U.S. patent application number 11/081427 was filed with the patent office on 2006-09-21 for linear accelerometer.
Invention is credited to John C. Christenson, Seyed R. Zarabadi.
Application Number | 20060207327 11/081427 |
Document ID | / |
Family ID | 36427422 |
Filed Date | 2006-09-21 |
United States Patent
Application |
20060207327 |
Kind Code |
A1 |
Zarabadi; Seyed R. ; et
al. |
September 21, 2006 |
Linear accelerometer
Abstract
A linear accelerometer is provided having a support substrate,
fixed electrodes having fixed capacitive plates, and a movable
inertial mass having movable capacitive plates capacitively coupled
to the fixed capacitive plates. Adjacent capacitive plates vary in
height. The accelerometer further includes support tethers for
supporting the inertial mass and allowing movement of the inertial
mass upon experiencing a linear acceleration along a sensing axis.
The accelerometer has inputs and an output for providing an output
signal which varies as a function of the capacitive coupling and is
indicative of both magnitude and direction of vertical acceleration
along the sensing Z-axis. A microsensor fabrication process is also
provided which employs a top side mask and etch module.
Inventors: |
Zarabadi; Seyed R.; (Kokomo,
IN) ; Christenson; John C.; (Kokomo, IN) |
Correspondence
Address: |
DELPHI TECHNOLOGIES, INC.
M/C 480-410-202
PO BOX 5052
TROY
MI
48007
US
|
Family ID: |
36427422 |
Appl. No.: |
11/081427 |
Filed: |
March 16, 2005 |
Current U.S.
Class: |
73/514.32 |
Current CPC
Class: |
G01P 2015/082 20130101;
G01P 15/18 20130101; G01P 15/125 20130101 |
Class at
Publication: |
073/514.32 |
International
Class: |
G01P 15/125 20060101
G01P015/125 |
Claims
1. A linear accelerometer comprising: a support substrate; a first
fixed electrode comprising one or more first fixed capacitive
plates fixed to the support structure and having a first height; a
second fixed electrode comprising one or more second fixed
capacitive plates fixed to the support structure and having a
second height; a movable inertial mass comprising one or more first
movable capacitive plates capacitively coupled to the one or more
first fixed capacitive plates and one or more second movable
capacitive plates capacitively coupled to the one or more second
fixed capacitive plates, wherein the one or more first movable
capacitive plates have a third height greater than the first height
of the one or more first fixed capacitive plates, and the one or
more second movable capacitive plates have a fourth height less
than the second height of the one or more second fixed capacitive
plates; a support structure for supporting the movable inertial
mass and allowing linear movement of the inertial mass relative to
the support substrate upon experiencing a linear acceleration along
a sensing axis; an input for providing input signals to one of the
first and second fixed and first and second movable capacitive
plates; and an output for receiving an output signal from the other
of the first and second fixed and first and second movable
capacitive plates which varies as a function of capacitive coupling
and is indicative of magnitude and direction of acceleration along
the sensing axis.
2. The linear accelerometer as defined in claim 1, wherein the
input comprises a first input signal applied to the first fixed
electrode and a second input signal applied to the second fixed
electrode, and wherein the output is electrically coupled to the
movable inertial mass.
3. The linear accelerometer as defined in claim 1, wherein the
first fixed capacitive plates are arranged substantially parallel
to the first movable capacitive plates, and the second fixed
capacitive plates are arranged substantially parallel to the second
movable capacitive plates.
4. The linear accelerometer as defined in claim 1, wherein the
support structure comprises a plurality of tethers.
5. The linear accelerometer as defined in claim 4, wherein the
plurality of tethers comprises four L-shaped tethers.
6. The linear accelerometer as defined in claim 1, wherein the
substrate comprises a silicon substrate.
7. The linear accelerometer as defined in claim 1, wherein the
first and second fixed capacitive plates each comprises a plurality
of capacitive plates, and wherein the first and second movable
capacitive plates each comprises a plurality of capacitive
plates.
8. The linear accelerometer as defined in claim 7 further
comprising: a third fixed electrode comprising one or more third
fixed capacitive plates; and a fourth fixed electrode comprising
one or more fourth fixed capacitive plates, wherein the movable
mass further comprises one or more third movable capacitive plates
forming a capacitive coupling with the third fixed capacitive
plates, and one or more fourth movable capacitive plates forming a
capacitive coupling with the fourth fixed capacitive plates, and
further wherein a height of the third fixed capacitive plates is
greater than a height of the third movable capacitive plates and a
height of the fourth fixed capacitive plates is less than a height
of the fourth movable capacitive plates.
9. The accelerometer as defined in claim 1, wherein the sensing
axis is perpendicular to the support substrate.
10. The accelerometer as defined in claim 1, wherein adjacent fixed
and movable capacitive plates are separated by first and second
gaps.
11. A linear accelerometer comprising: a support substrate; a first
bank of variable capacitors formed of one or more first fixed
capacitive plates and one or more first movable capacitive plates,
wherein the first fixed capacitive plates have a height greater
than a height of the first movable capacitive plates; a second bank
of variable capacitors formed of one or more second fixed
capacitive plates and one or more second movable capacitive plates,
wherein the second movable capacitor plates have a height greater
than a height of the second fixed capacitive plates; an inertial
mass that is linearly movable in response to linear acceleration
along a sensing axis, wherein the inertial mass includes the first
and second movable capacitive plates; a support structure for
supporting the inertia mass and allowing linear movement of the
inertial mass upon experiencing a linear acceleration along the
sensing axis; a first input for supplying an input signal to the
first bank of variable capacitors; a second input for supplying an
input signal to the second bank of variable capacitors; and an
output for sensing an output signal from the first and second
variable capacitors indicative of magnitude and direction of linear
acceleration sensed along the sensing axis in response to linear
movement of the inertial mass.
12. The linear accelerometer as defined in claim 11 further
comprising: a third bank of variable capacitors formed of a third
plurality of fixed capacitive plates and a third plurality of
movable capacitive plates, wherein the third plurality of fixed
capacitive plates have a height greater than a height of the third
plurality of movable capacitive plates; and a fourth bank of
variable capacitors formed of a fourth plurality of fixed
capacitive plates and a fourth plurality of movable capacitive
plates, wherein the fourth plurality of movable capacitive plates
have a height greater than a height of the fourth plurality of
fixed capacitive plates.
13. The linear accelerometer as defined in claim 11, wherein the
first and second input signals are clocked signals that are
out-of-phase with each other.
14. The linear accelerometer as defined in claim 11, wherein the
first fixed capacitive plates are arranged substantially parallel
to adjacent ones of the first movable capacitive plates and the
second fixed capacitive plates are arranged substantially parallel
to adjacent ones of the second movable capacitive plates.
15. The linear accelerometer as defined in claim 11, wherein the
support structure comprises a plurality of tethers.
16. The linear accelerometer as defined in claim 15, wherein the
plurality of tethers comprises four L-shaped tethers.
17. The linear accelerometer as defined in claim 11, wherein the
substrate comprises a silicon substrate.
18. The accelerometer as defined in claim 11, wherein the sensing
axis is perpendicular to the support substrate.
19. The accelerometer as defined in claim 11, wherein adjacent
fixed and movable capacitive plates are separated by first and
second gaps.
Description
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;
[0020] FIG. 7 is a exemplary block/circuit diagram illustrating
processing of the sensed capacitance output;
[0021] FIG. 8 is a block/circuit diagram further illustrating
processing self-test circuitry coupled to the accelerometer;
[0022] FIG. 9 is a flow diagram illustrating process steps for
fabricating the accelerometer according to the present
invention;
[0023] FIGS. 10A-10H are cross-sectional views further illustrating
the process steps for fabricating the accelerometer according to
the present invention; and
[0024] FIG. 11 is a top view of a portion of the accelerometer
illustrating a mask and etch module for forming the capacitive
plates of different heights.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Linear Accelerometer
[0025] Referring to FIGS. 1-4, an accelerometer 10 is illustrated
according to one embodiment of the present invention for sensing
magnitude and direction of acceleration along at least one sensing
axis, which is shown as the vertical Z-axis. The accelerometer 10
is shown and described herein as a single-axis linear
bi-directional accelerometer for sensing linear acceleration in
both upward and downward directions of the vertical Z-axis,
according to one embodiment. The Z-axis extends perpendicular to
the plane defined by the X- and Y-axes. Alternately, the
accelerometer 10 may be employed as a multi-axis accelerometer for
sensing acceleration in multiple axes. For example, the
accelerometer 10 may be employed as a three-axis accelerometer for
sensing linear acceleration in any of the X-, Y- and Z-axes.
Further, it should be appreciated that the accelerometer 10 could
be employed to sense angular acceleration or angular velocity, such
as angular acceleration or angular velocity about the Z-axis, as
well as linear acceleration along the vertical Z-axis.
[0026] The bi-directional linear accelerometer 10 is a
micro-machined MEMS accelerometer formed with a top-side etching
process described herein according to one embodiment. The linear
accelerometer 10 is fabricated on a supporting substrate 14, which
may include a silicon substrate, according to one embodiment. The
substrate 14 may be formed from a handle wafer having a bond oxide
layer 52 formed on the top surface. Various electrical and
mechanical components of the device are formed in an epitaxial
(EPI) device layer above the substrate 14. An overlying cover 54 is
shown in FIGS. 2 and 3 positioned on top to enclose the
accelerometer 10 to prevent contamination and damage, such as that
caused by moisture and particles.
[0027] Formed on top of the supporting substrate 14 is an inertial
proof mass 12 which extends over a cavity 50. The inertial mass 12
is shown in FIG. 1 having a central portion extending to each of
four corner quadrants. However, the inertial mass 12 may be formed
in any of a number of shapes and sizes. Inertial mass 12 is
suspended from the support substrate 14 via a support structure
shown, which, according to the embodiment includes four bent
generally L-shaped tethers 16A-16D, such that the inertial mass 12
is movable at least upward and downward relative to substrate 14 at
least in a direction along the Z-axis when subjected to vertical
acceleration. Tethers 16A-16D may have any appropriate shape.
[0028] The four generally L-shaped tethers 16A-16D extend between
the inertial mass 12 at one end and support anchored cantilevers
18A-18D, respectively, at the other end. Support anchored
cantilevers 18A-18D are rigidly fixed to and cantilevered from the
substrate 14 by respective anchors 19A-19D which are shown by
hidden lines. The tethers 16A-16D have a length, width, depth and
shape selected to achieve a desired resilient spring structure that
flexes to allow inertial mass 12 to move a distance within a
desired range when subjected to vertical acceleration. Together,
the inertial mass 12 and tethers 16A-16D act as a spring mass
system. It should be appreciated any one or more supporting
structures may be employed to support the mass 12 according to
other embodiments. For example, four folded beam tethers could be
employed.
[0029] The movable inertial mass 12 has a plurality of rigid
comb-like conductive fingers 20A and 20B that form movable
capacitive plates. The movable inertial mass 12 includes first and
third movable capacitive plates 20A and 20C each extending
lengthwise in a direction along the Y-axis, and second and fourth
movable capacitive plates 20B and 20D each extending lengthwise in
a direction along the X-axis. The inertial mass 12 with the
comb-like conductive fingers (plates) 20A-20D forms a movable
electrode that moves at least linearly in the sensing Z-axis when
subjected to a vertical acceleration along the sensing Z-axis. For
purposes of discussion herein, the X-axis and Y-axis are defined as
shown oriented in FIG. 1, and the vertical Z-axis is defined as
shown in FIGS. 2 and 3.
[0030] The linear accelerometer 10 also includes four fixed
electrodes 22A-22D shown generally located at ninety degree
(90.degree.) increments. The fixed electrodes 22A-22D generally
extend from and are fixed to the support substrate 14, and thus do
not move relative to the support substrate 14. Each of the fixed
electrodes 22A-22D includes a plurality of fixed capacitive plates
30A-30D, respectively, which are generally formed as a plurality of
rigid comb-like conductive fingers. The fixed capacitive plates
30A-30D are formed to be interdigitated with the movable capacitive
plates 20A-20D, respectively, to form four banks of variable
capacitors. That is, the movable capacitive plates 20A-20D are
oriented parallel to and interdigitated with the plurality of fixed
capacitive plates 30A-30D, respectively, so that adjacent
capacitive plates face each other in a juxtaposition such that a
capacitive coupling is provided.
[0031] The first plurality of fixed capacitive plates 30A of the
first fixed electrode 22A are interdisposed between adjacent first
movable capacitive plates 20A of inertial mass (movable electrode)
12 generally in a first quadrant of the inertial mass 12. The first
fixed electrode 22A has a signal input line 24A for receiving an
input clocked signal CLK applied to input pad 26. The input signal
CLK is a clocked signal, such as a square wave signal according to
one embodiment. The capacitive plates 20A and 30A thereby form a
first bank of variable capacitors.
[0032] The third fixed electrode 22C likewise includes a third
plurality of fixed capacitive plates 30C interdisposed between
adjacent third movable capacitive plates 20C of inertial mass 12
generally in the third quadrant of inertial mass 12 to provide a
third bank of variable capacitors. The third fixed electrode 22C
has a signal input line 24C for also receiving the input clocked
signal CLK applied to input pad 26. The bank of variable capacitors
formed by capacitive plates 20C and 30C is generally symmetric with
the first bank of variable capacitors formed by capacitive plates
20A and 30A.
[0033] The second fixed electrode 22B includes a second plurality
of fixed capacitive plates 30B interdisposed between adjacent
second movable capacitive plates 20B generally in the second
quadrant of inertial mass 12 to provide a second bank of variable
capacitors. The second fixed electrode 22B has a signal input line
24B for receiving an input clocked signal CLKB applied to input pad
28. Clocked signal CLKB is one hundred eighty degrees (180.degree.)
out-of-phase, i.e., inverse, as compared to clocked signal CLK,
according to one embodiment.
[0034] The fourth fixed electrode 22D includes fourth fixed
capacitive plates 30D interdisposed between adjacent fourth movable
capacitive plates 20D generally in the fourth quadrant of inertial
mass 12 to provide a fourth bank of variable capacitors. The fourth
fixed electrode 22D has a signal input line 24D for also receiving
the input clocked signal CLKB applied to input pad 28. The fourth
bank of variable capacitors is generally symmetric with the second
bank of variable capacitors.
[0035] Fixed electrodes 22A-22D are electrically conductive and are
electrically energized with out-of-phase input clocked signals CLK
and CLKB. Clocked signals CLK and CLKB may include other
out-of-phase signal waveforms, such as triangular or sine
waveforms. Adjacent fixed electrodes 22A-22D are dielectrically
isolated from each other via isolation trenches 40 within the
structure.
[0036] The sensed signal output line 32 is electrically coupled to
inertial mass (movable electrode) 12 via the second bent tether
16B. The output line 32 is further connected to output pad 34 for
supplying thereto the sensed output voltage (charge). The sensed
output signal is the sensed voltage generated on inertial mass 12
due to changes in capacitance in any of the four banks of variable
capacitors caused by acceleration. The sensed output signal is
further processed to determine the magnitude and direction of the
sensed vertical acceleration.
[0037] The electrical components formed in the EPI device layer
over substrate 14 are formed by an etching process which removes
material in the EPI layer, such as to form trenches. The input
lines 24A-24D, input pads 26 and 28, output line 32, output pad 34,
tethers 16A-16D, isolators 36, and gaps between adjacent capacitive
plates are formed as trenches 40 as shown in FIG. 4. Trenches 40
provide both physical separation and electrical isolation. The
reduced height for certain capacitive plates is formed by partially
etching the capacitive plates on the EPI layer from the top side
with a vertical mask and etch module to achieve the desired
height.
[0038] With particular reference to FIG. 4, the adjacent fixed and
movable capacitive plates 30A-30B and 20A-20B are shown spaced from
each other by etched trenches 40 which provide dielectric air gaps.
The gaps allow the movable capacitive plates 20A-20B to move
relative to the fixed capacitive plates 30A-30B. The adjacent fixed
and movable capacitive plates 30A-30B and 20A-20B are separated by
a greater distance on one side only in region 45, which enables the
capacitors formed thereby to serve as self-test capacitors that
enable testing of the accelerometer 10 with self-test processing
circuitry. The remaining adjacent fixed and movable capacitive
plates 30A-30B and 20A-20B in region 43, which is outside of region
45, are spaced from each other on each side by equal distances,
according to one embodiment. Capacitive plates 30C-30D and 20C-20D
are similarly spaced from each other.
[0039] The linear accelerometer 10 according to the present
invention employs fixed and movable capacitive plates interdisposed
between adjacent opposing plates to form multiple banks of variable
capacitors that sense both magnitude and direction of acceleration
in the sensing Z-axis. Adjacent fixed and movable capacitive plates
are configured having different heights to enable both the
magnitude and direction of acceleration to be sensed. That is, the
linear accelerometer 10 is able to sense not only magnitude of
acceleration, but also the direction of the acceleration, e.g.,
upward or downward direction of vertical acceleration.
[0040] With particular reference to FIG. 2, the first and third
movable capacitive plates 20A and 20C are shown formed having a
height that is less than the height of the first and third fixed
capacitive plates 30A and 30C. The reduced height of movable
capacitive plates 20A and 20C is realized by etching the EPI layer
on the top surface of the capacitive plates 20A and 20C to a
reduced height. This height variance is further illustrated with
capacitive plates 20A and 30A in FIGS. 5A-5C. Capacitive plates 20C
and 30C are formed similar to capacitive plates 20A and 30A.
[0041] As seen in FIG. 5A, capacitive plates 20A and 30A are formed
so that the bottom edge of each adjacent plate is substantially at
the same elevation when there is no vertical acceleration present.
The fixed capacitive plates 30A have a height that is higher than
the reduced height movable capacitive plates 20A by a predetermined
distance Dr. The capacitive plates 20A-20D and 30A-30D may have a
uniform doping (e.g., P+ or N+) or two different dopings (e.g.,
P+/N+ or P+(P++)/N+(N++)), according to one embodiment.
[0042] Capacitive plates 20A and 30A have an effective overlapping
area that determines the amount of capacitance generated by that
bank of capacitors. The maximum area of the resulting capacitors is
functionally the area of the smallest plate. The capacitance
therefore is a function of the overlapping height D.sub.C of
adjacent opposing capacitor plates. When the inertial mass 12 moves
upward by distance D due to downward acceleration, as seen in FIG.
5B, the overlapping height D.sub.C and area of the capacitor plates
20A and 30A remains the same (i.e., unchanged). When this happens
there is no change in capacitance generated by these capacitive
plates. When the inertial mass 12 moves downward by distance D due
to upward acceleration, as seen in FIG. 5C, the overlapping height
D.sub.C and area of the capacitor plates 20A and 30A is reduced.
This causes a reduction in the capacitance generated by these
capacitive plates. Thus, a change in capacitance of capacitive
plates 20A and 30A is indicative of the direction as well as
magnitude of the sensed acceleration.
[0043] With reference to FIG. 3, the second and fourth movable
capacitive plates 20B and 20D are shown having a height that is
greater than the height of the second and fourth fixed capacitive
plates 30B and 30D. The reduced height of the fixed capacitive
plates 30B and 30D is realized by etching the EPI layer on the top
surface of the capacitive plates connected to the second and fourth
fixed electrodes 22B and 22D to a reduced height. This height
variance is further illustrated with capacitive plates 20B and 30B
in FIGS. 6A-6C. Capacitive plates 20D and 30D are formed similar to
capacitive plates 20A and 30A.
[0044] As seen in FIG. 6A, capacitive plates 20B and 30B are formed
so that the bottom edge of each adjacent plate is substantially at
the same elevation when there is no vertical acceleration present.
The movable capacitive plates 20B have a height that extends higher
than the reduced height fixed capacitive plates 30B by a
predetermined distance D.sub.r.
[0045] Capacitive plates 20B and 30B have an effective overlapping
area that determines the amount of capacitance generated by that
bank of capacitors. The maximum area of the resulting capacitors is
functionally the area of the smallest plate and, therefore, the
capacitance is a function of the overlapping height D.sub.C of
adjacent opposing capacitor plates. When the inertial mass 12 moves
upward by distance D due to downward acceleration, as seen in FIG.
6B, the overlapping height D.sub.C and area of the capacitor plates
20B and 30B is reduced. This causes a reduction in the capacitance
generated by these capacitive plates. Thus, a change in capacitance
of capacitive plates 20B and 30B is indicative of the direction as
well as magnitude of the sensed acceleration. When the inertial
mass 12 moves downward by distance D due to upward acceleration, as
seen in FIG. 6C, the overlapping height D.sub.C and area of the
capacitor plates 20B and 30B remains the same (i.e., unchanged).
When this happens there is no change in capacitance generated by
these capacitive plates. Thus, no signal contribution to direction
or magnitude is provided by this set of capacitors.
[0046] The capacitive plates 20A-20D and 30A-30D may be configured
in various shapes and sizes. According to one embodiment,
capacitive plates 20A-20D and 30A-30D are generally rectangular.
The reduced height capacitive plates may be reduced in height up to
one-half the height of the extended height capacitive plates,
according to one embodiment. In one example, the reduced height
capacitive plates have a height of twenty-eight micrometers (28
.mu.m) as compared to a height of thirty micrometers (30 .mu.m) for
the extended height capacitive plates.
[0047] Referring to FIG. 7, a simplified representation of the
accelerometer 10 is shown electrically coupled to signal processing
circuitry, according to one embodiment. The accelerometer 10 is
generally represented as an electrical equivalent circuit having
four electromechanical capacitors C1-C4 representing the four banks
of variable capacitors. Capacitor C1 is formed by capacitive plates
20A and 30A, capacitor C2 is formed by capacitive plates 20C and
30C, capacitor C3 is formed by capacitive plates 20B and 30B, and
capacitor C4 is formed by capacitive plates 20D and 30D. Thus,
capacitors C1 and C2 receive input clocked signal CLK and
capacitors C2 and C3 receive input clocked signal CLKB.
[0048] The sensed output signal received at output pad 34 is input
to a charge amplifier 72 and is further processed by a demodulator
75 shown receiving clocked signal CLK. The feedback path, C.sub.F
in the charge amplifier 72, serves to prevent overloads in the high
frequency front-end amplifier section and to minimize signal
distortions due to high frequency signal components. The charge
amplifier 72 output voltage V.sub.O is inputted to the demodulation
circuit 75 to generate the output voltage denoted by V.sub.OUT. The
amplitude and sign of voltage V.sub.OUT represent the amplitude and
direction of the vertical acceleration applied to the accelerometer
10.
[0049] The output voltage V.sub.O may be represented by the
following equation: V.sub.O=[(C3+C4)-(C1+C2)]/C.sub.F. When the
inertial mass 12 moves downward, output voltage V.sub.O may be
represented by the following simplified equation:
V.sub.O=--(2*.DELTA.C)*CLK/C.sub.F, where A C represents the change
in capacitance of capacitors C3 and C4. When the inertial mass 12
moves upward, then the output voltage V.sub.O may be represented by
the following simplified equation: V.sub.O=+(2*.DELTA.C)*CLK/CF,
where .DELTA.C represents the change in capacitance of capacitors
C1 and C2.
[0050] Accordingly, the accelerometer 10 of the present invention
advantageously measures acceleration applied in either direction
along the vertical sensing axis. By employing movable capacitive
plates having a height different than the adjacent fixed capacitive
plates, the accelerometer 10 senses magnitude of acceleration as
well as the direction of the acceleration along the sensing axis.
This is achieved by applying clocked signals CLK and CLKB, which
are one hundred eighty degrees (180.degree.) out-of-phase with each
other, as inputs to the variable capacitors. The accelerometer 10
advantageously provides high gain to linear acceleration sensed
along the sensing Z-axis, while maintaining very low other linear
and rotational cross-axis sensitivities.
[0051] While the accelerometer 10 is shown and described herein as
a single-axis linear accelerometer, it should be appreciated that
accelerometer 10 may be configured to sense acceleration in other
sensing axes, such as the X- and Y-axes. Thus, the accelerometer 10
could be configured as a three-axis accelerometer. It should
further be appreciated that the accelerometer 10 may be configured
to sense angular acceleration or angular velocity.
[0052] The accelerometer 10 may be tested following its fabrication
by employing a self-test circuit as shown in FIG. 8, according to
one embodiment. The accelerometer 10 is generally illustrated
having normal operation capacitive plates forming variable
capacitors in region 43 and normal operation plus self-test
capacitive plates forming variable capacitors in region 45. As
mentioned above, the variable capacitors in region 43 are formed of
capacitive plates that are linear and have equal gap spacings
between each of the adjacent movable capacitive plates and the
fixed capacitive plates. This gap spacing arrangement results in no
response from motion along the X- and Y-axes, and allows for a
change in capacitance when subjected to the vertical acceleration
along the Z-axis. The normal plus self-test capacitive plates in
conjunction with the input clock arrangement maximizes the main
axis response while minimizing off axis responses.
[0053] The self-test operation can be performed by applying a
clocked signal CLK to variable capacitor C1 and applying its clock
compliment CLKB (one hundred eighty degrees (180.degree.)
out-of-phase) to variable capacitor C2. The average value of the
clocked signal CLK and its compliment signal CLKB is designed to be
different just as the self-test initiated and these average values
are chosen for a desired electrostatically induced inertial mass
displacement in the X-axis or Y-axis direction. Therefore, the X-
and Y-mode shape may be designed in relation to the Z-mode shape
such that an optimum trade-off is realized between the main sensing
axis and cross-axis responses. Similarly, clocked signals CLK and
CLKB are applied across variable capacitors C3 and C4. The sensed
output voltage is further processed as explained in connection with
FIG. 7 to generate an output voltage output V.sub.OUT.
[0054] The accelerometer 10 shown provides four variable capacitors
arranged in four symmetric quarters. However, it should be
appreciated that two or more variable capacitors may be provided in
other symmetries, such as one-half symmetries. It should also be
appreciated that additional signal pads may be formed on the
accelerometer 10. This may include a low impedance electrical
ground connection to minimize electrical feedthrough components, an
isolation pad, and a pad to create pseudo-differential electrical
connection(s) between the sensor 10 and readout electronic
circuitry of the signal of signal processing integrated circuitry
(IC).
Process of Manufacturing Microsensor
[0055] Referring now to FIG. 9 and FIGS. 10A-10H, a process 100 is
illustrated for fabricating a microsensor, such as the linear
accelerometer 10 described above. The microsensor is a
micro-electromechanical system (MEMS) sensor that is fabricated on
the crystal silicon substrate 14. While the fabrication process 100
is described herein according to one example to form the linear
accelerometer 10, it should be appreciated the fabrication process
may be used to form other microsensors.
[0056] The fabrication process 100 employs a trench etching
process, such as deep reactive ion etching (DRIE) and bond-etch
back process. The etching process generally includes etching out a
pattern from a doped material in EPI device layer 56 suspended over
subsurface cavity 50 formed in substrate 14. The fabrication
process 100 according to the embodiment shown provides for a top
side etching process which employs a vertical mask and etch module
for performing a top side mask and etch to remove material from the
top side surface of epitaxial (EPI) device layer 56 to achieve a
reduced height dimension.
[0057] The sequence of the steps for fabricating the microsensor
according to the fabrication process 100 are illustrated in FIG. 9,
according to one embodiment. FIGS. 10A-10H further illustrate the
fabrication process of FIG. 9 for forming a specific microsensor,
particularly the linear accelerometer 10. Process 100 is shown
beginning with step 110 of forming subsurface cavity 50 in the top
surface of handle wafer substrate 14, and mating the handle wafer
substrate 14 with the device EPI layer wafer 56. This step is
generally shown in FIG. 10A. The handle wafer substrate 14 may
include silicon or any other suitable support substrate. Grown on
top surface of substrate 14 is an oxide bond layer 52. Oxide bond
layer 52 may include silicon dioxide or any other suitable
dielectric material for forming a silicon bond.
[0058] The EPI device layer 56 is shown having an upper wafer
substrate 58 formed on the top surface thereof. EPI device layer 56
may include a single crystal EPI layer of silicon, according to one
embodiment. In one example, EPI layer 56 is thirty (30) micrometers
thick. The upper wafer substrate 58 allows for ease in handling the
EPI device layer 56 during the fabrication process.
[0059] Fabrication process 100 includes step 112 of bonding the
handle wafer substrate 14 to the EPI layer 56 and etching back the
upper wafer substrate 58 to leave the EPI layer 56 formed over the
cavity substrate. This step 112 is illustrated in FIG. 10B. Any
appropriate silicon bonding method may be employed as is well-known
in the art. Presumably, the EPI layer wafer 56 is etched back to
form a requisite device thickness. Etch stops for chemical etch
back of bonded silicon layers are known in the art. Dopent
concentration-dependent silicon etches are also known in the art.
Concentration-dependent selective removal of a layer of one doping
type material from the top of the second doping typing material
(i.e., P+ removed selectively from N-type material or N-type
material selectively removed from on top of P-type material) is
further known in the art.
[0060] The fabricated microsensor, particularly the linear
accelerometer 10, may employ a P-type device EPI layer 56,
according to one embodiment. The etch stop material may be a
counter-doped P++ layer, according to one embodiment. Process steps
of removing the P++ layer from the underlying P-type EPI layer are
well-known in the art.
[0061] Once the EPI layer wafer 56 is etched back to the desired
device layer thickness (e.g., thirty (30) micrometers), fabrication
process 100 forms a dielectric oxide layer 60 on the top surface of
the EPI device layer 56 in step 114. This step is seen in FIG. 10C.
Layer 60 may include silicon dioxide or any other suitable
dielectric medium.
[0062] Next, as seen in FIG. 10D, contacts 62 in the form of
openings are formed in the oxide layer according to step 116. The
contacts 62 may be formed by etching. In step 118, a metal
conductor 64 is deposited and patterned in the contacts 62 formed
on oxide layer 60, as seen in FIG. 10E. Additionally, the patterned
metal 64 may further be passivated in step 120. The patterned metal
64 may be aluminium or alloys of aluminium and silicon and other
advantageous materials deposited by known sputtering or evaporation
techniques. Accordingly, metal conductors 64 are appropriately
routed on top of the EPI device layer 56.
[0063] In step 122, microsensor fabrication process 100 patterns
the oxide layer 60 to expose portions of the top surface of device
silicon EPI layer 56. This exposes regions 66 on the top side of
EPI layer 60 as seen in FIG. 10F. The exposed regions 66 include
the overlying region where device components, such as capacitive
plates and isolation trenches, are to be formed.
[0064] Referring to FIG. 10G and step 124 of the fabrication
process 100 shown in FIG. 9, the silicon EPI device layer is masked
and etched using a vertical mask and etch module on the top side to
form areas of reduced height EPI layer 56. The reduced height EPI
layer 56 is formed in regions where reduced height capacitive
fingers or plates are desired, according to the embodiment shown.
The reduced height regions are shown by etched portions 68. The
step of masking and etching EPI layer 56 to form reduced height
regions 68 is achieved by employing a mask and etch module 70 seen
in FIG. 11. In the embodiment shown, the vertical etch and mask
module 70 is placed on top of the region 68 that is to be etched to
form the reduced height region(s). In this particular example, the
vertical mask and etch module 70 is placed on top of the movable
capacitive plates 20A to etch and remove material from the EPI
layer to form reduced height capacitive plates. It should be
appreciated that the vertical etching mask module 70 likewise is
placed on top of movable capacitive plates 20C and fixed capacitive
plates 30B and 30D, to form the reduced height capacitive plates
described in connection with the linear accelerometer 10.
[0065] The vertical mask and etch module 70 is a top side processed
module that may employ a photoresist mask applied to the EPI layer
56, followed by a shallow silicon etch. The vertical mask and etch
module 70 creates a step in the silicon on the top side. The
silicon etch step may include any of dry, wet or vapor phase
etches, as is known in the art. The etch may be isotropic or
anisotropic. If an isotropic etch is employed, the trenches that
separate capacitive plates on the accelerometer 10, as well as the
air gap distance between adjacent capacitive plates 40, should be
sized appropriately. Once the etch is performed, the photoresist
mask may be stripped from the surface, as is known in the art.
[0066] According to one example, the vertical mask and etch step
may include spinning a photoresist, and masking areas to expose
areas to be etched. This may include rinsing the photoresist
material away from the areas to be etched. An etchant, such
hydrofluoric (HF) acid according to a wet etching embodiment, is
then applied to the areas to be etched. A desired depth etch may be
achieved based on the etch rate of the etchant acid by controlling
the time that the etchant acid is applied to the non-masked
surface. Any appropriate silicon etch, such as a wet, dry or vapor
etch, may be used as is known in the art.
[0067] The fabrication process 100 further includes step 126 of
masking and etching the silicon EPI device layer to simultaneously
form the isolation trenches 40, and delineate the various features
of the micro-machined device in step 126. These features may
include forming the inertial proof mass 12, tether springs 16A-16D,
fixed electrodes and capacitive plates and the movable electrode
and capacitive plates. These various features may be formed by
masking and etching and is known in the art. According to one
embodiment, this process step uses DRIE etching to do this etch due
to its anisotropic characteristic and high aspect ratio
(depth-to-surface width) ability. Following completion of
fabrication process 100, the fabricated microsensor may be capped
with an overlying cover to prevent contamination and moisture
intrusion.
[0068] Accordingly, the process 100 of the present invention
advantageously provides for a top side microsensor fabrication
technique. The process advantageously allows for the formation of
different height structures on the device layer, without requiring
added bottom side processing steps and equipment.
[0069] It will be understood by those who practice the invention
and those skilled in the art, that various modifications and
improvements may be made to the invention without departing from
the spirit of the disclosed concept. The scope of protection
afforded is to be determined by the claims and by the breadth of
interpretation allowed by law.
* * * * *