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JP2010272956

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DESCRIPTION JP2010272956
PROBLEM TO BE SOLVED: In a capacitive electromechanical transducer, the sacrificial layer
etching rate with a large area and a narrow electrode gap (gap) can be made fast and stable, and
the productivity (uniformity, yield) of array elements is improved. A substrate, a cavity formed of
a vibrating film held at a predetermined distance from a substrate by a support portion disposed
on the substrate, a first electrode whose surface is exposed to the cavity, and a cavity And the
first electrode is provided on the surface of the substrate or the lower surface of the vibrating
film, and the second electrode is opposed to the first electrode. Is a capacitive electromechanical
transducer provided on the surface of a vibrating film or on the surface of a substrate, wherein
fine particles of an oxide film of a substance forming the first electrode are disposed on the
surface of the first electrode. The present invention provides a capacitive electromechanical
transducer characterized in that the diameter of the [Selected figure] Figure 1
Method of manufacturing capacitive type electro-mechanical transducer
[0001]
The present invention relates to a method of manufacturing a capacitive electromechanical
transducer used as an ultrasonic transducer or the like.
[0002]
BACKGROUND ART In recent years, capacitive electromechanical transducers manufactured
using a micromachining process have been actively studied.
04-05-2019
1
A typical capacitive electromechanical transducer has a vibrating membrane supported at a
predetermined distance from a lower electrode, and an upper electrode disposed on the surface
of the vibrating membrane. This is used as, for example, a capacitive ultrasonic transducer
(CMUT: Capacitive-Micromachined-Ultrasonic-Transducer), which is one form of a capacitive
electromechanical transducer.
[0003]
The capacitive ultrasonic transducer may use a lightweight diaphragm and transmit an ultrasonic
wave by vibrating the diaphragm by applying an electric field of a predetermined frequency
between the upper electrode and the lower electrode. it can. In addition, when the vibrating
membrane vibrates due to ultrasonic waves, ultrasonic waves can be received and detected as
electrical signals by detecting a change in capacitance between the upper electrode and the lower
electrode. As such a capacitive ultrasonic transducer, one having excellent wide band
characteristics in liquid and air can be easily obtained. For example, if this CMUT is applied in the
medical field, diagnosis with higher accuracy than conventional medical diagnosis can be
performed, and it is attracting attention as a promising technology.
[0004]
Next, the operation principle of the capacitive electromechanical transducer will be described.
When transmitting an elastic wave (typically, an ultrasonic wave), an AC potential (voltage) is
applied to a DC potential (voltage) between the lower electrode which is the first electrode and
the upper electrode which is the second electrode. ) Is superimposed and applied. By applying an
electric field between the first electrode and the second electrode in this manner, the vibrating
film is vibrated by the electrostatic force acting between the first electrode and the second
electrode, and elasticity including ultrasonic waves is generated. A wave is generated. On the
other hand, when receiving an ultrasonic wave, the diaphragm is deformed by the ultrasonic
wave, so that a signal is detected by the change in capacitance between the lower electrode and
the upper electrode due to the deformation. By such principle, mechanical energy and electrical
energy can be converted. The theoretical sensitivity of the capacitive electromechanical
transducer is inversely proportional to the square of the distance between the electrodes (also
referred to as a gap). In order to manufacture a highly sensitive device, it is preferable to set a
gap of 100 nm or less.
04-05-2019
2
[0005]
On the other hand, as a method of forming a gap of a capacitive electromechanical transducer, a
sacrificial layer having a thickness equal to a desired electrode distance is provided, a vibrating
film is formed on the sacrificial layer, and the sacrificial layer is removed. But are generally
adopted. Such techniques are disclosed in Patent Document 1 and Non-Patent Document 1.
[0006]
U.S. Patent No. 6,426,582
[0007]
IEEE Transactionson
Ultrasonics,Ferroelectrics,and Frequency
Control,Vol.52,No.12,Dec.2005,p.2242−2258.
[0008]
As described above, in order to enhance the electromechanical conversion efficiency of the
capacitive electromechanical transducer, it is preferable to narrow the electrode interval (for
example, to 100 nm or less).
[0009]
However, in order to narrow the gap between the electrodes, the thickness of the sacrificial layer
must be reduced correspondingly to the gap.
When the thickness of the sacrificial layer is reduced, removal of the sacrificial layer (for
example, composed of Si, SiO 2, metal, etc.) becomes very difficult.
The reason is that in order to etch the sacrificial layer present between the electrodes, the
etchant needs to cause an etching reaction with the sacrificial layer, but if the gap between the
electrodes becomes smaller than a certain distance, the gap between the electrodes It is because
it becomes difficult for etchant to penetrate.
04-05-2019
3
Furthermore, when a gas such as hydrogen is generated during the etching reaction, it takes time
for the etchant to penetrate further. For example, as described in Non-Patent Document 1, the
etching process may take several days to about one week at low temperature. In such a case,
when immersed in the etchant for a long time, the vibrating film itself of the capacitive
electromechanical transducer may be damaged by the etchant, and the yield may be lowered.
[0010]
On the other hand, it is possible to increase the etching rate to a certain extent by raising the
temperature of the etchant. However, if the etching reaction is promoted in such a manner, the
vibrating film having low mechanical strength may be broken into bubbles formed along with the
high temperature etching reaction, and the yield may be reduced. As described above, since the
sacrificial layer etching with a large area and a narrow electrode distance has low productivity
due to the diffusion limitation of the etchant, realization of high-speed etching is desired. On the
other hand, in order to etch the sacrificial layer, it is necessary to provide an inlet for the etchant.
The greater the number and the greater the number of etchant inlets, ie, the greater the exposed
surface of the sacrificial layer, the faster the etch rate. However, in the micro mechanicalelectrical conversion element, if a large hole or a large number of holes are provided in the
mechanical structure as an inlet of the etching solution, the performance of the original
mechanical structure is adversely affected, and the design performance, life, stability, and
reliability of the element May be impaired. For example, in the capacitive electromechanical
transducer, providing a large hole or a large number of holes in the vibrating film has a great
effect on the vibrating mass, stress of the vibrating portion, vibration frequency, vibration node,
vibration displacement and the like. For this reason, in such a capacitive electromechanical
transducer, it is preferable to minimize the size and number of etching solution inlets.
[0011]
As another method for etching the sacrificial layer, when etching the sacrificial layer between the
upper and lower electrodes, a method of applying a magnetic field in a direction perpendicular to
the current flowing in the etching solution can be considered. However, in this method, the
exposed surface (the inlet of the etching solution) of the sacrificial layer to the etching solution is
on the lateral side, and wide, multidirectional exposure is required. If the etchant has a small inlet
or a small size, the effect is limited.
04-05-2019
4
[0012]
In the case of the narrow electrode spacing, when taking out from the etching solution, the upper
and lower electrodes adhere due to the surface tension of the liquid, which may cause element
variation in the substrate surface by a so-called sticking phenomenon.
[0013]
The present invention has been completed as a result of intensive studies to solve the above
problems, and the main point of the present invention is to manufacture a capacitive
electromechanical transducer that converts mechanical energy and electrical energy by
displacement of a vibrating membrane. A method, comprising: forming a first electrode on a
substrate; forming a sacrificial layer on the first electrode; and forming a vibrating film provided
with a second electrode on the sacrificial layer A step of forming an opening for communicating
the sacrificial layer with the outside, and applying an electric field between the first electrode and
the third electrode provided outside in the electrolytic solution. Forming a cavity by electrolytic
etching, the sacrificial layer is formed of a conductive material having a smaller dissolution
potential than the first electrode, and in the step of forming the cavity, the first electrode and the
third electrode are formed. Apply between electrodes Position is characterized in that said
greater than the dissolution potential of the sacrificial layer, and a smaller potential than the
dissolution potential of the first electrode.
[0014]
According to the manufacturing method of the present invention, since the opening or opening
as described above is formed, the sacrificial layer can be etched at a relatively high speed by
electrolytic etching without depending on diffusion limitation, and the cavity can be formed well.
In addition, even if the size or the number of the openings or the openings is not increased,
uniform or even high-speed and stable etching rates can be realized by the anode potential
(voltage) of the first electrode.
And, even with a large-area capacitive electromechanical transducer (e.g., CMUT) or an array
capacitive electromechanical transducer having a plurality of element portions, productivity (e.g.,
shortening of production time, yield) and performance (e.g., e.g. The uniformity of the device
performance and the high sensitivity of the device can be improved.
04-05-2019
5
[0015]
FIG. 2 is a cross-sectional view showing an example of a capacitive electromechanical transducer.
The characteristic-curve figure which shows the current versus voltage dependence of various
electrodes. The figure which shows the manufacturing process of the capacitive-type
electromechanical transducer by one Embodiment of this invention. FIG. 7 is a diagram showing
a current versus time change curve of the electrolytic etching process in the first embodiment of
the present invention. The figure which shows the EDS analysis result of this invention The
element photograph after the electrolytic-etching process in 1st Example of this invention. The
expanded sectional view which shows the cavity area ¦ region 23 of this invention. The figure
which shows the manufacturing process of 2nd Example and embodiment of the manufacturing
method of this invention. The figure which shows the manufacturing process of 3rd Example and
embodiment of the manufacturing method of this invention. The figure which shows the
manufacturing process of 4th Example and embodiment of the manufacturing method of this
invention. The figure which shows the manufacturing process of 5th Example and embodiment of
the manufacturing method of this invention. The figure which shows the manufacturing process
of 6th Example and embodiment of the manufacturing method of this invention. The figure which
shows the manufacturing process of 7th Example and embodiment of the manufacturing method
of this invention. The figure which shows the manufacturing process of the 8th Example and
embodiment of the manufacturing method of this invention.
[0016]
The capacitive electromechanical transducer of the present invention is a capacitive
electromechanical transducer that converts mechanical energy and electrical energy by
displacement of a vibrating film. FIG. 1 is a cross-sectional view showing the basic configuration
of an element obtained by the method of manufacturing a capacitive electromechanical
transducer of the present invention. The lower electrode 8, which is a first electrode, is disposed
on the substrate 5, and the vibrating membrane support 2 on the lower electrode 8 supports the
vibrating membrane 3 and is fixed to the substrate 4. A cavity (space) 10 is formed surrounded
by the substrate 5, the vibrating membrane 3 and the vibrating membrane support 2. In the
present embodiment, at least a portion of the lower electrode 8 is provided to be exposed to the
cavity 10.
[0017]
04-05-2019
6
In the case where the substrate 5 is an insulating material (for example, a glass substrate), as
shown in FIG. 1, by providing a substrate penetrating wiring 22 penetrating the substrate and an
electrode pad 29 which is an electrical connection portion on the back surface of the substrate.
The upper and lower electrodes can be taken out to the back surface of the substrate. The upper
and lower electrodes can also be taken out of the substrate surface. In FIG. 1, the wiring of the
upper and lower electrodes and the electrode pad thereof are omitted. Furthermore, the upper
electrode 1 is disposed on the upper surface of the vibrating membrane 3. The upper electrode 1
is formed to face the lower electrode 8 with the insulating diaphragm 3 interposed therebetween,
and constitutes the capacitive electromechanical transducer of this embodiment. It is preferable
to apply a DC bias voltage between the upper electrode 1 and the lower electrode 8 during
operation in order to increase the conversion coefficient between mechanical energy and
electrical energy of the capacitive electromechanical transducer. The action of the DC bias
voltage causes the electrostatic attraction to pull the upper electrode 1 to the lower electrode 8
side, and a downward displacement occurs at the central portion of the vibrating film 3.
However, once the DC bias voltage exceeds a certain voltage, the vibrating membrane 3 may
break and contact the lower electrode 8 (collapse), and the electromechanical conversion
coefficient may be reduced. This constant voltage is referred to as the Collapse voltage. The bias
voltage is adjusted so that such collapse does not occur when driving in the collapsed state
(collapse driving). Therefore, when the upper electrode 1 is formed on the lower surface of the
vibrating film 3, it is necessary to provide an insulating film on the surface of the upper electrode
1 facing the lower electrode 8 or on the lower electrode 8. . In short, in order to prevent short
circuiting of the upper and lower electrodes, it is necessary to provide an insulating dielectric
between the upper and lower electrodes.
[0018]
In the present invention, the sacrificial layer is formed of a conductive material having a smaller
dissolution potential than the first electrode (lower electrode 8) which is an electrode provided
on the substrate. The potential applied between the first electrode and the third electrode
(external electrode) in the step of forming the cavity is larger than the dissolution potential of the
sacrificial layer and is more than the dissolution potential of the first electrode. The potential is
small. With such a configuration, the sacrificial layer is selectively etched in a short time when
electrolytic etching is performed. Here, the external electrode is an electrode disposed opposite
to the first electrode via the sacrificial layer outside the cavity. At this time, it is preferable to
provide an opening communicating with the outside for etching the sacrificial layer at an
appropriate position of the wall surface forming the cavity, such as the side wall of the vibrating
film or the cavity.
04-05-2019
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[0019]
As described above, when the relationship between the dissolution potential of the electrode
material and the sacrificial layer and the potential applied during electrolytic etching is not
satisfied, it becomes difficult to selectively etch the sacrificial layer in a short time during
electrolytic etching. For example, when the potential at the time of electrolytic etching (potential
applied between the first electrode and the third electrode) is smaller than the dissolution
potential of the sacrificial layer, the etching of the sacrificial layer does not proceed. When the
dissolution potential of the first electrode is smaller than the dissolution potential of the
sacrificial layer, the etching of the sacrificial layer proceeds and the etching solution reaches the
first electrode, the first electrode itself rather than the sacrificial layer Etching proceeds with
priority.
[0020]
20.0 ohms / square or less are preferable, as for the sheet resistance of the lower electrode 8, 5.0
ohms / square or less are more preferable, and 1.0 ohms / square or less are the most preferable.
[0021]
The material of the lower electrode 8 is preferably Ti.
The dissolution potential of Ti (also referred to as a dissolution voltage) is sufficiently large, for
example, 18 V when the concentration of a saline solution which is an electrolytic solution is 5 M
(mol / l). Therefore, the potential applied between the electrodes at the time of electrolytic
etching can be controlled in a sufficiently wide voltage range. However, since the oxidation
potential of Ti is about 4 V, it is preferable to perform the etching reaction at a potential of 4 V or
less when the oxide film is formed on the surface of the Ti electrode to adversely affect the
etching reaction of the sacrificial layer.
[0022]
The material of the lower electrode 8 is not limited to metal Ti, and other low resistance
materials can be used as long as the relationship between the solution potential of the electrode
04-05-2019
8
material and the sacrificial layer and the applied potential at the time of electrolytic etching is
satisfied. For example, a doped single crystal Si substrate, or a single crystal Si substrate having a
doped well region as a lower electrode, doped amorphous Si, doped polycrystalline Si, or a metal
or oxide having a higher dissolution potential than the sacrificial layer 11 described later A
semiconductor can be used.
[0023]
The dissolution potential of Si is −5 V or less. Therefore, when etching a metal sacrificial layer
(for example, Al, Cu, Cr, etc.), it is possible to use the single crystal Si substrate itself as the lower
electrode 8. In the case of the lower electrode 8 of Si, Si is not etched under the voltage condition
of the dissolution potential region of the sacrificial layer.
[0024]
When the substrate 5 is used as the lower electrode 8 (the substrate also serves as the lower
electrode), in order to increase the detection current of the element, it is preferable to reduce the
series resistance in the element circuit. Note that it is preferable to form the lower electrode 8 of
low resistance by doping the Si substrate to be the lower electrode 8 with an impurity according
to speeding up of electrolytic etching and ease of charge transfer. The surface impurity
concentration is preferably 10 <14> cm <-3> or more, more preferably 10 <16> cm <-3> or more,
and most preferably 10 <18> cm <-3> or more. At this time, the upper limit of the impurity
concentration is not particularly limited as long as other properties are not adversely affected.
[0025]
Furthermore, in order to provide the above-mentioned Hole, P type is suitable for a Si substrate.
Therefore, the impurity doping source is preferably B, Ga or the like. The surface roughness of
the lower electrode 8 may change the crystallinity of the electrode depending on the thermal
history of the process, and the surface roughness may increase. The surface roughness is in
proportion to the thickness of the lower electrode 8. If the surface roughness of the lower
electrode 8 is too large, there is a risk of discharge when applying a bias voltage to the device, so
it is necessary to set the surface roughness in an appropriate range. According to the knowledge
of the present inventor, the thickness of the lower electrode 8 is preferably in the range of 10 nm
to 500 nm. In order to further enhance the conductivity and process stability, the thickness is
04-05-2019
9
preferably in the range of 50 nm to 200 nm.
[0026]
When the vibrating film 3 is formed on the sacrificial layer, it is important that the sacrificial
layer is not oxidized in the process of forming the vibrating film 3. The reason is that, if it were
oxidized in the process of forming the vibrating film 3, the surface roughness of the sacrificial
layer would be sharply increased. The surface roughness is reflected in the vibrating film 3 and
the upper electrode 1 in contact with the sacrificial layer, and the surface roughness of the
vibrating film 3 and the upper electrode 1 is increased. As a result, defects, cracks and the like
may occur in the vibrating film 3, and the leak current of the upper electrode 1 may further
increase. For this reason, this sacrificial layer preferably has an ambient temperature of 200 ° C.
to 400 ° C. in the process of forming the vibrating film 3. When the vibrating film is formed of
silicon nitride (SiN) by a plasma CVD method, it is necessary to set conditions not to be oxidized
during the plasma CVD process.
[0027]
Further, the dissolution potential of the sacrificial layer needs to be smaller than that of Ti. For
example, selecting Cr as the sacrificial layer is preferable because its dissolution potential is
about 0.75V. The applied voltage for dissolving the sacrificial layer Cr in electrolytic etching is in
the range of 0.75 V to 4 V, which is a potential higher than the dissolution potential of the
sacrificial layer and smaller than the dissolution potential of the first electrode. It is preferable to
set to. The range of 2 V to 3 V is more preferable because the etching rate is increased.
[0028]
In theory, the conversion coefficient of this electromechanical transducer is inversely
proportional to the square of the distance between the upper and lower electrodes. Therefore,
the thinner the thickness of the sacrificial layer, the higher the conversion performance of this
electromechanical transducer. On the other hand, since the distance between the vibrating
membrane and the cavity bottom (there is a case where the substrate bottom also serves as the
lower electrode) becomes smaller as the sacrificial layer becomes thinner, sticking of the
vibrating membrane may occur. Therefore, the thickness of the sacrificial layer is preferably 20
to 500 nm, more preferably 50 to 300 nm, and most preferably in the range of 100 nm to 200
04-05-2019
10
nm.
[0029]
In the present invention, in order to suppress the sticking when the upper electrode and the
lower electrode due to the collapse described above come into contact (including when the
contact is made via the insulating layer), the following configuration may be used as necessary.
Can. That is, on the surface of the first electrode, fine particles made of an oxide film of a
substance forming the first electrode are disposed as a buffer structure. The diameter of the fine
particles is preferably in the range of 2 nm to 200 nm because the diameter of the fine particles
is in contact during normal vibration of the vibrating film and adversely affects the vibration
when the diameter of the fine particles is too small.
[0030]
The capacitive electromechanical transducer of the present invention can be manufactured by
the following manufacturing method. In FIG. 1, a lower electrode 8 which is a first electrode is
formed on a substrate 5, a sacrificial layer is formed on the first electrode, and a vibrating film
including an upper electrode 1 which is a second electrode on the sacrificial layer. 3 and
providing an opening for communicating the artificial layer with the outside of the cavity as an
inlet for the etchant on a part (typically a vibrating film) of the peripheral side wall surrounding
the sacrificial layer (the area to be the cavity). . Then, an electric field is applied between the first
electrode and the external electrode (third electrode) provided outside in the electrolytic solution
through the opening, and the sacrificial layer is removed by etching by electrolytic etching. Form
a cavity. At this time, it is more preferable that the region of the sacrificial layer is entirely
included in the region of the first electrode to be energized. With such a relationship, the electric
field can be applied uniformly or efficiently to the region to be electrolytically etched, so that
efficient etching can be performed. In this manner, the sacrificial layer is etched away to form the
cavity 10, and then the cavity is sealed by sealing the opening as the etchant inlet. A capacitive
electromechanical transducer including a plurality of such element portions can be manufactured
by the following manufacturing method. That is, at least a part of the sacrificial layers of the first
element portion and the second element portion disposed adjacent to or close to each other is
connected. Then, an opening is provided to connect the sacrificial layer of the portion to be the
cavity of the first element portion or the sacrificial layer to be the cavity of the second element
portion to the outside, and an electrode provided to the outside through this opening An electric
field is applied between the (third electrode) and the second electrode which is the lower
electrode. In this way, the first electrode, the sacrificial layers in the cavity portions of the first
04-05-2019
11
and second element portions, and the sacrificial layer connecting them are continuously etched
away. Thus, the sacrificial layer connecting the first element portion and the second element
portion is removed by etching in the electrolytic etching process, and the etching solution
connects the cavity of the first element portion and the cavity of the second element portion.
Function as a flow path for As a result, the cavity of the first element portion and the cavity of the
second element portion can be collectively formed by providing an opening communicating with
the sacrificial layer to be one of the cavities. In this case, it is more preferable that the region of
the sacrificial layer is entirely included in the region of the first electrode to be energized.
[0031]
According to the manufacturing method of the present embodiment, the sacrificial layer can be
etched at a relatively high speed without depending on diffusion limitation, and the cavity can be
formed sufficiently thin. In addition, generation of air bubbles can be suppressed, and damage to
the vibrating membrane can be prevented. And, even if the size or the number of the openings or
the openings are not increased, uniform or even high speed and stable etching speed can be
realized by the anode voltage of the first electrode. Therefore, even with a large-area capacitive
electromechanical transducer or an array capacitive electromechanical transducer, it is possible
to realize shortening of manufacturing time, uniformity of device performance, high sensitivity of
devices, improvement of yield, and the like. In the present invention, the electrolytic etching
solution is not limited to saline solution (NaCl aqueous solution), and it is also possible to use a
solution containing other electrolytic solution such as NaBr, NaClO4, NaOH, NaNO3 and the like.
A normal etching solution is a strong acid or a strong alkali, and there is a possibility that
damage to the lower electrode may occur due to a finite etching selectivity, so the lower
electrode is often covered with a protective film. Due to the provision of this protective film, the
distance between the upper and lower electrodes may be increased, and the sensitivity of the
capacitive electromechanical transducer may be lowered. However, according to the present
invention, since the pH neutral etchant can be employed, there is no damage to the lower
electrode. Furthermore, since there is no protective film, the distance between the upper and
lower electrodes can be shortened, and the sensitivity of the capacitive electromechanical
transducer can be increased.
[0032]
From the viewpoint of cost, NaCl aqueous solution is cheaper than other etching solutions, and
the equipment is simple, contamination, danger and so on are also low, and the use of NaCl is
advantageous. The concentration of the NaCl solution is preferably in the range of 0.01 M (mol /
04-05-2019
12
l) or more and the saturation concentration or less at room temperature, and is 0.2 M or more
and 2.5 M or less in order to provide chlorine ions essential for the electrolytic etching reaction.
Is more preferred. In the present invention, the final inter-electrode distance (the distance
between the lower electrode 8 and the upper electrode described later) of this element is
determined by the thickness of the sacrificial layer 11. The thinner the sacrificial layer 11, the
higher the electromechanical conversion coefficient of the device. However, if the distance
between the electrodes is too small, the risk of dielectric breakdown increases. The thickness of
the sacrificial layer 11 is preferably in the range of 5 nm to 4000 nm, more preferably in the
range of 10 nm to 1000 nm, and most preferably in the range of 20 nm to 500 nm, depending
on the conditions of the electrolytic etching and the drying step.
[0033]
In the present invention, when the plasma CVD method is performed, the pressure in the step is
preferably in the following range. For example, in the case of sealing with a silicon nitride
(referred to as SiNx) film formed by plasma CVD, the vibrating film 3 is deformed downward by
the atmospheric pressure to be in a concave state. Therefore, the range of pressure in the sealed
cavity 10 is preferably 1 Pa to 70,000 Pa, more preferably 10 Pa to 15,000 Pa, and most
preferably 20 Pa to 3000 Pa.
[0034]
In the present invention, the stress of silicon nitride by the plasma CVD can be adjusted by the
discharge electrode arrangement of the film forming apparatus of plasma CVD, discharge
frequency, gas composition, and temperature. If the stress is compressible, a Buckling
phenomenon (also referred to as a buckling phenomenon) easily occurs, and the vibrating
membrane 3 may be convex. A horizontal state is shown in FIG. Therefore, the internal residual
stress of the vibrating film 3 including the sealing film is preferably -200 to +200 MPa, more
preferably -100 to +100 MPa, and most preferably -50 to +50 MPa.
[0035]
Hereinafter, the present invention will be described in more detail by way of examples, but the
present invention is not limited by these descriptions.
[0036]
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First Example The dissolution potential of a metal will be described by electrolytic etching.
A metal film (film thickness 200 nm) of Al, Cu, Cr, and Ti is deposited on a Pyrex (registered
trademark) glass substrate by an electron beam method. Thereafter, the above-described
substrate with a metal film is locally immersed in an electrolyte solution having a concentration
of 2 M saline. Next, the metal film not immersed in the electrolyte is connected to the working
electrode of a potentiostat. In each embodiment of the present invention, HZ-5000
(manufactured by Hokuto Denko Corporation) is used as a potentiostat. Then, an Ag / AgCl
reference electrode (Reference electrode) and a platinum (Pt) counter electrode (Counter
electrode) are also immersed in the above-mentioned electrolytic solution and installed.
Hereinafter, the same potentiostat, reference electrode, and counter electrode are used for
electrolytic etching. The electrolytic etching experiments described below are performed at room
temperature.
[0037]
Next, the potential of the working electrode is swept by a voltage sweeper, the potential of the
working electrode is made -1 volts smaller than the natural potential, then raised to a
predetermined potential, and finally returned to the natural potential. The results are shown in
the electrolysis IV curves of FIG. 2 (a) to FIG. 2 (d) (horizontal axis: voltage applied to the working
electrode, vertical axis: current flowing from the working electrode).
[0038]
As shown in FIG. 2A, substantially no current flows in the range of about -0.7 V or less of Al.
Then, the current rapidly increases from -0.7 V or more. That is, the electrolytic reaction is
activated in the range of -0.7 V or more to etch Al.
[0039]
In the present invention, the solution potential means a potential immediately before the current
value rapidly increases (a potential which starts to increase rapidly) when sweeping the kinetic
04-05-2019
14
potential from a negative value to a positive value. In the present invention, the rapid increase of
the current value is a potential at which the current is increased by two or more digits than the
generated current value at the natural potential. In the above description, the dissolution
potential of Al is -0.7V.
[0040]
Similarly, as shown in FIG. 2 (b), the dissolution potential of Cu is -0.25 V, and when a voltage of 0.25 V or more is applied, the elution of blue Cu ions from the Cu surface is visually confirmed it
can. Similarly, as shown in FIG. 2C, the dissolution potential of Cr is about +0.75 V, and when a
voltage of +0.75 V or more is applied, the elution of yellow-green Cr ions from the Cr surface is
visually confirmed it can. As shown in FIG. 2 (c), although the initial stage of the electrolysis IV
curve at this time shows a hysteresis characteristic, it is confirmed that when the sweep is
repeated 30 times, substantially the same curve is obtained with good reproducibility. From this
result, it is confirmed that dissolution starts from about +0.75 V (the solution potential is +0.75
V). In addition, when the maximum voltage of the said sweep is + 1V, the electrolysis IV curve of
Ti is substantially zero, and there is no electrolysis reaction. Then, when the maximum voltage is
set to +10 V and the sweep is performed, a hysteresis phenomenon is observed as shown in FIG.
At this time, an electrolytic reaction occurs from about +4 V to form a brown substance on the Ti
surface. When the resistance of the Ti surface is measured by a tester, the brown substance is
presumed to be an oxide of Ti because the resistance is very high. When the maximum voltage is
further increased, the color of the oxide changes to purple, but the hysteresis appears in the
same manner. From this result, it can be seen that, in the case of Ti, even if the voltage changes,
the surface does not dissolve and a passive oxide is formed.
[0041]
In addition, although drawing is abbreviate ¦ omitted, melt ¦ dissolution of Au generate ¦ occur ¦
produces in the range of about + 1.1V or more on the conditions of said room temperature salt
solution (electrolyte solution). Similarly, the dissolution of single crystal Si occurs in the range of
about -5 V or less.
[0042]
Furthermore, a potential (voltage) higher than the dissolution potential for each of the metals (Al,
04-05-2019
15
Cu, Cr) on the Pyrex (registered trademark) glass substrate is applied, and the metals (Al, Cu, Cr)
dissolve due to the oxidation reaction. It is confirmed (etched). However, the portion to be
immersed in the electrolytic solution (saline solution) is not completely etched, and island-like
unetched regions are scattered. The reason is that in-plane variation of the etching reaction
forms island-like metal regions, interrupting the supply path of holes (also referred to as holes /
Hole) necessary for electrolytic etching (oxidation reaction) and stopping the etching. It is
considered to be from. . Therefore, a Ti film (film thickness 50 nm) is provided between the Al,
Cu, Cr and Pyrex (registered trademark) glass substrate, and the electrolytic etching voltage is set
in the range of 0.75 V to 3 V. As a result, the part to be immersed in the electrolytic solution
(saline solution) is sufficiently etched and there is no island-like unetched area.
[0043]
In the case of completely electrolytically etching one layer of conductor, it is necessary to provide
not only a potential higher than the dissolution potential of the conductor but also a path for
providing holes necessary for the oxidation reaction. For example, it is necessary to place an
additional conductor in contact with the conductor to be etched. Furthermore, in order to
selectively electrolytically etch the two layers of conductors, it is important to electrolytically
etch at a potential greater than the dissolution potential of the conductor to be etched and less
than the dissolution potential of the conductor providing the holes. It is.
[0044]
Preferably, in order to secure the hole supply path, the entire area of the conductor to be etched
is included in the area of the conductor providing the H holes.
[0045]
At that time, it may be observed that bubbles of about 0.1 to 1 mm in diameter are generated on
the surface of the counter electrode Pt, but the bubbles are considered to be hydrogen due to a
reduction reaction.
In particular, the conductor connected to the working electrode is very important in the method
of manufacturing a capacitive electromechanical transducer according to the present invention
because generation of bubbles is suppressed in the electrolytic etching process. The following
steps will be further described for the above-mentioned electrolytic etching conditions. 3 (a) to 3
04-05-2019
16
(k) are cross-sectional views for explaining steps of a first embodiment of a method of
manufacturing a capacitive electromechanical transducer according to the present invention. In
the following description, the patterning step is performed in the order of coating, drying,
exposing, developing, etc. of the photoresist on the substrate, etching, removal of the photoresist,
cleaning of the substrate, and drying. It means the whole process. Moreover, although the board ¦
substrate 4 of a present Example uses Si, the board ¦ substrate of another material can also be
used. For example, substrates such as SiO 2 and sapphire can also be used.
[0046]
In the manufacturing method of this embodiment, first, as shown in FIG. 3A, a Si substrate 4 (for
example, P type, orientation (100), resistivity 1 to 20 Ωcm, diameter 4 inches) is prepared and
cleaned. . Next, as shown in FIG. 3B, a Ti layer to be the lower electrode 8 is formed on the
surface of the Si substrate 4 by sputtering. Since this Ti layer has a role of supplying Hole to the
electrolytic etching reaction described later, the rate of the electrolytic etching largely depends
on the resistance of the Ti layer. In addition, this Ti layer also has a role of causing current to
flow in a certain frequency range as the lower electrode of the present invention. Further, in
order to pattern the Ti layer with an etching solution containing hydrofluoric acid, the film
thickness of the Ti layer used as the lower electrode 8 is preferably 10 nm to 1000 nm, and most
preferably 50 nm to 500 nm.
[0047]
In addition, when electrolytically etching the sacrificial layer in a later step, it is preferable to
reduce the voltage drop due to the lower electrode 8 in order to obtain a uniform, stable, and
high etching rate. Therefore, although the specific area ¦ region used as the lower electrode 8 is
not shown in FIG.3 (b), it is also possible to etch this board ¦ substrate 4 by DRIE (Deep Reactive
Ion Etching), and to electrically isolate an element. .
[0048]
Next, as shown in FIG. 3C, a metal Cr film (film thickness 200 nm) serving as the sacrificial layer
11 is deposited by electron beam evaporation, and an etchant containing (NH 4) 2 Ce (NO 3) 6 is
used. Pattern it.
[0049]
04-05-2019
17
It is preferable to reduce the voltage drop in the sacrificial layer 11 so as to obtain a uniform and
stable etching rate when electrolytically etching the sacrificial layer 11 later.
In consideration of the dimensions of elements that can be manufactured by the current
microfabrication technology, the resistivity of the sacrificial layer 11 is preferably 10 <-1> Ωcm
or less, more preferably 10 <-3> Ωcm or less, and 10 <-5 > Ω cm or less is most preferable.
Therefore, it is preferable to use a metal material as the material of the sacrificial layer 11.
[0050]
Next, as shown in FIG. 3D, a SiN x film (500 nm in film thickness) is formed by plasma CVD using
the vibrating film 3. Due to the step of the sacrificial layer 11, the diaphragm supporting portion
2 is simultaneously formed.
[0051]
Since the vibrating membrane of the capacitive electromechanical transducer is a part of the
dielectric in the capacitive structure, it is preferable that the dielectric constant of the vibrating
membrane 3 be high. For example, at least one of dielectric materials such as silicon nitride (Six
NY) film, silicon oxide (Six OY) film, silicon oxynitride (denoted as SiN x OY) film, Y 2 O 3, HfO,
HfAlO, BST [(Ba, Sr) TiO 3] Is preferably selected and used.
[0052]
Next, as shown in FIG. 3E, the Si3N4 film of the vibrating film 3 is patterned by the RIE method
using CF4 gas plasma, and an opening communicating with the sacrificial film 11 which is the
inlet 13 of the etching solution in the vibrating film 3 Form The etchant inlet 13 is provided at
the end of the cavity in this embodiment, but other arrangements are possible. For example, the
etchant inlet 13 may be provided at a fixed distance from the cavity, and another flow path may
be provided between the inlet 13 and the cavity. In addition, when providing the Stop layer of an
etching at the time of an etching, it is simple on a process of using hard-to-etch metal Cr.
04-05-2019
18
[0053]
Next, as shown in FIG. 3 (f), the back surface of the conductive substrate 4 and the working
electrode 16 of the potentiostat 15 are electrically connected. The sacrificial layer 11 is
electrically connected to the working electrode 16 via the lower electrode 8 and the conductive
substrate 4. Then, the electrical connection portion 35 is formed. The electrical connection
portion 35 is not limited to the back surface of the conductive substrate 4, and can be taken out
from the front surface of the substrate 4. In order to reduce the contact resistance of the
electrical connection 35, it is preferable to provide a metal film such as Ti (film thickness 20 nm
to 1000 nm) on the back surface of the substrate 4.
[0054]
Note that, in order to prevent the electric connection portion from being etched when the
electrolytic etching is performed, a protective insulating film is preferably provided on the outer
surface of the electrolytic etching portion. For example, silicon resin, photoresist or the like can
be employed. Alternatively, the back surface of the substrate 4 and the electrical connection
portion 35 can also be protected by a single-sided protection jig for wet etching.
[0055]
In addition, materials, such as Pt, Ni, C, can be used suitably for the material of the counter
electrode 18, for example.
[0056]
In the present invention, since the sacrificial layer 11 (Cu) is a low resistance material, the
amount of potential drop in the sacrificial layer 11 is relatively small.
Therefore, the potential of the sacrificial layer 11 and the potential of the lower electrode 8 have
substantially the same value. Thus, an electric circuit in which the sacrificial layer 11 and the
lower electrode 8 are the anode and the counter electrode 18 is the cathode is configured.
04-05-2019
19
[0057]
In the present embodiment, the reference electrode 17 is disposed at a distance of about 1 mm
from the surface of the substrate 4, and the counter electrode 18 is also disposed opposite to the
surface of the substrate 4 at a distance of about 10 mm.
[0058]
After providing the circuit configuration as described above, as shown in FIG. 3F, the substrate 4
protected on the back surface, the reference electrode 17 and the counter electrode 18 are
electrolytes called saline solution with a concentration of 2 M (mol / l). Immerse in
A voltage is applied between the counter electrode 18 (cathode) and the lower electrode 8 in the
electrolytic etching solution through the electric wiring 34 by the potentiostat 15. Thus, the
electrolytic etching reaction starts from the inlet 13 of the etching solution. In the case where the
sacrificial layer 11 is wet etched without using the electrolytic reaction, the etching is stopped by
diffusion limitation in a short time. This decrease is significant as the thickness of the sacrificial
layer decreases, the cross-sectional area of the sacrificial layer decreases. However, according to
the electrolytic etching method according to the present embodiment, the sacrificial layer 11 (Cr)
can be selectively removed in a relatively short time.
[0059]
The voltage applied to the electrode at the time of electrolytic etching is selected to be a potential
which is larger than the dissolution potential of the sacrificial layer 11 and smaller than the
dissolution potential of the lower electrode 8. That is, the electrolytic potential is applied in the
range of the dissolution potential of 0.75 V or more of the sacrificial layer 11 (Cr) and the
oxidation voltage of 4 V or less of the lower electrode 8 (Ti). For example, in the case of a 12 ×
12 piece 70 mm diameter sacrificial layer Cr pattern (Cr film thickness 200 nm) in a 20 mm
square chip at 2 V electrolytic etching, a curve of current versus time measured by a potentiostat
is shown in FIG. . The reason why the current increases at the first moment is because an
interface (Helmholtz layer) is formed at which ions present in the electrolyte are adsorbed on the
electrode surface. Thereafter, a substantially constant current flows, and the electrolytic etching
reaction proceeds stably. After that, the current is rapidly reduced by about two digits in about
160 seconds, and observation with an optical microscope shows that the etching of the sacrificial
layer is completed. At the completion of the etch, the sacrificial layer is sufficiently etched, so
that no charge is drained from the anode of the potentiostat. By using this relationship, the end
04-05-2019
20
point of etching can be electrically detected. As a result, it is very advantageous for the device
manufacturing process and the yield.
[0060]
As etching of the sacrificial layer proceeds, a cavity is formed. According to the observation of
the above-mentioned optical microscope, no bubbles are observed in the cavity in the etching
process, but the generation of bubbles is observed on the surface of platinum serving as the
counter electrode 18. In the soft vibrating film of the capacitive element as in the present
invention, since there are no bubbles in the cavity, it is possible to avoid the breakage of the
vibrating film by the bubbles.
[0061]
After completion of electrolytic etching, remove the electrolytic etching apparatus, jig, electrical
connection part 35, etc., and pure water (surface tension 72 dynes / cm), IPA solution (surface
tension 20.8 dynes / cm), HFE 7100 solution (3M company, Soak in order of surface tension (13
dynes / cm), wash, and finally air dry. Then, as shown in FIG. 3G, the vibrating membrane 3 is
formed with a cavity 10 by maintaining a predetermined distance from the lower electrode 8 by
the support portion 2. In the case of the narrow electrode spacing, when taking out from the
etching solution, the upper and lower electrodes adhere due to the surface tension of the liquid,
and the sticking phenomenon may occur to cause the element variation in the substrate surface.
For this reason, as described above, in the cleaning step, it is necessary to treat the cleaning
solutions (pure water, IPA solution, HFE 7100) in the order from the cleaning solution having the
large surface tension to the cleaning solution having the small surface tension.
[0062]
When observed with an electron microscope in the cavity area 23 at a position equidistant from
the etching solution inlet 13 adjacent to FIG. 3G, fine particles in the range of about 10 nm to
200 nm are scattered on the lower surface of the vibrating film 3 Is confirmed.
[0063]
Analysis by energy dispersive spectroscopy (EDS) shows that the fine particles contain chromium
04-05-2019
21
and oxygen as shown in FIG. 5 (a).
Furthermore, when analyzed by X-ray photoelectron spectroscopy, it is found that the fine
particles containing chromium and oxygen are chromium oxide (described as CrOx) whose main
component is Cr2O3. Then, when observed with an optical microscope, as shown in FIG. 6,
interference fringes are confirmed on the vibrating membrane 3, and it is understood from the
appearance of the interference fringes that sticking is not caused in the central portion of the
vibrating membrane 3. Therefore, it is understood that this particle group has an effect of
preventing sticking.
[0064]
The fine particles are formed in the vicinity of the cavity area 23 at an equal distance from the
adjacent etchant inlet 13. This cause will be described with reference to FIG. 7 which is an
enlarged view of the vicinity of the cavity area 23 in FIG. 3 (g). As shown in FIG. 7, the contact
interface 41 between the etchant and the sacrificial layer extends isotropically from the inlet of
the etchant. At this time, since the lower electrode 8 supplies holes, electrolytic etching is started
from the contact interface.
[0065]
As etching progresses and the contact interface 42 between the etchant and the sacrificial layer
from the inlet of the adjacent etchant contacts each other as shown in FIG. 7, the Hole supply
path to the sacrificial layer is blocked. And the subsequent etchings do not progress (or progress
is slow). At the end of the etching, fine particles (group) 43 made of the oxide of the sacrificial
layer 11 are formed on the lower surface of the SiNx film as the vibrating film 3 due to
nonuniformity and variation in the substrate surface.
[0066]
On the other hand, when the lower surface of the vibrating film 3 is analyzed by EDS, as shown in
FIG. 5B, the peak of chromium is on the lower surface of the SiNx other than the fine particles
(group) 43 made of oxide of the sacrificial layer 11. Not clearly observed.
[0067]
04-05-2019
22
After the electrolytic etching, it is observed that fine particles in the range of about 5 nm to 50
nm are scattered on the surface of the Ti lower electrode 8 by an electron microscope.
When the fine particles are analyzed by EDS, as shown in FIG. 5C, the peaks of Ti and oxygen O
are significantly confirmed. Accordingly, it can be seen that fine particles containing Ti and
oxygen O are formed on the surface of the lower electrode 8. Furthermore, analysis by X-ray
photoelectron spectroscopy shows that the fine particles containing Ti and oxygen O are titanium
oxide (TiOx) whose main component is TiO2.
[0068]
Similarly, analysis by X-ray photoelectron spectroscopy and transmission electron microscopy
shows that a titanium oxide layer with a thickness of about 10 nm is formed on the surface of the
Ti lower electrode 8 after electrolytic etching.
[0069]
Next, as shown in FIG. 3H, a SiNx film is formed as the sealing film 14 by plasma CVD.
The SiNx film seals the inlet 13 of the etching solution to form a sealing portion 20. The film
used in the sealing step may be at least one of a nitride film, an oxide film, a nitride oxide film, a
polymer resin film, a metal, an alloy and the like by CVD and PVD. A part of the film by this
process is covered on the upper surface of the vibrating film 3 and can be considered as a part of
the vibrating film.
[0070]
In order to seal the cavity 10 by the sealing portion 20, the thickness of the sealing SiNx film is
preferably 1/2 or more of the thickness of the sacrificial layer 11, more preferably the thickness
or more of the sacrificial layer 11, 1.2 times or more of the thickness of the layer 11 is most
preferable. This sealing step can form a sealed cavity 10.
04-05-2019
23
[0071]
The pressure in the plasma CVD process is preferably in the range of 0.1 Torr to several tens of
Torr.
[0072]
Next, as shown in FIG. 3I, the sealing film 14 is patterned, and Al is formed and patterned by PVD
(Physical Vapor Deposition) as the upper electrode 1.
Then, the upper electrode 1 and its wiring extraction pad 9 are formed on the vibrating film 3.
Since the sealing film 14 is sealed only in the vicinity of the inlet of the etching solution, the
influence of mechanical rigidity on the vibrating film 3 can be minimized. At this time, if the
sealing film 14 is not patterned, the distance between the upper and lower electrodes may
increase depending on the thickness of the sealing film, which may degrade the device
performance. In the present embodiment, the upper electrode 1 is formed of a kind of material
selected from metal, low resistance amorphous Si, and low resistance oxide semiconductor. In
order to prevent oxidation of the surface of the upper electrode or diffusion of metal due to heat,
it is also possible to provide two or more conductive layers. That is, the material of the upper
electrode is composed of two or more conductive layers. For example, Al / Cr, Mo / Ni, Cr / Al, Cr
/ Cu or the like can be used. The pad 9 and the upper electrode 1 shown in FIG. 3 (i) are
electrically connected.
[0073]
Next, as shown in FIG. 3J, the protective film 12 is formed on the upper electrode 1. The Si3N4
film of the protective film 12 is formed by plasma CVD. Instead of the Si3N4 film of the
protective film 12, a SiO2 film, SiOxNy, a polymer resin film (for example, polydimethylsiloxane
film, parylene film) or the like can be used.
[0074]
Finally, as shown in FIG. 3 (k), the protective film 12 and the Si3N4 film of the vibrating film 3
are patterned by a dry etching method called RIE using plasma of CF4 gas to form the upper
electrode pad 9 and the lower electrode pad 31. Form When the protective film 12 is a polymer
04-05-2019
24
resin film (for example, a polydimethylsiloxane film, a parylene film), it is possible to form the
pad 9 and the pad 31 by etching using oxygen plasma. Then, the manufacturing process of the
capacitive electromechanical transducer of this embodiment is completed.
[0075]
In this embodiment, the insulating film 3, the protective film 12, and the sealing film used as the
sealing portion 20 in FIG. 3 are all insulating silicon nitride (SiN x) films. Thus, by selecting the
same insulating material for the vibrating film 3, the sealing film to be the sealing portion 20,
and the protective film 12, the vibrating film 3 to be integrated and the sealing film to be the
sealing portion 20 are integrated. , And the protective film 12 function as a vibrating film as a
whole.
[0076]
In the present invention, the above-mentioned sealing part is not essential, and is provided as
necessary. However, when an acoustic wave is emitted into the air, an element not sealed may
have a decrease in the amplitude increase coefficient (also referred to as a Q value) of resonance
due to the damping effect of the air. On the other hand, when the capacitive electromechanical
transducer of the present invention is used in the liquid phase, the large damping effect of the
liquid and the low compressibility make the performance of the element unsealed when
transmitting and receiving in the liquid. Since it may decrease, it is very preferable to set it as the
structure which provides a sealing part.
[0077]
Second Embodiment FIGS. 8A to 8G are cross-sectional views for explaining steps of a second
embodiment of a capacitive electromechanical transducer according to the present invention. The
manufacturing process of the device of this embodiment is substantially the same as that of the
first embodiment except that the conductive substrate 4 also serves as the lower electrode. In the
present embodiment, as shown in FIG. 8G, the sealing portion 20, the lower electrode pad 31,
and the upper electrode pad 9 are simultaneously formed when the upper electrode 1 is formed.
It can be made more easily than in the case. Further, since there is no sealing film, the vibrating
film 3 can be thinner, the distance between the upper and lower electrodes can be shortened, and
the sensitivity can be improved.
04-05-2019
25
[0078]
In the present embodiment, the sealing portion 20 is a conductor, and the potential thereof is the
same as that of the substrate 4 which is the lower electrode. Therefore, the pattern of the sealing
portion 20 is separated from the pattern of the upper electrode 1 so as not to short circuit the
upper and lower electrodes.
[0079]
In the case of the present embodiment in which the main body of the substrate 4 is the lower
electrode, in order to increase the detection current of the element, it is preferable to reduce the
series resistance in the element circuit. For this reason, 20.0 ohms / square or less are preferable,
as for the sheet resistance of the board ¦ substrate 4 which functions as a lower electrode, 5.0
ohms / square or less are more preferable, and 1.0 ohms / square or less are the most preferable.
Note that it is preferable to form the lower electrode of low resistance by doping the impurity to
the Si substrate which is the lower electrode due to the speeding up of the electrolytic etching
and the ease of charge transfer. The surface impurity concentration is preferably 10 <14> cm <3> or more, more preferably 10 <16> cm <-3> or more, and most preferably 10 <18> cm <-3> or
more. Furthermore, a P-type Si substrate is preferred in order to provide the aforementioned
holes. Therefore, as the impurity dopant, boron, gallium, etc., which are a group III element, are
preferable.
[0080]
It is possible to electrically isolate the device by etching the substrate 4 by DRIE. As described
above, in the present embodiment, the lower electrode can be configured such that the substrate
doubles as an electrode by using a substrate whose surface has a low resistance at least.
[0081]
By providing the conductive film 27 on the back surface of the substrate 4, the contact resistance
between the electrical connection portion 35 and the conductive substrate 4 can be largely
04-05-2019
26
reduced when the electrolytic etching is performed. Furthermore, in order to eliminate an
unnecessary voltage drop, it is preferable to make an ohmic contact between the conductive film
27 and the conductive substrate 4 (for example, Si). Specifically, it is possible to form a Ti layer
(for example, with a film thickness of 100 to 500 nm) on the surface of the low resistance Si
substrate and anneal it.
[0082]
Although FIG. 8 (g) shows an example in which the lower electrode pad 31 is installed beside the
cavity 10 of the element, it is also possible to install the lower electrode pad 31 on the back
surface of the substrate 4. The other points are the same as in the first embodiment.
[0083]
Although the case where there is no protective film is shown in this embodiment, when the
protective film is provided, it can be installed in the same manner as the first embodiment. In
particular, when the wiring is taken out from the electrode pad, the polymer resin film (for
example, polydimethylsiloxane film, parylene film) and the like which can be formed at the low
temperature is more preferable.
[0084]
Third Embodiment FIG. 9A to FIG. 9K are cross-sectional views for explaining steps of a third
embodiment of a capacitive electromechanical transducer according to the present invention.
[0085]
The manufacturing process of the device of this embodiment is substantially the same as that of
the first and second embodiments, except that a conductive substrate is not used and a substrate
(for example, glass) having an insulating property is used.
As shown in FIG. 9A, a substrate 5 (glass substrate, 4 inches in diameter) is prepared and
cleaned. Next, as shown in FIG. 9B, a through wiring conductive portion 22 penetrating the
substrate 5 is provided.
04-05-2019
27
[0086]
As a substrate having such a substrate through wiring, for example, when a substrate through
hole is provided by using photosensitive glass (product name PEG3 manufactured by HOYA), the
substrate through hole is filled with metal Cu or Ni by plating. You can also After such a metal
through wiring is formed, the substrate surface can be polished by chemical mechanical
polishing (CMP) to form a substrate having the substrate through wiring. In the capacitive
element of the present invention, the surface roughness of the substrate adversely affects the
design of the cavity if the roughness is large (for example, the convex portion of the rough
surface of the substrate comes in contact with the diaphragm when the diaphragm vibrates).
Because of the presence, the average roughness Ra is preferably 10 nm or less, and more
preferably 2 nm or less. Moreover, 1 mm or less is preferable and, as for the level ¦ step
difference of the exposed surface of the said substrate through wiring 22 and the surface of the
board ¦ substrate 5, 0.2 mm or less is more preferable.
[0087]
On the other hand, it is also possible to form the substrate penetration wiring portion 22 by
thermal oxidation and LPCVD by penetrating etching the Si substrate by the DRIE method. In this
case, the insulation of the through wiring conductive portion 22 can be formed by thermal
oxidation. Also, the through wiring conductive portion 22 can be formed of doped LPCVD
polycrystalline Si.
[0088]
The processes of FIGS. 9 (a) to 9 (f) are the same as those of the first embodiment and the second
embodiment. Then, as shown in FIG. 9G, on the back surface of the substrate 5, a single
conductive film 27, for example, Ti (100 nm in film thickness) is formed. Thereafter, an electrical
connection portion 35 is formed in contact with the working electrode 16 on the conductive film
27 on the back surface of the substrate using a single-sided etching jig (not shown). Then, as in
the first embodiment, the substrate is immersed in the electrolytic solution to electrolytically etch
the sacrificial layer 11 in contact with the lower electrode 8 through the conductive film 27 and
the substrate through wiring 22.
04-05-2019
28
[0089]
Next, as in the first embodiment, as shown in FIG. 9H, after electrolytic etching, a drying step is
performed to form a cavity 10 which is open to the atmosphere.
[0090]
After that, as in the first embodiment, as shown in FIG. 9I, the inlet 13 of the etching solution is
sealed with plasma CVD SiN x to form the sealing portion 20.
Next, the connection wiring portion 28 between the substrate through wiring and the upper
electrode is provided with an opening by RIE. Then, as shown in FIG. 9J, a metal layer is formed
and patterned to simultaneously form the upper electrode 1 and the wiring portion 28 connected
to the substrate through wiring 22.
[0091]
Next, as in the first embodiment, as shown in FIG. 9K, a protective film 12 having an insulating
property is provided on the upper electrode 1. Finally, the conductive film 27 on the back surface
of the substrate 5 is patterned to form an electrode pad of the through-substrate wiring on the
back surface of the substrate. By doing this, the lower electrode 8 and the upper electrode 1 can
be taken out on the back surface of the substrate 5 as shown in FIG. 9 (k). In particular, this
method is important in high-density device array fabrication.
[0092]
In FIG. 9 (k) showing the present embodiment, the through wiring conductive portion 22 is
shown connected to the lower electrode 8 and the upper electrode 1, but one of the lower
electrode 8 and the upper electrode 1 is The other electrode may be connected to the throughwire conductive portion 22 and may be connected to the surface of the substrate.
[0093]
Fourth Embodiment FIG. 10A to FIG. 10E are cross-sectional views for explaining steps of a
fourth embodiment of a capacitive electromechanical transducer according to the present
invention.
04-05-2019
29
[0094]
The manufacturing process of the element of this embodiment is the same as that of the third
embodiment except that the sealing portion, the connection wiring portion, and the stock
electrode pad are simultaneously formed.
In the present embodiment, the sealing portion 20, the connection wiring portion 28, and the
lower electrode pad 9 are simultaneously formed when the upper electrode 1 is formed as in the
second embodiment without using SiNx in the sealing step. So it can be made more easily.
[0095]
As shown in FIG. 10D, the lower electrode pad 9 is described as being formed on the surface of
the substrate in order to measure the device halfway, but this is not essential.
It is also possible to form the pad of the upper electrode on the substrate surface.
[0096]
Fifth Embodiment FIGS. 11A to 10D are cross-sectional views for explaining steps of a fifth
embodiment of a capacitive electromechanical transducer according to the present invention. In
the device structure of this embodiment, the holes required for the electrolytic etching reaction
are not supplied to the lower electrode, but are provided to the upper electrode through the
conductive substrate. The other manufacturing processes are the same as in the first
embodiment.
[0097]
As shown in FIG. 11A, for example, the lower electrode 8 made of Ti metal is provided on the
conductive substrate 4 so as to be sandwiched by the insulating layer 6 (for example, SiNx or
04-05-2019
30
SiOx). Thereafter, the opening 32 to the substrate is formed by the RIE method. Next, a sacrificial
layer 11 (for example, metal Cr) is formed on the outermost surface of the substrate.
[0098]
Next, as shown in FIG. 11B, the upper electrode 1 (for example, Al) is formed on the sacrificial
layer 11 and patterned to simultaneously form a connection wiring portion between the
substrate and the upper electrode. .
[0099]
Next, the vibrating film 3 is formed on the upper electrode 1 by plasma CVD, and as shown in
FIG. 11C, the inlet 13 of the etching solution can be provided from the outermost surface of the
substrate to the sacrificial layer 11 is there.
For example, in the case of the Cr sacrificial layer 11, the Ti upper electrode 1, and the SiNx
vibrating film 3, SiNx can be etched by plasma of CF 4 gas by the RIE method as in the first
embodiment. Furthermore, it is also possible to etch an etching solution containing hydrofluoric
acid (for example, 49% HF solution: 30% H 2 O 2: H 2 O = 1: 1: 20). By etching under such
conditions, Cr can be etched according to the top surface position of the Cr sacrificial layer
without being etched.
[0100]
Thereafter, as shown in FIG. 12C, since the upper electrode 1 is electrically connected to the
substrate 4 through the connection wiring portion 30, electrolytic etching can be performed
through the substrate 4 having conductivity.
[0101]
Next, a drying step is performed to form an insulating sealing film 14 (for example, plasma CVD
Si3N4 or plasma CVD SiO2) as shown in FIG. 11 (d) and seal the inlet 13 of the etching solution.
Do.
Then, the sealing film 14 is patterned to form a sealing portion 20 for sealing the inlet 13 of the
04-05-2019
31
etching solution.
[0102]
In the case where the sealing film is not patterned, the sealing film can also be used as a
protective film as it is, or can be a vibrating film portion added on the original vibrating film 3.
[0103]
In FIG. 11D, the protective film, the pad of the lower electrode 8, and the upper electrode pad
taken out from the substrate 4 are omitted.
The electrode pad can be formed in the same manner as in the first embodiment, as required. The
other configuration is the same as that of the first embodiment.
[0104]
Sixth Embodiment FIGS. 12 (a) to 12 (d) are cross-sectional views for explaining steps of a sixth
embodiment of a capacitive electromechanical transducer according to the present invention. The
manufacturing process of the element of this embodiment is an element structure in which holes
(holes) necessary for electrolytic etching are supplied directly to the upper electrode through the
upper electrode pad without passing through the conductive substrate, regarding the structure of
the element. The other configuration is the same as that of the fifth embodiment.
[0105]
As shown in FIG. 12A, the lower electrode 8, the silicon nitride (SiNx) insulating layer 6, the
patterned Cr sacrificial layer 11, the Ti upper electrode 1, the silicon nitride (SiNx) vibrating film
3 on the conductive substrate Install in the order of Next, as shown in FIG. 12B, the SiNx
vibration film 3 is patterned to form the upper electrode pad 9. In forming the etching solution
inlet 13 leading to the upper electrode 1, the patterning process of the etching solution inlet 13
is performed in the same manner as the SiNx vibrating film 3 of the fifth embodiment and the Ti
upper electrode patterning process.
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[0106]
As shown in FIG. 12C, since the pad 9 of the upper electrode 1 is electrically connected to the
upper electrode 1, electrolytic etching can be performed through the upper electrode pad 9.
[0107]
Next, as shown in FIG. 12 (d), the lower electrode 8 is covered with the insulating film 6.
Thereafter, a sealing film 14 having an insulating property, for example, plasma CVD Si 3 N 4 or
plasma CVD SiO 2 is formed to seal the inlet 13 of the etching solution. Then, the sealing film 14
is patterned to form a sealing portion 20 for sealing the inlet 13 of the etching solution.
[0108]
When the sealing film is not patterned, the sealing film can also be used as a protective film as it
is, or can be a vibrating film portion added on the original vibrating film 3.
[0109]
In FIG. 12D, the pad of the upper electrode 1, the pad of the lower electrode 8, and the protective
film are omitted.
The protective film can be formed in the same manner as in the first embodiment, as required.
[0110]
Seventh Embodiment FIGS. 13 (a) to 13 (e) are cross-sectional views for explaining steps of a
seventh embodiment of a capacitive electromechanical transducer according to the present
invention. As in the fifth and sixth embodiments, the structure of the device of this embodiment
does not pass holes necessary for electrolytic etching through the lower electrode, but directly
through the through-substrate wiring and the upper electrode connection portion to the upper
electrode Is an element structure for supplying The other element configurations and element
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manufacturing processes are the same as in the third embodiment.
[0111]
As shown in FIG. 13C, in forming the etching solution inlet 13 leading to the upper electrode 1,
the patterning process of the etching solution inlet 13 is performed using the SiNx vibrating film
3 of the fifth embodiment and the Ti upper portion. It is the same as the electrode patterning
step.
[0112]
Thereafter, as shown in FIG. 13D, the sacrificial layer 11 in contact with the upper electrode 1 is
electrolytically etched through the substrate through wiring 22 to form a cavity 10.
The electrolytic etching process is the same as in the first, fifth and sixth embodiments.
[0113]
Finally, as shown in FIG. 13E, the inlet 13 of the etching solution is sealed with a sealing film 14
(for example, plasma CVD Si3N4 or plasma CVD SiO2). By so doing, it is also possible to use the
sealing film 14 and the protective film in common.
[0114]
Also in FIG. 13E, the through wiring pad 29 and the protective film on the back surface of the
substrate are not shown. By the way, in the first to seventh embodiments described above, one
element portion and the configuration around it are extracted and described. In order to produce
a large area element array, the element portions of the configurations shown in these figures
may be regularly or periodically arranged on the substrate. Its configuration and manufacturing
method are essentially the same as those described above.
[0115]
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If a flow path for connection is provided between the plurality of cavities 10, the cavity group
common to the flow path can share an electrode for providing an etching solution inlet and a
hole. Thus, for example, the number of openings formed in the vibrating membrane 3 can be
reduced, and the mechanical characteristics of the capacitive electromechanical transducer can
be further stabilized.
[0116]
In the embodiment, the vibrating portion is a laminated film including the vibrating film 3, the
upper electrode 1, the sealing film, and the like. In the above embodiment, the sealing film or the
like is a part of the vibrating film 3 and the protective film 12 or the like is a film different from
the vibrating film 3 in order to facilitate the description. It is also possible to integrate with a part
of
[0117]
In the first to fourth embodiments, the SiNx vibrating film 3 functions as an insulating film to
prevent shorting of the upper and lower electrodes. In the fifth to seventh embodiments, since
the short circuit of the upper and lower electrodes can be avoided by the insulating film 6, the
vibrating film 3 need not necessarily have the insulating property. For this reason, if an
insulating protective film for securing the insulating property is separately provided, the
vibrating film 3 having conductivity can also be provided.
[0118]
Eighth Embodiment FIGS. 14A to 14D are cross-sectional views for explaining steps of an eighth
embodiment of a capacitive electromechanical transducer according to the present invention. The
structure of the device of this embodiment is a device structure in which holes required for
electrolytic etching are supplied directly to the upper electrode through the upper electrode pad
without passing through the conductive substrate. The initial manufacturing process of this
embodiment is the same as that of the fifth and sixth embodiments. In this embodiment, the case
where the conductive substrate 4 is Si and the lower electrode 8 is Highly Doped Si will be
described.
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[0119]
As shown in FIG. 14A, the lower electrode 8, the SiNx insulating layer 6, the patterned Cr
sacrificial layer 11, the Ti upper electrode 1, and the SiNx vibrating film 3 are disposed in this
order on the conductive substrate 4.
[0120]
Next, as shown in FIG. 14B, the SiNx vibration film 3 is patterned to form the upper electrode pad
9.
Then, plasma dry etching (deep RIE etcher manufactured by STS, Inc.) using SF6 gas for deep
trenching is performed from the back surface of the substrate to the Cr sacrificial layer 11
through the Si substrate 4, the lower electrode 8 and the insulating layer 6. In the plasma by the
SF 6 gas, Cr has a sufficiently high etching selectivity with Si and SiN x, so the etching reaction
reaches the lower surface of the Cr sacrificial layer to form the substrate backside through hole
36.
[0121]
In FIG. 14 (b), an etching mask for deep trenching, for example, a thick film photoresist, an SiO2
film, or a Cr film is omitted.
[0122]
Thereafter, as in the first and third embodiments, when the substrate 4 fixed by the single-sided
etching jig is immersed in the electrolytic etching solution, the electrolytic etching solution flows
into the substrate back surface through hole 36.
[0123]
As shown in FIG. 14C, since the pad 9 of the upper electrode 1 is electrically connected to the
upper electrode 1, electrolytic etching is performed through the upper electrode pad 9 to
perform a drying step to form a cavity 10 Do.
[0124]
Next, as shown in FIG. 14D, a sealing film 14 having insulating properties, for example, plasma
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CVD Si3N4 or plasma CVD SiO2 is formed on the back surface of the substrate to seal the
substrate back surface through-hole 3. , Complete the device.
[0125]
The pad of the lower electrode 8, a protective film, etc. are abbreviate ¦ omitted in FIG.12 (d).
[0126]
DESCRIPTION OF SYMBOLS 1 upper electrode 2 support part 3 vibrating film 4
electroconductive substrate 5 insulating substrate 6 insulating layer 8 lower electrode 9
electrode pad of upper electrode 10 cavity (space) 11 sacrificial layer 12 protective film 13 inlet
of etching liquid 14 sealing film
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