Optical: systems and elements – Deflection using a moving element – By moving a reflective element
Reexamination Certificate
1999-09-23
2001-07-10
Schuberg, Darren (Department: 2872)
Optical: systems and elements
Deflection using a moving element
By moving a reflective element
C359S225100, C359S872000
Reexamination Certificate
active
06259548
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a micro-mirror device, which can be used as a scanning mirror to be incorporated, for example, in a light scanning type sensor for form recognition, a bar code reader, or a laser printer, and to a method for producing such a micro-mirror device.
2. Description of the Prior Art
FIG. 17
is a schematic plan view of the micro-mirror device in the prior art.
FIG. 18
is a cross sectional view of the micro-mirror device in
FIG. 18
, along the line
18
—~of
FIG. 7. A
mirror portion
101
is disposed on a surface of a mirror forming substrate
102
. The mirror portion is formed by a thin aluminum layer or a thin gold layer. The mirror forming substrate
102
can rotate around its center axis. A torsion beam
103
extends along the center axis of the mirror forming substrate
102
. The torsion beam
103
is supported by a pair of anchors
104
, which are fixed to a base substrate
106
. A pair of driving electrodes
105
are disposed on the base substrate
106
.
There are gaps of distance g
0
between the driving electrodes
105
and the mirror forming substrate
102
. A voltage is applied to either of the driving electrodes
105
, so that the mirror forming substrate
102
is driven to rotate by the electrostatic force. The mirror forming substrate
102
, torsion beam
103
and the anchors
104
are made from, for example, single crystal silicon, poly-crystalline silicon, or nickel plating. The base substrate
106
is made from, for example, silicon or glass.
The function of the micro-mirror device in the prior art is explained below.
When a voltage is applied to either of the driving electrodes
105
, an attracting force is generated between the mirror forming substrate
102
and the driving electrode
105
, depending on the voltage and the electrostatic capacity between them. The mirror forming substrate
102
rotates around its center portion, until the mirror portion
101
inclines at an angle &thgr;s (scanning angle). The mirror portion
101
can be driven to rotate and swing simultaneously, when a voltage, for example, a superposition of a biasing direct voltage Vdc and alternating voltages Vac, having phases difference of 180 degree to each other, as shown in
FIG. 19
, is applied to the driving electrodes
105
. The scanning angle of the mirror portion
101
and the scanning angle of a light beam can be controlled, depending on the imposing voltages.
When the micro-mirror device in the prior art is used, the theoretical maximum scanning angle &thgr;smax of the scanning angle &thgr;s is given by the following mathematical expression (1):
sin (&thgr;smax)=
g
0
/L
(1)
where L is a distance between the center and the side end of the mirror forming substrate
102
, as shown in FIG.
18
. For example, assuming that L is 1 mm and the required maximum scanning angle &thgr;smax is 15 degrees, the necessary distance go of the gap is calculated to be 259 &mgr;m, using this mathematical expression (1).
However, the mirror portion
101
in an actual micro-mirror device in the prior art can not be rotated up to the full span, i.e., the theoretical maximum scanning degree. This is because the relation between the distance g
0
of the gap and the electrostatic force to rotate the mirror portion
101
is non-linear, when the electrostatic force is used. Specifically, because the magnitude of the electrostatic attractive force is proportional to the inverse square of the distance g
0
of the gap, when the inclination angle of the mirror forming substrate
102
is large and the distance go of the gap between the mirror forming substrate
102
and one of the driving electrodes
105
is small, the electrostatic attractive force between the mirror forming substrate
102
and the driving electrode
105
becomes larger than the restoring force due to the torsion of the torsion beam
103
at a large torsion angle. As a result, the mirror forming substrate
102
is fixed to one of the driving electrodes
105
and does not move. This phenomenon is called “Pulled-in Phenomenon”. For avoiding this phenomenon, the scanning angle &thgr;s of a light beam of the mirror portion
101
is restricted to be within a stable region, which is, in general, about a half of the theoretical maximum scanning angle &thgr;smax.
The micro-mirror device in the prior art has the drawback that it is difficult to increase the maximum scanning angle &thgr;smax, due to the Pull-in Phenomenon.
The other drawback of the micro-mirror device in the prior art is that it is difficult to design a micro-mirror device, which can scan at a wide range of scanning angle of a light beam, a low driving voltage is used. Because, assuming that the characteristics of the torsional oscillation, for example, shear modulus or the Q-value of the oscillation are constant, the range of a stable scanning angle and the corresponding driving voltage are determined by the size of the mirror forming substrate
102
with the mirror portion, and the distance g
0
of the gap between the mirror forming substrate
102
and the driving electrodes
105
, which are disposed under the mirror forming substrate
102
.
SUMMARY OF THE INVENTION
An object of the present invention is to eliminate the drawbacks in the micro-mirror device in the prior art.
Another object is to propose a micro-mirror device, which can scan the light beam in an increased scanning angle, using a low driving voltage.
Another object is to propose a method for producing such a micro-mirror device.
The objects are attained by a micro-mirror device according to the present invention, comprising a driving frame which is separated from the mirror forming substrate, and the mirror forming substrate is not driven directly, but is driven indirectly through the driving frame.
More specifically, the object is attained by a micro-mirror device according to the present invention
1
comprising: a supporting substrate; a mirror forming substrate, on which a mirror portion is formed, a pair of first torsion beams disposed on a pair of the opposing sides of the mirror forming substrate, which are perpendicular to those sides and support the mirror forming substrate; a first driving frame surrounding at least one side of the mirror forming substrate and connected to the mirror forming substrate through a first link beam, which is disposed in parallel to the longitudinal direction of the first torsion beams; and a first driving force generating means for driving the first driving frame to move so that the movement is transmitted to the mirror forming substrate through the first link beam.
In an embodiment of the present invention, the ends of the first torsion beams are supported by a pair of first anchor portions projecting from the supporting substrate.
In an embodiment of the present invention, the ends of the first torsion beam are supported by the first driving frame.
In an embodiment of the present invention, further comprising: a second driving frame; a second driving force generating means for driving the second driving frame to move; wherein the ends of the first torsion beams are supported by the first driving frame; the first driving frame has third torsion beams, disposed on one side of the first driving frame opposing to the first link beam, perpendicular to the first torsion beam; the ends of the third torsion beams are supported by a pair of third anchor portions projecting from the supporting substrate; the second driving frame is connected to the first driving frame through a second link beam, which is disposed at one side of the second driving frame in parallel to the longitudinal direction of the third torsion beams.
In an embodiment of the present invention, the first driving frame has a pair of second torsion beams disposed on a pair of the opposing sides of the first driving frame, the second torsion beam are parallel to the first torsion beam; and the ends of the second torsion beam are supported by a pair of second anchor portions projecting from the supporting substrate.
In an em
Fujita Hiroyuki
Konno Nobuaki
Mita Makoto
Mita Yoshio
Toshiyoshi Hiroshi
Leydig, Voit& Mayer, L
Mitsubishi Denki & Kabushiki Kaisha
Schuberg Darren
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