, and are located on the metal electrode pattern 915 at areas in which the bottom cap overshock bumpers 820 come in contact with the metal electrode pattern 915.
The reduced-thickness recesses 970b preferably reduce stiction between the bottom cap overshock bumpers 820 and the metal electrode pattern 915 by reducing the amount of imprinting in the metal electrode pattern 915 that occurs when the bottom cap overshock bumpers 820 come in contact with the metal electrode pattern 915. In another preferred embodiment, as illustrated in FIG. 9ad, the metal electrode pattern 915 includes one or more cavities 980b. The cavities 980b in the metal electrode pattern 915 are preferably designed to eliminate stiction between the bottom cap overshock bumpers 820 and the metal electrode pattern 915. The cavities 980b may be formed using any suitable method for forming cavities in the metal electrode pattern 915. In a preferred embodiment, the cavities 980b are formed by selectively removing the gold layer and the titanium layer from the metal electrode pattern 915 to expose the underlying bottom measurement mass half 415. The cavities 980b may have any shape suitable for reducing stiction within the accelerometer 305. In a preferred embodiment, the cavities 980b are wider than the width w2 of the bottom cap overshock bumpers 820, and are located on the metal electrode pattern 915 at areas in which the bottom cap overshock bumpers 820 come in contact with the metal electrode pattern 915. The cavities 980b preferably reduce stiction between the bottom cap overshock bumpers 820 and the metal electrode pattern 915 by eliminating imprinting in the metal electrode pattern 915 that occurs when the bottom cap overshock bumpers 820 come in contact with the metal electrode pattern 915. The operation of the metal electrode pattern 915 is substantially as that described in U.S. patent application Ser. No. 09/936,630, filed on Sep. 14, 2001, the disclosure of which is incorporated herein by reference.
The bond ring 925 preferably facilitates bonding of the bottom measurement mass half 415 to the bottom cap wafer 420. The bond ring 925 may include any number of conventional commercially available materials suitable for creating a bond ring such as, for example, gold, aluminum, or silver. In a preferred embodiment, the bond ring 925 is made of a combination of gold and titanium. In a preferred embodiment, the combination of gold and titanium includes a layer of gold located on top of a layer of titanium. The layer of titanium preferably improves the adhesion of the gold to silicon and silicon dioxide. The bond ring 925 is preferably located on the lower surface 416 of the bottom measurement mass half 415, adjacent to the inner edge of the housing 913.
The bottom mass contact pad 935 is preferably used to create an electrical contact to the bottom measurement mass half 415. The bottom mass contact pad 935 may be located anywhere on the lower surface 416 of the housing 913. In a preferred embodiment, the bottom mass contact pad 935 is located on the outer edge of the lower surface 416 of the housing 913, away from the metal electrode pattern 915. The bottom mass contact pad 935 may include any number of conventional commercially available materials suitable for creating a contact pad such as, for example, aluminum, silver, or gold. In a preferred embodiment, the bottom mass contact pad 935 is made of a combination of gold and titanium. In a preferred embodiment, the combination of gold and titanium includes a layer of gold located on top of a layer of titanium. The layer of titanium preferably improves the adhesion of the gold to silicon and silicon dioxide. The bottom mass contact pad 935 may include any dimensions suitable for a contact pad. In a preferred embodiment, the bottom mass contact pad 935 is sufficiently large for enabling conventional wire bonding.
The groove 945 forms a passage 950 when the bottom measurement mass half 415 is bonded to the top measurement mass half 410. The passage 950 is preferably used to remove air from a cavity within the accelerometer 305, creating a vacuum within the accelerometer 305 when the accelerometer 305 is sealed within a vacuum package. The groove 945 may be shaped in any way suitable for creating a passage for venting air. In a preferred embodiment, the groove 945 is V-shaped. In a preferred embodiment, the groove 945 is designed to allow for the fluidic flow of air from within the accelerometer 305 during a vacuum pump down. The shape of the groove 945 is preferably substantially identical to the shape of the groove 940, as described above. The groove 945 is preferably located on the upper surface 417 of the housing 913 and extends from the outer edge of the housing 913 to the inner edge of the housing 913. The bottom measurement mass half 415 may include any number of grooves 945. In a preferred embodiment, the bottom measurement mass half 415 includes two grooves 945. In an alternative embodiment, the bottom measurement mass half 415 includes one groove 945. In an alternative embodiment, the bottom measurement mass half 415 includes a plurality of grooves 945. In an alternative embodiment, the bottom measurement mass half 415 includes no groove 945.
Referring to FIGS. 10, 11a, 11b, 11c, 11d, 11e, 11f, 11g, 11h, 11ha, 11hb, 11hc, 11hd, 11he, 11hf, 11hg, 11hh, 11hi, 11hj, 11i, 11j, 12a, 12b, 12c, and 13, a method 1000 of fabricating the accelerometer 305 will now be described. In a preferred embodiment, the method 1000 of fabricating the accelerometer 305 includes: acquiring two starting cap wafers in step 1005, shaping the two starting wafers using a cap wafer process in step 1010, acquiring two starting mass wafers in step 1020, shaping the two starting mass wafers using a mass wafer process in step 1025, bonding the wafers to form the accelerometer 305 using a bonding process in step 1035, making dicing cuts on the accelerometer 305 in step 1040, and packaging the accelerometer 305 in step 1045.
As illustrated in FIG. 11a, in step 1005 the two starting cap wafers 1105a and 1105b are fabricated. In a preferred embodiment, the two starting cap wafers 1105a and 1105b are identically sized and shaped. The starting cap wafers 1105a and 1105b may be fabricated from any number of conventional commercially available materials. In a preferred embodiment, the starting cap wafers 1105a and 1105b are made of silicon.
As illustrated in FIG. 11b, in step 1010 the two starting cap wafers 1105aand 1105b undergo a cap wafer process. In a preferred embodiment, the cap wafer process transforms the starting cap wafers 1105a and 1105b into the top cap wafer 405 and the bottom cap wafer 420, respectively. In an alternative embodiment, the cap wafer process includes a merged mask micro-machining process substantially as disclosed in one or more of the following: U.S. patent application Ser. No. 09/352,835, filed on Jul. 13, 1999, and U.S. patent application Ser. No. 09/352,025, filed on Jul. 13, 1999, the disclosures of which are incorporated herein by reference.
As illustrated in FIG. 11c, in step 1020 the two starting mass wafers 1120a and 1120b are fabricated. In a preferred embodiment, the two starting mass wafers 1120a and 1120b are identically sized and shaped. The starting mass wafers 1120a and 1120b may be fabricated from any number of conventional commercially available materials. In a preferred embodiment, the starting mass wafers 1120a and 1120b are made of silicon. In a preferred embodiment, each of the starting mass wafers 1120a and 1120b includes an etch-stop layer 1140a and 1140b, respectively. In a preferred embodiment, each of the starting mass wafers 1120a and 1120b includes an etch-masking layer 1150a and 1150b, respectively.
As illustrated in FIGS. 11id, 11e, 11f, 11g, 11h, 11ha, 11hb, 11hc, 11hd, 11he, 11hf, 11hg, 11hh, 11hi, 11hj and 11i, in step 1025 the two starting mass wafers 1120a and 1120b undergo a mass wafer process that transforms the two starting mass wafers 1120a and 1120b into the top measurement mass half 410 and the bottom measurement mass half 415, respectively. In a preferred embodiment, the mass wafer process is substantially as that described in U.S. Pat. No. 5,484,073, the disclosure of which is incorporated herein by reference. In an alternative embodiment, the mass wafer process includes a merged mask micromachining process substantially as disclosed in U.S. patent application Ser. No. 09/352,835, filed on Jul. 13, 1999, and U.S. patent application Ser. No. 09/352,025, filed on Jul. 13, 1999, the disclosures of which are incorporated herein by reference.
As illustrated in FIG. 11d, the mass wafer process of step 1025 begins by photolithigraphically patterning the etch-masking layer 1150a to create an area of exposure 1160 on the etch-masking layer 1150a. In a preferred embodiment, the etch-masking layer 1150a is photolithigraphically patterned to create the area of exposure 1160 in the shape of the top measurement mass 906, the housing 907, and the grooves 940. In a preferred embodiment, the photolithigraphically patterned area of exposure 1160 includes corner compensation structures X and Y.
In a preferred embodiment, as illustrated in FIG. 11e, an etching process is performed to shape the starting mass wafer 1120a into the top measurement mass half 410. The etching process may include any number of conventional commercially available processes suitable for etching. In a preferred embodiment, the etching process begins by removing the etch-masking layer 1150a from the starting mass wafer 1120 within the area of exposure 1160. The etch-masking layer 1150a may be removed using any suitable process for removing an etch-masking layer, such as, for example, plasma etching. In a preferred embodiment, the etch-masking layer 1150a is removed from the starting mass wafer 1120a within the area of exposure 1160 by using an etchant. In a preferred embodiment, removal of the etch-masking layer 1150a exposes the material from which the starting mass wafer 1120a is fabricated. In a preferred embodiment, the material from which the starting mass wafer 1120a is fabricated is silicon. In a preferred embodiment, the corner compensation structures X prevent the etchant from attacking and corroding convex corners within the area of exposure 1160. The corner structures Y preferably allow the grooves 940 to be simultaneously formed during the etching process used to define the measurement mass 906 and the housing 907. In a preferred embodiment, the corner compensation structures Y reduce etchant-induced corner erosion at an intersection between the grooves 940 and the area of exposure 1160.
In a preferred embodiment, a wet etching chemical is then applied to the exposed silicon on the starting mass wafer 1120a. The wet etching chemical may be any number of conventional commercially available wet etching chemicals suitable for etching silicon. In a preferred embodiment, the wet etching chemical is potassium hydroxide (KOH). The KOH preferably controllably etches through the silicon and terminates at the etch-stop layer 1140a of the starting mass wafer 1120a. In a preferred embodiment, as illustrated in FIG. 11f, the KOH etches the starting mass wafer 1120a into the shape of the top measurement mass 406, the housing 407, and the groove 940. In a preferred embodiment, the etch-stop layer 1140a remains on the backside surface of the springs 905 after the wet chemical etching process has been completed. In an alternative embodiment, the etch-stop layer 1140a is removed from the springs 905 during the wet chemical etching process.
Following the wet etching process, the remaining etch-masking layer 1150a on the starting mass wafer 1120a is removed from the starting mass wafer 1120a using a standard wet etchant.
An identical etching process is preferably used on the second starting mass wafer 1120b to shape the second starting mass wafer 1120b into the bottom measurement mass half 415.
In a preferred embodiment, as illustrated in FIG. 11g, the top measurement mass half 410 and the bottom measurement mass half 415 are bonded together to form a mass wafer pair 1130. The wafer bonding process may be any number of bonding processes suitable for bonding the top measurement mass half 410 and the bottom measurement mass half 415. In a preferred embodiment, the wafer bonding process is a fusion bonding process. In a preferred embodiment, the groove 940 in the top measurement mass half 410 is aligned with the groove 945 in the bottom measurement mass half 415 during the wafer bonding process to form the passage 950.
In a preferred embodiment, a metal layer 1142 is deposited onto the upper surface of the mass wafer pair 1150, which corresponds to the upper surface 411 of the top measurement mass half 410. Additionally, a metal layer 1143 is deposited onto the lower surface of the mass wafer pair 1130, which corresponds to the lower surface 416 of the bottom measurement mass half 415. The metal layers 1142 and 1143 may include any number of conventional commercially available materials suitable for creating a metal layer such as, for example, aluminum, silver, or gold. In a preferred embodiment, the metal layers 1142 and 1143 are fabricated from a combination of gold and titanium. In a preferred embodiment, the combination of gold and titanium includes a layer of gold located on top of a layer of titanium. The layer of titanium preferably improves the adhesion of the gold to silicon and silicon dioxide. The metal layers 1142 and 1143 are preferably patterned using an etch-masking layer. The etch-masking layer may be any etch-masking layer suitable for patterning metal layers. In a preferred embodiment, the etch-masking layer is photoresist. The metal layers 1142 and 1143 may be shaped into any pattern suitable for use within the accelerometer 305. In a preferred embodiment, as illustrated in FIG. 11h, the metal layer 1142 on the upper surface of the mass wafer pair 1130 is shaped to form the metal electrode pattern 910, the bond ring 920, and the top mass contact pad 930. In a preferred embodiment, as illustrated in FIG. 11h, the metal layer 1143 on the lower surface of the mass wafer pair 1130 is shaped to form the metal electrode pattern 915, the bond ring 925, and the bottom mass contact pad 935.
In a preferred embodiment, as illustrated in FIG. 11ha, the metal electrode pattern 910 includes a pattern 960a designed to reduce stiction between the metal electrode pattern 910 and the top cap overshock bumpers 720 during the operation of the accelerometer 305. In a preferred embodiment, as illustrated in FIG. 11hb, the metal electrode pattern 915 includes a pattern 960b designed to reduce stiction between the metal electrode pattern 915 and the bottom cap overshock bumpers 820 during the operation of the accelerometer 305. The patterns 960a and 960b may be created on the metal electrode patterns 910 and 915 using any number of methods suitable for creating patterns on the metal electrode patterns 910 and 915. In a preferred embodiment, as illustrated in FIG. 11ha, the pattern 960a is created by etching a pattern into the upper surface 411 of the top measurement mass half 410 to create a patterned surface 1165a, and depositing the metal layer 1142 onto the patterned surface 1165a. The metal layer 1142 preferably molds into the metal electrode 910 including the pattern 960a. In a preferred embodiment, as illustrated in FIG. 11hb, the pattern 960b is created by etching a pattern into the lower surface 416 of the bottom measurement mass half 415 to create a patterned surface 1165b, and depositing the metal layer 1143 onto the patterned surface 1165b. The metal layer 1143 preferably molds into the metal electrode 915 including the pattern 960b. The patterned surface 1165a etched into the upper surface 411 of the top measurement mass half 410 and the patterned surface 1165b etched into the lower surface 416 of the bottom measurement mass half 415 may include any number of patterns suitable for reducing the stiction between the metal electrode patterns 910 and 915 and the overshock protection bumpers 720 and 820, respectively. In a preferred embodiment, as illustrated in FIGS. 11hc and 11hf, the patterned surfaces 1165a and 1165b include a plurality of geometrically arranged squares. In another preferred embodiment, as illustrated in FIGS. 11hd and 11hg, the patterned surfaces 1165a and 1165b include a plurality of geometrically arranged circles. In another preferred embodiment, as illustrated in FIG. 11he, the patterned surfaces 1165a and 1165b include a series of concentric circles. In another preferred embodiment, as illustrated in FIG. 11hh, the patterned surfaces 1165a and 1165b include a series of geometrically arranged rectangles. In another preferred embodiment, as illustrated in FIGS. 11hi and 11hj, the patterned surfaces 1165a and 1165b include a series of geometrically arranged pie-shaped segments.,
In a preferred embodiment, as illustrated in FIG. 11i, the springs 905 are formed to couple the top measurement mass 906 to the housing 907, and the springs 911 are formed to couple the bottom measurement mass 912 to the housing 913. The springs 905 and 911 may be formed using any number of conventional commercially available methods suitable for creating spring members in the mass wafer pair 1130. In a preferred embodiment, the springs 905 and 911 are formed using a DRIE plasma etching technique. In a preferred embodiment, the etching technique is substantially as that described in U.S. Pat. No. 5,484,073, the disclosure of which is incorporated herein by reference. The springs 905 and 911 are preferably linear L-shaped springs, the design of which is described in U.S. Pat. Nos. 5,652,384 and 5,777,226, the disclosures of which are incorporated herein by reference. The springs 905 and 911 are preferably designed to maintain cross-axis rejection while providing lateral shock protection for the top measurement mass 906 and the bottom measurement mass 911, respectively. In a preferred embodiment, the etch-stop layers 1140a and 1140b remain on backside surfaces of the springs 905 and 911, respectively, after the plasma etching process has been completed. The etch-stop layers 1140a and 1140b on the springs 905 and 911 preferably improve the uniformity of the thickness of the springs 905 and 911. In addition, the etch-stop layers 1140a and 1140b on the springs 905 and 911 preferably improve the dimensional control of the springs 905 during the operation of the accelerometer 305. In another preferred embodiment, the etch-stop layers 1140a and 1140b are removed from the springs 905 and 911, respectively, during the plasma etching process.
As illustrated in FIG. 11j, in step 1035 the top cap wafer 405, the bottom cap wafer 420, and the mass wafer pair 1130 preferably undergo a bonding process to form the accelerometer 305. The bonding process of step 1035 may be any number of bonding processes such as, for example, fusion bonding, thermocompression, eutectic bonding, anodic bonding, or glass frit bonding. In a preferred embodiment, the bonding process of step 1035 is a thermocompression bonding process.
During the bonding process of step 1035, the top cap wafer 405 is bonded to the upper surface of the mass wafer pair 1130, which corresponds to the upper surface 411 of the top measurement mass half 410. In a preferred embodiment, the top bond ring 707 bonds with the bond ring 920, coupling the top cap wafer 405 and the top measurement mass half 410. The top bond ring 707 and the bond ring 920 are preferably bonded using the thermocompression bonding process.
The top bond oxide ring 710 preferably extends below the bottom surface 408 of the top cap wafer body 406. As a result, the bonding process preferably creates a narrow capacitor electrode gap between the top capacitor electrode 705 and the metal electrode pattern 910. During the bonding process, bond forces are preferably applied to the upper surface 407 of the top cap wafer 405, away from the top cap press frame recess 725. In a preferred embodiment, the top cap press frame recess 725 is positioned on the upper surface 407 of the top cap wafer 405 in a location that ensures that bond forces applied during the bonding process are localized to the bond ring regions and away from the narrow capacitor electrode gap region.
Also during the bonding process of step 1035, the bottom cap wafer 420 is bonded to the lower surface of the mass wafer pair 1130, which corresponds to the lower surface 416 of the bottom measurement mass half 415. In a preferred embodiment, the bottom bond ring 807 bonds with the bond ring 925, coupling the bottom cap wafer 420 and the bottom measurement mass half 415. The bottom bond ring 807 and the bond ring 925 are preferably bonded using the thermocompression bonding process.
The bottom bond oxide ring 810 preferably extends above the upper surface 423 of the bottom cap wafer body 421. As a result, the bonding process preferably creates a narrow capacitor electrode gap between the bottom capacitor electrode 805 and the metal electrode pattern 915. During the bonding process, bond forces are preferably applied to the bottom surface 422 of the bottom cap wafer 420, away from bottom cap press frame recess 825. In a preferred embodiment, the bottom cap press frame recess 825 is positioned on the bottom surface 422 of the bottom cap wafer 420 in a location that ensures that bond forces applied during the bonding process are localized to the bond ring regions and away from the narrow capacitor electrode gap region.
As illustrated in FIGS. 12a, 12b, and 12c, in step 1040 the accelerometer 305 undergoes a dicing process. Dicing cuts 1205, 1210, 1215, 1220 are preferably made at predetermined locations on the accelerometer 305. The dicing cuts 1205, 1210, 1215, 1220 serve a variety of purposes. In a preferred embodiment, the dicing cuts 1205, 1215, 1220 are made to separate the accelerometer 305 die from a wafer 1235, expose electrical leads from the electrodes 910 and 915, separate the electrical leads, and expose the passage 950. In another preferred embodiment, the dicing cut 1210 is made in addition to the dicing cuts 1205, 1215, 1220 to separate the accelerometer 305 die from the wafer 1235, expose electrical leads from the electrodes 910 and 915, separate the electrical leads, and expose the passage 950.
In a preferred embodiment, a cut 1205 is made on the top cap wafer 405. The cut 1205 preferably extends vertically through the top cap wafer body 406, resulting in the removal of a section of the top cap wafer body 406. In a preferred embodiment, the cut 1205 exposes the top mass contact pad 930. The cut 1205 may be performed using any number of conventional commercially available methods of performing a dicing cut such as, for example, using a diamond blade wafer saw. In a preferred embodiment, the cut 1205 is made by using a diamond blade wafer saw.
In a preferred embodiment, a cut 1215 is made extending vertically through the top cap wafer body 406 and into the housing 907 of the top measurement mass half 410. The cut 1215 is preferably stopped within the housing 907 before the cut 1215 reaches the passage 950. The cut 1215 may be stopped any distance before reaching the passage 950. In a preferred embodiment, the cut 1215 is stopped more than about 2 mils from the passage 950. The cut 1215 may be performed using any number of conventional commercially available methods of performing a dicing cut such as, for example, using a diamond blade wafer saw. In a preferred embodiment, the cut 1215 is made by using a diamond blade wafer saw.
In a preferred embodiment, a cut 1220 is made extending vertically through the bottom cap wafer body 421 and into the housing 913 of the bottom measurement mass half 415. The cut 1220 is preferably stopped within the housing 913 before the cut 1220 reaches the passage 950. The cut 1220 may be stopped any distance before reaching the passage 950. In a preferred embodiment, the cut 1220 is stopped more than about 2 mils from the passage 950. The cut 1220 may be performed using any number of conventional commercially available methods of performing a dicing cut such as, for example, using a diamond blade wafer saw. In a preferred embodiment, the cut 1215 is made by using a diamond blade wafer saw.
In an alternative preferred embodiment, a cut 1210 is made on the bottom cap wafer body 421. The cut 1210 preferably extends vertically through the bottom cap wafer body 421, resulting in the removal of a section of the bottom cap wafer body 421. In a preferred embodiment, the cut 1210 exposes the bottom mass contact pad 935. The cut 1210 may be performed using any number of conventional commercially available methods of performing a dicing cut such as, for example, using a diamond blade wafer saw. In a preferred embodiment, the cut 1210 is made by using a diamond blade wafer saw.
The cuts 1205, 1210, 1215, 1220 may be performed individually, or the cuts 1205, 1210, 1215, 1220 may be made in any combination to achieve the accelerometer 305 shape most suitable for a particular application. In a preferred embodiment, as illustrated in FIG. 12b, cuts 1205, 1215, and 1220 are performed on the accelerometer 305. In an alternative embodiment, cut 1210 is performed on the accelerometer 305 in addition to the cuts 1205, 1215, and 1220. Cut 1205 preferably exposes the top mass contact pad 930. Cut 1210 preferably exposes the bottom mass contact pad 935. Cuts 1215, 1220 preferably create a scribe lane 1230 surrounding the passage 950. The scribe lane 1230 is preferably attached to another die 1235.
During the dicing process, the scribe lane 1230 may remain attached to the accelerometer 305 and die 1235 to keep the accelerometer 305 hermetically sealed, or the scribe lane 1230 may be snapped to expose the passage 950 and separate the accelerometer 305 from the die 1235. In a preferred embodiment, as illustrated in FIG. 12c, the scribe lane 1230 is removed to expose the passage 950 and separate the accelerometer 305 from the die 1235. The exposed passage 950 is preferably used as a channel for removing air from within the accelerometer 305 to create a vacuum within the accelerometer 305 during packaging.
As illustrated in FIG. 13, in step 1045 the accelerometer 305 is packaged within a package 1305. The package 1305 may include any number of packages suitable for storing the accelerometer 305. In a preferred embodiment, the package 1305 is a housing. In another preferred embodiment, the package 1305 is a substrate.
The housing 1305 may be any number of housings suitable for storing the accelerometer 305. In a preferred embodiment, the housing 1305 includes a body 1310 and a lid 1315. The housing 1305 is preferably a conventional multi-layered ceramic package.
The accelerometer 305 is preferably placed within the body 1310 of the housing 1305. The accelerometer 305 may be placed within the housing 1305 using any number of methods suitable for securing the accelerometer 305 within the housing 1305. In a preferred embodiment, the accelerometer 305 is placed within the housing 1305 using a solder-die attachment process substantially as disclosed in U.S. patent application Ser. No. 09/914,421, filed on Mar. 15, 2000, the disclosure of which is incorporated herein by reference.
The lid 1315 is then preferably fastened to the body 1310 to seal the accelerometer 305 within the housing 1305. In a preferred embodiment, a vacuum process is used to remove air from the housing prior to fastening the lid 1315 to the body 1310, creating a vacuum or a low-pressure environment within the housing 1305. When the passage 950 is exposed, air is removed from within the accelerometer 305 during the vacuum process, creating a vacuum within the accelerometer 305 in the housing 1305.
In another preferred embodiment, the bonding process of step 1035 is performed in a vacuum environment, creating a vacuum within the cavity in the accelerometer 305 during the bonding process. In this embodiment, the passage 950 is preferably removed from the design of the accelerometer 305. The vacuum-sealed accelerometer 305 is then preferably placed in the housing 1305, and the housing is sealed by fastening the lid 1315 to the body 1310.
Although illustrative embodiments of the invention have been shown and described, a wide range of modification, changes and substitution is contemplated in the foregoing disclosure. In some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.

## Claims

1. A method of fabricating a sensor element, comprising:
using a first process to fabricate a measurement mass wafer for detecting acceleration, the measurement mass wafer including a mass housing having a cavity, and a spring mass assembly positioned within the cavity; 
fabricating a top cap wafer using the first process; 
fabricating a bottom cap wafer using the first process; 
bonding the top cap wafer to a side of the measurement mass wafer using a bonding process; 
bonding the bottom cap wafer to another side of the measurement mass wafer using the bonding process; and 
making one or more dicing cuts at predetermined locations on the sensor element.

2. The method of claim 1 further comprising etching a surface of the measurement mass wafer, applying a metal layer on the etched surface, and molding the metal layer to form a stiction-reducing electrode pattern.

3. The method of claim 1, wherein fabricating the measurement mass wafer further includes fabricating a passage for venting air from the cavity.

4. The method of claim 3, wherein the passage comprises a V-shaped groove.

5. The method of claim 3, wherein the dicing cuts are made through the top cap wafer and the bottom cap wafer and into the measurement mass wafer, stopping at a predetermined distance from the passage within the measurement mass wafer.

6. The method of claim 5 further comprising opening the passage after the dicing cuts are made to open an air vent to the passage.

7. The method of claim 6 further comprising a second process to expose the passage within the measurement mass wafer, wherein air is removed from the cavity through the passage to create a low pressure environment in the cavity, and wherein the passage is sealed to maintain the low pressure environment within the cavity.

8. The method of claim 6, further comprising packaging the sensor element in a sensor housing and using a vacuum process to remove substantially all air from the sensor housing during packaging to create a low pressure environment within the sensor housing; wherein air is removed from the accelerometer though the passage during the vacuum process; and wherein the sensor housing is sealed to maintain the low pressure environment.

9. The method of claim 1, wherein fabricating the top cap wafer further comprises forming a press frame recess in the top cap wafer.

10. The method of claim 1, wherein fabricating the bottom cap wafer further comprises forming a press frame recess in the bottom cap wafer.

11. The method of claim 1, wherein the dicing cuts penetrate through the top cap wafer, the bottom cap wafer, and at least partially through the measurement mass wafer.

12. The method of claim 1, wherein the top cap wafer includes a balanced metal pattern on an upper surface of the top cap wafer.

13. The method of claim 1, wherein the bottom cap wafer includes a balanced metal pattern on a lower surface of the bottom cap wafer.

14. The method of claim 1, wherein the spring-mass assembly comprises springs.

15. The method of claim 14, wherein the springs include an etch-stop layer on one or more surfaces of the springs.

16. The method of claim 1, wherein the measurement mass wafer includes one or more mass contact pads; and wherein the dicing cuts are made through the top cap wafer to expose the mass contact pads on the measurement mass wafer.

17. The method of claim 1, wherein the measurement mass wafer includes one or more mass contact pads; and wherein the dicing cuts are made through the bottom cap wafer to expose the mass contact pad on the measurement mass wafer.

18. The method of claim 1, wherein the measurement mass includes one or more mass contact pads and the dicing cuts are made:
through the top cap wafer to expose the mass contact pads on the measurement mass wafer; and 
through the bottom cap wafer to expose the mass contact pads on the measurement mass wafer.