Electrostatic actuator

Details Electrostatic actuator Electrostatic actuator

2002-10-23

2004-05-11

Jackson, Jerome (Department: 2815)

Active solid-state devices (e.g., transistors, solid-state diode

Responsive to non-electrical signal

C257S401000, C257S415000, C257S421000

Reexamination Certificate

active

06734512

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an electrostatic actuator which is manufactured using an MEMS (Micro Electro-Mechanical Systems) technique and, more particularly, to an electrostatic actuator which is applied to a micro switch for turning on or off a wide band signal frequency of from DC to several hundreds of GHz, a light switch for switching the direction of a light signal according to the inclination of the mirror, a scanner for switching the direction of a relevant wireless antenna, etc.
BACKGROUND OF THE INVENTION
A conventional technique will now be explained by taking up as an example thereof a technique and device that are described in a treatise entitled “A Micro-Machined Microwave Antenna Integrated with its Electrostatic Spatial Scanning” (Proceedings of IEEE Micro Electro Mechanical Systems, Nagoya, pp. 84-89, 1997), pronounced in the IEEE 10th Micro Electro Mechanical Systems International Conference by Dominique Chauver et al. of Tokyo Univ. LIMMS/SNRS-II.
A perspective view of this device is illustrated in FIG.
1
. In this device, a quartz substrate
610
is machined to form a torsional vibration plate
611
and springs
613
that support both ends of the vibration plate
611
. On the upside surface of the torsional vibration plate
611
, there is provided an upper electrode
612
consisting of a chrome/gold material, and this upper electrode
612
is electrically connected to a contact pad
614
through the intermediary of a wiring
615
, on the other hand, with respect to a silicon substrate
620
, there is formed an inclination structure
621
. Chauver et al. formed the inclination structure
621
having two inclined surfaces the angle of inclination of that is 35.3° by performing anisotropic wet etching with respect to a silicon substrate having a (110) Si crystal face. They formed two electrode patterns, lower electrodes
622
a
and
622
b
each made of chrome, respectively, on those two inclined surfaces. These lower electrodes
622
a
and
622
b
are respectively electrically connected to contact pads
624
a
and
624
b
. These quartz substrate
610
and silicon substrate
620
are bonded together in the way of being aligned with each other such that the torsional vibration plate
611
may be located over the inclination structure
621
(provided, however, that the method of bonding is not described).
Applying a voltage between the upper electrode
612
and the lower electrode
622
a
or
622
b
, due to the electrostatic attracting force an attractive force that acts toward the substrate (downside) occurs in the torsional vibration plate
611
. For this reason, the springs
613
are torsion-deformed (twisted), with the result that the torsional vibration plate
611
rotates about the springs
613
and gets inclined. By varying the voltage applied between the upper electrode
612
and the lower electrode
622
a
or
622
b
, it is possible to adjust the rotation angle of the torsional vibration plate
611
. Also, by selecting which of the lower electrodes
622
a
and
622
b
a voltage is applied to, it is possible to change the rotation direction of the torsional vibration plate
611
.
In this conventional technique, the application of the device to an antenna that changes the transmission direction or reception direction of a radio signal by varying the rotation direction of the torsional vibration plate
611
was stated. What is particularly noticeable is that by forming the lower electrode into an inclination structure it is possible to decrease the voltage that is applied. This is based on the principle that, since an electrostatic attracting force decreases in inverse proportion to the square of the distance between two structures, if the device can be designed so as to make small the distance between the upper electrode and the lower electrode, the voltage that is applied can be made small. When the rotation angle of the torsional vibration plate
611
is zero, a large electrostatic attracting force occurs between the upper electrode region and the lower electrode
622
a
/
622
b
region the lower electrode portion of that is provided at the position that is near the apex of the inclination structure
621
. As the torsional vibration plate
611
rotates, a large electrostatic attracting force also goes on occurring in the other regional portion, as well, of the lower electrode
622
a
/
622
b
. If the lower electrode
622
a
/
622
b
is provided on a flat surface having no inclination structure
621
, since the distance between the upper electrode and the lower electrode is large, a high level of voltage is needed for the purpose of rotating the torsional vibration plate
611
. Although Chauver et al. do not concretely state that effect of the inclination structure, calculating the electrostatic attracting force in relation to the inclination structure of 35.3°, it proved that the voltage that is applied can be decreased approximately 30% with respect to the flat structure.
Also, although Chauver et al. do not state, the second effect of the inclination structure
621
is to make more likely to occur the rotational movement about the springs
613
of the torsional vibration plate
611
. When applying a voltage between the upper electrode
612
and the lower electrode
622
a
/
622
b
, a force that acts toward the lower electrode occurs in the upper electrode
612
. However, in a case where the rigidity of the bending deformation of the springs
613
is smaller than the rigidity of the rotation (torsion), the tendency to deform toward the silicon substrate
620
side perpendicularly with respect thereto becomes more likely to occur than the tendency to rotate. The inclination structure
621
plays the role of preventing that perpendicular deformation and causing only the rotational movement alone to occur in the torsional vibration plate
611
.
FIGS. 2A
to
2
D are sectional views illustrating a method of manufacturing the structure on the silicon substrate side according to the above-described conventional technique. A silicon nitride film
72
a
and a silicon nitride film
72
b
are deposited on both surfaces, respectively, of a silicon substrate
71
the (110) Si crystal face of that serves as a principal surface by using a low-pressure vapor phase epitaxy (LP-CVD). And, with respect to one surface of them, patterning of the nitride film
72
a
is performed using a photolithography technique (the same figure A). This substrate is put into a 33% solution of KOH, thereby performing anisotropic etching with respect to the silicon substrate
71
. As a result of this, an inclination structure
73
having an inclination of 35.3° with respect to the flat surface is formed (the same figure B). Subsequently, on the surface of the silicon substrate having this inclination structure
73
, by sputtering, a silicon oxide film is deposited. A metal mask
76
is disposed on this resulting substrate, then chrome is deposited. At this time, through the openings formed in the metal mask
76
, the chrome is deposited on the inclination structure, thereby a lower electrode
75
can be formed (the same figure C). Thereafter, again, by sputtering, a silicon oxide film
77
is deposited on the chrome lower electrode
75
(the same figure D). Finally, a torsional vibration plate formed by machining a quartz substrate is bonded onto that silicon substrate
71
, thereby the device illustrated in
FIG. 1
is manufactured.
In this conventional technique, the torsional vibration plate had a dimension of 1×2×0.1 mm. Especially, for the reason why the torsional vibration plate having a width as great as 2 mm is designed to be inclined ±10, it was necessary to construct so that the height of the inclination structure may be equal to or more than 175 &mgr;m. For forming the lower electrode pattern on the substrate having a level difference that is as great as that height, Chauver et al. adopted the chrome deposition method utilizing a metal mask
76
such as that illustrated in FIG.
2
C. However, due to the existence of a clearance between

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