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JP2011166633

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DESCRIPTION JP2011166633
The present invention provides a technology capable of producing an electromechanical
transducer such as a two-dimensional capacitive electromechanical transducer having good
sensitivity in a desired frequency band without defects or with a high yield. An electromechanical
transducer has a plurality of elements 1, 2, 3 including at least one cell. The plurality of elements
1, 2, 3 are separately disposed independently of each other for the corresponding processing
circuits on the integrated circuit substrate 5 on which the plurality of processing circuits are
formed, and each element has a signal Each is mechanically and electrically coupled to a plurality
of corresponding processing circuits so that input and output can be performed. [Selected figure]
Figure 1
Electromechanical converter and method of manufacturing the same
[0001]
The present invention relates to an electromechanical transducer such as a capacitive ultrasonic
transducer that performs at least one of reception and transmission of an elastic wave such as an
ultrasonic wave, and a method of manufacturing the same. Technology applicable to the
realization of
[0002]
In recent years, capacitive electromechanical transducers manufactured using a micromachining
process have been actively studied.
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1
A typical capacitive electromechanical transducer has a cell including a vibrating membrane
supported at a distance from a lower electrode, and an upper electrode disposed on the surface
of the vibrating membrane. This is used, for example, as a capacitive ultrasonic transducer
(CMUT: Capacitive-Micromachined-Ultrasonic-Transducer). In general, a CMUT is composed of a
plurality of (usually about 100 to 3000) cells as one element (one element) and composed of
about 200 to 4000 elements, and the CMUT itself has a size of about several mm. The CMUT
transmits and receives ultrasonic waves using a lightweight diaphragm, and can easily obtain a
wide band characteristic in liquid and air. Use of this CMUT makes it possible to perform
diagnosis with higher accuracy than conventional medical diagnosis, and is attracting attention
as a promising technology.
[0003]
The operation principle of the capacitive electromechanical transducer will be described. When
transmitting an elastic wave such as an ultrasonic wave, a DC voltage is superimposed on a
minute AC voltage and applied between the lower electrode and the upper electrode. As a result,
the vibrating film vibrates to generate an elastic wave. When receiving an elastic wave, since the
vibrating membrane is deformed by the elastic wave, the signal of the elastic wave is detected by
the capacitance change between the lower electrode and the upper electrode due to the
deformation. Generally, in order to transmit and receive ultrasonic waves and sound waves, an
electromechanical transducer in which a plurality of elements are arrayed is used.
[0004]
As technology related to arraying of the electromechanical transducer, one in which a
piezoelectric body and a circuit board are combined is proposed (refer to Patent Document 1).
Further, an assembly of an electronic element array provided with a sensor array and an
integrated circuit module There is a proposal (see Patent Document 2).
[0005]
JP, 2000-298119, A JP, 2008-147622, A
[0006]
However, in order to manufacture a two-dimensional capacitive electromechanical transducer
having a desired sensitivity in a desired frequency band with a high yield, the above-described
arraying technology is not sufficient.
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2
[0007]
In view of the above problems, an electromechanical transducer according to the present
invention having a plurality of elements each including a cell constituted by a first electrode and
a second electrode provided with a gap between the first electrode and the first electrode. Has
the following features.
That is, the plurality of elements are arranged separately from each other with respect to the
corresponding processing circuits on the integrated circuit substrate on which the plurality of
processing circuits are formed.
Also, they are mechanically and electrically coupled to the corresponding plurality of processing
circuits so that signals can be input / output for each element.
[0008]
Further, in view of the above problems, an electric machine according to the present invention
having a plurality of elements including at least a cell including a first electrode and a second
electrode provided with a gap between the first electrode and the first electrode. The method of
manufacturing the conversion device is characterized by including the following steps.
A manufacturing process in which a plurality of elements are arranged in multiple planes on a
substrate. Out of the plurality of elements fabricated on the substrate, a good element is selected
and cut out, and each of the processing circuits on the integrated circuit substrate on which the
plurality of processing circuits are formed is separated from each other and independently
Placement process to place. A coupling step of mechanically and electrically coupling the
plurality of elements to the corresponding plurality of processing circuits so that signals can be
input / output for each element.
[0009]
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According to the present invention, out of a plurality of chips (elements) manufactured on a
substrate, non-defective chips inspected in advance can be selected and cut out, and connected to
corresponding processing circuits on an integrated circuit substrate. Therefore, it becomes
possible to produce a two-dimensional capacitive electromechanical transducer or the like having
good sensitivity in a desired frequency band without defects or with high yield.
[0010]
BRIEF DESCRIPTION OF THE DRAWINGS Sectional drawing of the electromechanical transducer
which concerns on Embodiment 1 of this invention. 5A to 5D illustrate a manufacturing process
of Embodiment 1. FIG. 6 is a table showing conditions of SiN film formation of the sensor chip
used in the first embodiment. 5A to 5D illustrate a manufacturing process of Embodiment 1. FIG.
5A to 5D illustrate a manufacturing process of Embodiment 1. FIG. 5A to 5D illustrate a
manufacturing process of Embodiment 1. FIG. 6 is a graph showing resonant frequency
characteristics of the plurality of sensor chips of Embodiment 1 and resonant frequency
characteristics of the electromechanical transducer of Embodiment 1. 5A to 5D illustrate a
manufacturing process of Embodiment 2 of the present invention. 7 is a graph showing resonant
frequency characteristics of the electromechanical transducer of the second embodiment.
[0011]
Hereinafter, embodiments of the present invention will be described. In the present invention, a
good chip is selected from a plurality of chips (elements) manufactured on a substrate, cut out,
and connected to a corresponding processing circuit on an integrated circuit substrate to
construct an electromechanical transducer. It is. Based on this concept, the basic form of the
electromechanical transducer of the present invention and the method of manufacturing the
same have the configuration as described above. Based on this basic form, the following
embodiments are possible.
[0012]
The plurality of elements (elements) may include a plurality of elements having different
resonance frequency characteristics. Generally, in ultrasound imaging, the frequency to be
detected differs depending on the target object, and a wider band is required. The resonance
frequency is determined by the size of the cavity constituting the CMUT or the like, the gap
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between the electrodes, the physical property values (Young's modulus, Poisson's ratio) and the
film thickness of the membrane material. From this, in order to give different resonance
frequencies to the configuration on the same substrate, the method of changing the cavity
diameter is simple. However, in this method, it is necessary to arrange the total area of the
transmitting and receiving structures according to the respective cavity diameters differently
within a certain area or to adopt an inefficient arrangement. The configuration including a
plurality of elements having different resonance frequency characteristics as described above
solves these problems and answers the above-mentioned requirements. According to this
configuration, it is possible to manufacture plural types of sensor chips having sensitivity
characteristics in different frequency bands by changing the hardness (corresponding to the
Young's modulus in physical property values) of the membrane film (vibration film) with the
same cavity diameter. . In this way, it is possible to manufacture a highly sensitive twodimensional capacitive electromechanical transducer or the like with a desired frequency band
without defects or with a high yield.
[0013]
In addition, a plurality of elements can be directly connected to corresponding processing
circuits on the integrated circuit substrate. Generally, the load of electrical mounting increases
with two-dimensionalization, and means such as wire bonding can not achieve high density.
Furthermore, in a CMUT or the like, since the vibration signal by ultrasonic waves is converted
into a change in capacitance value, if the parasitic capacitance of the device is large, the
sensitivity characteristic will be deteriorated that much, and the reduction of the parasitic
capacitance is a major issue. The structure directly connected on the integrated circuit substrate
solves such problems.
[0014]
Hereinafter, embodiments of the present invention will be described with reference to the
drawings. Embodiment 1 Embodiment 1 of the present invention will be described. The
electromechanical transducer of this embodiment shown in FIG. 1 has a plurality of sensor chips
which are elements including at least one cell. Each cell includes a lower electrode, which is a
first electrode, and an upper electrode, which is a second electrode opposed to each other with a
cavity (gap) therebetween. An element consists of one or more of the above-mentioned cells, and
when there are a plurality of cells, each cell in the element is electrically connected in parallel.
The sensor chip includes a sensor chip 1 having a resonance frequency of 1 MHz, a sensor chip 2
having a resonance frequency of 5 MHz, and a sensor chip 3 having a resonance frequency of 10
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MHz. These sensor chips are electrically mounted by the bumps 4 and electrically connected to
the integrated circuit substrate 5. Further, the gap between the sensor chips 1 to 3 and the
integrated circuit substrate 5 and the gap between the sensor chips 1 to 3 are filled with the
underfill 6 of the insulating material, and the common upper wiring 7 is ink jet drawn on the
upper layer thereof. The top electrode of the cell is used as a common electrode, drawn directly
by the machine. The plurality of sensor chips 1 to 3 are separately disposed independently and
are directly connected to the processing circuit of the corresponding element unit on the
integrated circuit substrate 5 by the bumps 4, so that the electromechanical transducer is a
parasitic It has almost no capacity.
[0015]
A method of manufacturing an electromechanical transducer according to the present
embodiment will be described. FIG. 2 (a) is a top view showing the arrangement of the sensor
chip 9 used in the fabrication of this embodiment on the silicon substrate 8, and FIG. 2 (b) is a
sacrifice for forming the cavity 11 of each cell of the sensor chip 9. It is a top view which shows
the formation process of a layer. Moreover, FIG.2 (c) is A-A 'sectional drawing of the part used as
the cell of FIG.2 (b). As shown in FIG. 2A, a sensor chip 9 of 2 mm square element unit is
arranged in multiple planes on a 4-inch silicon substrate 8 having a specific resistance value of
0.02 Ω · cm. This fabrication is performed according to the following surface type MEMS
method. In the step of forming the sacrificial layer to be the cavity 11 in FIGS. 2B and 2C, the
chromium film 10 is formed on the silicon substrate 8 with a thickness of 200 nm, and the cavity
11 of 25 μmφ is formed by photolithography. Pattern the sacrificial layer. Then, as shown in
FIG. 2C, a SiN film 12 to be a membrane film (vibration film) is formed 400 nm thick on the
silicon substrate 8 by a plasma CVD apparatus. At this time, physical property values of the SiN
film 12 can be controlled by SiH 4 gas flow rate, NH 3 gas flow rate, N 2 gas flow rate, frequency
of radio frequency (RF) power source, RF power and substrate temperature as film forming
parameters. The Young's modulus decreases as the amount of Si increases in the component ratio
of Si to N. Therefore, the Young's modulus can be controlled by the flow ratio of SiH 4 gas and
NH 3 gas. In the present embodiment, the silicon substrate 8 itself is used as the lower electrode.
In addition, the Young's modulus of the SiN film 12 to be a membrane film is set to 100 GPa, 170
GPa, and 240 GPa, and the resonance frequencies of the sensor chip are set to 1 MHz, 5 MHz,
and 10 MHz. That is, with the recipes A, B and C shown in FIG. 3, different SiN films 12 were
formed on the three wafer substrates 8 respectively.
[0016]
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FIG. 4A and FIG. 4B show a cavity forming process. Here, fine etching holes 13 communicating
with the respective chromium films 10 are opened by dry etching in a part of the SiN film 12
formed on the patterned chromium film 10 which is a sacrificial layer for forming the cavities 11.
. Then, the substrate is immersed in a solution of cerium ammonium nitrate as an etchant to etch
away the chromium sacrificial layer 10, and after washing and drying, a cavity 14 is formed.
Next, FIGS. 5A and 5B show the upper electrode forming step. Here, an aluminum film is formed
to a thickness of 400 nm on the substrate by sputtering, and the seal 16 of the etching hole 13
and the upper electrode pattern 17 are formed.
[0017]
FIG. 6A shows a process of forming an UBM (Under Bump Metal) layer for performing electrical
mounting. Here, on the back surface of the wafer (substrate) 8, the holes formed in the stencil
mask 18 are made to correspond to the respective sensor chips 9, and the stencil mask 18 is
aligned and set. Then, titanium / nickel / gold (= UBM layer) 19 is continuously formed on each
portion of the wafer back surface corresponding to the sensor chip 9. In this manner, a
manufacturing process of arranging the plurality of elements 9 on the substrate 8 in multiple
planes is performed.
[0018]
After that, a short circuit between the upper electrode and the lower electrode, a defect of
removing chromium, and the like are inspected with respect to the sensor chips 9 arranged in
plural on the wafer. Then, the sensor chip 9 in which all the cells are non-defective products was
cut out and separated by dicing. Here, about 80% of non-defective sensor chips (elements) 9 were
selected. FIG. 6 (b) shows the form at the end of the electrical mounting. In this electrical
mounting process, cream solder is transferred by printing to the UBM layer 19 of the nondefective chip 9, and the bump 4 is formed through a reflow furnace. On the other hand, cream
solder is printed on the UBM layer 21 on the side of the integrated circuit board 5 in which a
plurality of processing circuits 20 are arranged in element units. Then, after the sensor chip 9 is
aligned and mounted on the UBM layer 21 of the corresponding processing circuit 20 by a flip
chip bonder, the electrical connection is completed through a reflow furnace. Further, underfill
material 6 is filled between sensor chip 9 and integrated circuit substrate 5 and between the
plurality of elements using capillary action, and heat curing is performed to complete the
mechanical bonding. As described above, a good element is selected from the plurality of
elements 9 fabricated on the substrate 8 and cut out, and the corresponding processing circuits
on the integrated circuit substrate 5 on which the plurality of processing circuits 20 are formed
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are selected. Thus, placement steps are performed, which are placed separately and
independently of one another. In addition, a coupling step is performed to mechanically and
electrically couple the plurality of elements 9 with the corresponding plurality of processing
circuits 20 so that signals can be input / output for each element.
[0019]
FIG. 6 (c) shows the final step. Here, in order to use the upper electrode of each cell of the
electromechanical transducer as a common electrode, the conductive paste 23 is discharged by
the ink jet drawing machine 22 so that electrical connection with the upper electrode pattern 17
of each sensor chip 9 can be made. Common wiring 7 is drawn. Then, the common upper wiring
7 is heated and cured to complete the electrical mounting process. Thus, on the surface of the
filled insulating material 6, a wire in which one electrode of each cell of the plurality of elements
9 is a common electrode can be formed by ink jet drawing. The reception characteristics of the
two-dimensional capacitive electromechanical transducer of this embodiment thus produced
were measured by using a calibrated hydrophone in a castor oil-filled system. As a result, as the
frequency band of the capacitive electromechanical transducer including the sensor chip 9
having the individual frequency band shown in FIG. 7A, the broadened frequency band shown in
FIG. 7B is measured.
[0020]
Second Embodiment Next, a second embodiment of the present invention will be described. The
present embodiment is an embodiment in which the resonance frequency characteristics are
controlled closer to each other. In the electromechanical transducer according to the present
embodiment, the resonance frequencies of the sensor chips are 1 MHz, 2 MHz, and 3 MHz. To
realize such a resonance frequency, a method of changing the film thickness of a membrane film
(vibration film) having the same Young's modulus is effective. In this embodiment, in order to use
<100> single crystal silicon having a Young's modulus of 130 GPa as a membrane film of each
cell, an electromechanical transducer is manufactured according to a silicon direct bonding type
MEMS method.
[0021]
The manufacturing method of this embodiment will be described. FIG. 8 (a) is a top view showing
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the multi-face arrangement on the silicon substrate 24 of the sensor chip (element) 25 used in
the fabrication of this embodiment, and FIG. 8 (b) is a formation of the oxide film 26 of the
sensor chip 25. It is sectional drawing which shows a process. 8 (c) is a cross-sectional view
showing the etching process of the oxide film 26 of the sensor chip 25, FIG. 8 (d) is a crosssectional view showing the wafer bonding process of the sensor chip 25, and FIG. FIG. 18 is a
cross-sectional view showing the step of forming the membrane film 31 of the sensor chip 25.
Here, as shown in FIG. 8A, a sensor chip 25 of 2 mm square element unit is arranged in multiple
planes on a 4-inch silicon substrate 24 with a specific resistance value of 0.02 Ω · cm. This
fabrication is performed according to the following junction type MEMS method. In the oxide film
formation step of FIG. 8B, a 200 nm thick oxide film 26 is formed on the silicon substrate 24 by
pyrogenic oxidation (1000 ° C., 90 minutes) in a thermal oxidation furnace. In the oxide film
etching step of FIG. 8C, the oxide film 26 in the portion 27 to be a cavity is removed by RIE
etching using CF 4 gas. The wafer bonding step shown in FIG. 8D is a bonding step with the SOI
substrate 28 having a silicon active layer with a crystal orientation of <100>, and the respective
substrates 24 and 28 are cleaned in advance and exposed to N2 plasma for 10 minutes. The
activated surface is bonded in a vacuum bonding apparatus in an atmosphere of about 0.4 Pa.
Thereafter, heating is performed at 1100 ° C. for 2 hours to complete bonding of the two silicon
substrates 24 and 28. Here, bonding was performed using three types of SOI substrates 28 in
which the film thickness of the silicon active layer 29 is different from 400 nm, 800 nm, and
1100 nm.
[0022]
In the formation step of FIG. 8E, thereafter, the back grinder and KOH solution are used to
remove the BOX (buried oxide film) layer 30 from the back surface of the SOI substrate 28, and
finally the BOX layer 30 is removed with hydrofluoric acid. Then, a membrane film (vibrating
film) 31 of single crystal silicon is formed. Furthermore, as in the first embodiment, a UBM layer
for electrical mounting is formed and inspected, and a two-dimensional capacitive
electromechanical transducer is manufactured using about 60% non-defective chips having three
types of resonance frequencies. did. Similarly, as a reception characteristic, high sensitivity
reception characteristic was confirmed in a band of 5 MHz or less as shown in FIG. An
electromechanical transducer having a high sensitivity resonant frequency characteristic in a
relatively low frequency band as described above is considered effective as a sensor device used
for early detection of breast cancer and the like. According to the present embodiment, it is
possible to manufacture a sensor device that meets the purpose without element defects or with
high yield.
[0023]
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Although the sensor chip is manufactured using a silicon substrate in the above embodiment, the
present invention is not limited to this, and a chip using another material (such as a glass
substrate) may be used. In addition, although bump connection is used in electrical mounting, a
mounting technique using a conductive adhesive, an anisotropic conductive film, or the like can
also be employed. Furthermore, the upper wiring formation method using the ink jet drawing
apparatus can easily form the wiring without disconnection even on the relatively stepped
surface, and can form the wiring without using a large apparatus such as a film forming
apparatus or photolithography. As it is, it is a very effective method.
[0024]
1, 2, 3, 9, 25 elements (sensor chip), 5 integrated circuit substrate, 8 24 substrates (silicon
substrates) 12, 31 membrane films (vibrating films) 14 27 cavities (cavity) 17) Upper electrode
20 Processing circuit
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