Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
1998-05-05
2001-03-06
Williams, Hezron (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
Reexamination Certificate
active
06196067
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of micromachined accelerometers and seismometers, and in particular to accelerometers and seismometers which can withstand high shock loads.
2. Description of the Prior Art
The measurement of the vibration of celestial bodies is one of the primary methods for determining their mechanical structure. At the planetary level, seismology has already successfully elucidated the internal structure of the Earth and moon. A long-running goal of planetary exploration has been similarly to determine the internal structure of Mars. However, an unambiguous seismic survey of the global structure of the planet requires a long-lived network distributed across the planet. The associated costs of deploying and running conventional seismometers precludes their use in any kind of Martian application.
Seismometers can also determine structure at a more local level and are used on Earth for surveying petroleum reservoirs. Proposals also exist for local seismic surveys of buried liquid water deposits on Mars. For small objects, determination of the vibration spectrum from a single sensor can be used to determine the mechanical structure of the object. Such an application is being considered to determine the relative thickness of the possible ice layer which has been observed on Europa, one of the major moons of Jupiter. A seismic investigation of a comet has been also planned under the name of the Rosetta Mission, which is projected to involve the first landing on the comet.
Common to all these applications is the need for a small, robust and low-powered seismometer with performance comparable to presently available terrestrial seismometers, namely sensitivity to signals below 1 ng/{square root over (Hz)}. In order to meet this sensitivity, terrestrial seismometers have low-resonant-frequency suspensions, and this is resulted in bulky, massive instruments which are extremely delicate. Raising the resonant frequency of this seismometer allows a more compact instrument, but is achieved only at the expense of sensitivity.
BRIEF SUMMARY OF THE INVENTION
The invention is a differential capacitive sensor. The sensor comprises a proof mass micromachined in a first silicon substrate. A frame is micromachined in a first silicon substrate to be adjacent to the proof mass. At least one peripheral spring is connected between the frame and the proof mass to suspend the proof mass in the frame substantially without tilt. At least one movable capacitive plate is defined on the proof mass. At least one fixed capacitive plate is defined on at least one separate substrate. The fixed capacitive plate is separately fabricated on the separate substrate. The separate substrate is assembled on the frame and on at least one side of the proof mass thereby opposing the proof mass. The fixed capacitor plate on the substrate is positioned at a predetermined gap distance away from the movable plate. The fixed capacitive plate is disposed on the separate substrate prior to assembly of the separate substrates opposing the proof mass and frame. As a result, the gap distance between the movable and fixed capacitive plates may be closely controlled.
In a first embodiment the spring is coupled between the proof mass and the frame at a plurality of peripherally symmetric positions. In addition in the illustrated embodiment two substrates are employed opposing each side of the proof mass and frame. The proof mass and frame are sandwiched between the pair of substrates. Two fixed capacitive plates are employed. One of the two fixed capacitive plates is disposed on each of the two substrates. The sensor lies substantially in a plane and the proof mass has a six-fold symmetry about a center axis perpendicular to the plane. There are six springs coupled between the frame and proof mass in a six-fold symmetric array.
In a second embodiment the proof mass is a cylindrical disk and the spring is one or more continuous membranes extending between the frame and the proof mass. The proof mass has an upper and lower surface. The spring comprises in particular two continuous membranes extending across the upper and lower surface of the proof mass and between the proof mass and the frame. The proof mass has a mass of at least one gram and the spring provides the sensor with a resonant frequency of less than 100 Hz. More specifically, the resonant frequency is approximately 10 Hz.
Where the proof mass is a circular disk having an upper and lower surface, the frame comprises an angular ring with the peripheral spring comprising an upper and lower continuous membrane extending across the upper and lower surface of the proof mass between the angular ring and the circular disk-shaped proof mass.
The substrate has a cavity defined therein with a predetermined depth such that when the substrate is bonded to the frame. The cavity opposes the proof mass and the gap between the movable plate and the fixed plate is defined only by the depth of the cavity in the fixed substrate. The membranes are formed on separate wafers from the proof mass and the membranes are wafer-bonded to the frame and proof mass.
The invention is also defined as a method of fabricating a differential capacitive sensor comprising the steps of defining or micromachining a suspended proof mass assembly in a first substrate, and micromachining a cavity of predetermined depth in each of a second and third substrates separate from the first substrate. Capacitive plates are provided within the defined cavities in the second and third substrate and on the suspended proof mass assembly. The second and third substrates are assembled onto the first substrate so that the capacitive plates on the second and third substrates oppose the capacitive plate on the suspended proof mass assembly and are separated therefrom by predetermined distance equal to the predetermined depth of the cavity defined in the second and third substrates.
In the step of defining or micromachining the fixed capacitive plates on the second and third substrates, the cavity is etched or defined into the second and third substrates and the capacitive plates provided therein. The gap in the assembled sensor is defined by depth of the cavities defined in the second and third substrates.
In one embodiment the step of defining or micromachining the suspended proof mass assembly comprises forming a spacing layer on the proof mass assembly to define the gap between the capacitive plates when the second and third substrates are bonded to the spacing layer.
The step of defining or micromachining the suspended proof mass assembly comprises micromachining the proof mass, frame and symmetrically extending springs between the frame and proof mass from the first substrate or a peripherally extending membrane formed in a separate wafer. Where the peripherally extending spring is a continuous membrane, it extends between the frame and proof mass at all peripheral points. In the illustrated embodiment two continuous membranes are formed from separate wafers and bonded to each side of the proof mass.
A specifically illustrated embodiment of the invention is a differential capacity of sensor comprising an angular ring-shaped frame. A circular cylindrical proof mass of at least one gram is disposed within the angular ring-shaped frame. A pair of continuous membranes is disposed between the frame and the proof mass. The proof mass is coupled to the pair of continuous membranes, and opposing upper and lower surfaces of the proof mass are coupled to the membranes at a central location of the proof mass. Movable capacitive plates disposed on the membranes. An upper and lower substrate having fixed capacitive plates is disposed within cavities defined therein. The fixed capacitive plates oppose the movable capacitive plates on the membranes. As a result, a robust, compact sensor is provided with high shock resistance and small electrode spacing for increased sensitivity.
The invention, now having been briefly summarized, is illustrated in the following drawings, w
Martin Richard D.
Pike W. Thomas
California Institute of Technology
Dawes Daniel L.
Moller Richard A.
Williams Hezron
LandOfFree
Silicon micromachined accelerometer/seismometer and method... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Silicon micromachined accelerometer/seismometer and method..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Silicon micromachined accelerometer/seismometer and method... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2480186