U.S. patent application number 13/609703 was filed with the patent office on 2013-01-24 for manufacturing method of mems device, and substrate used therefor.
This patent application is currently assigned to FUJITSU LIMITED. The applicant listed for this patent is Hiroaki Inoue, Tadashi Nakatani, Satoshi Ueda. Invention is credited to Hiroaki Inoue, Tadashi Nakatani, Satoshi Ueda.
Application Number | 20130022790 13/609703 |
Document ID | / |
Family ID | 44560378 |
Filed Date | 2013-01-24 |
United States Patent
Application |
20130022790 |
Kind Code |
A1 |
Inoue; Hiroaki ; et
al. |
January 24, 2013 |
MANUFACTURING METHOD OF MEMS DEVICE, AND SUBSTRATE USED
THEREFOR
Abstract
A method for manufacturing a MEMS device, includes: preparing a
substrate provided with a first substrate in which a cavity is
formed, and a second substrate that is bonded to a side of the
first substrate and includes a slit to delimit a movable portion in
a position corresponding to the cavity, the second substrate,
including a first surface thereof facing the first substrate, being
provided with a thermally-oxidized film selectively formed on the
first surface in a position corresponding to the movable portion;
forming a first electrode layer on a second surface opposite to the
first surface; forming a sacrifice lager on the first electrode
layer and the second substrate; forming a second electrode layer on
the sacrifice layer; and removing the sacrifice layer and the
thermally-oxidized film after the second electrode layer is
formed.
Inventors: |
Inoue; Hiroaki; (Kawasaki,
JP) ; Nakatani; Tadashi; (Kawasaki, JP) ;
Ueda; Satoshi; (Kawasaki, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Inoue; Hiroaki
Nakatani; Tadashi
Ueda; Satoshi |
Kawasaki
Kawasaki
Kawasaki |
|
JP
JP
JP |
|
|
Assignee: |
FUJITSU LIMITED
KAWASAKI
JP
|
Family ID: |
44560378 |
Appl. No.: |
13/609703 |
Filed: |
September 11, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13012104 |
Jan 24, 2011 |
8293557 |
|
|
13609703 |
|
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Current U.S.
Class: |
428/161 |
Current CPC
Class: |
H01H 59/0009 20130101;
Y10T 428/24521 20150115; Y10T 428/24562 20150115 |
Class at
Publication: |
428/161 |
International
Class: |
B32B 3/30 20060101
B32B003/30 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 12, 2010 |
JP |
JP2010-056565 |
Claims
1-4. (canceled)
5. A substrate used to manufacture a MEMS device, comprising: a
first substrate; a second substrate; and an insulating layer
provided between the first substrate and the second substrate,
wherein the first substrate is provided with a cavity extending to
a surface of the first substrate on a side of the insulating layer,
and the second substrate is provided with a thermally-oxidized film
formed selectively in a position corresponding to the cavity.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is based upon and claims the benefit of
priority of the prior Japanese Patent Application No. 2010-056565,
filed on Mar. 12, 2010, the entire contents of which are
incorporated herein by reference.
FIELD
[0002] The embodiments discussed herein are directed to a
manufacturing method of a MEMS device, and a substrate used
therefor.
BACKGROUND
[0003] In recent years, devices having a micro structure and
produced by a micro-machining technology, which is sometimes called
"MEMS (Micro Electro Mechanical Systems) technology", have been put
into applications in a variety of fields.
[0004] The MEMS devices include such types as a MEMS switch, a MEMS
capacitor, a MEMS sensor, and so on for a high-frequency circuit.
For example, the MEMS switch has an advantageous feature, as
compared with a conventional semiconductor switch, such as a small
loss, high insulating properties, and good distortion
properties.
[0005] As a conventional technology, Japanese Laid-open Patent
Publication No. 2005-293918 proposes a MEMS switch in which a
movable portion is formed on a substrate, and a contact provided to
the movable portion makes contact with a contact electrode provided
in a fixed manner relative to the substrate.
[0006] In a MEMS device, the movable portion is fabricated by
using, for example, an ordinary SOI wafer and applying a D-RIE
process only to the active layer (device layer) thereof.
Alternatively, the movable portion is sometimes fabricated by
laminating Poly-Si, Poly-SiGe, or the like on the wafer as a device
layer, and applying an etching process or removing a sacrifice
layer. Depending on the MEMS device, there is also a method to
fabricate the movable portion by bonding a layer to a base wafer,
and applying a D-RIE process. Among these processes, the process of
removing the sacrifice layer to make a structure laminated on lower
and upper layers of the sacrifice layer movable is called a surface
MEMS process.
[0007] FIG. 13 is a plan view illustrating an example of a MEMS
switch 80j, and FIG. 14 is a cross sectional view of the MEMS
switch 80j illustrated in FIG. 13 taken along a line J-J.
[0008] Referring to FIGS. 13 and 14, the MEMS switch 80j includes a
substrate 81, a lower contact electrode 82, an upper contact
electrode 83, a lower driving electrode 84, an upper driving
electrode 85, and so on, all of which are formed on the substrate
81. The lower contact electrode 82 and the lower driving electrode
84 are integrally provided to a movable portion KBj that
constitutes a cantilever.
[0009] An SOI substrate is used as the substrate 81. The movable
portion KBj is formed by cutting off the active layer of the SOI
substrate by a slit SL. The lower contact electrode 82 and the
lower driving electrode 84 are formed on the active layer by
plating.
[0010] When a driving voltage is applied between the upper driving
electrode 85 and the lower driving electrode 84, an electrostatic
attractive force is generated therebetween, with which the lower
driving electrode 84 is attracted toward and moved to the upper
driving electrode 85. In this way, the movable portion KBj and the
lower contact electrode 82 that are integrated with the lower
driving electrode 84 move, and the lower contact electrode 82
touches the upper contact electrode 83 so that the contacts close.
At this time, if the driving voltage is set at zero, the contacts
return to the positions separated from each other due to the
elasticity of the movable portion KBj.
[0011] The MEMS switch 80j described above has a structure in which
a cavity is present below the lower surface of the movable portion
KBj, and only one end of the movable portion KBj is connected to
and supported by the substrate 81. The movable portion KBj is
capable of bending upward and downward with the supported portion
serving as a fulcrum point.
[0012] During a process of manufacturing the MEMS switch 80j, when
an electrode having a coefficient of thermal expansion larger than
that of the base material is laminated on the upper surface of the
movable portion KBj, and when the temperature goes down to a room
temperature, a stress is generated to cause the movable portion KBj
to warp upwardly. When a sacrifice layer such as SiO.sub.2 is
further laminated thereon, the laminated sacrifice layer generates
a stress which causes the movable portion KBj to warp downwardly.
Although the warpage of the movable portion KBj caused by the
electrode is small, for example, about 0.3 .mu.m, the downward
warpage of the movable portion KBj caused by the sacrifice layer
sometimes becomes, for example, about 1 .mu.m of which the
influence is great.
[0013] In other words, during a process of manufacturing the MEMS
switch 80j, a half etching of the sacrifice layer is performed to
form the contact of the upper contact electrode 83. However, if the
movable portion KBj largely warps, the adjustment or the control of
the etching depth can not be accurately performed. For this reason,
the accuracy of the interelectrode gap between the contact of the
upper contact electrode 83 and the lower contact electrode 82 after
the sacrifice layer is removed is worsened. Accordingly, desired
switching properties may not be obtained.
[0014] In addition, if large downward warpage of the movable
portion KBj is caused, there are sometimes cases where the upper
surface portion of the slit SL may not be completely filled with
the sacrifice layer. In such a case, the resist or polymer may
infiltrate into a gap of the slit SL during a post-process, which
makes it difficult to remove such a substance by cleaning, and
reduces yields.
SUMMARY
[0015] According to an aspect of the invention (embodiment), a
method for manufacturing a MEMS device, includes: preparing a
substrate provided with a first substrate in which a cavity is
formed, and a second substrate that is bonded to a side of the
first substrate on which the cavity is formed and includes a slit
to delimit a movable portion in a position corresponding to the
cavity, the second substrate, including a first surface thereof
facing the first substrate, being provided with a
thermally-oxidized film selectively formed on the first surface in
a position corresponding to the movable portion; forming a first
electrode layer on a second surface opposite to the first surface
on which the thermally-oxidized film for the movable portion is
formed; forming a sacrifice layer on the first electrode layer and
the second substrate; forming a second electrode layer on the
sacrifice layer; and removing the sacrifice layer and the
thermally-oxidized film after the second electrode layer is
formed.
[0016] The object and advantages of the invention will be realized
and attained by means of the elements and combinations particularly
pointed out in the claims.
[0017] It is to be understood that both the foregoing general
description and the following detailed description are exemplary
and explanatory and are not restrictive of the invention, as
claimed.
BRIEF DESCRIPTION OF DRAWINGS
[0018] FIG. 1 is a plan view of a MEMS switch according to the
present embodiment;
[0019] FIGS. 2A and 2B are cross sectional views of the MEMS switch
illustrated in FIG. 1;
[0020] FIGS. 3A, 3B, and 3C are diagrams illustrating a
manufacturing process of the MEMS switch according to the present
embodiment;
[0021] FIGS. 4A, 4B, and 4C are diagrams illustrating the
manufacturing process of the MEMS switch according to the present
embodiment;
[0022] FIGS. 5A and 5B are diagrams illustrating the manufacturing
process of an SOI substrate;
[0023] FIGS. 6A, 6B, and 6C are diagrams illustrating a
manufacturing process of the SOI substrate;
[0024] FIGS. 7A and 7B are diagrams illustrating the manufacturing
process of the SOI substrate;
[0025] FIGS. 8A, 8B, and 8C are diagrams illustrating the
manufacturing process of the SOI substrate;
[0026] FIGS. 9A and 9B are diagrams illustrating the manufacturing
process of the SOI substrate;
[0027] FIGS. 10A and 10B are diagrams illustrating the
manufacturing process of the SOI substrate;
[0028] FIGS. 11A, 11E, 11C, and 11D are diagrams illustrating
comparative examples of manufacturing processes of the MEMS
switch;
[0029] FIG. 12 is a diagram depicting an outline of the
manufacturing method of the MEMS switch;
[0030] FIG. 13 is a plan view illustrating an example of a MEMS
switch; and
[0031] FIG. 14 is a cross sectional view illustrating the MEMS
switch illustrated in FIG. 13 taken along a line J-J.
DESCRIPTION OF EMBODIMENTS
[0032] [MEMS Switch]
[0033] In this embodiment, a MEMS switch 1 is taken as an example
of a MEMS device, and a description will be given thereof. Various
structures may be employed as a MEMS switch other than those in the
examples described hereinafter. Manufacturing methods described
later can also be applied to various types of MEMS devices such as
a MEMS capacitor other than a MEMS switch.
[0034] FIG. 1 is a plan view of a MEMS switch 1 according to one
embodiment. FIG. 2A is a cross sectional view taken along a line
A-A in FIG. 1. FIG. 2B is a cross sectional view of the MEMS switch
1 illustrated in FIG. 1 including a portion taken along a step-like
line and partially taken along in a revolving manner. To be
specific, FIG. 2B is a revolved sectional view, including a portion
i) taken along a line starting from "A" indicated in the left side
of FIG. 1 and ending at a point at which a line A-A intersects with
a line X-X, a portion ii) taken along a line staring from the point
at which the line A-A intersects with the line X-X and ending at a
point at which the line X-X intersects with a line C-C, and a
portion iii) starting from the point at which the line X-X
intersects with the line C-C and ending at a point where "C" is
indicated in the right side of FIG. 1. However, the illustration of
the portion ii) is partially omitted. It should be noted that FIGS.
3A-3C, 4A-4C, and 11A-11D of which descriptions will be given later
are also illustrated in a manner similar to FIG. 2B.
[0035] Referring to FIGS. 1, 2A, and 2B, the MEMS switch 1 includes
an SOI substrate 11, a movable contact electrode 12, a fixed
contact electrode 13, a movable driving electrode 14, a fixed
driving electrode 15, a wall portion 17, a support portion 18, and
so on.
[0036] The SOI substrate 11 is a three-layer SOI (Silicon On
Insulator) substrate formed of a support substrate (handle layer)
11a, a BOX layer (intermediate oxide film layer) 11b, and an active
layer (device layer) 11c. The support substrate 11a is made of
silicon having a thickness of about 500 .mu.m. The BOX layer 11b is
an insulating layer made of SiO.sub.2 having a thickness of about 4
.mu.m. The active layer 11c is a silicon thin film having a
thickness of about 15 .mu.m.
[0037] The active layer 11c is provided with a slit 16 having a
horizontal U-shape in a front view (plan view). This means that the
movable portion KB is delimited by the slit 16. The support
substrate 11a is provided with a cavity (space) 21 corresponding to
a region including the movable portion KB.
[0038] In other words, the cavity 21 is provided in a manner to
extend to an inner surface of the active layer 11c (lower side of
the active layer 11c in the illustration) in the support substrate
11a. Here, during the manufacturing process of the MEMS switch 1,
although an oxide film layer having been subjected to patterning is
formed on a surface of the active layer 11c in the cavity 21, the
oxide film layer will be removed later.
[0039] In addition, a layer similar to the BOX layer 11b may be
formed continuously from the BOX layer on a surface (surrounding
surface) other than that of the active layer 11c in the cavity 21.
The manufacturing process of the MEMS switch 1 will be described in
detail later.
[0040] The movable portion KB constitutes a cantilever with a
portion in which the slit 16 is not provided serving as a fulcrum,
warps with the fulcrum or the vicinity thereof serving as a center
of warpage, and an end portion opposite to the fulcrum can move in
upper and lower directions in FIGS. 2A and 2B. Electrode portions
12a and 14a, which will be described later, are formed in intimate
contact with the surface of the movable portion KB.
[0041] The movable contact electrode 12 includes the electrode
portion 12a that is thin and elongated and formed in intimate
contact with the movable portion KB, and the anchor portion 12b
formed on one end portion of the electrode portion 12a.
[0042] The fixed contact electrode 13 includes an electrode base
portion 13a formed in intimate contact with the active layer 11c,
and a fixed contact portion 13b provided continuously from the
electrode base portion 13a in a manner to oppose thereto above the
electrode portion 12a. The fixed contact portion 13b is provided
with a contact portion ST.
[0043] An openable and closable contact is formed between the
electrode portion 12a and the contact portion ST of the fixed
contact portion 13b. The contact closes when the movable portion KB
warps upward to thereby cause the electrode portion 12a to make
contact with the fixed contact portion 13b. A signal line SL is
formed of the movable contact electrode 12 and the fixed contact
electrode 13. When the contact closes, the signal line SL passes a
high-frequency signal therethrough.
[0044] The movable driving electrode 14 includes an electrode
portion 14a formed of an elongated portion formed in intimate
contact with the movable portion KB and a rectangular portion
formed continuously from a front end portion of the elongated
portion, and an anchor portion 14b formed on one end portion of the
electrode portion 14a.
[0045] The fixed driving electrode 15 is formed of electrode base
portions 15a and 15c that are formed in intimate contact with the
active layer 11c, and an electrode opposing portion 15b that is
supported by the electrode base portions 15a and 15c and forms a
bridge straddling the movable portion KB thereabove. The electrode
opposing portion 15b faces the rectangular portion of the electrode
portion 14a thereabove.
[0046] The wall portion 17 is provided, on the SOI substrate 11, in
a rectangular frame shape so as to surround the movable contact
electrode 12, the fixed contact electrode 13, the movable driving
electrode 14, the fixed driving electrode 15, and so on. The height
of the wall portion 17 is the same as or higher than the other
electrodes.
[0047] A metallic material, for example, gold is used as a material
for the movable contact electrode 12, the fixed contact electrode
13, the movable driving electrode 14, the fixed driving electrode
15, and the wall portion 17.
[0048] Sometimes, a membrane material 20 is bonded onto the wall
portion 17 to seal space including functional portion KN such as
the movable contact electrode 12, the fixed contact electrode 13,
the movable driving electrode 14, the fixed driving electrode 15,
and the like, that is, the space surrounded by the wall portion 17,
against outside.
[0049] [Manufacturing Method of MEMS Switch]
[0050] Next, a description will be given of a manufacturing method
of the MEMS switch 1.
[0051] As illustrated in FIG. 3A, the SOI substrate 11 is prepared.
As described before, the SOI substrate 11 includes the support
substrate 11a, the BOX layer 11b, and the active layer 11c.
According to the SOI substrate 11 used in this embodiment, the
cavity 21 is further provided to the support substrate 11a, and an
oxide film layer 22 is formed on a surface in the cavity 21 on the
side of the active layer 11c.
[0052] The cavity 21 and the oxide film layer 22 are formed during
the course of the production of the SOI substrate 11. Referring to
FIG. 6A, the cavity 21, in plan view, has a shape including a
region corresponding to the movable portion KB of the MEMS switch 1
and a region correspond to the slit 16. The depth of the cavity 21
is, for example, about a few um to a few dozens .mu.m.
[0053] Referring to FIG. 5B, the oxide film layer 22, in plan view,
has the same shape as that of the movable portion KB of the MEMS
switch 1. Referring to FIG. 7A, alternatively, the shape of the
oxide film layer 22 in plan view may be arranged identical to the
shape of the lower electrode layer formed on a side of an upper
surface of the movable portion KB, that is a combination of a shape
of the electrode portion 12a and a shape of the electrode portion
14a. Yet alternatively, the shape of the oxide film layer 22 in
plan view may be arranged as a shape corresponding to the
above-mentioned shape but not identical. The oxide film layer 22
is, for example, a thermally-oxidized film made of, for example,
SiO.sub.2 and having a thickness of about 0.1 .mu.m to a few .mu.m,
e.g., about 0.1 .mu.m to 2 .mu.m.
[0054] Concave portions 11d for positioning are provided on the
lower side of the outer surface of the support substrate 11a.
[0055] Next, a metallic layer serving as the lower electrode layer
is formed by performing sputtering or the like using a metallic
material on the surface of the active layer 11c of the SOI
substrate 11. Then, as illustrated in FIG. 3B, patterning is
performed on the metallic layer thus formed through a process of
RIE of the like to form the electrode portion 12a, the electrode
portion 14a, and the like.
[0056] Further, the slit 16 is formed along a pattern of the
cantilever of the movable portion KB by performing
photolithography, D-RIE, and the like on the active layer 11c. The
width of the slit 16 is, for example, about 1 .mu.m to 2 .mu.m.
[0057] When the slit 16 is formed, the slit 16 is connected to the
cavity 21 to thereby form the movable portion KB which serves as a
cantilever. In addition, space KK which is sufficient for the
movable portion KB to be operated and deformed therein is formed by
the cavity 21.
[0058] When the electrode portion 12a and the electrode portion 14a
are formed in the movable portion KB, slight upward warpage is
caused in the movable portion KB due to a difference between
coefficients of thermal expansion of the metallic material and the
material for the active layer 11c, and also changes in the
temperature during the process. Specifically, when the temperature
during the process goes down to a room temperature, a tensile
stress of the metallic material having a larger coefficient of
thermal expansion exceeds that of the active layer 11c. This
generates a stress that causes warpage toward the side of the
electrode portion 12a, that is, toward upper side in the
drawing.
[0059] Since the material used for the oxide film layer 22 has a
coefficient of thermal expansion larger than that of the material
used for the active layer 11c, the presence of the oxide film layer
22 causes an action of the warpage to become larger toward the
upper side of the movable portion KB. However, such warpage can be
figured out in terms of scale by managing the process. This makes
it possible to perform control for correcting the warpage as
required in the post-process.
[0060] Next, as illustrated in FIG. 3C, the sacrifice layer 31 is
formed by lamination on the active layer 11, the electrode portions
12a and 13a, and the like by using SiO.sub.2 etc. The temperature
during the formation of the sacrifice layer 31 is, for example,
about 150.degree. C. The thickness of the sacrifice layer 31 is
about a few .mu.m to a few dozens .mu.m, for example, about 5
.mu.m.
[0061] By forming the sacrifice layer 31, a stress is generated to
cause the movable portion KB to warp downwardly because of the
difference in the coefficient of thermal expansion and the change
in the temperature. However, since the oxide film layer 22 is
formed on the lower surface of the movable portion KB, the stress
causing the sacrifice layer 31 to warp downwardly is reduced or
cancelled by the stress generated by the oxide film layer 22 which
causes the upward warpage.
[0062] To be specific, the combined stress resulted from the stress
caused by the oxide film layer 22 and the stress caused by the
electrode portions 12a and 14a etc. is the stress that acts on the
movable portion KB and causes the upward warpage. On the other
hand, the stress caused by the sacrifice layer 31 is the stress
that acts on the movable portion KB and causes the downward
warpage. Thus, the stress that causes the movable portion KB to
warp downward is reduced or cancelled by the stress that causes the
movable portion KB to warp upward. To put it differently, these
stresses balance with each other to substantially maintain the
horizontal condition of the movable portion KB. As a result, the
warpage caused by the formation of the sacrifice layer 31
disappears or reduces.
[0063] The presence of the oxide film layer 22 greatly influences
the reduction of the warpage of the movable portion KB caused by
the formation of the sacrifice layer 31. Therefore, such an oxide
film layer 22 that reduces or cancels the warpage of the movable
portion KB caused by the formation of the sacrifice layer 31 is
selectively formed in advance.
[0064] Since the warpage of the movable portion KB caused by the
formation of the sacrifice layer 31 is reduced, the sacrifice layer
31 can be continuously formed without interruptions on the upper
potion of the slit 16. For this reason, the resist or polymer does
not infiltrate into the slit 16 contrary to the conventional case.
Here, the sacrifice layer 31 does not come into the cavity 21.
[0065] Next, as illustrated in FIG. 4A, half-etching is performed
the required number of times, and subsequently patterning is
performed on the sacrifice layer 31 to selectively reduce the film
thickness of the sacrifice layer 31. The depth of the half-etching
performed on the sacrifice layer 31 is controlled to thereby adjust
an interelectrode gap GP2 between the electrode portion 12a and the
contact portion ST of the fixed contact portion 13b which will be
formed later.
[0066] Next, as illustrated in FIG. 4B, a seed layer is formed, as
necessary, on the electrode portions 12a and 14a, the sacrifice
layer 31, and the like, and plating or the like is performed using
a metallic material. Through this process, a metallic layer serving
as an upper electrode layer such as for the fixed contact portion
13b and the electrode opposing portion 15b, and as a structural
body such as for the anchor portion 14b, the wall portion 17, or
the support portion 18.
[0067] Subsequently, as illustrated in FIG. 4C, the sacrifice layer
31 and the oxide film layer 22 are removed by etching using HF
(hydrofluoric acid) vapor etc. Through this process, the functional
portion KN of the MEMS switch 1 is completed and ready for
operation as the MEMS switch 1.
[0068] The membrane material 20 is bonded onto the wall portion 17
as necessary. In the case where the SOI substrate 11 is a
disc-shaped wafer, a plurality of pieces of MEMS switch 1 formed on
the SOI substrate 11 are cut out into individual pieces of MEMS
switch 1 by dicing along the wall portion 17.
[0069] In this way, by using the SOI substrate 11 having the
support substrate 11a in which the cavity 21 is provided, and the
oxide film layer 22 formed on a surface in the cavity 21 on the
side of the active layer 11c, it is possible to reduce the warpage
of the movable portion KB caused when the sacrifice layer 31 is
formed as much as possible.
[0070] Furthermore, since the warpage of the movable portion KB
caused when the sacrifice layer 31 is formed is small, the
half-etching of the sacrifice layer 31 can be accurately performed,
and the size of the interelectrode gap GP2 etc. between the
electrode portion 12a and the contact portion ST of the fixed
contact portion 13b can be accurately adjusted.
[0071] For example, if the oxide film layer 22 is not provided on
the inner surface of the cavity 21j, the downward warpage of the
movable portion KBj caused when the sacrifice layer 31 is formed
becomes larger, for example, as illustrated in FIG. 11A. For
example, there is sometimes a case where the movable portion KBj
sags by about 1 .mu.m from the surface of the active layer 11c. For
this reason, there may be a case where the sacrifice layer 31 sinks
in the upper portion of the slit 16 and breaks. The resist or
polymer may infiltrate into such a portion. Instead, the thickness
of the sacrifice layer 31 in the vicinity of the slit 16 may
fluctuate.
[0072] In addition, for example, as illustrated in FIG. 11B, the
depth of a hole STA for the contact portion STj of the fixed
contact portion 13b, when the sacrifice layer 31 is half-etched,
can not be accurately controlled. As a result, for example, as
illustrated in FIG. 11C, the accuracy of the interelectrode gap GP
between the contact portion STj and the electrode portion 12j is
worsened when the metallic layer is formed by plating.
[0073] For example, as illustrated in FIG. 11D, after the sacrifice
layer 31 is released, the movable portion KBj may warp upwardly as
a reaction of the downward warpage thereof. If this occurs, the
electrode portion 12j may be constantly kept in contact with the
contact portion STj. In such a case, the MEMS switch 1 is
determined faulty, which reduces yields.
[0074] [Manufacturing Method of SOI Substrate]
[0075] Referring to FIGS. 5A-10B, a description will be given of
the manufacturing method of the SOI substrate 11.
[0076] First, a description will be given of an upper substrate BK1
and a lower substrate BK2 that are components for manufacturing the
SOI substrate 11.
[0077] FIGS. 5A and 5B illustrate the upper substrate BK1 to be
used for producing the SOI substrate 11. FIG. 5A is a sectional
side view, and FIG. 5B is a bottom view. FIGS. 6A-6C illustrate the
lower substrate BK2 to be used for producing the SOI substrate 11.
FIG. 6A is a plan view, and FIGS. 6B and 6C are cross sectional
views.
[0078] Referring to FIGS. 5A and 5B, the upper substrate BK1 is
resulted from forming a thermally-oxidized film 42 on a lower
surface of a silicon plate 41. The silicon plate 41 is a portion to
be polished and serves as the active layer 11c later, and the
thermally-oxidized film 42 is to serve as the BOX layer 11b
later.
[0079] As illustrated in FIG. 5B, the portion of the
thermally-oxidized film 42 which will serve as the movable portion
KB later is patterned in a shape identical to that of the movable
portion KB on which the oxide film layer 22 is formed.
[0080] Referring to FIGS. 6A and 6B, the lower substrate BK2 is
resulted from forming the cavity 21 in the upper surface of the
silicon plate 43 by D-RIE, wet etching, or the like. The planar
shape of the cavity 21 is a shape that corresponds to a region
including a portion to be turned to the movable portion KB. The
silicon plate 43 is a portion that turns to be the support
substrate 11a later.
[0081] FIG. 6C illustrates a variation example of the lower
substrate BK2B. As the lower substrate BK2B illustrated in FIG. 6C,
the oxide film layers 23 and 24 formed of SiO2 etc. may be formed
on the entire upper and lower surfaces of the silicon plate 43. The
entire upper and lower surfaces of the silicon plate 43 including
the wall surface of the cavity 21B are covered with the insulating
layer by the oxide film layers 23 and 24.
[0082] In the manufacturing process of the SOI substrate 11, the
upper substrate BK1 and the lower substrate BK2 are bonded together
so that the surface of the oxide film layer 22 coincides with a
surface of the silicon plate 43 in which the cavity 21 is
provided.
[0083] Alternatively, as illustrated in FIGS. 7A and 7B, the shape
of the oxide film layer 22 of the upper substrate BK1 may be made
identical with the shapes of the electrode portions 12a and 14a
formed on the upper side of the movable portion KB.
[0084] FIG. 7B illustrates, in plan view, the shapes of the
electrode portions 12a and 14a formed in the movable portion KB,
and FIG. 7A illustrates, in bottom view, the patterning for the
oxide film layer 22B formed on the thermally-oxidized film 42 of
the upper substrate BK1B. In these illustrations, the shapes of the
electrode portions 12a and 14b and the shape of the oxide film
layer 22B are in a mirror image relationship.
[0085] Next, the manufacturing process of the SOI substrate 11 will
be described.
[0086] As illustrated in FIG. 8A, the cavity 21 is formed on one
side of the silicon plate 43 which is to serve as the lower
substrate BK2, and the concave portion (alignment marker) 43d for
positioning is also formed. As illustrated in FIG. 8B, another
concave portion 43d is also formed on the other side of the silicon
plate 43 to serve as the lower substrate BK2.
[0087] As illustrated in FIG. 8C, the oxide film layers 23 and 24
are individually formed on two sides of the silicon plate 43
entirely as necessary to thereby form the lower substrate BK2B.
[0088] As illustrated in FIG. 9A, the upper substrate BK1
illustrated in FIGS. 5A and 5B or, alternatively, the upper
substrate BK1B illustrated in FIGS. 7A and 7B is bonded to the
upper surface of the lower substrate BK2 illustrated in FIG. 8B. In
this bonding process, for example, hydrophilic processing is
performed on the bonding surfaces, and two surfaces are placed
together which are then subjected to an annealing treatment at a
high temperature of about 1000.degree. C.
[0089] Next, as illustrated in FIG. 9B, the surface of the silicon
plate 41 is polished to a predetermined thickness required as the
active layer 11c.
[0090] Through this process, the thermally-oxidized film 42 turns
to be the BOX layer 11b, and the silicon plate 43 turns to be the
support substrate 11a. The cavity 21 extends to the surface inside
the active layer 11c in the support substrate 11a where the oxide
film layer 22 which has been subjected to patterning is formed.
[0091] Further, as illustrated in FIG. 10A, the upper substrate BK1
illustrated in FIGS. 5A and 5B or, alternatively, the upper
substrate BK1B illustrated in FIGS. 7A and 7B is bonded to the
upper surface of the lower substrate BK2B illustrated in FIG. 8C.
Next, as illustrated in FIG. 10B, the surface of the silicon plate
41 is polished to a predetermined thickness required as the active
layer 11c.
[0092] Through this process, the thermally-oxidized film 42 and the
oxide film layer 23 turn to be the BOX layer 11b, and the silicon
plate 43 turns to be the support substrate 11a. The cavity 21
extends to the surface inside the active layer 11c in the support
substrate 11a where the oxide film layer 22, which has been
subjected to patterning, is formed. The oxide film layer 23 is
formed in the other portion of the inner surface of the cavity
21.
[0093] As described above, the SOI substrate 11 is produced by
bonding together the lower substrate BK2 having the cavity 21 and
the upper substrate BK1 having the oxide film layer 22 that has
undergone the patterning. During this process, an oxide film layer
22 is formed and subjected to patterning so that the oxide film
layer 22 causes a stress of the same quality as and equivalent to a
stress that will be caused when the sacrifice layer 31 is formed
later. This arrangement makes it possible to reduce the warpage
that will be caused otherwise after the movable portion KB is
formed.
[0094] Consequently, it is possible to suppress the warpage or
depression of the movable portion KB during the manufacturing
process of the MEMS switch 1 and perform accurate control of the
dimensions during the formation of the electrode by applying
half-etching to the sacrifice layer 31. Therefore, it is possible
to manufacture the MEMS switch 1 having the desired driving
properties at a higher yield rate.
[0095] In addition, since it is possible to adopt a process using a
wafer of the SOI substrate 11 having the cavity 21, it is easy to
arrange it in a wafer level package (WLP) structure that has a low
profile and is implementable. Specifically, a single membrane
material 20 is bonded onto an entire area in which a plurality of
MEMS switches 1 are formed on the SOI substrate 11, and dicing is
preformed thereafter. In this way, it is possible to manufacture
individual MEMS switches 1 having a low profile in large
quantity.
[0096] Hereinafter, a description will be given of the outline
procedure of the manufacturing process of the MEMS switch 1 using
the SOI substrate 11 referring to a flowchart.
[0097] Referring to FIG. 12, an SOI substrate 11 is prepared. In
the SOI substrate 11, the support substrate 11a is provided with a
cavity 21, and the oxide film layer 22 is formed on the surface of
the active layer 11c in the cavity 21 (step #11). Then, the slit 16
is arranged to form the movable portion KB (#12).
[0098] The lower electrodes such as the electrode portions 12a and
14a are formed on the movable portion KB (#13), and the sacrifice
layer 31 is provided thereon (#14). Half-etching is performed on
the sacrifice layer 31 to thereby perform patterning (#15). An
upper electrode such as the fixed contact portion 13b is formed on
the sacrifice layer 31 (#16). Then, the sacrifice layer 31 and the
oxide film layer 22 are removed (#17).
[0099] According to the foregoing embodiment, during the
manufacturing of the MEMS switch 1, the SOI substrate 11 is used.
The SOI substrate 11 includes the support substrate 11a to which
the cavity 21 is provided, and the oxide film layer 22 that is
patterned on the inner surface of the active layer 11c. However, it
is also possible to manufacture the MEMS switch 1 without using the
above-mentioned SOI substrate 11 but using a different type of SOI
substrate.
[0100] For example, it is possible to use an SOI substrate formed
of the support substrate 11a, the BOX layer 11b, and the active
layer 11c without having the cavity 21 formed therein. In this
case, the cavity is produced from the rear side of the active layer
11c after the device structure is formed on the active layer
11c.
[0101] According to the foregoing embodiment, since the movable
portion is fixed relative to the BOX layer when the side of the
active layer is being processed, the movable portion KB is not
caused to warp when the sacrifice layer 31 is formed. Therefore, it
is possible to perform accurate control on the dimensions of the
interelectrode gap GP2 between the electrode portion 12a and the
contact portion ST of the fixed contact portion 13b. Instead of the
distance between the electrode portion 12a and the contact portion
ST or a distance between electrodes that make contact with each
other, a distance between two electrodes that do not make contact
with each other may be taken as the interelectrode gap GP2. This
means that it is also possible to perform accurate control on
dimensions of an interelectrode gap between the electrodes that do
not make contact with each other.
[0102] In the foregoing embodiment, the overall configurations of
the other portions such as the SOI substrate 11, the electrode
portions 12a and 14a, the fixed contact portion 13b, the contact
portion ST, the slit 16, the cavity 21, the oxide film layer 22,
the sacrifice layer 31, the movable portion KB, and the MEMS switch
1, the configurations of various parts thereof, the structure, the
shape, the material, the quantity, the layout, the temperature, the
production method, and the like may be altered as required in
accordance with the subject matter of the present invention.
[0103] All examples and conditional language recited herein are
intended for pedagogical purposes to aid the reader in
understanding the invention and the concepts contributed by the
inventor to furthering the art, and are to be construed as being
without limitation to such specifically recited examples and
conditions, nor does the organization of such examples in the
specification relate to a showing of the superiority and
inferiority of the invention. Although the embodiments of the
present invention have been described in detail, it should be
understood that the various changes, substitutions, and alterations
could be made hereto without departing from the spirit and scope of
the invention.
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