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JP2015501594

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This translation is machine-generated. It cannot be guaranteed that it is intelligible, accurate,
complete, reliable or fit for specific purposes. Critical decisions, such as commercially relevant or
financial decisions, should not be based on machine-translation output.
DESCRIPTION JP2015501594
The present invention relates to a pre-crushed volume micromachined transducer cell 10 having
a substrate 12 and a membrane 14 covering a total membrane area Atotal. The cavity 20 is
formed between the membrane 14 and the substrate 12. The membrane has a hole 15 and an
edge portion 14 a surrounding the hole 15. Cell 10 further comprises a stress layer 17 on
membrane 14. The stress layer 17 has a predetermined stress value on the membrane 14. The
stress layer 17 is configured to provide a bending moment on the membrane 14 in the direction
towards the substrate 12 such that the edge portion 14 a of the membrane 14 is crushed against
the substrate 12. The invention further relates to a method of manufacturing such a pre-crushing
capacity micromachined transducer cell 10.
Pre-crushed capacity micromachined transducer cell with stress layer
[0001]
The present invention relates to pre-crushing capacity micromachined transducer cells, in
particular to capacitive micromachined ultrasonic transducer (cMUT) cells or capacitive
micromachined pressure sensor cells and methods of manufacturing the same.
[0002]
In recent years, micromachined ultrasonic transducers (MUTs) have been developed.
Micromachined ultrasound transducers were manufactured in two design approaches. One is
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using a semiconductor layer having a piezoelectric property (pMUT), and the other is a
membrane (or diaphragm) and a substrate provided with an electrode (or electrode plate) for
forming a capacitor. The so-called capacitive micromachined ultrasound transducer (cMUT) used.
[0003]
The cMUT cell has a cavity below the membrane. When receiving ultrasound, the ultrasound
causes the membrane to move or vibrate. Variations in capacitance between the electrodes can
be detected. The ultrasound is thereby converted into a corresponding electrical signal.
Conversely, an electrical signal applied to the electrodes causes the membrane to move or
vibrate. Thereby, an ultrasonic wave is transmitted.
[0004]
Initially, cMUT cells were created to operate in a mode known as "non-crushing" mode.
Conventional "non-crushing" cMUT cells are essentially non-linear devices. Here, the efficiency
strongly depends on the bias voltage applied between the electrodes.
[0005]
To solve this problem, so-called "pre-compressed" cMUT cells have recently been developed. In
pre-crushed cMUT cells, portions of the membrane are permanently crushed or fixed to the
bottom of the cavity (or substrate). Above a certain bias voltage, the efficiency of the pre-crushed
cMUT cell is substantially independent of the bias voltage. This makes this cMUT cell much more
linear.
[0006]
In pre-crushed cMUT cells, the membrane can be crushed using different methods, for example
using electrical or mechanical crushing.
[0007]
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Electrical breakdown can be achieved, for example, using a bias voltage.
WO 2009/037655 A2 discloses a method of producing cMUTs, which is the step of providing
mostly completed cMUTs, which mostly completed cMUTs are (i) substrate layer, (ii) electrode
Defining one or more cMUT elements comprising a plate, (iii) membrane layer and (iv) electrode
ring, defining at least one hole through the membrane layer for each cMUT element, and in the
substrate layer Applying a bias voltage across the membrane and substrate layer of the one or
more cMUT elements to crush the membrane layer against it and applying a case layer to fix the
crushed membrane layer to the substrate layer And sealing.
[0008]
Mechanical crush can be achieved, for example, using ambient air pressure. WO 2010/097729
A1 is a cMUT cell having a substrate, a first electrode attached to the substrate, a movable
membrane formed in a spaced relationship to the first electrode, a second electrode attached to
the membrane, and a holding member. Disclose. The holding member functions to hold the
membrane in its pre-crushed state in the absence of a bias voltage when the membrane is in the
pre-crushed state and overlaps with the movable membrane. In one example, the holding
member is cast across the cMUT transducer cell while the membrane is pre-crushed by the
application of (atmospheric) pressure to the membrane.
[0009]
As disclosed in WO 2010/097729 A1, pre-crushed cMUT cells have been successfully
manufactured as low frequency cMUT cells with membranes of relatively large diameter. The
crush pressure is low and the cMUT cell is pre-crushed by ambient air pressure (ie, the
membrane touches the bottom of the cavity). However, for high frequency cMUT cells, the
retention members disclosed in WO 2010/097729 A1 can not be applied. Because the crush
pressure is very large, for example 5 bar or even 10 bar can easily be exceeded. In this case, the
retention layer disclosed in WO 2010/097729 A1 is not strong enough to hold the membrane in
place. Thus, the problem with the cMUT cell disclosed in WO 2010/097729 A1 is that although
it is essentially a "large membrane" solution, it does not work for high frequency cMUT cells with
small membrane diameters.
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[0010]
There is a need to improve such pre-crushing capacity micromachined transducer cells,
especially for high frequencies.
[0011]
The object of the present invention is to provide an improved pre-crushing capacity
micromachined transducer cell and a method of manufacturing the same, in particular for high
frequency pre-crushing capacity micromachined transducers.
[0012]
In a first aspect of the invention, a pre-crushed capacity micromachined transducer cell is
provided, which is a substrate and a membrane covering a total membrane area, wherein a cavity
is formed between the membrane and the substrate, And a membrane including an edge portion
surrounding the hole and the hole.
The cell is further a stress layer on the membrane, having a predetermined stress value on the
membrane, the membrane in a direction towards the substrate such that an edge portion of the
membrane is crushed against the substrate. And a stress layer configured to provide a bending
moment thereon.
[0013]
In another aspect of the present invention, a pre-crushed volume micromachined transducer cell
is provided which has a membrane and a cavity formed between the membrane and a substrate.
The membrane has a hole and an edge portion surrounding the hole. The edge portion of the
membrane is crushed against the substrate. The cell further has a stress layer formed on the
membrane. This stress layer has a predetermined stress or stress value on the membrane.
[0014]
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In a further aspect of the invention, there is provided a method of manufacturing a pre-crushed
capacity micromachined transducer cell, the method comprising the steps of providing a
substrate and providing a membrane covering the entire membrane area. A cavity is formed
between the membrane and the substrate. The method further comprises the steps of providing a
stress layer on the membrane, the stress layer having a predetermined stress value on the
membrane, and providing a hole in the membrane The membrane includes an edge portion
surrounding the hole, and the stress layer is configured to provide a bending moment on the
membrane in a direction towards the substrate such that the edge portion of the membrane is
crushed against the substrate And having a step.
[0015]
The basic idea of the invention is to provide a simple solution to provide pre-crushing capacity
micromachined transducer cells, in particular high frequency pre-crushing capacity
micromachined transducer cells. A stress layer (or at least that portion of the final cell) with a
particular stress or stress value on the membrane is present or formed on the membrane. The
stress layer is adapted or configured to provide a bending moment (or membrane refraction) on
the membrane in a direction towards the substrate such that an edge portion of the membrane is
crushed against the substrate. In other words, the bending moment is large enough to cause the
substrate to crush the edge portion. The stress layer is used to bring the edge portion of the
membrane to the substrate (or the bottom of the cavity). In other words, the stress layer provides
a bending moment that causes the membrane to collapse (in particular, a bending moment large
enough to bend the membrane relative to the bottom of the substrate or cavity). The stress layer
thus causes the membrane to collapse. In particular, the amplitude of refraction (i.e. without
substrate) due to refraction or forcing should exceed the height (or gap distance) of the cavity so
that the membrane is crushed against the substrate. In particular, if the stress layer is arranged
on the side of the membrane facing away from the substrate, the stress value should be negative
and thus compressive stress or force. In this case, the stress layer has a predetermined amount of
compressive stress. Alternatively, if the stress layer is placed on the side of the membrane facing
the substrate, the stress value should be positive and thus tensile. In this case, the stress layer
has a predetermined amount of tension.
[0016]
An additional stress layer is applied or introduced into the cell (formed on the membrane), which
has a specific stress value or level (intentionally made) to collapse the membrane, preferably a
specific In position. Also, the position of the stress layer can serve to provide a bending moment
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on the membrane in the direction towards the substrate (or to create a displacement of the
membrane in the direction towards the substrate). The stress value and possibly the position of
the stress layer can also be selected such that the edge portion of the membrane is crushed
against the substrate when the holes are provided in the membrane. The stress layer can be
present temporarily (e.g. only during manufacture) or permanently (e.g. in the final cell to be
manufactured).
[0017]
Preferred embodiments of the invention are defined in the dependent claims. It is to be
understood that the manufacturing method as claimed in the claims has similar and / or identical
preferred embodiments as the cells as claimed in the claims and the dependent claims.
[0018]
In one embodiment, the stress layer extends beyond the total membrane area. Thus, the position
of the stress layer provides a bending moment on the membrane in the direction towards the
substrate. For example, in the case of circular shaped cells and membranes, the total membrane
area can be defined by the diameter of the membrane (or cavity). In such cases, the outer radius
of the stressed layer may be greater than the radius of the total membrane area.
[0019]
In another embodiment, the stressed layer has holes. Thus, holes in the membrane can be easily
provided (especially in the middle of the total membrane area). The holes in the stress layer can
also be in particular in the middle of the total membrane area. Preferably, the centers of the holes
in the stress layer and the centers of the holes in the membrane are aligned.
[0020]
In a variant of this embodiment, the holes in the stressed layer are larger than the holes in the
membrane. Thus, the position of the stress layer helps to provide a bending moment on the
membrane in the direction towards the substrate, especially when used in combination with the
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previous embodiments. Such holes in the stress layer have a beneficial effect on the refractive
profile of the membrane. The size of the holes can be optimized to achieve the greatest effect. For
example, in the case of circular shaped cells and membranes, the inner radius of the stressed
layer (or the edge of the edge portion) may be larger than the radius of the holes in the
membrane.
[0021]
In a further embodiment, the stress layer is made of metal or metal alloy. These materials have
been shown to provide the desired stress values in a simple manner.
[0022]
In a further embodiment, the stress layer is made of at least one material selected from the group
comprising tungsten (W), titanium-tungsten (TiW), molybdenum (Mo) and molybdenumchromium (MoCr). These materials have been shown to provide desired stress values in an
advantageous manner. Because they provide a high melting point. From these metals (alloys), the
stress values can be tuned to the required values.
[0023]
Still further, in one embodiment, the crush pressure of the membrane is greater than 1 Bar. In a
variant of this embodiment, the crush pressure of the membrane is greater than 5 Bar. In another
variation of this embodiment, the crush pressure of the membrane is greater than 10 Bar.
Membranes with a crush pressure greater than 1 Bar (or 5 Bar or even 10 Bar) are not crushed
by environmental pressure. However, a stress layer is required to provide a crush.
[0024]
In another embodiment, the diameter of the membrane is less than or equal to 150 μm, in
particular less than or equal to 100 μm. Thus, a high frequency pre-crushing capacity
micromachined transducer cell is provided. The center frequency may for example be more than
8 MHz, in particular more than 10 MHz.
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[0025]
In a further embodiment, the cell further comprises a plug disposed in the hole of the membrane.
This plug is arranged only in the subarea of the total membrane area covered by the membrane.
[0026]
In a further embodiment, the cell further comprises a cover layer disposed on the membrane and
/ or the plug. In this way, matching of the cell or membrane thickness to the specific resonant
frequency of the cell (thus providing acoustic property control) or to the operating range can be
realized. The cover layer can also provide chemical passivation.
[0027]
In another embodiment, the cell further comprises a first electrode on or in the substrate and / or
a second electrode on or in the membrane. Thus, a capacity cell can be provided in a simple
manner.
[0028]
In a further embodiment, the second electrode is an annular electrode. In another embodiment,
the cavity is an annular cavity. In any of these embodiments, the cells can be circular shaped
cells. The circular shape is an advantageous cell shape. Because it provides a fairly good filling of
the available space and / or in particular, unwanted signals which compete with the desired
mode for the energy to be transmitted or which obscure the desired received signal. This is
because it hardly provides the higher order vibrational modes that it produces.
[0029]
In a further embodiment, the method further comprises the step of removing the stress layer.
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Thus, a cell with improved thermal properties is provided. This in particular reduces the
temperature dependence (for example when the stress layer is made of metal). In particular, the
entire stress layer can be removed, or a substantial portion of the stress layer (e.g. leaving only a
portion of the stress layer) can be removed.
[0030]
In another embodiment, stress values are provided when additional layers are provided on the
membrane. Thus, stress values can be provided in a simple manner.
[0031]
In one embodiment, the cell is a capacitive micromachined ultrasound transducer (cMUT) cell for
transmission and / or reception of ultrasound. In an alternative embodiment, the cell is a
capacitive micromachined pressure transducer (or sensor) cell that measures pressure.
[0032]
FIG. 1 shows a schematic cross section of a pre-crushing capacity micromachined transducer cell
according to a first embodiment. Fig. 2 shows a schematic cross section of a pre-crushing
capacity micromachined transducer cell according to a second embodiment. FIG. 6 illustrates
different manufacturing steps of a method of manufacturing a crushed capacity micromachined
transducer cell according to the first embodiment or the second embodiment. FIG. 6 illustrates
different manufacturing steps of a method of manufacturing a crushed capacity micromachined
transducer cell according to the first embodiment or the second embodiment. FIG. 6 illustrates
different manufacturing steps of a method of manufacturing a crushed capacity micromachined
transducer cell according to the first embodiment or the second embodiment. FIG. 6 illustrates
different manufacturing steps of a method of manufacturing a crushed capacity micromachined
transducer cell according to the first embodiment or the second embodiment. FIG. 6 illustrates
different manufacturing steps of a method of manufacturing a crushed capacity micromachined
transducer cell according to the first embodiment or the second embodiment. FIG. 5 illustrates
the fabrication steps of a method of fabricating a pre-crushed capacity micromachined
transducer cell according to a second embodiment. FIG. 5 illustrates the fabrication steps of a
method of fabricating a pre-crushed capacity micromachined transducer cell according to a
second embodiment. FIG. 7 shows a top view of a set of masks for a pre-crushed capacity
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micromachined transducer cell according to an embodiment.
[0033]
These and other aspects of the invention will be apparent from and will be elucidated with
reference to the embodiments described hereinafter.
[0034]
It can be shown that the collapsing pressure Pc (i.e. the static air pressure or water pressure at
which the membrane just touches the bottom of the substrate or cavity) is equivalent to:
Where g is the height of the cavity (also referred to as the gap), r is the radius of the membrane, t
is the membrane thickness, E is the Young's modulus, and v is Poison It is a ratio.
[0035]
As can be seen from the above equation, the crush pressure can be scaled as <img class =
"EMIRef" id = "269562528-00004" />. Here, r is the radius of the membrane. The smaller
diameter of the membrane implies a much higher crush pressure. For many practical ultrasound
devices, for example, for a 10 MHz ultrasound probe, the crush pressure easily exceeds 5 Bar or
10 Bar. This is especially true for high frequency cells, for example when the center frequency
exceeds about 8 MHz. In such a case, the holding member or layer as disclosed in, for example,
WO 2010/097729 can not maintain the crush mode.
[0036]
FIG. 1 shows a schematic cross section of a pre-crushing capacity micromachined transducer cell
10 according to a first embodiment, and FIG. 2 shows a schematic cross-section of a precrushing
capacity micromachined transducer cell 10 according to a second embodiment. Show. The cell
10 described herein may in particular be a high frequency pre-crushing capacity micromachined
transducer cell, for example, a membrane diameter of less than 150 μm (especially less than
100 μm) and / or a center frequency of 8 MHz or more, eg It can be a cell that you have. For
example, a transducer cell having a frequency of about 10 MHz has a membrane diameter of
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about 60 μm. However, it should be understood that the cells described herein can also be
applied to lower frequencies.
[0037]
The cell 10 of FIG. 1 or 2 has a substrate 12. The substrate 12 can be made of, for example,
silicon, but is not limited thereto. The substrate 12 may, for example, be electrically connected to
the cell 10 and may carry an ASIC that provides the outer electrical connection.
[0038]
The cell 10 further comprises a movable or flexible membrane 14 (or diaphragm) covering the
total membrane area Atotal (in or parallel to the plane of the substrate). The cavity 20 is formed
between the membrane 14 and the substrate 12. The membrane 14 has a hole 15 and an (inner)
edge portion 14 a surrounding the hole 15. The (inner) edge portion 15 forms a step or shelf or
ridge. In other words, the top surface of the edge portion 14a is higher than the top surface of
the membrane 14 (or its electrode). The holes 15 of the membrane 14 are arranged at the center
or central area of the total membrane area Atotal. Edge portion 14a is crushed relative to
substrate 12, thus providing a pre-crushed cell. In other words, edge portion 14 a (or membrane
14) contacts substrate 12 (or the bottom of cavity 20).
[0039]
The cell 10 of the first embodiment shown in FIG. 1 or the second embodiment shown in FIG. 2
further comprises a first electrode 16 formed on or in a substrate 12 and a membrane 14 (or
And a second electrode 18 formed (embedded therein). In other words, the substrate 12 has a
first electrode in or on it, and the membrane 14 has a second electrode 18 there. In particular,
the first electrode 16 can be displayed as being part of the substrate 12 and the second electrode
18 can be displayed as being part of the membrane 14. Thus, a capacity cell is provided. The cell
10 may in particular be a capacitive micromachined ultrasound transducer for the transmission
and / or reception of ultrasound. With respect to the reception of ultrasound, the ultrasound
causes the membrane 14 (and its electrode 18) to move or vibrate, and variations in the
capacitance between the first electrode 16 and the second electrode 18 are detected Can be The
ultrasound is thereby converted into a corresponding electrical signal. Conversely, the electrical
signals applied to the electrodes 16, 18 cause the membrane 14 (and its electrodes 18) to move
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or vibrate, thereby transmitting ultrasound. Alternatively, the cell can be any other suitable
capacitive micromachined transducer cell. For example, it can be a capacitive micromachined
pressure transducer (or sensor) cell that measures pressure.
[0040]
In the embodiment described herein, the membrane 14 has multiple (eg, two) layers. This is in
particular an electrically insulating layer or a dielectric layer (e.g. an ONO-layer), with a second
electrode embedded therein or between. For example, each ONO layer can have a thickness of
about 0.25 μm each, but is not limited thereto. Furthermore, for example, the diameter of the
membrane 14 can be between 25 and 150 μm, in particular between 50 and 150 μm, or
between 40 and 90 μm, or between 60 and 90 μm. Also, for example, the height of the cavity
(gap height) can be between 0.25 and 0.5 μm. However, it should be understood that any other
suitable membrane (eg, a single layer membrane) or dimensions can be used. Furthermore, in the
embodiments described herein, the second (upper) electrode 18 is an annular electrode (or an
annular shaped electrode). It has a hole at its center or center. However, it should be understood
that any other suitable second electrode can be used.
[0041]
Compared to the second embodiment of FIG. 2, the cell 10 of the first embodiment of FIG. 1
further comprises a (permanent) stress layer 17 formed on the membrane. This stress layer 17
has a predetermined (especially non-zero) stress or stress value on the membrane 14. The stress
layer is configured to provide a bending moment (or force) on the membrane 14 (and thus the
refraction of the membrane 14) in a direction towards the substrate 12 (downward in FIG. 1). As
a result, the edge portion 14 a of the membrane 14 is crushed against the substrate 12. The
bending moment is large enough to crush the edge portion 14 a relative to the substrate 12. In
the first embodiment of FIG. 1, the stress layer 17 is permanently present and thus in the final
cell to be manufactured. Thus, in this embodiment, the stressed layer 17 is also movable or
flexible so that it can move or vibrate with the membrane 14.
[0042]
In the first embodiment of FIG. 1, the position of stress layer 17 also serves to provide a bending
moment (or refraction) on the membrane in the direction towards substrate 12. As can be seen
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from FIG. 1, the stress layer 17 extends beyond the total membrane area Atotal. The stress layer
17 further comprises holes 19. The holes 19 in the stressed layer 17 are in the center or central
area of the total membrane area Atotal and are aligned with the holes 15 in the membrane 14.
However, the holes 19 in the stressed layer 17 are larger than the holes 15 in the membrane 14.
[0043]
With respect to the choice of stress layer material, many materials can have built-in stress when
deposited. This is due, for example, to the chemical composition, the thermal contraction between
the deposition temperature and the ambient temperature, or a combination of both. As the
material layer is deposited, the deposition state can determine the stress value. For example, the
stressed layer can be deposited by sputtering (e.g. for the deposition of a metal stressed layer). In
such cases, for example, the gas pressure during sputtering can determine the stress value.
[0044]
The stress layer 17 is made in particular of at least one material selected from the group
comprising metals or metal alloys, in particular tungsten (W), titanium-tungsten (TiW),
molybdenum (Mo) and molybdenum-chromium (MoCr). be able to. These materials have been
shown to provide desired stress values in an advantageous manner. Because they provide a high
melting point. From these metals (alloys), the stress values can be tuned to the required values. In
another example, stress layer 17 can be made of a combination of compressive nitride and etch
stop layer (preferably metal). Alternatively, the stress layer 17 can also be made of non-metallic
material. For example, stress layer 17 can be made of Si3 N4 (silicon nitride), which is deposited
particularly under "stress conditions".
[0045]
For example, stress layer 17 (eg, made of Si 3 N 4) can be deposited by plasma enhanced
chemical vapor deposition. As an example, when silicon nitride is deposited in a plasma enhanced
chemical vapor deposition system, and when the operating parameters of the system (eg
pressure, temperature, plasma power, RF settings or gas flow rates for both elements) are
adjusted , The ratio of Si to N can be varied (eg, changing from an exact 3: 4 ratio). This can be
used, for example, to induce built-in stress in the stress layer.
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[0046]
In the embodiment of FIG. 1, the stress layer 17 is disposed on the side of the membrane 14
facing away from the substrate (at the top of the membrane in FIG. 1). Thus, to provide a crush
condition, the stress value should be negative and thus compressive. In other words, the stress
layer 17 of FIG. 1 has a predetermined amount of compressive stress. However, it should be
understood that alternatively the stress layer can also be arranged on the side of the membrane
facing the substrate. The stress value should then be positive, and thus tensile, to provide a crush
condition. In this case, the stress layer has a predetermined amount of tensile stress.
[0047]
The stress value depends on the geometry, in particular the thickness t of the membrane, the
diameter (or radius) of the membrane and / or the height h 20 of the cavity 20 (also called gap
value g). Thus, a certain amount of refraction is required. The stress values are chosen in
particular such that the amplitude of refraction exceeds the (maximum) height h 20 of the cavity
20. As a result, the membrane 14 is crushed against the substrate 12. For example, stress values
can be on the order of two to three times -100 megapascals (MPa). The aforementioned metals
can be tuned, for example, to -1000 MPa. In particular, the crush pressure Pc (see equation
above) of the membrane 14 (and its electrodes 18) can be greater than 1 Bar, or 5 Bar, or 10 Bar.
[0048]
The layers of membrane 14 (including its electrodes 18), the cover layer 40, and in the
embodiment of FIG. 1, the stress layer 17 move or vibrate. These layers determine the overall
stiffness of the membrane or vibrating element. The overall stiffness, as well as the membrane
diameter and the gap height h20, is an important factor for the transducer characteristics (e.g.
resonant frequency and electrical (crush) voltage).
[0049]
Compared to the first embodiment of FIG. 1, the cell of the second embodiment of FIG. 2 does not
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14
have a stress layer in the final cell 10 to be manufactured. However, it should be understood that
such a stress layer may be present temporarily and thus only during manufacturing and not in
the final product.
[0050]
The second embodiment of FIG. 2 is a preferred embodiment. This is described below. When
metal is used as stress layer 17, the stress value is temperature dependent due to the difference
in thermal expansion coefficient. If the stressed layer 17 remains in the final cell 10 or in the
final product, then the temperature dependent properties of the (especially cMUT) cell result,
which may lead to thermal drift of, for example, the collapse voltage. To this end, the stress layer
17 is removed in the second preferred embodiment of FIG. If an additional metal layer is required
for acoustic reasons (to improve the acoustic impedance of the membrane), it has to be added as
the last layer covering the entire membrane. Here, the thermal drift is expected to be quite small
(in theory, it is exactly zero since there is no moment).
[0051]
In another embodiment (not shown), only a portion (or the remainder) of stress layer 17 may be
present in the final cell 10 or final product. In this case, the stress layer 17 is removed by a
considerable amount during manufacture. However, the remainder of the stress layer 17 is
present (or at least visible), in particular in the middle of the cell.
[0052]
The cell 10 of the first embodiment shown in FIG. 1 or the second embodiment shown in FIG. 2
further comprises a plug 30 disposed in the hole 15 of the membrane 14. The plug 30 is
arranged only in the subarea Asub of the total membrane area Atotal covered by the membrane
14. The total membrane area Atotal is defined by the diameter 2 * R14 of the membrane 14 (or
the cavity 20). The plug 30 contacts or is fixed to the substrate 12. The plug 30 is stationary
(immobile). The height and / or width of the plug 30 can determine the strength of the plug. For
example, a minimum height on the order of 1 μm can be required. The plug 30 can in particular
be made of nitride. In another example, the plug 30 is made of silicon dioxide or a combination of
nitride and silicon dioxide. However, any other suitable material is possible.
04-05-2019
15
[0053]
In the first embodiment of FIG. 1 or the second embodiment of FIG. 2, the plug 30 has a
“mushroom-like” shape. Thus, the plug 30 is disposed on (and in contact with or on) the stem
portion 30a disposed on (and in contact with or secured to) the substrate 12 and on the edge
portion 14a of the membrane. And a fixed head portion 30b. The sub area Asub (where the plug
30 is disposed) is smaller than the area defined by the hole of the annular (or annular shaped)
second electrode 18. In other words, the plug 30 (in the sub area Asub) is inside the hole of the
electrode ring of the second electrode 18. This is because the plug 30 is stationary (immobile)
and the second electrode 18 should be located in the movable area of the membrane 14. If the
second electrode 18 is arranged in an immovable area (e.g. a sub area Asub in which the plug 30
is arranged), this impairs the conversion performance of the cell. Thus, in this way, the second
electrode 18 is disposed in the movable area of the membrane 14 and not in the immovable area.
As a result, good conversion performance of the cell is maintained.
[0054]
The plug 30 is disposed only in a sub-area of the total membrane area or covers only this area.
Thus, it is not a retention layer that is disposed on or covers all of the total membrane area (and
possibly extends beyond the total membrane area). Contrary to the plug 30, such a retaining
layer is somewhat like a spring. Because it keeps the membrane against the surface. However, if a
strong enough force (eg, pulling) upwards (away from the substrate) is applied to the membrane,
the membrane will still move. This process is reversible. For example, at ambient pressure (1
Bar), it can be imagined that such a holding layer is strong enough to hold the membrane but in
vacuum the membrane is released. Conversely, the plug 30 really secures (or nails) the
membrane to the substrate surface. The only way to release the membrane is to break the plug
30.
[0055]
In the case of the second embodiment of FIG. 2, if stressed layer 17 is temporarily present (only
during manufacturing) as described above, plug 30 will have a recess formed by removing
stressed layer 17 It can have. This recess is a characteristic pattern in the plug 30 (in particular
made of nitride) in the form of an overhang structure which is provided by the removal of the
stress layer 17.
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[0056]
The cell 10 of the first embodiment shown in FIG. 1 or the second embodiment shown in FIG. 2
further includes a cover layer 40 disposed on the membrane 14 (or stress layer 17) and on the
plug 30. Have. The cover layer 40 is also movable or flexible so that it can move or vibrate with
the membrane 14. However, it should be understood that such a cover layer is optional. In the
case of a cMUT cell, the cover layer 40 provides alignment of the cell 10 to a specific resonant
frequency of the cell, or more particularly, alignment of the cell or membrane thickness. In the
case of a pressure sensor cell, the cover layer 40 provides alignment to the operating range.
Further optionally, additional layers or coatings may be applied, for example a coating of
Parylene-C or of an acoustic lens material (for example silicon).
[0057]
FIG. 4 shows a set of (etched) masks (or (etched) masks or reticles layout for the pre-crushed
capacitive micromachined transducer 10 according to an embodiment, in particular according to
the first or second embodiment described above. Show a top view of the containing layers). As
can be seen from FIG. 4, the cell 10 is a circular shaped cell. The membrane 14 is an annular
membrane. Thus, the total membrane area Atotal is an area of circular shape and is defined (or
limited) by the (outer) diameter 2 * R 14 of the membrane 14. A plug 30 (not shown in FIG. 4) of
maximum diameter 2 * R 30 is arranged in the hole 15 of the membrane 14 (having a diameter 2
* R 15). This plug 30 is arranged only in the subarea Asub of the total membrane area Atotal
(indicated by a broken line in FIG. 4). Optionally, as shown in FIG. 4, in addition to the central
hole 15, a plurality of etching holes 50 (three etching holes 50 in FIG. 4) can be present at the
end of the membrane 14.
[0058]
In FIG. 4, the hole of the annular second electrode 18 has a diameter of 2 * R 18 or is called the
inner diameter of the second electrode 18. In the example shown in FIG. 4, the outer diameter of
the second electrode 18 extends beyond the total membrane area Atotal. In other words, in this
example, the outer diameter of the second electrode 18 is larger than the outer diameter of the
membrane 14. However, for example, as shown in the embodiment of FIG. 1 or 2, the outer
diameter of the second electrode 18 can be smaller than the outer diameter of the membrane 14
04-05-2019
17
(or included in the total membrane area Atotal) Please understand the point.
[0059]
In FIG. 4, a plurality (four) of additional cells are shown around the middle cell 10. The cells can
form an array of cells or transducer elements. The intermediate cell 10 (or its electrode) is
electrically connected to the other cells by an electrical connection 60.
[0060]
In the case of a circular shaped cell, referring back to FIG. 1 or 2, the second electrode 18 is an
annular electrode. The cavity 20 is an annular cavity. In the case of such circular shaped cells,
the stress layer 17 is again an annular layer. In this case, as can be seen from FIG. 1, the outer
radius R o of the stressed layer 17 can be larger than the radius R 14 of the membrane 14 or the
total membrane area Atotal. Thus, as mentioned above, the stressed layer 17 can extend beyond
the total membrane area Atotal. Alternatively, the outer radius R o of the stress layer 17 can also
be smaller than the radius R 14, in theory, as long as the required bending moment is provided.
Furthermore, in this case, as can be seen from FIG. 1, the inner radius R i of the stressed layer 17
can be larger than the radius R 15 of the hole 15 of the membrane 14. Thus, as mentioned
above, the holes 19 (with a diameter 2 * R i) of the stressed layer 17 can be larger than the holes
15 (with a diameter 2 * R 15) of the membrane 14.
[0061]
In the case of such a circular shaped cell, the plug 30 is a circular shaped plug 30. The plug 30 is
smaller than the hole in the annular second electrode 18 (having a diameter 2 * R 18). In other
words, as can be seen from FIG. 1 or FIG. 2, the radius R 30 of the circular shaped plug 30 is the
radius R 18 of the hole in the annular second electrode 18 (or the inner radius R 18 of the
second electrode 18). Less than). Therefore, as described above, the sub area Asub (where the
plug 30 is disposed) is smaller than the area defined by the hole of the annular second electrode
18. The shape of the cell is advantageously a circular shaped cell. However, it should be
understood that any other suitable cell shape is possible.
[0062]
04-05-2019
18
Figures 3a to 3i respectively show the different manufacturing steps of the method of
manufacturing the collapsible capacity micromachined transducer cell 10 according to the first
or second embodiment. The explanations made in connection with FIGS. 1, 2 and 4 also apply to
the method shown in FIG. 3 and vice versa.
[0063]
In the initial step shown in FIG. 3a, the substrate 12 is provided first. Here, the first electrode 16
is present at or on the substrate. Then, the membrane 14 (covering the total membrane area
Atotal) is provided on the substrate 12. As mentioned above, the membrane 14 has two layers
(eg ONO-layer or ON-layer or O-layer or N-layer or combinations thereof), in or between which
the second electrode 18 is Be embedded. As can be seen from FIG. 3 a, in this example a
sacrificial layer 21 of thickness h 20 is provided on the substrate 12. The sacrificial layer 21 is
used to form the cavity 20 when the sacrificial layer 21 is removed (e.g., dry or wet etched). The
membrane 14 is provided on the sacrificial layer 21. However, it should be understood that any
other suitable manner of providing the cavity 20 can be used.
[0064]
In a further step, as shown in FIG. 3 b, a stress layer 17 is provided or formed (eg, applied or
deposited) on the membrane 14. The stressed layer 17 has a predetermined stress value on the
membrane 14 as described above in connection with the first embodiment. The stressed layer 17
shown in FIG. 3 b has a well-defined inner radius R i and an outer radius R o. Preferably, the
outer diameter 2 * R o of the stressed layer 17 exceeds the diameter 2 * R 14 of the membrane
14. Alternatively, in theory, the outer diameter 2 * R o of the stress layer 17 can also be smaller
than the diameter 2 * R 14. The purpose is to induce a bending moment large enough to bend
the membrane 14 to the bottom of the substrate 12 or cavity 20 once the membrane 14 is
released.
[0065]
Then, referring to FIG. 3 c, the membrane 14 is released by providing (eg, etching) the holes 15
in the membrane 14. In this example using the sacrificial layer 21, the membrane 14 is released
04-05-2019
19
by providing the holes 15 and performing a sacrificial etch of the sacrificial layer 21. After
providing the hole 15, the membrane 14 has an edge portion 14a surrounding the hole 15. The
edge portion 14 a of the membrane 14 collapses against the substrate 12 (or the bottom of the
cavity 20). More particularly, the edge portion 14 a of the membrane 14 collapses against the
substrate 12 when the hole 15 is provided in the membrane 14 or after this. This is due to the
fact that the stressed layer 17 provides a bending moment on the membrane 14 in the direction
towards the substrate 12 as described above. At this time, the membrane 14 contacts the
substrate 12 (or the bottom of the cavity 20).
[0066]
In this example, a cavity 20 having a height h 20 is formed between membrane 14 and substrate
12 by removing (eg, etching) sacrificial layer 21. Here, this is realized in the step in which the
holes 14 are provided in the membrane 14 or following the step in which the holes 14 are
provided. In particular, in the first etching step holes 15 in the membrane 14 can be provided
and in the subsequent etching step the sacrificial layer 21 can be removed. The holes 15 thus
also function as etching holes. Optionally, additional etch holes can be present at the end of the
membrane, for example etch holes 50 in FIG.
[0067]
The steps shown in FIGS. 3 d and 3 e are used to provide a plug 30 disposed in the hole 15 of the
membrane 14 as described above. Plugs 30 are arranged only in sub area Asub of total
membrane area Atotal. First, referring to FIG. 3d, an additional layer 29 (eg made of nitride) is
provided on the membrane 14 at least in the total membrane area Atotal (in all of the total
membrane area Atotal). In FIG. 3 d an additional layer 29 extends beyond the total membrane
area Atotal. The additional layer 29 seals the cavity 20 from its periphery and permanently
secures the membrane 14 to the substrate 12 (or the bottom of the cavity 20). Also, the etch
holes 50 can be closed by the additional layer 29. At this point, the cell is protected from external
contamination.
[0068]
In order to provide the plug, referring to FIG. 3e, the additional layer 29 is removed except for
the layer portion which is arranged in the sub area Asub. Thus, a plug 30 (for example made of
04-05-2019
20
nitride) is provided. Thus, the additional layer 29 is patterned and exists only in the subarea
Asub. This is at the center of the membrane 14. In particular, the height of the plug 30 can be the
height of the additional layer 29 (eg made of nitride). The membrane 14 is permanently fixed to
the substrate 12 (or the bottom of the cavity 20) by the plug 30.
[0069]
As a specific example, if the additional layer 29 (or plug layer) is made of nitride, the deposition
of the additional layer 29 is usually at 300 ° C. to 400 ° C. Thus, stress is the stress value at
that temperature (not at room temperature). In such a particular example, tungsten is a good
choice as a stress layer material.
[0070]
Up to this point, the manufacture of the cell according to the first embodiment shown in FIG. 1
and the second embodiment shown in FIG. 2 is identical. In the following, further manufacturing
steps of the second embodiment of FIG. 2 will be described. FIGS. 3 f and 3 g respectively show
the manufacturing steps of the method of manufacturing the pre-crushed capacity
micromachined transducer cell according to the second embodiment. As shown in FIG. 3 f, the
method comprises the step of removing the stress layer 17. This can be performed, for example,
by selective etching of the membrane 14 (eg ONO layer). The membrane 14 can not be flipped
because it is permanently fixed to the substrate 12 or the bottom of the cavity 20 by the plug 30
(made for example of nitride). In FIG. 3f, the entire stress layer 17 is removed. However, it should
be understood that only a substantial portion of the stress layer may be removed (e.g. leaving
only a few of the stress layers). For example, a wet etch process (isotropic) can remove all of the
stress layer (eg, made of metal). As another example, a dry etch process (directivity or
anisotropy) can remove only a substantial portion of the stress layer, leaving the rest (in
particular, the rest in the recess of the plug 30). .
[0071]
Optionally, referring to FIG. 3g, a cover layer 40 can be provided or disposed on the membrane
14 and the plug 30 (e.g. using N-deposition). Such cover layer 40 provides for matching the cell
10, or more particularly the thickness of the cell or membrane, to a particular resonant
frequency of the cell.
04-05-2019
21
[0072]
Furthermore, optionally, several additional processing steps can be performed. For example,
electrical connections to the power supply of cell 10 (eg, for powering bias and RF) or electrical
connections between different cells of an array of cells can be provided. For example, some
layers (e.g., nitrided layers) can be removed from the bond pad to create a conductive path to the
electrode. Furthermore, as another example, a protective layer or coating for electrical insulation
(e.g. parylene-C) can be applied.
[0073]
From a technical point of view, the pre-crushing capacity micromachined transducer cell (in
particular cMUT) of the invention is in principle manufactured in the same or similar manner as a
conventional "non-crushing" capacity micromachined transducer cell (in particular cMUT) be able
to. This is for example described in detail in WO 2010/032156, which is included here by
reference. This has, for example, the advantage of CMOS compatibility. As a result, the cMUT can
be combined with an ASIC, in particular a so-called microbeamformer.
[0074]
The present invention can be further described based on the following embodiments.
[0075]
In one embodiment, the cell or cMUT cell has a membrane with an embedded annular electrode.
For example, as described in detail in WO 2010/032156, the stack comprises aluminum for the
electrode, ONO for the membrane and nitride.
[0076]
04-05-2019
22
In another embodiment, deposition of the temporary patterned stress layer is followed by etching
of the sacrificial layer. When the membrane is released, the stress layer provides a bending
moment that causes the membrane to collapse.
[0077]
In another embodiment, a nitride layer is used to permanently secure the membrane to the
bottom of the cavity. Here, the cell or cMUT cell is pre-crushed. The nitride layer is patterned and
a significant percentage removed, leaving only a central plug or rivet of nitride.
[0078]
In a further embodiment, the temporary patterned stress layer is completely removed (preferred
embodiment).
[0079]
In another embodiment, the pre-crushed cell or cMUT cell is finished with a final nitrided layer.
Here, the membrane thickness is matched to the desired characteristic, for example the
resonance frequency.
[0080]
The invention is applicable in any cMUT application, in particular including ultrasound
applications, but in principle it is applicable to any other pre-collapsed capacity micromachined
transducer, for example a pressure sensor or pressure transducer. When applied to pressure
sensors, linearity is improved at the expense of sensitivity.
[0081]
A capacitive micromachined pressure sensor or transducer measures the capacitance value
04-05-2019
23
between the electrodes. For two flat electrodes separated by a distance d and having an area A,
the capacitance value C is <img class = "EMIRef" id = "269562528-000005" />. For the sake of
simplicity, the presence of a dielectric insulating layer between the electrodes is omitted in this
equation.
[0082]
In one example, electronically, the pressure sensor can be part of an electronic oscillator circuit.
Here, the oscillator frequency f is <img class = "EMIRef" id = "269562528-000006" />, where R
is the resistance of some external resistor. In this case, the pressure sensor output is the
frequency of the electronic circuit and is a linear distance at distance d. It should be noted that
this frequency has nothing to do with the mechanical resonance frequency of the membrane.
Thus, as the pressure is increased, the two plates move towards each other, the capacitance value
increases and the frequency decreases. The pressure P causes the membrane to move downward
by the amount of h which can be expressed as <img class = "EMIRef" id = "269562528-000007"
/>. Here, r is the radius of the membrane, and D is a constant. Hereinafter, the distance d between
the electrodes is <img class = "EMIRef" id = "269562528-000008" />. This is because the gap g
is reduced by the amount h or becomes <img class = "EMIRef" id = "269562528-000009" />.
Thus, the pressure is approximately linear with frequency until the collapse of the membrane.
However, in practice the shape of the electrode or membrane is not flat. The membrane bends.
This gives a variation in the distance across the electrodes. The best linearity is thus obtained at
the expense of having to measure small capacitance values when the electrodes are small. In fact,
the electrode with a radius of 50% compared to the membrane radius is already very linear.
[0083]
Here, one example of measuring the capacitance value has been described. However, it should be
understood that the capacitance value can also be measured in any other suitable manner.
[0084]
While the present invention has been illustrated and described in detail in the drawings and
foregoing description, such illustration and description are to be considered illustrative or
exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other
variations to the disclosed embodiments can be understood and effected by those skilled in the
04-05-2019
24
art in practicing the claimed invention, from a study of the drawings, the disclosure and the
appended claims.
[0085]
In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite
article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the
functions of several items recited in the claims. The mere fact that certain measures are recited in
mutually different dependent claims does not indicate that a combination of these measures can
not be used to advantage.
[0086]
Any reference signs in the claims should not be construed as limiting the scope of the invention.
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