U.S. patent application number 13/610144 was filed with the patent office on 2013-01-03 for electromechanical transducer.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to Takehiko Kawasaki, Yoshitaka Zaitsu.
Application Number | 20130000116 13/610144 |
Document ID | / |
Family ID | 42933810 |
Filed Date | 2013-01-03 |
United States Patent
Application |
20130000116 |
Kind Code |
A1 |
Zaitsu; Yoshitaka ; et
al. |
January 3, 2013 |
ELECTROMECHANICAL TRANSDUCER
Abstract
When the initial displacement greatly varies among cells in an
element, there is a need to reduce a bias voltage to be applied
between electrodes. This decreases the sensitivity. An
electromechanical transducer of the present invention includes an
element having a plurality of cells. Each of the cells includes a
first electrode and a second electrode that are provided with a
cavity being disposed therebetween. A groove is provided at a
position at a predetermined distance from the cavity of the cell on
the outermost periphery of the element.
Inventors: |
Zaitsu; Yoshitaka;
(Ichikawa-shi, JP) ; Kawasaki; Takehiko;
(Kamakura-shi, JP) |
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
42933810 |
Appl. No.: |
13/610144 |
Filed: |
September 11, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
12753782 |
Apr 2, 2010 |
8299550 |
|
|
13610144 |
|
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Current U.S.
Class: |
29/846 |
Current CPC
Class: |
Y10T 29/49155 20150115;
Y10T 29/49117 20150115; B06B 1/0292 20130101 |
Class at
Publication: |
29/846 |
International
Class: |
H05K 3/10 20060101
H05K003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 10, 2009 |
JP |
2009-096145 |
Claims
1. A method for preparing an electromechanical transducer
comprising an element including a plurality of cells, the cells
each including a first electrode, a second electrode provided with
a cavity being disposed between the first electrode and a vibrating
film to be deformed by a potential difference between the first
electrode and the second electrode, comprising the step of: forming
a groove at a predetermined position distant from the cavity of the
cell provided on the outermost periphery of the element, such that
when the element is deformed by a potential difference between the
first electrode and the second electrode, a difference in
displacement amount between an outermost cell of the plurality of
cells and an innermost cell of the plurality of cells is less than
the displacement amount when the groove is not provided.
2. The method for preparing an electromechanical transducer
according to claim 1, wherein the groove is provided in a thin film
that forms a vibrating film.
3. The method for preparing an electromechanical transducer
according to claim 2, wherein the groove penetrates the thin film
into a support portion configured to support the thin film.
4. The method for preparing an electromechanical transducer
according to claim 1, wherein the groove is formed on a position
that the predetermined distance from the cavity of the cell on the
outermost periphery of the element is shorter than or equal to a
distance between the cavities of the cells.
5. The method for preparing an electromechanical transducer
according to claim 2, wherein the groove is also provided in a
portion of the thin film between the cavities.
6. The method for preparing an electromechanical transducer
according to claim 1, wherein the groove continuously extends on an
outer side of the cell on the outermost periphery of the
element.
7. A method for preparing an electromechanical transducer
comprising an element including a plurality of cells, the cells
each including a first electrode and a second electrode provided
with a cavity being disposed between the first electrode,
comprising the step of: forming a groove at a predetermined
position distant from the cavity of the cell provided on the
outermost periphery of the element, wherein the length of the
groove, in a direction parallel to the surface on which the cells
are arranged, is longer than the length of the cavity.
8. The method for preparing an electromechanical transducer
according to claim 7, wherein the groove is formed on the thin film
that forms the vibrating film to be deformed by a potential
difference between the first electrode and the second
electrode.
9. The method for preparing an electromechanical transducer
according to claim 8, wherein the groove penetrates the thin film
into a support portion configured to support the thin film.
10. The method for preparing an electromechanical transducer
according to claim 7, wherein the groove is formed on a position
that the predetermined distance from the cavity of the cell on the
outermost periphery of the element is shorter than or equal to a
distance between the cavities of the cells.
11. The method for preparing an electromechanical transducer
according to claim 8, wherein the groove is also provided in a
portion of the thin film between the cavities.
12. The method for preparing an electromechanical transducer
according to claim 1, wherein the groove continuously extends on an
outer side of the cell on the outermost periphery of the element.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application is a continuation of U.S. patent
application Ser. No. 12/753,782, filed on Apr. 2, 2010, the content
of which is expressly incorporated by reference herein in its
entirety. This application also claims the benefit of Japanese
Application No. 2009-096145 filed Apr. 10, 2009, which is hereby
incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an electromechanical
transducer.
[0004] 2. Description of the Related Art
[0005] In recent years, electromechanical transducers produced by a
micromachining process have been researched actively. In
particular, capacitive electromechanical transducers called
capacitive micromachined ultrasonic transducers (CMUT) have
attracted attention, because they can transmit and receive
ultrasonic waves with a lightweight membrane and can obtain wider
band characteristics than piezoelectric electromechanical
transducers of the related art.
[0006] A CMUT includes a plurality of elements arranged in an array
in a one-dimensional or two-dimensional direction. Elements serve
to transmit and receive ultrasonic waves. FIG. 11A is a schematic
top view of a CMUT of the related art. An element 301 shown in FIG.
11A includes a plurality of cells 311. By simultaneously applying a
driving voltage signal to the cells 311 of the element 301,
ultrasonic waves are output from the element 301. Further,
ultrasonic detection signals received by the cells 311 of the
element 301 are added by upper electrodes 315 and a lower electrode
(not shown) that is common to the cells 311, and the sum serves as
an ultrasonic detection signal received by the element 301. The
upper electrodes 315 in the cells 311 are electrically connected by
lines 307.
[0007] U.S. Pat. No. 6,958,255 discloses an example of a CMUT
having such an element structure. In this CMUT, a substrate
penetrating line 304 is provided in a support substrate 303, as
shown in FIG. 11B. A circuit board 305 is electrically connected to
a lower electrode 316 by the substrate penetrating line 304, and is
electrically connected to upper electrodes 315 by lines, an
insulating-layer penetrating line, and the substrate penetrating
line 304. In the circuit board 305, driving voltage signals are
generated to output an ultrasonic wave from an element, and an
ultrasonic signal generated by an ultrasonic wave received by the
element is subjected to processing such as amplification and delay
addition.
[0008] Unfortunately, the displacement amount of the membrane
varies among the cells of the element. It can be conceived that
this variation among the cells is caused by warping of the
substrate due to the difference in coefficient of thermal expansion
between the membrane and the insulating layer and internal stresses
of the membrane and the insulating layer. The variation is
undesirable because it appears as differences in transmission
efficiency and detection sensitivity for the ultrasonic wave.
[0009] The transmission efficiency and detection sensitivity of the
CMUT increase as the gap between the upper and lower electrodes
decreases. Since electrostatic attractive force between the upper
and lower electrodes is increased by increasing the bias voltage,
the transmission efficiency and detection sensitivity of the CMUT
can be enhanced by increasing the bias voltage. However, when the
bias voltage excessively increases, the upper electrode is
attracted to the lower electrode together with the membrane the
instant that the bias voltage reaches a certain voltage, so that it
is difficult to obtain a desired vibration characteristic. This
phenomenon is referred to as a pull-in, and a voltage at which a
pull-in occurs is referred to as a pull-in voltage. A pull-in
voltage is determined by the initial displacement amount of the
membrane. Thus, since the upper limit value of the bias voltage
applied between the upper and lower electrodes is limited by
variation in initial displacement amount of the membrane among the
cells, the receiving sensitivity of the CMUT is limited.
SUMMARY OF THE INVENTION
[0010] The present invention provides an electromechanical
transducer that reduces variation in displacement amount of a
membrane among cells.
[0011] An electromechanical transducer according to an aspect of
the present invention includes an element. The element includes a
plurality of cells each including a first electrode and a second
electrode provided with a cavity being disposed therebetween. A
groove is provided at a predetermined distance from the cavity of
the cell on the outermost periphery of the element.
[0012] The presence of the groove on the outer side of the cell on
the outermost periphery of the element can provide an
electromechanical transducer that reduces variation in displacement
amount of a membrane among cells.
[0013] Further features of the present invention will become
apparent from the following description of exemplary embodiments
with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIGS. 1A and 1B are schematic views illustrating a structure
of an element in a CMUT according to a first embodiment of the
present invention.
[0015] FIGS. 2A and 2B are schematic views illustrating a structure
of an element in a CMUT according to a second embodiment of the
present invention.
[0016] FIG. 3 is a schematic view illustrating a structure of an
element in a CMUT according to a third embodiment of the present
invention.
[0017] FIG. 4 is a schematic view illustrating a structure of an
element in a CMUT according to a fourth embodiment of the present
invention.
[0018] FIGS. 5A to 5G are schematic views illustrating a method for
producing a CMUT to which the present invention can be applied.
[0019] FIGS. 6B1 and 6B2 are schematic views illustrating another
method for producing a CMUT to which the present invention can be
applied.
[0020] FIG. 7 is a schematic view illustrating an initial
displacement amount provided when no groove is formed.
[0021] FIG. 8 is a schematic view illustrating a structure in which
a groove is formed.
[0022] FIGS. 9A to 9C are graphs showing advantages obtained when a
groove is formed on the outer periphery of the element.
[0023] FIGS. 10A and 10B are graphs showing advantages obtained
when grooves are formed around the cells.
[0024] FIG. 11A and 11B are schematic views showing a structure of
an element in a CMUT of the related art.
DESCRIPTION OF THE EMBODIMENTS
[0025] An electromechanical transducer according to the present
invention includes an element having a plurality of cells. A groove
is provided at a position at a predetermined distance from a cavity
of a cell on the outermost periphery of the element. In each of the
cells, a lower electrode serving as a first electrode and an upper
electrode serving as a second electrode are provided with a cavity
being disposed therebetween. Further, a thin film (hereinafter
referred to as a membrane) serving as a vibrating film to be
deformed by the potential difference between the upper and lower
electrodes is provided on the cavity.
[0026] In the present invention, the term "a position at a
predetermined distance" refers to a position that satisfies the
following two conditions. The first condition is that the position
is provided on an outer side of a cell on the outermost periphery
of the element. The second condition is that, when a groove is
formed at the position, a difference in initial displacement amount
of the membrane between the outermost cell and a center cell in the
element is smaller than when the groove is not formed. Although
details will be described below, the distance from the cavity of
the cell on the outermost periphery is preferably within a range of
50 to 200% of the inter-cavity distance. Further, the term "groove"
refers to a structure that meets any of the following four
definitions (1) to (4): (1) a recess formed in the membrane from an
upper surface of the membrane (a surface opposite the cavity); (2)
a recess formed in the membrane and an insulating layer serving as
a support portion supporting the membrane; (3) a recess defined by
the absence of the membrane around the cell on the outermost
periphery of the element; and (4) a recess defined in an upper
surface of the support portion (a surface opposite the bottom of
the cavity) by the absence of the membrane on the outer periphery
of the element. That is, in the electromechanical transducer of the
present invention, a portion of the membrane provided at the
position at a predetermined distance from the cavity of the cell on
the outermost periphery of the element is thinner than a portion of
the membrane provided on the cavity, or is removed.
[0027] In the present invention, the term "membrane" refers not
only to a vibrating portion provided on the cavity, but also to a
portion provided between the cavities and a portion provided on the
outer side of the cell on the outermost periphery, because they are
formed as one thin film.
[0028] In the present invention, the upper electrode can be formed
by a film made of a choice from metal, a low-resistance amorphous
silicon, and a low-resistance oxide semiconductor. The membrane may
also function as the upper electrode. Further, when the upper
electrode is provided at the membrane, it may be located on any of
the upper and lower sides of the membrane, or may be provided
between membranes.
[0029] The lower electrode can be formed of any material that has a
low electrical resistance, for example, a doped single-crystal
silicon substrate, a doped polycrystal silicon film, a
single-crystal silicon substrate having a doped region serving as a
lower electrode, a doped amorphous silicon, an oxide semiconductor,
or a metal film. The substrate can also function as the lower
electrode.
[0030] It is conceivable that variation in displacement amount of
the membrane among the cells is reduced by the configuration of the
electromechanical transducer of the present invention for the
following reason: In a peripheral edge portion of the cell on the
outermost periphery of the element, the structures of the membrane
and the insulating layer (e.g., the joint area between the membrane
and the insulating layer) are identical or close to those of the
other cells. Thus, the distribution of internal stress of the
membrane in the cell on the outermost periphery is identical or
close to that of the other cells. Hence, it is conceivable that the
effect of reducing variation in displacement amount of the membrane
among the cells can be obtained by forming a groove in a portion of
the membrane on the outer periphery of the element.
[0031] In the following first to fourth embodiments, a groove is
provided around each cell (grooves are provided between cavities).
However, in the present invention, the difference in initial
displacement amount can be reduced as long as a groove is provided
on an element basis (the groove is provided at a position at a
predetermined distance from the cavities of the cells on the
outermost periphery of the element), instead of being provided on a
cell basis.
First Embodiment
[0032] A first embodiment of the present invention will be
described below. FIG. 1A is a top view of an element 101 of the
first embodiment, and FIG. 1B is a cross-sectional view taken along
broken line IB-IB of FIG. 1A. Referring to FIGS. 1A and 1B, the
element 101 includes cells 102 arranged in a plane. Each cell 102
includes a membrane 103, an insulating layer 104, a cavity 105
formed by a recess provided in the insulating layer 104, an upper
electrode 106, and a lower electrode 107. The upper electrode 106
and the lower electrode 107 in each cell 102 are connected
electrically. All upper electrodes 106 are electrically connected
by lines 108, and the lower electrodes 107 are electrically
isolated from one another. Further, grooves 109 are provided in an
upper surface of the membrane 103 and on the outer peripheries of
the cells 102 (in other words, grooves 109 are provided between the
cavities 105). In the first embodiment, the grooves 109 are
connected to surround the cavities 105 at peripheral edge portions
of the cells 102, and the depth of the grooves 109 is smaller than
the thickness of the membrane 103. Since the joint area between the
membrane 103 and the insulating layer 104 does not change,
variation in displacement amount of the membrane among the cells
can be reduced without decreasing the joint strength between the
membrane 103 and the insulating layer 104 supporting the membrane
103.
[0033] To verify the advantages of the present invention, the
initial displacement amount of the membrane was calculated using a
finite element method. The initial displacement amount of the
membrane is the amount of displacement caused by a resultant force
of the internal stress in the membrane and the pressure applied by
the difference in atmospheric pressure between the interior and
exterior of the cavity (about one atmospheric pressure=101325 Pa).
As the internal stress to be applied, a thermal contraction stress
generated by the temperature difference caused between the times
before and after formation of the membrane was assumed. A model of
an element in which eleven cells were arranged along each side was
prepared, and the amounts of initial displacement of the membrane
caused in the cells when the internal stress due to thermal
contraction was applied to the membrane and the insulating layer
were calculated. Analysis using the finite element method was
performed by commercially available software (ANSYS 11.0 from
ANSYS, Inc.).
[0034] FIG. 7 shows the initial displacement amounts of the
membrane in the cells that are calculated when a groove is not
provided in the membrane. This calculation result shows that the
initial displacement amount of the cell on the outermost periphery
(endmost cell) is larger than those of the other cells when a
groove is not provided.
[0035] Next, it was examined how the variation in initial
displacement amount of the membrane was changed by the difference
in shape of a groove formed on the outer side of the cell on the
outermost periphery of the element. FIGS. 9A and 9B show the
results of comparison of differences in initial displacement amount
between the center cell and the cell on the outermost periphery of
the element (hereinafter simply referred to as difference in
initial displacement amount).
[0036] FIG. 9A shows the relationship between the depth of the
groove and the difference in initial displacement amount. As shown
in FIG. 8, the depth of the groove represents a length of the
groove in a direction perpendicular to a surface on which the cells
are arranged, and the width of the groove represents a length of
the groove in a direction parallel to the surface on which the
cells are arranged. In FIG. 9A, the vertical axis indicates the
ratios of the difference in initial displacement in different
conditions provided in a case in which the difference in initial
displacement amount made when the depth of the groove is zero (no
groove) is one. The horizontal axis indicates the value obtained by
dividing the depth of the groove by the thickness of the membrane.
In this case, the width of the groove is fixed (fixed at 0.25 times
the inter-cavity distance) in all conditions, and the distance
between the groove and the cavity of the cell on the outermost cell
is set to be equal to the distance between the cavities
(inter-cavity distance). This result shows that the difference in
initial displacement amount decreases as the depth of the groove
increases. Accordingly, it is preferable that the groove penetrate
the membrane into the insulating layer serving as the support
portion.
[0037] FIG. 9B shows the relationship between the width of the
groove and the difference in initial displacement amount. The
vertical axis indicates the ratios of the difference in initial
displacement in different conditions provided in a case in which
the difference in initial displacement amount made when the depth
of the groove is zero (no groove) is one. The horizontal axis
indicates the value obtained by dividing the width of the groove by
the inter-cavity distance. The depth of the groove is fixed (fixed
at 1.5 times the thickness of the membrane) in all conditions, and
the distance between the groove and the cavity of the cell on the
outermost periphery is set to be equal to the inter-cavity
distance. This result shows that the difference in initial
displacement amount decreases as the width of the groove increases.
Further, the difference in initial displacement amount can be
reduced by about 40% by setting the width of the groove to be 10%
of the inter-cavity distance.
[0038] In addition, the difference in initial displacement
corresponding to the distance between the groove and the cell on
the outermost periphery ("cavity-groove distance" in FIG. 8) was
compared. FIG. 9C shows the result of comparison. The vertical axis
indicates the ratios of the difference in initial displacement
provided in a case in which the difference in initial displacement
amount made when there is no groove is one. The horizontal axis
indicates the value obtained by dividing the distance between the
groove and the outermost cell by the inter-cavity distance (gap).
In all conditions, the depth of the groove is fixed (fixed at 1.5
times the thickness of the membrane), and the width of the groove
is also fixed (fixed at 0.25 times the inter-cavity distance). This
result shows that the effect of reducing the difference in initial
displacement amount increases as the distance between the groove
and the outermost cell decreases. However, in order to prevent the
strength of the support portion supporting the membrane of the
outermost cell from decreasing, the distance between the groove and
the outermost cell is preferably more than or equal to 50% of the
inter-cavity distance. Further, in order to reduce the difference
in initial displacement amount, the distance between the groove and
the outermost cell is preferably less than or equal to 200% of the
inter-cavity distance, more preferably, less than or equal to the
inter-cavity distance.
[0039] In the present invention, as shown in FIG. 1, grooves may be
provided in portions of the membrane between the cavities. That is,
the groove may be formed not only on the outer periphery of the
element, but also on the outer periphery of each cell. FIGS. 10A
and 10B show the results of comparison of differences in initial
displacement amount according to differences in groove shape caused
when a groove is provided on the outer periphery of each cell. FIG.
10A shows the comparison result obtained in a case in which the
difference in initial displacement amount made when the depth of
the groove is zero (no groove) is one. The horizontal axis
indicates the value obtained by dividing the depth of the groove by
the thickness of the membrane. In this case, in all conditions, the
width of the groove is fixed at 0.25 times the inter-cavity
distance, and the distance between the groove and the cell on the
outermost periphery is set to be equal to the inter-cavity
distance. This result shows that the difference in initial
displacement amount decreases as the depth of the groove increases.
Further, the difference in initial displacement amount can be
reduced even when the groove does not penetrate the membrane.
[0040] FIG. 10B shows the comparison result obtained in a case in
which the difference in initial displacement amount made when the
width of the groove is zero (no groove) is one. The horizontal axis
indicates the value obtained by dividing the width of the groove by
the inter-cavity distance (gap). In all conditions, the depth of
the groove is fixed at 1.5 times the thickness of the membrane, and
the distance between the groove and the cell on the outermost
periphery is set to be equal to the inter-cavity distance. This
result shows that the difference in initial displacement amount
decreases as the width of the groove increases. By setting the
width of the groove to be 10% of the inter-cavity distance, the
difference in initial displacement amount can be reduced by about
40%.
Second Embodiment
[0041] Referring to FIGS. 2A and 2B, in a second embodiment,
grooves 109 are intermittently provided in portions of lines
surrounding cavities 105 on peripheral edge portions of cells 102.
FIG. 2A is a top view of an element of the second embodiment, and
FIG. 2B is a cross-sectional view taken along broken line IIB-IIB
of FIG. 2A. The grooves 109 penetrate a membrane 103, but are not
provided in an insulating layer 104 serving as a support portion.
In the second embodiment, since the insulating layer 104 can be
used as an etching stop layer during formation of the grooves 109,
the grooves 109 can be formed relatively easily.
Third Embodiment
[0042] In a third embodiment, grooves 109 are intermittently
provided in portions of lines surrounding cavities 105 on
peripheral edge portions of cells 102, as in the second embodiment
shown in FIG. 2A. Further, as shown in FIG. 3, the grooves 109
penetrate a membrane 103 and reach an insulating layer 104 under
the membrane 103. In the third embodiment, since the grooves 109
can have a depth larger than the thickness of the membrane 103, a
great effect of reducing the differences in displacement amount of
the membrane among the cells can be achieved.
Fourth Embodiment
[0043] In a fourth embodiment, the present invention is applied to
a CMUT in which cells 102 have a shape different from the square
shape. When the cells 102 are circular, as shown in FIG. 4, grooves
109 each shaped like an arc having a radius larger than that of the
cell 102 and being concentric with the cell 102 are provided in
peripheral edge portions of the cells 102.
Production Method
[0044] With reference to FIGS. 5A to 5G, a description will be
given of a production method for a CMUT including grooves provided
in peripheral edge portions of cells, as in the above-described
first embodiment. This production method is based on the CMUT
production method disclosed in U.S. Pat. No. 6,958,255. The
production method for the electromechanical transducer of the
present invention is not limited to this production method. For
example, a sacrifice layer may be formed on a substrate, a membrane
may be formed on the sacrifice layer, and the sacrifice layer may
be etched to form cavities (surface micromachining). FIGS. 5A to 5G
correspond to the following steps (a) to (g), respectively. [0045]
(a) Silicon oxide layers 202 and 203 are respectively formed on
opposite surfaces of a SOI (Silicon On Insulator) substrate 201.
[0046] (b) Through holes 204 are formed in portions of the silicon
oxide layer 202 where cavities of cells are to be formed, thereby
forming a device substrate 205. [0047] (c) A silicon oxide layer
210 is formed on an upper surface of a through line substrate 209
including a lower electrode 206, a through line 207, and a pad 208.
[0048] (d) The portion of the silicon oxide layer 202 remaining on
the device substrate 205 is joined to the silicon oxide layer 210
on the upper surface of the through line substrate 209. [0049] (e)
The layers other than the silicon oxide layer 202 of the device
substrate 205 and a membrane 211 of the SOI substrate 201 are
removed so that the silicon oxide layer 202 and the device layer
211 remain on the through line substrate 209, and upper electrodes
212 are formed on an upper surface of the membrane 211. [0050] (f)
Portions of the membrane 211 on the outer peripheries of the cells
are at least partly etched to form grooves 215. In this case, the
depth of the grooves 215 is smaller than the thickness of the
membrane 211. To form grooves 215 having a desired depth, for
example, the etching time can be adjusted in accordance with the
etching rate of the membrane 211 checked beforehand. [0051] (g) The
pad 208 on a lower surface of the through line substrate 209 is
joined to a pad 214 on an upper surface of a circuit board 213.
[0052] In the above-described step (f), the grooves 215 can be
formed so as to have a depth equal to the thickness of the membrane
211, as shown in FIG. 6B1. When the membrane 211 is formed of
single-crystal silicon and the silicon oxide layer 202 is formed of
silicon oxide, an etching material, such as carbon tetrafluoride,
which is insensitive to silicon oxide and sensitive to
single-crystal silicon is used. This allows grooves penetrating the
membrane 211 to be formed easily. By further etching the silicon
oxide layer 202 subsequently to the step shown in FIG. 6B1, deeper
grooves 215 can be formed as shown in FIG. 6B2. When the membrane
211 and the silicon oxide layer 202 are formed by the same
materials as above, an etching material, such as silicon
hexafluoride, which is insensitive to single-crystal silicon and
sensitive to silicon oxide is used. This allows grooves 215 to be
formed by etching portions of the silicon oxide layer 202 under the
grooves 215 in the membrane 211, with the membrane 211 used as a
mask.
[0053] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all such modifications and
equivalent structures and functions.
[0054] This application claims the benefit of Japanese Patent
Application No. 2009-096145 filed Apr. 10, 2009, which is hereby
incorporated by reference herein in its entirety.
* * * * *