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JP2004056351

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DESCRIPTION JP2004056351
The present invention provides a piezoelectric microphone using a ferroelectric thin film
obtained by sintering a high purity PZT thin film or the like on a thin sintered substrate of about
10 μm. A temporary firing layer 6c is formed by laminating on a substrate 6e and then firing at
a temperature lower than or equal to the phase transition temperature of the ferroelectric
starting material. The ferroelectric thin film 6c sintered at a high temperature higher than the
phase transition temperature of the ferroelectric raw material is used as the diaphragm 6 after
the application of the ferroelectric raw material to the above or without applying the ferroelectric
raw material. [Selected figure] Figure 1
Piezoelectric microphone
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a
piezoelectric microphone, and more particularly to a piezoelectric microphone using as an
oscillating plate a polarization-processed very thin ferroelectric thin film. [0002] Conventionally,
patent applicants are referred to as "condenser microphones" (Japanese Patent Laid-Open No.
2001-8293) (hereinafter referred to as conventional examples). Invented. In this prior art
example, <claims> <claim 1> An IC element comprising a diaphragm, a back electrode, and a FET
for a microphone is fixed in order from the top surface side in a casing having a sound hole in
the top plate. In the condenser microphone in which the terminal substrate is accommodated
with maintaining a predetermined distance, a contact ring serving as a spacer made of a
conductive material is interposed between the back surface of the back electrode and the
terminal substrate surface, and the outer peripheral surface of the contact ring A capacitor
microphone characterized in that an insulation film is interposed between the housing and the
inner peripheral surface of the casing. According to a second aspect of the present invention, the
insulating film is made of an insulating material integrally including an insulating cylindrical
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portion along the inner peripheral surface of the casing and a spacer ring portion integrally
protruded inside the top of the insulating cylindrical portion, the spacer ring 2. The condenser
microphone according to claim 1, wherein the diaphragm is interposed between the diaphragm
and the back electrode, and the back electrode, the contact ring and the casing are insulated by
the insulating cylindrical portion. According to a third aspect of the present invention, there is
provided the condenser microphone according to the first aspect, wherein the insulating film is
constituted by an insulating film attached to the inner surface of the casing. As shown in FIG. 5,
the upper surface of the casing 120 forming the casing is formed by drilling a plurality of sound
holes 122 in the top plate 121. Further, in the casing 120, a diaphragm 124 is provided by being
sandwiched between a spacer ring 129 having the same shape as the plate-shaped diaphragm
ring 125 having a donut shape disposed on the lower surface of the top plate 121. Furthermore,
a back electrode 126 is provided below the spacer ring 129 with a small gap from the diaphragm
124. In the conventional example, when the diaphragm 124 provided in this manner is vibrated
by air vibration propagating from the sound hole 122 provided in the top plate 121, the back
electrode and the vibration plate are not vibrated by the air vibration. The voltage across the
electrical capacitor formed by 124 will change. The electrical change is electrically output by the
IC element 133 fixed to the terminal plate 131 in the casing 120. On the other hand, a
ferroelectric thin film subjected to polarization processing is known as a device that generates a
voltage when stress is applied to a dielectric and causes a voltage change depending on the
amount of deformation.
A conventional ferroelectric thin film, for example, a ferroelectric PZT thin film, is formed on a
metal substrate to be a sintered substrate of about 1.5 mm to 0.04 mm. That is, conventionally,
there has been an example in which a method of forming a thin film using a sol-gel method on a
foil serving as a metal substrate is used, and a PZT thin film of 650 nm is formed on titanium foil
(Ti) of 0.05 mm thickness. Hereinafter, a conventional example of firing a PZT thin film on a
titanium foil will be described. First, a metal substrate made of titanium foil is placed on a
rotating table for PZT raw material application, and suction and adsorption are fixed. The PZT
raw material is applied by spin coating on the metal substrate placed in this manner to form a
film of the PZT raw material. Subsequently, the metal substrate on which the film of the PZT raw
material is formed is heated to a high temperature of 500 ° C. to 600 ° C., which is a sintering
temperature of PZT, to sinter the PZT raw material to form a PZT thin film. If it is desired to
increase the thickness of the PZT thin film, it is desirable to repeat the process of applying a PZT
raw material by spin coating on the sintered PZT thin film and sintering again at a high
temperature of 500 ° C. to 600 ° C. or more. The film thickness of the PZT thin film was
obtained. As described above, the metal substrate for sintering the PZT raw material is not only
titanium foil but also each metal substrate such as 0.04 mm stainless (SUS 304) foil, 1.2 mm
brass sheet, 1.5 mm nickel alloy, etc. Each was used to form a PZT thin film. However, in the
condenser microphone shown in the prior art, the diaphragm 124 and the back electrode 126
are essential for the condenser for detecting the change in sound. In addition, in order to detect a
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change in sound with high accuracy, it is required that the diaphragm 124 vibrates due to the
vibration of air that is sound, but the back electrode 126 must not vibrate due to this. That is, the
diaphragm 124 is required to vibrate even by a slight change in sound or vibration, and in order
to realize this, the substrate is required to be thin. On the other hand, the back electrode 126 is
required to be robust because it must not vibrate against air vibration. And in order to form the
back electrode 126 firmly, it was necessary to thicken the back electrode. As described above, in
the conventional condenser microphone, the thickness of the back electrode 126 limits the
miniaturization of the entire microphone. On the other hand, in a conventional ferroelectric thin
film, even a metal substrate which is a sintered substrate having a minimum thickness is about
0.04 mm (40 μm), and a PZT thin film can not be formed on a thin metal substrate of 10 μm or
less The
That is, if the film thickness of the metal substrate is reduced to about 10 μm, during spin
coating when applying the PZT material, irregularities are generated on the metal substrate when
attracted and adsorbed on the rotating table, and the PZT material is There was a problem that
wrinkles were generated when it was sintered. Furthermore, if the sintering of the PZT raw
material at 500 ° C. to 600 ° C. is repeated several times in order to obtain the thickness of the
PZT thin film, the initially generated wrinkles further increase and the strain is also accumulated.
Not only the strain but also the crack is generated, and the integrity of the generated PZT thin
film which is a high dielectric thin film is lowered, so that a high purity PZT thin film can not be
formed. As described above, since a thin film ferroelectric can not be obtained, there is a problem
that it is difficult to form a piezoelectric microphone that is compact and has good tracking.
Therefore, it is an object of the present invention to provide a piezoelectric microphone in which
a ferroelectric thin film obtained by sintering a high purity PZT thin film or the like on a thin
sintered substrate of about 10 μm is subjected to polarization treatment. According to the
present invention, a temporary firing layer is formed by laminating on a substrate a ferroelectric
material and then firing at a temperature lower than the sintering temperature of the
ferroelectric material. A ferroelectric thin film which is sintered at a temperature at which the
ferroelectric raw material is sintered after applying the ferroelectric raw material on the fired
layer or without applying the ferroelectric raw material is subjected to polarization treatment to
form a diaphragm. A piezoelectric microphone characterized in that: a casing having a sound hole
in a top plate; an upper diaphragm ring and a lower diaphragm ring, the upper diaphragm ring
being on the top plate side A diaphragm provided inside the casing of the top plate, a terminal
substrate provided inside the casing facing the top plate, to which an IC element comprising a
microphone FET is fixed, a conductive material, and a lower diaphragm ring and a terminal base
The diaphragm comprises a contact ring serving as a spacer between the surfaces and an
insulating film interposed to insulate between the outer peripheral surface of the contact ring
and the inner peripheral surface of the casing, and the diaphragm is coated with a ferroelectric
material on the substrate After that, a temporary firing layer to be fired at a temperature lower
than the sintering temperature of the ferroelectric material is laminated, and after applying the
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ferroelectric material on the temporary firing layer or without applying the ferroelectric material,
the ferroelectric is A piezoelectric microphone comprising: a ferroelectric thin film which is
sintered at a temperature at which a body material sinters is subjected to polarization treatment.
Then, in the present invention, in order to solve the above-mentioned problems, in a casing
having a sound hole in the top plate, an IC element comprising a diaphragm, a contact ring, and
an FET for a microphone in this order from the top surface side. A fixed terminal substrate is
accommodated with a predetermined distance maintained, a diaphragm is sandwiched between
an upper diaphragm ring and a lower diaphragm ring, and a spacer made of a conductive
material between the upper diaphragm ring and the terminal substrate surface A dual purpose
contact ring is interposed, and the outer circumferential surface of the contact ring and the inner
circumferential surface of the casing are insulated with an insulating film interposed.
The ferroelectric thin film forming the diaphragm is formed by applying a ferroelectric material
on a substrate, and repeating pre-baking of the substrate to which the ferroelectric material is
applied at a temperature lower than the sintering temperature. Form a pre-fired layer of the
body. By these series of steps, a calcined layer which has been temporarily sintered is gradually
formed thick on the substrate. Then, when the fired layer on the substrate has a desired
thickness, the temporarily fired layer is sintered at the sintering temperature of the ferroelectric
material to obtain a ferroelectric thin film. The ferroelectric thin film thus obtained is subjected
to polarization processing to be provided with piezoelectricity and used as a vibrating plate, and
when an external stress is applied to the piezoelectric thin film due to the vibration of air, a
voltage characteristic of the piezoelectric is generated. The change in voltage caused by the
vibration of air is detected to obtain the waveform information of the sound wave in the form of
an electrical signal. The insulating film may be constituted by an insulating film attached to the
inner peripheral surface of the casing. Embodiments of the present invention will be described
below based on the drawings. FIG. 1 is a longitudinal cross-sectional view of a piezoelectric
microphone according to an embodiment of the present invention, FIG. 2 is a cross-sectional view
of the same, and FIG. 3 is an explanation showing a procedure for forming a diaphragm
according to an embodiment of the present invention. It is a figure, FIG. 4 is explanatory drawing
showing the coating process of FIG. The piezoelectric microphone 1 has a casing 2 made of
aluminum or nickel white that is formed into a cylindrical shape with a bottom by drawing
processing as a housing, and in the casing 2, a top plate 3 forming an upper surface forms a
bottom wall. A plurality of sound holes 4 are bored in the top plate 3 or the sound from the
outside of the casing 2 is formed to propagate inside. In the lower part of the top plate 3 inside
the casing 2, a diaphragm 6 for detecting the vibration of the sound transmitted from the sound
hole 4 is provided. The diaphragm 6 is formed of a piezoelectric thin film obtained by polarizing
a ferroelectric thin film, and is stretched by the diaphragm rings 7 and 26 in contact with the
inner surface of the top plate 3 whose center is opened like a donut ring. It is nipped from above
and below and fixed. Thus, by being held by the diaphragm rings 7 and 26, the diaphragm 6 is
accommodated at a distance from the top plate 3 by the diaphragm rings 7 and 26, and can be
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vibrated by the sound transmitted from the sound hole 4 . Moreover, the casing 2 bend | folds
and crimps the foot part 5 of a cylindrical surface inside, and has stopped each member of the
diaphragm 6 grade | etc., Accommodated in the inside falling off. The diaphragm rings 7 and 26
positioned above and below the diaphragm 6 are accommodated in a cylindrical insulating
material 9 inserted in contact with the inner surface of the casing 2 to insulate the casings 2
from each other.
The insulating material 9 is formed in a cylindrical shape to form an insulating film, and
maintains the lower diaphragm ring 8 and the casing 2 in an insulating state. The lower end of
the lower diaphragm ring 8 is in contact with the upper end of the cylindrical contact ring 10,
the terminal board 11 is in contact with the lower end of the contact ring 10, and the skirt of the
casing 2 is formed on the lower surface of the terminal board 11. 5 is crimped. At this time,
although the contact ring 10 and the casing 2 are insulated, the cylindrical insulating material 9
may be extended to the skirt portion 5 and the insulating material 9 may be formed so as to
maintain the insulation state. good. The inside of the contact ring 10 forms a back space 12, and
electronic components such as an IC element 13 composed of FETs (field effect transistors) are
fixed and accommodated on the upper surface of the terminal substrate 11 in the back space 12
The ring 10 doubles as a spacer between the lower diaphragm ring 8 and the terminal substrate
11. The details of the diaphragm 6 will be described below. The vibrating plate 6 is made of a
polarized piezoelectric thin film. The piezoelectric thin film forming the vibrating plate 6
generates a voltage characteristic of the piezoelectric when external stress is applied by air
vibration. Then, by detecting and electrically converting the change in voltage of the piezoelectric
thin film caused by the vibration of air, it is possible to reproduce the sound applied to the
diaphragm 6. The diaphragm 6 is a piezoelectric thin film made of a PZT thin film, and as shown
in part A of FIG. 1, titanium foil 6a which forms the basis of a substrate which is a metal
substrate and platinum formed on the top of titanium foil 6a. Lead zirconate titanate (hereinafter
referred to as “PZT”) is formed by sintering on the electrode layer 6 b and the platinum
electrode layer 6 b. A thin film 6c and a gold electrode layer 6d formed as an upper electrode on
the top of the PZT thin film 6c. The titanium foil 6 a is formed as a substrate of the diaphragm 6
and has a metal foil shape with a thickness of 10 μm or less, the surface of which is thermally
oxidized. In this embodiment, although the titanium foil 6a is 10 μm or less, the film thickness
may be 10 μm or more such as 18 μm depending on the application of the diaphragm 6, and
may be formed to a desired film thickness. A thickness may be appropriately selected and used
from the strength, the response when vibrating by air, and the like. The platinum electrode layer
6 b is formed in a layer by sputtering on the upper surface of the titanium foil 6 a and functions
as a lower electrode which is one of the electrodes of the diaphragm 6. The platinum electrode
layer 6b and the titanium foil 6a form a substrate 6e. The PZT thin film 6c is formed by applying
and sintering a raw material of PZT, which is a ferroelectric material in which lead titanate
(PbTiO3) and lead zirconate (PbZrO3) are solid-solved on the upper surface of the substrate 6e.
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Therefore, in the case of spin coating, the PZT raw material is dropped as a PZT solution in which
the PZT raw material is mixed in a solution, and is sintered at a high temperature after spin
coating to form the PZT thin film 6c. The formation of the PZT thin film 6c is performed by the
formation step S of the diaphragm 6 which forms the diaphragm 6, as shown in FIG. The
formation step S is composed of a coating step S1, a temporary firing step S2, a repeated
judgment step S3, and a final sintering step S4. The application step S1 is a step of applying a
PZT raw material to the titanium foil 6a provided with the platinum electrode layer 6b by spin
coating. In the coating step S1, the titanium foil 6a provided with the platinum electrode layer 6b
is placed and fixed on the coating table 16 of the spin coating apparatus 14 shown in FIG. 4 so
that the platinum electrode layer 6b is on the top. At this time, the substrate 6 e is made to
permeate the solution 18 between the coating table 16 and the titanium foil 6 a so as to be fixed
by liquid surface adsorption so that the platinum electrode layer 6 b is on the upper surface. In
general, in an adsorption method such as vacuum suction performed by the spin coat apparatus
14, concave and convex deformation occurs in the thin titanium foil 6 a of about 10 μm as in
this embodiment due to the adsorption force, and firing of the PZT thin film 6 c in the
subsequent steps is performed as it is If you do, you will get wrinkles. However, as in the present
invention, the mounting and fixing to the spin coating device 14 is adsorbed on the liquid surface
by the solution 18, whereby the titanium foil 6a can not be made uneven, and the good PZT thin
film 6c can be fired. The solution 18 is a solution such as alcohol, but may be another solution as
long as it does not affect the composition of the vibrating plate 6 as well as the coating table 16.
The coating table drive unit 15 of the spin coating apparatus 14 placed on the coating table 16
by liquid surface adsorption is driven to rotate the coating table 16. Then, the PZT solution is
coated on the top surface of the platinum electrode layer 6 b on the titanium foil 6 a by dropping
the PZT solution from the PZT raw droplet dropping device 17. Next, it is determined whether
the application step S1 has been repeated a predetermined number of times in the repeated
determination step S3. The repetitive determination step S3 is merely determined by the
operator, but may be configured to automatically determine whether or not the number of times
set in advance has been reached for automatic manufacture. Since the substrate 6e coated with
the PZT solution has not been subjected to spin coating a predetermined number of times, it is
subsequently pre-baked in the pre-baking step S2. That is, in the pre-firing step S2, firing is
performed at a temperature lower than or equal to the phase transition temperature of the PZT
raw material forming the diaphragm 6 at a temperature of 300 ° C. to 350 ° C.
The temporary firing step S2 is repeated until it reaches a predetermined number of times in the
repeated determination step S3 to obtain the PZT thin film 6c on which the temporary fired layer
having a desired thickness is formed. After the PZT thin film 6c has a desired thickness, a final
sintering step S4 is then performed. In the final sintering step S4, the PZT thin film 6c spincoated by the coating step S1 in the final round with the temporary firing layer temporarily fired
in the temporary firing step S2 is 500 ° C., which is equal to or higher than the sintering
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temperature of PZT. Sinter at -600 ° C. Then, since the PZT thin film 6c is sintered after
reaching the sintering temperature, it becomes a good PZT thin film 6c on ceramic. As described
above, even if the PZT thin film 6c is formed by spin coating a plurality of times, no strain is
accumulated, cracks are less likely to be generated, and a uniform sintered PZT thin film 6c with
high purity can be obtained. In this embodiment, after the PZT thin film 6c is pre-sintered to
form a pre-sintered layer, the PZT solution is applied again in the application step S1 and then
the final sintering step S4 is performed. You may form so that final sintering process S4 may be
performed in the state in which the thin film 6c formed the temporary baking layer. The gold
electrode layer 6 d is formed on the PZT thin film 6 c formed as described above to form an
upper electrode. The upper electrode is not limited to gold, and may be formed of another
material such as silver, platinum, iridium, palladium or the like. If chemical reaction with PZT or
mutual diffusion of components is a concern, a buffer layer may be provided between the PZT
layer and the electrode to suppress chemical reaction or mutual diffusion, such as In2O3-SnO2
(ITO), ZnO, etc. Alternatively, the electrode may be formed using a base oxide such as Cu oxide or
Cu or Ni. The gold electrode layer 6 d is formed by a general method in the same manner as the
conventional diaphragm 6. The gold electrode layer 6d may be performed after the final sintering
step S4, and is placed on the spin-coated PZT solution before the final sintering step S4, and
integrated with the PZT thin film 6c by the final sintering step S4. It may be formed to In the
diaphragm 6, as described above, the lower electrode is formed of the platinum electrode layer
6b, the upper electrode is formed of the gold electrode layer 6d, and the sintered PZT thin film
6c is formed in the middle portion. After that, polarization treatment is performed to obtain the
diaphragm 6. The platinum electrode layer 6b, which is the upper electrode, is in contact with the
upper diaphragm ring 7 and further in contact with the casing 2, and is connected to the
terminal substrate 11 via the skirt 5 at the lower part of the casing 2 and is a lower electrode The
gold electrode layer 6d is in contact with the lower diaphragm ring 8 and further in contact with
the contact ring 10, and the lower portion of the contact ring 10 is connected to the terminal
substrate 11 to form an electric circuit.
Then, the diaphragm 6 polarizes the PZT thin film 6c formed as described above. In the
piezoelectric microphone 1 configured as described above, the diaphragm 6 vibrates due to the
vibration of the sound transmitted through the sound hole 4 and intruding the air, and the
movement thereof is appropriate because the diaphragm 6 itself is a thin film And the
compliance of the back space 12 of an appropriate size, and the change of the diaphragm 6 itself
due to the vibration of the diaphragm 6 becomes the generated voltage difference, and this
change is fixed on the terminal substrate 11 The element 13 converts it into low impedance and
outputs it electrically. Although PZT is used as the ferroelectric thin film in this embodiment, it is
possible to form a ferroelectric thin film by other materials such as PLZT and ZnO by the above
method. According to the present invention, a ferroelectric thin film is formed on a 2 μm to 4
μm or less substrate of 10 μm or less and polarized to form a diaphragm, so that a
conventional conventional condenser microphone can be obtained. In addition to being able to
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downsize the whole as a whole, it is possible to manufacture a piezoelectric microphone with
higher sensitivity and good f characteristics. And since the ferroelectric thin film forming the
diaphragm can obtain the ferroelectric characteristics if it is below the phase transition
temperature, the deterioration with respect to the temperature of about 260 degrees Celsius
such as the solder reflow is better than the conventional condenser microphone. Not get a
microphone. Similarly, it can be used for high temperature sites that could not be used
conventionally due to high temperatures. Further, since the back electrode in the condenser
microphone is not necessary, the size and thickness can be further reduced. BRIEF DESCRIPTION
OF THE DRAWINGS FIG. 1 is a longitudinal sectional view of a piezoelectric microphone
according to an embodiment of the present invention. FIG. 2 is a transverse sectional view of the
same. FIG. 3 forms a diaphragm according to an embodiment of the present invention.
Explanatory drawing showing the procedure [Fig. 4] Explanatory drawing showing the coating
process of Fig. 3 [Fig. 5] Conventional example Fig. 1 piezoelectric microphone 2 casing 3 top
plate 4 sound hole 5 skirt 6 diaphragm 6a titanium foil 6b platinum electrode layer 6c PZT thin
film 6d gold electrode layer 6e substrate 7 upper diaphragm ring 8 lower diaphragm ring 9
insulating material 10 contact ring 11 terminal substrate 12 back space 13 IC element 14 spin
coating device 15 coating table driving unit 16 coating table 17 PZT Raw Droplet Device 18
Solution Forming process S1 coating process S2 calcination process S3 repeated determining
step S4 final sintering step
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