Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2000-05-31
2001-08-07
Epps, Georgia (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C359S290000, C359S292000, C359S295000
Reexamination Certificate
active
06271955
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing spatial light modulator and electronic device employing it.
2. Description of the Related Art
This type of spatial light modulator is disclosed, for example Japanese Patent Application Laid-Open Nos. 4-230722, 5-188308, and 5-196880. An improved form of these devices is also described in the March 1994 issue of “Nikkei Microdevice” as a “DMD” (Digital Micromirror Device).
This DMD has, as shown in
FIG. 22
, a three-layer construction comprising an upper layer
800
, an intermediate layer
810
, and a lower layer
830
.
The upper layer
800
comprises a mirror
802
and a mirror support post
804
joined to the center of the lower surface of the mirror
802
. In connection with the fabrication process of the mirror
802
, in a position opposite to the mirror support post
804
is formed a depression
806
.
The intermediate layer
810
has a mirror support plate
812
which is coupled to the mirror support post
804
, and which is supported at opposite ends by hinges
814
so as to be able to be driven in an inclining manner. To provide the space for this mirror support plate
812
for driving in an inclining manner, the hinges
814
have on their lower sides hinge support posts
816
.
The intermediate layer
810
further is provided with first and second address electrodes
818
and
820
on opposing sides of the hinges
814
, each supported by electrode supporting posts
826
. Furthermore, outside this are provided a first mirror contact electrode
822
and second mirror contact electrode
824
, each supported by electrode supporting posts
826
.
The lower layer
830
comprises four electrodes
832
a
to
832
d
coupled to the electrode supporting posts
826
of the first and second address electrodes
818
and
820
, and a common electrode
834
coupled to the first and second mirror contact electrodes
822
and
824
.
This DMD, as shown in
FIG. 23
, has a bias voltage Va applied to the mirror
802
and the first and second mirror contact electrodes
822
and
824
. Then when for example a negative voltage is applied to the first address electrode
818
, and a positive voltage is applied to the second address electrode
820
, a Coulomb force acts between the mirror
802
and the first address electrode
818
, and the mirror
802
is driven to an inclined position as shown by the dot-dash line in FIG.
23
. By reversing the polarity of the voltage applied to the first and second address electrodes
818
and
820
, an inclined position as shown by the dot-dot-dash line in
FIG. 23
can be established.
The inclined position of the mirror
802
shown by a dot-dash line in
FIG. 23
is taken to be the “ON” position in which light is reflected toward a certain position, and the inclined position shown by a dot-dot-dash line is taken to be the “OFF” position in which light is reflected in a different direction. By varying the time between switches, a 256-gradation display can be obtained.
The DMD shown in
FIG. 22
is hypothetically manufacturable by a fabrication process as shown in
FIGS. 24A
to
24
H and
FIGS. 25A
to
25
F.
FIGS. 24A
to
24
H show the steps in the formation of the intermediate layer
810
on an already formed lower layer
830
, and
FIGS. 25A
to
25
F show the steps in the formation of the upper layer
800
on the intermediate layer
810
, and the formation of the interlayer spaces.
As shown in
FIG. 24A
, a substrate
840
on which an SRAM (static random access memory) is formed as the lower layer
830
is provided. Next, as shown in
FIG. 24B
, a resist
842
is coated on this substrate
840
, and in the stage shown in
FIG. 24C
a pattern corresponding to the hinge support posts
816
and electrode supporting posts
826
is formed.
As shown in
FIG. 24D
, an aluminum (Al) film is formed by vapor deposition over the surface of the resist
842
and trench portion, and then further as shown in
FIG. 24E
an aluminum oxide film
846
is formed over the surface.
Further after vapor deposition of an aluminum film
848
as shown in
FIG. 24F
, as shown in
FIG. 24G
a resist
850
is applied in a pattern. Thereafter, as shown in
FIG. 24H
, the aluminum film
848
is etched, whereby mirror support plate
812
, hinges
814
, and hinge support posts
816
are formed.
By the process shown in
FIGS. 25A
to
25
F, the upper layer
800
shown in
FIG. 22
is formed. For this purpose, as shown in
FIG. 25A
a resist
852
is applied thickly, and is formed in a pattern as shown in FIG.
25
B. Further, an aluminum film
854
is formed by vapor deposition, and after an aluminum oxide film
856
is formed over a part of the surface thereof, the extremities of the aluminum film
854
are removed by etching, whereby the mirror
802
and mirror support post
804
are formed. (See
FIGS. 25C
to
25
E.)
Finally, as shown in
FIG. 25F
, by removing the resist
842
and
852
, a space between the upper layer
800
and intermediate layer
810
is formed, and moreover a space between the intermediate layer
810
and lower layer
830
is formed.
However, in the above process, there is the problem that the DMD cannot be obtained with a high yield. One reason for this is that the factor determining the angle of inclination of the mirror
802
, that is, the distance between the lower surface of the mirror
802
and the mirror contact electrodes
822
and
824
depends on the thickness of the resist
852
in the resist step shown in FIG.
25
A.
In general, such a resist is formed by the spin coating method, and while it is difficult in itself to improve the uniformity of a resist layer thickness, when the spin coating method is used it is extremely difficult to make the resist
852
of a uniform thickness.
Moreover, in the conventional spin coating method, the larger the surface area of the wafer, the more difficult it is to ensure uniformity within the area of the resist film, and further to make the thickness of the resist film constant is for a large diameter semiconductor wafer almost impossible. Thus, it is difficult to form a plurality of devices simultaneously from a single semiconductor wafer, and the throughput is reduced.
In addition to the above problems, a further one is that in the stage of removing the resist shown in
FIG. 25F
, it is difficult to completely remove the resist from the furthest recesses of the underside of the mirror
802
and hinges
814
. If foreign objects are thus left behind, the mirror
802
and address electrodes
818
and
820
may short-circuit, or the inclination of the mirror may be obstructed, or the mirror contact electrodes
822
and
824
and address electrodes
818
and
820
may short-circuit.
Another problem with the above described construction of a DMD is that the depression
806
is formed in the center region of the mirror
802
. In the aluminum vapor deposition step of
FIG. 25C
, when aluminum is vapor deposited in the trench portion, the position opposing this trench is inevitably concave, and the forming of the depression
806
cannot be prevented.
In this three-layer DMD, since the hinges
814
are not in the same plane as the mirror
802
, the exposed surface area of the mirror
802
is increased, and the benefit is obtained of an increased light utilization ratio.
However, since the depression
806
is formed in the center of the large area mirror
802
, with this depression
806
in the line of a powerful beam of light, the light utilization ratio is actually reduced by the diffuse reflection. Alternatively, the diffusely reflected light may be input as information pertaining to another pixel, resulting in the problem of reduced image quality. Moreover, even if the side walls of the depression
806
are processed so as to be vertical, the area which is optically effective is reduced.
A further problem is that the above described spatial light modulator is formed on a substrate
840
on which an SRAM is formed, and the overall yield is the product of the yield of the SRAM and the yield of the spatial light modu
Atobe Mitsuro
Koeda Hiroshi
Yotsuya Shinichi
Epps Georgia
Seiko Epson Corporation
Spector David N.
Watson Mark P.
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