Optical: systems and elements – Deflection using a moving element – By moving a reflective element
Reexamination Certificate
2001-10-31
2004-09-21
Dunn, Drew A. (Department: 2872)
Optical: systems and elements
Deflection using a moving element
By moving a reflective element
C359S223100, C359S225100, C359S226200
Reexamination Certificate
active
06795225
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a micromirror unit used in optical apparatus for the purposes of changing the direction of light. In particular, it relates to a micromirror unit of the type which is advantageously incorporated in an optical disk apparatus (for writing to or reading data from an optical disk), an optical switching apparatus (for selectively connecting one optical fiber to another to provide a light passage), etc.
2. Description of the Related Art
A micromirror unit is provided with a reflective mirror member which is pivotable for changing the direction of reflected light. A popular technique for actuating the mirror member is to utilize electrostatic force. Micromirror units of this type (referred to as “static driving type” hereinafter) may have several structures. Such micromirror units are generally classified into two groups, depending on fabrication methods. One of the methods employs a “surface micro-machining” technique, whereas the other employs a “bulk micro-machining” technique.
In accordance with the surface micro-machining, patterned material layers in lamination may be formed on a base substrate, thereby providing required components such as a support, a mirror member and electrodes. In this layer forming process, a dummy layer (or sacrificial layer), which will be removed later, may also be formed on the substrate. A conventional micromirror unit of the static driving type by the surface micro-machining is disclosed in JP-A-7(1995)-287177 for example.
In accordance with the bulk micro-machining, on the other hand, a base substrate itself is subjected to etching, thereby providing required components such as a frame and a mirror forming base. Then, a mirror member and electrodes may be formed on the etched substrate by a thin-film forming technique. Micromirror units of the static driving type by the bulk micro-machining are disclosed in JP-A-9(1997)-146032, JP-A-9-146034, JP-A-10(1998)-62709 and JP-A-2000-13443.
One of the technically significant factors desired in a micromirror unit is a high flatness of the reflective mirror member. According to the above-mentioned surface micro-machining technique, however, the thickness of the resulting mirror member is rendered very small, so that the mirror member is liable to warp. To avoid this and ensure a high flatness, the mirror member should be made so small that its respective edges are less than 100 &mgr;m in length. In accordance with the bulk micro-machining, on the other hand, a rather thick substrate is processed, thereby providing a sufficiently rigid mirror forming base to support the mirror member. Thus, a relatively large mirror member having a high flatness can be obtained. Due to this advantage, the bulk micro-machining technique is widely used to fabricate a micromirror unit having a large mirror member whose edges are more than 100 &mgr;m in length.
FIG. 10
of the accompanying drawings shows an example of conventional micromirror unit fabricated by the bulk micro-machining technique. The illustrated micromirror unit
400
is of the static driving type, and includes a lamination of a mirror substrate
410
and a base substrate
420
. As shown in
FIG. 11
, the mirror substrate
410
includes a mirror forming base
411
and a frame
413
. The mirror forming base
411
has an obverse surface upon which a mirror member
411
a
is formed. The mirror forming base
411
is supported by the frame
413
via a pair of torsion bars
412
. The mirror forming base
411
has an reverse surface upon which a pair of electrodes
414
a
and
414
b
is formed. As shown in
FIG. 10
, the base substrate
420
is provided with a pair of electrodes
421
a
and
421
b
which faces the above-mentioned pair of electrodes
414
a
and
414
b
of the mirror forming base
411
.
With the above arrangement, the electrodes
414
a
,
414
b
of the mirror forming base
411
may be positively charged, whereas the electrode
421
a
of the base substrate
420
may be negatively charged. As a result, an electrostatic force is generated between these electrodes, thereby turning the mirror forming base
411
in the N
3
-direction shown in
FIG. 10
as the torsion bars
412
are being twisted. The rotation angle of the mirror forming base
411
is determined by the balance between the inter-electrode electrostatic force and the restoring force of the twisted torsion bars
412
. To rotate the mirror forming base
411
in the opposite direction, the other electrode
421
b
of the substrate
420
may be negatively charged. As readily understood, when the mirror forming base
411
is turned clockwise or counterclockwise, as required, the light reflected on the mirror member
411
a
is directed in the desired direction.
As noted above, the mirror forming base
411
is rotated through an angle which is defined by the balance between the inter-electrode electrostatic force and the restoring force of the twisted torsion bars
412
. Thus, it is possible to adjust the rotation angle of the base
411
by controlling the static electricity to be generated in correlation with the restoring force of the torsion bars
412
.
Generally, a micromirror unit is a structure whose minimum dimension is about several hundred micrometers. This is rather large size, and therefore the restoring force of the torsion bars tends to exceed the inter-electrode electrostatic force in strength. Thus, conventionally, the area of each electrode is rendered large (for generating a great electrostatic force), whereas each torsion bar is made uniformly thin along its length (for weakening the restoring force). In the prior art micromirror unit
410
(FIG.
11
), each torsion bar
412
has a constant small width L along the entire length.
In the above manner, however, the mirror forming base
411
is supported by the thin torsion bars
412
. Accordingly, it is difficult to hold the mirror forming base
411
stable (i.e., nonrotatable) about the normal N
3
(the line at right angles to the surface). If unstable about the normal N
3
, the mirror forming base
411
is liable to unduly swivel about the normal N
3
when the base
411
is supposed to rotate only about the axis defined by the torsion bars
412
. When such an unwanted swivel occurs, it is difficult or even impossible to precisely control the operation of the micromirror unit.
SUMMARY OF THE INVENTION
The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide a micromirror unit which does not suffer from the above-noted problems. Specifically, an object of the present invention is to provide a micromirror unit which is provided with torsion bars of reduced restoring force and still can exert excellent stability against undesired swiveling.
According to a first aspect of the present invention, there is provided a micromirror unit which includes: a first frame; a mirror forming base provided with a mirror surface; and a first torsion connector which includes a first end connected to the mirror forming base and a second end connected to the first frame. The torsion connector defines a first axis about which the mirror forming base is rotated relative to the first frame. The torsion connector has a width measured in a direction which is parallel to the mirror surface and perpendicular to the first axis. The width of the first torsion connector is relatively great at the first end and becomes gradually smaller from the first end toward the second end.
In a preferred embodiment, a micromirror unit further includes a second frame and a second torsion connector. The second torsion connector connects the second frame to the first frame and defines a second axis about which the first frame and the mirror forming base are rotated relative to the second frame.
In another preferred embodiment, the second torsion connector has a width measured in a direction which is parallel to the mirror surface and perpendicular to the second axis, wherein the width of the second torsion connecto
Mizuno Yoshihiro
Okuda Hisao
Sawaki Ippei
Tsuboi Osamu
Ueda Satoshi
Armstrong Kratz Quintos Hanson & Brooks, LLP
Dunn Drew A.
Fujitsu Limited
Pritchett Joshua L
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