Measuring and testing – Speed – velocity – or acceleration – Acceleration determination utilizing inertial element
Reexamination Certificate
1999-11-02
2001-07-10
Kwok, Helen (Department: 2856)
Measuring and testing
Speed, velocity, or acceleration
Acceleration determination utilizing inertial element
Reexamination Certificate
active
06257060
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to accelerometers, and in particular to structures for mounting the same, whereby external stress sources are isolated from active accelerometer components.
BACKGROUND OF THE INVENTION
Accelerometers generally measure acceleration forces applied to a body. Accelerometers are typically mounted directly onto a surface of the accelerated body. Such direct mounting ensures the immediate detection of even subtle forces exerted on the body. The directly mounted accelerometer is, however, also exposed to various extraneous shock, vibration and thermal stresses experienced by the accelerated body. The accelerometer measures the forces induced by such external stresses in combination with the applied acceleration forces and renders confused and inaccurate acceleration measurements. Generally, isolation mechanisms between the accelerometer and the accelerated body, typically integral to the accelerometer housing, protect the accelerometer from forces induced by stresses within the accelerated body.
While stresses experienced by the accelerated body are isolated from the accelerometer, sensitive accelerometers can suffer from error sources caused by subtle forces induced by stresses internal to the accelerometer but external to the acceleration sensing mechanism. In monolithic micro-machined accelerometers having vibrating beam force detectors suspended between a movable proof mass and an accelerometer frame, such forces are caused by, for example, bonding stresses between a silicon cover plate and the sensor frame or other assembly stresses. Other such forces are caused by, for example, thermal stresses resulting from a mismatch of thermal expansion coefficients between materials within the sensor. External thermal stresses may be induced by the typical mechanical coupling of the sensor frame to the silicon cover plate and by the mechanical coupling of the silicon cover plate to a ceramic or metal mounting plate. Because the cover and mounting plates are typically fabricated from different materials, they usually have substantially different coefficients of thermal expansion. When heated, the mismatch in thermal expansion coefficients generally causes undesirable stresses which induce distortion and strain in the sensor frame.
Bias performance and stability of monolithic silicon based accelerometers is based on proof mass sizing, commonly referred to as pendulousity, and on the degree of stress isolation in the mechanical die stack. Monolithic micro-machined vibrating beam accelerometers are typically targeted for small size which limits the proof mass size and generally requires special care in providing isolation from external stresses. Historically, the accelerometer frame is suspended from a second outer frame by flexures that permit the accelerometer frame to move relative to the outer frame, as shown and described in allowed U.S. patent application Ser. No. 08/735,299. Such isolation structure designs as have been possible using a potassium hydroxide (KOH) etching solution in a bulk process to cost effectively fabricate monolithic micro-machined vibrating beam accelerometers effectively minimize the distortion of the accelerometer frame and decrease the effects of the thermal coefficient mismatch. However, the orientation of the natural etch planes in silicon at 54.7 degrees from horizontal requires relatively large amounts of physical space when using a KOH etching solution, thus limiting both the pendulousity, i.e., possible proof mass size, and the possible isolation structure designs and requiring major compromises and trade-offs in proof mass sizing and isolation structure design in very small applications.
Furthermore, although some monolithic micro-machined vibrating beam accelerometers have included isolation structure in the one cover plate by which the sensor mechanism is mounted to the ceramic or metal mounting plate, to date, no monolithic micro-machined vibrating beam accelerometers have included isolation structure in the both cover plates for reducing or eliminating residual stresses caused by die bowing. Additionally, to date, none has provided a large centralized mounting area surrounded with a self-caging structure for surviving high shock loads.
SUMMARY OF THE INVENTION
The present invention overcomes the accelerometer and proof mass sizing constraints of the prior art by providing an acceleration sensor formed of a monocrystalline silicon substrate having essentially parallel opposing surfaces using a bulk straight wall deep reaction ion etching (DRIE) process. The acceleration sensor includes an outer frame member with an acceleration sensing mechanism disposed within the outer frame member and suspended therefrom by multiple flexures for de-coupling the sensitive elements of the acceleration sensing mechanism from stresses in the outer frame member. The flexures are formed with essentially parallel opposing walls extending between the opposing surfaces of the substrate and essentially perpendicular thereto. The opposing walls of the flexures are disposed in a self-caging relationship to each of the outer frame and the accelerometer frame member to protect the acceleration sensing mechanism from external shocks, especially lateral shocks input perpendicular to the accelerometer's input axis.
According to one aspect of the invention, the acceleration sensing mechanism is generally enclosed between two cover plates, bottom cover plate and a top cover plate, each also formed of monocrystalline silicon substrates having essentially parallel opposing surfaces using a bulk straight wall deep reaction ion etching (DRIE) process. Each of the top and bottom plates is formed with an outer frame portion suspended from an inner cover portion corresponding to the acceleration sensing mechanism. An accelerometer die stack is formed by mounting the acceleration sensor between the top and bottom cover plates. The outer frame portions of the top and bottom cover plates support, and are preferably adhesively attached to, the outer frame member surrounding the acceleration sensing mechanism, whereby the inner cover portions of the top and bottom cover plates serve, among other functions, to protect the sensitive components of the acceleration sensing mechanism from external shocks, especially shocks input along the accelerometer's input axis.
According to another aspect of the invention, the flexures suspending the outer frame portions of the top and bottom cover plates from the respective inner cover portions are formed in a self-caging relationship with each of the outer frame portions and the inner cover portions. The flexures are formed by very narrow overlapping slots cut using a bulk DRIE process and formed with essentially straight walls extending between and perpendicular to the opposing surfaces of the cover plate substrate. The walls of the resulting flexures and the opposing walls of the outer frame members and inner cover portions are similarly essentially straight and perpendicular to the opposing surfaces of the substrate. The flexures de-couple the outer frame portions from shocks and stresses experienced by the inner cover portions. The narrow slots and essentially parallel opposing relationship between the surfaces of the flexures and the surfaces of the outer frame portions and the inner cover portions restrict the lateral motions of the outer frame portions relative to the inner cover portions, such that a self-caging relationship is developed. Thus, when the accelerometers die stack is mounted to a mounting plate by an adhesive bond between the inner cover portion of either the top or bottom cover plate, the flexures in the cover plates further de-couple the acceleration sensor from externally induced stresses and protect the mechanism from later shock loads. Such de-coupling further allows the bond between the inner cover portion and the mounting plate to involve essentially all of the area of the inner cover portion.
According to yet another aspect of the invention, the top and bottom cover pla
Blake Graeme A.
Leonardson Ronald B.
Allied-Signal Inc.
Kwok Helen
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