Net zero isolator

Details Net zero isolator Net zero isolator

2001-10-31

2003-12-16

Chapman, John E. (Department: 2856)

Measuring and testing

Speed, velocity, or acceleration

Structural installation or mounting means

C073S514290, C248S604000

Reexamination Certificate

active

06662655

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 are typically integrated into the accelerometer housing to protect the accelerometer from forces induced by stresses within the accelerated body.
Additionally, 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, mounting stresses between a silicon cover plate and the sensor frame or other assembly stresses. Other such stresses include, 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. Since the cover and mounting plates are typically fabricated of materials different from the sensor frame, they usually have substantially different coefficients of thermal expansion. When operated at elevated temperatures, 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, now U.S. Pat. No. 5,948,981 to Woodruff entitled,
VIBRATING BEAM ACCELEROMETER
, issued Sep. 7, 1999. 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 57.4 degrees from vertical using a KOH etching solution requires relatively large amounts of physical space, 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.
In prior art devices, the flexures that suspend the accelerometer frame from the second outer frame are commonly compliant beam or spring isolators. These compliant beam or spring isolators are used to reduce the stresses caused by mounting displacements to a small value. These isolators obey a simple spring equation, given by:
Force (
F
)=spring constant(
k
)*displacement(
d
).
Thus, for a given mounting displacement, the force applied to the sensor is reduced through the isolator spring constant, which is designed to be as low as possible. The resulting strain in the sensor is thus reduced through the spring constant of the isolator.
A typical example of such compliant beam or spring isolators is found in the twin beam suspension system illustrated in FIG.
1
. The accelerometer illustrated in
FIG. 1
has a conventional isolation structure formed of compliant beam or spring isolators embodied as flexures. In
FIG. 1
the accelerometer
10
is a miniature structure fabricated from a substrate
12
of semiconductor material by conventional micromachining techniques. The substrate
12
is formed of a monocrystalline silicon material. The silicon substrate
12
often includes an upper silicon or active layer
14
that is electrically isolated from an underlying substrate
16
by an insulating layer
18
or an insulating layer is applied to active layer
14
, as shown and described in incorporated U.S. Pat. No. 5,948,981. The insulating layer
18
is may be a thin layer, e.g., about 0.1 to 10.0 micrometers, of oxide, such as silicon oxide. The silicon substrate
12
is usually formed by oxidizing active layer
14
and underlying substrate
16
, and adhering the two layers together. A portion of active layer
14
may be removed to bring the layer
14
to the desired thickness. The silicon oxide layer
18
retains its insulating properties over a wide temperature range to ensure effective mechanical resonator performance at high operating temperatures on the order of 100 degrees Celsius. In addition, the insulating layer
18
inhibits undesirable etching of the active layer
14
during etching.
The accelerometer
10
includes an acceleration sensor mechanism
20
having one or more flexures
22
pliantly suspending a proof mass
24
from a sensor frame or plate
26
for movement of the proof mass
24
along an input axis I normal to the proof mass
24
. The flexures
22
are preferably etched near or at the center of the underlying substrate
16
, i.e., substantially centered between the opposing upper and lower surfaces of the underlying substrate
16
. Optionally, the flexures
22
are formed by anistropically etching in a suitable etchant, such as potassium hydroxide (KOH). The flexures
22
define a hinge axis H about which the proof mass
24
moves in response to an applied force, such as the acceleration of the accelerated body, for example, a vehicle, aircraft or other moving body having the accelerometer
10
mounted thereon. The sensor mechanism
20
includes a pair of mechanical resonators
28
formed from the active silicon layer
14
and coupled between the proof mass
24
and the sensor plate
26
for measuring forces applied to the proof mass
24
. An oscillator circuit (not shown) drives the mechanical resonators
28
at their resonance frequency. In response to an applied force, the proof mass
24
rotates about the hinge axis H, causing axial forces, either compressive or tensile, to be applied to the mechanical resonators
28
. The axial forces change the frequency of vibration of the mechanical resonators
28
, and the magnitude of this change serves as a measure of the applied force or acceleration.
External stresses and strains may be induced in the sensitive acceleration sensor mechanism
20
by, for example, the typical mechanical coupling of the accelerometer sensor plate
26
to a silicon cover plate
30
. The silicon cover plate
30
is in turn typically connected to a ceramic or metal mounting plate
32
. Since the mounting
32
and cover plates
30
are fabricated from different materials, they will usually have substantially different coefficients of thermal expansion when cooled or heated during operation. This mismatch in thermal coefficients may cause undesirable stresses and strains at the interface of the inner

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