U.S. patent application number 15/918502 was filed with the patent office on 2018-12-27 for manufacturing method of mems sensor.
The applicant listed for this patent is Hitachi, Ltd.. Invention is credited to Atsushi ISOBE, Masaharu KINOSHITA, Kazuo ONO, Noriyuki SAKUMA, Tomonori SEKIGUCHI, Keiji WATANABE.
Application Number | 20180370793 15/918502 |
Document ID | / |
Family ID | 64691965 |
Filed Date | 2018-12-27 |
United States Patent
Application |
20180370793 |
Kind Code |
A1 |
KINOSHITA; Masaharu ; et
al. |
December 27, 2018 |
Manufacturing Method of Mems Sensor
Abstract
A manufacturing method of a MEMS sensor includes a step of, by
irradiating a first hole formed in a second layer on a
semiconductor substrate with a focused ion beam for a first
predetermined time, forming a first sealing film, which seals the
first hole, on the first hole, and a step of, by irradiating a
second hole formed in the second layer with a focused ion beam for
a second predetermined time, forming a second sealing film, which
seals the second hole, on the second hole. At this time, each of
the first predetermined time and the second predetermined time is a
time in which thermal equilibrium of the second layer is
maintainable, and the step of forming the first sealing film and
the step of forming the second sealing film are performed
repeatedly.
Inventors: |
KINOSHITA; Masaharu; (Tokyo,
JP) ; ISOBE; Atsushi; (Tokyo, JP) ; ONO;
Kazuo; (Tokyo, JP) ; SAKUMA; Noriyuki; (Tokyo,
JP) ; SEKIGUCHI; Tomonori; (Tokyo, JP) ;
WATANABE; Keiji; (Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Hitachi, Ltd. |
Tokyo |
|
JP |
|
|
Family ID: |
64691965 |
Appl. No.: |
15/918502 |
Filed: |
March 12, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B81B 3/0021 20130101;
B81B 2201/0292 20130101; B81C 2201/0143 20130101; B81B 2203/0315
20130101; B81C 1/00293 20130101; G01N 29/2406 20130101; B81C
2203/0145 20130101; B06B 1/0292 20130101; B81B 2201/0271 20130101;
B81C 1/00047 20130101; B81C 1/00817 20130101 |
International
Class: |
B81C 1/00 20060101
B81C001/00; B81B 3/00 20060101 B81B003/00; B06B 1/02 20060101
B06B001/02; G01N 29/24 20060101 G01N029/24 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 26, 2017 |
JP |
2017-123905 |
Claims
1. A manufacturing method of a MEMS sensor, comprising the steps
of: (a) preparing a substrate on which a first layer and a second
layer on the first layer via a cavity are formed and including a
first hole and a second hole that are formed in the second layer in
such a way as to communicate with the cavity; (b) after the step
(a), by irradiating the first hole with a focused ion beam for a
first predetermined time, forming a first sealing film that seals
the first hole on the first hole; and (c) after the step (b), by
irradiating the second hole with a focused ion beam for a second
predetermined time, forming a second sealing film that seals the
second hole on the second hole, wherein each of the first
predetermined time and the second predetermined time is a time in
which thermal equilibrium of the second layer is maintainable, and
wherein the step (b) and the step (c) are performed repeatedly.
2. The manufacturing method of a MEMS sensor according to claim 1,
wherein the MEMS sensor includes a plurality of first electrodes
and a plurality of second electrodes that are disposed opposite to
each other with a plurality of the cavities interposed therebetween
in a film thickness direction.
3. The manufacturing method of a MEMS sensor according to claim 1,
wherein each of the first predetermined time and the second
predetermined time is an irradiation time of the focused ion beam
that keeps a temperature of the second layer upon irradiation with
the focused ion beam below a melting point of the second layer.
4. The manufacturing method of a MEMS sensor according to claim 1,
wherein each of the first layer and the second layer is a silicon
nitride film, wherein the first hole and the second hole are formed
in the silicon nitride film serving as the second layer, and
wherein each of the first sealing film and the second sealing film
is a film containing a metal.
5. The manufacturing method of a MEMS sensor according to claim 1,
wherein a flow rate of a gas supplied when the first sealing film
and the second sealing film are formed is controlled to control a
sealing pressure of the first sealing film and the second sealing
film.
6. The manufacturing method of a MEMS sensor according to claim 1,
wherein a focused ion beam device that emits the focused ion beam
includes a mask having a double-layer structure composed of a first
mask and a second mask, and wherein, at at least any of the step
(b) or the step (c), when the focused ion beam is emitted, the
focused ion beam is caused to pass through a third opening formed
by overlapping a first opening of the first mask with a second
opening of the second mask.
7. The manufacturing method of a MEMS sensor according to claim 1,
wherein a focused ion beam device that emits the focused ion beam
includes a sample stage holding the substrate, the sample stage
having a substrate holding surface tilted relative to a horizontal
direction, and wherein, at at least any of the step (b) or the step
(c), the focused ion beam is emitted onto the substrate held on the
sample stage such that the substrate is tilted relative to the
horizontal direction.
8. A manufacturing method of a MEMS sensor, comprising the step of:
on a substrate, forming a first layer and a second layer on the
first layer via a cavity, and by irradiating each of a plurality of
holes formed in the second layer in such a way as to communicate
with the cavity with a focused ion beam having a predetermined beam
current density, forming a plurality of sealing films each sealing
each of the plurality of holes, on the plurality of holes,
respectively, wherein the predetermined beam current density is a
beam current density at which thermal equilibrium of the second
layer is maintainable, and wherein the plurality of sealing films
are formed simultaneously.
9. The manufacturing method of a MEMS sensor according to claim 8,
wherein the MEMS sensor includes a plurality of first electrodes
and a plurality of second electrodes that are disposed opposite to
each other with a plurality of the cavities interposed therebetween
in a film thickness direction.
10. The manufacturing method of a MEMS sensor according to claim 8,
wherein the predetermined beam current density is a current density
of the focused ion beam that keeps a temperature of the second
layer upon irradiation with the focused ion beam below a melting
point of the second layer.
11. The manufacturing method of a MEMS sensor according to claim 8,
wherein each of the first layer and the second layer is a silicon
nitride film, wherein the plurality of holes are formed in the
silicon nitride film serving as the second layer, and wherein each
of the plurality of sealing films is a film containing a metal.
12. The manufacturing method of a MEMS sensor according to claim 8,
wherein a flow rate of a gas supplied when the plurality of sealing
films are formed is controlled to control a sealing pressure of the
plurality of sealing films.
13. The manufacturing method of a MEMS sensor according to claim 8,
wherein a focused ion beam device that emits the focused ion beam
includes a mask having a double-layer structure composed of a first
mask and a second mask, and wherein, upon irradiation with the
focused ion beam at the step of forming the plurality of sealing
films, the focused ion beam is caused to pass through a third
opening formed by overlapping a first opening of the first mask
with a second opening of the second mask.
14. The manufacturing method of a MEMS sensor according to claim 8,
wherein a focused ion beam device that emits the focused ion beam
includes a sample stage holding the substrate, the sample stage
having a substrate holding surface tilted relative to a horizontal
direction, and wherein, at the step of forming the plurality of
sealing films, the focused ion beam is emitted onto the substrate
held on the sample stage such that the substrate is tilted relative
to the horizontal direction.
15. A manufacturing method of a MEMS sensor, comprising the steps
of: (a) on a substrate, forming a first layer, a second layer on
the first layer, and a third layer between the first layer and the
second layer; (b) after the step (a), by irradiating a first hole
formation spot of the second layer with a focused ion beam for a
first predetermined time, forming a first hole in the second layer
such that the first hole reaches the third layer; (c) after the
step (b), by irradiating a second hole formation spot of the second
layer with a focused ion beam for a second predetermined time,
forming a second hole in the second layer such that the second hole
reaches the third layer; (d) after the step (c), removing the third
layer through the first hole and the second hole and then forming a
cavity between the first layer and the second layer in such a way
as to communicate with the first hole and the second hole; and (e)
after the step (d), forming a sealing film on each of the first
hole and the second hole, wherein each of the first predetermined
time and the second predetermined time is a time in which thermal
equilibrium of the second layer is maintainable, and wherein,
before the step (d), the step (b) and the step (c) are performed
repeatedly.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] The present application claims priority from Japanese Patent
Application No. 2017-123905 filed on Jun. 26, 2017, the content of
which is hereby incorporated by reference into this
application.
TECHNICAL FIELD OF THE INVENTION
[0002] The present invention relates to a manufacturing method of a
MEMS sensor.
BACKGROUND OF THE INVENTION
[0003] For manufacture of a MEMS (Micro Electro Mechanical System)
sensor, a sensor manufacturing process through direct modeling by
use of a focused ion beam (FIB) has been studied to achieve a
manufacturing process with a shorter TAT (turn-around time).
Manufacturing a MEMS sensor having a cavity, in particular,
requires a process of sealing/protecting an element of the sensor
for the purpose of making the cavity vacuum to improve the
performance of the sensor.
[0004] Japanese Patent Application Laid-Open Publication No.
2009-4591 (Patent Document 1) discloses a technique in which a
micro-sampling piece is extracted and a corresponding sampling hole
formed in a semiconductor wafer is repaired by use of a focused ion
beam.
SUMMARY OF THE INVENTION
[0005] In a MEMS sensor such as an ultrasonic sensor, when a cavity
formed between an upper electrode and a lower electrode is sealed
by a focused ion beam, the number of sealing spots is large (a
total sealing volume is large), thereby causing a longer
manufacturing time.
[0006] Meanwhile, in order to eliminate the above problem of the
longer manufacturing time, a deposition rate by the focused ion
beam is increased, whereby another problem that the MEMS sensor is
destroyed by heat is caused.
[0007] Note that the above patent document (Japanese Patent
Application Laid-Open Publication No. 2009-4591) does not describe
structural destruction by heat, and describes only the method of
filling up a single hole without describing a method of efficiently
filling up a plurality of holes.
[0008] An object of the present invention is to provide a technique
in which, in manufacture of a MEMS sensor, a TAT can be shortened
and thermal destruction of the MEMS sensor can be prevented.
[0009] Other objects and novel characteristics of the present
invention will be apparent from the description of the present
specification and the accompanying drawings.
[0010] The typical ones of the embodiments disclosed in the present
application will be briefly described as follows.
[0011] A manufacturing method of a MEMS sensor according to one
embodiment, the method includes the steps of:
[0012] (a) preparing a substrate on which a first layer and a
second layer on the first layer via a cavity are formed and
including a first hole and a second hole that are formed in the
second layer in such a way as to communicate with the cavity;
[0013] (b) after the step (a), by irradiating the first hole with a
focused ion beam for a first predetermined time, forming a first
sealing film that seals the first hole on the first hole; and
[0014] (c) after the step (b), by irradiating the second hole with
a focused ion beam for a second predetermined time, forming a
second sealing film that seals the second hole on the second hole.
In this method, each of the first predetermined time and the second
predetermined time is a time in which thermal equilibrium of the
second layer is maintainable, and the step (b) and the step (c) are
performed repeatedly.
[0015] A manufacturing method of a MEMS sensor according to another
embodiment, the method includes the step of:
[0016] on a substrate, forming a first layer and a second layer on
the first layer via a cavity, and by irradiating each of a
plurality of holes formed in the second layer in such a way as to
communicate with the cavity with a focused ion beam having a
predetermined beam current density, forming a plurality of sealing
films each sealing each of the plurality of holes, on the plurality
of holes, respectively. Further, the predetermined beam current
density is a beam current density at which thermal equilibrium of
the second layer is maintainable, and the plurality of sealing
films are formed simultaneously.
[0017] Also, a manufacturing method of a MEMS sensor according to
another embodiment, the method includes the steps of:
[0018] (a) on a substrate, forming a first layer, a second layer on
the first layer, and a third layer between the first layer and the
second layer;
[0019] (b) after the step (a), by irradiating a first hole
formation spot of the second layer with a focused ion beam for a
first predetermined time, forming a first hole in the second layer
such that the first hole reaches the third layer;
[0020] (c) after the step (b), by irradiating a second hole
formation spot of the second layer with a focused ion beam for a
second predetermined time, forming a second hole in the second
layer such that the second hole reaches the third layer;
[0021] (d) after the step (c), removing the third layer through the
first hole and the second hole and then forming a cavity between
the first layer and the second layer in such a way as to
communicate with the first hole and the second hole; and
[0022] (e) after the step (d), forming a sealing film on each of
the first hole and the second hole. Each of the first predetermined
time and the second predetermined time is a time in which thermal
equilibrium of the second layer is maintainable, and before the
step (d), the step (b) and the step (c) are performed
repeatedly.
[0023] Effects obtained by the typical ones of the inventions
disclosed in the present application will be briefly described as
follows.
[0024] In manufacture of a MEMS sensor, the TAT can be shortened,
and thermal destruction of the MEMS sensor can be prevented.
BRIEF DESCRIPTIONS OF THE DRAWINGS
[0025] FIG. 1 is a plan view of a principle portion of a MEMS
sensor according to a first embodiment of the present
invention;
[0026] FIG. 2 is a cross-sectional view of a structure taken along
a line A-A of FIG. 1;
[0027] FIG. 3 is a schematic diagram of a focused ion beam device
used in the first embodiment;
[0028] FIG. 4 is a graph indicating a relation between a sealed
number and a manufacturing time, which has been studied by the
inventors;
[0029] FIG. 5 is a graph indicating a relation between a beam
current density and a temperature increase during a film-forming
process, which has been studied by the inventors;
[0030] FIG. 6 is a graph indicating a relation between an ion beam
irradiation time and a temperature increase, which has been studied
by the inventors;
[0031] FIG. 7 is a graph indicating a relation between an ion beam
irradiation time and a beam current density according to a
manufacturing method of the MEMS sensor of the first
embodiment;
[0032] FIG. 8 is a graph indicating a relation between an ion beam
irradiation time and a beam current density according to a first
modification example of the manufacturing method of the MEMS sensor
of the first embodiment;
[0033] FIG. 9 is a graph indicating a manufacturing time taken by
each manufacturing method of the MEMS sensor according to the first
embodiment;
[0034] FIG. 10 is a cross-sectional view of a structure of a
principle portion of a MEMS sensor according to a second
modification example of the first embodiment;
[0035] FIG. 11 is a cross-sectional view of a structure of a
principle portion of a MEMS sensor according to a third
modification example of the first embodiment before a sealing
process;
[0036] FIG. 12 is a cross-sectional view of the structure of the
principle portion of the MEMS sensor according to the third
modification example of the first embodiment after the sealing
process;
[0037] FIG. 13 is a schematic diagram of a structure of a focused
ion beam device according to a fourth modification example of the
first embodiment;
[0038] FIG. 14 is a plan view showing a method of using a
projection mask in the focused ion beam device of FIG. 13;
[0039] FIG. 15 is a schematic diagram of a structure of a sampling
stage in a focused ion beam device according to a fifth
modification example of the first embodiment;
[0040] FIG. 16 is a cross-sectional view of a principle portion in
a base forming process in a manufacturing method of a MEMS sensor
according to a second embodiment of the present invention;
[0041] FIG. 17 is a cross-sectional view of a principle portion in
a hole forming process in the manufacturing method of the MEMS
sensor according to the second embodiment of the present
invention;
[0042] FIG. 18 is a cross-sectional view of a principle portion in
a sacrifice layer removing process in the manufacturing method of
the MEMS sensor according to the second embodiment of the present
invention; and
[0043] FIG. 19 is a cross-sectional view of a principle portion in
a sealing process in the manufacturing method of the MEMS sensor
according to the second embodiment of the present invention.
DESCRIPTIONS OF THE PREFERRED EMBODIMENTS
First Embodiment
[0044] A structure of an ultrasonic sensor, which is one embodiment
of a MEMS sensor of the present invention, will be described with
reference to the drawings.
[0045] <Structure of Ultrasonic Sensor>
[0046] FIG. 1 is a plan view of a principle portion of a MEMS
sensor according to a first embodiment of the present invention.
FIG. 2 is a cross-sectional view of a structure taken along a line
A-A of FIG. 1.
[0047] An ultrasonic sensor (MEMS sensor) 4 according to the first
embodiment is a capacitive micro-machined ultrasonic transducer
(CMUT) to which a semiconductor technique is applied.
[0048] The ultrasonic sensor 4 of the first embodiment shown in
FIGS. 1 and 2 is manufactured on a substrate such as a
semiconductor substrate 1 by patterning a plurality of micro
sensors by lithography. Each micro sensor is structured such that a
cavity 8 having a vacuum atmosphere is formed in an insulating
layer, with electrodes being formed below and above the cavity
8.
[0049] An electrostatic force is applied between these electrodes
to cause an electrode film to vibrate, thereby transmitting an
ultrasonic signal. Upon receiving a signal, the micro sensor
converts a displacement amount of the electrode film into a change
in capacitance to detect a signal.
[0050] More specifically, in the structure of the ultrasonic sensor
4 of the first embodiment shown in FIGS. 1 and 2, a plurality of
micro sensors (ultrasonic sensors 4, which are also called cells)
each made into hexagon in a plan view are disposed in the
insulating layer on the semiconductor substrate 1 to spread across
the insulating layer.
[0051] In the first embodiment, description will be given of a case
in which each ultrasonic sensor 4 has a silicon oxide film
(SiO.sub.2 film) 9 which is the insulating layer formed on the
semiconductor substrate 1 and composed of a first layer and a
second layer. Specifically, a silicon oxide film (SiO.sub.2 film)
9a serving as the first layer is formed on the semiconductor
substrate 1, and a silicon oxide film (SiO.sub.2 film) 9b serving
as the second layer is formed on the silicon oxide film 9a via the
cavity 8. Further, in the silicon oxide film 9a, a lower electrode
(first electrode) 6 is formed. Meanwhile, in the silicon oxide film
9b, an upper electrode (second electrode) 7 having a hexagonal
shape in a plan view is formed.
[0052] That is, the ultrasonic sensor 4 includes a plurality of the
lower electrodes 6 and a plurality of the upper electrodes 7 that
are disposed opposite to each other with a plurality of the
cavities 8 interposed therebetween in a film thickness
direction.
[0053] Note that, as shown in FIG. 1, each pair of upper electrodes
7 adjacent to each other are electrically connected through a
wiring 2, so that one upper electrode 7 and another upper electrode
7 distant therefrom are also electrically connected through any of
the wirings 2.
[0054] In addition, as shown in FIG. 2, the cavity 8 formed between
the lower electrode 6 and the upper electrode 7 has a plurality of
holes 3 formed therein in such a way as to communicate with the
cavity 8. For example, in the silicon oxide film 9b on the cavity
8, a first hole 3a communicating with the cavity 8 is formed, and
similarly, a second hole 3b communicating with the cavity 8 is
formed in the silicon oxide film 9b on an opposite side to the
first hole 3a.
[0055] Then, a sealing film 5 which seals each of the plurality of
holes 3 is formed on an opening which opens in a front surface of
the silicon oxide film 9b of each of the holes 3. Specifically, the
opening of the first hole 3a which opens in the front surface of
the silicon oxide film 9b is covered with a first sealing film 5a
formed to seal the first hole 3a, and similarly, the opening of the
second hole 3b which opens in the front surface of the silicon
oxide film 9b is covered with a second sealing film 5b formed to
seal the second hole 3b.
[0056] In the ultrasonic sensor 4 shown in FIGS. 1 and 2, the upper
electrode 7 is formed into a hexagon in a plan view, and the cavity
8 corresponding to the upper electrode 7 is also formed into a
hexagon in a plan view. As a result, the holes 3 each communicating
with the cavity 8 are formed at locations close to the six corners
of the hexagonal upper electrode 7, respectively. Accordingly, the
plurality of sealing films 5, each of which seals each
corresponding hole 3, are also formed at locations close to the six
corners of the hexagonal upper electrode 7, respectively. By way of
example, in a plan view, each of the holes 3 and each of the
sealing films 5 sealing each hole 3 are provided on a diagonal line
L of the hexagonal upper electrode 7. Thus, in each of the
ultrasonic sensors 4, the plurality of holes 3 and the plurality of
sealing films 5 can be disposed efficiently relative to the
hexagonal upper electrode 7 and the hexagonal cavity 8.
[0057] In this case, the sealing film 5 such as the first sealing
film 5a and the second sealing film 5b is a film deposited by a
focused ion beam (FIB), which will be described later.
[0058] As described above, the plurality of ultrasonic sensors 4,
which are the plurality of micro sensors, are formed on the
semiconductor substrate 1. Each of the ultrasonic sensors 4 has the
plurality of holes 3 and the plurality of sealing films 5 sealing
the corresponding holes 3.
[0059] <Focused Ion Beam Device>
[0060] A schematic configuration of a focused ion beam device that
emits a focused ion beam for forming the sealing film 5 of the
first embodiment will then be described with reference to FIG. 3.
FIG. 3 is a schematic diagram of the focused ion beam device used
in the first embodiment to form the sealing film 5.
[0061] A focused ion beam device 20 shown FIG. 3 includes a vacuum
chamber (not shown) in which a focused ion beam 21 is emitted onto
a sample (a substrate such as the semiconductor substrate 1) 26.
The vacuum chamber houses an ion source 22 that discharges gallium
ions, an aperture 23 and a condenser lens 24 that condense an ion
beam, an objective lens 25 that focuses the ion beam on a front
surface of the sample 26, and a sample stage 27 holding the sample
26.
[0062] The ion beam which is condensed and focused on the front
surface of the sample 26 is emitted onto the front surface of the
sample 26 to process the sample 26.
[0063] <Matters Studied by Inventors of Present
Application>
[0064] FIG. 4 is a graph indicating a relation between a sealed
number and a manufacturing time, which has been studied by the
inventors. FIG. 5 is a graph indicating a relation between a beam
current density and a temperature increase during a film-forming
process, which has been studied by the inventors. FIG. 6 is a graph
indicating a relation between an ion beam irradiation time and a
temperature increase, which has been studied by the inventors.
[0065] FIG. 4 is a graph indicating temperature increases during a
film-forming process using focused ion beams having three types of
beam current densities, in terms of the relation between the sealed
number and the manufacturing time. Three types of beam current
densities include, for example, 15 nA/.mu.m.sup.2, 30
nA/.mu.m.sup.2, and 100 nA/.mu.m.sup.2. The beam current density of
15 nA/.mu.m.sup.2 does not cause thermal structural destruction (a
curve marked with OK in FIG. 4). The beam current densities of 30
nA/.mu.m.sup.2 and 100 nA/.mu.m.sup.2, however, cause thermal
structural destruction (curves marked with NG in FIG. 4).
[0066] Note that, in the case of the ultrasonic sensor 4, a test
cell has about 17,280 sealing spots, and one semiconductor chip has
about 331,776 sealing spots, which means the number of sealing
spots is significantly large. Under such circumstance, the curve
representing the beam current density of 15 nA/.mu.m.sup.2 that
does not cause thermal structural destruction (OK) defines a
manufacturing time for 17,280 sealing spots of the test cell as 144
hours, which is a long manufacturing time to take.
[0067] Next, FIG. 5 is a graph indicating the relation between a
beam current density and a temperature increase during the
film-forming process for the case of the MEMS sensor having the
cavity and the case of the MEMS sensor having no cavity. FIG. 5
indicates that, when the beam current density is 15 nA/.mu.m.sup.2,
a temperature increase in each case of the MEMS sensor having the
cavity and the MEMS sensor having no cavity is lower than
1600.degree. C., which is the melting point of the silicon oxide
film, and in both cases, thermal destruction of the MEMS sensors
does not occur (OK).
[0068] When the beam current density is 30 nA/.mu.m.sup.2, however,
the temperature increase of the MEMS sensor having no cavity does
not reach 1600.degree. C., while the temperature increase of the
MEMS sensor having the cavity significantly exceed 1600.degree. C.,
which is indicated as NG in the graph.
[0069] It is therefore understood from FIG. 5 that, in the MEMS
sensor having the cavity, the cavity prevents heat conduction
through the MESE sensor to slow down heat conduction, resulting in
faster temperature increase and high temperature. In contrast, in
the MEMS sensor having no cavity, heat conduction through the MEMS
sensor is not prevented and performed faster, resulting in slow
temperature increase and low temperature.
[0070] Next, FIG. 6 is a graph indicating the relation between an
irradiation time and a temperature increase for each beam current
density. In the case of the beam current density of 15
nA/.mu.m.sup.2 (curve A), the temperature increase saturates in a
low-temperature range below 1600.degree. C. In each case of the
beam current density of 30 nA/.mu.m.sup.2 (curve B) and beam
current densities of 100 nA/.mu.m.sup.2 (curve C), 1
.mu.A/.mu.m.sup.2 (curve D), and 10 .mu.A/.mu.m.sup.2 (curve E)
which are larger than 30 nA/.mu.m.sup.2, however, a saturation
temperature gets higher and higher.
[0071] In view of this, according to a manufacturing method of the
MEMS sensor of the first embodiment, the focused ion beam 21 is
emitted under a condition defined in a range F of FIG. 6 (hatched
range where a temperature increase is low). Specifically, the range
F where the temperature of the MEMS sensor does not reach
1600.degree. C. by applying a beam having a large beam current
density for a short time is used.
[0072] A manufacturing method of the MEMS sensor of the first
embodiment will then be described in detail. FIG. 7 is a graph
indicating a relation between an ion beam irradiation time and a
beam current density.
[0073] As shown in FIGS. 1 to 7, for example, the focused ion beam
21 is emitted onto sealing spots P and Q repeatedly to deposit
thereon. In this process, the current density of the focused ion
beam 21 is made larger, and the application time of the focused ion
beam 21 is made shorter at each round of beam irradiation.
[0074] More specifically, the semiconductor substrate 1 is prepared
first, the semiconductor substrate 1 including the first hole 3a
and the second hole 3b that are formed in the silicon oxide film 9b
such that they communicate with the cavity 8 formed between the
lower electrode 6 and the upper electrode 7 on the semiconductor
substrate 1 shown in FIG. 2. Subsequently, the focused ion beam
device 20 shown in FIG. 3 irradiates the first hole 3a of the
semiconductor substrate 1 with the focused ion beam 21 for a first
predetermined time.
[0075] This process forms the first sealing film 5a, which seals
the first hole 3a, on the first hole 3a. This is equivalent to the
first sealing spot P shown in FIG. 7, for example.
[0076] Following the deposition on the first sealing spot P, the
focused ion beam device 20 shown in FIG. 3 irradiates the second
hole 3b shown in FIG. 2 with the focused ion beam 21 for a second
predetermined time in the same manner.
[0077] This process forms the second sealing film 5b, which seals
the second hole 3b, on the second hole 3b. This is equivalent to
the deposition on the second sealing spot Q, which is performed
after the deposition on the first sealing spot P shown in FIG.
7.
[0078] In these processes, each of the first predetermined time and
the second predetermined time is the time in which thermal
equilibrium of the silicon oxide film 9b can be maintained, that
is, the ion beam irradiation time to such an extent that the
silicon oxide film 9b is not destroyed by heat. Specifically, each
of the first predetermined time and the second predetermined time
is the irradiation time of the focused ion beam 21 that prevents
the temperature of the silicon oxide film 9b upon irradiation with
the focused ion beam from reaching 1600.degree. C. (that keeps the
temperature of the silicon oxide film 9b below 1600.degree. C.),
which is the melting point of the silicon oxide film 9b. In other
words, film formation is performed in the range F shown in FIG. 6.
For example, the current density of the focused ion beam 21 is 10
.mu.A/.mu.m.sup.2, and the beam irradiation time, which is
equivalent to each of the first predetermined time and the second
predetermined time, is about 1E-8 (sec.) (E: exponential
function).
[0079] Under such a condition, as shown in FIG. 7, sequential
deposition on the sealing spots P, Q, R, S, P, . . . , Q, and R is
repeated. As a result, the sealing film 5 is formed on each hole 3
shown in FIG. 2 at the sealing spots P, Q, R, S, etc., shown in
FIG. 1.
[0080] Thus, according to the manufacturing method of the MEMS
sensor of the first embodiment indicated in FIG. 7, an ion beam
having a large beam current density is applied to a sealing spot
for a short time, and this irradiation process is repeatedly
performed in order on a plurality of sealing spots. In other words,
high-rate deposition is performed using the range in which there is
no temperature increase of the silicon oxide film 9b (range F in
FIG. 6), for example.
[0081] As a result, in manufacture of the MEMS sensor (ultrasonic
sensor 4), thermal destruction of the MEMS sensor can be prevented,
while, at the same time, the TAT (Turn-Around Time) is shortened to
allow the MEMS sensor to be manufactured efficiently.
[0082] A manufacturing method of the MEMS sensor according to
modification examples of the first embodiment will then be
described.
[0083] FIG. 8 is a graph indicating a relation between an ion beam
irradiation time and a beam current density according to a first
modification example of the manufacturing method of the MEMS sensor
of the first embodiment. This graph indicates the relation between
the irradiation time and the current density of the focused ion
beam 21 which forms one sealing film 5 sealing one hole 3.
[0084] In the first modification example, by irradiating each of
the plurality of holes 3 formed in the silicon oxide film 9b in
such a way as to communicate with the cavity 8 shown in FIG. 2 with
the focused ion beam 21 having a predetermined beam current density
shown in FIG. 3, the plurality of sealing films 5 respectively
sealing the plurality of holes 3 are formed on the plurality of
holes 3, respectively. At this time, according to the first
modification example, the plurality of sealing films are formed
simultaneously on the plurality of holes 3, respectively. That is,
the plurality of sealing films 5 are formed all at once. FIG. 8 is
the graph indicating the relation between the irradiation time and
the current density of the focused ion beam 21 for forming one
sealing film 5 (sealing spot P) in the case of simultaneously
forming the plurality of sealing films 5. Accordingly, each of the
plurality of holes 3 is irradiated simultaneously with the focused
ion beam 21 at the current density and for the irradiation time
indicated in FIG. 8.
[0085] Note that, to cause the focused ion beam device 20 to
irradiate the plurality of holes 3 with the focused ion beam 21
simultaneously, a plurality of openings corresponding to the
plurality of holes 3 are formed in a mask serving as the aperture
23 so as to allow the focused ion beam 21 to pass through each of
the openings. Thus, the focused ion beam 21 can be simultaneously
emitted onto the plurality of holes 3.
[0086] Also, the above predetermined beam current density set in
the first modification example is the beam current density at which
the thermal equilibrium of the silicon oxide film 9b can be
maintained. That is, in the range shown in FIG. 6 where the
temperature of the silicon oxide film 9b does not reach the melting
point of 1600.degree. C. (thermal equilibrium range, i.e., the
range of beam current density in which the temperature of the
silicon oxide film 9b is below 1600.degree. C.), the focused ion
beam 21 having a low current density (e.g., 15 nA/.mu.m.sup.2) is
emitted simultaneously onto the plurality of holes 3 (continuously)
for a long time, as indicated in FIG. 8 (parallel deposition).
[0087] According to the first modification example, irradiation
with the ion beam having a low current density simultaneously forms
the plurality of sealing films 5, and as a result, in the
manufacture of the MEMS sensor (ultrasonic sensor 4), thermal
destruction of the MEMS sensor can be prevented, and the TAT is
shortened, so that the MEMS sensor can be manufactured
efficiently.
[0088] FIG. 9 is a graph indicating a manufacturing time taken by
each manufacturing method of the MEMS sensor according to the first
embodiment. That is, the graph indicates the results of comparison
of the MEMS sensor manufacturing times taken by respective
manufacturing methods.
[0089] In FIG. 9, "study technique" indicates a MEMS sensor
manufacturing time in the case of forming the plurality of
(predetermined number of) sealing films 5 by adopting the beam
current density of 15 nA/.mu.m.sup.2 (curve A) indicated in FIG. 6.
In this case, the manufacturing time is 144 hours.
[0090] "Short time deposition" in FIG. 9 is the case of adopting
the method described with reference to FIG. 7, and in this method,
an ion beam having a large beam current density is emitted onto a
sealing spot for a short time and this irradiation process is
repeatedly performed in order on a plurality of sealing spots to
perform deposition. When the plurality of (predetermined number of)
sealing films 5 are formed by this "short time deposition" method,
the MEMS sensor manufacturing time is 0.35 hour, and the
manufacturing time can be reduced to 1/411 of the manufacturing
time taken by the "study technique." That is, adopting the "short
time deposition" method reduces the TAT, so that the MEMS sensor
can be manufactured efficiently.
[0091] Also, "parallel deposition" in FIG. 9 indicates the case of
adopting the method described with reference to FIG. 8, and in this
method, an ion beam having a low beam current density is emitted
simultaneously onto a plurality of sealing spots continuously (for
a long time) to form the plurality of sealing films 5 all at once.
When the plurality of (predetermined number of) sealing films 5 are
formed by this "parallel deposition" method, the MEMS sensor
manufacturing time is 0.6 hour, and the manufacturing time can be
reduced to 1/240 of the manufacturing time taken by the "study
technique." That is, adopting the "parallel deposition" method also
reduces the TAT, so that the MEMS sensor can be manufactured
efficiently, as in the case of adopting the "short time deposition"
method.
[0092] Note that the manufacturing time indicated by "parallel
deposition" in FIG. 9 is the manufacturing time taken when, for
example, about 240 sealing films 5 are formed in a 800
.mu.m.times.800 .mu.m area.
[0093] A second modification example of the first embodiment will
then be described.
[0094] FIG. 10 is a cross-sectional view of a structure of a
principle portion of a MEMS sensor according to the second
modification example of the first embodiment.
[0095] In the second modification example, as shown in FIG. 10, a
silicon nitride film (SiN film) 10, which is denser than the
silicon oxide film 9 shown in FIG. 2, is formed as an insulating
film formed on the semiconductor substrate 1. Specifically, the
MEMS sensor of the second modification example includes a silicon
nitride film (an SiN film or the first layer) 10a formed on the
semiconductor substrate 1, and a silicon nitride film (an SiN film
or the second layer) 10b formed on the silicon nitride film 10a.
The silicon nitride film 10a has the lower electrode 6 formed
therein, and the silicon nitride film 10b has the upper electrode
7, the first hole 3a, and the second hole 3b formed therein.
[0096] Moreover, each of the first sealing film 5a and the second
sealing film 5b formed on each hole 3, is a film containing a
metal. In the structure shown in FIG. 10, each of the first sealing
film 5a and the second sealing film 5b includes the silicon oxide
film 9 or the silicon nitride film 10, and a metal film 11 covering
the silicon oxide film 9 or the silicon nitride film 10. In other
words, the hole 3 is sealed with the silicon oxide film 9 or the
silicon nitride film 10, and the silicon oxide film 9 or the
silicon nitride film 10 is covered with the metal film 11. Note
that the metal film 11 is, for example, a tungsten film.
[0097] In this manner, the silicon nitride film 10 is adopted as
the insulating film formed on the semiconductor substrate 1, and
the sealing film 5 sealing the hole 3 is partially made of the
metal film 11, so that penetration of moisture from outside can be
prevented. As a result, reliability of the MEMS sensor (ultrasonic
sensor 4) can be improved.
[0098] A third modification example of the first embodiment will
then be described.
[0099] FIG. 11 is a cross-sectional view of a structure of a
principle portion of a MEMS sensor according to the third
modification example of the first embodiment before the sealing
process. FIG. 12 is a cross-sectional view of the structure of the
principle portion of the MEMS sensor according to the third
modification example of the first embodiment after the sealing
process.
[0100] In the third modification example, by controlling a pressure
in forming the sealing films 5 by the focused ion beam 21, an inner
pressure of the cavity 8 is controlled when the cavity 8 is sealed
up with the sealing films 5.
[0101] For example, in the structure before the sealing process
shown in FIG. 11, the inner pressure of the chamber (not shown) of
the focused ion beam device 20 shown in FIG. 3 is controlled by
controlling a flow rate of a gas supplied into the chamber. By this
process, a sealing pressure of the first sealing film 5a and the
second sealing film 5b to be formed, shown in FIG. 12, is
controlled. The above gas may be a gas used for film forming or may
be an inert gas supplied into the chamber.
[0102] Specifically, the flow rate of the gas supplied into the
chamber is controlled before the sealing process to control the
inner pressure of the cavity 8 as well as an external pressure to
the MEMS sensor, to 10 Pa, which is equal to a pressure at film
formation. The sealing process is performed under this condition,
that is, the first sealing film 5a and the second sealing film 5b
are formed. Hence, the inner pressure of the cavity 8 can be
controlled to 10 Pa.
[0103] Controlling the inner pressure of the cavity 8 in this
manner improves a performance of the MEMS sensor. The inner
pressure of the cavity 8 is, for example, related to a quality
factor. For this reason, controlling the inner pressure of the
cavity 8 is important to improve the performance of the MEMS
sensor.
[0104] A fourth modification example of the first embodiment will
then be described.
[0105] FIG. 13 is a schematic diagram of a structure of a focused
ion beam device according to the fourth modification example of the
first embodiment. FIG. 14 is a plan view showing a method of using
a projection mask in the focused ion beam device of FIG. 13.
[0106] In the fourth modification example, a focused ion beam
device 28 shown in FIG. 13 includes a mask having a double-layer
structure composed of a first mask and a second mask. Specifically,
this focused ion beam device 28 is provided with a first projection
mask (first mask) 12 and a second projection mask (second mask) 13
which are stacked one on top of another, in place of the aperture
23 shown in FIG. 3.
[0107] Note that, as shown in FIG. 14, the first projection mask 12
has a plurality of first openings 12a formed in such a way as to
correspond to respective locations of the holes 3 to be formed,
shown in FIG. 2, and the second projection mask 13 has a plurality
of second openings 13a formed in the same manner as the first
openings 12a. In this case, the first openings 12a and the second
openings 13a are slightly shifted in position to each other.
[0108] When the focused ion beam 21 is emitted onto the sample 26,
the plurality of first openings 12a of the first projection mask 12
are overlapped with the plurality of second openings 13a of the
second projection mask 13 to form a plurality of third openings 14
(hatched portions), through which the ion beam passes to be
condensed, as shown in FIG. 14. Overlapping the first openings 12a
of the first projection mask 12 with the second openings 13a of the
second projection mask 13 can form the third openings 14 each
smaller in area than each of the first openings 12a and the second
openings 13a.
[0109] Accordingly, causing the ion beam to pass through the third
opening 14 can make a beam diameter of the focused ion beam 21
smaller.
[0110] Using the mask having the double-layer structure in which
the first projection masks 12 and the second projection mask 13 are
overlapped with each other in this manner improves a degree of
freedom in changing a film-forming condition without increasing a
type of mask.
[0111] A fifth modification example of the first embodiment will
then be described.
[0112] FIG. 15 is a schematic diagram of a structure of a sampling
stage in a focused ion beam device according to the fifth
modification example of the first embodiment.
[0113] According to the fifth modification example, in the focused
ion beam device 20 shown in FIG. 3, a substrate holding surface 27a
of the sample stage 27 shown in FIG. 15 is tilted at an angle
.theta. relative to a horizontal direction X. Specifically, the
substrate holding surface 27a of the sample stage 27 which holds
the sample 26 is formed such that the substrate holding surface 27a
and the horizontal direction X make a predetermined angle .theta.
(e.g., .theta.=15.degree. or 30.degree.).
[0114] With this structure, in the process of forming the plurality
of sealing films 5 shown in FIG. 2, it is possible to irradiate the
sample 26 (e.g., the semiconductor substrate 1) held on the sample
stage 27 such that the sample 26 is tilted relative to the
horizontal direction X at a predetermined angle, with the focused
ion beam 21. Accordingly, when the plurality of sealing films 5 are
formed, a pitch between the films can be made smaller than a pitch
between the plurality of sealing films 5 formed on the horizontally
held sample 26.
[0115] Note that, since the plurality of holes 3 shown in FIG. 2
are not always arranged at equal intervals, allowing for adjustment
of the pitch between the sealing films 5 improves a degree of
freedom in forming the plurality of sealing films 5.
Second Embodiment
[0116] FIG. 16 is a cross-sectional view of a principle portion in
a base forming process in a manufacturing method of a MEMS sensor
according to a second embodiment of the present invention. FIG. 17
is a cross-sectional view of a principle portion in a hole forming
process in the manufacturing method of the MEMS sensor according to
the second embodiment of the present invention. FIG. 18 is a
cross-sectional view of a principle portion in a sacrifice layer
removing process in the manufacturing method of the MEMS sensor
according to the second embodiment of the present invention. FIG.
19 is a cross-sectional view of a principle portion in a sealing
process in the manufacturing method of the MEMS sensor according to
the second embodiment of the present invention.
[0117] In the second embodiment, main processes including the base
forming process to the sealing process according to the
manufacturing method of the MEMS sensor will be described.
[0118] First, the base forming process shown in FIG. 16 will be
described. On a semiconductor substrate 1, a silicon oxide film (a
first layer) 9a, a silicon oxide film (a second layer) 9b on the
silicon oxide film 9a, and a sacrifice layer 15 (a third layer)
between the silicon oxide film 9a and the silicon oxide film 9b are
formed. More specifically, the silicon oxide film (the first layer)
9a is formed as an insulating layer on the semiconductor substrate
1, and a lower electrode 6 is further formed on the silicon oxide
film 9a. Subsequently, on the lower electrode 6, another silicon
oxide film 9a is formed. Accordingly, a structure in which the
lower electrode 6 is formed in the silicon oxide film 9a is
provided. Then, the sacrifice layer 15 is further formed on an
upper layer of the silicon oxide film 9a on the lower electrode
6.
[0119] Note that the sacrifice layer 15 (the third layer) is
preferably a metal film made of, for example, titanium, tungsten,
or molybdenum. Forming the sacrifice layer 15 with the metal film
allows for highly precise formation of a cavity 8 (see FIG. 18) in
a subsequent process where the cavity 8 is formed by removing the
sacrifice layer 15.
[0120] Subsequently, the silicon oxide film (second layer) 9b
serving as the insulating layer is formed on the sacrifice layer
15, and an upper electrode 7 is further formed on the silicon oxide
film 9b. Another silicon oxide film 9b is then formed on the upper
electrode 7. Accordingly, a structure in which the upper electrode
7 is formed in the silicon oxide film 9b is provided. Hence, the
sacrifice layer 15 is formed between the lower electrode 6 and the
upper electrode 7 via the insulating layer.
[0121] Subsequently, by irradiating a first hole formation spot of
the silicon oxide film 9b with a focused ion beam 21 shown in FIG.
3 for a first predetermined time, a first hole 3a which reaches the
sacrifice layer 15 is formed in the silicon oxide film 9b, as shown
in FIG. 17. Also, by irradiating a second hole formation spot of
the silicon oxide film 9b with a focused ion beam 21 for a second
predetermined time, a second hole 3b which reaches the sacrifice
layer 15 as well is formed in the silicon oxide film 9b.
[0122] Note that, when a plurality of holes 3 are formed by cutting
processing with the focused ion beam 21 irradiated, the cutting
processing can be performed in the same manner as the ion beam
irradiation conditions shown in FIG. 7. That is, each of the first
predetermined time and the second predetermined time is the time in
which thermal equilibrium of the silicon oxide film (the second
layer) 9b can be maintained, that is, the ion beam irradiation time
to such an extent that the silicon oxide film 9b is not destroyed
by heat. Specifically, the first predetermined time and the second
predetermined time each represent the irradiation time of the
focused ion beam 21 that prevents the temperature of the silicon
oxide film 9b upon irradiation with the focused ion beam from
increasing to 1600.degree. C. (that keeps the temperature of the
silicon oxide film 9b below 1600.degree. C.), which is the melting
point of the silicon oxide film 9b.
[0123] Under the above irradiation conditions of the focused ion
beam 21, a step of forming the first hole 3a by the focused ion
beam 21 and a step of forming the second hole 3b by the focused ion
beam 21 are repeatedly performed in order.
[0124] Thus, by using the manufacturing method (ion beam
irradiation conditions) indicated in FIG. 7, irradiation with an
ion beam having a large beam current density is applied to each
hole formation spot for a short time, and this irradiation is
repeatedly applied to a plurality of hole formation spots in order
to perform cutting processing of the holes.
[0125] Accordingly, in processing the plurality of holes in the
manufacture of the MEMS sensor (ultrasonic sensor 4), thermal
destruction of the MEMS sensor can be prevented, and the TAT
(Turn-Around Time) is shortened, so that hole processing can be
performed efficiently.
[0126] Note that, as the method of forming the plurality of holes 3
according to the second embodiment, a method may be adopted in
which, by using the manufacturing method (ion beam irradiation
conditions) indicated in FIG. 8, continuous irradiation with the
focused ion beam 21 having a low current density for a long time is
simultaneously applied to the plurality of hole formation spots to
form the plurality of holes 3 all at once.
[0127] When this method is adopted for processing the plurality of
holes in the manufacture of the MEMS sensor (ultrasonic sensor 4),
thermal destruction of the MEMS sensor can also be prevented, and
the TAT (Turn-Around Time) is shortened, so that the hole
processing can be performed efficiently, as in the above case.
[0128] Note that formation of the plurality of holes 3 such as the
first hole 3a and the second hole 3b may be performed not by
cutting processing with the focused ion beam 21 but by dry etching
in such a way that the silicon oxide film 9b is etched to form
holes reaching the sacrifice layer 15.
[0129] Subsequently, after the plurality of holes 3 such as the
first hole 3a and the second hole 3b are formed in such a way as to
reach the sacrifice layer 15, the sacrifice layer 15 is removed
through the first hole 3a and the second hole 3b to form the cavity
8, which communicates with the first hole 3a and the second hole
3b, between the silicon oxide film 9a and the silicon oxide film
9b.
[0130] In this process, for example, the sacrifice layer 15 is
removed by wet etching through the first hole 3a and the second
hole 3b to form the cavity 8.
[0131] Subsequently, as shown in FIG. 19, the sealing film 5 (the
first sealing film 5a and the second sealing film 5b) sealing each
hole 3 is formed on each of the first hole 3a and the second hole
3b.
[0132] Formation of the plurality of sealing films 5 according to
the second embodiment is performed by using the irradiation
conditions of the focused ion beam 21 indicated in FIG. 7, which
has been described above in the first embodiment, and adopting the
short time deposition method indicated in FIG. 9 or by using the
irradiation conditions of the focused ion beam 21 indicated in FIG.
8 and adopting the parallel deposition method indicated in FIG.
9.
[0133] Note that the present invention is not limited to the
above-described embodiment, and various modifications are included.
For example, the above-described embodiment has been described in
detail so that the present invention is easily understood, and is
not limited to the one necessarily including all configurations
described.
[0134] Also, a part of the configuration of an embodiment can be
replaced with the configuration of other embodiments, and the
configuration of other embodiments can be added to the
configuration of an embodiment. In addition, other configurations
can be added to, deleted from, or replaced with the part of the
configuration of each embodiment. Note that each member described
in the drawings and a relative size is simplified and idealized so
that the present invention is easily understood, actual
implantation is more complicated in shape.
[0135] Further, in the first embodiment and the second embodiment
described above, description has been given of a case in which the
substrate is the semiconductor substrate 1, byway of example.
However, the substrate may be a glass substrate.
* * * * *