U.S. patent application number 16/016867 was filed with the patent office on 2018-10-18 for relative and absolute pressure sensor combined on chip.
The applicant listed for this patent is Melexis Technologies NV. Invention is credited to Appolonius Jacobus VAN DER WIEL.
Application Number | 20180299303 16/016867 |
Document ID | / |
Family ID | 52349838 |
Filed Date | 2018-10-18 |
United States Patent
Application |
20180299303 |
Kind Code |
A1 |
VAN DER WIEL; Appolonius
Jacobus |
October 18, 2018 |
RELATIVE AND ABSOLUTE PRESSURE SENSOR COMBINED ON CHIP
Abstract
A method for manufacturing a system in a wafer for measuring an
absolute and a relative pressure includes etching a shallow and a
deep cavity in the wafer. A top wafer is applied and the top wafer
is thinned for forming a first respectively second membrane over
the shallow respectively deep cavity, and for forming in the top
wafer first respectively second bondpads at the first respectively
second membrane resulting in a first respectively second sensor.
Back grinding the wafer results in an opened deep cavity and a
still closed shallow cavity. The first bondpads of the first sensor
measure an absolute pressure and the second bondpads of the second
sensor measure a relative pressure. The etching in the first step
defines the edges of the first membrane and of the second membrane
in respectively the sensors formed from the shallow and the deep
cavity.
Inventors: |
VAN DER WIEL; Appolonius
Jacobus; (Duisburg, BE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Melexis Technologies NV |
Tessenderlo |
|
BE |
|
|
Family ID: |
52349838 |
Appl. No.: |
16/016867 |
Filed: |
June 25, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
14956770 |
Dec 2, 2015 |
10031003 |
|
|
16016867 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G01F 15/14 20130101;
B81B 2201/0264 20130101; B81B 7/04 20130101; B81C 1/00214 20130101;
G01F 1/34 20130101; G01L 15/00 20130101; G01L 9/008 20130101 |
International
Class: |
G01F 1/34 20060101
G01F001/34; G01L 15/00 20060101 G01L015/00; G01L 9/00 20060101
G01L009/00; G01F 15/14 20060101 G01F015/14; B81B 7/04 20060101
B81B007/04; B81C 1/00 20060101 B81C001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 2, 2014 |
GB |
1421436.5 |
Claims
1. A pressure measurement system for measuring both an absolute and
a relative pressure, the system comprising: a base wafer comprising
a closed shallow cavity with a first membrane over the shallow
cavity forming an absolute pressure sensor, and on the same base
wafer, an open deep cavity, with and a second membrane over the
deep cavity forming a relative second pressure sensor, wherein the
edge of the first membrane respectively second membrane is
determined by the shallow cavity respectively the open deep cavity
forming the sensors.
2. A pressure measurement system according to claim 1, wherein the
edges of the shallow cavity and of the deep cavity have an angle of
between 80.degree. and 100.degree. with the vertical axis.
3. A pressure measurement system according to claim 1, wherein the
horizontal cross-section of the shallow and deep cavities is
circular.
4. A pressure measurement system according to claim 1, wherein the
shallow cavity has a bottom and the deep cavity comprises at least
one pillar, wherein the top of the pillar has the same height as
the bottom of the shallow cavity.
5. A pressure measurement system according to claim 1, wherein the
area of the horizontal cross-section of the deep cavity has a
different size than the area of the horizontal cross-section of the
shallow cavity.
6. A pressure measurement system according to claim 1, the relative
and absolute pressure sensors comprising bondpads, wherein the
bondpads are substantially made of platinum, copper, aluminium or
gold.
7. A flow meter for measuring a gas flow, the flow meter comprising
a housing for housing a pressure measurement system according to
claim 1, wherein the housing comprises: a tube having a first
opening and a second opening a cavity in communication with the
first opening and the second opening wherein the system can be
positioned such that the cavity is separated in a first part in
communication with the first opening and a second part in
communication with the second opening, one side of the second
membrane of the relative pressure sensor of the system is in the
first part of the cavity, the other side of the second membrane of
the relative pressure sensor being in the second part of the
cavity.
8. A flow meter according to claim 7, wherein the first membrane of
the absolute pressure sensor is in the first part of the cavity.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to the field of micro machined
pressure sensors. More specifically it relates to methods and
systems for measuring an absolute and relative pressure and to
methods for manufacturing those systems.
BACKGROUND OF THE INVENTION
[0002] Micromechanical pressure sensors for measuring the pressure
in a medium have been designed and also methods for manufacturing
micromechanical sensors are described in literature. Often the
substrate in which those sensors are produced is made of
semiconductor material.
[0003] Differential or relative pressure sensors measure the
pressure difference between two pressure environments. Typically,
one of the pressure environments can be the environment's ambient
pressure (as opposed to a second, pressurized environment).
Absolute pressure sensors measure pressure with respect to a zero
pressure reference value (vacuum reference).
[0004] A cutting-edge precision pressure-sensing technology of
interest is a piezo resistive measurement technique. Piezo
resistive materials have the ability to convert mechanical stress
into a change of electrical properties. For a pressure sensor, a
semiconductor membrane is provided, and a p-type diffusion layer of
piezo resistive material is formed on top of it to make a resistive
layer on the membrane. The pressure on the semiconductor membrane
causes the surface stress on the semiconductor material changes the
resistance value. The signal changes are then amplified and
interpreted as pressure readings.
[0005] Relative pressure sensors have an open back side under the
membrane and the pressure sensor gives an indication of the
pressure difference between the front and the back side of the
membrane.
[0006] For absolute pressure sensors the back side of the membrane
is vacuum sealed. The pressure in the vacuum cavity at one side of
the membrane is the reference pressure which allows to make an
absolute pressure measurement. Absolute pressure sensors are for
example useful for measuring the pressure in a tire.
[0007] For certain applications, however, the need exists to
measure an absolute and a relative pressure at the same time.
Therefore, there is a need for a robust absolute pressure sensor as
well as a robust relative pressure sensor present on a same die and
for methods of manufacturing such systems.
SUMMARY OF THE INVENTION
[0008] It is an object of embodiments of the present invention to
provide an absolute and a relative pressure sensor on a same die,
producible by one and the same process.
[0009] It is an advantage of embodiments of the present invention
that co-integration of an absolute and a relative pressure sensor
allows obtaining an absolute and relative pressure sensor with
matched performance.
[0010] It is an advantage of embodiments of the present invention
that efficient manufacturing of a pressure sensor can be
obtained.
[0011] The above objective is accomplished by a method and device
according to embodiments of the present invention.
[0012] The present invention relates to a method for manufacturing
a pressure measurement system out of silicon wafers for measuring
an absolute and a relative pressure, the method comprising: [0013]
a first step wherein a shallow cavity and a deep cavity are etched
in the base wafer, such that the depth of the shallow cavity is
smaller than the depth of the deep cavity, [0014] a second step
wherein a top wafer is applied to the base wafer, [0015] a third
step wherein the top wafer is thinned for forming a first membrane
over the shallow cavity and for forming a second membrane over the
deep cavity, and wherein elements are formed in the top wafer, the
elements allowing performing pressure measurements resulting in a
first sensor and a second sensor, respectively, and [0016] a fourth
step wherein back thinning is applied on the base wafer such that
the deep cavity is opened from the backside of the base wafer and
that the shallow cavity is still closed by the backside of the base
wafer, such that the system can be used for measuring an absolute
pressure using the first sensor and a relative pressure using the
second sensor and wherein the etching in the first step defines the
edge of the membranes created by the subsequent steps.
[0017] It is an advantage that this implies that both sensors have
the same temperature characteristic. Therefore both sensors are
matched with regard to temperature changes. Also the membrane
thickness is matched for the two sensors so that the ratio of
sensitivity is only defined by the ratio of membrane or cavity
diameter. It is an advantage that the membrane diameter can be
accurately defined in the first etching step only. It is an
advantage of embodiments of the present invention that the
temperature calibration of only one sensor is required. It is an
advantage of embodiments of the present invention that the first
cavity and the second cavity can be etched from the same side of
the substrate.
[0018] The etching in the first step may be deep reactive-ion
etching.
[0019] It is an advantage of embodiments of the present invention
that Deep Reactive-Ion Etching (DRIE) is used. DRIE etching has
steep edges, for example compared with anisotropic etching such as
Potassium Hydroxide etching (KOH etching). Because of the steep
edges it is possible to make smaller sensors, for example than
those made using KOH. It is thus an advantage of embodiments of the
present invention that the depth of the cavity can be deeper than
what would be the case when using KOH as etching process. As the
walls are steeper the area at the bottom part is bigger for a DRIE
etching process than for a KOH etching process when the depth of
both cavities is the same. It is an advantage of embodiments of the
present invention that the opening at the bottom of the cavity is
bigger when DRIE etching is applied than when for example KOH
etching is applied. If the opening is too small, e.g. smaller than
three times the depth, it is difficult to protect the cavity with
gel against humidity. When humidity enters the sensor (e.g. through
condensation), this humidity in the sensor might freeze thereby
destroying the complete sensor. It is an advantage of embodiments
of the present invention that the form of the cavity is not bound
to the crystal structure as the anisotropy is not caused by the
crystal structure of the silicon but by the kinetic energy of the
etch gas (properties plasma). It is an advantage of embodiments of
the present invention that as well round as square shapes can be
chosen for the membrane shapes. It is an advantage of embodiments
of the present invention that the shape of the membrane can be
optimized with regard to the intended application. For example when
applied as a pressure sensor, the shape can have an influence on
the linearity properties of the sensor. Because of the steep sizes
and because of the independence on the crystal structure very small
sensors with a round membrane can be made.
[0020] The etching in the first step may comprise etching such that
the horizontal cross-section of the cavities is circular. It is an
advantage of embodiments of the present invention that round
membranes are possible, which allows small sensor sizes. It is an
advantage of embodiments of the present invention that less surface
is needed for the membrane. It is an advantage of embodiments of
the present invention that CMOS circuitry can be placed in the
areas that otherwise would be used by the corners of a square
membrane. It is an advantage of embodiments of the present
invention that arrays of pressure sensors can be made.
[0021] The first step may comprise two sub-steps: [0022] in a first
sub-step the deep cavity is etched leaving at least one pillar
inside the cavity, the pillar subdividing the cavity, [0023] in a
second substep the shallow cavity and a part of the at least one
pillar are etched, wherein both are etched to the same depth.
[0024] It is an advantage of embodiments of the present invention
that at least one pillar is present below the membrane of the
relative pressure sensor. When the pressure difference across the
membrane of the relative pressure sensor is so high that the
membrane touches the at least one pillar, the one or more pillars
will protect the membrane from breaking with higher pressures. At
least the membrane will break at a higher pressure difference than
when no such a pillar would be present. Similarly the bottom of the
shallow cavity protects the membrane of the absolute pressure
sensor. It is therefore an advantage of embodiments of the present
invention that the burst pressure can be increased with regard to
embodiments lacking such a pillar in the deep cavity.
[0025] The etching in the first step may comprise etching such that
the area of the horizontal cross-section of the deep cavity has a
different size than the area of the horizontal cross-section of the
shallow cavity.
[0026] It is an advantage of embodiments of the present invention
that the size of the membrane can be different between the absolute
and relative pressure sensor. This allows to optimize for different
pressure ranges on one chip. The piezo resistors and membrane
thickness will be the same (matched) of the two sensors as they are
processed next to each other on the base wafer. Therefore the ratio
of sensitivity is given by the square of the ratio of the membrane
diameters.
[0027] The formation of the elements allowing performing pressure
measurements may be performed on the stack of the base substrate
and the top substrate, after they have been applied to each
other.
[0028] The formation of the elements may be performed by CMOS like
processing.
[0029] Applying the base substrate on the top substrate may
comprise bonding said base substrate and said top substrate in a
vacuum environment.
[0030] The second step of applying a top wafer to the base wafer
may comprise joining the top wafer to the base wafer using fusion
bonding. The fusion bonding may be performed at a temperature above
900.degree. C., e.g. at a temperature above 1000.degree. C., for
obtaining a high quality bonding. It is an advantage of embodiments
of the present invention that no interconnect needs to be
positioned between the membrane and the base wafer comprising the
pressure ports.
[0031] It is an advantage of embodiments of the present invention
that a robust and stress free joint between the wafers can be
obtained. It is an advantage that no interconnect needs to be
provided between the membrane and the base wafer comprising the
pressure cavities.
[0032] The present invention also relates to a pressure measurement
system for measuring both an absolute and a relative pressure, the
system comprising: [0033] a base wafer comprising a closed shallow
cavity with a first membrane over the shallow cavity forming an
absolute pressure sensor, and [0034] on the same base wafer, an
open deep cavity, with and a second membrane over the deep cavity
forming a relative second pressure sensor, [0035] wherein the edge
of the first membrane respectively second membrane is determined by
the shallow cavity respectively the open deep cavity of the base
wafer.
[0036] The edges of the shallow cavity and of the deep cavity may
have an angle of between 80.degree. and 100.degree. with the
vertical axis.
[0037] The horizontal cross-section of the shallow and deep
cavities may be circular.
[0038] The shallow cavity may have a bottom and the deep cavity may
comprise at least one pillar, [0039] wherein the top of the pillar
may have the same height as the bottom of the shallow cavity.
[0040] The area of the horizontal cross-section of the deep cavity
may have a different size than the area of the horizontal
cross-section of the shallow cavity.
[0041] The relative and absolute pressure sensors may comprise
bondpads, wherein the bondpads can be made of platinum, aluminum or
gold.
[0042] The joint between the membrane and the base wafer of the
pressure cavities may be a joint obtained by fusing bonding, e.g. a
joint obtained by fusion bonding above 900.degree. C., e.g. above
1000.degree. C. The membrane and the base wafer of the pressure
cavities may be directly positioned to each other, without the need
for an interconnecting means.
[0043] It is an advantage of embodiments of the present invention
that the sensors can be used in reactive environments (e.g. for
inlet and exhaust applications).
[0044] The present invention also relates to a flow meter for
measuring a gas flow, the flow meter comprising a housing for
housing a pressure measurement system as described above,
[0045] wherein the housing comprises a tube having a first opening
and a second opening a cavity in communication with the first
opening and the second opening, wherein the system is positionable
such that the cavity is separated in a first part in communication
with the first opening and a second part in communication with the
second opening,
[0046] one side of the second membrane of the relative pressure
sensor of the system is in the first part of the cavity, the other
side of the second membrane of the relative pressure sensor being
in the second part of the cavity.
[0047] The first membrane of the absolute pressure sensor may be in
the first part of the cavity.
[0048] It is an advantage of embodiments of the present invention
that both the absolute pressure sensor as well as the relative
pressure sensor are present on the same die. It is an advantage of
embodiments of the present invention that with one device both the
absolute as well as the relative pressure can be measured. As a
consequence it is an advantage of embodiments of the present
invention that with one device a flow meter can be realised.
[0049] Particular and preferred aspects of the present invention
are set out in the accompanying independent and dependent claims.
Features from the dependent claims may be combined with features of
the independent claims and with features of other dependent claims
as appropriate and not merely as explicitly set out in the
claims.
[0050] The above and other aspects of the present invention will be
apparent from and elucidated with reference to the embodiment(s)
described hereinafter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] FIG. 1 shows a flow chart illustrating steps in an exemplary
method for making a system on a single die for measuring both an
absolute and a relative pressure according to embodiments of the
present invention.
[0052] FIG. 2 shows a schematic vertical cross-section of a system
according to embodiments of the present invention, comprising both
an absolute and a relative pressure sensor on a single die.
[0053] FIG. 3 illustrates a wafer and a mask for etching according
to embodiments of the present invention.
[0054] FIG. 4 illustrates a shallow and a deep cavity after etching
according to embodiments of the present invention.
[0055] FIG. 5 illustrates a schematic vertical cross-section of an
etched base wafer and top wafer in a method step according to
embodiments of the present invention.
[0056] FIG. 6 illustrates a schematic vertical cross-section of two
pressure sensors in a method step according to embodiments of the
present invention, before back grinding of the etched base
wafer.
[0057] FIG. 7 illustrates a wafer and two masks for etching
according to alternative embodiments of the present invention.
[0058] FIG. 8 illustrates a deep cavity with pillar after etching
according to embodiments of the present invention.
[0059] FIG. 9 illustrates a shallow cavity and a deep cavity with
pillar after etching according to embodiments of the present
invention.
[0060] FIG. 10 illustrates a schematic vertical cross-section of an
etched wafer as in FIG. 9, provided with a top wafer, in a method
step according to embodiments of the present invention.
[0061] FIG. 11 illustrates a schematic vertical cross-section of
two partially finished pressure sensors before back grinding
according to embodiments of the present invention.
[0062] FIG. 12 shows a schematic vertical cross-section of a system
comprising an absolute and a relative pressure sensor according to
embodiments of the present invention.
[0063] FIG. 13 is a top view of a particular embodiment of a
shallow cavity and a deep cavity for an absolute and a relative
pressure sensor according to embodiments of the present
invention.
[0064] FIG. 14 is a schematic representation of a flow sensor with
an absolute and a relative pressure sensor according to embodiments
of the present invention.
[0065] FIG. 15 shows a chart illustrating the relationship between
the air flow and the relative pressure measurement and the
dependency on the absolute pressure of this relationship.
[0066] FIG. 16 shows an overview of a set of wafers with different
cavity depths and membrane thicknesses in accordance with
embodiments of the present invention.
[0067] FIG. 17 illustrates a further embodiment of a pressure
sensor according to an embodiment of the present invention, whereby
the cavity for the relative sensor does not have a fixed width.
[0068] FIG. 18 illustrates the linearity of sensors having a square
shaped membrane and sensors having a round membrane, with matched
sensitivity.
[0069] The drawings are only schematic and are non-limiting. In the
drawings, the size of some of the elements may be exaggerated and
not drawn on scale for illustrative purposes.
[0070] Any reference signs in the claims shall not be construed as
limiting the scope.
[0071] In the different drawings, the same reference signs refer to
the same or analogous elements.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
[0072] The present invention will be described with respect to
particular embodiments and with reference to certain drawings but
the invention is not limited thereto but only by the claims. The
drawings described are only schematic and are non-limiting. In the
drawings, the size of some of the elements may be exaggerated and
not drawn on scale for illustrative purposes. The dimensions and
the relative dimensions do not correspond to actual reductions to
practice of the invention.
[0073] The terms first, second and the like in the description and
in the claims, are used for distinguishing between similar elements
and not necessarily for describing a sequence, either temporally,
spatially, in ranking or in any other manner. It is to be
understood that the terms so used are interchangeable under
appropriate circumstances and that the embodiments of the invention
described herein are capable of operation in other sequences than
described or illustrated herein.
[0074] Moreover, the terms top, under and the like in the
description and the claims are used for descriptive purposes and
not necessarily for describing relative positions. It is to be
understood that the terms so used are interchangeable under
appropriate circumstances and that the embodiments of the invention
described herein are capable of operation in other orientations
than described or illustrated herein.
[0075] It is to be noticed that the term "comprising", used in the
claims, should not be interpreted as being restricted to the means
listed thereafter; it does not exclude other elements or steps. It
is thus to be interpreted as specifying the presence of the stated
features, integers, steps or components as referred to, but does
not preclude the presence or addition of one or more other
features, integers, steps or components, or groups thereof. Thus,
the scope of the expression "a device comprising means A and B"
should not be limited to devices consisting only of components A
and B. It means that with respect to the present invention, the
only relevant components of the device are A and B.
[0076] Reference throughout this specification to "one embodiment"
or "an embodiment" means that a particular feature, structure or
characteristic described in connection with the embodiment is
included in at least one embodiment of the present invention. Thus,
appearances of the phrases "in one embodiment" or "in an
embodiment" in various places throughout this specification are not
necessarily all referring to the same embodiment, but may.
Furthermore, the particular features, structures or characteristics
may be combined in any suitable manner, as would be apparent to one
of ordinary skill in the art from this disclosure, in one or more
embodiments.
[0077] Similarly it should be appreciated that in the description
of exemplary embodiments of the invention, various features of the
invention are sometimes grouped together in a single embodiment,
figure, or description thereof for the purpose of streamlining the
disclosure and aiding in the understanding of one or more of the
various inventive aspects. This method of disclosure, however, is
not to be interpreted as reflecting an intention that the claimed
invention requires more features than are expressly recited in each
claim. Rather, as the following claims reflect, inventive aspects
lie in less than all features of a single foregoing disclosed
embodiment. Thus, the claims following the detailed description are
hereby expressly incorporated into this detailed description, with
each claim standing on its own as a separate embodiment of this
invention.
[0078] Furthermore, while some embodiments described herein include
some but not other features included in other embodiments,
combinations of features of different embodiments are meant to be
within the scope of the invention, and form different embodiments,
as would be understood by those in the art. For example, in the
following claims, any of the claimed embodiments can be used in any
combination.
[0079] In the description provided herein, numerous specific
details are set forth. However, it is understood that embodiments
of the invention may be practiced without these specific details.
In other instances, well-known methods, structures and techniques
have not been shown in detail in order not to obscure an
understanding of this description.
[0080] Where in embodiments of the present invention reference is
made to orientation indications such as vertical and horizontal it
is assumed that the wafer is in a horizontal plane. This means for
example that the cavities are etched in the vertical direction in
the horizontal wafer.
[0081] In a first aspect the present invention relates to a method
100 for manufacturing a pressure sensing system 200 in a base wafer
210 for measuring an absolute and a relative pressure. An
embodiment of such a method is schematically illustrated in FIG.
1.
[0082] The method 100 comprises a first step 110 wherein a shallow
cavity 220 and a deep cavity 230 are etched in the base wafer 210,
such that the depth of the shallow cavity 220 is smaller than the
depth of the deep cavity 230. The etching in the first step also
will determine the edges of the membranes that will be used in the
pressure sensors. In embodiments of the present invention a second
step 120 comprises applying a top wafer 510 on top of the base
wafer 210 into which the shallow and deep cavities 220, 230 have
been etched. In embodiments of the present invention, the top wafer
and base wafer can be joined by fusion bonding. This can occur at
high temperature, such as above 900.degree. C. or above
1000.degree. C., such that the bonding is robust and stress
free.
[0083] In a third step 125, the top wafer 510 is thinned, to the
thickness of a membrane, for forming a first membrane 262 over the
shallow cavity 220 and for forming a second membrane 272 over the
deep cavity 230.
[0084] Moreover, in the top wafer 510 elements, such as first
bondpads 261 for Piezo-resistivity measurements over the first
membrane 262, and second bondpads 271 for Piezo-resistivity
measurements over the second membrane 272 typically are formed. It
is to be noticed that a part of the electrical connections required
can be common. Thereby, the basis for a first pressure sensor 260
and a second pressure sensor 270 is realised.
[0085] In embodiments of the present invention the fourth step 130
comprises back thinning, for instance back grinding of the base
wafer 210 such that the deep cavity 230 is opened from the backside
of the base wafer 210 and such that the shallow cavity 220 is still
closed by the backside of the base wafer 210.
[0086] In embodiments of the present invention the result of the
method 100 is a pressure sensing system 200 according to
embodiments of the present invention, comprising a first membrane
262 over the shallow cavity 220 with a vacuum reference pressure at
the back of the first membrane 262, inside the shallow cavity 220,
and, on the same die, a second membrane 272 over the deep cavity
230 with a connection to the outside world. The cavities in the
base wafer thereby define the edges of the membranes used in the
pressure sensors. The pressure range of the cavity of the absolute
pressure sensor is between 0 and 0.25 bar and is defined during the
bonding. The bonding process allows in principle internal pressures
from 0 to 1 bar upto 400.degree. C. Vacuum is preferred as it will
push the wafers together. In embodiments of the present invention
the first membrane 262 and/or the second membrane 272 have a
thickness between 5 .mu.m and 100 .mu.m for example about 15 or 20
.mu.m. An overview of a set of exemplary wafers with different
cavity depths and membrane thicknesses is shown in FIG. 16.
[0087] Uniformity of the membrane thickness over the entire wafer
is important to minimise the sensitivity variation on a wafer. For
that reason it may be of interest to use a SOI wafer for the top
wafer and use the buried oxide as etch stop for the removal of the
bulk silicon. Using the buried oxide as etch stop also has the
advantage that the thickness variation of the bulk wafer does not
influence the membrane thickness variation. Without using such an
etch stop layer thinner parts of the bulk wafer will result in
relatively thicker membranes.
[0088] In embodiments of the present invention the result of the
method 100 is a pressure sensing system 200 according to
embodiments of the present invention that can be used for measuring
an absolute pressure using the first bondpads 261 of the first
sensor 260 and a relative pressure using the second bondpads 271 of
the second sensor 270. In embodiments of the present invention the
etching applied during the cavity etching step 110 may be
anisotropic wet etching. For example potassium hydroxide (KOH)
etching, might be applied. KOH etching has an etch rate which is
dependent on the crystal directions. Practically no etching occurs
perpendicular to the <111> planes. Such anisotropic etching
has as a consequence that the shallow cavity 220 or deep cavity 230
always has a rectangular horizontal cross section with the largest
diameter at the surface with the etch mask. Because of the
anisotropic etching it is not possible to create circular
membranes. Moreover, because of the anisotropic etching the area of
the horizontal cross section of the cavity becomes smaller when
going deeper into the wafer.
[0089] In alternative embodiments of the present invention
anisotropic plasma etching can be applied such as for example deep
reactive ion etching (DRIE). Using DRIE it is possible to realise
shallow cavities 220 and deep cavities 230 with vertical walls. An
example of this process is shown in FIG. 3, showing the base wafer
210 and a first mask 310 on the left and the wafer and a second
mask on the right. One starts with etching the wafer in the right
window by covering the left window with a mask. After the deep etch
the left window is opened and the shallow etch is performed, thus
using the mask 312 as shown on the right hand side. FIG. 4 shows
the shallow cavity 220 and the deep cavity 230 after applying DRIE.
In embodiments of the present invention the first mask 310 on the
backside of the wafer comprises optional openings (illustrated in
FIG. 3), which after etching will result in etched holes 410 (shown
in FIG. 4) that may serve as an alignment basis.
[0090] In these embodiments, both the shallow cavity 220 as well as
the deep cavity 230 have substantially vertical edges. Therefore,
the area of the cross-section of the cavity remains the same at any
depth within the wafer 210. Therefore also, the bottom opening of
the deep cavity 230 after back grinding is bigger with DRIE than
for example when anisotropic etching such as for instance KOH
etching is applied. In some embodiments, RIE etching also allows to
make the bottom of the cavity created even wider than the top. FIG.
2 illustrates an exemplary embodiment of the present invention. The
opening after back grinding is also shown in FIG. 2. In embodiments
of the present invention where for example KOH etching is applied
the opening that is created by back grinding is much smaller than
the membrane surface. Such an anisotropically etched opening is not
suitable for applications where water condensation at the back of
the membrane can take place. Such a shape may cause membrane cracks
when for instance water freezes near the membrane, as the shape of
the cavity does not allow the ice to move away from the membrane.
An important feature of the steep sidewalls obtained by DRIE
etching is that when water condensates and freezes at the membrane
272 it is not trapped in a tapered structure (like for example with
anisotropic etching) that might cause the membrane to break when
the water expands due to freezing. Therefore it is an advantage of
embodiments of the present invention that vertical walls can be
created (e.g. by DRIE) which result in an opening at the back of
the obtained system 200 that is similar to the membrane dimensions.
This cannot be achieved with KOH etching on standard wafers with a
surface in the <100> direction. Nevertheless, wafers with a
<110> orientation can be used, with a limited aspect
ratio.
[0091] In embodiments of the present invention both the absolute as
well as the relative pressure sensor are created using a single
fabrication process, comprising a first step 110 of cavity etching,
a second step 120 of applying the top wafer, a third step 125 of
thinning the top wafer and processing the electronic circuit with
the piezo resistors and a fourth step 130 of applying back grinding
for opening the deep cavity thereby realizing a relative pressure
sensor. In some embodiments, the cavities can be formed such that
they are not tapered. In embodiments of the present invention the
properties of the absolute and relative sensors are matched. In
embodiments of the present invention the membrane size of the
absolute pressure sensor 260 might be different from the membrane
size of the relative pressure sensor 270. It is an advantage of
embodiments of the present invention that the sensitivity for each
sensor 260, 270 can be optimized with regard to the application by
designing another membrane size. The sensors 260, 270 can for
example be optimized for different pressure ranges on one chip. The
size of the membranes, determined by the membrane outer edges, is
determined by the pressure cavities in the base wafer.
[0092] In embodiments of the present invention the etching step 110
comprises etching such that the horizontal cross-section of the
cavities is circular. In embodiments of the present invention both
round as well as square membranes can be defined. Hereby the
membranes may have the same form as the horizontal cross-section of
the cavity. In embodiments of the present invention DRIE is used to
create cavities with steep sidewalls independent of the crystal
lattice. FIG. 18 shows the linearity of matched sensors with a
round membrane and with a rectangular membrane. It can be seen that
the linearity for sensors with a round membrane is substantially
better than the linearity of the sensors with a rectangular
membrane. The sensors used are matched, meaning that they make use
of the same resistors and the same membrane thickness. The membrane
diameters can be different to achieve that the sensors have
different sensitivity although having the same membrane thickness
and the same piezoresistors to assure that both sensors give the
same output when the same pressure signal is applied. A second
consequence is that one can make very small sensors with a round
membrane and the bondpads in the corners of a die. FIG. 13 shows a
top view of a system 200 according to an embodiment of the present
invention, with two circular-shaped sensors. It shows a shallow
cavity 220 with a round horizontal cross-section. Applying DRIE in
combination with back grinding allows creating cavities with round
openings and with vertical walls thus avoiding membrane breakage
due to frozen condensated water, as explained above.
[0093] In embodiments of the present invention the etching step 110
may comprise two sub-steps, as illustrated in FIG. 7 to FIG. 9. In
a first sub-step--see FIG. 8--the deep cavity 230 is etched wherein
not all of the wafer material within the cavity 230 to be formed is
etched away such that at least one pillar 810 remains, the pillar
810 subdividing the cavity 230. Alternatively worded: a plurality
of cavities 230 may be formed close to one another, leaving a
structure of pillars 810, possibly forming a kind of walls, in
between them. In a second sub-step--see FIG. 9--the shallow cavity
220 and a top part of the pillar(s) 810 are etched, wherein both
are etched to the same depth. FIG. 13 shows an exemplary embodiment
of the present invention realized using method steps according to
the present invention. It shows a deep cavity 230 subdivided by
three pillars 810. The presence of the pillars 810 may limit the
membrane deflection when submitted to a high pressure, thus
protecting the membrane 272 from bursting. In embodiments of the
present invention the number of pillars 810 can vary or any other
structure supporting the membrane when the deflection exceeds a
certain threshold is possible. It is an advantage of embodiments of
the present invention that the burst pressure can be increased with
regard to a pressure sensor lacking the supporting pillar(s) 810.
In FIG. 13 also the first bondpads 261 and second bondpads 271 are
shown.
[0094] The different method steps according to an exemplary
embodiment of the present invention are illustrated in FIG. 7 to
FIG. 11. FIG. 7 shows a, for instance, 525 .mu.m thick base wafer
and a masking deposition with two level patterning. An exemplary
embodiment of the first sub-step of the etching step 110 is
illustrated in FIG. 8. During this step deep cavities are etched by
means of a bottom mask 310, leaving pillars 810 across the area
which later will be covered by the second membrane 272. A top view
of these pillars 810, in this embodiment forming walls, is also
shown in FIG. 13. In a second sub-step of the etching step 110 the
top mask 710 is transferred to the surface to etch the shallow
cavity. This is illustrated in FIG. 9. Using the top mask 710, free
parts of the bottom mask 310 are removed, as well as thereafter
accessible semiconductor material of the base wafer 210. During the
second step 120 a top wafer (e.g. an oxide strip) 510 is applied,
as illustrated in FIG. 10.
[0095] In some embodiments, both membrane openings are first made
in an oxide mask, one of these openings is consequently covered
with resist and RIE etching is then performed with hardly any
etching of the resist. After the first etch forming at least part
of the first opening, the resist can be stripped and then both
openings are etched together.
[0096] An example thereof is illustrated in FIGS. 5 and 6 or
similarly in FIG. 10 and FIG. 11. In FIG. 5 the top wafer, which
can for instance be a silicon on insulator wafer, may be applied
through fusion bonding, after which thinning of the top wafer may
be applied to form the membranes 262, 272. The latter is
encompassed in step 125 which further may encompass the formation
of elements such as electrical contacts. FIG. 6 shows an exemplary
result after applying processing steps for creating the electrical
contacts of a piezoelectric sensor. It thereby is an advantage that
CMOS compatible processing can be performed.
[0097] FIG. 12 shows an exemplary result after applying the third
step 130 (i.e. back grinding) on the wafer. In the exemplary
embodiment of FIG. 12 the thickness of the base wafer is reduced
from 525 .mu.m to 400 .mu.m by the back grinding. As can be seen on
FIG. 12 this allowed to open the deep cavity 230 while the shallow
cavity 220 still remained closed. The typical wafer thickness
varies between 1 mm and 100 .mu.m and backgrinding is performed for
allowing opening of the deep cavity.
[0098] In embodiments of the present invention the system 200
comprises an absolute pressure sensor 260 (with closed cavity 220)
and a relative pressure sensor 270 (with opened cavity 230) on a
single chip.
[0099] In embodiments of the present invention the depth of the
shallow cavity 220, for the absolute sensor 260, may be chosen in
such a way that the bottom of the cavity supports the membrane 262
before the membrane 262 possibly bursts under outside pressure.
Therefore it is an advantage of embodiments of the present
invention that the pressure sensor according to embodiments of the
present invention can withstand higher pressures than would be the
case for an equivalent sensor without support for the membrane from
the bottom of the cavity. The depth of the shallow cavity is
therefore between 2 .mu.m and 20 .mu.m. The smaller the shallow
cavity, the more sensitive it is for cavity pressure variations. In
a second aspect, the present invention relates to a system 200 for
measuring an absolute and a relative pressure. The system 200
comprises a base wafer 210 comprising a shallow cavity 220 and a
deep cavity 230. The depth of the shallow cavity 220 is smaller
than the depth of the deep cavity 230. The base wafer also may be
referred to as bottom wafer.
[0100] The system 200 moreover comprises a top wafer 510 on top of
the wafer 210. The top wafer 510 comprises elements forming a first
sensor 260 and a second sensor 270. The first sensor 260 comprises
a first membrane 262 over the shallow cavity 220 and first bondpads
261 for Piezo-resistivity measurements over the first membrane 262.
The second sensor 270 comprises a second membrane 272 over the deep
cavity 230 and second bondpads 271 for Piezo-resistivity
measurements over the second membrane 272.
[0101] The back side of the base wafer 210 is removed such that the
deep cavity 230 is opened but that the shallow cavity 220 is still
closed by the backside of the base wafer 210. Therefore the system
200 can be used for measuring an absolute pressure with the first
sensor 260 through the first bondpads 261 and a relative pressure
with the second sensor 270 through the second bondpads 271.
[0102] FIG. 2 is a schematic drawing of an exemplary embodiment of
the present invention. It shows the pressure measurement system 200
comprising a first sensor 260 formed by a closed shallow cavity
220, a membrane 262 above it and bondpads 261 for piezo-restivity
measurement over the first membrane 262. The pressure measurement
system 200 furthermore comprises a second sensor 270 formed by a
deep open cavity 230, a membrane 272 above it, and bondpads 271 for
piezo-resistivity measurements over the second membrane 272.
[0103] In embodiments of the present invention the edges of the
shallow cavity 220 and of the deep cavity 230 have an angle between
80.degree. and 100.degree., such as for example 100.degree., with
the vertical axis. The exemplary embodiment of FIG. 2 shows
cavities with vertical walls. For the exemplary embodiment of FIG.
2 DRIE was used as etching method in the etching step 110.
[0104] In embodiments of the present invention the horizontal
cross-section of the cavities is circular. An exemplary embodiment
thereof is shown in FIG. 13. FIG. 13 shows the top view of a system
200. A shallow cavity 220 with circular cross section is shown.
[0105] In embodiments of the present invention the deep cavity 230
comprises at least one vertical pillar 810 wherein the top of the
pillar reaches up to the same height as the bottom of the shallow
cavity 220. An exemplary embodiment thereof is shown in FIG. 12 and
in FIG. 13. FIG. 12 shows a vertical cross section of a system 200.
In the deep cavity 230 a pillar 810 is shown. FIG. 13 also shows
the deep cavity 230 and the pillars 810.
[0106] In embodiments of the present invention the area of the
horizontal cross-section of the deep cavity 230 may have a
different size than the area of the horizontal cross-section of the
shallow cavity 220.
[0107] In embodiments of the present invention the first bondpads
261 and/or the second bondpads 271 are made of platinum or another
metal suitable for wire bonding.
[0108] In still other embodiments, the open cavity forming the
relative pressure sensor does not have a constant diameter. In some
embodiments, the open cavity is formed of a more broad first
subcavity lying deeper in the base substrate having a first
diameter and a second subcavity, in connection with the first
subcavity, which has a second diameter being smaller than the first
diameter. An example thereof is shown in FIG. 17.
[0109] In this way it is also possible to create the opening to the
back essentially outside the membrane area and to etch the channel
between the hole and the membrane cavity during the membrane cavity
etch.
[0110] In a third aspect, the present invention relates to a flow
meter 1400 for measuring a gas flow. The flow meter 1400 comprises
a housing 1410 for housing a pressure measurement system 200
according to embodiments of the present invention. The housing 1410
comprises a tube 1420 having an inlet and an outlet, wherein the
tube has a first opening 1430 and a second opening 1440 along its
length. The tube contains a venturi opening 1435 between the inlets
1430 and 1440 which causes a pressure difference between the two
inlets when a flow is present in the tube. Both openings are
oriented towards a cavity 1450 also comprised in the housing 1410.
The pressure measurement system 200 according to embodiments of the
present invention is positionable, and in the embodiment
illustrated in FIG. 14 is positioned, in the cavity 1450.
Positioning of the pressure measurement system 200 in the cavity
1450 separates the cavity 1450 in a first part 1460 communicating
with the first opening and a second part 1470 communicating with
the second opening. The positioning of the system 200 moreover is
such that one side of the second membrane 272 of the relative
pressure sensor 270 of the system 200 is in the first part 1460 of
the cavity 1450, and that the open end of the deep cavity 230 of
the relative pressure sensor is in the second part 1470 of the
cavity 1450. The positioning of the system 200 moreover is such
that the first membrane 262 of the absolute pressure sensor 260 is
in the first part 1460 of the cavity 1450.
[0111] The pressure measurement system 200 according to embodiments
of the present invention is thus mounted in the flow meter 1400
according to embodiments of the present invention such that a
differential pressure can be measured between the first opening
1430 of the tube 1420 and the second opening 1440 thereof, by the
relative pressure sensor 270 of the pressure measurement system
200. Moreover the absolute pressure at the first opening 1430 of
the tube 1420 can be measured by the absolute pressure sensor 260
of the pressure measurement system 200. A connector 1480, connected
with the first bondpads 261, may extend outside the housing 1410
thereby enabling measurements from outside the housing 1410. In
embodiments of the present invention the housing 1410 may be made
of plastic. It is convenient to place the read-out electronics in
the housing also. Such read-out electronics can be provided by a
CMOS circuit. The measurement principle on which the flow meter is
based is the Bernouilli principle. It is an advantage of
embodiments of the present invention that an absolute and relative
pressure sensor integrated on one die can be applied for building a
flow meter according to the Bernouilly principle. According to
embodiments of the present invention, the circuitry for the piezo
resistors can be a full blown CMOS process where the interface
electronics are co-integrated with the piezo resistors.
[0112] A chip according to embodiments of the present invention
with an absolute and a relative sensor with different sensitivity
is advantageous. For instance, flow meters exist where the absolute
pressure sensor has a range of about 1 bar and the relative
pressure sensor has a range of 0.5 bar. The present invention
allows to have absolute and relative pressure sensors on a single
chip, each optimized for their sensitivity range.
[0113] FIG. 15 shows the output of an orifice flow sensor in
function of the air flow. The individual graphs are each for a
different absolute pressure:
[0114] graph 1610 corresponds with an absolute pressure of 800
mBar,
[0115] graph 1620 corresponds with an absolute pressure of 975
mBar,
[0116] graph 1630 corresponds with an absolute pressure of 1150
mBar,
[0117] graph 1640 corresponds with an absolute pressure of 1500
mBar.
[0118] The graphs of FIG. 15 therefore illustrate the importance of
measuring the absolute pressure for flow measurements.
* * * * *