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Linear accelerometerLinear accelerometer description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060207327, Linear accelerometer. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATION [0001] This application is related to U.S. application Ser. No. [Docket No. DP-312388] entitled "METHOD OF MAKING MICROSENSOR," filed on the same date as the present application, the entire disclosure of which is hereby incorporated herein by reference. TECHNICAL FIELD [0002] The present invention generally relates to acceleration sensors (i.e. accelerometers) and, more particularly, relates to a micro-machined capacitively coupled linear accelerometer for sensing magnitude and direction of linear acceleration. BACKGROUND OF THE INVENTION [0003] Accelerometers are commonly employed to measure the second derivative of displacement with respect to time. In particular, linear accelerometers measure linear acceleration along a particular sensing axis. Linear accelerometers are frequently employed to generate an output signal (e.g., voltage) proportional to linear acceleration for use in any of a number of vehicle control systems. For example, the sensed output from a linear accelerometer may be used to control safety-related devices on an automotive vehicle, such as front and side impact air bags. According to other examples, accelerometers may be used in automotive vehicles for vehicle dynamics control and suspension control applications. [0004] Conventional linear accelerometers often employ an inertial mass suspended from a support frame by multiple support beams. The mass, support beams and frame generally act as a spring mass system, such that the displacement of the mass is proportional to the linear acceleration applied to the frame. The displacement of the mass generates a voltage proportional to linear acceleration which, in turn, is used as a measure of the linear acceleration. [0005] One type of an accelerometer is a micro-electromechanical structure (MEMS) sensor that employs a capacitive coupling between interdigitated fixed and movable capacitive plates that are movable relative to each other in response to linear acceleration. An example of a capacitive type single-axis linear accelerometer is disclosed in U.S. Pat. No. 6,761,070, entitled "MICROFABRICATED LINEAR ACCELEROMETER," the entire disclosure of which is hereby incorporated herein by reference. An example of a capacitive type dual-axis accelerometer is disclosed in U.S. application Ser. No. 10/832,666, filed on Apr. 27, 2004, entitled "DUAL-AXIS ACCELEROMETER," the entire disclosure of which is also hereby incorporated herein by reference. [0006] Some conventional capacitive type accelerometers employ a vertical stacked structure to sense linear acceleration in the vertical direction. The stacked vertical structure typically has an inertial proof mass suspended between upper and lower fixed capacitive plates. The inertial proof mass moves upward or downward responsive to vertical acceleration. The measured change in capacitance between the proof mass and the fixed capacitive plates is indicative of the sensed acceleration. The vertical stacked structure employed in the aforementioned conventional linear accelerometer generally requires significant process complexities in the fabrication of the device using bulk and surface micro-machining techniques. As a consequence, conventional vertical sensing accelerometers typically suffer from high cost and undesired packaging sensitivity. [0007] Additionally, the manufacturing process for fabricating conventional linear accelerometers typically involves a two-sided etch fabrication process which processes both the bottom and top of the patterned wafer. Conventional two-sided process fabrication typically uses a trench etching process, such as deep reactive ion etching (DRIE) and bond-etch back process. The etching process typically includes etching a pattern from a doped material suspended over a cavity to form a conductive pattern that is partially suspended over a cavity. The conventional etching processes typically require etching the patterned wafer from both the top and bottom sides. One example of a conventional etching approach is disclosed in U.S. Pat. No. 6,428,713, issued on Aug. 6, 2002, entitled "MEMS SENSOR STRUCTURE AND MICROFABRICATION PROCESS THEREFOR," which is hereby incorporated herein by reference. Another example of an accelerometer fabrication process is disclosed in U.S. Pat. No. 5,006,487, entitled "METHOD OF MAKING AN ELECTROSTATIC SILICON ACCELEROMETER," the entire disclosure of which is also hereby incorporated herein by reference. [0008] The conventional two-sided fabrication process generally requires additional equipment to pattern and etch the top and bottom sides of two wafers and to achieve proper alignment and bonding of the two wafers. This equipment adds to the costs of the device. Additionally, since the patterned top and bottom wafers are aligned and bonded together, the device may suffer from misalignment and bond degradation. [0009] Accordingly, it is therefore desirable to provide for a linear accelerometer and method of manufacturing a micro-machine microsensor that does not suffer undesired packaging sensitivity and other drawbacks of prior known sensors. In particular, it is desirable to provide for a cost-effective linear accelerometer that may sense vertical acceleration including both magnitude and direction of acceleration. It is further desirable to provide for a method of manufacturing a microsensor, such as a vertical linear accelerometer, that does not suffer from the above-described drawbacks of the prior known microsensor fabrication techniques. SUMMARY OF THE INVENTION [0010] In accordance with the teachings of the present invention, a linear accelerometer is provided. The accelerometer includes a support substrate, a first fixed electrode having one or more first fixed capacitive plates having a first height, and a second fixed electrode having one or more second fixed capacitive plates having a second height. The accelerometer also has a movable inertial mass including one or more first movable capacitive plates capacitively coupled to the first fixed capacitive plates and one or more second movable capacitive plates capacitively coupled to the second fixed capacitive plates. The first movable capacitive plates have a third height greater than the first height of the first fixed capacitive plates, and the second movable capacitive plates have a fourth height less than the second height of the second fixed capacitive plates. The accelerometer further includes a support structure for supporting the movable inertial mass and allowing linear movement of the inertial mass upon experiencing a linear acceleration along a sensing axis. The accelerometer has an input for providing input signals to one of the fixed and movable capacitive plates, and an output for providing an output signal from the other of the fixed and movable capacitive plates which varies as a function of the capacitive coupling and is indicative of magnitude and direction of linear acceleration along the sensing axis. [0011] By employing fixed and movable capacitive plates arranged to provide capacitive coupling with a height variation between opposing fixed and movable capacitive plates, the linear accelerometer measures a signal indicative of both the magnitude and the direction of acceleration. The accelerometer is particularly well-suited to measure vertical acceleration, according to one embodiment. [0012] These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. BRIEF DESCRIPTION OF THE DRAWINGS [0013] The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: [0014] FIG. 1 is a top view of a linear accelerometer shown with the overlying cover removed according to one embodiment of the present invention; [0015] FIG. 2 is a partial cut away sectional view of the accelerometer taken through lines II-II of FIG. 1; [0016] FIG. 3 is a partial cut away sectional view of the accelerometer taken through lines III-III of FIG. 1; [0017] FIG. 4 is an enlarged view of section IV of FIG. 1; [0018] FIGS. 5A-5C are cross-sectional views taken through lines V-V of FIG. 4 illustrating the fixed and movable capacitive plates subjected to no vertical acceleration in FIG. 5A, downward acceleration in FIG. 5B, and upward acceleration in FIG. 5C; [0019] FIGS. 6A-6C are cross-sectional views taken through lines VI-VI of FIG. 4 illustrating the fixed and movable capacitive plates subjected to no acceleration in FIG. 6A, downward acceleration in FIG. 6B, and upward acceleration in FIG. 6C; Continue reading about Linear accelerometer... 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