U.S. patent application number 09/742448 was filed with the patent office on 2001-07-26 for semiconductor mechanical sensor.
This patent application is currently assigned to DENSO Corporation. Invention is credited to Tmai, Masahito.
Application Number | 20010009110 09/742448 |
Document ID | / |
Family ID | 27302347 |
Filed Date | 2001-07-26 |
United States Patent
Application |
20010009110 |
Kind Code |
A1 |
Tmai, Masahito |
July 26, 2001 |
Semiconductor mechanical sensor
Abstract
A semiconductor mechanical sensor having a new structure in
which a S/N ratio is improved. In the central portion of a silicon
substrate 1, a recess portion 2 is formed which includes a beam
structure. A weight is formed at the tip of the beam, and in the
bottom surface of the weight in the bottom surface of the recess
portion 2 facing the same, an electrode 5 is formed. An alternating
current electric power is applied between the weight portion 4 and
the electrode 5 so that static electricity is created and the
weight is excited by the static electricity. In an axial direction
which is perpendicular to the direction of the excitation of the
weight, an electrode 6 is disposed to face one surface of the
weight and a wall surface of the substrate which faces the same. A
change in a capacitance between the facing electrodes is
electrically detected, and therefore, a change in a physical force
acting in the same direction is detected.
Inventors: |
Tmai, Masahito; (Chita-shi,
JP) |
Correspondence
Address: |
Cushman, Darby & Cushman,
Ninth Floor
1100 New York Avenue, N.W.,
Washington
DC
20005-3918
US
|
Assignee: |
DENSO Corporation
|
Family ID: |
27302347 |
Appl. No.: |
09/742448 |
Filed: |
December 22, 2000 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
09742448 |
Dec 22, 2000 |
|
|
|
09181615 |
Oct 28, 1998 |
|
|
|
6227050 |
|
|
|
|
09181615 |
Oct 28, 1998 |
|
|
|
08834129 |
Apr 14, 1997 |
|
|
|
5872024 |
|
|
|
|
08834129 |
Apr 14, 1997 |
|
|
|
08508170 |
Jul 27, 1995 |
|
|
|
5627318 |
|
|
|
|
08508170 |
Jul 27, 1995 |
|
|
|
08109504 |
Aug 20, 1993 |
|
|
|
5461916 |
|
|
|
|
Current U.S.
Class: |
73/514.32 ;
73/514.01 |
Current CPC
Class: |
Y10T 29/49002 20150115;
B81B 7/0006 20130101; G01P 15/125 20130101; G01C 19/5719 20130101;
G01P 2015/0817 20130101; Y10T 29/49004 20150115; G01P 2015/0828
20130101; B81B 2201/0235 20130101; G01C 19/56 20130101; C07C 303/44
20130101; C07C 309/17 20130101; C07C 303/32 20130101; C07C 309/17
20130101; G01C 19/5656 20130101; G01P 15/0802 20130101; C07C 303/32
20130101; C07C 303/44 20130101 |
Class at
Publication: |
73/514.32 ;
73/514.01 |
International
Class: |
G01P 015/00; G01P
015/125 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 21, 1992 |
JP |
4-223072 |
Oct 12, 1992 |
JP |
4-273202 |
Apr 2, 1993 |
JP |
5-77151 |
Claims
What is claimed is:
1. A semiconductor mechanical sensor comprising: a semiconductor
substrate; a beam structure extending in spaced relation over said
semiconductor substrate; a weight connected to said beam structure
and including a first mechanical force detect electrode, said
weight being movable along a predetermined direction; a second
mechanical force detect electrode facing said first mechanical
force detect electrode of said weight; an oscillation member for
oscillating said weight; and an AM modulation means for
superimposing a signal from said first mechanical force detect
electrode and said second mechanical force detect electrode on a
carrier wave; wherein movement of said weight produces a change in
capacitance between said first mechanical force detect electrode
and said second mechanical force detect electrode to enable said
sensor to detect mechanical forces acting thereon.
2. A semiconductor mechanical sensor in accordance with claim 1,
wherein said oscillation member includes a third electrode and a
fourth electrode, said third electrode being formed on said weight
and said fourth electrode facing said third electrode.
3. A semiconductor mechanical sensor in accordance with claim 1,
which is adapted to be used as a yaw rate sensor.
4. A semiconductor mechanical sensor in accordance with claim 1,
wherein said oscillation member oscillates said weight in a
direction generally perpendicular to said predetermined direction.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a semiconductor mechanical
sensor and Method of Manufacture of manufacturing the same, and
more particularly, to an acceleration sensor or a yaw rate sensor
and a method of manufacturing the same.
[0003] 2. Description of the Related Art
[0004] As a semiconductor mechanical sensor such as an acceleration
sensor or a yaw rate sensor, sensors using piezoelectric ceramics
are in wide use for attitude control of an automobile and to
prevent jitter in a commercial video camera. In addition, Japanese
Patent Publication Gazette No. 3-74926 discloses that two
piezoelectric resistor elements arranged in parallel to a
longitudinal axis of the cantilever, and in a side-by-side
configuration, detects a force which corresponds to a rotation
speed. In other words, without detecting deformation due to
vibration of the cantilever, only deformation due to twisting of
the cantilever is detected by the piezoelectric resistor
element.
[0005] However, regarding accuracy, cost, etc., existing yaw rate
sensors are not satisfactory, which restricts their application to
other purposes.
SUMMARY OF THE INVENTION
[0006] It is an object of the present invention to solve such a
problem and to offer a semiconductor mechanical sensor having a new
structure.
[0007] A further object of the present invention is to improve the
S/N ratio in such a semiconductor mechanical sensor having a new
structure.
[0008] A still further object of the present invention is to offer
a semiconductor mechanical sensor using a beam deflection type
capacity detection method and a method of manufacturing the same,
and to offer a semiconductor mechanical sensor which can detect
mechanical changes in two or three directions (when two such
semiconductor mechanical sensors are used) and a method of
manufacturing the same.
[0009] To achieve these objects, basically, a semiconductor
mechanical sensor according to the present invention has a
structure as follows. That is, the semiconductor mechanical sensor
comprises:
[0010] a semiconductor substrate;
[0011] a beam which is formed on the semiconductor substrate, the
beam having a weight; a first pair of electrodes one of which is
formed on a first surface of the weight and another one of which is
formed on a first surface of a wall of the substrate opposite to
the same surface of the weight; and a second pair of electrodes
which arranged perpendicular to the first pair of electrodes and
one of which is formed on a second surface of the weight different
from the first surface thereof and another one of which is formed
on a second surface of a wall different from the first surface of
the wall of the substrate, and opposite to the same surface of the
weight.
[0012] In other aspect of the present invention, in addition to the
above structure, the semiconductor mechanical sensor comprises: an
AM modulation circuit for superimposing a signal from the physical
force detect electrode onto a carrier wave; and a band pass filter
for passing a signal from the AM modulation circuit whose center
frequency coincides with the carrier wave.
[0013] In a further aspect of the present invention, a method of
manufacturing such a semiconductor mechanical sensor comprises the
steps of:
[0014] a fist step of forming a groove of a predetermined depth in
a main surface of a monocrystalline silicon substrate and
perpendicular to the main surface thereof, to thereby form a beam
which has a weight;
[0015] a second step of forming a pair of electrodes which face
each other, one of which is provided on a side surface of the
weight formed in a surface layer of the substrate and another one
of which is provided on an inner surface of the groove opposite to
the side surface of the weight, and forming another electrode on a
surface of the weight in a direction which is perpendicular to the
groove;
[0016] a third step of filling the groove with a filling material,
forming an electrode on a bottom surface of the groove and opposite
to the other electrode which is formed on the surface of the weight
with the filling material interposed therebetween to thereby form
another pair of electrodes, and of smoothing the major surface of
the monocrystalline silicon substrate;
[0017] a forth step of combining the main surface of the
monocrystalline silicon substrate with a separately prepared
substrate;
[0018] a fifth step of polishing a back surface of the
monocrystalline silicon substrate to remove a predetermined amount
thereof to thereby make the monocrystalline silicon substrate thin;
and
[0019] a sixth step of etching the filling material in the groove
in the monocrystalline silicon substrate to thereby form the beam
which has the weight.
[0020] In other words, in the semiconductor mechanical sensor
according to the present invention, the weight which is formed at
the tip of the beam is excited due to static electricity which is
created by applying an alternating current electric power to a side
wall of the substrate which faces one surface of the weight. In
such a state, in the axial direction which is perpendicular to the
excitation direction of the weight, a change in the capacitance
value between two electrodes arranged oppositely to each other is
electrically detected so that a mechanical force which acts and
changes in the same direction such as a yaw rate, an acceleration
or the like is detected.
[0021] More precisely, in the semiconductor mechanical sensor
according to the present invention, the weight is excited by static
electricity due to alternating current electric power, and in the
axial direction which is perpendicular to the direction of the
excitation, a change in the capacitance value between the two
electrodes arranged oppositely to each other, is electrically
detected. The detected signal is superimposed on the carrier wave
in the AM modulation circuit so that the carrier wave is AM
modulated. Further, the signal from the AM modulation circuit is
passed through the band pass filter which has a center frequency
which coincides with the frequency of the carrier wave.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a plan view of a semiconductor mechanical
sensor;
[0023] FIG. 2 is a view showing a cross section of FIG. 1 taken
along the line A-A;
[0024] FIG. 3 is a view showing an electric circuit of a
semiconductor mechanical sensor;
[0025] FIG. 4 is a view showing the waveform of an input
signal;
[0026] FIG. 5 is a view showing a quantity of displacement;
[0027] FIG. 6 is a view showing a signal waveform;
[0028] FIG. 7 is a view showing a signal waveform;
[0029] FIG. 8 is a view showing a quantity of displacement;
[0030] FIG. 9 is a view showing a signal waveform;
[0031] FIG. 10 is a plan view of a semiconductor mechanical sensor
according to other example.
[0032] FIG. 11 is a view showing a cross section of FIG. 10 taken
along the line B-B;
[0033] FIG. 12 is an explanatory diagram showing the principles of
the present invention;
[0034] FIG. 13 is a view showing an electric circuit of a
semiconductor mechanical sensor;
[0035] FIG. 14 is a plan view of a semiconductor mechanical
sensor;
[0036] FIG. 15 is a view showing a cross section of FIG. 14 taken
along the line A-A;
[0037] FIG. 16 is a cross-sectional view of a semiconductor
mechanical sensor according to other embodiment of the present
invention;
[0038] FIG. 17 is a schematic plan view of the semiconductor
mechanical sensor according to the embodiment shown in FIG. 16;
[0039] FIG. 18 is a plan view of the semiconductor mechanical
sensor according to the embodiment shown in FIG. 16;
[0040] FIGS. 19 to 31 are cross-sectional views each showing a
configuration of an intermediate material in respective
manufacturing steps;
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0041] In the following, semiconductor mechanical sensors according
to embodiments of the present invention will be described with
reference to the drawings.
[0042] FIG. 1 is a plan view of a semiconductor mechanical sensor
according to an embodiment of the present invention and FIG. 2 is a
view showing a cross section of FIG. 1 taken along the line A-A. In
the description hereinafter, to explain three dimensional
directions, a right-to-left direction will be referred to as the
X-axis direction, an up-down direction will be referred to as the
Y-axis direction and a direction which is perpendicular to the
drawing sheets will be referred to as the Z-axis direction.
[0043] FIG. 1 is a plan view showing a basic structure of a
semiconductor mechanical sensor according to the present invention,
and the semiconductor mechanical sensor comprises: a semiconductor
substrate 1; a beam 3 which is formed on the semiconductor
substrate 1, the beam having a weight 4; a first pair of electrodes
5 which is formed on one surface of the weight 4 and a wall surface
which corresponds to the weight surface; and a second pair of
electrodes 6 which is formed on one surface of the weight 4 and a
wall surface which corresponds to the weight surface in an axial
direction of the weight 4 which is perpendicular to the first pair
of electrodes 5.
[0044] More particularly, as clearly shown in FIGS. 1 and 2, the
silicon substrate 1 is a flat plate having a rectangular shape. In
the central portion of the silicon substrate 1, a rectangular
recess portion 2 is formed (depth; T). Within the recess portion 2,
the beam 3 which has a narrow width (width; W.sub.B) extends from
the left wall of the recess. At the tip of the beam 3, the weight
portion 4 is formed with a width greater than the beam 3 and a
square shape. The beam 3 and the weight portion 4 have the same
thickness. Further, one side surface of the weight portion 4 (the
top surface in FIG. 1) and the inner wall of the recess portion 2
are spaced away from each other by a small distance (distance d1).
In a similar manner, the other side surface of the weight portion 4
(the bottom surface in FIG. 1) and the inner wall of the recess
portion 2 are spaced away from each other by the same small
distance (distance d1). Similarly, the bottom surface of the weight
portion 4 and the beam 3 (the bottom surface in FIG. 2) and the
bottom surface of the recess portion 2 are spaced away from each
other by a small distance (distance d2).
[0045] Thus, the sensor has a cantilever structure. In this
structure, the space having the distance d2 is created by etching a
layer which is predeterminedly designed to be removed by a surface
micro machining technique.
[0046] In addition, the beam 3 forms a wiring region for the weight
portion 4 which serves as an electrode.
[0047] In the bottom surface of the recess portion 2, at a region
where the recess portion 2 faces the weight portion 4, the
electrode portion 5 is formed, and a portion which faces the
electrode portion 5, i.e., the weight portion 4 serves as an
electrode. Further, the electrode portion 6 is formed in a surface
of the inner wall of the recess portion 2 facing a side of the
weight portion 4 (i.e., the upper surface of the recess portion 2
in FIG. 1), and a portion facing the electrode portion 6, i.e., the
weight portion 4 serves as an electrode. The electrode 5 is an
electrode which provides static electricity. The electrode 6 is an
electrode which detects a displacement of the weight portion 4 and
forms a capacitance with the weight portion 4. In this structure,
the weight portion 4 and the electrodes 5 and 6 are insulated from
each other.
[0048] FIG. 3 is a view showing an electrical circuit which is used
in the semiconductor mechanical sensor according to the present
invention.
[0049] That is, as shown in FIG. 3, a circuit for effectively
operating the semiconductor mechanical sensor according to the
present invention comprises: oscillation means 8 which is connected
to a capacitor portion 7 which is formed by an electrode 4' which
is disposed on a side wall portion of the weight portion 4 and an
electrode 6 which is disposed on a wall surface of the substrate
facing the weight portion 4; impedance matching means 12 which is
connected to the capacitor portion 7; inverting amplifier means 13
which is connected to the impedance matching means 12; clock signal
generation means 17; and sample-and-hold means 26 which is
connected to the inverting amplifier means 13 and clock signal
generation means 17. In response to sample-and-hold periods which
are determined based on a clock signal which is output by the clock
signal generation means 17, the sample-and-hold means 26 records a
peak output value of the inversion amplifier means during each
sample-and-hold period and calculates a difference between the peak
values in different sample-and-hold periods. Differential amplifier
means 35 is provided for amplifying the difference value.
[0050] In other words, in the electrical circuit which is used in
the present invention, the capacitor portion 7 is formed by the
electrode 6 and the weight portion 4, and the oscillator 8 is
connected to the weight portion 4 side of the capacitor portion 7.
An impedance Z.sub.L is formed by a capacitor 9 and a resistor 10
connected to the electrode 6 side of the capacitor portion 7. A
power source 11 is connected to the capacitor 9.
[0051] To one end of the impedance Z.sub.L, the impedance matching
means 12, comprising an operational amplifier, is connected at a
point a which is created by a change in the capacitance value of
the capacitor portion 7. Here, an alternating current voltage
source V.sub.S (=V.multidot.sin .omega..sub.St) shown in FIG. 4 is
applied between the electrode 5 and the weight portion (electrode)
4 of FIG. 2. In such a state, when the weight portion 4 is
displaced by Coriolis deflection as shown in FIG. 5, a waveform as
shown in FIG. 6 appears at a non-inverted input terminal of the
impedance matching means 12 (the point .alpha. in FIG. 3).
[0052] The output of the impedance matching means 12 of FIG. 3 is
coupled to the inverting amplifier circuit 13. The inverting
amplifier means 13 is formed by an operational amplifier 14 and
resistors 15 and 16. A signal from the impedance matching means 12
is inverted and amplified by the inverting amplifier means 13.
[0053] The clock signal generation means 17 is comprised of a
voltage adjustor 18, two comparators 19 and 20, power sources 21
and 22, a NOR gate 23, a resistor 24 and a capacitor 25. In the
clock signal generation means 17, sample-and-hold periods T1 and T2
shown in FIG. 6 are generated.
[0054] The sample-and-hold circuit 26 is formed by two operational
amplifiers 27 and 28, switches 29, 30, 31 and 32 and capacitors 33,
34, 47 and 48. In the sample-and-hold periods T1 and T2 shown in
FIG. 6 generated by the clock signal generation means 17, the
switches 29, 30, 31 and 32 are opened and closed, whereby a
sample-and-hold operation is performed during these periods.
[0055] The differential amplifier circuit 35 is formed by
operational amplifiers 36, 37 and 38, resistors 39, 40, 41, 42, 43,
44 and 45 and a power source 46. From an output value available
from the sample-and-hold circuit 26, a difference between peak
values during the sample-and-hold periods T1 and T2 is calculated
(i.e., .DELTA. in FIG. 6) and amplified.
[0056] At the output terminal of the operational amplifier 38, a
sensor output V.sub.out is obtained.
[0057] Next, functions of a semiconductor mechanical sensor having
a construction as explained above will be described with reference
to FIG. 12.
[0058] As shown in FIG. 12, in the present invention, the beam
structure 3 is formed in a portion of the semiconductor substrate 1
spaced away from the semiconductor substrate 1, and an alternating
current electric power is applied to a wall surface of the
substrate which faces one surface of the weight 4 which is formed
at the tip of the beam 3, so as to generate static electricity and
excite the weight. In the axial direction which is perpendicular to
the direction of the excitation of the weight, the electrodes are
disposed in a facing relation with each other on the wall surfaces
of the substrate which face the one surface of the weight and the
surface of the beam. A change in the capacitance value between the
facing electrodes is electrically detected so that a mechanical
forces which act thereto in the same direction is detected.
[0059] Between the electrode 5 and the weight portion (electrode) 4
of FIG. 12, and alternating current voltage V.sub.S
(=V.multidot.sin .omega..sub.St) is applied where .omega..sub.S is
a rotational angular velocity. As a result, static electric power
F.sub.E as defined by Equation 1 below is created.
F.sub.E=.epsilon..sub.0.multidot.S.multidot.V.sub.S.sup.2/2d.sup.2
(1)
[0060] In the direction Z, a displacement as defined by Equation 2
below is generated. 1 D Z = F E L 3 3 E I Z + F E L 2 L m 2 E I Z (
2 )
[0061] where .epsilon..sub.0 is a dielectric constant, S is a
facing area of the electrodes, d is a distance between the
electrodes, L is the length of the beam, L.sub.m is the length of
the weight portion 4, I.sub.Z is a secondary moment of area of the
beam 3 in the Z-axis direction, and E is a Young's modulus.
[0062] Differentiating Equation 2 by time t, the velocity V.sub.Z
vibrates as:
V.sub.Z=dD.sub.Z/dt (3)
[0063] At this stage, with a rotational angular velocity .omega.
applied to the axis X which is perpendicular to the axis Z, the
Coriolis effect Fc defined by
Fc=2mV.sub.Z.omega. (4)
[0064] is created in the axis-Y direction.
[0065] In Equation 4, m is the mass of the weight portion 4.
[0066] Due to the Coriolis effect Fc, a displacement D.sub.Y which
is expressed by Equation 5 below is generated in the Y-axis
direction. 2 D Y = F C L 3 3 E IY + F C L 2 L m 2 E IY ( 5 )
[0067] where IY is a secondary moment of area in the axis-Z
direction. Hence, a capacitance between the electrodes C.sub.Y is
expressed by Equation 6 below. 3 C Y = 0 S y dy + DY ( 2 )
[0068] where S.sub.y is the faced area of the electrodes and
d.sub.y is the distance between the electrodes.
[0069] Due to a change in the value C.sub.y, a voltage V.omega.
defined by Equation 7 is created at the output terminal (output
voltage) V.sub.out. 4 V = Z Z + 1 / s C y V S ( 7 )
[0070] In other words, the output V.omega. changes in accordance
with the rotational angular velocity .omega. and the angular
velocity .omega. is calculated as the change in the value
V.omega..
[0071] Next, a description will be given of now the signal is
processed in the circuit with reference to FIG. 3.
[0072] The input waveform applied to the weight portion 4 is a
sinusoidal wave as shown in FIG. 4. Because of the Coriolis effect,
the weight portion 4 is displaced in accordance with a sinusoidal
wave which has a frequency double that of the input signal as can
be seen from Eq. 5 and FIG. 5. This creates a waveform at the
non-inverted input terminal a of the impedance matching means 12 of
FIG. 3, as shown in FIG. 6.
[0073] The most largely deformed portions of the input waveform of
the capacitor portion 7 during the sample-and-hold periods T1 and
T2 shown in FIG. 6, i.e., the portions corresponding to the peak
displacement of the weight portion 4 are peak-held by the
operational amplifiers 27 and 28, and the resultant difference is
amplified by the operational amplifiers 36 and 37, whereby the
voltage output V.sub.out which corresponds to the angular velocity
.omega. is calculated.
[0074] Next, we assume that an acceleration of a frequency fa (in
the direction Y) is applied as a disturbance noise. Here, if the
relation
fa<<2.pi..omega..sub.S (8)
[0075] holds, with respect to the input waveform shown in FIG. 7,
the acceleration is regarded as a displacement only on one side as
shown in FIG. 8, and therefore, the output waveform shown in FIG. 9
does not include a deformed portion.
[0076] In the processing in the circuit shown in FIG. 3, this
waveform is cancelled. For instance, where the characteristic
frequency of the cantilever 3 is 4 KHz and .omega..sub.S/2.pi.=3
KHz, since the frequency component of acceleration of an automobile
is around 300 Hz at maximum, Eq. 8 holds.
[0077] Further, since the frequency component is even smaller for
displacement due to temperature, Eq. 8 holds satisfactorily.
[0078] In this manner, in a sensing operation, the processing
circuit cancels most noises interfering with detection or the
angular velocity. Hence, the angular velocity is detected
accurately.
[0079] In addition, in the electrical circuit as above according to
the present invention, since a deformed waveform of the beam due to
acceleration and a deformed waveform of the beam due to a yaw rate
are different from each other and clearly distinguishable from each
other, the semiconductor mechanical sensor according to the present
invention can be used as both an acceleration sensor and a yaw rate
sensor, as well as for other sensors.
[0080] As described above, in the above example of the present
invention, the beam structure is formed in a portion of the silicon
substrate 1 (semiconductor substrate) spaced away from the silicon
substrate 1, and an alternating current electric power is applied
to a wall surface of the substrate which faces one surface of the
weight which is formed at the tip of the beam, so as to deflect the
weight by static electricity. In the axial direction perpendicular
to the direction of the excitation of the weight, the electrodes 6
are disposed in a facing relation on the wall surfaces of the
substrate facing the one surface of the weight and the surface of
the beam. A change in the capacitance value between the facing
electrodes is electrically detected so that mechanical forces which
act in the same direction, i.e., an acceleration or a yaw rate, is
detected. Thus, the semiconductor mechanical sensor has a new
structure.
[0081] The present invention is not limited to the example above.
For example, as shown in FIGS. 10 and 11, as a portion to which
static electricity is to be applied, an excitation electrode 48 may
be disposed in one side wall of the recess portion 2, and a detect
electrode 49 may be disposed on the bottom surface of the recess
portion 2.
[0082] As hereinabove described in detail, the present invention
provides a semiconductor mechanical sensor which has a new
structure.
[0083] Incidentally, the semiconductor mechanical sensor structure
as above has an inconvenience that in amplifying a signal of the
sensing part, noise (e.g., thermal noise, 1/f noise) is also
amplified, which makes it difficult to improve the S/N ratio.
[0084] As a result of study devoted to solving this problem, the
inventor of the present invention has come to the conclusion that
the problem can be solved if the semiconductor mechanical sensor
described above further comprises an AM modulation circuit for
superimposing a signal from the physical force detecting electrode
onto a carrier wave, and a band pass filter for passing a signal
from the AM modulation circuit whose center frequency coincides
with the carrier wave.
[0085] In the following, an embodiment of a circuit structure of
the example above according to the present invention will be
described with reference to the drawings. FIG. 13 is a plan view of
an electrical circuit according to the present invention, FIG. 14
is a plan view showing a semiconductor mechanical sensor, and FIG.
15 is a view showing a cross section of FIG. 14 taken along the
line A-A. In the description below, to explain three dimensional
directions, a right-to-left direction will be referred to as the
X-axis direction, an up-down direction will be referred to as the
Y-axis direction and a direction which is perpendicular to the
drawing sheets will be referred to as the Z-axis direction.
[0086] FIG. 14 shows an example where a semiconductor mechanical
sensor device comprises two semiconductor mechanical sensors
according to the present invention disposed as a pair. In such a
structure, a change in a certain physical force and a change in a
different physical force can be separately detected and detection
of a change in the physical force can be achieved accurately, for
instance.
[0087] In FIG. 14, a silicon substrate 51 is a flat plate and
includes a rectangular recess portion 52 (depth; T). Within the
recess portion 52, two beams 53 extend from the left side of FIG.
14. At the tips of the beams 53, a weight 55 is formed. On the
other hand, within the recess portion 52, two beams 54 extend from
the right side of FIG. 14, and at the tips of the beams 54, a
weight 56 is formed. The weights 55 and 56 are wider than the beams
53 and 54 and each is shaped in a rectangular shape. The beams 53
and 54 and the weights 55 and 56 have the same thickness.
[0088] In addition, one side surface of the weight 55 (the top
surface in FIG. 14) and the inner wall of the recess portion 52 are
spaced away from each other by a small distance (distance a). In a
similar manner, the other side surface of the weight 55 (the bottom
surface in FIG. 14) and the inner wall of the recess portion 52 are
spaced away from each other by the same small distance a.
Similarly, the bottom surface of the weight 55 (the bottom surface
in FIG. 15) and the bottom surface of the recess portion 52 are
spaced away from each other by a small distance (distance d1).
[0089] On the other hand, one side surface of the weight 56 (the
top surface in FIG. 14) and the inner wall of the recess portion 52
are spaced away from each other by the same small distance. In a
similar manner, the other side surface of the weight 56 (the bottom
surface in FIG. 14) and the inner wall of the recess portion 52 are
spaced away from each other by the same small distance a.
Similarly, the bottom surface of the weight 56 (the bottom surface
in FIG. 15) and the bottom surface of the recess portion 52 are
spaced away from each other by the small distance d1.
[0090] Thus, the illustrated sensor has a cantilever structure. In
this structure, the distance d1 is created by etching a layer which
is predeterminedly designed to be removed, by a surface micro
machining technique.
[0091] In FIG. 15, in the bottom surface of the recess portion 52
where the recess portion 52 faces the weights 55 and 56, electrodes
57 and 58 are formed. In portions of the weights 55 and 56 where
they face the electrodes 57 and 58, electrodes 59 and 60 are
formed. Further, in an inner wall surface of the recess portion 52
where the recess portion 52 faces the weights 55 and 56 (i.e., in
the upper surface of the recess portion 52 in FIG. 14), electrodes
159 and 160 are formed, and in portions of the weights 55 and 56
where they face the electrodes 159 and 160, electrodes 61 and 62
are formed.
[0092] In an inner wall surface of the recess portion 52 where the
recess portion 52 faces the weights 55 and 56 (i.e., in the lower
surface of the recess portion 52 in FIG. 14), electrodes 63 and 64
are formed, and in portions of the spineless 55 and 56 where they
face the electrodes 63 and 64, electrodes 65 and 66 are formed.
[0093] In addition, in this structure, the electrodes 57, 58, 59,
60, 61, 62, 63, 64, 159 and 160 are insulated from each other.
[0094] A capacitor C.sub.s+ is created by the electrodes 59 and 57,
a capacitor C.sub.s- is created by the electrodes 60 and 58, a
capacitor C.sub.d+ is created by the electrodes 159 and 61, a
capacitor C.sub.d- is created by the electrodes 64 and 66, a
capacitor C.sub.e+ is created by the electrodes 65 and 63, and a
capacitor C.sub.e- is created by the electrodes 160 and 62.
[0095] The beams 53 and 54 form wiring regions for the electrodes
59 (61, 65) and 60 (62, 66), respectively.
[0096] For clarity of explanation, although the electrodes 59, 61
and 65 are described as different electrodes, they are one and the
same electrode (same potential). Likewise, although described as
different electrodes for clarity of explanation, the electrodes 60,
62 and 66 are one and the same electrodes (same potential).
[0097] FIG. 13 shows an electrical circuit of the semiconductor
mechanical sensor according to the present invention.
[0098] The processing circuit of the sensor comprises an oscillator
67, a sensing part 68, a differential amplifier 69, a band pass
filter 70, a sample-and-hold circuit 71 and a subsequent stage
amplifier 72.
[0099] A capacitor Cr of FIG. 13 is not shown in FIGS. 14 and 15.
However, the capacitor Cr is connected in parallel with a resistor
R and has a fixed capacitance value Cr=C.sub.s+=C.sub.s-.
[0100] The capacitors C.sub.e+ and C.sub.e- drive the weights 55
and 56 by static electric force Fe. The capacitors C.sub.s+ and
C.sub.s- are capacitors for detecting the amount of displacement of
the weights 55 and 56 in the Z-axis direction due to the Coriolis
effect Fc.
[0101] The capacitors C.sub.d+ and C.sub.d- shown in FIG. 14 are
monitor capacitors for detecting the amount of movement of the
weights 55 and 56 in the Y-axis direction due to the drive
capacitors C.sub.e+ and C.sub.e-.
[0102] Next, the structure shown in FIG. 13, except for the sensing
part 68, will be described.
[0103] The oscillator 67 has an oscillation frequency of 10 KHz and
provides a voltage (alternating current electric power) for driving
the weights 55 and 56 and a signal (carrier wave) to the capacitors
C.sub.s+ and C.sub.s-. The resistor R applies a bias voltage to any
one of connection portions between the capacitors C.sub.s- or
C.sub.s+ and Crs, and has a resistance R>>1/.omega.Cr. By
applying a bias, each one of the resistors R makes subsequent
signal processing possible.
[0104] The differential amplifier 69 amplifies a difference voltage
between inputs (capacitors C.sub.s+ and C.sub.s-). The band pass
filter 70 has a center frequency of 10 KHz which coincides with the
frequency of the carrier wave. In addition, the band pass filter 70
attenuates signals other than those having a predetermined
frequency band (near the center frequency). In this example, the
band pass filter 70 is formed by a switched-capacitor filter
(S.C.F.).
[0105] The sample-and-hold circuit 71 (detector circuit)
demodulates a signal which is AM modulated as will be described
later. An operational amplifier 73 and resistors 74 and 75 form a
reference voltage for use within the processing circuit. The
subsequent stage amplifier 72 amplifies a detected signal. The
subsequent stage amplifier 72 may be omitted.
[0106] In this example, the electrodes 57, 58, 59 and 60 form a yaw
rate detecting electrode while the oscillator 67 and the
differential amplifier 69 form an AM modulation circuit.
[0107] Next, the functions of a semiconductor mechanical sensor
having the construction described above will be described.
[0108] When the oscillator 67 applies a voltage V.sub.IN
(=V.sub.CM.multidot.cos .omega..sub.ct) to the capacitors C.sub.e-
and C.sub.e+, static electric force Fe as defined by Equation 9
below is created.
Fe=(.epsilon..sub.0S/2a.sup.2).multidot.V.sub.IN.sup.2 (9)
[0109] where
[0110] .epsilon..sub.0; a dielectric constant
[0111] a; a distance between the capacitors C.sub.e- and
C.sub.e+
[0112] S; a faced electrode area of the capacitors C.sub.e- and
C.sub.e+
[0113]
[0114] Due to the static electric force Fe, the weights 55 and 56
are displaced in the Y-axis direction. Assuming that the amounts of
the displacements are Dy, the relationship shown in Equation 10 is
created.
Dy=KFe (10)
[0115] where K: a constant which is determined by the cantilever.
Here, it is to be noted that the weights 55 and 56 move in
different directions.
[0116] From Eqs. 9 and 10, where the velocities in the Y-axis
direction of the weights 55 and 56 are V.sub.y55 and V.sub.y56,
respectively, the following equation (11) is obtained.
V.sub.y55=-V.sub.y56=K.multidot.(.epsilon..sub.0S/4a.sup.2).multidot.V.sub-
.CM.sup.22.omega..sub.c.multidot.sin 2.omega..sub.c.sup.t (11)
[0117] At this stage, if the axis X is the rotation axis, and when
the weight is rotated with respect to the axis X rotates at the
angular velocity .omega., Coriolis effect
F.sub.c55=2mV.sub.y55.omega., F.sub.c56=2mV.sub.y56.omega. are
created at the axis z.
[0118] As a result, the weights 55 and 56 are displaced in the
Z-axis direction. Assuming that the displacements are D.sub.z55 and
D.sub.z56,
D.sub.z55=L.sub.55.multidot.F.sub.c55
D.sub.z56=L.sub.56.multidot.F.sub.c56 (12)
[0119] where L.sub.55, L.sub.56 are constants which are determined
by the cantilever.
[0120] If the weights 55 and 56 and the cantilever are formed to
have the same dimensions, L.sub.55=L.sub.56, and hence,
.vertline.D.sub.z55.vertline.=.vertline.D.sub.z56.vertline.=.DELTA.d.
[0121] In other words, the capacitance values of C.sub.s+ and
C.sub.s- are
C.sub.s+=(.epsilon..sub.0.multidot.S)/(d+.DELTA.d)
C.sub.s-=(.epsilon..sub.0.multidot.S)/(d-.DELTA.d) (13)
[0122] Hence, an output V.sub.pre of the differential amplifier 69
is
V.sub.pre=V.sub.IN.multidot.{C.sub.S+/(C.sub.S++Cr)-C.sub.S-/(C.sub.S-+Cr)-
}AV1.apprxeq.V.sub.IN.multidot.(-.DELTA.d/2d)AV1 (14)
[0123] where VA1 is an amplification factor of the differential
amplifier 67.
[0124] From Eqs. 11 and 12, .DELTA.d is
.DELTA.d=L.sub.552m.multidot.K(.epsilon..sub.0.multidot.S/4a.sup.2).multid-
ot.V.sub.CM.sup.2.multidot.2.omega..sub.c.multidot..omega..multidot.sin
2.omega..sub.ct (15)
[0125] On the other hand, from Eqs. 14 and 15,
V.sub.pre=AV1.multidot.V.sub.CM.sup.3.multidot.L.sub.55.multidot.2m.multid-
ot.K(.epsilon..sub.0.multidot.S/4a.sup.2).multidot..omega..sub.c.multidot.-
.omega..multidot.(sin .omega..sub.ct+sin 3.omega..sub.ct) (16)
[0126] In Eq. 16,
VCM3.multidot.L.sub.55.multidot.2m.multidot.K(.epsilon.0-
.multidot.S/4a2).multidot..omega..sub.c on the right side is a
constant which is determined by the structure of the cantilever and
a condition of the input voltage. From Eq. 16, it is understood
that the value V.sub.pre indicates a voltage which is in proportion
to the angular velocity .omega. which is to be detected. The value
V.sub.pre is expressed as a voltage output which is AM modulated to
the frequency of the input signal f.sub.IN=.omega..sub.c/2.pi. and
a frequency which is triple the same.
[0127] The foregoing has referred to a detected signal alone.
However, noise may be generated by circuit elements of the
differential amplifier 69 when a signal is processed in the
differential amplifier 69, and noise may be introduced into the
power source system from outside. These noises are also amplified
by the differential amplifier 69. Hence, from Eq. 16,
V.sub.pre=AV1.multidot.V.sub.CM.sup.3.multidot.L.sub.55.multidot.2m.multid-
ot.K(.epsilon..sub.0.multidot.S/4a.sup.2).multidot..omega..sub.c.multidot.-
.omega..multidot.(sin .omega..sub.ct+sin
3.omega..sub.ct)+AV1.multidot.V.s- ub.N (17)
[0128] Thus, AV1.multidot.V.sub.N is created which expresses a
noise which degrades the S/N ratio of the angular velocity .omega.
to be detected.
[0129] To deal with this, as shown in Eq. 17, signal data
concerning the angular velocity to be detected, is AM modulated by
a certain modulator and passed through the band pass filter 70,
having a center frequency f.sub.c=.omega..sub.c2.pi., whereby the
S/N ratio is improved.
[0130] Assume that an output of the band pass filter 70 having
5.sub.c=.omega..sub.c/2.pi. is V.sub.BPF,
V.sub.BPF=AV1.multidot.V.sub.CM.sup.3.multidot.L.sub.55.multidot.2m.multid-
ot.K(.epsilon..sub.0S/4a.sup.2).multidot..omega..sub.c.multidot..omega..mu-
ltidot.sin .omega..sub.ct+AV1.multidot.V.sub.N(f.sub.c) (18)
[0131] The value V.sub.BPF is expressed as shown in Fq. 18, and
therefore, only AV1.multidot.V.sub.N(f.sub.c), i.e., an noise
component whose frequency component is equal to f.sub.c is left.
Hence,
AV1.multidot.V.sub.N>>AV1.multidot.V.sub.N(f.sub.c) (19)
[0132] Thus, an output which is in proportion to the angular
velocity .omega. and which has a high S/N ratio is obtained. By
processing this output in the sample-and-hold circuit 71 (detector
circuit) if necessary, an output V.sub.out which is in proportion
to the angular velocity .omega. is obtained as below.
V.sub.out.apprxeq.AV1.multidot.V.sub.CM.sup.3.multidot.L.sub.55.multidot.2-
m.multidot.K(.epsilon..sub.0.multidot.S/4a.sup.2).multidot..omega..sub.c.m-
ultidot..omega. (20)
[0133] This output is amplified, if necessary, in the subsequent
stage amplifier 72.
[0134] As described above, in the present embodiment, the
oscillator 67 and the differential amplifier 69 (AM modulation
circuit) superimpose signals from the electrodes 57, 59 and 58, 60
(yaw rate detect electrodes) on a carrier wave, and a signal from
the differential amplifier 69 is passed through the band pass
filter 70 which has a center frequency which coincides with that of
the carrier wave. Hence, in processing a signal by the differential
amplifier 69, even if noise is generated in a circuit element of
the differential amplifier 69 when a signal is processed in the
differential amplifier 69 and other noise is introduced into the
power source system from outside, these noises are removed. That
is, noise (e.g., a thermal noises, a 1/f noise) is deenphasized and
therefore the S/N ratio is improved.
[0135] As described above, the present embodiment provides an
improved S/N ratio.
[0136] However, with respect to a semiconductor mechanical sensor
such as the semiconductor yaw rate sensor above which is movable in
two directions, the example described above is insufficient in
terms of structure. To manufacture the sensor, an efficient
manufacturing method for a high productivity has not been proposed
yet.
[0137] To deal with this, in addition to the examples described
above, the present invention offers a semiconductor mechanical
sensor which has an optimum structure and methods of efficiently
manufacturing the semiconductor mechanical sensors according to the
examples described above. That is, according to an other example of
the present invention, a semiconductor mechanical sensor comprises:
a thin monocrystalline silicon substrate which is joined onto a
substrate through an insulation film; a beam which is formed in the
monocrystalline silicon substrate and which has a weight; a first
electrode which is formed in one surface of said weight and a wall
surface which corresponds to said weight surface; and a second
electrode which is formed in one surface of the weight and a wall
surface which corresponds to the weight surface in an axial
direction of the weight which is perpendicular to the electrode,
and either one of the electrodes is preferably formed on the major
surface of the monocrystalline silicon substrate in parallel with
the monocrystalline silicon substrate.
[0138] Further, all electrode contacting portions are preferably
formed on the same surface of the thin monocrystalline silicon
substrate.
[0139] Describing the semiconductor mechanical sensor according to
the present invention in more detail, the semiconductor mechanical
sensor has a structure in which a plurality of groove portions 201
are formed in the tip portion 139 of a weight portion 139, an
electrode is disposed on an inner wall portion of each of groove
portions 201, and a fixed member 202 extends in each groove portion
201 and an other electrode is disposed on a side surface portion
which faces the inner wall portion of the groove portion of the
weight portion 4 of the fixed member 202.
[0140] In this example, a first electrode and a second electrode
which is disposed in an axial direction perpendicular to th first
electrode detect a mechanical quantity which is applied to a beam
having a weight.
[0141] Now, a semiconductor mechanical sensor having such a
structure according to the present invention will be described with
reference to FIGS. 16 to 18.
[0142] FIG. 17 is a schematic plan view of the semiconductor
mechanical sensor according to the present example. That is, in the
illustrated sensor, a cantilever 102 is formed in a monocrystalline
silicon substrate 101 so as to include a weight 139 at the tip. In
a tip portion 200 of the weight 139, three projections 103, 104 and
105 are formed spaced from each other to extend along the
elongation of the beam, and a groove portion 201 is formed between
the three projections 103, 104 and 105. On the monocrystalline
silicon substrate 101 side facing the tip portion surface 200 of
the cantilever 102 (weight 139), between the projections 103 and
104, two projections 106 and 107 are formed spaced from each other
to extend in parallel with the projections 103 and 104, thereby
forming a fixed portion 202. In a similar manner, on the
monocrystalline silicon substrate 101 side facing the tip portion
surface of the cantilever 102 (weight 139), between the projections
104 and 105, two projections 108 and 109 are formed spaced from
each other to extend parallel to the projections 104 and 105.
[0143] FIG. 18 is a plan view showing the semiconductor mechanical
sensor including the electrodes. FIG. 16 is a view showing a cross
section of FIG. 18 taken along the line A-A. In the drawings, an IC
circuit, wires and the like formed in an SOI circuit are omitted
and external contacting aluminum electrodes alone are shown as an
electrode for contacting a capacitance, an electrode for as
cillating the weight and the like in the sensor. In other words,
all electrode contacting portions are formed on the major surface
of the monocrystalline silicon substrate 101.
[0144] As shown in FIG. 16, the monocrystalline silicon substrate
101 is joined to a monocrystalline silicon substrate 110 through an
SiO.sub.2 film 111. In this monocrystalline silicon substrate 101,
the beam structure described earlier is formed.
[0145] In FIGS. 16 and 18, in a surface of the weight 139 of the
cantilever 102, a movable electrode 112 is formed. The movable
electrode 112 includes the three projections 103, 104 and 105 of
the weight 139. In addition, two electrodes 113 and 114 are formed
below the weight 139. The excitation electrode 114 receives an
alternating current electric power and excites the weight 139 by
the static electricity. In short, the movable electrode 112 and the
excitation electrode 114 form excitation electrodes.
[0146] The sense electrode 113 detects excitation of the weight
139, based on an output signal which is generated in response to
excitation of the weight 139, and feedback control is performed to
thereby achieve predetermined excitation of the weight 139. That
is, the movable electrode 112 and the sense electrode 113 form
electrodes for excitation feedback.
[0147] As shown in FIG. 18, on both sides of the projection 103 of
the cantilever 102, fixed electrodes 133 and 134 (projection 106)
are formed while on both sides of the projection 104, fixed
electrodes 135 (projection 107) and 136 (projection 108) are
formed. Further, on both sides of the projection 105, fixed
electrodes 137 (projection 109) and 138 are formed. In other words,
the projection 103 (movable electrode 112) and the fixed electrodes
133 and 134 form electrodes while the projection 104 (movable
electrode 112) and the fixed electrodes 135 and 136 form
electrodes. In addition, the projection 105 (movable electrode 112)
and the fixed electrodes 137 and 138 form faced electrodes.
[0148] FIGS. 19 to 23 show manufacturing steps. In the following,
the manufacturing steps will be described.
[0149] As shown in FIG. 19, an n type (100) monocrystalline silicon
substrate 101 of 1 to 20.OMEGA..multidot.cm is prepared, and a
recess portion 115 is etched in a major surface of the
monocrystalline silicon substrate 101 by dry etching or wet etching
to a predetermined depth, e.g., 0.1 to 5 .mu.m. An SiO.sub.2 film
is formed on the major surface of the monocrystalline silicon
substrate 101 and patterned by a photolithographic method.
Following this, in the major surface of the monocrystalline silicon
substrate 101 including the bottom portion of the recess portion
115, a trench 116 of a depth of about 0.1 to 30 .mu.m is formed by
dry etching or other suitable technique.
[0150] In this embodiment, a groove is formed by the recess portion
115 and the trench 116.
[0151] On the major surface of the monocrystalline silicon
substrate 101 including an inner wall of the trench 116, an n.sup.+
type diffusion layer 117 is formed which will be then covered with
an SiO.sub.2 film 118 by thermal oxidization.
[0152] Following this, as shown in FIG. 20, a polysilicon film 119
is buried in the recess portion 115 and the trench 116 by an LPCVD
method.
[0153] The surface of the polysilicon film 119 is then polished
using the SiO.sub.2 film 118 as a stopper to smooth the surface of
the polysilicon film 119. At this stage, the surfaces of the
polysilicon film 119 and the SiO.sub.2 film 118 are preferably
smoothed.
[0154] Then, in the surfaces, an SiO.sub.2 film 120 is formed to a
thickness of about 0.3 to 2 .mu.m by a CVD method or other suitable
method, and a bottom contact 121 is formed at a predetermined
location for electrical connection with the n.sup.+ type diffusion
layer 117.
[0155] Further, an n.sup.+ polysilicon 122 doped with As and P
(phosphorus) is formed to a thickness of 0.2 to 1 .mu.m which will
serve as an electrode pattern and a shield layer.
[0156] Next, a BGSP film 123 which serves as an insulation film,
for instance, is formed to a thickness of 0.2 to 1 .mu.m in the
surface. The surface of the BGSP film 123 is then polished and
flattened.
[0157] On the other hand, as shown in FIG. 21, a silicon substrate
110 is prepared and an SiO.sub.2 film 111 is grown into a thickness
0.2 to 1 .mu.m in a surface of the silicon substrate 110 by thermal
oxidization.
[0158] Following this, as shown in FIG. 22, the silicon substrates
101 and 110 are joined to each other through the SiO.sub.2 film 111
within N.sub.2 at a temperature of 1000.degree. C., for instance. A
back surface of the monocrystalline silicon substrate 101 is then
selectively polished using the SiO.sub.2 film 118 as a stopper. As
a result, the polysilicon 119 and an isolated region of the silicon
substrate 101 are exposed to the surface.
[0159] An IC board and other devices (not shown) are them formed in
the region of the monocrystalline silicon substrate 101 by a known
method, and an aluminum wire, a passivation film and a pad window
(these elements are not shown) are formed as well.
[0160] Next, as shown in FIG. 23, the SiO.sub.2 film 118 is removed
at a predetermined region, and the polysilicon film 119 is removed
at a predetermined region using an etching hole 124 which is shown
in FIG. 18. An etching solution may be TMAH
(tetramethylammoniumhidroxide), for example. As a result of
etching, a movable electrode (beam portion) is formed.
[0161] In the semiconductor mechanical sensor fabricated in this
manner, the thin monocrystalline silicon substrate 101 is joined
onto the monocrystalline silicon substrate 110 through the
SiO.sub.2 film 111, and in the monocrystalline silicon substrate
101, the cantilever 102 which has the weight 139 is formed at the
tip. Further, in one surface of the weight 139 (the bottom surface
in FIG. 16), the n.sup.+ type diffusion layer 117 is formed with
the bottom surface of the monocrystalline silicon substrate 101
facing the surface of the weight, and the n.sup.+ type polysilicon
122 (excitation electrode 114) is formed so that the n.sup.+ type
diffusion layer 117 and the n.sup.+ type polysilicon 122 form an
excitation electrode. By applying an alternating current electric
power to this excitation electrode, static electricity is created
which excites the weight 139. In addition, in the axial direction
which is perpendicular to the direction of the excitation of the
weight 139, the n.sup.+ type diffusion layer 117 is formed in one
surface of the weight 139 while the n.sup.+ type diffusion layer
117 is formed in a wall surface of the monocrystalline silicon
substrate 101 facing the surface of the weight 139 so that the
n.sup.+ type diffusion layer 117 of the weight 139 side and the
n.sup.+ type diffusion layer 117 on the side of the wall surface of
the monocrystalline silicon substrate 101 form a detecting
electrode for detecting a change in a physical quantity. The
physical quantity change detecting electrode detects a change in
the electrical capacitance and hence a change in a physical
quantity which acts in the same direction such as a yaw rate.
[0162] That is, an alternating current electric power is applied to
the excitation electrode (i.e., the n.sup.+ type diffusion layer
117 and the n.sup.+ type polysilicon 122) to create static
electricity and the weight is excited by the static electricity.
Under this condition, the yaw rate detecting electrode (i.e., the
n.sup.+ type diffusion layer 117 of the weight 139 side and the
n.sup.+ type diffusion layer 117 on the side of the wall surface of
the monocrystalline silicon substrate 101), for example, detects a
change in an electrical capacitance in the axial direction which is
perpendicular to the direction of the excitation of the weight 139,
whereby a change in a physical quantity which acts in the same
direction, such as a yaw rate, is detected.
[0163] Thus, in this embodiment, the recess portion 115 and the
trench 116 are formed as a groove of a predetermined depth in the
major surface of the monocrystalline silicon substrate 101 to
thereby form the cantilever 102 which has the weight 139 (first
step). In inner walls of the recess portion 115 and the trench 116
which surround a substrate surface region which serves as the
weight 139 and the weight 139, a pair of electrodes are formed
facing each other on the opposite sides of the trench 116 in the
direction of the surface of the substrate (a left-to-right
direction in FIG. 19), namely, the n.sup.+ type diffusion layer
117. At the same time, in a substrate surface region which will
serve as the weight 139, in the direction which is perpendicular to
the direction of the surface of the substrate (up-to-down direction
of FIG. 20; the direction of the thickness of the silicon substrate
101), the n.sup.+ type diffusion layer 117 (first electrode) is
formed (second step). Next, the recess portion 115 and the trench
116 are filled with a filling material, i.e., the polysilicon film
119, and the n.sup.+ type polysilicon 122 (electrode) is formed on
the opposite side of the polysilicon film 119 so as to face the
n.sup.+ type diffusion layer 117 (first electrode), followed by
smoothing of the major surface of the monocrystalline silicon
substrate 101 (third step). The major surface of the
monocrystalline silicon substrate 101 and the silicon substrate 110
are then joined to each other (fourth step). Thereafter, the back
surface side of the monocrystalline silicon substrate 101 is then
polished by a predetermined amount to thereby make the
monocrystalline silicon substrate 101 thin (fifth step). The
polysilicon film 119 is then etched from the back surface side of
the monocrystalline silicon substrate 101, whereby the cantilever
102 which has the weight 139 is formed (sixth step).
[0164] As a result, the semiconductor mechanical sensor comprises
the thin monocrystalline silicon substrate 101 which is joined onto
the monocrystalline silicon substrate 110 through the SiO.sub.2
film 111 (insulation film), the cantilever 102 which is formed in
the monocrystalline silicon substrate 101 and which has the weight
139, the movable electrode 112 which is formed in one surface of
the weight 139 and a wall surface which corresponds to the same,
the excitation electrode 114 (first electrode), the movable
electrode 112 of the weight 139, the projections 103 to 105 which
are formed one surface of the weight 139 and a wall surface which
corresponds to the same in the axial direction which is
perpendicular to the excitation electrode 114, and the fixed
electrodes 133 to 138 (second electrode).
[0165] Either one of the electrodes, namely, the movable electrode
112 or the excitation electrode 114 is formed parallel to the major
surface of the monocrystalline silicon substrate 101.
[0166] Further, all electrode contacting portions are formed on the
same surface of the thin monocrystalline silicon substrate 101.
[0167] Thus, the semiconductor mechanical sensor comprises the thin
monocrystalline silicon substrate 101 joined to the monocrystalline
silicon substrate 110 through the SiO.sub.2 film 111, the
cantilever 102 which is formed in the monocrystalline silicon
substrate 101 and which has the weight 139 at the tip, the
excitation electrode which is formed in one surface of the weight
139 and a wall surface of the monocrystalline silicon substrate 110
facing the weight, the excitation electrode creating static
electricity and exciting the weight when an alternating current
electric power is applied thereto, and the detecting electrode
which is formed in one surface of the weight 139 and a wall surface
of the monocrystalline silicon substrate 110 facing the weight in
the axial direction which is perpendicular to the direction of
excitation of the weight 139, the detecting electrode detecting a
change in an electrical capacitance and hence a change in a
physical quantity which acts in the same direction.
[0168] In this manner, processes are performed stably and a device
which is stable and accurate is manufactured without contamination
by using a surface micro machining technique, without performing a
thermal treatment and a photolithographic process during a wafer
forming process, especially during fabrication of an IC circuit, in
a condition where a wafer recess portion, a through hole and the
like have been already formed.
[0169] Although the foregoing has described the present embodiment
in relation to the case where the excitation electrode and the
sense electrode are buried in the substrate, the sense electrode
may be omitted to reduce cost, in which case, the silicon substrate
as it is may be used as the excitation electrode, unlike the
structure described above.
[0170] In addition, although the electrodes which are formed
parallel to the wafer surface are used as the sense electrode and
the excitation electrode and the electrodes which are disposed in
the vertical direction are used as the fixed electrodes for
detecting the Coriolis effect, in the present embodiment, the
opposite is also possible. That is, one of the fixed electrodes
which are disposed in the vertical direction in the silicon
substrate 101 may be used as the excitation electrode, and the
other one of the fixed electrodes may be used as the sense
electrode for performing feedback, while the electrodes which are
formed parallel to the wafer surface may be used as electrodes for
detecting the Coriolis effect.
[0171] Further, as the polysilicon film 119 for filling the recess
portion 115 and the trench 116 (i.e., a polycrystalline silicon
film), an amorphous silicon film or a silicon film in which a
polycrystalline portion and an amorphous portion are mixed may be
used.
[0172] Next, still another example of the present invention will be
described with reference to FIGS. 24 to 30.
[0173] This example is intended to further increase output as
compared with the preceding example and to prevent destruction of
the beam by excessive shock and the like.
[0174] FIGS. 24 to 30 show steps for manufacturing the sensor. In
the following, the manufacturing steps will be described.
[0175] In the example of FIG. 19, as shown in FIG. 24, an
Si.sub.3N.sub.4 film 125 having a thickness of 200 to 2000 .ANG. is
formed by the LPCVD method after formation of the SiO.sub.2 film
118. In this example, the thickness of the Si.sub.3N.sub.4 film 125
is 500 .ANG..
[0176] In processes similar to those of the above example,
polishing and flattening of the surface as shown in FIG. 22 in
relation to the above example are performed.
[0177] Following this, a resist 126 of FIG. 24 is patterned to a
predetermined pattern by a photolithographic technique, and a
region which will serve as the sense part of the monocrystalline
silicon substrate 101 is removed by dry etching or other suitable
method as shown in FIG. 25.
[0178] Next, using the resist 126 as a mask, the SiO.sub.2 film 118
is removed by wet etching, for example, which primarily uses
hydrofluoric acid as an etchant, followed by removal of the resist
126.
[0179] In the following, for clarity of explanation, an enlarged
view of a portion of the sensor part B of FIG. 25 will be referred
to.
[0180] FIG. 26 shows the enlarged portion.
[0181] As shown in FIG. 27, using the Si.sub.3N.sub.4 film 125 as a
mask, an SiO.sub.2 film 127 is grown to a thickness of 500 to 10000
.ANG. by thermal oxidization. In this embodiment, the thickness of
the SiO.sub.2 film 127 is 1000 .ANG..
[0182] Next, as shown in FIG. 28, the Si.sub.3N.sub.4 film 125 used
as a mask during thermal oxidization is removed by plasma etching
or etching using heated phosphoric acid. A polysilicon 128 is then
grown by the LPCVD method or other suitable method, on the surface.
The surface of the polysilicon 128 is then selectively polished and
removed using the SiO.sub.2 film 127 as a stopper.
[0183] Further, the surface is treated with a TMAH
(tetramethylammoniumhid- roxide) solution. At this stage, in a
peripheral portion, an IC circuit and the like are formed (not
shown).
[0184] Thereafter, as shown in FIG. 29, an Si.sub.3N.sub.4 film 129
having a thickness of 500 to 2000 .ANG. is formed on the surface,
and an n.sup.+ type polysilicon layer 130 is formed which will
serve as a stopper against excessive amplitudes of the electrode
layer and the sensor. Following this, a BPSG film 131 is formed as
a surface protection film. This film may be formed by an
Si.sub.3N.sub.4 film or the like. A window portion 132 is then
formed.
[0185] Then, as shown in FIG. 30, the polysilicon 119 and the
polysilicon 128 are etched through the window portion 132 with the
TMAH solution.
[0186] In this manner, a sensor which comprises a movable portion
(cantilever) which is entirely surrounded by an electrode and a
stopper is obtained. In such a structure, when the weight portion
is excited in a direction which is perpendicular to the substrate,
as shown in FIG. 30, since a>b and b is within the range of a,
there will be almost no capacitance change created during detection
of a yaw rate due to excitation. the relation a>b is attainable
in the first embodiment as well.
[0187] FIG. 31 is a view which clearly shows more detail of the
overall structure.
[0188] As described above, in the present example, since the
stopper member 130 is disposed above the cantilever 102, output is
further increased, as compared with the above example, and
destruction of the cantilever by excessive shock and the like is
prevented.
[0189] That is, in the present example, in the first step, a groove
of a predetermined depth is formed in the major surface of the
monocrystalline silicon substrate to thereby form the beam which
has the weight. In the second step, a pair of electrodes are formed
which faced each other on the opposite sides of the groove in a
substrate surface region and an inner wall of the groove which
surrounds the weight in the direction of the surface of the
substrate, while the first electrode is formed in a substrate
surface region which will serve as the weight in a direction which
is perpendicular to the surface of the substrate. In the third
step, the groove is filled with a filling material and an electrode
which faces the first electrode through the filling material is
formed, and the major surface of the monocrystalline silicon
substrate is smoothed. Next, in the fourth step, the major surface
of the monocrystalline silicon substrate and the silicon substrate
are joined to each other. In the fifth step, the back surface side
of the monocrystalline silicon substrate 101 is polished by a
predetermined amount to thereby make the monocrystalline silicon
substrate thin. Lastly, in the sixth step, the filling material is
etched from the back surface side of the monocrystalline silicon
substrate, whereby the beam which has the weight is formed. As a
result, the semiconductor mechanical sensor according to the
present invention is completed.
[0190] It is to be noted that the present invention is not limited
to the embodiments described above. Rather, two pairs of the sensor
units may be arranged in directions perpendicular to each other in
order to detect yaw rates in the two axial directions. Further, the
present invention is not limited to a cantilever. The present
invention is also not limited to detection of a yaw rate. For
instance, the excitation electrode of the embodiments above may be
replaced with an electrode which detects a capacitance of
displacement in an up-to-down direction so that the present
invention is applied to a mechanical sensor which is capable of
detecting displacements in two directions.
[0191] As heretofore described in detail, the present invention
creates effects by which a yaw rate sensor of the beam excitation
type capacity detection method and a method of manufacturing the
same are obtained, and a semiconductor mechanical sensor which can
detect movement in two or three directions and a method of
manufacturing the same are obtained.
* * * * *