JP2010251847

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DESCRIPTION JP2010251847
PROBLEM TO BE SOLVED: When the variation in initial displacement of the membrane in each
cell in the element is large, the bias voltage applied between the electrodes must be reduced, and
the sensitivity is lowered. An electromechanical transducer according to the present invention
includes a plurality of cells each including a first electrode and a second electrode provided with
a gap between the first electrode and the first electrode. Is formed. A groove is formed at a
predetermined distance from the gap of the cell provided at the outermost periphery of the
element. [Selected figure] Figure 1
Mechanical change element
[0001]
The present invention relates to a mechanical-electrical conversion element.
[0002]
In recent years, electromechanical transducers manufactured using a micromachining process
have been actively studied.
In particular, capacitive-type electromechanical transducers called CMUT (Capacitive
Micromachined Ultrasonic Transducers) can transmit and receive ultrasonic waves using
lightweight thin films, and are compared with piezoelectric electromechanical transducers used
in the past. It is noted that wide band characteristics can be easily obtained.
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[0003]
The CMUT is composed of a plurality of elements arranged in an array in one or two dimensions.
An element is an element that transmits or receives ultrasonic waves. FIG. 11 is a schematic view
showing a conventional CMUT, FIG. 11 (a) is a top view, and FIG. 11 (b) is a cross-sectional view
along a broken line Y1-Y2. As shown in FIG. 11A, the element 302 is composed of a plurality of
cells 311. By simultaneously applying a drive voltage signal to each of the cells 311 constituting
the element 302, an ultrasonic wave is output from the element 302. In addition, the ultrasonic
detection signals received by each of the cells 311 constituting the element 302 are added by the
upper electrode 315 and the lower electrode 316 common to each cell in the element, and the
sum thereof is the ultrasonic detection signal received by the element 302 It becomes. The upper
electrode 315 of each cell is electrically connected by the wiring 307.
[0004]
As an example of the structure of such an element, CMUT described in patent document 1 is
mentioned. The CMUT disclosed in Patent Document 1 has a structure in which a substrate
penetration wiring 304 is formed inside a support substrate 303 as shown in FIG. 11B. The
circuit board 305 and the lower electrode 316 are electrically connected by the board
penetration wiring 304. The circuit board 305 and the upper electrode 315 are electrically
connected by the wiring 307, the insulating layer through wiring 306, and the substrate through
wiring 304. On the circuit board 305, generation of a drive voltage signal for outputting an
ultrasonic wave in the element 302 and signal processing such as amplification of an ultrasonic
signal generated by receiving the ultrasonic wave in the element 302 and delay addition are
performed.
[0005]
U.S. patent registration publication no.6958255
[0006]
However, there are variations in the amount of membrane displacement of each cell in the
element.
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This cell-to-cell variation is considered to be caused by the difference in the thermal expansion
coefficients of the membrane and the insulating layer, and the warpage of the substrate due to
the influence of the internal stress of the membrane and the insulating layer. Since the variation
in displacement amount between cells appears as a difference between the transmission
efficiency and the detection sensitivity of ultrasonic waves, it is not preferable.
[0007]
In addition, the transmission efficiency and reception sensitivity of the CMUT become higher as
the gap between the upper and lower electrodes is narrower. If the bias voltage is increased, the
electrostatic attraction between the upper and lower electrodes is increased, so that the
transmission efficiency and reception sensitivity of the CMUT can be improved by increasing the
bias voltage. However, if the bias voltage is increased too much, the upper electrode is adsorbed
to the lower electrode together with the membrane at the moment the voltage reaches a certain
voltage, and desired vibration characteristics can not be obtained. This phenomenon is called
pull-in, and the voltage at which pull-in occurs is called pull-in voltage. The pull-in voltage is a
value determined by the initial displacement of the membrane. Therefore, since the upper limit
value of the bias voltage applied between the upper and lower electrodes is limited by the
magnitude of the variation of the initial displacement of the membrane of each cell, the reception
sensitivity of the CMUT is limited.
[0008]
An object of the present invention is to provide a electromechanical transducer in which the
variation in membrane displacement amount between cells is reduced in order to solve the
above-mentioned problems.
[0009]
In view of the above problems, the electromechanical transducer according to the present
invention includes a plurality of cells each including a first electrode and a second electrode
provided with a gap between the first electrode and the first electrode. A mechanical-electrical
conversion element in which an element having the element is formed, wherein a groove is
formed at a predetermined distance from the gap of the cell provided on the outermost periphery
of the element.
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[0010]
By forming the groove on the outside of the outermost cell of the element, it is possible to
provide the electromechanical transducer in which the variation of the membrane displacement
amount between cells is reduced.
[0011]
The schematic diagram which shows the structure of the element of CMUT in 1st embodiment of
this invention.
The schematic diagram which shows the structure of the element of CMUT in 2nd embodiment
of this invention.
Sectional drawing which shows the structure of the element of CMUT in 3rd embodiment of this
invention.
The schematic diagram which shows the structure of the element of CMUT in 4th embodiment of
this invention. The schematic diagram which shows an example of the method of producing
CMUT which can apply this invention. The schematic diagram which shows another example of
the method of producing CMUT which can apply this invention. The schematic diagram which
shows the initial stage displacement amount when not forming a groove ¦ channel. The schematic
diagram in the case of forming a groove. The graph which shows the effect at the time of forming
a slot in the perimeter of an element. The graph which shows the effect at the time of forming a
slot around a cell. The schematic diagram which shows the structure of the element of the
conventional CMUT.
[0012]
The electromechanical transducer of the present invention comprises an element having a
plurality of cells (hereinafter referred to as an element), and a groove is formed at a
predetermined distance from the cavity of the cell provided on the outermost periphery of the
element. The cell has a lower electrode as a first electrode and an upper electrode as a second
electrode formed with a gap (hereinafter referred to as a cavity) interposed. In addition, a thin
film (hereinafter referred to as a membrane) that constitutes a vibrating film that is deformed by
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the potential difference between the upper electrode and the lower electrode is formed on the
cavity.
[0013]
In the present invention, "a predetermined distance position" means a position that satisfies the
following two conditions. The first is that the position is outside the element rather than the cell
located at the outermost periphery of the element. The second problem is that when the groove
is formed at that position, the difference in initial displacement of the membrane between the
outermost cell of the element and the central cell is smaller than before forming the groove.
Although the details will be described later, it is preferable that the distance from the cavity of
the outermost peripheral cell be a distance of 50% or more and 200% or less of the distance
between cavities. Further, groove means a structure corresponding to any one of the
definitions shown in the following (1) to (4). (1) A recess formed in the membrane from the
upper surface of the membrane (the surface opposite to the cavity). (2) A recess formed in the
insulating layer as a membrane and a support portion for supporting the membrane. (3) A recess
formed by not forming a membrane on the outer periphery of the element outermost periphery
cell. (4) A recess formed on the upper surface (the surface opposite to the bottom of the cavity)
of the support by not forming a membrane on the outer periphery of the element. That is, in the
electromechanical transducer of the present invention, the membrane formed at a predetermined
distance from the cavity of the cell provided on the outermost periphery of the element becomes
thinner than the membrane formed on the cavity (above the gap) Or removed.
[0014]
In the present invention, the membrane means not only vibrating parts formed on the cavities,
but also parts formed between the cavities (gaps) or parts formed outside the outermost
peripheral cells. Because they are formed as two thin films, they are also called membranes.
[0015]
In the present invention, the upper electrode can be formed of a material selected from metals,
amorphous Si having low resistance, and oxide semiconductors having low resistance, and the
membrane may also serve as the upper electrode.
When the upper electrode is formed on the membrane, the upper electrode may be formed on
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either the upper or lower side of the membrane, or may be sandwiched by the membrane.
[0016]
The material of the lower electrode 8 may be an electrically low resistance material. For example,
a doped single crystal Si substrate, a doped polycrystalline Si film, or a single crystal Si substrate
having a doped region as a lower electrode, a doped amorphous Si, an oxide semiconductor, a
metal film, or the like may be used. It is also possible that the substrate doubles as the lower
electrode.
[0017]
The reason why the displacement amount of the membrane for each cell is reduced by the
configuration of the electromechanical transducer of the present invention is that the structure of
the membrane and the insulating layer (membrane and insulating layer at the periphery of the
cell located at the outermost periphery of the element It can be considered as a factor that the
junction area with the cell is identical or similar to that of other cells. Thereby, the distribution of
the internal stress of the membrane in the cell positioned at the outermost periphery and the
other cells is in the same or close state. Therefore, by providing the groove in the membrane
formed on the outer periphery of the element, it is considered that the effect of reducing the
variation of the membrane displacement amount for each cell can be obtained.
[0018]
First Embodiment A first embodiment of the present invention will be described below. As shown
in FIG. 1, the element 101 consists of cells 102 arranged side by side in a plane. FIG. 1A is a top
view, and FIG. 1B is a cross-sectional view taken along a broken line A1-A2. The cell 102
comprises a membrane 103, an insulating layer 104, a cavity 105 consisting of a recess in the
insulating layer 104, an upper electrode 106, and a lower electrode 107, and the upper electrode
106 and lower electrode 107 of each cell are electrically connected. All the elements of the upper
electrode 105 are electrically connected to each other by the wiring 108, and the lower electrode
107 is electrically isolated between the respective elements. Furthermore, in the present
embodiment, the groove 109 is formed on the outer periphery of each cell from the upper
surface of the membrane 103 (that is, the groove is formed between the cavities). The shape of
the groove 109 in the present embodiment is continuously formed so as to surround the cavity
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105 in the peripheral portion of the cell 102, and the depth thereof is shallower than the
thickness of the membrane 103. In the present embodiment, since the bonding area between the
membrane 103 and the insulating portion 104 does not change, the membrane displacement
amount for each cell without decreasing the bonding strength between the membrane 103 and
the insulating layer 104 as a support portion supporting the membrane. The effect of reducing
the difference between
[0019]
Next, in order to confirm the effect of the present invention, calculation of the initial deformation
of the membrane was performed using the finite element method. The initial displacement
amount of the membrane was a displacement generated by the combined force of the internal
stress of the membrane and the pressure applied by the pressure difference (approximately 1
atmospheric pressure = 101325 Pa) inside and outside the cavity. As an internal stress, a thermal
contraction stress generated due to a temperature difference between at the time of membrane
formation and after formation was assumed to be applied. As a model to be calculated, an
element consisting of 11 cells aligned on one side was assumed, and the initial deformation of
the membrane in each cell was calculated when internal stress was applied to the membrane and
the insulating layer due to heat contraction. Also, analysis using the finite element method is
commercially available software (ANSYS 11.0 ANSYS Inc. ) Was performed.
[0020]
First, FIG. 7 shows the result of calculating the initial deformation amount of the membrane in
each cell when the groove is not provided. From this result, it was found that when the groove is
not provided in the membrane, the initial displacement amount of the outermost (innermost) cell
is larger than that of the other cells.
[0021]
Next, a groove was provided on the outside of the outermost cell of the element, and it was
investigated how the variation of the initial displacement of the membrane changes due to the
difference in the shape of the groove. The result of comparing the difference in the initial
displacement of the membrane between the cell at the center of the element and the cell at the
outermost periphery of the element (hereinafter simply referred to as the difference in the initial
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displacement) is shown in FIG.
[0022]
FIG. 9A shows the relationship between the depth of the groove and the difference between the
initial displacement amounts. As shown in FIG. 8, the depth of the groove indicates the length of
the groove in the direction perpendicular to the arranged surface of the cells, and the width of
the groove indicates the groove in the direction parallel to the arranged surface of the cells
Indicates the length. The vertical axis represents the ratio of the difference of the initial
displacement under each condition when the difference of the initial displacement is 1 when the
depth of the groove is 0 (without the groove). The abscissa represents the groove depth divided
by the thickness of the membrane. In this case, the width of the groove is constant (fixed at 0.25
times the distance between cavities) under all conditions, and the distance between the groove
and the cavity of the outermost peripheral cell is the same value as the distance between cavities
(distance between cavities) I assume. From this result, it was shown that the deeper the groove,
the smaller the difference in the initial displacement amount. Therefore, it is preferable that the
groove is also formed in the insulating layer which is a support portion through the membrane.
[0023]
FIG. 9 (b) shows the relationship between the width of the groove and the difference in the initial
displacement. The vertical axis represents the ratio of the difference in initial displacement under
each condition, where the difference in initial displacement is 1 when the groove width is 0
(without a groove). The horizontal axis shows the width of the groove divided by the distance
between cavities. Under all conditions, the depth of the groove is constant (fixed at 1.5 times the
membrane thickness), and the distance between the groove and the cavity of the outermost cell is
the same value as the distance between cavities. From this result, it was found that the difference
in initial displacement decreases as the width of the groove is wider. Further, by forming a
groove having a width of 10% of the distance between cavities, the difference of the initial
displacement amount can be reduced by about 40%.
[0024]
Furthermore, the difference of the initial displacement amount according to the distance between
the groove and the cavity of the outermost peripheral cell ( cavity-groove distance shown in
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FIG. 8) was compared. The results are shown in FIG. 9 (c). The vertical axis shows the ratio of the
difference of the initial displacement when the difference of the initial displacement between the
cell at the center of the element and the outermost cell is 1 when there is no groove. The
horizontal axis indicates the distance between the groove and the cavity of the outermost cell
divided by the distance between the cavities (gap). In all conditions, the depth of the groove was
constant (fixed at 1.5 times the membrane thickness), and the width of the groove was constant
(fixed at 0.25 times the distance between cavities). As a result, it was found that the closer the
distance between the groove and the cavity of the outermost peripheral cell is, the greater the
effect of reducing the difference in the initial displacement amount. However, the distance
between the groove and the cavity of the outermost cell is preferably 50% or more of the
distance between the cavities, in order to prevent the strength of the supporting portion
supporting the membrane of the outermost cell from falling. Further, from the viewpoint of
reducing the difference in the initial displacement, it is preferably 200% or less of the distance
between cavities, and more preferably less than the distance between cavities.
[0025]
In the present invention, as shown in FIG. 1, grooves may be provided also in the membrane
formed between the cavities. That is, the groove may be provided on the outer periphery of the
cell not on an element basis but on a cell basis. The result of having compared the difference of
the initial stage displacement amount by the difference in the shape of the groove ¦ channel at
the time of forming a groove ¦ channel in the outer periphery of each cell is shown to Fig.10 (a),
(b). FIG. 10A shows a comparison result when the difference in initial displacement amount is 1
when the groove depth is 0 (no groove). The abscissa represents the groove depth divided by the
thickness of the membrane. In this case, the width of the groove is fixed at 0.25 times the
distance between cavities under all conditions, and the distance between the groove and the
cavity of the outermost peripheral cell is the same value as the distance between cavities. From
this result, it was found that the deeper the groove, the smaller the difference in the initial
displacement amount. Moreover, even if the groove did not penetrate the membrane, it was
possible to reduce the difference in initial displacement.
[0026]
FIG. 10 (b) shows the comparison result when the difference in initial displacement is 1 when the
groove width is 0 (no groove). The horizontal axis indicates the width of the groove divided by
the distance between the cavities (gap). In all conditions, the depth of the groove is fixed at 1.5
times the membrane thickness, and the distance between the groove and the cavity of the
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outermost cell is the same as the distance between the cavities. As a result, it can be seen that the
difference in initial displacement decreases as the width of the groove increases. By forming a
groove having a width of 10% of the distance between cavities, it is possible to reduce the
difference of the initial displacement amount by about 40%.
[0027]
Hereinafter, in Embodiments 1 to 4, a specific example will be described in which a groove is
provided for each cell (a groove is provided between cavities). However, in the present invention,
without forming a groove for each cell, if a groove is formed on an element basis (if a groove is
formed at a predetermined distance from the cavity of a cell provided on the outermost
periphery of the element) There is an effect of reducing the difference in displacement.
[0028]
Embodiment 2 As shown in FIG. 2, in the present embodiment, the groove 109 is intermittently
formed in a part of a line surrounding the cavity 105 at the peripheral portion of the cell 102.
FIG. 2 (a) is a top view, and FIG. 2 (b) is a cross-sectional view taken along a broken line B1-B2.
The groove 109 penetrates the membrane 103, and the groove is not formed in the insulating
layer as a support portion. In the present embodiment, the insulating layer 104 can be used as an
etching stop layer when forming the groove 109, so the formation of the groove 109 is relatively
easy.
[0029]
Embodiment 3 In the present embodiment, as shown in FIG. 2A of Embodiment 2, the groove
109 is intermittently formed in a part of a line surrounding the cavity 105 at the peripheral
portion of the cell 102. Furthermore, as shown in FIG. 3, the groove 109 penetrates through the
membrane 103 and reaches the insulating layer 104 below it. In the present embodiment, since
the groove 109 deeper than the thickness of the membrane 103 can be formed, it is possible to
obtain a high effect of reducing the difference in membrane displacement between cells.
[0030]
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Embodiment 4 This embodiment is an example in which the present invention is applied to a
CMUT in which the shape of the cell 102 is other than a square. When the cell 102 is circular as
shown in FIG. 4, a groove 109 having an arc shape larger than the radius of the cell 102 and
whose center coincides with the center of the cell 102 is formed in the peripheral portion of the
cell 102 do it.
[0031]
<< Manufacturing Method >> Next, based on the manufacturing method of the CMUT disclosed in
Patent Document 1, an example of the manufacturing method of a CMUT having a groove at the
peripheral portion of the cell as in the first embodiment will be described with reference to FIG.
Explain. However, the method of manufacturing the electro-mechanical transducer of the present
invention is not limited to this embodiment, and a sacrificial layer is formed on a substrate, a
membrane is formed thereon, and a cavity is formed by etching the sacrificial layer. A forming
method (surface micro machining) may be used. The following steps (a) to (f) correspond to (a) to
(f) in FIG. (A) Silicon oxide layers 202 and 203 are formed on both sides of an SOI (Silicon On
Insulator) substrate 201. (B) A through hole 204 is formed in a portion of the silicon oxide layer
202 where the cell cavity and the dummy cell cavity are to be produced, and the device substrate
205 is produced. (C) A silicon oxide layer 210 is formed on the upper surface of the through
wiring substrate having the lower electrode 206, the through wiring 207, and the pad 208. (D)
The silicon oxide layer 202 remaining on the top surface of the device substrate 205 and the
silicon oxide layer 210 on the top surface of the through wiring substrate 209 are bonded. (E)
Remove layers other than the silicon oxide film layer 202 and the device layer 211 so that the
silicon oxide layer 202 of the device substrate 205 and the device layer 211 of the SOI substrate
remain on the through wiring substrate 209. An upper electrode 212 is formed on the top
surface. (F) At least a part of the portion of the membrane 211 corresponding to the outer
periphery of the cell is etched to form a groove 215. Here, an example in which the depth of the
groove 215 is shallower than the thickness of the membrane 211 is shown. In order to form the
groove 215 having a desired depth, for example, a method of checking the etching rate of the
membrane 211 in advance and adjusting the etching time can be used. (E) The pads 208 on the
lower surface of the through wiring substrate 209 and the pads 214 on the upper surface of the
circuit substrate 213 are bonded.
[0032]
In the step (f), the groove 215 can be formed such that the depth of the groove 215 is equal to
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the thickness of the membrane 211 as shown in (b1) of FIG. When the membrane 211 is made of
single crystal silicon and the insulating layer 202 is made of silicon oxide, the membrane can be
made by using an etching material such as carbon tetrafluoride which hardly reacts with silicon
oxide and easily reacts with single crystal silicon. The groove passing through 211 can be easily
formed. Further, subsequent to (b1) of FIG. 6, as shown in (b2) of FIG. 6, the insulating layer 202
can be further etched to form a deeper groove 215. When the membrane 211 and the insulating
layer 202 are made of the same material as described above, the membrane 211 is used as a
mask by using an etching material such as silicon hexafluoride which hardly reacts with single
crystal silicon and easily reacts with silicon oxide. The insulating layer 202 located under the
groove 215 formed in the above may be etched to form the groove 215.
[0033]
DESCRIPTION OF SYMBOLS 101 element 102 cell 103 membrane 104 insulating layer 105
cavity 106 upper electrode 107 lower electrode 108 wiring 109 groove
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