U.S. patent application number 13/466354 was filed with the patent office on 2012-08-30 for mems-scanning mirror device and method for manufacturing the same.
Invention is credited to Makoto Fujino, Hirotake Maruyama, Hirokazu Tamura.
Application Number | 20120218613 13/466354 |
Document ID | / |
Family ID | 42117211 |
Filed Date | 2012-08-30 |
United States Patent
Application |
20120218613 |
Kind Code |
A1 |
Maruyama; Hirotake ; et
al. |
August 30, 2012 |
MEMS-SCANNING MIRROR DEVICE AND METHOD FOR MANUFACTURING THE
SAME
Abstract
An MEMS-scanning mirror device includes an electrostatic comb
actuator, in which a mirror surface is formed below a top surface
(TOPS) of the mirror device. In a method for manufacturing the
MEMS-scanning mirror device having the electrostatic comb actuator,
a mirror surface (10BS) of a mirror plate (10B) is formed by
removing an insulating layer (I) thereon.
Inventors: |
Maruyama; Hirotake;
(Itabashi-ku, JP) ; Fujino; Makoto; (Itabashi-ku,
JP) ; Tamura; Hirokazu; (Itabashi-ku, JP) |
Family ID: |
42117211 |
Appl. No.: |
13/466354 |
Filed: |
May 8, 2012 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
12588804 |
Oct 28, 2009 |
|
|
|
13466354 |
|
|
|
|
Current U.S.
Class: |
359/200.6 |
Current CPC
Class: |
G02B 26/0841
20130101 |
Class at
Publication: |
359/200.6 |
International
Class: |
G02B 26/10 20060101
G02B026/10 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 29, 2008 |
JP |
2008-277897 |
Claims
1. An MEMS-scanning mirror device comprising: an electrostatic comb
actuator; and a mirror portion driven by the electrostatic comb
actuator, wherein a mirror surface of the mirror portion is
positioned below a surface of the electrostatic comb actuator.
2. The MEMS-scanning mirror device according to claim 1, wherein
the device has a structure including at least three layers of
silicon, and the mirror surface is formed in a surface of any one
of the silicon layers other than the uppermost layer.
3. The MEMS-scanning mirror device according to claim 1, wherein
the device includes the electrostatic comb actuator, a silicon
wafer is further provided on a device layer of an SOI wafer, and
the mirror surface is formed in a surface of the device layer of
the SOI wafer.
4-6. (canceled)
7. The MEMS-scanning mirror device according to claim 1, further
comprising: a handle layer; and at least two device layers provided
above the handle layer, wherein the device layer that is the
uppermost layer forms the surface of the electrostatic comb
actuator, and the device layer below the uppermost layer forms the
mirror surface.
8. (canceled)
Description
CROSS-REFERENCE TO THE RELATED APPLICATION
[0001] This application is based on and claims the priority benefit
of Japanese Patent Application No. 2008-277897, filed on Oct. 29,
2008, the disclosure of which is incorporated herein by reference
in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an MEMS (micro
electromechanical system)-scanning mirror device and a method for
manufacturing the mirror device.
[0004] 2. Description of the Related Art
[0005] Along with the recent improvement in the multifunctionality
and operation speed of optical devices, high-speed optical path
switching and vector drawing of desired patterns have been
required. For example, in an electronic distance meter, in order to
compensate for measurement errors, alternate switching is performed
between an external optical path for measuring a distance from the
electronic distance meter to an external target and an internal
optical path provided inside the device.
[0006] For example, a variable optical attenuator in a distance
measuring equipment disclosed in Japanese Patent Application
Publication No. 2008-65293 requires optical path switching for
attenuation. Moreover, in capturing a target by the electronic
distance meter, a light beam has to be projected at a specified
angle. Furthermore, in order to draw a desired pattern, a line
drawing display device using laser beam scanning is required to be
able to perform optical scanning corresponding to the pattern.
[0007] In either case, it is important to improve the operation
speed and power consumption. For that purpose, application of an
MEMS-scanning mirror to changing a light reflection direction and
scanning has been examined. Moreover, in addition to the
improvement in the operation speed of an actuator and the power
consumption thereof, improvement in the specularity of a mirror
portion configured to reflect light and in manufacturing yield has
been the primary focus in this development field. Here, the
specularity corresponds to the roughness of a surface of the mirror
portion, and a smooth surface having small surface roughness is
said to have high specularity.
[0008] FIG. 6 is a schematic view showing a conventional example of
an MEMS-scanning mirror 27 having stepped electrostatic comb
actuators. As shown in FIG. 6, certain steps are provided between
movable combteeth 27e and fixed combteeth 27fA and 27fB.
[0009] The MEMS-scanning mirror 27 having the stepped electrostatic
comb actuators includes a disc-shaped mirror plate 27a. In the
mirror plate 27a, a pair of radially-extended shaft portions 27b
are formed. The pair of shaft portions 27b are connected to fixed
portions 27d through spring portions 27c, respectively.
[0010] The movable combteeth 27e are formed in a middle of each of
the shaft portions 27b. The fixed combteeth 27fA and 27fB are
allowed to face the movable combteeth 27e. The movable combteeth
27e and the fixed combteeth 27fA and 27fB form a pair of left and
right electrostatic actuators.
[0011] The pairs of left and right electrostatic actuators are used
to drive and rotate the mirror plate 27a. Specifically, the mirror
plate 27a is driven to be rotated in an arrow F direction by
applying a voltage to the pairs of left and right fixed combteeth
27fA and 27fB. Note that there are two kinds of drive systems: DC
drive for performing rotation from a first angle to a second angle
by applying a DC voltage between the movable combteeth and one of
or both of the left and right fixed combteeth; and resonant drive
for performing rotation based on resonance by applying a periodic
voltage such for example as an AC voltage.
[0012] There have been known various methods for manufacturing of
the MEMS-scanning mirror having the stepped electrostatic comb
actuators. For example, U.S. Pat. No. 6,758,983 discloses a process
for manufacturing an MEMS-scanning mirror which includes two layers
of silicon wafers and can be driven to be rotated around one axis.
The process includes the steps of:
(a) forming fixed combteeth structures in a surface of a first
silicon wafer; (b) bonding a second silicon wafer to the surface of
the first silicon wafer where the fixed combteeth structures are
formed; (c) after the step (b), reducing a thickness of the second
silicon wafer to a predetermined thickness and then performing
grinding or polishing to obtain specularity; (d) after the step
(c), removing a portion corresponding to a lower portion of a
mirror plate from a surface of the first silicon wafer, the surface
being opposite to the surface where the fixed combteeth structures
are formed; (e) after the step (d), forming movable combteeth and
the mirror plate in the second silicon wafer; and (f) after the
step (e), removing an oxide layer between the first and second
silicon wafers to obtain an MEMS-scanning mirror having stepped
electrostatic comb actuators. Note that the mirror plate may be
coated with gold, aluminum or the like by deposition or the like in
order to improve its optical characteristics.
[0013] In addition, U.S. Pat. No. 6,914,710 discloses a process for
manufacturing an MEMS-scanning mirror which can be driven to be
rotated around two axes. The process includes the steps of:
(a) forming a lower layer structure of the MEMS-scanning mirror
having fixed combteeth, in a device layer in a silicon-on-insulator
(SOI) wafer; (b) forming a concave portion in another silicon
wafer; (c) after the step (b), bonding the device layer of the SOI
wafer to the silicon wafer in a state where the lower layer
structure and the concave portion face each other; (d) after the
step (c), reducing a thickness of a handle layer of the SOI wafer
to a predetermined thickness and then performing mirror finishing;
(e) after the step (d), forming an upper layer structure of the
MEMS-scanning mirror having movable combteeth and a mirror plate,
in the handle layer; and (f) after the step (e), removing an
exposed silicon oxide layer to obtain stepped electrostatic comb
actuators.
[0014] Note that the mirror is completed by forming a film of gold
with chromium as an underlayer, on the mirror plate formed in the
step (e).
[0015] Moreover, there has also been known a technology related to
an MEMS structure described in U.S. Pat. No. 6,641,273, for
example.
[0016] However, in each of the conventional MEMS-scanning mirrors
having the stepped electrostatic comb actuators described in U.S.
Pat. Nos. 6,758,983 and 6,914,710, for example, the mirror plate is
formed on a top surface of the device. Thus, the mirror has the
following problems, which lead to a situation where it is difficult
to obtain a mirror having high specularity and to improve
manufacturing yield.
[0017] Specifically, in formation of the mirror plate, processing
such as grinding, lapping and polishing has heretofore been
performed on the uppermost layer of the device in order to improve
the specularity. In this event, as in the case of the manufacturing
method disclosed in U.S. Pat. No. 6,914,710, for example, when
structures such as combteeth and hollows are previously formed in a
lower portion of the silicon wafer to be subjected to such
processing, especially when a hollow is formed below the mirror
plate, it is difficult to secure uniformity of the processing. As a
result, scratches are left on the mirror surface, which makes it
difficult to obtain high specularity with good yield.
[0018] Furthermore, in conventional cases, since the mirror portion
is formed on the top surface of the device, damage to the mirror
portion due to handling, internal stress and the like, scratches on
the mirror surface, and the like are likely to occur in a
processing step subsequent to processing steps of mask formation,
etching, mask removal, film formation and the like after formation
of the mirror portion. These are also obstacles to achievement of
good specularity and high manufacturing yield.
[0019] Such problems are significant in a subsequent processing
step particularly when a hollow is formed below the mirror plate
portion.
[0020] Moreover, a concrete manufacturing method is not disclosed
in U.S. Pat. No. 6,641,273, for example. Therefore, it is
impossible to achieve a high manufacturing yield according to a
manufacturing method including: exposing the surface of the mirror
by removing silicon oxide films covering the entire front and back
surfaces thereof; forming stepped combteeth structures with a
handle layer present below the mirror plate; or exposing the
surface of the mirror plate by removing silicon oxide films after
forming the stepped combteeth structures with the handle layer
present below the mirror plate.
SUMMARY OF THE INVENTION
[0021] A first object of the present invention is to provide an
MEMS-scanning mirror device including a mirror surface having
excellent specularity.
[0022] A second object of the present invention is to provide a
method for manufacturing the MEMS-scanning mirror device with
efficient manufacturing yield.
[0023] To achieve the first object above, an MEMS-scanning mirror
device according to an embodiment of the present invention includes
an electrostatic comb actuator and a mirror portion driven by the
electrostatic comb actuator. In the device, a mirror surface of the
mirror portion is positioned below a surface of the electrostatic
comb actuator.
[0024] To achieve the second object above, a method for
manufacturing an MEMS-scanning mirror device according to an
embodiment of the present invention includes the steps of:
preparing a wafer in which a handle layer and a device layer having
a mirror-finished surface are bonded to each other with a silicon
oxide film interposed therebetween, the handle layer being a layer
in which a hollow is to be formed; forming, in the device layer, a
lower layer structure to be formed in the device layer, the
structure corresponding to a lower layer component of the
MEMS-scanning mirror; bonding a wafer for forming an upper device
layer to the device layer of the wafer after formation of the lower
layer structure, with a silicon oxide film interposed therebetween;
forming the upper device layer having a predetermined thickness by
processing the upper wafer; forming, in the upper device layer, an
upper layer structure to be formed in the upper device layer, the
structure corresponding to an upper layer component of the
MEMS-scanning mirror; forming the hollow in the handle layer; and
completing the MEMS-scanning mirror by removing the silicon oxide
films on the lower layer structure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1A to 1C are explanatory views showing a first
embodiment of an MEMS-scanning mirror according to the present
invention. FIG. 1A is a plan view, FIG. 1B is a cross-sectional
view taken along the line A-A' in FIG. 1A, and FIG. 1C is a
cross-sectional view taken along the line B-B' in FIG. 1A.
[0026] FIG. 2A to 2C are explanatory views showing a second
embodiment of an MEMS-scanning mirror according to the present
invention. FIG. 2A is a plan view, FIG. 2B is a cross-sectional
view taken along the line B-B' in FIG. 2A, and FIG. 2C is a
cross-sectional view taken along the line A-A' in FIG. 2A.
[0027] FIG. 3A to 3C are explanatory views showing a third
embodiment of an MEMS-scanning mirror according to the present
invention. FIG. 3A is a plan view, FIG. 3B is a cross-sectional
view taken along the line B-B' in FIG. 3A, and FIG. 3C is a
cross-sectional view taken along the line A-A' in FIG. 3A.
[0028] FIGS. 4A to 4E are explanatory views showing an embodiment
of a method for manufacturing an MEMS-scanning mirror according to
the present invention. FIG. 4A is an explanatory view of a
preparation step, FIG. 4B is an explanatory view of a step of
forming structures in a device layer, FIG. 4C is an explanatory
view of a silicon wafer bonding step, FIG. 4D is an explanatory
view of a step of forming an uppermost device layer, and FIG. 4E is
an explanatory view of a step of forming a mask on a handle
layer.
[0029] FIGS. 5A to 5D are explanatory views showing the embodiment
of the method for manufacturing an MEMS-scanning mirror according
to the present invention. FIG. 5A is an explanatory view of a step
of forming structures in the uppermost device layer, FIG. 5B is an
explanatory view of a step of forming hollows in the handle layer,
FIG. 5C is an explanatory view of a step of removing a silicon
oxide film, and FIG. 5D is an explanatory view of a step of forming
films on the respective device layers.
[0030] FIG. 6 is a perspective view showing a schematic
configuration of a conventional MEMS-scanning mirror having stepped
electrostatic comb actuators.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] With reference to the accompanying drawings, preferred
embodiments of the present invention will be described in detail
below.
First Embodiment
[0032] FIGS. 1A to 1C show an MEMS-scanning mirror device having
electrostatic comb actuators according to a first embodiment of the
present invention. The MEMS-scanning mirror device may be
configured to include, for example, at least one stepped
electrostatic comb actuator. In the illustrated embodiment, as
shown in FIGS. 1A to 1C, the MEMS-scanning mirror device includes
the electrostatic comb actuators and a mirror portion or mirror
plate 10B driven by the electrostatic comb actuators. The
MEMS-scanning mirror device has, for example, a three-layer
structure including an uppermost device layer T, a lowermost handle
layer H and a device layer B therebetween.
[0033] The device layer T and the device layer B are made of n type
or p type doped conductive silicon. For the handle layer H,
insulating or conductive silicon or glass is used. The device layer
T, the device layer B and the handle layer H are electrically
insulated from each other by insulating layers I (see FIG. 1C). The
insulating layers I are preferably a silicon oxide when the handle
layer H is made of silicon.
[0034] In the handle layer H, a hollow HO is formed (see FIG. 1B).
Components of the electrostatic comb actuators, which are formed in
the device layers T and B, respectively, are located above the
hollow HO. The components of the electrostatic comb actuators,
which are formed in the device layers T and B, will be described
while attaching subscripts to the symbols T and B denoting the
respective layers.
[0035] The components of the electrostatic comb actuators formed in
the device layer B include the mirror plate 10B, movable combteeth
11B, shafts 13B, springs 14B and frames 15B. All these components
are integrated together and electrically equipotential.
[0036] In the mirror plate 10B, a pair of the shafts 13B extended
in one direction are formed. The pair of shafts 13B are connected
to the frames 15B through the springs 14B, respectively. In a
middle of each of the shafts 13B, one or more movable combteeth 11B
are formed. Note that a film made of gold, aluminum or the like may
be formed on a surface of the mirror plate 10B.
[0037] In the device layer T, one or more fixed combteeth
corresponding to the movable combteeth 11B are formed. In the
embodiment shown in FIGS. 1A to 1C, the fixed combteeth include
right fixed combteeth 12T1, left fixed combteeth 12T2, a right base
16T1 and a left base 16T2. The right fixed combteeth 12T1 and the
left fixed combteeth 12T2 are integrally formed with the right base
16T1 and the left base 16T2, respectively, and have electrically
the same potentials as those of the bases. The potentials of the
right base 16T1 and the left base 16T2 can be set independently of
each other.
[0038] For the device layer B, a silicon wafer having a fine mirror
surface can be used. A material obtained by bonding the device
layer B and the handle layer H with the insulating layer I
interposed therebetween can be provided for formation of the
MEMS-scanning mirror. Alternatively, an SOI wafer may also be used,
in which the device layer B, the insulating layer I, and the handle
layer H are previously integrated together. A surface of the device
layer B in the SOI wafer can be finished in a fine mirror surface
state.
[0039] In manufacturing of the MEMS-scanning mirror by using any of
the materials described above, the mirror plate 10B having good
specularity can be achieved by using the fine mirror surface of the
device layer B as it is.
[0040] In the MEMS-scanning mirror device, as shown in FIGS. 1A and
1B, a surface 10BS of the mirror plate 10B is located below a top
surface TOPS that is a surface of the device layer T for the
electrostatic comb actuators. This structure is obtained because
the top surface of each electrostatic comb actuator is formed of
the surface of the device layer T positioned at the uppermost layer
and because the surface of the mirror portion is formed of the
surface of the device layer B located below the device layer T.
Therefore, a step is formed between the top surface of each comb
actuator and the surface of the mirror portion. This also means
that, basically, at least one step is formed between an upper
surface of at least one movable combtooth and a surface of at least
one fixed combtooth. In the embodiment shown in FIGS. 1A to 1C,
steps are formed between the upper surfaces of the multiple movable
combteeth and the surfaces of the multiple fixed combteeth. Such a
structure is formed by use of a manufacturing method to be
described later.
[0041] In a case of a conventional MEMS-scanning mirror having
electrostatic comb actuators, a mirror is formed on a top surface
and, after formation of a mirror plate, subsequent processing is
performed while exposing a surface of the mirror plate to the
outside. On the other hand, in a case of the MEMS-scanning mirror
device according to the present invention, the surface 10BS of the
device layer B can be protected by the insulating layer I such as a
silicon oxide film until immediately before completion of the
device. Thus, in steps of mask formation, etching, mask removal,
film formation and the like to be described later, the surface 10BS
of the mirror plate 10B can be prevented from being scratched
during handling or the like and from being contaminated.
[0042] The movable combteeth 11B are inserted between the right
fixed combteeth 12T1 and between the left fixed combteeth 12T2.
When a voltage is applied between the movable combteeth 11B and the
fixed combteeth 12T1 and 12T2, a torque around an X-X' axis is
generated in the movable combteeth 11B, that is to say, the mirror
plate 10B so as to lower electrostatic energy.
[0043] A method for driving the MEMS-scanning mirror device
according to the present invention is the same as the method for
driving the conventional MEMS-scanning mirror. The mirror plate 10B
is rotated around the X-X' axis shown in FIG. 1A. It is preferable
that the entire device layer B be grounded (denoted by reference
symbol GND) and the MEMS-scanning mirror device be driven by
applying voltages V1 and V2 to the fixed combteeth 12T1 and 12T2.
For example, in the case of DC drive, the mirror plate 10B can be
rotated by applying a DC voltage to any one of the left and right
fixed combteeth 12T1 and 12T2 or by applying different DC voltages
to the both fixed combteeth 12T1 and 12T2, respectively. Moreover,
in the case of resonant drive, for example, a periodic voltage such
as an AC voltage is applied to any one of the left and right fixed
combteeth 12T1 and 12T2 or to the both fixed combteeth 12T1 and
12T2.
Second Embodiment
[0044] FIGS. 2A to 2C show a second embodiment of an MEMS-scanning
mirror having two sets of stepped electrostatic comb actuators
according to the present invention. While the mirror plate 10B in
the first embodiment is rotated around one axis, a mirror plate in
this embodiment can be rotated around two axes, an X-X' axis and a
Y-Y' axis shown in FIGS. 2A and 2B.
[0045] As shown in FIGS. 2A to 2C, the MEMS-scanning mirror also
has a three-layer structure as in the case of the first embodiment,
and includes an uppermost device layer T, a lowermost handle layer
H and a device layer B therebetween.
[0046] The device layer T and the device layer B are made of n type
or p type doped conductive silicon. For the handle layer H,
insulating or conductive silicon or glass is used. These layers
described above are electrically insulated from each other by
insulating layers I. The insulating layers I are preferably a
silicon oxide when the handle layer H is made of silicon.
[0047] In the handle layer H, a hollow HO is formed. Components of
the electrostatic comb actuators, which are formed in the device
layers T and B, respectively, are located above the hollow HO. The
components of the electrostatic comb actuators, which are formed in
the device layers T and B, will be described while attaching
subscripts to the symbols T and B denoting the respective
layers.
[0048] The components of the electrostatic comb actuators formed in
the device layer B include a mirror plate 30B, movable combteeth
31B around the X axis, springs 33B around the X axis, an internal
frame 34B, an external frame 38B, movable combteeth 36B around the
Y axis and spring lower portions 311B around the Y axis. All the
components are integrated together and electrically
equipotential.
[0049] In the mirror plate 30B, a pair of the springs 33B around
the X axis extended along the X-X' axis are formed. Moreover, in
the mirror plate 30B, the movable combteeth 31B around the X axis
are formed in a direction perpendicular to the X-X' axis.
[0050] These components 30B and 31B are located inside the internal
frame 34B. The mirror plate 30B and the internal frame 34B are
connected to each other through the springs 33B around the X axis.
Thus, the mirror plate 30B is integrated with the internal frame
34B.
[0051] In the internal frame 34B, the movable combteeth 36B around
the Y axis perpendicular to the Y-Y' axis and a pair of the spring
lower portions 311B around the Y axis extended along the Y-Y' axis
are formed. The internal frame 34B and the external frame 38B are
connected to each other through the spring lower portions 311B
around the Y axis. By this structure, all the components formed in
the device layer B are integrated together. Note that a film made
of gold, aluminum or the like may be formed on a surface of the
mirror plate 30B.
[0052] The components of the electrostatic comb actuators formed in
the device layer T include upper fixed combteeth 32T1 around the X
axis, lower fixed combteeth 32T2 around the X axis, an inner upper
base 35T1, an inner lower base 35T2, left fixed combteeth 37T1
around the Y axis, right fixed combteeth 37T2 around the Y axis, an
outer upper base 39T1, an outer lower base 39T2, an outer left base
310T1, an outer right base 310T2, an upper spring upper portion
312T1 around the Y axis and a lower spring upper portion 312T2
around the Y axis.
[0053] The upper fixed combteeth 32T1 around the X axis, the inner
upper base 35T1, the upper spring upper portion 312T1 around the Y
axis and the outer upper base 39T1 are integrated together. The
lower fixed combteeth 32T2 around the X axis, the inner lower base
35T2, the lower spring upper portion 312T2 around the Y axis and
the outer lower base 39T2 are integrated together. The left fixed
combteeth 37T1 around the Y axis and the outer left base 310T1 are
integrated together. The right fixed combteeth 37T2 around the Y
axis and the outer right base 310T2 are integrated together.
Voltages independent of each other can be set respectively for the
integrated components.
[0054] For the device layer B, a silicon wafer having a fine mirror
surface can be used as in the case of the first embodiment. A
material obtained by bonding the device layer B and the handle
layer H with the insulating layer I interposed therebetween can be
provided for manufacturing of the MEMS-scanning mirror device.
Alternatively, an SOI wafer may also be used, in which the device
layer B, the insulating layer I and the handle layer H are
previously integrated together. A surface of the device layer B in
the SOI wafer can be finished in a fine mirror surface state.
[0055] In manufacturing of the MEMS-scanning mirror device by using
any of the materials described above, the mirror plate 30B having
good specularity can be achieved by using the fine mirror surface
of the device layer B as it is.
[0056] In the MEMS-scanning mirror device of the second embodiment
too, as shown in FIGS. 2A and 2B, a surface 30BS of the mirror
plate 30B is located below a top surface TOPS formed in the device
layer T.
[0057] In a case of the conventional MEMS-scanning mirror having
the electrostatic comb actuators, a mirror surface is formed on a
top surface and, after formation of a mirror plate, subsequent
processing is performed while exposing the mirror surface of the
mirror plate to the outside. On the other hand, in a case of the
MEMS-scanning mirror device according to this embodiment, the
surface 30BS of the device layer B can be protected by the
insulating layer I such as a silicon oxide film until immediately
before completion of the device, as in the case of the first
embodiment. Thus, in steps of mask formation, etching, mask
removal, film formation and the like to be described later, the
surface 30BS of the mirror plate 30B can be prevented from being
scratched during handling or the like and from being
contaminated.
[0058] The movable combteeth 31B around the X axis are inserted
between the upper fixed combteeth 32T1 around the X axis and
between the lower fixed combteeth 32T2 around the X axis. When a
voltage is applied between the movable combteeth 31B and the upper
and lower fixed combteeth 32T1 and 32T2, a torque around the X-X'
axis is generated in the movable combteeth 31B around the X axis,
that is to say, the mirror plate 30B so as to lower electrostatic
energy.
[0059] Moreover, the movable combteeth 36B around the Y axis are
inserted between the left fixed combteeth 37T1 around the Y axis
and between the right fixed combteeth 37T2 around the Y axis. When
a voltage is applied between the movable combteeth 36B and the left
and right fixed combteeth 37T1 and 37T2, a torque around the Y-Y'
axis is generated in the movable combteeth 36B around the Y axis,
that is to say, the internal frame 34B having the mirror plate 30B
so as to lower electrostatic energy.
[0060] The MEMS-scanning mirror according to the second embodiment
can be driven around the two axes, the X-X' axis and the Y-Y' axis.
It is preferable that the entire device layer B be grounded and the
MEMS-scanning mirror device be driven by applying voltages Vx1,
Vx2, Vy1 and Vy2 to the upper and lower fixed combteeth 32T1 and
32T2 around the X axis and the left and right fixed combteeth 37T1
and 37T2 around the Y axis, respectively.
[0061] According to the second embodiment, any of the following
drive systems can be adopted, including DC drive for the both two
axes, DC drive for one axis and resonant drive for the other axis,
and resonant drive for the both two axes. For example, when Vx1,
Vx2, Vy1 and Vy2 are all DC voltages, DC drive is adopted for the
both two axes. Alternatively, when all the voltages are AC
voltages, resonant drive is adopted for the both two axes. Still
alternatively, for example, when Vx1 and Vx2 are DC voltages and
Vy1 and Vy2 are AC voltages, it is possible to achieve DC drive
around the X-X' axis and resonant drive around the Y-Y' axis.
Third Embodiment
[0062] FIGS. 3A to 3C show schematic views of a third embodiment of
an MEMS-scanning mirror having one set of stepped electrostatic
comb actuators according to the present invention. In the third
embodiment, as in the case of the second embodiment, a mirror plate
can be rotated around two axes, an X-X' axis and a Y-Y' axis, but
drive around the Y-Y' axis is limited to resonant drive.
[0063] The MEMS-scanning mirror also has a three-layer structure as
in the cases of the first and second embodiments, and includes an
uppermost device layer T, a lowermost handle layer H and a device
layer B therebetween. The device layer T and the device layer B are
made of n type or p type doped conductive silicon. For the handle
layer H, insulating or conductive silicon or glass is used. These
layers described above are electrically insulated from each other
by insulating layers I. The insulating layers I are preferably a
silicon oxide when the handle layer H is made of silicon.
[0064] In the handle layer H, a hollow HO is formed. Components of
the electrostatic comb actuators, which are formed in the device
layers T and B, respectively, are located above the hollow HO. The
components of the electrostatic comb actuators, which are formed in
the device layers T and B, will be described while attaching
subscripts to the symbols T and B denoting the respective
layers.
[0065] The components of the electrostatic comb actuators formed in
the device layer B include a mirror plate 40B, movable combteeth
41B around the X axis, springs 43B around the X axis, an internal
frame 44B, movable combteeth 46B around the Y axis, left fixed
combteeth 47B1 around the Y axis, right fixed combteeth 47B2 around
the Y axis, an external frame 48B, spring lower portions 410B
around the Y axis, an outer left base 412B1, an outer right base
412B2 and grooves 413.
[0066] In the mirror plate 40B, a pair of the springs 43B around
the X axis extended along the X-X' axis are formed. Moreover, in
the mirror plate 40B, the movable combteeth 41B around the X axis
are formed in a direction perpendicular to the X-X' axis. These
components 40B and 41B are located inside the internal frame 44B.
The mirror plate 40B and the internal frame 44B are connected to
each other through the springs 43B around the X axis. Thus, the
mirror plate 40B is integrated with the internal frame 44B. In the
internal frame 44B, the movable combteeth 46B around the Y axis
perpendicular to the Y-Y' axis and a pair of the spring lower
portions 410B around the Y axis extended along the Y-Y' axis are
formed. In the external frame 48B, the grooves 413 are formed.
[0067] Thus, the outer left base 412B1 and the outer right base
412B2 are formed, which are insulated from the external frame 48B.
In the outer left and right bases 412B1 and 412B2, the left fixed
combteeth 47B1 around the Y axis and the right fixed combteeth 47B2
around the Y axis are formed, respectively.
[0068] The components 40B, 41B, 43B, 44B, 46B, 48B and 410B formed
in the device layer B are integrated together and electrically
equipotential.
[0069] The left and right fixed combteeth 47B1 and 47B2 around the
Y axis are connected respectively to the outer left and right bases
412B1 and 412B2 electrically isolated from the external frame 48B
by the grooves 413. Thus, voltages can be independently applied to
the external frame 48B and the outer left and right bases 412B1 and
412B2, respectively. Note that a film made of gold, aluminum or the
like may be formed on a surface of the mirror plate 40B.
[0070] The components of the electrostatic comb actuators formed in
the device layer T include upper fixed combteeth 42T1 around the X
axis, lower fixed combteeth 42T2 around the X axis, an inner upper
base 45T1, an inner lower base 45T2, an outer upper base 49T1, an
outer lower base 49T2, an upper spring upper portion 411T1 around
the Y axis and a lower spring upper portion 411T2 around the Y
axis.
[0071] The upper fixed combteeth 42T1 around the X axis, the inner
upper base 45T1, the upper spring upper portion 411T1 around the Y
axis and the outer upper base 49T1 are integrated together. The
lower fixed combteeth 42T2 around the X axis, the inner lower base
45T2, the lower spring upper portion 411T2 around the Y axis and
the outer lower base 49T2 are integrated together. Voltages
independent of each other can be set respectively for the
integrated components.
[0072] For the device layer B, a silicon wafer having a fine mirror
surface can be used as in the case of the second embodiment. A
material obtained by bonding the device layer B and the handle
layer H with the insulating layer I interposed therebetween can be
provided for manufacturing of the MEMS-scanning mirror device.
Alternatively, an SOI wafer may also be used, in which the device
layer B, the insulating layer I and the handle layer H are
previously integrated together. A surface of the device layer B in
the SOI wafer can be finished in a fine mirror surface state.
[0073] In manufacturing of the MEMS-scanning mirror device by using
any of the materials described above, the mirror plate 40B having
good specularity can be achieved by using the fine mirror surface
of the device layer B as it is.
[0074] In the MEMS-scanning mirror device of the third embodiment
too, as shown in FIGS. 3A and 3B, a surface 40BS of the mirror
plate 40B is located below a top surface TOPS defined by the device
layer T. In a case of the conventional MEMS-scanning mirror having
the electrostatic comb actuators, a mirror surface is formed on a
top surface and, after formation of a mirror plate, subsequent
processing is performed while exposing the mirror surface of the
mirror plate to the outside. On the other hand, in a case of the
MEMS-scanning mirror device according to this embodiment, the
surface 40BS of the device layer B can be protected by the
insulating layer I such as a silicon oxide film until immediately
before completion of the device, as in the case of the first
embodiment. Thus, in steps of mask formation, etching, mask
removal, film formation and the like to be described later, the
surface 40BS of the mirror plate 40B can be prevented from being
scratched during handling or the like and from being
contaminated.
[0075] The movable combteeth 41B around the X axis are inserted
between the upper fixed combteeth 42T1 around the X axis and
between the lower fixed combteeth 42T2 around the X axis. When a
voltage is applied between the movable combteeth 41B and the upper
and lower fixed combteeth 42T1 and 42T2, a torque around the X-X'
axis is generated in the movable combteeth 41B around the X axis,
that is to say, the mirror plate 40B so as to lower electrostatic
energy. Moreover, the left and right fixed combteeth 47B1 and 47B2
around the Y axis are located so as to form interdigital patterns
with the movable combteeth 46B around the Y axis. When an AC
voltage is applied between the movable combteeth 46B and the left
and right fixed combteeth 47B1 and 47B2, oscillation around the
Y-Y' axis is excited in the internal frame 44B having the mirror
plate 40B.
[0076] The MEMS-scanning mirror according to the third embodiment
can be driven around the two axes, the X-X' axis and the Y-Y' axis.
It is preferable that the entire device layer B be grounded and the
MEMS-scanning mirror (device) be driven by applying voltages Vx1,
Vx2, Vy1 and Vy2 to the upper and lower fixed combteeth 42T1 and
42T2 around the X axis and the left and right fixed combteeth 47B1
and 47B2 around the Y axis, respectively. Here, while the fixed
combteeth 42T1 and 42T2 and movable combteeth 41B around the X axis
form a pair of stepped electrostatic toothed combs, the fixed
combteeth 47B1 and 47B2 and movable combteeth 46B around the Y axis
form a pair of non-stepped electrostatic toothed combs.
Accordingly, only resonant drive can be adopted around the Y
axis.
[0077] In the third embodiment, it is possible to adopt any of the
following drive systems including a DC drive around the X-X' axis,
resonant drive around the Y-Y' axis, resonant drive for the both
two axes, or the like. For example, when Vx1 and Vx2 are DC
voltages and Vy1 and Vy2 are AC voltages, obtained are DC drive
around the X-X' axis and resonant drive around the Y-Y' axis.
Alternatively, when all the voltages are AC voltages, resonant
drive is adopted for the both two axes.
Fourth Embodiment
[0078] With reference to FIGS. 4A to 4E, description will be given
of an example of a method for manufacturing an MEMS-scanning mirror
having one set of stepped electrostatic comb actuators according to
the first embodiment. Each of schematic views shown in FIGS. 4A to
4E corresponds to the cross-section taken along the line A-A' in
FIG. 1.
[0079] Note that the method for manufacturing the MEMS-scanning
mirror can be applied to methods for manufacturing the
MEMS-scanning mirrors according to the second and third
embodiments.
1. Preparation Step
[0080] FIG. 4A is a cross-sectional view taken along the line A-A'
in FIG. 1, showing a wafer MM before formation of structures
corresponding to components of the MEMS-scanning mirror.
[0081] In the wafer MM, the device layer B and the handle layer H
are integrally formed with the insulating layer I interposed
therebetween. A surface BS of the device layer B is previously
mirror-finished before processing. The device layer B and the
handle layer H are preferably made of silicon and the insulating
layer I is a silicon oxide film. Here, a silicon oxide film
(insulating layer I) is also formed on a surface HS of the handle
layer H.
[0082] As the wafer MM, a commercially available
silicon-on-insulator (SOI) wafer can be used. In the silicon oxide
film (insulating layer I), a first alignment mark (not shown) to be
utilized in subsequent processing is formed. This first alignment
mark can be formed by: previously forming an alignment pattern on
the silicon oxide film (insulating layer I) by use of a
photosensitive resist; and then removing the silicon oxide film in
a portion not covered with the photosensitive resist, by etching
with hydrogen fluoride or buffered hydrogen fluoride. The
photosensitive resist can be removed by cleaning with acetone, for
example, after the etching.
2. Step of Forming Lower Layer Structure Corresponding to Lower
Layer Components of MEMS-Scanning Mirror in Device Layer B
[0083] In the device layer B, structures 10B', 11B', 13B', 14B' and
15B' corresponding respectively to the mirror plate 10B, the
movable combteeth 11B, the shafts 13B, the springs 14B and the
frame 15B are formed. FIG. 4B shows the structures 10B', 11B' and
15B'.
[0084] The processing described above can be performed by deep
reactive ion etching (DRIE) using the Bosch process after patterns
corresponding to the structures are previously formed with a
photosensitive resist on a surface of the device layer B by using
the first alignment mark. Note that FIG. 4B shows a state where the
photosensitive resist is removed after the DRIE.
3. Step of Bonding Silicon Wafer to Device Layer B
[0085] As shown in FIG. 4C, a silicon wafer 52 having silicon oxide
films (insulating layers I) formed on its surfaces is bonded to the
device layer B. This bonding is performed by bringing a top surface
of the device layer B and a surface of the silicon wafer 52 into
contact with each other and then heating the both to 1000.degree.
C. to 1100.degree. C., for example.
4. Step of Grinding and Lapping Silicon Wafer
[0086] A thickness of the silicon wafer 52 shown in FIG. 4C is
reduced by grinding and lapping the silicon wafer 52. Accordingly,
as shown in FIG. 4D, the upper device layer T having a
predetermined thickness is formed.
[0087] In a case of the conventional MEMS-scanning mirror, a
surface obtained by subjecting the top surface of the attached
wafer to grinding, lapping and polishing serves as a mirror.
[0088] Since a surface condition after polishing determines
specular performance (mirror performance), surface accuracy
tolerated in the conventional processing is stringent. Moreover,
when a hollow is formed below the silicon wafer attached to the
bottom of the silicon wafer subjected to the conventional
processing, particularly, below a portion corresponding to the
mirror plate, uniformity of the processing in the wafer surface is
impaired, which makes it difficult to obtain a mirror plate having
excellent specularity with good yield.
[0089] On the other hand, according to the embodiments of the
present invention, the mirror is formed not on the top surface of
the device but on the previously mirror-finished surface of the
wafer. Thus, polishing for mirror finishing can be omitted and
required accuracy of surface roughness tolerated in grinding and
lapping is reduced compared with the conventional case.
[0090] Moreover, as described later, the previously mirror-finished
surface of the wafer is protected by the silicon oxide film until
immediately before completion of the device. Thus, according to the
present invention, excellent specularity of the surface of the
mirror plate 10B is secured. Therefore, the MEMS-scanning mirror
device according to the present invention can improve manufacturing
yield.
5. Step of Forming Mask on Handle Layer H
[0091] To form a mask 51 to be used in an etching step for forming
a hollow in the handle layer H, a part of the silicon oxide film
(insulating layer I) on the surface HS of the handle layer H is
removed as shown in FIG. 4E. The first alignment mark is used for
formation of the mask 51. The mask 51 is obtained by: previously
forming a pattern on the silicon oxide film (insulating layer I) by
use of a photosensitive resist; and then removing the silicon oxide
film in a portion not covered with the photosensitive resist, by
etching with hydrogen fluoride or buffered hydrogen fluoride.
[0092] Note that the photosensitive resist is removed by cleaning
with acetone or the like after the etching. Note that the mask 51
used in the etching is formed so as not to cover a portion below
the structures corresponding to at least the mirror plate 10B and
the movable combteeth 11B.
[0093] Note that, in order to shorten the etching time in the step
of forming the mask 51 on the handle layer H, the mask 51 may be
previously formed in the preparation step 1. In this way, a thin
silicon oxide film (insulating layer I) in a mask opening caused in
the step 3 of bonding the silicon wafer to the device layer B can
be removed by brief etching.
6. Step of Forming Upper Layer Structure Corresponding to Upper
Layer Components of MEMS-Scanning Mirror in Device Layer T
[0094] Next, in the device layer T, structures 12T1', 12T2', 16T1'
and 16T2' corresponding respectively to the right fixed combteeth
12T1, the left fixed combteeth 12T2, the right base 16T1 and the
left base 16T2 are formed. FIG. 5A shows the structure 12T2'.
[0095] In this formation step, although stepped combteeth
structures (FIG. 5A shows 12T2' and 11B') are formed, the surface
of the mirror plate 10B is not yet exposed but protected by the
silicon oxide film (insulating layer I). Moreover, the handle layer
H exists below the structure corresponding to the mirror plate 10B
and maintains mechanical strength.
[0096] For the step of forming the upper layer structure
corresponding to the upper layer components of the MEMS-scanning
mirror in the device layer T, the same processing method as that
used for the step of forming the lower layer structure
corresponding to the lower layer components of the MEMS-scanning
mirror in the device layer B is used. Note that the first alignment
mark can be used also in this processing step.
[0097] However, when it is conceivable that the first alignment
mark is blurred by going through the processing from Step 2 to Step
5 and sufficient alignment accuracy cannot be obtained, an
unillustrated second alignment mark is formed in the device layer B
in Step 2 and a through-hole reaching the device layer B is formed
in the device layer T, in order for a circumference of the
through-hole to surround the second alignment mark by the Bosch
process using the first alignment mark, prior to Step 6. Thus, the
processing can be accurately performed by use of the second
alignment mark.
7. Step of Forming Hollow in Handle Layer H by Etching
[0098] The handle layer H is etched by the Bosch process using the
mask 51 formed in Step 5. Thus, as shown in FIG. 5B, the hollow HO
is formed below the lower layer structure corresponding to at least
the mirror plate 10B and the movable combteeth 11B in the
device.
[0099] Accordingly, in a rotation operation of the mirror plate 10B
and the movable combteeth 11B, collision thereof with the handle
layer H is avoided. In this Step 7, each of the movable combteeth
11B and the mirror plate 10B is still fixed by the silicon oxide
films (insulating layers I) and is not released.
8. Step of Removing Silicon Oxide Films
[0100] The silicon oxide films (insulating layers I) in exposed
portions formed on the surfaces of the handle layer H, having the
hollow formed in Step 7, and the device layers T and B are removed
by etching as shown in FIG. 5C. As an etchant, hydrogen fluoride
can be used. At this stage, the fixing of the movable combteeth 11B
and the mirror plate 10B by the silicon oxide films (insulating
layers I) is released. Thus, the MEMS-scanning mirror device is
completed.
9. Step of Forming Films on Surfaces of Device Layers B and T
[0101] On the surfaces of the device layers B and T, as shown in
FIG. 5D, films 53 such as gold, aluminum and dielectric multilayer
films are preferably formed by a method such as deposition and
sputtering. Thus, mirror reflectivity, that is, mirror
characteristic is improved. It is even more preferable to select a
material or optically design and employ a multilayer film having
suitable reflection and absorption characteristics, in accordance
with a wavelength of light to be reflected by the mirror.
[0102] According to the method for manufacturing the MEMS-scanning
mirror device having at least one set of stepped electrostatic comb
actuators according to the present invention, the mirror is formed
below the top surface and the surface 10BS of the mirror plate 10B
is not exposed until the silicon oxide films (insulating layers I)
are removed. Thus, the MEMS-scanning mirror device having excellent
specularity can be manufactured with good yield.
[0103] Furthermore, the etching using the Bosch process after Step
6 is performed in the state where the entire surface 10BS and back
surface of the mirror plate 10B are covered with the silicon oxide
films (insulating layers I). Accordingly, the entire surface 10BS
and back surface of the mirror plate 10B are protected by the
silicon oxide films (insulating layers I) during the etching. Thus,
the MEMS-scanning mirror having excellent specularity can be
manufactured with good yield.
[0104] Moreover, also in the step of forming the structures
corresponding to the components of the MEMS-scanning mirror in the
device layer T, the handle layer H is present below the mirror
plate 10B in the structure having a portion including at least
three layers of silicon. Accordingly, mechanical strength of the
portion of the mirror plate 10B is improved. Thus, the
MEMS-scanning mirror device having excellent specularity can be
manufactured with good yield.
[0105] According to the MEMS-scanning mirror device using the
electrostatic comb actuators and the manufacturing method thereof
according to the present invention, the mirror is formed at a
position below the top surface of the device and mirror surface
formation by grinding, lapping or polishing is not performed after
wafer bonding. Thus, good specularity of the mirror is
achieved.
[0106] Moreover, since no hollow is provided below the mirror plate
until immediately before completion of the device. Thus, damage or
scratches on the surface of the mirror plate caused during
processing can be prevented. Furthermore, the requirement for
finishing roughness of the top surface of the device can be
reduced. Therefore, the MEMS-scanning mirror having at least one
set of stepped electrostatic comb actuators with excellent
specularity can be manufactured with good yield.
[0107] Although the preferred embodiments of the present invention
have been described, it should be understood that the present
invention is not limited to those embodiments, but various changes
and modifications can be made to those embodiments.
* * * * *