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JP2012039197

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DESCRIPTION JP2012039197
An electromechanical transducer capable of being mounted on a circuit board by correcting a
warp of an electromechanical transducer and a method of manufacturing the same. In an
electromechanical transducer, circuit boards (103, 104) and an electromechanical transducer
(101) are electrically connected and laminated via a conductive adhesive (102). The layer
configuration of the circuit boards 103 and 104 is two or more layers, and the layer
configuration is asymmetric with respect to the direction perpendicular to the circuit board, with
the respective layers 103 and 104 made of materials with different coefficients of thermal
expansion. [Selected figure] Figure 5
Electromechanical converter and method of manufacturing the same
[0001]
The present invention relates to an electromechanical transducer including an electromechanical
transducer such as a capacitive ultrasonic transducer and a circuit board, and a method of
manufacturing the same.
[0002]
An ultrasonic transducer converts an electrical signal to an ultrasonic wave and converts an
ultrasonic wave to an electric signal, and is used as a probe for medical imaging, nondestructive
inspection, and the like.
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One of the ultrasonic transducers is a CMUT (Capacitive Micromachined Ultrasuound
Transducer). The CMUT has one or more cells composed of a substrate having a lower electrode,
a membrane (vibrator) supported by a support formed on the substrate, and an upper electrode.
The cavity (air gap) of the cell is formed surrounded by the lower electrode, the membrane and
the upper electrode (see Patent Document 1). The CMUT emits an ultrasonic wave by vibrating
the membrane by a voltage applied between the lower electrode and the upper electrode. Also,
the membrane vibrates due to the ultrasonic wave, thereby changing the capacitance between
the lower electrode and the upper electrode, and detecting the ultrasonic wave is detected.
[0003]
Conventionally, in the case of CMUT mounting, when the two-dimensional array structure is not
used, each cell is connected to a terminal on the periphery of the device using wire bonding to
exchange electrical signals. However, when two-dimensional arraying is performed, the method
of wire bonding is very difficult because the number of terminals is large and the density is also
high. Therefore, for example, it is necessary to use a BGA (Ball Grid Array) type in which
terminals are arranged on the back side of the device. For mounting on a circuit board such as a
printed circuit board, the device can be connected to the printed circuit board by soldering by
passing through a reflow oven as in a general GBA type IC device.
[0004]
On the other hand, the printed circuit board on which the device is mounted may be housed in a
probe case and used. In the probe case, a device such as a CMUT is disposed to face the object to
be measured, with the ultrasonic wave transmitting material and the ultrasonic wave transmitting
sheet interposed therebetween. At this time, it is preferable to keep the distance between the
device and the ultrasonic transmission sheet at a constant distance from the viewpoint of the
transmission efficiency of ultrasonic waves, and the surface of the device is flat or kept within a
predetermined curvature. Is desirable.
[0005]
JP, 2006-319712, A
[0006]
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2
However, a surface-type CMUT fabricated by a method of forming a sacrificial layer in a cavity
shape on a substrate and forming a membrane layer on the sacrificial layer and then etching the
sacrificial layer to form a cavity is a film stress during device fabrication, etc. It tends to be
warped by
The amount may be a non-negligible size with respect to the distance between the acoustic wave
transmission sheet and the CMUT, which is calculated and set from the wavelength of an acoustic
wave such as an object ultrasonic wave. In such a case, the distance between the surface from the
acoustic wave transmission sheet is different between the center and the end of the surface of
the device, and the strength and the arrival of the acoustic wave reaching each position
compared to the case where the distance between the surfaces is constant. Time can be different.
In the present specification, an acoustic wave includes what is called a sound wave, an ultrasonic
wave, or a photoacoustic wave, and is generated inside a measuring object by irradiating light
(electromagnetic waves) such as near infrared rays into the measuring object, for example. It
includes an acoustic wave and a reflected acoustic wave that is transmitted inside the object to be
measured and reflected inside the object to be measured.
[0007]
In view of the above problems, the electromechanical transducer according to the present
invention in which the circuit board and the electromechanical transducer are electrically
connected and laminated via the conductive adhesive has the following features. The layer
configuration of the circuit board is two or more layers, and the layer configuration is
asymmetric with respect to the direction perpendicular to the circuit board, the layers being
made of materials having different coefficients of thermal expansion.
[0008]
Further, in view of the above problems, the manufacturing method of the present invention in
which the circuit board and the electromechanical transducer are electrically connected and
stacked has the following steps. Manufacturing the electromechanical transducer. A step of
laminating a plurality of layers made of materials having different coefficients of thermal
expansion to produce the circuit board. A step of positioning and overlapping the conductive
adhesive between the circuit board and the electromechanical transducer; Heating the circuit
board and the electromechanical transducer stacked to melt the adhesive, and lowering the
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3
temperature to about the temperature at which the adhesive hardens again to connect the two.
[0009]
According to the present invention, since the layer configuration of two or more layers of the
circuit board is made asymmetric with respect to the direction perpendicular to the circuit board
because the respective layers are made of materials of different thermal expansion coefficients,
the warp of the electromechanical transducer is corrected. Can be mounted on a circuit board.
[0010]
BRIEF DESCRIPTION OF THE DRAWINGS The figure which shows the external appearance
perspective view and sectional drawing of the Example of the electro-mechanical transducer of
this invention.
Sectional drawing which shows the state in which the circuit board and the electromechanical
transducer were laminated | stacked. Sectional drawing which shows an example of an
electromechanical transducer. Process sectional drawing which shows an example of the
preparation methods of an electromechanical transducer. Sectional drawing explaining the
lamination | stacking process of a circuit board and an electromechanical transducer. The figure
explaining the adjustment method of the curvature amount of the circuit board which has a layer
structure of two or more layers.
[0011]
The feature of the present invention is to make the circuit board of the two or more layer
constitution composed of materials having different coefficients of thermal expansion
asymmetric with respect to the vertical direction, and to connect the circuit board and the
electromechanical transducer through the conductive adhesive. It is in connecting electrically
and laminating. As a result, the asymmetry of the layer configuration of the circuit board is
adjusted to equalize the amount of warping of the electromechanical transducer and the amount
of warping of the circuit board at the time of bonding by the conductive adhesive, and the circuit
board becomes flat at room temperature. Can be used to correct the warping of
electromechanical transducers. Based on this concept, the electromechanical transducer and the
method of the present invention have the basic configuration as described in the section for
solving the above problems. The electromechanical transducer according to the present invention
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comprises at least an electromechanical transducer and a circuit board.
[0012]
Hereinafter, more specific examples will be described using the drawings. However, the scope of
the present invention is not limited to the configuration of the embodiment. For example, in the
embodiment, the electromechanical transducer is an ultrasonic transducer such as a CMUT, but
any method (a MMUT using a magnetic film, a PMUT using a piezoelectric thin film, etc.) that
may cause a warp It may be EXAMPLE FIG. 1 shows the entire ultrasound probe including an
ultrasound transducer. (A) is a perspective view, (b) is a cross-sectional view. As shown in FIG. 1B,
the gap 202 in the protection means 2 of the ultrasonic probe having the protection means 2 in
which the upper part is the ultrasonic wave transmission part and the grip part 3 and the cable
part 4 is a sheet 201. , The ultrasonic transducer 1 is disposed. The gap 202 covered by the
ultrasonic wave transmission sheet 201 is configured to efficiently transmit ultrasonic waves,
and is filled with air, oil such as castor oil, or the like. The distance between the surfaces of the
ultrasonic transducer 1 and the ultrasonic transmitting sheet 201 is set to, for example, a quarter
of the wavelength of the target ultrasonic wave so as to maximize the transmission efficiency of
the ultrasonic wave.
[0013]
FIG. 2 shows a connection form of the ultrasonic transducer 101 and a printed circuit board
which is a circuit board, and shows a state of connection already. The ultrasonic transducer 101
and the printed circuit board are electrically connected by a solder 102 which is a conductive
adhesive. This connection is performed, for example, as follows. Electrode pads of a printed
circuit board including processing circuits for processing signals and electrode pads are aligned
with the lower electrode of the ultrasonic transducer 101 and the like, and the two boards are
joined by the conductive solder 102. Thus, a CMUT or the like capable of signal processing of
transmission and reception of ultrasonic waves is configured. In the present embodiment, the
printed circuit board has a laminated structure of substrate material A (103) and substrate
material B (104) having different thermal expansion coefficients.
[0014]
As shown in FIG. 3, the ultrasonic transducer includes, for example, a lower electrode 12 which is
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a first electrode on a substrate 11, an air gap 15, a vibrating portion 14 supported by a support
portion, and an upper electrode 16 which is a second electrode. With one or more cells. The
lower electrode 12 and the support portion are disposed on the main surface of the substrate 11,
and the vibrating portion 14 having the upper electrode 16 is movably supported by the support
portion with the air gap 15 opposed to the substrate 11 and the lower electrode 12. This
operation is performed as follows. When the ultrasonic signal p (t) emitted from the
measurement target 27 is incident on the vibration unit 14, the vibration unit 14 vibrates
according to the waveform of the ultrasonic signal p (t). At this time, when the bias voltage Vb is
applied between the lower electrode 12 and the upper electrode 16 using the DC voltage source
28, a current signal i (t) corresponding to the vibration of the vibrating portion 14 is generated.
Therefore, the waveform of the ultrasonic signal p (t) is obtained by measuring the current signal
i (t). When transmitting ultrasonic waves, a DC voltage and a minute AC voltage are
superimposed and applied between the lower electrode 12 and the upper electrode 16. Thereby,
the vibration part 14 vibrates and an ultrasonic wave is generated.
[0015]
When measuring the internal state of the measuring object using an ultrasonic transducer, the
ultrasonic wave transmission sheet 201 of the ultrasonic probe is scanned along the measuring
object. At this time, scanning is performed by directly or indirectly contacting the object to be
measured. Indirect means scanning through a shape maintaining member such as a compression
plate for maintaining the shape of the object to be measured. In this embodiment, as described
above, the ultrasonic transducer is scanned along the object to be measured via the ultrasonic
wave transmitting sheet 201 of the ultrasonic probe. However, for example, as an ultrasonic
transducer provided with a protective layer of substantially uniform thickness over the entire
measurement surface, this may be in direct or indirect contact with the object to be measured for
scanning. Even in such a case, the warping of the ultrasonic transducer causes the reduction of
the transmission efficiency of the ultrasonic waves. Therefore, it is preferable to correct the
warping to improve the transmission efficiency.
[0016]
In general, an ultrasonic transducer is used as an array device composed of a plurality of
elements (about 200 to 4000) with a plurality of (usually about 100 to 3000) cells as one
element (one element). The height of the air gap 15 is, for example, 100 nm to 200 nm. The
diameter of the air gap 15 is, for example, in the range of 10 μm to 200 μm. Further, the upper
electrode 16 and the lower electrode 12 are formed of materials such as Al, Cr, Ti, Au, Pt, and Cu.
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The vibrating portion 14 is formed of an insulating material such as silicon nitride. From the
viewpoint of sensitivity and the like, it is preferable to keep the air gap 15 in a reduced pressure
state with respect to the atmospheric pressure and to make the vibrating portion 14 concave.
[0017]
As a method of producing an ultrasonic transducer, a method of producing warpage using
surface micromachining is described. FIG. 4 is a schematic view explaining an example of the
manufacturing process. First, for example, the Si substrate 11 is prepared. Next, a conductor film,
for example, a metal or a doped semiconductor is deposited by vacuum evaporation, sputtering,
CVD or the like, and the lower electrode 12 is formed by photolithography and etching (FIG. 4A).
Next, the sacrificial layer 13 is formed. First, for example, 100 nm of amorphous silicon is formed
by PECVD. A pattern of the sacrificial layer 103 to be an air gap is formed by photolithography
and etching (FIG. 4 (b)). Next, the vibration part and the support part are formed. For example,
the vibrating portion 14 which is a silicon nitride film and the support portion are formed to a
thickness of 100 nm by PECVD (FIG. 4C). Next, etching holes (not shown) are formed in the
silicon nitride film 14 by photolithography and etching. This is an inlet for introducing an etching
solution into the sacrificial layer. Next, for example, the substrate is immersed in Tetramethyl
Ammonium Hydroxide (TMAH). Thus, the TMAH etches the amorphous silicon 103 which is a
sacrificial layer. Thus, the air gap 15 is formed. Next, a metal such as aluminum is deposited, and
the upper electrode 106 is patterned by photolithography and etching (FIG. 4D).
[0018]
As mentioned above, ultrasonic transducers made by surface micromachining are prone to
warping. Therefore, in the present embodiment, the ultrasonic transducer and the circuit board
are connected to correct this warp. The circuit board is manufactured by laminating a plurality of
layers made of materials having different coefficients of thermal expansion. FIG. 5 shows the
state before the connection state of FIG. As shown in FIG. 5, the ultrasonic transducer 101 is in a
warped state. In the connection process, a reflow furnace is passed in the state of FIG. 5 (a). That
is, the solder 102 of the conductive adhesive is sandwiched between the circuit board and the
ultrasonic transducer, and both are positioned and superposed, and the circuit board and
ultrasonic transducer thus superposed are heated to melt the solder 102. FIG. 5 (b) shows a state
in which the solder 102 of the adhesive is melted and the temperature of the adhesive is lowered
again. Here, due to the difference in thermal expansion between the substrate material A (103) of
the printed circuit board and the substrate material B (104), the printed circuit board is warped.
The amount of warpage is previously adjusted to be the same as the amount of warpage
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possessed by the ultrasonic transducer 101. As a result, when the temperature is returned to
room temperature, the connection state is as shown in FIG.
[0019]
Here, a method of calculating the amount of warpage of the printed circuit board for correcting
the warpage of the ultrasonic transducer 101 will be described. As shown in FIG. 6, it is
considered that the printed circuit board is warped from a flat shape into a circular arc due to the
warp near the temperature at which the solder 102 melts and hardens again. In FIG. 6A, d is the
thickness of the printed circuit board, L is the length of the printed circuit board, and R and θ
are the imaginary circle radius and the angle at which the printed circuit board is viewed.
Assuming that the thermal expansion coefficients of the substrate material A and the substrate
material B are αA and αB, respectively, the lengths at the temperature T around the
temperature at which the solder 102 solidifies again are L × (1 + αA × T), L × (1 + αB × T) ).
The interface between the two substrate materials is considered to be the average elongation.
Then, it becomes as follows. R × θ = L × (1 + αA × T) (R + d) × θ = L × (1 + αB × T) From
these relationships, θ = L × (αB−αA) × T / d.
[0020]
From the relationship shown in FIG. 6 (b), the amount of warpage h is as follows. h = (L / 2) ×
tan (θ / 2) And, if θ is approximated as small, it is as follows. h = (L / 2) <2> × (αB−αA) × T
/d
[0021]
For example, as the substrate material A and the substrate material B, those having thermal
expansion coefficients αA and αB of 12 ppm and 20 ppm, respectively, the hardening
temperature T of the solder 102 is 200 ° C., the length L of the printed board is 20 mm, When
the thickness d is 1.6 mm, the amount of warpage h is 100 μm. In this manner, the warping
state of the printed board near the temperature at which the solder 102 melts and hardens again
is adjusted so as to be substantially equal to the warp of the ultrasonic transducer 101, and the
two are joined. At this time, the ultrasonic transducer 101 is originally warped and is not warped
further. Further, the rigidity of the printed circuit board is adjusted so as to be able to force the
warp of the ultrasonic transducer 101. As a result, when the temperature is returned to room
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temperature, the connection state of the both becomes the state of FIG. 2 and the warpage is
corrected. The thermal expansion coefficient of the material used for the substrate material of
the printed circuit board is about 20 ppm to 12 ppm of glass epoxy and 38 ppm of polyimide
and the like. The thermal expansion coefficient of the material used for the wiring material of the
printed circuit board is 16 ppm of copper or the like. By adjusting the combination of these
expansion coefficients and the thickness and length, it is possible to obtain a desired amount of
warpage combined with the warpage of the ultrasonic transducer. When it is necessary to
consider the thermal expansion coefficient of the material of the circuit wiring when considering
the amount of warpage of the circuit board, the amount of warpage is calculated considering that
the layer of the circuit wiring is also one layer. In the present embodiment, the number of
laminations is two, but in the case of three or more layers, the combination of the thermal
expansion coefficient, the thickness and the length may be adjusted according to the above
calculation method. In addition, when it is necessary to treat layers embedded in one layer as
substantially laminated layers, such layers may be counted in the number of laminated layers.
[0022]
According to an electromechanical transducer including the capacitance type electromechanical
transducer of the present invention and a method of manufacturing the same, for example, in the
case of using the electromechanical transducer in a package and using it, the transmission
efficiency of acoustic waves such as ultrasonic waves Can be improved. The electro-mechanical
transducer including the produced capacitive-type electro-mechanical transducer etc. can be
used as an ultrasonic probe etc. of fields, such as construction, material, medical treatment.
[0023]
1, 101 ... ultrasonic transducer (electromechanical transducer), 102 ... solder (conductive
adhesive), 103 ... circuit board substrate material A, 104 ... circuit substrate substrate material B,
201 ... ultrasonic wave transmission sheet (ultrasonic wave Transmission part), 202 ... gap
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