Microactuator

Details Microactuator Microactuator

1993-06-29

2002-10-29

Budd, Mark O. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Charge accumulating

Reexamination Certificate

active

06472794

ABSTRACT:

II. BACKGROUND OF THE INVENTION
The present invention relates to a microactuator that can be fabricated according to the IC fabrication processes characterized by etching and lithography, for example, and can be used as a micropositioner for the multiple probe head of a scanning probe microscope and the pickup head for a signal recording and reproduction equipment.
As a first prior art example of a microactuator, there is an electrostatic micro wobble motor introduced in a paper authored by Mehregany et al. (“Operation of microfabricated harmonic and ordinary side-drive motors”, Proceedings of the third IEEE Workshop on Micro Electro Mechanical Systems, Napa Valley, Calif., USA, Feb. 11-14, 1990, pp. 1-8)
FIG. 50
is a schematic plan view to show how this prior art electrostatic micro wobble motor is structured and
FIG. 51
is a cross-sectional view of the foregoing motor.
In FIG.
50
and
FIG. 51
, item
1
is a bearing, item
2
is a rotor of about 100 &mgr;m in diameter and items
3
a
through
3
h
are eight electrodes arranged around the Periphery of the rotor
2
. (On a photograph of the motor in the above referenced paper, there are 12 static poles observed.) Although not shown in the drawings, these electrodes are connected by wires with a voltage supply source and can be applied with voltages arbitrarily selected.
As indicated in
FIG. 50
, the rotor
2
is shaped like a ring and between the inner circumference thereof and the bearing
1
there exists a clearance C. Therefore, in contrast to an ordinary motor, the rotor
2
does not rotate around the bearing
1
with the bearing serving as the axis of rotation. As voltages are applied to electrodes
3
a
through
3
h
in succession, the rotor
2
revolves since it is attracted sequentially towards the excited electrodes
3
a
through
3
h.
At the same time, however, since the rotor
2
moves while it is in rolling contact with the bearing
1
at the contact point of
2
a,
the rotor
2
rotates by the difference between the outer circumference of the bearing
1
and the inner circumference of the rotor
2
. A detailed description on this performance will be made later.
The rotor
2
is held by a flange
1
a
so that it does not slip off from the bearing
1
. The electrodes
3
a
through
3
h
(only
3
a
and
3
e
are shown in
FIG. 51
) are almost of the same height as the rotor
2
. On the bottom surface of the rotor
2
, there are a plurality dot-like mounds
2
b,
not a ring-like mound, which slide on and contact electrically with a shield layer
4
.
FIG.
52
through
FIG. 56
are the cross-sectional illustrations to show the fabrication processes (a) through (e), respectively, for the electrostatic micro wobble motor, which will be described hereunder. The fabrication processes employ the ordinary IC fabrication methods such as etching, lithography or the like.
(a) As illustrated in
FIG. 52
, an insulating layer
6
is first formed on a silicon substrate
5
by depositing in succession an oxide layer of 1 &mgr;m in thickness grown thermally and a silicon nitride layer of 1 &mgr;m in thickness formed by means of a low pressure chemical vapor deposition method (LPCVD).
Then, a polysilicon thin film of 3500 Å thick with phosphorus diffused sufficiently therein is formed by LPCVD and patterning is applied thereto to complete an electric shield layer
4
.
Further, a low temperature oxide layer (LTO)
7
of 2.2 &mgr;m thick is deposited to make a first sacrificial layer and then patterning is applied by 2 steps, the one for forming a base
7
a
of the electrodes
3
a
through
3
h
and the other for forming a hollow
7
b
in preparation of creating a mound
2
b
on the bottom of the rotor
2
.
(b) As illustrated in
FIG. 53
, a polysilicon layer of 2.5 &mgr;m thick diffused with phosphorus sufficiently is deposited by LPCVD and then the rotor
2
and the electrodes
3
a
through
3
h
(only
3
a
and
3
e
are shown here) as indicated in FIG.
50
and
FIG. 51
were formed by means of a reactive ion etching method (RIE). As shown in
FIG. 53
, the electrodes
3
a
through
3
h
are fixed on the silicon substrate
5
and a plurality of the mound
2
b
are formed on the bottom of the rotor
2
. On account of a thermal oxidation layer after patterning used as the mask for the reactive ion etching of the foregoing polysilicon layer, the thickness of the rotor
2
as well as the electrodes
3
a
through
3
h
is approximately 2.2 &mgr;m at this stage.
(c) As illustrated in
FIG. 54
, an LTO layer
8
to make a second sacrificial layer of about 0.3 &mgr;m thick is deposited for retaining the clearance C between the bearing
1
and the rotor
2
. At the same time, an anchor
8
a
for the bearing
1
is formed by patterning.
Although the diameter of the bearing
1
is about 36 &mgr;m, the smallest possible diameter is 26 &mgr;m due to the restrictions imposed by the process employed here.
(d) As illustrated in
FIG. 55
, a polysilicon layer of 1 &mgr;m thick with phosphorus diffused sufficiently is deposited by LPCVD and the bearing
1
provided with the flange
1
a
is formed.
(e) As illustrated in
FIG. 56
, the LTO layers
7
and
8
serving as the first and second sacrificial layers respectively are dissolved by buffered hydrogen fluoride (HF) and the rotor is released completely to realize the structure as shown in FIG.
51
.
The operational principle of the prior art electrostatic micro wobble motor having a structure as described above will be explained in the following with the help of FIG.
50
. As stated before, the rotor
2
does not rotate around the bearing
1
with the bearing serving as the axis of rotation. Instead, the rotor revolves as it is attracted by the excited electrodes
3
a
through
3
h
sequentially and at the same time it rotates by the difference between the outer circumference of the bearing
1
and the inner circumference of the rotor
2
while it is in rolling contact with the bearing
1
at the contact point
2
a.
In other words, suppose the electrodes are excited in the direction X as indicated in
FIG. 50
in an order of the electrodes
3
a,
3
b,
3
c
and so forth, then the rotor
2
is first attracted by the excited electrode
3
a.
Next, it will be attracted by the electrode
3
b
and then by the electrode
3
c
and so forth, resulting in revolving of the rotor
2
also in the direction X.
Since the clearance C between the rotor
2
and the bearing
1
is set up to be smaller than the gap between the rotor
2
and the electrodes
3
a
through
3
h,
there will be the contact point
2
a
where the rotor
2
will come into contact with the bearing
1
. Besides, the correct gap between the rotor
2
and the electrodes
3
a
through
3
h
corresponds to G+E as indicated in
FIG. 50
, where the E means an effective gap length that produces the motor's torque, and the following relationship is established inherently from the structure of the motor:
E=G−C>
0
(This is obvious when the state of the electrode
3
e
being excited has been observed.) As the gap between the rotor
2
and the electrodes
3
a
through
3
h,
G will be dealt with for convenience because of the possibilities in reducing the effective gap length E to the minimum.
Now, as the rotor
2
revolves in the direction X, the contact point
2
a
also is to move likewise in the direction X. Since the bearing
1
is fixed in position, slipping at the contact point
2
a
by the amount of the difference between the outer circumference of the bearing
1
and the inner circumference of the rotor
2
is taking place unless the rotor
2
revolves. However, attracting force is applied to the rotor
2
in the direction of pressing the bearing
1
and practically any slipping hardly occurs at the contact point
2
a.
Therefore, as the rotor
2
revolves in the same direction as the shifting direction (the direction of X) of the voltages applied to the electrodes
3
a
through
3
h,
the rotor
2
is consequently to rotate in the same direction (the X direction of
FIG. 50
) by the amount corresponding t

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