U.S. patent application number 09/791965 was filed with the patent office on 2002-02-21 for production method for integrated angular speed sensor device.
Invention is credited to Cuccia, Aurea, Ferrari, Paolo, Ferrera, Marco, Foroni, Mario, Montanini, Pietro, Vigna, Benedetto.
Application Number | 20020022291 09/791965 |
Document ID | / |
Family ID | 8230820 |
Filed Date | 2002-02-21 |
United States Patent
Application |
20020022291 |
Kind Code |
A1 |
Ferrari, Paolo ; et
al. |
February 21, 2002 |
PRODUCTION METHOD FOR INTEGRATED ANGULAR SPEED SENSOR DEVICE
Abstract
An angular speed sensor comprises a pair of mobile masses which
are formed in an epitaxial layer and are anchored to one another
and to the remainder of the device by anchorage elements. The
mobile masses are symmetrical with one another, and have first
mobile excitation electrodes which are intercalated with respective
first fixed excitation electrodes and second mobile detection
electrodes which are intercalated with second fixed detection
electrodes. The first mobile and fixed excitation electrodes extend
in a first direction and the second mobile and fixed detection
electrodes extend in a second direction which is perpendicular to
the first direction and is disposed on a single plane parallel to
the surface of the device.
Inventors: |
Ferrari, Paolo; (Gallarate,
IT) ; Vigna, Benedetto; (Potenza, IT) ;
Foroni, Mario; (Milano, IT) ; Cuccia, Aurea;
(Milano, IT) ; Ferrera, Marco; (Domodossola,
IT) ; Montanini, Pietro; (Melegnano, IT) |
Correspondence
Address: |
SEED INTELLECTUAL PROPERTY LAW GROUP PLLC
701 FIFTH AVE
SUITE 6300
SEATTLE
WA
98104-7092
US
|
Family ID: |
8230820 |
Appl. No.: |
09/791965 |
Filed: |
February 22, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09791965 |
Feb 22, 2001 |
|
|
|
09178285 |
Oct 23, 1998 |
|
|
|
6209394 |
|
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Current U.S.
Class: |
438/50 ;
257/E21.552; 257/E21.573; 438/489 |
Current CPC
Class: |
H01L 21/76202 20130101;
G01C 19/574 20130101; G01C 19/5769 20130101; H01L 21/764 20130101;
G01P 15/0802 20130101 |
Class at
Publication: |
438/50 ;
438/489 |
International
Class: |
H01L 021/00; H01L
021/36; H01L 021/20 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 23, 1997 |
EP |
97830537.3 |
Claims
1. A method for production of an integrated angular speed sensor
device, comprising: forming a sacrificial region on a substrate of
semiconductor material; growing a pseudo-epitaxial layer including
a polycrystalline epitaxial region in contact with the sacrificial
region and a monocrystalline epitaxial region in contact with the
substrate; forming electronic elements in the monocrystalline
epitaxial region; removing selective portions of the
polycrystalline epitaxial region thereby forming excitation
electrodes that are intercalated with fixed excitation electrodes
that extend in a first direction of extension; forming mobile
detection electrodes that are intercalated with fixed detection
electrodes that extend in a second direction of extension which is
substantially perpendicular to the first direction; and forming a
trench separating from one another the mobile and fixed excitation
and detection electrodes; and removing the sacrificial region
through the trench.
2. A method according to claim 1 wherein removing the sacrificial
region comprises removing a silicon oxide region.
3. A method according to claim 1 wherein the pseudo-epitaxial layer
and the substrate have a first conductivity type, the method
further comprising: forming in the substrate buried contact regions
with a second conductivity type before growing the pseudo-epitaxial
layer; forming regions of electrically isolating material extending
above the buried contact regions and delimiting between one another
selective contact portions of the buried contact regions; forming a
well region having the second conductivity type above the
sacrificial region after growing the pseudo-epitaxial layer; and
forming sinker contact regions having the second conductivity type
and extending from a surface of the pseudo-epitaxial layer as far
as the buried contact regions thereby forming sinker contacts,
after growing the pseudo-epitaxial layer.
4. A method according to claim 1, further comprising: forming a
first mask of silicon carbide above the pseudo-epitaxial layer, the
first mask protecting regions beneath during the step of removing
the sacrificial region; and forming a second mask of silicon oxide
above the first mask, the second mask protecting the first mask
during the step of formation of the trench.
5. A method for production of an integrated angular speed sensor
device, the method comprising: forming a mobile structure flexibly
anchored to a semiconductor body, the mobile structure including
mobile excitation electrodes and mobile detection electrodes;
forming on the semiconductor body fixed excitation electrodes
alternating with the mobile excitation electrodes, the mobile and
fixed excitation electrodes having a first direction of extension;
forming on the semiconductor body fixed detection electrodes
alternating with the mobile detection electrodes, the mobile and
fixed detection electrodes having a second direction of extension
at an angle to the first direction, the mobile and fixed excitation
and detection electrodes all being in the plane of the surface; and
forming in the semiconductor body, prior to forming the mobile and
fixed excitation and detection electrodes, a buried contact region
that is doped to provide a conductive path, wherein one of the
fixed excitation and detection electrodes is formed in connection
with the buried contact region.
6. The method of claim 5 wherein the semiconductor body is a
monocrystalline substrate, the method further comprising: growing a
monocrystalline epitaxial region in contact with the substrate and
the buried contact region; and forming electronic elements in the
monocrystalline epitaxial region.
7. The method of claim 6, further comprising forming a conductive
sinker contact region extending through the monocrystalline
epitaxial region as far as the buried contact region, after growing
the monocrystalline epitaxial region.
8. The method of claim 5 wherein forming the mobile structure and
fixed excitation and detection electrodes includes: forming a
sacrificial region on the semiconductor body; growing a
polycrystalline epitaxial region in contact with the sacrificial
region; removing selective portions of the polycrystalline
epitaxial region to form the mobile structure and fixed excitation
and detection electrodes and a trench separating from one another
the mobile and fixed excitation and detection electrodes; and
removing the sacrificial region through the trench to release the
mobile structure from the semiconductor body.
9. A method according to claim 8, further comprising: forming a
first mask of silicon carbide above the polycrystalline epitaxial
region, the first mask protecting regions beneath the first mask
during the step of removing the sacrificial region; and forming a
second mask of silicon oxide above the first mask, the second mask
protecting the first mask during the step of formation of the
trench.
10. The method of claim 8 wherein the semiconductor body is a
monocrystalline substrate, the method further comprising: growing a
monocrystalline epitaxial region in contact with the substrate and
the buried contact region; and forming electronic elements in the
monocrystalline epitaxial region.
Description
TECHNICAL FIELD
[0001] The present invention relates to an integrated angular speed
sensor device and the production method thereof.
[0002] As is known, an angular speed sensor, or gyroscope, or yaw
sensor, is a device which can measure the variation of direction of
the speed vector of a moving body.
BACKGROUND OF THE INVENTION
[0003] Angular sensors can be used in the car industry, for ABS,
active suspensions, ASR, dynamic control of the vehicle and
inertial navigation systems; in consumer goods, for image
stabilization systems in cinecameras, in sports equipment, in
three-dimensional "mice"; in industrial process control, for
example in the control of industrial machines, in robotics; in the
medical field; and in the military field, for new weapons
systems.
[0004] Conventional gyroscopes, which are based on conservation of
the angular moment of a rotating mass, are too costly and bulky,
and are insufficiently reliable for the new applications. In
addition, although optical fiber and laser gyroscopes have
excellent performance levels, they are too costly for the
applications indicated.
[0005] The increasing need for small, inexpensive gyroscopes has
stimulated development activity in many industrial and academic
research centers. In about the 1950s, the first vibrating
gyroscopes were thus produced: they measure the angular speed of
the systems on which they are fitted, by detecting the effect of
the Coriolis force on a mass which vibrates in the non-inertial
rotating system. In these sensors it is essential for the sensing
mass to be kept moving by means of an actuation mechanism. The
first gyroscope produced in the 1950s used a magnetic field for
excitation of the sensing mass and detection of the Coriolis force;
subsequently in the 1960s, the piezoelectric effect, which is now
the most commonly used type, was employed (see for example B.
Johnson, "Vibrating Rotation Sensors", Sensors and Actuators, 1995,
SAE, SP-1066, pages 41-47).
[0006] At present, there is need for vibrating gyroscopes in which
the motion-sensing device comprises a silicon microstructure. In
fact, the possibility of using machinery and production processes
which are typical of the microelectronics industry should make it
possible to produce gyroscopes in large volumes and at a low cost,
which are essential requirements for car industry and consumer
goods applications.
SUMMARY OF THE INVENTION
[0007] In one aspect, the present invention provides a
motion-sensing device that is a vibrating-type, integrated angular
speed sensor and a production method that results in a
motion-sensing device at a low cost, and with a high level of
performance and reliability.
[0008] In one aspect, the integrated angular speed sensor device
includes a mobile structure anchored to a semiconductor material
body and having first mobile excitation electrodes which are
intercalated with first fixed excitation electrodes. The first
mobile and first fixed excitation electrodes have a first direction
of extension. The speed sensor device also includes second mobile
detection electrodes which are intercalated with second fixed
detection electrodes. The second mobile and second fixed detection
electrodes have a second direction of extension which is
substantially perpendicular to said first direction.
[0009] In another aspect, the present invention includes a method
for production of an integrated angular speed sensor device. The
method includes forming a mobile structure having first mobile
excitation electrodes which are intercalated with first fixed
excitation electrodes and which extend in a first direction of
extension and forming second mobile detection electrodes which are
intercalated with second fixed detection electrodes which extend in
a second direction of extension which is substantially
perpendicular to said first direction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] For the understanding of the present invention, a preferred
embodiment is now described, purely by way of non-limiting example,
with reference to the attached drawings, in which:
[0011] FIG. 1 is a simplified plan view of part of an integrated
circuit incorporating a motion-sensing device, in accordance with
embodiments of the present invention;
[0012] FIG. 2 is an enlarged view of a portion of the integrated
circuit of FIG. 1, in accordance with embodiments of the present
invention;
[0013] FIG. 3 is a simplified isometric cross-section of a portion
of the motion-sensing device of FIG. 1, in accordance with
embodiments of the present invention;
[0014] FIG. 4 is a simplified cross-sectional view of the
motion-sensing device, taken along the plane IV-IV of FIG. 3, in
accordance with embodiments of the present invention;
[0015] FIGS. 5-12 are simplified cross-sectional views through a
wafer of semiconductor material incorporating the motion-sensing
device, in accordance with embodiments of the present invention;
and
[0016] FIG. 13 is a simplified plan view of the motion-sensing
device, showing the shape of buried regions which are formed in an
intermediate step of the present method, in accordance with
embodiments of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0017] In one embodiment, the present device includes a
motion-sensing device 1 and related signal processing circuitry.
The motion-sensing device 1, which is shown in detail in FIGS. 1-3,
has a structure which is symmetrical with respect to a horizontal
central axis indicated at A in FIG. 1, in which, consequently, only
approximately half of the motion-sensing device 1 is shown. The
motion-sensing device 1 comprises two mobile masses 2a and 2b,
which are connected to one another and are anchored to a bulk
region 12 of N.sup.+-doped polycrystalline silicon by anchorage
elements 3 and 4.
[0018] In detail, as viewed from above, the mobile masses 2a, 2b
have substantially the shape of two adjacent squares or rectangles
which have first sides 10a facing one another, second sides 10b are
parallel to the first sides, and third and fourth sides 10c, 10d
perpendicular to the first and second sides 10a, 10b. From the
first sides 10a of the mobile masses 2a, 2b (which face one
another) there extend the anchorage elements 4. From the second,
third and fourth sides 10b-10d of each of the mobile masses 2a, 2b,
there extend elongate extensions which form mobile electrodes 6a,
6b of the sensor, and specifically, the mobile electrodes 6a extend
from the third and fourth sides 10c, 10d of each of the mobile
masses 2a, 2b, perpendicularly to the sides 10c, 10d, and form
mobile excitation electrodes, whereas the mobile electrodes 6b
extend from the second side 10b of each mobile mass 2a, 2b,
perpendicularly to the second side 10b, and form mobile detection
electrodes.
[0019] The anchorage elements 3 extend from the comers of the
mobile masses 2a, 2b between the third side 10c, the second side
10b and the fourth side 10d of the mobile masses 2a, 2b. The
anchorage elements 3 are L-shaped and comprise, starting from the
mobile mass 2a, 2b, a first section 3a which is parallel to the
mobile electrodes 6a, and a second section 3b which is parallel to
the mobile electrodes 6b. The second section 3b extends away from
the mobile electrodes 6a. On the other hand, the anchorage elements
4 extend from the center of the first sides 10a of the mobile
masses 2a, 2b. The anchorage elements 4, starting from the mobile
masses 2a, 2b, comprise first sections 4a which are parallel to the
mobile electrodes 6a; second sections 4b which are U-shaped, with
concavities which face one another, and a third section 4c which is
common to the two anchorage elements 4 which face one another,
thereby forming two forks 5 which face one another and extend
between the two mobile masses 2a, 2b.
[0020] The mobile electrodes 6a, 6b are intercalated or
interdigitated (alternate) with fixed electrodes 7a.sub.1, 7a.sub.2
and 7b.sub.1, 7b.sub.2, starting from respective fixed regions
8a.sub.1, 8a.sub.2 and 8b.sub.1, 8b.sub.2. In particular, the fixed
electrodes 7a.sub.1 and 7a.sub.2 are interdigitated with the mobile
electrodes 6a and are adjacent to one another. For both the third
and fourth sides 10c, 10d of the mobile masses 2a, 2b, the fixed
electrodes 7a.sub.1 are disposed in the vicinity of the anchorage
elements 4 and the fixed electrodes 7a.sub.2 are disposed in the
vicinity of the anchorage elements 3. The fixed electrodes 7b.sub.1
and 7b.sub.2 are interdigitated with the mobile electrodes 6b and
are adjacent to one another. The fixed electrodes 7b.sub.1 are
disposed on the left in FIG. 1, and the fixed electrodes 7b.sub.2
are disposed on the right. The fixed electrodes 7a.sub.2 are biased
to a positive voltage with respect to the fixed electrodes
7a.sub.1, as symbolized in the Figure by the voltages V.sup.+ and
V.sup.-. In one embodiment, the voltage V.sup.+ applied to the
fixed electrodes 7a.sub.2 is a square wave, and is in counter-phase
or phase opposition for the two mobile masses 2a, 2b, such as to
generate a direct force alternately towards the top and towards the
bottom of FIG. 1. This imparts a vibratory movement in the
direction of the axis Y to the mobile masses 2a, 2b. The fixed
electrodes 7b.sub.1 and 7b.sub.2 represent the electrodes for
detection of the signal generated by the mobile masses 2a, 2b, as
described hereinafter.
[0021] The distance between each mobile electrode 6a, 6b and the
two facing fixed electrodes 7a.sub.1, 7a.sub.2, 7b.sub.1, 7b.sub.2
in the static condition (in the absence of vibrations) is not the
same, as shown in the enlarged detail in FIG. 2 in which the mobile
electrodes are simply indicated as 6 and the fixed electrodes as 7.
Thereby, together with the two fixed electrodes 7 which faces it,
each mobile electrode 6 forms two capacitors which are parallel
with one another, one of which (the one which is defined by the
mobile electrode 6 and by the fixed electrode 7 at a shorter
distance) constitutes the capacitor which determines vibration of
the two mobile masses 2a and 2b or generation of the signal that is
detected and processed to determine the angular speed of the
device.
[0022] In one embodiment of the motion-sensing device 1, the
various regions which form the mobile masses 2a, 2b, the mobile
electrodes 6a, 6b, the anchorage elements 3, 4, the fixed regions
8a.sub.1, 8a.sub.2, 8b.sub.1, 8b.sub.2 and the fixed electrodes
7a.sub.1, 7a.sub.2, and 7b.sub.1, 7b.sub.2 (which, in one
embodiment, all comprise polycrystalline silicon of N.sup.+-type)
are separated from one another and from the bulk region 12 by a
trench 13, the overall shape of which is shown in FIG. 1. In one
embodiment, the bulk region 12 is surrounded by a first P-type
polycrystalline epitaxial region 14, which in turn is surrounded by
a second P-type polycrystalline epitaxial region 17. The two
polycrystalline epitaxial regions 14, 17 are separated from one
another by a second trench 15 with a closed rectangular shape,
which electrically insulates the motion-sensing device 1 from the
remainder of the device.
[0023] The fixed regions 8a.sub.1, 8a.sub.2, 8b.sub.1, 8b.sub.2 and
the bulk region 12 are biased by buried contacts, as shown in FIG.
3 for the regions 8a.sub.2 and 8b.sub.1. In one embodiment, the
regions 8a.sub.2 and 8b.sub.1 extend above a P-type substrate 20,
and are electrically insulated therefrom by a nitride region 18c, a
nitride region 18a, and thick oxide regions 21a, 21b, as shown in
FIG. 13. In one embodiment, shown in FIG. 3, the thick oxide
regions 21a, 21b have in their center an aperture 23, at which the
fixed regions 8a.sub.2 and 8b.sub.1 are in electrical contact with
respective N.sup.+ buried contact regions 24a, 24b extending along
the upper surface 30 of the substrate 20. In one embodiment, the
buried contact regions 24a, 24b extend from the apertures 23, below
a series of insulating regions which include the nitride region
18a, a thick oxide region 21c and a nitride region 18b, as shown
near the sensor area for the buried contact region 24a, which is
also shown in FIG. 4. The buried contact regions 24a, 24b extend
beyond the confines of the polycrystalline epitaxial region 17,
below a monocrystalline epitaxial region 37", where the buried
contact regions are in electrical contact with corresponding deep
contact or sinker regions, as shown in FIGS. 10-12 for the buried
contact region 24a, which is electrically connected to the sinker
region 26, extending from a surface 41 of the wafer. As shown,
contact with the other fixed regions 8a.sub.1, 8a.sub.2, 8b.sub.2
and with the bulk region 12 (which is electrically coupled to the
mobile masses 2a, 2b) is obtained in a manner similar to that
described for the fixed region 8a.sub.2, and in particular the bulk
region 12 is connected by a buried contact region (not shown)
extending parallel to the region 24a, along a plane parallel to
that of the cross-sectional view shown in FIG. 3.
[0024] As can be seen in FIG. 3, the trench 13 extends from the
surface 41 of the device as far as an air gap 16 in the area of the
mobile masses 2a, 2b of the mobile electrodes 6a, 6b, of the fixed
electrodes 7a.sub.1, 7a.sub.2, and 7b.sub.1, 7b.sub.2 and of the
anchorage elements 3, 4, and as far as the insulating nitride
regions 18a in the area of the fixed regions 8a.sub.1, 8a.sub.2 and
8b.sub.1, 8b.sub.2. The trench 15 extends from the surface 41 of
the device as far as the insulating nitride region 18b.
[0025] In the above-described embodiment of the motion-sensing
device 1, the presence of the two mobile masses 2a, 2b and of the
anchorage forks 5 makes it possible to eliminate, by suitable
signal processing, effects caused by apparent inertial forces to
which the two mobile masses are subjected. In fact, if the
non-inertial system does not rotate, but is subject to linear
acceleration A, the two mobile masses 2a, 2b (which have the same
mass m) are subjected to a force F.sub.a which is the same for
both. On the other hand, the Coriolis force F.sub.c is dependent on
the direction of the speed vector, and by subtracting the signals
detected by the fixed electrodes 7b.sub.1, 7b.sub.2, it is possible
to eliminate the common effect caused by the inertial force
F.sub.a. If W is the angular speed of the non-inertial system, and
A is its linear acceleration, the Coriolis force F.sub.c which acts
on the mobile mass with a mass m moving with a speed V relative to
the rotating system, is provided by the vector product:
F.sub.c=2m(W.times.V),
[0026] whereas the inertial force F.sub.a caused by the effect of
the acceleration A is:
F.sub.a=mA.
[0027] Since the two mobile masses 2a, 2b move in phase opposition,
an overall force F.sub.a+F.sub.c acts on one of the two mobile
masses, and an overall force F.sub.a-F.sub.c acts on the other. If
the two signals resulting from these two forces are then
subtracted, a measurement is obtained of the effect induced by the
Coriolis force F.sub.c.
[0028] In the structure shown, both excitation and detection are
electrostatic. The pairs of electrodes 6a, 7a.sub.1, 7.sub.2 make
the mobile masses 2a, 2b oscillate along the axis Y at their
resonance frequency, thus optimizing the conversion of the
electrical energy into mechanical energy. In contrast, the pairs of
electrodes 6b, 7b.sub.1, 7b.sub.2, owing to the effect of the
vibration (aforementioned force F.sub.a) and of the rotation
(Coriolis force F.sub.c), which give rise to variation of the
distance of the electrodes in the direction X, detect a variation
of the capacitance associated with the pairs of electrodes 6b,
7b.sub.1, 7b.sub.2, and generate a corresponding signal which can
be processed by the associated circuitry.
[0029] In some embodiments, the motion-sensing device 1 is
manufactured in the manner described hereinafter with reference to
FIGS. 5-13, in which the thicknesses of the various layers of
material are not shown to scale, and some layers are not shown in
all Figures, for clarity of representation and ease of
understanding.
[0030] As shown in the embodiment of FIG. 5, a sensor area 32 and a
circuitry area 34 are defined in the P-type monocrystalline silicon
substrate 20 by conventional photomasking and ion implantation
techniques. The N.sup.+ buried contact regions are similarly formed
(only the buried contact region 24a is shown in FIG. 5). A pad
oxide layer 31 is formed, for example thermally grown, on the
surface 30 of the substrate 20 with a thickness of 200-900 .ANG.. A
silicon nitride layer 18 is deposited, with a thickness of 700-3000
.ANG., above the pad oxide layer 31. The silicon nitride layer 18
is then photolithographically defined to have the shape shown in
FIG. 13. The portions of the surface 30 of the substrate 20 which
are not covered by the nitride layer 18 are then locally oxidized,
with formation of thick oxide regions which comprise a sacrificial
region 33 and the buried oxide regions 21a, 21b and 21c (only the
regions 33, 21a and 21c are shown in FIG. 6), as well as similar
regions for buried contact with the other electrodes. Reference is
also made to FIG. 13, showing the various nitride and oxide regions
which are present in this step.
[0031] Subsequently, after a photolithography step, plasma etching
removes the portions of the layers 31, 18 in the sensor area 32
where the buried contacts of the motion-sensing device 1 are to be
formed (apertures 23), and removes the silicon nitride layer 18 in
the circuitry area 34. This provides the structure of FIG. 7,
wherein the nitride regions 18a, 18b and 18c can be seen, but the
underlying pad oxide regions are not shown.
[0032] A polycrystalline or amorphous silicon layer 35 is then
deposited as shown in FIG. 8. By a photolithography and plasma
etching step, the polycrystalline or amorphous silicon layer 35 is
removed, with the exception of the sensor area 32, forming a
silicon region 35' which represents the nucleus for a subsequent
epitaxial growth step. Then, by an etching step, the pad oxide
layer 31 is removed where it is exposed, and epitaxial growth is
carried out, with formation of a so-called pseudo-epitaxial P-type
layer 37. The pseudo-epitaxial P-type layer 37 has a
polycrystalline structure in the sensor area 32 (polycrystalline
region 37'), and has a monocrystalline structure elsewhere
(monocrystalline region 37"). Thereby a wafer 38 is obtained, which
is shown in FIG. 9.
[0033] Subsequently, the pseudo-epitaxial layer 37 is doped N-type
by conventional ion implantation in order to produce sinker
regions. In one embodiment, as shown in FIG. 10, a portion of the
wafer 38 is shown which is slightly displaced to the left compared
with FIGS. 5-9. The sinker region 26 is formed in the
monocrystalline region 37", and a N.sup.+ well region 42 is formed
in the polycrystalline region 37', to accommodate the
motion-sensing device 1, and to form the mobile masses 2a, 2b, the
mobile and fixed electrodes 6, 7, and the bulk region 12. In
particular, the well region 42 electrically contacts the buried
contact region 24a in the position of the aperture 23 in the buried
oxide region 21a.
[0034] Subsequently, the electronic elements of the circuitry are
formed in the circuitry area 34 by conventional processing steps.
In the example shown, an N-type collector well 44 is formed
extending from the surface 41 of the wafer 38 to the substrate 20.
An NPN transistor 45 is formed in the collector well 44, having a
N.sup.+ collector contact region 46, a P base region 47, and a
N.sup.+ emitter region 48.
[0035] On the surface 41 of the wafer 38, a dielectric layer 50 is
then deposited for contact opening, for example of BPSG (Boron
Phosphorous Silicon Glass). Then, by a masking and selective
removal step, the contacts are opened in the circuitry area 34 and
on the sinker region 26, the dielectric layer 50 is removed from
the sensor area 32, and a metal layer is deposited and shaped,
forming contacts 51 for the transistor 45 and for the
motion-sensing device 1.
[0036] A passivation dielectric layer 52 is then deposited. The
passivation dielectric layer 52 is removed in the area of the
contact pads in order to permit electrical contacts to be made to
the device, and in the sensor area 32, resulting in the structure
of FIG. 10.
[0037] Then, a layer of silicon carbide 53 and an oxide layer 54
are then deposited and defined, to form a mask for the subsequent
step of excavating the polycrystalline region 37'. In one
embodiment, the oxide layer 54 is formed by conventional TEOS. In
one embodiment, the oxide layer 54 forms the masking layer for
subsequent etching of the trenches, whereas the carbide layer 53
forms the masking layer during the step of removing the sacrificial
regions. Then, etching separates the fixed electrodes and the
mobile electrodes, for separation of the fixed regions 8a.sub.1,
8a.sub.2 and 8b.sub.1, 8b.sub.2 from one another and from the
remainder of the well 42, in order to form the anchorage elements
3, 4 for forming holes 56 (see FIGS. 1 and 3) inside the mobile
masses 2a and 2b, and for insulation of regions which have
different potential. In this step, the nitride regions 18a, 18b
protect the substrate 20 and the buried contact region 24a against
etching. Thereby, the holes 56 and the trenches 13 and 15 are
formed, and the P-type polycrystalline epitaxial region 37' is
divided into the regions 14 and 17. Therefore, the structure is
obtained which is shown in cross-section in FIG. 11, taken along
the same cross-sectional plane as in FIG. 3.
[0038] Finally, the sacrificial region 33 is removed by wet etching
with hydrofluoric acid or with hydrofluoric acid vapors, through
the trenches 13 and 15 and the holes 56, and the area which was
previously occupied by the sacrificial region 33 forms the air gap
16 which separates the mobile masses 2a, 2b, the corresponding
anchorage elements 3 and 4, and the fixed electrodes 7, from the
underlying substrate 20. In this step, the oxide layer 54 is also
removed, whereas the silicon carbide layer 53 protects the
polycrystalline silicon regions beneath, as well as the passivation
dielectric layer 52. Thereby, the structure of FIG. 12 is obtained.
By subsequent plasma etching, the silicon carbide layer 53 is
removed from the entire wafer 38, thus providing the structure
shown in FIGS. 1-4, which has previously been described.
[0039] Some advantages of the described device and production
method are as follows. The structure of the above-described sensor
element, with detection electrodes extending perpendicularly to the
excitation electrodes, make it possible to directly detect the
signal generated by virtue of speed variations of the device.
Additionally, it is compatible with the process steps for
production of integrated circuits, permitting integration of the
motion-sensing device 1 and of the circuitry which processes the
signal generated by the latter on a single integrated circuit. In
this respect, it is particularly advantageous that an electrostatic
solution is used both for excitation of the motion-sensing device
1, and for detection of the response. In the structure described,
the connection of the two mobile masses 2a, 2b by means of two
forks 5 ensures matching of the respective resonance frequencies.
In turn, this permits elimination in a simple manner of the
inertial acceleration effect.
[0040] Production of the two mobile masses 2a and 2b by means of
epitaxial processing makes it possible to obtain better sensitivity
than in structures in which the mobile mass is produced from a
polycrystalline silicon layer deposited on the wafer, owing to the
greater mass which can be obtained, and to the greater surface area
of the actuation and detection capacitors (associated with the
depth of the pseudo-epitaxial layer). The use of process steps
which are typical of the microelectronics industry makes it
possible to produce the sensor at a low cost, and to guarantee a
high level of reliability.
[0041] Finally, it will be appreciated that many modifications and
variants can be made to the device and the method described here,
all of which are within the context of the inventive concept, as
defined in the attached claims. In particular, the types of doping
of the various regions can be inverted with one another; the
circuitry can comprise both bipolar and MOS devices, and the
details can be replaced by others which are technically
equivalent.
[0042] From the foregoing it will be appreciated that, although
specific embodiments of the invention have been described herein
for purposes of illustration, various modifications may be made
without deviating from the spirit and scope of the invention.
Accordingly, the invention is not limited except as by the appended
claims.
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