Method of manufacturing three-dimensional structure and...

Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Physical stress responsive

Reexamination Certificate

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C438S052000, C438S738000, C216S099000

Reexamination Certificate

active

06730534

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-223479, filed Jul. 24, 2001, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing a three-dimensional structure including a plurality of portions different in thickness, for example, an oscillator applied to an optical scanner.
2. Description of the Related Art
U.S. Pat. No. 6,188,504 discloses an optical scanner including a three-dimensional structure called as an oscillator, which is produced by selectively etching a semiconductor substrate.
FIG. 18
shows the configuration of this optical scanner.
As shown in
FIG. 18
, the optical scanner comprises an oscillator
510
having a movable plate
512
, a support frame
514
surrounding the movable plate
512
, and a pair of elastic members
516
connecting the movable plate
512
and the support frame
514
, a coil
522
extending along the periphery of the movable plate, a pair of wires
524
extending through the elastic members
516
, respectively, a pair of feeding pads
526
formed on the support frame
514
, and magnets
532
fixed to the support frame
514
. The movable plate
512
has a reflecting surface
528
formed thereon to reflect a beam of light. The wires
524
have ends connected to the ends of the coil
522
and the other ends connected to the feeding pads
526
. The elastic members
516
comprise an insulating elastic film such as polyimide resin. The insulating elastic film extends also over the movable plate
512
to function as an interlayer insulating film, which insulates the coil
522
from the wires
524
.
In
FIG. 18
, when an AC voltage is applied to the pair of feeding pads
526
, an AC current flows through the coil
522
. Then, Lorentz force is generated owing to the interaction between the current flowing through the coil
522
and magnetic fields generated by the magnets
532
. Thus, the movable plate
512
is subjected to a couple of forces exerted around an axis thereof passing through the interior of the elastic members
516
. The directions of these forces depend on the direction of the current flowing through the coil
522
. Since the AC current flows through the coil
522
, the movable plate
512
oscillates around the axis passing through the interior of the elastic members
516
. The oscillation of the movable plate
512
scans a beam of light reflected by the reflecting surface
528
of the movable plate
512
.
Now, the process steps of manufacturing the oscillator
510
of this optical scanner
510
will be described with reference to
FIGS. 19
to
22
.
First, as shown in
FIG. 19
, a silicon nitride film
544
is formed on the major surfaces (top and bottom surfaces) of a silicon substrate
542
. Then, the silicon nitride film on the bottom surface side is selectively etched to form a mask
548
used to form the movable plate and the support frame.
Then, as shown in
FIG. 20
, the coil
522
, a polyimide film
552
, the wires
524
and feeding pads
526
, a polyimide film
554
, and a polyimide etching mask
556
are sequentially formed on the silicon nitride film on the top surface side of the silicon substrate
542
. An end of each of the wires
524
is electrically connected to a corresponding end of the coil
522
through a corresponding one of via holes formed in the polyimide film
552
.
Subsequently, as shown in
FIG. 21
, with the top surface side of the silicon substrate
542
sealed, the silicon substrate
542
is selectively etched through the mask
548
from the bottom surface side with TMAH (Tetramethyl ammonium hydroxide) or the like, so that its portion that is not covered by the mask
548
is removed, to form the movable plate
512
and the support frame
514
.
Furthermore, the polyimide films
552
and
554
are etched through the polyimide etching mask
556
to form the elastic members
516
(see FIG.
22
). Finally, the polyimide etching mask
556
and the remaining silicon nitride film
544
and
548
are removed to obtain the oscillator
510
for an optical scanner, shown in FIG.
22
.
In the oscillator
510
for an optical scanner, shown in
FIG. 22
, the thickness of each of the movable plate
512
and the support frame
514
is always the same as that of the silicon substrate
542
, as is apparent from the method of manufacturing the oscillator. If the movable plate
512
is miniaturized, i.e. a dimension of the movable plate
512
such as the width W or length A thereof is reduced, the area of the coil decreases relatively to the volume of the movable plate
512
. Consequently, the oscillator or scanner is less efficiently driven.
As the movable plate
512
is miniaturized, the dimension of the movable plate
512
such as the width W or length A thereof approaches the thickness of the silicon substrate
542
. Accordingly, the movable plate
512
is shaped like a block as shown in FIG.
23
. As a result, the position of the center of gravity
564
of the movable plate deviates from an oscillation axis
562
. That is, as the movable plate
512
is miniaturized, the distance D from the oscillation axis
562
to the position of the center of gravity increases. This may cause unwanted vibration modes to be generated during driving.
Further, with the TMAH-based etching, which is most commonly applied to silicon etching, etching speed varies depending on the plane direction of silicon. Accordingly, the plane direction of silicon is selected according to the shape of a structure to be produced. A wafer with a plane direction (
100
) is used to form the movable plate described previously. In this case, the sides of the movable plate are tapered as shown in FIG.
23
. The width WL or length LL of the top surface of the movable plate, in which the coil is formed, equals the width WS or length LS, respectively, of a mask
566
used to form the movable plate plus double the width (length) LT of the tapered portion. Therefore, the etching process with TMAH does not enable the formation of a movable plate in which the width WL or length LL of the top side is smaller than 2×LT.
The value of LT depends on the thickness of the silicon substrate. Accordingly, a thin silicon substrate
542
may be used to form a small movable plate. However, the thin silicon substrate
542
is not stiff and is thus not strong enough for handling during production. Further, the support frame
514
formed is as thick as the silicon substrate
542
used. Consequently, the produced oscillator
510
is not strong and is thus difficult to handle.
Therefore, with the method of manufacturing the oscillator
510
(three-dimensional structure) described previously, it is difficult to produce a very small movable plate (member).
BRIEF SUMMARY OF THE INVENTION
It is a main object of the present invention to provide a method of manufacturing a three-dimensional structure, the method allowing portions different in thickness to be formed by a single etching process.
It is another object of the present invention to provide a method of manufacturing a three-dimensional structure, such as an oscillator, the method allowing a very small member, such as a movable plate, to be formed by wet etching.
It is yet another object of the present invention to provide a method of manufacturing an oscillator, the method allowing production of an oscillator that can be efficiently driven even with a small movable plate.
It is still another object of the present invention to provide a method of manufacturing an oscillator, which hardly generates unwanted vibration modes even with a small movable plate.
The present invention provides a method of manufacturing a three-dimensional structure (for example, an oscillator applied to an optical scanner or an acceleration sensor) having portions different in thickness, the method comprising: forming a laminated structure, which comprises at least two layers to be processed and at least one inner mas

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