Optical: systems and elements – Optical modulator – Light wave temporal modulation
Reexamination Certificate
2001-07-11
2004-08-24
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Optical modulator
Light wave temporal modulation
C291S027000
Reexamination Certificate
active
06781742
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a deformable mirror device (DMD), and more specifically, the present invention relates to a method of driving a DMD.
2. Description of the Related Art
The development of high intensity, high definition flat panel displays (FPDs) has been advancing in recent years. Displays such as liquid crystal displays, EL (electroluminescence) displays, and plasma displays can be given as examples of FPDs.
Furthermore, in addition to the above FPDs, digital micromirror devices (hereafter referred to as DMDs) have been in the spotlight. Techniques related to DMDs have been disclosed by Texas Instruments, Inc., in patent applications such as: Japanese Patent Application Laid-open No. Hei 5-150173, Japanese Patent Application Laid-open No. Hei 5-183851, Japanese Patent Application Laid-open No. Hei 7-240891, Japanese Patent Application Laid-open No. Hei 8-334709, Japanese Patent Application Laid-open No. Hei 8-227044, Japanese Patent Application Laid-open No. Hei 8-051586, and Japanese Patent Application Laid-open No. Hei 8-227044.
A plurality of micromirrors approximately 16 &mgr;m×16 &mgr;m in size are formed having a pitch of 17 m&mgr; on a CMOS SRAM formed on a silicon substrate, and each of the micromirrors corresponds to a screen pixel, in the DMD. The number of micromirrors reaches 480,000 for SVGA, 786,000 for XGA, and 1,300,000 for SXGA.
The angle of the micromirror changes by an angle &thgr; with respect to the substrate if a digital signal having image information (digital video signal) is input to the SRAM of the DMD, in accordance with an electric field effect due to voltage from the SRAM. If the angle of the micromirror with respect to the substrate changes by an amount &thgr; (where 0<&thgr;<90°), then light from a light source is separated into two directions when reflected in the micromirror. One of the lights separated into two directions is absorbed by a light absorber, while the other arrives at a screen and forms an image.
Note that the term digital signal denotes a signal having two voltage valuesin this specification. Of the two voltage values, the higher is indicated by the term HI, while the lower is indicated by the term LO.
Schematic diagrams of a structure of a general DMD pixel are shown in
FIGS. 20A and 20B
.
FIG. 20A
is a perspective diagram of a DMD pixel, and
FIG. 20B
is a cross sectional diagram of the DMD pixel of
FIG. 20A. A
plurality of pixels are formed on a substrate
901
, and each pixel has a first electrode (first address electrode)
902
a
, a second electrode (second address electrode)
902
b
, landing sites
903
, a micromirror
904
, a hinge
905
, and hinge support posts
906
.
The angle of the micromirror
904
with respect to the substrate
901
is changed by an amount &thgr; with the hinge
905
acting as a rotational axis. Thehinge
905
is fixed on the substrate
901
by the hinge posts
906
.
A portion of the micromirror
904
contacts the landing site
903
when the micromirror
904
is inclined to an angle greater than &thgr; with respect to thesubstrate with the hinge
905
as an axis of rotation. The landing site
903
is maintained at the same electric potential as that of the mirror
904
, or has insulating properties.
The electric potential of a digital video signal input to the pixel is imparted to the first address electrode
902
a
. Further, the digital video signal is inverted with electric potential of ground as a standard point, and the inverted signal is imparted to the second address electrode
902
b
as an inverted digital video signal.
A fixed electric potential (standard electric potential) is imparted to the micromirror
904
. The micromirror
904
is then inclined by an angle &thgr; to thefirst address electrode
902
a
side if the electric potential difference between the standard electric potential and that of the digital video signal is greater than the size of the electric potential difference between the standard electric potential and that of the inverted digital video signal. Conversely, if the electric potential difference between the standard electric potential and that of the digital video signal is smaller than the size of the electric potential difference between the standard electric potential and that of the inverted digital video signal, then the micromirror
904
is inclined by an angle &thgr; to the second address electrode
902
b
side.
Digital light processing (DLP) with a projector using a DMD having the above structure differs from a projector using liquid crystals, and there is no light loss from a polarizing plate, and the aperture ratio is equal to or greater than 90%; the efficiency of utilizing light is therefore high. Further, this is a reflective type device, differing from general transmission type liquid crystal panels, and therefore the spacing between pixels, namely the spacing between the micromirrors, is small at about 0.8 &mgr;m, and a high definition image can easily be obtained even a projection is enlarged on the screen. In addition, no thermal problem develops like that of liquid crystal panels using thin film transistors because DMDs have superior cooling efficiency, and it is possible to use a high power light source, and therefore making projectors high definition becomes easy.
A drive circuit of a pixel in a conventional DMD is shown next in FIG.
21
. Reference numeral
911
denotes a data driver, reference numeral
912
denotes a scanning driver, and reference numeral
914
denotes a pixel portion. The pixel portion
914
has a plurality of pixels
913
.
The digital driver
911
inputs a digital video signal into a plurality of data lines
918
, and the scanning driver
912
inputs a scanning signal into a plurality of scanning lines
917
. Regions having one data line
918
and one scanning line
917
correspond to the pixels
913
for the case of the DMD shown by FIG.
21
.
The pixels
913
each have a switching transistor
915
, a SRAM
916
having a plurality of transistors. A gate electrode of the switching transistor
915
is connected to the scanning line
917
. Further, one of a source region and a drain region of the switching transistor
915
is connected to the data line
918
, and the other is connected to an input terminal Vin of the SRAM
916
and to the first address electrode
902
a.
Note that the term SRAM denotes a static RAM having no transfer gates throughout this specification. If HI input is imparted to the SRAM, then LO output is obtained, and if LO input is imparted to the SRAM, then HI output is obtained. Conversely, if a HI output is imparted to the SRAM, then a LO input is obtained, and if a LO output is imparted to the SRAM, then a HI input is obtained.
Note that, throughout this specification, the term transistor denotes an electric field effect transistor, which functions as a switching element.
An out put terminal Vout of the SRAM
916
is connected to the second address electrode
902
b
. Further, Vddh denotes a high voltage side electric power source, and Vss denotes a low voltage side electric power source.
The switching transistor
915
is selected in the DMD shown in
FIG. 21
by the scanning signal input to the scanning line
917
from the scanning driver
912
. Note that, in this specification, the term selection of a wiring denotes a state in which all transistors whose gate electrode is connected to the wiring are on.
The digital video signal is then input to the data line
918
from the data driver
911
. The input digital video signal is input to the input terminal Vin of the SRAM
916
, and to the first address electrode
902
a
, through the switching transistor
915
in an ON state. The digital video signal input to the input terminal Vin of the SRAM
916
is inverted, with the ground electric potential as a standard, and is then output from the output terminal Vout as an inverted digital video signal, and input to the second address electrode
902
b.
If the digital video signal and the inverted digital video signal are input to
Koyama Jun
Yamazaki Shunpei
Fish & Richardson P.C.
Schwartz Jordan M.
Semiconductor Energy Laboratory Co,. Ltd.
Stultz Jessica
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