JP2011130038

Patent Translate
Powered by EPO and Google
Notice
This translation is machine-generated. It cannot be guaranteed that it is intelligible, accurate,
complete, reliable or fit for specific purposes. Critical decisions, such as commercially relevant or
financial decisions, should not be based on machine-translation output.
DESCRIPTION JP2011130038
The present invention provides an electromechanical transducer such as a CMUT in which the
insulating effect between electrodes separated by a groove is enhanced. An electro-mechanical
transducer includes a conductive substrate, a plurality of elements disposed on the side of a first
surface of the substrate and having cells, a groove, and an insulating film. The groove 107
extends from the side of the second surface opposite to the first surface of the substrate to the
side of the first surface so as to electrically isolate the plurality of elements from one another to
divide the substrate into a first The electrode 103 is formed. The insulating film 109 is formed on
the side wall of the first electrode opposite to each other with the groove interposed
therebetween. The width between the insulating films 109 on the second surface side of the
substrate is smaller than the width between the insulating films 109 on the first surface side of
the substrate. In the insulating film 109, the thickness on the side of the second surface is larger
than the thickness on the side of the first surface. [Selected figure] Figure 1
Electromechanical converter and method of manufacturing the same
[0001]
The present invention relates to an electromechanical transducer such as an ultrasonic
transducer and a method of manufacturing the same. More particularly, the present invention
relates to a technique for improving the reliability of an electromechanical transducer.
[0002]
04-05-2019
1
The ultrasonic transducer converts an electric signal into an ultrasonic wave and converts an
ultrasonic wave into an electric signal, and is used as a probe for medical imaging, nondestructive
inspection, and the like. One form of ultrasound transducer is the Capacitive Micromachined
Ultrasound Transducer (CMUT). A CMUT generally comprises a substrate having a lower
electrode, a membrane supported by a support formed on the substrate, and an upper electrode.
The CMUT vibrates the membrane with a voltage applied between the lower electrode and the
upper electrode to emit an ultrasonic wave. Also, the membrane is vibrated by the received
ultrasonic wave, and the ultrasonic wave is detected by the change in capacitance between the
lower electrode and the upper electrode.
[0003]
Conventionally, CMUTs have been manufactured using so-called surface micromachining, bulk
micromachining and the like. Also, as a wiring method, there is a method of connecting elements
to a circuit board by using one or more membranes and cavities (that is, one or more cells) on a
silicon substrate as one element, and using the silicon substrate itself as a lower electrode and a
through wiring. is there. For example, when the elements of the CMUT are two-dimensionally
arrayed, there is known a method in which grooves for dividing the lower electrode are provided
for each element to electrically separate the elements and to draw out the respective wires from
the back surface Patent Documents 1 and 2). It is also known to fill grooves with an insulating
material such as epoxy in order to obtain high insulation between adjacent elements.
[0004]
US-A1-20060075818 Specification US-A1-20070013269 specification
[0005]
However, in the conventional CMUT having a groove for dividing the lower electrode as
described above, there is a possibility that conductive foreign matter may be mixed in the groove
during the manufacturing process or in use, thereby bridging the lower electrodes and causing a
short circuit. .
[0006]
In view of the above problems, an electromechanical transducer according to the present
04-05-2019
2
invention comprises: a first electrode made of a conductive substrate; and a second electrode
provided opposite to the first surface of the substrate with a gap therebetween. And the substrate
is separated for each of the elements by a groove penetrating from the first side to the second
side opposite to the first side. There is.
Then, an insulating film is provided on the side wall of the substrate facing each other across the
groove, and the width between the insulating films sandwiching the groove on the second surface
side is the width of the first surface side of the substrate. The width is smaller than the width
between the insulating films, and the thickness of the insulating film on the second surface side
of the substrate is larger than the thickness of the insulating film on the first surface side of the
substrate.
[0007]
Further, in view of the above problems, a manufacturing method for manufacturing the electromechanical transducer according to the present invention is provided by facing a conductive
substrate as a first electrode and a first surface of the substrate with a gap therebetween. A
method of manufacturing an electromechanical transducer comprising a plurality of elements
having one or more cells including the second electrode, comprising the following steps.
Forming a groove in the substrate and forming the first electrode for each of the elements. An
insulating film is formed on the side wall of the first electrode facing the groove, and a thickness
of the second surface opposite to the first surface of the substrate on which the plurality of
elements are disposed is the thickness The width between the insulating films sandwiching the
groove on the side of the second surface of the substrate is greater than the thickness on the side
of the first surface of the substrate, and the width of the insulating film between the insulating
films on the side of the first surface of the substrate is And a step of forming so as to be narrower
than the width between the insulating films sandwiching.
[0008]
According to the present invention, since the side wall of the first electrode divided by the groove
is covered with the insulating film, short circuit between the electrodes is prevented even if
foreign matter is mixed in the groove which is a gap. Can do. In addition, the width between the
insulating films on the side walls of the first electrode is such that the opening side of the groove
04-05-2019
3
(that is, the side of the second surface opposite to the first surface of the substrate) is the bottom
side of the groove (that is, the side , Side of the first surface of the substrate). Therefore, the
foreign substance which can enter the groove is smaller than the width of the inner groove and
does not reach the opposite side wall surfaces of the opposing first electrode, so a higher interelectrode insulation effect is expected, and the reliability of the electromechanical transducer It is
possible to improve the quality.
[0009]
Sectional drawing of CMUT which is an example of the electro-mechanical transducer which can
apply this invention. FIG. 7 is a process flow diagram of the CMUT manufacturing method of the
second embodiment.
[0010]
Hereinafter, embodiments of the present invention will be described. The important point in the
electromechanical transducer of the present invention is that the insulating film is placed on the
side wall of the first electrode formed by the groove of the substrate and the insulating film
sandwiching the groove on the first surface side of the substrate on which the element is
disposed. The width between the insulating films on the side of the second surface opposite to
the first surface is narrower than the width between the two. In addition, an insulating film is
formed on the side wall of the first electrode facing across the groove, so that the thickness on
the opposite side of the second surface is larger than the thickness on the side of the first surface
of the substrate. It is to be.
[0011]
Based on the above concept, the basic form of the electromechanical transducer and the method
of manufacturing the same according to the present invention have the configuration as
described in the section for solving the problems. Based on this basic form, embodiments as
described below are possible. For example, the substrate on which the first electrode (the lower
substrate described later) is formed may be made of silicon, and the insulating film may be
formed of a thermal oxide film (see Embodiment 1 described later). In addition, the element can
be made to be a capacitive electromechanical transducer (see Embodiment 1 described later). The
grooves can be formed by, for example, dry etching such as Deep-RIE, or a method of forming the
04-05-2019
4
grooves in a silicon substrate by alkaline wet etching. The first electrode can be connected to the
circuit board directly or via a relay substrate having through hole wiring. Hereinafter,
embodiments will be described using the drawings.
[0012]
First Embodiment A first embodiment according to a CMUT, which is an electromechanical
transducer to which the present invention can be applied, will be described. However, the present
invention is applicable not only to CMUT, but also to an electromechanical transducer having a
similar structure (a structure in which a substrate is divided by a groove to form a first electrode
for each element). For example, the present invention can be applied to an ultrasonic transducer
(a so-called piezoelectric transducer (PMUT), a magnetic transducer (MMUT) or the like) using
distortion, a magnetic field, or light. That is, the electromechanical transducer to which the
present invention can be applied is not limited to the one in which the structure on the lower
electrode 103 which is the first electrode described later is described later.
[0013]
As shown in FIG. 1 which is a schematic view showing the cross-sectional structure of the CMUT
of this embodiment, the element substrate 101 of the CMUT of this embodiment is electrically
connected to the circuit board 102. The element substrate 101 and the circuit board 102 are
fixed to each other, and the circuit board is disposed below the element substrate. In the element
substrate 101, elements having a plurality of cells are two-dimensionally arranged. Each cell
includes an upper electrode 104 which is a second electrode, a membrane 105, a support portion
106 made of an insulator, and a lower electrode 103 which is a first electrode, and there is a
space between the upper electrode and the lower electrode. A cavity 108 is formed. The lower
electrode 103 is separated for each element by forming a groove 107 penetrating from the
second surface to the first surface in the thickness direction of the conductive electrode
substrate. In the present embodiment, the surface of the side wall of the lower electrode 103
opposed to the groove 107 is covered with the insulating film 109. The side wall of the lower
electrode is the surface of the lower electrode newly exposed along the groove by forming the
groove 107 in the conductive electrode substrate. The width between the insulating film 109 on
the side wall surface of the lower electrode 103 is from the side of the first surface of the
substrate which is the bottom side of the groove 107 to the circuit board 102 side (the second
side of the substrate which is the opening side of the groove 107). The structure is narrower as
you go to the side of the The inside of the groove 107 is an air gap and does not take a form
filled with an insulating material or the like.
04-05-2019
5
[0014]
In each element, the cavities 108 of the plurality of cells may be sealed independently or in
communication with each other. A plurality of cells are electrically connected in parallel to form
an element. Each element may include one or more cells, and the number of cells in each
element, the arrangement form of the cells, the form of the cavity 108 and the like are free as
long as the electromechanical conversion function can be achieved. Further, the arrangement
method and the number of the elements are not limited to those in the present embodiment, and
only the desired number of elements may be provided in a desired arrangement. The groove 107
may be formed in a form corresponding to the arrangement form of the element or the like.
Further, the upper electrode may double as a membrane (vibration membrane). The circuit board
102 includes a processing circuit (not shown) for processing a signal and an electrode pad 110,
and the electrode pad 110 is electrically connected to the lower electrode 103 by a solder bump
111.
[0015]
The above-described structure CMUT operates, for example, as follows. When ultrasonic waves
are received, the membrane 105 is displaced, and the gap between the upper electrode 104 and
the lower electrode 103 changes. A signal processing circuit of the circuit board 102 detects the
amount of change in capacitance due to this, and an ultrasonic image can be obtained by
processing the signal. Further, in the case of transmitting an ultrasonic wave, a voltage is applied
from the circuit board 102 to the upper electrode 104 or the lower electrode 103 to vibrate the
membrane 105 and transmit the ultrasonic wave. The present embodiment can be manufactured
by a bonding type (bulk micromachining), a surface type (surface micro machining), or the like.
In the bonding method, for example, a cavity is formed in a silicon substrate, and a membrane is
formed by bonding an SOI substrate (see Embodiment 2 described later). In the surface type
method, a membrane is formed on a sacrificial layer, the sacrificial layer is etched through an
etching hole formed in the membrane, and finally the etching hole is filled with a silicon nitride
film or the like to form a cavity.
[0016]
Next, the lower electrode 103 and the insulating film 109 on the side wall of the lower electrode
04-05-2019
6
in the present embodiment will be described in detail. The width (distance) between the
insulating films 109 of the grooves 107 separating the lower electrodes is narrower toward the
opening, and the larger the inclination angle of the side walls of the insulating film 109 in the
grooves 107 with the substrate 106, the better. The width of the bottom between the insulating
films 109 of the groove 107 is wider than the opening, and even if foreign matter enters the
groove, the particulate foreign matter can not bridge the side wall of the opposing lower
electrode 103. is there. In addition to the side wall of the lower electrode 103, it is more
preferable that the insulating film 109 be formed on the bottom surface of the lower electrode
103 to be bonded to the circuit board 102 except for the wiring to the circuit board. This is
because a short circuit can be prevented even in the case where the lower electrodes 103 are
connected due to the inclusion of elongated linear foreign matter.
[0017]
The lower electrode 103 used in the present embodiment is preferably formed of a
semiconductor substrate of silicon or the like which is easily microfabricated. The resistivity of
the lower electrode 103 is preferably less than 0.02 Ω · cm. This is because the smaller the
wiring resistance of the lower electrode, the smaller the signal loss. The lower thickness of the
lower electrode 103 is preferably 100 μm or less because the parasitic capacitance generated
between adjacent lower electrodes can be reduced and the wiring resistance can be reduced. The
insulating film 109 formed on the side wall of the lower electrode is preferably a thermal oxide
film of silicon. In the case of an oxide film, there is little risk of contamination with particles and
the like in the processing process, and there is no need to take into consideration peeling and
adhesion of the film. In the case of an oxide film, when the lower electrode is thermally oxidized
after the groove dividing the lower electrode 103 is formed, for example, perpendicularly to the
substrate surface, the oxide film is formed thicker on the surface of the electrode 103 toward the
opening of the groove 107. Be done. Therefore, it is possible to easily form a device structure in
which the width between the insulating films 109 formed on the side wall of the lower electrode
becomes narrower toward the opening. In this case, even if a groove structure having a narrower
width toward the opening is formed, the width of the bottom of the groove on the side of the first
surface does not have to be greatly extended if the groove for dividing the lower electrode is
formed vertically. Therefore, the separation between elements can be narrowed. Although a
cavity can not be disposed in the groove portion, if the separation width is narrowed, the number
of cavities that can be disposed per unit area of the substrate can be increased, and the
sensitivity of the entire device can be enhanced (the fill factor can be increased).
[0018]
04-05-2019
7
On the other hand, in the case of forming the insulating film by thermal oxidation, it is preferable
that the groove has a narrow width because foreign matter can be prevented from being mixed.
Further, it is preferable that the aspect ratio is high because a gradient is generated in the
concentration of oxygen gas diffused in the depth direction of the groove at the time of thermal
oxidation, and the oxide film thickness becomes thicker toward the opening. Since the depth of
the groove 103 depends on the thickness of the lower electrode 103, the width of the groove
satisfying this condition is preferably 5 μm or less, more preferably 3 μm or less. Further, it is
preferable that the thermal oxide film 109 in the present embodiment has a thicker film
thickness on the opening side so that the opening side of the groove 107 defining the distance
between the adjacent lower electrodes 103 becomes narrower. Preferably, the film thickness of
the bottom side wall surface of the groove 107 is 0.4 to 0.5 μm, and the film thickness on the
opening side of the groove 107 is 1 μm or more. However, the film thickness mentioned here is
somewhat different depending on the oxidation conditions.
[0019]
By forming the insulating film as thick as the opening of the groove on the surface of the lower
electrode separated by the groove of the above structure, it is possible to prevent the foreign
matter from being mixed in the groove. Further, even if foreign matter enters the groove, the
width between the insulating films in the groove is wider than the opening, so that it is
impossible to bridge the adjacent lower electrode at least in the case of particles. Thus, a high
short circuit preventing effect is expected as compared with the prior art.
[0020]
Second Embodiment A second embodiment according to a method of manufacturing a CMUT,
which is an electromechanical transducer to which the present invention can be applied, will be
described. The CMUT is manufactured as follows by the manufacturing method of this
embodiment. The lower electrode is separated by a groove for each element, and the SOI
substrate is bonded on the upper surface of the Si substrate with the insulating film formed on
the surface of the lower electrode facing the groove to form a membrane, and between the lower
electrode and the circuit substrate Wiring An insulating film is formed on the side wall of the
lower electrode divided by the groove by thermal oxidation.
[0021]
04-05-2019
8
When the lower electrode is thermally oxidized, an oxide film is formed thicker on the side wall
of the lower electrode closer to the opening of the groove, and a device shape in which the width
of the groove becomes narrower closer to the opening can be easily obtained. In the case of oxide
film formation, there is no fear of contamination with particles or the like in the processing
process, and there is no need to take into consideration peeling or adhesion of the film. In
addition, since the oxide film can be simultaneously formed on the bottom of the lower electrode
other than the side wall of the lower electrode, insulation other than the side wall can be
simultaneously performed. However, in order to wire the lower electrode to the circuit board, it is
necessary to ensure conduction without oxidizing the entire surface or a part of the lower
electrode bottom. In the present embodiment, before forming the insulating film, the region for
ensuring the conduction from the lower electrode is covered with SiN, and then the insulating
film is formed by thermal oxidation, and finally the SiN is peeled off to ensure the conduction of
the lower electrode. . Since the Si substrate prepared first becomes the lower electrode later, one
having a low resistivity is preferable. In the present embodiment, a Si substrate having a specific
resistance of less than 0.02 Ω · cm is used. The thickness of the Si substrate is preferably 100
μm to 525 μm.
[0022]
The process flow of this embodiment is shown in FIG. First, the Si substrate 201 is thermally
oxidized to form an oxide film 202. Next, a resist pattern for a cavity pattern is formed by
photolithography. Further, the oxide film 202 is etched with buffered hydrofluoric acid (BHF)
using the resist pattern as a mask to form a cavity recess 203. FIG. 2A is a cross-sectional view
after the formation of the recess 203 for the cavity. Next, in order to insulate the bottom of the
cavity, the Si substrate is thermally oxidized again.
[0023]
Next, the substrate surfaces of the Si substrate and the SOI substrate 204 in which the concave
portions for cavities are formed are cleaned, and both substrate surfaces are activated and then
bonded under vacuum. A cavity is formed in this process. FIG. 2B is a cross-sectional view after
wafer bonding. Next, on the bottom of the Si substrate, a groove pattern for separating the lower
electrode for each element and a lead-out wiring pattern for the lower electrode are formed by
photolithography. First, SiN is formed on the bottom of the Si substrate in order to secure a
wiring pattern for electrically connecting the lower electrode to the circuit substrate.
04-05-2019
9
Furthermore, a wiring pattern is formed with a resist by photolithography. Thereafter, SiN is
etched using the resist pattern as a mask, and then the resist is removed. Next, in order to form a
groove in the lower electrode, Cr 206 is deposited on the Si substrate 201 where the SiN pattern
205 remains. FIG. 2C is a cross-sectional view after Cr film formation.
[0024]
Furthermore, a groove pattern is formed with a resist by photolithography. The Cr is etched
using the resist pattern as a mask, and then the resist is removed. Next, the groove 201 is formed
by dry etching the Si 201 using the remaining Cr pattern as a mask. At this time, the grooves 207
are formed vertically by Deep-RIE or the like. By this process, the lower electrode is separated for
each element. After the groove 207 is formed, Cr is removed. FIG. 2D is a cross-sectional view
after forming the groove.
[0025]
Next, the Si substrate 201 is thermally oxidized again to form an oxide film 208 on the side wall
of the lower electrode after the groove formation. At this time, the remaining area of the SiN 205
is not oxidized. FIG. 2E is a cross-sectional view of the substrate 201 after thermal oxidation. At
the time of thermal oxidation, a gradient of oxygen concentration occurs in the depth direction of
the groove 207 inside the groove, and the oxide film 208 is formed thicker on the side wall of the
lower electrode toward the opening of the groove. Therefore, it is preferable to perform thermal
oxidation under the condition that the film thickness gradient of the oxide film 208 formed on
the side wall of the lower electrode becomes large along the depth direction of the groove 207.
The temperature is from 1000 ° C. to 1100 ° C. while maintaining the oxygen concentration
gradient. By this process, the oxide film 208 is formed to be thicker toward the opening, and the
vertical groove has a shape in which the width becomes narrower toward the opening. At this
time, since the oxide film 208 is formed on the surface of the Si substrate 201 and also in the
inside thereof, it has a cross-sectional shape as shown in FIG. 2E. There is no problem if the
groove partially closes on the opening side. The SiN 205 is removed after oxide film formation.
[0026]
Next, the support substrate layer 209 and the buried oxide film layer 210 of the SOI substrate
are removed by etching. A membrane is formed by this process. FIG. 2F is a cross-sectional view
04-05-2019
10
after membrane formation. The processes after the formation of the groove 207 may be
appropriately reinforced if it is determined that the device strength is insufficient. Next, the
formation part 211 of upper electrode lead-out wiring is formed. On the membrane, a resist
pattern of the upper electrode lead-out forming portion 211 is formed by photolithography. The
membrane and the support portion 202 are etched using this resist as a mask. FIG. 2G is a crosssectional view after forming the wiring formation portion of the upper electrode.
[0027]
Next, Al 212 is vapor-deposited on the formation portion 211 for the upper electrode lead-out
wiring. A resist pattern of the upper electrode is formed on the surface on which Al is vapordeposited by photolithography. The upper electrode is formed by wet etching Al 212 using this
resist pattern as a mask. FIG. 2H is a cross-sectional view after forming the upper electrode.
Finally, the electrode pad 214 of the circuit substrate 213 and the lower electrode 201 are
aligned, and the two substrates are joined by the solder bump 215. Thereby, a CMUT capable of
signal processing of transmission and reception of ultrasonic waves is manufactured. FIG. 2I is a
cross-sectional view of a CMUT completed by this process.
[0028]
DESCRIPTION OF SYMBOLS 101 ... Element substrate, 103, 201 ... Lower electrode (1st
electrode, electroconductive board ¦ substrate, Si substrate) 104, 212 ... Upper electrode (2nd
electrode, Al) 107, 207 ... Groove ¦ channel, 109, 208 ... Insulating film
04-05-2019
11